Absence of autoantigen Ku in mature human neutrophils and human promyelocytic leukemia line (HL-60) cells and lymphocytes undergoing apoptosis

iTRAQ-based quantitative proteomics reveals biomarkers/pathways in psoriasis that can predict the efficacy of methotrexate

BACKGROUND

Methotrexate (MTX) is the first-line medicine to treat psoriasis. So far, there has been less research on protein biomarkers to predict its efficacy by the proteomic technique.

OBJECTIVES

To evaluate differentially expressed proteins in peripheral mononuclear cells (PBMCs) between good responders (GRs) and non-responders (NRs) after MTX treatment, compared with normal controls (NCs).

METHODS

We quantified protein expression of PBMCs with 4 GRs and 4 NRs to MTX and 4 NCs by isobaric tags for relative and absolute quantification (iTRAQ), analyzing and identifying proteins related to efficacy of MTX in 18 psoriatic patients.

RESULTS

A total of 3,177 proteins had quantitative information, and 403 differentially expressed proteins (fold change ≥ 1.2, p < .05) were identified. Compared to NCs, upregulated proteins (ANXA6, RPS27A, EZR, XRCC6), participating in the activation of NF-κB, the JAK-STAT pathway, and neutrophil degranulation were detected in GRs. The proteins (GPV, FN1, STOM), involving platelet activation, signaling and aggregation as well as neutrophil degranulation were significantly downregulated in GRs. These proteins returned to normal levels after MTX treatment. Furthermore, Western blotting identified the expression of ANXA6 and STAT1 in PBMCs, which were significantly downregulated in GRs, but not in NRs.

CONCLUSIONS

We identified seven differentially expressed and regulated proteins (ANXA6, GPV, FN1, XRCC6, STOM, RPS27A, and EZR) as biomarkers to predict MTX efficacy in NF-κB signaling, JAK-STAT pathways, neutrophil degranulation, platelet activation, signaling and aggregation.

Inhibition of DNA Repair Protein Ku70 in High-Glucose Environment Aggravates the Neurotoxicity Induced by Bupivacaine in SH-SY5Y Cells

Bupivacaine, a common local anesthetic, causes serious nerve injury, especially in diabetic patients, as high glucose has been reported to enhance bupivacaine-induced neurotoxicity. However, the key regulator for synergism remains unknown. To our surprise, the expression of repair protein Ku70 is suppressed, while the high-glucose environment induces DNA oxidative damage in neurons. Here, we aim to investigate whether the inhibition of Ku70 by high-glucose conditions aggrandized bupivacaine-induced DNA damage. Consistent with previous results, bupivacaine induced reactive oxygen species production and upregulated Ku70 and cleaved caspase-3 expressions at both transcript and protein levels and ultimately caused nucleic acid damage and apoptosis in human neuroblastoma (SH-SY5Y) cells. High-glucose treatment inhibited the expression of Ku70 and enhanced bupivacaine-induced neurotoxicity. In contrast, the overexpression of Ku70 mitigated DNA damage and apoptosis triggered by bupivacaine and high glucose. In conclusion, our data indicated that local anesthetics may aggravate nerve toxicity in a high-glucose environment.

Chromatin association of XRCC5/6 in the absence of DNA damage depends on the XPE gene product DDB2

Damaged DNA-binding protein 2 (DDB2), a nuclear protein, participates in both nucleotide excision repair and mRNA transcription. The transcriptional regulatory function of DDB2 is significant in colon cancer, as it regulates metastasis. To characterize the mechanism by which DDB2 participates in transcription, we investigated the protein partners in colon cancer cells. Here we show that DDB2 abundantly associates with XRCC5/6, not involving CUL4 and DNA-PKcs. A DNA-damaging agent that induces DNA double-stranded breaks (DSBs) does not affect the interaction between DDB2 and XRCC5. In addition, DSB-induced nuclear enrichment or chromatin association of XRCC5 does not involve DDB2, suggesting that the DDB2/XRCC5/6 complex represents a distinct pool of XRCC5/6 that is not directly involved in DNA break repair (NHEJ). In the absence of DNA damage, on the other hand, chromatin association of XRCC5 requires DDB2. We show that DDB2 recruits XRCC5 onto the promoter of SEMA3A, a DDB2-stimulated gene. Moreover, depletion of XRCC5 inhibits SEMA3A expression without affecting expression of VEGFA, a repression target of DDB2. Together our results show that DDB2 is critical for chromatin association of XRCC5/6 in the absence of DNA damage and provide evidence that XRCC5/6 are functional partners of DDB2 in its transcriptional stimulatory activity.

Reptin regulates DNA double strand breaks repair in human hepatocellular carcinoma

Reptin/RUVBL2 is overexpressed in most hepatocellular carcinomas and is required for the growth and viability of HCC cells. Reptin is involved in several chromatin remodeling complexes, some of which are involved in the detection and repair of DNA damage, but data on Reptin involvement in the repair of DNA damage are scarce and contradictory. Our objective was to study the effects of Reptin silencing on the repair of DNA double-strand breaks (DSB) in HCC cells. Treatment of HuH7 cells with etoposide (25 μM, 30 min) or γ irradiation (4 Gy) increased the phosphorylation of H2AX by 1.94 ± 0.13 and 2.0 ± 0.02 fold, respectively. These values were significantly reduced by 35 and 65 % after Reptin silencing with inducible shRNA. Irradiation increased the number of BRCA1 (3-fold) and 53BP1 foci (7.5 fold). Depletion of Reptin reduced these values by 62 and 48%, respectively. These defects in activation and/or recruitment of repair proteins were not due to a decreased number of DSBs as measured by the COMET assay. All these results were confirmed in the Hep3B cell line. Protein expression of ATM and DNA-PKcs, the major H2AX kinases, was significantly reduced by 52 and 61 % after Reptin depletion whereas their mRNA level remained unchanged. Phosphorylation of Chk2, another ATM target, was not significantly altered. Using co-immunoprecipitation, we showed an interaction between Reptin and DNA-PKcs. The half-life of newly-synthesized DNA-PKcs was reduced when Reptin was silenced. Finally, depletion of Reptin was synergistic with etoposide or γ irradiation to reduce cell growth and colony formation. In conclusion, Reptin is an important cofactor for the repair of DSBs. Our data, combined with those of the literature suggests that it operates at least in part by regulating the expression of DNA-PKcs by a stabilization mechanism. Overexpression of Reptin in HCC could be a factor of resistance to treatment, consistent with the observed overexpression of Reptin in subgroups of chemo-resistant breast and ovarian cancers.

Effect of Ku70 expression on radiosensitivity in renal carcinoma 786-O cells

Background

Radiotherapy plays an important role in cancer therapy. However, the radioresistance of some human cancers, particularly renal carcinoma, often results in radiotherapy failure. The Ku protein is essential for the repair of a majority of DNA double-strand breaks in mammalian cells, but effect of Ku70 expression on radiosensitivity in renal carcinoma is unclear. Here, we investigate the impact of Ku70 on radiosensitivity in renal carcinoma cells through regulating the expression of Ku70.

Methods

The stable overexpression of Ku70 or suppression of Ku70 in renal carcinoma cell line (786-O) was generated by retrovirus-mediated Ku70 cDNA or shRNA targeting Ku70. Ku70 expression was determined by RT-PCR and Western blot analysis, the apoptosis of the stable cells was assayed with flow cytometry and TUNEL assay and the effect of radiation on the livability of stable cells was assessed by MTT assay.

