Dexamethasone induction of hypertension and diabetes is PPAR-alpha dependent in LDL receptor-null mice

Identification of a physiologically relevant endogenous ligand for PPARalpha in liver

The nuclear receptor PPARalpha is activated by drugs to treat human disorders of lipid metabolism. Its endogenous ligand is unknown. PPARalpha-dependent gene expression is impaired with inactivation of fatty acid synthase (FAS), suggesting that FAS is involved in generation of a PPARalpha ligand. Here we demonstrate the FAS-dependent presence of a phospholipid bound to PPARalpha isolated from mouse liver. Binding was increased under conditions that induce FAS activity and displaced by systemic injection of a PPARalpha agonist. Mass spectrometry identified the species as 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine (16:0/18:1-GPC). Knockdown of Cept1, required for phosphatidylcholine synthesis, suppressed PPARalpha-dependent gene expression. Interaction of 16:0/18:1-GPC with the PPARalpha ligand-binding domain and coactivator peptide motifs was comparable to PPARalpha agonists, but interactions with PPARdelta were weak and none were detected with PPARgamma. Portal vein infusion of 16:0/18:1-GPC induced PPARalpha-dependent gene expression and decreased hepatic steatosis. These data suggest that 16:0/18:1-GPC is a physiologically relevant endogenous PPARalpha ligand.

The constitutive androstane receptor is an anti-obesity nuclear receptor that improves insulin sensitivity

Obesity and type 2 diabetes are related metabolic disorders of high prevalence. The constitutive androstane receptor (CAR) was initially characterized as a xenobiotic receptor regulating the responses of mammals to xenotoxicants. In this study, we have uncovered an unexpected role of CAR in preventing obesity and alleviating type 2 diabetes. Using a high fat diet (HFD)-induced obesity model, we showed that treatment of wild type mice with the CAR agonist 1,4-bis[2-(3,5 dichloropyridyloxy)] benzene (TCPOBOP) efficiently prevented obesity from happening or reversed preinduced obesity. Treatment with TCPOBOP improved insulin sensitivity in both the HFD-induced type 2 diabetic model and the ob/ob mice. In contrast, CAR null mice maintained on a chow diet showed spontaneous insulin insensitivity, which cannot be relieved by TOPOBOP treatment. The hepatic steatosis in HFD-treated mice and ob/ob mice was markedly reduced by the TCPOBOP treatment. The metabolic benefits of CAR activation may have resulted from the combined effect of inhibition of lipogenesis, very low density lipoprotein secretion and export of triglycerides, and gluconeogenesis as well as increases in brown adipose tissue energy expenditure and peripheral fat mobilization. Moreover, the skeletal muscle of CAR-activated mice showed a decreased incomplete oxidation, despite having a lower expression level of peroxisome proliferator-activated receptor alpha and its target genes involved in fatty acid oxidation. In summary, our results have revealed an important metabolic function of CAR and may establish this "xenobiotic receptor" as a novel therapeutic target for the prevention and treatment of obesity and type 2 diabetes.

PPARalpha blocks glucocorticoid receptor alpha-mediated transactivation but cooperates with the activated glucocorticoid receptor alpha for transrepression on NF-kappaB

Glucocorticoid receptor alpha (GRalpha) and peroxisome proliferator-activated receptor alpha (PPARalpha) are transcription factors with clinically important immune-modulating properties. Either receptor can inhibit cytokine gene expression, mainly through interference with nuclear factor kappaB (NF-kappaB)-driven gene expression. The present work aimed to investigate a functional cross-talk between PPARalpha- and GRalpha-mediated signaling pathways. Simultaneous activation of PPARalpha and GRalpha dose-dependently enhances transrepression of NF-kappaB-driven gene expression and additively represses cytokine production. In sharp contrast and quite unexpectedly, PPARalpha agonists inhibit the expression of classical glucocorticoid response element (GRE)-driven genes in a PPARalpha-dependent manner, as demonstrated by experiments using PPARalpha wild-type and knockout mice. The underlying mechanism for this transcriptional antagonism relies on a PPARalpha-mediated interference with the recruitment of GRalpha, and concomitantly of RNA polymerase II, to GRE-driven gene promoters. Finally, the biological relevance of this phenomenon is underscored by the observation that treatment with the PPARalpha agonist fenofibrate prevents glucocorticoid-induced hyperinsulinemia of mice fed a high-fat diet. Taken together, PPARalpha negatively interferes with GRE-mediated GRalpha activity while potentiating its antiinflammatory effects, thus providing a rationale for combination therapy in chronic inflammatory disorders.

Novel insights into glucocorticoid-mediated diabetogenic effects: towards expansion of therapeutic options?

At pharmacological concentrations, glucocorticoids (GCs) display potent anti-inflammatory effects, and are therefore frequently prescribed by physicians to treat a wide variety of diseases. Despite excellent efficacy, GC therapy is hampered by their notorious metabolic side effect profile. Chronic exposure to increased levels of circulating GCs is associated with central adiposity, dyslipidaemia, skeletal muscle wasting, insulin resistance, glucose intolerance and overt diabetes. Remarkably, many of these side-effects of GC treatment resemble the various components of the metabolic syndrome (MetS), in which indeed subtle disturbances in the hypothalamic-pituitary-adrenal (HPA) axis and/or increased tissue sensitivity to GCs have been reported. Recent developments have led to renewed interest in the mechanisms of GC's diabetogenic effects. First, 'selective dissociating glucocorticoid receptor (GR) ligands', which aim to segregate GC's anti-inflammatory and metabolic actions, are currently being developed. Second, at present, selective 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) inhibitors, which may reduce local GC concentrations by inhibiting cortisone to cortisol conversion, are evaluated in clinical trials as a novel treatment modality for the MetS. In this review, we provide an update of the current knowledge on the mechanisms that underlie GC-induced dysmetabolic effects. In particular, recent progress in research into the role of GCs in the pathogenesis of insulin resistance and beta-cell dysfunction will be discussed.

Role of nuclear receptors in the modulation of insulin secretion in lipid-induced insulin resistance

In healthy individuals, a hyperbolic relationship exists between whole-body insulin-sensitivity and insulin secretion. Thus, for any difference in insulin-sensitivity, a reciprocal proportionate change occurs in insulin secretion. Such a feedback loop is evident in healthy individuals ingesting diets high in saturated fat and in late pregnancy where, despite lipid-induced insulin resistance, glucose tolerance is maintained through augmented GSIS (glucose-stimulated insulin secretion). NRs (nuclear receptors) are members of a superfamily of ligand-regulated and orphan transcription factors. On activation by a cognate ligand, many ligand-activated NRs recruit the RXR (retinoid X receptor) for heterodimer formation. Such NRs include the PPARs (peroxisome-proliferator-activated receptors), which are involved in lipid sensing and liporegulation. PPARs exert important lipid-lowering effects in vivo, thereby opposing the development of lipid-induced insulin resistance by relieving the inhibition of insulin-stimulated glucose disposal by muscle and lowering the necessity for augmented GSIS to counter lipid-induced insulin resistance. Long-chain fatty acids are proposed as natural PPAR ligands and some specific endogenous pathways of lipid metabolism are believed to generate PPAR agonists. Other NRs, e.g. the LXR (liver X receptor), which senses expansion of the metabolically active pool of cholesterol, and the FXR (farnesoid X receptor; NR1H4), which, like the LXR, is involved in sterol metabolism, also modulate systemic lipid levels and insulin-sensitivity. In this review, we discuss how these NRs impact insulin secretion via effects on the insulin-sensitivity-insulin secretion feedback loop and, in some cases, via direct effects on the islet itself. In addition, we discuss interactions between these nutrient/metabolite-responsive NRs and NRs that are central to the action of metabolically important hormones, including (i) the glucocorticoid receptor, critical for maintaining glucose homoeostasis in stress, inflammation and during fasting, and (ii) the thyroid hormone receptors, vital for maintenance of oxidative functions. We present data indicating that the RXR occupies a key role in directly modulating islet function and that its heterodimerization with at least two of its partners modulates GSIS.

