Abstract
Indoxyl sulfate (IS) is accumulated during severe renal insufficiency and known for its nephrotoxic properties. Transient receptor potential vanilloid 1 (TRPV1) is present in the kidney and acts as a renal sensor. However, the mechanism underlying IS-mediated renal tubular damage in view of TRPV1 is lacking. Here, we demonstrated that TRPV1 was expressed in tubular cells of Lilly Laboratories cell-porcine kidney 1 (LLC-PK1) and Madin-Darby canine kidney cells (MDCK). IS treatment in both cells exhibited tubular damage with increased LDH release and reduced cell viability in dose- and time-dependent manners. MDCK, however, was more vulnerable to IS. We, therefore, investigated MDCK cells to explore a more detailed mechanism. Interestingly, IS-induced tubular damage was markedly attenuated in the presence of selective TRPV1 blockers. IS showed no effect on TRPV1 expression but significantly increased arachidonate 12-lipoxygenase (ALOX12) protein, mRNA expression, and 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE) amounts in a dose-dependent manner, indicating that the ALOX12/12(S)-HETE pathway induced TRPV1 hyperfunction in IS-mediated tubulotoxicity. Blockade of ALOX12 by cinnamyl-3,4-dihydroxy-α-cyanocinnamate or baicalein attenuated the effects of IS. Since aryl hydrocarbon receptor (AhR) activation after IS binding is crucial in mediating cell death, here, we found that the AhR blockade not only ameliorated tubular damage but also attenuated ALOX12 expression and 12(S)-HETE production caused by IS. The uremic toxic adsorbent AST-120, however, showed little effect on ALOX12 and 12(S)-HETE, as well as IS-induced cell damage. These results clearly indicated that IS activated AhR and then upregulated ALOX12, and this induced endovanilloid 12(S)-HETE synthesis and contributed to TRPV1 hyperfunction in IS-treated tubular cells. Further study on TRPV1 may attenuate kidney susceptibility to the functional loss of end-stage kidney disease via IS.
Highlights
As a highly perfused organ, the kidney plays an important role in the maintenance of body homeostasis via tubular function to reabsorb water, nutrients, and electrolytes back to circulation and to excrete metabolic waste products
The Transient receptor potential vanilloid 1 (TRPV1) mRNA showed similar expression to that in protein levels (Figure 2B). These results demonstrated that transient receptor potential vanilloid 1 (TPRV1) in Madin-Darby canine kidney cells (MDCK) cells was more abundant than that in Laboratories cell-porcine kidney 1 (LLC-PK1) cells and indicated that MDCK was more vulnerable to Indoxyl sulfate (IS) if TRPV1 was the downstream pathway in this cytotoxicity
Compared to the IS-treated cells, a selective ALOX12 blocker cinnamyl-3,4-dihydroxy-α-cyanocinnamate (CDC) significantly lowered lactate dehydrogenase (LDH) release and enhanced cell viability. Another ALOX12 blocker baicalein (Bai) to the IS-treated cells showed similar effects as those of the CDC by reducing LDH release and increasing cell viability. These results suggested that IS-induced tubular cell damage was dependent on the effect of ALOX12-mediated 12(S)-Hydroxyeicosatetraenoic Acid (HETE) generation, which is known as an endogenous ligand for TRPV1 activation
Summary
As a highly perfused organ, the kidney plays an important role in the maintenance of body homeostasis via tubular function to reabsorb water, nutrients, and electrolytes back to circulation and to excrete metabolic waste products. TRPV1 is found mainly in the nociceptive neuron of the peripheral nervous system and acts as a sensor in detecting noxious stimuli. It is a non-selective cation channel that is exclusively localized in mammalian tissue, predominantly in the nervous system, which allows for the influx of calcium and sodium into cells to depolarize neurons, leading to the perception of pain. Activation of TRPV1 in the isolated kidney decreases renal perfusion pressure and increases the glomerular filtration rate (GFR) and diuretic response [5]. These results indicate that TRPV1 plays an important role in maintaining renal hemodynamics and excretory function for keeping body fluid homeostasis. TRPV1 is found to be expressed in renal tubules [4]; the effect of tubular TRPV1 distribution is still unclear
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