Hypoxia, which commonly associates with respiratory and cardiovascular diseases, provokes an acute inflammatory response. However, underlying mechanisms are not well understood. Here we report that red blood cells (RBCs) induce hypoxic inflammation by producing reactive oxygen species (ROS) that diffuse to endothelial cells of adjoining blood vessels. Real-time fluorescence imaging of rat and mouse lungs revealed that in the presence of RBC-containing vascular perfusion, hypoxia increased microvascular ROS, and cytosolic Ca 2؉ , leading to Pselectin-dependent leukocyte recruitment. However, in the presence of RBCfree perfusion, all hypoxia-induced responses were completely inhibited. Because hemoglobin (Hb) autoxidation causes RBC superoxide formation that readily dismutates to H 2 O 2 , hypoxia-induced responses were lost when we inhibited Hb autoxidation with CO or nitrite, or when the H 2 O 2 inhibitor, catalase was added to the infusion to neutralize the RBC-derived ROS. By contrast, perfusion with RBCs from BERK-trait mice that are more susceptible to Hb autoxidation and to hypoxia-induced superoxide production enhanced the hypoxia-induced responses. We conclude that in hypoxia, increased Hb autoxidation augments superoxide production in RBCs. Conse
IntroductionHypoxia is associated with inflammatory diseases, such as acute lung injury (ALI) or cardiovascular conditions with low cardiac output. It is therefore capable of inducing an inflammatory response 1 that could further impair organ function through leukocyte accumulation and increased capillary leak. 2 However, the mechanisms directly linking hypoxia to inflammation remain unclear. Possible mechanisms include hypoxia-induced activation of the transcription factor, hypoxia-inducible factor-1 (HIF-1), triggering the expression of proinflammatory genes, 3 and increased production of mitochondrial reactive oxygen species (ROS), which activate endothelial secretion of leukocyte adhesion receptors, thereby promoting the inflammatory response. 4,5 As reviewed, 6 the processes that are generally considered a source for ROS include the mitochondrial, the xanthine/ xanthine oxidase and the nicotinamide adenine dinucleotide phosphate oxidase systems.Another potential source for ROS that is frequently neglected is the red blood cell (RBC). RBCs contain the largest pool of O 2 and are in intimate contact with lung capillaries. The RBC source for ROS is particularly relevant under hypoxic conditions, which causes a dramatic increase in the rate of hemoglobin (Hb) autoxidation, 7,8 resulting in increased RBC production of superoxide and hydrogen peroxide. It has been shown that, although RBCs contain an extensive antioxidant system, including superoxide dismutase, catalase, glutathione peroxidase, and thioredoxin, under hypoxic conditions RBC-derived ROS can damage the cell membrane 9 and leak of the RBC. 10 Under hypoxic conditions, these effects of RBC-generated ROS have been attributed to the increased ROS production coupled with the increased affinity of Hb for...
