Fatty Acid Synthase binds endogenous dsRNAs and dampens the innate immune response to exogenous dsRNAs by limiting their accumulation

In mammalian cells, the presence of double-stranded RNA (dsRNA) in the cytoplasm is a danger signal indicative of viral infection. Cells establish and maintain an antiviral state via the production of interferon (IFN) in response to the detection of viral dsRNAs by specialized RNA-binding proteins (RBPs). In addition to viral dsRNA, endogenous (endo) dsRNAs can also be expressed and recognized either causing autoimmune diseases or regulating physiological processes, depending on their abundance, localization and modification profile. Although evidence on the regulatory role of endo-dsRNAs has increasingly been reported, the mechanisms behind their association with host RBPs and their immunogenicity remains largely misunderstood. To identify new endo-dsRNA-associated proteins and regulatory pathways, we induced the accumulation of endo-dsRNAs in wild type (WT) and ADAR1 KO HCT116 cells treated with the DNA methyltransferase inhibitor 5-azacytidin (5-AZA) and isolated the endo-dsRNA-associated proteome. Among the interactors, we identified the Fatty Acid Synthase (FASN), a key enzyme involved in lipid metabolism. We demonstrated that FASN associates with endo-dsRNAs already in untreated WT HCT116 cells and that this interaction increases upon 5-AZA treatment and in the absence of ADAR1. Moreover, FASN depletion leads to the accumulation of cytoplasmic endo-dsRNAs in close proximity to mitochondria, induces the expression of IFN-stimulated genes (ISGs) such as IFIT1, increases type III IFN secretion and MAVS aggregation. This correlates to a strong reduction of Sindbis virus (SINV) replication in FASN KO HCT116 cells, where ISG expression is significantly induced. This increase in antiviral response is also observed upon poly I:C treatment of FASN KO HCT116 cells. Overall, our data indicate that the accumulation of endo-dsRNAs in FASN-depleted cells induce a stronger innate immune response to exogenous dsRNAs, suggesting a positive feedback loop dependent on FASN expression.