Glucocorticoids antagonize estrogens by glucocorticoid receptor-mediated activation of estrogen sulfotransferase

Glucocorticoids and estrogens are two classes of steroid hormones that have essential but distinct physiologic functions. Estrogens also represent a risk factor for breast cancer. It has been suggested that glucocorticoids can attenuate estrogen responses, but the mechanism by which glucocorticoids inhibit estrogenic activity is unknown. In this study, we show that activation of glucocorticoid receptor (GR) by dexamethasone (DEX) induced the expression and activity of estrogen sulfotransferase (SULT1E1 or EST), an enzyme important for the metabolic deactivation of estrogens, because sulfonated estrogens fail to activate the estrogen receptor. Treatment with DEX lowered circulating estrogens, compromised uterine estrogen responses, and inhibited estrogen-dependent breast cancer growth in vitro and in a xenograft model. We further showed that the mouse and human SULT1E1 genes are transcriptional targets of GR and deletion of Sult1e1/Est in mice abolished the DEX effect on estrogen responses. These findings have revealed a novel nuclear receptor-mediated and metabolism-based mechanism of estrogen deprivation, which may have implications in therapeutic development for breast cancers. Because glucocorticoids and estrogens are widely prescribed drugs, our results also urge caution in avoiding glucocorticoid-estrogen interactions in patients.

Acute dexamethasone-induced increase in cardiac lipoprotein lipase requires activation of both Akt and stress kinases

Following dexamethasone (DEX), cardiac energy generation is mainly through utilization of fatty acids (FA), with DEX animals demonstrating an increase in coronary lipoprotein lipase (LPL), an enzyme that hydrolyzes lipoproteins to FA. We examined the mechanisms by which DEX augments cardiac LPL. DEX was injected in rats, and hearts were removed, or isolated cardiomyocytes were incubated with DEX (0-8 h), for measurement of LPL activity and Western blotting. Acute DEX induced whole body insulin resistance, likely an outcome of a decrease in insulin signaling in skeletal muscle, but not cardiac tissue. The increase in luminal LPL activity after DEX was preceded by rapid nongenomic alterations, which included phosphorylation of AMPK and p38 MAPK, that led to phosphorylation of heat shock protein (HSP)25 and actin cytoskeleton rearrangement, facilitating LPL translocation to the myocyte cell surface. Unlike its effects in vivo, although DEX activated AMPK and p38 MAPK in cardiomyocytes, there was no phosphorylation of HSP25, nor was there any evidence of F-actin polymerization or an augmentation of LPL activity up to 8 h after DEX. Combining DEX with insulin appreciably enhanced cardiomyocyte LPL activity, which closely mirrored a robust elevation in phosphorylation of HSP25 and F-actin polymerization. Silencing of p38 MAPK, inhibition of PI 3-kinase, or preincubation with cytochalasin D prevented the increases in LPL activity. Our data suggest that, following DEX, it is a novel, rapid, nongenomic phosphorylation of stress kinases that, together with insulin, facilitates LPL translocation to the myocyte cell surface.

DiRE: identifying distant regulatory elements of co-expressed genes

Regulation of gene expression in eukaryotic genomes is established through a complex cooperative activity of proximal promoters and distant regulatory elements (REs) such as enhancers, repressors and silencers. We have developed a web server named DiRE, based on the Enhancer Identification (EI) method, for predicting distant regulatory elements in higher eukaryotic genomes, namely for determining their chromosomal location and functional characteristics. The server uses gene co-expression data, comparative genomics and profiles of transcription factor binding sites (TFBSs) to determine TFBS-association signatures that can be used for discriminating specific regulatory functions. DiRE's unique feature is its ability to detect REs outside of proximal promoter regions, as it takes advantage of the full gene locus to conduct the search. DiRE can predict common REs for any set of input genes for which the user has prior knowledge of co-expression, co-function or other biologically meaningful grouping. The server predicts function-specific REs consisting of clusters of specifically-associated TFBSs and it also scores the association of individual transcription factors (TFs) with the biological function shared by the group of input genes. Its integration with the Array2BIO server allows users to start their analysis with raw microarray expression data. The DiRE web server is freely available at http://dire.dcode.org.

The tangle of nuclear receptors that controls xenobiotic metabolism and transport: crosstalk and consequences

The expression of many genes involved in xenobiotic/drug metabolism and transport is regulated by at least three nuclear receptors or xenosensors: aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR). These receptors establish crosstalk with other nuclear receptors or transcription factors controlling signaling pathways that regulate the homeostasis of bile acids, lipids, glucose, inflammation, vitamins, hormones, and others. These crosstalks are expected to modify profoundly our vision of xenobiotic/drug disposition and toxicity. They provide molecular mechanisms to explain how physiopathological stimuli affect xenobiotic/drug disposition, and how xenobiotics/drugs may affect physiological functions and generate toxic responses. In addition, the possibility that xenosensors may control other signaling pathways opens the way to new pharmacological opportunities.

Absence of peroxisome proliferator-activated receptor-alpha abolishes hypertension and attenuates atherosclerosis in the Tsukuba hypertensive mouse

Peroxisome proliferator-activated receptor-alpha is widely distributed in the vasculature where it is believed to exert pleiotropic antiatherogenic effects. Its role in the regulation of blood pressure is still unresolved; however, some evidence suggests that it may affect the renin-angiotensin system. We investigated its role in angiotensin II-induced hypertension in the Tsukuba hypertensive mouse (THM). This is a model of hypertension and atherosclerosis because of high angiotensin II and aldosterone levels as a result of the transgenic expression of the entire human renin-angiotensin system. Making the THM animals deficient in Peroxisome proliferator-activated receptor-alpha (THM/PPARKO) totally abolished hypertension and myocardial hypertrophy. This was accompanied by a reduction in plasma human active renin in THM/PPARKO mice compared with THM animals from 3525+/-128 mU/L to 1910+/-750 mU/L (P<0.05) and by a normalization of serum aldosterone (1.6+/-0.29 nmol/L versus 3.4+/-0.69 nmol/L; P=0.003). In the THM/PPARKO mice, the extent of atherosclerosis at the aortic sinus after a 12-week period on an atherogenic diet was decreased by >80%. In addition, the spontaneous formation of foam cells from peritoneal macrophages, a blood pressure-independent event, was reduced by 92% in the THM/PPARKO mice, suggesting protection from the usual oxidative stress in these animals, possibly because of lower prevailing angiotensin II levels. Finally, chronic fenofibrate treatment further elevated blood pressure in THM animals but not in THM/PPARKO animals. Taken together, these data indicate that peroxisome proliferator-activated receptor-alpha may regulate the renin-angiotensin system. They raise the possibility that its activation may aggravate hypertension and hasten atherosclerosis in the context of an activated renin-angiotensin system.

