Metabolic Functions of Peroxisome Proliferator-Activated Receptor beta/delta in Skeletal Muscle

PPARδ Attenuates Alcohol-Mediated Insulin Resistance by Enhancing Fatty Acid-Induced Mitochondrial Uncoupling and Antioxidant Defense in Skeletal Muscle

Alcohol consumption leads to the dysfunction of multiple organs including liver, heart, and skeletal muscle. Alcohol effects on insulin resistance in liver are well evidenced, whereas its effects in skeletal muscle remain controversial. Emerging evidence indicates that alcohol promotes adipose tissue dysfunction, which may induce organ dysregulation. We show that consumption of ethanol (EtOH) reduces the activation of 5′AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) as well as the protein of carnitine palmitoyltransferase 1 (CPT1) and glucose transporter type 4 (GLUT4) in C₂C₁₂ myotube. We observed that chronic EtOH consumption increases free fatty acid levels in plasma and triglyceride (TG) accumulation in skeletal muscle and that these increases induce insulin resistance and decrease glucose uptake. Hence, ethanol dysregulates metabolic factors and induces TG accumulation. We found peroxisome proliferator-activated receptor β/δ (PPARδ) activation recovers AMPK activation and increases carnitine-acylcarnitine translocase (CACT) protein. These effects may contribute to enhance mitochondrial activation via uncoupling protein 3 (UCP3) when fatty acids are used as a substrate, thus reduces EtOH-induced increases in TG levels in skeletal muscle. In addition, PPARδ activation recovered EtOH-induced loss of protein kinase B (AKT) phosphorylation at serine 473 via rapamycin-insensitive companion of mammalian target of rapamycin (Rictor) activation. Importantly, PPARδ activation enhanced mitochondrial uncoupling via UCP3. Taken together, the study shows PPARδ enhances fatty acid utilization and uncoupled respiration via UCP3 and protects against EtOH-induced lipotoxicity and insulin resistance in skeletal muscle.

Nuclear receptors and nonalcoholic fatty liver disease

Nuclear receptors are transcription factors which sense changing environmental or hormonal signals and effect transcriptional changes to regulate core life functions including growth, development, and reproduction. To support this function, following ligand-activation by xenobiotics, members of subfamily 1 nuclear receptors (NR1s) may heterodimerize with the retinoid X receptor (RXR) to regulate transcription of genes involved in energy and xenobiotic metabolism and inflammation. Several of these receptors including the peroxisome proliferator-activated receptors (PPARs), the pregnane and xenobiotic receptor (PXR), the constitutive androstane receptor (CAR), the liver X receptor (LXR) and the farnesoid X receptor (FXR) are key regulators of the gut:liver:adipose axis and serve to coordinate metabolic responses across organ systems between the fed and fasting states. Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease and may progress to cirrhosis and even hepatocellular carcinoma. NAFLD is associated with inappropriate nuclear receptor function and perturbations along the gut:liver:adipose axis including obesity, increased intestinal permeability with systemic inflammation, abnormal hepatic lipid metabolism, and insulin resistance. Environmental chemicals may compound the problem by directly interacting with nuclear receptors leading to metabolic confusion and the inability to differentiate fed from fasting conditions. This review focuses on the impact of nuclear receptors in the pathogenesis and treatment of NAFLD. Clinical trials including PIVENS and FLINT demonstrate that nuclear receptor targeted therapies may lead to the paradoxical dissociation of steatosis, inflammation, fibrosis, insulin resistance, dyslipidemia and obesity. Novel strategies currently under development (including tissue-specific ligands and dual receptor agonists) may be required to separate the beneficial effects of nuclear receptor activation from unwanted metabolic side effects. The impact of nuclear receptor crosstalk in NAFLD is likely to be profound, but requires further elucidation. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

Inactivation of PPARβ/δ adversely affects satellite cells and reduces postnatal myogenesis

Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is a ubiquitously expressed gene with higher levels observed in skeletal muscle. Recently, our laboratory showed (Bonala S, Lokireddy S, Arigela H, Teng S, Wahli W, Sharma M, McFarlane C, Kambadur R. J Biol Chem 287: 12935-12951, 2012) that PPARβ/δ modulates myostatin activity to induce myogenesis in skeletal muscle. In the present study, we show that PPARβ/δ-null mice display reduced body weight, skeletal muscle weight, and myofiber atrophy during postnatal development. In addition, a significant reduction in satellite cell number was observed in PPARβ/δ-null mice, suggesting a role for PPARβ/δ in muscle regeneration. To investigate this, tibialis anterior muscles were injured with notexin, and muscle regeneration was monitored on days 3, 5, 7, and 28 postinjury. Immunohistochemical analysis revealed an increased inflammatory response and reduced myoblast proliferation in regenerating muscle from PPARβ/δ-null mice. Histological analysis confirmed that the regenerated muscle fibers of PPARβ/δ-null mice maintained an atrophy phenotype with reduced numbers of centrally placed nuclei. Even though satellite cell numbers were reduced before injury, satellite cell self-renewal was found to be unaffected in PPARβ/δ-null mice after regeneration. Previously, our laboratory had showed (Bonala S, Lokireddy S, Arigela H, Teng S, Wahli W, Sharma M, McFarlane C, Kambadur R. J Biol Chem 287: 12935-12951, 2012) that inactivation of PPARβ/δ increases myostatin signaling and inhibits myogenesis. Our results here indeed confirm that inactivation of myostatin signaling rescues the atrophy phenotype and improves muscle fiber cross-sectional area in both uninjured and regenerated tibialis anterior muscle from PPARβ/δ-null mice. Taken together, these data suggest that absence of PPARβ/δ leads to loss of satellite cells, impaired skeletal muscle regeneration, and postnatal myogenesis. Furthermore, our results also demonstrate that functional antagonism of myostatin has utility in rescuing these effects.

