Biomedical subjects

Richard A Heyman

Publications and source records attributed to Richard A Heyman.

17 recordsLinked to original sources

PubMed2005

Biological characterization of a heterodimer-selective retinoid X receptor modulator: potential benefits for the treatment of type 2 diabetes.

Specific retinoid X receptor (RXR) agonists, such as LG100268 (LG268), and the thiazolidinedione (TZD) PPARgamma agonists, such as rosiglitazone, produce insulin sensitization in rodent models of insulin resistance and type 2 diabetes. In sharp contrast to the TZDs that produce significant increases in body weight gain, RXR agonists reduce body weight gain and food consumption. Unfortunately, RXR agonists also suppress the thyroid hormone axis and generally produce hypertriglyceridemia. Heterodimer-selective RXR modulators have been identified that, in rodents, retain the metabolic benefits of RXR agonists with reduced side effects. These modulators bind specifically to RXR with high affinity and are RXR homodimer partial agonists. Although RXR agonists activate many heterodimer partners, these modulators selectively activate RXR:PPARalpha and RXR:PPARgamma, but not RXR:RARalpha, RXR:LXRalpha, RXR:LXRbeta, or RXR:FXRalpha. We report the in vivo characterization of one RXR modulator, LG101506 (LG1506). In Zucker fatty (fa/fa) rats, LG1506 is a potent insulin sensitizer that also enhances the insulin-sensitizing activities of rosiglitazone. Administration of LG1506 reduces both body weight gain and food consumption and blocks the TZD-induced weight gain when coadministered with rosiglitazone. LG1506 does not significantly suppress the thyroid hormone axis in rats, nor does it elevate triglycerides in Sprague Dawley rats. However, LG1506 produces a unique pattern of triglycerides elevation in Zucker rats. LG1506 elevates high-density lipoprotein cholesterol in humanized apolipoprotein A-1-transgenic mice. Therefore, selective RXR modulators are a promising approach for developing improved therapies for type 2 diabetes, although additional studies are needed to understand the strain-specific effects on triglycerides.

Analysis of Variance↗

PubMed2005

FXR, a therapeutic target for bile acid and lipid disorders.

The farnesoid X receptor (FXR) is a nuclear receptor expressed in tissues exposed to high concentrations of bile acids such as the liver, kidney and intestine and functions as a bile acid sensor. FXR regulates the expression of various transport proteins and biosynthetic enzymes crucial to the physiological maintenance of lipids, cholesterol and bile acid homeostasis. The concept of reverse endocrinology, whereby the receptor is identified first, followed by the identification of ligands and the sequential elucidation of the physiological role of the receptor has been widely used for a number of orphan nuclear receptors. The design of synthetic high affinity ligands acting via these receptors not only helps to decipher the function of the receptor, but also should lead to the development of novel and highly specific drugs. The bile acid receptor FXR is a perfect example where this strategy helped with understanding the role of this receptor in cholesterol and bile acid homeostasis. Regulation of FXR through small-molecule drugs represents a promising therapy for diseases resulting from lipid, cholesterol and bile acid abnormalities.

Animals↗

PubMed2004

Macrophage liver X receptor is required for antiatherogenic activity of LXR agonists.

OBJECTIVE: Complications of atherosclerotic cardiovascular disease due to elevated blood cholesterol levels are the major cause of death in the Western world. The liver X receptors, LXRalpha and LXRbeta (LXRs), are ligand-dependent transcription factors that act as cholesterol sensors and coordinately control transcription of genes involved in cholesterol and lipid homeostasis as well as macrophage inflammatory gene expression. LXRs regulate cholesterol balance through activation of ATP-binding cassette transporters that promote cholesterol transport and excretion from the liver, intestine, and macrophage. Although LXR agonists are known to delay progression of atherosclerosis in mouse models, their ability to abrogate preexisting cardiovascular disease by inducing regression and stabilization of established atherosclerotic lesions has not been addressed. METHODS AND RESULTS: We demonstrate that LXR agonist treatment increases ATP-binding cassette transporter expression within preexisting atherosclerotic lesions, resulting in regression of these lesions as well as remodeling from vulnerable to stable lesions and a reduction in macrophage content. Further, using macrophage-selective LXR-deficient mice created by bone marrow transplantation, we provide the first evidence that macrophage LXR expression is necessary for the atheroprotective actions of an LXR agonist. CONCLUSIONS: These data substantiate that drugs targeting macrophage LXR activity may offer therapeutic benefit in the treatment of atherosclerotic cardiovascular disease.

