Endothelial ERα promotes glucose tolerance by enhancing endothelial insulin transport to skeletal muscle

The estrogen receptor (ER) designated ERα has actions in many cell and tissue types that impact glucose homeostasis. It is unknown if these include mechanisms in endothelial cells, which have the potential to influence relative obesity, and processes in adipose tissue and skeletal muscle that impact glucose control. Here we show that independent of impact on events in adipose tissue, endothelial ERα promotes glucose tolerance by enhancing endothelial insulin transport to skeletal muscle. Endothelial ERα-deficient male mice are glucose intolerant and insulin resistant, and in females the antidiabetogenic actions of estradiol (E2) are absent. The glucose dysregulation is due to impaired skeletal muscle glucose disposal that results from attenuated muscle insulin delivery. Endothelial ERα activation stimulates insulin transcytosis by skeletal muscle microvascular endothelial cells. Mechanistically this involves nuclear ERα-dependent upregulation of vesicular trafficking regulator sorting nexin 5 (SNX5) expression, and PI3 kinase activation that drives plasma membrane recruitment of SNX5. Thus, coupled nuclear and non-nuclear actions of ERα promote endothelial insulin transport to skeletal muscle to foster normal glucose homeostasis.

Macrophage-to-endothelial cell crosstalk by the cholesterol metabolite 27HC promotes atherosclerosis in male mice

Hypercholesterolemia and vascular inflammation are key interconnected contributors to the pathogenesis of atherosclerosis. How hypercholesterolemia initiates vascular inflammation is poorly understood. Here we show in male mice that hypercholesterolemia-driven endothelial activation, monocyte recruitment and atherosclerotic lesion formation are promoted by a crosstalk between macrophages and endothelial cells mediated by the cholesterol metabolite 27-hydroxycholesterol (27HC). The pro-atherogenic actions of macrophage-derived 27HC require endothelial estrogen receptor alpha (ERα) and disassociation of the cytoplasmic scaffolding protein septin 11 from ERα, leading to extranuclear ERα- and septin 11-dependent activation of NF-κB. Furthermore, pharmacologic inhibition of cyp27a1, which generates 27HC, affords atheroprotection by reducing endothelial activation and monocyte recruitment. These findings demonstrate cell-to-cell communication by 27HC, and identify a major causal linkage between the hypercholesterolemia and vascular inflammation that partner to promote atherosclerosis. Interventions interrupting this linkage may provide the means to blunt vascular inflammation without impairing host defense to combat the risk of atherosclerotic cardiovascular disease that remains despite lipid-lowering therapies.

Abstract 450: Monocyte-to-endothelial Cell Crosstalk By 27-hydroxycholesterol Promotes Monocyte Vascular Recruitment And Thereby Advances Atherogenesis

How hypercholesterolemia invokes the vascular inflammation that is critical to atherogenesis is poorly understood. Sterol 27-hydroxylase (cyp27a1) converts cholesterol to 27-hydroxycholesterol (27HC), and we previously showed that 27HC is an estrogen receptor alpha (ERα) ligand. Here we determined how 27HC produced by monocytes impacts atherogenesis. In the absence of changes in circulating lipids, monocyte cyp27a1 deletion in apolipoprotein E -/- mice (apoE -/- ) caused a 35% decline in atherosclerotic lesion severity; this was related to decreased macrophage accumulation due to a reduction in lesion monocyte recruitment, which likely occurred as a result of decreases in endothelial cell VCAM-1 and ICAM-1. Experiments in cultured endothelial cells revealed that 27HC liganding of ERα activates NF-κB by causing the disassociation of the cytoplasmic scaffolding protein septin 11 from ERα and resulting septin 11 activation of Jnk kinase signaling. Endothelial septin 11 deletion in apoE -/- mice phenocopied the impact of monocyte cyp27a1 loss, yielding a 37% decrease in lesion severity that was similarly explained by declines in endothelial cell VCAM-1 and ICAM-1, lesion monocyte recruitment and macrophage accumulation. In bone marrow transplant studies the reintroduction of monocyte cyp27a1 caused a 71% increase in lesions which was fully dependent on endothelial cell ERα and septin 11. Furthermore, in apoE -/- mice pharmacologic inhibition of cyp27a1 decreased lesion severity by 41% without altering circulating lipids, causing declines in endothelial cell VCAM-1 and ICAM-1, monocyte recruitment and macrophage accumulation. There was no additivity with monocyte cyp27a1 deletion and cyp27a1 inhibition, indicating that the treatment-related atheroprotection occurs via monocyte cyp27a1 inhibition. These cumulative findings reveal that through a unique pro-inflammatory mechanism involving endothelial cell ERα and septin 11, monocyte-to-endothelial cell crosstalk by the cholesterol metabolite 27HC promotes monocyte vascular recruitment to advance atherogenesis. Interventions attenuating the crosstalk may complement LDL-directed strategies in the battle against atherosclerosis.

