extfieldsofstudy: Biology Medicine

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Mutations in THAP1/DYT6 reveal that diverse dystonia genes disrupt similar neuronal pathways and functions

Dystonia is characterized by involuntary muscle contractions. Its many forms are genetically, phenotypically and etiologically diverse and it is unknown whether their pathogenesis converges on shared pathways. Mutations in THAP1 [THAP (Thanatos-associated protein) domain containing, apoptosis associated protein 1], a ubiquitously expressed transcription factor with DNA binding and protein-interaction domains, cause dystonia, DYT6. There is a unique, neuronal 50-kDa Thap1-like immunoreactive species, and Thap1 levels are auto-regulated on the mRNA level. However, THAP1 downstream targets in neurons, and the mechanism via which it causes dystonia are largely unknown. We used RNA-Seq to assay the in vivo effect of a heterozygote Thap1 C54Y or ΔExon2 allele on the gene transcription signatures in neonatal mouse striatum and cerebellum. Enriched pathways and gene ontology terms include eIF2α Signaling, Mitochondrial Dysfunction, Neuron Projection Development, Axonal Guidance Signaling, and Synaptic LongTerm Depression, which are dysregulated in a genotype and tissue-dependent manner. Electrophysiological and neurite outgrowth assays were consistent with those enrichments, and the plasticity defects were partially corrected by salubrinal. Notably, several of these pathways were recently implicated in other forms of inherited dystonia, including DYT1. We conclude that dysfunction of these pathways may represent a point of convergence in the pathophysiology of several forms of inherited dystonia.

Introduction
Dystonia is a brain disorder that causes disabling involuntary muscle contractions and abnormal postures. When this is the only feature, it is termed isolated dystonia. The pathogenic molecular mechanisms underlying the neuronal dysfunction that leads to dystonia remain to be elucidated and current treatments are unsatisfactory. The advent of next generation sequencing is rapidly expanding the list of genes causing isolated dystonia, including dominant mutations in THAP1 (DYT6), TOR1A (DYT1), GNAL (DYT25), ANO3 (DYT24), CIZ1 (DYT23) and TUBB4A (DYT4), and recessive mutations in HPCA (DYT2), COL6A3 (DYT27) and PRKRA (DYT16) [1][2][3][4][5][6], although some of these are still pending confirmation. Apart from rare inherited defects in dopamine synthesis [7], there is no known biological pathway that causally links genetic forms of dystonia. Phenotypic similarities between some inherited dystonias, including the most common DYT1 and DYT6, may suggest a shared underlying pathogenic mechanism. Uncovering such mechanisms would be a significant milestone, and potentially widely applicable for therapeutic development.
DYT6 is caused by mutations in THAP1 [Thanatos-associated (THAP) domain-containing apoptosis-associated protein] [8,9], encoding a ubiquitously expressed transcription factor [10,11]. Similar to DYT1, caused by a mutation in TOR1A encoding TorsinA, the disease phenotype is restricted to the central nervous system despite widespread expression of the mutated protein. Disease-causing mutations in THAP1 are dispersed throughout the coding regions, but most are missense and located in the DNA-binding domain (DBD) [12]. THAP domain DNA-binding activity is zinc-dependent, and the four metal-coordinating residues of the C2CH module are crucial for functional activity [13]. Nonsense mutations, equivalent to a null allele, likely result in the generation of small mRNA species that are subject to nonsense-mediated decay [8]. Little is known about the biological function of THAP1, particularly in neurons, although there is a neuron-specific DNA-binding THAP1-like-immunoreactive species [10]. There is also an alternatively spliced form lacking Exon2 which functionally does not substitute for the full-length protein [14] and is normally present at very low levels in the brain [15].
DYT6 is inherited in an autosomal dominant manner with reduced penetrance. Few brains from DYT6 patients have been examined and, to date, they do not exhibit any characteristic neuropathologic lesions [16]. Structural and functional neuroimaging in DYT6 manifesting and non-manifesting carriers (NMCs) demonstrates abnormalities in cerebello-thalamo-cortical and cortico-striato-pallido-thalamo-cortical pathways [17]. Genetically engineered mice with heterozygote Thap1 mutations, either C54Y or ΔExon2, display structural abnormalities of the deep cerebellar nuclei and deficits on motor tasks without overt dystonia [18]. Both mutations are early embryonic lethal when homozygote [18], and in mouse embryonic stem cells, lead to decreased viability and neuroectodermal differentiation [14]. The cell cycle is the major dysregulated pathway that emerged from microarray assays of HUVECs with up-or down-regulation of THAP1 and of human lymphoblasts harboring a disease-associated intronic variant of THAP1 [19,20], but was not enriched in ES cells [14].
To study the role of Thap1 in brain, we performed unbiased transcriptomic, RNA-Seq profiling of postnatal day 1 striatal and cerebellar tissue in two genetic mouse models of THAP1/ DYT6 harboring mutations that alter the DNA binding domain, either (1) Thap1 C54Y , a constitutive knock in (KI) of the C54Y causative mutation in the DNA binding domain (DBD) of THAP1 and (2) Thap1 -, a constitutive knockout (KO) of exon 2 (ΔExon2) [18]. This was followed by functional studies to validate dysregulated molecular pathways with a focus on those that were either "top hits" and/or overlapped with other dystonias. We, and others, [21] have found that molecular abnormalities are present in the developmental stage, but their relationship to the emergence and persistence of the phenotype remains to be determined.

Differential gene expression analysis in the striatum and cerebellum of Thap1 +/and Thap1 C54Y /+ vs wild-type mice
Thap1 C54Y /+ KI mice carry a point mutation in one of three cysteine residues that are part of the zinc binding motif [19], altering binding of THAP1 to DNA [8,22]. Thap1 KO mice, i.e. ΔExon2, deletes part of the DBD, and is referred to herein as Thap1 +/- [18]. We performed RNA-Seq analysis of cerebellar and striatal tissue dissected from postnatal day 1 (P1) Thap1 +/-(ΔExon2) , Thap1 C54Y/+ and wild-type (WT) controls (N = 4/genotype, all males). At this age, neurons outnumber glia and the limited studies in Ruiz et al. [18] showed greater changes in mRNA levels than in the adult. There was a higher number of differentially expressed genes (DEGs) in Thap1 +/than in Thap1 C54Y/+ in both structures (Fig 1 and S1-S4 Tables). 55 striatal DEGs overlapped between genotypes (Fig 1C, S5 Table) and 35 overlapped in the cerebellum (Fig 1F, S5 Table). The highest ranked DAVID Gene Ontology (GO) terms for the striatal overlapping DEGs were positive regulation of signal transduction, proteasomemediated ubiquitin-dependent protein catabolic process and lipid storage, while those for overlapping DEGs in the cerebellum included cellular response to amino acid stimulus, and DNA-templated transcription. Among the overlapping DEGs, Cdh4 and Phf13 (up-regulated in striatum), Wibg and Rsph1 (down-regulated in striatum), Ppan (differentially regulated in cerebellum), and Nid2 (down-regulated in cerebellum) contain presumptive THABS motifs (S5 Table), supporting the notion that Thap1 may act as either an activator or repressor [13].
To determine if any of the DEGs are linked to dystonia or related disorders, we crossmatched them with the Emory University genetic dystonia panel (http://geneticslab.emory. edu/tests/MM550) and with the Mount Sinai Genetic Testing Laboratory Movement Disorders and Neuromuscular Disease Panel (S6 Table). We found 59 cross-matched striatal Thap1 +/-DEGs and 5 Thap1 C54Y/+ DEGs. Cryab and Fus appeared in both sets. In cerebellum, we identified 54 cross-matched DEGs in the Thap1 +/and 7 in Thap1 C54Y/+ , including 3 overlapping genes in both genotypes, Thap1, Lama1 and Sacs.
As the RNA for this study was derived from whole tissue, we utilized an RNA-sequencing transcriptome and splicing database annotating glia, neurons, and vascular cells from the mouse brain [23], in order to investigate the cell subtype origin of the DEGs in the striatum and cerebellum of the Thap1 +/or Thap1 C54Y/+ vs WT mice (S7 Table). The largest cell-type fraction of both the up-and down-regulated genes in the Thap1 +/striatum were neuronal, while the greatest fraction of the up-and down-regulated genes in the cerebellum were Venn diagrams show the number of overlapping DEGs (total, up-regulated or down-regulated) between Thap1 +/and Thap1 C54Y/+ relative to WT striatum. Diagrams show number of total DEGs as well expressed in endothelial and oligodendrocyte progenitor cells, respectively. The largest celltype fraction of both the up-and down-regulated genes in the Thap1 C54Y/+ striatum were neuronal and astrocytic in origin, while most of the up-and down-regulated genes in the cerebellum were derived from endothelial and microglial cells (S7 Table). It should be noted that P1 mice will have a greater enrichment for cell growth and gene regulatory pathways. Zhang et al. [23] showed that oligodendrocyte-lineage cell isolation does not occur until P17, the earliest time point when the full collection of oligodendrocyte-lineage cells is present. Therefore, one potential limitation of this study is age at which the transcriptomes were assayed but it is also a strength in terms of looking at altered pathways during this critical period.
Ingenuity Pathway Analysis (IPA) and DAVID GO terms identified biological functions and pathways enriched in DEGs from each dataset. The highest ranked IPA canonical pathways and GO Terms for each set of DEGs are shown in Fig 2 and S1 and S2 Tables. There were marked genotype-dependent differences, but there were overlaps between striatum and cerebellum within each genotype. Based on what were either the most significantly enriched pathways and terms, and/or those which are related to identified abnormalities in DYT6 and DYT1 models, we chose to functionally explore the eIF2α pathway, neuron projection development, synaptic plasticity [long term depression (LTD) and potentiation (LTP)], and mitochondrial Complex I.
To determine if any of the DEGs from the RNA-Seq datasets were bound directly by THAP1, we compared them against two publicly available THAP1 ChIP-Seq datasets, one from human ENCODE K562 cells, and the other from mouse ES cells [14]. The Thap1 +/cerebellum had the greatest number of overlapping genes when compared to both ChIP-Seq datasets, with a total of 32 overlapping genes when compared to the mouse ES cell data and 39 overlapping genes when compared to the K562 dataset (S11 Table). The highest ranking biological functions enriched in the 32-member gene set as identified by DAVID GO are: cellular macromolecules metabolic process, ribosomal small subunit assembly, and negative regulation of protein kinase activity (S11 Table). The highest ranking biological functions in the 39-member gene set as identified by DAVID GO are: cellular process, cellular metabolic process and gene expression (S11 Table).

