Inhibiting toll-like receptor 4 signaling ameliorates pulmonary fibrosis during acute lung injury induced by lipopolysaccharide: an experimental study

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Lipopolysaccharide (LPS)-induced proliferation of lung fibroblasts is closely correlated with loss of gene expression of thymocyte differentiation antigen-1 (Thy-1), accompanied with deacetylation of histones H3 and H4 at the Thy-1 gene promoter region; however, the mechanism remains enigmatic. We report here that LPS downregulates Thy-1 gene expression by activating histone deacetylases (HDACs) via Toll-like receptor 4 (TLR4) signaling. Treatment of primary cultured mouse lung fibroblasts with LPS resulted in significant upregulation of TLR4 and enhanced cell proliferation that was abolished by silencing TLR4 with lentivirus-delivered TLR4 shRNA. Interestingly, LPS increased the mRNA and protein levels of HDAC-4, -5, and -7, an effect that was abrogated by HDAC inhibitor trichostatin A (TSA) or TLR4-shRNA-lentivirus. Consistent with these findings, Ace-H3 and Ace-H4 were decreased by LPS that was prevented by TSA. Most importantly, chromosome immunoprecipitation (ChIP) analysis demonstrated that LPS decreased the association of Ace-H4 at the Thy-1 promoter region that was efficiently restored by pretreatment with TSA. Accordingly, LPS decreased the mRNA and protein levels of Thy-1 that was inhibited by TSA. Furthermore, silencing the Thy-1 gene by lentivirus-delivered Thy-1 shRNA could promote lung fibroblast proliferation, even in the absence of LPS. Conversely, overexpressing Thy-1 gene could inhibit lung fibroblast proliferation and reduce LPS-induced lung fibroblast proliferation. Our data suggest that LPS upregulates and activates HDACs through TLR4, resulting in deacetylation of histones H3 and H4 at the Thy-1 gene promoter that may contribute to Thy-1 gene silencing and lung fibroblast proliferation. Pulmonary fibrosis is characterized by aberrant proliferation and activation of lung fibroblasts. 1,2 The phenotypic transition of lung fibroblasts during the process of pulmonary fibrosis can be stimulated by various pathological conditions, including lipopolysaccharide (LPS), a component of bacteria membranes and an important player during the development of pulmonary fibrosis. 3,4 LPS-induced proliferation of lung fibroblast is associated with the silencing of thymocyte differentiation antigen-1 (Thy-1) and deacetylation of histones H3 and H4 at the Thy-1 gene promoter; 5,6 however, the mechanism by which LPS promotes the deacetylation of histones H3 and H4 leading to downregulation of Thy-1 gene expression is largely unknown.Thy-1 is a cell surface glycoprotein that is present in normal lung fibroblasts but absent from the fibroblastic foci of idiopathic pulmonary fibrosis (IPF). 7 Heterogeneous surface expression of Thy-1 in fibroblasts results in the specialization of fibroblast, including Thy-1 (+) and Thy-1 (− ) subsets. Hagood et al 5,8,9 reported that lack of Thy-1 expression on lung fibroblasts contributed to IPF. Recently, it was found that epigenetic mechanisms, such as promoter hypermethylation and histone modification, play an important role in Thy-1 gene silencing in lung fibroblasts. 6,7 Our...

Section: Discussionsupporting

confidence: 92%

mentioning

confidence: 99%

HDAC is essential for epigenetic regulation of Thy-1 gene expression during LPS/TLR4-mediated proliferation of lung fibroblasts

Xing

Nie

et al. 2015

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“…3,5,28 Our finding that HMGB1 can enhance LPSinduced lung fibroblast proliferation is novel, yet consistent with findings of cooperative effects in other types of cells, including synovial fibroblasts, macrophages, and human monocytes. [29][30][31] For example, Wahamaa et al 32 showed that HMGB1 increased LPS-induced secretion of cytokines, such as tumor necrosis factor-α (TNF-α), interleukin (IL)-6, and IL-8, in synovial fibroblasts.…”

Section: Discussionsupporting

confidence: 85%

High-mobility group box 1 accelerates lipopolysaccharide-induced lung fibroblast proliferation in vitro: involvement of the NF-κB signaling pathway

