Abstract 4377: HDAC inhibitors enhance ESR2 expression and augment response to ESR2 agonist therapy

Background: Glioblastomas (GBM) are the most common and lethal primary brain tumors that have dismal survival rates. The estrogen receptor β (ESR2) functions as a tumor suppressor in glioblastoma (GBM). However, ESR2 expression is commonly suppressed during glioma progression with low expression in higher grades. In this study, we explored the hypothesis that aberrant epigenetic changes may contribute to loss of expression of the ESR2 and drugs that reverse these modifications will enhance ESR2 agonist-mediated tumor suppression by upregulating ESR2 expression. Methods: We tested utility of epigenetic drugs currently in clinical trials which acts as inhibitor of HDAC (Belinostat, Givinostat, panobinostat, romidepsin), DNMT (Decitabine), histone methylases (BIX-01294) and Bet enzymes (I-BET151) in upregulating ESR2 using both established and primary GBM models. The effect of combination therapy of HDAC inhibitors (HDACIs) along ESR2 agonist was evaluated using cell viability, colony formation, Caspase 3/7, TUNEL, and neurosphere formation assays. Mechanistic studies were performed using Western blot, qRT-PCR, ChIP and ERE reporter assays. The efficacy of combination therapy in vivo was examined using orthotopic models of GBM and mouse survival was determined using Kaplan-Meier survival curves. Results: Of all inhibitors tested, only two HDACis (panobinostat and romidepsin) showed the potential to increase the expression of ESR2 in established GBM model cells (U87 and U251). Upregulation of ESR2 by HDACi was also validated using primary GBM cells. Treatment with panobinostat or romidepsin uniquely upregulated ESR2 isoform-1 expression that functions as a tumor suppressor but not the ERS2 isoform-5 that drives oncogenic functions. Further, combination therapy of panobinostat with ESR2 agonist LY500307 synergistically reduced cell viability, colony formation and enhanced apoptosis. Mechanistic studies showed that panobinostat induced ESR2 is functional, as it enhanced ESR2 mediated ERE-, SP1-, AP1- reporter activities and ESR target genes involved in apoptosis. ChIP analysis confirmed alteration in the histone acetylation in the ESR2 promoter region. In orthotopic GBM models, combination therapy of panobinostat and LY500307 enhanced survival of tumor-bearing mice. Conclusions: Our results suggest that the combination therapy of HDACIs and LY500307 provides therapeutic utility in overcoming the suppression of ESR2 expression that commonly occurs in GBM progression. Supported by NIH grant CA178499 (RKV) Citation Format: Uday P. Pratap, Gangadhara R. Sareddy, Suryavathi Viswanadhapalli, Weiwei Tang, Prabhakar Pitta Venkata, Junhao Liu, Rajeshwar R. Tekmal, Andrew Brenner, Ratna K. Vadlamudi. HDAC inhibitors enhance ESR2 expression and augment response to ESR2 agonist therapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4377.