Cigarette smoke extract inhibits the NLRP3 inflammasome in human macrophages and impairs cell metabolism via NLRP3-independent activation of Caspase-1

Abstract

The NLRP3 inflammasome is a protein complex formed by the immune sensor NLRP3, the adaptor protein ASC and pro-Caspase-1. Assembly of the inflammasome triggers activation of Caspase-1, which in turn cleaves pro-IL-1β and pro-IL-18 into their secreted forms. The role of NLRP3 in chronic obstructive pulmonary disease (COPD) is still controversial. Cigarette smoke is a major risk factor for several chronic lung diseases, including COPD. Herein we investigated the effects of cigarette smoke extract (CSE) on the NLRP3 inflammasome in human monocyte-derived macrophages (hMDM) and in THP-1 cells. More specifically, we measured the release of mature IL-1β and IL-18, the mRNA levels of pro-IL-1β, IL-18 and NLRP3 as well as the protein levels of pro-IL-1β, pro-Caspase-1, ASC and NLRP3. We also quantified the activity of Caspase-1. Intracellular ATP, mitochondrial membrane potential and extracellular lactate were measured to assess cell metabolic status. We report that CSE reduced the release of IL-1β and IL-18 induced by LPS priming followed by ATP stimulation. This was paralleled by a reduction of the IL-1β precursor and NLRP3 both at protein and mRNA levels. CSE also reduced basal expression of IL-18 mRNA. Interestingly, despite causing inhibition of the NLRP3 inflammasome, CSE increased the activity of Caspase-1 via a NLRP3-independent and Caspase-8-dependent mechanism. Active Caspase-1 led to impairment of cellular metabolism. Overall, data herein reported unveil new mechanisms of macrophage dysfunction that are triggered upon exposure to cigarette smoke.