Regulation of Toll-like receptor 4 expression and its signaling by hypoxia in cultured microglia.
Hypoxia is an important biological signal that regulates a wide variety of physiological responses. At the same time, hypoxia is involved in multiple pathological situations. In particular, hypoxia is closely associated with neural injury in the brain. Hypoxia has been recently proposed as a neuroinflammatogen, as it can induce the inflammatory activation of microglia, a major cellular source of inflammatory mediators in the brain. In this article, we present evidence that hypoxia enhances Toll-like receptor 4 (TLR4) expression in cultured microglia and differentially regulates the downstream signaling pathways of TLR4. Hypoxia up-regulated TLR4 expression at the mRNA and protein levels in a microglia cell line, as well as in primary microglia cultures. Hypoxia, however, differentially regulated MyD88-dependent and -independent pathways of TLR4 signaling: Hypoxia enhanced lipopolysaccharide (LPS)-induced interferon regulatory factor-3 (IRF-3) activation and the subsequent expression of IFNbeta (MyD88-independent pathway), whereas it suppressed LPS-induced NF-kappaB activation (MyD88-dependent pathway). Hypoxia did not affect IFNgamma signaling, which was represented by signal transducer and activator of transcription-1 (STAT1) activation and interferon-regulatory factor-1 (IRF-1) induction. Taken together, although hypoxia up-regulates TLR4 expression, its downstream signaling pathways appear to be differentially modulated by hypoxia.
Ock
Jiyeon
J
Department of Pharmacology, Kyungpook National University School of Medicine, Daegu, Korea.
Jeong
Jaeyoon
J
Choi
Wan Sung
WS
Lee
Won-Ha
WH
Kim
Sang-Hyun
SH
Kim
In Kyeom
IK
Suk
Kyoungho
K
D000818:Animals; D015153:Blotting, Western; D015687:Cell Hypoxia / Q000502:physiology*; D002478:Cells, Cultured; D024202:Electrophoretic Mobility Shift Assay; D005434:Flow Cytometry; D005786:Gene Expression Regulation / Q000502:physiology; D007202:Indicators and Reagents; D007249:Inflammation / Q000473:pathology; D007371:Interferon-gamma / Q000502:physiology; D008262:Macrophage Activation / Q000502:physiology; D051379:Mice; D008813:Mice, Inbred ICR; D017628:Microglia / Q000378:metabolism*; D053594:Myeloid Differentiation Factor 88 / Q000096:biosynthesis / Q000235:genetics; D012313:RNA / Q000032:analysis / Q000096:biosynthesis; D015398:Signal Transduction / Q000502:physiology*; D051195:Toll-Like Receptor 2 / Q000096:biosynthesis / Q000235:genetics; D051197:Toll-Like Receptor 4 / Q000096:biosynthesis*; D015854:Up-Regulation / Q000502:physiology
D007202:Indicators and Reagents; D053594:Myeloid Differentiation Factor 88; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D012313:RNA; D007371:Interferon-gamma