GGsTop, a novel and specific γ-glutamyl transpeptidase inhibitor, protects hepatic ischemia-reperfusion injury in rats

Cellular and molecular mechanisms of hepatic ischemia-reperfusion injury: The role of oxidative stress and therapeutic approaches

Ischemia-reperfusion (IR) or reoxygenation injury is the paradoxical exacerbation of cellular impairment following restoration of blood flow after a period of ischemia during surgical procedures or other conditions. Acute interruption of blood supply to the liver and subsequent reperfusion can result in hepatocyte injury, apoptosis, and necrosis. Since the liver requires a continuous supply of oxygen for many biochemical reactions, any obstruction of blood flow can rapidly lead to hepatic hypoxia, which could quickly progress to absolute anoxia. Reoxygenation results in the increased generation of reactive oxygen species and oxidative stress, which lead to the enhanced production of proinflammatory cytokines, chemokines, and other signaling molecules. Consequent acute inflammatory cascades lead to significant impairment of hepatocytes and nonparenchymal cells. Furthermore, the expression of several vascular growth factors results in the heterogeneous closure of numerous hepatic sinusoids, which leads to reduced oxygen supply in certain areas of the liver even after reperfusion. Therefore, it is vital to identify appropriate therapeutic modalities to mitigate hepatic IR injury and subsequent tissue damage. This review covers all the major aspects of cellular and molecular mechanisms underlying the pathogenesis of hepatic ischemia-reperfusion injury, with special emphasis on oxidative stress, associated inflammation and complications, and prospective therapeutic approaches.

Multitargeting HDAC Inhibitors Containing a RAS/RAF Protein Interfering Unit

In this work, a series of multitargeting histone deacetylase (HDAC) inhibitors capable of regulating the signal transduction between RAS protein and downstream effectors were obtained by introducing a zinc-ion-binding group into the framework of rigosertib via different linkers. Among them, two representative compounds, XSJ-7 and XSJ-10, not only showed stronger antiproliferative activity against many types of cancer cells including solid tumor cells but also presented more potent inhibition on different subtypes of HDAC than suberoylanilide hydroxamic acid (SAHA). Significantly, XSJ-10 presented moderate pharmacokinetic behaviors and showed stronger antitumor activity than oxaliplatin, SAHA, and rigosertib in the HT-29 xenograft mouse models without significant systemic toxicity. Research on the anticancer mechanism of XSJ-10 revealed that it can effectively induce the apoptosis of cancer cells and suppress the tumor by strongly inhibiting the RAS-RAF-MEK-ERK signaling pathway and the acetylation level of HDAC3.

Targeting gamma-glutamyl transpeptidase: A pleiotropic enzyme involved in glutathione metabolism and in the control of redox homeostasis

Gamma-glutamyl transpeptidase (GGT) is an enzyme located on the outer membrane of the cells where it regulates the metabolism of glutathione (GSH), the most abundant intracellular antioxidant thiol. GGT plays a key role in the control of redox homeostasis, by hydrolyzing extracellular GSH and providing the cell with the recovery of cysteine, which is necessary for de novo intracellular GSH and protein biosynthesis. Therefore, the upregulation of GGT confers to the cell greater resistance to oxidative stress and the advantage of growing fast. Indeed, GGT is upregulated in inflammatory conditions and in the progression of various human tumors and it is involved in many physiological disorders related to oxidative stress, such as cardiovascular disease and diabetes. Currently, increased GGT expression is considered a marker of liver damage, cancer, and low-grade chronic inflammation. This review addresses the current knowledge on the structure-function relationship of GGT, focusing on human GGT, and provides information on the pleiotropic biological role and relevance of the enzyme as a target of drugs aimed at alleviating oxidative stress-related diseases. The development of new GGT inhibitors is critically discussed, as are the advantages and disadvantages of their potential use in clinics. Considering its pleiotropic activities and evolved functions, GGT is a potential "moonlighting protein".

Design and evaluation of novel analogs of 2-amino-4-boronobutanoic acid (ABBA) as inhibitors of human gamma-glutamyl transpeptidase

Inhibitors of gamma-glutamyl transpeptidase (GGT1, aka gamma-glutamyl transferase) are needed for the treatment of cancer, cardiovascular illness and other diseases. Compounds that inhibit GGT1 have been evaluated in the clinic, but no inhibitor has successfully demonstrated specific and systemic GGT1 inhibition. All have severe side effects. L-2-amino-4‑boronobutanoic acid (l-ABBA), a glutamate analog, is the most potent GGT1 inhibitor in vitro. In this study, we have solved the crystal structure of human GGT1 (hGGT1) with ABBA bound in the active site. The structure was interrogated to identify interactions between the enzyme and the inhibitor. Based on these data, a series of novel ABBA analogs were designed and synthesized. Their inhibitory activity against the hydrolysis and transpeptidation activities of hGGT1 were determined. The lead compounds were crystalized with hGGT1 and the structures solved. The kinetic data and structures of the complexes provide new insights into the critical role of protein structure dynamics in developing compounds for inhibition of hGGT1.

