Hepatocyte-Specific Ablation or Whole-Body Inhibition of Xanthine Oxidoreductase in Mice Corrects Obesity-Induced Systemic Hyperuricemia Without Improving Metabolic Abnormalities

Uric Acid Inhibits Mice Pancreatic Steatosis via the Glycerophospholipid Pathway

Background: despite evidence for mutually reinforcing effects of serum uric acid (SUA) and lipids, the effects of uric levels on pancreatic steatosis are not well-established. In this study, the relationship between low concentrations of uric acid and pancreatic steatosis was evaluated. Methods: forty C57BL/6J mice were fed a diet of high uric acid (HU), high fat (HF), high uric acid and high fat (HUHF), and normal control (NC) (10 mice in each group). Weight was measured weekly. Ultrasonography was performed to observe the pancreatic echo intensity of all mice before the end of feeding. Subsequently, peripheral blood was taken for biochemical examination. Intact pancreatic tissues were taken, part of which was used for pathological examination, part of which was used for PCR experiments and Western Blot experiments to obtain glycerophospholipid-associated mRNA data and protein levels. Results: body weight was significantly higher in the HF group than in the other three groups. Higher uric acid matched lower total cholesterol and triglyceride, matched higher low-density lipoprotein, and matched equal high-density lipoprotein. Ultrasound images and HE staining of pancreatic tissues of mice showed that higher uric acid matched lower fat content. The mRNA levels of phospholipase A2 group IB were highest in high uric acid group, while relative protein expression levels were lowest in high uric acid and control groups. Phospholipase A2 group IIA showed the opposite patterns. Conclusions: elevated serum uric acid at low concentrations can inhibit pancreatic steatosis, which is modulated via the glycerophospholipid metabolic pathway.

Gender Influence on XOR Activities and Related Pathologies: A Narrative Review

Taking into account the patient's gender is the first step towards more precise and egalitarian medicine. The gender-related divergences observed in purine catabolism and their pathological consequences are good examples of gender medicine differences. Uric acid is produced by the activity of xanthine oxidoreductase (XOR). The serum levels of both XOR activity and uric acid differ physiologically between the genders, being higher in men than in women. Their higher levels have been associated with gout and hypertension, as well as with vascular, cardiac, renal, and metabolic diseases. The present review analyzes the gender-related differences in these pathological conditions in relation to increases in the serum levels of XOR and/or uric acid and the opportunity for gender-driven pharmacological treatment.

Hemin and iron increase synthesis and trigger export of xanthine oxidoreductase from hepatocytes to the circulation

We recently reported a previously unknown salutary role for xanthine oxidoreductase (XOR) in intravascular heme overload whereby hepatocellular export of XOR to the circulation was identified as a seminal step in affording protection. However, the cellular signaling and export mechanisms underpinning this process were not identified. Here, we present novel data showing hepatocytes upregulate XOR expression/protein abundance and actively release it to the extracellular compartment following exposure to hemopexin-bound hemin, hemin or free iron. For example, murine (AML-12 cells) hepatocytes treated with hemin (10 μM) exported XOR to the medium in the absence of cell death or loss of membrane integrity (2.0 ± 1.0 vs 16 ± 9 μU/mL p < 0.0001). The path of exocytosis was found to be noncanonical as pretreatment of the hepatocytes with Vaculin-1, a lysosomal trafficking inhibitor, and not Brefeldin A inhibited XOR release and promoted intracellular XOR accumulation (84 ± 17 vs 24 ± 8 hemin vs 5 ± 3 control μU/mg). Interestingly, free iron (Fe2+ and Fe3+) induced similar upregulation and release of XOR compared to hemin. Conversely, concomitant treatment with hemin and the classic transition metal chelator DTPA (20 μM) or uric acid completely blocked XOR release (p < 0.01). Our previously published time course showed XOR release from hepatocytes likely required transcriptional upregulation. As such, we determined that both Sp1 and NF-kB were acutely activated by hemin treatment (∼2-fold > controls for both, p < 0.05) and that silencing either or TLR4 with siRNA prevented hemin-induced XOR upregulation (p < 0.01). Finally, to confirm direct action of these transcription factors on the Xdh gene, chromatin immunoprecipitation was performed indicating that hemin significantly enriched (∼5-fold) both Sp1 and NF-kB near the transcription start site. In summary, our study identified a previously unknown pathway by which XOR is upregulated via SP1/NF-kB and subsequently exported to the extracellular environment. This is, to our knowledge, the very first study to demonstrate mechanistically that XOR can be specifically targeted for export as the seminal step in a compensatory response to heme/Fe overload.