Thap1 mutant mice show evidence of abnormal eIF2α signaling
The eIF2α signaling pathway was one of the top differentially regulated signaling pathways in the striatum and cerebellum of Thap1 +/mice. The eIF2α pathway is a key effector of the cellular response to several stressors, including the accumulation of misfolded proteins in the endoplasmic reticulum (ER), and was linked to TorsinA function soon after TOR1A was identified as the causative gene in DYT1 [24][25][26]. DYT16 is caused by mutations in PRKRA, a stress-activated modulator of the eIF2α kinase PKR, with evidence of abnormal phosphorylation of PKR and eIF2α in patient fibroblasts under ER stress [27,28]. In addition, a proteomics-based study identified abnormal eIF2α pathway activation in DYT1 mouse and rat brains, which correlated with assays in human brains [24]. Therefore, given the previous evidence suggesting a role of eIF2a signaling dysregulation in dystonia, and our own RNA-seq data, we investigated UPR genes and proteins to assay the baseline status of the UPR in Thap1 recombinant mice. as the number of up-or down-regulated genes in the (D) Thap1 +/cerebellum vs WT (E) Thap1 C54Y/+ cerebellum vs WT. (F) Venn diagrams show the number of overlapping DEGs (total, up-regulated or down-regulated) between Thap1 +/and Thap1 C54Y/+ relative to WT cerebellum. Cogged gears in panels A, B, D and E represent the number and the direction of the differentially expressed genes for each genotype and brain region, as follows: up-regulated genes (turquoise color, upward right arrow); down-regulated genes (dark blue color, downward left arrow); total [number] of genes (purple color, downward right arrow). https://doi.org/10.1371/journal.pgen.1007169.g001 Initially, we assayed the relative mRNA levels for members of the eIF2α signaling pathway in P1 Thap1 +/cerebellum and striatum by RT-qPCR. We assessed changes in genes known to play a role in UPR or eIF2α signaling pathways using DEGs found to be dysregulated directly from the RNA-seq analysis (eIF3K, eIF2a, eIF4A, eIF4B), as well as upstream and downstream mediators of eIF2α (BiP, Chop, Rsp6, XBP1s, and total XBP1) in Fig 3A. There was dysregulated expression of most components of this signaling pathway in cerebellum (S9 Table), and in the Thap1 +/striatum significant differences were observed in Atf4 expression, and XBP1s/ total XBP1 ratios (Fig 3, S9 Table). These data show that there are baseline abnormalities of the eIF2α signaling pathway; however, their contribution to the genotype-dependent phenotypes remains to be determined.
The eIF2α signaling pathway is involved in translational regulation, and notably, the DEGs were enriched for a second translational control pathway mediated by mTOR. These kinase cascades regulate protein function via phosphorylation and protein levels. We assayed  of key genes from the eIF2α signaling pathway were assayed with quantitative real-time PCR (RT-qPCR) using cerebellar samples. Data normalized relative to WT. Data are presented as means ± Standard Error of the Mean (SEM); n = 8 for each genotype and brain region with separate littermate WT controls, Student's t test ( Ã p<0.05; ÃÃ p<0.01; ÃÃÃ p<0.005). See also S9 Table. (B) Levels of protein expression in the striatum and cerebellum of P1 THAP1 +/mice relative to WT were assayed by western blot. Densitometry measurements (arbitrary units) are normalized to the housekeeping gene GAPDH, or for phosphoproteins, relative to GAPDH and their respective holoprotein level. Data are presented as means ± SEM; n = 4 for each genotype and region with separate, littermate WT controls, Student's t test. Ã p<0.05. See also S9 Table. https://doi.org/10.1371/journal.pgen.1007169.g003 Mutations in THAP1/DYT6 disrupt similar neuronal pathways and functions components of both pathways at basal level by western blotting of protein lysates derived from P1 Thap1 +/striatum and cerebellum. Consistent with the RNA-Seq and RT-qPCR data, the key effector of the eIF2α pathway, ATF4, was reduced by 25% in the striatum (Fig 3, right  panel). Overall, the data lend further support to the presence of dysfunction of the eIF2α pathway in DYT6 brain. Notably, screening designed to identify genes implicated in the response to ER stress in human B cells via genetic interactions identified THAP1 as the top hit and all changes were at the level of protein interactions [29].
To assay the function of the eIF2α pathway in the presence of a Thap1 mutation, we challenged P4 Thap1 +/and WT pups with tunicamycin, which induces ER stress and the unfolded protein response (UPR) in liver, cerebral cortex, and cerebellum at this age [30]. There was a clear engagement of the main ER chaperone, BiP, in striatum of Thap1 +/mice, and cerebellum of both genotypes (Fig 4). We detected genotype-dependent differences in the expression levels of ATF4 at basal level (dextrose-only controls) in the striatum of Thap1 +/mice as compared WT mice, and the most notable genotype-dependent difference following challenge with tunicamycin was a decrease in ATF4 striatal expression in Thap1 +/but not in WT mice (Fig 4). No differences were observed amongst the different groups when we assessed p-eIF2α/eIF2α and p-eIF2α/GAPDH expression levels (S3 Fig). Therefore, we could not find consistent up or down changes in the UPR with Thap1 baseline status and tunicamycin treatments. Nonetheless, our data suggest a dysregulation of the eIF2α signaling pathway. Therefore, we went on to perform physiologic/functional experiments in Figs 5-7.

Synaptic plasticity is impaired in Thap1 +/and Thap1 C54Y/+ mouse striatum
Synaptic plasticity is a specific neuronal function predicted by IPA and GO to be disrupted in the striatum of Thap1+/-mice, particularly long-term depression (LTD) and the related phenomenon of synaptic depotentiation (S1 Table). This association is again reminiscent of the deficit in striatal LTD in mouse models of DYT1 [34,35], suggesting that this might be a convergent feature among different types of dystonia and might even be related to dysfunction of the eIF2α pathway [26]. Although differential gene expression for Thap1 C54Y/+ striatum was less predictive of a synaptic plasticity phenotype (S2 Table), we tested whether persistent synaptically-induced forms of plasticity at glutamatergic synapses-both LTD and long-term potentiation (LTP)-were altered in the striatum of both mouse lines. Extracellular recordings in acute slices from Thap1 +/mice stimulated with high-frequency synaptic stimulation (HFS) revealed greatly reduced LTD, similar to that reported for DYT1 mouse models (Fig 5A 1 , S9 Table). In contrast, LTP was normal in Thap1 +/striatum (Fig 5A 2 ). The plasticity phenotype was different for the Thap1 C54Y/+ mice: LTD showed a non-significant trend towards enhancement (Fig 5B 1 , S9 Table), but LTP was reduced (Fig 5B 2 , S9 Table).
The potential for plasticity at inhibitory synapses could distort the interpretation of our striatal field recordings. For example, reduced LTD at excitatory synapses is also consistent with increased GABA-A mediated currents. To address this possibility, we repeated the LTD experiment in the presence of the GABA-A antagonist gabazine (S2 Fig), and confirmed that LTD was reduced in slices from Thap1 +/mice. Moreover, the Thap1 C54Y/+ mutation led to significantly enhanced LTD, in agreement with the trend observed in the absence of gabazine (Fig 5B 1 , S9 Table).
These results indicate that synaptic plasticity is susceptible to the deletion of Thap1 Exon2 or expression of a DYT6-related mutation of Thap1, while the specific nature of the disruption differs between these two manipulations. Note that paired-pulse ratio was not affected in either Tunicamycin challenge in P4 Thap1 +/and WT pups increases eiF2α signaling pathway proteins and reveals genotype-dependent differences in striatum and cerebellum. Western blot analysis of (A) striatal and (B) cerebellar lysates from Thap1 +/and WT littermates for BiP, ATF4 and CHOP were performed 24 hrs after subcutaneous tunicamycin (TM) diluted in 150mM dextrose (or dextrose-only control; DEX). Protein expression levels represent normalization to the housekeeping gene GAPDH. Data are presented as means ± SEM; n = 5 for each genotype and region, data normalized to WT (dextrose-only) controls. Statistical differences were assessed by two-way ANOVAs with Tukey's post hoc tests. Ã p < 0.05; ÃÃ p < 0.01; ÃÃÃ p < 0.001. See also S9 Table. https://doi.org/10.1371/journal.pgen.1007169.g004 the Thap1 +/or Thap1 C54Y/+ striatum (Fig 5C, S9 Table), indicating that presynaptic function was normal. However, basal synaptic efficiency was enhanced in Thap1 C54Y/+ mice [stimulus strength to evoke 1 mV field excitatory postsynaptic potential (EPSP): 0.88 ± 0.11 μA (n = 24) for wildtype, 0.87 ± 0.10 μA (n = 21) for Thap1 +/-, and 0.49 ± 0.05 μA (n = 23) for Thap1 C54Y/+ ; S9 Table]. The increased efficiency in Thap1 C54Y/+ striatum would be consistent with elevated In Thap1 C54Y/+ mice, LTD was not significantly reduced (B 1 ), but LTP was deficient (B 2 ; p < .05). Note that wildtype data are the same as for panel A, and that all genotypes were analyzed together. (C) Paired-pulse ratio was not altered in Thap1 +/and Thap1 C54Y/+ mice. The traces show averaged EPSPs recorded at inter-stimulus interval = 50 ms (thin and thick lines show responses to first and second stimuli, respectively). All graphs show group means ± SEM, and the number of slices/mice for each group are shown in parentheses. Data were analyzed by ANOVAs performed over the final 5 minutes of recording (panels A and B) or on averaged paired-pulse data for each interval (panel C), followed where appropriate by Newman-Keuls post-hoc tests. Ã p<0.05 See also S9 Table. https://doi.org/10.1371/journal.pgen.1007169.g005 synapse number. Alternatively, postsynaptic function could be up-regulated in Thap1 C54Y/+ mice. Since striatal LTP at excitatory synapses is expressed postsynaptically [36], basal upregulation of postsynaptic function might limit the extent to which these synapses can be further potentiated, consistent with the reduced LTP seen in these mice.
These extracellular recordings reveal plasticity phenotypes that must be expressed by a substantial fraction of MSNs. Future patch-clamp experiments, performed on MSNs expressing either the D1 or D2 receptor, will be useful to determine if the Thap1 +/and Thap1 C54Y/+ mutations differentially affect LTD or LTP in MSN subtypes.

