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-estradiol (E(2))-dependent gene activity in yeast expressing the mammalian estrogen receptor although these compounds bind to receptors isolated from these cells. Various explanations have been offered for antiestrogen resistance in yeast systems including differences in cell-specific components and lack of permeability of the yeast cell wall to these compounds. We have used a strain of Saccharomyces cerevisiae transformed with the human estrogen receptor gene, and two estrogen response elements linked to a lacZ reporter gene to study the pharmacology of estrogen agonists and antagonists. The rank order of potency of estrogen agonists 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 extracts of CY525 results in a similar order of potency with a reverse order for E(1) and E(3). The pure estrogen antagonist ICI164,384 also binds to the receptor from cell-free extracts of CY525 with an IC50 of approximately 14nM. As in mammalian cells ICI164,384 does not induce E(2)-dependent gene activity. However, unlike mammalian cells, E(2)-induced 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 competed for binding. Unlabeled DES and E(1) were also found to compete, however excess unlabeled ICI164,384, E(3) and the second generation antagonist ICI182,720 were unable to displace H-3-E(2) binding in intact cells. These results indicate that certain compounds enter the intact yeast cell more readily than others and offer an explanation for antagonist resistance in these organisms.