Pete Taylor

The Notch Target Hes1 Directly Modulates Gli1 Expression and Hedgehog Signaling: A Potential Mechanism of Therapeutic Resistance

Purpose: Multiple developmental pathways including Notch, Hedgehog, and Wnt are active in malignant brain tumors such as medulloblastoma and glioblastoma (GBM). This raises the possibility that tumors might compensate for therapy directed against one pathway by upregulating a different one. We investigated whether brain tumors show resistance to therapies against Notch, and whether targeting multiple pathways simultaneously would kill brain tumor cells more effectively than monotherapy. Experimental Design: We used GBM neurosphere lines to investigate the effects of a gamma-secretase inhibitor (MRK-003) on tumor growth, and chromatin immunoprecipitation to study the regulation of other genes by Notch targets. We also evaluated the effect of combined therapy with a Hedgehog inhibitor (cyclopamine) in GBM and medulloblastoma lines, and in primary human GBM cultures. Results: GBM cells are at least partially resistant to long-term MRK-003 treatment, despite ongoing Notch pathway suppression, and show concomitant upregulation of Wnt and Hedgehog activity. The Notch target Hes1, a repressive transcription factor, bound the Gli1 first intron, and may inhibit its expression. Similar results were observed in a melanoma-derived cell line. Targeting Notch and Hedgehog simultaneously induced apoptosis, decreased cell growth, and inhibited colony-forming ability more dramatically than monotherapy. Low-passage neurospheres isolated from freshly resected human GBMs were also highly susceptible to coinhibition of the two pathways, indicating that targeting multiple developmental pathways can be more effective than monotherapy at eliminating GBM-derived cells. Conclusions: Notch may directly suppress Hedgehog via Hes1 mediated inhibition of Gli1 transcription, and targeting both pathways simultaneously may be more effective at eliminating GBMs cells. Clin Cancer Res; 16(24); 6060–70. ©2010 AACR.

Reactivation of death receptor 4 (DR4) expression sensitizes medulloblastoma cell lines to TRAIL

Object Apoptosis, a key cellular response to therapeutic agents is often inactivated in tumor cells. In this study, we evaluated the expression of the tumor necrosis family of death receptors, DR4 and DR5, in medulloblastoma tumor samples and cell lines to determine if epigenetic modulation of gene expression could sensitize tumor cell lines to TRAIL-mediated apoptosis. Methods Human medulloblastoma samples and cell lines were analyzed for DR4 and DR5 expression by quantitative PCR and immunofluorescence assays. Cell lines with downregulated expression of one or both genes were treated with the histone deacetylase inhibitor, MS-275, and the expression of DR4 and DR5 measured by quantitative PCR, Western blotting, flow cytometry and chromatin immunoprecipitation assays. Induction of apoptosis in the presence of MS-275 was evaluated by TUNEL assay and its ability to augment TRAIL-mediated cytotoxicity was determined by MTT assays, Western blotting and flow cytometry. Results Compared to normal cerebellum, DR4, but not DR5 expression was consistently downregulated in medulloblastoma tumor samples and in Daoy and D283 cell lines. Interestingly, MS-275 decreased cell growth and induced apoptosis in Daoy and D283 cells. In Daoy cells, this coincided with increased histone H3 and H4 acetylation at the DR4 promoter and enhanced DR4 gene and protein expression as well as elevated Caspase-8 activity. The involvement of DR4 in the cellular response to MS-275 was further confirmed by the observation that knockdown of DR4 and FADD abrogated apoptosis. Further, addition of TRAIL to MS-275 treated cells resulted in an enhancement of apoptosis, suggesting that the upregulated death receptors were functional. Conclusion Our study provides an understanding of the role of DR4 in apoptosis of medulloblastoma cell lines and suggests a potential contribution of aberrant histone deacetylation to the resistance of medulloblastoma cells to therapeutic death.

REST Is a Novel Prognostic Factor and Therapeutic Target for Medulloblastoma

Medulloblastoma is a malignant pediatric brain tumor. Current treatment following patient stratification into standard and high-risk groups using clinical features has improved survival. However, a subset of patients with standard risk features have unanticipated aggressive disease, underscoring the need for a better understanding of tumor biology and the development of novel treatments. Poor differentiation, a hallmark of medulloblastomas is associated with elevated expression levels of the repressor of neuronal differentiation called repressor element 1-silencing transcription factor (REST). Here, we assessed whether elevated REST expression levels had prognostic significance and whether its pharmacologic manipulation would promote neurogenesis and block tumor cell growth. REST levels in patient tumors were measured by immunohistochemistry and stratified into negative, low/moderate- (+/++/+++), and high-REST (+++++) groups. Kaplan–Meier curves revealed that patients with high-REST tumors had worse overall and event-free survival compared with patients with REST-negative or REST-low tumors. Because histone deacetylases (HDAC) are required for REST-dependent repression of neurogenesis, we evaluated a panel of HDAC inhibitors (HDACI) for their effects on growth and differentiation of established and primary REST-positive cell lines. MS-275, trichostatin-A (TSA), valproic acid (VPA), and suberoylanilide hydroxamic acid (SAHA) upregulated expression of the REST-target neuronal differentiation gene, Syn1, suggesting a potential effect of these HDACIs on REST function. Interestingly, VPA and TSA substantially increased histone acetylation at the REST promoter and activated its transcription, whereas SAHA unexpectedly promoted its proteasomal degradation. A REST-dependent decrease in cell growth was also observed following SAHA treatment. Thus, our studies suggest that HDACIs may have therapeutic potential for patients with REST-positive tumors. This warrants further investigation. Mol Cancer Ther; 11(8); 1713–23. ©2012 AACR.

