ATR- and ATM-Mediated DNA Damage Response Is Dependent on Excision Repair Assembly during G1 but Not in S Phase of Cell Cycle

Modulating Excited State Properties and Ligand Ejection Kinetics in Ruthenium Polypyridyl Complexes Designed to Mimic Photochemotherapeutics

Ruthenium(II) polypyridyl complexes have gained significant interest as photochemotherapeutics (PCTs) due to their synthetic viability, strong light absorption, well understood excited state properties, and high phototoxicity indexes. Herein, we report the synthesis, characterization, electrochemical, spectrochemical, and preliminary cytotoxicity analyses of three series of ruthenium(II) polypyridyl complexes designed to mimic PCTs. The three series have the general structure of [Ru(bpy)2(N-N)]2+ (Series 1), [Ru(bpy)(dmb)(N-N)]2+ (Series 2), and [Ru(dmb)2(N-N)]2+ (Series 3, where N-N is a bidentate polypyridyl ligand, bpy = 2,2'-bipyridine, and dmb = 6,6'-dimethyl-2,2'-bipyridine). In the three series, the N-N ligand was systematically modified to incorporate increased conjugation and/or electronegative heteroatoms to increase dπ-π* backbonding, red-shifting the lowest energy metal-to-ligand charge transfer (MLCT) absorptions from λmax = 454 to λmax = 580 nm, nearing the therapeutic window for PCTs (600-1100 nm). In addition, steric bulk was systematically introduced through the series, distorting the Ru(II) octahedra, making the dissociative 3dd* state thermally accessible at room and body temperatures. This resulted in a 4 orders of magnitude increase in photoinduced ligand ejection kinetics, and demonstrates the ability to modulate both the MLCT* and dd* manifolds in the complexes, which is critical in PCT drug design. Preliminary cell viability assays suggest that the increased steric bulk to lower the 3dd* states may interfere with the cytotoxicity mechanism, limiting photoinitiated toxicity of the complexes. This work demonstrates the importance of understanding both the MLCT* and dd* manifolds and how they impact the ability of a complex to act as a PCT agent.

DNA Double Strand Break and Response Fluorescent Assays: Choices and Interpretation

Accurately characterizing DNA double-stranded breaks (DSBs) and understanding the DNA damage response (DDR) is crucial for assessing cellular genotoxicity, maintaining genomic integrity, and advancing gene editing technologies. Immunofluorescence-based techniques have proven to be invaluable for quantifying and visualizing DSB repair, providing valuable insights into cellular repair processes. However, the selection of appropriate markers for analysis can be challenging due to the intricate nature of DSB repair mechanisms, often leading to ambiguous interpretations. This comprehensively summarizes the significance of immunofluorescence-based techniques, with their capacity for spatiotemporal visualization, in elucidating complex DDR processes. By evaluating the strengths and limitations of different markers, we identify where they are most relevant chronologically from DSB detection to repair, better contextualizing what each assay represents at a molecular level. This is valuable for identifying biases associated with each assay and facilitates accurate data interpretation. This review aims to improve the precision of DSB quantification, deepen the understanding of DDR processes, assay biases, and pathway choices, and provide practical guidance on marker selection. Each assay offers a unique perspective of the underlying processes, underscoring the need to select markers that are best suited to specific research objectives.

Bi-allelic TTI1 variants cause an autosomal-recessive neurodevelopmental disorder with microcephaly

Telomere maintenance 2 (TELO2), Tel2 interacting protein 2 (TTI2), and Tel2 interacting protein 1 (TTI1) are the three components of the conserved Triple T (TTT) complex that modulates activity of phosphatidylinositol 3-kinase-related protein kinases (PIKKs), including mTOR, ATM, and ATR, by regulating the assembly of mTOR complex 1 (mTORC1). The TTT complex is essential for the expression, maturation, and stability of ATM and ATR in response to DNA damage. TELO2- and TTI2-related bi-allelic autosomal-recessive (AR) encephalopathies have been described in individuals with moderate to severe intellectual disability (ID), short stature, postnatal microcephaly, and a movement disorder (in the case of variants within TELO2). We present clinical, genomic, and functional data from 11 individuals in 9 unrelated families with bi-allelic variants in TTI1. All present with ID, and most with microcephaly, short stature, and a movement disorder. Functional studies performed in HEK293T cell lines and fibroblasts and lymphoblastoid cells derived from 4 unrelated individuals showed impairment of the TTT complex and of mTOR pathway activity which is improved by treatment with Rapamycin. Our data delineate a TTI1-related neurodevelopmental disorder and expand the group of disorders related to the TTT complex.

