Decreased sensory nerve excitation and bone pain associated with mouse Lewis lung cancer in TRPV1-deficient mice

Bone pain is one of the most common and life-limiting complications of cancer metastasis to bone. Although the mechanism of bone pain still remains poorly understood, bone pain is evoked as a consequence of sensitization and excitation of sensory nerves (SNs) innervating bone by noxious stimuli produced in the microenvironment of bone metastases. We showed that bone is innervated by calcitonin gene-related protein (CGRP)⁺ SNs extending from dorsal root ganglia (DRG), the cell body of SNs, in mice. Mice intratibially injected with Lewis lung cancer (LLC) cells showed progressive bone pain evaluated by mechanical allodynia and flinching with increased CGRP⁺ SNs in bone and augmented SN excitation in DRG as indicated by elevated numbers of pERK- and pCREB-immunoreactive neurons. Immunohistochemical examination of LLC-injected bone revealed that the tumor microenvironment is acidic. Bafilomycin A1, a selective inhibitor of H⁺ secretion from vacuolar proton pump, significantly alleviated bone pain, indicating that the acidic microenvironment contributes to bone pain. We then determined whether the transient receptor potential vanilloid 1 (TRPV1), a major acid-sensing nociceptor predominantly expressed on SNs, plays a role in bone pain by intratibially injecting LLC cells in TRPV1-deficient mice. Bone pain and SN excitation in the DRG and spinal dorsal horn were significantly decreased in TRPV1 ^-/- mice compared with wild-type mice. Our results suggest that TRPV1 activation on SNs innervating bone by the acidic cancer microenvironment in bone contributes to SN activation and bone pain. Targeting acid-activated TRPV1 is a potential therapeutic approach to cancer-induced bone pain.

Inhibition of pH regulation as a therapeutic strategy in hypoxic human breast cancer cells

Hypoxic cancer cells exhibit resistance to many therapies. This study compared the therapeutic effect of targeting the pH regulatory proteins (CAIX, NHE1 and V-ATPase) that permit cancer cells to adapt to hypoxic conditions, using both 2D and 3D culture models. Drugs targeting CAIX, NHE1 and V-ATPase exhibited anti-proliferative effects in MCF-7, MDA-MB-231 and HBL-100 breast cancer cell lines in 2D. Protein and gene expression analysis in 2D showed that CAIX was the most hypoxia-inducible protein of the 3 targets. However, the expression of CAIX differed between the 3 cell lines. This difference in CAIX expression in hypoxia was consistent with a varying activity of FIH-1 between the cell lines. 3D expression analysis demonstrated that both CAIX and NHE1 were up-regulated in the hypoxic areas of multicellular tumor spheroids. However, the induction of CAIX expression in hypoxia was again cell line dependent. 3D invasion assays conducted with spheroids showed that CAIX inhibition significantly reduced the invasion of cells. Finally, the capability of both NHE1 and CAIX inhibitors to combine effectively with irradiation was exhibited in clonogenic assays. Proteomic-mass-spectrometric analysis indicated that CAIX inhibition might be combining with irradiation through stimulating apoptotic cell death. Of the three proteins, CAIX represents the target with the most promise for the treatment of breast cancer.

Some assembly required: Contributions of Tom Stevens' lab to the V-ATPase field

Tom Stevens' lab has explored the subunit composition and assembly of the yeast V-ATPase for more than 30 years. Early studies helped establish yeast as the predominant model system for study of V-ATPase proton pumps and led to the discovery of protein splicing of the V-ATPase catalytic subunit. The Vma^- phenotype, characteristic of loss-of-V-ATPase activity in yeast was key in determining the enzyme's subunit composition via yeast genetics. V-ATPase subunit composition proved to be highly conserved among eukaryotes. Genetic screens for new vma mutants led to identification of a set of dedicated V-ATPase assembly factors and helped unravel the complex pathways for V-ATPase assembly. In later years, exploration of the evolutionary history of several V-ATPase subunits provided new information about the enzyme's structure and function. This review highlights V-ATPase work in the Stevens' lab between 1987 and 2017.

