Guanosine triphosphatase-activating protein-associated protein, but not src-associated protein p68 in mitosis, is a part of insulin signaling complexes

Sam68 associates with the SH3 domains of Grb2 recruiting GAP to the Grb2-SOS complex in insulin receptor signaling

The 68 kDa Src substrate associated during mitosis (Sam68) is an RNA binding protein with Src homology (SH) 2 and 3 domain binding sites. We have recently found that Sam68 is a substrate of the insulin receptor (IR) that translocates from the nucleus to the cytoplasm and that Tyr-phosphorylated Sam68 associates with the SH2 domains of p85 PI3K and GAP, in vivo and in vitro. In the present work, we have further demonstrated the cytoplasmic localization of Sam68, which is increased in cells overexpressing IR. Besides, we sought to further study the association of Sam68 with the Ras-GAP pathway by assessing the interactions with SH3 domains of Grb2. We employed GST-fusion proteins containing the SH3 domains of Grb2 (N or C), and recombinant Sam68 for in vitro studies. In vivo studies of protein-protein interaction were assessed by co-immunoprecipitation experiments with specific antibodies against Sam68, GAP, Grb2, SOS, and phosphotyrosine; and by affinity precipitation with the fusion proteins (SH3-Grb2). Insulin stimulation of HTC-IR cells promotes phosphorylation of Sam68 and its association with the SH2 domains of GAP. Sam68 is constitutively associated with the SH3 domains of Grb2 and it does not change upon insulin stimulation, but Sam68 is Tyr-phosphorylated and promotes the association of GAP with the Grb2-SOS complex. In vitro studies with fusion proteins showed that Sam68 association with Grb2 is preferentially mediated by the C-terminal SH3 domains of Grb2. In conclusion, Sam68 is a substrate of the IR and may have a role as a docking protein in IR signaling, recruiting GAP to the Grb2-SOS complex, and in this way it may modulate Ras activity.

Diversification of cardiac insulin signaling involves the p85 alpha/beta subunits of phosphatidylinositol 3-kinase

Ventricular cardiomyocytes and cardiac tissue of lean and genetically obese (fa/fa) Zucker rats were used 1) to study the role of the p85 regulatory subunit isoforms p85 alpha and p85 beta for insulin signaling through the phosphatidylinositol (PI) 3-kinase pathway, and 2) to elucidate the implications of these mechanisms for cardiac insulin resistance. Western blot analysis of cardiomyocyte lysates revealed expression of p85 alpha and p85 beta but no detectable amounts of the splice variants of p85 alpha. Essentially no p85 alpha subunit of PI 3-kinase was found to be associated with insulin receptor substrate (IRS)-1 or IRS-2 in basal and insulin-stimulated (5 min) cardiomyocytes. Instead, insulin produced a twofold increase in p85 beta associated with IRS-1, leading to a three- to fourfold increase in p85 beta-associated PI 3-kinase activity. This response was significantly reduced in obese animals. Comparable results were obtained in the intact heart after in vivo stimulation. In GLUT-4-containing vesicles, an increased abundance (3.7 +/- 0.7-fold over basal) of p85 alpha was observed after insulin stimulation of lean animals, with no significant effect in the obese group. No p85 beta could be detected in GLUT-4-containing vesicles. Recruitment of the p110 catalytic subunit of PI 3-kinase and a twofold increase in enzyme activity in GLUT-4-containing vesicles by insulin was observed only in lean rats. We conclude that, in the heart, p85 alpha recruits PI 3-kinase activity to GLUT-4 vesicles, whereas p85 beta represents the main regulator of IRS-1- and IRS-2-mediated PI 3-kinase activation. Furthermore, multiple defects of PI 3-kinase activation, involving both the p85 alpha and the p85 beta adaptor subunits, may contribute to cardiac insulin resistance.

