ATM is activated by mitotic stress and suppresses centrosome amplification in primary but not in tumor cells

The role of AMPK in metabolism and its influence on DNA damage repair

One of the most complex health disproportions in the human body is the metabolic syndrome (MetS). It can result in serious health consequences such as type 2 diabetes mellitus, atherosclerosis or insulin resistance. The center of energy regulation in human is AMP-activated protein kinase (AMPK), which modulates cells' metabolic pathways and protects them against negative effects of metabolic stress, e.g. reactive oxygen species. Moreover, recent studies show the relationship between the AMPK activity and the regulation of DNA damage repair such as base excision repair (BER) system, which is presented in relation to the influence of MetS on human genome. Hence, AMPK is studied not only in the field of counteracting MetS but also prevention of genetic alterations and cancer development. Through understanding AMPK pathways and its role in cells with damaged DNA it might be possible to improve cell's repair processes and develop new therapies. This review presents AMPK role in eukaryotic cells and focuses on the relationship between AMPK activity and the regulation of BER system through its main component-8-oxoguanine glycosylase (OGG1).

Genetic variation and radiation quality impact cancer promoting cellular phenotypes in response to HZE exposure

There exists a wide degree of genetic variation within the normal human population which includes disease free individuals with heterozygote defects in major DNA repair genes. A lack of understanding of how this genetic variation impacts cellular phenotypes that inform cancer risk post heavy ion exposure poses a major limitation in developing personalized cancer risk assessment astronauts. We initiated a pilot study with Human Mammary Epithelial Cell strains (HMEC) derived from wild type, a p16 silenced derivative of wild type, and various genetic variants that were heterozygote for DNA repair genes; BRCA1, BRCA2 and ATM. Cells strains were exposed to different high and low LET radiation qualities to generate both simple and complex lesions and centrosome aberrations were examined as a surrogate marker of genomic instability and cancer susceptibility post different exposures. Our results indicate that centrosome aberration frequency is higher in the genetic variants under study. The aberration frequency increases with dose, complexity of the lesion generated by different radiation qualities and age of the individual. This increase in genomic instability correlates with elevated check-point activation post radiation exposure. These studies suggest that the influence of individual genetics on cell cycle regulation could modify the degree of early genomic instability in response to complex lesions and potentially define cancer predisposition in response to HZE exposure. These results will have significant implications in estimating cancer susceptibility in genetically variant individuals exposed to HZE particles.

Spindle Assembly Disruption and Cancer Cell Apoptosis with a CLTC-Binding Compound

AK3 compounds are mitotic arrest agents that induce high levels of γH2AX during mitosis and apoptosis following release from arrest. We synthesized a potent AK3 derivative, AK306, that induced arrest and apoptosis of the HCT116 colon cancer cell line with an EC₅₀ of approximately 50 nmol/L. AK306 was active on a broad spectrum of cancer cell lines with total growth inhibition values ranging from approximately 25 nmol/L to 25 μmol/L. Using biotin and BODIPY-linked derivatives of AK306, binding to clathrin heavy chain (CLTC/CHC) was observed, a protein with roles in endocytosis and mitosis. AK306 inhibited mitosis and endocytosis, while disrupting CHC cellular localization. Cells arrested in mitosis by AK306 showed the formation of multiple microtubule-organizing centers consisting of pericentrin, γ-tubulin, and Aurora A foci, without apparent centrosome amplification. Cells released from AK306 arrest were unable to form bipolar spindles, unlike nocodazole-released cells that reformed spindles and completed division. Like AK306, CHC siRNA knockdown disrupted spindle formation and activated p53. A short-term (3-day) treatment of tumor-bearing APC-mutant mice with AK306 increased apoptosis in tumors, but not normal mucosa. These findings indicate that targeting the mitotic CHC complex can selectively induce apoptosis and may have therapeutic value.Implication: Disruption of clathrin with a small-molecule inhibitor, AK306, selectively induces apoptosis in cancer cells by disrupting bipolar spindle formation. Mol Cancer Res; 16(9); 1361-72. ©2018 AACR.

