Lethal and Non-Lethal Functions of Caspases in the DNA Damage Response

Abstract

Members of the caspase family are well known for their roles in the initiation and execution of cell death. Due to their function in the removal of damaged cells that could otherwise become malignant, caspases are important players in the DNA damage response (DDR), a network of pathways that prevent genomic instability. However, emerging evidence of caspases positively or negatively impacting the accumulation of DNA damage in the absence of cell death demonstrates that caspases play a role in the DDR that is independent of their role in apoptosis. This review highlights the apoptotic and non-apoptotic roles of caspases in the DDR and how they can impact genomic stability and cancer treatment.

Keywords: DNA damage; apoptosis; caspases; cell cycle; genomic instability.

Figure 1

The DNA damage response. Upon DNA damage, a network of pathways known as the DNA damage response (DDR) is activated. Double strand breaks (DSBs) are detected by the Ku70/Ku80 heterodimer or the MRN complex (MRE11, RAD50, and NBS1). These sensors then recruit and activate the apical kinases DNA-dependent protein kinase (DNA-PK) through the Ku70/Ku80 complex or ataxia–telangiectasia-mutated (ATM) through the MRN complex. Activated DNA-PKcs induces DNA repair through non-homologous end-joining (NHEJ). Activated DNA-PKcs can phosphorylate H2AX and checkpoint kinase 2 (Chk2). ATM phosphorylates H2AX, amplifying the DDR by recruiting more DNA repair proteins, such as the mediator of DNA damage checkpoint 1 (MDC1) to the sites of DNA damage; Chk2-inducing cell cycle arrest; and breast cancer 1 (BRCA1)-inducing DNA repair by homologous recombination (HR). In the presence of single strand DNA (ssDNA), replication protein A (RPA) coats the ssDNA, resulting in the recruitment of ATRIP/ataxia–telangiectasia and Rad3-related (ATR) complexes, resulting in ATR activation, which is amplified by the further recruitment of TopBP1, 9-1-1, and Rad17. ATR induces cell cycle arrest through the activation of checkpoint kinase 1 (Chk1) and phosphorylation of p53 and promotes DNA repair by HR through Rad51 activation.

Figure 2

The extrinsic and intrinsic apoptotic pathways. The activation of the extrinsic pathway is initiated by the binding of ligands to receptors located on the plasma membrane. Upon activation of these receptors, the death-inducing signaling complex (DISC), composed of Fas-associated protein with death domain (FADD) and caspase-8, is formed, activating caspase-8. Active caspase-8 then cleaves caspase-3, which cleaves downstream substrates, culminating in apoptosis. DNA damage activates the intrinsic pathway, leading to the activation of BH3-only proteins and allowing the oligomerization of BAX and BAK and the release of cytochrome c (Cyt c) from the mitochondria, following the mitochondria outer membrane permeabilization (MOMP). Cytosolic Cyt c forms a complex with apoptotic peptidase activating factor 1 (APAF-1) and caspase-9 called the apoptosome, resulting in the activation of caspase-9, caspase-3, and apoptosis. DNA damage can also induce PIDDosome assembly, a complex comprised of p53-induced protein with a death domain (PIDD), RIP-associated ICH-1/CAD-3 homologous protein with a death domain (RAIDD), and caspase-2, resulting in caspase-2 activation. Once activated, caspase-2 cleaves the pro-apoptotic protein BID to tBID, inducing MOMP and apoptosis. Alternatively, the activation of caspase-8 by extrinsic signals can engage the activation of the intrinsic pathway by cleavage of BID.

Figure 3

Sublethal activation of the apoptotic pathways promotes DNA damage. The intrinsic and extrinsic apoptotic pathways can be activated in the absence of cell death in the presence of sub-lethal levels of extrinsic or intrinsic stimuli. Under these conditions, caspase-8 and caspase-9 are activated by their recruitment to their activation platforms, death-inducing signaling complex (DISC), and the apoptosome, respectively. Caspase-8 and caspase-9 then activate caspase-3, which cleaves the inhibitor of caspase-activated DNAse (ICAD), resulting in the activation of caspase-activated DNAse (CAD). As an endonuclease, CAD fragments DNA, which induces DNA damage. In addition to CAD, endonuclease G (EndoG) is another downstream effector of caspase-3 that can induce DNA fragmentation under sublethal activation of the apoptotic pathways.

Figure 4

Non-apoptotic roles of caspases in the DNA damage response. Upon cytokinesis failure, caspase-2 induces p53-dependent cell cycle arrest through the cleavage of mouse double minute 2 homolog (MDM2). Under DNA damage conditions, caspase-2 regulates the activation of p53 in a MDM2-independent manner. Similarly, caspase-8 activates p53 through an unknown mechanisms and functional outcome. Caspase-8 is also involved in the phosphorylation of H2AX through the formation of a complex with receptor-interacting serine/threonine-protein kinase 1 (RIPK1), receptor-interacting serine/threonine-protein kinase 3 (RIPK3), Fas-associated protein with death domain (FADD), and the cellular-FLICE inhibitory protein (c-FLIP) known as the RIPoptosome. Unlike caspase-2 and caspase-8 that are involved in the activation of components of the DDR, the expression of a dominant negative caspase-9 (caspase-9b) results in the inhibition of Chk1 phosphorylation through the activation of caspase-7. Caspase-2 is also involved in the prevention of genomic instability by preventing stalled replication forks that could collapse and become double strand breaks. Likewise, during mitosis, the formation of the RIPoptosome results in cleavage of polo like kinase 1 (PLK1), which is crucial for the prevention of mitotic defects and genomic instability. In contrast, caspase-8 can promote the accumulation of DNA damage and genomic instability by cleaving ubiquitin specific peptidase 28 (USP28) and preventing the activation of p53.