Cooperative electrogenic proton transport pathways in the plasma membrane of the proton-secreting osteoclast

A proton is a ubiquitous signaling ion. Many transmembrane H⁺ transport pathways either maintain pH homeostasis or generate acidic compartments. The osteoclast is a bone-resorbing cell, which degrades bone tissues by secreting protons and lysosomal enzymes into the resorption pit. The plasma membrane facing bone tissue (ruffled border), generated partly by fusion of lysosomes, may mimic H⁺ flux mechanisms regulating acidic vesicles. We identified three electrogenic H⁺-fluxes in osteoclast plasma membranes-a vacuolar H⁺-ATPase (V-ATPase), a voltage-gated proton channel (Hv channel) and an acid-inducible H⁺-leak-whose electrophysiological profiles and regulation mechanisms differed. V-ATPase and Hv channel, both may have intracellular reservoirs, but the recruitment/internalization is regulated independently. V-ATPase mediates active H⁺ efflux, acidifying the resorption pit, while acid-inducible H⁺ leak, activated at an extracellular pH < 5.5, diminishes pit acidification, possibly to protect bone from excess degradation. The two-way H⁺ flux mechanisms in opposite directions may have advantages in fine regulation of pit pH. Hv channel mediates passive H⁺ efflux. Although its working ranges are limited, the amount of H⁺ extrusion is 100 times larger than those of the V-ATPase and may support reactive oxygen species production during osteoclastogenesis. Extracellular Ca²⁺, H⁺ and inorganic phosphate, which accumulate in the resorption pit, will either stimulate or inhibit these H⁺ fluxes. Skeletal integration is disrupted by too much or too less of bone resorption. Diversities in plasma membrane H⁺ flux pathways, which may co-operate or compete, are essential to adjust osteoclast functions in variable conditions.

V-ATPase inhibition increases cancer cell stiffness and blocks membrane related Ras signaling - a new option for HCC therapy

Hepatocellular carcinoma (HCC) is the fifth most frequent cancer worldwide and the third leading cause of cancer-related death. However, therapy options are limited leaving an urgent need to develop new strategies. Currently, targeting cancer cell lipid and cholesterol metabolism is gaining interest especially regarding HCC. High cholesterol levels support proliferation, membrane-related mitogenic signaling and increase cell softness, leading to tumor progression, malignancy and invasive potential. However, effective ways to target cholesterol metabolism for cancer therapy are still missing. The V-ATPase inhibitor archazolid was recently shown to interfere with cholesterol metabolism. In our study, we report a novel therapeutic potential of V-ATPase inhibition in HCC by altering the mechanical phenotype of cancer cells leading to reduced proliferative signaling. Archazolid causes cellular depletion of free cholesterol leading to an increase in cell stiffness and membrane polarity of cancer cells, while hepatocytes remain unaffected. The altered membrane composition decreases membrane fluidity and leads to an inhibition of membrane-related Ras signaling resulting decreased proliferation in vitro and in vivo. V-ATPase inhibition represents a novel link between cell biophysical properties and proliferative signaling selectively in malignant HCC cells, providing the basis for an attractive and innovative strategy against HCC.

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.

pH homeostasis links the nutrient sensing PKA/TORC1/Sch9 ménage-à-trois to stress tolerance and longevity

The plasma membrane H⁺-ATPase Pma1 and the vacuolar V-ATPase act in close harmony to tightly control pH homeostasis, which is essential for a vast number of physiological processes. As these main two regulators of pH are responsive to the nutritional status of the cell, it seems evident that pH homeostasis acts in conjunction with nutrient-induced signalling pathways. Indeed, both PKA and the TORC1-Sch9 axis influence the proton pumping activity of the V-ATPase and possibly also of Pma1. In addition, it recently became clear that the proton acts as a second messenger to signal glucose availability via the V-ATPase to PKA and TORC1-Sch9. Given the prominent role of nutrient signalling in longevity, it is not surprising that pH homeostasis has been linked to ageing and longevity as well. A first indication is provided by acetic acid, whose uptake by the cell induces toxicity and affects longevity. Secondly, vacuolar acidity has been linked to autophagic processes, including mitophagy. In agreement with this, a decline in vacuolar acidity was shown to induce mitochondrial dysfunction and shorten lifespan. In addition, the asymmetric inheritance of Pma1 has been associated with replicative ageing and this again links to repercussions on vacuolar pH. Taken together, accumulating evidence indicates that pH homeostasis plays a prominent role in the determination of ageing and longevity, thereby providing new perspectives and avenues to explore the underlying molecular mechanisms.

