The Uremic Toxin Indoxyl Sulfate Accelerates Senescence in Kidney Proximal Tubule Cells

Abstract

Kidney fibrosis is the common final pathway of nearly all chronic and progressive nephropathies. One cause may be the accumulation of senescent cells that secrete factors (senescence associated secretory phenotype, SASP) promoting fibrosis and inflammation. It has been suggested that uremic toxins, such as indoxyl sulfate (IS), play a role in this. Here, we investigated whether IS accelerates senescence in conditionally immortalized proximal tubule epithelial cells overexpressing the organic anion transporter 1 (ciPTEC-OAT1), thereby promoting kidney fibrosis. Cell viability results suggested that the tolerance of ciPTEC-OAT1 against IS increased in a time-dependent manner at the same dose of IS. This was accompanied by SA-β-gal staining, confirming the accumulation of senescent cells, as well as an upregulation of p21 and downregulation of laminB1 at different time points, accompanied by an upregulation in the SASP factors IL-1β, IL-6 and IL-8. RNA-sequencing and transcriptome analysis revealed that IS accelerates senescence, and that cell cycle appears to be the most relevant factor during the process. IS accelerates senescence via TNF-α and NF-ĸB signalling early on, and the epithelial-mesenchymal transition process at later time points. In conclusion, our results suggest that IS accelerates cellular senescence in proximal tubule epithelial cells.

Immunosenescence and Immune Exhaustion Are Associated with Levels of Protein-Bound Uremic Toxins in Patients on Hemodialysis

“…IxS proved to be the most important PBUT in our study, acting as an independent factor for senescent phenotypic alterations of lymphocytes. A recent experiment showed that the incubation of proximal tubular epithelial cells in the presence of IxS leads to the upregulation in the SASP factors, accelerates the cellular senescence of epithelial cells, and finally promotes kidney fibrosis via TNF-α, NF-ĸB signaling pathways, and the epithelial–mesenchymal transition process [ 38 ]. No clinical studies have proven these results, but it seems extremely promising that IxS may be implicated in the senescent progression of other cell types, a hypothesis that has to be investigated.…”

Protein-Bound Uremic Toxins in Senescence and Kidney Fibrosis

“…ROS triggers DDR, and DDR promotes ROS production by activating its downstream effectors, including p53 and p21 [ 145 ]. Recent research has suggested that PBUTs may accelerate senescence via cell cycle arrest and inhibition of cell proliferation [ 146 , 147 ]. Others have suggested that PCS and IS upregulate p21 and increase the number of cells positive for senescence-associated beta-galactosidase [ 9 ].…”

“…During CKD progression, the released inflammatory (SASP) factors activate different pathways and initiate various processes, including senescence and EMT, in tubular epithelial cells [ 146 , 149 ]. As discussed, PBUT accumulation-induced inflammation might be one reason for senescence development.…”

Dietary Omega-3 Polyunsaturated Fatty Acids and Amelioration of CKD: Possible Cellular Mechanisms

“…Because IS is a protein-bound uremic toxin, the authors assumed that DHA might act as a competitor for IS binding, thereby enhancing its clearance, reducing its accumulation in proximal tubular epithelial cells (PTECs), and ameliorating IS-induced oxidative stress, thus providing protection against tubular injury. Given that IS directly accelerates cellular senescence in PTECs, 8 which is associated with tubulointerstitial injury, IS accumulation in the tubules and higher serum IS levels may contribute to a vicious cycle that leads to CKD progression. Further investigations are required to determine whether DHA supplementation directly reduces IS accumulation and tubulotoxicity in the pathogenesis of CKD.…”