The fine-tuning of TRAF2-GSTP1-1 interaction: effect of ligand binding and in situ detection of the complex

Inflammation and cancer cell survival: TRAF2 as a key player

TNF receptor-associated factor 2 (TRAF2) plays a crucial role in both physiological and pathological processes. It takes part in the regulation of cell survival and death, tissue regeneration, development, endoplasmic reticulum stress response, autophagy, homeostasis of the epithelial barrier and regulation of adaptive and innate immunity. Initially identified for its interaction with TNF receptor 2 (TNFR2), TRAF2 contains a TRAF domain that enables homo- and hetero-oligomerization, allowing it to interact with multiple receptors and signaling molecules. While best known for mediating TNFR1 and TNFR2 signaling, TRAF2 also modulates other receptor pathways, including MAPK, NF-κB, and Wnt/β-catenin cascades. By regulating NF-κB-inducing kinase (NIK), TRAF2 is a key activator of the alternative NF-κB pathway, linking it to inflammatory diseases, immune dysfunction, and tumorigenesis. In the innate immune system, TRAF2 influences macrophage differentiation, activation, and survival and stimulates natural killer cell cytotoxicity. In the adaptive immune system, it represses effector B- and T-cell activity while sustaining regulatory T-cell function, thus promoting immune suppression. The lack of fine-tuning of TRAF2 activity leads to excessive NF-kB activation, driving chronic inflammation and autoimmunity. Although TRAF2 can act as a tumor suppressor, it is predominantly described as a tumor promoter, as its expression has been correlated with increased metastatic potential and poorer prognosis in several types of cancer. Targeting TRAF2 or TRAF2-dependent signaling pathways might represent a promising anti-cancer therapeutic strategy.

Glutathione S-transferase: A keystone in Parkinson's disease pathogenesis and therapy

Parkinson's disease is a progressive neurodegenerative disorder that predominantly affects motor function due to the loss of dopaminergic neurons in the substantia nigra. It presents significant challenges, impacting millions worldwide with symptoms such as tremors, rigidity, bradykinesia, and postural instability, leading to decreased quality of life and increased morbidity. The pathogenesis of Parkinson's disease is multifaceted, involving complex interactions between genetic susceptibility, environmental factors, and aging, with oxidative stress playing a central role in neuronal degeneration. Glutathione S-Transferase enzymes are critical in the cellular defense mechanism against oxidative stress, catalysing the conjugation of the antioxidant glutathione to various toxic compounds, thereby facilitating their detoxification. Recent research underscores the importance of Glutathione S-Transferase in the pathophysiology of Parkinson's disease, revealing that genetic polymorphisms in Glutathione S-Transferase genes influence the risk and progression of the disease. These genetic variations can affect the enzymatic activity of Glutathione S-Transferase, thereby modulating an individual's capacity to detoxify reactive oxygen species and xenobiotics, which are implicated in Parkinson's disease neuropathological processes. Moreover, biochemical studies have elucidated the role of Glutathione S-Transferase in not only maintaining cellular redox balance but also in modulating various cellular signalling pathways, highlighting its neuroprotective potential. From a therapeutic perspective, targeting Glutathione S-Transferase pathways offers promising avenues for the development of novel treatments aimed at enhancing neuroprotection and mitigating disease progression. This review explores the evident and hypothesized roles of Glutathione S-Transferase in Parkinson's disease, providing a comprehensive overview of its importance and potential as a target for therapeutic intervention.

Investigation of the inhibitory effects of the telomere-targeted compounds on glutathione S-transferase P1

Glutathione S-transferase P1 (GSTP1) plays a significant role in cancer progression and chemotherapy resistance, with its overexpression diminishing chemotherapeutic efficacy across various tumor types. This study evaluates the inhibitory effects of 6-thio-2'-deoxyguanosine (6-thio-dG) and its dimeric form (6-thio-2'-dG-Dimer) on GSTP1. Enzyme inhibition assays with recombinant human GSTP1, kinetic analysis, molecular docking, and molecular dynamic simulations were employed. Enzymatic assays were performed in 0.1 M phosphate buffer (pH 6.5) at 30 °C, containing 1 mM EDTA, 1 mM GSH, and 1 mM CDNB. The compounds 6-thio-dG and its dimer were dissolved in 2.5% DMSO for the experiments. The IC₅₀ values indicated that the dimer exhibited a higher potency (IC₅₀: 0.339 μM) than the monomer (IC₅₀: 15.14 μM). Kinetic analysis revealed noncompetitive inhibition with glutathione (Ki: 12.26 μM) and mixed inhibition with CDNB (Ki: 11.41 μM) for the monomer, whereas the dimer showed mixed inhibition with glutathione (Ki: 0.972 μM) and competitive inhibition with CDNB (Ki: 0.723 μM). Molecular docking confirmed the higher binding affinity of the dimer (binding energy: - 7.9 kcal/mol, Ki: 1.595 μM) compared to the monomer (binding energy: - 6.2 kcal/mol, Ki: 28.21 μM). The dimer form of 6-thio-dG shows strong potential to enhance chemotherapeutic efficacy by effectively inhibiting GSTP1 and overcoming drug resistance. Its superior inhibitory properties make it a valuable candidate for targeted cancer therapies.

