Nicotinamide adenine dinucleotide (NAD+) is a key cofactor required for essential metabolic oxidation-reduction reactions. It also regulates various cellular activities, including gene expression, signaling, DNA repair and calcium homeostasis. Intracellular NAD+ levels are tightly regulated and often respond rapidly to nutritional and environmental changes. Numerous studies indicate that elevating NAD+ may be therapeutically beneficial in the context of numerous diseases. However, the role of NAD+ on skeletal muscle exercise performance is poorly understood. CD38, a multi-functional membrane receptor and enzyme, consumes NAD+ to generate products such as cyclic-ADP-ribose. CD38 knockout mice show elevated tissue and blood NAD+ level. Chronic feeding of high-fat, high-sucrose diet to wild type mice leads to exercise intolerance and reduced metabolic flexibility. Loss of CD38 by genetic mutation protects mice from diet-induced metabolic deficit. These animal model results suggest that elevation of tissue NAD+ through genetic ablation of CD38 can profoundly alter energy homeostasis in animals that are maintained on a calorically-excessive Western diet.

We have investigated the changes in skeletal muscle mass and exercise capacity with age, and the incidence and impact on muscle mass and exercise capacity of the spontaneous elevation of fasting blood glucose during old age, in male C57BL/6J mice. In mice ranging in age from 5 to 32 months, gastrocnemius muscle mass decreased significantly (↓27% at 32 months, ↓36% when normalized to body weight; ↓23% at 22 months normalized to body weight) compared to 5 months. In mice ranging in age from 7 to 31 months, exercise capacity (exercise to exhaustion on an incline treadmill protocol, expressed as total work) decreased significantly (↓65% at 31 months (↓23% at 22 months)) compared to 7 months. In 672 mice ranging in age from 18 to 26 months, peak incidence of elevated fasting blood glucose occurred at 21 months, at which age 53% of mice were below 110 mg glucose/dl blood, 33% of mice were between 110 and 140 mg/dl, and 14% were over 140 mg/dl. Comparing groups of 22 month‐old mice with high (>170 mg/dl, n=9) or low (<90 mg/dl, n=11) fasting blood glucose, the high group had significantly less body weight (↓4%), less gastrocnemius muscle weight (↓5%), and less exercise capacity (↓17%). We conclude that elevated fasting blood glucose is associated with increased sarcopenia and decreased exercise capacity in old age mice.

The peroxisome proliferator-activated receptors (PPARs) are well established to be important in modulating the fibrogenic response to liver injury. PPARγ plays a role in hepatic fibrosis, presumably by virtue of its expression in hepatic stellate cells, which are key effectors of fibrosis. In this study, we evaluated whether the potent nonthiozolidinedione PPARγ agonist, GW570, had effects on isolated stellate cells and hepatic fibrosis in vivo.

Activation of peroxisome proliferator-activated receptor (PPAR) alpha, delta, and gamma subtypes increases expression of genes involved in fatty acid transport and oxidation and alters adiposity in animal models of obesity and type-2 diabetes. PPARpan agonists which activate all three receptor subtypes have antidiabetic activity in animal models without the weight gain associated with selective PPARgamma agonists. Herein we report the effects of selective PPAR agonists (GW9578, a PPARalpha agonist, GW0742, a PPARdelta agonist, GW7845, a PPARgamma agonist), combination of PPARalpha and delta agonists, and PPARpan (PPARalpha/gamma/delta) activators (GW4148 or GW9135) on body weight (BW), body composition, food consumption, fatty acid oxidation, and serum chemistry of diet-induced obese AKR/J mice. PPARalpha or PPARdelta agonist treatment induced a slight decrease in fat mass (FM) while a PPARgamma agonist increased BW and FM commensurate with increased food consumption. The reduction in BW and food intake after cotreatment with PPARalpha and delta agonists appeared to be synergistic. GW4148, a PPARpan agonist, induced a significant and sustained reduction in BW and FM similar to an efficacious dose of rimonabant, an antiobesity compound. GW9135, a PPARpan agonist with weak activity at PPARdelta, induced weight loss initially followed by rebound weight gain reaching vehicle control levels by the end of the experiment. We conclude that PPARalpha and PPARdelta activations are critical to effective weight loss induction. These results suggest that the PPARpan compounds may be expected to maintain the beneficial insulin sensitization effects of a PPARgamma agonist while either maintaining weight or producing weight loss.

