Elevated expression of the Na⁺/H⁺ exchanger NHE1 has been correlated with tumor malignancy. In contrast, the tumor suppressor protein Merlin protects from cancerogenesis. Merlin is highly related to the ERM (Moesin-Ezrin-Radixin) protein family that is directly attached to and functionally linked with NHE1. Besides, NHE1 activity can be stimulated via the Na⁺/H⁺ exchanger regulatory factor 1 (NHERF1) that, in turn, is a common interaction partner for Merlin.

Do NHE1 expression and activity affect Merlin or, vice versa, does Merlin have an impact on NHE1?

Human melanoma (MV3) cells were stably transfected with NHE1. Western blots and immunofluorescence confirmed NHE1 overexpression. Accordingly, NHE1 activity was elevated as determined by measuring the pH recovery rate upon cytosolic acidification (NH₄Cl prepulse method). Furthermore, we used atomic force microscopy (AFM) in order to assess the cell stiffness. The cortex of NHE1 overexpressing cells was significantly stiffer whereas the bulk stiffness remained unaffected. Migratory activity analysed by video microscopy was not significantly changed, whereas cell branching was clearly raised. Neither NHE1 expression nor its activity had an impact on Merlin expression. In contrast, Merlin-deficient MV3 cells, created by transient transfection with siRNA, showed a decreased NHE1 expression and, paradoxically, an increase in NHE1 activity. Interestingly, loss of Merlin also led to a higher cell volume and a clearly elevated cell motility that could be further increased by combining the Merlin deficiency with NHE1 overexpression.

These results suggest a novel tumor suppressor function of Merlin by inhibition of the proto-oncogenic NHE1 activity in melanoma cells.