Human progesterone A-receptors can be synthesized intracellularly and are biologically functional

Progesterone receptors in normal breast development and breast cancer

Progesterone receptors (PR) play a pivotal role in many female reproductive tissues such as the uterus, the ovary, and the mammary gland (MG). Moreover, PR play a key role in breast cancer growth and progression. This has led to the development and study of different progestins and antiprogestins, many of which are currently being tested in clinical trials for cancer treatment. Recent reviews have addressed the role of PR in MG development, carcinogenesis, and breast cancer growth. Thus, in this review, in addition to making an overview on PR action in normal and tumor breast, the focus has been put on highlighting the still unresolved topics on hormone treatment involving PR isoforms and breast cancer prognosis.

Myometrial Progesterone responsiveness and the Control of Human Parturition

OBJECTIVE

The goal of this review is to assess the body of literature addressing the mechanism of progesterone withdrawal in the control of human parturition and in particular the recent advances in testing the hypothesis that human parturition is initiated by decreased myometrial responsiveness to progesterone, ie, functional progesterone withdrawal.

METHODS

Published studies of progesterone responsiveness of the pregnant human myometrium in the context of parturition control were reviewed.

RESULTS

Advances in understanding the molecular basis for progesterone receptor (PR)-mediated control of progesterone responsiveness has led to the hypothesis that functional progesterone withdrawal in human parturition is mediated by specific changes in myometrial PR expression, function, or both. The human PR exists as two major subtypes, PR-A and PR-B. As PR-A represses progesterone actions mediated by PR-B, the extent of progesterone responsiveness is inversely related to the PR-A/PR-B expression ratio. In women, the onset of term labor is associated with a significant increase in the myometrial PR-A/PR-B expression ratio that may facilitate functional progesterone withdrawal. Interestingly, expression of the estrogen receptor-alpha (ERalpha) increases concordantly with the PR-A/PR-B expression ratio in nonlaboring myometrium. This finding indicates that functional estrogen activation and functional progesterone withdrawal are linked.

CONCLUSION

Functional progesterone withdrawal in human parturition is likely mediated by an increase in the myometrial PR-A/PR-B expression ratio and possibly by modulation of coactivator and corepressor proteins. Functional progesterone withdrawal appears to induce functional estrogen activation. Thus, for most of pregnancy, progesterone may decrease myometrial estrogen responsiveness by inhibiting ERalpha expression. Such an interaction would explain why the human myometrium is refractory to the high levels of circulating estrogens for most of pregnancy. At term, functional progesterone withdrawal removes the suppression of ERalpha expression leading to an increase in ERalpha and a concomitant increase in myometrial estrogen responsiveness. Estrogen can then act to transform the myometrium to a contractile phenotype. This model explains why disruption of progesterone action alone triggers the full parturition cascade. The link between functional progesterone withdrawal and functional estrogen activation may be a critical mechanism for the endocrine control of human parturition.

Sex steroids in uterine endometrial cancers

Some uterine endometrial cancers conserve estrogen dependency in advancement. However, the concept of advancement in tumor is complicated, because it involves simple growth in primary tumor and secondary spreading. The expression manner of estrogen receptor alpha exon 5 splicing variant, ER beta, progesterone receptor-A (N-terminus deletion mutant) is associated with metastatic potential in uterine endometrial cancers. Increased estrogen-related receptor alpha expression is related to tumor advancement with the loss of estrogen dependency. Steroid receptor coactivator-3 contributes to tumor progression and can be used as a treatment target for advanced uterine endometrial cancers. Estrogen responsive oncogenes, c-jun and c-Ha-ras, are not modi-fied by progestin in uterine endometrial cancer cells and are considered to be an instinct phenotype as such cancers. By contrast, metastatic potential of estrogen-dependent uterine endometrial cancers can be partially controlled by progestin via metastasis-related genes, E-cadherin/catenins, plasminogen activator inhibitor-1, vascular endothelial growth factor. Thus, sex steroids related phenomena are impress-ive in the advancement of uterine endometrial cancers.

