S-phase sensing of DNA-protein crosslinks triggers TopBP1-independent ATR activation and p53-mediated cell death by formaldehyde

Monoubiquitinated H2B, a Main Chromatin Target of Formaldehyde, Is Important for S-Phase Checkpoint Signaling and Genome Stability

Formaldehyde (FA) is a human carcinogen with ubiquitous environmental exposures and significant endogenous formation. Genotoxic activity of FA stems from its reactivity with DNA-NH2 groups. Histone lysines are another source of aldehyde-reactive amino groups in chromatin, however, chromatin/histone damage responses to FA and their biological significance are poorly understood. We examined histone posttranslational modifications in FA-treated human lung cells and found that the majority of the most prominent small lysine modifications associated with active or inactive chromatin were unchanged. FA moderately decreased H3K9 and H3K27 acetylation and H2A-K119 monoubiquitination but caused surprisingly severe losses of H2B-K120 monoubiquitination, especially in primary and stem-like cells. H2Aub1 decreases reflected its slower ubiquitination linked to a lower ubiquitin availability due to K48-polyubiquitination of FA-damaged proteins. Depletion of H2Bub1 resulted from its rapid deubiquitination in part by ATXN7L3-associated deubiquitinases and was independent on DNA damage signaling, indicating a direct chromatin damage response. Manipulations of H2Bub1 abundance showed that it was important for robust ATM and ATR signaling, efficient S-phase checkpoint, and suppression of mitotic transmission of unreplicated DNA and formation of micronuclei. Our findings identified H2B deubiquitination as a major FA-induced chromatin damage response that regulates S-phase checkpoint signaling and genome stability.

A new role for human papillomavirus 16 E2: Mitotic activation of the DNA damage response to promote viral genome segregation

Human papillomaviruses (HPV) are causative agents in around 5% of all human cancers. To identify and develop new targeted HPV therapeutics we must enhance our understanding of the viral life cycle and how it interacts with the host. The HPV E2 protein dimerizes and binds to 12bp target sequences in the viral genome and segregates the viral genome during mitosis. In this function, E2 binds to the viral genome and the host chromatin simultaneously, ensuring viral genomes reside in daughter nuclei following cell division. We have demonstrated that a mitotic interaction between E2 and the DNA damage response (DDR) protein TOPBP1 is required for E2 segregation function. In non-infected cells, following DNA damage, TOPBP1 is recruited to the mitotic host genome via interaction with MDC1 and this interaction protects DNA integrity during mitosis. Recently we demonstrated that the E2-TOPBP1 interaction activates the DNA damage response (DDR) during mitosis independently from external stimuli, promoting TOPBP1 interaction with mitotic chromatin and therefore segregation of the viral genome. Therefore, the virus has hijacked an existing host mechanism in order to segregate the viral genome. This intricate E2 function will be described and discussed.

Ubiquitin signaling and the proteasome drive human DNA-protein crosslink repair

DNA-protein crosslinks (DPCs) are large cytotoxic DNA lesions that form following exposure to chemotherapeutic drugs and environmental chemicals. Nucleotide excision repair (NER) and homologous recombination (HR) promote survival following exposure to DPC-inducing agents. However, it is not known how cells recognize DPC lesions, or what mechanisms selectively target DPC lesions to these respective repair pathways. To address these questions, we examined DPC recognition and repair by transfecting a synthetic DPC lesion comprised of the human oxoguanine glycosylase (OGG1) protein crosslinked to double-stranded M13MP18 into human cells. In wild-type cells, this lesion is efficiently repaired, whereas cells deficient in NER can only repair this lesion if an un-damaged homologous donor is co-transfected. Transfected DPC is subject to rapid K63 polyubiquitination. In NER proficient cells, the DPC is subject to K48 polyubiquitination, and is removed via a proteasome-dependent mechanism. In NER-deficient cells, the DNA-conjugated protein is not subject to K48 polyubiquitination. Instead, the K63 tag remains attached, and is only lost when a homologous donor molecule is present. Taken together, these results support a model in which selective addition of polyubiquitin chains to DNA-crosslinked protein leads to selective recruitment of the proteasome and the cellular NER and recombinational DNA repair machinery.

Atypical genotoxicity of carcinogenic nickel(II): Linkage to dNTP biosynthesis, DNA-incorporated rNMPs, and impaired repair of TOP1-DNA crosslinks

Cancer is a genetic disease requiring multiple mutations for its development. However, many carcinogens are DNA-unreactive and nonmutagenic and consequently described as nongenotoxic. One of such carcinogens is nickel, a global environmental pollutant abundantly emitted by burning of coal. We investigated activation of DNA damage responses by Ni and identified this metal as a replication stressor. Genotoxic stress markers indicated the accumulation of ssDNA and stalled replication forks, and Ni-treated cells were dependent on ATR for suppression of DNA damage and long-term survival. Replication stress by Ni resulted from destabilization of RRM1 and RRM2 subunits of ribonucleotide reductase and the resulting deficiency in dNTPs. Ni also increased DNA incorporation of rNMPs (detected by a specific fluorescent assay) and strongly enhanced their genotoxicity as a result of repressed repair of TOP1-DNA protein crosslinks (TOP1-DPC). The DPC-trap assay found severely impaired SUMOylation and K48-polyubiquitination of DNA-crosslinked TOP1 due to downregulation of specific enzymes. Our findings identified Ni as the human carcinogen inducing genome instability via DNA-embedded ribonucleotides and accumulation of TOP1-DPC which are carcinogenic abnormalities with poor detectability by the standard mutagenicity tests. The discovered mechanisms for Ni could also play a role in genotoxicity of other protein-reactive carcinogens.

ATR activation by Cr-DNA damage is a major survival response establishing late S and G2 checkpoints after Cr(VI) exposure

Inhalation exposure to hexavalent chromium is known to cause lung cancer and other pulmonary toxicity. Cellular metabolism of chromium(VI) entering cells as chromate anion produces different amounts of reactive Cr(V) intermediates and finally yields Cr(III). Direct reduction of Cr(VI) by ascorbate (Asc), the dominant metabolic reaction in vivo but not in standard cell cultures, skips production of Cr(V) but still permits extensive formation of Cr-DNA damage. To understand the importance of different forms of biological injury in Cr(VI) toxicity, we examined activation of several protein- and DNA damage-sensitive stress responses in human lung cells under Asc-restored conditions. We found that Asc-restored cells suppressed upregulation of oxidant-sensitive stress systems by Cr(VI) but showed a strong activation of the apical DNA damage-responsive kinase ATR. ATR signaling was triggered in late S phase and persisted upon entry of cells into G2 phase. Inhibition of ATR prevented the establishment of late-S and G2 cell cycle checkpoints and did not lead to a compensatory activation of a related kinase ATM. Inactivation of ATR also strongly impaired viability of Cr(VI)-treated lung cells including stem-like cells and revealed a significant formation of toxic Cr-DNA damage at low Cr(VI) doses. Our findings identified a major Cr(VI) resistance mechanism involving sensing of Cr-DNA damage by ATR in late S phase and a subsequent establishment of protective cell cycle checkpoints.

