PPARß/δ ameliorates fructose-induced insulin resistance in adipocytes by preventing Nrf2 activation.

We studied whether PPARß/δ deficiency modifies the effects of high fructose intake (30% fructose in drinking water) on glucose tolerance and adipose tissue dysfunction, focusing on the CD36-dependent pathway that enhances adipose tissue inflammation and impairs insulin signaling. Fructose intake for 8 weeks significantly increased body and liver weight, and hepatic triglyceride accumulation in PPARß/δ-deficient mice but not in wild-type mice. Feeding PPARß/δ-deficient mice with fructose exacerbated glucose intolerance and led to macrophage infiltration, inflammation, enhanced mRNA and protein levels of CD36, and activation of the JNK pathway in white adipose tissue compared to those of water-fed PPARß/δ-deficient mice. Cultured adipocytes exposed to fructose also exhibited increased CD36 protein levels and this increase was prevented by the PPARß/δ activator GW501516. Interestingly, the levels of the nuclear factor E2-related factor 2 (Nrf2), a transcription factor reported to up-regulate Cd36 expression and to impair insulin signaling, were increased in fructose-exposed adipocytes whereas co-incubation with GW501516 abolished this increase. In agreement with Nrf2 playing a role in the fructose-induced CD36 protein level increases, the Nrf2 inhibitor trigonelline prevented the increase and the reduction in insulin-stimulated AKT phosphorylation caused by fructose in adipocytes. Protein levels of the well-known Nrf2 target gene NAD(P)H: quinone oxidoreductase 1 (Nqo1) were increased in water-fed PPARß/δ-null mice, suggesting that PPARß/δ deficiency increases Nrf2 activity; and this increase was exacerbated in fructose-fed PPARß/δ-deficient mice. These findings indicate that the combination of high fructose intake and PPARß/δ deficiency increases CD36 protein levels via Nrf2, a process that promotes chronic inflammation and insulin resistance in adipose tissue.

Enhanced fatty acid oxidation in adipocytes and macrophages reduces lipid-induced triglyceride accumulation and inflammation.

Lipid overload in obesity and type 2 diabetes is associated with adipocyte dysfunction, inflammation, macrophage infiltration, and decreased fatty acid oxidation (FAO). Here, we report that the expression of carnitine palmitoyltransferase 1A (CPT1A), the rate-limiting enzyme in mitochondrial FAO, is higher in human adipose tissue macrophages than in adipocytes and that it is differentially expressed in visceral vs. subcutaneous adipose tissue in both an obese and a type 2 diabetes cohort. These observations led us to further investigate the potential role of CPT1A in adipocytes and macrophages. We expressed CPT1AM, a permanently active mutant form of CPT1A, in 3T3-L1 CARΔ1 adipocytes and RAW 264.7 macrophages through adenoviral infection. Enhanced FAO in palmitate-incubated adipocytes and macrophages reduced triglyceride content and inflammation, improved insulin sensitivity in adipocytes, and reduced endoplasmic reticulum stress and ROS damage in macrophages. We conclude that increasing FAO in adipocytes and macrophages improves palmitate-induced derangements. This indicates that enhancing FAO in metabolically relevant cells such as adipocytes and macrophages may be a promising strategy for the treatment of chronic inflammatory pathologies such as obesity and type 2 diabetes.

PPARß/δ prevents endoplasmic reticulum stress-associated inflammation and insulin resistance in skeletal muscle cells through an AMPK-dependent mechanism.

