Modulation by vitamin B6 of glucocorticoid receptor-mediated gene expression requires transcription factors in addition to the glucocorticoid receptor

Epidermal glucocorticoid and mineralocorticoid receptors act cooperatively to regulate epidermal development and counteract skin inflammation

Endogenous and synthetic glucocorticoids (GCs) regulate epidermal development and combat skin inflammatory diseases. GC actions can be mediated through the GC receptor (GR) and/or the mineralocorticoid receptor (MR), highly homologous ligand-activated transcription factors. While the role of GR as a potent anti-inflammatory mediator is well known, that of MR is not as clear, nor is whether these receptors cooperate or antagonize each other in the epidermis. To address this, we generated mice with epidermal-specific loss of both receptors (double knockout, DKO), and analyzed the phenotypical and functional consequences relative to single KOs or controls (CO). At birth, DKO epidermis displayed a phenotype of defective differentiation and inflammation, which was more severe than in either single KO, featuring neutrophil-containing infiltrates, and gene dysregulation characteristic of human psoriatic lesions. This phenotype resolved spontaneously. However, in adulthood, single or combined loss of GC receptors increased susceptibility to inflammation and hyperproliferation triggered by phorbol ester which, different to CO, was not effectively counteracted by GC treatment. Also, DKOs were more susceptible to imiquimod-induced psoriasis than CO showing severe defective epidermal differentiation and microabcesses while single KOs showed an intermediate response. Immortalized DKO keratinocytes featured increased proliferation kinetics and reduced cell size, a unique phenotype relative to single KO cells. The lack of GR and MR in keratinocytes, individual or combined, caused constitutive increases in p38 and ERK activities, which were partially reversed upon reinsertion of receptors into DKO cells. DKO keratinocytes also displayed significant increases in AP-1 and NF-κB transcriptional activities, which were partially rescued by ERK and p38 inhibition, respectively. Reinsertion of GR and MR in DKO keratinocytes resulted in physical and cooperative functional interactions that restored the transcriptional response to GCs. In conclusion, our data have revealed that epidermal GR and MR act cooperatively to regulate epidermal development and counteract skin inflammation.

Primary aldosteronism patients show skin alterations and abnormal activation of glucocorticoid receptor in keratinocytes

Primary aldosteronism (PA) is a disease characterized by high aldosterone levels caused by benign adrenal tumors being the most frequent cause of secondary hypertension. Aldosterone plays vital physiological roles through the mineralocorticoid receptor (MR) but in certain cell types, it can also activate the glucocorticoid (GC) receptor (GR). Both MR and GR are structurally and functionally related and belong to the same family of ligand-dependent transcription factors that recognize identical GC regulatory elements (GREs) on their target genes. GCs play key roles in skin pathophysiology acting through both GR and MR; however, the effects of aldosterone and the potential association of PA and skin disease were not previously addressed. Skin samples from PA revealed histopathological alterations relative to control subjects, featuring epidermal hyperplasia, impaired differentiation, and increased dermal infiltrates, correlating with increased NF-κB signaling and up-regulation of TNF-A and IL-6 cytokines. PA skin samples also showed significantly higher expression of MR, GR, and HSD11B2. In cultured keratinocytes, aldosterone treatment increased GRE transcriptional activity which was significantly inhibited by co-treatment with GR- and MR-antagonists. This study demonstrates that high levels of aldosterone in PA patients correlate with skin anomalies and inflammatory features associated with abnormal GR/MR activation in epidermal keratinocytes.

Ginkgo biloba Extract Prevents Female Mice from Ischemic Brain Damage and the Mechanism Is Independent of the HO1/Wnt Pathway

It is well known that gender differences exist in experimental or clinical stroke with respect to brain damage and loss of functional outcome. We have previously reported neuroprotective properties of Ginkgo biloba/EGb 761® (EGb 761) in transient and permanent mouse models of brain ischemia using male mice, and the mechanism of action was attributed to the upregulation of the heme oxygenase 1 (HO1)/Wnt pathway. Here, we sought to investigate whether EGb 761's protective effect in ovariectomized female mice following stroke is also mediated by the HO1/Wnt pathway. Female mice were ovariectomized (OVX) to remove the protective effect of estrogen and were treated with EGb 761 for 7 days prior to inducing permanent middle cerebral artery occlusion (pMCAO) and allowed to survive for an additional 7 days. At day 8, animals were sacrificed, and the brains were harvested for infarct volume analysis, western blots, and immunohistochemistry. The OVX female mice treated with EGb 761 showed significantly lower infarct size as compared to Veh/OVX animals. EGb 761 treatment in female mice inhibited apoptosis by preventing caspase-3 cleavage and blocking the extrinsic apoptotic pathway. EGb 761 pretreatment significantly enhanced neurogenesis in OVX mice as compared to the Veh/OVX group and significantly upregulated androgen receptor expression with no changes in HO1/Wnt signaling. These results suggest that EGb 761 prevented brain damage in OVX female mice by improving grip strength and neurological deficits, and the mechanism of action is not through HO1/Wnt but via blocking the extrinsic apoptotic pathway.

Modulation of gene expression by vitamin B6

The physiologically active form of vitamin B6, pyridoxal 5'-phosphate (PLP), is known to function as a cofactor in many enzymic reactions in amino acid metabolism. Recent studies have shown that, apart from its role as a coenzyme, PLP acts as a modulator of steroid hormone receptor-mediated gene expression. Specifically, elevation of intracellular PLP leads to a decreased transcriptional response to glucocorticoid hormones, progesterone, androgens, and oestrogens. For example, the induction of cytosolic aspartate aminotransferase (cAspAT) in rat liver by hydrocortisone is suppressed by the administration of pyridoxine. The suppression of the cAspAT induction by pyridoxine is caused by a decrease in the expression of the cAspAT gene, which is brought about by inactivation of the binding activity of the glucocorticoid receptor to the glucocorticoid-responsive element in the regulatory region of the cAspAT gene. Vitamin B6 has recently been found to modulate gene expression not only for steroid hormone-responsive or PLP-dependent enzymes but also for steroid- and PLP-unrelated proteins such as serum albumin. Albumin gene expression was found to be modulated by vitamin B6 through a novel mechanism that involves inactivation of tissue-specific transcription factors, such as HNF-1 or C/EBP, by direct interaction with PLP in a similar manner to glucocorticoid receptor. Enhancement of albumin gene expression in the liver by an increased supply of amino acids can be explained by elevated binding of HNF-1 and C/EBP to their DNA-binding sites which, in turn, is caused by a decrease in the intracellular level of PLP by the increased amino acid supply. These findings that vitamin B6 acts as a physiological modulator of gene expression add a new dimension to the hitherto recognized function of vitamin B6 as a cofactor of enzyme action.

Comparison of 7α-methyl-19-nortestosterone effectiveness alone or combined with progestins on androgen receptor mediated-transactivation

7α-methyl-19-nortestosterone (MENT) is an androgen with potent gonadotropin inhibitory activity and prostate-sparing effects. These attributes give MENT advantages over testosterone as a male contraceptive, but, as in the case of testosterone, a partial dose-dependent suppression of spermatogenesis has been observed. Combination of testosterone or MENT with synthetic progestins improves the rate of azoospermia; however, it is unknown whether these combinations affect hormone androgenicity or exert synergistic effects via progestational or androgenic interaction. Herein, using transactivation assays, we examined the ability of MENT alone or combined with several 19-nor-derived synthetic progestins to activate androgen receptor (AR)-dependent gene transcription. In addition, the capability of 7α-methyl-estradiol (7α-methyl-E2), an aromatized metabolite of MENT, to transactivate gene transcription via estrogen receptor α (ERα; ESR1) or ERβ (ESR2) was also investigated. As expected, MENT induced gene transactivation through either the progesterone receptor (PGR) or the AR. MENT was as efficient as progesterone in activating PGR-mediated reporter gene expression, but it was ten times more potent than testosterone and dihydrotestoterone in activating of AR-driven gene expression. The addition of increasing concentrations of other 19-nortestosterone derivatives (norethisterone or levonorgestrel) did not affect, in a significant manner, the ability of MENT to activate AR-dependent reporter gene transcription. The same results were obtained with different cell lines. 7α-Methyl-E2 resulted in potent estrogen activity via both ER subtypes with efficiency similar to natural E2. These results suggest that the addition of 19-nortestosterone-derived progestins, as a hormonal adjuvant in male fertility strategies for effective spermatogenic suppression, does not display any detrimental effect that would interfere with MENT androgenic transcriptional activity.