Glucocorticoids, metabolism and metabolic diseases

Since the discovery of the beneficial effects of adrenocortical extracts for treating adrenal insufficiency more than 80 years ago, glucocorticoids (GC) and their cognate, intracellular receptor, the glucocorticoid receptor (GR) have been characterized as critical components of the delicate hormonal control system that determines energy homeostasis in mammals. Whereas physiological levels of GCs are required for proper metabolic control, excessive GC action has been tied to a variety of pandemic metabolic diseases, such as type II diabetes and obesity. Highlighted by its importance for human health, the investigation of molecular mechanisms of GC/GR action has become a major focus in biomedical research. In particular, the understanding of tissue-specific functions of the GC-GR pathway has been proven to be of substantial value for the identification of novel therapeutic options in the treatment of severe metabolic disorders. Therefore, this review focuses on the role of the GC-GR axis for metabolic homeostasis and dysregulation, emphasizing tissue-specific functions of GCs in the control of energy metabolism.

Macrophage expression of peroxisome proliferator-activated receptor-alpha reduces atherosclerosis in low-density lipoprotein receptor-deficient mice

BACKGROUND

The peroxisome proliferator-activated receptor-alpha (PPARalpha) plays important roles in lipid metabolism, inflammation, and atherosclerosis. PPARalpha ligands have been shown to reduce cardiovascular events in high-risk subjects. PPARalpha expression by arterial cells, including macrophages, may exert local antiatherogenic effects independent of plasma lipid changes.

METHODS AND RESULTS

To examine the contribution of PPARalpha expression by bone marrow-derived cells in atherosclerosis, male and female low-density lipoprotein receptor-deficient (LDLR(-/-)) mice were reconstituted with bone marrow from PPARalpha(-/-) or PPARalpha(+/+) mice and challenged with a high-fat diet. Although serum lipids and lipoprotein profiles did not differ between the groups, the size of atherosclerotic lesions in the distal aorta of male and female PPARalpha(-/-) --> LDLR(-/-) mice was significantly increased (44% and 46%, respectively) compared with controls. Male PPARalpha(-/-) --> LDLR(-/-) mice also had larger (44%) atherosclerotic lesions in the proximal aorta than male PPARalpha(+/+) --> LDLR(-/-) mice. Peritoneal macrophages from PPARalpha(-/-) mice had increased uptake of oxidized LDL and decreased cholesterol efflux. PPARalpha(-/-) macrophages had lower levels of scavenger receptor B type I and ABCA1 protein expression and an accelerated response of nuclear factor-kappaB-regulated inflammatory genes. A laser capture microdissection analysis verified suppressed scavenger receptor B type I and increased nuclear factor-kappaB gene expression levels in vivo in atherosclerotic lesions of PPARalpha(-/-) --> LDLR(-/-) mice compared with the lesions of control PPARalpha(+/+) --> LDLR(-/-) mice.

CONCLUSIONS

These data demonstrate that PPARalpha expression by macrophages has antiatherogenic effects via modulation of cell cholesterol trafficking and inflammatory activity.

PPARs and their metabolic modulation: new mechanisms for transcriptional regulation?

Peroxisome proliferator-activated receptors (PPARs) as ligand-activated nuclear receptors involved in the transcriptional regulation of lipid metabolism, energy balance, inflammation, and atherosclerosis are at the intersection of key pathways involved in the pathogenesis of diabetes and cardiovascular disease. Synthetic PPAR agonists like fibrates (PPAR-alpha) and thiazolidinediones (PPAR-gamma) are in therapeutic use to treat dyslipidaemia and diabetes. Despite strong encouraging in vitro, animal model, and human surrogate marker studies with these agents, recent prospective clinical cardiovascular trials have yielded mixed results, perhaps explained by concomitant drug use, study design, or a lack of efficacy of these agents on cardiovascular disease (independent of their current metabolic indications). The use of PPAR agents has also been limited by untoward effects. An alternative strategy to PPAR therapeutics is better understanding PPAR biology, the nature of natural PPAR agonists, and how these molecules are generated. Such insight might also provide valuable information about pathways that protect against the metabolic problems for which PPAR agents are currently indicated. This approach underscores the important distinction between the effects of synthetic PPAR agonists and the unequivocal biologic role of PPARs as key transcriptional regulators of metabolic and inflammatory pathways relevant to diabetes and atherosclerosis.

Adiposity and cardiovascular disorders: disturbance of the regulatory system consisting of humoral and neuronal signals

Obesity, a major healthcare issue, is associated with significant cardiovascular morbidities, including hypertension and atherosclerosis. Numerous intensive studies conducted this decade have revealed that adipose tissue is a major endocrine organ that secretes a variety of bioactive substances, termed adipocytokines. Adipocytokine secretion profiles are altered as obesity develops, which may increase the risk of obesity-related cardiovascular disorders. For instance, leptin is upregulated in obese subjects and plays important roles in the pathophysiology of obesity-related atherogenesis through multiple mechanisms, such as its proliferative, proinflammatory, prothrombotic, and prooxidant actions. In contrast, adiponectin, which is downregulated in obese subjects, has protective effects against cardiovascular disorders at various atherogenic stages. In addition to these factors secreted by adipose tissue, neuronal circuits involving autonomic nerves are now being recognized as an important metabolic regulatory system and have thus attracted considerable attentions. Alterations in fat accumulation in intraabdominal organs, such as visceral adipose tissue and the liver, send afferent neuronal signals to the brain, leading to modulation of sympathetic tonus and thereby affecting the vasculature. Moreover, these humoral and neuronal signaling pathways communicate with each other, resulting in cooperative metabolic regulation among tissues/organs throughout the body. Further elucidation of these regulatory systems is anticipated to lead to new approaches to devising therapeutic strategies for the metabolic syndrome.

Peroxisome proliferator-activated receptors as molecular targets in relation to obesity and Type 2 diabetes

The three isotypes of peroxisome proliferator-activated receptors (PPARs) are currently perceived as major regulatory nodes (or hubs) of metabolic pathway networks, linking most prevalent diseases including Type 2 diabetes, obesity, dyslipidemia and atherosclerosis. The integrative functions of PPARs are also reflected in their ecogenetic profile, when the variants underlying pharmacogenetic interactions were also shown to modulate the effect of lifestyle factors. Despite their extensive clinical use, there are many outstanding issues, especially concerning their safety. Critical pharmacogenomic assessment is warranted for the new potent ligands of multiple PPAR isoforms as many have displayed serious side-effects in a limited number of treated subjects. Nevertheless, the advent of genomic, transcriptomic and system biology-level approaches, integrating knowledge from model systems and human biology, should greatly facilitate the transition to individualized PPAR-based therapies.

