Hormone-dependent processing of the avian progesterone receptor

Small-animal PET of steroid hormone receptors predicts tumor response to endocrine therapy using a preclinical model of breast cancer

Estrogen receptor-α (ERα) and progesterone receptor (PR) are expressed in most human breast cancers and are important predictive factors for directing therapy. Because of de novo and acquired resistance to endocrine therapy, there remains a need to identify which ERα-positive (ERα(+))/PR-positive (PR(+)) tumors are most likely to respond. The purpose of this study was to use estrogen- and progestin-based radiopharmaceuticals to image ERα and PR in mouse mammary tumors at baseline and after hormonal therapy and to determine whether changes in these imaging biomarkers can serve as an early predictive indicator of therapeutic response.

METHODS

Mammary adenocarcinomas that spontaneously develop in aged female mice deficient in signal transducer and activator of transcription-1 (STAT1) were used. Imaging of ERα and PR in primary tumor-bearing mice and mice implanted with mammary cell lines (SSM1, SSM2, and SSM3) derived from primary STAT1-deficient (STAT1(-/-)) tumors was performed. Hormonal treatments consisted of estradiol, an ER agonist; letrozole, an aromatase inhibitor; and fulvestrant, a pure ER antagonist. Small-animal PET/CT was performed using (18)F-fluoroestradiol ((18)F-FES) for ER, (18)F-fluoro furanyl norprogesterone ((18)F-FFNP) for PR, and (18)F-FDG for glucose uptake. Tracer uptake in the tumor was quantified and compared with receptor concentration determined by in vitro assays of resected tumors.

RESULTS

Primary STAT1(-/-) mammary tumors and implanted SSM2 and SSM3 tumors showed high (18)F-FES and (18)F-FFNP uptake and were confirmed to be ERα(+)/PR(+). Classic estrogen-induced regulation of the progesterone receptor gene was demonstrated by increased (18)F-FFNP uptake of estradiol-treated SSM3 tumors. Treatment with fulvestrant decreased (18)F-FFNP, (18)F-FES, and (18)F-FDG uptake and inhibited growth of SSM3 tumors but decreased only (18)F-FES uptake in SSM2 tumors, with no effect on growth, despite both tumors being ERα(+)/PR(+). Decreased (18)F-FFNP uptake by SSM3 tumors occurred early after initiation of treatment, before measurable tumor growth inhibition.

CONCLUSION

Using small-animal PET, a profile was identified that distinguished fulvestrant-sensitive from fulvestrant-resistant ERα(+)/PR(+) tumors before changes in tumor size. This work demonstrates that imaging baseline tumoral (18)F-FES uptake and initial changes in (18)F-FFNP uptake in a noninvasive manner is a potentially useful strategy to identify responders and nonresponders to endocrine therapy at an early stage.

Morphological relationships and leukocyte influence on steroid production in the epigonal organ-ovary complex of the skate, Leucoraja erinacea

In elasmobranchs, a unique association exists between an immune tissue, the epigonal organ (EO), and the gonads. In this study, the histological and vascular relationships of the EO and ovarian follicles of the little skate, Leucoraja erinacea, were assessed. Perfusions of Evans blue dye and Batson's monomer showed a shared vascular pathway from the gonadal artery into the epigonal-ovary complex, with blood first entering the EO and then perfusing the ovarian follicles. Histological studies demonstrated direct cellular contact between epigonal leukocytes and the follicle wall (FW), as well as the presence of leukocytes between the steroidogenic theca and granulosa cells. In vitro analyses demonstrated that epigonal cells co-cultured with FW cells cause a dose-dependent inhibition of estrogen (E2) and testosterone (T) production. In contrast, conditioned media from epigonal leukocytes, stimulated or unstimulated with lipopolysaccharide (10 microg/ml), increase the production of E2 and T from FW cells of the ovaries. These studies provide a basis for further investigations of leukocyte secreted factors and cell contact modulation of follicular steroid production.

Influence of reproductive activity, sex steroids, and seasonality on epigonal organ cellular proliferation in the skate (Leucoraja erinacea)

In elasmobranchs, the epigonal organ, a unique leukopoietic immune tissue, is associated with the gonads. As the ovaries increase in size during reproductive activity, the overall mass of the epigonal organ does not change. However, immunohistochemistry (proliferating cell nuclear antigen Ab) demonstrated more proliferative activity and extravasation of epigonal leukocytes from blood vessels in reproductively active (RA) skates (Leucoraja erinacea) than in non-reproductively active (NRA) skates. In addition, [(3)H]thymidine incorporation was greater in epigonal leukocytes from RA skates than in leukocytes from NRA skates. Plasma from RA skates, but not from NRA skates, increased proliferation of epigonal leukocytes in vitro, an effect that was not seen using steroid-free plasma. In contrast to the stimulatory effect of plasma on leukocyte proliferation, addition of steroids (estrogen, progesterone, testosterone, and dexamethasone) in vitro decreased [(3)H]thymidine incorporation. While the inhibitory response to steroids was seasonally variable, (3)[H]thymidine incorporation was always highest in RA animals, in which plasma steroid levels were also consistently highest. These studies suggest functional interactions between reproductive and immune tissues in the skate, and that cellular turnover in epigonal tissue may be influenced by gonadal activity.

Effects of reproductive activity and sex hormones on apoptosis in the epigonal organ of the skate (Leucoraja erinacea)

In elasmobranchs, a unique association exists between an immune tissue, the epigonal organ, and the gonads. The intimate morphological relationship between these tissues suggests functional interactions. In this study, we used apoptosis to assess differences between epigonal tissues of reproductively active (RA) and non-reproductively active (NRA) skates (Leucoraja erinacea). Plasma steroid levels were significantly higher in RA than in NRA animals, and TUNEL analysis showed that epigonal tissue of RA skates had greater DNA fragmentation than NRA skates. Addition of steroids to epigonal leukocytes in vitro demonstrated that progesterone, testosterone, and dexamethasone, but not estrogen, induced apoptosis of epigonal leukocytes as evidenced by DNA laddering and caspase-3 antibody labeling. This study supports recent evidence that cellular homeostasis of epigonal lymphomyeloid tissue may be influenced by gonadal activity and reproductive steroids in a representative of the most basal gnathastome group.

