Certain transition metals, including nickel, chromium, cadmium, and copper, are carcinogenic to humans and/or animals. They induce cancer of the respiratory tract in humans exposed to metal-containing aerosols and also increase the incidence of childhood malignancies in progeny of the exposed fathers. However, mechanisms of the carcinogenic activity of these metals remain obscure. In recent years, we have been testing a hypothesis that the mechanisms involve metal-mediated structural and/or oxidative damage to chromatin and some enzymes, including DNA repair proteins and non-heme oxidases.
In 2005/2006 we continued investigations based on the above hypothesis. Our research on transition metals' interactions with chromatin components were focused on Ni(II)-assisted modifications of core histones, especially histones H2A and H2B. As we have found before, Ni(II) mediates hydrolytic cleavage of the C-terminal tail of histone H2A. To check biological consequences of such cleavage, we obtained constructs of the truncated and full-length histones H2A, transfected them to cultured cells, and found clusters of genes expressed differently in cells bearing the truncated histone H2A. Cells cultured with Ni(II) also showed extensive modification of histone H2B, a close partner of H2A in chromatin, including truncation and generation of two novel effects by Ni(II): oxidation of methionine and deamidation of glutamine residues. In addition, Ni(II) was found to affect the ubiquitination of histones H2A and H2B and methylation of histone H3 that may dysregulate gene expression and DNA repair.
Following our search for toxic metals' effects on enzymatic proteins, we continued investigating the role of Ni(II), Cr(VI), Cr(III), Co(II), and some other metals on HIF-1alpha prolyl hydroxylase (PH). PH is activated by Fe(II) and may therefore be sensitive to interference by other transition metals. This enzyme is crucial for up-regulation of the hypoxia-inducible factor HIF-1alpha that mediates cellular response to hypoxia. Our previous investigations in cultured cells showed that this effect was elicited through inhibition of PH by the metals. The major mechanistic reason for this inhibition was depletion of ascorbic acid, a co-activator of PH. Metals like Ni(II), Co(II) and Cr(VI) have the capacity to mediate ascorbate destruction through oxidation and hydrolysis of both ascorbic and dehydroascorbic acids. An alternative mode of PH inhibition through substitution of Fe(II)by the other metal ions was also considered. This was investigated using the computerized quantum chemical modeling approach. As we found, the tight binding of Fe(II) in the active site of PH makes it impossible for other ions to substitute for Fe(II).
Our studies on the biological implications of metal-mediated ascorbate depletion are being continued on in vitro and animal models, including gulonolactone oxidase knock-out mice. These mice, like humans, cannot synthesize ascorbic acid. Preliminary tests revealed their higher sensitivity to acute toxicity of Ni(II) as manifested by increased lethality and kidney damage, depending on the level of dietary ascorbate intake, as compared with wild-type mice of the same strain (C57BL). Long-term carcinogenesis assays to test if they are also more sensitive to the industrial carcinogen nickel subsulfide are under way.
Very importantly, the ascorbate depletion and HIF-1alpha inactivation by Ni(II) and other metals can be prevented by ascorbic acid supplementation. This should help to develop prevention strategies against human cancer in metal industries.
In addition to the above, our research provides an experimental basis for collaborative studies on oxidative DNA damage and enzyme inhibition by transition metals and other chemical carcinogens and toxicants led by other PIs, Drs. L.M. Anderson, M.P. Waalkes, J.M. Phang, and L.K. Keefer.
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