Glutathione transport is a unique function of the ATP-binding cassette protein ABCG2

Cadmium transport by mammalian ATP-binding cassette transporters

Cellular responses to toxic metals depend on metal accessibility to intracellular targets, reaching interaction sites, and the intracellular metal concentration, which is mainly determined by uptake pathways, binding/sequestration and efflux pathways. ATP-binding cassette (ABC) transporters are ubiquitous in the human body-usually in epithelia-and are responsible for the transfer of indispensable physiological substrates (e.g. lipids and heme), protection against potentially toxic substances, maintenance of fluid composition, and excretion of metabolic waste products. Derailed regulation and gene variants of ABC transporters culminate in a wide array of pathophysiological disease states, such as oncogenic multidrug resistance or cystic fibrosis. Cadmium (Cd) has no known physiological role in mammalians and poses a health risk due to its release into the environment as a result of industrial activities, and eventually passes into the food chain. Epithelial cells, especially within the liver, lungs, gastrointestinal tract and kidneys, are particularly susceptible to the multifaceted effects of Cd because of the plethora of uptake pathways available. Pertinent to their broad substrate spectra, ABC transporters represent a major cellular efflux pathway for Cd and Cd complexes. In this review, we summarize current knowledge concerning transport of Cd and its complexes (mainly Cd bound to glutathione) by the ABC transporters ABCB1 (P-glycoprotein, MDR1), ABCB6, ABCC1 (multidrug resistance related protein 1, MRP1), ABCC7 (cystic fibrosis transmembrane regulator, CFTR), and ABCG2 (breast cancer related protein, BCRP). Potential detoxification strategies underlying ABC transporter-mediated efflux of Cd and Cd complexes are discussed.

Enhancing of cerebral Abeta clearance by modulation of ABC transporter expression: a review of experimental approaches

Clearance of amyloid-beta (Aβ) from the brain is impaired in both early-onset and late-onset Alzheimer's disease (AD). Mechanisms for clearing cerebral Aβ include proteolytic degradation, antibody-mediated clearance, blood brain barrier and blood cerebrospinal fluid barrier efflux, glymphatic drainage, and perivascular drainage. ATP-binding cassette (ABC) transporters are membrane efflux pumps driven by ATP hydrolysis. Their functions include maintenance of brain homeostasis by removing toxic peptides and compounds, and transport of bioactive molecules including cholesterol. Some ABC transporters contribute to lowering of cerebral Aβ. Mechanisms suggested for ABC transporter-mediated lowering of brain Aβ, in addition to exporting of Aβ across the blood brain and blood cerebrospinal fluid barriers, include apolipoprotein E lipidation, microglial activation, decreased amyloidogenic processing of amyloid precursor protein, and restricting the entrance of Aβ into the brain. The ABC transporter superfamily in humans includes 49 proteins, eight of which have been suggested to reduce cerebral Aβ levels. This review discusses experimental approaches for increasing the expression of these ABC transporters, clinical applications of these approaches, changes in the expression and/or activity of these transporters in AD and transgenic mouse models of AD, and findings in the few clinical trials which have examined the effects of these approaches in patients with AD or mild cognitive impairment. The possibility that therapeutic upregulation of ABC transporters which promote clearance of cerebral Aβ may slow the clinical progression of AD merits further consideration.

Natural Compounds and Glutathione: Beyond Mere Antioxidants

The tripeptide glutathione plays important roles in many cell processes, including differentiation, proliferation, and apoptosis; in fact, disorders in glutathione homeostasis are involved both in the etiology and in the progression of several human diseases, including cancer. Natural compounds have been found to modulate glutathione levels and function beyond their role as mere antioxidants. For example, certain compounds can upregulate the expression of glutathione-related enzymes, increase the availability of cysteine, the limiting amino acid for glutathione synthesis, or directly interact with glutathione and modulate its function. These compounds may have therapeutic potential in a variety of disease states where glutathione dysregulation is a contributing factor. On the other hand, flavonoids' potential to deplete glutathione levels could be significant for cancer treatment. Overall, while natural compounds may have potential therapeutic and/or preventive properties and may be able to increase glutathione levels, more research is needed to fully understand their mechanisms of action and their potential benefits for the prevention and treatment of several diseases. In this review, particular emphasis will be placed on phytochemical compounds belonging to the class of polyphenols, terpenoids, and glucosinolates that have an impact on glutathione-related processes, both in physiological and pathological conditions. These classes of secondary metabolites represent the most food-derived bioactive compounds that have been intensively explored and studied in the last few decades.

Understanding and targeting resistance mechanisms in cancer

Resistance to cancer therapies has been a commonly observed phenomenon in clinical practice, which is one of the major causes of treatment failure and poor patient survival. The reduced responsiveness of cancer cells is a multifaceted phenomenon that can arise from genetic, epigenetic, and microenvironmental factors. Various mechanisms have been discovered and extensively studied, including drug inactivation, reduced intracellular drug accumulation by reduced uptake or increased efflux, drug target alteration, activation of compensatory pathways for cell survival, regulation of DNA repair and cell death, tumor plasticity, and the regulation from tumor microenvironments (TMEs). To overcome cancer resistance, a variety of strategies have been proposed, which are designed to enhance the effectiveness of cancer treatment or reduce drug resistance. These include identifying biomarkers that can predict drug response and resistance, identifying new targets, developing new targeted drugs, combination therapies targeting multiple signaling pathways, and modulating the TME. The present article focuses on the different mechanisms of drug resistance in cancer and the corresponding tackling approaches with recent updates. Perspectives on polytherapy targeting multiple resistance mechanisms, novel nanoparticle delivery systems, and advanced drug design tools for overcoming resistance are also reviewed.

Cellular Compartmentalization, Glutathione Transport and Its Relevance in Some Pathologies

Reduced glutathione (GSH) is the most abundant non-protein endogenous thiol. It is a ubiquitous molecule produced in most organs, but its synthesis is predominantly in the liver, the tissue in charge of storing and distributing it. GSH is involved in the detoxification of free radicals, peroxides and xenobiotics (drugs, pollutants, carcinogens, etc.), protects biological membranes from lipid peroxidation, and is an important regulator of cell homeostasis, since it participates in signaling redox, regulation of the synthesis and degradation of proteins (S-glutathionylation), signal transduction, various apoptotic processes, gene expression, cell proliferation, DNA and RNA synthesis, etc. GSH transport is a vital step in cellular homeostasis supported by the liver through providing extrahepatic organs (such as the kidney, lung, intestine, and brain, among others) with the said antioxidant. The wide range of functions within the cell in which glutathione is involved shows that glutathione’s role in cellular homeostasis goes beyond being a simple antioxidant agent; therefore, the importance of this tripeptide needs to be reassessed from a broader metabolic perspective.

Transport Mechanisms at the Blood–Brain Barrier and in Cellular Compartments of the Neurovascular Unit: Focus on CNS Delivery of Small Molecule Drugs

Ischemic stroke is a primary origin of morbidity and mortality in the United States and around the world. Indeed, several research projects have attempted to discover new drugs or repurpose existing therapeutics to advance stroke pharmacotherapy. Many of these preclinical stroke studies have reported positive results for neuroprotective agents; however, only one compound (3K3A-activated protein C (3K3A-APC)) has advanced to Phase III clinical trial evaluation. One reason for these many failures is the lack of consideration of transport mechanisms at the blood–brain barrier (BBB) and neurovascular unit (NVU). These endogenous transport processes function as a “gateway” that is a primary determinant of efficacious brain concentrations for centrally acting drugs. Despite the knowledge that some neuroprotective agents (i.e., statins and memantine) are substrates for these endogenous BBB transporters, preclinical stroke studies have largely ignored the role of transporters in CNS drug disposition. Here, we review the current knowledge on specific BBB transporters that either limit drug uptake into the brain (i.e., ATP-binding cassette (ABC) transporters) or can be targeted for optimized drug delivery (i.e., solute carrier (SLC) transporters). Additionally, we highlight the current knowledge on transporter expression in astrocytes, microglia, pericytes, and neurons with an emphasis on transport mechanisms in these cell types that can influence drug distribution within the brain.

