UbcH5c-dependent activation of DNA-dependent protein kinase in response to replication-mediated DNA double-strand breaks

Camptothecin (CPT) exhibits strong cytotoxicity by inducing DNA double-strand breaks (DSBs) through DNA replication. Unlike radiation-induced DSBs, which have two DNA ends, CPT-induced DSBs are considered to have only one DNA end. However, the differences in cellular responses to one-ended and two-ended DSBs are not well understood. Our previous study showed that proteasome inhibitor treatment suppressed CPT-induced activation of DNA-PK, a factor required for non-homologous end-joining in DSB repair, suggesting that the ubiquitin-proteasome pathway is involved in DNA-PK activation in response to one-ended DSBs. To identify the ubiquitination factors required for DNA-PK activation, we screened an siRNA library against E2 ubiquitin-conjugating enzymes and identified UbcH5c. Knockdown of UbcH5c suppressed DNA-PK activation caused by CPT, but not by the radio-mimetic drug neocarzinostatin. UbcH5c-dependent DNA-PK activation occurred independent of DNA end resection. Furthermore, loss of UbcH5c reduced DNA-PK-dependent chromosomal aberrations and suppressed the activation of cell cycle checkpoint in response to CPT. These results suggest that UbcH5c regulates DNA-PK activation in response to one-ended DSBs caused by replication fork collapse. To our knowledge, this is the first report of a DSB repair-related factor that is differentially involved in the response to one- and two-ended DSBs.

Camptothecin compromises transcription recovery and cell survival against cisplatin and ultraviolet irradiation regardless of transcription-coupled nucleotide excision repair

DNA-damaging anti-cancer drugs are used clinically to induce cell death by causing DNA strand breaks or DNA replication stress. Camptothecin (CPT) and cisplatin are commonly used anti-cancer drugs, and their combined use enhances the anti-tumour effects. However, the mechanism underlying this enhanced effect has not been well studied. In this study, we analysed the combined effect of CPT and cisplatin or ultraviolet (UV) and found that CPT suppresses transcription recovery after UV damage and induces the disappearance of the Cockayne syndrome group B (CSB) protein, a transcription-coupled nucleotide excision repair (TC-NER) factor. This CPT-induced disappearance of CSB expression was suppressed by proteasome and transcription inhibitors. Moreover, CSB ubiquitination was detected after CPT treatment in a transcription-dependent manner, suggesting that the transcription stress caused by CPT induces CSB ubiquitination, resulting in CSB undetectability. However, Cockayne syndrome group A (CSA) and CUL4A were not involved in the CPT-induced CSB undetectability, suggesting that CSB ubiquitination caused by CPT is regulated differently from the UV response. However, cisplatin or UV sensitivity was enhanced by CPT even in CSB- or CSA-knockout cells. Furthermore, the excessive CSB expression, which suppressed CSB ubiquitination, did not cancel the combined effect of CPT. These results suggest that CPT blocks the repair of cisplatin or UV-induced DNA damage regardless of TC-NER status. CPT possibly compromised the alternative repair pathways other than TC-NER, leading to the suppression of transcription recovery and enhancement of cell killing.

DNA damage in human glomerular endothelial cells induces nodular glomerulosclerosis via an ATR and ANXA2 pathway

Collagen type VI (COL6) deposition occurs in various glomerular diseases, causing serious pathological damage like nodular lesions. However, the mechanisms underlying the deposition of COL6 remain unclear. In renal biopsy samples, immunohistochemical analyses revealed that COL6 and phosphorylated histone H2AX (γ-H2AX), a DNA damage marker, were detected mainly in diabetic nodular glomerulosclerosis, in which the γ-H2AX-positive area was identified as the independent factor significantly associated with the COL6-positive area (β: 0.539, t = 2.668). In in vitro studies, COL6 secretion from human renal glomerular endothelial cells (HRGECs) was assessed by measuring the decrease in the cytoplasmic COL6-positive cells and an increase in the amount of COL6 in the culture medium. Mitomycin C (MMc) treatment of HRGECs increased the number of γ-H2AX-positive cells and COL6 secretion, which were suppressed by a specific inhibitor of ataxia telangiectasia and Rad3-related (ATR). MMc-induced COL6 secretion was also suppressed by Annexin A2 (ANXA2) siRNA transfection. Moreover, the inhibition of ATR activity did not induce any extra suppression in the MMc-induced COL6 secretion by ANXA2 siRNA transfected cells. These results confirm that nodular glomerulosclerosis partially results from DNA damage in the glomerulus and that DNA damage-induced COL6 secretion from HRGECs occurs through an ATR and ANXA2-mediated pathway.