Oncoprotein SET dynamically regulates cellular stress response through nucleocytoplasmic transport in breast cancer

SETβ is the predominant isoform of oncoprotein SE translocation (SET) in various breast cancer cell lines. Interactome-transcriptome analysis has shown that SETβ is intimately associated with cellular stress response. Among various exogenous stimuli, formaldehyde (FA) causes distinct biological effects in a dose-dependent manner. In response to FA at different concentrations, SET dynamically shuttles between the nucleus and cytoplasm, performing diverse biofunctions to restore homeostasis. At a low concentration, FA acts as an epidermal growth factor (EGF) and activates the HER2 receptor and downstream signaling pathways in HER2+ breast cancer cells, resulting in enhanced cell proliferation. Nucleocytoplasmic transport of SETβ is controlled by the PI3K/PKCα/CK2α axis and depletion or blockade of the transport of SETβ suppresses EGF-induced activation of AKT and ERK. SETβ also inhibits not only stress-induced activation of p38 MAPK signaling pathway, but also assembly of stress granules by hindering formation of the G3BP1-RNA complex. Our findings suggest that SET functions as an important regulator which modulates cellular stress signaling pathways dynamically.

Alfaleh M, H. Abdulaal W, Bakhaidar RB, Alselami MO, Zahrani MAL, Alqarni HM, F. Alghaith A, Alshehri S, Badr-Eldin SM, Aldawsari HM, Al-hejaili OD, Aldhabi BM, Mahdi WA. Wasp venom peptide improves the proapoptotic activity of alendronate sodium in A549 lung cancer cells

BACKGROUND

Lung cancer in men and women is considered the leading cause for cancer-related mortality worldwide. Anti-cancer peptides represent a potential untapped reservoir of effective cancer therapy.

METHODOLOGY

Box-Behnken response surface design was applied for formulating Alendronate sodium (ALS)-mastoparan peptide (MP) nanoconjugates using Design-Expert software. The optimization process aimed at minimizing the size of the prepared ALS-MP nanoconjugates. ALS-MP nanoconjugates' particle size, encapsulation efficiency and the release profile were determined. Cytotoxicity, cell cycle, annexin V staining and caspase 3 analyses on A549 cells were carried out for the optimized formula.

RESULTS

The results revealed that the optimized formula was of 134.91±5.1 nm particle size. The novel ALS-MP demonstrated the lowest IC50 (1.3 ± 0.34 μM) in comparison to ALS-Raw (37.6 ± 1.79 μM). Thus, the results indicated that when optimized ALS-MP nanoconjugate was used, the IC50 of ALS was also reduced by half. Cell cycle analysis demonstrated a significantly higher percentage of cells in the G2-M phase following the treatment with optimized ALS-MP nanoconjugates.

CONCLUSION

The optimized ALS-MP formula had significantly improved the parameters related to the cytotoxic activity towards A549 cells, compared to control, MP and ALS-Raw.

Phyto-Phospholipid Conjugated Scorpion Venom Nanovesicles as Promising Carrier That Improves Efficacy of Thymoquinone against Adenocarcinoma Human Alveolar Basal Epithelial Cells

Lung cancer is a dangerous type of cancer in men and the third leading cause of cancer-related death in women, behind breast and colorectal cancers. Thymoquinone (THQ), a main compound in black seed essential oils, has a variety of beneficial effects, including antiproliferative, anti-inflammatory, and antioxidant properties. On the other hand, scorpion venom peptides (SV) induce apoptosis in the cancer cells, making it a promising anticancer agent. THQ, SV, and Phospholipon® 90H (PL) were incorporated in a nano-based delivery platform to assess THQ's cellular uptake and antiproliferative efficacy against a lung cancer cell line derived from human alveolar epithelial cells (A549). Several nanovesicles were prepared and optimized using factorial experimental design. The optimized phytosome formulation contained 79.0 mg of PL and 170.0 mg of SV, with vesicle size and zeta potential of 209.9 nm and 21.1 mV, respectively. The IC50 values revealed that A549 cells were significantly more sensitive to the THQ formula than the plain formula and THQ. Cell cycle analysis revealed that THQ formula treatment resulted in significant cell cycle arrest at the S phase, increasing cell population in this phase by 22.1%. Furthermore, the THQ formula greatly increased cell apoptosis (25.17%) when compared to the untreated control (1.76%), plain formula (11.96%), or THQ alone (13.18%). The results also indicated that treatment with THQ formula significantly increased caspase-3, Bax, Bcl-2, and p53 mRNA expression compared to plain formula and THQ. In terms of the inflammatory markers, THQ formula significantly reduced the activity of TNF-α and NF-κB in comparison with the plain formula and THQ only. Overall, the findings from the study proved that a phytosome formulation of THQ could be a promising therapeutic approach for the treatment of lung adenocarcinoma.

NAD+ metabolism controls growth inhibition by HIF1 in normoxia and determines differential sensitivity of normal and cancer cells

The hypoxia-induced transcription factor HIF1 inhibits cell growth in normoxia through poorly understood mechanisms. A constitutive upregulation of hypoxia response is associated with increased malignancy, indicating a loss of antiproliferative effects of HIF1 in cancer cells. To understand these differences, we examined a control of cell cycle in primary human cells with activated hypoxia response in normoxia. Activated HIF1 caused a global slowdown of cell cycle progression through G1, S and G2 phases leading to the loss of mitotic cells. Cell cycle inhibition required a prolonged HIF1 activation and was not associated with upregulation of p53 or the CDK inhibitors p16, p21 or p27. Growth inhibition by HIF1 was independent of its Asn803 hydroxylation or the presence of HIF2. Antiproliferative effects of hypoxia response were alleviated by inhibition of lactate dehydrogenase and more effectively, by boosting cellular production of NAD⁺, which was decreased by HIF1 activation. In comparison to normal cells, various cancer lines showed several fold-higher expression of NAMPT which is a rate-limiting enzyme in the main biosynthetic pathway for NAD⁺. Inhibition of NAMPT activity in overexpressor cancer cells sensitized them to antigrowth effects of HIF1. Thus, metabolic changes in cancer cells, such as enhanced NAD⁺ production, create resistance to growth-inhibitory activity of HIF1 permitting manifestation of its tumor-promoting properties.AbbreviationsDMOG: dimethyloxalylglycine, DM-NOFD: dimethyl N-oxalyl-D-phenylalanine, NMN: β-nicotinamide mononucleotide.

The mechanism of cytotoxicity of 4-nonylphenol in a human hepatic cell line involves ER-stress, apoptosis, and mitochondrial dysfunction

4-Nonylphenol (4-NP) is an emerging environmental pollutant widely diffused in waters and sediments. It mainly derives from the degradation of alkyl phenol ethoxylates, compounds commonly employed as industrial surfactants. 4-NP strongly contaminates foods and waters for human use; thus, it displays a wide range of toxic effects not only for aquatic organisms but also for mammals and humans. After ingestion through the diet, it tends to accumulate in body fluids and tissues. One of the main organs where 4-NP and its metabolites are concentrated is the liver, where it causes, even at low doses, oxidative stress and apoptosis. In the present study, we analyzed the effects of 4-NP on a human hepatic cell line (HepG2) to deepen the knowledge of its cytotoxic mechanism. We found that 4-NP, in a range of concentration from 50 to 100 μM, significantly reduced cell viability; it caused a partial block of proliferation and induced apoptosis with activation of caspase-3 and overexpression of p53. Moreover, 4-NP induced-apoptosis seemed to involve both an ER-stress response, with the appearance of high level of GRP78, CHOP and the spliced XBP1, and a dysregulation of mitochondrial physiology, characterized by an overexpression of main markers of mitochondrial dynamics. Our data support the idea that a daily consumption of 4-NP-contaminated foods may lead to local damages at the level of gastrointestinal system, including liver, with negative consequences for the organ physiology.

