Chronic Exposure to Particulate Chromate Induces Premature Centrosome Separation and Centriole Disengagement in Human Lung Cells

Particulate hexavalent chromium inhibits global transcription of genes in DNA repair pathways, particularly targeting homologous recombination repair, base excision repair, mismatch repair and microhomology-mediated end-joining

Hexavalent chromium [Cr(VI)] is a human lung carcinogen with widespread exposure. How Cr(VI) causes cancer is poorly understood, but chromosome instability plays a central role. Inhibition of DNA repair pathways leads to chromosome instability; however, despite the importance of these pathways in the mechanism of Cr(VI)-induced lung carcinogenesis, there are no data considering in-depth analysis on the transcriptional changes of genes involved in them. This study characterized the global transcriptional changes of mRNA expression after Cr(VI) exposure focusing on DNA repair pathways. The repair pathways considered included homologous recombination repair, non-homologous end joining, microhomology-directed end-joining, single strand annealing, mismatch repair, base excision repair, nucleotide excision repair and crosslink repair. Normal human lung fibroblast cells were exposed to increasing zinc chromate concentrations for 24, 72 or 120 h then RNA was extracted and sequenced. Our results indicate Cr(VI) causes differential expression of genes in lung cancer pathways and downregulates expression of some genes in all 8 DNA repair pathways. Homologous recombination repair, mismatch repair, base excision repair and microhomology-directed end-joining were the most affected pathways. This study provides a critical in-depth analysis of the effects of Cr(VI) on DNA repair pathways and contributes new insights into the mechanism of Cr(VI)-carcinogenesis.

Carcinogenic Mechanisms of Hexavalent Chromium: From DNA Breaks to Chromosome Instability and Neoplastic Transformation

PURPOSE OF REVIEW

Hexavalent chromium [Cr(VI)] is a well-established human carcinogen, yet the mechanisms by which it leads to carcinogenic outcomes is still unclear. As a driving factor in its carcinogenic mechanism, Cr(VI) causes DNA double strand breaks and break-repair deficiency, leading to the development of chromosome instability. Therefore, the aim of this review is to discuss studies assessing Cr(VI)-induced DNA double strand breaks, chromosome damage and instability, and neoplastic transformation including cell culture, experimental animal, human pathology and epidemiology studies.

RECENT FINDINGS

Recent findings confirm Cr(VI) induces DNA double strand breaks, chromosome instability and neoplastic transformation in exposed cells, animals and humans, emphasizing these outcomes as key steps in the mechanism of Cr(VI) carcinogenesis. Moreover, recent findings suggest chromosome instability is a key phenotype in Cr(VI)-neoplastically transformed clones and is an inheritable and persistent phenotype in exposed cells, once more suggesting chromosome instability as central in the carcinogenic mechanism. Although limited, some studies have demonstrated DNA damage and epigenetic modulation are also key outcomes in biopsies from chromate workers that developed lung cancer. Additionally, we also summarized new studies showing Cr(VI) causes genotoxic and clastogenic effects in cells from wildlife, such as sea turtles, whales, and alligators. Overall, across the literature, it is clear that Cr(VI) causes neoplastic transformation and lung cancer. Many studies measured Cr(VI)-induced increases in DNA double strand breaks, the most lethal type of breaks clearly showing that Cr(VI) is genotoxic. Unrepaired or inaccurately repaired breaks lead to the development of chromosome instability, which is a common phenotype in Cr(VI) exposed cells, animals, and humans. Indeed, many studies show Cr(VI) induces both structural and numerical chromosome instability. Overall, the large body of literature strongly supports the conclusion that Cr(VI) causes DNA double strand breaks, inhibits DNA repair and chromosome instability, which are key to the development of Cr(VI)-induced cell transformation.

