Editors' ChoiceApoptosis

Sphingolipids Lower the Death Threshold

Science Signaling  13 Mar 2012:
Vol. 5, Issue 215, pp. ec75
DOI: 10.1126/scisignal.2003033

Mitochondria play a central role in the apoptotic pathway by releasing cytochrome c, which nucleates a caspase-activation complex, in response to stimuli that promote mitochondrial outer membrane permeabilization (MOMP). The proapoptotic BCL-2 family members BAK and BAX oligomerize in response to prodeath signals to form pores that contribute to MOMP (see Hollville and Martin). BID, another BCL-2 family member, interacts with BAK or BAX to promote their oligomerization, MOMP, and cytochrome c release. Mitochondria interact with other organelles, such as the endoplasmic reticulum, where the initial steps in sphingolipid synthesis occur. Ceramide is a proapoptotic sphingolipid, as well as a precursor to other bioactive sphingolipids, such as sphingosine 1-phosphate (S1P). Chipuk et al. found that “dirty” mitochondrial preparations that had some heterotypic membranes (in that they were positive for endoplasmic reticulum) had a lower threshold for cytochrome c release in response to recombinant proapoptotic cleaved BID or a BID fragment than did “clean” mitochondrial preparations lacking contaminating membranes. Fractions of the portion that was removed from the clean preparations and that restored sensitivity to BID when added to the clean preparations were enriched in sphingomyelinases (SMases). In vitro transcribed and translated SMases cooperated with BID to promote MOMP from clean mitochondrial preparations. Pharmacological inhibitors confirmed the importance of SMase activity, and the BAK inhibitor BCL-xL confirmed the importance of BAK. Because the cells used for the BAK analysis do not have BAX, similar types of experiments were performed with clean mitochondrial preparations from bak, bax double-negative cells that had been reconstituted with BAX and showed a similar cooperative effect of SMases on apoptosis induced by BID. A battery of pharmacological inhibitors of the sphingomyelin metabolism pathway revealed that S1P promoted BAK-mediated MOMP and that hexadecenal (hex; a molecule derived from S1P) promoted BAX-mediated MOMP. SIP and hex promoted structural rearrangements in BAK and BAX, respectively, that occur during oligomerization as detected with conformation-specific antibodies. Lipid-binding assays confirmed a direct interaction between BAK and S1P and between BAX and hex. Overexpression of SMases enhanced the sensitivity to a death-promoting condition, and knockdown of sphingosine kinase activity conferred resistance to apopotosis, thus confirming the importance of sphingolipid metabolism in setting the threshold for cell death. Thus, the authors propose that lipids delivered from the endoplasmic reticulum or other membranous compartments provide the precursors for mitochondrial sphingolipid metabolism that controls the threshold for the induction of apoptosis.

J. E. Chipuk, G. P. McStay, A. Bharti, T. Kuwana, C. J. Clarke, L. J. Siskind, L. M. Obeid, D. R. Green, Sphingolipid metabolism cooperates with BAK and BAX to promote the mitochondrial pathway of apoptosis. Cell 148, 988–1000 (2012). [PubMed]

E. Hollville, S. J. Martin, Greasing the path to BAX/BAK activation. Cell 148, 845–846 (2012). [PubMed]