The Anti-inflammasome Effect of Lactate and the Lactate GPR81-Receptor in Pancreatic and Liver Inflammation

Abstract

See “Lactate reduces liver and pancreatic injury in Toll-like receptor– and inflammasome-mediated inflammation via GPR81-mediated suppression of innate immunity,” by Hoque R, Farooq A, Ghani A, et al, on page 1763. See “Lactate reduces liver and pancreatic injury in Toll-like receptor– and inflammasome-mediated inflammation via GPR81-mediated suppression of innate immunity,” by Hoque R, Farooq A, Ghani A, et al, on page 1763. When organ injury occurs in the absence of an adequate oxygen supply, the glucose metabolism for energy generation is diverted to lactate, rather than pyruvate, and serum lactate levels rise. Any clinician regards this as an ominous sign for the patient's prognosis and suspect behind rising lactate levels organ ischemia or microcirculatory failure of some sort. He or she will also be aware that this is often paralleled by progressive acidification (a falling pH) and systemic inflammation. Recent cell biological studies have, indeed, suggested that lactate has a direct pro-inflammatory effect on the Toll-like receptor (TLR)/nuclear factor (NF)-κB/inflammasome signaling cascade that ultimately drives systemic inflammation via the activation of interleukin (IL)–1β.1Samuvel D.J. Sundararaj K.P. Nareika A. et al.Lactate boosts TLR4 signaling and NF-kappaB pathway-mediated gene transcription in macrophages via monocarboxylate transporters and MD-2 up-regulation.J Immunol. 2009; 182: 2476-2484Crossref PubMed Scopus (129) Google Scholar, 2Nareika A. He L. Game B.A. et al.Sodium lactate increases LPS-stimulated MMP and cytokine expression in U937 histiocytes by enhancing AP-1 and NF-kappaB transcriptional activities.Am J Physiol Endocrinol Metab. 2005; 289: E534-E542Crossref PubMed Scopus (56) Google Scholar Acute liver injury and acute pancreatitis are often associated with significant fluid loss into the interstitial space and affected patients accordingly require large intravenous fluid volumes for replacement.3IAP/APA evidence-based guidelines for the management of acute pancreatitis.Pancreatology. 2013; 13: e1-e15PubMed Scopus (0) Google Scholar For pancreatitis, a recent randomized clinical trial has unequivocally shown that Ringer lactate solution is much superior to saline infusion when used as a fluid replacement therapy.4Wu B.U. Hwang J.Q. Gardner T.H. et al.Lactated Ringer's solution reduces systemic inflammation compared with saline in patients with acute pancreatitis.Clin Gastroenterol Hepatol. 2011; 9: 710-717.e1Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar This seems to contradict the first statement on the negative role and harmful effects of lactate in systemic inflammation and organ failure. Hoque et al5Hoque R. Farooq A. Ghani A. et al.Lactate reduces liver and pancreatic injury in Toll-like receptor– and inflammasome-mediated inflammation via GPR81-mediated suppression of innate immunity.Gastroenterology. 2014; 146: 1763-1774Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar now provide an explanation for this apparent paradox in an elegant experimental study in this issue of Gastroenterology. They focused their investigation on macrophages/monocytes, which are known to be critically involved in liver and pancreatic inflammation6Perides G. Weiss E.R. Michael E.S. et al.TNF-alpha-dependent regulation of acute pancreatitis severity by Ly-6C(hi) monocytes in mice.J Biol Chem. 2011; 286: 13327-13335Crossref PubMed Scopus (39) Google Scholar, 7Sendler M. Dummer A. Weiss F.U. et al.Tumour necrosis factor alpha secretion induces protease activation and acinar cell necrosis in acute experimental pancreatitis in mice.Gut. 2013; 62: 430-439Crossref PubMed Scopus (121) Google Scholar and used animal disease models for hepatitis (lipopolysaccharide [LPS] and d-galactosamine) and pancreatitis (LPS and cerulein) to test their hypothesis that lactate has anti-inflammatory effects and mediates them through a recently discovered lactate receptor, the plasma membrane Gi protein-coupled receptor 81 (GPR81).8Cai T.Q. Ren N. Jin L. et al.Role of GPR81 in lactate-mediated reduction of adipose lipolysis.Biochem Biophys Res Commun. 