Background: Uremic toxins have been suggested to promote progression of chronic renal failure. We have shown that organic anion transporter-mediated uptake of uremic toxins induces oxidative stress in opossum kidney renal tubular cells overexpressing the transporter. Plasminogen activator inhibitor-1 (PAI-1) and nuclear factor-kappa B (NF-kappaB) are major factors known to promote tubulointerstitial fibrosis. The present study examined the signaling pathway that is activated by uremic toxins to induce PAI-1 and activate NF-kappaB in human renal proximal tubular cells (HK-2).
Methods: Uremic toxins in the form of organic anion were examined their ability to induce oxidative stress, PAI-1 gene expression, and NF-kappaB activation in HK-2. PAI-1 expression was measured by enzyme-linked immunosorbent assay (ELISA) and the Northern blotting. Human PAI-1 promoter activity was estimated by luciferase reporter gene (NKkappaB-luc) assay. NF-kappaB activation was measured by the pNFkappaB-luc reporter gene and electrophretic gel mobility shift assay.
Results: Among organic anion species tested, indoxyl sulfate and indoleacetic acid induced free radical production in HK-2. A nonspecific transporter inhibitor (probenecid) suppressed the IS-stimulated radical production. Indoxyl sulfate and indoleacetic acid dose dependently increased the expressions of PAI-1 mRNA and protein in these cells. The luciferase reporter gene assay revealed that indoxyl sulfate and indoleacetic acid dose dependently activated NF-kappaB and PAI-1 promoter. Activation of NF-kappaB was also confirmed by an electrophoretic gel mobility shift assay. Both antioxidant and NF-kappaB inhibitors dose dependently inhibited the activation of PAI-1 promoter by indoxyl sulfate.
Conclusion: Uremic toxins induce free radical production by renal tubular cells and activate NF-kappaB which, in turn, up-regulates PAI-1 expression. Thus, progression of chronic renal failure may be promoted by PAI-1 up-regulation induced by uremic toxins.