Gilles Simard

OPA1 R445H mutation in optic atrophy associated with sensorineural deafness

The heterozygous R445H mutation in OPA1 was found in five patients with optic atrophy and deafness. Audiometry suggested that the sensorineural deafness resulted from auditory neuropathy. Skin fibroblasts showed hyperfragmentation of the mitochondrial network, decreased mitochondrial membrane potential, and adenosine triphosphate synthesis defect. In addition, OPA1 was found to be widely expressed in the sensory and neural cochlear cells of the guinea pig. Thus, optic atrophy and deafness may be related to energy defects due to a fragmented mitochondrial network. Ann Neurol 2005

Oxidative Stress and Metabolic Pathologies: From an Adipocentric Point of View

Oxidative stress plays a pathological role in the development of various diseases including diabetes, atherosclerosis, or cancer. Systemic oxidative stress results from an imbalance between oxidants derivatives production and antioxidants defenses. Reactive oxygen species (ROS) are generally considered to be detrimental for health. However, evidences have been provided that they can act as second messengers in adaptative responses to stress. Obesity represents a major risk factor for deleterious associated pathologies such as type 2 diabetes, liver, and coronary heart diseases. Many evidences regarding obesity-induced oxidative stress accumulated over the past few years based on established correlations of biomarkers or end-products of free-radical-mediated oxidative stress with body mass index. The hypothesis that oxidative stress plays a significant role in the development of metabolic disorders, especially insulin-resistance state, is supported by several studies where treatments reducing ROS production reverse metabolic alterations, notably through improvement of insulin sensitivity, hyperlipidemia, or hepatic steatosis. In this review, we will develop the mechanistic links between oxidative stress generated by adipose tissue in the context of obesity and its impact on metabolic complications development. We will also attempt to discuss potential therapeutic approaches targeting obesity-associated oxidative stress in order to prevent associated-metabolic complications.

Behaviour of phospholipase-modified HDL towards cultured hepatocytes. I. Enhanced transfers of HDL sterols and apoproteins.

Human HDL subfractions (HDL2, HDL3, or HDL separated by heparin affinity chromatography) were labelled either on their apolipoprotein moiety with 125I or on their sterols: unesterified [14C]cholesterol and [3H]cholesteryl linoleyl ether, a non-hydrolysable analog of esterified cholesterol. HDL subfractions were then treated with or without phospholipase A2 from Crotalus adamanteus in presence of albumin leading to a 72-82% phosphatidylcholine degradation. Control and treated HDL were reisolated and then addressed to cultured rat hepatocytes. (A) During incubations, unesterified [14C]cholesterol from HDL3 readily appeared in hepatocytes. The specific uptake of HDL esterified cholesterol calculated from [3H]cholesteryl ether was 2-4-times less important. Uptake of HDL cholesterol tended to saturate at 150-200 micrograms/ml HDL protein. A prior phospholipase treatment of HDL3 stimulated by 2-5-fold the uptake of [3H]cholesteryl ether, whereas the transfer of free [14C]cholesterol was minimally increased. The uptake of 3H/14C-labelled sterols from HDL2 was 2-3-times higher than from HDL3. (B) Parallel experiments were conducted with 125I-labelled HDL subfractions. At 37 degrees C, the specific uptake and degradation of HDL3 125I-apolipoprotein were about 2-fold enhanced following treatment of HDL3 with phospholipase A2. Uptakes of apolipoprotein and of esterified cholesterol were compared, indicating a preferential delivery of the sterol over apoprotein (X5). The dissociation was still more pronounced with phospholipase-treated HDL3. Competition experiments showed that 12-times more unlabelled HDL3 were required to half reduce the uptake of HDL3 [3H]cholesteryl ether than to impede similarly the HDL 125I-apolipoprotein recovered in cells. Uptake of 125I-labelled apolipoprotein from HDL2 was quantitatively comparable to that from HDL3

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.

Mitochondrial energy metabolism in a model of undernutrition induced by dexamethasone

The present investigation was undertaken to evaluate whether mitochondrial energy metabolism is altered in a model of malnutrition induced by dexamethasone (DEX) treatment (1.5 mg/kg per d for 5 d). The gastrocnemius and liver mitochondria were isolated from DEX-treated, pair-fed (PF) and control (CON) rats. Body weight was reduced significantly more in the DEX-treated group (-16%) than in the PF group (-9%). DEX treatment increased liver mass (+59% v. PF, +23% v. CON) and decreased gastrocnemius mass. Moreover, in DEX-treated rats, liver mitochondria had an increased rate of non-phosphorylative O2 consumption with all substrates (approximately +42%). There was no difference in enzymatic complex activities in liver mitochondria between rat groups. Collectively, these results suggest an increased proton leak and/or redox slipping in the liver mitochondria of DEX-treated rats. In addition, DEX decreased the thermodynamic coupling and efficiency of oxidative phosphorylation. We therefore suggest that this increase in the proton leak and/or redox slip in the liver is responsible for the decrease in the thermodynamic efficiency of energy conversion. In contrast, none of the variables of energy metabolism determined in gastrocnemius mitochondria was altered by DEX treatment. Therefore, it appears that DEX specifically affects mitochondrial energy metabolism in the liver.

Kinetics and control of oxidative phosphorylation in rat liver mitochondria after dexamethasone treatment.

