Directional Migration of MDA-MB-231 Cells Under O2/pH Gradients.

We hypothesized that cancer cells actively migrate toward intratumor microvessels, guided by tissue gradients of metabolic substrates (such as O2) and/or metabolites (such as CO2/H+). To test this hypothesis, we developed an in vitro model in which cellular energy metabolism establishes gradients of O2/nutrient/metabolite in monolayer cells cultured in a conventional culture dish. When gradients of O2 ranging from 3% to ~0% were produced, MDA-MB-231 cells located at 300, 500 and 1500 µm downstream in the gradient demonstrated significant directional migrations (Rayleigh z test). We also found a similar directionality in cell migration at the same location even when the initial O2 level in the O2 gradient was raised from 3% to 21%. Interestingly, such directionalities were no longer demonstrated when the cell density was lowered from 1.8 × 106 to 0.9 × 106 cells/ml. In the former, the magnitude of the extracellular pH gradient in regions 300 and 500 µm downstream in the gradient was significantly larger. Thus, the direction of cell migrations appeared to depend on the gradient of extracellular pH rather than on O2.

Directional Migration of MDA-MB-231 Cells Under Oxygen Concentration Gradients.

To elucidate the initial mechanism of hematogenous metastasis of cancer cells, we hypothesized that cancer cells migrate toward regions with higher oxygen concentration such as intratumor micro vessels along the oxygen concentration gradient. To produce gradients of oxygen concentration in vitro, we devised the gap cover glass (GCG). After placing a GCG onto cultured MDA-MB-231 cells (a metastatic breast cancer cell line), the migration of individual cells under the GCG was tracked up to 12 h at 3 % oxygen in the micro incubator. We quantified the migration of individual cells using forward migration index (FMI). The cell migration perpendicular to the oxygen gradients was random in the direction whereas FMIs of the cell located at 300, 500, 700, and 1500 µm from the oxygen inlet were positive (p < 0.05) indicating a unidirectional migration toward the oxygen inlet. Present results are consistent with our hypothesis that MDA-MB-231 cells migrate toward regions with higher oxygen concentration.