Tumor promotion by metanil yellow and malachite green during rat hepatocarcinogenesis is associated with dysregulated expression of cell cycle regulatory proteins.

Metanil yellow (MY) and malachite green (MG) are textile dyes, which, despite a ban, are used as food-coloring agents. MY and MG have promoter effects on the development of hepatic preneoplastic lesions induced by N-nitrosodiethylamine (DEN). Tumor-promoting agents are not mutagenic but may alter the expression of genes whose products are associated with hyper-proliferation, tissue remodeling, and inflammation. Cell cycle controls normally function to ensure the integrity of the genome and arrest of cells at G1/S or G2/M checkpoints until all the prerequisite events are completed. In order to understand the mechanism(s) of tumor promotion by MY and MG, we have studied the levels of PCNA, a marker of cell proliferation and cell cycle regulatory proteins, cyclin D1, and its associated kinase, cdk4, cyclin B1, and associated kinase, cdc2. Immunohistochemical staining showed an elevated level of PCNA in animals administered MY and MG subsequent to DEN treatment. Western and Northern blot hybridization showed an increased expression of both cyclin D1 and its associated kinase cdk4, and cyclin B1 and its associated kinase cdc2, in livers of rats administered MY and MG after administration of DEN as compared to untreated or DEN controls. The increased level of mRNA was due to the increased rate of transcription of these genes as studied by run-on transcription assay. These data obtained by the in vivo model of liver tumor development provide strong evidence for a link between dysregulation of the two critical checkpoints of the cell cycle as one of the possible mechanism(s) during tumor promotion by malachite green and metanil yellow.