Thyroid hormone induces myocardial matrix degradation by activating matrix metalloproteinase-1

Hyperthyroid patients develop left ventricular hypertrophy associated with alterations of several cardiac parameters such as heart rate, cardiac output, cardiac contraction and hemodynamic overload leading to cardiac complications. Although cardiac hypertrophy and contractile abnormality occur, interstitial fibrosis in the heart usually does not take place in hyperthyroid condition. Therefore, in the present study, the mechanism regulating myocardial extracellular matrix (ECM) remodeling in hyperthyroid condition was investigated. Cardiac hypertrophy was developed in Sprague-Dawley rats by administration of 3,5,3'-triiodo-L-thyronine (triiodothyronine, 8 microg/100g BW, ip, SID) and glucocorticosteroid, dexamethasone (DEX, 35 microg/100g BW, po, SID), which is also an inducer of hypertrophy for 15 days. Heart/Body weight ratio and atrial and brain natriuretic peptide mRNAs were significantly increased in both triiodothyronine- and DEX-treated rats compared to control. Collagens-I and -III deposition in the left ventricular sections was reduced in triiodothyronine-treated rats, whereas in DEX-treated animals those were increased compared to control. While mRNA and protein levels of procollagens-I and -III were increased with triiodothyronine (p<0.01), the levels of mature collagens-I and -III were decreased. The levels of the mature collagens were increased with DEX compared to control. MMP-1 activity in the serum and left ventricle was higher with reduced levels of TIMPs-3 and -4 in the left ventricle of triiodothyronine-treated rats. The results suggest that accelerated breakdown of collagens-I and -III by MMP-1 due to suppression of the endogenous TIMPs plays an important role in regulating the ECM in myocardium of hyperthyroid rat.

Fasting glucose in acute myocardial infarction: incremental value for long-term mortality and relationship with left ventricular systolic function

OBJECTIVE

Elevation of blood glucose is a common metabolic disorder among patients with acute myocardial infarction (AMI) and is associated with adverse prognosis. However, few data are available concerning the long-term prognostic value of elevated fasting glucose during the acute phase of infarction.

RESEARCH DESIGN AND METHODS

We prospectively studied the relationship between fasting glucose and long-term mortality in patients with AMI. Fasting glucose was determined after an >/=8 h fast within 24 h of admission. The median duration of follow-up was 24 months (range 6-48). All multivariable Cox models were adjusted for the Global Registry of Acute Coronary Events (GRACE) risk score.

RESULTS

In nondiabetic patients (n = 1,101), compared with patients with normal fasting glucose (<100 mg/dl), the adjusted hazard ratio for mortality progressively increased with higher tertiles of elevated fasting glucose (first tertile 1.5 [95% CI 0.8-2.9], P = 0.19; second tertile 3.2 [1.9-5.5], P < 0.0001; third tertile 5.7 [3.5-9.3], P < 0.0001). The c statistic of the model containing the GRACE risk score increased when fasting glucose data were added (0.8 +/- 0.02-0.85 +/- 0.02, P = 0.004). Fasting glucose remained an independent predictor of mortality after further adjustment for ejection fraction. Elevated fasting glucose did not predict mortality in patients with diabetes (n = 462).

CONCLUSIONS

Fasting glucose is a simple robust tool for predicting long-term mortality in nondiabetic patients with AMI. Fasting glucose provides incremental prognostic information when added to the GRACE risk score and left ventricular ejection fraction. Fasting glucose is not a useful prognostic marker in patients with diabetes.

Glucocorticoids produce whole body insulin resistance with changes in cardiac metabolism

Insulin resistance is viewed as an insufficiency in insulin action, with glucocorticoids being recognized to play a key role in its pathogenesis. With insulin resistance, metabolism in multiple organ systems such as skeletal muscle, liver, and adipose tissue is altered. These metabolic alterations are widely believed to be important factors in the morbidity and mortality of cardiovascular disease. More importantly, clinical and experimental studies have established that metabolic abnormalities in the heart per se also play a crucial role in the development of heart failure. Following glucocorticoids, glucose utilization is compromised in the heart. This attenuated glucose metabolism is associated with altered fatty acid supply, composition, and utilization. In the heart, elevated fatty acid use has been implicated in a number of metabolic, morphological, and mechanical changes and, more recently, in "lipotoxicity". In the present article, we review the action of glucocorticoids, their role in insulin resistance, and their influence in modulating peripheral and cardiac metabolism and heart disease.

Peroxisome proliferator-activated receptors as transcriptional nodal points and therapeutic targets

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors involved in the transcriptional regulation of key metabolic pathways such as lipid metabolism, adipogenesis, and insulin sensitivity. More recent work implicates all 3 PPAR isotypes (alpha, gamma, and delta, also known as beta or beta/delta) in inflammatory and atherosclerotic pathways. Because these nuclear receptors are activated by extracellular signals and control multiple gene targets, PPARs can be seen as nodes that control multiple inputs and outputs involved in energy balance, providing insight into how metabolism and the vasculature may be integrated. The ongoing clinical use of fibrates, which activate PPARalpha, and thiazolidinediones, which activate PPARgamma, establishes these receptors as viable drug targets, whereas considerable in vitro animal model and human surrogate marker studies suggest that PPAR activation may limit inflammation and atherosclerosis. Together, these various observations have stimulated intense interest in PPARs as therapeutic targets and led to large-scale cardiovascular end-point trials with PPAR agonists. The first of these studies has generated mixed results that require careful review, especially in anticipation of additional clinical trial data and ongoing attempts to develop novel PPAR modulators. Such analysis of the existing PPAR data, the appropriate use of currently approved PPAR agonists, and continued progress in PPAR therapeutics will be predicated on a better understanding of PPAR biology.

Regulation of inflammation by PPARs: a future approach to treat lung inflammatory diseases?

Lung inflammatory diseases, such as acute lung injury (ALI), asthma, chronic obstructive pulmonary disease (COPD) and lung fibrosis, represent a major health problem worldwide. Although glucocorticoids are the most potent anti-inflammatory drug in asthma, they exhibit major side effects and have poor activity in lung inflammatory disorders such as ALI or COPD. Therefore, there is growing need for the development of alternative or new therapies to treat inflammation in the lung. Peroxisome proliferator-activated receptors (PPARs), including the three isotypes PPARalpha, PPARbeta (or PPARdelta) and PPARgamma, are transcription factors belonging to the nuclear hormone receptor superfamily. PPARs, and in particular PPARalpha and PPARgamma, are well known for their critical role in the regulation of energy homeostasis by controlling expression of a variety of genes involved in lipid and carbohydrate metabolism. Synthetic ligands of the two receptor isotypes, the fibrates and the thiazolidinediones, are clinically used to treat dyslipidaemia and type 2 diabetes, respectively. Recently however, PPARalpha and PPARgamma have been shown to exert a potent anti-inflammatory activity, mainly through their ability to downregulate pro-inflammatory gene expression and inflammatory cell functions. The present article reviews the current knowledge of the role of PPARalpha and PPARgamma in controlling inflammation, and presents different findings suggesting that PPARalpha and PPARgamma activators may be helpful in the treatment of lung inflammatory diseases.

