The induction of a catabolic phenotype in human primary osteoblasts and osteocytes by polyethylene particles

Polyethylene (PE) wear particles are associated with the osteolysis seen in aseptic loosening that leads to orthopaedic implant failure. While cells of the monocyte/macrophage lineage are implicated, evidence is now emerging that osteoblastic cells may also be affected by PE. In this study we investigated the effect of PE particles on osteoblasts, using a novel in vitro cell culture system that was developed to juxtapose cells and PE particles, replicating the 3-dimensional (3D) environment near implants. This system allowed normal human bone-derived cells (NHBC) to undergo differentiation into a mature osteocyte-like phenotype over a 21-28-day culture period. PE particles induced an increase in mRNA expression of the osteocyte markers E11, DMP-1 and SOST/sclerostin. NHBC responded to PE particles by increasing the mRNA expression of several genes associated with osteoclast formation and activity (RANKL, IL-8 and M-CSF) and decreased the expression of the osteoclast antagonist, OPG. PE also appeared to induce a switch in the RUNX2 control of gene expression from that of promoting matrix production (type I collagen) to inducing the expression of pro-osteoclastogenic genes. These results suggest that PE particles switch mature osteoblastic cells from an anabolic to a more catabolic phenotype. This concept was further supported by the finding that PE-induced expression of RANKL mRNA in the mouse osteocyte cell line, MLO-Y4. Overall, our results suggest that PE particles directly induce a change in the phenotype of mature osteoblasts and osteocytes, consistent with the net loss of bone near orthopaedic implants.