Isozyme-specific inhibition of GSTP1-1: a crucial element in cancer-targeting drugs

Selectively targeting cancer cells has been a main challenge in cancer therapy. The purpose is to spare normal cells and minimize side effects. Targeting the antioxidant enzymes (i.e. GST) for the purpose of selectively killing cancer cells has attracted much attention in the past few decades. The intention of lowering the antioxidant enzymes is "tipping" the ROS concentrations to levels above the cytotoxic threshold. This would result in extensive damage to the cellular macromolecules and organelles leading to cell death. Here we focused on the glutathione S-transferase pi 1 (GSTP1), because it is one of the overexpressed antioxidant enzymes in cancer and has been targeted for the purpose of killing cancer cells. However, most available GSTP1 inhibitors do not show selectivity towards the isozyme. This can potentially lead to many side effects. Therefore, the search for optimal selective GSTP1 inhibitors is still underway. The novelty of this review stems from highlighting the significance of selectively targeting GSTP1. We also addressed the structural feature of the enzyme which challenges the design of novel selective GSTP1 inhibitors. We then provide guidelines to help resolve these challenges to help design future compounds. The first objective of this review is to present a brief literature review to highlight the importance of selectively targeting GSTP1. Briefly, the lack of selectivity towards GSTP1 has resulted in extensive side effects which limited reaching advanced clinical trials. We screened publications on many potential inhibitors, including some that reached phase I and II clinical trials, for their ability to bind with GSTP1, GSTM, and GSTA. All compounds appear to bind different GST isozymes (at least to some extent). The second objective is to present differences in the structures of GST isotypes (GSTP1, GSTM, GSTA) which could allow selectively targeting a certain isotype. Our modelling results highlight the importance of certain structural moieties for better selective binding to GSTP1.

Glutathione S-transferase: A versatile and dynamic enzyme

The dynamic and versatile group of enzymes referred to as glutathione S-transferases (GSTs) play diverse roles in cellular detoxification, safeguarding hosts from oxidative damage, and performing various other functions. This review explores different classes of GST, existence of polymorphisms in GST, functions of GST and utilizations of GST inhibitors in treatment of human diseases. The study indicates that the cytosolic GSTs, mitochondrial GSTs, microsomal GSTs, and bacterial proteins that provide resistance to Fosfomycin are the major classes. Given a GST, variation in its expression and function among individuals is due to the presence of polymorphic alleles that encode it. Genetic polymorphism might result in the modification of GST activity, thereby increasing individuals' vulnerability to harmful chemical compounds. GSTs have been demonstrated to play a regulatory function in cellular signalling pathways through kinases, S-Glutathionylation, and in detoxification processes. Various applications of bacterial GSTs and their potential roles in plants were examined. Targeting GSTs, especially GSTP1-1, is considered a potential therapeutic strategy for treating cancer and diseases linked to abnormal cell proliferation. Their role in cancer cell growth, differentiation, and resistance to anticancer agents makes them promising targets for drug development, offering prospects for the future.

Glutathione transferase P1 is modified by palmitate

Glutathione transferase P1 (GSTP1) is a multi-functional protein that protects cells from electrophiles by catalyzing their conjugation with glutathione, and contributes to the regulation of cell proliferation, apoptosis, and signalling. GSTP1, usually described as a cytosolic enzyme, can localize to other cell compartments and we have reported its strong association with the plasma membrane. In the current study, the hypothesis that GSTP1 is palmitoylated and this modification facilitates its dynamic localization and function was investigated. Palmitoylation is the reversible post-translational addition of a 16-C saturated fatty acid to proteins, most commonly on Cys residues through a thioester bond. GSTP1 in MCF7 cells was modified by palmitate, however, GSTP1 Cys to Ser mutants (individual and Cys-less) retained palmitoylation. Treatment of palmitoylated GSTP1 with 0.1 N NaOH, which cleaves ester bonds, did not remove palmitate. Purified GSTP1 was spontaneously palmitoylated in vitro and peptide sequencing revealed that Cys48 and Cys102 undergo S-palmitoylation, while Lys103 undergoes the rare N-palmitoylation. N-palmitoylation occurs via a stable NaOH-resistant amide bond. Analysis of subcellular fractions of MCF7-GSTP1 cells and a modified proximity ligation assay revealed that palmitoylated GSTP1 was present not only in the membrane fraction but also in the cytosol. GSTP1 isolated from E. coli, and MCF7 cells (grown under fatty acid free or regular conditions), associated with plasma membrane-enriched fractions and this association was not altered by palmitoyl CoA. Overall, GSTP1 is modified by palmitate, at multiple sites, including at least one non-Cys residue. These modifications could contribute to regulating the diverse functions of GSTP1.

Glutathione-Dependent Pathways in Cancer Cells

The most abundant tripeptide-glutathione (GSH)-and the major GSH-related enzymes-glutathione peroxidases (GPxs) and glutathione S-transferases (GSTs)-are highly significant in the regulation of tumor cell viability, initiation of tumor development, its progression, and drug resistance. The high level of GSH synthesis in different cancer types depends not only on the increasing expression of the key enzymes of the γ-glutamyl cycle but also on the changes in transport velocity of its precursor amino acids. The ability of GPxs to reduce hydroperoxides is used for cellular viability, and each member of the GPx family has a different mechanism of action and site for maintaining redox balance. GSTs not only catalyze the conjugation of GSH to electrophilic substances and the reduction of organic hydroperoxides but also take part in the regulation of cellular signaling pathways. By catalyzing the S-glutathionylation of key target proteins, GSTs are involved in the regulation of major cellular processes, including metabolism (e.g., glycolysis and the PPP), signal transduction, transcription regulation, and the development of resistance to anticancer drugs. In this review, recent findings in GSH synthesis, the roles and functions of GPxs, and GST isoforms in cancer development are discussed, along with the search for GST and GPx inhibitors for cancer treatment.

