Proliferating cell nuclear antigen in regulation of androgen receptor signalings in castration-resistant prostate cancer cells

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The overexpression of the full-length androgen receptor (AR-FL) and/or the constitutively active AR splicing variants (AR-Vs), such as AR-V7 and ARv567es lacking all or part of the ligand binding domain, contributes to the development and progression of castration resistant prostate cancers (CRPC). Androgen deprivation therapy (ADT) is not effective in CRPC overexpressing the AR-Vs and can even induce AR-V overexpression. Proliferating cell nuclear antigen (PCNA), preferentially overexpressed in all tumor cells, is a non-oncogenic protein essential for DNA replication and repair as well as cell growth and survival. Native PCNA is a ring-shaped homotrimer located mainly in the nucleoplasm. To be functional, PCNA must be linearized or monomerized for relocalization to chromatin, cytoplasm, or cell membrane, and serves as a platform for and executes its function through interaction with partner proteins containing the PCNA-interacting protein box (PIP-box) and other motifs. The PI’s group identified a consensus sequence of the PIP-box at the N-terminus of AR. PCNA complexes with AR-FL and AR-V7, which can be attenuated by a PIP-box-specific inhibitor T2AA. PCNA also binds to ARv567es and directly to recombinant AR protein. PCNA-I1S, a small molecule PCNA inhibitor developed by PI’s group, binds at the interface of two monomers, stabilizes trimer structure, and attenuates PCNA association to chromatin. PCNA-I1S and T2AA inhibit AR transcriptional activity and expression of AR target genes in CRPC LNCaP-AI and 22Rv1 cells. The knockdown expression of PCNA reduces dihydrotestosterone-stimulated AR transcriptional activity and abolishes the inhibitory effects of PCNA-I1S on AR activity. More importantly, an AR- specific PIP-box peptide inhibitor R9-AR-PIP, developed recently in the PI’s lab, binds to PCNA and inhibits AR transcriptional activity, expression of AR target genes, and growth of AR-positive cells. Based on these observations, the PI hypothesizes that PCNA interacts with AR-FL and AR-Vs through the AR PIP-box and enhances AR transcriptional activities and that targeting the PCNA-AR interaction will attenuate AR-FL- and AR-V-mediated signaling and inhibit growth of CRPC overexpressing AR-FL and/or AR-Vs. Two specific aims are proposed. Studies in aim 1 will further elucidate the role of the AR PIP-box in the interaction of AR-FL and AR-Vs with PCNA by protein mutagenesis and investigate the inhibitory effects of PCNA-I1S, T2AA, and R9-AR-PIP on the colocalization and chromatin association of PCNA with AR-FL and AR-Vs, the role of PCNA-AR interaction in recruitment of AR-FL and AR-Vs to chromatin, and the selective inhibitory effects of R- 9-AR-PIP on AR-PCNA interaction. Studies in aim 2 will determine the inhibitory effects of PCNA-I1S, T2AA, and R9-AR-PIP on the occupancy of the androgen response elements by AR-FL and AR-Vs, the transcriptional activity of AR-FL and AR-V7, and expression of AR-V-specific genes. Moreover, the cytotoxic effects of R9-AR- PIP on CRPC cells in culture, the therapeutic effects of R9-AR-PIP against CRPC xenograft tumors in mice, and the selective inhibitory effects of R9-AR-PIP on AR signaling in the CRPC tumors will be investigated.