The glutathione S-transferases (abbr. GST; EC 2.5.1.18) are a family of catalytic and binding proteins that detoxify many chemical carcinogens. In murine liver, the major enzyme family of four proteins is encoded by a family of genes which are expressed with efficiencies differing by more than 10-fold. The GST enzymes are inducible; administration of BHA, or other dietary antioxidants, increases the level of GST 10-fold by increasing GST mRNA more than 20-fold. These anti-oxidants were first identified as inducers because of their ability to protect rodents against powerful chemical carcinogens. The GST genes provide an opportunity to examine the control of a differentially expressed, inducible family of enzymes which are abundant in normal tissues. To purify the GST genes, pGT55, a cDNA plasmid homologous to the major mouse liver GST mRNA, will be used to identify homologous DNA fragments from a library of mouse genomic DNA in phage lambda. The genomic clones will be partially restriction mapped, so that fragments which hybridize to the cDNA can be identified and sequenced. This DNA sequence data will define gene specific regions for each clone, and S-1 mapping or reverse transcription of RNA hybridized to these regions will be used to identify the genes expressed in murine liver. These genes will be used for studies of transcription rates and mRNA levels, before and after induction by anti-oxidants. These studies will show whether or not increases in GST mRNA are due to increased transcription, and if the increased transcription is due to activation of new genes. In addition, comparative DNA sequence analysis of these genes will show regions of conservation and divergence which may be responsible for differing GST substrate specificities and specific activities. To identify the DNA sequences responsible for the differential expression of members of the GST family, promoter regions from the 5' -ends of the active GST genes will be fused to heterologous genes and levels of promoter-dependent expression will be measured. The role of the chromosomal environment will be examined with DNase I hypersensitivity and methylation pattern mapping. To begin to study the factors responsible for induction, cultured cells will be surveyed for induction in vitro. Antioxidants induce a variety of detoxication enzymes that project tissues against chemical carcinogens. Characterization of GST expression is a first step towards understanding the processes responsible for induction.
| Pearson, W R; Reinhart, J; Sisk, S C et al. (1988) Tissue-specific induction of murine glutathione transferase mRNAs by butylated hydroxyanisole. J Biol Chem 263:13324-32 |