Association of serum uric acid with hepatic steatosis detected by controlled attenuation parameter in the United States population

BACKGROUND

The relationship between serum uric acid (SUA) and nonalcoholic fatty liver disease (NAFLD) has been previously reported. Controlled attenuation parameter (CAP) has better diagnostic performance than ultrasonography for assessing hepatic steatosis. The association of SUA with hepatic steatosis detected by CAP is worth further study.

METHODS

The US population aged 20 years or older from the National Health and Nutrition Examination Survey (NHANES) was assessed. Hepatic steatosis was evaluated by the controlled attenuation parameter (CAP). NAFLD status was defined as CAP values of 268 dB/m without hepatitis B or C virus infection or considerable alcohol consumption. Multiple imputations were performed to fill in the missing covariate values. Linear regression, logistic regression, and smooth curve fitting were used to examine the association.

RESULTS

In total, 3919 individuals participated in this study. There was a positive association between SUA (µmol/L) and CAP (β = 0.14, 95% CI: 0.12-0.17, P < 0.01). After stratification by sex, a significant relationship between SUA and CAP existed in both males (β = 0.12, 95% CI: 0.09-0.16, P < 0.01) and females (β = 0.17, 95% CI: 0.14-0.20, P < 0.01) after multiple imputation. The inflection points of the threshold effect of SUA on CAP were 487.7 µmol/L in males and 386.6 µmol/L in females. There was a positive association between SUA (mg/dL) and NAFLD (OR = 1.30, 95% CI: 1.23-1.37, P < 0.01). After stratification by race, positive relationships were also observed. Meanwhile, a positive relationship existed between hyperuricemia and NAFLD (OR = 1.94, 95% CI: 1.64-2.30, P < 0.01). The positive relationship was more significant in females than in males (P for interaction < 0.01).

CONCLUSIONS

There was a positive association between SUA and CAP, as well as between SUA and NAFLD. Subgroup studies stratified by sex and ethnicity demonstrated that the effects were consistent.

Release of hepatic xanthine oxidase (XO) to the circulation is protective in intravascular hemolytic crisis

Xanthine oxidase (XO) catalyzes the catabolism of hypoxanthine to xanthine and xanthine to uric acid, generating oxidants as a byproduct. Importantly, XO activity is elevated in numerous hemolytic conditions including sickle cell disease (SCD); however, the role of XO in this context has not been elucidated. Whereas long-standing dogma suggests elevated levels of XO in the vascular compartment contribute to vascular pathology via increased oxidant production, herein, we demonstrate, for the first time, that XO has an unexpected protective role during hemolysis. Using an established hemolysis model, we found that intravascular hemin challenge (40 μmol/kg) resulted in a significant increase in hemolysis and an immense (20-fold) elevation in plasma XO activity in Townes sickle cell phenotype (SS) sickle mice compared to controls. Repeating the hemin challenge model in hepatocyte-specific XO knockout mice transplanted with SS bone marrow confirmed the liver as the source of enhanced circulating XO as these mice demonstrated 100% lethality compared to 40% survival in controls. In addition, studies in murine hepatocytes (AML12) revealed hemin mediates upregulation and release of XO to the medium in a toll like receptor 4 (TLR4)-dependent manner. Furthermore, we demonstrate that XO degrades oxyhemoglobin and releases free hemin and iron in a hydrogen peroxide-dependent manner. Additional biochemical studies revealed purified XO binds free hemin to diminish the potential for deleterious hemin-related redox reactions as well as prevents platelet aggregation. In the aggregate, data herein reveals that intravascular hemin challenge induces XO release by hepatocytes through hemin-TLR4 signaling, resulting in an immense elevation of circulating XO. This increased XO activity in the vascular compartment mediates protection from intravascular hemin crisis by binding and potentially degrading hemin at the apical surface of the endothelium where XO is known to be bound and sequestered by endothelial glycosaminoglycans (GAGs).

Xanthine oxidase mediates chronic stress-induced cerebrovascular dysfunction and cognitive impairment

Xanthine oxidase (XO) mediates vascular function. Chronic stress impairs cerebrovascular function and increases the risk of stroke and cognitive decline. Our study determined the role of XO on stress-induced cerebrovascular dysfunction and cognitive decline. We measured middle cerebral artery (MCA) function, free radical formation, and working memory in 6-month-old C57BL/6 mice who underwent 8 weeks of control conditions or unpredictable chronic mild stress (UCMS) with or without febuxostat (50 mg/L), a XO inhibitor. UCMS mice had an impaired MCA dilation to acetylcholine vs. controls (p < 0.0001), and increased total free radical formation, XOR protein levels, and hydrogen peroxide production in the liver compared to controls. UCMS increased hydrogen peroxide production in the brain and cerebrovasculature compared to controls. Working memory, using the y-maze test, was impaired (p < 0.05) in UCMS mice compared to control mice. However, blocking XO using febuxostat prevented the UCMS-induced impaired MCA response, while free radical production and hydrogen peroxide levels were similar to controls in the liver and brain of UCMS mice treated with febuxostat. Further, UCMS + Feb mice did not have a significant reduction in working memory. These data suggest that the cerebrovascular dysfunction associated with chronic stress may be driven by XO, which leads to a reduction in working memory.

