Regulation of articular chondrocyte catabolic genes by growth factor interaction

Osteoarthritis is characterized by a loss of articular cartilage homeostasis in which degradation exceeds formation. Several growth factors have been shown to promote cartilage formation by augmenting articular chondrocyte anabolic activity. This study tests the hypothesis that such growth factors also play an anticatabolic role. We transferred individual or combinations of the genes encoding insulin‐like growth factor‐I, bone morphogenetic protein‐2, bone morphogenetic protein‐7, transforming growth factor‐β1, and fibroblast growth factor‐2, into adult bovine articular chondrocytes and measured the expression of catabolic marker genes encoding A disintegrin and metalloproteinase with thrombospondin motifs‐4 and ‐5, matrix metalloproteinases‐3 and ‐13, and interleukin‐6. When delivered individually, or in combination, these growth factor transgenes differentially regulated the direction, magnitude, and time course of expression of the catabolic marker genes. In concert, the growth factor transgenes regulated the marker genes in an interactive fashion that ranged from synergistic inhibition to synergistic stimulation. Synergistic stimulation prevailed over synergistic inhibition, reaching maxima of 15.2‐ and 2.7‐fold, respectively. Neither the magnitude nor the time course of the effect of the transgene combinations could be predicted on the basis of the individual transgene effects. With few exceptions, the data contradict our hypothesis. The results demonstrate that growth factors that are traditionally viewed as chondrogenic tend also to promote catabolic gene expression. The competing actions of these potential therapeutic agents add an additional level of complexity to the selection of regulatory factors for restoring articular cartilage homeostasis or promoting repair. This study found that transgenes encoding growth factors that are known to regulate the anabolic activity of articular chondrocytes also regulate catabolic functions in these cells. The data further demonstrate that certain growth factor transgenes interact to synergistically stimulate or inhibit chondrocyte catabolic gene expression. These findings are relevant to efforts to identify therapeutic agents for articular cartilage repair.