Synergistic role for pituitary growth hormone in the regulation of hepatic estrogen and progesterone receptors and vitellogenesis in female freshwater turtles, Chrysemys picta

Previous studies using the freshwater turtle Chrysemys picta have demonstrated that estradiol, progesterone (P), testosterone (T), and growth hormone (GH) regulate hepatic vitellogenin synthesis, suggesting a multihormonal regulation of vitellogenesis in the turtle. In this study we further investigated the interaction between estradiol-17beta (E) and growth hormone in the regulation of vitellogenin (vtg) in hypophysectomized post-reproductive female turtles (C. picta). Northern blot analysis was used to monitor the changes in vtg mRNA, ER mRNA, and PR mRNA expression; Western blot to determine changes in PR isoform expression and a homologous ELISA for measurement of plasma vtg. Compared to sham-operated controls, hypophysectomy did not reduce the hepatic levels of any parameters below the seasonal norm. Changes in these parameters in hypophysectomized animals after administration of GH alone, estrogen alone, or GH in combination with estrogen were well correlated. The effect of estrogen alone was greater than that of GH, and elevated all end-points analyzed. With the exception of plasma vitellogenin, the effect of GH plus estrogen was significantly greater than either hormone alone. In contrast to changes in ER mRNA, vtg mRNA, and vtg protein, the effect of estrogen and GH plus estrogen treatments on PRA mRNA and PRC mRNA, although significant, was relatively modest. However, changes in PRA and PRB protein were large (5- to 10-fold), and of similar magnitude to the changes in ER mRNA, vtg mRNA, and plasma vtg. Further, PRA and PRB protein levels appeared to be differentially affected. Thus, in sham and normal animals, only PRB was detected, and the levels were similar. After GH administration, PRB increased 4- to 5-fold, and PRA became detectable. Estrogen increased both isoforms of PR equally (approximately 6-fold), and some synergism was apparent when the two hormones were administered together, PR levels being the highest in this group (about an 8-fold increase). The results suggest that although estrogen is the primary regulatory factor involved in activation of vitellogenin synthesis, basal levels of all primary components of signaling pathways involved in vitellogenesis measured here may be maintained in the absence of either estrogen or GH, and that GH alone can activate transcription of some of these important transcription factors.

Effect of gonadal steroids on progesterone receptor, estrogen receptor, and vitellogenin expression in male turtles (Chrysemys picta)

Hepatic vitellogenin (vtg) is a yolk precursor protein sequestered in follicular oocytes as nutrient supply for developing embryos in nonmammalian vertebrates. In prior research studies we have demonstrated that both progesterone (P) and testosterone (T) inhibit estrogen (E)-induced vitellogenesis in the male fresh water turtle (Chrysemys picta), and have suggested that these hormones may be involved in multihormonal regulation of vitellogenesis in the female turtle. However, the modes of action of progesterone and testosterone on estrogen-induced vitellogenesis are not known. We have proposed that progesterone inhibits vitellogenesis by modulation of progesterone receptor A (PRA) or B (PRB) isoforms and/or estrogen receptor (ER) gene transcription. In this study, we compare the vitellogenic responses of reproductively inactive male turtles to estradiol 17beta in the presence of exogenous testosterone or progesterone. Northern blot analysis was used to monitor the changes in vtg mRNA, ER mRNA, and PR mRNA expression; Western blotting to determine changes in PR isoform expression and a homologous ELISA for measurement of plasma vtg. Progesterone and testosterone reduced estrogen-induced vtg mRNA expression, but plasma vtg was not significantly reduced by these steroids. PRA and PRB were transcribed even though ER mRNA could not be detected, suggesting constitutive PR expression. However, in the presence of estradiol 17beta, both PR isoforms and mRNA transcripts were increased as a correlate of ER mRNA transcription, suggesting both transcriptional and translational effects; these effects were inhibited by testosterone and progesterone treatments. Since ER mRNA was sharply reduced by both testosterone and progesterone, and estradiol 17 beta increased PR mRNA transcription and translation, it is likely that the action of progesterone in reducing vtg mRNA is indirect via down regulation of ER mRNA, thus ER. This study provides further information on the role of progesterone and testosterone in the regulation of hepatic vitellogenesis, suggesting regulation of vitellogenesis mainly via modulation of hepatic ER mRNA.

Progesterone receptor: some viewpoints on hypothalamic seasonal fluctuations in a lower vertebrate

Steroids secreted by the ovary, specifically estrogen and progesterone, influence the expression of behaviors associated with reproduction by interacting with a specific binding protein, or receptor, located in target cells in certain hypothalamic nuclei. The present paper reviews the progesterone receptor studies in the vertebrates brain, the progesterone receptor fluctuations throughout the reproductive cycle and suggests a role for progesterone receptors in the regulation of hypothalamic functions in amphibians. Furthermore, we report here a combined biochemical and immunohistochemical analysis of the hypothalamic progesterone receptor during the reproductive cycle of a lower vertebrate, the female amphibian anura Rana esculenta. 3H-Progesterone binding activity was found in both cytosol and nuclear extract samples. The progesterone binding moiety showed typical characteristics of a true receptor, such as high affinity, low capacity and specificity for progesterone. Further characterization was performed by using monoclonal antiserum raised against both the subunits A and B of the chicken progesterone receptor. Immunostained neurons were located mainly in two specific regions of the hypothalamus: the preoptic area and the infundibular hypothalamus. An immunoreactive band of about 67 kDa was observed using Western blotting, both in the cytosol and in the nuclear extract. Progesterone receptor levels fluctuated throughout the cycle along with plasma steroids and vitellogenin synthesis.

