Mitochondrial damage is involved in the pathogenesis of osteoarthritis. Metformin, one of the most common prescriptions for patients with type 2 diabetes, can reportedly activate Sirtuin 3 (SIRT3) expression which protects mitochondria from oxidative stress. In this study, we investigated the inhibitory property of metformin on mitochondrial damage by focusing on the interleukin-1 beta (IL-1β)-stimulated osteoarthritis model by using primary murine chondrocytes. Our results demonstrated that SIRT3 was downregulated in chondrocytes under IL-1β stimulation, where its expression was positively correlated with mitochondrial damage and reactive oxygen species (ROS) production. Metformin treatment upregulated SIRT3 expression and mitigated loss of cell viability and decreased the generation of mitochondria-induced ROS in chondrocytes stimulated with IL-1β. Metformin also attenuated IL-1β-induced expressions of catabolic genes such as matrix metalloproteinase-3 (MMP3) and MMP13 and enhanced the anabolic indicator Collagen Ⅱ. These effects were mediated by phosphatase and tensin homolog (PTEN)-induced putative kinase protein 1 (PINK1)/Parkindependent mitophagy and the autophagic elimination of damaged mitochondria. Further, the SIRT3 inhibitor 3-TYP effectively inhibited the initiation of mitophagy, as decreased expression of PINK1 and Parkin, decreased the LC3II/LC3I, enhanced the expression of MMP3 and MMP13, and decreased the expression of Collagen Ⅱ. Overall, our findings provide evidence that metformin suppresses IL-1β-induced oxidative and osteoarthritis-like inflammatory changes by enhancing the SIRT3/PINK1/Parkin signaling pathway, thereby indicating metformin's potential in prevention and treatment of osteoarthritic joint disease.

Chondrocyte dysfunction is a key mechanism underlying osteoarthritis. Metformin has shown protective effects in many diseases. The present study aimed to investigate the effects of metformin on autophagy and apoptosis in the process of osteoarthritis. A mouse osteoarthritis model was set up by surgically destabilizing medial meniscus in the knee. Intraarticular injection of metformin or vehicle was applied in the right knee for eight weeks. Mouse articular chondrocytes were isolated and passaged for in vitro experiments. Small interfering RNA (siRNA) transfection was used to silence target genes. Western blotting, immunohistochemistry, transmission electron microscopy were used. After eight weeks, metformin restored surgery-induced upregulation of MMP13 and downregulation of type II collagen in the joint cartilage. In cultured primary murine chondrocytes, IL-1b aggravated apoptosis and catabolic response in a dose-dependent manner. In the presence of IL-1b, metformin increased phosphorylated levels of AMPKa and upregulated SIRT1 protein expression, leading to an increase in autophagy as well as a decrease in catabolism and apoptosis. Inactivating AMPKa or inhibiting SIRT1 prevented the augmented autophagy in the presence of metformin. Silencing AMPKa2, but not AMPKa1, reduced SIRT1 expression and downregulated autophagy in cultured chondrocytes. Metformin protects against IL-1b-induced extracellular matrix (ECM) degradation in cultured chondrocytes and in mouse osteoarthritis model through activating AMPKa/SIRT1 signaling. Metformin shed light on the treatment of osteoarthritis.