**6.2 PPARβ/δ, inflammation and insulin resistance in skeletal muscle cells**

FFAs may cause insulin resistance in skeletal muscle through several mechanisms, including effects on metabolism (Roden et al., 1996; Haber et al., 2003), signaling (Hirabara et al., 2007; Silveira et al., 2008) and mitochondrial function (Schrauwen et al., 2010; Hirabara et al., 2010). In addition, FFAs activate pro-inflammatory pathways, linking the development of this pathology to a chronic low-grade systemic inflammatory response (Wellen & Hotamisligil, 2005). In addition to FFA-induced inflammation through TLR, an additional pathway leads to FFA-mediated inflammation. This pathway involves intracellular accumulation of fatty acid derivatives. Once fatty acids are taken up by skeletal muscle cells they are either stored as fatty acid derivatives or undergo β-oxidation in the mitochondria. In the presence of high plasma FFA, fatty acid flux in skeletal muscle cells exceeds its oxidation, which leads to the accumulation of fatty acid derivatives, such as diacylglycerol (DAG), which can then activate a number of different serine kinases that negatively regulate insulin action. Thus, DAG is a potent allosteric activator of protein kinase Cθ (PKCθ), which is the most abundant PKC isoform in skeletal muscle (Griffin et al., 1999; Cortright et al., 2000; Itani et al., 2000). This PKC isoform inhibits the action of insulin by phosphorylating certain serine residues on insulin receptor substrate 1 (IRS1), including Ser307 in the rodent IRS-1 protein (reviewed in ref. (Gual et al., 2005)). This phosphorylation impairs insulinreceptor signaling through several distinct mechanisms (Hotamisligil et al., 1996). PKC also impairs insulin sensitivity by activating another serine kinase, IκB kinase β (IKKβ) (Perseghin et al., 2003). In addition to phosphorylating IRS-1 in Ser307, IKKβ phosphorylates IκB. Thus, it activates the pro-inflammatory transcription factor NF-κB, which has been linked to fatty acid-induced impairment of insulin action in skeletal muscle in rodents (Kim et al., 2001; Yuan et al., 2001). Once activated, NF-κB regulates the expression of multiple inflammatory mediators, including IL-6. This cytokine correlates strongly with insulin resistance and type 2 diabetes (Pickup et al., 1997; Kern et al., 2001; Pradhan et al., 2001) and its plasma levels are 2-3 times higher in patients with obesity and type 2 diabetes than in lean control subjects (Kern et al., 2001).

Accumulation of fatty acid derivatives can be attenuated by mitochondrial β-oxidation. The rate-limiting step for β-oxidation of long-chain fatty acids is their transport into mitochondria via CPT-1. The activity of this enzyme is inhibited by malonyl-CoA, the

Peroxisome Proliferator-Activated

**8. Acknowledgments** 

Advisory Service for its help.

2006;124:783-801.

2007;298:309-16.

*Chem* 1999;274:6718-25.

1997;46:3-10.

8.

**9. References** 

Receptor β/δ (PPARβ/δ) as a Potential Therapeutic Target for Dyslipidemia 227

receptor has been considered a therapeutic target to prevent and treat dyslipidemia. However, as with any drug designed for human therapy, a great deal of research will be needed on the efficacy and safety of PPARβ/δ activators before they reach clinical use.

The author's work that is summarized in this review was supported by grants from the Ministerio de Ciencia e Innovación of Spain (SAF2009-06939). CIBER de Diabetes y Enfermedades Metabólicas Asociadas (CIBERDEM) is an Instituto de Salud Carlos III project. L.S-M. and L.S. were supported by FPI grants from the Spanish Ministerio de Ciencia e Innovación. We would like to thank the University of Barcelona's Language

Third Report of the National Cholesterol Education Program (NCEP) Expert Panel on

Adiels M, Olofsson SO, Taskinen MR, Boren J. Overproduction of very low-density

Akira S, Uematsu S, Takeuchi O. Pathogen recognition and innate immunity. *Cell*

Akiyama TE, Lambert G, Nicol CJ et al. Peroxisome proliferator-activated receptor

Auwerx J, Baulieu E, Beato M et al. A unified nomenclature system for the nuclear receptor

Bansal S, Buring JE, Rifai N, Mora S, Sacks FM, Ridker PM. Fasting compared with

Barak Y, Liao D, He W et al. Effects of peroxisome proliferator-activated receptor delta on

