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 alpha and beta 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 alpha 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 alpha 1 subunit. It remains to be determined whether a tyrosine phosphorylation mechanism contributes to the low activity of Na,K-ATPase in lens fibers.
Delamere
Nicholas A
NA
Department of Ophthalmology and Visual Sciences, University of Louisville, School of Medicine, Louisville, Kentucky 40292, USA. delamere@louisville. edu
D000818:Animals; D001345:Autoradiography; D015153:Blotting, Western; D003593:Cytoplasm / Q000737:chemistry; D004591:Electrophoresis, Polyacrylamide Gel; D004848:Epithelium / Q000502:physiology; D005786:Gene Expression Regulation / Q000502:physiology*; D007908:Lens, Crystalline / Q000502:physiology*; D010766:Phosphorylation / Q000187:drug effects; D011233:Precipitin Tests; D051381:Rats; D000254:Sodium-Potassium-Exchanging ATPase / Q000096:biosynthesis* / Q000187:drug effects / Q000502:physiology*; D013917:Thrombin / Q000494:pharmacology; D014443:Tyrosine / Q000187:drug effects
D014443:Tyrosine; D013917:Thrombin; D000254:Sodium-Potassium-Exchanging ATPase