RIG-I and IL-6 are negative-feedback regulators of STING induced by double-stranded DNA.

The stimulator of interferon genes (STING) protein has emerged as a critical signal transduction molecule in the innate immune response. Sustained activation of the STING signaling induced by cytosolic DNA has been considered to be the cause of a variety of autoimmune diseases characterized by uncontrolled inflammation. Therefore, it is important to understand the molecular basis of the regulation of STING signaling pathway. Here we demonstrate that the STING protein undergoes a proteasome-mediated degradation in human diploid cell (HDC) lines including MRC-5 following the transfection of double-stranded DNA (dsDNA). The degradation of STING is accompanied by the increased expression of both RIG-I and IL-6. Employing the RIG-I siRNA knockdown and an IL-6 neutralizing antibody greatly inhibits the degradation of STING induced by dsDNA. We further demonstrate that both IL-6 and RIG-I are downstream molecules of STING along the DNA sensor pathway. Therefore, STING degradation mediated by RIG-I and IL-6 may serve as a negative feedback mechanism to limit the uncontrolled innate immune response induced by dsDNA. We have further shown that RIG-I and IL-6 promote STING degradation by activating/dephosphorylating UNC-51-like kinase (ULK1). Interestingly, the STING protein is not significantly affected by dsDNA in non-HDC HEK293 cells. Our study thus has identified a novel signaling pathway for regulating STING in HDCs.