Developmental regulation of murine mammary progesterone receptor gene expression

Progesterone signaling in uterine leiomyoma biology: Implications for potential targeted therapy

Uterine leiomyomas (ULs) are the most common benign smooth muscle cell steroid-dependent tumors that occur in women of reproductive age. Progesterone (P4) is a major hormone that promotes the ULs development and growth. P4 action in ULs is mediated mainly by its nuclear progesterone receptors (PGRs), although rapid non-genomic responses have also been observed. Data on the membrane progesterone receptors (mPRs) regulated signaling pathways in ULs in the available literature is still very limited. One of the essential characteristics of ULs is the excessive production of extracellular matrix (ECM). P4 has been shown to stimulate ECM production and collagen synthesis in ULs. Recent research demonstrated that, despite their benign nature, ULs may present with abnormal vasculature. P4 has been shown to regulate angiogenesis in ULs through the upregulation of vascular endothelial growth factor (VEGF) and by controlling the secretion of permeability factors. This review summarizes the key findings regarding the role of PGRs and mPRs in ULs, especially highlighting the potential ECM and angiogenesis modulation by P4. An increased understanding of this mechanistic role of nuclear and specifically mPRs in the biology of P4-modulated ECM and angiogenesis in the growth of ULs could turn out to be fundamental for developing effective targeted therapies for ULs.

Infant Formula Feeding Changes the Proliferative Status in Piglet Neonatal Mammary Glands Independently of Estrogen Signaling

BACKGROUND

Soy infant formula contains isoflavones, which are able to bind to and activate estrogen receptor (ER) pathways. The mammary gland is sensitive to estrogens, raising concern that the use of soy formulas may promote premature development.

OBJECTIVE

We aimed to determine if soy formula feeding increases mammary gland proliferation and differentiation in comparison to other infant postnatal diets.

METHODS

White-Dutch Landrace piglets aged 2 d received either sow milk (Sow), or were provided milk formula (Milk), soy formula (Soy), milk formula supplemented with 17-beta-estradiol (2 mg/(kg·d); M + E2), or milk formula supplemented with genistein (84 mg/L of diet; M + G) until day 21. Mammary gland proliferation and differentiation was assessed by histology, and real-time RT-PCR confirmation of differentially expressed genes identified by microarray analysis.

RESULTS

Mammary terminal end bud numbers were 19-31% greater in the Milk, Soy, and M + G groups relative to the Sow and M + E2, P <0.05. Microarray analysis identified differentially expressed genes between each formula-fed group relative to the Sow (±1.7-fold, P <0.05). Real-time RT-PCR confirmed 2- to 4-fold increases in mRNA transcripts of genes involved in cell proliferation, insulin-like growth factor 1 (IGF1), fibroblast growth factor 10 (FGF10), and fibroblast growth factor 18 (FGF18), in all groups relative to the Sow, P <0.05. In contrast, genes involved in cell differentiation and ductal morphogenesis, angiotensin II receptor type 2 (AGTR2), microtubule associated protein 1b (MAP1B), and kinesin family member 26b (KIF26B), were significantly upregulated by 2-, 4-, and 13-fold, respectively, in the M + E2 group. Additionally, mRNA expression of ER-specific gene targets, progesterone receptor (PGR), was increased by 12-fold, and amphiregulin (AREG) and Ras-like estrogen regulated growth inhibitor (RERG) expression by 1.5-fold in the M + E2 group, P <0.05. In the soy and M + G groups, mRNA expressions of fatty acid synthesis genes were increased 2- to 4-fold.

CONCLUSIONS

Our data indicate soy formula feeding does not promote ER-signaling in the piglet mammary gland. Infant formula feeding (milk- or soy-based) may initiate proliferative pathways independently of estrogenic signaling.

Estradiol Preferentially Induces Progestin Receptor-A (PR-A) Over PR-B in Cells Expressing Nuclear Receptor Coactivators in the Female Mouse Hypothalamus

Estrogens act in brain to profoundly influence neurogenesis, sexual differentiation, neuroprotection, cognition, energy homeostasis, and female reproductive behavior and physiology through a variety of mechanisms, including the induction of progestin receptors (PRs). PRs are expressed as two isoforms, PR-A and PR-B, that have distinct functions in physiology and behavior. Because these PR isoforms cannot be distinguished using cellular resolution techniques, the present study used isoform-specific null mutant mice that lack PR-A or PR-B for the first time to investigate whether 17β-estradiol benzoate (EB) regulates the differential expression of the PR isoforms in the ventromedial nucleus of the hypothalamus (VMN), arcuate nucleus, and medial preoptic area, brain regions that are rich in EB-induced PRs. Interestingly, EB induced more PR-A than PR-B in all three brain regions, suggesting that PR-A is the predominant isoform in these regions. Given that steroid receptor coactivator (SRC)-1 and SRC-2 are important in estrogen receptor (ER)-dependent transcription in brain, including PR induction, we tested whether the expression of these coactivators was correlated with PR isoform expression. The majority of EB-induced PR cells expressed both SRC-1 and SRC-2 in the three brain regions of all genotypes. Interestingly, the intensity of PR-A immunoreactivity correlated with SRC-2 expression in the VMN, providing a potential mechanism for selective ER-mediated transactivation of PR-A over PR-B in a brain region-specific manner. In summary, these novel findings indicate that estrogens differentially regulate PR-A and PR-B expression in the female hypothalamus, and provide a mechanism by which steroid action in brain can selectively modulate behavior and physiology.

Mammary gland development

The mammary gland develops through several distinct stages. The first transpires in the embryo as the ectoderm forms a mammary line that resolves into placodes. Regulated by epithelial–mesenchymal interactions, the placodes descend into the underlying mesenchyme and produce the rudimentary ductal structure of the gland present at birth. Subsequent stages of development—pubertal growth, pregnancy, lactation, and involution—occur postnatally under the regulation of hormones. Puberty initiates branching morphogenesis, which requires growth hormone (GH) and estrogen, as well as insulin-like growth factor 1 (IGF1), to create a ductal tree that fills the fat pad. Upon pregnancy, the combined actions of progesterone and prolactin generate alveoli, which secrete milk during lactation. Lack of demand for milk at weaning initiates the process of involution whereby the gland is remodeled back to its prepregnancy state. These processes require numerous signaling pathways that have distinct regulatory functions at different stages of gland development. Signaling pathways also regulate a specialized subpopulation of mammary stem cells that fuel the dramatic changes in the gland occurring with each pregnancy. Our knowledge of mammary gland development and mammary stem cell biology has significantly contributed to our understanding of breast cancer and has advanced the discovery of therapies to treat this disease.

Progesterone reverts LPS-reduced FAAH activity in murine peripheral blood mononuclear cells by a receptor-mediated fashion

Increased anandamide concentrations are associated with pregnancy failure. Anandamide levels are regulated by the fatty acid amide hydrolase (FAAH). The aim of the study was to investigate the role of progesterone (P) on FAAH modulation in murine peripheral blood mononuclear cells (PBMC) under septic conditions. We observed that in vivo administration of LPS to non-pregnant (NP) mice decreased FAAH activity of PBMC while in pregnant mice no changes in FAAH activity were observed. NP animals administered with P had a similar response to LPS as the pregnant animals. Also, NP mice injected with P antagonist and P showed that the effect of P on LPS-reduced FAAH activity was impaired. Furthermore, LPS produced a decrease in the ratio of PR-B/PR-A in NP animals. Our results showed that, in our model the endotoxin decreased PBMC's FAAH activity and this condition was reverted by P in a receptor-mediated fashion.

