Aline Couturier

Regular endurance exercise improves the diminished hepatic carnitine status in mice fed a high-fat diet

SCOPE Metabolic stress induced by chronic high-fat (HF) diet feeding or genetically induced diabetes impairs carnitine status. Herein, we tested the hypothesis that regular endurance exercise (EE) improves the HF diet-induced impairment of carnitine status through stimulating the expression of hepatic genes involved in carnitine synthesis and uptake. METHODS AND RESULTS Eighteen male C57BL/6 mice were assigned to three groups: group S received a standard diet, group HF received a HF diet, and group HF+EE received an HF diet and was regularly exercised on a treadmill. After 10 wk, mice of the HF and the HF+EE groups were highly obese and insulin resistant compared with mice of the S group (p<0.05), but mice of the HF+EE group were less insulin resistant than those of the HF group (p<0.05). The HF group had lower carnitine concentrations and mRNA and protein levels of genes involved in carnitine synthesis and uptake in the liver than the S group (p<0.05), whereas these parameters did not differ between the S group and the HF+EE group. CONCLUSION These findings indicate that regular EE reverses an HF diet-induced impairment of hepatic carnitine content by stimulating hepatic carnitine synthesis and uptake.

Supplementation of carnitine leads to an activation of the IGF-1/PI3K/Akt signalling pathway and down regulates the E3 ligase MuRF1 in skeletal muscle of rats

BackgroundRecently, it has been shown that carnitine down-regulates genes involved in the ubiquitin-proteasome system (UPS) in muscle of pigs and rats. The mechanisms underlying this observation are yet unknown. Based on the previous finding that carnitine increases plasma IGF-1 concentration, we investigated the hypothesis that carnitine down-regulates genes of the UPS by modulation of the of the IGF-1/PI3K/Akt signalling pathway which is an important regulator of UPS activity in muscle.MethodsMale Sprague–Dawley rats, aged four weeks, were fed either a control diet with a low native carnitine concentration or the same diet supplemented with carnitine (1250 mg/kg diet) for four weeks. Components of the UPS and IGF-1/PI3K/Akt signalling pathway in skeletal muscle were examined.ResultsRats fed the diet supplemented with carnitine had lower mRNA and protein levels of MuRF1, the most important E3 ubiquitin ligase in muscle, decreased concentrations of ubiquitin-protein conjugates in skeletal muscle and higher IGF-1 concentration in plasma than control rats (P < 0.05). Moreover, in skeletal muscle of rats fed the diet supplemented with carnitine there was an activation of the PI3K/Akt signalling pathway, as indicated by increased protein levels of phosphorylated (activated) Akt1 (P < 0.05).ConclusionThe present study shows that supplementation of carnitine markedly decreases the expression of MuRF1 and concentrations of ubiquitinated proteins in skeletal muscle of rats, indicating a diminished degradation of myofibrillar proteins by the UPS. The study moreover shows that supplementation of carnitine leads to an activation of the IGF-1/PI3K/Akt signalling pathway which in turn might contribute to the observed down-regulation of MuRF1 and muscle protein ubiquitination.

Treatment with pharmacological PPARα agonists stimulates the ubiquitin proteasome pathway and myofibrillar protein breakdown in skeletal muscle of rodents.

BACKGROUND Treatment of hyperlipidemic patients with fibrates, agonists of peroxisome proliferator-activated receptor α (PPARα), provokes muscle atrophy as a side effect. The molecular mechanism underlying this phenomenon is still unknown. We tested the hypothesis that activation of PPARα leads to an up-regulation of the ubiquitin proteasome system (UPS) which plays a major role in protein degradation in muscle. METHODS Rats, wild-type and PPARα-deficient mice (PPARα(-/-)) were treated with synthetic PPARα agonists (clofibrate, WY-14,643) to study their effect on the UPS and myofibrillar protein breakdown in muscle. RESULTS In rats and wild-type mice but not PPARα(-/-) mice, clofibrate or WY-14,643 caused increases in mRNA and protein levels of the ubiquitin ligases atrogin-1 and MuRF1 in muscle. Wild-type mice treated with WY-14,643 had a greater 3-methylhistidine release from incubated muscle and lesser muscle weights. In addition, wild-type mice but not PPARα(-/-) mice treated with WY-14,643 had higher amounts of ubiquitin-protein conjugates, a decreased activity of PI3K/Akt1 signalling, and an increased activity of FoxO1 transcription factor in muscle. Reporter gene and gel shift experiments revealed that the atrogin-1 and MuRF1 promoter do not contain functional PPARα DNA-binding sites. CONCLUSIONS These findings indicate that fibrates stimulate ubiquitination of proteins in skeletal muscle which in turn stimulates protein degradation. Up-regulation of ubiquitin ligases is probably not mediated by PPARα-dependent gene transcription but by PPARα-dependent inhibition of the PI3K/Akt1 signalling pathway leading to activation of FoxO1. GENERAL SIGNIFICANCE PPARα plays a role in the regulation of the ubiquitin proteasome system.

