Pathogenesis of Gout Hyon K. Choi, MD, DrPH; David B. Mount, MD; and Anthony M. Reginato, MD, PhD adapted from Anna

1. Pathogenesis of GoutHyon K. Choi, MD, DrPH; David B. Mount, MD; and Anthony M. Reginato, MD, PhDadapted from  Annals of Internal Medicine 2005;143:499-516 ????????? ????? ?????

2. Introduction Gout is a type of inflammatory arthritis that is triggered by the crystallization of uric acid within the joints and is often associated with hyperuricemia (Figure 1). Acute gout is typically intermittent, constituting one of the most painful conditions experienced by humans. Chronic tophaceous gout usually develops after years of acute intermittent gout, although tophi occasionally can be part of the initial presentation.

4. Figure 1 Gout is mediated by the supersaturation and crystallization of uric acid within the joints. The amount of urate in the body depends on the balance between dietary intake, synthesis, and excretion. Hyperuricemia results from the overproduction of urate (10%), from underexcretion of urate (90%), or often a combination of the two. Approximately one third of urate elimination in humans occurs in the gastrointestinal tract, with the remainder excreted in the urine.

5. Introduction In addition to the morbidity that is attributable to gout itself, the disease is associated with such conditions as the insulin resistance syndrome, hypertension, nephropathy, and disorders associated with increased cell turnover. The overall disease burden of gout remains substantial and may be increasing. The prevalence of self-reported, physician-diagnosed gout in the Third National Health and Nutrition Examination Survey was found to be greater than 2% in men older than 30 years of age and in women older than 50 years of age. The prevalence increased with increasing age and reached 9% in men and 6% in women older than 80 years of age.

6. Introduction Furthermore, the incidence of primary gout (that is, patients without diuretic exposure) doubled over the past 20 years, according to the Rochester Epidemiology Project. Dietary and lifestyle trends and the increasing prevalence of obesity and the metabolic syndrome may explain the increasing incidence of gout. Researchers have recently made great advances in defining the pathogenesis of gout, including elucidating its risk factors tracing the molecular mechanisms of renal urate transport crystal-induced inflammation.

7. I.ABSENCE OF URICASE IN HUMANS Humans are the only mammals in whom gout is known to develop spontaneously, probably because hyperuricemia only commonly develops in humans. In most fish, amphibians, and nonprimate mammals, uric acid that has been generated from purine metabolism undergoes oxidative degradation through the uricase enzyme, producing the more soluble compound allantoin. In humans, the uricase gene is crippled by 2 mutations that introduce premature stop codons.

8. ABSENCE OF URICASE IN HUMANS The absence of uricase, combined with extensive reabsorption of filtered urate, results in urate levels in human plasma that are approximately 10 times those of most other mammals(30 to 59 �mol/L). The evolutionary advantage of these findings is unclear, but urate may serve as a primary antioxidant in human blood because it can remove singlet oxygen and radicals as effectively as vitamin C. Of note, levels of plasma uric acid (about 300 �M) are approximately 6 times those of vitamin C in humans. Other potential advantages of the relative hyperuricemia in primate species have been speculated. However, hyperuricemia can be detrimental in humans, as demonstrated by its proven pathogenetic roles in gout and nephrolithiasis and by its putative roles in hypertension and other cardiovascular disorders.

9. II.THE ROLE OF URATE LEVELS Uric acid is a weak acid (pKa, 5.8) that exists largely as urate, the ionized form, at physiologic pH. Population studies indicate a direct positive association between serum urate levels and a future risk for gout as shown in Figure 2. Conversely, the use of antihyperuricemic medication is associated with an 80% reduced risk for recurrent gout, confirming the direct causal relationship between serum uric acid levels and risk for gouty arthritis.

11. Figure 2 Annual incidence of gout was less than 0.1% for men with serum uric acid levels less than 416 �mol/L, 0.4% for men with levels of 416 to 475 �mol/L, 0.8% for men with levels of 476 to 534 �mol/L, 4.3% for men with levels of 535 to 594 �mol/L, and 7.0% for men with levels greater than 595 mol/L, according to the Normative Aging Study. The solid line denotes these data points; the dotted line shows an exponential projection of the data points.