Protein Phosphatase 2A Activation Via ApoER2 in Trophoblasts Drives Preeclampsia in a Mouse Model of the Antiphospholipid Syndrome

[Figure: see text].

Abstract 102: Apolipoprotein E Receptor 2 In Endothelium Affords Protection From Both Atherosclerosis And Insulin Resistance

Introduction: Apolipoprotein E Receptor 2 (ApoER2) is an LDL receptor family member that mediates the actions of apolipoprotein E (apoE) and other ligands. The LRP8 gene that encodes ApoER2 is a major gene locus for premature atherosclerosis and myocardial infarction. Studies were designed to determine how ApoER2 in endothelial cells impacts cardiovascular and metabolic health.

Hypothesis: Based on prior cell culture studies, the hypothesis was that endothelial ApoER2 is atheroprotective. How endothelial ApoER2 impacts adiposity or glucose homeostasis is more difficult to predict.

Methods: In vivo studies were done in control ApoER2 floxed mice (ApoER2 fl/fl ) and mice deficient in ApoER2 in endothelial cells (ApoER2 ΔEC ). Studies of endothelial cell insulin uptake and transcytosis were performed in human aortic and skeletal muscle endothelial cells (HAEC and HSMEC).

Results: Atherosclerotic lesion severity on LDLR -/- background was increased in ApoER2 ΔEC mice compared to ApoER2 fl/fl , and this was not related to differences in circulating lipids. Intravital microscopy revealed that leukocyte-endothelial cell adhesion is increased in ApoER2 ΔEC , and vascular TNFα and IL-1β expression was exaggerated. There was no impact of endothelial ApoER2 silencing on adiposity. However, standard chow-fed ApoER2 ΔEC mice displayed marked glucose intolerance and insulin resistance. Both pancreatic insulin secretion and hepatic insulin sensitivity were normal. Alternatively, in ApoER2 ΔEC mice skeletal muscle glucose disposal was decreased by 41%, and this was due to a 55% fall in insulin delivery to muscle. Using human apoE3 ex vivo, apoE stimulated a 279% increase in insulin uptake in HAEC, and the finding was confirmed in HSMEC. Insulin transcytosis rose 209% with apoE, and apoE enhancement of insulin transport was fully ApoER2-dependent.

Conclusions: Endothelial ApoER2 affords atheroprotection by decreasing leukocyte-endothelial cell adhesion and inflammatory gene expression in the vessel wall. Endothelial ApoER2 also has antidiabetic action, and this is due to the promotion of endothelial insulin transport to skeletal muscle. Targeting of the related mechanisms may provide protection from both cardiovascular and metabolic disease.

Abstract MP40: Breakpoint Cluster Region Protein Function As A Kinase Mediates The Atheroprotective Actions Of Hdl In Vascular Endothelium

Introduction: HDL has direct atheroprotective actions on endothelial cells (EC) that include the attenuation of vascular inflammation and promotion of endothelial monolayer integrity. These processes are mediated by scavenger receptor class B, type 1 (SR-BI) and its adaptor protein PDZK1. We have identified the breakpoint cluster region protein (BCR) as a PDZK1 interacting protein in EC. Best known as a gene in the BCR-ABL complex associated with the Philadelphia chromosome, BCR structure predicts both adaptor protein and kinase functions.

Hypothesis: Via interaction with PDZK1 and kinase activity, BCR mediates the atheroprotective actions of HDL on EC.

