Export of miRNAs from activated macrophages is cooperative and HuR-dependent.

MiRNA export is a tightly regulated process crucial for maintaining balanced miRNA and target gene expression levels in metazoan cells. RNA-interacting proteins such as HuR play a key role in the selectivity and specificity of miRNA export, thereby enabling context-dependent release of gene-repressing miRNAs from mammalian cells. Our results demonstrate that activated macrophages cooperatively export miRNAs, where hepatic miR-122 significantly enhances the export of miR-146a and other miRNAs. We also observe that this cooperative export causes a synchronized increase in the expression of pro-inflammatory target genes in activated macrophages. In investigating the molecular mechanisms, we found that the miRNA-binding protein HuR cooperatively binds to miRNAs and promotes their entry into endosomes, thereby facilitating their cooperative export. This highlights the selective and cooperative nature of endosome targeting happening in activated macrophages as a prerequisite for extracellular vesicle-mediated miRNA export. Impact statement This study uncovers a new mechanism for miRNA export regulation in mammalian cells, driven by the RNA-binding protein HuR. In macrophages, HuR cooperatively binds with miRNAs to export both low- and high-affinity substrates, thereby influencing macrophage activation. This cooperative export method is a common strategy for coordinated miRNA release across different cells.