Effects of hormone and cellular modulators of protein phosphorylation on transcriptional activity, DNA binding, and phosphorylation of human progesterone receptors

Ovarian stimulation with FSH reduces phosphorylation of gonadotrope progesterone receptor and LH secretion in the rat

Administration of human FSH (hFSH) to cyclic rats during the dioestrous phase attenuates progesterone receptor (PR)-dependent events of the preovulatory LH surge in pro-oestrus. The increased bioactivity of the putative ovarian gonadotropin surge inhibiting/attenuating factor induced by hFSH treatment is not associated with a decrease in PR protein expression, and the possibility of its association at a PR posttranslational effect has been raised. The present experiments aimed to analyse PR phosphorylation status in the gonadotrope of rats with impaired LH secretion induced byin vivohFSH injection. Two experimental approaches were used. First, incubated pro-oestrous pituitaries from hFSH-injected cycling and oestrogen-treated ovariectomized (OVX) rats were used to analyze the effect of calyculin, an inhibitor of intracellular phosphatases, on PR-dependent LH release, which was measured in the incubation medium by RIA. Second, pituitaries taken from hFSH-injected intact cycling and OVX rats and later incubated with P or GNRH1 were used to assess the phosphorylation rate of gonadotrope. The latter was analysed in formalin-fixed, paraffin-embedded tissue sections by immunohistochemistry using a MAB that recognizes the phosphorylated (p) form of PR at Ser294. Calyculin reduced the ovary-mediated inhibition of hFSH in GNRH1-stimulated LH secretion. In addition, the immunohistochemical expression of pSer294 PR was significantly reduced after ovarian stimulation with hFSH in pituitaries from pro-oestrous rats incubated with P or GNRH1. Altogether, these results suggested that the ovarian-dependent inhibitory effect of FSH injection on the preovulatory LH secretion in the rat may involve an increase in dephosphorylation of PR.

Cyclin dependent kinase 2 and the regulation of human progesterone receptor activity

The function of the S phase kinase cyclin A/Cdk2 in maintaining and regulating cell cycle kinetics is well established. However an alternative role in the regulation of progesterone receptor (PR) signaling is emerging. PR and its coactivators are phosphoproteins. Cyclin A/Cdk2 phosphorylates several of the PR phosphorylation sites in vitro and there is evidence that it participates in PR phosphorylation in vivo. Cyclin A/Cdk2 also functions as a PR coactivator. Overexpression increases PR transcriptional activity independent of PR phosphorylation. In the presence of hormone, cyclin A/Cdk2 is recruited to PR bound to DNA of target genes. Inhibition of Cdk activity prevents recruitment of the p160 coactivator steroid receptor coactivator-1 (SRC-1), suggesting that Cdk2 phosphorylates SRC-1. Consistent with this finding, phosphatase treatment of SRC-1 reduces its ability to interact with PR in vitro. Moreover, PR transcriptional activity is highest in S phase where cyclin A is expressed. In G1, PR activity is reduced and the capacity to recruit SRC-1 to a progestin responsive promoter is diminished. Future studies will focus on the importance of cyclin A/Cdk2 phosphorylation of other components of the PR transcription complex, such as the p160 coactivator SRC-1, and the specific role of Cdk2 target sites in the regulation of PR activity.

Fine structural alterations induced by cortisol administration in non-adrenalectomized/non-fasted rat hepatocytes

AIM

To investigate the impact of exogenously administered cortisol on non-adrenalectomized and non-fasted rat hepatocytes morphology, by means of electron microscopy. Emphasis has been given to alterations concerning particular organelles such as the nucleus, the endoplasmic reticulum, mitochondria and lysosomes.

MATERIAL AND METHOD

Accordingly to their treatment, 50 male Wistar rats were divided into two experimental groups: Group I (N=30): Hydrocortisol 8 mg/100 g of body weight intraperitoneally. Group II (N=20): Controls, given 2 ml of normal saline intraperitoneally. Animals were sacrificed 30 and 180 min after injection and liver specimens were taken for morphological study.

RESULTS

In cortisol-treated cells: At 30 min, a significant decondensation of chromatin fibers was noticed, whereas the nuclear envelope and the nucleoli remained almost intact. Endoplasmic reticulum increased in size and complexity, as well as the number of mitochondria. The number of lysosomes slightly decreased. At 180 min, chromatin remained fully decondensated. Large nucleoli were apparent within the nucleus and the external membrane of the nuclear envelope was devoid of ribosomes.