ATM and p53 combined analysis predicts survival in glioblastoma multiforme patients: A clinicopathologic study

Glioblastoma is one of the most malignant cancers, with a distinguishing dismal prognosis: surgery followed by chemo- and radiotherapy represents the current standard of care, and chemo- and radioresistance underlie disease recurrence and short overall survival of patients suffering from this malignancy. ATM is a kinase activated by autophosphorylation upon DNA doublestrand breaks arising from errors during replication, byproducts of metabolism, chemotherapy or ionizing radiations; TP53 is one of the most popular tumor suppressor, with a preeminent role in DNA damage response and repair. To study the effects of the immunohistochemical expression of p-ATM and p53 in glioblastoma patients, 21 cases were retrospectively examined. In normal brain tissue, p-ATM was expressed only in neurons; conversely, in tumors cells, the protein showed a variable cytoplasmic expression (score: +,++,+++), with being completely undetectable in three cases. Statistical analysis revealed that high p-ATM score (++/+++) strongly correlated to shorter survival (P = 0.022). No difference in overall survival was registered between p53 normally expressed (NE) and overexpressed (OE) glioblastoma patients (P = 0.669). Survival analysis performed on the results from combined assessment of the two proteins showed that patients with NE p53 /low pATM score had longer overall survival than the NE p53/ high pATM score counterpart. Cox-regression analysis confirmed this finding (HR = 0.025; CI 95% = 0.002-0.284; P = 0.003). Our study outlined the immunohistochemical expression of p-ATM/p53 in glioblastomas and provided data on their possible prognostic/predictive of response role. A "non-oncogene addiction" to ATM for NEp53 glioblastoma could be postulated, strengthening the rationale for development of ATM inhibiting drugs.

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.

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.

Modulation of NKG2D ligand expression and metastasis in tumors by spironolactone via RXRγ activation

The diuretic drug spironolactone up-regulates NKG2D ligand expression in colon cancer cells via activation of the ATM–Chk2–mediated checkpoint pathway to enhance the antitumor function of NK cells.

Tumor metastasis and lack of NKG2D ligand (NKG2DL) expression are associated with poor prognosis in patients with colon cancer. Here, we found that spironolactone (SPIR), an FDA-approved diuretic drug with a long-term safety profile, can up-regulate NKG2DL expression in multiple colon cancer cell lines by activating the ATM–Chk2-mediated checkpoint pathway, which in turn enhances tumor elimination by natural killer cells. SPIR can also up-regulate the expression of metastasis-suppressor genes TIMP2 and TIMP3, thereby reducing tumor cell invasiveness. Although SPIR is an aldosterone antagonist, its antitumor effects are independent of the mineralocorticoid receptor pathway. By screening the human nuclear hormone receptor siRNA library, we identified retinoid X receptor γ (RXRγ) instead as being indispensable for the antitumor functions of SPIR. Collectively, our results strongly support the use of SPIR or other RXRγ agonists with minimal side effects for colon cancer prevention and therapy.

DNA damage associated with mitosis and cytokinesis failure

Mitosis is a highly dynamic process, aimed at separating identical copies of genomic material into two daughter cells. A failure of the mitotic process generates cells that carry abnormal chromosome numbers. Such cells are predisposed to become tumorigenic upon continuous cell division and thus need to be removed from the population to avoid cancer formation. Cells that fail in mitotic progression indeed activate cell death or cell cycle arrest pathways; however, these mechanisms are not well understood. Growing evidence suggests that the formation of de novo DNA damage during and after mitotic failure is one of the causal factors that initiate those pathways. Here, we analyze several distinct malfunctions during mitosis and cytokinesis that lead to de novo DNA damage generation.

Tightrope act: autophagy in stem cell renewal, differentiation, proliferation, and aging

Autophagy is a constitutive lysosomal catabolic pathway that degrades damaged organelles and protein aggregates. Stem cells are characterized by self-renewal, pluripotency, and quiescence; their long life span, limited capacity to dilute cellular waste and spent organelles due to quiescence, along with their requirement for remodeling in order to differentiate, all suggest that they require autophagy more than other cell types. Here, we review the current literature on the role of autophagy in embryonic and adult stem cells, including hematopoietic, mesenchymal, and neuronal stem cells, highlighting the diverse and contrasting roles autophagy plays in their biology. Furthermore, we review the few studies on stem cells, lysosomal activity, and autophagy. Novel techniques to detect autophagy in primary cells are required to study autophagy in different stem cell types. These will help to elucidate the importance of autophagy in stem cells during transplantation, a promising therapeutic approach for many diseases.

