Extra-cellular thiol metabolism in clones of human metastatic melanoma with different gamma-glutamyl transpeptidase expression: implications for cell response to platinum-based drugs

Induction of Gamma-Glutamyltransferase Activity and Consequent Pro-oxidant Reactions in Human Macrophages Exposed to Crocidolite Asbestos

Asbestos is the main causative agent of malignant pleural mesothelioma. The variety known as crocidolite (blue asbestos) owns the highest pathogenic potential, due to the dimensions of its fibers as well as to its content of iron. The latter can in fact react with macrophage-derived hydrogen peroxide in the so called Fenton reaction, giving rise to highly reactive and mutagenic hydroxyl radical. On the other hand, hydroxyl radical can as well originate after thiol-dependent reduction of iron, a process capable of starting its redox cycling. Previous studies showed that glutathione (GSH) is one such thiol, and that cellular gamma-glutamyltransferase (GGT) can efficiently potentiate GSH-dependent iron redox cycling and consequent oxidative stress. As GGT is expressed in macrophages and is released upon their activation, the present study was aimed at verifying the hypothesis that GSH/GGT-dependent redox reactions may participate in the oxidative stress following the activation of macrophages induced by crocidolite asbestos. Experiments in acellular systems confirmed that GGT-mediated metabolism of GSH can potentiate crocidolite-dependent production of superoxide anion, through the production of highly reactive dipeptide thiol cysteinyl-glycine. Cultured THP-1 macrophagic cells, as well as isolated monocytes obtained from healthy donors and differentiated to macrophages in vitro, were investigated as to their expression of GGT and the effects of exposure to crocidolite. The results show that crocidolite asbestos at subtoxic concentrations (50-250 ng/1000 cells) can upregulate GGT expression, which raises the possibility that macrophage-initiated, GSH/GGT-dependent pro-oxidant reactions may participate in the pathogenesis of tissue damage and inflammation consequent to crocidolite intoxication.

Redox Modulation of NF-κB Nuclear Translocation and DNA Binding in Metastatic Melanoma. The Role of Endogenous and γ-Glutamyl Transferase-Dependent Oxidative Stress

Aims and background

The transcription factor NF-κB is implicated in the expression of genes involved in cell proliferation, apoptosis and metastasis. In melanoma, high constitutive levels of NF-κB activation are usually observed. NF-κB is regulated by oxidation/reduction (redox) processes, and the occurrence of constitutive oxidative stress in melanoma cells has been documented. Recent studies of our laboratories showed that the membrane-bound gamma-glutamyl transferase (GGT) enzyme activity – expressed by a number of malignancies, including melanoma – can act as a basal source of superoxide, hydrogen peroxide and other prooxidants.

Methods

In the present study we utilized the 2/60 clone of Me665/2 human metastatic melanoma, which displays high levels of GGT activity, in order to verify if the presence of this enzyme - through the promotion of redox processes - may influence the activation status of NF-κB. The latter was evaluated by determining the nuclear translocation of the p65 subunit (by immunoblot), the DNA binding of NF-kB (by elec-trophoretic mobility shift assay) and its transcriptional activity (by gene transactivation studies).

Results

Me665/2/60 cells displayed a basal production of hydrogen peroxide. Stimulation of GGT activity by its substrates glutathione and glycyl-glycine caused additional production of hydrogen peroxide, up to levels approx. double the basal levels. Nuclear translocation of the NF-κB p65 subunit, DNA-binding and gene transactivation were thus investigated in Me665/2/60 cells whose GGT activity was modulated by means of substrates or inhibitors. Stimulation of GGT activity resulted in increased nuclear translocation of p65, while on the other hand NF-κB DNA binding and gene transactivation were paradoxically decreased. NF-κB DNA binding could be restored by treating cell lysates with the thiol-re-ducing agent dithiothreitol (DTT). Treatment of cells with exogenous hydrogen peroxide did not affect NF-κB activation status.