CONCLUSIONS

Corticosteroids seem to promptly induce changes in hepatocytes ultrastructucture. The alterations mainly concern the size of nucleoli, the number of mitochondria and the complexity of endoplasmic reticulum, thus being consistent with an increased etabolic activity and protein synthesis.

Evaluation of in vitro methods for detecting the effects of various chemicals on the human progesterone receptor, with a focus on pyrethroid insecticides

The progesterone receptor (PR) is associated with physiological events such as implantation and the maintenance of pregnancy. Recently, it has become a social concern that chemicals may exert agonistic or antagonistic effects on hormone receptors. Therefore, we examined the effects of various chemicals on the human PR, with a focus on pyrethroid insecticides, using three in vitro methods. Eight pyrethroid insecticides (fenvalerate, d-allethrin, d-phenothrin, prallethrin, empenthrin, permethrin, cypermethrin and imiprothrin), examples of environmental pollutants and positive control chemicals were subjected to a reporter gene assay (luciferase assay) using human breast cancer T-47D cells, a two-hybrid assay and a binding assay using the same whole cells or receptors (cell-free). In none of these did the eight pyrethroid insecticides show any binding to the PR, agonistic or antagonistic effects.

Hormonal regulation of tumor suppressor proteins in breast cancer cells

This laboratory is studying hormonal regulation of tumor suppressor proteins, p53 and retinoblastoma (pRB). Estrogen receptor and progesterone receptor positive human breast cancer cell lines, T47D and MCF-7, were utilized for determining influence of hormonal and antihormonal agents on the level of expression of p53, state of phosphorylation of pRB, and rate of cell proliferation. The expression of p53 in T47D cells grown for 4-5 days in culture medium containing charcoal-treated (stripped) fetal bovine serum declined gradually to 10% of the level seen in control (whole serum, non charcoal-treated) groups. Supplementation of culture medium containing stripped serum with 0.1-1 nM estradiol (E(2)) restored p53 to its level seen in the control within 6-24 h. Under above conditions, treatment of cells with R5020 or RU486 reduced (15-30%) the level of p53. Incubation of cells in E(2)-containing growth medium caused cell proliferation and hyperphosphorylation of pRB; the latter effect was seen maximally between 24-72 h. The E(2)-induced hyperphosphorylation of pRB and increase in the level of p53 were sensitive to the presence of ICI and 4-hydroxy tamoxifen (OHT). T47D and MCF-7 cells were also transiently transfected with a P1CAT reporter plasmid containing c-Myc responsive element and the levels of chloramphenicol acetyltransferase (CAT) activity were observed in response to various treatments. E(2) and OHT caused P1CAT induction as seen by increased CAT activity: E(2) caused an endogenous increase in the expression of an ICI-sensitive c-Myc form. These data suggest that estrogen upregulates p53 expression while progesterone downregulates this process. Further, E(2) regulates p53 level and pRB activity in a coordinated manner.

Modulation of androgen receptor (AR)-mediated transcriptional activity by EGF in the developing mouse reproductive tract primary cells

Recently, a role for epidermal growth factor in male sexual differentiation was reported from different laboratories. We demonstrated that androgen receptor (AR) mediates the EGF-induced effects. The mechanism, by which EGF modulates AR mediated activity, is not known and the current studies were designed to investigate the role of AR. Using mesenchymal cell preparation from the 18-day fetal reproductive tract, first, we determined whether EGF induced sexual differentiation by enhancing AR gene expression. Thus, AR mRNA and AR protein levels were measured in response to EGF-treatment using RT-PCR based analysis of AR mRNA and Western blot analysis of AR protein level respectively. Both of these analysis detected presence of AR gene expression in this cell preparation, however no effect of EGF was observed in AR protein or AR mRNA expression. Next, we examined whether EGF enhanced AR mediated transcriptional activity in the developing tract. Using the mesenchymal cells, co-transfected with AR expression vector (pSV-AR) and androgen response element linked to luciferase reporter vector (pMAMneoLUC) we assessed AR-mediated transcriptional activity in response to EGF treatment in the presence and absence of testosterone. The results showed that androgen stimulated the luciferase activity in a dose dependent manner, as expected. EGF also enhanced such activity; however, the response was significantly lower than that by androgen. EGF, however, produced striking enhancement of the androgen-induced transcriptional activity when used with androgen. EGF and testosterone produced no stimulation of the luciferase activity either alone or in combination in the cells lacking AR expression vector, suggesting a role for AR in the effect of EGF and testosterone. Flutamide, an AR antagonist, also blocked the enhancement of luciferase activity induced by EGF and testosterone, further confirming the role of AR in the effect of EGF and testosterone. Thus, it appears that EGF-modulation of sexual differentiation involves enhancement of AR-mediated transcriptional activity and not enhancement of AR gene expression. Additionally, it appears that EGF modulates sexual differentiation in the presence of testosterone possibly by potentiating the testosterone-effect.

