Endoglin regulates nitric oxide‐dependent vasodilatation

Identification of serum endoglin as a novel prognostic marker after acute myocardial infarction

Endoglin is a proliferation-associated and hypoxia-inducible protein expressed in endothelial cells. The levels of soluble circulating endoglin and their prognostic significance in patients with acute myocardial infarction (AMI) are not known. In this observational prospective study serum endoglin levels were measured by ELISA in 183 AMI patients upon admission to hospital and 48 hrs later and in 72 healthy controls. Endoglin levels in AMI patients on admission were significantly lower than in healthy controls (4.25 ± 0.99 ng/ml versus 4.59 ± 0.87 ng/ml; P= 0.013), and decreased further in the first 48 hours (3.65 ± 0.76 ng/ml, P < 0.001). Upon follow-up (median 319 days), patients who died had a significantly greater decrease in serum endoglin level over the first 48 hrs than those who survived (1.03 ± 0.91 versus 0.54 ± 0.55 ng/ml; P= 0.025). Endoglin decrease was an independent predictor of short-term (30 days) (hazard ratio 2.33;95% CI = 1.27–4.23; P= 0.006) cardiovascular mortality, and also predicts overall cardiovascular mortality during the follow-up (median 319 days) in AMI patients (hazard ratio 2.13;95% CI = 1.20–3.78; P= 0.01). In conclusion, early changes in serum endoglin may predict mortality after AMI.

A mouse model for Costello syndrome reveals an Ang II-mediated hypertensive condition

Germline activation of H-RAS oncogenes is the primary cause of Costello syndrome (CS), a neuro-cardio-facio-cutaneous developmental syndrome. Here we describe the generation of a mouse model of CS by introduction of an oncogenic Gly12Val mutation in the mouse H-Ras locus using homologous recombination in ES cells. Germline expression of the endogenous H-RasG12V oncogene, even in homozygosis, resulted in hyperplasia of the mammary gland. However, development of tumors in these mice was rare. H-RasG12V mutant mice closely phenocopied some of the abnormalities observed in patients with CS, including facial dysmorphia and cardiomyopathies. These mice also displayed alterations in the homeostasis of the cardiovascular system, including development of systemic hypertension, extensive vascular remodeling, and fibrosis in both the heart and the kidneys. This phenotype was age dependent and was a consequence of the abnormal upregulation of the renin-Ang II system. Treatment with captopril, an inhibitor of Ang II biosynthesis, prevented development of the hypertension condition, vascular remodeling, and heart and kidney fibrosis. In addition, it partially alleviated the observed cardiomyopathies. These mice should help in elucidating the etiology of CS symptoms, identifying additional defects, and evaluating potential therapeutic strategies.

Endoglin (CD105): a marker of tumor vasculature and potential target for therapy

Endoglin (CD105) is an accessory protein of the transforming growth factor-beta receptor system expressed on vascular endothelial cells. Mutation of the endoglin gene is associated with hereditary hemorrhagic telangiectasias, or Osler-Weber-Rendu syndrome, and has been studied extensively in the context of this disease. The expression of endoglin is elevated on the endothelial cells of healing wounds, developing embryos, inflammatory tissues, and solid tumors. Endoglin is a marker of activated endothelium, and its vascular expression is limited to proliferating cells. Recent studies identified endoglin expression in several solid tumor types, with the level of expression correlating with various clinicopathologic factors including decreased survival and presence of metastases. Attempts to target endoglin and the cells that express this protein in tumor-bearing mice have yielded promising results.

Novel biochemical pathways of endoglin in vascular cell physiology

The broad role of the transforming growth factor beta (TGFbeta) signaling pathway in vascular development, homeostasis, and repair is well appreciated. Endoglin is emerging as a novel, complex, and poorly understood regulatory component of the TGFbeta receptor complex, whose importance is underscored by its recognition as the site of mutations causing hereditary hemorrhagic telangiectasia (HHT) [McAllister et al., 1994]. Extensive analyses of endoglin function in normal developmental mouse models [Bourdeau et al., 1999; Li et al., 1999; Arthur et al., 2000] and in HHT animal models [Bourdeau et al., 2000; Torsney et al., 2003] exemplify the importance of understanding endoglin's biochemical functions. However, novel mechanisms underlying the regulation of these pathways continue to emerge. These mechanisms include modification of TGFbeta receptor signaling at the ligand and receptor activation level, direct effects of endoglin on cell adhesion and migration, and emerging roles for endoglin in the determination of stem cell fate and tissue patterning. The purpose of this review is to highlight the cellular and molecular studies that underscore the central role of endoglin in vascular development and disease.

