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.
Keywords: Cancer metastasis; MDA-MB-231 cell; Migration; Oxygen gradients; pH gradients.