13. Urate crystallizes as a monosodium salt in oversaturated tissue fluids. Its crystallization depends on the concentrations of both urate and cation levels. Alteration in the extracellular matrix leading to an increase in nonaggregated proteoglycans, chondroitin sulfate, insoluble collagen fibrils, and other molecules in the affected joint may serve as nucleating agents. Furthermore, monosodium urate (MSU) crystals can undergo spontaneous dissolution depending on their physiochemical environments. Chronic cumulative urate crystal formation in tissue fluids leads to MSU crystal deposition (tophus) in the synovium and cell surface layer of cartilage. Synovial tophi are usually walled off, but changes in the size and packing of the crystal from microtrauma or from changes in uric acid levels may loosen them from the organic matrix. This activity leads to �crystal shedding� and facilitates crystal interaction with synovial cell lining and residential inflammatory cells, leading to an acute gouty flare.

14. II.THE ROLE OF URATE LEVELS1. Urate Balance Furthermore, these factors may explain the predilection of gout in the first metatarsal phalangeal joint (a peripheral joint with a lower temperature) and osteoarthritic joints (degenerative joints with nucleating debris) and the nocturnal onset of pain (because of intra-articular dehydration). Hyperuricemia results from urate overproduction (10%), underexcretion (90%), or often a combination of the two. The purine precursors come from exogenous (dietary) sources or endogenous metabolism (synthesis and cell turnover).

15. II.2. The Relationship between Purine Intake and Urate levels The dietary intake of purines contributes substantially to the blood uric acid. For example, the institution of an entirely purine-free diet over a period of days can reduce blood uric acid levels of healthy men from an average of 297 �mol/L to 178 �mol/L . The bioavailable purine content of particular foods would depend on their relative cellularity and the transcriptional and metabolic activity of the cellular content. Little is known, however, about the precise identity and quantity of individual purines in most foods, especially when cooked or processed.

16. The Relationship between Purine Intake and Urate levels When a purine precursor is ingested, pancreatic nucleases break its nucleic acids into nucleotides, phosphodiesterases break oligonucleotides into simple nucleotides, pancreatic and mucosal enzymes remove phosphates and sugars from nucleotides. The addition of dietary purines to purine free dietary protocols has revealed a variable increase in blood uric acid levels. RNA has a greater effect than DNA, ribomononucleotides have a greater effect than nucleic acid, and adenine has a greater effect than guanine

17. The Relationship between Purine Intake and Urate levels(2) A recent large prospective study showed that men in the highest quintile of meat intake had a 41% higher risk for gout compared with the lowest quintile, and men in the highest quintile of seafood intake had a 51% higher risk compared with the lowest quintile. In a nationally representative sample of men and women in the United States, higher levels of meat and seafood consumption were associated with higher serum uric acid levels. However, consumption of oatmeal and purine-rich vegetables (for example, peas, beans, lentils, spinach, mushrooms, and cauliflower) was not associated with an increased risk for gout. The variation in the risk for gout associated with different purine-rich foods may be explained by varying amounts and type of purine content and their bioavailability for metabolizing purine to uric acid.

18. The Relationship between Purine Intake and Urate levels At the practical level, these data suggest that dietary purine restriction in patients with gout or hyperuricemia may be applicable to purines of animal origin but not to purine-rich vegetables, which are excellent sources of protein, fiber, vitamins, and minerals. Similarly, implications of the recent findings in the management of hyperuricemia or gout were consistent with the new dietary recommendations for the general public, with the exception of the guidelines for fish intake (Figure 4).

20. Data on the relationship between diet and the risk for gout are primarily derived from the recent Health Professionals Follow-Up Study. Implications of these findings in the management of hyperuricemia or gout are generally consistent with the new Healthy Eating Pyramid, except for fish intake. The use of plant-derived ?-3 fatty acids or supplements of eicosapentaenoic acid and docosahexaenoic acid in place of fish consumption could be considered to provide patients the benefit of these fatty acids without increasing the risk for gout. Use of ?-3 fatty acids may have anti-inflammatory effect against gouty flares. Vitamin C intake exerts a uricosuric effect. Red arrows denote an increased risk for gout, solid green arrows denote a decreased risk, and yellow arrows denote no influence on risk. Broken green arrows denote potential effect but without prospective evidence for the outcome of gout.

