Modulation of base hydroxylation by bile acids and salicylates in a model of human colonic mucosal DNA: putative implications in colonic cancer.

Bile acids are believed to be involved in the formation of colonic cancer, and 5-aminosalicylic acid and other salicylates may have a protective role. The precise mechanisms of both actions are not known, but modifications (stimulation or inhibition) of basal or oxygen-radical induced DNA base hydroxylation as potential early events in tumor formation by these compounds may be involved in such actions. We, therefore, investigated whether: (1) bile acids in concentrations as they occur systemically or intraluminally are able to enhance basal or OH*-radical-stimulated base hydroxylation in DNA from calf thymus; (2) 5-aminosalicylic acid, its main intestinal metabolite N-acetyl-aminosalicylic acid and salicylate, the main aspirin metabolite, are able to inhibit this hydroxylation; and (3) DNA from calf thymus can be used as a model by comparing its base composition and hydroxylation with DNA from normal human colonic mucosa. We found an enhancement of the OH*-radical-induced DNA hydroxylation especially 8-OH adenine with 214.0%. On the other hand 5-ASA, N-acetyl-ASA, and salicylate showed a concentration-dependent inhibition of OH*-stimulated hydroxylation with IC50 between 0.04 +/- 0.01 mM (X +/- SD) and 1.3 +/- 0.1 mM. No effects were observed on basal hydroxylation. Electron spin resonance spectroscopy studies showed reduction of the corresponding base signals pointing to a scavenger mechanism. In DNA isolated from normal human colonic mucosa (N = 7) a similar base distribution was found as in calf thymus; hydroxylation was < or = 1.0% in both systems. From our results we conclude that DNA from calf thymus may serve as a model for human colonic mucosal DNA and that one of the carcinogenic actions of bile acids may be enhancement of oxygen-radical-induced DNA base hydroxylation, especially 8-OH adenine. The absence of effects under unstimulated conditions supports their role as cocarcinogens. The concentration-dependent inhibition of OH*-stimulated DNA hydroxylation by 5-ASA, salicylate, and N-acetyl-ASA may be a possible mechanism of chemoprevention.