Similarly, when human liver cytosol was used in this study, 17-sulfate was the predominant product with 38-fold less 3-sulfate, and the addition of celecoxib did not affect the product profile. SULT1A1-catalyzed sulfonation of 10 nM estrone and 4 M studies suggested that the presence of the bulky tryptophan residue in the substrate-binding site of the rat SULT2A homolog instead of glycine as in human SULT2A1 may explain HILDA this species difference. model and inhibited growth of breast epithelial cells [10, 11]. Combinations of the ether analog of RRR-alpha-tocopherol (vitamin E) with celecoxib 48740 RP synergistically inhibited tumor volume compared to individual treatments with either drug in nude mice . Studies in rats have found that a combination of the aromatase inhibitor exemestane with celecoxib was better at slowing the growth of breast cancer than either drug used alone . A human clinical trial reported anti-tumor effects of celecoxib treatment in breast cancer patients . It was speculated that COX-2 inhibition may contribute to the prevention and/or treatment of breast cancer by celecoxib [13C15] however, the mechanisms underlying its antitumor activity are not fully understood. The ability of celecoxib to switch the dominant product of 17-E2 sulfonation from 3-sulfate to 17-sulfate with human recombinant SULT2A1 as well as in human liver cytosol  suggested that 17-E2 levels in breast tissue would be reduced by celecoxib, as the 17-sulfate was resistant to sulfatase hydrolysis [16, 17]. This suggests another mechanism by which celecoxib could be beneficial in treating estrogen 48740 RP receptor positive breast cancer. The effects of celecoxib on the position and rate of sulfonation of several steroids, shown in Figure 1, have been examined with human recombinant SULT2A1 . They are dehydroepiandrosterone (DHEA; (3)-3-hydroxyandrost-5-en-17-one), androstenediol (AD; (3,17)-androst-5-en-3,17-diol), epitestosterone (Epi-T; (17)-17-hydroxyandrost-4-en-3-one), testosterone (T; (17)-17-hydroxyandrost-4-en-3-one), 17-estradiol (17-E2; (17)-estra-1,3,5(10)-triene-3,17-diol), estrone (E1; 3-hydroxyestra-1(10),2,4-trien-17-one), 3-methyl ether of 17-E2 (3Me-E2), 6-dehydroestradiol (6D-E2; estra-1,3,5(10),6-tetraene-3,17-diol), 17-dihydroequilenin (17-Eqn; 1,3,5(10)6,8-estrapentaen-3,17 -diol),17-dihydroequilin (17-Eq; 1,3,5(10)7-estratetraen-3, 17-diol) and 9-dehydroestradiol (9D-E2; (17)-estra-1,3,5(10),9(11)-tetraene-3,17-diol). With 17-E2, 6D-E2, 9D-E2 and the equine estrogens 17-Eqn and 17-Eq that are found in hormone replacement therapy preparations such as premarin , celecoxib switched the preferred position of sulfonation by SULT2A1 from the 3-OH to the 17-OH. The modulation of SULT2A1 activity observed was proposed to be due to a conformational change of the SULT2A1 dimer upon celecoxib binding to the PAPS (3-phosphoadenosine-5-phosphosulfate) binding site in one of the constituent monomers such that the 17-estrogens bound with the 17-OH in a favorable position for sulfonation. The stimulation of 17-sulfonation occurred regardless of 48740 RP the order of addition of celecoxib or PAPS to assays. The effect of celecoxib in human liver cytosol, which is an important site of steroid sulfonation, is of interest for understanding the likely effects of celecoxib on metabolism of physiologically important estrogens and the equine estrogens commonly used in hormone replacement therapy. Open in a separate window Figure 1 Chemical structures of steroids tested for the effect of celecoxib on their sulfonation by human liver cytosol. The structural differences with 17-E2 are highlighted in red. 48740 RP It was previously shown that the ratio of 17-E2C17-sulfate/17-E2C3-sulfate was higher for human SULT2A1 than for human liver cytosol at a given concentration of celecoxib, and this was thought to be due to the contributions of SULT1A1 and SULT1E1 to 17-E2C3-sulfate formation in liver . It is known that several xenobiotics including dietary and environmental chemicals are better inhibitors of SULT1A1 and SULT1E1 than of SULT2A1 [20, 21]. We hypothesized that the presence of specific phenol SULT inhibitors as well as celecoxib in incubations with human liver cytosol would result in increases in the ratio of 17-sulfate/3-sulfate. In this study, we investigated the sulfonation of 17-E2 in the presence of both celecoxib and the phenol SULT inhibitors, triclosan or quercetin. Quercetin was reported as a potent inhibitor of SULT1A1 [20C23] while triclosan inhibited SULT1E1 and 1A1 [20, 21, 24, 25]. The structures of inhibitors are shown in Figure 2. Open in a separate window Figure 2 Structure of 17-E2 with carbon atoms numbered and rings named according to steroid nomenclature. Structure of celecoxib and the SULT inhibitors triclosan and quercetin are also shown. The switching of the.