Transcriptional activation of the membrane-bound progesterone receptor (mPR) by dioxin, in endocrine-responsive tissues

Pleiotropy of Progesterone Receptor Membrane Component 1 in Modulation of Cytochrome P450 Activity

Progesterone receptor membrane component 1 (PGRMC1) is one of few proteins that have been recently described as direct modulators of the activity of human cytochrome P450 enzymes (CYP)s. These enzymes form a superfamily of membrane-bound hemoproteins that metabolize a wide variety of physiological, dietary, environmental, and pharmacological compounds. Modulation of CYP activity impacts the detoxification of xenobiotics as well as endogenous pathways such as steroid and fatty acid metabolism, thus playing a central role in homeostasis. This review is focused on nine main topics that include the most relevant aspects of past and current PGRMC1 research, focusing on its role in CYP-mediated drug metabolism. Firstly, a general overview of the main aspects of xenobiotic metabolism is presented (I), followed by an overview of the role of the CYP enzymatic complex (IIa), a section on human disorders associated with defects in CYP enzyme complex activity (IIb), and a brief account of cytochrome b5 (cyt b5)'s effect on CYP activity (IIc). Subsequently, we present a background overview of the history of the molecular characterization of PGRMC1 (III), regarding its structure, expression, and intracellular location (IIIa), and its heme-binding capability and dimerization (IIIb). The next section reflects the different effects PGRMC1 may have on CYP activity (IV), presenting a description of studies on the direct effects on CYP activity (IVa), and a summary of pathways in which PGRMC1's involvement may indirectly affect CYP activity (IVb). The last section of the review is focused on the current challenges of research on the effect of PGRMC1 on CYP activity (V), presenting some future perspectives of research in the field (VI).

Glycyrrhizin Derivatives Suppress Cancer Chemoresistance by Inhibiting Progesterone Receptor Membrane Component 1

Progesterone receptor membrane component 1 (PGRMC1) is highly expressed in various cancer cells and contributes to tumor progression. We have previously shown that PGRMC1 forms a unique heme-stacking functional dimer to enhance EGF receptor (EGFR) activity required for cancer proliferation and chemoresistance, and the dimer dissociates by carbon monoxide to attenuate its biological actions. Here, we determined that glycyrrhizin (GL), which is conventionally used to ameliorate inflammation, specifically binds to heme-dimerized PGRMC1. Binding analyses using isothermal titration calorimetry revealed that some GL derivatives, including its glucoside-derivative (GlucoGL), bind to PGRMC1 potently, whereas its aglycone, glycyrrhetinic acid (GA), does not bind. GL and GlucoGL inhibit the interaction between PGRMC1 and EGFR, thereby suppressing EGFR-mediated signaling required for cancer progression. GL and GlucoGL significantly enhanced EGFR inhibitor erlotinib- or cisplatin (CDDP)-induced cell death in human colon cancer HCT116 cells. In addition, GL derivatives suppressed the intracellular uptake of low-density lipoprotein (LDL) by inhibiting the interaction between PGRMC1 and the LDL receptor (LDLR). Effects on other pathways cannot be excluded. Treatment with GlucoGL and CDDP significantly suppressed tumor growth following xenograft transplantation in mice. Collectively, this study indicates that GL derivatives are novel inhibitors of PGRMC1 that suppress cancer progression, and our findings provide new insights for cancer treatment.

Sex Hormone-Dependent Physiology and Diseases of Liver

Sexual dimorphism is associated not only with somatic and behavioral differences between men and women, but also with physiological differences reflected in organ metabolism. Genes regulated by sex hormones differ in expression in various tissues, which is especially important in the case of liver metabolism, with the liver being a target organ for sex hormones as its cells express estrogen receptors (ERs: ERα, also known as ESR1 or NR3A; ERβ; GPER (G protein-coupled ER, also known as GPR 30)) and the androgen receptor (AR) in both men and women. Differences in sex hormone levels and sex hormone-specific gene expression are mentioned as some of the main variations in causes of the incidence of hepatic diseases; for example, hepatocellular carcinoma (HCC) is more common in men, while women have an increased risk of autoimmune liver disease and show more acute liver failure symptoms in alcoholic liver disease. In non-alcoholic fatty liver disease (NAFLD), the distinction is less pronounced, but increased incidences are suggested among men and postmenopausal women, probably due to an increased tendency towards visceral fat accumulation.

