Unraveling the molecular principles by which ceramides commit cells to death

Abstract

Ceramides are central intermediates of sphingolipid metabolism that can activate a variety of tumor suppressive cellular programs, including cell cycle arrest, senescence and apoptosis. Indeed, perturbations in ceramide generation and turnover are frequently linked to cancer cell survival and resistance to chemotherapy. Consequently, the potential of ceramide-based therapeutics in the treatment of cancer has become a major focus of interest. A growing body of evidence indicates that ceramides can act directly on mitochondria to trigger apoptotic cell death. However, molecular details of the underlying mechanism are scarce. In our recent study (Dadsena S et al., 2019, Nat Commun 10:1832), we used a photoactivatable ceramide probe combined with computer simulations and functional studies to identify the voltage-dependent anion channel VDAC2 as a critical effector of ceramide-induced mitochondrial apoptosis. Collectively, our findings provide a novel molecular framework for how ceramides execute their widely acclaimed anti-neoplastic activities.

Keywords: ceramide transfer protein; hexokinase; mitochondrial apoptosis; photo-activatable lipids; tumor suppressor lipid; voltage-dependent anion channel.

Figure 1. FIGURE 1: Mitochondrial targeting of CERT induces Bax-dependent apoptosis.

Ceramides are synthesized through N-acylation of long-chain bases (LCB) by ceramide synthases (CerS) on the cytosolic surface of the endoplasmic reticulum (ER). The bulk of newly synthesized ceramides are transferred to the Golgi by the ceramide transfer protein CERT for metabolic conversion into sphingomyelin by a Golgi-resident SM synthase (SMS). Expression of mitoCERT, a CERT variant in which the Golgi-targeting pleckstrin homology domain was replaced by an outer mitochondrial membrane anchor, triggers Bax-dependent apoptosis by diverting the biosynthetic flow of ceramides to mitochondria. Figure adapted from Dadsena et al. (2019), Nat Commun 10:1832. doi: 10.1038/s41467-019-09654-4

Figure 2. FIGURE 2: Model of how ceramides may exert their tumor suppressor activities.

(A) Under non-stressed conditions, the bulk of hexokinases (HK) is bound to VDACs. This interaction antagonizes cell death by blocking mitochondrial translocation of Bax and renders cancer cells hyperglycolytic by enhancing the phosphorylation of glucose, the rate-limiting step in glycolysis. As ceramides bind VDACs at a site that overlaps with the binding site for HK, the arrival of these stress mediators in mitochondria may promote dissociation of HK from VDACs. This would revert the hyperglycolytic state of cancer cells as well as sensitize them to Bax-dependent apoptosis. (B) Binding of HK and ceramide to VDACs in each case is critically dependent on a Glu residue that faces the hydrophobic membrane interior. We postulate that ceramide binding facilitates VDAC dimerization/oligomerization, a process that hinders the association of HK and helps create mitochondrial platforms for Bax translocation. See text for further details.