Inhibitors of V-ATPases: old and new players

The curious case of vacuolar ATPase: regulation of signaling pathways

The Vacuolar ATPase (V-ATPase) is a proton pump responsible for controlling the intracellular and extracellular pH of cells. The structure of V-ATPase has been highly conserved among all eukaryotic cells and is involved in diverse functions across species. V-ATPase is best known for its acidification of endosomes and lysosomes and is also important for luminal acidification of specialized cells. Several reports have suggested the involvement of V-ATPase in maintaining an alkaline intracellular and acidic extracellular pH thereby aiding in proliferation and metastasis of cancer cells respectively. Increased expression of V-ATPase and relocation to the plasma membrane aids in cancer modulates key tumorigenic cell processes like autophagy, Warburg effect, immunomoduation, drug resistance and most importantly cancer cell signaling. In this review, we discuss the direct role of V-ATPase in acidification and indirect regulation of signaling pathways, particularly Notch Signaling.

Lactoferrin selectively triggers apoptosis in highly metastatic breast cancer cells through inhibition of plasmalemmal V-H+-ATPase

Breast cancer is the most common type of cancer affecting women. Despite the good prognosis when detected early, significant challenges remain in the treatment of metastatic breast cancer. The recruitment of the vacuolar H⁺-ATPase (V-H⁺-ATPase) to the plasma membrane, where it mediates the acidification of the tumor microenvironment (TME), is a recognized feature involved in the acquisition of a metastatic phenotype in breast cancer. Therefore, inhibitors of this pump have emerged as promising anticancer drugs. Lactoferrin (Lf) is a natural pro-apoptotic iron-binding glycoprotein with strong anticancer activity whose mechanism of action is not fully understood. Here, we show that bovine Lf (bLf) preferentially induces apoptosis in the highly metastatic breast cancer cell lines Hs 578T and MDA-MB-231, which display a prominent localisation of V-H⁺-ATPase at the plasma membrane, but not in the lowly metastatic T-47D or in the non-tumorigenic MCF-10-2A cell lines. We also demonstrate that bLf decreases the extracellular acidification rate and causes intracellular acidification in metastatic breast cancer cells and, much like the well-known proton pump inhibitors concanamycin A and bafilomycin A1, inhibits V-H⁺-ATPase in sub-cellular fractions. These data further support that bLf targets V-H⁺-ATPase and explain the selectivity of bLf for cancer cells, especially for highly metastatic breast cancer cells. Altogether, our results pave the way for more rational in vivo studies aiming to explore this natural non-toxic compound for metastatic breast cancer therapy.

Rethinking the Combination of Proton Exchanger Inhibitors in Cancer Therapy

Microenvironmental acidity is becoming a key target for the new age of cancer treatment. In fact, while cancer is characterized by genetic heterogeneity, extracellular acidity is a common phenotype of almost all cancers. To survive and proliferate under acidic conditions, tumor cells up-regulate proton exchangers and transporters (mainly V-ATPase, Na⁺/H⁺ exchanger (NHE), monocarboxylate transporters (MCTs), and carbonic anhydrases (CAs)), that actively extrude excess protons, avoiding intracellular accumulation of toxic molecules, thus becoming a sort of survival option with many similarities compared with unicellular microorganisms. These systems are also involved in the unresponsiveness or resistance to chemotherapy, leading to the protection of cancer cells from the vast majority of drugs, that when protonated in the acidic tumor microenvironment, do not enter into cancer cells. Indeed, as usually occurs in the progression versus malignancy, resistant tumor clones emerge and proliferate, following a transient initial response to a therapy, thus giving rise to more malignant behavior and rapid tumor progression. Recent studies are supporting the use of a cocktail of proton exchanger inhibitors as a new strategy against cancer.

The pH stability of foot-and-mouth disease virus

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This review summarized the molecular determinants of the acid stability of FMDV in order to explore the uncoating mechanism of FMDV and improve the acid stability of vaccines.

Background

The foot-and-mouth disease virus (FMDV) capsid is highly acid labile and tends to dissociate into pentameric subunits at acidic condition to release viral RNA for initiating virus replication. However, the acid stability of virus capsid is greatly required for the maintenance of intact virion during the process of virus culture and vaccine production. The conflict between the acid lability in vivo and acid stability in vitro of FMDV capsid promotes the selection of a series of amino acid substitutions which can confer resistance to acid-induced FMDV inactivation. In order to explore the uncoating activity of FMDV and enhance the acid stability of vaccines, we summarized the available works about the pH stability of FMDV.

Main body of the abstract

In this review, we analyzed the intrinsic reasons for the acid instability of FMDV from the structural and functional aspects. We also listed all substitutions obtained by different research methods and showed them in the partial capsid of FMDV. We found that a quadrangle region in the viral capsid was the place where a great many pH-sensitive residues were distributed. As the uncoating event of FMDV is dependent on the pH-sensitive amino acid residues in the capsid, this most pH-sensitive position indicates a potential candidate location for RNA delivery triggered by the acid-induced coat disassociation.

Short conclusion

This review provided an overview of the pH stability of FMDV. The study of pH stability of FMDV not only contributes to the exploration of molecule and mechanism information for FMDV uncoating, but also enlightens the development of FMDV vaccines, including the traditionally inactivated vaccines and the new VLP (virus-like particle) vaccines.

F-actin reorganization by V-ATPase inhibition in prostate cancer

The vacuolar ATPase (V-ATPase) proton pump sustains cellular pH homeostasis, and its inhibition triggers numerous stress responses. However, the cellular mechanisms involved remain largely elusive in cancer cells. We studied V-ATPase in the prostate cancer (PCa) cell line PC-3, which has characteristics of highly metastatic PCa. V-ATPase inhibitors impaired endo-lysosomal pH, vesicle trafficking, migration, and invasion. V-ATPase accrual in the Golgi and recycling endosomes suggests that traffic of internalized membrane vesicles back to the plasma membrane was particularly impaired. Directed movement provoked co-localization of V-ATPase containing vesicles with F-actin near the leading edge of migrating cells. V-ATPase inhibition prompted prominent F-actin cytoskeleton reorganization. Filopodial projections were reduced, which related to reduced migration velocity. F-actin formed novel cytoplasmic rings. F-actin rings increased with extended exposure to sublethal concentrations of V-ATPase inhibitors, from 24 to 48 h, as the amount of alkalinized endo-lysosomal vesicles increased. Studies with chloroquine indicated that F-actin rings formation was pH-dependent. We hypothesize that these novel F-actin rings assemble to overcome widespread traffic defects caused by V-ATPase inhibition, similar to F-actin rings on the surface of exocytic organelles.