Arabidopsis GAAPs interacting with MAPR3 modulate the IRE1-dependent pathway upon endoplasmic reticulum stress

Cell viability requires the maintenance of intracellular homeostasis through the unfolded protein response mediated by receptors localized on the endoplasmic reticulum (ER) membrane. The receptor IRE1 mediates not only various adaptive outputs but also programmed cell death (PCD) under varying stress levels. However, little is known about the mechanism by which the same receptors trigger different responses in plants. Arabidopsis Golgi anti-apoptotic protein 1 (GAAP1) and GAAP3 resist PCD upon ER stress and negatively modulate the adaptive response of the IRE1-bZIP60 pathway through IRE1 association. To elucidate the mechanism underlying the anti-PCD activity of GAAPs, we attempted to isolate interactors of GAAPs by yeast two-hybrid screening. Membrane-associated progesterone receptor 3 (MAPR3) was isolated as one of the factors interacting with GAAP. Mutations in GAAP1/GAAP3 and/or MAPR3 enhanced the sensitivity of seedlings to ER stress. Whole-transcriptome analysis combined with quantitative reverse transcription-PCR and cellular analysis showed that regulated IRE1-dependent decay (RIDD) and autophagy were impaired in mutants mapr3, gaap1mapr3, and gaap3mapr3. MAPR3, GAAP1, and GAAP3 interacted with IRE1B as determined by protein interaction assays. These data suggest that the interaction of GAAP1/GAAP3 with MAPR3 mitigates ER stress to some extent through regulating IRE10-mediated RIDD and autophagy.

Sex dimorphic regulation of osteoprogenitor progesterone in bone stromal cells

Increasing peak bone mass is a promising strategy to prevent osteoporosis. A mouse model of global progesterone receptor (PR) ablation showed increased bone mass through a sex-dependent mechanism. Cre-Lox recombination was used to generate a mouse model of osteoprogenitor-specific PR inactivation, which recapitulated the high bone mass phenotype seen in the PR global knockout mouse mode. In this work, we employed RNA sequencing analysis to evaluate sex-independent and sex-dependent differences in gene transcription of osteoprogenitors of wild-type and PR conditional knockout mice. PR deletion caused marked sex hormone-dependent changes in gene transcription in male mice as compared to wild-type controls. These transcriptional differences revealed dysregulation in pathways involving immunomodulation, osteoclasts, bone anabolism, extracellular matrix interaction and matrix interaction. These results identified many potential mechanisms that may explain our observed high bone mass phenotype with sex differences when PR was selectively deleted in the MSCs.

Sex Hormones and Their Receptors Regulate Liver Energy Homeostasis

The liver is one of the most essential organs involved in the regulation of energy homeostasis. Hepatic steatosis, a major manifestation of metabolic syndrome, is associated with imbalance between lipid formation and breakdown, glucose production and catabolism, and cholesterol synthesis and secretion. Epidemiological studies show sex difference in the prevalence in fatty liver disease and suggest that sex hormones may play vital roles in regulating hepatic steatosis. In this review, we summarize current literature and discuss the role of estrogens and androgens and the mechanisms through which estrogen receptors and androgen receptors regulate lipid and glucose metabolism in the liver. In females, estradiol regulates liver metabolism via estrogen receptors by decreasing lipogenesis, gluconeogenesis, and fatty acid uptake, while enhancing lipolysis, cholesterol secretion, and glucose catabolism. In males, testosterone works via androgen receptors to increase insulin receptor expression and glycogen synthesis, decrease glucose uptake and lipogenesis, and promote cholesterol storage in the liver. These recent integrated concepts suggest that sex hormone receptors could be potential promising targets for the prevention of hepatic steatosis.