Conclusions

Altogether, the data obtained indicate that GGT activity may impair the redox status of thiols that is critical for NF-κB DNA binding and gene transactivation, through the production of prooxidant species allegedly distinct from hydrogen peroxide. GGT activity therefore appears to be an additional factor in modulation of NF-κB transcriptional activity in melanoma, capable of hindering NF-κB DNA binding even in conditions where continuous oxidative stress would favor NF-κB nuclear translocation.

Understanding the interaction of an antitumoral platinum(II) 7-azaindolate complex with proteins and DNA

The reactivity of the [Pt(dmba)(aza-N1)(dmso)] complex 1, (a potential antitumoral drug with lower IC50 than cisplatin in several tumoral cell lines) with different proteins and oligonucleotides is investigated by means of mass spectrometry (ESI-TOF MS). The results obtained show a particular binding behaviour of this platinum(II) complex. The interaction of 1 with the assayed proteins apparently takes place by Pt-binding to the most accessible coordinating amino acids, presumably at the surface of the protein -this avoiding protein denaturation or degradation- with the subsequent release of one or two ligands of 1. The specific reactivity of 1 with distinct proteins allows to conclude that the substituted initial ligand (dmso or azaindolate) is indicative of the nature of the protein donor atom finally bound to the platinum(II) centre, i.e. N- or S-donor amino acid. Molecular modeling calculations suggest that the release of the azaindolate ligand is promoted by a proton transfer to the non-coordinating N present in the azaindolate ring, while the release of the dmso ligand is mainly favoured by the binding of a deprotonated Cys. The interaction of complex 1 with DNA takes always place through the release of the azaindolate ligand. Interestingly, the interaction of 1 with DNA only proceeds when the oligonucleotides are annealed forming a double strand. Complex 1 is also capable to displace ethidium bromide from DNA and it also weakly binds to DNA at the minor groove, as shown by Hoechst 33258 displacement experiments. Furthermore, complex 1 is also a good inhibitor of cathepsin B (an enzyme implicated in a number of cancer related events). Therefore, although compound 1 is definitely able to bind proteins that can hamper its arrival to the nuclear target, it should be taken into consideration as a putative anticancer drug due to its strong interaction with oligonucleotides and its effective inhibition of cat B.

In vivo labeling of B16 melanoma tumor xenograft with a thiol-reactive gadolinium based MRI contrast agent

Murine melanoma B16 cells display on the extracellular side of the plasma membrane a large number of reactive protein thiols (exofacial protein thiols, EPTs). These EPTs can be chemically labeled with Gd-DO3A-PDP, a Gd(III)-based MRI contrast agent bearing a 2-pyridinedithio chemical function for the recognition of EPTs. Uptake of gadolinium up to 10(9) Gd atoms per cell can be achieved. The treatment of B16 cells ex vivo with a reducing agent such as tris(2-carboxyethyl)phosphine (TCEP) results in an increase by 850% of available EPTs and an increase by 45% of Gd uptake. Blocking EPTs with N-ethylmaleimide (NEM) caused a decrease by 84% of available EPTs and a decrease by 55% of Gd uptake. The amount of Gd taken up by B16 cells is therefore dependent upon the availability of EPTs, whose actual level in turn changes according to the extracellular redox microenvironment. Then Gd-DO3A-PDP has been assessed for the labeling of tumor cells in vivo on B16.F10 melanoma tumor-bearing mice. Gd-DO3A-PDP (or Gd-DO3A as the control) has been injected directly into the tumor region at a dose level of 0.1 μmol and the signal enhancement in MR images followed over time. The washout kinetics of Gd-DO3A-PDP from tumor is very slow if compared to that of control Gd-DO3A, and 48 h post injection, the gadolinium-enhancement is still clearly visible. Therefore, B16 cells can be labeled ex vivo as well as in vivo according to a common EPTs-dependent route, provided that high levels of the thiol reactive probe can be delivered to the tumor.