The present investigation was undertaken in order to evaluate the contributions of ATP synthesis and proton leak reactions to the rate of active respiration of liver mitochondria, which is altered following dexamethasone treatment (1.5 mg/kg per day for 5 days). We applied top-down metabolic control analysis and its extension, elasticity analysis, to gain insight into the mechanisms of glucocorticoid regulation of mitochondrial bioenergetics. Liver mitochondria were isolated from dexamethasone-treated, pair-fed and control rats when in a fed or overnight fasted state. Injection of dexamethasone for 5 days resulted in an increase in the fraction of the proton cycle of phosphorylating liver mitochondria, which was associated with a decrease in the efficiency of the mitochondrial oxidative phosphorylation process in liver. This increase in proton leak activity occurred with little change in the mitochondrial membrane potential, despite a significant decrease in the rate of oxidative phosphorylation. Regulation analysis indicates that mitochondrial membrane potential homoeostasis is achieved by equal inhibition of the mitochondrial substrate oxidation and phosphorylation reactions in rats given dexamethasone. Our results also suggest that active liver mitochondria from dexamethasone-treated rats are capable of maintaining phosphorylation flux for cellular purposes, despite an increase in the energetic cost of mitochondrial ATP production due to increased basal proton permeability of the inner membrane. They also provide a complete description of the effects of dexamethasone treatment on liver mitochondrial bioenergetics.

Dexamethasone treatment specifically increases the basal proton conductance of rat liver mitochondria

We investigated the role that mitochondrial proton leak may play in the glucocorticoid‐induced hypermetabolic state. Sprague–Dawley rats were injected with dexamethasone over a period of 5 days. Liver mitochondria and gastrocnemius subsarcolemmal and intermyofibrillar mitochondria were isolated from dexamethasone‐treated, pair‐fed and control rats. Respiration and membrane potential were measured simultaneously using electrodes sensitive to oxygen and to the potential‐dependent probe triphenylmethylphosphonium, respectively. Five days of dexamethasone injection resulted in a marked increase in the basal proton conductance of liver mitochondria, but not in the muscle mitochondrial populations. This effect would have a modest impact on energy expenditure in rats.

Systems biology of antioxidants.

Understanding the role of oxidative injury will allow for therapy with agents that scavenge ROS (reactive oxygen species) and antioxidants in the management of several diseases related to free radical damage. The majority of free radicals are generated by mitochondria as a consequence of the mitochondrial cycle, whereas free radical accumulation is limited by the action of a variety of antioxidant processes that reside in every cell. In the present review, we provide an overview of the mitochondrial generation of ROS and discuss the role of ROS in the regulation of endothelial and adipocyte function. Moreover, we also discuss recent findings on the role of ROS in sepsis, cerebral ataxia and stroke. These results provide avenues for the therapeutic potential of antioxidants in a variety of diseases.

Cancer cachexia: Measured and predicted resting energy expenditures for nutritional needs evaluation

OBJECTIVE Cancer cachexia is associated with weight loss, poor nutritional status, and systemic inflammation. Accurate nutritional support for patients is calculated on resting energy expenditure (REE) measurement or prediction. The present study evaluated the agreement between measured and predicted REE (mREE and pREE, respectively) and the influence of acute phase response (APR) on REE. METHODS Thirty-six patients with cancer were divided into weight-stable (WS; weight loss <2%) and weight-losing (WL; weight loss >5%) patients. Measured REE was measured by indirect calorimetry and adjusted for fat-free mass (FFM). The Bland-Altman approach was used to assess the agreement between mREE and pREE from the Harris-Benedict equations (HBE). Blood levels of C-reactive protein were assessed. RESULTS There was no difference in mREE between groups (WS 1677 +/- 273, WL 1521 +/- 305) even when mREE was adjusted for FFM (WS 1609 +/- 53, WL 1589 +/- 53). In WL patients, FFM-adjusted REE correlated with blood C-reactive protein levels (r = 0.471, P = 0.048). HBEs tend to underestimate REE in both groups. CONCLUSION WL and WS patients with cancer had similar REEs but were different in terms of APR. APR could contribute to weight loss through enhancing REE. In a clinical context, HBE was in poor agreement with mREE in both groups.

Propionyl-L-carnitine Corrects Metabolic and Cardiovascular Alterations in Diet-Induced Obese Mice and Improves Liver Respiratory Chain Activity

Aims Obesity is a primary contributor to acquired insulin resistance leading to the development of type 2 diabetes and cardiovascular alterations. The carnitine derivate, propionyl-L-carnitine (PLC), plays a key role in energy control. Our aim was to evaluate metabolic and cardiovascular effects of PLC in diet-induced obese mice. Methods C57BL/6 mice were fed a high-fat diet for 9 weeks and then divided into two groups, receiving either free- (vehicle-HF) or PLC-supplemented water (200 mg/kg/day) during 4 additional weeks. Standard diet-fed animals were used as lean controls (vehicle-ST). Body weight and food intake were monitored. Glucose and insulin tolerance tests were assessed, as well as the HOMAIR, the serum lipid profile, the hepatic and muscular mitochondrial activity and the tissue nitric oxide (NO) liberation. Systolic blood pressure, cardiac and endothelial functions were also evaluated. Results Vehicle-HF displayed a greater increase of body weight compared to vehicle-ST that was completely reversed by PLC treatment without affecting food intake. PLC improved the insulin-resistant state and reversed the increased total cholesterol but not the increase in free fatty acid, triglyceride and HDL/LDL ratio induced by high-fat diet. Vehicle-HF exhibited a reduced cardiac output/body weight ratio, endothelial dysfunction and tissue decrease of NO production, all of them being improved by PLC treatment. Finally, the decrease of hepatic mitochondrial activity by high-fat diet was reversed by PLC. Conclusions Oral administration of PLC improves the insulin-resistant state developed by obese animals and decreases the cardiovascular risk associated to this metabolic alteration probably via correction of mitochondrial function.