Methods: Newly-developed kinase assays that employ time-resolved Forster resonance energy transfer were used to detect kinase activity and identify kinase substrates. In vivo studies were done in ApoAI -/- versus ApoAI -/- ;BCR -/- mice administered control AAV or AAV-ApoAI. Leukocyte-EC interaction and carotid artery reendothelialization were evaluated by intravital microscopy and thermal injury, respectively. Atherosclerosis was invoked using AAV8-PCSK9. BCR expression was compared in human atherosclerotic and normal arteries in two independent publicly available patient cohorts.

Results: In cultured EC, HDL activation of eNOS, blunting of EC-monocyte adhesion, and promotion of EC migration all required BCR, and reconstitution studies with BCR mutants revealed requirements for both BCR-PDZK1 interaction and kinase activity. In response to HDL, BCR directly phosphorylated Akt-Ser473 and it provoked Akt-Thr308 phosphorylation by PDK1. ApoAI restoration in vivo in ApoAI -/- mice prompted SR-BI-PDZK1-BCR protein complex formation and BCR phosphorylation of Akt-Ser473 in mouse aorta. It also blunted leukocyte-endothelial cell adhesion and enhanced carotid artery reendothelialization in BCR +/+ , but not in BCR -/- mice. Furthermore, the atheroprotective effect of ApoAI restoration was demonstrable in BCR +/+ , but not in BCR -/- mice. In both human cohorts BCR expression was decreased in atherosclerotic versus normal arteries.

Conclusion: BCR function as a novel direct and indirect kinase of Akt mediates the atheroprotective actions of HDL in vascular endothelium.

Apolipoprotein E receptor 2 deficiency decreases endothelial adhesion of monocytes and protects against autoimmune encephalomyelitis

Under normal conditions, the blood-brain barrier effectively regulates the passage of immune cells into the central nervous system (CNS). However, under pathological conditions such as multiple sclerosis (MS), leukocytes, especially monocytes, infiltrate the CNS where they promote inflammatory demyelination, resulting in paralysis. Therapies targeting the immune cells directly and preventing leukocyte infiltration exist for MS but may compromise the immune system. Here, we explore how apolipoprotein E receptor 2 (ApoER2) regulates vascular adhesion and infiltration of monocytes during inflammation. We induced experimental autoimmune encephalitis in ApoER2 knockout mice and in mice carrying a loss-of-function mutation in the ApoER2 cytoplasmic domain. In both models, paralysis and neuroinflammation were largely abolished as a result of greatly diminished monocyte adherence due to reduced expression of adhesion molecules on the endothelial surface. Our findings expand our mechanistic understanding of the vascular barrier, the regulation of inflammation and vascular permeability, and the therapeutic potential of ApoER2-targeted therapies.

Reelin Depletion Protects Against Atherosclerosis by Decreasing Vascular Adhesion of Leukocytes

[Figure: see text].

Reelin depletion protects against autoimmune encephalomyelitis by decreasing vascular adhesion of leukocytes

Neuroinflammation as a result of immune cell recruitment into the central nervous system (CNS) is a key pathogenic mechanism of multiple sclerosis (MS). However, current anti-inflammatory interventions depleting immune cells or directly targeting their trafficking into the CNS can have serious side effects, highlighting a need for better immunomodulatory strategies. We detected increased Reelin concentrations in the serum of patients with MS, resulting in increased endothelial permeability to leukocytes through increased nuclear factor κB-mediated expression of vascular adhesion molecules. We thus investigated the prophylactic and therapeutic potential of Reelin immunodepletion in experimental autoimmune encephalomyelitis (EAE) and further validated the results in Reelin knockout mice. Removal of plasma Reelin by either approach protected against neuroinflammation and largely abolished the neurological consequences by reducing endothelial permeability and immune cell accumulation in the CNS. Our findings suggest Reelin depletion as a therapeutic approach with an inherent good safety margin for the treatment of MS and other diseases where leukocyte extravasation is a major driver of pathogenicity.

