title: Differential regulation of acid sphingomyelinase in macrophages stimulated with oxidized low-density lipoprotein (LDL) and oxidized LDL immune complexes: role in phagocytosis and cytokine release.
abstract: Oxidized low-density lipoprotein (oxLDL) and oxLDL-containing immune complexes (oxLDL-IC) contribute to the formation of lipid-laden macrophages (foam cells). Fcγ receptors mediate uptake of oxLDL-IC, whereas scavenger receptors internalize oxLDL. We have previously reported that oxLDL-IC, but not free oxLDL, activate macrophages and prolong their survival. Sphingomyelin is a major constituent of cell membranes and lipoprotein particles and acid sphingomyelinase (ASMase) hydrolyses sphingomyelin to generate the bioactive lipid ceramide. ASMase exists in two forms: lysosomal (L-ASMase) and secretory (S-ASMase). In this study we examined whether oxLDL and oxLDL-IC regulate ASMase differently, and whether ASMase mediates monocyte/macrophage activation and cytokine release. The oxLDL-IC, but not oxLDL, induced early and consistent release of catalytically active S-ASMase. The oxLDL-IC also consistently stimulated L-ASMase activity, whereas oxLDL induced a rapid transient increase in L-ASMase activity before it steadily declined below baseline. Prolonged exposure to oxLDL increased L-ASMase activity; however, activity remained significantly lower than that induced by oxLDL-IC. Further studies were aimed at defining the function of the activated ASMase. In response to oxLDL-IC, heat-shock protein 70B' (HSP70B') was up-regulated and localized with redistributed ASMase in the endosomal compartment outside the lysosome. Treatment with oxLDL-IC induced the formation and release of HSP70-containing and IL-1β-containing exosomes via an ASMase-dependent mechanism. Taken together, the results suggest that oxLDL and oxLDL-IC differentially regulate ASMase activity, and the pro-inflammatory responses to oxLDL-IC are mediated by prolonged activation of ASMase. These findings may contribute to increased understanding of mechanisms mediating macrophage involvement in atherosclerosis.
authors.affiliation: Department of Regenerative Medicine and Cell Biology, Medical University of South Carolina, Charleston, SC 29425, USA.
authors.forename: Jean-Philip
authors.initials: JP
authors.lastname: Truman
authors.forename: Mohammed M
authors.initials: MM
authors.lastname: Al Gadban
authors.forename: Kent J
authors.initials: KJ
authors.lastname: Smith
authors.forename: Russell W
authors.initials: RW
authors.lastname: Jenkins
authors.forename: Nalini
authors.initials: N
authors.lastname: Mayroo
authors.forename: Gabriel
authors.initials: G
authors.lastname: Virella
authors.forename: Maria F
authors.initials: MF
authors.lastname: Lopes-Virella
authors.forename: Alicja
authors.initials: A
authors.lastname: Bielawska
authors.forename: Yusuf A
authors.initials: YA
authors.lastname: Hannun
authors.forename: Samar M
authors.initials: SM
authors.lastname: Hammad
mesh_terms: D000818:Animals; D002460:Cell Line; D016207:Cytokines / Q000276:immunology / Q000378:metabolism*; D055354:Exosomes / Q000276:immunology / Q000378:metabolism; D006801:Humans; D008077:Lipoproteins, LDL / Q000276:immunology*; D008247:Lysosomes / Q000276:immunology / Q000378:metabolism; D008264:Macrophages / Q000201:enzymology* / Q000276:immunology* / Q000378:metabolism; D051379:Mice; D010587:Phagocytosis; D013108:Sphingomyelin Phosphodiesterase / Q000276:immunology* / Q000378:metabolism
chemical_list: D016207:Cytokines; D008077:Lipoproteins, LDL; C099252:oxidized low density lipoprotein; D013108:Sphingomyelin Phosphodiesterase