Molecular and cellular evidence of a direct interaction between the TRAF2 C-terminal domain and ganglioside GM1

TNF receptor-associated factor 2 (TRAF2) is involved in different cellular processes including signal transduction and transcription regulation. We here provide evidence of a direct interaction between the TRAF domain of TRAF2 and the monosialotetrahexosylganglioside (GM1). Previously, we showed that the TRAF domain occurs mainly in a trimeric form in solution, but it can also exist as a stable monomer when in the nanomolar concentration range. Here, we report that the quaternary structure of the TRAF domain is also affected by pH changes, since a weakly acidic pH (5.5) favors the dissociation of the trimeric TRAF domain into stable monomers, as previously observed at neutral pH (7.6) with the diluted protein. The TRAF domain-GM1 binding was similar at pH 5.5 and 7.6, suggesting that GM1 interacts with both the trimeric and monomeric forms of the protein. However, only the monomeric protein appeared to cause membrane deformation and inward vesiculation in GM1-containing giant unilamellar vesicles (GUVs). The formation of complexes between GM1 and TRAF2, or its TRAF domain, was also observed in cultured human leukemic HAP1 cells expressing either the truncated TRAF domain or the endogenous full length TRAF2. The GM1-protein complexes were observed after treatment with tunicamycin and were more concentrated in cells undergoing apoptosis, a condition which is known to cause cytoplasm acidification. These findings open the avenue for future studies aimed at deciphering the physiopathological relevance of the TRAF domain-GM1 interaction.

Overexpression of Glutathione S-Transferases in Human Diseases: Drug Targets and Therapeutic Implications

Glutathione S-transferases (GSTs) are a major class of phase II metabolic enzymes. Besides their essential role in detoxification, GSTs also exert diverse biological activities in the occurrence and development of various diseases. In the past few decades, much research interest has been paid to exploring the mechanisms of GST overexpression in tumor drug resistance. Correspondingly, many GST inhibitors have been developed and applied, solely or in combination with chemotherapeutic drugs, for the treatment of multi-drug resistant tumors. Moreover, novel roles of GSTs in other diseases, such as pulmonary fibrosis and neurodegenerative diseases, have been recognized in recent years, although the exact regulatory mechanisms remain to be elucidated. This review, firstly summarizes the roles of GSTs and their overexpression in the above-mentioned diseases with emphasis on the modulation of cell signaling pathways and protein functions. Secondly, specific GST inhibitors currently in pre-clinical development and in clinical stages are inventoried. Lastly, applications of GST inhibitors in targeting cell signaling pathways and intracellular biological processes are discussed, and the potential for disease treatment is prospected. Taken together, this review is expected to provide new insights into the interconnection between GST overexpression and human diseases, which may assist future drug discovery targeting GSTs.

A GSTP1-mediated lactic acid signaling promotes tumorigenesis through the PPP oxidative branch

Lactic acidosis is a feature of solid tumors and plays fundamental role(s) rendering cancer cells to adapt to diverse metabolic stresses, but the mechanism underlying its roles in redox homeostasis remains elusive. Here we show that G6PD is phosphorylated at tyrosine 249/322 by the SRC through the formation of a GSTP1-G6PD-SRC complex. Lactic acid attenuates this formation and the phosphorylation of G6PD by non-covalently binding with GSTP1. Furthermore, lactic acid increases the activity of G6PD and facilitates the PPP (NADPH production) through its sensor GSTP1, thereby exhibiting resistance to reactive oxygen species when glucose is scarce. Abrogating a GSTP1-mediated lactic acid signaling showed attenuated tumor growth and reduced resistance to ROS in breast cancer cells. Importantly, positive correlations between immuno-enriched SRC protein and G6PD Y249/322 phosphorylation specifically manifest in ER/PR positive or HER negative types of breast cancer. Taken together, these results suggest that GSTP1 plays a key role in tumor development by functioning as a novel lactate sensor.

The Multifaceted Role of Glutathione S-Transferases in Health and Disease

In humans, the cytosolic glutathione S-transferase (GST) family of proteins is encoded by 16 genes presented in seven different classes. GSTs exhibit remarkable structural similarity with some overlapping functionalities. As a primary function, GSTs play a putative role in Phase II metabolism by protecting living cells against a wide variety of toxic molecules by conjugating them with the tripeptide glutathione. This conjugation reaction is extended to forming redox sensitive post-translational modifications on proteins: S-glutathionylation. Apart from these catalytic functions, specific GSTs are involved in the regulation of stress-induced signaling pathways that govern cell proliferation and apoptosis. Recently, studies on the effects of GST genetic polymorphisms on COVID-19 disease development revealed that the individuals with higher numbers of risk-associated genotypes showed higher risk of COVID-19 prevalence and severity. Furthermore, overexpression of GSTs in many tumors is frequently associated with drug resistance phenotypes. These functional properties make these proteins promising targets for therapeutics, and a number of GST inhibitors have progressed in clinical trials for the treatment of cancer and other diseases.

NBDHEX re-sensitizes adriamycin-resistant breast cancer by inhibiting glutathione S-transferase pi

PURPOSE

Adriamycin is a novel chemotherapeutic agent of great benefit for treating breast cancer. However, adriamycin -resistance remains a major obstacle. The vital Glutathione transferase P1 (GSTPi) inhibitor 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio) hexanol (NBDHEX) has recently shown antitumor activity in various cancers. In this study, we analyzed the effect of NBDHEX and adriamycin combination against breast cancer in vitro and in vivo.

METHODS

CCK-8 assay was performed to test cell viability. The location and expression level of GSTpi was determined by immunofluorescence and Western blot in cells and immunohistochemistry staining in tissues. The enzyme activity test was applied to detect the effect of NBDHEX on the activity of GSTpi. The apoptosis related proteins' expression was tested using Western blot. The phosphorylation sites of GSTpi were detected by mass spectrometry. Antitumor effects of single treatment or co-administration of adriamycin and NBDHEX were evaluated in nude mice.

RESULTS

NBDHEX treatment inhibited GSTpi enzyme activity and co-administration of adriamycin and NBDHEX promoted apoptosis of adriamycin-resistance breast cancer cell. Moreover, drug combination of NBDHEX and adriamycin significantly enhanced tumor growth inhibition compared with single agent.

CONCLUSION

NBDHEX serves as a good candidate for combination with adriamycin, offering new insights for breast cancer treatment.