The deubiquitylase USP37 links REST to the control of p27 stability and cell proliferation

The RE1 silencing transcription factor (REST) is a repressor of neuronal differentiation and its elevated expression in neural cells blocks neuronal differentiation. In this study, we demonstrate a role for REST in the control of proliferation of medulloblastoma cells. REST expression decreased the levels of cyclin-dependent kinase (CDK)NIB/p27, a CDK inhibitor and a brake of cell proliferation in these cells. The reciprocal relationship between REST and p27 was validated in human tumor samples. REST knockdown in medulloblastoma cells derepessed a novel REST target gene encoding the deubiquitylase ubiquitin (Ub)-specific peptidase 37 (USP37). Ectopically expressed wild-type USP37 formed a complex with p27, promoted its deubiquitination and stabilization and blocked cell proliferation. Knockdown of REST and USP37 prevented p27 stabilization and blocked the diminution in proliferative potential that normally accompanied REST loss. Unexpectedly, wild-type USP37 expression also induced the expression of REST-target neuronal differentiation genes even though REST levels were unaffected. In contrast, a mutant of USP37 carrying a site-directed change in a conserved cysteine failed to rescue REST-mediated p27 destabilization, maintenance of cell proliferation and blockade to neuronal differentiation. Consistent with these findings, a significant correlation between USP37 and p27 was observed in patient tumors. Collectively, these findings provide a novel connection between REST and the proteasomal machinery in the control of p27 and cell proliferation in medulloblastoma cells.

Chromatin remodelling at the topoisomerase II-beta promoter is associated with enhanced sensitivity to etoposide in human neuroblastoma cell lines

Etoposide, an inhibitor of topoisomerase II, promotes DNA damage and apoptosis of cancer cells and is a component of standard therapy for neuroblastoma. Resistance to etoposide has been observed in neural tumour cells expressing lower levels of topoisomerase II. In the present study, we have examined the contribution of epigenetic modulation of gene expression in the potentiation of etoposide-mediated cytotoxicity in neuroblastoma cells. Specifically, we studied the effects of histone deacetylase inhibition with valproic acid on topoisomerase II gene expression and apoptosis in response to etoposide. Using human neuroblastoma cell lines SK-N-AS and SK-N-SH, we show that although the combination of valproic acid and etoposide promoted a reduction in growth compared to either drug alone in both cells, the effect was substantially enhanced in SK-N-AS compared to SK-N-SH cells. An increase in histone H3 acetylation and p21 expression was observed in both cell lines, however, upregulation of topoisomerase II-beta gene expression and an increase in PARP cleavage was observed in SK-N-AS cells only. Furthermore, chromatin immunoprecipitation assays revealed an increase in acetylation of histone H3 at the cognate topoisomerase II-beta gene after treatment with valproic acid in SK-N-AS cells. These results suggest a potential epigenetic mechanism of regulation of the topoisomerase II-beta gene and a possible role for its increased expression in the sensitivity of SK-N-AS neuroblastoma cells to etoposide.

Regulation of USP37 Expression by REST-Associated G9a-Dependent Histone Methylation

The deubiquitylase (DUB) USP37 is a component of the ubiquitin system and controls cell proliferation by regulating the stability of the cyclin-dependent kinase inhibitor 1B, (CDKN1B/p27Kip1). The expression of USP37 is downregulated in human medulloblastoma tumor specimens. In the current study, we show that USP37 prevents medulloblastoma growth in mouse orthotopic models, suggesting that it has tumor-suppressive properties in this neural cancer. Here, we also report on the mechanism underlying USP37 loss in medulloblastoma. Previously, we observed that the expression of USP37 is transcriptionally repressed by the RE1 silencing transcription factor (REST), which requires chromatin remodeling factors for its activity. Genetic and pharmacologic approaches were employed to identify a specific role for G9a, a histone methyltransferase (HMT), in promoting methylation of histone H3 lysine-9 (H3K9) mono- and dimethylation, and surprisingly trimethylation, at the USP37 promoter to repress its gene expression. G9a inhibition also blocked the tumorigenic potential of medulloblastoma cells in vivo. Using isogenic low- and high-REST medulloblastoma cells, we further showed a REST-dependent elevation in G9a activity, which further increased mono- and trimethylation of histone H3K9, accompanied by downregulation of USP37 expression. Together, these findings reveal a role for REST-associated G9a and histone H3K9 methylation in the repression of USP37 expression in medulloblastoma. Implications: Reactivation of USP37 by G9a inhibition has the potential for therapeutic applications in REST-expressing medulloblastomas. Mol Cancer Res; 15(8); 1073–84. ©2017 AACR.