Using yeast to study glucocorticoid receptor phosphorylation

The glucocorticoid receptor (GR) is a phosphoprotein and a member of the steroid/thyroid receptor superfamily of ligand dependent transcription factors. When the glucocorticoid receptor is expressed in yeast (Saccharomyces cerevisiae), it is competent for signal transduction and transcriptional regulation. We have studied the glucocorticoid receptor phosphorylation in yeast and demonstrated that the receptor is phosphorylated in both the absence and presence of hormone, on serine and threonine residues. This phosphorylation occurs within 15 min upon addition of radioactivity in both hormone treated and untreated cells. As reported for mammalian cells, additional phosphorylation occurs upon hormone binding and this phosphorylation is dependent on the type of the ligand. We have followed the hormone dependent receptor phosphorylation by electrophoretic mobility shift assay, and have shown that this mobility change is sensitive to phosphatase treatment. In addition, the appearance of hormone dependent phosphoisoforms of the receptor depends on the potency of the agonist used. Using this method we show that the residues contributing to the hormone dependent mobility shift are localized in one of the transcriptional activation domains, between amino acids 130-247. We altered the phosphorylation sites within this domain that correspond to the amino acids phosphorylated in mouse hormone treated cells. Using phosphopeptide maps we show that hormone changes the peptide pattern of metabolically labelled receptor, and we identify peptides which are phosphorylated in hormone dependent manner. Then we determine that phosphorylation of residues S224 and S232 is increased in the presence of hormone, whereas phosphorylation of residues T171 and S246 is constitutive. Finally, we show that in both yeast and mammalian cells the same residues on the glucocorticoid receptor are phosphorylated. Our results suggest that yeast cells would be a suitable system to study glucocorticoid receptor phosphorylation. The genetic manipulability of yeast cells, together with conservation of the phosphorylation of GR in yeast and mammalian cells and identification of hormone dependent phosphorylation, would facilitate the isolation of molecules involved in the glucocorticoid receptor phosphorylation pathway and further our understanding of this process.

Differential expression of uterine progesterone receptor forms A and B during the menstrual cycle

Recent studies suggest that the progesterone receptor isoforms (PR-A and PR-B) activate genes differentially and that PR-A may act as a repressor of PR-B function. Hence, the absolute and relative expression of the two isoforms will determine the response to progesterone. We have measured their relative expression in the uterus of cycling women who underwent endometrial biopsy. PR isoforms were identified on blots of SDS-PAGE gels by reaction with the AB-52 antibody after immunoprecipitation from endometrial extract. Both isoforms were highest in the peri-ovulatory phase, but levels of PR-A were always higher than those of PR-B. The ratio of PR-A to PR-B changed during the menstrual cycle. Between days 2 and 8, PR-B is almost undetectable and the A:B ratio is >10:1. From days 9 to 13, the ratio is about 5:1, and it is about 2:1 between days 14 and 16. Thereafter, PR-B dwindles rapidly and is virtually undetectable at the end of the cycle. In various hypoestrogenic environments, PR-B expression was reduced. However, exogenous estrogens in the follicular phase in the form of oral contraceptives, enhanced PR-B expression. These data support the possibility that progesterone acts through cycle-specific PR isoforms.