Roles of trans-lesion synthesis (TLS) DNA polymerases in tumorigenesis and cancer therapy

DNA damage tolerance and mutagenesis are hallmarks and enabling characteristics of neoplastic cells that drive tumorigenesis and allow cancer cells to resist therapy. The 'Y-family' trans-lesion synthesis (TLS) DNA polymerases enable cells to replicate damaged genomes, thereby conferring DNA damage tolerance. Moreover, Y-family DNA polymerases are inherently error-prone and cause mutations. Therefore, TLS DNA polymerases are potential mediators of important tumorigenic phenotypes. The skin cancer-propensity syndrome xeroderma pigmentosum-variant (XPV) results from defects in the Y-family DNA Polymerase Pol eta (Polη) and compensatory deployment of alternative inappropriate DNA polymerases. However, the extent to which dysregulated TLS contributes to the underlying etiology of other human cancers is unclear. Here we consider the broad impact of TLS polymerases on tumorigenesis and cancer therapy. We survey the ways in which TLS DNA polymerases are pathologically altered in cancer. We summarize evidence that TLS polymerases shape cancer genomes, and review studies implicating dysregulated TLS as a driver of carcinogenesis. Because many cancer treatment regimens comprise DNA-damaging agents, pharmacological inhibition of TLS is an attractive strategy for sensitizing tumors to genotoxic therapies. Therefore, we discuss the pharmacological tractability of the TLS pathway and summarize recent progress on development of TLS inhibitors for therapeutic purposes.

The XPA Protein-Life under Precise Control

Nucleotide excision repair (NER) is a central DNA repair pathway responsible for removing a wide variety of DNA-distorting lesions from the genome. The highly choreographed cascade of core NER reactions requires more than 30 polypeptides. The xeroderma pigmentosum group A (XPA) protein plays an essential role in the NER process. XPA interacts with almost all NER participants and organizes the correct NER repair complex. In the absence of XPA's scaffolding function, no repair process occurs. In this review, we briefly summarize our current knowledge about the XPA protein structure and analyze the formation of contact with its protein partners during NER complex assembling. We focus on different ways of regulation of the XPA protein's activity and expression and pay special attention to the network of post-translational modifications. We also discuss the data that is not in line with the currently accepted hypothesis about the functioning of the XPA protein.

The Endoplasmic Reticulum Stress Sensor IRE1α Regulates the UV DNA Repair Response through the Control of Intracellular Calcium Homeostasis

The unfolded protein response is activated by UVB irradiation, but the role of a key mediator, IRE1α, is not clear. In this study, we show that mice with an epidermal IRE1α deletion are sensitized to UV with increased apoptosis, rapid loss of UV-induced cyclopyrimidine dimer‒positive keratinocytes, and sloughing of the epidermis. In vitro, Ire1α-deficient keratinocytes have increased UVB sensitivity, reduced cyclopyrimidine dimer repair, and reduced accumulation of γH2AX and phosphorylated ATR, suggesting defective activation of nucleotide excision repair. Knockdown of XBP1 or pharmacologic inhibition of the IRE1α ribonuclease did not phenocopy Ire1α deficiency. The altered UV response was linked to elevated intracellular calcium levels and ROS, and this was due to dysregulation of the endoplasmic reticulum calcium channel InsP3R. Pharmacologic, genetic, and biochemical studies linked the regulation of the Ins3PR, intracellular calcium, and normal UV DNA damage response to CIB1 and the IRE1α‒TRAF2‒ASK1 complex. These results suggest a model where IRE1α activation state drives CIB1 binding either to the InsP3R or ASK1 to regulate endoplasmic reticulum calcium efflux, ROS, and DNA repair responses after UV irradiation.

Long-wavelength UVA enhances UVB-induced cell death in cultured keratinocytes: DSB formation and suppressed survival pathway

Solar UV radiation consists of both UVA and UVB. The wavelength-specific molecular responses to UV radiation have been studied, but the interaction between UVA and UVB has not been well understood. In this study, we found that long-wavelength UVA, UVA1, augmented UVB-induced cell death, and examined the underlying mechanisms. Human keratinocytes HaCaT were exposed to UVA1, followed by UVB irradiation. Irradiation by UVA1 alone showed no effect on cell survival, whereas the UVA1 pre-irradiation remarkably enhanced UVB-induced cell death. UVA1 delayed the repair of pyrimidine dimers formed by UVB and the accumulation of nucleotide excision repair (NER) proteins to damaged sites. Gap synthesis during NER was also decreased, suggesting that UVA1 delayed NER, and unrepaired pyrimidine dimers and single-strand breaks generated in the process of NER were left behind. Accumulation of this unrepaired DNA damage might have led to the formation of DNA double-strand breaks (DSBs), as was detected using gel electrophoresis analysis and phosphorylated histone H2AX assay. Combined exposure enhanced the ATM-Chk2 signaling pathway, but not the ATR-Chk1 pathway, confirming the enhanced formation of DSBs. Moreover, UVA1 suppressed the UVB-induced phosphorylation of Akt, a survival signal pathway. These results indicated that UVA1 influenced the repair of UVB-induced DNA damage, which resulted in the formation of DSBs and enhanced cell death, suggesting the risk of simultaneous exposure to high doses of UVA1 and UVB.

