Powering senescence: The ugly side of mitochondria

Senescence is a cellular response, whereby damaged cells cease to proliferate following exposure to endogenous and exogenous stressors, including telomere dysfunction, oncogene activation, genotoxic stress and chromatin remodelling. Despite being a tumor suppressor mechanism by preventing proliferation of potentially harmful cells, senescence is also a driver of agerelated tissue dysfunction and pathology. This is in part due to the development of a powerful secretory phenotype involving overproduction of growth factors, proteases and pro-inflammatory cytokines, collectively known as the senescence-associated secretory phenotype (SASP). Moreover, these cells also produce and secrete higher levels of reactive oxygen species (ROS). Both the SASP and ROS can stabilize senescence and induce senescence in neighboring cells. Thus, there is increasing interest in understanding the pathways that regulate senescence, particularly those that influence its pro-oxidant and proinflammatory phenotypes. Mitochondria have been implicated in cellular senescence, mostly since they are considered the main generators of ROS. However, reports have also emphasized the importance of nonmitochondrial ROS sources, redox stress and deficits in antioxidant defenses in senescence induction. Therefore, it remained unknown whether mitochondria are truly necessary for senescence and, if so, what mechanisms are involved. Answering these questions is challenging for several reasons: i) conventional assays to assess intracellular ROS are relatively unspecific and with several methodological pitfalls; ii) processes occurring within mitochondria are diverse and complex and iii) intervening in individual aspects of mitochondrial biology results unequivocally in adaptive responses which alter mitochondrial function in different ways. In our study, published in The EMBO Journal, we designed a proof-of-principle experiment that ultimately disclosed the requirement of mitochondria for senescence (Fig. 1). We utilised the property of the ubiquitin E3 ligase Parkin to induce widespread mitophagy upon mitochondrial membrane depolarization in order to generate human fibroblasts without mitochondria. Furthermore, we optimised culture conditions to keep mitochondria-depleted cells viable for a sufficient amount of time to investigate their role in senescence. The physical loss of mitochondria was confirmed by 3D electron microscopy and undetectable mitochondrial respiration, proteins or mitochondrial DNA. Using this powerful method, we found that following a variety of senescence triggers (e.g. oxidative stress, oncogene activation and replicative exhaustion), features of cellular senescence were significantly reduced in mitochondria-depleted cells, particularly the pro-oxidant and pro-inflammatory phenotypes, while energy levels were preserved with increased compensatory glycolysis. Importantly, global transcriptome analysis revealed that a substantial fraction of senescence-associated changes were mitochondria-dependent. Notably, while expression of cyclin-dependent kinase inhibitors p16 and p21 was reduced, mitochondria-depleted senescent cells did not resume proliferation. Based on this data, we suggest that mitochondria may be highly promising targets for anti-senescence therapies. By targeting them, we may be able to suppress the detrimental SASP known to induce aging and cancer, while maintaining the tumor suppressor capabilities of senescent cells.