Applying genome-wide CRISPR to identify known and novel genes and pathways that modulate formaldehyde toxicity

Formaldehyde (FA), a ubiquitous environmental pollutant, is classified as a Group I human carcinogen by the International Agency for Research on Cancer. Previously, we reported that FA induced hematotoxicity and chromosomal aneuploidy in exposed workers and toxicity in bone marrow and hematopoietic stem cells of experimental animals. Using functional toxicogenomic profiling in yeast, we identified genes and cellular processes modulating eukaryotic FA cytotoxicity. Although we validated some of these findings in yeast, many specific genes, pathways and mechanisms of action of FA in human cells are not known. In the current study, we applied genome-wide, loss-of-function CRISPR screening to identify modulators of FA toxicity in the human hematopoietic K562 cell line. We assessed the cellular genetic determinants of susceptibility and resistance to FA at 40, 100 and 150 μM (IC10, IC20 and IC60, respectively) at two time points, day 8 and day 20. We identified multiple candidate genes that increase sensitivity (e.g. ADH5, ESD and FANC family) or resistance (e.g. FASN and KDM6A) to FA when disrupted. Pathway analysis revealed a major role for the FA metabolism and Fanconi anemia pathway in FA tolerance, consistent with findings from previous studies. Additional network analyses revealed potential new roles for one-carbon metabolism, fatty acid synthesis and mTOR signaling in modulating FA toxicity. Validation of these novel findings will further enhance our understanding of FA toxicity in human cells. Our findings support the utility of CRISPR-based functional genomics screening of environmental chemicals.

L-Carnitine reduces the negative effects of formalin on sperm parameters, chromatin condensation and apoptosis in mice: An experimental study

Background

Formalin is commonly applied as an antiseptic and tissue fixative. It has reactive molecules that lead to its cytotoxic effects. According to recent studies, formalin causes a change in the testicular and sperm structure and L-carnitine (LC) acts as an antioxidant to counteract its effects.

Objective

This study aimed to investigate the protective effects of LC on the parameters, chromatin condensation and apoptosis of mice sperm exposed to formalin.

Materials and Methods

In this experimental study, 24 balb/c mice (25-40 gr,10-12 wk) were divided into three groups (n = 8/each): group I without any injections or gavage; group II, received 10 mg/ kg formalin intraperitoneally (I.P); and group III was exposed to formalin and LC, where a dose of 10 mg/kg formalin was injected I.P daily and LC the dose of 100 mg/kg was kept in a solvent solution. After 31 days, the sperm examination was performed as follows: to evaluate chromatin and DNA quality of the sperm, we applied aniline blue (AB), toluidine blue (TB), chromomycin A3 (CMA3), and terminal transferase-mediated deoxy uridine triphosphate biotin end labeling (TUNEL) tests.

Results

Sperm parameters such as count, motility, morphology, and viability displayed a significant decrease in the formalin group. While the data exhibited a considerable augment in sperm parameters in the formalin + LC than the formalin and control groups (p < 0.001), significant differences were detected between groups with respect to TB staining, TUNEL test, CMA3 test and AB staining in the formalin and formalin + LC groups.

Conclusion

LC can reduce the negative effects of formalin on sperm parameters, chromatin stability, and percentage of apoptosis in an animal model.

Tandem Deubiquitination and Acetylation of SPRTN Promotes DNA-Protein Crosslink Repair and Protects against Aging

DNA-protein crosslinks (DPCs) are highly toxic DNA lesions that threaten genomic integrity. Recent findings highlight that SPRTN, a specialized DNA-dependent metalloprotease, is a central player in proteolytic cleavage of DPCs. Previous studies suggest that SPRTN deubiquitination is important for its chromatin association and activation. However, the regulation and consequences of SPRTN deubiquitination remain unclear. Here we report that, in response to DPC induction, the deubiquitinase VCPIP1/VCIP135 is phosphorylated and activated by ATM/ATR. VCPIP1, in turn, deubiquitinates SPRTN and promotes its chromatin relocalization. Deubiquitination of SPRTN is required for its subsequent acetylation, which promotes SPRTN relocation to the site of chromatin damage. Furthermore, Vcpip1 knockout mice are prone to genomic instability and premature aging. We propose a model where two sequential post-translational modifications (PTMs) regulate SPRTN chromatin accessibility to repair DPCs and maintain genomic stability and a healthy lifespan.

TNFAIP1 Mediates Formaldehyde-Induced Neurotoxicity by Inhibiting the Akt/CREB Pathway in N2a Cells

Formaldehyde (FA) is a common air pollutant. Exposure to exogenous FA can cause damage to the nervous system, such as learning and memory impairment, balance dysfunction, and sleep disorders. Excessive production of endogenous FA also causes memory impairment and is thought to be associated with Alzheimer's disease (AD). Tumor necrosis factor alpha-induced protein 1 (TNFAIP1) plays a crucial role in neurodevelopment and neurological diseases. However, the role of TNFAIP1 in FA-induced neurotoxicity is unclear. Herein, using a mouse neuroblastoma cell line (N2a cells), we explored the mechanism of TNFAIP1 in FA-induced neurotoxicity, the involvement of the Akt/CREB signaling pathway, and how the expression of TNFAIP1 is regulated by FA. We found that exposure to 100 μM or 200 μM FA for 24 h led to decreased cell viability, increased cell apoptosis and neurite retraction, increased reactive oxygen species (ROS) levels, upregulated protein expression of TNFAIP1 and decreased the levels of phosphorylated Akt and CREB in the Akt/CREB pathway. Knockdown of TNFAIP1 using a TNFAIP1 small interfering RNA (siRNA) expression vector prevented FA from inhibiting the Akt/CREB pathway, thus reducing cell apoptosis and restoring cell viability and neurite outgrowth. Clearance of ROS by vitamin E (Vit E) repressed the FA-mediated upregulation of TNFAIP1 expression. These results suggest that FA increases the expression of TNFAIP1 by inducing oxidative stress and that upregulated TNFAIP1 then inhibits the Akt/CREB pathway, consequently leading to cell apoptosis and neurite retraction. Therefore, TNFAIP1 is a potential target for alleviating FA-induced neurotoxicity and related neurological disorders.

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.

p53 activation by Cr(VI): a transcriptionally limited response induced by ATR kinase in S-phase

Cellular reduction of carcinogenic chromium(VI) causes several forms of Cr-DNA damage with different genotoxic properties. Chromate-treated cultured cells have shown a strong proapoptotic activity of the DNA damage-sensitive transcription factor p53. However, induction of p53 transcriptional targets by Cr(VI) in rodent lungs was weak or undetectable. We examined Cr(VI) effects on the p53 pathway in human cells with restored levels of ascorbate that acts as a principal reducer of Cr(VI) in vivo but is nearly absent in standard cell cultures. Ascorbate-restored H460 and primary human cells treated with Cr(VI) contained higher levels of p53 and its Ser15 phosphorylation, which were induced by ATR kinase. Cr(VI)-stimulated p53 phosphorylation occurred in S-phase by a diffusible pool of ATR that was separate from the chromatin-bound pool targeting DNA repair substrates at the sites of toxic mismatch repair of Cr-DNA adducts. Even when more abundantly present than after exposure to the radiomimetic bleomycin, Cr(VI)-stabilized p53 showed a much more limited activation of its target genes in two types of primary human cells. No increases in mRNA were found for nucleotide excision repair factors and a majority of proapoptotic genes. A weak transcription activity of Cr(VI)-upregulated p53 was associated with its low lysine acetylation in the regulatory C-terminal domain, resulting from the inability of Cr(VI) to activate ATM in ascorbate-restored cells. Thus, p53 activation by ascorbate-metabolized Cr(VI) represents a limited genome-protective response that is defective in upregulation of DNA repair genes and proapoptotic transcripts for elimination of damaged cells.