Obese female mice do not exhibit overt hyperuricemia despite hepatic steatosis and impaired glucose tolerance

Recent reports have clearly demonstrated a tight correlation between obesity and elevated circulating uric acid levels (hyperuricemia). However, nearly all preclinical work in this area has been completed with male mice, leaving the field with a considerable gap in knowledge regarding female responses to obesity and hyperuricemia. This deficiency in sex as a biological variable extends beyond unknowns regarding uric acid (UA) to several important comorbidities associated with obesity including nonalcoholic fatty liver disease (NAFLD). To attempt to address this issue, herein we describe both phenotypic and metabolic responses to diet-induced obesity (DIO) in female mice. Six-week-old female C57BL/6J mice were fed a high-fat diet (60% calories derived from fat) for 32 weeks. The DIO female mice had significant weight gain over the course of the study, higher fasting blood glucose, impaired glucose tolerance, and elevated plasma insulin levels compared to age-matched on normal chow. While these classic indices of DIO and NAFLD were observed such as increased circulating levels of ALT and AST, there was no difference in circulating UA levels. Obese female mice also demonstrated increased hepatic triglyceride (TG), cholesterol, and cholesteryl ester. In addition, several markers of hepatic inflammation were significantly increased. Also, alterations in the expression of redox-related enzymes were observed in obese mice compared to lean controls including increases in extracellular superoxide dismutase (Sod3), heme oxygenase (Ho)-1, and xanthine dehydrogenase (Xdh). Interestingly, hepatic UA levels were significantly elevated (~2-fold) in obese mice compared to their lean counterparts. These data demonstrate female mice assume a similar metabolic profile to that reported in several male models of obesity in the context of alterations in glucose tolerance, hepatic steatosis, and elevated transaminases (ALT and AST) in the absence of hyperuricemia affirming the need for further study.

Redox imbalance in COVID-19 pathophysiology

Background

The pathophysiologic significance of redox imbalance is unquestionable as numerous reports and topic reviews indicate alterations in redox parameters during corona virus disease 2019 (COVID-19). However, a more comprehensive understanding of redox-related parameters in the context of COVID-19-mediated inflammation and pathophysiology is required.

Methods

COVID-19 subjects (n = 64) and control subjects (n = 19) were enrolled, and blood was drawn within 72 h of diagnosis. Serum multiplex assays and peripheral blood mRNA sequencing was performed. Oxidant/free radical (electron paramagnetic resonance (EPR) spectroscopy, nitrite-nitrate assay) and antioxidant (ferrous reducing ability of serum assay and high-performance liquid chromatography) were performed. Multivariate analyses were performed to evaluate potential of indicated parameters to predict clinical outcome.

Results

Significantly greater levels of multiple inflammatory and vascular markers were quantified in the subjects admitted to the ICU compared to non-ICU subjects. Gene set enrichment analyses indicated significant enhancement of oxidant related pathways and biochemical assays confirmed a significant increase in free radical production and uric acid reduction in COVID-19 subjects. Multivariate analyses confirmed a positive association between serum levels of VCAM-1, ICAM-1 and a negative association between the abundance of one electron oxidants (detected by ascorbate radical formation) and mortality in COVID subjects while IL-17c and TSLP levels predicted need for intensive care in COVID-19 subjects.

Conclusion

Herein we demonstrate a significant redox imbalance during COVID-19 infection affirming the potential for manipulation of oxidative stress pathways as a new therapeutic strategy COVID-19. However, further work is requisite for detailed identification of oxidants (O₂^•-, H₂O₂ and/or circulating transition metals such as Fe or Cu) contributing to this imbalance to avoid the repetition of failures using non-specific antioxidant supplementation.