Therapeutic wavelengths of ultraviolet B radiation activate apoptotic, circadian rhythm, redox signalling and key canonical pathways in psoriatic epidermis

Ultraviolet B radiation (UVB) exerts pleiotropic effects on human skin. DNA damage response and repair pathways are activated by UVB; if damage cannot be repaired, apoptosis ensues. Although cumulative UVB exposure predisposes to skin cancer, UVB phototherapy is widely used as an effective treatment for psoriasis. Previous studies defined the therapeutic action spectrum of UVB and showed that psoriasis is resistant to apoptosis. This study aimed to investigate early molecular responses within psoriasis plaques following irradiation with single equi-erythemogenic doses of clinically-effective (311 nm, narrow-band) compared to clinically-ineffective (290 nm) UVB. Forty-eight micro-dissected epidermal samples from 20 psoriatic patients were analyzed using microarrays. Our bioinformatic analysis compared gene expression between 311 nm irradiated, 290 nm irradiated and control psoriasis epidermis to specifically identify 311 nm UVB differentially expressed genes (DEGs) and their upstream regulatory pathways. Key DEGs and pathways were validated by immunohistochemical analysis. There was a dynamic induction and repression of 311 nm UVB DEGs between 6 h and 18 h, only a limited number of DEGs maintained their designated expression status between time-points. Key disease and function pathways included apoptosis, cell death, cell migration and leucocyte chemotaxis. DNA damage response pathways, NRF2-mediated oxidative stress response and P53 signalling were key nodes, interconnecting apoptosis and cell cycle arrest. Interferon signalling, dendritic cell maturation, granulocyte adhesion and atherosclerotic pathways were also differentially regulated. Consistent with these findings, top transcriptional regulators of 311 nm UVB DEGs related to: a) apoptosis, DNA damage response and cell cycle control; b) innate/acquired immune regulation and inflammation; c) hypoxia/redox response and angiogenesis; d) circadian rhythmicity; f) EGR/AP1 signalling and keratinocyte differentiation; and g) mitochondrial biogenesis. This research provides important insights into the molecular targets of 311 nm UVB, underscoring key roles for apoptosis and cell death. These and the other key pathways delineated may be central to the therapeutic effects of 311 nm in psoriasis.

ATM and ATR Activation Through Crosstalk Between DNA Damage Response Pathways

Cells losing the ability to self-regulate in response to damage are a hallmark of cancer. When a cell encounters damage, regulatory pathways estimate the severity of damage and promote repair, cell cycle arrest, or apoptosis. This decision-making process would be remarkable if it were based on the total amount of damage in the cell, but because damage detection pathways vary in the rate and intensity with which they promote pro-apoptotic factors, the cell's real challenge is to reconcile dissimilar signals. Crosstalk between repair pathways, crosstalk between pro-apoptotic signaling kinases, and signals induced by damage by-products complicate the process further. The cell's response to [Formula: see text] and UV radiation neatly illustrates this concept. While these forms of radiation produce lesions associated with two different pro-apoptotic signaling kinases, ATM and ATR, recent experiments show that ATM and ATR react to both forms of radiation. To simulate the pro-apoptotic signal induced by [Formula: see text] and UV radiation, we construct a mathematical model that includes three modes of crosstalk between ATM and ATR signaling pathways: positive feedback between ATM/ATR and repair proteins, ATM and ATR mutual upregulation, and changes in lesion topology induced by replication stress or repair. We calibrate the model to agree with 21 experimental claims about ATM and ATR crosstalk. We alter the model by adding or removing specific processes and then examine the effects of each process on ATM/ATR crosstalk by recording which claims the altered model violates. Not only is this the first mathematical model of ATM/ATR crosstalk, it provides a strong argument for treating pro-apoptotic signaling as a holistic effort rather than attributing it to a single dominant kinase.

RHOAming Through the Nucleotide Excision Repair Pathway as a Mechanism of Cellular Response Against the Effects of UV Radiation

Typical Rho GTPases include the enzymes RhoA, Rac1, and Cdc42 that act as molecular switches to regulate essential cellular processes in eukaryotic cells such as actomyosin dynamics, cell cycle, adhesion, death and differentiation. Recently, it has been shown that different conditions modulate the activity of these enzymes, but their functions still need to be better understood. Here we examine the interplay between RhoA and the NER (Nucleotide Excision Repair) pathway in human cells exposed to UVA, UVB or UVC radiation. The results show high levels and accumulation of UV-induced DNA lesions (strand breaks and cyclobutane pyrimidine dimers, CPDs) in different cells with RhoA loss of function (LoF), either by stable overexpression of negative dominant RhoA (RhoA-N19 mutant), by inhibition with C3 toxin or by transient silencing with siRNA. Cells under RhoA LoF showed reduced levels of γH2AX, p-Chk1 (Ser345) and p-p53 (Ser15) that reflected causally in their accumulation in G1/S phases, in low survival rates and in reduced cell proliferation, also in accordance with the energy of applied UV light. Even NER-deficient cells (XPA, XPC) or DNA translesion synthesis (TLS)-deficient cells (XPV) showed substantial hypersensitivity to UV effects when previously submitted to RhoA LoF. In contrast, analyses of apoptosis, necrosis, autophagy and senescence revealed that all cells displaying normal levels of active RhoA (RhoA-GTP) are more resistant to UV-promoted cell death. This work reaffirms the role of RhoA protein signaling in protecting cells from damage caused by UV radiation and demonstrates relevant communicating mechanisms between actin cytoskeleton and genomic stability.

