Heritable high frequency modulation of antigen expression in neoplastic cells exposed to 5-aza-2'-deoxycytidine or hydroxyurea: analysis and implications.

A study was undertaken to determine the ability of the DNA hypomethylating drug 5-aza-2'-deoxycytidine (5-azadCyd) to induce antigen expression in a high proportion of treated tumor cells and to evaluate if this effect could be mimicked by the drug hydroxyurea (HU) which is a genomic DNA hypermethylating agent. Induction of heritable changes in gene expression by 5-azacytidine or the 2'-deoxy analogue (5-azadCyd), at least in some cases, may not be necessarily due to their hypomethylating properties, but to some other induced high frequency genetic change which occurs when DNA synthesis or repair is perturbed. A comparison of 5-azadCyd and HU effects on human and murine tumors was chosen for this study. The phenotypic properties examined with the above treatments were (a) induction of a Mr 110,000 antigen, detected with M111 antibody, in a variant subpopulation (SMeLus7) of human melanoma cells which fail to maximally express this antigen (M111). The parent cell line, MeWo, and other MeWo-derived variant cell cell lines do not demonstrate a similarly inducible phenotype; and (b) induction of class I major histocompatibility complex antigens in a population of mouse mammary adenocarcinoma cells which normally fail to express these antigens. The results showed that 5-azadCyd was effective in inducing antigen expression in both systems whereas HU was effective (and equally so) only in the human melanoma cell line system. In these treatments 5-azadCyd was demonstrated to transiently hypomethylate the same human melanoma cell line whereas HU hypermethylated genomic cytosines. The results suggest that some of the reported effects of 5-azacytidine or 5-azadCyd in inducing very high frequency heritable phenotypic alterations may not necessarily be related to the drugs' ability to cause DNA hypomethylation. The implications of these results are discussed in terms of the use of 5-azacytidine or 5-azadCyd and the effects of chemotherapy on tumor progression and metastasis.