Mammary gland morphology and gene expression signature of weanling male and female rats following exposure to exogenous estradiol

In order to characterize the actions of xenoestrogens, it is essential to possess a solid portrait of the physiological effects of exogenous estradiol. We assessed effects of three doses of exogenous estradiol (E2) (0.1, 1.0 and 10 µg/kg/day) given between postnatal days 21 and 33 on the mammary gland morphology and gene expression profiles of male and female rats compared to vehicle-treated controls. The male mammary gland was more responsive to E2 treatment than in females, with 509 genes regulated >2-fold in a dose-dependent manner in males and only 174 in females. In males, E2 treatment significantly (P < 0.01) increased the number of terminal end buds (TEBs) and the expression of proliferating cell nuclear antigen (PCNA) protein (P < 0.05), both of which are indicators of proliferation. This change was linked to a significant increase (P < 0.05) in the expression of the gene encoding amphiregulin, which is known to induce TEB formation. There was also a dose-dependent increase (P < 0.001) in the estrogen-regulated gene encoding the progesterone receptor. In intact females, despite lack of changes in mammary morphology, we observed a dose-dependent increase (P < 0.05) in the expression of genes encoding three milk proteins: whey acidic protein, casein beta and casein kappa. There was a significant (P < 0.05) downregulation of both estrogen receptors in response to E2 treatment. These results suggest that mammary glands of male rats are very sensitive to exogenous E2 during development post-weaning. The dose-dependent increase observed in amphiregulin and progesterone receptor gene expression was linked to morphological changes and represents a reliable and sensitive tool to evaluate estrogenicity. In contrast, intact weanling female rats were less responsive.

Epimorphin is a novel regulator of the progesterone receptor isoform-a

Epimorphin/syntaxin-2 is a membrane-tethered protein localized extracellularly (Epim) and intracellularly (Stx-2). The extracellular form Epim stimulates morphogenic processes in a range of tissues, including in murine mammary glands where its overexpression in luminal epithelial cells is sufficient to drive hyperplasia and neoplasia. We analyzed WAP-Epim transgenic mice to gain insight into how Epim promotes malignancy. Ectopic overexpression of Epim during postnatal mammary gland development led to early side-branching onset, precocious bud formation, and increased proliferation of mammary epithelial cells. Conversely, peptide-based inhibition of Epim function reduced side branching. Because increased side branching and hyperplasia occurs similarly in mice upon overexpression of the progesterone receptor isoform-a (Pgr-a), we investigated whether Epim exhibits these phenotypes through Pgr modulation. Epim overexpression indeed led to a steep upregulation of both total Pgr mRNA and Pgr-a protein levels. Notably, the Pgr antagonist RU486 abrogated Epim-induced ductal side branching, mammary epithelial cell proliferation, and bud formation. Evaluation of Epim signaling in a three-dimensional ex vivo culture system showed that its action was dependent on binding to its extracellular receptor, integrin-αV, and on matrix metalloproteinase 3 activity downstream of Pgr-a. These findings elucidate a hitherto unknown transcriptional regulator of Pgr-a, and shed light on how overexpression of Epim leads to malignancy.

Progestogens used in postmenopausal hormone therapy: differences in their pharmacological properties, intracellular actions, and clinical effects

The safety of progestogens as a class has come under increased scrutiny after the publication of data from the Women's Health Initiative trial, particularly with respect to breast cancer and cardiovascular disease risk, despite the fact that only one progestogen, medroxyprogesterone acetate, was used in this study. Inconsistency in nomenclature has also caused confusion between synthetic progestogens, defined here by the term progestin, and natural progesterone. Although all progestogens by definition have progestational activity, they also have a divergent range of other properties that can translate to very different clinical effects. Endometrial protection is the primary reason for prescribing a progestogen concomitantly with postmenopausal estrogen therapy in women with a uterus, but several progestogens are known to have a range of other potentially beneficial effects, for example on the nervous and cardiovascular systems. Because women remain suspicious of the progestogen component of postmenopausal hormone therapy in the light of the Women's Health Initiative trial, practitioners should not ignore the potential benefits to their patients of some progestogens by considering them to be a single pharmacological class. There is a lack of understanding of the differences between progestins and progesterone and between individual progestins differing in their effects on the cardiovascular and nervous systems, the breast, and bone. This review elucidates the differences between the substantial number of individual progestogens employed in postmenopausal hormone therapy, including both progestins and progesterone. We conclude that these differences in chemical structure, metabolism, pharmacokinetics, affinity, potency, and efficacy via steroid receptors, intracellular action, and biological and clinical effects confirm the absence of a class effect of progestogens.

Molecular mechanisms of steroid receptor-mediated actions by synthetic progestins used in HRT and contraception

Synthetic progestins are used by millions of women as contraceptives and in hormone replacement therapy (HRT), although their molecular mechanisms of action are not well understood. The importance of investigating these mechanisms, as compared to those of progesterone, has been highlighted by clinical evidence showing that medroxyprogesterone acetate (MPA), a first generation progestin, increases the risk of breast cancer and coronary heart disease in HRT users. A diverse range of later generation progestins with varying structures and pharmacological properties is available for therapeutic use and it is becoming clear that different progestins elicit beneficial and adverse effects to different extents. These differences in biological activity are likely to be due to many factors including variations in dose, metabolism, pharmacokinetics, bioavailability, and regulation of, and/or binding, to serum-binding proteins and steroidogenic enzymes. Since the intracellular effects on gene expression and cell signaling of steroids are mediated via intracellular steroid receptors, differential actions via the progesterone and other steroid receptors and their isoforms, are likely to be the major cause of differential intracellular actions of progestins. Since many progestins bind not only to the progesterone receptor, but also to the glucocorticoid, androgen, mineralocorticoid, and possibly the estrogen receptors, it is plausible that synthetic progestins exert therapeutic actions as well as side-effects via some of these receptors. Here we review the molecular mechanisms of intracellular actions of old (MPA, norethisterone, levonorgestrel, gestodene) vs. new (drospirenone, dienogest, trimegestone) generation progestins, via steroid receptors.

16α-hydroxyprogesterone: origin, biosynthesis and receptor interaction

The metabolism of progesterone (PROG) by cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1) results in the formation of both 17α-hydroxyprogesterone (17-OHPROG) and 16α-hydroxyprogesterone (16-OHPROG) in humans. Unlike 17-OHPROG, 16-OHPROG is not metabolised further in steroidogenic tissue. While this metabolite can be readily detected in serum and urine, its physiological role remains unclear. This paper reviews the production of 16-OHPROG by human CYP17A1 by providing insight into the catalysis of PROG by CYP17A1 and highlights the role of Ala105 in the 16α-hydroxylation reaction. As 16-OHPROG has been putatively linked to reproductive function, we investigated the interaction of this steroid metabolite with both isoforms of the human progesterone receptor (hPR). We show for the first time that 16-OHPROG can bind to both hPR-A and hPR-B and act as an agonist for both receptors.

