SELECTIVE UPTAKE OF ESTROGENIC COMPOUNDS BY SACCHAROMYCES-CEREVISIAE - A MECHANISM FOR ANTIESTROGEN RESISTANCE IN YEAST EXPRESSING THE MAMMALIAN ESTROGEN-RECEPTOR

Estrogen antagonists such as ICI164,384 do not inhibit 17 beta-estradi ol (E(2))-dependent gene activity in yeast expressing the mammalian es trogen receptor although these compounds bind to receptors isolated fr om these cells. Various explanations have been offered for antiestroge n resistance in yeast systems including differences in cell-specific c omponents and lack of permeability of the yeast cell wall to these com pounds. We have used a strain of Saccharomyces cerevisiae transformed with the human estrogen receptor gene, and two estrogen response eleme nts linked to a lacZ reporter gene to study the pharmacology of estrog en agonists and antagonists. The rank order of potency of estrogen ago nists in this strain (CY525) is similar to that in estrogen-dependent mammalian cells: DES greater than or equal to E(2)>E(1)>E(3)=zeranol. Competitive binding with H-3-E(2) by these compounds in cell-free extr acts of CY525 results in a similar order of potency with a reverse ord er for E(1) and E(3). The pure estrogen antagonist ICI164,384 also bin ds to the receptor from cell-free extracts of CY525 with an IC50 of ap proximately 14nM. As in mammalian cells ICI164,384 does not induce E(2 )-dependent gene activity. However, unlike mammalian cells, E(2)-induc ed gene activity in CY525 is not inhibited by ICI164,384. Intact CY525 cells incubated with H-3-17 beta estradiol were found to specifically bind the labeled ligand since excess unlabeled E(2) effectively compe ted for binding. Unlabeled DES and E(1) were also found to compete, ho wever excess unlabeled ICI164,384, E(3) and the second generation anta gonist ICI182,720 were unable to displace H-3-E(2) binding in intact c ells. These results indicate that certain compounds enter the intact y east cell more readily than others and offer an explanation for antago nist resistance in these organisms.