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SMRT, Sin3, and N-CoR establish protein-to-protein link between deacetylases and DNA-bound repressors hence, the term corepressors. These corepressors interact with each other and influence nuclear receptor to remodel chromatin structure without altering the regulatory complex through the help of hormone dependent adaptor HDAC. Transcriptional repression is achieved through regulation of chromatin structure resulting in an opposed cellular differentiation.

While regulation of receptor function is known to occur at many levels (e.g. transcriptional, post-translational), it is generally perceived that a tissue either expresses or does not express a particular receptor in an all-ornone fashion. Many pathological (e.g. tissue injury) and physiological (e.g. angiogenesis) processes have, however, been shown to be associated with the transcriptional induction of specific receptors. Induced receptors are not confined to any particular class, but range from G protein-coupled receptors to receptor tyrosine kinases. The potential implications of de novo receptor expression are profound with respect to potential novel therapeutic targets in specific disease states. Further, this observation may explain unexpected side-effects in the pharmacotherapy of existing disease states. In this article Lucy Donaldson, Michael Hanley and Amparo Villablanca discuss circumstances under which de novo receptor induction has been described, potential mechanisms of induction and the implications for pharmacology.

The cellular handling of cholecystokinin (CCK) receptor molecules during CCK-stimulated signaling was analyzed with polyclonal rabbit antiserum directed toward the peptide domain within the COOH terminus of CCK receptor. Immunolocalization of CCK receptor molecules with the CCK antiserum exhibited the recycling of large proportion of intenalized CCK receptors involving induced conformational changes. Furthermore, the internalization of CCK receptors was not affected by receptor phosphorylation.

The effect of immunoneutralization of gonadotropin-releasing hormone (GnRH) on LH secretion and concentrations of GnRH receptor, GnRH receptor mRNA, and gonadotropin subunit mRNA in pituitary tissue of orchidectomized sheep (wethers) was assessed. Thirty-six wethers were assigned at random to one of six treatment groups (six wethers per group). Thirty wethers (groups 2 to 6) received 200 mL (i.v.) of anti-GnRH antisera at passive immunization (PI). Anterior pituitary tissue was collected .5, 1, 2, 4, or 8 d after PI from wethers in groups 2 to 6, respectively. Pituitary tissue was also collected from unimmunized wethers (Group 1). Intravenous administration of anti-GnRH sera increased anti-GnRH activity to 69.1 [+ or -] .7% (percentage of total 125I-labeled GnRH bound by a 1:1,000 serum: GEL-PBS dilution) within 1 h of PI. Anti-GnRH activity declined gradually during the period after PI, and 8 d after PI anti-GnRH activity was 57.2 [+ or -] 1.7%. Serum concentration of LH was significantly reduced, relative to the pretreatment (16.1 [+ or -] 1.8 ng/mL) level, within 4 h (7.6 [+ or -] 1.5 ng/mL) of PI, and the LH level was 10% of the pretreatment concentration 8 d after PI (1.6 [+ or -] 0.2 ng/mL). Steady-state concentration of GnRH receptor mRNA decreased progressively during the period after PI and was significantly reduced, relative to the level in unimmunized control wethers (.44 [+ or -] .03 pg/[[micro]gram] total RNA), 4 (.33 [+ or -] .03 pg/[[micro]gram] total RNA), and 8 (.31 [+ or -] .02 pg/[[micro]gram] total RNA) d after PI. Tissue concentrations of GnRH receptor and mRNA for the [Alpha], LH[Beta], and FSH[Beta] subunits were also reduced (P < .05) by PI. These data indicate that maintenance of steady-state concentrations of GnRH receptor and GnRH receptor mRNA requires continued GnRH stimulation. Key Words: Receptors, mRNA, Sheep, GnRH, Passive Immunization, Gonadotropins

Past studies have shown that epidermal growth factor (EGF) is able to mimic the uterotropic effects of estrogen in the rodent. These studies have suggested a 'cross-talk' model in which EGF receptor (EGF-R) signaling results in activation of nuclear estrogen receptor (ER) and its target genes in an estrogen-independent manner. Furthermore, in vitro studies have indicated the requirement for ER in this mechanism. To verify the requirement for ER in an in vivo system, EGF effects were studied in the uteri of ER knockout (ERKO) mice, which lack functional ER. The EGF-R levels, autophosphorylation, and c-fos induction were observed at equivalent levels in both genotypes indicating that removal of ER did not disrupt the EGF responses. Induction of DNA synthesis and the progesterone receptor gene in the uterus were measured after EGF treatment of both ERKO and wild-type animals. Wild-type mice showed increases of 4.3-fold in DNA synthesis, as well as an increase in PR mRNA after EGF treatment. However, these responses were absent in ERKO mice, confirming that the estrogen-like effects of EGF in the mouse uterus do indeed require the ER. These data conclusively demonstrate the coupling of EGF and ER signaling pathways in the rodent reproductive tract.

