Alteration in the agonist/antagonist balance of antiestrogens by activation of protein kinase A signaling pathways in breast cancer cells: Antiestrogen selectivity and promoter dependence

We find that stimulation of the protein kinase A (PKA) signaling pathway in MCF-7 human breast cancer cells changes the agonist/antagonist activity of tamoxifen and related antiestrogens; it activates or enhances their estrogen agonist activity and reduces their ability to antagonize the effects of estradiol (E₂). In MCF-7 human breast cancer cells which contain high levels of endogenous estrogen receptor (ER), the antiestrogen trans-hydroxy- tamoxifen (TOT) fails to stimulate transcription of the estrogen-responsive promoter-reporter constructs estrogen response element (ERE)-TATA- chloramphenicol acetyl transferase (CAT), (ERE)₂-TATA-CAT, and pS2-CAT. However, when cells are treated with isobutyl methylxanthine plus cholera toxin (which increases intracellular cAMP approximately 10-fold), or with 8- bromo-cAMP, or are transfected with expression vectors for the PKA catalytic subunits, the transcriptional activity of the antiestrogen-ER complex is now increased, to levels 20-75% that of E₂, and TOT also becomes much less effective in antagonizing the stimulation of transcription by E₂. Although this alteration in the agonist and antagonist activity of TOT is observed with three promoter-reporter constructs, containing a simple TATA promoter or a more complex, pS2 promoter, elevation of cAMP did not enhance the transcription by either TOT or E₂ of the reporter plasmid ERE-thymidine kinase-CAT. Thus, this phenomenon is promoter specific. The maximal stimulatory effects of isobutylmethylxanthine plus cholera toxin and PKA catalytic subunits on TOT and E₂ transcriptional enhancement were not additive, consistent with the hypothesis that they are both acting via stimulation of the same signal transduction pathway. By contrast, cAMP and PKA catalytic subunit transfection failed to evoke any transcription by the more pure antiestrogen ICI164,384 with any of the four promoter-reporter constructs tested. Our findings, documenting that stimulation of the PKA signaling pathway activates the agonist activity of tamoxifen-like antiestrogens, may in part explain the development of tamoxifen resistance by some ER-containing breast cancers. They also suggest that the use of antiestrogens like ICI164,384, that fail to activate ER transcription in the presence of cAMP, may prove more effective for long-term antiestrogen therapy in breast cancer.