Hemolysis in sickle cell mice causes pulmonary hypertension due to global impairment in nitric oxide bioavailability

Abstract

Pulmonary hypertension is a highly prevalent complication of sickle cell disease and is a strong risk factor for early mortality. However, the pathophysiologic mechanisms leading to pulmonary vasculopathy remain unclear. Transgenic mice provide opportunities for mechanistic studies of vascular pathophysiology in an animal model. By microcardiac catheterization, all mice expressing exclusively human sickle hemoglobin had pulmonary hypertension, profound pulmonary and systemic endothelial dysfunction, and vascular instability characterized by diminished responses to authentic nitric oxide (NO), NO donors, and endothelium-dependent vasodilators and enhanced responses to vasoconstrictors. However, endothelium-independent vasodilation in sickle mice was normal. Mechanisms of vasculopathy in sickle mice involve global dysregulation of the NO axis: impaired constitutive nitric oxide synthase activity (NOS) with loss of endothelial NOS (eNOS) dimerization, increased NO scavenging by plasma hemoglobin and superoxide, increased arginase activity, and depleted intravascular nitrite reserves. Light microscopy and computed tomography revealed no plexogenic arterial remodeling or thrombi/ emboli. Transplanting sickle marrow into wild-type mice conferred the same phenotype, and similar pathobiology was observed in a nonsickle mouse model of acute alloimmune hemolysis. Although the time course is shorter than typical pulmonary hypertension in human sickle cell disease, these results demonstrate that hemolytic anemia is sufficient to produce endothelial dysfunction and global dysregulation of NO. IntroductionPulmonary hypertension is a highly prevalent complication of sickle cell disease that is associated with early mortality. [1][2][3][4] The putative mechanisms responsible for pulmonary hypertension are the focus of intense current research and remain incompletely defined. 5 One mechanism proposed is that hemolytic anemia and decompartmentalization of erythrocyte hemoglobin and arginase into plasma leads to nitric oxide (NO) scavenging and arginine degradation, limiting the bioavailability of NO. 3,[6][7][8][9][10] This process would ultimately lead to acute changes in pulmonary vascular endothelial and vasomotor function and chronic pathologic intimal and smooth muscle hyperplasia. Alternatively, chronic lung disease caused by recurrent pulmonary infarction, pneumonia, acute chest syndrome, and thromboembolism could lead to chronic hypoxemia, pulmonary fibrosis, thrombotic vascular obliteration, and secondary pulmonary hypertension. [11][12][13][14][15] Pulmonary hypertension could also arise from chronic hypoxia or chronic nocturnal hypoxia. [16][17][18] Additional factors contributing to pulmonary hypertension include right-heart failure secondary to a chronic high cardiac output as compensation for chronic anemia and left ventricular diastolic dysfunction secondary to cardiac tissue microinfarction and/or iron overload. [19][20][21][22] In short, is exposure to hemoglobin S (HbS) erythrocytes sufficient to cause pulmonar...

Cardiac involvement in sickle β-thalassemia

“…The recent observation that PHT is a common finding in various forms of chronic hemolytic anemias, both hereditary and acquired ones [37], raised the concept that PHT should somehow be related to hemolysis. Indeed, intravascular hemolysis is now well recognized to reduce NO bioavailability through different mechanisms [38]. Reduced NO availability leads to a continuous process of vascular deregulation, consisting of increased in situ thrombogenicity, vasoconstriction, and, particularly in the case of sickling syndromes, increased erythrocyte adhesion, delayed transient time, and hence increased predisposition to sickling [39].…”

Conjunctival and pulmonary hemodynamic properties in sickle cell disease subjects with and without pulmonary hypertension

“…One possible explanation could be higher microvascular network resistance due to increased adherence of red and white blood cells, and platelet to the endothelium and to each other resulting from hypoxia in the SCD subjects with PH 17. Interestingly, the conjunctival D was within the normal range in the SCD subject with PH in this study despite lower than normal conjunctival V. This finding may suggest a diminished vasodilatory response to hypoxia in this SCD subject due to vasculopathy 18, increased consumption of nitric oxide, or decreased activity of soluble guanylate cyclase and other downstream messengers in vascular smooth muscle 19. Future studies in a larger population are required to investigate the relationships between severity of PH and microvascular velocity in SCD subjects.…”

Endothelial nitric oxide synthase and nitric oxide regulate endothelial tissue factor expression in vivo in the sickle transgenic mouse

“…For example, eNOS uncoupling leads not only to loss of enzyme NO-producing activity but also to production of superoxide rather than NO. This can occur due to several events: enzyme oxidation [21]; deficiency of co-factor NADPH [22]; deficiency of arginine due to diet, release of arginase from red cells [23,24], or metabolic changes accompanying renal disease [25]; deficiency of tetrahydrobiopterin; presence of the elevated levels of the endogenous inhibitor asymmetric dimethylarginine [26]; and abnormal eNOS monomerization [27] caused by abnormal enzyme S-nitrosylation [28]. Notably, tetrahydrobiopterin can be relatively deficient in situations where amount of eNOS enzyme is increased, as may be the case in sickle mice [29].…”