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

The abnormal sodium content of many cataracts suggests Na,K-ATPase is vital for maintenance of eye lens transparency. Since tyrosine phosphorylation is considered a possible regulatory mechanism for Na,K-ATPase, experiments were conducted to test the influence of protein tyrosine phosphatase-1B (PTP-1B) on Na,K-ATPase activity. Membrane material was isolated separately from porcine lens epithelium and fiber cells. Tyrosine phosphoproteins, Na,K-ATPase alpha1 polypeptide and PTP-1B were examined by Western blot. Na,K-ATPase activity was determined by measuring ATP hydrolysis in the presence or absence of ouabain. Western blot analysis revealed tyrosine phosphorylation of multiple membrane proteins in both lens cell types, the differentiated fiber cells and non-differentiated epithelium. When membrane material was subjected to immunoprecipitation using an antibody directed against Na,K-ATPase alpha1, a colocalized phosphotyrosine band was detected in lens fibers but not epithelium. Incubation with PTP-1B caused a approximately 50% increase of Na,K-ATPase activity in fiber membrane material. Na,K-ATPase activity in lens epithelium membrane material was not significantly altered by PTP-1B treatment even though PTP-1B was demonstrated to cause dephosphorylation of multiple membrane proteins in the epithelium as well as fibers. While endogenous PTP-1B was detected in both cell types, endogenous tyrosine phosphatase activity was low in both epithelium and fiber membrane material. The results illustrate endogenous tyrosine phosphorylation of Na,K-ATPase alpha1 polypeptide in fibers. Na,K-ATPase alpha1 in lens fibers may be a potential target for PTP-1B.