NLRP3 and IL-1ß in macrophages as critical regulators of metabolic diseases.

The activation of the NLRP3 inflammasome leads to the autocleavage and activation of caspase-1. Caspase-1 cleaves several substrates, including the pro-inflammatory cytokine IL-1ß. Inflammation, in particular IL-1ß, has long been associated with the progression of metabolic disorders, and recent evidence suggests that the NLRP3 inflammasome plays a critical role in this inflammation. This review concentrates on the activation of NLRP3 during the development of metabolic disorders and the effect this activation has on the inflammatory state as well as the metabolic state of the cell.

Succinate is an inflammatory signal that induces IL-1ß through HIF-1α.

Macrophages activated by the Gram-negative bacterial product lipopolysaccharide switch their core metabolism from oxidative phosphorylation to glycolysis. Here we show that inhibition of glycolysis with 2-deoxyglucose suppresses lipopolysaccharide-induced interleukin-1ß but not tumour-necrosis factor-α in mouse macrophages. A comprehensive metabolic map of lipopolysaccharide-activated macrophages shows upregulation of glycolytic and downregulation of mitochondrial genes, which correlates directly with the expression profiles of altered metabolites. Lipopolysaccharide strongly increases the levels of the tricarboxylic-acid cycle intermediate succinate. Glutamine-dependent anerplerosis is the principal source of succinate, although the 'GABA (γ-aminobutyric acid) shunt' pathway also has a role. Lipopolysaccharide-induced succinate stabilizes hypoxia-inducible factor-1α, an effect that is inhibited by 2-deoxyglucose, with interleukin-1ß as an important target. Lipopolysaccharide also increases succinylation of several proteins. We therefore identify succinate as a metabolite in innate immune signalling, which enhances interleukin-1ß production during inflammation.

SCCA2 inhibits TNF-mediated apoptosis in transfected HeLa cells. The reactive centre loop sequence is essential for this function and TNF-induced cathepsin G is a candidate target.

The squamous cell carcinoma antigens, SCCA1 and SCCA2, are members of the serine protease inhibitors (serpin) superfamily and are transcribed by two tandomly arrayed genes. A number of serpins are known to inhibit apoptosis in mammalian cells. In this study we demonstrate the ability of SCCA2 to inhibit tumor necrosis factor-alpha (TNF alpha)-induced apoptosis. HeLa cells stably transfected with SCCA2 cDNA had increased percentage cell survival and reduced DNA fragmentation. We investigated if the reactive centre loop (RCL) was necessary to allow SCCA2 to inhibit TNF alpha-mediated apoptosis. The RCL amino acids (E353Q, L354G, S355A), flanking the predicted cleavage site, were mutated and the resulting SCCA2 lost both the ability to inhibit cathepsin G and to protect stably transfected cells from TNF alpha-induced apoptosis. The presence of SCCA2 caused a decrease in the activation of caspase-3 upon induction with TNF alpha but no direct inhibition of caspases by SCCA2 has been found. Expression of cathepsin G was found to be induced in HeLa cells following treatment with TNF alpha. This protease has recently been shown to have a role in apoptosis through cleavage of substrates, so maybe the relevant target for SCCA2 in this system.