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The regulation of the metabolic insulin response by mouse growth factor receptor-binding protein 10 (Grb10) has been addressed in this report. We find mouse Grb10 to be a critical component of the insulin receptor (IR) signaling complex that provides a functional link between IR and p85 phosphatidylinositol (PI) 3-kinase and regulates PI 3-kinase activity. This regulatory mechanism parallels the established link between IR and p85 via insulin receptor substrate (IRS) proteins. A direct association was demonstrated between Grb10 and p85 but was not observed between Grb10 and IRS proteins. In addition, no effect of mouse Grb10 was observed on the association between IRS-1 and p85, on IRS-1-associated PI 3-kinase activity, or on insulin-mediated activation of IR or IRS proteins. A critical role of mouse Grb10 was observed in the regulation of PI 3-kinase activity and the resulting metabolic insulin response. Dominant-negative Grb10 domains, in particular the SH2 domain, eliminated the metabolic response to insulin in differentiated 3T3-L1 adipocytes. This was consistently observed for glycogen synthesis, glucose and amino acid transport, and lipogenesis. In parallel, the same metabolic responses were substantially elevated by increased levels of Grb10. A similar role of Grb10 was confirmed in mouse L6 cells. In addition to the SH2 domain, the Pro-rich amino-terminal region of Grb10 was implicated in the regulation of PI 3-kinase catalytic activity. These regulatory roles of Grb10 were extended to specific insulin mediators downstream of PI 3-kinase including PKB/Akt, glycogen synthase kinase, and glycogen synthase. In contrast, a regulatory role of Grb10 in parallel insulin response pathways including p70 S6 kinase, ubiquitin ligase Cbl, or mitogen-activated protein kinase p38 was not observed. The dissection of the interaction of mouse Grb10 with p85 and the resulting regulation of PI 3-kinase activity should help elucidate the complexity of the IR signaling mechanism.

Insulin stimulation results in a considerable spectrum of cellular responses, only part of which have been firmly correlated with the activation of established insulin receptor (ER) targets such as ERS-1, IRS-2, and She. Many responses may be transduced by alternative direct IR targets, some of which may still be unknown, may act in parallel to but independently of ERS-1, ERS-2, and She, and may be members of the growing family of SH2 domain-containing signaling adaptors. An SH2 domain-coding region of a protein termed PSM was cloned based on its interaction with an activated IR cytoplasmic fragment in a yeast two-hybrid screen. When used as a hybridization probe this region led to the isolation of a protein-coding cDNA which is expressed with a wide tissue distribution and exists in several variant forms. A pleckstrin homology domain and three Pro-rich regions including a putative SH3 domain binding site were identified in addition to the SH2 domain in the deduced 756 amino acid sequence. They imply a role of PSM in tyrosine kinase and phosphatase-mediated signaling pathways. A similar sequence termed SH2-B had been reported in an earlier study, which may represent the rat homolog of PSM. A role of PSM specifically in insulin action is suggested by the interaction of its SH2 domain with an activated but not with an inactive catalytic fragment of the ER in the yeast two-hybrid system in vivo, by the insulin-dependent association of a glutathione S-transferase (GST) PSM SH2 domain fusion protein with purified ER in vitro, and by the insulin-dependent association of GST PSM SH2 with the IR in cell extracts. In contrast, PSM was not found to associate with the established ER substrate ERS-1 under any conditions and appears to act independently of ERS-1. All of our findings are compatible with a putative role of PSM in insulin action. [ABSTRACT FROM AUTHOR]