The structure of progesterone receptors

Novel concepts on pregnancy clocks and alarms: redundancy and synergy in human parturition

The signals and mechanisms that synchronize the timing of human parturition remain a mystery and a better understanding of these processes is essential to avert adverse pregnancy outcomes. Although our insights into human labor initiation have been informed by studies in animal models, the timing of parturition relative to fetal maturation varies among viviparous species, indicative of phylogenetically different clocks and alarms; but what is clear is that important common pathways must converge to control the birth process. For example, in all species, parturition involves the transition of the myometrium from a relaxed to a highly excitable state, where the muscle rhythmically and forcefully contracts, softening the cervical extracellular matrix to allow distensibility and dilatation and thus a shearing of the fetal membranes to facilitate their rupture. We review a number of theories promulgated to explain how a variety of different timing mechanisms, including fetal membrane cell senescence, circadian endocrine clocks, and inflammatory and mechanical factors, are coordinated as initiators and effectors of parturition. Many of these factors have been independently described with a focus on specific tissue compartments.In this review, we put forth the core hypothesis that fetal membrane (amnion and chorion) senescence is the initiator of a coordinated, redundant signal cascade leading to parturition. Whether modified by oxidative stress or other factors, this process constitutes a counting device, i.e. a clock, that measures maturation of the fetal organ systems and the production of hormones and other soluble mediators (including alarmins) and that promotes inflammation and orchestrates an immune cascade to propagate signals across different uterine compartments. This mechanism in turn sensitizes decidual responsiveness and eventually promotes functional progesterone withdrawal in the myometrium, leading to increased myometrial cell contraction and the triggering of parturition. Linkage of these processes allows convergence and integration of the gestational clocks and alarms, prompting a timely and safe birth. In summary, we provide a comprehensive synthesis of the mediators that contribute to the timing of human labor. Integrating these concepts will provide a better understanding of human parturition and ultimately improve pregnancy outcomes.

Distribution of progesterone receptor in female mouse tissues

Two novel antibodies against the mammalian progesterone receptor (PR) were raised and characterized to study the distribution of PR and the effect of estrogen on PR expression in various female murine tissues by immunohistochemistry. There were estrogen-independent constitutive PR expressions in the smooth muscle cells of uterus, uterine blood vessels, urinary bladder, duodenum, and jejunum of ovariectomized mice. Uterine stromal cells, capsular cells of kidney and adrenal gland, and the epithelial cells of submandibular gland expressed PR constitutively. PR expression was detected in some thymic cells and the number of PR-positive thymic cells increased markedly after estrogen treatment. Estrogen induced PR expression in the epithelial cells of uterus, vagina, urethra, and skin and the stromal cells of vagina, urethra, and pancreatic ducts, as well as the smooth muscle cells of some blood vessels. These results suggest cell-specific progesterone actions in the urinary tract, skin, and gastrointestinal organs, on the immune functions, and on the regulation of local blood flow.

An N-terminally truncated third progesterone receptor protein, PR(C), forms heterodimers with PR(B) but interferes in PR(B)-DNA binding

Multiple progesterone receptor (PR) isoforms may explain in part the complex and diverse biological actions of progestins. Recent studies demonstrate that the human 116 kDa B-receptor (PR[B]) and the 94 kDa A-receptor (PR[A]) can have very different transcriptional functions that are cell- and promoter-specific. Additionally, we have shown the existence of a smaller N-terminally truncated 60 kDa progestin-specific binding protein, called the C-receptor (PR[C]), that has unique transcriptional potentiating properties. In the presence of the other two PR isoforms, PR(C) enhances the transcriptional activities of the larger PR proteins. In order to determine the mechanism of action for the transcriptional promoting abilities of PR(C), the structural and functional properties of PR(C) were analysed and compared to those of PR(A) and PR(B). PR(C) consistently displayed a dissociation constant (Kd) approximately 5 times higher than that for PR(B) and PR(A), suggesting that the N-terminal truncation of PR(C) results in a conformation different from the two larger PR isoforms, that affects the hormone-binding region and its interaction with hormone. Despite this change, PR(C) is still capable of forming heterodimers with the larger PR(B) in solution, as determined by co-immunoprecipitation studies, but PR(C) interferes in tight PR(B) binding to DNA in gel-shift assays. Surprisingly, progestin and antiprogestin autoregulation of PR(C) protein levels parallel those for PR(B) and PR(A).