Welcome to this special issue of PPAR Research dedicated to “PPARs and Obesity.” Obesity and the interrelated disorders of the metabolic syndrome are a global health epidemic. To address this major problem, it is essential to understand the mechanisms regulating energy metabolism, and it has been known for years that PPARs play an important role in many facets of energy homeostasis. This is a very active and exciting field of research that, without a doubt, justifies a special issue. The genetic, molecular, and physiological aspects of PPARs as well as the metabolic effects of recently developed PPAR and RXR agonists are among the topics discussed. The issue begins with a review of key observations made in human subjects harboring genetic variations in PPARγ and a thorough overview of the metabolic effects of PPARs in genetically modified animal models. Over the last five years, the knowledge on PPAR delta biology has literally exploded and the potential therapeutic usefulness of this receptor in metabolic syndrome is now recognized. The interaction of PPARs with uncoupling proteins regulating energy expenditure is reviewed as are recent developments with RXR agonists. A closely related topic addresses the molecular and physiological functions of PPAR coactivators and corepressors in relationship to adipocyte energy metabolism. The selective PPAR modulator concept has attracted the attention of the field for over a decade; however the molecular bases underlying their differential mode of action have only begun to emerge recently. In addition to selective PPAR agonists, compounds that simultaneously activate two (dual agonists) or three (pan agonists) PPAR isoforms are in development. The potential advantages of these new combinations are discussed. The intriguing possibility that PPARs may mediate effects of caloric restriction on longevity is also considered. Finally, a growing body of evidence indicates that inflammation is a key feature of the obese state and that PPARs display strong anti-inflammatory properties. The evidence that PPARs may be interesting therapeutic targets to modulate obesity-induced inflammation is also reviewed. While these reviews just scratch the surface of PPAR/RXR interactions and regulation of energy balance, this special issue is packed with exciting, high quality reports from recognized experts in the field. We hope that the ideas presented here will generate further interest from the scientific community in this rapidly expanding area of research. Francine M. Gregoire Sander Kersten Wallace Harrington

The peroxisome proliferator-activated receptors (PPARs) impart diverse cellular effects in biological systems. Because stellate cell activation during liver injury is associated with declining PPARgamma expression, we hypothesized that its expression is critical in stellate cell-mediated fibrogenesis. We therefore modulated its expression during liver injury in vivo. PPARgamma was depleted in rat livers by using an adenovirus-Cre recombinase system. PPARgamma was overexpressed by using an additional adenoviral vector (AdPPARgamma). Bile duct ligation was utilized to induce stellate cell activation and liver fibrosis in vivo; phenotypic effects (collagen I, smooth muscle alpha-actin, hydroxyproline content, etc.) were measured. PPARgamma mRNA levels decreased fivefold and PPARgamma protein was undetectable in stellate cells after culture-induced activation. During activation in vivo, collagen accumulation, assessed histomorphometrically and by hydroxyproline content, was significantly increased after PPARgamma depletion compared with controls (1.28 +/- 0.14 vs. 1.89 +/- 0.21 mg/g liver tissue, P < 0.03). In isolated stellate cells, AdPPARgamma overexpression resulted in significantly increased adiponectin mRNA expression and decreased collagen I and smooth muscle alpha-actin mRNA expression compared with controls. During in vivo fibrogenesis, rat livers exposed to AdPPARgamma had significantly less fibrosis than controls. Collagen I and smooth muscle alpha-actin mRNA expression were significantly reduced in AdPPARgamma-infected rats compared with controls (P < 0.05, n = 10). PPARgamma-deficient mice exhibited enhanced fibrogenesis after liver injury, whereas PPARgamma receptor overexpression in vivo attenuated stellate cell activation and fibrosis. The data highlight a critical role for PPARgamma during in vivo fibrogenesis and emphasize the importance of the PPARgamma pathway in stellate cells during liver injury.

Peroxisome proliferator-activated receptor-gamma (PPARgamma) agonists have been shown to have significant therapeutic benefits such as desirable glycemic control in type 2 diabetic patients; however, these agents may cause fluid retention in susceptible individuals. Since PPARgamma is expressed selectively in distal nephron epithelium, we studied the mechanism of PPARgamma agonist-induced fluid retention using male Sprague-Dawley rats treated with either vehicle or GI262570 (farglitazar), a potent PPARgamma agonist. GI262570 (20 mg/kg/day) induced a plasma volume expansion. The plasma volume expansion was accompanied by a small but significant decrease in plasma potassium concentration. Small but significant increases in plasma sodium and chloride concentrations were also observed. These changes in serum electrolytes suggested an activation of the renal mineralocorticoid response system; however, GI262570-treated rats had lower plasma levels of aldosterone compared with vehicle-treated controls. mRNA levels for a group of genes involved in distal nephron sodium and water absorption are changed in the kidney medulla with GI262570 treatment. In addition, due to a possible rebound effect on epithelial sodium channel (ENaC) activity, a low dose of amiloride did not prevent GI262570-induced fluid retention. On the contrary, the rebound effect after amiloride treatment potentiated GI262570-induced plasma volume expansion. This is at least partially due to a synergistic effect of GI262570 and the rebound from amiloride treatment on ENaCalpha expression. In summary, our current data suggest that GI262570 can increase water and sodium reabsorption in distal nephron by stimulating the ENaC and Na,K-ATPase system. This may be an important mechanism for PPARgamma agonist-induced fluid retention.