Endoglin in angiogenesis and vascular diseases

Endoglin is a transmembrane auxillary receptor for transforming growth factor-beta (TGF-beta) that is predominantly expressed on proliferating endothelial cells. Endoglin deficient mice die during midgestation due to cardiovascular defects. Mutations in endoglin and activin receptor-like kinase 1 (ALK1), an endothelial specific TGF-beta type I receptor, have been linked to hereditary hemorrhagic telangiectasia (HHT), an autosomal dominant vascular dysplasia characterized by telangiectases and arteriovenous malformations. Endoglin heterozygote mice develop HHT-like vascular abnormalities, have impaired tumor and post-ischemic angiogenesis and demonstrate an endothelial nitric oxide synthase-dependent deterioration in the regulation of vascular tone. In pre-eclampsia, placenta-derived endoglin has been shown to be strongly upregulated and high levels of soluble endoglin are released into the circulation. Soluble endoglin was found to cooperate with a soluble form of vascular endothelial growth factor receptor 1 in the pathogenesis of pre-eclampsia by inducing endothelial cell dysfunction. Endoglin is highly expressed in tumor-associated endothelium, and endoglin antibodies have been successfully used to target activated endothelial cells and elicit anti-angiogenic effects in tumor mouse models. These exciting advances provide opportunities for the development of new therapies for diseases with vascular abnormalities.

Long-term nebivolol administration reduces renal fibrosis and prevents endothelial dysfunction in rats with hypertension induced by renal mass reduction

OBJECTIVES

D/L-Nebivolol is a lypophilic beta1-adrenergic antagonist which is devoid of intrinsic sympathomimetic activity and can increase nitric oxide (NO) bioavailability with its subsequent vasodilating properties. The purpose of the present work was to assess the effect of long-term nebivolol administration on both renal damage and endothelial dysfunction induced by renal mass reduction (RMR) in rats. Atenolol, which does not increase NO bioavailability, was included in the study as a comparative beta-adrenoceptor antagonist.

METHODS

Rats were subjected to both right nephrectomy and surgical removal of two-thirds of the left kidney in order to retain approximately one-sixth of the total renal mass. One week after ablation, rats were distributed randomly according to the following experimental groups: control group containing RMR rats without treatment; RMR rats treated daily with nebivolol for 6 months (drinking water, 8 mg/kg per day); and RMR rats treated daily with atenolol for 6 months (drinking water, 80 mg/kg per day). A group of sham-operated animals was also included.

RESULTS

Administration of either nebivolol or atenolol similarly reduced arterial pressure in comparison with RMR untreated animals; however, animals receiving nebivolol presented lower levels of collagen type I expression as well as lower glomerular and interstitial fibrosis than those receiving atenolol. Urinary excretion of oxidative stress markers were also lower in animals receiving nebivolol than in rats treated with atenolol. Furthermore, nebivolol prevented RMR-induced endothelial dysfunction more efficiently than atenolol.

CONCLUSIONS

Nebivolol protects against renal fibrosis, oxidative stress and endothelial dysfunction better than equivalent doses, in terms of arterial pressure reduction, of atenolol in a hypertensive model of renal damage induced by RMR.

Pathophysiology of hypertension during preeclampsia: linking placental ischemia with endothelial dysfunction

Studies over the last decade have provided exciting new insights into potential mechanisms underlying the pathogenesis of preeclampsia. The initiating event in preeclampsia is generally regarded to be placental ischemia/hypoxia, which in turn results in the elaboration of a variety of factors from the placenta that generates profound effects on the cardiovascular system. This host of molecules includes factors such as soluble fms-like tyrosine kinase-1, the angiotensin II type 1 receptor autoantibody, and cytokines such as tumor necrosis factor-alpha, which generate widespread dysfunction of the maternal vascular endothelium. This dysfunction manifests as enhanced formation of factors such as endothelin, reactive oxygen species, and augmented vascular sensitivity to angiotensin II. Alternatively, the preeclampsia syndrome may also be evidenced as decreased formation of vasodilators such as nitric oxide and prostacyclin. Taken together, these alterations cause hypertension by impairing renal pressure natriuresis and increasing total peripheral resistance. Moreover, the quantitative importance of the various endothelial and humoral factors that mediate vasoconstriction and elevation of arterial pressure during preeclampsia remains to be elucidated. Thus identifying the connection between placental ischemia/hypoxia and maternal cardiovascular abnormalities in hopes of revealing potential therapeutic regimens remains an important area of investigation and will be the focus of this review.

