Aurora-B mediated ATM serine 1403 phosphorylation is required for mitotic ATM activation and the spindle checkpoint

Pharmaceutical inhibition of the Chk2 kinase mitigates cone photoreceptor degeneration in an iPSC model of Bardet-Biedl syndrome

Bardet-Biedl syndrome (BBS) is a syndromic ciliopathy leading to progressive blindness starting in childhood, but the mechanism of photoreceptor degeneration remains unclear. The basal body of the photoreceptor primary cilium originates from the centrosome's mother centriole, and BBS-related proteins form a complex at basal body. Centrosomes also organize microtubules of the mitotic spindle. We show here that photoreceptors from Bbs10 -/- mouse pups present a DNA damage response (DDR) that becomes persistent and localizes to the basal body. In patient-derived induced pluripotent stem cells (iPSCs) carrying BBS10 mutations, BBS retinal progenitor cells (RPCs) present a DDR that correlates with activation of the mitotic spindle checkpoint. Pharmaceutical inhibition of the Chk2 kinase in BBS RPCs mitigates cell death and genomic instability and restores the phospho-proteome. Drug treatment of BBS retinal organoids improves tissue organization, cone survival, and outer segment maturation, thus opening a possible therapeutic avenue to delay photoreceptor degeneration in BBS.

Small Molecular Inhibitors That Target ATM for Drug Discovery: Current Research and Potential Prospective

The protein kinase ataxia telangiectasia mutated (ATM) is a constituent of the phosphatidylinositol 3-kinase-related kinase (PIKK) family, exerting a pivotal influence on diverse cellular processes, notably the signaling of double-strand DNA breaks (DSB) and stress response. The dysregulation of ATM is implicated in the pathogenesis of cancer and other diseases such as neurodegeneration. Hence, ATM is deemed a promising candidate for potential therapeutic interventions across a spectrum of diseases. Presently, while ATM small molecule inhibitors are not commercially available, various selective inhibitors have progressed to the clinical research phase. Specifically, AZD1390, WSD0628, SYH2051, and ZN-B-2262 are under investigation in clinical studies pertaining to glioblastoma multiforme and advanced solid tumors, respectively. In this Perspective, we encapsulate the structure, biological functions, and disease relevance of ATM. Subsequently, we concentrate on the design concepts and structure-activity relationships (SAR) of ATM inhibitors, delineating potential avenues for the development of more efficacious ATM-targeted inhibitors.

RNA demethylase FTO participates in malignant progression of gastric cancer by regulating SP1-AURKB-ATM pathway

Gastric cancer (GC) is the 5th most prevalent cancer and the 4th primary cancer-associated mortality globally. As the first identified m6A demethylase for removing RNA methylation modification, fat mass and obesity-associated protein (FTO) plays instrumental roles in cancer development. Therefore, we study the biological functions and oncogenic mechanisms of FTO in GC tumorigenesis and progression. In our study, FTO expression is obviously upregulated in GC tissues and cells. The upregulation of FTO is associated with advanced nerve invasion, tumor size, and LNM, as well as the poor prognosis in GC patients, and promoted GC cell viability, colony formation, migration and invasion. Mechanistically, FTO targeted specificity protein 1 and Aurora Kinase B, resulting in the phosphorylation of ataxia telangiectasia mutated and P38 and dephosphorylation of P53. In conclusion, the m6A demethylase FTO promotes GC tumorigenesis and progression by regulating the SP1-AURKB-ATM pathway, which may highlight the potential of FTO as a diagnostic biomarker for GC patients' therapy response and prognosis.

Overexpression of Aurora Kinase B Is Correlated with Diagnosis and Poor Prognosis in Hepatocellular Carcinoma

Aurora kinase B (AURKB) overexpression promotes tumor initiation and development by participating in the cell cycle. In this study, we focused on the mechanism of AURKB in hepatocellular carcinoma (HCC) progression and on AURKB's value as a diagnostic and prognostic biomarker in HCC. We used data from The Cancer Genome Atlas (TCGA) and the Gene Expression Omnibus (GEO) to analyze AURKB expression in HCC. We found that the expression levels of AURKB in HCC samples were higher than those in the corresponding control group. R packages were used to analyze RNA sequencing data to identify AURKB-related differentially expressed genes (DEGs), and these genes were found to be significantly enriched during the cell cycle. The biological function of AURKB was verified, and the results showed that cell proliferation was slowed down and cells were arrested in the G2/M phase when AURKB was knocked down. AURKB overexpression resulted in significant differences in clinical symptoms, such as the clinical T stage and pathological stage. Kaplan-Meier survival analysis, Cox regression analysis, and Receiver Operating Characteristic (ROC) curve analysis suggested that AURKB overexpression has good diagnostic and prognostic potential in HCC. Therefore, AURKB may be used as a potential target for the diagnosis and cure of HCC.

Phosphorylation of Bub1 by Mph1 promotes Bub1 signaling at the kinetochore to ensure accurate chromosome segregation

Bub1 is a conserved mitotic kinase involved in signaling of the spindle assembly checkpoint. Multiple phosphorylation sites on Bub1 have been characterized, yet it is challenging to understand the interplay between the multiple phosphorylation sites due to the limited availability of phosphospecific antibodies. In addition, phosphoregulation of Bub1 in Schizosaccharomyces pombe is poorly understood. Here we report the identification of a new Mph1/Mps1-mediated phosphorylation site, i.e., Ser532, of Bub1 in Schizosaccharomyces pombe. A phosphospecific antibody against phosphorylated Bub1-Ser532 was developed. Using the phosphospecific antibody, we demonstrated that phosphorylation of Bub1-Ser352 was mediated specifically by Mph1/Mps1 and took place during early mitosis. Moreover, live-cell microscopy showed that inhibition of the phosphorylation of Bub1 at Ser532 impaired the localization of Bub1, Mad1, and Mad2 to the kinetochore. In addition, inhibition of the phosphorylation of Bub1 at Ser532 caused anaphase B lagging chromosomes. Hence, our study constitutes a model in which Mph1/Mps1-mediated phosphorylation of fission yeast Bub1 promotes proper kinetochore localization of Bub1 and faithful chromosome segregation.

SF3B1 mutation and ATM deletion codrive leukemogenesis via centromeric R-loop dysregulation

RNA splicing factor SF3B1 is recurrently mutated in various cancers, particularly in hematologic malignancies. We previously reported that coexpression of Sf3b1 mutation and Atm deletion in B cells, but not either lesion alone, leads to the onset of chronic lymphocytic leukemia (CLL) with CLL cells harboring chromosome amplification. However, the exact role of Sf3b1 mutation and Atm deletion in chromosomal instability (CIN) remains unclear. Here, we demonstrated that SF3B1 mutation promotes centromeric R-loop (cen-R-loop) accumulation, leading to increased chromosome oscillation, impaired chromosome segregation, altered spindle architecture, and aneuploidy, which could be alleviated by removal of cen-R-loop and exaggerated by deletion of ATM. Aberrant splicing of key genes involved in R-loop processing underlay augmentation of cen-R-loop, as overexpression of the normal isoform, but not the altered form, mitigated mitotic stress in SF3B1-mutant cells. Our study identifies a critical role of splice variants in linking RNA splicing dysregulation and CIN and highlights cen-R-loop augmentation as a key mechanism for leukemogenesis.

