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 beta 1-integrin subunit inhibited attachment to fibronectin, MGP, and type II collagen, and a MAb to the beta 3-integrin 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 beta 1 and beta 3 subunit families. The modulation of the chondrocyte phenotype during monolayer culture may be related to activation or increased expression of integrins.