Strontium promotes osteogenic differentiation of mesenchymal stem cells through the RAS/ERK 1/2 MAPK signaling pathway

The cellular and molecular mechanism underlying the anabolic effect of strontium on bone remains to be elucidated. The present study thus aimed to investigate the effect of strontium on the osteoblast lineage differentiation of mesenchymal stem cells (MSCs) and the possible involvement of the mitogen activation protein kinase (MAPK) signaling pathway. Primary murine bone marrow MSCs and MSClike C3H10T1/2 cells were induced to differentiate into osteoblasts in the presence of strontium chloride. Strontium significantly increased the cellular ALP activity at day 14, 21 of induction and dose-dependently promoted calcium deposition in MSCs cultures. Three osteogenic marker genes, RUNX2, osteopontin (OPN) and osteocalcin (OCN) were also significantly up-regulated in response to strontium treatment. Accompanying the enhanced osteogenic differentiation, the increased phosphorylation of ERK1/2, but not p38 and JNK MAPKs, was detected in strontium-treated MSCs. PD98059, a selective inhibitor of ERK1/2 kinase, on the other hand, exhibited significant inhibitory effects on strontium-induced enhancement of ERK1/2 phosphorylation, Runx2 gene expression and ALP activity. Furthermore, Rous sarcoma kinase (RAS), an upstream regulator of ERK1/2, was also found to be essential in the process as Ras was activated by strontium treatment and SiRNA mediated Ras knockdown inhibited strontium-stimulated expression of osteogenic markers. Taken together, these results indicated that strontium can potentate osteogenesis of progenitor cells through a mechanism mediated by the Ras/ERK1/2 MAPK signalling pathway