Mid-aged and aged wild-type and progestin receptor knockout (PRKO) mice demonstrate rapid progesterone and 3alpha,5alpha-THP-facilitated lordosis

RATIONALE

Progesterone (P) and its 5alpha-reduced metabolite, 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-THP), facilitate sexual behavior of rodents via agonist-like actions at intracellular progestin receptors (PRs) and membrane GABA(A)/benzodiazepine receptor complexes (GBRs), respectively.

OBJECTIVE

Given that ovarian secretion of progestins declines with aging, whether or not senescent mice are responsive to progestins was of interest.

METHODS

Homozygous PR knockout (PRKO) or wild-type mice that were between 10-12 (mid-aged) or 20-24 (aged) months of age were administered P or 3alpha,5alpha-THP, and the effect on lordosis were examined. Effects of a progestin-priming regimen that enhances PR-mediated (experiment 1) or more rapid, PR-independent effects of progestins (experiments 2 and 3) on sexual behavior were examined. Levels of P, 3alpha,5alpha-THP, and muscimol binding were examined in tissues from aged mice (experiment 4).

RESULTS

Wild-type, but not PRKO, mice were responsive when primed with 17beta-estradiol (E(2); 0.5 microg) and administered P (500 microg, subcutaneously). Mid-aged wild-type mice demonstrated greater increases in lordosis 6 h later compared to their pre-P, baseline test than did aged wild-type mice (experiment 1). Lordosis of younger and older wild-type, but not PRKO, mice was significantly increased within 5 min of intravenous (IV) administration of P (100 ng), compared with E(2)-priming alone (experiment 2). However, wild-type and PRKO mice demonstrated significant increases in lordosis 5 min after IV administration of 3alpha,5alpha-THP, an effect which was more pronounced in mid-aged than in aged animals (100 ng-experiment 3). In tissues from aged wild-type and PRKO mice, levels of P, 3alpha,5alpha-THP, and muscimol binding were increased by P administration (experiment 4). PR binding was lower in the cortex of PRKO than that of wild-type mice.

CONCLUSIONS

Mid-aged and aged PRKO and wild-type mice demonstrated rapid P or 3alpha,5alpha-THP-facilitated lordosis that may be, in part, independent of activity at PRs.

Enhanced sexual behaviors and androgen receptor immunoreactivity in the male progesterone receptor knockout mouse

Reproductive and behavioral functions of progesterone receptors (PRs) in males were assessed by examining consequences of PR gene deletion. Basal hormone levels were measured in male progesterone receptor knockout (PRKO) mice and compared to wild-type (WT) counterparts. RIA of serum LH, testosterone, and progesterone levels revealed no significant differences. Levels of FSH were moderately but significantly lower and inhibin levels were higher in PRKOs; these differences were not accompanied by gross differences in testicular weight or morphology. PRKOs exhibited significant alterations in sexual behavior. In initial tests PRKOs exhibited reduced latency to mount, compared with WT. In second sessions, PRKOs again showed a significantly reduced latency to mount and increased likelihood of achieving ejaculation. RU486 treatment in WT produced increased mount and intromission frequency and decreased latency to intromission. In anxiety-related behavior tests, PRKO mice exhibited intermediate anxiety levels, compared with WT, suggesting that enhanced sexual behavior in PRKOs is not secondary to reduced anxiety. Immunohistochemical analysis revealed significantly enhanced androgen receptor expression in the medial preoptic nucleus and bed nucleus of the stria terminalis of PRKO. We conclude that testicular development and function and homeostatic regulation of the hypothalamic-pituitary testicular axis are altered to a lesser extent by PR gene deletion. In contrast, PR appears to play a substantial role in inhibiting the anticipatory/motivational components of male sexual behavior in the mouse. The biological significance of this inhibitory mechanism and the extent to which it is mediated by reduced androgen receptor expression remain to be clarified.