DCTPP1 prevents a mutator phenotype through the modulation of dCTP, dTTP and dUTP pools

To maintain dNTP pool homeostasis and preserve genetic integrity of nuclear and mitochondrial genomes, the synthesis and degradation of DNA precursors must be precisely regulated. Human all-alpha dCTP pyrophosphatase 1 (DCTPP1) is a dNTP pyrophosphatase with high affinity for dCTP and 5'-modified dCTP derivatives, but its contribution to overall nucleotide metabolism is controversial. Here, we identify a central role for DCTPP1 in the homeostasis of dCTP, dTTP and dUTP. Nucleotide pools and the dUTP/dTTP ratio are severely altered in DCTPP1-deficient cells, which exhibit an accumulation of uracil in genomic DNA, the activation of the DNA damage response and both a mitochondrial and nuclear hypermutator phenotype. Notably, DNA damage can be reverted by incubation with thymidine, dUTPase overexpression or uracil-DNA glycosylase suppression. Moreover, DCTPP1-deficient cells are highly sensitive to down-regulation of nucleoside salvage. Our data indicate that DCTPP1 is crucially involved in the provision of dCMP for thymidylate biosynthesis, introducing a new player in the regulation of pyrimidine dNTP levels and the maintenance of genomic integrity.

DNA damage signaling in the cellular responses to mustard vesicants

Mustard vesicants, including sulfur mustard (2,2'-dichlorodiethyl sulfide, SM) and nitrogen mustard (bis(2-chloroethyl)methylamine, HN2) are cytotoxic blistering agents synthesized for chemical warfare. Because they contain highly reactive electrophilic chloroethyl side chains, they readily react with cellular macromolecules like DNA forming monofunctional and bifunctional adducts. By targeting DNA, mustards can compromise genomic integrity, disrupt the cell cycle, and cause mutations and cytotoxicity. To protect against genotoxicity following exposure to mustards, cells initiate a DNA damage response (DDR). This involves activation of signaling cascades including ATM (ataxia telangiectasia mutated), ATR (ataxia telangiectasia and Rad3-related) and DNA-PKcs (DNA-dependent protein kinase, catalytic unit). Signaling induced by the DDR leads to the recruitment and activation of repair related proteins such as phospho H2AX and phospho p53 to sites of DNA lesions. Excessive DNA modifications by mustards can overwhelm DNA repair leading to single and double strand DNA breaks, cytotoxicity and tissue damage, sometimes leading to cancer. Herein we summarize DDR signaling pathways induced by SM, HN2 and the half mustard, 2-chloroethyl ethyl sulfide (CEES). At the present time, little is known about how mustard-induced DNA damage leads to the activation of DDR signaling. A better understanding of mechanisms by which mustard vesicants induce the DDR may lead to the development of countermeasures effective in mitigating tissue injury.

Gene Prioritization through Consensus Strategy, Enrichment Methodologies Analysis, and Networking for Osteosarcoma Pathogenesis

Osteosarcoma is the most common subtype of primary bone cancer, affecting mostly adolescents. In recent years, several studies have focused on elucidating the molecular mechanisms of this sarcoma; however, its molecular etiology has still not been determined with precision. Therefore, we applied a consensus strategy with the use of several bioinformatics tools to prioritize genes involved in its pathogenesis. Subsequently, we assessed the physical interactions of the previously selected genes and applied a communality analysis to this protein-protein interaction network. The consensus strategy prioritized a total list of 553 genes. Our enrichment analysis validates several studies that describe the signaling pathways PI3K/AKT and MAPK/ERK as pathogenic. The gene ontology described TP53 as a principal signal transducer that chiefly mediates processes associated with cell cycle and DNA damage response It is interesting to note that the communality analysis clusters several members involved in metastasis events, such as MMP2 and MMP9, and genes associated with DNA repair complexes, like ATM, ATR, CHEK1, and RAD51. In this study, we have identified well-known pathogenic genes for osteosarcoma and prioritized genes that need to be further explored.