Characterization of insulin receptor substrate 3 in rat liver derived cells

In rat liver derived HTC cells transfected with and expressing human insulin receptors, there are multiple p60-70 proteins that are tyrosine phosphorylated following insulin treatment of cells. Employing antibodies to insulin receptor substrate 3 (alpha-IRS-3), we found that IRS-3 is a major p60 phosphoprotein that is tyrosine phosphorylated following insulin treatment of cells and interacts with phosphatidylinositol-3-kinase (PI3K). Majority of IRS-3 when phosphorylated appears to interact with PI3K. Tyrosine phosphorylation of IRS-3 is robust at 2 min and steadily increases up to 30-90 min of insulin treatment. Following insulin treatment of cells, some high molecular weight phosphoproteins are coimmunoprecipitated with alpha-IRS-3. In summary, IRS-3 is the major p60 protein that is tyrosine phosphorylated and interacts with PI3K in HTC rat liver derived cells following insulin treatment of cells. Unlike related IRS-1/2 that is transiently phosphorylated, IRS-3 shows robust and prolonged tyrosine phosphorylation upon insulin treatment of cells and may play a role in delayed and/or prolonged insulin actions.

Stimulation of glycogen synthesis by insulin requires S6 kinase and phosphatidylinositol-3-kinase in HTC-IR cells

In order to study the role of phosphatidylinositol-3-kinase (PI3K), PKB, FRAP, S6 kinase, and MAP kinase in insulin-stimulated glycogen synthesis, we used a specific inhibitor of PI3K, LY294002, the immunosuppressant inhibitor of FRAP, rapamycin, and the inhibitor of MAPK kinase (MEK)/MAPK, PD98059, in rat HTC hepatoma cells overexpressing human insulin receptors. The PI3K inhibitor LY294002 completely blocks insulin-stimulated glycogen synthesis by inhibiting glycogen synthase, PKB (Akt-1), and FRAP (RAFT) autophosphorylation, as well as p70 S6 kinase activation, whereas insulin receptor substrates tyrosine phosphorylation and MEK activity were not affected. However, rapamycin only partially blocks insulin-stimulated glycogen synthesis by partial inhibition of glycogen synthase, whereas it completely blocks S6 kinase activation and FRAP autophosphorylation, but does not affect either PKB autophosphorylation, MEK activity, or insulin receptor tyrosine phosphorylation. Insulin-stimulated glycogen synthesis and glycogen synthase were not affected by the MEK/MAPK inhibitor PD98059. These data suggest that the PI3K, and not the MAPK pathway plays an important role in the insulin-stimulated glycogen synthesis in the hepatocyte, partly mediated by FRAP and S6 kinase activation. However, the inhibition of FRAP and S6 kinase activation is not sufficient to block insulin-stimulated glycogen synthesis, suggesting an important role of a branching pathway upstream of S6 kinase and downstream of PI3K, which is probably mediated by PKB in the signaling of the insulin receptor in hepatoma HTC cells.

p68 Sam is a substrate of the insulin receptor and associates with the SH2 domains of p85 PI3K

The 68 kDa Src substrate associated during mitosis is an RNA binding protein with Src homology 2 and 3 domain binding sites. A role for Src associated in mitosis 68 as an adaptor protein in signaling transduction has been proposed in different systems such as T-cell receptors. In the present work, we have sought to assess the possible role of Src associated in mitosis 68 in insulin receptor signaling. We performed in vivo studies in HTC-IR cells and in vitro studies using recombinant Src associated in mitosis 68, purified insulin receptor and fusion proteins containing either the N-terminal or the C-terminal Src homology 2 domain of p85 phosphatidylinositol-3-kinase. We have found that Src associated in mitosis 68 is a substrate of the insulin receptor both in vivo and in vitro. Moreover, tyrosine-phosphorylated Src associated in mitosis 68 was found to associate with p85 phosphatidylinositol-3-kinase in response to insulin, as assessed by co-immunoprecipitation studies. Therefore, Src associated in mitosis 68 may be part of the signaling complexes of insulin receptor along with p85. In vitro studies demonstrate that Src associated in mitosis 68 associates with the Src homology 2 domains of p85 after tyrosine phosphorylation by the activated insulin receptor. Moreover, tyr-phosphorylated Src associated in mitosis 68 binds with a higher affinity to the N-terminal Src homology 2 domain of p85 compared to the C-terminal Src homology 2 domain of p85, suggesting a preferential association of Src associated in mitosis 68 with the N-terminal Src homology 2 domain of p85. This association may be important for the link of the signaling with RNA metabolism.