Hemolysis in Sickle Cell Mice Causes Pulmonary Hypertension Due to Global Impairment in Nitric Oxide Bioavailability.

Pulmonary hypertension is a highly prevalent complication of sickle cell disease and is a strong risk factor for early mortality. However, the pathophysiological mechanisms by which sickle hemoglobin leads to pulmonary vasculopathy remain unclear. Transgenic mice provide opportunities for mechanistic studies of vascular pathophysiology in an animal model of severe sickle cell disease. Using micro-cardiac catheterization we found that all mice expressing exclusively human sickle hemoglobin develop pulmonary hypertension. Recognizing that the NO pathways can have complex abnormalities in other conditions of pulmonary hypertension, the NO axis in sickle mice was assessed by multiple methods. From a mechanistic standpoint the mice exhibit profound pulmonary and systemic endothelial dysfunction and vascular instability characterized by diminished responses to authentic nitric oxide (NO), NO donors and endothelium-dependent pulmonary vasodilators, and enhanced responses to vaso-constrictors. However, endothelium-independent vasodilation in the sickle mice was normal. Mechanisms of vasculopathy in sickle mice involve global dysregulation of the NO-axis: impaired constitutive nitric oxide synthase activity with loss of eNOS coupling (dimerization), increased NO scavenging by plasma hemoglobin and superoxide, increased arginase activity, and depleted intravascular nitrite reserves. Consistent with a functional rather than structural defect, light microscopy and computed tomography of the lungs revealed no plexogenic arterial remodeling, thrombi/emboli, or inflammation. Transplanting sickle marrow into wild-type mice conferred the same phenotype. Similar pathobiology was observed in a non-sickle mouse model of acute alloimmune hemolysis, supporting a major role of hemolysis as a mechanism for this dysregulation of NO and vasculopathy. In this study, alloimmune hemolytic mice were chosen for comparison in order to generalize beyond hemoglobinopathies. Future analogous studies with thalassemic mice may be useful to model pulmonary hypertension in human thalassemia intermedia. In conclusion, this animal model extends the evidence for global impairment in NO responsiveness and NO production in sickle cell disease, and suggests that hemolytic anemia is associated with endothelial dysfunction and pulmonary hypertension.