Signal transduction involved in cell cycle regulation of normal hepatocyte and hepatoma cells

The cell cycle is controlled by cyclin/cyclin-dependent kinase (CDK) complexes and CDK inhibitors. The expression and function of these molecules are induced and modulated by signal transduction governing cell growth and differentiation. We found that when normal rat hepatocyte growth was induced by hepatocyte growth factor (HGF), HGF receptor (MET) was activated, with association of MET with growth factor receptor-bound protein (GRB)-2, which is the initial step of activation of the mitogen-activated protein kinase (MAPK) cascade. We also report that the MAPK cascade was sequentially activated, followed by hepatocyte growth. Before cell density reached confluence, p21/WAF1, a CDK inhibitor, prevented Cdk4 activity, resulting in cell cycle arrest. Thus, in normal hepatocyte growth, the cell cycle is strictly controlled. In contrast, in human hepatocellular carcinomas (HCCs), MAPK/ERK (extracellular signal regulated kinase) was constitutively upregulated. Enhanced association of GRB-2 with growth factor receptors or Sos, a ras activator, was also observed, which may account for MAPK activation. On the other hand, p21/WAF1 in human HCCs was deficient or overwhelmed by CDKs in excess stoichiometry. These observations indicate that constitutively active MAPK and/or disturbance of cell cycle regulators contributes much to unrestricted cell growth in human HCCs. Finally, we introduced deletion mutants of GRB-2 into HepG2 cells, resulting in reduction of MAPK/ERK activity and prevention of cell growth. In addition, when we introduced the p21/WAF1 expression vector into HepG2 cells, the cell cycle was arrested at G(1)/S transition with inhibition of cell growth. These results may provide a new strategy for anticancer therapy by modulating cell cycle machinery.