Discovery of glucocorticoid receptor-beta in mice with a role in metabolism

Glucocorticoid hormones control diverse physiological processes, including metabolism and immunity, by activating the major glucocorticoid receptor (GR) isoform, GRalpha. However, humans express an alternative isoform, human (h)GRbeta, that acts as an inhibitor of hGRalpha to produce a state of glucocorticoid resistance. Indeed, evidence exists that hGRbeta contributes to many diseases and resistance to glucocorticoid hormone therapy. However, rigorous testing of the GRbeta contribution has not been possible, because rodents, especially mice, are not thought to express the beta-isoform. Here, we report expression of GRbeta mRNA and protein in the mouse. The mGRbeta isoform arises from a distinct alternative splicing mechanism utilizing intron 8, rather than exon 9 as in humans. The splicing event produces a form of beta that is similar in structure and functionality to hGRbeta. Mouse (m)GRbeta has a degenerate C-terminal region that is the same size as hGRbeta. Using a variety of newly developed tools, such as a mGRbeta-specific antibody and constructs for overexpression and short hairpin RNA knockdown, we demonstrate that mGRbeta cannot bind dexamethasone agonist, is inhibitory of mGRalpha, and is up-regulated by inflammatory signals. These properties are the same as reported for hGRbeta. Additionally, novel data is presented that mGRbeta is involved in metabolism. When murine tissue culture cells are treated with insulin, no effect on mGRalpha expression was observed, but GRbeta was elevated. In mice subjected to fasting-refeeding, a large increase of GRbeta was seen in the liver, whereas mGRalpha was unchanged. This work uncovers the much-needed rodent model of GRbeta for investigations of physiology and disease.

Pyridoxal 5'-phosphate deficiency causes a loss of aromatic L-amino acid decarboxylase in patients and human neuroblastoma cells, implications for aromatic L-amino acid decarboxylase and vitamin B(6) deficiency states

Pyridoxal 5'-phosphate, the active form of vitamin B(6), is an essential cofactor for multiple enzymes, including aromatic l-amino acid decarboxylase that catalyses the final stage in the production of the neurotransmitters dopamine and serotonin. In two patients with inherited disorders of vitamin B(6) metabolism, we observed reductions in plasma aromatic l-amino acid decarboxylase activity. In one patient, this change was related to an increase in K(m) for pyridoxal 5'-phosphate. Furthermore, pyridoxal 5'-phosphate-deficient human SH-SY5Y neuroblastoma cells were found to exhibit reduced levels of aromatic l-amino acid decarboxylase activity and protein but with no alteration in expression. Further reductions in activity and protein were observed with the addition of the vitamin B(6) antagonist 4-deoxypyridoxine, which also reduced aromatic l-amino acid decarboxylase mRNA levels. Neither pyridoxal 5'-phosphate deficiency nor the addition of 4-deoxypyridoxine affected aromatic l-amino acid decarboxylase stability over 8 h with protein synthesis inhibited. Increasing extracellular availability of pyridoxal 5'-phosphate was not found to have any significant effect on intracellular pyridoxal 5'-phosphate concentrations or on aromatic l-amino acid decarboxylase. These findings suggest that maintaining adequate pyridoxal 5'-phosphate availability may be important for optimal treatment of aromatic l-amino acid decarboxylase deficiency and l-dopa-responsive conditions.

Mechanism of action of bolandiol (19-nortestosterone-3beta,17beta-diol), a unique anabolic steroid with androgenic, estrogenic, and progestational activities

Bolandiol is a synthetic anabolic steroid that increases lean body mass and bone mineral density without significant stimulation of sex accessory glands in castrate adult male rats. Since bolandiol suppresses gonadotropins and endogenous testosterone (T) production, we investigated its mechanism of action. We compared the potency of bolandiol in vitro and in vivo with T, 5alpha-dihydrotestosterone (DHT), 19-nortestosterone (19-NT) and estradiol (E(2)). Bolandiol bound with lower affinity to the recombinant rat androgen receptor (AR) than the other androgens and had low, but measurable, affinity for recombinant human progestin receptors (PR-A, PR-B), and estrogen receptors (ERalpha and beta-1). Functional agonist activity was assessed in transcription assays mediated by AR, PR, or ER. Bolandiol was stimulatory in all these assays, but only 4-9% as potent as T, DHT, and 19-NT via AR, 1% as potent as progesterone via PR, and 3% and 1% as potent as E(2) acting through ERalpha or ERbeta, respectively. In immature castrate rats, bolandiol was equipotent to T in stimulating growth of the levator ani muscle but less potent than T in stimulating growth of the sex accessory glands. Bolandiol also stimulated uterine weight increases in immature female rats, which were partly blocked by ICI 182,780, but it was not aromatized in vitro by recombinant human aromatase. In contrast to T, stimulation of sex accessory gland weights by bolandiol was not inhibited by concomitant treatment with the dual 5alpha-reductase inhibitor dutasteride. As bolandiol exhibits tissue selectivity in vivo, it may act via AR, PR, and/or ER, utilize alternative signaling pathway(s) or transcriptional coregulators, and/or be metabolized to a more potent selective steroid.

[Molecular aspects of glucocorticoid sensitivity]

Glucocorticoids play an essential role in maintaining basal and stress-related homeostasis. Most known effects of glucocorticoids are mediated by the intracellular glucocorticoid receptors. The glucocorticoid sensitivity seems to depend on the amount of receptors expressed and the efficiency of glucocorticoid receptor-mediated signal transduction. Glucocorticoid resistance or hypersensitivity, seen in autoimmune-inflammatory diseases and in metabolic syndrome respectively, can represent the variability of several steps that influence the signaling cascade of glucocorticoid action. The recognition of these steps could provide the understanding of the clinical phenotype and course of such diseases as well as their responsiveness to glucocorticoid therapy. The comprehension of these pathophysiological mechanisms can also improve the possible therapeutic interventions. In this review, we have summarized the multiple factors that have been shown to be involved in this signaling cascade and, thus, to influence glucocorticoid sensitivity.

Liver-specific increase of UTP and UDP-sugar concentrations in rats induced by dietary vitamin B6-deficiency and its relation to complex N-glycan structures of liver membrane-proteins

This is the first known report on the influence of vitamin B6-deficiency on the concentration of UDP-sugars and other uracil nucleotides in rats. Animals aged 3 weeks or 2 months were fed a vitamin B6-free diet for periods varying from 3 days to 7 weeks. Nucleotides were quantified by enzymatic-photometry and by SAX-high precision liquid chromatography. In 3 week-old rats, vitamin B6-deficiency resulted in an up to 6.3-fold increase in the concentrations of UTP, UDP, UMP and UDP-sugars and less of CTP in rat liver, while no changes were observed in older rats. In young rats, the concentration of uracil nucleotides started to increase after 1 week diet, with a maximum after 2 weeks. After 5 weeks, the concentrations returned to normal values. In heart, lungs, kidney and brain, concentrations were measured after 2 weeks diet in young rats. In contrast to liver, the heart muscle uracil nucleotide concentrations were decreased by 40%. In kidney, the sum of UTP, UDP and UMP showed a decrease of 40%, whereas UDP-sugars were increased 1.4-fold. In the lungs, nucleotide concentrations were mostly unaffected by vitamin B6-deficiency, despite a 70% increase of UDP-GA. In brain, UDP-Glc, UDP-Gal and the sum of CTP and CDP showed an increase of 30-50%. We became surprised that the increased UDP-sugar concentrations did not influence the structure of liver plasma membrane-N-glycans. Despite the 4 to 6-fold increase of UTP and UDP-sugars, no changes in the complexity or sialylation of these N-glycans could be detected. This study demonstrates that, especially in liver, pyridoxal phosphate is closely involved in the control of uracil nucleotides during a defined period of development. In contrast to in vitro experiments, in vivo N-glycan biosynthesis in liver is regulated by a more complex or higher mechanism than substrate concentrations.

Time course of changes in pyridoxal 5'-phosphate (vitamin B6 active form) and its neuroprotection in experimental ischemic damage

In the present study, we investigated ischemia-induced changes of pyridoxal 5'-phosphate synthesizing enzyme and degrading enzyme and neuroprotective effects and roles of pyridoxal 5'-phosphate against ischemic damage in the gerbil hippocampal CA1 region. Pyridoxal 5'-phosphate oxidase and pyridoxal phosphate phosphatase immunoreactivities were changed in neurons up to 2 days after ischemia, while 4 days after ischemia their immunoreactivities were expressed in astrocytes. Pyridoxal 5'-phosphate oxidase immunoreactivity and its protein level were highest 12 h after ischemia, while those in pyridoxal phosphate phosphatase were highest 2 days after ischemia. Total activities of these enzymes were changed after ischemia, but specific activities of the enzymes were not altered. Treatment with pyridoxal 5'-phosphate into brains (4 microg/5 microl, i.c.v.) at 30 min before transient ischemia protected about 80% of CA1 pyramidal cells 4 days after ischemia and induced elevation of glutamic acid decarboxylase 67 immunoreactivity in the CA1 region. However, pyridoxal 5'-phosphate treatment into ischemic brains decreased GABA transaminase immunoreactivity in the CA1 region after ischemia. These results indicate that pyridoxal 5'-phosphate may be associated with the inhibitory discharge of GABA in the hippocampal CA1 neurons, and the increased level of GABA may protect hippocampal CA1 pyramidal cells from ischemic damage.

