Lithium chloride attenuates BMP-2 signaling and inhibits osteogenic differentiation through a novel WNT/GSK3- independent mechanism

Lithium inhibition of glycogen synthase kinase3 (GSK3) activity has been shown to mimic the canonical WNT signaling. Analogous to WNT, lithium prevents GSK3-mediated phosphorylation of cytosolic transcription factor β-catenin and its subsequent degradation by the proteasome complex. Although stabilization of β-catenin in osteoblasts has been shown to promote bone mass accrual in a mouse model, several studies reported inhibitory effects of lithium supplements on the osteogenic differentiation of cultured mesenchymal stem cells. One possible explanation for these apparent contradictory findings might be that lithium affects the differentiation of osteoblast progenitors through additional signaling events, which independently or in concert with WNT signaling, affect the bone resorption activities in vivo. In the current study, we used murine MC3T3-E1 pre-osteoblasts and a pluripotent mesenchymal cell line C2C12 to investigate lithium effects during the early stages of osteoblast differentiation. We demonstrate here that lithium inhibits BMP-2 signaling to affect osteogenic differentiation in both cell lines. Lithium treatment reduces BMP-2-induced SMAD 1,5,8 phosphorylation in both MC3T3-E1 and C2C12 cells without affecting their dephosphorylation. Additionally, in MC3T3-E1 cells, lithium attenuates BMP-2-induced osteogenic differentiation through GSK3 inhibition; while in C2C12 cells, these negative effects of lithium ions on BMP-2 signaling do not rely on GSK3 inhibition or activation of canonical WNT signaling. Our work suggests the presence of a novel GSK3/WNT-independent mechanism of lithium action during the early stages of osteogenic differentiation.