Acidosis-mediated inhibition of cancer cell dissemination is restored by hypoxia via regulation of NHE1-dependent signaling

Tumors develop an acidic pH due to shifts in metabolism towards glycolysis and lactic acid production, a phenomenon known as the Warburg effect. Multiple studies have shown that acidosis promotes an aggressive tumor phenotype, characterized by higher migratory, invasive, and metastatic potentials1-3. We instead demonstrate that cancer cell exposure to acidic extracellular pH (pHe=6.4) suppresses proliferation, cell dissociation from tumor spheroids, and migration. Acidosis acutely inhibits motility by downregulating the activity of sodium-hydrogen exchanger isoform-1 (NHE1), which in turn suppresses phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt). Inhibition of PI3K/Akt then blocks Yes-associated protein (YAP) translocation to the nucleus, thereby reducing NHE1 expression. Although cells rapidly adjust their intracellular pH in response to pHe changes, prolonged cell exposure to acidosis downregulates NHE1 expression, which is restored in a delayed fashion upon switching to physiological pHe. This lag phase may explain the reduced extravasation of tumor cells preconditioned at acidic pHe prior to their injection in chick embryos and mice. The reduced migratory potential due to acidosis is rescued by hypoxia. Cumulatively, acidosis alone suppresses aggressive tumor phenotypes but has minimal effect in the presence of hypoxia. Overall design: Cells (500,000) were plated into T75 flasks and allowed to reach 70-80% confluence using cell culture medium with pHe=7.4. At that point, cells were washed with 2.5 mL of cell culture media with pH of 7.4, 6.7 or 6.4, and then incubated with media of prescribed pH for 24 h.