Results

Up-regulation of Ku70 expression of 786-O cells could inhibit cell apoptosis and reduce susceptibility to radiation. On the contrary, 786-O cells with suppression of Ku70 expression could induce cell apoptosis and significantly enhance the sensitivity to radiation.

Conclusions

These findings indicated that Ku70 might play an important role in radioresistance of renal carcinoma, and inhibition of Ku70 can increase the radiosensitivity of 786-O cells by enhancing apoptosis, suggesting down-regulation of Ku70 expression combined with radiotherapy will be a potential strategy for renal cell carcinoma therapy.

Ratio of γ-H2AX level in lymphocytes to that in granulocytes detected using flow cytometry as a potential biodosimeter for radiation exposure

This study aims to assess utilisation of the ratio of γ-H2AX in lymphocytes to that in granulocytes (RL/G of γ-H2AX) in blood as a rapid method for population triage and dose estimation during large-scale radiation emergencies. Blood samples from healthy volunteers exposed to 0-10 Gy of (60)Co irradiation were collected. The samples were cultured for 0-24 h and then analysed using flow cytometry to measure the levels of γ-H2AX in lymphocytes and granulocytes. The basal RL/G levels of γ-H2AX in healthy human blood, the response of RL/G of γ-H2AX to ionising radiation and its relationship with doses, time intervals after exposure and individual differences were also analysed. The level of γ-H2AX in lymphocytes increased in a dose-dependent manner after irradiation, whereas the level in granulocytes was not affected. A linear dose-effect relationship with low inter-experimental and inter-individual variations was observed. The RL/G of γ-H2AX may be used as a biomarker for population triage and dose estimation during large-scale radiation emergencies if blood samples can be collected within 24 h.

Methyl angolensate from callus of Indian redwood induces cytotoxicity in human breast cancer cells

AIM

Natural products discovered from medicinal plants have played an important role in the treatment of cancer. Methyl angolensate (MA), a tetranortriterpenoid obtained from the root callus of Indian Redwood tree, Soymida febrifuga Roxb. (A.Juss) was tested for its anticancer properties on breast cancer cells.

METHODS

Cell viability was tested using trypan blue, MTT and LDH assays. Tritiated thymidine assay and flowcytometry were used to study effect of MA on cell proliferation. The activation of apoptosis was checked by annexin V and JC-1 staining followed by FACS analysis. Immunoblotting analysis was used for studying expression of apoptotic and DNA double strand break repair proteins.

RESULTS

We find that MA inhibited the growth of breast cancer cell line, T47D in a time- and dose-dependent manner. MA treatment led to the inhibition of cell proliferation as detected by tritiated thymidine assay and flowcytometry. Further, MA treated cells exhibited typical apoptotic morphological changes and led to the accumulation of subG1 peak in cell cycle distribution. The induction of apoptosis was further confirmed both by annexin V staining and JC1 staining. We also find that MA activates MAP kinase pathway to induce apoptosis. Besides, we find a time dependent activation followed by degradation of DNA double-strand break repair proteins upon treatment with MA.

CONCLUSION

These results suggest that MA induces cytotoxicity in breast cancer cells. Further, the altered expression of DSB repair proteins in MA treated cells may control the induction of apoptosis in these cancer cells.

Positive regulation of DNA double strand break repair activity during differentiation of long life span cells: the example of adipogenesis

Little information is available on the ability of terminally differentiated cells to efficiently repair DNA double strand breaks (DSBs), and one might reasonably speculate that efficient DNA repair of these threatening DNA lesions, is needed in cells of long life span with no or limited regeneration from precursor. Few tissues are available besides neurons that allow the study of DNA DSBs repair activity in very long-lived cells. Adipocytes represent a suitable model since it is generally admitted that there is a very slow turnover of adipocytes in adult. Using both Pulse Field Gel Electrophoresis (PFGE) and the disappearance of the phosphorylated form of the histone variant H2AX, we demonstrated that the ability to repair DSBs is increased during adipocyte differentiation using the murine pre-adipocyte cell line, 3T3F442A. In mammalian cells, DSBs are mainly repaired by the non-homologous end-joining pathway (NHEJ) that relies on the DNA dependent protein kinase (DNA-PK) activity. During the first 24 h following the commitment into adipogenesis, we show an increase in the expression and activity of the catalytic sub-unit of the DNA-PK complex, DNA-PKcs. The increased in DNA DSBs repair activity observed in adipocytes was due to the increase in DNA-PK activity as shown by the use of DNA-PK inhibitor or sub-clones of 3T3F442A deficient in DNA-PKcs using long term RNA interference. Interestingly, the up-regulation of DNA-PK does not regulate the differentiation program itself. Finally, similar positive regulation of DNA-PKcs expression and activity was observed during differentiation of primary culture of pre-adipocytes isolated from human sub-cutaneous adipose tissue.

Our results show that DNA DSBs repair activity is up regulated during the early commitment into adipogenesis due to an up-regulation of DNA-PK expression and activity. In opposition to the general view that DNA DSBs repair is decreased during differentiation, our results demonstrate that an up-regulation of this process might be observed in post-mitotic long-lived cells.

Hydrogen sulfide‐induces DNA damage and changes in apoptotic gene expression in human lung fibroblast cells

Hydrogen sulfide (H2S) has been shown previously to exert proapoptotic activity. However, the mechanism(s) by which H2S affects cell growth and function have not been addressed adequately. In this study, cultured human lung fibroblasts were treated with the H2S donor NaHS (10-75 microM; 12-48 h). NaHS caused a concentration-dependent increase in micronuclei formation (indicating DNA damage) and cell cycle arrest (G1 phase). NaHS increased expression of ku 70 and ku 80 but did not affect the expression of other DNA repair proteins such as proliferating cell nuclear antigen (PCNA) or replication protein A (rNase protection assay). NaHS treatment also resulted in stabilization of p53 coupled with induction of downstream proteins such as p21, Bax, and cytochrome c, as well as translocation of Bax from the cytosol to the mitochondria and release of cytochrome c from mitochondria. NaHS did not up-regulate cell levels of the antiapoptotic protein, Bcl-2. We propose that the genotoxic action of H2S propels the cell toward apoptotic death triggered initially by stabilization of p53 and subsequently involving a cascade of downstream products. These results are of significance as they uncover a hitherto unknown and very fundamental role for H2S in determining cell fate.

DNA repair Ku proteins in gastric cancer cells and pancreatic acinar cells

The DNA repair protein Ku acts as a heterodimer of Ku70 and Ku80 that binds to the DNA ends, nicks, or single-to-double-strand transition. It has a crucial role for DNA double-strand-break repair. Either Ku70 or Ku80 itself may have a unique function that is independent of the other Ku subunit. In this review, the role of Ku on cell proliferation and apoptosis will be discussed. Ku acts as a regulator of transcription by interacting with the recombination signal binding protein Jkappa and the NF-kappaB p50 homodimer to up-regulate p50 expression, which may regulate the proliferation of gastric cancer cells. Both Ku70 and Ku80 expressions are mediated by constitutively activated NF-kappaB and constitutively expressed cyclooxygenase-2 in gastric cancer cells, which may be related to gastric cell proliferation and carcinogenesis. In addition, nuclear loss of Ku may underlie the mechanism of apoptosis in pancreatic acinar cells after oxidative stress.