Maintaining Genome Integrity: Protein Kinases and Phosphatases Orchestrate the Balancing Act of DNA Double-Strand Breaks Repair in Cancer

DNA double-strand breaks (DSBs) are the most lethal DNA damages which lead to severe genome instability. Phosphorylation is one of the most important protein post-translation modifications involved in DSBs repair regulation. Kinases and phosphatases play coordinating roles in DSB repair by phosphorylating and dephosphorylating various proteins. Recent research has shed light on the importance of maintaining a balance between kinase and phosphatase activities in DSB repair. The interplay between kinases and phosphatases plays an important role in regulating DNA-repair processes, and alterations in their activity can lead to genomic instability and disease. Therefore, study on the function of kinases and phosphatases in DSBs repair is essential for understanding their roles in cancer development and therapeutics. In this review, we summarize the current knowledge of kinases and phosphatases in DSBs repair regulation and highlight the advancements in the development of cancer therapies targeting kinases or phosphatases in DSBs repair pathways. In conclusion, understanding the balance of kinase and phosphatase activities in DSBs repair provides opportunities for the development of novel cancer therapeutics.

Microarray and in silico analysis of DNA repair genes between human testis of patients with nonobstructive azoospermia and normal cells

DNA repair processes are critical to maintaining genomic integrity. As a result, dysregulation of repair genes is likely to be linked with health implications, such as an increased prevalence of infertility and an accelerated rate of aging. We evaluated all the DNA repair genes (322 genes) by microarray. This study has provided insight into the connection between DNA repair genes, including RAD23B, OBFC2A, PMS1, UBE2V1, ERCC5, SMUG1, RFC4, PMS2L5, MMS19, SHFM1, INO80, PMS2L1, CHEK2, TRIP13, and POLD4. The microarray analysis of six human cases with different nonobstructive azoospermia revealed that RAD23B, OBFC2A, PMS1, UBE2V1, ERCC5, SMUG1, RFC4, PMS2L5, MMS19, SHFM1, and INO80 were upregulated, and expression of PMS2L1, CHEK2, TRIP13, and POLD4 was downregulated versus the normal case. For this purpose, Enrich Shiny GO, STRING, and Cytoscape online evaluation was applied to predict proteins' functional and molecular interactions and then performed to recognize the master pathways. Functional enrichment analysis revealed that the biological process (BP) terms "base-excision repair, AP site formation," "nucleotide-excision repair, DNA gap filling," and "nucleotide-excision repair, preincision complex assembly" was significantly overexpressed in upregulated differentially expressed genes (DEGs). BP analysis of downregulated DEGs highlighted "histone phosphorylation," "DNA damage response, detection DNA response," "mitotic cell cycle checkpoint signaling," and "double-strand break repair." Overrepresented molecular function (MF) terms in upregulated DEGs included "Oxidized base lesion DNA N-glycosylase activity," "uracil DNA N-glycosylase activity," "bubble DNA binding" and "DNA clamp loader activity." Interestingly, MF investigation of downregulated DEGs showed overexpression in "heterotrimeric G-protein complex," "5'-deoxyribose-5-phosphate lyase activity," "minor groove of adenine-thymine-rich DNA binding," and "histone kinase activity." Our findings suggest that these genes and their interacting hub proteins could help determine the pathophysiology of germ cell abnormalities and infertility.

A function for ataxia telangiectasia and Rad3-related (ATR) kinase in cytokinetic abscission

Abscission, the final stage of cytokinesis, occurs when the cytoplasmic canal connecting two emerging daughter cells is severed either side of a large proteinaceous structure, the midbody. Here, we expand the functions of ATR to include a cell-cycle-specific role in abscission, which is required for genome stability. All previously characterized roles for ATR depend upon its recruitment to replication protein A (RPA)-coated single-stranded DNA (ssDNA). However, we establish that in each cell cycle ATR, as well as ATRIP, localize to the midbody specifically during late cytokinesis and independently of RPA or detectable ssDNA. Rather, midbody localization and ATR-dependent regulation of abscission requires the known abscission regulator-charged multivesicular body protein 4C (CHMP4C). Intriguingly, this regulation is also dependent upon the CDC7 kinase and the known ATR activator ETAA1. We propose that in addition to its known RPA-ssDNA-dependent functions, ATR has further functions in preventing premature abscission.

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ATR localises non-canonically to the midbody during late cytokinesis

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Absence of ATR function results in faster abscission and increased binucleates

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CDC7 kinase and the ESCRT protein, CHMP4C are required for ATR midbody localisation

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ATR functions upstream of known abscission regulators, CHMP4B and ANCHR

NEDD9 Restrains dsDNA Damage Response during Non-Small Cell Lung Cancer (NSCLC) Progression

DNA damaging modalities are the backbone of treatments for non-small cell lung cancer (NSCLC). Alterations in DNA damage response (DDR) in tumor cells commonly contribute to emerging resistance to platinating agents, other targeted therapies, and radiation. The goal of this study is to identify the previously unreported role of NEDD9 scaffolding protein in controlling DDR processes and sensitivity to DNA damaging therapies. Using a siRNA-mediated approach to deplete NEDD9 in a group of human and murine KRAS/TP53-mutant NSCLC cell lines, coupled with a set of cell viability and clonogenic assays, flow cytometry analysis, and Western blotting, we evaluated the effects of NEDD9 silencing on cellular proliferation, DDR and epithelial-to-mesenchymal transition (EMT) signaling, cell cycle, and sensitivity to cisplatin and UV irradiation. Using publicly available NSCLC datasets (TCGA) and an independent cohort of primary NSCLC tumors, subsequent in silico and immunohistochemical (IHC) analyses were performed to assess relevant changes in NEDD9 RNA and protein expression across different stages of NSCLC. The results of our study demonstrate that NEDD9 depletion is associated with the increased tumorigenic capacity of NSCLC cells. These phenotypes were accompanied by significantly upregulated ATM-CHK2 signaling, shifting towards a more mesenchymal phenotype in NEDD9 depleted cells and elevated sensitivity to UV-irradiation. IHC analyses revealed an association between reduced NEDD9 protein expression and a decrease in overall (OS) and progression-free survival (PFS) of the NSCLC patients. These data, for the first time, identified NEDD9 as a negative regulator of ATM kinase activity and related DDR signaling in numerous KRAS/TP53 mutated NSCLC, with its effects on the regulation of DDR-dependent EMT signaling, sensitivity to DNA damaging modalities in tumor cells, and the survival of the patients.

Phosphorylation of MAD2 at Ser195 Promotes Spindle Checkpoint Defects and Sensitizes Cancer Cells to Radiotherapy in ATM Deficient Cells

The spindle assembly checkpoint (SAC) is a critical monitoring device in mitosis for the maintenance of genomic stability. Specifically, the SAC complex comprises several factors, including Mad1, Mad2, and Bub1. Ataxia-telangiectasia mutated (ATM) kinase, the crucial regulator in DNA damage response (DDR), also plays a critical role in mitosis by regulating Mad1 dimerization and SAC. Here, we further demonstrated that ATM negatively regulates the phosphorylation of Mad2, another critical component of the SAC, which is also involved in DDR. Mechanistically, we found that phosphorylation of Mad2 is aberrantly increased in ATM-deficient cells. Point-mutation analysis further revealed that Serine 195 mainly mediated Mad2 phosphorylation upon ATM ablation. Functionally, the phosphorylation of Mad2 causes decreased DNA damage repair capacity and is related to the resistance to cancer cell radiotherapy. Altogether, this study unveils the key regulatory role of Mad2 phosphorylation in checkpoint defects and DNA damage repair in ATM-deficient cells.

