Acute organ injury, such as acute kidney injury (AKI) and disease (AKD), are major causes of morbidity and mortality worldwide. Hyperuricemia (HU) is common in patients with impaired kidney function but the impact of asymptomatic HU on the different phases of AKI/AKD is incompletely understood. We hypothesized that asymptomatic HU would attenuate AKD because soluble, in contrast to crystalline, uric acid (sUA) can attenuate sterile inflammation. In vitro, 10 mg/dL sUA decreased reactive oxygen species and interleukin-6 production in macrophages, while enhancing fatty acid oxidation as compared with a physiological concentration of 5 mg/dL sUA or medium. In transgenic mice, asymptomatic HU of 7–10 mg/dL did not affect post-ischemic AKI/AKD but accelerated the recovery of kidney excretory function on day 14. Improved functional outcome was associated with better tubular integrity, less peritubular inflammation, and interstitial fibrosis. Mechanistic studies suggested that HU shifted macrophage polarization towards an anti-inflammatory M2-like phenotype characterized by expression of anti-oxidative and metabolic genes as compared with post-ischemic AKI-chronic kidney disease transition in mice without HU. Our data imply that asymptomatic HU acts as anti-oxidant on macrophages and tubular epithelial cells, which endorses the recovery of kidney function and structure upon AKI.

Asymptomatic hyperuricemia (HU) is considered a pathogenic factor in multiple disease contexts but a causative role is only proven for the crystalline form of uric acid in gouty arthritis and urate nephropathy. Epidemiological studies document a robust association of HU with hypertension, cardiovascular disease, and CKD progression, but CKD-related impaired uric acid clearance and the use of diuretics which further impair uric acid clearance likely accounts for these associations. Interpreting the available trial evidence is further complicated by referring to xanthine oxidase inhibitors as urate-lowering treatment, although these drugs inhibit other substrates, so that attributing their effects only to HU is problematic. In this review, we provide new mechanistic insights on the biological effects of soluble and crystalline UA, and discuss clinical evidence on the role of asymptomatic HU in CKD, cardiovascular disease, and sterile inflammation. We identify research areas with gaps in experimental and clinical evidence, specifically on infectious complications that represent the second common cause of death in CKD patients, referred to as secondary immunodeficiency related to kidney disease (SIDKD). In addition, we address potential therapeutic approaches on how and when to treat asymptomatic HU in patients with kidney disease and where further interventional studies are required.

Background and Aims Currently, it is unknown whether CKD patients with hyperuricemia (HU)-related uric acid (UA) crystalluria, also known as chronic UA nephropathy, would benefit from targeting HU, UA crystallization, and/or inflammation to slow down the progressive decline of kidney function. Although large multi-center RCTs with xanthine oxidase inhibitors (XOIs) such as allopurinol and two Mendelian randomization studies have disproven a causal link between HU and CKD progression, one cannot rule out that XOIs may still represent a viable therapeutic approach for patients with HU-related UA crystalluria in order to prevent CKD progression. This issue requires clarification. To address this, we used our well-established CKD mouse model of HU with UA crystalluria and tested numerous therapeutic approaches including XOI and anakinra. Method Alb-creERT2;Glut9lox/lox (male and female) mice were injected with tamoxifen and placed on an acidogenic diet with the purine inosine to induce HU and crystalluria-related CKD. After chronic UA nephropathy was established on day 14, mice were treated either 1) with the XOIs allopurinol or febuxostat, 2) with sodium bicarbonate supplementation to neutralize urinary pH to prevent UA crystallization, or 3) with anakinra to inhibit IL-1β-driven inflammation, or 4) a combination of the aforementioned therapies for 2 weeks. Saline was used as control. On day 0, 14, and 28, GFR (as primary endpoint) as well as serum UA levels and urinary UA crystals (as secondary endpoints) were determined. At the end of the study, we quantified kidney injury, immune cell infiltration, inflammation, and interstitial fibrosis using histological analysis, ELISA, RT-qPCR, and flow cytometry. Results Therapy with XOIs but not with sodium bicarbonate and anakinra reduced serum UA levels in mice with HU and UA crystalluria. Treating mice with febuxostat improved kidney function (increase in GFR) by preventing kidney injury, UA crystalluria, UA crystal granuloma formation, and interstitial fibrosis compared with the control group, while allopurinol worsened the outcomes of chronic UA nephropathy. On the other hand, sodium bicarbonate supplementation had no effects on serum UA levels, kidney inflammation, immune cell infiltration and interstitial fibrosis but improved kidney function due to less urinary UA crystal deposition (neutralization of urine pH) and granuloma formation as compared with saline-treated mice. As expected, anakinra did not slow down the progression of chronic UA nephropathy as noticed by a similar GFR decline compared with the control group. Although, anakinra reduced the number of infiltrating immune cells and the mRNA expression of inflammatory mediators in the kidney as well as the serum IL-1β concentrations, we observed more interstitial fibrosis. In combination, allopurinol with anakinra did not improve the outcomes of chronic UA nephropathy despite the inhibitory effect of anakinra on the inflammatory response because allopurinol triggered tubulointerstitial nephritis and vasodilation. Interestingly, both combination therapies febuxostat with anakinra and febuxostat with sodium bicarbonate were protective by improving kidney function and preventing UA crystal deposition, kidney injury, inflammation, interstitial fibrosis and UA crystal granuloma formation compared with the single therapies. Conclusion Our interventional study is the first pre-clinical study, which reveals that the XOIs allopurinol and febuxostat have differential effects on the outcomes of HU-related UA crystalluria. While allopurinol contributes to the progression of CKD by causing tubulointerstitial nephritis in mice, febuxostat rather seems to be renoprotective similar to urinary pH neutralization with sodium bicarbonate. We identified both combination therapies febuxostat with anakinra or sodium bicarbonate as most beneficial in order to prevent CKD progression in mice. Thus, febuxostat in combination with IL-1β inhibition or prevention of urinary UA crystallization may represent therapeutic approaches for patients with chronic UA nephropathy.