Abstract B5: Identification of a role for REST in promoting preneoplastic events in a novel genetically engineered mouse model

Medulloblastoma is a pediatric cerebellar tumor that arises from undifferentiated neuronal stem/progenitor cells (NSCs). The Repressor Element-1 (RE1) Silencing Transcription Factor (REST) negatively regulates neurogenesis by repressing numerous genes involved in terminal neuronal differentiation. We previously showed that REST levels were abnormally elevated in medulloblastoma samples and that this was associated with poor prognosis for patients. Previous studies from two groups including ours showed that REST knockdown in human tumor cells blocked their tumorigenic potential in mouse orthotopic models, suggesting that REST was required for tumor maintenance. In addition, constitutively elevated expression of REST in neural progenitors in the context of Myc overexpression promoted tumorigenesis in the murine brain, implicating REST in tumor progression. However, a role for REST in tumor initiation has not been demonstrated. To examine if elevated REST expression in neuronal progenitors was sufficient to promote pre-neoplastic events such as proliferation, delayed differentiation and genetic instability, we generated a novel transgenic (Tg) mouse model in which human REST Tg expression could be conditionally elevated in the post-natal cerebellar progenitor cells. These studies revealed that cerebellar progenitors harvested from REST Tg mice, had sustained neurosphere formation and proliferation potential compared to progenitors from age-matched wild type littermates. Progenitors from REST Tg mice also exhibited substantially decreased expression of REST target neuronal differentiation genes such as Syn1 and SCG10 and displayed delayed onset of neurogenesis compared to wild type controls. Interestingly, progenitors with elevated REST expression had significantly perturbed telomere homeostasis associated with altered telomerase levels and increased genomic instability compared to wild type controls. The above findings were recapitulated in vivo where cerebella from REST Tg mice exhibited prolonged Ki-67 staining accompanied by lack of differentiation and accumulation of genetic instability. Furthermore, REST mediated genetic instability was associated with increased sensitivity to DNA damaging reagents such as etoposide compared to normal wild type progenitors. Thus, we provide the first demonstration of a role for elevated REST expression in neural progenitors in promoting tumor initiating events such as hyperproliferation, blockade of differentiation and telomere defects. We also show that the genetic instability contributes to the accelerated response of these cells to clinically relevant DNA damage-inducing agents. These observations may have therapeutic implications for high REST medulloblastomas. Citation Format: Tara Dobson, Ellen Busschers, Pete Taylor, Julianne Humphries, Sadhan Majumder, Vidya Gopalakrishnan. Identification of a role for REST in promoting preneoplastic events in a novel genetically engineered mouse model. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr B5.

Abstract 14: Targeting Notch in malignant brain tumors: Crosstalk with Hedgehog as a potential mechanism of treatment resistance

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC Developmental signaling cascades such as Notch, Hedgehog, and Wnt are increasingly being investigated as novel therapeutic targets in malignant brain tumors. These pathways are thought to play particularly important roles in stem-like cancer cells, which seem resistant to standard chemo- and radiation therapies. Indeed, inhibiting Notch signaling in medulloblastoma or glioblastoma (GBM) cultures decreases the proportion of stem-like cells. However, we thought that other developmental stem cell maintenance pathways might compensate for the pharmacological inhibition of Notch signaling, and therefore investigated Hedgehog and Wnt. Interestingly, we found that levels of the Hedgehog targets Gli1 and Patched1B increased almost two-fold in response to pharmacological inhibition of Notch in neurosphere lines derived from malignant gliomas. The Wnt target Axin2 was also upregulated in some lines, but in a less reproducible fashion. We hypothesized that there might be direct transcriptional regulation of Hedgehog targets by the Notch pathway, and used chromatin immunoprecipitation to evaluate whether the repressive factor Hes1 bound to the first intron of Gli1. We found that Hes1 does bind to multiple N-boxes in this intron, consistent with the concept that it acts as a transcriptional repressor for Gli1. Together, our data suggest a negative feedback loop between Notch and Hedgehog by which GBMs could escape from pharmacological Notch inhibition by up-regulating Hedgehog signaling. We also investigated whether targeting both Notch and Hedgehog signaling simultaneously would kill brain tumor cells more effectively than as monotherapies. As we hypothesized, while pharmacological inhibition of Notch (using gamma-secretase inhibitor) or Hedgehog (using cyclopamine) signaling alone inhibited the growth and clonogenicity of GBM neurosphere cell lines in a dose-dependent fashion, co-treatment resulted in significantly more robust inhibition of cell growth and colony-forming ability. Moreover, when neurosphere lines capable of forming intracranial tumors were pretreated with one or both pathway inhibitors before injection into mice, co-treatment significantly prolonged survival. This is consistent with our model that inhibiting both pathways more effectively kills the xenograft-initiating cells within the culture. We next investigated whether these findings applied to primary human GBMs and found that freshly cultured neurospheres were highly susceptible to co-treatment but more refractory to monotherapy. Our findings suggest that targeting multiple developmental pathways may be necessary to fully ablate the stem-like population in malignant brain tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 14.