Induction of osteoarthritis and metabolic inflammation by a very high-fat diet in mice: effects of short-term exercise

Abstract

Objective: To test the hypotheses that obesity due to a very high-fat diet induces knee osteoarthritis (OA), and that short-term wheel-running exercise protects against obesity-induced knee OA by reducing systemic inflammation and metabolic dysregulation.

Methods: Male C57BL/6J mice were fed either a control diet (13.5% kcal from fat) or a very high-fat diet (60% kcal from fat) from age 12 weeks to age 24 weeks. From 20 to 24 weeks of age, half of the mice were housed with running wheels. The severity of knee OA was determined by assessing histopathologic features, and serum cytokines were measured using a multiplex bead immunoassay and enzyme-linked immunosorbent assays. Body composition was quantified by dual-energy x-ray absorptiometry, and insulin resistance was assessed by glucose tolerance testing.

Results: Feeding mice with a very high-fat diet increased knee OA scores and levels of serum leptin, adiponectin, KC (mouse analog of interleukin-8 [IL-8]), monokine induced by interferon-γ (CXCL9), and IL-1 receptor antagonist to an extent in proportion to the gain in body fat (3-fold increase in percent body fat compared to controls). Wheel-running exercise reduced progression of OA in the medial femur of obese mice. In addition, exercise disrupted the clustering of cytokine expression and improved glucose tolerance, without reducing body fat or cytokine levels.

Conclusion: Obesity induced by a very high-fat diet in mice causes OA and systemic inflammation in proportion to body fat. Increased joint loading is not sufficient to explain the increased incidence of knee OA with obesity, as wheel running is protective rather than damaging. Exercise improves glucose tolerance and disrupts the coexpression of proinflammatory cytokines, suggesting that increased aerobic exercise may act independently of weight loss in promoting joint health.

Figure 1

Effect of a high-fat (HF) diet and activity on body composition and glucose tolerance. A HF diet significantly increased body weight (A) and percent body fat (B) in both sedentary and exercised animals, despite similar nightly running distances (C). Blood glucose levels following a glucose tolerance test (D) showed that a HF diet significantly decreased glucose clearance from the blood in both sedentary and exercised mice, as indicated by increased blood glucose area under the curve (AUC) levels. (E) Exercise reduced the blood glucose AUC in HF fed mice. AUC units are mg/dl•min. Values are mean ± SEM. *p<0.05, **p<0.01, ***p<0.001.

Figure 2

Representative sagittal knee sections from the medial compartment in sedentary or exercised mice fed a control or high-fat diet. Sections were stained with hematoxylin, fast green, and Safranin-O. A high-fat diet increased the modified Mankin OA score in the medial femoral condyle and tibial plateau in sedentary animals. The medial femoral condyle in high-fat sedentary animals was characterized by a loss of Safranin-O staining, indicating a loss of glycosaminoglycan content. Scale bars = 500 μm.

Figure 3

Site-specific modified Mankin OA scores (A–D) as a function of diet and activity. A very high-fat (HF) diet increased OA in the medial femoral condyle and tibial plateau in sedentary animals. 4 wks of wheel running prevented this diet effect in the medial femoral condyle but not in the medial tibial plateau. Changes in subchondral bone thickness and the loss of Safranin-O staining intensity are shown as a function of diet and activity for the medial femur (E) and tibia (F). A high-fat diet increased subchondral bone thickness in a site and activity specific manner, and it also increased cartilage GAG loss in the medial femur, which was recovered with exercise. Cartilage GAG loss was calculated relative to the pixel intensities of Safranin-O staining in the growth plate (0% loss) and the subchondral bone (100% loss). RIU=relative intensity units. Values = mean ± SEM. *p<0.05.

Figure 4

Correlation cluster diagrams of serum chemokine and cytokine concentrations within (A) sedentary (N=10: 5 control and 5 HF) and (B) exercised mice (N=10, 5 control and 5 HF). Filled circles and symbols identify cytokines that are positively correlated with the metabolic parameters indicated in the legend. Sedentary mice express a highly correlated cytokine cluster associated with increased adiposity and glucose dysregulation. In exercised mice, this cytokine cluster is significantly disrupted; monocyte-induced interferon-γ (MIG) is co-expressed, and adiponectin (ACRP30) and IL-1Ra are no longer associated with adiposity or glucose dysregulation. Numerical values represent the pairwise Pearson product-moment correlations. Only correlations with p<0.05 are shown. Line thickness is proportional to correlation strength.