Possible role of insulin resistance in activation of plasma xanthine oxidoreductase in health check-up examinees

Association of xanthine oxidoreductase inhibitor use with insulin secretory capacity in patients with type 2 diabetes

AIM/INTRODUCTION

Xanthine oxidoreductase (XOR) inhibitor treatment, which reduces reactive oxygen species (ROS) production and increases adenosine triphosphate (ATP) synthesis, has been reported to improve glycemic control. The possible protective effects of XOR inhibitor treatment on insulin secretory capacity were investigated in patients with type 2 diabetes.

MATERIALS AND METHODS

This retrospective cross-sectional study included 428 patients with type 2 diabetes. Insulin secretory capacity was assessed based on fasting serum C-peptide concentration (CPR) and C-peptide index (CPI) in all subjects, while insulin resistance in non-insulin users (n = 312) was determined using the homeostasis model assessment of insulin resistance (HOMA-IR) index.

RESULTS

Median values for CPR and CPI in all subjects were 2.4 ng/mL and 1.5, respectively, while that for HOMA-IR in non-insulin users was 3.2. The XOR inhibitor users (n = 72) had significantly (P < 0.001) higher CPR and CPI levels than non-users (n = 356). Multivariable regression analyses showed XOR inhibitor use was positively associated with CPR (β = 0.153, P = 0.001) and CPI (β = 0.144, P = 0.001). Similar results were observed in propensity score analyses. In subgroup analyses of patients with a preserved estimated glomerular filtration rate (≥60 mL/min/1.73 m2) and non-insulin users, these associations remained significant. Furthermore, the associations were significant in patients with lower (≤6.0 mg/dL) but not with higher (>6.0 mg/dL) uric acid levels (P for interaction <0.05). On the other hand, XOR inhibitor use showed no significant association with HOMA-IR.

CONCLUSIONS

The results of XOR inhibitor treatment, especially a sufficient reduction in serum uric acid level, may provide protective effects on insulin secretory capacity in patients with type 2 diabetes.

Dysuricemia

Gout results from elevated serum urate (SU) levels, or hyperuricemia, and is a globally widespread and increasingly burdensome disease. Recent studies have illuminated the pathophysiology of gout/hyperuricemia and its epidemiology, diagnosis, treatment, and complications. The genetic involvement of urate transporters and enzymes is also proven. URAT1, a molecular therapeutic target for gout/hyperuricemia, was initially derived from research into hereditary renal hypouricemia (RHUC). RHUC is often accompanied by complications such as exercise-induced acute kidney injury, which indicates the key physiological role of uric acid. Several studies have also revealed its physiological role as both an anti-oxidant and a pro-oxidant, acting as both a scavenger and a generator of reactive oxygen species (ROSs). These discoveries have prompted research interest in SU and xanthine oxidoreductase (XOR), an enzyme that produces both urate and ROSs, as status or progression biomarkers of chronic kidney disease and cardiovascular disease. The notion of "the lower, the better" is therefore incorrect; a better understanding of uric acid handling and metabolism/transport comes from an awareness that excessively high and low levels both cause problems. We summarize here the current body of evidence, demonstrate that uric acid is much more than a metabolic waste product, and finally propose the novel disease concept of "dysuricemia" on the path toward "normouricemia", or optimal SU level, to take advantage of the dual roles of uric acid. Our proposal should help to interpret the spectrum from hypouricemia to hyperuricemia/gout as a single disease category.

Association of plasma xanthine oxidoreductase activity with vascular endothelial function independent of serum uric acid level: MedCity21 health examination registry

Background

Xanthine oxidoreductase (XOR) inhibitor administration, known to reduce uric acid and reactive oxygen species (ROS) production, also improves vascular endothelial function (VEF). This cross-sectional study examined our hypothesis that XOR contributes to impaired VEF through ROS but not uric acid production.

Methods

In 395 subjects (196 males, 199 females) without urate-lowering agent administration who underwent a health examination, plasma XOR activity was determined using our highly sensitive assay based on [13C2,15N2] xanthine and liquid chromatography/triple quadrupole mass spectrometry. For VEF evaluation, flow-mediated dilatation (FMD) in the brachial artery was determined by ultrasound, with physical and laboratory measurements also obtained.

Results

The median values for plasma XOR activity, serum uric acid, and FMD were 26.6 pmol/h/mL, 5.4 mg/dL, and 6.2%, respectively. Simple regression analysis showed weak correlations of both log plasma XOR activity and serum uric acid level with FMD (r = -0.213, p < 0.001 and r = -0.139, p = 0.006, respectively). However, multivariable linear regression analyses revealed that log plasma XOR activity but not serum uric acid level remained associated with FMD (β = -0.116, p = 0.037 and β = 0.041, p = 0.549, respectively) after adjustments for various clinical parameters, with no remarkable inconsistencies for the association observed in subgroups divided based on sex or uric acid level. Finally, a series of mediation analyses showed that serum uric acid level did not meet the criteria for mediator of the association of plasma XOR activity with FMD (p = 0.538).

Conclusions

These findings suggest the possibility that XOR contributes to the pathophysiology of impaired VEF through ROS but not uric acid production.