Progesterone action in human tissues: regulation by progesterone receptor (PR) isoform expression, nuclear positioning and coregulator expression

Progesterone is a critical regulator of normal female reproductive function, with diverse tissue-specific effects in the human. The effects of progesterone are mediated by its nuclear receptor (PR) that is expressed as two isoforms, PRA and PRB, which are virtually identical except that PRA lacks 164 amino acids that are present at the N-terminus of PRB. Considerable in vitro evidence suggests that the two PRs are functionally distinct and in animals, tissue-specific distribution patterns of PRA and PRB may account for some of the diversity of progesterone effects. In the human, PRA and PRB are equivalently expressed in most target cells, suggesting that alternative mechanisms control the diversity of progesterone actions. PR mediates the effects of progesterone by association with a range of coregulatory proteins and binding to specific target sequences in progesterone-regulated gene promoters. Ligand activation of PR results in redistribution into discrete subnuclear foci that are detectable by immunofluorescence, probably representing aggregates of multiple transcriptionally active PR-coregulator complexes. PR foci are aberrant in cancers, suggesting that the coregulator composition and number of complexes is altered. A large family of coregulators is now described and the range of proteins known to bind PR exceeds the complement required for transcriptional activation, suggesting that in the human, tissue-specific coregulator expression may modulate progesterone response. In this review, we examine the role of nuclear localization of PR, coregulator association and tissue-specific expression in modulating progesterone action in the human.

Strain-specific differences in the mechanisms of progesterone regulation of murine mammary gland development

Progesterone (P) is required for normal mammary gland development, and is implicated in the etiology of mammary cancer in rodents and humans. We analyzed mammary gland developmental responses to P and estrogen (E) in two strains of mice (BALB/c and C57BL/6) that exhibit differences in ductal development at sexual maturity and alveologenesis during pregnancy. C57BL/6 mice exhibited reduced proliferative and morphological responses to P. Analysis of known mediators of sidebranching and alveologenesis revealed that reduced P-induced expression of P receptor isoform B and receptor activator of nuclear factor-kappaB ligand (RANKL), as well as altered expression and regulation of cyclin D1, CCAAT/enhancer binding protein beta, and the downstream effectors of RANKL, nuclear Id2 and p21, contribute significantly to the reduced P responsiveness of the C57BL/6 mammary gland. In contrast, E responsiveness was greater in C57BL/6 than in BALB/c glands. E may play a compensatory role in C57BL/6 alveologenesis through its effect on the induction and activation of signal transducer and activator of transcription 5a, a known regulator of RANKL. These observations suggest that in human populations with heterogeneous genetic backgrounds, individuals may respond differentially to the same hormone. Thus, genetic diversity may have a role in determining the effects of P in normal mammary development and tumorigenesis.

Loss of caveolin-3 induces a lactogenic microenvironment that is protective against mammary tumor formation

Here, we show that functional loss of a single gene is sufficient to confer constitutive milk protein production and protection against mammary tumor formation. Caveolin-3 (Cav-3), a muscle-specific caveolin-related gene, is highly expressed in muscle cells. We demonstrate that Cav-3 is also expressed in myoepithelial cells within the mammary gland. To determine whether genetic ablation of Cav-3 expression affects adult mammary gland development, we studied the phenotype(s) of Cav-3(-/-)-null mice. Interestingly, Cav-3(-/-) virgin mammary glands developed lobulo-alveolar hyperplasia, akin to the changes normally observed during pregnancy and lactation. Genome-wide expression profiling revealed up-regulation of gene transcripts associated with pregnancy/lactation, mammary stem cells, and human breast cancers, consistent with a constitutive lactogenic phenotype. Expression levels of three key transcriptional regulators of lactation, namely Elf5, Stat5a, and c-Myc, were also significantly elevated. Experiments with pregnant mice directly showed that Cav-3(-/-) mice underwent precocious lactation. Finally, using orthotopic tumor cell implantation, we demonstrated that virgin Cav-3(-/-) mice were dramatically protected against mammary tumor formation. Thus, Cav-3(-/-) mice are a novel preclinical model to study the protective effects of a lactogenic microenvironment on mammary tumor onset and progression. Our current studies have broad implications for using the lactogenic microenvironment as a paradigm to discover new therapies for the prevention and/or treatment of human breast cancers.

Regulation of gene expression in the bovine mammary gland by ovarian steroids

It is well established that estrogen is required for mammary epithelial cell proliferation and ductal development in the growing animal, and that lobuloalveolar development during gestation is dependent on progesterone. The effects of these steroid hormones on gene expression in the mammary gland are mediated primarily by their respective nuclear hormone receptors, which function as hormone-bound transcription factors. To gain insight into how estrogen and progesterone regulate mammary gland growth and function in cattle, we and others have characterized the expression patterns of their cognate nuclear hormone receptors in the bovine mammary gland throughout development, pregnancy, and lactation. This work has identified a lack of expression of estrogen receptor beta and a greater abundance of progesterone receptor during lactation in the bovine mammary gland, compared with the rodent gland. We speculate that interactions among the estrogen receptor isoforms that regulate progesterone receptor expression may contribute to these species differences. Further, demonstrated expression of substantial quantities of estrogen receptor within the prepubertal bovine mammary fat pad, along with coordinated insulin-like growth factor-I expression, suggests that this tissue may stimulate parenchymal growth via an estrogen-responsive paracrine mechanism. In addition, the recent availability of bovine genomic sequence information and microarray technologies has permitted the study of global gene expression in the mammary gland in response to the steroid environment. We have identified more than 100 estrogen-responsive genes, of which the majority are novel estrogen gene targets. Estrogen-induced changes in gene expression were consistent with increased mammary epithelial cell proliferation, increased extracellular matrix turnover in parenchyma, and increased extracellular matrix deposition in the fat pad. A comparison of estrogen-responsive genes in the mammary glands of humans, mice, and cattle suggests considerable variation among species, as well as potential differences in regulatory elements in common estrogen receptor gene targets. Continuing studies using advanced molecular techniques should assist in elucidating the complex regulation of mammary function at the transcript level.

Overlapping and distinct expression of progesterone receptors A and B in mouse uterus and mammary gland during the estrous cycle

In rodents, progesterone receptors (PRs) A and B have different and often nonoverlapping roles, and this study asked whether different activities of the PR proteins in mouse are related to differences in their expression in reproductive tissues. The individual expression of PRA and PRB was determined immunohistochemically in mammary gland and uterus during the estrous cycle or in response to endocrine manipulation. In the mammary gland, PRA and PRB were colocated in PR+ epithelial cells, with little change during the estrous cycle. In the uterus, PRA was not detected in luminal epithelium at any stage of the cycle, and PR+ luminal cells expressed only PRB. In the stroma and myometrium, PRA and PRB levels fluctuated with cyclical systemic hormone exposure. Observation of functional end points suggested that augmented stromal and/or myometrial PRA in proestrus inhibited estrogen receptor expression and epithelial proliferation. Colocation of PRA and PRB was hormonally regulated, and ovariectomy did not reproduce the expression of PRA and PRB in the uterus during the estrous cycle. Whereas PRB was the only PR in the luminal epithelium in cycling mice, ovariectomy restored PRA expression, resulting in PRA-PRB colocation. In stroma and myometrium, PRA and PRB colocated in PR+ cells, but ovariectomy reduced PRA levels more than PRB, resulting in PRB-only-expressing cells. This study has shown that nonoverlapping PRA and PRB expression in the uterus, in particular the lack of PRA, and expression of PRB only in the luminal epithelium throughout the estrous cycle, is likely to contribute to the distinct roles of PRA and PRB in the adult mouse.

