Endothelial cell expression of haemoglobin α regulates nitric oxide signalling

Abstract

This study presents a new model for the regulation of nitric oxide signalling in endothelial cells; the oxidation state of endothelial haemoglobin α, controlled by cytochrome B5 reductase 3, regulates nitric oxide bioactivity and diffusion towards its vascular smooth muscle targets. This manuscript provides evidence for a new model of the regulation of nitric oxide signalling in myoendothelial gap junctions. The oxidation state of endothelial haemoglobin α, controlled by cytochrome b5 reductase 3, regulates nitric oxide bioavailability and diffusion towards its vascular smooth muscle targets by promoting either the formation of vasodilator S-nitrosothiols or the scavenging of nitric oxide. This endothelial regulation of nitric oxide modulates the effects of agents involved in the control of vascular tone, such as α-adrenergic agonists or substances that evoke endothelium-dependent relaxations. Models of unregulated nitric oxide (NO) diffusion do not consistently account for the biochemistry of NO synthase (NOS)-dependent signalling in many cell systems1,2,3. For example, endothelial NOS controls blood pressure, blood flow and oxygen delivery through its effect on vascular smooth muscle tone4, but the regulation of these processes is not adequately explained by simple NO diffusion from endothelium to smooth muscle3,5. Here we report a new model for the regulation of NO signalling by demonstrating that haemoglobin (Hb) α (encoded by the HBA1 and HBA2 genes in humans) is expressed in human and mouse arterial endothelial cells and enriched at the myoendothelial junction, where it regulates the effects of NO on vascular reactivity. Notably, this function is unique to Hb α and is abrogated by its genetic depletion. Mechanistically, endothelial Hb α haem iron in the Fe3+ state permits NO signalling, and this signalling is shut off when Hb α is reduced to the Fe2+ state by endothelial cytochrome b5 reductase 3 (CYB5R3, also known as diaphorase 1)6. Genetic and pharmacological inhibition of CYB5R3 increases NO bioactivity in small arteries. These data reveal a new mechanism by which the regulation of the intracellular Hb α oxidation state controls NOS signalling in non-erythroid cells. This model may be relevant to haem-containing globins in a broad range of NOS-containing somatic cells7,8,9,10,11,12,13.