Summary: V-ATPase activates multiple stress responses. In prostate cancer, sub-lethal concentrations of V-ATPase inhibitors trigger widespread traffic defects. F-actin assembles into rings that mimic those seen during regulated exocytosis.

Reiterative epoxide-based strategies for the synthesis of stereo-n-ads and application to polypropionate synthesis. A Personal Account

The enantioselective synthesis of polypropionates continues to be an attractive realm for the synthetic chemists mostly due to the challenges presented by the number of consecutive stereogenic centers contained within the aliphatic chain. Over the years, our laboratory has developed an epoxide-based three-step reiterative methodology for the construction of these targets, with the ultimate goal that the approach could be extended to the synthesis of polypropionate-containing natural products. The key steps include the diastereoselective epoxidation of allylic and homoallylic alcohols, and the regioselective cleavage of 2-methyl-3,4-epoxy alcohols. The choice of the organometallic reagent, and the cis/trans geometry of the chiral epoxide can be used to control both the relative and absolute configuration of the resulting propionate unit, allowing our approach to be applied in the synthesis of advanced fragments. Additionally, the combination of our first- and second-generation methodologies permits the incorporation of different variations at the methyl moiety.

Nanoparticulate vacuolar ATPase blocker exhibits potent host-targeted antiviral activity against feline coronavirus

Feline infectious peritonitis (FIP), caused by a mutated feline coronavirus, is one of the most serious and fatal viral diseases in cats. The disease remains incurable, and there is no effective vaccine available. In light of the pathogenic mechanism of feline coronavirus that relies on endosomal acidification for cytoplasmic entry, a novel vacuolar ATPase blocker, diphyllin, and its nanoformulation are herein investigated for their antiviral activity against the type II feline infectious peritonitis virus (FIPV). Experimental results show that diphyllin dose-dependently inhibits endosomal acidification in fcwf-4 cells, alters the cellular susceptibility to FIPV, and inhibits the downstream virus replication. In addition, diphyllin delivered by polymeric nanoparticles consisting of poly(ethylene glycol)-block-poly(lactide-co-glycolide) (PEG-PLGA) further demonstrates an improved safety profile and enhanced inhibitory activity against FIPV. In an in vitro model of antibody-dependent enhancement of FIPV infection, diphyllin nanoparticles showed a prominent antiviral effect against the feline coronavirus. In addition, the diphyllin nanoparticles were well tolerated in mice following high-dose intravenous administration. This study highlights the therapeutic potential of diphyllin and its nanoformulation for the treatment of FIP.

Direct interaction of the Golgi V-ATPase a-subunit isoform with PI(4)P drives localization of Golgi V-ATPases in yeast

PI(4)P directly interacts with the cytosolic domain of yeast Golgi vacuolar H+-ATPase (V-ATPase) a-isoform, Stv1, and the human Golgi a-subunit isoform. Lys-84 of Stv1 is essential for PI(4)P interaction, and localization of Stv1-containing V-ATPases in vivo requires the PI(4)P interaction. We propose that phosphatidylinositol binding exerts organelle-specific control over V-ATPases.

Luminal pH and phosphoinositide content are fundamental features of organelle identity. Vacuolar H⁺-ATPases (V-ATPases) drive organelle acidification in all eukaryotes, and membrane-bound a-subunit isoforms of the V-ATPase are implicated in organelle-specific targeting and regulation. Earlier work demonstrated that the endolysosomal lipid PI(3,5)P₂ activates V-ATPases containing the vacuolar a-subunit isoform in Saccharomyces cerevisiae. Here we demonstrate that PI(4)P, the predominant Golgi phosphatidylinositol (PI) species, directly interacts with the cytosolic amino terminal (NT) domain of the yeast Golgi V-ATPase a-isoform Stv1. Lysine-84 of Stv1NT is essential for interaction with PI(4)P in vitro and in vivo, and interaction with PI(4)P is required for efficient localization of Stv1-containing V-ATPases. The cytosolic NT domain of the human V-ATPase a2 isoform specifically interacts with PI(4)P in vitro, consistent with its Golgi localization and function. We propose that NT domains of V_o a-subunit isoforms interact specifically with PI lipids in their organelles of residence. These interactions can transmit organelle-specific targeting or regulation information to V-ATPases.

The interaction of the bioinsecticide PA1b (Pea Albumin 1 subunit b) with the insect V-ATPase triggers apoptosis

PA1b (Pea Albumin 1, subunit b) peptide is an entomotoxin, extracted from Legume seeds, with a lethal activity towards several insect pests, such as mosquitoes, some aphids and cereal weevils. This toxin acts by binding to the subunits c and e of the plasma membrane H+-ATPase (V-ATPase) in the insect midgut. In this study, two cereal weevils, the sensitive Sitophilus oryzae strain WAA42, the resistance Sitophilus oryzae strain ISOR3 and the insensitive red flour beetle Tribolium castaneum, were used in biochemical and histological experiments to demonstrate that a PA1b/V-ATPase interaction triggers the apoptosis mechanism, resulting in insect death. Upon intoxication with PA1b, apoptotic bodies are formed in the cells of the insect midgut. In addition, caspase-3 enzyme activity occurs in the midgut of sensitive weevils after intoxication with active PA1b, but not in the midgut of resistant weevils. These biochemical data were confirmed by immuno-histochemical detection of the caspase-3 active form in the midgut of sensitive weevils. Immuno-labelling experiments also revealed that the caspase-3 active form and V-ATPase are close-localized in the insect midgut. The results concerning this unique peptidic V-ATPase inhibitor pave the way for the utilization of PA1b as a promising, more selective and eco-friendly insecticide.

Twelve-membered macrolactones: privileged scaffolds for the development of new therapeutics

Natural products commonly produced as secondary metabolites of various plants and micro-organisms represent a diverse chemical space of compounds. The diversity of natural products makes them an attractive target for interrogation by both chemists and biologists alike. Indeed, the study of 12-membered macrolactones has already led to the discovery of lead drug compounds and new biological targets, which has motivated the development of diverted total synthetic routes to libraries of analogs. This review explores the discovery, biological characterization, and synthesis of several 12-membered macrolactones, exploiting examples that underscore their importance in the drug discovery field. It is our hope that this review will motivate further interest in this class of natural products, a group of molecules that we think merit the classification of 'privileged scaffolds' within the medicinal chemistry community, to further investigate and develop novel compounds with promising bioactivity.