Atp2b2, encoding plasma membrane Ca2+-ATPase type 2, (PMCA2) exhibits tissue-specific first exon usage in hair cells, neurons, and mammary glands of mice

Atp2b2 encodes the plasma membrane Ca(2+)-ATPase type 2 (PMCA2) expressed in various tissues, including stereocilia of cochlear and vestibular hair cells, cerebellar Purkinje cells, and lactating mammary epithelia. Mutations of the gene lead to deafness, ataxia, and reduced Ca(2+) levels in milk. Heterozygous mutants also have abnormal hearing, suggesting that precise regulation of Atp2b2 is required for normal function. In this study, we describe Atp2b2 5'-untranslated region genomic structure and transcript usage in mice. Using 5'-rapid amplification of cDNA ends, we observed four transcripts: types alpha, beta, mu and delta, each splicing into a common ATG-containing exon. Types alpha and beta correspond to previously published mammalian cDNA sequences. Types mu and delta constitute novel 5'-untranslated region sequences, and were observed at high levels only in lactating mammary gland. Using real-time reverse transcriptase polymerase chain reaction, we quantified relative transcript usage across several tissues. We show that alpha and beta are abundant throughout the CNS, as well as the cochlea. When we microdissected the cochlea into hair cell and spiral ganglion containing fractions, we found that cochlear hair cell expression is mediated through the type alpha transcript. In situ hybridization studies in cerebellum using exon-specific probes revealed that alpha dominates in Purkinje neurons, while beta is enriched in cerebellar granule neurons. We compared 5'-untranslated region sequence across multiple species, and found high conservation around the first exons for alpha and beta in mammals, but not other species. The regions around the mu and delta first exons are highly conserved between rat and mouse, but less so with other species. Our results show that expression of Atp2b2 is highly regulated, using four different transcriptional start regions, two of which are differentially expressed in neuronal tissue. This suggests that unique regulatory mechanisms are used to control Atp2b2 expression in different types of cells.

Progesterone receptor isoforms A and B: temporal and spatial differences in expression during murine mammary gland development

Progesterone is a potent mitogen in the mammary gland. Based on studies using cells and animals engineered to express progesterone receptor (PR) isoforms A or B, PRA and PRB are believed to have different functions. Using an immunohistochemical approach with antibodies specific for PRA only or PRB only, we show that PRA and PRB expression in mammary epithelial cells is temporally and spatially separated during normal mammary gland development in the BALB/c mouse. In the virgin mammary gland when ductal development is active, the only PR protein isoform expressed was PRA. PRA levels were significantly lower during pregnancy, suggesting a minor role at this stage of development. PRB was abundantly expressed only during pregnancy, during alveologenesis. PRA and PRB colocalization occurred in only a small percentage of cells. During pregnancy there was extensive colocalization of PRB with 5-bromo-2'-deoxyuridine (BrdU) and cyclin D1; 95% of BrdU-positive cells and 83% of cyclin D1-positive cells expressed PRB. No colocalization of PRA with either BrdU or cyclin D1 was observed at pregnancy. In the virgin gland, PRA colocalization with BrdU or cyclin D1 was low; only 27% of BrdU-positive cells and 4% of cyclin D1-positive cells expressed PRA. The implication of these findings is that different actions of progesterone are mediated in PRB positive vs. PRA-positive cells in vivo. The spatial and temporal separation of PR isoform expression in mouse mammary gland provides a unique opportunity to determine the specific functions of PRA vs. PRB in vivo.

Proliferation of estrogen receptor-alpha-positive mammary epithelial cells is restrained by transforming growth factor-beta1 in adult mice

Transforming growth factor (TGF)-beta1 is a potent inhibitor of mammary epithelial proliferation. In human breast, estrogen receptor (ER)-alpha cells rarely co-localize with markers of proliferation, but their increased frequency correlates with breast cancer risk. To determine whether TGF-beta1 is necessary for the quiescence of ER-alpha-positive populations, we examined mouse mammary epithelial glands at estrus. Approximately 35% of epithelial cells showed TGF-beta1 activation, which co-localized with nuclear receptor-phosphorylated Smad 2/3, indicating that TGF-beta signaling is autocrine. Nuclear Smad co-localized with nuclear ER-alpha. To test whether TGF-beta inhibits proliferation, we examined genetically engineered mice with different levels of TGF-beta1. ER-alpha co-localization with markers of proliferation (ie, Ki-67 or bromodeoxyuridine) at estrus was significantly increased in the mammary glands of Tgf beta1 C57/bl/129SV heterozygote mice. This relationship was maintained after pregnancy but was absent at puberty. Conversely, mammary epithelial expression of constitutively active TGF-beta1 via the MMTV promoter suppressed proliferation of ER-alpha-positive cells. Thus, TGF-beta1 activation functionally restrains ER-alpha-positive cells from proliferating in adult mammary gland. Accordingly, we propose that TGF-beta1 dysregulation may promote proliferation of ER-alpha-positive cells associated with breast cancer risk in humans.

SEX STEROID HORMONE RECEPTORS IN THE DEVELOPING FEMALE REPRODUCTIVE TRACT OF LABORATORY RODENTS

Many chemicals released into the environment potentially disrupt the endocrine system in wildlife and humans. Some of these chemicals exhibit estrogenic activity by binding to the estrogen receptors. The developing organism is particularly sensitive to estrogenic chemicals during the critical period in which the induction of long-term changes and persistent molecular alterations in female reproductive tracts occur. Perinatal mouse and rat models can be utilized as indicators for determining the consequences of exposure to exogenous estrogenic agents, including possible xenoestrogens or environmental endocrine disruptors. Estrogen receptors (ER) and estrogen responsive genes, therefore, need to be identified in order to understand the molecular basis of estrogenic actions. Recent identifications of ER subtypes and isoforms make understanding target organ responses to these estrogenic chemicals even more difficult. Indeed, many reports suggest that these chemicals do affect the reproductive and developmental processes of female laboratory rodents that had been perinatally exposed, and that interactions between sex steroid hormone receptors occur. Much information concerning the expression of sex steroid receptors in rodents has been reported concerning the normal development of the Müllerian duct. Thus, accumulated information on the expression of ER subtypes and isoforms as well as that of progesterone and androgen receptors in laboratory rodents is herein reviewed, in addition to the presentation of our own data.

Reproductive tissue selective actions of progesterone receptors

The steroid hormone, progesterone, plays a central coordinate role in diverse events associated with female reproduction. In humans and other vertebrates, the biological activity of progesterone is mediated by modulation of the transcriptional activity of two progesterone receptors, PR-A and PR-B. These receptors arise from the same gene and exhibit both overlapping and distinct transcriptional activities in vitro. To delineate the individual roles of PR-A and PR-B in vivo, we have generated mouse models in which expression of a single PR isoform has been ablated. Analysis of the reproductive phenotypes of these mice has indicated that PR-A and PR-B mediate mostly distinct but partially overlapping reproductive responses to progesterone. While selective ablation of the PR-A protein (PR-A knockout mice, PRAKO mice) shows normal mammary gland response to progesterone but severe uterine hyperplasia and ovarian abnormalities, ablation of PR-B protein (PRBKO mice) does not affect biological responses of the ovary or uterus to progesterone but results in reduced pregnancy-associated mammary gland morphogenesis. The distinct tissue-specific reproductive responses to progesterone exhibited by these isoforms are due to regulation of distinct subsets of progesterone-dependent target genes by the individual PR isoforms. This review will summarize our current understanding of the selective contribution of PR isoforms to the cellular and molecular actions of progesterone in reproductive tissues.

