Acidosis Drives Vasculogenic Mimicry in PDAC CSCs via Na+/H+ Exchanger Isoform 1 (NHE1) and Calcium Entry

Vasculogenic mimicry (VM) is the ability of cancer stem cells (CSCs) to express an endothelial-like phenotype and participate in tumor neovascularization via the formation of a blood-conducting, matrix-rich network. We previously reported that pancreatic ductal adenocarcinoma (PDAC) CSCs develop their VM phenotype via two interacting and coordinated factors that support the formation of the VM network: (i) the overexpression of genes for endothelial factors and vascular receptors and (ii) the very high secretion of numerous pro-angiogenic/growth factors. While microenvironmental acidosis (low pHe) is an important driver of tumor metastasis, especially in PDAC, and is a component of the CSC niche, its role in VM and the ion transporters involved remains unknown. As normal stem cell differentiation is regulated by Na⁺/H⁺ exchanger 1 (NHE1)-driven pH, we investigated the role of NHE1 and the intracellular signaling involved in the acidosis-induced VM using a platform of 3D organotypic cultures composed of Matrigel with increasing concentrations of Collagen I. VM was highest on 90% Matrigel:10% Collagen I, representative of an early tumor ECM, and it decreased with increasing concentrations of Collagen I, representative of advanced tumors. In all ECM compositions, VM capacity increased stepwise with pHe acidification, and both basal and acid-stimulated VM were dependent on NHE1 activity. Acidification also decreased resting pHi and increased NHE1 proton extrusion activity, NHE1/ß1 integrin co-expression, and intracellular Ca²⁺. The stimulation of VM by extracellular acidosis depended on the transport of extracellular Ca²⁺ into the cell and the consequent increase in intracellular Ca²⁺. Altogether, these data demonstrate that extracellular acidification triggers cellular mechanisms that upregulate VM to overcome the constraints imposed by ECM composition, thereby permitting VM in ECMs where this phenotype is not expressed and extending the VM phenotype towards the tumor center to further drive metastasis.