Functional inhibition of acid sphingomyelinase by Fluphenazine triggers hypoxia-specific tumor cell death

Owing to lagging or insufficient neo-angiogenesis, hypoxia is a feature of most solid tumors. Hypoxic tumor regions contribute to resistance against antiproliferative chemotherapeutics, radiotherapy and immunotherapy. Targeting cells in hypoxic tumor areas is therefore an important strategy for cancer treatment. Most approaches for targeting hypoxic cells focus on the inhibition of hypoxia adaption pathways but only a limited number of compounds with the potential to specifically target hypoxic tumor regions have been identified. By using tumor spheroids in hypoxic conditions as screening system, we identified a set of compounds, including the phenothiazine antipsychotic Fluphenazine, as hits with novel mode of action. Fluphenazine functionally inhibits acid sphingomyelinase and causes cellular sphingomyelin accumulation, which induces cancer cell death specifically in hypoxic tumor spheroids. Moreover, we found that functional inhibition of acid sphingomyelinase leads to overactivation of hypoxia stress-response pathways and that hypoxia-specific cell death is mediated by the stress-responsive transcription factor ATF4. Taken together, the here presented data suggest a novel, yet unexplored mechanism in which induction of sphingolipid stress leads to the overactivation of hypoxia stress-response pathways and thereby promotes their pro-apoptotic tumor-suppressor functions to specifically kill cells in hypoxic tumor areas.

Covalent Modulators of the Vacuolar ATPase

The vacuolar H⁺ ATPase (V-ATPase) is a complex multisubunit machine that regulates important cellular processes through controlling acidity of intracellular compartments in eukaryotes. Existing small-molecule modulators of V-ATPase either are restricted to targeting one membranous subunit of V-ATPase or have poorly understood mechanisms of action. Small molecules with novel and defined mechanisms of inhibition are thus needed to functionally characterize V-ATPase and to fully evaluate the therapeutic relevance of V-ATPase in human diseases. We have discovered electrophilic quinazolines that covalently modify a soluble catalytic subunit of V-ATPase with high potency and exquisite proteomic selectivity as revealed by fluorescence imaging and chemical proteomic activity-based profiling. The site of covalent modification was mapped to a cysteine residue located in a region of V-ATPase subunit A that is thought to regulate the dissociation of V-ATPase. We further demonstrate that a previously reported V-ATPase inhibitor, 3-bromopyruvate, also targets the same cysteine residue and that our electrophilic quinazolines modulate the function of V-ATPase in cells. With their well-defined mechanism of action and high proteomic specificity, the described quinazolines offer a powerful set of chemical probes to investigate the physiological and pathological roles of V-ATPase.

Anti-leukemic effects of the V-ATPase inhibitor Archazolid A

Prognosis for patients suffering from T-ALL is still very poor and new strategies for T-ALL treatment are urgently needed. Our study shows potent anti-leukemic effects of the myxobacterial V-ATPase inhibitor Archazolid A. Archazolid A reduced growth and potently induced death of leukemic cell lines and human leukemic samples. By inhibiting lysosomal acidification, Archazolid A blocked activation of the Notch pathway, however, this was not the mechanism of V-ATPase inhibition relevant for cell death induction. In fact, V-ATPase inhibition by Archazolid A decreased the anti-apoptotic protein survivin. As underlying mode of action, this work is in line with recent studies from our group demonstrating that Archazolid A induced S-phase cell cycle arrest by interfering with the iron metabolism in leukemic cells. Our study provides evidence for V-ATPase inhibition as a potential new therapeutic option for T-ALL.

DMXL2 drives epithelial to mesenchymal transition in hormonal therapy resistant breast cancer through Notch hyper-activation

The acquisition of endocrine therapy resistance in estrogen receptor α (ERα) breast cancer patients represents a major clinical problem. Notch signalling has been extensively linked to breast cancer especially in patients who fail to respond to endocrine therapy. Following activation, Notch intracellular domain is released and enters the nucleus where activates transcription of target genes. The numerous steps that cascade after activation of the receptor complicate using Notch as biomarker. Hence, this warrants the development of reliable indicators of Notch activity. DMXL2 is a novel regulator of Notch signalling not yet investigated in breast cancer. Here, we demonstrate that DMXL2 is overexpressed in a subset of endocrine therapy resistant breast cancer cell lines where it promotes epithelial to mesenchymal transition through hyper-activation of Notch signalling via V-ATPase dependent acidification. Following DMXL2 depletion or treatment with Bafilomycin A1, both EMT targets and Notch signalling pathway significantly decrease. We show for the first time that DMXL2 protein levels are significantly increased in ERα positive breast cancer patients that progress after endocrine therapy. Finally, we demonstrate that DMXL2 is a transmembrane protein with a potential extra-cellular domain. These findings identify DMXL2 as a novel, functional biomarker for ERα positive breast cancer.

Isolation of Intact and Functional Melanosomes from the Retinal Pigment Epithelium

Melanosomes of retinal pigment epithelium (RPE) have many vision supporting functions. Melanosome research would benefit from a method to isolate pure and characterized melanosomes. Sucrose gradient centrifugation is the most commonly used method for isolation of RPE melanosomes, but the isolated products are insufficiently characterized and their quality is unclear. Here we introduce a new gentle method for fractionation of porcine RPE that produces intact functional melanosomes with minimal cross-contamination from other cell organelles. The characterization of isolated organelles was conducted with several methods confirming the purity of the isolated melanosomal fraction (transmission electron microscopy, immunoblotting) and presence of the melanosomal membrane (fluorescence staining of melanosomal membrane, zeta potential measurement). We demonstrate that our isolation method produces RPE melanosomes with the ability to generate free phosphate (Pi) from ATP thereby proving that many membrane proteins remain functional after isolation. The isolated porcine RPE melanosomes represented V-type H+ATPase activity that was demonstrated with bafilomycin A1, a specific V-ATPase inhibitor. We anticipate that the isolation method described here can easily be optimized for the isolation of stage IV melanosomes from other pigmented cell types and tissues.

