Activation of the progesterone receptor of rabbit uterus

Steroid hormone receptors

In the three decades since the original discovery of receptors for steroid hormones, much has been learned about the biochemical processes by which these regulatory agents exert their effects in target tissues. The intracellular receptor proteins are potential transcription factors, needed for optimal gene expression in hormone-dependent cells. They are present in an inactive form until association with the hormone converts them to a functional state that can react with target genes. Transformation of the receptor protein to the nuclear binding form appears to involve the removal of both macromolecular and micromolecular factors that act to keep the receptor form reacting with DNA. Much of the native receptor is present in the nucleus, loosely bound and readily extractable, but for some and possibly all steroid hormones, some receptor is in the cytoplasm, perhaps in equilibrium with a nuclear pool. Methods have been developed for the stabilization, purification, and characterization of receptor proteins, and through cloning and sequencing of their cDNAs, primary structures for these receptors are now known. This has led to the recognition of structural similarities among the family of receptors for the different steroid hormones and to the identification of regions in the protein molecule responsible for the various aspects of their function. Monoclonal antibodies recognizing specific molecular domains are available for most receptors. Despite the knowledge that has been acquired, many important questions remain unsolved. How does association with the steroid remove factors keeping the receptor protein in its native state, and how does binding of the transformed receptor to the response element in the promoter region enhance gene transcription? Once it has converted the receptor to the nuclear binding state, is there a further role for the steroid in modulating transcription? Still not entirely clear is the involvement of phosphorylation and/or dephosphorylation in hormone binding, receptor transformation, and transcriptional activation. Less vital to basic understanding but important in the overall picture is whether the native receptors for gonadal hormones are entirely confined to the nucleus or whether there is an intracellular distribution equilibrium. With the effort now being devoted to this field, and with the application of new experimental techniques, especially those of molecular biology, our understanding of receptor function is progressing rapidly. The precise mechanism of steroid hormone action should soon be completely established.

Two progesterone receptors in the oviduct of the freshwater turtle Chrysemys picta: possible homology to mammalian and avian progesterone receptor systems

Two progesterone receptors in the oviduct of the freshwater turtle Chrysemys picta: possible homology to mammalian and avian receptor systems. Here we report the characterization of two specific progesterone receptors in nuclear extracts of the turtle oviduct. The receptors differ in dissociation constants (2.8 nM vs 27 nM) which can be separated on DEAE-Sepharose, the former eluting at 0.08 M KCl and the latter at 0.20 M KCl. [3H]R5020 photoaffinity labeling SDS-PAGE revealed that the 2.8 nM moiety migrates with an apparent molecular weight of 80 +/- 5 kDa and the 27 nM moiety migrates with an apparent molecular weight of 120 +/- 5 kDa. These receptors are termed PR-A and PR-B due to their molecular mass and elution profiles. DNA-cellulose chromatographic studies show that both bind DNA-cellulose with the PR-A eluting at 0.09 M NaCl and PR-B eluting between 0.20-0.21 M NaCl. In reproductively inactive turtles (from the months of January and February) estradiol is undetectable, and PR-B is absent as determined by Scatchard analysis, [3H]R5020 photoaffinity labeling electrophoretic studies and DEAE-Sepharose and DNA-cellulose chromatography. In these animals PR-B can be replenished by estrogen treatment, suggesting a physiological role for both PR-A and PR-B and dependence of PR-B on estradiol.

Glucocorticoid and progesterone receptors bind to the same sites in two hormonally regulated promoters

The glucocorticoid receptor of rat liver recognizes nucleotide sequences near the promoter of mouse mammary tumour virus (MMTV) required for hormonal induction in gene transfer experiments. Similar nucleotide sequences have been found in the human metallothionein gene IIA and in the chicken lysozyme gene, the later induced also by oestrogen, progesterone and androgens. In microinjection experiments, deletion of only 44 base pairs (bp) of the lysozyme promoter (from -208 to -164) results in coordinated loss of progesterone and glucocorticoid-dependent gene expression. We show here that purified glucocorticoid receptor from rat liver and progesterone receptor from rabbit uterus yield similar or overlapping exonuclease III footprints in the promoter regions of MMTV and chicken lysozyme. Thus, the regulatory elements for different steroid hormones may be similar or at least share structural features.

