title: Role of janus kinase-2 in insulin-mediated phosphorylation and inactivation of protein phosphatase-2A and its impact on upstream insulin signalling components.
abstract: Our recent studies indicate that insulin rapidly inactivates serine/threonine protein phosphatase-2A (PP-2A) by increasing tyrosine phosphorylation on the catalytic subunit. The exact mechanism of PP-2A inactivation by insulin in vivo is unclear. The Janus kinase (JAK) family of non-receptor protein tyrosine kinases constitute a novel type of signal-transduction pathway which is activated in response to a wide variety of polypeptide ligands, including insulin. In this study we investigated the potential role of JAK-2 in insulin-mediated tyrosine phosphorylation and inactivation of PP-2A using the rat skeletal muscle cell line L6. Co-immunoprecipitation studies revealed that PP-2A is associated with JAK-2 in the basal state. Insulin treatment did not alter JAK-2 association with PP-2A, but did increase JAK-2-mediated tyrosine phosphorylation of the PP-2A catalytic subunit and therefore inhibited PP-2A enzymic activity. Furthermore, PP-2A is associated with phosphoinositide 3-kinase (PI-3K) in the basal state and insulin treatment increases the catalytic activity of PI-3K bound to PP-2A. Pretreatment with AG-490, a specific JAK-2 inhibitor, and SpcAMP, a cAMP agonist, prevented the insulin-mediated increase in (i) JAK-2 kinase activity, (ii) PP-2A tyrosine phosphorylation, (iii) PP-2A inactivation and restored the enzyme activity to control levels, and (iv) PP-2A and JAK-2-associated PI-3K activity. These observations, together with the fact that insulin rapidly activates JAK-2 in L6 cells, and that this is accompanied by an increase in tyrosine phosphorylation of PP-2A in JAK-2 immunoprecipitates, suggest that insulin controls the activation status of PP-2A by tyrosine phosphorylation via JAK-2. PP-2A inactivation may result in an amplification of insulin-generated signals at the level of PI-3K.
authors.affiliation: The Diabetes Research Laboratory, Winthrop University Hospital, 259 First St, Mineola, NY 11501, USA. nbegum@winthrop.org
authors.forename: N
authors.initials: N
authors.lastname: Begum
authors.forename: L
authors.initials: L
authors.lastname: Ragolia
mesh_terms: D017667:Adipocytes / Q000201:enzymology; D000818:Animals; D002460:Cell Line; D000242:Cyclic AMP / Q000819:agonists / Q000494:pharmacology; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007328:Insulin / Q000494:pharmacology*; D053614:Janus Kinase 2; D018482:Muscle, Skeletal; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010766:Phosphorylation; D011233:Precipitin Tests; D054648:Protein Phosphatase 2; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D011518:Proto-Oncogene Proteins; D051381:Rats; D015398:Signal Transduction; D020032:Tyrphostins / Q000494:pharmacology
chemical_list: D004791:Enzyme Inhibitors; D007328:Insulin; D011518:Proto-Oncogene Proteins; D020032:Tyrphostins; C095512:alpha-cyano-(3,4-dihydroxy)-N-benzylcinnamide; D000242:Cyclic AMP; D019869:Phosphatidylinositol 3-Kinases; D011505:Protein-Tyrosine Kinases; C507927:Jak2 protein, rat; D053614:Janus Kinase 2; D010749:Phosphoprotein Phosphatases; D054648:Protein Phosphatase 2