Regulated CYP19 aromatase transcription in breast stromal fibroblasts

Extraglandular estrogen synthesis mediates the proliferation of estrogen-re sponsive breast cancer in postmenopausal women. Aromatase, the cytochrome P 450 Cyp19 enzyme, catalyzes the rate-limiting step in estrogen biosynthesis . Activity is present in both normal and neoplastic breast tissue, and Cyp1 9 protein is localized by immunohistochemistry predominantly in breast stro mal fibroblasts. In cultured breast stromal fibroblasts, both activity and Cyp19 messenger ribonucleic acid are increased to a substantial degree by h ormonal and growth factor regulators of transcription. Transcriptional regu lation of CYP19 is complex in breast tissues, in which exon switching in th e usage of alternative first exons occurs from predominantly EI.4 in breast tissue from cancer-fi ee women to predominantly EI.3 and PII in breast tum ors and quadrants with or without tumor. The present study questioned wheth er the first exon switch occurs as a result of an inherent difference betwe en fibroblasts in normal and tumor tissues or because of differences in loc al regulators between these tissues. To distinguish between these two possi bilities, we examined fibroblasts cultured from breast tumor, benign breast , and reduction mammoplasty tissues for the ability of various CYP19 transc riptional regulators to modulate first exon EI.3, EI.4, and PII usage. A se miquantitative RT-PCR method was used to identify transcripts containing si x of the nine known CYP19 first exons. Combinations of cAMP and Dex regulat ed transcription from first exons EI.3, EI.4, and PII in fibroblasts cultur ed from ail tissues, but not in reduction mammoplasty epithelial cells. The se results provide evidence that the fibroblasts from these breast tissues are not inherently different in transcriptional regulation of CYP19 first e xon usage and that transcriptional regulatory molecules are likely to media te the exon switch phenomenon.