Supplementation With the Sialic Acid Precursor N-Acetyl-D-Mannosamine Breaks the Link Between Obesity and Hypertension

BACKGROUND

Obesity-related hypertension is a common disorder, and attempts to combat the underlying obesity are often unsuccessful. We previously revealed that mice globally deficient in the inhibitory immunoglobulin G (IgG) receptor FcγRIIB are protected from obesity-induced hypertension. However, how FcγRIIB participates is unknown. Studies were designed to determine if alterations in IgG contribute to the pathogenesis of obesity-induced hypertension.

METHODS

Involvement of IgG was studied using IgG μ heavy chain-null mice deficient in mature B cells and by IgG transfer. Participation of FcγRIIB was interrogated in mice with global or endothelial cell-specific deletion of the receptor. Obesity was induced by high-fat diet (HFD), and blood pressure (BP) was measured by radiotelemetry or tail cuff. The relative sialylation of the Fc glycan on mouse IgG, which influences IgG activation of Fc receptors, was evaluated by Sambucus nigra lectin blotting. Effects of IgG on endothelial NO synthase were assessed in human aortic endothelial cells. IgG Fc glycan sialylation was interrogated in 3442 human participants by mass spectrometry, and the relationship between sialylation and BP was evaluated. Effects of normalizing IgG sialylation were determined in HFD-fed mice administered the sialic acid precursor N-acetyl-D-mannosamine (ManNAc).

RESULTS

Mice deficient in B cells were protected from obesity-induced hypertension. Compared with IgG from control chow-fed mice, IgG from HFD-fed mice was hyposialylated, and it raised BP when transferred to recipients lacking IgG; the hypertensive response was absent if recipients were FcγRIIB-deficient. Neuraminidase-treated IgG lacking the Fc glycan terminal sialic acid also raised BP. In cultured endothelial cells, via FcγRIIB, IgG from HFD-fed mice and neuraminidase-treated IgG inhibited vascular endothelial growth factor activation of endothelial NO synthase by altering endothelial NO synthase phosphorylation. In humans, obesity was associated with lower IgG sialylation, and systolic BP was inversely related to IgG sialylation. Mice deficient in FcγRIIB in endothelium were protected from obesity-induced hypertension. Furthermore, in HFD-fed mice, ManNAc normalized IgG sialylation and prevented obesity-induced hypertension.

CONCLUSIONS

Hyposialylated IgG and FcγRIIB in endothelium are critically involved in obesity-induced hypertension in mice, and supportive evidence was obtained in humans. Interventions targeting these mechanisms, such as ManNAc supplementation, may provide novel means to break the link between obesity and hypertension.

Hyposialylated IgG activates endothelial IgG receptor FcγRIIB to promote obesity-induced insulin resistance

Type 2 diabetes mellitus (T2DM) is a common complication of obesity. Here, we have shown that activation of the IgG receptor FcγRIIB in endothelium by hyposialylated IgG plays an important role in obesity-induced insulin resistance. Despite becoming obese on a high-fat diet (HFD), mice lacking FcγRIIB globally or selectively in endothelium were protected from insulin resistance as a result of the preservation of insulin delivery to skeletal muscle and resulting maintenance of muscle glucose disposal. IgG transfer in IgG-deficient mice implicated IgG as the pathogenetic ligand for endothelial FcγRIIB in obesity-induced insulin resistance. Moreover, IgG transferred from patients with T2DM but not from metabolically healthy subjects caused insulin resistance in IgG-deficient mice via FcγRIIB, indicating that similar processes may be operative in T2DM in humans. Mechanistically, the activation of FcγRIIB by IgG from obese mice impaired endothelial cell insulin transcytosis in culture and in vivo. These effects were attributed to hyposialylation of the Fc glycan, and IgG from T2DM patients was also hyposialylated. In HFD-fed mice, supplementation with the sialic acid precursor N-acetyl-D-mannosamine restored IgG sialylation and preserved insulin sensitivity without affecting weight gain. Thus, IgG sialylation and endothelial FcγRIIB may represent promising therapeutic targets to sever the link between obesity and T2DM.

