Molecular mechanisms of action of steroid/thyroid receptor superfamily members

Drosophila Sgs genes: stage and tissue specificity of hormone responsiveness

The up- and down-regulation of the salivary gland secretion protein (Sgs) genes during the third larval instar of Drosophila melanogaster are controlled by fluctuations of the titre of the steroid hormone 20-hydroxyecdysone (20E). Induction of these genes by a low hormone titre is a secondary response to 20E mediated by products of 20E-induced 'early' genes. Surprisingly, in the case of the Sgs-4 gene this response also requires a direct contribution of the 20E-receptor complex. A model is presented which proposes that the Sgs genes, and other 20E-regulated genes with similar temporal expression profiles, are regulated by complex hormone response units. The hormonal signal is effectively transmitted by these response units only after binding of additional factors, e.g. secretion enhancer binding proteins, which act together in a synergistic manner with the 20E receptor and early gene products to establish a stage- and tissue-specific expression pattern.

Analysis of mechanisms that determine dominant negative estrogen receptor effectiveness

To analyze the mechanisms by which estrogen receptor (ER) activity is suppressed by dominant negative mutants, we examined the role of specific ER functions and domains in transcriptional repression. We previously described three transcriptionally inactive human ER mutants (the frameshift mutant S554fs, the point mutant L540Q, and the truncated receptor ER1-530), which act as effective dominant negative mutants, inhibiting the activity of wild type ER when they are coexpressed in mammalian cells. After additional mutational modifications, the ability of the ER mutants to suppress the activity of wild type ER was analyzed in cotransfection assays of the dominant negative mutants and wild type ER and an estrogen-responsive reporter gene (2ERE-TATA-CAT or 2ERE-pS2-CAT). Eliminating the ability of the three dominant negative mutants to bind to estrogen response element (ERE) DNA (by introducing three point mutations in their DNA binding domains) dramatically reduced, but did not completely abolish, the dominant negative activity of the ER mutants. The mutation G521R, which rendered the three mutants incapable of binding estradiol, also reduced, but did not abolish, their dominant negative activity. Immunoprecipitation with monoclonal or flag antibodies followed by Western blotting demonstrated that each of the original dominant negative ER mutants formed heterodimers with wild type ER. Rendering the dominant negative mutants dimerization deficient by the mutation L507R strongly reduced, but did not eliminate, their dominant negative activity. Deletion of the N-terminal A/B domain resulted in the nearly complete loss of inhibitory activity of the three dominant negative mutants. However, these double mutants retained their ability to heterodimerize with wild type ER, suggesting that dominant negative interference also occurs at an additional step beyond dimerization. Our data indicate that competition for ERE binding, formation of inactive heterodimers, and specific transcriptional silencing can all contribute to the dominant negative phenotype and that these receptors suppress the activity of wild type ER by acting at multiple steps in the ER-response pathway.

Androgen and glucocorticoid regulation of androgen receptor cDNA expression

Androgen receptor (AR) levels are regulated by androgens, other steroids and non-steroidal hormones via complex, tissue-specific processes. Since alterations in receptor levels may influence cellular sensitivity to androgens, understanding AR regulation is of fundamental and potentially therapeutic significance. In most target tissues and AR-containing cell lines, AR mRNA is down-regulated in response to androgens. We have reconstituted this androgen-mediated down-regulation of AR mRNA in COS 1 cells transfected with a human AR cDNA under the control of the cytomegalovirus (CMV) promoter. The sequences mediating receptor mRNA down-regulation are represented within the AR cDNA and not within the CMV promoter. Androgenic down-regulation of AR cDNA expression was time- and dose-dependent, resembling native AR mRNA down-regulation. In addition, androgenic regulation of the receptor cDNA was not dependent on protein synthesis suggesting that AR and/or another pre-existing protein(s) is involved in this process. In COS 1 cells co-transfected with androgen and glucocorticoid receptor cDNAs, dexamethasone mimicked the action of androgen in down-regulating AR mRNA. This response depended on glucocorticoid receptors. Androgen had little effect on steady-state levels of AR protein consistent with reports that androgen down-regulates AR mRNA but increases AR protein half-life (Kemppainen et al. (1992) J. Biol. Chem. 267, 968-974; Zhou et al. (1995) Mol. Endocrinol. 9, 208-218). However, glucocorticoids decreased AR protein levels in cells that co-expressed androgen and glucocorticoid receptors. These results indicate that sequences represented in the AR cDNA mediate AR mRNA down-regulation by both androgens and glucocorticoids. Inhibition of AR mRNA and protein by glucocorticoids suggests that these steroids may modulate androgen action in tissues, such as mammary gland and prostate, which express both androgen and glucocorticoid receptors.

Thyroid hormone receptors bind to the promoter of the mouse histone H10 gene and modulate its transcription

It has been shown that the mouse histone H10 promoter contains a DNA element, composed of a direct repeat of the sequence GGTGACC separated by 7 nt, which is able to bind retinoic acid receptors and to modulate transcription of reporter genes following treatment with retinoic acid. We have now investigated whether this DNA motif is also responsive to thyroid hormone. We co-transfected CV-1 monkey kidney cells with chloramphenicol acetyltransferase (CAT) expression plasmids containing either 740 bp of the H10 wild-type promoter or five copies of the repeat element cloned in front of the thymidine kinase promoter and expression vectors for human thyroid hormone receptors (TRs) alpha or beta and retinoid X receptor alpha (RXR alpha). Treatment of transfected cells with triiodothyronine led to a dose-dependent increase in CAT activity. Transfection experiments with increasing amounts of expression vectors for either TR alpha or RXR alpha resulted in up to 6-fold enhancement of CAT transcription. Furthermore, point mutations within the half-sites of the response element of the H10 promoter, as well as deletions within the interspace region, lowered CAT activity to 60-80% of that of the wild-type control. Electrophoretic mobility shift assays showed that the repeat element was able to form retarded complexes with TR alpha homodimers, as well as with TR alpha-RXR alpha heterodimers. Our results suggest that thyroid hormone receptors are involved in the regulation of mouse histone H10 expression.

Ligand-dependent, transcriptionally productive association of the amino- and carboxyl-terminal regions of a steroid hormone nuclear receptor

The estrogen receptor (ER), a 66-kDa protein that mediates the actions of estrogens in estrogen-responsive tissues, is a member of a large superfamily of nuclear hormone receptors that function as ligand-activated transcription factors. ER shares a conserved structural and functional organization with other members of this superfamily, including two transcriptional activation functions (AFs), one located in its amino-terminal region (AF-1) and the second located in its carboxyl-terminal, ligand-binding region (AF-2). In most promoter contexts, synergism between AF-1 and AF-2 is required for full ER activity. In these studies, we demonstrate a functional interaction of the two AF-containing regions of ER, when expressed as separate polypeptides in mammalian cells, in response to 17 beta-estradiol (E2) and antiestrogen binding. The interaction was transcriptionally productive only in response to E2, and was eliminated by point or deletion mutations that destroy AF-1 or AF-2 activity or E2 binding. Our results suggest a definitive mechanistic role for E2 in the activity of ER--namely, to alter receptor conformation to promote an association of the amino- and carboxyl-terminal regions, leading to transcriptional synergism between AF-1 and AF-2. The productive re assembly of two portions of ER expressed in cells as separate polypeptides demonstrates the evolutionarily conserved modular structural and functional organization of the nuclear hormone receptors. The ligand-dependent interaction of the two AF-containing regions of ER allows for the assembly of a complete activation function from two distinct regions within the same protein, providing a mechanism for hormonally regulated transcription.

