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Superinduction of c-fos gene expression by estrogen in cultured guinea-pig endometrial cells requires priming by a cycloheximide-dependent mechanism.

The c-fos gene expression is rapidly induced by various mitogenic agents and protein synthesis inhibitors in many cell types. Estradiol-17 beta can induce c-fos gene expression in breast cancer cell lines and in the uterus in vivo, but not in cultured guinea-pig endometrial cells. Using this model, we investigated whether a protein synthesis inhibitor, cycloheximide, could induce the c-fos gene and permit a superinduction by estrogens. In the presence of cycloheximide (10 micrograms/ml), protein synthesis was inhibited at 95% within the first hour. From 190 min after the addition of estradiol-17 beta or diethylstilbestrol (10(-8) M) and cycloheximide (10 micrograms/ml), there was a significant increase (ranging from 3- to 5-fold) of the c-fos messenger RNA level (2.2 kilobase in size), compared with the level in cells treated with cycloheximide alone. Nonestrogenic steroid hormones and estradiol-17 alpha were unable to induce c-fos gene expression in the presence of cycloheximide. The effect of estradiol-17 beta observed in the presence of cycloheximide was completely abolished by 4-hydroxy-tamoxifen or by Ly 156758 or by ICI 164384 (10(-6) M). The c-fos mRNAs were rather stable in cells treated with cycloheximide for 2 h (half-life = 51 +/- 6 min) and there was no further increase in the c-fos messenger RNA stability after the addition of cycloheximide plus estradiol-17 beta (half-life = 40 +/- 3 min). The overall results suggest a response at the transcriptional level. In conclusion, cycloheximide transmits activating signals to the c-fos gene which act as priming elements to allow the estrogen action in cultured guinea-pig endometrial cells.