The RNA helicase RIG-I plays a key role in sensing pathogen-derived RNA. Double-stranded RNA structures bearing 5′-tri- or diphosphates are commonly referred to as activating RIG-I ligands. However, endogenous RNA fragments generated during viral infection via RNase L also activate RIG-I. Of note, RNase-digested RNA fragments bear a 5′-hydroxyl group and a 2′,3′-cyclic phosphate. How endogenous RNA fragments activate RIG-I despite the lack of 5′-phosphorylation has not been elucidated. Here we describe an endogenous RIG-I ligand (eRL) that is derived from the internal transcribed spacer 2 region (ITS2) of the 45S ribosomal RNA after partial RNase A digestion in vitro, RNase A protein transfection or RNase L activation. The immunostimulatory property of the eRL is dependent on 2′,3′-cyclic phosphate and its sequence is characterized by a G-quadruplex containing sequence motif mediating guanosine-5′-triphosphate (GTP) binding. In summary, RNase generated self-RNA fragments with 2′,3′-cyclic phosphate function as nucleotide-5′-triphosphate binding aptamers activating RIG-I.

Inhibition of the androgen receptor (AR) is a major target of prostate cancer (PCa) therapy. However, prolonged androgen deprivation results eventually in castration-resistant PCa (CRPC) with metastasis and poor survival. Emerging evidence suggests that epithelial-mesenchymal transition (EMT) may facilitate castration-resistance and cancer metastasis in PCa. The human androgen-dependent, castration-sensitive prostate cancer (CSPC) cell line LNCaP and the CRPC cell line C4-2 are often used as a model system for human PCa. However, the role of the AR and the effect of AR antagonist (antiandrogen) treatment on the RNA expression of key factors of EMT including the long non-coding RNAs (lncRNAs) DRAIC in PCa cells remain elusive. Although as expected the established AR target genes PSA and FKBP5 are strongly induced by androgens in both cell lines, both E-cadherin and vimentin mRNA levels are upregulated by androgens in LNCaP but not in C4-2 cells by short- and long-term treatments. The mRNA levels of E-cadherin and vimentin remain unchanged by antiandrogen treatment in both cell lines. The expression of transcription factors that regulate EMT including Slug, Snail and ZEB1 and the lncRNA DRAIC were affected by androgen treatment in both cell lines. The mRNA level of Slug is upregulated by androgens and interestingly downregulated by antiandrogens in both cell lines. On the other hand, ZEB1 mRNA levels are strongly upregulated by androgens but remain unchanged by antiandrogens. In contrast, Snail mRNA levels are repressed by androgen treatment similar to DRAIC RNA levels. However, while antiandrogen treatment seems not to change Snail mRNA levels, antiandrogen treatments induce DRAIC RNA levels. Moreover, despite the strong upregulation of Zeb1 mRNA, no significant increase of the ZEB1 protein was observed indicating that despite androgen upregulation, posttranscriptional regulation of EMT controlling transcription factors occurs. SLUG protein was enhanced in both cell lines by androgens and reduced by antiandrogens. Taken together, our data suggest that the ligand-activated AR regulates the expression of several EMT key factors and antiandrogens counteract AR activity only on selected genes.

Defects in nucleic acid metabolizing enzymes can lead to spontaneous but selective activation of either cGAS/STING or RIG-like receptor (RLR) signaling, causing type I interferon–driven inflammatory diseases. In these pathophysiological conditions, activation of the DNA sensor cGAS and IFN production are linked to spontaneous DNA damage. Physiological, or tonic, IFN signaling on the other hand is essential to functionally prime nucleic acid sensing pathways. Here, we show that low-level chronic DNA damage in mice lacking the Aicardi-Goutières syndrome gene SAMHD1 reduced tumor-free survival when crossed to a p53-deficient, but not to a DNA mismatch repair-deficient background. Increased DNA damage did not result in higher levels of type I interferon. Instead, we found that the chronic interferon response in SAMHD1-deficient mice was driven by the MDA5/MAVS pathway but required functional priming through the cGAS/STING pathway. Our work positions cGAS/STING upstream of tonic IFN signaling in Samhd1-deficient mice and highlights an important role of the pathway in physiological and pathophysiological innate immune priming.