V-ATPase Localization Mediates Mutant RAS–Induced Macropinocytosis

Oncogenic mutations in RAS proteins can enhance macropinocytosis, a cellular process—implicated in both tumorigenesis and metastasis—by which certain nutrients are endocytosed from the environment. Ramirez, Hauser, and colleagues found that vacuolar ATPase (V-ATPase), a transmembrane protein complex responsible for transducing protons across cellular and organellar membranes, is vital for RAS-mediated macropinocytosis. Expression of oncogenic RAS mutant proteins (HRAS^G12V or KRAS^G12V) caused redistribution of V-ATPase from the cytoplasm to the plasma membrane, a critical step in macropinocytosis. Cells in which plasma membrane V-ATPase was depleted exhibited abnormal cholesterol trafficking, reducing the membrane association of RAC1, the proper localization of which is required for the membrane-ruffling step of macropinocytosis. Further experiments revealed that macropinocytosis caused by expression of oncogenic RAS mutants was dependent on the soluble adenylate cyclase (sAC)–protein kinase A pathway. The fact that sAC activation is highly dependent on bicarbonate availability prompted an investigation of the source of bicarbonate in cells exhibiting mutant RAS–mediated macropinocytosis. Experiments aimed at addressing this question showed that the process depended on mutant RAS–induced upregulation of the expression of SLC4A7, a sodium bicarbonate cotransporter. In a mouse xenograft model of KRAS-mutant pancreatic cancer, shRNA-mediated SLC4A7 knockdown reduced tumor growth and, in some cases, was even associated with tumor regression. Notably, this effect was not seen in mice xenografted with human pancreatic cancer cells expressing wild-type KRAS, providing further evidence for the specific role of SLC4A7 in pancreatic cancer harboring mutations affecting RAS proteins. Collectively, this work provides a foundational understanding of a key pathway underlying macropinocytosis in cancer cells and demonstrates its relevance in a mouse model of pancreatic cancer.

Ramirez C, Hauser AD, Vucic EA, Bar-Sagi D. Plasma membrane V-ATPase controls oncogenic RAS-induced macropinocytosis. Nature 2019;576:477–81.

• ©2020 American Association for Cancer Research.