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Inhibition of PP2A, but not PP5, mediates p53 activation by low levels of okadaic acid in rat liver epithelial cells
Author(s) -
Messner Donald J.,
Romeo Charles,
Boynton Alton,
Rossie Sandra
Publication year - 2006
Publication title -
journal of cellular biochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.028
H-Index - 165
eISSN - 1097-4644
pISSN - 0730-2312
DOI - 10.1002/jcb.20919
Subject(s) - okadaic acid , protein phosphatase 2 , phosphatase , gene knockdown , cancer research , apoptosis , mdm2 , phosphorylation , cell cycle , cell cycle checkpoint , biology , chemistry , microbiology and biotechnology , biochemistry
The microbial toxin okadaic acid (OA) specifically inhibits PPP‐type ser/thr protein phosphatases. OA is an established tumor promoter with numerous cellular effects that include p53‐mediated cell cycle arrest. In T51B rat liver epithelial cells, a model useful for tumor promotion studies, p53 activation is induced by tumor‐promoting (low nanomolar) concentrations of OA. Two phosphatases sensitive to these concentrations of OA, PP2A and protein phosphatase 5 (PP5), have been implicated as negative regulators of p53 . In this study we examined the respective roles of these phosphatases in p53 activation in non‐neoplastic T51B cells. Increases in p53 activity were deduced from levels of p21 (cip1) and/or the rat orthologue of mdm2, two p53‐regulated gene products whose induction was blocked by siRNA‐mediated knockdown of p53. As observed with 10 nM OA, both phospho‐ser15‐p53 levels and p53 activity were increased by 10 µM fostriecin or SV40 small t‐antigen. Both of these treatments selectively inhibit PP2A but not PP5. siRNA‐mediated knockdown of PP2A, but not PP5, also increased p53 activity. Finally, adenoviral‐mediated over‐expression of an OA‐resistant form of PP5 did not prevent increased phospho‐ser15‐p53, p53 protein, or p53 activity caused by 10 nM OA. Together these results indicate that PP5 blockade is not responsible for OA‐induced p53 activation and G 1 arrest in T51B cells. In contrast, specific blockade of PP2A mimics p53‐related responses to OA in T51B cells, suggesting that PP2A is the target for this response to OA. J. Cell. Biochem. © 2006 Wiley‐Liss, Inc.