Particulate air pollution exaggerates diet-induced insulin resistance through NLRP3 inflammasome in mice

Air particulate matter 2.5 (PM_2.5) has been demonstrated to exaggerate insulin resistance in both human and animal studies. However, the exact molecular mechanisms remain elusive. This study sought to assess the role of NLRP3 inflammasome in PM_2.5 exposure-induced insulin resistance and explore the underlying mechanisms. Wild-type (WT), Nlrp3^-/-, Tlr4^Lps-d, or Nrf2^-/- mice, on a normal diet or high-fat diet (HFD), were exposed to PM_2.5 or filtered air (FA) in a whole-body exposure facility. Priming (first signal) and assembly (second signal) of NLRP3 inflammasome activation were assessed by measuring the transcription of Nlrp3/Il-1β and detecting the activity of caspase-1 and secretion of IL-1β. We found PM_2.5 exposure exaggerated insulin resistance and increased IL-1β production in the HFD-fed WT mice, but not Nlrp3^-/- mice. Gene expressions of Nlrp3 and Il-1β in the lungs and peritoneal macrophages were upregulated in WT mice exposed to PM_2.5. When stimulated with LPS (first signal) or monosodium urate (second signal), PM_2.5 exposure was able to enhance the activity of caspase-1 and IL-1β secretion, suggesting that PM_2.5 may serve as a stimulus of either the first or second signal for NLRP3 inflammasome activation. Effects of PM_2.5 on caspase-1 activation and IL-1β secretion were partially blocked in Tlr4^Lps-d mice. Reactive oxygen species (ROS), co-localization of NLRP3 and mitochondria, and secondary lysosomes in macrophages were increased after PM_2.5 exposure, while deficiency of antioxidant gene Nrf2 in mice significantly enhanced PM_2.5-induced secretion of IL-1β. Imaging flow cytometry and transmission electron microscopy demonstrated an engulfment of PM_2.5 particles by macrophages, while suppression of phagocytosis by cytochalasin D abolished PM_2.5-induced transcription of Nlrp3/Il-1β. Our results demonstrated a critical role of NLRP3 inflammasome in PM_2.5 exaggerated insulin resistance, and multiple pathways in the first and second signals of NLRP3 inflammasome activation may be involved.