Vitamin B6 conjugation to nuclear corepressor RIP140 and its role in gene regulation

Pyridoxal 5'-phosphate (PLP), the biologically active form of vitamin B6, is an important cofactor in amino acid metabolism, and supplementary vitamin B6 has protective effects in many disorders. Other than serving as a cofactor, it can also modulate the activities of steroid hormone receptors and transcription factors. However, the molecular basis of this modulation is unclear. Here, we report that mouse nuclear receptor interacting protein 140 (RIP140) can be modified by PLP conjugation. We mapped the modification site to Lys613 by LC-ESI-MS/MS analysis. This modification enhanced its transcriptional corepressive activity and its physiological function in adipocyte differentiation. We attribute this effect to increased interaction of RIP140 with histone deacetylases and nuclear retention of RIP140. This study uncovers a new physiological role of vitamin B6 in gene regulation by PLP conjugation to a transcriptional coregulator, which represents a new function of an old form of protein post-translational modification that has important biological consequences.

A germline variation in the progesterone receptor gene increases transcriptional activity and may modify ovarian cancer risk

Recently, we and others have detected a haplotype of the human progesterone receptor gene (PR). This haplotype consists of a 320-bp insertion in intron G together with point mutations in exons 4 and 5 and was named PROGINS. Whereas the exon 5 mutation is silent, the mutation in exon 4 results in a V660L substitution. Interestingly, this genetic polymorphism was seen to cosegregate with an increased risk of sporadic ovarian cancer in different ethnic groups. Our data provide evidence for the existence of an epidemiological link between a mutated progesterone receptor allele and ovarian cancer (odds ratio, 3.02; 95% confidence interval, 1.86-4.91). Functional characterization of the mutated receptor protein revealed a greater transcriptional activity compared with the wild-type receptor. By contrast, hormone binding and hormone dissociation rates were similar in both receptor proteins. We found that the increased transcriptional activity was due to increased stability resulting in higher expression of the mutant protein. Thus, the long-lasting hyperactivation of progesterone receptor-driven genes secondary to the increased transcriptional activity of the mutated progesterone receptor may participate in ovarian carcinogenesis. This is of special interest, because only a few genetic markers are available for the majority of women diagnosed with sporadic ovarian cancer.

A single amino acid change in the first zinc finger of the DNA binding domain of the glucocorticoid receptor regulates differential promoter selectivity

Mammalian species are well known to differ in their sensitivity to glucocorticoids, but the molecular basis for this difference remains largely uncharacterized. To address this issue, the transcriptional activity of the mouse and human glucocorticoid receptor (GR) was analyzed on two model glucocorticoid-responsive promoters. Mouse GR (mGR) displayed unique promoter discrimination in response to a range of glucocorticoids, with enhanced activity on a simple glucocorticoid response element (GRE)-based promoter and diminished activity on the complex mouse mammary tumor virus promoter compared with human GR (hGR). Promoter discrimination between mGR and hGR was mapped to a single amino acid change at residue 437 (glycine to valine) of mGR and to sequence differences within individual GREs of the different promoters. Mouse GR displayed higher activation on GREs with a guanine rather than a thymine at the -6 position. Binding studies indicated mGR (mGR437V) displayed a weaker affinity for GREs containing a thymine at the -6 position than a mGR mutant containing a glycine at residue 437 (mGR437G). Despite distinct transcriptional activities, both receptors had similar affinities for response elements that contain a guanine at the -6 position. Our findings support a model by which the presence of a valine residue at position 437 of mGR induces a conformational change that leads to alterations in affinity and/or transcriptional activation in a promoter-dependent context.

Comparative evaluation of androgen and progesterone receptor transcription selectivity indices of 19-nortestosterone-derived progestins

Synthetic 19-nortestosterone-derived progestins show affinity for the androgen receptor (AR) and retain varying degrees of androgenic activity. In this study, AR- and progesterone receptor (PR)-dependent transcriptional activation induced by norethisterone (NET), levonorgestrel (LNG) and gestodene (GSD), and their 5alpha-reduced derivatives, including limited trypsin digestion of AR in the presence of natural and synthetic progestins were investigated. The results confirmed the progestogenic activity of the three 19-nortestosterone derivatives, which decreases after reduction of the 4-ene-double bound. These compounds were able to activate AR-dependent reporter gene expression, LNG and GSD being the stronger activators. 5alpha-Reduction of LNG and GSD did not change their androgenic transcriptional activity; however, the activation of AR by 5alpha-NET was four-fold higher than NET. The highest selectivity transcriptional index, as a measure of progestogenicity versus androgenicity, was obtained for NET. The 5alpha-reduced derivatives had values significantly lower than those of their parent compounds. Non-reduced and 5alpha-reduced 19-nortestosterone progestins induced virtually identical proteolysis fragmentation patterns of the AR to those observed with DHT.

Ligand-independent interactions of p160/steroid receptor coactivators and CREB-binding protein (CBP) with estrogen receptor-alpha: regulation by phosphorylation sites in the A/B region depends on other receptor domains

Estrogen receptor (ER)alpha and ERbeta are transcription factors that can be activated by both ligand binding and growth factor signaling. Estradiol increases ER activity in part by enhancing interactions between its carboxy-terminal, ligand-binding domain (LBD) and the p160/SRC (steroid receptor coactivator) and p300/CBP (cAMP response element binding protein (CREB) binding protein) families of coactivators. In the absence of ligand and the LBD, these cofactors can also interact with the amino-terminal (A/B) domain of ERs in vitro. SRC-1 also enhances the ligand-independent activity of the full-length receptor. Both ligand-independent and estradiol-induced ER activity are increased by phosphorylation at specific serine (Ser) residues in the A/B domain (Ser104/106 and Ser118 in ERalpha). In the case of ERbeta, phosphorylation enhances the ligand-independent recruitment and action of SRC-1. We show here that unliganded ERalpha can activate endogenous gene expression in MCF-7 cells, and that this activation is mediated in part by a p160 coactivator. In transfected HeLa cells, we show that the full-length ERalpha can interact physically and functionally with p160/SRCs and CBP in the absence of ligand and that mutation of Ser104/106/118 affects these interactions. Accordingly, ERalpha dephosphorylation decreases its ligand-independent interaction with SRC-1 and CBP in vitro. In HeLa cells, both Ser104/106 and Ser118 impinge on SRC-1 action by two mechanisms: 1) a seemingly indirect effect on SRC-1 recruitment that requires other receptor domains in addition to the A/B, consistent with our finding that the ligand-independent interaction between the A/B and the LBD and its enhancement by SRC-1 depend in part on Ser104/106/118; and 2) an effect on SRC-1 coactivation that can be observed in the absence of the LBD. Ser104/106/118 can also affect coactivation by a subset of coactivators in the presence of hormone, albeit to a lesser extent than in its absence. Altogether, our observations suggest that the enhancement of ERalpha activity by p160/SRCs and CBP can be regulated by phosphorylation and stress the importance of using full-length receptors to assess the role of the activation function-1 in cofactor recruitment.

Molecular origins for the dominant negative function of human glucocorticoid receptor beta

This study molecularly elucidates the basis for the dominant negative mechanism of the glucocorticoid receptor (GR) isoform hGRbeta, whose overexpression is associated with human glucocorticoid resistance. Using a series of truncated hGRalpha mutants and sequential mutagenesis to generate a series of hGRalpha/beta hybrids, we find that the absence of helix 12 is neither necessary nor sufficient for the GR dominant negative phenotype. Moreover, we have localized the dominant negative activity of hGRbeta to two residues and found that nuclear localization, in addition to heterodimerization, is a critical feature of the dominant negative activity. Molecular modeling of wild-type and mutant hGRalpha and hGRbeta provides structural insight and a potential physical explanation for the lack of hormone binding and the dominant negative actions of hGRbeta.

Structure and mechanism of Escherichia coli pyridoxine 5'-phosphate oxidase

Escherichia coli pyridoxine 5'-phosphate oxidase (PNPOx) catalyzes the oxidation of either pyridoxine 5'-phosphate (PNP) or pyridoxamine 5'-phosphate (PMP), forming pyridoxal 5'-phosphate (PLP). This reaction serves as the terminal step in the de novo biosynthesis of PLP in E. coli and as a part of the salvage pathway of this coenzyme in both E. coli and mammalian cells. Recent studies have shown that in addition to the active site, PNPOx contains a noncatalytic site that binds PLP tightly. The crystal structures of PNPOx with one and two molecules of PLP bound have been determined. In the active site, the PLP pyridine ring is stacked almost parallel against the re-face of the middle ring of flavin mononucleotide (FMN). A large protein conformational change occurs upon binding of PLP. When the protein is soaked with excess PLP an additional molecule of this cofactor is bound about 11 A from the active site. A possible tunnel exists between the two sites. Site mutants were made of all residues at the active site that make interactions with the substrate. Stereospecificity studies showed that the enzyme is specific for removal of the proR hydrogen atom from the prochiral C4' carbon of PMP. The crystal structure and the stereospecificity studies suggest that the pair of electrons on C4' of the substrate are transferred to FMN as a hydride ion.

