The Effects of Endoxifen and Other Major Metabolites of Tamoxifen on the Sulfation of Estradiol Catalyzed by Human Cytosolic Sulfotransferases hSULT1E1 and hSULT1A1*1
作者:Edwin J. Squirewell、Michael W. Duffel
DOI:10.1124/dmd.115.063206
日期:2015.6
Tamoxifen is successfully used for both treatment and prevention of estrogen-dependent breast cancer, yet side effects and development of resistance remain problematic. Endoxifen is a major active metabolite of tamoxifen that is being investigated for clinical use. We hypothesized that endoxifen and perhaps other major metabolites of tamoxifen may affect the ability of human estrogen sulfotransferase 1E1 (hSULT1E1) and human phenol sulfotransferase 1A1 isoform 1 (hSULT1A1*1) to catalyze the sulfation of estradiol, an important mechanism in termination of estrogen signaling through loss of activity at estrogen receptors. Our results indicated that endoxifen, N -desmethyltamoxifen (N-desTAM), 4-hydroxytamoxifen (4-OHTAM), and tamoxifen- N -oxide were weak inhibitors of hSULT1E1 with K i values ranging from 10 μ M to 38 μ M (i.e., over 1000 times higher than the 8.1 nM K m value for estradiol as substrate for the enzyme). In contrast to the results with hSULT1E1, endoxifen and 4-OHTAM were significant inhibitors of the sulfation of 2.0 µ M estradiol catalyzed by hSULT1A1*1, with IC50 values (9.9 μ M and 1.6 μ M, respectively) that were similar to the K m value (1.5 μ M) for estradiol as substrate for this enzyme. Additional investigation of the interaction of these metabolites with the two sulfotransferases revealed that endoxifen, 4-OHTAM, and N-desTAM were substrates for hSULT1E1 and hSULT1A1*1, although the relative catalytic efficiencies varied with both the substrate and the enzyme. These results may assist in future elucidation of cell- and tissue-specific effects of tamoxifen and its metabolites.
他莫昔芬被成功地用于治疗和预防雌激素依赖性乳腺癌,但副作用和耐药性的产生仍然是个问题。恩多昔芬是他莫昔芬的一种主要活性代谢物,目前正在对其临床应用进行研究。我们假设,恩多昔芬和他莫昔芬的其他主要代谢物可能会影响人雌激素硫基转移酶 1E1(hSULT1E1)和人酚硫基转移酶 1A1 异构体 1(hSULT1A1*1)催化雌二醇硫酸化的能力,而雌二醇硫酸化是通过雌激素受体失去活性来终止雌激素信号传导的重要机制。我们的研究结果表明,endoxifen、N-desmethyltamoxifen(N-desTAM)、4-hydroxytamoxifen(4-OHTAM)和 tamoxifen- N -oxide 是 hSULT1E1 的弱抑制剂,其 K i 值从 10 μ M 到 38 μ M 不等(即比作为该酶底物的雌二醇的 8.1 nM K m 值高出 1000 多倍)。与 hSULT1E1 的结果相反,内酯雌酚和 4-OHTAM 是 hSULT1A1*1 催化的 2.0 μ M 雌二醇硫酸化的显著抑制剂,其 IC50 值(分别为 9.9 μ M 和 1.6 μ M)与该酶底物雌二醇的 K m 值(1.5 μ M)相似。对这些代谢物与两种磺基转移酶相互作用的进一步研究表明,内昔芬、4-OHTAM 和 N-desTAM 是 hSULT1E1 和 hSULT1A1*1 的底物,尽管相对催化效率因底物和酶而异。这些结果可能有助于今后阐明他莫昔芬及其代谢物对细胞和组织的特异性影响。