LPS induction of gene expression in human monocytes

doi:10.1016/S0898-6568(00)00149-2

Cellular Signalling

Volume 13, Issue 2, February 2001, Pages 85-94

https://doi.org/10.1016/S0898-6568(00)00149-2 Get rights and content

Abstract

Lipopolysaccharide (LPS [endotoxin]) is the principal component of the outer membrane of Gram-negative bacteria. Recent studies have elucidated how LPS is recognized by monocytes and macrophages of the innate immune system. Human monocytes are exquisitely sensitive to LPS and respond by expressing many inflammatory cytokines. LPS binds to LPS-binding protein (LBP) in plasma and is delivered to the cell surface receptor CD14. Next, LPS is transferred to the transmembrane signaling receptor toll-like receptor 4 (TLR4) and its accessory protein MD2. LPS stimulation of human monocytes activates several intracellular signaling pathways that include the IkappaB kinase (IKK)–NF-κB pathway and three mitogen-activated protein kinase (MAPK) pathways: extracellular signal-regulated kinases (ERK) 1 and 2, c-Jun N-terminal kinase (JNK) and p38. These signaling pathways in turn activate a variety of transcription factors that include NF-κB (p50/p65) and AP-1 (c-Fos/c-Jun), which coordinate the induction of many genes encoding inflammatory mediators.

Section snippets

Septic shock

LPS is the principal component of the outer membrane Gram-negative bacteria (Fig. 1). Humans have evolved to detect low levels of LPS to combat infection. Monocytes orchestrate the innate immunity response to LPS by expressing a variety of inflammatory cytokines that include tumor necrosis factor-alpha (TNFα). However, over-reaction to the presence of LPS can lead to sepsis, septic shock, or systemic inflammatory response syndrome. Despite much effort there is no good therapy for this problem.

LBP and CD14

LPS and LPS-containing particles (including intact bacteria) form complexes with a plasma protein known as LPS-binding protein (LBP) [1] (Fig. 2). In humans, LBP is present in plasma at 3–10 μg/ml but levels rise dramatically after an acute phase response. In a survey of monoclonal antibodies to macrophage surface antigens it was noted that antibodies to CD14 abrogated the binding of LBP-opsonized particles to the cells, suggesting that CD14 is the receptor for LBP-opsonized particles [2] (Fig.

The TLR family

In both insects and vertebrates, the rapid response to pathogens is called innate immunity [13], [14]. The first event in an innate immune response in mammals is the activation of macrophages, which engulf invading pathogens and secrete cytokines that activate other aspects of the immune response. In Drosophila, Toll recognizes pathogens and induces an innate immune response via activation of the Rel transcription factors Dorsal, Dif, and Relish [15], which are homologous to mammalian NF-κB.

TLR4

Other studies used a genetic approach to identify the LPS signaling receptor. These studies made use of two strains of mice, C3H/HeJ and C57BL/10ScCr, which contained a mutation in the Lps gene that made them hyporesponsive to LPS. It was determined that defective LPS signaling in these mouse strains was due to mutations in TLR4, indicating that TLR4 was the LPS signaling receptor [21], [22] (Fig. 2). These studies are elegantly reviewed by Beutler [23]. Macrophages from mice with a targeted

MD-2

In contrast to the studies of Kirschning et al. [20], Chow et al. [27] showed that TLR4 mediated LPS activation of NF-κB in 293 cells. Therefore, there was still controversy about whether TLR4 was sufficient to confer LPS responsiveness to 293 cells. It was suggested that additional proteins may be required for LPS activation of cells via TLR4. RP105 is a B cell-specific receptor that contains LRR repeats similar to Drosophila Toll and is involved in B cell activation [28]. A new protein called

The IKK–NF-κB pathway

Many studies have demonstrated that LPS activates NF-κB in human monocytes and monocytic cell lines [33]. However, until recently the intracellular signaling molecules that mediate this activation had not been elucidated. For instance, LPS activation of NF-κB in human THP-1 monocytic cells is inhibited by dominant-negative mutants of MyD88, IL-1 receptor-associated kinase (IRAK), and TNFα receptor-associated factor (TRAF6) but not by dominant-negative TRAF2 [34]. In addition, IRAK does not

Regulation of nuclear NF-κB activity

A second level of NF-κB activation has been reported that involves regulation of the transactivation activity of p65 and c-Rel. We and others showed that D609, a phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor, reduced nuclear NF-κB-dependent transcription without affecting the cytokine-induced nuclear translocation and DNA binding of NF-κB [56], [57]. Similarly, LPS induction of NF-κB-dependent gene expression in THP-1 cells was blocked by the protein tyrosine kinase inhibitor

