A precursor form of human kidney gamma-glutamyl transferase in normal and cancerous tissues, and its possible post-translational modification

Detection of distinct glycosylation patterns on human γ-glutamyl transpeptidase 1 using antibody-lectin sandwich array (ALSA) technology

Background

γ-Glutamyl transpeptidase 1 (GGT1) is an N-glycosylated membrane protein that catabolizes extracellular glutathione and other γ-glutamyl-containing substrates. In a variety of disease states, including tumor formation, the enzyme is shed from the surface of the cell and can be detected in serum. The structures of the N-glycans on human GGT1 (hGGT1) have been shown to be tissue-specific. Tumor-specific changes in the glycans have also been observed, suggesting that the N-glycans on hGGT1 would be an important biomarker for detecting tumors and monitoring their progression during treatment. However, the large quantities of purified protein required to fully characterize the carbohydrate content poses a significant challenge for biomarker development. Herein, we investigated a new antibody-lectin sandwich array (ALSA) platform to determine whether this microanalytical technique could be applied to the characterization of N-glycan content of hGGT1 in complex biological samples.

Results

Our data show that hGGT1 can be isolated from detergent extracted membrane proteins by binding to the ALSA platform. Probing hGGT1 with lectins enables characterization of the N-glycans. We probed hGGT1 from normal human liver tissue, normal human kidney tissue, and hGGT1 expressed in the yeast Pichia pastoris. The lectin binding patterns obtained with the ALSA platform are consistent with the hGGT1 N-glycan composition obtained from previous large-scale hGGT1 N-glycan characterizations from these sources. We also validate the implementation of the Microcystis aeruginosa lectin, microvirin, in this platform and provide refined evidence for its efficacy in specifically recognizing high-mannose-type N-glycans, a class of carbohydrate modification that is distinctive of hGGT1 expressed by many tumors.

Conclusion

Using this microanalytical approach, we provide proof-of-concept for the implementation of ALSA in conducting high-throughput studies aimed at investigating disease-related changes in the glycosylation patterns on hGGT1 with the goal of enhancing clinical diagnoses and targeted treatment regimens.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-014-0101-0) contains supplementary material, which is available to authorized users.

Glutathione S-conjugates as prodrugs to target drug-resistant tumors

Living organisms are continuously exposed to xenobiotics. The major phase of enzymatic detoxification in many species is the conjugation of activated xenobiotics to reduced glutathione (GSH) catalyzed by the glutathione-S-transferase (GST). It has been reported that some compounds, once transformed into glutathione S-conjugates, enter the mercapturic acid pathway whose end products are highly reactive and toxic for the cell responsible for their production. The cytotoxicity of these GSH conjugates depends essentially on GST and gamma-glutamyl transferases (γGT), the enzymes which initiate the mercapturic acid synthesis pathway. Numerous studies support the view that the expression of GST and γGT in cancer cells represents an important factor in the appearance of a more aggressive and resistant phenotype. High levels of tumor GST and γGT expression were employed to selectively target tumor with GST- or γGT-activated drugs. This strategy, explored over the last two decades, has recently been successful using GST-activated nitrogen mustard (TLK286) and γGT-activated arsenic-based (GSAO and Darinaparsin) prodrugs confirming the potential of GSH-conjugates as anticancer drugs.

γ-Glutamyl transpeptidase is a heavily N-glycosylated heterodimer in HepG2 cells

The cell surface enzyme γ-glutamyl transpeptidase (GGT) is expressed by human hepatocellular carcinomas (HCCs). HCCs arise from malignant transformation of hepatocytes and are the most common form of primary liver cancer. Identification of tumor-specific, post-translational modifications of GGT may provide novel biomarkers for HCC. The HepG2 cell line, derived from a human HCC, has been used extensively in studies of liver cancer. However, the use of this cell line for studies of GGT have been stymied by reports that HepG2 cells do not process the GGT propeptide into its heterodimeric subunits. The data in this study demonstrate that HepG2 cells do, in fact, produce the mature heterodimeric form of GGT. Immunohistochemical and immunoaffinity analyses provide direct evidence that, in HepG2 cells, GGT is properly localized to the bile canaliculi. Three independent, experimental approaches demonstrate that GGT in HepG2 cells is comprised of two subunits that are more heavily N-glycosylated than GGT from normal human liver tissue. These data directly contradict the dogma in the field. These data support the use of HepG2 cells as a model system for analyzing tumor-specific changes in the post-translational modifications of GGT.

Biochemical and structural properties of gamma-glutamyl transpeptidase from Geobacillus thermodenitrificans: an enzyme specialized in hydrolase activity

Gamma-glutamyltranspeptidases (gamma-GTs) catalyze the transfer of the gamma-glutamyl moiety of glutathione and related gamma-glutamyl amides to water (hydrolysis) or to amino acids and peptides (transpeptidation) and play a key role in glutathione metabolism. Recently, gamma-GTs have been considered attractive pharmaceutical targets for cancer and useful tools to produce gamma-glutamyl compounds. To find out gamma-GTs with special properties we have chosen microorganisms belonging to Geobacillus species which are source of several thermostable enzymes of potential interest for biotechnology. gamma-GT from Geobacillus thermodenitrificans (GthGT) was cloned, expressed in Escherichia coli, purified to homogeneity and characterized. The enzyme, synthesized as a precursor homotetrameric protein of 61-kDa per subunit, undergoes an internal post-translational cleavage of the 61 kDa monomer into 40- and 21-kDa shorter subunits, which are then assembled into an active heterotetramer composed of two 40- and two 21-kDa subunits. The kinetic characterization of the hydrolysis reaction using L-glutamic acid gamma-(4-nitroanilide) as the substrate reveals that the active enzyme has K(m) 7.6 microM and V(max) 0.36 micromol min/mg. The optimum pH and temperature for the hydrolysis activity are 7.8 and 52 degrees C, respectively. GthGT hydrolyses the physiological antioxidant glutathione, suggesting an involvement of the enzyme in the cellular defense mechanism against oxidative stress. Unlike other gamma-GTs, the mutation of the highly conserved catalytic nucleophile, Thr353, abolishes the post-translational cleavage of the pro-enzyme, but does not completely block the hydrolytic action. Furthermore, GthGT does not show any transpeptidase activity, suggesting that the enzyme is a specialized gamma-glutamyl hydrolase. The GthGT homology-model structure reveals peculiar structural features, which should be responsible for the different functional properties of the enzyme and suggests the structural bases of protein thermostability.

