Lipid mixtures containing a very high proportion of saturated fatty acids only modestly impair insulin signaling in cultured muscle cells

Physiological and pathophysiological concentrations of fatty acids induce lipid droplet accumulation and impair functional performance of tissue engineered skeletal muscle

Fatty acids (FA) exert physiological and pathophysiological effects leading to changes in skeletal muscle metabolism and function, however, in vitro models to investigate these changes are limited. These experiments sought to establish the effects of physiological and pathophysiological concentrations of exogenous FA upon the function of tissue engineered skeletal muscle (TESkM). Cultured initially for 14 days, C2C12 TESkM was exposed to FA-free bovine serum albumin alone or conjugated to a FA mixture (oleic, palmitic, linoleic, and α-linoleic acids [OPLA] [ratio 45:30:24:1%]) at different concentrations (200 or 800 µM) for an additional 4 days. Subsequently, TESkM morphology, functional capacity, gene expression and insulin signaling were analyzed. There was a dose response increase in the number and size of lipid droplets within the TESkM (p < .05). Exposure to exogenous FA increased the messenger RNA expression of genes involved in lipid storage (perilipin 2 [p < .05]) and metabolism (pyruvate dehydrogenase lipoamide kinase isozyme 4 [p < .01]) in a dose dependent manner. TESkM force production was reduced (tetanic and single twitch) (p < .05) and increases in transcription of type I slow twitch fiber isoform, myosin heavy chain 7, were observed when cultured with 200 µM OPLA compared to control (p < .01). Four days of OPLA exposure results in lipid accumulation in TESkM which in turn results in changes in muscle function and metabolism; thus, providing insight ito the functional and mechanistic changes of TESkM in response to exogenous FA.

Long-chain acyl-CoA synthetase-1 mediates the palmitic acid-induced inflammatory response in human aortic endothelial cells

Saturated fatty acid (SFA) induces proinflammatory response through a Toll-like receptor (TLR)-mediated mechanism, which is associated with cardiometabolic diseases such as obesity, insulin resistance, and endothelial dysfunction. Consistent with this notion, TLR2 or TLR4 knockout mice are protected from obesity-induced proinflammatory response and endothelial dysfunction. Although SFA causes endothelial dysfunction through TLR-mediated signaling pathways, the mechanisms underlying SFA-stimulated inflammatory response are not completely understood. To understand the proinflammatory response in vascular endothelial cells in high-lipid conditions, we compared the proinflammatory responses stimulated by palmitic acid (PA) and other canonical TLR agonists [lipopolysaccharide (LPS), Pam3-Cys-Ser-Lys4 (Pam₃CSK₄), or macrophage-activating lipopeptide-2)] in human aortic endothelial cells. The expression profiles of E-selectin and the signal transduction pathways stimulated by PA were distinct from those stimulated by canonical TLR agonists. Inhibition of long-chain acyl-CoA synthetases (ACSL) by a pharmacological inhibitor or knockdown of ACSL1 blunted the PA-stimulated, but not the LPS- or Pam₃CSK₄-stimulated proinflammatory responses. Furthermore, triacsin C restored the insulin-stimulated vasodilation, which was impaired by PA. From the results, we concluded that PA stimulates the proinflammatory response in the vascular endothelium through an ACSL1-mediated mechanism, which is distinct from LPS- or Pam₃CSK₄-stimulated responses. The results suggest that endothelial dysfunction caused by PA may require to undergo intracellular metabolism. This expands the understanding of the mechanisms by which TLRs mediate inflammatory responses in endothelial dysfunction and cardiovascular disease.

Emerging Pharmacological Targets for the Treatment of Nonalcoholic Fatty Liver Disease, Insulin Resistance, and Type 2 Diabetes

Type 2 diabetes (T2D) is characterized by persistent hyperglycemia despite hyperinsulinemia, affects more than 400 million people worldwide, and is a major cause of morbidity and mortality. Insulin resistance, of which ectopic lipid accumulation in the liver [nonalcoholic fatty liver disease (NAFLD)] and skeletal muscle is the root cause, plays a major role in the development of T2D. Although lifestyle interventions and weight loss are highly effective at reversing NAFLD and T2D, weight loss is difficult to sustain, and newer approaches aimed at treating the root cause of T2D are urgently needed. In this review, we highlight emerging pharmacological strategies aimed at improving insulin sensitivity and T2D by altering hepatic energy balance or inhibiting key enzymes involved in hepatic lipid synthesis. We also summarize recent research suggesting that liver-targeted mitochondrial uncoupling may be an attractive therapeutic approach to treat NAFLD, nonalcoholic steatohepatitis, and T2D.

