Dose-dependent histopathological and immunohistochemical effects of metformin in experimental osteoarthritis

To evaluate the dose-dependent effects of metformin on joint pathology and inflammatory and apoptotic processes in a monosodium iodoacetate (MIA)-induced rat model of osteoarthritis.

Forty male Wistar albino rats were randomized into four groups (n = 10): sham control, OA, MIA + metformin 100 mg/kg, and MIA + metformin 200 mg/kg. OA was induced by intra-articular injection of 3.5 mg MIA. Metformin was administered intraperitoneally for 30 days. Spontaneous pain-related behavior was scored with a semi-quantitative, non-reflexive system. Histopathological parameters and IL-1β, IL-6, TNF-α, caspase-3, and LC3 expression were examined.

Compared with control, the OA group worsened across all histopathological parameters (p < 0.01). Immunohistochemically, IL-1β, IL-6, TNF-α, and caspase-3 expression increased, whereas LC3 expression decreased (p < 0.01). Histopathological severity and marker profiles changed in an ordered manner across the untreated OA and both metformin groups (Jonckheere-Terpstra, two-sided p < 0.001). After adjustment for six pairwise comparisons, the 200 mg/kg group differed from untreated OA on every tissue parameter, whereas the 100 mg/kg group did not reach the adjusted threshold for histopathology despite large effect sizes (Cliff's delta 0.85 to 1.00). Pain-related behavior improved from day 3 to day 30 in both metformin groups (100 mg/kg: p = 0.005; 200 mg/kg: p = 0.028), whereas at day 30 only the 200 mg/kg group differed from untreated OA (adjusted p = 0.002).

Metformin showed chondroprotective and anti-inflammatory effects with reduced apoptotic marker expression and altered autophagy-related signaling, attenuating cartilage degeneration in a dose-dependent manner. Improvement in pain-related behavior compared with untreated OA was significant only at 200 mg/kg. Key Points • Metformin started on the day of MIA induction attenuated structural joint damage at day 30. • Histopathological and immunohistochemical outcomes showed an ordered dose-related pattern across untreated OA, 100 mg/kg, and 200 mg/kg groups. • Treatment was accompanied by reduced inflammatory and apoptotic marker expression and altered autophagy-related signaling. • At day 30, improvement in pain-related behavior was significant compared with untreated OA only at 200 mg/kg.