Xanthine oxidoreductase activity is correlated with hepatic steatosis

The enzyme xanthine oxidoreductase (XOR) catalyzes the synthesis of uric acid (UA) from hypoxanthine and xanthine, which are products of purine metabolism starting from ribose-5-phosphate. Several studies suggested a relationship between hyperuricemia and hepatic steatosis; however, few previous studies have directly examined the relationship between XOR activity and hepatic steatosis. A total of 223 subjects with one or more cardiovascular risk factors were enrolled. The liver-to-spleen (L/S) ratio on computed tomography and the hepatic steatosis index (HSI) were used to assess hepatic steatosis. We used a newly developed highly sensitive assay based on [¹³C₂, ¹⁵N₂] xanthine and liquid chromatography/triple quadrupole mass spectrometry to measure plasma XOR activity. Subjects with the L/S ratio of < 1.1 and the HSI of < 36 had increased XOR activity and serum UA levels. Independent of insulin resistance and serum UA levels, multivariate logistic regression analysis revealed that plasma XOR activity was associated with the risk of hepatic steatosis as assessed by the L/S ratio and HSI. According to the findings of this study, plasma XOR activity is associated with hepatic steatosis independent of insulin resistance and serum UA levels.

Racial Differences in XO (Xanthine Oxidase) and Mitochondrial DNA Damage-Associated Molecular Patterns in Resistant Hypertension

BACKGROUND

We previously reported increased plasma XO (xanthine oxidase) activity in patients with resistant hypertension. Increased XO can cause mitochondrial DNA damage and promote release of fragments called mitochondrial DNA damage-associated molecular patterns (mtDNA DAMPs). Here, we report racial differences in XO activity and mtDNA DAMPs in Black and White adults with resistant hypertension.

METHODS

This retrospective study includes 91 resistant hypertension patients (44% Black, 47% female) with blood pressure >140/90 mm Hg on ≥4 medications and 37 normotensive controls (30% Black, 54% female) with plasma XO activity, mtDNA DAMPs, and magnetic resonance imaging of left ventricular morphology and function.

RESULTS

Black-resistant hypertension patients were younger (mean age 52±10 versus 59±10 years; P=0.001), with higher XO activity and left ventricular wall thickness, and worse diastolic dysfunction than White resistant hypertension patients. Urinary sodium excretion (mg/24 hour per kg) was positively related to left ventricular end-diastolic volume (r=0.527, P=0.001) and left ventricular mass (r=0.394, P=0.02) among Black but not White resistant hypertension patients. Patients with resistant hypertension had increased mtDNA DAMPs versus controls (P<0.001), with Black mtDNA DAMPS greater than Whites (P<0.001). Transmission electron microscopy of skeletal muscle biopsies in resistant hypertension patients demonstrates mitochondria cristae lysis, myofibrillar loss, large lipid droplets, and glycogen accumulation.

CONCLUSIONS

These data warrant a large study to examine the role of XO and mitochondrial mtDNA DAMPs in cardiac remodeling and heart failure in Black adults with resistant hypertension.

Association between circulating cystatin C and hyperuricemia: a cross-sectional study

INTRODUCTION/OBJECTIVES

Circulating cystatin C has reportedly been related to cardiovascular disease, diabetes, and metabolic syndrome, apart from its traditional role in estimating the glomerular filtration rate. However, whether circulating cystatin C is related to hyperuricemia remains unclear.

METHOD

We included 2406 men and 1273 women who attended their annual health checkups in this study. Anthropometric and biochemical parameters were measured. Hyperuricemia was diagnosed as fasting serum uric acid > 420 µmol/L in men and women.

RESULTS

A total of 695 (18.9%) participants were diagnosed with hyperuricemia. Hyperuricemic patients had significantly higher serum cystatin C levels than healthy controls (0.91 (0.83-1.02) versus 0.82 (0.72-0.92) mg/L, P < 0.001). Serum cystatin C levels were positively related to the prevalence of hyperuricemia, which was 5.18%, 14.76%, 22.66%, and 31.24% in participants with serum cystatin C levels in the first, second, third, and fourth quartiles, respectively (P < 0.001 for trend). In stepwise multivariate logistic regression analysis, participants with serum cystatin C in the fourth quartile had a more than twofold increased risk of hyperuricemia (OR 2.262, 95% CI 1.495-3.422; P < 0.001) compared with those with serum cystatin C in the first quartile. In subgroup analyses, the fourth quartile of cystatin C was related to increased risks of hyperuricemia in both non-obese and obese participants (OR 4.405, 95% CI 1.472-13.184, P = 0.008; OR 1.891, 95% CI 1.228-2.911, P = 0.004, respectively), in non-metabolic syndrome participants (OR 3.043, 95% CI 1.692-5.473; P < 0.001) but not in metabolic syndrome participants (OR 1.689, 95% CI 0.937-3.045; P = 0.081), and in non-non-alcoholic fatty liver disease (non-NAFLD) (OR 2.128, 95% CI 1.424-3.180; P < 0.001, respectively) and young and middle-aged participants (OR 2.235, 95% CI 1.492-3.348, P < 0.001) but not in NAFLD and elderly participants.