Immunohistochemical labelling of steroid receptors in normal and malignant human endometrium

For several years it was generally believed that only a single estrogen receptor (ER) and progesterone receptor (PR) existed. However, the discovery of a new ER (ERbeta) with specificity for estrogens has induced new insights in the estrogen signalling system. Moreover, PR is expressed as two major isoforms, PR-A and PR-B that arise from alternative transcriptional starting sites within the same gene. Although PR-A and PR-B were thought to occur in similar amounts, it is now clear that they are differentially expressed and thus have distinct functions in several human tissues, including human endometrium. The ER and PR expression and distribution pattern might play an important role in normal endometrial function and pathogenesis and the expression and relationship of the two distinct ER's and PR's could be of essential clinical implications. Moreover, the imbalance in ERalpha/ERbeta expression and the PR-A/PR-B ratio might play an important role in endometrial transition and subsequently influence endometrial pathogenesis. The knowledge of the pattern of steroid receptors in human endometrial tissue is of extreme importance, since it might start a new field in hormone therapy of endometrial cancer.

Systematic expression analysis and antibody screening do not support the existence of naturally occurring progesterone receptor (PR)-C, PR-M, or other truncated PR isoforms

Functional progesterone withdrawal associated with human parturition has been ascribed to various mechanisms modulating the function of the classical progesterone receptors (PRs), B and A, in utero. These include up-regulation of the inhibitory PR-C isoform, described as a 60-kDa protein occurring from translation initiation at codon 595. Our initial attempts to detect PR-C yielded uninterpretable results. To systematically validate antibodies for immunodetection of PR isoforms, we generated expression vectors for PR variants originating from putative start codons AUG-289, -301, -595, -632, and -692 in addition to those for PR-B and PR-A, and for alternative splice variants PR-T, PR-S, and PR-M. All constructs were subjected to in vitro and in vivo translation and immunoblotting with a panel of 13 PR antibodies. Antibodies raised against full-length PR were generally not capable of detecting N-terminally truncated forms, whereas C-terminal antibodies did not or only weakly reacted with PR-B and PR-A but produced prominent nonspecific signals. Thus, immunodetection of N-terminally truncated PR isoforms is prone to artifacts. Proteins of about 64 kDa were expressed from PR-289 and -301, but no corresponding endogenous forms were observed. PR-T, PR-S, and PR-M cDNAs yielded no detectable translation products. No protein was translated from AUG-595 in our PR-C expression vector unless a Kozak sequence was introduced, and the product was not 60 but 38 kDa in size. Thus, the 60-kDa protein called PR-C does not originate from AUG-595 and is not a naturally occurring PR isoform.

Alternative splicing and the progesterone receptor in breast cancer

Progesterone receptor status is a marker for hormone responsiveness and disease prognosis in breast cancer. Progesterone receptor negative tumours have generally been shown to have a poorer prognosis than progesterone receptor positive tumours. The observed loss of progesterone receptor could be through a range of mechanisms, including the generation of alternatively spliced progesterone receptor variants that are not detectable by current screening methods. Many progesterone receptor mRNA variants have been described with deletions of various whole, multiple or partial exons that encode differing protein functional domains. These variants may alter the progestin responsiveness of a tissue and contribute to the abnormal growth associated with breast cancer. Absence of specific functional domains from these spliced variants may also make them undetectable or indistinguishable from full length progesterone receptor by conventional antibodies. A comprehensive investigation into the expression profile and activity of progesterone receptor spliced variants in breast cancer is required to advance our understanding of tumour hormone receptor status. This, in turn, may aid the development of new biomarkers of disease prognosis and improve adjuvant treatment decisions.

