Vacuolar-type H+-ATPase (V-ATPase) is a multi-subunit enzyme that has important roles in the acidification of a variety of intracellular compartments and some extracellular milieus. Four isoforms for the membrane-intrinsic subunit (subunit a) of the V-ATPase have been identified in mammals, and they confer distinct cellular localizations and activities on the proton pump. We found that V-ATPase with the a3 isoform is highly expressed in pancreatic islets, and is localized to membranes of insulin-containing secretory granules in β-cells. oc/oc mice, which have a null mutation at the a3 locus, exhibited a reduced level of insulin in the blood, even with high glucose administration. However, islet lysates contained mature insulin, and the ratio of the amount of insulin to proinsulin in oc/oc islets was similar to that of wild-type islets, indicating that processing of insulin was normal even in the absence of the a3 function. The insulin contents of oc/oc islets were reduced slightly, but this was not significant enough to explain the reduced levels of the blood insulin. The secretion of insulin from isolated islets in response to glucose or depolarizing stimulation was impaired. These results suggest that the a3 isoform of V-ATPase has a regulatory function in the exocytosis of insulin secretion.

In the epididymis and vas deferens, the vacuolar H(+)ATPase (V-ATPase), located in the apical pole of narrow and clear cells, is required to establish an acidic luminal pH. Low pH is important for the maturation of sperm and their storage in a quiescent state. The V-ATPase also participates in the acidification of intracellular organelles. The V-ATPase contains many subunits, and several of these subunits have multiple isoforms. So far, only subunits ATP6V1B1, ATP6V1B2, and ATP6V1E2, previously identified as B1, B2, and E subunits, have been described in the rat epididymis. Here, we report the localization of V-ATPase subunit isoforms ATP6V1A, ATP6V1C1, ATP6V1C2, ATP6V1G1, ATP6V1G3, ATP6V0A1, ATP6V0A2, ATP6V0A4, ATP6V0D1, and ATP6V0D2, previously labeled A, C1, C2, G1, G3, a1, a2, a4, d1, and d2, in epithelial cells of the rat epididymis and vas deferens. Narrow and clear cells showed a strong apical staining for all subunits, except the ATP6V0A2 isoform. Subunits ATP6V0A2 and ATP6V1A were detected in intracellular structures closely associated but not identical to the TGN of principal cells and narrow/clear cells, and subunit ATP6V0D1 was strongly expressed in the apical membrane of principal cells in the apparent absence of other V-ATPase subunits. In conclusion, more than one isoform of subunits ATP6V1C, ATP6V1G, ATP6V0A, and ATP6V0D of the V-ATPase are present in the epididymal and vas deferens epithelium. Our results confirm that narrow and clear cells are well fit for active proton secretion. In addition, the diverse functions of the V-ATPase may be established through the utilization of specific subunit isoforms. In principal cells, the ATP6V0D1 isoform may have a physiological function that is distinct from its role in proton transport via the V-ATPase complex.

The vacuolar-type H؉ -ATPase (V-ATPase) translocates protons across membranes. Here, we have identified a mouse cDNA coding for a fourth isoform (a4) of the membrane sector subunit a of V-ATPase. This isoform was specifically expressed in kidney, but not in the heart, brain, spleen, lung, liver, muscle, or testis. Immunoprecipitation experiments, together with sequence similarities for other isoforms (a1, a2, and a3), indicate that the a4 isoform is a component of V-ATPase. Moreover, histochemical studies show that a4 is localized in the apical and basolateral plasma membranes of cortical ␣-and ␤-intercalated cells, respectively. These results suggest that the V-ATPase, with the a4 isoform, is important for renal acid/base homeostasis. A ubiquitous vacuolar-type Hϩ -ATPase (V-ATPase) 1 translocates protons across membranes utilizing the energy of ATP hydrolysis (for reviews, see Refs. 1-5). The organellar acidic pH generated by V-ATPase is responsible for various processes including zymogen activation, receptor-mediated endocytosis, macromolecule degradation, and protein sorting. The enzyme is also found in plasma membranes, where it transports protons outside cells such as osteoclasts (6, 7), renal-intercalated cells (8), and epithelial cells of the seminal duct and bladder (9, 10).V-ATPase has a membrane peripheral V 1 sector for ATP hydrolysis and an integral V o for proton translocation (1-5). The V o sector consists of at least five subunits (a, c, cЈ, cЉ, and d) (11). Subunit a is the largest (116 kDa) of the V-ATPase subunits, and its isoforms have been found in yeast (12), chicken (13), mouse (7, 14, 15), cow (16, 17), and human (18). These isoforms exhibit different distributions in organelles and tissues. Yeast isoforms (Vph1p and Stv1p) are localized in vacuoles and Golgi/endosomes, respectively (12). Three isoforms (a1, a2, and a3) have been found previously in mammals (7, 14 -18). These isoforms may be important for the localization of V-ATPase in various organelles or plasma membranes.In this study, we have isolated a mouse cDNA coding for a fourth isoform (a4) of V-ATPase subunit a. Isoform a4 exhibits high similarities with the a1, a2, and a3 isoforms and is expressed exclusively in kidney. Furthermore, a4 was localized immunochemically in the apical and basolateral plasma membranes of cortical ␣-and ␤-intercalated cells, respectively. These results suggest that the V-ATPase with the a4 isoform is a kidney-specific proton pump important for acid/base homeostasis.

Accumulating evidence indicates that the acidic microenvironments critically influence malignant behaviors of cancer including invasiveness, metastasis, and chemoresistance. Because the vacuolar-type H þ -ATPase (V-ATPase) has been shown to cause extracellular acidification by pumping protons, we studied the role of V-ATPase in distant metastasis. Real-time PCR analysis revealed that the high-metastatic B16-F10 melanoma cells strongly expressed the a3 isoform V-ATPase compared to the low-metastatic B16 parental cells. Consistent with this, B16-F10 cells created acidic environments in lung metastases by acridine orange staining and strong a3 V-ATPase expression in bone metastases by immunohistochemistry. Immunocytochemical analysis showed B16-F10 cells expressed a3 V-ATPase not only in cytoplasm but also plasma membrane, whereas B16 parental cells exhibited its expression only in cytoplasm. Of note, knockdown of a3 V-ATPase suppressed invasiveness and migration with reduced MMP-2 and MMP-9 expression in B16-F10 cells and significantly decreased lung and bone metastases, despite that tumor growth was not altered. Importantly, administration of a specific V-ATPase a3 inhibitor FR167356 reduced bone metastasis of B16-F10 cells. These results suggest that a3 V-ATPase promotes distant metastasis of B16-F10 cells by creating acidic environments via proton secretion. Our results also suggest that inhibition of the development of cancer-associated acidic environments by suppressing a3 V-ATPase could be a novel therapeutic approach for the treatment of cancer metastasis. Mol Cancer Res; 9(7); 845-55. Ó2011 AACR.