The DNA-PK catalytic subunit regulates Bax-mediated excitotoxic cell death by Ku70 phosphorylation

NU7441, a selective inhibitor of DNA-PKcs, alleviates intracerebral hemorrhage injury with suppression of ferroptosis in brain

Neuronal apoptosis, oxidative stress, and ferroptosis play a crucial role in the progression of secondary brain injury following intracerebral hemorrhage (ICH). Although studies have highlighted the important functions of DNA-dependent protein kinase catalytic subunit (DNA-PKcs) in various experimental models, its precise role and mechanism in ICH remain unclear. In this study, we investigated the effects of DNA-PKcs on N2A cells under a hemin-induced hemorrhagic state in vitro and a rat model of collagenase-induced ICH in vivo. The results revealed a notable increase in DNA-PKcs levels during the acute phase of ICH. As anticipated, DNA-PKcs and γ-H2AX had consistent upregulations after ICH. Administration of NU7441, a selective inhibitor of DNA-PKcs, alleviated neurological impairment, histological damage, and ipsilateral brain edema in vivo. Mechanistically, NU7441 attenuated neuronal apoptosis both in vivo and in vitro, alleviated oxidative stress by decreasing ROS levels, and suppressed ferroptosis by enhancing GPX4 activity. These results suggest that inhibition of DNA-PKcs is a promising therapeutic target for ICH.

The multifaceted functions of DNA-PKcs: implications for the therapy of human diseases

The DNA-dependent protein kinase (DNA-PK), catalytic subunit, also known as DNA-PKcs, is complexed with the heterodimer Ku70/Ku80 to form DNA-PK holoenzyme, which is well recognized as initiator in the nonhomologous end joining (NHEJ) repair after double strand break (DSB). During NHEJ, DNA-PKcs is essential for both DNA end processing and end joining. Besides its classical function in DSB repair, DNA-PKcs also shows multifaceted functions in various biological activities such as class switch recombination (CSR) and variable (V) diversity (D) joining (J) recombination in B/T lymphocytes development, innate immunity through cGAS-STING pathway, transcription, alternative splicing, and so on, which are dependent on its function in NHEJ or not. Moreover, DNA-PKcs deficiency has been proven to be related with human diseases such as neurological pathogenesis, cancer, immunological disorder, and so on through different mechanisms. Therefore, it is imperative to summarize the latest findings about DNA-PKcs and diseases for better targeting DNA-PKcs, which have shown efficacy in cancer treatment in preclinical models. Here, we discuss the multifaceted roles of DNA-PKcs in human diseases, meanwhile, we discuss the progresses of DNA-PKcs inhibitors and their potential in clinical trials. The most updated review about DNA-PKcs will hopefully provide insights and ideas to understand DNA-PKcs associated diseases.

DNA-PKcs interacts with and phosphorylates Fis1 to induce mitochondrial fragmentation in tubular cells during acute kidney injury

The catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) regulates cell death. We sought to determine whether DNA-PKcs played a role in the tubular damage that occurs during acute kidney injury (AKI) induced by LPS injection (to mimic sepsis), cisplatin administration, or renal ischemia/reperfusion injury. Although DNA-PKcs normally localizes to the nucleus, we detected cytoplasmic DNA-PKcs in mouse kidney tissues and urinary sediments of human patients with septic AKI. Increased cytoplasmic amounts of DNA-PKcs correlated with renal dysfunction. Tubule cell-specific DNA-PKcs deletion attenuated AKI-mediated tubular cell death and changes in the abundance of various proteins with mitochondrial functions or roles in apoptotic pathways. DNA-PKcs interacted with Fis1 and phosphorylated it at Thr³⁴ in its TQ motif, which increased the affinity of Fis1 for Drp1 and induced mitochondrial fragmentation. Knockin mice expressing a nonphosphorylatable T34A mutant exhibited improved renal function and histological features and reduced mitochondrial fragmentation upon induction of AKI. Phosphorylation of Thr³⁴ in Fis1 was detectable in urinary sediments of human patients with septic AKI and correlated with renal dysfunction. Our findings provide insight into the role of cytoplasmic DNA-PKcs and phosphorylated Fis1 in AKI development.

