Selective uptake of estrogenic compounds by Saccharomyces cerevisiae: a mechanism for antiestrogen resistance in yeast expressing the mammalian estrogen receptor

Abstract

Estrogen antagonists such as ICI164,384 do not inhibit 17 beta-estradiol (E2)-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 > or = E2 > E1 > E3 = zeranol. Competitive binding with 3H-E2 by these compounds in cell-free extracts of CY525 results in a similar order of potency with a reverse order for E1 and E3. 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 E2-dependent gene activity. However, unlike mammalian cells, E2-induced gene activity in CY525 is not inhibited by ICI164,384. Intact CY525 cells incubated with 3H-17 beta estradiol were found to specifically bind the labeled ligand since excess unlabeled E2 effectively competed for binding. Unlabeled DES and E1 were also found to compete, however, excess unlabeled ICI164,384, E3 and the second generation antagonist ICI182,720 were unable to displace 3H-E2 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.