Abstract
Detachment of cells from the primary tumour precedes metastatic progression by facilitating cell release into the tissue. Solid tumours exhibit altered pH homeostasis with extracellular acidification. In human melanoma, the Na+/H+ exchanger NHE1 is an important modifier of the tumour nanoenvironment. Here we tested the modulation of cell-cell-adhesion by extracellular pH and NHE1. MV3 tumour spheroids embedded in a collagen matrix unravelled the efficacy of cell-cell contact loosening and 3D emigration into an environment mimicking physiological confinement. Adhesive interaction strength between individual MV3 cells was quantified using atomic force microscopy and validated by multicellular aggregation assays. Extracellular acidification from pHe7.4 to 6.4 decreases cell migration and invasion but increases single cell detachment from the spheroids. Acidification and NHE1 overexpression both reduce cell-cell adhesion strength, indicated by reduced maximum pulling forces and adhesion energies. Multicellular aggregation and spheroid formation are strongly impaired under acidification or NHE1 overexpression. We show a clear dependence of melanoma cell-cell adhesion on pHe and NHE1 as a modulator. These effects are opposite to cell-matrix interactions that are strengthened by protons extruded via NHE1. We conclude that these opposite effects of NHE1 act synergistically during the metastatic cascade.
Highlights
Even be reversed so that pHe of solid tumours is more acidic than pHi and may be as low as pHe6.76–8
Based on this knowledge and the fact that upregulation of Na+/H+ exchanger isoform 1 (NHE1) has been correlated with tumour malignancy and NHE1 function through increased proton efflux with tumour cell invasiveness[26], we hypothesised that NHE1 affects cell-cell adhesion in human melanoma
Using single cell force spectroscopy (SCFS) with atomic force microscopy (AFM) we found a reduction of cell-cell-adhesion forces upon extracellular acidification and NHE1 overexpression
Summary
Even be reversed so that pHe of solid tumours is more acidic than pHi and may be as low as pHe6.76–8. We had shown that melanoma cell migration strongly depends on pHe and NHE1 activity It is inhibited by extracellular acidification below pHe7.0 and/or NHE1 inhibition[15,19]. NHE1 was predominantly expressed at the wound periphery, where low pHe values occur, providing an explanation of how NHE1 could contribute to centrifugal pHe-gradients in chronic wounds[24]. The above cited studies suggest that an acidic pHe in the tumour microenvironment is likely to play an important role in different steps of the metastatic cascade Based on this knowledge and the fact that upregulation of NHE1 has been correlated with tumour malignancy and NHE1 function through increased proton efflux with tumour cell invasiveness[26], we hypothesised that NHE1 affects cell-cell adhesion in human melanoma. Using single cell force spectroscopy (SCFS) with atomic force microscopy (AFM) we found a reduction of cell-cell-adhesion forces upon extracellular acidification and NHE1 overexpression
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