Thap1 +/deficiency in mGluR-LTD is normalized by inhibiting the eIF2α phosphatase
Normal regulation of eIF2α by phosphorylation is required for multiple forms of LTD. In the striatum, inhibition of the eIF2α kinase PERK prevents synaptically-induced LTD [26], and eIF2α has been implicated in hippocampal LTD induced by pharmacological stimulation of metabotropic glutamate receptors (mGluR-LTD) [37,38]. To test whether the LTD deficits in Thap1 +/striatum could be due to dysregulation of eIF2α, we evaluated if abnormal LTD were rescued by Sal003, a selective inhibitor of the eIF2α phosphatase (Fig 6). We found differential effects of Sal003 on mGluR-LTD (induced by the group 1 agonist DHPG) and synapticallyinduced LTD (induced by high-frequency stimulation; HFS). Sal003 restored mGluR-LTD to wildtype levels (Fig 6A, S9 Table), in agreement with the finding that eIF2α phosphorylation is required for this form of plasticity in the hippocampus [37]. However, Sal003 had no effect on HFS-induced LTD (Fig 6B, S9 Table). This finding contrasts with the ability of Sal003 to restore LTD in DYT1 mutant mice [26], indicating that Thap1 deletion can interfere with LTD by affecting signaling mechanisms in addition to eIF2α dysregulation. Since HFS-induced LTD in striatum depends on the synaptic activation of mGluRs [39,40], it is possible that, independent of its effect on eIF2α signaling,Thap1 deletion interferes with the ability of HFS to activate postsynaptic mGluRs. In fact, we identified numerous dysregulated genes that participate in synaptic transmission and conduction of nerve impulses in Thap1 +/striatum (S1 Table).

Neurite outgrowth is decreased in Thap1 +/and Thap1 C54Y/+ striatal neurons in vitro
Neurite development was another enriched pathway identified across regions in Thap1 +/mice. TorsinA has a proposed role in neuritogenesis, and diffusion tensor imaging abnormalities detected by MRI are seen in different brain regions of patients with DYT6, DYT1 and other dystonias [41][42][43]. Signaling by the Rho family of GTPases is the top pathway in the Thap1 C54Y/+ striatum, and this pathway plays a critical role in neuritogenesis and axonal pathfinding [44]. To investigate the influence of Thap1 on this process, we assayed neurite development in striatal MSNs in vitro. We cultured striatal E16 (embryonic day 16) neurons from individual Thap1 +/and WT embryos and quantified neurite length after 24 hours. Thap1 +/striatal neurons exhibited shorter processes as compared to WT controls. The phenotype was present but less severe in Thap1 C54Y/+ striatal neurons (Fig 7, S9 Table). The total number of cells was equal between the genotypes, suggesting normal survival of the heterozygote mutant neurons following plating at equal densities.

Discussion
We used an unbiased RNA-Seq approach to identify dysregulated genes and pathways in mice harboring either a Thap1 C54Y (disease causing) or a ΔExon2, i.e. null allele. A major goal was to determine if these pathways and resultant phenotypes overlap with abnormalities observed in other dystonias, particularly DYT1 [45], due to their clinical, electrophysiological, structural and functional neuroimaging similarities. RNA-Seq was performed in two brain regions, striatum and cerebellum, consistently implicated in dystonia pathogenesis [46][47][48][49][50], and at a developmental time point when Thap1 levels and transcriptional abnormalities peak [10,15,18], supporting the notion that dystonia is a developmental disorder [51,52]. These initial experiments yielded data consistent with the proposed hypothesis, further supported by ensuing functional validation.

Thap1 +/and Thap1 C54Y/+ mice are not equivalent
The data herein support our previous conclusion that the C54Y mutation does not equal a DNA-binding loss-of-function mutation. First, there were a far greater number of DEGs in Thap1 +/than Thap1 C54Y/+ mice (Fig 1). Second, fold changes were overall lower in Thap1 C54Y/+ in comparison to Thap1 +/-(S5 Table). Third, only a small number of DEGs were altered in both  Table. https://doi.org/10.1371/journal.pgen.1007169.g006 genotypes (S5 Table). For both mutations, the log2 values are low for what would be predicted for a transcription factor [53], perhaps in the presence of the C54Y mutation due to compensatory auto-up-regulation of Thap1 [18,54]. Reports of dystonia patients with homozygous THAP1 mutations with non-manifesting heterozygous parents [55,56] highlight dosage dependent effects of THAP1 mutations.
THAP1 mutations occur in all domains of the protein, but genotype-phenotype correlations have proven difficult to establish due to the small number of patients with each mutation. The C54Y protein, unlike the WT protein, does not bind to the Tor1a promoter [22], but it may bind DNA at other sites and perhaps aberrantly expands DNA binding beyond THAB motifs [57] and/or alter cofactor binding [58]. Thus, some DYT6 mutations represent a partial or total loss of function, whereas others could lead to a combination of haploinsufficiency and gain of function, accounting for the genotype-dependent DEG disparity. Shared transcriptomic and phenotypic features, despite the many differences between mice carrying the C54Y and null alleles, also support this possibility. Thus, there was overlap of key disordered biological processes and biofunctions in the striatum and cerebellum between genotypes. These The total number of nuclei (DAPI stain) was independent of genotype after plating equal numbers of neurons in all wells. Neurons were derived from 3 independent platings. Data are presented as means ± SEM, minimum of 25 neurons per well. Statistical differences were assessed by ANOVA with Student's post hoc t-test. Ã p < 0.05; ÃÃ p < 0.01; ÃÃÃ p < 0.001. See also S9 Table. https://doi.org/10.1371/journal.pgen.1007169.g007 results indicate that the choice of animal model must be carefully considered in DYT6 research, as different mutations can yield divergent results, although sometimes leading to dysregulation of the same pathways and processes.

EIF2α signaling dysfunction: A link between DYT6 and other dystonias?
These new data together with published DYT1 studies [24,26], suggest a point of convergence of neuronal dysfunction on the eIF2α pathway in DYT6 and DYT1. To a lesser extent, other translational control pathways (mTOR and eIF4/p70S6K) are also implicated in the dysregulation produced by mutations in Thap1. Notably, as a transcription factor, THAP1 regulates TOR1A transcription in artificial systems, but this has not been verified in vivo [18,22,59].
A proteomics-based study identified abnormal eIF2α pathway activation in DYT1 mouse and rat brain, which correlated with assays in human brain [24]. A second group designed an RNAi-based functional genomic screening in HEK293T cells that also implicated the eIF2α pathway in DYT1 biology. Moreover, pharmacological manipulation of eIF2α signaling restored absent cortico-striatal LTD in DYT1 knock-in mice [26]. Together, these reports support the presence of abnormal eIF2α signaling in DYT1 brain and its possible causal link to DYT1 synaptic defects. EIF2α signaling provides a potential point of merger with another, rarer form of primary dystonia. DYT16 is caused by mutations in PRKRA, an activator of the eIF2α kinase PKR, with evidence of abnormal eIF2α phosphorylation in patient fibroblasts [27,28]. Coding variants in ATF4, a direct target of eIF2α, were found in patients with focal dystonia [26] and lastly, a recent gene-expression analysis in adult cerebellar tissue from a mouse model of DYT11 dystonia also identified genes associated with protein translation among the top down-regulated mRNAs [60].
We report eIF2α-pathway-related molecular and electrophysiological findings in DYT6 mice that have some similarities with those in DYT1, including abnormalities in baseline ATF4 and in LTD. Taken together, these reports suggest that efforts in dystonia research should include the unravelling of the mechanisms underlying these observations, addressing causality and reversibility.

