Monika Chabicovsky

Effects of cruciferous vegetables and their constituents on drug metabolizing enzymes involved in the bioactivation of DNA-reactive dietary carcinogens

Epidemiological studies give evidence that cruciferous vegetables (CF) protect humans against cancer, and also results from animal experiments show that they reduce chemically induced tumor formation. These properties have been attributed to alterations in the metabolism of carcinogens by breakdown products of glucosinolates, which are constituents of CF. The present article gives an overview on the present state of knowledge on the impact of CF and their constituents on enzymes that are involved in the metabolism of DNA-reactive carcinogens. The development of in vitro models with metabolically competent cell lines led to the detection of potent enzyme inducers contained in CF such as sulforaphane. Recently, we showed that Brassica juices induce glutathione-S-transferases (GST) and cytochrome P-450 1A2 in human hepatoma cells (HepG2) and protect against the genotoxic effects of B(a)P and other carcinogens. Earlier in vivo experiments with rodents indicated that indoles and isothiocyanates, two major groups of glucosinolate breakdown products, attenuate the effects of polycyclic aromatic hydrocarbons (PAHs) and nitrosamines via induction of GST and inhibition of cytochrome-P450 isoenzymes, respectively. Our own investigations showed that CF are also protective towards heterocyclic amines (HAs): Brussels sprouts- and garden cress juices attenuated IQ-induced DNA-damage and preneoplastic lesions in colon and liver of rats. These effects were paralleled by induction of uridine-di-phospho-glucuronosyl transferase (UDPGT) which is very probably the mechanism of protection against HAs by cruciferous vegetables. There is also evidence that consumption of CF might protect humans against cancer. In matched control intervention studies with these vegetables, it was shown that they induce GST-activities in humans but overall, results were inconclusive. Recently, we carried out crossover intervention studies and found pronounced GST-induction upon consumption of Brussels sprouts and red cabbage, whereas no effects were seen with white cabbage and broccoli. Furthermore, we found that the isoenzyme induced was GST-pi which plays an important role in protection against breast, bladder, colon and testicular cancer. No induction of the GST-alpha isoform could be detected. Urinary mutagenicity experiments gave further evidence that CF affect drug metabolism in humans. Consumption of red cabbage led to changes in the pattern of meat-derived urinary mutagenicity. Overall, CF are among the most promising chemopreventive dietary constituents and further elucidation of their protective mechanisms and the identification of active constituents may contribute to the development of highly protective Brassica varieties.

Follistatin overexpression in rodent liver tumors: A possible mechanism to overcome activin growth control

The activin‐follistatin system is a potent growth regulatory system of liver tissue homeostasis. Activin A inhibits hepatocellular DNA synthesis and induces cell death. Follistatin binds activin and sequesters it from the signaling pathway. Consistently, follistatin has been reported to act as an inducer of DNA synthesis in the liver. Using RNase protection analysis, we studied the expression of follistatin in rat and mouse liver tumors as a possible mechanism to overcome activin growth control. Approximately 40% of the tumors (nine of 24 each), most of them hepatocellular carcinomas, displayed increased levels of follistatin mRNA when compared to tumor‐surrounding liver tissue. The degree of overexpression was highly variable but independent of the carcinogen treatment that animals had received. It was also independent from the histological stage of malignancy and further found in rat liver adenomas. Follistatin expression was also observed in cell lines derived from human hepatocellular carcinomas. Overexpression of follistatin may represent a unique strategy of hepatic tumors to overcome the inhibitory action of a growth factor, activin, by decreasing its local bioavailability.

Nuclear Ras: Unexpected subcellular distribution of oncogenic forms

The Harvey‐ras gene encodes small guanine nucleotide binding proteins, mutant forms of which are associated with a number of human malignancies. Based on studies with truncated forms of the protein it is known that correct post‐translational processing of Ras is essential for cytoplasmic membrane localization and function. Surprisingly, immunofluorescence analysis provided evidence that in addition to its cytosolic localization, activated H‐RasVal 12 was also localized in the nuclei of transformed cells both in vitro and in vivo. Immunoblot analysis of nuclear fractions was consistent with results found by immunohistochemistry. Moreover, inhibition of protein farnesylation prevented the nuclear targeting of activated H‐RasVal 12 and NFκB. Alterations in subcellular distribution pattern and phosphorylation of the cell cycle inhibitor p27, which is involved in Ras driven tumor growth, coincided with nuclear localization of H‐RasVal 12. Proteins are often not functional until they are transported to their final destination. Indeed, Ras was found to complex with NTF2 a factor involved in nuclear protein import and export. Therefore it is suggested that NTF2 is the actual carrier for oncogenic Ras. In view of these observations the question arises whether the nuclear localization of H‐RasVal 12 in tumors is important in oncogenic activation or whether it is a response to apoptosis. J. Cell. Biochem. Suppl. 36: 1‐11, 2001.

Hepatocarcinogenesis in the Context of Strain Differences in Energy Metabolism Between Inbred Strains of Mice (C57BL/6J and C3H/He)

Liver neoplasia in mice is one of the most frequent tumor target tissue endpoints observed in 2-year carcinogenicity studies of the US National Toxicology Program (NTP). B6C3F1 mice are the hybrids of male C3H/He, which are highly sensitive (hepatomas in 7291% at 14 months), and C57B1/6J females, which exhibit a low susceptibility to spontaneous and chemically induced hepatocarcinogenesis (Heston, 1963; Drinkwater and Bennett, 1991; Dragani et al. 1995; Wastl et al. 1998). Thus, the validity of mouse liver tumor endpoint in assessing the potential hazards of chemical exposure to humans is a controversial issue, since tumor susceptibility varies even within inbred strains of mice. Therefore our studies aimed to elucidate strain specifities that may modulate tumor development to improve the assessment for the relevance of mouse liver tumor induction in toxicological testing of chemicals.