Ebook: Nuclear Receptors as Molecular Targets for Cardiometabolic and Central Nervous System Diseases

“The Solvay Pharmaceuticals Conferences: where industry meets academia in a search for novel therapies”

Nuclear Receptors Take Off

The cloning of the first nuclear receptor cDNA encoding the human glucocorticoid receptor was described in 1985 by the team of Evans [1]. Over the next 25 years a dramatic growth of knowledge on nuclear receptors followed this discovery [2]. The knowledge on nuclear receptors has delivered novel therapies for lipid control and hormone replacement, and for management of cancers and diabetes, and millions of humans were subjected to therapies with nuclear receptor modulators over last decades [2,3].

Nuclear receptors are a family of transcription factors consisting of 49 members identified in the human genome [3]. Nuclear receptors regulate transcription by binding to response elements in the regulatory regions of target genes and thereby affect expression of genes involved in differentiation, growth, lipid homeostasis, inflammation and immunity. Therefore, nuclear receptors are attractive molecular targets for design of therapy for diabetes, obesity, atherosclerosis, cancer, inflammation and neurodegeneration.

Many drugs from the armamentarium of contemporary physicians are acting on nuclear receptors: estrogens for hormone replacement therapy, anti-estrogens for treatment of cancer, steroids for treatment of inflammatory disorders, fibrates for treatment of dyslipidemia, and thiazolidinediones for therapy of diabetes [2].

Through their distinct tissue distribution and specific target gene activation, the peroxisome proliferator-activated receptors (PPARs) α, γ and δ modulate diverse aspects of fatty acid metabolism, energy balance, insulin sensitivity, glucose homeostasis and inflammatory responses. Two types of PPARs are marketed: PPARα is the target for fibrates (hypolipidemic drugs), PPARγ is the target for thiazolidinediones (anti-diabetic drugs).

The Liver X Receptors (LXRs) modulate macrophage cholesterol efflux and repress the expression of pro-inflammatory genes. Therefore, LXRs are considered as a target for the treatment of atherosclerosis (prevention and reversal). LXRs are key players in inflammatory conditions such as rheumatoid arthritis, inflammatory bowel diseases and diabetes. Through action on both cholesterol homeostasis and inflammatory processes, LXRs are considered as prospective targets for design of novel therapies for Alzheimer's disease.

Thyroid hormone signals are transduced by two distinct nuclear receptors: TRα and TRβ. TRα mediates the effects of thyroid hormones on heart rate whereas TRβ mediates cholesterol lowering effects. Therapeutic use of these receptors has not substantiated yet, but both are carefully considered by drug developers.

In addition to transcriptional regulation of metabolic pathways, nuclear receptors regulate the expression of genes participating in inflammatory cascades as well as genes promoting cellular growth and differentiation. Therefore, nuclear receptors continue to be important for the development of novel therapies of inflammation, cancer and neurodegeneration.

This volume contains papers from the Eight Solvay Pharmaceuticals Conference on Nuclear Receptors as Molecular Targets for Cardiometabolic and Central Nervous System Diseases held in Nice (France) April 11–13, 2007.

It has been the aim of these conferences to bring together scientists from academia and from industry in order to stimulate dialog between them in a congenial setting. The focus of this conference centered on the mechanistic involvement of nuclear receptors in cardiological, metabolic and neurological disorders, on possible explanation of pathways involved in pathogenesis, on susceptibility to and prevention of metabolic and neurological disorders and on the aspects of drug finding including chemistry and rational drug design. New technologies were highlighted including gene expression, novel approaches towards epigenetics, physiological monitoring and prospective use of novel therapeutics.

W. Cautreels, C. Steinborn, L. Turski

References

[1] S.M. Hollenberg, C. Weinberger C, E.S. Ong et al. Primary structure and expression of a functional human glucocorticoid receptor cDNA. Nature 318 (1985) 635–641.

[2] R.M. Evans. The nuclear receptor superfamily: a rosetta stone for physiology. Mol. Endocrinol. 19 (2005) 1429–1438.

[3] D.L. Morganstein and M.G. Parker. Role of nuclear receptor coregulators in metabolism. Exp. Rev. Endocrinol. Metab. 2 (2007) 797–807.

Nuclear receptors (NRs) are major targets for drug discovery and play key roles in development and homeostasis, as well as in many diseases such as obesity, diabetes and cancer. This review provides a general overview of the mechanism of action of nuclear receptors and explores the various factors that are instrumental in modulating their pharmacology. One of the most promising aspects of nuclear receptor pharmacology is that it is now possible to develop ligands with a large spectrum of full, partial or inverse agonist or antagonist activities, but also compounds, called selective nuclear receptor modulators, that activate only a subset of the functions induced by the cognate ligand or that act in a cell-type-selective manner.

