Formation of 8-hydroxydeoxyguanosine and cell-cycle arrest in the rat liver via generation of oxidative stress by phenobarbital: association with expression profiles of p21(WAF1/Cip1), cyclin D1 and Ogg1

To evaluate the risk of exposure to so-called non-genotoxic chemicals and elucidate mechanisms underlying their promoting activity on rat liver carcinogenesis the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG), cytochrome P-450 (P-450) and hydroxyl radicals induction, DNA repair and alteration to cellular proliferation and apoptosis in the rat liver were investigated during 2 weeks of phenobarbital (PB) administration at a dose of 0.05%. Significant increase of hydroxyl radical levels by day 4 of PB exposure accompanied the accumulation of 8-OHdG in the nucleus and P-450 isoenzymes CYP2B1/2 and CYP3A2 in the cytoplasm of hepatocytes. Conspicuous elevation of 8-OHdG and apoptosis in the liver tissue were associated with reduction of the proliferating cell nuclear antigen (PCNA) index after 8 days of PB application. Thereafter, 8-OHdG levels decreased with an increase in mRNA expression for the 8-OHdG repair enzyme, DNA glycosylase 1 (Ogg1). Analysis with LightCycler quantitative 2-step RT-PCR demonstrated induction of cyclin D1 (CD1) and p21(WAF1/Cip1) mRNA expression on days 4 and 6, respectively, preceding marked elevation of PCNA and apoptotic indices. These results suggest that similar to genotoxic, non-genotoxic chemicals might induce reversible alteration to nuclear 8-OHdG in the rat liver after several days of continuous application; however, by a different mechanism. Increased 8-OHdG formation is caused by developing oxidative stress or apoptotic degradation of DNA and coordinated with enhanced expression of CD1 mRNA and cell proliferation, subsequent increase of p21(WAF1/Cip1) mRNA expression, cell-cycle arrest and apoptosis, while activation of 8-OHdG repair mechanisms contributes to protection of tissue against reactive oxygen species-induced cell death.