V-ATPase functions in normal and disease processes

V-ATPase inhibitors and implication in cancer treatment

Acidity is one of the main features of the tumors. The V-ATPase is the primary responsible for the control of tumor microenvironment by proton extrusion to the extracellular medium. The acid environment favors tissue damage, activation of destructive enzymes in the extracellular matrix, the acquisition of metastatic cell phenotypes as well as increasing the destructive capacity. The application of specific inhibitors of V-ATPases, can decrease the acidity of tumor and may allow the reduction of tumor metastasis, acting on the survival of tumor cells and prevent the phenomena of chemoresistance. Among the most important inhibitors can be distinguished benzolactone enamides (salicylihalamide), lobatamide A and B, apicularen, indolyls, oximidine, macrolactone archazolid, lobatamide C, and cruentaren. The latest generation of inhibitors includes NiK12192, FR202126, and PPI SB 242784. The purpose of this paper is to describe the latest advances in the field of V-ATPase inhibitors, describe further developments related to the classic inhibitors, and discuss new potential applications of these drugs in cancer treatment.

Loss of the apical V-ATPase a-subunit VHA-6 prevents acidification of the intestinal lumen during a rhythmic behavior in C. elegans

In Caenorhabditis elegans, oscillations of intestinal pH contribute to the rhythmic defecation behavior, but the acid-base transport mechanisms that facilitate proton movement are not well understood. Here, we demonstrate that VHA-6, an intestine-specific a-subunit of the H(+)-K(+)-ATPase complex (V-ATPase), resides in the apical membrane of the intestinal epithelial cells and is required for luminal acidification. Disruption of the vha-6 gene led to early developmental arrest; the arrest phenotype could be complemented by expression of a fluorescently labeled vha-6 transgene. To study the contribution of vha-6 to pH homeostasis in larval worms, we used a partial reduction of function through postembryonic single-generation RNA interference. We demonstrate that the inability to fully acidify the intestinal lumen coincides with a defect in pH recovery of the intestinal epithelial cells, suggesting that VHA-6 is essential for proton pumping following defecation. Moreover, intestinal dipeptide accumulation and fat storage are compromised by the loss of VHA-6, suggesting that luminal acidification promotes nutrient uptake in worms, as well as in mammals. Since acidified intracellular vesicles and autofluorescent storage granules are indistinguishable between the vha-6 mutant and controls, it is likely that the nutrient-restricted phenotype is due to a loss of plasma membrane V-ATPase activity specifically. These data establish a simple genetic model for proton pump-driven acidification. Since defecation occurs at 45-s intervals in worms, this model represents an opportunity to study acute regulation of V-ATPase activity on a short time scale and may be useful in the study of alternative treatments for acid-peptic disorders.

Inhibitors of V-ATPases: old and new players

V-ATPases constitute a ubiquitous family of heteromultimeric, proton translocating proteins. According to their localization in a multitude of eukaryotic endomembranes and plasma membranes, they energize many different transport processes. Currently, a handful of specific inhibitors of the V-ATPase are known, which represent valuable tools for the characterization of transport processes on the level of tissues, single cells or even purified proteins. The understanding of how these inhibitors function may provide a basis to develop new drugs for the benefit of patients suffering from diseases such as osteoporosis or cancer. For this purpose, it appears absolutely essential to determine the exact inhibitor binding site in a target protein on the one side and to uncover the crucial structural elements of an inhibitor on the other side. However, even for some of the most popular and long known V-ATPase inhibitors, such as bafilomycin or concanamycin, the authentic structures of their binding sites are elusive. The aim of this review is to summarize the recent advances for the old players in the inhibition game, the plecomacrolides bafilomycin and concanamycin, and to introduce some of the new players, the macrolacton archazolid, the benzolactone enamides salicylihalamide, lobatamide, apicularen, oximidine and cruentaren, and the indolyls.

The relationship between spotted fever group Rickettsiae and ixodid ticks

Spotted fever group Rickettsiae are predominantly transmitted by ticks. Rickettsiae have developed many strategies to adapt to different environmental conditions, including those within their arthropod vectors and vertebrate hosts. The tick-Rickettsiae relationship has been a point of interest for many researchers, with most studies concentrating on the role of ticks as vectors. Unfortunately, less attention has been directed towards the relationship of Rickettsiae with tick cells, tissues, and organs. This review summarizes our current understanding of the mechanisms involved in the relationship between ticks and Rickettsiae and provides an update on the recent methodological improvements that have allowed for comprehensive studies at the molecular level.

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.

Regulation of the V-ATPase along the endocytic pathway occurs through reversible subunit association and membrane localization

The lumen of endosomal organelles becomes increasingly acidic when going from the cell surface to lysosomes. Luminal pH thereby regulates important processes such as the release of internalized ligands from their receptor or the activation of lysosomal enzymes. The main player in endosomal acidification is the vacuolar ATPase (V-ATPase), a multi-subunit transmembrane complex that pumps protons from the cytoplasm to the lumen of organelles, or to the outside of the cell. The active V-ATPase is composed of two multi-subunit domains, the transmembrane V₀ and the cytoplasmic V₁. Here we found that the ratio of membrane associated V₁/Vo varies along the endocytic pathway, the relative abundance of V₁ being higher on late endosomes than on early endosomes, providing an explanation for the higher acidity of late endosomes. We also found that all membrane-bound V-ATPase subunits were associated with detergent resistant membranes (DRM) isolated from late endosomes, raising the possibility that association with lipid-raft like domains also plays a role in regulating the activity of the proton pump. In support of this, we found that treatment of cells with U18666A, a drug that leads to the accumulation of cholesterol in late endosomes, affected acidification of late endosome. Altogether our findings indicate that the activity of the vATPase in the endocytic pathway is regulated both by reversible association/dissociation and the interaction with specific lipid environments.