Heat shock protein 90 and the nuclear transport of progesterone receptor

Steroid receptors exist as large oligomeric complexes in hypotonic cell extracts. In the present work, we studied the nuclear transport of the 2 major components of the oligomeric complex, the receptor itself and the heat shock protein 90 (Hsp90), by using different in vitro transport systems: digitonin permeabilized cells and purified nuclei. We demonstrate that the stabilized oligomeric complex of progesterone receptor (PR) cannot be transported into the nucleus and that unliganded PR salt dissociated from Hsp90 is transported into the nucleus. When nonstabilized PR oligomer was introduced into the nuclear transport system, the complex dissociated and the PR but not the Hsp90 was transported into the nucleus. If PR exists as an oligomeric form after synthesis, as suggested by the experiments with reticulocyte lysate, the present results suggest that the complex is short-lived and is dissociated before or during nuclear transport. Thus, the role of Hsp90 in PR action is likely to reside in the Hsp90-assisted chaperoning process of PR preceding nuclear transport of the receptor.

Evidence of a progesterone receptor in the liver of the green frog Rana esculenta and its down-regulation by 17 beta estradiol and progesterone

Progesterone is a versatile hormone showing an ample variety of effects. One of the numerous functions attributed to progesterone is the modulation of vitellogenesis in oviparous vertebrates. As a prerequisite for the possible involvement of progesterone in vitellogenesis modulation, we investigated the presence of a progesterone receptor (PR) in the liver of the female green frog Rana esculenta. 3H-Progesterone (3H-P) binding activity was found in both cytosol and nuclear extract of the liver of Rana esculenta. The progesterone-binding moiety showed the typical characteristics of a true receptor, such as high affinity, low capacity, and specificity for progesterone. It also bound to DNA-cellulose and was eluted with a linear salt gradient at a concentration of 0.05 M of NaCl. The progesterone-binding moiety was down regulated by steroid hormones, in that ovariectomy resulted in a significant increase, in both cytosol and nuclear extract, of 3H-P binding activity with respect to intact females. On the contrary, 3H-P binding activity was almost undetectable after estradiol and/or progesterone treatment. The progesterone binding moiety of Rana esculenta was analyzed by Western blotting with the aid of a monoclonal antibody raised against the subunits A and B of the chicken PR. An immunoreactive band of about 67 kDa was observed in the liver of both intact and treated females. The 67 kDa band showed an increased intensity in ovariectomized animals, while it was faint following treatment with estradiol and/or progesterone. This is the first report on the presence of a progesterone receptor (PR) in the liver of an amphibian. PR of Rana esculenta is down regulated by estradiol and/or progesterone and shows peculiar immunological and biochemical characteristics, which make it rather different from the PR of other vertebrates.

Evidence for enhanced ubiquitin-mediated proteolysis of the chicken progesterone receptor by progesterone

Genomic actions of progesterone are mediated via A and B isoforms of the progesterone receptor (PR). One major factor controlling PR level is progesterone causing negative autoregulation (down-regulation) of the receptor protein. In this work we studied the mechanism whereby progesterone exerts its effects on PR level in the chicken oviduct. We found that progesterone does not markedly regulate PR mRNA expression. Furthermore, we demonstrate here for the first time that PR is a target for ubiquitylation and that the proportion of ubiquitylated PR is increased by progesterone treatment. Our data suggest that ligand-induced down-regulation of PR involves enhanced degradation of receptor protein by ubiquitin-proteasome system in vivo.

Progesterone receptor in the reproductive system of the female of Octopus vulgaris: characterization and immunolocalization

In this study for the first time we have characterized a progesterone receptor in the reproductive system of the female of Octopus vulgaris. Scatchard analysis revealed that one binding component with high affinity and low capacity for the ligand was present only in the nuclear extract. Competition experiments showed that the progesterone receptor was strictly specific for progesterone. DNA-cellulose binding and DEAE-Sephacel both confirmed the presence of one 3H-progesterone binding component which eluted at a salt concentration of 0.14 +/- 0.05 M NaCl and 0.15 +/- 0.05 M NaCl respectively. By using monoclonal antibodies against chicken progesterone receptor (subunits A and B), we have localized on Western Blot one band of about 70 kDa. Immunoreactivity for progesterone binding molecules has been localized in the nuclei of the follicle cells of the ovary, of the proximal portion of the oviduct and of the outer region of the nidimental gland. These data, taken together, provide evidence that in Octopus vulgaris the progesterone receptor has biochemical and immunohistochemical characteristics resembling those of progesterone receptor in vertebrates.

Characterization of progesterone-binding moieties in the little skate Raja erinacea

