Carnitine supplementation in high-fat diet-fed rats does not ameliorate lipid-induced skeletal muscle mitochondrial dysfunction in vivo

Experimental models of lipid overload and their relevance in understanding skeletal muscle insulin resistance and pathological changes in mitochondrial oxidative capacity

It remains essential to decipher some of the pathological mechanisms that link obesity with deteriorating human health. Insulin resistance, due to enhanced free fatty acid substrate delivery, results in disrupted glucose homeostasis and altered mitochondrial oxidative capacity, which is a characteristic feature of an obese state. In fact, as a major site for regulating glucose homeostasis and energy production in response to insulin, the skeletal muscle has become an interesting target tissue to understand the impact of lipid overload on the development of insulin resistance and impaired mitochondrial respiratory function. In addition to systematically retrieving the discussed data, the current review brings an essential perspective in understanding the relevance of experimental models of lipid overload such as high fat diets in understanding the pathological link between insulin resistance and pathological changes in mitochondrial oxidative capacity. Importantly, inclusion of evidence from transgenic model highlights some of the unique molecular targets that are implicated in the development of insulin resistance and inefficient mitochondrial respiration processes within an obese state. Importantly, saturation with lipid products such as ceramides and diacylglycerols, especially within the skeletal muscle, appears to be instrumental in paving the path leading to worsening of metabolic complications. These metabolic consequences mostly interfere with the efficiency of the mitochondrial electron transport chain, leading to overproduction of toxic reactive oxygen species. Therefore, therapeutic agents that reverse the effects of lipid overload by improving insulin sensitivity and mitochondrial oxidative capacity are crucial for the management or even treatment of metabolic diseases.

L-carnitine infusion does not alleviate lipid-induced insulin resistance and metabolic inflexibility

BACKGROUND

Low carnitine status may underlie the development of insulin resistance and metabolic inflexibility. Intravenous lipid infusion elevates plasma free fatty acid (FFA) concentration and is a model for simulating insulin resistance and metabolic inflexibility in healthy, insulin sensitive volunteers. Here, we hypothesized that co-infusion of L-carnitine may alleviate lipid-induced insulin resistance and metabolic inflexibility.

METHODS

In a randomized crossover trial, eight young healthy volunteers underwent hyperinsulinemic-euglycemic clamps (40mU/m2/min) with simultaneous infusion of saline (CON), Intralipid (20%, 90mL/h) (LIPID), or Intralipid (20%, 90mL/h) combined with L-carnitine infusion (28mg/kg) (LIPID+CAR). Ten volunteers were randomized for the intervention arms (CON, LIPID and LIPID+CAR), but two dropped-out during the study. Therefore, eight volunteers participated in all three intervention arms and were included for analysis.

RESULTS

L-carnitine infusion elevated plasma free carnitine availability and resulted in a more pronounced increase in plasma acetylcarnitine, short-, medium-, and long-chain acylcarnitines compared to lipid infusion, however no differences in skeletal muscle free carnitine or acetylcarnitine were found. Peripheral insulin sensitivity and metabolic flexibility were blunted upon lipid infusion compared to CON but L-carnitine infusion did not alleviate this.

CONCLUSION

Acute L-carnitine infusion could not alleviated lipid-induced insulin resistance and metabolic inflexibility and did not alter skeletal muscle carnitine availability. Possibly, lipid-induced insulin resistance may also have affected carnitine uptake and may have blunted the insulin-induced carnitine storage in muscle. Future studies are needed to investigate this.

Novel Markers of the Metabolic Impact of Exogenous Retinoic Acid with A Focus on Acylcarnitines and Amino Acids

Treatment with all-trans retinoic acid (ATRA), the carboxylic form of vitamin A, lowers body weight in rodents by promoting oxidative metabolism in multiple tissues including white and brown adipose tissues. We aimed to identify novel markers of the metabolic impact of ATRA through targeted blood metabolomics analyses, with a focus on acylcarnitines and amino acids. Blood was obtained from mice treated with a high ATRA dose (50 mg/kg body weight/day, subcutaneous injection) or placebo (controls) during the 4 days preceding collection. LC-MS/MS analyses with a focus on acylcarnitines and amino acids were conducted on plasma and PBMC. Main results showed that, relative to controls, ATRA-treated mice had in plasma: increased levels of carnitine, acetylcarnitine, and longer acylcarnitine species; decreased levels of citrulline, and increased global arginine bioavailability ratio for nitric oxide synthesis; increased levels of creatine, taurine and docosahexaenoic acid; and a decreased n-6/n-3 polyunsaturated fatty acids ratio. While some of these features likely reflect the stimulation of lipid mobilization and oxidation promoted by ATRA treatment systemically, other may also play a causal role underlying ATRA actions. The results connect ATRA to specific nutrition-modulated biochemical pathways, and suggest novel mechanisms of action of vitamin A-derived retinoic acid on metabolic health.