Elevated levels of the hormone resistin, which is secreted by fat cells, are proposed to cause insulin resistance and to serve as a link between obesity and type 2 diabetes. In this report we show that resistin expression is significantly decreased in the white adipose tissue of several different models of obesity including the ob/ob, db/db, tub/tub, and KKAy mice compared with their lean counterparts. Furthermore, in response to several different classes of antidiabetic peroxisome proliferator-activated receptor γ agonists, adipose tissue resistin expression is increased in both ob/ob mice and Zucker diabetic fatty rats. These data demonstrate that experimental obesity in rodents is associated with severely defective resistin expression, and decreases in resistin expression are not required for the antidiabetic actions of peroxisome proliferator-activated receptor γ agonists.

Peroxisome proliferator-activated receptor gamma (PPAR gamma) agonists, including the glitazone class of drugs, are insulin sensitizers that reduce glucose and lipid levels in patients with type 2 diabetes mellitus. To more fully understand the molecular mechanisms underlying their therapeutic actions, we have characterized the effects of the potent, tyrosine-based PPAR gamma ligand GW1929 on serum glucose and lipid parameters and gene expression in Zucker diabetic fatty rats. In time-course studies, GW1929 treatment decreased circulating FFA levels before reducing glucose and triglyceride levels. We used a comprehensive and unbiased messenger RNA profiling technique to identify genes regulated either directly or indirectly by PPAR gamma in epididymal white adipose tissue, interscapular brown adipose tissue, liver, and soleus skeletal muscle. PPAR gamma activation stimulated the expression of a large number of genes involved in lipogenesis and fatty acid metabolism in both white adipose tissue and brown adipose tissue. In muscle, PPAR gamma agonist treatment decreased the expression of pyruvate dehydrogenase kinase 4, which represses oxidative glucose metabolism, and also decreased the expression of genes involved in fatty acid transport and oxidation. These changes suggest a molecular basis for PPAR gamma-mediated increases in glucose utilization in muscle. In liver, PPAR gamma activation coordinately decreased the expression of genes involved in gluconeogenesis. We conclude from these studies that the antidiabetic actions of PPAR gamma agonists are probably the consequence of 1) their effects on FFA levels, and 2), their coordinate effects on gene expression in multiple insulin-sensitive tissues.

The discovery that peroxisome proliferator-activated receptor (PPAR)-gamma was the molecular target of the thiazolidinedione class of antidiabetic agents suggested a key role for PPAR-gamma in the regulation of carbohydrate and lipid metabolism. Through the use of high-throughput biochemical assays, GW1929, a novel N-aryl tyrosine activator of human PPAR-gamma, was identified. Chronic oral administration of GW1929 or troglitazone to Zucker diabetic fatty (ZDF) rats resulted in dose-dependent decreases in daily glucose, free fatty acid, and triglyceride exposure compared with pretreatment values, as well as significant decreases in glycosylated hemoglobin. Whole body insulin sensitivity, as determined by the euglycemic-hyperinsulinemic clamp technique, was significantly increased in treated animals. Comparison of the magnitude of glucose lowering as a function of serum drug concentrations showed that GW1929 was 2 orders of magnitude more potent than troglitazone in vivo. These data were consistent with the relative in vitro potencies of GW1929 and troglitazone. Isolated perfused pancreas studies performed at the end of the study confirmed that pancreata from vehicle-treated rats showed no increase in insulin secretion in response to a step change in glucose from 3 to 10 mmol/l. In contrast, pancreata from animals treated with GW1929 showed a first- and second-phase insulin secretion pattern. Consistent with the functional data from the perfusion experiments, animals treated with the PPAR-gamma agonist had more normal islet architecture with preserved insulin staining compared with vehicle-treated ZDF rats. This is the first demonstration of in vivo efficacy of a novel nonthiazolidinedione identified as a high-affinity ligand for human PPAR-gamma. The increased potency of GW1929 compared with troglitazone both in vitro and in vivo may translate into improved clinical efficacy when used as monotherapy in type 2 diabetic patients. In addition, the significant improvement in daily meal tolerance may impact cardiovascular risk factor management in these patients.