2-(2-Cholesteroxyethoxyl)ethyl 3'-S-glutathionylpropionate and its self-assembled micelles for brain delivery: Design, synthesis and evaluation

The blood-brain barrier (BBB) is a barrier that prevents almost all large and most small exogenous molecules from reaching the brain. The barrier is the major cause of treatment failure for most brain diseases. Extensive efforts have been made to facilitate drug molecules to cross the BBB. One of the approaches is to employ an endogenous ligand or ligand analogue that can enter the brain through its transporter or receptor at the BBB as a brain-targeting agent. Glutathione (GSH) transporters are richly expressed at the BBB with limited presence in other tissues except kidneys. 2-(2-Cholesteroxyethoxyl)ethyl 3'-S-glutathionylpropionate (COXP), formed by connecting GSH with cholesterol through a linker, was designed as a GSH transporter-mediated brain targeting molecule. The amphiphilic nature of COXP enables the molecule to self-assemble to form micelles with a CMC value of 3.9 μM. By using DiR as a fluorescence tracking agent and the whole-body fluorescence imaging technique, the brain distribution of DiR delivered by COXP micelles in mice was 20 folds higher when compared with free DiR. Interestingly, the brain targeting effect was further enhanced by co-administration of GSH. The low CMC value and effective brain targeting make COXP micelles a promising drug delivery system to the brain.

Triphenylphosphonium derivatives disrupt metabolism and inhibit melanoma growth in vivo when delivered via a thermosensitive hydrogel

Despite dramatic improvements in outcomes arising from the introduction of targeted therapies and immunotherapies, metastatic melanoma is a highly resistant form of cancer with 5 year survival rates of <35%. Drug resistance is frequently reported to be associated with changes in oxidative metabolism that lead to malignancy that is non-responsive to current treatments. The current report demonstrates that triphenylphosphonium(TPP)-based lipophilic cations can be utilized to induce cytotoxicity in pre-clinical models of malignant melanoma by disrupting mitochondrial metabolism. In vitro experiments demonstrated that TPP-derivatives modified with aliphatic side chains accumulated in melanoma cell mitochondria; disrupted mitochondrial metabolism; led to increases in steady-state levels of reactive oxygen species; decreased total glutathione; increased the fraction of glutathione disulfide; and caused cell killing by a thiol-dependent process that could be rescued by N-acetylcysteine. Furthermore, TPP-derivative-induced melanoma toxicity was enhanced by glutathione depletion (using buthionine sulfoximine) as well as inhibition of thioredoxin reductase (using auranofin). In addition, there was a structure-activity relationship between the aliphatic side-chain length of TPP-derivatives (5–16 carbons), where longer carbon chains increased melanoma cell metabolic disruption and cell killing. In vivo bio-distribution experiments showed that intratumoral administration of a C¹⁴-TPP-derivative (12-carbon aliphatic chain), using a slow-release thermosensitive hydrogel as a delivery vehicle, localized the drug at the melanoma tumor site. There, it was observed to persist and decrease the growth rate of melanoma tumors. These results demonstrate that TPP-derivatives selectively induce thiol-dependent metabolic oxidative stress and cell killing in malignant melanoma and support the hypothesis that a hydrogel-based TPP-derivative delivery system could represent a therapeutic drug-delivery strategy for melanoma.

Cryo-electron microscopy structure of human ABCB6 transporter

Human ATP-binding cassette transporter 6 of subfamily B (ABCB6) is an ABC transporter involved in the translocation toxic metals and anti-cancer drugs. Using cryo-electron microscopy, we determined the molecular structure of full-length ABCB6 in an apo state. The structure of ABCB6 unravels the architecture of a full-length ABCB transporter that harbors two N-terminal transmembrane domains which is indispensable for its ATPase activity in our in vitro assay. A slit-like substrate binding pocket of ABCB6 may accommodate the planar shape of porphyrins, and the existence of a secondary cavity near the mitochondrial intermembrane space side would further facilitate substrate release. Furthermore, the ATPase activity of ABCB6 stimulated with a variety of porphyrin substrates showed different profiles in the presence of glutathione (GSH), suggesting the action of a distinct substrate translocation mechanism depending on the use of GSH as a cofactor.

Role of nitric oxide in the response to photooxidative stress in prostate cancer cells

A continuous state of oxidative stress during inflammation contributes to the development of 25% of human cancers. Epithelial and inflammatory cells release reactive oxygen species (ROS) and reactive nitrogen species (RNS) that can damage DNA. ROS/RNS have biological implications in both chemoresistance and tumor recurrence. As several clinically employed anticancer drugs can generate ROS/RNS, we have addressed herein how inducible nitric oxide synthase and nitric oxide (iNOS/^•NO) affect the molecular pathways implicated in the tumor response to oxidative stress. To mimic the oxidative stress associated with chemotherapy, we used a photosensitizer (pheophorbide a) that can generate ROS/RNS in a controlled manner. We investigated how iNOS/^•NO modulates the tumor response to oxidative stress by involving the NF-κB and Nrf2 molecular pathways. We found that low levels of iNOS induce the development of a more aggressive tumor population, leading to survival, recurrence and resistance. By contrast, high levels of iNOS/^•NO sensitize tumor cells to oxidative treatment, causing cell growth arrest. Our analysis showed that NF-κB and Nrf2, which are activated in response to oxidative stress, communicate with each other through RKIP. For this critical role, RKIP could be an interesting target for anticancer drugs. Our study provides insight into the complex signaling response of cancer cells to oxidative treatments as well as new possibilities for the rational design of new therapeutic strategies.

Phytochemical Profile of Capsicum annuum L. cv Senise, Incorporation into Liposomes, and Evaluation of Cellular Antioxidant Activity

Overproduction of oxidants in the human body is responsible for oxidative stress, which is associated with several diseases. High intake of vegetables and fruits can reduce the risk of chronic diseases, as they are sources of bioactive compounds capable of contrasting the free radical effects involved in cancer, obesity, diabetes, and neurodegenerative and cardiovascular diseases. Capsicum annuum L. cv Senise is a sweet pepper that is grown in the Basilicata region (Italy). It is an important source of polyphenols, carotenoids, and capsinoids and can play a key role in human health. In this study, an ethanol extract was obtained from C. annuum dried peppers and the analysis of the phytochemical composition was performed by LC-ESI/LTQ Orbitrap/MS. The extract was incorporated into liposomes, which showed small size (~80 nm), good homogeneity, negative surface charge, and good stability in storage. The biological activity of the extract was evaluated in the human hepatoma (HepG2) cell line, used as model cells. The extract showed no cytotoxic activity and reduced the intracellular reactive oxygen species (ROS) level in stressed cells. The antioxidant activity was further improved when the extract was loaded into liposomes. Moreover, the extract promoted the expression of endogenous antioxidants, such as catalase, superoxide dismutase, and glutathione peroxidase through the Nrf-2 pathway evaluated by RT-PCR.

