Implications of PI3K/AKT inhibition on REST protein stability and neuroendocrine phenotype acquisition in prostate cancer cells

Treatment-induced neuroendocrine prostate cancer (t-NEPC) is an aggressive subtype of prostate cancer (PCa) that arises as a consequence of rigorous androgen receptor (AR) pathway inhibition (ARPI) therapies. While the PI3K/AKT pathway has been investigated as a co-therapeutic target with ARPI for advanced PCa, whether this strategy can prevent tumor progression to t-NEPC remains unknown. Here, we report that PI3K/AKT inhibition alone reduces RE-1 silencing transcription factor (REST) protein expression and induces multiple NE markers in PCa cells. The loss of REST by PI3K/AKT inhibition is through protein degradation mediated by the E3-ubiquitin ligase beta-TRCP and REST phosphorylations at the S1024, S1027, and S1030 sites. Since AR inhibition can also deplete REST, the combinational inhibition of PI3K/AKT and AR further aggravated REST protein reduction. We profiled the transcriptomes of AKT and AR inhibitions in the LNCaP cells. The Gene Set Enrichment Analysis (GSEA) showed that these transcriptomes are highly correlated with the REST-regulated gene signature. Co-targeting AKT and AR resulted in a higher correlation comparing to those of single treatment. Comparing these transcriptomes to the t-NEPC gene signature in patients by GSEA, we observed that adding AKT inhibition to AR blockade enhanced the expression of neurogenesis-related genes and resulted in a stronger and broader upregulation of REST-regulated genes specific to t-NEPC. These results indicate that AKT pathway inhibition can induce neuroendocrine differentiation of PCa cells via REST protein degradation. It delineates a potential risk for the AR and PI3K/AKT co-targeting strategy as it may further facilitate t-NEPC development.