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Adhesion Molecules and Endothelial Dysfunction in Vaso-Occlusive Crisis

Review Article | DOI: https://doi.org/10.31579/2639-4162/209

Adhesion Molecules and Endothelial Dysfunction in Vaso-Occlusive Crisis

  • Emmanuel Ifeanyi Obeagu

Department of Medical Laboratory Science, Kampala International University, Uganda.

*Corresponding Author: Emmanuel Ifeanyi Obeagu, Department of Medical Laboratory Science, Kampala International University, Uganda.

Citation: Emmanuel I. Obeagu, (2024), Adhesion Molecules and Endothelial Dysfunction in Vaso-Occlusive Crisis, J. General Medicine and Clinical Practice, 7(14); DOI:10.31579/2639-4162/209

Copyright: © 2024, Emmanuel Ifeanyi Obeagu. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Received: 29 July 2024 | Accepted: 12 August 2024 | Published: 22 August 2024

Keywords: adhesion molecules; endothelial dysfunction; vaso-occlusive crisis; sickle cell disease, vascular endothelium; icam-1; vcam-1; e-selectin, p-selectin

Abstract

Vaso-occlusive crisis (VOC) is a severe and painful complication of sickle cell disease (SCD) characterized by the obstruction of blood flow due to interactions between sickled erythrocytes, leukocytes, platelets, and the vascular endothelium. Central to the pathogenesis of VOC are adhesion molecules, such as ICAM-1, VCAM-1, E-selectin, and P-selectin, which facilitate the adhesion of blood cells to the endothelium, initiating inflammatory responses and promoting vascular occlusion. Endothelial dysfunction, a condition marked by impaired vasodilation, increased permeability, and a pro-inflammatory state, plays a pivotal role in VOC. The reduced bioavailability of nitric oxide (NO) due to hemolysis and the presence of oxidative stress further exacerbate endothelial activation and adhesion molecule expression. Elevated inflammatory cytokines in SCD contribute to these processes, creating a vicious cycle of endothelial injury and vaso-occlusion. Therapeutic strategies targeting these underlying mechanisms, such as inhibition of adhesion molecules, enhancement of NO bioavailability, antioxidant therapy, and anti-inflammatory agents, offer potential for reducing the frequency and severity of VOC. 

Introduction

Vaso-occlusive crisis (VOC) is a primary and debilitating complication of sickle cell disease (SCD), a genetic disorder characterized by the presence of abnormal hemoglobin S. This condition leads to the deformation of red blood cells (RBCs) into a sickle shape, which impedes their flow through blood vessels and causes recurrent episodes of acute pain and organ damage. The pathophysiology of VOC is complex, involving a cascade of events that begin with the adhesion of sickled RBCs to the vascular endothelium, followed by interactions with leukocytes and platelets, leading to vascular occlusion and ischemia.1-3 The endothelium, a thin layer of cells lining the blood vessels, plays a critical role in maintaining vascular homeostasis by regulating blood flow, vascular tone, and the balance between coagulation and fibrinolysis. In SCD, the integrity and function of the endothelium are severely compromised, contributing to the pathogenesis of VOC. Endothelial cells in patients with SCD exhibit increased expression of adhesion molecules, a hallmark of endothelial activation and dysfunction.4-6 Adhesion molecules are cell surface proteins that mediate the attachment of cells to each other and to the extracellular matrix. In the context of VOC, these molecules facilitate the adhesion of sickled RBCs, leukocytes, and platelets to the endothelium, promoting vascular inflammation and occlusion. Key adhesion molecules involved in this process include intracellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), E-selectin, and P-selectin.7-8 ICAM-1 is expressed on endothelial cells and is upregulated in response to inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β). It binds to leukocyte function-associated antigen-1 (LFA-1) on leukocytes, facilitating their firm adhesion and transmigration across the endothelium. This interaction is crucial for the recruitment of leukocytes to sites of vascular injury and inflammation in VOC. [9-10]