MAPK pathways

Many upstream activators are involved in LPS signaling in monocytes. For instance, LPS stimulation of monocytes/macrophages induces the rapid tyrosine phosphorylation of proteins, including the src tyrosine kinase family members p53/56^lyn, p58/64^c-fgr [83], [84], [85]. LPS also activates typical (PKCβ) and atypical (PCKζ) isoforms of protein kinase C [73], [85]. In addition, pertussis toxin inhibited LPS-induced responses in the P388D1 mouse macrophage line, indicating that the G-proteins Gi

The ERK pathway

Many studies have shown that LPS activates the extracellular signal-regulated kinase (ERK1/2) in monocytes/macrophages [88], [89], [90], [91], [92], [93]. The kinase domains of ERK1/2 have a TEY motif, which is activated by phosphorylation. It has been suggested that LPS activation of tyrosine kinases leads to activation of the MEK–ERK1/2 pathway in a Raf-1-dependent manner [94]. Dominant-negative repressors of both Ras and c-Raf inhibited LPS induction of the TNFα promoter in RAW 264.7

The JNK pathway

LPS stimulation of THP-1 cells and RAW 264.7 cells rapidly activates the JNK pathway [106]. This pathway contains the c-Jun N-terminal kinase (JNK), which is also known as the stress-activated protein kinase (SAPK1) [107], [108]. The two isoforms of JNK (54 and 46 kDa) phosphorylate the N-terminus of c-Jun. The JNK kinases have TPY motifs in their kinase domains that are required for activation [109], [110], [111], [112], [113]. Upstream protein kinases such as mPAK3 (p21-activated kinase) [114]

The p38 pathway

The third MAPK pathway includes isoforms of p38/SAPK2/RK (reactivating kinase) [128]. p38 was originally identified in LPS-stimulated murine macrophages [129]. p38 isoforms contain TGY motifs in their kinase domains [109]. Cdc42, PAK, and Rac1 are upstream signaling molecules that activate the p38 pathway [120], [130], [131]. MAPKKKs involved in the p38 signaling cascade include protein kinase RNA-regulated (PKR) [132] MLK3/SPRK [118], [120], ASK1 [133], and TAK1 [134], [135]. The MAPKKs

LPS induction of gene expression

LPS stimulation of monocytes and macrophages induces many genes that express inflammatory mediators, such as cytokines and chemokines (Table 1). LPS responsive cis-acting DNA promoter elements have been characterized in the 5′ flanking region of many of these genes (reviewed in Ref. [87]). The transcription factors that bind to these LPS response elements include NF-κB/Rel proteins, AP-1 proteins, and NF-IL-6. These transcription factors are activated by phosphorylation, which permits the rapid

Summary

The recent discovery of TLR4 as the LPS signaling receptor has stimulated great excitement in the field of LPS biology. Future studies with mice containing knock-outs in the various signaling intermediates will identify those molecules that mediate LPS induction of gene expression in human monocytes. These signaling molecules may represent novel targets for the treatment of patients with septic shock.

Acknowledgements

This work was supported by a grant from the NIH (HL48872). We would like to acknowledge past and present members of the laboratory and colleagues at The Scripps Research Institute for simulating discussions. J. Robertson and B. Parker are acknowledged for the preparation of the manuscript.

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Review ArticleLPS induction of gene expression in human monocytes

Abstract

Section snippets

Septic shock

LBP and CD14

The TLR family

TLR4

MD-2

The IKK–NF-κB pathway

Regulation of nuclear NF-κB activity

MAPK pathways

The ERK pathway

The JNK pathway

The p38 pathway

LPS induction of gene expression

Summary

Acknowledgements

Blood

Curr Opin Microbiol

Blood

Curr Opin Immunol

Immunity

J Biol Chem

J Biol Chem

Immunobiology

J Biol Chem

Immunity

J Biol Chem

Cell

Cell

J Biol Chem

FEBS Lett

Mol Cell

J Biol Chem

Blood

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

Cell

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

Blood

J Biol Chem

Science

Science

J Immunol

Nucleic Acids Res

Nucleic Acids Res

J Immunol

Proc Natl Acad Sci U S A

J Exp Med

Nature

Proc Natl Acad Sci U S A

Prog Clin Biol Res

Annu Rev Cell Dev Biol

Nature

Proc Natl Acad Sci U S A

Nature

Nature

Review Article
LPS induction of gene expression in human monocytes