Role of Glutathione in the Multidrug Resistance in Cancer

Multidrug resistance is the main problem in anticancer therapy. Cancer cells use many defense strategies in order to survive chemotherapy. Among known multidrug resistance mechanisms the most important are: drug detoxification inside the cell using II phase detoxifying enzymes and active transport of the drug to the extracellular environment. Cancer cells may be also less sensitive to proapoptotic signals and have different intracellular drug distribution, which makes them more resistant to anticancer drugs. Role of glutathione in multidrug resistance is the object of interest of many scientists, however, defining it’s function in these processes still remains a challenge. In this paper, properties of glutathione and it’s role in multidrug resistance in cancer cells were described.

Degradation of bidentate-coordinated platinum(II)-based DNA intercalators by reduced L-glutathione

We have examined the interaction of [(5,6-dimethyl-1,10-phenanthroline)(1S,2S-diaminocyclohexane)platinum(II)] (2+) (1, 56MESS), [(5-methyl-1,10-phenanthroline)(1S,2S-diaminocyclohexane)platinum(II)] (2+) (2, 5MESS), [(5,6-dimethyl-1,10-phenanthroline)(1R,2R-diaminocyclohexane)platinum(II)] (2+) (3, 56MERR), and [(5,6-dimethyl-1,10-phenanthroline)(ethylenediamine)platinum(II)] (2+) (4, 56MEEN) with reduced L-glutathione and L-methionine. Both thiols degrade all four complexes, mainly by displacing the ancillary ligand and forming a doubly bridged dinuclear complex. The degradation half-life of all the complexes with methionine is >7 days, indicating that these reactions are not biologically relevant. The rate of degradation by glutathione appears to be particularly important and shows an inverse correlation to cytotoxicity. The least active complex, 4 (t 1/2 glutathione: 20 h), degrades fastest, followed by 3 (31 h), 2 (40 h), and 1 (68 h). The major degradation product, [bis-mu-{reduced L-glutathione}bis{5,6-dimethyl-1,10-phenanthroline}bis{platinum(II)}] (2+) (5, 56MEGL), displays no cytotoxicity and is excluded as the source of the anticancer activity. Once bound by glutathione, these metal complexes do not then form coordinate bonds with guanosine. Partial encapsulation of the complexes within cucurbit[n]urils is able to stop the degradation process.

Cellular response to oxidative stress and ascorbic acid in melanoma cells overexpressing gamma-glutamyltransferase

The extracellular gamma-glutamyltransferase-mediated metabolism of glutathione has been implicated in prooxidant events which may have impact on cellular functions including drug resistance. This study was performed in two GGT-transfected melanoma clones to explore the hypothesis that GGT expression in tumour cells is implicated in modulation of cell behaviour under stress conditions. Our results show that GGT-overexpression in melanoma cells was associated with resistance to oxidative stress produced by prooxidant agents such as hydrogen peroxide and ascorbic acid. In GGT-overexpressing cells, ability to tolerate oxidative stress was evidenced by the presence of a moderate level of ROS and lack of DNA damage response following treatment with H(2)O(2). Cellular response to oxidative stress induced by ascorbic acid was detectable only in the clone with low GGT activity which also exhibited an increased susceptibility to apoptosis. The increased resistance of the GGT-overexpressing clone was not related to intracellular GSH content but rather to the increased expression of catalase and to a reduced efficiency of iron-mediated formation of toxic free radicals. Taken together, these findings are consistent with a contribution of GGT in the mechanisms of drug resistance, because induction of oxidative stress is a relevant event in the apoptotic response to cytotoxic agents.

