HIF1α Protects from Osteoarthritis by Inhibiting MMP13 Expression Through WNT/ Β-Catenin Signaling

Purpose: Articular cartilage is an avascular and highly hypoxic tissue. HIF1α is a crucial hypoxia factor involved in chondrocyte activity and survival. The role of hypoxia in the regulation of cartilage remodeling and maintenance in osteoarthritis (OA) remains unknown. We and others have demonstrated the important role of Wnt/β-catenin pathway as a major chondrocyte regulator, the down-regulation of which preserving cartilage from damage. Here we investigated the level of hypoxia during OA and how this interferes with the Wnt/β-catenin signaling to modulate cartilage loss. Methods: To determine the role of hypoxia in OA, fl/fl HIF1α mice underwent destabilization of the medial meniscus (DMM). Hypoxyprobe assay was used to quantify the level of hypoxia in OA cartilage. Conditional Col2CreERT;fl/fl HIF1α (ΔHIF1αchon) mice were used to investigate the role of HIF1α. Knees were fixed and embedded in paraffin to assess cartilage loss using safranin-O staining and the expression of MMp-13 using immunohistochemistry. Primary chondrocyte cultures were used for understanding cellular mechanisms using RT-qPCR, Western Blot analysis and co-immunoprecipitation as well chip assay. Results: In WT mice (fl/fl), DMM downregulates hypoxia levels in the articular cartilage along with decreased HIF1α expression. Conditional deletion of HIF1α promoted cartilage breakdown and MMP-13 expression compared to WT. Hypoxia inhibits the expression of catabolic genes (Mmp-13, Mmp3) induced by Wnt/β-catenin activation in primary chondrocytes. Co-immunoprecipitations and Chip assays demonstrated the formation of β-catenin/HIF1α complexes in the nucleus. Moreover, this complex reduced β-catenin/TCF4 transcriptional activity resulting to an inhibition of Mmp13 expression. This effect was totally abolished when HIF1α was knocked-down by SiRNA strategy. Finally, blockade of β-catenin/TCF4 interaction by PKF 118-310, a specific inhibitor of this binding, alleviated the OA phenotype and downregulated Mmp13 expression in ΔHIF1αchon mice. Conclusions: Loss of hypoxia reduces HIF1α/β-catenin interaction in favor of β-catenin/TCF4 in OA. This complex acts as a positive regulator of MMP13 thus increasing cartilage degradation. Moreover, loss of HIF1α is rescued by inhibiting the β-catenin/TCF4 complex. Our study sheds light on a new role of HIF1α/β-catenin interaction in cartilage and brings new insights into the impact of hypoxia in articular cartilage and OA.