Dicer‐independent snRNA/snoRNA‐derived nuclear RNA 3 regulates tumor‐associated macrophage function by epigenetically repressing inducible nitric oxide synthase transcription

BACKGROUND: Small RNAs (sRNAs) extensively mediate gene‐specific chromatin regulation in lower organisms. As a dominant type of functional sRNAs in mature mammals, microRNAs mainly regulate gene expression at post‐transcription level in the cytoplasm. Currently, whether there exists a type of nuclear‐localized sRNAs mediating gene‐specific epigenetic regulation in mature mammalian cells remains largely unclear. Here, we profiled sRNAs enriched in the nucleus and investigated their function in mediating gene‐specific epigenetic regulation in anti‐tumor immunity. METHODS: We established cytoplasmic and nuclear transcriptomes of sRNAs of dendritic cells (DCs) using high‐throughput sequencing. Transcription abundances of sRNAs and mRNAs were analyzed by reverse transcription‐quantitative polymerase chain reaction (RT‐qPCR) assay. The associations between sRNAs and Argonaute (AGO) proteins were detected by RNA immunoprecipitation analysis. Synthesized sRNAs and locked nucleic acid (LNA) ‐modified sRNA inhibitors were used to screen the function of sRNAs in innate immune cells. The effect of sRNA on the enrichment of either chromatin remodeler or histone modification at the gene promoter was analyzed by chromatin immunoprecipitation (ChIP)‐qPCR assay. Chromatin accessibility qPCR assay was used to detect the accessibility of gene promoters. A B16 melanoma‐bearing mouse model was established to determine the function of sRNAs in tumor‐associated macrophages (TAMs) and their effect on tumor growth. RESULTS: We identified a new class of nucleus‐localized sRNAs, named snRNA/snoRNA‐derived nuclear RNAs (sdnRNAs). Some sdnRNAs were Dicer‐independent and had no association with Argonaute proteins. sdnRNA‐3, the most abundant Dicer‐independent sdnRNAs identified in our analysis, was selected as a representative to examine the biological function of sdnRNAs. sdnRNA‐3 selectively inhibited the transcription of Nos2 in macrophages during innate immune response by repressing the chromatin accessibility at Nos2 gene promoter. sdnRNA‐3 promoted the enrichments of repressive chromatin‐remodeling regulator Mi‐2β and the repressive histone modification H3K27me3 at Nos2 gene promoter. In the B16 melanoma mouse model, we found higher expression of sdnRNA‐3 in M2 TAMs than M1 TAMs and DCs. Transfer of sdnRNA‐3‐silenced macrophages inhibited tumor growth with increased expression of inducible nitric oxide synthase (iNOS) in TAMs. CONCLUSIONS: Our results demonstrated that the sdnRNA‐3 repressed the transcription of Nos2 by repressing chromatin accessibility at the promoter, providing new insights into the regulation of macrophage function in tumor immunity.