Age-related loss of muscle protein may involve a decreased response to anabolic factors of muscle protein synthesis through dysregulation of translation factors. To verify this hypothesis, we simultaneously investigated muscle protein synthesis and expression of some factors implicated in insulin signal transduction during hyperinsulinemia and hyperaminoacidemia in 6 young (25+/-1 year; mean+/-sem) and 8 elderly subjects (72+/-2 year). Incorporation of L-[1-13C] leucine in muscle proteins (fractional synthesis rate, FSR) was measured in vastus lateralis, before and during a euglycemic hyperinsulinemic hyperaminoacidemic clamp, together with Western blot analysis of protein kinase B (PKB), mTOR, 4E-BP1, and S6K1 phosphorylation. In basal state, muscle protein FSR was reduced in elderly in comparison with young subjects (0.061+/-0.004% per hour) vs 0.082+/-0.010% per hour, elderly vs. young, P<0.05). During clamp, muscle protein FSR was stimulated in young (0.119+/-0.006% per hour; P<0.05), but this response was significantly lower in elderly subjects (0.084+/-0.005% per hour, P<0.05 vs young subjects). Phosphorylation of PKB, mTOR, and 4E-BP1 were similarly increased by insulin and amino acid in both groups, except for S6K1 phosphorylation, which was not stimulated in elderly subjects. In conclusion, 1) response of muscle protein synthesis to insulin and amino acid is impaired in elderly humans; 2) a defect in S6K1 pathway activation may be responsible for this alteration. This modification is a mechanistic basis of sarcopenia development during aging.

The primary gene mutated in Charcot-Marie-Tooth type 2A is mitofusin-2 (Mfn2). Mfn2 encodes a mitochondrial protein that participates in the maintenance of the mitochondrial network and that regulates mitochondrial metabolism and intracellular signaling. The potential for regulation of human Mfn2 gene expression in vivo is largely unknown. Based on the presence of mitochondrial dysfunction in insulin-resistant conditions, we have examined whether Mfn2 expression is dysregulated in skeletal muscle from obese or nonobese type 2 diabetic subjects, whether muscle Mfn2 expression is regulated by body weight loss, and the potential regulatory role of tumor necrosis factor (TNF)␣ or interleukin-6. We show that mRNA concentration of Mfn2 is decreased in skeletal muscle from both male and female obese subjects. Muscle Mfn2 expression was also reduced in lean or in obese type 2 diabetic patients. There was a strong negative correlation between the Mfn2 expression and the BMI in nondiabetic and type 2 diabetic subjects. A positive correlation between the Mfn2 expression and the insulin sensitivity was also detected in nondiabetic and type 2 diabetic subjects. To determine the effect of weight loss on Mfn2 mRNA expression, six morbidly obese subjects were subjected to weight loss by bilio-pancreatic diversion. Mean expression of muscle Mfn2 mRNA increased threefold after reduction in body weight, and a positive correlation between muscle Mfn2 expression and insulin sensitivity was again detected. In vitro experiments revealed an inhibitory effect of TNF␣ or interleukin-6 on Mfn2 expression in cultured cells. We conclude that body weight loss upregulates the expression of Mfn2 mRNA in skeletal muscle of obese humans, type 2 diabetes downregulates the expression of Mfn2 mRNA in skeletal muscle, Mfn2 expression in skeletal muscle is directly proportional to insulin sensitivity and is inversely proportional to the BMI, TNF␣ and interleukin-6 downregulate Mfn2 expression and may participate in the dysregulation of Mfn2 expression in obesity or type 2 diabetes, and the in vivo modulation of Mfn2 mRNA levels is an additional level of regulation for the control of muscle metabolism and could provide a molecular mechanism for alterations in mitochondrial function in obesity or type 2 diabetes. Diabetes 54: [2685][2686][2687][2688][2689][2690][2691][2692][2693] 2005