Is Glyceryl Trinitrate, a Nitric Oxide Donor Responsible for Ameliorating the Chemical-Induced Tissue Injury In Vivo?

Oxidative stress induced by well-known toxins including ferric nitrilotriacetate (Fe-NTA), carbon tetrachloride (CCl₄) and thioacetamide (TAA) has been attributed to causing tissue injury in the liver and kidney. In this study, the effect of glyceryl trinitrate (GTN), a donor of nitric oxide and NG-nitroarginine methyl ester (l-NAME), a nitric oxide inhibitor on TAA-induced hepatic oxidative stress, GSH and GSH-dependent enzymes, serum transaminases and tumor promotion markers such as ornithine decarboxylase (ODC) activity and [³H]-thymidine incorporation in rats were examined. The animals were divided into seven groups consisting of six healthy rats per group. The six rats were injected intraperitoneally with TAA to evaluate its toxic effect, improvement in its toxic effect if any, or worsening in its toxic effect if any, when given in combination with GTN or l-NAME. The single necrogenic dose of TAA administration caused a significant change in the levels of both hepatic and serum enzymes such as glutathione S-transferase (GST), glutathione reductase (GR), glutathione peroxidase (GPx), γ-glutamyl transpeptidase (GGT), glucose 6-phosphate dehydrogenase (G6PD), alanine aminotransferase (AST) and aspartate aminotransferase (ALT). In addition, treatment with TAA also augmented malondialdehyde (MDA), ornithine decarboxylase (ODC) activity and [³H]-thymidine incorporation in rats liver. Concomitantly, TAA treatment depleted the levels of GSH. However, most of these changes were alleviated by the treatment of animals with GTN dose-dependently. The protective effect of GTN against TAA was also confirmed histopathologically. The present data confirmed our earlier findings with other oxidants including Fe-NTA and CCl₄. The GTN showed no change whatsoever when administered alone, however when it was given along with TAA then it showed protection thereby contributing towards defending the role against oxidants-induced organ toxicity. Overall, GTN may contribute to protection against TAA-induced oxidative stress, toxicity, and proliferative response in the liver, according to our findings.

Epigallocatechin-3-gallate inhibits osteopontin expression and prevents experimentally induced hepatic fibrosis

Osteopontin (OPN) is a matricellular cytokine and a stress-induced profibrogenic molecule that promotes activation of stellate cells during the pathogenesis of hepatic fibrosis. We studied the protective effects of epigallocatechin-3-gallate (EGCG) to suppress oxidative stress, inhibit OPN expression, and prevent experimentally induced hepatic fibrosis. Liver injury was induced with intraperitoneal injections of N-nitrosodimethylamine (NDMA) in a dose of 1 mg/100 g body weight on 3 consecutive days of a week for 28 days. A group of rats received 0.2 mg EGCG/100 g body weight orally everyday during the study. The animals were sacrificed on day 28th from the beginning of exposure. Serum levels of AST, ALT, OPN, malondialdehyde, collagen type IV, and hyaluronic acid were measured. Immunohistochemistry and/or real-time PCR were performed for α-SMA, 4-HNE, OPN, collagen type I, and type III. Serial administrations of NDMA produced well developed fibrosis and early cirrhosis in rat liver. Treatment with EGCG significantly reduced serum/plasma levels of AST, ALT, OPN, malondialdehyde, collagen type IV, and hyaluronic acid and prevented deposition of collagen fibers in the hepatic tissue. Protein and/or mRNA levels demonstrated marked decrease in the expression of α-SMA, 4-HNE, OPN, collagen type I, and type III. Treatment with EGCG prevented excessive generation of reactive oxygen species, suppressed oxidative stress, significantly reduced serum and hepatic OPN levels, and markedly attenuated hepatic fibrosis. The results indicated that EGCG could be used as a potent therapeutic agent to prevent hepatic fibrogenesis and related adverse events.