The effects of epidermal growth factor, interleukin-1, and phenytoin, alone and in combination, on C19 steroid conversions in fibroblasts

The formation of the biologically active metabolite 5 alpha-dihydrotestosterone (DHT) from testosterone in response to phenytoin (Ph), interleukin-1 (IL-1), and epidermal growth factor (EGF) was investigated. The androgen DHT stimulates matrix synthesis in connective tissue and bone. Duplicate incubations were performed with confluent human gingival fibroblasts, 14C-testosterone, and optimal stimulatory concentrations of IL-1 (5 IU/ml), EGF (10 ng/ml), Ph (5 micrograms/ml), Ph + EGF, and Ph + IL-1 respectively for 24 hours in Eagle's MEM at 37 degrees C. The medium was then analyzed for radioactive metabolites. Similar incubations were performed with human gingival tissue using 14C-4-androstenedione as substrate in the presence or absence of EGF, Ph, and EGF + Ph. In the cell lines studied, EGF stimulated DHT and 4-androstenedione synthesis by 20% (n = 5; P < 0.01; Wilcoxon signed rank statistic for paired observations). IL-1 stimulated DHT and 4-androstenedione synthesis by 2-fold (n = 6; P < 0.01). Ph stimulated DHT and 4-androstenedione synthesis by 2-fold increases (n = 3; P < 0.01). Combinations of phenytoin and EGF stimulated DHT and 4-androstenedione synthesis by 33% and 37% greater than the effect of phenytoin alone (n = 3; P < 0.01). Combinations of Ph and IL-1 caused a 45% increase in the amount of DHT formed and a 66% increase in 4-androstenedione when compared to the effect of phenytoin alone (n = 3; P < 0.01). 14C-4-androstenedione was converted to DHT and testosterone by human gingival tissue. There were 2-fold, 4-fold, and 2.5-fold increases in DHT synthesis and 5-fold, 2-fold, and 6-fold increases in the formation of testosterone in response to EGF, Ph, and EGF + Ph respectively (n = 3; P < 0.01). EGF and IL-1 present in inflammatory exudate may have implications on phenytoin-induced overgrowth via the steroid metabolic pathway.

Protein kinase C pathway potentiates androgen-mediated gene expression of the mouse vas deferens specific aldose reductase-like protein (MVDP)

Transcription of the mouse vas deferens protein (MVDP) gene, a member of the aldo-keto reductase superfamily, is stimulated by androgens via the androgen responsive element (ARE) located in the proximal promoter (-111 to -97). We investigated interaction between androgens and the protein kinase C (PKC) signalling pathway. Transcriptional regulation was determined by analysis of chloramphenicol acetyltransferase (CAT). T47D cells were transiently transfected with 5' flanking MVDP DNA promoter sequences (-1804 to +41; -510 to +41 and -121 to +41) fused to the reporter (CAT) gene. Androgen-induced transcriptional activity can be enhanced from 6 (1.8 and 0.5 kb MVDP-CAT constructs) to 18 fold (0.16 kb MVDP-CAT construct), in a time and dose-dependent manner, by the PKC activator 12-o-tetradecanoylphorbol-13 acetate (TPA). A mutation in the proximal ARE abolished both androgen and TPA-dependent gene enhancement. TPA influenced minimally MMTV promoter in T47D cells and MVDP promoter in CV1 cells suggesting that the effects of the PKC activator are probably promoter and cell-specific. In contrast, activation of protein kinase A (PKA) via addition of dibutyryl-cAMP (db-cAMP) reduced androgen induction of the MVDP gene.

The synthesis of 5-alpha-dihydrotestosterone from androgens by human gingival tissues and fibroblasts in culture in response to TGF-beta and PDGF

The effects of TGF-beta and PDGF on the metabolic conversion of 14C-testosterone by human gingival tissue (HGT) from 5 subjects was investigated. The metabolic conversions in response to TGF-beta and PDGF were also studied in 4-6 cell-lines of cultured gingival fibroblasts, using 14C-testosterone and 14C-4-androstenedione as substrates. Duplicate incubations of HGT were performed in Eagle's MEM + 10% FCS and optimal stimulatory concentrations of TGF-beta/PDGF for 24 h. Similar incubations were performed in duplicate with cell-lines of cultured gingival fibroblasts, TGF-beta/PDGF, 14C-testosterone/14C-4-androstenedione in Eagle's MEM + 10% FCS. The radioactive metabolites were extracted, separated and quantified. With HGT, TGF-beta and PDGF caused 2.5/2-fold increases in DHT synthesis (p < 0.1; Wilcoxon signed rank test) and 3.4/2-fold increases in 4-androstenedione formation (p < 0.1) from 14C-testosterone. PDGF increased DHT and testosterone synthesis from 14C-4-androstenedione by 3-fold in gingivae (p < 0.1). With cell-lines, average values of duplicate incubations showed 2.8/2-fold increases in DHT synthesis from 14C-testosterone in response to TGF-beta/PDGF (p < 0.1; p < 0.2) and 2.4/2-fold increases in 4-androstenedione synthesis (p < 0.1; p < 0.2). With 14C-4-androstenedione as substrate, TGF-beta/PDGF caused 1.6/1.9-fold increases in DHT synthesis compared with controls (p < 0.05; p < 0.1) and 1.7/1.5-fold increases in testosterone formation from this substrate (p < 0.05; p < 0.1). Due to the strong implications of TGF-beta/PDGF and anabolic androgens on matrix repair, significant increases in DHT synthesis from 2 androgenic substrates in response to TGF-beta and PDGF are of particular relevance to inflammatory repair processes.