Increased levels of circulating endothelial progenitor cells and circulating endothelial nitric oxide synthase in patients with gliomas

OBJECTIVE

Gliomas are among the highest vascularized tumors. We hypothesized that patients with gliomas have increased levels of circulating endothelial progenitor cells (EPCs) and circulating endothelial nitric oxide synthase (eNOS).

METHODS

The fraction of EPCs was quantified by fluorescence-activated cell sorter analysis using anti-CD34, -CD133 and -KDR (kinase insert domain receptor) monoclonal antibodies in unselected peripheral blood samples of 32 patients with gliomas. Control groups included 47 patients with other central nervous system tumors or diseases, 10 patients with recent ischemic strokes, and 19 healthy blood donors. The circulating eNOS concentration of plasma was measured by a colorimetric assay in the same samples. In addition, CD34(+)CD105(+) KDR(+) and CD34(+)CD146(+)KDR(-) cell fractions were measured.

RESULTS

The percentage of CD34(+)CD133(+)KDR(+) EPCs in the blood of glioma patients is significantly greater than that in the blood of patients with other central nervous system tumors or diseases (p = 0.003), stroke patients (p = 0.005), or healthy donors (p = 0.013). The plasma eNOS concentration is also significantly greater in glioma patients compared with each of the control groups (p < 0.001 for all groupwise comparisons). No significant differences in the levels of the EPCs or eNOS between any of the control groups were demonstrated. In the glioma patients, the level of eNOS correlated with the fraction of CD34(+)CD105(+)KDR(+) cells (r = 0.748; p = 0.008).

INTERPRETATION

The data are suggestive of increased mobilization of EPCs contributing to neoplastic vasculogenesis in glioma. The increased levels of EPCs and eNOS in the peripheral blood of glioma patients trigger further investigations as to their value as independent parameters for use in clinical practice.

Endothelial expression of endoglin in normocholesterolemic and hypercholesterolemic C57BL/6J mice before and after atorvastatin treatment

Endoglin (CD105) is a homodimeric transmembrane glycoprotein strongly related to transforming growth factor (TGF)-β signaling and many pathological states. In this study, we wanted to evaluate whether endoglin is expressed in normocholesterolemic and hypercholesterolemic C57BL/6J mice as well as whether it is affected by atorvastatin treatment in these mice. C57BL/6J mice were fed with chow diet or an atherogenic diet for 12 weeks after weaning. In 2 atorvastatin-treated groups, mice were fed the same diets (chow or atherogenic) as described above except atorvastatin was added at the dosage of 10 mg·kg–1·day–1for the last 8 weeks before euthanasia. Biochemical analysis of blood samples revealed that administration of atherogenic diet significantly increased levels of total cholesterol, VLDL, LDL, and decreased levels of HDL. Atorvastatin treatment resulted in a significant decrease in total cholesterol and VLDL only in mice fed by atherogenic diet. Quantitative stereological analysis revealed that atorvastatin significantly decreased endothelial expression of endoglin in C57BL/6J mice fed the atherogenic diet. In conclusion, we demonstrated that endothelial expression of endoglin is upregulated by hypercholesterolemia and decreased by the hypolipidemic effect of atorvastatin in C57BL/6J mice, suggesting that endoglin expression could be involved in atherogenesis.

Correlation between soluble endoglin, vascular endothelial growth factor receptor-1, and adipocytokines in preeclampsia

CONTEXT

Recent reports have demonstrated that soluble endoglin (sEng), an antiangiogenic protein thought to impair TGF-beta binding to receptors, and soluble vascular endothelial growth factor receptor (sVEGFR)-1 play important roles in the pathophysiology of preeclampsia (PE). Moreover, insulin resistance, which is greatly influenced by adipocytokines, characterizes PE.

OBJECTIVES

We examined possible links between sEng, VEGF, sVEGFR, and adipocytokines in the pathophysiology of PE.

STUDY DESIGN

We performed a cross-sectional study in 30 PE patients and controls matched for gestational age and body mass index. Blood samples were collected soon after disease onset. We measured serum concentrations of leptin, adiponectin, sEng, VEGF, placental growth factor (PlGF), and sVEGFR [soluble fms-like tyrosine kinase 1 (sFlt-1) and soluble fetal liver kinase 1 (sFlk-1)], and examined the placental protein content of sEng and sFlt-1.

