The life and death of DNA-PK

Abstract

Double-strand breaks (DSBs) arise endogenously during normal cellular processes and exogenously by genotoxic agents such as ionizing radiation (IR). DSBs are one of the most severe types of DNA damage, which if left unrepaired are lethal to the cell. Several different DNA repair pathways combat DSBs, with nonhomologous endjoining (NHEJ) being one of the most important in mammalian cells. Competent NHEJ catalyses repair of DSBs by joining together and ligating two free DNA ends of little homology (microhomology) or DNA ends of no homology. The core components of mammalian NHEJ are the catalytic subunit of DNA protein kinase (DNA-PK cs ), Ku subunits Ku70 and Ku80, Artemis, XRCC4 and DNA ligase IV. DNA-PK is a nuclear serine/threonine protein kinase that comprises a catalytic subunit (DNA-PK cs ), with the Ku subunits acting as the regulatory element. It has been proposed that DNA-PK is a molecular sensor for DNA damage that enhances the signal via phosphorylation of many downstream targets. The crucial role of DNA-PK in the repair of DSBs is highlighted by the hypersensitivity of DNA-PK À/À mice to IR and the high levels of unrepaired DSBs after genotoxic insult. Recently, DNA-PK has emerged as a suitable genetic target for molecular therapeutics such as siRNA, antisense and novel inhibitory small molecules. This review encompasses the recent literature regarding the role of DNA-PK in the protection of genomic stability and focuses on how this knowledge has aided the development of specific DNA-PK inhibitors, via both small molecule and directed molecular targeting techniques. This review promotes the inhibition of DNA-PK as a valid approach to enhance the tumor-cell-killing effects of treatments such as IR. The double-strand break (DSB) is generally regarded as the most lethal of all DNA lesions, which if unrepaired severely threatens not only the integrity of the genome but also survival of the organism (Hoeijmakers, 2001;van Gent et al., 2001;Vilenchik and Knudson, 2003). DSBs can arise endogenously by cellular processes such as cleavage during immunoglobulin gene rearrangement (V(D)J recombination) and meiotic recombination. DSBs are also produced by exposure to ionizing radiation (IR), radiomimetic drugs, such as bleomycin, and the collapse of replication forks when the replication machinery encounters singlestranded breaks (SSBs) (Haber, 2000;Karran, 2000;Norbury and Hickson, 2001;Bassing et al., 2002). Unrepaired DSBs can activate cell cycle checkpoint arrests and signal for cell death (Jackson, 2001;Norbury and Hickson, 2001). Possibly even more detrimental to the cell are the unrepaired or misrepaired DSBs that lead to genomic rearrangements which ultimately destabilize the genome, a phenotype observed in a large number of malignancies (Lengauer et al., 1998;Hoeijmakers, 2001;Elliott and Jasin, 2002;Thompson and Schild, 2002;Shiloh, 2003;Vilenchik and Knudson, 2003). To complicate matters further, the location of the DSBs and the structure of the damaged DNA ends can vary depending on the damaging agent....

DNA-PKcs and ATM influence generation of ionizing radiation-induced bystander signals

“…ATM is reported to play a role in telomere maintenance as well (Pandita, 2002;Denchi and de Lange, 2007). In addition to DNA-PKcs and ATM's well-established roles in repair and intracellular signaling, (Nagasawa et al, 2003(Nagasawa et al, , 2005Collis et al, 2005;Lavin and Kozlov, 2007), our findings indicate a role for these proteins in intercellular signaling of the ionizing radiation-induced BSE. We designed a cell transfer strategy that enables us to differentiate between the generation versus the reception of bystander signals.…”

“…We focused on the repair proteins DNAPKcs (DNA-dependent Protein Kinase catalytic subunit) and ATM (Ataxia Telangectasia Mutated). DNA-PK is a primary component of the non-homologous end-joining (NHEJ) DNA repair pathway, consisting of the Ku 70/80 heterodimer and DNA-PKcs (Collis et al, 2005). DNA-PK is critical for double-strand break (DSB) repair and for V(D)J recombination (Jackson and Jeggo, 1995).…”

Ataxia-telangiectasia and Rad3-related and DNA-dependent protein kinase cooperate in G2 checkpoint activation by the DNA strand-breaking nucleoside analogue 2′-C-cyano-2′-deoxy-1-β-d-arabino-pentofuranosylcytosine

“…12), DNA-PK is one of the core components of mammalian repair of DSBs through the nonhomologous end-joining pathway. It has been proposed to sense DNA damage and enhance signal transduction via phosphorylation of downstream targets, including H2AX and p53 (13). However, a role for DNA-PK in the activation of transducers of the DNA damage signaling pathway in cellular responses to genotoxic stress has not been established.…”

DNA-PK phosphorylation sites on Oct-1 promote cell survival following DNA damage

“…DNA-PK is essential in maintaining genomic integrity, as mice lacking either DNA-PKcs or Ku suffer from profound radiosensitivity, immunodeficiency and age prematurely (Ferguson and Alt, 2001;Lieber et al, 2003). At the cellular level, DNA-PK deficiency results in defective repair of DSBs and defects in telomere maintenance that lead to telomeric fusions and translocations (Lieber et al, 2003;Bailey and Goodwin, 2004;Collis et al, 2005).…”

“…Suggested substrates for DNA-PK include several factors of the NHEJ pathway (Meek et al, 2004;Collis et al, 2005). DNA-PK is also known to autophosphorylate (Chan et al, 2002;Douglas et al, 2002;Soubeyrand et al, 2003).…”