Membrane progesterone receptor expression in mammalian tissues: a review of regulation and physiological implications

The recent discovery of a novel, membrane localized progestin receptor (mPR) unrelated to the classical progesterone receptor (PR) in fishes and its subsequent identification in mammals suggests a potential mediator of non-traditional progestin actions, particularly in tissues where PR is absent. While early studies on mPR focused on final oocyte maturation in fishes, more current studies have examined mPRs in multiple mammalian systems in both reproductive and non-reproductive tissues as well as in diseased tissues. Here we review the current data on mPR in mammalian systems including male and female reproductive tracts, liver, neuroendocrine tissues, the immune system and breast and ovarian cancer. We also provide new data demonstrating mPR expression in the RAW 264.7 immune cell line and bone marrow-derived macrophages as well as mPR expression and downstream gene regulation in ovarian cancer cells.

PGRMC1 (progesterone receptor membrane component 1): a targetable protein with multiple functions in steroid signaling, P450 activation and drug binding

Hormone signaling is important in a number of disease states, and hormone receptors are effective therapeutic targets. PGRMC1 (progesterone receptor membrane component 1) is a member of a multi-protein complex that binds to progesterone and other steroids, as well as pharmaceutical compounds. In spite of its name, PGRMC1 shares homology with cytochrome b5-related proteins rather than hormone receptors, and heme binding is the sole biochemical activity of PGRMC1. PGRMC1 and its homologues regulate cholesterol synthesis by activating the P450 protein Cyp51/lanosterol demethylase, and the cholesterol synthetic pathway is an important target in cardiovascular disease and in treating infections. PGRMC1 binding partners include multiple P450 proteins, PAIR-BP1, Insig, and an uncharacterized hormone/drug-binding protein. PGRMC1 is induced in a spectrum of cancers, where it promotes cell survival and damage resistance, and PGRMC1 is also expressed in the nervous system and tissues involved in drug metabolism, cholesterol synthesis and hormone synthesis and turnover. One of the appealing features of PGRMC1 and its associated protein complex is its affinity for steroids and drugs. Together with its biological role in promoting tumor survival, PGRMC1 is an attractive target for therapeutic intervention in cancer and related malignancies.

Characteristics of membrane progestin receptor alpha (mPRalpha) and progesterone membrane receptor component 1 (PGMRC1) and their roles in mediating rapid progestin actions

Rapid, progestin actions initiated at the cell surface that are often nongenomic have been described in a variety of reproductive tissues, but until recently the identities of the membrane receptors mediating these nonclassical progestins actions remained unclear. Evidence has been obtained in the last 4-5 years for the involvement of two types of novel membrane proteins unrelated to nuclear steroid receptors, progesterone membrane receptors (mPRs) and progesterone receptor membrane component 1 (PGMRC1), in progestin signaling in several vertebrate reproductive tissues and in the brain. The mPRs, (M(W) approximately 40 kDa) initially discovered in fish ovaries, comprise at least three subtypes, alpha, beta and gamma and belong to the seven-transmembrane progesterone adiponectin Q receptor (PAQR) family. Both recombinant and wildtype mPRs display high affinity (K(d) approximately 5 nM), limited capacity, displaceable and specific progesterone binding. The mPRs are directly coupled to G proteins and typically activate pertussis-sensitive inhibitory G proteins (G(i)), to down-regulate adenylyl cyclase activity. Recent studies suggest the alpha subtype (mPRalpha) has important physiological functions in variety of reproductive tissues. The mPRalpha is an intermediary in progestin induction of oocyte maturation and stimulation of sperm hypermotility in fish. In mammals, the mPRalphas have been implicated in progesterone regulation of uterine function in humans and GnRH secretion in rodents. The single-transmembrane protein PGMRC1 (M(W) 26-28 kDa) was first purified from porcine livers and its cDNA was subsequently cloned from porcine smooth muscle cells and a variety of other tissues by different investigators. PGMRC1 and the closely-related PGMRC2 belong to the membrane-associated progesterone receptor (MAPR) family. The PGMRC1 protein displays moderately high binding affinity for progesterone which is 2- to 10-fold greater than that for testosterone and glucocorticoids, and also can bind to other molecules such as heme, cholesterol metabolites and proteins. The signal transduction pathways induced by binding of progesterone to PGMRC1 have not been described to date, although motifs for tyrosine kinase, kinase binding, SH2 and SH3 have been predicted from the amino acid sequence. Evidence has been obtained that PGMRC1 mediates the antiapoptotic affects of progesterone in rat granulosa cells. The PGMRC1 protein may also be an intermediary in the progesterone induction of the acrosome reaction in mammalian sperm. Despite these recent advances, many aspects of progestin signaling through these two families of novel membrane proteins remain unresolved. Biochemical characterization of the receptors has been hampered by rapid degradation of the partially purified proteins. A major technical challenge has been to express sufficient amounts of the recombinant receptors on the plasma membranes in eukaryotic systems to permit investigations of their progestin binding and signal transduction characteristics. Additional basic information on the molecular and cellular mechanisms by which mPRs and PGMRC1 interact with progestins, signal transductions pathways and other proteins will be required to establish a comprehensive model of nontraditional progestin actions mediated through these novel proteins.

