Abstract
The role of endogenous regucalcin in the regulation of deoxyribonuleic acid (DNA) synthesis in the nuclei of the cloned rat hepatoma cells (H4‐II‐E) with proliferative cells was investigated. Cells were cultured for 6–96 h in a α‐minimum essential medium (α‐MEM) containing fetal bovine serum (FBS; 1 or 10%). Cell number was significantly increased between 24 and 96 h after culture with 10% FBS; cell proliferation was markedly stimulated by culture with 10% FBS as compared with that of 1% FBS. In vitro DNA synthesis activity in the nuclei of cells was significantly elevated 6 h after culture with 10% FBS and its elevation was remarkable at 12 and 24 h after the culture. Nuclear DNA synthesis activity was significantly reduced in the presence of various protein kinase inhibitors (PD98059, staurosprine, or trifluoperazine) in the reaction mixture containing the nuclei of cells cultured for 12 and 24 h with FBS (1 and 10%). The addition of regucalcin (10^−7^ and 10^−6^M) in the reaction mixture caused a significant inhibition of nuclear DNA synthesis activity. The presence of anti‐regucalcin monoclonal antibody (25–100 ng/ml) in the reaction mixture containing the nuclei of cells cultured for 24 h with 10% FBS resulted in a significant increase in nuclear DNA synthesis activity. This increase was completely blocked by the addition of regucalcin (10^−6^ M). The effect of anti‐regucalcin antibody (100 ng/ml) in increasing nuclear DNA synthesis activity was significantly inhibited in the presence of various protein kinase inhibitors. DNA synthesis activity was significantly enhanced in the presence of anti‐regucalcin antibody (100 ng/ml) in the reaction mixture containing the nuclei of cells cultured for 24 h with 10% FBS in the presence of Bay K 8644 (2.5 × 10^−6^ M). Culture with Bay K 8644 did not cause a significant increase in DNA synthesis activity in the absence of anti‐regucalcin antibody. The present study demonstrates that endogenous regucalcin plays a suppressive role in the enhancement of nuclear DNA synthesis with proliferative cells. J. Cell. Biochem. 82: 704–711, 2001. © 2001 Wiley‐Liss, Inc.