Abstract
ObjectiveLimiting purine intake, inhibiting xanthine oxidoreductase (XOR) and inhibiting urate reabsorption in proximal tubule by uricosuric drugs, to reduce serum uric acid (UA) levels, are recognized treatments for gout. However, the mechanism of increased how XOR expression and activity in hyperuricemia and gout remains unclear. This study aims to explore whether exogenous purines are responsible for increased XOR expression and activity.MethodsHepG2 and Bel-7402 human hepatoma cells were stimulated with exogenous purine, or were exposed to conditioned growth medium of purine-stimulated Jurkat cells, followed by measurement of XOR expression and UA production to determine the effect of lymphocyte-secreted cytokines on XOR expression in hepatocytes. The expression of STAT1, IRF1 and CBP and their binding on the XDH promoter were detected by western blotting and ChIP-qPCR. The level of DNA methylation was determined by bisulfite sequencing PCR. Blood samples from 117 hyperuricemia patients and 119 healthy individuals were collected to analyze the correlation between purine, UA and IFN-γ concentrations.ResultsExcess of purine was metabolized to UA in hepatocyte metabolism by XOR that was induced by IFN-γ secreted in the conditioned growth medium of Jurkat cells in response to exogenous purine, but it did not directly induce XOR expression. IFN-γ upregulated XOR expression due to the enhanced binding of STAT1 to IRF1 to further recruit CBP to the XDH promoter. Clinical data showed positive correlation of serum IFN‐γ with both purine and UA, and associated risk of hyperuricemia.ConclusionPurine not only acts as a metabolic substrate of XOR for UA production, but it induces inflammation through IFN-γ secretion that stimulates UA production through elevation of XOR expression.
Highlights
Hyperuricemia is a metabolic disease caused by abnormal purine metabolism and due to an excessively increased serum urate concentration that can occur as a result of overproduction of uric acid (UA) and/or over reabsorption of urate, possibly driven by intracellular nicotinate [1]
We demonstrate that excessive purine generates UA without directly affecting Xanthine oxidoreductase (XOR) expression and activity in hepatic cells, instead indirectly elevates XOR expression induced by IFN-g that is secreted form lymphocyte (Jurkat cells) in response to excessive purines
These results indicate that exogenous purine elevates intracellular UA production in cultured hepatocytes and that UA production increased with increasing concentrations of substrate and plateaued at substrate concentration of 100 mM, which could be the result of saturation of substrate binding site of XOR leading to a steady state
Summary
Hyperuricemia is a metabolic disease caused by abnormal purine metabolism and due to an excessively increased serum urate concentration that can occur as a result of overproduction of uric acid (UA) and/or over reabsorption of urate, possibly driven by intracellular nicotinate [1]. Elevated serum UA levels caused by a high purine diet is the most common cause of acute gout. It has become almost a medical consensus that gout patients should avoid alcohol and sweet beverages, as well as high purine foods [13,14,15]. Whether enhanced catalytic conversion of excess purine into UA through upregulation of XOR in hyperuricemia has not been reported. The upregulation of XOR expression, by cytokines IL-1, IL-6 and IFN-g, has been reported in lung, renal, and mammary epithelial cells [20,21,22,23], but their effect on XOR expression and UA production in hepatocytes remained unknown. We hypothesized that purine may directly or indirectly via inflammatory cytokines regulate XOR expression
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