AIM/HYPOTHESIS: Endoplasmic reticulum (ER) stress, which is involved in the link between inflammation and insulin resistance, contributes to the development of type 2 diabetes mellitus. In this study, we assessed whether peroxisome proliferator-activated receptor (PPAR)ß/δ prevented ER stress-associated inflammation and insulin resistance in skeletal muscle cells. METHODS: Studies were conducted in mouse C2C12 myotubes, in the human myogenic cell line LHCN-M2 and in skeletal muscle from wild-type and PPARß/δ-deficient mice and mice exposed to a high-fat diet. RESULTS: The PPARß/δ agonist GW501516 prevented lipid-induced ER stress in mouse and human myotubes and in skeletal muscle of mice fed a high-fat diet. PPARß/δ activation also prevented thapsigargin- and tunicamycin-induced ER stress in human and murine skeletal muscle cells. In agreement with this, PPARß/δ activation prevented ER stress-associated inflammation and insulin resistance, and glucose-intolerant PPARß/δ-deficient mice showed increased phosphorylated levels of inositol-requiring 1 transmembrane kinase/endonuclease-1α in skeletal muscle. Our findings demonstrate that PPARß/δ activation prevents ER stress through the activation of AMP-activated protein kinase (AMPK), and the subsequent inhibition of extracellular-signal-regulated kinase (ERK)1/2 due to the inhibitory crosstalk between AMPK and ERK1/2, since overexpression of a dominant negative AMPK construct (K45R) reversed the effects attained by PPARß/δ activation. CONCLUSIONS/INTERPRETATION: Overall, these findings indicate that PPARß/δ prevents ER stress, inflammation and insulin resistance in skeletal muscle cells by activating AMPK.

Tau hyperphosphorylation and increased BACE1 and RAGE levels in the cortex of PPARß/δ-null mice.

The role of peroxisome proliferator activator receptor (PPAR)ß/δ in the pathogenesis of Alzheimer's disease has only recently been explored through the use of PPARß/δ agonists. Here we evaluated the effects of PPARß/δ deficiency on the amyloidogenic pathway and tau hyperphosphorylation. PPARß/δ-null mice showed cognitive impairment in the object recognition task, accompanied by enhanced DNA-binding activity of NF-κB in the cortex and increased expression of IL-6. In addition, two NF-κB-target genes involved in ß-amyloid (Aß) synthesis and deposition, the ß site APP cleaving enzyme 1 (Bace1) and the receptor for advanced glycation endproducts (Rage), respectively, increased in PPARß/δ-null mice compared to wild type animals. The protein levels of glial fibrillary acidic protein (GFAP) increased in the cortex of PPARß/δ-null mice, which would suggest the presence of astrogliosis. Finally, tau hyperphosphorylation at Ser199 and enhanced levels of PHF-tau were associated with increased levels of the tau kinases CDK5 and phospho-ERK1/2 in the cortex of PPARß/δ(-/-) mice. Collectively, our findings indicate that PPARß/δ deficiency results in cognitive impairment associated with enhanced inflammation, astrogliosis and tau hyperphosphorylation in the cortex.

TNF-α inhibits PPARß/δ activity and SIRT1 expression through NF-κB in human adipocytes.

The mechanisms linking low-grade chronic inflammation with obesity-induced insulin resistance have only been partially elucidated. PPARß/δ and SIRT1 might play a role in this association. In visceral adipose tissue (VAT) from obese insulin-resistant patients we observed enhanced p65 nuclear translocation and elevated expression of the pro-inflammatory cytokines TNF-α and IL-6 compared to control subjects. Inflammation was accompanied by a reduction in the levels of SIRT1 protein and an increase in PPARß/δ mRNA levels. Stimulation of human mature SGBS adipocytes with TNF-α caused similar changes in PPARß/δ and SIRT1 to those reported in obese patients. Unexpectedly, PPAR DNA-binding activity and the expression of PPARß/δ-target genes was reduced following TNF-α stimulation, suggesting that the activity of this transcription factor was inhibited by cytokine treatment. Interestingly, the PPARß/δ ligand GW501516 prevented the expression of inflammatory markers and the reduction in the expression of PPARß/δ-target genes in adipocytes stimulated with TNF-α. Consistent with a role for NF-κB in the changes caused by TNF-α, treatment with the NF-κB inhibitor parthenolide restored PPAR DNA-binding activity, the expression of PPARß/δ-target genes and the expression of SIRT1 and PPARß/δ. These findings suggest that the reduction in PPARß/δ activity and SIRT1 expression caused by TNF-α stimulation through NF-κB helps perpetuate the inflammatory process in human adipocytes.