Combined cytotoxicity of co-exposure to aldehyde mixtures on human bronchial epithelial BEAS-2B cells

Aldehydes are well-known air pollutants and often studied alone, while co-exposure to aldehyde mixtures is more common than single aldehydes. Unfortunately, it has been very little known about the (mechanism of) combined toxicity of aldehyde mixtures. Here, formaldehyde and acrolein were selected as the typical representatives of common aldehydes, and were used to explore to get in-depth insight into the mechanism of combined toxicity of aldehyde mixtures. The NOECs (non-observed effect concentrations) are 60 μmoL/L for formaldehyde, and 0.5 μmoL/L for acrolein, so acrolein is more toxic than formaldehyde. Formaldehyde and acrolein mixtures showed significant cytotoxicity and synergistic effects in a concentration/time-dependent way on BEAS-2B cells based on acute and chronic cytotoxicity assay. Acrolein was dominant in aldehyde mixtures in inducing cytotoxicity at environmentally relevant doses because of higher toxicity. Moreover, aldehyde mixtures significantly synergistically increased the intracellular reactive oxygen species (ROS), malondialdehyde (MDA) and lactate dehydrogenase (LDH) leakage, while caused an antagonistic effects on glutathione (GSH). Besides, formaldehyde could significantly potentiated the activation of environmental stress sensitive Nrf2 pathway induced by acrolein, even at doses at which formaldehyde treatment alone had no any response. Furthermore, as the downstream components of Nrf2 pathway, catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPX) and heme oxygenase-1 (HO-1) were significantly synergistically induced by formaldehyde and acrolein mixtures. Antioxidants N-acetylcysteine and reduced glutathione could significantly suppress the acute and chronic combined cytotoxicity of acrolein and formaldehyde mixtures, and changed their interactions (synergism) on cytotoxicity. Taken together, aldehyde mixtures have higher toxicity than that expected for additivity based on single aldehydes even at environmentally relevant concentrations, and the combined cytotoxicity may be enhanced through oxidative stress and the related Nrf2 pathway. Prolonged exposure to pollutants containing aldehyde mixtures through inhalation may have more serious threat to respiratory system in animal and human.

Vitamin C increases DNA breaks and suppresses DNA damage-independent activation of ATM by bleomycin

Bleomycin is a redox-active drug with anticancer and other clinical applications. It is also frequently used as a tool in fundamental research on cellular responses to DNA double-strand breaks (DSBs). A conversion of bleomycin into its DNA-breaking form requires Fe, one-electron donors and O2. Here, we examined how a major biological antioxidant ascorbate (reduced vitamin C), which is practically absent in standard cell culture, impacts cellular responses to bleomycin. We found that restoration of physiological levels of vitamin C in human cancer cells increased their killing by bleomycin in 2D cultures and 3D tumor spheroids. Higher cytotoxicity of bleomycin occurred in cells with normal and shRNA-depleted p53. Cellular vitamin C enhanced the ability of bleomycin by produce DSBs, which was established by direct measurements of these lesions in three cell lines. Vitamin C-restored cancer cells also showed a higher sensitivity to killing by low-dose bleomycin in combination with inhibitors of DSB repair-activating ATM or DNA-PK kinases. The presence of ascorbate in bleomycin-treated cells suppressed a DSB-independent activation of the ATM-CHK2 axis by blocking superoxide radical. In vitro studies detected a greatly superior ability of ascorbate over other cellular reducers to catalyze DSB formation by bleomycin. Ascorbate was faster than other antioxidants in promoting two steps in activation of bleomycin. Our results demonstrate strong activation effects of vitamin C on bleomycin, shifting its toxicity further toward DNA damage and making it more sensitive to manipulations of DNA repair.

Toxicological Antagonism among Welding Fume Metals: Inactivation of Soluble Cr(VI) by Iron

Epidemiological studies in chromate production have established hexavalent chromium as a potent lung carcinogen. Inhalation of chromium(VI) most often occurs in mixtures with other metals as among stainless steel welders, which is the largest occupational group with Cr(VI) exposure. Surprisingly, carcinogenicity of Cr(VI)-containing welding fumes is moderate and not consistently higher than that of Cr-free welding. Here, we investigated interactions between chromate and three other metal ions [Fe(III), Mn(II), Ni(II)] that are typically released from stainless steel welding particles. In human lung epithelial cells with physiological levels of ascorbate and glutathione, Cr(VI) was by far the most cytotoxic metal in single exposures. Coexposure with Fe(III) suppressed cytotoxicity and genotoxicity of Cr(VI), which resulted from a severe inhibition of Cr uptake by cells and required extracellular ascorbate/glutathione. Chemically, detoxification of Cr(VI) occurred via its rapid extracellular reduction by Fe(II) that primarily originated from ascorbate-reduced Fe(III). Glutathione was a significant contributor to reduction of Cr(VI) by Fe only in the presence of ascorbate. We further found that variability in Cr(VI) metabolism among common cell culture media was caused by their different Fe content. Ni(II) and Mn(II) had no detectable effects on metabolism, cellular uptake or cytotoxicity of Cr(VI). The main biological findings were confirmed in three human lung cell lines, including stem cell-like and primary cells. We discovered extracellular detoxification of carcinogenic chromate in coexposures with Fe(III) ions and identified the underlying chemical mechanism. Our findings established an important case when exposure to mixtures causes inactivation of a potent human carcinogen.

A pH-sensitive methenamine mandelate-loaded nanoparticle induces DNA damage and apoptosis of cancer cells

Methenamine mandelate is a urinary antibacterial agent, which can be converted to formaldehyde in urine that has a relatively low pH of average 5.5-6.8. Here, we prepare a pH-sensitive PLGA-based nanoparticle containing both methenamine mandelate and NaHCO₃. Methenamine mandelate/NaHCO₃-coloaded nanoparticle could enter cells via endosome/lysosome pathway. The pH in lysosomes and endo-lysosomes is approximately 5.0. In the acidic environment, NaHCO₃ reacts with proton and produce CO₂ bubbles, which burst nanoparticles and lead to the rapidly release of methenamine mandelate. Meanwhile, methenamine mandelate was then quickly converted to a sufficient amount of formaldehyde in this acidic environment, which induced DNA damage and DNA damage response (DDR). Consequently, methenamine mandelate/NaHCO₃-coloaded nanoparticles caused cell cycle arrest, cell growth inhibition and apoptosis of cancer cells. Moreover, methenamine mandelate/NaHCO₃-coloaded nanoparticles also show intensive inhibitory effect on the growth of MCF-7 xenograft tumor in vivo. Therefore, methenamine mandelate/NaHCO₃-coloaded nanoparticle is a promising type of formulation for the treatment of cancer, which could give the "old drug" methenamine mandelate a new anti-cancer function in clinical.

STATEMENT OF SIGNIFICANCE

Methenamine mandelate is a urinary antibacterial agent, which can be converted to formaldehyde in urine that has a relatively low pH of average 5.5-6.8. Here, we prepare a pH-sensitive PLGA-based nanoparticle containing both methenamine mandelate and NaHCO₃. Methenamine mandelate/NaHCO₃-coloaded nanoparticle could enter cells via endosome/lysosome pathway. The pH in lysosomes and endo-lysosomes is approximately 5.0. In the acidic environment, NaHCO₃ reacts with proton and produce CO₂ bubbles, which burst nanoparticles and lead to the rapidly release of methenamine mandelate. Meanwhile, methenamine mandelate was then quickly converted to a sufficient amount of formaldehyde in this acidic environment, which induced DNA damage and DNA damage response (DDR). Methenamine mandelate/NaHCO₃-coloaded nanoparticle is a promising type formulation for the treatment of cancer, which could give the "old drug" methenamine mandelate a new anti-cancer function in clinical.

