Intracellular and extracellular moesins differentially regulate Src activity and β-catenin translocation to the nucleus in breast cancer cells.

It is increasingly recognized that a single protein can have multiple, sometimes paradoxical, roles in cell functions as well as pathological conditions depending on its cellular locations. Here we report that moesins (MSNs) in the intracellular and extracellular domains present opposing roles in pro-tumorigenic signaling in breast cancer cells. Using live cell imaging with fluorescence resonance energy transfer (FRET)- and green fluorescent protein (GFP)-based biosensors, we investigated the molecular mechanism underlying the cellular location-dependent effect of MSN on Src and β-catenin signaling in MDA-MB-231 breast cancer cells. Inhibition of intracellular MSN decreased the activities of Src and FAK, whereas overexpression of intracellular MSN increased them. By contrast, extracellular MSN decreased the activities of Src, FAK, and RhoA, as well as β-catenin translocation to the nucleus. Consistently, Western blotting and MTT-based analysis showed that overexpression of intracellular MSN elevated the expression of oncogenic genes, such as p-Src, β-catenin, Lrp5, MMP9, Runx2, and Snail, as well as cell viability, whereas extracellular MSN suppressed them. Conditioned medium derived from MSN-overexpressing mesenchymal stem cells or osteocytes showed the anti-tumor effects by inhibiting the Src activity and β-catenin translocation to the nucleus as well as the activities of FAK and RhoA and MTT-based cell viability. Conditioned medium derived from MSN-inhibited cells increased the Src activity, but it did not affect the activities of FAK and RhoA. Silencing CD44 and/or FN1 in MDA-MB-231 cells blocked the suppression of Src activity and β-catenin accumulation in the nucleus by extracellular MSN. Collectively, the results suggest that cellular location-specific MSN is a strong regulator of Src and β-catenin signaling in breast cancer cells, and that extracellular MSN exerts tumor-suppressive effects via its interaction with CD44 and FN1.