Nutritional regulation and role of peroxisome proliferator-activated receptor delta in fatty acid catabolism in skeletal muscle

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors primarily involved in lipid homeostasis. PPARdelta displays strong expression in tissues with high lipid metabolism, such as adipose, intestine and muscle. Its role in skeletal muscle remains largely unknown. After a 24-h starvation period, PPARdelta mRNA levels are dramatically up-regulated in gastrocnemius muscle of mice and restored to control level upon refeeding. The rise of PPARdelta is accompanied by parallel up-regulations of fatty acid translocase/CD36 (FAT/CD36) and heart fatty acid binding protein (H-FABP), while refeeding promotes down-regulation of both genes. To directly access the role of PPARdelta in muscle cells, we forced its expression and that of a dominant-negative PPARdelta mutant in C2C12 myogenic cells. Differentiated C2C12 cells responds to 2-bromopalmitate or synthetic PPARdelta agonist by induction of genes involved in lipid metabolism and increment of fatty acid oxidation. Overexpression of PPARdelta enhanced these cellular responses, whereas expression of the dominant-negative mutant exerts opposite effects. These data strongly support a role for PPARdelta in the regulation of fatty acid oxidation in skeletal muscle and in adaptive response of this tissue to lipid catabolism.     

Role of AMP kinase and PPARdelta in the regulation of lipid and glucose metabolism in human skeletal muscle

The peroxisome proliferator-activated receptor (PPAR)delta has been implicated in the regulation of lipid metabolism in skeletal muscle. Furthermore, activation of PPARdelta has been proposed to improve insulin sensitivity and reduce glucose levels in animal models of type 2 diabetes. We recently demonstrated that the PPARdelta agonist GW501516 activates AMP-activated protein kinase (AMPK) and stimulates glucose uptake in skeletal muscle. However, the underlying mechanism remains to be clearly identified. In this study, we first confirmed that incubation of primary cultured human muscle cells with GW501516 induced AMPK phosphorylation and increased fatty acid transport and oxidation and glucose uptake. Using small interfering RNA, we have demonstrated that PPARdelta expression is required for the effect of GW501516 on the intracellular accumulation of fatty acids. Furthermore, we have shown that the subsequent increase in fatty acid oxidation induced by GW501516 is dependent on both PPARdelta and AMPK. Concomitant with these metabolic changes, we provide evidence that GW501516 increases the expression of key genes involved in lipid metabolism (FABP3, CPT1, and PDK4) by a PPARdelta-dependent mechanism. Finally, we have also demonstrated that the GW501516-mediated increase in glucose uptake requires AMPK but not PPARdelta. In conclusion, the PPARdelta agonist GW501516 promotes changes in lipid/glucose metabolism and gene expression in human skeletal muscle cells by PPARdelta- and AMPK-dependent and -independent mechanisms.     

The PPARdelta agonist, GW501516, promotes fatty acid oxidation but has no direct effect on glucose utilisation or insulin sensitivity in rat L6 skeletal muscle cells

Peroxisome proliferator-activated receptor-delta (PPARdelta) activation enhances skeletal muscle fatty acid oxidation and improves whole body glucose homeostasis and insulin sensitivity. Recently, GW501516, a selective PPARdelta agonist, was reported to increase glucose uptake in human skeletal myotubes by an AMPK-dependent mechanism that may contribute to the improved glucose tolerance. Here, we demonstrate that whilst GW501516 increases expression of PGC-1alpha and CPT-1 and stimulates fatty-acid oxidation in L6 myotubes, it fails to enhance insulin sensitivity, AMPK activity or glucose uptake and storage. Our findings exclude sarcolemmal glucose transport as a potential target for the therapeutic action of PPARdelta agonists in skeletal muscle.     

