Modifications of the hepatocyte growth factor/c-met pathway by constitutive expression of transforming growth factor-alpha in rat liver epithelial cells.

We have previously shown that rat liver epithelial cells (RLEC) transfected with and constitutively expressing transforming growth factor-alpha (TGF-alpha) have an enhanced mitogenic response to hepatocyte growth factor (HGF). In the study reported here, we examined tumor clones derived from the TGF-alpha transfectants with respect to mitogenic response to HGF. Tumor cell lines that expressed TGF-alpha responded to HGF with a greater increase in DNA synthesis than did the nontransfected parental RLEC (pRLEC). The tumor clones had also acquired a lower threshold for HGF response, which enabled them to undergo significant DNA synthesis at a low concentration of HGF that did not evoke a response in the pRLEC or TGF-alpha transfectants. We investigated the mechanisms by which TGF-alpha expression may influence the HGF/c-met pathway. We showed that most TGF-alpha transfectants and tumor cells displayed increases in c-met mRNA and protein, indicating that the enhanced HGF response may be due in part to an increase in the amount of receptor present. However, in all transfectants and tumor clones that constitutively expressed TGF-alpha, c-met was tyrosine phosphorylated in the absence of ligand (HGF) or other exogenous growth factors. These data suggest that induction of c-met mRNA and transactivation of c-met may be a sequela of the constitutive expression of TGF-alpha and that constitutive activation of the epidermal growth factor receptor pathway leads to phosphorylation and activation of c-met. These studies provide evidence for a novel mechanism of communication between epidermal growth factor receptor and c-met pathways that may partially explain the synergistic effects reported between TGF-alpha and HGF.