Characterization of the adrenal-specific antigen IZA (inner zone antigen) and its role in the steroidogenesis

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.

Breast cancer proteomics by laser capture microdissection, sample pooling, 54-cm IPG IEF, and differential iodine radioisotope detection

The presence of progesterone receptor (PR) in estrogen receptor (ER)-positive breast cancer is associated with a good prognosis, and indicates that tumors are likely to respond to tamoxifen. However, ER+/PR- tumors respond less well. To reveal the potential molecular mechanism of this phenomenon, we sought to identify differential protein abundances between invasive ductal carcinoma cells from cryopreserved ER+/PR+ and ER+/PR- mammary tumor specimens. Because current proteomics methods are hampered in the examination of most primary human tumor samples by the extreme tissue heterogeneity, we used laser capture microdissection (LCM) to isolate tumor cells and developed a sample pooling strategy to analyze small sample protein lysates. Proteins from LCM-harvested tumors were pooled into four sub-pools from each condition of three tumors/sub-pool, and proteins from respective paired sub-pools were co-electrophoresed by 2-DE using 54-cm IEF over pH 4-9. Abundance ratios were accurately quantified by a differential multiplex radioactive ProteoTope method at low attomole levels ( approximately 3.6 microg protein per labeling reaction, <180 ng per multiplex protein sample per 54-cm gel). Applying this approach, differentially displayed proteins were identified by MS using comigrating non-radioactively labeled tumor proteins. They include decreased cytochrome b5 and transgelin, and more abundant CRABP-II, cyclophilin A, Neudesin, and hemoglobin in ER+/PR+ tumors versus ER+/PR- providing a possible explanation for differential susceptibility against tamoxifen as a result of deregulated cytochrome b5-dependent metabolism. This study demonstrates the potential of ProteoTope and LCM to enable extremely sensitive and precise differential analyses from well-defined primary clinical specimen.

Progesterone-induced neuroprotection

Estrogen and progesterone are two steroid hormones whose biology has been greatly studied within the confines of reproductive function. As a consequence, the effects of these hormones on the brain have focused primarily on the hypothalamus. Growing evidence, however, forces us to recognize that various extrahypothalamic brain regions, including the cerebral cortex and hippocampus, are equally important targets of these hormones. As such, hormones are involved in numerous aspects of brain function, and elicit effects ranging from the regulation of mood and cognition to the regulation of neuronal survival. While estrogen exerts neuroprotective effects in various experimental models, the potential for progesterone as a protective agent has, until recently, been greatly understudied. Here, we review the data from various laboratories including our own that support the protective role of progesterone and describe the multiplicity of mechanisms by which progesterone elicits these protective effects. Finally, we contrast the neurobiology of progesterone with that of the clinically used progestin, medroxyprogesterone acetate (MPA), and suggest that the "natural" progesterone may be the better choice when considering which progestin to use for future therapeutic/ palliative purposes in CNS-related disorders.

Spectroscopic and biochemical characterization of heme binding to yeast Dap1p and mouse PGRMC1p

Yeast damage-associated response protein (Dap1p) and mouse progesterone receptor membrane component-1 protein (mPGRMC1p) belong to a highly conserved class of putative membrane-associated progesterone binding proteins (MAPR), with Dap1p and inner zone antigen (IZA), the rat homologue of mPGRMC1p, recently being reported to bind heme. While primary structure analysis reveals similarities to the cytochrome b(5) motif, neither of the two axial histidines responsible for ligation to the heme is present in any of the MAPR proteins. In this paper, EPR, MCD, CD, UV-vis, and general biochemical methods have been used to characterize the nature of heme binding in both Dap1p and a His-tagged, membrane anchor-truncated mPGRMC1p. As isolated, Dap1p is a tetramer which can be converted to a dimer upon addition of 150 mM salt. The heme is noncovalently attached, with a maximal, in vitro, heme loading of approximately 30%, for both proteins. CD and fluorescence spectroscopies indicate a well-ordered structure, suggesting the low level of heme loading is probably not due to improperly folded protein. EPR confirmed a five-coordinate, high-spin, ferric resting state for both proteins, indicating one axial amino acid ligand, in contrast to the six-coordinate, low-spin, ferric state of cytochrome b(5). The MCD spectrum confirmed this conclusion for Dap1p and indicated the axial ligand is most likely a tyrosine and not a histidine, or a cysteine; however, an aspartic acid residue could not be conclusively ruled out. Potential axial ligands, which are conserved in all MAPRs, were mutated (Y78F, D118A, and Y138F) and purified to homogeneity. The Y78F and D118A mutants were found to bind heme; however, Y138F did not. This result is consistent with the MCD data and indicates that Tyr138 is most likely the axial ligand to the heme in Dap1p.

