Combined Actions and Interactions of Chemicals in Mixtures

Nickel compounds are carcinogenic to humans and animals. However, the
mechanism leading to tumour formation is still not fully understood since the
mutagenic potential is rather weak. Nickel(II) is mostly non-mutagenic in bacterial
test systems and only weakly mutagenic in mammalian cell lines. In contrast to the
weak mutagenic activity Ni(II) enhance cytotoxicity and mutagenicity of several
other DNA damaging agents. In mammalian cells in culture Ni(II) enhanced the
UV-induced cytoxicity, mutagenicity and SCE in Chinese hamster V79 cells
(Christie 1989). In combination with benzo(a)pyrene, nickel(II) enhances the
frequency of mutations and cell transformation in Syrian hamster embryo cells
(Rivedal & Sanner 1980). A possible mechanism for the synergistic effect of Ni(II)
could be interference with excision repair processes. Hartwig et al. (1994) used
UVC light to study the interaction of Ni(II) with DNA repair. By different
techniques they were able to show that non-cytotoxic doses of Ni(II) delayed the
incision step in nucleotide excision repair in mammalian cells after low, biological
relevant UV doses. The interference with DNA repair is partly reversible by the
addition of magnesium(II), providing further evidence that the competition with
essential metal ion may be an important mechanism for the toxic action of Ni(II).
Similar interactions with DNA repair of UV-induced DNA damage have been
observed with other carcinogenic and/or mutagenic metal ions.
Co(II) is weakly mutagenic at the HPRT locus and enhances the frequency of SCE
in Chinese hamster V79 cells. Additionally, at both endpoints the metal ion
enhanced the genotoxicity of UV light. Analyses of the kinetics of strand-break
induction and closure after UV irradiation by nucleotide sedimentation reveals an
accumulation of strand breaks in the presence of Co(II). This indicates that either
the polymerisation or ligation step in excision is affected (Hartwig 1991). It has
also been shown that Co(II), at non-cytotoxic doses, inhibits both the incision and
polymerisation step of the repair of UV induced DNA lesions, as measured by the
alkaline unwinding technique in human fibroblasts (Kasten et al. 1997). It was
demonstrated by competition experiments that one possible mechanism is the
exchange of essential magnesium (II) ions by cobalt (II), since the cobalt(II)
induced inhibition of the polymerisation step was completely reversed by the
addition of magnesium(II).
Using the COMET assay on isolated human lymphocytes De Boeck et al (1998)
could demonstrate that cobalt metal was able to inhibit the repair of
methylmethansulfonate (MMS) induced DNA damage. The majority of the MMS
induced lesions are processed via the base excision-repair pathway. After
recognition and excision of the MMS-alkylated bases, an apurinic site (AP) occurs
as intermediate. When cobalt inhibits the action of the polymerase, the persisting
alkali labile AP sites will give rise to single strand breaks, which will result in
DNA fragmentation /migration in the COMET assay.
An analysis of DNA strand breaks by alkaline elution indicates that DNA repair of
Chinese hamster ovary cells treated with MMS was inhibited by sodium arsenite
(Lee-Chen 1993). The enhancing effect was mainly due to the inhibition of the
excision of alkali-labile sites.
The above-mentioned investigations indicate that interaction with DNA repair
might be a common mechanism of metal genotoxicity.
It is important to note that large differences exist in the efficiency of various DNA
repair systems between different types of mammalian cells, e.g. between rodent
cells and human cells. Chinese hamster ovary (CHO) cells clearly differ from
human cells concerning the relative resistance to methylating and ethylating agents,
and the persistence of the chromosomal aberrations induced by such agents. CHO
94