21. III.PURINE METABOLISM AND GOUT The vast majority of patients with endogenous overproduction of urate have the condition as a result of salvaged purines arising from increased cell turnover in proliferative and inflammatory disorders (for example, hematologic cancer and psoriasis), from pharmacologic intervention resulting in increased urate production (such as chemotherapy), or from tissue hypoxia. Only a small proportion of those with urate overproduction (10%) have the well-characterized inborn errors of metabolism (for example, superactivity of 5�-phosphoribosyl-1-pyrophosphate synthetase and deficiency of hypoxanthine� guanine phosphoribosyl transferase). These genetic disorders have been extensively reviewed in textbooks.

23. The de novo synthesis starts with 5�-phosphoribosyl 1-pyrophosphate (PRPP), which is produced by addition of a further phosphate group from adenosine triphosphate (ATP) to the modified sugar ribose-5-phosphate. This step is performed by the family of PRPP synthetase (PRS) enzymes. In addition, purine bases derived from tissue nucleic acids are reutilized through the salvage pathway. The enzyme hypoxanthine� guanine phosphoribosyl transferase (HPRT) salvages hypoxanthine to inosine monophosphate (IMP) and guanine to guanosine monophosphate (GMP). Only a small proportion of patients with urate overproduction have the well-characterized inborn errors of metabolism, such as superactivity of PRS and deficiency of HPRT. Furthermore, conditions associated with net ATP degradation lead to the accumulation of adenosine diphosphate (ADP) and adenosine monophosphate (AMP), which can be rapidly degraded to uric acid. These conditions are displayed in left upper corner. Plus sign denotes stimulation, and minus sign denotes inhibition..

24. PURINE METABOLISM AND GOUT Ethanol administration has been shown to increase uric acid production by net ATP degradation to AMP. Decreased urinary excretion as a result of dehydration and metabolic acidosis. A large-scale prospective study confirmed that the effect of ethanol on urate levels can be translated into the risk for gout. Compared with abstinence, daily alcohol consumption of 10 to 14.9 g increased the risk for gout by 32%; daily consumption of 15 to 29.9 g, 30 to 49.9 g, and 50 g or greater increased the risk by 49%, 96%, and 153%, respectively. Furthermore, the study also found that this risk varied according to type of alcoholic beverage: Beer conferred a larger risk than liquor, whereas moderate wine drinking did not increase risk.

25. PURINE METABOLISM AND GOUT Correspondingly, a national U.S. survey demonstrated parallel associations between these alcoholic beverages and serum urate levels. These findings suggest that certain nonalcoholic components that vary among these alcoholic beverages play an important role in urate metabolism. Ingested purines in beer, such as highly absorbable guanosine, may produce an effect on blood uric acid levels that is sufficient to augment the hyperuricemic effect of alcohol itself, thereby producing a greater risk for gout than liquoror wine. Whether other nonalcoholic offending factors exist remains unclear, particularly in regard to beer; instead, protective factors in wine may be mitigating the alcohol effect on the risk for gout.

26. PURINE METABOLISM AND GOUT Fructose is the only carbohydrate that has been shown to exert a direct effect on uric acid metabolism. Fructose phosphorylation in the liver uses ATP, and the accompanying phosphate depletion limits regeneration of ATP from ADP. The subsequent catabolism of AMP serves as a substrate for uric acid formation. Thus, within minutes after fructose infusion, plasma (and later urinary) uric acid concentrations are increased.

27. PURINE METABOLISM AND GOUT In conjunction with purine nucleotide depletion, rates of purine synthesis de novo are accelerated, thus potentiating uric acid production. Oral fructose may also increase blood uric acid levels, especially in patients with hyperuricemia or a history of gout . Fructose has also been implicated in the risk for the insulin resistance syndrome and obesity, which are closely associated with gout. Furthermore, hyperuricemia resulting from ATP degradation can occur in acute, severe illnesses, such as the adult respiratory distress syndrome, myocardial infarction, or status epilepticus.

