A p53-independent G1 cell cycle checkpoint induced by the suppression of protein kinase C alpha and theta isoforms.

The protein kinase C (PKC) family consists of multiple isoforms that are involved in the regulation of diverse cellular responses. Suppression of PKC induces growth arrest in various types of cells. However, the underlying molecular mechanisms have not been thoroughly investigated. In this report, we demonstrated that the concurrent inhibition, rather than separate inhibition, of phorbol ester-dependent PKC alpha and theta isoforms is crucial for the induction of G1 cell cycle arrest and that this negative cell cycle regulation is via p53-independent mechanisms. PKC suppression-mediated growth arrest is associated with the induction of cell cycle inhibitor p21WAF1/CIP1 and the occurrence of hypophosphorylated Rb. The G1 checkpoint induced by the suppression of PKC occurs not only in murine Swiss3T3 but also in p53-deficient cells and human lung cancer cells containing mutated p53. Luciferase and nuclear run-off assays demonstrated that p21WAF1/CIP1 is, in part, transcriptionally regulated in response to the suppression of PKC alpha and theta. However, the stability of p21 mRNA is also augmented after the addition of PKC alpha and theta antisense oligonucleotides, indicating the involvement of post-transcriptional mechanisms in p21WAF1/CIP1 expression. These data suggest the existence of a cell cycle checkpoint pathway regulated by PKC alpha and theta isoforms. Furthermore, our findings support the notion that G1 checkpoint control can be restored in tumor cells containing abnormal p53, by targeting the PKC-regulated p21WAF1/CIP1 induction.