Involvement of a specificity proteins-binding element in regulation of basal and estrogen-induced transcription activity of the BRCA1 gene

INTRODUCTION

Increased estrogen level has been regarded to be a risk factor for breast cancer. However, estrogen has also been shown to induce the expression of the tumor suppressor gene, BRCA1. Upregulation of BRCA1 is thought to be a feedback mechanism for controlling DNA repair in proliferating cells. Estrogens enhance transcription of target genes by stimulating the association of the estrogen receptor (ER) and related coactivators to estrogen response elements or to transcription complexes formed at activator protein (AP)-1 or specificity protein (Sp)-binding sites. Interestingly, the BRCA1 gene lacks a consensus estrogen response element. We previously reported that estrogen stimulated BRCA1 transcription through the recruitment of a p300/ER-alpha complex to an AP-1 site harbored in the proximal BRCA1 promoter. The purpose of the study was to analyze the contribution of cis-acting sites flanking the AP-1 element to basal and estrogen-dependent regulation of BRCA1 transcription.

METHODS

Using transfection studies with wild-type and mutated BRCA1 promoter constructs, electromobility binding and shift assays, and DNA-protein interaction and chromatin immunoprecipitation assays, we investigated the role of Sp-binding sites and cAMP response element (CRE)-binding sites harbored in the proximal BRCA1 promoter.

RESULTS

We report that in the BRCA1 promoter the AP-1 site is flanked upstream by an element (5'-GGGGCGGAA-3') that recruits Sp1, Sp3, and Sp4 factors, and downstream by a half CRE-binding motif (5'-CGTAA-3') that binds CRE-binding protein. In ER-alpha-positive MCF-7 cells and ER-alpha-negative Hela cells expressing exogenous ER-alpha, mutation of the Sp-binding site interfered with basal and estrogen-induced BRCA1 transcription. Conversely, mutation of the CRE-binding element reduced basal BRCA1 promoter activity but did not prevent estrogen activation. In combination with the AP-1/CRE sites, the Sp-binding domain enhanced the recruitment of nuclear proteins to the BRCA1 promoter. Finally, we report that the MEK1 (mitogen-activated protein kinase kinase-1) inhibitor PD98059 attenuated the recruitment of Sp1 and phosphorylated ER-alpha, respectively, to the Sp and AP-1 binding element.

CONCLUSION

These cumulative findings suggest that the proximal BRCA1 promoter segment comprises cis-acting elements that are targeted by Sp-binding and CRE-binding proteins that contribute to regulation of BRCA1 transcription.