Environmental perturbations that increase plasma thyroid hormone ([T.sub.3]) concentrations also profoundly affect female reproductive behavior and physiology. We explored whether these effects were mediated by interactions between [T.sub.3] receptor (TR) and estrogen receptor (ER). This hypothesis was of interest because the half-site of a consensus [T.sub.3] response element DNA sequence is identical to an ER response element (ERE), and TRs bind to a consensus ERE. Molecular data presented in the accompanying paper [Zhu, Y.-S., Yen, P.M., Chin, W. W. & Pfaff, D. W. (1996) Proc. Natl. Acad. Sci. USA 93, 12587-12592] demonstrate that TRs and ERs are both present in rat hypothalamic nuclear extracts and that both can bind to the promoter the hypothalamic gene preproenkephalin and that interactions between liganded TRs and ERs affect preproenkephalin transcription. In this paper, we show that molecular interactions between TRs and ERs are sufficient to mediate environmental effects on estrogen-controlled reproductive behavior. Ovariectomized (OVX) rats treated with high doses of [T.sub.3] showed significantly lower levels of lordosis behavior in response to estradiol benzoate (EB) compared with OVX females treated with EB alone. Conversely, thyroidectomized/OVX females treated with EB showed significantly greater levels of lordosis behavior compared with OVX females treated with EB, showing the effect of endogenous [T.sub.3]. Thyroid hormone interference with EB-induced behavior could not be explained by a reduction in plasma [E.sub.2] concentrations or by a general reduction in responsiveness of EB-sensitive tissues. Moreover, numbers of hypothalamic ER-immunoreactive cells increased dramatically following [T.sub.3] treatment. These data suggest that [T.sub.3] may reduce EB-dependent sexual behavior through interactions between TR and ER in the nuclei of behaviorally relevant hypothalamic neurons, envisioning for the first time a functional consequence of interactions between two nuclear hormone receptors in brain. These results also open up the possibility of molecular interactions on DNA encoding environmental signals, a new field for the study of neuronal integration.

The RaKCaR Ca2+/polyvalent cation-sensing receptor was cloned to determine changes in the concentrations of extracellular Ca2+/polyvalent cations in kidney cells. Glomeruli cells exhibited RakCaR messenger ribonucleic acid and parathyroid hormone/parathyroid hormone-related protein coexpression. The coexpression of the two receptors in the glumeruli regulate the reabsorption of divalent cations and inhibited the stimulatory effects of parathyroid hormone.

Increasing prolactin levels or increasing responsiveness to prolactin may contribute to reproductive aging by influencing the secretory patterns of hypothalamic GnRH, pituitary gonadotropins, and/or ovarian steroids. Some studies have documented changes in the levels of prolactin in peripheral plasma. The goal of this study was to determine whether prolactin receptor mRNA levels in the brain change with aging, which may lead to increasing responsiveness to prolactin. Young (2-4 months) and middle-aged (9-11 months), demonstrating 3 consecutive estrous cycles, and old (16-19 months) and very old (20-21 months) rats, exhibiting repeated pseudopregnancies, were bilaterally ovariectomized. They were implanted with Silastic capsules containing estradiol-17[Beta] one week later, and killed 2 days after capsule implantation. Changes in prolactin receptor gene expression were assessed using in situ hybridization. The level of prolactin receptor=mRNA in choroid plexus, periventricular area of the preoptic nucleus, and arcuate nucleus increased significantly by the time the animals were old. In the lateral ventromedial nucleus, prolactin receptor gene expression did not change significantly during aging, even in the oldest group of rats. These findings suggest that changes in the prolactin receptor gene may influence the ability of prolactin to exert effects and may allow older animals to be more responsive to prolactin than young rats.

Studies have suggested that the presence of corticosteroids may induce a downregulation of their respective receptors in various organs. Herein, animals that were depleted of cortiosteroid hormones by adrenalectomy, and received aldosterone or dexamethasone at physiological doses, showed no significant changes in the levels of both mineralocorticoid and glucocorticoid receptor mRNA expression in the kidney, heart and colon. It was concluded that the regulation of hormone receptor expression does not depend on hormonal status.