Animals↗

PubMed2004

Regulation of PPARgamma coactivator 1alpha (PGC-1alpha) signaling by an estrogen-related receptor alpha (ERRalpha) ligand.

Peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha (PGC-1alpha) is a transcriptional coactivator that is a key component in the regulation of energy production and utilization in metabolic tissues. Recent work has identified PGC-1alpha as a strong coactivator of the orphan nuclear receptor estrogen-related receptor alpha (ERRalpha), implicating ERRalpha as a potential mediator of PGC-1alpha action. To understand the role of ERRalpha in PGC-1alpha signaling, a parallel approach of high-throughput screening and gene-expression analysis was used to identify ERRalpha small-molecule regulators and target genes. We report here the identification of a potent and selective ERRalpha inverse agonist that interferes effectively with PGC-1alpha/ERRalpha-dependent signaling. This inverse agonist inhibits the constitutive activity of ERRalpha in both biochemical and cell-based assays. Also, we demonstrate that monoamine oxidase B is an ERRalpha target gene whose expression is regulated by PGC-1alpha and ERRalpha and inhibited by the ERRalpha inverse agonist. The discovery of potent and selective ERRalpha modulators and their effect on PGC-1alpha signaling provides mechanistic insight into gene regulation by PGC-1alpha. These findings validate ERRalpha as a promising therapeutic target in the treatment of metabolic disorders, including diabetes and obesity.

Animals↗

PubMed2004

The selective estrogen receptor modulator arzoxifene and the rexinoid LG100268 cooperate to promote transforming growth factor beta-dependent apoptosis in breast cancer.

We show that the selective estrogen receptor modulator arzoxifene (Arz) and the rexinoid LG100268 (268) synergize to promote apoptosis in a rat model of estrogen receptor-positive breast carcinoma and in estrogen receptor-positive human breast cancer cells in culture. We also show that it is not necessary to administer Arz and 268 continuously during tumor progression to prevent cancer in the rat model because dosing of these drugs in combination for relatively short periods, each followed by drug-free rests, is highly effective. This new approach to chemoprevention uses high doses of drugs that are too toxic for long-term administration. However, when given for short periods, the agents are nontoxic and still induce apoptosis in breast cancer cells. We also show that the ability of the two drugs to induce apoptosis is the combined result of induction of transforming growth factor beta by Arz, together with inhibition of the prosurvival nuclear factor kappaB and phosphatidylinositol 3' kinase signaling pathways by 268. The new protocol we have developed for chemoprevention allows the efficacious and safe administration of 268 and Arz, and these agents now should be considered for clinical use.

Animals↗

PubMed2004

Erralpha and Gabpa/b specify PGC-1alpha-dependent oxidative phosphorylation gene expression that is altered in diabetic muscle.

Recent studies have shown that genes involved in oxidative phosphorylation (OXPHOS) exhibit reduced expression in skeletal muscle of diabetic and prediabetic humans. Moreover, these changes may be mediated by the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha). By combining PGC-1alpha-induced genome-wide transcriptional profiles with a computational strategy to detect cis-regulatory motifs, we identified estrogen-related receptor alpha (Erralpha) and GA repeat-binding protein alpha as key transcription factors regulating the OXPHOS pathway. Interestingly, the genes encoding these two transcription factors are themselves PGC-1alpha-inducible and contain variants of both motifs near their promoters. Cellular assays confirmed that Erralpha and GA-binding protein a partner with PGC-1alpha in muscle to form a double-positive-feedback loop that drives the expression of many OXPHOS genes. By using a synthetic inhibitor of Erralpha, we demonstrated its key role in PGC-1alpha-mediated effects on gene regulation and cellular respiration. These results illustrate the dissection of gene regulatory networks in a complex mammalian system, elucidate the mechanism of PGC-1alpha action in the OXPHOS pathway, and suggest that Erralpha agonists may ameliorate insulin-resistance in individuals with type 2 diabetes mellitus.

