We have shown previously that insulin at the physiological concentration suppresses hepatitis B surface antigen (HBsAg) gene expression in cultured human hepatoma Hep3B cells, and this suppression phenomenon is concomitant with the stimulation of cell proliferation. We have now examined whether these two distinct insulin actions on the Hep3B cells are mediated through the same or different signaling pathways. After prolonged treatment with high concentration of tumor promoter, 12-O-tetradecanoyl phorbol-13-acetate (TPA), the protein kinase C-alpha (PKC-alpha) level in the Hep3B cells was diminished and could not be detected by Western blot analysis. Under this condition, TPA treatment has no effect on the number or affinity of the insulin receptor on Hep3B cells. However, insulin-stimulated cell proliferation was completely abolished in the PKC-alpha depleted cells. In contrast, insulin still suppressed HBsAg gene expression with the same ED50 (approximately 0.5 nM) as the control cell. The induction of proto-oncogene egr-1 (early-growth-regulatory-1) by insulin and TPA under similar conditions were also examined. Both insulin and TPA stimulated egr-1 gene expression up to 10-fold in the control cell, but neither of these two agents showed any effect on egr-1 gene expression in the PKC-alpha down-regulated Hep3B cells. These observations indicate that the PKC-alpha may be involved in the insulin induced egr-1 expression and cell proliferation but not in the insulin suppressed HBsAg gene expression in human hepatoma cells.