Chondrocyte AMP-Activated Protein Kinase Activity Suppresses Matrix Degradation Responses to Proinflammatory Cytokines Interleukin-1β and Tumor Necrosis Factor α

Objective. Interleukin-1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF alpha) stimulate chondrocyte matrix catabolic responses, thereby compromising cartilage homeostasis in osteoarthritis (OA). AMP-activated protein kinase (AMPK), which regulates energy homeostasis and cellular metabolism, also exerts antiinflammatory effects in multiple tissues. This study was undertaken to test the hypothesis that AMPK activity limits chondrocyte matrix catabolic responses to IL-1 beta and TNF alpha. Methods. Expression of AMPK subunits was examined, and AMPK alpha activity was ascertained by the phosphorylation status of AMPK alpha Thr(172) in human knee articular chondrocytes and cartilage by Western blotting and immunohistochemistry, respectively. Procatabolic responses to IL-1 beta and TNF alpha, such as release of glycosaminoglycan, nitric oxide, and matrix metalloproteinases 3 and 13 were determined by dimethylmethylene blue assay, Griess reaction, and Western blotting, respectively, in cartilage explants and chondrocytes with and without knockdown of AMPK alpha by small interfering RNA. Results. Normal human knee articular chondrocytes expressed AMPK alpha 1, alpha 2, beta 1, beta 2, and gamma 1 subunits. AMPK activity was constitutively present in normal articular chondrocytes and cartilage, but decreased in OA articular chondrocytes and cartilage and in normal chondrocytes treated with IL-1 beta and TNF alpha. Knockdown of AMPK alpha resulted in enhanced catabolic responses to IL-1 beta and TNF alpha in chondrocytes. Moreover, AMPK activators suppressed cartilage/chondrocyte procatabolic responses to IL-1 beta and TNF alpha and the capacity of TNF alpha and CXCL8 (IL-8) to induce type X collagen expression. Conclusion. Our findings indicate that AMPK activity is reduced in OA cartilage and in chondrocytes following treatment with IL-1 beta or TNF alpha. AMPK activators attenuate dephosphorylation of AMPK alpha and procatabolic responses in chondrocytes induced by these cytokines. These observations suggest that maintenance of AMPK activity supports cartilage homeostasis by protecting cartilage matrix from inflammation-induced degradation.