28. IV.ADIPOSITY, INSULIN RESISTANCE, AND GOUT Increased adiposity and the insulin resistance syndrome are both associated with hyperuricemia. Body mass index, waist-to-hip ratio, and weight gain have all been associated with the risk for incident gout in men. Conversely, small, open-label interventional studies showed that weight reduction was associated with a decline in urate levels and risk for gout. Reduced de novo purine synthesis was observed in patients after weight loss, resulting in decreased serum urate levels.

29. ADIPOSITY, INSULIN RESISTANCE, AND GOUT Exogenous insulin can reduce the renal excretion of urate in both healthy and hypertensive persons. Insulin may enhance renal urate reabsorption through stimulation of the urate�anion exchanger uratetransporter-1 (URAT1) (63) or through the sodium-dependent anion cotransporter in brush-border membranes of the renal proximal tubule. Because serum levels of leptin and urate tend to increase together, some investigators have also suggested that leptin may affect renal reabsorption.

30. ADIPOSITY, INSULIN RESISTANCE, AND GOUT In the insulin resistance syndrome, impaired oxidative phosphorylation may increase systemic adenosine concentrations by increasing the intracellular levels of coenzyme A esters of long-chain fatty acids. Increased adenosine, in turn, can result in renal retention of sodium, urate, and water. Some researchers have speculated that increased extracellular adenosine concentrations over the long term may also contribute to hyperuricemia by increasing urate production. The growing �epidemic� of obesity and the insulin resistance syndrome present a substantial challenge in the prevention and management of gout.

31. V.HYPERTENSION, CARDIOVASCULAR DISORDERS, AND GOUT Associations between hypertension and the incidence of gout have been observed, but researchers were previously unable to determine whether hypertension was independently associated or if it only served as a marker for associated risk factors, such as dietary factors, obesity, diuretic use, and renal failure. A recent prospective study, however, has confirmed that hypertension is associated with an increased risk for gout independent of these potential confounders.

32. HYPERTENSION, CARDIOVASCULAR DISORDERS, AND GOUT Renal urate excretion was found to be inappropriately low relative to glomerular filtration rates in patients with essential hypertension. Reduced renal blood flow with increased renal and systemic vascular resistance may also contribute to elevated serum uric acid levels. Hyperuricemia in patients with essential hypertension may reflect early nephrosclerosis, thus implying renal morbidity in these patients. Furthermore, studies have suggested that hyperuricemia may be associated with incident hypertension or cardiovascular disorders.

33. VI.RENAL TRANSPORT OF URATE Renal urate transport is typically explained by a 4-component model: glomerular filtration, a near-complete reabsorption of filtered urate, subsequent secretion, and postsecretory reabsorption in the remaining proximal tubule. This model evolved from an interpretation of the effects of �uricosuric� and �antiuricosuric� agents; drugs and compounds known to affect serum urate levels are summarized in the Table.

36. RENAL TRANSPORT OF URATE The urate secretion step was incorporated into the model to explain the potent antiuricosuric effect of pyrazinamide. However, direct inhibition of proximal tubular urate secretion by pyrazinoate, the relevant metabolite, has never been demonstrated. Indeed, pyrazinamide has no effect in animal species that eliminate urate through net secretion, and direct effects of the drug on human urate secretion are largely unsubstantiated Rather, studies utilizing renal brush-border membrane vesicles have shown that pyrazinoate activates the reabsorption of urate through indirect stimulation of apical urate exchange (Figure 5). Similar mechanisms underlie the clinically relevant hyperuricemic effects of lactate, ketoacids, and nicotinate

37. VI.1.The Renal Urate�Anion Exchanger URAT1 Enomoto and colleagues(63) recently identified the molecular target for uricosuric agents, an anion exchanger responsible for the reabsorption of filtered urate by the renal proximal tubule (Table). The authors searched the human genome database for novel gene sequences within the organic anion transporter (OAT) gene family and identified URAT1 (SLC22A12), a novel transporter expressed at the apical brush border of the proximal nephron.

38. The Renal Urate�Anion Exchanger URAT1 Urate�anion exchange activity similar to that of URAT1 was initially described in brushborder membrane vesicles from urate-reabsorbing species, such as rats and dogs, and was subsequently confirmed in human kidneys. Frog eggs (Xenopus oocytes) injected with URAT1-encoding RNA transport urate and exhibit pharmacologic properties consistent with data from human brush-border membrane vesicles.

