Tamoxifen is critical as inducer of Cre activity in mouse models for hepatotoxicity studies

Tamoxifen (TAM) is intensively used for cell-specific Cre recombinase-induced gene (in)activation and reporter gene expression for fate mapping and tracing at predefined time frames. Despite extensive studies using the TAM-cre system, the influence of TAM per se remains to be elucidated. To unravel this influence, male C57Bl/6N mice were subjected to TAM and hepatotoxic (carbon tetrachloride; CCl4) injury. Surprisingly, the level of transaminases (ALT and AST) and necrosis index in TAM/CCl4 mice decreased significantly as compared with the vehicle/CCl4 group. No obvious alterations were reported without damage induction between the groups. mRNA expression of a critical phase I metabolizing enzyme, CYP2E1 was downregulated (p = 0.0433) in the TAM pretreated group, as measured by real time RT-PCR. However, protein expression of CYP2E1 as measured by immunostaining and immunoblotting was not influenced by TAM pretreatment. In contrast, anti-oxidants e.g. ascorbate and catalase, as well as some thiol containing metabolites, e.g. methionine, were upregulated in the TAM group upon damage. Furthermore, TAM increases presence of resident macrophages and recruitment of leucocytes in injured areas as indicated by F4/80 and CD45 positive staining. Our findings suggest that TAM has no clear impact in the control groups; however, upon liver intoxication induces a hepatoprotection via (1) downregulating of CYP2E1 in mRNA level, (2) increasing the availability of antioxidants and thiol group containing metabolites, and (3) recruiting restorative macrophages. This must be considered before conclusions based on this inducible Cre system can be drawn.