Thap1 mutations alter synaptic plasticity and neuritogenesis
Multiple dysregulated pathways highlighted in the GO and IPA analyses of the RNA-Seq data may contribute to the deficits in synaptic plasticity and neuritogenesis described herein. These include eIF2α signaling, which in addition to being a key component of ER stress responses, regulates important physiological events under homeostatic conditions, including synaptic plasticity [61][62][63][64][65]. ATF4 plays a specific role in neuronal plasticity, postsynaptic development, mushroom spine density, memory, neuronal survival, caspase activation and dopaminergic neuron degeneration [66][67][68][69]. Selective knockdown of ATF4 impairs hippocampal LTP in vitro and in vivo [68] and as noted above, pharmacological inhibition of eIF2α dephosphorylation rescues cortico-striatal LTD defects in DYT1 KI mice [26]. Notably, eIF2α signaling was not among the top functional pathways in C54Y mice, and these mice exhibited reduced LTP but normal LTD. This highlights the presence of a synaptic plasticity defect in both dystonia models, but with genotype-driven differences as discussed earlier. Furthermore, these data suggest intriguing similarities between the different dystonia models that deserve further study, as there are other IPA and GO-enriched pathways in both genotypes that impinge on synaptic plasticity. These include LTD and LTP themselves in the ΔExon2 mice, the mTOR translation control pathway [70,71], Dopamine-DARPP-32 feedback in cAMP signaling [72], and G-protein/second messenger/cAMP gene groups.
Finally, we identify deficits in neuritogenesis in vitro as a possible convergence point between Tor1a and Thap1 mutations. Mutant TorsinA inhibits neurite extension in cultured cells [73] and DYT1 mice have thinner and less complex dendrites in Purkinje cells and striatal medium spiny neurons [42,43]. The deficit in neuritogenesis is present in both ΔExon2 and C54Y striatal neurons. Many of the same highlighted GO and IPA pathways that could alter synaptic plasticity may also contribute to defective neuritogenesis, including the overlapping GO neurite projection terms and IPA Axonal Guidance Signaling, eIF2α/ATF4 pathway [74], Rho GTPase signaling, and G-protein and cAMP signaling pathways [75]. The fact that in 9-week-old mice, basal synaptic efficiency was enhanced in corticostriatal inputs of Thap1 C54Y mice yet normal in Thap1 +/mice, suggests that any effects on neuritogenesis that persist into adulthood might be offset by additional synaptic changes. Future studies will need to address the temporal and biological relationship between the neurodevelopmental and plasticity phenotypes uncovered in this study.

Additional insights into THAP1/DYT6 neurobiology
Other disordered pathways previously implicated in DYT1 and observed in the study reported herein are related to mitochondrial dysfunction [76,77] and lipid metabolism [78,79]. Interestingly, many of the Thap1 DEGs which are included in the Emory University and Mount Sinai Genetic Testing Movement Disorders and Neuromuscular Disease Panels are also linked to those biological processes, suggesting that lipid metabolism and mitochondrial function may deserve further investigation in different forms of dystonia. Furthermore, torsins have recently been implicated as essential regulators of cellular lipid metabolism [78].
In conclusion, using an unbiased transcriptomic analysis in two brain regions from two mouse models of DYT6, we identified eIF2α dysregulation as a potential point of convergence between different forms of dystonia, possibly through its influence on key homeostatic neurodevelopmental events. The identification of similar eIF2α dysregulation and synaptic plasticity defects as previously described in DYT1 mice and rats in the DYT6 animals is a key convergence of biological mechanisms among inherited dystonias, perhaps adding the group of translational dysregulation-associated dystonias (DYT1, DYT3, DYT6, and perhaps DYT16 [24,26,80] to those linked to dopamine dysfunction (DYT5, DYT11, DYT25) [7]. Moreover, the disordered post-synaptic DARPP-32/G-protein/cAMP signaling system(s) potentially overlaps with other dystonias, particularly DYT25 caused by mutations in GNAL [8], suggesting that there are multiple pathways which may contribute to this phenotype. The consolidation of multiple types of dystonia into specific pathogenic mechanisms could facilitate focused research into the etiology of dystonia and the rational design of targeted therapies applicable to this group of movement disorders.

Ethics statement
Experimental procedures were carried out in compliance with the United States Public Health Service's Policy on Humane Care and Use of Experimental Animals and were approved by the Institutional Animal Care and Use Committee (IACUC) at Icahn School of Medicine at Mount Sinai (Protocol #07-0483).

Animals
The Thap1 C54Y/+ and Thap1 +/mice used in this study were congenic C57Bl. The generation of the original mice has been previously provided in detail in Ruiz et al. [18] and as the mutations are early embryonic lethal, breeding strategy was always heterozygote X WT. Animals were maintained on a 12:12 light: dark cycle with ad libitum access to food and water throughout the course of the entire experiment.

Real-time qPCR, library preparation, and sequencing
Postnatal day 1 (P1) pups were euthanized by decapitation, the brain was dissected, snap-frozen striatal and cerebellar tissues were homogenized in QIAzol Lysis Reagent (Qiagen). Total RNA purification was performed with the miRNeasy mini kit (Qiagen), and was carried out according to the manufacturer's instructions. Five hundred nanograms of RNA were reversedtranscribed using the High Capacity RNA-to-cDNA Kit (Applied Biosystems, Foster City, CA, USA). The cDNA solution was subjected to real-time qPCR in a Step-One Plus system (Applied Biosystems) using the PerfeCTa SYBR Green FastMix ROX (Quanta BioSciences). Quantitative PCR consisted of 40 cycles, 15 s at 95˚C and 30 s at 60˚C each, followed by dissociation curve analysis. Primer sequences can be found in S8 Table. Total RNA from P1 striatal and cerebellar tissues (2-3 μg/ sample) were submitted for further processing to the Genomics Core Facility at the Icahn School of Medicine at Mount Sinai. Sequencing libraries were prepared with the TruSeq RNA Sample Prep Kit v2 protocol (Illumina, San Diego, CA, USA). Briefly, ribosomal RNA was removed using the Ribo-Zero rRNA Removal Kit (Human/Mouse/Rat) (Illumina), the remaining RNA fragmented and cDNA synthesized with random hexamers, end-repaired and ligated with appropriate adaptors for sequencing. After size selection and purification using AMPure XP beads (Beckman Coulter, CA, USA), 6 bp barcode bases were introduced at one end of the adaptors. The size and concentration of the RNA-Seq libraries was measured by Bioanalyzer and Qubit fluorometry (Life Technologies, Grand Island, NY, USA), and the rRNA-depleted libraries sequenced on the Illumina HiSeq 2500 System with 100 nucleotide paired-end reads.

RNA-Seq data analysis
Fastq files were aligned to the Ensembl release 88 (GRCm38.75) version of Human Reference genome (mm10) [81], using STAR read aligner [82]. Accepted mapped reads were summarized separately to gene and exon levels using the featureCounts function of subread [83,84], and used to generate gene and exon count matrices. We examined gene expression data for each sample, and found that the total number of mapped reads [Total Reads (Mean): 39,708,979.5; Uniquely Mapped Reads (Mean): 36,244,152.41] were similar across all samples (S10 Table). There were no obvious outlier samples on visual inspection of principal component analysis or hierarchically clustering of gene expression, and all samples were retained for downstream analyses.

Differential gene expression analysis
For each of the primary comparisons within the study, the assembled count matrix was filtered to remove transcripts without any counts in any sample. Counts were adjusted by total library size and normalized using DESeq2 [85]. P-values were adjusted using the Benjamini-Hochberg method [86]. Due to the small number of DEGs present in striatum and cerebellum of Thap1 C54Y/+ mice, p-value of 0.05 was used as cut off.

Treatment with tunicamycin
Two subcutaneous injections of 3 μg/g tunicamycin (T7765, Sigma-Aldrich) diluted in 150mM dextrose (or dextrose-only control) were applied two hours apart on postnatal day 4 as described [30]. Mice were euthanized 24 hours later, the striatum and cerebellum dissected and snap frozen.

Western blotting
Snap-frozen striatal and cerebellar tissues were homogenized in RIPA buffer with N-ethylmaleimide and protease/phosphatase inhibitors as described previously [24,87]. Protein concentrations were determined using the BCA assay (23225, ThermoFisher Scientific), 30 μg protein lysates were resolved in 10% or 12% Bis/Trisacrylamide gels (BioRad), transferred to nitrocellulose membranes and western blot completed and quantified using the following antibodies:

Electrophysiology
Nine-week-old mice were anesthetized with isoflurane, their brains rapidly removed from the skull and placed in ice-cold modified solution (aCSF) containing (in mM): 215 sucrose, 2.5 KCl, 1.6 NaH 2 PO 4 , 4 MgSO4, 1 CaCl 2 , 4 MgCl 2 , 20 glucose, 26 NaHCO 3 (pH = 7.4, equilibrated with 95% O 2 and 5% CO 2 ) and 250 μm thick coronal slices containing the striatum prepared with a Vibratome VT1000S (Leica Microsystems), incubated at 31˚C for 30 min and then at room temperature for ! 1h in normal aCSF containing (in mM): 120 NaCl, 3.3 KCl, 1.2 Na 2 HPO 4 , 26 NaHCO 3 , 1.3 MgSO 4 , 1.8 CaCl 2 , 11 glucose (pH = 7.4 equilibrated with 95% O 2 and 5% CO 2 ). Hemi-slices were transferred to a recording chamber constantly oxygenated and perfused with aCSF at~4mL/min using a peristaltic pump (Masterflex C/L); experiments were performed at 28.0 ± 0.1˚C. Recordings were acquired with a GeneClamp 500B amplifier (Axon Instruments) and Digidata 1440A (Molecular Devices). All signals were low-pass filtered at 2 kHz and digitized at 10 kHz. For field EPSP recordings, a patch pipette was fabricated on a micropipette puller (Sutter Instruments), filled with normal aCSF, and placed in the dorsomedial striatum for LTP or dorsolateral striatum for LTD. A concentric bipolar electrode (FHC) was positioned immediately above the corpus callosum. Before and after HFS, the stimulus intensity was set evoke an EPSP that was 50% of the maximal obtainable response. During HFS, stimulus intensity was increased to evoke a maximal response. LTD or LTP was induced by the following high-frequency stimulation (HFS) protocol: four 1-s duration, 100 Hz trains, separated by 10 s. mGluR LTD was induced by a 5-min bath application of 100 μM DHPG [(S)-3,5-Dihydroxyphenylglycine]. Sal003, when present, was applied for 5-10 min before DHPG, and washed out with DHPG. Gabazine, when present, was applied at 10 μM for at least 20 min before the delivery of HFS, and remained in the superfusate for the remainder of the recording. Square-wave current pulses (100 μs pulse width) were delivered through a stimulus isolator (Isoflex, AMPI). Paired-pulse ratio was measured by delivering two stimuli at 20, 50, 100 ms inter-stimulus intervals. Each inter-stimulus interval was repeated three times and the resulting EPSPs were averaged.
NeuriteTracer, a neurite tracing plugin for the freely available image-processing program ImageJ was used to analyze fluorescence microscopy images of neurites and nuclei of cultured primary neurons. The plugin was used to count neuronal nuclei, and traces and measure neurite length as described [88]. Ten randomly selected images of each neuronal culture type were processed. DAPI staining was employed as a nuclear stain. The average length was obtained by dividing the total length of the traces by the number of nuclear counts.

