Differentiation of chick lens epithelial cells: Involvement of the epidermal growth factor receptor and endogenous ligand

PURPOSE. To characterize the constitutively activated epidermal growth factor receptor in a lens epithelial cell population experiencing initial stages of lens fiber formation, the chick lens annular pad. METHODS. Phosphotyrosine levels of the receptor were examined with western blot analysis and immunoprecipitation after Ligand stimulation. Endogenous receptor ligands were immunologically identified in whole cell lysates of freshly isolated cells. The expression of lens fiber-specific differentiation marker proteins was examined with western blot analysis and enzyme-linked immunosorbent assay (ELISA) in short-term primary cultures of annular pad cells exposed to ligand. RESULTS. The major phosphotyrosine-containing protein in annular pad cells comigrated with the epidermal growth factor receptor and increased its phosphotyrosine content after epidermal growth factor treatment. Both time- and dose-dependent responses were noted. The constitutive activation of the receptor was determined in the presence of phosphatase inhibitors. Endogenous transforming growth factor-alpha, but nor epidermal growth factor, was detected in freshly isolated cells. Transforming growth factor-alpha (TGF-alpha) treatment produced greater increases in receptor phosphotyrosine levels than equimolar levels of epidermal growth factor. Finally, TGF-alpha treatment induced increased expression of the beaded filament protein filensin when compared with control cells. Filensin expression was increased further when cells were costimulated with TGF-alpha and cAMP analogs. CONCLUSIONS. At least in the postnatal lens, endogenous TGF-alpha may affect overall growth patterns by modulating differentiation-specific protein expression. Furthermore, signaling pathways elicited by TGF-alpha and cAMP analogs converge to cooperatively enhance lens fiber differentiation.