Saccharomyces cerevisiae As a Model System to Study Steroid Hormone Receptors

Abstract

The human estrogen (ER) and progesterone receptor (PR) are members of the steroid receptor superfamily of ligand-dependent transcription factors (Evans, 1988). The cloning of the cDNAs for these receptors — ER in 1986 (Green et al, 1986) and hPR in 1987 (Misrahi et al, 1987) — and their subsequent expression in heterologous cell types permitted the reconstitution of hormone-responsive transcription systems that have been amenable to genetic analysis. The availability of these systems, coupled with existing biochemical techniques, has in the last ten years advanced our understanding of these transcription factors considerably. Currently, we believe that the steroid hormone receptor signal transduction pathway is comprised of multiple steps that can be genetically or pharmacologically separated The human estrogen (ER) and progesterone receptor (PR) are members of the steroid receptor superfamily of ligand-dependent transcription factors (Evans, 1988). The cloning of the cDNAs for these receptors — ER in 1986 (Green et al, 1986) and hPR in 1987 (Misrahi et al, 1987) — and their subsequent expression in heterologous cell types permitted the reconstitution of hormone-responsive transcription systems that have been amenable to genetic analysis. The availability of these systems, coupled with existing biochemical techniques, has in the last ten years advanced our understanding of these transcription factors considerably. Currently, we believe that the steroid hormone receptor signal transduction pathway is comprised of multiple steps that can be genetically or pharmacologically separated (McDonnell, 1995) A general outline of the proposed pathway is shown in Figure 1. In the absence of hormone, the sex steroid receptors (SRs) reside in a transcriptionally latent form in the nuclei of target cells (Beato et al, 1987). These inactive receptors have been shown biochemically to be sequestered in large macromolecular complex containing the receptor and a series of associated heat-shock proteins (Bagchi et al, 1991; Pratt, 1990; Smith and Toft, 1993). The role of these associated proteins in vivo is unknown. However, it has been postulated that they are involved in the folding of the receptor and in maintaining it in a transcriptionally inactive state within target cells (Smith and Toft, 1993). Upon interaction with their cognate hormone, the SRs undergo a conformational change (Allan et al, 1992; McDonnell et al, 1995). This event promotes the displacement of heat -shock proteins and other associated proteins, permitting dimerization and the subsequent association of the activated SRs with specific DNA response elements (SREs) located within the regulatory regions of target gene promoters (Beato et al, 1987; Kumar and Chambon, 1988). Coincident with this conformational change, and possibly as a consequence of it, the SRs undergo a series of phosphorylations (Bagchi et al, 1992; Denner et al, 1989; Takimoto et al, 1992). The resulting biological response to the ligand is determined by the cell and promoter context of the DNA-bound receptor (Tora et al, 1988; Tzukerman et al, 1994). The precise mechanism by which the DNA-bound receptors affect target gene transcription is unknown, although it has been shown that hormone-activated PR can stabilize the formation of the transcription pre-initiation complex when assayed in vitro (Klein-Hitpass et al, 1990)