2008; 377: 987-991Crossref PubMed Scopus (181) Google Scholar The authors found that low concentrations of lactate reduce organ injury in both disease models via binding to GPR81. In vitro and in vivo they could show that lactate signaling in macrophages/monocytes is dependent on GPR81 and its adaptor protein ARRB2, and that small interfering RNA blockage of the expression of either protein can regulate this process. Moreover, through this signaling pathway lactate directly inhibited the action of the NLRP3 inflammasome, which is activated via TLR-4 and the action of caspase 1. The consequences were a reduced NF-κB action and diminished conversion of pro–IL-1β to active IL-1β—with a significant beneficial effect on disease severity in both models. Simply put: Lacate at physiologic concentrations is a terrific anti-inflammatory agent and one with proven efficiency not only in human disease but also in experimental models of inflammation. This study is intriguing for several reasons. For acute pancreatitis and acute hepatitis, the early impact of inflammatory cell activation has recently been highlighted and macrophages seem to play a critical role in the disease initiation and severity.6Perides G. Weiss E.R. Michael E.S. et al.TNF-alpha-dependent regulation of acute pancreatitis severity by Ly-6C(hi) monocytes in mice.J Biol Chem. 2011; 286: 13327-13335Crossref PubMed Scopus (39) Google Scholar, 7Sendler M. Dummer A. Weiss F.U. et al.Tumour necrosis factor alpha secretion induces protease activation and acinar cell necrosis in acute experimental pancreatitis in mice.Gut. 2013; 62: 430-439Crossref PubMed Scopus (121) Google Scholar, 9Neuhofer P. Liang S. Einwachter H. et al.Deletion of IkappaBalpha activates RelA to reduce acute pancreatitis in mice through up-regulation of Spi2A.Gastroenterology. 2013; 144: 192-201Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar To activate the NLRP3 inflammasome, a 2-checkpoint model of priming and activation has been proposed in which TLR-4 (or TLR-9) represent the second checkpoint. Although the role of TLRs in bacterial infection is unequivocal, research continues on its importance for sterile inflammation, such as the early phase of pancreatitis.10Sharif R. Dawra R. Wasiluk K. et al.Impact of toll-like receptor 4 on the severity of acute pancreatitis and pancreatitis-associated lung injury in mice.Gut. 2009; 58: 813-819Crossref PubMed Scopus (123) Google Scholar, 11Szabo G. Mandrekar P. Oak S. et al.Effect of ethanol on inflammatory responses. Implications for pancreatitis.Pancreatology. 2007; 7: 115-123Abstract Full Text PDF PubMed Scopus (62) Google Scholar, 12Guenther A. Aghdassi A. Muddana V. et al.Toll-like receptor 4 polymorphisms in German and US patients are not associated with occurrence or severity of acute pancreatitis.Gut. 2010; 59: 1154-1155Crossref PubMed Scopus (13) Google Scholar The net effect of NLRP3 inflammasome activation is the proteolytic cleavage and activation of the strongly pro-inflammatory cytokine IL-1β, which regulates most of the systemic inflammatory response syndrome. Constitutive overexpression of IL-1β in the pancreas alone was found to be sufficient to induce spontaneous pancreatitis.13Marrache F. Tu S.P. Bhagat G. et al.Overexpression of interleukin-1beta in the murine pancreas results in chronic pancreatitis.Gastroenterology. 2008; 135: 1277-1287Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar Reducing the action of the NLRP3 inflammasome would therefore be expected to have a beneficial effect on either the onset or the severity of the disease. Inflammasomes assemble to key signaling platforms that detect pathogenic microorganisms and sterile stressors before leading to downstream activation of IL-1β. The 2-step model of priming and activation of the canonical NLRP3 inflammasome predicts that the basal expression of its components is insufficient for its activation in resting cells. An initial activating signal transmitted via pattern recognition receptor on the cell surface results in up-regulation of the NLRP3 components and represents the priming reaction. Only thereafter can the NLRP3 inflammasome be activated via LPS stimulation of its receptor, the TLR-4, which then induces activation of TRIF, the binding of NLRP3 to ASC, and the subsequent caspase 1–dependent induction of pro–IL-β1 expression and its activation. Although LPS-stimulated TLR-4 activation is probably the best investigated mechanisms of NLRP3 inflammasome activation, other mechanisms involving bacterial membrane components, the release of cathepsin B from lysosomes, low potassium levels, high intracellular calcium concentrations, or the release of reactive oxygen species from mitochondria have all been implicated in this process (Figure 1). Lactate has long been regarded as critical for energy (ATP) generation when oxygen is in short supply (anaerobic glycolysis) and rising levels in the blood were regarded as an early sign of microcirculatory failure. It was only recently discovered that lactate is also a signaling molecule and a ligand for a dedicated lactate receptor.8Cai T.Q. Ren N. Jin L. et al.Role of GPR81 in lactate-mediated reduction of adipose lipolysis.Biochem Biophys Res Commun. 