Contrary Roles of Wnt/β-Catenin Signaling in BMP9-Induced Osteogenic and Adipogenic Differentiation of 3T3-L1 Preadipocytes

Our previous study revealed that 3T3-L1 preadipocytes can differentiate to either osteoblasts or adipocytes in response to bone morphogenic protein 9 (BMP9). In the present study, we try to further investigate whether the Wnt/beta-catenin signaling plays a crucial role in this process. It was found that BMP9 effectively activated the Wnt/beta-catenin signaling, and induced the expression levels of certain canonical Wnt ligands and their receptors in preadipocytes. Exogenous expression of beta-catenin, Wnt1, Wnt3a, and Wnt10b potentiated BMP9-induced alkaline phosphatase (ALP) activity, while beta-catenin knockdown or Dickkopf 1 (Dkk1) diminished BMP9-induced ALP activity. Moreover, it was demonstrated that beta-catenin overexpression promoted BMP9-induced mineralization, and increased the expression levels of late osteogenic markers osteopontin and osteocalcin. Furthermore, beta-catenin inhibited BMP9-induced lipid accumulation and the adipogenic marker adipocyte fatty acid binding protein (aP2). The cell-implantation assay results identified that beta-catenin not only augmented BMP9-induced ectopic bone formation, but also blocked adipogenesis in vivo. Mechanistically, it was found that beta-catenin and BMP9 synergistically stimulated the osteogenic transcription factors runt-related transcription factor 2 (Runx2) and Osterix (OSX). However, BMP9-induced adipogenic transcription factors, peroxisome proliferator-activated receptor gamma (PPAR gamma) and CCAAT enhancer-binding protein alpha (C/EBP alpha), were inhibited by beta-catenin. Therefore, these findings suggested that the Wnt/beta-catenin signaling, potentially via the modulation of osteogenic and adipogenic transcriptional factors, exerts an opposite effect on BMP9-induced osteogenic and adipogenic differentiation in preadipocytes.