Alternatively activated macrophages cause DNA damage through an arginase 1-mediated bystander effect and promote epithelial cell transformation.

Colorectal oncogenesis involves progressive genetic and epigenetic alterations in colon epithelial cells. Classically activated M1 macrophages can generate bystander-induced mutations and neoplastic transformation in epithelial cells. Herein, we report a novel mechanism by which alternatively activated M2 macrophages also induce a bystander effect. Similar to M1 macrophages, M2-activated macrophages cause double-strand DNA breaks in targeted epithelial cells, activate multiple signaling pathways, induce a DNA repair response, and transform epithelial cells in vitro. The M2-induced bystander effect occurs through an arginase 1-mediated mechanism. This enzyme is secreted by M2 macrophages and increases extracellular L-ornithine that is imported by target cells. This leads to increased polyamine metabolism and intracellular hydrogen peroxide causing DNA damage. Arginase 1-positive M2-like macrophages were frequently detected in preneoplastic colon adenomas using multiplex immunofluorescence. A greater proportion of activated macrophages was noted in histologically normal colon than in adenomas or stage I colorectal cancer. In addition to M1-like and M2-like activated macrophages, many macrophages in the colon biopsies expressed a hybrid M1/M2 phenotype. Activated macrophages significantly correlated with double-strand DNA damage in colon epithelial cells and DNA damage repair response. These findings define a novel M2-induced bystander effect and support a role for activated macrophages in colorectal cancer initiation and progression.