Targeting GGT1 Eliminates the Tumor-Promoting Effect and Enhanced Immunosuppressive Function of Myeloid-Derived Suppressor Cells Caused by G-CSF

Myeloid-derived suppressor cells (MDSCs) are major immunosuppressive cells that accumulate in tumor-bearing hosts. Since MDSCs suppress anti-tumor immunity and promote tumor progression, they are promising targets for cancer immunotherapy. Granulocyte colony-stimulating factor (G-CSF) is an agent used for the treatment of chemotherapy-induced febrile neutropenia (FN) in patients with cancer. However, several reports have revealed that G-CSF plays crucial immune-related adverse roles in tumor progression through MDSCs. In this study, we showed that MDSCs differentiated in the presence of G-CSF in vitro exhibited enhanced proliferation and immunosuppressive activity compared to those differentiated without G-CSF. RNA sequencing analysis demonstrated that G-CSF enhanced the immunosuppressive function of MDSCs by upregulating gamma-glutamyltransferase (GGT) 1. Moreover, in the EL4 lymphoma-bearing neutropenic mouse model, administration of recombinant G-CSF increased the number of MDSCs and attenuated the anti-cancer effect of chemotherapy. We showed that the combination of GGsTop, a GGT inhibitor, could prevent G-CSF-induced tumor growth, without affecting the promotion of myelopoiesis by G-CSF. These results suggest that targeting GGT1 can mitigate G-CSF-induced enhanced immunosuppressive functions of MDSCs and can eliminate the tumor-promoting effect of G-CSF. Furthermore, GGsTop could be an attractive combination agent during G-CSF treatment for FN in patients with cancer.

Effects of GGsTop® on Collagen and Glutathione in the Oral Mucosa Using a Rat Model of 5-Fluorouracil-Induced Oral Mucositis

BACKGROUND/AIM

To evaluate the usefulness of GGsTop^® for oral mucositis, a quantitative study focusing on oral mucosal tissues is necessary. In this study, we aimed to quantify collagen and glutathione using a rat model of 5-fluorouracil-induced oral mucositis.

MATERIALS AND METHODS

Changes in ulcer area and erythrocyte count were measured to confirm the usefulness of GGsTop^® for oral mucositis. The effect of GGsTop on collagen was evaluated by observing oral mucosal tissue sections and measuring the collagen concentration in the tissues. The total glutathione concentration and the oxidized glutathione concentration were measured, and the concentration of the reduced form was calculated.

RESULTS

GGsTop^® shortened the treatment period for oral mucositis without affecting the white blood cell count. In addition, GGsTop^® promoted collagen production and alleviated oxidative stress conditions.

CONCLUSION

GGsTop affects collagen and glutathione in the treatment of oral mucositis.

Crystal structures of glutathione- and inhibitor-bound human GGT1: critical interactions within the cysteinylglycine binding site

Overexpression of γ-glutamyl transpeptidase (GGT1) has been implicated in an array of human diseases including asthma, reperfusion injury, and cancer. Inhibitors are needed for therapy, but development of potent, specific inhibitors of GGT1 has been hampered by a lack of structural information regarding substrate binding and cleavage. To enhance our understanding of the molecular mechanism of substrate cleavage, we have solved the crystal structures of human GGT1 (hGGT1) with glutathione (a substrate) and a phosphate-glutathione analog (an irreversible inhibitor) bound in the active site. These are the first structures of any eukaryotic GGT with the cysteinylglycine region of the substrate-binding site occupied. These structures and the structure of apo-hGGT reveal movement of amino acid residues within the active site as the substrate binds. Asn-401 and Thr-381 each form hydrogen bonds with two atoms of GSH spanning the γ-glutamyl bond. Three different atoms of hGGT1 interact with the carboxyl oxygen of the cysteine of GSH. Interactions between the enzyme and substrate change as the substrate moves deeper into the active site cleft. The substrate reorients and a new hydrogen bond is formed between the substrate and the oxyanion hole. Thr-381 is locked into a single conformation as an acyl bond forms between the substrate and the enzyme. These data provide insight on a molecular level into the substrate specificity of hGGT1 and provide an explanation for seemingly disparate observations regarding the enzymatic activity of hGGT1 mutants. This knowledge will aid in the design of clinically useful hGGT1 inhibitors.

Inhibition of γ-glutamyltransferase ameliorates ischaemia-reoxygenation tissue damage in rats with hepatic steatosis

BACKGROUND AND PURPOSE

Hepatic steatosis may be associated with an increased γ-glutamyltransferase (γ-GT) levels. Ischaemia-reoxygenation (IR) injury causes several deleterious effects. We evaluated the protective effects of a selective inhibitor of γ-GT in experimentally induced IR injury in rats with obesity and steatosis.