RESULTS

sEng concentrations in PE patients (60.9 +/- 28.8 ng/ml) were significantly higher than those in controls (11.2 +/- 4.4 ng/ml). There was a significant correlation between sEng and sFlt-1 or PlGF. Moreover, there were significant differences in mean blood pressure between the high and low sEng groups, and in proteinuria between the high and low sFlt-1 groups, and significant differences in placental sEng and sFlt-1 contents between patients with and without severe hypertension or proteinuria. sEng was also correlated positively with adiponectin levels and negatively with the leptin to adiponectin ratio.

CONCLUSIONS

Along with sFlt-1 and PlGF, sEng might play a role in the pathophysiology of PE, especially in elevating blood pressure, while the association with hypoadiponectinemia and the high leptin to adiponectin ratio in pregnancy seem to be risk factors for PE.

Elevated serum soluble endoglin (sCD105) decreased during extracorporeal elimination therapy for familial hypercholesterolemia

Extracorporeal elimination is a method of LDL-lowering therapy that is used in severe familial hypercholesterolemia (FH) after other therapeutic approaches have failed. There are currently no universally accepted biomarkers that would allow determining necessary intensity of therapy and frequency of future therapeutic interventions. An ideal tool for immediate evaluation would be a readily measurable serum marker. We hypothesized that soluble endoglin (sCD105), a recently described indicator of endothelial dysfunction, may represent such a tool. Eleven patients with FH (three homozygous, eight heterozygous; Fredrickson type IIa, IIb) that have been monitored for 4.5+/-2.8 years were treated; eight by LDL-apheresis and three by hemorheopheresis. 40 sCD105 measurements were done, before and after two consecutive elimination procedures. Baseline serum sCD105 levels were significantly higher in the patients (5.74+/-1.47 microg/l in series I, 6.85+/-1.85 microg/l in series II) than in the control group (3.85+/-1.25 microg/l). They decreased to normal after LDL-elimination (p=0.0003) in all except for one patient. This return to normal was not due to a non-specific capture of endoglin in adsorption or filtration columns as demonstrated by measurement of sCD105 before and after passage through the elimination media. We conclude that the soluble endoglin levels in patients with severe FH remain elevated despite long-term intensive therapy and that they decrease after extracorporeal elimination. Endoglin can therefore serve as a marker for evaluation of the treatment efficacy and of the decreased atherosclerotic activity in patients with FH treated by extracorporeal LDL-cholesterol elimination.

Gene expression fingerprinting for human hereditary hemorrhagic telangiectasia

Hereditary hemorrhagic telangiectasia (HHT) or Osler-Weber-Rendu syndrome is an autosomal dominant vascular disorder characterized by telangiectases and internal arteriovenous malformations. It is caused by mutations in elements of the transforming growth factor-beta (TGF-beta) receptor complex: endoglin, a co-receptor, responsible for HHT1, or ALK1 (activin receptor-like kinase 1), a type I receptor leading to HHT2. Recently, we have established cultures of HHT endothelial cells, primary targets of the disease. These cells showed deficient TGF-beta signaling and angiogenesis, representing a useful human model to study the molecular mechanism of this disease. To understand the pathogenic mechanism underlying HHT, we have used total RNA probes to compare HHT versus non-HHT cells by expression microarrays. This work represents a systematic study to identify target genes affected in HHT cells. Given the similarity of symptoms in HHT1 and HHT2, special interest has been put on the identification of common targets for both HHT types. As a result, 277 downregulated and 63 upregulated genes were identified in HHT versus control cells. These genes are involved in biological processes relevant to the HHT pathology, such as angiogenesis, cytoskeleton, cell migration, proliferation and NO synthesis. The type of misregulated genes found in HHT endothelial cells lead us to propose a model of HHT pathogenesis, opening new perspectives to understand this disorder. Moreover, as the disease is originated by mutations in proteins of the TGF-beta receptor complex, these results may be useful to find out targets of the TGF-beta pathway in endothelium.

Life history of eNOS: partners and pathways

The complex regulation of eNOS (endothelial nitric oxide synthase) in cardiovascular physiology occurs at multiple stages. eNOS mRNA levels are controlled both at the transcriptional and post-transcriptional phases, and epigenetic mechanisms appear to modulate tissue-specific eNOS expression. The eNOS enzyme reversibly associates with a diverse family of protein partners that regulate eNOS sub-cellular localization, catalytic function, and biological activity. eNOS enzyme activity and sub-cellular localization are intimately controlled by post-translational modifications including phosphorylation, nitrosylation, and acylation. The multiple extra-cellular stimuli affecting eNOS function coordinate their efforts through these key modifications to dynamically control eNOS and NO bioactivity in the vessel wall. This review will focus on the biochemical partners and perturbations of the eNOS protein as this vital enzyme undergoes modulation by diverse signal transduction pathways in the vascular endothelium.

