Oncogene (2013) 32, 3789–3797 & 2013 Macmillan Publishers Limited All rights reserved 0950-9232/13 www.nature.com/onc REVIEW Various modes of cell death induced by DNA damage O Surova and B Zhivotovsky The consequences of DNA damage depend on the cell type and the severity of the damage. Mild DNA damage can be repaired with or without cell-cycle arrest. More severe and irreparable DNA injury leads to the appearance of cells that carry mutations or causes a shift towards induction of the senescence or cell death programs. Although for many years it was argued that DNA damage kills cells via apoptosis or necrosis, technical and methodological progress during the last few years has helped to reveal that this injury might also activate death by autophagy or mitotic catastrophe, which may then be followed by apoptosis or necrosis. The molecular basis underlying the decision-making process is currently the subject of intense investigation. Here, we review current knowledge about the response to DNA damage and subsequent signaling, with particular attention to cell death induction and the molecular switches between different cell death modalities following damage. Oncogene (2013) 32, 3789–3797; doi:10.1038/onc.2012.556; published online 3 December 2012 Keywords: DNA damage; cell death; p53; senescence; response INTRODUCTION cell-cycle arrest, whereas severe and irreparable injury shifts the Genomic integrity and stability is constantly threatened by DNA cellular response towards induction of the senescence or cell damage, which may arise from numerous environmental and death programs, such as apoptosis, mitotic catastrophe (MC), intrinsic sources. Failure to repair DNA damage can cause autophagy and necrosis. The molecular basis underlying the mutations, genomic instability, premature aging, mental multiplex decision-making process is currently the subject of intense retardation and other developmental disorders as well as cancer. investigation. Here, we review current knowledge about the Consequently, eukaryotes have evolved a complex set of DDR with particular attention to cell death induction and the mechanisms to sense different types of DNA damage and molecular switches between different cell death modalities safeguard the genome, the so-called DNA damage response following DNA damage. (DDR). The DDR comprises a multiple networks of signal transduction pathways initiated by sensing damage, followed by signal transmission to effectors through activation of a cascade of CELL SENESCENCE IN RESPONSE TO DNA DAMAGE phosphorylation events. A group of proteins, including mediator Cellular senescence is one of the consequences of severe or of DNA damage checkpoint protein 1 (Chk1; MDC1/NFBD1), 53BP1 irreparable DNA damage, especially DNA DSBs, and it is (p53-binding protein 1), the Mre11-Rad50-NBS1 (MRN) and the characterized by an irreversible growth arrest of cells in the G1 Rad9-Rad1-Hus1 (9-1-1) complexes, have been implicated as DNA phase. Cellular senescence limits the proliferation of DNA- damage sensors and recognize both double- and single-strand damaged cells that are at risk of neoplastic transformation. breaks (DSBs and SSBs, respectively) as well as regions of However, these cells remain metabolically active, showing a replication stress.1–4 Binding of the MRN and 9-1-1 complexes to greatly altered pattern of gene expression. The outcomes of the sites of DNA damage in chromatin activates the signal cellular senescence following DNA damage are highly dependent transducing kinase ataxia-telangiectasia mutated (ATM), the ATM on the cell type and physiological context. Cells of epithelial and and Rad3-related (ATR) kinase and the DNA-dependent protein stromal origin undergo senescence following severe DNA damage, kinase (DNA-PK). Once activated, ATR and ATM amplify the while lymphoid cells undergo apoptosis. In most cell types, damage signal by phosphorylating various substrates, including activation of p53 is crucial for initiating the senescence response key DDR proteins H2AX, BRCA1, NBS1, SMC1 (structural following DNA damage, and in certain cells p53 is also important maintenance of chromosome protein 1), MDC1 and 53BP1, as for maintaining the senescent growth arrest. p21waf1/cip1,asa well as repair proteins Ku70/80, Artemis and Rad-like proteins.5,6 crucial target of p53 and mediator of senescence, inhibits a Additionally, in order to allow time for DNA repair, ATM spectrum of cyclin-dependent kinases that regulate the phos- temporarily arrests the cell cycle by phosphorylation and phorylation of the retinoblastoma protein (pRb). The hypopho- activation of cell-cycle checkpoints. p53, as one of the main sphorylated form of pRb binds to the transcription factor E2F, targets of ATM/ATR, is a universal