Increased M1 macrophages in young miR-15a/16-/- mice with tumour grafts or dextran sulphate sodium-induced colitis.

M1 macrophages are involved in inflammation by producing proinflammatory cytokines, whereas M2 macrophages are associated with wound healing and tissue regeneration by producing anti-inflammatory cytokines. MicroRNAs are involved in macrophage polarization. To evaluate whether miR-15a/16 is involved in macrophage polarization under tumour or inflammation microenvironments, we observed the growth of transplanted hepatic cancer (H22) cells or severity of dextran sulphate sodium (DSS)-induced colitis in 8-week-old miR-15a/16 knockout (KO) mice. Compared with littermate controls, the miR-15a/16-/- mice exhibited retarded tumour growth and increased sensibility to DSS-induced colitis. Meanwhile, the M1 cell frequencies were higher in tumour tissues and inflamed colons of KO mice than of littermate controls. Macrophages with miR-15a/16 deletion revealed an enhanced NF-κB transcription under the physiological state and lipopolysaccharide (LPS) or high mobility group box 1 (HMGB1) stimulation. STAT3 expression was also significantly increased in miR-15a/16-/- macrophages under LPS or HMGB1 stimulation. The polarization of M1 macrophages can be associated with the coactivation of NF-κB and STAT3. Results indicated that miR-15a/16 deficiency in the macrophages directs M1 polarization for tumour suppression and proinflammation. Thus, miR-15a/16 deletion in macrophages holds a distinct biological significance from that of the microRNA deficiency in tumour cells.