Metformin's antitumour and anti-angiogenic activities are mediated by skewing macrophage polarization

Beneficial effects of metformin on cancer risk and mortality have been proved by epidemiological and clinical studies, thus attracting research interest in elucidating the underlying mechanisms. Recently, tumour-associated macrophages (TAMs) appeared to be implicated in metformin-induced antitumour activities. However, how metformin inhibits TAMs-induced tumour progression remains ill-defined. Here, we report that metformin-induced antitumour and anti-angiogenic activities were not or only partially contributed by its direct inhibition of functions of tumour and endothelial cells. By skewing TAM polarization from M2- to M1-like phenotype, metformin inhibited both tumour growth and angiogenesis. Depletion of TAMs by clodronate liposomes eliminated M2-TAMs-induced angiogenic promotion, while also abrogating M1-TAMs-mediated anti-angiogenesis, thus promoting angiogenesis in tumours from metformin treatment mice. Further in vitro experiments using TAMs-conditioned medium and a coculture system were performed, which demonstrated an inhibitory effect of metformin on endothelial sprouting and tumour cell proliferation promoted by M2-polarized RAW264.7 macrophages. Based on these results, metformin-induced inhibition of tumour growth and angiogenesis is greatly contributed by skewing of TAMs polarization in microenvironment, thus offering therapeutic opportunities for metformin in cancer treatment.