Identification of murine uterine genes regulated in a ligand-dependent manner by the progesterone receptor

Progesterone (P4) acting through its cognate receptor, the progesterone receptor (PR), plays an important role in uterine physiology. The PR knockout (PRKO) mouse has demonstrated the importance of the P4-PR axis in the regulation of uterine function. To define the molecular pathways regulated by P4-PR in the mouse uterus, Affymetrix MG U74Av2 oligonucleotide arrays were used to identify alterations in gene expression after acute and chronic P4 treatments. PRKO and wild-type mice were ovariectomized and then treated with vehicle or 1 mg P4 every 12 h. Mice were killed either 4 h after the first injection (acute P4 treatment) or after the fourth injection of P4 (chronic P4 treatment). At the genomic level, the major change in gene expression after acute P4 treatment was an increase in the expression of 55 genes. Conversely, the major change in gene expression after chronic P4 treatment was an overall reduction in the expression of 102 genes. In the analysis, retinoic acid metabolic genes, cytochrome P 450 26a1 (Cyp26a1), alcohol dehydrogenase 5, and aldehyde dehydrogenase 1a1 (Aldh1a1); kallikrein genes, Klk5 and Klk6; and specific transcription factors, GATA-2 and Cited2 [cAMP-corticosterone-binding protein/p300-interacting transactivator with glutamic acid (E) and aspartic acid (D)-rich tail], were validated as regulated by the P4-PR axis. Identification and analysis of these responsive genes will help define the role of PR in regulating uterine biology.

Cre-mediated recombination in cell lineages that express the progesterone receptor

Using gene-targeting methods, a progesterone receptor Cre knockin (PR-Cre) mouse was generated in which Cre recombinase was inserted into exon 1 of the PR gene. The insertion positions the Cre gene downstream (and under the specific control) of the endogenous PR promoter. As for heterozygotes for the progesterone receptor knockout (PRKO) mutation, mice heterozygous for the Cre knockin insertion are phenotypically indistinguishable from wildtype. Crossing the PR-Cre with the ROSA26R reporter revealed that Cre excision activity is restricted to cells that express PR in progesterone-responsive tissues such as the uterus, ovary, oviduct, pituitary gland, and mammary gland. Initial characterization of the PR-Cre mouse underscores the utility of this model to precisely ablate floxed target genes specifically in cell lineages that express the PR. In the wider context of female reproductive tissue ontology, this model will be indispensable in tracing the developmental fate of cell lineages that descend from PR positive progenitors.

Revealing Progesterone's Role in Uterine and Mammary Gland Biology: Insights from the Mouse

Apart from distinguishing the in vivo effects of progesterone (P) from those of estrogen (E), the progesterone receptor knockout (PRKO) mouse has furnished unprecedented access to novel cell-signaling paradigms, hitherto unsuspected. Along with providing new cellular principles by which P influences proliferative and differentiative programs obligate for tissue development and tumor progression, the PRKO in conjunction with transcript profiling has begun to uncover the transcriptional cascades underlying these processes. Moreover, studies on isoform-specific knockouts for PR-A (PR-AKO) and PR-B (PR-BKO) have clearly defined distinct physiological roles for the two subtypes of PR, providing essential physiological support for previous in vitro observations. Although the PR-AKO exhibits an infertility phenotype, the PR-BKO displays normal fecundity. Conversely, although normal mammary morphogenesis can manifest in the PR-AKO, pregnancy-associated mammary morphogenesis is severely impaired in the PR-BKO. By virtue of its ability to suppress E-induced and PR-B-mediated uterine and mammary proliferation, the PR-A isoform is likely to be an attractive drug target for the next generation of selective PR modulators in the treatment of uterine and mammary gland hyperplasia. Along with defining the dynamic interplay between E and P responses and physiological events mediated by PR-A and PR-B, further studies on these models should provide a broader conceptual framework for understanding abnormal progestin responses in vivo, with attendant implications for the management of female reproductive health and for the diagnosis and/or treatment of breast cancer.

Intact progesterone receptors are essential to counteract the proliferative effect of estradiol in a genetically engineered mouse model of endometriosis

OBJECTIVE

To determine the role of P and its nuclear receptor PR in the growth of ectopic uterine tissue of mice with or without a disrupted PR gene.

DESIGN

Animal study.

SETTING

Academic medical center.

ANIMAL(S)

Female wild-type (WT) and transgenic knockout mice for P receptor (PRKO).

INTERVENTION(S)

Endometriosis was induced in the following groups of ovariectomized adult mice: [1] untreated WT, [2] estradiol (E(2))-treated WT, [3] P-treated WT, [4] E(2) + P-treated WT, [5] untreated PRKO, [6] E(2)-treated PRKO, and [7] E(2) + P-treated PRKO (n = 5 in each group).