In this study we report evidence of a [3H]progesterone-binding moiety in the liver and oviduct of the little skate Raja erinacea. It is characterized by high affinity, low capacity and DNA-cellulose-binding activity. Furthermore Western blot analysis revealed that monoclonal antibodies against the chicken progesterone receptor (PR) subunits A and B cross-reacted with a 110-kDa band in the liver and a 80-kDa band in the oviduct. When analyzed by DEAE-Sepharose ion-exchange column chromatography, [3H]progesterone-binding molecules resolved into two peaks, one nonadherent and one adherent to the column. The liver adherent peak eluted in a linear gradient at a NaCl concentration of about 0.07 M and resolved on Western blot as a single band of a 110 kDa. The oviduct adherent peak eluted at about 0.14 M NaCl and resolved on Western blot as a single band of 80 kDa. Competition studies showed that the progesterone-binding moiety in the cytosol was specific for progesterone. On the contrary, the nuclear component is not specific for progesterone; it also binds testosterone and estradiol 17 beta in the oviduct, and progesterone, testosterone, dihydrotestosterone, estradiol 17 beta, mibolerone, and R5020 in the liver. The [3H]progesterone-binding activity was monitored in both liver and oviduct of females in different reproductive stages. Females were separated into three groups; laying, nonlaying, and immature. [3H]Progesterone-binding activity levels were higher in the liver of immature than of nonlaying skates, and it was undetectable in laying skates. [3H]Progesterone binding was higher in the oviduct of laying and nonlaying skates than of immature skates. This PR-binding moiety has many characteristics of a true receptor: high affinity, low capacity, binds to DNA, and cross-reacts with antibodies against chicken PR. However, while the cytosolic form of this progesterone-binding component was quite specific for P, nuclear extracted material was nonspecific. If these progesterone-binding components are homologous with the PR A and PR B forms of other vertebrates, as we believe, it is clear that there are species differences that probably relate to phylogenetic level and physiology of the organism.

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

Expression of the chicken progesterone receptor forms A and B is differentially regulated by estrogen in vivo

The chicken progesterone receptor (cPR), like its human counterpart (hPR), exists as two isoforms, PR-A and PR-B, displaying different biological activities depending upon cellular and promoter contexts. Here we show that the ratio of PR isoforms observed in the immature chicken oviduct is changed during estrogen-induced differentiation from PR-B dominancy to that of PR-A. This is the first report describing that the expression ratio of PR isoforms is altered by upregulation of PR-A by estrogen action in vivo. This result provides a plausible explanation to the differences in oviduct's response to progesterone depending on hormonal and developmental status of the animal.

Hormone-dependent changes in A and B forms of progesterone receptor

The influence of different estrogen and/or progesterone treatments on concentrations of A and B forms of progesterone receptor (PR-A and PR-B) in the different cell types of chick oviduct was studied. A semiquantitative immunohistochemical assay for cellular PR concentrations was developed using a computer-assisted image analysis system. The staining intensity of nuclear PR in the basal layer of epithelial cells, glandular, smooth muscle and mesothelial cells was analysed separately using two monoclonal antibodies, PR6 and PR22. The measured concentrations of PR varied between different cell types and from cell to cell. A significant decrease in PR concentration, as noted by a decrease in staining intensity, was observed in all cell types studied 2 or 6 h after a single injection of progesterone with or without simultaneous estrogen administration. The decrease was also verified with immunoblotting and an immunoenzymometric assay (IEMA) for chicken PR. After down-regulation the concentration of PR recovered to the control level within 48 h after progesterone or estrogen administration. Estrogen administration alone was observed to cause changes in the concentration of PR-A only, having little or no effect on PR-B concentration depending on the cell type studied. These findings indicate that estrogen and progesterone cause cell-specific changes not only to the total concentration of PR but also to the cellular ratio of PR-A and PR-B.

Mechanisms of action of sex steroid hormones: basic concepts and clinical correlations

The review deals with the clinically important aspects of the basic mechanisms of sex steroid hormones. Steroids can act through two basic mechanisms: genomic and non-genomic. The classical genomic action is mediated by specific intracellular receptors, whereas the primary target for the non-genomic one is the cell membrane. Many clinical symptoms seem to be mediated through the non-genomic route. Furthermore, membrane effects of steroid and other factors can interfere with the intranuclear receptor system inducing or repressing steroid-and receptor-specific genomic effects. These signalling pathways may lead to unexpected hormonal or anti-hormonal effects in patients treated with certain drugs. Steroid receptors (SRs) are members of a large family of nuclear transcription factors that regulate gene expression by binding to their cognate steroid ligands, to the specific enhancer sequences of DNA (steroid response elements) and to the basic transcription machinery. SRs are phosphoproteins, which are further phosphorylated after ligand binding. The role of phosphorylation in receptor transaction is complex and may not be uniform to all SRs. However, phosphorylation/dephosphorylation is believed to be a key event regulating the transcriptional activity of steroid receptors. SR activities can be affected by the amount of SR in the cell nuclei, which is modified by the rate of transcription and translation of the SR gene as well as by proteolysis of the SR protein. There is an auto- and heteroregulation of receptor levels. Some of the SRs appear to bind specific protease inhibitors and exhibit protease activity. The physiological significance of this weak proteolytic activity is not clear. Some SRs are expressed as two or more isoforms, which may have different effects on transcription. Receptor isoforms are different translation or transcription products of a single gene. Isoform A of the progesterone receptor is a truncated form of PR isoform B originating from the same gene, but it is able to suppress not only the gene enhancing activity of PR-B but also that of other steroid receptors. From the clinical point of view, it is important to note that the final hormonal effect in a target tissue is dependent on the cross talk between different nuclear steroid receptors and on expression of receptor isoforms.

Reptilian (Chrysemys picta) hepatic progesterone receptors: relationship to plasma steroids and the vitellogenic cycle

In non-mammals, estrogen-induced yolk precursors produced by the adult female liver are the main nutritional source for development. Evidence exists that progesterone exerts counter-regulatory effects on estrogen-induced vitellogenesis, and we have used the turtle model (Chrysemys picta) to study changes in hepatic progesterone receptor during the vitellogenic cycle. Using radioligand methods, we show that high and lower affinity binding sites are present in the cytosolic but not nuclear extracts. The lower affinity site is detectable at all times of the year; the high affinity site is mainly observed during non-vitellogenic periods and does not correlate with plasma estrogen. DNA-cellulose chromatography shows that PR-A is present in spring, summer, and winter, and that PR-B is down-regulated except in animals which recently laid eggs. Western blots confirm the presence of PR in all months, but PR-A (88 kDa) is the dominant isoform. PR-B (125 kDa) is well correlated with the luteal phase, winter and fall. Immunocytochemical studies show that PR is nuclear in location, and nuclear heat shock protein 90 (hsp90) is present. Competitive binding studies of both sites reveal that progesterone is the most effective ligand for both, followed by pregnenolone, deoxycorticosterone, and R5020. RU 486 does not bind to the high affinity site but binds moderately well to the lower affinity site. This study suggests that progesterone receptor isoforms are differentially expressed and may be involved as transcriptional regulators of hepatic function outside the periods of active vitellogenesis in the turtle.