New Insights in the Pathophysiology of Antiphospholipid Syndrome

The antiphospholipid syndrome (APS) is an autoimmune disorder characterized by an elevated risk for arterial and venous thrombosis and pregnancy-related morbidity. Since the discovery of the disease in 1980s, numerous studies in cell culture systems, in animal models, and in patient populations have been reported, leading to a deeper understanding of the pathogenesis of APS. These studies have determined that circulating autoantibodies, collectively called antiphospholipid antibodies (aPL), the majority of which recognize cell surface proteins attached to the plasma membrane phospholipids, play a causal role in the development of the disease. The binding of aPL to the cell surface antigens triggers interaction of the complex with transmembrane receptors to initiate intracellular signaling in critical cell types, including platelets, monocytes, endothelial cells, and trophoblasts. Subsequent alteration of various cell functions results in inflammation, thrombus formation, and pregnancy complications. Apolipoprotein E receptor 2 (apoER2), a lipoprotein receptor family member, has been implicated as a mediator for aPL actions in platelets and endothelial cells. Nitric oxide (NO) is a signaling molecule known to exert potent antithrombotic, anti-inflammatory, and anti-atherogenic effects. NO insufficiency and oxidative stress have been linked to APS pathogenesis. This review will focus on the recent findings on how apoER2 and dysregulation of NO production contribute to aPL-mediated pathologies in APS.

Identification of a Monoclonal Antibody That Attenuates Antiphospholipid Syndrome-Related Pregnancy Complications and Thrombosis

In the antiphospholipid syndrome (APS), patients produce antiphospholipid antibodies (aPL) that promote thrombosis and adverse pregnancy outcomes. Current therapy with anticoagulation is only partially effective and associated with multiple complications. We previously discovered that aPL recognition of cell surface β2-glycoprotein I (β2-GPI) initiates apolipoprotein E receptor 2 (apoER2)-dependent signaling in endothelial cells and in placental trophoblasts that ultimately promotes thrombosis and fetal loss, respectively. Here we sought to identify a monoclonal antibody (mAb) to β2-GPI that negates aPL-induced processes in cell culture and APS disease endpoints in mice. In a screen measuring endothelial NO synthase (eNOS) activity in cultured endothelial cells, we found that whereas aPL inhibit eNOS, the mAb 1N11 does not, and instead 1N11 prevents aPL action. Coimmunoprecipitation studies revealed that 1N11 decreases pathogenic antibody binding to β2-GPI, and it blocks aPL-induced complex formation between β2-GPI and apoER2. 1N11 also prevents aPL antagonism of endothelial cell migration, and in mice it reverses the impairment in reendothelialization caused by aPL, which underlies the non-thrombotic vascular occlusion provoked by disease-causing antibodies. In addition, aPL inhibition of trophoblast proliferation and migration is negated by 1N11, and the more than 6-fold increase in fetal resorption caused by aPL in pregnant mice is prevented by 1N11. Furthermore, the promotion of thrombosis by aPL is negated by 1N11. Thus, 1N11 has been identified as an mAb that attenuates APS-related pregnancy complications and thrombosis in mice. 1N11 may provide an efficacious, mechanism-based therapy to combat the often devastating conditions suffered by APS patients.

ApoE Receptor 2 Mediation of Trophoblast Dysfunction and Pregnancy Complications Induced by Antiphospholipid Antibodies in Mice

OBJECTIVE

Pregnancies in women with the antiphospholipid syndrome (APS) are frequently complicated by fetal loss and intrauterine growth restriction (IUGR). How circulating antiphospholipid antibodies (aPL) cause pregnancy complications in APS is poorly understood. We sought to determine whether the low-density lipoprotein receptor family member apolipoprotein E receptor 2 (ApoER2) mediates trophoblast dysfunction and pregnancy complications induced by aPL.

METHODS

Placental and trophoblast ApoER2 expression was evaluated by immunohistochemistry and immunoblotting. Normal human IgG and aPL were purified from healthy individuals and APS patients, respectively. The role of ApoER2 in aPL-induced changes in trophoblast proliferation and migration and in kinase activation was assessed using RNA interference in HTR-8/SVneo cells. The participation of ApoER2 in aPL-induced pregnancy loss and IUGR was evaluated in pregnant ApoER2(+/+) and ApoER2(-/-) mice injected with aPL or normal human IgG.