Endoglin increases eNOS expression by modulating Smad2 protein levels and Smad2-dependent TGF-beta signaling

The endothelial nitric oxide synthase (eNOS) is a critical regulator of cardiovascular homeostasis, whose dysregulation leads to different vascular pathologies. Endoglin is a component of the transforming growth factor beta (TGF-beta) receptor complex present in endothelial cells that is involved in angiogenesis, cardiovascular development, and vascular homeostasis. Haploinsufficient expression of endoglin has been shown to downregulate endothelium-derived nitric oxide in endoglin(+/-) (Eng(+/-)) mice and cultured endothelial cells. Here, we find that TGF-beta1 leads to an increased vasodilatation in Eng(+/+) mice that is severely impaired in Eng(+/-) mice, suggesting the involvement of endoglin in the TGF-beta regulated vascular homeostasis. The endoglin-dependent induction of eNOS occurs at the transcriptional level and is mediated by the type I TGF-beta receptor ALK5 and its downstream substrate Smad2. In addition, Smad2-specific signaling is upregulated in endoglin-induced endothelial cells, whereas it is downregulated upon endoglin gene suppression with small interference RNA (siRNA). The endoglin-dependent upregulation of Smad2 was confirmed using eNOS and pARE promoters, whose activities are known to be Smad2 dependent, as well as with the interference of Smad2 with siRNA, Smurf2, or a dominant negative form of Smad2. Furthermore, increased expression of endoglin in endoglin-inducible endothelial cells or in transfectants resulted in increased levels of Smad2 protein without affecting the levels of Smad2 mRNA. The increased levels of Smad2 appear to be due to a decreased ubiquitination and proteasome-dependent degradation leading to stabilization of Smad2. These results suggest that endoglin enhances Smad2 protein levels potentiating TGF-beta signaling, and leading to an increased eNOS expression in endothelial cells.

Loss of Vav2 proto-oncogene causes tachycardia and cardiovascular disease in mice

The Vav family is a group of signal transduction molecules that activate Rho/Rac GTPases during cell signaling. Experiments using knockout mice have indicated that the three Vav proteins present in mammals (Vav1, Vav2, and Vav3) are essential for proper signaling responses in hematopoietic cells. However, Vav2 and Vav3 are also highly expressed in nonhematopoietic tissues, suggesting that they may have additional functions outside blood cells. Here, we report that this is the case for Vav2, because the disruption of its locus in mice causes tachycardia, hypertension, and defects in the heart, arterial walls, and kidneys. We also provide physiological and pharmacological evidence demonstrating that the hypertensive condition of Vav2-deficient mice is due to a chronic stimulation of the renin/angiotensin II and sympathetic nervous systems. Together, these results indicate that Vav2 plays crucial roles in the maintenance of cardiovascular homeostasis in mice.

Reduced tumor growth and angiogenesis in endoglin-haploinsufficient mice

Endoglin is a transforming growth factor-beta(1) (TGF-beta(1)) accessory receptor which is highly expressed in tumor vessels. To study the role of endoglin in tumor growth and angiogenesis we induced a highly vascularized tumor in mice heterozygous for endoglin (Eng+/-) and in their control littermates (Eng+/+) by injecting 10(6) Lewis lung carcinoma (3LL) cells subcutaneously. Nine days after injection, the tumor was removed and weighed. Capillary density (CD31 immunohistochemistry), hemoglobin content and vascular cell adhesion molecule-1 (VCAM-1) expression were used to assess tumor vascularization. Tumor perfusion rate was measured by laser-Doppler technique. Expression of the hypoxia-inducible factor (HIF), endothelial nitric oxide synthase (eNOS) and vascular endothelial growth factor (VEGF) were determined by Western blot analysis. The aerobic metabolism and oxygen dependency were inferred from the measurement of ATP in tumoral tissue. Tumor weight, capillary density, hemoglobin and VCAM-1 were reduced by about 30% in Eng+/- compared to Eng+/+ littermates. The protein levels of eNOS and phosphorylated eNOS were significantly reduced in Eng+/- compared to Eng+/+ mice. HIF expression was slightly reduced whereas VEGF level was slightly increased in Eng+/- compared to Eng+/+. Tumor tissue levels of ATP and ADP were similar in both types of mice. These data demonstrate that endoglin plays a major role in tumor neoangiogenesis.