A structural role for hormone in the thyroid hormone receptor

The crystal structure of the rat alpha 1 thyroid hormone receptor ligand-binding domain bound with a thyroid hormone agonist reveals that ligand is completely buried within the domain as part of the hydrophobic core. In addition, the carboxy-terminal activation domain forms an amphipathic helix, with its hydrophobic face constituting part of the hormone binding cavity. These observations suggest a structural role for ligand, in establishing the active conformation of the receptor, that is likely to underlie hormonal regulation of gene expression for the nuclear receptors.

A protein that interacts with members of the nuclear hormone receptor family: identification and cDNA cloning

In search of proteins which interact with activated steroid hormone receptors, we screened a human liver lambda gt11 expression library with the glucocorticoid receptor. We identified and cloned a cDNA sequence of 1322 bp that encodes a protein of 274 aa. This protein consists predominantly of hydrophilic amino acids and contains a putative bipartite nuclear localization signal. The in vitro translated receptor-associating protein runs in SDS/polyacrylamide gels with an apparent molecular mass of 46 kDa. By use of the bacterially expressed fusion protein with glutathione S-transferase we have found that interaction is not limited to the glucocorticoid receptor but included other nuclear receptors--most notably, the estrogen and thyroid receptors. Binding also occurs with the glucocorticoid receptor complexed with the antiglucocorticoid RU 38486, with the estrogen receptor complexed with the antiestrogen 4-hydroxytamoxifen or ICI 164,384, and even with receptors not complexed with ligand. Association with steroid hormone receptors depends on prior receptor activation--i.e., release from heat shock proteins. The sequence identified here appears to be a general partner protein for nuclear hormone receptors, with the gene being expressed in a variety of mammalian tissues.

Specific mutations in the ligand binding domain selectively abolish the silencing function of human thyroid hormone receptor beta

Although most nuclear hormone receptors are ligand-dependent transcriptional activators, certain members of this superfamily, such as thyroid hormone receptor (TR) and retinoic acid receptor (RAR), are involved in transcriptional repression. The silencing function of these receptors has been localized to the ligand binding domain (LBD). Previously, we demonstrated that overexpression of either the entire LBD or only the N-terminal region of the LBD (amino acids 168-259) is able to inhibit the silencing activity of TR. From this result we postulated the existence of a limiting factor (corepressor) that is necessary for TR silencing activity. To support this hypothesis, we identified amino acids in the N-terminal region of the LBD of TR that are important for the corepressor interaction and for the silencing function of TR. The silencing activity of TR was unaffected by overexpression of the LBD of mutant TR (V174A/D177A), suggesting that valine at position 174 and/or aspartic acid at position 177 are important for corepressor interaction. This mutant receptor protein, V174/D177, also lost the ability to silence target genes, suggesting that these amino acids are important for silencing function. Control experiments indicate that this mutant TR maintains its wild-type hormone binding and transactivation functions. These findings further strengthen the idea that the N-terminal region of the LBD of TR interacts with a putative corepressor protein(s) to achieve silencing of basal gene transcription.

Steroid hormones and temperature induce changes of binding parameters of their receptors in intact cells

When MCF-7 cells were treated with 17 beta-estradiol, dexamethasone, or promegestone at 37 degrees C, the KD of receptors for their cognate ligands was found to decrease as compared to that measured at 2 degrees C. Cell incubation with hormone at 37 degrees C did not affect the Bmax of glucocorticoid and progesterone receptors, but caused a 40% increase of that of estrogen receptor. This increase required the presence of ligand, was insensitive to cycloheximide, and was completed within 10 min of cell incubation at physiological temperature. We conclude that an early step of estrogen action is the recruitment of pre-existing receptor molecules through activation of their binding capacity.

Activation of the estrogen receptor through phosphorylation by mitogen-activated protein kinase

The phosphorylation of the human estrogen receptor (ER) serine residue at position 118 is required for full activity of the ER activation function 1 (AF-1). This Ser118 is phosphorylated by mitogen-activated protein kinase (MAPK) in vitro and in cells treated with epidermal growth factor (EGF) and insulin-like growth factor (IGF) in vivo. Overexpression of MAPK kinase (MAPKK) or of the guanine nucleotide binding protein Ras, both of which activate MAPK, enhanced estrogen-induced and antiestrogen (tamoxifen)-induced transcriptional activity of wild-type ER, but not that of a mutant ER with an alanine in place of Ser118. Thus, the activity of the amino-terminal AF-1 of the ER is modulated by the phosphorylation of Ser118 through the Ras-MAPK cascade of the growth factor signaling pathways.

Steroid hormone signalling system and fungi

Three components of the steroid hormone signalling system, 17 beta-hydroxysteroid dehydrogenase, androgen binding proteins and steroid hormone signalling molecule testosterone were determined in the filamentous fungus Cochliobolus lunatus for the first time in a fungus. Their possible role in C. lunatus is discussed in comparison with their role in mammalian steroid hormone signalling system. The results are in accordance with the hypothesis, that the elements of primordial signal transduction system should exist in present day eukaryotic microorganisms.

Progesterone receptor structure and function altered by geldanamycin, an hsp90-binding agent

The assembly of progesterone receptor (PR) heterocomplexes in vitro involves at least eight components of the molecular chaperone machinery, and as earlier reports have shown, these proteins exhibit complex, dynamic, but ordered, interactions with one another and PR. Using the selective hsp90 binding agent geldanamycin (GA), we have found that PR assembly in vitro can be arrested at a previously observed intermediate assembly step. Like mature PR complexes, the intermediate complexes contain hsp90, but they differ from mature complexes by the presence of hsp70, p60, and p48 and the absence of immunophilins and p23. Arrest of PR assembly is likely due to GA's ability to directly block binding of p23 to hsp90. An important functional consequence of GA-mediated assembly arrest in vitro is the inability of the resulting PR complexes to bind progesterone, despite the presence of hsp90 in the receptor complexes. The biological significance of the in vitro observations is demonstrated by GA's ability to (i) rapidly block PR's hormone binding capacity in intact cells and (ii) alter the composition of COS cell PR complexes in a manner similar to that observed during in vitro reconstitutions. An updated model for the cyclic assembly pathway of PR complexes that incorporates the present findings with earlier results is presented.

