Characterization of gene-specific DNA repair by primary cultures of rat hepatocytes.

At present, almost all the information on gene-specific DNA repair in mammals comes from studies with transformed cell lines and proliferating primary cells obtained from rodents and humans. In the present study, we measured the repair of specific DNA regions in primary cultures of nondividing rat hepatocytes (parenchymal cells). DNA damage was induced by irradiating the primary cultures of hepatocytes with ultraviolet (UV) light, and the presence of cyclobutane pyrimidine dimers (CPDs) was measured by using T4 endonuclease V in the following: a 21-kb BamHI fragment containing the albumin gene, a 14-kb BamHI fragment containing the H-ras gene, and the genome overall. The frequency of CPDs in the two BamHI fragments and the genome overall were similar and ranged from 0.5 to 1.3 CPDs per 10 kb for UV doses of 5-30 J/m2. However, the removal of CPDs from the DNA fragment containing the albumin gene was significantly higher than from that of the genome overall and the DNA fragment containing the H-ras gene. Within 24 hr, approximately 67% of the CPDs was removed from the DNA fragment containing the albumin gene versus less than 40% for the genome overall and the DNA fragment containing the H-ras gene. The lower repair observed for the 14-kb fragment containing the H-ras gene is probably indicative of repair of the nontranscribed region of this fragment because the H-ras gene makes up only 2.4 kb of the 14-kb fragment. Primary cultures of hepatocytes removed CPDs from the transcribed strand of albumin fragment more efficiently than from the nontranscribed strand; however, no differences were observed in the repair of the two strands of the fragment containing the H-ras gene. These results demonstrate that primary cultures of nondividing rat hepatocytes show differential repair of UV-induced DNA damage that is comparable to what has been reported for transformed, proliferating mammalian cell lines.