Expression of DNA-dependent protein kinase in human granulocytes

Human polymorphonuclear leukocytes (PMN) have been reported to completely lack of DNA-dependent protein kinase (DNA-PK) which is composed of Ku protein and the catalytic subunit DNA-PKcs, needed for nonhomologous end-joining (NHEJ) of DNA double-strand breaks. Promyelocytic HL-60 cells express a variant form of Ku resulting in enhanced radiation sensitivity. This raises the question if low efficiency of NHEJ, instrumental for the cellular repair of oxidative damage, is a normal characteristic of myeloid differentiation. Here we confirmed the complete lack of DNA-PK in PMN protein extracts, and the expression of the truncated Ku86 variant form in HL-60. However, this degradation of DNA-PK was shown to be due to a DNA-PK-degrading protease in PMN and HL-60. In addition, by using a protease-resistant whole cell assay, both Ku86 and DNA-PKcs could be demonstrated in PMN, suggesting the previously reported absence in PMN of DNA-PK to be an artefact. The levels of Ku86 and DNA-PKcs were much reduced in PMN, as compared with that of the lymphocytes, whereas HL-60 displayed a markedly elevated DNA-PK concentration. In conclusion, our findings provide evidence of reduced, not depleted expression of DNA-PK during the mature stages of myeloid differentiation.

Polyethyleneimine grafted with pluronic P85 enhances Ku86 antisense delivery and the ionizing radiation treatment efficacy in vivo

In an effort to improve the efficacy of antisense delivery, we evaluated polyethyleneimine (PEI, 2 kDa) alone or grafted with nonionic amphiphilic block copolymer Pluronic (P85) as a carrier for Ku86 antisense oligonucleotide (ASO) delivery. Ku86 is an abundant nuclear protein that plays an important role in nonhomologous DNA end joining and has implications in tumorigenesis and acquired drug resistance. Transfection of adherent and suspension cell lines with Ku86 ASOs complexed with P85-g-PEI (2 kDa) conjugates was associated with a specific decrease in Ku86 mRNA levels (EC50<75 nM and EC50<250 nM, respectively, n=3). More importantly, no requirement for reduced serum conditions was necessary during transfection. In contrast, whereas Ku86 ASOs complexed with PEI (2 kDa) alone were effective in decreasing Ku86 mRNA levels in adherent cell lines (EC50<75 nM, n=3), the formulation did not produce any detectable decrease in Ku86 mRNA levels in suspension cell lines. Transfection of adherent cell lines with 500 nM Ku86 ASOs formulated with P85-g-PEI (2 kDa) was associated with a specific decrease (<10% remaining of control) in Ku86 protein expression and a two-fold increased cell death after treatment with ionizing radiation (IR). In athymic nude mice bearing subcutaneous human HT29 colon adenocarcinoma xenografts, Ku86 ASO-P85-g-PEI (2 kDa) administration (15 mg/kg, subcutaneously) with a Q1D x 7 treatment schedule, when combined with a single dose of IR (6 Gy), caused a significant inhibition of HT29 tumor growth compared with mismatch- and naked antisense-pretreated control groups (time from 200 to 1000 mm3, 126.9 versus 84.18 and 87.76 days, P<0.005). A potentiation of the antitumor activity was observed in all mice treated with Ku86 ASO-P85-g-PEI (2 kDa) formulation; however, tumor growth inhibition was reversible upon treatment cessation. No morbidity/mortality or changes in histopathology were observed under this treatment regiment. Our results indicate that P85-g-PEI (2 kDa) conjugates may increase the efficacy of Ku86 ASO delivery in management of resistant malignancies, thus providing a rationale for their evaluation in cancer patients in combination with conventional anticancer therapies.

The membrane form of the DNA repair protein Ku interacts at the cell surface with metalloproteinase 9

The Ku heterodimer (Ku70/Ku80) plays a central role in DNA double-strand breaks repair. Ku is also expressed on the cell surface of different types of cells where its function remains poorly understood. From a yeast two-hybrid screen, we have identified a specific interaction between the core region of Ku80 and the hemopexin domain of metalloproteinase 9 (MMP-9), a key enzyme involved in the degradation of extracellular matrix (ECM) components. Ku associates with MMP-9 on the surface of leukemic cells as demonstrated by co-immunoprecipitation experiments in membrane extracts and double-label immunofluorescence studies. In normal and tumoral migratory cells, Ku80 and MMP-9 colocalize at the periphery of leading edge of cells and cellular invasion of collagen IV matrices was blocked by antibodies directed against Ku70 or Ku80 subunits as well as by Ku80-specific antisense oligonucleotides. Our results indicate that Ku and MMP-9 interact at the cell membrane of highly invasive hematopoietic cells of normal and tumoral origin and document the unexpected importance of the membrane-associated form of Ku in the regulation of ECM remodelling.

Acute promyelocytic leukemia: PML/RARalpha and the leukemic stem cell

Acute promyelocytic leukemia (APL) is distinguished from other acute myeloid leukemias (AMLs) by cytogenetic, clinical, as well as biological characteristics. The hallmark of APL is the t(15;17), which leads to the expression of the PML/RARalpha fusion protein. PML/RARalpha is the central leukemia-inducing lesion in APL and is directly targeted by all trans retinoic acid (t-RA) as well as by arsenic, both compounds able to induce complete remissions. This review focuses on potential stem cell involvement in APL outlining the knowledge about the APL-initiating stem cell and the influence of PML/RARalpha on the biology of the hematopoietic stem cell. Moreover, the importance of the blockage of t-RA signaling by the PML/RARalpha for the pathogenesis of APL is discussed, taking the relevance of the t-RA signaling pathway for the global hematopoiesis into account.

Oxidative stress induces nuclear loss of DNA repair proteins Ku70 and Ku80 and apoptosis in pancreatic acinar AR42J cells

Cell death linked to oxidative DNA damage has been implicated in acute pancreatitis. The severe DNA damage, which is beyond the capacity of the DNA repair proteins, triggers apoptosis. It has been hypothesized that oxidative stress may induce a decrease in the Ku70 and Ku80 levels and apoptosis in pancreatic acinar cells. In this study, it was found that oxidative stress caused by glucose oxidase (GO) acting on beta-d-glucose, glucose/glucose oxidase (G/GO), induced slight changes in cytoplasmic Ku70 and Ku80 but drastically induced a decrease in nuclear Ku70 and Ku80 both time- and concentration-dependently in AR42J cells. G/GO induced apoptosis determined by poly(ADP-ribose) polymerase cleavage, an increase in expression of p53 and Bax, and a decrease in Bcl-2 expression. G/GO-induced apoptosis was in parallel with the loss of nuclear Ku proteins in AR42J cells. Caspase-3 inhibitor prevented G/GO-induced nuclear Ku loss and cell death. G/GO did not induce apoptosis in the cells transfected with either the Ku70 or Ku80 expression gene but increased apoptosis in those transfected with the Ku dominant negative mutant. Pulse and pulse-chase results show that G/GO induced Ku70 and Ku80 syntheses, even though Ku70 and Ku80 were degraded both in cytoplasm and nucleus. G/GO-induced decrease in Ku binding to importin alpha and importin beta reflects possible modification of nuclear import of Ku proteins. The importin beta level was not changed by G/GO. These results demonstrate that nuclear decrease in Ku70 and Ku80 may result from the decrease in Ku binding to nuclear transporter importins and the degradation of Ku proteins. The nuclear loss of Ku proteins may underlie the mechanism of apoptosis in pancreatic acinar cells after oxidative stress.

