5-hydroxytryptamine2A receptor inverse agonists as antipsychotics

We have used a cell-based functional assay to define the pharmacological profiles of a wide range of central nervous system active compounds as agonists, competitive antagonists, and inverse agonists at almost all known monoaminergic G-protein-coupled receptor (GPCR) subtypes. Detailed profiling of 40 antipsychotics confirmed that as expected, most of these agents are potent competitive antagonists of the dopamine D2 receptor. Surprisingly, this analysis also revealed that most are potent and fully efficacious 5-hydroxytryptamine (5-HT)2A receptor inverse agonists. No other molecular property was shared as universally by this class of compounds. Furthermore, comparisons of receptor potencies revealed that antipsychotics with the highest extrapyramidal side effects (EPS) liability are significantly more potent at D2 receptors, the EPS-sparing atypical agents had relatively higher potencies at 5-HT2A receptors, while three were significantly more potent at 5-HT2A receptors. Functional high-throughput screening of a diverse chemical library identified 530 ligands with inverse agonist activity at 5-HT2A receptors, including several series of compounds related to known antipsychotics, as well as a number of novel chemistries. An analog of one of the novel chemical series, AC-90179, was pharmacologically profiled against the remaining monoaminergic GPCRs and found to be a highly selective 5-HT2A receptor inverse agonist. The behavioral pharmacology of AC-90179 is characteristic of an atypical antipsychotic agent.

A selective peroxisome proliferator-activated receptor-gamma (PPARgamma) modulator blocks adipocyte differentiation but stimulates glucose uptake in 3T3-L1 adipocytes

Peroxisome proliferator-activated receptor-gamma (PPARgamma) agonists such as the thiazolidinediones are insulin sensitizers used in the treatment of type 2 diabetes. These compounds induce adipogenesis in cell culture models and increase weight gain in rodents and humans. We have identified a novel PPARgamma ligand, LG100641, that does not activate PPARgamma but selectively and competitively blocks thiazolidinedione-induced PPARgamma activation and adipocyte conversion. It also antagonizes target gene activation as well as repression in agonist-treated 3T3-L1 adipocytes. This novel PPARgamma antagonist does not block adipocyte differentiation induced by a ligand for the retinoid X receptor (RXR), the heterodimeric partner for PPARgamma, or by a differentiation cocktail containing insulin, dexamethasone, and 1-methyl-3-isobutylxanthine. Surprisingly, LG100641, like the PPARgamma agonist rosiglitazone, increases glucose uptake in 3T3-L1 adipocytes. Such selective PPARgamma antagonists may help determine whether insulin sensitization by thiazolidinediones is mediated solely through PPARgamma activation, and whether there are PPARgamma-ligand-independent pathways for adipocyte differentiation. If selective PPARgamma modulators block adipogenesis in vivo, they may prevent obesity, lower insulin resistance, and delay the onset of type 2 diabetes.

Synthesis of retinoid X receptor-specific ligands that are potent inducers of adipogenesis in 3T3-L1 cells

A novel series of oxime ligands has been synthesized that displays potent, specific activation of the retinoid X receptors (RXRs). The oximes of 3-substituted (tetramethyltetrahydronaphthyl)carbonylbenzoic acids are readily available by condensation with hydroxyl- or methoxylamine; alkylation of the hydroxyl oxime provides a variety of analogues. Oximes and variously substituted oxime derivatives demonstrate high binding affinity for the RXRs and specific RXR activation and, hence, are called rexinoids. These oxime rexinoids are activators of the RXR:PPARgamma heterodimer and are potent inducers of differentiation of 3T3-L1 preadipocytes to adipocytes. We have recently reported that ligands which activate the RXR:PPARgamma heterodimer in this manner are effective in the treatment of type II diabetes (non-insulin-dependent diabetes mellitus, NIDDM). Thus, these new oxime rexinoids are potential therapeutic agents for the treatment of metabolic disorders, such as obesity and diabetes.

