Distinct roles for PP1 and PP2A in phosphorylation of the retinoblastoma protein. PP2a regulates the activities of G(1) cyclin-dependent kinases.
The function of the retinoblastoma protein (pRB) in controlling the G(1) to S transition is regulated by phosphorylation and dephosphorylation on serine and threonine residues. While the roles of cyclin-dependent kinases in phosphorylating and inactivating pRB have been characterized in detail, the roles of protein phosphatases in regulating the G(1)/S transition are not as well understood. We used cell-permeable inhibitors of protein phosphatases 1 and 2A to assess the contributions of these phosphatases in regulating cyclin-dependent kinase activity and pRB phosphorylation. Treating asynchronously growing Balb/c 3T3 cells with PP2A-selective concentrations of either okadaic acid or calyculin A caused a time- and dose-dependent decrease in pRB phosphorylation. Okadaic acid and calyculin A had no effect on pRB phosphatase activity even though PP2A was completely inhibited. The decrease in pRB phosphorylation correlated with inhibitor-induced suppression of G(1) cyclin-dependent kinases including CDK2, CDK4, and CDK6. The inhibitors also caused decreases in the levels of cyclin D2 and cyclin E, and induction of the cyclin-dependent kinase inhibitors p21(Cip1) and p27(Kip1). The decrease in cyclin-dependent kinase activities were not dependent on induction of cyclin-dependent kinase inhibitors since CDK inhibition still occurred in the presence of actinomycin D or cycloheximide. In contrast, selective inhibition of protein phosphatase 1 with tautomycin inhibited pRB phosphatase activity and maintained pRB in a highly phosphorylated state. The results show that protein phosphatase 1 and protein phosphatase 2A, or 2A-like phosphatases, play distinct roles in regulating pRB function. Protein phosphatase 1 is associated with the direct dephosphorylation of pRB while protein phosphatase 2A is involved in pathways regulating G(1) cyclin-dependent kinase activity.
Yan
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Department of Pharmacology, The University of Texas Southwestern Medical Center, Dallas, Texas 75235-9041, USA.
Mumby
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D016475:3T3 Cells; D000818:Animals; D000935:Antifungal Agents / Q000494:pharmacology; D056746:Cyclin G; D056766:Cyclin G1; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007527:Isoenzymes / Q000378:metabolism*; D008387:Marine Toxins; D051379:Mice; D008807:Mice, Inbred BALB C; D019319:Okadaic Acid / Q000494:pharmacology; D010080:Oxazoles / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D010766:Phosphorylation; D054645:Protein Phosphatase 1; D054648:Protein Phosphatase 2; D011714:Pyrans; D016160:Retinoblastoma Protein / Q000378:metabolism*; D013141:Spiro Compounds
D000935:Antifungal Agents; C534703:Ccng1 protein, mouse; D056746:Cyclin G; D056766:Cyclin G1; D016213:Cyclins; D004791:Enzyme Inhibitors; D007527:Isoenzymes; D008387:Marine Toxins; D010080:Oxazoles; D011714:Pyrans; D016160:Retinoblastoma Protein; D013141:Spiro Compounds; C053079:tautomycin; D019319:Okadaic Acid; C059041:calyculin A; D018844:Cyclin-Dependent Kinases; D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1; D054648:Protein Phosphatase 2