A whale of a tale: whale cells evade the driving mechanism for hexavalent chromium-induced chromosome instability

Chromosome instability, a hallmark of lung cancer, is a driving mechanism for hexavalent chromium [Cr(VI)] carcinogenesis in humans. Cr(VI) induces structural and numerical chromosome instability in human lung cells by inducing DNA double-strand breaks and inhibiting homologous recombination repair and causing spindle assembly checkpoint (SAC) bypass and centrosome amplification. Great whales are long-lived species with long-term exposures to Cr(VI) and accumulate Cr in their tissue, but exhibit a low incidence of cancer. Data show Cr(VI) induces fewer chromosome aberrations in whale cells after acute Cr(VI) exposure suggesting whale cells can evade Cr(VI)-induced chromosome instability. However, it is unknown if whales can evade Cr(VI)-induced chromosome instability. Thus, we tested the hypothesis that whale cells resist Cr(VI)-induced loss of homologous recombination repair activity and increased SAC bypass and centrosome amplification. We found Cr(VI) induces similar amounts of DNA double-strand breaks after acute (24 h) and prolonged (120 h) exposures in whale lung cells, but does not inhibit homologous recombination repair, SAC bypass, or centrosome amplification, and does not induce chromosome instability. These data indicate whale lung cells resist Cr(VI)-induced chromosome instability, the major driver for Cr(VI) carcinogenesis at a cellular level, consistent with observations that whales are resistant to cancer.

Hexavalent Chromium Targets Securin to Drive Numerical Chromosome Instability in Human Lung Cells

Hexavalent chromium [Cr(VI)] is a known human lung carcinogen with widespread exposure in environmental and occupational settings. Despite well-known cancer risks, the molecular mechanisms of Cr(VI)-induced carcinogenesis are not well understood, but a major driver of Cr(VI) carcinogenesis is chromosome instability. Previously, we reported Cr(VI) induced numerical chromosome instability, premature centriole disengagement, centrosome amplification, premature centromere division, and spindle assembly checkpoint bypass. A key regulator of these events is securin, which acts by regulating the cleavage ability of separase. Thus, in this study we investigated securin disruption by Cr(VI) exposure. We exposed human lung cells to a particulate Cr(VI) compound, zinc chromate, for acute (24 h) and prolonged (120 h) time points. We found prolonged Cr(VI) exposure caused marked decrease in securin levels and function. After prolonged exposure at the highest concentration, securin protein levels were decreased to 15.3% of control cells, while securin mRNA quantification was 7.9% relative to control cells. Additionally, loss of securin function led to increased separase activity manifested as enhanced cleavage of separase substrates; separase, kendrin, and SCC1. These data show securin is targeted by prolonged Cr(VI) exposure in human lung cells. Thus, a new mechanistic model for Cr(VI)-induced carcinogenesis emerges with centrosome and centromere disruption as key components of numerical chromosome instability, a key driver in Cr(VI) carcinogenesis.

Prolonged particulate hexavalent chromium exposure induces RAD51 foci inhibition and cytoplasmic accumulation in immortalized and primary human lung bronchial epithelial cells

Hexavalent chromium [Cr(VI)] is a human lung carcinogen with widespread exposure risks. Cr(VI) causes DNA double strand breaks that if unrepaired, progress into chromosomal instability (CIN), a key driving outcome in Cr(VI)-induced tumors. The ability of Cr(VI) to cause DNA breaks and inhibit repair is poorly understood in human lung epithelial cells, which are extremely relevant since pathology data show Cr(VI)-induced tumors originate from bronchial epithelial cells. In the present study, we considered immortalized and primary human bronchial epithelial cells. Cells were treated with zinc chromate at concentrations ranging 0.05 to 0.4μg/cm2 for acute (24 h) and prolonged (120 h) exposures. DNA double strand breaks (DSBs) were measured by neutral comet assay and the status of homologous recombination repair, the main pathway to fix Cr(VI)-induced DSBs, was measured by RAD51 foci formation with immunofluorescence, RAD51 localization with confocal microscopy and sister chromatid exchanges. We found acute and prolonged Cr(VI) exposure induced DSBs. Acute exposure induced homologous recombination repair, but prolonged exposure inhibited it resulting in chromosome instability in immortalized and primary human bronchial epithelial cells.