Turtle oviduct progesterone receptor: radioligand and immunocytochemical studies of changes during the seasonal cycle

In order to determine the regulation of the oviduct progesterone receptor inChrysemys picta, radioligand binding studies were performed to determine changes in the high and lower affinity binding sites during the seasonal cycle. Lower affinity sites were present in both cytosolic and nuclear fractions during the cycle and peaked during the peri-ovulatory/early luteal periods. The high affinity sites, present exclusively in the nuclear fraction, increased following the preovulatory peak in plasma estradiol, remained elevated during the early luteal phase following the post-ovulatory peak in progesterone, and declined to non-detectable levels just before egg-laying. DNA-cellulose affinity chromatography showed that both high and low affinity binding sites were integral to both progesterone receptor B and A isoforms. Western blot analysis confirmed the binding studies and showed that PR-B (115 kDa) was present in greatest amounts during the peri-ovulatory and luteal periods, whereas PR-A (88 kDa) increased during those periods and was present following egg-laying. Immunocytochemical analysis revealed increased progesterone receptor immunostaining from the winter to the peri-ovulatory period in the three major zones (luminal epithelium, submucosal glands and the myometrium) following the preovulatory peak in estradiol, a decrease in all three zones, especially the myometrium, in the late luteal period following the post-ovulatory peak in progesterone, and an increase again during fall recrudescence. Competition studies demonstrated that progesterone was the most effective competitor followed by pregnenolone, R5020 and deoxycorticosterone. RU 486 does not bind to the high affinity site, but binds quite well to the lower affinity site. This study suggests that progesterone receptor isoforms in the turtle oviduct may be under the regulation of changing estrogen/progesterone ratios during the cycle.

Progesterone receptor does not form oligomeric (8S), non-DNA-binding complex in intact cell nuclei

We raised a polyclonal antibody, alpha D, against a synthetic peptide (amino acids 522-535) of chicken progesterone receptor (PR). The sequence is located between the DNA-binding domain and the hormone-binding domain in the region within the sequences required for stability of the oligomeric form of PR. In the immunoblot, alpha D reacted with both A and B forms of PR. In the sucrose gradient and dot-blot the antibody did not recognize the so-called 8S form of PR, which is an oligomeric complex of PR and other proteins. When the oligomeric complex was dissociated by salt treatment, the antibody recognized the resulting 4S form of PR. This would suggest that the epitope is masked in the 8S form of PR and exposed in the 4S form. To study whether a similar complex exists in vivo, we used the antibody for immunohistochemistry. Two different fixation techniques were employed, freeze-drying-vapor fixation and liquid fixation. In the animals not treated with progesterone, intensive nuclear staining was detected independent of the fixation technique. When receptor from similarly treated animals was analyzed by sucrose gradient, all of the receptor molecules were in the oligomeric complex (8S). Ligand binding is known to promote a dissociation of this complex. Thus progesterone treatment should lead to an increased immunodetection of the epitope; however, progesterone treatment decreased the intensity of PR immunostaining. These results suggest that the oligomeric complex (8S), present in tissue extracts, does not exist in intact cell nuclei. They also call into question the proposed role of hsp90 in regulating progesterone receptor function.

Effects of luteinizing hormone and follicle-stimulating hormone on the progesterone receptor induction in chicken granulosa cells in vivo

Progesterone receptor (PR) is increased in the granulosa cells prior to ovulation. In order to investigate the factors that induce PR in granulosa cells, we examined whether the PR is induced in granulosa cells by the injection of chickens with ovine luteinizing hormone (oLH) or porcine follicle-stimulating hormone (pFSH) by Western blot analysis. In the oLH-stimulated birds, PR of 79 kDa and 110 kDa were recognized in the granulosa cells of the largest follicle, whereas a PR of 110 kDa was observed in those of the smaller follicles (the second and third largest follicles). No immunoreaction product was observed in the granulosa cells of both the largest and the smaller follicles in pFSH-stimulated birds. In the control birds that were injected with saline, no immunoprecipitate was recognized in the granulosa cells of the largest or smaller follicles. These results suggest that LH may be involved in the induction of PR in the granulosa cells prior to ovulation.

H-7 reduces the nuclear binding of [3H]dexamethasone in rat liver slices but does not affect the phosphorylation of glucocorticoid receptor

The effect of 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), an inhibitor of protein kinase C, on the nuclear binding of [3H]dexamethasone and on the phosphorylation of glucocorticoid receptor was studied in rat liver slices to ascertain the role of protein kinase C in the expression of glucocorticoid action. H-7 reduces the nuclear binding of [3H]dexamethasone in rat liver slices. It does not affect the extent of phosphorylation of glucocorticoid receptor both in the absence or in the presence of glucocorticoid. These findings indicate that protein kinase C may be involved in the nuclear binding of glucocorticoid receptor but does not directly influence the receptor phosphorylation.