EXPERIMENTAL APPROACH

Otsuka Long-Evans Tokushima Fatty (OLETF) rats with hepatic steatosis were used in the current study. The portal vein and hepatic artery of left lateral and median lobes were clamped to induce ischaemia. Before clamping, 1 ml of saline (IR group) or 1-ml saline containing 1 mg·kg^-1 body weight of GGsTop (γ-GT inhibitor; IR-GGsTop group) was injected into the liver via the inferior vena cava. Blood flow was restored after at 30 min of the start of ischaemia. Blood was collected before, at 30 min after ischaemia and at 2 h and 6 h after reoxygenation. All the animals were killed at 6 h and the livers were collected.

KEY RESULTS

Treatment with GGsTop resulted in significant reduction of serum ALT, AST and γ-GT levels and hepatic γ-GT, malondialdehyde, 4-hydroxy-2-nonenal and HMGB1 at 6 h after reoxygenation. Inhibition of γ-GT retained normal hepatic glutathione levels. There was prominent hepatic necrosis in IR group, which is significantly reduced in IR-GGsTop group.

CONCLUSION AND IMPLICATIONS

Treatment with GGsTop significantly increased hepatic glutathione content, reduced hepatic MDA, 4-HNE and HMGB1 levels and, remarkably, ameliorated hepatic necrosis after ischaemia-reoxygenation. The results indicated that GGsTop could be an appropriate therapeutic agent to reduce IR-induced liver injury in obesity and steatosis.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Recombinant thrombomodulin prevented hepatic ischemia-reperfusion injury by inhibiting high-mobility group box 1 in rats

Recombinant thrombomodulin (rTM) is a novel anticoagulant and anti-inflammatory agent that inhibits secretion of high-mobility group box 1 (HMGB1) from liver. We evaluated the protective effects of rTM on hepatic ischemia-reperfusion injury in rats. Ischemia was induced by clamping the portal vein and hepatic artery of left lateral and median lobes of the liver. At 30 min before ischemia and at 6 h after reperfusion, 0.3 ml of saline (IR group) or 0.3 ml of saline containing 6 mg/kg body weight of rTM (IR-rTM group) was injected into the liver through inferior vena cava or caudate vein. Blood flow was restored at 60 min of ischemia. Blood was collected 30 min prior to induction of ischemia and before restoration of blood flow, and at 6, 12, and 24 h after reperfusion. All the animals were euthanized at 24 h after reperfusion and the livers were harvested and subjected to biochemical and pathological evaluations. Serum levels of ALT, AST, and HMGB1 were significantly lower after reperfusion in the IR-rTM group compared to IR group. Marked hepatic necrosis was present in the IR group, while necrosis was almost absent in IR-rTM group. Treatment with rTM significantly reduced the expression of TNF-α and formation of 4-hydroxynonenal in the IR-rTM group compared to IR group. The results of the present study indicate that rTM could be used as a potent therapeutic agent to prevent IR-induced hepatic injury and the related adverse events.

Synthesis and evaluation of the inhibitory activity of the four stereoisomers of the potent and selective human γ-glutamyl transpeptidase inhibitor GGsTop

2-Amino-4-{[3-(carboxymethyl)phenoxy](methoxy)phosphoryl}butanoic acid (GGsTop) is a potent, highly selective, nontoxic, and irreversible inhibitor of γ-glutamyl transpeptidase (GGT). GGsTop has been widely used in academic and medicinal research, and also as an active ingredient (Nahlsgen) in commercial anti-aging cosmetics. GGsTop consists of four stereoisomers due to the presence of two stereogenic centers, i.e., the α-carbon atom of the glutamate mimic (l/d) and the phosphorus atom (R_P/S_P). In this study, each stereoisomer of GGsTop was synthesized stereoselectively and their inhibitory activity against human GGT was evaluated. The l- and d-configurations of each stereoisomer were determined by a combination of a chiral pool synthesis and chiral HPLC analysis. The synthesis of the four stereoisomers of GGsTop used chiral synthetic precursors that were separated by chiral HPLC on a preparative scale. With respect to the configuration of the α-carbon atom of the glutamate mimic, the l-isomer (k_on=174M^-1s^-1) was ca. 8-fold more potent than the d-isomer (k_on=21.5M^-1s^-1). In contrast, the configuration of the phosphorus atom is critical for GGT inhibitory activity. Based on a molecular modeling approach, the absolute configuration of the phosphorus atom of the active GGsTop isomers was postulated to be S_P. The S_P-isomers inhibited human GGT (k_on=21.5-174M^-1s^-1), while the R_P-isomers were inactive even at concentrations of 0.1mM.