Inhibition of cell growth by bafilomycin A1, a selective inhibitor of vacuolar H(+)-ATPase

3D ex vivo tissue platforms to investigate the early phases of influenza A virus- and SARS-CoV-2-induced respiratory diseases

Pandemic outbreaks of viruses such as influenza virus or SARS-CoV-2 are associated with high morbidity and mortality and thus pose a massive threat to global health and economics. Physiologically relevant models are needed to study the viral life cycle, describe the pathophysiological consequences of viral infection, and explore possible drug targets and treatment options. While simple cell culture-based models do not reflect the tissue environment and systemic responses, animal models are linked with huge direct and indirect costs and ethical questions. Ex vivo platforms based on tissue explants have been introduced as suitable platforms to bridge the gap between cell culture and animal models. We established a murine lung tissue explant platform for two respiratory viruses, influenza A virus (IAV) and SARS-CoV-2. We observed efficient viral replication, associated with the release of inflammatory cytokines and the induction of an antiviral interferon response, comparable to ex vivo infection in human lung explants. Endolysosomal entry could be confirmed as a potential host target for pharmacological intervention, and the potential repurposing potentials of fluoxetine and interferons for host-directed therapy previously seen in vitro could be recapitulated in the ex vivo model.

The V-ATPases in cancer and cell death

Transmembrane ATPases are membrane-bound enzyme complexes and ion transporters that can be divided into F-, V-, and A-ATPases according to their structure. The V-ATPases, also known as H⁺-ATPases, are large multi-subunit protein complexes composed of a peripheral domain (V1) responsible for the hydrolysis of ATP and a membrane-integrated domain (V0) that transports protons across plasma membrane or organelle membrane. V-ATPases play a fundamental role in maintaining pH homeostasis through lysosomal acidification and are involved in modulating various physiological and pathological processes, such as macropinocytosis, autophagy, cell invasion, and cell death (e.g., apoptosis, anoikis, alkaliptosis, ferroptosis, and lysosome-dependent cell death). In addition to participating in embryonic development, V-ATPase pathways, when dysfunctional, are implicated in human diseases, such as neurodegenerative diseases, osteopetrosis, distal renal tubular acidosis, and cancer. In this review, we summarize the structure and regulation of isoforms of V-ATPase subunits and discuss their context-dependent roles in cancer biology and cell death. Updated knowledge about V-ATPases may enable us to design new anticancer drugs or strategies.

Entry of the Varicellovirus Canid herpesvirus 1 into Madin-Darby canine kidney epithelial cells is pH-independent and occurs via a macropinocytosis-like mechanism but without increase in fluid uptake

Canid herpesvirus 1 (CHV-1) is a Varicellovirus that causes self-limiting infections in adult dogs but morbidity and mortality in puppies. Using a multipronged approach, we discovered the CHV-1 entry pathway into Madin-Darby canine kidney (MDCK) epithelial cells. We found that CHV-1 triggered extensive host cell membrane lamellipodial ruffling and rapid internalisation of virions in large, uncoated vacuoles, suggestive of macropinocytosis. Treatment with inhibitors targeting key macropinocytosis factors, including inhibitors of Na⁺ /H⁺ exchangers, F-actin, myosin light-chain kinase, protein kinase C, p21-activated kinase, phosphatidylinositol-3-kinase and focal adhesion kinase, significantly reduced viral replication. Moreover, the effect was restricted to exposure to the inhibitors early in infection, confirming a role for the macropinocytic machinery during entry. The profile of inhibitors also suggested a role for signalling via integrins and receptor tyrosine kinases in viral entry. In contrast, inhibitors of clathrin, caveolin, microtubules and endosomal acidification did not affect CHV-1 entry into MDCK cells. We found that the virus colocalised with the fluid-phase uptake marker dextran; however, surprisingly, CHV-1 infection did not enhance the uptake of dextran. Thus, our results indicate that CHV-1 uses a macropinocytosis-like, pH-independent entry pathway into MDCK cells, which nevertheless is not based on stimulation of fluid uptake. TAKE AWAYS: CHV-1 enters epithelial cells via a macropinocytosis-like mechanism. CHV-1 induces extensive lamellipodial ruffling. CHV-1 entry into MDCK cells is pH-independent.

Unraveling and targeting RAS-driven metabolic signaling for therapeutic gain

RAS mutations are among the most frequent oncogenic drivers observed in human cancers. With a lack of available treatment options, RAS-mutant cancers account for many of the deadliest cancers in the United States. Recent studies established that altered metabolic requirements are a hallmark of cancer, and many of these alterations are driven by aberrant RAS signaling. Specifically, RAS-driven cancers are characterized by upregulated glycolysis, the differential channeling of glycolytic intermediates, upregulated nutrient scavenging pathways such as autophagy and macropinocytosis, and altered glutamine utilization and mitochondrial function. This unique metabolic landscape promotes tumorigenesis, proliferation, survival in nutrient deficient environments and confers resistance to conventional cytotoxic and targeted therapies. Emerging work demonstrates how these dependencies can be therapeutically exploited in vitro and in vivo with many metabolic inhibitors currently in clinical trials. This review aims to outline the unique metabolic requirements induced by aberrant RAS signaling and how these altered dependencies present opportunities for therapeutic intervention.

Estradiol analogs attenuate autophagy, cell migration and invasion by direct and selective inhibition of TRPML1, independent of estrogen receptors

The cation channel TRPML1 is an important regulator of lysosomal function and autophagy. Loss of TRPML1 is associated with neurodegeneration and lysosomal storage disease, while temporary inhibition of this ion channel has been proposed to be beneficial in cancer therapy. Currently available TRPML1 channel inhibitors are not TRPML isoform selective and block at least two of the three human isoforms. We have now identified the first highly potent and isoform-selective TRPML1 antagonist, the steroid 17β-estradiol methyl ether (EDME). Two analogs of EDME, PRU-10 and PRU-12, characterized by their reduced activity at the estrogen receptor, have been identified through systematic chemical modification of the lead structure. EDME and its analogs, besides being promising new small molecule tool compounds for the investigation of TRPML1, selectively affect key features of TRPML1 function: autophagy induction and transcription factor EB (TFEB) translocation. In addition, they act as inhibitors of triple-negative breast cancer cell migration and invasion.

