Astilbin improves potassium oxonate-induced hyperuricemia and kidney injury through regulating oxidative stress and inflammation response in mice

Abstract

Astilbin is a flavonoid compound derived from the rhizome of Smilax china L. The effects and possible molecular mechanisms of astilbin on potassium oxonate-induced hyperuricemia mice were investigated in this study. Different dosages of astilbin (5, 10, and 20 mg/kg) were administered to induce hyperuricemic mice. The results demonstrated that the serum uric acid (Sur) level was significantly decreased by increasing the urinary uric acid (Uur) level and fractional excretion of urate (FEUA) with astilbin, related with suppressing role in meditation of Glucose transporter 9 (GLUT9), Human urate transporter 1 (URAT1) expression and up-regulation of ABCG2, Organic anion transporter 1/3 (OAT1/3) and Organic cation transporter 1 (OCT1). In addition, kidney function parameters, including serum creatinine (Scr) and blood urea nitrogen (BUN) were restored in astilbin-treated hyperuricemic rats. Further investigation indicated that astilbin prevented the renal damage against the expression of Thioredoxin-interacting protein (TXNIP) and its related inflammation signal pathway, including NLR pyrin domain-containing 3/Nuclear factor κB (NLRP3/NF-κB), which is associated with the up-regulation of interleukin-1β (IL-1β) and interleukin-18 (IL-18), and also presented a renal protective role by suppression oxidative stress. Moreover, astilbin inhibited activation of the Janus kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3) cascade and over-expression of suppressor of cytokine signaling 3 (SOCS3) in the kidneys of potassium oxonate-induced mice. These findings provide potent evidence and therapeutic strategy for astilbin as a safe and promising compound in the development of a disease-modifying drug due to its function against hyperuricaemia and renal injury induced by potassium oxonate.

Introduction

Hyperuricemia is characterized by a persistent up-regulation of serum urate concentrations above 400 mM, in which monosodium urate monohydrate crystals may be deposited in tissues causing several pathological conditions, including acute gouty arthritis, urolithiasis and obstructive uropathy [1]. Uric acid, also known as urate, which is the final breakdown product of purines in humans, caused by a dysfunction in metabolism of purine or a down-regulation in uric acid production, is created from the oxidation of hypoxanthine and xanthine in the liver. Its homeostasis relies on the balance of the production and excretion of urate, and a large number of urate is excreted in the urine through four main steps, which could be presented as ultrafiltration, reabsorption, secretion, and reabsorption of post-secretion [2].

Renal urate under-excretion regulated by organic anion transporters in kidney is considered to be the major cause of hyperuricaemia. Glucose transporter 9 (GLUT9), which is localized in apical as well as basolateral membranes of the distal nephron is responsible for urate reabsorption. Mutations in GLUT9 cause urate reabsorption reduction, resulting in hyperuricaemia [3]. Human urate transporter 1 (URAT1), localized in the apical membrane of proximal tubules, also controls renal urate reabsorption. Organic anion transporter 1 (OAT1) at the basolateral membrane of renal proximal tubules contributes to urate excretion rather than reabsorption, whose decreasing is found in rats with hyperuricaemia. Urate transporter (UAT) excretes urate from renal proximal tubules to lumen both in humans and rats. Furthermore, organic cation transporter 1 (OCT1) and organic cation transporter 2 (OCT2) in the basolateral membrane of renal proximal tubules are related to the exclusion of organic cations or toxic wastes, and also their down-regulated expression levels have been found in streptozotocin-induced diabetic kidney injury of rats [4], [5], [6], [7]. Therefore, these renal organic ion transporters are likely to be potential targets for drugs affecting urate homoeostasis and kidney function.

Thioredoxin-interacting protein (TXNIP), also known as thioredoxin-binding protein-2 (TBP-2) or VD3 up-regulated protein 1 (VDUP1), a very crucial negative regulator of thioredoxin (Trx) system, which interacts with thioredoxin to block antioxidant function, is most abundant in kidney glomeruli [8]. Renal TXNIP over-expression is observed in rats and patients with diabetic nephropathy. TXNIP links oxidative stress to the NLR pyrin domain-containing 3 (NLRP3) inflammasome activation in a reactive oxygen species (ROS)-sensitive manner. This inflammasome activation potently increases interleukin-1β (IL-1β) maturation and secretion [9], and initiates early harmful events in podocytes and glomeruli to cause kidney injury. Increasing evidences suggest that uric acid-induced inflammation is the central mechanism for renal dysfunction in hyperuricemic rodents and patients, in which NF-kappa B (NF-κB) pathway plays an important role. Actually, NF-κB is a key transcription factor in regulating the expression of pro-inflammatory cytokine IL-1β mediated [10], [11].

Oxidative stress has been found to be a major contributing factor to the renal injury and it is typically related to a down-regulation in the antioxidant defense [12]. ROS has been implicated in many disorders [13]. Furthermore, TXNIP is the endogenous inhibitor of ROS elimination by binding to the active cysteine residue of Trx, resulting in oxidative stress [14]. And also, the up-regulated ROS could trigger a large number of cytokines and inflammatory factors, such as NF-κB, TXNIP, and inflammasome [15]. Recently, some studies have indicated that NF-κB regulated the ROS-induced NLRP3 inflammasome by accelerating NLRP3 inflammasome transcription [16], [17]. TXNIP can combine with NLRP3 directly and result in NLRP3 inflammasome assembly under oxidative stress [18], demonstrating a relationship between TXNIP/NLRP3/NF-κB and oxidative stress. However, further study should be conducted to clarify the mechanism of TXNIP-related ROS and inflammation response. Furthermore, oxidative damage on bronchial epithelial cells, and then regulate JAK/STAT signaling pathway and relative cytokines of epithelial cells through oxidative stress [19]. A study also indicated that activation of the JAK2–STAT3 cascade was linked with renal injury [20]. Thus, we further investigated the mechanism of JAK2–STAT3 in renal dysfunction.

Astilbin, as an active flavonoid compound, is isolated from the rhizome of Smilax china L. (Smilaceae), which is widely applied in the traditional Chinese medical treatment [21], and has been used for anti-arthritic, anti-hepatic and anti-renal injury [22], [23], [24]. A study has shown that astilbin significantly improve myocardial function in the diabetic rats treatment with during myocardial ischemia and reperfusion (I/R) challenge [25]. This was indicated by a decreasing in infarct size and histopathological score. Meanwhile, astilbin was also shown to be related with the stimulation of cardiodynamics and inhibition of myocardial cell necrosis directly [26]. All of the results demonstrated above supposed that treatment with astilbin leaded to the myocardial function recovery (Fig. 1).

However, in this study, astilbin was firstly investigated to study its effects on serum and renal uric acid levels, renal action, oxidative stress as well as inflammation response in potassium oxonate-induced hyperuricemic mice by exploring renal protein levels of organic ion transporters, TXNIP-related ROS and inflammation-associated signaling pathway.