Protein Phosphatase-1 and -2A Dephosphorylate p53 at Ser-20 to Control Growth-Factor-Induced Differentiation and Stress-Induced Apoptosis of Lens Epithelial Cells

Abstract

Purpose: The tumor suppressor is an important regulator in the ocular lens. Inactivation of p53 by viral gene expression or knockout leads to cataractogenesis. Mechanistically, p53 regulates both lens cell differentiation and apoptosis. For the control of lens differentiation, p53 directly control both c-Maf and Prox-1 to control lens differentiation. For the control of apoptosis, p53 regulates Bak and a panel of other genes. In the present study, we present evidence to show that PP1 and PP-2A dephosphorylate p53 at Ser-20 to control its transactivity through which PP-1 and PP-2A controls lens differentiation and pathogenesis.

Methods: Human lens epithelial cells, alphaTN4-1 mouse lens epithelial cells, wild type and p53 knockout mice were used as testing systems. Co-immunoprecipitation assays, in vitro dephosphorylation assay, inhibition of phosphatase activity, overexpression and knockdown of various subunits from PP-1 and PP-2A were used to determine the specific dephosphorylation of p53 by PP-1 and PP-2A. Reverse transcription polymerase chain reaction, Western-blot analysis and reporter gene activity assays were used to study the expression of the related genes under various conditions.

Results: During mouse lens development, p53 was differentially phosphorylated at Ser-20 and other sites. Both PP-1 and PP-2A participate in control of p53 transactivity through direct dephosphorylation at Ser-20. Changes in the phosphorylation status of p53 at Ser-20 leads to altered patterns of induced lens differentiation and apoptosis of lens epithelial cells.

Conclusions: PP-1 and -2A regulates lens differentiation and survival through modulation of p53 functions. Dephosphorylation of p53 at Ser-20 by PP-1 and PP-2A displays profound effects on p53 functions in the ocular lens. (Supported by EY018380, CSC, Zhongshan Ophthalmic Center and HNU)

Keywords: 646 phosphorylation • 714 signal transduction • 426 apoptosis/cell death