A role for Hsp90 in retinoid receptor signal transduction

The ubiquitous heat shock protein Hsp90 appears to participate directly in the function of a broad range of cellular signal transduction components, including steroid hormone receptors; however, an evolutionarily related subclass of intracellular receptors, exemplified by the retinoid receptors RAR and RXR, had been inferred from biochemical studies to function independently of Hsp90. To examine this issue genetically, we measured mammalian and avian retinoid receptor activity in a Saccharomyces cerevisiae strain in which the expression of the yeast Hsp90 homologue could be conditionally repressed approximately 20-fold relative to wild type. We tested transcriptional activation by RAR or RXR-RAR, from two types of retinoic acid response elements, triggered by three different agonist ligands. In every condition, we found that activation was severely compromised under conditions of low Hsp90 expression. We showed that the defect was in signal transduction rather than transcription activation per se, and that high affinity hormone binding was abolished in extracts of cells producing low levels of Hsp90. We suggest that Hsp90 may function in at least one step of signal transduction by all members of the intracellular receptor superfamily.

Regulation of EGF-induced tenascin-C by steroids in tenascin-C-non-producing human carcinoma cells

Tenascin-C, a 6-armed extracellular matrix glycoprotein, is expressed in a temporally and spatially restricted pattern during tumorigenesis in association with stromal-epithelial interactions. We have previously shown that de novo synthesis of tenascin-C is induced by the diffusible factor EGF in tenascin-C-non-producing human epidermoid carcinoma cells in stromal-epithelial interactions. We now demonstrate that the addition of human tenascin-C or tenascin-C peptides to the culture medium of these cells had little effect on the induction of tenascin-C. The physiological regulators of tenascin-C induction through the EGF receptor, however, have not yet been characterized. We show that steroid hormones down-regulate EGF-induced tenascin-C glycoprotein and its mRNA in these tenascin-C-non-producing carcinoma cells. Of the steroids examined, hydrocortisone most effectively inhibited the secretion of tenascin-C. These steroids did not affect EGF-induced autophosphorylation or de novo synthesis of EGF receptors, nor did they compete for the binding of EGF to its receptor. Our results indicate that the induction of tenascin-C by EGF and its down-regulation by steroids might proceed in these carcinoma cells through separate signal transduction pathways.

Ligand-induced conformational alterations of the androgen receptor analyzed by limited trypsinization. Studies on the mechanism of antiandrogen action

Limited proteolysis of in vitro produced human androgen receptor was used to probe the different conformations of the receptor after binding of androgens and several antiandrogens. The results provide evidence for five different conformations of the receptor, as detected by the formation of proteolysis resisting fragments: 1) an initial conformation of the unoccupied receptor not resisting proteolytic attack; and receptor conformations characterized by 2) a 35-kDa proteolysis resisting fragment spanning the ligand binding domain and part of the hinge region, obtained with most antagonists, and in an initial step after agonist binding; 3) a 29-kDa proteolysis resisting fragment spanning the ligand binding domain, obtained in the presence of agonists after an activation process; 4 and 5) 30- and 25-kDa fragments, derived from 2 and 3, but missing part of the C terminus, obtained with RU486 (RU486 has antiandrogenic properties, besides its effects as an antiprogestagen/antiglucocorticoid). Concomitantly with the change from 2 to 3 (and of 4 to 5 for RU486), dissociation of the 8 S complex of receptor with associated proteins occurred. With a mutant receptor (LNCaP cell mutation in C-terminal region), some antagonists activated transcription analogous to agonists, and induced the activated receptor conformation 3. A mutant lacking the C-terminal 12 amino acids bound RU486 but not androgens, and formed with RU486 conformation 5. These data imply that, after the initial rapid binding of ligand, androgens induce a conformational change of the receptor, a process that also involves release of associated proteins. RU486 induces an inappropriate conformation of the C-terminal end, similar as found for its effect on the progesterone receptor. In contrast, the other antiandrogens act at a different step in the mechanism of action: they do not induce an abnormal conformation, but act earlier and prevent a conformation change by stabilizing a complex with associated proteins.

Isolation, regulation, and DNA-binding properties of three Drosophila nuclear hormone receptor superfamily members

We have designed a rapid cloning and screening strategy to identify new members of the nuclear hormone receptor superfamily that are expressed during the onset of Drosophila metamorphosis. Using this approach, we isolated three Drosophila genes, designated DHR38, DHR78, and DHR96. All three genes are expressed throughout third-instar larval and prepupal development. DHR38 is the Drosophila homolog of NGFI-B and binds specifically to an NGFI-B response element. DHR78 and DHR96 are orphan receptor genes. DHR78 is induced by 20-hydroxyecdysone (20E) in cultured larval organs, and its encoded protein binds to two AGGTCA half-sites arranged as either direct or palindromic repeats. DHR96 is also 20E-inducible, and its encoded protein binds selectively to the hsp27 20E response element. The 20E receptor can bind to each of the sequences recognized by DHR78 and DHR96, indicating that these proteins may compete with the receptor for binding to a common set of target sequences.

A role for nucleosome assembly in both silencing and activation of the Xenopus TR beta A gene by the thyroid hormone receptor

We have assembled the thyroid hormone-inducible promoter of the Xenopus thyroid hormone receptor (TR)beta A gene into chromatin using replication-coupled and -independent assembly pathways in vivo. We establish that heterodimers of TR and 9-cis retinoic acid receptors (RXR) can bind to their recognition sites within chromatin both in vivo and in vitro and alternately repress or activate transcription dependent on the absence or presence of thyroid hormone. Maximal transcriptional repression requires the presence of unliganded TR/RXR heterodimers during replication-coupled chromatin assembly. We demonstrate an increase in transcription directed by the TR beta A promoter of over two orders of magnitude in vivo, following the addition of thyroid hormone. This increase in transcription involves the relief of the repressed state that is established by the unliganded TR/RXR heterodimer during replication-coupled chromatin assembly. The association of thyroid hormone with the chromatin-bound TR/RXR heterodimer leads to the disruption of local chromatin structure in a transcription-independent process. Thus, chromatin structure has multiple roles in the regulation of TR beta A gene expression in vivo: The TR/RXR heterodimer recognizes the response element within chromatin, TR/RXR makes use of the chromatin assembly process to silence transcription more efficiently, and TR/RXR directs the disruption of local chromatin structure in response to thyroid hormone.

