Background:

The Na⁺/H⁺ exchanger isoform 1 (NHE1) is believed to be essential for directional cell migration during wound healing and tumour invasiveness. However, in the intestinal epithelium, which displays very rapid cellular restitution of surface wounds, other NHE isoforms (NHE2, NHE3) are expressed far more strongly.

Aim and methods:

We studied the role of NHE isoforms 1, 2 and 3 on migration speed during experimental would healing in RGM1 cells, a nontransformed gastric surface cell line and in Caco2bbe cells, a well differentiating colonic tumour cell line. Stable NHE3 overexpression was performed in Caco2bbe cells, while lentiviral transient NHE2 overexpression was achieved in RGM-1 cells. Migration velocity was studied during growth factor and low pH-stimulated as well as serum starvation-inhibited wound healing. To assess cell migration speed objectively, a fluorescent dye-based plate reader assay was used to assess experimental wound closure velocity. NHE isoforms were selectively inhibited by HOE642 and S1611 in different concentrations, and NHE overexpression was employed.

Results:

In confluent monolayers, RGM1 cells express NHE1 mRNA in far higher levels than other NHEs, whereas NHE2 mRNA expression is higher than NHE1 and NHE3 in Caco2bbe cells. Both cell lines displayed cell migration after experimental wounding even after serum starvation and in alkaline pH, and this migratory speed was independent of NHE activity in either cell line. Fetal calf serum (FCS) as well as epidermal growth factor (EGF) significantly stimulated cell migration in a partly NHE1-dependent fashion in RGM-1 cells, but in a NHE-independent fashion in Caco2bbe cells. Preincubation at low pH also stimulated migratory speed in a NHE1-dependent fashion in both RGM1 and Caco2bbe cells. An influence of NHE2 activity on cell migration could not be shown in NHE2 overexpressing RGM1 or in Caco2bbe cells. NHE3 inhibition also did not result in a change in migratory speed.

Conclusions:

Directed cell migration during wound closure is possible even in the absence of NHE functional activity in gastrointestinal epithelial cells. Dependency of cell migration on NHE1 varies among cell types. Low pH enhances cell migration in a NHE1-dependent fashion, whereas stimulation by growth factors is predominantly NHE1-independent. Therapeutic NHE1 inhibition for gastrointestinal malignancy is therefore likely to have a highly variable effect on tumour invasiveness depending on the cell type.