ATR kinase inhibition sensitizes quiescent human cells to the lethal effects of cisplatin but increases mutagenesis

The ATR protein kinase is known to protect cells from DNA damage induced during the replicative phase of the cell cycle. Small molecule ATR kinase inhibitors have therefore been developed to improve the effectiveness of DNA damage-based chemotherapy regimens aimed at killing rapidly proliferating tumor cells. However, whether ATR functions in a similar manner in non-replicating cells has not been examined and is important considering the fact that most cells in the body, including cancer stem cells in solid tumors, normally reside in either a quiescent or differentiated non-replicating state. Using cultured human cell lines maintained in a quiescent or slowly growing state in vitro, ATR was found to be activated following treatment with the common anti-cancer drug cisplatin in a manner dependent on the nucleotide excision repair (NER) system. Moreover, treatment with the ATR kinase inhibitors VE-821 and AZD6738 enhanced quiescent cell killing and apoptotic signaling induced by cisplatin. However, ATR kinase inhibition in quiescent cells treated with a low concentration of cisplatin also elevated the level of mutagenesis at the hypoxanthine phosphoribosyltransferase locus and resulted in increased levels of PCNA mono-ubiquitination. These results suggest that the excision gaps generated by NER may require a greater utilization of potentially mutagenic translesion synthesis polymerases in the absence of ATR kinase function. Thus, though ATR kinase inhibitors can aid in the killing of cisplatin-treated quiescent cells, such treatments may also result in a greater reliance on alternative mutagenic DNA polymerases to complete the repair of cisplatin-DNA adducts.

Formaldehyde inhibits UV-induced phosphorylation of histone H2AX

Formaldehyde (FA) is widely known to cause DNA damage. Recently, our study showed that FA can also inhibit a repair process of DNA damage, nucleotide excision repair (NER). DNA damage response (DDR) involving activation of phosphorylation pathways is important for the accuracy of the repair process, and the inhibition of the accurate repair would raise mutation rate, leading to cancer. We herein investigated whether FA influences phosphorylation of histone H2AX (γ-H2AX), an intermediate player of DDR signaling pathways. Human keratinocytes HaCaT were treated with FA and then exposed to UV known to generate clear γ-H2AX signal. UV-induced γ-H2AX was inhibited by FA in a dose-dependent manner. The repair of pyrimidine dimers was inhibited by FA, while the recruitments of γ-H2AX-related proteins, Mre11 and 53BP1, to damaged sites were also delayed. Mre11, Nbs-1, H2AX and ATM were not degraded after treatment with FA as opposed to NER-related protein, TFIIH. On the other hand, FA inhibited phosphorylation of ATM which acts upstream of γ-H2AX. These results suggest that FA can affect the repair of DNA damage via inhibition of the phosphorylation pathways of H2AX.

Polynuclear ruthenium organometallic compounds induce DNA damage in human cells identified by the nucleotide excision repair factor XPC

Ruthenium organometallic compounds represent an attractive avenue in developing alternatives to platinum-based chemotherapeutic agents. While evidence has been presented indicating ruthenium-based compounds interact with isolated DNA in vitro, it is unclear what effect these compounds exert in cells. Moreover, the antibiotic efficacy of polynuclear ruthenium organometallic compounds remains uncertain. In the present study, we report that exposure to polynuclear ruthenium organometallic compounds induces recruitment of damaged DNA sensing protein Xeroderma pigmentosum Group C into chromatin-immobilized foci. Additionally, we observed one of the tested polynuclear ruthenium organometallic compounds displayed increased cytotoxicity against human cells deficient in nucleotide excision repair (NER). Taken together, these results suggest that polynuclear ruthenium organometallic compounds induce DNA damage in cells, and that cellular resistance to these compounds may be influenced by the NER DNA repair phenotype of the cells.

Damage removal and gap filling in nucleotide excision repair

The nucleotide excision repair (NER) system removes a variety of types of helix-distorting lesions from DNA through a dual incision mechanism, in which the damaged nucleotide bases are excised in the form of a small, excised, damage-containing single-stranded DNA oligonucleotide (sedDNA). Damage removal leaves a gap in the DNA template that must then be filled in by the action of a DNA polymerase and ligated to the downstream phosphodiester backbone in the DNA to complete the repair reaction. Defects in damage removal, sedDNA processing, or gap filling have the potential to be mutagenic and lethal to cells, and thus several human pathologies, including cancer and aging, are associated with defects in NER. This review summarizes our current understanding of NER with a focus on the enzymes that excise sedDNAs and restore the duplex DNA to its native state in human cells.

Discovery and Characterization of 4-Hydroxy-2-pyridone Derivative Sambutoxin as a Potent and Promising Anticancer Drug Candidate: Activity and Molecular Mechanism

Sambutoxin, a representative derivative of 4-hydroxy-2-pyridone, was isolated from Hericium alpestre for the first time in this study. The possible correlation between the sambutoxin-induced suppression of tumor growth and its influence on cell-cycle arrest and apoptosis was investigated. The effects of sambutoxin on reactive oxygen species (ROS) production, DNA damage, mitochondrial transmembrane potential, cell apoptosis, and the expression of related proteins were evaluated. An in vitro cell viability study demonstrated that sambutoxin could inhibit the proliferation of various cancer cells. Treatment with sambutoxin induced the production of ROS, which caused DNA damage. Furthermore, the subsequent sambutoxin-induced activation of ATM and Chk2 resulted in G2/M arrest, accompanied by decreased expression of cdc25C, cdc2, and cyclin B1. Sambutoxin induced apoptosis by activating the mitochondrial apoptosis pathway through an increased Bax/Bcl-2 ratio, loss of mitochondrial membrane potential (ΔΨm), cytochrome (Cyt) c release, caspase-9 and caspase-3 activation, and poly(ADP-ribose) polymerase (PARP) degradation. The ROS elevation induced the sustained phosphorylation of c-Jun N-terminal kinase (JNK), while SP600125, a JNK inhibitor, nearly completely reversed sambutoxin-induced apoptosis. Accordingly, an in vivo study showed that sambutoxin exhibited potential antitumor activity in a BALB/c nude mouse xenograft model without significant systemic toxicity. Moreover, the expression changes in proteins related to the G2/M phase, DNA damage, and apoptosis in vivo were consistent with those in vitro. Importantly, sambutoxin has remarkable antiproliferative effects and is a promising anticarcinogen candidate for cancer treatment.

