5-Azacytidine potentiates initiation induced by carcinogens in rat liver

To test the validity of the hypothesis that hypomethylationof DNA plays an important role in the initiation of carcinogenicprocess, 5-azacytidine (5-AzC) (10 mg/kg), an inhibitor of DNAmethylation, was given to rats during the phase of repair synthesisinduced by the three carcinogens, benzoa]-pyrene (200 mg/kg),N-methyl-N-nltrosourea (60 mg/kg) and 1,2-dimethylhydrazine(1,2-DMH) (100 mg/kg). The initiated hepatocytes in the liverwere assayed as the -glutamyltransferase (-GT) positive fociformed following a 2-week selection regimen consisting of dietary0.02% 2-acetylaminofluorene coupled with a necrogenic dose ofCCl₄. The results obtained indicate that with all three carcinogens,administration of 5-AzC during repair synthesis increased theincidence of initiated hepatocytes, for example 10–20foci/cm² in 5-AzC and carcinogen-treated rats compared with3–5 foci/cm² in rats treated with carcinogen only. Administrationof ³H]-5-azadeoxycytidine during the repair synthesis inducedby 1,2-DMH further showed that 0.019 mol% of cytosine residuesin DNA were substituted by the analogue, indicating that incorporationof 5-AzC occurs during repair synthesis. In the absence of thecarcinogen, 5-AzC given after a two thirds partial hepatectomy,when its incorporation should be maximum, failed to induce any-GT positive foci. The results suggest that hypomethylationof DNA per se may not be sufficient for initiation. Perhapstwo events might be necessary for initiation, the first causedby the carcinogen and a second involving hypomethylation ofDNA.