An afferent vagal nerve pathway links hepatic PPARalpha activation to glucocorticoid-induced insulin resistance and hypertension

Glucocorticoid excess causes insulin resistance and hypertension. Hepatic expression of PPARalpha (Ppara) is required for glucocorticoid-induced insulin resistance. Here we demonstrate that afferent fibers of the vagus nerve interface with hepatic Ppara expression to disrupt blood pressure and glucose homeostasis in response to glucocorticoids. Selective hepatic vagotomy decreased hyperglycemia, hyperinsulinemia, hepatic insulin resistance, Ppara expression, and phosphoenolpyruvate carboxykinase (PEPCK) enzyme activity in dexamethasone-treated Ppara(+/+) mice. Selective vagotomy also decreased blood pressure, adrenergic tone, renin activity, and urinary sodium retention in these mice. Hepatic reconstitution of Ppara in nondiabetic, normotensive dexamethasone-treated PPARalpha null mice increased glucose, insulin, hepatic PEPCK enzyme activity, blood pressure, and renin activity in sham-operated animals but not hepatic-vagotomized animals. Disruption of vagal afferent fibers by chemical or surgical means prevented glucocorticoid-induced metabolic derangements. We conclude that a dynamic interaction between hepatic Ppara expression and a vagal afferent pathway is essential for glucocorticoid induction of diabetes and hypertension.

Corticotropin-releasing hormone neurons in hypertensive patients are activated in the hypothalamus but not in the brainstem

The corticotrophin-releasing hormone (CRH)-expressing neurons were studied in the hypothalamus and brainstem of individuals who suffered from essential hypertension and had died due to acute myocardial infarction or brain hemorrhage. Healthy normotensive individuals who died in accidents made up the control group. In hypertensive patients we found extremely high expression of CRH in all parts of the hypothalamic paraventricular nucleus (Pa). In addition, CRH neuronal profiles were observed in the caudal hypothalamic area and dorsal parts of the extended amygdala. In the control group, CRH neurons were found only in the Pa and in much smaller numbers than in hypertensive patients. Also, in contrast to the controls, we found in hypertensives a very high number of CRH fibers running from the most rostral part of the Pa to the median eminence and innervating the caudal part of the suprachiasmatic nucleus (SCh). A quantitative evaluation showed that the area covered by CRH fibers in the SCh of hypertensives was about three times larger than that in the control SCh. Linear regression analysis demonstrated a negative correlation between the area of CRH fibers and the number of vasopressin (VP) or neurotensin (NT) neurons within the SCh. This relationship occurred particularly in hypertensive patients in whose SCh a larger CRH fiber area and a smaller number of VP or NT neurons were observed. We found a few CRH neuronal profiles and fibers in brainstem nuclei that are involved in cardiovascular regulation, but no apparent difference was observed between the control and hypertensive group.

Drug-Induced Endocrine and Metabolic Disorders

Complex interactions exist amongst the various components of the neuroendocrine system in order to maintain homeostasis, energy balance and reproductive function. These components include the hypothalamus-pituitary- adrenal and -gonadal axes, the renin-angiotensin-aldosterone system, the sympathetic nervous system and the pancreatic islets. These hormones, peptides and neurotransmitters act in concert to regulate the functions of many organs, notably the liver, muscles, kidneys, thyroid, bone, adrenal glands, adipocytes, vasculature, intestinal tract and gonads, through many intermediary pathways. Endocrine and metabolic disorders can arise from imbalance amongst numerous hormonal factors. These disturbances may be due to endogenous processes, such as increased secretion of hormones from a tumour, as well as exogenous drug administration. Drugs can cause endocrine abnormalities via different mechanisms, including direct alteration of hormone production, changes in the regulation of the hormonal axis, effects on hormonal transport, binding, and signalling, as well as similar changes to counter-regulatory hormone systems. Furthermore, drugs can affect the evaluation of endocrine parameters by causing interference with diagnostic tests. Common drug-induced endocrine and metabolic disorders include disorders of carbohydrate metabolism, electrolyte and calcium abnormalities, as well as drug-induced thyroid and gonadal disorders. An understanding of the proposed mechanisms of these drug effects and their evaluation and differential diagnosis may allow for more critical interpretation of the clinical observations associated with such disorders, better prediction of drug-induced adverse effects and better choices of and rationales for treatment.

Peroxisome proliferator-activated receptor-alpha and glucocorticoids interactively regulate insulin secretion during pregnancy

We evaluated the impact of peroxisome proliferator-activated receptor (PPAR)alpha activation and dexamethasone treatment on islet adaptations to the distinct metabolic challenges of fasting and pregnancy, situations where lipid handling is modified to conserve glucose. PPARalpha activation (24 h) in vivo did not affect glucose-stimulated insulin secretion (GSIS) in nonpregnant female rats in the fasted state, although fasting suppressed GSIS. Dexamethasone treatment (5 days) of nonpregnant rats lowered the glucose threshold and augmented GSIS at high glucose; the former effect was selectively opposed by PPARalpha activation. Pregnancy-induced changes in GSIS were opposed by PPARalpha activation at day 19 of pregnancy. Dexamethasone treatment from day 14 to 19 of pregnancy did not modify the GSIS profile of perifused islets from 19-day pregnant rats but rendered the islet GSIS profile refractory to PPARalpha activation. During sustained hyperglycemia in vivo, dexamethasone treatment augmented GSIS in nonpregnant rats but limited further modification of GSIS by pregnancy. We propose that the effect of PPARalpha activation to oppose lowering of the glucose threshold for GSIS by glucocorticoids is important as part of the fasting adaptation, and modulation of the islet GSIS profile by glucocorticoids toward term facilitates the transition of maternal islet function from the metabolic demands of pregnancy to those imposed after parturition.

PPARalpha activation elevates blood pressure and does not correct glucocorticoid-induced insulin resistance in humans

Fibrates, activators of the nuclear receptor PPARalpha, improve dyslipidemia, but their effects on insulin resistance and vascular disease are unresolved. To test the hypothesis that PPARalpha activation improves insulin resistance and vascular function, we determined the effects of fenofibrate in healthy adults with insulin resistance induced by short-term glucocorticoid administration. Eighteen normal-weight subjects were studied in four stages: at baseline, after 21 days of fenofibrate (160 mg/day) alone, after 3 days of dexamethasone (8 mg/day) added to fenofibrate, and after 3 days of dexamethasone added to placebo (dexamethasone alone). Dexamethasone alone caused hyperinsulinemia, increased glucose, decreased glucose disposal, and reduced insulin-induced suppression of hepatic glucose production as determined by hyperinsulinemic euglycemic clamp and increased systolic blood pressure as determined by ambulatory monitoring, features associated with an insulin-resistant state. Fenofibrate improved fasting LDL and total cholesterol in the setting of dexamethasone treatment but had no significant effect on levels of insulin or glucose, insulin-stimulated glucose disposal, or insulin suppression of glucose production during clamps, or ambulatory monitored blood pressure. In the absence of dexamethasone, fenofibrate lowered fasting triglycerides and cholesterol but unexpectedly increased systolic blood pressure by ambulatory monitoring. These data suggest that PPARalpha activation in humans does not correct insulin resistance induced by glucocorticoids and may adversely affect blood pressure.

