Background DNA repair synthesis in rat hepatocyte cultures used for genotoxicity testing

DNA repair synthesis in primary rat hepatocyte cultures (HPC) was investigated using the bromodeoxyuridine (BrdUrd) density-shift method and autoradiography. Analysis by density-gradient centrifugation of DNA labelled with BrdUrd and [(3)H]deoxycytidine ([(3)H]dCyd) for 18-20 hr showed that considerable DNA repair replication occurs in HPC even in the absence of an added genotoxic agent. Repair was demonstrated to be most probably a consequence of DNA damage caused by the collagenase perfusion of the liver during hepatocyte isolation, and by beta-decay of the (3)H-labelled DNA precursor. Autoradiographic analysis of the distribution of silver grains over nuclei in intact non-S-phase cells, and over isolated nuclei from HPC exposed to [(3)H]dCyd for 18-20 hr, showed that the vast majority of the radioactivity was incorporated into the nuclei themselves, and not into overlying cytoplasm or mitochondria. In addition, direct localization of mitochondria in hepatocytes using the mitochondrion-specific dye rhodamine 123 showed that very few mitochondria were actually located over the nucleus. The results suggest that cytosolic labelling of control HPC in autoradiographs is mainly caused by mitochondrial DNA synthesis whereas nuclear labelling essentially reflects repair synthesis. They call into question the commonly applied practice of evaluating unscheduled DNA synthesis (UDS) in HPC by subtracting the number of cytosolic silver grains from nuclear grains and expressing repair synthesis as net grains per nucleus.