Biomedical subjects

Todd Davidson

Publications and source records attributed to Todd Davidson.

10 recordsLinked to original sources

PubMed2006

Alterations of histone modifications and transgene silencing by nickel chloride.

Although it has been well established that insoluble nickel compounds are potent carcinogens and soluble nickel compounds are less potent, the mechanisms remain unclear. Nickel compounds are weakly mutagenic, but cause epigenetic effects in cells. Previous studies have shown that insoluble nickel compounds enter cells by phagocytosis and silence gene expression, but the entry of soluble nickel compounds and their effects on gene silencing have not been well studied. Here, we have demonstrated, using a dye that fluoresces when nickel ions bind, that soluble nickel compounds were taken up by cells. Nickel ions localized initially in the cytoplasm, but later entered the nucleus and eventually silenced a transgene. In addition, we described three major changes in histone modification of cells exposed to soluble nickel compounds: (i) loss of acetylation of H2A, H2B, H3 and H4; (ii) increases of H3K9 dimethylation; and (iii) substantial increases of the ubiquitination of H2A and H2B. These effects were observed at nickel exposure conditions that had minimum effects on cell cytotoxicity. Moreover, we demonstrated that nickel-induced transgene silencing was associated with similar changes of histone modifications in their nuclesomes. This study is the first to show that nickel compounds increase histone ubiquitination in cells. These new findings will further our understanding of the epigenetic mechanisms of nickel-mediated carcinogenesis.

Animals↗

PubMed2006

DMT1: which metals does it transport?

DMT1-Divalent Metal (Ion) Transporter 1 or SLC11A2/DCT1/Nramp2 - transports Fe2+ into the duodenum and out of the endosome during the transferrin cycle. DMTI also is important in non-transferrin bound iron uptake. It plays similar roles in Mn2+ trafficking. Voltage clamping showed that six other metals evoked currents, but it is unclear if these metals are substrates for DMT1. This report summarizes progress on which metals DMT1 transports, focusing on results from the authors' labs. We recently cloned 1A/+IRE and 2/-IRE DMT1 isoforms to generate HEK293 cell lines that express them in a tetracycline-inducible fashion, then compared induced expression to uninduced expression and to endogenous DMT1 expression. Induced expression increases approximately 50x over endogenous expression and approximately 10x over uninduced levels. Fe2+, Mn2+, Ni2+ and Cu1+ or Cu2+ are transported. We also explored competition between metal ions using this system because incorporation essentially represents DMT1 transport and find this order for transport affinity: Mn>?Cd>?Fe>Pb-Co-Ni>Zn. The effects of decreased DMT1 also could be examined. The Belgrade rat has diminished DMT1 function and thus provides ways of testing. A series of DNA constructs that generate siRNAs specific for DMT1 or certain DMT1 isoforms yield another way to test DMT1-based transport.

Animals↗

PubMed2005

Soluble nickel interferes with cellular iron homeostasis.

Soluble nickel compounds are likely human carcinogens. The mechanism by which soluble nickel may contribute to carcinogenesis is unclear, though several hypotheses have been proposed. Here we verify the ability of nickel to enter the cell via the divalent metal ion transporter 1 (DMT1) and disturb cellular iron homeostasis. Nickel may interfere with iron at both an extracellular level, by preventing iron from being transported into the cell, and at an intracellular level, by competing for iron sites on enzymes like the prolyl hydroxylases that modify hypoxia inducible factor-1alpha (HIF-1alpha). Nickel was able to decrease the binding of the Von Hippel-Lindau (VHL) protein to HIF-1alpha, indicating a decrease in prolyl hydroxylase activity. The ability of nickel to affect various iron dependent processes may be an important step in nickel dependent carcinogenesis. In addition, understanding the mechanisms by which nickel activates the HIF-1alpha pathway may lead to new molecular targets in fighting cancer.