Peroxisome-proliferator-activated receptor delta activates fat metabolism to prevent obesity

In contrast to the well-established roles of PPARgamma and PPARalpha in lipid metabolism, little is known for PPARdelta in this process. We show here that targeted activation of PPARdelta in adipose tissue specifically induces expression of genes required for fatty acid oxidation and energy dissipation, which in turn leads to improved lipid profiles and reduced adiposity. Importantly, these animals are completely resistant to both high-fat diet-induced and genetically predisposed (Lepr(db/db)) obesity. As predicted, acute treatment of Lepr(db/db) mice with a PPARdelta agonist depletes lipid accumulation. In parallel, PPARdelta-deficient mice challenged with high-fat diet show reduced energy uncoupling and are prone to obesity. In vitro, activation of PPARdelta in adipocytes and skeletal muscle cells promotes fatty acid oxidation and utilization. Our findings suggest that PPARdelta serves as a widespread regulator of fat burning and identify PPARdelta as a potential target in treatment of obesity and its associated disorders.     

Chronic fetal hypoglycemia inhibits the later steps of stimulus-secretion coupling in pancreatic beta-cells

Peroxisome proliferator-activated receptor-delta (PPARdelta) activation results in upregulation of genes associated with skeletal muscle fatty acid oxidation and mitochondrial uncoupling. However, direct, noninvasive assessment of lipid metabolism and mitochondrial energy coupling in skeletal muscle following PPARdelta stimulation has not been examined. Therefore, in this study we examined the response of a selective PPARdelta agonist (GW610742X at 5 or 100 mg.kg(-1).day(-1) for 8 days) on skeletal-muscle lipid metabolism and mitochondrial coupling efficiency in rats by using in vivo magnetic resonance spectroscopy (MRS). There was a decrease in the intramyocellular lipid-to-total creatine ratio as assessed by in vivo (1)H-MRS in soleus and tibialis anterior muscles by day 7 (reduced by 49 and 46%, respectively; P < 0.01) at the high dose. Following the (1)H-MRS experiment (day 8), [1-(13)C]glucose was administered to conscious rats to assess metabolism in the soleus muscle. The relative fat-vs.-carbohydrate oxidation rate increased in a dose-dependent manner (increased by 52 and 93% in the 5 and 100 mg.kg(-1).day(-1) groups, respectively; P < 0.05). In separate experiments where mitochondrial coupling was assessed in vivo (day 7), (31)P-MRS was used to measure hindlimb ATP synthesis and (13)C-MRS was used to measure the hindlimb tricarboxylic acid cycle flux (V(tca)). There was no alteration, at either dose, in mitochondrial coupling efficiency measured as the ratio of unidirectional ATP synthesis flux to V(tca). Soleus muscle GLUT4 expression was decreased by twofold, whereas pyruvate dehydrogenase kinase 4, carnitine palmitoyl transferase 1a, and uncoupling protein 2 and 3 expression was increased by two- to threefold at the high dose (P < 0.05). In summary, these are the first noninvasive measurements illustrating a selective PPARdelta-mediated decrease in muscle lipid content that was consistent with a shift in metabolic substrate utilization from carbohydrate to lipid. However, the mitochondrial-energy coupling efficiency was not altered in the presence of increased uncoupling protein expression.     

Emerging roles of PPARdelta in metabolism

PPARdelta differs from the other two PPAR isotypes (alpha and gamma) by its more wide-spread tissue-specific expression pattern, its involvement in developmental processes and its profound impact on muscle and heart fat metabolism. Activation of PPARdelta modulates inflammatory responses of macrophages and is linked to altered lipoprotein metabolism, most importantly a significant raise of HDL cholesterol. PPARdelta activation in the liver decreases hepatic glucose output, thereby contributing to improved glucose tolerance and insulin sensitivity. Several studies have shown that PPARdelta polymorphisms are associated with plasma lipid levels, body mass index and the risk for diabetes and coronary heart disease. These findings support that high affinity PPARdelta agonists may be promising drugs of the future to treat the metabolic syndrome which is an expanding overweight-related health threat characterized by insulin resistance, hyperglycemia, dyslipidemia, hypertension, and accelerated atherosclerosis.     

