ADAR1 downregulation by autophagy drives senescence independently of RNA editing by enhancing p16 levels.

Cellular senescence plays a causal role in ageing and, in mice, depletion of p16-expressing senescent cells delays ageing-associated disorders. Adenosine deaminases acting on RNA (ADARs) are RNA-editing enzymes that are also implicated as important regulators of human ageing, and ADAR inactivation causes age-associated pathologies such as neurodegeneration in model organisms. However, the role, if any, of ADARs in cellular senescence is unknown. Here we show that ADAR1 is post-transcriptionally downregulated by autophagic degradation to promote senescence through p16 upregulation. The ADAR1 downregulation is sufficient to drive senescence in both in vitro and in vivo models. Senescence induced by ADAR1 downregulation is p16-dependent and independent of its RNA-editing function. Mechanistically, ADAR1 promotes SIRT1 expression by affecting its RNA stability through HuR, an RNA-binding protein that increases the half-life and steady-state levels of its target mRNAs. SIRT1 in turn antagonizes translation of mRNA encoding p16. Hence, downregulation of ADAR1 and SIRT1 mediates p16 upregulation by enhancing its mRNA translation. Finally, Adar1 is downregulated during ageing of mouse tissues such as brain, ovary and intestine, and Adar1 expression correlates with Sirt1 expression in these tissues in mice. Together, our study reveals an RNA-editing-independent role for ADAR1 in the regulation of senescence by post-transcriptionally controlling p16 expression.