Binding, Competitive↗

PubMed2004

Nickel decreases cellular iron level and converts cytosolic aconitase to iron-regulatory protein 1 in A549 cells.

Nickel (Ni) compounds are well-established carcinogens and are known to initiate a hypoxic response in cells via the stabilization and transactivation of hypoxia-inducible factor-1 alpha (HIF-1alpha). This change may be the consequence of nickel's interference with the function of several Fe(II)-dependent enzymes. In this study, the effects of soluble nickel exposure on cellular iron homeostasis were investigated. Nickel treatment decreased both mitochondrial and cytosolic aconitase (c-aconitase) activity in A549 cells. Cytosolic aconitase was converted to iron-regulatory protein 1, a form critical for the regulation of cellular iron homeostasis. The increased activity of iron-regulatory protein 1 after nickel exposure stabilized and increased transferrin receptor (Tfr) mRNA and antagonized the iron-induced ferritin light chain protein synthesis. The decrease of aconitase activity after nickel treatment reflected neither direct interference with aconitase function nor obstruction of [4Fe-4S] cluster reconstitution by nickel. Exposure of A549 cells to soluble nickel decreased total cellular iron by about 40%, a decrease that likely caused the observed decrease in aconitase activity and the increase of iron-regulatory protein 1 activity. Iron treatment reversed the effect of nickel on cytosolic aconitase and iron-regulatory protein 1. To assess the mechanism for the observed effects, human embryonic kidney (HEK) cells over expressing divalent metal transporter-1 (DMT1) were compared to A549 cells expressing only endogenous transporters for inhibition of iron uptake by nickel. The inhibition data suggest that nickel can enter via DMT1 and compete with iron for entry into the cell. This disturbance of cellular iron homeostasis by nickel may have a great impact on the ability of the cell to regulate a variety of cell functions, as well as create a state of hypoxia in cells under normal oxygen tension. These effects may be very important in how nickel exerts phenotypic selection pressure to convert a normal initiated cell into a cancer cell.

Aconitate Hydratase↗

PubMed2004

Exposure to chromium (VI) in the drinking water increases susceptibility to UV-induced skin tumors in hairless mice.

Hexavalent chromium (Cr (VI)) is a well known-human carcinogen with exposures occurring in both occupational and environmental settings. Although lung carcinogenicity has been well documented for occupational exposure via inhalation, the carcinogenic hazard of drinking water exposure to Cr (VI) has yet to be established. We used a hairless mouse model to study the effects of K(2)CrO(4) in the drinking water on ultraviolet radiation (UVR)-induced skin tumors. Hairless mice were unexposed or exposed to UVR alone (1.2 kJ/m(2)), K(2)CrO(4) alone at 2.5 and 5.0 ppm, or the combination of UVR and K(2)CrO(4) at 0.5, 2.5, and 5.0 ppm. Mice were observed on a weekly basis for the appearance of skin tumors larger than 2 mm. All the mice were euthanized on day 182. The skin tumors were excised and subsequently analyzed microscopically for malignancy by histopathology. There was a total absence of observable skin tumors in untreated mice and in mice exposed to chromate alone. However, there was a dose-dependent increase in the number of skin tumors greater than 2 mm in mice exposed to K(2)CrO(4) and UV compared with mice exposed to UV alone. The increase in tumors larger than 2 mm was statistically significant (P < 0.05) for UV and K(2)CrO(4) at the two highest K(2)CrO(4) doses (2.5 and 5.0 ppm), and there was a statistically significant increase in the numbers of malignant tumors per mouse in the UVR plus K(2)CrO(4) (5 ppm) group compared with UV alone. The data presented here indicate that K(2)CrO(4) increases the number of UV-induced skin tumors in a dose-dependent manner, and these results support the concern that regulatory agencies have relative to the carcinogenic health hazards of widespread human exposure to Cr (VI) in drinking water.