The peroxisome proliferator-activated receptor beta/delta agonist, GW501516, regulates the expression of genes involved in lipid catabolism and energy uncoupling in skeletal muscle cells

Lipid homeostasis is controlled by the peroxisome proliferator-activated receptors (PPARalpha, -beta/delta, and -gamma) that function as fatty acid-dependent DNA-binding proteins that regulate lipid metabolism. In vitro and in vivo genetic and pharmacological studies have demonstrated PPARalpha regulates lipid catabolism. In contrast, PPARgamma regulates the conflicting process of lipid storage. However, relatively little is known about PPARbeta/delta in the context of target tissues, target genes, lipid homeostasis, and functional overlap with PPARalpha and -gamma. PPARbeta/delta, a very low-density lipoprotein sensor, is abundantly expressed in skeletal muscle, a major mass peripheral tissue that accounts for approximately 40% of total body weight. Skeletal muscle is a metabolically active tissue, and a primary site of glucose metabolism, fatty acid oxidation, and cholesterol efflux. Consequently, it has a significant role in insulin sensitivity, the blood-lipid profile, and lipid homeostasis. Surprisingly, the role of PPARbeta/delta in skeletal muscle has not been investigated. We utilize selective PPARalpha, -beta/delta, -gamma, and liver X receptor agonists in skeletal muscle cells to understand the functional role of PPARbeta/delta, and the complementary and/or contrasting roles of PPARs in this major mass peripheral tissue. Activation of PPARbeta/delta by GW501516 in skeletal muscle cells induces the expression of genes involved in preferential lipid utilization, beta-oxidation, cholesterol efflux, and energy uncoupling. Furthermore, we show that treatment of muscle cells with GW501516 increases apolipoprotein-A1 specific efflux of intracellular cholesterol, thus identifying this tissue as an important target of PPARbeta/delta agonists. Interestingly, fenofibrate induces genes involved in fructose uptake, and glycogen formation. In contrast, rosiglitazone-mediated activation of PPARgamma induces gene expression associated with glucose uptake, fatty acid synthesis, and lipid storage. Furthermore, we show that the PPAR-dependent reporter in the muscle carnitine palmitoyl-transferase-1 promoter is directly regulated by PPARbeta/delta, and not PPARalpha in skeletal muscle cells in a PPARgamma coactivator-1-dependent manner. This study demonstrates that PPARs have distinct roles in skeletal muscle cells with respect to the regulation of lipid, carbohydrate, and energy homeostasis. Moreover, we surmise that PPARbeta/delta agonists would increase fatty acid catabolism, cholesterol efflux, and energy expenditure in muscle, and speculate selective activators of PPARbeta/delta may have therapeutic utility in the treatment of hyperlipidemia, atherosclerosis, and obesity.     

Alterations of peroxisome proliferator-activated receptor delta activity affect fatty acid-controlled adipose differentiation

Fatty acids have been postulated to regulate adaptation of adipose mass to nutritional changes by controlling expression of genes implicated in lipid metabolism via activation of nuclear receptors. Ectopic expression of the nuclear receptors PPARgamma or PPARdelta promotes adipogenesis in fibroblastic cells exposed to thiazolidinediones or long-chain fatty acids. To investigate the role of PPARdelta in fatty acid regulation of gene expression and adipogenesis in a preadipose cellular context, we studied the effects of overexpressing the native receptor or the dominant-negative PPARdelta mutant in Ob1771 and 3T3-F442A cells. Overexpression of PPARdelta enhanced fatty acid induction of the adipose-related genes for fatty acid translocase, adipocyte lipid binding protein, and PPARgamma and fatty acid effects on terminal differentiation. A transactivation-deficient form of PPARdelta mutated in the AF2 domain severely reduced these effects. Findings are similar in Ob1771 or 3T3-F442A preadipose cells. These data demonstrate that PPARdelta plays a central role in fatty acid-controlled differentiation of preadipose cells. Furthermore, they suggest that modulation of PPARdelta expression or activity could affect adaptive responses of white adipose tissue to nutritional changes.     

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.     

Overexpression of UCP-3 in skeletal muscle of mice results in increased expression of mitochondrial thioesterase mRNA