ATM-dependent suppression of stress signaling reduces vascular disease in metabolic syndrome

Metabolic syndrome is associated with insulin resistance and atherosclerosis. Here, we show that deficiency of one or two alleles of ATM, the protein mutated in the cancer-prone disease ataxia telangiectasia, worsens features of the metabolic syndrome, increases insulin resistance, and accelerates atherosclerosis in apoE-/- mice. Transplantation with ATM-/- as compared to ATM+/+ bone marrow increased vascular disease. Jun N-terminal kinase (JNK) activity was increased in ATM-deficient cells. Treatment of ATM+/+apoE-/- mice with low-dose chloroquine, an ATM activator, decreased atherosclerosis. In an ATM-dependent manner, chloroquine decreased macrophage JNK activity, decreased macrophage lipoprotein lipase activity (a proatherogenic consequence of JNK activation), decreased blood pressure, and improved glucose tolerance. Chloroquine also improved metabolic abnormalities in ob/ob and db/db mice. These results suggest that ATM-dependent stress pathways mediate susceptibility to the metabolic syndrome and that chloroquine or related agents promoting ATM activity could modulate insulin resistance and decrease vascular disease.

Genotype-by-sex interaction in the aetiology of type 2 diabetes mellitus: support for sex-specific quantitative trait loci in Hypertension Genetic Epidemiology Network participants

AIMS/HYPOTHESIS

While there are sex-related differences in both the prevalence of type 2 diabetes mellitus and disease risk factors, there is only limited research on sex-specific influences on type 2 diabetes aetiology within the same study population. Thus, we assessed genotype-by-sex interaction using a liability threshold model in an attempt to localise sex-specific type 2 diabetes quantitative trait loci (QTLs).

SUBJECTS, MATERIALS AND METHODS

Hypertensive siblings and their offspring and/or parents in the Hypertension Genetic Epidemiology Network of the Family Blood Pressure Program were recruited from five field centres. The diabetic phenotype was adjusted for race, study centre, age and non-linear age effects. In total, 567 diabetic individuals were identified in 385 families. Variance component linkage analyses in the combined sample and stratified by sex and race were performed (SOLAR program) using race-specific marker allele frequencies derived from a random sample of participants at each centre.

RESULTS

We observed a QTL-specific genotype-by-sex interaction (p=0.009) on chromosome 17 at 31 cM, with females displaying a robust adjusted logarithm of odds (LOD) of 3.0 compared with 0.2 in males and 1.3 in the combined sample. Three additional regions demonstrating suggestive evidence for linkage were detected: chromosomes 2 and 5 in the female sample and chromosome 22 (adjusted LOD=1.9) in the combined sample.

CONCLUSIONS/INTERPRETATION

These findings suggest that multiple genes may regulate susceptibility to type 2 diabetes, demonstrating the importance of considering the interaction of genes and environment in the aetiology of common complex traits.

Haploinsufficiency in the PPARalpha and LDL receptor genes leads to gender- and age-specific obesity and hyperinsulinemia

When preparing peroxisome proliferator-activated receptor (PPAR)alpha:low-density lipoprotein receptor (LDLR) (-/-) double knockout mice, we unexpectedly found a unique gender- and age-specific obesity in the F1 generation, PPARalpha (+/-):LDLR (+/-), even in mice fed standard chow. Body weights of the male heterozygous mice increased up to about 60 g at 75 weeks of age, then decreased by about 30 g at 100 weeks of age. More than 95% of the heterozygous mice between 35- and 75-week-olds were overweight. Of interest, the obese heterozygous mice also exhibited hyperinsulinemia correlating with moderate insulin resistance. Hepatic gene expression of LDLR was lower than expected in the heterozygous mice, particularly at 50 and 75 weeks of age. In contrast, the hepatic expression of PPARalpha was higher than expected in obese heterozygous mice, but decreased in non-obese older heterozygous mice. Modulated expression of these genes may be partially associated with the onset of the hyperinsulinemia.

Lipin 1 is an inducible amplifier of the hepatic PGC-1alpha/PPARalpha regulatory pathway

Perturbations in hepatic lipid homeostasis are linked to the development of obesity-related steatohepatitis. Mutations in the gene encoding lipin 1 cause hepatic steatosis in fld mice, a genetic model of lipodystrophy. However, the molecular function of lipin 1 is unclear. Herein, we demonstrate that the expression of lipin 1 is induced by peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha (PGC-1alpha), a transcriptional coactivator controlling several key hepatic metabolic pathways. Gain-of-function and loss-of-function strategies demonstrated that lipin selectively activates a subset of PGC-1alpha target pathways, including fatty acid oxidation and mitochondrial oxidative phosphorylation, while suppressing the lipogenic program and lowering circulating lipid levels. Lipin activates mitochondrial fatty acid oxidative metabolism by inducing expression of the nuclear receptor PPARalpha, a known PGC-1alpha target, and via direct physical interactions with PPARalpha and PGC-1alpha. These results identify lipin 1 as a selective physiological amplifier of the PGC-1alpha/PPARalpha-mediated control of hepatic lipid metabolism.

Signalling mechanisms linking hepatic glucose and lipid metabolism

Fatty liver and hepatic triglyceride accumulation are strongly associated with obesity, insulin resistance and type 2 diabetes, and are subject to nutritional influences. Hepatic regulation of glucose and lipid homeostasis is influenced by a complex system of hormones, hormonally regulated signalling pathways and transcription factors. Recently, considerable progress has been made in elucidating molecular pathways and potential factors that are affected in insulin-resistant states. In this review we discuss some of the key factors that are involved in both the regulation of glucose and lipid metabolism in the liver. Understanding the molecular network that links hepatic lipid accumulation and impaired glucose metabolism may provide targets for dietary or pharmacological interventions.

Insulin resistance and atherosclerosis

Considerable evidence supports the association between insulin resistance and vascular disease, and this has led to wide acceptance of the clustering of hyperlipidemia, glucose intolerance, hypertension, and obesity as a clinical entity, the metabolic syndrome. While insulin resistance, by promoting dyslipidemia and other metabolic abnormalities, is part of the proatherogenic milieu, it is possible that insulin resistance itself in the vascular wall does not promote atherosclerosis. Recent findings suggest that insulin resistance and atherosclerosis could represent independent and ultimately maladaptive responses to the disruption of cellular homeostasis caused by the excess delivery of fuel.

Mechanisms underlying regulation of the expression and activities of the mammalian pyruvate dehydrogenase kinases

The mechanisms that control mammalian pyruvate dehydrogenase complex (PDC) activity include its phosphorylation (inactivation) by a family of pyruvate dehydrogenase kinases (PDKs 1 - 4). Here we review new developments in the regulation of the activities and expression of the PDKs, in particular PDK2 and PDK4, in relation to glucose and lipid homeostasis. This review describes recent advances relating to the acute and long-term modes of regulation of the PDKs, with particular emphasis on the regulatory roles of nuclear receptors including peroxisome proliferator-activated receptor (PPAR) alpha and Liver X receptor (LXR), PPAR gamma coactivator alpha (PGC-1alpha) and insulin, and the impact of changes in PDK activity and expression in glucose and lipid homeostasis. Since PDK4 may assist in lipid clearance when there is an imbalance between lipid delivery and oxidation, it may represent an attractive target for interventions aimed at rectifying abnormal lipid as well as glucose homeostasis in disease states.

