Acute and long-term effects of carbon tetrachloride on DNA modifications (I-compounds) in male mouse liver

I-compounds are recently discovered species and tissue dependent covalent DNA modifications which are detectable by the ³²P-postlabeling assay for DNA adducts and tend to increase with the animal's age. The effects of the hepatocarcinogen carbon tetrachloride (CCl₄) on hepatic I-compounds were studied in 10-12-month-old male ICR mice using the ³²P-postlabeling assay. CCl₄ was dissolved in corn oil (20%, v/v) and intraperitoneally (i.p.) injected in doses of 0.75 ml/kg (0.375 ml/100 g body weight, 20% CCl₄ in corn oil) while control mice received corn oil only (0.375 ml/100 g body wt). Twenty-four h after a single injection of CCl₄, the intensity of non-polar I-spots in liver DNA was significantly increased as compared with corn oil treated controls, while the level of one polar I-compound was reduced at 24 h. DNA synthesis (as indicated by [³H]thymidine incorporation) was not significantly affected at 24 h after a single dose of CCl₄. To study the long-term effects of CCl₄, five groups of mice were given two consecutive weekly injections of 0.75 ml/kg CCl₄ (as above) and were sacrificed 1, 4, 8, 12 and 22 weeks after the second treatment. In these groups the total liver I-compound levels were reduced to 17.3-49.0% compared with corresponding controls. The maximum decline was observed at 4 weeks (17.3% of control). Comparison of thymidine incorporation showed no significant increase between control and treated liver DNAs at 1, 4 and 8 weeks after CCl₄, suggesting that the decrease in I-compound levels was probably not a secondary effect of increased DNA synthesis during postnecrotic proliferation. Even though there was a trend of recovery between 8 and 22 weeks, I-compound levels still remained significantly lower at 22 weeks (49.0%). Since I-compounds appear to be normal DNA modifications, the results suggest that persistent reduction of I-compound levels contributes to the hepatocarcinogenic effect of CCl₄.