Mizuho Tamura

Uric Acid Secretion from Adipose Tissue and Its Increase in Obesity

Background: Purine metabolism in adipose tissue is largely unknown. Results: Adipose tissue has abundant xanthine oxidoreductase activity. Uric acid is secreted from adipose tissues and cells, and the secretion is augmented in obese mice. Conclusion: Adipose tissue can secrete uric acid in mice. Significance: Dysfunction of obese adipose tissue could be related to overproduction of uric acid. Obesity is often accompanied by hyperuricemia. However, purine metabolism in various tissues, especially regarding uric acid production, has not been fully elucidated. Here we report, using mouse models, that adipose tissue could produce and secrete uric acid through xanthine oxidoreductase (XOR) and that the production was enhanced in obesity. Plasma uric acid was elevated in obese mice and attenuated by administration of the XOR inhibitor febuxostat. Adipose tissue was one of major organs that had abundant expression and activities of XOR, and adipose tissues in obese mice had higher XOR activities than those in control mice. 3T3-L1 and mouse primary mature adipocytes produced and secreted uric acid into culture medium. The secretion was inhibited by febuxostat in a dose-dependent manner or by gene knockdown of XOR. Surgical ischemia in adipose tissue increased local uric acid production and secretion via XOR, with a subsequent increase in circulating uric acid levels. Uric acid secretion from whole adipose tissue was increased in obese mice, and uric acid secretion from 3T3-L1 adipocytes was increased under hypoxia. Our results suggest that purine catabolism in adipose tissue could be enhanced in obesity.

Xanthine Oxidase Inhibition by Febuxostat Attenuates Experimental Atherosclerosis in Mice

Atherosclerosis is a chronic inflammatory disease due to lipid deposition in the arterial wall. Multiple mechanisms participate in the inflammatory process, including oxidative stress. Xanthine oxidase (XO) is a major source of reactive oxygen species (ROS) and has been linked to the pathogenesis of atherosclerosis, but the underlying mechanisms remain unclear. Here, we show enhanced XO expression in macrophages in the atherosclerotic plaque and in aortic endothelial cells in ApoE−/− mice, and that febuxostat, a highly potent XO inhibitor, suppressed plaque formation, reduced arterial ROS levels and improved endothelial dysfunction in ApoE−/− mice without affecting plasma cholesterol levels. In vitro, febuxostat inhibited cholesterol crystal-induced ROS formation and inflammatory cytokine release in murine macrophages. These results demonstrate that in the atherosclerotic plaque, XO-mediated ROS formation is pro-inflammatory and XO-inhibition by febuxostat is a potential therapy for atherosclerosis.

Febuxostat, an Inhibitor of Xanthine Oxidase, Suppresses Lipopolysaccharide-Induced MCP-1 Production via MAPK Phosphatase-1-Mediated Inactivation of JNK

Excess reactive oxygen species (ROS) formation can trigger various pathological conditions such as inflammation, in which xanthine oxidase (XO) is one major enzymatic source of ROS. Although XO has been reported to play essential roles in inflammatory conditions, the molecular mechanisms underlying the involvement of XO in inflammatory pathways remain unclear. Febuxostat, a selective and potent inhibitor of XO, effectively inhibits not only the generation of uric acid but also the formation of ROS. In this study, therefore, we examined the effects of febuxostat on lipopolysaccharide (LPS)-mediated inflammatory responses. Here we show that febuxostat suppresses LPS-induced MCP-1 production and mRNA expression via activating MAPK phosphatase-1 (MKP-1) which, in turn, leads to dephosphorylation and inactivation of JNK in macrophages. Moreover, these effects of febuxostat are mediated by inhibiting XO-mediated intracellular ROS production. Taken together, our data suggest that XO mediates LPS-induced phosphorylation of JNK through ROS production and MKP-1 inactivation, leading to MCP-1 production in macrophages. These studies may bring new insights into the novel role of XO in regulating inflammatory process through MAPK phosphatase, and demonstrate the potential use of XO inhibitor in modulating the inflammatory processes.

Intracellular ATP Decrease Mediates NLRP3 Inflammasome Activation upon Nigericin and Crystal Stimulation

Activation of the nucleotide-binding oligomerization domain–like receptor family, pyrin domain–containing 3 (NLRP3) inflammasome initiates an inflammatory response, which is associated with host defense against pathogens and the progression of chronic inflammatory diseases such as gout and atherosclerosis. The NLRP3 inflammasome mediates caspase-1 activation and subsequent IL-1β processing in response to various stimuli, including extracellular ATP, although the roles of intracellular ATP (iATP) in NLRP3 activation remain unclear. In this study, we found that in activated macrophages artificial reduction of iATP by 2-deoxyglucose, a glycolysis inhibitor, caused mitochondrial membrane depolarization, leading to IL-1β secretion via NLRP3 and caspase-1 activation. Additionally, the NLRP3 activators nigericin and monosodium urate crystals lowered iATP through K+- and Ca2+-mediated mitochondrial dysfunction, suggesting a feedback loop between iATP loss and lowering of mitochondrial membrane potential. These results demonstrate the fundamental roles of iATP in the maintenance of mitochondrial function and regulation of IL-1β secretion, and they suggest that maintenance of the intracellular ATP pools could be a strategy for countering NLRP3-mediated inflammation.

Activity of xanthine oxidase in plasma correlates with indices of insulin resistance and liver dysfunction in patients with type 2 diabetes mellitus and metabolic syndrome: A pilot exploratory study

There is controversy as to whether hyperuricemia is an independent risk factor for cardiometabolic diseases. The serum level of uric acid is affected by a wide variety of factors involved in its production and excretion. In contrast, evidence has accumulated that locally‐ and systemically‐activated xanthine oxidase (XO), a rate‐limiting enzyme for production of uric acid, is linked to metabolic derangement in humans and rodents. We therefore explored the clinical implication of plasma XO activity in patients with type 2 diabetes mellitus and metabolic syndrome (MetS).

Febuxostat ameliorates secondary progressive experimental autoimmune encephalomyelitis by restoring mitochondrial energy production in a GOT2-dependent manner

Oxidative stress and mitochondrial dysfunction are important determinants of neurodegeneration in secondary progressive multiple sclerosis (SPMS). We previously showed that febuxostat, a xanthine oxidase inhibitor, ameliorated both relapsing-remitting and secondary progressive experimental autoimmune encephalomyelitis (EAE) by preventing neurodegeneration in mice. In this study, we investigated how febuxostat protects neuron in secondary progressive EAE. A DNA microarray analysis revealed that febuxostat treatment increased the CNS expression of several mitochondria-related genes in EAE mice, most notably including GOT2, which encodes glutamate oxaloacetate transaminase 2 (GOT2). GOT2 is a mitochondrial enzyme that oxidizes glutamate to produce α-ketoglutarate for the Krebs cycle, eventually leading to the production of adenosine triphosphate (ATP). Whereas GOT2 expression was decreased in the spinal cord during the chronic progressive phase of EAE, febuxostat-treated EAE mice showed increased GOT2 expression. Moreover, febuxostat treatment of Neuro2a cells in vitro ameliorated ATP exhaustion induced by rotenone application. The ability of febuxostat to preserve ATP production in the presence of rotenone was significantly reduced by GOT2 siRNA. GOT2-mediated ATP synthesis may be a pivotal mechanism underlying the protective effect of febuxostat against neurodegeneration in EAE. Accordingly, febuxostat may also have clinical utility as a disease-modifying drug in SPMS.