l-carnitine alleviates synovitis in knee osteoarthritis by regulating lipid accumulation and mitochondrial function through the AMPK-ACC-CPT1 signaling pathway

BACKGROUND: Knee osteoarthritis (KOA) is a disability-associated condition that is rapidly growing with the increase in obesity rates worldwide. There is a pressing need for precise management and timely intervention in the development of KOA. l-carnitine has been frequently recommended as a supplement to increase physical activity in obese individuals due to its role in fatty acid metabolism, immune disorders, and in maintaining the mitochondrial acetyl-CoA/CoA ratio. In this study, we aimed to investigate the anti-inflammatory effects of l-carnitine on KOA and delineate a potential molecular mechanism. METHODS: Lipopolysaccharide-stimulated primary rat fibroblast-like synoviocytes (FLS) were treated with an AMP-activated protein kinase (AMPK) inhibitor or siRNA and carnitine palmitoyltransferase 1 (CPT1) siRNA to examine the synovial protective effects of l-carnitine. An anterior cruciate ligament transection model of rats was treated with an AMPK agonist (metformin) and CPT1 inhibitor (etomoxir) to define the therapeutic effects of l-carnitine. RESULTS: l-carnitine displayed a protective effect against synovitis of KOA in vitro and in vivo experiments. Specifically, l-carnitine treatment can reduce synovitis by inhibiting AMPK-ACC-CPT1 pathway activation and showed an increase in fatty acid β-oxidation, a lower lipid accumulation, and a noticeable improvement in mitochondrial function. CONCLUSIONS: Our data suggested that l-carnitine can mitigate synovitis in FLS and synovial tissue, and the underlying mechanism may be related to improving mitochondrial function and reducing lipid accumulation via the AMPK-ACC-CPT1 signaling pathway. Therefore, l-carnitine may be a potential treatment strategy for KOA.