Requirement of androgen-dependent activation of protein kinase Cζ for androgen-dependent cell proliferation in LNCaP cells and its roles in transition to androgen-independent cells

A cell line that we designed, AILNCaP, proliferated in androgen-depleted medium after emerging from long-term androgen-depleted cultures of an androgen-sensitive prostate cancer cell line, LNCaP. Using this cell line as a model of progression to androgen independence, we demonstrated that the activity of the mammalian target of rapamycin/p70 S6 kinase transduction pathway is down-regulated after androgen depletion in LNCaP, whereas its activation is related to transition of this cell line to androgen-independent proliferation. Kinase activity of protein kinase C zeta is regulated by androgen stimulation in LNCaP cells, whereas it is activated constitutively in AILNCaP cells under androgen-depleted conditions. Treatment with a protein kinase C zeta pseudosubstrate inhibitor reduced p70 S6 kinase activity and cell proliferation in both cell lines. We identified that both protein kinase C zeta and p70 S6 kinase were associated in LNCaP cells and this association was enhanced by the androgen stimulation. We examined the expression of phosphoprotein kinase C zeta and phospho-p70 S6 kinase in hormone-naive prostate cancer specimens and found that the expression of both kinases was correlated with each other in those specimens. Significant correlation was observed between the expression of both kinases and Ki67 expression. Most of the prostate cancer cells that survived after prior hormonal treatment also expressed both kinases. This is the first report that shows the significance of this pathway for both androgen-dependent and -independent cell proliferation in prostate cancer. Our data suggest that protein kinase C zeta/mammalian target of rapamycin/S6 kinase pathway plays an important role for the transition of androgen-dependent to androgen-independent prostate cancer cells.