Biomechanical regulation of matrix metalloproteinase‐9 in cultured chondrocytes

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Abstract
Abnormal mechanical loading of joints may induce degeneration of articular cartilage. Shear stress is one mode of mechanical loading that may regulate chondrocyte metabolism. We investigated the mechanism by which shear stress induces the gene encoding matrix metalloproteinase‐9, a mediator of the progressive degradation of articular cartilage in osteoarthritis. In vitro experiments using passaged rabbit chondrocytes in monolayer culture subjected to a shear stress of 16 dyn/cm2 (1.6 Pa) in a flow channel showed increased expression of the matrix metalloproteinase‐9 gene. The induction of matrix metalloproteinase‐9 appeared to depend on a region in the 5′ promoter of the gene that contains a 12–0‐tetradecanoylphorbol 13‐acetateresponsive element. Transfection experiments using a construct containing a luciferase reporter driven by a 12–0‐tetradecanoylphorbol 13‐acetate‐responsive element indicated that shear stress activated a 12–0‐tetradecanoylphorbol 13‐acetate‐responsive element‐mediated transcription in chondrocytes. Similar experiments showed that shear stress induced a matrix metalloproteinase‐9 promoter construct (matrix metalloproteinase‐9‐luciferase). Shear stress activated c‐Jun NH2‐terminal kinase, extracellular signal‐regulated kinase, and p38. Transfection o, matrix metalloproteinase‐9‐luciferase together with the dominant negative mutant of c‐Jun NH2‐terminal kinase, but not with that of extracellular signal‐regulated kinase or p38, attenuated the shearinduced matridx metalloproteinase‐9 promoter activity. In addition, transfection of constructs encoding dominant negative mutants of Ras, Rac, and Cdc42 attenuated the induction of c‐Jun transcriptional activity by shear stres. Thus, shear stimulation of chondrocytes stimulates Ras, Rac, and Cdc42, which subsequently activate c‐Jun NH2‐terminal kinase to induce a 12–0‐tetradecanoylphorbol 13‐acetate‐responsive elementmediated expression of matrix metalloproteinase‐9.

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