ZK98299, a novel antiprogesterone that does not interact with chicken oviduct progesterone receptor

Steroid antagonists, at receptor level, are valuable tools for elucidating the mechanism of steroid hormone action. We have examined and compared the interaction of avian and mammalian progesterone receptors with progestins; progesterone and R5020, and a newly synthesized antiprogesterone ZK98299. In the chicken oviduct cytosol, [3H]R5020 binding to macromolecule(s) could be eliminated with prior incubation of cytosol with excess radioinert steroids progesterone or R5020 but not ZK98299. Alternatively, [3H]ZK98299 binding in the chicken oviduct was not abolished in the presence of excess progesterone, R5020, or ZK98299. In the calf uterine cytosol, [3H]R5020 or [3H]ZK98299 binding was competeable with progesterone, R5020 and ZK98299 but not estradiol, DHT or cortisol. Furthermore, immunoprecipitation and protein A-Sepharose adsorption analysis revealed that in the calf uterine cytosol, the [3H]R5020-receptor complexes were recognized by anti-progesterone receptor monoclonal antibody PR6. This antibody, however, did not recognize [3H]ZK98299-receptor complexes. When phosphorylation of progesterone receptor was attempted in the chicken oviduct mince, presence of progesterone resulted in an increased phosphorylation of the known components A (79 kDa) and B (110 kDa) receptor proteins. Presence of ZK98299 neither enhanced the extent of phosphorylation of A and B proteins nor did it reverse the progesterone-dependent increase in the phosphorylation. The avian progesterone receptor, therefore, has unique steroid binding site(s) that exclude(s) interaction with ZK98299. The lack of immunorecognition of calf uterine [3H]ZK98299-receptor complexes, suggests that ZK98299 is either interacting with macromolecule(s) other than the progesterone receptor or with another site on the same protein. Alternatively, the antisteroid binds to the R5020 binding site but the complex adopts a conformation that is not recognized by the PRG antibodies.

Molecular cloning, sequence analyses, and expression of complementary DNA encoding murine progesterone receptor

Progesterone receptors exist in two molecular forms commonly designated as "A" and "B" forms, the relative proportion of which can vary among species. In murine tissues, progesterone receptor exists predominantly as the "A" form which, in mammary glands, is also under developmental regulation [Shyamala et al. (1990) Endocrinology 126, 2882-2889]. Therefore, toward resolving the molecular mechanisms responsible for the predominance of the "A" form of progesterone receptor in murine tissues and its developmental regulation, we have isolated, sequenced, and expressed the complementary DNA corresponding to the mouse progesterone receptor. Nucleotide sequence analysis revealed two in-frame ATG codons, such that the largest open reading frame beginning with the first codon could encode a polypeptide with an estimated molecular weight of 99,089, while the shorter open reading frame beginning with the second codon could produce a polypeptide with a calculated molecular weight of 81,829. The murine progesterone receptor had complete identity for the DNA binding domain of human and rabbit progesterone receptors and 99% homology with the chicken progesterone receptor; for the steroid binding domain, it had 96% homology with human and rabbit progesterone receptors and 86% homology with chicken progesterone receptors. Expression of the complete complementary DNA in Chinese hamster ovary cells yielded a protein which bound the synthetic progestin promegestone with an equilibrium dissociation constant of approximately 1 nM, and in Western blot analyses revealed both "A" and "B" forms of immunoreactive receptor.

Localization of progesterone receptors in the shell gland of laying and nonlaying chickens

The aim of the present study was to localize the progesterone receptor (PR) in the shell gland of the chicken and to determine if the localization and the amount of PR in the shell gland change with egg laying activity. White Leghorn hens laying regularly or out of lay for more than 2 wk were used. Localization of PR in the shell gland was done by immunocytochemistry. The amount of PR in the shell gland mucosal tissues was measured by Western blot analysis. Monoclonal antibodies to the chicken PR, PR6, and PR13 were used in both experiments. The PR were present in the nuclei of the surface epithelial cells, tubular gland cells, stromal fibroblasts, and smooth muscle cells in the arterial wall and myometrium of laying hens. The PR were localized in the same tissues of the shell gland of nonlaying hens as in laying hens. However, the density of PR-positive cells in nonlaying hens was greater, possibly because the tissues were atrophied. Western blot analysis indicated that the amount of PR was greater in nonlaying hens than in laying hens when equal amounts of proteins were used, whereas the molecular weights of PR of laying and nonlaying hens were identical. These results suggest that progesterone acting through its receptor may regulate the secretory activity of the surface epithelium and tubular gland cells for shell formation and the contractile activity of the myometrium. Moreover, once the cells of the shell gland differentiate and become responsive to progesterone, these cells maintain this responsiveness even during the nonlaying period.

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.

Co-operation of progestational steroids with epidermal growth factor in activation of gene expression in mammary tumor cells

The progesterone receptor belongs to a class of ligand binding transcription factors that regulate transcription by interacting with specific DNA sequences on hormone regulated genes. In human mammary tumor T47D cells that contain both progesterone and epidermal growth factor (EGF) receptors, the progestin-induced transactivation at various hormone regulated promoters is enhanced by EGF. The effect of EGF is rapid and does not require new protein synthesis. EGF treatment does not alter the DNA binding activity of the progesterone receptor nor does it affect the total ligand-dependent phosphorylation of this receptor. These results suggest that EGF enhances the transactivation property of the progesterone receptor through mechanisms other than those involving a direct interaction of this receptor with its cognate binding sites.

Androgen receptor heterogeneity and phosphorylation in human LNCaP cells

Androgen receptor heterogeneity and phosphorylation were studied in the human LNCaP cell line. Fluorography after photoaffinity labeling as well as immunoblotting with a specific polyclonal antibody revealed that the human androgen receptor migrated as a closely spaced 110 kD doublet on SDS-polyacrylamide gels. A time-dependent change in the ratio between the two isoforms was not observed after R1881 treatment of intact cells. In nuclear extracts of LNCaP cells that were incubated with [32P]orthophosphate in the presence of 10 nM R1881, a 110 kD phosphorylated protein was demonstrated after immunopurification using a monoclonal antibody against the human androgen receptor. Only a very small amount of this phosphoprotein was detected in the nuclear fraction from cells not treated with R1881. These results indicate that the human androgen receptor in LNCaP cells can be phosphorylated.