Expression of Ku80 in cervical cancer correlates with response to radiotherapy and survival

To reveal the genes relevant for prediction of cervical cancer after radiotherapy, we previously carried out cDNA microarray experiments on primary cervical cancer comparing patients with a complete response (CR) and those with no change (NC). Some of these genes had already been associated with the radiation response, such as x-ray repair cross-complementing 5 (XRCC5), which was found more in radioresistant tumors than in radiosensitive ones. The aim of this study was to confirm the possible roles of XRCC5 mRNA levels by a real-time polymerase chain reaction method in 20 cervical cancers, and Ku80 protein, which is the gene product of XRCC5, using a histopathologic method of formalin-fixed sections of tumor biopsies in determining tumor response to radiotherapy and survival in 89 patients with cervical cancer. The levels of XRCC5 mRNA were 10(4.82) +/- 10(0.2) copies/microg total RNA in tumor tissues in the CR group (mean +/- standard deviation) and 10(4.95) +/- 10(0.32) copies/microg total RNA in those in the NC group. The levels of XRCC5 mRNA were not significantly different between the CR and NC groups. Histopathologic methods revealed 29.2% (26 of 89) of the patients to be Ku80-negative, with Ku80-positive findings in 70.8% (63 of 89). Of the Ku80-negative patients, 19 had CR, 3 had a partial response (PR), and 4 had NC. Of the Ku80-positive patients, 25 had CR, 22 had PR, and 16 had NC. Ku80-negative tumors showed significantly better responses than Ku80-positive ones, comparing CR and PR/NC responses (p = 0.01). In addition, overall survival was significantly better in the Ku80-negative patients as compared with those who were Ku80-positive (p = 0.04). The results of this study suggest that a low expression of Ku80 protein leads to radiosensitivity in cervical cancer and that Ku80 might play a role in treatment outcome.

Cell cycle-dependent regulation of the DNA-dependent protein kinase

Human DNA-dependent protein kinase (DNA-PK) is a nuclear-localized serine/threonine protein kinase. The holoenzyme consists of a catalytic subunit with a molecular mass of 465 kDa and a DNA-binding heterodimer Ku86/70. The kinase has been implicated in a variety of nuclear processes including V(D)J recombination, double-strand break repair, and transcription. Cells with defective DNA-PK activity show increased radiosensitivity and lack of V(D)J recombination. To study DNA-PK activity during the cell cycle, HeLa cells were separated by elutriation centrifugation into different cell cycle compartments based on cellular size. DNA-PK activity was found to vary during the cell cycle. The kinase activity was lowest during G1 phase and increased dramatically as the cells entered S phase and remained high during the G2-phase. The subcellular distribution of DNA-PKcs is relocalized from the cytoplasm during M and G1 phases to the nucleus during G1-S phase transition and S phase. Expression of both the catalytic subunit and the Ku86/70 heterodimer was found to be constant throughout the cell cycle. This study demonstrates that DNA-PK activity as well as its subcellular localization fluctuates during the cell cycle. In addition, the distribution of DNA-PK during M phase corresponds with low DNA-PK activity.

Human neutrophils isolated from peripheral blood contain Ku protein but not DNA-dependent protein kinase

Ku protein, a heterodimer of 70kDa (Ku70) and 86kDa (Ku86) polypeptides, is involved in non-homologous DNA end-joining (NHEJ) of DNA double-strand break repair and V(D)J recombination in combination with the catalytic component of DNA-dependent protein kinase (p470). Although Ku protein is known to be ubiquitously present in eukaryotic cells, it was previously reported to be absent in mature neutrophils. Using a mixture of protease inhibitors in the isolation procedure of neutrophils from human peripheral blood, we were able to detect Ku in the neutrophils by immunoblot and flow-cytometric analyses. Transcripts of Ku70 and Ku86 genes were also detected by the reverse transcriptase-polymerase chain reaction (RT-PCR), and Ku protein was shown to be localized in the nucleus of neutrophils as a heterodimer. Like poly(ADP-ribose) polymerase-1, neither mRNA nor protein of p470 was detected in the neutrophils. These results suggest that Ku is involved independently of p470 in DNA metabolism and signal transduction.

Heterogeneous expression of Ku70 in human tissues is associated with morphological and functional alterations of the nucleus

Ku70 is a subunit of DNA-protein kinase complex and involved in diverse intranuclear events including the repair of double-stranded DNA breaks. Ku70 is rich in the interphase nucleus of cultured cells. In human tissues, however, the distribution of Ku70 has not yet been systematically examined. To characterize the difference of Ku70 distribution between cells of human tissues and cultured cells, the expression of Ku70 was examined in various normal and neoplastic human tissues by immunohistochemistry and immunoblot. In addition, the role of Ku70 in the cellular response against ionizing radiation (IR) was analysed in fibroblasts after exposure to 5 Gy IR and apoptotic indices were examined in Ku70-overexpressed fibroblasts from an ataxia telangiectasia patient and in normal fibroblasts, before and after irradiation. In contrast to cultured cells, Ku70 was not detected in some interphase cells of human tissues and was distributed heterogeneously, even in the same nucleus. Ku70 expression was strikingly low in terminally differentiated cells such as neutrophils, eosinophils, glomerular capillary endothelial cells and fibroblasts, and was absent in spermatids. In spermatocytes, Ku70 was tightly integrated with chromosome filaments, unlike other somatic cells under mitosis. After exposure to IR, Ku70 expression was not increased in ataxia telangiectasia fibroblasts, but was significantly increased in normal fibroblasts. Most of the increased Ku70 was of soluble nuclear protein fraction. Furthermore, overexpression of Ku70 increased radiation resistance both in ataxia telangiectasia fibroblasts and normal fibroblasts. The presented data indicate that the distribution of Ku70 in cells of human tissues is closely associated with the cell cycle, cellular differentiation, nuclear shape and the process of repair of DNA damage caused by IR.

Mutation in the glucose-6-phosphate dehydrogenase gene leads to inactivation of Ku DNA end binding during oxidative stress

Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme of the oxidative pentose phosphate cycle, regulates the NADPH/NADP(+) ratio in eukaryotic cells. G6PD deficiency is one of the most common mutations in humans and is known to cause health problems for hundreds of millions worldwide. Although it is known that decreased G6PD functionality can result in increased susceptibility to oxidative stress, the molecular targets of this stress are not known. Using a Chinese hamster ovary G6PD-null mutant, we previously demonstrated that exposure to a thiol-specific oxidant, hydroxyethyldisulfide, caused enhanced radiation sensitivity and an inability to repair DNA double strand breaks. We now demonstrate a molecular mechanism for these observations: the direct inhibition of DNA end binding activity of the Ku heterodimer, a DNA repair protein, by oxidation of its cysteine residues. Inhibition of Ku DNA end binding was found to be reversible by treatment of the nuclear extract with dithiothreitol, suggesting that the homeostatic regulation of reduced cysteine residues in Ku is a critical function of G6PD and the oxidative pentose cycle. In summary, we have discovered a new layer of DNA damage repair, that of the functional maintenance of repair proteins themselves. In view of the rapidly escalating number of roles ascribed to Ku, these results may have widespread ramifications.

