HDAC inhibition results in suppression of FOXM1 and genes associated with poor prognosis prostate cancer through p53 dependent and independent mechanisms

Abstract

Prostate cancer is a disease of remarkable biological heterogeneity and is now the most commonly diagnosed malignancy in men worldwide. The standard of care combination of ADT and radiotherapy is highly effective in patients with locally advanced prostate cancer yet a third of patients relapse within 3 years. Therefore, strategies to treat or prevent development of ADT/radiotherapy-refractory disease are becoming increasingly important.

Despite p53 being mutated in 50% of tumours, it has been reported that only 12% of primary Prostate Cancer (PCa) harbour a p53 alteration. Emerging data have identified epigenetic changes in gene expression as a major differentiator between indolent and aggressive disease, wherein treatment-induced, epigenetic driven transcriptional changes underpin adaptive tumour cell survival responses and immune escape mechanisms, ultimately leading to patient relapse. Recent data from our group has associated this molecular subgroup of poor prognostic patients to an up-regulation of oncogenic transcription factor FOXM1, also known to be suppressed by p53.

HDACi are an emerging class of epigenetic modifying anti-cancer agents that we hypothesise will enhance p53 mediated cell death, whilst also altering patterns of gene expression to target p53-dependent and independent vulnerabilities in PCa. Using novel clinically relevant models of advanced prostate cancer and metastatic castrate resistant disease, our data demonstrates that the Class I HDAC inhibitor, Entinostat, augments the effects of both the p53 activating MDM2 inhibitor, Nutlin-3A and radiotherapy, resulting in a synergistic induction of cell death dependent on WT p53. Further exploration into the mechanism of cell death demonstrated induction of pro-apoptotic BIM and PUMA alongside repression of anti-apoptotic c-FLIP, previously shown by us to be a direct target of p53. We further provide evidence that Entinostat in the absence of p53, induces expression of CDK inhibitor p21 to result in cell cycle arrest through suppression of critical cell cycle genes such as Cyclin B1 and FOXM1. Therefore, Entinostat induces a p53-independent suppressive role of p21 that essentially mimics the activation of p53.

Genome-wide transcriptomic analyses confirmed that Entinostat mimics p53 mediated suppression of cell-cycle genes, in particular those activated by FOXM1, to suppress molecular signalling we previously linked to poor prognosis metastatic disease. Specifically, Entinostat impacts expression of genes such as MYBL2, UHRF1, RRM2 and BRCA1 that are central in cell cycle and DNA repair, through suppression of FOXM1, a nodal regulator of these processes.

These results uncover a multi-modal mechanism of action for Entinostat, whereby induction of p21, independent of p53, suppresses cell cycle and DNA repair through downregulation of critical regulator FOXM1 whilst enhancing efficacy of SOC radiotherapy to significantly increase apoptotic cell death. Thus, we may have evidence to support the use of Entinostat to enhance current prostate cancer SOC to prevent or treat castrate resistant prostate cancer and potentially delay exposure to systemic treatments that impact quality of life.

Date of Award	Dec 2021
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Prostate Cancer UK
Supervisor	Ian Mills (Supervisor), Simon McDade (Supervisor) & Richard Kennedy (Supervisor)