Two unrelated putative membrane-bound progestin receptors, progesterone membrane receptor component 1 (PGMRC1) and membrane progestin receptor (mPR) beta, are expressed in the rainbow trout oocyte and exhibit similar ovarian expression patterns

BACKGROUND

In lower vertebrates, steroid-induced oocyte maturation is considered to involve membrane-bound progestin receptors. Two totally distinct classes of putative membrane-bound progestin receptors have been reported in vertebrates. A first class of receptors, now termed progesterone membrane receptor component (PGMRC; subtypes 1 and 2) has been studied since 1996 but never studied in a fish species nor in the oocyte of any animal species. A second class of receptors, termed membrane progestin receptors (mPR; subtypes alpha, beta and gamma), was recently described in vertebrates and implicated in the progestin-initiated induction of oocyte maturation in fish.

METHODS

In the present study, we report the characterization of the full coding sequence of rainbow trout PGMRC1 and mPR beta cDNAs, their tissue distribution, their ovarian expression profiles during oogenesis, their hormonal regulation in the full grown ovary and the in situ localization of PGMRC1 mRNA in the ovary.

RESULTS

Our results clearly show, for the first time in any animal species, that rainbow trout PGMRC1 mRNA is present in the oocyte and has a strong expression in ovarian tissue. In addition, we show that both mPR beta and PGMRC1, two members of distinct membrane-bound progestin receptor classes, exhibit highly similar ovarian expression profiles during the reproductive cycle with maximum levels during vitellogenesis and a down-expression during late vitellogenesis. In addition, the mRNA abundance of both genes is not increased after in vitro hormonal stimulation of full grown follicles by maturation inducing hormones.

CONCLUSION

Together, our findings suggest that PGMRC1 is a new possible participant in the progestin-induced oocyte maturation in fish. However, its participation in the process of oocyte maturation, which remains to be confirmed, would occur at post-transcriptional levels.

CYP17- and CYP11B-dependent steroid hydroxylases as drug development targets

Steroid hormone biosynthesis is catalyzed by the action of a series of cytochrome P450 enzymes as well as reductases. Defects in steroid hydroxylating P450s are the cause of several severe defects such as the adrenogenital syndrome (AGS), corticosterone methyl oxidase (CMO) I or II deficiencies, or pseudohermaphroditism. In contrast, overproduction of steroid hormones can be involved in breast or prostate cancer, in hypertension, and heart fibrosis. Besides inhibiting the action of the steroid hormones on the level of steroid hormone receptors by using antihormones, which often is connected with severe side effects, more recently the steroid hydroxylases themselves turned out to be promising new targets for drug development. Since the 3-dimensional structures of steroid hydroxylases are not yet available, computer models of the corresponding CYPs may help to develop new inhibitors of these enzymes. During the past years, the necessary test systems have been developed and new compounds have been synthesized, which displayed selective and specific inhibition of CYP17, CYP11B2, and CYP11B1. With some of these potential new drugs, clinical trials are under way. It can be expected that in the near future some of these compounds will contribute to our arsenal of new and selective drugs.

