Caveolin-1 phosphorylation is required for stretch-induced EGFR and Akt activation in mesangial cells
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Increased glomerular hydrostatic pressure is an important determinant of glomerulosclerosis and can be modeled in vitro by exposure of mesangial cells (MC) to cyclic mechanical strain. We have recently shown that Akt mediates the stretch-induced production of type I collagen, an important contributor to sclerosis, in MC. Here we studied the upstream mediators of Akt activation. Primary rat MC were exposed to 1 Hz cyclic strain for 10 min, previously shown to induce maximal Akt activation. Neither the integrin inhibitor GRDGSP nor cytoskeletal disruptors had any effect on stretch-induced Akt activation. Akt activation was, however, mediated by transactivation of the epidermal growth factor receptor (EGFR), and this required receptor kinase activity since Akt activation did not occur in cells expressing kinase-dead EGFR (K721A). Src was further shown to be upstream of the EGFR, with its inhibitor SU6656 preventing both EGFR and Akt activation. The membrane microdomains caveolae were found to be required for this signaling to occur. Chemical disruption of caveolae with cyclodextrin or filipin prevented Akt activation, and both EGFR and Akt activation were lost in caveolin-1 (cav-1) knockout MC. The latter was rescued with reexpression of cav-1. Further, Src-mediated phosphorylation of cav-1 on Y14 was required for stretch-induced EGFR and Akt activation, since these were abrogated in MC expressing the nonphosphorylatable cav-1 Y14A mutant. Thus, mechanical strain-induced activation of Akt in MC is independent of integrin activation and the actin cytoskeleton, but depends upon EGFR transactivation. EGFR transactivation requires intact caveolae and the Src-mediated phosphorylation of cav-1 on Y14. These studies define a novel function for cav-1 and caveolae in EGFR transactivation leading to Akt activation by mechanical stress.
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