Up-regulation of gamma-glutamyl transpeptidase (GGT) activity in growth perturbed C6 astrocytes

Activity of gamma-glutamyl transpeptidase (GGT) was studied in astrocyte-like C6 glial cells modulated in growth and maturation by different concentration of serum and dibutyryl cyclic AMP (Db-cAMP) supplement in culture medium. After reduction of serum concentration from 10% to 0.1%, the number of GGT positive cells determined histochemically increased 3.1 times and the GGT activity/mg protein in whole cell lysates was 5.1 times higher. In cultures with 0.1% serum + Db-cAMP, the histochemically and biochemically assayed GGT activity exceeded 5.1 and 7.9 times the values measured in control 10% serum cultures, respectively. The up-regulation of GGT was accompanied by an inhibition of proliferation, enhanced differentiation and hypertrophy of cells. In addition, the process of metabolic perturbation and/or cellular stress was revealed in these cultures by the (i) growth-support release followed by shrinkage and death of a small number of cells and (ii) higher oxidation of 2'7'dichlorofluorescein diacetate to its fluorescent form in the adherent/viable cells. The observed up-regulation of GGT is considered to primarily reflect increased metabolism of glutathione and/or the maintenance of the redox potential in cells stressed by sub-optimal concentration of serum and Db-cAMP supplement. The concomitant cellular hypertrophy and differentiation and their relationship to increased activity of GGT await further investigation. The study suggests that up-regulation of GGT can contribute to adaptation of astrocytic cells to metabolic and/or oxidative perturbances occurring under various pathological conditions, including radiation- and drug-induced toxicity.

Serial lectin affinity chromatography demonstrates altered asparagine-linked sugar-chain structures of prostate-specific antigen in human prostate carcinoma

Differences between prostate carcinoma (PCA) and benign prostatic hyperplasia (BPH) in asparagine (N)-linked sugar-chain structures of prostate-specific antigen (PSA) were investigated using serial lectin affinity chromatography. The amounts of PSA passing through columns of concanavalin A (Con A), phytohaemagglutinin E4 (PHA-E4) and PHA-L4 were significantly greater for PCA-derived PSA than BPH. We propose that the sugar moiety structure of PSA which is increased in PCA is a multiantennary complex type with branched N-acetylglucosamine beta(1->4) mannose. We suggest that N-linked sugar chains in PSA are altered during oncogenesis in the human prostate and may serve as diagnostic tools for PCA.

Serum gamma-glutamyltransferase and alkaline phosphatase during experimental liver metastases. Detection of tumour-specific isoforms and factors affecting their serum levels

Tumour-specific isoenzymes and tumour markers in serum are potentially useful in the detection and monitoring of liver metastases. An experimental rat model was used in the search for such isoenzymes and to study factors affecting their serum levels. Splenic injection of CC531 colon carcinoma cells in syngeneic WagRij rats caused liver metastases after 3 weeks with concomitant and significant increases in serum levels of gamma-glutamyltransferase (GT) and alkaline phosphatase (ALP). The presence of tumour-specific isoforms of both enzymes, as well as increased amounts of the liver isoform of ALP, were demonstrated in serum. The serum levels of the tumour variants were clearly related to their elimination rates from the circulation. Thus, the slow clearance of the tumour ALP resulted in high serum levels of this isoform, compared with the more rapid elimination of tumour GT and its lower serum level. When using another colon carcinoma cell line (DHD/K12), metastatic to liver in BD IX rats, no increases in serum GT were detected. This was related to the rapid elimination from the circulation of the GT variant from the DHD/K12 metastatic tissue. The relatively high amount of the tumour ALP isoform detected in serum during growth of the CC531 liver metastases indicated that this isoform could be useful as a marker of tumour growth.

gamma-Glutamyl transpeptidase, a glutathionase: its expression and function in carcinogenesis

gamma-Glutamyl transpeptidase (GGT) is found throughout the plant and animal kingdoms. It is a cell surface glycoprotein that cleaves gamma-glutamyl amide bonds. The most abundant physiologic substrates for the enzyme are glutathione and glutathione-conjugated compounds. GGT initiates the cleavage of extracellular glutathione into its constituent amino acids which can then be transported into the cell. It also catalyzes the initial step in the conversion of glutathione-conjugated compounds to mercapturic acids. GGT is expressed at high levels in many human tumors and in many carcinogen-induced tumors in animals. These observations have lead an increased focus on the role of the enzyme in the development and treatment of tumors. This chapter begins with an overview of the structure and function of GGT in normal tissues. A summary of its expression in neoplastic tissues and the ways in which GGT effects the response of tumors to chemotherapy follows. The chapter concludes with a discussion of strategies for using GGT to activate and target chemotherapy drugs to tumors as a means of improving treatment for common human malignancies.