Formation and degradation of nitrogen mustard-induced MGMT-DNA crosslinking in 16HBE cells

N-methyl-2,2-di(chloroethyl)amine (HN2) is a kind of bifunctional alkyltating agent, which can react with nucleophilic groups in DNA and/or protein to form HN2-bridged crosslinking of target molecules, such as DNA-protein crosslinkings (DPC). O⁶-methylguanine-DNA methyltransferase (MGMT) is a DNA damage repair enzyme which solely repairs alkyl adduct on DNA directly. However, MGMT was detected to act as a protein cross-linked with DNA via alkylation in presence of HN2, and unexpectedly turned into a DNA damage enhancer in the form of MGMT-DNA cross-link (mDPC). Present study aimed to explore the possible ways to lessen the incorporation of MGMT into DPC as well as to save it for DNA repair. To find out the influencing factors of mDPC formation and cleavage, human bronchial epithelial cell line 16HBE was exposed to HN2 and the factors related with MGMT expression and degradation were investigated. When c-Myc, a negative transcriptional factor of MGMT was inhibited by 10058-F4, MGMT expression and mDPC formation were increased, and more γ-H2AX was also detected. Sustained treatment with O⁶BG, a specific exogenous substrate and depleter of MGMT, could reduce the level of MGMT and mDPC formation. In contrast, a transient 1h pre-treatment of O⁶GB before HN2 exposure would cause a high MGMT and mDPC level. MGMT was increasingly ubiquitinated after HN2 exposure in a time-dependent manner. At the same time, MGMT was also SUMOylated with a downward time-dependent manner compared to its ubiquitination. Inhibitors of E1, E2 or E3 ligases of ubiqutination all led to the accumulation of mDPC and total-DPC (tDPC) with the difference as that mDPC was sensitive to E1 inhibitor while tDPC more sensitive to E2 and E3 inhibitor. Our results demonstrated the control of mDPC level could be realized through transcription inhibitory effect of c-Myc, O⁶GB application, and the acceleration of mDPC ubiquitination and subsequent degradation.

Nickel-induced HIF-1α promotes growth arrest and senescence in normal human cells but lacks toxic effects in transformed cells

Nickel is a human carcinogen that acts as a hypoxia mimic by activating the transcription factor HIF-1α and hypoxia-like transcriptomic responses. Hypoxia and elevated HIF-1α are typically associated with drug resistance in cancer cells, which is caused by increased drug efflux and other mechanisms. Here we examined the role of HIF-1α in uptake of soluble Ni(II) and Ni(II)-induced cell fate outcomes using si/shRNA knockdowns and gene deletion models. We found that HIF-1α had no effect on accumulation of Ni(II) in two transformed (H460, A549) and two normal human cell lines (IMR90, WI38). The loss of HIF-1α also produced no significant impact on p53-dependent and p53-independent apoptotic responses or clonogenic survival of Ni(II)-treated transformed cells. In normal human cells, HIF-1α enhanced the ability of Ni(II) to inhibit cell proliferation and cause a permanent growth arrest (senescence). Consistent with its growth-suppressive effects, HIF-1α was important for upregulation of the cell cycle inhibitors p21 (CDKN1A) and p27 (CDKN1B). Irrespective of HIF-1α status, Ni(II) strongly increased levels of MYC protein but did not change protein expression of the cell cycle-promoting phosphatase CDC25A or the CDK inhibitor p16. Our findings indicate that HIF-1α limits propagation of Ni(II)-damaged normal cells, suggesting that it may act in a tumor suppressor-like manner during early stages of Ni(II) carcinogenesis.

20S immunoproteasomes remove formaldehyde-damaged cytoplasmic proteins suppressing caspase-independent cell death

Immunoproteasomes are known for their involvement in antigen presentation. However, their broad tissue presence and other evidence are indicative of nonimmune functions. We examined a role for immunoproteasomes in cellular responses to the endogenous and environmental carcinogen formaldehyde (FA) that binds to cytosolic and nuclear proteins producing proteotoxic stress and genotoxic DNA-histone crosslinks. We found that immunoproteasomes were important for suppression of a caspase-independent cell death and the long-term survival of FA-treated cells. All major genotoxic responses to FA, including replication inhibition and activation of the transcription factor p53 and the apical ATM and ATR kinases, were unaffected by immunoproteasome inactivity. Immunoproteasome inhibition enhanced activation of the cytosolic protein damage sensor HSF1, elevated levels of K48-polyubiquitinated cytoplasmic proteins and increased depletion of unconjugated ubiquitin. We further found that FA induced the disassembly of 26S immunoproteasomes, but not standard 26S proteasomes, releasing the 20S catalytic immunoproteasome. FA-treated cells also had higher amounts of small activators PA28αβ and PA28γ bound to 20S particles. Our findings highlight the significance of nonnuclear damage in FA injury and reveal a major role for immunoproteasomes in elimination of FA-damaged cytoplasmic proteins through ubiquitin-independent proteolysis.

The toxicity of continuous long-term low-dose formaldehyde inhalation in mice

Although the toxicity of high-dose formaldehyde (FA) inhalation has been extensively analyzed in animals, the effect of continuous long-term exposure to low-dose FA has not been well documented. This study aims to evaluate the toxicity of continuous long-term low-dose FA inhalation in mice. Forty-eight Kunming male mice were equally randomized to three groups according to the dose of FA inhalation exposure: a control (0 mg/m³) group, a low-dose (0.08 mg/m³) group and a high-dose (0.8 mg/m³) group. The mice have been selected to expose to FA for different consecutive days at 24 h/day. The learning and memory functions, pathological changes in the lung and liver, and the percentage of CD4 ⁺ T and CD8 ⁺ T cells were observed and analyzed. It was found that continuous long-term inhalation of FA at relatively low doses could impair the learning and memory functions and induce pathological changes in the lung and liver, but did not seem to significantly affect the number of immune (CD4 ⁺ T and CD8 ⁺ T) cells.

Melamine and/or formaldehyde exposures affect steroidogenesis via alteration of StAR protein and testosterone synthetic enzyme expression in male mice

The reproductive effects of melamine and formaldehyde, either alone or in combination, on mature male Swiss mice were investigated. The animals were orally administered melamine (50mg/kg/day), formaldehyde (25mg/kg/day), a mixture of melamine and formaldehyde, or a vehicle control for 65 consecutive days. As a result, the deterioration of sperm characteristics and inhibition of testicular enzyme activity were observed in the melamine- and formaldehyde-exposed groups. In addition, testosterone and luteinizing hormone levels were significantly reduced in the melamine but not in the formaldehyde-exposed group, which correlated with down-regulation of transcription levels of steroidogenic-related genes. Histopathologically, both compounds caused lesions in the testes. However, the co-exposure reduced the induced alterations in spermatogenesis, steroidogenesis, and testicular architecture that were obviously observed in the melamine-exposed group. Consequently, we demonstrated that melamine exhibited more pronounced reproductive impact in comparison with formaldehyde. In addition, formaldehyde was able to substantially temper the melamine -induced reproductive toxic effect.

Is it possible to use biomonitoring for the quantitative assessment of formaldehyde occupational exposure?

The European classification, labeling and packaging classified formaldehyde as human carcinogen Group 1B and mutagen 2, fostering the re-evaluation of the exposure risk in occupational settings. Although formaldehyde exposure is traditionally measured in air, many efforts were made to identify specific exposure biomarkers: urinary formaldehyde, formic acid and DNA damage indicators. Though used in combination, none of these seems satisfactory. The influence of the metabolism on exogenous formaldehyde levels, the exposure to other xenobiotics, the difference in genetic background and metabolism efficiency, misled the relationship between genotoxicity and exposure data. Nevertheless, the limitation of adverse effects to the local contact sites hampers biomonitoring. Here we discuss the feasibility of formaldehyde biomonitoring and the use of DNA, DNA-protein cross-links and protein adducts as potential biomarkers.

