Proteoglycan depletion alone is not sufficient to stimulate proteoglycan synthesis in cultured bovine cartilage explants

Abstract

Articular cartilage comprises a small number of cells embedded within a matrix primarily composed of collagen and proteoglycan (PG). The functional integrity of the tissue is highly dependent on the maintenance of matrix structure, which, in turn, is controlled by the chondrocytes. In normal tissue there is a slow but steady turnover of matrix components such that their levels remain constant. In certain diseased states the equilibrium is upset, resulting in a net loss of matrix components. The object of the present study was to artificially upset the synthetic/loss equilibrium by enzymic depletion and assess the ability of chondrocytes to respond by increasing PG synthesis. Cultured bovine articular cartilage explants were depleted using enzymes as follows; 10 U/ml Streptomyces hyaluronidase (induced a 30% loss of PG), 2000 U/ml testicular hyaluronidase (70% loss of PG) and 100 U/ml collagenase (35% loss of PG) and control (6% loss of PG). Collagenase also induced a 50% loss of collagen. Collagenase treatment induced a 50% stimulation of PG synthesis above control levels. Elevated synthesis levels were maintained for 9 days. Testicular hyaluronidase induced a brief elevation in PG synthesis on day 3 of culture. Streptomyces hyaluronidase treatment did not cause an alteration in the rate of PG synthesis above control levels. Histological examination indicated that collagenase-treated explants formed outgrowths consisting of rounded chondrocytes within a fine fibrous matrix which stained intensely with safranin-O, indicating a high concentration of PG. The production of repair-like outgrowths may explain the elevated PG synthesis rates measured. It appears, therefore, that collagen and matrix organization is more important than PG levels in the control of PG synthesis in articular cartilage explant cultures.