The membrane-associated progesterone-binding protein 25-Dx: Expression, cellular localization and up-regulation after brain and spinal cord injuries

Progesterone has neuroprotective effects in the injured and diseased spinal cord and after traumatic brain injury (TBI). In addition to intracellular progesterone receptors (PR), membrane-binding sites of progesterone may be involved in neuroprotection. A first putative membrane receptor of progesterone, distinct from the classical intracellular PR isoforms, with a single membrane-spanning domain, has been cloned from porcine liver. Homologous proteins were cloned in rats (25-Dx), mice (PGRMC1) and humans (Hpr.6). We will refer to this progesterone-binding protein as 25-Dx. The distribution and regulation of 25-Dx in the nervous system may provide some clues to its functions. In spinal cord, 25-Dx is localized in cell membranes of dorsal horn neurons and ependymal cells lining the central canal. A role of 25-Dx in mediating the protective effects of progesterone in the spinal cord is supported by the observation that its mRNA and protein are up-regulated by progesterone in dorsal horn of the injured spinal cord. In contrast, the classical intracellular PRs were down-regulated under these conditions. In brain, 25-Dx is particularly abundant in the hypothalamic area, circumventricular organs, ependymal cells of the ventricular walls, and the meninges. Interestingly, it is co-expressed with vasopressin in neurons of the paraventricular, supraoptic and retrochiasmatic nuclei. In response to TBI, 25-Dx expression is up-regulated in neurons and induced in astrocytes. The expression of 25-Dx in structures involved in cerebrospinal fluid production and osmoregulation, and its up-regulation after brain damage, point to a potentially important role of this progesterone-binding protein in the maintenance of water homeostasis after TBI. Our observations suggest that progesterone's actions may involve different signaling mechanisms depending on the pathophysiological context, and that 25-Dx may be involved in the neuroprotective effect of progesterone in the injured brain and spinal cord.

Progesterone receptor membrane component 1: an integrative review

Progesterone receptor membrane component 1 (PGRMC1) contains a cytochrome b5 domain fold and belongs to the so-called membrane-associated progesterone receptor (MAPR) protein family that is widespread in eukaryotes. PGRMC1 and the related PGRMC2 mammalian family member diverged sometime after the evolution of segmented metazoan body plan and the appearance of vertebrates. Therefore PGRMC1 might be expected to be involved in some ancient eukaryotic processes, as well as more modern functions related to multicellularity and tissue interactions. Perhaps this explains the perplexing diversity of contexts where PGRMC1 has been observed, apparently being involved in different cellular processes at various sub-cellular locations. This review attempts to collate and interpret these observations. Ironically, despite being the archetypal member of the MAPR family, it has yet to be demonstrated that PGRMC1 exhibits specific progesterone binding. Potential roles of heme and steroid/sterol ligands are reviewed, as well as the implications of apparent target sequences within PGRMC1 for binding by SH2- and SH3-domain proteins as well as kinases. These motifs are modelled using the cytochrome b5 domain NMR structure of the Arabidopsis protein 1J03, implicating a possible function for PGRMC1 as an adaptor protein involved in regulating protein interactions and intracellular signal transduction and/or membrane trafficking. This interpretation is supported by the apparent presence of immunoreceptor tyrosine-based activation motif/ITAM sequences that are involved in endocytosis and vesicle targeting, and the colocalisation of PGRMC1 with caveolin and at the cytoplasmic membrane. Evidence for roles in disease, especially cancer, is also discussed.

Unexpected diversity of aryl hydrocarbon receptors in non-mammalian vertebrates: insights from comparative genomics

Ligand-activated receptors are well-known targets of environmental chemicals that disrupt endocrine signaling. Genomic approaches are providing new opportunities to understand the comparative biology and molecular evolution of these receptors. One example of this is the aryl hydrocarbon receptor (AHR), a basic-helix-loop-helix (bHLH)-Per-Arnt-Sim (PAS) transcription factor through which planar aromatic hydrocarbons cause altered gene expression and toxicity. In contrast to humans and other mammals, which possess a single AHR, teleosts such as the Atlantic killifish (Fundulus heteroclitus) have at least two AHRs (AHR1 and AHR2). Analysis of sequenced genomes has revealed additional, unexpected AHR diversity in non-mammalian vertebrates, including the chicken Gallus gallus (three predicted AHR genes), bony fishes such as the pufferfish Takifugu (formerly Fugu) rubripes (five AHR genes) and zebrafish Danio rerio (three AHR genes), and cartilaginous fishes such as the spiny dogfish Squalus acanthias (three AHR genes). In contrast, invertebrates appear to possess single AHRs that do not bind typical ligands of vertebrate AHRs. We suggest that AHR diversity in vertebrates arose through both gene and whole-genome duplications combined with lineage-specific gene loss, and that sensitivity to the developmental toxicity of planar aromatic hydrocarbons may have had its origin in the evolution of the ligand-binding capacity of the AHR in the chordate lineage. Comparative molecular and genomic studies are providing new insights into AHR diversity and function in non-mammalian species, revealing additional complexity in mechanisms by which environmental chemicals interfere with receptor-dependent signaling.

