Wnt signaling stimulates osteoblastogenesis of mesenchymal precursors by suppressing CCAAT/enhancer-binding protein α and peroxisome proliferator-activated receptor γ

Mesenchymal precursor cells have the potential to differentiate into several cell types, including adipocytes and osteoblasts. Activation of Wnt/beta-catenin signaling shifts mesenchymal cell fate toward osteoblastogenesis at the expense of adipogenesis; however, molecular mechanisms by which Wnt signaling alters mesenchymal cell fate have not been fully investigated. Our prior work indicates that multipotent precursors express adipogenic and osteoblastogenic transcription factors at physiological levels and that ectopic expression of Wnt10b in bipotential ST2 cells suppresses expression of CCAAT/enhancer-binding protein alpha (C/EBP alpha) and peroxisome proliferator-activated receptor gamma (PPAR gamma) and increases expression of Runx2, Dlx5, and osterix. Here, we demonstrate that transient activation of Wnt/beta-catenin signaling rapidly suppresses C/EBP alpha and PPAR gamma, followed by activation of osteoblastogenic transcription factors. Enforced expression of C/EBP alpha or PPAR gamma partially rescues lipid accumulation and decreases mineralization in ST2 cells expressing Wnt10b, suggesting that suppression of C/EBP alpha and PPAR gamma is required for Wnt/beta-catenin to alter cell fate. Furthermore, knocking down expression of C/EBP alpha, PPAR gamma, or both greatly reduces adipogenic potential and causes spontaneous osteoblastogenesis in ST2 cells and mouse embryonic fibroblasts, suggesting that Wnt signaling alters the fate of mesenchymal precursor cells primarily by suppressing C/EBP alpha and PPAR gamma.