Correlation of alpha-fetoprotein expression in normal hepatocytes during development with tyrosine phosphorylation and insulin receptor expression.

The molecular mechanism of hepatic cell growth and differentiation is ill defined. In the present study, we examined the putative role of tyrosine phosphorylation in normal rat liver development and in an in vitro model, the alpha-fetoprotein-producing (AFP+) and AFP-nonproducing (AFP-) clones of the McA-RH 7777 rat hepatoma. We demonstrated in vivo and in vitro that the AFP+ phenotype is clearly associated with enhanced tyrosine phosphorylation, as assessed by immunoblotting and flow cytometry. Moreover, immunoprecipitation of proteins with anti-phosphotyrosine antibody showed that normal fetal hepatocytes expressed the same phosphorylation pattern as stable AFP+ clones and likewise for adult hepatocytes and AFP- clones. The tyrosine phosphorylation of several proteins, including the beta-subunit of the insulin receptor, insulin receptor substrate-1, p85 regulatory subunit of phosphatidylinositol-3-kinase, and ras-guanosine triphosphatase-activating protein, was observed in AFP+ clones, whereas the same proteins were not phosphorylated in AFP- clones. We also observed that fetal hepatocytes and the AFP+ clones express 4 times more of the insulin receptor beta-subunit compared with adult hepatocytes and AFP- clones and, accordingly, that these AFP+ clones were more responsive to exogenous insulin in terms of protein tyrosine phosphorylation. Finally, growth rate in cells of AFP+ clones was higher than that measured in cells of AFP- clones, and inhibition of phosphatidylinositol-3-kinase by LY294002 and Wortmannin blocked insulin- and serum-stimulated DNA synthesis only in cells of AFP+ clones. These studies provide evidences in support of the hypothesis that signaling via insulin prevents hepatocyte differentiation by promoting fetal hepatocyte growth.