Formaldehyde Is a Potent Proteotoxic Stressor Causing Rapid Heat Shock Transcription Factor 1 Activation and Lys48-Linked Polyubiquitination of Proteins

Endogenous and exogenous formaldehyde (FA) has been linked to cancer, neurotoxicity, and other pathophysiologic effects. Molecular and cellular mechanisms that underlie FA-induced damage are poorly understood. In this study, we investigated whether proteotoxicity is an important, unrecognized factor in cell injury caused by FA. We found that irrespective of their cell cycle phases, all FA-treated human cells rapidly accumulated large amounts of proteins with proteasome-targeting K48-linked polyubiquitin, which was comparable with levels of polyubiquitination in proteasome-inhibited MG132 controls. Both nuclear and cytoplasmic proteins were damaged and underwent K48-polyubiquitination. There were no significant changes in the nonproteolytic K63-polyubiquitination of soluble and insoluble cellular proteins. FA also rapidly induced nuclear accumulation and Ser326 phosphorylation of the main heat shock-responsive transcription factor HSF1, which was not a result of protein polyubiquitination. Consistent with the activation of the functional heat shock response, FA strongly elevated the expression of HSP70 genes. In contrast to the responsiveness of the cytoplasmic protein damage sensor HSF1, FA did not activate the unfolded protein response in either the endoplasmic reticulum or mitochondria. Inhibition of HSP90 chaperone activity increased the levels of K48-polyubiquitinated proteins and diminished cell viability after FA treatment. Overall, our results indicate that FA is a strong proteotoxic agent, which helps explain its diverse pathologic effects, including injury in nonproliferative tissues.

Bifunctional alkylating agent-mediated MGMT-DNA cross-linking and its proteolytic cleavage in 16HBE cells

Nitrogen mustard (NM), a bifunctional alkylating agent (BAA), contains two alkyl arms and can act as a cross-linking bridge between DNA and protein to form a DNA-protein cross-link (DPC). O(6)-methylguanine-DNA methyltransferase (MGMT), a DNA repair enzyme for alkyl adducts removal, is found to enhance cell sensitivity to BAAs and to promote damage, possibly due to its stable covalent cross-linking with DNA mediated by BAAs. To investigate MGMT-DNA cross-link (mDPC) formation and its possible dual roles in NM exposure, human bronchial epithelial cell line 16HBE was subjected to different concentrations of HN2, a kind of NM, and we found mDPC was induced by HN2 in a concentration-dependent manner, but the mRNA and total protein of MGMT were suppressed. As early as 1h after HN2 treatment, high mDPC was achieved and the level maintained for up to 24h. Quick total DPC (tDPC) and γ-H2AX accumulation were observed. To evaluate the effect of newly predicted protease DVC1 on DPC cleavage, we applied siRNA of MGMT and DVC1, MG132 (proteasome inhibitor), and NMS-873 (p97 inhibitor) and found that proteolysis plays a role. DVC1 was proven to be more important in the cleavage of mDPC than tDPC in a p97-dependent manner. HN2 exposure induced DVC1 upregulation, which was at least partially contributed to MGMT cleavage by proteolysis because HN2-induced mDPC level and DNA damage was closely related with DVC1 expression. Homologous recombination (HR) was also activated. Our findings demonstrated that MGMT might turn into a DNA damage promoter by forming DPC when exposed to HN2. Proteolysis, especially DVC1, plays a crucial role in mDPC repair.

ATM and KAT5 safeguard replicating chromatin against formaldehyde damage

Many carcinogens damage both DNA and protein constituents of chromatin, and it is unclear how cells respond to this compound injury. We examined activation of the main DNA damage-responsive kinase ATM and formation of DNA double-strand breaks (DSB) by formaldehyde (FA) that forms histone adducts and replication-blocking DNA-protein crosslinks (DPC). We found that low FA doses caused a strong and rapid activation of ATM signaling in human cells, which was ATR-independent and restricted to S-phase. High FA doses inactivated ATM via its covalent dimerization and formation of larger crosslinks. FA-induced ATM signaling showed higher CHK2 phosphorylation but much lower phospho-KAP1 relative to DSB inducers. Replication blockage by DPC did not produce damaged forks or detectable amounts of DSB during the main wave of ATM activation, which did not require MRE11. Chromatin-monitoring KAT5 (Tip60) acetyltransferase was responsible for acetylation and activation of ATM by FA. KAT5 and ATM were equally important for triggering of intra-S-phase checkpoint and ATM signaling promoted recovery of normal human cells after low-dose FA. Our results revealed a major role of the KAT5-ATM axis in protection of replicating chromatin against damage by the endogenous carcinogen FA.

Effects of formaldehyde on mitochondrial dysfunction and apoptosis in SK-N-SH neuroblastoma cells

Methanol ingestion is neurotoxic in humans due to its metabolites, formaldehyde and formic acid. Here, we compared the cytotoxicity of methanol and its metabolites on different types of cells. While methanol and formic acid did not affect the viability of the cells, formaldehyde (200-800 μg/mL) was strongly cytotoxic in all cell types tested. We investigated the effects of formaldehyde on oxidative stress, mitochondrial respiratory functions, and apoptosis on the sensitive neuronal SK-N-SH cells. Oxidative stress was induced after 2 h of formaldehyde exposure. Formaldehyde at a concentration of 400 μg/mL for 12 h of treatment greatly reduced cellular adenosine triphosphate (ATP) levels. Confocal microscopy indicated that the mitochondrial membrane potential (MMP) was dose-dependently reduced by formaldehyde. A marked and dose-dependent inhibition of mitochondrial respiratory enzymes, viz., NADH dehydrogenase (complex I), cytochrome c oxidase (complex IV), and oxidative stress-sensitive aconitase was also detected following treatment with formaldehyde. Furthermore, formaldehyde caused a concentration-dependent increase in nuclear fragmentation and in the activities of the apoptosis-initiator caspase-9 and apoptosis-effector caspase-3/-7, indicating apoptosis progression. Our data suggests that formaldehyde exerts strong cytotoxicity, at least in part, by inducing oxidative stress, mitochondrial dysfunction, and eventually apoptosis. Changes in mitochondrial respiratory function and oxidative stress by formaldehyde may therefore be critical in methanol-induced toxicity.

Different ATM Signaling in Response to Chromium(VI) Metabolism via Ascorbate and Nonascorbate Reduction: Implications for in Vitro Models and Toxicogenomics

BACKGROUND

Carcinogenic hexavalent chromium [Cr(VI)] requires cellular reduction to generate DNA damage. Metabolism of Cr(VI) by its principal reducer ascorbate (Asc) lacks a Cr(V) intermediate, which is abundant in reactions with a minor reducing agent, glutathione. Cultured cells are widely used in mechanistic studies of Cr(VI) toxicity; however, they typically contain < 1% of normal Asc levels. Asc deficiency is also expected to diminish protection against reactive oxygen species.

OBJECTIVES

We assessed how the presence of Asc in cells affects their stress signaling and survival responses to chromate.

METHODS

We investigated the effects of Asc restoration in human lung H460 cells and normal human lung fibroblasts on the activation and functional role of ATM kinase, which controls DNA damage responses involving several hundreds of proteins.