Human androgen insensitivity mutation does not alter oligonucleotide recognition by the androgen receptor-DHT complex

We studied the binding of dihydrotestosterone-receptor complexes (DHT-R) from human genital skin fibroblasts to oligodeoxyribonucleotides and DNA. Following incubation of fibroblasts with 2 nM [3H]DHT (45 min, 37 degrees C), DHT-R were prepared as total fibroblast sonicates (sonication of cells in 0.5 M KCl), intact fibroblast cytosol (100000 X g supernatant) or intact fibroblast nuclear extract (sonication of nuclei in 0.5 M KCl). DHT-R were also prepared by incubation of fractionated fibroblast cytosol with 4 nM [3H]DHT (4 h, 0 degrees C). Optimal conditions were established for binding of DHT-R from total fibroblast sonicates to oligo-dT cellulose: 60 min, 0 degrees C, low salt (0.05-0.10 M KCl), linearity with DHT-R concentration, and nucleotide saturation. With total fibroblast sonicates the rank order of DHT-R binding was oligo-dT approximately equal to -dG greater than DNA greater than -dC greater than or equal to -dA approximately equal to -dI. Intact fibroblast cytosol displayed a similar preference of DHT-R binding to oligo-dT and -dG but the binding was quantitatively higher than for total fibroblast sonicates, the binding for fractionated fibroblast cytosolic DHT-R formed at 0 degrees C being quantitatively lower. However, binding of DHT-R from cytosol (0 degrees C) to DNA-cellulose was equal to that for DHT-R from cytosol (37 degrees C). Binding of DHT-R from intact fibroblast nuclear extracts was lower than for total fibroblast sonicates. Preparations from cells of patients with receptor-negative, complete androgen insensitivity lacked both DHT-R formation and specific oligonucleotide binding. Binding of oligonucleotides to DHT-R from cells of patients with receptor-positive, complete androgen insensitivity could not be distinguished from that of normal cells. These results suggest: (a) androgen receptor-steroid complexes recognize and bind to certain preferred deoxyribonucleotides; (b) various factors affect the quantitative binding of DHT-R from different cellular preparations to deoxyribonucleotides; and (c) neither qualitative nor quantitative abnormalities for DHT-R of complete androgen-insensitive patients were detectable from oligonucleotide or DNA binding.

Hydrophobic interaction chromatography of the rabbit uterine progesterone receptor

The chromatographic behavior of the rabbit uterine progesterone receptor interaction on several different hydrophobic matrices was characterized. Receptor, prepared in 0.6 M NaCl, exhibited a progressive retardation of elution, followed by retention, on a series of alkyl agarose columns as the length of the alkyl chain [(CH2)nH-] increased (n = 0-10), reflecting the presence of hydrophobic regions on the protein. Adsorption did not occur directly at the steroid binding site of the molecule and did not require activation to the DNA-binding form. Elution could be achieved by a decrease in the ionic strength of the buffer or the addition of glycerol, resulting in partial purification of receptor. Receptor bound tightly to phenyl agarose, although elution of the receptor under mild conditions (decreasing salt gradient, increasing glycerol gradient) resulted in poor yield and only modest purification. Passage of the non-activated progesterone receptor over Reactive Blue Sepharose effectively removed albumin, presumably by a hydrophobic interaction, although receptor was not retained. In the activated form, approximately 25% of receptor was bound to Reactive Blue Sepharose, reflecting an interaction of the Cibacron Blue dye with the polynucleotide binding site of the receptor. Hydrophobic chromatography may be an important adjunct to methods for purification of the progesterone receptor.

Transformation of a rabbit progesterone receptor from an 8S form to 5.5S and 4S forms

The 8S form of the rabbit uterine progesterone receptor transforms slowly at 0 degree C to a 4S form with an intermediate 5.5S form. The transformation is accelerated by either heat, increased ionic strength and dilution of cytosol. The transformation, which is reversibly inhibited by sodium molybdate, is unrelated to total cytosolic alkaline phosphatase activity. The transformation is accompanied by a positive change in receptor surface charge and a decrease in the rate of progesterone dissociation. The stability of the 8S progesterone-receptor complex is reduced and the sedimentation coefficient increased by acidic conditions; acid does not affect the 4S receptor as drastically.