2008; 377: 987-991Crossref PubMed Scopus (181) Google Scholar The GPR81 is a 7-transmembrane receptor and binding of lactate leads to recruitment of the intracellular adaptor ARRB2 before downstream signaling is transmitted. From Hoque et al,5Hoque R. Farooq A. Ghani A. et al.Lactate reduces liver and pancreatic injury in Toll-like receptor– and inflammasome-mediated inflammation via GPR81-mediated suppression of innate immunity.Gastroenterology. 2014; 146: 1763-1774Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar we learn that this signal ultimately reduces the activation of the NLRP3 inflammasome, with all its beneficial effects on macrophages/monocytes and 2 experimental inflammatory diseases of the liver and pancreas. The exact mechanism through which GPR81 and ARRB2 counteract the NLRP3 and TLR pathways is currently unknown (Figure 1) and will require further study. Lactate possesses several chemical and metabolic effects outside this signaling cascade and these include a reduction in pH. A lower pH, on the other hand, could have several unwanted consequences as far as the pancreas is concerned: The pancreatic acinar cell stores its proteases in acidic secretory vesicles and when pancreatitis is induced the pH in intracellular compartment becomes more acidic and a premature protease activation begins.14Kruger B. Lerch M.M. Tessenow W. Direct detection of premature protease activation in living pancreatic acinar cells.Lab Invest. 1998; 78: 763-764PubMed Google Scholar For this premature intracellular protease activation, 2 mechanisms have been proposed and involve trypsinogen activation by cathepsin B or trypsinogen autoactivation, both of which are pH dependent.15Hirano T. Saluja A. Ramarao P. et al.Apical secretion of lysosomal enzymes in rabbit pancreas occurs via a secretagogue regulated pathway and is increased after pancreatic duct obstruction.J Clin Invest. 1991; 87: 865-869Crossref PubMed Scopus (64) Google Scholar, 16Halangk W. Lerch M.M. Brandt-Nedelev B. et al.Role of cathepsin B in intracellular trypsinogen activation and the onset of acute pancreatitis.J Clin Invest. 2000; 106: 773-781Crossref PubMed Scopus (449) Google Scholar Chloroquine, an agent that raises the intracellular pH, reduces premature zymogen activation and ameliorates the course of experimental pancreatitis.17Lerch M.M. Saluja A.K. Dawra R. et al.The effect of chloroquine administration on two experimental models of acute pancreatitis.Gastroenterology. 1993; 104: 1768-1779Abstract Full Text PDF PubMed Scopus (0) Google Scholar A proton pump, the vacuolar ATPase, regulates zymogen activation in the acinar cell18Gorelick F.S. Shugrue C.A. Kolodecik T.R. et al.Vacuolar adenosine triphosphatase and pancreatic acinar cell function.J Gastroenterol Hepatol. 2006; 21: S18-S21Crossref PubMed Scopus (1) Google Scholar and an acid challenge sensitizes the pancreatic acinar cell to secretagogue-induced zymogen activation and injury.19Bhoomagoud M. Jung T. Atladottir J. et al.Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats.Gastroenterology. 2009; 137: 1083-1092Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar All of these observations would suggest that lactate, at concentrations that reduce pH or lead to supraphysiologic lactate concentrations, would have a negative effect on the disease onset or severity. However, the experimental concentrations used by Hoque et al5Hoque R. Farooq A. Ghani A. et al.Lactate reduces liver and pancreatic injury in Toll-like receptor– and inflammasome-mediated inflammation via GPR81-mediated suppression of innate immunity.Gastroenterology. 2014; 146: 1763-1774Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar would only result in lactate levels of 0.3 mmol/L, which are well within the physiologic range and therefore unlikely to affect pH or have pathologic effects. Beside its role in NF-κB and IL-1β activation, the NRLP3 pathway is closely involved in another essential pathophysiologic mechanism such as the cellular calcium homeostasis. Full activation of the NRLP3 inflammasome requires decreased extracellular K+ concentrations resulting in decreased osmotic pressure with cell swelling. Cell swelling, in turn, induces a regulatory volume decrease response through transient receptor potential cation channels (TRPM2/7 and TRPV2) that trigger intracellular Ca++ mobilization.20Latz E. Xiao T.S. Stutz A. Activation and regulation of the inflammasomes.Nat Rev Immunol. 2013; 13: 397-411Crossref PubMed Scopus (1903) Google Scholar The mobilized Ca++ has many molecular targets including, in the pancreas, the premature activation of proteases, a hallmark of pancreatitis. Whether a NRLP3 inflammasome triggered disease mechanism and its lactate-induced, GPR81-mediated down-regulation is operative in the acinar cell itself is presently unknown. The study of Hoque et al5Hoque R. Farooq A. Ghani A. et al.Lactate reduces liver and pancreatic injury in Toll-like receptor– and inflammasome-mediated inflammation via GPR81-mediated suppression of innate immunity.Gastroenterology. 