The role of endomembrane-localized VHA-c in plant growth

In plant cells, the vacuolar-type H+-ATPase (V-ATPase), a large multis`ubunit endomembrane proton pump, plays an important role in acidification of subcellular organelles, pH and ion homeostasis, and endocytic and secretory trafficking. V-ATPase subunit c (VHA-c) is essential for V-ATPase assembly, and is directly responsible for binding and transmembrane transport of protons. In previous studies, we identified a PutVHA-c gene from Puccinellia tenuiflora, and investigated its function in plant growth. Subcellular localization revealed that PutVHA-c is mainly localized in endosomal compartments. Overexpression of PutVHA-c enhanced V-ATPase activity and promoted plant growth in transgenic Arabidopsis. Furthermore, the activity of V-ATPase affected intracellular transport of the Golgi-derived endosomes. Our results showed that endomembrane localized-VHA-c contributes to plant growth by influencing V-ATPase-dependent endosomal trafficking. Here, we discuss these recent findings and speculate on the VHA-c mediated molecular mechanisms involved in plant growth, providing a better understanding of the functions of VHA-c and V-ATPase.

Testosterone up-regulates vacuolar ATPase expression and functional activities in vas deferens of orchidectomized rats

Precise regulation of vas deferens fluid pH is essential for sperm. However, the mechanisms underlying effect of testosterone on vas deferens fluid pH have never been identified, which could involve changes in expression and functional activity of vacoular (V)-ATPase.

METHODS

Orchidectomized, adult male Sprague-Dawley rats were treated subcutaneously with 125 μg/kg/day and 250 μg/kg/day testosterone with or without flutamide (androgen receptor blocker) and finasteride (5α-reductase inhibitor) for seven (7) days. Following treatment completion, in vivo perfusion of vas deferens lumen was performed and changes in fluid secretion rate, pH and HCO₃^- content were measured with and without bafilomycin, a V-ATPase inhibitor. Rats were then sacrificed and vas deferens were harvested and subjected for V-ATPase A1 and B1/2 protein expression and distribution analysis by western blotting and immunohistochemistry, respectively.

RESULTS

In sham-operated and testosterone-treated orchidectomized rats, higher fluid secretion rate, which was not antagonized by bafilomycin but lower HCO₃^- content and pH which were antagonized by bafilomycin were observed when compared to orchidectomized-only and orchidectomized, testosterone-treated rats receiving flutamide or finasteride, respectively. Bafilomycin had no effect on fluid secretion rate, HCO₃^- content and pH in orchidectomized and testosterone-treated orchidectomized rats receiving flutamide and finasteride. V-ATPase A1 and B1/2 proteins were expressed at high levels in vas deferens and were highly distributed at the apical membrane of luminal epithelium and in muscle layer of this organ, mainly in sham and testosterone-treated orchidectomized rats.

CONCLUSIONS

V-ATPase is involved in acidification of vas deferens fluid under testosterone influence.

Identification and in silico characterization of a novel p.P208PfsX1 mutation in V-ATPase a3 subunit associated with autosomal recessive osteopetrosis in a Pakistani family

Background

Osteopetrosis is a rare inherited bone disorder mainly described as an increased bone density caused by defective osteoclastic bone resorption. To date, genetic variants of eleven genes have been reported so far to be associated with different types of osteopetrosis. However, malignant infantile osteopetrosis, a lethal form of the disease, is mostly (50%) caused by mutation(s) in TCIRG1 gene. In this study, we investigated a consanguineous Pakistani family clinically and genetically to elucidate underlying molecular basis of the infantile osteopetrosis.

Methods

DNA samples from five family members were subjected to SNP-array based whole genome homozygosity mapping. Data was analyzed and potentially pathogenic mutation was identified by Sanger sequencing of two affected as well as three phenotypically healthy individuals in the family. The significance of identified pathogenic variation and its impact on protein structure and function was studied using various bioinformatics tools.

