Contribution of acidic melanoma cells undergoing epithelial-to-mesenchymal transition to aggressiveness of non-acidic melanoma cells.

Tumor cell plasticity largely depends on epithelial-to-mesenchymal transition (EMT) and its reversion. It was ascertained that EMT characterizes disease progression, including melanoma malignancy. As most solid tumors, melanoma shows extracellular acidosis, we analyse the impact of acidic environment on the EMT development in human melanoma cells. Melanoma cells were exposed to an acidic extracellular environment (pH 6.7) and tested for EMT markers. We found that acidic cells express a significant up-regulation of mesenchymal markers (N-cadherin, Vimentin), transcription factors (Twist, NF-κB) and a significant, although modest, reduction of E-cadherin expression. Acidic cell also express an increased invasiveness through Matrigel associated with an up-regulation of MMP-9 activity. When we injected acidic cells intravenously into immunodeficient animals, we found a number of lung micrometastases not different from non-acidic cells. Indeed, they show a partial G1 cell cycle arrest, which might interfere with the growth of lung colonies. When we investigated the in vitro invasiveness and lung colonization of a mixed population of acidic and non acidic melanoma cells, we found that acidic cells promote in vitro invasiveness of non-acidic cells and this cooperation leads to an higher migration rate than acidic cells. Moreover, acidic cells cooperate for a better lung colonization of non-acidic cells, that represent the greater part of cells participating to lung micrometastases. We found evidence that acidity triggers in melanoma cells an EMT program, which although “incomplete”, potentiates migration rate and development of lung colonies into immunodeficient host of cells grown in standard pH.