Endothelial cells (ECs) are critically involved in the pathogenesis of atherosclerosis by producing inflammatory mediators, including interleukin-1 beta (IL-1β). However, its mechanism of externalisation is yet to be elucidated. This study investigates the effect of neutrophil elastase (NE) on ECs in terms of IL-1β release and explores the underlying mechanism. Human coronary artery endothelial cells (HCAEC) were treated with a combination of tumour necrosis factor-alpha and interleukin-1 alpha then incubated with NE for 2 or 6 h. ELISA, western blotting, flow cytometry and live cell imaging were used. Paraffin-embedded sections of aortic sinus lesions of apoe-/- mice were immunostained for NE and VWF (Von Willebrand factor). NE is predominantly expressed in ECs in experimental atherosclerosis. In vitro, NE caused significant IL-1β release into supernatants after 6h (579 ± 90 vs. 80 ± 19 pg/ml in control; n = 4, p < 0.0001). The release was via microparticle shedding which is significantly attenuated by neutrophil elastase inhibitor III (62.64 ± 10 vs. 579.3 ± 90 pg/ml in NE-primed cells; n = 3, p<0.05); but, the levels of IL-1β in the supernatant or lysate did not change in the presence of caspase-1 inhibitor. NE enters ECs, cleaves proIL-1β (31kDa) inside cells resulting in release of active isoforms of IL-1β in microparticles. No remarkable effects were seen on NALP3 or caspase-1 expression in EC lysates. This is the first description of the secretion of IL-1β by microvesicle shedding from ECs which is caspase-1 independent. NE is detected in the endothelium of murine atherosclerotic plaques and, therefore, this is a plausible mechanism to promote local IL-1β release in the vasculature.
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