PRESENCE OF DISTINCT AP-1 DIMERS IN NORMAL AND TRANSFORMED RAT HEPATOCYTES UNDER BASAL CONDITIONS AND AFTER EPIDERMAL GROWTH-FACTOR STIMULATION

Activation of the transcriptional regulator AP-1, a dimeric complex fo rmed of various combinations of Fos and Jun proteins, is a key step in the cellular response to mitogens, Because different dimers are belie ved to display different regulatory functions, we hypothesized that tr ansformed cells that lack normal growth constraints might display AP-1 dimers that are different from those of normal cells, We therefore co mpared in primary and transformed rat hepatocytes (1) the composition of AP-1 dimers under basal conditions and (2) AP-1 induction by epider mal growth factor (EGF), Under basal conditions, AP-I contained predom inantly Jun homodimers in both cell types. However, whereas normal hep atocytes contained only JunD, both JunD and JunB were present in the A P-1 complex of 7777 cells. EGF treatment triggered almost identical pr ograms of fos and jun gene activation at the messenger RNA (mRNA) leve l in both cell types, with an early accumulation of c-fos, c-jun, and junB mRNAs, but no change in junD mRNA levels, In both cell types, c-F os and Fra-1 proteins increased after EGF treatment, but differences i n the induction of Jun proteins were noted, with an increase of c-Jun in hepatocytes and an increase of JunB in 7777 cells. In both cell typ es, activation of AP-1 DNA binding activity by EGF was accompanied by the recruitment of Fra-1 into AP-1, detected earlier in 7777 cells tha n in hepatocytes, and with the transient appearance of c-Fos in 7777 c ells only, Finally, EGF activated AP-1-dependent transcription in 7777 cells but not in hepatocytes. These data indicate important differenc es in the functional activity of AP-1 in transformed hepatocytes.