The effect of mitohormetic adaptation on the response to inflammatory stress in healthy and osteoarthritic chondrocytes; in-vitro study

Osteoarthritis (OA) is the most common musculoskeletal disease worldwide, causing chronic pain and limited mobility. Increasing evidence suggests that mitochondria are involved in the pathogenesis of OA. Impaired mitochondrial function and quality control mechanisms contribute to senescence and chondrocyte apoptosis in OA pathogenesis, leading to OA progression. Mitohormesis describes the preconditioning in which non-lethal mitochondrial stress triggers a hormetic response and increases the cell's resistance to oxidative stress. Recent studies have shown that activation of mitochondrial stress pathways and hormetic mechanisms prolong life span, increasing the interest in this field daily. "Metformin", a widely used anti-hyperglycemic agent worldwide, is thought to exert its beneficial effects by inhibiting the complex-1 activity of the mitochondrial electron transport chain, thereby increasing oxidative stress and triggering adaptation mechanisms. The aim of this study was to investigate for the first time in the literature whether osteoarthritic and healthy chondrocytes acquire resistance to inflammation after mitohormetic preconditioning. Healthy and osteoarthritic chondrocytes obtained during surgery from patients with stage 3 OA, according to Kelgren-Lawrence Classification, who had undergone total knee replacement surgery, were cultured. Oxidative stress was induced by inhibiting complex 1 of the electron transport chain elements through metformin. After cell viability and toxic doses and durations of the applications were evaluated by MTT test, the expression of FGF21, SOD2, Nrf2, mitochondrial unfolded protein response elements ATF4, Lonp1, Trap1 and Hsp10, apoptosis markers Caspase3 and Parp1 were analyzed by Western Blot method. Changes in the amount of reactive oxygen products (ROS) were evaluated by ROS analysis. Metformin preconditioning decreases ROS levels by increasing mitochondria-specific SOD2 synthesis against oxidative stress in the inflammatory microenvironment induced by IL-1β in OA chondrocytes. The mitohormetic adaptation triggered by this process activates the mtROS-PERK-eIF2α-ATF4-FGF21 pathway through the "Integrated Stress Response" in coordination with the ER stress response to inflammatory stress. After metformin preconditioning, OA chondrocytes exhibit an adaptive behaviour towards survival by showing a stronger response to the new inflammatory stress. Thus, metformin preconditioning triggers a mitohormetic response in OA chondrocytes, making these cells more resistant to future inflammatory stresses and ensuring their survival.