Loss of nuclear p53 protein in preneoplastic rat hepatocytes is accompanied by Mdm2 and Bcl-2 overexpression and by defective response to DNA damage in vivo

Previous studies have indicated that isolated preneoplastic rat hepatocytes in vitro fail to induce nuclear p53 protein and fail to block replication in response to genotoxic compounds. This suggests that defects in the prote ction of genomic integrity are part of their premalignant character. In the present study, we have investigated if similar defects occur in vivo. Pren eoplastic glutathione-S-transferase (GST) 7-7-positive foci were induced in male Wistar rats by diethylnitrosamine (DEN) initiation and promotion with 2-acetylaminofluorene (2-AAF)/partial hepatectomy (PH), The response to ge notoxic damage was studied by X-irradiation, p53 protein was moderately exp ressed in nuclei in surrounding hepatocytes. This nuclear p53 staining had decreased 2 weeks after 2-AAF treatment. In foci, the protein was detected in the cytoplasm whereas the nuclei were negative. Levels of p21(waf1/cip1) protein were high in nuclei and cytoplasm of surrounding hepatocytes, wher eas the expression in foci was low. A low level of Mdm2 in nuclei was obser ved in surrounding liver, while both Mdm2 and Bcl-2 protein were strongly e xpressed in the cytoplasm in foci. X-ray exposure further induced nuclear e xpression of p53, p21(waf1/cip1), and Mdm2 in surrounding hepatocytes, but focal nuclei were still negative. DNA replication was strongly reduced by X -irradiation in surrounding hepatocytes, but only partially reduced in the foci, These results indicate that the p53 pathway of response to genomic st ress is impaired in preneoplastic cells in vivo. This may support their clo nal expansion and their further malignant transformation because protection against genetic damage is diminished.