The influence of insulin-like growth factor (IGF) on C19 steroid conversions by human gingiva and in cultured gingival fibroblasts

The metabolic conversion of 14C-testosterone by human gingival tissue in response to IGF was studied. Androgen metabolic studies were also performed in 5 to 7 cell-lines of cultured gingival fibroblasts, using 14C-testosterone and 14C-4-androstenedione as initial substrates. Duplicate incubations of gingival tissue were performed after establishing the wet weight, in Eagle's MEM + 10% FCS and optimal stimulatory concentrations of IGF for 24 hours. Similar incubations were performed in duplicate with cell-lines of gingival fibroblasts, control/IGF, and 14C-testosterone/14C-4-androstenedione. At the end of the incubation period, the radioactive metabolites were extracted, evaporated, subjected to thin layer chromatography for their separation, and quantified by scanning in a Berthold's linear analyzer. With the gingival tissue samples, IGF caused a 4-fold increase in 5 alpha-dihydrotestosterone (DHT) synthesis (n = 5; P < 0.1, Wilcoxon signed rank test for paired observations) and a 3.5-fold increase in 4-androstenedione formation (n = 5; P < 0.1) from 14C-testosterone. When similar incubations were performed with cell-lines of fibroblasts and 14C-testosterone, average values of duplicate incubations showed a 2.5-fold increase in DHT synthesis in response to IGF (n = 7; P < 0.002) and a 2.3-fold increase in 4-androstenedione formation (n = 7; P < 0.002). With 14C-4-androstenedione as substrate, IGF stimulated a 2.7-fold increase in DHT synthesis (n = 5; P < 0.1) compared with controls and a 1.8-fold increase in testosterone formation (n = 5; P < 0.1). Since both DHT and IGF are implicated in protein turnover by fibroblasts, significant stimulation of DHT synthesis by IGF in gingiva and cultured fibroblasts is suggestive of a possible mechanism for mediating inflammatory repair via the androgen metabolic pathway.

Phosphorylation and progesterone receptor function

Four phosphorylation sites have been identified in the chicken progesterone receptor. Two of these sites exhibit basal phosphorylation which is enhanced upon treatment with hormone and two of the sites are phosphorylated in response to hormone. Mutation of one of these hormone dependent sites, Ser530 to Ala530, causes a decrease in transcriptional activation at low concentrations of hormone, but the activity is unaffected at high concentrations. However, the hormone binding of the mutant is unaffected suggesting that phosphorylation of Ser530 plays a role in facilitating the response of the receptor to low concentrations of hormone. The chicken progesterone receptor can be activated by modulators of kinases in the absence of hormone. The finding that signals initiated by tyrosine phosphorylation (through treatment with EGF) or through the dopamine receptor suggests that there are multiple means of activating chicken progesterone receptor. In contrast, the human progesterone receptor does not exhibit ligand independent activation; however, its activity in the presence of the agonist R5020 is enhanced by treatment with 8-Br-cAMP, an activator of protein kinase A, and treatment with 8-Br-cAMP causes the antagonist, RU486, to act as an agonist.

