Indoxyl Sulfate, a Tubular Toxin, Contributes to the Development of Chronic Kidney Disease

Abstract

Indoxyl sulfate (IS), a uremic toxin, causes chronic kidney disease (CKD) progression via its tubulotoxicity. After cellular uptake, IS directly induces apoptotic and necrotic cell death of tubular cells. Additionally, IS increases oxidative stress and decreases antioxidant capacity, which are associated with tubulointerstitial injury. Injured tubular cells are a major source of transforming growth factor-β1 (TGF-β1), which induces myofibroblast transition from residual renal cells in damaged kidney, recruits inflammatory cells and thereby promotes extracellular matrix deposition in renal fibrosis. Moreover, IS upregulates signal transducers and activators of transcription 3 phosphorylation, followed by increases in TGF-β1, monocyte chemotactic protein-1 and α-smooth muscle actin production, which participate in interstitial inflammation, renal fibrosis and, consequently, CKD progression. Clinically, higher serum IS levels are independently associated with renal function decline and predict all-cause mortality in CKD. The poor removal of serum IS in conventional hemodialysis is also significantly associated with all-cause mortality and heart failure incidence in end-stage renal disease patients. Scavenging the IS precursor by AST-120 can markedly reduce tubular IS staining that attenuates renal tubular injury, ameliorates IS-induced oxidative stress and rescues antioxidant glutathione activity in tubular epithelial cells, thereby providing a protective role against tubular injury and ultimately retarding renal function decline.

Keywords: chronic kidney disease; indoxyl sulfate; oxidative stress; renal fibrosis; tubular injury.

Figure 1

The mechanisms by which IS induces renal tubular cell death. After cellular uptake of IS by OAT1/3 in renal tubules, IS binds to AhR in the cytoplasm and exerts direct tubulotoxicity via necrotic or apoptotic cell death. IS upregulates 12-lipoxygenase and 12(S)-HETE synthesis and induces TRPV1 hyperfunction, which leads to necrosis of tubular cells. Additionally, IS induces apoptosis of tubular cells through the activation of the proapoptotic Bax protein, disrupting the mitochondrial membrane potential, or the activation of ERK 1/2 and p38 MAPK. IS, indoxyl sulfate; OAT, organic anion transporter; AhR, aryl hydrocarbon receptor; 12(S)-HETE, endovanilloid 12-hydroxyeicosatetraenoic acid; TRPV1, transient receptor potential vanilloid 1; Bax, Bcl-2-associated X protein; ERK 1/2, extracellular signal-regulated kinases 1/2; MAPK, mitogen-activated protein kinase.

Figure 2

The mechanisms by which IS induces oxidative stress and is associated with tubular cell injury and interstitial inflammation. After cellular uptake of IS by OAT1/3 in renal tubules, IS binds to AhR in the cytoplasm and induces tubular cell injury and interstitial inflammation through the downregulation of Nrf2, and this increased oxidative stress induces CREB and NF-κB expression, resulting in increased NOX4-induced ROS production and mitochondrial oxidative phosphorylation, and decreases antioxidant SOD and glutathione activity. IS, indoxyl sulfate; OAT, organic anion transporter; AhR, aryl hydrocarbon receptor; Nrf2, nuclear factor erythroid 2-related factor 2; NOX4, NADPH oxidase 4; ROS, reactive oxygen species; NF-κB, nuclear factor-κB; CREB, cAMP response element binding protein; SOD, superoxide dismutase.

Figure 3

Injured renal tubular cells overproduce TGF-β1 and lead to interstitial inflammation and renal fibrosis. Injured renal tubules increase the overproduction of TGF-β1 and Stat3 phosphorylation and lead to interstitial inflammation and renal fibrosis, which ultimately hasten CKD progression. After TGF-β1 signaling activation, HSP90 and Smad 2/3 are phosphorylated and cause myofibroblast transition of fibroblasts, increase renal ECM production and also reduce TIMP-1 expression, an inhibitor of matrix metalloproteinases, which are responsible for ECM degradation. TGF-β1 also stimulates ROS generation and increases ICAM-1, which is an adhesion molecule that promotes monocyte/macrophage adhesion. The activation of Stat3 phosphorylation in tubules by IS stimulation leads to increases in MCP-1, which recruits inflammatory cells, and tubular α-SMA, which aggravates renal fibrosis. TGF-β1, transforming growth factor-β1; HSP90, heat shock proteins 90; ECM, extracellular matrix; TIMP-1, tissue inhibitor of metalloproteinase-1; ROS, reactive oxygen species; NF-κB, nuclear factor-κB; p53, protein 53; ICAM-1, intercellular adhesion molecule-1; MCP-1, monocyte chemotactic protein-1; α-SMA: α-smooth muscle actin; Stat 3, signal transducers and activators of transcription.