EXPRESSION OF ESTROGEN SULFOTRANSFERASE IN MCF-7 CELLS BY CDNA TRANSFECTION SUPPRESSES THE ESTROGEN RESPONSE - POTENTIAL ROLE OF THE ENZYMEIN REGULATING ESTROGEN-DEPENDENT GROWTH OF BREAST EPITHELIAL-CELLS

Estrogen sulfotransferase (EST) is a cytosolic enzyme that catalyzes t he sulfonation of estrogens at the 3-hydroxyl position by use of 3'-ph osphoadenosine-5'-phosphosulfate as an activated sulfate donor. Althou gh largely known and studied as a phase II metabolic enzyme with promi nent expression in the liver, the high substrate specificity of EST (w ith a high V-max/K-m value for estrogen) suggests that expression of t he enzyme in extrahepatic, estrogen target tissues, such as the breast epithelium, may constitute an effective mechanism for local estrogen regulation as well. In this study, we have evaluated the physiological significance of EST expression by cDNA transfection studies with use of the estrogen-dependent MCF-7 breast cancer cell line as a model sys tem. We show that expression of EST in MCF-7 cells effectively reduces the cells' response to physiological concentrations of estradiol (10 nM) by up to 70% as determined in an estrogen-responsive reporter gene assay. In addition, we demonstrate that expression of EST similarly i nhibits estrogen-stimulated DNA synthesis and cell proliferation by 21 % and 46%, respectively. (The thymidine incorporation rate was measure d 3 days after and the cell numbers were counted 8 days after transfec tion.) These results provide direct evidence for the functional signif icance of in situ EST expression in the breast epithelium and suggest that abnormal regulation of the enzyme may have pathological implicati ons in the development and maintenance of hormone-dependent breast car cinomas.