Barish GD, Narkar VA, Evans RM. PPAR delta: a dagger in the heart of the metabolic

Barroso E, Rodriguez-Calvo R, Serrano-Marco L et al. The PPAR{beta}/{delta} Activator

Barter P, Gotto AM, LaRosa JC et al. HDL cholesterol, very low levels of LDL cholesterol,

Berger J, Leibowitz MD, Doebber TW et al. Novel peroxisome proliferator-activated receptor

Boden G. Role of fatty acids in the pathogenesis of insulin resistance and NIDDM. *Diabetes*

Increased Fatty Acid Oxidation. *Endocrinology* 2011;152:1848-59.

and cardiovascular events. *N Engl J Med* 2007;357:1301-10.

Treatment Panel III) final report. *Circulation* 2002;106:3143-421.

*Arterioscler Thromb Vasc Biol* 2008;28:1225-36.

superfamily. *Cell* 1999;97:161-63.

syndrome. *J Clin Invest* 2006;116:590-597.

mice on a Western diet. *J Biol Chem* 2004;279:20874-81.

Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult

lipoproteins is the hallmark of the dyslipidemia in the metabolic syndrome.

beta/delta regulates very low density lipoprotein production and catabolism in

nonfasting triglycerides and risk of cardiovascular events in women. *JAMA*

placentation, adiposity, and colorectal cancer. *Proc Natl Acad Sci U S A* 2002;99:303-

GW501516 Prevents the Down-Regulation of AMPK Caused by a High-Fat Diet in Liver and Amplifies the PGC-1{alpha}-Lipin 1-PPAR{alpha} Pathway Leading to

(PPAR) gamma and PPARdelta ligands produce distinct biological effects. *J Biol* 

product of acetyl-CoA carboxylase, which, in turn, is inhibited by AMPK. This kinase is a metabolic sensor that detects low ATP levels and increases oxidative metabolism (Reznick & Shulman, 2006), by reducing the levels of malonyl-CoA. Interestingly, activation of fatty acid oxidation by overexpressing CPT-1 in cultured skeletal muscle cells (Sebastian et al., 2007) and in mouse skeletal muscle (Bruce et al., 2009) improves lipid-induced insulin resistance. Hence, this approach may provide a valid therapeutic strategy to prevent this pathology. Activation of PPARβ/δ by its ligands (including GW501516) enhances fatty acid catabolism in adipose tissue and skeletal muscle, thereby delaying weight gain (for a review see (Barish et al., 2006)). This increase in fatty acid oxidation in human skeletal muscle cells following PPARβ/δ activation by GW501516 is dependent on both PPARβ/δ and AMPK (Kramer et al., 2007). AMPK is activated by GW501516 by modulating the ATP:AMP ratio (Kramer et al., 2007). Despite these data, little information was available on whether the increase in fatty acid oxidation attained after PPARβ/δ activation prevented fatty acid-induced inflammation and insulin resistance in skeletal muscle cells. However, we have recently reported that the PPARβ/δ ligand GW501516 prevented palmitate-induced inflammation and insulin resistance in skeletal muscle cells (Coll et al., 2010b). Treatment with GW501516 enhanced the expression of two-well known PPARβ/δ-target genes involved in fatty acid oxidation, CPT-1 and pyruvate dehydrogenase kinase 4 (PDK-4), and increased the phosphorylation of AMPK. This prevented the reduction in fatty acid oxidation caused by palmitate exposure. In agreement with these changes, GW501516 treatment reversed the increase in DAG and PKCθ activation caused by palmitate. These effects were abolished in the presence of the CPT-1 inhibitor etomoxir, thereby implicating increased fatty acid oxidation in the changes. Consistent with these findings, PPARβ/δ activation by GW501516 blocked palmitate-induced NF-κB DNA-binding activity. Likewise, drug treatment inhibited the increase in IL-6 expression caused by palmitate in C2C12 myotubes and human skeletal muscle cells, as well as the protein secretion of this cytokine. Overall, these findings indicate that PPARβ/δ attenuates fatty acid-induced NF-κB activation and the subsequent development of insulin resistance in skeletal muscle cells by reducing DAG accumulation. Interestingly, it has been suggested that the hypotrigliceridemic effect of GW501516 in humans is dependent of the increase in CPT-1 expression observed in skeletal muscle (Riserus et al., 2008).