The H+-ATPase (V-ATPase): from proton pump to signaling complex in health and disease

A primary function of the H⁺-ATPase (or V-ATPase) is to create an electrochemical proton gradient across eukaryotic cell membranes, which energizes fundamental cellular processes. Its activity allows for the acidification of intracellular vesicles and organelles, which is necessary for many essential cell biological events to occur. In addition, many specialized cell types in various organ systems such as the kidney, bone, male reproductive tract, inner ear, olfactory mucosa, and more, use plasma membrane V-ATPases to perform specific activities that depend on extracellular acidification. It is, however, increasingly apparent that V-ATPases are central players in many normal and pathophysiological processes that directly influence human health in many different and sometimes unexpected ways. These include cancer, neurodegenerative diseases, diabetes, and sensory perception, as well as energy and nutrient-sensing functions within cells. This review first covers the well-established role of the V-ATPase as a transmembrane proton pump in the plasma membrane and intracellular vesicles and outlines factors contributing to its physiological regulation in different cell types. This is followed by a discussion of the more recently emerging unconventional roles for the V-ATPase, such as its role as a protein interaction hub involved in cell signaling, and the (patho)physiological implications of these interactions. Finally, the central importance of endosomal acidification and V-ATPase activity on viral infection will be discussed in the context of the current COVID-19 pandemic.

Differential Effects of Rapamycin and Metformin in Combination With Rapamycin on Mechanisms of Proteostasis in Cultured Skeletal Myotubes

mTOR inhibition extends life span in multiple organisms. In mice, when metformin treatment (Met) is added to the mTOR inhibitor rapamycin (Rap), median and maximal life span is extended to a greater degree than with Rap or Met alone. Treatments that extend life span often maintain proteostasis. However, it is less clear how individual tissues, such as skeletal muscle, maintain proteostasis with life span-extending treatments. In C2C12 myotubes, we used deuterium oxide (D2O) to directly measure two primary determinants of proteostasis, protein synthesis, and degradation rates, with Rap or Met+Rap treatments. We accounted for the independent effects of cell growth and loss, and isolated the contribution of autophagy and mitochondrial fission to obtain a comprehensive assessment of protein turnover. Compared with control, both Rap and Met+Rap treatments lowered mitochondrial protein synthesis rates (p < .001) and slowed cellular proliferation (p < .01). These changes resulted in greater activation of mechanisms promoting proteostasis for Rap, but not Met+Rap. Compared with control, both Rap and Met+Rap slowed protein breakdown. Autophagy and mitochondrial fission differentially influenced the proteostatic effects of Rap and Met+Rap in C2C12 myotubes. In conclusion, we demonstrate that Met+Rap did not increase protein turnover and that these treatments do not seem to promote proteostasis through increased autophagy.

The viral protein U (Vpu)-interacting host protein ATP6V0C down-regulates cell-surface expression of tetherin and thereby contributes to HIV-1 release

Host proteins with antiviral activity have evolved as first-line defenses to suppress viral replication. The HIV-1 accessory protein viral protein U (Vpu) enhances release of the virus from host cells by down-regulating the cell-surface expression of the host restriction factor tetherin. However, the exact mechanism of Vpu-mediated suppression of antiviral host responses is unclear. To further understand the role of host proteins in Vpu's function, here we carried out yeast two-hybrid screening and identified the V0 subunit C of vacuolar ATPase (ATP6V0C) as a Vpu-binding protein. To examine the role of ATP6V0C in Vpu-mediated tetherin degradation and HIV-1 release, we knocked down ATP6V0C expression in HeLa cells and observed that ATP6V0C depletion impairs Vpu-mediated tetherin degradation, resulting in defective HIV-1 release. We also observed that ATP6V0C overexpression stabilizes tetherin expression. This stabilization effect was specific to ATP6V0C, as overexpression of another subunit of the vacuolar ATPase, ATP6V0C″, had no effect on tetherin expression. ATP6V0C overexpression did not stabilize CD4, another target of Vpu-mediated degradation. Immunofluorescence localization experiments revealed that the ATP6V0C-stabilized tetherin is sequestered in a CD63- and lysosome-associated membrane protein 1 (LAMP1)-positive intracellular compartment. These results indicate that the Vpu-interacting protein ATP6V0C plays a role in down-regulating cell-surface expression of tetherin and thereby contributes to HIV-1 assembly and release.

Iron-Doping of Copper Oxide Nanoparticles Lowers Their Toxic Potential on C6 Glioma Cells

Copper oxide nanoparticles (CuO-NPs) are well known for their cytotoxicity which in part has been attributed to the release of copper ions from CuO-NPs. As iron-doping has been reported to reduce the susceptibility of CuO-NPs to dissolution, we have compared pure CuO-NPs and CuO-NPs that had been doped with 10% iron (CuO-Fe-NPs) for copper release and for their toxic potential on C6 glioma cells. Physicochemical characterization revealed that dimercaptosuccinate (DMSA)-coated CuO-NPs and CuO-Fe-NPs did not differ in their size or zeta potential. However, the redox activity and liberation of copper ions from CuO-Fe-NPs was substantially slower compared to that from CuO-NPs, as demonstrated by cyclic voltammetry and by the photometric quantification of the copper ion-bathocuproine complex, respectively. Exposure of C6 cells to these NPs caused an almost identical cellular copper accumulation and each of the two types of NPs induced ROS production and cell toxicity. However, the time- and concentration-dependent loss in cell viability was more severe for cells that had been treated with CuO-NPs compared to cells exposed to CuO-Fe-NPs. Copper accumulation and toxicity after exposure to either CuO-NPs or CuO-Fe-NPs was prevented in the presence of copper chelators, while neutralization of the lysosomal pH by bafilomycin A1 prevented toxicity without affecting cellular copper accumulation or ROS production. These data demonstrate that iron-doping does not affect cellular accumulation of CuO-NPs and suggests that the intracellular liberation of copper ions from CuO-NPs is slowed by the iron doping, which in turn lowers the cell toxic potential of iron-doped CuO-NPs.