ZT-25, a new vacuolar H(+)-ATPase inhibitor, induces apoptosis and protective autophagy through ROS generation in HepG2 cells

The vacuolar H(+)-ATPase (V-ATPase) has recently been proposed as a key target for new strategies in cancer treatment. Our previous work has proved that diphyllin glycoside is a novel inhibitor of V-ATPase. Here the cytotoxic effects of ZT-25, the most potent diphyllin glycoside derivatives, were studied and some of the underlying mechanisms were elucidated. ZT-25 displayed strong cytotoxicity on several cancer cell lines and relatively low cytotoxicity on human fetal hepatic cells (WRL-68) at submicromolar concentrations. In human hepatoma cells HepG2, ZT-25 induced G1/G0 phase arrest and apoptosis, as well as mitochondrial membrane potential (MMP) dissipation and ATP depletion. Furthermore, Bcl-2 protein decreased, while Bax protein and cleaved caspase-3 protein increased upon ZT-25 treatment. Benzyloxycarbony (Cbz)-l-Val-Ala-Asp (OMe)-fluoromethylketone (Z-VAD-FMK), a well-known pan-caspase inhibitor, attenuated ZT-25-induced cell death, suggesting the involvement of caspase-dependent pathway. Intriguingly, ZT-25 induced autophagy in HepG2 cells as characterized by increased the conversion of LC3 I to LC3 II, Beclin-1 expression and autophagosome formation. Meanwhile, p-mTOR expression was decreased which indicated that ZT-25-induced autophagy might be mediated through the suppression of mTOR pathway. Inhibition of autophagy by 3-methyladenine (3-MA) and chloroquine (CQ) obviously promoted ZT-25-induced cell death, suggesting the protective role of autophagy. Increased intracellular ROS level was found to be the early event in ZT-25-treated HepG2 cells. Inhibition of ROS generation by N-acetyl-l-cysteine (NAC) attenuated ZT-25-induced cell death and autophagy. Together, these results provide key insights into the ZT-25-induced cytotoxicity in HepG2 cells, which will have a great impact on the further development of diphyllin derivatives.

NR1D1 ameliorates Mycobacterium tuberculosis clearance through regulation of autophagy

NR1D1 (nuclear receptor subfamily 1, group D, member 1), an adopted orphan nuclear receptor, is widely known to orchestrate the expression of genes involved in various biological processes such as adipogenesis, skeletal muscle differentiation, and lipid and glucose metabolism. Emerging evidence suggests that various members of the nuclear receptor superfamily perform a decisive role in the modulation of autophagy. Recently, NR1D1 has been implicated in augmenting the antimycobacterial properties of macrophages and providing protection against Mycobacterium tuberculosis infection by downregulating the expression of the IL10 gene in human macrophages. This antiinfective property of NR1D1 suggests the need for an improved understanding of its role in other host-associated antimycobacterial pathways. The results presented here demonstrate that in human macrophages either ectopic expression of NR1D1 or treatment with its agonist, GSK4112, enhanced the number of acidic vacuoles as well as the level of MAP1LC3-II, a signature molecule for determination of autophagy progression, in a concentration- and time-dependent manner. Conversely, a decrease in NR1D1 in knockdown cells resulted in the reduced expression of lysosomal-associated membrane protein 1, LAMP1, commensurate with a decrease in the level of transcription factor EB, TFEB. This is indicative of that NR1D1 may have a regulatory role in lysosome biogenesis. NR1D1 being a repressor, its positive regulation on LAMP1 and TFEB is suggestive of an indirect byzantine mechanism of action. Its role in the modulation of autophagy and lysosome biogenesis together with its ability to repress IL10 gene expression supports the theory that NR1D1 has a pivotal antimycobacterial function in human macrophages.

Vacuolar-ATPase (V-ATPase) Mediates Progesterone-Induced Uterine Fluid Acidification in Rats

We hypothesized that progesterone-induced decrease in uterine fluid pH involves V-ATPase. In this study, expression and functional activity of V-ATPase in uterus were investigated under progesterone influence. Ovariectomized adult female rats received subcutaneous injection of estradiol-17β (1 µg/kg/day) or progesterone (20 mg/kg/day) for 3 days or 3 days estradiol-17β followed by 3 days vehicle, progesterone, or estradiol-17β plus progesterone. Mifepristone, a progesterone receptor blocker, was concomitantly given to the rats which received progesterone. A day after last injection, rate of uterine fluid secretion, its HCO3 (-) concentration, and pH were determined via in vivo uterine perfusion in rats under anesthesia. V-ATPase inhibitor, bafilomycin, was introduced into the perfusion buffer, and changes in these parameters were observed. Expression of V-ATPase A1 and B1/2 proteins and mRNAs in uterus were quantified by Western blotting and real-time PCR, respectively. Distribution of these proteins was observed by immunohistochemistry. Our findings showed that under progesterone influence, uterine fluid secretion rate, HCO3 (-) concentration, and pH were significantly reduced. Administration of bafilomycin did not cause significant changes in fluid secretion rate; however, HCO3 (-) concentration and pH were significantly elevated. In parallel with these changes, expression of V-ATPase A1 and B1/2 proteins and mRNAs were significantly increased with these proteins highly distributed in uterine luminal and glandular epithelia. In conclusion, increased expression and functional activity of V-ATPase were most likely responsible for the decreased in uterine fluid pH observed under progesterone influence.

Synthesis and structure-activity studies of the V-ATPase inhibitor saliphenylhalamide (SaliPhe) and simplified analogs

An efficient total synthesis of the potent V-ATPase inhibitor saliphenylhalamide (SaliPhe), a synthetic variant of the natural product salicylihalamide A (SaliA), has been accomplished aimed at facilitating the development of SaliPhe as an anticancer and antiviral agent. This new approach enabled facile access to derivatives for structure-activity relationship studies, leading to simplified analogs that maintain SaliPhe's biological properties. These studies will provide a solid foundation for the continued evaluation of SaliPhe and analogs as potential anticancer and antiviral agents.