The mineralocorticoid receptor (MR) antagonist, RU-28318, suppresses the enhanced rate of energy gain produced by ovariectomy (OVX) in rats. The suppression of OVX-related changes is due to an increase in the energetic efficiency, rather than a decrease in food intake or increase in energy consumption. A similar effect for surgery and RU-28318 is evident on body energy gain, energetic efficiency, and fat gain. The glucocorticoid receptor antagonist, RU-486, has no effect on the energy changes induced by OVX.

Peroxisome proliferators cause rapid and coordinated transcriptional activation of genes encoding peroxisomal [beta]-oxidation system enzymes by activating peroxisome some proliferator-activated receptor (PPAR) isoform(s). Since the thyroid hormone ([T.sub.3]; 3,3',5-triiodothyronine) receptor (TR), another member of the nuclear hormone receptor superfamily, regulates a subset of fatty acid metabolism genes shared with PPAR, we examined the possibility of interplay between peroxisome proliferator and [T.sub.3] signaling pathways. [T.sub.3] inhibited ciprofibrate-induced luciferase activity as well as the endogenous peroxisomal [beta]-oxidation enzymes in transgenic mice carrying a 3.2-kb 5'-flanking region of the rat peroxisomal enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase gene fused to the coding region of luciferase. Transfection assays in hepatoma H4-II-E-C3 and CV-1 cells indicated that this inhibition is mediated by TR in a ligand-dependent fashion. Gel shift assays revealed that modulation of PPAR action by TR occurs through titration of limiting amounts of retinoid X receptor (RXR) required for PPAR activation. increasing amounts of RXR partially reversed the inhibition in a reciprocal manner; PPAR also inhibited TR activation. Results with heterodimerization-deficient TR and PPAR mutants further confirmed that interaction between PPAR and TR signaling systems is indirect. These results suggest that a convergence of the peroxisome proliferator and [T.sub.3] signaling pathways occurs through their common with the heterodimeric partner RXR.

Anterior pituitary cells were cultured for 2 days from 6-,14-, and 23-25-month-old C57BL/6NNia mice. The cells were then stimulated with one of three biolinylated GnRHs (biolinyl-Lys4]-[D-Lys6] [GnRH, [biolinyl-[Ser.syp.4]]-[D-[Lys.sup.6]]GnRH, [biolinyl-[Ser.sup.4]]-[D-[Trp.sup.6], des-[Gly.sup.10]]GnRH) at 4 [degrees] C for 1 h. Some of these cells were processed unfixed, attaching avidin-fluorescein and examined with fluorescent microscopy. Other cells were fixed and the biolinylated GnRH coupled to avidin-gold (20 nm particies), which was subsequently silver-enhanced (70 nm particles) and examined with light and scanning electron microscopy (SEM). To enhance the location of receptor sites for quantitation, a back-scattering electron detector was employed with the SEM. Gonadotropes from all three age groups ranged in size from 3 to 13 [mu] m in diameter. The Iargest gonadotropes in each age group displayed the highest number of GnRH receptors, but the concentration/surface area ([mu][m.sup.2]) was less than for smaller cells. Smaller gonadotropes (3-6 [mu]m diameter) from 14- and 23-25-month-old mice contained more GnRH receptor sites/[mu][m.sup.2] (p

This study was conducted to determine whether dietary zinc status affects the expression of the insulin-like growth factor I and growth hormone receptor/growth hormone binding protein genes in the liver of growing rats. Weanling male Sprague-Dawley rats were randomly allotted to zinc-deficient, pair-fed or ad libitum-fed dietary treatments and fed diets containing no added zinc for 14 d. Zinc acetate was added to the deionized, distilled water (30 mg/L) provided to pair-fed and ad libitum-fed rats. As expected, zinc deficiency significantly reduced growth rate by 60% and was associated with a significantly lower serum insulin-like growth factor I concentration (46 and 67% lower than pair-fed and ad libitum-fed rats, respectively). The reduction in serum insulin-like growth factor I concentration was associated with a decrease in insulin-like growth factor I gene expression. The abundance of the 7.5-kb insulin-like growth factor I mRNA transcript in zinc-deficient and pair-fed rats was 14 and 31% that of the ad libitum-fed rats. The 0.8-1.2-kb insulin-like growth factor I transcript also was significantly lower in the zinc-deficient and pair-fed rats. In contrast, the abundance of the 1.8-kb insulin-like growth factor I transcript was unaffected by zinc deficiency. The growth hormone receptor mRNA levels of zinc-deficient and pair-fed rats were 17 and 50% and their growth hormone binding protein mRNA levels were 46 and 65% those of the ad libitum-fed rats. In summary, zinc deficiency markedly decreases expression of the insulin-like growth factor I and growth hormone receptor genes. The mechanisms that account for the decreases are obscure. Nonetheless, our results indicate that the growth retardation caused by zinc deficiency is associated with defects in the growth hormone receptor signaling pathway. INDEXING KEY WORDS: growth hormone binding protein; rats; insulin-like growth factor I; growth hormone receptor; zinc

Analysis of genomic DNA from 32 unrelated families with male-limit precocious puberty subjected to Asp578 to Gly, Met571 to Ile and Thr577 to Ile mutations in transmembrane helix 6 of the human luteinizing hormone receptor (hLHR) shows that heterogeneous point mutations occur mainly in the region covering nt 1624-1741 of exon 11. The mutations trigger hLHR to result in male-limited precocious puberty.

