Capsaicin Improves Fatty Acid Oxidation in Skeletal Muscle Cells Through the TRPV1/TFEB-Dependent Lipophagy Enhancement

Background: Excess intramyocellular lipid induces lipotoxicity, which has been implicated in the development of insulin resistance and metabolic diseases. Although dietary capsaicin has been shown to reduce intramyocellular lipid content in mice, the underlying mechanisms remain unclear. This study aims to investigate the effects of capsaicin on lipophagy and fatty acid metabolism in skeletal muscle cells and explore the underlying mechanisms. Methods: Wild-type and transient receptor potential vanilloid channel 1 (TRPV1) knock-out (KO) mice were fed a high-fat diet (HD) with or without capsaicin for 4 months. They underwent a fasting exercise to induce autophagy before measurement. C2C12 myotubes were cultured with palmitate and treated with or without capsaicin, specific antagonists, or transcription factor EB (TFEB) RNAi. Respiratory exchange ratio (RER), metabolic parameters and muscle mitochondrial fatty acid oxidation were assessed. Autophagic flux was detected using the Premo Autophagy Tandem Sensor. TFEB transcriptional activity was detected by luciferase assays. RNA-sequencing, Western blotting, and immunofluorescence were conducted to analyze the expression of lipophagy-related genes. Results: Lipid overload inhibited fatty acid oxidation and impaired autophagy flux in muscles of HD-fed wild-type mice and myotubes cultured with palmitate. Although capsaicin significantly attenuated this inhibition (p < 0.05), the effect was not replicated in the TRPV1 KO mice (p > 0.05). In cultured myotubes, capsaicin dose-dependently promoted the expression of TFEB and also enhanced TFEB transcriptional activity (p < 0.05). This led to increased acidic lysosomes, degradation of p62 and perilipin 2, and improved fatty acid oxidation (p < 0.05). The TRPV1 antagonist, the calcineurin inhibitor, or TFEB siRNA significantly inhibited the effects of capsaicin (p < 0.05). Conclusions: Capsaicin ameliorates impaired lipophagy and fatty acid oxidation in lipid-overloaded skeletal muscle cells through a TRPV1/TFEB-dependent pathway.