BACKGROUND & AIMS The 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, GPR81, regulate TLR induction of signal 1 and limit inflammasome activation and organ injury. METHODS Primary mouse macrophages and human monocytes were incubated with TLR4 agonists and lactate and assayed for levels of pro-IL1β, NLRP3, and CASP1; release of IL1β; and activation of NFκB and caspase 1. Small interfering (si)RNAs were used to reduce levels of GPR81andARRB2, and an NFκB luciferase reporter transgene was transfected in RAW 264.7 cells. Cell lysates were analyzed by immunoprecipitation with an antibody against GPR81. Acute hepatitis was induced in C56BL/6N mice by administration of lipopolysaccharaide (LPS) and D-galactosamine. Acute pancreatitis was induced by administration of LPS and caerulein. Some mice were given intraperitoneal injections of sodium lactate or siRNA against Gpr81. Activation of NFκB in tissue macrophages was assessed in mice that express a reporter transgene. RESULTS In macrophages and monocytes, increasing concentrations of lactate reduced TLR4-mediated induction of Il1B, Nlrp3, and Casp1; activation of NFκB; release of IL1β; and cleavage of CASP1. GPR81 and ARRB2 physically interacted and were required for these effects. Administration of lactate reduced inflammation and organ injury in mice with immune hepatitis; this reduction required Gpr81 dependence in vivo. Lactate also prevented activation of NFκB in macrophages of mice, and when given following injury, reduced the severity of acute pancreatitis and acute liver injury. CONCLUSIONS Lactate negatively regulates TLR induction of the NLRP3 inflammasome and production of IL1β, via ARRB2 and GPR81. Lactate could be a promising immunomodulatory therapy for patients with acute organ injury.

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Accumulation of neurotoxic βamyloid (Aβ) is a major hallmark of Alzheimer's disease (AD)1. Formation of Aβ is catalyzed by γsecretase, a protease with numerous substrates2,3. Little is known about the molecular mechanisms that confer substrate specificity on this potentially promiscuous enzyme. Knowledge of the mechanisms underlying its selectivity is critical for the development of clinically effective γ-secretase inhibitors that can reduce Aβ formation without impairing cleavage of other γ-secretase substrates, especially Notch, which is essential for normal biological functions3,4. Here we report the discovery of a novel γ-secretase activating protein (gSAP), which dramatically and selectively increases Aβ production through a mechanism involving its interactions with both γsecretase and its substrate, the amyloid precursor protein C-terminal fragment (APP-CTF). gSAP does not interact with Notch nor does it affect its cleavage. Recombinant gSAP stimulates Aβ production in vitro. Reducing gSAP levels in cell lines decreases Aβ levels. Knockdown of gSAP in a mouse model of Alzheimers disease reduces levels of Aβ and plaque development. gSAP represents a new type of γ-secretase regulator that directs enzyme specificity by interacting with a specific substrate. We demonstrate that imatinib, an anti-cancer drug previously found to inhibit Aβ formation without affecting Notch cleavage5, achieves its Aβ-lowering effect by preventing gSAP interaction with the γ-secretase substrate, APP-CTF. Thus, gSAP can serve as an Aβ-lowering therapeutic target without affecting other key functions of γ-secretase.