Estrogen and phorbol esters regulate amphiregulin expression by two separate mechanisms in human breast cancer cell lines.

The actions of 17 beta-estradiol (E2) and protein kinase C (PKC) appear to converge in the regulation of expression of certain growth modulatory genes, such as the growth factor amphiregulin (AR). AR is known to modulate cell growth by binding to the epidermal growth factor receptor. In the current report we established the mechanisms of the PKC-activating phorbol ester tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) and the steroid hormone E2 on the induction of AR expression in human breast carcinoma cell lines. TPA (100 nM) and E2 (1 nM) induce AR messenger RNA (mRNA) expression by 6- to 8-fold and 3- to 6-fold, respectively, in a time- and dose-dependent manner. In addition, immunoreactive AR protein is induced by both TPA and E2 by 6- to 8-fold and 2- to 4-fold, respectively. The PKC-modulating drugs, bryostatin and H-7, and antiestrogens (ICI 164,384 and 4-hydroxytamoxifen) interfere with AR induction by TPA and estrogen, respectively. The effects of TPA and E2 on the induction of AR mRNA were both closely associated with enhanced transcription of the AR gene. However, TPA had an additional effect at the posttranscriptional level by stabilizing the AR mRNA. The protein synthesis inhibitor, cycloheximide, prevented AR induction by TPA, suggesting that a component of the TPA induction of AR is indirect and dependent upon protein synthesis. Conversely, the E2 induction of AR transcription was found to be a direct response, independent of protein synthesis. The results presented herein thus demonstrate that TPA and E2 are able to stimulate AR gene transcription by two separate mechanisms.