Roles of UVA radiation and DNA damage responses in melanoma pathogenesis

The growing incidence of melanoma is a serious public health issue that merits a thorough understanding of potential causative risk factors, which includes exposure to ultraviolet radiation (UVR). Though UVR has been classified as a complete carcinogen and has long been recognized for its ability to damage genomic DNA through both direct and indirect means, the precise mechanisms by which the UVA and UVB components of UVR contribute to the pathogenesis of melanoma have not been clearly defined. In this review, we therefore highlight recent studies that have addressed roles for UVA radiation in the generation of DNA damage and in modulating the subsequent cellular responses to DNA damage in melanocytes, which are the cell type that gives rise to melanoma. Recent research suggests that UVA not only contributes to the direct formation of DNA lesions but also impairs the removal of UV photoproducts from genomic DNA through oxidation and damage to DNA repair proteins. Moreover, the melanocyte microenvironment within the epidermis of the skin is also expected to impact melanomagenesis, and we therefore discuss several paracrine signaling pathways that have been shown to impact the DNA damage response in UV-irradiated melanocytes. Lastly, we examine how alterations to the immune microenvironment by UVA-associated DNA damage responses may contribute to melanoma development. Thus, there appear to be multiple avenues by which UVA may elevate the risk of melanoma. Protective strategies against excess exposure to UVA wavelengths of light therefore have the potential to decrease the incidence of melanoma. Environ. Mol. Mutagen. 59:438-460, 2018. © 2018 Wiley Periodicals, Inc.

Kaempferol Suppresses Proliferation and Induces Cell Cycle Arrest, Apoptosis, and DNA Damage in Breast Cancer Cells

Kaempferol is a flavonoid that has been extensively investigated owing to its antitumor effects. Nevertheless, little is known about its underlying mechanisms of action. We aimed to explore the role of kaempferol in breast cancer (BC), and thus we investigated how kaempferol suppresses the growth of BC cells. The cells were treated with kaempferol, and the effects on multiple cancer-associated pathways were evaluated. The MTS assay was used to study the cell growth inhibition induced by kaempferol. The cell cycle was analyzed by flow cytometry. Western blotting was used to analyze cellular apoptosis and DNA damage. We found that the proliferation of the triple-negative BC (TNBC) MDA-MB-231 cells was suppressed effectively by kaempferol. Interestingly, the suppressive effect of kaempferol on cell proliferation was stronger in MDA-MB-231 cells than in the estrogen receptor-positive BT474 cell line. Furthermore, after the treatment with kaempferol for 48 h, the population of cells in the G₁ phase was significantly reduced, from 85.48% to 51.35%, and the population of cells in the G₂ phase increased markedly from 9.27% to 37.5%, which indicated that kaempferol contributed to the induction of G₂/M arrest. Kaempferol also induced apoptosis and DNA damage in MDA-MB-231 cells. Kaempferol increased the expression levels of γH2AX, cleaved caspase 9, cleaved caspase 3, and p-ATM compared to those of the control group. Collectively, these results showed that kaempferol may be a potential drug for the effective treatment of TNBC.

Loss of Genome Fidelity: Beta HPVs and the DNA Damage Response

While the role of genus alpha human papillomaviruses in the tumorigenesis and tumor maintenance of anogenital and oropharyngeal cancers is well-established, the role of genus beta human papilloviruses (β-HPVs) in non-melanoma skin cancers (NMSCs) is less certain. Persistent β-HPV infections cause NMSCs in sun-exposed skin of people with a rare genetic disorder, epidermodysplasia verruciformis. However, β-HPV infections in people without epidermodysplasia verruciformis are typically transient. Further, β-HPV gene expression is not necessary for tumor maintenance in the general population as on average there is fewer than one copy of the β-HPV genome per cell in NMSC tumor biopsies. Cell culture, epidemiological, and mouse model experiments support a role for β-HPV infections in the initiation of NMSCs through a "hit and run" mechanism. The virus is hypothesized to act as a cofactor, augmenting the genome destabilizing effects of UV. Supporting this idea, two β-HPV proteins (β-HPV E6 and E7) disrupt the cellular response to UV exposure and other genome destabilizing events by abrogating DNA repair and deregulating cell cycle progression. The aberrant damage response increases the likelihood of oncogenic mutations capable of driving tumorigenesis independent of a sustained β-HPV infection or continued viral protein expression. This review summarizes what is currently known about the deleterious effects of β-HPV on genome maintenance in the context of the virus's putative role in NMSC initiation.