Inflammatory effects of hexavalent chromium in the lung: A comprehensive review

Besides smoking, lung cancer can be caused by other factors, including heavy metals such as cadmium, nickel, arsenic, beryllium and hexavalent chromium [Cr(VI)], which is used in multiple settings, resulting in widespread environmental and occupational exposures as well as heavy use. The mechanism by which Cr(VI) causes lung cancer is not completely understood. Currently, it is admitted chromosome instability is a key process in the mechanism of Cr(VI)-induced cancer, and previous studies have suggested Cr(VI) impacts the lung tissue in mice by triggering tissue damage and inflammation. However, the mechanism underlying Cr(VI)-induced inflammation and its exact role in lung cancer are unclear. Therefore, this review aimed to systematically examine previous studies assessing Cr(VI)-induced inflammation and to summarize the major inflammatory pathways involved in Cr(VI)-induced inflammation. In cell culture studies, COX2, VEGF, JAK-STAT, leukotriene B4 (LTB4), MAPK, NF-ҡB and Nrf2 signaling pathways were consistently upregulated by Cr(VI), clearly demonstrating that these pathways are involved in Cr(VI)-induced inflammation. In addition, Akt signaling was also shown to contribute to Cr(VI)-induced inflammation, although discrepant findings were reported. Few mechanistic studies were performed in animal models, in which Cr(VI) upregulated oxidative pathways, NF-kB signaling and the MAPK pathway in the lung tissue. Similar to cell culture studies, opposite effects of Cr(VI) on Akt signaling were reported. This work provides insights into the mechanisms by which Cr(VI) induces lung inflammation. However, discrepant findings and other major issues in study design, both in cell and animal models, suggest that further studies are required to unveil the mechanism of Cr(VI)-induced inflammation and its role in lung cancer.

Particulate matter (PM10) destabilizes mitotic spindle through downregulation of SETD2 in A549 lung cancer cells

Air pollution represents an environmental problem, impacting negatively in human health. Particulate matter of 10 μm or less in diameter (PM₁₀) is related to pulmonary diseases, including lung cancer. Mitotic spindle is made up by chromosome-microtubule (MT) interactions, where SETD2 plays an important role in MT stability. SETD2 binds and activates α-TUBULIN sub-unit and promotes MT polymerization. Alongside this mechanism, the spindle assembly checkpoint (SAC) senses the adequate mitotic progression through proteins such as BUBR1, AURORA B and SURVIVIN. Alterations in MT dynamics as well as in SAC cause aneuploidy and chromosomal instability, a common phenotype in cancer cells. In this study, we evaluated the effect of PM₁₀ in the expression and protein levels of SETD2, as well as the effect in the expression and protein levels of SAC and mitotic components involved in chromosomal segregation/mitosis, using the A549 lung cancer cell line. A549 cell cultures were exposed to PM₁₀ (10 μg/cm²) for 24 h to evaluate the expression and protein levels of SETD2 (SETD2), TUBA1A (α-TUBULIN), CCNB1 (CYCLIN B1), BUB1B (BUBR1), AURKB (AURORA B) and BIRC5 (SURVIVIN). We observed that PM₁₀ decreases the expression and protein levels of SETD2, α-TUBULIN and BUBR1 and increases the levels of AURORA B and SURVIVIN in A549 cells, compared with non-treated cells. PM₁₀ also caused a decrease in mitotic index and in the percentage of cells in G2/M when compared with control group. Co-localization of SETD2/α -TUB was lower in PM₁₀-treated cells in comparison with non-treated cells. Finally, micronuclei (MN) frequency was higher in PM₁₀-treated cells in contrast with non-treated cells, being whole chromosomes more common in PM₁₀-treated MN than in non-treated MN. Our results suggest that PM₁₀ causes missegregation and aneuploidy through downregulation of SETD2 and SAC components, inducing aneuploidy and predisposing to the generation of chromosomal instability in transformed cells.