Mechanistic differences in the activation of estrogen receptor-alpha (ER alpha)- and ER beta-dependent gene expression by cAMP signaling pathway(s)

Although increases in intracellular cAMP can stimulate estrogen receptor-alpha (ER alpha) activity in the absence of exogenous hormone, no studies have addressed whether ER beta can be similarly regulated. In transient transfections, forskolin plus 3-isobutyl-1-methylxanthine (IBMX), which increases intracellular cAMP, stimulated the transcriptional activities of both ER alpha and ER beta. This effect was blocked by the protein kinase A inhibitor H89 (N-(2-(p-bromocinnamylamino)-ethyl)-5-isoquinolinesulfonamide) and was dependent on an estrogen response element. A 12-O-tetradecanoylphorbol-13-acetate response element (TRE) located 5' to the estrogen response element was necessary for cAMP-dependent activation of gene expression by ER beta but not ER alpha, indicating that the former subtype requires a functional interaction with TRE-interacting factor(s) to stimulate transcription. Both p160 and CREB-binding protein coactivators stimulated cAMP-induced ER alpha and ER beta transcriptional activity. However, mutation of the two cAMP-inducible SRC-1 phosphorylation sites important for cAMP activation of chicken progesterone receptor or all seven known SRC-1 phosphorylation sites did not specifically impair cAMP activation of ER alpha. The E/F domains of ER alpha are sufficient for activation by forskolin/IBMX, and this is accompanied by an increase in receptor phosphorylation. In contrast, cAMP signaling reduces the phosphorylation of the corresponding region of ER beta, and this correlates with the lack of forskolin/IBMX stimulated transcriptional activity. Our data suggest that cAMP activation of ER alpha transcriptional activity is associated with receptor instead of SRC-1 phosphorylation. Moreover, differences in the cofactor requirements, domains of ER alpha and ER beta sufficient for forskolin/IBMX activation, and the effect of cAMP on receptor phosphorylation indicate that this signaling pathway utilizes distinct mechanisms to stimulate ER alpha and ER beta transcriptional activity.

Molecular analysis of the androgen receptor in ten prostate cancer specimens obtained before and after androgen ablation

Hormonal or androgen-ablation (AA) therapy is the predominant form of systemic treatment for metastatic prostate cancer. Although an initial response to AA is observed in 70%-80% of patients with advanced disease, most tumors eventually progress to androgen-independent growth, and only a minority of affected individuals are alive 5 years following initiation of treatment. Because AA induces a dramatic change in the hormonal milieu of the patient and because these tumors maintain the ability to proliferate, it is possible that this treatment selects a population of cells with mutated androgen receptors (ARs) that sustain growth despite the absence of circulating androgen. To test this hypothesis we investigated the molecular structure of the AR in 10 prostate cancer specimens obtained before and after AA. Tumors (coded A through L) were microdissected to uniquely enrich genomic DNA from cancer cells. Exons 1-8 of the AR were screened by polymerase chain reaction, single-stranded conformational polymorphism, and sequence analysis. A mutation consisting of an expansion of the polyglutamine (poly-Q) repeat from 20 (found in 100% of the sequences of specimens obtained before AA) to 26 (found in 70% of the sequences of specimens obtained after AA) was detected in patient F. The 26 glutamine (Q26) AR readily translocated to the nucleus upon addition of androgen, and did not show significant differences in its ability to bind (3)[H]-dihydrotestosterone compared to its wild-type counterpart. Nevertheless, analysis of transcriptional activity showed that the Q66 AR was transcriptionally 30%-50% less active than the wild-type molecule. Because clones of AR with an expanded poly-Q tract were detected only in the specimen from patient F obtained after AA, we conclude that in specific circumstances, AA treatments can select variant forms of the AR in the prostate of patients affected by prostate cancer. Further experiments are needed to conclusively determine whether the Q26 clone was responsible for sustaining survival of prostate cancer cells in the androgen-depleted milieu of this patient.

Antagonism of glucocorticoid receptor transactivity and cell growth inhibition by transforming growth factor-beta through AP-1-mediated transcriptional repression

We have examined the interaction of the glucocorticoid receptor (GR) and transforming growth factor-beta (TGF-beta) signal pathways because of their mutual involvement in the regulation of cell growth, development and differentiation. Most studies of this cross-talk event have focused on the effects of glucocorticoids (GCs) on TGF-beta responses. In this work, we show that TGF-beta can antagonize dexamethasone (Dex)-mediated growth suppression in mouse fibrosarcoma L929 cells. TGF-beta also repressed GR-mediated reporter (pMMTV-CAT) gene expression in a concentration-dependent manner, with an IC(50) of 5 ng/ml of TGF-beta. Maximal inhibition (76%) was observed at 10 ng/ml of TGF-beta. Conversely, Dex inhibited TGF-beta-mediated promoter (p3TP-Lux) activity in these same cells. As TGF-beta inhibition of GR-mediated gene expression occurred after Dex-mediated nuclear translocation of GR, we conclude that TGF-beta inhibition of GR signaling occurs at the level of GR-mediated transcription activity. However, TGF-beta did not repress GR-mediated gene expression using the pGRE(2)E1B-CAT minimal promoter construct, suggesting that TGF-beta did not inhibit intrinsic GR activity but, rather, required DNA-binding factor(s) distinct from GR. As the MMTV promoter contains several putative AP-1 binding sites, we hypothesized that AP-1, a transcription factor composed of c-jun and c-fos proteins, might be involved in the TGF-beta inhibition of GR functions. Curcumin, a potent inhibitor of AP-1 expression, completely abolished the inhibitory effect of TGF-beta on GR-mediated gene expression without affecting GR activity in the absence of TGF-beta, and this drug blocked TGF-beta-induced binding of AP-1 to a response element derived from the MMTV sequence. Furthermore, curcumin abolished TGF-beta inhibition of Dex-induced growth suppression. Taken as a whole, our data suggest that TGF-beta can antagonize the growth inhibitory properties of GR by blocking GR transactivity at complex promoters through a mechanism involving transcriptional repression by DNA-bound AP-1.

A novel androgen receptor mutant, A748T, exhibits hormone concentration-dependent defects in nuclear accumulation and activity despite normal hormone-binding affinity

Functional analysis of androgen receptor (AR) gene mutations isolated from prostate cancer has led to the identification of residues that play important roles in the structure and function of the receptor. Here we report the characteristics of a novel AR mutation A748T located in helix 5 of the ligand-binding domain, which was identified in metastatic prostate cancer. Despite a normal hormone-binding affinity, A748T causes hormone concentration-dependent defects in nuclear accumulation and transcriptional activation. Moreover, when equivalent amounts of DNA are transfected, the mutant is expressed at much lower levels than the wild-type AR (ARWT). Treatment with geldanamycin to disrupt receptor-heat shock protein complexes rapidly decreases the levels of ARWT but not A748T, suggesting that the lower expression and rapid degradation rate of A748T is due to weaker interactions with heat shock proteins. Further analysis revealed that hormone dissociates from A748T five times faster than from ARWT. Loss of the ability to form stable amino/carboxyl-terminal interactions causes accelerated dissociation rates in some AR mutants. However, A748T exhibits normal amino/carboxyl-terminal interactions at high hormone concentrations, suggesting that the mutation alters interactions with ligand. Consistent with this conclusion, our structural model predicts that A748T disrupts crucial contact points with ligand, thereby altering the conformation of the ligand-binding domain.

SKF-82958 is a subtype-selective estrogen receptor-alpha (ERalpha ) agonist that induces functional interactions between ERalpha and AP-1

The transcriptional activity of estrogen receptors (ERs) can be regulated by ligands as well as agents such as dopamine, which stimulate intracellular signaling pathways able to communicate with these receptors. We examined the ability of SKF-82958 (SKF), a previously characterized full dopamine D1 receptor agonist, to stimulate the transcriptional activity of ERalpha and ERbeta. Treatment of HeLa cells with SKF-82958 stimulated robust ERalpha-dependent transcription from an estrogen-response element-E1b-CAT reporter in the absence of estrogen, and this was accompanied by increased receptor phosphorylation. However, induction of ERbeta-directed gene expression under the same conditions was negligible. In our cell model, SKF treatment did not elevate cAMP levels nor enhance transcription from a cAMP-response element-linked reporter. Control studies revealed that SKF-82958, but not dopamine, competes with 17beta-estradiol for binding to ERalpha or ERbeta with comparable relative binding affinities. Therefore, SKF-82958 is an ERalpha-selective agonist. Transcriptional activation of ERalpha by SKF was more potent than expected from its relative binding activity, and further examination revealed that this synthetic compound induced expression of an AP-1 target gene in a tetradecanoylphorbol-13-acetate-response element (TRE)-dependent manner. A putative TRE site upstream of the estrogen-response element and the amino-terminal domain of the receptor contributed to, but were not required for, SKF-induced expression of an ERalpha-dependent reporter gene. Overexpression of the AP-1 protein c-Jun, but not c-Fos, strongly enhanced SKF-induced ERalpha target gene expression but only when the TRE was present. These studies provide information on the ability of a ligand that weakly stimulates ERalpha to yield strong stimulation of ERalpha-dependent gene expression through cross-talk with other intracellular signaling pathways producing a robust combinatorial response within the cell.

