Using Diet-induced Obesity to Examine the Role for Advanced Glycation End-products in Metabolic Osteoarthritis Pathogenesis in Young Rats

Abstract

Osteoarthritis (OA) is the most common form of arthritis and currently treatment is restricted to pain medication and ultimately joint replacement. Recent studies have identified a new osteoarthritic phenotype termed metabolic syndrome-associated/ metabolic OA, in which metabolic disorders and low-grade inflammation play a key mechanistic role in the disruption of joint homeostasis and cartilage degradation. One potential mediator of low-grade inflammation associated with metabolic OA may be advanced glycation end-products (AGEs). AGEs are a heterogeneous group of carbohydrate-binding proteins which can form within the body, and are commonly found in cooked food and animal-based products. Serum and tissue AGEs concentrations are increased by a high-fat (HF) diet and are reduced by exercise in humans and rodents. In hyaline articular cartilage (HAC), AGEs accumulate within the extracellular matrix, crosslink with collagen, and reduce mechanical function. They have also been shown to upregulate proinflammatory cytokine release from chondrocytes and synoviocytes. However, it is not known how HF diet affects AGE accumulation and cytokine expression in articular cartilage. Therefore this study aims to test the hypothesis that a HF diet increases systemic and articular cartilage AGE accumulation and catabolic cytokine expression. Further, given that exercise is known to upregulate anti-inflammatory pathways, the study examined the effect of voluntary exercise on inflammation and AGE accumulation. Male Sprague-Dawley rats were fed either a control diet (Chow) or a high-fat (HF) diet from D21. From D63 of age, half of the rats were provided running wheels for voluntary wheel exercise (EX) up to D121, in contrast to the other half that were allowed only spontaneous cage activity (SED). Metabolic assessment at D121 included mass, percentage body fat and plasma insulin, leptin and glucose. Plasma cytokines (IFN , IL1 , IL1 , IL6, IL10, TNF ) and AGE concentration were also assessed. AGE content in Chow and HF-Chow, along with AGE concentration in, liver and kidney were assessed using an enzyme-linked immunosorbent assay (ELISA). Hyaline articular cartilage of the femoro-tibial joint was immunohistochemically examined for the distribution of AGEs and carboxymethyllysine (CML), and the expression of receptor of advanced glycation end-products (RAGE). Using humeral head explant cultures, early cartilage degradation was assessed using a glycosaminoglycan release assay after 24h in culture. Explant cytokine secretion was also assessed using ELISAs and a multiplex bead immunoassay. Explants were further challenged with AGE-BSA to evaluate the effects of AGEs on glycosaminoglycan release and cytokine secretion. HF diet resulted in significant increases in percentage body fat which was significantly reduced with exercise. Plasma TNFHF- SED group compared to standard chow diet alone or in combination with exercise, However, no other cytokines showed significant changes in response to HF diet or exercise. Voluntary exercise was found to reduce AGE accumulation and enhance AGE elimination, but HF diet was found to diminish these effects. No differences in AGE accumulation and RAGE expression in the hyaline articular cartilage were found, but variable CML distribution was found in HF rats without voluntary exercise, suggesting early cartilage changes are associated with metabolic OA. HF diet induced greater glycosaminoglycan and IL6 release from the explants, which was ameliorated by voluntary exercise. Glycosaminoglycan release, independent of weight and percentage body fat, was associated with concomitant TNF ants of the HF rats, but not in the Chow rats. Greater IL10 secretion was only observed in HF rats exposed to voluntary exercise. AGE challenge resulted in greater cartilage degradation in explants, especially in the HF-EX group, but no changes in cytokine concentrations were observed. AGE accumulation was dependent of the nutritional and physical activity environments. Early joint changes such as cartilage degradation, proinflammatory cytokine release and CML distribution were found in the HF rats, and these effects could be ameliorated by voluntary exercise. The results presented in this thesis suggest that, exposure to a HF diet early in life can lead to systemic exposure to AGEs and provides further evidence to support a role of AGEs in the development of metabolic OA.