MicroRNA-17/20a/106a modulate macrophage inflammatory responses through targeting signal-regulatory protein α

Background: Signal-regulatory protein α (SIRPα) is an essential signaling molecule that modulates leukocyte inflammatory responses. However, the regulation of selective SIRPα synthesis and its dynamic changes in leukocytes under inflammatory stimulation remain incompletely understood.

Objective: We sought to identify the microRNAs (miRNAs) that posttranscriptionally regulate SIRPα synthesis and their roles in modulating macrophage inflammatory responses.

Methods: SIRPα was induced in SIRPα-negative promyelocytic cells by retinoic acid or phorbol 12-myristate 13-acetate, and the differential expression of miRNAs was assessed by means of microarray and quantitative RT-PCR assays. The roles of identified miRNAs in controlling SIRPα synthesis in leukocytes and leukocyte inflammatory responses were determined.

Results: We identified SIRPα as a common target gene of miR-17, miR-20a, and miR-106a. During SIRPα induction, levels of these 3 miRNAs were all reduced, and their downregulation by retinoic acid or phorbol 12-myristate 13-acetate occurred through suppression of the c-Myc signaling pathway. All miR-17, miR-20a, and miR-106a specifically bound to the same seed sequence within the SIRPα 3' untranslated region and correlated inversely with SIRPα protein levels in various cells. In macrophages upregulation of miR-17, miR-20a, and miR-106a by LPS served as the mechanism underlying LPS-induced SIRPα reduction and macrophage activation. Both in vitro and in vivo assays demonstrate that miR-17, miR-20a, and miR-106a regulate macrophage infiltration, phagocytosis, and proinflammatory cytokine secretion through targeting SIRPα.

Conclusion: These findings demonstrate for the first time that miR-17, miR-20a, and miR-106a regulate SIRPα synthesis and SIRPα-mediated macrophage inflammatory responses in a redundant fashion, providing a novel pathway in which a panel of miRNAs can modulate immune polarization through regulation of macrophage activation.