Is skeletal muscle mitochondrial dysfunction a cause or an indirect consequence of insulin resistance in humans?

The precise cause of insulin resistance and type 2 diabetes is unknown. However, there is a strong association between insulin resistance and lipid accumulation — and, in particular, lipotoxic fatty acid metabolites — in insulin-target tissues. Such accumulation is known to cause insulin resistance, particularly in skeletal muscle, by reducing insulin-stimulated glucose uptake. Reduced fat-oxidation capacity appears to cause such lipid accumulation and, over the past few years, many studies have concluded that decreased mitochondrial oxidative phosphorylation could be the initiating cause of lipid deposition and the development of insulin resistance. The aim of this review is to summarize the latest findings regarding the link between skeletal muscle mitochondrial dysfunction and insulin resistance in humans. At present, there are too few studies to definitively conclude that, in this context, mitochondria are functionally impaired (dysfunction in the respiratory chain). Indeed, insulin resistance could also be related to a decrease in the number of mitochondria or to a combination of this and mitochondrial dysfunction. Finally, we also consider whether or not these aberrations could be the cause of the development of the disease or whether mitochondrial dysfunction may simply be the consequence of an insulin-resistant state

Fatty liver and insulin resistance in obese Zucker rats: No role for mitochondrial dysfunction

The relationship between insulin resistance and mitochondrial function is of increasing interest. Studies looking for such interactions are usually made in muscle and only a few studies have been done in liver, which is known to be a crucial partner in whole body insulin action. Recent studies have revealed a similar mechanism to that of muscle for fat-induced insulin resistance in liver. However, the exact mechanism of lipid metabolites accumulation in liver leading to insulin resistance is far from being elucidated. One of the hypothetical mechanisms for liver steatosis development is an impairment of mitochondrial function. We examined mitochondrial function in fatty liver and insulin resistance state using isolated mitochondria from obese Zucker rats. We determined the relationship between ATP synthesis and oxygen consumption as well as the relationship between mitochondrial membrane potential and oxygen consumption. In order to evaluate the quantity of mitochondria and the oxidative capacity we measured citrate synthase and cytochrome c oxidase activities. Results showed that despite significant fatty liver and hyperinsulinemia, isolated liver mitochondria from obese Zucker rats display no difference in oxygen consumption, ATP synthesis, and membrane potential compared with lean Zucker rats. There was no difference in citrate synthase and cytochrome c oxidase activities between obese and lean Zucker rats in isolated mitochondria as well as in liver homogenate, indicating a similar relative amount of hepatic mitochondria and a similar oxidative capacity. Adiponectin, which is involved in bioenergetic homeostasis, was increased two-fold in obese Zucker rats despite insulin resistance. In conclusion, isolated liver mitochondria from lean and obese insulin-resistant Zucker rats showed strictly the same mitochondrial function. It remains to be elucidated whether adiponectin increase is involved in these results

Effects of the cannabinoid CB1 antagonist rimonabant on hepatic mitochondrial function in rats fed a high-fat diet

The aim of this study was to investigate the effect of rimonabant treatment on hepatic mitochondrial function in rats fed a high-fat diet. Sprague-Dawley rats fed a high-fat diet (35% lard) for 13 wk were treated with rimonabant (10 mg·kg−1·day−1) during the last 3 wk and matched with pair-fed controls. Oxygen consumption with various substrates, mitochondrial enzyme activities on isolated liver mitochondria, and mitochondrial DNA quantity were determined. Body weight and fat mass were decreased in rats treated with rimonabant compared with pair-fed controls. Moreover, the serum adiponectin level was increased with rimonabant. Hepatic triglyceride content was increased, while serum triglycerides were decreased. An increase of mitochondrial respiration was observed in rats treated with rimonabant. The increase of mitochondrial respiration with palmitoyl-CoA compared with respiration with palmitoyl-l-carnitine stating that the entry of fatty acids into mitochondria via carnitine palmitoyltransferase I was increased in rats treated with rimonabant. Moreover, rimonabant treatment led to a reduction in the enzymatic activity of ATP synthase, whereas the quantity of mitochondrial DNA and the activity of citrate synthase remained unchanged. To summarize, rimonabant treatment leads to an improvement of hepatic mitochondrial function by increasing substrate oxidation and fatty acid entry into mitochondria for the β-oxidation pathway and by increasing proton leak. However, this increase of mitochondrial oxidation is regulated by a decrease of ATP synthase activity in order to have only ATP required for the cell function

Dynamic regulation of mitochondrial network and oxidative functions during 3T3-L1 fat cell differentiation

