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

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.

The influence of protein tyrosine phosphatase‐1B on Na,K‐ATPase activity in lens

“…The lens epithelium may express different tyrosine phosphatases. It has been reported elsewhere that the abundance of tyrosine phosphatase protein does not necessarily reflect tyrosine phosphatase activity since the enzyme requires activation (Blanquet and Croquet, 1995a). Different mechanisms for PTP‐1B activation have been proposed.…”

“…In most cells, tyrosine phosphorylation is reversible. Regulation of tyrosine phosphorylation occurs through the opposing activities of tyrosine kinases and protein tyrosine phosphatases (PTPases) that become activated in response to a variety of cellular signals (Blanquet and Croquet, 1995a). Thus, a decreased level of protein tyrosine phosphorylation in differentiating HL‐60 cells is observed following PTPase activation (Frank and Sartorelli, 1986).…”

Glutathione and Glutaredoxin in Redox Regulation and Cell Signaling of the Lens

“…This enzyme is only 18-kDa in size. It has been found in the bovine lens and, interestingly, it has more activity in older lens epithelial cells, perhaps a reason for a slower cell proliferation or migration during aging [ 104 ]. LMW-PTP has been purified from chick lens, but the authors could never keep the enzyme in an active state due to its spontaneous inactivation by the oxygen present in the air [ 105 ].…”

Low molecular weight protein tyrosine phosphatase (LMW-PTP) and its possible physiological functions of redox signaling in the eye lens

“…Blanquet and Croquet first detected PTPase activity in bovine lens [11] and observed an increased PTPase activity with aging of the lens [12]. Umeda et al [13] purified an 18-kDa PTPase from the lenses of chick embryos and characterized it to be LMW-PTP.…”