Leptin down‐regulates insulin action through phosphorylation of serine‐318 in insulin receptor substrate 1

Abstract

Insulin resistance in skeletal muscle is found in obesity and type 2 diabetes. A mechanism for impaired insulin signaling in peripheral tissues is the inhibition of insulin action through serine phosphorylation of insulin receptor substrate (Irs) proteins that abolish the coupling of Irs proteins to the activated insulin receptor. Recently, we described serine-318 as a protein kinase C (PKC)-dependent phosphorylation site in Irs1 (Ser-318) activated by hyperinsulinemia. Here we show in various cell models that the adipose hormone leptin, a putative mediator in obesity-related insulin resistance, promotes phosphorylation of Ser-318 in Irs1 by a janus kinase 2, Irs2, and PKC-dependent pathway. Mutation of Ser-318 to alanine abrogates the inhibitory effect of leptin on insulin-induced Irs1 tyrosine phosphorylation and glucose uptake in L6 myoblasts. In C57Bl/6 mice, Ser-318 phosphorylation levels in muscle tissue were enhanced by leptin and insulin administration in lean animals while in diet-induced obesity Ser-318 phosphorylation levels were already up-regulated in the basal state, and further stimulation was diminished. In analogy, in lymphocytes of obese hyperleptinemic human subjects basal Ser-318 phosphorylation levels were increased compared to lean individuals. During a hyperinsulinemic euglycemic clamp, the increment in Ser-318 phosphorylation observed in lean individuals was absent in obese. In summary, these data suggest that phosphorylation of Ser-318 in Irs1 mediates the inhibitory signal of leptin on the insulin-signaling cascade in obese subjects.

Phosphorylation of Ser357 of Rat Insulin Receptor Substrate-1 Mediates Adverse Effects of Protein Kinase C-δ on Insulin Action in Skeletal Muscle Cells

“…Data of the novel PKC isoform ␦ mirrored this positive and negative modulation of insulin signaling; PKC-␦ is shown to be important for insulin-stimulated glucose uptake (36), and it participates in the insulin-dependent activation of Akt (38). On the other hand, we and others have shown that activation of PKC-␦ by lipids and leptin is involved in the impairment of insulin signaling (35,47) and in the induction of apoptosis of insulin-secreting cells (48). Serine phosphorylation of IRS-1 appears to be a major mechanism for the adverse effects of PKC-␦ on insulin action (32,41,47).…”

“…On the other hand, we and others have shown that activation of PKC-␦ by lipids and leptin is involved in the impairment of insulin signaling (35,47) and in the induction of apoptosis of insulin-secreting cells (48). Serine phosphorylation of IRS-1 appears to be a major mechanism for the adverse effects of PKC-␦ on insulin action (32,41,47). Although in vitro at least 18 PKC-␦-dependent phosphorylation sites in IRS-1 have been identified (41), so far only two sites could be demonstrated to be phosphorylated in vivo and to be functional active, these are Ser 24 and Ser 318 (32,42,47).…”

“…Serine phosphorylation of IRS-1 appears to be a major mechanism for the adverse effects of PKC-␦ on insulin action (32,41,47). Although in vitro at least 18 PKC-␦-dependent phosphorylation sites in IRS-1 have been identified (41), so far only two sites could be demonstrated to be phosphorylated in vivo and to be functional active, these are Ser 24 and Ser 318 (32,42,47). Neither of these sites were shown to be phosphorylated upon insulin stimulation by PKC-␦, although insulin could activate PKC-␦ and induce its association to IRS-1 (40).…”

Protein kinase C-δ is involved in the inflammatory effect of IL-6 in mouse adipose cells

“…In another study, isolated adipocytes from B6 mice fed a high-fat diet showed impaired insulin-induced glucose uptake, oxidative stress and activation of PKCδ, which were suggested to be important for obesity-induced insulin resistance in the adipose tissue [28]. It has also been shown that PKCδ is involved in insulin-induced Ser-318 phosphorylation of IRS-1, thus impairing insulin signalling [29]. In this study, the role of several kinases (e.g.…”

New 4-[(5-arylidene-2-arylimino-4-oxo-3-thiazolidinyl)methyl]benzoic acids active as protein tyrosine phosphatase inhibitors endowed with insulinomimetic effect on mouse C2C12 skeletal muscle cells

“…This condition is mediated by adipose-derived cytokines and hormones (among them leptin) that influence insulin action in liver, fat and skeletal muscle [26,27].…”