Pathogenesis of Gout

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Pathogenesis of Gout

• Hyon K. Choi et al. ACP
• 21st April 2006
• ?????

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Clinical principles

• Overall disease burden of gout is substantial and
  increasing
• More scientific data available need to be
  integrate
• Hyperuricemia and gout associated with insulin
  resistance syndrome and related comorbid
  conditions
• Lifestyle modification recommended
• Effective management of risk factors for gout
• Urate-anion exchanger urate transporter-1 (URAT1)
  is specific target of action
• Long-term health effect need to be clarified

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Pathophysiologic principles

• Direct causal relationship between urate levels
  risk for gout
• Lifestyle factors (adiposity and dietary habits)
  contributes
• Urate reabsorbed from proximal tubule via
  brush-border URAT-1
• Sodium-dependent reabsorption of anions increases
  conc in proximal tubules, resulting increased
  urate exchange via URAT-1, increased urate
  reabsorption by kidney, and hyperuricemia
• Genetic variation in renal urate transporters or
  upstream regulatory factors explain hereditary
  susceptibility, transporters also serve as
  targets for future drug development
• Urate crystals directly initiate, amplify,
  sustain an intense inflammatory attack because of
  ability to stimulate the synthesis and release of
  humoral and cellular inflammatory mediators
• Cytokines, chemokines, protease, and oxidants
  involved in acute urate crystal-induced
  inflammation also contribute to chronic
  inflammation leads to chronic gouty synovitis,
  cartilage loss, and bone erosion

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Introduction

• Gout--type of inflammatory arthritis-triggered by
  crystallization of UA within joints and
  associated with hyperuricemia (Figure 1)
• Acute gout-typically intermittent
• Chronic tophaceous gout-develops after years of
  acute intermittent gout, although tophi
  occasionally can be part of initial presentation.
• Gout is associated with insulin resistance
  syndrome, hypertension, nephropathy, and disorder
  with increased cell turnover.
• NHANES III gt2 in men gt30 y/o, women gt50 y/o 9
  in men gt80 y/o, 6 in women gt80 y/o
• Rochester Epidemiology Project (REP) Incidence
  of primary gout double over past 20 yrs
• Dietary and lifestyle trends increasing
  prevalence of obesity and metabolic syndrome
  explain.
• Recently, advance in defining pathogenesis,
  elucidating risk factors, tracing molecular
  mechanisms of renal urate transport and crystal
  induced inflammation

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Absence of Uricase in humans

• Humans the only mammals gout develop
  spontaneously
• In most fish, amphibians and nonprimate mammals,
  UA generated from purine metabolism undertgoes
  oxidative degradation through uricase enzyme,
  producing more soluble compound allantoin.
• In humans, uricase gene is crippled by 2
  mutations that introduce premature stop codons
• Absence of uricase, combined extensive
  reabsorption of filtered urate, resulting urate
  levels in human plasma approximately 10 times
  than other mammals (30-59 mmol/L)
• Urate may serve as primary antioxidant in human
  blood, remove singlet oxygen and radicals as
  effectively as vitamin C. Level of plasma uric
  acid (300 mM) are approximately 6 times those of
  vitamin C in humans
• Hyperuricemia has detrimental in
  humans-pathogenetic roles in gout and
  nephrolithiasis and putative roles in
  hypertension and other CV disorders.

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The role of urate levels

• Uric acid-weak acid (pKa, 5.8)-exists largely as
  urate, ionized form, at physiologic pH
• Urate increase, risk for supersaturation and
  crystal formation increase
• Positive association between serum urate levels
  future risk for gout (Figure 2)
• Antihyperuricemic medicaiton 80 reduced risk for
  recurrent gout arthritis
• Solubility of urate in joint fluids influenced by
  other factors in joint, (Figure 3) Variation in
  these factors account for difference in risk for
  gout with urate elevation
• Predilection of gout in 1st MTP joint-peripheral
  joint with lower temperature, osteoarthritic
  joints-degenerative joints with nucleating
  debris, nocturnal onset of pain-intra-articular
  dehydration

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Urate balance

• Amount of urate in body depends on the balance
  between dietary intake, synthesis, and the rate
  of excretion
• Hyperuricemia results from urate overproduction
  (10), underexcretion (90), or often combination
• Purine precursors come from exogenous (dietary)
  sources or endogenous metabolism (synthesis and
  cell turnover)i

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The relationship between purine intake and urate
levels

