LSC - 2020 - Caspase-1 activation by cigarette smoke occurs via TLR4/TRIF/caspase-8 axis leading to metabolic alterations in human macrophages

Abstract

Activation of the NLRP3 inflammasome has been related to chronic inflammation. Cigarette smoke is a risk factor for several inflammatory diseases. However, the impact of cigarette smoke on NLRP3-dependent responses is unclear. Herein we investigated the effects of cigarette smoke extract (CSE) on the NRLP3 inflammasome and its downstream effectors in human primary macrophages at baseline and in the presence of LPS, alone or with extracellular ATP. We focused on transcriptional control of inflammasome components, activation of caspases (enzymatic assay, immunofluorescence and western blot of supernatants) and cell metabolism (intracellular ATP (iATP), mitochondrial membrane potential (MMP) and lactate release as readout of glycolytic flux). We found that CSE inhibited the NLRP3 inflammasome at transcriptional level. Surprisingly, CSE activated caspase-1 via a NLRP3-independent and TLR4-TRIF-caspase-8 dependent mechanism. Previous findings showed that caspase-1 regulates cell metabolism. Therefore, we evaluated the impact of CSE and active caspase-1 on cell metabolism and energetics. We found that CSE reduced MMP, iATP and glycolysis at baseline and after stimulation with LPS. Inhibition of caspase-1 by z-YVAD-fmk restored iATP levels and glycolytic flux but not MMP. Our data show that CSE inhibits the NLRP3 inflammasome and causes dysregulation of TLR4-dependent responses by promoting a shift towards TRIF-dependent activation of caspase-8 and caspase-1. Active caspase-1 impairs glycolytic flux. Overall, herein we unveil novel mechanism of immune-metabolic alteration leading to macrophage dysfunction upon exposure to cigarette smoke.