Sodium thiocyanate: a probe for the conformations of the androgen-receptor complex

In the presence of sodium thiocyanate (NaCNS), partially purified 5 alpha-dihydrotestosterone-receptor (DHT-R) complexes extracted from normal genital skin fibroblasts (GSF) dissociate with complex (biphasic) kinetics. The rate constant of the 'fast' component and the magnitude of the 'slow' component vary with temperature (29-37 degrees C) and NaCNS concentration (0.1-0.4 M). Equimolar sodium bromide is much less effective; potassium chloride up to 1 M has no effect. DHT-R complexes from the GSF of a subject with partial androgen insensitivity (PAI) yield biphasic dissociation profiles that differ from normal and are influenced by NaCNS. Together with the temperature-dependent, first-order (monophasic) dissociative behavior of normal DHT-R complexes in the absence of NaCNS (Kaufman et al., 1982), the foregoing data have been used to construct a kinetic model involving the dissociation of DHT from 3 conformationally related forms of the androgen-receptor complex: (1) dysactivated; (2) preactivated; (3) activated.

Extraction of nuclear androgen receptors from rat prostate with different reagents

After in vitro labelling of androgen receptors in prostate tissue from castrated rats, about 70% of the labelled androgens in nuclei could be extracted with buffer solutions with 0.4 M KCl, or 10 mM pyridoxal phosphate, or 0.4 mM Cibacron blue, or heparin (0.2 mg/ml). In the nuclear extracts, 60-80% of the steroid was recovered as steroid-receptor complex. Sedimentation values of the receptors, on sucrose gradients containing 0.4 M KCl, were 3-4 S for the KCl extract and 4-5 S for pyridoxal phosphate and heparin extracts. The Cibacron blue extract contained an aggregated form of the receptor (5-7 S). The presence of the protease inhibitor di-isopropylfluorophosphate during isolation caused a small increase in S value of the receptors. However, the differences in sedimentation values between KCl, pyridoxal phosphate and heparin extracts remained. The receptors in the KCl extracts could be precipitated with protamine sulphate only after addition of 10 mM pyridoxal phosphate to the extracts. On the basis of these results it is concluded that the androgen receptors in rat prostate can be effectively extracted from nuclei by certain reagents which have in common a strongly negatively charged group and a less polar or hydrophobic region. These reagents form complexes with nuclear receptors and influence the sedimentation values and precipitability with protamine sulphate of these receptors.

Photoaffinity labeling of steroid binding proteins with unmodified ligands

Photoactivation of the alpha,beta-unsaturated ketones of natural and synthetic steroid molecules by light of lambda greater than or equal to 330 nm allows their covalent attachment to steroid-binding proteins. The general validity of this method is demonstrated with two steroid hormone receptors and the steroid-binding protein uteroglobin. Progesterone can be covalently attached to the partially purified progesterone receptor and to uteroglobin, and comigrates with the binding proteins upon electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate. Similarly the synthetic glucocorticoid triamcinolone acetonide can be covalently bound to the partially purified glucocorticoid of rat liver. This method allows the identification of steroid hormone receptors after electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate. Labeling with radioactive steroids is specific since it can be prevented by the addition of an excess of non-radioactive ligand. Digestion of the labeled binding proteins with trypsin or chymotrypsin yields a defined pattern of radioactive peptides, demonstrating that covalent attachment takes place at specific binding sites.