Statistical analyses
GraphPad software (GraphPad Prism 5) was used to perform Student's t-tests for the qPCR and western blot densitometry, two-way ANOVAs followed by Tukey's post hoc tests were used to analyze the tunicamycin western blot densitometry. Statistical differences in neurite length / outgrowth were assessed by ANOVA with Student's post hoc t-test. Statistical significance was deemed to be achieved if P < 0.05. Values are presented as mean ± SEM. Electrophysiological data were analyzed by one-way ANOVAs followed, where appropriate, by Newman-Keuls post hoc tests.

Accession numbers
All next generation sequencing data are deposited in NCBI-Gene Expression Omnibus database and are accessible through GEO Series accession number GSE98839. The accession number for human ENCODE ChIP-Seq data from K562 cells used in the manuscript for THAP1 is GEO: GSM803408. The accession number for the mouse embryonic stem (ES) cells ChIP-Seq data for Thap1 used in the manuscript is GEO: GSE86911. Right: Summary data over final 5 min of recording (mean ± sem). LTD, measured over the final 5 min, was reduced in slices from Thap +/mice, and enhanced in slices from Thap1 C54Y/+ mice (ANOVA followed by Neman-Keuls post hoc tests). See also S9 Table. (TIF)

S3 Fig.
Western blot analysis of (A) striatal and (B) cerebellar lysates from Thap1 +/and WT littermates for p-eIF2α/eIF2α, and p-eIF2α/GAPDH were performed 24 hrs after subcutaneous tunicamycin (TM) diluted in 150mM dextrose (or dextrose-only control; DEX). Data are presented as means ± SEM; n = 3 for each genotype and region; data normalized to WT (dextrose-only) controls. Statistical differences were assessed by two-way ANOVAs with Tukey's post hoc tests. See also S9 Table. (TIF) S1 Table Table. Putative THABs motifs in DEGs overlapping in the striatum and cerebellum of Thap1 +/and Thap1 C54Y/+ vs WT mice. In silico analysis was used to identification putative THABS motifs in 1KB of sequence upstream of the translation start codon. Putative THABs motif indicated by "+" symbol. (XLSX) S6 Table. Cross-match of the lists of DEGs found to be dysregulated in either of the Thap1 mutant striatum or cerebellum (Thap1 C54Y/+ or Thap1 +/vs WT) with disease genes that have been previously linked to dystonia or other movement disorders, using the Emory University dystonia sequencing panel and the Mount Sinai Genetic Testing Laboratory Movement Disorders and Neuromuscular Disease Panel. (XLSX) S7 Table. List of brain specific cell types and their respective DEGs in the striatum and cerebellum of Thap1 +/or Thap1 C54Y/+ relative to WT mice. (XLSX) S8 Table. List of primer sequences for RT-qPCR. Primer sequences for the constituents of eIF2α signaling pathway, and the top 5 DEGs from the striatum and cerebellum of Thap1 +/or Thap1 C54Y/+ relative to WT. (XLSX) S9 Table. Table detailing the statistical analyses for all significant data,


extfieldsofstudy: Chemistry Medicine

s2fieldsofstudy: Biology Chemistry

text:
DOX-Vit D, a Novel Doxorubicin Delivery Approach, Inhibits Human Osteosarcoma Cell Proliferation by Inducing Apoptosis While Inhibiting Akt and mTOR Signaling Pathways

Doxorubicin (DOX) is a very potent and effective anticancer agent. However, the effectiveness of DOX in osteosarcoma is usually limited by the acquired drug resistance. Recently, Vitamin D (Vit-D) was shown to suppress the growth of many human cancer cells. Taken together, we synthesized DOX-Vit D by conjugating Vit-D to DOX in order to increase the delivery of DOX into cancer cells and mitigate the chemoresistance associated with DOX. For this purpose, MG63 cells were treated with 10 µM DOX or DOX-Vit D for 24 h. Thereafter, MTT, real-time PCR and western blot analysis were used to determine cell proliferation, genes and proteins expression, respectively. Our results showed that DOX-Vit D, but not DOX, significantly elicited an apoptotic signal in MG63 cells as evidenced by induction of death receptor, Caspase-3 and BCLxs genes. Mechanistically, the DOX-Vit D-induced apoptogens were credited to the activation of p-JNK and p-p38 signaling pathway and the inhibition of proliferative proteins, p-Akt and p-mTOR. Our findings propose that DOX-Vit D suppressed the growth of MG63 cells by inducing apoptosis while inhibiting cell survival and proliferative signaling pathways. DOX-Vit D may serve as a novel drug delivery approach to potentiate the delivery of DOX into cancer cells.

Introduction
Osteosarcoma (OS) is one of the most widespread and lethal forms of childhood primary bone cancer [1]. In Canada, OS accounts for about 5% of all tumors in pediatric patients with an incidence rate of 8 cases per million each year especially in adolescents [2,3]. Despite the substantial progress in chemotherapies against OS, the mortality rate of OS patients has not been changed significantly due to chemoresistance and other factors [4].
One of those standard therapies for OS is doxorubicin (DOX), an effective anthracycline antibiotic [5]. The combination of DOX with other chemotherapeutic agents such as cisplatin, ifosfamide and methotrexate cured 60-76% of newly diagnosed non-metastatic OS [6]. Although DOX has improved survival rates in cancer patients, the effectiveness of DOX in OS is usually limited by      under N 2 for 24 h in the dark. The crude mixture was dried under reduced pressure, and purified with a Combiflash RF system (hexane/ethyl acetate, 50/50) to afford product as an orange solid with a yield of 60%. 1 (Figures 1 and 2).

Cell Culture and Treatments
The human osteosarcoma cancer cell line, MG63 cells, (ATCC, Manassas, VA, USA) was maintained according to the ATCC's instructions.

Effect of DOX and DOX-Vit D on MG63 Cell Proliferation
The effect of DOX and DOX-Vit D on MG63 cell proliferation was determined by measuring the capacity of reducing enzymes to convert 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazoliumbromide (MTT) to colored formazan crystals as described previously [21,22]. The percentage of cell proliferation was calculated relative to control wells designated as 100% viable cells using the following formula: (1)

RNA Extraction and cDNA Synthesis
Total RNA was extracted using TRIzol reagent (Invitrogen ® , Carlsbad, CA, USA) as described previously [20].

Determination of Reactive Oxygen Species (ROS) Production
ROS was measured fluorometrically using 2,7-dichlorofluorescein diacetate (DCF-DA) assay as described previously [24]. Briefly, MG63 cells were treated for 24 h with 10 µM DOX-Vit D. Thereafter, cells were washed with PBS before incubated for 30 min in fresh media containing 10 µM DCF-DA. The fluorescence was directly measured using excitation and emission wavelengths of 485 and 535 nm,

Protein Extraction from MG63 Cells
MG63 cells were treated for 24 h with 10 µM DOX-Vit D or DOX, then the total cellular protein was extracted from the cells as described previously [20].

Immuno Blot Analysis
Cell lysates were analyzed by SDS-PAGE and immunoblotting were performed as described previously [20].

Determination of MAPKs Signaling Pathway
The protein phosphorylation of MAPKs was measured using the PhosphoTracer MAPK ELISA Kit (Abcam, Cambridge, UK) according to manufacturer's instructions and as described previously [20].

Extration of Nuclear Protein
MG63 cells were treated for 2 h with 10 µM DOX-Vit D or DOX, then the nuclear protein was extracted from the cells as described previously [20,25].

Statistical Analysis
Results are shown as mean ± SEM. Statistical analysis was carried out using SigmaPlot ® for Windows (Systat Software, Inc., San Jose, CA, USA). One-way analysis of variance (ANOVA) followed by Tukey-Kramer multiple comparison tests or unpaired two-sided student t-test was carried out. A probability value obtained less than 0.05 is considered significant.

Physiochemical Properities of DOX-Vit D in Comaprison to DOX
Given that the main purpose of the current study is to improve the lipopilicity of DOX, we investigated the physiochemical properties of DOX-Vit D in comparison to DOX using ACD iLab and VCCLAB software (https://www.acdlabs.com/resources/ilab/) as described previously [27]. Perhaps the better predictor of lipophilicity is the distribution coefficient at pH 7.4 (LogD 7.4 ) since it considers the ionizable group at certain pH in addition to the estimated partition coefficient (LogP).
Of interest, Table 2 shows that DOX-Vit D has clear higher predicted values of LogP and LogD 7.4 in comparison to DOX. This was consistent with a low predicted water solubility of DOX-Vit D (0.0029 µg/mL) in comparison to DOX (0.49 mg/mL) and a higher LogS value for DOX-Vit D. Together, it is reasonable to assume that our novel DOX derivative, DOX-Vit D, is more lipophilic than DOX.

Effect of DOX and DOX-Vit D on MG63 Cells Proliferation
To determine the cytotoxic effect of DOX and DOX-Vit D on OS, MG63 cells were exposed to 10 µM DOX and DOX-Vit D for 24 h. Thereafter, the cell proliferation was determined using MTT assay. Our results showed that a 10 µM DOX did not significantly affect cell proliferation at 24 h ( Figure 3). However, 10 µM DOX-Vit D significantly decreased the cell proliferation by approximately 50% in comparison to control ( Figure 3). Effect of DOX and DOX-Vit D on MG63 cells proliferation. MG63 cells were exposed to 10 µM DOX and DOX-Vit D for 24 h. Thereafter, the cell proliferation was determined using MTT assay. The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control.

