Lactate secreted by esophageal cancer cells induces M2 macrophage polarization via the AKT/ERK pathway

Abstract

Background: Elevated lactate results in an acidic tumor microenvironment (TME), which stimulates the progression of esophageal cancer (EC). Tumor-associated macrophages (TAMs) are an essential component of the TME. However, the regulatory mechanisms of lactate secreted by EC on TAMs and the effects of EC advancement are unclear.

Methods: Proteins and mRNA expression were determined by western blot and RT-qPCR. Cell metastasis and growth were assessed by scratch assay, transwell and BrdU assays. Lactate in cells was quantified using a lactate kit. A mouse model was constructed for validation in vivo.

Results: First, we determined that lactate upgraded the M2-type polarization marker levels of macrophages. Cell function assays confirmed that lactate-activated M2 macrophages accelerated EC cell migration and proliferation in vitro. However, the lactate inhibitor - oxamate hampered the level of lactate in TE-1 cells. Oxamate abolished the facilitation of macrophage polarization by lactate. In addition, we discovered that phosphorylated AKT and phosphorylated ERK was obviously raised in lactate-stimulated macrophages, and oxamate addition reversed this change, implying that AKT and ERK signaling pathways were involved in macrophage polarization. Response experiments proved that attenuation of AKT/ERK signaling markedly returned the lactate-induced promotion of EC migration and proliferation by macrophages. Finally, mouse tumor models demonstrated that lactate enhanced EC growth by inducing M2 macrophage polarization.

Conclusion: EC-secreted lactate stimulated macrophage M2 polarization via the AKT/ERK pathway thereby boosting the growth of EC.

Keywords: AKT; EC; ERK; M2 macrophages; lactate.