Characterization and partial purification of androgen receptors from ram seminal vesicles

An androgen receptor has been demonstrated in the cytosol and in the nuclear fraction of ram seminal vesicles. The cytosol receptor was stabilized by sodium molybdate and 2 distinct [3H]methyltrienolone-binding proteins, one sedimenting at 9S and one sedimenting at 3S, could be demonstrated by sucrose-gradient centrifugation in the presence of 50 mM molybdate. The slower sedimenting form could be partially purified by ADP-sepharose chromatography. The purified receptor still sedimented at 3S after centrifugation on sucrose gradients containing either 0.6 M KCl or 50 mM molybdate. The receptor was destroyed by heating at 50 degrees C for 30 min and its complex with [3H]methyltrienolone dissociated slowly at low temperatures. The apparent equilibrium-dissociation constant (KD) for the purified receptor was: 3.8 x 10(-10) M. The relative affinities for different steroids decreased in the following sequence: 5 alpha-dihydrotestosterone greater than or equal to methyltrienolone greater than testosterone much greater than estradiol greater than R5020 greater than progesterone greater than diethylstilbestrol. The nuclear androgen receptor sedimented at 3S on sucrose gradients containing 0.6 M KCl. At pH 7.4 it behaved as an acidic protein with an electrophoretic mobility towards the anodic region of the agar gel. Because of the relatively large content of cytoplasmic and nuclear androgen receptors and the availability of large amounts of tissue the ram seminal vesicles could be a suitable source for large-scale purification of these receptors.

Transformation of the estrogen-receptor complex from chick oviduct in 2 steps

Salt treatment of the cytoplasmic estradiol-receptor complex from chick oviduct induces a strong affinity of the complex for DNA-cellulose and phenyl-sepharose. This process is called activation. Binding to heparin- and lysozyme-sepharose is also observed with the untreated complex. But, the salt treatment, additional binding of the complex to these adsorbents is seen. The increased ability of the complex to bind to polyanions and polycations is destroyed by mild trypsination. The binding to the hydrophobic adsorbent is not affected by this treatment. Neither a change of the sedimentation constant nor of the size of the receptor protein is observed after salt treatment in the cold. After binding of the salt-activated estradiol-receptor complex to DNA-cellulose in the cold, an increase of its sedimentation constant and its size, as measured by density-gradient centrifugation and agarose gel chromatography, resp., becomes apparent. A similar phenomenon is observed after binding to DEAE-cellulose and to some extent after binding to heparin-sepharose. The nuclear complex seems to have the same sedimentation constant as the cytoplasmic complex eluted from DNA-cellulose. The sedimentation constant of the nuclear complex is not changed after DNA-cellulose chromatography. The cytoplasmic progesterone-receptor complex from the same tissue, i.e. the oviduct, does not show any change of size. Thus the well-known process of transformation can now be separated into 2 steps. (1) Activation of the estradiol-receptor complex for its binding to various adsorbents in vitro and probably to its acceptor site(s) in vivo. (2) Increase of receptor size. This second step seems to be a special property of the estradiol-receptor complex. Its physiological significance is unclear.

The general validity of the subunit model of the progesterone receptor from chick oviduct appears questionable. Comparison of progesterone and estrogen receptor

Estrogen and progesterone receptors from chick oviduct were compared with respect to their chromatographic behaviour on DEAE-cellulose and their DNA-binding ability to test the general validity of the subunit model from O'Malley and coworkers. Both hormone-receptor complexes can be separated on DEAE-cellulose into 2 components A and B. The 2 progesterone-receptor components appear to occur in equimolar amounts, whereas in the case of the estrogen receptor the amount of component A is always significantly larger. After trypsin treatment the estrogen component B disappears. The remaining A is a receptor fragment with reduced molecular weight. This and other data indicate that the estrogen component B represents an aggregated form of the estrogen receptor and not a receptor subunit. The trypsinated progesterone-receptor fragments, however, are still separable into 2 components, even though also reduced in molecular weight. Our DNA-binding data of the progesterone-receptor components are almost consistent with earlier data from O'Malley and coworkers, even though we find some DNA-binding ability also for component B. Both estrogen-receptor components exhibit affinity for DNA and significantly more than 50% (up to 80%) of the total estrogen-receptor complex are able to bind to DNA. Furthermore we could show that the estrogen-receptor from chick oviduct can be transformed from a DNA-non-binding to a DNA-binding form, similar to other steroid-hormone receptors. This is not compatible with pre-existing receptor subunits in equimolar amounts, one with and the other without affinity for DNA.