Differential gene expression and regulation of the bone morphogenic protein antagonists follistatin and gremlin in normal and osteoarthritic human chondrocytes and synovial fibroblasts

Objective

To compare gene expression and regulation of the bone morphogenic protein (BMP) antagonists follistatin, gremlin, chordin and noggin in human normal and osteoarthritis (OA) chondrocytes and synovial fibroblasts.

Methods

Follistatin protein production was measured using a specific ELISA, and localization of follistatin and gremlin in cartilage was determined by immunohistochemistry. Basal and induced gene expression was determined using real-time PCR. Gene expression was monitored following treatment with inflammatory, antiinflammatory, growth and developmental factors.

Results

All BMP antagonists, except noggin, were expressed in chondrocytes and synovial fibroblasts. Follistatin and gremlin were significantly upregulated in OA chondrocytes, but not in OA synovial fibroblasts. Chordin was weakly expressed in normal and OA cells. Production of follistatin protein paralleled the gene expression pattern. Follistatin was expressed preferentially by the chondrocytes at the superficial zone of cartilage. Gremlin was not detected in normal cartilage; in OA it was found at the superficial zone, not at the very superficial layers of cartilage but rather at the upper intermediary layers. Tumor necrosis factor alpha and interferon gamma stimulated follistatin expression, but downregulated gremlin. IL-1β had no effect on follistatin, but reduced gremlin. Conversely, BMP-2 and BMP-4 significantly stimulated gremlin, but downregulated follistatin. IL-13, dexamethasone, transforming growth factor beta1, basic fibroblast growth factor, platelet-derived growth factor BB and epidermal growth factor down-regulated the expression of both antagonists.

Conclusion

We show, for the first time, the involvement of the BMP antagonists follistatin and gremlin in OA pathophysiology. Data suggest that follistatin and gremlin expression is timed with specific stages in the progression of OA. The balance of BMP antagonist levels during the OA process may play a critical role in influencing the progression of OA, making these antagonists interesting new targets for the treatment of this disease.