A putative step in steroid hormone action involves insertion of steroid ligands into DNA facilitated by receptor proteins

The hypothesis is advanced that hormonal ligands in the steroid/thyroid superfamily act through insertion between base pairs in partially unwound DNA. Using published X-ray coordinates of the complex of the glucocorticoid hormone response element (GRE) with the glucocorticoid receptor DNA binding domain, the interface between the protein and the gene was examined. The site 5'-TG-3'-5'-CA-3' previously shown to accommodate cortisol was found in the first two bases of the GRE half sites, 5'-TGTTCT-3'. These base pairs were sufficiently exposed at the receptor-gene interface to permit access by the steroid. Docking of cortisol into the receptor/DNA complex resulted in a favorable van der Waals energy. Given the general lack of correlation of receptor binding with hormonal activity, we propose that hormone action involves an additional step in which the receptor protein in concert with other transcription factors inserts the hormone into the DNA. This notion provides an explanation for earlier paradoxical observations including structural analogies between base pairs and steroid hormones. The insertion hypothesis suggests that receptor bound ligand facilitates DNA unwinding, stereospecific control of donor/acceptor functional groups on the DNA followed by insertion and release of the ligand between base pairs at 5'-TG-3'-5'-CA-3'.

Integrated control of seed maturation and germination programs by activator and repressor functions of Viviparous-1 of maize

The Viviparous-1 (VP1) transcriptional activator of maize is required for abscisic acid induction of maturation-specific genes late in seed development leading to acquisition of desiccation tolerance and arrest in embryo growth. Here, we show that VP1 also inhibits induction of the germination-specific alpha-amylase genes in aleurone cells of the developing seed and thereby appears to be involved in preventing precocious hydrolyzation of storage compounds accumulating in the endosperm. In developing seeds of the somatically instable vp1-m2 mutant, hydrolase activity was derepressed specifically in endosperm sectors underlying vp1 mutant aleurone. A barley alpha-amylase promoter-GUS reporter construct (Amy-GUS) was induced in developing vp1 mutant aleurone cells but not in wild-type aleurone cells. Moreover, transient expression of recombinant VP1 and vp1 mutant aleurone cells strongly inhibited expression of Amy-GUS and thus effectively complemented this aspect of the mutant phenotype. VP1 specifically repressed induction of Amy-GUS by the hormone gibberellic acid in aleurone of germinating barley seeds. Deletion of the acidic transcriptional activation domain of VP1 did not affect the inhibitory activity, indicating that VP1 has a discrete repressor function. Hence, physically combining activator and repressor functions in one protein may provide a mechanism to integrate the control of two normally consecutive developmental programs, seed maturation and seed germination.

A movable and regulable inactivation function within the central region of a temperature-sensitive p53 mutant

p53 is the most frequently mutated gene in human cancer. Naturally occurring mutations of p53 are mainly located within a region containing residues 100-300 and are predominantly of missense type, resulting in loss of the protein's DNA binding activity. Here we show that this type of mutation also represses the p53 N-terminal activation domain. The repression activity is localized in the central region of mutant p53 containing residues 101-318. Interestingly, the central region of a temperature-sensitive mutant p53N247I possesses a movable and regulable inactivation function. It represses other activities present on the same polypeptide chain without strict regard to the configuration of that polypeptide only at the nonpermissive temperature (37 degrees C) and not at the permissive temperature (30 degrees C). Furthermore, this mutant p53 region exhibits no other activity, and its function is independent of endogenous p53 status.

Role of transcriptional activation of I kappa B alpha in mediation of immunosuppression by glucocorticoids

Glucocorticoids are potent immunosuppressive drugs, but their mechanism is poorly understood. Nuclear factor kappa B (NF-kappa B), a regulator of immune system and inflammation genes, may be a target for glucocorticoid-mediated immunosuppression. The activation of NF-kappa B involves the targeted degradation of its cytoplasmic inhibitor, I kappa B alpha, and the translocation of NF-kappa B to the nucleus. Here it is shown that the synthetic glucocorticoid dexamethasone induces the transcription of the I kappa B alpha gene, which results in an increased rate of I kappa B alpha protein synthesis. Stimulation by tumor necrosis factor causes the release of NF-kappa B from I kappa B alpha. However, in the presence of dexamethasone this newly released NF-kappa B quickly reassociates with newly synthesized I kappa B alpha, thus markedly reducing the amount of NF-kappa B that translocates to the nucleus. This decrease in nuclear NF-kappa B is predicted to markedly decrease cytokine secretion and thus effectively block the activation of the immune system.

Spinal and bulbar muscular atrophy: a trinucleotide-repeat expansion neurodegenerative disease

Spinal and bulbar muscular atrophy (SBMA) is an X-linked, adult-onset motor neuronopathy that is caused by expansion of a trinucleotide (CAG) repeat in the androgen-receptor gene. The length of this repeat varies as it is passed down through SBMA families, and correlates inversely with the age of onset of the disease. The motor-neuron degeneration that occurs in this disease is probably caused by a toxic gain of function in the androgen-receptor protein. Subsequent to the identification of the mutation in SBMA, other inherited neurodegenerative diseases have been found to be caused by the expansion of CAG repeats in the coding regions of other genes. Because these diseases probably share a common pathogenesis, investigation of SBMA might help to determine a general mechanism of neuronal degeneration.

Mice lacking progesterone receptor exhibit pleiotropic reproductive abnormalities

Although progesterone has been recognized as essential for the establishment and maintenance of pregnancy, this steroid hormone has been recently implicated to have a functional role in a number of other reproductive events. The physiological effects of progesterone are mediated by the progesterone receptor (PR), a member of the nuclear receptor superfamily of transcription factors. In most cases the PR is induced by estrogen, implying that many of the in vivo effects attributed to progesterone could also be the result of concomitantly administered estrogen. Therefore, to clearly define those physiological events that are specifically attributable to progesterone in vivo, we have generated a mouse model carrying a null mutation of the PR gene using embryonic stem cell/gene targeting techniques. Male and female embryos homozygous for the PR mutation developed normally to adulthood. However, the adult female PR mutant displayed significant defects in all reproductive tissues. These included an inability to ovulate, uterine hyperplasia and inflammation, severely limited mammary gland development, and an inability to exhibit sexual behavior. Collectively, these results provide direct support for progesterone's role as a pleiotropic coordinator of diverse reproductive events that together ensure species survival.

Simian virus 40 late gene expression is regulated by members of the steroid/thyroid hormone receptor superfamily

Transcription of the late genes of simian virus 40 (SV40) is repressed during the early phase of the lytic cycle of infection of binding of cellular factors, called IBP-s, to the SV40 late promoter; repression is relieved after the onset of viral DNA replication by titration of these repressors. Preliminary data indicated that one of the major components of IBP-s was human estrogen-related receptor 1 (hERR1). We show here that several members of the steroid/thyroid hormone receptor superfamily, including testis receptor 2, thyroid receptor alpha 1 in combination with retinoid X receptor alpha, chicken ovalbumin upstream promoter transcription factors 1 and 2 (COUP-TF1 and COUP-TF2), as well as hERR1, possess the properties of IBP-s. These receptors bind specifically to hormone receptor binding sites present in the SV40 major late promoter. Recombinant COUP-TF1 specifically represses transcription from the SV40 major late promoter in a cell-free transcription system. Expression of COUP-TF1, COUP-TF2, or hERR1 in monkey cells results in repression of the SV40 late promoter, but not the early promoter, in the absence of the virally encoded large tumor antigen. Overexpression of COUP-TF1 leads to a delay in the early-to-late switch in SV40 gene expression during the lytic cycle of infection. Thus, members of this superfamily can play major direct roles in regulating expression of SV40. Possibly, natural or synthetic ligands to these receptors can serve as antiviral drugs. Our findings also provide the basis for the development of assays to screen for the ligands to testis receptor 2 and hERR1.

