Human periodontal fibroblast response to a nanostructured hydroxyapatite bone replacement graft in vitro

The efficacy of nanostructured hydroxyapatite (NHA) for the treatment of osseous defects has been demonstrated in recent studies, even though the underlining biological mechanism is still poorly known. This study examined the alterations in cellular adhesion and mitogenic responses in human periodontal ligament (PDL) cells treated with a novel nanostructured hydroxyapatite bone graft substitute and characterized associated changes in cellular signalling pathways.

Cultured PDL cells were stimulated with NHA in a surface coated form. Proliferation was determined by bromodeoxyuridine (BrdU) incorporation and cell adhesion was analysed by a colorimetric assay. In order to understand altered adhesion properties of PDL fibroblasts their integrin profile was analysed and the phosphorylation status of focal adhesion kinase (FAK) and beta1 integrin was determined by immunoblotting. In order to understand the signalling mechanisms of increased cell proliferation of PDL cells caused by NHA, the phosphorylation status of the serine/threonine protein kinase Akt, of the signal regulated kinases ERK1/2 and of the epidermal growth factor receptor (EGFR) was analysed by western blot using phospho-specific antibodies.

The results indicated that NHA is a strong stimulator of PDL cell attachment and proliferation. Mechanistically, alpha5beta1 integrin-mediated cellular adhesion of PDL fibroblasts, which resulted in altered phosphorylation and activation levels of FAK. Proliferation mediated by NHA was mechanistically caused by activation of the epidermal growth factor receptor (EGFR) pathway and its downstream targets ERK1/2 and Akt.

In sum, our findings present evidence that alpha5beta1 integrin-mediated cellular adhesion of NHA to PDL fibroblasts, whereas proliferation was caused by activation of the epidermal growth factor receptor (EGFR) and the MAP kinase (ERK1/2) and Akt pathways.