The in vivo mutation assay using the X-linked phosphatidylinositol glycan class A gene (Pig-a in rodents, PIG-A in humans) is a promising tool for evaluating the mutagenicity of chemicals. Approaches for measuring Pig-a mutant cells have focused on peripheral red blood cells (RBCs) and reticulocytes (RETs) from rodents. The recently developed PIGRET assay is capable of screening >1×10 RETs for Pig-a mutants by concentrating RETs in whole blood prior to flow cytometric analysis. Additionally, due to the characteristics of erythropoiesis, the PIGRET assay can potentially detect increases in Pig-a mutant frequency (MF) sooner after exposure compared with a Pig-a assay targeting total RBCs (RBC Pig-a assay). In order to test the merits and limitations of the PIGRET assay as a short-term genotoxicity test, an interlaboratory trial involving 16 laboratories was organized by the Mammalian Mutagenicity Study Group of the Japanese Environmental Mutagenicity Society (MMS/JEMS). First, the technical proficiency of the laboratories and transferability of the assay were confirmed by performing both the PIGRET and RBC Pig-a assays on rats treated with single doses of N-nitroso-N-ethylurea. Next, the collaborating laboratories used the PIGRET and RBC Pig-a assays to assess the mutagenicity of a total of 24 chemicals in rats, using a single treatment design and mutant analysis at 1, 2, and 4 weeks after the treatment. Thirteen chemicals produced positive responses in the PIGRET assay; three of these chemicals were not detected in the RBC Pig-a assay. Twelve chemicals induced an increase in RET Pig-a MF beginning 1 week after dosing, while only 3 chemicals positive for RBC Pig-a MF produced positive responses 1 week after dosing. Based on these results, we conclude that the PIGRET assay is useful as a short-term test for in vivo mutation using a single-dose protocol.

Methimazole (MTZ), an anti-thyroid drug, is known to cause liver injury in humans. It has been demonstrated that MTZ-induced liver injury in Balb/c mice is accompanied by T helper (Th) 2 cytokine-mediated immune responses; however, there is little evidence for immune responses associated with MTZ-induced liver injury in rats. To investigate species differences in MTZ-induced liver injury, we administered MTZ with a glutathione biosynthesis inhibitor, L-buthionine-S,R-sulfoximine (BSO), to F344 rats and subsequently observed an increase in plasma alanine aminotransferase (ALT) and high-mobility group box 1 (HMGB1), which are associated with hepatic lesions. The hepatic mRNA expression of innate immune-related genes significantly increased in BSO- and MTZ-treated rats, but the change in Th2-related genes was not much greater than the change observed in the previous mouse study. Moreover, an increase in Kupffer cells and an induction of the phosphorylation of extracellular signal-regulated kinase (ERK)/c-Jun N-terminal kinase (JNK) proteins were accompanied by an increase in Toll-like receptor 4 (TLR4) expression, indicating that Kupffer cell activation occurs through HMGB1-TLR4 signaling. To elucidate the mechanism of liver injury in rats, gadolinium chloride, which inactivates the function of Kupffer cells, was administered before BSO and MTZ administration. The gadolinium chloride treatment significantly suppressed the increased ALT, which was accompanied by decreased hepatic mRNA expression related to innate immune responses and ERK/JNK phosphorylation. In conclusion, Kupffer cell-mediated immune responses are crucial factors for the exacerbation of MTZ-induced liver injury in rats, indicating apparent species differences in the immune-mediated exacerbation of liver injury between mice and rats.

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