Impaired mitochondrial quality control and stress signalling machinery modulate senescence induction by genotoxic...

Cellular senescence is a hallmark of ageing and age-related disease and is closely associated with mitochondrial dysfunction and the accumulation of DNA damage. However, the contribution of mitochondria-nucleus communication, mitochondrial quality control (mtQC) and stress signalling to senescence remains incompletely understood. Here, we investigated the interplay between mtQC pathways and cellular stress responses in DNA damage-induced senescence using mouse embryonic fibroblasts (MEFs). MEFs deficient in the mitochondrial protease HtrA2 (proteostasis), the transcription factor Chop (integrated stress response; ISR) or the mitophagy regulator Pink1 were exposed to three mechanistically distinct DNA-damaging agents: bleomycin, etoposide and doxorubicin. Senescence was characterised using multiple complementary markers, including the proportion of high senescence-associated β-galactosidase-positive cells, nuclear size, total and nuclear p21 abundance, and transcriptional analysis of p16, p21 and genes associated with cell-cycle regulation and stress signalling. Mitochondrial dysfunction through mtQC impairment enhanced sensitivity to senescence with HtrA2 and Pink1 loss promoting increased senescence under DNA damage. Although DNA damage response (DDR) was activated as seen by changes in p21 homeostasis, this did not always correlate with senescence levels, which indicates that DDR alone cannot account for all senescence characteristics. The ISR played a modulatory role in the senescence induction, with Chop loss of function reducing senescence induction following DNA damage despite DDR activation. The different DNA damaging drugs produced different senescence outcomes, thus highlighting the importance of the stressor context in addition to the cellular homeostasis mechanisms in the overall senescence profile. This approach allowed, for the first time, to identify senescence subtypes dependent of mtQC and ISR integrity in the context of genotoxic stress.