Microsomal antiestrogen-binding site ligands induce growth control and differentiation of human breast cancer cells through the modulation of cholesterol metabolism

Abstract

The microsomal antiestrogen-binding site (AEBS) is a highaffinity membranous binding site for the antitumor drug tamoxifen that selectively binds diphenylmethane derivatives of tamoxifen such as PBPE and mediates their antiproliferative properties. The AEBS is a hetero-oligomeric complex consisting of 3B-hydroxysterol-# 8 -# 7 -isomerase and 3B-hydroxysterol-# 7 -reductase. High-affinity AEBS ligands inhibit these enzymes leading to the massive intracellular accumulation of zymostenol or 7-dehydrocholesterol (DHC), thus linking AEBS binding to the modulation of cholesterol metabolism and growth control. The aim of the present study was to gain more insight into the control of breast cancer cell growth by AEBS ligands. We report that PBPE and tamoxifen treatment induced differentiation in human breast adenocarcinoma cells MCF-7 as indicated by the arrest of cells in the G 0 -G 1 phase of the cell cycle, the increase in the cell volume, the accumulation and secretion of lipids, and a milk fat globule protein found in milk. These effects were observed with other AEBS ligands and with zymostenol and DHC. Vitamin E abrogates the induction of differentiation and reverses the control of cell growth produced by AEBS ligands, zymostenol, and DHC, showing the importance of the oxidative processes in this effect. AEBS ligands induced differentiation in estrogen receptor-negative mammary tumor cell lines SKBr-3 and MDA-MB-468 but with a lower efficiency than observed with MCF-7. Together, these data show that AEBS ligands exert an antiproliferative effect on mammary cancer cells by inducing cell differentiation and growth arrest and highlight the importance of cholesterol metabolism in these effects.

Identification and pharmacological characterization of cholesterol-5,6-epoxide hydrolase as a target for tamoxifen and AEBS ligands

“…We recently established that AEBS ligands induce differentiation and apoptosis in breast cancer cells through a mechanism involving the production of sterol autoxidation products, and that vitamin E inhibits these effects (3,4,12). Consistent with these data, previous reports have shown a limited clinical outcome with Tam treatment when cholesterol metabolism enzymes (28) or antioxidant enzymes (29) are overexpressed in breast tumors.…”

“…ChEH is the last member of the epoxide hydrolase family with an unidentified coding gene. The results from these studies, along with the relationship that we established between sterol autoxidation products and the functions of the AEBS (3,4,12), suggested to us a pharmacological and structural link between the AEBS and ChEH, which we investigated in the present study. Our results indicate that the enzymes that form the AEBS are involved in the catalytic activity of ChEH.…”

“…The AEBS has no affinity for estrogens, but binds selective ER modulators (SERMs); these ligands contain a hydrophobic core that mimics the steroid backbone of estrogens, grafted to a dialkylaminoalkyl side chain. Thus, in addition to Tam, SERMs, such as raloxifene, 4-OH-Tam, and RU-39411, are ligands of the AEBS (3)(4)(5). The AEBS selectively binds diphenylmethane derivatives of Tam, including (4-benzyl-phenoxy)-ethyl-N-pyrrolidine (PBPE) and tesmilifene (6) and it also binds σ-receptor ligands and inhibitors of cholesterol biosynthesis, such as triparanol and AY-9944 (5, 7).…”

“…AEBS expression is ubiquitous, and AEBS is highly expressed in proliferative cells, such as tumor cells, and in cholesterogenic tissues, such as the liver and brain, in accordance with its relationship with cholesterol metabolism (5,10,11). We recently established that AEBS ligands induce breast cancer cell differentiation and apoptosis through a mechanism involving the production of sterol autoxidation products (3,4,12). Cholestan-5α,6α-epoxy-3β-ol (α-CE), cholestan-5β,6β-epoxy-3β-ol (β-CE), and 7-ketocholesterol are among the major autoxidation products of cholesterol (13).…”

Differences in Sterol Composition of Gonads of the Lottiid Limpets Nipponacmea concinna and Nipponacmea fuscoviridis from Northeastern Japan

“…As compared to our previous results 7 , the sterol composition in gonads of N. concinna and N. fuscoviridis was Interestingly, the resumption of meiosis in an in vitro maturation and fertilization mouse model has been reported to be induced by zymosterol. Recently, it has been reported that zymostenol inhibits cell growth 18 , whereas desmosterol was shown to stimulate cell proliferation and survival 19 .…”