Results

DNA samples from five family members were subjected to genome-wide SNP array genotyping and homozygosity mapping which identified ~4 Mb region on chr11 harboring the TCIRG1 gene. Sanger sequencing unveiled a novel homozygous deletion c. 624delC in exon 6 of the TCIRG1 gene encodes a3 subunit of V-ATPase complex. The identified deletion resulted in a frame shift producing a truncated protein of 208 aa. In silico analysis of premature termination of the a3 subunit of V-ATPase complex revealed deleterious effects on the protein structure, predicting impaired or complete loss of V-ATPase function causing infantile osteopetrosis.

Conclusions

Since a3 subunit of V-ATPase complex plays a crucial role in bone resorption process, structurally abnormal a3 subunit might have adversely affected bone resorption process, leading to infantile osteopetrosis in Pakistani family. Therefore, the present study not only expands the genotypic spectrum of osteopetrosis but also improve understandings of the role of V-ATPase a3 subunit in bone resorption process. Moreover, our findings should help in genetic counseling and provide further insight into the disease pathogenesis and potential targeted therapy.

Electronic supplementary material

The online version of this article (10.1186/s12881-017-0506-4) contains supplementary material, which is available to authorized users.

Mammary epithelium-specific inactivation of V-ATPase reduces stiffness of extracellular matrix and enhances metastasis of breast cancer

Extracellular matrix (ECM) critically impacts tumor progression and is influenced by both cancer and host tissue cells. While our understanding of cancer cell ECM remodeling is widespread, the importance of host tissue ECM, which provides initial congenial environment for primary tumor formation, is partly understood. Here, we report a novel role of epithelial cell-associated vacuolar ATPase 'a2' isoform (a2V) in regulating breast tissue ECM stiffness to control metastasis. Using a mammary gland-specific a2V-knockout model, we show that in the absence of a2V, breast tumors exhibit atypically soft tumor phenotype, less tumor rigidity, and necrotic tumor microenvironment. These tumors contain a decreased number of cancer cells at primary tumor site, but showed extensive metastases compared to control. Nanomechanical evaluation of normal breast tissues revealed a decrease in stiffness and collagen content in ECM of a2V-deleted breast tissues. Mechanistically, inhibition of a2V expression caused dispersed Golgi morphology with relocation of glycosyltransferase enzymes to early endosomes in mammary epithelial cells. This resulted in defective glycosylation of ECM proteins and production of compromised ECM that further influenced tumor metastasis. Clinically, in patients with cancer, low a2V expression levels in normal breast tissue correlated with lymph node metastasis. Thus, using a new knockout mouse model, we have identified a2V expression in epithelial cells as a key requirement for proper ECM formation in breast tissue and its expression levels can significantly modulate breast tumor dissemination. Evaluation of a2V expression in normal breast tissues can help in identifying patients with high risk of developing metastases.

Extracellular Adenosine Stimulates Vacuolar ATPase-Dependent Proton Secretion in Medullary Intercalated Cells

Acidosis is an important complication of AKI and CKD. Renal intercalated cells (ICs) express the proton pumping vacuolar H⁺-ATPase (V-ATPase) and are extensively involved in acid-base homeostasis. H⁺ secretion in type A intercalated cells (A-ICs) is regulated by apical vesicle recycling and stimulated by cAMP. In other cell types, cAMP is increased by extracellular agonists, including adenosine, through purinergic receptors. Adenosine is a Food and Drug Administration-approved drug, but very little is known about the effect of adenosine on IC function. Therefore, we investigated the role of adenosine in the regulation of V-ATPase in ICs. Intravenous treatment of mice with adenosine or agonists of ADORA2A and ADORA2B purinergic P1 receptors induced V-ATPase apical membrane accumulation in medullary A-ICs but not in cortical A-ICs or other IC subtypes. Both receptors are located in A-IC apical membranes, and adenosine injection increased urine adenosine concentration and decreased urine pH. Cell fractionation showed that adenosine or an ADORA2A or ADORA2B agonist induced V-ATPase translocation from vesicles to the plasma membrane and increased protein kinase A (PKA)-dependent protein phosphorylation in purified medullary ICs that were isolated from mice. Either ADORA2A or ADORA2B antagonists or the PKA inhibitor mPKI blocked these effects. Finally, a fluorescence pH assay showed that adenosine activates V-ATPase in isolated medullary ICs. Our study shows that medullary A-ICs respond to luminal adenosine through ADORA2A and ADORA2B receptors in a cAMP/PKA pathway-dependent mechanism to induce V-ATPase-dependent H⁺ secretion.

