Alessandro Corti

Cellular pathways for transport and efflux of ascorbate and dehydroascorbate

The mechanisms allowing the cellular transport of ascorbic acid represent a primary aspect for the understanding of the roles played by this vitamin in pathophysiology. Considerable research effort has been spent in the field, on several animal models and different cell types. Several mechanisms have been described to date, mediating the movements of different redox forms of ascorbic acid across cell membranes. Vitamin C can enter cells both in its reduced and oxidized form, ascorbic acid (AA) and dehydroascorbate (DHA), utilizing respectively sodium-dependent transporters (SVCT) or glucose transporters (GLUT). Modulation of SVCT expression and function has been described by cytokines, steroids and post-translational protein modification. Cellular uptake of DHA is followed by its intracellular reduction to AA by several enzymatic and non-enzymatic systems. Efflux of vitamin C has been also described in a number of cell types and different pathophysiological functions were proposed for this phenomenon, in dependence of the cell model studied. Cellular efflux of AA is mediated through volume-sensitive (VSOAC) and Ca(2+)-dependent anion channels, gap-junction hemichannels, exocytosis of secretory vesicles and possibly through homo- and hetero-exchange systems at the plasma membrane level. Altogether, available data suggest that cellular efflux of ascorbic acid - besides its uptake - should be taken into account when evaluating the cellular homeostasis and functions of this important vitamin.

γ-Glutamyltransferase activity in human atherosclerotic plaques—Biochemical similarities with the circulating enzyme

BACKGROUND AND PURPOSE Serum gamma-glutamyltransferase (GGT) activity has been identified as a predictor of complications of atherosclerosis, with a prognostic value for cardiovascular diseases and stroke. Human atherosclerotic lesions contain active GGT, which can give rise to pro-oxidant molecular species; thus a direct contribution of GGT to atherosclerosis progression is conceivable. The relationship between plaque and serum GGT is however unclear. METHODS AND RESULTS Human carotid plaques obtained from 18 consecutive patients undergoing carotid endoarteriectomy were analyzed, of which 6 were used for anion exchange and gel filtration chromatography/western blot studies, 7 for beta-lipoprotein precipitation, and 5 for RNA extraction and determination of low molecular weight thiols. Mean GGT activity in crude plaque homogenates was 60.9+/-21.5 (S.D.) mU/g tissue. The characteristics of GGT activity were compared in plaque homogenates and in serum obtained from controls (healthy blood donors). The methods employed (anion exchange and gel chromatography, western blot) showed the presence in plaque homogenates of two distinct complexes containing GGT activity, one of which comparable with plasma LDL/GGT complexes. Accordingly, precipitation of beta-lipoproteins from plaque homogenates resulted in removal of GGT activity. RT-PCR indicated in plaques the presence of GGT mRNA transcribed from GGT-I gene. Analysis of plaque extracts also revealed the presence of enzyme product cysteinyl-glycine both as free and protein-bound form, confirming that GGT-dependent pro-oxidant reactions may occur within the plaque environment. CONCLUSIONS The results obtained suggest the presence in plaques of a serum-like GGT protein, indicating that a direct contribution of serum GGT to enzyme activity found within atherosclerotic lesions is possible. Data also indicate the occurrence of GGT-mediated redox reactions within plaque environment, which might influence plaque progression.

Prooxidant reactions promoted by soluble and cell-bound gamma-glutamyltransferase activity

Recent studies have provided evidence for the prooxidant roles played by molecular species originating during the catabolism of glutathione (GSH) effected by gamma-glutamyltransferase (GGT), an enzyme normally present in serum and on the outer surface of numerous cell types. The reduction of metal ions by GSH catabolites is capable of inducing the redox cycling processes, leading to the production of reactive oxygen species and other free radicals. Through the action of these reactive compounds, cell membrane GGT activity can ultimately produce oxidative modifications on a variety of molecular targets, involving oxidation and/or S-thiolation of protein thiol groups in the first place. This chapter is a survey of the procedures most suitable to reveal GGT-dependent prooxidant reactions and their effects at the cellular and extracellular level, including methods in histochemistry, cytochemistry, and biochemistry, with special reference to methods for the evaluation of protein thiol redox status.