MAIN OUTCOME MEASURE(S)

The size of ectopic uterine tissue in WT and PRKO mice were compared between the groups subjected to treatments with P or E(2). Tissue proliferating cell nuclear antigen (PCNA) levels were compared among these groups.

RESULTS

Treatment with P only significantly decreased the size of WT ectopic uterine tissue. The untreated PRKO ectopic uterine tissue was significantly larger than WT tissue. Estradiol increased the size of ectopic uterine tissues, and this E(2)-dependent growth could be suppressed by P in WT tissues but not in PRKO tissues. Finally, the hormone-dependent changes in ectopic uterine tissue size were accompanied by comparable alterations in PCNA levels.

CONCLUSION(S)

Intact PR in ectopic uterine tissue is essential to abolish E(2)-dependent or -independent proliferation. We also suggest that ectopic uterine tissue is associated with significantly increased resistance to P action and increased predisposition to E(2)-dependent proliferation in the absence of PR. Overall, these findings support the hypothesis that P resistance in human endometriosis may be due to the absence of sufficient levels of functional PR in this tissue.

Hormonal defect in maspin heterozygous mice reveals a role of progesterone in pubertal ductal development

Progesterone and PR are mainly thought to affect tertiary ductal side branching and alveologenesis in late stage of mammary gland development. Here, we present evidence that they also play a role in early ductal development. This conclusion derived from our analysis of maspin heterozygous (Mp+/-) mice that showed defective ductal development at puberty. The defect was due to a reduced systemic level of progesterone. We show that treatment of Mp+/- mice with progesterone rescued the defect of ductal development. When both wild-type and Mp+/- mice were ovariectomized at 4 wk of age, treatment with progesterone alone can stimulate their ductal growth. In addition, treatment of wild-type mice with the progesterone inhibitor RU486 slowed ductal development in a dose-dependent manner. To confirm that progesterone receptor (PR) was required for progesterone action in ductal development at pubertal stage, we treated ovariectomized PR-deficient (PRKO) and wild-type mice with progesterone and examined ductal development at 7 wk of age. Whereas wild-type mammary glands displayed abundant ductal growth after progesterone treatment, there was a significant retardation of ductal growth in PRKO mice. Furthermore, we observed reduced ductal development in intact PRKO mice at 7 wk of age compared with that of wild-type mice. However, the defect was rescued at late stage of mammary development in PRKO mice. These data demonstrate that progesterone signaling, which is mediated by PR, plays an important role in early ductal development. In PRKO mice, a compensatory mechanism occurs that rescues the ductal defect at a late stage of mammary development.

Progesterone induction of calcitonin expression in the murine mammary gland

Progesterone, via its nuclear receptor, is mandatory not only for the induction and specification of mammary gland ductal side-branching and lobuloalveologenesis but also for carcinogen-induced mammary tumorigenesis. Notwithstanding these recent advances, a more comprehensive molecular explanation of progesterone-induced mammary morphogenesis is contingent upon the identification and characterization of mammary molecular targets that are responsive to the progesterone signal. Toward this goal, we report that calcitonin, a 32 amino acid peptide hormone involved in calcium homeostasis, is exclusively expressed in, and secreted from, luminal epithelial cells within the mammary gland of the pregnant mouse, and, importantly, its expression is progesterone-dependent. Conversely, the calcitonin receptor is present during all stages of post-natal mammary development examined, is localized to the myoepithelial cell lineage, and is not regulated by progesterone. Because calcitonin induction spatiotemporally correlates with increases in progesterone-induced mammary gland proliferation and structural remodeling, we posit that calcitonin - through its receptor - may be involved in one or both of these progesterone-dependent processes.

Progesterone-dependent regulation of female reproductive activity by two distinct progesterone receptor isoforms