Local anesthetics induce autophagy in young permanent tooth pulp cells

Pulp cells are essential for tooth development, and dentin repair and regeneration. In addition these cells have been identified as an important stem cell source. Local anesthetics are widely used in dental clinics, as well as the other clinical disciplines and have been suggested to interfere with human permanent tooth development and induce tooth agenesis through unknown mechanisms. Using pig model and human young permanent tooth pulp cells, our research has identified that the local anesthetics commonly used in clinics can affect cell proliferation. Molecular pathway profiling suggested that LC3II is one of the earliest molecules induced by the agents and p62 is the only common downstream target identified for all the drugs tested. The effect of the drugs could be partially recovered by V-ATPase inhibitor only if early intervention is performed. Our results provide novel evidence that local anesthetics could affect tooth cell growth that potentially can have impacts on tooth development.

Single-Flask Multicomponent Palladium-Catalyzed α,γ-Coupling of Ketone Enolates: Facile Preparation of Complex Carbon Scaffolds

A three-component palladium-catalyzed reaction sequence has been developed in which γ-substituted α,β-unsaturated products are obtained in a single flask by an α-alkenylation with either a subsequent γ-alkenylation or γ-arylation of a ketone enolate. Coupling of a variety of electronically and structurally different components was achieved in the presence of a Pd/Q-Phos catalyst (2 mol %), usually at 22 °C with yields of up to 85 %. Most importantly, access to these products is obtained in one simple operation in place of employing multiple reactions.

The vacuolar-type H⁺-ATPase at a glance - more than a proton pump

The vacuolar H(+)-ATPase (V-ATPase) has long been appreciated to function as an electrogenic H(+) pump. By altering the pH of intracellular compartments, the V-ATPase dictates enzyme activity, governs the dissociation of ligands from receptors and promotes the coupled transport of substrates across membranes, a role often aided by the generation of a transmembrane electrical potential. In tissues where the V-ATPase is expressed at the plasma membrane, it can serve to acidify the extracellular microenvironment. More recently, however, the V-ATPase has been implicated in a bewildering variety of additional roles that seem independent of its ability to translocate H(+). These non-canonical functions, which include fusogenicity, cytoskeletal tethering and metabolic sensing, are described in this Cell Science at a Glance article and accompanying poster, together with a brief overview of the conventional functions of the V-ATPase.

Transmembrane proteins--Mining the cattle tick transcriptome

Managing the spread and load of pathogen-transmitting ticks is an important task worldwide. The cattle tick, Rhipicephalus microplus, not only impacts the economy through losses in dairy and meat production, but also raises concerns for human health in regards to the potential of certain transmitted pathogens becoming zoonotic. However, novel strategies to control R. microplus are hindered by lack of understanding tick biology and the discovery of suitable vaccine or acaricide targets. The importance of transmembrane proteins as vaccine targets are well known, as is the case in tick vaccines with Bm86 as antigen. In this study, we describe the localization and functional annotation of 878 putative transmembrane proteins. Thirty proteins could be confirmed in the R. microplus gut using LC-MS/MS analysis and their roles in tick biology are discussed. To the best of our knowledge, 19 targets have not been reported before in any proteomics study in various tick species and the possibility of using the identified proteins as targets for tick control are discussed. Although tissue expression of identified putative proteins through expansive proteomics is necessary, this study demonstrates the possibility of using bioinformatics for the identification of targets for further evaluation in tick control strategies.

A novel lung explant model for the ex vivo study of efficacy and mechanisms of anti-influenza drugs

Influenza A virus causes considerable morbidity and mortality largely because of a lack of effective antiviral drugs. Viral neuraminidase inhibitors, which inhibit viral release from the infected cell, are currently the only approved drugs for influenza, but have recently been shown to be less effective than previously thought. Growing resistance to therapies that target viral proteins has led to increased urgency in the search for novel anti-influenza compounds. However, discovery and development of new drugs have been restricted because of differences in susceptibility to influenza between animal models and humans and a lack of translation between cell culture and in vivo measures of efficacy. To circumvent these limitations, we developed an experimental approach based on ex vivo infection of human bronchial tissue explants and optimized a method of flow cytometric analysis to directly quantify infection rates in bronchial epithelial tissues. This allowed testing of the effectiveness of TVB024, a vATPase inhibitor that inhibits viral replication rather than virus release, and to compare efficacy with the current frontline neuraminidase inhibitor, oseltamivir. The study showed that the vATPase inhibitor completely abrogated epithelial cell infection, virus shedding, and the associated induction of proinflammatory mediators, whereas oseltamivir was only partially effective at reducing these mediators and ineffective against innate responses. We propose, therefore, that this explant model could be used to predict the efficacy of novel anti-influenza compounds targeting diverse stages of the viral replication cycle, thereby complementing animal models and facilitating progression of new drugs into clinical trials.

Discovery of a Small-Molecule Probe for V-ATPase Function

Lysosomes perform a critical cellular function as a site of degradation for diverse cargoes including proteins, organelles, and pathogens delivered through distinct pathways, and defects in lysosomal function have been implicated in a number of diseases. Recent studies have elucidated roles for the lysosome in the regulation of protein synthesis, metabolism, membrane integrity, and other processes involved in homeostasis. Complex small-molecule natural products have greatly contributed to the investigation of lysosomal function in cellular physiology. Here we report the discovery of a novel, small-molecule modulator of lysosomal acidification derived from diversity-oriented synthesis through high-content screening.

Design and synthesis of analogues of natural products

In this article strategies for the design and synthesis of natural product analogues are summarized and illustrated with some selected examples.

Novel techniques in the development of osteoporosis drug therapy: the osteoclast ruffled-border vacuolar H(+)-ATPase as an emerging target

INTRODUCTION

Bone loss occurs in many diseases, including osteoporosis, rheumatoid arthritis and periodontal disease. For osteoporosis alone, it is estimated that 75 million people are afflicted worldwide, with high risks of fractures and increased morbidity and mortality. The demand for treatment consumes an ever-increasing share of healthcare resources. Successive generations of antiresorptive bisphosphonate drugs have reduced side effects, minimized frequency of dosing, and increased efficacy in halting osteoporotic bone loss, but their shortcomings have remained significant to the extent that a monoclonal antibody antiresorptive has recently taken a significant market share. Yet this latter, paradigm-shifting approach has its own drawbacks.

AREAS COVERED

This review summarizes recent literature on bone-remodeling cell and molecular biology and the background for existing approaches and emerging therapeutics and targets for treating osteoporosis. The authors discuss vacuolar H(+)-ATPase (V-ATPase) molecular biology and the recent advances in targeting the osteoclast ruffled-border V-ATPase (ORV) for the development of novel antiresorptive drugs. They also cover examples from the V-ATPase-targeted drug discovery literature, including conventional molecular biology methods, in silico drug discovery, and gene therapy in more detail as proofs of concept.

