Stimulation of Superficial Zone Protein Accumulation by Hedgehog and Wnt Signaling in Surface Zone Bovine Articular Chondrocytes

Objective. To determine the roles of the hedgehog and Wnt signaling pathways in accumulation of superficial zone protein (SZP) in surface zone articular chondrocytes. Methods. Explant cultures of disks of surface zone cartilage or isolated chondrocytes from the surface zone of articular cartilage of bovine stifle joints were cultured in serum-free chemically defined medium. Accumulation of SZP in the culture medium, in response to hedgehog proteins (sonic hedgehog [SHH] and Indian hedgehog [IHH]), Wnt proteins (Wnt-3a, Wnt-5a, and Wnt-11), agonists of the Wnt/beta-catenin pathway (glycogen synthase kinase 3 beta [GSK-3 beta] inhibitors), and antagonists of the Wnt/beta-catenin pathway, was investigated. The interaction between transforming growth factor beta 1 (TGF beta 1) and hedgehog proteins or antagonists of the Wnt/beta-catenin pathway was also investigated. Results. Hedgehog proteins stimulated SZP accumulation. Activation of the Wnt/beta-catenin pathway by Wnt-3a and GSK-3 beta inhibitors led to inhibition of SZP accumulation; however, Wnt-5a and Wnt-11 had no influence on SZP accumulation. Conversely, antagonists of the Wnt/beta-catenin pathway stimulated SZP accumulation. In addition, there were combinatorial effects of TGF beta 1 and hedgehog proteins or antagonists of the Wnt/beta-catenin pathway on SZP accumulation. Conclusion. SHH and IHH signaling has a stimulatory effect on SZP accumulation in surface zone cartilage and isolated articular chondrocytes. These findings provide insight into the regulatory mechanisms of articular cartilage homeostasis and maintenance by morphogens.