Immunological and physiological analysis of human breast cancer progesterone receptor heterogeneity, following KCl dissociation and size exclusion chromatography

Progesterone receptor (PR) levels were determined in 69 human breast cancer specimens by both radioligand assay (RLA) and enzyme immunoassay (EIA). These methods did not detect the same number of sites, and for each tumor there was a constant ratio between epitopes and PR-binding sites corresponding to 1/4, 2/4, ... 8/4. High performance size exclusion chromatography was performed to separate the various PR isoforms, and the ability of these isoforms to interact with the monoclonal antibodies was assessed. Determination of PR in the chromatographic fractions by EIA and RLA showed that the various isoforms isolated by chromatography presented variable quantities of steroid-binding sites and epitopes, thus confirming the differences observed in the cytosol assays. The dissociation of molybdate-stabilized PR by KCl and measurement by RLA and EIA of the isoforms obtained showed two different types of chromatographic patterns, particularly in the 8S polymeric form where the monoclonal antibodies appeared to detect mainly the heavier 8S fraction, which may correspond to the 8S-B form of the progesterone receptor. The monoclonal antibodies also detected an intermediate PR polymeric form (236 kDa) which was not always detected by the tritiated ligand. Our results suggest that breast cancer PR exhibit a marked molecular heterogeneity which may partially explain the differences in response to hormonal therapy, particularly for tumors with high receptor levels which nonetheless fail to respond to treatment.

Loss of proliferative potential during terminal differentiation coincides with the decreased abundance of a subset of heterogeneous ribonuclear proteins

The decrease in abundance of a subset of highly conserved basic nuclear proteins is established to correlate with the loss of proliferative potential in association with the process of terminal differentiation in murine mesenchymal stem cells and human keratinocytes. These proteins, designated P2Ps for proliferation potential proteins, have apparent molecular masses of 30-40 kD, are associated with the 30-40S substructures of nuclear hnRNP complexes, and are recognized by antibodies made against core proteins of hnRNP particles. They also share an epitope in common with heat shock protein-90 (hsp90) and are recognized by two mAbs against hsp90. Two-dimensional electrophoretic Western blots furthermore show that P2Ps make up a subset of hnRNP proteins. Cells that possess these proteins express the potential to proliferate whether or not they are traversing the cell cycle. These include rapidly growing cells, reversibly growth-arrested cells, and nonterminally differentiated cells. In contrast, cells that have irreversibly lost their proliferative potential, such as terminally differentiated cells, show a marked reduction in the abundance of P2Ps as determined by immunodetection on Western blots. A correlation, therefore, exists between the presence of this subset of nuclear proteins and the proliferative potential in two cell types. These results raise the possibility that as a subset of hnRNP proteins, P2Ps may mediate posttranscriptional control of the processing of specific RNAs required for cell proliferation.

Binding of heat shock proteins to the avian progesterone receptor

The protein composition of the avian progesterone receptor was analyzed by immune isolation of receptor complexes and gel electrophoresis of the isolated proteins. Nonactivated cytosol receptor was isolated in association with the 90-kilodalton (kDa) heat shock protein, hsp90, as has been described previously. A 70-kDa protein was also observed and was shown by Western immunoblotting to react with an antibody specific to the 70-kDa heat shock protein. Thus, two progesterone receptor-associated proteins are identical, or closely related, to heat shock proteins. When the two progesterone receptor species, A and B, were isolated separately in the absence of hormone, both were obtained in association with hsp90 and the 70-kDa protein. However, activated receptor isolated from oviduct nuclear extracts was associated with the 70-kDa protein, but not with hsp90. A hormone-dependent dissociation of hsp90 from the cytosolic form of the receptor complex was observed within the first hour of in vivo progesterone treatment, which could explain the lack of hsp90 in nuclear receptor complexes. In a cell-free system, hsp90 binding to receptor was stabilized by molybdate but disrupted by high salt. These treatments, however, did not alter the binding of the 70-kDa protein to receptor. Association of the 70-kDa protein with the receptor could be disrupted by the addition of ATP at elevated temperatures (23 degrees C). The receptor-associated 70-kDa protein is an ATP-binding protein, as demonstrated by its affinity labeling with azido[32P]ATP. These results indicate that the two receptor-associated proteins interact with the progesterone receptor by different mechanisms and that they are likely to affect the structure or function of the receptor in different ways.

Hormone specific phosphorylation and transformation of chicken oviduct progesterone receptor

Phosphorylation of chick progesterone receptor (PR) was attempted by incubating tissue minces from estrogen-primed oviducts with ortho [32P]phosphate in the absence and presence of different steroids. The phosphorylated PR was immunopurified from the cytosol using anti-PR monoclonal antibody alpha PR22 (Sullivan et al., 1986). Although all three known peptides of PR, peptides B (110K), A (79K) and the 90 kDa nonhormone binding peptide (heat shock protein, hsp-90), were phosphorylated, the presence of only progesterone increased the degree of phosphorylation of receptor peptides A and B and the dissociation of the hsp-90 from the PR heterooligomer. Other steroids, cortisol, estradiol and dihydrotestosterone (DHT) had no effect on the phosphorylation or on the dissociation of hsp-90 from the PR. Incubation of phosphorylated PR at 23 degrees C or at 4 degrees C with 0.3 M KCl or 10 mM ATP also caused dissociation of the hsp-90. Presence of progesterone in vitro increased dissociation of the hsp-90 and the subsequent PR binding to DNA-cellulose. Transformation in vivo or under cell free conditions did not alter the degree of phosphorylation of PR peptides A and B. Our results demonstrate that PR is phosphorylated in a hormone-specific manner and that its transformation by various agents leads to loss of the hsp-90 from the oligomeric structure without an apparent involvement of dephosphorylation.