The good Samaritan glutathione-S-transferase P1: An evolving relationship in nitric oxide metabolism mediated by the direct interactions between multiple effector molecules

Glutathione-S-transferases (GSTs) are phase II detoxification isozymes that conjugate glutathione (GSH) to xenobiotics and also suppress redox stress. It was suggested that GSTs have evolved not to enhance their GSH affinity, but to better interact with and metabolize cytotoxic nitric oxide (NO). The interactions between NO and GSTs involve their ability to bind and store NO as dinitrosyl-dithiol iron complexes (DNICs) within cells. Additionally, the association of GSTP1 with inducible nitric oxide synthase (iNOS) results in its inhibition. The function of NO in vasodilation together with studies associating GSTM1 or GSTT1 null genotypes with preeclampsia, additionally suggests an intriguing connection between NO and GSTs. Furthermore, suppression of c-Jun N-terminal kinase (JNK) activity occurs upon increased levels of GSTP1 or NO that decreases transcription of JNK target genes such as c-Jun and c-Fos, which inhibit apoptosis. This latter effect is mediated by the direct association of GSTs with MAPK proteins. GSTP1 can also inhibit nuclear factor kappa B (NF-κB) signaling through its interactions with IKKβ and Iκα, resulting in decreased iNOS expression and the stimulation of apoptosis. It can be suggested that the inhibitory activity of GSTP1 within the JNK and NF-κB pathways may be involved in crosstalk between survival and apoptosis pathways and modulating NO-mediated ROS generation. These studies highlight an innovative role of GSTs in NO metabolism through their interaction with multiple effector proteins, with GSTP1 functioning as a "good Samaritan" within each pathway to promote favorable cellular conditions and NO levels.

Head or tail? A molecular dynamics approach to the complex structure of TNF-associated factor TRAF2

Tumor necrosis factor receptor-associated factor proteins (TRAFs) are trimeric proteins that play a fundamental role in signaling, acting as intermediaries between the tumor necrosis factor (TNF) receptors and the proteins that transmit the downstream signal. The monomeric subunits of all the TRAF family members share a common tridimensional structure: a C-terminal globular domain and a long coiled-coil tail characterizing the N-terminal section. In this study, the dependence of the TRAF2 dynamics on the length of its tail was analyzed in silico. In particular, we used the available crystallographic structure of a C-terminal fragment of TRAF2 (168 out of 501 a.a.), TRAF2-C, and that of a longer construct, addressed as TRAF2-plus, that we have re-constructed using the AlphaFold2 code. The results indicate that the longer N-terminal tail of TRAF2-plus has a strong influence on the dynamics of the globular regions in the protein C-terminal head. In fact, the quaternary interactions among the TRAF2-C subunits change asymmetrically in time, while the movements of TRAF2-plus monomers are rather limited and more ordered than those of the shorter construct. Such findings shed a new light on the dynamics of TRAF subunits and on the protein mechanism in vivo, since TRAF monomer-trimer equilibrium is crucial for several reasons (receptor recognition, membrane binding, hetero-oligomerization).

Glutathione S-Transferase Alpha 4 Promotes Proliferation and Chemoresistance in Colorectal Cancer Cells

Glutathione S-transferase alpha 4 (GSTA4) is a phase II detoxifying enzyme that is overexpressed in colorectal cancer (CRC) and regulated by the oncogenic transcription factor AP-1. However, the role of GSTA4 in these CRC cells remains unclear. In this study, we investigated the roles of GSTA4 in the CRC cells by inactivating GSTA4 in HCT116 human CRC cells (Defined as HCT116^ΔGSTA4) using the CRISPR/Cas9 gene editing. Cell proliferation, clonogenicity, and susceptibility to chemotherapeutic drugs were analyzed in vitro and in a xenograft model. The results showed that loss of GSTA4 significantly decreased cell proliferation and clonogenicity, whereas it increased intracellular reactive oxygen species and cell susceptibility to 5-fluorouracil (5-FU) and oxaliplatin. Additionally, exposure of HCT116^ΔGSTA4 cells to 5-FU increased the expression of γH2AX, a hallmark of double-stranded DNA breaks. In contrast, no remarkably increased γH2AX was noted in oxaliplatin-treated HCT116^ΔGSTA4 cells compared with HCT116 cells. Moreover, loss of GSTA4 blocked the AKT and p38 MAPK pathways, leading to proliferative suppression. Finally, the xenograft model showed decreased tumor size for HCT116^ΔGSTA4 cells compared with HCT116 cells, confirming in vitro findings. These findings suggest that GSTA4 is capable of promoting proliferation, tumorigenesis, and chemoresistance and is a potential target for CRC therapy.

The Odd Faces of Oligomers: The Case of TRAF2-C, A Trimeric C-Terminal Domain of TNF Receptor-Associated Factor

TNF Receptor Associated Factor 2 (TRAF2) is a trimeric protein that belongs to the TNF receptor associated factor family (TRAFs). The TRAF2 oligomeric state is crucial for receptor binding and for its interaction with other proteins involved in the TNFR signaling. The monomer-trimer equilibrium of a C- terminal domain truncated form of TRAF2 (TRAF2-C), plays also a relevant role in binding the membrane, causing inward vesiculation. In this study, we have investigated the conformational dynamics of TRAF2-C through circular dichroism, fluorescence, and dynamic light scattering, performing temperature-dependent measurements. The data indicate that the protein retains its oligomeric state and most of its secondary structure, while displaying a significative increase in the heterogeneity of the tyrosines signal, increasing the temperature from ≈15 to ≈35 °C. The peculiar crowding of tyrosine residues (12 out of 18) at the three subunit interfaces and the strong dependence on the trimer concentration indicate that such conformational changes mainly involve the contact areas between each pair of monomers, affecting the oligomeric state. Molecular dynamic simulations in this temperature range suggest that the interfaces heterogeneity is an intrinsic property of the trimer that arises from the continuous, asymmetric approaching and distancing of its subunits. Such dynamics affect the results of molecular docking on the external protein surface using receptor peptides, indicating that the TRAF2-receptor interaction in the solution might not involve three subunits at the same time, as suggested by the static analysis obtainable from the crystal structure. These findings shed new light on the role that the TRAF2 oligomeric state might have in regulating the protein binding activity in vivo.