VCAM-1 is another critical adhesion molecule expressed on endothelial cells. It interacts with very late antigen-4 (VLA-4) on leukocytes and sickled RBCs, promoting their adhesion to the endothelium. This interaction is particularly significant in SCD, as it not only contributes to the recruitment of leukocytes but also to the adhesion of sickled RBCs, exacerbating vascular occlusion and ischemic injury. E-selectin and P-selectin are selectins expressed on activated endothelial cells and platelets. They mediate the initial rolling of leukocytes and sickled RBCs on the endothelial surface, a critical step in the adhesion cascade. P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes and RBCs binds to P-selectin, while E-selectin interacts with various glycoproteins on leukocytes. These interactions are essential for the recruitment of cells to sites of vascular injury and inflammation in VOC. [11-15] Endothelial dysfunction, a condition characterized by impaired endothelium-dependent vasodilation, increased endothelial permeability, and a pro-inflammatory and pro-thrombotic state, plays a pivotal role in the pathogenesis of VOC. In SCD, endothelial dysfunction is both a cause and consequence of the vaso-occlusive events. The reduced bioavailability of nitric oxide (NO) due to hemolysis and increased oxidative stress further exacerbates endothelial activation and the expression of adhesion molecules. Nitric oxide is a potent vasodilator produced by endothelial cells that plays a key role in maintaining vascular homeostasis. In SCD, hemolysis releases free hemoglobin into the bloodstream, which scavenges NO, reducing its availability. This results in vasoconstriction, increased endothelial activation, and upregulation of adhesion molecules, promoting VOC. [16-20] Oxidative stress, resulting from chronic hemolysis and ischemia-reperfusion injury, also contributes to endothelial dysfunction in SCD. Reactive oxygen species (ROS) generated during these processes damage endothelial cells, further reducing NO bioavailability and increasing the expression of adhesion molecules. This oxidative damage exacerbates the inflammatory response and promotes vascular occlusion. Inflammatory cytokines, such as TNF-α and IL-1β, are elevated in SCD and play a critical role in endothelial dysfunction. These cytokines upregulate the expression of adhesion molecules on endothelial cells, enhance leukocyte adhesion and transmigration, and contribute to the pro-inflammatory state in VOC. The interplay between these inflammatory mediators and endothelial cells creates a vicious cycle of endothelial injury and vascular occlusion. [21-25]

Adhesion Molecules in Vaso-Occlusive Crisis

Adhesion molecules are pivotal in the pathophysiology of vaso-occlusive crisis (VOC), playing a central role in the interaction between sickled red blood cells (RBCs), leukocytes, and the vascular endothelium. These molecules facilitate the adhesion and migration of cells across the endothelium, promoting vascular inflammation and occlusion. Key adhesion molecules involved in VOC include intracellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), E-selectin, and P-selectin. Intracellular Adhesion Molecule-1 (ICAM-1) is a transmembrane protein expressed on endothelial cells and various other cell types, including leukocytes. In the context of VOC, ICAM-1 expression is significantly upregulated in response to inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β). This upregulation facilitates the firm adhesion of leukocytes to the endothelium through interactions with leukocyte function-associated antigen-1 (LFA-1) on leukocytes. The ICAM-1/LFA-1 interaction is crucial for leukocyte transmigration across the endothelium, contributing to vascular inflammation and endothelial damage in VOC. [26-30] Vascular Cell Adhesion Molecule-1 (VCAM-1) is another critical adhesion molecule expressed on endothelial cells, especially during inflammatory states. It interacts with very late antigen-4 (VLA-4) on leukocytes and sickled RBCs. The binding of VCAM-1 to VLA-4 promotes the adhesion of these cells to the endothelium, exacerbating vascular occlusion and ischemic injury in VOC. VCAM-1 plays a particularly significant role in the recruitment of leukocytes and the adhesion of sickled RBCs, making it a crucial target in understanding and potentially mitigating VOC. E-selectin and P-selectin are selectin family adhesion molecules expressed on activated endothelial cells and platelets. These molecules mediate the initial rolling of leukocytes and sickled RBCs on the endothelial surface, a critical step in the adhesion cascade leading to VOC. P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes and RBCs binds to P-selectin, while E-selectin interacts with various glycoproteins on leukocytes. The initial rolling mediated by selectins is followed by firm adhesion and transmigration, leading to vascular occlusion. [31-35]

P-selectin is stored in Weibel-Palade bodies of endothelial cells and alpha granules of platelets, P-selectin is rapidly translocated to the cell surface upon activation by inflammatory stimuli. P-selectin plays a critical role in the early stages of VOC by mediating the rolling and adhesion of sickled RBCs and leukocytes to the endothelium. E-selectin is induced on endothelial cells in response to inflammatory cytokines, E-selectin mediates the adhesion of leukocytes to the endothelium. It plays a complementary role to P-selectin in the recruitment of leukocytes to sites of vascular injury and inflammation. Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1), also known as CD31, is expressed on endothelial cells and plays a role in the transmigration of leukocytes through the endothelium. Although not as extensively studied in VOC as ICAM-1, VCAM-1, and selectins, PECAM-1 is involved in the inflammatory response and may contribute to endothelial dysfunction in SCD. [36-40] Integrins, such as LFA-1 and VLA-4, are expressed on leukocytes and sickled RBCs. These integrins interact with their respective ligands (ICAM-1 and VCAM-1) on endothelial cells, mediating firm adhesion and transmigration. The role of integrins in VOC highlights the importance of leukocyte and RBC-endothelial interactions in the pathogenesis of this condition. [41-42]

