Metabolic Dysregulation Accelerates Progression in Early Degenerative Rat Knee Joints Driven by Local Inflammatory Changes

Abstract

Purpose: Obesity is a well-known and important risk factor for osteoarthritis (OA). Moreover, obesity is highly associated with the metabolic syndrome (MetS), which is defined as the simultaneous occurrence of the following factors: obesity, insulin resistance, abnormal blood lipid levels and hypertension. Growing evidence indicates that both OA and MetS are low-grade inflammatory conditions with elevation in systemic inflammatory markers. Nonetheless, it is unclear whether MetS low-grade inflammation induces OA, or contributes to the disease. The aim of this study was to determine the contribution of metabolic alterations, induced by a High-Fat Diet (HFD), on the onset or progression of OA in a rat model of local cartilage damage. Methods: Forty Wistar rats (12 weeks old, male; Charles-River, Sulzfeld, Germany), were randomly divided over two groups: twenty rats were fed a HFD (60% of the kcal contained fat: D12492i, Research Diets Inc., New Brunswick, USA) while the other animals received a standard diet. After 12 weeks, local articular cartilage damage was induced on the femoral condyles, in one knee joint according to the groove model in 14 rats of each diet group. Remaining animals served as a control group in each arm. At week 24, serum was collected, subchondral bone was assessed by μCT scan (Quantum FX, PerkinElmar, USA) to identify diet and/or surgery related changes. In addition, OA severity was evaluated by rat OARSI histopathology score and in parallel macrophage activity with CD68 immunostaining from histological sections was assessed. Results: HFD feeding resulted in metabolic dysregulation as indicated by significantly increased weight (2,6 g/day vs. 1,9 g/day; p<0.001), fasting insulin (4.2 ± 1.7 mU/L vs. 2.8 ± 1.6 mU/L; p=0.003) and total cholesterol (152 ± 30 mg/dL vs. 116 ± 32 mg/dL; p=0.003) compared to the standard fed rats. HFD feeding alone resulted in very mild cartilage degeneration (2 ± 1.1 vs 0.58 ± .7; p=0.06) and synovial inflammation (1.0 ± 0.6 vs 0.3 ± 0.5; p=0.075) both subscores of the rat OARSI histopathology score without changes on the subchondral bone parameters. However, when HFD feeding is combined with the surgical model of applied local cartilage damage, OA severity is statistically significant increased compared to the local cartilage damage group on a standard diet (6.2 ± 2.1 vs 3.4 ± 1.4; p=0.001). Synovial membrane inflammation (1.3 ± 0.9vs 0.5 ± 0.5; p=0.011; Figure 1A+D) and multiple large osteophyte formation, demonstrated by histology (0.9 ± 1 vs 0.2 ± 0.4; p=0.04; Figure 1B) and quantified on μCT (328 ± 349 μm³ vs 7 ± 14 μm³; p<0.001; see Figure 1C+E), contributes most to this increased OA severity. OA related subchondral bone changes were observed on the tibia compartment by μCT, independent for the diet. Immunohistochemical CD68 expression as observed on both the synovial membrane as well as in the subchondral bone as shown in Figure 2 can explain the increase in selected inflammatory parameters when groove surgery is combined with a HFD. Conclusions: This study shows that a high-fat diet induces metabolic alterations and increases the inflammatory state of the joint. This by itself does not result in severe osteoarthritis. However, when adding a HFD to a mild cartilage damage model of OA, joint degeneration is significantly increased 12 weeks post-surgery. This progression of joint degeneration appears to be driven mainly by the inflammatory response as demonstrated by an increased CD68 expression in both the subchondral bone and synovium membrane and enhanced synovitis and osteophytosis. Hence, our findings indicate that systemic metabolic and subsequent inflammatory factors need an additional trigger to contribute to the progression of the OA.View Large Image Figure ViewerDownload Hi-res image Download (PPT)