RESULTS

We found that ApoER2 is abundant in human and mouse placental trophoblasts and in multiple trophoblast-derived cell lines, including HTR-8/SVneo cells. ApoER2 and its interaction with the cell surface protein β2 -glycoprotein I were required for aPL-induced inhibition of cultured trophoblast proliferation and migration. In parallel, aPL antagonism of Akt kinase activation by epidermal growth factor in trophoblasts was mediated by ApoER2. Furthermore, in a murine passive-transfer model of pregnancy complications of APS, ApoER2(-/-) mice were protected from both aPL-induced fetal loss and aPL-induced IUGR.

CONCLUSION

ApoER2 plays a major role in the attenuation of trophoblast function by aPL, and the receptor mediates aPL-induced pregnancy complications in vivo in mice. ApoER2-directed interventions can now potentially be developed to combat the pregnancy complications associated with APS.

Endothelial Fcγ Receptor IIB Activation Blunts Insulin Delivery to Skeletal Muscle to Cause Insulin Resistance in Mice

Modest elevations in C-reactive protein (CRP) are associated with type 2 diabetes. We previously revealed in mice that increased CRP causes insulin resistance and mice globally deficient in the CRP receptor Fcγ receptor IIB (FcγRIIB) were protected from the disorder. FcγRIIB is expressed in numerous cell types including endothelium and B lymphocytes. Here we investigated how endothelial FcγRIIB influences glucose homeostasis, using mice with elevated CRP expressing or lacking endothelial FcγRIIB. Whereas increased CRP caused insulin resistance in mice expressing endothelial FcγRIIB, mice deficient in the endothelial receptor were protected. The insulin resistance with endothelial FcγRIIB activation was due to impaired skeletal muscle glucose uptake caused by attenuated insulin delivery, and it was associated with blunted endothelial nitric oxide synthase (eNOS) activation in skeletal muscle. In culture, CRP suppressed endothelial cell insulin transcytosis via FcγRIIB activation and eNOS antagonism. Furthermore, in knock-in mice harboring constitutively active eNOS, elevated CRP did not invoke insulin resistance. Collectively these findings reveal that by inhibiting eNOS, endothelial FcγRIIB activation by CRP blunts insulin delivery to skeletal muscle to cause insulin resistance. Thus, a series of mechanisms in endothelium that impairs insulin movement has been identified that may contribute to type 2 diabetes pathogenesis.

Abstract 98: Antiphospholipid Antibodies Induce Thrombosis by Activating Endothelial PP2A via ApoER2-Dab2-PSD95 Complex Formation

In the antiphospholipid syndrome (APS), patients generate antiphospholipid antibodies (aPL) that promote thrombosis. We previous showed that aPL recognition of the endothelial cell surface protein β2-Glycoprotein I (β2GPI) causes β2GPI to interact with the LDL receptor family member ApoER2 and thereby antagonize endothelial NO synthase (eNOS). The decrease in bioavailable NO then leads to exaggerated thrombus formation. In the present work we sought to determine how aPL and the β2GPI-ApoER2 tandem antagonize eNOS. Initial experiments employed co-immunoprecipitation, RNAi-based gene silencing and adenoviral introduction of mutant proteins into human aortic endothelial cells. We discovered that in contrast to normal human IgG from healthy subjects (NHIgG), aPL invoke the formation of a complex between the cytoplasmic tail of ApoER2 and Dab-2 and PSD95, and that the formation of the complex is required for eNOS antagonism. We also found that ApoER2-Dab2-PSD95 complex formation in response to aPL potently activates the serine/threonine phosphatase PP2A, which in turn dephosphorylates eNOS-S1179, thereby extinguishing eNOS enzymatic activity. Furthermore, we found that upon aPL treatment, the PP2A catalytic and regulatory subunits are recruited to the ApoER2-Dab2-PSD95 complex. To test if these processes are operative in APS-related thrombosis in vivo, intravital microscopy of the mesenteric microcirculation was employed to evaluate thrombus formation in mice. In wild-type mice, aPL administration caused exaggerated thrombus formation compared to treatment with NHIgG. In contrast, ApoER2 did not promote thrombosis in knock-in ApoER2-EIG mice harboring a mutant receptor incapable of interacting with Dab-2. Moreover, in wild-type mice aPL treatment caused a dramatic increase in PP2A activity in the aorta, and administration of the PP2A inhibitor Endothall fully prevented thrombus formation induced by aPL. Having revealed the molecular underpinnings of the disorder, current treatment of APS with anticoagulation, which is often ineffective and fraught with complications, can potentially be replaced by new mechanism-based therapies targeting ApoER2 complex formation or PP2A.

