Site-specific genotoxicity of rubiadin: localization and histopathological changes in the kidneys of rats

Rubiadin (Rub) is a genotoxic component of madder color (MC) that is extracted from the root of Rubia tinctorum L. MC induces renal tumors and preneoplastic lesions that are found in the proximal tubule of the outer stripe of the outer medulla (OSOM), suggesting that the renal carcinogenicity of MC is site specific. To clarify the involvement of Rub in renal carcinogenesis of MC, we examined the distribution of Rub in the kidney of male gpt delta rats that were treated with Rub for 28 days. We used desorption electrospray ionization quadrupole time-of-flight mass spectrometry imaging (DESI-Q-TOF-MSI), along with the histopathological analysis, immunohistochemical staining, and reporter gene mutation assays of the kidney. DESI-Q-TOF-MSI revealed that Rub and its metabolites, lucidin and Rub-sulfation, were specifically distributed in the OSOM. Histopathologically, karyomegaly characterized by enlarged nuclear and microvesicular vacuolar degeneration occurred in proximal tubule epithelial cells in the OSOM. The ɤ-H2AX- and p21-positive cells were also found in the OSOM rather than the cortex. Although dose-dependent increases in gpt and Spi^- mutant frequencies were observed in both the medulla and cortex, the mutant frequencies in the medulla were significantly higher. The mutation spectra of gpt mutants showed that A:T-T:A transversion was predominant in Rub-induced gene mutations, consistent with those of MC. Overall, the data showed that the distribution of Rub and its metabolites resulted in site-specific histopathological changes, DNA damage, and gene mutations, suggesting that the distribution of genotoxic components and metabolites is responsible for the site-specific renal carcinogenesis of MC.