Progesterone receptor mRNA levels during pregnancy, labor, lactation and the estrous cycle in rat uterus

Progesterone plays important roles in the regulation of female reproduction. In this study, progesterone receptor (PR) mRNA levels in rat uterus during pregnancy, labor, lactation and the estrous cycle were examined by competitive RT-PCR. During pregnancy and lactation, PR mRNA levels had decreased on day 20 of pregnancy (P20) and P21 compared with P15 but increased during labor. After a decline on day 1 of lactation (L1), PR mRNA levels had increased again on L3 and L14 compared with P15, P18, P20, P21 and P21pm (at 2200-2300 h on P21). There was no significant change in the PR mRNA level during the estrous cycle. The PR mRNA level did not change during 1 week of progesterone treatment or afterwards. Injection of 17beta-estradiol did not affect PR mRNA levels in rats treated with progesterone or those without any injections. In rats on P18, 17beta-estradiol injection did not change PR mRNA levels after sham-operation but induced an increase in PR mRNA levels of rats ovariectomized 6 h before the treatment. These results suggest that uterine PR mRNA levels are differently regulated during late pregnancy, labor and lactation, and during labor estrogen is one of the essential factors for the increase in PR mRNA levels.

Expression of progesterone receptors A and B in the mouse ovary during the estrous cycle

Progesterone plays a central role in the regulation of ovarian function. The progesterone receptor (PR) has been shown to be essential for ovulation because mice lacking PR fail to ovulate and are infertile. PR is expressed as two isoforms, PRA and PRB, which have been shown to have different functional activities. In this study, we investigated the cellular distribution of PRA and PRB in the ovaries and oviducts of cycling mice using immunohistochemistry with isoform-specific monoclonal antibodies. In the ovary, on the evening of proestrus before ovulation, both the granulosa and theca cells of the preovulatory follicles expressed both PR isoforms. PRA and PRB staining was also observed in the theca cells of preantral and antral follicles, whereas only PRB was observed in the granulosa cells of primary, preantral, and antral follicles and in the corpus luteum. In the oviduct, PRA was the predominant isoform observed, expressed in both the epithelial and stromal cells, whereas PRB was only detected in the epithelial cells. The differences in PRA and PRB localization in the ovary and oviduct may reflect diverse functions for PRA and PRB in reproductive tissues and may have important implications in understanding the mechanisms of progesterone action.

Sex steroids and growth factors in the regulation of mammary gland proliferation, differentiation, and involution

The mammary gland is subjected to major morphological and biochemical changes during the lactation cycle. It is therefore not surprising that this dynamic process is strictly controlled. The importance of the sex steroid hormones 17beta-estradiol and progesterone for normal development of the mammary gland was recognized several decades ago and has been unequivocally confirmed since. Furthermore, it is now also established that the influence of sex steroids is not restricted to mammogenesis, but that these hormones also control involution. Another important regulatory role is played by growth factors that have been shown to modulate survival (epidermal growth factor, amphiregulin, transforming growth factor alpha, insulin like growth factor, and tumor necrosis factor alpha) or apoptosis (tumor necrosis factor alpha, transforming growth factor beta) of mammary cells. However, the molecular mechanism underlying the influence of sex steroid hormones and/or growth factors on the development and function of the mammary gland remains largely unknown to date. Also scarce is information on the interaction between both groups of modulators. Nevertheless, based on the current indications compiled in this review, an important functional role for sex steroid hormones in the lactation cycle in co-operation with growth factors can be suggested.

Defective mammary gland morphogenesis in mice lacking the progesterone receptor B isoform

Progesterone (P) regulates female reproduction via two nuclear receptors, PR-A and PR-B. Although both receptors display overlapping and distinct transcription regulatory properties, their individual physiological roles are unclear. To address the physiological role of PR-A, we generated a mouse model in which expression of PR-B was specifically ablated (PRBKO-/-). We show that selective activation of PR-A in PRBKO-/- mice is sufficient to elicit normal ovarian and uterine responses to P but results in reduced mammary gland morphogenesis. In the absence of PR-B, pregnancy-associated ductal sidebranching and lobuloalveolar development are markedly reduced due to decreased ductal and alveolar epithelial cell proliferation and decreased survival of alveolar epithelium. In an effort to elucidate the molecular genetic signaling pathways that are differentially regulated by PRs in the mammary gland, we have identified receptor activator of nuclear factor kappa B ligand (RANKL) as a paracrine mediator of P-dependent alveologenesis. Further, we demonstrate that the defects in PRBKO-/- mice are associated with an inability of PR-A to activate the RANKL signaling pathway in response to P. Our data indicate that functional interaction between PR-A and PR-B is not required for reproductive activity and that selective modulation of PR-A activity by progestin agonists may have a protective effect against both uterine and mammary gland hyperplasias.

Progesterone receptor expression in medroxyprogesterone acetate-induced murine mammary carcinomas and response to endocrine treatment

Using medroxyprogesterone acetate (MPA) as a carcinogen, we were able to induce in BALB/c female mice, several progestin-dependent mammary ductal carcinomas that regress completely with estrogen or antiprogestins and are maintained by serial transplantations in syngeneic mice. Progestin-independent variants were subsequently generated or appeared spontaneously. Based on their response to estrogen or antiprogestins, we subdivided them into responsive progestin-independent (R-PI) variants which regress completely and unresponsive progestin-independent (UR-PI) carcinomas which are resistant to both families of compounds. In this study we have investigated progesterone receptor (PR) expression in six responsive progestin-dependent, six R-PI, and three UR-PI tumors. Progestin-dependent and R-PI tumors disclosed a higher expression of the PR(A) isoform as compared with PR(B), as well as an additional band of 78 kDa that was not detected in uterine tissue; all were down-regulated by progestins. UR-PI tumors expressed lower levels of all bands in western blots, but were highly reactive by immunohistochemistry. PR RNA expression was detected in both, UR-PI and R-PI tumors. PR binding was comparable in progestin-dependent and R-PI tumors. In the three UR-PI tumors, only 29/61 (48%) of the samples evaluated showed low binding levels, the rest were negative. This report is the first to describe in an experimental model of breast cancer the expression of PR isoforms and their distribution. Our results suggest the expression of functionally altered isoforms in a subgroup of mammary carcinomas, which may explain their lack of hormone response.

The effects of estradiol and selective estrogen receptor modulators on gene expression and messenger RNA stability in immortalized sheep endometrial stromal cells and human endometrial adenocarcinoma cells

The purpose of this study was to identify an endometrial cell line that maintained the E2 up-regulation of estrogen receptor (ER) mRNA by enhanced message stability and to assess its dependence on ER protein. Estradiol (E2) effects on gene expression were measured in three cell lines: one immortalized from sheep endometrial stroma (ST) and two from human endometrial adenocarcinomas (Ishikawa and ECC-1). E2 up-regulated ER mRNA levels in ST and Ishikawa cells, but down-regulated ER mRNA levels in ECC-1 cells. E2 up-regulated progesterone receptor (PR), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and transforming growth factor-alpha (TGF-alpha) in both Ishikawa and ECC-1 cells. The selective estrogen receptor modulator ICI 182,780 antagonized the E2-induced up-regulation of ER and/or PR mRNA levels in all three cells, while another, GW 5638, antagonized the up-regulation of PR mRNA in Ishikawa and ECC-1 cells. In mechanistic studies, E2 had no effect on ER mRNA stability in ST cells and it destabilized ER mRNA in ECC-1 cells. Thus, Ishikawa cells appear to be the most physiologically relevant cell line in which to study the up-regulation of ER mRNA levels by enhanced mRNA stability. Its antagonism by ICI 182,780 reveals that ER protein is involved in this E2 response.

