MicroRNA‐146a and RBM4 form a negative feed‐forward loop that disrupts cytokine mRNA translation following TLR4 responses in human THP‐1 monocytes

• Published in: Immunology and cell biology Vol. 91; no. 8; pp. 532 - 540
• Main Authors: Brudecki, Laura, Ferguson, Donald A, McCall, Charles E, El Gazzar, Mohamed
• Format: Journal Article
• Language: English
• Published: United States Nature Publishing Group 01.09.2013; Blackwell Science Ltd
• Subjects: Adaptive Immunity; Argonaute Proteins - metabolism; Cell Differentiation - genetics; Cell Line; Cell Nucleus - metabolism; Cytosol - metabolism; endotoxin tolerance; Endotoxins - immunology; Feedback, Physiological; Gene Expression Regulation - genetics; Homeostasis; Humans; Immune Tolerance; Inflammation Mediators - metabolism; Interleukin-6 - genetics; Interleukin-6 - metabolism; microRNA; MicroRNAs - genetics; Monocytes - immunology; p38 Mitogen-Activated Protein Kinases - metabolism; Protein Binding - genetics; Protein Biosynthesis - genetics; Protein Transport - genetics; RNA, Messenger - metabolism; RNA-Binding Proteins - metabolism; RNA‐binding protein; sepsis; Sepsis - immunology; severe systemic inflammation; Toll-Like Receptor 4 - metabolism; translational repression; Tumor Necrosis Factor-alpha - genetics; Tumor Necrosis Factor-alpha - metabolism
• ISSN: 0818-9641; 1440-1711
• DOI: 10.1038/icb.2013.37

Within hours after its initiation, the severe systemic inflammatory response of sepsis shifts to an adaptive anti‐inflammatory state with coincident immunosuppression. This anti‐inflammatory phenotype is characterized by diminished proinflammatory cytokine gene expression in response to toll‐like receptor (TLR) stimulation with bacterial endotoxin/lipopolysaccharide (LPS), also known as endotoxin tolerance/adaptation. Our and other studies have established that gene‐specific reprogramming following TLR4 responses independently represses transcription and translation of proinflammatory genes such as tumor necrosis factor alpha (TNFα). We also previously demonstrated that TNFα and interleukin (IL)‐6 mRNA translation is repressed in endotoxin‐adapted THP‐1 human monocytes by an miRNA‐based mechanism involving the argonaute family protein argonaute 2 (Ago2). Here, we further define the molecular nature of reprogramming translation by showing that TLR4‐induced microRNA‐146 promotes a feed‐forward loop that modifies the subcellular localization of the RNA‐binding protein RBM4 (RNA‐binding motif protein 4) and promotes its interaction with Ago2. This interaction results in the assembly of a translation‐repressor complex that disrupts TNFα and IL‐6 cytokine synthesis in endotoxin‐adapted THP‐1 monocytes. This novel molecular path prevents the phosphorylation of RBM4 on serine‐309 by p38 MAPK (mitogen‐activated protein kinase), which leads to RBM4 accumulation in the cytosol and interaction with Ago2. We further find that microRNA‐146a knockdown by antagomirs or protein phosphatase inhibition by okadaic acid increases p38 MAPK phosphorylation and results in RBM4 serine‐309 phosphorylation and nuclear relocalization, which disrupts RBM4 and Ago2 interactions and restores TLR4‐dependent synthesis of TNFα and IL‐6. We conclude that miR‐146a has a diverse and critical role in limiting an excessive acute inflammatory reaction.