Nitric oxide and prostaglandin E₂ production in response to ultra-high molecular weight polyethylene particles depends on osteoblast maturation state

Background: Recent studies have shown that osteoblast-like cells respond directly to ultra-high molecular weight polyethylene particles in culture, suggesting that they may be involved in aseptic loosening of endoprostheses. We tested the hypothesis that the state of cell maturation plays a role in the response of osteogenic cells to ultra-high molecular weight polyethylene particles. Methods: MG63 cells (immature osteoblast-like cells), OCT-1 cells (mature secretory osteoblast-like cells), and MLO-Y4 cells (osteocyte-like cells) were treated for twenty-four hours with commercial ultra-high molecular weight polyethylene particles with an average diameter of 1 μm. The effect of particle treatment on cell proliferation was assessed by measuring the number of cells, whereas the effects on differentiation and local factor production were assessed by measuring the production of osteocalcin, prostaglandin E₂, and nitric oxide. The effect of particles on apoptosis was also evaluated. Results: The addition of ultra-high molecular weight polyethylene particles increased the number of MG63 cells, did not affect the number of OCT-1 cells, and led to a decrease in the number of MLO-Y4 cells. The observed changes in cell number were not due to programmed cell death, as no more than 3% of the cells in cultures treated with the highest concentration of particles were undergoing apoptosis. Osteocalcin production was not affected by the addition of particles. Prostaglandin E₂ production was increased in all three types of cultures, but the effect was greatest in OCT-1 cell cultures, as was the absolute amount of prostaglandin E₂ produced. Nitric oxide production was unaffected in MG63 cell cultures, but it was stimulated in OCT-1 and MLO-Y4 cell cultures. Conclusions: The results of the present study support the hypothesis that osteoblast cell maturation state plays an important role in the response to ultra-high molecular weight polyethylene particles and that the terminally differentiated osteocyte may be involved in the bone response to wear debris in vivo. Clinical Relevance: Because maturation state in the osteoblast lineage is an important factor in the cellular response to wear debris and the osteocyte is surprisingly sensitive to ultra-high molecular weight polyethylene particles, and because these cells have been shown to be in direct contact with the bone-implant interface via canaliculi, their role in aseptic loosening needs to be considered.