Activation of protein kinase C alpha inhibits growth of pancreatic cancer cells via p21(cip)-mediated G(1) arrest

We have analyzed human pancreatic cancer cells to explore the growth regula tory function of protein kinase C (PKC)alpha, PKC alpha subcellular redistr ibution, activation kinetics and downregulation were examined in detail and correlated to immediate and delayed effects on cell-cycle regulatory pathw ays. TPA treatment resulted in transient PKC alpha activation accompanied b y translocation of the enzyme into membrane and nuclear compartments, and w as followed by subsequent downregulation, TPA-induced inhibition of DNA syn thesis was prevented by a PKC-antagonist and was reproduced by microinjecti on of recombinant PKC alpha, indicating that activation of this isoenzyme w as required and sufficient for growth inhibitory effects, PKC alpha activat ion arrested cells in the G(1) phase of the cell cycle as a consequence of selective inhibition of cyclin dependent kinase (CDK)2 activity with concom itant hypophosphorylation of Rb, The inhibition of CDK2 activity resulted f rom induction of p21(cip1) cyclin-dependent kinase inhibitors. Levels of p2 1(cip1) remained elevated and CDK2 activity repressed in spite of PKC alpha downregulation, indicating that downstream effecters of PKC alpha are the primary determinants for the duration of PKC-mediated growth inhibition. Th e PKC alpha-induced block in cell proliferation persisted even though cells mere kept in the presence of growth factors, suggesting that induction of PKCa results in a permanent,withdrawal of pancreatic cancer cells from the cell cycle.