Mitogenic stimulation of hepatocellular proliferation in rodents following 1,4-dichlorobenzene administration

Abstract

1,4-Dichlorobenzene (DCB), a non-DNA-reactive compound, induced hepatocellular carcinomas at 600 mg/kg/day, but not 300 mg/kg/day in male and female B6C3F1 mice in a National Toxicology Program (NTP) bioassay. Cell proliferation studies were performed under conditions of the NTP bioassay to determine the mode of DCB-induced hepatocellular proliferation and whether this proliferative response may be related to the carcinogenic activity of DCB. The percentage of cells in S-phase (labeling index; LI) was measured using immunohistochemical detection of 5-bromo-2'-deoxyuridine. Time-course and dose-response studies revealed a sharp increase in LI 24 h after treatment in female mice and rats, and at 48 h in male mice with no increases in liver-associated plasma enzymes at up to twice the highest bioassay dose. During 13 weeks of DCB administration under bioassay conditions, a statistically significant transient peak of hepatocellular proliferation was observed during week 1 at 600 mg/kg/day, but not at 300 mg/kg/day, in male and female mice. Hepatocellular proliferation was also observed in female rats, which were reported as exhibiting no increased liver tumor incidence when compared to controls in the NTP bioassay. An increase in liver weight as a percentage of body weight compared to controls was observed in high dose male and female mice, and female rats at all time points. No significant elevations in liver-associated plasma enzymes were found at any time point, indicating a lack of overt hepatotoxicity. Histopathological evaluation revealed no evidence of hepatocellular necrosis in all groups. These data indicate an early mitogenic stimulation of cell proliferation, rather than regeneration secondary to cytolethality, in the livers of DCB-treated mice, which correlates with previously observed tumor formation in a dose-dependent manner. The mode by which a chemical induces cell proliferation is an important consideration in mechanistic studies and the risk assessment process. The demonstrated mitogenic activity of DCB raises the possibility that this early proliferative response may be sufficient for liver tumor formation in the B6C3F1 mouse, or that DCB may provide a selective growth advantage to preneoplastic cells in the mouse liver upon long-term treatment. The observed induction of cell proliferation by DCB in the rat in the absence of a tumorigenic response suggests important species differences and complexities in the relationship between cell proliferation and carcinogenesis, and indicates that caution be applied in equating cell proliferation to cancer.

Prediction of rodent nongenotoxic carcinogenesis: evaluation of biochemical and tissue changes in rodents following exposure to nine nongenotoxic NTP carcinogens.

“…In the rat, liver weight and liver hypertrophy were clearly increased by DEHP, CINN and DCB, as reported by others (30)(31)(32). MON, ETU, and LIM also gave weak increases in liver weight that were sometimes accompanied by liver hypertrophy.…”

“…The noncarcinogens CINN, DCB, ETU, TMTU, and LIM also gave increases in labeling index; only DETU had no effect. Increases in labeling index produced by some of these compounds have been shown previously (27,30,31,34). All of the compounds tested gave increases in cytochrome P450, determined either as total (spectral) P450 or as specific isoforms (Western blots).…”

“…In the mouse liver, weight, labeling index, and liver hypertrophy were clearly increased by the hepatocarcinogens DEHP, CINN, DCB, and ETU. Some of the effects of DEHP, CINN, and DCB have been shown previously (30,31,41). CEA, MON, and DETU gave little or no increases in liver weight but did produce some hypertrophy, which was observed microscopically.…”

“…During the early stages of this simulation, we evaluated a range of acute/subacute markers of presumed nongenotoxic rodent carcinogenesis. The markers used in this study have been reported by others for the rodent liver (27,28,30,31,41), kidney (9,26,33,39,40), and thyroid gland (9,37,38). The results of the study for individual markers are shown schematically in Figure 1.…”

Thirteen‐week Inhalation Toxicity of p‐Dichlorobenzene in Mice and Rats

“…Medical case reports on the health of p-DCB-exposed humans [5][6][7][8][9][10] revealed that clinical signs and symptoms induced by excessive exposure were central nervous system effects, eye and nose irritation and hepatic, renal and hematological disorders. Studies on experimental toxicology demonstrated that p-DCB induced hepatic, renal and hematological toxicity in mice and rats 6,[11][12][13][14][15][16][17][18][19][20][21][22][23] . Toxicity data from the inhalation exposure of experimental animals to p-DCB vapor are more relevant for assessing the health risks of p-DCB-exposed humans than those from oral administration because inhalation exposure is the principal route of humans, exposure to p-DCB.…”