This study addressed the contribution of acidic sphingomyelinase (ASMase) in TNF-α–mediated hepatocellular apoptosis. Cultured hepatocytes depleted of mitochondrial glutathione (mGSH) became sensitive to TNF-α, undergoing a time-dependent apoptotic cell death preceded by mitochondrial membrane depolarization, cytochrome c release, and caspase activation. Cyclosporin A treatment rescued mGSH-depleted hepatocytes from TNF-α–induced cell death. In contrast, mGSH-depleted hepatocytes deficient in ASMase were resistant to TNF-α–mediated cell death but sensitive to exogenous ASMase. Furthermore, although in vivo administration of TNF-α or LPS to galactosamine-pretreated ASMase+/+ mice caused liver damage, ASMase–/– mice exhibited minimal hepatocellular injury. To analyze the requirement of ASMase, we assessed the effect of glucosylceramide synthetase inhibition on TNF-α–mediated apoptosis. This approach, which blunted glycosphingolipid generation by TNF-α, protected mGSH-depleted ASMase+/+ hepatocytes from TNF-α despite enhancement of TNF-α–stimulated ceramide formation. To further test the involvement of glycosphingolipids, we focused on ganglioside GD3 (GD3) because of its emerging role in apoptosis through interaction with mitochondria. Analysis of the cellular redistribution of GD3 by laser scanning confocal microscopy revealed the targeting of GD3 to mitochondria in ASMase+/+ but not in ASMase–/– hepatocytes. However, treatment of ASMase–/– hepatocytes with exogenous ASMase induced the colocalization of GD3 and mitochondria. Thus, ASMase contributes to TNF-α–induced hepatocellular apoptosis by promoting the mitochondrial targeting of glycosphingolipids.
Carmen García-Ruiz, Anna Colell, Montserrat Marí, Albert Morales, María Calvo, Carlos Enrich, José C. Fernández-Checa
Guidelines: The Editorial Board will only consider letters that we deem relevant and of interest to our readers. We will not post data that have not been subjected to peer review, nor will we post letters that are essentially a reiteration of another letter. We reserve the right to edit any letter for length, content, and clarity. Authors will be notified by e-mail if their letters were accepted. No appeals will be considered.
Specific requirements: All letters must be 400 words or fewer. You may enter the letter as plain text or HTML. The author's name and e-mail address are required, and will be posted with the letter. All possible conflicts of interest must be noted, even if they are not posted. If you wish to include a figure (keep in mind that non-peer-reviewed data will not be posted), please contact the editors directly at firstname.lastname@example.org.