Endoglin modulation of TGF-beta1-induced collagen synthesis is dependent on ERK1/2 MAPK activation

BACKGROUND/AIMS

Transforming growth factor-beta1 (TGF-beta1) plays a pivotal role in the extracellular matrix accumulation observed in fibrotic diseases. Endoglin is an important component of the TGF-beta receptor complex highly expressed in tissues undergoing fibrotic processes. Endoglin expression regulates the effect of TGF-beta on extracellular matrix synthesis. The purpose of our study has been to understand the molecular mechanism by which endoglin exerts its effects on fibrosis and the possible role of MAP kinases in these effects.

METHODS

We have assessed in mock and in endoglin-transfected L6E9 myoblasts the effect of TGF-beta1 on collagen mRNA by Northern blot and effect of TGF-beta1 on collagen content in the cultured medium by [(3)H]-Proline incorporation into collagen proteins. Total and activated MAPK and their role on collagen synthesis were assessed by Western blot.

RESULTS

TGF-beta1 induced an increase on alpha(2) (I) collagen mRNA expression and collagen accumulation in mock-transfected myoblasts, whereas the response was much lower in endoglintransfected cells. TGF-beta1 activated the ERK1/2 and p38 MAPK pathways but not the JNK pathway in L6E9 myoblasts. TGF-beta1-induced alpha(2) (I) collagen mRNA expression and collagen accumulation were completely inhibited by SB203580, in either mock or endoglintransfected myoblasts. PD98059 increased TGF-beta1 induced-collagen synthesis and accumulation in endoglin-transfected myoblasts but not in mock cells.

CONCLUSION

Our studies demonstrate that TGF-beta1- induced collagen synthesis is mediated by p38 MAPK activation in L6E9 myoblasts. Furthermore, endoglin expression reduces basal and TGF-beta1 induced collagen synthesis when ERK1/2 pathway is operating.

Mice Deficient in Telomerase Activity Develop Hypertension Because of an Excess of Endothelin Production

Background— Telomere shortening has been related to vascular dysfunction and hypertension. In the present study, we analyzed the influence of telomerase deficiency and telomere shortening on arterial pressure (AP).

Methods and Results— AP was evaluated in 6-month-old mice lacking the RNA component of the telomerase ( terc −/− ) at the first generation and third generation (G3). First generation and G3 mice showed higher AP than wild-type (WT) mice. To analyze the mechanisms involved, mean AP and vascular resistance in response to vasoactive substances were measured in G3 and WT mice. These mice showed similar responses to acetylcholine, N G -nitro- l -arginine methyl ester, angiotensin II, and losartan administration. Mean AP did not increase after endothelin-1 (ET-1) administration in G3 mice, but it did in WT animals. Bosentan treatment decreased mean AP only in G3 mice. Serum and urine concentrations of ET-1 were higher in terc −/− than in WT mice. Endothelin-converting enzyme (ECE-1) mRNA expression was higher in terc −/− animals than in the WT group. FR901533, an ECE antagonist, decreased blood pressure in conscious G3 mice. Studies in mouse embryonic fibroblasts from G3 mice suggest that ECE-1 overexpression could be mediated by reactive oxygen species in an AP-1–dependent mechanism, in which some kinases such as PI3-kinase, Akt, erk1/2, and Jun Kinase could be involved. An increased activity of nicotinamide adenine dinucleotide phosphate oxidase seems to be the main source of reactive oxygen species.

Conclusions— Mice lacking telomerase activity show hypertension as a result of an increase in plasma ET-1 levels, which is a consequence of ECE-1 overexpression. A direct link between telomerase activity and hypertension is reported.

Endoglin regulates cyclooxygenase-2 expression and activity

The endoglin heterozygous (Eng(+/-)) mouse, which serves as a model of hereditary hemorrhagic telangiectasia (HHT), was shown to express reduced levels of endothelial NO synthase (eNOS) with impaired activity. Because of intricate changes in vasomotor function in the Eng(+/-) mice and the potential interactions between the NO- and prostaglandin-producing pathways, we assessed the expression and function of cyclooxygenase (COX) isoforms. A specific upregulation of COX-2 in the vascular endothelium and increased urinary excretion of prostaglandin E(2) were observed in the Eng(+/-) mice. Specific COX-2 inhibition with parecoxib transiently increased arterial pressure in Eng(+/-) but not in Eng(+/+) mice. Transfection of endoglin in L6E9 myoblasts, shown previously to stimulate eNOS expression, led to downregulation of COX-2 with no change in COX-1. In addition, COX-2 promoter activity and protein levels were inversely correlated with endoglin levels, in doxycyclin-inducible endothelial cells. Chronic NO synthesis inhibition with N(omega)-nitro-l-arginine methyl ester induced a marked increase in COX-2 only in the normal Eng(+/+) mice. N(omega)-nitro-l-arginine methyl ester also increased COX-2 expression and promoter activity in doxycyclin-inducible endoglin expressing endothelial cells, but not in control cells. The level of COX-2 expression following transforming growth factor-beta1 treatment was less in endoglin than in mock transfected L6E9 myoblasts and was higher in human endothelial cells silenced for endoglin expression. Our results indicate that endoglin is involved in the regulation of COX-2 activity. Furthermore, reduced endoglin levels and associated impaired NO production may be responsible, at least in part, for augmented COX-2 expression and activity in the Eng(+/-) mice.