Augmenter of Liver Regeneration Protects against Ethanol-Induced Acute Liver Injury by Promoting Autophagy

Alcoholic liver disease is associated with high morbidity and mortality, and treatment options are limited to date. Augmenter of liver regeneration (ALR) may protect against hepatic injury from chemical poisons, including ethanol. Autophagy appears to positively influence survival in cases of liver dysfunction, although the mechanisms are poorly understood. Herein, we investigated effects of ALR-induced autophagy in vitro and in vivo in an ethanol-induced model of acute liver injury. Decreased serum levels of alanine aminotransferase and aspartate aminotransferase and reduced histologic lesions revealed that mice overexpressing ALR experienced less liver damage than wild-type. ALR-knockdown mice experienced more severe liver damage than wild-type. ALR-transfected HepG2 cells showed increased survival rates, improved maintenance of mitochondrial membrane potential, and increased ATP levels after ethanol treatment. The observed protection was associated with up-regulation of autophagy-markers, including light chain 3II, beclin-1, and autophagy-related gene 5, and down-regulation of p62 by ALR. Autophagy was inhibited in ALR-knockdown mice and HepG2 cells, and autophagy inhibitor bafilomycin A1 attenuated the protective effects of ALR. Results showed phosphorylated mammalian target of rapamycin (mTOR) was down-regulated when ALR was overexpressed and up-regulated when ALR was knocked down. These data show that ALR is protective against ethanol-induced acute liver injury by promoting autophagy, probably via repressing the mTOR pathway. These results have potential implications for the clinical treatment of alcoholic liver disease patients.

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.

Ion Trapping of Amines in Protozoa: A Novel Removal Mechanism for Micropollutants in Activated Sludge

To optimize removal of organic micropollutants from the water cycle, understanding the processes during activated sludge treatment is essential. In this study, we hypothesize that aliphatic amines, which are highly abundant among organic micropollutants, are partly removed from the water phase in activated sludge through ion trapping in protozoa. In ion trapping, which has been extensively investigated in medical research, the neutral species of amine-containing compounds diffuse through the cell membrane and further into acidic vesicles present in eukaryotic cells such as protozoa. There they become trapped because diffusion of the positively charged species formed in the acidic vesicles is strongly hindered. We tested our hypothesis with two experiments. First, we studied the distribution of the fluorescent amine acridine orange in activated sludge by confocal fluorescence imaging. We observed intense fluorescence in distinct compartments of the protozoa, but not in the bacterial biomass. Second, we investigated the distribution of 12 amine-containing and eight control micropollutants in both regular activated sludge and sludge where the protozoa had been inactivated. In contrast to most control compounds, the amine-containing micropollutants displayed a distinctly different behavior in the noninhibited sludge compared to the inhibited one: (i) more removal from the liquid phase; (ii) deviation from first-order kinetics for the removal from the liquid phase; and (iii) higher amounts in the solid phase. These results provide strong evidence that ion trapping in protozoa occurs and that it is an important removal mechanism for amine-containing micropollutants in batch experiments with activated sludge that has so far gone unnoticed. We expect that our findings will trigger further investigations on the importance of this process in full-scale wastewater treatment systems, including its relevance for accumulation of ammonium.

Autophagy Regulates Proteasome Inhibitor-Induced Pigmentation in Human Embryonic Stem Cell-Derived Retinal Pigment Epithelial Cells

The impairment of autophagic and proteasomal cleansing together with changes in pigmentation has been documented in retinal pigment epithelial (RPE) cell degeneration. However, the function and co-operation of these mechanisms in melanosome-containing RPE cells is still unclear. We show that inhibition of proteasomal degradation with MG-132 or autophagy with bafilomycin A1 increased the accumulation of premelanosomes and autophagic structures in human embryonic stem cell (hESC)-derived RPE cells. Consequently, upregulation of the autophagy marker p62 (also known as sequestosome-1, SQSTM1) was confirmed in Western blot and perinuclear staining. Interestingly, cells treated with the adenosine monophosphatedependent protein kinase activator, AICAR (5-Aminoimidazole-4-carboxamide ribonucleotide), decreased the proteasome inhibitor-induced accumulation of premelanosomes, increased the amount of autophagosomes and eradicated the protein expression of p62 and LC3 (microtubule-associated protein 1A/1B-light chain 3). These results revealed that autophagic machinery is functional in hESC-RPE cells and may regulate cellular pigmentation with proteasomes.

Identification of V-ATPase as a molecular sensor of SOX11-levels and potential therapeutic target for mantle cell lymphoma

Background

Mantle cell lymphoma (MCL) is an aggressive disease with short median survival. Molecularly, MCL is defined by the t(11;14) translocation leading to overexpression of the CCND1 gene. However, recent data show that the neural transcription factor SOX11 is a disease defining antigen and several involved signaling pathways have been pin-pointed, among others the Wnt/β-catenin pathway that is of importance for proliferation in MCL. Therefore, we evaluated a compound library focused on the Wnt pathway with the aim of identifying Wnt-related targets that regulate growth and survival in MCL, with particular focus on SOX11-dependent growth regulation.

Methods

An inducible SOX11 knock-down system was used to functionally screen a library of compounds (n = 75) targeting the Wnt signaling pathway. A functionally interesting target, vacuolar-type H⁺-ATPase (V-ATPase), was further evaluated by western blot, siRNA-mediated gene silencing, immunofluorescence, and flow cytometry.

Results

We show that 15 out of 75 compounds targeting the Wnt pathway reduce proliferation in all three MCL cell lines tested. Furthermore, three substances targeting two different targets (V-ATPase and Dkk1) showed SOX11-dependent activity. Further validation analyses were focused on V-ATPase and showed that two independent V-ATPase inhibitors (bafilomycin A1 and concanamycin A) are sensitive to SOX11 levels, causing reduced anti-proliferative response in SOX11 low cells. We further show, using fluorescence imaging and flow cytometry, that V-ATPase is mainly localized to the plasma membrane in primary and MCL cell lines.

Conclusions

We show that SOX11 status affect V-ATPase dependent pathways, and thus may be involved in regulating pH in intracellular and extracellular compartments. The plasma membrane localization of V-ATPase indicates that pH regulation of the immediate extracellular compartment may be of importance for receptor functionality and potentially invasiveness in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2550-4) contains supplementary material, which is available to authorized users.

