Vasculopathy and pulmonary hypertension in sickle cell disease

Abstract

Sickle cell disease (SCD) is an autosomal recessive disorder in the gene encoding the β-chain of hemoglobin. Deoxygenation causes the mutant hemoglobin S to polymerize, resulting in rigid, adherent red blood cells that are entrapped in the microcirculation and hemolyze. Cardinal features include severe painful crises and episodic acute lung injury, called acute chest syndrome. This population, with age, develops chronic organ injury, such as chronic kidney disease and pulmonary hypertension. A major risk factor for developing chronic organ injury is hemolytic anemia, which releases red blood cell contents into the circulation. Cell free plasma hemoglobin, heme, and arginase 1 disrupt endothelial function, drive oxidative and inflammatory stress, and have recently been referred to as erythrocyte damage-associated molecular pattern molecules (eDAMPs). Studies suggest that in addition to effects of cell free plasma hemoglobin on scavenging nitric oxide (NO) and generating reactive oxygen species (ROS), heme released from plasma hemoglobin can bind to the toll-like receptor 4 to activate the innate immune system. Persistent intravascular hemolysis over decades leads to chronic vasculopathy, with ∼10% of patients developing pulmonary hypertension. Progressive obstruction of small pulmonary arterioles, increase in pulmonary vascular resistance, decreased cardiac output, and eventual right heart failure causes death in many patients with this complication. This review provides an overview of the pathobiology of hemolysis-mediated endothelial dysfunction and eDAMPs and a summary of our present understanding of diagnosis and management of pulmonary hypertension in sickle cell disease, including a review of recent American Thoracic Society (ATS) consensus guidelines for risk stratification and management.

Keywords: cell free hemoglobin; nitric oxide; pulmonary hypertension; sickle cell disease.

Fig. 1.

A: normal vascular conditions. Red blood cells (RBC) circulate avoiding the cell free zone under laminar flow. Platelets accompany RBC and are inhibited by endothelial derived nitric oxide (NO). Endothelial NO synthase (eNOS) produces NO from substrate l-arginine (l-Arg), making l-citrulline (l-Cit) and NO. NO has a paracrine effect diffusing into smooth muscle cells (SMC), which then activates soluble guanylate cyclase (sGC) forming cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP), leading to vasodilation. B: sickled cells undergo intravascular hemolysis releasing arginase, ADP, and hemoglobin tetramers into the vascular space. Cell free hemoglobin (CFH) enters the cell free zone scavenging NO and forming reactive oxygen species (ROS). Arginase consumes substrate l-Arg. ADP activates platelets via P2Y receptors. C: hypoxia/reoxygenation/hemolysis activates oxidase activity. Ischemia-reperfusion activates endothelial xanthine oxidase (XO) in addition to mobilizing soluble hepatic XO which binds to vascular endothelial cells and forms ROS from excess purine nucleotides released from increased RBC formation/enucleation. Leukocyte and endothelium-derived NADPH oxidase (NOX) forms ROS. ROS inhibits sGC via oxidation, additional superoxide (O₂·⁻) formation by eNOS uncoupling and O₂·⁻ scavenging of NO within the cell free zone to form peroxynitrite (ONOO⁻). D: intravascular hemolysis activates the inflammasome. Uric acid formed by XO binds to the intracellular nucleotide-binding oligomerization domain-like receptor (NLRs) which promotes release of IL1-β. Cell free heme (CFH) activates the TLR-4 receptors. Both mechanisms lead to sterile inflammation. ROS and cell free heme activate polymorphonuclear cells (PMN), which release DNA NETs. Products of intravascular hemolysis, such as hemoglobin, cell free heme, uric acid, and ATP are thus considered danger-associated molecular pattern molecules (DAMPs).

Fig. 2.

Vasoconstriction and vasodilation pathways and therapies for pulmonary hypertension (PH). Activation of soluble guanylate cyclase (sGC) by nitric oxide (NO) increases cyclic guanosine monophosphate (cGMP) levels and leads to vasodilation and inhibition of cellular proliferation in smooth muscle cells (SMC) through the activation of sGC by NO. Inhaled NO, oral nitrite and nitrate, oral l-arginine or tetrahydrobiopterin (BH₄), and phosphodiesterase 5 (PDE5) inhibitors (sildenafil/tadalafil) modulate PH through the NO-sGC-cGMP pathway. Prostacyclins, produced from arachidonic acid by cyclooxygenase (COX) in endothelial cells, target the prostanoid receptor (IP) on smooth muscle cells leading to the activation of adenylate cyclase, which forms cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP). cAMP promotes vasodilation and inhibits cellular proliferation in SMC. Epoprostenol is a prostanoid derivative and selexipag is a new IP receptor agonist that can treat PH through the prostacyclin pathway. Endothelin-1 (ET-1) promotes vasoconstriction and vascular remodeling through binding to ET-A and -B receptors of the SMC. Ambrisentan binds to ET-A and bosentan binds to both ET-A and B. Both ET-1 blockers have been used to treat PH in the endothelin pathway.

Fig. 3.

New therapies that target the NO signaling pathway. Soluble guanylate cyclase (sGC) within smooth muscle cells (SMC) can be stimulated by NO, which binds to the heme moiety within the heterodimer, activating its cyclase function, producing cyclic guanosine monophosphate (cGMP) and promoting vasodilation. sGC can be oxidized by reactive oxygen species (ROS) such as superoxide (O₂·⁻). The oxidized form does not respond to NO and is quickly degraded by the proteosome. sGC activator targets the oxidized (ferric or heme-free) form of sGC and salvages its degradation while activating the cyclase and inducing production of cGMP promoting vasodilation.