Abstract 16: BcrKinase is a Novel Akt Kinase That Modulates Scavenger Receptor BI- and PDZK1-dependent Actions of HDL in Endothelium

High density lipoprotein cholesterol (HDL) has direct atheroprotective actions on endothelium. These are mediated by scavenger receptor class B, type I (SR-BI) and its adaptor protein PDZK1, and they entail the activation of Akt kinase, which phosphorylates and thereby stimulates endothelial nitric oxide synthase (eNOS). In the present work we sought to determine how PDZK1 couples HDL/SR-BI to Akt and eNOS to modulate endothelial function. Using tandem affinity purification (TAP) following the infection of the human endothelial cell line EAhy926 with adenovirus expressing TAP-tagged PDZK1, we identified Breakpoint Cluster Region (Bcr) kinase as a PDZK1 interacting protein in endothelium. Whereas Bcr is well-known as a component of the Bcr-Abl fusion protein that results from translocation of the Philadelphia chromosome in chronic myelogenous leukemia, little is known of its function in endothelial cells. Bcr contains several distinctive domains including a C-terminal PDZ binding motif and a serine/threonine protein kinase domain. In primary human aortic endothelial cells (HAEC), we determined that endogenous Bcr interacts with PDZK1 in an HDL-dependent manner, and that Bcr is required for HDL-induced activation of eNOS and HDL stimulation of endothelial cell migration, which underlies the ability of the lipoprotein to promote endothelial monolayer integrity. Studies of mutant forms of Bcr with disruption of PDZK1 binding or kinase activity introduced into endothelial cells further revealed that Bcr-PDZK1 interaction and its kinase function are required for HDL activation of Akt and eNOS. Using a novel kinase assay that we recently developed that employs time-resolved Forster resonance energy transfer, we found that via SR-BI and PDZK1, HDL stimulates Bcr kinase activity in endothelial cells more than 20-fold. In addition, using Akt-based peptides in studies of the two known kinases for Akt, mTOR and PDK1, we determined that HDL activates Bcr kinase to directly phosphorylate Akt-Ser473 in an mTOR independent manner, and that Akt-Thr308 is a direct substrate of PDK1. These collective findings have identified Bcr to be a novel kinase for Akt, and they have revealed that Bcr is critically involved in HDL modulation of endothelial cell phenotype.

A Protein Kinase C Phosphorylation Motif in GLUT1 Affects Glucose Transport and is Mutated in GLUT1 Deficiency Syndrome

Protein kinase C has been implicated in the phosphorylation of the erythrocyte/brain glucose transporter, GLUT1, without a clear understanding of the site(s) of phosphorylation and the possible effects on glucose transport. Through in vitro kinase assays, mass spectrometry, and phosphospecific antibodies, we identify serine 226 in GLUT1 as a PKC phosphorylation site. Phosphorylation of S226 is required for the rapid increase in glucose uptake and enhanced cell surface localization of GLUT1 induced by the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Endogenous GLUT1 is phosphorylated on S226 in primary endothelial cells in response to TPA or VEGF. Several naturally occurring, pathogenic mutations that cause GLUT1 deficiency syndrome disrupt this PKC phosphomotif, impair the phosphorylation of S226 in vitro, and block TPA-mediated increases in glucose uptake. We demonstrate that the phosphorylation of GLUT1 on S226 regulates glucose transport and propose that this modification is important in the physiological regulation of glucose transport.

