Vacuolar ATPase ‘a2’ isoform exhibits distinct cell surface accumulation and modulates matrix metalloproteinase activity in ovarian cancer

Abstract

Tumor associated vacuolar H+-ATPases (V-ATPases) are multi-subunit proton pumps that acidify tumor microenvironment, thereby promoting tumor invasion. Subunit ‘a’ of its V0 domain is the major pH sensing unit that additionally controls sub-cellular targeting of V-ATPase and exists in four different isoforms. Our study reports an elevated expression of the V-ATPase-V0a2 isoform in ovarian cancer(OVCA) tissues and cell lines(A2780, SKOV-3 and TOV-112D). Among all V0’a’ isoforms, V0a2 exhibited abundant expression on OVCA cell surface while normal ovarian epithelia did not. Sub-cellular distribution of V-ATPase-V0a2 confirmed its localization on plasma-membrane, where it was also co-associated with cortactin, an F-actin stabilizing protein at leading edges of cancer cells. Additionally, V0a2 was also localized in early and late endosomal compartments that are sites for modulations of several signaling pathways in cancer. Targeted inhibition of V-ATPase-V0a2 suppressed matrix metalloproteinase activity(MMP-9 & MMP-2) in OVCA cells. In conclusion, V-ATPase-V0a2 isoform is abundantly expressed on ovarian tumor cell surface in association with invasion assembly related proteins and plays critical role in tumor invasion by modulating the activity of matrix-degrading proteases. This study highlights for the first time, the importance of V-ATPase-V0a2 isoform as a distinct biomarker and possible therapeutic target for treatment of ovarian carcinoma.

The Function of V-ATPases in Cancer

“…Subsequent work demonstrated that the V-ATPase is present on the plasma membrane of invasive breast cancer cells, but not noninvasive breast cancer cells, and participates in control of cytoplasmic pH (175). The V-ATPase has since been found to be expressed on the plasma membranes of numerous invasive cancer cell types, including melanoma, Ewing sarcoma, and breast, liver, lung, ovarian, esophageal, prostate, and pancreatic cancers (9,21,23,24,63,71,90,107,125,136,176,216). Furthermore, overexpression of particular V-ATPase subunits has been observed in both human cancer cell lines and human tumor samples.…”

“…Similar results have been obtained with other cancer cell types. The invasion and migration of pancreatic cancer cells are reduced upon treatment with concanamycin A (23), and V-ATPase inhibition decreases in vitro invasion of prostate cancer cells and migration of ovarian cancer cells (90,125). Finally, knockdown of subunit c of V 0 and subunit A of V 1 using siRNA treatment, which results in complete ablation of V-ATPase activity, reduces the invasiveness of liver and gastric cancer cells, respectively (101,108,216).…”

Vacuolar ATPase as a potential therapeutic target and mediator of treatment resistance in cancer

“…Other studies have described expression of V‐ATPase/specific subunits in pancreatic cancer,32, 33 nonsmall cell lung cancer,34 oral squamous cell carcinoma,35, 36 ovarian cancer,37 cervical cancer,38 breast cancer,39, 40 gliomas,41 and hepatocellular carcinoma 42. A summary of studies investigating clinical significance of V‐ATPase in cancer is provided in Table 1.…”

“…Increased cancer cell invasive activity is frequently associated with aberrant V‐ATPase plasma membrane localization suggesting that increased acidification of the extracellular space plays an important role 29, 33, 34, 37, 47, 48, 49, 50, 51, 52…”

“…Once activated, these proteases increase extracellular matrix degradation and facilitate migration of cancer cells. For example, in ovarian cancer cells, V o a2 was found to be localized in endosomes and inhibition showed a decrease in MMP‐9 and ‐2 activity 37. A study in hepatocellular carcinoma (HCC) found that inhibiting the V o c subunit led to decreased MMP‐2 expression and extracellular MMP‐2 activity 56…”