Adult Human Vascular Smooth Muscle Cells on 3D Silk Fibroin Nonwovens Release Exosomes Enriched in Angiogenic and Growth-Promoting Factors

BACKGROUND

Our earlier works showed the quick vascularization of mouse skin grafted Bombyx mori 3D silk fibroin nonwoven scaffolds (3D-SFnws) and the release of exosomes enriched in angiogenic/growth factors (AGFs) from in vitro 3D-SFnws-stuck human dermal fibroblasts (HDFs). Here, we explored whether coronary artery adult human smooth muscle cells (AHSMCs) also release AGFs-enriched exosomes when cultured on 3D-SFnws in vitro.

METHODS

Media with exosome-depleted FBS served for AHSMCs and human endothelial cells (HECs) cultures on 3D-SFnws or polystyrene. Biochemical methods and double-antibody arrays assessed cell growth, metabolism, and intracellular TGF-β and NF-κB signalling pathways activation. AGFs conveyed by CD9+/CD81+ exosomes released from AHSMCs were double-antibody array analysed and their angiogenic power evaluated on HECs in vitro.

RESULTS

AHSMCs grew and consumed D-glucose more intensely and showed a stronger phosphorylation/activation of TAK-1, SMAD-1/-2/-4/-5, ATF-2, c-JUN, ATM, CREB, and an IκBα phosphorylation/inactivation on SFnws vs. polystyrene, consistent overall with a proliferative/secretory phenotype. SFnws-stuck AHSMCs also released exosomes richer in IL-1α/-2/-4/-6/-8; bFGF; GM-CSF; and GRO-α/-β/-γ, which strongly stimulated HECs' growth, migration, and tubes/nodes assembly in vitro.

CONCLUSIONS

Altogether, the intensified AGFs exosomal release from 3D-SFnws-attached AHSMCs and HDFs could advance grafts' colonization, vascularization, and take in vivo-noteworthy assets for prospective clinical applications.

The phosphatase CTDSPL2 is phosphorylated in mitosis and a target for restraining tumor growth and motility in pancreatic cancer

Carboxy-terminal domain (CTD) small phosphatase like 2 (CTDSPL2), also known as SCP4 or HSPC129, is a new member of the small CTD phosphatase (SCP) family and its role in cancers remains unclear. Here, we used a Phos-tag technique to screen a series of phosphatases and identified CTDSPL2 as a mitotic regulator. We demonstrated that CTDSPL2 was phosphorylated at T86, S104, and S134 by cyclin-dependent kinase 1 (CDK1) in mitosis. Depletion of CTDSPL2 led to mitotic defects and prolonged mitosis. Resultantly, CTDSPL2 deletion restrained proliferation, migration, and invasion in pancreatic cancer cells. We further confirmed the dominant negative effects of a phosphorylation-deficient mutant form of CTDSPL2, implying the biological significance of CTDSPL2 mitotic phosphorylation. Moreover, RT² cell cycle array analysis revealed p21 and p27 as downstream regulators of CTDSPL2, and inhibition of p21 and/or p27 partially rescued the phenotype in CTDSPL2-deficient cell lines. Importantly, both CTDSPL2 depletion and phosphorylation-deficient mutant CTDSPL2 hindered tumor growth in xenograft models. Together, our findings for the first time highlight the novel role of CTDSPL2 in regulating cell mitosis, proliferation and motility in pancreatic cancer and point out the implications of CTDSPL2 in regulating two critical cell cycle participants (p21 and p27), providing an alternative molecular target for pancreatic cancer treatment.

Dual-functional significance of ATM-mediated phosphorylation of spindle assembly checkpoint component Bub3 in mitosis and the DNA damage response

Both the DNA damage response (DDR) and the mitotic checkpoint are critical for the maintenance of genomic stability. Among proteins involved in these processes, the Ataxia-Telangiectasia Mutated (ATM) kinase is required for both activation of the DDR and the spindle assembly checkpoint (SAC). In mitosis without DNA damage, the enzymatic activity of ATM is enhanced; however, substrates of ATM in mitosis are unknown. Using Stable Isotope Labeled Amino Acid in cell culture (SILAC)-mass spectrometry analysis, we identified a number of proteins that can potentially be phosphorylated by ATM during mitosis. This list is highly enriched in proteins involved in cell cycle regulation and the DDR. Among them, we further validated that ATM phosphorylated Budding Uninhibited by Benzimidazoles 3 (Bub3), a major component of the SAC, on serine 135 both in vitro and in vivo. During mitosis, this phosphorylation promoted activation of another SAC component, Bub1. Mutation of Bub3 serine 135 to alanine led to a defect in SAC activation. Furthermore, we found that ATM-mediated phosphorylation of Bub3 on serine 135 was also induced by ionizing radiation-induced DNA damage. However, this event resulted in independent signaling involving interaction with the Ku70-Ku80-DNA-PKcs sensor/kinase complex, leading to efficient non-homologous end joining repair. Taken together, we highlight the functional significance of the crosstalk between the kinetochore-oriented signal and double strand break repair pathways via ATM phosphorylation of Bub3 on serine 135.

Mitotic and DNA Damage Response Proteins: Maintaining the Genome Stability and Working for the Common Good

Cellular function is highly dependent on genomic stability, which is mainly ensured by two cellular mechanisms: the DNA damage response (DDR) and the Spindle Assembly Checkpoint (SAC). The former provides the repair of damaged DNA, and the latter ensures correct chromosome segregation. This review focuses on recently emerging data indicating that the SAC and the DDR proteins function together throughout the cell cycle, suggesting crosstalk between both checkpoints to maintain genome stability.

The Abscission Checkpoint: A Guardian of Chromosomal Stability

The abscission checkpoint contributes to the fidelity of chromosome segregation by delaying completion of cytokinesis (abscission) when there is chromatin lagging in the intercellular bridge between dividing cells. Although additional triggers of an abscission checkpoint-delay have been described, including nuclear pore defects, replication stress or high intercellular bridge tension, this review will focus only on chromatin bridges. In the presence of such abnormal chromosomal tethers in mammalian cells, the abscission checkpoint requires proper localization and optimal kinase activity of the Chromosomal Passenger Complex (CPC)-catalytic subunit Aurora B at the midbody and culminates in the inhibition of Endosomal Sorting Complex Required for Transport-III (ESCRT-III) components at the abscission site to delay the final cut. Furthermore, cells with an active checkpoint stabilize the narrow cytoplasmic canal that connects the two daughter cells until the chromatin bridges are resolved. Unsuccessful resolution of chromatin bridges in checkpoint-deficient cells or in cells with unstable intercellular canals can lead to chromatin bridge breakage or tetraploidization by regression of the cleavage furrow. In turn, these outcomes can lead to accumulation of DNA damage, chromothripsis, generation of hypermutation clusters and chromosomal instability, which are associated with cancer formation or progression. Recently, many important questions regarding the mechanisms of the abscission checkpoint have been investigated, such as how the presence of chromatin bridges is signaled to the CPC, how Aurora B localization and kinase activity is regulated in late midbodies, the signaling pathways by which Aurora B implements the abscission delay, and how the actin cytoskeleton is remodeled to stabilize intercellular canals with DNA bridges. Here, we review recent progress toward understanding the mechanisms of the abscission checkpoint and its role in guarding genome integrity at the chromosome level, and consider its potential implications for cancer therapy.