Binding of heat shock proteins to the avian progesterone receptor

The protein composition of the avian progesterone receptor was analyzed by immune isolation of receptor complexes and gel electrophoresis of the isolated proteins. Nonactivated cytosol receptor was isolated in association with the 90-kilodalton (kDa) heat shock protein, hsp90, as has been described previously. A 70-kDa protein was also observed and was shown by Western immunoblotting to react with an antibody specific to the 70-kDa heat shock protein. Thus, two progesterone receptor-associated proteins are identical, or closely related, to heat shock proteins. When the two progesterone receptor species, A and B, were isolated separately in the absence of hormone, both were obtained in association with hsp90 and the 70-kDa protein. However, activated receptor isolated from oviduct nuclear extracts was associated with the 70-kDa protein, but not with hsp90. A hormone-dependent dissociation of hsp90 from the cytosolic form of the receptor complex was observed within the first hour of in vivo progesterone treatment, which could explain the lack of hsp90 in nuclear receptor complexes. In a cell-free system, hsp90 binding to receptor was stabilized by molybdate but disrupted by high salt. These treatments, however, did not alter the binding of the 70-kDa protein to receptor. Association of the 70-kDa protein with the receptor could be disrupted by the addition of ATP at elevated temperatures (23 degrees C). The receptor-associated 70-kDa protein is an ATP-binding protein, as demonstrated by its affinity labeling with azido[32P]ATP. These results indicate that the two receptor-associated proteins interact with the progesterone receptor by different mechanisms and that they are likely to affect the structure or function of the receptor in different ways.

Regulation of glucocorticoid receptor activity

Four levels of regulation of glucocorticoid receptor (GR) activity have been investigated. (1) Phosphorylation of the GR was studied in NIH 3T3 cells metabolically labeled with [32P]orthophosphate. A highly specific antiserum against the GR was used to immunoprecipitate 32P-labeled GR, and protein blotting was used to determine the GR concentration. Comparison of the relative specific activities of non-activated and activated receptor revealed a 3-4-fold increase in GR phosphorylation within 60 min upon hormone activation. (2) The affinity of the GR for its hormone response element (GRE) was quantitated in in vitro binding and gel shift experiments. The comparison of monomers, dimers and trimers of the GRE showed that GR binding affinity to multimers is much higher than the affinity for a GRE monomer. (3) The concentration of the GR was determined in quantitative protein blot assays as a function of time after hormone treatment of NIH 3T3 cells. A down-regulation of GR was observed. Only 30% of the maximal GR concentration observed in the absence of hormone remained after 24 h of hormone treatment. (4) The effect of the presence of hormone on the subcellular location of the GR was studied. Hormone treatment and withdrawal experiments indicated that the presence of hormone is not only required to initiate the cascade of events resulting in transcriptional trans-activation. GR translocated to the nucleus upon hormone addition returns rapidly to the cytoplasm upon hormone withdrawal. This indicated an active role for the hormone in the tight nuclear binding of GR.

Progesterone receptor concentration differences in the chick oviduct cells and apparent down-regulation by ligand. A semiquantitative immunohistochemical study

A semiquantitative immunohistochemical technique was developed for identification of chick progesterone receptor (PR). The mouse monoclonal antibody PR6 was used. The nuclear PR concentration was analyzed with Leitz Orthoplan MPV-3 light microscope. The target tissue was chick oviduct, with epithelial, glandular, mesenchymal, smooth muscle and peritoneal cells analyzed separately. PR concentration varied between different cell types and also from cell to cell within a single cell type. A significant decrease of PR concentration, as noted by decrease in staining, was also observed in all studied cell types, 6 h after a single injection of progesterone. This technique allows for histological identification of biochemical events that should help lead to the understanding of the role of PR changes in a variety of experimental situations.

Progesterone receptor is constitutively expressed in chicken intestinal mesothelium and smooth muscle

We have previously shown that progesterone receptor (PR) is expressed in the mesothelium of the chick oviduct and ovary and in the smooth muscle cells of the oviduct and the bursa of Fabricius. Here, we investigated the presence of PR in different parts of the peritoneum and abdominal organs using an immunohistochemical staining based on monoclonal antibodies against chicken PR. In 4-week-old sexually immature chicks, PR expression was located in the mesothelial cells of different parts of the peritoneum, in a thin layer of muscle cells of the ileum and throughout the muscle tissue of the colon and cloaca. In chicks of the same age treated with estrogen, PR was demonstrated similarly in the peritoneum and in the smooth muscle cells of the ileum, colon and cloaca. Using 25-week-old mature chickens, PR was also detected in identical tissues. Immunoblotting of the cloacal cytosol revealed the B form, but no A form of PR, both of which were found in the oviduct samples. Muscle cells of the duodenum and jejunum were not found to contain PR. Estrogen treatment was not needed to stimulate the production of PR in any of the tissues examined. We therefore conclude that the B form of PR is constitutively expressed in the mesothelial cells in different parts of the peritoneum and also in the smooth muscle cells of the ileum, colon and cloaca.

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.

Studies with antibodies against the conserved cysteine region of progesterone receptor

Polyclonal antibodies were generated against two synthetic peptides corresponding to sequences from the DNA-binding domain of steroid receptors. The sequence for peptide 1 (13 amino acids) lies between the two putative metal-binding loops of the conserved cysteine region while the sequence for peptide 2 (12 amino acids) lies within one loop. Peptide antibodies were generated by injecting rabbits with peptide conjugated to bovine serum albumin. By Western blot analysis, antibodies to peptide 2 recognized chick and human progesterone receptor and human glucocorticoid receptor, but peptide 1 antibodies did not. No cross-reactivity with native chick progesterone receptor was detected with either anti-peptide. These findings suggest that the epitopes for peptide 2 antibodies, and possibly for peptide 1 antibodies, are inaccessible to antibody in the native receptor.