Endothelial Dysfunction in Vaso-Occlusive Crisis

Endothelial dysfunction is a critical component of the pathophysiology of vaso-occlusive crisis (VOC) in sickle cell disease (SCD). It refers to a state of impaired endothelial function characterized by reduced vasodilatory capacity, increased permeability, and a pro-inflammatory and pro-thrombotic environment. The dysfunction of the endothelium plays a significant role in the initiation and perpetuation of VOC, contributing to the overall severity and frequency of these painful episodes. One of the primary roles of the endothelium is to regulate vascular tone through the production of vasodilators such as nitric oxide (NO). In SCD, endothelial dysfunction is marked by decreased bioavailability of NO due to the effects of hemolysis. When sickled RBCs undergo hemolysis, free hemoglobin is released into circulation. This free hemoglobin can scavenge NO, leading to reduced levels of this critical vasodilator. As a result, the ability of blood vessels to dilate is compromised, leading to increased vascular resistance and impaired blood flow. This lack of adequate vasodilation contributes to the development of VOC by promoting further sickling of RBCs and enhancing the risk of vascular occlusion. [43-47].  Endothelial dysfunction is also associated with increased permeability of the endothelial barrier. In SCD, inflammatory mediators, including cytokines such as TNF-α and IL-1β, induce the expression of adhesion molecules and increase the permeability of the endothelium. This heightened permeability allows for the extravasation of plasma proteins and leukocytes into the surrounding tissue, exacerbating inflammation and tissue injury. The infiltration of inflammatory cells further activates the endothelium, creating a vicious cycle of endothelial activation and dysfunction. Increased endothelial permeability can also contribute to edema and tissue ischemia, worsening the clinical presentation of VOC. Endothelial dysfunction in SCD is characterized by a shift toward a pro-inflammatory and pro-thrombotic state. In normal physiology, the endothelium maintains a delicate balance between pro-coagulant and anti-coagulant factors. However, in SCD, this balance is disrupted. The activated endothelium releases pro-inflammatory cytokines, chemokines, and adhesion molecules that promote leukocyte recruitment and activation. This inflammatory response is a significant contributor to the pathogenesis of VOC, as it leads to increased leukocyte adhesion to the endothelium and enhanced interactions between leukocytes, sickled RBCs, and the vascular wall. [48-52].

Moreover, the endothelium in SCD tends to produce more von Willebrand factor (vWF) and tissue factor (TF), both of which promote coagulation. The increased expression of vWF enhances platelet adhesion and aggregation, while TF is a potent pro-coagulant that initiates the extrinsic pathway of the coagulation cascade. This pro-thrombotic environment increases the risk of thrombus formation, which can further compromise blood flow and contribute to the development of VOC. Oxidative stress plays a crucial role in endothelial dysfunction in SCD. Chronic hemolysis and ischemia-reperfusion injury result in the generation of reactive oxygen species (ROS), which cause damage to endothelial cells. Oxidative stress leads to the oxidation of lipids, proteins, and DNA, impairing endothelial function and promoting inflammation. The oxidative environment also contributes to the depletion of NO, as ROS can react with NO to form peroxynitrite, further reducing its bioavailability. In addition, oxidative stress can upregulate the expression of adhesion molecules on endothelial cells, enhancing the adhesion of leukocytes and sickled RBCs and perpetuating the cycle of inflammation and endothelial dysfunction. The interplay between oxidative stress and endothelial dysfunction underscores the complexity of VOC pathophysiology in SCD. [53-57]. Inflammatory cytokines are central to the development of endothelial dysfunction in SCD. Elevated levels of TNF-α, IL-1β, and other cytokines promote the activation of endothelial cells, leading to increased expression of adhesion molecules and enhanced permeability. These cytokines also stimulate the production of reactive oxygen species, further exacerbating oxidative stress and endothelial injury. The persistent inflammation associated with SCD results in chronic endothelial activation, creating an environment that is conducive to the development of VOC [58].

