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 ei cosanoids, which contribute to periprosthetic osteolysis. The particle-indu ced activation of macrophages and monocytes has been extensively studied, b ut only limited information is available on the response of osteoblasts to particulate wear debris. This study examines the effects of particulate wea r debris, proinflammatory cytokines, and growth factors on osteoblast funct ions. Methods: MG-63 osteoblasts were treated with metal particles (titanium, tit anium alloy, and chromium orthophosphate) or polymeric particles (polyethyl ene and polystyrene) of phagocytosable sizes or were treated with exogenous cytokines and growth factors. The kinetics of particle phagocytosis and th e number of engulfed particles were assessed with use of fluoresceinated pa rticles. Cell proliferation was determined according to [H-3]thymidine inco rporation, and cell viability was determined by either fluorescein diacetat e uptake or trypan blue exclusion. Expressions of osteoblast-specific genes were quantified with Northern blot hybridization, and the secretions of os teoblast-specific proteins and cytokines were analyzed by enzyme-linked imm unosorbent assays. Results: MG-63 osteoblasts phagocytosed particles and became saturated afte r twenty-four hours. A maximum of forty to sixty particles per cell were ph agocytosed. Each type of particle significantly suppressed procollagen alph a1[1] gene expression (p < 0.05), whereas other osteoblast-specjfic genes ( osteonectin, osteocalcin, and alkaline phosphatase) did not show significan t changes. Particle-stimulated osteoblasts released interleukin-6 (p < 0.05 ) and a smaller amount of transforming growth factor-pr. Particles reduced cell proliferation in a dose-dependent manner without affecting cell viabil ity (p < 0.05). Exogenous tumor necrosis factor-a also enhanced the release of interleukin-6 (p < 0.01) and transforming growth factor-beta1 (p < 0.05 ), whereas the secretion of transforming growth factor-<beta>1 was increase d by insulin-like growth factor-1 and prostaglandin E2 as well. Insulin-lik e growth factor-1 and transforming growth factor-beta1 significantly increa sed procollagen alpha1[1] gene expression in osteoblasts (p < 0.05), while tumor necrosis factor-<alpha> and prostaglandin E2 significantly suppressed procollagen alpha1[1] gene expression (p < 0.01), In contrast, neither exo genous nor endogenous interleukin-6 had any effect on other cytokine secret ion, on proliferation, or on procollagen <alpha>1[1] gene expression. The t ranscription inhibitor actinomycin D reduced both procollagen alpha1[1] tra nscription and interleukin-6 production. Inhibitors of protein synthesis (c yclohexamide) and intracellular protein transport (brefeldin A and monensin ) blocked the release of interleuhin-6, but none of these compounds influen ced the suppressive effect of titanium on procollagen alpha1[1] gene expres sion. Conclusions: MG-63 osteoblasts phagocytose particulate wear debris, and thi s process induces interleukin-6 production and suppresses type-1 collagen s ynthesis. Osteoblast-derived interleukin-6 may induce osteoclast differenti ation and/or activation, but the resorbed bone cannot be replaced by new bo ne because of diminished osteoblast function (reduced type-1 collagen synth esis). Exogenous cytokines (tumor necrosis factor-a and interleukin-1 beta) , growth factors (insulin-like growth factor-1 and transforming growth fact or-beta1), 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 insul in-like growth factor-1 or transforming growth factor-beta1. These growth f actors, if delivered locally, may have therapeutic potential to prevent or reverse periprosthetic osteolysis.