In vivo quantitative phosphoproteomic profiling identifies novel regulators of castration-resistant prostate cancer growth

Prostate cancer remains a leading cause of cancer-related mortality worldwide owing to our inability to treat effectively castration-resistant tumors. To understand the signaling mechanisms sustaining castration-resistant growth, we implemented a mass spectrometry-based quantitative proteomic approach and use it to compare protein phosphorylation in orthotopic xenograft tumors grown in either intact or castrated mice. This investigation identified changes in phosphorylation of signaling proteins such as MEK, LYN, PRAS40, YAP1 and PAK2, indicating the concomitant activation of several oncogenic pathways in castration-resistant tumors, a notion that was confirmed by tumor transcriptome analysis. Further analysis demonstrated that the activation of mTORC1, PAK2 and the increased levels of YAP1 in castration-resistant tumors can be explained by the loss of androgen inhibitory actions. The analysis of clinical samples demonstrated elevated levels of PAK2 and YAP1 in castration-resistant tumors, whereas knockdown experiments in androgen-independent cells demonstrated that both YAP1 and PAK2 regulate cell colony formation and cell invasion activity. PAK2 also influenced cell proliferation and mitotic timing. Interestingly, these phenotypic changes occur in the absence of obvious alterations in the activity of AKT, MAPK or mTORC1 pathways, suggesting that PAK2 and YAP1 may represent novel targets for the treatment of castration-resistant prostate cancer. Pharmacologic inhibitors of PAK2 (PF-3758309) and YAP1 (Verteporfin) were able to inhibit the growth of androgen-independent PC3 xenografts. This work demonstrates the power of applying high-resolution mass spectrometry in the proteomic profiling of tumors grown in vivo for the identification of novel and clinically relevant regulatory proteins.