LRRC8/VRAC channels exhibit a noncanonical permeability to glutathione, which modulates epithelial-to-mesenchymal transition (EMT)

Volume-regulated anion channels (VRAC) are chloride channels activated in response to osmotic stress to regulate cellular volume and also participate in other cellular processes, including cell division and cell death. Recently, members of the LRRC8 family have been identified as the main contributors of VRAC conductance. LRRC8/VRAC is permeable to chloride ions but also exhibits significant permeability to various substrates that vary strongly in charge and size. In this study, we explored the intriguing ability of LRRC8/VRAC to transport glutathione (GSH), the major cellular reactive oxygen species (ROS) scavenger, and its involvement in epithelial-to-mesenchymal transition (EMT), a cellular process in which cellular oxidative status is a crucial step. First, in HEK293-WT cells, we showed that a hypotonic condition induced LRRC8/VRAC-dependent GSH conductance (P_GSH/P_Cl of ~0.1) and a marked decrease in intracellular GSH content. GSH currents and GSH intracellular decrease were both inhibited by DCPIB, an inhibitor of LRRC8/VRAC, and were not observed in HEK293-LRRC8A KO cells. Then, we induced EMT by exposing renal proximal tubule epithelial cells to the pleiotropic growth factor TGFβ1, and we measured the contribution of LRRC8/VRAC in this process by measuring (i) EMT marker expression (assessed both at the gene and protein levels), (ii) cell morphology and (iii) the increase in migration ability. Interestingly, pharmacologic targeting of LRRC8/VRAC (DCPIB) or RNA interference-mediated inhibition (LRRC8A siRNA) attenuated the TGFβ1-induced EMT response by controlling GSH and ROS levels. Interestingly, TGFβ1 exposure triggered DCPIB-sensitive chloride conductance. These results suggest that LRRC8/VRAC, due to its native permeability to GSH and thus its ability to modulate ROS levels, plays a critical role in EMT and might contribute to other physiological and pathophysiological processes associated with oxidative stress.

Highlights in Resistance Mechanism Pathways for Combination Therapy

Combination chemotherapy has been a mainstay in cancer treatment for the last 60 years. Although the mechanisms of action and signaling pathways affected by most treatments with single antineoplastic agents might be relatively well understood, most combinations remain poorly understood. This review presents the most common alterations of signaling pathways in response to cytotoxic and targeted anticancer drug treatments, with a discussion of how the knowledge of signaling pathways might support and orient the development of innovative strategies for anticancer combination therapy. The ultimate goal is to highlight possible strategies of chemotherapy combinations based on the signaling pathways associated with the resistance mechanisms against anticancer drugs to maximize the selective induction of cancer cell death. We consider this review an extensive compilation of updated known information on chemotherapy resistance mechanisms to promote new combination therapies to be to discussed and tested.

Prognostic implication of ABC transporters and cancer stem cell markers in patients with stage III colon cancer receiving adjuvant FOLFOX-4 chemotherapy

Cancer stem cell (CSC) and ATP-binding cassette (ABC) transporters are associated with treatment resistance and outcomes of patients with cancer. The present study investigated the prognostic implications of pre-therapeutic expression of ABC transporters and CSC markers in patients with colon cancer (CC) who received adjuvant 5-fluorouracil, leucovorin and oxaliplatin combination therapy (FOLFOX-4). The immunohistochemical expression of 3 ABC transporters, including ABC subfamily C member 2 (ABCC2), ABCC3 and ABC subfamily G member 2 (ABCG2), and 3 CSC markers, including sex determining region Y-box 2 (SOX2), leucine-rich repeat-containing G protein-coupled receptor 5 and aldehyde dehydrogenase 1, were determined in 164 CC tissues from patients with stage III CC, who underwent postoperative FOLFOX-4 chemotherapy. The association between the protein expression and patients' prognoses was statistically analyzed. ABCG2 was associated with favorable overall survival rate (OS; P=0.001), and ABCC2, ABCG2 and SOX2 were associated with increased disease-free survival rate (DFS; P=0.001, 0.002 and 0.013, respectively). In multivariate analyses, ABCG2 was an independent prognostic factor for OS [hazard ratio (HR)=2.877; P=0.046], and ABCC2 and SOX2 were independent prognostic factors for DFS (HR=2.831; P=0.014; HR=2.558, P=0.020, respectively). ABCC2, ABCG2 and SOX2 may be promising prognostic markers for patients with CC receiving FOLFOX-4 therapy.

Thiocoraline mediates drug resistance in MCF-7 cells via PI3K/Akt/BCRP signaling pathway

Thiocoraline, a depsipeptide bisintercalator with potent antitumor activity, was first isolated from marine actinomycete Micromonospora marina. It possesses an intense toxicity to MCF-7 cells at nanomolar concentrations in a dose-dependent manner evaluated by MTT assay and crystal violet staining. We established a human breast thiocoraline-resistant cancer subline of MCF-7/thiocoraline (MCF-7/T) to investigate the expression variation of breast cancer resistance proteins (BCRP) and its subsequent influence on drug resistance. Colony-forming assay showed that the MCF-7 cells proliferated faster than the MCF-7/T cells in vitro. Western blot analysis demonstrated that thiocoraline increased the phosphorylation of Akt. Additionally, the sensitivity of tumor cells to thiocoraline was reduced with a concurrent rise in phosphorylation level of Akt and of BCRP expression.These studies indicated that thiocoraline probably mediated the drug resistance via PI3K/Akt/BCRP signaling pathway. MK-2206 dihydrochloride, a selective phosphorylation inhibitor of Akt, significantly decreased MCF-7 cell viability under exposure to thiocoraline compared to the control. However, it was not obviously able to decrease MCF-7/T cell viability when cells were exposed to thiocoraline.

Drosophila strain specific response to cisplatin neurotoxicity

Drosophila melanogaster has recently been developed as a simple, in vivo, genetic model of chemotherapy-induced peripheral neuropathy. Flies treated with the chemotherapy agent cisplatin display both a neurodegenerative phenotype and cell death in rapidly dividing follicles, mimicking the cell specific responses seen in humans. Cisplatin induces climbing deficiencies and loss of fertility in a dose dependent manner. Drosophila sensitivity to cisplatin in both cell types is affected by genetic background. We show that mutation or RNAi-based knockdown of genes known to be associated with CIPN incidence in humans affect sensitivity of flies to CIPN. Drosophila is a promising model with which to study the effect of genetics on sensitivity to CIPN.

NOS1 upregulates ABCG2 expression contributing to DDP chemoresistance in ovarian cancer cells

Nitric oxide synthase 1 (NOS1) has been reported to promote various cancer processes including chemoresistance. However, the role of NOS1 in chemoresistance has remained unclear. ATP-binding cassette, subfamily G, member 2 (ABCG2) has been identified as a molecular cause of multidrug resistance in a number of cancer types, including ovarian cancer. The present study observed that in ovarian cancer cells, the expression of ABCG2 was significantly upregulated in response to cis-diamminedichloroplatinum (cisplatin/DDP) treatment, in addition the expression of NOS1 exhibited an increasing trend. Additionally, the levels of NOS1 and ABCG2 in chemoresistant ovarian cancer profiles in Gene Expression Omnibus datasets (GSE26712 and GSE51373) were higher than in chemosensitive profiles. Furthermore, overexpression of NOS1 could upregulate ABCG2 expression, and expression of ABCG2 was inhibited by NOS1 selective inhibitor (N-PLA). In assays of cell survival, NOS1 appeared to increase the potential for DDP resistance, and this effect was reversed by addition of ABCG2 inhibitor (verapamil). The present study indicated that NOS1-induced chemoresistance was partly mediated by the upregulation of ABCG2 expression. This result suggests a link between the expression of NOS1 and the ABCG2-associated chemoresistance in ovarian cancer.

Glutathione: subcellular distribution and membrane transport 1

Glutathione (γ-l-glutamyl-l-cysteinylglycine) is a small tripeptide found at millimolar concentrations in nearly all eukaryotes as well as many prokaryotic cells. Glutathione synthesis is restricted to the cytosol in animals and fungi and to the cytosol and plastids in plants. Nonetheless, glutathione is found in virtually all subcellular compartments. This implies that transporters must exist that facilitate glutathione transport into and out of the various subcellular compartments. Glutathione may also be exported and imported across the plasma membrane in many cells. However, in most cases, the molecular identity of these transporters remains unclear. Whilst glutathione transport is essential for the supply and replenishment of subcellular glutathione pools, recent evidence supports a more active role for glutathione transport in the regulation of subcellular glutathione redox homeostasis. However, our knowledge of glutathione redox homeostasis at the level of specific subcellular compartments remains remarkably limited and the role of glutathione transport remains largely unclear. In this review, we discuss how new tools and techniques have begun to yield insights into subcellular glutathione distribution and glutathione redox homeostasis. In particular, we discuss the known and putative glutathione transporters and examine their contribution to the regulation of subcellular glutathione redox homeostasis.