Heregulin induces transcriptional activation of the progesterone receptor by a mechanism that requires functional ErbB-2 and mitogen-activated protein kinase activation in breast cancer cells

The present study addresses the capacity of heregulin (HRG), a ligand of type I receptor tyrosine kinases, to transactivate the progesterone receptor (PR). For this purpose, we studied, on the one hand, an experimental model of hormonal carcinogenesis in which the synthetic progestin medroxyprogesterone acetate (MPA) induced mammary adenocarcinomas in female BALB/c mice and, on the other hand, the human breast cancer cell line T47D. HRG was able to exquisitely regulate biochemical attributes of PR in a way that mimicked PR activation by progestins. Thus, HRG treatment of primary cultures of epithelial cells of the progestin-dependent C4HD murine mammary tumor line and of T47D cells induced a decrease of protein levels of PRA and -B isoforms and the downregulation of progesterone-binding sites. HRG also promoted a significant increase in the percentage of PR localized in the nucleus in both cell types. DNA mobility shift assay revealed that HRG was able to induce PR binding to a progesterone response element (PRE) in C4HD and T47D cells. Transient transfections of C4HD and T47D cells with a plasmid containing a PRE upstream of a chloramphenicol acetyltransferase (CAT) gene demonstrated that HRG promoted a significant increase in CAT activity. In order to assess the molecular mechanisms underlying PR transactivation by HRG, we blocked ErbB-2 expression in C4HD and T47D cells by using antisense oligodeoxynucleotides to ErbB-2 mRNA, which resulted in the abolishment of HRG's capacity to induce PR binding to a PRE, as well as CAT activity in the transient-transfection assays. Although the inhibition of HRG binding to ErbB-3 by an anti-ErbB-3 monoclonal antibody suppressed HRG-induced PR activation, the abolishment of HRG binding to ErbB-4 had no effect on HRG activation of PR. To investigate the role of mitogen-activated protein kinases (MAPKs), we used the selective MEK1/MAPK inhibitor PD98059. Blockage of MAPK activation resulted in complete abrogation of HRG's capacity to induce PR binding to a PRE, as well as CAT activity. Finally, we demonstrate here for the first time that HRG-activated MAPK can phosphorylate both human and mouse PR in vitro.

P21-activated kinase-1 phosphorylates and transactivates estrogen receptor-alpha and promotes hyperplasia in mammary epithelium

Stimulation of p21-activated kinase-1 (Pak1) induces cytoskeleton reorganization and signaling pathways in mammary cancer cells. Here, we show that inhibition of Pak1 kinase activity by a dominant-negative fragment or by short interference RNA markedly reduced the estrogen receptor-alpha (ER) transactivation functions. To understand the role of Pak1 in mammary glands, we developed a murine model expressing constitutively active Thr423 glutamic acid Pak1 driven by the beta-lactoglobulin promoter. We show that mammary glands from these mice developed widespread hyperplasia associated with apocrine metaplasia and lobuloalveolar hyperdevelopment during lactation. Mammary tissues with active Pak1 also exhibited an increased activation of mitogen-activated protein kinase and stimulated transactivation functions of the ER and expression of endogenous ER target genes. Furthermore, Pak1 directly phosphorylated the activation function-2 domain of the ER at the N-terminal residue Ser305, and its mutation to Ala (S305A) abolished the Pak1-mediated phosphorylation and transactivation functions of the ER, while its mutation to glutamic acid (S305E) promoted transactivation activity of ER. These findings reveal a novel role for the Pak1-ER pathway in promoting hyperplasia in mammary epithelium.

Concurrent pregnancy retards mammary involution: effects on apoptosis and proliferation of the mammary epithelium after forced weaning of mice

The effect of pregnancy on postweaning mammary gland involution was investigated in mice. On the third day after forced weaning at Lactation Day 10, the apoptotic index was 56% lower in mammary tissue of mice that were pregnant at the time of weaning than in nonpregnant mice. Conversely, the bromodeoxyuridine-labeling index was increased sevenfold in pregnant mice compared to nonpregnant controls (3.5% vs. 0.5%, respectively). Structure of mammary alveoli was largely maintained in postweaning pregnant mice. The effect of pregnancy on three specific mammary epithelial cell survival pathways was also examined. First, pregnancy blocked the loss of Stat5a phosphorylation during involution. Significantly, loss of Stat5a phosphorylation during involution was not correlated with loss of Stat5a nuclear localization. Second, pregnancy maintained nuclear-localized progesterone receptor during lactation. Third, pregnancy was associated with increased expression of bfl-1 during involution but had little effect on the expression of other bcl-2 family members. The data indicate that pregnancy inhibits mammary cell apoptosis after weaning while permitting proliferation of the mammary epithelium, and they support the hypothesis that Stat5a and progesterone-signaling pathways act in concert to mediate this effect.

Mechanisms of action of estrogen and progesterone

Estrogen and progesterone are steroid hormones that play a pivotal role in the regulation of mammalian reproduction. One primary action of these hormones is to regulate the development and function of the uterus. These hormones act by regulating the transcription of specific genes in the uterus. The actions of these hormones are mediated by their specific hormone receptors. These receptors are nuclear transcription factors, whose transcriptional regulatory activity is mediated by the binding of the specific steroid to these molecules. Once these receptors bind hormone, they can bind to specific cis-acting sequences in the promoter region of responsive genes and regulate transcription of these genes. In the regulation of transcription, these receptors interact with specific cofactors to activate the transcriptional machinery. A second gene family, the Steroid Receptor Coactivator (SRC) family, has been identified that serves to modulate the transcriptional activity of the hormone receptors. To date, three members of the SRC family have been identified. During the last decade, gene targeting technology has been used to identify the role of these receptors in the regulation of reproduction and uterine biology.

Cellular expression of estrogen and progesterone receptors in mammary glands: regulation by hormones, development and aging

At present, there is an extensive body of literature documenting the participation of estrogen receptors (ER) and progesterone receptors (PR) in mammary gene expression. Yet, the precise roles of these receptors in regulating mammary development, carcinogenesis and the growth of a subset of tumors still remain unclear. Mammary glands are composed of various cell types with different developmental potentials. Further, ultimately, that it is their mutual interactions which dictate the behavior of mammary epithelial cells. Therefore, to resolve the roles of ER and PR in normal mammary growth, differentiation and carcinogenesis, analyses for the expression of these receptors at the level of individual cell types is of paramount importance. Accordingly, in the present studies using immunolocalization techniques, we document the ontogeny and cellular distribution of ER and PR during mammary development and in response to ovarian hormones and aging. In addition, we discuss the potential biological significances of the expression patterns of ER and PR during various physiological states. We believe that the observations reported here should provide a conceptual framework(s) for elucidating the roles of ER and PR in normal and neoplastic mammary tissues.

