Tumour cell migration is a major process in tumour metastasis and requires distinct cell-matrix- interactions at focal adhesion sites. It has been shown that one mechanism of modulating the cell-matrix-interaction in human melanoma cells (MV3) involves the activity of the Na⁺/H⁺-exchanger NHE1. This antiporter accumulates at the cell front and creates an extracellular pH-gradient at the cell surface with the leading edge being more acidic by up to 0.2 pH-units. Protons expelled by the NHE1 stabilise integrin-matrix-adhesions at the leading edge of the cell thus enabling the cell to create propulsive forces and to migrate directionally. We were now able to show that the pH gradient at the outer cell surface correlates with a complementary cytosolic pH gradient, which also appears to be NHE1-dependent. Ratiometric measurements of the intracellular pH (pHi) using the proton-sensitive, fluorescent dye BCECF revealed that the leading edge of MV3 cells is 0.15 pH units more alkaline than the rear end (DpH=0.15; n= 48; p <10–5). Inhibition of NHE1 either with cariporide (HOE 642) or by exposing the cells to Na⁺-free Ringer solution acidifies the cells and causes the gradient to flatten (DpH=0.08 with cariporide and 0.07 in Na⁺-free solution). These results strongly suggest an NHE1-dependency of the intracellular pH-gradient of human melanoma cells.

The observed pHi gradient was also present in murine melanoma cells (B16) (DpH=0.13; n=3) whereas transformed renal epithelial cells (MDCK-F1) showed no significant pH-gradient (n=5). Furthermore, the overall pHi of tumour cells was generally more alkaline (pHi 7.45 for MV3 cells and pHi 7.58 for B16 cells) than the one of MDCK-F1 cells (pHi 7.21). Altogether these findings imply that a higher activity of NHE1 in tumour cells causes a more alkaline cytosolic environment and a distinct pHi gradient, which may facilitate tumour cell migration.