To investigate if the estrogen control of the tumorigenic phenotype of breast cancer cells was mediated through activation of the c-fos protooncogene, we examined the expression of this oncogene in MCF-7 cells. In cells synchronized by double thymidine blockade, the peptide growth factors transforming growth factor alpha and epidermal growth factor increased c-fos mRNA levels 6-fold above controls after 30 min of treatment. The phorbol ester, 12-O-tetradecanoylphorbol-13-acetate, increased

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... ate, increased c-fos mRNA levels 4- to 5-fold above control. 17 beta-Estradiol, a growth stimulator, increased c-fos mRNA levels less than 2-fold above control levels, while progesterone, vitamin D3, dihydrotestosterone, and dexamethasone had little effect on c-fos mRNA levels. In contrast, 17 beta-estradiol treatment initially diminished the c-myc RNA level after 30 min of treatment and resulted in an elevation of c-myc by 2.5 h after initiation of treatment. We conclude that c-fos induction in these cells is growth related and accompanies stimulation by transforming growth factor alpha and epidermal growth factor. 17 beta-Estradiol, on the other hand, induced much smaller increases in c-fos mRNA levels, suggesting an alternative or more complex mechanism of cellular stimulation.