An ATM-Chk2-INCENP pathway activates the abscission checkpoint

During cell division, in response to chromatin bridges, the chromosomal passenger complex (CPC) delays abscission to prevent chromosome breakage or tetraploidization. Here, we show that inhibition of ATM or Chk2 kinases impairs CPC localization to the midbody center, accelerates midbody resolution in normally segregating cells, and correlates with premature abscission and chromatin breakage in cytokinesis with trapped chromatin. In cultured human cells, ATM activates Chk2 at late midbodies. In turn, Chk2 phosphorylates human INCENP-Ser91 to promote INCENP binding to Mklp2 kinesin and CPC localization to the midbody center through Mklp2 association with Cep55. Expression of truncated Mklp2 that does not bind to Cep55 or nonphosphorylatable INCENP-Ser91A impairs CPC midbody localization and accelerates abscission. In contrast, expression of phosphomimetic INCENP-Ser91D or a chimeric INCENP protein that is targeted to the midbody center rescues the abscission delay in Chk2-deficient or ATM-deficient cells. Furthermore, the Mre11–Rad50–Nbs1 complex is required for ATM activation at the midbody in cytokinesis with chromatin bridges. These results identify an ATM–Chk2–INCENP pathway that imposes the abscission checkpoint by regulating CPC midbody localization.

Cellular functions of the protein kinase ATM and their relevance to human disease

The protein kinase ataxia telangiectasia mutated (ATM) is a master regulator of double-strand DNA break (DSB) signalling and stress responses. For three decades, ATM has been investigated extensively to elucidate its roles in the DNA damage response (DDR) and in the pathogenesis of ataxia telangiectasia (A-T), a human neurodegenerative disease caused by loss of ATM. Although hundreds of proteins have been identified as ATM phosphorylation targets and many important roles for this kinase have been identified, it is still unclear how ATM deficiency leads to the early-onset cerebellar degeneration that is common in all individuals with A-T. Recent studies suggest the existence of links between ATM deficiency and other cerebellum-specific neurological disorders, as well as the existence of broader similarities with more common neurodegenerative disorders. In this Review, we discuss recent structural insights into ATM regulation, and possible aetiologies of A-T phenotypes, including reactive oxygen species, mitochondrial dysfunction, alterations in transcription, R-loop metabolism and alternative splicing, defects in cellular proteostasis and metabolism, and potential pathogenic roles for hyper-poly(ADP-ribosyl)ation.

A Novel Ruthenium(II) Complex With Lapachol Induces G2/M Phase Arrest Through Aurora-B Kinase Down-Regulation and ROS-Mediated Apoptosis in Human Prostate Adenocarcinoma Cells

Lapachol is a well-studied natural product that has been receiving great interest due to its anticancer properties that target oxidative stress. In the present work, two novel lapachol-containing ruthenium(II) complexes [Ru(Lap)(dppm)(bipy)]PF6 (1) and [Ru(Lap)(dppm)(phen)]PF6 (2) [Lap = lapachol, dppm = 1,1'-bis(diphosphino)methane, bipy = 2,2'-bipyridine, phen = 1,10-phenantroline] were synthesized, fully characterized, and investigated for their cellular and molecular responses on cancer cell lines. We found that both complexes exhibited a potent cytotoxic effect in a panel of cancer cell lines in monolayer cultures, as well as in a 3D model of multicellular spheroids formed from DU-145 human prostate adenocarcinoma cells. Furthermore, the complex (2) suppressed the colony formation, induced G2/M-phase arrest, and downregulated Aurora-B. The mechanism studies suggest that complex (2) stimulate the overproduction of reactive oxygen species (ROS) and triggers caspase-dependent apoptosis as a result of changes in expression of several genes related to cell proliferation and caspase-3 and -9 activation. Interestingly, we found that N-acetyl-L-cysteine, a ROS scavenger, suppressed the generation of intracellular ROS induced by complex (2), and decreased its cytotoxicity, indicating that ROS-mediated DNA damage leads the DU-145 cells into apoptosis. Overall, we highlighted that coordination of lapachol to phosphinic ruthenium(II) compounds considerably improves the antiproliferative activities of resulting complexes granting attractive selectivity to human prostate adenocarcinoma cells. The DNA damage response to ROS seems to be involved in the induction of caspase-mediated cell death that plays an important role in the complexes' cytotoxicity. Upon further investigations, this novel class of lapachol-containing ruthenium(II) complexes might indicate promising chemotherapeutic agents for prostate cancer therapy.

Mitotic kinases as drivers of the epithelial-to-mesenchymal transition and as therapeutic targets against breast cancers

Biological therapies against breast cancer patients with tumors positive for the estrogen and progesterone hormone receptors and Her2 amplification have greatly improved their survival. However, to date, there are no effective biological therapies against breast cancers that lack these three receptors or triple-negative breast cancers (TNBC). TNBC correlates with poor survival, in part because they relapse following chemo- and radio-therapies. TNBC is intrinsically aggressive since they have high mitotic indexes and tend to metastasize to the central nervous system. TNBCs are more likely to display centrosome amplification, an abnormal phenotype that results in defective mitotic spindles and abnormal cytokinesis, which culminate in aneuploidy and chromosome instability (known causes of tumor initiation and chemo-resistance). Besides their known role in cell cycle control, mitotic kinases have been also studied in different types of cancer including breast, especially in the context of epithelial-to-mesenchymal transition (EMT). EMT is a cellular process characterized by the loss of cell polarity, reorganization of the cytoskeleton, and signaling reprogramming (upregulation of mesenchymal genes and downregulation of epithelial genes). Previously, we and others have shown the effects of mitotic kinases like Nek2 and Mps1 (TTK) on EMT. In this review, we focus on Aurora A, Aurora B, Bub1, and highly expressed in cancer (Hec1) as novel targets for therapeutic interventions in breast cancer and their effects on EMT. We highlight the established relationships and interactions of these and other mitotic kinases, clinical trial studies involving mitotic kinases, and the importance that represents to develop drugs against these proteins as potential targets in the primary care therapy for TNBC.

ATM inhibition overcomes resistance to histone deacetylase inhibitor due to p21 induction and cell cycle arrest

The antiproliferative effect induced by histone deactylase inhibitors (HDACi) is associated with the up-regulated expression of the cyclin-dependent kinase inhibitor p21. Paradoxically, the increased expression of p21 correlates with a reduced cell killing to the drug. The direct targeting of p21 is not feasible. An alternate approach could selectively target factors upstream or downstream of p21 that affect one or more specific aspects of p21 function. HDAC inhibitors appear to activate p21 expression via ataxia telangiectasia mutated (ATM) activity. KU60019, a specific ATM inhibitor, has shown to decrease the p21 protein levels in a concentration dependent manner. We explored the potential synergistic interaction of the ATM inhibitor with romidepsin, given the potential complementary impact around p21. A synergistic cytotoxic effect was observed in all lymphoma cell lines examined when the HDACi was combined with KU60019. The increase in apoptosis correlates with decreased expression of p21 due to the ATM inhibitor. KU60019 decreased expression of the cyclin-dependent kinase inhibitor at the transcriptional level, compromising the ability of HDACi to induce p21 and cell cycle arrest and ultimately facilitating a shift toward the apoptotic phase. Central to the increased apoptosis observed when romidepsin is combined with KU60019 is the reduced expression of p21 and the absence of a G2/M cell cycle arrest that would be exploited by the tumor cells to evade the cytotoxic effect of the HDAC inhibitor. We believe this strategy may offer a promising way to identify rational combinations for HDACi directed therapy, improving their activity in malignant disease.