Endurance and Resistance Training Affect High Fat Diet-Induced Increase of Ceramides, Inflammasome Expression, and Systemic Inflammation in Mice

The study aimed to investigate the effects of differentiated exercise regimes on high fat-induced metabolic and inflammatory pathways. Mice were fed a standard diet (ST) or a high fat diet (HFD) and subjected to regular endurance training (ET) or resistance training (RT). After 10 weeks body weight, glucose tolerance, fatty acids (FAs), circulating ceramides, cytokines, and immunological mediators were determined. The HFD induced a significant increase in body weight and a disturbed glucose tolerance (p < 0.05). An increase of plasma FA, ceramides, and inflammatory mediators in adipose tissue and serum was found (p < 0.05). Both endurance and resistance training decreased body weight (p < 0.05) and reduced serum ceramides (p < 0.005). While RT attenuated the increase of NLRP-3 (RT) expression in adipose tissue, ET was effective in reducing TNF-α and IL-18 expression. Furthermore, ET reduced levels of MIP-1γ, while RT decreased levels of IL-18, MIP-1γ, Timp-1, and CD40 in serum (p < 0.001), respectively. Although both exercise regimes improved glucose tolerance (p < 0.001), ET was more effective than RT. These results suggest that exercise improves HFD-induced complications possibly through a reduction of ceramides, the reduction of inflammasome activation in adipose tissues, and a systemic downregulation of inflammatory cytokines.

Niacin in pharmacological doses alters microRNA expression in skeletal muscle of obese Zucker rats.

Administration of pharmacological niacin doses was recently reported to have pronounced effects on skeletal muscle gene expression and phenotype in obese Zucker rats, with the molecular mechanisms underlying the alteration of gene expression being completely unknown. Since miRNAs have been shown to play a critical role for gene expression through inducing miRNA-mRNA interactions which results in the degradation of specific mRNAs or the repression of protein translation, we herein aimed to investigate the influence of niacin at pharmacological doses on the miRNA expression profile in skeletal muscle of obese Zucker rats fed either a control diet with 30 mg supplemented niacin/kg diet or a high-niacin diet with 780 mg supplemented niacin/kg diet for 4 wk. miRNA microarray analysis revealed that 42 out of a total of 259 miRNAs were differentially expressed (adjusted P-value <0.05), 20 being down-regulated and 22 being up-regulated, between the niacin group and the control group. Using a biostatistics approach, we could demonstrate that the most strongly up-regulated (log2 ratio ≥0.5) and down-regulated (log2 ratio ≤−0.5) miRNAs target approximately 1,800 mRNAs. Gene-term enrichment analysis showed that many of the predicted target mRNAs from the most strongly regulated miRNAs were involved in molecular processes dealing with gene transcription such as DNA binding, transcription regulator activity, transcription factor binding and in important regulatory pathways such as Wnt signaling and MAPK signaling. In conclusion, the present study shows for the first time that pharmacological niacin doses alter the expression of miRNAs in skeletal muscle of obese Zucker rats and that the niacin-regulated miRNAs target a large set of genes and pathways which are involved in gene regulatory activity indicating that at least some of the recently reported effects of niacin on skeletal muscle gene expression and phenotype in obese Zucker rats are mediated through miRNA-mRNA interactions.

Ingestion of frying fat leads to activation of the endoplasmic reticulum stress-induced unfolded protein response in the duodenal mucosa of pigs

SCOPE Ingestion of oxidized fats (OF) causes activation of stress signaling pathways such as nuclear factor-erythroid 2-related factor 2 (Nrf2) in the intestine. Activation of this pathway is mediated by ER stress-induced unfolded protein response (UPR). Herein, we hypothesized that ingestion of OF causes ER stress-induced UPR in duodenal mucosa of pigs. METHODS AND RESULTS Six-wk-old cross-bred pigs received either a control diet with fresh fat or a diet with OF for 29 days. Pigs fed OF exhibited increased phosphorylation of the ER stress downstream target eukaryotic translation initiation factor 2α and induction of several genes involved in ER stress-induced UPR and Nrf2 target genes in duodenal mucosa. No signs of an impaired intestinal barrier function or a systemic inflammatory response could be found in pigs fed OF. CONCLUSION Activation of ER stress-induced UPR by OF in the duodenal mucosa in pigs might be interpreted as a cytoprotective response to stress factors associated with ingestion of OF. Given the similarities between pigs and humans with respect to digestive physiology, it is possible that regular ingestion of fried foods containing OF activates the ER stress-induced UPR also in the intestinal mucosa of humans.

Dietary Fish Oil Inhibits Pro-Inflammatory and ER Stress Signalling Pathways in the Liver of Sows during Lactation

Lactating sows have been shown to develop typical signs of an inflammatory condition in the liver during the transition from pregnancy to lactation. Hepatic inflammation is considered critical due to the induction of an acute phase response and the activation of stress signaling pathways like the endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR), both of which impair animal´s health and performance. Whether ER stress-induced UPR is also activated in the liver of lactating sows and whether dietary fish oil as a source of anti-inflammatory effects n-3 PUFA is able to attenuate hepatic inflammation and ER stress-induced UPR in the liver of sows is currently unknown. Based on this, two experiments with lactating sows were performed. The first experiment revealed that ER stress-induced UPR occurs also in the liver of sows during lactation. This was evident from the up-regulation of a set of genes regulated by the UPR and numerically increased phosphorylation of the ER stress-transducer PERK and PERK-mediated phosphorylation of eIF2α and IκB. The second experiment showed that fish oil inhibits ER stress-induced UPR in the liver of lactating sows. This was demonstrated by decreased mRNA levels of a number of UPR-regulated genes and reduced phosphorylation of PERK and PERK-mediated phosphorylation of eIF2α and IκB in the liver of the fish oil group. The mRNA levels of various nuclear factor-κB-regulated genes encoding inflammatory mediators and acute phase proteins in the liver of lactating sows were also reduced in the fish oil group. In line with this, the plasma levels of acute phase proteins were reduced in the fish oil group, although differences to the control group were not significant. In conclusion, ER stress-induced UPR is present in the liver of lactating sows and fish oil is able to inhibit inflammatory signaling pathways and ER stress-induced UPR in the liver.