EXPERT OPINION

Existing therapeutic options for osteoporosis have limitations and inherent drawbacks. Thus, the search for novel approaches to osteoporosis drug discovery remains relevant. Targeting the ORV may be one of the more selective means of regulating bone resorption. Furthermore, this approach may be effective without removing active osteoclasts from the finely balanced osteoclast-osteoblast coupling required for normal bone remodeling.

Mutations in the proteolipid subunits of the vacuolar H+-ATPase provide resistance to indolotryptoline natural products

Indolotryptoline natural products represent a small family of structurally unique chromopyrrolic acid-derived antiproliferative agents. Like many prospective anticancer agents before them, the exploration of their potential clinical utility has been hindered by the limited information known about their mechanism of action. To study the mode of action of two closely related indolotryptolines (BE-54017, cladoniamide A), we selected for drug resistant mutants using a multidrug resistance-suppressed (MDR-sup) Schizosaccharomyces pombe strain. As fission yeast maintains many of the basic cancer-relevant cellular processes present in human cells, it represents an appealing model to use in determining the potential molecular target of antiproliferative natural products through resistant mutant screening. Full genome sequencing of resistant mutants identified mutations in the c and c′ subunits of the proteolipid substructure of the vacuolar H⁺-ATPase complex (V-ATPase). This collection of resistance-conferring mutations maps to a site that is distant from the nucleotide-binding sites of V-ATPase and distinct from sites found to confer resistance to known V-ATPase inhibitors. Acid vacuole staining, cross-resistance studies, and direct c/c′ subunit mutagenesis all suggest that indolotryptolines are likely a structurally novel class of V-ATPase inhibitors. This work demonstrates the general utility of resistant mutant selection using MDR-sup S. pombe as a rapid and potentially systematic approach for studying the modes of action of cytotoxic natural products.

Vacuolin-1 potently and reversibly inhibits autophagosome-lysosome fusion by activating RAB5A

Autophagy is a catabolic lysosomal degradation process essential for cellular homeostasis and cell survival. Dysfunctional autophagy has been associated with a wide range of human diseases, e.g., cancer and neurodegenerative diseases. A large number of small molecules that modulate autophagy have been widely used to dissect this process and some of them, e.g., chloroquine (CQ), might be ultimately applied to treat a variety of autophagy-associated human diseases. Here we found that vacuolin-1 potently and reversibly inhibited the fusion between autophagosomes and lysosomes in mammalian cells, thereby inducing the accumulation of autophagosomes. Interestingly, vacuolin-1 was less toxic but at least 10-fold more potent in inhibiting autophagy compared with CQ. Vacuolin-1 treatment also blocked the fusion between endosomes and lysosomes, resulting in a defect in general endosomal-lysosomal degradation. Treatment of cells with vacuolin-1 alkalinized lysosomal pH and decreased lysosomal Ca²⁺ content. Besides marginally inhibiting vacuolar ATPase activity, vacuolin-1 treatment markedly activated RAB5A GTPase activity. Expression of a dominant negative mutant of RAB5A or RAB5A knockdown significantly inhibited vacuolin-1-induced autophagosome-lysosome fusion blockage, whereas expression of a constitutive active form of RAB5A suppressed autophagosome-lysosome fusion. These data suggest that vacuolin-1 activates RAB5A to block autophagosome-lysosome fusion. Vacuolin-1 and its analogs present a novel class of drug that can potently and reversibly modulate autophagy.

Intracellular trafficking of AIP56, an NF-κB-cleaving toxin from Photobacterium damselae subsp. piscicida

AIP56 (apoptosis-inducing protein of 56 kDa) is a metalloprotease AB toxin secreted by Photobacterium damselae subsp. piscicida that acts by cleaving NF-κB. During infection, AIP56 spreads systemically and depletes phagocytes by postapoptotic secondary necrosis, impairing the host phagocytic defense and contributing to the genesis of infection-associated necrotic lesions. Here we show that mouse bone marrow-derived macrophages (mBMDM) intoxicated by AIP56 undergo NF-κB p65 depletion and apoptosis. Similarly to what was reported for sea bass phagocytes, intoxication of mBMDM involves interaction of AIP56 C-terminal region with cell surface components, suggesting the existence of a conserved receptor. Biochemical approaches and confocal microscopy revealed that AIP56 undergoes clathrin-dependent endocytosis, reaches early endosomes, and follows the recycling pathway. Translocation of AIP56 into the cytosol requires endosome acidification, and an acidic pulse triggers translocation of cell surface-bound AIP56 into the cytosol. Accordingly, at acidic pH, AIP56 becomes more hydrophobic, interacting with artificial lipid bilayer membranes. Altogether, these data indicate that AIP56 is a short-trip toxin that reaches the cytosol using an acidic-pH-dependent mechanism, probably from early endosomes. Usually, for short-trip AB toxins, a minor pool reaches the cytosol by translocating from endosomes, whereas the rest is routed to lysosomes for degradation. Here we demonstrate that part of endocytosed AIP56 is recycled back and released extracellularly through a mechanism requiring phosphoinositide 3-kinase (PI3K) activity but independent of endosome acidification. So far, we have been unable to detect biological activity of recycled AIP56, thereby bringing into question its biological relevance as well as the importance of the recycling pathway.

Cloning, expression and purification of subunit H of vacuolar H⁺-ATPase from Mythimna separata Walker (Lepidoptera: Noctuidae)

The vacuolar (H+)-ATPase (V-ATPase) of insect, which is composed of membrane-bound V0 complex and peripheral V1 complex, participates in lots of important physiological process. Subunit H, as a subunit of V1 complex, plays a vital role in bridging the communication between V1 and V0 complexes and interaction with other proteins. Yeast subunit H has been successfully crystallized through expression in E. coli, but little is known about the structure of insect subunit H. In this study, we cloned, expressed and purified the subunit H from midgut of Mythimna separata Walker. Through RACE (rapidly amplification of cDNA ends) technique, we got 1807 bp full length of subunit H, and to keep the nature structure of subunit H, we constructed Baculovirus expression vector with His-tag in the C-terminal and expressed the recombinant protein in insect sf9 cells, thereafter, purified the recombinant protein by Ni-NTA columns. Results of SDS-PAGE, western blotting and mass spectrometry showed that the recombinant protein was successfully expressed. The method of expressing and purifying M. separata subunit H will provide a foundation for obtaining the crystal of subunit H and further study of the design of novel insecticides based on its structure and function.

