Clinical Applications of Mifepristone (RU486) and Other Antiprogestins Assessing the Science and Recommending a Research Agenda (1993) / Chapter Skim
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B2. Overview and Background: Mechanism of Action of Antiprogestins
B2. Overview and Background: Mechanism of Action of Antiprogestins
Pages 120-138
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From page 120... ...
The first and most highly conserved is termed C1. It encodes the DNA binding domain and contains two zinc finger structures (Evans, 1988~.
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From page 121... ...
These receptor forms share common hormone binding and DNA binding domains. Either form can activate transcription of a target gene in cells co-transfected with the corresponding expression vector for PR-A or PR-B, as well as a suitable reporter plasmid (Bocquel et al., 1989~.
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In addition, this domain is important for transcriptional activation of steroid receptors. The DNA Binding Domain The DNA binding domain is a highly conserved region that is responsible for the DNA binding specificity of the receptor.
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There is evidence, in the case of the glucocorticoid receptor, that the heat shock protein complex is important for maintaining the integrity of the hormone binding site in the absence of ligand (Pratt et al., 1990) , but the progesterone receptor does not appear to require the heat shock proteins to maintain ligand binding activity.
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From page 124... ...
have shown that the human progesterone receptor undergoes a similar DNA-dependent phosphorylation in vitro, suggesting that this enzyme may be responsible for the DNA-dependent phosphorylation observed in T47D breast cancer cells. In addition to the phosphorylations that are common to both PR-A and PR-B, PR-B has at least two additional phosphorylation sites as judged by mobility on SDS gels and by peptide mapping (Sheridan et al., 1989; Beck et al., 1992~.
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From page 125... ...
Treatment with hormone results in dissociation of the receptor-heat shock protein complex, dimerization of the receptor, and binding to specific DNA response elements termed PREs, or progesterone response elements. Since the glucocorticoid receptor recognizes the same DNA sequence, these elements are also termed GREs or PRE/GREs.
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From page 127... ...
, which are representative of the classes of antagonists that promote DNA binding and prevent DNA binding, respectively. with progesterone itself and with progesterone agonists for binding to the progesterone receptor, and with dexamethasone and triamcinolone acetonide for binding to the glucocorticoid receptor (Philibert et al., 1981; Elger et al., 1986; Raynaud and Ojasoo, 1986; Kalimi, 1987~.
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From page 128... ...
Although there are many amino acid differences in the ligand binding domains of the progesterone receptors, a single amino acid substitution is sufficient to convert the chicken progesterone receptor from a protein that does not bind RU 486 to a protein that does bind it (Benhamou et al., 1992~. This exquisite sensitivity suggests that studies of antagonists for potential clinical applications will have to be done with human steroid receptors, since receptors from other species may respond somewhat differently.
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From page 129... ...
had suggested that RU 486 inhibited dissociation of the receptor-heat shock protein complexes and that the mechanism for antagonism might be due to failure to produce DNA binding complexes. Studies have shown that RU 486 does not block dissociation of the progesterone receptor-heat shock protein complex and that RU 486 does permit receptor dimerization and DNA binding (El-Ashry-Stowers et al., 1989~.
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From page 130... ...
Whether the ZK 98 299 prevents the dissociation of the receptor-heat shock protein complexes, blocks dimerization, or simply alters the conformation of the dimer to reduce the affinity for DNA has not been reported. Effects of Antagonists on Transcriptional Activation The effects of antagonists on transcriptional activation of progesterone receptors have been examined by using endogenous receptors in
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and TAF-2 (the hormone binding domain)
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In vitro transcription mediated by the human progesterone receptor requires the presence of a ligand that produces the DNA binding form of the receptor (Bagchi et
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Thus, this same antagonist may be acting through different mechanisms in inhibiting the progesterone and the glucocorticoid receptors. Treatment of T47D breast cancer cells with ZK 98 299 results in some enhancement of receptor phosphorylation, but the phosphorylation that alters the mobility on SDS gels is blocked (Takimoto et al., 1992~.
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The studies utilizing antagonists that promote DNA binding suggest that in some fashion the conformation of the receptor is inappropriate to interact productively with other transcription factors to initiate transcription. However, almost nothing is known about the detailed mechanism of transcriptional activation by steroid receptors.
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Effects of hormone and cellular modulators of protein phosphorylation on transcriptional activity, DNA binding, and phosphorylation of human progesterone receptors. Molecular Endocrinology 6:607-620, 1992.
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In situ photolinked nuclear progesterone receptors of human breast cancer cells: Subunit molecular weights after transformation and translocation. Endocrinology 113:2195-2201, 1983.
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Phosphotryptic peptide analysis of human progesterone receptor: New phosphorylated sites formed in nuclei after hormone treatment. Journal of Biological Chemistry 264:6520-6528, 1989.
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Chicken progesterone receptor is phosphorylated by a DNA-dependent protein kinase during in vitro transcription assays. Molecular Endocrinology 6:~14, 1992.
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