Glutathione S-Transferases in Cancer

In humans, the glutathione S-transferases (GST) protein family is composed of seven members that present remarkable structural similarity and some degree of overlapping functionalities. GST proteins are crucial antioxidant enzymes that regulate stress-induced signaling pathways. Interestingly, overactive GST proteins are a frequent feature of many human cancers. Recent evidence has revealed that the biology of most GST proteins is complex and multifaceted and that these proteins actively participate in tumorigenic processes such as cell survival, cell proliferation, and drug resistance. Structural and pharmacological studies have identified various GST inhibitors, and these molecules have progressed to clinical trials for the treatment of cancer and other diseases. In this review, we discuss recent findings in GST protein biology and their roles in cancer development, their contribution in chemoresistance, and the development of GST inhibitors for cancer treatment.

Protein Lipoxidation: Basic Concepts and Emerging Roles

Protein lipoxidation is a non-enzymatic post-translational modification that consists of the covalent addition of reactive lipid species to proteins. This occurs under basal conditions but increases in situations associated with oxidative stress. Protein targets for lipoxidation include metabolic and signalling enzymes, cytoskeletal proteins, and transcription factors, among others. There is strong evidence for the involvement of protein lipoxidation in disease, including atherosclerosis, neurodegeneration, and cancer. Nevertheless, the involvement of lipoxidation in cellular regulatory mechanisms is less understood. Here we review basic aspects of protein lipoxidation and discuss several features that could support its role in cell signalling, including its selectivity, reversibility, and possibilities for regulation at the levels of the generation and/or detoxification of reactive lipids. Moreover, given the great structural variety of electrophilic lipid species, protein lipoxidation can contribute to the generation of multiple structurally and functionally diverse protein species. Finally, the nature of the lipoxidised proteins and residues provides a frameshift for a complex interplay with other post-translational modifications, including redox and redox-regulated modifications, such as oxidative modifications and phosphorylation, thus strengthening the importance of detailed knowledge of this process.

In Vitro and in Vivo Efficacy of NBDHEX on Gefitinib-resistant Human Non-small Cell Lung Cancer

Gefitinib, a first-generation EGFR tyrosine kinase inhibitor (EGFR-TKI), is recommended for treatment of non-small cell lung cancer (NSCLC) patients who harbor activating EGFR mutations. However, the tumors of most patients initially sensitive to gefitinib will develop resistance within several months of therapy. Drug resistance is a major obstacle to NSCLC treatment. The novel glutathione transferase P1 (GSTPi) inhibitor 6-(7-nitro-2, 1, 3-benzoxadiazol-4-ylthio) hexanol (NBDHEX) has recently been shown to be active against tumors. In this study, we investigated the in vitro and in vivo efficacy of NBDHEX against NSCLC. Treatment with NBDHEX inhibited GSTpi enzymatic activity and promoted apoptosis of gefinitb-resistant NSCLC cells. Moreover, NBDHEX reduced tumor growth in mice. These findings indicated that NBDHEX is a good candidate for treatment of NSCLC patients, and that NBDHEX offers a new approach to cancer therapy.

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.

Effects of Glutathione Transferase-Targeting Nitrobenzoxadiazole Compounds in Relation to PD-L1 Status in Human Melanoma Cells

BACKGROUND

PD-L1 is a membrane protein with inhibitory effects on immune responses, whose expression has been correlated with high aggressiveness and the propensity of melanoma to metastasize. The nitrobenzoxadiazole (NBD) NBDHEX and its analog MC3181 are endowed with strong antitumor activity towards melanoma and a significant ability to reduce its adhesion and invasiveness. Therefore, we investigated whether PD-L1 status could affect cell sensitivity to the cytotoxic effects of NBDs. We then evaluated the effects of NBDHEX on PD-L1 expression and autophagy in melanoma cells. We used the BRAF-mutated A375 melanoma cell line and an A375 variant population enriched for PD-L1+ cells as a model. The cytotoxic effects of NBDs were evaluated in comparison to those of the BRAF inhibitor vemurafenib and the autophagy inhibitor chloroquine.

METHODS

The effect of NBDHEX on autophagy was determined by measuring LC3-II and p62 protein levels by Western blot. The cytotoxic activity of the compounds was evaluated by sulforhodamine B assay. PD-L1 expression and plasma membrane localization were analyzed by FACS and Western blot analysis.

RESULTS

NBDHEX behaves as a late-autophagy inhibitor in A375 melanoma cells, as previously found in other tumor cell lines. NBDHEX and MC3181 showed strong and comparable cytotoxic activity in both parental and PD-L1+ A375 cells, with IC50 values in the sub-micromolar range. Conversely, cells sorted for high PD-L1 expression had lower sensitivity to both the BRAF inhibitor vemurafenib and the autophagy inhibitor chloroquine. NBDHEX treatment did not change the total expression and cell surface localization of PD-L1 in both parental and PD-L1+ A375 cells.

CONCLUSIONS

Our data suggest that NBDs may represent a promising treatment strategy for melanoma with elevated PD-L1 expression.