Vav3 proto-oncogene deficiency leads to sympathetic hyperactivity and cardiovascular dysfunction

Although much is known about environmental factors that predispose individuals to hypertension and cardiovascular disease, little information is available regarding the genetic and signaling events involved. Indeed, few genes associated with the progression of these pathologies have been discovered despite intensive research in animal models and human populations. Here we identify Vav3, a GDP-GTP exchange factor that stimulates Rho and Rac GTPases, as an essential factor regulating the homeostasis of the cardiovascular system. Vav3-deficient mice exhibited tachycardia, systemic arterial hypertension and extensive cardiovascular remodeling. These mice also showed hyperactivity of sympathetic neurons from the time of birth. The high catecholamine levels associated with this condition led to the activation of the renin-angiotensin system, increased levels of kidney-related hormones and the progressive loss of cardiovascular and renal homeostasis. Pharmacological studies with drugs targeting sympathetic and renin-angiotensin responses confirmed the causative role and hierarchy of these events in the development of the Vav3-null mouse phenotype. These observations uncover the crucial role of Vav3 in the regulation of the sympathetic nervous system (SNS) and cardiovascular physiology, and reveal a signaling pathway that could be involved in the pathophysiology of human disease states involving tachycardia and sympathetic hyperactivity with unknown etiologies.

Endoglin regulates renal ischaemia-reperfusion injury

BACKGROUND

Renal ischaemia-reperfusion (I-R) can cause acute tubular necrosis and chronic renal deterioration. Endoglin, an accessory receptor for Transforming Growth Factor-beta1 (TGF-beta1), is expressed on activated endothelium during macrophage maturation and implicated in the control of fibrosis, angiogenesis and inflammation.

METHODS

Endoglin expression was monitored over 14 days after renal I-R in rats. As endoglin-null mice are not viable, the role of endoglin in I-R was studied by comparing renal I-R injury in haploinsufficient mice (Eng(+/-)) and their wild-type littermates (Eng(+/+)). Renal function, morphology and molecular markers of acute renal injury and inflammation were compared.

RESULTS

Endoglin mRNA up-regulation in the post-ischaemic kidneys of rats occurred at 12 h after I-R; endoglin protein levels were elevated throughout the study period. Expression was initially localized to the vascular endothelium, then extended to fibrotic and inflamed areas of the interstitium. Two days after I-R, plasma creatinine elevation and acute tubular necrosis were less marked in Eng(+/-) than in Eng(+/+) mice. Significant up-regulation of endoglin protein was found only in the post-ischaemic kidneys of Eng(+/+) mice and coincided with an increased mRNA expression of the TGF-beta1 and collagen IV (alpha1) chain genes. Significant increases in vascular cell adhesion molecule-1 (VCAM-1) and inducible nitric oxide synthase (iNOS) expression, nitrosative stress, myeloperoxidase activity and CD68 staining for macrophages were evident in post-ischaemic kidneys of Eng(+/+), but not Eng(+/-) mice, suggesting that impaired endothelial activation and macrophage maturation may account for the reduced injury in post-ischaemic kidneys of Eng(+/-) mice.

CONCLUSIONS

Endoglin is up-regulated in the post-ischaemic kidney and endoglin-haploinsufficient mice are protected from renal I-R injury. Endoglin may play a primary role in promoting inflammatory responses following renal I-R.

Interaction and functional interplay between endoglin and ALK-1, two components of the endothelial transforming growth factor-beta receptor complex

Transforming growth factor-beta (TGF-beta) signaling in endothelial cells is able to modulate angiogenesis and vascular remodeling, although the underlying molecular mechanisms remain poorly understood. Endoglin and ALK-1 are components of the TGF-beta receptor complex, predominantly expressed in endothelial cells, and mutations in either endoglin or ALK-1 genes are responsible for the vascular dysplasia known as hereditary hemorrhagic telangiectasia. Here we find that the extracellular and cytoplasmic domains of the auxiliary TGF-beta receptor endoglin interact with ALK-1 (a type I TGF-beta receptor). In addition, endoglin potentiates TGF-beta/ALK1 signaling, with the extracellular domain of endoglin contributing to this functional cooperation between endoglin and ALK-1. By contrast, endoglin appears to interfere with TGF-beta/ALK-5 signaling. These results suggest that the functional association of endoglin with ALK-1 is critical for the endothelial responses to TGF-beta.

