SIGNIFICANCE OF GGT EXPRESSION IN TUMORS

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The enzyme gamma-glutamyl transpeptidase (GGT) is expressed at 50 to 100 times the normal level in preneoplastic lesions from a variety of epithelial tissues including liver, breast lung and colon. The physiological function of GGT and the role it may play in tumor formation have been subjects of controversy for many years. In the liver, expression of GGT has been viewed as an indicator of dedifferentiation with no functional significance in tumor progression. However, we have proposed that GGT is part of a detoxification system that enables the cells to survive in the toxic environment created by many carcinogens. GGT has been cloned by several laboratories. It is now possible to directly test our hypothesis regarding the function of GGT. We have constructed an expression vector containing the cDNA for human GGT. We have transfected the DNA into two GGT-negative cell lines and obtained stable transformants. We will use these lines to answer the following questions: Does expression of GGT enable cells to use extracellular glutathione as a source of cysteine? Does expression of GGT effect the intracellular level of glutathione? The recent report of a consensus sequence for antioxidant genes and the presence of this sequence in the promoter region of the GGT gene in the rat supports the theory that GGT is part of a detoxification system. We will use GGT-positive clones to determine whether GGT is being induced as one of a set of antioxidant genes or if it is being turned on with other fetal genes. If GGT is part of a detoxication system than expression of GGT may be a useful clinical marker in predicting the therapeutic efficacy of specific drugs. We will use both the liver cell lines transfected with human cDNA and the liver cells in which GGT has been induced with ras to determine whether GGT-positive cells are more resistant to various classes of chemotherapeutic agents. Finally, we plan to investigate the species differences in the induction of GGT in the liver. GGT is induced in rat hepatocytes when they are put in primary culture but preliminary data indicates that it is not induced in mouse hepatocytes. Using primary cultures of rat and mouse hepatocytes we will investigate the effect of media components and the intracellular level of glutathione on the induction of GGT. GGT is aberrantly expressed in many epithelial tumors. An increased understanding of this enzyme and its regulation may lead to improved treatment of GGT-positive tumors.