Insulin receptor substrate-1 as a signaling molecule for focal adhesion kinase pp125(FAK) and pp60(src)

Role of the actin cytoskeleton in insulin action

Insulin has diverse effects on cells, including stimulation of glucose transport, gene expression, and alterations of cell morphology. The hormone mediates these effects by activation of signaling pathways which utilize, 1) adaptor molecules such as the insulin receptor substrates (IRS), the Src and collagen homologs (Shc), and the growth factor receptor binding protein 2 (Grb2); 2) lipid kinases such as phosphatidylinositol 3-kinase (PI 3-Kinase); 3) small G proteins; and 4) serine, threonine, and tyrosine kinases. The activation of such signaling molecules by insulin is now well established, but we do not yet fully understand the mechanisms integrating these seemingly diverse pathways. Here, we discuss the involvement of the actin cytoskeleton in the propagation and regulation of insulin signals. In muscle cells in culture, insulin induces a rapid actin filament reorganization that coincides with plasma membrane ruffling and intense accumulation of pinocytotic vesicles. Initiation of these effects of insulin requires an intact actin cytoskeleton and activation of PI 3-kinase. We observed recruitment PI 3-kinase subunits and glucose transporter proteins to regions of reorganized actin. In both muscle and adipose cells, actin disassembly inhibited early insulin-induced events such as recruitment of glucose transporters to the cell surface and enhanced glucose transport. Additionally, actin disassembly inhibited more prolonged effects of insulin, including DNA synthesis and expression of immediate early genes such as c-fos. Intact actin filaments appear to be essential for mediation of early events such as association of Shc with Grb2 in response to insulin, which leads to stimulation of gene expression. Preliminary observations support a role for focal adhesion signaling complexes in insulin action. These observations suggest that the actin cytoskeleton facilitates propagation of the morphological, metabolic, and nuclear effects of insulin by regulating proper subcellular distribution of signaling molecules that participate in the insulin signaling pathway.

Extracellular matrix selectively modulates the response of mammary epithelial cells to different soluble signaling ligands

In adherent cells, cell-substratum interactions are essential for the propagation of some growth factor signaling events. However, it has not been resolved to what extent different types of extracellular matrix regulate the signals elicited by different soluble ligands. Our previous work has shown that prolactin signaling in mammary epithelium requires a specific cell interaction with the basement membrane and does not occur in cells plated on collagen I. We have now investigated whether the proximal signaling pathways triggered by insulin, epidermal growth factor (EGF), and interferon-gamma are differentially regulated in primary mammary epithelial cell cultures established on basement membrane and collagen I. Two distinct signaling pathways triggered by insulin exhibited a differential requirement for cell-matrix interactions. Activation of insulin receptor substrate (IRS) and phosphatidylinositol 3-kinase was restricted to cells contacting basement membrane, whereas the phosphorylation of Erk occurred equally in cells on both substrata. The amplitude and duration of insulin-triggered IRS-1 phosphorylation and its association with phosphatidylinositol 3-kinase were strongly enhanced by cell-basement membrane interactions. The mechanism for inhibition of IRS-1 phosphorylation in cells cultured on collagen I may in part be mediated by protein-tyrosine phosphatase activity since vanadate treatment somewhat alleviated this effect. In contrast to the results with insulin, cell adhesion to collagen I conferred greater response to EGF, leading to higher levels of tyrosine phosphorylation of the EGF receptor and Erk. The mechanism for increased EGF signaling in cells adhering to collagen I was partly through an increase in EGF receptor expression. The interferon-gamma-activated tyrosine phosphorylation of Jak2 and Stat3 was independent of the extracellular matrix. It is well recognized that the cellular environment determines cell phenotype. We now suggest that this may occur through a selective modulation of growth factor signal transduction resulting from different cell-matrix interactions.