Tyrosine Phosphorylation by Src-Family Tyrosine Kinases Alters Lens Na,K-ATPase Activity

Abstract

Abstract: : Purpose: Na,K-ATPase is essential for the regulation of cytoplasmic Na₊ and K₊ levels in lens cells. Insufficient Na,K-ATPase activity is associated with cataract. Based on earlier studies in which tyrosine kinase inhibitors were found to suppress Na,K-ATPase activity changes that occur in response to thrombin, endothelin or dopamine, we hypothesize that regulation of Na,K-ATPase activity might occur through phosphorylation of the Na,K-ATPase α1 (catalytic) subunit. Methods: Western blot studies confirmed the expression of Src, Fyn and Lyn tyrosine kinases in lens cells. To examine phosphotyrosine modification of Na,K-ATPase α1, membrane material from porcine lens epithelium was incubated for 20 minutes in ATP-containing solution in the presence of partially purified Lyn kinase. After this, membrane proteins were separated by electrophoresis and analyzed by Western blot to identify Na,K-ATPase α1 and tyrosine phosphoproteins. Results: Na,K-ATPase α1 protein was detected at 100 kDa. Lyn kinase increased tyrosine phosphorylation of a co-localized 100 kDa band. Na, K-ATPase activity was reduced by 80% in membrane material treated for 20 minutes with Lyn kinase. Different Src family tyrosine kinases, Src and Fyn, also caused phosphorylation of the 100 kDa putative Na,K-ATPase α1 band. However, Fyn kinase treatment significantly increased Na,K-ATPase activity while Src kinase caused little change of Na,K-ATPase activity. Conclusion: These results suggest tyrosine phosphorylation of the Na,K-ATPase α1 subunit is capable of modifying Na,K-ATPase activity. Activation of different tyrosine kinases appears to have the potential to cause different changes in Na, K-ATPase activity.

Keywords: 482 NaK ATPase • 446 ion transporters • 515 phosphorylation