Although the irreversible cell-cycle arrest is traditionally believed as the major function of senescent cells, recent studies have unveiled some additional functions of senescent cells.  Most noteworthy among them is the increased secretion of various pro-inflammatory proteins, such as inflammatory cytokines, chemokines and growth factors, into the surrounding extracellular space.  This newly recognized senescent phenotype, termed the senescence-associated secretory phenotype (SASP), reportedly contributes to tumour suppression, tissue regeneration, embryonic development and even tumorigenesis promotion, depending on the biological context.  Thus, controlling the induction of SASP could profoundly affect the maintenance of homeostasis and disease control.  However, although persistent activation of the DNA damage response (DDR), which is believed to drive the cell senescence program, is known to play key roles in the onset of SASP (Rodier et al., Nat. Cell Biol., 2009; Takahashi et al., Mol. Cell, 2012), the precise mechanisms underlying this process remain largely unclear.

In eukaryotic cells, the localization of self DNA is restricted to the nucleus and mitochondria, and thereby the self DNA is sequestered from the cytoplasmic DNA sensing machineries, which activate pro-inflammatory cytokine pathways.  In normal, healthy cells, DNase2 and TREX1 (DNase3), cytoplasmic DNases that target double stranded (ds)DNA and single stranded (ss)DNA for degradation, respectively, prevent the cytoplasmic accumulation of free DNA.  In senescent cells, however, DNA fragments of nuclear origin reportedly accumulated in the cytoplasm.  Moreover, it has recently become apparent that DNA damage causes the cytoplasmic accumulation of nuclear DNA in various cell types (Takahashi et al., Nat. Commun. 2017 *1).  These reports, together with our previous observations that senescent cells express increased levels of interferon (IFN)-b, a pro-inflammatory cytokine known to be induced by the cytoplasmic DNA sensing pathway (Tahara et al., Oncogene 1995), led us to the idea that persistent DDR activation may provoke SASP through the aberrant activation of the cytoplasmic DNA sensing pathway, at least to a certain extent, in senescent cells.

In the present study, we reveal that although both dsDNA and ssDNA are constitutively emanating from the nucleus to the cytoplasm, DNase2 and TREX1 rapidly remove the exported nuclear DNA before it accumulate in the cytoplasm, thereby preventing aberrant activation of cytoplasmic DNA sensing pathway and consequent SASP in pre-senescent cells.   However, in senescent cells, the downregulation of DNase2 and TREX1 appear to cause the cytoplasmic accumulation of nuclear DNA, thus provoking SASP through the aberrant activation of cGAS-STING cytoplasmic DNA sensing machinery (see Figure1).  Interestingly, moreover, the blockage of this pathway prevents SASP in senescent hepatic stellate cells, accompanied by a decline of obesity-associated hepatocellular carcinoma development in mice.  These results strongly suggest that the down-regulation of DNase2/TREX1 is contributing to the activation of the cGAS/STING pathway and the consequent induction of SASP, at least to a certain extent in senescent in vivo.  These findings provide valuable new insights into the roles and mechanisms of SASP and possibilities for their control.

This article was published in Nature Communications on March 28, 2018.

Downregulation of cytoplasmic DNases is implicated in cytoplasmic DNA accumulation and SASP in senescent cells

Akiko Takahashi, Tze Mun Loo, Ryo Okada, Fumitaka Kamachi, Yoshihiro Watanabe, Masahiro Wakita, Sugiko Watanabe, Shimpei Kawamoto, Kenichi Miyata, Glen N. Barber, Naoko Ohtani & Eiji Hara