Background Activation of Wnt/β-catenin pathway triggers chondrocyte catabolism and MMP13 activation that contribute to osteoarthritis (OA). The mechanism of down-regulation of Wnt/β-catenin pathway in cartilage homeostasis is unknown. Because chondrocytes are in a hypoxic environment that is lost in OA, we speculated that Hypoxia Inducible Factor 1α (HIF1α) inhibits Wnt signaling and chondrocyte catabolism.
Objectives We here investigated the interaction of β-catenin/TCF4 and the molecular regulation of Mmp-13 in articular chondrocytes and in OA mice.
Methods Primary murine chondrocytes from WT and ΔHIF1α were cultured with Wnt3a in normoxic (21% O2) and hypoxic (1% O2) conditions and analyzed the expression of catabolic markers. The binding of TCF4 to Mmp-13 regulatory regions was assessed in Chip assay in both conditions. To determine the impact of the interaction of Wnt and HIF1α in healthy cartilage and in OA, ΔHIF1αchon and fl/fl HIF1α underwent DMM and received articular injection of PKF 118-310, an inhibitor of β-catenin and TCF4 interaction.
Results Hypoxia prevented the increase in Mmp-13 and the decrease in Col2 and ACAN expression induced by Wnt. Blocking HIF1α by siRNA or cre-recombinase enhanced the expression of Mmp-13, but not Col2 expression suggesting that HIF1α regulated catabolic rather than anabolic genes. In hypoxic chondrocytes, Chip assay revealed that HIF1α lowered β-catenin/TCF4 transcriptional activity and the expression of Mmp13. Indeed, HIF1α interacted with β-catenin which displaced TCF4 from Mmp13 regulatory sequences. Moreover, DMM resulted in decreased HIF1α expression in articular cartilage of control mice. Furthermore, cartilage lesions were higher in ΔHIF1αchon mice submitted to DMM along with higher expression of β-catenin and Mmp13. Local administration of PKF 118-310 in ΔHIF1αchon reduced Mmp13 expression and prevented cartilage lesions. These results showed that HIF1α limited cartilage breakdown by blocking the binding of β-catenin to TCF4 subsequently leading to a lower Mmp-13 activation.
Conclusions We show here that the HIF1α/β-catenin complex acts as a negative regulator of Wnt signaling responsible for Mmp13 transcription in chondrocytes and in mice osteoarthritic joints. Therefore, targeting the HIF1α/β-catenin interaction is a new approach to reduce chondrocyte catabolism and prevent from osteoarthritis.
Disclosure of Interest None declared