P0122THE MECHANISM OF DNA INJURY INDUCED COLLAGEN TYPE VI EXCRETION IN GLOMERULAR ENDOTHERIAL CELLS

Background and Aims

Collagen deposition is observed in various part of kidney tissue and is serious pathological damage in kidney. Nodular glomerulosclerosis is one of a phenotype of collagen deposition in glomerular lesion, which affects the prognosis of renal function, but there is no effective treatment at present. Recently, we identified that glomerular endothelial cells with DNA double-strand breaks (DSB) are involved in the long-term accumulation of ECM molecules, especially collagen type VI(COL6). However, the mechanism of glomerular COL6 accumulation is still unclear.

Method

We examined the phospho-histone H2AX (γ-H2AX) of the DSB marker and COL6 accumulation in biopsied human kidneys to investigate their association. In addition, we investigated in vitro the relationship between DSB and COL6 excretion and the intracellular signal pathways in human glomerular endothelial cells (HRGEc) using mitomycin C (MMc)-induced DNA damage. In this study, we focused on the expression of DSB response signal pathways, i.e. ATM, ATR and DNA-PK using their specific kinase inhibitors (KU55933, VE-821, Nu7441).

Results

COL6 deposits and γ-H2AX expression in nuclei were detected in human glomerular capillary loop and glomerular nodular lesions. The multiple regression analysis showed that γ-H2AX positive area is the most independent factor for COL6 accumulation in glomeruli (β: 0.539, t=2.668, p=0.014). Furthermore, COL6 was the independent factor of nodular lesion in various pathologic diseases such as diabetic nephropathy, hypertensive glomerulosclerosis and the long-term transplanted allografts (β: 0.373, t=2.268, p=0.032).

In vitro study, COL6 excretion detected by the decrease of COL6 positive cells was suppressed only in the ATR-inhibited group (control vs. kinase inhibitors; 2 h, 40.1 ± 23.5 vs. 59.0 ± 23.2%, p <0.01; 24 h, 18.8 ± 23.6 vs. 56.3 ± 16.5%, p <0.001).

Conclusion

This study suggested that nodular glomerulosclerosis is induced by DNA damage and COL6 deposition through ATR pathway in glomerular capillary endothelial cells. These findings will provide great insights into the development of new therapies that would suppress glomerularsclerosis in various kidney diseases.

Preliminary quantitative evaluation of radiation-induced DNA damage in peripheral blood lymphocytes after cardiac dual-isotope imaging

DNA double-strand breaks (DSBs) of peripheral blood lymphocyte were prospectively assessed in 9 patients who were injected with ²⁰¹Tl-chloride and ¹²³I-beta-methyl-p-iodophenyl-pentadecanoic acid in dual-isotope imaging. Phosphorylated H2AX (γH2AX) was used as a biomarker for detecting DSBs, and the mean number of γH2AX foci per cell was measured microscopically. Mean γH2AX foci before administration of radiopharmaceuticals and at 3, 6, and 24 h following administration were 0.22 ± 0.34, 0.10 ± 0.14, 0.59 ± 0.46, and 0.52 ± 0.40, respectively (p = n.s. for all combinations).

