Regulation of Na,K-ATPase Function in the Lens

The two cell types in the lens, epithelium and fiber, have a very different specific activity of Na,K-ATPase; activity is much higher in the epithelium. However, judged by Western blot, fibers and epithelium express a similar amount of both Na,K-ATPase α and β subunit proteins. Na,K-ATPase protein abundance does not tally with Na,K-ATPase activity. Studies were conducted to examine whether protein synthesis plays a role in maintenance of the high Na,K-ATPase activity in lens epithelium. An increase of cytoplasmic sodium was found to increase Na,K-ATPase protein expression in the epithelium, but not in the fibers. The findings illustrate the ability of lens epithelium to synthesize new Na,K-ATPase protein as a way to boost Na,K-ATPase in response to cell damage or pathological events. Methionine incorporation studies suggested Na,K-ATPase synthesis may also play a role in day to day preservation of high Na,K-ATPase activity. Na,K-ATPase protein in lens epithelial cells appeared to be continually synthesized and degraded. Experiments with cycloheximide suggest that specific activity of Na,K-ATPase in the lens epithelium may depend on the ability of the cells to continuously synthesize fresh Na,K-ATPase proteins. However, other factors such as phosphorylation of Na,K-ATPase α subunit may also influence Na,K-ATPase activity. When intact lenses were exposed to the agonist thrombin, Na,K-ATPase activity was diminished, but the response was suppressed by inhibitors of the Src family of non-receptor tyrosine kinases. Thrombin elicited tyrosine phosphorylation of lens epithelium membrane proteins, including a 100 kDa protein band thought to be the Na,K-ATPase α 1 subunit. It remains to be determined whether a tyrosine phosphorylation mechanism contributes to the low activity of Na,K-ATPase in lens fibers.