Abstract PR-06: PKC∈-activated Stat3 and subsequent AR interaction are potential molecular targets for prevention of late-stage prostate cancer

Prostate cancer (PCa) is the second most common malignancy in men and the second leading cause of cancer-related deaths in the Western World. Androgen ablation therapy remains the gold standard for the treatment of PCa. Despite the initial success of androgen ablation therapy, resistance to anti-androgen therapy manifests by progression to castration-resistant prostate cancer (CRPC). CRPC is the end stage that accounts for the majority of PCa patient deaths. An understanding of the mechanisms linked to the emergence of CRPC is the key to the prevention of this disease. Overwhelming evidence supports the concept that development of CRPC is usually related to continued activation of androgen receptor (AR). Several mechanisms are proposed for androgen-dependent (AD) and androgen-independent (AI) activation of AR in CRPC. Gene amplification and mutations in AR are frequently observed in recurrent PCa, which may account for the hypersensitivity of the AR to low castrate level of androgens and altered ligand specificity. In the AR-dependent pathway, the functions of AR are modified significantly and involve AR interaction with many co-activators and co-repressors. Increased AR activity in CRPC is perhaps caused by cross talk of AR with multiple intracellular signaling cascades. We present here that PKC∈ and Stat3, which correlate with the aggressiveness of PCa, may play roles in both AI AR activation and promotion of PCa cell survival for emergence and progression of CRPC. Stat3 has two conserved amino acid residues (Tyr705 and Ser727), which have to be phosphorylated for maximum transcriptional activity. PKC∈, which is constitutively activated in PCa, mediates phosphorylation of both Stat3Tyr705 and Stat3Ser727. Specific observations are our finding that PKC∈ deletion in TRAMP mice, which prevents PCa development and metastasis, inhibits the expression levels of IL-6, pStat3Ser727 and pStat3Tyr705. PKC∈ overexpression is sufficient to transform AD LNCaP cells into an AI variant. We further explored the possibility that PKC∈− activated Stat3 is a non-androgen activator of AR. In this experiment, human PCa LNCaP (AD) cells were infected with adenoviral vector containing PKC∈ or empty vector; 48 h later, cells were harvested and whole cell lysates were prepared. The effect of PKC∈ overexpression on the interaction of Stat3 with AR was determined by performing reciprocal immunoprecipitation/blotting experiments. Overexpression of PKC∈ increased the interaction of Stat3 with AR. We also found similar interaction of Stat3 with AR in human PCa C4–2 (AI) cells treated with IL-6. To determine the link of PKC∈ in Stat3 interaction with AR and the growth of C4–2 cells, we used plumbagin (PL). PL is a unique plant-derived chemopreventive agent, which selectively inhibits expression of PKC∈. PL administration delayed the growth of C4–2 cells xenograft tumors as determined by both tumor weight and volume. In a reciprocal immunoprecipation/blotting experiments, PL inhibited Stat3-AR interaction. We conclude that: 1) PKC∈-mediated Stat3Ser727 phosphorylation and subsequent Stat3-AR interaction are essential components of the emergence and progression of CRPC, 2) Progression of CRPC is the result of cross-talk of PKC∈-Stat3 signaling with other signaling pathways (AKT and MAPK) which activate AR as well as promote cell survival and 3) PKC∈-activated Stat3 and subsequent AR interaction are potential molecular targets for prevention of late stage PCa. Support: NIH Grant CA138761. Citation Information: Cancer Prev Res 2011;4(10 Suppl):PR-06.