Animals↗

PubMed2004

Nickel-induced 1,4-alpha-glucan branching enzyme 1 up-regulation via the hypoxic signaling pathway.

Using the mouse Affymetrix gene chip, we found that 1,4-alpha-glucan branching enzyme 1 (GBE1) was one of the most up-regulated genes following nickel exposure. This result was confirmed by Northern blot in two mouse cell lines, four mouse tissues, and three human cell lines. We further found that this gene was also up-regulated by cobalt, hypoxia, the iron chelator (deferoxamine, or DFO), and the prolyl hydroxylase (PH) inhibitor (dimethyloxalyglycine, DMOG), suggesting that hypoxia inducible factor-1alpha (HIF-1alpha) was involved in the up-regulation of this gene. Experiments using HIF-1alpha +/+ and HIF-1alpha -/- mouse cells demonstrated this gene was up-regulated through a HIF-1alpha-dependent hypoxic signaling pathway. Because the hypoxic signaling pathway is believed to be important in the initiation and progression of carcinogenesis, it is important to study genes regulated by this pathway.

1,4-alpha-Glucan Branching Enzyme↗

PubMed2003

The involvement of hypoxia-inducible transcription factor-1-dependent pathway in nickel carcinogenesis.

Nickel is a potent environmental pollutant in industrial countries. Because nickel compounds are carcinogenic, exposure to nickel represents a serious hazard to human health. The understanding of how nickel exerts its toxic and carcinogenic effects at a molecular level may be important in risk assessment, as well as in the treatment and prevention of occupational diseases. Previously, using human and rodent cells in vitro, we showed that hypoxia-inducible signaling pathway was activated by carcinogenic nickel compounds. Acute exposure to nickel resulted in the accumulation of hypoxia-inducible transcription factor (HIF)-1, which strongly activated hypoxia-inducible genes, including the recently discovered tumor marker NDRG1 (Cap43). To further identify HIF-1-dependent nickel-inducible genes and to understand the role of the HIF-dependent signaling pathway in nickel-induced transformation, we used the Affymetrix GeneChip to compare the gene expression profiles in wild-type cells or in cells from HIF-1 alpha knockout mouse embryos exposed to nickel chloride. As expected, when we examined 12,000 genes for expression changes, we found that genes coding for glycolytic enzymes and glucose transporters, known to be regulated by HIF-1 transcription factor, were induced by nickel only in HIF-1 alpha-proficient cells. In addition, we found a number of other hypoxia-inducible genes up-regulated by nickel in a HIF-dependent manner including BCL-2-binding protein Nip3, EGLN1, hypoxia-inducible gene 1 (HIG1), and prolyl 4-hydroxylase. Additionally, we found a number of genes induced by nickel in a HIF-independent manner, suggesting that Ni activated other signaling pathways besides HIF-1. Finally, we found that in HIF-1 alpha knockout cells, nickel strongly induced the expression of the whole group of genes that were not expressed in the presence of HIF-1. Because the majority of modulated genes were induced or suppressed by nickel in a HIF-1-dependent manner, we elucidated the role of HIF-1 transcription factor in cell transformation. In HIF-1 alpha-proficient cells, nickel exposure increased soft agar growth, whereas it decreased soft agar growth in HIF-1 alpha-deficient cells. We hypothesize that the induction of HIF-1 transcription factor by nickel may be important during the nickel-induced carcinogenic process.

Animals↗

PubMed2003

GeneChip analysis of signaling pathways effected by nickel.

