Further Evidence that Gradients of Extracellular pH Direct Migration of MDA-MB-231 Cells In Vitro

We hypothesised that concentration gradients of O₂/H⁺ within tissue guide migration of primary cancer cells toward intra-tumour microvessels, thus promoting intravasation and eventual haematogenous metastasis of cancer cells. Previously, we demonstrated in vitro that MDA-MB-231 cells under pH and O₂ gradients (0.2-0.3 units/mm and ~ 6%/mm, respectively) migrate toward higher pH/O₂ regions. The present study was designed to address questions yet unanswered in the previous one, i.e., (1) whether extracellular O₂ gradients could be a cue for directional cell migration in physiologically relevant O₂ environments, and (2) whether average pH level in the bulk extracellular medium affects directional cell migration. In the absence of pH gradients, directional cell migration was not demonstrated at a physiological O₂ level (<5%). We demonstrated that both the migration velocity and directionality are significantly affected by the average extracellular pH level. This result is consistent with our model for directional cell migration that does not necessitate sensing of pH gradient at a single cell level. Thus, in this study, we demonstrated further evidence that gradients of extracellular pH direct migration of MDA-MB-231 cells in vitro.