The effect of modulation of gamma-glutamyl transpeptidase and nitric oxide synthase activity on GSH homeostasis in HepG2 cells

High glutathione (GSH) level and elevated gamma-glutamyl transpeptidase (gammaGT) activity are hallmarks of tumor cells. Toxicity of drugs and radiation to the cells is largely dependent on the level of thiols. In the present studies, we attempted to inhibit gammaGT activity in human hepatoblastoma (HepG2) cells to examine whether the administration of gammaGT inhibitors, acivicin (AC) and 1,2,3,4-tetrahydroisoquinoline (TIQ) influences cell proliferation and enhances cytostatic action of doxorubicin (DOX) and cisplatin (CP) on HepG2 cells. The effects of these inhibitors were determined by 1-(4,5-dimethylthiazol-2-yl)-3,5-diphenylformazan (MTT), BrdU and lactate dehydrogenase (LDH) tests and by estimation of GSH level. Additionally, we investigated the changes in caspase-3 activity, which is a marker of apoptosis. The obtained results showed that the gammaGT inhibitors introduced to the medium alone elicited cytotoxic effect, which was accompanied by an increase in GSH level in the cells. TIQ concomitantly increased caspase-3 activity. Doxorubicin and CP proved to be cytotoxic, and both inhibitors augmented this effect. As well DOX as CP radically decreased GSH levels, whereas gammaGT inhibitors had diverse effects. Therefore, the obtained results confirm that gammaGT inhibitors can enhance pharmacological action of DOX and CP, which may permit clinicians to decrease their doses thereby alleviating side effects. Aminoguanidine (nitric oxide synthase inhibitor) given alone was little cytotoxic to HepG2 cells, while its introduction to the medium together with DOX and CP significantly increased their cytotoxicity. Aminoguanidine on its own did not show any effect on GSH level in HepG2 cells, but markedly and significantly elevated its concentration when added in combination with CP but not with DOX. This indicates that when CP was used as a cytostatic, GSH level rose after treatment with its combination with both AC and aminoguanidine.

Redox-sensitive contrast agents for MRI based on reversible binding of thiols to serum albumin

DOTA-based complexes of gadolinium (Gd) bearing a thiol moiety on a propyl or hexyl arm were synthesized. It was hypothesized that these complexes would form reversible covalent linkages with human serum albumin (HSA), which contains a reactive thiol at cysteine-34. The binding constant of the hexyl complex to HSA was measured to be 64 mM(-1) and decreased to 17, 6.1, and 3.6 mM(-1) in the presence of 0.5, 1, and 2 mM homocysteine, respectively. The binding constant of the propyl complex to HSA was significantly lower (5.0 mM(-1)) and decreased to 2.0, 1.5, and 0.87 mM(-1) in the presence of 0.5, 1, and 2 mM homocysteine, respectively. The longitudinal water-proton relaxivities of the hexyl and propyl complexes at 37 degrees C and 4.7 T were 2.3 and 2.9 mM(-1) s(-1), respectively, in saline. The relaxivities of the HSA-bound forms of the hexyl and propyl complexes were calculated to be 5.3 and 4.5 mM(-1) s(-1), respectively. The in vivo pharmacokinetics of both thiol complexes were altered by a chase of homocysteine but not saline, while the washout of GdDTPA was unaffected by either chase. Such redox-sensitive reversible binding of Gd complexes to plasma albumin can be exploited for imaging tissue redox and the blood-pool by MRI.

Modulation of cell growth and cisplatin sensitivity by membrane γ-glutamyltransferase in melanoma cells

The plasma membrane enzyme gamma-glutamyltransferase (GGT) is regarded as critical for the maintenance of intracellular levels of glutathione (GSH). GGT expression has been implicated in drug resistance through elevation of intracellular GSH. The dependence of intracellular GSH on GGT expression was not conclusively ascertained. The present study was designed to investigate the role of GGT and of intracellular GSH levels in modulating proliferation and sensitivity to cisplatin of melanoma cells. GGT transfection resulted in increased growth, both in vitro and in tumour xenografts. In addition, GGT-transfected cells exhibited reduced sensitivity to cisplatin associated with lower DNA platination. A decrease in intracellular GSH levels, rather than an increase, was observed in GGT-transfected cells; moreover, in cysteine-deficient conditions, the expression of GGT did not provide transfected cells with the ability of utilising extracellular GSH. In conclusion, these results indicate that GGT activity confers a growth advantage unrelated with intracellular glutathione supply, and are consistent with the interpretation that cisplatin resistance is the consequence of modifications of cellular pharmacokinetics as a result of extracellular drug inactivation by thiol metabolites originated by GGT-mediated GSH cleavage.

