Research ArticleCELL DEATH

OAS1/RNase L executes RIG-I ligand–dependent tumor cell apoptosis

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Science Immunology  16 Jul 2021:
Vol. 6, Issue 61, eabe2550
DOI: 10.1126/sciimmunol.abe2550

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Executing tumor cell death

There is increasing interest in developing cancer immunotherapies that target the innate immune pathways regulating cytokine production and cell death, but the interplay between these two closely connected processes is not well understood. In mouse and human cancer cell lines, Boehmer et al. demonstrate that cytokine production and apoptosis induced by retinoic acid–inducible gene I (RIG-I) ligands, including 5′-triphosphate RNA (3p-RNA), are two separable events in which RIG-I is required for production of type I interferon but not execution of apoptosis. Mass spectrometry and loss-of-function assays showed that 3p-RNA directly activates OAS1 and RNase L, which promoted translational arrest and depletion of antiapoptotic MCL-1. These results demonstrate that RIG-I–mediated apoptosis involves priming and effector stages, reminiscent of inflammasome activation, both of which could serve as potential targets for cancer immunotherapy.

Abstract

Cytoplasmic double-stranded RNA is sensed by RIG-I–like receptors (RLRs), leading to induction of type I interferons (IFN-Is), proinflammatory cytokines, and apoptosis. Here, we elucidate signaling mechanisms that lead to cytokine secretion and cell death induction upon stimulation with the bona fide RIG-I ligand 5′-triphosphate RNA (3p-RNA) in tumor cells. We show that both outcomes are mediated by dsRNA-receptor families with RLR being essential for cytokine production and IFN-I–mediated priming of effector pathways but not for apoptosis. Affinity purification followed by mass spectrometry and subsequent functional analysis revealed that 3p-RNA bound and activated oligoadenylate synthetase 1 and RNase L. RNase L–deficient cells were profoundly impaired in their ability to undergo apoptosis. Mechanistically, the concerted action of translational arrest triggered by RNase L and up-regulation of NOXA was needed to deplete the antiapoptotic MCL-1 to cause intrinsic apoptosis. Thus, 3p-RNA–induced apoptosis is a two-step process consisting of RIG-I–dependent priming and an RNase L–dependent effector phase.

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