Secondary pulmonary hypertension (PH) is a leading cause of mortality and morbidity in patients with hemolytic anemias, including sickle cell disease (SCD) and thalassemia. Asthma is another common co-morbidity in SCD that has been linked to early death. The high frequency of asthma in this population cannot be attributed to genetic predisposition alone, and likely reflects in part, the contribution of overlapping mechanisms shared between these otherwise distinct disorders. There is accumulating evidence that dysregulated arginine metabolism and elevated arginase activity contributes to a number of pulmonary conditions. Patients with hemolytic disorders are at risk for lung complications triggered or worsened by hemolysis-driven release of erythrocyte arginase and low nitric oxide (NO) bioavailability. Inactivation of NO correlates with hemolytic rate and is associated with the erythrocyte release of cell-free hemoglobin, which consumes NO directly, and the simultaneous release of the arginine-metabolizing enzyme arginase, which limits bioavailability of the NO synthase substrate arginine during the process of intravascular hemolysis. Rapid consumption of NO is accelerated further by oxygen radicals in a milieu of oxidative stress common to hemolytic disorders. Once released into circulation, arginase will convert arginine to ornithine, which in turn is the precursor to proline, an amino acid involved in collagen formation, lung fibrosis, airway remodeling and vascular smooth muscle proliferation, common features of pulmonary dysfunction in thalassemia and SCD. Evidence supporting the role of arginase in sickle cell lung disease will be reviewed and relevance to other hemolytic disorders including the thalassemia syndromes will be considered.
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