Punicalagin from pomegranate promotes human papillary thyroid carcinoma BCPAP cell death by triggering ATM-mediated DNA damage response

Punicalagin (PUN), a component derived from pomegranate, is well known for its anticancer activity. Our previous work revealed that PUN induces autophagic cell death in papillary thyroid carcinoma cells. We hypothesized that PUN triggers DNA damage associated with cell death because DNA damage was reported as an inducer of autophagy. Our results showed that PUN treatment caused DNA breaks as evidenced by the significant enhancement in the phosphorylation of H2A.X. However, reactive oxygen species and DNA conformational alteration, 2 common inducing factors in DNA damage, were not involved in PUN-induced DNA damage. The phosphorylation of ataxia-telangiectasia mutated gene-encoded protein (ATM) but not ataxia telangiectasia and Rad3-related protein (ATR) was up-regulated in a time- and dosage-dependent manner after PUN treatment. KU-55933, an inhibitor of ATM, inhibited the phosphorylation of ATM induced by PUN and reversed the decreased cell viability caused by PUN. Thus, we demonstrated that PUN induces cell death of papillary thyroid carcinoma cells by triggering ATM-mediated DNA damage response, which provided novel mechanisms and potential targets for the better understanding of the anticancer actions of PUN.

Endosulfan induces cell dysfunction through cycle arrest resulting from DNA damage and DNA damage response signaling pathways

Our previous study showed that endosulfan increases the risk of cardiovascular disease. To identify toxic mechanism of endosulfan, we conducted an animal study for which 32 male Wistar rats were randomly and equally divided into four groups: Control group (corn oil only) and three treatment groups (1, 5 and 10mgkg^-1·d^-1). The results showed that exposure to endosulfan resulted in injury of cardiac tissue with impaired mitochondria integrity and elevated 8-OHdG expression in myocardial cells. Moreover, endosulfan increased the expressions of Fas, FasL, Caspase-8, Cleaved Caspase-8, Caspase-3 and Cleaved Caspase-3 in cardiac tissue. In vitro, human umbilical vein endothelial cells (HUVECs) were treated with different concentrations of endosulfan (1, 6 and 12μgmL^-1) for 24h. An inhibitor for Ataxia Telangiectasia Mutated Protein (ATM) (Ku-55933, 10μM) was added in 12μgmL^-1 group for 2h before exposure to endosulfan. Results showed that endosulfan induced DNA damage and activated DNA damage response signaling pathway (ATM/Chk2 and ATR/Chk1) and consequent cell cycle checkpoint. Furthermore, endosulfan promoted the cell apoptosis through death receptor pathway resulting from oxidative stress. The results provide a new insight for mechanism of endosulfan-induced cardiovascular toxicity which will be helpful in future prevention of cardiovascular diseases induced by endosulfan.

DNA damage-induced ATM- and Rad-3-related (ATR) kinase activation in non-replicating cells is regulated by the XPB subunit of transcription factor IIH (TFIIH)

The role of the DNA damage response protein kinase ataxia telangiectasia-mutated (ATM)- and Rad-3-related (ATR) in the cellular response to DNA damage during the replicative phase of the cell cycle has been extensively studied. However, little is known about ATR kinase function in cells that are not actively replicating DNA and that constitute most cells in the human body. Using small-molecule inhibitors of ATR kinase and overexpression of a kinase-inactive form of the enzyme, I show here that ATR promotes cell death in non-replicating/non-cycling cultured human cells exposed to N-acetoxy-2-acetylaminofluorene (NA-AAF), which generates bulky DNA adducts that block RNA polymerase movement. Immunoblot analyses of soluble protein extracts revealed that ATR and other cellular proteins containing SQ motifs become rapidly and robustly phosphorylated in non-cycling cells exposed to NA-AAF in a manner largely dependent on ATR kinase activity but independent of the essential nucleotide excision repair factor XPA. Although the topoisomerase I inhibitor camptothecin also activated ATR in non-cycling cells, other transcription inhibitors that do not directly damage DNA failed to do so. Interestingly, genetic and pharmacological inhibition of the XPB subunit of transcription factor IIH prevented the accumulation of the single-stranded DNA binding protein replication protein A (RPA) on damaged chromatin and severely abrogated ATR signaling in response to NA-AAF and camptothecin. Together, these results reveal a previously unknown role for transcription factor IIH in ATR kinase activation in non-replicating, non-cycling cells.

