Renal prostacyclin biosynthesis is reduced in children with hemolytic-uremic syndrome in the context of systemic platelet activation

Shiga Toxin 2a-Induced Endothelial Injury in Hemolytic Uremic Syndrome: A Metabolomic Analysis

BACKGROUND

Endothelial dysfunction plays a pivotal role in the pathogenesis of postenteropathic hemolytic uremic syndrome (HUS), most commonly caused by Shiga toxin (Stx)-producing strains of Escherichia coli.

METHODS

To identify new treatment targets, we performed a metabolomic high-throughput screening to analyze the effect of Stx2a, the major Stx type associated with HUS, on human renal glomerular endothelial cells (HRGEC) and umbilical vein endothelial cells (HUVEC). Cells were treated either with sensitizing tumor necrosis factor α (TNF-α) or Stx2a, a sequence of both or remained untreated.

RESULTS

We identified 341 metabolites by combined liquid chromatography/tandem mass spectrometry and gas chromatography/mass spectrometry. Both cell lines exhibited distinct metabolic reaction profiles but shared elevated levels of free fatty acids. Stx2a predominantly altered the nicotinamide adenine dinucleotide (NAD) cofactor pathway and the inflammation-modulating eicosanoid pathway, which are associated with lipid metabolism. In HRGEC, Stx2a strongly diminished NAD derivatives, leading to depletion of the energy substrate acetyl coenzyme A and the antioxidant glutathione. HUVEC responded to TNF-α and Stx2a by increasing production of the counteracting eicosanoids prostaglandin I2, E1, E2, and A2, while in HRGEC only more prostaglandin I2 was detected.

CONCLUSIONS

We conclude that disruption of energy metabolism and depletion of glutathione contributes to Stx-induced injury of the renal endothelium and that the inflammatory response to Stx is highly cell-type specific.

Shiga toxin 2 and lipopolysaccharide induce human microvascular endothelial cells to release chemokines and factors that stimulate platelet function

Shiga toxins (Stxs) produced by Shigella dysenteriae type 1 and enterohemorrhagic Escherichia coli are the most common cause of hemolytic-uremic syndrome (HUS). It is well established that vascular endothelial cells, mainly those located in the renal microvasculature, are targets for Stxs. The aim of the present research was to evaluate whether E. coli-derived Shiga toxin 2 (Stx2) incubated with human microvascular endothelial cells (HMEC-1) induces release of chemokines and other factors that might stimulate platelet function. HMEC-1 were exposed for 24 h in vitro to Stx2, lipopolysaccharide (LPS), or the Stx2-LPS combination, and chemokine production was assessed by immunoassay. More interleukin-8 was released than stromal cell-derived factor 1alpha (SDF-1alpha) or SDF-1beta and RANTES. The Stx2-LPS combination potentiated chemokine release, but Stx2 alone caused more release of SDF-1alpha at 24 h than LPS or Stx2-LPS did. In the presence of low ADP levels, HMEC-1 supernatants activated platelet function assessed by classical aggregometry, single-particle counting, granule secretion, P-selectin exposure, and the formation of platelet-monocyte aggregates. Supernatants from HMEC-1 exposed only to Stx2 exhibited enhanced exposure of platelet P-selectin and platelet-THP-1 cell interactions. Blockade of platelet cyclooxygenase by indomethacin prevented functional activation. The chemokine RANTES enhanced platelet aggregation induced by SDF-1alpha, macrophage-derived chemokine, or thymus and activation-regulated chemokine in the presence of very low ADP levels. These data support the hypothesis that microvascular endothelial cells exposed to E. coli O157:H7-derived Stx2 and LPS release chemokines and other factors, which when combined with low levels of primary agonists, such as ADP, cause platelet activation and promote the renal thrombosis associated with HUS.

Thrombotic microangiopathy, hemolytic uremic syndrome, and thrombotic thrombocytopenic purpura

The term thrombotic microangiopathy (TMA) defines a lesion of vessel wall thickening (mainly arterioles or capillaries), intraluminal platelet thrombosis, and partial or complete obstruction of the vessel lumina. Depending on whether renal or brain lesions prevail, two pathologically indistinguishable but somehow clinically different entities have been described: the hemolytic uremic syndrome (HUS) and the thrombotic thrombocytopenic purpura (TTP). Injury to the endothelial cell is the central and likely inciting factor in the sequence of events leading to TMA. Loss of physiological thromboresistance, leukocyte adhesion to damaged endothelium, complement consumption, abnormal von Willebrand factor release and fragmentation, and increased vascular shear stress may then sustain and amplify the microangiopathic process. Intrinsic abnormalities of the complement system and of the von Willebrand factor pathway may account for a genetic predisposition to the disease that may play a paramount role in particular in familial and recurrent forms. Outcome is usually good in childhood, Shiga toxin-associated HUS, whereas renal and neurological sequelae are more frequently reported in adult, atypical, and familial forms of HUS and in TTP. Plasma infusion or exchange is the only treatment of proven efficacy. Bilateral nephrectomy and splenectomy may serve as rescue therapies in very selected cases of plasma resistant HUS or recurrent TTP, respectively.

