Wnt-mediated regulation of chondrocyte maturation:: Modulation by TGF-β

Wnt proteins are expressed during limb morphogenesis, yet their role and mechanism of action remains unclear during long bone growth. Writ expression, effects and modulation of signaling events by BMP and transforming growth factor-beta (TGF-beta) were evaluated in chick embryonic chondrocytes. Chondrocyte cell cultures underwent spontaneous maturation with increased expression of coIX and this was associated with an increase in the expression of multiple Wnts, including Wnts 4, 5a, 8c, and 9a. Both parathyroid hormone related peptide (PTHrP) and TGF-beta inhibited coIX, but had disparate effects on Wnt expression. While TGF-beta strongly inhibited all Writs, PTHrP did not inhibit either Wnt8c or Wnt9a and had lesser effects on the expression of the other Writs. BMP-2 induced coIX expression, and also markedly increased Wnt8c expression. Overexpression of beta-catenin and/or T cell factor (TCF)-4 also induced the type X collagen promoter. Overexpression of Wnt8c induced maturation, as did overexpression of P-catenin. The Wnt8c/beta-catenin maturational effects were enhanced by BMP-2 and inhibited by TGF-beta. TGF-beta also inhibited activation of the Topflash reporter by beta-catenin, suggesting a direct inhibitory effect since the Topflash reporter contains only P-catenin binding sequences. In turn beta-catenin inhibited activation of the p3TP-Luc reporter by TGF-beta, although the effect was partial. Thus, Wnt/beta-catenin signaling is a critical regulator of the rate of chondrocyte differentiation. Moreover, this pathway is modulated by members of the TGF-beta family and demonstrates the highly integrated nature of signals controlling enclochondral ossification. J. Cell. Biochem.