Sequence of the promoter region of the mouse gene encoding ornithine aminotransferase

We have isolated and sequenced the promoter region of the mouse gene (mOAT) encoding ornithine aminotransferase. A comparison of these mOAT sequences with previously reported sequences for the rat and human genes encoding OAT, rOAT and hOAT, respectively, revealed a 256-bp region flanking the transcription start point that is highly conserved between the three genes. This region contains sequence motifs resembling binding sites for general transcription factors, as well as other trans-acting regulatory proteins.

Identification of a transactivation function in the progesterone receptor that interacts with the TAFII110 subunit of the TFIID complex

Transcriptional activation of target genes by the human progesterone receptor is thought to involve direct or indirect protein-protein interactions between the progesterone receptor and general transcription factors. A key role in transcription plays the general factors. A key role in transcription plays the general transcription factor TFIID, a multiprotein complex consisting of the TATA-binding protein and several tightly associated factors (TAFs). TAFs have been shown to be required for activated transcription and are, thus, potential targets of activator proteins. Using in vitro interaction assays, we could identify specific interactions between the progesterone receptor and the TATA-binding protein-associated factor dTAFII110. The dTAFII110 domain responsible for the interaction is distinct from that reported to suffice for binding to Sp1. Somewhat surprisingly, deletion analysis indicated that the previously identified activation functions 1 and 2 of the progesterone receptor are not required for this interaction but pointed to an important role of the DNA binding domain. In cotransfection experiments and an in vitro transcription assay, the DNA binding domain of the progesterone receptor displayed significant activation potential. These findings, taken together, suggest that an interaction between the progesterone receptor and TAFII110 may represent an important step in the mechanism of activation.

Exclusive homodimerization of the orphan receptor hepatocyte nuclear factor 4 defines a new subclass of nuclear receptors

Hepatocyte nuclear factor 4 (HNF-4), a highly conserved member of the steroid hormone receptor superfamily critical for development and liver-specific gene expression, is very similar to another superfamily member, retinoid X receptor alpha (RXR alpha), in overall amino acid sequence and DNA binding specificity. Since RXR alpha is known to heterodimerize with many other nuclear receptors, the formation of heterodimers between HNF-4 and RXR alpha was examined. With the electrophoretic mobility shift assay, coimmunoprecipitation, and transient transfection assays, it is shown that, unlike other nuclear receptors, HNF-4 does not form heterodimers with RXR alpha either in the presence or in the absence of DNA. We also show that in vitro-translated HNF-4 does not form heterodimeric complexes on DNA with a number of other receptors, including RXR beta, RXR gamma, retinoic acid receptor alpha, or thyroid hormone receptor alpha. To investigate the hypothesis that the lack of heterodimerization between HNF-4 and RXR alpha is due to a strong homodimerization activity of HNF-4, glycerol gradient sedimentation and kinetic analysis were used to show that HNF-4 is in fact a stable homodimer in solution. Finally, immunohistochemistry is used to show that the HNF-4 protein is found exclusively in the nuclei in both HepG2 cells, which express endogenous HNF-4, and transfected COS cells, which overexpress HNF-4. These findings lead us to propose that HNF-4 defines a new subclass of nuclear receptors which reside primarily in the nucleus and which bind DNA and regulate transcription as homodimers.

Drosophila hormone receptor 38: a second partner for Drosophila USP suggests an unexpected role for nuclear receptors of the nerve growth factor-induced protein B type

In Drosophila the response to the hormone ecdysone is mediated in part by Ultraspiracle (USP) and ecdysone receptor (EcR), which are members of the nuclear receptor superfamily. Heterodimers of these proteins bind to ecdysone response elements (EcREs) and ecdysone to modulate transcription. Herein we describe Drosophila hormone receptor 38 (DHR38) and Bombyx hormone receptor 38 (BHR38), two insect homologues of rat nerve growth factor-induced protein B (NGFI-B). Although members of the NGFI-B family are thought to function exclusively as monomers, we show that DHR38 and BHR38 in fact interact strongly with USP and that this interaction is evolutionarily conserved. DHR38 can compete in vitro against EcR for dimerization with USP and consequently disrupt EcR-USP binding to an EcRE. Moreover, transfection experiments in Schneider cells show that DHR38 can affect ecdysone-dependent transcription. This suggests that DHR38 plays a role in the ecdysone response and that more generally NGFI-B type receptors may be able to function as heterodimers with retinoid X receptor type receptors in regulating transcription.

Coordinated regulation of and transcriptional activation by Xenopus thyroid hormone and retinoid X receptors

Thyroid hormone (T3) plays a causative role in amphibian metamorphosis. This regulation is thought to be mediated by heterodimers of T3 receptors (TRs) and retinoid X receptors (RXRs). We report here that Xenopus TRs can indeed form strong heterodimers with Xenopus RXRs on the T3 response element (TRE) present in Xenopus TR beta genes. Using a T3-responsive in vivo transcription system established by introducing TRs and RXRs into Xenopus oocytes, we demonstrated that TR-RXR heterodimers repressed TR beta gene promoter in the absence of T3 and activated the promoter in the presence of the hormone. Furthermore, by analyzing the expression of TR and RXR genes, we showed that TR and RXR genes were coordinately regulated in different tissues during metamorphosis. Thus high levels of their mRNAs are present in the limb during early stages of limb development when morphogenesis occurs and in the tail toward the end of metamorphosis when it is being resorbed. Such correlations coupled with our TRE-binding and in vivo transcriptional activation experiments provide strong evidence that TRs and RXRs function together to mediate the effects of T3 during metamorphosis. These results further suggest a possible molecular basis for the temporal regulation of tissue-specific metamorphosis.

Norepinephrine-independent regulation of GRII mRNA in vivo by a tricyclic antidepressant

Desipramine (DMI), a tricyclic antidepressant drug used in the treatment of depression, has been shown to increase steady-state levels of glucocorticoid receptor type II (GRII) mRNA in vitro and in vivo. To determine whether this effect is secondary to norepinephrine (NE) reuptake inhibition i.e., increases in synaptic NE induced by DMI, GRII mRNA levels were assayed in rat hippocampus following neurotoxic lesioning of NE neurons with DSP4. Chronic DMI treatment significantly increased GRII mRNA levels to the same degree in lesioned and non-lesioned animals. In contrast to DMI, the non-tricyclic antidepressant fluoxetine had no effect on GRII mRNA. These results provide evidence which demonstrates that a tricyclic antidepressant can regulate steady-state mRNA levels in vivo by a mechanism which is independent of its effects on synaptic monoamine levels.

