Exploring the anti-proliferative impact of PBX1 perturbation in enzalutamide-resistant CRPC

Advanced prostate cancer (PCa) is often driven by upregulated activity of the androgen receptor (AR) and is clinically targeted by second-generation AR-antagonists such as Enzalutamide. Though patients initially respond to treatment, many develop resistance, leading to disease advancement and poor prognosis. We have focused on a mechanism involving an AR cistrome switch from a canonical to a noncanonical cistrome as PCa becomes Enzalutamide-resistant. As collaborating protein partners drive transcriptional activity, we employed core regulatory circuitry analysis in Enzalutamide-resistant PCa cell line models to identify novel TF candidates that may mediate non-canonical AR cistrome reprogramming, including Pre-B-cell leukemia homeobox 1 (PBX1), a TALE homeodomain TF. We previously showed that PBX1 is highly expressed in castrate-resistant and Enzalutamide-resistant (EnzaR-CRPC) cell line models compared with sensitive models. We have also demonstrated that PBX1 inhibition has an anti-proliferative effect in a high-PBX1-expressing EnzaR-CRPC cell line, with synergism observed with continuous Enzalutamide treatment. In the current study, we perturbed PBX1 expression in an EnzaR-CRPC cell line by shRNA (PBX1 KD) and CRISPR-Cas9 (PBX1 KO). Western blotting, RT-qPCR, and genomic DNA analysis validated a significant reduction in PBX1 protein and gene levels. Similar to previous observations with PBX1 inhibitors, PBX1 KD and PBX1 KO cells showed an initial, prominent decrease in proliferation compared to their respective controls. Interestingly, PBX1 KO clones after several passages regained increased proliferation. On the contrary, the PBX1 KD cells continue to demonstrate anti-proliferation. Ongoing transcriptomic and genome-wide DNA analysis will provide a clear understanding of the biological pathways affected by PBX1 perturbation and, most importantly, of changes in the AR and PBX1 cistrome, promoter, and enhancer landscape at low levels of PBX1 (PBX1 KD) vs. complete elimination of PBX1 (PBX1 KO). This would provide a better insight into (i) AR-PBX1 transcriptional axis and PBX1's role in promoting treatment-resistant, advanced PCa, (ii) the long-term implications for PBX1 suppression, and (iii) the best treatment strategy for targeting both PBX1 and AR for a durable positive outcome in advanced treatment-resistant CRPC.