The BACE1 product sAPPß induces ER stress and inflammation and impairs insulin signaling.

OBJECTIVE: ß-secretase/ß-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1) is a key enzyme involved in Alzheimer's disease that has recently been implicated in insulin-independent glucose uptake in myotubes. However, it is presently unknown whether BACE1 and the product of its activity, soluble APPß (sAPPß), contribute to lipid-induced inflammation and insulin resistance in skeletal muscle cells. MATERIALS/METHODS: Studies were conducted in mouse C2C12 myotubes, skeletal muscle from Bace1-/-mice and mice treated with sAPPß and adipose tissue and plasma from obese and type 2 diabetic patients. RESULTS: We show that BACE1 inhibition or knockdown attenuates palmitate-induced endoplasmic reticulum (ER) stress, inflammation, and insulin resistance and prevents the reduction in Peroxisome Proliferator-Activated Receptor γ Co-activator 1α (PGC-1α) and fatty acid oxidation caused by palmitate in myotubes. The effects of palmitate on ER stress, inflammation, insulin resistance, PGC-1α down-regulation, and fatty acid oxidation were mimicked by soluble APPß in vitro. BACE1 expression was increased in subcutaneous adipose tissue of obese and type 2 diabetic patients and this was accompanied by a decrease in PGC-1α mRNA levels and by an increase in sAPPß plasma levels of obese type 2 diabetic patients compared to obese non-diabetic subjects. Acute sAPPß administration to mice reduced PGC-1α levels and increased inflammation in skeletal muscle and decreased insulin sensitivity. CONCLUSIONS: Collectively, these findings indicate that the BACE1 product sAPPß is a key determinant in ER stress, inflammation and insulin resistance in skeletal muscle and gluconeogenesis in liver.

Targeting endoplasmic reticulum stress in insulin resistance.

The endoplasmic reticulum (ER) is involved in the development of insulin resistance and progression to type 2 diabetes mellitus (T2DM). Disruption of ER homeostasis leads to ER stress, which activates the unfolded protein response (UPR). This response is linked to different processes involved in the development of insulin resistance (IR) and T2DM, including inflammation, lipid accumulation, insulin biosynthesis, and ß-cell apoptosis. Understanding the mechanisms by which disruption of ER homeostasis leads to IR and its progression to T2DM may offer new pharmacological targets for the treatment and prevention of these diseases. Here, we examine ER stress, the UPR, and downstream pathways in insulin sensitive tissues, and in IR, and offer insights towards therapeutic strategies.

PPARß/δ attenuates palmitate-induced endoplasmic reticulum stress and induces autophagic markers in human cardiac cells.

BACKGROUND: Chronic endoplasmic reticulum (ER) stress contributes to the apoptotic cell death in the myocardium, thereby playing a critical role in the development of cardiomyopathy. ER stress has been reported to be induced after high-fat diet feeding in mice and also after saturated fatty acid treatment in vitro. Therefore, since several studies have shown that peroxisome proliferator-activated receptor (PPAR)ß/δ inhibits ER stress, the main goal of this study consisted in investigating whether activation of this nuclear receptor was able to prevent lipid-induced ER stress in cardiac cells. METHODS AND RESULTS: Wild-type and transgenic mice with reduced PPARß/δ expression were fed a standard diet or a high-fat diet for two months. For in vitro studies, a cardiomyocyte cell line of human origin, AC16, was treated with palmitate and the PPARß/δ agonist GW501516. Our results demonstrate that palmitate induced ER stress in AC16 cells, a fact which was prevented after PPARß/δ activation with GW501516. Interestingly, the effect of GW501516 on ER stress occurred in an AMPK-independent manner. The most striking result of this study is that GW501516 treatment also upregulated the protein levels of beclin 1 and LC3II, two well-known markers of autophagy. In accordance with this, feeding on a high-fat diet or suppression of PPARß/δ in knockout mice induced ER stress in the heart. Moreover, PPARß/δ knockout mice also displayed a reduction in autophagic markers. CONCLUSION: Our data indicate that PPARß/δ activation might be useful to prevent the harmful effects of ER stress induced by saturated fatty acids in the heart by inducing autophagy.

