RESUMO
Exposure of monocytes and macrophages to endotoxin/lipopolysaccharide (LPS) from Gram-negative bacteria activates the NF-κB signaling pathway. At early times, this leads to their production of proinflammatory cytokines, but subsequently, they produce anti-inflammatory interleukin-10 (IL-10) to quell the immune response. LPS-mediated induction of IL10 gene expression requires the p40 isoform of the RNA-binding protein AUF1. As LPS exerts modest effects upon IL10 mRNA stability, we hypothesized that AUF1 controls the expression of signaling proteins. Indeed, knockdown of AUF1 impairs LPS-mediated p38 mitogen-activated protein kinase (MAPK) and NF-κB signaling, and the expression of an RNA interference-refractory p40(AUF1) cDNA restores both signaling pathways. To define the molecular mechanisms by which p40(AUF1) controls IL10 expression, we focused on the NF-κB pathway in search of AUF1-regulated targets. Here, we show that p40(AUF1) serves to maintain proper levels of the kinase TAK1 (transforming growth factor-ß-activated kinase), which phosphorylates the IKKß subunit within the IκB kinase complex to activate NF-κB-regulated genes. However, p40(AUF1) does not control the TAK1 mRNA levels but instead promotes the translation of the mRNA. Thus, p40(AUF1) regulates a critical node within the NF-κB signaling pathway to permit IL10 induction for the anti-inflammatory arm of an innate immune response.
Assuntos
Ribonucleoproteínas Nucleares Heterogêneas Grupo D/metabolismo , Quinase I-kappa B/metabolismo , Interleucina-10/genética , Monócitos/metabolismo , Sequência de Bases , Linhagem Celular , Primers do DNA/genética , Ativação Enzimática , Expressão Gênica/efeitos dos fármacos , Técnicas de Silenciamento de Genes , Ribonucleoproteína Nuclear Heterogênea D0 , Ribonucleoproteínas Nucleares Heterogêneas Grupo D/antagonistas & inibidores , Ribonucleoproteínas Nucleares Heterogêneas Grupo D/genética , Humanos , Imunidade Inata/genética , Imunidade Inata/fisiologia , Mediadores da Inflamação/metabolismo , Interferon Tipo I/metabolismo , Lipopolissacarídeos/farmacologia , MAP Quinase Quinase Quinases/genética , MAP Quinase Quinase Quinases/metabolismo , Monócitos/efeitos dos fármacos , NF-kappa B/metabolismo , Processamento Pós-Transcricional do RNA , Estabilidade de RNA , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Transdução de Sinais , Proteínas Quinases p38 Ativadas por Mitógeno/metabolismoRESUMO
IL-10 is an immunomodulatory cytokine that regulates inflammatory responses of mononuclear phagocytes (monocytes and macrophages). Mononuclear cells exposed to microbes or microbial products secrete a host of proinflammatory cytokines followed by delayed onset of anti-inflammatory IL-10. IL-10 suppresses immune responses by inhibiting cytokine production by mononuclear phagocytes. Using THP-1, a human promonocytic leukemia cell line, we show that endotoxin/lipopolysaccharide (LPS) exposure induces IL10 expression while IFN-gamma blocks this LPS-mediated effect. IFN-gamma is an important modulator of IL-10 production during infectious diseases. We show that LPS and IFN-gamma regulate IL10 expression in THP-1 cells in part through posttranscriptional mechanisms. Our results demonstrate that 3'-untranslated region (3'-UTR) AU-rich elements (AREs) decrease expression of a chimeric luciferase reporter gene in THP-1 cells. The ARE-binding protein AUF1 binds the IL10 3'-UTR. Depletion of AUF1 by RNAi suppresses LPS-mediated induction of IL10 mRNA and protein without affecting LPS-mediated stabilization of IL10 mRNA. Upon complementation with either RNAi-refractory p37 or p40 AUF1 plasmids, only p40 restores LPS-mediated induction of IL10 mRNA and protein to near normal levels. Thus, the p40 AUF1 isoform selectively plays a critical, positive role in IL10 expression upon LPS exposure.
