Cell cycle regulation and p53 activation by protein phosphatase 2Cα

Protein phosphatase 2C (PP2C) dephosphorylates a broad range of substrates, regulating stress response and growth-related pathways in both prokaryotes and eukaryotes. We now demonstrate that PP2Calpha, a major mammalian isoform,, inhibits cell growth and activates the p53 pathway. In 293 cell clones, in which PP2Calpha expression is regulated by a tetracycline-inducible promoter, PP2Calpha overexpression led to G(2)/M cell cycle arrest and apoptosis. Furthermore, PP2Calpha induced the expression of endogenous p53 and the p53-responsive gene p21. Activation of the p53 pathway by PP2Calpha took place both in cells harboring endogenous p53, as well as in p53-null cells transfected with exogenous p53. Induction of PP2Calpha resulted in an increase in the overall levels of p53 protein as-well as an augmentation of p53 transcription activity. The dephosphorylation activity of PP2Calpha is essential to the described phenomena, as none of these effects was detected when an enzymatically inactive PP2Calpha mutant was overexpressed. p53 plays an important role in PP2Calpha-directed cell cycle arrest and apoptosis because perturbation of p53 expression in human 293 cells by human papillomavirus E6 led to a significant increase in cell survival. The role of PP2Calpha in p53 activation is discussed.