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

The ATM kinase plays a critical role in the maintenance of genetic stability. ATM is activated in response to DNA damage and is essential for cell-cycle checkpoints. Here, we report that ATM is activated in mitosis in the absence of DNA damage. We demonstrate that mitotic ATM activation is dependent on the Aurora-B kinase and that Aurora-B phosphorylates ATM on serine 1403. This phosphorylation event is required for mitotic ATM activation. Further, we show that loss of ATM function results in shortened mitotic timing and a defective spindle checkpoint, and that abrogation of ATM Ser1403 phosphorylation leads to this spindle checkpoint defect. We also demonstrate that mitotically activated ATM phosphorylates Bub1, a critical kinetochore protein, on Ser314. ATM-mediated Bub1 Ser314 phosphorylation is required for Bub1 activity and is essential for the activation of the spindle checkpoint. Collectively, our data highlight mechanisms of a critical function of ATM in mitosis.

Mitosis hit with an ATM transaction fee: aurora B-mediated activation of ATM during mitosis

In this issue of Molecular Cell, Yang et al. (2011) demonstrate that Aurora B phosphorylates ATM, leading to its mitotic activation and ability to phosphorylate Bub1 and regulate the spindle checkpoint, thus maintaining genomic integrity.

Diverse system stresses: common mechanisms of chromosome fragmentation

Chromosome fragmentation (C-Frag) is a newly identified MCD (mitotic cell death), distinct from apoptosis and MC (mitotic catastrophe). As different molecular mechanisms can induce C-Frag, we hypothesize that the general mechanism of its induction is a system response to cellular stress. A clear link between C-Frag and diverse system stresses generated from an array of molecular mechanisms is shown. Centrosome amplification, which is also linked to diverse mechanisms of stress, is shown to occur in association with C-Frag. This led to a new model showing that diverse stresses induce common, MCD. Specifically, different cellular stresses target the integral chromosomal machinery, leading to system instability and triggering of MCD by C-Frag. This model of stress-induced cell death is also applicable to other types of cell death. The current study solves the previously confusing relationship between the diverse molecular mechanisms of chromosome pulverization, suggesting that incomplete C-Frag could serve as the initial event responsible for forms of genome chaos including chromothripsis. In addition, multiple cell death types are shown to coexist with C-Frag and it is more dominant than apoptosis at lower drug concentrations. Together, this study suggests that cell death is a diverse group of highly heterogeneous events that are linked to stress-induced system instability and evolutionary potential.

The role of LKB1 and AMPK in cellular responses to stress and damage

The LKB1 and AMPK proteins participate in an energy sensing cascade that responds to depletion of ATP, serving as a master regulator of metabolism that inhibits anabolic processes and stimulates catabolic processes. However in recent years, LKB1 and AMPK have been implicated in a variety of other cellular processes, both cytoplasmic and nuclear, such as control of cell polarity and regulation of gene transcription. In this review, we summarize the most recent discoveries regarding participation of LKB1 and AMPK in signaling pathways that respond to cellular stress and damage, and the relevance of this signaling for disease and therapy.

The transcription factor SP1 regulates centriole function and chromosomal stability through a functional interaction with the mammalian target of rapamycin/raptor complex

Specificity protein 1 (SP1) is an essential transcription factor implicated in the regulation of genes that control multiple cellular processes, including cell cycle, apoptosis, and DNA damage. Very few nontranscriptional roles for SP1 have been reported thus far. Using confocal microscopy and centrosome fractionation, we identified SP1 as a centrosomal protein. Sp1-deficient mouse embryonic fibroblasts and cells depleted of SP1 by RNAi have increased centrosome number associated with centriole splitting, decreased microtubule nucleation, chromosome misalignment, formation of multipolar mitotic spindles and micronuclei, and increased incidence of aneuploidy. Using mass spectrometry, we identified P70S6K, an effector of the mTOR/raptor (mTORC1) kinase complex, as a novel interacting protein of SP1. We found that SP1-deficient cells have increased phosphorylation of the P70S6K effector ribosomal protein S6, suggesting that SP1 participates in the regulation of the mTORC1/P70S6K/S6 signaling pathway. We previously reported that aberrant mTORC1 activation leads to supernumerary centrosomes, a phenotype rescued by the mTORC1 inhibitor rapamycin. Similarly, treatment with rapamycin rescued the multiple centrosome phenotype of SP1-deficient cells. Taken together, these data strongly support the hypothesis that SP1 is involved in the control of centrosome number via regulation of the mTORC1 pathway, and predict that loss of SP1 function can lead to aberrant centriole splitting, deregulated mTORC1 signaling, and aneuploidy, thereby contributing to malignant transformation.