RESULTS

Treatment of standard cultures with Cr(VI) strongly activated ATM, as indicated by its automodification at Ser1981 and by phosphorylation of checkpoint kinase 2 (CHK2) and chromatin/transcription regulator KRAB-associated protein 1 (KAP1). Confirming the importance of activated ATM, its inhibition impaired replication recovery and clonogenic survival. In contrast, fully Asc-restored cells lacked ATM activation by Cr(VI), and ATM silencing produced no significant effects on p53 stabilization, apoptosis, replication recovery, or clonogenic survival. Dose dependence studies found a close correlation between ATM activation and the extent of Cr(VI) reduction by glutathione.

CONCLUSIONS

Asc restoration in cultured cells dramatically altered their stress responses to Cr(VI) by preventing activation of the oxidant-sensitive ATM network. We suggest that toxicogenomic and other cell response-based approaches likely underestimate Cr(VI) genotoxicity when standard ATM-activating carcinogens are used as references.

CITATION

Luczak MW, Green SE, Zhitkovich A. 2016. Different ATM signaling in response to chromium(VI) metabolism via ascorbate and nonascorbate reduction: implications for in vitro models and toxicogenomics. Environ Health Perspect 124:61-66; http://dx.doi.org/10.1289/ehp.1409434.

Cordycepin induces cell cycle arrest and apoptosis by inducing DNA damage and up-regulation of p53 in Leukemia cells

Cordycepin, an adenosine analog derived from Cordyceps militaris has been shown to exert anti-tumor activity in many ways. However, the mechanisms by which cordycepin contributes to the anti-tumor still obscure. Here our present work showed that cordycepin inhibits cell growth in NB-4 and U937 cells by inducing apoptosis. Further study showed that cordycepin increases the expression of p53 which promotes the release of cytochrome c from mitochondria to the cytosol. The released cytochrome c can then activate caspase-9 and trigger intrinsic apoptosis. Cordycepin also blocks MAPK pathway by inhibiting the phosphorylation of ERK1/2, and thus sensitizes the apoptosis. In addition, our results showed that cordycepin inhibits the expression of cyclin A2, cyclin E, and CDK2, which leads to the accumulation of cells in S-phase. Moreover, our study showed that cordycepin induces DNA damage and causes degradation of Cdc25A, suggesting that cordycepin-induced S-phase arrest involves activation of Chk2-Cdc25A pathway. In conclusion, cordycepin-induced DNA damage initiates cell cycle arrest and apoptosis which leads to the growth inhibition of NB-4 and U937 cells.

Proteasome activity is important for replication recovery, CHK1 phosphorylation and prevention of G2 arrest after low-dose formaldehyde

Formaldehyde (FA) is a human carcinogen with numerous sources of environmental and occupational exposures. This reactive aldehyde is also produced endogenously during metabolism of drugs and other processes. DNA-protein crosslinks (DPCs) are considered to be the main genotoxic lesions for FA. Accumulating evidence suggests that DPC repair in high eukaryotes involves proteolysis of crosslinked proteins. Here, we examined a role of the main cellular proteolytic machinery proteasomes in toxic responses of human lung cells to low FA doses. We found that transient inhibition of proteasome activity increased cytotoxicity and diminished clonogenic viability of FA-treated cells. Proteasome inactivation exacerbated suppressive effects of FA on DNA replication and increased the levels of the genotoxic stress marker γ-H2AX in normal human cells. A transient loss of proteasome activity in FA-exposed cells also caused delayed perturbations of cell cycle, which included G2 arrest and a depletion of S-phase populations at FA doses that had no effects in control cells. Proteasome activity diminished p53-Ser15 phosphorylation but was important for FA-induced CHK1 phosphorylation, which is a biochemical marker of DPC proteolysis in replicating cells. Unlike FA, proteasome inhibition had no effect on cell survival and CHK1 phosphorylation by the non-DPC replication stressor hydroxyurea. Overall, we obtained evidence for the importance of proteasomes in protection of human cells against biologically relevant doses of FA. Biochemically, our findings indicate the involvement of proteasomes in proteolytic repair of DPC, which removes replication blockage by these highly bulky lesions.

Inhaled formaldehyde induces bone marrow toxicity via oxidative stress in exposed mice

Formaldehyde (FA) is an economically important chemical, and has been found to cause various types of toxic damage to the body. Formaldehyde-induced toxic damage involves reactive oxygen species (ROS) that trigger subsequent toxic effects and inflammatory responses, which may increase risk of cancer. Therefore, in the present study, we aimed to investigate the possible toxic mechanism in bone marrow caused by formaldehyde. In accordance with the principle of randomization, the mice were divided into four groups of 6 mice per group. One group was exposed to ambient air and the other three groups were exposed to different concentrations of formaldehyde (20, 40, 80 mg/m3) for 15 days in the respective inhalation chambers, 2h a day. At the end of the 15-day experimental period, all mice were killed. Bone marrow cells were obtained. Some of those were used for the determination of blood cell numbers, bone marrow karyote numbers, CFU-F, superoxide dismutase (SOD) activity and malondialdehyde (MDA) content; others were used for the determination of mitochondrial membrane potential (MMP), cell cycle and Bcl-2, Bax, CytC protein expression. WBC and PLT numbers in median and high dose groups were obvious reduced, but there was no change on RBC numbers. There was also reduced numbers of bone marrow karyotes and CFU-F in the high dose group. SOD activity was decreased, but MDA content was increased. MMP and Bcl-2 expression were decreased with increasing formaldehyde concentration, while expression of Bax and Cyt C was increased. We also observed change in cell cycling, and found that there was S phase arrest in the high dose group. Our study suggested that a certain concentration of formaldehyde could have toxic effects on the hematopoietic system, with oxidative stress as a critical effect.

BLM protein mitigates formaldehyde-induced genomic instability

Formaldehyde is a reactive aldehyde that has been classified as a class I human carcinogen by the International Agency for Cancer Research. There are growing concerns over the possible adverse health effects related to the occupational and environmental human exposures to formaldehyde. Although formaldehyde-induced DNA and protein adducts have been identified, the genomic instability mechanisms and the cellular tolerance pathways associated with formaldehyde exposure are not fully characterized. This study specifically examines the role of a genome stability protein, Bloom (BLM) in limiting formaldehyde-induced cellular and genetic abnormalities. Here, we show that in the absence of BLM protein, formaldehyde-treated cells exhibited increased cellular sensitivity, an immediate cell cycle arrest, and an accumulation of chromosome radial structures. In addition, live-cell imaging experiments demonstrated that formaldehyde-treated cells are dependent on BLM for timely segregation of daughter cells. Both wild-type and BLM-deficient formaldehyde-treated cells showed an accumulation of 53BP1 and γH2AX foci indicative of DNA double-strand breaks (DSBs); however, relative to wild-type cells, the BLM-deficient cells exhibited delayed repair of formaldehyde-induced DSBs. In response to formaldehyde exposure, we observed co-localization of 53BP1 and BLM foci at the DSB repair site, where ATM-dependent accumulation of formaldehyde-induced BLM foci occurred after the recruitment of 53BP1. Together, these findings highlight the significance of functional interactions among ATM, 53BP1, and BLM proteins as responders associated with the repair and tolerance mechanisms induced by formaldehyde.

