Differential mutagenicity of riddelliine in liver endothelial and parenchymal cells of transgenic big blue rats.

Riddelliine is a naturally occurring pyrrolizidine alkaloid that induces liver hemangiosarcomas in rats and mice. We previously reported higher levels of DNA adducts in liver endothelial cells than in liver parenchymal cells of riddelliine-treated mice and rats [Cancer Lett. 193 (2003) 119], suggesting that the tumor specificity is due to higher levels of DNA damage in the cells that form hemangosarcomas. In the present study, we evaluated the cell-specificity of riddelliine mutagenicity in rat liver. Female transgenic Big Blue rats were treated by gavage with 0.3 mg riddelliine per kg body weight, 5 days a week for 12 weeks. One day after the last treatment, the rats were sacrificed and liver parenchymal and endothelial cell fractions were isolated and purified. DNA was extracted from the cell fractions and used to assay for mutant frequency (MF) in the cII transgene. While there was no difference in the cII MFs of liver parenchymal cells in control and riddelliine-treated rats, the cII MF of liver endothelial cells from treated rats was significantly greater than the cII MF of endothelial cells from control rats. Molecular analysis of the mutants in liver endothelial cells indicated that G:C-->T:A transversion, a mutation that is characteristically induced by riddelliine, accounted for only 9% of all mutations in control rats, but made up 17% of mutations in treated rats. In contrast, G:C-->A:T transition, the major mutation in control rats where it made up 54% of all mutations, was reduced to 40% of mutations in riddelliine-treated rats. These results suggest that the relatively high mutagenicity of riddelliine in rat liver endothelial cells may be partially responsible for the tumorigenic specificity of this agent.