Relationship between morphological transformation and [3H]thymidine incorporation stimulated by a chemical carcinogen in postconfluent cultures of hamster embryo cells.

Postconfluent cultures of Syrian hamster embryo cell strains were exposed to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) or to combinations of MNNG, hydroxyurea (HU), caffeine (CF), and/or theophylline. At various intervals, [3H]thymidine incorporation into DNA was determined by autoradiography and liquid scintillation spectrometry. In parallel cultures, drug- and/or carcinogen-elicited cytotoxicity and carcinogen-induced morphological transformation were determined. The results indicate that: (a) MNNG induces, in a relatively small proportion of cells, a round of HU- and CF-suppressible [3H]thymidine incorporation superimposed on, or immediately followed by, a wave of inhibitor-resistant incorporation; (b) the transformation frequency varies with the detectable [3H]thymidine in the acid-insoluble fractions and the relative size of cell subpopulations undergoing HU- and CF-suppressible DNA replication; (c) the cells exposed to 1 mM CF or theophylline during the treatment with MNNG (0.5 microgram/ml), as well as for the first 24 hr after being plated for focus formation, develop statistically significant fewer (p less than 0.001) transformed foci, without measurable cytotoxic effects; and (d) at the doses used, HU inhibited [3H]thymidine incorporation of carcinogen-stimulated cells. It is concluded that MNNG-stimulated [3H]thymidine incorporation into DNA that is suppressible by both HU and CF may be associated with the initiation event(s) of morphological transformation; however, the nature of suppression by HU and CF appears to be different.