Previous studies demonstrated that NELL-1 promoted osteogenic differentiation of bone marrowl stromal cells (BMSCs). BMSC differentiation into osteoblasts and the signaling pathways involved are poorly understood. We hypothesized that NELL-1 binds to Integrin and promotes osteogenic differentiation through FAK signaling pathway.

To test this hypothesis, we examined the physical interaction of NELL-1 with Integrin beta-1 and inhibition of NELL-1's cell binding and adhesion using knockdown of Integrin beta-1 siRNA. We also assayed for phosphorylation of FAK as well as expression of osteogenic differentiation markers (OCN, OPN, ALP staining) and adiopogenic differentiation markers (ORO staining) in BMSC.

We found that NELL-1 physically binds to Integrin beta-1 and Integrin beta-1 siRNA knockdown reduced NELL-1 cell binding significantly. NELL-1 stimulated expression of Integrin beta-1 and activated FAK phosphorylation. Finally, NELL-1 enhanced expression of osteogenic genes, whereas it inhibited expression of adiopogenic genes. Taken together, our results suggest that NELL-1 may initiates transduction signals through Integrin, and FAK pathway may play an important role in regulating NELL-1 induced osteogenic and adipogenic differentiation of BMSC.