Early decrease in dna repair proteins, Ku70 and Ku86, and subsequent DNA fragmentation after transient focal cerebral ischemia in mice

BACKGROUND AND PURPOSE

Ku70 and Ku86, multifunctional DNA repair proteins, bind to broken DNA ends, including double-strand breaks, and trigger a DNA repair pathway. To investigate the involvement of these proteins in DNA fragmentation after ischemia/reperfusion, Ku protein expression was examined before and after transient focal cerebral ischemia (FCI) in mice.

METHODS

Adult male CD-1 mice were subjected to 60 minutes of FCI by intraluminal suture blockade of the middle cerebral artery. Ku protein expression was studied by immunohistochemistry and Western blot analysis. DNA fragmentation was evaluated by gel electrophoresis and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL). The spatial relationship between Ku expression and DNA fragmentation was examined by double labeling with Ku and TUNEL after reperfusion.

RESULTS

Immunohistochemistry showed constitutive expression of Ku proteins in control brains. The number of Ku-expressing cells was decreased in the entire middle cerebral artery territory as early as 4 hours after reperfusion and remained reduced until 24 hours. Western blot analyses confirmed the significant reduction of these proteins (59.4% and 57.7% reduction in optical density at 4 hours of reperfusion from the normal level of Ku70 and Ku86 bands, respectively; P<0.001). DNA gel electrophoresis demonstrated DNA laddering 24 hours after reperfusion, but not at 4 hours. Double staining with Ku and TUNEL showed a concomitant loss of Ku immunoreactivity and TUNEL-positive staining.

CONCLUSIONS

These results suggest that the early reduction of Ku proteins and the loss of defense against DNA damage may underlie the mechanism of DNA fragmentation after FCI.

Hypoxia-activated ligand HAL-1/13 is lupus autoantigen Ku80 and mediates lymphoid cell adhesion in vitro

Hypoxia is known to induce extravasation of lymphocytes and leukocytes during ischemic injury and increase the metastatic potential of malignant lymphoid cells. We have recently identified a new adhesion molecule, hypoxia-activated ligand-1/13 (HAL-1/13), that mediates the hypoxia-induced increases in lymphocyte and neutrophil adhesion to endothelium and hypoxia-mediated invasion of endothelial cell monolayers by tumor cells. In this report, we used expression cloning to identify this molecule as the lupus antigen and DNA-dependent protein kinase-associated nuclear protein, Ku80. The HAL-1/13-Ku80 antigen is present on the surface of leukemic and solid tumor cell lines, including T and B lymphomas, myeloid leukemias, neuroblastoma, rhabdomyosarcoma, and breast carcinoma cells. Transfection and ectopic expression of HAL-1/13-Ku80 on (murine) NIH/3T3 fibroblasts confers the ability of these normally nonadhesive cells to bind to a variety of human lymphoid cell lines. This adhesion can be specifically blocked by HAL-1/13 or Ku80-neutralizing antibodies. Loss of expression variants of these transfectants simultaneously lost their adhesive properties toward human lymphoid cells. Hypoxic exposure of tumor cell lines resulted in upregulation of HAL-1/13-Ku80 expression at the cell surface, mediated by redistribution of the antigen from the nucleus. These studies indicate that the HAL-1/13-Ku80 molecule may mediate, in part, the hypoxia-induced adhesion of lymphocytes, leukocytes, and tumor cells.

Potentiation of chemosensitivity in multidrug-resistant human leukemia CEM cells by inhibition of DNA-dependent protein kinase using wortmannin

DNA-dependent protein kinase (DNA-PK) is activated by DNA strand breaks and participates in DNA repair. Its regulatory subunit, Ku autoantigen, binds to DNA and recruits the catalytic subunit (DNA-PKcs). We show here a new role of DNA-PK in the development of multidrug resistance (MDR). The Ku-DNA binding activity, the levels of Ku70/Ku80 and DNA-PKcs in MDR variants, CEM/VLB(10-2), CEM/VLB(55-8) and CEM/VLB100 were higher than those in their parental drug-sensitive CEM cells in a drug resistance-dependent fashion. Also, CEM/VLB100 cells showed about 3-fold increase of DNA-PK enzyme activity as compared with CEM cells. Similar results were observed in another MDR cell line, FM3A/M mouse mammary carcinoma cells. Moreover, we observed that CEM/VLB100 cells were about 11-fold sensitive to wortmannin, which inhibits DNA-PK, compared with the CEM cells, and sensitized the MDR cells when combined with either bleomycin or vincristine, but have a little effect on CEM cells. Wortmannin was shown to inhibit DNA-PK and Ku-DNA binding activity in CEM/VLB100 cells dose dependently but had a little or no effect on their parental cells. Our results suggested that enhanced expression of DNA-PK participates in the development of MDR, and the use of DNA-PK inhibitors such as wortmannin is likely to improve the effectiveness of anticancer drugs and thus could partially overcome drug resistance in MDR cells, through its ability to inhibit Ku/DNA-PK activity.

Caspase-mediated degradation of AMPA receptor subunits: a mechanism for preventing excitotoxic necrosis and ensuring apoptosis

Activation of ionotropic glutamate receptors of the AMPA and NMDA subtypes likely contributes to neuronal injury and death in various neurodegenerative disorders. Excitotoxicity can manifest as either apoptosis or necrosis, but the mechanisms that determine the mode of cell death are not known. We now report that levels of AMPA receptor subunits GluR-1 and GluR-4 are rapidly decreased in cultured rat hippocampal neurons undergoing apoptosis in response to withdrawal of trophic support (WTS), whereas levels of NMDA receptor subunits NR1, NR2A, and NR2B are unchanged. Exposure of isolated synaptosomal membranes to "apoptotic" cytosolic extracts resulted in rapid degradation of AMPA receptor subunits. Treatment of cells and synaptosomal membranes with the caspase inhibitors prevented degradation of AMPA receptor subunits, demonstrating a requirement for caspases in the process. Calcium responses to AMPA receptor activation were reduced after withdrawal of trophic support and enhanced after treatment with caspase inhibitors. Vulnerability of neurons to excitotoxic necrosis was decreased after withdrawal of trophic support and potentiated by treatment with caspase inhibitors. Our data indicate that caspase-mediated degradation of AMPA receptor subunits occurs during early periods of cell stress and may serve to ensure apoptosis by preventing excitotoxic necrosis.