Synthesis and characterisation of a new benzamide-containing nitrobenzoxadiazole as a GSTP1-1 inhibitor endowed with high stability to metabolic hydrolysis

The antitumor agent 6-((7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)thio)hexan-1-ol (1) is a potent inhibitor of GSTP1-1, a glutathione S-transferase capable of inhibiting apoptosis by binding to JNK1 and TRAF2. We recently demonstrated that, unlike its parent compound, the benzoyl ester of 1 (compound 3) exhibits negligible reactivity towards GSH, and has a different mode of interaction with GSTP1-1. Unfortunately, 3 is susceptible to rapid metabolic hydrolysis. In an effort to improve the metabolic stability of 3, its ester group has been replaced by an amide, leading to N-(6-((7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)thio)hexyl)benzamide (4). Unlike 3, compound 4 was stable to human liver microsomal carboxylesterases, but retained the ability to disrupt the interaction between GSTP1-1 and TRAF2 regardless of GSH levels. Moreover, 4 exhibited both a higher stability in the presence of GSH and a greater cytotoxicity towards cultured A375 melanoma cells, in comparison with 1 and its analog 2. These findings suggest that 4 deserves further preclinical testing.

4-Bromo-1,8-naphthalimide derivatives as fluorogenic substrates for live cell imaging of glutathione S-transferase (GST) activity

Fluorogenic substrates are used to visualize the activity of cancer-associated enzymes and to interpret biological events. Certain types of glutathione S-transferase (GST), such as Pi class GST (referred to as GSTP1), are more highly expressed in a wide variety of human cancer tissues compared to their corresponding normal tissues. Pi class GST is thus a cancer cell molecular marker and potential target for overcoming resistance to chemotherapy. Here, we report that 4-bromo-1,8-naphthalimide (BrNaph) is a practical fluorogenic GST substrate. We have found that HE-BrNaph, an N-hydroxyethyl derivative, shows remarkable fluorescence enhancement upon GST-catalyzed S_NAr replacement of the bromo group with a glutathionyl group. This substitution was highly selective and occurred only in the presence of GSH/GSTs; no non-enzymatic reaction was observed. We demonstrated that HE-BrNaph allows visualization of GST activity in living cells and enables to distinguish cancer cells from normal cells. Further, various N-substitutions in BrNaph retain susceptibility to enzymatic activity and isozyme selectivity, suggesting the applicability of BrNaph derivatives. Thus, BrNaph and its derivatives are GST substrates useful for fluorescence imaging and the intracellular detection of GSTP1 activity in living cells.

Glutathione S-transferase π: a potential role in antitumor therapy

Glutathione S-transferase π (GSTπ) is a Phase II metabolic enzyme that is an important facilitator of cellular detoxification. Traditional dogma asserts that GSTπ functions to catalyze glutathione (GSH)-substrate conjunction to preserve the macromolecule upon exposure to oxidative stress, thus defending cells against various toxic compounds. Over the past 20 years, abnormal GSTπ expression has been linked to the occurrence of tumor resistance to chemotherapy drugs, demonstrating that this enzyme possesses functions beyond metabolism. This revelation reveals exciting possibilities in the realm of drug discovery, as GSTπ inhibitors and its prodrugs offer a feasible strategy in designing anticancer drugs with the primary purpose of reversing tumor resistance. In connection with the authors' current research, we provide a review on the biological function of GSTπ and current developments in GSTπ-targeting drugs, as well as the prospects of future strategies.

6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio) hexanol: a promising new anticancer compound

The 7-nitro-2,1,3-nitrobenzoxadiazole (NBD) derivatives are a series of compounds containing the NBD scaffold that are not glutathione (GSH) peptidomimetics, and result in a strong inhibition of glutathione S-transferases (GSTs). Growing evidences highlight their pivotal roles and outstanding anticancer activity in different tumor models. In particular, 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio) hexanol (NBDHEX) is extensively studied, which is a very efficient inhibitor of GSTP1-1. It triggers apoptosis in several tumor cell lines and this cytotoxic activity is observed at micro and submicromolar concentrations. Importantly, studies have shown that NBDHEX acts as an anticancer drug by inhibiting GSTs catalytic activity, avoiding inconvenience of the inhibitor extrusion from the cell by specific pumps and disrupting the interaction between the GSTP1-1 and key signaling effectors. Additionally, some researchers also have discovered that NBDHEX can act as late-phase autophagy inhibitor, which opens new opportunities to fully exploit its therapeutic potential. In this review, we summarize the advantages, anticancer mechanisms, and analogs of this compound, which will establish the basis on the usage of NBDHEX in clinical applications in future.

Glutathione transferases: substrates, inihibitors and pro-drugs in cancer and neurodegenerative diseases

Glutathione transferase classical GSH conjugation activity plays a critical role in cellular detoxification against xenobiotics and noxious compounds as well as against oxidative stress. However, this feature is also exploited by cancer cells to acquire drug resistance and improve their survival. As a result, various members of the family were found overexpressed in a number of different cancers. Moreover several GST polymorphisms, ranging from null phenotypes to point mutations, were detected in members of the family and found to correlate with the onset of neuro-degenerative diseases. In the last decades, a great deal of research aimed at clarifying the role played by GSTs in drug resistance, at developing inhibitors to counteract this activity but also at exploiting GSTs for prodrugs specific activation in cancer cells. Here we summarize some of the most important achievements reached in this lively area of research.

New insight into the interaction of TRAF2 C-terminal domain with lipid raft microdomains

In this study we provide the first evidence of the interaction of a truncated-TRAF2 with lipid raft microdomains. We have analyzed this interaction by measuring the diffusion coefficient of the protein in large and giant unilamellar vesicles (LUVs and GUVs, respectively) obtained both from synthetic lipid mixtures and from natural extracts. Steady-state fluorescence measurements performed with synthetic vesicles indicate that this truncated form of TRAF2 displays a tighter binding to raft-like LUVs with respect to the control (POPC-containing LUVs), and that this process depends on the protein oligomeric state. Generalized Polarization measurements and spectral phasor analysis revealed that truncated-TRAF2 affects the membrane fluidity, especially when vesicles are heated up at physiological temperature. The addition of nanomolar concentration of TRAF2 in GUVs also seems to exert a mechanical action, as demonstrated by the formation of intraluminal vesicles, a process in which ganglioside GM1 plays a crucial role.