PIAS3 induction of PRB sumoylation represses PRB transactivation by destabilizing its retention in the nucleus

Progesterone receptor (PR) plays a critical role in cell proliferation and differentiation, and its transcriptional activity is known to be modulated by cofactor proteins. In the present study, we demonstrated that in the presence of progesterone, protein inhibitor of activated STAT-3 (PIAS3) significantly inhibited the PR transcriptional activity and the expression of progesterone-responsive genes. Reduction of endogenous PIAS3 by PIAS3 small-interfering RNA enhanced PR transactivation in a ligand-dependent manner. PIAS3 interacted with PR both in vitro and in vivo and the interaction was enhanced by progesterone. Furthermore, our findings suggested that PIAS3 strongly induced PRB sumoylation at three sites, Lys-7, Lys-388 and Lys-531. In addition, novel roles in PRB nuclear retention and transactivation were identified for these sites. Our data also suggested that PIAS3 was recruited in a largely hormone-dependent manner in response to a progesterone-responsive promoter. Finally, we demonstrated that PIAS3 inhibited the DNA-binding activity of PR and influenced its nuclear export as well as PR transactivation. Taken together, these data strongly suggested that PIAS3 played an important physiological role in PR function.

Progesterone receptor isoform identification and subcellular localization in endometrial cancer

OBJECTIVE

These studies were undertaken to characterize the subcellular localization of the two major isoforms of progesterone receptors (PR), PRA and PRB, in endometrial cancer.

METHODS

Immunohistochemistry, immunoprecipitation, and confocal microscopy were performed using Hec50co and KLE endometrial cancer cell models expressing PRA or PRB as a consequence of transduction. The location of PRB compared to PRA was determined, and antibodies were tested for specificity with respect to PR isoform recognition. Immunohistochemical analyses of PR expression and subcellular compartmentalization were also performed on 20 formalin-fixed endometrial cancer tumors.

RESULTS

Morphological and biochemical evaluations demonstrated that PRA is localized to the nucleus, even in the absence of progesterone. In contrast, a large proportion of PRB is cytoplasmic in the absence of ligand, but is rapidly translocated to the nucleus in the presence of progesterone. The differential distribution of PRA and PRB proved to be a hallmark of malignant and nonmalignant epithelia in 20 samples of archival endometrial tissue from women with the pre-operative diagnosis of endometrial cancer. All endometrial cancer specimens demonstrated cytoplasmic PRB in 50% or more of the cells, and five of the seven tumors that were moderately to poorly differentiated demonstrated no PRB staining in the nuclei. Nuclear PRB was significantly associated with increasing tumor differentiation (P = 0.031).

CONCLUSION

In the absence of ligand, PRA is nuclear and PRB is largely cytoplasmic. This suggests that PRA may exert ligand-independent nuclear effects, while PRB may have nongenomic cytoplasmic actions in endometrial cancer cells.

Molecular mechanism of estrogen receptor (ER)alpha-specific, estradiol-dependent expression of the progesterone receptor (PR) B-isoform

The physiological effects of progesterone are mediated by the progesterone receptor (PR) isoforms PRA and PRB, transcribed from a single gene, under control of two distinct promoters. Both the isoforms display different, promoter- and cell line-specific transactivation properties. Upregulation of both isoforms in response to estradiol stimulation has been described, although the two promoters contain no classical estrogen response element (ERE). Therefore, we decided to investigate the regulation of PRB-expression through distinct estrogen receptor (ER)-isoforms: ERalpha and ERbeta We demonstrate, that in HeLa cells treated with E2, PRB promoter activity was enhanced (five-fold) by ERalpha, but not by ERbeta. ERbeta was also unable to stimulate activity of the PRB promoter in BT20 and Ishikawa cells, where ERalpha induced reporter activity by two-fold. Deletion of the AF1-but not AF2 domain from ERalpha resulted in loss of the transactivation potential in all cell lines tested. Furthermore, in BT20 cells deletion of the AF2 domain of ERalpha resulted in stronger transcriptional activation than that mediated through wild-type ERalpha. In SK-BR-3 cells both ERs repressed PRB promoter activity and this repression was enhanced by co-transfection of SRC1. However, strong estrogen-dependent stimulation was observed after deletion of AF2. We conclude that PRB expression is stimulated by ERalpha but not ERbeta in an unique, AF1-dependent but AF2-independent mechanism.

