Abstract
The role of regucalcin, which is a regulatory protein of calcium signaling, in the regulation of protein phosphatase activity in the nuclei of rat kidney cortex was investigated. Protein phosphatase activity towards phosphotyrosine, phosphoserine, and phosphothreonine was found in the nuclei. The enzyme activity towards three phosphoamino acids was significantly increased by the addition of calcium chloride (10–50 μM) in the enzyme reaction mixture. This increase was significantly inhibited by trifluoperazine (25 or 50 μM), an antagonist of calmodulin. The presence of regucalcin (50 or 100 nM) in the enzyme reaction mixture caused a significant decrease in protein phosphatase activity towards three phosphoamino acids. This effect was also seen in the presence of calcium (25 μM) and/or calmodulin (5 μg/ml). Protein phosphatase activity towards three phosphoamino acids was significantly increased in the presence of anti‐regucalcin monoclonal antibody (25 or 50 ng/ml) in the enzyme reaction mixture. This effect was completely blocked by the addition of regucalcin (100 nM). The effect of antibody (25 ng/ml) in increasing protein phosphatase activity towards phosphotyrosine was significantly inhibited by vanadate (10^−4^ M). Also, the antibody's effect towards phosphoserine and phosphothreonine was significantly inhibited by cyclosporin A (10^−5^ M). Endogenous regucalcin was found in the nuclei of rat kidney cortex using Western blot analysis. Nuclear regucalcin level was significantly reduced by the administration of saline (0.9% NaCl) for seven days in rats. Protein phosphatase activity towards three phosphoamino acids was significantly decreased by saline administration. The effect of anti‐regucalcin monoclonal antibody (25 ng/ml) in increasing protein phosphatase activity towards three phosphoamino acids was weakened in the renal cortex nuclei of saline‐administrated rats. The present study demonstrates that endogenous regucalcin plays a suppressive role in the regulation of protein phosphatase activity in the nuclei of rat kidney cortex cells. © 2001 Wiley‐Liss, Inc.