Exploring a Ring-Closing Metathesis Approach to the Archazolid Macrocycle

The synthesis of a new "eastern domain" of a proposed dihydroarchazolid analogue is described along with initial results from attempted macrocyclization by RCM. This work has revealed a competing backbiting process that may be preventable by alcohol functionalization and installation of a metathesis relay.

Targeting V-ATPase in primary human monocytes by archazolid potently represses the classical secretion of cytokines due to accumulation at the endoplasmic reticulum

The macrolide archazolid inhibits vacuolar-type H(+)-ATPase (V-ATPase), a proton-translocating enzyme involved in protein transport and pH regulation of cell organelles, and potently suppresses cancer cell growth at low nanomolar concentrations. In view of the growing link between inflammation and cancer, we investigated whether inhibition of V-ATPase by archazolid may affect primary human monocytes that can promote cancer by sustaining inflammation through the release of tumor-promoting cytokines. Human primary monocytes express V-ATPase, and archazolid (10-100nM) increases the vesicular pH in these cells. Archazolid (10nM) markedly reduced the release of pro-inflammatory (TNF-α, interleukin-6 and -8) but also of anti-inflammatory (interleukin-10) cytokines in monocytes stimulated with LPS, without affecting cell viability up to 1000nM. Of interest, secretion of interleukin-1β was increased by archazolid. Comparable effects were obtained by the V-ATPase inhibitors bafilomycin and apicularen. The phosphorylation of p38 MAPK and ERK-1/2, Akt, SAPK/JNK or of the inhibitor of NFκB (IκBα) as well as mRNA expression of IL-8 were not altered by archazolid in LPS-stimulated monocytes. Instead, archazolid caused endoplasmic reticulum (ER) stress response visualized by increased BiP expression and accumulation of IL-8 (and TNF-α) at the ER, indicating a perturbation of protein secretion. In conclusion, by interference with V-ATPase, archazolid significantly affects the secretion of cytokines due to accumulation at the ER which might be of relevance when using these agents for cancer therapy.

Palladium-catalyzed alkenylation of ketone enolates under mild conditions

A protocol for a mild, catalytic, intermolecular alkenylation of ketone enolates has been developed using a Pd/Q-Phos catalyst. Efficient intermolecular coupling of a variety of ketones with alkenyl bromides was achieved with a slight excess of LiHMDS and temperatures down to 0 °C.

PA1b inhibitor binding to subunits c and e of the vacuolar ATPase reveals its insecticidal mechanism

The vacuolar ATPase (V-ATPase) is a 1MDa transmembrane proton pump that operates via a rotary mechanism fuelled by ATP. Essential for eukaryotic cell homeostasis, it plays central roles in bone remodeling and tumor invasiveness, making it a key therapeutic target. Its importance in arthropod physiology also makes it a promising pesticide target. The major challenge in designing lead compounds against the V-ATPase is its ubiquitous nature, such that any therapeutic must be capable of targeting particular isoforms. Here, we have characterized the binding site on the V-ATPase of pea albumin 1b (PA1b), a small cystine knot protein that shows exquisitely selective inhibition of insect V-ATPases. Electron microscopy shows that PA1b binding occurs across a range of equivalent sites on the c ring of the membrane domain. In the presence of Mg·ATP, PA1b localizes to a single site, distant from subunit a, which is predicted to be the interface for other inhibitors. Photoaffinity labeling studies show radiolabeling of subunits c and e. In addition, weevil resistance to PA1b is correlated with bafilomycin resistance, caused by mutation of subunit c. The data indicate a binding site to which both subunits c and e contribute and inhibition that involves locking the c ring rotor to a static subunit e and not subunit a. This has implications for understanding the V-ATPase mechanism and that of inhibitors with therapeutic or pesticidal potential. It also provides the first evidence for the position of subunit e within the complex.

Vas deferens neuro-effector junction: from kymographic tracings to structural biology principles

The vas deferens is a simple bioassay widely used to study the physiology of sympathetic neurotransmission and the pharmacodynamics of adrenergic drugs. The role of ATP as a sympathetic co-transmitter has gained increasing attention and furthered our understanding of its role in sympathetic reflexes. In addition, new information has emerged on the mechanisms underlying the storage and release of ATP. Both noradrenaline and ATP concur to elicit the tissue smooth muscle contractions following sympathetic reflexes or electrical field stimulation of the sympathetic nerve terminals. ATP and adenosine (its metabolic byproduct) are powerful presynaptic regulators of co-transmitter actions. In addition, neuropeptide Y, the third member of the sympathetic triad, is an endogenous modulator. The peptide plus ATP and/or adenosine play a significant role as sympathetic modulators of transmitter's release. This review focuses on the physiological principles that govern sympathetic co-transmitter activity, with special interest in defining the motor role of ATP. In addition, we intended to review the recent structural biology findings related to the topology of the P2X1R based on the crystallized P2X4 receptor from Danio rerio, or the crystallized adenosine A2A receptor as a member of the G protein coupled family of receptors as prototype neuro modulators. This review also covers structural elements of ectonucleotidases, since some members are found in the vas deferens neuro-effector junction. The allosteric principles that apply to purinoceptors are also reviewed highlighting concepts derived from receptor theory at the light of the current available structural elements. Finally, we discuss clinical applications of these concepts.

The NuRD complex cooperates with DNMTs to maintain silencing of key colorectal tumor suppressor genes

Many tumor suppressor genes (TSGs) are silenced through synergistic layers of epigenetic regulation including abnormal DNA hypermethylation of promoter CpG islands, repressive chromatin modifications and enhanced nucleosome deposition over transcription start sites. The protein complexes responsible for silencing of many of such TSGs remain to be identified. Our previous work demonstrated that multiple silenced TSGs in colorectal cancer cells can be partially reactivated by DNA demethylation in cells disrupted for the DNA methyltransferases 1 and 3B (DNMT1 and 3B) or by DNMT inhibitors (DNMTi). Herein, we used proteomic and functional genetic approaches to identify additional proteins that cooperate with DNMTs in silencing these key silenced TSGs in colon cancer cells. We discovered that DNMTs and the core components of the NuRD (Mi-2/nucleosome remodeling and deacetylase) nucleosome remodeling complex, chromo domain helicase DNA-binding protein 4 (CHD4) and histone deacetylase 1 (HDAC1) occupy the promoters of several of these hypermethylated TSGs and physically and functionally interact to maintain their silencing. Consistent with this, we find an inverse relationship between expression of HDAC1 and 2 and these TSGs in a large panel of primary colorectal tumors. We demonstrate that DNMTs and NuRD cooperate to maintain the silencing of several negative regulators of the WNT and other signaling pathways. We find that depletion of CHD4 is synergistic with DNMT inhibition in reducing the viability of colon cancer cells in correlation with reactivation of TSGs, suggesting that their combined inhibition may be beneficial for the treatment of colon cancer. Since CHD4 has ATPase activity, our data identify CHD4 as a potentially novel drug target in cancer.