39. The Renal Urate�Anion Exchanger URAT1 These and other experiments indicate that uricosuric compounds (for example, probenecid, benzbromarone, sulfinpyrazone, and losartan) directly inhibit URAT1 from the apical side of tubular cells (�cis-inhibition�). Conversely, antiuricosuric substances (for example, pyrazinoate, nicotinate, and lactate) serve as the exchanging anion from inside cells (Figure 6), thereby stimulating anion exchange and urate reabsorption (�trans-stimulation�).

42. The Renal Urate�Anion Exchanger URAT1 In addition to urate, URAT1 has particular affinity for aromatic organic anions, such as nicotinate and pyrazinoate, followed by lactate, �-hydroxybutyrate, acetoacetate, and inorganic anions, such as chloride and nitrate. Enomoto and colleagues (63) provided unequivocal genetic proof that URAT1 is crucial for urate homeostasis: --A handful of patients with �familial renal hypouricemia� were shown to carry loss of-function mutations in the human SLC22A12 gene encoding URAT1, indicating that this exchanger is essential for proximal tubular reabsorption. Furthermore, pyrazinamide, benzbromarone, and probenecid failed to affect urate clearance in patients with homozygous loss-of-function mutations in SLC22A12, indicating that URAT1 isessential for the effect of both uricosuric and antiuricosuric agents.

43. VI.2.Secondary Sodium Dependency of Urate Reabsorption Antiuricosuric agents exert their effect by stimulating renal reabsorption rather than inhibiting tubular secretion. The mechanism appears to involve a �priming� of renal urate reabsorption through the sodium-dependent loading of proximal tubular epithelial cells with anions capable of a trans-stimulation of urate reabsorption . Studies from several laboratories have indicated that a transporter in the proximal tubule brush border mediates sodium-dependent reabsorption of pyrazinoate, nicotinate, lactate, pyruvate, �-hydroxybutyrate, and acetoacetate, monovalent anions that are also substrates for URAT1 (63).

44. Secondary Sodium Dependency of Urate Reabsorption Increased plasma concentrations of these antiuricosuric anions result in their increased glomerular filtration and greater reabsorption by the proximal tubule. The augmented intraepithelial concentrations in turn induce the reabsorption of urate by promoting the URAT1-dependent anion exchange of filtered urate (trans-stimulation) (Figure 6).

45. Secondary Sodium Dependency of Urate Reabsorption Urate reabsorption by the proximal tubule thus exhibits a form of secondary sodium dependency, in that sodium dependent loading of proximal tubular cells stimulates brush-border urate exchange; urate itself is not a substrate for the sodium�anion transporter. The molecular identity of the relevant sodium-dependent anion cotransporter or cotransporters remains unclear; however, a leading candidate gene is SLC5A8, which encodes a sodium-dependent lactate and butyrate cotransporter.

46. Secondary Sodium Dependency of Urate Reabsorption Preliminary data indicate that the SLC5A8 protein can also transport both pyrazinoate and nicotinate, potentiating urate transport in Xenopus oocytes that co-express URAT1. The antiuricosuric mechanism explains the long-standing clinical observation that hyperuricemia is induced by increased b-hydroxybutyrate and acetoacetate levels in diabetic ketoacidosis (95), increased lactic acid levels in alcohol intoxication (45), or increased nicotinate and pyrazinoate levels in niacin and pyrazinamide therapy, respectively (96).

47. Secondary Sodium Dependency of Urate Reabsorption Urate retention is also known to be provoked by a reduction in extracellular fluid volume and by excesses of angiotensin II, insulin, and parathyroid hormone; URAT1 and the sodium-dependent anion cotransporter or cotransporters may be targets for these stimuli.

48. VI.3.Dose-Dependent Dual Response in Urate Excretion A conundrum(??) in the pathophysiology of gout has been how certain anions can exhibit either uricosuric or antiuricosuric properties, depending on the dose administered. Monovalent anions that interact with URAT1 have the dual potential to increase or decrease renal urate excretion because they can both trans-stimulate and cis-inhibit apical urate exchange in the proximal tubule. For example, a low concentration of pyrazinoate stimulates urate reabsorption as a consequence of trans-stimulation, whereas a higher concentration reduces urate reabsorption through extracellular cis-inhibition of URAT1(Figure 7).

