Ceramide Produces Apoptosis Through Induction of p27kip1 by Protein Phosphatase 2A-dependent Akt Dephosphorylation in PC-3 Prostate Cancer Cells

Ceramide induces cell cycle arrest and apoptotic cell death associated with increased levels of p27kip1. The aim of this study was to examine the effects of ceramide on p27kip1 protein levels as a measure of cell cycle arrest and apoptosis. Results showed that ceramide increased p27kip1 protein levels through activation of protein phosphatase 2A (PP2A) in PC-3 prostate cancer cells. Treatment of cells with the PP2A inhibitor okadaic acid or with PP2A-C siRNA inhibited ceramide-induced enhanced p27kip1 protein expression and Akt dephosphorylation, and prevented Skp2 downregulation. Overexpression of constitutively active Akt attenuated ceramide-induced Skp2 downregulation and p27kip1 upregulation. In addition, ceramide stimulated binding of the PP2A catalytic subunit PP2A-C to Akt as assessed by immunoprecipitation experiments, indicating that PP2A is involved in the induction of p27kip1 via inhibition of Akt pathway. Finally, whether PP2A can regulate p27kip1 expression independently of Akt pathway was determined. Knockdown of PP2A-C with siRNA reduced p27kip1 levels in the presence of Akt inhibitor. These data reveal that PP2A is a regulator of ceramide-induced p27kip1 expression via Akt-dependent and Akt-independent pathways.