Specific interactions of progestins and anti-progestins with progesterone antibodies, plasma binding proteins and the human recombinant receptor

This structure-activity study compares the affinity of a series of progestins, progesterone metabolites and anti-progestins for a panel of monoclonal antibodies to progesterone, coypu (Myocastor coypus) or guinea pig plasma progesterone-binding proteins (PPBPs) and the human recombinant progesterone receptor A form (PR-A). The compounds tested were progesterone, Promegestone (R5020), Mifepristone (RU486), ZK98,734, Onapristone (ZK98,299), 11 alpha-hydroxyprogesterone, 11 alpha-progesterone hemisuccinate, androsterone, etiocholanolone, 5 alpha- and 5 beta-pregnane-3,20-diones, and 20 alpha- and 20 beta-hydroxyprogesterones. The Ki values for these ligands were determined by competitive binding assays using radiolabelled progesterone as the binding site ligand. For anti-progesterone antibodies (e.g. DB3 and 11/32), only progesterone (3.6-8.8 nM), the 11 alpha-derivatives (1.0-5.5 nM) used to prepare the immunogen and the two 5-pregnanediones (20.9-45.1 nM) were bound with high affinity. For PR-A, high affinity binding was found with receptor agonists (Ki = 1.1-6.2 nM), both 5- and 20-reduced metabolites, and antagonists (0.6-28.0 nM), but not with the 11 alpha-derivatives (950 nM-1.0 microM). In contrast, the PPBPs displayed high affinity interactions with progesterone (3.5-4.2 nM) and both 5 alpha- and 20 alpha-reduced metabolites (2.4-3.4 nM). Binding with the beta-isomers and R5020 was less pronounced (22-170 nM) and there was no evidence of high affinity binding with PR antagonists (> 1.0 microM). Analogs with the 17-keto group did not bind to any of the binders studied. Thus, commonalities among the three types of protein binders were their comparable binding affinities for progesterone (3.5-8.8 nM) and 5-pregnanedione isomers (2.4-330 nM), and a lack of binding for two C17-keto steroids (androsterone and etiocholanolone). The results imply that the tertiary features of the binding domain of these three types of proteins are sufficiently different to result in unique binding structures.

Glucocorticoid antagonist RU 486 reverses agonist-induced apoptosis and c-myc repression in human leukemic CEM-C7 cells

A synthetic glucocorticoid dexamethasone (DEX) inhibits proliferation and induces apoptosis of clone CEM-C7 cells, but not in clone CEM-C1 cells, even though they contain glucocorticoid receptors (GR). We previously showed that suppression of c-myc is a critical step in glucocorticoid-induced cell lysis of C7 cells. It is not reduced in C1 cells. In this study we review the basis for this conclusion and present evidence that the glucocorticoid antagonist RU 486 rescues DEX-treated C7 cells from cell death. An increase in DEX-repressed c-myc mRNA levels precedes the recovery of cell growth. A threshold level of Myc expression appears to be required to maintain growth and viability of C7 cells. Although C1 cells are highly resistant to lysis by glucocorticoids, addition of forskolin, an inducer of protein kinase A, synergizes to evoke complete apoptosis. This synergistic effect is prevented by RU 486, indicating direct involvement of the GR.

The N-terminal region (A/B) of rat thyroid hormone receptors alpha 1, beta 1, but not beta 2 contains a strong thyroid hormone-dependent transactivation function

In this study we have investigated the role of the N-terminal region of thyroid hormone receptors (TRs) in thyroid hormone (TH)-dependent transactivation of a thymidine kinase promoter containing TH response elements composed either of a direct repeat or an inverted palindrome. Comparison of rat TR beta 1 with TR beta 2 provides an excellent model since they share identical sequences except for their N termini. Our results show that TR beta 2 is an inefficient TH-dependent transcriptional activator. The degree of transactivation corresponds to that observed for the mutant TR delta N beta 1/2, which contains only those sequences common to TR beta 1 and TR beta 2. Thus, TH-dependent activation appears to be associated with two separate domains. The more important region, however, is embedded in the N-terminal domain. Furthermore, the transactivating property of TR alpha 1 was also localized to the N-terminal domain between amino acids 19 and 30. Using a coimmunoprecipitation assay, we show that the differential interaction of the N terminus of TR beta 1 and TR beta 2 with transcription factor IIB correlates with the TR beta 1 activation function. Hence, our results underscore the importance of the N-terminal region of TRs in TH-dependent transactivation and suggest that a transactivating signal is transmitted to the general transcriptional machinery via a direct interaction of the receptor N-terminal region with transcription factor IIB.

Role of the protein chaperone YDJ1 in establishing Hsp90-mediated signal transduction pathways

The substrate-specific protein chaperone Hsp90 (heat shock protein 90) from Saccharomyces cerevisiae functions in diverse signal transduction pathways. A mutation in YDJ1, a member of the DnaJ chaperone family, was recovered in a synthetic-lethal screen with Hsp90 mutants. In an otherwise wild-type background, the ydj1 mutation exerted strong and specific effects on three Hsp90 substrates, derepressing two (the estrogen and glucocorticoid receptors) and reducing the function of the third (the tyrosine kinase p60v-src). Analysis of one of these substrates, the glucocorticoid receptor, indicated that Ydj1 exerts its effects through physical interaction with Hsp90 substrates.

Regulation of apoptosis by steroid hormones

Steroid hormones play major roles in regulation in growth, development, homeostasis, and cell death. Together with other hormones and growth factors, steroids regulate both the function and cellular composition of organs throughout the body. In this article we will discuss the mechanisms of steroid hormone regulated apoptosis. Emphasis will be placed on the effect of glucocorticoids on lymphoid cells.

Ligand-dependent repression of the erythroid transcription factor GATA-1 by the estrogen receptor

High-dose estrogen administration induces anemia in mammals. In chickens, estrogens stimulate outgrowth of bone marrow-derived erythroid progenitor cells and delay their maturation. This delay is associated with down-regulation of many erythroid cell-specific genes, including alpha- and beta-globin, band 3, band 4.1, and the erythroid cell-specific histone H5. We show here that estrogens also reduce the number of erythroid progenitor cells in primary human bone marrow cultures. To address potential mechanisms by which estrogens suppress erythropoiesis, we have examined their effects on GATA-1, an erythroid transcription factor that participates in the regulation of the majority of erythroid cell-specific genes and is necessary for full maturation of erythrocytes. We demonstrate that the transcriptional activity of GATA-1 is strongly repressed by the estrogen receptor (ER) in a ligand-dependent manner and that this repression is reversible in the presence of 4-hydroxytamoxifen. ER-mediated repression of GATA-1 activity occurs on an artificial promoter containing a single GATA-binding site, as well as in the context of an intact promoter which is normally regulated by GATA-1. GATA-1 and ER bind to each other in vitro in the absence of DNA. In coimmunoprecipitation experiments using transfected COS cells, GATA-1 and ER associate in a ligand-dependent manner. Mapping experiments indicate that GATA-1 and the ER form at least two contacts, which involve the finger region and the N-terminal activation domain of GATA-1. We speculate that estrogens exert effects on erythropoiesis by modulating GATA-1 activity through protein-protein interaction with the ER. Interference with GATA-binding proteins may be one mechanism by which steroid hormones modulate cellular differentiation.

