Cells were treated and lysed because already described here. Insulin resistance happens throughout life; but when combined with impaired insulin secretion, it contributes to type 2 diabetes (1, 2). Initially, cells of the pancreas boost insulin secretion to compensate for moderate hyperglycemia owing to insulin resistance; however, prolonged hyperinsulinemia and hyperglycemia might impair insulin action and exacerbate the demand for insulin Tfpi and promote the development of diabetes (2C4). Recent evidence reveals a detailed relationship between pancreatic -cell function and insulin signaling. Disruption of the insulin receptor in pancreatic cells impairs glucose-stimulated insulin secretion (5). Moreover, mice missing IRS-2 develop uncompensated peripheral insulin Bay 60-7550 resistance owing at least partially to reduced -cell mass, suggesting that IRS-2 is usually a common element for insulin action and -cell proliferation or survival (6). Insulin binds to the Bay 60-7550 subunit of the insulin receptor to promote tyrosine autophosphorylation of the subunit, which activates the catalytic domain name and stimulates phosphorylation of cellular substrates, including the IRS proteins and Shc (7, 8). Phosphorylation of IRS-1 or IRS-2 on multiple tyrosine residues creates an active signaling complex by recruiting numerous proteins, including the phosphatidylinositol 3 (PI3) kinase, Grb2, Nck, Crk, Fyn, SHP2, and possibly others (9, 10). In humans, mutations of the insulin receptor contribute hardly ever to type 2 diabetes, and IRS-protein mutations that are directly responsible for diabetes in people are difficult to find (11); however, dysregulation of the insulin receptor and IRS proteins are common occurrences in type 2 diabetes (11C13). A number of mechanisms might be involved in the inhibition of insulin-stimulated tyrosine phosphorylation of the insulin receptor and the IRS proteins, including proteasome-mediated degradation (14, 15); phosphatase-mediated dephosphorylation (16, 17), or kinase-mediated serine/threonine phosphorylation (18). Acute and chronic stress causes insulin resistance, which might be mediated through cytokine-stimulated protein kinase cascades, including those triggered by TNF- or IL1- (19). TNF-, a potential mediator of insulin resistance, promotes serine/threonine phosphorylation of IRS-1 and IRS-2 (20C22). Serine/threonine phosphorylation of IRS-1 impairs the ability of IRS-1 to connect with the insulin receptor, which inhibits subsequent insulin-stimulated tyrosine phosphorylation (23). Moreover, during serine phosphorylation, IRS-1 might inhibit insulin-stimulated tyrosine phosphorylation of the insulin receptor itself (20C22, 24, 25). However, the phosphorylation sites in IRS-1 that mediate the inhibitory effects of TNF- are hard to determine. The c-Jun NH2-terminal kinase (JNK) is usually activated by varied inflammatory stimuli, including TNF-, IL1-, double-stranded RNA and lipopolysaccharide. Recently, we reported that JNK-1 forms a stable complex with IRS-1 and phosphorylates Ser307, which inhibits insulin-stimulated tyrosine phosphorylation of IRS-1 (26). The phosphorylation of Ser307 is usually hard to study in intact cells or tissues because the methods used to radioactively label cultured cells induce JNK activity (26). To investigate the mechanism of Ser307 phosphorylation Bay 60-7550 in cells, we developed a phosphospecific polyclonal antibody that specifically immunoblots the phosphorylated Ser307 residue in IRS-1. By using this antibody, we demonstrate that insulin/IGF-1 and TNF- use unique mechanisms in 3T3-L1 cells to mediate phosphorylation of IRS-1 on Ser307. Moreover, insulin-stimulated phosphorylation of Ser307 is likely to be physiologically important, as it happens in skeletal muscle mass of mice, rats, and humans in response to insulin. Given that phosphorylation of Ser307 inhibits insulin-stimulated tyrosyl phosphorylation of IRS-1, we propose that Ser307 integrates feedback and heterologous signals to attenuate IRS-1Cmediated signals and contribute to insulin resistance. Methods Reagents. Murine TNF- was purchased from R&S Systems Inc. (Minneapolis, Minnesota, USA). Human being insulin and IGF-1 were a gift from Eli Lilly and Co. (Indianapolis, Indiana, USA). Protein A-agarose was purchased from RepliGen Corp. (Needham, Massachusetts, USA). The 3-isobutylmethylxanthine, dexamethasone, aprotinin, and leupeptin were purchased from Sigma Chemical Co. (St. Louis, Missouri, USA). An enhanced chemiluminescence (ECL) detection system was purchased from Amersham Pharmacia Biotech Inc. (Piscataway, New Jersey, USA). Fugene 6 was purchased from Roche Molecular Biochemicals (Indianapolis, Indiana, USA). Okadaic acid, PD98059, LY294002, wortmannin, and.