Repression of adenosine triphosphate-binding cassette transporter ABCG2 by estrogen increases intracellular glutathione in brain endothelial cells following ischemic reperfusion injury

The adenosine triphosphate-binding cassette efflux transporter ABCG2, which is located in the blood-brain barrier limits the entry of endogenous compounds and xenobiotics into the brain, and its expression and activity are regulated by estrogen. This study was aimed to define the role of ABCG2 in estrogen-mediated neuroprotection against ischemic injury. ABCG2 protein levels before and after ischemic stroke were increased in the brain of female mice by ovariectomy, which were reversed by estrogen replacement. In brain endothelial cell line bEnd.3, estrogen reduced the basal ABCG2 protein level and efflux activity and protected cells from ischemic injury without inducing ABCG2 expression. When bEnd.3 cells were transfected with ABCG2 small interfering RNA, ischemia-induced cell death was reduced, and the intracellular concentration of glutathione, an antioxidant that is transported by ABCG2, was increased. In addition, after ischemic stroke in ovariectomized mice, estrogen prevented the reduction of intracellular glutathione level in brain microvessels. These data suggested that the suppression of ABCG2 by estrogen is involved in neuroprotection against ischemic injury by increasing intracellular glutathione, and that the modulation of ABCG2 activity offers a therapeutic target for brain diseases in estrogen-deficient aged women.

Glutathione-mediated antioxidant response and aerobic metabolism: two crucial factors involved in determining the multi-drug resistance of high-risk neuroblastoma

Neuroblastoma, a paediatric malignant tumor, is initially sensitive to etoposide, a drug to which many patients develop chemoresistance. In order to investigate the molecular mechanisms responsible for etoposide chemoresistance, HTLA-230, a human MYCN-amplified neuroblastoma cell line, was chronically treated with etoposide at a concentration that in vitro mimics the clinically-used dose. The selected cells (HTLA-Chr) acquire multi-drug resistance (MDR), becoming less sensitive than parental cells to high doses of etoposide or doxorubicin. MDR is due to several mechanisms that together contribute to maintaining non-toxic levels of H₂O₂. In fact, HTLA-Chr cells, while having an efficient aerobic metabolism, are also characterized by an up-regulation of catalase activity and higher levels of reduced glutathione (GSH), a thiol antioxidant compound. The combination of such mechanisms contributes to prevent membrane lipoperoxidation and cell death. Treatment of HTLA-Chr cells with L-Buthionine-sulfoximine, an inhibitor of GSH biosynthesis, markedly reduces their tumorigenic potential that is instead enhanced by the exposure to N-Acetylcysteine, able to promote GSH synthesis.

Collectively, these results demonstrate that GSH and GSH-related responses play a crucial role in the acquisition of MDR and suggest that GSH level monitoring is an efficient strategy to early identify the onset of drug resistance and to control the patient's response to therapy.

A role for ABCG2 beyond drug transport: Regulation of autophagy

The ABC drug transporters, including ABCG2, are well known for their ability to efflux a wide spectrum of chemotherapeutic agents, thereby conferring a multidrug-resistant phenotype. However, studies over the past several years suggest that the ABC transporters may play additional role(s) in cell survival in the face of stress inducers that are not ABCG2 substrates (i.e., nutrient deprivation, ionizing radiation, rapamycin). The mechanism by which this occurs is largely unknown. In the present study, using several cancer cell lines and their ABCG2-overexpressing sublines, we show that cells overexpressing ABCG2 were more resistant to these stressors. This resistance was associated with an elevated level of autophagy flux, as measured by a higher rate of SQSTM1/p62 degradation and greater accumulation of LC3-II when compared to parental cells. Knockdown of ABCG2 reduced autophagic activity in resistant cells to a level similar to that observed in parental cells, confirming that the enhanced autophagy was ABCG2-dependent. Moreover, using cell viability, apoptosis, and clonogenic assays, we demonstrated that the ABCG2-expressing cells were more resistant to amino acid starvation and radiation-induced cell death. Importantly, knockdown of the critical autophagy factors ATG5 and ATG7 greatly reduced cell survival, verifying that enhanced autophagy was critical for this effect. Taken together, these data indicate that autophagy induced by various stressors is enhanced/accelerated in the presence of ABCG2, resulting in delayed cell death and enhanced cell survival. This defines a new role for this transporter, one with potential clinical significance.

ATP Binding Cassette Sub-family Member 2 (ABCG2) and Xenobiotic Exposure During Early Mouse Embryonic Stem Cell Differentiation

BACKGROUND

ATP binding cassette sub-family member 2 (ABCG2) is a well-defined efflux transporter found in a variety of tissues. The role of ABCG2 during early embryonic development, however, is not established. Previous work which compared data from the ToxCast screening program with that from in-house studies suggested an association exists between exposure to xenobiotics that regulate Abcg2 transcription and differentiation of mouse embryonic stem cells (mESC), a relationship potentially related to redox homeostasis.

METHODS

mESC were grown for up to 9 days. Pharmacological inhibitors were used to assess transporter function with and without xenobiotic exposure. Proliferation and differentiation were evaluated using RedDot1 and quantiative reverse transcriptase-polymerase chain reaction, respectively. ABCG2 activity was assessed using a Pheophorbide a-based fluorescent assay. Protein expression was measured by capillary-based immunoassay.

RESULTS

ABCG2 activity increased in differentiating mESC. Treatment with K0143, an inhibitor of ABCG2, had no effect on proliferation or differentiation. As expected, mitoxantrone and topotecan, two chemotherapeutics, displayed increased toxicity in the presence of K0143. Exposure to K0143 in combination with chemicals predicted by ToxCast to regulate ABCG2 expression did not alter xenobiotic-induced toxicity. Moreover, inhibition of ABCG2 did not shift the toxicity of either tert-Butyl hydroperoxide or paraquat, two oxidative stressors.

CONCLUSION

As previously reported, ABCG2 serves a protective role in mESC. The role of ABCG2 in regulating redox status, however, was unclear. The hypothesis that ABCG2 plays a fundamental role during mESC differentiation or that regulation of the receptor by xenobiotics may be associated with altered mESC differentiation could not be supported. Birth Defects Research, 110:35-47, 2018. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.

Leishmania LABCG2 transporter is involved in ATP-dependent transport of thiols

The Leishmania LABCG2 transporter has a key role in the redox metabolism of these protozoan parasites. Recently, the involvement of LABCG2 in virulence, autophagy and oxidative stress has been described. Null mutant parasites for LABCG2 present an increase in the intracellular levels of glutathione (GSH) and trypanothione [T(SH)₂]. On the other hand, parasites overexpressing LABCG2 transporter export non-protein thiols to the extracellular medium. To explore if LABCG2 may mediate an active transport of non-protein thiols, the effect of these molecules on ATPase activity of LABCG2 as well as the ability of LABCG2 to transport them was determined using a baculovirus-Sf9 insect cell system. Our results indicate that all thiols tested [GSH, T(SH)₂] as well as their oxidized forms GSSG and TS₂ (trypanothione disulfide) stimulate LABCG2-ATPase basal activity. We have measured the transport of [³H]-GSH in inside-out Sf9 cell membrane vesicles expressing LABCG2-GFP (green fluorescence protein), finding that LABCG2 was able to mediate a rapid and concentration-dependent uptake of [³H]-GSH in the presence of ATP. Finally, we have analyzed the ability of different thiol species to compete for this uptake, T(SH)₂ and TS₂ being the best competitors. The IC₅₀ value for [³H]-GSH uptake in the presence of increasing concentrations of T(SH)₂ was less than 100 μM, highlighting the affinity of this thiol for LABCG2. These results provide the first direct evidence that LABCG2 is an ABC transporter of reduced and oxidized non-protein thiols in Leishmania, suggesting that this transporter can play a role in the redox metabolism and related processes in this protozoan parasite.

