ABSTRACT
Estrogen receptor-alpha (ERalpha) and its ligand estradiol (E2) has critical roles in breast cancer growth and are key therapeutic targets. In this study, we report a novel dual role of the adenosine A1 receptor (Adora1) as an E2/ERalpha target and a regulator of ERalpha transcriptional activity. In ERalpha-positive breast cancer cells, E2 upregulated Adora1 messenger RNA (mRNA) and protein levels, an effect that was reversed by the E2 antagonist ICI 182 780. Small interference RNA ablation of Adora1 in ERalpha-positive cells reduced basal and E2-dependent proliferation, whereas Adora1 over-expression in an ERalpha-negative cell line induced proliferation. The selective Adora1 antagonist, DPCPX, reduced proliferation, establishing Adora1 as a mediator of E2/ERalpha-dependent breast cancer growth. Intriguingly, Adora1 ablation decreased both mRNA and protein levels of ERalpha and, consequently, estrogen-responsive element-dependent ERalpha transcriptional activity. Moreover, Adora1 ablation decreased binding activity of ERalpha to the promoter of its target gene TFF1 and led to reduced TFF1 promoter activity and mRNA levels, suggesting that Adora1 is required for full transcriptional activity of ERalpha on E2 stimulation. Taken together, we showed a short feed-forward loop involving E2, ERalpha and Adora1 that favors breast cancer growth. These data suggest that Adora1 may represent an important target for therapeutic intervention in hormone-dependent breast cancer.
Subject(s)
Breast Neoplasms/metabolism , Estradiol/metabolism , Estrogen Receptor alpha/physiology , Estrogens/pharmacology , Receptor, Adenosine A1/physiology , Breast Neoplasms/pathology , Cell Line, Tumor , Estrogen Receptor alpha/agonists , Estrogen Receptor alpha/genetics , Estrogen Receptor alpha/metabolism , Female , Humans , Receptor, Adenosine A1/genetics , Receptors, Estrogen/metabolism , Trefoil Factor-1 , Tumor Suppressor Proteins/physiologyABSTRACT
Epithelial-stromal interactions play key roles for aromatase expression and estrogen production in breast cancer tissue. Upregulated aromatase expression in breast fibroblasts increases the tissue concentration of estradiol (E2), which then activates a large number of carcinogenic genes via estrogen receptor-alpha (ERalpha) in malignant epithelial cells. This clinically pertains, since aromatase inhibitors (AIs) are the most effective hormonal treatment of ERalpha-positive breast tumors. A single gene encodes aromatase, the key enzyme in estrogen biosynthesis, the inhibition of which by an AI effectively eliminates E2 production. Since alternative promoters regulated by distinct signaling pathways control aromatase expression, it is possible to target these pathways and inhibit estrogen production in a tissue-selective fashion. We and others previously found that the majority of estrogen production in breast cancer tissue was accounted for by the aberrant activation of the proximal promoter I.3/II region. PGE(2) that is secreted in large amounts by malignant breast epithelial cells is the most potent known natural inducer of this promoter region in breast adipose fibroblasts. Signaling effectors/transcriptional regulators that mediate PGE(2) action include the activator pathways p38/CREB-ATF and JNK/jun and the inhibitory factor BRCA1 in breast adipose fibroblasts. Selective inhibition of this promoter region may treat breast cancer while permitting aromatase expression via alternative promoters in the brain and bone and thus obviate the key side effects of the current AIs. The signaling pathways that mediate the regulation of the promoter I.3/II region in undifferentiated fibroblasts in malignant breast tumors are reviewed.