H-dependent inorganic phosphate transporter in breast cancer cells: Possible functions in the tumor microenvironment.

Tumor microenvironment has a high concentration of inorganic phosphate (Pi), which is actually a marker for tumor progression. Regarding Pi another class of transporter has been recently studied, an H-dependent Pi transporter, that is stimulated at acidic pH in Caco2BBE human intestinal cells. In this study, we characterized the H-dependent Pi transport in breast cancer cell (MDA-MB-231) and around the cancer tissue. MDA-MB-231 cell line presented higher levels of H-dependent Pi transport as compared to other breast cell lines, such as MCF-10A, MCF-7 and T47-D. The Pi transport was linear as a function of time and exhibited a Michaelis-Menten kinetic of K = 1.387 ± 0.1674 mM Pi and V = 198.6 ± 10.23 Pi × h × mg protein hence reflecting a low affinity Pi transport. H-dependent Pi uptake was higher at acidic pH. FCCP, Bafilomycin A1 and SCH28080, which deregulate the intracellular levels of protons, inhibited the H-dependent Pi transport. No effect on pHi was observed in the absence of inorganic phosphate. PAA, an H-dependent Pi transport inhibitor, reduced the Pi transport activity, cell proliferation, adhesion, and migration. Arsenate, a structural analog of Pi, inhibited the Pi transport. At high Pi conditions, the H-dependent Pi transport was five-fold higher than the Na-dependent Pi transport, thus reflecting a low affinity Pi transport. The occurrence of an H-dependent Pi transporter in tumor cells may endow them with an alternative path for Pi uptake in situations in which Na-dependent Pi transport is saturated within the tumor microenvironment, thus regulating the energetically expensive tumor processes.