title: Lipid rafts uncouple surface expression of transmembrane TNF-alpha from its cytotoxicity associated with ICAM-1 clustering in Raji cells.
abstract: Since transmembrane tumor necrosis factor-alpha (tmTNF-alpha) has been reported to have a palmitoylated site at Cys(-47), and therefore its functions may be linked to lipid raft membrane microdomains. The present study tested a hypothesis that lipid rafts may serve as a signaling platform to mediate the bioactivity of tmTNF-alpha. We found that destruction of lipid rafts with methyl-beta-cyclodextrin (MCD) in Raji cells almost completely blocked the cytotoxicity of tmTNF-alpha, as did an anti-TNF-alpha antibody. Although a proportion of tmTNF-alpha was colocated with lipid rafts, either the replacement of Cys at -47 by Ala, destructing its possible lipid rafts-attaching site or the displacement of its cytoplasmic domain by the C-terminal sequence (131-157) of caveolin-1, making all tmTNF-alpha target to lipid rafts, had no effect on tmTNF-alpha cytotoxicity. The data suggest that the cytotoxicity of tmTNF-alpha is not associated with its lipid rafts location. Unparallel to decreased cytotoxicity, moreover, MCD significantly increased tmTNF-alpha expression on the cell surface, and these increased tmTNF-alpha molecules were capable of binding to sTNFR1. To further explore the mechanism of lipid rafts-mediated cytotoxicity of tmTNF-alpha, we demonstrated that MCD led to a marked decrease in adhesion of Raji cells to T24 cells, which was due to dissociation of adhesion molecule ICAM-1 from lipid rafts. These results indicate that lipid rafts importantly participate in the cytotoxicity of tmTNF-alpha through ICAM-1 clustering and consequent enhancement of the cell-cell contact. The data suggest that lipid rafts are essential for the killing of tmTNF-alpha through the cell-cell contact mediated by ICAM-1. However, lipid rafts may limit exposure of tmTNF-alpha to the cell surface.
authors.affiliation: Department of Immunology, Tongji Medical College, Huazhong University of Science and Technology, 13 Hangkong Road, Wuhan, Hubei 430030, China.
authors.forename: Shu
authors.initials: S
authors.lastname: Zhang
authors.forename: Tao
authors.initials: T
authors.lastname: Liu
authors.forename: Huifang
authors.initials: H
authors.lastname: Liang
authors.forename: Hailong
authors.initials: H
authors.lastname: Zhang
authors.forename: Dan
authors.initials: D
authors.lastname: Yan
authors.forename: Nidan
authors.initials: N
authors.lastname: Wang
authors.forename: Xiaodan
authors.initials: X
authors.lastname: Jiang
authors.forename: Wei
authors.initials: W
authors.lastname: Feng
authors.forename: Jing
authors.initials: J
authors.lastname: Wang
authors.forename: Pinlan
authors.initials: P
authors.lastname: Li
authors.forename: Zhuoya
authors.initials: Z
authors.lastname: Li
mesh_terms: D000954:Antigens, Surface / Q000378:metabolism; D002448:Cell Adhesion / Q000276:immunology; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000378:metabolism*; D003602:Cytotoxicity, Immunologic / Q000502:physiology; D006367:HeLa Cells; D006801:Humans; D007111:Immunity, Cellular / Q000502:physiology; D018799:Intercellular Adhesion Molecule-1 / Q000378:metabolism*; D021962:Membrane Microdomains / Q000502:physiology*; D011485:Protein Binding; D055503:Protein Multimerization / Q000276:immunology; D021381:Protein Transport; D018124:Receptors, Tumor Necrosis Factor / Q000378:metabolism; D014409:Tumor Necrosis Factor-alpha / Q000276:immunology* / Q000378:metabolism* / Q000502:physiology
chemical_list: D000954:Antigens, Surface; D018124:Receptors, Tumor Necrosis Factor; D014409:Tumor Necrosis Factor-alpha; D018799:Intercellular Adhesion Molecule-1