Mitochondria have been shown to be impaired in insulin resistance-related diseases but have not been extensively studied during the first steps of adipose cell development. This study was designed to determine the sequence of changes of the mitochondrial network and function during the first days of adipogenesis. 3T3-L1 preadipocytes were differentiated into adipocytes without using glitazone compounds. At days 0, 3, 6, 9, and 12, mitochondrial network imaging, mitochondrial oxygen consumption, membrane potential, and oxidative phosphorylation efficiency were assessed in permeabilized cells. Gene and protein expressions related to fatty acid metabolism and mitochondrial network were also determined. Compared to preadipocytes (day 0), new adipocytes (days 6 and 9) displayed profound changes of their mitochondrial network that underwent fragmentation and redistribution around lipid droplets. Drp1 and mitofusin 2 displayed a progressive increase in their gene expression and protein content during the first 9 days of differentiation. In parallel with the mitochondrial network redistribution, mitochondria switched to uncoupled respiration with a tendency towards decreased membrane potential, with no variation of mtTFA and NRF1 gene expression. The expression of PGC1α and NRF2 genes and genes involved in lipid oxidation (UCP2, CD36, and CPT1) was increased. Reactive oxygen species (ROS) production displayed a nadir at day 6 with a concomitant increase in antioxidant enzyme gene expression. This 3T3-L1-based in vitro model of adipogenesis showed that mitochondria adapted to the increased number of lipid droplets by network redistribution and uncoupling respiration. The timing and regulation of lipid oxidation-associated ROS production appeared to play an important role in these changes

MRI versus histological methods for time course monitoring of steatosis amount in a murine model of NAFLD

International audiencePURPOSE: Hepatic steatosis is an increasingly frequent disease with potentially severe complications. A simple quantification method is required for pretherapeutic studies to allow steatosis monitoring. This study aimed at evaluating steatosis quantification via a standard 1.5T MRI machine in a murine model.MATERIALS AND METHODS: Eleven groups of two rats received a choline methionine deficient diet. MRI was performed at days 0, 2, 4, 5, 6, 7 and 8, and weeks 2, 3, 4 and 5. A phased array surface coil system was used to acquire a GE T1 in- and out-of-phase multi-echo sequence, with neither cardiac nor respiratory synchronization. Steatosis was calculated with the 3-echoes method. Histological quantifications were performed both by optical analysis (percentage of fatty hepatocytes) and by automated measurement of the area of steatosis (AOS). The reference was total intrahepatic triglycerides (TIT). Protocol was approved by the ethic committee.RESULTS: Steatosis without inflammation, increasing with diet duration, was obtained. MRI provided better agreement (intraclass correlation coefficient) with TIT (0.889, p&lt;0.001) than did AOS (0.629, p=0.001) or optical analysis (0.280, p=0.098). MRI permitted closer monitoring of TIT over time than did AOS or optical analysis. By multivariate analysis, MRI was an independent predictor of TIT on first step and ALT on second step. A model combining these 2 variables provided excellent agreement with TIT (0.953, p&lt;0.001) and permitted excellent monitoring of steatosis over time.CONCLUSION: MRI is reliable, easy, fast and superior to histological techniques for the assessment of hepatic steatosis in a murine model.</p

Fatty acid flux and oxidation are increased by rimonabant in obese women.

This study aimed to determine in obese women if endocannabinoid receptor antagonism has effects on fatty acid and triglyceride metabolism and insulin sensitivity which are independent from the metabolic effects of weight loss. Fourteen obese (BMI=33.0±0.5 kg/m(2)) (mean±SEM) Caucasian post-menopausal women, aged 57.8±4.7 years were studied. The women were randomised to 2 groups, one group received the endocannabinoid receptor antagonist rimonabant (20 mg/d) for 12 weeks. A control group achieved the same weight loss by a hypocaloric dietary intervention over 12 weeks. Palmitate production rate (Ra), a measure of lipolysis, and palmitate oxidation rate, and VLDL(1) and VLDL(2) triglyceride (TG) kinetics, were measured using isotopic tracers before and after the intervention. Weight loss was not different in the 2 groups; 2.6±0.5 kg with rimonabant and 3.1±1.0 kg in the control group. Palmitate Ra increased with rimonabant with no change in the control group (p=0.03 between groups). Palmitate oxidation rate increased with rimonabant but decreased in the control group (p=0.005 between groups). VLDL(1) TG secretion rate decreased in the control group and increased in the rimonabant group (p=0.008 between groups). There was no significant effect on insulin sensitivity. This study suggests that endocannabinoid receptor antagonism for 12 weeks in obese women increased lipolysis and fatty acid oxidation. The increase in VLDL(1) TG secretion rate may be due to the increase in lipolysis which exceeded the increase in fatty acid oxidation