The biological effects of extracellular matrix can in large part be attributed to the ability of integrins to promote cell adhesion, regulate cell migration, and profoundly influence cell proliferation and differentiation. To elucidate the effects of the extracellular matrix on cellular behavior, we have focused on the intracellular signaling pathways activated by beta1- and alphav-subunit containing integrins. Our Preliminary Studies have indicated that, upon binding to extracellular ligand, a subgroup of integrins which includes alpha1beta1, alpha5beta1 and alphavbeta3, but not alpha2beta1, alpha3beta1 and alpha6beta1, combines sequentially with the adaptors Shc and Grb2, and thereby activates Ras-Extracellular signal Regulated Kinase (ERK) signaling and Serum Response Element (SRE)-dependent transcription. In contrast, Focal Adhesion Kinase, which is activated by all Beta1- and alphav-integrins, is not sufficient to cause these events. Interestingly, ligation of integrins linked to Shc allows primary cells to progress through G1 in response to mitogens, while ligation of other integrins causes exit from the cell cycle and, in the absence of intercellular contact, apoptotic death. Thus, the coupling of specific integrins to Shc may function as a binary switch to control cell survival and cell cycle progression in response to the extracellular matrix. We have recently shown that the recruitment of Shc is indirect and specified by the transmembrane domain of integrin alpha subunit. Coimmunoprecipitation studies have indicated that this portion of the integrin interacts with the transmembrane adaptor caveolin and thereby with the tyrosine kinase Fyn. Biochemical experiments conducted on caveolin-negative and caveolin-transfected thyroid cells as well as Fyn-knock out and Fyn-reconstituted embryo fibroblasts have provided compelling evidence that caveolin and Fyn are both necessary for the recruitment of Shc and Grb2 and the activation of Ras-ERK signaling in response to integrin ligation. In this proposal, we plan to: 1) examine the molecular basis of interaction between integrins and caveolin; 2) elucidate the mechanism by which caveolin mediates the recruitment and tyrosine phosphorylation of Shc; 3) examine the relative roles of Shc and FAK in protection from apoptotic cell death; and 4) examine the role of Shc and FAK in the control of cell cycle progression. We hope that these studies will increase our understanding of the mechanisms by which the extracellular matrix regulates cell survival and cell proliferation.
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