The development of platinum compounds and their possible combination

Cisplatin plays a central role in cancer chemotherapy in spite of its toxicity. To circumvent this toxicity and to enhance its therapeutic index a lot of preclinical and clinical studies have been conducted and several thousand analogues have been synthesized. Much more analysis remains to be done, but nowadays, the absence of any definitive, biologically interpretable molecular predictor of activity is consistent with the idea that platinum compounds have multiple intracellular targets and that cells can have multiple mechanisms of resistance. This review analyses a part of these platinum compounds analyzed to date, their mechanism of action, resistance and the future trends in this sector.

Long term smoking with age builds up excessive oxidative stress in bronchoalveolar lavage fluid

BACKGROUND

Epithelial lining fluid plays a critical role in protecting the lung from oxidative stress, in which the oxidised status may change by ageing, smoking history, and pulmonary emphysema.

METHODS

Bronchoalveolar lavage (BAL) was performed on 109 young and older subjects with various smoking histories. The protein carbonyls, total and oxidised glutathione were examined in BAL fluid.

RESULTS

By Western blot analysis, the major carbonylated protein in the BAL fluid was sized at 68 kDa, corresponding to albumin. The amount of carbonylated albumin per mg total albumin in BAL fluid was four times higher in older current smokers and three times higher in older former smokers than in age matched non-smokers (p<0.0001, p=0.0003, respectively), but not in young smokers. Total glutathione in BAL fluid was significantly increased both in young (p=0.006) and older current smokers (p=0.0003) compared with age matched non-smokers. In contrast, the ratio of oxidised to total glutathione was significantly raised (72%) only in older current smokers compared with the other groups. There was no significant difference in these parameters between older smokers with and without mild emphysema.

CONCLUSIONS

Oxidised glutathione associated with excessive protein carbonylation accumulates in the lung of older smokers with long term smoking histories even in the absence of lung diseases, but they are not significantly enhanced in smokers with mild emphysema.

The S-thiolating activity of membrane gamma-glutamyltransferase: formation of cysteinyl-glycine mixed disulfides with cellular proteins and in the cell microenvironment

Previous studies have documented that activity of the plasma membrane enzyme gamma-glutamyltransferase (GGT) is accompanied by prooxidant processes, with production of reactive oxygen species and oxidation of cellular protein thiols. The present work was aimed to verify the occurrence and extent of S-thiolation mediated by GGT and characterize the molecular species involved in mixed disulfide formation. Experiments show that the cysteinyl-glycine (CG) originating from cellular GGT-mediated glutathione (GSH) metabolism can efficiently thiolate cellular proteins, as well as proteins present in the extracellular environment. With cells presenting high levels of GGT expression, basal levels of CG-containing protein mixed disulfides are detectable, in cellular proteins, as well as in proteins of the culture medium. Stimulation of GGT activity in these cells by administration of substrates results in an increase of CG mixed disulfide formation and a concomitant decrease of GSH-containing disulfides, likely due to GGT-dependent removal of GSH from the system. The findings reported suggest that binding of CG ("protein S-cysteylglycylation") may represent an as yet unrecognized function of membrane GGT, likely playing a regulatory role(s) in the cell and its surroundings.

Different mechanisms for gamma-glutamyltransferase-dependent resistance to carboplatin and cisplatin

