Tumor-associated macrophages found at early tumor sites exhibit altered metabolism and contribute to tumor progression

Previously, we reported that macrophages at early tumor sites, before the tumors are even palpable, have a mixed phenotype with production of both pro- and anti-inflammatory cytokines, decreased production of reactive oxygen species, and increased dependence on oxidative phosphorylation. Here we show that depleting these tumor associated macrophages (TAM) impacted subsequent growth of the EMT6 tumor and metastasis of the 4T1 tumor supporting the contention that macrophages found at early tumor sites contribute to later stages of tumor progression. Since these TAM could not be clearly defined as M1 or M2 we set out to characterize them further focusing on their metabolism. We found that TAM present at early sites of 168, EMT6 and 4T1 tumors had elevated expression of pkm2, glut1 and ampk, and transwell assays revealed that these tumors could induce expression of pkm2, glut1 and ampk in bone marrow derived macrophages. Since tumor-derived lactate is known to induce alterations in white blood cells we investigated lactate production by these tumors and whether lactate could induce expression of these same genes. The data revealed that the 168, EMT6 and 4T1 tumors produced significant amounts of lactate and that lactate induced expression of pkm2, glut1 and ampk in bone marrow derived macrophages. Finally, we found that targeting lactate dehydrogenase or eIF2 at early tumor sites impacted growth of the EMT6 tumor, and targeting AMPK or GCN2 impacted metastasis of the 4T1 tumor. Collectively, these data support the contention that alterations in macrophages are already present at the early stages of tumor growth, before a tumor is even palpable, and that targeting metabolism at these early tumor sites can impact later stages of tumor progression.