A functional screen for copper homeostasis genes identifies a pharmacologically tractable cellular system

BACKGROUND

Copper is essential for the survival of aerobic organisms. If copper is not properly regulated in the body however, it can be extremely cytotoxic and genetic mutations that compromise copper homeostasis result in severe clinical phenotypes. Understanding how cells maintain optimal copper levels is therefore highly relevant to human health.

RESULTS

We found that addition of copper (Cu) to culture medium leads to increased respiratory growth of yeast, a phenotype which we then systematically and quantitatively measured in 5050 homozygous diploid deletion strains. Cu's positive effect on respiratory growth was quantitatively reduced in deletion strains representing 73 different genes, the function of which identify increased iron uptake as a cause of the increase in growth rate. Conversely, these effects were enhanced in strains representing 93 genes. Many of these strains exhibited respiratory defects that were specifically rescued by supplementing the growth medium with Cu. Among the genes identified are known and direct regulators of copper homeostasis, genes required to maintain low vacuolar pH, and genes where evidence supporting a functional link with Cu has been heretofore lacking. Roughly half of the genes are conserved in man, and several of these are associated with Mendelian disorders, including the Cu-imbalance syndromes Menkes and Wilson's disease. We additionally demonstrate that pharmacological agents, including the approved drug disulfiram, can rescue Cu-deficiencies of both environmental and genetic origin.

CONCLUSIONS

A functional screen in yeast has expanded the list of genes required for Cu-dependent fitness, revealing a complex cellular system with implications for human health. Respiratory fitness defects arising from perturbations in this system can be corrected with pharmacological agents that increase intracellular copper concentrations.

Attenuation of Candida albicans virulence with focus on disruption of its vacuole functions

The objective of the present review is to discuss if the yeast vacuole can be used as a target for attenuation of Candida albicans virulence. Literature searches were made electronically using predetermined inclusion criteria. The main searches were made through a systematic strategy in PubMed and authoritative journals in microbiology. It appeared that C. albicans virulence may be reduced by inhibiting vacuolar proton-translocating ATPase (V-ATPase) functions and acidification of the yeast vacuole by V-ATPase inhibitors, by seeking the synergistic effect of antifungals and non-antifungals affecting yeast vacuolar functions, and by inhibiting filament production – also regulated by the vacuole. Accordingly, we may impair C. albicans virulence by inhibiting functions of its vacuole, which plays essential roles during colonization and invasion of the host. Except for drugs where indications for clinical use can be redefined, such interventions may be closer to theory than to reality at the moment. But since the yeast is so difficult to eradicate by antifungal treatment, it could be rewarding to seek new strategies for reducing its virulence rather than trying to eradicate it completely from the microbiota, which often turns out to be impossible.

Advances in targeting the vacuolar proton-translocating ATPase (V-ATPase) for anti-fungal therapy

Vacuolar proton-translocating ATPase (V-ATPase) is a membrane-bound, multi-subunit enzyme that uses the energy of ATP hydrolysis to pump protons across membranes. V-ATPase activity is critical for pH homeostasis and organelle acidification as well as for generation of the membrane potential that drives secondary transporters and cellular metabolism. V-ATPase is highly conserved across species and is best characterized in the model fungus Saccharomyces cerevisiae. However, recent studies in mammals have identified significant alterations from fungi, particularly in the isoform composition of the 14 subunits and in the regulation of complex disassembly. These differences could be exploited for selectivity between fungi and humans and highlight the potential for V-ATPase as an anti-fungal drug target. Candida albicans is a major human fungal pathogen and causes fatality in 35% of systemic infections, even with anti-fungal treatment. The pathogenicity of C. albicans correlates with environmental, vacuolar, and cytoplasmic pH regulation, and V-ATPase appears to play a fundamental role in each of these processes. Genetic loss of V-ATPase in pathogenic fungi leads to defective virulence, and a comprehensive picture of the mechanisms involved is emerging. Recent studies have explored the practical utility of V-ATPase as an anti-fungal drug target in C. albicans, including pharmacological inhibition, azole therapy, and targeting of downstream pathways. This overview will discuss these studies as well as hypothetical ways to target V-ATPase and novel high-throughput methods for use in future drug discovery screens.

Vacuolar H+-ATPase: An Essential Multitasking Enzyme in Physiology and Pathophysiology

Vacuolar H+-ATPases (V-ATPases) are large multisubunit proton pumps that are required for housekeeping acidification of membrane-bound compartments in eukaryotic cells. Mammalian V-ATPases are composed of 13 different subunits. Their housekeeping functions include acidifying endosomes, lysosomes, phagosomes, compartments for uncoupling receptors and ligands, autophagosomes, and elements of the Golgi apparatus. Specialized cells, including osteoclasts, intercalated cells in the kidney and pancreatic beta cells, contain both the housekeeping V-ATPases and an additional subset of V-ATPases, which plays a cell type specific role. The specialized V-ATPases are typically marked by the inclusion of cell type specific isoforms of one or more of the subunits. Three human diseases caused by mutations of isoforms of subunits have been identified. Cancer cells utilize V-ATPases in unusual ways; characterization of V-ATPases may lead to new therapeutic modalities for the treatment of cancer. Two accessory proteins to the V-ATPase have been identified that regulate the proton pump. One is the (pro)renin receptor and data is emerging that indicates that V-ATPase may be intimately linked to renin/angiotensin signaling both systemically and locally. In summary, V-ATPases play vital housekeeping roles in eukaryotic cells. Specialized versions of the pump are required by specific organ systems and are involved in diseases.