Steroid hormone binding in the Harderian gland of birds: characteristics of the androgen, estrogen, and progestin receptors of Anas platyrhynchos and Gallus domesticus

A series of experiments was carried out in immature female chicks and ducks to establish whether the avian Harderian gland contains specific receptors for sex steroids. Cytosol preparations of Harderian glands were submitted to hormone saturation analysis using radiolabeled estradiol, ORG-2058, and dimethylnortestosterone as ligands. In addition, the sedimentation characteristics of the hormone-receptor complexes were studied by ultracentrifugation of linear sucrose gradients. The presence of high affinity binding sites for estrogens (Kd = 2.4 and 1.6 nM), progestins (0.8 and 1.0 nM), and androgens (1.0 and 1.0 nM) was indicated in the chick and duck glands, respectively. The sedimentation coefficients were 7-7.5 S, 7-8 S, and 8 S for estrogen, progestin, and androgen receptor-ligand complexes, respectively. The concentration of the androgen receptor was significantly higher in chick than in duck Harderian glands whereas the estrogen and progestin receptor concentrations were similar in both species. A striking finding was the presence of progestin receptors, which apparently do not exist in the glands of many mammals. Priming with estrogens did not modify the concentration of ORG-2058 binding sites in either species studied, indicating that gland progestin receptor is not estrogen-regulated. Overall the data suggest intracellular mechanisms whereby circulating gonadal hormones regulate avian Harderian gland function.

Structural differences between the glucocorticoid, dioxin and oxysterol receptors from rat liver cytosol

The rat hepatic glucocorticoid, dioxin and oxysterol receptors were subjected to high performance liquid chromatography on size-exclusion and anion-exchange columns. Both the glucocorticoid receptor and the dioxin receptor had a Stokes radius Rs approximately 7.5 nm, expected value for heteromeric complexes containing a dimer of the Mr approximately 90,000 heat shock protein, hsp90 (Rs approximately 7.0 nm). The oxysterol receptor represented a much smaller entity (Rs approximately 6.0 nm). When analyzed on a Mono Q anion-exchange column, the molybdate-stabilized glucocorticoid receptor and dioxin receptor eluted as single peaks at approximately 0.30 M and 0.26-0.28 M NaCl, respectively, whereas the oxysterol receptor represented a less negatively charged species (0.11-0.14 M NaCl). Following washing of the Mono Q column with molybdate-free buffer, the activated monomeric glucocorticoid receptor was detected (0.10-0.12 M NaCl). In contrast, no modification in the elution pattern of the dioxin receptor and the oxysterol receptor was observed. These data demonstrate differences in the physico-chemical properties of the glucocorticoid, dioxin and oxysterol receptors, respectively, which might reflect structural differences.

Species crossreactivity of human progesterone receptor monoclonal antibodies: Western blot analysis

The crossreactivity of monoclonal antibodies (hPRa 1, 2, 3 and 6) generated against human progesterone receptor was examined in six mammalian and an avian species using the techniques of sodium-dodecylsulfate polyacrylamide gel electrophoresis and Western blot analysis. Immunoreactive bands were detected on protein blots of receptor-containing preparations from human endometrial carcinoma grown in nude mice, human T47D breast cancer cells, rabbit, cow and mouse uteri, and chick oviduct. No receptor-associated, immunoreactive bands were detected in rat, guinea pig or hamster uteri. The number and molecular weights of the receptor subunits detected varied between species, and only human progesterone receptor displayed electrophoretic microheterogeneity in its high molecular weight subunit. These data demonstrate that the human progesterone receptor antibodies recognize epitopes not common to all species.