CD95/Fas ligand mRNA is toxic to cells through more than one mechanism

Abstract

CD95/Fas ligand induces apoptosis through binding of the protein to the CD95 receptor. However, CD95L mRNA also induces toxicity in the absence of CD95. Dying cells exhibit features of DISE (Death Induced by Survival Gene Elimination), a form of cell death mediated by RNA interference (RNAi). DISE relies on targeting mediated by six nucleotides of complementarity between positions 2-7, the 6mer seed sequence of a RISC-bound (R-sRNA), and the 3UTR of an mRNA, a feature that allows to predict the effect of 6mer seed sequences on cell viability. We now report that CD95L mRNA processing generates an sRNA nearly identical to shL3, a commercial CD95L-targeting shRNA that led to the discovery of DISE. Neither of the miRNA biogenesis proteins Drosha or Dicer are required for CD95L mRNA processing. Interestingly, CD95L toxicity depends on the core component of the RISC, Ago 2, in some cell lines, but not in others. In the HCT116 colon cancer cell line, Ago 1-4 appear to function redundantly in RNAi. In fact, Ago 1/2/3 knockout cells retained sensitivity to CD95L mRNA toxicity. Toxicity was only blocked by mutation of all in-frame start codons in the CD95L ORF. Expression of a toxic CD95L mRNA caused an enrichment for R-sRNAs with toxic 6mer seed sequences, while expression of the nontoxic CD95L mutant enriched for loading of R-sRNAs with nontoxic 6mer seeds. However, CD95L was not the only source of these R-sRNAs. We found that CD95L mRNA may induce DISE directly and indirectly, and that alternate mechanisms may underlie CD95L mRNA processing and toxicity.