Kinetochore protein MAD1 participates in the DNA damage response through ataxia-telangiectasia mutated kinase-mediated phosphorylation and enhanced interaction with KU80

Objective: Mitotic arrest-deficient protein 1 (MAD1) is a kinetochore protein essential for the mitotic spindle checkpoint. Proteomic studies have indicated that MAD1 is a component of the DNA damage response (DDR) pathway. However, whether and how MAD1 might be directly involved in the DDR is largely unknown. Methods: We ectopically expressed the wild type, or a phosphorylation-site--mutated form of MAD1 in MAD1 knockdown cells to look for complementation effects. We used the comet assay, colony formation assay, immunofluorescence staining, and flow cytometry to assess the DDR, radiosensitivity, and the G2/M checkpoint. We employed co-immunoprecipitation followed by mass spectrometry to identify MAD1 interacting proteins. Data were analyzed using the unpaired Student's t-test. Results: We showed that MAD1 was required for an optimal DDR, as knocking down MAD1 resulted in impaired DNA repair and hypersensitivity to ionizing radiation (IR). We found that IR-induced serine 214 phosphorylation was ataxia-telangiectasia mutated (ATM) kinase-dependent. Mutation of serine 214 to alanine failed to rescue the phenotypes of MAD1 knockdown cells in response to IR. Using mass spectrometry, we identified a protein complex mediated by MAD1 serine 214 phosphorylation in response to IR. Among them, we showed that KU80 was a key protein that displayed enhanced interaction with MAD1 after DNA damage. Finally, we showed that MAD1 interaction with KU80 required serine 214 phosphorylation, and it was essential for activation of DNA protein kinases catalytic subunit (DNA-PKcs). Conclusions: MAD1 serine 214 phosphorylation mediated by ATM kinase in response to IR was required for the interaction with KU80 and activation of DNA-PKCs.

Aurora kinases and DNA damage response

It is well established that Aurora kinases perform critical functions during mitosis. It has become increasingly clear that the Aurora kinases also perform a myriad of non-mitotic functions including DNA damage response. The available evidence indicates that inhibition Aurora kinase A (AURKA) may contribute to the G₂ DNA damage checkpoint through AURKA's functions in PLK1 and CDC25B activation. Both AURKA and Aurora kinase B (AURKB) are also essential in mitotic DNA damage response that guard against DNA damage-induced chromosome segregation errors, including the control of abscission checkpoint and prevention of micronuclei formation. Dysregulation of Aurora kinases can trigger DNA damage in mitosis that is sensed in the subsequent G₁ by a p53-dependent postmitotic checkpoint. Aurora kinases are themselves linked to the G₁ DNA damage checkpoint through p53 and p73 pathways. Finally, several lines of evidence provide a connection between Aurora kinases and DNA repair and apoptotic pathways. Although more studies are required to provide a comprehensive picture of how cells respond to DNA damage, these findings indicate that both AURKA and AURKB are inextricably linked to pathways guarding against DNA damage. They also provide a rationale to support more detailed studies on the synergism between small-molecule inhibitors against Aurora kinases and DNA-damaging agents in cancer therapies.

Clinical Implications of Chromosomal Instability (CIN) and Kinetochore Abnormalities in Breast Cancers

Genetic instability is a defining property of cancer cells and is the basis of various lesions including point mutations, copy number alterations and translocations. Chromosomal instability (CIN) is part of the genetic instability of cancer and consists of copy number alterations in whole or parts of cancer cell chromosomes. CIN is observed in differing degrees in most cancers. In breast cancer, CIN is commonly part of the genomic landscape of the disease and has a higher incidence in aggressive sub-types. Tumor suppressors that are commonly mutated or disabled in cancer, such as p53 and pRB, play roles in protection against CIN, and as a result, their dysfunction contributes to the establishment or tolerance of CIN. Several structural and regulatory proteins of the centromeres and kinetochore, the complex structure that is responsible for the correct distribution of genetic material in the daughter cells during mitosis, are direct or, mostly, indirect transcription targets of p53 and pRB. Thus, despite the absence of structural defects in genes encoding for centromere and kinetochore components, dysfunction of these tumor suppressors may have profound implications for the correct function of the mitotic apparatus contributing to CIN. CIN and its prognostic and therapeutic implications in breast cancer are discussed in this article.

TGFBR2 mediated phosphorylation of BUB1 at Ser-318 is required for transforming growth factor-β signaling

BUB1 (budding uninhibited by benzimidazoles-1) is required for efficient TGF-β signaling, through its role in stabilizing the TGFBR1 and TGFBR2 complex. Here we demonstrate that TGFBR2 phosphorylates BUB1 at Serine-318, which is conserved in primates. S318 phosphorylation abrogates the interaction of BUB1 with TGFBR1 and SMAD2. Using BUB1 truncation domains (1–241, 241–482 and 482–723), we demonstrate that multiple contact points exist between BUB1 and TGF-β signaling components and that these interactions are independent of the BUB1 tetratricopeptide repeat (TPR) domain. Moreover, substitutions in the middle domain (241–482) encompassing S318 reveals that efficient interaction with TGFBR2 occurs only in its dephosphorylated state (241–482 S318A). In contrast, the phospho-mimicking mutant (241–482 S318D) exhibits efficient binding with SMAD2 and its over-expression results in a decrease in TGFBR1-TGFBR2 and TGFBR1-SMAD2 interactions. These findings suggest that TGFBR2 mediated BUB1 phosphorylation at S318 may serve as a switch for the dissociation of the SMAD2-TGFBR complex, and therefore represents a regulatory event for TGF-β signaling. Finally, we provide evidence that the BUB1-TGF-β signaling axis may mediate aggressive phenotypes in a variety of cancers.

DNA damage response proteins regulating mitotic cell division: double agents preserving genome stability

The DNA damage response recognizes DNA lesions and coordinates a cell cycle arrest with the repair of the damaged DNA, or removal of the affected cells to prevent the passage of genetic alterations to the next generation. The mitotic cell division, on the other hand, is a series of processes that aims to accurately segregate the genomic material from the maternal to the two daughter cells. Despite their great importance in safeguarding genomic integrity, the DNA damage response and the mitotic cell division were long viewed as unrelated processes, mainly because animal cells that are irradiated during mitosis continue cell division without repairing the broken chromosomes. However, recent studies have demonstrated that DNA damage proteins play an important role in mitotic cell division. This is performed through regulation of the onset of mitosis, mitotic spindle formation, correction of misattached kinetochore-microtubules, spindle checkpoint signaling, or completion of cytokinesis (abscission), in the absence of DNA damage. In this review, we summarize the roles of DNA damage proteins in unperturbed mitosis, analyze the molecular mechanisms involved, and discuss the potential implications of these findings in cancer therapy.

Direct Phosphorylation and Stabilization of MYC by Aurora B Kinase Promote T-cell Leukemogenesis

Deregulation of MYC plays an essential role in T cell acute lymphoblastic leukemia (T-ALL), yet the mechanisms underlying its deregulation remain elusive. Herein, we identify a molecular mechanism responsible for reciprocal activation between Aurora B kinase (AURKB) and MYC. AURKB directly phosphorylates MYC at serine 67, counteracting GSK3β-directed threonine 58 phosphorylation and subsequent FBXW7-mediated proteasomal degradation. Stabilized MYC, in concert with T cell acute lymphoblastic leukemia 1 (TAL1), directly activates AURKB transcription, constituting a positive feedforward loop that reinforces MYC-regulated oncogenic programs. Therefore, inhibitors of AURKB induce prominent MYC degradation concomitant with robust leukemia cell death. These findings reveal an AURKB-MYC regulatory circuit that underlies T cell leukemogenesis, and provide a rationale for therapeutic targeting of oncogenic MYC via AURKB inhibition.