ABC Transporters in Cancer Stem Cells: Beyond Chemoresistance

The efficacy of chemotherapy is one of the main challenges in cancer treatment and one of the major obstacles to overcome in achieving lasting remission and a definitive cure in patients with cancer is the emergence of cancer resistance. Indeed, drug resistance is ultimately accountable for poor treatment outcomes and tumour relapse. There are various molecular mechanisms involved in multidrug resistance, such as the change in the activity of membrane transporters primarily belonging to the ATP binding cassette (ABC) transporter family. In addition, it has been proposed that this common feature could be attributed to a subpopulation of slow-cycling cancer stem cells (CSCs), endowed with enhanced tumorigenic potential and multidrug resistance. CSCs are characterized by the overexpression of specific surface markers that vary in different cancer cell types. Overexpression of ABC transporters has been reported in several cancers and more predominantly in CSCs. While the major focus on the role played by ABC transporters in cancer is polarized by their involvement in chemoresistance, emerging evidence supports a more active role of these proteins, in which they release specific bioactive molecules in the extracellular milieu. This review will outline our current understanding of the role played by ABC transporters in CSCs, how their expression is regulated and how they support the malignant metabolic phenotype. To summarize, we suggest that the increased expression of ABC transporters in CSCs may have precise functional roles and provide the opportunity to target, particularly these cells, by using specific ABC transporter inhibitors.

5-Aminosalicylic Acid Modulates the Immune Response in Chronic Beryllium Disease Subjects

INTRODUCTION

Chronic beryllium disease (CBD) is characterized by accumulation of macrophages and beryllium-specific CD4⁺ T cells that proliferate and produce Th1 cytokines. 5-Amino salicylic acid (5-ASA) is currently used to treat inflammatory bowel disease and has both antioxidant and anti-inflammatory actions. We hypothesized that 5-ASA may be a beneficial therapeutic in CBD.

METHODS

Seventeen CBD patients were randomized 3:1 to receive 5-ASA 500-mg capsules or placebo four times daily for 6 weeks orally. Primary study endpoints included changes in beryllium lymphocyte proliferation (BeLPT). Secondary endpoints included changes in bronchoalveolar lavage (BAL) fluid, cells, serum, and blood cell glutathione (GSH) levels, BAL cell TNF-α levels, lung function, and quality of life measures.

RESULTS

5-ASA decreased BAL cell BeLPT by 20% within the 5-ASA treatment group. No significant changes were observed in serum, PBMCs, BALF, or BAL cell GSH levels in either the 5-ASA or placebo treatment group. 5-ASA treatment decreased ex vivo Be-stimulated BAL cell TNF-α levels within the 5-ASA group and when compared to placebo. Significant improvements were noted in quality of life measurements with 5-ASA treatment.

CONCLUSIONS

5-ASA's ability to decrease BAL cell BeLPT and Be-stimulated BAL cell TNF-α levels suggests that 5-ASA may impact the beryllium-specific immune response in CBD. 5-ASA use in other non-infectious granulomatous lung diseases, such as sarcoidosis, may prove to be a useful alternative treatment to corticosteroids for those with mild to moderate disease.

Deregulated angiogenesis in chronic lung diseases: a possible role for lung mesenchymal progenitor cells (2017 Grover Conference Series)

Chronic lung disease (CLD), including pulmonary fibrosis (PF) and chronic obstructive pulmonary disease (COPD), is the fourth leading cause of mortality worldwide. Both are debilitating pathologies that impede overall tissue function. A common co-morbidity in CLD is vasculopathy, characterized by deregulated angiogenesis, remodeling, and loss of microvessels. This substantially worsens prognosis and limits survival, with most current therapeutic strategies being largely palliative. The relevance of angiogenesis, both capillary and lymph, to the pathophysiology of CLD has not been resolved as conflicting evidence depicts angiogenesis as both reparative or pathologic. Therefore, we must begin to understand and model the underlying pathobiology of pulmonary vascular deregulation, alone and in response to injury induced disease, to define cell interactions necessary to maintain normal function and promote repair. Capillary and lymphangiogenesis are deregulated in both PF and COPD, although the mechanisms by which they co-regulate and underlie early pathogenesis of disease are unknown. The cell-specific mechanisms that regulate lung vascular homeostasis, repair, and remodeling represent a significant gap in knowledge, which presents an opportunity to develop targeted therapies. We have shown that that ABCG2^pos multipotent adult mesenchymal stem or progenitor cells (MPC) influence the function of the capillary microvasculature as well as lymphangiogenesis. A balance of both is required for normal tissue homeostasis and repair. Our current models suggest that when lymph and capillary angiogenesis are out of balance, the non-equivalence appears to support the progression of disease and tissue remodeling. The angiogenic regulatory mechanisms underlying CLD likely impact other interstitial lung diseases, tuberous sclerosis, and lymphangioleiomyomatosis.

Kinetic Modeling of ABCG2 Transporter Heterogeneity: A Quantitative, Single-Cell Analysis of the Side Population Assay

The side population (SP) assay, a technique used in cancer and stem cell research, assesses the activity of ABC transporters on Hoechst staining in the presence and absence of transporter inhibition, identifying SP and non-SP cell (NSP) subpopulations by differential staining intensity. The interpretation of the assay is complicated because the transporter-mediated mechanisms fail to account for cell-to-cell variability within a population or adequately control the direct role of transporter activity on staining intensity. We hypothesized that differences in dye kinetics at the single-cell level, such as ABCG2 transporter-mediated efflux and DNA binding, are responsible for the differential cell staining that demarcates SP/NSP identity. We report changes in A549 phenotype during time in culture and with TGFβ treatment that correlate with SP size. Clonal expansion of individually sorted cells re-established both SP and NSPs, indicating that SP membership is dynamic. To assess the validity of a purely kinetics-based interpretation of SP/NSP identity, we developed a computational approach that simulated cell staining within a heterogeneous cell population; this exercise allowed for the direct inference of the role of transporter activity and inhibition on cell staining. Our simulated SP assay yielded appropriate SP responses for kinetic scenarios in which high transporter activity existed in a portion of the cells and little differential staining occurred in the majority of the population. With our approach for single-cell analysis, we observed SP and NSP cells at both ends of a transporter activity continuum, demonstrating that features of transporter activity as well as DNA content are determinants of SP/NSP identity.

The LABCG2 Transporter from the Protozoan Parasite Leishmania Is Involved in Antimony Resistance

Treatment for leishmaniasis, which is caused by Leishmania protozoan parasites, currently relies on a reduced arsenal of drugs. However, the significant increase in the incidence of drug therapeutic failure and the growing resistance to first-line drugs like antimonials in some areas of Northern India and Nepal limit the control of this parasitic disease. Understanding the molecular mechanisms of resistance in Leishmania is now a matter of urgency to optimize drugs used and to identify novel drug targets to block or reverse resistant mechanisms. Some members of the family of ATP-binding cassette (ABC) transporters in Leishmania have been associated with drug resistance. In this study, we have focused our interest to characterize LABCG2's involvement in drug resistance in Leishmania. Leishmania major parasites overexpressing the ABC protein transporter LABCG2 were generated in order to assess how LABCG2 is involved in drug resistance. Assays of susceptibility to different leishmanicidal agents were carried out. Analysis of the drug resistance profile revealed that Leishmania parasites overexpressing LABCG2 were resistant to antimony, as they demonstrated a reduced accumulation of Sb(III) due to an increase in drug efflux. Additionally, LABCG2 was able to transport thiols in the presence of Sb(III) Biotinylation assays using parasites expressing LABCG2 fused with an N-terminal green fluorescent protein tag revealed that LABCG2 is partially localized in the plasma membrane; this supports data from previous studies which suggested that LABCG2 is localized in intracellular vesicles that fuse with the plasma membrane during exocytosis. In conclusion, Leishmania LABCG2 probably confers antimony resistance by sequestering metal-thiol conjugates within vesicles and through further exocytosis by means of the parasite's flagellar pocket.