Caspase-mediated cleavage of X-ray repair cross-complementing group 4 promotes apoptosis by enhancing nuclear translocation of caspase-activated DNase

X-ray repair cross-complementing group 4 (XRCC4), a repair protein for DNA double-strand breaks, is cleaved by caspases during apoptosis. In this study, we examined the role of XRCC4 in apoptosis. Cell lines, derived from XRCC4-deficient M10 mouse lymphoma cells and stably expressing wild-type XRCC4 or caspase-resistant XRCC4, were established and treated with staurosporine (STS) to induce apoptosis. In STS-induced apoptosis, expression of wild-type, but not caspase-resistant, XRCC4 in XRCC4-deficient cells enhanced oligonucleosomal DNA fragmentation and the appearance of TUNEL-positive cells by promoting nuclear translocation of caspase-activated DNase (CAD), a major nuclease for oligonucleosomal DNA fragmentation. CAD activity is reportedly regulated by the ratio of two inhibitor of CAD (ICAD) splice variants, ICAD-L and ICAD-S mRNA, which, respectively, produce proteins with and without the ability to transport CAD into the nucleus. The XRCC4-dependent promotion of nuclear import of CAD in STS-treated cells was associated with reduction of ICAD-S mRNA and protein, and enhancement of phosphorylation and nuclear import of serine/arginine-rich splicing factor (SRSF) 1. These XRCC4-dependent, apoptosis-enhancing effects were canceled by depletion of SRSF1 or SR protein kinase (SRPK) 1. In addition, overexpression of SRSF1 in XRCC4-deficient cells restored the normal level of apoptosis, suggesting that SRSF1 functions downstream of XRCC4 in activating CAD. This XRCC4-dependent, SRPK1/SRSF1-mediated regulatory mechanism was conserved in apoptosis in Jurkat human leukemia cells triggered by STS, and by two widely used anti-cancer agents, Paclitaxel and Vincristine. These data imply that the level of XRCC4 expression could be used to predict the effects of apoptosis-inducing drugs in cancer treatment.

Aquarius is required for proper CtIP expression and homologous recombination repair

Accumulating evidence indicates that transcription is closely related to DNA damage formation and that the loss of RNA biogenesis factors causes genome instability. However, whether such factors are involved in DNA damage responses remains unclear. We focus here on the RNA helicase Aquarius (AQR), a known R-loop processing factor, and show that its depletion in human cells results in the accumulation of DNA damage during S phase, mediated by R-loop formation. We investigated the involvement of Aquarius in DNA damage responses and found that AQR knockdown decreased DNA damage-induced foci formation of Rad51 and replication protein A, suggesting that Aquarius contributes to homologous recombination (HR)-mediated repair of DNA double-strand breaks (DSBs). Interestingly, the protein level of CtIP, a DSB processing factor, was decreased in AQR-knockdown cells. Exogenous expression of Aquarius partially restored CtIP protein level; however, CtIP overproduction did not rescue defective HR in AQR-knockdown cells. In accordance with these data, Aquarius depletion sensitized cells to genotoxic agents. We propose that Aquarius contributes to the maintenance of genomic stability via regulation of HR by CtIP-dependent and -independent pathways.

DNA double-strand breaks induced intractable glomerular fibrosis in renal allografts

BACKGROUNDS

The relationship between DNA damage and glomerular fibrosis in renal allografts remains unclear.

METHODS

We examined renal allograft specimens from 35 patients in which DNA double-strand breaks (DSBs) and glomerular fibrosis were detected by phospho-histone H2A.X (γ-H2AX) expression and collagen (COL) types III, IV, and VI accumulation. We also examined the in vitro relationship between DNA damage and COL accumulation by mitomycin C (MMc)-induced DNA damage in human glomerular endothelial cells (HRGEc).

RESULTS

The γ-H2AX and COL type VI, which mainly accumulated in the subendothelial and mesangial regions, were positively correlated with the duration of the post-renal transplant (RT) period. In multiple regression analysis, the duration of the post-RT period and cg in the Banff '07 classification were identified as a significant predictor of COL type VI accumulation and γ-H2AX expression in the glomerular capillaries. In addition, the γ-H2AX-positive area was also identified as a predictor of glomerular accumulation of COL type VI. COL type VI was detected in the cytoplasm of the HRGEc, which was secreted into the supernatant after MMc stimulation with γ-H2AX expression. The number of γ-H2AX (-)/COL type VI (+) cells was inversely associated with the number of γ-H2AX (+)/COL type VI (-) cells during 24-h MMc treatment.

CONCLUSIONS

Our findings suggest that the long-term RT induces DSBs and HRGEc-secreted COL type VI accumulation in the glomerular capillaries, which might progress to intractable glomerular fibrosis.