Calreticulin modulates the in vitro DNA binding but not the in vivo transcriptional activation by peroxisome proliferator-activated receptor/retinoid X receptor heterodimers

Calreticulin is a ubiquitous calcium binding/storage protein found primarily in the endoplasmic reticulum. Calreticulin has been shown to inhibit DNA binding and transcriptional activation by glucocorticoid and androgen hormone receptors by binding to the conserved sequence KXFF(K/R)R, present in the DNA-binding domains of all known members of the steroid/nuclear hormone receptor superfamily. To determine whether calreticulin might be a general regulator of hormone-responsive pathways, we examined its effect on DNA binding in vitro and transcriptional activation in vivo by heterodimers of the peroxisome proliferator-activated receptor (PPAR) and the 9-cis retinoic acid receptor (RXR alpha). We show here that purified calreticulin inhibits the binding of PPAR/RXR alpha heterodimers and of other nuclear hormone receptors, to peroxisome proliferator-responsive DNA elements in vitro. However, overexpression of calreticulin in transiently transfected cultured cells had little or no effect on transactivation mediated by PPAR/RXR alpha. Therefore, while calreticulin inhibits the binding of both nuclear and steroid hormone receptors to cognate response elements in vitro, our findings suggest that calreticulin does not necessarily play an important role in the regulation of all classes of hormone receptors in vivo.

Regulation of Xenopus laevis estrogen receptor gene expression is mediated by an estrogen response element in the protein coding region

To investigate the 17 beta-estradiol induction of the mRNA coding for the Xenopus laevis estrogen receptor (XER), we cloned the promoter and the 5'-flanking region of the ER gene. Transcription initiation sites were identified by primer extension, and confirmed by the polymerase chain reaction. The promoter and 5'-flanking region contain an imperfect TATA box and a potential CAAT box at -51. Sequence analysis and transfections indicated that no functional estrogen response element (ERE) was present in approximately 3 kb of 5'-flanking region. An imperfect ERE, GGTCAGTTTGACG, which differs from the consensus ERE sequence by 1 nucleotide, was detected in the protein coding region of the gene, approximately 480 nucleotides downstream of the transcription initiation site. In transient transfections using a simple promoter containing two copies of this Xenopus estrogen receptor ERE (XERE), we observed an estrogen-dependent increase in CAT activity of four- to five-fold, to a level approximately 20-fold greater than the activity of the control plasmid lacking the XEREs. In competition gel mobility-shift assays, the XERE exhibited a weak, but clearly detectable, ability to compete for binding of human ER to a labeled consensus ERE. Because it exhibits sequence-specific binding to the ER in competition gel mobility-shift assays, and is able to confer estrogen-dependent transcription on a simple synthetic promoter, the novel XERE, located in the protein coding region of the XER gene appears to represent a weak, but functional, ERE.

Cell fate control in the Drosophila retina by the orphan receptor seven-up: its role in the decisions mediated by the ras signaling pathway

Drosophila seven-up is an orphan receptor of the steroid receptor family that is required to specify photoreceptor neuron subtypes in the developing compound eye. Expression of seven-up is confined to four of the eight photoreceptor precursors, R3/R4/R1/R6. We show that misexpression of seven-up in any of the other cell types within the developing ommatidium interferes with their differentiation. Each cell type responds differently to seven-up misexpression. For example, ectopic expression in the non-neuronal cone cells using the sevenless promoter/enhancer (sev-svp) causes the cone cells to take on a neuronal identity. Ectopic expression of seven-up in R2/R5 using the rough enhancer (ro-svp) causes these neurons to lose aspects of their photoreceptor subtype identity while remaining neuronal. Each cell type appears to have a different developmental time window that is sensitive to misexpressed seven-up. The temporal order of responsiveness of each cell type to misexpressed seven-up is similar but not identical to the order of neuronal differentiation. This suggests that there are processes of specification that are distinct from the specification to become a photoreceptor neuron. We have identified members of the ras signaling pathway as suppressors of the cone cell to R7 neuron transformation caused by sev-svp. Suppression of the sev-svp phenotype can be achieved by decreasing the gene-dosage of any of the members of the ras-pathway. This suggests that the function of seven-up in the cone cells requires ras signaling. However, a decrease in ras signaling results in enhancement of the phenotype caused by the ro-svp transgene. We discuss the relationship between decisions controlled by seven-up and those controlled by ras signaling.

The c-jun proto-oncogene down-regulates the rat alpha-fetoprotein promoter in HepG2 hepatoma cells without binding to DNA

The effects of a phorbol ester (TPA) and of members of the Jun and Fos oncoprotein family on the activity of the rat alpha-fetoprotein (AFP) promoter were checked by using transient expression experiments in HepG2 hepatoma cells. TPA blocked the activity of the rat AFP promoter in a dose-dependent manner. Overexpression of c-Jun specifically repressed the rat AFP promoter but not the albumin promoter. JunB and JunD were poorer inhibitors. c-Fos expression did not potentiate the negative effect of Jun. The Jun-induced repression does not require binding of c-Jun to the AFP promoter. DNase 1 footprinting experiments did not display any high affinity binding site for Jun on the AFP promoter. Integrity of the c-Jun DNA binding domain is not required for the c-Jun protein to block the AFP promoter. The N-terminal part of Jun, which contains the activating domain, is responsible for the repression as shown by using Jun-Gal4 chimera. Jun likely exerts its negative control on the AFP promoter via protein-protein interactions with a not yet identified trans-activating factor within the -134 to +6 region or with a component of the general machinery of transcription. Jun proteins can thus be key intermediates in regulatory cascades which result in the differential modulation of the AFP and albumin gene expression in the course of liver development and carcinogenesis.

Cavity depth and width effects on cyclophane-steroid recognition: molecular complexation of cholesterol and progesterone in aqueous solution

BACKGROUND

Recent X-ray crystal structures show that steroid-binding proteins contain deep hydrophobic cavities defined by aromatic amino-acid side chains which encapsulate steroid molecules. These cavities resemble the binding site of a synthetic macrotricyclic cyclophane receptor which we recently reported to form complexes with cholesterol in aqueous solution. The binding affinity of the cyclophane-cholesterol complex (Ka approximately 10(6) M-1, 295 K) is similar to that measured for the cholesterol complex of steroid-transport proteins such as sterol carrier protein-2 (SCP-2). Here we describe synthesis and binding studies of a related receptor with a cavity that is wider and 2 A deeper than that of the previous cyclophane, and a comparison of the steroid-binding affinity and selectivity of the two synthetic receptors.