Reassessing the Role of Diacylglycerols in Insulin Resistance

Skeletal muscle (SM) insulin resistance (IR) plays an important role in the burden of obesity, particularly because it leads to glucose intolerance and type 2 diabetes. Among the mechanisms thought to link IR to obesity is the accumulation, in muscle cells, of different lipid metabolites. Diacylglycerols (DAGs) are subject of particular attention due to reported interactions with the insulin signaling cascade. Given that SM accounts for the majority of insulin-stimulated glucose uptake, this review integrates recent observational and mechanistic works with the sole focus on questioning the role of DAGs in SM IR. Particular attention is given to the subcellular distributions and specific structures of DAGs, highlighting future research directions towards reaching a consensus on the mechanistic role played by DAGs.

Robust intrinsic differences in mitochondrial respiration and H2O2 emission between L6 and C2C12 cells

Rat L6 and mouse C2C12 cell lines are commonly used to investigate myocellular metabolism. Mitochondrial characteristics of these cell lines remain poorly understood despite mitochondria being implicated in the development of various metabolic diseases. To address this need, we performed high-resolution respirometry to determine rates of oxygen consumption and H₂O₂ emission in suspended myoblasts during multiple substrate-uncoupler-inhibitor titration protocols. The capacity for oxidative phosphorylation supported by glutamate and malate, with and without succinate, or supported by palmitoyl-l-carnitine was lower in L6 compared with C2C12 myoblasts (all P < 0.01 for L6 vs. C2C12). Conversely, H₂O₂ emission during oxidative phosphorylation was greater in L6 than C2C12 myoblasts (P < 0.01 for L6 vs. C2C12). Induction of noncoupled respiration revealed a significantly greater electron transfer capacity in C2C12 compared with L6 myoblasts, regardless of the substrate(s) provided. Mitochondrial metabolism was also investigated in differentiated L6 and C2C12 myotubes. Basal rates of oxygen consumption were not different between intact, adherent L6, and C2C12 myotubes; however, noncoupled respiration was significantly lower in L6 compared with C2C12 myotubes (P = 0.01). In summary, L6 myoblasts had lower respiration rates than C2C12 myoblasts, including lesser capacity for fatty acid oxidation and greater electron leak toward H₂O₂. L6 cells also retain a lower capacity for electron transfer compared with C2C12 following differentiation to form fused myotubes. Intrinsic differences in mitochondrial metabolism between these cell lines should be considered when modeling and investigating myocellular metabolism.

Lipid-Induced Insulin Resistance in Skeletal Muscle: The Chase for the Culprit Goes from Total Intramuscular Fat to Lipid Intermediates, and Finally to Species of Lipid Intermediates

The skeletal muscle is the largest organ in the body. It plays a particularly pivotal role in glucose homeostasis, as it can account for up to 40% of the body and for up to 80%-90% of insulin-stimulated glucose disposal. Hence, insulin resistance (IR) in skeletal muscle has been a focus of much research and review. The fact that skeletal muscle IR precedes β-cell dysfunction makes it an ideal target for countering the diabetes epidemic. It is generally accepted that the accumulation of lipids in the skeletal muscle, due to dietary lipid oversupply, is closely linked with IR. Our understanding of this link between intramyocellular lipids (IMCL) and glycemic control has changed over the years. Initially, skeletal muscle IR was related to total IMCL. The inconsistencies in this explanation led to the discovery that particular lipid intermediates are more important than total IMCL. The two most commonly cited lipid intermediates for causing skeletal muscle IR are ceramides and diacylglycerol (DAG) in IMCL. Still, not all cases of IR and dysfunction in glycemic control have shown an increase in either or both of these lipids. In this review, we will summarise the latest research results that, using the lipidomics approach, have elucidated DAG and ceramide species that are involved in skeletal muscle IR in animal models and human subjects.