Todd Davidson

Hypoxic stress induces dimethylated histone H3 lysine 9 through histone methyltransferase G9a in mammalian cells.

Dimethylated histone H3 lysine 9 (H3K9me2) is a critical epigenetic mark for gene repression and silencing and plays an essential role in embryogenesis and carcinogenesis. Here, we investigated the effects of hypoxic stress on H3K9me2 at both global and gene-specific level. We found that hypoxia increased global H3K9me2 in several mammalian cell lines. This hypoxia-induced H3K9me2 was temporally correlated with an increase in histone methyltransferase G9a protein and enzyme activity. The increase in H3K9me2 was significantly mitigated in G9a-/- mouse embryonic stem cells following hypoxia challenge, indicating that G9a was involved in the hypoxia-induced H3K9me2. In addition to the activation of G9a, our results also indicated that hypoxia increased H3K9me2 by inhibiting H3K9 demethylation processes. Hypoxic mimetics, such as deferoxamine and dimethyloxalylglycine, were also found to increase H3K9me2 as well as G9a protein and activity. Finally, hypoxia increased H3K9me2 in the promoter regions of the Mlh1 and Dhfr genes, and these increases temporally correlated with the repression of these genes. Collectively, these results indicate that G9a plays an important role in the hypoxia-induced H3K9me2, which would inhibit the expression of several genes that would likely lead to solid tumor progression.

Soluble nickel inhibits HIF-prolyl-hydroxylases creating persistent hypoxic signaling in A549 cells

Soluble nickel compounds are carcinogenic to humans although the mechanism by which they cause cancer remains unclear. One major consequence of exposure to nickel is the stabilization of hypoxia inducible factor‐1α (HIF‐1α), a protein known to be overexpressed in a variety of cancers. In this study, we report a persistent stabilization of HIF‐1α by nickel chloride up to 72 h after the removal of nickel from the culture media. In addition, we show that the HIF‐prolyl hydroxylases (PHDs) are inhibited when cells are exposed to nickel and that they remain repressed for up to 72 h after nickel is removed. We then show that nickel can inhibit purified HIF‐PHDs 2 in vitro, through direct interference with the enzyme. Through theoretical calculations, we also demonstrate that nickel may be able to replace the iron in the active site of this enzyme, providing a plausible mechanism for the persistent inhibition of HIF‐PHDs by nickel. The data presented suggest that nickel can interfere with HIF‐PHD directly and does not inhibit the enzyme by simply depleting cellular factors, such as iron or ascorbic acid. Understanding the mechanisms by which nickel can inhibit HIF‐PHDs and stabilize HIF‐1α may be important in the treatment of cancer and ischemic diseases.

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-1α (HIF-1α). Nickel was able to decrease the binding of the Von Hippel–Lindau (VHL) protein to HIF-1α, 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-1α pathway may lead to new molecular targets in fighting cancer.

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.

Selected Molecular Mechanisms of Metal Toxicity and Carcinogenicity

Abstract This chapter will summarize some of the molecular responses exhibited by cells that come into contact with toxic metals. We will consider the transport of toxic metals into cells and how this interferes with the transport of essential metals. Toxic metals also interfere with the intracellular action of essential metals and may cause toxicity and cancer by this mechanism. Some metals have very specialized effects on enzymes and there are proteins that can bind toxic metals such as metallothionein, which will also be discussed. A number of metals are also slightly mutagenic or genotoxic and these effects will be reviewed. Since most metals that are carcinogenic, such as arsenic (As), beryllium (Be), cadmium (Cd), and nickel (Ni), with the exception of chromium (Cr), do not interact with DNA and are not mutagenic, we discuss other mechanisms such as epigenetic effects to account for their carcinogenic activity, as well as how they affect the expression of genes. Metals also interfere with cell signaling and we will discuss the hypoxic signaling pathway, in particular, as well as those involving PI3K, Akt, reactive oxygen species mitogen-activated protein kinase (MAPK), NF-κB, NF-AT, and AP-1. Finally, the basis of the interaction of toxic metals with all cellular constituents is their coordination with biological ligands, which will be addressed throughout the chapter.