Profibrotic Effect of Interleukin-18 in HK-2 Cells Is Dependent on Stimulation of the Toll-like Receptor 4 (TLR4) Promoter and Increased TLR4 Expression

Abstract

IL-18 induces profibrotic changes in TECs independent of TGF-β1 activity. IL-18 stimulates the TLR4 promoter via AP-1 activation to increase TLR4 expression in TECs and stimulates profibrotic changes in TECs through increased TLR4 expression/signaling. The profibrotic effect of IL-18 in TECs is mediated through stimulation of TLR4 expression via activation of AP-1. This represents a novel fibrotic signaling pathway in TECs independent of TGF-β1. IL-18 is an important mediator of obstruction-induced renal fibrosis and tubular epithelial cell injury independent of TGF-β1 activity. We sought to determine whether the profibrotic effect of IL-18 is mediated through Toll-like receptor 4 (TLR4). Male C57BL6 wild type and mice transgenic for human IL-18-binding protein were subjected to left unilateral ureteral obstruction versus sham operation. The kidneys were harvested 1 week postoperatively and analyzed for IL-18 production and TLR4 expression. In a separate arm, renal tubular epithelial cells (HK-2) were directly stimulated with IL-18 in the presence or absence of a TLR4 agonist, TLR4 antagonist, or TLR4 siRNA knockdown. Cell lysates were analyzed for TLR4, α-smooth muscle actin, and E-cadherin expression. TLR4 promotor activity, as well as AP-1 activation and the effect of AP-1 knockdown on TLR4 expression, was evaluated in HK-2 cells in response to IL-18 stimulation. The results demonstrate that IL-18 induces TLR4 expression during unilateral ureteral obstruction and induces TLR4 expression in HK-2 cells via AP-1 activation. Inhibition of TLR4 or knockdown of TLR4 gene expression in turn prevents IL-18-induced profibrotic changes in HK-2 cells. These results suggest that IL-18 induces profibrotic changes in tubular epithelial cells via increased TLR4 expression/signaling.