Activation of Aromatase Expression by Retinoic Acid Receptor-related Orphan Receptor (ROR) α in Breast Cancer Cells IDENTIFICATION OF A NOVEL ROR RESPONSE ELEMENT

Estrogen is a key regulator of the proliferation and differentiation of breast cancer cells. In addition to the estrogen supply from the ovary, estrogen is produced locally from androgen by aromatase. However, the regulation of aromatase gene expression in breast cancer has not yet been fully clarified. Retinoic acid receptor-related orphan receptor (ROR) alpha plays an important role in the differentiation of many organs by regulating the transcription of target genes. Because aromatase and ROR alpha are expressed in breast cancer, the effect of ROR alpha on aromatase gene expression was studied. ROR alpha significantly augmented the expression of aromatase mRNA, particularly those containing exon I.4, in MCF7 cells, and aromatase activities in T47D and MCF7 cells. ROR alpha also stimulated the proliferation of these cells. Transient transfection-based reporter gene assays using the promoter at exon I. 4 showed that ROR alpha augmented the transcription. A series of truncated mutation studies revealed that ROR alpha activated the transcription through -147 to +14 bp of the promoter I.4. Furthermore, ROR alpha bound to the fragment containing -119 to -107 bp of the promoter in vitro, indicating that this region may contain a novel ROR response element. Chromatin immunoprecipitation assay showed that ROR alpha bound to the region containing this site of the promoter I. 4 in MCF7 cells. Moreover, we examined clinical samples and found a correlation between ROR alpha and aromatase expression. These results suggest that ROR alpha directly activates the aromatase expression to accelerate the local production of estrogen, which results in the proliferation of breast cancer cells.