Differential requirements for ras and the retinoblastoma tumor suppressor protein in the androgen dependence of prostatic adenocarcinoma cells

Prostate cells are dependent on androgen for proliferation, but during tumo r progression prostate cancer cells achieve independence from the androgen requirement. We report that androgen withdrawal fails to inhibit cell cycle progression or influence the expression of cyclin-dependent kinase (CDK)/c yclins in androgen-independent prostate cancer cells, indicating that these cells signal for cell cycle progression in the absence of androgen. Howeve r, phosphorylation of the retinoblastoma tumor suppressor protein (RB) is s till required for G(1)-S progression in androgen-independent cells, since t he expression of constitutively active RE (PSM-RB) or p16ink4a caused cell cycle arrest and mimicked the effects of androgen withdrawal on downstream targets in androgen-dependent LNCaP cells. Since Ras is known to mediate mi togenic signaling to RE, we hypothesized that active V12Ras would induce an drogen-independent cell cycle progression in LNCaP cells. Although V12Ras w as able to stimulate ERK phosphorylation and induce cyclin D1 expression in the absence of androgen, it was not sufficient to promote androgen-indepen dent cell cycle progression. Similarly, ectopic expression of CDK4/cyclin D 1, which stimulated RE phosphorylation in the presence of androgen, was inc apable of inactivating RE or driving cell cycle progression in the absence of androgen. We show that androgen regulates both CDK4/cyclin D1 and CDK2 c omplexes to inactivate RE and initiate cell cycle progression. Together, th ese data show that androgen independence is achieved via deregulation of th e androgen to RE signal, and that this signal can only be partially initiat ed by the Ras pathway in androgen-dependent cells.