Studies on Cadmium-induced carcinogenesis

Abstract

Cadmium is a potent carcinogen in rodents and has recently been accepted by the International Agency for Research on Cancer as a category 1 (human) carcinogen, but the molecular mechanism of its action remains largely unclear. It has however been suggested that cadmium-induced carcinogenesis may involve either direct or indirect interaction of Cd2+ with DNA. In this study, it is found that when Cd2+ is allowed to interact with adenine and guanine, there is a marked change in the HPLC retention time for adenine but not for guanine. Since Cd is believed to bind covalently to adenine and guanine, the changes in retention time but absence of any cadmium in the peak fraction point to the following: (1) lability of cadmium-nucleobase adducts, (2) introduction of some kind of chemical modification in adenine but not in guanine as a result of covalent binding. This result is different from that for Ni2+ in which case a change in retention time was observed for guanine but not for adenine. It is believed that the interaction with Cd"- causes a shift in tautomeric equilibrium of adenine and adenine nucleotides towards the imino form followed by hydroxylation of the dominant form. This would provide an explanation as to why cadmium is mutagenic but would not explain why cadmium is comutagenic. Cd" is believed to bind covalently with N7 centres of adenine and guanine. At low concentrations (< 50 mM) Cd is found to react with plasmid DNA to produce a mixture of Form I and Form II bands whereas higher concentrations (> 100 mM) of Cd2+ cause extensive damage to DNA at solution pH of 5.8 for cadmium(II) nitrate. Within the range 0 to 100 mM (when pH is adjusted to 7.4 by adding NaOH) an increase in concentration of Cd2+ is found to cause a decrease in the gel mobility rate of plasmid and an increase in the intensity of the Form II band. When plasmid DNA is digested with BamHl, only the Form III band is observed both in the presence and absence of Cd . However, the mobility of the band is found to decrease with the increase in the concentration of Cd2+. When the enzyme Sspl, which cuts plasmid DNA at the AT sites, is used instead of BamHl, two bands are observed in the presence of cadmium as against one band in the absence of cadmium. These results either suggest that Cd binds covalently with DNA l (possibly at G, A and T centres) and can form intrastrand bifunctional AT adducts but not the GG adducts or it may be that neither GG nor AT adducts are formed and yet Sspl digestion is prevented because of a structural modification introduced in adenine by its interaction with Cd2+. In the presence of antioxidants such as cysteine, glutathione and ascorbate (especially cysteine and ascorbate), DNA damage is found to be greater than that expected for the combined effects of the antioxidant and Cd2+. The increased DNA damage is believed to be due to the formation of reactive oxygen species (ROS). It is found that Cd" binds with ascorbate to form 1:1 adduct in solution in 0.10 M NaNOi, forming chelates in which ascorbate acts as a polydentate ligand. It is believed that it is this binding with Cd2+ that activates ascorbate so that it is more susceptible to attack by molecular oxygen. Reactive oxygen species that are produced, then damage DNA. If the results of the present in vitro experiments are also applicable to in in vivo situations, it would mean that when there is a sufficient body burden of cadmium, ascorbate which normally prevents damage to DNA by acting as a free radical scavenger, can promote damage to DNA by producing free radicals.