Insight into the Mode of Action of Celangulin V on the Transmembrane Potential of Midgut Cells in Lepidopteran Larvae

Celangulin V (CV) is the main insecticidal constituent of Celastrus angulatus. The V-ATPase H subunit of the midgut cells of lepidopteran larvae is the putative target protein of CV. Here, we compared the effects of CV on the midgut membrane potentials of Mythimna separata and Agrotis ipsilon larvae with those of the Cry1Ab toxin from Bacillus thuringiensis and with those of inactive CV-MIA, a synthetic derivative of CV. We investigated the changes in the apical membrane potentials (V_am) and basolateral membrane potentials (V_bm) of the midguts of sixth-instar larvae force-fed with the test toxins. We also measured the V_am and V_bm of larval midguts that were directly incubated with the test toxins. Similar to the effect of Cry1Ab, the V_am of CV-treated midguts rapidly decayed over time in a dose-dependent manner. By contrast, CV-MIA did not influence V_am. Meanwhile, the V_am of A. ipsilon larval midguts directly incubated with CV decayed less than that of M. separata larval midguts, whereas that of larvae force-fed with CV did not significantly change. Similar to Cry1Ab, CV did not affect the V_bm of isolated midguts. CV significantly inhibited V-ATPase activity in a dose-dependent manner. Therefore, CV initially inhibits V-ATPase in the apical membrane and affects intracellular pH, homeostasis, and nutrient transport mechanisms in lepidopteran midgut cells.

Acidification of uterine epithelium during embryo implantation in mice

Uterine luminal epithelium (LE) is essential for establishing uterine receptivity. Previous microarray analysis revealed upregulation of Atp6v0d2 in gestation day 4.5 (D4.5) LE in mice. Realtime PCR showed upregulation of uterine Atp6v0d2 starting right before embryo attachment ∼D4.0. In situ hybridization demonstrated specific uterine localization of Atp6v0d2 in LE upon embryo implantation. Atp6v0d2 encodes one subunit for vacuolar-type H+-ATPase (V-ATPase), which regulates acidity of intracellular organelles and extracellular environment. LysoSensor Green DND-189 detected acidic signals in LE and glandular epithelium upon embryo implantation, correlating with Atp6v0d2 upregulation in early pregnant uterus. Atp6v0d2-/- females had significantly reduced implantation rate and marginally reduced delivery rate from first mating only, but comparable number of implantation sites and litter size compared to control and comparable fertility to control from subsequent matings, suggesting a nonessential role of Atp6v0d2 subunit in embryo implantation. Successful implantation in both control and Atp6v0d2-/- females was associated with uterine epithelial acidification. No significant compensatory upregulation of Atp6v0d1 mRNA was detected in D4.5 Atp6v0d2-/- uteri. To determine the role of V-ATPase instead of a single subunit in embryo implantation, a specific V-ATPase inhibitor bafilomycin A1 (2.5 μg/kg) was injected via uterine fat pad on D3 18:00 h. This treatment resulted in reduced uterine epithelial acidification, delayed implantation, and reduced number of implantation sites. It also suppressed oil-induced artificial decidualization. These data demonstrate uterine epithelial acidification as a novel phenomenon during embryo implantation and V-ATPase is involved in uterine epithelial acidification and uterine preparation for embryo implantation.

Vacuolar ATPase as a possible therapeutic target in human acute myeloid leukemia

INTRODUCTION

V-ATPase is a proton pump expressed both in the membrane of intracellular organelles (e.g. endosomes, lysosomes, Golgi structures) and the plasma membrane. It is an important regulator of organellar functions, intracellular molecular trafficking, intercellular communication and intracellular signaling. It is therefore considered as a possible therapeutic target in the treatment of human malignancies. Areas covered: Relevant publications were identified through literature searches in the PubMed database. We searched for original articles and reviews describing the possible importance of V-ATPase for leukemogenesis and chemosensitivity in human myeloid cells, especially acute myeloid leukemia (AML) cells. Expert commentary: The expression of V-ATPase in the primary human AML cells varies between patients, and high levels are associated with high constitutive release of a wide range of soluble mediators. Several of the molecules included in the V-ATPase interactome may also be important in leukemogenesis and/or development of chemoresistance in human AML. Therapeutic targeting of V-ATPase should therefore be regarded as a possible therapeutic strategy in human AML, but the efficiency of such targeting will probably differ between patients. The possibility of toxicity, especially hematological toxicity and immunosuppression, also has to be clarified.

