Pancreatic cancer is extremely aggressive and often has a poor prognosis, so understanding the underlying molecular mechanism that keeps these particular cancer cells alive is critical to improving patient outcomes. Now, a new study at the NYU Grossman School of Medicine reveals the mechanism that helps pancreatic cancer cells avoid starvation within dense tumors by hijacking a process that pulls nutrients in from their surroundings.
The new research—published in Nature through an article titled “Plasma membrane V-ATPase controls oncogenic RAS-induced macropinocytosis”— explains how changes in the RAS gene, known to encourage the abnormal growth seen in 90% of pancreatic cancer patients, also accelerates a process that supplies the building blocks required for that growth.
“We found a mechanism related to nutrient supply that we believe could be used to deny RAS mutant tumor cells of a key survival mechanism,” remarked lead study investigator Craig Ramirez, PhD, a postdoctoral fellow in the department of biochemistry and molecular pharmacology at NYU School of Medicine.
The process the NYU team was studying, called macropinocytosis, engulfs proteins and fats, which can be broken down into amino acids and metabolites used to build new proteins, DNA strands, and cell membranes. Cancer cells cannot multiply without these resources on hand, noted the study authors.
The research team found that RAS mutations further activate the protein SLC4A7, which enables the protein called bicarbonate-dependent soluble adenylate cyclase to activate the enzyme protein kinase A. This, in turn, was found to change the location of a protein called v-ATPase.
“We identify vacuolar ATPase (V-ATPase) as an essential regulator of RAS-induced micropinocytosis,” the authors wrote. “Oncogenic RAS promotes the translocation of V-ATPase from intracellular membranes to the plasma membrane via a pathway that requires the activation of protein kinase A by a bicarbonate-dependent soluble adenylate cyclase. Accumulation of V-ATPase at the plasma membrane is necessary for the cholesterol-dependent plasma-membrane association of RAC1, a prerequisite for the stimulation of membrane ruffling and macropinocytosis.”
By shifting where v-ATPase operates from the depths of cells to areas near their outer membranes, the reaction positions the enzyme to deliver the cholesterol needed by RAC1 to attach to cell membranes, the researchers said. A build-up of v-ATPase near outer membranes, and the related positioning of Rac1, enable membranes to temporarily bulge, roll over on themselves, and form nutrient-vesicles during macropinocytosis.
Interestingly, in cell culture studies, treatment of mutant RAS cells with the SLC4 family inhibitor S0859 led to a significant reduction in RAS-dependent v-ATPase localization to outer membranes, as well as to the inhibition of micropinocytosis. Moreover, analysis of molecular data from human pancreatic ductal adenocarcinoma (PDAC) tissue revealed that the gene for SLC4A7 is expressed four-fold higher in tumors than in normal nearby pancreatic tissue.
The study team also showed that silencing the gene for SLC4A7 in pancreatic cancer cells slowed down or shrunk tumors in mice. After 14 days, 62% of tumors with silenced SLC4A7 showed reduced growth in mice compared with tumors with the active gene, and 31% of tumors showed shrinkage.
“We are now searching for drug candidates that might inhibit the action of SLC4A7 or v-ATPase as potential future treatments that block macropinocytosis,” concluded senior study investigator Dafna Bar-Sagi, PhD, senior vice president, vice dean for science, and chief scientific officer at NYU Langone Health. “Both of these proteins are in principle good targets because they’re linked to cancer growth and operate near the cancer cell surfaces, where a drug delivered through the bloodstream could reach them.”