DNA-PKcs promotes cardiac ischemia reperfusion injury through mitigating BI-1-governed mitochondrial homeostasis

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is a novel inducer to promote mitochondrial apoptosis and suppress tumor growth in a variety of cells although its role in cardiovascular diseases remains obscure. This study was designed to examine the role of DNA-PKcs in cardiac ischemia reperfusion (IR) injury and mitochondrial damage. Cardiomyocyte-specific DNA-PKcs knockout (DNA-PKcs^CKO) mice were subjected to IR prior to assessment of myocardial function and mitochondrial apoptosis. Our data revealed that IR challenge, hypoxia-reoxygenation (HR) or H₂O₂-activated DNA-PKcs through post-transcriptional phosphorylation in murine hearts or cardiomyocytes. Mice deficient in DNA-PKcs in cardiomyocytes were protected against cardiomyocyte death, infarct area expansion and cardiac dysfunction. DNA-PKcs ablation countered IR- or HR-induced oxidative stress, mPTP opening, mitochondrial fission, mitophagy failure and Bax-mediated mitochondrial apoptosis, possibly through suppression of Bax inhibitor-1 (BI-1) activity. A direct association between DNA-PKcs and BI-1 was noted where DNA-PKcs had little effect on BI-1 transcription but interacted with BI-1 to promote its degradation. Loss of DNA-PKcs stabilized BI-1, thus offering resistance of mitochondria and cardiomyocytes against IR insult. Moreover, DNA-PKcs ablation-induced beneficial cardioprotection against IR injury was mitigated by concurrent knockout of BI-1. Double deletion of DNA-PKcs and BI-1 failed to exert protection against global IR injury and mitochondrial damage, confirming a permissive role of BI-1 in DNA-PKcs deletion-elicited cardioprotection against IR injury. DNA-PKcs serves as a novel causative factor for mitochondrial damage via suppression of BI-1, en route to the onset and development of cardiac IR injury.

Inhibition of TRIB3 Protects Against Neurotoxic Injury Induced by Kainic Acid in Rats

Epilepsy refers to a group of neurological disorders of varying etiologies characterized by recurrent seizures, resulting in brain dysfunction. Endoplasmic reticulum (ER) stress is highly activated in the process of epilepsy-related brain injury. However, the mechanisms by which ER stress triggers neuronal apoptosis remain to be fully elucidated. Tribbles pseudokinase 3 (TRIB3) is a pseudokinase that affects a number of cellular functions, and its expression is increased during ER stress. Here, we sought to clarify the role of TRIB3 in neuronal apoptosis mediated by ER stress. In the kainic acid (KA) (10 mg/kg)-induced rat seizure model, we characterized neuronal injury and apoptosis after KA injection. KA induced an ER stress response, as indicated by elevated expression of glucose-regulated protein 78 (GRP78) and C/EBP homologous protein (CHOP). TRIB3 protein was upregulated concomitantly with the downregulation of phosphorylated-protein kinase B (p-AKT) in rats following KA administration. In rat cortical neurons treated with KA, TRIB3 knockdown by siRNA reduced the number of dying neurons, decreased the induction of GRP78 and CHOP and the activation of caspase-3, and blocked the dephosphorylation of AKT after KA treatment. Our findings indicate that TRIB3 is involved in neuronal apoptosis occurring after KA-induced seizure. The knockdown of TRIB3 effectively protects against neuronal apoptosis in vitro, suggesting that TRIB3 may be a potential therapeutic target for the treatment of epilepsy.