Effects of Disruption of PMC1 in the tfp1∆/∆ Mutant on Calcium Homeostasis, Oxidative and Osmotic Stress Resistance in Candida albicans

The vacuolar-type H⁺-ATPase (V-ATPase) is essential for many cell processes. Our previous study has demonstrated that Tfp1 is a putative subunit of V-ATPase, loss of which causes disorders in calcium homeostasis and decreased resistance to oxidative stress. In this study, we found that further deletion of PMC1, a vacuolar calcium pump, in tfp1∆/∆ mutant led to more severe dysregulation of calcium homeostasis. Besides, the tfp1∆/∆pmc1∆/∆ mutant was more sensitive to H₂O₂ and had a higher ROS level. As is known, V-ATPase mutants are sensitive to NaCl, and PMC1 mutant is resistant against NaCl. However, the tfp1∆/∆pmc1∆/∆ mutant exhibited sensitivity to NaCl. Mechanism study demonstrated that their sensitivity was associated with reduced osmotic resistance caused by relatively low expression of GPD1. In addition, we first found that NaCl addition significantly declined ROS levels in tfp1∆/∆ and tfp1∆/∆pmc1∆/∆ mutants. In tfp1∆/∆ mutant, decreased ROS levels were relevant to enhanced antioxidant activities. However, in tfp1∆/∆pmc1∆/∆ mutant, reduced ROS resulted from decreased total calcium content, revealing that NaCl affected ROS levels in the two mutants through different mechanisms. Taken together, our data indicated that loss of both TFP1 and PMC1 further affected calcium homeostasis and other cellular processes in Candida albicans and provides a potential antifungal target.

Molecular Insights into the Potential Insecticidal Interaction of β-Dihydroagarofuran Derivatives with the H Subunit of V-ATPase

Celangulin V (CV), one of dihydroagarofuran sesquiterpene polyesters isolated from Chinese bittersweet (Celastrus angulatus Maxim), is famous natural botanical insecticide. Decades of research suggests that is displays excellent insecticidal activity against some insects, such as Mythimna separata Walker. Recently, it has been validated that the H subunit of V-ATPase is one of the target proteins of the insecticidal dihydroagarofuran sesquiterpene polyesters. As a continuation of the development of new pesticides from these natural products, a series of β-dihydroagarofuran derivatives have been designed and synthesized. The compound JW-3, an insecticidal derivative of CV with a p-fluorobenzyl group, exhibits higher insecticidal activity than CV. In this study, the potential inhibitory effect aused by the interaction of JW-3 with the H subunit of V-ATPase c was verified by confirmatory experiments at the molecular level. Both spectroscopic techniques and isothermal titration calorimetry measurements showed the binding of JW-3 to the subunit H of V-ATPase was specific and spontaneous. In addition, the possible mechanism of action of the compound was discussed. Docking results indicated compound JW-3 could bind well in ‘the interdomain cleft’ of the V-ATPase subunit H by the hydrogen bonding and make conformation of the ligand–protein complex become more stable. All results are the further validations of the hypothesis, that the target protein of insecticidal dihydroagarofuran sesquiterpene polyesters and their β-dihydroagarofuran derivatives is the subunit H of V-ATPase. The results also provide new ideas for developing pesticides acting on V-ATPase of insects.