The steroid hormone, progesterone, is a central coordinator of all aspects of female reproductive activity. The physiological effects of progesterone are mediated by interaction of the hormone with specific intracellular progesterone receptors (PRs) that are expressed from a single gene as two protein isoforms and that are members of the nuclear receptor superfamily of transcription factors. Analysis of the structural and functional relationships of each isoform using in vitro systems has demonstrated that the PR-A and PR-B proteins have different transcription activation properties when liganded to progesterone. More recently, selective ablation of the PR-A and PR-B proteins in mice had facilitated examination of the contribution of the individual PR isoforms to the pleiotropic reproductive activities of progesterone. Analysis of the phenotypic consequences of these mutations on female reproductive function has provided proof of concept that the distinct transcriptional responses to PR-A and PR-B observed in cell-based transactivation assays are reflected in a distinct tissue-selective contribution of the individual isoforms to the reproductive activities of progesterone. In PR-A knock-out mice, in which the expression of the PR-A isoform is selectively ablated (PRAKO), the PR-B isoform functions in a tissue-specific manner to mediate a subset of the reproductive functions of PRs. Ablation of PR-A does not affect response of the mammary gland or thymus to progesterone but results in severe abnormalities in ovarian and uterine function leading to female infertility. More recent studies using PR-B knock-out (PRBKO) mice have shown that ablation of PR-B does not affect either ovarian, uterine or thymic responses to progesterone but results in reduced mammary ductal morphogenesis and alveologenesis during pregnancy. Thus, PR-A is both necessary and sufficient to elicit the progesterone-dependent reproductive responses necessary for female fertility, while the PR-B isoform is required to elicit normal proliferative and differentiative responses of the mammary gland to progesterone. This review will summarize our current understanding of the selective contribution of the two PR isoforms to progesterone action.

Progesterone involvement in breast development and tumorigenesis--as revealed by progesterone receptor "knockout" and "knockin" mouse models

In light of recent clinical trials, the debate concerning the risks and benefits of progestin-based postmenopausal hormone replacement therapy (HRT) has reached a renewed level of urgency. Irrespective of the position taken, the consensus is that more basic research needs to be performed to address progesterone's fundamental role in mammary development and tumorigenesis. Towards this end, the progesterone receptor knockout (PRKO) mouse demonstrated that progesterone is essential for pregnancy-associated mammary gland ductal side-branching and alveologenesis and that these morphological changes are dependent on progesterone-induced mammary epithelial proliferation. Importantly, the PRKO mouse showed that the progesterone-proliferative signal significantly contributes to mammary tumor susceptibility in an established mammary tumor model. Insight into the cellular mechanism(s) by which progesterone affects mammary morphogenesis has been disclosed by a new PR-LacZ knockin mouse, which revealed that PR's spatial expression pattern undergoes precise choreographed distributional changes that precede key stages in postnatal mammary development. In the case of early pregnancy, the segregation of cells undergoing progesterone-induced proliferation from those that express PR implicates a paracrine mode of action for progesterone-induced mammary epithelial proliferation, whereas the preparturient decline of PR expression underscores the need to remove this signal for full functional differentiation of this tissue. Our findings support the proposal that the mammary gland's normal response to the progesterone-signal is dependent upon specific spatial organizational patterns of PR expression and that derailment in these cellular processes may contribute to abnormal mammary development, including cancer. This review concludes by emphasizing the need to identify the downstream molecular targets that mediate progesterone's effects in this tissue. Identification of such targets will not only enhance our mechanistic understanding of progesterone's role in mammary development and cancer, but may also facilitate the formulation of new design strategies in breast cancer diagnosis and/or treatment.

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 receptors in mammary gland development and tumorigenesis

The steroid hormone, progesterone (P), is a central coordinator of all aspects of female reproductive activity and plays a key role in pregnancy-associated mammary gland morphogenesis and mammary tumorigenesis. The effects of P on the mammary gland are mediated by two structurally and functionally distinct nuclear receptors PR-A and PR-B that arise from a single gene. Null mutation of both receptors in PR knockout (PRKO) mice has demonstrated a critical role for PRs in mediating pregnancy-associated mammary ductal branching and lobuloalveolar differentiation and in initiation of mammary tumors in response to carcinogen. Analysis of the molecular genetic pathways disrupted in PRKO mice has recently yielded important insights into the molecular mechanisms of regulation of mammary gland morphogenesis by PRs. In addition to its essential role in regulating proliferative and differentiative responses of the adult mammary gland during pregnancy, P plays a critical role in the protection against mammary tumorigenesis afforded by early parity. Thus, the effects of P on postnatal developmental plasticity of the mammary gland differ between young and adult glands. This review will summarize recent advances in our understanding of 1) the molecular mechanisms by which PRs mediate pregnancy-associated mammary gland morphogenesis, 2) the role of PRs in mediating tumorigenic responses of the adult mammary gland to carcinogen, and 3) the role of P in long-term protection of the juvenile mammary gland against tumorigenesis. In addition, we will summarize recent insights into the isoform selective contributions to some of these activities of PRs obtained from comparative analysis of P-dependent mammary gland development in PR isoform specific knockout mice lacking either the PR-A (PRAKO) or PR-B (PRBKO).