Caspase-mediated degradation of AMPA receptor subunits: a mechanism for preventing excitotoxic necrosis and ensuring apoptosis

Activation of ionotropic glutamate receptors of the AMPA and NMDA subtypes likely contributes to neuronal injury and death in various neurodegenerative disorders. Excitotoxicity can manifest as either apoptosis or necrosis, but the mechanisms that determine the mode of cell death are not known. We now report that levels of AMPA receptor subunits GluR-1 and GluR-4 are rapidly decreased in cultured rat hippocampal neurons undergoing apoptosis in response to withdrawal of trophic support (WTS), whereas levels of NMDA receptor subunits NR1, NR2A, and NR2B are unchanged. Exposure of isolated synaptosomal membranes to "apoptotic" cytosolic extracts resulted in rapid degradation of AMPA receptor subunits. Treatment of cells and synaptosomal membranes with the caspase inhibitors prevented degradation of AMPA receptor subunits, demonstrating a requirement for caspases in the process. Calcium responses to AMPA receptor activation were reduced after withdrawal of trophic support and enhanced after treatment with caspase inhibitors. Vulnerability of neurons to excitotoxic necrosis was decreased after withdrawal of trophic support and potentiated by treatment with caspase inhibitors. Our data indicate that caspase-mediated degradation of AMPA receptor subunits occurs during early periods of cell stress and may serve to ensure apoptosis by preventing excitotoxic necrosis.

Chemosensitivity of B cell chronic lymphocytic leukemia and correlated expression of proteins regulating apoptosis, cell cycle and DNA repair

B cell chronic lymphocytic leukemia (B-CLL) cannot be cured with conventional chemotherapy. This clinical enigma appears to be at least partially due to the fact that B-CLL cells are resistant to programmed cell death (apoptosis) and that they are arrested in G0/G1 phase of the cell cycle. The reasons for the dysregulation of these two key cellular events in B-CLL are unclear. The present study aimed at determining correlations between the expression levels of proteins regulating apoptosis, cell cycle and DNA repair in B-CLL cells and normal B cells. In addition, the differential sensitivity of B-CLL cells to drug-induced apoptosis was quantified. We show that in B-CLL cells levels of the death-suppressor Bcl-2 correlated positively with those of the pro-apoptotic protein Bax and of the cyclin-dependent kinase (cdk) inhibitor p27Kip1. In B-CLL cells levels of the anti-apoptotic Bcl-xL showed a positive correlation with levels of the 80 kDa regulatory component (Ku80) of the DNA-dependent protein kinase that is involved in DNA double-stranded break repair. These correlations were not detected in normal B cells. The sensitivity of leukemic cells to FLUD but not to ADM, CPM or to DEX was reduced in pre-treated patients. These data support the hypothesis that in B-CLL cells death-modulators and molecules modulating cell cycle and DNA repair are regulated in a coordinated manner. Leukemia (2000) 14, 40-46.

Evidence for caspase-mediated cleavage of AMPA receptor subunits in neuronal apoptosis and Alzheimer's disease

In Alzheimer's disease (AD) synapses degenerate and neurons die in brain regions involved in learning and memory processes. Although the cellular and molecular mechanisms underlying the neurodegenerative process in AD are unclear, increasing evidence suggests roles for amyloid beta-peptide (Abeta) and biochemical cascades associated with a form of programmed cell death called apoptosis. Cysteine proteases of the caspase family are activated in neurons undergoing apoptosis and apparently play a major role in the cell death process by cleaving yet-to-be-identified substrates. We now report that caspase activity is increased in brain tissue and neurons from AD patients, and in cultured hippocampal neurons undergoing apoptosis after exposure to amyloid beta-peptide (Abeta). Western blot analyses using antibodies against different subunits of 2-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) types of ionotropic glutamate receptors indicate that AMPA receptor subunits (GluR1, GluR2/3, and GluR4), but not NMDA receptor subunits (NR1 and NR2A), are proteolytically cleaved after exposure of hippocampal neurons to apoptotic insults, including Abeta, and that the caspase inhibitor zVAD-fmk suppresses such cleavage. Western blot analysis of brain tissue from AD patients and age-matched controls revealed evidence for increased proteolysis of AMPA receptor subunits in AD. Our data suggest roles for caspase-mediated cleavage of AMPA receptor subunits in modifying neuronal responsivity to glutamate and in the neurodegenerative process in AD.

Changes in expression of the DNA repair protein complex DNA-dependent protein kinase after ischemia and reperfusion

Reperfusion of ischemic tissue causes an immediate increase in DNA damage, including base lesions and strand breaks. Damage is reversible in surviving regions indicating that repair mechanisms are operable. DNA strand breaks are repaired by nonhomologous end joining in mammalian cells. This process requires DNA-dependent protein kinase (DNA-PK), composed of heterodimeric Ku antigen and a 460,000 Da catalytic subunit (DNA-PKcs). In this study, a rabbit spinal cord model of reversible ischemia was used to demonstrate the effect of acute CNS injury on the activity and expression of DNA-dependent protein kinase. The DNA-binding activity of Ku antigen, analyzed by an electrophoretic mobility shift assay, increased during reperfusion after a short ischemic insult (15 min of occlusion), from which the animals recover neurological function. After severe ischemic injury (60 min of occlusion) and reperfusion that results in permanent paraplegia, Ku DNA binding was reduced. Protein levels of the DNA-PK components-Ku70, Ku80, and DNA-PKcs-were monitored by immunoblotting. After 60 min of occlusion, the amount of DNA-PKcs and the enzyme poly(ADP-ribose) polymerase (PARP) decreased with the same time course during reperfusion. Concurrently 150 and 120 kDa fragments were immunostained by an anti-DNA-PKcs monoclonal antibody. This antibody was shown to cross-react with alpha-fodrin breakdown products. The 120 kDa fodrin peptide is associated with caspase-3 activation during apoptosis. Both DNA-PKcs and PARP are also substrates for caspase-3-like activities. The results are consistent with a model in which after a short ischemic insult, DNA repair proteins such as DNA-PK are activated. After severe ischemic injury, DNA damage overwhelms repair capabilities, and cell death programs are initiated.

Changes in expression of the DNA repair protein complex DNA-dependent protein kinase after ischemia and reperfusion

Reperfusion of ischemic tissue causes an immediate increase in DNA damage, including base lesions and strand breaks. Damage is reversible in surviving regions indicating that repair mechanisms are operable. DNA strand breaks are repaired by nonhomologous end joining in mammalian cells. This process requires DNA-dependent protein kinase (DNA-PK), composed of heterodimeric Ku antigen and a 460,000 Da catalytic subunit (DNA-PKcs). In this study, a rabbit spinal cord model of reversible ischemia was used to demonstrate the effect of acute CNS injury on the activity and expression of DNA-dependent protein kinase. The DNA-binding activity of Ku antigen, analyzed by an electrophoretic mobility shift assay, increased during reperfusion after a short ischemic insult (15 min of occlusion), from which the animals recover neurological function. After severe ischemic injury (60 min of occlusion) and reperfusion that results in permanent paraplegia, Ku DNA binding was reduced. Protein levels of the DNA-PK components-Ku70, Ku80, and DNA-PKcs-were monitored by immunoblotting. After 60 min of occlusion, the amount of DNA-PKcs and the enzyme poly(ADP-ribose) polymerase (PARP) decreased with the same time course during reperfusion. Concurrently 150 and 120 kDa fragments were immunostained by an anti-DNA-PKcs monoclonal antibody. This antibody was shown to cross-react with alpha-fodrin breakdown products. The 120 kDa fodrin peptide is associated with caspase-3 activation during apoptosis. Both DNA-PKcs and PARP are also substrates for caspase-3-like activities. The results are consistent with a model in which after a short ischemic insult, DNA repair proteins such as DNA-PK are activated. After severe ischemic injury, DNA damage overwhelms repair capabilities, and cell death programs are initiated.