RAD50 regulates mitotic progression independent of DNA repair functions

The Mre11A/RAD50/NBN complex (MRN) is an essential regulator of the cellular damage response after DNA double-strand breaks (DSBs). More recent work has indicated that MRN may also impact on the duration of mitosis. We show here that RAD50-deficient fibroblasts exhibit a marked delay in mitotic progression that can be rescued by lentiviral transduction of RAD50. The delay was observed throughout all mitotic phases in live cell imaging using GFP-labeled H2B as a fluorescent marker. In complementation assays with RAD50 phosphorylation mutants, modifications at Ser635 had little effect on mitotic progression. By contrast with RAD50, fibroblast strains deficient in ATM or NBN did not show a significant slowing of mitotic progression. Ataxia-telangiectasia-like disorder (ATLD) fibroblasts with nuclease-deficient MRE11A (p.W210C) tended to show slower mitosis, though by far not as significant as RAD50-deficient cells. Inhibitor studies indicated that ATM kinase activity might not grossly impact on mitotic progression, while treatment with MRE11A inhibitor PFM39 modestly prolonged mitosis. Inhibition of ATR kinase significantly prolonged mitosis but this effect was mostly independent of RAD50 status. Taken together, our data unravel a mitotic role of RAD50 that can be separated from its known functions in DNA repair.

Building bridges between chromosomes: novel insights into the abscission checkpoint

In the presence of chromatin bridges, mammalian cells delay completion of cytokinesis (abscission) to prevent chromatin breakage or tetraploidization by regression of the cleavage furrow. This abscission delay is called "the abscission checkpoint" and is dependent on Aurora B kinase. Furthermore, cells stabilize the narrow cytoplasmic canal between the two daughter cells until the DNA bridges are resolved. Impaired abscission checkpoint signaling or unstable intercellular canals can lead to accumulation of DNA damage, aneuploidy, or generation of polyploid cells which are associated with tumourigenesis. However, the molecular mechanisms involved have only recently started to emerge. In this review, we focus on the molecular pathways of the abscission checkpoint and describe newly identified triggers, Aurora B-regulators and effector proteins in abscission checkpoint signaling. We also describe mechanisms that control intercellular bridge stabilization, DNA bridge resolution, or abscission checkpoint silencing upon satisfaction, and discuss how abscission checkpoint proteins can be targeted to potentially improve cancer therapy.

Mitosis-specific MRN complex promotes a mitotic signaling cascade to regulate spindle dynamics and chromosome segregation

The MRE11-RAD50-NBS1 (MRN) complex is well known for participating in DNA damage response pathways in all phases of cell cycle. Here, we show that MRN constitutes a mitosis-specific complex, named mMRN, with a protein, MMAP. MMAP directly interacts with MRE11 and is required for optimal stability of the MRN complex during mitosis. MMAP colocalizes with MRN in mitotic spindles, and MMAP-deficient cells display abnormal spindle dynamics and chromosome segregation similar to MRN-deficient cells. Mechanistically, both MMAP and MRE11 are hyperphosphorylated by the mitotic kinase, PLK1; and the phosphorylation is required for assembly of the mMRN complex. The assembled mMRN complex enables PLK1 to interact with and activate the microtubule depolymerase, KIF2A, leading to spindle turnover and chromosome segregation. Our study identifies a mitosis-specific version of the MRN complex that acts in the PLK1-KIF2A signaling cascade to regulate spindle dynamics and chromosome distribution.

Selective human inhibitors of ATR and ATM render Leishmania major promastigotes sensitive to oxidative damage

All cellular processes, including those involved in normal cell metabolism to those responsible for cell proliferation or death, are finely controlled by cell signaling pathways, whose core proteins constitute the family of phosphatidylinositol 3-kinase-related kinases (PIKKs). Ataxia Telangiectasia Mutated (ATM) and Ataxia Telangiectasia and Rad3 related (ATR) are two important PIKK proteins that act in response to DNA damage, phosphorylating a large number of proteins to exert control over genomic integrity. The genus Leishmania belongs to a group of early divergent eukaryotes in evolution and has a highly plastic genome, probably owing to the existence of signaling pathways designed to maintain genomic integrity. The objective of this study was to evaluate the use of specific human inhibitors of ATR and ATM in Leishmania major. Bioinformatic analyses revealed the existence of the putative PIKK genes ATR and ATM, in addition to mTOR and DNA-PKcs in Leishmania spp. Moreover, it was possible to suggest that the inhibitors VE-821 and KU-55933 have binding affinity for the catalytic sites of putative L. major ATR and ATM, respectively. Promastigotes of L. major exposed to these inhibitors show slight growth impairment and minor changes in cell cycle and morphology. It is noteworthy that treatment of promastigotes with inhibitors VE-821 and KU-55933 enhanced the oxidative damage caused by hydrogen peroxide. These inhibitors could significantly reduce the number of surviving L. major cells following H₂O₂ exposure whilst also decreasing their evaluated IC₅₀ to H₂O₂ to less than half of that observed for non-treated cells. These results suggest that the use of specific inhibitors of ATR and ATM in Leishmania interferes in the signaling pathways of this parasite, which can impair its tolerance to DNA damage and affect its genome integrity. ATR and ATM could constitute novel targets for drug development and/or repositioning for treatment of leishmaniases.

ATM directs DNA damage responses and proteostasis via genetically separable pathways

The protein kinase ATM is a master regulator of the DNA damage response but also responds directly to oxidative stress. Loss of ATM causes ataxia telangiectasia, a neurodegenerative disorder with pleiotropic symptoms that include cerebellar dysfunction, cancer, diabetes, and premature aging. We genetically separated the activation of ATM by DNA damage from that by oxidative stress using separation-of-function mutations. We found that deficient activation of ATM by the Mre11-Rad50-Nbs1 complex and DNA double-strand breaks resulted in loss of cell viability, checkpoint activation, and DNA end resection in response to DNA damage. In contrast, loss of oxidative activation of ATM had minimal effects on DNA damage-related outcomes but blocked ATM-mediated initiation of checkpoint responses after oxidative stress and resulted in deficiencies in mitochondrial function and autophagy. In addition, expression of a variant ATM incapable of activation by oxidative stress resulted in widespread protein aggregation. These results indicate a direct relationship between the mechanism of ATM activation and its effects on cellular metabolism and DNA damage responses in human cells and implicate ATM in the control of protein homeostasis.

Aurora Kinase B, a novel regulator of TERF1 binding and telomeric integrity

AURKB (Aurora Kinase B) is a serine/threonine kinase better known for its role at the mitotic kinetochore during chromosome segregation. Here, we demonstrate that AURKB localizes to the telomeres in mouse embryonic stem cells, where it interacts with the essential telomere protein TERF1. Loss of AURKB function affects TERF1 telomere binding and results in aberrant telomere structure. In vitro kinase experiments successfully identified Serine 404 on TERF1 as a putative AURKB target site. Importantly, in vivo overexpression of S404-TERF1 mutants results in fragile telomere formation. These findings demonstrate that AURKB is an important regulator of telomere structural integrity.

DNA damage response and hematological malignancy

DNA damage is a serious threat to cellular homeostasis. Damaged DNA leads to genomic instability, mutation, senescence, and/or cell death. DNA damage triggers a cellular response called the DNA damage response (DDR), followed by activation of the DNA repair machinery. DDR both maintains cellular homeostasis and prevents cancer development. Germ line mutation of DDR-associated genes can lead to cancer-susceptible syndromes. Somatic mutation of DDR-associated genes has also been reported in various tumors, including hematological malignancies. Therapeutic approaches that target the DDR and DNA repair are thus now being developed. Understanding the mechanism(s) underlying DDR and DNA repair will increase our knowledge of cancer etiology and facilitate development of cancer therapies.