Atherosclerotic coronary artery disease is still the leading cause of mortality in industrialised countries. This is largely related to the current tremendous increase of the prevalence of obesity, metabolic syndrome and diabetes. Therefore new strategies have to be developed for stopping such an epidemic situation. Drugs acting through the nuclear receptor LXR may offer an additional benefit or an alternative approach to current therapies since LXR receptors modulate not only the genes controlling the Reverse Cholesterol Transport (RCT) but also genes involved in pathways which are altered in metabolic diseases.

Originally identified as orphan members of the nuclear receptor superfamily, Liver X Receptors exist as two isoforms, LXRα and LXRβ. Oxysterols were identified as the putative physiological ligands for the LXRs, and additional studies have demonstrated that these receptors act as sensors for these cholesterol metabolites and are essential components of a physiological feedback loop regulating cholesterol metabolism and transport. LXR pathway may have also an important role in glucose metabolism since many reports now have shown that LXR-activation can be protective in genetic diabetes models in rodent, improve glucose tolerance and facilitate pancreas insulin secretion.

However the usefulness of LXRs as pharmacological targets has been questioned by the effect of systemic LXR-activation on the expression of hepatic lipogenic genes directly and via activation of hepatic sterol regulatory element-binding protein-1C (SREBP-1C) leading to hypertriglyceridemia and hepatic steatosis. Successful development of LXR-based therapeutics will therefore require methods to exploit the beneficial aspects of LXR-activation whereas avoiding these unwanted side effects.

Using a functional genomic approach, we have recently shown that the orphan nuclear receptors ERRα and γ coordinate a broad transcriptional program controlling energy production and utilization in the heart. In addition, both ERRs appear to be critical for normal heart function as several of their target genes are known to be associated with human cardiomyopathies. The ability to regulate the activity of ERRα and/or ERRγ using synthetic ligands suggests the potential for new therapeutic approaches to prevent and manage cardiovascular diseases.

Bile acids are the natural agonists for the nuclear receptor Farnesoid X Receptor (FXR). Studies utilizing natural and synthetic FXR-agonists and FXR null mice indicate that FXR controls numerous metabolic pathways, including those involved in bile acid, lipid and glucose homeostasis. In addition, FXR functions to control bacterial growth in the intestine, gallstone formation, hepatic regeneration and tumorogenesis. Thus, FXR may represent a novel target for pharmaceutical intervention that may influence various metabolic disorders or diseases.

PPARs are important regulators of lipid and glucose metabolism. Clinical trials assessing the efficacy of fibrates and thiazolidinediones in the treatment of dyslipidemia and insulin resistance and recent genetic studies evaluating the impact of genetic variation in genes encoding PPARs on prediabetic phenotypes, such as insulin resistance, β-cell dysfunction, subclinical inflammation, and ectopic lipid deposition, revealed the importance of these nuclear hormone receptors in human metabolic disease. These findings as well as novel aspects of the role of PPARs in human metabolism are summarized herein.

The liver is considered the major “control center” for maintenance of whole-body cholesterol homeostasis. This organ is the main site for de novo cholesterol synthesis, clearing cholesterol-containing chylomicron remnants and low-density lipoprotein (LDL) particles from plasma and is the major contributor to high-density lipoprotein (HDL) formation. The liver has a central position in the classical definition of the reverse cholesterol transport pathway by taking up periphery-derived cholesterol from lipoprotein particles followed by conversion into bile acids or its direct secretion into bile for eventual removal via the feces. During the past couple of years, however, an additional important role of the intestine in maintenance of cholesterol homeostasis and regulation of plasma cholesterol levels has become apparent. Firstly, molecular mechanisms of cholesterol absorption have been elucidated and novel pharmacological compounds have been identified that interfere with the process and positively impact plasma cholesterol levels. Secondly, it is now evident that the intestine itself contributes to fecal neutral sterol loss as a cholesterol-secreting organ: selective modulation of this process may provide an effective means to accelerate cholesterol turnover. Finally, very recent work has unequivocally demonstrated that the intestine contributes significantly to plasma HDL cholesterol levels and that intestine-specific activation of LXR leads to “clinically relevant” elevation of plasma HDL levels in animal models. Thus, the intestine is a potential target for novel anti-atherosclerotic treatment strategies that, in addition to interference with cholesterol absorption, modulate direct cholesterol excretion and plasma HDL cholesterol levels.