Role of the progesterone receptor (PR) in susceptibility of mouse mammary gland to 7,12-dimethylbenz[a]anthracene-induced hormone-independent preneoplastic lesions in vitro

Glands of wild-type (WT) and progesterone receptor knockout (PRKO) mice were exposed to 7,12-dimethylbenz[a]anthracene (DMBA) while cultured in serum-free medium containing insulin, prolactin, aldosterone, and cortisol. Glands of WT but not PRKO mice responded to DMBA with epithelial hyperplasia after 10 days in this medium. After culture without prolactin and adrenocortical hormones for an additional 14 days, hyperplastic lesions were present only in glands of WT mice. We conclude that in the absence of PR, epithelial structures are resistant to the carcinogenic action of DMBA.

Disruption of steroid and prolactin receptor patterning in the mammary gland correlates with a block in lobuloalveolar development

Targeted deletion of the bZIP transcription factor, CCAAT/enhancer binding protein-beta (C/EBPbeta), was shown previously to result in aberrant ductal morphogenesis and decreased lobuloalveolar development, accompanied by an altered pattern of progesterone receptor (PR) expression. Here, similar changes in the level and pattern of prolactin receptor (PrlR) expression were observed while screening for differentially expressed genes in C/EBPbeta(null) mice. PR patterning was also altered in PrlR(null) mice, as well as in mammary tissue transplants from both PrlR(null) and signal transducer and activator of transcription (Stat) 5a/b-deficient mice, with concomitant defects in hormone-induced proliferation. Down-regulation of PR and activation of Stat5 phosphorylation were seen after estrogen and progesterone treatment in both C/EBPbeta(null) and wild-type mice, indicating that these signaling pathways were functional, despite the failure of steroid hormones to induce proliferation. IGF binding protein-5, IGF-II, and insulin receptor substrate-1 all displayed altered patterns and levels of expression in C/EBPbeta(null) mice, suggestive of a change in the IGF signaling axis. In addition, small proline-rich protein (SPRR2A), a marker of epidermal differentiation, and keratin 6 were misexpressed in the mammary epithelium of C/EBPbeta(null) mice. Together, these data suggest that C/EBPbeta is a master regulator of mammary epithelial cell fate and that the correct spatial pattern of PR and PrlR expression is a critical determinant of hormone-regulated cell proliferation.

A novel LacZ reporter mouse reveals complex regulation of the progesterone receptor promoter during mammary gland development

To further our understanding of progesterone (P) as an endocrine mammogen, a PR(lacz) knockin mouse was generated in which the endogenous progesterone receptor (PR) promoter directly regulated lacZ reporter expression. The PR(lacz) mouse revealed PR promoter activity was restricted to the epithelial compartment during the prenatal and postnatal stages of mammary gland development. At puberty, PR promoter activity was unexpectedly robust and restricted to the body cells within the terminal end buds and to the luminal epithelial cells in the subtending ducts. In the adult, the preferential localization of PR(lacz) positive cells to the distal regions of ductal side branches provided a cellular context to the recognized mandatory role of P in ductal side-branching, and segregation of these cells from cells that undergo proliferation supported an intraepithelial paracrine mode of action for P in branching morphogenesis. Toward the end of pregnancy, the PR(lacz) mouse disclosed a progressive attenuation in PR promoter activity, supporting the postulate that the preparturient removal of the proliferative signal of P is a prerequisite for the emergence of a functional lactating mammary gland. The data suggest that PR expression before pregnancy is to ensure the specification and spatial organization of ductal and alveolar progenitor cell lineages, whereas abrogation of PR expression before lactation is required to enable terminal differentiation of the mammary gland.

