Toll-like receptor 4 modulates skeletal muscle substrate metabolism.

Toll-like receptor 4 (TLR4), a protein integral to innate immunity, is elevated in skeletal muscle of obese and type 2 diabetic humans and has been implicated in the development of lipid-induced insulin resistance. The purpose of this study was to examine the role of TLR4 as a modulator of basal (non-insulin-stimulated) substrate metabolism in skeletal muscle with the hypothesis that its activation would result in reduced fatty acid oxidation and increased partitioning of fatty acids toward neutral lipid storage. Human skeletal muscle, rodent skeletal muscle, and skeletal muscle cell cultures were employed to study the functional consequences of TLR4 activation on glucose and fatty acid metabolism. Herein, we demonstrate that activation of TLR4 with low (metabolic endotoxemia) and high (septic conditions) doses of LPS results in increased glucose utilization and reduced fatty acid oxidation in skeletal muscle and that these changes in metabolism in vivo occur in concert with increased circulating triglycerides. Moreover, animals with a loss of TLR4 function possess increased oxidative capacity in skeletal muscle and present with lower fasting levels of triglycerides and nonesterified free fatty acids. Evidence is also presented to suggest that these changes in substrate metabolism under metabolic endotoxemic conditions are independent of skeletal muscle-derived proinflammatory cytokine production. This report illustrates that skeletal muscle is a target for circulating endotoxin and may provide critical insight into the link between a proinflammatory state and dysregulated metabolism as observed with obesity, type 2 diabetes, and metabolic syndrome.

Low levels of lipopolysaccharide modulate mitochondrial oxygen consumption in skeletal muscle.

OBJECTIVE: We have previously demonstrated that activation of toll-like receptor 4 (TLR4) in skeletal muscle results in an increased reliance on glucose as an energy source and a concomitant decrease in fatty acid oxidation under basal conditions. Herein, we examined the effects of lipopolysaccharide (LPS), the primary ligand for TLR4, on mitochondrial oxygen consumption in skeletal muscle cell culture and mitochondria isolated from rodent skeletal muscle. MATERIALS/METHODS: Skeletal muscle cell cultures were exposed to LPS and oxygen consumption was assessed using a Seahorse Bioscience extracellular flux analyzer. Mice were also exposed to LPS and oxygen consumption was assessed in mitochondria isolated from skeletal muscle. RESULTS: Acute LPS exposure resulted in significant reductions in Carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP)-stimulated maximal respiration (state 3u) and increased oligomycin induced state 4 (state 4O) respiration in C2C12 and human primary myotubes. These findings were observed in conjunction with increased mRNA of uncoupling protein 3 (UCP3), superoxide dismutase 2 (SOD2), and pyruvate dehydrogenase activity. The LPS-mediated changes in substrate oxidation and maximal mitochondrial respiration were prevented in the presence of the antioxidants N-acetylcysteine and catalase, suggesting a potential role of reactive oxygen species in mediating these effects. Mitochondria isolated from red gastrocnemius and quadriceps femoris muscle from mice injected with LPS also demonstrated reduced respiratory control ratio (RCR), and ADP- and FCCP-stimulated respiration. CONCLUSION: LPS exposure in skeletal muscle alters mitochondrial oxygen consumption and substrate preference, which is absent when antioxidants are present.