Vanadate increases glucocorticoid receptor-mediated gene expression: a novel mechanism for potentiation of a steroid receptor

Transition metal oxyanions, such as molybdate, tungstate and vandadate, have been shown to prevent in vitro hormone-induced activation of the glucocorticoid receptor (GR) by blocking dissociation of the GR/heat shock protein heterocomplex. In this work, we report a novel effect of vanadate: in vivo potentiation of GR-mediated gene expression. In cells stably-transfected with complex (mouse mammary tumor virus (MMTV)) or minimal GR-regulated CAT reporters, treatment with 500muM vanadate caused CAT gene expression to dramatically increase, even at saturating concentrations of dexamethasone; while no such effect was seen in response to RU486 antagonist. Similar treatment with molybdate had no effect on GR activity, suggesting that the response to vanadate was not a general property of transition metal oxyanions. Treatment with vanadate after hormone-induced nuclear translocation of the GR also caused potentiation, demonstrating that vanadate was acting on a post-transformation event, perhaps by affecting the transactivation function of DNA-bound GR. Paradoxically, vanadate caused an apparent but temporary "loss" of GR protein immediately after treatment (as measured by loss of reactivity to BuGR2 antibody and of hormone-binding capacity) that returned to normal at approximately 8h post-treatment, suggesting that potentiation of GR transactivation function (as measured by our CAT assays) was probably occurring during the later stages (8-24h) of this assay. However, gel shift analyses revealed that vanadate could induce binding of the hormone-free GR to glucocorticoid response element (GRE)-containing oligonucleotides immediately after treatment. Thus, the rapid vanadate-induced "loss" of GR was not due to degradation of GR protein. Yet, vanadate in the absence of hormone had no effect on CAT reporter expression, demonstrating that this form of the GR still requires agonist for its enhanced transcriptional activity. As an indication of the potential mechanism of vanadate action, vanadate was found to dramatically stimulate the mitogen-activated protein kinases, ERK-1 and ERK-2. In addition, vanadate potentiation of GR reporter gene expression was completely blocked by the tyrosine kinase inhibitor herbimycin A. Taken as a whole, our results suggest that vanadate can have dramatic and complex effects on GR structure and function, resulting in hormone-free activation of GR DNA-binding function, as well as alterations to the BuGR2 epitope and hormone-binding domains--while at the same time stimulating tyrosine phosphorylation pathways controlling GR-mediated gene transcription.

Pyridoxal 5'-phosphate inhibits DNA binding of HNF1

An efficient Escherichia coli expression system was constructed for the production of a variant form of HNF1 protein having the additional five amino acid residues (Asp-Arg-Trp-Gly-Ser) at the NH(2)-terminal. The cDNA encoding HNF1 was ligated to 6 x His tag and inserted into an inducible bacterial expression vector pRSET A. After expression in E. coli, the recombinant product was purified by Ni-NTA affinity column chromatography. The purified product showed expected NH(2)-terminal sequence and specific binding to the HNF1 site. The effect of pyridoxal 5'-phosphate and its analogues on the binding activity of recombinant HNF1 was examined and found that only pyridoxal 5'-phosphate effectively inhibited the DNA binding. The concentration of pyridoxal 5'-phosphate that inhibited 50% of DNA binding was around 100 microM. Furthermore, we identified Lys197 of HNF1 molecule as the essential residue of DNA binding. These observations suggest that pyridoxal 5'-phosphate directly interacts with tissue-specific transcription factor HNF1 and modulates the binding to DNA.

Proteasome-mediated glucocorticoid receptor degradation restricts transcriptional signaling by glucocorticoids

Ligand-dependent down-regulation of the glucocorticoid receptor (GR) has been shown to limit hormone responsiveness, but the mechanisms involved in this process are poorly understood. The glucocorticoid receptor is a phosphoprotein that upon ligand binding becomes hyperphosphorylated, and recent evidence indicates that phosphorylation status of the glucocorticoid receptor plays a prominent role in receptor protein turnover. Because phosphorylation is a key signal for ubiquitination and proteasomal catabolism of many proteins, we evaluated whether the ubiquitin-proteasomal pathway had a role in glucocorticoid receptor down-regulation and the subsequent transcriptional response to glucocorticoids. Pretreatment of COS-1 cells expressing mouse glucocorticoid receptor with the proteasome inhibitor MG-132 effectively blocks glucocorticoid receptor protein down-regulation by the glucocorticoid dexamethasone. Interestingly, both MG-132 and a second proteasome inhibitor beta-lactone significantly enhanced hormone response of transfected mouse glucocorticoid receptor toward transcriptional activation of glucocorticoid receptor-mediated reporter gene expression. The transcriptional activity of the endogenous human glucocorticoid receptor in HeLa cells was also enhanced by MG-132. Direct evidence for ubiquitination of the glucocorticoid receptor was obtained by immunoprecipitation of cellular extracts from proteasome-impaired cells. Examination of the primary sequence of mouse, human, and rat glucocorticoid receptor has identified a candidate PEST degradation motif. Mutation of Lys-426 within this PEST element both abrogated ligand-dependent down-regulation of glucocorticoid receptor protein and simultaneously enhanced glucocorticoid receptor-induced transcriptional activation of gene expression. Unlike wild type GR, proteasomal inhibition failed to enhance significantly transcriptional activity of K426A mutant GR. Together these findings suggest a major role of the ubiquitin-proteasome pathway in regulating glucocorticoid receptor protein turnover, thereby providing a mechanism to terminate glucocorticoid responses.

A-ring reduced metabolites of 19-nor synthetic progestins as subtype selective agonists for ER alpha

It has previously been demonstrated that 19-nor contraceptive progestins undergo in vivo and in vitro enzyme-mediated A-ring double bond hydrogenation. Bioconversion of 19-nor progestins to their corresponding tetrahydro derivatives results in the loss of progestational activity and acquisition of estrogenic activities and binding to the ER. Herein, we report subtype-selective differences in ligand binding and transcriptional potency of nonphenolic synthetic 19-nor derivatives between ER alpha and ER beta. In this study, we have examined both ER- and PR-mediated transcriptional activity of a number of A-ring chemically reduced derivatives of norethisterone and Gestodene. Double bond hydrogenation decreased the transcriptional potency of norethisterone and Gestodene through both PR isoforms with a 100- to 1,000-fold difference, respectively. In terms of the effects of norethisterone and Gestodene and their corresponding 5 alpha-dihydro (5 alpha-norethisterone and 5 alpha-Gestodene), or 3 alpha,5 alpha-tetrahydro or 3 beta,5 alpha-tetrahydro derivatives (3 alpha,5 alpha-norethisterone/3 alpha,5 alpha-Gestodene and 3 beta,5 alpha-norethisterone/3beta,5 alpha-Gestodene, respectively) on estrogen-mediated transcriptional regulation, the 3 beta,5 alpha-tetrahydro derivatives of both norethisterone and Gestodene showed the highest induction when HeLa cells were transiently transfected with an expression vector for ER alpha. This activity could be inhibited with tamoxifen. These compounds did not activate gene transcription via ER beta, and none of them showed antagonistic activities through either ER subtype. The 3 beta,5 alpha-tetrahydro derivatives of both norethisterone and Gestodene were active in other cells in addition to HeLa cells and activated reporter expression through the oxytocin promoter. In summary, two ER alpha selective agonists have been identified. These compounds, with ER alpha vs. ER beta selective agonist activity, may be useful in evaluating the distinct role of these receptors as well as in providing useful insights into ER action.

Proinflammatory cytokines regulate human glucocorticoid receptor gene expression and lead to the accumulation of the dominant negative beta isoform: a mechanism for the generation of glucocorticoid resistance

Inflammatory responses in many cell types are coordinately regulated by the opposing actions of NF-kappaB and the glucocorticoid receptor (GR). The human glucocorticoid receptor (hGR) gene encodes two protein isoforms: a cytoplasmic alpha form (GRalpha), which binds hormone, translocates to the nucleus, and regulates gene transcription, and a nuclear localized beta isoform (GRbeta), which does not bind known ligands and attenuates GRalpha action. We report here the identification of a tumor necrosis factor (TNF)-responsive NF-kappaB DNA binding site 5' to the hGR promoter that leads to a 1.5-fold increase in GRalpha mRNA and a 2.0-fold increase in GRbeta mRNA in HeLaS3 cells, which endogenously express both GR isoforms. However, TNF-alpha treatment disproportionately increased the steady-state levels of the GRbeta protein isoform over GRalpha, making GRbeta the predominant endogenous receptor isoform. Similar results were observed following treatment of human CEMC7 lymphoid cells with TNF-alpha or IL-1. The increase in GRbeta protein expression correlated with the development of glucocorticoid resistance.

The androgen receptor (AR) amino-terminus imposes androgen-specific regulation of AR gene expression via an exonic enhancer

Androgen and glucocorticoid receptor (AR, GR), two closely related members of the nuclear receptor superfamily, can recognize a similar cis-acting DNA sequence, or hormone response element (HRE). Despite this apparent commonality, these receptors regulate distinct target genes in vivo. The AR gene itself is regulated by AR but not GR in a variety of cell types, including osteoblast-like cells, as shown here. To understand this specificity, we first identified the DNA sequences responsible for androgen-mediated up-regulation of AR messenger RNA. A 6.5-kb region encompassing exon D, intron 4, and exon E of the AR gene contains four exonic HREs and exhibits cell type-specific, AR-mediated transcriptional enhancement when placed upstream of a heterologous promoter and reporter gene. A 350-bp fragment consisting of just exons D and E exhibits the same cell- and androgen-specificity as the 6.5-kb region, as well as the native AR gene. Consistent with a role for the exonic HREs, androgen regulation via this intragenic enhancer requires the HREs as well as a functional receptor DNA binding domain. A panel of AR/GR chimeric receptors was used to test which AR domains (amino-terminal, DNA binding or ligand binding) confer androgen-specific regulation of the 350-bp enhancer. Only chimeric receptors containing the amino-terminus of AR induced reporter gene activity from the AR gene enhancer. Further, a constitutively active AR consisting of only the AR amino-terminus and DNA binding domain (AA phi) retained the capacity to activate the internal responsive region, unlike a constitutively active chimera harboring the GR amino-terminus and AR DNA binding domain (GA phi). Thus, the AR amino terminus is the sole determinant for androgen-specific regulation of the AR gene internal enhancer. These findings support a model in which the amino termini of ARs bound to HREs within the AR gene interact with an exclusive auxiliary factor(s) to elicit androgen-specific regulation of AR messenger RNA. This is the first example of androgen-specific response in which the necessary and sufficient distinguishing capacity resides within the AR amino terminus.

