Introduction: 

Within a narrow range of extracellular pH human melanoma cells (MV3) show integrin-mediated adhesion and migration. They are even able to establish their own pH-nanoenvironment at the cell surface, which is essential for their migration. Yet, practically nothing is known about their ability to utilize extracellular pH-gradients, as they can be found in solid tumors, for directional migration.

Methods: 

In order to assess MV3 cell migration in pH-gradients we established an in vitro model. Using the pH-sensitive fluorescein conjugate BCECF and a ratiometric measurement method we validated the stability of the gradient over several hours. Employing PCR, western blot, and immunofluorescence analyses we studied the expression and subcellular distribution of potential proton sensors. Cytosolic Ca²⁺ was measured with FURA-2.

Results: 

Migration experiments on MV3 cells indicate a directional migration away from physiological towards more acidic pH. The proton sensitive G-protein coupled receptors OGR1, GPR4 and TDAG8 were expressed at mRNA level and the expression of OGR1 and GPR4 could be confirmed at the protein level. OGR1 and GPR4 were evenly distributed all over the cell. Since Ca²⁺ measurements did not reveal a signal from intracellular stores upon extracellular acidification the involvement of OGR1 in pH-sensing remains uncertain.

Conclusion: 

As migration along a pH-gradient has never been observed before in eukaryotic cells, we expect the pH-taxis of tumor cells to become an interesting topic in both biological and clinical research. The established pH-gradient should turn out to be useful to further study pH-taxis and its underlying molecular mechanisms.