HBXIP, a binding protein of HBx, regulates maintenance of the G2/M phase checkpoint induced by DNA damage and enhances sensitivity to doxorubicin-induced cytotoxicity

To maintain the integrity of the genome, cells need to detect and repair DNA damage before they complete cell division. Hepatitis B x-interacting protein (HBXIP), a binding protein of HBx (Hepatitis B virus × protein), is aberrantly overexpressed in human cancer cells and show to promote cell proliferation and inhibit apoptosis. The present study is designed to investigate the role of HBXIP on the DNA damage response. Our results show that HBXIP acts as an important regulator of G2/M checkpoint in response to DNA damage. HBXIP knockdown increases phospho-histone H2AX expression and foci formation after treatment with ionizing radiation (IR). HBXIP regulates the ATM-Chk2 pathway following DNA damage. Depletion of HBXIP abrogates IR-induced G2/M cell cycle checkpoints, accompanying decrease the expression of phospho-Cdc25C, phospho-Cdc2 (Tyr15) and p27. We also show that downregulation of HBXIP expression sensitizes cancer cells to chemotherapy, as evidenced by an increase in apoptosis and cleavage of caspase-3 and caspase-9. Our data suggest that HBXIP can function as a mediator protein for DNA damage response signals to activate the G2/M checkpoint to maintain genome integrity and prevent cell death.

Xeroderma Pigmentosa Group A (XPA), Nucleotide Excision Repair and Regulation by ATR in Response to Ultraviolet Irradiation

The sensitivity of Xeroderma pigmentosa (XP) patients to sunlight has spurred the discovery and genetic and biochemical analysis of the eight XP gene products (XPA-XPG plus XPV) responsible for this disorder. These studies also have served to elucidate the nucleotide excision repair (NER) process, especially the critical role played by the XPA protein. More recent studies have shown that NER also involves numerous other proteins normally employed in DNA metabolism and cell cycle regulation. Central among these is ataxia telangiectasia and Rad3-related (ATR), a protein kinase involved in intracellular signaling in response to DNA damage, especially DNA damage-induced replicative stresses. This review summarizes recent findings on the interplay between ATR as a DNA damage signaling kinase and as a novel ligand for intrinsic cell death proteins to delay damage-induced apoptosis, and on ATR's regulation of XPA and the NER process for repair of UV-induced DNA adducts. ATR's regulatory role in the cytosolic-to-nuclear translocation of XPA will be discussed. In addition, recent findings elucidating a non-NER role for XPA in DNA metabolism and genome stabilization at ds-ssDNA junctions, as exemplified in prematurely aging progeroid cells, also will be reviewed.

Role of DNA Repair Factor Xeroderma Pigmentosum Protein Group C in Response to Replication Stress As Revealed by DNA Fragile Site Affinity Chromatography and Quantitative Proteomics

Replication stress (RS) fuels genomic instability and cancer development and may contribute to aging, raising the need to identify factors involved in cellular responses to such stress. Here, we present a strategy for identification of factors affecting the maintenance of common fragile sites (CFSs), which are genomic loci that are particularly sensitive to RS and suffer from increased breakage and rearrangements in tumors. A DNA probe designed to match the high flexibility island sequence typical for the commonly expressed CFS (FRA16D) was used as specific DNA affinity bait. Proteins significantly enriched at the FRA16D fragment under normal and replication stress conditions were identified using stable isotope labeling of amino acids in cell culture-based quantitative mass spectrometry. The identified proteins interacting with the FRA16D fragment included some known CFS stabilizers, thereby validating this screening approach. Among the hits from our screen so far not implicated in CFS maintenance, we chose Xeroderma pigmentosum protein group C (XPC) for further characterization. XPC is a key factor in the DNA repair pathway known as global genomic nucleotide excision repair (GG-NER), a mechanism whose several components were enriched at the FRA16D fragment in our screen. Functional experiments revealed defective checkpoint signaling and escape of DNA replication intermediates into mitosis and the next generation of XPC-depleted cells exposed to RS. Overall, our results provide insights into an unexpected biological role of XPC in response to replication stress and document the power of proteomics-based screening strategies to elucidate mechanisms of pathophysiological significance.

C-Terminal Binding Protein is Involved in Promoting to the Carcinogenesis of Human Glioma

C-terminal binding protein (CtBP) is responsible for regulating the pathogenesis of a lot of cancer types. However, whether CtBP1/2 is involved in regulating the growth and development of human glioma is still obscure. In the present study presented here, our results firstly reveal that CtBP1/2 deficiency, induced by siRNA interference, disrupts the functional integrity of the MRN complex that is responsible for DNA repair in human glioma cells. The dysfunction of the MRN complex further contributes to the up-regulation of ATM and Rad3-related kinase (ATR) and Chk1 signaling pathway, which inhibits cell cycle progression mediated by CDK2, preparing for the initiation of DNA repair. Under the condition of hypoxia, hypoxia-inducible factor-1α (HIF-1α) can be directly regulated by CDK2 on protein level, playing coordinately regulatory role in the carcinogenesis of human glioma cells. Overall, our findings reveal that CtBP1/2 is essential to promote to human glioma cell growth through maintaining the DNA stability regulated by the MRN/ATR/Chk1/CDK2/HIF-1α signaling pathway.