Okadaic acid-induced lens epithelial cell apoptosis requires inhibition of phosphatase-1 and is associated with induction of gene expression including p53 and bax.

Abstract

It is well established that phosphorylation and dephosphorylation are key cellular events which regulate important metabolic activities such as gene expression, cell cycle progression, and apoptosis. The polyether fatty acid, okadaic acid has been shown previously to activate apoptosis in a variety of cell lines. Although this marine sponge toxin is known to inhibit protein phosphatase (PP)-2A and PP-1, it is not certain in most cases whether inhibition of PP-1 or PP-2A is necessary to activate apoptosis. Furthermore, it is not clear how inhibition of these phosphatases leads to apoptosis. Here we present evidence that inhibition of PP-2A by okadaic acid does not activate apoptosis in the lens system. However, when PP-1 is inhibited by okadaic acid, rabbit lens epithelial cells undergo rapid apoptosis. Associated with this process is the several-fold up-regulation of the tumor suppressor gene p53 and the pro-apoptotic gene bax at both mRNA and protein levels. Analyses of the temporal pattern of expression of the two genes reveal that the up-regulation is maximized in a few hours after treatment with okadaic acid, when the majority of the treated cells become committed to apoptosis. A brief treatment of the cells with a protein synthesis inhibitor can abolish okadaic acid-induced up-regulation of both P53 and Bax proteins. Concomitant with this inhibition, okadaic acid-induced apoptosis is also temporarily blocked. These results suggest that okadaic acid-induced expression of p53, bax, and other genes are necessary for the activation of the apoptotic programs in lens systems.