CONCLUSIONS

This study revealed a positive association of circulating cystatin C with hyperuricemia. Key Points • Serum cystatin C is associated with an increased risk of hyperuricemia. • Serum cystatin C is a useful biomarker in distinguishing patients at high risk of having hyperuricemia.

URAT1-selective inhibition ameliorates insulin resistance by attenuating diet-induced hepatic steatosis and brown adipose tissue whitening in mice

Objective

Accumulating evidence indicates that high uric acid (UA) is strongly associated with obesity and metabolic syndrome and drives the development of nonalcoholic fatty liver disease (NAFLD) and insulin resistance. Although urate transporter-1 (URAT1), which is primarily expressed in the kidneys, plays a critical role in the development of hyperuricemia, its pathophysiological implication in NAFLD and insulin resistance remains unclear. We herein investigated the role and functional significance of URAT1 in diet-induced obese mice.

Methods

Mice fed a high-fat diet (HFD) for 16–18 weeks or a normal-fat diet (NFD) were treated with or without a novel oral URAT1-selective inhibitor (dotinurad [50 mg/kg/day]) for another 4 weeks.

Results

We found that URAT1 was also expressed in the liver and brown adipose tissue (BAT) other than the kidneys. Dotinurad administration significantly ameliorated HFD-induced obesity and insulin resistance. HFD markedly induced NAFLD, which was characterized by severe hepatic steatosis as well as the elevation of serum ALT activity and tissue inflammatory cytokine genes (chemokine ligand 2 (Ccl2) and tissue necrosis factor α (TNFα)), all of which were attenuated by dotinurad. Similarly, HFD significantly increased URAT1 expression in BAT, resulting in lipid accumulation (whitening of BAT), and increased the production of tissue reactive oxygen species (ROS), which were reduced by dotinurad via UCP1 activation.

Conclusions

In conclusion, a novel URAT1-selective inhibitor, dotinurad, ameliorates insulin resistance by attenuating hepatic steatosis and promoting rebrowning of lipid-rich BAT in HFD-induced obese mice. URAT1 serves as a key regulator of the pathophysiology of metabolic syndrome and may be a new therapeutic target for insulin-resistant individuals, particularly those with concomitant NAFLD.

•

URAT1 is expressed in the liver and brown adipose tissue other than in the kidneys.

•

URAT1-selective inhibitor ameliorates HFD-induced insulin resistance.

•

URAT1-selective inhibitor improves NAFLD through the inhibition of Ccl2 and TNFα.

•

URAT1-selective inhibitor promotes rebrowning of HFD-induced lipid-rich BAT.

•

URAT1 serves as a key regulator of the pathophysiology of metabolic syndrome.

Xanthine oxidoreductase: A leading actor in cardiovascular disease drama

Cardiovascular diseases (CVD) are the leading cause of global mortality and their pathogenesis lies mainly in the atherosclerotic process. There are close connections linking oxidative stress and inflammation to endothelial dysfunction, atherosclerosis and, consequently, to CVD. This review focuses on the role of xanthine oxidoreductase (XOR) and its products on the development of chronic inflammation and oxidative stress, responsible for atheromatous plaque formation. Evidence is reported that an excessive level of XOR products favors inflammatory response and plaque development, thereby promoting major cardiovascular risk factors. Also, the relationship between hyperuricemia and hypertension as well as between XOR activity and CVD is confirmed. In spite of the increasing number of clinical studies investigating the output of cardiovascular patients treated with urate-lowering therapies (including uricosuric drugs, XOR inhibitors and recombinant uricase) the results are still uncertain. The inhibition of XOR activity appears more promising than just the control of uricemia level in preventing cardiovascular events, possibly because it also reduces the intracellular accumulation of urate, as well as the production of reactive oxygen species. However, XOR inhibition also reduces the availability of the multifaced mediator nitric oxide and, at present, can be recommended only in hyperuricemic patients.

Novel role of xanthine oxidase-dependent H2O2 production in 12/15-lipoxygenase-mediated de novo lipogenesis, triglyceride biosynthesis and weight gain

12/15-lipoxygenase (12/15-LOX) plays an essential role in oxidative conversion of polyunsaturated fatty acids into various bioactive lipid molecules. Although 12/15-LOX's role in the pathophysiology of various human diseases has been well studied, its role in weight gain is controversial and poorly clarified. Here, we demonstrated the role of 12/15-LOX in high-fat diet (HFD)-induced weight gain in a mouse model. We found that 12/15-LOX mediates HFD-induced de novo lipogenesis (DNL), triglyceride (TG) biosynthesis and the transport of TGs from the liver to adipose tissue leading to white adipose tissue (WAT) expansion and weight gain via xanthine oxidase (XO)-dependent production of H₂O₂. 12/15-LOX deficiency leads to cullin2-mediated ubiquitination and degradation of XO, thereby suppressing H₂O₂ production, DNL and TG biosynthesis resulting in reduced WAT expansion and weight gain. These findings infer that manipulation of 12/15-LOX metabolism may manifest a potential therapeutic target for weight gain and obesity.