A new nitrobenzoxadiazole-based GSTP1-1 inhibitor with a previously unheard of mechanism of action and high stability

CONTEXT

The nitrobezoxadiazole derivative NBDHEX is a potent inhibitor of glutathione transferase P1-1 (GSTP1-1) endowed with outstanding anticancer activity in different tumor models.

OBJECTIVE

To characterize by in vitro biochemical and in silico studies the NBDHEX analogues named MC2752 and MC2753.

MATERIALS AND METHODS

Synthesis of MC2752 and MC2753, biochemical assays and in silico docking and normal-mode analyses.

RESULTS

The presence of a hydrophobic moiety in the side chain of MC2753 confers unique features to this molecule. Unlike its parent drug NBDHEX, MC2753 does not require GSH to trigger the dissociation of the complex between GSTP1-1 and TRAF2, and displays high stability towards the nucleophilic attack of the tripeptide under physiological conditions.

DISCUSSION AND CONCLUSION

MC2753 may represent a lead compound for the development of novel GSTP1-1 inhibitors not affected in their anticancer action by fluctuations of cellular GSH levels, and characterized by an increased half-life in vivo.

A novel orally active water-soluble inhibitor of human glutathione transferase exerts a potent and selective antitumor activity against human melanoma xenografts

We designed and synthesized two novel nitrobenzoxadiazole (NBD) analogues of the anticancer agent 6-((7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)thio)hexan-1-ol (NBDHEX). The new compounds, namely MC3165 and MC3181, bear one and two oxygen atoms within the hydroxy-containing alkyl chain at the C4 position of the NBD scaffold, respectively. This insertion did not alter the chemical reactivity with reduced glutathione, while it conferred a remarkable increase in water solubility. MC3181 was more selective than NBDHEX towards the target protein, glutathione transferase P1-1, and highly effective in vitro against a panel of human melanoma cell lines, with IC50 in the submicromolar-low micromolar range. Interestingly, the cellular response to MC3181 was cell-type-specific; the compound triggered a JNK-dependent apoptosis in the BRAF-V600E-mutated A375 cells, while it induced morphological changes together with an increase in melanogenesis in BRAF wild-type SK23-MEL cells. MC3181 exhibited a remarkable therapeutic activity against BRAF-V600E-mutant xenografts, both after intravenous and oral administration. Outstandingly, no treatment-related signs of toxicity were observed both in healthy and tumor-bearing mice after single and repeated administrations. Taken together, these results indicate that MC3181 may represent a potential novel therapeutic opportunity for BRAF-mutated human melanoma, while being safe and water-soluble and thus overcoming all the critical aspects of NBDHEX in vivo.

c-Jun N-terminal kinase activation by nitrobenzoxadiazoles leads to late-stage autophagy inhibition

BACKGROUND

Nitrobenzoxadiazole derivatives (NBDs), including NBDHEX and the recently developed MC3181, are promising anticancer agents able to target glutathione transferase and inhibit both its catalytic activity and ability to sequester TNF-receptor associated factor 2 (TRAF2) and c-Jun N-terminal kinase (JNK). NBDs have been shown to impair the growth and survival of a broad-spectrum of tumor types, in vitro and in vivo. Herein, we evaluated the effects of the new compound MC3181 on U-2OS osteosarcoma cells and investigated the impact of both NBDHEX and MC3181 on autophagy.

METHODS

Cell viability was evaluated by sulforhodamine B assay. The dissociation of the TRAF2-GSTP1-1 complex was detected by proximity ligation assay, while the phospho-activation of JNK was assessed by western blotting. The effects of NBDs on autophagy were evaluated by GFP-LC3 puncta formation, western blotting for LC3-II and p62, and LC3 turnover assay in the presence of bafilomycin A1. The role of JNK in the reduction of autophagic flux caused by NBDs was investigated using JNK1 shRNA-transfected cells. Fluorogenic caspase activity assay and flow cytometric analysis of DNA content were used to determine the cytotoxic effects of NBDs on JNK1-silenced cells.

RESULTS

Similar to NBDHEX, MC3181 reduced viability and activated TRAF2/JNK signaling in U-2OS cells. Moreover, NBDs induced the accumulation of autophagic vesicles and LC3-II while reducing both basal and nutritional stress-induced autophagic flux. Furthermore, increased levels of both LC3-II and the autophagy selective substrate p62 were observed in different tumor cell lines treated with NBDs, the concurrent increase of these markers being consistent with an impairment of autophagosome clearance. Autophagy inhibition by NBDs required JNK activity: NBDs caused autophagy inhibition and caspase-3 activation in JNK-positive U-2OS, but no autophagic flux inhibition or caspase-3 activation in JNK-silenced cells.

CONCLUSIONS

Our demonstration that NBDs can act as late-phase autophagy inhibitors opens new opportunities to fully exploit their therapeutic potential. This may not rely solely on their effectiveness in inducing cell cycle arrest and apoptosis, but also on their ability to weaken the capacity of tumor cells to endure stress conditions via autophagy. In addition, this study provides evidence that JNK can participate in impairing autophagy.

TNFR-Associated Factor-2 (TRAF2): Not Only a Trimer

TNF receptor-associated factors (TRAFs) are characterized by an oligomeric structure that plays a fundamental role in the binding process with membrane receptors. In this work, we studied the trimer-to-monomer (T ↔ 3M) equilibrium transition of the TRAF2 C-terminal domain using both chemical (dilution/guanidinium hydrochloride) and mechanical stress (high pressure) to induce the dissociation of the native protein into subunits. The experimental results and computer simulations indicate that stable monomers exist and that their population accounts for 15% of the total TRAF2 molecules already at a physiological intracellular concentration (≈1 μM), being instead the predominant species in the nanomolar concentration range. Because the total amount of TRAF2 changes during a cell cycle, the monomer-trimer equilibrium can be crucial for regulating the activities of TRAF2 in vivo.

