Extreme Activation of Androgen Receptor for Prostate Cancer Therapy

Abstract

Prostate cancer (PCa) is the second most common cancer worldwide in men and one of the major causes of cancer-related death among men in Australia. In PCa cells, the androgen receptor (AR) is the key driver of cell proliferation, cell cycle progression, and metabolism; thus, blocking AR activity with androgen deprivation therapy (ADT) is a standard-of-care treatment for metastatic PCa. However, ADT is never curative, with all patients eventually relapsing with lethal castration-resistant prostate cancer (CRPC). In a paradoxical phenomenon, potent activation of AR with high doses of androgens can also inhibit the growth of PCa tumours. However, the exact mechanism(s) by which activation of AR can block PCa growth is poorly understood. Therefore, in my PhD project, I explored the mechanisms underlying PCa growth suppression in response to extreme activation of AR using a potent androgen, methyltestosterone (MeT). I have found that methyl-testosterone (MeT), a synthetic androgen, can potently transactivate AR and suppress the proliferation of AR-positive prostate cancer cells (LNCaP, C42B, MR49F, and 22RV1) but not an AR-negative cell line (PC3) or a PCa model expressing a version of the AR lacking the ligand-binding domain (R1-D567), suggesting that the growth-inhibitory effects of MeT are AR-dependent. Mechanistically, MeT acts much like high-dose dihydrotestosterone (DHT) in terms of genome-wide AR binding (evaluated by ChIP-seq) and the transcriptional program activated via AR (evaluated by RNA-seq). However, these analyses showed that MeT only extends the AR cistrome and enables AR to act as a potent transcriptional repressor of genes associated with cell cycle, DNA replication, and DNA damage responses. Unexpectedly, our RNA-seq data revealed that MeT dysregulates the expression of transposable elements, including endogenous retroviruses (ERVs). Mechanistically, we found that MeT suppresses the expression of DNA methyl-transferases (DNMTs) and EZH2, which are considered to be key factors repressing the expression of transposable elements. Consistent with the proposed hypothesis, my PhD work showed that MeT caused global hypomethylation of DNA and re-distribution of H3K27me3. More specifically, my research supports a model whereby DNA hypomethylation was linked to the induction of endogenous retroviruses (ERVs). Interestingly, I found that ERV induction was associated with a “viral mimicry” response characterised by activation of pattern recognition receptors RIG-I and STING and subsequent activation of interferon (IFN) signalling. Importantly, I also observed increased expression of MHC class I genes with MeT treatment, suggesting that it can enhance tumour immunogenicity. Validating this finding, co-culture of a murine model of PCa (RM1) with tumour-specific CD8+ T cells revealed that MeT promoted enhanced recognition and functional cytokine production by T cells. Collectively, my work has provided a greater understanding of growth-inhibitory effects of androgens on PCa tumours and uncovers a potential new role for high-dose androgen therapy as an immunosensitisation agent.