Coexpression of c-myc and transforming growth factor alpha (TGFalpha) as transgenes in mouse liver resulted in an acceleration of neoplastic development as compared to either transgene expressed alone. In an attempt to identify a functional relationship between the expression of the c-myc transgene and signalling by the growth factor TGFalpha, we analyzed the role of the epidermal growth factor (EGF) receptor (EGFR), a tyrosine kinase receptor which binds and is activated by TGFalpha, in primary hepatocytes from the c-myc transgenic mouse. Western blot analysis of the EGFR and Scatchard analysis with [125-I]EGF demonstrated a 5- to 10-fold increase in the number of high affinity EGFR/cell in the c-myc mouse. The higher number of EGFR was maintained even after 72 hours of exposure to EGF, while exposure of normal hepatocytes to EGF for 72 hours reduced the total number of EGFR to below 50% of that of untreated cells. To investigate tyrosine phosphorylation following EGF treatment in these cells, whole cell lysates were analyzed by Western blot analysis with antibodies to anti-phosphotyrosine. Very similar patterns of phosphorylated proteins were detected during a 30 minute time course of EGF treatment. To determine changes in total phosphorylation, cultured hepatocytes were incubated with [32-P]orthophosphate and then triggered with EGF. The patterns of phosphorylated proteins following EGF treatment were again very similar, but the total [32-P] incorporation was increased in proteins from c-myc hepatocytes as compared to normal mouse hepatocytes. The increased expression and aberrant regulation of the EGFR and the increased amount of total phosphorylation after EGF treatment in the c-myc mouse constitutes a possible mechanism for the synergistic effect in neoplastic development seen in the c-myc/TGFalpha double transgenic mouse.
