DNA methylating activity in murine lymphoma cells xenogenized by triazene derivatives.

We investigated whether epigenetic rather than mutational events may be involved in the induction of novel immunogenicity ("xenogenization") in murine lymphoma treated with triazene derivatives. To this end, we assessed the DNA methylation pattern of tumor cells during the course of in vivo or in vitro xenogenization by 2 triazenes, and compared it with the changes induced by the DNA hypomethylating agent 5-azacytidine (5-aza). While all of the tested agents were able to increase tumor cell immunogenicity, their effects on DNA methylating activity were opposite. In particular, the novel immunogenicity conferred by 5-aza treatment correlated well with the extent of hypomethylation induced, whereas xenogenization by triazenes was accompanied by a limited increase in DNA methylating activity. Interestingly, the antimutagenic compound quinacrine, which is known to block the xenogenizing activity of triazenes, was incapable of hypermethylating effects, whereas the hypermethylating agent cytosine arabinoside (ara-C) was apparently unable to interfere with the induction of novel immunogenicity by triazenes.