Nitrobenzoxadiazole-based GSTP1-1 inhibitors containing the full peptidyl moiety of (pseudo)glutathione

CONTEXT

The inhibition of glutathione S-transferase P1-1 (GSTP1-1) is a sound strategy to overcome drug resistance in oncology practice.

OBJECTIVE

The nitrobenzoxadiazolyl (NBD) S-conjugate of glutathione and the corresponding γ-oxa-glutamyl isostere (compounds 1 and 5, respectively) have been disclosed as GST inhibitors. The rationale of their design is discussed in juxtaposition to non-peptide NBD thioethers.

MATERIALS AND METHODS

Synthesis of derivatives 1 and 5 and in vitro evaluation on human GSTP1-1 and M2-2 are reported.

RESULTS

Conjugates 1 and 5 were found to be low micromolar inhibitors of both isoforms. Furthermore, they display a threefold reduction in selectivity for GSTM2-2 over the P1-1 isozyme in comparison with the potent non-peptide inhibitor nitrobenzoxadiazolyl-thiohexanol (NBDHEX).

DISCUSSION AND CONCLUSIONS

Spectroscopic data are congruent with the formation of a stable sigma-complex between GSH and the inhibitors in the protein active site. Conjugate 5 is suitable for in vivo modulation of GST activity in cancer treatment.

A new water soluble MAPK activator exerts antitumor activity in melanoma cells resistant to the BRAF inhibitor vemurafenib

Recovery of mitogen activated protein kinase (MAPK) or activation of alternative pathways, such as the PI3K/AKT/mTOR, are involved in acquired resistance to BRAF inhibitors which represent the first-line treatment of BRAF-mutated metastatic melanoma. We recently demonstrated that 6-((7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)thio)hexan-1-ol (NBDHEX) and its water soluble analog 2-(2-(2-((7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)thio)ethoxy)ethoxy)ethanol (MC3181) trigger apoptosis in BRAF V600E mutated melanoma cells through activation of the MAPK c-Jun N-terminal kinase (JNK). Herein, we investigated whether NBDHEX and MC3181 might exert antitumor activity against BRAF V600E mutated human melanoma cells rendered resistant to the BRAF inhibitor vemurafenib. To this aim we generated a subline of A375 melanoma resistant in vitro and in vivo to vemurafenib (A375-VR8) and characterized by NRAS G13R mutation, high basal levels of CRAF protein and phospho-activation of AKT. In these cells ERK phosphorylation was not significantly down-modulated by vemurafenib concentrations capable of abrogating ERK phosphorylation in sensitive A375 cells. Both NBDHEX and MC3181 induced marked antiproliferative and apoptotic effects in A375-VR8 cells and, at equitoxic concentrations, caused a strong phosphorylation of JNK, p38, and of the downstream mediators of apoptosis ATF2 and p53. Drug treatment further increased ERK phosphorylation, which was required for the cellular response to the NBD derivatives, as apoptosis was antagonized by the ERK inhibitor FR180204. Finally, in vivo administration of MC3181 provoked JNK activation at the tumor site and markedly reduced A375-VR8 growth. These evidences strongly suggest that the activation of multiple pro-apoptotic MAPK pathways by MC3181 might represent a new strategy for the treatment of melanoma resistant to BRAF inhibitors.

Inactivation of human salivary glutathione transferase P1-1 by hypothiocyanite: a post-translational control system in search of a role

Glutathione transferases (GSTs) are a superfamily of detoxifying enzymes over-expressed in tumor tissues and tentatively proposed as biomarkers for localizing and monitoring injury of specific tissues. Only scarce and contradictory reports exist about the presence and the level of these enzymes in human saliva. This study shows that GSTP1-1 is the most abundant salivary GST isoenzyme, mainly coming from salivary glands. Surprisingly, its activity is completely obscured by the presence of a strong oxidizing agent in saliva that causes a fast and complete, but reversible, inactivation. Although salivary α-defensins are also able to inhibit the enzyme causing a peculiar half-site inactivation, a number of approaches (mass spectrometry, site directed mutagenesis, chromatographic and spectrophotometric data) indicated that hypothiocyanite is the main salivary inhibitor of GSTP1-1. Cys47 and Cys101, the most reactive sulfhydryls of GSTP1-1, are mainly involved in a redox interaction which leads to the formation of an intra-chain disulfide bridge. A reactivation procedure has been optimized and used to quantify GSTP1-1 in saliva of 30 healthy subjects with results of 42±4 mU/mg-protein. The present study represents a first indication that salivary GSTP1-1 may have a different and hitherto unknown function. In addition it fulfills the basis for future investigations finalized to check the salivary GSTP1-1 as a diagnostic biomarker for diseases.

Synthesis and structure--activity relationship of new cytotoxic agents targeting human glutathione-S-transferases

The 6-((7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)thio)hexan-1-ol (NBDHEX, 1), a "suicide inhibitor" of the glutathione-S-transferase GSTP1-1, showed pro-apoptotic properties in tumor cells, but in vivo studies were limited by poor bioavailability and high affinity towards GSTM2-2, expressed in many non-cancerous tissues. Here we describe the synthesis and biological characterization of new 1 analogs (2-40), in which the hydroxyhexyl portion at the C4-sulfur atom has been replaced with phenyl-containing moieties as well as substituted alkyl chains. Some of the new compounds displayed 10-100 times increased water-solubility (8, 11, 17, 26-28, 34, 35), and most of them showed higher GSTP1-1 selectivity (2-20, 23-26, 31-33, 35) than 1. The presence of a phenyl ring with polar substituents is in general associated, with some exceptions (23, 24) to low cytotoxicity in osteosarcoma U-2OS cells. Differently, some alkyl derivatives possess cytotoxicity comparable (26, 34, 35) or higher (30, 32) than 1. Among the novel compounds, selected ones (26, 27, 34, and 35) deserve further investigation for their anticancer potential.