DNA-PK, the DNA-activated protein kinase, is differentially expressed in normal and malignant human tissues

DNA-PK is a nuclear, serine/threonine protein kinase required for repairing DNA double-strand breaks and for V(D)J recombination. To determine the distribution of DNA-PK in human tissues, we assayed paraffin-embedded sections of normal and cancerous tissues for DNA-PKcs and Ku80 by immunohistochemistry. We also assayed for Brca2, a human tumor suppressor gene that is implicated in the repair of DNA strand-breaks. Brca2 was strongly expressed in epithelial cells of the breast, endometrium, and thymus, in tingible body macrophages of follicular germinal centers of lymphoid tissue, and in reticuloendothelial cells in the spleen. DNA-PKcs and Ku80 expression was usually parallel, but both were expressed in a highly cell- and tissue-specific manner. The highest levels were observed in spermatogenic cells (but not in spermatozoa), and in neurons and glial cells of the central and autonomic nervous system. Neither protein was consistently expressed in liver nor in resting mammary epithelium, but lactating breast epithelium was strongly positive for DNA-PKcs and Ku80. In contrast to established human cell cultures, expression between cells in the same tissue was highly selective in the epidermis, exocrine pancreas, renal glomeruli, the red pulp of the spleen, and within cellular compartments of tonsils, lymph nodes, and thymus. Most cancerous tissues were consistently positive for DNA-PKcs and Ku80, except invasive carcinoma of the breast. DNA-PKcs, Ku80, and Ku70 mRNAs were expressed in all normal tissues with relatively little variation in levels. Our results suggest that the apparent absence of DNA-PKcs and Ku80 from some cells or tissues is a consequence of post-transcriptional mechanisms that regulate protein levels.

The Ku70 autoantigen interacts with p40phox in B lymphocytes

Ku70, a regulatory component of the DNA-dependent protein kinase, was identified by a yeast two-hybrid screen of a B lymphocyte cDNA library as a partner of p40phox, a regulatory component of the O2--producing NADPH oxidase. Truncated constructs of p40phox and Ku70 were used to map the interacting sites. The 186 C-terminal amino acids (aa) of Ku70 were found to interact with two distinct regions of p40phox, the central core region (aa 50–260) and the C-terminal extremity (aa 260–339). In complementary experiments, it was observed that Ku70 binds to immobilized recombinant p40phox fusion protein and that p40phox and Ku70 from a B lymphocyte cell extract comigrate in successive chromatographies on Q Separose, Superose 12 and hydroxylapatite columns. Moreover, we report that Ku70 and p40phox colocalize in B lymphocytes and in transfected Cos-7 cells. We also show that the two NADPH oxidase activating factors, p47phox and p67phox are substrates for DNA-PK in vitro and that they are present together with p40phox in the nucleus of B cells. These results may help solve the paradox that the phox protein triad, p40phox, p47phox and p67phox, is expressed equally in B lymphocytes and neutrophils, whereas the redox component of the NADPH oxidase, a flavocytochrome b, which is well expressed in neutrophils, is barely detectable in B lymphocytes.

Regulation of the DNA-dependent protein kinase (DNA-PK) activity in eukaryotic cells

The DNA-dependent protein kinase (DNA-PK) is a trimeric nuclear serine/threonine protein kinase consisting of a large catalytic sub-unit and the Ku heterodimer that regulates kinase activity by its association with DNA. DNA-PK is a major component of the DNA double strand break repair apparatus, and cells deficient in one of its component are hypersensitive to ionizing radiation. DNA-PK is also required to lymphoid V(D)J recombination and its absence confers in mice a severe combined immunodeficiency phenotype. The purpose of this review is to summarize the current knowledge on the mechanisms that contribute to regulate DNA-PK activity in vivo or in vitro and relates them to the role of DNA-PK in cellular functions. Finally, the studies devoted to drug-inhibition of DNA-PK in order to enhance cancer therapy by DNA-damaging agents are presented.

Caspase-mediated proteolysis during apoptosis: insights from apoptotic neutrophils

Apoptosis is initiated by activation of caspases (interleukin 1beta-converting enzyme homologues), which cause coordinated cleavage of several death substrates that function in structural or homeostatic pathways. The relationship between substrate cleavage and apoptosis is not yet known, nor is it clear whether cleavage of specific substrates is a critical requirement for apoptosis. The human neutrophil provides novel insights into the roles of proteolysis of specific substrates during apoptosis, since only a subset of caspase substrates are present in mature neutrophils. Of the death substrates we screened, PARP, the nuclear mitotic apparatus protein (NuMA), the 70 kDa subunit of the U1 small ribonucleoprotein (U1-70kDa) and the catalytic subunit of DNA-dependent protein kinase (DNA-PK(CS)) were not detected in non-apoptotic neutrophils; in contrast, lamin B and fodrin were present in amounts similar to those found in other cells. Caspase-3 activity was absent in freshly isolated neutrophils, but was detected when neutrophils were aged in vitro, coincident with the onset of morphologic and biochemical apoptosis. The absence of PARP, NuMA, U1-70kDa and DNA-PK(CS) in non-apoptotic neutrophils suggests that these are not critical anti-apoptotic proteins, and that their fragments are not required components of the neutrophil apoptotic pathway. These studies highlight the conserved role of caspase activation in the apoptotic mechanism, and focus attention on several conserved structural substrates as potential transducers of the proteolytic signal in apoptosis.

The promoters for human DNA-PKcs (PRKDC) and MCM4: divergently transcribed genes located at chromosome 8 band q11

A 30-kb genomic segment containing the promoter and first 9 exons of PRKDC, the gene encoding the catalytic subunit (DNA-PKcs) of the human DNA-activated protein kinase, DNA-PK, was isolated and partially sequenced. Sequence comparison with the NCBI nonredundant database revealed the locations of the first 13 exons of the upstream gene, MCM4. MCM4 is an essential component of a protein complex that prevents DNA from being replicated more than once per cell cycle. The MCM4 and DNA-PKcs promoters are in CpG islands separated by approximately 700 bp, and transcription from each initiates at multiple, closely spaced sites. Both promoters lack TATA boxes, and the MCM4 promoter also lacks an initiator (Inr) element but has an inverted CCAAT box. The DNA-PKcs promoter has an Inr-like sequence as well as a downstream MED-1 element. The two promoters appear to function independently, as sequences required for core promoter activity do not overlap, and sequences extending into the 5' region of each gene had little or no effect on transcription of the other gene, as shown in transient transfection assays. The arrangement of the PRKDC/MCM4 gene pair is similar to that of the ATM/E14(NPAT) gene pair. ATM, the product of the gene mutated in ataxia telangiectasia, and DNA-PKcs function in pathways that detect or repair DNA damage and are members of a family of large, serine/threonine kinases that are closely related to phosphatidylinositol 3 kinases.

