Suppression of hepatocyte apoptosis and induction of DNA synthesis by the rat and mouse hepatocarcinogen diethylhexylphlathate (DEHP) and the mouse hepatocarcinogen 1,4-dichlorobenzene (DCB)

Nongenotoxic rodent hepatocarcinogens do not damage DNA but cause liver tum ours in the rat and mouse, associated with the induction of hepatic DNA syn thesis. Previously, we have demonstrated that nongenotoxic hepatocarcinogen s such as phenobarbitone and the peroxisome proliferator (PP), nafenopin, a lso suppress rat hepatocyte apoptosis. The nongenotoxic chemicals 1,4-dichl orobenzene (DCB) and the PP, diethylhexyl phthalate (DEHP), both induce hig h levels of DNA synthesis in rat liver in vivo, but only DEHP is hepatocarc inogenic in this species. Here, we investigate whether the difference in ra t carcinogenicity of these two hepatic mitogens may be due to differences i n their ability to suppress hepatocyte apoptosis. In rat hepatocytes in vit ro, MEHP (the active metabolite of DEHP) induced DNA synthesis 2.5-fold (P = 0.001) and suppressed 10- and 4-fold, respectively both spontaneous (P = 0.0008) and transforming growth factor beta 1 (TGF beta 1)-induced (P = 0.0 001) apoptosis. DCB gave a small (1.7-fold) increase in DNA synthesis (P = 0.03) and a small (1.7- to 2-fold) suppression of both spontaneous (P = 0.0 22) and TGF beta 1-induced (P = 0.015) apoptosis. We next analysed the indu ction of DNA synthesis and the suppression of apoptosis in rat liver in viv o. Both DEHP and DCB were able to induce DNA synthesis although, as seen in vitro, the induction by DCB (4.2-fold; P = 0.023) was less marked than tha t with DEHP (13.4-fold; P = 0.007). Similarly, DEHP and DCB were both able to suppress rat hepatocyte apoptosis in vivo but the magnitude of the suppr ession was comparable; apoptosis was reduced to undetectable Levels in four out of five animals with DCB and three out of five with DEHP. Since both c hemicals suppressed apoptosis and induced DNA synthesis in rat liver but, o verall, DCB was less potent, the disparate hepatocarcinogenic potential of these two chemicals could arise from differences in the magnitude of growth perturbation. To test this hypothesis, we repeated the studies in mouse, a species where both DCB and DEHP are hepatocarcinogenic. Both in vitro and in vivo, DCB and DEHP/MEHP were able to suppress apoptosis and induce hepat ocyte DNA synthesis in the mouse with comparable potencies. The data suppor t the hypothesis that the carcinogenicity of nongenotoxic hepatocarcinogens is associated strongly with the ability to perturb hepatocyte growth regul ation. However, the ability to effect such changes is not unique to nongeno toxic carcinogens and is common to some noncarcinogenic chemicals, such as DCB, suggesting that the growth perturbation may need to exceed a threshold for carcinogenesis.