Macrophage-induced tumor angiogenesis is regulated by the TSC2-mTOR pathway

Tumor-associated macrophages (TAM) have multifaceted roles in tumor development but they have been associated particularly closely with tumor angiogenesis. However, while the accumulation of TAM (M2 phenotype) promotes tumor angiogenesis the mechanism through which monocytes differentiate to generate TAM is unclear. Here we report that the mTOR pathway is a critical element in the regulation of monocyte differentiation to TAM. In human peripheral monocytes stimulated by lipopolysaccharide (LPS), mTOR was inhibited by rapamycin or activated by RNAi-mediated knockdown of the mTOR repressor TSC2. Rapamycin caused the monocytes to differentiate into M1 macrophages releasing more IL-12 and less IL-10, whereas TSC2 knockdown caused the monocytes to differentiate into M2 macrophages releasing less IL-12 and more IL-10. In parallel fashion, angiogenic properties were promoted or reduced in HUVEC cells co-cultured with TSC2-deficient monocytes or rapamycin-treated monocytes, respectively. Furthermore, tumor angiogenesis and growth in murine xenografts were promoted or reduced by infusion of hosts with TSC2-deficient or TSC2-overexpressing monocytes, respectively. Lastly, in vivo depletion of macrophages was sufficient to block the anti-angiogenesis effects of rapamycin on tumors. Our results define the TSC2-mTOR pathway as a key determinant in the differentiation of monocytes into M2 phenotype TAM that promote angiogenesis.