Activation of phosphotyrosine phosphatase activity is associated with decreased differentiation in adult bovine lens

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Abstract
The postnatal vertebrate eye lens provides an opportunity to study possible involvement of reversible protein phosphorylation in the differentiation process of epithelial cells. Epithelial cells at the lens equator, indeed, differentiate continuously into fiber cells throughout life but this capacity progressively decreases with age. Here we describe the characterization of a phosphotyrosine‐protein phosphatase(s) (PTPase(s)) in the equatorial epithelium of bovine lens which exhibits a high level of specific activity. PTPase(s) is detected in cellular detergent extracts using phospholabeled synthetic peptides, p‐nitrophenyl phosphate, and lens epithelial membranes as substrates. We show that activity of this PTPase(s) is increased in the equatorial epithelium as the age is increased. We also show that this enzyme(s) exerts its dephosphorylating activity predominantly on a calpactin‐like protein associated with lens epithelial membranes. Dephosphorylation of this protein is only obtained when membranes are subjected to extracts in the presence of fibroblast growth factor (FGF). It is suggested that an FGF‐activated PTPase(s) might conceivably counteract effects of differentiation stimulatory factors for limiting differentiation of lens throughout life.

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