Analysis of two transcript isoforms of vacuolar ATPase subunit H in mouse and zebrafish

ATP6V1H encodes the subunit H of vacuolar ATPase (V-ATPase) and has been recently proved to regulate osteoclast function. The alternative splicing of ATP6V1H gene results in two isoforms, and it is not clear whether and how the two isoforms function differently. In this report, we used bioinformatics methods to compare the differences of two isoforms in different species. The distributions and amounts of two isoforms were analyzed in eleven kinds of mouse tissues and mouse osteoclasts using RT-PCR, Q-PCR, western blot and immunohistochemical staining methods, respectively. In order to observe the in vivo biological differences of two isoforms during development, the zebrafish mRNA of two wild type atp6v1h transcripts as well as their mutant forms were also injected into zebrafish embryos, respectively. Bioinformatic analysis revealed that two isoforms were quite different in many ways, especially in protein size, internal space, phosphorylation state and H-bond binding. The amounts of two transcripts and the ratio of long and short transcript varied a lot from tissue to tissue or cell to cell, and osteoclasts were the cells only expressing long isoform among the tissues or cells we detected. The in vivo selective expression of two subunit H splice variants showed their different effects on the craniofacial development of zebrafish. The short isoform reduced the size of zebrafish head and did not play a complete function compared with the long isoform. We propose that long isoform of subunit H is necessary for the normal craniofacial bone development and the lack of short transcript might be necessary for the normal osteoclastic function.

Repositioning of proton pump inhibitors in cancer therapy

Drug repositioning, as a smart way to exploit new molecular targets of a known drug, has been gaining increasing attention in the discovery of anti-cancer drugs. Proton pump inhibitors (PPIs) as benzimidazole derivatives, which are essentially H⁺-K⁺-ATPases inhibitors, are commonly used in the treatment of acid-related diseases such as gastric ulcer. In recent years, exploring the new application of PPIs in anti-cancer field has become a hot research topic. Interestingly, cancer cells display an alkaline intracellular pH and an acidic extracellular pH. The extracellular acidity of tumors can be corrected by PPIs that are selectively activated in an acid milieu. It is generally believed that PPIs might provoke disruption of pH homeostasis by targeting V-ATPase on cancer cells, which is the theoretical basis for PPIs to play an anti-cancer role. Numerous studies have shown specialized effects of the PPIs on tumor cell growth, metastasis, chemoresistance, and autophagy. PPIs may really represent new anti-cancer drugs due to better safety and tolerance, the potential selectivity in targeting tumor acidity, and the ability to inhibit mechanism pivotal for cancer homeostasis. In this review, we focus on the new therapeutic applications of PPIs in multiple cancers, explaining the rationale behind this approach and providing practical evidence.

pH homeostasis in yeast; the phosphate perspective

Recent research further clarified the molecular mechanisms that link nutrient signaling and pH homeostasis with the regulation of growth and survival of the budding yeast Saccharomyces cerevisiae. The central nutrient signaling kinases PKA, TORC1, and Sch9 are intimately associated to pH homeostasis, presumably allowing them to concert far-reaching phenotypical repercussions of nutritional cues. To exemplify such repercussions, we briefly describe consequences for phosphate uptake and signaling and outline interactions between phosphate homeostasis and the players involved in intra- and extracellular pH control. Inorganic phosphate uptake, its subcellular distribution, and its conversion into polyphosphates are dependent on the proton gradients created over different membranes. Conversely, polyphosphate metabolism appears to contribute in determining the intracellular pH. Additionally, inositol pyrophosphates are emerging as potent determinants of growth potential, in this way providing feedback from phosphate metabolism onto the central nutrient signaling kinases. All these data point towards the importance of phosphate metabolism in the reciprocal regulation of nutrient signaling and pH homeostasis.

Pah1p negatively regulates the expression of V-ATPase genes as well as vacuolar acidification

In yeast, PAH1 plays an important role in cell homeostasis and lipid biosynthesis. PAH1 encodes for the PA phosphatase, Pah1p, which is responsible for de novo TAG and phospholipid synthesis. It has been suggested that the lack of Pah1p causes irregular vacuolar morphology and dysfunctional V-ATPase pump activity. However, the molecular connection between Pah1p and V-ATPase activity has remained unclear. Through real-time PCR, we have shown that PAH1 is maximally induced at the stationary stage in the presence of inositol. We also found that vacuoles were less fragmented when PAH1 is maximally expressed. Subsequently, we observed that vacuoles from pah1Δ cells were more acidic than those in WT cells. Furthermore, V-ATPase genes were upregulated in the absence of Pah1p. These results suggest that Pah1p plays an important role in vacuolar activity by negatively regulating the expression of V-ATPase genes. As such, we provide evidence to show the role of Pah1p in vacuolar acidification and fragmentation.