• Dietary purines contributes to blood uric acid
• Purine free diet reduce blood uric acid level
  from 297 to 178 mmol/L
• Bioavailable purine content of particular food
  depend on relative cellularity, transcriptional,
  metabolic activity of cellular content.
• Little is known the precise identity quantity
  of individual purines in most food, esp when
  cooked or processed
• Purine precursor ingested, pancreatic nucleases
  break its nucleic acids into nucleotides,
  phosphodiesterases break oligonucleotides into
  simple nucleotides, pancreatic and mucosal
  enzymes remove phosphates and sugar from
  nucleotides
• Addition dietary purine to purine-free dietary
  protocols revealed variable increase in blood
  uric acid, depending on the formulation and dose
  of purines administered
• RNA has greater effect than DNA,
  ribomononucleotides greater effect than nucleic
  acid, adenine than guanine

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• Men highest quintile of meat intake had 41
  higher risk for gout , highest quintile seafood
  intake had 51 higher risk. (also in US men and
  women)
• Consumption of oatmeal and purine-rich vegetables
  (peas, beans, lentils, spinach, mushrooms, and
  cauliflower) was not.
• explained by varying amounts and type of purine
  content and bioavailability for metabolizing
  purine to uric acid.
• Dietary purine restriction in gout or
  hyperuricemia may be applicable to purines of
  animal origin
• Plant-derived w-3 fatty acids or supplements of
  eicosapentasenoic acid and docosahexaenoic acid
  instead of fish consumption considered provide
  benefit of these fatty acids without increasing
  risk for gout.

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Purine metabolism and gout

• Vast majority endogenous overproduction of urate
  arising from increased cell turnover in
  proliferative and inflammatory disorders
  (hematologic cancer psoriasis), from
  pharmacologic intervention (chemotherapy), or
  from tissue hypoxia.
• Only 10 urate overproduction have
  well-characterized inborn errors of metabolism
  (superactivity of 5-phosphoribosyl-1-pyrophosphat
  e synthetase and deficiency of hypoxanthine-quanin
  e phosphoribosyl transferase)
• Condition with ATP degradation lead to
  accumulation of ADP and AMP, rapidly degraded to
  uric acid
• Ethanol increase UA production by net ATP
  degradation to AMP, and decreased urinary
  excretion as dehydration and metabolic acidosis
  contribute to hyperuricemia

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• Daily alcohol consumption 10-14.9 g increased
  risk for gout 32, 15-29.9 g, 30-49.9 g, gt 50g,
  increased risk by 49, 96, and 153.
• Beer conferred a larger risk than liquor,
  moderate wine drinking did not increase risk
• National US survey showed certain nonalcoholic
  components that vary among these alcoholic
  beverages play important role in purine
  metabolism,
• Highly absorbable guanosine (purine in beer),
  augment the hyperuricemic effect of alcohol,
  producing a greater risk for gout than liquor or
  wine

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• Fructose-only CHO to exert a direct effect on UA
  metabolism
• Fructose phosphrylation in liver uses ATP,
  accompanying phosphate depletion limits
  regeneration ATP from ADP, subsequent catabolism
  of AMP serve as a substrate for UA
• Within minutes of fructose infusion, plasma
  (lately urine) UA increased
• Purine nucleotide depletion, rates of purine
  synthesis denovo accelerated, thus potentiating
  uric acid production
• Oral fructose also increase blood UA, especially
  in pt with hyperuricemia or history of gout
• Fructose has also been implicated in the risk for
  insulin r4esistance syndrome and obesity, which
  are closely with gout
• Hyperuricemia resulting from ATP degradation can
  occur in acute, severe illness, such as ARDS, MI,
  or status epilepticus.

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Adiposity, insulin resistance, and gout

• BMI, waist-to-hip ratio, weight gain associated
  with risk for incident gout in men
• Weight reduction decline in urate level risk
  for gout, reduced de novo purine synthesis,
  resulting in decreased serum urate levels
• Exogenous insulin reduce renal urate excretion in
  both healthy and hypertensive persons
• Insulin enhance renal urate reabsorption through
  stimulation of the URAT1 or through
  sodium-dependent anion cotransporter in
  brush-border membranes of the renal proximal
  tubule
• Serum leptin and urate tend to increase together,
  leptin may affect renal reabsoprion.
• Insulin resistance syndrome, impaired oxidative
  phosphorylation increase systemic adenosine by
  increasing the intracellular level of coenzyme A
  esters of long-chain fatty acids
• Increased adenosine, result in renal retention of
  sodium, urate, and water, long-term may
  contribute to hyperuricemia

