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[Extracellular histones are involved in lipopolysaccharide-induced alveolar macrophage injury by activating the TWIK2-NLRP3 pathway].

OBJECTIVE
To explore the role and mechanism of extracellular histones involved in lipopolysaccharide (LPS)-induced alveolar macrophage injury.


METHODS
The mouse alveolar macrophage cell line (MH-S) was cultured in vitro and passaged, and the cells were cultured to 80% of cells for cell proliferation. The cells were stimulated with 1 mg/L LPS for 3 hours and 50 mg/L exogenous histones for 3, 6, 12, and 24 hours, respectively (LPS+histones 3, 6, 12, 24 h groups), and other groups included phosphate buffered saline (PBS) control group (PBS group), LPS alone stimulation group (LPS group), the exogenous histones alone stimulation group (histones group) and heparin pretreatment histones group (heparin+LPS+histones group). The cells in each group were challenged with different reagent, the expression of lactate dehydrogenase (LDH) and inflammatory factors in the supernatant were detected by enzyme linked immunosorbent assay (ELISA), and the change of intracellular K+ concentration was detected by FluxORTM II green potassium channel. The proteins such as potassium channel protein (TWIK2), inflammasome (NLRP3), and apoptosis associated speck like protein containing a CARD (ASC) were determined by Western Blot.


RESULTS
Compared with the PBS group, the levels of LDH and inflammatory factors such as interleukin (IL-1β, IL-18) and tumor necrosis factor-α (TNF-α) were significantly increased after LPS stimulation group. Compared with the LPS group, the levels of LDH and inflammatory factors were significantly increased after the treatment with exogenous histones, and reached a peak after 3 hours of the histones stimulation [LDH (U/L): 123.10±1.83 vs. 85.32±1.66, IL-1β (mg/L): 40.75±2.60 vs. 18.78±1.37, IL-18 (mg/L): 49.94±2.45 vs. 30.19±1.82, TNF-α (mg/L): 36.51±1.56 vs. 20.84±1.61, all P < 0.01]. Western Blot results showed that compared with the LPS group, NLRP3, ASC and TWIK2 protein expression were significantly up-regulated in the LPS+histones group (NLRP3/GAPDH: 0.80±0.02 vs. 0.57±0.02, ASC/GAPDH: 0.57±0.02 vs. 0.38±0.01, TWIK2/GAPDH: 0.65±0.01 vs. 0.41±0.01, all P < 0.01), and the expression of the above proteins were significantly down-regulated after heparin pretreatment (NLRP3/GAPDH: 0.28±0.02 vs. 0.80±0.02, ASC/GAPDH: 0.25±0.02 vs. 0.57±0.02, TWIK2/GAPDH: 0.35±0.01 vs. 0.65±0.01, all P < 0.01), indicating that histones could activate NLRP3 through TWIK2 to participate in inflammatory reaction. In addition, intracellular K+ concentration in LPS+histones group decreased significantly compared with the LPS group (fluorescence intensity: 35.48±2.53 vs. 83.92±3.11, P < 0.01). Compared with LPS+histones group, K+ concentration increased significantly after pretreatment with heparin (fluorescence intensity: 72.10±1.78 vs. 35.48±2.53, P < 0.01), indicating that extracellular histones could cause K+ massive efflux through TWIK2, and thus mediate NLRP3 activation and participate in inflammatory injury of alveolar macrophages.


CONCLUSIONS
Extracellular histones can cause inflammatory damage in alveolar macrophages, and its mechanism may be related to the activation of NLRP3 by extracellular histones activation of TWIK2 channel to promote K+ efflux.