Mechanisms of estrogen action

Our appreciation of the physiological functions of estrogens and the mechanisms through which estrogens bring about these functions has changed during the past decade. Just as transgenic mice were produced in which estrogen receptors had been inactivated and we thought that we were about to understand the role of estrogen receptors in physiology and pathology, it was found that there was not one but two distinct and functional estrogen receptors, now called ER alpha and ER beta. Transgenic mice in which each of the receptors or both the receptors are inactive have revealed a much broader role for estrogens in the body than was previously thought. This decade also saw the description of a male patient who had no functional ER alpha and whose continued bone growth clearly revealed an important function of estrogen in men. The importance of estrogen in both males and females was also demonstrated in the laboratory in transgenic mice in which the aromatase gene was inactivated. Finally, crystal structures of the estrogen receptors with agonists and antagonists have revealed much about how ligand binding influences receptor conformation and how this conformation influences interaction of the receptor with coactivators or corepressors and hence determines cellular response to ligands.

Quantitative analysis of estrogen receptor proteins in rat mammary gland

Estrogen receptor alpha and beta proteins (ERalpha and ERbeta) at various stages of development of the rat mammary gland were quantified by Western blotting. ERalpha and ERbeta recombinant proteins were used as standards, and their molar concentrations were measured by ligand binding assays. In 3-week-old pregnant, lactating, and postlactating rats the ERalpha content ranged from 0.30-1.55 fmol/microg total protein (mean values). The ERbeta content of the same samples ranged between 1.06-7.50 fmol/microg total protein. At every developmental stage, the ERbeta content of the mammary gland was higher than that of ERalpha. When receptor levels were normalized against beta-actin, it was evident that ER expression changed during development, with maximum expression of both receptors during the lactation period. With an antibody raised against the 18-amino acid insert of the ERbeta variant, originally called ERbeta2 but named ERbetains in this paper, Western blots revealed that ERbetains protein was up-regulated during the lactation period. RT-PCR showed that the levels of messenger RNA of ERbetains paralleled those of the protein. Double immunohistochemical staining with anti-ERalpha and anti-ERbetains antibodies revealed that ERbetains protein colocalized with ERalpha in 70-80% of the ERalpha-expressing epithelial cells during lactation and with 30% of these cells during pregnancy. These observations indicate that expression of ERbetains is regulated not only quantitatively, but also with regard to its cellular distribution. As ERbetains acts as the dominant repressor of ERalpha, we suggest that its coexpression with ERalpha quenches ERalpha function and may be one of the factors that contribute to the previously described insensitivity of the mammary gland to estrogens during lactation.

Reproductive functions of the progesterone receptor isoforms: lessons from knock-out mice

Progesterone plays a central coordinate role in diverse reproductive events associated with establishment and maintenance of pregnancy. In humans and other vertebrates, the biological activities of progesterone are mediated by two proteins, A (PR-A) and B (PR-B) that arise from the same gene and function as progesterone activated transcription factors that exhibit different transcription regulatory activities in vitro. Mice lacking both PR isoforms (PRKO mice) exhibit pleiotropic reproductive abnormalities. To address the physiological role of the individual isoforms, we have selectively ablated PR-A expression in mice (PRAKO). We have demonstrated that PR-B mediates a subset of the reproductive functions of P. Ablation of PR-A does not affect responses of the mammary gland or thymus to P but results in severe abnormalities in ovarian and uterine function. Analysis of urine function of PRAKP mice reveals an unexpected P-dependent proliferative activity of PR-B in the epithelium and provides evidence that the tissue-specific reproductive effects of this isoform are due to specificity of target gene transactivation rather than differences in tissue-specific expression relative to PR-A. Taken together, our data indicate that PR-A and PR-B act in vivo as two functionally distinct transcription factors.

Inability of overexpressed des(1-3)human insulin-like growth factor I (IGF-I) to inhibit forced mammary gland involution is associated with decreased expression of IGF signaling molecules

Overexpression of des(1-3) human insulin-like growth factor I (IGF-I) in the mammary glands of transgenic mice (WAP-DES) inhibits apoptosis during natural, but not forced, mammary involution. We hypothesized that this differential response would correlate with the expression of IGF signal transducers. Forced and natural involution were analyzed in nontransgenic and WAP-DES mice beginning on day 16 postpartum. During natural involution, mammary gland wet weight was higher and apoptosis was lower in WAP-DES than in nontransgenic mice. The WAP-DES transgene had no effect on these parameters during forced involution. Mammary tissue concentrations of the transgene protein were 2- to 10-fold higher than those of endogenous IGF-I. Western blot analysis of pooled mammary tissue extracts demonstrated only slightly higher phosphorylation of the IGF signal transducers insulin receptor substrate-1 (IRS-1) and Akt in the WAP-DES than in nontransgenic mice. Dramatic early reductions in phospho-IRS-1, phospho-Akt, IRS-1, IRS-2, and Akt proteins occurred during forced, but not natural, involution. The abundance of the IGF-I receptor and the messenger RNAs for the IGF-I receptors, IRS-1 and -2, were not affected by either genotype or involution. These findings support the conclusions that mammary cells lose their responsiveness to insulin-like signals during forced involution, and that posttranscriptional or posttranslational regulation of IRS-1 and IRS-2 may play a role in this loss.

Ductal growth is impeded in mammary glands of C-neu transgenic mice

The steroid hormone, estradiol, is essential for both the growth of normal breast and induction of mammary carcinomas. The growth promoting effects of estrogen are presumed to be mediated by growth factors, in particular, epidermal growth factor, which mediates its effects through erbB receptors, erbB1 and erbB2/C-neu. C-neu is amplified and over-expressed in a large number of human cancers and transgenic mice over-expressing C-neu also develop mammary tumors. However, as yet, the impact of C-neu over-expression on estrogen action during normal mammary development and hence, its precise role in carcinogenesis, remains unclear. In the present studies, we demonstrate that estradiol-dependent mammary ductal growth accompanying puberty is impaired in transgenic mice expressing wild type Cneu, and is intrinsic to the tissue. The impairment is not due to an overall impairment in estrogen action, since progesterone receptor expression is unaffected in C-neu mice. It is also not due to an intrinsic inability of the epithelial cells to proliferate, since impeded ductal growth co-exists with alveolar growth during pregnancy. Therefore, we propose that, depending on the physiological state, C-neu may either promote or inhibit the growth of mammary epithelial cells, and discuss its potential significance to carcinogenesis.

Subgroup of reproductive functions of progesterone mediated by progesterone receptor-B isoform

Progesterone regulates reproductive function through two intracellular receptors, progesterone receptor-A (PR-A) and progesterone receptor-B (PR-B), that arise from a single gene and function as transcriptional regulators of progesterone-responsive genes. Although in vitro studies show that PR isoforms can display different transcriptional regulatory activities, their physiological significance is unknown. By selective ablation of PR-A in mice, we show that the PR-B isoform modulates a subset of reproductive functions of progesterone by regulation of a subset of progesterone-responsive target genes. Thus, PR-A and PR-B are functionally distinct mediators of progesterone action in vivo and should provide suitable targets for generation of tissue-selective progestins.

Central role of peroxisome proliferator-activated receptors in the actions of peroxisome proliferators

Peroxisome proliferators (PPs) are a large class of structurally dissimilar chemicals that have diverse effects in rodents and humans. Most, if not all, of the diverse effects of PPs are mediated by three members of the nuclear receptor superfamily called peroxisome proliferator-activated receptors (PPARs). In this review, we define the molecular mechanisms of PPs, including PPAR binding specificity, alteration of gene expression through binding to DNA response elements, and cross talk with other signaling pathways. We discuss the roles of PPARs in growth promotion in rodent hepatocarcinogenesis and potential therapeutic effects, including suppression of cancer growth and inflammation.