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Hypertension, CV disorders, and gout

• Recent prospective study confirmed that
  hypertension associated with increased risk for
  gout indepnedently of potential confounders
  (dietary, obesity, diuretic use, renal failure)
• Renal urate excretion was found to be
  inappropriately low relative to GFR in pt with
  essential hypertension
• Reduce renal blood flow with increased renal and
  systemic vascular resistance may also contribute
• Hyperuricemia in hypertension may reflect early
  nephrosclerosis, implying renal morbidity
• Hyperuricemia may be associated with incident
  hypertension or CV disorders

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Renal transport of urate

• 4-component model
• Glomerular filtration
• near-complete reabsorption of filtered urate
• subsequent secretion
• postsecretory reabsorption in remaining
  proximal tubule.
• Uricosuric and antiuricosuric agents (table)
• Pyrazinamide-potent antiuricosuric effect
• Pyrazinoate activates the reabsorption of urate
  through indirect stimulation of apical urate
  exchange
• Similar mechanisms underlie the clinically
  relevant hyperuricemic effects of lactate,
  ketoacids, and nicotinate

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The renal urate-anion exchanger URAT1

• Enomoto identified molecular target for
  uricosuric agents, an anion exchanger responsible
  for the reabsorption of filtered urate by renal
  proximal tubule.
• URAT1 (SLC22A12), organic anion transporter (OAT)
  gene family, a novel transporter expressed at the
  apical brush border of the proximal nephron
• Uricosuric compounds (probenecid, benzbromarone,
  sulfinpyrazone, and losartan) directly inhibit
  URAT1 from the apical side of tubular cells
  (cis-inhibition).
• Antiuricosuric substances (pyrazinoate,
  nicotinate, and lactate) serve as exchanging
  anion from inside cells, stimulating anion
  exchange and urate reabsorption
  (trans-stimulation) (Figure 6)
• URAT1 has particularly affinity for aromatic
  organic anions, such as nicotinate, purazinoate,
  lactate, b-hydroxybutyrate, acetoacetate, and
  inorganic anions, such as chloride and nitrate.

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• Enomoto provide unequivocal genetic proof that
  URAT1 is crucial for urate homeostasis
• Pts with familial renal hypouricemia shown to
  carry loss-of-function mutations in human
  SLC22A12 gene encoding URAT1, indicating that
  this exchanger is essential for proximal tubular
  reabsorption
• Pyrazinamide, benzbromarone, probenecid failed to
  affect urate clearance in pt with homogenous
  loss-of-function mutation in SLC22A12, indicating
  that URAT1is essential for the effect of both
  uricosuric and antiuricosuric agents.

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Secondary sodium dependency of urate reabsorption

• Antiuricosuric agents exert effect by stimulating
  renal reabsorption
• Involve priming of renal urate reabsorption
  through sodium-dependent loading of proximal
  tubular epithelial cells with anions capable of
  trans-stimulation of urate reabsorption (Figure
  6)
• Transporter in the proximal tubular brush border
  mediates sodium-dependent reabsorption of
  pyrazinoate, nicotinate, lactate, pyruvate,
  b-hydroxybutyrate, and acetoacetate, monovalent
  anions are also substrates for URAT1
• Increased plasma antiuricosuric anions increased
  GFR and greater reabsorption by proximal tubule
• Augmented intraepithelial conc in turn induce the
  reabsorption of urate by promoting the
  URAT1-dependent anion exchange of filtered urate
  (trans-stimulation)

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• Urate reabsorption by proximal tubule exhibits a
  form of secondary sodium dependency,
  sodium-dependent loading of proximal tubular
  cells stimulates brush-border urate exchange
  urate itself is not a substrate for sodium-anion
  transporter
• Leading candidate gene is SLC5A8, which encodes a
  sodium-dependent lactate and butyrate
  cotransporter, also transport both pyrazinoate
  and nicotinate
• Antiuricosuric mechanism explains the
  long-standing clinical observation that
  hyperuricemia is induced by increased
  b-hydroxybutyrate and acetoacetate in DKA,
  increased lactic acids in alcohol intoxication,
  or increased nicotinate pyrazinoate levels in
  niacin pyrazinamide therapy
• Urate retention also provokes by reduction in
  extracellular fluid volume and by excess of
  angiotensin II, insulin, and parathyroid hormone
• URAT1 and sodium-dependent anion cotransporter or
  cotransporters may be targets for stimuli

