RESUMEN
For growth stimulation of liver cells by hepatocyte growth factor (HGF) or transforming growth factor alpha (TGFalpha) via receptor tyrosine kinases, c-fos/c-jun has been considered a point of intersection for cross-talk between the different signal transduction pathways. Recent evidence strongly implicates translocation of pro-TGFalpha into the nucleus as an important step preceding the initiation of hepatic DNA synthesis. We asked whether an active c-jun is required for the nuclear translocation of pro-TGFalpha and its stimulatory effect on DNA synthesis. For this purpose we used mice with c-jun inactivated post partum in hepatocytes by the Cre-loxP recombination system (c-jun(Deltaliver)). Nuclear fractions from control and c-jun(Deltaliver) mouse livers contained TGFalpha as pro-form of 17 kDa and the epidermal growth factor receptor (erbb-1) with molecular weights of 170 and 150 kDa (truncated form). Hepatocytes were isolated by collagenase perfusion and cultivated. A lack of c-jun did not alter the apoptotic activity but significantly suppressed DNA synthesis in the cultured hepatocytes. In control and c-jun(Deltaliver) cells DNA synthesis was almost always associated with nuclear presence of pro-TGFalpha. 76.5 +/- 6.8% of hepatocytes with pro-TGFalpha positive nuclei and only 4.52 +/- 1.31% of hepatocytes with negative nuclei exhibited DNA replication. About 85% of the pro-TGFalpha positive nuclei also contained erbb-1. Treatment of cultures with mature TGFalpha or HGF elevated the frequency of pro-TGFalpha positive nuclei replicating DNA; HGF and TGFalpha-induced nuclear pro-TGFalpha and DNA synthesis significantly more in c-jun(Deltaliver) than in control hepatocytes. These results suggest that (i) a lack of c-jun suppresses basal rates of DNA replication in hepatocytes; (ii) c-jun deficient hepatocytes show a pronounced growth response towards HGF or TGFalpha; (iii) nuclear translocation of pro-TGFalpha together with erbb-1 and its association with DNA synthesis are independent of c-jun.