Diminished hepatic gluconeogenesis via defects in tricarboxylic acid cycle flux in peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha)-deficient mice

The peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha (PGC-1alpha) is a highly inducible transcriptional coactivator implicated in the coordinate regulation of genes encoding enzymes involved in hepatic fatty acid oxidation, oxidative phosphorylation, and gluconeogenesis. The present study sought to assess the effects of chronic PGC-1alpha deficiency on metabolic flux through the hepatic gluconeogenic, fatty acid oxidation, and tricarboxylic acid cycle pathways. To this end, hepatic metabolism was assessed in wild-type (WT) and PGC-1alpha(-/-) mice using isotopomer-based NMR with complementary gene expression analyses. Hepatic glucose production was diminished in PGC-1alpha(-/-) livers coincident with reduced gluconeogenic flux from phosphoenolpyruvate. Surprisingly, the expression of PGC-1alpha target genes involved in gluconeogenesis was unaltered in PGC-1alpha(-/-) compared with WT mice under fed and fasted conditions. Flux through tricarboxylic acid cycle and mitochondrial fatty acid beta-oxidation pathways was also diminished in PGC-1alpha(-/-) livers. The expression of multiple genes encoding tricarboxylic acid cycle and oxidative phosphorylation enzymes was significantly depressed in PGC-1alpha(-/-) mice and was activated by PGC-1alpha overexpression in the livers of WT mice. Collectively, these findings suggest that chronic whole-animal PGC-1alpha deficiency results in defects in hepatic glucose production that are secondary to diminished fatty acid beta-oxidation and tricarboxylic acid cycle flux rather than abnormalities in gluconeogenic enzyme gene expression per se.

Manipulation of inflammation modulates hyperlipidemia in apolipoprotein E-deficient mice: a possible role for interleukin-6

There are increasing evidences showing that inflammation participates in atherosclerosis. Therefore, the therapeutic use of anti-inflammatory agents should be considered. We have induced chronic, aseptic inflammation upon the injection of turpentine and tested the effect of dexamethasone on lipoprotein metabolism and, consequently, atherosclerosis in apolipoprotein E-deficient mice. Aseptic inflammation caused a significant decrease in hyperlipidemia. Treatment with dexamethasone elicited the opposite effect increasing hyperlipidemia through mechanisms related to the increase in the synthesis of triglyceride-rich lipoproteins. Changes in plasma lipids correlated with those observed in the size of atherosclerotic lesions. Our data suggest the presence of a common mechanism present in both observations and which is probably related to the cytokine secretion. Among the candidates, we chose to test the effect of interleukin-6 because it is involved in both processes, atherosclerosis and inflammation, and its expression is efficiently repressed by corticosteroids. The injection of recombinant interleukin-6 in our mice elicited the same effects observed in our model of inflammation. We conclude that manipulation of inflammation-related mechanisms modulates lipid homeostasis and development of atherosclerotic plaque in rodents.

Sorting out the roles of PPAR alpha in energy metabolism and vascular homeostasis

PPARalpha is a nuclear receptor that regulates liver and skeletal muscle lipid metabolism as well as glucose homeostasis. Acting as a molecular sensor of endogenous fatty acids (FAs) and their derivatives, this ligand-activated transcription factor regulates the expression of genes encoding enzymes and transport proteins controlling lipid homeostasis, thereby stimulating FA oxidation and improving lipoprotein metabolism. PPARalpha also exerts pleiotropic antiinflammatory and antiproliferative effects and prevents the proatherogenic effects of cholesterol accumulation in macrophages by stimulating cholesterol efflux. Cellular and animal models of PPARalpha help explain the clinical actions of fibrates, synthetic PPARalpha agonists used to treat dyslipidemia and reduce cardiovascular disease and its complications in patients with the metabolic syndrome. Although these preclinical studies cannot predict all of the effects of PPARalpha in humans, recent findings have revealed potential adverse effects of PPARalpha action, underlining the need for further study. This Review will focus on the mechanisms of action of PPARalpha in metabolic diseases and their associated vascular pathologies.

PGC-1 coactivators: inducible regulators of energy metabolism in health and disease

Members of the PPARgamma coactivator-1 (PGC-1) family of transcriptional coactivators serve as inducible coregulators of nuclear receptors in the control of cellular energy metabolic pathways. This Review focuses on the biologic and physiologic functions of the PGC-1 coactivators, with particular emphasis on striated muscle, liver, and other organ systems relevant to common diseases such as diabetes and heart failure.

From molecular action to physiological outputs: peroxisome proliferator-activated receptors are nuclear receptors at the crossroads of key cellular functions

Peroxisome proliferator-activated receptors (PPARs) compose a family of three nuclear receptors which act as lipid sensors to modulate gene expression. As such, PPARs are implicated in major metabolic and inflammatory regulations with far-reaching medical consequences, as well as in important processes controlling cellular fate. Throughout this review, we focus on the cellular functions of these receptors. The molecular mechanisms through which PPARs regulate transcription are thoroughly addressed with particular emphasis on the latest results on corepressor and coactivator action. Their implication in cellular metabolism and in the control of the balance between cell proliferation, differentiation and survival is then reviewed. Finally, we discuss how the integration of various intra-cellular signaling pathways allows PPARs to participate to whole-body homeostasis by mediating regulatory crosstalks between organs.

Dexamethasone and cardiac potassium currents in the diabetic rat

Experiments were designed to compare effects of dexamethasone on transient (Ipeak) and sustained (Isus) K+ currents in control and diabetic rat myocytes. Ventricular myocytes were isolated from control or type 1 streptozotocin (STZ)-induced diabetic male and female rats. Currents were measured using whole-cell voltage-clamp methods. Incubation of cells from control males or females with 100 nM dexamethasone (5-9 h) significantly (P<0.005) augmented Isus (by 28-31%). Ipeak was unchanged. Isus augmentation was abolished by cycloheximide or cytochalasin D, but not by inhibition of protein kinases A or C. Inhibition of tyrosine kinases by genistein (but not its inactive analog genistin) prevented the increase of Isus by dexamethasone. In marked contrast, dexamethasone had a significantly (P<0.015) smaller effect on Isus (11% increase) in cells from male STZ-diabetic rats, as compared to control cells. However, Isus augmentation in cells from female STZ-diabetic rats was normal (31% increase). In ovariectomized-diabetic rats, Isus was unchanged by dexamethasone. The reduced effect in diabetic males might be due to preactivation of tyrosine kinases linking dexamethasone to current modulation. In conclusion, type I diabetes is associated with gender-specific changes in sensitivity of K+ currents to glucocorticoids, linked to alterations in tyrosine-phosphorylated proteins.