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Dose-dependent dual response in urate excretion

• Monocovalent anions interact with URAT1 have 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
• Low conc of pyrazinoate stimulates urate
  reabsorption by tran-stimulation, higher conc.
  reduce through extracellular cis-inhibition of
  URAT1
• Biphasic effects on urate excretion
  (antiuricosuric at low doses, uricosuria at high
  doses) are particularly well described for
  salicylate
• Salicylate cis-inhibits URAT1 by intracellular
  salicylate, which is evidently a substrate for
  sodium-pyrazinoate transporter
• Minimal doses of salicylate-75, 150, and 325 mg
  daily-were shown to increase serum uric acids
  levels by 16, 12, and 2 mmol/L

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Other renal urate transporters

• Basolateral membrane of proximal tubular cells,
  entry of urate from surrounding interstitium
  driven by sodium-dependent uptake of divalent
  anions, such as a-ketoglutarate
• Candidate proteins-both OAT1 and OAT3, each of
  which functions as anion-dicarboxylate exchangers
  
• These proteins facilitate the basolateral influx
  or efflux of urate
• Several molecular candidates proposed
• urate transporter/channel (UAT, also known as
  galectin-9) voltage-driven organic anion
  transporter-1 (OATV1)
• Apical ATP-driven anion transporter multidrug
  
• resistance protein4 (MRP4)

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Urate crystal-induced inflammation

• Urate crystals are directly able to initiate, to
  amplify, and to sustain an intense inflammatory
  attack because of their ability to stimulate the
  synthesis and release of humoral and cellular
  inflammatory mediators (Figure 8)

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Urate crystal-induced cell activation and
signaling

• Urate crystals interact with phagocyte through 2
  mechanisms
• Activate cells through conventional route as
  opsonized and phagocytosed particles, eliciting
  the stereotypical phagocyte response of lysosomal
  fusion, respiratory burst, and release of
  inflamatory mediators
• Interact directly with lipid membranes proteins
  through cell membrane perturbation and
  cross-linking of membrane glycoproteins in the
  phagocyte-leads to the activation of several
  signal tranduction pathways, including G
  proteins, phospholipase c and D, Src tyrosine
  kinases, the mitogen-activated protein kinases
  ERK1/ERK2, c-Jun N-terminal kinase, and p38
  mitogen-activated protein kinase
• gtThese steps are critical for crystal-induced
  interleukin (IL)-8 expression in monocytic cells,
  which plays a key role in the neutrophil
  accumulation

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Crystal-induced cellular response

• Monocytes and mast cells participate the early
  phase of inflammation, Neutrophil infiltrates
  occur late
• Phagocytes from noninflamed joints may contain
  urate crystals, most of these phagocytes are
  macrophages
• State of differentiation of mononuclear
  phagocytes determines whether the crystal will
  trigger an inflammatory response
• Less differentiated cell lines, TNF-a synthesis
  endothelial cell activation occur after urate
  crystal phagocytosis,
• Well-defferentiated macrophages failed to induce
  TNF-a or activate endothelial cells
• Freshly isolated human monocytes lead to a
  vigorous response by induction of TNF-a, IL-1b,
  IL-6, IL-8, and cyclooxygenase-2 secretion, human
  macrophages differentiated failed to secrete
  cytokines
• Monocytes play a central role in stimulating
  acute attack of gout, differentiated macrophage
  play an anti-inflammatory role in terminating an
  acute attack and preserving asymptomatic state

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• Experimental animal models suggest mast cells
  involved in early phase of crystal-induced
  inflammation, they also release inflammatory
  mediators, such as histamine, in response to C3a,
  C5a, and IL-1. the vasodilatation, increased
  vascular permeability, and pain also mediated by
  kinins, complement cleavage peptides, and other
  vasoactive prostaglandins

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Neutrophil influx and amplification

• Neutrophilic synovitis-the hallmark of acute
  gouty attack
• Neutrophilic-endothelial cell interaction leading
  to neutrophilic influx be an important event in
  this inflammation and represents a major locus
  for pharmacologic effect of colchicine
• Neutrophilic influx promoted by
  endothelial-neutrophil adhesion triggered by
  IL-1, TNF-a, and several chemokines, such as IL-8
  and neutrophil chemoatractant protein-1 (MCP-1)
• Neutrophil migration-mediated by cytokine-induced
  clustering of E-selectin on endothelial cells.
• Colchicine interferes with the interaction by
  altering number and distribution of selectins on
  endothelial cells and neutrophils in response to
  IL-1 or TNF-a

