Hypoxic gene activation by lipopolysaccharide in macrophages: implication of hypoxia-inducible factor 1α

Abstract

Hypoxia-inducible factor 1 (HIF-1) regulates many genes induced by low oxygen conditions. The expression of important hypoxic genes such as glucose transporter 1 and vascular endothelial growth factor are increased in macrophages during wound healing and in the presence of the endotoxin, lipopolysaccharide (LPS). Recent studies have demonstrated that nonhypoxic stimuli can also activate HIF-1 in a cell-specific manner. Here, we demonstrate that in macrophages, LPS can control the activation of hypoxia-regulated genes through the HIF-1 pathway. We show that in these cells, protein expression levels of HIF-1␣ are strongly increased to levels comparable to hypoxic induction. HIF-1␣ mRNA levels are markedly increased following LPS stimulation, suggesting a transcriptional induction. In functional studies, the LPS-induced HIF-1 complex could specifically bind to the HIF-1 DNA-binding motif. Additionally, when cells were transfected with an HIF-1-specific reporter construct, LPS could strongly activate the expression of the reporter to levels that surpassed those observed after hypoxic induction. This induction was blocked by the cotransfection of a dominant-negative form of HIF-1␣. These results indicate that the HIF-1 complex is involved in macrophage gene activation following LPS exposure and identify a novel pathway that could play a determinant role during inflammation and wound healing. IntroductionHypoxia activates a number of genes that are important in the cellular and tissue adaptation to low oxygen conditions. 1 These genes include erythropoietin, glucose transporters, glycolytic enzymes, and vascular endothelial growth factor (VEGF). The hypoxic expression of these different genes is controlled at the transcriptional level by the ubiquitously expressed transcription factor, hypoxia-inducible factor 1 (HIF-1). HIF-1 is a member of the basic helix-loop-helix superfamily of transcription factors. Only active as a heterodimer, HIF-1 is composed of 2 subunits: HIF-1␣ and HIF-1␤. 2 Whereas the HIF-1␤ protein is readily found in all cells, HIF-1␣ is virtually undetectable in normal oxygen conditions. When cells are subjected to hypoxic conditions (1% oxygen), levels of the HIF-1␣ subunit are rapidly increased. Instead of acting on HIF-1␣ transcription or translation, hypoxia increases HIF-1␣ protein levels by inhibiting the rapid ubiquitination and degradation of HIF-1␣ by the proteasome. An elegant series of studies has shown that when oxygen is present, HIF-1␣ is modified by prolyl hydroxylation, which permits the binding of the von Hippel-Lindau protein (pVHL), a recognition component of the E3 ligase complex. This promotes the ubiquitin degradation of HIF-1␣. [3][4][5][6][7][8] In a very recent study, acetylation of HIF-1␣ in normoxic conditions also targets HIF-1␣ for proteasomal degradation. 9 Under hypoxic conditions, prolyl hydroxylation of HIF-1␣ is blocked and acetylation is down-regulated, which permits HIF-1␣ protein stabilization. HIF-1␣ is then free to bind with HIF-1␤ to form the HIF-1 transcripti...

HIF2α signaling inhibits adherens junctional disruption in acute lung injury

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TCR Engagement Increases Hypoxia-Inducible Factor-1α Protein Synthesis via Rapamycin-Sensitive Pathway under Hypoxic Conditions in Human Peripheral T Cells

“…However, this is not the case in human peripheral blood T cells (22). Moreover, it is reported that, in monocyte/macrophage, lipopolysaccharides enhance HIF-1␣ expression via increased expression of HIF-1␣ mRNA (15). Hudson et al (33) showed that rapamycin interferes with HIF-1␣ activation in hypoxic PC-3 cells by increasing the rate of HIF-1␣ degradation.…”

“…In addition to this hypoxia-dependent mechanism, recent studies have demonstrated non-hypoxic pathways for regulation of HIF-1␣ activity (9,10) and multiple regulatory mechanisms of HIF-1␣ protein level, may enable cells to adapt to diverse alteration of environments and preserve homeostasis in a tissue-dependent manner. For example, receptor-mediated regulation of HIF-1␣ expression is thought to be cell and signal specific, and a number of reports have suggested that receptor-mediated factors such as growth factors, hormones, and cytokines induce HIF-1␣ protein expression through various signalings including PI3K/mammalian target of rapamycin (mTOR) pathway (11)(12)(13)(14)(15).…”

A Myeloid Hypoxia-inducible Factor 1α-Krüppel-like Factor 2 Pathway Regulates Gram-positive Endotoxin-mediated Sepsis

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