Heat and chemical shock potentiation of glucocorticoid receptor transactivation requires heat shock factor (HSF) activity. Modulation of HSF by vanadate and wortmannin

Heat shock and other forms of stress increase glucocorticoid receptor (GR) activity in cells, suggesting cross-talk between the heat shock and GR signal pathways. An unresolved question concerning this cross-talk is whether heat shock factor (HSF1) activity is required for this response. We addressed this issue by modulating HSF1 activity with compounds acting by distinct mechanisms: sodium vanadate (SV), an inhibitor of protein phosphatases; and wortmannin, an inhibitor of DNA-dependent protein kinase. Using HSF1- and GR-responsive CAT reporters, we demonstrate that SV inhibits both HSF1 activity and the stress potentiation of GR, while having no effect on the hormone-free GR or HSF1. Paradoxically, SV increased hormone-induced GR activity in the absence of stress. In contrast, wortmannin increased HSF1 activity in stressed cells and had no effect on HSF1 in the absence of stress. Using the pMMTV-CAT reporter containing the negative regulatory element 1 site for DNA-dependent protein kinase, wortmannin was found to increase the GR response. However, in cells expressing a minimal promoter lacking negative regulatory element 1 sites, wortmannin had no effect on the GR in the absence of stress but increased the stress potentiation of GR. Our results show that the mechanism by which GR activity is increased in stressed cells requires intrinsic HSF1 activity.

Roles of the NFI/CTF gene family in transcription and development

The Nuclear Factor I (NFI) family of site-specific DNA-binding proteins (also known as CTF or CAAT box transcription factor) functions both in viral DNA replication and in the regulation of gene expression. The classes of genes whose expression is modulated by NFI include those that are ubiquitously expressed, as well as those that are hormonally, nutritionally, and developmentally regulated. The NFI family is composed of four members in vertebrates (NFI-A, NFI-B, NFI-C and NFI-X), and the four NFI genes are expressed in unique, but overlapping, patterns during mouse embryogenesis and in the adult. Transcripts of each NFI gene are differentially spliced, yielding as many as nine distinct proteins from a single gene. Products of the four NFI genes differ in their abilities to either activate or repress transcription, likely through fundamentally different mechanisms. Here, we will review the properties of the NFI genes and proteins and their known functions in gene expression and development.

A C619Y mutation in the human androgen receptor causes inactivation and mislocalization of the receptor with concomitant sequestration of SRC-1 (steroid receptor coactivator 1)

Androgen ablation therapy is a primary treatment for advanced prostate cancer, but tumors become refractive to therapy. Consequently, the role of the androgen receptors (ARs) and of mutations in the AR in prostate cancer has been a subject of much concern. In the course of analyzing tumors for mutations, we identified a somatic mutation that substitutes tyrosine for a cysteine at amino acid 619 (C619Y), which is near the cysteines that coordinate zinc in the DNA binding domain in the AR. The mutation was re-created in a wild-type expression vector and functional analyses carried out using transfection assays with androgen-responsive reporters. The mutant is transcriptionally inactive and unable to bind DNA. In response to ligand treatment, AR619Y localizes abnormally in numerous, well circumscribed predominantly nuclear aggregates in the nucleus and cytoplasm. Interestingly, these aggregates also contain the bulk of the coexpressed steroid receptor coactivator SRC-1, suggesting, in analogy to AR in spinal bulbar muscular atrophy, that this mutant may alter cellular physiology through sequestration of critical proteins. Although many inactivating mutations have been identified in androgen insensitivity syndrome patients, to our knowledge, this is the first characterization of an inactivating mutation identified in human prostate cancer.

7alpha-Iodo and 7alpha-fluoro steroids as androgen receptor-mediated imaging agents

We have synthesized several 7alpha-fluoro (F) and 7alpha-iodo (I) analogues of 5alpha-dihydrotestosterone (5alpha-DHT) and 19-nor-5alpha-dihydrotestosterone (5alpha-NDHT) and tested them for binding to the androgen receptor and for their biological activity in an in vitro assay with cells that have been engineered to respond to androgens. The relative binding affinity to the androgen receptor determined in competition assays showed that in the androstane series the fluoro steroids have the highest affinity and that F-17alpha-CH3-DHT (4) has a higher affinity than 5alpha-DHT. All other steroids were somewhat less potent than 5alpha-DHT with F-DHT (2) = I-17alpha-CH3-DHT (3) >/= F-NDHT (6) > F-17alpha-CH3-NDHT (8) = I-DHT (1) >/= I-NDHT (5) > I-17alpha-CH3-NDHT (7). The relative biological activity in cells transfected with the androgen receptor and an androgen responsive reporter gene is 4 >> 5alpha-DHT > 2 > 6 > 3 >/= 1 >/= 8 >/= 5 > 7. The iodinated compound, I-17alpha-CH3-DHT (3), with the highest binding activity was synthesized labeled with 125I and was shown to bind with high affinity, Ka = 1.9 x 10(10) L/mol, and low nonspecific binding to the androgen receptor in rat prostatic cytosol. However, when radiolabeled [125I]-17alpha-CH3-DHT ([125I]3) was injected into castrated male rats, it showed very poor androgen receptor-mediated uptake into the rat prostate. This was unexpected in light of its superior receptor binding properties and its protection by the 17alpha-methyl group from metabolic oxidation at C-17. However, the biological potency of I-17alpha-CH3-DHT (3) was not as high as would have been expected. When I-DHT (1) and I-17alpha-CH3-DHT (3) were incubated in aqueous media at 37 degrees C they rapidly decomposed, but they were stable at 0 degrees C. The fluorinated analogue 4 treated similarly at 37 degrees C was completely stable. The products of the decomposition reaction of I-DHT (1) at 37 degrees C were identified as iodide and principally 17beta-hydroxy-5alpha-androst-7-en-3-one. The temperature dependence of this elimination reaction explains the inconsistency between the high binding to the androgen receptor (measured at 0 degrees C) and the low biological activity, as well as the poor androgen receptor mediated concentration in vivo. The fluorinated analogue F-17alpha-CH3-DHT (4) has both high affinity for the androgen receptor and high stability in aqueous media. Of the compounds tested, 4 has the highest affinity for the androgen receptor as well as the highest androgenic activity. Thus it is likely that F-17alpha-CH3-DHT 4 labeled with 18F will be an excellent receptor-mediated diagnostic imaging agent.

CREB binding protein coordinates the function of multiple transcription factors including nuclear factor I to regulate phosphoenolpyruvate carboxykinase (GTP) gene transcription

Nuclear factor I (NFI) binds to a region of the phosphoenolpyruvate carboxykinase (GTP) (PEPCK) gene promoter adjacent to the cAMP regulatory element (CRE) and inhibits the induction of transcription from the gene promoter caused by the catalytic subunit of protein kinase A. In vivo footprinting studies demonstrated that both the CRE and the NFI-binding site are occupied by transcription factors, regardless of the presence of factors that stimulate (dibutyryl cAMP or dexamethasone) or inhibit (insulin) transcription from the PEPCK gene promoter. The NFI effects on transcription from the PEPCK gene promoter were observed even in the absence of the NFI binding site, suggesting the possibility of other weaker binding sites on the promoter or an interaction of NFI with a transcriptional co-activator. A mammalian two-hybrid system was used to demonstrate direct interaction between the transactivation domain of NFI-C and the CREB binding domain of the CREB-binding protein (CBP). Overexpression of a gene fragment encoding the CREB binding domain of CBP stimulates transcription from the PEPCK gene promoter. The inhibitory effect of NFI on transcription of the PEPCK gene induced by the catalytic subunit of protein kinase A appears to be the result of an interaction between NFI and the CREB-binding protein in which NFI competes with CREB for binding to the CREB-binding site on CBP. In contrast, glucocorticoids and thyroid hormone use the steroid hormone receptor binding domain of CBP to stimulate transcription from the PEPCK gene promoter. NFI-A combines with dexamethasone or thyroid hormone in an additive manner to stimulate PEPCK gene transcription. We conclude that CBP coordinates the action of the multiple factors known to control transcription of the PEPCK gene.