LmABCB3, an atypical mitochondrial ABC transporter essential for Leishmania major virulence, acts in heme and cytosolic iron/sulfur clusters biogenesis

Background

Mitochondria play essential biological functions including the synthesis and trafficking of porphyrins and iron/sulfur clusters (ISC), processes that in mammals involve the mitochondrial ATP-Binding Cassette (ABC) transporters ABCB6 and ABCB7, respectively. The mitochondrion of pathogenic protozoan parasites such as Leishmania is a promising goal for new therapeutic approaches. Leishmania infects human macrophages producing the neglected tropical disease known as leishmaniasis. Like most trypanosomatid parasites, Leishmania is auxotrophous for heme and must acquire porphyrins from the host.

Methods

LmABCB3, a new Leishmania major protein with significant sequence similarity to human ABCB6/ABCB7, was identified and characterized using bioinformatic tools. Fluorescent microscopy was used to determine its cellular localization, and its level of expression was modulated by molecular genetic techniques. Intracellular in vitro assays were used to demonstrate its role in amastigotes replication, and an in vivo mouse model was used to analyze its role in virulence. Functional characterization of LmABCB3 was carried out in Leishmania promastigotes and Saccharomyces cerevisiae. Structural analysis of LmABCB3 was performed using molecular modeling software.

Results

LmABCB3 is an atypical ABC half-transporter that has a unique N-terminal extension not found in any other known ABC protein. This extension is required to target LmABCB3 to the mitochondrion and includes a potential metal-binding domain. We have shown that LmABCB3 interacts with porphyrins and is required for the mitochondrial synthesis of heme from a host precursor. We also present data supporting a role for LmABCB3 in the biogenesis of cytosolic ISC, essential cofactors for cell viability in all three kingdoms of life. LmABCB3 fully complemented the severe growth defect shown in yeast lacking ATM1, an orthologue of human ABCB7 involved in exporting from the mitochondria a gluthatione-containing compound required for the generation of cytosolic ISC. Indeed, docking analyzes performed with a LmABCB3 structural model using trypanothione, the main thiol in this parasite, as a ligand showed how both, LmABCB3 and yeast ATM1, contain a similar thiol-binding pocket. Additionally, we show solid evidence suggesting that LmABCB3 is an essential gene as dominant negative inhibition of LmABCB3 is lethal for the parasite. Moreover, the abrogation of only one allele of the gene did not impede promastigote growth in axenic culture but prevented the replication of intracellular amastigotes and the virulence of the parasites in a mouse model of cutaneous leishmaniasis.

Conclusions

Altogether our results present the previously undescribed LmABCB3 as an unusual mitochondrial ABC transporter essential for Leishmania survival through its role in the generation of heme and cytosolic ISC. Hence, LmABCB3 could represent a novel target to combat leishmaniasis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1284-5) contains supplementary material, which is available to authorized users.

Glutathione Depletion Accelerates Cigarette Smoke-Induced Inflammation and Airspace Enlargement

The study objective was to assess age-related changes in glutathione (GSH) adaptive response to cigarette smoke (CS) exposure. Older cigarette smokers show a decline (67%) in lung epithelial lining fluid (ELF) GSH and a 1.8-fold decreased GSH adaptive response to cigarette smoking with a concomitant elevation (47%) of exhaled nitric oxide compared with younger smokers. In order to isolate the changes in tissue GSH from other age-related effects, pharmacological inhibition of the rate limiting step in GSH synthesis was employed to examine the lung's response to CS exposure in young mice. The γ-glutamylcysteine ligase inhibitor L-buthionine-sulfoximine (BSO) was administered in the drinking water (20 mM) to decrease by half the in vivo GSH levels to those found in aged mice and humans. Mice were then exposed to CS (3 h/day) for 5 or 15 days. Biochemical analysis of the ELF and lung tissue revealed an inhibition of the CS-induced GSH adaptive response by BSO with a concurrent increase in mixed protein-GSH disulfides indicating increased cysteine oxidation. The prevention of the GSH adaptive response led to an increase in pro-inflammatory cytokines present in the lung. Airspace enlargement is a hallmark of lung emphysema and was observed in mice treated with BSO and exposed to CS for as little as 15 days, whereas these types of changes normally take up to 6 months in this model. BSO treatment potentiated both lung elastase and matrix metalloproteinase activity in the CS group. These data suggest that age-related decline in the GSH adaptive response can markedly accelerate many of the factors thought to drive CS-induced emphysema.

Antioxidant Mechanisms and ROS-Related MicroRNAs in Cancer Stem Cells

Increasing evidence indicates that most of the tumors are sustained by a distinct population of cancer stem cells (CSCs), which are responsible for growth, metastasis, invasion, and recurrence. CSCs are typically characterized by self-renewal, the key biological process allowing continuous tumor proliferation, as well as by differentiation potential, which leads to the formation of the bulk of the tumor mass. CSCs have several advantages over the differentiated cancer cell populations, including the resistance to radio- and chemotherapy, and their gene-expression programs have been shown to correlate with poor clinical outcome, further supporting the relevance of stemness properties in cancer. The observation that CSCs possess enhanced mechanisms of protection from reactive oxygen species (ROS) induced stress and a different metabolism from the differentiated part of the tumor has paved the way to develop drugs targeting CSC specific signaling. In this review, we describe the role of ROS and of ROS-related microRNAs in the establishment and maintenance of self-renewal and differentiation capacities of CSCs.

Sulfur as a signaling nutrient through hydrogen sulfide

Hydrogen sulfide (H₂S) has emerged as an important signaling molecule with beneficial effects on various cellular processes affecting, for example, cardiovascular and neurological functions. The physiological importance of H₂S is motivating efforts to develop strategies for modulating its levels. However, advancement in the field of H₂S-based therapeutics is hampered by fundamental gaps in our knowledge of how H₂S is regulated, its mechanism of action, and its molecular targets. This review provides an overview of sulfur metabolism; describes recent progress that has shed light on the mechanism of H₂S as a signaling molecule; and examines nutritional regulation of sulfur metabolism, which pertains to health and disease.

Multi-Drug Resistance ABC Transporter Inhibition Enhances Murine Ventral Prostate Stem/Progenitor Cell Differentiation

Multi-drug resistance (MDR)-ATP binding cassette (ABC) transporters, ABCB1, ABCC1, and ABCG2 participate in the efflux of steroid hormones, estrogens, and androgens, which regulate prostate development and differentiation. The role of MDR-ABC efflux transporters in prostate epithelial proliferation and differentiation remains unclear. We hypothesized that MDR-ABC transporters regulate prostate differentiation and epithelium regeneration. Prostate epithelial differentiation was studied using histology, sphere formation assay, and prostate regeneration induced by cycles of repeated androgen withdrawal and replacement. Embryonic deletion of Abcg2 resulted in a decreased number of luminal cells in the prostate and increased sphere formation efficiency, indicating an imbalance in the prostate epithelial differentiation pattern. Decreased luminal cell number in the Abcg2 null prostate implies reduced differentiation. Enhanced sphere formation efficiency in Abcg2 null prostate cells implies activation of the stem/progenitor cells. Prostate regeneration was associated with profound activation of the stem/progenitor cells, indicating the role of Abcg2 in maintaining stem/progenitor cell pool. Since embryonic deletion of Abcg2 may result in compensation by other ABC transporters, pharmacological inhibition of MDR-ABC efflux was performed. Pharmacological inhibition of MDR-ABC efflux enhanced prostate epithelial differentiation in sphere culture and during prostate regeneration. In conclusion, Abcg2 deletion leads to activation of the stem/progenitor cells and enhances differentiating divisions; and pharmacological inhibition of MDR-ABC efflux leads to epithelial differentiation. Our study demonstrates for the first time that MDR-ABC efflux transporter inhibition results in enhanced prostate epithelial cell differentiation.