Bafilomycin A1 targets both autophagy and apoptosis pathways in pediatric B-cell acute lymphoblastic leukemia

B-cell acute lymphoblastic leukemia is the most common type of pediatric leukemia. Despite improved remission rates, current treatment regimens for pediatric B-cell acute lymphoblastic leukemia are often associated with adverse effects and central nervous system relapse, necessitating more effective and safer agents. Bafilomycin A1 is an inhibitor of vacuolar H(+)-ATPase that is frequently used at high concentration to block late-phase autophagy. Here, we show that bafilomycin A1 at a low concentration (1 nM) effectively and specifically inhibited and killed pediatric B-cell acute lymphoblastic leukemia cells. It targeted both early and late stages of the autophagy pathway by activating mammalian target of rapamycin signaling and by disassociating the Beclin 1-Vps34 complex, as well as by inhibiting the formation of autolysosomes, all of which attenuated functional autophagy. Bafilomycin A1 also targeted mitochondria and induced caspase-independent apoptosis by inducing the translocation of apoptosis-inducing factor from mitochondria to the nucleus. Moreover, bafilomycin A1 induced the binding of Beclin 1 to Bcl-2, which further inhibited autophagy and promoted apoptotic cell death. In primary cells from pediatric patients with B-cell acute lymphoblastic leukemia and a xenograft model, bafilomycin A1 specifically targeted leukemia cells while sparing normal cells. An in vivo mouse toxicity assay confirmed that bafilomycin A1 is safe. Our data thus suggest that bafilomycin A1 is a promising candidate drug for the treatment of pediatric B-cell acute lymphoblastic leukemia.

Undecylprodigiosin induced apoptosis in P388 cancer cells is associated with its binding to ribosome

Prodigiosins (PGs) are a family of natural red pigments with anticancer activity, and one member of the family has entered clinical phase II trials. However, the anticancer mechanisms of PGs remain largely unclear. This study was designed to investigate the molecular basis of anticancer activity of UP, a derivative of PGs, in P388 cells. By introducing pharmacological inhibitors and utilizing a variety of analytical approaches including western blotting, flow cytometry and confocal laser microscopy, we found that UP inhibited proliferation of P388 via arresting cells at G2/M phase and inducing cells apoptosis, which was related to the activation of P38, JNK rather than ERK1/2 signaling. ROS regeneration and acidification in cells appear not involved in UP induced apoptosis. Furthermore, utilizing mass spectrometry, sucrose density gradient fractionation and immunofluorescence staining, we discovered that UP was apparently located at ribosome. These results together indicate that ribosome may be the potential target of UP in cancer cells, which opened a new avenue in delineating the anticancer mechanism of PGs.

Pyk2 regulates H+-ATPase-mediated proton secretion in the outer medullary collecting duct via an ERK1/2 signaling pathway

Acid-secreting intercalated cells respond to changes in systemic pH through regulation of apical H(+) transporters. Little is known about the mechanism by which these cells sense changes in extracellular pH (pH(o)). Pyk2 is a nonreceptor tyrosine kinase activated by autophosphorylation at Tyr402 by cell-specific stimuli, including decreased pH, and is involved in the regulation of MAPK signaling pathways and transporter activity. We examined whether the Pyk2 and MAPK signaling pathway mediates the response of transport proteins to decreased pH in outer medullary collecting duct cells. Immunoblot analysis of phosphorylated Pyk2 (Tyr402), ERK1/2 (Thr202/Tyr204), and p38 (Thr180/Tyr182) was used to assay protein activation. To examine specificity of kinase activation and its effects, we used Pyk2 small interfering RNA to knockdown Pyk2 expression levels, the Src kinase inhibitor 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]-pyrimidine (PP 1) to inhibit Pyk2 phosphorylation, and the MEK inhibitor U0126 to inhibit ERK1/2 phosphorylation. The pH-sensitive fluorescent probe 2'-7'-bis(carboxyethyl)-5(6)-carboxyfluorescein-acetoxymethyl ester (BCECF-AM) was used to assay H(+) transporter activity. The activity of H(+) transporters was measured as the rate of intracellular pH (pH(i)) recovery after an NH(4)Cl prepulse. We show that Pyk2 is endogenously expressed and activated by acid pH in mouse-derived outer medullary collecting duct (mOMCD1) cells. Incubation of mOMCD1 cells in acid media [extracellular pH (pH(o)) 6.7] increased the phosphorylation of Pyk2, ERK1/2, and p38. Reduction in pH(i) induced by an NH(4)Cl prepulse also increased the phosphorylation of Pyk2, ERK1/2, and p38. Consistent with our previous studies, we found that mOMCD1 cells exhibit H(+)-ATPase and H(+),K(+)-ATPase activity. Pyk2 inhibition by Pyk2 siRNA and PP 1 prevented Pyk2 phosphorylation as well as H(+)-ATPase-mediated recovery in mOMCD1 cells. In addition, ERK1/2 inhibition by U0126 prevented acid-induced ERK1/2 phosphorylation and H(+)-ATPase-mediated pH(i) recovery but not phosphorylation of p38. We conclude that Pyk2 and ERK1/2 are required for increasing H(+)-ATPase, but not H(+),K(+)-ATPase, activity at decreased pH(i) in mOMCD1 cells.

Low-pH triggering of human metapneumovirus fusion: essential residues and importance in entry

Human metapneumovirus (HMPV) is a significant respiratory pathogen classified in the Pneumovirinae subfamily of the paramyxovirus family. Recently, we demonstrated that HMPV F protein-promoted cell-cell fusion is stimulated by exposure to low pH, in contrast to what is observed for other paramyxovirus F proteins. In the present study, we examined the potential role of histidine protonation in HMPV F fusion and investigated the role of low pH in HMPV viral entry. Mutagenesis of the three ectodomain histidine residues of the HMPV F protein demonstrated that the mutation of a histidine in the heptad repeat B linker domain (H435) ablated fusion activity without altering cell surface expression or proteolytic processing significantly. Modeling of the HMPV F protein revealed several basic residues surrounding this histidine residue, and the mutation of these residues also reduced fusion activity. These results suggest that electrostatic repulsion in the heptad repeat B linker region may contribute to the triggering of HMPV F. In addition, we examined the effect of inhibitors of endosomal acidification or endocytosis on the entry of a recombinant green fluorescent protein-expressing HMPV. Interestingly, chemicals that raise the pH of endocytic vesicles resulted in a 30 to 50% decrease in HMPV infection, while the inhibitors of endocytosis reduced infection by as much as 90%. These data suggest that HMPV utilizes an endocytic entry mechanism, in contrast to what has been hypothesized for most paramyxoviruses. In addition, our results indicate that HMPV uses the low pH of the endocytic pathway to enhance infectivity, though the role of low pH likely differs from classically described mechanisms.

