The Transmembrane Protein TM4SF3 Interacts with AR and AR-V7 and is Recruited to AR Target Genes.

Prostate cancer transitions from an early treatable form to the lethal castration-resistant prostate cancer (CRPC). Androgen receptor (AR) and constitutively active AR splice variants, like AR-V7, may be major drivers of CRPC. Our lab recently identified a novel mechanism of AR regulation via the transmembrane protein TM4SF3 (Transmembrane 4 superfamily 3), which exhibits a physical interaction, nuclear co-localization, and mutual stabilization with AR. Here we have mapped the interaction domains within AR and TM4SF3 and discovered that TM4SF3 also physically interacts with AR-V7, regulating its protein stability and the viability of CRPC cells expressing AR-V7. Ubiquitination of TM4SF3 and AR-V7 was detected for the first time and TM4SF3 interaction with either AR or AR-V7 resulted in mutual de-ubiquitination of both proteins, showing that mutual stabilization results from de-ubiquitination. Interestingly, nuclear TM4SF3 was co-recruited to the promoters of AR- and AR-V7-regulated genes and required for their expression, showing that TM4SF3 interaction is critical for their transcriptional functions. The results collectively show the multiple critical regulatory functions of TM4SF3 on AR or AR-V7 in prostate cancer cells.