title: Alpha particles induce apoptosis through the sphingomyelin pathway.
abstract: The sphingomyelin pathway involves the enzymatic cleavage of sphingomyelin to produce ceramide, a second messenger that serves as a key mediator in the rapid apoptotic response to various cell stressors. Low-linear energy transfer (LET) γ radiation can initiate this pathway, independent of DNA damage, via the cell membrane. Whether short-ranged, high-LET α particles, which are of interest as potent environmental carcinogens, radiotherapies and potential components of dirty bombs, can act through this mechanism to signal apoptosis is unknown. Here we show that irradiation of Jurkat cells with α particles emitted by the ²²⁵Ac-DOTA-anti-CD3 IgG antibody construct results in dose-dependent apoptosis. This apoptosis was significantly reduced by pretreating cells with cholesterol-depleting nystatin, a reagent known to inhibit ceramide signaling by interfering with membrane raft coalescence and ceramide-rich platform generation. The effects of nystatin on α-particle-induced apoptosis were related to disruption of the ceramide pathway and not to microdosimetry alterations, because similar results were obtained after external irradiation of the cells with a broad beam of collimated α particles using a planar ²⁴¹Am source. External irradiation allowed for more precise control of the dosimetry and geometry of the irradiation, independent of antibody binding or cell internalization kinetics. Mechanistically consistent with these findings, Jurkat cells rapidly increased membrane concentrations of ceramide after external irradiation with an average of five α-particle traversals per cell. These data indicate that α particles can activate the sphingomyelin pathway to induce apoptosis.
authors.affiliation: a  Memorial Sloan-Kettering Cancer Center, Department of Chemistry and Molecular Pharmacology, New York, New York 10065.
authors.forename: Jonathan H
authors.initials: JH
authors.lastname: Seideman
authors.forename: Branka
authors.initials: B
authors.lastname: Stancevic
authors.forename: Jimmy A
authors.initials: JA
authors.lastname: Rotolo
authors.forename: Michael R
authors.initials: MR
authors.lastname: McDevitt
authors.forename: Roger W
authors.initials: RW
authors.lastname: Howell
authors.forename: Richard N
authors.initials: RN
authors.lastname: Kolesnick
authors.forename: David A
authors.initials: DA
authors.lastname: Scheinberg
mesh_terms: D000186:Actinium / Q000009:adverse effects; D000512:Alpha Particles / Q000009:adverse effects*; D017209:Apoptosis / Q000187:drug effects / Q000528:radiation effects*; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism; D007553:Isotope Labeling; D019169:Jurkat Cells; D009761:Nystatin / Q000494:pharmacology; D011874:Radiometry; D013109:Sphingomyelins / Q000378:metabolism*
chemical_list: D007074:Immunoglobulin G; D013109:Sphingomyelins; D009761:Nystatin; D000186:Actinium