•

12/15-LOX-12(S)-HETE axis via activation of CREB-Egr1 enhances TG biosynthesis.

•

12/15-LOX-12(S)-HETE axis via activation of SREBP1c triggers DNL.

•

H₂O₂ mediates 12/15-LOX-12(S)-HETE axis-induced DNL and TG biosynthesis.

•

12/15-LOX via TG biosynthesis leads to WAT expansion and body weight gain.

•

Downstream to 12/15-LOX, H₂O₂-mediates WAT expansion and body weight gain.

Human and rodent red blood cells do not demonstrate xanthine oxidase activity or XO-catalyzed nitrite reduction to NO

A number of molybdopterin enzymes, including xanthine oxidoreductase (XOR), aldehyde oxidase (AO), sulfite oxidase (SO), and mitochondrial amidoxime reducing component (mARC), have been identified as nitrate and nitrite reductases. Of these enzymes, XOR has been the most extensively studied and reported to be a substantive source of nitric oxide (NO) under inflammatory/hypoxic conditions that limit the catalytic activity of the canonical NOS pathway. It has also been postulated that XOR nitrite reductase activity extends to red blood cell (RBCs) membranes where it has been immunohistochemically identified. These findings, when combined with countervailing reports of XOR activity in RBCs, incentivized our current study to critically evaluate XOR protein abundance/enzymatic activity in/on RBCs from human, mouse, and rat sources. Using various protein concentrations of RBC homogenates for both human and rodent samples, neither XOR protein nor enzymatic activity (xanthine → uric acid) was detectable. In addition, potential loading of RBC-associated glycosaminoglycans (GAGs) by exposing RBC preparations to purified XO before washing did not solicit detectable enzymatic activity (xanthine → uric acid) or alter NO generation profiles. To ensure these observations extended to absence of XOR-mediated contributions to overall RBC-associated nitrite reduction, we examined the nitrite reductase activity of washed and lysed RBC preparations via enhanced chemiluminescence in the presence or absence of the XOR-specific inhibitor febuxostat (Uloric®). Neither addition of inhibitor nor the presence of the XOR substrate xanthine significantly altered the rates of nitrite reduction to NO by RBC preparations from either human or rodent sources confirming the absence of XO enzymatic activity. Furthermore, examination of the influence of the age (young cells vs. old cells) of human RBCs on XO activity also failed to demonstrate detectable XO protein. Combined, these data suggest: 1) that XO does not contribute to nitrite reduction in/on human and rodent erythrocytes, 2) care should be taken to validate immuno-detectable XO by demonstrating enzymatic activity, and 3) XO does not associate with human erythrocytic glycosaminoglycans or participate in nonspecific binding.

Xanthine dehydrogenase as a prognostic biomarker related to tumor immunology in hepatocellular carcinoma

Background

Xanthine dehydrogenase (XDH) is a critical enzyme involved in the oxidative metabolism of purines, pterin and aldehydes and a central component of the innate immune system. However, the prognostic value of XDH in predicting tumor-infiltrating lymphocyte abundance, the immune response, and survival in different cancers, including hepatocellular carcinoma (HCC), is still unclear.

Methods

XDH expression was analyzed in multiple databases, including Oncomine, the Tumor Immune Estimation Resource (TIMER), the Kaplan–Meier plotter database, the Gene Expression Profiling Interactive Analysis (GEPIA) database, and The Cancer Genome Atlas (TCGA). XDH-associated transcriptional profiles were detected with an mRNA array, and the levels of infiltrating immune cells were validated by immunohistochemistry (IHC) of HCC tissues. A predictive signature containing multiple XDH-associated immune genes was established using the Cox regression model.

Results

Decreased XDH mRNA expression was detected in human cancers originating from the liver, bladder, breast, colon, bile duct, kidney, and hematolymphoid system. The prognostic potential of XDH mRNA expression was also significant in certain other cancers, including HCC, breast cancer, kidney or bladder carcinoma, gastric cancer, mesothelioma, lung cancer, and ovarian cancer. In HCC, a low XDH mRNA level predicted poorer overall survival, disease-specific survival, disease-free survival, and progression-free survival. The prognostic value of XDH was independent of the clinical features of HCC patients. Indeed, XDH expression in HCC activated several immune-related pathways, including the T cell receptor, PI3K-AKT, and MAPK signaling pathways, which induced a cytotoxic immune response. Importantly, the microenvironment of XDH^high HCC tumors contained abundant infiltrating CD8 + T cells but not exhausted T cells. A risk prediction signature based on multiple XDH-associated immune genes was revealed as an independent predictor in the TCGA liver cancer cohort.

