Nit1 and Fhit tumor suppressor activities are additive

The fragile histidine triad gene (human FHIT, mouse Fhit) has been shown to act as a tumor suppressor gene. Nit1 and Fhit form a fusion protein, encoded by the NitFhit gene in flies and worms, suggesting that mammalian Nit1 and Fhit proteins, which are encoded by genes on different chromosomes in mammals, may function in the same signal pathway(s). A previous study showed that Nit1 deficiency in knockout mice confers a cancer prone phenotype, as does Fhit deficiency. We have now assessed the tumor susceptibility of Fhit(-/-)Nit1(-/-) mice and observed that double knockout mice develop more spontaneous and carcinogen-induced tumors than Fhit(-/-) mice, suggesting that the extent of tumor susceptibility due to Nit1 and Fhit deficiency is additive, and that Nit1 and Fhit affect distinct signal pathways in mammals. Nit1, like Fhit, is present in cytoplasm and mitochondria but not nuclei. Because Fhit deficiency affects responses to replicative and oxidative stress, we sought evidence for Nit1 function in response to such stresses in tissues and cultured cells: when treated with hydroxyurea, the normal kidney-derived double-deficient cells appear not to activate the pChk2 pathway and when treated with H(2)O(2), show little evidence of DNA damage, compared with wild type and Fhit(-/-) cells. The relevance of Nit1 deficiency to human cancers was examined in human esophageal cancer tissues, and loss of Nit1 expression was observed in 48% of esophageal adenocarcinomas.