The inhibitory function in human progesterone receptor N termini binds SUMO-1 protein to regulate autoinhibition and transrepression

Although most studies of progesterone receptors (PR) and their two isoforms, PR-A and PR-B, focus on transcriptional stimulation, the receptors exhibit important inhibitory properties. Autoinhibition refers to an inhibitory function located in the PR N terminus, whose deletion increases transcriptional activity at least 6-10-fold. Transrepression refers to the ability of PR-A to suppress the transcriptional activity of PR-B and other nuclear receptors, including estrogen receptors. Self-squelching refers to the observation in transient transfection assays that increasing receptor concentrations paradoxically decrease transcriptional activity. Using a series of N-terminal deletion mutants constructed in both PR isoforms, we have mapped their autoinhibitory and transrepressor activities to a small ubiquitin-like modifier (SUMO-1) protein consensus-binding motif, (387)IKEE, located in the N terminus upstream of AF1. Self-squelching does not involve this site. SUMO-1 binds PR covalently at (387)IKEE, but only if the C-terminal, liganded, hormone-binding domain is also present. A single point K388R mutation within the (387)IKEE motif in either PR-A or PR-B leads to a loss of autoinhibitory and transrepressor functions of the liganded, full-length receptors. We conclude that autoinhibition and transrepression involve N-terminal sumoylation combined with intramolecular N/C-terminal communication.

Progesterone receptor isoform expression in human meningiomas

The majority of meningiomas express the progesterone receptor (PR), and therefore meningiomas are considered to be progesterone-responsive. In addition, an association has been reported between PR and prognosis. At least two PR isoforms exist, PR-B (116--120 kDa) and PR-A (81 kDa), each of which are likely to have different biological functions. Knowledge of the differential expression of both isoforms is necessary to understand the effects of progesterone on meningioma growth. Therefore, in this study, PR-A and PR-B expression levels were determined in 61 human meningiomas by immunoblotting. Total PR expression levels were determined with a ligand binding assay (LBA) (total PR(LBA)). Both PR isoforms and an additional PR 78 kDa protein (PR-78) were expressed in the meningiomas. Meningiomas expressing more PR-A than PR-B had significantly higher total PR(LBA) levels (P<0.001). The PR-78 band intensity was negatively associated with that of PR-B (r(s)=-0.76, P<0.0001). PR-78 may represent an endogenous degradation product, but a similar regulation pathway in the biogenesis of both PR-B and PR-78 is not excluded. Meningiomas contain both PR isoforms, but in highly variable ratios and this variability may have some biological significance. Most meningiomas express more PR-A than PR-B. Therefore in meningioma, assuming that PR-B is more transcriptionally active than PR-A, progesterone responsiveness could be based on transrepression rather than on transactivation of target genes, and progesterone blockade may only be effective in certain subsets of meningiomas.

Steroid receptors and metastatic potential in endometrial cancers

The relative overexpression of estrogen receptor (ER)-alpha exon 5 splicing variant, the disrupted synchronization of ER-beta and ER-alpha expressions, and the suppression of progesterone receptor (PR) form A expression as a transcriptional repressor might be related to metastatic potential of uterine endometrial cancers, leading to poor patient prognosis related to estrogen refractoriness.

An N-terminal inhibitory function, IF, suppresses transcription by the A-isoform but not the B-isoform of human progesterone receptors

The B-isoform of human progesterone receptors (PR) contains three activation functions (AF3, AF1, and AF2), two of which (AF1 and AF2) are shared with the A-isoform. AF3 is in the B-upstream segment (BUS), the far N-terminal 164 amino acids of B-receptors; AF1 is in the 392-amino acid N-terminal region common to both receptors; and AF2 is in the C-terminal hormone binding domain. B-receptors are usually stronger transactivators than A-receptors due to transcriptional synergism between AF3 and one of the two downstream AFs. We now show that the N terminus of PR common to both isoforms contains an inhibitory function (IF) located in a 292-amino acid segment lying upstream of AF1. IF represses the activity of A-receptors but is not inhibitory in the context of B-receptors due to constraints imparted by BUS. As a result, IF inhibits AF1 or AF2 but not AF3, regardless of the position of IF relative to BUS. IF is functionally independent and strongly represses transcription when it is fused upstream of estrogen receptors. These data demonstrate the existence of a novel, transferable inhibitory function, mapping to the PR N terminus, which begins to assign specific roles to this large undefined region.

