The Effects of Particulate Wear Debris, Cytokines, and Growth Factors on the Functions of MG-63 Osteoblasts

Background: Particle-challenged cells release cytokines, chemokines, and eicosanoids, which contribute to periprosthetic osteolysis. The particle-induced activation of macrophages and monocytes has been extensively studied, but only limited information is available on the response of osteoblasts to particulate wear debris. This study examines the effects of particulate wear debris, proinflammatory cytokines, and growth factors on osteoblast functions. Methods: MG-63 osteoblasts were treated with metal particles (titanium, titanium alloy, and chromium orthophosphate) or polymeric particles (polyethylene and polystyrene) of phagocytosable sizes or were treated with exogenous cytokines and growth factors. The kinetics of particle phagocytosis and the number of engulfed particles were assessed with use of fluoresceinated particles. Cell proliferation was determined according to [3H]-thymidine incorporation, and cell viability was determined by either fluorescein diacetate uptake or trypan blue exclusion. Expressions of osteoblast-specific genes were quantified with Northern blot hybridization, and the secretions of osteoblast-specific proteins and cytokines were analyzed by enzyme-linked immunosorbent assays. Results: MG-63 osteoblasts phagocytosed particles and became saturated after twenty-four hours. A maximum of forty to sixty particles per cell were phagocytosed. Each type of particle significantly suppressed procollagen a1[I] gene expression (p < 0.05), whereas other osteoblast-specific genes (osteonectin, osteocalcin, and alkaline phosphatase) did not show significant changes. Particle-stimulated osteoblasts released interleukin-6 (p < 0.05) and a smaller amount of transforming growth factor-b1. Particles reduced cell proliferation in a dose-dependent manner without affecting cell viability (p < 0.05). Exogenous tumor necrosis factor-a also enhanced the release of interleukin-6 (p < 0.01) and transforming growth factor-b1 (p < 0.05), whereas the secretion of transforming growth factor-b1 was increased by insulin-like growth factor-I and prostaglandin E2 as well. Insulin-like growth factor-I and transforming growth factor-b1 significantly increased procollagen a1[I] gene expression in osteoblasts (p < 0.05), while tumor necrosis factor-a and prostaglandin E2 significantly suppressed procollagen a1[I] gene expression (p < 0.01). In contrast, neither exogenous nor endogenous interleukin-6 had any effect on other cytokine secretion, on proliferation, or on procollagen a1[I] gene expression. The transcription inhibitor actinomycin D reduced both procollagen a1[I] transcription and interleukin-6 production. Inhibitors of protein synthesis (cyclohexamide) and intracellular protein transport (brefeldin A and monensin) blocked the release of interleukin-6, but none of these compounds influenced the suppressive effect of titanium on procollagen a1[I] gene expression. Conclusions: MG-63 osteoblasts phagocytose particulate wear debris, and this process induces interleukin-6 production and suppresses type-I collagen synthesis. Osteoblast-derived interleukin-6 may induce osteoclast differentiation and/or activation, but the resorbed bone cannot be replaced by new bone because of diminished osteoblast function (reduced type-I collagen synthesis). Exogenous cytokines (tumor necrosis factor-a and interleukin-1b), growth factors (insulin-like growth factor-I and transforming growth factor-b1), and prostaglandin E2 can modify particulate-induced alterations of osteoblast functions. Clinical Relevance: Altered osteoblast functions probably contribute to the progression of periprosthetic osteolysis. Suppressed osteoblast functions, however, could be compensated for by certain growth factors, such as insulin-like growth factor-I or transforming growth factor-b1. These growth factors, if delivered locally, may have therapeutic potential to prevent or reverse periprosthetic osteolysis.