Effect of DOX and DOX-Vit D on MG63 Cells Proliferation
To determine the cytotoxic effect of DOX and DOX-Vit D on OS, MG63 cells were exposed to 10 µM DOX and DOX-Vit D for 24 h. Thereafter, the cell proliferation was determined using MTT assay. Our results showed that a 10 µM DOX did not significantly affect cell proliferation at 24 h ( Figure 3). However, 10 µM DOX-Vit D significantly decreased the cell proliferation by approximately 50% in comparison to control ( Figure 3). Effect of DOX and DOX-Vit D on MG63 cells proliferation. MG63 cells were exposed to 10 µM DOX and DOX-Vit D for 24 h. Thereafter, the cell proliferation was determined using MTT assay. The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control.

Effect of DOX and DOX-Vit D on MG63 Cells Proliferation
To determine the cytotoxic effect of DOX and DOX-Vit D on OS, MG63 cells were exposed to 10 µM DOX and DOX-Vit D for 24 h. Thereafter, the cell proliferation was determined using MTT assay. Our results showed that a 10 µM DOX did not significantly affect cell proliferation at 24 h ( Figure 3). However, 10 µM DOX-Vit D significantly decreased the cell proliferation by approximately 50% in comparison to control ( Figure 3). Effect of DOX and DOX-Vit D on MG63 cells proliferation. MG63 cells were exposed to 10 µM DOX and DOX-Vit D for 24 h. Thereafter, the cell proliferation was determined using MTT assay. The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control.

Effect of DOX and DOX-Vit D on MG63 Cells Proliferation
To determine the cytotoxic effect of DOX and DOX-Vit D on OS, MG63 cells were exposed to 10 µM DOX and DOX-Vit D for 24 h. Thereafter, the cell proliferation was determined using MTT assay. Our results showed that a 10 µM DOX did not significantly affect cell proliferation at 24 h ( Figure 3). However, 10 µM DOX-Vit D significantly decreased the cell proliferation by approximately 50% in comparison to control ( Figure 3).

Effect of DOX and DOX-Vit D on MG63 Cells Proliferation
To determine the cytotoxic effect of DOX and DOX-Vit D on OS, MG63 cells were exposed to 10 µM DOX and DOX-Vit D for 24 h. Thereafter, the cell proliferation was determined using MTT assay. Our results showed that a 10 µM DOX did not significantly affect cell proliferation at 24 h ( Figure 3). However, 10 µM DOX-Vit D significantly decreased the cell proliferation by approximately 50% in comparison to control ( Figure 3). Effect of DOX and DOX-Vit D on MG63 cells proliferation. MG63 cells were exposed to 10 µM DOX and DOX-Vit D for 24 h. Thereafter, the cell proliferation was determined using MTT assay. The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control. Figure 3. Effect of DOX and DOX-Vit D on MG63 cells proliferation. MG63 cells were exposed to 10 µM DOX and DOX-Vit D for 24 h. Thereafter, the cell proliferation was determined using MTT assay. The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control.

Effect of DOX and DOX-Vit D on Proapoptotic Genes
To investigate whether the inhibitory effect of DOX-Vit D on MG63 cell proliferation and growth is an apoptosis-dependent mechanism, MG63 cells were treated for 24 h with 10 µM DOX and DOX-Vit D. Thereafter, the mRNA levels of proapoptotic genes, Caspase-3, p53 and BCLxs, were determined by real time-PCR. Figure 4 shows that DOX-Vit D caused a significant induction of Caspase-3 and BCLxs genes expression by approximately 250% and 400%, respectively, in comparison to control. On the other hand, DOX significantly decreased the expression of Caspase-3, BCLxs and P53 by about 50%, 20% and 30%, respectively, in comparison to control.
In light of our findings, DOX-Vit D seems to inhibit the growth of MG63 cells through an apoptosis-dependent mechanism. Next, we questioned whether the DOX-Vit D elicited an apoptotic signal in MG63 cells is mediated extrinsically through the activation of death receptor and/or intrinsically by the induction of oxidative stress. Therefore, a series of independent experiments were conducted as follows.

Effect of DOX and DOX-Vit D on Proapoptotic Genes
To investigate whether the inhibitory effect of DOX-Vit D on MG63 cell proliferation and growth is an apoptosis-dependent mechanism, MG63 cells were treated for 24 h with 10 µM DOX and DOX-Vit D. Thereafter, the mRNA levels of proapoptotic genes, Caspase-3, p53 and BCLxs, were determined by real time-PCR. Figure 4 shows that DOX-Vit D caused a significant induction of Caspase-3 and BCLxs genes expression by approximately 250% and 400%, respectively, in comparison to control. On the other hand, DOX significantly decreased the expression of Caspase-3, BCLxs and P53 by about 50%, 20% and 30%, respectively, in comparison to control.
In light of our findings, DOX-Vit D seems to inhibit the growth of MG63 cells through an apoptosis-dependent mechanism. Next, we questioned whether the DOX-Vit D elicited an apoptotic signal in MG63 cells is mediated extrinsically through the activation of death receptor and/or intrinsically by the induction of oxidative stress. Therefore, a series of independent experiments were conducted as follows.

Effect of DOX and DOX-Vit D on the Expression of DR-4
In order to determine the capacity of DOX and DOX-Vit D to modulate the expression of DR-4 mRNA, MG63 cells were treated for 24 h with 10 µM DOX and DOX-Vit D. Thereafter, the mRNA levels of DR-4 was determined by real time-PCR. Figure 5 shows that treatment of MG63 cell with DOX-Vit D caused a significant induction of DR-4 by about 170% in comparison to control. On the other hand, DOX significantly inhibit the expression of DR-4 by approximately 60% in comparison to control. Thereafter, total RNA was isolated using TRIzol reagent, and the mRNA levels of (A) Caspase-3, (B) p53 and (C) BCLxs were quantified using real time-PCR and normalized to a β-actin housekeeping gene. The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control. * p < 0.05 compared to DOX.

Effect of DOX and DOX-Vit D on the Expression of DR-4
In order to determine the capacity of DOX and DOX-Vit D to modulate the expression of DR-4 mRNA, MG63 cells were treated for 24 h with 10 µM DOX and DOX-Vit D. Thereafter, the mRNA levels of DR-4 was determined by real time-PCR. Figure 5 shows that treatment of MG63 cell with DOX-Vit D caused a significant induction of DR-4 by about 170% in comparison to control. On the other hand, DOX significantly inhibit the expression of DR-4 by approximately 60% in comparison to control. Thereafter, total RNA was isolated using TRIzol reagent, and the mRNA level of DR-4 was quantified using real time-PCR and normalized to a β-actin housekeeping gene. The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control. * p < 0.05 compared to DOX.

Effect of DOX and DOX-Vit D on the Oxidative Stress
The involvement of intrinsic apoptotic pathway was addressed by two approaches. Firstly, we determined the effect of DOX-Vit D and DOX on the mRNA expression of oxidative stress markers. Figure 6 shows that treatment of cells with 10 µM DOX-Vit D caused a significant induction of NQO-1 and HO-1 by approximately 250% and 6000%, respectively, in comparison to control. In contrast, DOX significantly inhibited the expression of HO-1 by about 70%, whereas no significant changes were observed with NQO-1 ( Figure 6A,B).
The second approach was to investigate the effect of DOX and DOX-Vit D on the generation of ROS using DCF assay. The incubation of MG63 cells with DOX and DOX-Vit D for 24 h caused a significant increase in the formation of ROS by about 400% and 350%, respectively, in comparison to control ( Figure 6C).
Our findings suggest an involvement of both extrinsic and intrinsic pathways in the induction of proapoptotic genes by DOX-Vit D. The induction of the aforementioned pathways are known to trigger apoptosis through the activation of MAPK signaling pathway. Thus, we have investigated whether or not DOX-Vit D induces proapoptotic genes through MAPK signaling pathway.
(A) Figure 5. Effect of DOX and DOX-Vit D on the expression of DR-4. MG63 cells were treated for 24 h with 10 µM DOX and DOX-Vit D. Thereafter, total RNA was isolated using TRIzol reagent, and the mRNA level of DR-4 was quantified using real time-PCR and normalized to a β-actin housekeeping gene. The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control. * p < 0.05 compared to DOX.

Effect of DOX and DOX-Vit D on the Oxidative Stress
The involvement of intrinsic apoptotic pathway was addressed by two approaches. Firstly, we determined the effect of DOX-Vit D and DOX on the mRNA expression of oxidative stress markers. Figure 6 shows that treatment of cells with 10 µM DOX-Vit D caused a significant induction of NQO-1 and HO-1 by approximately 250% and 6000%, respectively, in comparison to control. In contrast, DOX significantly inhibited the expression of HO-1 by about 70%, whereas no significant changes were observed with NQO-1 ( Figure 6A,B).
The second approach was to investigate the effect of DOX and DOX-Vit D on the generation of ROS using DCF assay. The incubation of MG63 cells with DOX and DOX-Vit D for 24 h caused a significant increase in the formation of ROS by about 400% and 350%, respectively, in comparison to control ( Figure 6C).
Our findings suggest an involvement of both extrinsic and intrinsic pathways in the induction of proapoptotic genes by DOX-Vit D. The induction of the aforementioned pathways are known to trigger apoptosis through the activation of MAPK signaling pathway. Thus, we have investigated whether or not DOX-Vit D induces proapoptotic genes through MAPK signaling pathway.

