DNA replication stress triggers rapid DNA replication fork breakage by Artemis and XPF.

DNA replication stress (DRS) leads to the accumulation of stalled DNA replication forks leaving a fraction of genomic loci incompletely replicated, a source of chromosomal rearrangements during their partition in mitosis. MUS81 is known to limit the occurrence of chromosomal instability by processing these unresolved loci during mitosis. Here, we unveil that the endonucleases ARTEMIS and XPF-ERCC1 can also induce stalled DNA replication forks cleavage through non-epistatic pathways all along S and G2 phases of the cell cycle. We also showed that both nucleases are recruited to chromatin to promote replication fork restart. Finally, we found that rapid chromosomal breakage controlled by ARTEMIS and XPF is important to prevent mitotic segregation defects. Collectively, these results reveal that Rapid Replication Fork Breakage (RRFB) mediated by ARTEMIS and XPF in response to DRS contributes to DNA replication efficiency and limit chromosomal instability.

NEK5 interacts with topoisomerase IIß and is involved in the DNA damage response induced by etoposide.

Cells are daily submitted to high levels of DNA lesions that trigger complex pathways and cellular responses by cell cycle arrest, apoptosis, alterations in transcriptional response, and the onset of DNA repair. Members of the NIMA-related kinase (NEK) family have been related to DNA damage response and repair and the first insight about NEK5 in this context is related to its role in centrosome separation resulting in defects in chromosome integrity. Here we investigate the potential correlation between NEK5 and the DNA damage repair index. The effect of NEK5 in double-strand breaks caused by etoposide was accessed by alkaline comet assay and revealed that NEK5-silenced cells are more sensitive to etoposide treatment. Topoisomerase IIß (TOPIIß) is a target of etoposide that leads to the production of DNA breaks. We demonstrate that NEK5 interacts with TOPIIß, and the dynamics of this interaction is evaluated by proximity ligation assay. The complex NEK5/TOPIIß is formed immediately after etoposide treatment. Taken together, the results of our study reveal that NEK5 depletion increases DNA damage and impairs proper DNA damage response, pointing out NEK5 as a potential kinase contributor to genomic stability.

Resveratrol abrogates the temozolomide-induced G2 arrest leading to mitotic catastrophe and reinforces the temozolomide-induced senescence in glioma cells.

BACKGROUND: Temozolomide (TMZ) is the most widely used drug to treat glioblastoma (GBM), which is the most common and aggressive primary tumor of the Central Nervous System and one of the hardest challenges in oncotherapy. TMZ is an alkylating agent that induces autophagy, apoptosis and senescence in GBM cells. However, therapy with TMZ increases survival after diagnosis only from 12 to 14.4 months, making the development of combined therapies to treat GBM fundamental. One candidate for GBM therapy is Resveratrol (Rsv), which has additive toxicity with TMZ in several glioma cells in vitro and in vivo. However, the mechanism of Rsv and TMZ additive toxicity, which is the aim of the present work, is not clear, especially concerning cell cycle dynamics and long term effects. METHODS: Glioma cell lines were treated with Rsv and TMZ, alone or in combinations, and the induction and the role of autophagy, apoptosis, cell cycle dynamics, protein expression and phosphorylation status were measured. We further evaluated the long term senescence induction and clonogenic capacity. RESULTS: As expected, temozolomide caused a G2 cell cycle arrest and extensive DNA damage response. Rsv did not reduced this response, even increasing pATM, pChk2 and gammaH2Ax levels, but abrogated the temozolomide-induced G2 arrest, increasing levels of cyclin B and pRb(S807/811) and reducing levels of pWee1(S642) and pCdk1(Y15). This suggests a cellular state of forced passage through G2 checkpoint despite large DNA damage, a scenario that may produce mitotic catastrophe. Indeed, the proportion of cells with high nuclear irregularity increased from 6 to 26% in 48 h after cotreatment. At a long term, a reduction in clonogenic capacity was observed, accompanied by a large induction of senescence. CONCLUSION: The presence of Rsv forces cells treated with TMZ through mitosis leading to mitotic catastrophe and senescence, reducing the clonogenic capacity of glioma cells and increasing the chronic effects of temozolomide.

Nek1 silencing slows down DNA repair and blocks DNA damage-induced cell cycle arrest.

Never in mitosis A (NIMA)-related kinases (Nek) are evolutionarily conserved proteins structurally related to the Aspergillus nidulans mitotic regulator NIMA. Nek1 is one of the 11 isoforms of the Neks identified in mammals. Different lines of evidence suggest the participation of Nek1 in response to DNA damage, which is also supported by the interaction of this kinase with proteins involved in DNA repair pathways and cell cycle regulation. In this report, we show that cells with Nek1 knockdown (KD) through stable RNA interference present a delay in DNA repair when treated with methyl-methanesulfonate (MMS), hydrogen peroxide (H(2)O(2)) and cisplatin (CPT). In particular, interstrand cross links induced by CPT take much longer to be resolved in Nek1 KD cells when compared to wild-type (WT) cells. In KD cells, phosphorylation of Chk1 in response to CPT was strongly reduced. While WT cells accumulate in G(2)/M after DNA damage with MMS and H(2)O(2), Nek1 KD cells do not arrest, suggesting that G(2)/M arrest induced by the DNA damage requires Nek1. Surprisingly, CPT-treated Nek1 KD cells arrest with a 4N DNA content similar to WT cells. This deregulation in cell cycle control in Nek1 KD cells leads to an increased sensitivity to genotoxic agents when compared to WT cells. These results suggest that Nek1 is involved in the beginning of the cellular response to genotoxic stress and plays an important role in preventing cell death induced by DNA damage.