An androgen receptor mutation in the direct vicinity of the proposed C-terminal alpha-helix of the ligand binding domain containing the AF-2 transcriptional activating function core is associated with complete androgen insensitivity

Subjects with androgen insensitivity syndromes (AIS) are characterized by a 46, XY karyotype, presence of testes, normal or elevated androgen levels in blood, and impairment of the usual response to androgens associated with various aberrations of male differentiation and virilization ranging from slightly undervirilized men to phenotypic females. Here we describe a novel proline to serine mutation in codon 892 (exon 8) of the androgen receptor in a patient with complete androgen insensitivity. The mutation is located in the direct vicinity of the proposed C-terminal alpha-helix of the ligand binding domain containing the AF-2 transcriptional activating function core. Investigation of androgen binding in cultured testicular fibroblasts of the patient revealed a reduced AR binding capacity (11 fmol/mg protein) and a highly elevated Kd value (3.1 nM) in comparison to control genital skin fibroblasts. Cotransfection studies with an androgen-responsive reporter gene revealed a diminished transactivation property of the mutant androgen receptor.

Long duration electroporation for achieving high level expression of glucocorticoid receptors in mammalian cell lines

A method is presented that utilizes long duration electroporation (LDE) to more efficiently introduce DNA into mammalian cell lines than standard electroporation techniques. With SV40-based vectors, more than 550,000 glucocorticoid receptors (GRs) per cell could be obtained in COS-7 cells with good cell survival. In experiments with a CMV-driven vector expressing an enhanced Green Fluorescent Protein (EGFP), 54% of the cells were transfected, and 77% of EGFP positive cells expressed EGFP at moderate to high levels. In cell lines not containing the large T antigen, a CMV-driven vector for the GR was superior to the SV40-based vector. In EDR3, DG44, and CV-1 cell lines approximately 220,000, 190,000 and 150,000 GRs/cell were obtained, respectively. Transfection efficiency of the EGFP vector ranged from 44 to 55% for the three cell lines. Cortisol treatment of COS-7 and DG44 cultures cotransfected with vectors expressing the GR and a GRE driven luciferase gene produced 4 to 12 times more enzyme activity per plate with LDE than conventional electroporation protocols. LDE allows transient overexpression of proteins in COS-7 cells at the high levels generally achieved by mammalian overexpression systems only in stable cell lines.

Vitamin B-6 deficiency and level of dietary protein affect hepatic tyrosine aminotransferase activity in cats

Total activity [pyridoxal 5'-phosphate (PLP) added in the assay] of hepatic tyrosine aminotransferase (TAT) measured in cats at 0300, 0900, 1500 and 2100h was 10.3 +/-1.1, 14.0 +/- 0.7, 9.8 +/- 1.3 and 11.0 +/- 0.7 nkat/g liver, indicating little diurnal variation. Activity after 18 h of food deprivation was 10.0 +/- 0.3 nkat/g liver, also not different from cats that were eating ad libitum. These findings support the idea that cats have only limited changes in the activity of hepatic TAT compared with rats. Total TAT activity was measured in cats fed high protein (550 g/kg) and low protein (180 g/kg) diets for 4 wk. Cats fed a high protein diet had activities significantly higher (about twice) than cats fed the low protein diet. Hepatic TAT activity of vitamin B-6-deficient cats (diet without pyridoxine for 9 wk) was compared with cats given the same diet with 8 mg pyridoxine/kg. Total hepatic TAT activity in deficient cats was significantly (P < 0.05) lower per gram soluble or total protein (but not per gram liver) than control cats; holoenzyme activity and percentage of active enzyme in deficient cats were also significantly lower by 75 and 64%, respectively. The apparent Km of TAT from cats for tyrosine (2.1 mmol/L) was similar to that for rats (1.9 mmol/L), but higher for PLP in cats (0.16 micromol/L) than rats (0.034 micromol/L). Part of the reason for the higher plasma tyrosine in vitamin B-6-deficient cats than rats is the higher Km of TAT for PLP in cats than rats.

Identification of a novel RING finger protein as a coregulator in steroid receptor-mediated gene transcription

Using the DNA-binding domain of androgen receptor (AR) as a bait in a yeast two-hybrid screening, we have identified a small nuclear RING finger protein, termed SNURF, that interacts with AR in a hormone-dependent fashion in both yeast and mammalian cells. Physical interaction between AR and SNURF was demonstrated by coimmunoprecipitation from cell extracts and by protein-protein affinity chromatography. Rat SNURF is a highly hydrophilic protein consisting of 194 amino acid residues and comprising a consensus C3HC4 zinc finger (RING) structure in the C-terminal region and a bipartite nuclear localization signal near the N terminus. Immunohistochemical experiments indicated that SNURF is a nuclear protein. SNURF mRNA is expressed in a variety of human and rat tissues. Overexpression of SNURF in cultured mammalian cells enhanced not only androgen, glucocorticoid, and progesterone receptor-dependent transactivation but also basal transcription from steroid-regulated promoters. Mutation of two of the potential Zn2+ coordinating cysteines to serines in the RING finger completely abolished the ability of SNURF to enhance basal transcription, whereas its ability to activate steroid receptor-dependent transcription was maintained, suggesting that there are separate domains in SNURF that mediate interactions with different regulatory factors. SNURF is capable of interacting in vitro with the TATA-binding protein, and the RING finger domain is needed for this interaction. Collectively, we have identified and characterized a ubiquitously expressed RING finger protein, SNURF, that may function as a bridging factor and regulate steroid receptor-dependent transcription by a mechanism different from those of previously identified coactivator or integrator proteins.

Activation of androgen receptor function by a novel nuclear protein kinase

Androgen receptor (AR) belongs to the nuclear receptor superfamily and mediates the biological actions of male sex steroids. In this work, we have characterized a novel 130-kDa Ser/Thr protein kinase ANPK that interacts with the zinc finger region of AR in vivo and in vitro. The catalytic kinase domain of ANPK shares considerable sequence similarity with the minibrain gene product, a protein kinase suggested to contribute to learning defects associated with Down syndrome. However, the rest of ANPK sequence, including the AR-interacting interface, exhibits no apparent homology with other proteins. ANPK is a nuclear protein that is widely expressed in mammalian tissues. Its overexpression enhances AR-dependent transcription in various cell lines. In addition to the zinc finger region, ligand-binding domain and activation function AF1 of AR are needed, as the activity of AR mutants devoid of these domains was not influenced by ANPK. The receptor protein does not appear to be a substrate for ANPK in vitro, and overexpression of ANPK does not increase the extent of AR phosphorylation in vivo. In view of this, it is likely that ANPK-mediated activation of AR function is exerted through modification of AR-associated proteins, such as coregulatory factors, and/or through stabilization of the receptor protein against degradation.

Nuclear factor I regulates expression of the gene for phosphoenolpyruvate carboxykinase (GTP)

Nuclear factor-I (NFI) binds to the phosphoenolpyruvate carboxykinase (GTP) (PEPCK) gene promoter immediately 5' to the cAMP regulatory element (CRE). This suggests an interaction between NFI and factors that bind the CRE. Of the four NFI isoforms expressed in mammalian tissues, NFI-A and -B stimulate basal transcription from the PEPCK gene promoter in HepG2 cells, while NFI-C and -X are slightly inhibitory. All four NFI isoforms abrogate the 20-fold protein kinase Ac (PKAc)-mediated induction of transcription from the PEPCK gene promoter. Normal PKAc-mediated induction was noted when the CRE was moved 10 base pairs 3' of its original location. However if the CRE was moved 5 base pairs 3', placing it out of phase with the other elements in the promoter, or moved 5' to -285 (the P3(I) site in the promoter), some PKA-mediated stimulation was lost. The NFI-C isoform effectively inhibited PKAc induction regardless of the relative positions of the CRE and the NFI binding sites. NFI-C also abrogated cAMP regulatory element-binding protein (CREB)-induced activity of wild type and mutant PEPCK promoters. There was some cooperativity in the binding of CREB and NFI to their respective binding sites but this did not appear to be physiologically important.

A nuclear receptor corepressor modulates transcriptional activity of antagonist-occupied steroid hormone receptor

Synthetic steroid hormone antagonists are clinically important compounds that regulate physiological responses to steroid hormones. The antagonists bind to the hormone receptors, which are ligand-inducible transcription factors, and modulate their gene-regulatory activities. In most instances, a steroid receptor, such as progesterone receptor (PR) or estrogen receptor (ER), is transcriptionally inactive when complexed with an antagonist and competitively inhibits transactivation of a target steroid-responsive gene by the cognate hormone-occupied receptor. In certain cellular and promoter contexts, however, antagonist-occupied PR or ER acquires paradoxical agonist-like activity. The cellular mechanisms that determine the switch from the negative to the positive mode of transcriptional regulation by an antagonist-bound steroid receptor are unknown. We now provide strong evidence supporting the existence of a cellular inhibitory cofactor that interacts with the B form of human PR (PR-B) complexed with the antiprogestin RU486 to maintain it in a transcriptionally inactive state. In the presence of unliganded thyroid hormone receptor (TR) or ER complexed with the antiestrogen 4-hydroxytamoxifen, which presumably sequesters a limiting pool of the inhibitory cofactor, RU486-PR-B functions as a transcriptional activator of a progesterone-responsive gene even in the absence of hormone agonist. In contrast, hormone-occupied TR or ER fails to induce transactivation by RU486-PR-B. Recent studies revealed that a transcriptional corepressor, NCoR (nuclear receptor corepressor), interacts with unliganded TR but not with liganded TR. Interestingly, coexpression of NCoR efficiently suppresses the partial agonistic activity of antagonist-occupied PR-B but fails to affect transactivation by agonist-bound PR-B. We further demonstrate that RU486-PR-B interacts physically with NCoR in vitro. These novel observations suggest that the inhibitory cofactor that associates with RU486-PR-B and represses its transcriptional activity is either identical or structurally related to the corepressor NCoR. We propose that cellular mechanisms that determine the switch from the antagonistic to the agonistic activity of RU486-PR-B involve removal of the corepressor from the antagonist-bound receptor so that it can effect partial but significant gene activation.