Clinical implication of expression of progesterone receptor form A and B mRNAs in secondary spreading of gynecologic cancers

This study was designed to determine the clinical implication of expression of progesterone receptor form A (PR-A) and B (PR-B) mRNAs in secondary spreading of gynecologic cancers. Approximately equal expression of PR-A and PR-B mRNAs was designated as type AB and dominant expression of PR-B mRNA as type B. Alteration from type AB to type B in the metastatic cancers occurred in 3/8 cases of uterine endometrial cancers, 2/8 cases of uterine cervical cancers, and 2/8 cases of ovarian cancers. Other cancers revealed type B regardless of primary or metastatic status. Thus, all metastatic cancers studied revealed type B. These results suggest that transcription of PR-A mRNA may be damaged, which might lead to uncontrolled overexpression of PR-B mRNA in metastatic lesion, and that the type B status could reveal a highly malignant phenotype in these three gynecologic cancers.

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).

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.

Steroid receptors in breast cancer

Recent advances in steroid receptor structure and function now indicate that oestrogen binds to the oestrogen receptor (ER) molecule at a specific site, denoted region E. This allows binding of the oestrogen-ER complex to DNA via cysteine residues in region C of the ER molecule, which tetrahedrally co-ordinate zinc. This modulates transcription and stimulates cell growth. A number of newly discovered growth factors are also regulated by ER, as is the progesterone receptor. Steroid receptor concentrations in tissues can now be measured on smaller tissue samples using enzyme immunoassay or on cells obtained by fine needle aspiration using monoclonal antibody technology. The prognostic value of steroid receptor is limited, but still constitutes the best marker for predicting response to endocrine therapy. The role of steroid receptors in selecting patients for adjuvant therapy is discussed.

Two distinct estrogen-regulated promoters generate transcripts encoding the two functionally different human progesterone receptor forms A and B

The human progesterone receptor (hPR) cDNA, synthesized from T47D breast cancer cells, and the hPR gene 5'-flanking region were cloned and sequenced. Comparison of the cDNA-deduced amino acid sequence with other PR homologues demonstrated the modular structure characteristic of nuclear receptors. As in the case of the chicken homologue, there are two hPR forms, A and B, which originate from translational initiation at AUG2 (codon 165) and AUG1, respectively. Northern blot analysis of T47D mRNA using various cDNA derived probes identified two classes of hPR mRNAs, one of which could code for hPR form B, while the other one lacked the 5' region upstream of AUG1. S1 nuclease mapping and primer extension analyses confirmed that the second class of hPR transcripts are initiated between +737 and +842 and thus encode hPR form A, but not form B. By using the hPR gene 5'-flanking sequences as promoter region in chimeric genes, we show that a functional promoter (located between -711 and +31) directs initiation of hPR mRNAs from the authentic start sites located at +1 and +15. Most importantly, initiation of transcription from chimeric genes demonstrated the existence of a second promoter located between +464 and +1105. Transient co-transfection experiments with vectors expressing the human estrogen receptor showed that both promoters were estrogen inducible, although no classical estrogen responsive element was detected in the corresponding sequences. When transiently expressed, the two hPR forms similarly activated transcription from reporter genes containing a single palindromic progestin responsive element (PRE), while form B was more efficient at activating the PRE of the mouse mammary tumor virus long terminal repeat. Transcription from the ovalbumin promoter, however, was induced by hPR form A, but not by form B.

Evolving trends in steroid hormone receptor research

Recent advances have challenged classical notions regarding the nature of steroid hormone receptors in the cell including localization, activation, configuration, and stability. Molecular biology has revealed a remarkable similarity in the primary structure of a wide variety of receptor classes that goes beyond steroid action. Post-translational modification of a primary unit, expressed in response to genetic conservatism, would appear to assure receptor dynamics specific to hormone-, organ-, and tissue-dependent processes, and may even lead to toxicity and oncogenicity.