K+ deficiency caused defects in renal tubular cell proliferation, oxidative stress response, tissue repair and tight junction integrity, but enhanced energy production, proteasome function and cellular K+ uptake

Hypokalemia is a common electrolyte disorder in hospitalized patients and those with chronic diseases and is associated with renal tubular injury. Our recent expression proteomics study revealed changes in levels of several proteins in renal tubular cells during K⁺ deficiency. However, functional significance and mechanisms underlying such changes remained unclear. The present study, thus, aimed to investigate functional changes of renal tubular cells induced by K⁺ deficiency. MDCK cells were maintained in normal-K⁺ (ANK; [K⁺] = 5.0 mM), Low-K⁺ (ALK; [K⁺] = 2.5 mM), or K⁺-depleted (AKD; [K⁺] = 0 mM) medium. Cell count and cell death assay showed that ALK and AKD groups had marked decrease in cell proliferation without significant change in cell death. Other functional investigations revealed that AKD cells had significantly increased levels of carbonylated proteins (by OxyBlot assay), impaired tissue repair (by scratch assay), defective tight junction (by Western blotting, immunofluorescence staining and measuring transepithelial electrical resistance), increased intracellular ATP level (by ATP measurement), decreased levels of ubiquitinated proteins (by Western blotting), and increased level of Na⁺/K⁺-ATPase (by Western blotting), which was consistent with the increased cellular K⁺ uptake after K⁺ repletion. Our findings have shown that AKD caused defects in cell proliferation, oxidative stress response, tissue repair and tight junction integrity, but on the other hand, enhanced energy production, proteasome function and cellular K⁺ uptake. These findings may shed light onto cellular response to K⁺ deficiency and better understanding of both pathogenic and compensatory mechanisms in hypokalemic nephropathy.

p53 modulates the effect of ribosomal protein S6 kinase1 (S6K1) on cisplatin toxicity in chronic myeloid leukemia cells

Chronic myeloid leukemia (CML) is characterized by the expression of the oncoprotein, BCR-ABL. BCR-ABL inhibitors revolutionized CML chemotherapy while blast crisis (BC) CML patients are less responsive. Since suppression of ribosomal protein S6 kinase1 (S6K1) phosphorylation reverses the resistance to BCR-ABL inhibitor in CML cells and S6K1 inhibitors augment cisplatin toxicity in lung cancer cells, we speculated that combination of S6K1 inhibitor and cisplatin may be beneficial for eliminating BC CML cells. To our surprise, S6K1 inhibition decreased cisplatin-induced DNA damage and cell death only in p53^-/- BC CML cells but not in p53^+/+ BC CML cells. During the progression of CML, p53 expression either decreases or mutates. Moreover, the expression of p53 affects drug response of CML cells. Our results confirmed that S6K1 inhibition reversed cisplatin toxicity is dependent on p53 expression in CML cells. Moreover, p53 attenuated the phosphorylation and localization of S6K1 via attenuating 3-phosphoinositide dependent protein kinase-1 (PDK1) phosphorylation. Furthermore, S6K1 acts via DNA-PKcs to regulate H2AX phosphorylation and PARP cleavage, respectively. Taken together, our results suggest that p53/PDK1/S6K1 is a novel pathway regulating cisplatin toxicity in BC CML cells.

DNA-PKcs, ATM, and ATR Interplay Maintains Genome Integrity during Neurogenesis

The DNA damage response (DDR) orchestrates a network of cellular processes that integrates cell-cycle control and DNA repair or apoptosis, which serves to maintain genome stability. DNA-PKcs (the catalytic subunit of the DNA-dependent kinase, encoded by PRKDC), ATM (ataxia telangiectasia, mutated), and ATR (ATM and Rad3-related) are related PI3K-like protein kinases and central regulators of the DDR. Defects in these kinases have been linked to neurodegenerative or neurodevelopmental syndromes. In all cases, the key neuroprotective function of these kinases is uncertain. It also remains unclear how interactions between the three DNA damage-responsive kinases coordinate genome stability, particularly in a physiological context. Here, we used a genetic approach to identify the neural function of DNA-PKcs and the interplay between ATM and ATR during neurogenesis. We found that DNA-PKcs loss in the mouse sensitized neuronal progenitors to apoptosis after ionizing radiation because of excessive DNA damage. DNA-PKcs was also required to prevent endogenous DNA damage accumulation throughout the adult brain. In contrast, ATR coordinated the DDR during neurogenesis to direct apoptosis in cycling neural progenitors, whereas ATM regulated apoptosis in both proliferative and noncycling cells. We also found that ATR controls a DNA damage-induced G2/M checkpoint in cortical progenitors, independent of ATM and DNA-PKcs. These nonoverlapping roles were further confirmed via sustained murine embryonic or cortical development after all three kinases were simultaneously inactivated. Thus, our results illustrate how DNA-PKcs, ATM, and ATR have unique and essential roles during the DDR, collectively ensuring comprehensive genome maintenance in the nervous system.