Specific glutamic acid residues in targeted proteins induce exaggerated retardations in Phos-tag SDS-PAGE migration

We describe two unique proteins, Escherichia coli ClpX and human histone H2A, that show extremely retarded migrations relative to their molecular weights in Phos-tag SDS-PAGE, despite being nonphosphorylated. Although ClpX separated into multiple migration bands in Phos-tag gels, the separation was not due to phosphorylation. The N-terminal 47-61 region of ClpX was responsible for producing multiple phosphorylation-independent structural variants, even under denaturing conditions, and some of these variants were detected as highly up-shifted bands. By systematic Ala-scanning mutation analysis in the N-47-61 region, we concluded that the Glu-51 or Glu-54 residue was responsible for the appearance of exaggerated mobility-shifting bands. Histone H2A showed a much slower migration in Phos-tag gels in comparison with other major histones having similar molecular weights, and we found that the Glu-62 or Glu-65 residue caused the retarded migration. In addition, Phos-tag SDS-PAGE permitted us to detect a shift in the mobility of the phosphorylated form of histone H2A from that of the nonphosphorylated one. This is the first report showing that exaggerated retardation in the migration of a certain protein in Phos-tag SDS-PAGE is induced by interactions between the Phos-tag molecule and the carboxylate group of a specific Glu residue on the target.

Regulation of the DNA Damage Response by DNA-PKcs Inhibitory Phosphorylation of ATM

Ataxia-telangiectasia mutated (ATM) regulates the DNA damage response as well as DNA double-strand break repair through homologous recombination. Here we show that ATM is hyperactive when the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is chemically inhibited or when the DNA-PKcs gene is deleted in human cells. Pre-incubation of ATM protein with active DNA-PKcs also significantly reduces ATM activity in vitro. We characterize several phosphorylation sites in ATM that are targets of DNA-PKcs and show that phospho-mimetic mutations at these residues significantly inhibit ATM activity and impair ATM signaling upon DNA damage. In contrast, phospho-blocking mutations at one cluster of sites increase the frequency of apoptosis during normal cell growth. DNA-PKcs, which is integral to the non-homologous end joining pathway, thus negatively regulates ATM activity through phosphorylation of ATM. These observations illuminate an important regulatory mechanism for ATM that also controls DNA repair pathway choice.

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.

Skp2 is required for Aurora B activation in cell mitosis and spindle checkpoint

The Aurora B kinase plays a critical role in cell mitosis and spindle checkpoint. Here, we showed that the ubiquitin E3-ligase protein Skp2, also as a cell-cycle regulatory protein, was required for the activation of Aurora B and its downstream protein. When we restored Skp2 knockdown Hela cells with Skp2 and Skp2-LRR E3 ligase dead mutant we found that Skp2 could rescue the defect in the activation of Aurora B, but the mutant failed to do so. Furthermore, we discovered that Skp2 could interact with Aurora B and trigger Aurora B Lysine (K) 63-linked ubiquitination. Finally, we demonstrated the essential role of Skp2 in cell mitosis progression and spindle checkpoint, which was Aurora B dependent. Our results identified a novel ubiquitinated substrate of Skp2, and also indicated that Aurora B ubiquitination might serve as an important event for Aurora B activation in cell mitosis and spindle checkpoint.

HPV16-E2 induces prophase arrest and activates the cellular DNA damage response in vitro and in precursor lesions of cervical carcinoma

Cervical intraepithelial neoplasia (CIN) is caused by human papillomavirus (HPV) infection and is the precursor to cervical carcinoma. The completion of the HPV productive life cycle depends on the expression of viral proteins which further determines the severity of the cervical neoplasia. Initiation of the viral productive replication requires expression of the E2 viral protein that cooperates with the E1 viral DNA helicase. A decrease in the viral DNA replication ability and increase in the severity of cervical neoplasia is accompanied by simultaneous elevated expression of E6 and E7 oncoproteins. Here we reveal a novel and important role for the HPV16-E2 protein in controlling host cell cycle during malignant transformation. We showed that cells expressing HPV16-E2 in vitro are arrested in prophase alongside activation of a sustained DDR signal. We uncovered evidence that HPV16-E2 protein is present in vivo in cells that express both mitotic and DDR signals specifically in CIN3 lesions, immediate precursors of cancer, suggesting that E2 may be one of the drivers of genomic instability and carcinogenesis in vivo.

Mutation of the BRCA1 SQ-cluster results in aberrant mitosis, reduced homologous recombination, and a compensatory increase in non-homologous end joining

Mutations in the breast cancer susceptibility 1 (BRCA1) gene are catalysts for breast and ovarian cancers. Most mutations are associated with the BRCA1 N- and C-terminal domains linked to DNA double-strand break (DSB) repair. However, little is known about the role of the intervening serine-glutamine (SQ) - cluster in the DNA damage response beyond its importance in regulating cell cycle checkpoints. We show that serine-to-alanine alterations at critical residues within the SQ-cluster known to be phosphorylated by ATM and ATR result in reduced homologous recombination repair (HRR) and aberrant mitosis. While a S1387A BRCA1 mutant - previously shown to abrogate S-phase arrest in response to radiation - resulted in only a modest decrease in HRR, S1387A together with an additional alteration, S1423A (BRCA12P), reduced HRR to vector control levels and similar to a quadruple mutant also including S1457A and S1524A (BRCA14P). These effects appeared to be independent of PALB2. Furthermore, we found that BRCA14P promoted a prolonged and struggling HRR late in the cell cycle and shifted DSB repair from HRR to non-homologous end joining which, in the face of irreparable chromosomal damage, resulted in mitotic catastrophe. Altogether, SQ-cluster phosphorylation is critical for allowing adequate time for completing normal HRR prior to mitosis and preventing cells from entering G1 prematurely resulting in gross chromosomal aberrations.

Maintaining Genome Stability in Defiance of Mitotic DNA Damage

The implementation of decisions affecting cell viability and proliferation is based on prompt detection of the issue to be addressed, formulation and transmission of a correct set of instructions and fidelity in the execution of orders. While the first and the last are purely mechanical processes relying on the faithful functioning of single proteins or macromolecular complexes (sensors and effectors), information is the real cue, with signal amplitude, duration, and frequency ultimately determining the type of response. The cellular response to DNA damage is no exception to the rule. In this review article we focus on DNA damage responses in G2 and Mitosis. First, we set the stage describing mitosis and the machineries in charge of assembling the apparatus responsible for chromosome alignment and segregation as well as the inputs that control its function (checkpoints). Next, we examine the type of issues that a cell approaching mitosis might face, presenting the impact of post-translational modifications (PTMs) on the correct and timely functioning of pathways correcting errors or damage before chromosome segregation. We conclude this essay with a perspective on the current status of mitotic signaling pathway inhibitors and their potential use in cancer therapy.

MET inhibition overcomes radiation resistance of glioblastoma stem-like cells

Glioblastoma (GBM) contains stem-like cells (GSCs) known to be resistant to ionizing radiation and thus responsible for therapeutic failure and rapidly lethal tumor recurrence. It is known that GSC radioresistance relies on efficient activation of the DNA damage response, but the mechanisms linking this response with the stem status are still unclear. Here, we show that the MET receptor kinase, a functional marker of GSCs, is specifically expressed in a subset of radioresistant GSCs and overexpressed in human GBM recurring after radiotherapy. We elucidate that MET promotes GSC radioresistance through a novel mechanism, relying on AKT activity and leading to (i) sustained activation of Aurora kinase A, ATM kinase, and the downstream effectors of DNA repair, and (ii) phosphorylation and cytoplasmic retention of p21, which is associated with anti-apoptotic functions. We show that MET pharmacological inhibition causes DNA damage accumulation in irradiated GSCs and their depletion in vitro and in GBMs generated by GSC xenotransplantation. Preclinical evidence is thus provided that MET inhibitors can radiosensitize tumors and convert GSC-positive selection, induced by radiotherapy, into GSC eradication.

