Exposure to ionizing radiation stimulates the C. elegans p53-dependent and ceramide biosynthesis pathways to activate core mitochondrial apoptotic machinery.
The mechanisms underlying radiation-induced cell death remain somewhat controversial. One suggestion is that ceramide functions as a crucial transducer of apoptosis after exposure to ionizing radiation. However, until now, evidence for a direct link between ceramide synthesis and apoptosis pathways has been lacking. Xinzhu Deng and colleagues now clarify this relationship.
Using Caenorhabditis elegans to model radiation-induced apoptosis, Deng and colleagues carried out a screen to identify sphingolipid mutants in which gonadal germ cells were resistant to apoptosis after exposure to ionizing radiation. Only two mutants were found, and analyses revealed that these were deletion mutants that caused the disruption of the catalytic domains in two of the three ceramide synthase genes: hyl-1 and lagr-1. Although developmental somatic cell death was unaffected by these mutants (hyl-1(ok976) and lagr-1(gk327)), radiation-induced germ cell apoptosis was almost completely abrogated in both single and double mutants, reiterating the role for ceramide synthesis specifically in radiation-induced apoptosis. Microinjection of C16-ceramide — the species thought to be involved in apoptosis transduction according to work in mammalian models — qualitatively and quantitatively mimicked radiation-induced germ cell apoptosis in non-irradiated wild-type worms. Moreover, microinjection of low-dose C16-ceramide restored radiation-induced germ cell apoptosis in hyl-1(ok976);lagr-1(gk327) double mutants, showing that ceramide is a crucial effector of radiation-induced apoptosis.
So, how does ceramide mediate these effects? In the worm, radiation-induced DNA damage activates a transcriptional response that is dependent on CEP-1 (the homologue of p53), increasing the expression of the BH3-only proteins EGL-1 and CED-13. These in turn activate the core apoptotic machinery in the mitochondria. Surprisingly, the authors found that egl-1 and ced-13 transcripts were increased as normal after hyl-1(ok976);lagr-1(gk327) mutants were irradiated. Further, they showed that ceramide acts in parallel to the BH3-only proteins, upstream of the mitochondrial core apoptotic machinery, to mediate radiation-induced apoptosis. Consistently, they saw ceramide accumulation in the mitochondria after irradiation, which was not detectable in the hyl-1(ok976);lagr-1(gk327) mutant. In addition, they showed that CED-4 (a member of the core apoptotic machinery) was no longer relocalized to the nuclear membrane by EGL-1 in the hyl-1(ok976);lagr-1(gk327) mutant, and proposed that ceramide might create a permissive environment for CED-4 trafficking from the mitochondria. Therefore, the CEP-1-dependent and ceramide synthesis pathways coordinately activate the core apoptotic machinery in the mitochondria to commit germ cells to apoptosis after exposure to ionizing radiation.
Together, these data characterize a crucial cell death signalling pathway that is specific to responses to ionizing radiation exposure and is probably conserved in mammals. Further understanding of this process might lead to improvements in the efficacy of radiotherapy.
Gemma K. Alderton
References
Deng, X. et al. Ceramide biogenesis is required for radiation-induced apoptosis in the germ line of C. elegans. Science322, 110–115 (2008) | Article | PubMed |