Molecular identification of adrenal inner zone antigen as a heme-binding protein

The adrenal inner zone antigen (IZA), which reacts specifically with a monoclonal antibody raised against the fasciculata and reticularis zones of the rat adrenal, was previously found to be identical with a protein variously named 25-Dx and membrane-associated progesterone receptor. IZA was purified as a glutathione S-transferase-fused or His(6)-fused protein, and its molecular properties were studied. The UV-visible absorption and EPR spectra of the purified protein showed that IZA bound a heme chromophore in high-spin type. Analysis of the heme indicated that it is of the b type. Site-directed mutagenesis studies were performed to identify the amino-acid residues that bind the heme to the protein. The results suggest that two Tyr residues, Tyr107 and Tyr113, and a peptide stretch, D99-K102, were important for anchoring the heme into a hydrophobic pocket. The effect of IZA on the steroid 21-hydroxylation reaction was investigated in COS-7 cell expression systems. The results suggest that the coexistence of IZA with CYP21 enhances 21-hydroxylase activity.

Hpr6 (heme-1 domain protein) regulates the susceptibility of cancer cells to chemotherapeutic drugs

Cancer cells have varying levels of susceptibility to chemotherapeutic agents, and the proteins that direct drug susceptibility are promising targets for intervention in cancer. Hpr6 (heme-1 domain protein)/PGRMC1 (progesterone receptor membrane component 1) is overexpressed in tumors, and Hpr6 is the human homolog of a budding yeast damage resistance gene called Dap1p. Cells lacking Dap1p are damage-sensitive, and we have found that inhibition of Hpr6 expression by RNA inhibition (RNAi) increases sensitivity of breast cancer cells to chemotherapeutic drugs. Hpr6 is composed largely of a cytochrome b(5)-related heme-1 domain, and we have found that purified Hpr6 binds to heme, similar to its yeast and rodent homologues. We generated an aspartate 120-to-glycine (D120G) mutant of Hpr6 at a highly conserved site in the heme-1 domain and demonstrated that Hpr6-D120G cannot bind to heme. The Hpr6-D120G mutant was named Hpr6(hbd) for heme binding defective. We prepared an adenovirus encoding Hpr6(hbd) and found that adenovirus Hpr6(hbd) increases susceptibility of breast cancer cells to doxorubicin and camptothecin. Our findings support a model in which Hpr6, similar to its yeast homolog, binds to heme and regulates susceptibility to damaging agents.

A Novel Group of Genes Regulates Susceptibility to Antineoplastic Drugs in Highly Tumorigenic Breast Cancer Cells

Doxorubicin is an anthracycline antibiotic used for cancer chemotherapy. The utility of doxorubicin is limited by its inability to kill all of the cells within a tumor and by resistant cells emerging from the treated population. We have screened for genes that regulate doxorubicin susceptibility in highly tumorigenic breast cancer cells by cDNA microarray and RNA interference (RNAi) analysis, and we have identified genes associated with both proliferation and cell cycle arrest after doxorubicin treatment. We confirmed that MDA-MB-231 cells treated with doxorubicin induce the expression of carbonic anhydrase II (CAII), inhibitor of differentiation/DNA binding 2 (Id2), activating transcription factor 3 (Atf3), and the phosphatidylinositol 3-kinase 55-kDa regulatory subunit p55PIK. These genes were induced at different times and with varying specificities to different chemotherapeutic drugs. In addition to being induced at the transcriptional level, the CAII and clusterin proteins were elevated after doxorubicin treatment. CAII, Id2, p55PIK, and clusterin were not altered by doxorubicin in MCF-7 cells, a weakly tumorigenic cell line used in previous studies of doxorubicin-regulated gene expression. By inhibiting gene expression using RNAi, we found that CAII and clusterin increase cell survival after doxorubicin treatment, whereas Id2 increases susceptibility to doxorubicin. Our results support a model in which highly tumorigenic breast cancer cells induce a transcriptional response to doxorubicin that is distinct from less malignant cells. The induced genes regulate drug susceptibility positively and negatively and may be novel targets for therapeutic intervention.