Gonadotrophin-releasing hormone receptor agonist down-modulates pertussis toxin-insensitive G proteins G9-alpha and G11-alpha in alpha T3-1 pituitary gonadotrophs. Exposure to the agonist increases the turnover of G9 alpha and G11 alpha but not cause changes in mRNA levels. These results reflect enhanced proteolysis of the activated G proteins.

Experimental studies of a yeast two-hybrid system suggest the ability of the thyroid-hormone-receptor interacting protein (Trip1), which possesses structural homology with the transcriptional mediator Sug1, to react in vitro with the regulatory protein GAL4's activation domain and with the thyroid-hormone receptor. This implies a role of Trip1 as a transcription activator in yeast.

Coexpression of the cDNA for a member of the nuclear receptor family, ubiquitous receptor (UR), in COS-1 cells prevents the induction of chloramphenicol acetyltransferase (CAT) reporter gene expression by hRXRalpha and human retinoic acid receptor alpha, in the presence of all-trans-retinoic acid. UR, in combination with hRXRalpha, activates DR-4-CAT expression, which is reduced by the coexpression of thyroid hormone receptor. Normal growth and differentiation involve the regulation of gene activation in retinoic acid and thyroid hormone signaling pathways by UR.

The activation of G protein alpha subunit, a(s), by hormone receptor or a GTPase-inhibiting mutation results in activation-induced depalmitoylation and increased rate of attachment of the a(s) pool by (3H) palmitate. Regulation of palmitoylation is a feature of a number of proteins. Palmitoylation cycles control membrane activity and subcellular location.

A novel kinetic analysis model was used to study insulin receptor autophosphorylation. The fast reaction phase, slow reaction phase and a partition function for both phases are studied in the kinetic model of autophosphorylation. Insulin was found to enhance autophosphorylation through a 10-fold increase in the rate constant for the fast reaction phase and a two-fold increase in the rate constant of the partition function favoring the fast reaction phase. The juxtamembrane sites of the insulin receptor was found to react primarily during the slow reaction phase. The carboxy-terminal and central autophosphorylation domains react in the fast reaction phase.

Studies of hormone receptors and the transcription factors that they induce are providing new insights into the control of growth and metamorphosis of insects. The epidermis and the nervous system are used to illustrate the power of these new approaches. Many aspects of a tissue's response to the molting hormone are encoded at the level of the ecdysteroid receptor (EcR) itself. For example, there is a correlation between the presence of EcR and a particular tissue's response to ecdysteroids during larval molts. In preparation for metamorphosis around the time of commitment, all tissues express high levels of EcR as they are losing their larval identities and preparing to transform into the adult. Different forms of EcR are then associated with different programs of differentiation during the subsequent adult development. Tissue- and cell-specific aspects of responses to ecdysteroid are presumably also reflected in quantitative and qualitative alterations in the cascade of transcription factors that the hormone elicits. In Manduca sexta the ecdysteroid-induced transcription factor MHR3 shows a region-specific onset of expression in the epidermis that mirrors the spread of competence to form a new cuticle in response to the rise of ecdysteroid. Juvenile hormone (JH) prevents metamorphosis by modulating ecdysteroid action during the molt and thus may also influence the levels and forms of the EcR and the ecdysteroid-induced transcription factors. Studies of a putative nuclear receptor for JH have shown that its expression is influenced by both the ecdysteroid and JH titers.

The possibility that cholecystokinin (CCK) receptor can be a multiple kinase substrate and a receptor-specific kinase substrate was examined. Permeabilization experiments were conducted on dispersed rat pancreatic acinar cells. The existence of the two kinases that function on the CCK receptor and its analogy to enzymes in the beta-andrenergic receptor family of kinases were established.

