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.

Involvement of Protein Phosphatase 2A in the Interleukin-3-Stimulated Jak2-Stat5 Signaling Pathway

“…(29) Our in vitro data (Fig. 4) support the idea that Jak2 downregulates PP2A activity via tyrosine phosphorylation of the C subunit of PP2A.…”

“…Previous studies showed an IL-11-induced association of PP2A, PI3 kinase, and Yes kinase with Jak2 (28) and an increase in PP2A-Jak2 association in the insulin-stimulated rat skeletal muscle cell line L6. (29) In the latter case, insulin treatment did not increase the association between Jak2 and PP2A. In 32Dcl3 cells, the degree of association increased dramatically during agonist stimulation (Fig.…”

“…Various protein kinase activities are known to be regulated by PP2A in vitro, (16,17) and PP2A has been shown to form stable complexes with various protein kinases, including casein kinase 2a, (25) calmodulin (CaM) kinase IV, (26) p21-activating kinase 1 (PAK1), PAK3, p70 S6 kinase, (27) and Jak2. (28,29) In this paper, we investigate the agonist-dependent and timedependent association of PP2A and Jak2 in 32Dcl3 myeloid progenitor cells. Because tyrosine phosphorylation of PP2A has the potential to modify the serine/threonine phosphatase activity of the enzyme, we carried out experiments aimed at uncovering a possible role for PP2A in Jak-Stat signal transduction.…”

Oxidative stress induces inactivation of protein phosphatase 2A, promoting proinflammatory NF-κB in aged rat kidney

“…It has been reported that growth stimulation of cells in response to epidermal growth factor or serum [16], in response to IL-1 or TNFα [17], or in response to insulin [18] also promotes transient tyrosine phosphorylation and inactivation of PP2A. Thus, the increased phosphorylation of PP2A observed in intact cells in vitro implicates uncontrolled clinical conditions such as inflammatory or cancerous status [19].…”

Reduction of PP2A Cα stimulates adipogenesis by regulating the Wnt/GSK-3β/β-catenin pathway and PPARγ expression

“…Several studies reported that insulin 322 down-regulated PP2A Cα expression and PP2A activity in muscle 323[27][28][29], suggesting that insulin is mainly involved in the reduction of 324 PP2A Cα in C3H10T1/2 and 3T3-L1 in our experiment. These observa-325 tions led us to hypothesize that PP2A Cα regulates adipocyte differenti-326 ation by regulating the expression of adipocyte marker genes.…”