Induction of the transferrin receptor gene by benzo[a]pyrene in breast cancer MCF-7 cells: potential as a biomarker of PAH exposure

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental DNA-damaging agents regarded as risk factors for human disease, including lung and breast cancer. The biotransformation of PAHs to carcinogenic metabolites is mediated by the aromatic hydrocarbon receptor (AhR), which activates transcription at xenobiotic responsive elements (XREs = 5'-GCGTG-3') found in the promoter regions of genes encoding for detoxifying enzymes, including CYP1A1 and CYP1B1. In this study, we wished to identify novel biomarkers that may be useful in monitoring critical carcinogenic events of the breast induced by PAHs. Using a GeneMAP CancerArray, we analyzed in breast cancer MCF-7 cells the temporal effects of the AhR agonist benzo[a]pyrene (B[a]P), which is a prototype PAH and known environmental carcinogen. Genes upregulated at least threefold by B[a]P and containing potential XREs within their promoter regions included CYP1A1, CYP1B1, paired box gene 3 (PAX3), cortactin (CTTN/EMS1), beta-2-microglobulin (B2M), and transferrin receptor (TfR). The stimulatory effects of B[a]P on expression of these genes were abrogated by cotreatment with the AhR antagonist flavonoid, alpha-napthoflavone (ANF). The TfR gene was selected for further analysis as its promoter region contains two potential XREs and its expression has been shown to be increased in breast cancer cells. Accumulation of TfR mRNA in B[a]P-treated cells was confirmed by quantitative real time PCR. Transient transfection studies indicated that the transcriptional activity of the TfR promoter was stimulated by B[a]P, whereas ANF counteracted this induction. These results indicate that the TfR gene may be a potential biomarker of PAH exposure.

The ligand status of the aromatic hydrocarbon receptor modulates transcriptional activation of BRCA-1 promoter by estrogen

In sporadic breast cancers, BRCA-1 expression is down-regulated in the absence of mutations in the BRCA-1 gene. This suggests that disruption of BRCA-1 expression may contribute to the onset of mammary tumors. Environmental contaminants found in industrial pollution, tobacco smoke, and cooked foods include benzo(a)pyrene [B(a)P] and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which have been shown to act as endocrine disruptors and tumor promoters. In previous studies, we documented that estrogen (E2) induced BRCA-1 transcription through the recruitment of an activator protein-1/estrogen receptor-alpha (ER alpha) complex to the proximal BRCA-1 promoter. Here, we report that activation of BRCA-1 transcription by E2 requires occupancy of the BRCA-1 promoter by the unliganded aromatic hydrocarbon receptor (AhR). The stimulatory effects of E2 on BRCA-1 transcription are counteracted by (a) cotreatment with the AhR antagonist 3'-methoxy-4'-nitroflavone; (b) transient expression in ER alpha-negative HeLa cells of ER alpha lacking the protein-binding domain for the AhR; and (c) mutation of two consensus xenobiotic-responsive elements (XRE, 5'-GCGTG-3') located upstream of the ER alpha-binding region. These results suggest that the physical interaction between the unliganded AhR and the liganded ER alpha plays a positive role in E2-dependent activation of BRCA-1 transcription. Conversely, we show that the AhR ligands B(a)P and TCDD abrogate E2-induced BRCA-1 promoter activity. The repressive effects of TCDD are paralleled by increased recruitment of the liganded AhR and HDAC1, reduced occupancy by p300, SRC-1, and diminished acetylation of H4 at the BRCA-1 promoter region flanking the XREs. We propose that the ligand status of the AhR modulates activation of the BRCA-1 promoter by estrogen.