Mitochondrial (Dys)function and Insulin Resistance: From Pathophysiological Molecular Mechanisms to the Impact of Diet

Mitochondrial dysfunction has been implicated in the pathogenesis of insulin resistance, the hallmark of type 2 diabetes mellitus (T2DM). However, the cause-effect relationship remains to be fully elucidated. Compelling evidence suggests that boosting mitochondrial function may represent a valuable therapeutic tool to improve insulin sensitivity. Mitochondria are highly dynamic organelles, which adapt to short- and long-term metabolic perturbations by undergoing fusion and fission cycles, spatial rearrangement of the electron transport chain complexes into supercomplexes and biogenesis governed by peroxisome proliferator-activated receptor γ co-activator 1α (PGC 1α). However, these processes appear to be dysregulated in type 2 diabetic individuals. Herein, we describe the mechanistic link between mitochondrial dysfunction and insulin resistance in skeletal muscle alongside the intracellular pathways orchestrating mitochondrial bioenergetics. We then review current evidence on nutritional tools, including fatty acids, amino acids, caloric restriction and food bioactive derivatives, which may enhance insulin sensitivity by therapeutically targeting mitochondrial function and biogenesis.

Rapid regulation of substrate use for oxidative phosphorylation during a single session of high intensity interval or aerobic exercises in different rat skeletal muscles

Different exercise protocols lead to long-term adaptations that are related to increased mitochondrial content through the activation of mitochondrial biogenesis. However, immediate mitochondrial response to exercise and energetic substrate utilization is still unknown. We evaluate the mitochondrial physiology of two types rat skeletal muscle fibres immediately after a single session of high intensity interval exercise (HIIE) or aerobic exercise (AER). We found AER was able to reduce the ATP synthesis dependent oxygen flux in the tibialis (TA) when stimulated by complex I and II substrates. On the other hand, there was an increase of the maximum velocity (V_max) for glycerol-phosphate oxidation and V_max and affinity (K_M) of palmitoyl-carnitine oxidation (PC). The exercise did not affect oxygen flux coupled to ATP synthesis in red gastrocnemius (RG) but, surprisingly, reduced its affinity for PC, decreasing the apparent catalytic efficiency (V_max/K_M) of oxidation for PC. Neither exercise protocol was able to change the electron transfer system capacity of the mitochondria or markers of mitochondrial content. The AER group had increased H₂O₂ production compared to the SED and HIIE groups, with the mechanism being predominantly the escape of electrons through reverse flux in complex I and other sites in TA, and only through other sites in RG. There were no changes in the activities of antioxidant enzymes. Our results show that mitochondria from different muscles submitted to distinct exercise protocols show alterations in the specific fluxes of substrate utilization and oxygen metabolism, indicating that the dynamics of mitochondria are linked to the metabolic flexibility.

Defects in skeletal muscle subsarcolemmal mitochondria in a non-obese model of type 2 diabetes mellitus

Skeletal muscle resistance to insulin is related to accumulation of lipid-derived products, but it is not clear whether this accumulation is caused by skeletal muscle mitochondrial dysfunction. Diabetes and obesity are reported to have a selective effect on the function of subsarcolemmal and interfibrillar mitochondria in insulin-resistant skeletal muscle. The current study investigated the role of the subpopulations of mitochondria in the pathogenesis of insulin resistance in the absence of obesity. A non-obese spontaneous rat model of type 2 diabetes mellitus, (Goto-Kakizaki), was used to evaluate function and biochemical properties in both populations of skeletal muscle mitochondria. In subsarcolemmal mitochondria, minor defects are observed whereas in interfibrillar mitochondria function is preserved. Subsarcolemmal mitochondria defects characterized by a mild decline of oxidative phosphorylation efficiency are related to ATP synthase and structural alterations of inner mitochondria membrane but are considered unimportant because of the absence of defects upstream as shown with polarographic and spectrophometric assays. Fatty acid transport and oxidation is preserved in both population of mitochondria, whereas palmitoyl-CoA increased 25% in interfibrillar mitochondria of diabetic rats. Contrary to popular belief, these data provide compelling evidence that mitochondrial function is unaffected in insulin-resistant skeletal muscle from T2DM non-obese rats.