Involvement of FAK-mediated BMP-2/Smad pathway in mediating osteoblast adhesion and differentiation on nano-HA/chitosan composite coated titanium implant under diabetic conditions

Chitosan (CS)-based hydroxyapatite (HA) composites have emerged as a novel strategy for promoting bone regeneration. Here nanophase HA/CS composite coated porous titanium implants (nCT) were fabricated and their biological behavior under diabetic conditions was investigated. We proposed that the focal adhesion kinase (FAK)-mediated BMP-2/Smad pathway played a role in mediating the promotive effect of nCTs on osteoblast adhesion and differentiation under diabetes-induced high reactive oxygen species (ROS) condition. To confirm the hypothesis, rat osteoblasts on bare titanium implants (Ti) and nCT were subjected to normal serum (NS), diabetic serum (DS), DS + NAC (a potent ROS inhibitor) and DS + cytochalasin D (an actin polymerization inhibitor). In vivo on diabetic sheep implanted with Ti or nCT showed that diabetes-induced ROS overproduction impaired osteoblast adhesion, evidenced by immunostaining of F-actin and vinculin and morphological observation through inhibition of FAK phosphorylation, which contributed to suppressed BMP-2-dependent Smad1/5/8 phosphorylation. nCT substrate reactivated the FAK-BMP-2/Smad pathway, thus reversing osteoblast dysfunction, which exerted a similar effect to NAC treatment on Ti. These effects were further confirmed by improved osteointegration within nCT in diabetic sheep, evidenced by micro-CT and histological examinations. Our study demonstrated that reactivation of the FAK-BMP-2/Smad pathway was involved in improving osteoblast adhesion and differentiation by nano-HA/CS composite coating, potentially directing biomaterial modification and biofunctionalization under diabetic conditions.