Androgen receptor-mediated antagonism of estrogen-dependent low density lipoprotein receptor transcription in cultured hepatocytes

Postmenopausal women receiving hormone replacement therapy have a lower risk of coronary heart disease than women who do not receive hormone treatment. Multiple mechanisms are likely to underlie estrogen's cardioprotective action, including lowering of plasma low density lipoprotein (LDL) cholesterol. Using an in vitro system exhibiting normal regulation of LDL receptor (LDLR) gene transcription, we show that 17beta-estradiol activates the LDLR promoter in transiently transfected HepG2 cells. LDLR activation by estrogen in HepG2 cells is dependent on the presence of exogenous estrogen receptor, and the estrogen-responsive region of the LDLR promoter colocalizes with the sterol response element previously identified. The estrogen response is concentration dependent, saturable, and sensitive to antagonism by estrogen receptor antagonists. Further, we show that compounds with androgen receptor agonist activity attenuate the estrogen-induced up-regulation of LDLR in our model system. Progestins with androgen receptor agonist activity, such as medroxyprogesterone acetate, also suppress estrogen's effects on LDLR expression through their androgenic properties. Characterization of the interplay between these hormone receptors on the LDLR in vitro system may allow a better understanding of the actions of sex steroids on LDLR gene expression and their roles in cardiovascular disease.

Identification, characterization, and tissue distribution of human peroxisome proliferator-activated receptor (PPAR) isoforms PPARgamma2 versus PPARgamma1 and activation with retinoid X receptor agonists and antagonists

We describe the cloning, characterization, and tissue distribution of the two human peroxisome proliferator activated receptor isoforms hPPARgamma2 and hPPARgamma1. In cotransfection assays the two isoforms were activated to approximately the same extent by known PPARgamma activators. Human PPARgamma binds to DNA as a heterodimer with the retinoid X receptor (RXR). This heterodimer was activated by both RXR agonists and antagonists and the addition of PPARgamma ligands with retinoids resulted in greater than additive activation. Such heterodimer-selective modulators may have a role in the treatment of PPARgamma/RXR-modulated diseases like diabetes. Northern blot analysis indicated the presence of PPARgamma in skeletal muscle, and a sensitive RNase protection assay confirmed the presence of only PPARgamma1 in muscle that was not solely due to fat contamination. However, both PPARgamma1 and PPARgamma2 RNA were detected in fat, and the ratio of PPARgamma1 to PPARgamma2 RNA varied in different individuals. The presence of tissue-specific distribution of isoforms and the variable ratio of PPARgamma1 to PPARgamma2 raised the possibility that isoform expression may be modulated in disease states like non-insulin-dependent diabetes mellitus. Interestingly, a third protected band was detected with fat RNA indicating the possible existence of a third human PPARgamma isoform.

Activation of specific RXR heterodimers by an antagonist of RXR homodimers

Retinoid X receptor (RXR) plays a central role in the regulation of many intracellular receptor signalling pathways and can mediate ligand-dependent transcription, acting as a homodimer or as a heterodimer. Here we identify an antagonist towards RXR homodimers which also functions as an agonist when RXR is paired as a heterodimer to specific partners, including peroxisome proliferator-activated receptor and retinoic acid receptor. This dimer-selective ligand confers differential interactions on the transcription machinery: the antagonist promotes association with TAF110 (TATA-binding protein (TBP)-associated factor 110) and the co-repressor SMRT, but not with TBP, and these properties are distinct from pure RXR agonists. This unique class of RXR ligands will provide a means to control distinct target genes at the level of transcription and allow the development of retinoids with a new pharmacological action.

NF-kappa B activation by interleukin-1 (IL-1) requires an IL-1 receptor-associated protein kinase activity

An important function of interleukin-1 (IL-1) is activation of the transcription factor NF-kappa B, which is signaled via the type I IL-1 receptor (IL-1RI). By receptor mutagenesis studies, we have identified a region of the cytoplasmic domain of IL-1RI that is required for both IL-1-mediated NF-kappa B activation and IL-1-dependent activation of a receptor-associated protein kinase activity we term IRAK. No IL-1RI mutants were found that can activate NF-kappa B in the absence of IRAK activity. Therefore, we propose that IRAK activation is a necessary step in the activation of NF-kappa B by IL-1.