DNA Damage Response Is Involved in the Developmental Toxicity of Mebendazole in Zebrafish Retina

Intestinal helminths cause iron-deficiency anemia in pregnant women, associated with premature delivery, low birth weight, maternal ill health, and maternal death. Although benzimidazole compounds such as mebendazole (MBZ) are highly efficacious against helminths, there are limited data on its use during pregnancy. In this study, we performed in vivo imaging of the retinas of zebrafish larvae exposed to MBZ, and found that exposure to MBZ during 2 and 3 days post-fertilization caused malformation of the retinal layers. To identify the molecular mechanism underlying the developmental toxicity of MBZ, we performed transcriptome analysis of zebrafish eyes. The analysis revealed that the DNA damage response was involved in the developmental toxicity of MBZ. We were also able to demonstrate that inhibition of ATM significantly attenuated the apoptosis induced by MBZ in the zebrafish retina. These results suggest that MBZ causes developmental toxicity in the zebrafish retina at least partly by activating the DNA damage response, including ATM signaling, providing a potential adverse outcome pathway in the developmental toxicity of MBZ in mammals.

The ATM- and ATR-related SCD domain is over-represented in proteins involved in nervous system development

ATM and ATR are cellular kinases with a well-characterized role in the DNA-damage response. Although the complete set of ATM/ATR targets is unknown, they often contain clusters of S/TQ motifs that constitute an SCD domain. In this study, we identified putative ATM/ATR targets that have a conserved SCD domain across vertebrates. Using this approach, we have identified novel putative ATM/ATR targets in pathways known to be under direct control of these kinases. Our analysis has also unveiled significant enrichment of SCD-containing proteins in cellular pathways, such as vesicle trafficking and actin cytoskeleton, where a regulating role for ATM/ATR is either unknown or poorly understood, hinting at a much broader and overarching role for these kinases in the cell. Of particular note is the overrepresentation of conserved SCD-containing proteins involved in pathways related to neural development. This finding suggests that ATM/ATR could be directly involved in controlling this process, which may be linked to the adverse neurological effects observed in patients with mutations in ATM.

Phosphoproteomic Profiling Reveals Epstein-Barr Virus Protein Kinase Integration of DNA Damage Response and Mitotic Signaling

Epstein-Barr virus (EBV) is etiologically linked to infectious mononucleosis and several human cancers. EBV encodes a conserved protein kinase BGLF4 that plays a key role in the viral life cycle. To provide new insight into the host proteins regulated by BGLF4, we utilized stable isotope labeling by amino acids in cell culture (SILAC)-based quantitative proteomics to compare site-specific phosphorylation in BGLF4-expressing Akata B cells. Our analysis revealed BGLF4-mediated hyperphosphorylation of 3,046 unique sites corresponding to 1,328 proteins. Frequency analysis of these phosphosites revealed a proline-rich motif signature downstream of BGLF4, indicating a broader substrate recognition for BGLF4 than its cellular ortholog cyclin-dependent kinase 1 (CDK1). Further, motif analysis of the hyperphosphorylated sites revealed enrichment in ATM, ATR and Aurora kinase substrates while functional analyses revealed significant enrichment of pathways related to the DNA damage response (DDR), mitosis and cell cycle. Phosphorylation of proteins associated with the mitotic spindle assembly checkpoint (SAC) indicated checkpoint activation, an event that inactivates the anaphase promoting complex/cyclosome, APC/C. Furthermore, we demonstrated that BGLF4 binds to and directly phosphorylates the key cellular proteins PP1, MPS1 and CDC20 that lie upstream of SAC activation and APC/C inhibition. Consistent with APC/C inactivation, we found that BGLF4 stabilizes the expression of many known APC/C substrates. We also noted hyperphosphorylation of 22 proteins associated the nuclear pore complex, which may contribute to nuclear pore disassembly and SAC activation. A drug that inhibits mitotic checkpoint activation also suppressed the accumulation of extracellular EBV virus. Taken together, our data reveal that, in addition to the DDR, manipulation of mitotic kinase signaling and SAC activation are mechanisms associated with lytic EBV replication. All MS data have been deposited in the ProteomeXchange with identifier PXD002411 (http://proteomecentral.proteomexchange.org/dataset/PXD002411).

Epstein-Barr virus (EBV) is a herpesvirus that is associated with B cell and epithelial human cancers. Herpesviruses encode a protein kinase which is an important regulator of lytic virus replication and is consequently a target for anti-viral drug development. The EBV genome encodes for a serine/threonine protein kinase called BGLF4. Previous work on BGLF4 has largely focused on its cyclin-dependent kinase 1 (CDK1)-like activity. The range of BGLF4 cellular substrates and the full impact of BGLF4 on the intracellular microenvironment still remain to be elucidated. Here, we utilized unbiased quantitative phosphoproteomic approach to dissect the changes in the cellular phosphoproteome that are mediated by BGLF4. Our MS analyses revealed extensive hyperphosphorylation of substrates that are normally targeted by CDK1, Ataxia telangiectasia mutated (ATM), Ataxia telangiectasia and Rad3-related (ATR) proteins and Aurora kinases. The up-regulated phosphoproteins were functionally linked to the DNA damage response, mitosis and cell cycle pathways. Our data demonstrate widespread changes in the cellular phosphoproteome that occur upon BGLF4 expression and suggest that manipulation of the DNA damage and mitotic kinase signaling pathways are central to efficient EBV lytic replication.

CHMP4C Disruption Sensitizes the Human Lung Cancer Cells to Irradiation

Human lung cancer is highly invasive and the most malignant among human tumors. Adenocarcinoma as a specific type of non-small cell lung cancer occurs with high frequency and is also highly resistant to radiation therapy. Thus, how to avoid radiation resistance and improve radiotherapy effectiveness is a crucial question. In the present study, human lung cancer A549 and H1299 cells were irradiated using γ-rays from a Co60 irradiator. Protein expression was detected by Western blotting. Cell cycle and apoptosis were measured by flow cytometry. Surviving fraction was determined by colony formation assay. γH2AX and 53BP1 foci formation were examined by fluorescence microscopy. In the results, we show that CHMP4C, a subunit of Endosomal sorting complex-III (ESCRT-III), is involved in radiation-induced cellular response. Radiation-induced Aurora B expression enhances CHMP4C phosphorylation in non-small cell lung cancer (NSCLC) cells, maintaining cell cycle check-point and cellular viability as well as resisting apoptosis. CHMP4C depletion enhances cellular sensitivity to radiation, delays S-phase of cell cycle and reduces ionizing radiation (IR)-induced γH2AX foci formation. We found that Aurora B targets CHMP4C and inhibition of Aurora B exhibits similar effects with silencing of CHMP4C in radioresistance. We also confirm that CHMP4C phosphorylation is elevated after IR both in p53-positive and-negative cells, indicating that the close correlation between CHMP4C and Aurora B signaling pathway in mediating radiation resistance is not p53 dependent. Together, our work establishes a new function of CHMP4C in radiation resistance, which will offer a potential strategy for non-small cell lung cancer by disrupting CHMP4C.

ATM and ATR signaling at a glance

ATM and ATR signaling pathways are well conserved throughout evolution and are central to the maintenance of genome integrity. Although the role of both ATM and ATR in DNA repair, cell cycle regulation and apoptosis have been well studied, both still remain in the focus of current research activities owing to their role in cancer. Recent advances in the field suggest that these proteins have an additional function in maintaining cellular homeostasis under both stressed and non-stressed conditions. In this Cell Science at a Glance article and the accompanying poster, we present an overview of recent advances in ATR and ATM research with emphasis on that into the modes of ATM and ATR activation, the different signaling pathways they participate in - including those that do not involve DNA damage - and highlight their relevance in cancer.