2014; 146: 1763-1774Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar have shown this regulation to apply to inflammatory cell, specifically macrophages/monocytes, and the required protein machinery may not be expressed in the epithelial cells of the liver or pancreas. In both models investigated, the co-stimulatory signal for activation of the inflammasome was LPS. However, acute pancreatitis and hepatitis, at least in the initiating phase where fluid resuscitation with Ringer lactate is clinically indicated, are considered to be sterile, that is, non-bacterial, inflammatory processes. Although previous clinical trials suggest that treatment with Ringer lactate is beneficial in this early disease phase and can reduce the systemic pro-inflammatory response,4Wu B.U. Hwang J.Q. Gardner T.H. et al.Lactated Ringer's solution reduces systemic inflammation compared with saline in patients with acute pancreatitis.Clin Gastroenterol Hepatol. 2011; 9: 710-717.e1Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar it would be interesting to learn whether disease models of pancreatitis and hepatitis, that do not require bacterial LPS as co-stimulatory signal, also benefit from a lactate GPR81–mediated inhibition of the NLPR3 inflammasome. If this were the case, the pathophysiologic basis for the clinical use of Ringer lactate would be even sounder. At present, Paracelsus' principle that “the dose makes the poison” remains a consideration for the clinical use of Ringer lactate in patients with liver and pancreatic diseases. The hepatotoxic properties of lactate in animal models and the caveats from clinical trials that seem to confirm this potentially negative effect in liver disease21Karmaniolou I.I. Theodoraki K.A. Orfanos N.F. et al.Resuscitation after hemorrhagic shock: the effect on the liver–a review of experimental data.J Anesth. 2013; 27: 447-460Crossref PubMed Scopus (16) Google Scholar warrants some caution. In the absence of data from controlled trials and in light of the study by Hoque et al5Hoque R. Farooq A. Ghani A. et al.Lactate reduces liver and pancreatic injury in Toll-like receptor– and inflammasome-mediated inflammation via GPR81-mediated suppression of innate immunity.Gastroenterology. 2014; 146: 1763-1774Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar a current recommendation would encourage the use of Ringer lactate solution where rapid fluid replacement is required in inflammatory disease of the liver and pancreas. After the initial disease phase and before serum lactate levels rise above physiologic levels or the pH is reduced, the fluid replacement should probably be continued with other crystalloids to avoid impairing liver function and disturbed coagulation. Hoque et al5Hoque R. Farooq A. Ghani A. et al.Lactate reduces liver and pancreatic injury in Toll-like receptor– and inflammasome-mediated inflammation via GPR81-mediated suppression of innate immunity.Gastroenterology. 2014; 146: 1763-1774Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar have given us a much better understanding of how lactate works, through which mechanisms it transmits its anti-inflammatory function in monocytes, and why inflammatory disorders of the liver and pancreas may benefit from its clinical use. Lactate Reduces Liver and Pancreatic Injury in Toll-Like Receptor– and Inflammasome-Mediated Inflammation via GPR81-Mediated Suppression of Innate ImmunityGastroenterologyVol. 146Issue 7PreviewThe NACHT, LRR, and pyrin domain–containing protein 3 (NLRP3) inflammasome induces inflammation in response to organ injury, but little is known about its regulation. Toll-like receptors (TLRs) provide the first signal required for activation of the inflammasome and stimulate aerobic glycolysis to generate lactate. We examined whether lactate and the lactate receptor, Gi-protein–coupled receptor 81 (GPR81), regulate TLR induction of signal 1 and limit inflammasome activation and organ injury. Full-Text PDF Covering the CoverGastroenterologyVol. 146Issue 7PreviewAbnormalities in ion channels, such as voltage-gated sodium channels, that are directly involved in gastrointestinal motility and visceral pain may be a genetic factor in the pathogenesis of irritable bowel syndrome (IBS). One of these, the tetrodotoxin-resistant sodium channel NaV1.5 (encoded by SCN5A), is expressed in human smooth muscle cells and the interstitial cells of Cajal, the gastrointestinal pacemakers that provide an electrical stimulus for contraction. Inhibition of NaV1.5, which is also expressed in human myocardium, by the anti-anginal agent, ranolazine, is associated with constipation and patients with arrhythmia-predisposing mutations in SCN5A are more likely to have gastrointestinal symptoms compared with patients with other arrhythmia-associated ion channel defects. Full-Text PDF