ADAR1 downregulation contributes to p16INK4a upregulation independent of RNA editing during senescence

Cellular senescence plays a causal role in ageing and, in mouse, depletion of p16INK4a-expressing senescent cells delays ageing-associated disorders. Adenosine deaminases acting on RNA (ADARs) RNA editing enzymes 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 upregulating p16INK4a. ADAR1 is downregulated during senescence post-transcriptionally by autophagy-lysosomal pathway and the downregulation is sufficient to drive senescence in both in vitro and in vivo models. Senescence induced by ADAR1 downregulation is p16INK4a 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. And SIRT1, in turn, antagonizes translation of mRNA encoding p16INK4a. Hence, downregulation of ADAR1 and SIRT1 mediates p16INK4aupregulation 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 of ADAR1 in regulating senescence by post-transcriptionally controlling p16INK4a expression. RNA-seq experiment in ADAR control (Ctrl) and knock-down (KD) conditions in IMR90 cell lines. HuR RIP-seq in control, shADAR1 and shHuR cell lines.