Progesterone receptor and the mechanism of action of progesterone antagonists

Currently available progesterone antagonists have been suggested to fall into two categories based on differences in how they interact with and inactivate the progesterone receptor (PR). The anti-progestin ZK98299 (Type I) impairs PR association with DNA, while Type II compounds (RU486, ZK112993, ZK98734) promote PR binding to DNA. Type II agents, therefore, appear to inhibit receptor activity at a step downstream of DNA binding, presumably failing to induce conformational changes in PR structure requird for enhancement of transcription. This paper discusses both published and unpublished data supporting the concept of two types of progestin antagonists. Using PR-mediated induction of reporter genes in breast cancer cells as an assay for biological response, both types of anti-progestins, after correction for difference in steroid binding affinity, inhibit progestin induction substoichiometrically. However, Type II anti-progestins are more potent, inhibiting at lower ratios of antagonist to agonist than ZK98299. This suggests that in addition to behaving by classical competitive mechanisms these compounds (in particular Type II) may exhibit additional activity as transrepressors of PR in the same cell bound to hormone agonist. Transrepression may occur by the combined mechanisms of heterodimerization and competition for binding to DNA. In support of this, mixed ligand dimers form readily in solution between a PR subunit bound to agonist and another bound to either type of anti-progestin, whereas these mixed ligand dimers bind poorly, if at all, to specific progesterone response elements (PREs) in vitro. Additionally, when added as a single ligand, Type II agents increase PR dimerization in solution and PR affinity for PREs as compared with single ligand dimers formed by progestin agonist. This contrasts with ZK98299, when given as a single ligand, which reduces PR affinity for PREs without disrupting solution dimerization. Thus the higher affinity of PR for PREs may account for the greater biological potency of Type II compounds as compared with ZK98299. As a further distinction between types of antiprogestins, ZK98299 minimally stimulates phosphorylation of PR whereas RU486 increases site-specific phosphorylation of PR in a manner indistinguishable from that of hormone agonist. Additionally, ZK98299 is not susceptible in vivo to functional switching to a partial agonist by cross talk with cAMP signal transduction pathways, as occurs with Type II compounds. Thus, ZK98299 under certain conditions may be a more pure antagonist than Type II compounds.

Glucocorticoid receptor phosphorylation in v-mos-transformed cells

Nucleocytoplasmic shuttling of glucocorticoid receptors (GRs) is disrupted in v-mos-transformed cells leading to the redistribution of hormone-bound receptors from the nuclear to cytoplasmic compartments. We show here that GRs from v-mos-transformed cells are hyperphosphorylated on a specific peptide and maintain hormone-induced phosphorylations upon a prolonged hormone treatment that is associated with disruptions in its nucleocytoplasmic shuttling. Since similar effects on GR nucleocytoplasmic shuttling and phosphorylation were exerted upon treatment of nontransformed cells with the protein phosphatase inhibitor okadaic acid, we examined whether hyperphosphorylation of GRs in v-mos-transformed cells resulted from inhibition of receptor dephosphorylation. Protein phosphatase activity, measured using various substrates in vitro, was identical in cell-free extracts prepared from v-mos-transformed and nontransformed cells. Analysis of phosphate turnover in vivo from either the sum of all GR phosphorylation sites or from individual sites using pulse-chase analysis, did not reveal any significant difference between v-mos-transformed cells versus nontransformed cells. Thus, hyperphosphorylation of GR in v-mos-transformed cells does not appear to result from inhibition of GR dephosphorylation, but rather from stimulation of GR phosphorylation.

Synergism between androgens and protein kinase-C on androgen-regulated gene expression

Androgen (R1881) induced transcriptional activity of the human androgen receptor, stably expressed in CHO cells, can be stimulated an extra 2-fold by the addition of the protein kinase C activator, 4 beta-phorbol 12-myristate 13-acetate (PMA). This extra stimulation is not observed when the protein kinase A activator bromoadenosine 3':5'-cyclic monophosphate (8-BrcAMP) is used. The transcriptional activity was measured using a reporter plasmid containing the MMTV-promoter, coupled to the luciferase gene. The effect of PMA on R1881-induced transcription was not due to a higher expression level of the androgen receptor. Also, no extra phosphorylation of the androgen receptor could be measured after incubation with PMA. When GRE-tk-LUC and PSA-LUC reporters were used, the synergistic effect of PMA could not be observed. The findings on the composite MMTV-LTR promoter can be explained by either a direct synergistic interaction between occupied AP-1 like responsive elements and the androgen receptor or via an unknown transcription factor activated by the PKC pathway and interacting with the androgen receptor.