Collateral sensitivity: ABCG2-overexpressing cells are more vulnerable to oxidative stress

Multidrug resistance (MDR), which is the main obstacle to cancer chemotherapy, is mainly due to overexpression of ATP-binding cassette (ABC) transporters, especially ABCB1 (P-glycoprotein), ABCC1 (MRP1), and ABCG2 (BCRP). A novel idea to overcome MDR is that of collateral sensitivity, i.e., finding a treatment to which cells overexpressing ABC transporters are more sensitive than cells that do not overexpress them. In this study we demonstrate for the first time that MDCKII-BCRP cells, overexpressing ABCG2, are more vulnerable to exogenous oxidative stress induced by several oxidants, viz. paraquat, menadione, hydrogen peroxide, tert-butylperoxide, and 2,2-azobis(2-methylpropionamidine) dihydrochloride. MDCKII-BCRP cells have significantly decreased glutathione level and decreased activities of glutathione S-transferase and glutathione reductase, which may underlie their augmented vulnerability to oxidative stress. These results suggest the possibility of using agents that induce oxidative stress to selectively kill cells overexpressing BCRP.

Glutathione depletion regulates both extrinsic and intrinsic apoptotic signaling cascades independent from multidrug resistance protein 1

Glutathione (GSH) depletion is an important hallmark of apoptosis. We previously demonstrated that GSH depletion, by its efflux, regulates apoptosis by modulation of executioner caspase activity. However, both the molecular identity of the GSH transporter(s) involved and the signaling cascades regulating GSH loss remain obscure. We sought to determine the role of multidrug resistance protein 1 (MRP1) in GSH depletion and its regulatory role on extrinsic and intrinsic pathways of apoptosis. In human lymphoma cells, GSH depletion was stimulated rather than inhibited by pharmacological blockage of MRP1 with MK571. GSH loss was dependent on initiator caspases 8 and 9 activity. Genetic knock-down (>60 %) of MRP1 by stable transfection with short hairpin small interfering RNA significantly reduced MRP1 protein levels, which correlated directly with the loss of MRP1-mediated anion transport. However, GSH depletion and apoptosis induced by both extrinsic and intrinsic pathways were not affected by MRP1 knock-down. Interestingly, stimulation of GSH loss by MK571 also enhanced the initiator phase of apoptosis by stimulating initiator caspase 8 and 9 activity and pro-apoptotic BCL-2 interacting domain cleavage. Our results clearly show that caspase-dependent GSH loss and apoptosis are not mediated by MRP1 proteins and that GSH depletion stimulates the initiation phase of apoptosis in lymphoid cells.

Thiol/redox metabolomic profiling implicates GSH dysregulation in early experimental graft versus host disease (GVHD)

Graft-versus-host disease (GVHD) is a common complication of allogeneic bone marrow transplantation (BMT). Upregulation of inflammatory cytokines precedes the clinical presentation of GVHD and predicts its severity. In this report, thiol/redox metabolomics was used to identify metabolic perturbations associated with early preclinical (Day+4) and clinical (Day+10) stages of GVHD by comparing effects in Syngeneic (Syn; major histocompatibility complex- identical) and allogeneic transplant recipients (Allo BMT) in experimental models. While most metabolic changes were similar in both groups, plasma glutathione (GSH) was significantly decreased, and GSH disulfide (GSSG) was increased after allogeneic compared to syngeneic recipient and non-transplant controls. The early oxidation of the plasma GSH/GSSG redox couple was also observed irrespective of radiation conditioning treatment and was accompanied by significant rise in hepatic protein oxidative damage and ROS generation. Despite a significant rise in oxidative stress, compensatory increase in hepatic GSH synthesis was absent following Allo BMT. Early shifts in hepatic oxidative stress and plasma GSH loss preceded a statistically significant rise in TNF-α. To identify metabolomic biomarkers of hepatic GVHD injury, plasma metabolite concentrations analyzed at Day+10 were correlated with hepatic organ injury. GSSG (oxidized GSH) and β-alanine, were positively correlated, and plasma GSH cysteinylglycine, and branched chain amino acids were inversely correlated with hepatic injury. Although changes in plasma concentrations of cysteine, cystathionine (GSH precursors) and cysteinylglycine (a GSH catabolite) were not significant by univariate analysis, principal component analysis (PCA) indicated that accumulation of these metabolites after Allo BMT contributed significantly to early GVHD in contrast to Syn BMT. In conclusion, thiol/redox metabolomic profiling implicates that early dysregulation of host hepatic GSH metabolism and oxidative stress in sub-clinical GVHD before elevated TNF-α levels is associated with GVHD pathogenesis. Future studies will probe the mechanisms for these changes and examine the potential of antioxidant intervention strategies to modulate GVHD.

ABCG2 is not able to catalyze glutathione efflux and does not contribute to GSH-dependent collateral sensitivity

ABCG2 is a key human ATP-binding cassette (ABC) transporter mediating cancer cell chemoresistance. In the case of ABCC1, another multidrug transporter, earlier findings documented that certain modulators greatly increase ABCC1-mediated glutathione (GSH) efflux and, upon depletion of intracellular GSH, induce "collateral sensitivity" leading to the apoptosis of multidrug resistant cells. Recently, it has been suggested that ABCG2 may mediate an active GSH transport. In order to explore if ABCG2-overexpressing cells may be similarly targeted, we first looked for the effects of ABCG2 expression on cellular GSH levels, and for an ABCG2-dependent GSH transport in HEK293 and MCF7 cells. We found that, while ABCG2 overexpression altered intracellular GSH levels in these transfected or drug-selected cells, ABCG2 inhibitors or transport modulators did not influence GSH efflux. We then performed direct measurements of drug-stimulated ATPase activity and (3)H-GSH transport in inside-out membrane vesicles of human ABC transporter-overexpressing Sf9 insect cells. Our results indicate that ABCG2-ATPase is not modulated by GSH and, in contrast to ABCC1, ABCG2 does not catalyze any significant GSH transport. Our data suggest no direct interaction between the ABCG2 transporter and GSH, although a long-term modulation of cellular GSH by ABCG2 cannot be excluded.

HIF-2α suppresses p53 to enhance the stemness and regenerative potential of human embryonic stem cells

Human embryonic stem cells (hESCs) have been reported to exert cytoprotective activity in the area of tissue injury. However, hypoxia/oxidative stress prevailing in the area of injury could activate p53, leading to death and differentiation of hESCs. Here we report that when exposed to hypoxia/oxidative stress, a small fraction of hESCs, namely the SSEA3+/ABCG2+ fraction undergoes a transient state of reprogramming to a low p53 and high hypoxia inducible factor (HIF)-2α state of transcriptional activity. This state can be sustained for a period of 2 weeks and is associated with enhanced transcriptional activity of Oct-4 and Nanog, concomitant with high teratomagenic potential. Conditioned medium obtained from the post-hypoxia SSEA3+/ABCG2+ hESCs showed cytoprotection both in vitro and in vivo. We termed this phenotype as the “enhanced stemness” state. We then demonstrated that the underlying molecular mechanism of this transient phenotype of enhanced stemness involved high Bcl-2, fibroblast growth factor (FGF)-2, and MDM2 expression and an altered state of the p53/MDM2 oscillation system. Specific silencing of HIF-2α and p53 resisted the reprogramming of SSEA3+/ABCG2+ to the enhanced stemness phenotype. Thus, our studies have uncovered a unique transient reprogramming activity in hESCs, the enhanced stemness reprogramming where a highly cytoprotective and undifferentiated state is achieved by transiently suppressing p53 activity. We suggest that this transient reprogramming is a form of stem cell altruism that benefits the surrounding tissues during the process of tissue regeneration.