An overview of the crosstalk between inflammatory processes and metabolic dysregulation during diabetic cardiomyopathy.

Metabolic disorders such as obesity, insulin resistance and type 2 diabetes mellitus are all linked to cardiovascular diseases such as cardiac hypertrophy and heart failure. Diabetic cardiomyopathy in particular, is characterized by structural and functional alterations in the heart muscle of people with diabetes that finally lead to heart failure, and which is not directly attributable to coronary artery disease or hypertension. Several mechanisms have been involved in the pathogenesis of diabetic cardiomyopathy, such as alterations in myocardial energy metabolism and calcium signaling. Metabolic disturbances during diabetic cardiomyopathy are characterized by increased lipid oxidation, intramyocardial triglyceride accumulation, and reduced glucose utilization. Overall changes result in enhanced oxidative stress, mitochondrial dysfunction and apoptosis of the cardiomyocytes. On the other hand, the progression of heart failure and cardiac hypertrophy usually entails a local rise in cytokines in cardiac cells and the activation of the proinflammatory transcription factor nuclear factor (NF)-κB. Interestingly, increasing evidences are arising in the recent years that point to a potential link between chronic low-grade inflammation in the heart and metabolic dysregulation. Therefore, in this review we summarize recent new insights into the crosstalk between inflammatory processes and metabolic dysregulation in the failing heart during diabetes, paying special attention to the role of NF-κB and peroxisome proliferator activated receptors (PPARs). In addition, we briefly describe the role of the AMP-activated protein kinase (AMPK), sirtuin 1 (SIRT1) and other pathways regulating cardiac energy metabolism, as well as their relationship with diabetic cardiomyopathy.

Targeting PPARß/δ for the treatment of type 2 diabetes mellitus.

INTRODUCTION: The nuclear receptors Peroxisome Proliferator-Activated Receptors (PPAR)α and PPARγ are therapeutic targets for hypertriglyceridemia and insulin resistance, respectively. Evidence is now emerging that the PPARß/δ isotype is a potential pharmacological target for the treatment of insulin resistance and type 2 diabetes mellitus. AREAS COVERED: In this review, the capacity of PPARß/δ to prevent the development of insulin resistance and type 2 diabetes mellitus is discussed. Special emphasis is placed on preclinical studies and the molecular mechanisms responsible for its actions in the main cell types involved in these pathologies: adipocytes, ß-cells, skeletal muscle cells and hepatocytes. EXPERT OPINION: While several concerns remain for the development of PPARß/δ agonists, these drugs have demonstrated their efficacy in the treatment of insulin resistance and type 2 diabetes mellitus in preclinical studies, as well as in a few short clinical studies in humans. Although this data is promising, additional studies must be performed to confirm the efficacy and safety of these drugs in the treatment of type 2 diabetes mellitus.

The Role of Peroxisome Proliferator-Activated Receptor beta/delta on the Inflammatory Basis of Metabolic Disease.

The pathophysiology underlying several metabolic diseases, such as obesity, type 2 diabetes mellitus, and atherosclerosis, involves a state of chronic low-level inflammation. Evidence is now emerging that the nuclear receptor Peroxisome Proliferator-Activated Receptor (PPAR)beta/delta ameliorates these pathologies partly through its anti-inflammatory effects. PPARbeta/delta activation prevents the production of inflammatory cytokines by adipocytes, and it is involved in the acquisition of the anti-inflammatory phenotype of macrophages infiltrated in adipose tissue. Furthermore, PPARbeta/delta ligands prevent fatty acid-induced inflammation in skeletal muscle cells, avoid the development of cardiac hypertrophy, and suppress macrophage-derived inflammation in atherosclerosis. These data are promising and suggest that PPARbeta/delta ligands may become a therapeutic option for preventing the inflammatory basis of metabolic diseases.