Role of chromatin structure in the regulation of transcription by RNA polymerase II

Recent studies on chromatin have concentrated on the relationship between its structure and gene activity. This topic, which addresses the fundamental mechanisms by which genes are expressed, has become a controversial issue, and the present data support the hypothesis that the structure of chromatin is an important component of transcriptional regulation. Notwithstanding, the complexity of this problem suggests that the current models are probably only a rough approximation of the truth.

Mechanism of transcriptional antirepression by GAL4-VP16

Promoter- and enhancer-binding factors appear to function by facilitating the transcription reaction as well as by counteracting chromatin-mediated repression (antirepression). We have examined the mechanism by which a hybrid activator, GAL4-VP16, is able to counteract histone H1-mediated repression by using both H1-DNA complexes and reconstituted H1-containing chromatin templates. The GAL4 DNA binding domain alone was sufficient to disrupt local H1-DNA interactions, but a transcriptional region was additionally necessary for antirepression. GAL4-VP16-mediated antirepression required an auxiliary factor, denoted as a co-antirepressor, which was partially purified from Drosophila embryos. We have found that the co-antirepressor activity was sensitive to digestion with RNase A. Moreover, total RNA from Drosophila embryos could partially substitute for the co-antirepressor fraction, which indicated that the co-antirepressor may function as a histone acceptor ("histone sink"). These findings suggest a model for gene activation in which sequence-specific transcription factors disrupt H1-DNA interactions at the promoter to facilitate transfer of H1 to a histone acceptor, which then allows access of the basal transcription factors to the DNA template.

A general method for purification of H1 histones that are active for repression of basal RNA polymerase II transcription

H1 histones were purified from extracts of salt-treated nuclei as a co-product of RNA polymerase II transcription factors from both Drosophila embryos and HeLa cells by a simple and general method. This procedure was also used to purify H1 as co-product of the core histones from calf thymus. The key steps in this purification exploit the solubility of H1 in 2.26 M ammonium sulfate and the chromatographic properties of the highly charged H1 molecules on a phenyl-Sepharose resin. H1 that is prepared by this procedure is active for in vitro repression of basal RNA polymerase II transcription. This method provides a new means of purifying H1 by a mild procedure that is likely to be generally useful for studies of transcription and chromatin structure.

Sequence-specific antirepression of histone H1-mediated inhibition of basal RNA polymerase II transcription

To understand the principles of control and selectivity in gene expression, the biochemical mechanisms by which promoter- and enhancer-binding factors regulate transcription by RNA polymerase II were analyzed. A general observed repressor of transcription was purified and identified as histone H1. Since many aspects of H1 binding to naked DNA resemble its interaction with chromatin, purified H1 bound to naked DNA was used as a model for the repressed state of the DNA template. Three sequence-specific transcription factors, Sp1, GAL4-VP16, and GAGA factor, were shown to counteract H1-mediated repression (antirepression). In addition, Sp1 and GAL4-VP16, but not the GAGA factor, activated transcription in the absence of H1. Therefore, true activation and antirepression appear to be distinct activities of sequence-specific factors. Furthermore, transcription antirepression by GAL4-VP16 was sustained for several rounds of transcription. These findings, together with previous studies on H1, suggest that H1 participates in repression of the genome in the ground state and that sequence-specific transcription factors induce selected genes by a combination of true activation and release of basal repression that is mediated at least in part by H1.

Sequence-specific transcriptional antirepression of the Drosophila Krüppel gene by the GAGA factor

We have analyzed the proximal promoter of the Drosophila Krüppel (Kr) gene. A 44-base pair fragment containing the RNA start sites contains significant promoter activity, and this minimal promoter is flanked both upstream and downstream by binding sites for the GAGA factor. The GAGA factor is the predominant sequence-specific DNA binding factor that interacts with the Kr promoter region, and the purified protein activates Kr transcription in vitro. However, strong transcriptional activation of Kr as well as of Ultrabithorax, another GAGA factor-responsive gene, requires the presence of a DNA binding transcriptional repressor. The GAGA factor is able to relieve this repression in a binding site-dependent manner, and, thus, these data suggest that the GAGA factor functions as an antirepressor, rather than an activator, of the Kr gene.