The <i>N</i>⁶-methyladenosine (m⁶A)-forming enzyme METTL3 facilitates M1 macrophage polarization through the methylation of <i>STAT1</i> mRNA

Abstract

Compelling evidence indicates that epigenetic regulations orchestrate dynamic macrophage polarization. <i>N</i>⁶-methyladenosine (m⁶A) methylation is the most abundant epigenetic modification of mammalian mRNA, but its role in macrophage polarization is still completely unknown. Here, we show that the m⁶A-catalytic enzyme methyltransferase like 3 (METTL3) is specifically upregulated following the M1 polarization of mouse macrophages. Furthermore, METTL3 knockdown through siRNA transfection markedly inhibited M1, but enhanced M2, macrophage polarization. Conversely, its overexpression via plasmid transfection greatly facilitated M1, but attenuated M2, macrophage polarization. Further methylated RNA immunoprecipitation and in vitro m⁶A methylation assays suggested that METTL3 directly methylates mRNA encoding signal transducer and activator of transcription 1 (STAT1), a master transcription factor controlling M1 macrophage polarization, at its coding sequence and 3'-untranslated regions. In addition, METTL3-mediated <i>STAT1</i> mRNA methylation significantly increased mRNA stability and subsequently upregulated STAT1 expression. In conclusion, METTL3 drives M1 macrophage polarization by directly methylating <i>STAT1</i> mRNA, potentially serving as an anti-inflammatory target.