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• In synovial tissue, neutrophils follow conc.
  gradients of chemoattractants such as C5a,
  leukotriene B4, platelet-activating factor, IL-1,
  IL-8
• IL-8 and growth-related gene chemokines play
  central role in neutrophil invasion
• IL-8 alone account for 90 neutrophil chemotactic
  activity of human monocytes in response to urate
  crystals
• Neutralization of IL-8 or its receptor may
  substantially reduce IL-8-induced neutrophilic
  inflammatory process provide potential
  therapeutic target in gout
• Calcium-binding proteins (calgranulins) S100A8
  S100A9 also been shown involve in neutrophil
  migration induced by urate crystals

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Spontaneous resolution of acute gout

• Self-limited nature of acute gout involve several
  mechanisms
• Clearance of urate crystals by differentiated
  macrophages- inhibition of leukocyte and
  endothelial activation
• Neutrophil apoptosis and other apoptotic cell
  clearance-fundamental mechanism in resolution of
  acute inflammation
• Transforming growth factor-b abundant in acute
  gouty synovial fluid, inhibits IL-1 receptor
  expression IL-1-driven cellular inflammatory
  responses.
• Upregulation of IL-10-limit experimental
  urate-induced inflammation function as a native
  inhibitor of gouty inflammation
• Urate crystals induce PPAR-r in human
  monocytes-promote neutrophil and macrophage
  apoptosis
• Inactivation of inflammatory mediators by
  proteolytic cleavage, cross-desensitization of
  receptors for chemokines, release of lipoxins,
  IL-1 receptor antagonist, and other
  anti-inflamamtory mediators all facilitate the
  resolution of acute gout
• Increase vascular permeability allows entry of
  large molecules (Apo B and E) and other plasma
  proteins into synovial cavity, also contributes
  to spontaneous resolution of acute flares.

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Chronic gouty arthritis

• Typically develops in pts had gout for yrs.
• Cytokines, chemokines, proteases, oxidants also
  contribute to chronic inflammation-chronic
  synovitis, cartilage loss, bone erosion
• Even during remission of acute flares, low-grade
  synovitis persist with ongoing intra-articular
  phagocytosis of crystal by leukocytes
• Tophi on cartilage surface contribute to
  chondrolysis despite adequate Tx of both
  hyperuricemia and acute gouty attacks.
• Adherent chondrocytes phagocytoze microcrystals
  and produce active metalloproteinases
• Crystal-chondrocyte cell membrane interactions
  trigger chondrocyte activation, gene expression
  of IL-b and inducible NO synthase, NO release,
  and overexpression of matrix metalloproteinases-le
  ads to cartilage destruction
• The crystals also suppress the 1,25-VitD2-induced
  activity of alkaline phosphatase and osteocalcin,
  reduce the anabolic effects of osteoblasts,
  contributing to damage juxta-articular bone (Fig
  9)

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Summary

• Weight control, limits on red meat consumption,
  daily exercise are important foundations of
  lifestyle modification recommendation
• Pts with gout could consider using plant-derived
  w-3 fatty acids or supplements of
  eicosapentaenoic acid and decosahexanoic acid
  instead consuming fish for CV benefits
• Daily consumption of nuts and legumes as
  recommended by Harvard Healthy Eating Pyramid may
  also provide important health benefit without
  increasing risk for gout
• Daily glass of wine also benefit in contrast to
  beer or liquor consumption
• Lifestyle modifications are inexpensive and safe
  and combined with drug therapy, may result better
  control of gout.

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• Effective management of gout risk factors
  certain therapies for comorbid conditions may
  also aid
• Antihypertensive agents with uricosuric
  properties (losartan, amlodipine) have better
  risk-benefit ratio than diuretics for
  hypertension with gout
• Fenofibrate with uricosuric property have
  favorable risk-benefit ratio among pts with gout
  metabolic syndrome
• Recently elucidated molecular mechanism of renal
  urate transport has several important
  implications in condition association with high
  urate levels
• URAT1 anion exchanger provide a specific target
  of action for well-known substance affecting
  urate levels
• Genetic variation in renal transporters or
  upstream regulatory factors explain the tendency
  to develop condition associated with high urate
  levels and pts particular response to medications
• Transporter serve as target for future drug
  development
• Crystal-induced inflammation indicate gout shares
  many pathogenetic features with other. Potent
  anti-inflammatory medications may have
  therapeutic potential