Oncogenic transformation of C3H/10T1/2 Cl 8 mouse embryo fibroblasts by inhibitors of nucleotide metabolism.

Morphological or oncogenic transformation of mouse embryo, C3H/10T1/2 Cl 8 fibroblasts was induced by methotrexate, 5-fluorouracil and 5-fluorodeoxyuridine. It is known that these compounds cause inhibition of thymidylate synthetase and, hence, depletion of deoxythymidine triphosphate (dTTP) and an increased ratio of deoxycytidine triphosphate (dCTP) to dTTP in the deoxyribonucleotide pools that are used for DNA synthesis in mammalian cells. This ratio is, in effect, increased by treating mammalian cells with arabinosyl cytosine and 5-azacytidine, which are converted into analogs of dCTP in mammalian cells and also induce oncogenic transformation of C3H/10T1/2 cells. By contrast, trifluorothymidine, 5-bromodeoxyuridine and 5-iododeoxyuridine, which are analogs of thymidine that in effect reduce the dCTP:dTTP ratio, did not induce oncogenic transformation. Moreover, thymidine was selectively lethal to tumorigenic C3H/10T1/2 cells and inhibited oncogenic transformation in cells treated with 5-fluorodeoxyuridine. These observations suggest that treatments that effectively increase the dCTP:dTTP ratio in mammalian cells facilitate oncogenic transformation of C3H/10T1/2 cells, whereas treatments that have the effect of decreasing this ratio inhibit transformation. However, dCyd did not induce oncogenic transformation of C3H/10T1/2 cells, although it has been shown to increase the dCTP:dTTP ratio in mammalian cells. Thus, increasing this ratio may not be sufficient to cause the transformation.