Stimulation of the mitogen-activated protein kinase cascade and tyrosine phosphorylation of the epidermal growth factor receptor by hepatopoietin

Hepatopoietin (HPO) is a novel human hepatotrophic growth factor, which spe cifically stimulates proliferation of cultured primary hepatocytes in vitro and liver regeneration after liver partial hepatectomy in vivo. Recently, the identification of the mitogenic effect of HPO on hepatoma cell lines an d the existence of HPO-specific receptors indicate that HPO acts via its sp ecific cell surface receptor, However, the molecular mechanism of HPO actio n is not fully elucidated. In this report, we examined the signal transduct ion events induced by HPO in hepatoma cell line (HepG2), Our results demons trated that HPO induces phosphorylation of mitogen-activated protein kinase kinase and mitogen-activated protein kinase (MAPK) in a rapid and transien t manner. HPO stimulates tyrosine phosphorylation of epidermal growth facto r receptor (EGFR). Furthermore, we observed that both MAPK activation and t he mitogenic effect of HPO on HepG2 cells were completely blocked by AG1478 , a specific inhibitor of EGFR tyrosine kinase activity. However, the effec ts of HPO were not antagonized by an EGFR-blocking antibody, mAb528, which blocks the interaction between epidermal growth factor and EGFR, indicating that stimulation of tyrosine phosphorylation of EGFR by HPO was not mediat ed by epidermal growth factor. In contrast, genistein, a general tyrosine k inase inhibitor, significantly attenuated the tyrosine phosphorylation of E GFR in response to HPO, In conclusion, our results suggest that tyrosine ph osphorylation of EGFR may play a critical role in MAPK activation and mitog enic stimulation by HPO.