Here we report that in skeletal muscle cells the contribution to insulin resistance and inflammation of two common dietary long-chain fatty acids depends on the channeling of these lipids to distinct cellular metabolic fates. Exposure of cells to the saturated fatty acid palmitate led to enhanced diacylglycerol levels and the consequent activation of the protein kinase Cθ/nuclear factor κB pathway, finally resulting in enhanced interleukin 6 secretion and down-regulation of the expression of genes involved in the control of the oxidative capacity of skeletal muscle (peroxisome proliferator-activated receptor (PPAR)γ-coactivator 1α) and triglyceride synthesis (acyl-coenzyme A: diacylglycerol acyltransferase 2). In contrast, exposure to the monounsaturated fatty acid oleate did not lead to these changes. Interestingly, co-incubation of cells with palmitate and oleate reversed both inflammation and impairment of insulin signaling by channeling palmitate into triglycerides and by up-regulating the expression of genes involved in mitochondrial β-oxidation, thus reducing its incorporation into diacylglycerol. Our findings support a model of cellular lipid metabolism in which oleate protects against palmitate-induced inflammation and insulin resistance in skeletal muscle cells by promoting triglyceride accumulation and mitochondrial β-oxidation through PPARα- and protein kinase A-dependent mechanisms.

It has been reported that walnuts reduce serum cholesterol levels in normal young men. To assess the acceptability of walnuts and their effects on serum lipid levels and low-density lipoprotein (LDL) oxidizability in free-living hypercholesterolemic persons. Randomized, crossover feeding trial. Lipid clinic at a university hospital. 55 men and women (mean age, 56 years) with polygenic hypercholesterolemia. A cholesterol-lowering Mediterranean diet and a diet of similar energy and fat content in which walnuts replaced approximately 35% of the energy obtained from monounsaturated fat. Patients followed each diet for 6 weeks. Low-density lipoprotein fatty acids (to assess compliance), serum lipid levels, lipoprotein(a) levels, and LDL resistance to in vitro oxidative stress. 49 persons completed the trial. The walnut diet was well tolerated. Planned and observed diets were closely matched. Compared with the Mediterranean diet, the walnut diet produced mean changes of −4.1% in total cholesterol level, −5.9% in LDL cholesterol level, and −6.2% in lipoprotein(a) level. The mean differences in the changes in serum lipid levels were −0.28 mmol/L (95% CI, −0.43 to −0.12 mmol/L) (−10.8 mg/dL [−16.8 to −4.8 mg/dL]) (P < 0.001) for total cholesterol level, −0.29 mmol/L (CI, −0.41 to −0.15 mmol/L) (−11.2 mg/dL [−16.3 to −6.1 mg/dL]) (P < 0.001) for LDL cholesterol level, and −0.021 g/L (CI, −0.042 to −0.001 g/L) (P = 0.042) for lipoprotein(a) level. Lipid changes were similar in men and women except for lipoprotein(a) levels, which decreased only in men. Low-density lipoprotein particles were enriched with polyunsaturated fatty acids from walnuts, but their resistance to oxidation was preserved. Substituting walnuts for part of the monounsaturated fat in a cholesterol-lowering Mediterranean diet further reduced total and LDL cholesterol levels in men and women with hypercholesterolemia.

Fructose makes up a significant proportion of energy intake in westernized diets; its increased consumption has paralleled the growing prevalence of obesity and metabolic syndrome over the past two decades. In the current study, we demonstrate that fructose administration (10% wt/vol) in the drinking water of rats reduces the trans-activating and trans-repressing activity of the hepatic peroxisome proliferator-activated receptor α (PPARα). As a consequence, fructose decreases hepatic fatty oxidation and increases pro-inflammatory transcription factor nuclear factor κB (NF-κB) activity. These changes were not observed in glucose-administered rats (10% wt/vol), although both carbohydrates produced similar changes in plasma adiponectin and in the hepatic expression of transcription factors and enzymes involved in fatty acid synthesis. Fructose-fed, but not glucose-fed, rats were hyperleptinemic and exhibited increased tyrosine phosphorylation of the signal transducer and activator of transcription-3 (STAT-3) transcription factor, although they did not present a similar increase in the serine phosphorylation of nuclear STAT3. Thus, an impairment in the hepatic transduction of the leptin signal could be responsible for the observed alterations in PPARα activity in fructose-fed rats. Because PPARα activity is lower in human than in rodent liver, fructose ingestion in humans should cause even worse effects, which would partly explain the link between increased consumption of fructose and widening epidemics of obesity and metabolic syndrome. Conclusion : Hypertriglyceridemia and hepatic steatosis induced by fructose ingestion result from a reduction in the hepatic catabolism of fatty acids driven by a state of leptin resistance. (Hepatology 2007;45:778–788.)