The management of idiopathic thrombotic microangiopathy. Changing trends

Thrombotic microangiopathy, including the two related syndromes thrombotic thrombocytopenic purpura and hemolytic-uremic syndrome, is a rare and severe multisystem disorder, due to widespread deposition of intravascular microthrombi consisting mainly of platelets, with subsequent consumption thrombocytopenia, microangiopathic hemolytic anemia, renal abnormalities, and neurologic disturbances. The epidemic, verotoxin-induced hemolytic-uremic syndrome, typically associated with prodromal diarrhea, mainly affects young children in small outbreaks. By contrast, idiopathic thrombotic microangiopathy generally affects adults in a sporadic form; it has a more devastating course and a less favourable outcome. Over 90% of the reported cases in the adult, when untreated, have progressed to death within three months of diagnosis. Since the introduction of plasma exchange, a dramatic change in the prognosis of the disease has taken place, although the mortality rate still remains considerable. Indeed, improved survival is the most striking feature of adult thrombotic microangiopathy compared to some decades ago. In the present article we will focus on the evolving concepts able to exert a considerable impact in the management of the adult idiopathic form of thrombotic microangiopathy.

Endothelial cell function and thrombosis

The endothelium is pivotal in the control of haemostasis and thrombosis because it is the primary source of many of the major haemostatic regulatory molecules. Healthy endothelial cells, unlike extravascular cells, are anticoagulant and antithrombotic. This is due to the regulated secretion of antiplatelet agents, including prostacyclin and nitric oxide. Following vessel injury, platelet adhesion to exposed matrix requires von Willebrand Factor, another endothelial cell product. Local generation of thrombin causes a series of receptor-mediated endothelial cell functional responses, while the surface of the endothelium is additionally the site for inactivation of thrombin by antithrombin, and its conversion to a coagulation inhibitor by interaction with thrombomodulin. Endothelial cells are also the source of circulating tissue-type plasminogen activator and its inhibitor, and Tissue Factor pathway inhibitor. In disease states, many of these endothelial cell properties are perturbed towards a more procoagulant and prothrombotic phenotype.

Thrombotic microangiopathies associated with drugs and bone marrow transplantation

This type of thrombotic microangiopathy more commonly resembles the hemolytic-uremic syndrome (HUS) than thrombotic thrombocytopenic purpura (TTP). The syndrome has been associated with the use of cyclosporin, mitomycin C, combinations of other chemotherapeutic and immunosuppressive agents, and total body irradiation. Endothelial cell injury and von Willebrand factor may be involved in pathogenesis of the intravascular platelet aggregation and tissue (especially renal) ischemia and infarction that characterize the entity. The most effective therapy for thrombotic microangiopathy associated with drugs and bone marrow transplantation has not been determined.

Iloprost in the treatment of thrombotic microangiopathy: report of thirteen cases

Defective prostacyclin bioavailability seems to play a role in the pathogenesis of thrombotic microangiopathy, including thrombotic thrombocytopenic purpura and hemolytic uremic syndrome. Eight consecutive patients with a proven diagnosis of thrombotic microangiopathy were treated by Iloprost, a recently developed stable prostacyclin analogue; during follow-up, three of them relapsed and received further treatment. To our knowledge, this is the first report on a wide series of patients who received Iloprost for thrombotic microangiopathy. Soon after diagnosis, Iloprost was given by continuous intravenous infusion at a rate of 1.5-2 ng/kg/minute over 16-18 h/day for several days (mean 12 days; range 6-24) until the platelet count steadily increased. In addition, plasma exchange with fresh frozen plasma (average volume exchange 20-40 mL/kg for each session) was performed in 11 out of the 13 cases. No other antiplatelet agent was given. In all 13 cases, Iloprost administration coincided with achievement of remission. At present, all the patients are still maintaining remission. Our results indicate a useful role for Iloprost in the management of thrombotic microangiopathy.

The hemolytic uremic syndrome

HUS is the most common cause of acute renal failure in infants and young children and follows a diarrheal prodrome about 90% of the time. Persuasive evidence shows that virtually all of postdiarrheal cases are caused by EHEC infections, and that the great majority of cases in the United States are caused by the EHEC serotype O157:H7. Mortality is approximately 5%, and approximately 10% of survivors are left with severe sequelae. A much larger number (30%-50%) experience mild chronic renal damage. Public health strategies, including zero tolerance for fecal contamination in slaughter houses and additional public education on proper food handling and cooking, does much to decrease the prevalence of the syndrome. Efforts to further dissect the postdiarrheal pathogenic cascade should continue, and an animal model needs to be developed. Only then will researchers be positioned to develop effective intervention strategies. Preventing life-threatening extrarenal complications, especially of the CNS, is a major challenge. Idiopathic nondiarrheal HUS accounts for approximately 10% of cases and comprises a poorly understood composite of HUS subsets. Research directed toward a better understanding of these mysterious variants also is a priority for the years ahead.

Partial clinical improvement in Upshaw-Schulman syndrome following prostacyclin infusion

We report on a Japanese girl with Upshaw-Schulman syndrome, a congenital disorder characterized by recurrent thrombocytopenia, microangiopathic hemolytic anemia, proteinuria and hematuria that can be transiently improved by the transfusion of plasma or various plasma components. Unusually Large von Willebrand Factor (ULvWF) multimers were found during both relapse and remission phases. Serial plasma levels of 6-keto-prostaglandin F1 alpha (PGF1 alpha), the stable metabolite of prostacyclin (PGI2), were low at relapse. When the patient was treated with continuous PGI2 infusion, the microangiopathic hemolytic process gradually subsided within 10 days. These results suggest that PGI2 may be partly involved in the pathogenesis of this congenital disorder.

Endothelial cell function and thrombosis

Healthy endothelium is a metabolically active interface between the blood and extravascular tissues. Its intimal surface is anticoagulant and antithrombotic, and it secretes a variety of molecules involved in regulating platelet function and blood coagulation. The rapid interactions between platelets, their secreted components, or thrombin and endothelial cells at sites of vessel damage ensure the local secretion of mediators such as prostacyclin and nitric oxide that limit the intravascular growth of the haemostatic plug. There is considerable evidence that a decreased ability of endothelial cells to synthesize NO contributes to the pathogenesis of arterial disease. Local deficiency of PGI2 synthesis has also been implicated in the thrombotic problems in haemolytic uraemic syndrome. Endothelium is also the source of circulating von Willebrand factor, important for efficient platelet adhesion. Chronically elevated plasma levels of vWF in a series of diseases where there is vascular pathology apparently reflect endothelial cell damage or activation, and may contribute to the prothrombotic tendency they exhibit. They may be compounded by decreased levels of the surface anticoagulant thrombomodulin, if the increased concentrations of the soluble forms of thrombomodulin detected in the circulation under similar conditions are a reflection of loss from the endothelium. Further alterations of function in a procoagulant/prothrombotic direction take place when endothelial cells are exposed to certain cytokines or lipopolysaccharide. Tissue factor synthesis is induced, thrombomodulin expression is decreased, and there is enhanced sensitivity of vWF secretion. In addition, the balance of tissue-type plasminogen activator and plasminogen activator inhibitor type I secretion is changed in favour of the latter. These processes are each likely to contribute to the occurrence of disseminated intravascular coagulation which can accompany septic shock.

Prostacyclin in diarrhoea-associated haemolytic uraemic syndrome

The role of prostacyclin (PGI2) in the pathogenesis of haemolytic uraemic syndrome (HUS) is controversial. In part, confusion has been caused by failure to distinguish between two main sub-types of the syndrome: extrinsic, diarrhoea-associated HUS (D+ HUS), usually caused by infection with verocytotoxin-producing Escherichia coli or Shigella dysenteriae, and the heterogeneous group of non-prodromal forms where intrinsic factors predominate (D- HUS). This paper critically reviews data confined to D+ HUS. Two methods have been used to assess PGI2 synthesis; the generation of PGI2 from endothelium in the presence of HUS plasma in vitro and the measurement of stable metabolites in body fluids. No concensus could be reached with regard to the former. The reported increase of PGI2 stable metabolites in plasma may represent reduced clearance or increased carriage by plasma lipids. Apparent differences between studies of urinary excretion of PGI2 metabolites may reflect the way excretion was expressed. If the metabolite concentration is factored for urinary creatinine, it appears that renal excretion and thus renal synthesis of PGI2 is reduced. However, these are insufficient data on which to attribute the pathogenesis of D+ HUS to disordered PGI2 metabolism.