RESULTS

A new tricyclic cyclophane receptor with a 13 A deep cavity was synthesized to study the effect of increased cavity depth on receptor selectivity for steroids. NMR analysis demonstrated that this receptor provided increased steroidal side-chain encapsulation with a corresponding gain in binding free energy of 0.9 kcal mol-1 (in d4-methanol) as compared to our previously reported 11 A deep receptor. An unexpected consequence of the increase in cavity depth was a corresponding enlargement of the cavity width, as indicated both by steroid-binding studies and molecular modeling. This enlargement in cavity width increases binding affinity for saturated steroids while decreasing the association strength of unsaturated steroids such as cholesterol. In water, cholesterol binds to the new receptor with Ka approximately 1.5 x 10(5) M-1 and exhibits a significant complexation-mediated solubility increase.

CONCLUSIONS

Small changes in steroid receptor dimensions have resulted in large differences in steroid selectivity and binding affinity. These results indicate that potentially large gains in steroid-binding free energy may be obtainable from complete hydrophobic encapsulation of the flexible aliphatic steroidal side chain. These results have implications for the design of synthetic receptor mimics of natural steroid binding proteins.

Characterization of genes encoding members of the nuclear hormone receptor superfamily from Onchocerca volvulus

Several lines of evidence suggest that molting in parasitic nematodes is controlled through the action of steroid molting hormones, or ecdysones. In other organisms, the central mediator of steroid hormone action is the hormone receptor. These receptor molecules are members of a superfamily of proteins called the nuclear hormone receptor family. Using an oligonucleotide derived from the amino-acid sequence of the Drosophila melanogaster ecdysone receptor, genes encoding homologues of the nuclear hormone receptor family were identified in the genome of the human filarial parasite Onchocerca volvulus. The O. volvulus genome contains at least three genes that encode putative members of the nuclear hormone receptor superfamily. A complete cDNA for one of these genes, designated OvNHR-1, has been isolated and characterized. The OvNHR-1 cDNA was 2378 bp in length, and contained a single open reading frame of 1104 bp. The open reading frame encoded a peptide with all of the features characteristic of a member of the nuclear hormone receptor superfamily of proteins. OVNHR-1 appeared to be encoded by a single-copy gene. Expression of the mRNA corresponding to OvNHR-1 was developmentally regulated, with maximal expression occurring during early embryogenesis. The polypeptide encoded by the OvNHR-1 open reading frame is antigenic in a minority of individuals exposed to O. volvulus.

Prevention of hypoxic-ischemic damage with dexamethasone is dependent on age and not influenced by fasting

Pretreatment with the synthetic glucocorticoid dexamethasone prevents hypoxic-ischemic brain damage in 7-day-old neonatal rats. We presently characterize the response further by examining the effect of varying the age, the glucocorticoid, and the time of injection and by examining whether fasting can influence the response. Rats (n = 193) were randomized to one of 16 different treatment groups and subjected to hypoxia-ischemia (right carotid artery occlusion +8% O2 which was 3 h in duration for 7-day, 1 h for 2-week, and 30 min for 1-month-old animals). The brains were subsequently perfusion fixed and the area of infarction was measured from hematoxylin- and eosin-stained sections. Time dependence studies demonstrated that treatment with 0.1 mg/kg intraperitoneal dexamethasone 4 h prior to hypoxia reduced infarct size compared to vehicle-treated animals whereas pretreatment at either 48 h or 4 days was ineffective. Dexamethasone pretreatment (4 h) also provided neuroprotection against 4 h of hypoxia-ischemia. Fasted animals which received dexamethasone had reduced blood glucose levels yet markedly less damage than controls. Another glucocorticoid, methylprednisolone (0.7 mg/kg), also reduced infarction. In 2-week-old animals the area of infarction was reduced by pretreatment with dexamethasone, whereas in 1-month-old animals dexamethasone was ineffective. The results suggest that a glucocorticoid-mediated response intervenes in events leading to neuronal death in young animals but not older animals once myelination and synaptogenesis are complete.

Transcription factor TFIIB and the vitamin D receptor cooperatively activate ligand-dependent transcription

The active metabolite of vitamin D, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], regulates gene transcription through binding to the vitamin D receptor (VDR), a member of the nuclear hormone receptor superfamily. Sequence-specific transcription factors, including nuclear hormone receptors, are thought to interact with the basal transcription complex to regulate transcription. In glutathione S-transferase fusion-based protein-protein binding assays we found that VDR specifically binds to TFIIB, a component of the basal complex, and that the interaction requires select domains of each protein. To assess the functional significance of this interaction, transfection assays were performed with a 1,25(OH)2D3-responsive reporter construct. In P19 embryonal carcinoma cells cotransfection of VDR and TFIIB cooperatively activated reporter transcription, while each factor alone gave very low to no activation. This activation was dependent on 1,25(OH)2D3 and the dose of TFIIB and VDR transfected, demonstrating that a nuclear hormone receptor functionally interacts with TFIIB in vivo. In contrast, transfection of NIH 3T3 cells generated strong reporter activation by 1,25(OH)2D3 in the presence of VDR alone, and cotransfection of TFIIB led to specific dose-dependent repression of reporter activity. Taken together, these results indicate that TFIIB-nuclear hormone receptor interaction plays a critical role in ligand-dependent transcription, which is apparently modulated by a cell-type-specific accessory factor.

Emerging diversities in the mechanism of action of steroid hormones

The classical genomic action of steroid hormones acting through intracellular receptors is well recognized. Within this concept of action, questions regarding the ultimate fate of the hormone and lack of a tight correlation between tissue uptake and biological activity with receptor binding remain unanswered. Evidence has accumulated that steroid hormones can exert non-classical action that is characterized by rapid effect of short duration. In most of these cases, the hormone effects occurs at the membrane level and is not associated with entry into the cell. The possible mechanisms for these non-classical actions are: (a) changes in membrane fluidity; (b) steroid hormone acting on receptors on plasma membranes; (c) steroid hormones regulating GABAA receptors on plasma membranes; and (d) activation of steroid receptors by factors such as EGF, IGF-1 and dopamine. Data have also been obtained indicating that receptor-mediated insertion of steroid hormones into DNA may take place with the steroid acting as a transcription factor. These new proposed mechanism of action of steroid hormones should not be viewed as a challenge to the classical mechanism. These diverse modes of action provide for an integrated action of hormones which may be rapid and of short duration or prolonged to address the physiological needs of the individual.