Animals↗

PubMed2004

The flip side: Identifying small molecule regulators of nuclear receptors.

Members of the nuclear hormone receptor superfamily function as ligand-activated transcription factors to regulate genetic networks controlling cell growth and differentiation, inflammatory responses, and metabolism. The ability to modulate nuclear receptor-dependent gene expression with small molecules has made the superfamily a favored target for drug discovery. Not surprisingly, small molecules that regulate receptor activity are currently used to treat a number of human disorders. Over the last 10 years, the availability of a common platform of functional assays suitable for any nuclear receptor has facilitated the identification of endogenous and synthetic ligands that have been used as tools to uncover previously unanticipated endocrine signaling pathways. Recent progress in understanding the molecular basis for ligand-dependent gene regulation suggests that a new era of "designer" ligands with tissue- and/or gene-selective activity will quickly be upon us.

Animals↗

PubMed2004

Bile acids lower triglyceride levels via a pathway involving FXR, SHP, and SREBP-1c.

We explored the effects of bile acids on triglyceride (TG) homeostasis using a combination of molecular, cellular, and animal models. Cholic acid (CA) prevents hepatic TG accumulation, VLDL secretion, and elevated serum TG in mouse models of hypertriglyceridemia. At the molecular level, CA decreases hepatic expression of SREBP-1c and its lipogenic target genes. Through the use of mouse mutants for the short heterodimer partner (SHP) and liver X receptor (LXR) alpha and beta, we demonstrate the critical dependence of the reduction of SREBP-1c expression by either natural or synthetic farnesoid X receptor (FXR) agonists on both SHP and LXR alpha and LXR beta. These results suggest that strategies aimed at increasing FXR activity and the repressive effects of SHP should be explored to correct hypertriglyceridemia.

Animals↗

PubMed2003

Nurr1-RXR heterodimers mediate RXR ligand-induced signaling in neuronal cells.

The retinoid X receptor (RXR) is essential as a common heterodimerization partner of several nuclear receptors (NRs). However, its function as a bona fide receptor for endogenous ligands has remained poorly understood. Such a role would depend on the existence of RXR activating ligands in vivo and on the ability of such ligands to influence relevant biological functions. Here we demonstrate the presence of endogenous RXR ligands in the embryonic central nervous system (CNS) and show that they can activate heterodimers formed between RXR and the orphan NR Nurr1 in vivo. Moreover, RXR ligands increase the number of surviving dopaminergic cells and other neurons in a process mediated by Nurr1-RXR heterodimers. These results provide evidence for a role of Nurr1 as a ligand-independent partner of RXR in its function as a bona fide ligand-activated NR. Finally, our findings identify RXR-Nurr1 heterodimers as a potential target in the treatment of neurodegenerative disease.

Animals↗

PubMed2003

Activation of the retinoid X receptor suppresses appetite in the rat.

The retinoid X receptor (RXR), a ubiquitously expressed intracellular receptor, regulates pathways controlling glucose, triglycerides, cholesterol, and bile acid metabolism. In addition to its role in those metabolic pathways, we reported that RXR activation with a pan agonist [e.g. LG100268 (LG268)] decreases both body weight gain (BWG) and food consumption (FC) in obese, insulin-resistant rodents. In parallel with those changes in energy balance, we show here that activation of RXR pathways results in adipose tissue remodeling, particularly within sc fat where the rate of apoptosis is increased 5-fold. This change may underlie the selective decrease in fat mass observed in Zucker fatty rats treated with LG268 for 6 wk. Because FC is strongly correlated with BWG in treated animals, we hypothesized that regulation of FC might be the primary mechanism underlying reduced BWG during RXR agonist administration. Importantly, decreased FC is due to decreased meal size, suggestive of induced satiety rather than malaise and/or aversion to food. Furthermore, administration of LG268 directly into the brain via intracerebroventricular injection also reduces FC, BWG, and insulin, whereas the elevation in triglycerides observed after oral administration is absent. The latter observation suggests that RXR actions on energy balance and lipid homeostasis are separable. Therefore, ligand-mediated activation of either an RXR homodimer or an unidentified heterodimeric complex regulates pathways controlling energy balance at least in part via a central nervous system-mediated mechanism.