Induction of mammary gland development in estrogen receptor-alpha knockout mice

Mammary glands from the estrogen receptor-a knockout (alphaERKO) mouse do not undergo ductal morphogenesis or alveolar development. Disrupted ERalpha signaling may result in reduced estrogen-responsive gene products in the mammary gland or reduced mammotropic hormones that contribute to the alphaERKO mammary phenotype. We report that circulating PRL is reduced in the female alphaERKO mouse. Implantation of an age-matched, heterozygous ERalpha pituitary isograft under the renal capsule of 25-day-old or 12-week-old alphaERKO mice increased circulating PRL and progesterone levels, and induced mammary gland development. Grafted alphaERKO mice also possessed hypertrophied corpora lutea demonstrating that PRL is luteotropic in the alphaERKO ovary. By contrast, ovariectomy at the time of pituitary grafting prevented mammary gland development in alphaERKO mice despite elevated PRL levels. Hormone replacement using pellet implants demonstrated that pharmacological doses of estradiol induced limited mammary ductal elongation, and estradiol in combination with progesterone stimulated lobuloalveolar development. PRL alone or in combination with progesterone or estradiol did not induce alphaERKO mammary growth. Estradiol and progesterone are required for the structural development of the alphaERKO mammary gland, and PRL contributes to this development by inducing ovarian progesterone levels. Therefore, the manifestation of the alphaERKO mammary phenotype appears due to the lack of direct estrogen action at the mammary gland and an indirect contributory role of estrogen signaling at the hypothalamic/pituitary axis.

A reappraisal of progesterone action in the mammary gland

The ovarian hormones estrogen and progesterone and their respective receptors are essential for maintenance of postnatal developmental plasticity of the mammary gland and play a key role in mammary tumorigenesis. Mouse models in which expression of the progesterone receptors was genetically ablated have recently become available. Studies of these models have demonstrated that progesterone is specifically required for pregnancy associated ductal proliferation and lobuloalveolar differentiation of the mammary epithelium, but not for immediate postpubertal ductal morphogenesis. Use of these mice in combination with mammary gland transplantation indicates that developmental regulation by progesterone appears to occur through a paracrine mechanism in which progesterone receptor (PR) positive cells represent a subset of non-proliferating epithelial cells that are capable of directing proliferation and/or differentiation of neighboring receptor negative cells. The hierarchical organization of these receptors in the epithelium and their segregation from proliferating cells is a conserved feature in rodent and human mammary tissue. The identification of paracrine mediators of the progesterone response is now an imminent goal as is the delineation of the individual contributions of the two PR isoforms using similar approaches.

C/EBPbeta (CCAAT/enhancer binding protein) controls cell fate determination during mammary gland development

Deletion of the transcription factor CCAAT/enhancer binding protein (C/EBP)beta results in a severe inhibition of lobuloalveolar development in the mouse mammary gland. Because progesterone receptor (PR) is requisite for alveolar development, the expression of PR was investigated in C/EBPbeta-/- mice. Unexpectedly, the number of PR-positive cells, as well as the levels of PR mRNA, were elevated 3-fold in the mammary glands of C/EBPbeta-/- mice. Furthermore, in contrast to wild-type nulliparous mice, in which PR distribution shifted from a uniform to nonuniform pattern between 8-12 weeks of age, C/EBPbeta-/- mice exhibited uniform PR distribution throughout all stages of mammary development analyzed. No change in C/EBPbeta mRNA levels was observed in the mammary glands of PR-/- mice, suggesting that PR acts in a pathway either in parallel to or downstream of C/EBPbeta. The overexpression and disrupted cellular distribution of PR in C/EBPbeta-/- mice were coincident with a striking 10-fold decrease in cell proliferation after acute steroid hormone treatment, assayed by incorporation of bromodeoxyuridine. In wild-type mice, PR and bromodeoxyuridine-positive cells were adjacent to each other and rarely colocalized. No differences in the level or pattern of PR expression were observed in the uterus, suggesting that C/EBPbeta influences PR in a mammary-specific fashion. Together, these data suggest that C/EBPbeta may control cell fate decisions in the mammary gland through the appropriate temporal and spatial expression of molecular markers, such as PR, that induce the proliferation of alveolar progenitor cells via juxtacrine mechanisms.

Estrogen receptors alpha and beta in the rodent mammary gland

An obligatory role for estrogen in growth, development, and functions of the mammary gland is well established, but the roles of the two estrogen receptors remain unclear. With the use of specific antibodies, it was found that both estrogen receptors, ERalpha and ERbeta, are expressed in the rat mammary gland but the presence and cellular distribution of the two receptors are distinct. In prepubertal rats, ERalpha was detected in 40% of the epithelial cell nuclei. This decreased to 30% at puberty and continued to decrease throughout pregnancy to a low of 5% at day 14. During lactation there was a large induction of ERalpha with up to 70% of the nuclei positive at day 21. Approximately 60-70% of epithelial cells expressed ERbeta at all stages of breast development. Cells coexpressing ERalpha and ERbeta were rare during pregnancy, a proliferative phase, but they represented up to 60% of the epithelial cells during lactation, a postproliferative phase. Western blot analysis and sucrose gradient centrifugation confirmed this pattern of expression. During pregnancy, the proliferating cell nuclear antigen was not expressed in ERalpha-positive cells but was observed in 3-7% of ERbeta-containing cells. Because more than 90% of ERbeta-bearing cells do not proliferate, and 55-70% of the dividing cells have neither ERalpha nor ERbeta, it is clear that the presence of these receptors in epithelial cells is not a prerequisite for estrogen-mediated proliferation.

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.

Progesterone signaling and mammary gland morphogenesis

Progesterone was identified as a mammogenic hormone several years ago but until now its precise role in mammary development has remained obscure. Recently with the generation of several transgenic mouse models and development of reagents for analysis of progesterone receptor expression, the role of progesterone signaling in mammary development is becoming more clear. The most significant observations to emerge from these studies are (1) progesterone receptors (PR) are present in a heterogeneous manner in the epithelial cells and undetectable in the surrounding fat pad; (2) they are essential for lobuloalveolar and not for ductal morphogenesis; (3) progesterone signaling through progesterone receptors, leading to lobuloalveolar development, is initiated in the epithelium and may occur through paracrine mechanisms; and (4) a regulated expression of the two isoforms of progesterone receptor is critical for maintaining appropriate responsiveness to progesterone and hence, epithelial cell replicative homeostasis. These studies also reveal that the consequences of progesterone signaling through progesterone receptor may depend on the cell context, cell-cell and cell-extracellular matrix interactions, the dynamics of PR turnover and the fate of PR positive cells.

Mammary gland growth and development from the postnatal period to postmenopause: ovarian steroid receptor ontogeny and regulation in the mouse

Ovarian steroid hormones play a critical role in regulating mammary gland growth and development. The mammary gland sequentially acquires and cyclically exhibits proliferative responses to estrogen and/or progesterone from birth to postmenopause. The focus of this review is to present our current understanding of estrogen and progesterone receptor distribution in epithelial and stromal cells and their functions in relation to mammary gland development. Insights gained from the study of the normal mammary gland are relevant to our understanding of the conditions which may predispose women to the development of breast cancer as well as to alterations in hormonal regulation that occur in breast cancer.