Effect of orthovanadate on tyrosine phosphorylation of P120 GTPase-activating protein in rat liver macrophages (Kupffer cells).

GTPase-activating protein (GAP), a protein capable of regulating the activity of p21ras protein, is phosphorylated on tyrosine residues following the activation of tyrosine kinase(s) associated with several growth factor receptors. The present study was designed to examine potential role of phosphotyrosine phosphatase in tyrosine phosphorylation of GAP. Addition of orthovanadate, a phosphate analogue known to inhibit phosphotyrosine phosphatase, to cultured liver macrophages induced tyrosine phosphorylation of numerous cellular proteins with a range of molecular weights between 30-130 kDa; one tyrosine-phosphorylated protein was identified as the 120 kDa GAP. The effect of orthovanadate on the tyrosine phosphorylation of GAP was time- and concentration-dependent. Quantitated data indicated that approximately 4% of the total content of cellular GAP was tyrosine-phosphorylated upon orthovanadate treatment. These observations suggest a potential regulatory role of phosphotyrosine phosphatase in the tyrosine phosphorylation of GTPase-activating protein in cellular signaling mechanisms in the hepatic macrophages.