The membrane-associated progesterone-binding protein 25-Dx is expressed in brain regions involved in water homeostasis and is up-regulated after traumatic brain injury

After traumatic brain injury, progesterone has important neuroprotective effects in the nervous system. There is better functional outcome and less oedema formation in pseudopregnant rat females (high levels of endogenous progesterone) than in males. In addition to intracellular progesterone receptors, membrane binding sites of the hormone such as 25-Dx may also be involved in neuroprotection. In the present study we investigated the distribution of the membrane-associated progesterone-binding protein 25-Dx in rat brain. Immunohistochemical analysis showed that 25-Dx is particularly abundant in the hypothalamic area, circumventricular organs, and ependymal cells of the lateral walls of the third and lateral ventricles. A strong signal was also detected in the meninges. Double immunofluorescence immunolabelling and confocal microscopy showed that 25-Dx is co-expressed with vasopressin in neurones of the paraventricular, supraoptic and retrochiasmatic nuclei. Levels of 25-Dx expression were higher in pseudopregnant females than in males. After traumatic brain injury, 25-Dx expression was up-regulated in neurones and induced in astrocytes, which play an important role in regulating water and ion homeostasis. The expression of 25-Dx in structures involved in CSF production (choroid plexus) and in osmoregulation (circumventricular organs, hypothalamus and meninges), and its up-regulation after brain damage, point to a novel and potentially important role of this progesterone-binding protein in the maintenance of water homeostasis after traumatic brain injury.

Overexpression of the cytochrome p450 activator hpr6 (heme-1 domain protein/human progesterone receptor) in tumors

OBJECTIVE

Hpr6 (heme-1 domain protein/human progesterone receptor) is one of a family of proteins that are implicated in progesterone metabolism, resistance to genotoxic agents and steroid biosynthesis. Because these processes are frequently misregulated in tumors, we have examined the expression of Hpr6 in a group of clinical tumor samples and cancer cell lines.

METHODS

Hpr6 expression was analyzed by Western blot in extracts from breast, cervix, colon and thyroid cell lines and in nonmalignant and adjacent tumor tissue from breast, colon and thyroid. Hpr6 localization was determined by immunofluorescence.

RESULTS

Hpr6 expression is significantly elevated in breast tumors in comparison with matched nonmalignant tissue and demonstrated limited overexpression in colon and thyroid tumors. Hpr6 is strongly expressed in a panel of tumor cell lines originating from breast, thyroid and colon. Hpr6 localizes to the perinuclear region of the cell, consistent with a role in cell detoxification, signaling and/or sterol synthesis.

CONCLUSIONS

Hpr6 homologues regulate cytochrome P450 proteins implicated in hormone, steroid and xenobiotic chemical metabolism. These are the first studies linking Hpr6 expression to cancer progression and cellular survival. Our results suggest that Hpr6 is an important marker for cancer progression and a potential anticancer therapeutic target.

Dap1p, a heme-binding protein that regulates the cytochrome P450 protein Erg11p/Cyp51p in Saccharomyces cerevisiae

Alkylating agents chemically modify DNA and cause mutations that lead to cancer. In the budding yeast Saccharomyces cerevisiae, resistance to the alkylating agent methyl methanesulfonate (MMS) is mediated in part by Dap1p (damage resistance protein 1). Dap1p is related to cytochrome b5, which activates cytochrome P450 proteins, elevating the metabolism of lipids and xenobiotic compounds. We have found that Dap1p, like cytochrome b5, binds to heme and that Dap1p targets the cytochrome P450 protein Erg11p/Cyp51p. Genetic analysis indicates that Erg11p acts downstream of Dap1p. Furthermore, Dap1p regulates the stability of Erg11p, and Erg11p is stabilized in dap1Delta cells by the addition of heme. Thus, Dap1p utilizes heme to stabilize Erg11p, which in turn regulates ergosterol synthesis and MMS resistance. Dap1p homologues have been identified in numerous eukaryotes, including mammals, suggesting that the Dap1p-cytochrome P450 protein pathway is broadly conserved in eukaryotic species.