TLR5 activation induces expression of the pro-inflammatory mediator Urokinase Plasminogen Activator via NF-κB and MAPK signalling pathways in human dental pulp cells.

Aim: To explore the involvement of TLR5 in pulp inflammation and to examine the effects of TLR5 activation with its ligand, FlaB protein, on pro-inflammatory gene expression.
Methodology: TLR5 expression in dental pulp tissues and human dental pulp cells (hDPCs) were determined by immunohistochemistry, immunocytochemistry, Western blots and RT-PCR analyses. To examine the role of TLR5, hDPCs were treated with recombinant FlaB protein (500 ng mL ^-1 ) to activate the receptor or with a small interfering RNA against TLR5 (si-TLR5) to downregulate the receptor. After exposure to FlaB, the expression of inflammation-related proteins was screened using a protein array kit. Western blots or qRT-PCR analyses were performed to identify changes in the expression of uPA (urokinase plasminogen activator), TIMPs (tissue inhibitor of metalloproteinases), and IL-6 and to determine their signalling pathways. Statistical analysis was performed using one-way analysis of variance (anova) with Tukey post hoc test; P < 0.05 was considered statistically significant.
Result: TLR5 expression was identified in pulp tissues and hDPCs. In the protein array analysis, treatment with FlaB significantly increased uPA expression (P < 0.01) and significantly decreased TIMP1/4 (P < 0.05). FlaB treatment also significantly increased expression of the inflammatory marker IL-6 (P < 0.01). FlaB treatment increased phosphorylation of the NF-κB p65 subunit, JNK, p38 and ERK. Chemical inhibitors of NF-κB (Bay11-7082), p38 (SB202190) or ERK (U0126) decreased the FlaB induction of uPA expression. Downregulation of TLR5 expression by siRNA decreased the FlaB induction of uPA protein and p65 phosphorylation.
Conclusion: TLR5 activation with FlaB treatment induced the expression of uPA via the NF-κB and MAPK signalling pathways. Flagellin-bearing oral bacteria may cause pulp inflammation through TLR5. The findings provide new clues to control pulpal diseases by targeting TLR5 signalling pathways.
(© 2019 International Endodontic Journal. Published by John Wiley & Sons Ltd.)