Grb10 has been described as a cellular partner of several receptor tyrosine kinases, including the insulin receptor (IR) and the insulin-like growth factor I (IGF-I) receptor (IGF-IR).Its cellular role is still unclear and a positive as well as an inhibitory role in mitogenesis depending on the cell context has been implicated. We have tested other mitogenic receptor tyrosine kinases as putative Grb10 partners and have identified the activated forms of platelet-derived growth factor (PDGF) receptor ␤ (PDGFR␤), hepatocyte growth factor receptor (Met), and fibroblast growth factor receptor as candidates. We have mapped Y771 as a PDFGR␤ site that is involved in the association with Grb10 via its SH2 domain. We have further investigated the putative role of Grb10 in mitogenesis with four independent experimental strategies and found that all consistently suggested a role as a positive, stimulatory signaling adaptor in normal fibroblasts.

An SH2 domain originally termed SH2-B had been identified as a direct cellular binding target of a number of mostly mitogenic receptors. The complete cellular protein, termed PSM, and respective sequence variants share additional Pro-rich and PH regions, as well as similarities with APS and Lnk. A role of these mediators has been implicated in signaling pathways found downstream of growth hormone receptor and receptor tyrosine kinases, including the insulin, insulin-like growth factor-I (IGF-I), platelet-derived growth factor (PDGF), nerve growth factor, hepatocyte growth factor, and fibroblast growth factor receptors. As a result of this report a total of four PSM/SH2-B sequence variants termed ␣, ␤, ␥, and ␦ have now been identified in the mouse and have been compared with the available rat and human sequences. Variant differences are based on alternative splicing and define distinct last exons 7, 8, and 9 that result in reading frameshifts and unique carboxyl-terminal amino acid sequences. Variant sequences have been identified from cDNA libraries and directly by reverse transcription-polymerase chain reaction. Sequence analysis predicts four distinctly sized protein products that have been demonstrated after cDNA expression. All were found phosphorylated on tyrosine specifically in response to IGF-I and PDGF stimulation. cDNA expression of the four variants caused variant-dependent levels of stimulation of IGF-I-and PDGF-induced mitogenesis. The most pronounced increase in mitogenesis was consistently observed for the ␥ variant followed by ␦, ␣, and ␤ with decreasing responses. In contrast, the mitogenic response to epidermal growth factor consistently remained unaffected. The variants are expressed in most mouse tissues, typically, most strongly in pairs of ␣ and ␦ or ␤ and ␥. Our findings implicate differential roles of the PSM/SH2-B splice variants in specific mitogenic signaling pathways.

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 (IR) targets such as IRS-1, IRS-2, and Shc. 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 IRS-1, IRS-2, and Shc, 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 IR 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 IR 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 IR substrate IRS-1 under any conditions and appears to act independently of IRS-1. All of our findings are compatible with a putative role of PSM in insulin action.

Grb10 is a member of a recently identified family of adapter proteins that are thought to play a role in receptor tyrosine kinase-mediated signal transduction. We identified and isolated the Grb10 SH2 domain based on its interaction with the intracellular domain of the insulin receptor ␤-subunit using the yeast two-hybrid system. The interaction was specific for the insulin receptor and the insulin-like growth factor-1 receptor, and it required a catalytically active receptor kinase domain and an intact Grb10 SH2 domain. Glutathione S-transferase fusion proteins containing the Grb10 SH2 domain associated in an insulin-dependent manner with insulin receptors from cell lysates and with purified insulin receptors. Co-precipitation experiments revealed the association of cellular Grb10 with hormone-stimulated insulin receptors in cell extracts. The Grb10 SH2 domain did not bind to an insulin receptor lacking 43 amino acids at the carboxyl terminus, and it exhibited highest affinity for a phosphopeptide containing Tyr(P)-1322. Unlike p85 and Syp, which also bind to Tyr(P)-1322, Grb10 was not found to associate with insulin receptor substrate-1. These results suggest that Grb10 is a novel insulin receptor interactive protein and provide direct evidence for an insulin receptor substrate-1-independent function of the insulin receptor carboxyl terminus in protein binding.The insulin receptor plays important roles in metabolism and growth regulation of target tissues (1-3). Upon insulin stimulation the receptor becomes autophosphorylated on at least six or seven tyrosine residues located in the juxtamembrane, kinase, and carboxyl-terminal regions of its ␤-subunit (4 -7). Many signals are relayed through the insulin receptor substrate-1 (IRS-1), 1 which is phosphorylated by the receptor on various tyrosine residues (8). Phosphorylation of a tyrosine at position 960 in the juxtamembrane region of the insulin receptor is essential for signaling through IRS-1 (9, 10). Among the proteins that bind to IRS-1 are the p85 subunit of phosphatidylinositol 3-kinase (PI3-kinase), Grb2, the tyrosine phosphatase Syp/SHPTP2, and Nck (2). IRS-1 is required for insulin-mediated mitogenesis (11, 12); however, studies using homozygous IRS-1 knockout mice predict important IRS-1-independent insulin signaling pathways (13,14). A promising candidate, the structurally closely related protein IRS-2, was recently identified, which has signaling functions similar to IRS-1 (15, 16). The Shc proteins represent additional receptor tyrosine kinase substrates that mediate p21 ras activation in the mitogenic pathway of insulin action (17). We employed the yeast two-hybrid system to identify signaling mediators for alternative pathways analogous to signals emerging from other receptor tyrosine kinases. Here we describe the identification of Grb10 as an IRS-1-independent interactive protein of the activated insulin receptor, and we define the sites of interaction in the receptor and Grb10. MATERIALS AND METHODSAntibodies-The antibody against the insulin re...