Objective To investigate liver transcriptome changes in rats with oral exposure to low-dose benzobfluoranthene (BbF), and to explore the hepatotoxicity of BbF and its possible mechanism.

Methods A total of 16 male SD rats, with a body weight of 90-100 g, were selected, and were randomly divided into experimental group and control group based on body weight after adaptive feeding. The two groups were treated with solvent and 1 μg/kg BbF, respectively, by gavage, twice a week for 4 consecutive weeks. The rats were sacrificed at the end of exposure to collect blood and liver tissue samples for determination of serological parameters and oxidative stress levels. RNA sequencing was used to investigate changes of gene expression in livers, gene ontology (GO) enrichment analysis was used to investigate the main functions of differentially expressed genes, and the protein-protein interaction (PPI) network and Clue GO analysis were used to visualize the interaction between differentially expressed genes; quantitative real-time PCR(RT-PCR) was used to validate the results of transcriptome.

Results At the end of the exposure, the organ coefficient was 2.60%±0.23% in the control group and 3.12%±0.25% in the experimental group. Compared with the control group, the experimental group had a significantly higher organ coefficient of the liver (P < 0.01), while there was no significant difference in body weight between the two groups (P>0.05). The analysis of serological parameters showed that there were no significant differences in the contents of alanine transaminase(ALT) and aspartate aminotransferase(AST) between the two groups (P>0.05). In the experimental group, the content of hydrogen peroxide (H₂O₂) was 34.46±2.04 mmol/g, the activity of catalase (CAT) was 14.03±2.38 U/mg, and the activity of superoxide dismutase (SOD) was 40.70±3.74 U/mg. Compared with the control group, the experimental group had a significant increase in the content of H₂O₂ (P < 0.05) and significant reductions in activities of CAT and SOD (P < 0.01), and there was no significant difference in the content of malondialdehyde(MDA) between the two groups(P>0.05). Transcriptome sequencing showed that there were 394 differentially expressed genes between the two groups, of which there were 229 up-regulated genes and 165 down-regulated genes. GO enrichment analysis showed the highest number of differentially expressed genes enriched in the oxidation-reduction process, with marked enrichment in multiple protein folding-related terms. PPI network analysis and Clue GO analysis showed that Hspa5, Acox2, and Ehhadh were hub genes among the differentially expressed genes, and the hub gene network was mainly involved in the cellular response to unfolded protein and antioxidant capacity.

Conclusion Exposure to low-dose BbF can cause slight oxidative stress in rat liver and disrupt the function of the endoplasmic reticulum. The body protectively triggers unfolded protein response and the Nrf2-antioxidant pathway to alleviate liver damage.