Abstract
To understand how glomerular epithelial cell (GEC) proliferation may be regulated in health and disease, we studied the effects of type I collagen extracellular matrices (ECM) on EGF receptor (EGF-R) activation in cultured rat GEC. EGF stimulated proliferation of GEC adherent to ECM, but not of GEC on a plastic substratum. Significant and prolonged EGF-R tyrosine autophosphorylation (which reflects receptor kinase activation) was induced by EGF in GEC adherent to collagen, but EGF did not stimulate EGF-R autophosphorylation in GEC on plastic (at 37 degrees C). However, EGF-R autophosphorylation increased significantly in plastic-adherent GEC that were stimulated with EGF at 4 degrees C or in the presence of vanadate, an inhibitor of phosphotyrosine phosphatases. Furthermore, dephosphorylation of EGF-R was enhanced in GEC on plastic as compared with collagen. At 4 degrees C, [125I]EGF binding was not different between substrata, and there was negligible accumulation of intracellular [125I]EGF (which reflects EGF-R internalization). At 37 degrees C, EGF-R internalization was reduced significantly in collagen-adherent GEC as compared with GEC on plastic. Thus, contact with ECM facilitates proliferation and EGF-R activation in GEC. The enhanced activity of EGF-R tyrosine kinase may be due to ECM-induced reduction in EGF-R internalization and dephosphorylation by phosphotyrosine phosphatase(s). Signals from ECM to growth factor receptors may regulate cell turnover in the glomerulus under normal conditions and during immune glomerular injury.