Peroxisome proliferator-activated receptors (PPAR) are nuclear hormone receptors which function as transcription factors that control the expression of the acyl-CoA oxidase gene, thus regulating the peroxisomal beta-oxidation of fatty acids. A search for physiological activators of PPAR-alpha showed that retinoic X receptor-beta could bind with PPAR-alpha and the resulting heterodimer stimulates the acyl-CoA oxidase gene promoter. In addition, PPAR-alpha could also be activated by physiological concentrations of fatty acids and a synthetic arachidonic acid analog.

The peptide des-His-1-(Glu-9)glucagon amide is a full glucagon antagonist at receptors on hepatic plasma membranes. The interactions of this antagonist with intact and saponin-permeabilized canine hepatocytes were investigated. The results confirmed that des-His-1-(Glu-9)glucagon amide acts as a glucagon antagonist bybinding to glucagon receptors. Based on the results, a model for the mechanism of action of the glucagon antagonist is presented.

On his canvas entitled 'La trahison des Images' ('The Perfidy of Images'), Rene Magritte painted a tobacco pipe in a very realistic manner and added the words: 'Ceci n'est pas une pipe' ('This is not a pipe'). In similar style, it is of prime importance to state that the first three-dimensional (3D) models of G protein-coupled receptors (GPCRs) that have been defined and displayed are definitely not GPCRs and never will be. However, they probably represent a very important breakthrough in our understanding of GPCR structure and function as well as being a source of novel working hypotheses that can be experimentally explored. Several generations of scientists have elaborated their research on the concept of hormone-receptor interaction without having access to any structural representation of this molecular complex. Thus, for many years receptors remained only indirectly characterized through their functions or their binding properties. As a consequence, a number of important issues are still to be addressed. How does a hormone bind and activate its receptor? How does an antagonist inactivate a receptor? What is a partial agonist? What are the molecular mechanisms of signal transduction and modulation? Obviously, answers to these questions are of prime importance for medicinal chemists in their attempts to rationalize drug design.

A study was done on the composition of the androgen receptor protein complex during the process of hormone-induced receptor transformation in intact cells. Specific monoclonal antibodies were able to identify hsp90, hsp70 and hsp56. In addition, it was shown that the receptor complex undergoes large rearrangements prior to tight nuclear binding leading to the loss of different heat shock proteins. These findings suggest the presence of an antigenic epitope in the DNA-binding domain of the androgen receptor.

TOXICOLOGY Hormone Receptor Activity of PBDEs and Metabolites Polybrominated diphenyl ethers (PBDEs) have been shown to have estrogenic, antiestrogenic and/or antiandrogenic activities mediated through estrogen receptor [alpha] (ER[alpha]). However, relatively [...]

The ligand-binding domain of nuclear receptors contains a transcriptional activation function (AF-2) that mediates hormone-dependent binding of coactivator proteins. Scanning surface mutagenesis on the human thyroid hormone receptor was performed [...]

A single heterozygous nucleotide exchange in exon M2 of the gene encoding the parathyroid hormone-parathyroid hormone-related peptide (PTH-PTHrP) receptor was identified in a patient with Jansen-type metaphyseal chondrodysplasia, which changes a strictly conserved histidine residue at position 223 in the receptor's first intracellular loop to arginine. Constitutive, ligand-independent adenosine 3',5'-monophosphate accumulation was observed in COS-7 cells expressing the mutant PTH-PTHrP receptor but not in cells expressing the wild-type receptor. This finding explains the severe ligand-independent hypercalcemia and hypophosphatemia, and most likely the abnormal formation of endochondral bone, in this rare form of short-limbed dwarfism., Jansen-type metaphyseal chondrodysplasia (JMC) [1], a rare form of short-limbed dwarfism, is associated with asymptomatic but often profound hypercalcemia and hypophosphatemia [2, 3], despite the lack of parathyroid gland abnormalities [...]

Signal transmission by insulin involves tyrosine phosphorylation of a major insulin receptor substrate (IRS-1) and exchange of Ras-bound guanosine diphosphate for guanosine triphosphate. Proteins containing Src homology 2 and 3 (SH2 and SH3) domains, such as the p85 regulatory subunit of phosphatidylinositol-3 kinase and growth factor receptorbound protein 2 (GRB2), bind tyrosine phosphate sites on IRS-1 through their SH2 regions. Such complexes in COS cells were found to contain the heterologously expressed putative guanine nucleotide exchange factor encoded by the Drosophila son of sevenless gene (dSos). Thus, GRB2, p85, or other proteins with SH2-SH3 adapter sequences may link Sos proteins to IRS-1 signaling complexes as part of the mechanism by which insulin activates Ras., Insulin receptors as well as other tyrosine kinases can elicit biological actions by increasing the steady-state level of guanosine triphosphate (GTP)-bound Ras proteins (1-4). The mechanism of Ras activation by [...]