High uric acid directly inhibits insulin signalling and induces insulin resistance

Highlights

• Hyperuricemic mice showed impaired glucose tolerance with insulin resistance.

• Hyperuricemia inhibited insulin signaling in liver, muscle and fat tissues.

• HUA levels induced oxidative stress in HepG2 cells.

• NAC blocked the HUA-induced insulin signalling impairment in HepG2 cells.

Abstract

Background and aim

Accumulating clinical evidence suggests that hyperuricemia is strongly associated with abnormal glucose metabolism and insulin resistance. However, how high uric acid (HUA) level causes insulin resistance remains unclear. We aimed to determine the direct role of HUA in insulin resistance in vitro and in vivo in mice.

Methods

An acute hyperuricemia mouse model was created by potassium oxonate treatment, and the impact of HUA level on insulin resistance was investigated by glucose tolerance test, insulin tolerance test and insulin signalling, including phosphorylation of insulin receptor substrate 1 (IRS1) and Akt. HepG2 cells were exposed to HUA treatment and N-acetylcysteine (NAC), reactive oxygen species scavenger; IRS1 and Akt phosphorylation was detected by Western blot analysis after insulin treatment.

Results

Hyperuricemic mice showed impaired glucose tolerance with insulin resistance. Hyperuricemia inhibited phospho-Akt (Ser473) response to insulin and increased phosphor-IRS1 (Ser307) in liver, muscle and fat tissues. HUA induced oxidative stress, and the antioxidant NAC blocked HUA-induced IRS1 activation and Akt inhibition in HepG2 cells.

Conclusion

This study supplies the first evidence of HUA directly inducing insulin resistance in vivo and in vitro. Increased uric acid level may inhibit IRS1 and Akt insulin signalling and induce insulin resistance. The reactive oxygen species pathway plays a key role in HUA-induced insulin resistance.

Introduction

Uric acid is the end-product of purine metabolism, and in humans, the upper normal range of concentration is 6 mg/dl for women and 7 mg/dl for men [1]. In the last few decades, the prevalence of hyperuricemia has been rapidly increasing worldwide [2], [3]. Meanwhile, a large body of evidence has established the association of elevated serum uric acid level and various metabolic disorders, including gout, hypertension, atherosclerosis, chronic renal diseases, and metabolic syndrome [4], [5], [6].

Recently, evidence has emerged from several large epidemiological studies that hyperuricemia is related to insulin resistance [7], [8], [9]. High-sensitivity C-reactive protein (hs-CRP) level is often higher in hyperuricemic patients than in the general population, hs-CRP level was found to be an independent predictor of homeostatic model assesssment-insulin resistance [10], [11]. In addition, soluble uric acid could increase tissue levels of NADPH oxidase and the generation of reactive oxygen species (ROS) in mature adipose tissue [12]; oxidative-stressed adipose tissue shows decreased sensitivity to insulin as a risk factor of insulin resistance. However, the causal mechanism of hyperuricemia in the development of insulin resistance is still unclear.

Some studies have shown that high uric acid (HUA) levels increase oxidative stress in adipocytes, vascular smooth muscle cells and human umbilical vein endothelial cells [13], [14]. Our previous study showed increased ROS production with uric acid treatment in β-cells [15]. One attractive hypothesis is that many factors leading to insulin resistance are mediated by the generation of abnormal amounts of ROS. Emerging evidence also supports an important role of ROS in various forms of insulin resistance [16], [17].

Insulin resistance is defined as a profound dysregulation of the insulin signalling system and thus represents a state of impaired ability of peripheral tissues to respond to the physiological levels of insulin. Binding of insulin to the insulin receptor (IR) initiates signaling cascades by activating its receptor tyrosine kinase, thus leading to glucose transport activation and other metabolic effects. Most signals of the IR are transmitted through complexes assembled around IR substrate (IRS)-1/2, which is composed of multiple interaction domains and phosphorylation motifs [18], [19]. Because IRS1 is centrally located within the insulin signalling pathway, defects in IRS1 function significantly impair downstream responses of the IR [20], [21]. In particular, IRS1 Ser phosphorylation leads to decreased Tyr phosphorylation [22], and increased proteasome-mediated degradation [23]. Phosphorylation of human IRS1 (Ser312; corresponding to Ser307 in the rodent form) is a representative molecular marker of insulin resistance [24], [25].

Potassium oxonate was previously found to reduce the degradation of uric acid, thereby increasing uric acid level in rodents [26], [27]. Therefore, in this study, we used an acute hyperuricemia mouse model to dissect the effect of HUA level on insulin resistance. We investigated the effect of uric acid on glucose tolerance, insulin resistance and insulin signalling, as manifested by changes in the phosphorylation of IRS1 and Akt activity in mouse liver, muscle, and adipose tissues and human HepG2 cells. In view of the potential important role of ROS in insulin resistance, we examined whether antioxidants could prevent insulin resistance induced by HUA.