Conclusion

These findings suggest that XDH is a valuable prognostic biomarker in HCC and other cancers and indicate that it may function in tumor immunology. Loss of XDH expression may be an immune evasion mechanism for HCC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-02173-7.

Metabolism and Hepatotoxicity of Pyrazinamide, an Antituberculosis Drug

Pyrazinamide (PZA) is an important component of a standard combination therapy against tuberculosis. However, PZA is hepatotoxic, and the underlying mechanisms are poorly understood. Biotransformation of PZA in the liver was primarily suggested behind its hepatoxicity. This review summarizes the knowledge of the key enzymes involved in PZA metabolism and discusses their contributions to PZA hepatotoxicity. SIGNIFICANCE STATEMENT: This review outlines the current understanding of PZA metabolism and hepatotoxicity. This work also highlights the gaps in this field, which can be used to guide the future studies on PZA-induced liver injury.

Purine metabolites and complex diseases: role of genes and nutrients

PURPOSE OF REVIEW

Purines have several important physiological functions as part of nucleic acids and as intracellular and extracellular signaling molecules. Purine metabolites, particularly uric acid, have been implicated in congenital and complex diseases. However, their role in complex diseases is not clear and they have both beneficial and detrimental effects on disease pathogenesis. In addition, the relationship between purines and complex diseases is affected by genetic and nutritional factors. This review presents latest findings about the relationship between purines and complex diseases and the effect of genes and nutrients on this relationship.

RECENT FINDINGS

Evidence from recent studies show strong role of purines in complex diseases. Although they are causal in only few diseases, our knowledge about their role in other diseases is still evolving. Of all the purines, uric acid is the most studied. Uric acid acts as an antioxidant as well as a prooxidant under different conditions, thus, its role in disease also varies. Other purines, adenosine and inosine have been less studied, but they have neuroprotective properties which are valuable in neurodegenerative diseases.

SUMMARY

Purines are molecules with great potential in disease pathogenesis as either metabolic markers or therapeutic targets. More studies need to be conducted to understand their relevance for complex diseases.

Influence of xanthine oxidoreductase inhibitor, topiroxostat, on body weight of diabetic obese mice

Plasma xanthine oxidoreductase (XOR) activity is high in metabolic disorders such as diabetic mellitus, obesity, or overweight. Thus, this study investigated whether the XOR inhibitor, topiroxostat, affected body weight. Male db/db mice were fed standard diets with or without topiroxostat for 4 weeks. Body weight and food intake were constantly monitored, along with monitoring plasma biochemical markers, including insulin and XOR activity. Additionally, hepatic hypoxanthine and XOR activity were also documented. Single regression analysis was performed to determine the mechanism. Topiroxostat treatment suppressed weight gain relative to the vehicle without any impact on food intake. However, the weight of fat pads and hepatic and muscle triglyceride content did not change. Topiroxostat decreased the plasma uric acid and increased hepatic hypoxanthine in response to the inhibition of XOR activity. Plasma ketone body and free fatty acid were also increased. Moreover, fat weight was weakly associated with plasma XOR activity in the diabetic state and was negatively associated with ketone body by topiroxostat. These results suggested that topiroxostat amplified the burning of lipids and the salvage pathway, resulting in predisposing the body toward catabolism. The inhibition of plasma XOR activity may contribute to weight loss.

The Role of Oxidative Stress in Hyperuricemia and Xanthine Oxidoreductase (XOR) Inhibitors

Uric acid is the end product of purine metabolism in humans. Hyperuricemia is a metabolic disease caused by the increased formation or reduced excretion of serum uric acid (SUA). Alterations in SUA homeostasis have been linked to a number of diseases, and hyperuricemia is the major etiologic factor of gout and has been correlated with metabolic syndrome, cardiovascular disease, diabetes, hypertension, and renal disease. Oxidative stress is usually defined as an imbalance between free radicals and antioxidants in our body and is considered to be one of the main causes of cell damage and the development of disease. Studies have demonstrated that hyperuricemia is closely related to the generation of reactive oxygen species (ROS). In the human body, xanthine oxidoreductase (XOR) catalyzes the oxidative hydroxylation of hypoxanthine to xanthine to uric acid, with the accompanying production of ROS. Therefore, XOR is considered a drug target for the treatment of hyperuricemia and gout. In this review, we discuss the mechanisms of uric acid transport and the development of hyperuricemia, emphasizing the role of oxidative stress in the occurrence and development of hyperuricemia. We also summarize recent advances and new discoveries in XOR inhibitors.