Nuclear factor-κB (NF-κB) is a transcription factor induced by a wide range of stimuli, including hyperglycaemia and pro-inflammatory cytokines. It is associated with cardiac hypertrophy and heart failure. It was previously reported that the NF-κB-mediated inhibition of proliferator-activated receptor-γ coactivator-1α (PGC-1α) might explain the shift in glucose metabolism during cardiac pathological processes induced by pro-inflammatory stimuli, although the specific mechanisms remain to be elucidated. We addressed the specific mechanisms by which exposure to tumour necrosis factor-α (TNF-α) results in PGC-1α down-regulation in cardiac cells and, as a consequence, in the metabolic dysregulation that underlies heart dysfunction and failure. By using coimmunoprecipitation studies, we report for the first time that the p65 subunit of NF-κB is constitutively bound to PGC-1α in human cardiac cells and also in mouse heart, and that NF-κB activation by TNF-α exposure increases this binding. Overexpression and gene silencing analyses demonstrated that the main factor limiting the degree of this association is p65, because only the modulation of this protein modified the physical interaction. Our data show that the increased physical interaction between p65 and PGC-1α after NF-κB activation is responsible for the reduction in PGC-1α expression and subsequent dysregulation of glucose oxidation. On the basis of these data, we propose that p65 directly represses PGC-1α activity in cardiac cells, thereby leading to a reduction in pyruvate dehydrogenase kinase 4 (PDK4) expression and the subsequent increase in glucose oxidation observed during the proinflammatory state.

Inflammatory responses in the heart that are driven by sustained increases in cytokines have been associated with several pathological processes, including cardiac hypertrophy and heart failure. Emerging data suggest a link between cardiomyopathy and myocardial metabolism dysregulation. To further elucidate the relationship between a pro-inflammatory profile and cardiac metabolism dysregulation, a human cell line of cardiac origin, AC16, was treated with tumour necrosis factor-α (TNF-α). Exposure of AC16 cells to TNF-α inhibited the expression of peroxisome proliferator-activated receptor coactivator 1α (PGC-1α), an upstream regulator of lipid and glucose oxidative metabolism. Studies performed with cardiac-specific transgenic mice (Mus musculus) overexpressing TNF-α, which have been well characterized as a model of cytokine-induced cardiomyopathy, also displayed reduced PGC-1α expression in the heart compared with that of control mice. The mechanism by which TNF-α reduced PGC-1α expression in vitro appeared to be largely mediated via both p38 mitogen-activated protein kinase and nuclear factor-κB pathways. PGC-1α downregulation resulted in an increase in glucose oxidation rate, which involved a reduction in pyruvate dehydrogenase kinase 4 expression and depended on the DNA-binding activity of both peroxisome proliferator-activated receptor β/δ and estrogen-related receptor α transcription factors. These results point to PGC-1α downregulation as a potential contributor to cardiac dysfunction and heart failure in metabolic disorders with an inflammatory background.

The mechanisms responsible for increased expression of TNF-α in skeletal muscle cells in diabetic states are not well understood. We examined the effects of the saturated acid palmitate on TNF-α expression. Exposure of C2C12 skeletal muscle cells to 0.75 mm palmitate enhanced mRNA (25-fold induction, P < 0.001) and protein (2.5-fold induction) expression of the proinflammatory cytokine TNF-α. This induction was inversely correlated with a fall in GLUT4 mRNA levels (57% reduction, P < 0.001) and glucose uptake (34% reduction, P < 0.001). PD98059 and U0126, inhibitors of the ERK-MAPK cascade, partially prevented the palmitate-induced TNF-α expression. Palmitate increased nuclear factor (NF)-κB activation and incubation of the cells with the NF-κB inhibitors pyrrolidine dithiocarbamate and parthenolide partially prevented TNF-α expression. Incubation of palmitate-treated cells with calphostin C, a strong and specific inhibitor of protein kinase C (PKC), abolished palmitate-induced TNF-α expression, and restored GLUT4 mRNA levels. Palmitate treatment enhanced the expression of phospho-PKCθ, suggesting that this PKC isoform was involved in the changes reported, and coincubation of palmitate-treated cells with the PKC inhibitor chelerythrine prevented the palmitate-induced reduction in the expression of IκBα and insulin-stimulated Akt activation. These findings suggest that enhanced TNF-α expression and GLUT4 down-regulation caused by palmitate are mediated through the PKC activation, confirming that this enzyme may be a target for either the prevention or the treatment of fatty acid-induced insulin resistance.