Effect of DOX and DOX-Vit D on the Oxidative Stress
The involvement of intrinsic apoptotic pathway was addressed by two approaches. Firstly, we determined the effect of DOX-Vit D and DOX on the mRNA expression of oxidative stress markers. Figure 6 shows that treatment of cells with 10 µM DOX-Vit D caused a significant induction of NQO-1 and HO-1 by approximately 250% and 6000%, respectively, in comparison to control. In contrast, DOX significantly inhibited the expression of HO-1 by about 70%, whereas no significant changes were observed with NQO-1 ( Figure 6A,B).
The second approach was to investigate the effect of DOX and DOX-Vit D on the generation of ROS using DCF assay. The incubation of MG63 cells with DOX and DOX-Vit D for 24 h caused a significant increase in the formation of ROS by about 400% and 350%, respectively, in comparison to control ( Figure 6C).
Our findings suggest an involvement of both extrinsic and intrinsic pathways in the induction of proapoptotic genes by DOX-Vit D. The induction of the aforementioned pathways are known to trigger apoptosis through the activation of MAPK signaling pathway. Thus, we have investigated whether or not DOX-Vit D induces proapoptotic genes through MAPK signaling pathway. (A)

Effect of DOX and DOX-Vit D on MAPK Signaling Pathway
To assess the role of MAPK signaling pathway on the DOX-Vit D mediated induction of proapoptotic genes, MG63 cells were treated with 10 µM DOX-Vit D and DOX. Thereafter, phosphorylated MAPK levels were determined using a commercially available kit. Figure 7 shows that incubation of the cells with 10 µM of DOX-Vit D but not DOX significantly induced phosphorylation of p38 and JNK by approximately 250% and 160%, respectively, in comparison to control.

Effect of DOX and DOX-Vit D on MAPK Signaling Pathway
To assess the role of MAPK signaling pathway on the DOX-Vit D mediated induction of proapoptotic genes, MG63 cells were treated with 10 µM DOX-Vit D and DOX. Thereafter, phosphorylated MAPK levels were determined using a commercially available kit. Figure 7 shows that incubation of the cells with 10 µM of DOX-Vit D but not DOX significantly induced phosphorylation of p38 and JNK by approximately 250% and 160%, respectively, in comparison to control.
To further confirm whether activation of the MAPK pathways is required for the apoptotic cell death mediated by DOX-Vit D, MG63 cells were treated with 10 µM p38 inhibitor, SB203580, and JNK inhibitor, SP600125, in the presence and absence of DOX-Vit D. Thereafter, the cells proliferation were measured using MTT assay. Figure 8 shows that DOX-Vit D alone caused a significant inhibition of MG63 cell proliferation by about 50% in comparison to control. Importantly, treatment of cells with SB203580 and SP600125 partially but significantly protects the cells against the cytotoxic effect of DOX-Vit D. Our findings suggest that the activation of MAPK is essential for the cytotoxic effect of DOX-Vit D.

Effect of DOX and DOX-Vit D on MAPK Signaling Pathway
To assess the role of MAPK signaling pathway on the DOX-Vit D mediated induction of proapoptotic genes, MG63 cells were treated with 10 µM DOX-Vit D and DOX. Thereafter, phosphorylated MAPK levels were determined using a commercially available kit. Figure 7 shows that incubation of the cells with 10 µM of DOX-Vit D but not DOX significantly induced phosphorylation of p38 and JNK by approximately 250% and 160%, respectively, in comparison to control. To further confirm whether activation of the MAPK pathways is required for the apoptotic cell death mediated by DOX-Vit D, MG63 cells were treated with 10 µM p38 inhibitor, SB203580, and JNK inhibitor, SP600125, in the presence and absence of DOX-Vit D. Thereafter, the cells proliferation were measured using MTT assay. Figure 8 shows that DOX-Vit D alone caused a significant inhibition of MG63 cell proliferation by about 50% in comparison to control. Importantly, treatment of cells with SB203580 and SP600125 partially but significantly protects the cells against the cytotoxic effect of DOX-Vit D. Our findings suggest that the activation of MAPK is essential for the cytotoxic effect of DOX-Vit D.  To further confirm whether activation of the MAPK pathways is required for the apoptotic cell death mediated by DOX-Vit D, MG63 cells were treated with 10 µM p38 inhibitor, SB203580, and JNK inhibitor, SP600125, in the presence and absence of DOX-Vit D. Thereafter, the cells proliferation were measured using MTT assay. Figure 8 shows that DOX-Vit D alone caused a significant inhibition of MG63 cell proliferation by about 50% in comparison to control. Importantly, treatment of cells with SB203580 and SP600125 partially but significantly protects the cells against the cytotoxic effect of DOX-Vit D. Our findings suggest that the activation of MAPK is essential for the cytotoxic effect of DOX-Vit D.  Thereafter, the cell proliferation was determined using MTT assay. The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control. * p < 0.05 compared to DOX-Vit D.
In order to examine whether the inhibitory effect of DOX-Vit D on MG63 cell proliferation and growth is also attributed to the suppression of cell survival and proliferation pathways, we have determined the effect of DOX-Vit D on NF-κB, Akt and mTOR signaling pathways.

Effect of DOX and DOX-Vit D on NF-κB Signaling Pathway
The basal activity of the NF-κB transcription factor in OS seems to be crucial for their growth or resistance to chemotherapy. Therefore, we have investigated whether DOX-Vit D suppresses MG63 cell growth through the inhibition of NF-κB. For this purpose, MG63 cells were treated with 10 µM DOX and DOX-Vit D. Thereafter, NF-κB binding activity was determined using a commercially available kit. Figure 9 shows that neither DOX nor DOX-Vit D significantly affects the binding activity of NF-κB suggesting an NF-κB-independent mechanism.
Pharmaceutics 2018, 10, x 11 of 16 Thereafter, the cell proliferation was determined using MTT assay. The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control. * p < 0.05 compared to DOX-Vit D.
In order to examine whether the inhibitory effect of DOX-Vit D on MG63 cell proliferation and growth is also attributed to the suppression of cell survival and proliferation pathways, we have determined the effect of DOX-Vit D on NF-κB, Akt and mTOR signaling pathways.

Effect of DOX and DOX-Vit D on NF-κB Signaling Pathway
The basal activity of the NF-κB transcription factor in OS seems to be crucial for their growth or resistance to chemotherapy. Therefore, we have investigated whether DOX-Vit D suppresses MG63 cell growth through the inhibition of NF-κB. For this purpose, MG63 cells were treated with 10 µM DOX and DOX-Vit D. Thereafter, NF-κB binding activity was determined using a commercially available kit. Figure 9 shows that neither DOX nor DOX-Vit D significantly affects the binding activity of NF-κB suggesting an NF-κB-independent mechanism.
(A) (B) Figure 9. Effect of Effect of DOX and DOX-Vit D on NF-κB signaling pathway. MG63 cells were treated for 24 h with 10 µM DOX and DOX-Vit D. Thereafter, NF-κB binding activity was determined in nuclear extracts using a commercially available kit. The results are presented as the mean ± SEM (n = 6).

Effect of DOX and DOX-Vit D on Akt and mTOR Signaling Pathway
Since Akt and mTOR pathway promotes cell growth, proliferation and survival, we examined the effect of DOX-Vit D on Akt and mTOR signaling pathway. For this purpose, MG63 cells were treated with 10 µM DOX and DOX-Vit D. Thereafter, Akt and mTOR protein expression levels were determined using Western blot analysis. Figure 10 shows that DOX-Vit D caused a significant inhibition of p-Akt and p-mTOR protein expression by approximately 40% and 50%, respectively, in Figure 9. Effect of Effect of DOX and DOX-Vit D on NF-κB signaling pathway. MG63 cells were treated for 24 h with 10 µM DOX and DOX-Vit D. Thereafter, NF-κB binding activity was determined in nuclear extracts using a commercially available kit. The results are presented as the mean ± SEM (n = 6).

Effect of DOX and DOX-Vit D on Akt and mTOR Signaling Pathway
Since Akt and mTOR pathway promotes cell growth, proliferation and survival, we examined the effect of DOX-Vit D on Akt and mTOR signaling pathway. For this purpose, MG63 cells were treated with 10 µM DOX and DOX-Vit D. Thereafter, Akt and mTOR protein expression levels were determined using Western blot analysis. Figure 10 shows that DOX-Vit D caused a significant inhibition of p-Akt and p-mTOR protein expression by approximately 40% and 50%, respectively, in comparison to control suggesting an Akt/mTOR-dependent inhibition of cell growth by DOX-Vit D. In contrast, DOX did not significantly alter the expression of p-Akt and p-mTOR protein expression. comparison to control suggesting an Akt/mTOR-dependent inhibition of cell growth by DOX-Vit D.
In contrast, DOX did not significantly alter the expression of p-Akt and p-mTOR protein expression. Figure 10. Effect of DOX and DOX-Vit D on Akt and mTOR signaling pathway. MG63 cells were treated for 24 h with 10 µM DOX and DOX-Vit D. Thereafter, total and phosphorylated Akt and mTOR protein expression levels were determined by Western blot analysis and detected using the enhanced chemiluminescence method. The intensity of protein bands was normalized to the signals obtained for GAPDH protein and quantified using ImageJ ® . The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control. * p < 0.05 compared to DOX.