sensor of genotoxic stress and leading to transactivation of the genes essential for the G1-to-S has a key role in cellular responses to DNA damage, determining transition. On the other hand, p21 can mediate transient DNA the fate of cells towards either survival, which involves cell-cycle damage-induced growth arrest. It is still unclear what determines delay accompanied by repair of DNA damage, or cell death.7 whether cells senesce or arrest transiently. It is likely that rapid Depending on the cell type and the severity and extent of DNA DNA repair will quickly shut down p53–p21 signaling, whereas damage, the activated DDR can elicit different cellular responses incomplete or incorrect repair results in prolonged signaling and (Figure 1). Mild DNA damage normally leads to the induction of senescence.8–10 Division of Toxicology, Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden. Correspondence: Professor B Zhivotovsky, Institute of Environmental Medicine, Division of Toxicology, Karolinska Institutet, Nobels vag. 13, Box 210, Stockholm SE-171 77, Sweden. E-mail: [email protected] Received 27 August 2012; revised 2 October 2012; accepted 3 October 2012; published online 3 December 2012 DNA damage and cell death O Surova and B Zhivotovsky 3790 pathways are intact, have a preference toward senescence compared with transformed cells and that senescence is usually induced by lower levels of damage than those leading to apoptosis.13 Overall, p53 stands at the center of a complex DNA damage signal transduction pathway following DNA damage. MRN 9-1-1 Phosphorylation of p53 at Ser15, Thr18 and Ser376 and dephosphorylation at Ser392 has been detected in fibroblasts undergoing senescence.14 By contrast, phosphorylation at Ser46 sensing damage was shown to be associated with the induction of p53AIP1 (p53- G1 MDC1 BRCA1 regulated apoptosis-inducing protein 1) and apoptosis. A study on 53BP1 NBS1 MCF-7 cells demonstrated the role of different phosphorylation M S Chk1/Chk2 Ku70/80 ATM/ATR sites (Ser15 and Ser46), induction of p21 and high or low reactive oxygen species (ROS) levels in determining the dosage of G2 adriamycin that induced either apoptosis or senescence. Cells in cell cycle signal DNA repair checkpoints transductions these two states exhibited quite distinct time course profiles of the proteins p53, p21 and E2F1. As mentioned above, DNA damage can also activate Rb through the p16 pathway Cell Death to induce senescence. An in vivo study showed that, following TSC2/ PARP p53 cyclophosphamide treatment, a p53-dependent effect can also mTORC1 occur in an apoptosis-independent fashion by inducing cellular senescence in a p53- and p16-dependent manner.15 Thus, the genetic context, cell type or the specific activation of certain target genes upon DNA damage can favor either senescence or Necrosis Apoptosis Autophagy apoptosis. Figure 1. DNA damage response. DNA damage results from multiple environmental and intrinsic sources, activating a complex set of APOPTOSIS AS A FAST-ACTING RESPONSE TO DNA DAMAGE reactions called DNA damage response. This response is initiated by Like cellular senescence, apoptosis is activated in response to damage detection, followed by signal transmission to effectors and finally results in a number of cellular responses, such as induction of cellular stress and is executed when repair of DNA damage is slow cell-cycle arrest, repair of the lesions or, in case of non-repairable or incomplete. As mentioned above, it is still not fully understood damage, leads to activation of cell death program. A group of how cells decide whether to undergo senescence or apoptosis, proteins, including Mre11-Rad50-NBS1 (MRN complex) and the and despite the fact that most cells are capable of both, these Rad9-Rad1-Hus1 (9-1-1) complex, fulfilling a function of DNA processes seem to be mutually exclusive. Numerous studies have damage sensors, and recognized both DSBs and SSBs. Binding of clearly shown cross-talk between the processes of apoptosis and the MRN and 9-1-1 complexes to the sites of DNA damage in senescence, primarily at the level of p53, which is considered to be chromatin activates the signal transducing kinase ATM, preferen- a major player in the apoptotic response to genotoxins. The tially in response to DSBs and the disruption of chromatin structure. phosphorylation of p53 at Ser46 is the most prominent in priming By contrast, the ATR kinase is activating primarily in response to SSBs 16 and replication forks. The activation of DNA repair (orchestrated its apoptotic activity. Subsequently, homeodomain-interacting protein kinase 2 (HIPK2), a serine/threonine nuclear kinase, was by set of DNA damage response proteins, including BRCA1, NBS1, as 17,18 well as repair proteins Ku70/80