Uric Acid Promotes Apoptosis in Human Proximal Tubule Cells by Oxidative Stress and the Activation of NADPH Oxidase NOX 4

Abstract

Mild hyperuricemia has been linked to the development and progression of tubulointerstitial renal damage. However the mechanisms by which uric acid may cause these effects are poorly explored. We investigated the effect of uric acid on apoptosis and the underlying mechanisms in a human proximal tubule cell line (HK-2). Increased uric acid concentration decreased tubule cell viability and increased apoptotic cells in a dose dependent manner (up to a 7-fold increase, p<0.0001). Uric acid up-regulated Bax (+60% with respect to Ctrl; p<0.05) and down regulated X-linked inhibitor of apoptosis protein. Apoptosis was blunted by Caspase-9 but not Caspase-8 inhibition. Uric acid induced changes in the mitochondrial membrane, elevations in reactive oxygen species and a pronounced up-regulation of NOX 4 mRNA and protein (p<0.05). In addition, both reactive oxygen species production and apoptosis was prevented by the NADPH oxidase inhibitor DPI as well as by Nox 4 knockdown. URAT 1 transport inhibition by probenecid and losartan and its knock down by specific siRNA, blunted apoptosis, suggesting a URAT 1 dependent cell death. In summary, our data show that uric acid increases the permissiveness of proximal tubule kidney cells to apoptosis by triggering a pathway involving NADPH oxidase signalling and URAT 1 transport. These results might explain the chronic tubulointerstitial damage observed in hyperuricaemic states and suggest that uric acid transport in tubular cells is necessary for urate-induced effects.

Effect of uric acid on mitochondrial function and oxidative stress in hepatocytes

“…Previous studies have shown that ROS enhances apoptosis by inducing oxidative stress during neuronal injury (Loh et al, 2006). Another study proposed that apoptotic events occurring under high UA levels are dependent on ROS generation (Verzola et al, 2014). Moreover, another study reported that mitochondrial dysfunction results in the loss of CCO, inducing an increase in ROS expression and subsequently triggering the intrinsic apoptotic cascade protein (Circu and Aw, 2010), which is similar to the results of our experiment.…”

Nox4 has a crucial role in uric acid-induced oxidative stress and apoptosis in renal tubular cells

“…The main MAPKs are P38, ERK1/2 and c-Jun N-terminal kinase 1/2, whose activity is stimulated by phosphorylation. The ROS-induced activation of MAPKs has been reported in several kidney diseases (26)(27)(28), while the association between UA and MAPKs has largely remained elusive; however, previous studies have indicated the participation of MAPKs in UA-induced tubular cell apoptosis (29,30). As the underlying mechanisms required further elucidation, the present study examined the phosphorylation of P38 and ERK1/2 and found them significantly activated following exposure to UA.…”

“…In addition, the expression of Bax and Bcl-2, which can be regulated by MAPKs, was detected, revealing that high levels of UA caused a Bax/Bcl-2 imbalance, suggesting that apoptosis was induced through the mitochondrial pathway. A study by Verzola et al (30) also observed that UA promotes apoptosis in renal tubular cells by activating Nox4. Compared with this report, the present study provided further evidence to support their findings.…”

Zurampic Protects Pancreatic β-Cells from High Uric Acid Induced-Damage by Inhibiting URAT1 and Inactivating the ROS/AMPK/ERK Pathways

“…Accumulating literature has proposed that uric acid is involved in cell viability and apoptosis. A previous study once implied that uric acid decreased proximal tubular cell viability [22]. Another study gave the opposite evidence which proved suitable concentration of uric acid improved cell viability of HT22 hippocampal cells and BV-2 microglia [23].…”