Discussion
These investigations provide strong evidence that DOX-Vit D suppresses the growth of human OS, MG63 cell line, through the induction of apoptosis and the inhibition of cell survival and proliferative signaling pathways.
One of the strategies for treating OS and minimizing the development of chemoresistance associated with chemotherapeutic agents includes the induction of apoptosis and/or the attenuation of cell survival and proliferative signaling pathways. Studies using transgenic mice provide direct evidence that overexpression of cell survival pathways and/or disruption of apoptosis promote tumorigenesis, metastasis and contribute to chemoresistance [28,29]. Therefore, the development of a new chemotherapeutic agent that is able to attenuate the proliferation of OS while inducing apoptosis is an urgently needed to overcome chemoresistance.
DOX, a broad-spectrum anthracycline antibiotic, is one of those standard therapies for the treatment of OS [5]. Unfortunately, the effectiveness of DOX in OS is usually limited by the acquired drug resistance that leads to poor prognosis and suboptimal outcomes [30]. Recently, Vit-D has been shown to suppress the growth of many human cancer cells and reverse chemotherapy drug-resistant [30,31]. Taken together, we synthesized DOX-Vit D by conjugating Vit-D to DOX in order to mitigate the chemoresistance associated with DOX. The current study was conducted to investigate the antiproliferative and apoptogenic effects of 10 µM DOX-Vit D in comparison to 10 µM DOX and the possible mechanism(s) involved using the human OS cell line, MG63 cells. The concentration of DOX used in the current study was maintained within the therapeutic range of plasma concentration reported in human. For example, human subjects given a dose of 60-75 mg/m 2 DOX for the treatment of metastatic cancer had mean plasma concentrations range from 5 and 15 µM and an average halflife of ∼25 h [32,33]. In addition, several in vitro studies on human cancer cells to explore the cytotoxicity of DOX used concentrations range from 1 to 10 µM [32,33].
Initially, we have demonstrated that DOX-Vit D, but not DOX, was able to significantly suppress the MG63 cell proliferation and growth. Notably, the anticancer effect of DOX-Vit D is attributed to the induction of proapoptotic genes Caspase-3 and BCLxs. Activation of the proapoptotic genes plays a crucial role in the initiation of apoptosis through the cleavage of the key cellular proteins resulting in the irreversible commitment to cell death [29,34]. Similar to our observation, it has been reported that calciferol and its chemical derivative, MT19c, induce apoptosis and inhibit the growth and proliferation of many cancer cell lines through the activation of the Caspase-3 enzyme [35,36]. The Figure 10. Effect of DOX and DOX-Vit D on Akt and mTOR signaling pathway. MG63 cells were treated for 24 h with 10 µM DOX and DOX-Vit D. Thereafter, total and phosphorylated Akt and mTOR protein expression levels were determined by Western blot analysis and detected using the enhanced chemiluminescence method. The intensity of protein bands was normalized to the signals obtained for GAPDH protein and quantified using ImageJ ® . The results are presented as the mean ± SEM (n = 6). + p < 0.05 compared to control. * p < 0.05 compared to DOX.

Discussion
These investigations provide strong evidence that DOX-Vit D suppresses the growth of human OS, MG63 cell line, through the induction of apoptosis and the inhibition of cell survival and proliferative signaling pathways.
One of the strategies for treating OS and minimizing the development of chemoresistance associated with chemotherapeutic agents includes the induction of apoptosis and/or the attenuation of cell survival and proliferative signaling pathways. Studies using transgenic mice provide direct evidence that overexpression of cell survival pathways and/or disruption of apoptosis promote tumorigenesis, metastasis and contribute to chemoresistance [28,29]. Therefore, the development of a new chemotherapeutic agent that is able to attenuate the proliferation of OS while inducing apoptosis is an urgently needed to overcome chemoresistance.
DOX, a broad-spectrum anthracycline antibiotic, is one of those standard therapies for the treatment of OS [5]. Unfortunately, the effectiveness of DOX in OS is usually limited by the acquired drug resistance that leads to poor prognosis and suboptimal outcomes [30]. Recently, Vit-D has been shown to suppress the growth of many human cancer cells and reverse chemotherapy drug-resistant [30,31]. Taken together, we synthesized DOX-Vit D by conjugating Vit-D to DOX in order to mitigate the chemoresistance associated with DOX. The current study was conducted to investigate the antiproliferative and apoptogenic effects of 10 µM DOX-Vit D in comparison to 10 µM DOX and the possible mechanism(s) involved using the human OS cell line, MG63 cells. The concentration of DOX used in the current study was maintained within the therapeutic range of plasma concentration reported in human. For example, human subjects given a dose of 60-75 mg/m 2 DOX for the treatment of metastatic cancer had mean plasma concentrations range from 5 and 15 µM and an average half-life of ∼25 h [32,33]. In addition, several in vitro studies on human cancer cells to explore the cytotoxicity of DOX used concentrations range from 1 to 10 µM [32,33].
Initially, we have demonstrated that DOX-Vit D, but not DOX, was able to significantly suppress the MG63 cell proliferation and growth. Notably, the anticancer effect of DOX-Vit D is attributed to the induction of proapoptotic genes Caspase-3 and BCLxs. Activation of the proapoptotic genes plays a crucial role in the initiation of apoptosis through the cleavage of the key cellular proteins resulting in the irreversible commitment to cell death [29,34]. Similar to our observation, it has been reported that calciferol and its chemical derivative, MT19c, induce apoptosis and inhibit the growth and proliferation of many cancer cell lines through the activation of the Caspase-3 enzyme [35,36]. The inhibitory effect of DOX on proapoptotic genes might be attributed to the fact that our MG-63 cells are resistant to DOX. In agreement with our results, it has been shown that 10 µM DOX was neither elicit Caspase-3 activation or apoptosis in DOX resistant MG63 cells [37]. Importantly, promoting apoptosis has been shown to overcome the chemoresistance associated with DOX during the treatment of OS [38].
Apoptosis is known to be elicited by the activation of extrinsic and/or intrinsic signals. These signals are instructing the cells to undergo programmed cell death through the activation of proapoptotic genes [29,34]. Accordingly, we investigated whether the DOX-Vit D-induced apoptosis in MG63 cells is mediated through the extrinsic and/or intrinsic apoptotic pathway. The extrinsic signals induce apoptosis through binding of cell surface death receptors such as TNF/Fas-receptor with its ligand and subsequently activates proapoptotic enzymes [39]. Hence, we have tested whether DOX-Vit D triggers extrinsic apoptotic pathway by measuring the expression of DR-4. In this current study, we found that the induction of DR-4 mRNA in response to DOX-Vit D significantly contributes to the activation of proapoptotic genes. These findings are in agreement with the observation that calciferol increases the activity of proapoptotic genes through the induction of DR [36]. On the other hand, DOX seems to decrease the expression of proapoptotic genes through the downregulation of DR-4.
The intrinsic signal is another pivotal pathway that could initiate apoptosis through the oxidative stress and ROS-dependent mechanism [40]. Oxidative stress and ROS have been considered as a potent inducer of apoptosis [41]. In this context, the involvement of the intrinsic apoptotic pathway in the cytotoxic effect of DOX-Vit D was evidenced by the induction of the oxidative stress markers, NQO-1 and HO-1, in addition to the generation of ROS. In a manner similar to our observations, it has been reported that calciferol treatment results in the accumulation of ROS and subsequently coordinates proapoptotic genes activation [36].
Our findings suggest that DOX-Vit D upregulated the expression of proapoptotic genes through the activation of both extrinsic and intrinsic apoptotic pathways. The induction of the aforementioned pathways are known to trigger apoptosis through the activation of the MAPK signaling pathway. A wealth of information suggests the involvement of MAPK cascades in cell death and survival signaling [42,43]. In particular, it has demonstrated that persistent activation of p38 and JNK promotes apoptosis and cell death [42,43]. Taken together, the possibility that DOX-Vit D would induce apoptosis through the activation of MAPKs could not be ruled out. Thus, the third objective of the current study was to explore the role of DOX-Vit D on the MAPKs signaling pathway. Our results demonstrate that DOX-Vit D, but not DOX, significantly increased the protein expression level of p-p38 and p-JNK whereas no significant changes were observed on p-ERK1/2. The direct evidence for the involvement of p-p38 and p-JNK in the DOX-Vit D-induced cytotoxicity was supported by the observation that p-p38 inhibitor, SB203580, and JNK inhibitor, SP600125, significantly protect against DOX-Vit D-induced cell death suggesting a MAPK-dependent mechanism. The premise of this observation emerges from the finding that calciferol chemical derivative, MT19c, induces apoptosis through a p38 and JNK-dependent mechanism [35].
Apoptosis-mediated cell death might induce the turn off of survival pathways, such as NF-κB and Akt/mTOR pathways, that could otherwise interfere with the apoptotic response. The aberrant activation of those proliferating proteins in OS seems to be crucial for their growth or resistance to chemotherapy [44,45]. Thus, it is imperative to investigate the effect of DOX-Vit D on the aforementioned cell survival and proliferating pathways. Although DOX-Vit D did not significantly affect the binding activity level of NF-κB in MG63 cells, DOX-Vit D significantly downregulated the protein expression level of p-Akt and p-mTOR. In addition, DOX did not significantly alter the p-Akt/p-mTOR proteins expression. These results are in agreement with previous reports on OS showing that high Akt/mTOR activity was associated with poor clinical outcome and chemoresistence associated with DOX [46,47]. On the other hand, everolimus, an mTOR inhibitor, has been shown to decrease drug-induced resistance in OS [48,49]. Our findings not only suggest an Akt/mTOR-dependent inhibition of MG63 cell growth by DOX-Vit D but also support DOX-Vit D as a promising developmental strategy for the treatment of OS resistant to chemotherapy [50].
To reiterate, our results clearly demonstrated that DOX-Vit D, a novel DOX derivative, suppresses MG63 cell growth through the induction of apoptosis and the inhibition of Akt/mTOR signaling pathways. Such observation will raise the potential of developing DOX-Vit D analogues for the treatment of OS resistant to chemotherapy. Given that DOX-VitD has a higher lipophilicity compared to DOX, it is reasonable to assume that DOX-VitD may serve as a novel drug delivery approach to minimize the first-pass effect while increasing lymphatic exposure and ultimately improving overall systemic drug exposure. Additional studies are going to test the toxicity and the kinetic of DOX-VitD in rats. Our preliminary data have shown that DOX-VitD is tolerable upon oral and intravenous administration in rats. Given that our new derivative has a stoichiometry of 1 to 1 ratio (Dox-VitD), it might not necessarily be the most active one. Therefore, we will compare our new derivative with a combination of the free drugs at different ratios in order to confirm the mechanism(s) of action and discriminate additive effects vs synergistic effects.