Cell cycle regulation and p53 activation by protein phosphatase 2C alpha

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 PP2C alpha, a major mammalian isoform, inhibits cell growth and activates the p53 pathway. In 293 cell clones, in which PP2C alpha expression is regulated by a tetracycline-inducible promoter, PP2C alpha overexpression led to G(2)/M cell cycle arrest and apoptosis. Furthermore, PP2C alpha induced the expression of endogenous p53 and the p53-responsive gene p21. Activation of the p53 pathway by PP2C alpha took place both in cells harboring endogenous p53, as well as in p53-null cells transfected with exogenous p53. Induction of PP2C alpha resulted in an increase in the overall levels of p53 protein as well as an augmentation of p53 transcription activity. The dephosphorylation activity of PP2C alpha is essential to the described phenomena, as none of these effects was detected when an enzymatically inactive PP2C alpha mutant was overexpressed. p53 plays an important role in PP2C alpha-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 PP2C alpha in p53 activation is discussed.