The proton pump inhibitor inhibits cell growth and induces apoptosis in human hepatoblastoma

PURPOSE

In normal physiology, a vacuolar-type proton pump (V-ATPase) maintains an intracellular acid microenvironment in lysosome, endosome, and other endomembrane systems. Cancer cells overexpress V-ATPase compared with normal cells, and disturbances of the acid environment are thought to significantly impact the cancer cell infiltration and growth. Bafilomycin A1 (Baf-A1) is a specific inhibitor of the proton-pump inhibitor (PPI) V-ATPase. Neoplastic cells are reportedly more sensitive to Baf-A1 than normal cells, and the difference between the susceptibility to Baf-A1 in normal cells and that in cancer cells may become a target in the cancer therapy. With this in mind, we used cells of hepatoblastoma, the cancer type accounting for 80% of all childhood liver cancers, to investigate the effects of Baf-A1 as an inducer of cancer cell apoptosis and inhibitor of cancer cell reproduction

METHODS AND RESULTS

Electron microscopy showed significant morphological change of the hepatoblastoma cells of the Baf-A1-treated group compared with hepatoblastoma cells of the Baf-A1-free group. The rate of the apoptotic cell increased, and cell reproduction was inhibited. Moreover, the analysis of hepatoblastoma cells using the gene Chip gene expression analysis arrays showed that three of the 27 V-ATPase-related transcripts (ATP6V0D2, ATP6V1B1, and ATP6V0A1) were more weakly expressed in the Baf-A1-treated cells than in the Baf-A1-free cells. In normal human hepatic cells, on the other hand, the inhibition of cell growth of the Baf-A1-treated cells was negligible compared to that of the cells without Baf-A1 treatment. The result of apoptotic cell detection by morphological observations and flow cytometry revealed that Baf-A1 inhibits hepatoblastoma cellular reproduction by inducing apoptosis. On the other hand, the Baf-A1-conferred inhibition of cell growth was negligible in normal human hepatocytes

CONCLUSION

The V-ATPase inhibitor Baf-A1 has been proven to selectively inhibit the reproduction and induce the apoptosis of hepatoblastoma cells without adversely influencing normal hepatic cells. With these effects, V-ATPase inhibitors may hold promise as therapeutic agents for hepatoblastoma. Given that three V-ATPase-related genes (ATP6V0D2, ATP6V1B1, and ATP6V0A1) were more weakly expressed in the hepatoblastoma cells of the Baf-A1-treated group than in the Baf-A1-free cells, drug development targeting V-ATPase gene of hepatoblastomas is expected.

Energizing an Invertebrate Embryo: Bafilomycin‐Dependent Respiration and the Metabolic Cost of Proton Pumping by the V‐ATPase

We examine herein the contribution of V-ATPase activity to the energy budget of aerobically developing embryos of Artemia franciscana and discuss the results in the context of quiescence under anoxia. (31)P-NMR analysis indicates that intracellular pH and NTP levels are unaffected by acute incubation of dechorionated embryos with the V-ATPase inhibitor, bafilomycin A(1). Bafilomycin A(1) also has no significant effect on oxygen consumption by isolated mitochondria. Taken together, these data indicate that bafilomycin does not affect energy-producing pathways in the developing embryo. However, the V-ATPase inhibitor exhibits a concentration-dependent inhibition of oxygen consumption in aerobic embryos. A conservative analysis of respirometric data indicates that proton pumping by the V-ATPase, and processes immediately dependent on this activity, constitutes approximately 31% of the aerobic energy budget of the preemergent embryo. Given the complete absence of detectable Na(+)K(+)-ATPase activity during the first hours of aerobic development, it is plausible that the V-ATPase is performing a role in both the acidification of intracellular compartments and the energization of plasma membranes. Importantly, the high metabolic cost associated with maintaining these diverse proton gradients requires that V-ATPase activity be downregulated under anoxia in order to attain the almost complete metabolic depression observed in the quiescent embryo.

ATP6V0C competes with von Hippel-Lindau protein in hypoxia-inducible factor 1alpha (HIF-1alpha) binding and mediates HIF-1alpha expression by bafilomycin A1

HIF-1alpha not only enables cells to survive under hypoxic conditions but also promotes cell cycle arrest and apoptosis. Therefore, its expression should be controlled at optimal levels in growing tumors. We recently reported that bafilomycin A1 exorbitantly expressed HIF-1alpha and induced the p21(WAF1/Cip1)-mediated growth arrest of tumors (Mol Pharmacol 70:1856-1865, 2006). In the present study, we addressed the mechanism underlying bafilomycin-induced HIF-1alpha expression. Bafilomycin stabilized HIF-1alpha under normoxic conditions without changes in intracellular pH. However, when ATP6V0C, the target protein of bafilomycin, was knocked down, this bafilomycin effect was significantly attenuated. Inversely, ATP6V0C expression increased HIF-1alpha levels in a gene dose-dependent manner. ATP6V0C competed with Von Hippel-Lindau protein in HIF-1alpha binding by directly interacting with HIF-1alpha, which was stimulated by bafilomycin. In confocal images, ATP6V0C was normally present in the cytoplasm but was translocated in company with HIF-1alpha to the nucleus by bafilomycin. The N-terminal end (amino acids 1-16) of HIF-1alpha was identified as the ATP6V0C-interacting motif. These results suggest that ATP6V0C, a novel regulator of HIF-1alpha, mediates HIF-1alpha expression by bafilomycin.

Non-apoptotic concentrations of prodigiosin (H+/Cl− symporter) inhibit the acidification of lysosomes and induce cell cycle blockage in colon cancer cells

Prodigiosin (PG) is a bacterial red pigment with interesting immunosuppressive and apoptotic properties that have been partly attributed to its ability to uncouple V-ATPase through the promotion of the H+/Cl- symporter. In the present study, we investigate the effect of non-apoptotic concentrations of PG on the lysosomal-pH and on cell cycle progression in colon cancer cells. Lysosomal-pH was tested in DLD-1 cells using acridine orange vital staining. Orange granules, indicative of acidified lysosomes, decreased significantly in cells treated with 25 nM of PG for 1/2 h, and disappeared completely at 100 nM. This suggests that PG can induce lysosomal alkalinization without any apparent cytotoxic effect. Cell cycle progression was analysed in HT29 cells and we found that PG induces a blockage in the G1 phase. This blockage correlates with p21(WAF1/CIP1) induction, and it can be triggered either dependently or independently of p53. In conclusion, the reversible increase in lysosomal-pH and cytosol acidification induced by non-apoptotic concentrations of PG in colon cancer cells, suggests that the apoptotic process induced by PG can not be solely explained by changes in intracellular pH. The effect of intracellular acidification on cell cycle arrest must be analysed more exactly.

