Molecular Mechanism of Action at Estrogen Receptor Alpha of a New Clinically Relevant Antiestrogen (GW7604) Related to Tamoxifen

Endocrinology. 2001 Feb;142(2):838-46. doi: 10.1210/endo.142.2.7932.

Abstract

Tamoxifen is the endocrine treatment of choice for all stages of estrogen receptor (ER)-positive breast cancer, and it is the first drug approved to reduce the incidence of breast cancer in high-risk women. Unfortunately, tamoxifen also possesses some estrogen-like effects in the uterus that cause a modest increase in the risk of endometrial cancer. GW5638 is a tamoxifen derivative with a novel carboxylic acid side chain with no uterotropic activity in the rat (Willson et al., J Med Chem, 1994, 37:1550-1552). We have compared and contrasted the actions of 4-hydroxytamoxifen (4-OHT, the active metabolite of tamoxifen) with GW7604 [the presumed metabolite of GW5638 in breast (MCF-7) and endometrial (ECC-1) cell lines in vitro]. GW7604 did not cause the growth of ECC-1 cells at any concentration (10(-11)-10(-6) M), but 4-OHT was weakly estrogen-like at low concentrations (10(-11)-10(-10) M). Compounds (10(-7) M) blocked the growth promoting action of estradiol (10(-10) M) in both ECC-1 and MCF-7 cells. Western blotting was used to show that GW7604 and raloxifene did not affect ER levels significantly, compared with controls, in MCF-7 cells; whereas the pure antiestrogen ICI182,780 decreased ER levels (P < 0.05). An assay system was used that can classify compounds into tamoxifen-like, raloxifene-like, or pure antiestrogens. The assay depends on the activation of the transforming growth factor alpha (TGFalpha) gene in situ by wild-type or D351Y mutant ER stably transfected into MDA-MB-231 cells (MacGregor-Schafer et al., Cancer Res, 1999, 59:4308-4313). GW7604 inhibited both estradiol (10(-9) M) and 4-OHT (10(-8), 10(-7) M) induction of TGFalpha in a concentration related manner (10(-9)-10(-6) M). GW7604 and raloxifene stimulated TGFalpha with the D351Y ER. In contrast, ICI 182,780 (10(-6) M) did not initiate TGFalpha and blocked the induction of TGFalpha with GW7604, raloxifene, and 4-OHT in D351Y-transfected cells. Using computer-assisted molecular models of ER complexes, we found that the antiestrogenic side chain of 4-OHT weakly interacted with the surface amino acid 351 (aspartate), but the carboxylic acid of GW7604 caused a strong repulsion of aspartate 351. We propose that GW7604 is less estrogen-like than 4-OHT, because it disrupts the surface charge around aa351 required for coactivator docking in the 4-OHT:ER complex. This charge is restored in the D351Y ER, thus converting GW7604 from an antiestrogen to an estrogen-like molecule.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Carcinoma / pathology
  • Cell Division / drug effects
  • Cinnamates / chemistry
  • Cinnamates / pharmacology*
  • Endometrial Neoplasms / pathology
  • Estrogen Receptor Modulators / pharmacology*
  • Estrogen Receptor alpha
  • Estrogens / pharmacology
  • Female
  • Gene Expression Regulation
  • Humans
  • Models, Molecular
  • RNA, Messenger / metabolism
  • Receptors, Estrogen / drug effects*
  • Receptors, Estrogen / metabolism
  • Selective Estrogen Receptor Modulators / pharmacology
  • Stilbenes / chemistry
  • Stilbenes / pharmacology*
  • Tamoxifen / analogs & derivatives*
  • Tamoxifen / pharmacology
  • Transforming Growth Factor alpha / genetics
  • Tumor Cells, Cultured

Substances

  • Cinnamates
  • Estrogen Receptor Modulators
  • Estrogen Receptor alpha
  • Estrogens
  • GW 7604
  • RNA, Messenger
  • Receptors, Estrogen
  • Selective Estrogen Receptor Modulators
  • Stilbenes
  • Transforming Growth Factor alpha
  • Tamoxifen
  • afimoxifene