RGD Peptide Modulates Osteoblast Response to PEG-modified Surfaces

Objectives: Osteoblasts exhibit a more differentiated morphology on surfaces with rough microtopographies. Surface effects are mediated through integrins that bind the RGD motif in cell attachment proteins. We examined the response of MG63 immature osteoblast-like cells to rough Ti surfaces that were modified with adsorbed monomolecular layers of a comb-like graft copolymer, poly-(L-lysine)-g-poly(ethylene glycol) (PEG), functionalized with an integrin-receptor-active RGD peptide (PEG-RGD) and an inactive RDG control sequence (PEG-RDG). Methods: Ti surfaces were prepared by grit blasting and acid etching (SLA) and then modified by treatment with PEG graft copolymers with either 2% RGD or 2% RDG. Plastic and SLA disks + PEG were used as controls. To examine RGD dose-response, 0.144, 0.72, 3.6, and 18% RGD modified polymers were used. In addition, cells were cultured on disks coated with PEG or PEG-RGD(2%), and free RGDS was added to the media after the cells attached. Analyses were performed 24 h after cultures had achieved confluence on the plastic surface. Results: Cell number was reduced on SLA, and further decreased on surfaces coated with PEG or PEG-RDG. Cellular alkaline phosphatase specific activity (ALPase) was increased on SLA coated with PEG and PEG-RDG, but not with PEG-RGD. Cell layer homogenates, which contain ALP-enriched matrix vesicles, exhibited similar increases. Osteocalcin levels increased on SLA, PEG, and PEG-RDG (6X). TGF-b1 and PGE2 were increased in conditioned media of cells cultured on SLA, and there was a 20X increase on SLA+PEG. PEG effects were inhibited dose-dependently by addition of RGD to the surface. Free RGDS decreased the effect elicited by PEG surfaces. Conclusions: These unexpected results suggest that PEG may have osteogenic properties, perhaps correlated with effects that alter cell attachment and spreading, and promote a more differentiated morphology.