Mice overexpressing human UCP-3 in skeletal muscle (UCP-3tg) are lean despite overeating, have increased metabolic rate, and their skeletal muscle mitochondria show increased proton conductance. The true function of UCP-3 however, has yet to be determined. It is assumed that UCP-3tg mice have increased fatty acid beta-oxidation to fuel their increased metabolic rate. In this study we have quantified skeletal muscle mRNA levels of a number of genes involved in fatty acid metabolism. mRNA levels of uncoupling protein-2, carnitine palmitoyl transferase-1beta and fatty acid binding proteins, and transporters were unchanged when compared to wild-type mice. Lipoprotein lipase mRNA was slightly, but significantly, increased by 50%. The most notable change in gene expression was a threefold increase in mitochondrial thioesterase (MTE-1) expression. In the face of a chronic increase in mitochondrial uncoupling these changes suggest that increased flux of fatty acids through the beta-oxidation pathway does not necessarily require marked changes in expression of genes involved in fatty acid metabolism. The large increase in MTE-1 both confirms the importance of this gene in situations where mitochondrial beta-oxidation is increased and supports the hypothesis that UCP-3 exports fatty acids generated by MTE-1 in the mitochondrion.     

Hypothalamic lipotoxicity and the metabolic syndrome

Ectopic accumulation of lipids in peripheral tissues, such as pancreatic β cells, liver, heart and skeletal muscle, leads to lipotoxicity, a process that contributes substantially to the pathophysiology of insulin resistance, type 2 diabetes, steatotic liver disease and heart failure. Current evidence has demonstrated that hypothalamic sensing of circulating lipids and modulation of hypothalamic endogenous fatty acid and lipid metabolism are two bona fide mechanisms modulating energy homeostasis at the whole body level. Key enzymes, such as AMP-activated protein kinase (AMPK) and fatty acid synthase (FAS), as well as intermediate metabolites, such as malonyl-CoA and long-chain fatty acids-CoA (LCFAs-CoA), play a major role in this neuronal network, integrating peripheral signals with classical neuropeptide-based mechanisms. However, one key question to be addressed is whether impairment of lipid metabolism and accumulation of specific lipid species in the hypothalamus, leading to lipotoxicity, have deleterious effects on hypothalamic neurons. In this review, we summarize what is known about hypothalamic lipid metabolism with focus on the events associated to lipotoxicity, such as endoplasmic reticulum (ER) stress in the hypothalamus. A better understanding of these molecular mechanisms will help to identify new drug targets for the treatment of obesity and metabolic syndrome.     

Nutritional regulation and role of peroxisome proliferator-activated receptor δ in fatty acid catabolism in skeletal muscle

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors primarily involved in lipid homeostasis. PPARδ displays strong expression in tissues with high lipid metabolism, such as adipose, intestine and muscle. Its role in skeletal muscle remains largely unknown. After a 24-h starvation period, PPARδ mRNA levels are dramatically up-regulated in gastrocnemius muscle of mice and restored to control level upon refeeding. The rise of PPARδ is accompanied by parallel up-regulations of fatty acid translocase/CD36 (FAT/CD36) and heart fatty acid binding protein (H-FABP), while refeeding promotes down-regulation of both genes. To directly access the role of PPARδ in muscle cells, we forced its expression and that of a dominant-negative PPARδ mutant in C2C12 myogenic cells. Differentiated C2C12 cells responds to 2-bromopalmitate or synthetic PPARδ agonist by induction of genes involved in lipid metabolism and increment of fatty acid oxidation. Overexpression of PPARδ enhanced these cellular responses, whereas expression of the dominant-negative mutant exerts opposite effects. These data strongly support a role for PPARδ in the regulation of fatty acid oxidation in skeletal muscle and in adaptive response of this tissue to lipid catabolism.     

Intramuscular lipid metabolism, insulin action, and obesity

With the increasing prevalence of obesity, research has focused on the molecular mechanism(s) linking obesity and skeletal muscle insulin resistance. Metabolic alterations within muscle, such as changes in the cellular location of fatty acid transporter proteins, decreased mitochondrial enzyme activity, and defects in mitochondrial morphology, likely contribute to obesity and insulin resistance. These defects are thought to play a role in the reduced skeletal muscle fatty acid oxidation and increased intramuscular lipid (IMCL) accumulation that is apparent with obesity and other insulin-resistant states such as type 2 diabetes. Intramuscular triacylglycerol does not appear to be a ubiquitous marker of insulin resistance, although specific IMCL intermediates such as long-chain fatty acyl-CoAs, ceramide, and diacylglycerol may inhibit insulin signal transduction. In this review, we will briefly summarize the defects in skeletal muscle lipid metabolism associated with obesity, and discuss the proposed mechanisms by which these defects may contribute to insulin resistance.