Aberrant intracellular metabolism of T-DM1 confers T-DM1 resistance in human epidermal growth factor receptor 2-positive gastric cancer cells

Trastuzumab emtansine (T-DM1), an antibody-drug conjugate (ADC) consisting of human epidermal growth factor receptor 2 (HER2)-targeted mAb trastuzumab linked to antimicrotubule agent mertansine (DM1), has been approved for the treatment of HER2-positive metastatic breast cancer. Acquired resistance has been a major obstacle to T-DM1 treatment, and mechanisms remain incompletely understood. In the present study, we established a T-DM1-resistant N87-KR cell line from HER2-positive N87 gastric cancer cells to investigate mechanisms of acquired resistance and develop strategies for overcoming it. Although the kinetics of binding, internalization, and externalization of T-DM1 were the same in N87-KR cells and N87 cells, N87-KR was strongly resistant to T-DM1, but remained sensitive to both trastuzumab and DM1. T-DM1 failed to inhibit microtubule polymerization in N87-KR cells. Consistently, lysine-MCC-DM1, the active T-DM1 metabolite that inhibits microtubule polymerization, accumulated much less in N87-KR cells than in N87 cells. Furthermore, lysosome acidification, achieved by vacuolar H+ -ATPase (V-ATPase), was much diminished in N87-KR cells. Notably, treatment of sensitive N87 cells with the V-ATPase selective inhibitor bafilomycin A1 induced T-DM1 resistance, suggesting that aberrant V-ATPase activity decreases T-DM1 metabolism, leading to T-DM1 resistance in N87-KR cells. Interestingly, HER2-targeted ADCs containing a protease-cleavable linker, such as hertuzumab-vc-monomethyl auristatin E, were capable of efficiently overcoming this resistance. Our results show for the first time that a decrease in T-DM1 metabolites induced by aberrant V-ATPase activity contributes to T-DM1 resistance, which could be overcome by HER2-targeted ADCs containing different linkers, including a protease-cleavable linker. Accordingly, we propose that V-ATPase activity in lysosomes is a novel biomarker for predicting T-DM1 resistance.

Synthesis of a C1-C23 fragment of the archazolids and evidence for V-ATPase but not COX inhibitory activity

A convergent synthesis of a C1-C23 fragment of the archazolids has been completed based on a high yielding Stille coupling to costruct the substituted Z,Z,E-conjugated triene. After removal of the protecting groups, the resulting tetrol exhibited evidence for inhibition of the vacuolar-type ATPase (V-ATPase) but not cyclooxygenase (COX) inhibitory activity.

Cancer with low cathepsin D levels is susceptible to vacuolar (H+ )-ATPase inhibition

Vacuolar (H⁺)‐ATPases (V‐ATPases) have important roles in the supply of nutrients to tumors by mediating autophagy and the endocytic uptake of extracellular fluids. Accordingly, V‐ATPases are attractive therapeutic targets for cancer. However, the clinical use of V‐ATPase inhibitors as anticancer drugs has not been realized, possibly owing to their high toxicity in humans. Inhibition of V‐ATPase may be an appropriate strategy in highly susceptible cancers. In this study, we explored markers of V‐ATPase inhibitor sensitivity. V‐ATPase inhibitors led to pH impairment in acidic intracellular compartments, suppression of macropinocytosis, and decreased intracellular amino acid levels. The sensitivity of cells to V‐ATPase inhibitors was correlated with low cathepsin D expression, and cancer cells showed increased sensitivity to V‐ATPase inhibitors after pretreatment with a cathepsin D inhibitor and siRNA targeting the cathepsin D gene (CTSD). In addition, V‐ATPase inhibitor treatment led to the induction of the amino acid starvation response, upregulation of endoplasmic reticulum stress markers, and suppression of mammalian target of rapamycin (mTOR) signaling in cells expressing low levels of cathepsin D. Some colorectal cancer patients showed the downregulation of cathepsin D in tumor tissues compared with matched normal tissues. These findings indicate that V‐ATPase inhibitors are promising therapeutic options for cancers with downregulated cathepsin D.