Particulate hexavalent chromium alters microRNAs in human lung cells that target key carcinogenic pathways

Hexavalent chromium [Cr(VI)] is a global environmental pollutant and human lung carcinogen. However, the mechanisms of Cr(VI) carcinogenesis are not well defined. Cr(VI)-altered gene expression has been reported in the literature and is implicated in numerous mechanisms of Cr(VI) carcinogenesis. MicroRNAs (miRNAs) play a key role in controlling gene expression and are associated with carcinogenic mechanisms. To date no studies have evaluated global changes in miRNA expression in human cells after Cr(VI) exposure. We used RNA sequencing to evaluate how a particulate Cr(VI) compound (zinc chromate), the most potent form of Cr(VI), alters global miRNA expression after acute (24 h) or prolonged (72 and 120 h) exposure to 0.1, 0.2 and 0.3 μg/cm² zinc chromate in an immortalized, non-cancerous human lung cell line (WTHBF-6). Particulate Cr(VI) significantly affected expression of miRNAs at all time points and concentrations tested. We also found the number of significantly downregulated miRNAs increased in a time- and concentration-dependent manner and many miRNAs were upregulated after 24 h exposure at the intermediate concentration tested. Pathway analyses of the differentially expressed miRNAs predicted miRNAs target pathways of Cr(VI) carcinogenesis in a time- and concentration-dependent manner. These data are the first to evaluate global changes in miRNA expression in human lung cells after Cr(VI) exposure and indicate miRNAs may play a key role in pathways of Cr(VI) carcinogenesis.

Extract of bulbus of Fritillaria cirrhosa induces spindle multipolarity in human-derived colonic epithelial NCM460 cells through promoting centrosome fragmentation

Bulbus of Fritillaria cirrhosa D. Don (BFC), an outstanding antitussive and expectorant herbal drug used in China and many other countries, has potential but less understood genotoxicity. Previously, we have reported that aqueous extract of BFC compromised the spindle assembly checkpoint and cytokinesis in NCM460 cells. Here, we found that one remarkable observation in BFC-treated NCM460 cells was multipolar mitosis, a trait classically compromises the fidelity of chromosome segregation. More detailed investigation revealed that BFC-induced spindle multipolarity in metaphases and ana-telophases in a dose- and time-dependent manner, suggesting BFC-induced multipolar spindle conformation was not transient. The frequency of multipolar metaphase correlated well to that of multipolar ana-telophases, indicating that BFC-induced multipolar metaphases often persisted through anaphase. Unexpectedly, BFC blocked the proliferation of binucleated cells, suggesting spindle multipolarity was not downstream of BFC-induced cytokinesis failure. Exposure of BFC to early mitotic cells, rather than S/G2 cells, contributed greatly to spindle multipolarity, indicating BFC might disrupt centrosome integrity rather than induce centrosome overduplication. The immunofluorescence results showed that the centrosomes were severely fragmented by a short-term treatment of BFC and the extent of centrosome fragmentation in early mitotic cells was larger than this in S/G2 cells. Consistently, several genes (e.g. p53, Rb centrin-2, Plk-4, Plk-1 and Aurora-A) involved in regulating centrosome integrity were significantly deregulated by BFC. Together, our results suggest that BFC causes multipolar spindles primarily by inducing centrosome fragmentation. Coupling these results to our previous observations, we recommend the risk/benefit ratio should be considered in the practical use of BFC.

Levels, Spatial Distribution, and Impact Factors of Heavy Metals in the Hair of Metropolitan Residents in China and Human Health Implications

Chronic exposure to low levels of heavy metals threatens human health. However, few studies evaluated the health effects and spatial distributions of chronic exposure to heavy metals in metropolitan residents throughout mainland China using unified sampling methods and evaluation indicators at the national level. Here, the concentrations and spatial distributions of heavy metals (As, Cd, Cr, Sb, Pb, and Hg) in the hair of 1202 metropolitan residents from mainland China were analyzed, and differences in age and sex were evaluated. Most target metals exhibited higher concentrations in the hair of residents from South Central China. Generally, male hair had higher As and Se concentrations, whereas female hair had higher Cd and Pb levels (p < 0.05). A significant pairwise correlation existed between most metals in hair, especially Cd-Pb (r = 0.638, p < 0.05). The Se/heavy metal molar ratio is used as an indicator to assess the detoxification ability. The results demonstrated that protecting metropolitan residents in South Central China from heavy metals in their daily life is crucial, particularly for Hg, Pb, and Cr with Se/(Hg, Pb, or Cr) molar ratios < 1. This is the first study to comprehensively consider the antagonistic effects of Se and heavy metals using the molar ratio of Se/heavy metals to evaluate health implications and propose health management policies for metropolitan residents in China.

