Moderate Hypoxia Potentiates Interleukin-1β Production in Activated Human Macrophages

Rationale: Inflammation drives atherogenesis. Animal and human studies have implicated interleukin-1 beta (IL-1 beta) in this disease. Moderate hypoxia, a condition that prevails in the atherosclerotic plaque, may conspire with inflammation and contribute to the evolution and complications of atherosclerosis through mechanisms that remain incompletely understood. Objective: This study investigated the links between hypoxia and inflammation by testing the hypothesis that moderate hypoxia modulates IL-1 beta production in activated human macrophages. Methods and Results: Our results demonstrated that hypoxia enhances pro-IL-1 beta protein, but not mRNA, expression in lipopolysaccharide-stimulated human macrophages. We show that hypoxia limits the selective targeting of pro-IL-1 beta to autophagic degradation, thus prolonging its half-life and promoting its intracellular accumulation. Furthermore, hypoxia increased the expression of NLRP3, a limiting factor in NLRP3 inflammasome function, and augmented caspase-1 activation in lipopolysaccharide-primed macrophages. Consequently, hypoxic human macrophages secreted higher amounts of mature IL-1 beta than did normoxic macrophages after treatment with crystalline cholesterol, an endogenous danger signal that contributes to atherogenesis. In human atherosclerotic plaques, IL-1 beta localizes predominantly to macrophage-rich regions that express activated caspase-1 and the hypoxia markers hypoxia-inducible factor 1 alpha and hexokinase-2, as assessed by immunohistochemical staining of carotid endarterectomy specimens. Conclusions: These results indicate that hypoxia potentiates IL-1 beta expression in cultured human macrophages and in the context of atheromata, therefore unveiling a novel proinflammatory mechanism that may participate in atherogenesis.