Adipose Tissue↗

PubMed2003

Identification of the antineoplastic agent 6-mercaptopurine as an activator of the orphan nuclear hormone receptor Nurr1.

The purine anti-metabolite 6-mercaptopurine is one of the most widely used drugs for the treatment of acute childhood leukemia and chronic myelocytic leukemia. Developed in the 1950s, the drug is also being used as a treatment for inflammatory diseases such as Crohn's disease. The antiproliferative mechanism of action of this drug and other purine anti-metabolites has been demonstrated to be through inhibition of de novo purine synthesis and incorporation into nucleic acids. Despite the extensive clinical use and study of 6-mercaptopurine and other purine analogues, the cellular effects of these compounds remain relatively unknown. More recently, purine anti-metabolites have been shown to function as protein kinase inhibitors and to regulate gene expression. In an attempt to find small molecule regulators of the orphan nuclear receptor Nurr1, interestingly, we identified 6-mercaptopurine as a specific activator of this receptor. A detailed analysis of 6-mercaptopurine regulation of Nurr1 demonstrates that 6-mercaptopurine regulates Nurr1 through a region in the amino terminus. This activity can be inhibited by components of the purine biosynthesis pathway. These findings indicate that Nurr1 may play a role in mediating some of the antiproliferative effects of 6-mercaptopurine and potentially implicate Nurr1 as a molecular target for treatment of leukemias.

Amino Acid Sequence↗

PubMed2003

Promoter-specific roles for liver X receptor/corepressor complexes in the regulation of ABCA1 and SREBP1 gene expression.

Liver X receptors (LXRs) regulate the expression of genes involved in cholesterol and fatty acid homeostasis, including the genes for ATP-binding cassette transporter A1 (ABCA1) and sterol response element binding protein 1 (SREBP1). Loss of LXR leads to derepression of the ABCA1 gene in macrophages and the intestine, while the SREBP1c gene remains transcriptionally silent. Here we report that high-density-lipoprotein (HDL) cholesterol levels are increased in LXR-deficient mice, suggesting that derepression of ABCA1 and possibly other LXR target genes in selected tissues is sufficient to result in enhanced HDL biogenesis at the whole-body level. We provide several independent lines of evidence indicating that the repressive actions of LXRs are dependent on interactions with the nuclear receptor corepressor (NCoR) and the silencing mediator of retinoic acid and thyroid hormone receptors (SMRT). While dissociation of NCoR and SMRT results in derepression of the ABCA1 gene in macrophages, it is not sufficient for derepression of the SREBP1c gene. These findings reveal differential requirements for corepressors in the regulation of genes involved in cholesterol and fatty acid homeostasis and raise the possibility that these interactions may be exploited to develop synthetic ligands that selectively modulate LXR actions in vivo.

ATP Binding Cassette Transporter 1↗

PubMed2003

Orphan nuclear receptor modulators.

Nuclear hormone receptors are ligand-activated transcription factors that regulate gene expression and play a critical role in endocrine signaling. Orphan nuclear receptors belong to this gene super-family but their target genes and physiological function have not been completely elucidated. In recent years, the identification of natural ligands for these orphan receptors, their expression pattern in different tissues and studies with knock-out animals has delineated distinct regulatory functions for these proteins. The orphans belonging to the PPAR, LXR and FXR family function as lipid and bile-acid sensors while PXR and CAR function as xenobiotic sensors. This review will describe the discovery of natural and synthetic ligands for a number of these orphan receptors (excluding the PPARs) and the identification and characterization of novel signaling pathways and new hormone response systems linked to these targets. Small-molecule modulators of LXR and FXR control key genes involved in cholesterol and lipid metabolism. PXR is a highly promiscuous xenosensor that responds to xenobiotic ligands (antibiotics, statins, glucocorticoids) and induces the Cyp3A gene, thereby playing a role in hepatoprotection and bile acid metabolism. A related receptor from the gene subfamily, CAR, displays high ligand selectivity and modulation of its activity in humans may significantly alter metabolism of drugs and other xenobiotics. The role of the ER relatives, the ERRs will become more apparent as ligands are identified and linked to target genes and physiological function. These targets offer multiple opportunities for therapeutic intervention with small-molecule drugs, in diseases related to neuronal function, inflammation, lipid homeostasis, metabolic function and cancer.