Different roles of proteolipids and 70-kDa subunits of V-ATPase in growth and death of cultured human cells

BACKGROUND

The vacuolar-type proton-translocating adenosine triphosphatase (V-ATPase) plays important roles in cell growth and tumour progression. V-ATPase is composed of two distinct structures, a hydrophilic catalytic cytosolic sector (V(1)) and a hydrophobic transmembrane sector (V(0)). The V(1) sector is composed of 5-8 different subunits with the structure A(3)B(3)C(1)D(1)E(1)F(1)G(1)H(1). The V0 sector is composed of 5 different subunits with the structure 1161381191166. The over-expression of 16-kDa proteolipid subunit of V-ATPase in the perinuclear region of the human adventitial fibroblasts promotes phenotypic modulation that contributes to neointimal formation and medial thickening. A relationship between oncogenicity and the expression of the 16-kDa proteolipid has also been suggested in human pancreatic carcinoma tissue.

RESULTS

We found that the mRNA levels of the 16-kDa proteolipid but not of the 70-kDa subunit of V-ATPase in human myofibroblasts were more abundant in serum-containing medium (MF(+) cells) than serum-free medium (MF(-) cells). In HeLa cells, the levels of mRNA and protein of the 16-kDa, 21-kDa or 70-kDa were clearly suppressed when the corresponding anti-sense oligonucleotides were administered to the culture medium. The growth rate and viability (mostly due to necrosis) of HeLa cells were reduced markedly by the 16-kDa and 21-kDa anti-sense, but little by the 70-kDa anti-sense, and not at all by any sense oligonucleotides. The localization of 16-kDa/21-kDa proteolipid subunits was different from that of the 70-kDa subunit in HeLa cells.

CONCLUSION

These results suggest that the 16-kDa and 21-kDa proteolipid subunits of the V0 sector play crucial roles in growth and death of cultured human cells. Our results may provide new insights into the mechanism and therapeutic implications for vessel wall hyperplasia and tumorigenesis.

Poly(2-alkylacrylic acid) polymers deliver molecules to the cytosol by pH-sensitive disruption of endosomal vesicles

The permeability barrier posed by cell membranes represents a challenge for the delivery of hydrophilic molecules into cells. We previously proposed that poly(2-alkylacrylic acid)s are endocytosed by cells into acidified vesicles and are there triggered by low pH to disrupt membranes and release the contents of endosomes/lysosomes to the cytosol. If this hypothesis is correct, these polymers could be valuable in drug-delivery applications. The present paper reports functional comparisons of a family of three poly(2-alkylacrylic acid)s. Poly(2-propylacrylic acid) (PPAA), poly(2-ethylacrylic acid) (PEAA) and poly(2-methylacrylic acid) (PMAA) were compared in red-blood-cell haemolysis assays and in a lipoplex (liposome-DNA complex) assay. We also directly examined the ability of these polymers to disrupt endosomes and lysosomes in cultured human cells. Our results show that: (i) unlike membrane-disruptive peptides, the endosomal-disruptive ability of poly(2-alkylacrylic acid)s cannot necessarily be predicted from their haemolytic activity at low pH, (ii) PPAA (but not PEAA or PMAA) potently facilitates gene transfection by cationic lipoplexes and (iii) endocytosed poly(2-alkylacrylic acid)s are triggered by luminal acidification to selectively disrupt endosomes (not lysosomes) and release their contents to the cytosol. These results will facilitate the rational design of future endosomal-disrupting polymers for drug delivery.

In bafilomycin A1-resistant cells, bafilomycin A1 raised lysosomal pH and both prodigiosins and concanamycin A inhibited growth through apoptosis

In bafilomycin A(1)-resistant cells (Vero-317 and MC-3T3-E1), bafilomycin A(1) neither inhibited cell growth, induced cell death, nor activated caspase-3. However, 100 nM bafilomycin A(1) did raise the lysosomal pH similar to 10 mM NH(4)Cl. Prodigiosins, H(+)/Cl(-) symporters that raise the lysosomal pH, inhibited cell growth through apoptosis and caused the activation of caspase-3. Concanamycin A also inhibited the growth of these cells through apoptosis. 10 mM NH(4)Cl inhibited the growth of these cells as well, but cytostatically. These results suggest that plecomacrolides inhibited cell growth apoptotically through specific site(s), in contrast to the cytostatic effect of 10 mM NH(4)Cl, besides raising the lysosomal pH.

BE-18591 as a new H(+)/Cl(-) symport ionophore that inhibits immunoproliferation and gastritis

In our previous papers [e.g. Sato et al., J. Biol. Chem. 273 (1998) 21455-21462], we have shown that prodigiosins can uncouple various H(+)-ATPases through their H(+)/Cl(-) symport activity. BE-18591 is an enamine of 4-methoxy-2,2'-bipyrrole-5-carboxyaldehyde (tambjamine group antibiotics) which resembles the prodigiosins. We found that BE-18591 was a new group of antibiotics that uncouples various H(+)-ATPases: it inhibited proton pump activities with IC(50)s of about 1-2 nM (about 20 pmol/mg protein) for submitochondrial particles as well as gastric vesicles and of 230 nM (about 230 pmol/mg protein) for lysosomes, but it had little effect on their ATP hydrolyses (up to 10 microM), a property of H(+)/Cl(-) symport activity. At low concentrations (<1 microM), BE-18591 inhibited immunoproliferation, the IC(50) of lipopolysaccharide-stimulated mouse splenocytes was 38 nM, that of Concanavalin A-stimulated cells was 230 nM. Gastritis of rabbits was also inhibited. At higher concentrations (>1 microM), BE-18591 induced neurite outgrowth (15% induction in 48 h at 4 microM), inhibited bone resorption (approximately 35% in 48 h at 10 microM) and caused cell death (approximately 30% in 48 h at 4 microM) but with little apoptosis.