In this work, we investigated the effect of gamma-glutamyltransferase (GGT) overexpression on cell viability after carboplatin treatment and compared with cisplatin. Carboplatin challenge of HeLa cells induced GGT and glutamate-cystine ligase (GCL) activities by 2- and 1.4-fold, respectively and concomitantly increased the intracellular reduced glutathione (GSH) level (1.5-fold). To study the role of GGT, HeLa-GGT cells, a stably transfected cell line overexpressing GGT (120-150 mU/mg protein) and the parental HeLa cells (10-15 mU/mg protein) were used. Both cell lines exhibited comparable viability (IC(50) approximately 150 microM) after carboplatin treatment when cultured in standard (250 microM cystine) medium. Culture in low (50 microM) cystine medium resulted in a dramatic decrease (approximately 90%) of the intracellular GSH level and to a 2.5-fold increase of carboplatin cytotoxicity (IC(50) approximately 60 microM). When GSH (50 microM) was included in the culture medium, only HeLa-GGT cells exhibited increased resistance to carboplatin. Using partially purified GGT from HeLa-GGT cells, we show that cisplatin forms adducts with cysteinylglycine, depending only on GGT activity whereas carboplatin did not efficiently react with cysteinylglycine. Thus, in this model system, GGT activity can affect platinum drugs cytotoxocity by two different ways: cisplatin can be detoxified extracellularly after reaction with the -SH group of cysteinylglycine; in the case of carboplatin, the supply of GSH precursors, initiated by GGT, increases the intracellular level of the tripeptide and provides enhanced defensive mechanisms to the cell.

Possible role of membrane gamma-glutamyltransferase activity in the facilitation of transferrin-dependent and -independent iron uptake by cancer cells

BACKGROUND: The molecular mechanisms by which iron is physiologically transported trough the cellular membranes are still only partially understood. Several studies indicate that a reduction step of ferric iron to ferrous is necessary, both in the case of transferrin-mediated and transferrin-independent iron uptake. Recent studies from our laboratory described gamma-glutamyltransferase activity (GGT) as a factor capable to effect iron reduction in the cell microenvironment. GGT is located on the outer aspect of plasma membrane of most cell types, and is often expressed at high levels in malignant tumors and their metastases. The present study was aimed at verifying the possibility that GGT-mediated iron reduction may participate in the process of cellular iron uptake. RESULTS: Four distinct human tumor cell lines, exhibiting different levels of GGT activity, were studied. The uptake of transferrin-bound iron was investigated by using 55Fe-loaded transferrin, as well as by monitoring fluorimetrically the intracellular iron levels in calcein-preloaded cells. Transferrin-independent iron uptake was investigated using 55Fe complexed by nitrilotriacetic acid (55Fe-NTA complex).The stimulation of GGT activity, by administration to cells of the substrates glutathione and glycyl-glycine, was generally reflected in a facilitation of transferrin-bound iron uptake. The extent of such facilitation was correlated with the intrinsic levels of the enzyme present in each cell line. Accordingly, inhibition of GGT activity by means of two independent inhibitors, acivicin and serine/boric acid complex, resulted in a decreased uptake of transferrin-bound iron. With Fe-NTA complex, the inhibitory effect - but not the stimulatory one - was also observed. CONCLUSION: It is concluded that membrane GGT can represent a facilitating factor in iron uptake by GGT-expressing cancer cells, thus providing them with a selective growth advantage over clones that do not possess the enzyme.

Glutathione catabolism as a signaling mechanism

Glutathione (GSH) is the main intracellular thiol antioxidant, and as such participates in a number of cellular antitoxic and defensive functions. Nevertheless, non-antioxidant functions of GSH have also been described, e.g. in modulation of cell proliferation and immune response. Recent studies from our and other laboratories have provided evidence for a third functional aspect of GSH, i.e. the prooxidant roles played by molecular species originating during its catabolism by the membrane ectoenzyme gamma-glutamyl transpeptidase (GGT). The reduction of metal ions effected by GSH catabolites is capable to induce redox cycling processes leading to the production of reactive oxygen species (superoxide, hydrogen peroxide), as well as of other free radicals. Through the action of these reactive compounds, GSH catabolism can ultimately lead to oxidative modifications on a variety of molecular targets, involving oxidation and/or S-thiolation of protein thiol groups in the first place. Modulating effects of this kind have been observed on several important, redox-sensitive components of the signal transduction chains, such as cell surface receptors, protein phosphatase activities and transcription factors. Against this background, the prooxidant reactions induced by GSH catabolism appear to represent a novel, as yet unrecognized mechanism for modulation of cellular signal transduction.