In young men ingesting protein meals, slowly digested proteins (caseins: CAS) induce a higher protein gain than those that are rapidly digested (whey proteins: WP). Our aim was to assess whether or not this is true in elderly men receiving mixed meals. The effects of meals containing either CAS or two different amounts of WP (WP-iN: isonitrogenous with CAS, or WP-iL: providing the same amount of leucine as CAS) on protein metabolism (assessed by combining oral and intravenous leucine tracers) were compared in nine healthy, elderly (mean ± S.E.M. age 72 ± 1 years) and six young men (24 ± 1 years). In both age groups, WP-iL and WP-iN were digested faster than CAS (P < 0.001, ANOVA). Proteolysis was inhibited similarly whatever the meal and age groups (P = NS). Protein synthesis was higher with WP-iN than with CAS or WP-iL (P < 0.01), irrespective of age (P = NS). An age-related effect (P < 0.05) was found with postprandial leucine balance. Leucine balance was higher with CAS than with WP-iL (P < 0.01) in young men, but not in elderly subjects (P = NS). In isonitrogenous conditions, leucine balance was higher with WP-iN than with CAS (P < 0.001) in both age groups, but the magnitude of the differences was higher in the elderly men (P = 0.05). In conclusion, during aging, protein gain was greater with WP (rapidly digested protein), and lower with CAS (slowly digested protein). This suggests that a 'fast' protein might be more beneficial than a 'slow' one to limit protein losses during aging.

It has long been recognized that numerous dietary parameters, such as the amount and type of protein and nonprotein energy sources, affect protein metabolism. More recently, we demonstrated that the protein digestion rate is an independent factor regulating postprandial protein gain. Indeed, in young men, using a non-steady-state approach and intrinsically labeled milk protein fractions [whey protein (WP) and casein (CAS)] we showed that a slow digested dietary protein (CAS) induced a greater protein gain than a fast one (WP). The mechanisms of this gain also differed according to the protein rate of digestion. WP stimulated amino acid oxidation and protein synthesis without modifying proteolysis, whereas CAS increased amino acid oxidation and protein synthesis to a lesser extent and strongly inhibited proteolysis. These results led to the concept of "slow" and "fast" protein and were confirmed by further experiments during which the meals tested presented different digestion rates but were otherwise identical in terms of amino acid profile. We also analyzed the effects of fat and carbohydrates added to CAS and WP. Our preliminary results suggest that added nonprotein energy sources to CAS and WP attenuated the differences in both the protein digestion rate and protein gain. Finally, and in contrast to young subjects, a "fast" protein may be more beneficial than a "slow" one in elderly subjects, to limit body protein loss. However, long-term studies are needed to confirm this age-related effect.

Obesity and aging are characterized by decreased insulin sensitivity (IS) and muscle protein synthesis. Intramuscular ceramide accumulation has been implicated in insulin resistance during obesity. We aimed to measure IS, muscle ceramide level, protein synthesis, and activation of intracellular signaling pathways involved in translation initiation in male Wistar young (YR, 6-month) and old (OR, 25-month) rats receiving a low- (LFD) or a high-fat diet (HFD) for 10 weeks. A corresponding cellular approach using C2C12 myotubes treated with palmitate to induce intracellular ceramide deposition was taken. A decreased ability of adipose tissue to store lipids together with a reduced adipocyte diameter and a development of fibrosis were observed in OR after the HFD. Consequently, OR fed the HFD were insulin resistant, showed a strong increase in intramuscular ceramide level and a decrease in muscle protein synthesis associated with increased eIF2α phosphorylation. The accumulation of intramuscular lipids placed a lipid burden on mitochondria and created a disconnect between metabolic and regulating pathways in skeletal muscles of OR. In C2C12 cells, palmitate-induced ceramide accumulation was associated with a decreased protein synthesis together with upregulated eIF2α phosphorylation. In conclusion, a reduced ability to expand adipose tissues was found in OR, reflecting a lower lipid buffering capacity. Muscle mitochondrial activity was affected in OR conferring a reduced ability to oxidize fatty acids entering the muscle cell. Hence, OR were more prone to ectopic muscle lipid accumulation than YR, leading to decreased muscle protein anabolism. This metabolic change is a potential therapeutic target to counter sarcopenic obesity.

The obesity-related impairment of protein metabolism is characterized by 1) a reduced turnover rate of skeletal muscle proteins in PA; 2) a lack of stimulation of mitochondrial protein synthesis by insulin and amino acid; and 3) a lower inhibition of WB proteolysis by insulin and amino acid. Alterations of selective muscle protein kinetics may predispose obese subjects to muscle metabolic dysfunction leading to type 2 diabetes.

Two species of Listeria are pathogenic; L. monocytogenes infects humans and animals, and L. ivanovii has been considered to infect ruminants only. We report L. ivanovii–associated gastroenteritis and bacteremia in a man. This isolate was indistinguishable from prototypic ruminant strains. L. ivanovii is thus an enteric opportunistic human pathogen.

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