Untersuchung der zonalen Zellproliferation mittels BrdU in der Leber der B6C3F1 und C57BL Maus nach Verabreichung von drei nicht-genotoxischen Karzinogenen

Abstract

Investigation of zonal cell proliferation through BrdU in the liver of B6C3F1 and C57BL mice after treatment with three non-genotoxic carcinogens

The effects of three non-genotoxic hepatocarcinogens on cell proliferation in the liver of male B6C3F1 and C57BL mice were evaluated. Animals were treated with phenobarbital, chloroform and Wyeth 14,643 for 1, 4, and 13 weeks. Labeling of proliferating cells with BrdU was achieved using a 7 day mini pump.

Some compounds seem to have a proliferative effect only on certain populations of hepatocytes (CHEN et al., 1995; CONSTAN et al., 1995; BAHNEMANN, 2000). Cell proliferation was measured by the lobule-dependent zonal measurement method (LZM; BAHNEMANN and MELLERT, 1997) to detect increases in cell proliferation of hepatocytes in specific zones. The liver lobule was divided into three equal parts with the zone 1 representing the periportal zone, the zone 3 lying close to the central vein and the zone 2 between the two other zones. In control animals of both strains, zone 2 was found to be the zone with the highest proliferative activity. This finding is in contrast to the streaming liver concept suggested by ZAJICEK et al. (1985) for rat liver according to which zone 1 reveals the highest proliferative activity.

The B6C3F1 mouse showed a higher labeling index (LI) after 4 and 13 weeks compared to the C57BL mouse. This might correlate with the higher tumor prevelance in this strain. Phenobarbital induced in both strains a sustained increase in cell proliferation for up to 13 weeks. After 4 and 13 weeks elevated LI were found only in zone 3. When using a random evaluation scheme, these differences of the LI would not have been detected. B6C3F1 mice treated with chloroform showed a sustained increase in cell proliferation in all zones for up to 13 weeks. In C57BL mice treated with chloroform an elevated LI could only be detected after 1 week. Wyeth 14,643 induced in both mouse strains at all 3 timepoints and in all 3 zones a marked increase in cell proliferation with a predominance in zone 1. Mitotic and apoptotic rates were also evaluated using the LZM method. Zones with high proliferative activity also showed increases in the mitotic index. A marked increase in the apototic rate could only be seen after treatment with Wyeth 14,643.

The necessity of zonal dependent evaluation of cell proliferation in the liver could be demonstrated as a consequence of the induction of cell proliferation by some compounds in only a specific zone of the liver lobule. When measuring the LI randomly throughout the whole liver lobule, there is always the risk to miss these compound specific zonal effects. It was possible to show strain specific differences in cell proliferation that correlate with tumor prevalence in the different strains. Furthermore, this work yields cell proliferation data for a database for short-time carcinogenicity evaluating non-genotoxic carcinogens.