Reproductive functions of progesterone receptors

The steroid hormone progesterone plays a central role in the reproductive events associated with pregnancy establishment and maintenance. Physiological effects of progesterone are mediated by interaction of the hormone with specific intracellular progesterone receptors (PRs) that are expressed as two protein isoforms, PR-A and PR-B. Both proteins arise from the same gene and are members of the nuclear receptor superfamily of transcription factors. Since these two isoforms were identified in the early 1970s, extensive controversy has existed regarding the selective contributions of the individual PR proteins to the physiological functions of progesterone. During the past decade, significant progress has been made in this regard using two complimentary approaches. First, analysis of the structural and functional relationships of each isoform using in vitro systems has generated compelling evidence to support the conclusion that PR-A and PR-B have different transcription activation properties when liganded to progesterone. Second, the advent of gene-targeting approaches to introduce subtle mutations into the mouse genome has facilitated the evaluation of the significance of observations made in vitro in a physiological context. Selective ablation of PR-A and PR-B proteins in mice using these technologies has allowed us to address the spatiotemporal expression and contribution of the individual PR isoforms to the pleiotropic reproductive activities of progesterone. Analysis of the phenotypic consequences of these mutations on female reproductive function has provided proof of concept that the distinct transcriptional responses to PR-A and PR-B observed in cell-based transactivation assays are, indeed, reflected in an ability of the individual isoforms to elicit distinct, physiological responses to progesterone. In PR-A knockout mice, in which the expression of the PR-A isoform is selectively ablated (PRAKO), the PR-B isoform functions in a tissue-specific manner to mediate a subset of the reproductive functions of PRs. Ablation of PR-A does not affect responses of the mammary gland or thymus to progesterone but instead results in severe abnormalities in ovarian and uterine function, leading to female infertility. These tissue-selective activities of PR-B are due to this isoform's ability to regulate a subset of progesterone-responsive target genes in reproductive tissues rather than to differences in its spatiotemporal expression relative to the PR-A isoform. More recent studies using PR-B knockout (PRBKO) mice have shown that ablation of PR-B does not affect ovarian, uterine, or thymic responses to progesterone but rather results in reduced mammary ductal morphogenesis. Thus, PR-A is both necessary and sufficient to elicit the progesterone-dependent reproductive responses necessary for female fertility, while PR-B is required to elicit normal proliferative responses of the mammary gland to progesterone. This chapter will summarize recent progress in our understanding of the selective contribution of the two PR isoforms to progesterone action.

Progesterone's role in mammary gland development and tumorigenesis as disclosed by experimental mouse genetics

The progesterone receptor knockout mouse demonstrated progesterone's importance to parity-induced mammary tertiary branching and lobuloalveologenesis. Because early parity provides significant protection against breast cancer whereas prolonged exposure to premenopausal ovarian progesterone (or to postmenopausal supplementations thereof) has been linked to breast cancer risk, this steroid can be considered to exhibit contrasting roles in breast cancer etiology. This review describes the important mouse models that have contributed to our understanding of progesterone's role in mammary gland development and neoplasia. We conclude by emphasising the urgent need to identify the molecular targets of the progesterone receptor, and to determine whether these targets are modulated differently by the progesterone receptor isoforms (A and B) during mammary morphogenesis and tumorigenesis.

A complex role for the progesterone receptor in the response to vascular injury

Clinical studies of hormone replacement therapy to prevent cardiovascular diseases have heightened interest in the cardiovascular effects of progestins. However, the role of the progesterone receptor (PR) in vascular biology has not been studied in vivo. We studied ovariectomized female PR knockout (PRKO) mice and their wild-type (WT) littermates using the mouse carotid artery injury model. Placebo-treated PRKO mice showed significantly greater vascular medial hypertrophy and vascular smooth muscle cell (VSMC) proliferation in response to vascular injury than did WT mice. Progesterone had no significant effect in the PRKO mice, but worsened the response to injury in WT mice. VSMCs cultured from PRKO mouse aortae were markedly hyperproliferative, and their growth was not affected by progesterone. In contrast to the in vivo findings, progesterone inhibited proliferation of WT-derived VSMCs. Furthermore, reintroduction of PR into PRKO-derived VSMCs using adenoviral methods restored progesterone-mediated inhibition of proliferation to these cells. This effect was reversed by the PR antagonist, RU 486. Thus, the effects of PR and progesterone differ markedly between cultured VSMCs and intact blood vessels. These data demonstrate a direct role for the PR in regulating the response to vascular injury and VSMC proliferation.

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.

