Enhancement of protein tyrosine phosphatase activity in the proliferation of cloned rat hepatoma H4-II-E cells: suppressive role of endogenous regucalcin.

The role of endogenous regucalcin in the regulation of protein tyrosine phosphatase activity in the proliferation of the cloned rat H4-II-E hepatoma cells was investigated. Cells were cultured for 6 to 96 h in a medium containing 1.0 or 10% fetal bovine serum (FBS). Cell numbers were significantly raised by culture with 10% FBS in comparison with that of 1.0% FBS. Protein tyrosine phosphatase activity in the cells was significantly elevated by culture with 10% FBS for 24 to 96 h as compared with that of 1% FBS. Such an increase was not seen in protein phosphatase activity toward phosphoserine or phosphothreonine. The presence of anti-regucalcin monoclonal antibody (50 or 100 ng/ml) in the enzyme reaction mixture caused a remarkable elevation of protein tyrosine phosphatase activity in the cells obtained by culture with 1.0 or 10% FBS. This elevation was completely prevented by the addition of regucalcin (10-6 M). The effect of antibody in elevating protein tyrosine phosphatase activity in the cells was significantly inhibited by the addition of okadaic acid (10-6 M) or vanadate (10(-6) M), an inhibitor of protein phosphatase, in the reaction mixture. The present study suggests that protein tyrosine phosphatase activity in the cloned rat hepatoma cells is increased in serum-stimulated cell proliferation, and that endogenous regucalcin has a suppressive role in the enhancement of the enzyme activity in proliferative cells.