Background and Objectives Monocytes play essential roles in immune defense but are also implicated in pathological inflammatory responses due to excessive and prolonged activation. How survival and apoptotic regulators control monocyte lifespan to avoid pathological conditions associated with chronic inflammatory response such as observed in rheumatoid arthritis (RA) remains elusive. As part of the molecular rheostat machinery, micro (mi)RNAs represent an important class of regulatory molecules. MiR-125b, a highly conserved miRNA, has emerged as a key regulator of hematopoietic stem cells development and homeostasis. The aim of our study was to identify endogenous miR-125b target genes that contribute to altering monocyte responses.
Materials and Methods We used gain- and loss-of-function methods to evaluate the effect of miR-125b on the human monocytic cell line THP-1. A transcriptome analysis was performed to identify putative targets that were validated using a luciferase reporter construct containing the 3’UTR of candidate genes. Apoptosis was analysed by Caspase 3/7 assays. Monocyte oxygen consumption rate was investigated using the Seahorse Extracellular Flux Analyzer. TOM20 staining was used to monitor the mitochondrial network.
Results Transcriptome analysis showed deregulation of several genes involved in apoptosis. Among these, the BH3-only pro-apoptotic BIK and the mitochondrial fission process 1 (MTP18) genes were identified as putative miR-125b targets. Using reporter systems, the binding of miR-125b to both 3’UTR target sequences was validated. Enforced expression of miR-125b in THP-1 decreased BIK and MTP18 expression levels and monocyte viability though augmentation of caspase 3/7 activity. LPS-induced expression of miR-125b reduced mitochondrial respiration and triggered mitochondrial fusion through BIK and MTP18 silencing respectively. Importantly, BIK and MTP18 knock-down mimicked miR-125b over-expression phenotype while miR-125b inhibitor reversed phenotype.
Conclusions Our present work identified BIK and MTP18 as novel cellular targets of miR-125b in human monocytes. Our data also suggest a novel role for miR-125b in miR-125b in regulating metabolic adaptation of monocytes to inflammation, as well as mitochondrial fusion and fission, thus extending its involvement in cell biology and RA pathogenesis.
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