pH nanoenvironment at the surface of single melanoma cells

Effect of NHE1 antisense gene transfection on the biological behavior of SGC-7901 human gastric carcinoma cells

AIM: To study the effect of type 1 Na⁺/H⁺ exchanger (NHE1) antisense human gene transfection on the biological behavior of gastric carcinoma cell line SGC-7901.

METHODS: Antisense NHE1 eukaryotic expression on vector pcDNA3.1 was constructed by recombinant DNA technique and transfected into gastric carcinoma cell line SGC-7901 with DOTAP liposome transfection method. Morphological changes of cells were observed with optic and electron microscopes. Changes in cell proliferative capacity, apoptosis, intracellular pH (pH_i), cell cycle, clone formation in two-layer soft agar, and tumorigenicity in nude mice were examined.

RESULTS: Antisense eukaryotic expressing vectors were successfully constructed and transfected into SGC-7901. The transfectant obtained named 7901-antisense (7901-AS) stablely produced antisense NHE1. There was a significant difference between the pH_i of 7901-AS cells (6.77 ± 0.05) and that of 7901-zeo cells and SGC-7901 cells (7.24 ± 0.03 and 7.26 ± 0.03, P < 0.01). Compared with SGC-7901 and 7901-zeo cells, 7901-AS cells mostly showed cell proliferation inhibition, G₁/G₀ phase arrest, increased cell apoptotic rate, recovery of contact inhibition, and density contact. The tumorigenicity in nude mice and cloning efficiency in the two-layer soft agar were clearly inhibited.

CONCLUSION: NHE1 antisense gene significantly restrains the malignant behavior of human gastric carcinoma cells, suppresses cell growth and induces cell apoptosis, and partially reverses the malignant phenotypes of SGC-7901. These results suggest a potential role for human tumor gene therapy.

Protons extruded by NHE1: digestive or glue?

Many physiological and pathophysiological processes, such as embryogenesis, immune defense, wound healing, or metastasis, are based on cell migration and invasion. The activity of the ubiquitously expressed NHE1 isoform of the plasma membrane Na(+)/H(+) exchanger is one of the requirements for directed locomotion of migrating cells. The mechanisms by which NHE1 is involved in cell migration are multiple. NHE1 contributes to cell migration by affecting the cell volume, by regulating the intracellular pH and thereby the assembly and activity of cytoskeletal elements, by anchoring the cytoskeleton to the plasma membrane, by the organization of signal transduction and by regulating gene expression. The present review focuses on two additional, extracellular mechanisms by which NHE1 activity contributes to cell migration and invasion. Protons extruded by the NHE1 lead to local, extracellular acidification which, on the one hand, can create pH optima needed for the activity of proteinases at invadopodia/podosomes necessary for extracellular matrix digestion and, on the other hand, facilitates cell/matrix interaction and adhesion at the cell front.

Dynamics of single potassium channel proteins in the plasma membrane of migrating cells

Cell migration is an important physiological process among others controlled by ion channel activity. Calcium-activated potassium channels (K(Ca)3.1) are required for optimal cell migration. Previously, we identified single human (h)K(Ca)3.1 channel proteins in the plasma membrane by means of quantum dot (QD) labeling. In the present study, we tracked single-channel proteins during migration to classify their dynamics in the plasma membrane of MDCK-F cells. Single hK(Ca)3.1 channels were visualized with QD- or Alexa488-conjugated antibodies and tracked at the basal cell membrane using time-lapse total internal reflection fluorescence (TIRF) microscopy. Analysis of the trajectories allowed the classification of channel dynamics. Channel tracks were compared with those of free QD-conjugated antibodies. The size of the label has a pronounced effect on hK(Ca)3.1 channel diffusion. QD-labeled channels have a (sub)diffusion coefficient D(QDbound) = 0.067 microm(2)/s(alpha), whereas that of Alexa488-labeled channels is D(Alexa) = 0.139 microm(2)/s. Free QD-conjugated antibodies move much faster: D(QDfree) = 2.163 microm(2)/s(alpha). Plotting the mean squared distances (msd) covered by hK(Ca)3.1 channels as a function of time points to the mode of diffusion. Alexa488-labeled channels diffuse normally, whereas the QD-label renders hK(Ca)3.1 channel diffusion anomalous. Free QD-labeled antibodies also diffuse anomalously. Hence, QDs slow down diffusion of hK(Ca)3.1 channels and change the mode of diffusion. These results, referring to the role of label size and properties of the extracellular environment, suggest that the pericellular glycocalyx has an important impact on labels used for single molecule tracking. Thus tracking fluorescent particles within the glycocalyx opens up a possibility to characterize the pericellular nanoenvironment.

pH dependence of melanoma cell migration: protons extruded by NHE1 dominate protons of the bulk solution

Migration and morphology of human melanoma cells (MV3) depend on extracellular pH (pHe) and the activity of the Na+/H+ exchanger NHE1. To distinguish effects of NHE1 activity per se from effects of pHe we compared an NHE1-deficient mutant with rescued and wild-type cells. Time lapse video microscopy was used to investigate migratory and morphological effects caused by pHe and NHE1 activity, and a membrane-bound fluorescein conjugate was employed for ratiometric pH measurements at the outer leaflet of the cell membrane. As long as NHE1 remained inactive due to deficiency or inhibition by cariporide (HOE642) neither migration nor morphology was affected by changes in pHe. Under these conditions pH at the outer leaflet of the plasma membrane was uniform all over the cell surface. The typical pH dependence of MV3 cell migration and morphology could be reconstituted by restoring NHE1 activity. At the same time the proton gradient at the outer leaflet of the plasma membrane with the higher proton concentration at the leading edge and the lower one at the cell rear was re-established as well. Hence, NHE1 activity generates a proton gradient at the cell surface accompanied by the cells' ability to respond to changes in pHe (bulk pH). We conclude that NHE1 activity contributes to the generation of a well-defined cell surface pH by creating a proton gradient at the outer leaflet of the plasma membrane that is needed for (i) the development of a variety of morphologies including a distinct polarity and (ii) migration. A missing proton gradient at the cell surface cannot be compensated for by varying pHe.