The Distribution and Function of Collagen Receptors on Bovine Articular Chondrocytes

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Abstract

The purpose of this study was to investigate the distribution and function of putative collagen receptors on chondrocytes, released by protease digestion from bovine articular cartilage. The presence and distribution of receptors on chondrocytes were determined by antibodies. The functions of the receptors on chondrocytes were studied by examining the adhesion to immobilized collagens, the binding of soluble collagens, and the metabolic responses in agarose gel to the presence of exogenous collagens. In addition, the number and affinity of collagen binding sites on chondrocytes were determined by Scatchard analysis. Chondrocytes showed no difference in adhesion to either type I or type II collagen, but bound soluble type II collagen in preference to type I. In agarose suspension culture with exogenous collagens, type H collagen, but not type I collagen, inhibited synthesis of collagen early in culture, and caused a selective stimulation of decorin synthesis. Fluorescence cytometric analysis of chondrocytes using receptor antibodies showed twice the number of chondrocytes from the articular surface bound anti-annexin V compared to chondrocytes from the deep zone; only slight variations were detected with the integrin antibodies. In freshly isolated chondrocytes, integrins β1 and α5 were detected at low levels and increased with time in culture, while the expression of annexin V was initially high and diminished during culture. Collagenase treatment of chondrocytes after 24 hours in culture restored the binding of anti-annexin V to its original level, but caused no change in the binding of integrin antibodies. Competition studies confirmed that chondrocytes bound native bovine type II collagen in preference to other collagens tested, and a dissociation constant of 1.5 × 10⁻¹⁰ was calculated for native type II collagen. Anti-annexin V, but not anti-β1 integrin, completely inhibited the binding of native type n collagen to suspended chondrocytes. However, both integrin antibodies and anti-annexin V inhibited the attachment of chondrocytes to immobilized collagen. The results suggest that under the conditions tested, annexin V is the predominant receptor for native type II collagen on bovine articular chondrocytes, and further, that type II collagen, via interaction with its receptor, perturbs the metabolism of chondrocytes cultured in suspension.

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