Glutathione transferase omega 1-1 (GSTO1-1) plays an anti-apoptotic role in cell resistance to cisplatin toxicity

Several lines of evidence correlate the overexpression of glutathione S-transferase omega 1-1 (GSTO1-1) with the onset of drug resistance of cancer cells; however, no direct evidence is yet available. In order to investigate the mechanisms involved, stable transfection with GSTO1-1 complementary DNA was performed in HeLa cells, which spontaneously express very low levels of GSTO1-1. When transfected cells were seeded at low density, a sharp increase in GSTO1-1 expression was observed as compared with controls, along with an increased resistance against cisplatin cytotoxicity. When seeded at increasing densities, control untransfected cells also presented with an increase in GSTO1-1 expression, again accompanied by cisplatin resistance; the latter was significantly reduced after transfection with GSTO1-1 small interfering RNA. Cisplatin resistance of transfected cells was not accounted for by changes in the intracellular drug concentration nor in the amount of DNA cross-links or content of glutathione. Rather, transfected cells presented with a marked decrease of apoptosis as compared with controls, suggesting that GSTO1-1 overexpression may prevent cisplatin toxicity by interfering with the apoptotic process. Cisplatin treatment was in fact followed at early times (1-2 h) by activation of both Akt kinase and extracellular signal-regulated kinase (ERK)-1/2 in the transfected cells but not in controls. Conversely, in transfected cells, the strong activation of Jun N-terminal kinase (JNK)-1 induced by cisplatin at later times (10-20 h) was completely prevented. In conclusion, GSTO1-1 overexpression appears to be associated with activation of survival pathways (Akt and ERK1/2) and inhibition of apoptotic pathways (JNK1), as well as protection against cisplatin-induced apoptosis.

Mechanostructure and composition of highly reproducible decellularized liver matrices.

Despite the increasing number of papers on decellularized scaffolds, there is little consensus on the optimum method of decellularizing biological tissue such that the micro-architecture and protein content of the matrix are conserved as far as possible. Focusing on the liver, the aim of this study was therefore to develop a method for the production of well-characterized and reproducible matrices that best preserves the structure and composition of the native extra cellular matrix (ECM). Given the importance of matrix stiffness in regulating cell response, the mechanical properties of the decellularized tissue were also considered. The testing and analysis framework is based on the characterization of decellularized and untreated samples in the same reproducible initial state (i.e., the equilibrium swollen state). Decellularized ECM (dECM) were characterized using biochemical, histological, mechanical and structural analyses to identify the best procedure to ensure complete cell removal while preserving most of the native ECM structure and composition. Using this method, sterile decellularized porcine ECM with highly conserved intra-lobular micro-structure and protein content were obtained in a consistent and reproducible manner using the equilibrium swollen state of tissue or matrix as a reference. A significant reduction in the compressive elastic modulus was observed for liver dECM with respect to native tissue, suggesting a re-examination of design parameters for ECM-mimicking scaffolds for engineering tissues in vitro.

Membrane gamma-glutamyl transferase activity promotes iron-dependent oxidative DNA damage in melanoma cells.

A number of recent observations have suggested a potential role for membrane-bound gamma-glutamyltransferase (GGT) in tumor progression and appearance of more aggressive and resistant phenotypes, through redox interactions leading to production of reactive oxygen species. The present study was aimed to evaluate whether such pro-oxidant activity of GGT can promote oxidative DNA damage, thus contributing to cancer genomic instability. Human GGT-transfected melanoma cells were studied, and DNA damage was measured by using the alkaline comet assay. Our results indicate that higher levels of GGT activity are associated with higher levels of background DNA damage and oxidized bases. This association cannot be explained by differences in cell cycle distribution or apoptotic rates. GGT-over-expressing cells also presented with a markedly higher glucose uptake, a phenomenon potentially leading to higher metabolic rate and oxidative DNA damage. Anyway, when GGT-over-expressing cells were incubated in the presence of GGT substrates and a source of catalytic iron, increased levels of DNA damage and oxidized bases were observed, an effect completely prevented in the presence of GGT inhibitors or various antioxidants.The findings reported indicate that GGT activity is able to promote iron-dependent DNA oxidative damage, thus potentially representing an important mechanism in initiation/progression of neoplastic transformation.

Metabolism of the Tumor Angiogenesis Inhibitor 4-(N-(S-Glutathionylacetyl)amino)phenylarsonous Acid *

4-(N-(S-glutathionylacetyl)amino) phenylarsonous acid (GSAO) is a small, synthetic mitochondrial poison that targets angiogenic endothelial cells and is currently being tested in a Phase I/IIa clinical trial. The trivalent arsenical of GSAO reacts with and perturbs adenine nucleotide translocase of the inner mitochondrial membrane of endothelial cells, which leads to proliferation arrest. Three observations indicated that the γ-glutamyl residue of GSAO is cleaved at the endothelial cell surface by γ-glutamyl transpeptidase (γGT). GSAO was found to be an efficient substrate for γGT, endothelial cell accumulation and antiproliferative activity of GSAO was blunted by a competitive substrate and an active site inhibitor of γGT, and the level of cell surface γGT correlated strongly with the sensitivity of cells to GSAO. Using transport inhibitors, it was revealed that the resulting metabolite of GSAO cleavage by γGT, 4-(N-(S-cysteinylglycylacetyl)amino) phenylarsonous acid (GCAO), was transported across the plasma membrane by an organic anion transporter. Furthermore, GCAO is likely processed by dipeptidases in the cytosol to 4-(N-(S-cysteinylacetyl)amino) phenylarsonous acid (CAO), and it is this metabolite that reacts with mitochondrial adenine nucleotide translocase. Taken together, our findings indicate that γGT processing of GSAO at the cell surface is the rate-limiting step in its antiangiogenic activity. This information can explain the kidney toxicity at high doses of GSAO noted in preclinical studies and will aid in the anticipation of potential side effects in humans and in the design of better antimitochondrial cancer drugs.

