Carbon tetrachloride and chloroform induced toxicity and DNA strand breaks in primary Fisher 344 rat, B6C3F1 mouse, and human hepatocytes and three immortalized human liver epithelial cell lines

Abstract

Species differences in susceptibility to the carcinogenic potential of xenobiotics present problems for reliable interspecies extrapolation, particularly with respect to risk assessment. Carbon tetrachloride (CCl4) and chloroform (CHCl3) are two chemicals that have been demonstrated to be carcinogenic in chronic bioassays, but qualitative and quantitative species, strain, and sex differences have been observed. It has been postulated that the production of DNA strand breaks (SBs) may be a critical event in carcinogenesis and positive correlations have been observed with some chemicals between the production of SBs and carcinogenic potencies. Therefore, this research investigates the potential significance of SBs in the hepatotoxicity and carcinogenicity of CCl4 and CHCl3. The results obtained indicated that primary B6C3F1 mouse hepatocytes were more sensitive to the production of SBs than primary Fisher 344 rat hepatocytes and that these species differences may be related to differences observed in the carcinogenic effects of CCl4. Primary human hepatocytes and immortalized human liver epithelial (IHLE) cell lines responded similarly to primary rat hepatocytes in the induction of SBs and cytotoxicity when exposed to CCl4 and CHCl3, indicating that humans may also be at a reduced risk upon exposure to these chemicals than the B6C3F1 mouse. The production of SBs at cytotoxic doses of CCl4 and CHCl3 were likely a result of, not a cause of, cell death. However, because not all cytotoxicity leads to DNA damage, it is likely specific mechanisms of cell death lead to the formation of SBs. Although necrosis appeared to be the primary cause of DNA degradation at cytotoxic doses, calcium (Ca2+)-endonucleases were also likely involved. Because SBs were observed in mouse hepatocytes at levels that did not produce significant cytotoxicity, there appeared to be a second mechanism involving free radicals in the production of SBs, that was not a function of or a cause of cytotoxicity. Because the production of SBs has been linked to carcinogenicity, further exploration of the mechanism of SB formation at low doses in mouse hepatocytes may lead to an increased understanding of the mechanism of carcinogenicity of CCl4 and the reason for susceptibility of the B6C3F1 mouse to CCl4.