Inhibition of DMA Synthesis by Chemical Carcinogens in Cultures of Initiated and Normal Proliferating Rat Hepatocytes1

Rat hepatocytes in primary culture can be stimulated to repli cate under the influence of rat serum and sparse plating condi tions. Higher replication rates are induced by serum from twothirds 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 carcin ogens and noncarcinogens on the ability of hepatocytes to synthesize DMA were examined by measuring the incorporation of [3H]thymidine by liquid scintillation counting and autoradiography. Hepatocyte DMA 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 activation as well as the direct-acting carcinogen A/-methyl-/v"-nitro-A/-nitrosoguanidine interfered with DNA synthesis. Aflatoxin BT inhibited DNA synthesis by 50% (ID50)at concentrations between 1 x 10~8and 1 x 10~7 M. The ID50for 2-acetylaminofluorene was between 1 x 10~7 and 1 x 10~6 M. Benzo(a)pyrene and 3'-methyl-4-dimethylaminoazobenzene inhibited DNA synthesis 50% between 1 x 10~5and 1 x 10~4 M. Diethylnitrosamine and dimethylnitrosamine (ID50 between 1 x 10~4 and 5 x 10~4 M) and 1and 2-naphthylamine (ID60 between 1 x 10~5 and 5 x 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. 7-Glutamyltransferase-positive hepatocytes from diethylnitrosamine-treated rats were observed to be relatively insensitive to carcinogen inhibition of DNA synthesis.