Localization of β1-Integrins in Human Cartilage and Their Role in Chondrocyte Adhesion to Collagen and Fibronectin

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.