The Effects of Endoxifen and Other Major Metabolites of Tamoxifen on the Sulfation of Estradiol Catalyzed by Human Cytosolic Sulfotransferases hSULT1E1 and hSULT1A1*1

Abstract

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 mM to 38 mM (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 mM estradiol catalyzed by hSULT1A1*1, with IC 50 values (9.9 mM and 1.6 mM, respectively) that were similar to the K m value (1.5 mM) 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.

Hydroxylated and sulfated metabolites of commonly occurring airborne polychlorinated biphenyls inhibit human steroid sulfotransferases SULT1E1 and SULT2A1

“…Recombinant human SULT1E1 (Squirewell and Duffel, 2015) and SULT2A1 (Gulcan et al, 2008) were expressed in Escherichia coli BL21 (DE3) cells, purified, and characterized as described previously.…”

The Modulation of Drug Efflux Transporter by Curcumin in McF7 Breast Cancer Cells After Repeated Exposure of Endoxifen and Estradiol

“…Several studies have argued that epithelial-mesenchymal transition, which is characterized by increased vimentin expression, might play a significant role in the development of E resistance [7]. In addition, Squirewell et al [14] argued that E may affect the ability of human Drug efflux transporters are involved in the decreased sensitivity of BC cells to TMX [9,10]. In the present study, the sensitivity of BC cells to E was diminished, which was followed by increased activities of the drug efflux transporters P-glycoprotein, BCRP, and MRP1.…”

“…This effect may occur through the suppression of P-glycoprotein (MDR1) expression [16][17][18]. Squirewell et al [14] proposed a mechanism of the interactions of polyphenols, including CM, with ABC transporters and showed that polyphenols counteracted tumor cell chemoresistance by interacting with the ATP-binding domains of P-glycoprotein and inhibits its ATPase activity site and steroid interacting regions of the protein cytosolic domains [13]. However, for a flavonoid to interact with MRP1, it also needs transport stimulation of reduced glutathione [1,19].…”

The potential disruption of estrogen and androgen homeostasis and adipocyte differentiation by metabolites of common airborne polychlorinated biphenyls

“…Recombinant human hSULT1E1 [315] and hSULT2A1 [316] were expressed in Escherichia coli BL21 (DE3) cells, purified, and characterized as described previously.…”

“…Assays for the sulfation of estradiol catalyzed by SULT1E1 were conducted using a previously described method [317].…”