The alternative TrkAIII splice variant targets the centrosome and promotes genetic instability

The hypoxia-regulated alternative TrkAIII splice variant expressed by human neuroblastomas exhibits oncogenic potential, driven by in-frame exon 6 and 7 alternative splicing, leading to omission of the receptor extracellular immunoglobulin C(1) domain and several N-glycosylation sites. Here, we show that the TrkAIII oncogene promotes genetic instability by interacting with and exhibiting catalytic activity at the centrosome. This function depends upon intracellular TrkAIII accumulation and spontaneous interphase-restricted activation, in cytoplasmic tyrosine kinase (tk) domain orientation, predominantly within structures that closely associate with the fully assembled endoplasmic reticulum intermediate compartment and Golgi network. This facilitates TrkAIII tk-mediated binding of gamma-tubulin, which is regulated by endogenous protein tyrosine phosphatases and geldanamycin-sensitive interaction with Hsp90, paving the way for TrkAIII recruitment to the centrosome. At the centrosome, TrkAIII differentially phosphorylates several centrosome-associated components, increases centrosome interaction with polo kinase 4, and decreases centrosome interaction with separase, the net results of which are centrosome amplification and increased genetic instability. The data characterize TrkAIII as a novel internal membrane-associated centrosome kinase, unveiling an important alternative mechanism to "classical" cell surface oncogenic receptor tk signaling through which stress-regulated alternative TrkAIII splicing influences the oncogenic process.

Phosphorylation of p53 on Ser15 during cell cycle caused by Topo I and Topo II inhibitors in relation to ATM and Chk2 activation

The DNA topoisomerase I (topo1) inhibitor topotecan (TPT) and topo2 inhibitor mitoxantrone (MXT) damage DNA inducing formation of DNA double-strand breaks (DSBs). We have recently examined the kinetics of ATM and Chk2 activation as well as histone H2AX phosphorylation, the reporters of DNA damage, in individual human lung adenocarcinoma A549 cells treated with these drugs. Using a phospho-specific Ab to tumor suppressor protein p53 phosphorylated on Ser15 (p53-Ser15(P)) combined with an Ab that detects p53 regardless of the phosphorylation status and multiparameter cytometry we correlated the TPT- and MXT-induced p53-Ser15(P) with ATM and Chk2 activation as well as with H2AX phosphorylation in relation to the cell cycle phase. In untreated interphase cells, p53-Ser15(P) had "patchy" localization throughout the nucleoplasm while mitotic cells showed strong p53-Ser15(P) cytoplasmic immunofluorescence (IF). The intense phosphorylation of p53-Ser15, combined with activation of ATM and Chk2 (involving centrioles) as well as phosphorylation of H2AX seen in the untreated mitotic cells, suggest mobilization of the DNA damage detection/repair machinery in controlling cytokinesis. In the nuclei of cells treated with TPT or MXT, the expression of p53-Ser15(P) appeared as closely packed foci of intense IF. Following TPT treatment, the induction of p53-Ser15(P) was most pronounced in S-phase cells while no significant cell cycle phase differences were seen in cells treated with MXT. The maximal increase in p53-Ser15(P) expression, rising up to 2.5-fold above the level of its constitutive expression, was observed in cells treated with TPT or MXT for 4-6 h. This maximum expression of p53-Ser15(P) coincided in time with the peak of Chk2 activation but not with ATM activation and H2AX phosphorylation, both of which crested 1-2 h after the treatment with TPT or MXT. The respective kinetics of p53-Ser15 phosphorylation versus ATM and Chk2 activation suggest that in response to DNA damage by TPT or MXT, Chk2 rather than ATM mediates p53 phosphorylation.

Kinetics of histone H2AX phosphorylation and Chk2 activation in A549 cells treated with topotecan and mitoxantrone in relation to the cell cycle phase

