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SUMMARY Obesity-induced inflammation originating from expanding adipose tissue interferes with insulin sensitivity. Important metabolic effects have been recently attributed to IL-1β and IL-18, two members of the IL-1 family of cytokines. Processing of IL-1β and IL-18 requires cleavage by caspase-1, a cysteine protease regulated by a protein complex called the inflammasome. We demonstrate that the inflamma-some/caspase-1 governs adipocyte differentiation and insulin sensitivity. Caspase-1 is upregulated during adipocyte differentiation and directs adipocytes toward a more insulin-resistant phenotype. Treatment of differentiating adipocytes with recombinant IL-1β and IL-18, or blocking their effects by inhibitors, reveals that the effects of caspase-1 on adipocyte differentiation are largely conveyed by IL-1β. Caspase-1 and IL-1β activity in adipose tissue is increased both in diet-induced and genetically induced obese animal models. Conversely, mice deficient in caspase-1 are more insulin sensitive as compared to wild-type animals. In addition, differentiation of preadipocytes isolated from caspase-1−/− or NLRP3−/− mice resulted in more metabolically active fat cells. In vivo, treatment of obese mice with a caspase-1 inhibitor significantly increases their insulin sensitivity. Indirect calorimetry analysis revealed higher fat oxidation rates in caspase-1−/− animals. In conclusion, the inflammasome is an important regulator of adipocyte function and insulin sensitivity, and caspase-1 inhibition may represent a novel therapeutic target in clinical conditions associated with obesity and insulin resistance.

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IL-1␤-converting enzyme (ICE; caspase-1) is the intracellular protease that cleaves the precursors of IL-1␤ and IL-18 into active cytokines. In the present study, the effect of ICE deficiency was evaluated during experimental colitis in mice. In acute dextran sulfate sodium-induced colitis, ICE-deficient (ICE KO) mice exhibited a greater than 50% decrease of the clinical scores weight loss, diarrhea, rectal bleeding, and colon length, whereas daily treatment with IL-1 receptor antagonist revealed a modest reduction in colitis severity. To further characterize the function of ICE and its role in intestinal inflammation, chronic colitis was induced over a 30-day time period. During this chronic time course, ICE KO mice exhibited a near complete protection, as reflected by significantly reduced clinical scores and almost absent histological signs of colitis. Consistently, colon shortening occurred only in dextran sulfate sodium-exposed wild-type mice but not in ICE KO mice. Protection was accompanied by reduced spontaneous release of the proinflammatory cytokines IL-18, IL-1␤, and IFN-␥ from total colon cultures. In addition, flow cytometric analysis of isolated mesenteric lymph node cells revealed evidence of reduced cell activation in ICE KO mice as evaluated by surface expression of CD3 CD69 and CD4 CD25. We conclude that inhibition of ICE represents a novel anti-inflammatory strategy for intestinal inflammation.

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Interleukin (IL)-18, formerly called interferon ␥ (IFN-␥)-inducing factor, is biologically and structurally related to IL-1␤. A comparison of gene expression, synthesis, and processing of IL-18 with that of IL-1␤ was made in human peripheral blood mononuclear cells (PBMCs) and in human whole blood. Similar to IL-1␤, the precursor for IL-18 requires processing by caspase 1. In PBMCs, mature but not precursor IL-18 induces IFN-␥; in whole human blood stimulated with endotoxin, inhibition of caspase 1 reduces IFN-␥ production by an IL-1␤-independent mechanism. Unlike the precursor for IL-1␤, precursor for IL-18 was expressed constitutively in PBMCs and in fresh whole blood from healthy human donors. Western blotting of endotoxinstimulated PBMCs revealed processed IL-1␤ in the supernatants via an caspase 1-dependent pathway. However, in the same supernatants, only unprocessed precursor IL-18 was found. Unexpectedly, precursor IL-18 was found in freshly obtained PBMCs and constitutive IL-18 gene expression was present in whole blood of healthy donors, whereas constitutive IL-1␤ gene expression is absent. Similar to human PBMCs, mouse spleen cells also constitutively contained the preformed precursor for IL-18 and expressed steady-state IL-18 mRNA, but there was no IL-1␤ protein and no spontaneous gene expression for IL-1␤ in these same preparations. We conclude that although IL-18 and IL-1␤ are likely members of the same family, constitutive gene expression, synthesis, and processing are different for the two cytokines.

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