αvβ5 Integrin Promotes Dedifferentiation of Monolayer-Cultured Articular Chondrocytes

Objective. When cultured in monolayers, articular chondrocytes undergo an obvious phenotypic change. Although the involvement of integrins has been suggested, the exact mechanisms of the change have not been determined. This study was undertaken to clarify the mechanisms underlying the loss of chondrocyte phenotype early after plating. Methods. Primary cultured human articular chondrocytes were used for the experiments. Involvement of respective integrins in the phenotypic change was investigated in RNA interference (RNAi) experiments. A signaling pathway involved in the change was identified in experiments using specific inhibitors and adenoviruses encoding mutated genes involved in the pathway. Adenoviruses carrying mutated GTPases were used to determine the involvement of small GTPases in the process. Results. In monolayer-cultured chondrocytes, suppression of alpha v or beta 5 integrin expression by RNAi inhibited morphologic changes in the cells and increased (or prevented a reduction in) the expression of various cartilage matrix genes. Consistent results were obtained in experiments using a blocking antibody and a synthetic inhibitor of alpha v beta 5 integrin. The decrease in cartilage matrix gene expression in chondrocytes after plating was mediated by ERK signaling, which was promoted primarily by alpha v beta 5 integrin. In articular chondrocytes, the affinity of alpha v beta 5 integrin for ligands was regulated by the small GTPase R-Ras. R-Ras was gradually activated in monolayer-cultured chondrocytes after plating, which caused a gradual decline in cartilage matrix gene expression through enhanced alpha v beta 5 integrin activation and the subsequent increase in ERK signaling. Conclusion. Our findings indicate that alpha v beta 5 integrin may be involved in the change that occurs in monolayer-cultured chondrocytes after plating.