Inhibition of DNA synthesis by chemical carcinogens in cultures of initiated and normal proliferating rat hepatocytes

Rat hepatocytes in primary culture can be stimulated to replicate under the influence of rat serum and sparse plating conditions. Higher replication rates are induced by serum from two-thirds partially hepatectomized rats (Michalopoulos, G., Cianciulli, H. D., Novotny, A. R., Kligerman, A. D., Strom, S. C., and Jirtle, R. L. Cancer Res., 42: 4673–4682, 1982). The effects of carcinogens and noncarcinogens on the ability of hepatocytes to synthesize DNA were examined by measuring the incorporation of [3H]thymidine by liquid scintillation counting and autoradiography. Hepatocyte DNA synthesis was not decreased by ethanol or dimethyl sulfoxide at concentrations less than 0.5%. No effect was observed when 0.1 mM ketamine, Nembutal, hypoxanthine, sucrose, ascorbic acid, or benzo( e )pyrene was added to cultures of replicating hepatocytes. Estrogen, testosterone, tryptophan, and vitamin E inhibited DNA synthesis by approximately 50% at 0.1 mM, a concentration at which toxicity was noticeable. Several carcinogens requiring metabolic activiation as well as the direct-acting carcinogen N -methyl- N ′-nitro- N -nitrosoguanidine interfered with DNA synthesis. Aflatoxin B1 inhibited DNA synthesis by 50% (ID50) at concentrations between 1 × 10−8 and 1 × 10−7 M. The ID50 for 2-acetylaminofluorene was between 1 × 10−7 and 1 × 10−6 M. Benzo( a )pyrene and 3′-methyl-4-dimethylaminoazobenzene inhibited DNA synthesis 50% between 1 × 10−5 and 1 × 10−4 M. Diethylnitrosamine and dimethylnitrosamine (ID50 between 1 × 10−4 and 5 × 10−4 M) and 1- and 2-naphthylamine (ID50 between 1 × 10−5 and 5 × 10−4 M) caused inhibition of DNA synthesis at concentrations which overlapped with concentrations that caused measurable toxicity. The ability of hepatocytes to activate 2-acetylaminofluorene to reactive intermediates capable of binding to DNA and inhibiting new DNA synthesis decreased as a function of time in culture. γ-Glutamyl-transferase-positive hepatocytes from diethynitrosamine-treated rats were observed to be relatively insensitive to carcinogen inhibition of DNA synthesis.