Integrin‐mediated attachment of articular chondrocytes to extracellular matrix proteins

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Abstract
Objective. To investigate the interactions between chondrocytes and their extracellular matrix (ECM). An attachment assay was used to determine the extent of integrin‐mediated attachment of chondrocytes to a variety of ECM proteins and the effect of monolayer culturing on attachment activity.Methods. Bovine and human articular cartilage chondrocytes were grown in high‐density monolayer cultures for 3–21 days and used in the assays. Cell shape and production of 3H‐proline–labeled collagen were monitored to assess phenotypic changes with time in culture. Cultured chondrocytes were incubated in wells coated with purified proteins, with and without specific inhibitors of integrin‐mediated attachment, and cell attachment was determined.Results. Compared with bovine serum albumin, chondrocytes showed significant attachment to fibronectin, matrix Gla protein (MGP), osteopontin, bone sialoprotein II (BSP II), vitronectin, and types II and VI collagen. A synthetic peptide containing the integrin‐recognition sequence Arg‐Gly‐Asp inhibited attachment to all the proteins tested, except types II and VI collagen. A monoclonal antibody (MAb) to the β1integrin subunit inhibited attachment to fibronectin, MGP, and type II collagen, and a MAb to the β3integrin subunit inhibited attachment to BSP II and osteopontin. An increase in cell attachment was seen with time in culture, and this increase was followed by a change in the chondrocytes to flattened, type I collagen–producing cells.Conclusion. Chondrocytes can attach to a variety of cartilage and bone proteins; this attachment is mediated via integrins, including members of both the β1 and β3 subunit families. The modulation of the chondrocyte phenotype during monolayer culture may be related to activation or increased expression of integrins.