Cep57 and Cep57L1 maintain centriole engagement in interphase to ensure centriole duplication cycle

Centrioles duplicate in interphase only once per cell cycle. Newly formed centrioles remain associated with their mother centrioles. The two centrioles disengage at the end of mitosis, which licenses centriole duplication in the next cell cycle. Therefore, timely centriole disengagement is critical for the proper centriole duplication cycle. However, the mechanisms underlying centriole engagement during interphase are poorly understood. Here, we show that Cep57 and Cep57L1 cooperatively maintain centriole engagement during interphase. Codepletion of Cep57 and Cep57L1 induces precocious centriole disengagement in interphase without compromising cell cycle progression. The disengaged daughter centrioles convert into centrosomes during interphase in a Plk1-dependent manner. Furthermore, the centrioles reduplicate and the centriole number increases, which results in chromosome segregation errors. Overall, these findings demonstrate that the maintenance of centriole engagement by Cep57 and Cep57L1 during interphase is crucial for the tight control of centriole copy number and thus for proper chromosome segregation.

Mitotic Spindle Apparatus Abnormalities in Chronic Obstructive Pulmonary Disease Cells: A Potential Pathway to Lung Cancer

Chronic obstructive pulmonary disease (COPD) is a long-term lung disease characterized by irreversible lung damage resulting in airflow limitation, abnormal permanent air-space enlargement, and emphysema. Cigarette smoking is the major cause of COPD with 15% to 30% of smokers developing either disease. About 50% to 80% of patients with lung cancer have preexisting COPD and smokers who have COPD are at an increased risk for developing lung cancer. Therefore, COPD is considered an independent risk for lung cancer, even after adjusting for smoking. A crucial early event in carcinogenesis is the induction of the genomic instability through alterations in the mitotic spindle apparatus. To date, the underlying mechanism by which COPD contributes to lung cancer risk is unclear. We hypothesized that tobacco smoke carcinogens induce mitotic spindle apparatus abnormalities and alter expression of crucial genes leading to increased genomic instability and ultimately tumorigenesis. To test our hypothesis, we assessed the genotoxic effects of a potent tobacco-smoke carcinogen [4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, (NNK)] on bronchial epithelial cells from patients with COPD and normal bronchial epithelial cells and identified genes associated with mitotic spindle defects and chromosome missegregation that also overlap with lung cancer. Our results indicate that exposure to NNK leads to a significantly altered spindle orientation, centrosome amplification, and chromosome misalignment in COPD cells as compared with normal epithelial cells. In addition, we identified several genes (such as AURKA, AURKB, and MAD2L2) that were upregulated and overlap with lung cancer suggesting a potential common pathway in the transition from COPD to lung cancer.

Airborne particulate matter induces mitotic slippage and chromosomal missegregation through disruption of the spindle assembly checkpoint (SAC)

Particulate matter (PM) is a risk factor for lung cancer development and chromosomal missegregation and cell cycle disruptions are key cellular events that trigger tumorigenesis. We aimed to study the effect of PM₁₀ (PM with an aerodynamic diameter ≤10 μm) on mitotic arrest and chromosomal segregation, evaluating the spindle assembly checkpoint (SAC) protein dynamics in the human lung A549 adenocarcinoma cell line. For this purpose, synchronized cells were exposed to PM₁₀ for 24 h to obtain the frequency of micronucleated (MN) and trinucleated (TN) cells. Then, the efficiency of the mitotic arrest after PM₁₀ exposure was analyzed. To elucidate the effect of PM₁₀ in chromosomal segregation, the levels and subcellular localization of essential SAC proteins were evaluated. Results indicated that A549 cells exposed to PM₁₀ exhibited an increase in MN and TN cells and a decrease in mitotic indexes and G2/M phase. A549 cells treated with PM₁₀ showed reduced protein levels of MDC1 and NEK2 (38% and 35% respectively), which is required for MAD2 recruitment to kinetochores, MAD2 and BUBR1, effectors of the SAC (25% and 18% respectively), and CYCLIN B1, required during G2/M phase (35%). Besides, PM₁₀ exposure increase the levels of AURORA B and SURVIVIN, required for SAC activation through chromosome-microtubule attachment errors (85% and 74% respectively). We suggest that PM₁₀ causes mitotic slippage due to alterations in MAD2 localization. Thus, PM₁₀ causes inadequate chromosomal segregation and deficient mitotic arrest by altering SAC protein levels, predisposing A549 cells to chromosomal instability, a common feature observed in cancer.