Multiple glutathione disulfide removal pathways mediate cytosolic redox homeostasis

Glutathione is central to cellular redox chemistry. The majority of glutathione redox research has been based on the chemical analysis of whole-cell extracts, which unavoidably destroy subcellular compartment-specific information. Compartment-specific real-time measurements based on genetically encoded fluorescent probes now suggest that the cytosolic glutathione redox potential is about 100 mV more reducing than previously thought. Using these probes in yeast, we show that even during severe oxidative stress, the cytosolic glutathione disulfide (GSSG) concentration is much more tightly regulated than expected and provides a mechanistic explanation for the discrepancy with conventional measurements. GSSG that is not immediately reduced in the cytosol is rapidly transported into the vacuole by the ABC-C transporter Ycf1. The amount of whole-cell GSSG is entirely dependent on Ycf1 and uninformative about the cytosolic glutathione pool. Applying these insights, we identify Trx2 and Grx2 as efficient backup systems to glutathione reductase for cytosolic GSSG reduction.

Glutathione efflux and cell death

SIGNIFICANCE

Glutathione (GSH) depletion is a central signaling event that regulates the activation of cell death pathways. GSH depletion is often taken as a marker of oxidative stress and thus, as a consequence of its antioxidant properties scavenging reactive species of both oxygen and nitrogen (ROS/RNS).

RECENT ADVANCES

There is increasing evidence demonstrating that GSH loss is an active phenomenon regulating the redox signaling events modulating cell death activation and progression.

CRITICAL ISSUES

In this work, we review the role of GSH depletion by its efflux, as an important event regulating alterations in the cellular redox balance during cell death independent from oxidative stress and ROS/RNS formation. We discuss the mechanisms involved in GSH efflux during cell death progression and the redox signaling events by which GSH depletion regulates the activation of the cell death machinery.

FUTURE DIRECTIONS

The evidence summarized here clearly places GSH transport as a central mechanism mediating redox signaling during cell death progression. Future studies should be directed toward identifying the molecular identity of GSH transporters mediating GSH extrusion during cell death, and addressing the lack of sensitive approaches to quantify GSH efflux.

Glutathione transporters

BACKGROUND

Glutathione (GSH) is synthesized in the cytoplasm but there is a requirement for glutathione not only in the cytoplasm, but in the other organelles and the extracellular milieu. GSH is also imported into the cytoplasm. The transports of glutathione across these different membranes in different systems have been biochemically demonstrated. However the molecular identity of the transporters has been established only in a few cases.

SCOPE OF REVIEW

An attempt has been made to present the current state of knowledge of glutathione transporters from different organisms as well as different organelles. These include the most well characterized transporters, the yeast high-affinity, high-specificity glutathione transporters involved in import into the cytoplasm, and the mammalian MRP proteins involved in low affinity glutathione efflux from the cytoplasm. Other glutathione transporters that have been described either with direct or indirect evidences are also discussed.

MAJOR CONCLUSIONS

The molecular identity of a few glutathione transporters has been unambiguously established but there is a need to identify the transporters of other systems and organelles. There is a lack of direct evidence establishing transport by suggested transporters in many cases. Studies with the high affinity transporters have led to important structure-function insights.

GENERAL SIGNIFICANCE

An understanding of glutathione transporters is critical to our understanding of redox homeostasis in living cells. By presenting our current state of understanding and the gaps in our knowledge the review hopes to stimulate research in these fields. This article is part of a Special Issue entitled Cellular functions of glutathione.

Abcg2 deficiency augments oxidative stress and cognitive deficits in Tg-SwDI transgenic mice

Oxidative stress and neuroinflammation play important roles in Alzheimer's disease (AD). ABCG2 is a transporter protein expressed in the brain and involved in GSH transport. To study the roles of Abcg2 in oxidative stress and AD, we cross-bred Tg-SwDI and Abcg2-KO mice and generated Tg-SwDI/Abcg2-KO (double-tg) mice. Brain tissues from double-tg, Tg-SwDI, wild-type, and Abcg2-KO mice at various ages were analyzed. Aβ40 and Aβ42 were detected in Tg-SwDI and double-tg mice. Total brain GSH was decreased and levels of lipid/DNA oxidation were increased in 3-month double-tg compared to Tg-SwDI mice. Low brain GSH was still detected in 9-month double-tg mice. Increased HMOX-1 and MCP-5 expression was observed in 9-month double-tg mice but not in Tg-SwDI mice compared to WT and Abcg2-KO mice. Increased HMOX-1 and decreased ICAM-1 expression were observed in 12-month double-tg mice compared to Tg-SwDI mice. The levels of Nrf-2 expression and activity were decreased in 6-month double-tg mice. Behavioral tests show impaired cognitive/memory performance of 9-month double-tg compared to Tg-SwDI mice as well as WT and Abcg2-KO mice. These results suggest that Abcg2 deficiency increases oxidative stress and alters inflammatory response in the brain and exacerbates cognitive/memory deficit in double-tg mice at different developmental stages.

CFTR is the primary known apical glutathione transporter involved in cigarette smoke-induced adaptive responses in the lung

One of the most abundant antioxidants in the lung is glutathione (GSH), a low-molecular-weight thiol, which functions to attenuate both oxidative stress and inflammation. GSH is concentrated in the epithelial lining fluid (ELF) of the lung and can be elevated in response to the increased oxidant burden from cigarette smoke (CS). However, the transporter(s) responsible for the increase in ELF GSH with cigarette smoke is not known. Three candidate apical GSH transporters in the lung are CFTR, BCRP, and MRP2, but their potential roles in ELF GSH transport in response to CS have not been investigated. In vitro, the inhibition of CFTR, BCRP, or MRP2 resulted in decreased GSH efflux in response to cigarette smoke extract. In vivo, mice deficient in CFTR, BCRP, or MRP2 were exposed to either air or acute CS. CFTR-deficient mice had reduced basal and CS-induced GSH in the ELF, whereas BCRP or MRP2 deficiency had no effect on ELF GSH basal or CS-exposed levels. Furthermore, BCRP or MRP2 deficiency had little effect on lung tissue GSH. These data indicate that CFTR is predominantly involved in maintaining basal ELF GSH and increasing ELF GSH in response to CS.

A synthetic chalcone as a potent inducer of glutathione biosynthesis

Chalcones continue to attract considerable interest due to their anti-inflammatory and antiangiogenic properties. We recently reported the ability of 2',5'-dihydroxychalcone (2',5'-DHC) to induce both breast cancer resistance protein-mediated export of glutathione (GSH) and c-Jun N-terminal kinase-mediated increased intracellular GSH levels. Herein, we report a structure-activity relationship study of a series of 30 synthetic chalcone derivatives with hydroxyl, methoxyl, and halogen (F and Cl) substituents and their ability to increase intracellular GSH levels. This effect was drastically improved with one or two electrowithdrawing groups on phenyl ring B and up to three methoxyl and/or hydroxyl groups on phenyl ring A. The optimal structure, 2-chloro-4',6'-dimethoxy-2'-hydroxychalcone, induced both a potent NF-E2-related factor 2-mediated transcriptional response and an increased formation of glutamate cysteine ligase holoenzyme, as shown using a human breast cancer cell line stably expressing a luciferase reporter gene driven by antioxidant response elements.