SIGNIFICANCE STATEMENT

The DNA damage response (DDR) is essential for prevention of a broad spectrum of different human neurologic diseases. However, a detailed understanding of the DDR at a physiological level is lacking. In contrast to many in vitro cellular studies, here we demonstrate independent biological roles for the DDR kinases DNA-PKcs, ATM, and ATR during neurogenesis. We show that DNA-PKcs is central to DNA repair in nonproliferating cells, and restricts DNA damage accumulation, whereas ATR controls damage-induced G2 checkpoint control and apoptosis in proliferating cells. Conversely, ATM is critical for controlling apoptosis in immature noncycling neural cells after DNA damage. These data demonstrate functionally distinct, but cooperative, roles for each kinase in preserving genome stability in the nervous system.

Ku70 Serine 155 mediates Aurora B inhibition and activation of the DNA damage response

The Ku heterodimer (Ku70/Ku80) is the central DNA binding component of the classical non-homologous end joining (NHEJ) pathway that repairs DNA double-stranded breaks (DSBs), serving as the scaffold for the formation of the NHEJ complex. Here we show that Ku70 is phosphorylated on Serine 155 in response to DNA damage. Expression of Ku70 bearing a S155 phosphomimetic substitution (Ku70 S155D) in Ku70-deficient mouse embryonic fibroblasts (MEFs) triggered cell cycle arrest at multiple checkpoints and altered expression of several cell cycle regulators in absence of DNA damage. Cells expressing Ku70 S155D exhibited a constitutive DNA damage response, including ATM activation, H2AX phosphorylation and 53BP1 foci formation. Ku70 S155D was found to interact with Aurora B and to have an inhibitory effect on Aurora B kinase activity. Lastly, we demonstrate that Ku and Aurora B interact following ionizing radiation treatment and that Aurora B inhibition in response to DNA damage is dependent upon Ku70 S155 phosphorylation. This uncovers a new pathway where Ku may relay signaling to Aurora B to enforce cell cycle arrest in response to DNA damage.

Association between the XRCC6 polymorphisms and cancer risks: a systematic review and meta-analysis

A number of studies have been carried out to investigate the association of X-ray repair complementing defective repair in Chinese hamster cells 6 (XRCC6) polymorphisms and cancer risks, and the results remained inconsistent and inconclusive.To assess the effect of XRCC6 polymorphisms on cancer susceptibility, we conducted a meta-analysis, up to May 23rd 2014, 6267 cases with different types of tumor and 7536 controls from 20 published case-control studies. Summary odds ratios and corresponding 95% confidence intervals for XRCC6 polymorphism and cancer risk were estimated using fixed- or random-effects models when appropriate. Heterogeneity was assessed by chi-squared-based Q-statistic test, and the sources of heterogeneity were explored by subgroup analyses, logistic meta-regression analyses and Galbraith plot. Publication bias was evaluated by Begg funnel plot and Egger test. Sensitivity analyses were also performed.The rs2267437 polymorphism was associated with a significant increase in risks of overall cancers, breast cancer, renal cell carcinoma and hepatocellular carcinoma, and it could increase the cancer risk in Asian population; the rs5751129 polymorphism could increase the cancer risk in overall cancers; the rs132770 polymorphism was associated with the increased renal cell carcinoma risk; furthermore, the rs132793 polymorphism could decrease breast cancer risk and increase risks in "other cancers".Overall, the results provided evidences that the single nucleotide polymorphisms in XRCC6 promoter region might play different roles in various cancers, indicating different cancers have different tumorigenesis mechanisms. Our studies may perhaps supplement for the disease monitoring of cancers in the future, and additional studies to determine the exact molecular mechanism might provide us with interventions to protect the susceptible subgroups.

