Regulation of TLR4 in silica-induced inflammation: An underlying mechanism of silicosis

Abstract

Silicosis is an incurable lung disease affecting millions of workers in hazardous occupations. It is caused by chronic exposure to the dust that contains free crystalline silica. Silica-induced lung damage occurs by several main mechanisms including cell death by apoptosis, fibrosis and production of cytokines. However, the signal pathways involved in these mechanisms are not fully characterized. In this study, the toll-like receptor 4 (TLR4)-related signal pathway was examined in silica-treated U937-differentiated macrophages. The expression level of TLR4 was measured by both quantitative PCR and Western blot. Confirmation of the involvement of MyD88/TIRAP and NFκB p65 cascade was performed by Western blot. The secretion of cytokines IL-1β, IL-6, IL-10 and TNFα was measured by enzyme-linked immunosorbent assay. Our results showed that TLR4 and related MyD88/TIRAP pathway was associated with silica-exposure in U937-differentiated macrophages. Protein expression of TLR4, MyD88 and TIRAP was upregulated when the U937-differentiated macrophages were exposed to silica. However, the upregulation was attenuated when TLR4 inhibitor, TAK-242 was present. At different incubation times of silica exposure, it was found that NFκB p65 cascade was activated at 10-60 minutes. Release of cytokines IL-1β, IL-6, IL-10 and TNFα was induced by silica exposure and the induction of IL-1β, IL-6 and TNFα was suppressed by the addition of TAK-242. In conclusion, our study demonstrated that TLR4 and related MyD88/TIRAP pathway was involved in silica-induced inflammation in U937-differentiated macrophages. Downstream NFκB p65 cascade was activated within 1 hour when the U937-differentiated macrophages were exposed to silica. The better understanding of early stage of silica-induced inflammatory process may help to develop earlier diagnosis of silicosis.

The importance of differences; On environment and its interactions with genes and immunity in the causation of rheumatoid arthritis

“…Which immune reactions that will ultimately be triggered depend on both local conditions for immune activation and on genetic constitution, mainly in MHC genes expressed by the host. Of interest in the context of local immune activation is that smoke (containing char and other agents) as well as silica dust can activate dendritic cells as well as monocytes/macrophages as antigen-presenting cells by effects mediated via pattern recognition molecules, including TLR4 [167][168][169]. That such events happen also in vivo in patients is indicated from the demonstration of germinal centre-like structures in the lungs of ACPA-positive newly diagnosed RA patients [163,170].…”

Comparative Analysis of Global Gene Expression and Complement Components Levels in Umbilical Cord Blood from Preterm and Term Neonates: Implications for Significant Downregulation of Immune Response Pathways related to Prematurity

“…Toll-like receptors (TLRs) are highly conserved evolutionary receptors of the innate immunity, which constitute the first barrier against pathogens 49 - 51 . In our study we found significantly lower expression of not only TLR5 , which confirms previous studies, but also TLR1, 2, 4, 6, 8 and TLR10 were downregulated in our preterm group, when compared with term babies.…”

MyD88 regulates a prolonged adaptation response to environmental dust exposure-induced lung disease

“…The complexity and biodiversity of agriculture dust exposures is increasingly recognized, and animal and human studies have defined roles for TLR2/TLR4/TLR9signaling pathways [8][9][10][11][12]. The TLR/IL-1R/IL-18R adaptor protein myeloid differentiation factor 88 (MyD88) that is used by all TLRs except for TLR3 [13] has been shown to have a fundamental role in the acute inflammatory response to organic dust [10,14,15] as well as other inflammatory exposures [16,17]. MyD88 signaling mediates bleomycin-induced IL-17B expression in alveolar macrophages to promote pulmonary fibrosis [16].…”

“…MyD88 signaling mediates bleomycin-induced IL-17B expression in alveolar macrophages to promote pulmonary fibrosis [16]. In silica-induced fibrosis, silica dust increases MyD88 expression in macrophages [17]. In experimental asthma, MyD88 expression in epithelial cells mediates eosinophilia whereas MyD88 expression in conventional dendritic cells controls the neutrophilic response [18].…”