Fatty acids: ancestral ligands and modern co-regulators of the steroid hormone receptor cell signalling pathway

Long chain fatty acids derived from endogenous metabolism and/or nutrition are regulators of cell signalling pathways. They can be lipid second messengers of signal transduction systems or modulators and regulators of intracellular signalling pathways such as those involved in the mechanism of action of steroid hormones. Fatty acids have been shown to activate gene transcription under the control of some evolutionarily primitive members of the steroid/thyroid superfamily of receptors. They may represent ancestral ligands of this superfamily of receptors. Fatty acids are also known to regulate the activity of protein kinases, particularly protein kinase C, and thereby phosphorylation of intracellular proteins involved in regulation of gene transcription. Fatty acids may be co-regulators in the cross-talk between membrane-triggered signal transduction and the intracellular steroid hormone signalling pathway. Fatty acids are known to affect either negatively and/or positively the binding of steroid hormones to their specific plasma transport proteins and their specific intracellular receptors and, very recently, fatty acids have also been shown to co-regulate glucocorticoid-dependent gene expression. The mechanism of action of steroid hormones will be used as an illustration of how fatty acids can intervene at different levels of cellular organization to regulate biological activity, with a focus on the glucocorticoid receptor.

Steroid hormone receptors: activators of gene transcription

Over the past three decades, a great deal of evidence has accumulated in favor of the hypothesis that steroid hormones act via regulation of gene expression. The action is mediated by specific nuclear receptor proteins, which belong to a superfamily of ligand-modulated transcription factors that regulate homeostasis, reproduction, development and differentiation. This family includes receptors for steroid hormones, thyroid hormones, hormonal forms of vitamin A and D, peroxisomal activators, and ecdysone. Molecular cloning and structure/function analyses have revealed that all members of the steroid/thyroid hormone/retinoic acid receptor family have a similar functional domain structure: a variable N-terminal region, which is involved in modulation of gene expression; a short well-conserved DNA-binding domain, which is crucial for recognition of specific DNA sequences and for receptor dimerization; and a partially conserved C-terminal ligand-binding domain, which is important for hormone binding and also for receptor dimerization and transactivation. In contrast to other members of the receptor superfamily steroid hormone receptors form transient complexes with several heat shock proteins. This interaction promotes proper folding and stability of the receptor molecule. Hormone binding induces a conformational change in the receptor molecule and simultaneously a dissociation of all heat shock proteins, which results in DNA-binding of the hormone-receptor complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Nuclear accessory factors enhance the binding of progesterone receptor to specific target DNA

The human progesterone receptor (PR) is dependent upon hormone and a nuclear accessory factor(s) for maximal binding to progesterone response elements (PRES) in vitro. Recombinant full-length PR, expressed in a baculovirus system and purified to apparent homogeneity, was used as a substrate to isolate and identify the accessory factor(s). The major PRE binding enhancement activity present in nuclear extracts was shown to be associated with the high mobility group chromatin protein HMG-1. Moreover, HMG-1 was equally effective in enhancing the DNA binding of both the A and B isoforms of PR. Enhancement of PRE binding was highly selective for HMG-1 as a single purified protein and was not mimicked by a general protein stabilization effect. In gel mobility shift assays, it appeared that HMG-1 enhanced PRE binding without stably participating as a component of the final DNA-PR complex, suggesting that HMG-1 acts indirectly by modifying the PR protein or the target DNA. HMG-1 is a sequence-independent DNA binding protein that recognizes distorted DNA structures and is also able to promote further distortions by bending DNA. Enhancement of PRE binding was found to be intrinsic to the conserved DNA binding domain of HMG-1 suggesting that HMG-1 acts by promoting a structural alteration in the target PRE-DNA.

Progesterone receptor phosphorylation complexities in defining a functional role

All steroid receptors are phosphoproteins and several, including progesterone receptors (PRs), become hyperphosphorylated upon binding of ligand. PR phosphorylation is complex, occurring in different cellular compartments and perhaps requiring multiple serine kinases. A model that is emerging proposes that PR phosphorylation is progressive, occurring in at least a three-stage cascade. However, the functional significance of this phosphorylation cascade remains unclear.

Functional cross-talk between receptors for peptide and steroid hormones

Communication between a cell surface peptide hormone receptor and an intracellular steroid hormone receptor can take various routes, as dictated by the physiology of a particular cell type. There is increasing evidence for a novel route which requires that a peptide hormone receptor pathway converge on a steroid hormone receptor, leading to its activation. One consequence of such a process can be signal amplification for the peptide hormone receptor agonist. This is exemplified by the self-potentiating action of GnRH, which is a critical component in events leading to a surge in LH secretion and ovulation. One signaling pathway stimulated by the GnRH receptor may entail a phosphorylation cascade resulting in progesterone-independent modulation of progesterone receptor activity.