Wnt/β-Catenin and retinoic acid receptor signaling pathways interact to regulate chondrocyte function and matrix turnover

Abstract

Activation of the Wnt/β-catenin and retinoid signaling pathways is known to tilt cartilage matrix homeostasis toward catabolism. Here, we investigated possible interactions between these pathways. We found that all-trans-retinoic acid (RA) treatment of mouse epiphyseal chondrocytes in culture did increase Wnt/β-catenin signaling in the absence or presence of exogenous Wnt3a, as revealed by lymphoid enhancer factor/ T-cell factor/β-catenin reporter activity and β-catenin nuclear accumulation. This stimulation was accompanied by increased gene expression of Wnt proteins and receptors and was inhibited by co-treatment with Dickkopf-related protein-1, an extracellular inhibitor of Wnt/β-catenin signaling, suggesting that RA modulates Wnt signaling at Wnt cell surface receptor level. RA also enhanced matrix loss triggered by Wnt/β-catenin signaling, whereas treatment with a retinoid antagonist reduced it. Interestingly, overexpression of retinoic acid receptor γ (RARγ) strongly inhibited Wnt/β-catenin signaling in retinoid-free cultures, whereas small interfering RNA-mediated silencing of endogenous RARγ expression strongly increased it. Small interfering RNA-mediated silencing of RARα or RARβ had minimal effects. Co-immunoprecipitation and two-hybrid assays indicated that RARγ interacts with β-catenin and induces dissociation of β-catenin from lymphoid enhancer factor in retinoid-free cultures. The N-terminal domain (AF-1) of RARγ but not the C-terminal domain (AF-2) was required for association with β-catenin, whereas both AF-1 and AF-2 were necessary for inhibition of β-catenin transcriptional activity. Taken together, our data indicate that the Wnt and retinoid signaling pathways do interact in chondrocytes, and their cross-talks and cross-regulation play important roles in the regulation of cartilage matrix homeostasis. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.