Selective insensitivity of ZR-75-1 human breast cancer cells to 2-methoxyestradiol:: Evidence for type II 17β-hydroxysteroid dehydrogenase as the underlying cause

2-Methoxyestradiol (2-MeO-E-2), a nonpolar endogenous metabolite of 17 beta-estradiol, has strong antiproliferative, apoptotic, and antiangiogenic actions. Among the four human breast cancer cell lines tested (MCF-7, T-47D ZR-75-1, and MDA-MB-435s), the ZR-75-1 cells were selectively insensitive to the antiproliferative actions of 2-MeO-E-2, although these cells had a similar sensitivity as other cell lines to several other anticancer agents (5-fluorouracil, mitomycin C, doxorubicin, colchicine, vinorelbine, and paclitaxel). Mechanistically, this insensitivity is largely attributable to the presence of high levels of a steroid-selective metabolizing enzyme, the type II 17 beta-hydroxysteroid dehydrogenase (17 beta-14SD), in the ZR-75-1 cells, which rapidly converts 2-MeO-E-2 to the inactive 2-methoxyestrone, but this enzyme does not metabolically inactivate other nonsteroidal anticancer agents. The type II 17 beta-HSD-mediated conversion of 2-MeO-E-2 to 2-methoxyestrone in ZR-75-1 cells followed the first-order kinetics, with a very short half-life (similar to 2 hours). In comparison, the T-47D MCF-7, and MDA-MB-435s human breast cancer cells' which were highly sensitive to 2-MeO-E-2, had very low or undetectable catalytic activity for the conversion of 2-MeO-E-2 to 2-methoxyestrone. Reverse transcription-PCR analysis of the mRNA levels of three known oxidative 17 beta-HSD isozymes (types II, IV, and VIII) revealed that only the type II isozyme was selectively expressed in the ZR-75-1 cells, whereas the other two isozymes were expressed in all four cell lines. Taken together, our results showed, for the first time, that the high levels of type II 17 beta-HSD present in ZR-75-1 cells were largely responsible for the facile conversion of 2-MeO-E-2 to 2-methoxyestrone and also for the selective insensitivity to the antiproliferative actions of 2-MeO-E-2.