Regulation of the tumor suppressor pRB by protein kinase Cepsilon in human prostate cancer cells

Prostatic epithelial cells normally rely on androgens for their growth and survival. However, prostate cancer cells expressing PKCϵ have the potential of surviving in the absence of androgen, and a correlation between the androgen independent (AI) recurrence of prostate cancer and PKCϵ expression has been established. Ectopic PKCϵ expression is sufficient to transform the androgen dependent LNCaP prostate cancer cell line into an AI variant (LNCaP/PKCϵ), capable of forming tumors in castrated nude mice. In vitro analysis of LNCaP/PKCϵ cells revealed competency for autocrine growth, and a sharp increase in the rate of proliferation, associated with an accelerated G₁ transit and enhanced translational capacity. The G₁/S cell cycle control proteins govern the phosphorylation state and half-life of the tumor suppressor protein pRB. Following culture in androgen-free medium, pRB was hypophosphorylated in quiescent LNCaP cells and association with the transcription factor E2F-1 increased. LNCaP/PKCϵ cells continued to proliferate and maintained pRB in a hyperphosphorylated state. This inactivation of pRB in LNCaP/PKCϵ cells was dependent on increased basal expression of cyclin D1 and CDK4 and their stable dimerization. The CDK2 inhibitors p21 and p27, were predominately associated with CDK4, whose activity they promote. Basal expression of pRB decreased in LNCaP/PKCϵ cells, and evidence suggests the hyperactivity of CDK4 was leading to post-translational modifications resulting in pRB degradation.; The identified phenotype of PKCϵ expressing prostate cancer cells was targeted to evaluate potential therapeutic benefits of disabling PKCϵ and downstream effector mechanisms. PKCϵ expressing cells were markedly more sensitive than LNCaP cells to an inhibitor of CDK4, showing critical dependence on CDK4 for enhanced proliferation and growth. c-Myc, an oncogene and transcription factor upregulated by PKCϵ, was targeted for downregulation by anti-sense. Diminished c-Myc levels decreased proliferation and survival of LNCaP/PKCϵ cells but not LNCaP cells. Translational hyperactivity via a rapamycin-sensitive pathway(s) was also found to be important to the proliferation and growth of PKCϵ expressing cells. Understanding the mechanisms by which PKCϵ allows prostate cancer cells to survive and proliferate in the absence of androgen could lead to the design of chemotherapeutics that are more effectively targeted.