Transforming growth factor a suppression of articular chondrocyte phenotype and Sox9 expression in a rat model of osteoarthritis

Objective. To define the roles of transforming growth factor a (TGF alpha) in cartilage degradation. Methods. Primary rat articular chondrocytes and articular osteochondral explants were cultured with TGF alpha to assess the effects of TGF alpha on chondrocyte physiology and phenotype. Results. TGF alpha altered chondrocyte morphology through reorganization of the actin cytoskeleton and formation of stress fibers. Expression of anabolic genes, including aggrecan, type II collagen, and cartilage link protein, was reduced in response to TGF alpha. Proliferation of chondrocytes and formation of articular chondrocyte clusters was stimulated by TGF alpha. Expression of matrix metalloproteinase 13 and cathepsin C was increased by TGF alpha. We demonstrated the down-regulation of Sox9 messenger RNA and protein levels by TGF alpha. This was associated with reduced levels of phosphorylated and total SOX9 in cartilage explants upon TGF alpha treatment. In contrast, another growth factor identified in our microarrays, Kid, had no effects on the chondrocyte parameters tested. To examine correlations between the increased levels of TGF alpha in experimental knee osteoarthritis (OA) with the levels of TGF alpha in humans with knee OA, a microarray analysis of mRNA from 13 normal and 12 late-stage OA cartilage samples was performed. Seven OA samples showed TGFA mRNA levels similar to those in the normal controls, but expression was markedly increased in the other 5 OA samples. These data confirm that TGFA transcript levels are increased in a subset of patients with OA. Conclusion. This study adds TGF alpha to the list of dysregulated cytokines present in degrading cartilage in OA. Since TGF alpha inhibits articular chondrocyte anabolic capacity, increases catabolic factors, and contributes to the development of chondrocyte clusters, TGF alpha may be a potential target for therapeutic strategies in the treatment of OA.