Absence of autophagy results in reactive oxygen species-dependent amplification of RLR signaling

Abstract

Autophagy is a highly conserved process that maintains homeostasis by clearing damaged organelles and long-lived proteins. The consequences of deficiency in autophagy manifest in a variety of pathological states including neurodegenerative diseases, inflammatory disorders, and cancer. Here, we studied the role of autophagy in the homeostatic regulation of innate antiviral defense. Single-stranded RNA viruses are recognized by the members of the RIG-I-like receptors (RLRs) in the cytosol. RLRs signal through IPS-1, resulting in the production of the key antiviral cytokines, type I IFNs. Autophagy-defective Atg5 ؊/؊ cells exhibited enhanced RLR signaling, increased IFN secretion, and resistance to infection by vesicular stomatitis virus. In the absence of autophagy, cells accumulated dysfunctional mitochondria, as well as mitochondriaassociated IPS-1. Reactive oxygen species (ROS) associated with the dysfunctional mitochondria were largely responsible for the enhanced RLR signaling in Atg5 ؊/؊ cells, as antioxidant treatment blocked the excess RLR signaling. In addition, autophagy-independent increase in mitochondrial ROS by treatment of cells with rotenone was sufficient to amplify RLR signaling in WT cells. These data indicate that autophagy contributes to homeostatic regulation of innate antiviral defense through the clearance of dysfunctional mitochondria, and revealed that ROS associated with mitochondria play a key role in potentiating RLR signaling.innate immunity ͉ interferon ͉ reactive oxygen species ͉ virus infection ͉ mitochondria A utophagy is an ancient evolutionarily conserved pathway designed to maintain cellular homeostasis by degrading long-lived proteins and organelles in the cytosol. It has also been studied extensively as a critical survival mechanism during starvation conditions (1, 2). Recent studies demonstrated that autophagy is used by the cells of the innate and adaptive immune systems to combat viral infections (3, 4).Current paradigm suggests that autophagy or the molecules required for autophagy could regulate these innate viral recognition pathways in distinct ways. RNA viruses are recognized by two distinct innate sensors (5). In plasmacytoid dendritic cells (pDC), recognition of ssRNA viruses occurs in the endosomes via Toll-like receptors 7 and 8 (6-8). Autophagy plays a key role in the recognition of certain ssRNA viruses by delivering viral replication intermediate from the cytosol to the endosome, where it engages Toll-like receptor 7 activation in pDCs (9). In contrast to pDCs, most other cell types of the body use cytosolic sensors of viral replication via retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA-5) belonging to the RLR family (10-13). A recent study reported by Jounai et al. demonstrated that innate recognition of vesicular stomatitis virus (VSV) in mouse embryonic fibroblasts (MEFs) via the RIG-I pathway is negatively regulated by the Atg5-Atg12 conjugate (14). This study showed that type I interferons and cytokine production...

Autophagy: for better or for worse

“…In contrast, Th2 cytokines, such as interleukin (IL)-4 and IL-13, inhibit autophagy [105]. Conversely, cytokine production can be regulated by autophagy, as shown by the increased production of potent anti-viral factors upon RIG-I receptor activation in the absence of autophagy [106], enhanced IFN-β production in dsDNA-stimulated Atg9a-deleted mouse embryonic fibroblast cells [73], and increased IL-1β and IL-18 production in autophagy-deficient macrophages (Atg16 −/− , LC3B −/− and Beclin-1 +/− ) [74,107]. In relation to this, it has been reported that inhibition of autophagy by Beclin-1 or Atg5 knockdown results in reactive oxygen species (ROS) accumulation and sensitization of NLRP3 inflammasome activation [108].…”

Autophagy Negatively Regulates Cell Death by Controlling NPR1-Dependent Salicylic Acid Signaling during Senescence and the Innate Immune Response inArabidopsis

“…In yeast and mammals, cells lacking autophagy accumulate dysfunctional mitochondria, resulting in an increase in ROS production (Zhang et al, 2007;Tal et al, 2009). In Arabidopsis, however, high levels of H 2 O 2 in the atg2 and atg5 mutants might not be due to dysfunctional mitochondria.…”

Loss of autophagy enhances MIF/macrophage migration inhibitory factor release by macrophages

“…12,13 Previous reports have demonstrated that loss of autophagy in macrophages and dendritic cells (DCs) leads to increased secretion of pro-inflammatory cytokines, including IL6, type I IFN and the IL1 (interleukin 1) family members IL1A, IL1B and IL18. [14][15][16][17][18] Moreover, through effects on IL1B, loss of autophagy leads to increased IL23 secretion by macrophages and DCs, and supernatant fractions from these LPS-treated autophagy-defective cells, high in IL1B and IL23, enhance the secretion of IL17, IL22 and IFNG by innate gd T cells. 19 Similarly, mice with deletion of the autophagy gene Atg5 in myeloid cells produce more IL1A and IL17 in response to infection with Mycobacterium tuberculosis.…”