Modifying effects of disulfiram on DNA adduct formation and persistence of benzaldehyde in N-nitroso-N-methyl-benzylamine-induced carcinogenesis in rats

Abstract

Whereas disulfiram (DSF) is known to inhibit tumor formation resulting from a number of chemical carcinogens, such inhibition does not apply to nitrosamines. In the present study, biochemical and morphological findings were examined to elucidate the effect of DSF on long-term application of N-nitroso-N-methylbenzylamine (NMBA). HPLC and fluorescence detection were used to determine O⁶-methylguanine (O⁶-MG) in DNA obtained from the respiratory tract of rats subjected to long-term simultaneous application of DSF and NMBA. After 2 days of treatment, more O⁶-MG was detected in the proximal portion of the respiratory tract, including the trachea and main bronchi, than in the distal portion. The findings were reversed after 10 and 30 days, at which time formation of the DNA adduct was substantially higher in the distal portion of the respiratory tract, despite increases in both portions. The biochemical results corresponded to morphological findings. Initially, mereased numbers of metabolizing goblet cells appeared in mucous cell hyperplasia in the proximal respiratory tract. Subsequently, the hyperplasia migrated to distal regions of the respiratory tract; at this stage, the goblet cells disappeared from the proximal portion, which now revealed toxic degeneration, atrophy and subsequent squamous metaplasia of the mucous lining and squamous papillomas. At various times during a 40-day period, 2 to 7 times more O⁶-MG in pulmonary DNA was detected in rats treated with DSF and NMBA, than with NMBA alone, whereby distinct amounts of O⁶-MG were found in the latter animals. In contrast to the above-mentioned morphological findings, no morphological alterations occurred in the respiratory tract of the animals treated with NMBA alone. It is therefore conceivable that the above pathological lesions resulted not merely from the presence of DNA adducts, but also from an additional, previously unspecified effect. As benzaldehyde (BA) is formed in equimolar amounts in NMBA metabolism and DSF has been demonstrated to inhibit aldehyde metabolism, this aldehyde is a possible candidate for such an effect. In the present study, rats were therefore treated with BA, DSF, or NMBA, or combinations thereof. Histomorphological evaluation of these experiments revealed that long-term application of BA alone led to the following alterations in the respiratory tract: goblet cell hyperplasia, hyperplasia of the peribronchial lymphatic system, mucous epithelial atrophy and accompanying peerivasculitis — the same alterations seen under long-term application of NMBA and DSF. Furthermore, these changes were most pronounced in the group with concomitant application of NMBA, DSF, and BA. It is therefore conceivable that BA plays a role in pathological changes observed under the influence of NMBA.