Animals↗

PubMed2002

Identification of macrophage liver X receptors as inhibitors of atherosclerosis.

Recent studies have identified the liver X receptors (LXR alpha and LXR beta) as important regulators of cholesterol metabolism and transport. LXRs control transcription of genes critical to a range of biological functions including regulation of high density lipoprotein cholesterol metabolism, hepatic cholesterol catabolism, and intestinal sterol absorption. Although LXR activity has been proposed to be critical for physiologic lipid metabolism and transport, direct evidence linking LXR signaling pathways to the pathogenesis of cardiovascular disease has yet to be established. In this study bone marrow transplantations were used to selectively eliminate macrophage LXR expression in the context of murine models of atherosclerosis. Our results demonstrate that LXRs are endogenous inhibitors of atherogenesis. Additionally, elimination of LXR activity in bone marrow-derived cells mimics many aspects of Tangier disease, a human high density lipoprotein deficiency, including aberrant regulation of cholesterol transporter expression, lipid accumulation in macrophages, splenomegaly, and increased atherosclerosis. These results identify LXRs as targets for intervention in cardiovascular disease.

Animals↗

PubMed2002

A natural product that lowers cholesterol as an antagonist ligand for FXR.

Extracts of the resin of the guggul tree (Commiphora mukul) lower LDL (low-density lipoprotein) cholesterol levels in humans. The plant sterol guggulsterone [4,17(20)-pregnadiene-3,16-dione] is the active agent in this extract. We show that guggulsterone is a highly efficacious antagonist of the farnesoid X receptor (FXR), a nuclear hormone receptor that is activated by bile acids. Guggulsterone treatment decreases hepatic cholesterol in wild-type mice fed a high-cholesterol diet but is not effective in FXR-null mice. Thus, we propose that inhibition of FXR activation is the basis for the cholesterol-lowering activity of guggulsterone. Other natural products with specific biologic effects may modulate the activity of FXR or other relatively promiscuous nuclear hormone receptors.

Animals↗

PubMed2002

Redundant pathways for negative feedback regulation of bile acid production.

The orphan nuclear hormone receptor SHP has been proposed to have a key role in the negative feedback regulation of bile acid production. Consistent with this, mice lacking the SHP gene exhibit mild defects in bile acid homeostasis and fail to repress cholesterol 7-alpha-hydroxylase expression in response to a specific agonist for the bile acid receptor FXR. However, this repression is retained in SHP null mice fed bile acids, demonstrating the existence of compensatory repression pathways of bile acid signaling. We provide evidence for two such pathways, based on activation of the xenobiotic receptor PXR or the c-Jun N-terminal kinase JNK. We conclude that redundant mechanisms regulate this critical aspect of cholesterol homeostasis.

Animals↗

PubMed2002

Prevention and treatment of experimental breast cancer with the combination of a new selective estrogen receptor modulator, arzoxifene, and a new rexinoid, LG 100268.

The selective estrogen receptor modulator arzoxifene and the rexinoid LG 100268 were active not only as single agents for prevention and treatment of breast cancer in the rat model that uses nitrosomethylurea as the carcinogen but also showed striking synergy, both preventively and therapeutically, in a series of six experiments with a total of 465 rats. Mechanistic studies in cell culture reported here suggest that enhancement of stromal-epithelial interactions may contribute to this synergy. The possible clinical use of the combination of arzoxifene and LG 100268 for prevention of breast cancer in women at high risk, for treatment of women in the adjuvant setting, or for treatment of end-stage disease should now be considered.

Animals↗