Roles of calcium phosphate-mediated integrin expression and MAPK signaling pathways in the osteoblastic differentiation of mesenchymal stem cells

Abstract

Osteoinduction of calcium phosphate (CaP) ceramics has been widely confirmed and accepted, but the underlying mechanism has not been fully elucidated. This study investigates the early biomolecular events that contributed to the transduction of extracellular material cues outside in, when MSCs were cultured in osteoinductive biphasic calcium phosphate (BCP) ceramics, and explores their roles in BCP-induced osteogenesis. The results demonstrated that BCP ceramics had a strong adsorption affinity for serum proteins, which might favor cell adhesion and mediate the expression of the cell surface receptor – integrin. qRT-PCR analysis found that BCP ceramics significantly up-regulated integrin α₂ and α₃ genes under both in vitro and in vivo conditions. As integrins clustered together into focal contacts (cell adhesion sites), immunofluorescence staining showed that compared to glass surfaces, cells seeded in BCP ceramics formed a relatively short focal contact and exhibited a smaller cell size. Moreover, western blotting analysis indicated that BCP ceramics could activate down-stream MAPK signaling pathways, whereas blockage of either ERK or P38 signals could dramatically attenuate BCP-induced osteogenesis. These findings suggested that BCP ceramics might mediate cell adhesion through trans-membrane proteins – integrins – to realize the transduction of “outside-in signaling”, and subsequently trigger the intracellular MAPK signaling cascade to induce the osteogenic differentiation of MSCs. They offered a promising principle for designing and fabricating tissue-inducing biomaterials to provide appropriate cues for target cells.