Inhaled formaldehyde induces DNA-protein crosslinks and oxidative stress in bone marrow and other distant organs of exposed mice

Formaldehyde (FA), a major industrial chemical and ubiquitous environmental pollutant, has been classified as a leukemogen. The causal relationship remains unclear, however, due to limited evidence that FA induces toxicity in bone marrow, the site of leukemia induction, and in other distal organs. Although induction of DNA-protein crosslinks (DPC), a hallmark of FA toxicity, was not previously detected in the bone marrow of FA-exposed rats and monkeys in studies published in the 1980s, our recent studies showed increased DPC in the bone marrow, liver, kidney, and testes of exposed Kunming mice. To confirm these preliminary results, in the current study we exposed BALB/c mice to 0, 0.5, 1.0, and 3.0 mg m(-3) FA (8 hr per day, for 7 consecutive days) by nose-only inhalation and measured DPC levels in bone marrow and other organs of exposed mice. As oxidative stress is a potential mechanism of FA toxicity, we also measured glutathione (GSH), reactive oxygen species (ROS), and malondialdehyde (MDA), in the bone marrow, peripheral blood mononuclear cells, lung, liver, spleen, and testes of exposed mice. Significant dose-dependent increases in DPC, decreases in GSH, and increases in ROS and MDA were observed in all organs examined (except for DPC in lung). Bone marrow was among the organs with the strongest effects for DPC, GSH, and ROS. In conclusion, exposure of mice to FA by inhalation induced genotoxicity and oxidative stress in bone marrow and other organs. These findings strengthen the biological plausibility of FA-induced leukemogenesis and systemic toxicity.

Role of direct reactivity with metals in chemoprotection by N-acetylcysteine against chromium(VI), cadmium(II), and cobalt(II)

The antioxidant N-acetylcysteine (NAC) is widely used for the assessment of the role of reactive oxygen species (ROS) in various biological processes and adverse drug reactions. NAC has been found to effectively inhibit the toxicity of carcinogenic metals, which was attributed to its potent ROS-suppressive properties. However, the absence of redox activity among some metals and findings from genetic models suggested a more diverse, smaller role of oxidative stress in metal toxicity. Here, we examined mechanisms of chemoprotection by NAC against Cd(II), Co(II), and Cr(VI) in human cells. We found that NAC displayed a broad-spectrum chemoprotective activity against all three metals, including suppression of cytotoxicity, apoptosis, p53 activation, and HSP72 and HIF-1α upregulation. Cytoprotection by NAC was independent of cellular glutathione. NAC strongly inhibited the uptake of all three metals in histologically different types of human cells, explaining its high chemoprotective potential. A loss of Cr(VI) accumulation by cells was caused by NAC-mediated extracellular reduction of chromate to membrane-impermeative Cr(III). Suppression of Co(II) uptake resulted from a rapid formation of Co(II)-NAC conjugates that were unable to enter cells. Our results demonstrate that NAC acts through more than one mechanism in preventing metal toxicity and its chemoprotective activity can be completely ROS-independent. Good clinical safety and effectiveness in Co(II) sequestration suggest that NAC could be useful in the prevention of tissue accumulation and toxic effects of Co ions released by cobalt-chromium hip prostheses.

Uptake, p53 pathway activation, and cytotoxic responses for Co(II) and Ni(II) in human lung cells: implications for carcinogenicity

Cobalt(II) and nickel(II) ions display similar chemical properties and act as hypoxia mimics in cells. However, only soluble Co(II) but not soluble Ni(II) is carcinogenic by inhalation. To explore potential reasons for these differences, we examined responses of human lung cells to both metals. We found that Co(II) showed almost 8 times higher accumulation than Ni(II) in H460 cells but caused a less efficient activation of the transcriptional factor p53 as measured by its accumulation, Ser15 phosphorylation, and target gene expression. Unlike Ni(II), Co(II) was ineffective in downregulating the p53 inhibitor MDM4 (HDMX). Co(II)-treated cells continued DNA replication at internal doses that caused massive apoptosis by Ni(II). Apoptosis and the overall cell death by Co(II) were delayed and weaker than by Ni(II). Inhibition of caspases but not programmed necrosis pathways suppressed Co(II)-induced cell death. Knockdown of p53 produced 50%-60% decreases in activation of caspases 3/7 and expression of 2 most highly upregulated proapoptotic genes PUMA and NOXA by Co(II). Overall, p53-mediated apoptosis accounted for 55% cell death by Co(II), p53-independent apoptosis for 20%, and p53/caspase-independent mechanisms for 25%. Similar to H460, normal human lung fibroblasts and primary human bronchial epithelial cells had several times higher accumulation of Co(II) than Ni(II) and showed a delayed and weaker caspase activation by Co(II). Thus, carcinogenicity of soluble Co(II) could be related to high survival of metal-loaded cells, which permits accumulation of genetic and epigenetic abnormalities. High cytotoxicity of soluble Ni(II) causes early elimination of damaged cells and is expected to be cancer suppressive.

p53 activation by Ni(II) is a HIF-1α independent response causing caspases 9/3-mediated apoptosis in human lung cells

Hypoxia mimic nickel(II) is a human respiratory carcinogen with a suspected epigenetic mode of action. We examined whether Ni(II) elicits a toxicologically significant activation of the tumor suppressor p53, which is typically associated with genotoxic responses. We found that treatments of H460 human lung epithelial cells with NiCl2 caused activating phosphorylation at p53-Ser15, accumulation of p53 protein and depletion of its inhibitor MDM4 (HDMX). Confirming the activation of p53, its knockdown suppressed the ability of Ni(II) to upregulate MDM2 and p21 (CDKN1A). Unlike DNA damage, induction of GADD45A by Ni(II) was p53-independent. Ni(II) also increased p53-Ser15 phosphorylation and p21 expression in normal human lung fibroblasts. Although Ni(II)-induced stabilization of HIF-1α occurred earlier, it had no effect on p53 accumulation and Ser15 phosphorylation. Ni(II)-treated H460 cells showed no evidence of necrosis and their apoptosis and clonogenic death were suppressed by p53 knockdown. The apoptotic role of p53 involved a transcription-dependent program triggering the initiator caspase 9 and its downstream executioner caspase 3. Two most prominently upregulated proapoptotic genes by Ni(II) were PUMA and NOXA but only PUMA induction required p53. Knockdown of p53 also led to derepression of antiapoptotic MCL1 in Ni(II)-treated cells. Overall, our results indicate that p53 plays a major role in apoptotic death of human lung cells by Ni(II). Chronic exposure to Ni(II) may promote selection of resistant cells with inactivated p53, providing an explanation for the origin of p53 mutations by this epigenetic carcinogen.

Triptolide induced DNA damage in A375.S2 human malignant melanoma cells is mediated via reduction of DNA repair genes

Numerous studies have demonstrated that triptolide induces cell cycle arrest and apoptosis in human cancer cell lines. However, triptolide-induced DNA damage and inhibition of DNA repair gene expression in human skin cancer cells has not previously been reported. We sought the effects of triptolide on DNA damage and associated gene expression in A375.S2 human malignant melanoma cells in vitro. Comet assay, DAPI staining and DNA gel electrophoresis were used for examining DNA damage and results indicated that triptolide induced a longer DNA migration smear based on single cell electrophoresis and DNA condensation and damage occurred based on the examination of DAPI straining and DNA gel electrophoresis. The real-time PCR technique was used to examine DNA damage and repair gene expression (mRNA) and results indicated that triptolide led to a decrease in the ataxia telangiectasia mutated (ATM), ataxia-telangiectasia and Rad3-related (ATR), breast cancer 1, early onset (BRCA-1), p53, DNA-dependent serine/threonine protein kinase (DNA-PK) and O6-methylguanine-DNA methyltransferase (MGMT) mRNA expression. Thus, these observations indicated that triptolide induced DNA damage and inhibited DNA damage and repair-associated gene expression (mRNA) that may be factors for triptolide-mediated inhibition of cell growth in vitro in A375.S2 cells.