The NR4A subfamily of nuclear orphan receptors comprises three members Nur77, Nurr1 and NOR-1 that are each expressed in the vessel wall in response to injury and in atherosclerotic lesion macrophages. To study the function of NR4As in vascular smooth muscle cells, endothelial cells and monocyte/macrophages gain of function and siRNA-mediated knock-down experiments have been performed in cultured cells and in dedicated mouse models. Nur77 has been shown to inhibit the formation of smooth muscle cell-rich lesions, to promote endothelial cell survival and to modulate the inflammatory response of macrophages. Most recently, small-molecule activators of NR4As such as 6-mercaptopurine have been identified to modulate the transcriptional activity of these nuclear receptors in experimental model systems. In this chapter we will present the knowledge currently available on vascular actions of NR4As and the function of these nuclear receptors in metabolism will be reviewed briefly. A clinical perspective to approach NR4As as targets for intervention in vascular disease will be given as well as directions for future research.

At present there are about 250.000 patients with dementia in the Netherlands. Sixty to 70% of these are diagnosed as patients with Alzheimer's disease (AD). Considering the relative increase in the number of elderly people the prevalence of AD will only increase further.

One hundred years after the first description of AD the underlying molecular mechanisms that finally result in the loss of higher cognitive functions still remain to be clarified. At present there is no cure.

Accumulating evidence indicates a link between an aberrant brain cholesterol metabolism and AD. Therefore, modulation of cerebral cholesterol metabolism may be a possible novel strategy in the treatment of the disease. In the present paper the role of cholesterol in AD and the possibilities to use it as a target for treatment will be addressed.

The biology and role of peroxisome proliferator-activated receptors (PPARs) for physiological and pathophysiological processes has been primarily studied in peripherial organs and tissues. Little is known about the physiological role of PPARs for brain development, maintainance and function. Lessions from transgenic mouse models, however, provide evidence that PPARs may play pivotal roles for CNS development and performance. Thus, knock-out of the PPARβ/δ isoform results in disconnection of the two brain hemispheres and the expression pattern of PPARγ in late fetal development points to an important role for CNS development.

Recently it became clear, that PPARs play an important role for the pathogenesis of various disorders of the CNS. The finding that activation of PPARs, and in particular of the PPARγ isoform, suppresses inflammation in peripherial macrophages and in models of human autoimmune disease, instigated the experimental evaluation of these salutary actions for several CNS disorders that harbor an inflammatory component. Activation of all PPAR isoforms, but especially of PPARγ, has been found to be protective in murine in vitro and in vivo models of Multiple Sclerosis. The verification of these findings in human cells prompted the initiation of clinical studies evaluating PPARγ activation in Multiple Sclerosis patients. Likewise, Alzheimer's disease (AD) has a prominent inflammatory component that arises in response to neurodegeneration and in particular to extracellular deposition of β-amyloid peptides. The fact that non-steroidal anti-inflammatory drugs (NSAIDs) delay the onset and reduce the risk to develop AD, while they also bind to and activate PPARγ, led to the hypothesis that one dimension of NSAID protection in AD may be mediated by PPARγ. Several lines of evidence from in vitro and in vivo studies have supported this hypothesis, using AD-related transgenic cellular and animal models. Principally, anti-amyloidogenic, anti-inflammatory and insulin-sensitizing effects may account for the observed effects. A number of clinical trials have been communicated with promising results and further trials are in preparation, which aim to delineate the exact mechanism of interaction. Animal models of other neurodegenerative disease such as Parkinson's and Amyotrophic Lateral Sclerosis, both associated with a considerable degree of CNS inflammation, have been studied with a positive outcome. Yet, it is not clear whether reduction of inflammation or other, to date unknown mechanisms, account for the observed neuroprotection.

Circadian patterns of cardiovascular vulnerability have been well documented, with a peak incidence of cardiovascular events in the morning. Recent studies have outlined the importance of the “Clock genes” in the development of metabolic disorders predisposing to atherosclerosis. Rev-erbα is a nuclear receptor that regulates hepatic and adipose lipid metabolism as well as vascular inflammation. Recent findings identify Rev-erbα also as a major regulator of the circadian regulation of metabolic pathways. Moreover, cross-talk between Rev-erbα and other nuclear receptors well described as key regulators of atherosclerosis may converge to integrate metabolic and circadian signals.