Abstract
In the past, proteins have been described that may be involved in chondrocyte interactions with extracellular collagen, but little is known about the role of integrins in chondrocyte-collagen interactions. Here we report on the analysis of β1-integrin distribution in human fetal cartilage and on the expression of integrins on fetal chondrocytes, using monoclonal and polyclonal antibodies to integrin α- and β-chains. We show the presence of α2-, α5-, α6-, αv-, and β1-chains on freshly isolated chondrocytes by surface immunofluorescence in the fluorescence-activated cell sorter and by surface iodination followed by immunoprecipitation. Affinity chromatography of bovine chondrocyte membrane proteins on a collagen-Sepharose column followed by immunoprecipitation confirmed the presence of the collagen-binding α2β1-integrin on chondrocytes. Chondrocyte adhesion on native collagens I and II, on fibronectin, and on laminin was completely blocked by anti-β1; anti-α2 reduced chondrocyte binding to collagen by only 40-50%; similarly, anti-α1-antibodies were also able to reduce chondrocyte binding to collagen, although α1 could not be unequivocally identified on chondrocytes. Chondrocyte adhesion to fibronectin was Mg 2+- and Ca 2+-dependent and could be inhibited by anti-α5 and by RGD peptides. Chondrocyte adhesion to native collagens is Mg 2+-, but not Ca 2+-dependent and RGD-independent. Interestingly, although these data point to a role of α2β1 in chondrocyte-collagen interactions in vitro , α2 could not be visualized in sections of human fetal cartilage, in contrast to the β1-, αv-, and α5-chains which were present. This suggests that α2β1-integrin may be involved in the assembly of a pericellular collagen matrix in vitro, but may not be required for chondrocyte-collagen interactions in intact cartilage.
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