Extracellular acidity strengthens mesenchymal stem cells to promote melanoma progression

Mesenchymal stem cells (MSC) participate to tumor stroma development and several evidence suggests that they play a role in facilitating cancer progression. Because melanoma often shows extracellular pH low enough to influence host cell as tumor cell behavior, the aim of this study is to elucidate whether acidity affects cross talk between MSC and melanoma cells to disclose new liaisons promoting melanoma progression, and to offer new therapeutic opportunities. We found that MSC grown in a low pH medium (LpH-MSC) stimulate melanoma xenografts more than MSC grown in a standard pH medium. LpH-MSC express a higher level of TGFβ that is instrumental of epithelial-to-mesenchymal transition (EMT)-like phenotype induction in melanoma cells. LpH-MSC profile also shows a switching to an oxidative phosphorylation metabolism that was accompanied by a forced glycolytic pathway of melanoma cells grown in LpH-MSC-conditioned medium. Metformin, an inhibitor of mitochondrial respiratory chain was able to reconvert oxidative metabolism and abrogate TGFβ expression in LpH-MSC. In addition, esomeprazole, a proton pump inhibitor activated in acidosis, blocked TGFβ expression in LpH-MSC through the downregulation of IkB. Both agents, metformin and esomeprazole, inhibited EMT profile in melanoma cells grown in LpH-MSC medium, and reduced glycolytic markers. Thus, acidosis of tumor microenvironment potentiates the pro-tumoral activity of MSC and orchestrates for a new potential symbiosis, which could be target to limit melanoma progression.