The DNA topoisomerase I (topo1) inhibitor topotecan (TPT) and topo2 inhibitors doxorubicin, etoposide and mitoxantrone (MXT) are widely used antitumor drugs. They stabilize otherwise transient ("cleavable") complexes of topo1 or topo2 with DNA, respectively. Collisions of DNA replication forks (during replication) or progressing RNA polymerase molecules (during transcription) with these complexes convert them into double-strand DNA breaks (DSBs). Formation of DSBs triggers activation of ATM and phosphorylation of histone H2AX, the markers that have been used to correlate DNA damage with cell cycle phase or induction of apoptosis. In the present study we explored a relationship between H2AX phosphorylation and activation of checkpoint kinase 2 (Chk2) in human lung carcinoma A549 cells treated with TPT or with MXT. Activation of Chk2 was detected immunocytochemically using a phospho-specific (Thr68) Ab and measuring Chk2-Thr68(P)immunofluorescence (IF), concurrently with DNA content, by laser scanning cytometry. In the untreated cells, activated Chk2 was present predominantly in centrosomes. Upon treatment with TPT or MTX, the activated Chk2 presented itself in form of either minute or large IF foci in the cell's nucleoplasm. H2AX phosphorylation whether induced by TPT or MXT was rapid, with the maximal rate occurring during the initial 2 h and peaking at 2 h of treatment. TPT or MXT induced Chk2 activation occurred at a distinctly slower pace, peaking at 4 h. While TPT-induced H2AX phosphorylation and Chk2 activation were maximal in S-phase cells, Chk2 activation was also much pronounced in G(2)M cells; the least affected by TPT were G(1) cells. MTX-induced H2AX phosphorylation was maximal in G(1) cells while Chk2 activation was maximal in G(2)M and minimal in G(1) cells. The pattern of cell-cycle phase specific response to TPT or MXT by H2AX phosphorylation and Chk2 activation was different when measured either as integrated or maximal pixel of gammaH2AX or Chk2-Thr68(P) IF, the former reflecting total IF per nucleus the latter stressing the punctate (foci) character of expression of these phospho-modified proteins.

The presence of high-risk chromosome aberrations in chronic lymphocytic leukaemia does not correlate with centrosome aberrations

Chromosome aberrations are frequently found in B-cell chronic lymphocytic leukaemia (B-CLL), and specific chromosome aberrations identify poor prognostic subgroups. Almost all the aberrations identified in B-CLL involve loci where genes with a role in the regulation of centrosome duplication have been mapped. Centrosome aberrations have been described as a possible cause of numerical chromosome abnormalities in both solid and haematological tumours. However, little is known about the possible role of centrosome aberrations in B-CLL. To investigate whether centrosome aberrations do occur in B-CLL and correlate with cytogenetically defined prognostic subgroups, we examined a set of 64 B-CLL samples by immunofluorescent staining. B-CLL cases differed significantly from controls in the mean frequency of cells with centrosome aberrations, while no difference was found between subgroups with or without specific chromosome aberrations. Our results indicated that although centrosome aberrations were a common feature in B-CLL, they did not represent a reliable prognostic marker.

Genomic instability and increased expression of BUB1B and MAD2L1 genes in ductal breast carcinoma

In a series of invasive ductal breast carcinoma, we investigated the status of chromosomal and intrachromosomal instability by fluorescence in situ hybridisation and determined the level of mRNA expression for two genes involved in the mitotic spindle checkpoint pathway, BUB1B and MAD2L1. All breast cancers demonstrated higher chromosomal instability rates in tumor samples (average: 56.86%, range: 36.24-76.78%) than in controls (average: 11.54%, range: 9.91-14.84%) (P<0.0001). As well as intrachromosomal instability rates were elevated in tumor (average: 18.45% range: 8.34-35.8%) as compared with controls (average: 4.18% range: 3.47-4.81%) (P<0.0001). An increase in BUB1B and MAD2L1 transcripts was demonstrated in the majority of the tumor tested. BUB1B mRNA levels but not MAD2L1 levels correlated with intrachromosomal instability (r=0.722, P=0.018).

Centrosomal localization of DNA damage checkpoint proteins

During mitosis, the phosphatidylinositol-3 (PI-3) family-related DNA damage checkpoint kinases ATM and ATR were found on the centrosomes of human cells. ATRIP, an interaction partner of ATR, as well as Chk1 and Chk2, the downstream targets of ATR or ATM, were also localized to the centrosomes. Surprisingly, the DNA-PK inhibitor vanillin enhanced the level of ATM on centrosomes. Accordingly, DNA-PKcs, the catalytic subunit of DNA-PK, was also found on the centrosomes. Vanillin altered the phosphorylation of Chk2 in the centrosomes and in whole cell extracts. Nucleoplasmic ATM co-immunoprecipitated with Ku70/86, the DNA binding subunits of DNA-PK, while vanillin diminished this association. Vanillin did not affect microtubule polymerization at the centrosomes but, surprisingly, caused a transient enhancement of alpha-tubulin foci in the nucleus. Interestingly, gamma-tubulin was also present in the nucleus and co-immunoprecipitated with ATR or BRCA1. DNA damage led to a reduction of the mentioned checkpoint proteins on the centrosomes but increased the level of gamma-tubulin at this organelle. Taken together, these results indicate that DNA damage checkpoint proteins may control the formation of gamma-tubulin and/or the kinetics of microtubule formation at the centrosomes, and thereby couple them to the DNA damage response.