Unrevealed roles of extracellular enolase-1 (ENO1) in promoting glycolysis and pro-cancer activities in multiple myeloma via hypoxia-inducible factor 1α

The involvement of enolase-1 (ENO1), intracellularly or extracellularly, has been implicated in cancer development. Moreover, anticancer activities of an ENO1-targeting antibody has demonstrated the pathological roles of extracellular ENO1 (surface or secreted forms). However, although ENO1 was first identified as a glycolytic enzyme in the cytosol, to the best of our knowledge, extracellular ENO1 has not been implicated in glycolysis thus far. In the present study, the effects of extracellular ENO1 on glycolysis and other related pro-cancer activities were investigated in multiple myeloma (MM) cells in vitro and in vivo. Knockdown of ENO1 expression reduced lactate production, cell viability, cell migration and surface ENO1 expression in MM cells. Notably, addition of extracellular ENO1 protein in cancer cell culture enhanced glycolytic activity, hypoxia-inducible factor 1-alpha (HIF-1 alpha) expression, glycolysis-related gene (GRG) expression and pro-cancer activities, such as cell migration, cell viability and tumor-promoting cytokine secretion. Consistently, these extracellular ENO1-induced cellular effects were inhibited by an ENO1-specific monoclonal antibody (mAb). In addition, extracellular ENO1-mediated glycolysis, GRG expression and pro-cancer activities were also reduced by HIF-1 alpha silencing. Lastly, administration of an ENO1 mAb reduced tumor growth and serum lactate levels in an MM xenograft model. These results suggested that extracellular ENO1 (surface or secreted forms) enhanced a HIF-1 alpha-mediated glycolytic pathway, in addition to its already identified roles. Therefore, the results of the present study highlighted the therapeutic potential of ENO1-specific antibodies in treating MM, possibly via glycolysis inhibition, and warrant further studies in other types of cancer.