Characterization of murine S-endoglin isoform and its effects on tumor development

Endoglin is a transmembrane glycoprotein that acts as an auxiliary receptor for transforming growth factor-beta (TGF-beta) and modulates cellular responses to this pleiotropic cytokine. Endoglin is strongly expressed in endothelial cells, where it appears to exert a crucial role in vascular development and angiogenesis. Two endoglin isoforms (L and S), differing in their cytoplasmic domains, have been previously characterized in human tissues. We now demonstrate the existence of similar L- and S-endoglin variants in murine tissues with 47 and 35 amino acids, respectively, in their cytoplasmic tail. RT-PCR analysis showed that L is the predominant endoglin isoform expressed in mouse tissues, although S-endoglin mRNA is significantly expressed in liver and lung, as well as in endothelial cell lines. Furthermore, a protein of size equivalent to recombinant S-endoglin expressed in mammalian cells was detected in mouse endothelial cells by Western blot analysis. L- and S-endoglin isoforms can form disulfide-linked heterodimers, as demonstrated by cotransfection of L- and S-endoglin constructs. To address the role of S-endoglin in vivo, an S-Eng(+) transgenic mouse model that targets S-endoglin expression to the endothelium was generated. The lethal phenotype of endoglin-null (Eng(-/-)) mice was not rescued by breeding S-Eng(+) transgenic mice into the endoglin-null background. S-Eng(+) mice exhibited reduced tumor growth and neovascularization after transplantation of Lewis lung carcinoma cells. In addition, S-Eng(+) mice showed a drastic inhibition of benign papilloma formation when subjected to two-stage chemical skin carcinogenesis. These results point to S-endoglin as an antiangiogenic molecule, in contrast to L-endoglin which is proangiogenic. Oncogene (2005) 24, 4450-4461. doi:10.1038/sj.onc.1208644 Published online 4 April 2005.

A role for endoglin in coupling eNOS activity and regulating vascular tone revealed in hereditary hemorrhagic telangiectasia

Decreased endothelial NO synthase (eNOS)-derived NO bioavailability and impaired vasomotor control are crucial factors in cardiovascular disease pathogenesis. Hereditary hemorrhagic telangiectasia type 1 (HHT1) is a vascular disorder associated with ENDOGLIN (ENG) haploinsufficiency and characterized by venous dilatations, focal loss of capillaries, and arteriovenous malformations (AVMs). We report that resistance arteries from Eng+/- mice display an eNOS-dependent enhancement in endothelium-dependent dilatation and impairment in the myogenic response, despite reduced eNOS levels. We have found that eNOS is significantly reduced in endoglin-deficient endothelial cells because of decreased eNOS protein half-life. We demonstrate that endoglin can reside in caveolae and associate with eNOS, suggesting a stabilizing function of endoglin for eNOS. After Ca2+-induced activation, endoglin-deficient endothelial cells have reduced eNOS/Hsp90 association, produce less NO, and generate more eNOS-derived superoxide (O2-), indicating that endoglin also facilitates eNOS/Hsp90 interactions and is an important regulator in the coupling of eNOS activity. Treatment with an O2- scavenger reverses the vasomotor abnormalities in Eng(+/-) arteries, suggesting that uncoupled eNOS and resulting impaired myogenic response represent early events in HHT1 pathogenesis and that the use of antioxidants may provide a novel therapeutic modality.

Highlights on endoglin (CD105): from basic findings towards clinical applications in human cancer

Antibody targeting of tumor-associated vasculature is a promising therapeutic approach in human cancer; however, a specific cell membrane marker for endothelial cells of tumor vasculature has not been discovered yet. Endoglin (CD105) is a cell-surface glycoprotein most recently identified as an optimal indicator of proliferation of human endothelial cells. The finding that CD105 is over-expressed on vascular endothelium in angiogenetic tissues has prompted several pre-clinical studies designed to get a deeper understanding on the role of CD105 in angiogenesis, and to evaluate the most appropriate clinical setting(s) to utilize CD105 as a therapeutic target. In this review, the foreseeable clinical applications of CD105 in human cancer are discussed.