Establishment of a cell-free system of neuronal apoptosis: comparison of premitochondrial, mitochondrial, and postmitochondrial phases

Apoptosis is a fundamental process required for normal development of the nervous system and is triggered during neurodegenerative disease. To dissect the molecular events leading to neuronal cell death, we have developed a cell-free model of neuronal apoptosis. The model faithfully reproduces key elements of apoptosis, including chromatin condensation, DNA fragmentation, caspase activation/processing, and selective substrate cleavage. We report that cell-free apoptosis is activated in premitochondrial, mitochondrial, and postmitochondrial phases by tamoxifen, mastoparan, and cytochrome c, respectively, allowing a functional ordering of these proapoptotic modulators. Furthermore, this is the first report of mitochondrial-mediated activation of cell-free apoptosis in a cell extract. Although Bcl-2 blocks activation at the premitochondrial and mitochondrial levels, it does not affect the postmitochondrial level. The cell-free system described here provides a valuable tool to elucidate the molecular events leading to neuronal cell death.

On the role of DNA double-strand breaks in toxicity and carcinogenesis

DNA double-strand breaks are associated with various endogenous processes, such as transcription, recombination, replication, and with the process of active cell death, which aims to eliminate cells. In addition, DNA double-strand breaks can be induced by irradiation, exposure to chemicals, increased formation of reactive oxygen species, and, indirectly, during repair of other types of DNA damage or as a consequence of extranuclear lesions. In addition to the neutral filter elution of DNA, the recently introduced pulsed-field gel electrophoresis is capable of determining DNA double-strand breaks with higher accuracy and sensitivity and is expected to increase our knowledge on the frequency and the role of DNA breakage. Parallel determination of parameters for cytotoxicity is necessary to elucidate the causal primary lesion. Although the repair of DNA double-strand breaks is a complex task, cells are capable of repairing--with or without errors and up to a certain extent--and surviving this DNA lesion. Gene translocations, rearrangements, amplifications, and deletions arising during repair and misrepair of double-strand breaks may contribute to cell transformation and tumor development.

Attenuation of DNA-dependent protein kinase activity and its catalytic subunit by the herpes simplex virus type 1 transactivator ICP0

The DNA-dependent protein kinase (DNA-PK) is involved in several fundamental nuclear processes, including DNA double-strand break repair, V(D)J recombination, and transcription by RNA polymerases I and II. In this study, we show that infection of mammalian cells with herpes simplex virus type 1 attenuates DNA-PK activity by specifically depleting the p350/DNA-PKcs catalytic subunit. The half-life of the p350/DNA-PKcs protein decreases from greater than 24 h to less than 4 h following infection. The depletion of DNA-PK activity and p350/DNA-PKcs abundance is dependent on expression of the viral immediate-early protein ICP0. As ICP0 acts as a promoter-independent transactivator of gene expression, these data suggest that ICP0 may function by directly or indirectly targeting the p350/DNA-PKcs subunit of DNA-PK, thereby altering the inhibitory effects of DNA-PK on RNA polymerase II transcription.

DNA-dependent protein kinase is a target for a CPP32-like apoptotic protease

We demonstrate that the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs) is specifically, proteolytically cleaved in HL-60 cells treated with staurosporine (STS), a potent inducer of apoptosis. The proteolysis of DNA-PKcs correlated with or preceded apoptotic chromosomal DNA degradation. Cell-free extracts prepared from STS-treated HL-60 cells recapitulated the proteolysis of DNA-PKcs in an in vitro assay using purified DNA-PK as the substrate. Western blot analyses of the apoptotic cell extract showed that the 32-kDa precursor of CPP32 is expressed in HL-60 cells and processed following STS treatment. In addition, whereas the DNA-PKcs protease activity was not inhibitable by many conventional protease inhibitors, it was inhibitable by a highly selective peptide-derived inhibitor of CPP32. These data strongly suggest that CPP32, or a CPP32-like protease, is responsible for DNA-PKcs proteolysis. Finally, our results demonstrated that the cleavage of DNA-PKcs in vitro proceeded in the presence of Bcl-2, indicating that the function provided by Bcl-2 lies upstream the proteolysis of DNA-PKcs.

CPP32/Yama/apopain cleaves the catalytic component of DNA-dependent protein kinase in the holoenzyme

DNA-dependent protein kinase (DNA-PK) is composed of a 460-kDa catalytic component (p460) and a DNA-binding component Ku protein. Immunoblot analysis after treatment of Jurkat cells with anti-Fas antibody demonstrated the cleavage of p460 concomitantly with an increase in CPP32/Yama/apopain activity. Recombinant CPP32/Yama/apopain specifically cleaved p460 in the DNA-PK preparation that had been purified from Raji cells into 230- and 160-kDa polypeptides, the latter of which was detected in anti-Fas-treated Jurkat cells. The regulatory component Ku protein was not significantly affected by CPP32/Yama/apopain. DNA-PK activity was decreased with the disappearance of p460 in the incubation of DNA-PK with CPP32/Yama/apopain. These results suggest that the catalytic component of DNA-PK is one of the target proteins for CPP32/Yama/apopain in Fas-mediated apoptosis.

The DNA-dependent protein kinase, DNA-PK: 10 years and no ends in sight

The phenomenon of DNA-dependent protein phosphorylation was first described little over 10 years ago. Since then a DNA-dependent protein kinase, DNA-PK, has been purified from human cells and many of its biochemical properties have been characterized. DNA-PK is composed of a large catalytic subunit, DNA-PKcs, and a DNA-targeting protein, Ku. When assembled on a suitable DNA molecule, the DNA-PK holoenzyme acts as a serine/threonine protein kinase that in vitro phosphorylates many DNA binding and non-binding proteins and transcription factors. Recent genetic studies point strongly to functions in DNA double-strand break repair and V(D)J recombination. In addition, biochemical studies suggest a role in the regulation of transcription. Here we discuss, from a historical perspective, the events leading up to our current understanding of the function of DNA-PK, including recent results from our own studies suggesting the involvement of DNA-PK in apoptosis and in viral infection of human cells.

The DNA-dependent protein kinase: a matter of life and (cell) death

The catalytic subunit of the DNA-dependent protein kinase (DNA-PK) is a member of the phosphatidylinositol 3-kinase family. Recent genetic and biochemical studies indicate the involvement of DNA-PK in immunoglobulin/T-cell-receptor gene recombination, double-strand DNA break repair, the stress response and autoimmunity. A role in the suppression of apoptosis could link some of the enzyme's diverse functions.

DNA-dependent protein kinase is one of a subset of autoantigens specifically cleaved early during apoptosis

Proteolytic cleavage of key substrates appears to be an important biochemical mechanism underlying the apoptotic process, and the centrality of interleukin 1 beta-converting enzyme (ICE)-like proteases as mediators of apoptosis has been suggested. The identification of the relevant substrates of the ICE protease family during apoptosis therefore constitutes a major challenge. Using human autoantibodies, we demonstrate here that a subset of autoantigens is specifically cleaved early during apoptosis. One of these cleaved molecules is identified as the catalytic subunit of the DNA-dependent protein kinase. The time courses of all proteolytic cleavages are identical and coincide with the onset of morphologic apoptosis. Furthermore, all cleavages share the same inhibition characteristics, which implicate an ICE-like activity(ies). We propose that cleavage of these autoantigens targets these molecules for an autoimmune response by revealing immunocryptic fragments in a proimmune apoptotic setting. Study of the immunogenicity of these fragments may yield insights into the autoimmune targeting of molecules. Moreover, the autoantibodies described will be valuable tools for the elucidation of mechanistically important proteolytic steps along the apoptotic pathway.