Infectious entry by amphotropic as well as ecotropic murine leukemia viruses occurs through an endocytic pathway

Infectious entry of enveloped viruses is thought to proceed by one of two mechanisms. pH-dependent viruses enter the cells by receptor-mediated endocytosis and are inhibited by transient treatment with agents that prevent acidification of vesicles in the endocytic pathway, while pH-independent viruses are not inhibited by such agents and are thought to enter the cell by direct fusion with the plasma membrane. Nearly all retroviruses, including amphotropic murine leukemia virus (MuLV) and human immunodeficiency virus type 1, are classified as pH independent. However, ecotropic MuLV is considered to be a pH-dependent virus. We have examined the infectious entry of ecotropic and amphotropic MuLVs and found that they were equally inhibited by NH4Cl and bafilomycin A. These agents inhibited both viruses only partially over the course of the experiments. Agents that block the acidification of endocytic vesicles also arrest vesicular trafficking. Thus, partial inhibition of the MuLVs could be the result of virus inactivation during arrest in this pathway. In support of this contention, we found that that the loss of infectivity of the MuLVs during treatment of target cells with the drugs closely corresponded to the loss of activity due to spontaneous inactivation at 37 degrees C in the same period of time. Furthermore, the drugs had no effect on the efficiency of infection under conditions in which the duration of infection was held to a very short period to minimize the effects of spontaneous inactivation. These results indicate that the infectious processes of both ecotropic and amphotropic MuLVs were arrested rather than aborted by transient treatment of the cells with the drugs. We also found that infectious viruses were efficiently internalized during treatment. This indicated that the arrest occurred in an intracellular compartment and that the infectious process of both the amphotropic and ecotropic MuLVs very likely involved endocytosis. An important aspect of this study pertains to the interpretation of experiments in which agents that block endocytic acidification inhibit infectivity. As we have found with the MuLVs, inhibition of infectivity may be secondary to the block of endocytic acidification. While this strongly suggests the involvement of an endocytic pathway, it does not necessarily indicate a requirement for an acidic compartment during the infectious process. Likewise, a lack of inhibition during transient treatment with the drugs would not preclude an endocytic pathway for viruses that are stable during the course of the treatment.

Semisynthetic derivatives of concanamycin A and C, as inhibitors of V- and P-type ATPases: structure-activity investigations and developments of photoaffinity probes

V-type ATPases are inhibited by the plecomacrolides bafilomycin and concanamycin, which exert their inhibitory potential at nanomolar concentrations. In addition, some P-type ATPases are inhibited at micromolar concentrations. We initiated intensive structure-activity investigations with semisynthetic concanamycin derivatives to approach the following two questions: (i) What is the pharmacophor, the structural key element, of the plecomacrolides that leads to their inhibitory potential against V- and P-type ATPases? (ii) Where is the binding site within these two different types of ATPases? In a first step, we examined where chemical modifications (O-acylations, substitutions, eliminations) could be placed without seriously affecting the inhibitory potential of the macrolides. In a second step, we used the knowledge of these structure-activity investigations to introduce traceable elements (fluorescent or radioactive) or nitrene-generating azido or carbene-generating diazirine-groups able to bind the inhibitors to their target covalently. These studies led finally to the synthesis of two photoaffinity probes that were used in labeling experiments with the purified plasma membrane V-type ATPase of Manduca sexta (described in a following paper, Huss, M., Gassel, M., Ingenhorst, G., Dröse, S., Zeeck, A., Altendorf, K., Wieczorek, H., manuscript submitted).

Potential role of vacuolar H-adenosine triphosphatase in neointimal formation in cultured human saphenous vein

OBJECTIVE

Vacuolar H(+)-adenosine triphosphatase plays a pivotal role in pH regulation and molecular transport across the vacuolar membranes and is involved in cell proliferation and transformation. In the present study, possible involvement of vacuolar H(+)-adenosine triphosphatase in neointimal formation was investigated in an organ culture model of human saphenous vein.

METHODS AND RESULTS

Cultured saphenous vein segments developed neointimal formation and marked thickening of the media within 14 days. Neointimal formation and medial thickening were completely inhibited by 10 nmol/L bafilomycin A(1), a selective inhibitor of vacuolar H(+)-adenosine triphosphatase, although structurally related macrolide antibiotics FK-506 and erythromycin were without an effect. The neointimal cells were positive for alpha-smooth muscle actin and vimentin but negative for desmin, indicative of myofibroblasts. The emergence of myofibroblasts was inhibited, and endothelial cells were preserved in the saphenous vein segments treated with bafilomycin A(1). Uptake of bromodeoxyuridine, a proliferation marker, by myofibroblasts was abrogated in the saphenous vein segments treated with 10 nmol/L bafilomycin A(1). Detection of apoptotic cells by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling concomitant with identification of desmin-expressing smooth muscle cells demonstrated that neointimal myofibroblasts, but not medial smooth muscle cells, that expressed desmin underwent apoptosis by treatment with bafilomycin A(1).

CONCLUSIONS

These results suggest that vacuolar H(+)-adenosine triphosphatase may be involved in myofibroblast growth that contributes to neointimal formation and medial thickening in cultured human saphenous vein. Increased sensitivity of myofibroblasts, but not endothelial cells, and differentiated smooth muscle cells to bafilomycin A(1) may have potential therapeutic implications in the treatment for vein graft disease.

Stage-dependent redistribution of the V-ATPase during bovine implantation

The 16-kD subunit of the vacuolar H(+)-ATPase (V-ATPase), or ductin, is essential for the activity of this proton pump and has roles in intercellular communication and control of cell growth and differentiation. The V-ATPase is important for acidification-dependent degradation of tissue matrices through which some cell types move, and for pH regulation across some epithelial cell layers. Placentation involves intricate signaling, cell proliferation, and controlled invasion. We examined the distribution of three subunits of the V-ATPase in bovine trophoblast and endometrium at the time of implantation to determine the relationship of ductin expression to that of two other subunits, A (approximately 73 kD) and B (approximately 58 kD). Epithelial expression of all three subunits was observed, and in nonpregnant animals this expression was apical. As pregnancy proceeded, expression of all subunits became pericellular in luminal but not glandular epithelium, suggesting a redistribution of V-ATPase activity. The trophoblast expressed all three subunits during initial contact with the epithelium. In the stroma, ductin expression was reduced after implantation, and we discuss the possibility that ductin plays a role in the shifting communication between stromal and epithelial cells induced by embryo attachment. (J Histochem Cytochem 47:1247-1254, 1999)

beta(1) integrin binds the 16-kDa subunit of vacuolar H(+)-ATPase at a site important for human papillomavirus E5 and platelet-derived growth factor signaling

