Grb10 deletion enhances muscle cell proliferation, differentiation and GLUT4 plasma membrane translocation

Grb10 is an intracellular adaptor protein which binds directly to several growth factor receptors, including those for insulin and insulin-like growth factor receptor-1 (IGF-1), and negatively regulates their actions. Grb10-ablated (Grb10-/-) mice exhibit improved whole body glucose homeostasis and an increase in muscle mass associated specifically with an increase in myofiber number. This suggests that Grb10 may act as a negative regulator of myogenesis. In this study, we investigated in vitro, the molecular mechanisms underlying the increase in muscle mass and the improved glucose metabolism. Primary muscle cells isolated from Grb10-/- mice exhibited increased rates of proliferation and differentiation compared to primary cells isolated from wild-type mice. The improved proliferation capacity was associated with an enhanced phosphorylation of Akt and ERK in the basal state and changes in the expression of key cell cycle progression markers involved in regulating transition of cells from the G1 to S phase (e.g., retinoblastoma (Rb) and p21). The absence of Grb10 also promoted a faster transition to a myogenin positive, differentiated state. Glucose uptake was higher in Grb10-/- primary myotubes in the basal state and was associated with enhanced insulin signaling and an increase in GLUT4 translocation to the plasma membrane. These data demonstrate an important role for Grb10 as a link between muscle growth and metabolism with therapeutic implications for diseases, such as muscle wasting and type 2 diabetes

Mice with a Disruption of the Imprinted Grb10 Gene Exhibit Altered Body Composition, Glucose Homeostasis, and Insulin Signaling during Postnatal Life▿

The Grb10 adapter protein is capable of interacting with a variety of receptor tyrosine kinases, including, notably, the insulin receptor. Biochemical and cell culture experiments have indicated that Grb10 might act as an inhibitor of insulin signaling. We have used mice with a disruption of the Grb10 gene (Grb10Δ2-4 mice) to assess whether Grb10 might influence insulin signaling and glucose homeostasis in vivo. Adult Grb10Δ2-4 mice were found to have improved whole-body glucose tolerance and insulin sensitivity, as well as increased muscle mass and reduced adiposity. Tissue-specific changes in insulin receptor tyrosine phosphorylation were consistent with a model in which Grb10, like the closely related Grb14 adapter protein, prevents specific protein tyrosine phosphatases from accessing phosphorylated tyrosines within the kinase activation loop. Furthermore, insulin-induced IRS-1 tyrosine phosphorylation was enhanced in Grb10Δ2-4 mutant animals, supporting a role for Grb10 in attenuation of signal transmission from the insulin receptor to IRS-1. We have previously shown that Grb10 strongly influences growth of the fetus and placenta. Thus, Grb10 forms a link between fetal growth and glucose-regulated metabolism in postnatal life and is a candidate for involvement in the process of fetal programming of adult metabolic health

Grb10 and Grb14: enigmatic regulators of insulin action – and more?

The Grb proteins (growth factor receptor-bound proteins) Grb7, Grb10 and Grb14 constitute a family of structurally related multidomain adapters with diverse cellular functions. Grb10 and Grb14, in particular, have been implicated in the regulation of insulin receptor signalling, whereas Grb7 appears predominantly to be involved in focal adhesion kinase-mediated cell migration. However, at least in vitro, these adapters can bind to a variety of growth factor receptors. The highest identity within the Grb7/10/14 family occurs in the C-terminal SH2 (Src homology 2) domain, which mediates binding to activated receptors. A second well-conserved binding domain, BPS [between the PH (pleckstrin homology) and SH2 domains], can act to enhance binding to the IR (insulin receptor). Consistent with a putative adapter function, some non-receptor-binding partners, including protein kinases, have also been identified. Grb10 and Grb14 are widely, but not uniformly, expressed in mammalian tissues, and there are various isoforms of Grb10. Binding of Grb10 or Grb14 to autophosphorylated IR in vitro inhibits tyrosine kinase activity towards other substrates, but studies on cultured cell lines have been conflicting as to whether Grb10 plays a positive or negative role in insulin signalling. Recent gene knockouts in mice have established that Grb10 and Grb14 act as inhibitors of intracellular signalling pathways regulating growth and metabolism, although the phenotypes of the two knockouts are distinct. Ablation of Grb14 enhances insulin action in liver and skeletal muscle and improves whole-body tolerance, with little effect on embryonic growth. Ablation of Grb10 results in disproportionate overgrowth of the embryo and placenta involving unidentified pathways, and also impacts on hepatic glycogen synthesis, and probably on glucose homoeostasis. This review discusses the extent to which previous studies in vitro can account for the observed phenotype of knockout animals, and considers evidence that aberrant function of Grb10 or Grb14 may contribute to disorders of growth and metabolism in humans