Wnt signalling mediates the cross-talk between bone marrow derived pre-adipocytic and pre-osteoblastic cell populations

The mechanisms underlying the inverse relationship between osteogenic and adipogenic differentiation of bone marrow stromal cells (MSC) are not known in detail. We have previously established two cell lines from mouse bone marrow that are committed to either osteogenic (osteoblasts and chondrocytes) (mMSCBone) or adipogenic (mMSCAdipo) lineage. To identify the molecular mechanism determining the lineage commitment, we compared the basal gene expression profile of mMSCBone versus mMSCAdipo using Affymetrix GeneChip® MG430A 2.0 Array. Gene annotation analysis based on biological function revealed an over-representation of skeletal development genes in mMSCBone while genes related to lipid metabolism and immune response were highly expressed in mMSCAdipo. In addition, there was a significant up-regulation of canonical Wnt signalling genes in mMSCBone compared to mMSCAdipo (p < 0.006). Dual-luciferase assay and expression analysis of genes related to Wnt signalling demonstrated significant activation of Wnt signalling pathway in mMSCBone compared to mMSCAdipo. Reduced Wnt activity in mMSCAdipo was associated with increased expression of the Wnt inhibitor, secreted frizzled-related protein 1 (sFRP-1) at both mRNA and protein levels in mMSCAdipo. Interestingly, conditioned medium (CM) collected from mMSCAdipo (mMSC-CMAdipo) inhibited osteoblast differentiation of mMSC, while depletion of sFRP-1 protein from mMSC-CMAdipo abolished its inhibitory effect on osteoblast differentiation. Furthermore, treatment of mMSC with recombinant sFRP-1 resulted in a dose-dependent inhibition of osteoblast and stimulation of adipocyte differentiation. In conclusion, cross-talk exists between different populations of MSC in the bone marrow, and Wnt signalling functions as a molecular switch that determines the balance between osteoblastogenesis and adipogenesis.