50. The anti-uricosuric agent pyrazinoate (PZA), a metabolite of pyrazinamide, has dual effects on urate transport by the proximal tubule. Urate uptake by brush-border membrane vesicles isolated from canine kidney cortex is shown, in the presence of 100 mM sodium (Na) with 0.1 mM PZA, 0 PZA, or 5 mM PZA. The concentration results in Na-dependent uptake of PZA and a potentiation of urate uptake via urate transporter-1 (URAT1); in contrast, the higher concentration cis-inhibits URAT1, thus reducing urate uptake by the membrane vesicles. Paradoxical effects of pyrazinoate and nicotinate on urate transport in dog renal microvillus membranes. J Clin Invest. 1985;76:543-7.

51. Dose-Dependent Dual Response in Urate Excretion Dissenting(??) opinions notwithstanding, these observations remain consistent with the basic scheme of apical urate transport shown in Figure 6. Biphasic effects on urate excretion (that is, antiuricosuria at low doses and uricosuria at high doses) are particularly well described for salicylate. Salicylate cis-inhibits URAT1, explaining the high-dose uricosuric effect; low anti-uricosuria reflects a trans-stimulation of URAT1 by intracellular salicylate, which is evidently a substrate for the sodium�pyrazinoate transporter. Minimal doses of salicylate�75, 150, and 325 mg daily�were shown to increase serum uric acid levels by 16, 12, and 2 mol/L, respectively. However, the effect on the risk for gout of this salicylate-induced increase in the serum uric acid level has not been determined.

52. VI.4.Other Renal Urate Transporters At the basolateral membrane of proximal tubular cells, the entry of urate from the surrounding interstitium appears to be driven by sodium-dependent uptake of divalent anions, such as a-ketoglutarate, rather than monovalent carboxylates, such as pyrazinoate and lactate (Figure 6). Candidate proteins for this basolateral urate exchange activity include both OAT1 and OAT3, each of which function as anion1- -dicarboxylate2- exchangers at the basolateral membrane of the proximal tubule.

53. X.SUMMARY The disease burden of gout remains substantial and may be increasing. As more scientific data on modifiable risk factors and comorbidities of gout become available, integration of these data into gout care strategy may become essential, similar to the current care strategies for hypertension and type 2 diabetes. Recommendations for lifestyle modification to treat or to prevent gout are generally in line with those for the prevention or treatment of other major chronic disorders Weight control, limits on red meat consumption, and daily exercise are important foundations of lifestyle modification recommendations Plant-derived ?-3 fatty acids or supplements of eicosapentaenoic acid and docosahexanoic acid instead of consuming fish for cardiovascular benefits.

54. SUMMARY Further risk�benefit assessments in each specific clinical context would be helpful. Daily consumption of nuts and legumes as ecommended by the Harvard Healthy Eating Pyramid (32) may also provide important health benefits without increasing the risk for gout. Similarly, a daily glass of wine may benefit health without imposing an elevated risk for gout, especially in contrast to beer or liquor consumption. These lifestyle modifications are inexpensive and safe and, when combined with drug therapy, may result in better control of gout.

55. SUMMARY Effective management of gout risk factors (for example, hypertension) and the antihypertensive agents with uricosuric properties (for example, losartan or amlodipine could have a better risk� benefit ratio than diuretics for hypertension in hypertensive patients with gout. Similarly, the uricosuric property of fenofibrate may be associated with a favorable risk� benefit ratio among patients with gout and the metabolic syndrome.

56. SUMMARY The recently elucidated molecular mechanism of renal urate transport has several important implications in conditions that are associated with high urate levels. In particular, the molecular characterization of the URAT1 anion exchanger has provided a specific target of action for well known substances affecting urate levels. Genetic variation in these renal transporters or upstream regulatory factors may explain the genetic tendency to develop conditions associated with high urate levels and a patient�s particular response to medications. Furthermore, the transporters themselves may serve as targets for future drug development.