Integrins mediate adhesive interactions between cells and the extracellular matrix, and play a role in cell migration, proliferation, differentiation, cytoskeletal organization, and signal transduction. We have identified an interaction between the beta(1) integrin and the 16-kDa subunit of vacuolar H(+)-ATPase (16K). This interaction was first isolated in a yeast two-hybrid screen and confirmed by coimmunoprecipitation and in in vitro binding assays using bacterially expressed proteins. Immunofluorescent studies performed in L6 myoblasts expressing both native and epitope-tagged 16K demonstrate co-localization with beta(1) integrin in focal adhesions. Deletion of the fourth of four transmembrane helices in 16K results in loss of interaction with beta(1) integrin in vitro and in the two-hybrid system, and less prominent staining in focal adhesions. This helix is also required for ligand-independent activation of platelet-derived growth factor-beta receptor signaling by the human papillomavirus E5 oncoprotein. Overexpression of 16K or expression of 16K lacking this helix alters the morphology of myoblasts and fibroblasts, suggesting that the interaction of 16K with integrins could be important for cell growth control. We also discuss the possible role 16K might play in integrin movement.

Bafilomycin A1 induces apoptosis in the human pancreatic cancer cell line Capan-1

Bafilomycin A1, a specific inhibitor of vacuolar type H(+)-ATPase, can inhibit the growth of a variety of cultured cells in a dose-dependent manner, but its mechanism is unclear. The aim of this study was to examine whether bafilomycin A1 inhibits the growth of Capan-1 human pancreatic cancer cells through apoptosis. The effect of bafilomycin A1 on tumour growth in vitro and in vivo was examined using an MTT assay and an in vivo tumour model. The presence or absence of apoptosis was determined by morphology and DNA analysis of tumour cells. The concentration of bafilomycin A1 for 50 per cent inhibition of cell viability during 72 h by the MTT assay was 5 nm. In DNA analysis, a ladder of fragmented DNA was detected in Capan-1 cells treated with bafilomycin A1 at concentrations greater than 10 nm for 24 h. Nude mice bearing a xenografted Capan-1 cell line tumour received 4 weeks of bafilomycin A1 (1.0 mg/kg per day). This treatment significantly inhibited tumour growth compared with controls after 21 days (P < 0.05). Histopathological examination of tumour cells in the treated group demonstrated signs of apoptosis with chromatin condensation and cell shrinkage. These observations suggest that bafilomycin A1 inhibits the growth of Capan-1 human pancreatic cancer cells through apoptosis.

Bafilomycin A1 and intracellular multiplication of Legionella pneumophila

Multiplication of Legionella pneumophila in HeLa cells was found to be inhibited by noncytotoxic concentrations of bafilomycin A1, with blockage of bacterial growth at a concentration 15.6 nM. The inhibiting action was evident only when the antibiotic was present during the initial phase of intracellular multiplication, i.e., during the formation of the phagosome, whereas the addition of the drug did not affect microorganisms already actively multiplying within the phagosome.

Bafilomycin A1 induces apoptosis in PC12 cells independently of intracellular pH

PC12 cells growth-arrested with bafilomycin A1 died showing apoptotic chromatin condensation in the nuclei. The bafilomycin A1-induced chromatin condensation was preceded by neurite outgrowth (NOG), required higher concentrations of bafilomycin A1 than NOG, and was suppressed by cycloheximide and aurintricarboxylic acid. NH4Cl (10 mM), another acidotropic pH perturbing agent, neither induced apoptotic chromatin condensation by itself nor suppressed that induced by bafilomycin A1, suggesting that bafilomycin A1-induced apoptosis occurs independently of intracellular pH in PC12 cells.

Expression of 16 kDa proteolipid of vacuolar-type H(+)-ATPase in human pancreatic cancer

Recent studies have shown that bafilomycin A1-sensitive vacuolar-type H(+)-ATPase (V-ATPase) plays important roles in cell growth and differentiation. However, there is no published study that has focused on the expression of V-ATPase in human tumour tissues. This study was designed to examine the mRNA and protein levels for the 16 kilodalton (kDa) proteolipid of V-ATPase in human pancreatic carcinoma tissues. We first investigated the mRNA level for V-ATPase in six cases of invasive pancreatic cancers and two normal pancreases, using reverse transcription-polymerase chain reaction technique. Then, we examined immunohistochemically the level of V-ATPase protein in 49 pancreatic cancers and ten benign cystic neoplasms of the pancreas, using antisera raised against the 16 kDa proteolipid. There was a notable difference in the level of V-ATPase mRNA between normal and pancreatic carcinoma tissues, with no evident difference in the expression of the beta-actin gene. Immunohistochemically, 42 out of 46 invasive ductal cancers (92%) displayed a mild to marked immunoreactivity for V-ATPase in the cytoplasm, whereas neither non-invasive ductal cancers nor benign cystic neoplasms expressed detectable immunoreactive proteins. These findings suggest that the overexpression of V-ATPase protein is characteristic of invasive pancreatic tumours. V-ATPase may play some crucial roles in tumour progression.

Inhibition of mitogen-induced DNA synthesis by bafilomycin A1 in Swiss 3T3 fibroblasts

Quiescent cells (in G0) can be stimulated to enter the cell cycle and proceed to DNA synthesis in S-phase by a wide range of growth factors and mitogens. Activation of cell-surface growth factor receptors with intrinsic protein tyrosine kinase activity initiates autophosphorylation of the receptors and subsequent activation of signal transduction cascades. After activation the receptors undergo ligand-induced internalization to endosomes, which become acidified by the action of a vacuolar H(+)-ATPase (V-ATPase). The extent to which vesicular acidification plays a role in mitogenic signalling by receptors with intrinsic tyrosine kinase activity remains unknown. Here we have shown that bafilomycin A1, a specific inhibitor of V-ATPase, inhibits endosome acidification and mitogen-induced DNA synthesis in Swiss 3T3 fibroblasts. Addition of bafilomycin A1 at successively later times during G1 progressively decreased the inhibition of DNA synthesis such that no inhibition was observed when bafilomycin A1 was added at the onset of S-phase. Bafilomycin A1 also induced a dramatic but reversible change in the morphology of Swiss 3T3 cells. However, the rapid activation of c-fos mRNA accumulation by epidermal growth factor and insulin was unaffected by bafilomycin A1. Together, the results suggest that activation of the V-ATPase plays an important role in the mitogenic signalling pathways that occur during the G1 phase of the cell cycle but is not required for the initial epidermal growth factor and insulin-evoked signalling events that lead to c-fos mRNA expression.