Regulation of protein phosphatase activity by regucalcin localization in rat liver nuclei

Abstract

The regulatory role of regucalcin on protein phosphatase activity in isolated rat liver nuclei was investigated. Phosphatase activity toward phosphotyrosine and phosphoserine was significantly increased by the addition of CaCl(2) (10(-5) and 10(-4) M) in the enzyme reaction mixture. Trifluoperazine (25 and 50 microM), an antagonist of calmodulin, significantly inhibited protein phosphatase activity toward phosphoserine, while it had no effect on the enzyme activity toward phosphotysine and phosphothreonine. Cyclosporin A (10(-6)-10(-4) M), an inhibitor of Ca(2+)/calmodulin-dependent protein phosphatase activity toward phosphoserine, but not phosphotyrosine and phosphoserine. Thus, Ca(2+)/calmodulin-dependent phosphatases were present in liver nuclei. Regucalcin (0.25 and 0.5 microM) had an inhibitory effect on liver nuclear phosphatase activity toward phosphotyrosine, phosphoserine, and phosphothreonine. The presence of anti-regucalcin monoclonal antibody (25 and 50 ng/ml) in the enzyme reaction mixture caused a significant elevation of nuclear phosphatase activity toward three phosphoaminoacids. An analysis with sodium sulfate-polyacrylamide gel electrophoresis suggested a possibility of localization of regucalcin in liver nuclei. Moreover, regucalcin was determined in liver nuclei using enzyme-linked immunoadsorbent assay. The present study demonstrates that the endogenous regucalcin inhibits phosphatase activity in the liver nuclei.

Role of endogenous regucalcin in the regulation of Ca2+-ATPase activity in rat liver nuclei

“…These results indicate that Ca 2ϩ -ATPase activity is suppressed by the endogenous regucalcin in liver nuclei. Regucalcin has been demonstrated to localize in rat liver nuclei [Omura and Yamaguchi, 1999]. The present finding suggests that regucalcin plays a role in the control of Ca 2ϩ -ATPase activity in liver nuclei.…”

Suppressive Effect of Endogenous Regucalcin on Guanosine Triphosphatase Activity in Rat Liver Nucleus.

“…12,14) The presence of exogenous regucalcin with lower concentration (0.1 and 0.25 mM) used in the enzyme reaction mixture did not have a significant inhibitory effect on GTPase activity in liver nucleus. However, the enzyme activity was significantly decreased by the addition of 0.5 mM regucalcin.…”

“…10,11) Regucalcin in the cytoplasm of liver cells has been shown to transport to the nucleus, 12) and it may play a role in the regulation of nuclear function in liver cells. Regucalcin has been shown to have an inhibitory effect on Ca 2ϩ -dependent protein kinases, 13) protein phosphatases, 14) and deoxyribonucleic acid (DNA) 15) and ribonucleic acid (RNA) 16) synthesis in liver nucleus. The effect of regucalcin on liver nuclear function, however, remains to be elucidated.…”

Sexual Dimorphism of Rat Liver Nuclear Proteins

“…NF45, a doublestranded RNA binding protein, has been reported to stimulate gene expression in mammalian cells (52). Regulcacin is a regulatory protein of Ca 2ϩ -dependent signaling that can inhibit protein kinase and phosphatase activity (53)(54)(55)(56). Of all the proteins identified, NF45 and the three hnRNPs are potentially of greatest interest, insofar as they are sexually dimorphic, GH-regulated nuclear proteins with the ability to regulate gene transcription and/or translation, and conceivably could contribute to the GH-dependent regulation of gene expression seen in liver.…”