Therapeutic Implications

These strategies focus on modulating the inflammatory response, improving endothelial function, and inhibiting the pathological interactions that lead to vascular occlusion. Below are key therapeutic implications based on the pathophysiological mechanisms underlying VOC. Targeting adhesion molecules presents a promising approach to reduce the adhesion of sickled red blood cells (RBCs) and leukocytes to the endothelium, thereby mitigating VOC. Agents such as anti-P-selectin and anti-VCAM-1 antibodies can disrupt the interactions between these adhesion molecules and their ligands, effectively reducing cell adhesion and preventing vascular occlusion. Clinical trials exploring these agents are essential to determine their efficacy and safety in SCD patients.59 Small molecules that interfere with the signaling pathways that upregulate adhesion molecules on endothelial cells could help prevent their expression and subsequent cell adhesion. Research into these compounds is necessary to identify candidates that can be effective in clinical settings. Restoring the balance of nitric oxide (NO) is crucial for maintaining endothelial function and promoting vasodilation in SCD. Pharmacological agents that release NO, such as nitroglycerin or other NO donors, can be used to improve endothelial function and promote vasodilation. These agents may provide symptomatic relief during VOC episodes and help prevent future crises. Medications that inhibit phosphodiesterase, thereby increasing the levels of cyclic guanosine monophosphate (cGMP), may enhance NO signaling and improve endothelial function. This approach could also help alleviate the vasoconstriction associated with VOC. L-arginine is the precursor for NO synthesis. Supplementing with L-arginine may improve NO production and bioavailability, thereby supporting endothelial function and reducing the risk of VOC. Reducing oxidative stress is critical in restoring endothelial function and preventing the damage caused by reactive oxygen species (ROS). Potential antioxidant therapies include: N-acetylcysteine (NAC) is an antioxidant that has shown promise in reducing oxidative stress in SCD. It can replenish intracellular glutathione levels, thereby enhancing the body’s antioxidant defenses and protecting endothelial cells from oxidative damage. As a fat-soluble antioxidant, vitamin E can help neutralize free radicals and reduce oxidative stress in SCD. Studies investigating its role in improving endothelial function and reducing VOC severity could provide valuable insights. This medication has been widely used in SCD to increase fetal hemoglobin levels and reduce leukocyte counts. Hydroxyurea decreases the frequency of VOC by modulating inflammation and reducing the expression of adhesion molecules on endothelial cells. Although they are not commonly used in chronic management, corticosteroids may be beneficial during acute crises due to their potent anti-inflammatory effects. Their use should be carefully considered, balancing potential benefits with risks.60-61

Targeting specific inflammatory cytokines, such as TNF-α or IL-1β, with monoclonal antibodies may help reduce the inflammatory response in SCD. This approach could provide a more precise intervention in managing VOC. Low Molecular Weight Heparin (LMWH) can reduce the risk of thrombus formation during VOC. Its use in prophylactic or therapeutic settings should be explored to determine its impact on VOC frequency and severity. Emerging evidence suggests that DOACs may have a role in managing thrombosis in SCD. Clinical studies are needed to evaluate their safety and efficacy in this patient population. Incorporating these therapeutic strategies into a comprehensive care plan for SCD patients is essential. A multidisciplinary approach involving hematologists, primary care physicians, and supportive care specialists can optimize patient outcomes. Educating patients about recognizing early signs of VOC and the importance of hydration, pain management, and adherence to treatment regimens can empower them to manage their condition effectively. Regular monitoring of patients for early signs of VOC and evaluating treatment responses is crucial for adjusting therapies and preventing complications.61

Conclusion

Vaso-occlusive crisis (VOC) is a significant and painful complication of sickle cell disease (SCD), arising from a complex interplay of factors involving sickled red blood cells, leukocytes, platelets, and the vascular endothelium. Adhesion molecules and endothelial dysfunction are central to the pathophysiology of VOC, facilitating the adhesion and migration of cells, leading to vascular inflammation and obstruction. Key adhesion molecules, such as ICAM-1, VCAM-1, E-selectin, and P-selectin, play critical roles in mediating the interactions between sickled RBCs, leukocytes, and the endothelium, driving the inflammatory response that precipitates VOC.

Endothelial dysfunction, characterized by impaired vasodilation, increased permeability, and a pro-inflammatory and pro-thrombotic state, is a fundamental aspect of VOC. The reduced bioavailability of nitric oxide due to hemolysis, coupled with oxidative stress and elevated inflammatory cytokines, exacerbates endothelial activation and promotes the adhesion of blood cells to the vascular wall. This dysfunction creates a vicious cycle that not only contributes to the occurrence of VOC but also intensifies the severity of the episodes.

References

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