Estrogen metabolism and its relationship to chemical carcinogenesis. In breast epithelial cells, several SERMs were observed to modulate estrogen metabolism (as depicted by boxed arrows), in particular via modulation of detoxification enzymes: sulfotransferase, SULT; UDP-glucuronosyltransferase, UGT; NAPDH:quinone oxidoreductase, NQO1; glutathione-S-transferase-P1, GST. Catechol-O-methyl transferase (COMT) activity was not perturbed by SERMs. SERMs did not modulate expression of CYP450s in E2-treated cells.