Cross-talk between nuclear factor-kappa B and the steroid hormone receptors: mechanisms of mutual antagonism

Nuclear factor kappa B (NF-kappa B) is an inducible transcription factor that positively regulates the expression of proimmune and proinflammatory genes, while glucocorticoids are potent suppressors of immune and inflammatory responses. NF-kappa B and the glucocorticoid receptor (GR) physically interact, resulting in repression of NF-kappa B transactivation. In transient cotransfection experiments, we demonstrate a dose-dependent, mutual antagonism between NF-kappa B and GR. Functional dissection of the NF-kappa B p50 and p65 subunits and deletion mutants of GR indicate that the GR antagonism is specific to the p65 subunit of NF-kappa B heterodimer, whereas multiple domains of GR are essential to repress p65-mediated transactivation. Despite its repression of GR transactivation, p65 failed to block the transrepressive GR homologous down-regulation function. We also demonstrate that negative interactions between p65 and GR are not selective for GR, but also occur between NF-kappa B and androgen, progesterone B, and estrogen receptors. However, although each of these members of the steroid hormone receptor family is repressed by NF-kappa B, only GR effectively inhibits p65 transactivation. Further, in cotransfections using a chimeric estrogen-GR, the presence of the GR DNA-binding domain is insufficient to confer mutual antagonism to the p65-estrogen receptor interaction. Selectivity of p65 repression for each steroid receptor is demonstrated by I kappa B rescue from NF-kappa B-mediated inhibition. Together these data suggest that NF-kappa B p65 physically interacts with multiple steroid hormone receptors, and this interaction is sufficient to transrepress each steroid receptor. Further, the NF-kappa B status of a cell has the potential to significantly alter multiple steroid signaling pathways within that cell.

Interaction between the amino- and carboxyl-terminal regions of the rat androgen receptor modulates transcriptional activity and is influenced by nuclear receptor coactivators

Identical N-terminal deletions in the wild-type rat androgen receptor (rAR) and a constitutively active rAR (ARDelta641-902) devoid of the ligand-binding domain (LBD) resulted in dissimilar consequences in transcriptional activation: deletion of residues 149-295 abolished wild-type AR activity, but did not influence that of ARDelta641-902. The activity of the N-terminal transactivation domain is thus controlled by the hormone-occupied LBD, suggesting that the N- and C-terminal regions of rAR communicate. Consistent with this idea, a strong androgen-dependent interaction between the N-terminal region and LBD was demonstrated in a mammalian two-hybrid system using GAL4 and VP16 fusion proteins. This interaction can be direct or indirect. Several nuclear receptor coactivators (CBP, F-SRC-1, SRC-1, and RIP140) that interact with other steroid receptors were tested as potential mediators of the N- and C-terminal interaction of rAR using the mammalian two-hybrid system. CBP or F-SRC-1 not only enhanced AR-mediated transactivation, but also facilitated the androgen-dependent interaction between the N- and C-terminal domains, implying that part of the coactivator-dependent transcriptional activation occurs via this mechanism. In contrast, SRC-1, a coactivator for the progesterone receptor, inhibited both AR-mediated transactivation and interaction between the N and C termini. Recruitment of coregulators may involve AR domains other than the LBD, as F-SRC-1 and CBP enhanced, but SRC-1 repressed, the transcriptional activity of ARDelta641-902. Collectively, interplay between the N-terminal region and LBD of rAR results in the formation of a transactivation complex that includes coregulators and that is mandatory for optimal activation of androgen-induced promoters.

Trans-retinoic acid and glucocorticoids synergistically induce transcription from the mouse mammary tumor virus promoter in human embryonic kidney cells

Human embryonic kidney (K293) cells transfected with a mouse mammary tumor virus (MMTV) promoter-luciferase reporter construct (pHH-Luc) were utilized to investigate the potential effects of trans-retinoic acid (tRA), either by itself or in combination with glucocorticoid (GC) hormones, on a well-characterized, GC-sensitive transcriptional response. tRA or the synthetic GC hormone dexamethasone induced transcription from the MMTV promoter in a dose-dependent manner, with 1 micromol tRA and 1 micromol dexamethasone alone causing a four- to six-fold and a 40-fold induction of basal transcription, respectively. Simultaneous treatment with 1 micromol dexamethasone and 1 micromol tRA resulted in a synergistic transcriptional response that was 120-fold higher than basal level and 2.5 times the predicted response, based on a simple additive effect of both agonists. tRA does not appear to mediate this synergistic transcriptional response by enhancing GC receptor (GR) binding capacity, affinity, or nuclear translocation. tRA was unable to potentiate GC-induced transcriptional activity from a minimal GC response element (GRE), and GC were unable to potentiate tRA-induced transcriptional activity from a minimal retinoic acid response element (RARE). These data rule out direct protein-protein interactions between GC and retinoid receptors as a mechanism for the observed synergism. tRA also synergized with aldosterone-induced, mineralocorticoid receptor (MR)-mediated, transcriptional activation of the MMTV promoter, resulting in a response that was 1.7 times the predicted additive response. The MMTV GRE located between -187 and -165 was required for GC-induced and synergistic activation of the MMTV promoter, whereas sequences located within -151 to +5 were sufficient for tRA-induced transcription from the MMTV promoter. Mutation of a consensus RARE half-site (CCAAGT) identified at position -65 to -60 within the MMTV-LTR did not affect either tRA-induced transcriptional activation or synergism with GC. We propose that the tRA-induced transcriptional response from the MMTV promoter, as well as synergism with GC, may be mediated by the activation or induction of a factor(s) that either directly binds to the MMTV promoter or indirectly stabilizes binding of another transcription factor to these sequences.

Differential phosphorylation of chicken progesterone receptor in hormone-dependent and ligand-independent activation

Many steroid receptors, including chicken progesterone receptor, have been shown to be activated in the absence of their cognate ligands by modulators of kinases and phosphatases. To investigate the molecular mechanism of ligand-independent activation, chicken progesterone receptor mutants in which either one or all four of the previously identified phosphorylation sites have been changed to nonphosphorylatable alanine were analyzed for their ability to be activated by progesterone, 8-bromoadenosine 3':5'-cyclic monophosphate, or a dopamine agonist, SKF82958. Our current study shows that the receptor is differently phosphorylated in ligand-dependent and ligand-independent activation. The transcriptional activity of the receptor in response to 8-bromoadenosine 3':5'-cyclic monophosphate is affected by mutation of either Ser211 or Ser260. In addition, our data demonstrated that none of the four sites is absolutely required for the activation of the receptor by either 8-bromoadenosine 3':5'-cyclic monophosphate or the dopamine agonist. Treatment with 8-bromoadenosine 3':5'-cyclic monophosphate did not increase the overall level of receptor phosphorylation or cause phosphorylation of the receptor at alternate sites. These data raise the possibility that ligand-independent activation of the chicken progesterone receptor may be mediated through changes in the phosphorylation of coregulators or other protein factors interacting with the receptors.

Mouse glucocorticoid receptor phosphorylation status influences multiple functions of the receptor protein

Although studies have shown that the mouse glucocorticoid receptor (mGR) contains eight phosphorylation sites (Bodwell, J. E., Ortí, E. , Coull, J. M., Pappin, D. J. C., Smith, L. I., and Swift, F. (1991) J. Biol. Chem. 266, 7549-7555), the effect of phosphorylation on receptor function is unclear. We have examined the consequences of single or multiple phosphorylation site mutations on several properties of mGR including receptor expression, ligand-dependent nuclear translocation, hormone-mediated transactivation, ligand-dependent down-regulation of mGR, and receptor protein half-life. Mutations had little effect on receptor expression, subcellular distribution, ligand-dependent nuclear translocation, or on the ability to activate hormone-mediated transcription from a complex (murine mammary tumor virus) promoter. In contrast, the phosphorylation status of the mGR had a profound effect on the ability to transactivate a minimal promoter containing simple glucocorticoid response elements after hormone administration. Similarly, ligand-dependent down-regulation by glucocorticoids of both receptor mRNA and protein was abrogated in mutants containing three or more phosphorylation site alterations. Finally, we show that the phosphorylation status of mGR has a profound effect on the stability of the glucocorticoid receptor protein. Receptors containing seven or eight mutated sites have a markedly extended half-life and do not show the ligand-dependent destabilization seen with wild type receptor. These data show that receptor phosphorylation may play a crucial role in regulating receptor levels and hence control receptor functions.