Boldine: a potential new antiproliferative drug against glioma cell lines.

Malignant gliomas are the most common and devastating primary tumors of the central nervous system. Currently no efficient treatment is available. This study evaluated the effect and underlying mechanisms of boldine, an aporphine alkaloid of Peumus boldus, on glioma proliferation and cell death. Boldine decreased the cell number of U138-MG, U87-MG and C6 glioma lines at concentrations of 80, 250 and 500 muM. We observed that cell death caused by boldine was cell-type specific and dose-dependent. Exposure to boldine for 24 h did not activate key mediators of apoptosis. However, it induced alterations in the cell cycle suggesting a G(2)/M arrest in U138-MG cells. Boldine had no toxic effect on non-tumor cells when used at the same concentrations as those used on tumor cells. Based on these results, we speculate that boldine may be a promising compound for evaluation as an anti-cancer agent.

The balance between NRF2/GSH antioxidant mediated pathway and DNA repair modulates cisplatin resistance in lung cancer cells.

Lung cancer patients face a dismal prognosis mainly due to the low efficacy of current available treatments. Cisplatin is the first-line chemotherapy treatment for those patients, however, resistance to this drug is a common and yet not fully understood phenomenon. Aiming to shed new light into this puzzle, we used established normal and malignant lung cell lines displaying different sensitivity towards cisplatin treatment. We observed a negative correlation between cell viability and DNA damage induction upon cisplatin treatment. Interestingly, drug sensitivity in those cell lines was not due to either difference on DNA repair capacity, or in the amount of membrane ion channel commonly used for cisplatin uptake. Also, we noted that glutathione intracellular levels, and expression and activity of the transcription factor nuclear factor erythroid 2-related factor 2 (NRF2) were determinant for cisplatin cytotoxicity. Remarkably, analysis of gene expression in non-small cell lung cancer patients of the TCGA data bank revealed that there is a significant lower overall survival rate in the subset of patients bearing tumors with unbalanced levels of NRF2/KEAP1 and, as consequence, increased expression of NRF2 target genes. Thus, the results indicate that NRF2 and glutathione levels figure as important cisplatin resistance biomarkers in lung cancer.

The key role of UVA-light induced oxidative stress in human Xeroderma Pigmentosum Variant cells.

The UVA component of sunlight induces DNA damage, which are basically responsible for skin cancer formation. Xeroderma Pigmentosum Variant (XP-V) patients are defective in the DNA polymerase pol eta that promotes translesion synthesis after sunlight-induced DNA damage, implying in a clinical phenotype of increased frequency of skin cancer. However, the role of UVA-light in the carcinogenesis of these patients is not completely understood. The goal of this work was to characterize UVA-induced DNA damage and the consequences to XP-V cells, compared to complemented cells. DNA damage were induced in both cells by UVA, but lesion removal was particularly affected in XP-V cells, possibly due to the oxidation of DNA repair proteins, as indicated by the increase of carbonylated proteins. Moreover, UVA irradiation promoted replication fork stalling and cell cycle arrest in the S-phase for XP-V cells. Interestingly, when cells were treated with the antioxidant N-acetylcysteine, all these deleterious effects were consistently reverted, revealing the role of oxidative stress in these processes. Together, these results strongly indicate the crucial role of oxidative stress in UVA-induced cytotoxicity and are of interest for the protection of XP-V patients.

NEK1 kinase domain structure and its dynamic protein interactome after exposure to Cisplatin.

NEK family kinases are serine/threonine kinases that have been functionally implicated in the regulation of the disjunction of the centrosome, the assembly of the mitotic spindle, the function of the primary cilium and the DNA damage response. NEK1 shows pleiotropic functions and has been found to be mutated in cancer cells, ciliopathies such as the polycystic kidney disease, as well as in the genetic diseases short-rib thoracic dysplasia, Mohr-syndrome and amyotrophic lateral sclerosis. NEK1 is essential for the ionizing radiation DNA damage response and priming of the ATR kinase and of Rad54 through phosphorylation. Here we report on the structure of the kinase domain of human NEK1 in its apo- and ATP-mimetic inhibitor bound forms. The inhibitor bound structure may allow the design of NEK specific chemo-sensitizing agents to act in conjunction with chemo- or radiation therapy of cancer cells. Furthermore, we characterized the dynamic protein interactome of NEK1 after DNA damage challenge with cisplatin. Our data suggest that NEK1 and its interaction partners trigger the DNA damage pathways responsible for correcting DNA crosslinks.