Prostate cancer (PCa) is one of the most common cancers in men worldwide. Current treatments focus on targeting the Androgen Receptor (AR) with androgen deprivation therapies (ADT) such as enzalutamide. Although initially effective, ADT’s create a selection pressure resulting in treatment resistance in a form of advanced PCa called castration resistant prostate cancer (CRPC). This emphasises the need for alternative therapeutic approaches that will bypass this resistant mechanism, continue to disrupt AR signalling without targeting AR itself. One such method includes targeting co-regulators in the AR signalling network such as the Serum Response Factor (SRF), which has been identified as being crucial to PCa progression. Previous co-immunoprecipitation studies combined with mass spectrometry have identified common interactors between AR and SRF intracellular network, including HSP70, HSP90 and members of the PI3K/Akt pathway. Our research focuses on targeting the co-regulators within the AR-SRF network as a pharmacological approach for patients with CRPC.

We investigated targeting the AR-SRF interactome in four PCa cell lines: LNCaP (ADT-sensitive), LNCaP Abl and C4 (ADT-resistant), 22Rv1 (ADT-resistant and ARv7 positive) and in a non-cancerous prostate cell line (PWR1E). Small molecule inhibitors include SRF inhibitors (CCG1423, Lestaurtinib), AR/ARv7 inhibitors (Enzalutamide, EPI7170) and common co-factor inhibitors (VER-15508, JG-98, Ganetespib, Ipatasertib, Alpelisib). MTT assays and IncuCyte proliferation analysis were performed for single and combination treatments, with CompuSyn software analyzing combination profiles for antagonism, additivity, or synergy. Western blotting assessed phosphoproteomic changes after treatment.

We have shown that inhibiting AR, SRF and common co-factors, singly and in combination, decreases cell viability and proliferation in the PCa cell line panel. Furthermore, we show that the following combinations are synergistic, even at IC10 concentrations: Lestaurtinib+Ipatasertib, CCG1423+EPI7170, EPI7170+Ipatasertib and EPI7170+Lestaurtinib. Lestaurtinib and Ipatasertib were shown to alter the global phosphorylation landscape in 22Rv1. To better understand the molecular mechanisms behind the synergy, proteomic and phosphoproteomic analysis are currently ongoing.

To conclude, our results indicate that targeting the AR-SRF intracellular network holds promise as a treatment option for patients with CRPC. Combination treatments were shown to bypass resistance mechanisms in our CRPC cell line models and are synergistic at lower concentrations. Furthermore, by studying the proteomic and phosphor-proteomic landscape before and after treatments, we can further understand disease and resistance mechanisms in PCa as well as find potential novel targets which can be used for the disease.