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Sphingomyelinase mediates macrophage activation by titanium particles independent of phagocytosis: A role for free radicals, NFkB, and TNFα
Author(s) -
Soloviev Alexander,
Schwarz Edward M.,
Darowish Michael,
O'Keefe Regis J.
Publication year - 2005
Publication title -
journal of orthopaedic research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.041
H-Index - 155
eISSN - 1554-527X
pISSN - 0736-0266
DOI - 10.1016/j.orthres.2005.03.019.1100230604
Subject(s) - sphingomyelin , phagocytosis , microbiology and biotechnology , chemistry , lipid peroxidation , macrophage , ceramide , signal transduction , intracellular , gpx4 , glutathione , biochemistry , oxidative stress , biology , glutathione peroxidase , enzyme , membrane , apoptosis , in vitro
The manner in which wear debris initiates intracellular signaling and macrophage activation remains poorly understood. While particle phagocytosis has been implicated in this process, recent studies have shown that phagocytosis is not required for macrophage activation. We examined the hypothesis that titanium particles stimulate macrophages through membrane associated signaling events involving free radicals, sphingomyelinase, NFkB, and TNFα. Titanium particles stimulated peroxidation of linoleic acid, producing malondialdehyde, while neither lipopolysaccharide nor PBS pre‐incubated with particles did, suggesting that the increased peroxidation is related to the presence of the particles themselves. Furthermore, particles stimulated sphingomyelin metabolism in a neutral sphingomyelinase (NSmase) containing cell free system; this effect was inhibited by glutathione, indicating that NSmase activation was due to titanium induced free radicals. Titanium particles also stimulated NSmase activity in cultures of ANA‐1 murine macrophages. Addition of purified NSmase to ANA‐1 cell cultures stimulated NFkB binding, increased transcriptional activity in cells transfected with NFkB responsive promoters, and induced TNFα expression. These effects were also inhibited by addition of glutathione. Similarly, glutathione inhibited the ability of titanium particles to induce NFkB signaling and TNFα expression in ANA‐1 cells. The findings demonstrate that titanium particles generate free radicals and induce plasma membrane peroxidation and NSmase activation. NSmase, in turn, hydrolyzes sphingomyelin, with activation of the NFkB signaling pathway and induction of responsive genes, including TNFα. This study demonstrates a mechanism for phagocytosis‐independent macrophage activation and defines the sphingomyelin cycle as a potential therapeutic target for the prevention of wear debris induced osteolysis. © 2005 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved.