Increased in vivo production of thromboxane in patients with sickle cell disease is accompanied by an impairment of platelet functions to the thromboxane A2 agonist U46619

Thrombo-Inflammation in COVID-19 and Sickle Cell Disease: Two Faces of the Same Coin

People with sickle cell disease (SCD) are at greater risk of severe illness and death from respiratory infections, including COVID-19, than people without SCD (Centers for Disease Control and Prevention, USA). Vaso-occlusive crises (VOC) in SCD and severe SARS-CoV-2 infection are both characterized by thrombo-inflammation mediated by endothelial injury, complement activation, inflammatory lipid storm, platelet activation, platelet-leukocyte adhesion, and activation of the coagulation cascade. Notably, lipid mediators, including thromboxane A2, significantly increase in severe COVID-19 and SCD. In addition, the release of thromboxane A2 from endothelial cells and macrophages stimulates platelets to release microvesicles, which are harbingers of multicellular adhesion and thrombo-inflammation. Currently, there are limited therapeutic strategies targeting platelet-neutrophil activation and thrombo-inflammation in either SCD or COVID-19 during acute crisis. However, due to many similarities between the pathobiology of thrombo-inflammation in SCD and COVID-19, therapies targeting one disease may likely be effective in the other. Therefore, the preclinical and clinical research spurred by the COVID-19 pandemic, including clinical trials of anti-thrombotic agents, are potentially applicable to VOC. Here, we first outline the parallels between SCD and COVID-19; second, review the role of lipid mediators in the pathogenesis of these diseases; and lastly, examine the therapeutic targets and potential treatments for the two diseases.

Role of the hemostatic system on sickle cell disease pathophysiology and potential therapeutics

Recent studies suggest that sickle cell disease (SCD) is a hypercoagulable state contributing to vaso-occlusive events in the microcirculation, resulting in acute and chronic sickle cell-related organ damage. In this article, we review the existing evidence for contribution of hemostatic system perturbation to SCD pathophysiology. We also review the data showing increased risk of thromboembolic events, particularly newer information on the incidence of venous thromboembolism. Finally, the potential role of platelet inhibitors and anticoagulants in SCD is briefly reviewed.

Sickle cell disease and venous thromboembolism: what the anticoagulation expert needs to know

Venous thromboembolism (VTE) is common in patients with sickle cell disease (SCD). The etiology of increased risk of VTE in SCD patients is multifactorial and is related to both traditional factors and SCD-specific factors. Traditional risk factors such as central venous catheters, frequent hospitalization, orthopedic surgeries for avascular necrosis, and pregnancy may lead to increased incidence of VTE in the SCD population. In addition, SCD itself appears to be a hypercoagulable state, and many SCD-specific factors such as thrombophilic defects, genotype and splenectomy may modify the risk of VTE. SCD complications such as acute chest syndrome and pulmonary hypertension may also be related to VTE. Anticoagulation experts should be aware of these factors to help inform prophylaxis and treatment decisions.

Venous thromboembolism in adults with sickle cell disease: a serious and under-recognized complication

BACKGROUND

Sickle cell disease is recognized as a hypercoagulable state; however, the frequency and characteristics of venous thromboembolism in sickle cell patients have not been well defined. The purpose of this study was to establish the prevalence and risk factors for venous thromboembolism in a large cohort of patients with sickle cell disease and determine the relationship between venous thromboembolism and mortality.

METHODS

We performed a cross-sectional study of 404 sickle cell disease patients cared for at the Sickle Cell Center for Adults at Johns Hopkins. Demographic, sickle cell disease-specific comorbidity, and venous thromboembolism data were collected on all patients.

RESULTS

One hundred one patients (25%) had a history of venous thromboembolism with a median age at diagnosis of 29.9 years. A history of non-catheter-related venous thromboembolism was found in 18.8% of patients. Sickle variant genotypes conferred a higher risk of non-catheter-related venous thromboembolism compared with sickle cell anemia genotypes (SS/Sβ(0)) (relative risk [RR] 1.77; 95% confidence interval [CI], 1.18-2.66). Tricuspid regurgitant jet velocity ≥2.5 m/s also was associated with non-catheter-related venous thromboembolism (RR 1.65; 95% CI, 1.12-2.45). Thirty patients (7.4%) died during the study period. Adjusting for all variables, non-catheter-related venous thromboembolism was independently correlated with death (RR 3.63; 95% CI, 1.66-7.92).

CONCLUSION

Venous thromboembolism is common in adults with sickle cell disease. Sickle variant genotypes and tricuspid regurgitant jet velocity ≥2.5 m/s are associated with non-catheter-related venous thromboembolism. In addition, non-catheter-related venous thromboembolism appears to be an independent risk factor for death in our cohort. These results suggest that disease-specific prophylaxis and treatment strategies for venous thromboembolism should be investigated in sickle cell disease patients.

Whole blood viscosity assessment issues II: Prevalence in endothelial dysfunction and hypercoagulation

Background:

Virchow's triad in cardiovascular disease comprises blood viscosity, plasma D-dimer and homocysteine as indices of three associated but separate vascular phenomena.

Aims:

This work investigates prevalence of hyperviscosity in hyperhomocysteinaemia and positive D-dimer; and differences or similarities in stasis status among sub-populations of hyperhomocysteinaemia vs. normohomocysteinaemia and negative vs. positive D-dimer.

Patients and Methods:

10-years de-identified archived clinical pathology data for the period of January 1999 to December 2008 were audited. All cases tested for D-dimer (n=6845) and homocysteine (n=1665), which were concomitantly tested for haematocrit and total proteins, were extracted.

Results:

The results show a very low prevalence of hyperviscosity associated with a positive D-dimer sub-population (1.48%), which is not statistically different in comparison with the negative D-dimer sub-population. The prevalence of hyperviscosity associated with hyperhomocysteinaemia (5.04%) was statistically significantly higher in comparison to the normohomocysteinaemia sub-population (p = 0.05). The prevalence of low viscosity is significantly higher in the positive D-dimer sub-population relative to the negative D-dimer sub-population (p < 0.00001), but not different between hyperhomocysteinaemia vs. normohomocysteinaemia. Normoviscosity is statistically significantly commoner in normoviscosity relative to hyper-homocysteinaemia as well as in negative compared with positive D-dimer (p < 0.00001).

Conclusion:

The findings reported here suggest putting into perspective the specificity of whole blood viscosity relative to stasis, not necessarily sensitivity to disease conditions where it is implicated.

Beta-thalassaemia and sickle cell anaemia as paradigms of hypercoagulability

Thalassaemia and sickle cell disease (SCD) represent the most common forms of hereditary haemolytic anaemia and result from a partial or complete lack of synthesis of one of the major alpha- or beta-globin chains of haemoglobin A or from a single amino acid mutation (beta(6Glu-->Val)) of the beta-globin chain respectively. Although they have different pathophysiologies, patients with these conditions manifest both biochemical and clinical evidence of hypercoagulability. While the frequency of various thrombotic complications may vary in beta-thalassaemia and homozygous SCD [sickle cell anaemia (SCA)], patients with both diseases manifest decreased levels of natural anticoagulant proteins, as well as increased markers of thrombin generation and platelet activation. The abnormal phospholipid membrane assymetry present in the red blood cells of beta-thalassaemia and SCA patients, with resultant phosphatidylserine exposure appears to play a significant role in the aetiology of the observed hypercoagulable state. This review presents the available data on the aetiology and clinical manifestations of the coagulation and platelet activation that exist in both beta-thalassaemia and SCA, as well as the potential therapeutic implications resulting from this hypercoagulability.

Modulation of the expression and activity of cyclooxygenases in normal and accelerated erythropoiesis

OBJECTIVE

The present study was aimed at characterizing the expression and activity of cyclooxygenase (COX) isoenzymes in erythropoiesis.

METHODS

The expression and activity of cyclooxygenase (COX) and prostaglandin (PG) synthases were investigated in: 1) erythroblasts developed in culture from human CD34(+) hematopoietic progenitors, 2) erythroblasts in bone marrow specimens, and 3) peripheral erythrocytes isolated from healthy donors and from patients with a high regeneration rate of erythrocytes.

RESULTS

While COX-1 protein was observed at each stage of erythroblast development, COX-2 protein was induced at later stages through a p38/MAPK-dependent pathway. Both COX isoforms were also observed in mature erythroblasts of the bone marrow. Erythroblasts developed in culture synthesized significantly more PGE(2) than TXB(2) and indomethacin delayed erythroid maturation. COX-1 and COX-2 were also observed in erythrocytes by immunostainings, although COX expression was confined to a fraction of circulating erythrocytes. Peripheral erythrocytes synthesized low but detectable amounts of PGE(2) and TXB(2). Similarly to erythroblast progenitors, PGE(2) was the prevalent prostanoid released by erythrocytes. This biosynthetic capacity was significantly increased in erythrocytes from patients with accelerated erythropoiesis as compared to controls.

CONCLUSIONS

Both COX isoforms are present and enzymatically active during human erythropoiesis, although with different kinetics, and COX-derived prostanoids may play a role in erythroid maturation. Furthermore, peripheral erythrocytes retain in part the capacity of expressing COX and synthesizing prostanoids, which may contribute to the hemostatic/thrombotic response to vascular injury in different diseases, including congenital hemolytic disorders.

Inhibition of platelet aggregation by rat globin

This study was undertaken to identify and characterize the anti-aggregatory protein factor present in rat polymorphonuclear leukocytes (PMNs) supernatant. Since the purified protein exhibited sequence homology to beta globin, globin was also isolated from rat blood by acid-acetone precipitation and was purified on Superdex-75 column in FPLC. Elution of rat globin on the gel filtration column yielded two peaks of approximately 60 and 30 kDa as observed in the PMNs supernatant. Purity of globin and eluted fractions was further evaluated by SDS-PAGE. Platelet aggregation induced by agonists viz. adenosine-5'-diphosphate (ADP; 2-5 microM), arachidonic acid (AA; 10 microM), A23187 (2.50 microg/ml) was inhibited by globin and the purified fractions. ADP-induced rise in intracellular calcium levels and expression of CD62 on the platelets were reduced by both globin and active fraction of PMNs supernatant. Results obtained suggest that globin or globin-related protein present in the PMNs supernatant inhibits platelet aggregation response.

Eicosanoids in sickle cell disease: potential relevance of neutrophil leukotriene B4 to disease pathophysiology

Neutrophil activation with the release of intracellular granule contents has been observed in sickle cell disease (SCD). Because leukotriene B(4) (LTB(4)), a 5-lipoxygenase metabolite of arachidonic acid in neutrophils, is a chemoattractant and enhances neutrophil adhesion to endothelium, we assessed plasma levels of this metabolite in controls (n = 9) and individuals with SCD, SS genotype, both in basal "steady state" (n = 37) and during episodes of vaso-occlusion (n = 10) and acute chest syndrome (n = 5). Thirteen patients with SCD, SC genotype, in steady state were also studied. Although no significant differences were noted between the control (136 +/- 32 fmol/mL) and SC genotype (177 +/- 83 fmol/mL, P >.15), LTB(4) levels were markedly increased in patients with SS genotype in basal steady state (207 +/- 64 fmol/mL, P <.003) compared with those in controls. Values were further increased during vaso-occlusion (264 +/- 94 fmol/mL) and acute chest syndrome (363 +/- 124 fmol/mL). These levels were significantly different from measurements taken during steady state (P <.04 and P <.0001, respectively). No correlation was noted between LTB(4) level and total white cell or neutrophil count. Additionally, the significant correlation noted in SCD between increased levels of plasma LTB(4) and soluble L-selectin (P <.03) reflects neutrophil activation. We also observed an effect of LTB(4) on red cell-endothelial adhesion at concentrations that appear clinically relevant (1-10 pmol/mL) with concomitant up-regulation of mRNA for the endothelial vitronectin receptor. These properties of LTB(4) are relevant to disease pathophysiology, providing further evidence of the contribution of the neutrophil to the proinflammatory and proadhesive phenotype in SCD.

The hypercoagulable state in thalassemia

Thalassemia is a congenital hemolytic disorder caused by a partial or complete deficiency of alpha- or beta-globin chain synthesis. Homozygous carriers of beta-globin gene defects suffer from severe anemia and other serious complications from early childhood. The disease is treated by chronic blood transfusion. However, this can cause severe iron overload resulting in progressive organ failure. Some forms of alpha thalassemia are also associated with a similar clinical picture. Despite the difficulties associated with treatment, standards of care for thalassemic patients have improved in recent years, resulting in almost doubling of the average life expectancy. As a consequence, additional previously undescribed, complications are now being recognized. In particular, profound hemostatic changes have been observed in patients with beta-thalassemia major (beta-TM) and beta-thalassemia intermedia (beta-TI) and also in patients with alpha thalassemia (hemoglobin H disease). The presence of a higher than normal incidence of thromboembolic events, mainly in beta-TI, and the existence of prothrombotic hemostatic anomalies in the majority of the patients, even from a very young age, have led to the recognition of the existence of a chronic hypercoagulable state in thalassemic patients. Despite the appearance of numerous publications on the frequent occurrence of thromboembolic complications in thalassemia, this complication has not been emphasized or comprehensively reviewed. This review summarizes the current literature and discusses possible mechanisms of the lifelong hypercoagulable state that exists in thalassemia.

