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.
Allgayer
H
H
Oncology Department, Rehabilitation Clinic Ob der Tauber, Bad Mergentheim, Germany.
Kolb
M
M
Stuber
V
V
Kruis
W
W
D000818:Animals; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000494:pharmacology*; D001647:Bile Acids and Salts / Q000494:pharmacology*; D002417:Cattle; D003110:Colonic Neoplasms / Q000517:prevention & control; D004247:DNA / Q000187:drug effects* / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D008401:Gas Chromatography-Mass Spectrometry; D005765:Gastrointestinal Agents / Q000494:pharmacology*; D006801:Humans; D006900:Hydroxylation / Q000187:drug effects; D020128:Inhibitory Concentration 50; D007413:Intestinal Mucosa / Q000187:drug effects*; D012459:Salicylates / Q000494:pharmacology*; D013950:Thymus Gland / Q000187:drug effects*
D000894:Anti-Inflammatory Agents, Non-Steroidal; D001647:Bile Acids and Salts; D005765:Gastrointestinal Agents; D012459:Salicylates; D004247:DNA