Ceramide Produces Apoptosis Through Induction of p27^kip1 by Protein Phosphatase 2A-dependent Akt Dephosphorylation in PC-3 Prostate Cancer Cells

Abstract

Ceramide induces cell cycle arrest and apoptotic cell death associated with increased levels of p27^kip1. The aim of this study was to examine the effects of ceramide on p27^kip1 protein levels as a measure of cell cycle arrest and apoptosis. Results showed that ceramide increased p27^kip1 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-Cα siRNA inhibited ceramide-induced enhanced p27^kip1 protein expression and Akt dephosphorylation, and prevented Skp2 downregulation. Overexpression of constitutively active Akt attenuated ceramide-induced Skp2 downregulation and p27^kip1 upregulation. In addition, ceramide stimulated binding of the PP2A catalytic subunit PP2A-Cαβ to Akt as assessed by immunoprecipitation experiments, indicating that PP2A is involved in the induction of p27^kip1 via inhibition of Akt pathway. Finally, whether PP2A can regulate p27^kip1 expression independently of Akt pathway was determined. Knockdown of PP2A-Cα with siRNA reduced p27^kip1 levels in the presence of Akt inhibitor. These data reveal that PP2A is a regulator of ceramide-induced p27^kip1 expression via Akt-dependent and Akt-independent pathways.

This work was supported by a research grant from the Chung-Ang University.