A comparison of particulate hexavalent chromium cytotoxicity and genotoxicity in human and leatherback sea turtle lung cells from a one environmental health perspective

Evaluating health risks of environmental contaminants can be better achieved by considering toxic impacts across species. Hexavalent chromium [Cr(VI)] is a marine pollutant and global environmental contaminant. While Cr(VI) has been identified as a human lung carcinogen, health effects in marine species are poorly understood. Little is known about how Cr(VI) might impact humans and marine species differently. This study used a One Environmental Health Approach to compare the cytotoxicity and genotoxicity of particulate Cr(VI) in human and leatherback sea turtle (Dermochelys coriacea) lung fibroblasts. Leatherbacks may experience prolonged exposures to environmental contaminants and provide insight to how environmental exposures affect health across species. Since humans and leatherbacks may experience prolonged exposure to Cr(VI), and prolonged Cr(VI) exposure leads to carcinogenesis in humans, in this study we considered both acute and prolonged exposures. We found particulate Cr(VI) induced cytotoxicity in leatherback cells comparable to human cell data supporting current research that shows Cr(VI) impacts health across species. To better understand mechanisms of Cr(VI) toxicity we assessed the genotoxic effects of particulate Cr(VI) in human and leatherback cells. Particulate Cr(VI) induced similar genotoxicity in both cell lines, however, human cells arrested at lower concentrations than leatherback cells. We also measured intracellular Cr ion concentrations and found after prolonged exposure human cells accumulated more Cr than leatherback cells. These data indicate Cr(VI) is a health concern for humans and leatherbacks. The data also suggest humans and leatherbacks respond to chemical exposure differently, possibly leading to the discovery of species-specific protective mechanisms.

Hexavalent Chromium-Induced Chromosome Instability Drives Permanent and Heritable Numerical and Structural Changes and a DNA Repair-Deficient Phenotype

A key hypothesis for how hexavalent chromium [Cr(VI)] causes cancer is that it drives chromosome instability (CIN), which leads to neoplastic transformation. Studies show chronic Cr(VI) can affect DNA repair and induce centrosome amplification, which can lead to structural and numerical CIN. However, no studies have considered whether these outcomes are transient or permanent. In this study, we exposed human lung cells to particulate Cr(VI) for three sequential 24-hour periods, each separated by about a month. After each treatment, cells were seeded at colony-forming density, cloned, expanded, and retreated, creating three generations of clonal cell lines. Each generation of clones was tested for chromium sensitivity, chromosome complement, DNA repair capacity, centrosome amplification, and the ability to grow in soft agar. After the first treatment, Cr(VI)-treated clones exhibited a normal chromosome complement, but some clones showed a repair-deficient phenotype and amplified centrosomes. After the second exposure, more than half of the treated clones acquired an abnormal karyotype including numerical and structural alterations, with many exhibiting deficient DNA double-strand break repair and amplified centrosomes. The third treatment produced new abnormal clones, with previously abnormal clones acquiring additional abnormalities and most clones exhibiting repair deficiency. CIN, repair deficiency, and amplified centrosomes were all permanent and heritable phenotypes of repeated Cr(VI) exposure. These outcomes support the hypothesis that CIN is a key mechanism of Cr(VI)-induced carcinogenesis.Significance: Chromium, a major public health concern and human lung carcinogen, causes fundamental changes in chromosomes and DNA repair in human lung cells. Cancer Res; 78(15); 4203-14. ©2018 AACR.