Pathophysiologic Role of Nitric Oxide (NO) in Sickle Cell Acute Chest Syndrome (ACS) • 796

Abstract

NO, a potent cyto-protective signalling molecule, is produced by endothelial cells (ECs), and down-regulates EC expression of selective adhesion molecules. Recently, two case reports have suggested a role for NO inhalation in the treatment of ACS in patients with sickle cell disease (SCD). The data presented suggests a mechanism for the therapeutic use of NO in alleviating ACS. We have previously shown that hypoxia increased sickle red blood cell (RBC)-EC adhesion and that this effect was mediated via up-regulation of vascular cell adhesion molecule (VCAM)-1 on the EC surface. We now demonstrate that in ACS there is an increase in circulating soluble VCAM-1 (sVCAM-1) and a decrease in NO. We evaluated plasma levels of NO metabolites (assayed as nitrate and nitrite) and sVCAM-1 in 18 age-matched controls, 34 children with SCD in steady state, 13 patients in vaso-occlusive crisis (VOC) and 5 in ACS. Nitrate levels in controls, and patients with SCD in steady state, VOC and ACS were 22.4±1.6 (mean ± SE), 20.5±1.3, 14.3±0.4 and 11.1±1.4 μM, respectively. While there was no difference between the control group and the SCD patients in steady state, nitrate levels were significantly reduced in SCD patients in VOC and ACS with the greatest decrease in ACS (P<0.001). Circulatory levels of sVCAM-1 in controls, and patients with SCD in steady state, VOC and ACS were 491±90, 844±191, 856±64, and 1169±213 ng/ml, respectively. sVCAM-1 levels were significantly elevated in steady state, VOC and ACS (P<0.001) when compared to controls with the greatest increase occurring in ACS (significant increase over both steady state and VOC, P<0.02 to 0.001). Additionally, in in vitro experiments, the NO donor SIN-1 (n=6) inhibited hypoxia-induced sickle RBC adhesion to human lung micro-vascular ECs in a concentration-dependent manner with 57 and 73% inhibition noted at 10 and 100 μM, respectively. This effect was mediated via a concomitant down-regulation of hypoxia-induced VCAM-1 expression (66 and 85% inhibition). Our data shows that (1) hypoxia-induced VCAM-1 up-regulation on ECs occurs in vivo i.e. in ACS; (2) NO production is markedly decreased in ACS and (3) NO by attenuating hypoxia-induced RBC-EC adhesion may potentially modulate the clinical course of acute chest syndrome in patients with sickle cell disease.