Strontium ranelate rebalances bone marrow adipogenesis and osteoblastogenesis in senescent osteopenic mice through NFATc/Maf and Wnt signaling

With aging, bone marrow mesenchymal stromal cell (MSC) osteoblast differentiation decreases whereas MSC differentiation into adipocytes increases, resulting in increased adipogenesis and bone loss. Here, we investigated whether activation of cell signaling by strontium ranelate (SrRan) can reverse the excessive adipogenic differentiation associated with aging. In murine MSC cultures, SrRan increased Runx2 expression and matrix mineralization and decreased PPARγ2 expression and adipogenesis. This effect was associated with increased expression of the Wnt noncanonical representative Wnt5a and adipogenic modulator Maf and was abrogated by Wnt- and nuclear factor of activated T-cells (NFAT)c antagonists, implying a role for Wnt and NFATc/Maf signaling in the switch in osteoblastogenesis to adipogenesis induced by SrRan. To confirm this finding, we investigated the effect of SrRan in SAMP6 senescent mice, which exhibit decreased osteoblastogenesis, increased adipogenesis, and osteopenia. SrRan administration at a clinically relevant dose level increased bone mineral density, bone volume, trabecular thickness and number, as shown by densitometric, microscanning, and histomorphometric analyses in long bones and vertebrae. This attenuation of bone loss was related to increased osteoblast surface and bone formation rate and decreased bone marrow adipocyte volume and size. The restoration of osteoblast and adipocyte balance induced by SrRan was linked to increased Wnt5a and Maf expression in the bone marrow. The results indicate that SrRan acts on lineage allocation of MSCs by antagonizing the age-related switch in osteoblast to adipocyte differentiation via mechanisms involving NFATc/Maf and Wnt signaling, resulting in increased bone formation and attenuation of bone loss in senescent osteopenic mice.