The root bark of Morus alba L. regulates tumor-associated macrophages by blocking recruitment and M2 polarization of macrophages.

Tumor-associated macrophages (TAMs) promote tumor growth and metastasis, and are closely related with poor prognosis of cancers. Therefore, TAMs have been an attractive target in cancer therapy. This study investigated whether the root bark of Morus alba L. (MA) regulates TAMs. Methylene chloride extract of MA (MEMA) decreased the migration of RAW264.7 cells and THP-1 macrophages toward cancer cells via inhibition of focal adhesion kinase and Src activity. In addition, MEMA inhibited the phorbol myristate acetate-stimulated secretion of plasminogen activator inhibitor-1 from cancer cells, leading to the decreased chemotaxis of macrophages. Finally, MEMA-suppressed M2 macrophage polarization induced by interleukin (IL)-4/IL-13 or IL-6. MEMA downregulated the mRNA expression of M2 macrophage markers and decreased the phosphorylation of signal transducer and activator of transcription (STAT) 6 and STAT3 in RAW264.7 cells. Suppression of M2 polarization of macrophages by MEMA resulted in the reduced migration of Lewis lung carcinoma cells when the conditioned media from RAW264.7 cells was used as a chemoattractant. Taken together, our results demonstrate that MEMA regulates TAMs by blocking the recruitment of macrophages into tumor microenvironments and by inhibiting M2 polarization of macrophages.