Muscle development and obesity: Is there a relationship?

The formation of skeletal muscle from the epithelial somites involves a series of events triggered by temporally and spatially discrete signals resulting in the generation of muscle fibers which vary in their contractile and metabolic nature. The fiber type composition of muscles varies between individuals and it has now been found that there are differences in fiber type proportions between lean and obese animals and humans. Amongst the possible causes of obesity, it has been suggested that inappropriate prenatal environments may 'program' the fetus and may lead to increased risks for disease in adult life. The characteristics of muscle are both heritable and plastic, giving the tissue some ability to adapt to signals and stimuli both pre and postnatally. Given that muscle is a site of fatty acid oxidation and carbohydrate metabolism and that its development can be changed by prenatal events, it is interesting to examine the possible relationship between muscle development and the risk of obesity.

Lactobacillus rhamnosus accelerates zebrafish backbone calcification and gonadal differentiation through effects on the GnRH and IGF systems

Endogenous microbiota play essential roles in the host’s immune system, physiology, reproduction and nutrient metabolism. We hypothesized that a continuous administration of an exogenous probiotic might also influence the host’s development. Thus, we treated zebrafish from birth to sexual maturation (2-months treatment) with Lactobacillus rhamnosus, a probiotic species intended for human use. We monitored for the presence of L. rhamnosus during the entire treatment. Zebrafish at 6 days post fertilization (dpf) exhibited elevated gene expression levels for Insulin-like growth factors -I and -II, Peroxisome proliferator activated receptors -α and -β, VDR-α and RAR-γ when compared to untreated-10 days old zebrafish. Using a gonadotropin-releasing hormone 3 GFP transgenic zebrafish (GnRH3-GFP), higher GnRH3 expression was found at 6, 8 and 10 dpf upon L. rhamnosus treatment. The same larvae exhibited earlier backbone calcification and gonad maturation. Noteworthy in the gonad development was the presence of first testes differentiation at 3 weeks post fertilization in the treated zebrafish population -which normally occurs at 8 weeks- and a dramatic sex ratio modulation (93% females, 7% males in control vs. 55% females, 45% males in the treated group). We infer that administration of L. rhamnosus stimulated the IGF system, leading to a faster backbone calcification. Moreover we hypothesize a role for administration of L. rhamnosus on GnRH3 modulation during early larval development, which in turn affects gonadal development and sex differentiation. These findings suggest a significant role of the microbiota composition on the host organism development profile and open new perspectives in the study of probiotics usage and application.

The Functions of PPARs in Aging and Longevity

Peroxisome proliferator-activated receptors (PPARs) are key regulators in various age-associated pathophysiological processes related to energy metabolism and oxidative stress. A progressive rise of oxidative stress and related inflammatory reaction appears the hallmarks of the aging process and many age-related diseases. PPARs are important redox-sensitive transcription factors and their dyregulated activations seem to be major culprits for these pathological processes. Drugs targeting PPARs activity are already in widespread clinical use; however, based on these concepts, this review highlights the understanding of the role of PPARs in aging and indicates the necessary particular attention for the potential therapeutic uses of current PPAR agonists in age-associated diseases.

PPARs and the cardiovascular system

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone-receptor superfamily. Originally cloned in 1990, PPARs were found to be mediators of pharmacologic agents that induce hepatocyte peroxisome proliferation. PPARs also are expressed in cells of the cardiovascular system. PPAR gamma appears to be highly expressed during atherosclerotic lesion formation, suggesting that increased PPAR gamma expression may be a vascular compensatory response. Also, ligand-activated PPAR gamma decreases the inflammatory response in cardiovascular cells, particularly in endothelial cells. PPAR alpha, similar to PPAR gamma, also has pleiotropic effects in the cardiovascular system, including antiinflammatory and antiatherosclerotic properties. PPAR alpha activation inhibits vascular smooth muscle proinflammatory responses, attenuating the development of atherosclerosis. However, PPAR delta overexpression may lead to elevated macrophage inflammation and atherosclerosis. Conversely, PPAR delta ligands are shown to attenuate the pathogenesis of atherosclerosis by improving endothelial cell proliferation and survival while decreasing endothelial cell inflammation and vascular smooth muscle cell proliferation. Furthermore, the administration of PPAR ligands in the form of TZDs and fibrates has been disappointing in terms of markedly reducing cardiovascular events in the clinical setting. Therefore, a better understanding of PPAR-dependent and -independent signaling will provide the foundation for future research on the role of PPARs in human cardiovascular biology.

Muscle-Specific PPARbeta/delta Agonism May Provide Synergistic Benefits with Life Style Modifications

Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) has emerged as a powerful metabolic regulator in diverse tissues including fat, skeletal muscle, and the heart. It is now established that activation of PPARβ/δ promotes fatty acid oxidation in several tissues, such as skeletal muscle and adipose tissue. In muscle, PPARβ/δ appears to act as a central regulator of fatty acid catabolism. PPARβ/δ contents are increased in muscle during physiological situations such as physical exercise or long-term fasting, characterized by increased fatty acid oxidation. Targeted expression of an activated form of PPARβ/δ in skeletal muscle induces a switch to form increased numbers of type I muscle fibers resembling the fiber type transition by endurance training. Activation of PPARβ/δ also enhances mitochondrial capacity and fat oxidation in the skeletal muscle that resembles the effect of regular exercise. Therefore, it is hypothesized that muscle-specific PPARβ/δ agonists could be a key strategy to support the poor cardiorespiratory fitness associated with metabolic disorders.