title: Inhibition of PP2A, but not PP5, mediates p53 activation by low levels of okadaic acid in rat liver epithelial cells.
abstract: 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 microM 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 G1 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.
authors.affiliation: Department of Biochemistry, Purdue University, West Lafayette, Indiana 47907, USA.
authors.forename: Donald J
authors.initials: DJ
authors.lastname: Messner
authors.forename: Charles
authors.initials: C
authors.lastname: Romeo
authors.forename: Alton
authors.initials: A
authors.lastname: Boynton
authors.forename: Sandra
authors.initials: S
authors.lastname: Rossie
mesh_terms: D000475:Alkenes / Q000494:pharmacology; D000595:Amino Acid Sequence; D000818:Animals; D000957:Antigens, Viral, Tumor / Q000235:genetics / Q000378:metabolism; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism; D016193:G1 Phase / Q000187:drug effects; D008099:Liver / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D019319:Okadaic Acid / Q000494:pharmacology*; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D011090:Polyenes; D054648:Protein Phosphatase 2; D011753:Pyrones / Q000494:pharmacology; D034741:RNA, Small Interfering; D051381:Rats; D016159:Tumor Suppressor Protein p53 / Q000187:drug effects / Q000378:metabolism*
chemical_list: D000475:Alkenes; D000957:Antigens, Viral, Tumor; C493958:Cdkn1a protein, rat; D050759:Cyclin-Dependent Kinase Inhibitor p21; D004791:Enzyme Inhibitors; D009687:Nuclear Proteins; D011090:Polyenes; D011753:Pyrones; D034741:RNA, Small Interfering; D016159:Tumor Suppressor Protein p53; D019319:Okadaic Acid; D010749:Phosphoprotein Phosphatases; C522189:Ppp2r1b protein, rat; D054648:Protein Phosphatase 2; C089641:protein phosphatase 5; C040313:fostriecin