Certain transition metals, including nickel, chromium, cadmium, and copper, are carcinogenic to humans and/or animals. Their effects include cancer in the progeny of fathers exposed to welding fumes and other metal dusts. However, mechanisms of the carcinogenic activity of these metals remain obscure. In recent years, we have been testing a hypothesis that one such mechanism would involve metal-mediated conformational and/or oxidative damage to DNA and nuclear proteins. In 2000/2001 we continued testing that hypothesis. Our investigations of transition metals' interactions with nuclear proteins were focused on Ni(II) and Cu(II) binding to the -KTESHHKAKGK end-motif of the 34-amino acids-long C-terminal tail of histone H2A. As we have found, Ni(II), but not Cu(II), bound to this motif causes hydrolytic cleavage of its E-S peptide bond. This cleavage results in the formation of a new redox-active complex between Ni(II) and the cleaved-off octapeptide, -SHHKAKGK, capable of mediating oxidative DNA and protein damage. Unlike Ni(II), Cu(II) forms redox-active complex with the original motif. The above interactions may lead to oxidative DNA damage and chromatin remodeling, i.e., to both genotoxic and epigenetic effects. Another investigation focused on the MTH1 proteins, the oxidized purine nucleoside triphosphate pyrophosphohydrolases (also named 8-oxo-dGTPases), a class of mammalian enzymes preventing incorporation of promutagenic oxidized purine nucleotides, such as 8-oxo-dGTP, 8-oxo-dATP, 2-OH-dATP, 2-OH-ATP, 8-oxo-GTP, and 8-oxo-ATP into DNA or RNA. Electron-microscopic immunohistochemistry revealed for the first time MTH1's presence in the acrosomic vesicles of spermatocytes and sperm heads. The presence of MTH1 in the sperm indicates the importance of this enzyme to the process of fertilization and embryonal development. MTH1 sensitivity to metal inhibition, found by us previously, may thus be mechanistically relevant to metal-induced transgenerational carcinogenesis. Following our search for metal interactions with enzymatic proteins, we continued the investigations on the tumor suppressor protein Fhit, a diadenosine triphosphate phosphohydrolase, belonging, like MTH1, to the class of """"""""nudix"""""""" hydrolases. Recently, we have updated the list of metals inhibiting Fhit in vitro: Cu(II)>Fe(III)>Ni(II)>Zn(II)>>Cd(II)>Fe(II)>Cr(III)>>Co(II); As(III), As(V), and Be(II) are not inhibitory. The relatively strongest inhibition by Cu(II) (LC50, 0.4 mM) is due to prompt Fhit oxidation and dimerization by this metal, interacting specifically with the Cys-39 residue in the Fhit molecule. Fe(III) (LC50, 1.7 mM), but not the other metals (LC50, ranging from 3.5 to >400 mM), may enhance Fhit oxidation as well. Thus, the enzymatic activity of Fhit may be regulated by its redox status. In addition, our research provides an experimental basis for collaborative studies on oxidative DNA damage by other chemical carcinogens (see projects ZO1 BC 05352 and ZO1 BC 05488).
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