Mechanisms of vascular instability in a transgenic mouse model of sickle cell disease

We investigated a transgenic mouse model of sickle cell disease, homozygous for deletion of mouse beta-globin and containing transgenes for human beta(S) and beta(S-antilles) globins linked to the transgene for human alpha-globin. In these mice, basal cGMP production in aortic rings is increased, whereas relaxation to an endothelium-dependent vasodilator, A-23187, is impaired. In contrast, aortic expression of endothelial nitric oxide synthase (NOS) is unaltered in sickle mice, whereas expression of inducible NOS is not detected in either group; plasma nitrate/nitrite concentrations and NOS activity are similar in both groups. Increased cGMP may reflect the stimulatory effect of peroxides (an activator of guanylate cyclase), because lipid peroxidation is increased in aortae and in plasma in sickle mice. Despite increased vascular cGMP levels in sickle mice, conscious systolic blood pressure is comparable to that of aged-matched controls; sickle mice, however, evince a greater rise in systolic blood pressure in response to nitro-L-arginine methyl ester, an inhibitor of NOS. Systemic concentrations of the vasoconstrictive oxidative product 8-isoprostane are increased in sickle mice. We conclude that vascular responses are altered in this transgenic sickle mouse and are accompanied by increased lipid peroxidation and production of cGMP; we suggest that oxidant-inducible vasoconstrictor systems such as isoprostanes may oppose nitric oxide-dependent and nitric oxide-independent mechanisms of vasodilatation in this transgenic sickle mouse. Destabilization of the vasoactive balance in the sickle vasculature by clinically relevant states may predispose to vasoocclusive disease.

Thromboxanes: synthase and receptors

Thromboxane A2 is a biologically potent arachidonate metabolite through the cyclooxygenase pathway. It induces platelet aggregation and smooth muscle contraction and may promote mitogenesis and apoptosis of other cells. Its roles in physiological and pathological conditions have been widely documented. The enzyme that catalyzes its synthesis, thromboxane A2 synthase, and the receptors that mediate its actions, thromboxane A2 receptors, are the two key components critical for the functioning of this potent autacoid. Recent molecular biological studies have revealed the structure-function relationship and gene organizations of these proteins as well as genetic and epigenetic factors modulating their gene expression. Future investigation should shed light on detailed molecular signaling events specifying thromboxane A2 actions, and the genetic underpinning of the enzyme and the receptors in health and disease.

Eicosanoids in sickle cell disease: potential relevance of 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid to the pathophysiology of vaso-occlusion

The monohydroxyeicosanoid 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE), which is derived from oxygenation of arachidonic acid by 12-lipoxygenase, is one of the major metabolites in platelets. In a recent study, we have showed that this eicosanoid stimulated basal sickle-red-cell-endothelial-cell adhesion. To understand the pathophysiologic significance of 12-HETE, we measured the levels of this eicosanoid in plasma and urine from children with sickle cell disease. We found that as compared with controls, plasma 12-HETE levels are increased in patients with sickle-cell disease in the steady state, and are increased further during vaso-occlusive crises. Urinary 12-HETE levels were also increased during the steady state. We also assessed plasma levels of soluble P-selectin (another potential marker for platelet activation), and found changes in the levels of this marker similar to those seen with plasma 12-HETE. In additional studies, we found that 12-HETE enhanced hypoxia-induced sickle-red-cell-endothelial adherence, and that this effect was mediated by potentiation of agonist-induced upregulation of the expression of the mRNA for vascular cell adhesion molecule-1 (VCAM-1) in endothelial cells. Because 12-HETE appears to enhance both basal and agonist-induced sickle-red-cell adhesion, this metabolite could potentially play a role in the pathogenesis of the vaso-occlusive crisis (VOC) in sickle-cell disease.

Platelet thromboxane A2 receptor number and function in normal and hypertensive pregnancy

PROBLEM

Our study evaluated the number and function of platelets for thromboxane A2/prostaglandin H2 (TxA2/PGH2) and the platelet response to TxA2 receptor agonists in normal and hypertensive pregnancy. In addition, correlations between platelet membrane lipid composition and TxA2 receptor number and function were evaluated.

METHODS

Ten normotensive healthy pregnant women (GC), 10 hypertensive pregnant women (PIH), and 10 normotensive nonpregnant healthy women (C) were examined. Radioligand binding and aggregation studies were performed. Lipid composition of platelet membrane was assessed as cholesterol to phospholipids ratio (CH/PL) and normalized to protein content.

RESULTS

TxA2/PGH2 receptor binding sites and affinity did not differ among the groups. An enhanced response to TxA2 receptor agonist U46619 in GC and in PIH compared to C was observed, while platelet response to thrombin was not different among the groups. CH/PL was altered in PIH respect to GC and C. No significant correlation was found between CH/PL and number and function of platelet TxA2 receptors in PIH.

CONCLUSIONS

In PIH there is no alteration of number or function of TxA2 receptors on platelet membrane. However, the enhanced TxA2 production associated with the increased sensitivity of platelet to TxA2 in pregnancy may contribute to the microcirculatory thrombotic changes of PIH.