Lipopolysaccharide acting via toll-like receptor 4 transactivates the TGF-β receptor in vascular smooth muscle cells

Abstract

Toll-like receptors (TLRs) recognise pathogen‑associated molecular patterns, which allow the detection of microbial infection by host cells. Bacterial-derived toxin lipopolysaccharide activates TLR4 and leads to the activation of the Smad2 transcription factor. The phosphorylation of the Smad2 transcription factor is the result of the activation of the transforming growth factor-β receptor 1 (TGFBR1). Therefore, we sought to investigate LPS via TLR4-mediated Smad2 carboxy terminal phosphorylation dependent on the transactivation of the TGFBR1. The in vitro model used human aortic vascular smooth muscle cells to assess the implications of TLR4 transactivation of the TGFBR1 in vascular pathophysiology. We show that LPS-mediated Smad2 carboxy terminal phosphorylation is inhibited in the presence of TGFBR1 inhibitor, SB431542. Treatment with MyD88 and TRIF pathway antagonists does not affect LPS-mediated phosphorylation of Smad2 carboxy terminal; however, LPS-mediated Smad2 phosphorylation was inhibited in the presence of MMP inhibitor, GM6001, and unaffected in the presence of ROCK inhibitor Y27632 or ROS/NOX inhibitor DPI. LPS via transactivation of the TGFBR1 stimulates PAI-1 mRNA expression. TLRs are first in line to respond to exogenous invading substances and endogenous molecules; our findings characterise a novel signalling pathway in the context of cell biology. Identifying TLR transactivation of the TGFBR1 may provide future insight into the detrimental implications of pathogens in pathophysiology.

Role of microRNA, bta-miR-375 in Immune Sturdiness of Vechur: The Native Cattle Breed of Kerala, India #

“…In contrast to the our findings, there are reports suggesting LPS mediated suppression of TGF-β signalling which in turn overcomes the inhibitory effect of TGFβ on NF-kappa B activation followed by stimulation of the release of pro inflammatory cytokines [ 23 , 24 ]. But supporting the findings of the current study, there are reports explaining activation of TGF-β signalling through LPS mediated TLR4 signalling resulting in inhibition of the release of pro inflammatory cytokines from the activated T cell [ 22 ].…”

“…Signalling pathway involving transforming growth factor β (TGF-β; a pleiotropic cytokine secreted predominantly by immune cells) are having potent regulatory and anti-inflammatory activity have been reported for its association with lipopolysaccharide mediated TLR4 signalling and thereby influences NF-kappa B signalling [ 21 ]. Both NF-kappa B signalling and TGF-β signalling along with LPS mediated TLR4 signalling are central in regulation of immune responses and release of cytokines from immune cells [ 22 , 23 , 24 ]. Findings of our study, suggest that immune related pathways preferably, Toll-like receptor signalling pathway, I-kappa B kinase/NF-kappa B signalling, SMAD protein signal transduction and TGF-β receptor signalling are inter related and these pathways are significantly influenced by the target genes of bta-miR-375.…”

“…Controversial reports are there regarding the association of TGF-β signalling pathway and LPS mediated TLR4 signalling. It has been reported that LPS mediated TLR4 signalling is having an inhibitory effect on TGF-β receptor signalling through the coordinated action of SMAD proteins [ 23 ] even though, a stimulatory effect of the same pathway on TGF-β signalling has also been reported [ 22 ]. LPS-induced NF-kappa B activation was found to be inhibited in murine macrophages by TGF-β1 via Smad3 and Smad6 proteins thereby inhibiting the release of pro-inflammatory cytokines [ 23 ].…”

“…But this inhibitory effect of TGF-β1on NF-kappa B activation can be overcome by triggering Smad3 linker phosphorylation via TLR4–IRAK1-activated ERK1/2 and subsequent blocking Smad6 expression by LPS [ 23 ]. Lipopolysaccharide acting via TLR4, transactivating the TGF-β receptor in human vascular smooth muscle cells has been reported but the authors could not detect any LPS mediated inhibition of SMAD 6 expression in these cells [ 22 ]. Similar studies showed that LPS enhances Smad2/3-dependent TGF-β1 transcriptional responses in normal fibroblasts through TLR4 signalling [ 25 , 26 ].…”

Transforming growth factor-β receptors: versatile mechanisms of ligand activation

“…In human vascular smooth muscle cells, TLR4induced TGFBR activation involves MMPs but not ROCK-dependent pathways [115]. Employing pharmacological inhibitors of MMPs shows that TLR4-mediated TGFBR activation measured as phosphorylation of Smad2 carboxyl terminal in human vascular smooth muscle cells is dependent on MMP-2 but not MMP-9 dependent pathways [115]. Mitogen-activated protein kinases (MAPK) are common downstream mediators of receptor activation, including TGFBR, PAR-1 and TLR4 [135][136][137][138].…”