The carcinogenicity of nickel compounds has been shown in numerous epidemiological and animal studies. Carcinogenesis is generally considered as a multistep accumulation of genetic alterations. Nickel, however, being highly carcinogenic is only a weak mutagen. We hypothesize that nickel may act by modulating signaling pathways, and subsequently by reprogramming transcription factors. Insoluble nickel is considered to be more carcinogenic than soluble. In this study using GeneChip technology we compared changes in gene expression caused by soluble and insoluble nickel compounds. We found that both soluble and insoluble nickel compounds induce similar signaling pathways following 20 h of in vitro exposure. For example, both nickel compounds activated a number of transcription factors including hypoxia-inducible factor I (HIF-1) and p53. The induction of these important transcription factors exerts potent selective pressure leading to cell transformation. The obtained data are in agreement with our previous observations that acute nickel exposure activates HIF-1 and p53 transcription factors and in nickel-transformed cells, the ratio of HIF-I activity to p53 activity was shifted towards high HIF-I activity. The activation of the same signaling pathways by soluble and insoluble nickel compounds suggested that both nickel compounds have similar carcinogenic potential in vitro.

Animals↗

PubMed2003

Hypoxia inducible factor-1 alpha-independent suppression of aryl hydrocarbon receptor-regulated genes by nickel.

Aryl hydrocarbon receptor (AhR)-dependent enzymes are involved in the biotransformation of harmful xenobiotics into more easily excretable metabolites. Cross-talk between the AhR pathway and the hypoxia inducible factor-1alpha (HIF-1alpha) pathway has been demonstrated previously, although the mechanism remains unclear and quite controversial. Because nickel is known to mimic hypoxia, we investigated the effects of short-term nickel exposure on AhR-dependent gene expression. Gene-chip analysis identified several AhR-dependent genes that are suppressed by exposure to nickel. Using Northern blots, we then confirmed that nickel can down-regulate both the basal and benzo[a]pyrene-inducible expression of AhR-dependent genes in mouse and human cell lines. Using a HIF-1alpha knockout cell line and 3-[2-[4-(bis-(4-fluorophenyl) methylene]-1-piperidinyl)ethyl]-2,3-dihydro-2-thioxo-4(1H)quinazolinone (R59949), which blocks HIF-1alpha protein accumulation, we show HIF-1alpha-independent suppression of AhR-dependent genes by nickel. Desferrioxamine and hypoxia were also able to suppress the basal and inducible expression levels of AhR-regulated genes. Finally, dimethyloxalylglycine, an inhibitor of Fe(II)- and 2-oxoglutarate-dependent dioxygenases also inhibited AhR-dependent expression in a HIF-1alpha-independent manner. Our data suggest that an Fe(II)-, oxoglutarate-, and oxygen-dependent enzyme may directly or indirectly be involved in the regulation of AhR-dependent transcriptional activity by nickel and other hypoxia-mimicking agents.

Animals↗

PubMed2002

The role of hypoxia-inducible signaling pathway in nickel carcinogenesis.

Using human and rodent cells in vitro, we characterized a hypoxia-inducible signaling pathway as one of the pathways affected by carcinogenic nickel compounds. Acute exposure to nickel activates hypoxia-inducible transcription factor-1 (HIF-1), which strongly induces hypoxia-inducible genes, including the recently discovered tumor marker Cap43. This gene has been cloned based on its nickel inducibility and was found to be highly inducible by hypoxia. To identify other HIF-1-dependent/independent nickel-inducible genes, we used cells obtained from HIF-1 alpha null mouse embryos and analyzed gene expression changes using the microarray technique. We found that genes coding for glycolytic enzymes, known to be regulated by HIF-1, were also induced in nickel-exposed cells. In addition, we identified a number of new genes highly induced by nickel in an HIF-dependent manner. Elevated HIF-1 activity after acute nickel exposure might be selectively advantageous because nickel-transformed rodent and human cells possess increased HIF-1 transcriptional activity. Hypoxia plays an important role in tumor progression. It selects for cells with enhanced glycolytic activity, causing production of large amounts of lactic acid, one of the most common features of tumor cells (Warburg effect). Here, we hypothesize that exposure to nickel activates the hypoxia-inducible pathway and facilitates selection of cells with increased transcriptional activity of hypoxia-inducible genes, which may be important in the nickel-induced carcinogenic process.

Animals↗