DNA synthesis inhibition in HeLa cells as a simple test for agents that damage human DNA

Inhibition of HeLa cell DNA synthesis by DNA-damaging agents as a test for mutagenic carcinogens has been investigated further. Several mutagens and/or carcinogenic agents not previously assayed for DNA synthesis inhibition were tested and found to be positive. The effect of two DNA-damaging agents administered simultaneously was investigated: With ultraviolet light (UV) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) the effects were partly additive, whereas with MNNG and ICR-170 they were not. When UV was administered simultaneously with hydroxyurea (HU), a non-DNA-damaging inhibitor of DNA synthesis, the effects of the DNA-damaging agent were still evident after HU was removed and the HU-induced DNA synthesis inhibition reversed. Therefore, although the test probably cannot determine whethter there are one or more DNA-damaging agents in an unknown mixture, the presence of a powerful DNA synthesis inhibitor that is not a DNA-damaging agent will not mask the effects of a DNA-damaging agent in the same sample. Despite the fact that ICR-170 induces little DNA repair in xeroderma pigmentosum (XP) cells, which are deficient in excision repair, the inhibition of DNA synthesis induced by this agent was less in XP cells than in HeLa cells. The inhibition of DNA synthesis by MNNG was also less in XP than in HeLa cells. Therefore, contrary to expectations, the use of a repair-deficient cell did not increase the sensitivity of the test. Various in vitro mammalian cell tests are compared. Assays for unscheduled DNA synthesis are much less sensitive to intercalating agents, such as adriamycin, and to X-rays than are assays for inhibition of DNA synthesis but takes longer and requires more specialized skills than does measurement of DNA synthesis inhibition. Finally, the in vitro HeLa DNA synthesis inhibition test is compared with the in vivo DNA synthesis inhibition test with mouse testes.