No abstract

The estrogen receptor (ER) mixed agonists tamoxifen and raloxifene have been shown to protect against bone loss in ovariectomized rats. However, the mechanism by which these compounds manifest their activity in bone is unknown. We have used a series of in vitro screens to select for compounds that are mechanistically distinct from tamoxifen and raloxifene in an effort to define the properties of an ER modulator required for bone protection. Using this approach, we identified a novel high affinity ER antagonist, GW5638, which when assayed in vitro functions as an ER antagonist, inhibiting the agonist activity of estrogen, tamoxifen, and raloxifene and reversing the "inverse agonist" activity of the pure antiestrogen ICI182,780. Thus, GW5638 appears to function as an antagonist in these in vitro systems, although in a manner distinct from other known ER modulators. Predictably, therefore, GW5638 alone displays minimal uterotropic activity in ovariectomized rats, but will inhibit the agonist activity of estradiol in this environment. Unexpectedly, however, this compound functions as a full ER agonist in bone and the cardiovascular system. These data suggest that the mechanism by which ER operates in different cells is not identical, and that classical agonist activity is not required for the bone protective activity of ER modulators.

The promoter of the human C3 gene has been shown to be responsive to stimulation by both estrogen and tamoxifen-activated estrogen receptor (ER) in transcriptional assays reconstituted in mammalian cells. Using a series of deletions and point mutations, we have determined that the agonist activity of these two compounds was dependent upon the direct interaction of ER with each of three estrogen response elements (EREs) contained within this promoter. One of these sequences, ERE1 resembles the canonical vitellogenin A2-ERE whereas the other two, ERE2 and ERE3, do not display significant homology to known EREs. Using gene transfer studies it was shown that these sequences are necessary and sufficient for ER-mediated transcription. Interestingly, using in vitro receptor/DNA-binding assays we demonstrated that neither ERE1, ERE2, or ERE3 alone formed high-affinity complexes with purified ER; however when a promoter fragment containing all three sequences was used, specific, high-affinity ER-DNA interactions were observed. It was not surprising, therefore, that, when assayed individually on a heterologous promoter, these sequences function as weak EREs but together they act in a synergistic manner to create a strong ER-dependent enhancer. It has been suggested that tamoxifen mediates its partial agonist activity through AP-1 at target promoters. However, the fact that purified ER can bind directly to the estrogen-responsive sequences within the C3 promoter, and that tamoxifen activity on this promoter is unaffected by AP-1 coexpression, indicates that at least on some promoters tamoxifen can manifest partial agonist activity through a classical ER/ ERE- mediated mechanism.

Previously, we have shown that agonists and antagonists interact with distinct, though overlapping regions within the human progesterone receptor (hPR) resulting in the formation of structurally different complexes. Thus, a link was established between the structure of a ligand-receptor complex and biological activity. In this study, we have utilized a series of in vitro assays with which to study hPR pharmacology and have identified a third class of hPR ligands that induce a receptor conformation which is distinct from that induced by agonists or antagonists. Importantly, when assayed on PR-responsive target genes these compounds were shown to exhibit partial agonist activity; an activity that was influenced by cell context. Thus, as has been shown previously for estrogen receptor, the overall structure of the ligandreceptor complex is influenced by the nature of the ligand. It appears, therefore, that the observed differences in the activity of some PR and estrogen receptor ligands reflect the ability of the cellular transcription machinery to discriminate between the structurally different complexes that result following ligand interaction. These data support the increasingly favored hypothesis that different ligands can interact with different regions within the hormone binding domains of steroid hormone receptors resulting in different biologies.