Neuronal apoptosis in cerebral ischemia/reperfusion area following electrical stimulation of fastigial nucleus

Previous studies have indicated that electrical stimulation of the cerebellar fastigial nucleus in rats may reduce brain infarct size, increase the expression of Ku70 in cerebral ischemia/reperfusion area, and decrease the number of apoptotic neurons. However, the anti-apoptotic mechanism of Ku70 remains unclear. In this study, fastigial nucleus stimulation was given to rats 24, 48, and 72 hours before cerebral ischemia/reperfusion injury. Results from the electrical stimulation group revealed that rats exhibited a reduction in brain infarct size, a significant increase in the expression of Ku70 in cerebral ischemia/reperfusion regions, and a decreased number of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells. Double immunofluorescence staining revealed no co-localization of Ku70 with TUNEL-positive cells. However, Ku70 partly co-localized with Bax protein in the cytoplasm of rats with cerebral ischemia/reperfusion injury. These findings suggest an involvement of Ku70 with Bax in the cytoplasm of rats exposed to electrical stimulation of the cerebellar fastigial nucleus, and may thus provide an understanding into the anti-apoptotic activity of Ku70 in cerebral ischemia/reperfusion injury.

New insights into the link between DNA damage and apoptosis

SIGNIFICANCE

When lesions are unrepaired or there are defects in the DNA repair system, DNA damage is often correlated to apoptosis. However, different kinds of lesions and different degrees of lesion severity can trigger numerous signaling responses.

RECENT ADVANCES

DNA repair proteins involved in specific DNA repair pathways can modulate the function or activity of some apoptotic factors, further emphasizing the crosstalk between DNA damage and cell death.

CRITICAL ISSUES

Here, we discuss the signaling networks that link DNA damage to apoptosis, and we focus on post-translational modifications, leading to crucial changes in protein behavior, following various kinds of DNA damage. Moreover, we analyze the existence of apoptosis-related functions of typical repair proteins, leading to diverse, often-overlapping, DNA damage responses.

FUTURE DIRECTIONS

The better understanding of the regulation and the functionality of key DNA repair proteins, also involved in apoptosis regulation, has the potential of modulating the cell outcomes on DNA damage, particularly in the context of cancer treatment.

DNA-dependent protein kinase and DNA repair: relevance to Alzheimer's disease

The pathological hallmark of Alzheimer's disease (AD), the leading cause of senile dementia, involves region-specific neuronal death and an accumulation of neuronal and extracellular lesions termed neurofibrillary tangles and senile plaques, respectively. One of the biochemical abnormalities observed in AD is reduced DNA end-joining activity. The reduced capacity of post-mitotic neurons for some types of DNA repair is further compromised by aging. The predominant mechanism to repair double-strand DNA (dsDNA) breaks (DSB) is non-homologous end joining (NHEJ), which requires DNA-dependent protein kinase (DNA-PK) activity. DNA-PK is a holoenzyme comprising the p460 kDa DNA-PK catalytic subunit (DNA-PKcs) and the Ku heterodimer consisting of p86 (Ku 80) and p70 (Ku 70) subunits. Ku binds to DNA ends first and then recruits DNA-PKcs during NHEJ. However, in AD brains, reduced NHEJ activity has been reported along with reduced levels of DNA-PKcs and the Ku proteins, indicating a potential link between AD and dsDNA damage. Since age-matched control brains also show a reduction in these protein levels, whether there is a direct link between NHEJ ability and AD remains unknown. Possible mechanisms involving the role of DNA-PK in neurodegeneration, a benchmark of AD, are the focus of this review.