Deng

et al. 2015

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The mechanism underlying lipopolysaccharide (LPS)-induced aberrant proliferation of lung fibroblasts in Gram-negative bacilli-associated pulmonary fibrosis is unknown. High-mobility group box 1 (HMGB1) is a ubiquitous nuclear protein that is released from the nuclei of lung fibroblasts after LPS stimulation. It can exasperate LPS-induced inflammation and hasten cell proliferation. Thus, this study investigated the effects of LPS-and/or HMGB1-stimulating murine lung fibroblasts on gene expression using various assays in vitro. Thiazolyl-diphenyl-tetrazolium bromide (MTT) assay data showed that either LPS or HMGB1 could induce lung fibroblast proliferation. Endogenous HMGB1 secreted from lung fibroblasts was detected by enzyme-linked immunosorbent assay (ELISA) 48 h after LPS stimulation. Pretreatment with an anti-HMGB1 antibody inhibited the proliferative effects of LPS on lung fibroblasts. DNA microarray data showed that the NF-κB signaling genes were upregulated in cells after stimulated with LPS, HMGB1, or both. Secretion of matrix metalloproteinase (MMP)-2 and MMP-9, and tissue inhibitor of metalloproteinase 2 (TIMP-2) was significantly upregulated after treatment with LPS, HMGB1, or their combination. However, an NF-κB inhibitor was able to downregulate levels of these proteins. In addition, levels of Toll-like receptor 4 (TLR4), Toll-like receptor 2 (TLR2), and receptors for advanced glycation end products (RAGE) mRNA and proteins were also upregulated in these cells after LPS treatment and further upregulated by LPS plus HMGB1. In conclusion, the data from the current study demonstrate that LPS-induced lung fibroblast secretion of endogenous HMGB1 can augment the proproliferative effects of LPS and, therefore, may play a key role in exacerbation of pulmonary fibrosis. The underlying molecular mechanisms are related to the activation of the TLR4/NF-κB signaling pathway and its downstream targets.

“…HMGB1 activates inflammatory pathways by stimulating TLR4 in many types of tissue injuries. 42,43 Our previous work, [22][23][24] as well as the work of others, [25][26][27] indicated that TLR4 mediates many forms of ALI, such as hemorrhagic shock-induced ALI and LPS-induced ALI. Overexpression of TLR4 in the lung tissue amplifies the severity of LPS-induced ALI.…”

Section: Discussionmentioning

confidence: 94%

“…[19][20][21] We have previously demonstrated that the activation of TLR4 and its downstream intracellular signal-transduction pathways is crucial to lipopolysaccharide (LPS)-induced ALI. [22][23][24] Other studies also reported that TLR4 mutant (C3H/HeJ) mice developed less ALI after unresuscitated hemorrhagic shock or being challenged with LPS. 25,26 Conversely, the overexpression of TLR4 in transgenic mice aggravated ALI and pulmonary inflammation, endothelial cell damage, and the recruitment of neutrophils to the lung.…”

mentioning

confidence: 95%

TLR4 as receptor for HMGB1-mediated acute lung injury after liver ischemia/reperfusion injury

Yang

Deng

et al. 2013

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Acute lung injury (ALI) frequently occurs after liver transplantation and major liver surgery. Proinflammatory mediators released by damaged liver after liver ischemia/reperfusion (I/R) injury might contribute to this form of ALI, but the underlying mechanisms have not been well characterized. High-mobility group box protein 1 (HMGB1), a recently identified proinflammatory cytokine, was found to be significantly higher in the serum after liver I/R injury. This study investigated whether HMGB1 was involved as a stimulating factor, and whether its downstream Toll-like receptor 4 (TLR4), p38 mitogen-activated protein kinase (p38MAPK), and activator protein-1 (AP-1) signaling pathways act as mediators in the development of liver I/R injury-induced ALI. Extensive ALI and lung inflammation was induced in a rat model of liver I/R injury. Serum HMGB1 was significantly higher after liver I/R injury, and more importantly, expression of HMGB1 mRNA and protein in the lung tissue was also significantly increased. We further found that liver I/R injury enhanced the expression of TLR4 mRNA and protein, and the activity of p38MAPK and AP-1 in the lung tissue. Inhibition of TLR4 expression in the lung tissue by infection with pGCSIL-GFP-lentivirus-expressing small hairpin RNAs targeting the TLR4 gene (TLR4-shRNA lentivirus) significantly attenuated ALI, lung inflammation, and activity of p38MAPK and AP-1 in the lung tissue. These findings indicate that HMGB1 might contribute to the underlying mechanism for liver I/R injuryinduced ALI and that its downstream TLR4, p38MAPK, and AP-1 signaling pathways are potentially important mediators in the development of ALI.