Metformin promotes apoptosis of senescent chondrocytes by disrupting the Bcl-xL/Mcl-1-Bak interaction.

Introduction: The accumulation of senescent chondrocytes (SEC) beyond the body's tolerance capacity is a key contributor to osteoarthritis (OA) pathogenesis. Metformin (Met), a first-line oral medication for type 2 diabetes, has shown therapeutic potential in various diseases. This study aims to explore the therapeutic effects and underlying mechanisms of Met in OA. Methods: Cellular senescence was induced in immortalized human chondrocytes using doxorubicin (Dox), confirmed by β-galactosidase staining and western blotting. The effects of Met on SEC viability, mitochondrial membrane potential, and apoptosis were assessed. Protein expression and interactions were analyzed by western blot and live-cell fluorescence resonance energy transfer (FRET) analysis. Results: Met promoted death of SEC and reduced mitochondrial membrane potential, inducing caspase- and reactive oxygen species (ROS)-dependent apoptosis. Met upregulated Bak and Bim expression, downregulated Mcl-1, and activated Caspase-3, Caspase-8, and Caspase-9, leading to PARP cleavage. FRET analysis revealed that Met disrupted Mcl-1/Bak and Bcl-xL/Bak binding while facilitating Bim-Bak interaction, promoting Bak dimerization. Discussion: This study provides the first evidence that Met induces apoptosis of SEC by reducing Mcl-1-Bak and Bcl-xL-Bak interactions while enhancing Bim-Bak binding, suggesting a novel therapeutic strategy for OA by targeting senescent chondrocytes through mitochondrial apoptosis pathways.