Paracrine regulation of apoptosis by steroid hormones in the male and female reproductive system

In males, androgens are essential in maintaining the integrity of the prostate. Androgen-ablation induces apoptosis of the prostatic epithelium. In females, ovariectomy induces apoptosis in uterine epithelium while progesterone inhibits this process. The objective of this study was to determine whether androgen and progesterone inhibit apoptosis, respectively, in mouse prostatic and uterine epithelia via steroid receptors in the epithelium or in the stroma. To address this question, prostatic tissue recombinants were prepared with rat urogenital sinus mesenchyme plus bladder epithelium from wild-type or testicular feminization mutant (Tfm) mice. Thus, prostatic tissue was generated having androgen receptor (AR) in both epithelium and stroma or in the stroma only. Castration of hosts induced apoptosis in the AR-negative Tfm prostatic epithelium with an epithelial apoptotic index virtually identical to prostatic tissue recombinants containing wild-type epithelium. Moreover, this castration-induced prostatic epithelial apoptosis was blocked by testosterone and dihydrotestosterone in both wild-type and Tfm prostatic tissue recombinants. Likewise, uterine tissue recombinants were prepared in which epithelium and/or stroma was devoid of progesterone receptor (PR) by using uterine epithelium and stroma of wild-type and PR knockout mice. Progesterone inhibited uterine epithelial apoptosis only in tissue recombinants prepared with PR-positive stroma. The PR status of the epithelium did not affect epithelial apoptotic index. Therefore, the apoptosis in prostatic and uterine epithelia is regulated by androgen and progesterone via stromal AR and PR, respectively. In both cases, epithelial AR or PR is not required for hormonal regulation of epithelial apoptosis in prostatic and uterine epithelium.

A membrane-associated progesterone-binding protein, 25-Dx, is regulated by progesterone in brain regions involved in female reproductive behaviors

The ventromedial hypothalamus (VMH) plays a central role in the regulation of the female reproductive behavior lordosis, a behavior dependent upon the sequential activation of receptors for the ovarian steroid hormones estradiol (E) and progesterone (P). These receptors function as transcription factors to alter the expression of target genes. To discover behaviorally relevant genes targeted by E and P in the VMH, we used the differential display PCR to identify messenger RNAs that are differentially expressed in the hypothalamus of ovariectomized (ovx) rats treated with E alone compared with ovariectomized rats treated with E and P. We show here that one interesting mRNA within the hypothalamus that is repressed by P after E priming encodes the protein 25-Dx, the rat homolog of the human membrane-associated P-binding protein Hpr6.6. Neurons in the brain containing the highest levels of 25-Dx are located in several nuclei of the basal forebrain, including the VMH. 25-Dx expression is also higher in the hypothalamus of female P receptor "knockout" mice than in their wild-type littermates. These findings suggest a mechanism in which the activation of nuclear P receptor represses expression of a membrane P receptor, 25-Dx, during lordosis facilitation.

Progesterone receptors in reproduction: functional impact of the A and B isoforms

Progesterone (P) is a key regulator of female reproductive activity. The effects of P are mediated by two progesterone receptor (PR) proteins, termed A and B, that arise from a single gene and act as ligand-activated transcription factors to regulate the expression of reproductive target genes. Null mutation of the PR gene in mice (PRKO) leads to pleiotropic reproductive abnormalities. This paper will review the reproductive functions of PRs delineated using the PRKO mouse. Further, we will summarize the structure and functional properties of PRs and discuss how functional differences between the PR-A and PR-B isoforms are likely to impact on the overall physiological role of the receptor in reproductive systems.

Ovulation: a multi-gene, multi-step process

The luteinizing hormone (LH) surge initiates a cascade of proteolytic events that control ovulation. One of the genes induced by LH is the progesterone receptor (PR). Because mice with a mutant PR gene (PRKO) fail to ovulate and are infertile, we have used them as a model in which to determine PR target genes that might mediate the ovulatory process. The matrix metalloproteinases (MMPs: MMP2, MMP9, and MMP13) appear to be expressed in ovaries of PRKO mice in a manner similar to that in their wild-type littermates. However, the expression of two other types of proteases, cathepsin L (a member of the papain family) and ADAMTS-1 (A Disintegrin And Metalloproteinase with Thrombospondin-like motifs), are selectively induced in granulosa cells of preovulatory follicles by the LH surge. Maximal levels of these proteases are observed at 12-16 h after an LH surge, the time of ovulation. Furthermore, mRNAs encoding cathepsin L and ADAMTS-1 are reduced in the PRKO mice compared to their wild-type littermates. These novel observations indicate that these two proteases regulate some key step(s) controlling ovulation.