Takeshi Iidaka

Differential Effects of Partial Hepatectomy and Carbon Tetrachloride Administration on Induction of Liver Cell Foci in a Model for Detection of Initiation Activity

Differential effects of partial hepatectomy (PH) and carbon tetrachloride (CC14) administration on induction of glutathione S‐transferase placental form (GST‐P)‐positive foci were investigated in a model for detection of initiation activity. Firstly, we surveyed cell proliferation kinetics and fluctuation in cytochrome P450 (CYP) mRNA levels by means of relative‐quantitative real‐time reverse transcriptase‐polymerase chain reaction (RT‐PCR) and CYP 2E1 apoprotein amount by immuno‐blotting (experiment I) after PH or CC14 administration. Next, to assess the interrelationships among cell proliferation, fluctuation of CYPs after PH or CC14 administration and induction of liver cell foci, the non‐hepatocarcinogen, 1,2‐dimethylhydrazine (DMH) was administered to 7‐week‐old male F344 rats and initiated populations were selected using the resistant hepatocyte model (experiment II). In experiment I, the values of all CYP isozyme mRNAs after PH or CC14 administration were drastically decreased at the 12‐h tune point. From 72 h, mRNAs for all CYP isozymes began increasing, with complete recovery after 7 days. The CYP 2E1 apoprotein content in the PH group fluctuated weakly, whereas in the CC14 group it had decreased rapidly after 12 h and was still low at the 48 h point. In experiment II, induction of GST‐P‐positive foci was related to cell kinetics in the PH group, with about a 6‐h time lag between tune for carcinogen administration giving greatest induction of GST‐P‐positive foci and peaks in bromodeoxyuridine (BrdU) labeling, presumably due to the necessity for bioactivation of DMH. With CC14 administration, induction of foci appeared dependent on the recovery of CYP 2E1. In conclusion, PH was able to induce cell proliferation with maintenance of CYP 2E1, therefore being advantageous for induction of liver cell foci in models to detect initiation activity.

Summation of initiation activities in the liver after partial hepatectomy

Summation of initiation activities of different carcinogens in the liver after partial hepatectomy (PH) was investigated with reference to induction of glutathione S-transferase placental form (GST-P) positive foci. Firstly, effects of repeated administration of 1,2-dimethylhydradine (DMH) were compared with the results of a single administration of the same total dose (Expt. I). Subsequently, we studied summation of initiation potential with serial administration of DMH with diethylnitrosamine (DEN) or N-bis (2-hydroxpropyl)-nitrosamine (DHPN). In Expt. I, induction of GST-P-positive foci by multiple low-dose administration was equal to that with the single large-dose treatment. In order to avoid toxicity in hepatectomized rats, the low repeated-dose approach appeared superior. In Expt. II, the numbers of GST-P-positive foci in the groups treated with DMH plus DHPN or DMH plus DEN were significantly higher than those in the groups receiving the carcinogens singly. It is concluded that there is summation of initiation potential with doses of a single or multiple carcinogens. These results suggest that the present initiation assay model is useful to investigate summation of initiation activities of various environmental chemicals.