Xanthine Oxidase Drives Hemolysis and Vascular Malfunction in Sickle Cell Disease

OBJECTIVE

Chronic hemolysis is a hallmark of sickle cell disease (SCD) and a driver of vasculopathy; however, the mechanisms contributing to hemolysis remain incompletely understood. Although XO (xanthine oxidase) activity has been shown to be elevated in SCD, its role remains unknown. XO binds endothelium and generates oxidants as a byproduct of hypoxanthine and xanthine catabolism. We hypothesized that XO inhibition decreases oxidant production leading to less hemolysis. Approach and Results: Wild-type mice were bone marrow transplanted with control (AA) or sickle (SS) Townes bone marrow. After 12 weeks, mice were treated with 10 mg/kg per day of febuxostat (Uloric), Food and Drug Administration-approved XO inhibitor, for 10 weeks. Hematologic analysis demonstrated increased hematocrit, cellular hemoglobin, and red blood cells, with no change in reticulocyte percentage. Significant decreases in cell-free hemoglobin and increases in haptoglobin suggest XO inhibition decreased hemolysis. Myographic studies demonstrated improved pulmonary vascular dilation and blunted constriction, indicating improved pulmonary vasoreactivity, whereas pulmonary pressure and cardiac function were unaffected. The role of hepatic XO in SCD was evaluated by bone marrow transplanting hepatocyte-specific XO knockout mice with SS Townes bone marrow. However, hepatocyte-specific XO knockout, which results in >50% diminution in circulating XO, did not affect hemolysis levels or vascular function, suggesting hepatocyte-derived elevation of circulating XO is not the driver of hemolysis in SCD.

CONCLUSIONS

Ten weeks of febuxostat treatment significantly decreased hemolysis and improved pulmonary vasoreactivity in a mouse model of SCD. Although hepatic XO accounts for >50% of circulating XO, it is not the source of XO driving hemolysis in SCD.

Plasma xanthine oxidoreductase activity in Japanese patients with type 2 diabetes across hospitalized treatment

Aims/Introduction

Xanthine oxidoreductase (XOR) is an enzyme that catalyzes hypoxanthine and xanthine to xanthine and uric acid, respectively. Plasma XOR activity has recently been measured in humans. However, limited information is known about plasma XOR activity in patients with type 2 diabetes mellitus, and its changes after short‐term glycemic control treatment.

Materials and Methods

We enrolled 28 Japanese patients (10 men/18 women) with type 2 diabetes mellitus who were hospitalized to undergo medical treatment for diabetes. Plasma XOR activity, quantified using triple quadrupole mass spectrometry and liquid chromatography, and other clinical parameters were examined at admission and 2 weeks after treatment during hospitalization. Changes in plasma XOR activity after treatment during hospitalization and associated clinical parameters were assessed.

Results

At the time of admission, the median plasma XOR activity was 83.1 pmol/h/mL, with a wide range of 14.4–1150 pmol/h/mL. Multiple regression analysis identified serum aspartate transaminase and alanine transaminase levels as significant and independent factors correlating with the baseline plasma XOR. Two weeks of treatment during hospitalization was associated with a significant decrease in plasma XOR activity. Changes in serum aspartate transaminase were also the only significant and independent factor correlating with changes in plasma XOR activity.

Conclusions

A close relationship was observed between plasma XOR activity and liver transaminases in patients with type 2 diabetes mellitus, cross‐sectionally, and also across treatment during hospitalization.

Purine Metabolite Signatures and Type 2 Diabetes: Innocent Bystanders or Actionable Items?

PURPOSE OF REVIEW

This article reviews evidence linking cardiometabolic conditions with changes in purine metabolites, including increased serum uric acid (sUA), and discusses intervention studies that investigated the therapeutic relevance of these associations.

RECENT FINDINGS

Metabolic and epidemiological findings support a correlation between sUA and circulating levels of other purines with insulin resistance (IR) and risk factors for cardiovascular disease (CVD). In addition, increased activity of xanthine oxidoreductase (XOR), the rate-limiting enzyme for UA production, has been detected in tissues targeted by obesity. Yet, inhibition of XOR in pre-clinical and clinical studies generally failed to support a causal role for excess sUA in IR and CVD. The lack of efficacy of XOR inhibitors strongly suggests that UA is a marker of, rather than a direct contributory factor for, cardiometabolic diseases. Validation of the function of other purines will require a paradigm shift, from a "UA-centric" view to a more granular assessment of the entire purine network and its interaction with other pathways.