|Insulin receptor and IRS-1 co-immunoprecipitation with SOCS-3, and IKKα/β phosphorylation are increased in obese Zucker rat skeletal muscle.|
Zolotnik, IA; Figueroa, TY; Yaspelkis, BB
We evaluated if selected pro-inflammatory cytokines and/or the protein suppressor of cytokine signaling 3 (SOCS-3) could account for decreased insulin-stimulated phosphatidylinositol 3-kinase (PI3-K) activity in the skeletal muscle of the obese Zucker rat.Eight lean and eight obese Zucker rats ~4weeks of age were obtained and allowed to feed ad libitum for 4weeks before undergoing hind limb perfusion in the presence of 500μU/ml insulin.Insulin-stimulated skeletal muscle PI3-K activity and 3-O-methylglucose transport rates were reduced (Pless than 0.05) in obese compared to lean animals. IRS-1 concentration remained unchanged although IRS-1 tyrosine phosphorylation was decreased (Pless than 0.05), and IRS-1 serine phosphorylation (pS) was increased (Pless than 0.05) in obese animals compared to lean animals. IKKα/β pS and JNK theronine/tyrosine phosphorylation was increased (Pless than 0.05) in the obese animals. IκBα concentration was decreased (Pless than 0.05) and IκBα pS was increased (Pless than 0.05) in the obese compared to lean Zucker animals. SOCS-3 concentration and SOCS-3 co-immunoprecipitation with both insulin receptor β-subunit (IR-β) and IRS-1 were elevated (Pless than 0.05) in obese compared to lean animals. IRS-1 co-immunoprecipitation with IR-β was reduced 56% in the obese animals.Increased IKKα/β and JNK serine phosphorylation may contribute to increasing IRS-1 serine phosphorylation, while concurrent co-localization of SOCS-3 with both IR-β and IRS-1 may prevent IRS-1 from interacting with IR-β. These two mechanisms thusly may independently contribute to impairing insulin-stimulated PI3-K activation in the skeletal muscle of the obese Zucker rat.
|Leptin induces mitogenic effect on human choriocarcinoma cell line (JAr) via MAP kinase activation in a glucose-dependent fashion.|
G Bifulco, A Trencia, M Caruso, G A Tommaselli, C Miele, C di Carlo, F Beguinot, C Nappi, G Bifulco, A Trencia, M Caruso, G A Tommaselli, C Miele, C di Carlo, F Beguinot, C Nappi
Leptin and glucose effect on cell growth has been investigated in the JAr human choriocarcinoma cells. When JAr cells were cultured in the presence of 6m M glucose (LG), proliferation and thymidine incorporation were induced by serum but not by leptin. At variance, at 25m M glucose (HG), proliferation and thymidine incorporation were stimulated by leptin and serum to a comparable extent. HG culturing also enhanced leptin-stimulated insulin receptor substrate 1 (IRS1) and MAPK phosphorylation. Blockage of MAPK activity with PD98059 caused an inhibition of glucose- and leptin-dependent thymidine incorporation. At variance with HG conditions no effects were observed in cells cultured in 6m M glucose upon treatment with PD98059. Neither glucose nor leptin determined a modification in leptin receptors total content.In this study, we provide evidence that in placental cells, leptin, similarly to that observed with insulin, stimulates cell proliferation by inducing the IRS1/MAPK pathway in a glucose-dependent fashion.
|Phosphatidylinositol 3'-kinase and p70s6k are required for insulin but not bisperoxovanadium 1,10-phenanthroline (bpV(phen)) inhibition of insulin-like growth factor binding protein gene expression. Evidence for MEK-independent activation of mitogen-activated protein kinase by bpV(phen).|
Band, C J and Posner, B I
J. Biol. Chem., 272: 138-45 (1997)
The hormonal regulation of insulin-like growth factor binding protein (IGFBP)-1 and -4 mRNA was compared in serum-free primary rat hepatocyte cultures. The combination of dexamethasone and glucagon (Dex/Gluc) strongly increased IGFBP-1 and IGFBP-4 mRNA levels. Insulin suppressed Dex/Gluc-stimulated IGFBP-1 but not IGFBP-4 mRNA levels. In contrast, the peroxovanadium compound, bisperoxovanadium 1,10-phenanthroline (bpV(phen)), completely abrogated Dex/Gluc induction of both IGFBP mRNA species. Wortmannin and rapamycin blocked the inhibitory effect of insulin but not that of bpV(phen) on Dex/Gluc-stimulated IGFBP mRNA. Thus, although phosphatidylinositol 3'-kinase and p70s6k are necessary for insulin-mediated transcriptional inhibition of the IGFBP-1 gene, a signaling pathway, independent of phosphatidyloinositol 3'-kinase and p70s6k, is activated by bpV(phen) and mediates IGFBP-1 as well as IGFBP-4 mRNA inhibition. Mitogen-activated protein (MAP) kinase activity induced by insulin was suppressed to below basal levels in the presence of Dex/Gluc, whereas in response to bpV(phen), MAP kinase activity was high and unaffected by Dex/Gluc, consistent with a role of MAP kinases in bpV(phen)-mediated inhibition of IGFBP mRNA. The specific MAP kinase kinase (MEK) inhibitor, PD98059, inhibited insulin but not bpV(phen)-stimulated MAP kinase activity, suggesting that MAP kinases can be activated in a MEK-independent fashion. Peroxovanadium compounds are strong inhibitors of tyrosine phosphatases, which may inhibit specific tyrosine/threonine phosphatases involved in the negative regulation of MAP kinases.
|The type I interferon receptor mediates tyrosine phosphorylation of insulin receptor substrate 2.|
Platanias, L C, et al.
J. Biol. Chem., 271: 278-82 (1996)
Binding of interferon alpha (IFN alpha) to its receptor induces activation of the Tyk-2 and Jak-1 tyrosine kinases and tyrosine phosphorylation of multiple downstream signaling elements, including the Stat components of the interferon-stimulated gene factor 3 (ISGF-3). IFN alpha also induces tyrosine phosphorylation of IRS-1, the principle substrate of the insulin receptor. In this study we demonstrate that various Type I IFNs rapidly stimulate tyrosine phosphorylation of IRS-2. This is significant since IRS-2 is the major IRS protein found in hematopoietic cells. The IFN alpha-induced phosphorylated form of IRS-2 associates with the p85 regulatory subunit of the phosphatidylinositol 3'-kinase, suggesting that this kinase participates in an IFN alpha-signaling cascade downstream of IRS-2. We also provide evidence for an interaction of IRS-2 with Tyk-2, suggesting that Tyk-2 is the kinase that phosphorylates this protein during IFN alpha stimulation. A conserved region in the pleckstrin homology domain of IRS-2 may be required for the interaction of IRS-2 with Tyk-2, as shown by the selective binding of glutathione S-transferase (GST) fusion proteins containing the IRS-2-IH1PH or IRS-1-IH1PH domains to Tyk-2 but not other Janus kinases in vitro.