CLM3, a Multitarget Tyrosine Kinase Inhibitor With Antiangiogenic Properties, Is Active Against Primary Anaplastic Thyroid Cancer In Vitro and In Vivo

CONTEXT AND OBJECTIVE We have studied the antitumor activity of a pyrazolo[3,4-d]pyrimidine compound (CLM3) proposed for a multiple signal transduction inhibition [including the RET tyrosine kinase, epidermal growth factor receptor, and vascular endothelial growth factor (VEGF) receptor and with antiangiogenic activity] in primary anaplastic thyroid cancer (ATC) cells, in the human cell line 8305C (undifferentiated thyroid cancer), and in an ATC-cell line (AF). DESIGN AND MAIN OUTCOME MEASURES CLM3 was tested in primary ATC cells at the concentrations of 5, 10, 30, and 50 μM; in 8305C cells, in AF cells, at 1, 5, 10, 30, 50, or 100 μM; and in AF cells in CD nu/nu mice. RESULTS CLM3 significantly inhibited the proliferation of 8305C and AF cells, also inducing apoptosis. A significant reduction of proliferation with CLM3 in ATC cells (P < .01, ANOVA) was shown. CLM3 increased the percentage of apoptotic ATC cells dose dependently (P < .001, ANOVA) and inhibited migration (P < .01) and invasion (P < .001). The AF cell line was injected sc in CD nu/nu mice, and tumor masses became detectable 15 days later. CLM3 (50 mg/kg per die) significantly inhibited tumor growth (starting 16 d after the beginning of treatment). CLM3 significantly decreased the VEGF-A expression and microvessel density in AF tumor tissues. Furthermore, CLM3 inhibited epidermal growth factor receptor, AKT, and ERK1/2 phosphorylation and down-regulated cyclin D1 in 8305C and AF cells. CONCLUSIONS The antitumor and antiangiogenic activity of a pyrazolo[3,4-d]pyrimidine compound (CLM3) is very promising in anaplastic thyroid cancer, opening the way to a future clinical evaluation.

Contribution by Polymorphonucleate Granulocytes to Elevated Gamma-Glutamyltransferase in Cystic Fibrosis Sputum

Background Cystic fibrosis (CF) is an autosomal recessive disorder characterized by a chronic neutrophilic airways inflammation, increasing levels of oxidative stress and reduced levels of antioxidants such as glutathione (GSH). Gamma-glutamyltransferase (GGT), an enzyme induced by oxidative stress and involved in the catabolism of GSH and its derivatives, is increased in the airways of CF patients with inflammation, but the possible implications of its increase have not yet been investigated in detail. Principal Findings The present study was aimed to evaluate the origin and the biochemical characteristics of the GGT detectable in CF sputum. We found GGT activity both in neutrophils and in the fluid, the latter significantly correlating with myeloperoxidase expression. In neutrophils, GGT was associated with intracellular granules. In the fluid, gel-filtration chromatography showed the presence of two distinct GGT fractions, the first corresponding to the human plasma b-GGT fraction, the other to the free enzyme. The same fractions were also observed in the supernatant of ionomycin and fMLP-activated neutrophils. Western blot analysis confirmed the presence of a single band of GGT immunoreactive peptide in the CF sputum samples and in isolated neutrophils. Conclusions In conclusion, our data indicate that neutrophils are able to transport and release GGT, thus increasing GGT activity in CF sputum. The prompt release of GGT may have consequences on all GGT substrates, including major inflammatory mediators such as S-nitrosoglutathione and leukotrienes, and could participate in early modulation of inflammatory response.

Cultured human cells release soluble γ-glutamyltransferase complexes corresponding to the plasma b-GGT

Serum γ-glutamyltransferase (GGT) is thought to derive from the liver, but its values predict morbidity and mortality for several diseases, such as cardiac infarction, stroke, diabetes, renal failure and cancer. We assessed total GGT and its fractions in the culture supernatants of human cell lines (melanoma, prostate cancer, bronchial epithelium) by gel filtration chromatography. We also compared the GGT elution profile in plasma and the corresponding very-low-density lipoprotein (VLDL) fraction. All the cell lines tested released soluble GGT whose activity increased in parallel with the cell growth. Released GGT presented a molecular weight of 2000 kDa, identical to the b-GGT fraction of human plasma and corresponding to that of VLDL. But ultracentrifugation studies showed that b-GGT had a higher density than VLDL. The b-GGT present in human plasma can be produced by tissues other than the liver, thus explaining the increase of serum GGT observed in diseases of other organs.