Additional organizational features of the murine γ-glutamyl hydrolase gene

Gamma-glutamyl hydrolase and drug resistance

Gamma-glutamyl hydrolase (GGH) is a lysosomal enzyme involved in the metabolism of folates and anti-folates. It acts as an endo- and/or exo-peptidase to cleave gamma-polyglutamate chains that are attached to folates and anti-folates after they enter a mammalian cell. Whereas the addition of multiple glutamates is necessary to enable the cell to retain folates and anti-folates, hydrolysis of the polyglutamate tails by GGH has the opposite effect of making (anti)-folates exportable again. Thus, GGH plays an important role in the cellular homeostasis of folate. Furthermore, high levels of GGH have been associated with cellular resistance to anti-folates, in particular methotrexate. Consequently, GGH also has pharmacological importance. In addition to the intracellular GGH, carboxypeptidase II (also called intestinal folate conjugase, prostate specific membrane antigen or N-acetyl-alpha-linked acidic dipeptidase) is another enzyme with gamma-glutamyl hydrolase activity; it resides, however, in the cellular membrane. Although genetically and biochemically distinct, this enzyme too appears to play a major role in folate homeostasis, by cleaving polyglutamates from extracellular folate-polyglutamates, so that they can be imported into the cell. Finally, there have been reports suggesting that gamma-glutamyl hydrolase plays a role as a tumor marker in breast and lung cancer.

Characterization of the human gamma-glutamyl hydrolase promoter and its gene expression in human tissues and cancer cell lines

Human gamma-glutamyl hydrolase (hGH) plays an important role in the metabolism of folic acid and the pharmacology of antifolates such as methotrexate. We have previously cloned and characterized hGH cDNA and its gene. We report here that the levels of hGH gene expression are high in tissues such as liver, kidney, and placenta as determined by Northern blot and RT-PCR analyses. In contrast, hGH expression is relatively low in spleen, lung, small intestine, and peripheral blood leukocytes. In addition, high levels of hGH mRNA were detected in most cancer cell lines examined. There was no significant difference in hGH mRNA levels between breast cancer tissues and their normal counterparts, although breast cancer tissues generally appeared to have heterogeneous expression of hGH mRNA. Deletion analysis and transient transfection assays were performed. A positive regulatory element located from -52 to +4 relative to the transcriptional start site was found to be required for basal promoter activity in both HepG2 and MCF-7 cells. Also, transcriptional inhibitory elements were found at -96 to-52 and +88 to+104 in MCF-7 cells, but not in HepG2 cells. These data provide novel insights into the regulation of hGH gene transcription in HepG2 and MCF-7 cells.

Regulation of transcription of the murine gamma-glutamyl hydrolase gene. Delineation of core promoter A and the role of LYF-1, E2F and ETS-1 in determining tumor-specific expression

Our earlier studies (Gene 268 (2001) 183) showed that transcription of the mouse gamma-glutamyl hydrolase (gamma GH) gene is under the control of two separate promoters widely distributed within the genome. We now report on further studies examining the functional characteristics of the more efficient promoter (promoter A) which is contiguous with an exon 1 (exon A1a) alternate within the main body of the gene. Functional deletion analysis of promoter A in pGL3 transfected in NIH3T3 cells defined a 189 bp region of sequence 26 bp upstream of exon A1a as mediating core promoter transcriptional activity. Further functional deletion analysis and site-directed mutagenesis attributed the activity of this core region to the presence of five SP1 sites within the most upstream 130 bp of sequence and the presence of three cis-active elements, Ets-1, Lyf-1 and E2F in the remaining downstream 59 bp of sequence. Electrophoretic gel-mobility shift assays showed that differences in the binding of trans-acting factors to these three elements accounts for the tumor-specific expression of this gene. Finally, analysis by fluorescent in situ hybridization revealed that promoter A and the main body of the mouse gamma GH gene are located between A3 and A5 on chromosome 4. This is an interesting finding in light of our earlier results which located promoter B and two associated alternates of exon 1 of this gene on chromosome 17.