Nuclear receptors and their coregulators in kidney

Nuclear receptors are transcription factors that are essential in embryonic development, maintenance of differentiated cellular phenotypes, metabolism, and apoptosis. Dysfunction of nuclear receptor signaling leads to a wide spectra of proliferative, reproductive, and metabolic diseases, including cancers, infertility, obesity, and diabetes. In addition, many proteins have been identified as coregulators which can be recruited by DNA-binding nuclear receptors to affect transcriptional regulation. The cellular level of coregulators is crucial for nuclear receptor-mediated transcription and many coregulators have been shown to be targets for diverse intracellular signaling pathways and posttranslational modifications. This review provides a general overview of the roles and mechanism of action of nuclear receptors and their coregulators. Since progression of renal diseases is almost always associated with inflammatory processes and/or involve metabolic disorders of lipid and glucose, cell proliferation, hypertrophy, apoptosis, and hypertension, the importance of nuclear receptors and their coregulators in these contexts will be addressed.

Gene expression profiling of potential peroxisome proliferator-activated receptor (PPAR) target genes in human hepatoblastoma cell lines inducibly expressing different PPAR isoforms

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors and commonly play an important role in the regulation of lipid homeostasis. To identify human PPARs-responsive genes, we established tetracycline-regulated human hepatoblastoma cell lines that can be induced to express each human PPAR and investigated the gene expression profiles of these cells.

RESULTS

The expression of each introduced PPAR gene was investigated using the various concentrations of doxycycline in the culture media. We found that the expression of each PPAR subtype was tightly controlled by the concentration of doxycycline in these established cell lines. DNA microarray analyses using these cell lines were performed with or without adding each subtype ligand and provided much important information on the PPAR target genes involved in lipid metabolism, transport, storage and other activities. Interestingly, it was noted that while ligand-activated PPARdelta induced target gene expression, unliganded PPARdelta repressed these genes. The real-time RT-PCR was used to verify the altered expression of selected genes by PPARs and we found that these genes were induced to express in the same pattern as detected in the microarray analyses. Furthermore, we analysed the 5'-flanking region of the human adipose differentiation-related protein (adrp) gene that responded to all subtypes of PPARs. From the detailed analyses by reporter assays, the EMSAs, and ChIP assays, we determined the functional PPRE of the human adrp gene.

CONCLUSION

The results suggest that these cell lines are important tools used to identify the human PPARs-responsive genes.

Interactive influences of peroxisome proliferator-activated receptor alpha activation and glucocorticoids on pancreatic beta cell compensation in insulin resistance induced by dietary saturated fat in the rat

AIMS/HYPOTHESIS

We sought to elucidate whether excess glucocorticoids and increased dietary lipids act synergistically to impair glucose tolerance and, if so, whether activation of peroxisome proliferator-activated receptor alpha (PPARalpha) has an adverse or beneficial effect on glucose tolerance.

METHODS

Dexamethasone (100 microg kg(-1) body weight day(-1); 5 days) was administered to insulin-resistant rats fed a high-saturated-fat (HF) diet for 4 weeks. The PPARalpha agonist WY14643 was administered (50 mg kg(-1) body weight intraperitoneally) 24 h before sampling. Glucose-stimulated insulin secretion (GSIS) was assessed in vivo after an acute glucose bolus injection, and in vitro using step-up and step-down islet perifusions.

RESULTS

Although neither PPARalpha activation nor dexamethasone alone affected fasting glycaemia in the HF group, dexamethasone in combination with PPARalpha activation elicited marked postabsorptive hyperglycaemia. Dexamethasone treatment of HF rats had little effect on GSIS after an acute glucose challenge in vivo, but induced glucose intolerance. PPARalpha activation augmented GSIS in dexamethasone-treated HF rats in vivo, restoring glucose tolerance. Contrasting with data obtained in vivo, greatly enhanced peak rates of GSIS were observed ex vivo in perifusions of islets from dexamethasone-treated HF rats compared with those from untreated HF rats, an effect attenuated by antecedent PPARalpha activation.

CONCLUSIONS/INTERPRETATION

The study demonstrates that glucocorticoid excess precipitates the development of glucose intolerance in rats maintained on a high-saturated-fat diet. It does this by interrupting the negative feedback loop between insulin sensitivity and secretion in vivo, such that further enhancement of compensatory insulin secretion is not possible. PPARalpha activation restores the coupling between insulin secretion and action.

Glucocorticoids regulate transcription of the gene for phosphoenolpyruvate carboxykinase in the liver via an extended glucocorticoid regulatory unit

The hepatic transcriptional regulation by glucocorticoids of the cytosolic form of phosphoenolpyruvate carboxykinase (PEPCK-C) gene is coordinated by interactions of specific transcription factors at the glucocorticoid regulatory unit (GRU). We propose an extended GRU that consists of four accessory sites, two proximal AF1 and AF2 sites and their distal counterpart dAF1 (-993) and a new site, dAF2 (-1365); together, these four sites form a palindrome. Sequencing and gel shift binding assays of hepatic nuclear proteins interacting with these sites indicated similarity of dAF1 and dAF2 sites to the GRU proximal AF1 and AF2 sites. Chromatin immunoprecipitation assays demonstrated that glucocorticoids enhanced the binding of FOXO1 and peroxisome proliferator-activated receptor-alpha to AF2 and dAF2 sites and not to dAF1 site but enhanced the binding of hepatic nuclear transcription factor-4alpha only to the dAF1 site. Insulin inhibited the binding of these factors to their respective sites but intensified the binding of phosphorylated FOXO1. Transient transfections in HepG2 human hepatoma cells showed that glucocorticoid receptor interacts with several non-steroid nuclear receptors, yielding a synergistic response of the PEPCK-C gene promoter to glucocorticoids. The synergistic stimulation by glucocorticoid receptor together with peroxisome proliferator-activated receptor-alpha or hepatic nuclear transcription factor-4alpha requires all four accessory sites, i.e. a mutation of each of these markedly affects the synergistic response. Mice with a targeted mutation of the dAF1 site confirmed this requirement. This mutation inhibited the full response of hepatic PEPCK-C gene to diabetes by reducing PEPCK-C mRNA level by 3.5-fold and the level of circulating glucose by 25%.

"New" hepatic fat activates PPARalpha to maintain glucose, lipid, and cholesterol homeostasis

De novo lipogenesis is an energy-expensive process whose role in adult mammals is poorly understood. We generated mice with liver-specific inactivation of fatty-acid synthase (FAS), a key lipogenic enzyme. On a zero-fat diet, FASKOL (FAS knockout in liver) mice developed hypoglycemia and fatty liver, which were reversed with dietary fat. These phenotypes were also observed after prolonged fasting, similarly to fasted PPARalpha-deficiency mice. Hypoglycemia, fatty liver, and defects in expression of PPARalpha target genes in FASKOL mice were corrected with a PPARalpha agonist. On either zero-fat or chow diet, FASKOL mice had low serum and hepatic cholesterol levels with elevated SREBP-2, decreased HMG-CoA reductase expression, and decreased cholesterol biosynthesis; these were also corrected with a PPARalpha agonist. These results suggest that products of the FAS reaction regulate glucose, lipid, and cholesterol metabolism by serving as endogenous activators of distinct physiological pools of PPARalpha in adult liver.