PDZK1 prevents neointima formation via suppression of breakpoint cluster region kinase in vascular smooth muscle

Scavenger receptor class B, type I (SR-BI) and its adaptor protein PDZK1 mediate responses to HDL cholesterol in endothelium. Whether the receptor-adaptor protein tandem serves functions in other vascular cell types is unknown. The current work determined the roles of SR-BI and PDZK1 in vascular smooth muscle (VSM). To evaluate possible VSM functions of SR-BI and PDZK1 in vivo, neointima formation was assessed 21 days post-ligation in the carotid arteries of wild-type, SR-BI-/- or PDZK1-/- mice. Whereas neointima development was negligible in wild-type and SR-BI-/-, there was marked neointima formation in PDZK1-/- mice. PDZK1 expression was demonstrated in primary mouse VSM cells, and compared to wild-type cells, PDZK1-/- VSM displayed exaggerated proliferation and migration in response to platelet derived growth factor (PDGF). Tandem affinity purification-mass spectrometry revealed that PDZK1 interacts with breakpoint cluster region kinase (Bcr), which contains a C-terminal PDZ binding sequence and is known to enhance responses to PDGF in VSM. PDZK1 interaction with Bcr in VSM was demonstrated by pull-down and by coimmunoprecipitation, and the augmented proliferative response to PDGF in PDZK1-/- VSM was abrogated by Bcr depletion. Furthermore, compared with wild-type Bcr overexpression, the introduction of a Bcr mutant incapable of PDZK1 binding into VSM cells yielded an exaggerated proliferative response to PDGF. Thus, PDZK1 has novel SR-BI-independent function in VSM that affords protection from neointima formation, and this involves PDZK1 suppression of VSM cell proliferation via an inhibitory interaction with Bcr.

ETS-related gene (ERG) controls endothelial cell permeability via transcriptional regulation of the claudin 5 (CLDN5) gene

ETS-related gene (ERG) is a member of the ETS transcription factor family. Our previous studies have shown that ERG expression is highly enriched in endothelial cells (EC) both in vitro and in vivo. ERG expression is markedly repressed in response to inflammatory stimuli. It has been shown that ERG is a positive regulator of several EC-restricted genes including VE-cadherin, endoglin, and von Willebrand factor, and a negative regulator of other genes such as interleukin (IL)-8 and intercellular adhesion molecule (ICAM)-1. In this study we have identified a novel role for ERG in the regulation of EC barrier function. ERG knockdown results in marked increases in EC permeability. This is associated with a significant increase of stress fiber and gap formation in EC. Furthermore, we identify CLDN5 as a downstream target of ERG in EC. Thus, our results suggest that ERG plays a pivotal role in regulating EC barrier function and that this effect is mediated in part through its regulation of CLDN5 gene expression.

Molecular mechanisms controlling vascular lumen formation in three-dimensional extracellular matrices

Considerable progress has been made toward a molecular understanding of how cells form lumen and tube structures in three-dimensional (3D) extracellular matrices (ECM). This progress has occurred through work performed with endothelial and epithelial cell models using both in vitro and in vivo approaches. Despite the apparent similarities between endothelial and epithelial cell lumen and tube formation mechanisms, there are clear distinctions that directly relate to their functional differences. This review will focus on endothelial cell (EC) lumen formation mechanisms which control blood vessel formation during development and postnatal life. Of great interest is that an EC lumen signaling complex has been identified which controls human EC lumen and tube formation in 3D matrices and which coordinates integrin-ECM contacts, cell surface proteolysis, cytoskeletal rearrangements, and cell polarity. This complex consists of the collagen-binding integrin α2β1, the collagen-degrading membrane-type 1 matrix metalloproteinase (MT1-MMP), junction adhesion molecule (Jam)C, JamB, polarity proteins Par3 and Par6b, and the Rho GTPase Cdc42-GTP. These interacting proteins are necessary to stimulate 3D matrix-specific signaling events (including activation of protein kinase cascades that regulate the actin and microtubule cytoskeletons) to control the formation of EC lumens and tube networks. Also, EC lumen formation is directly coupled to the generation of vascular guidance tunnels, enzymatically generated ECM conduits that facilitate EC tube remodeling and maturation. Mural cells such as pericytes are recruited along EC tubes within these tunnel spaces to control ECM remodeling events resulting in vascular basement membrane matrix assembly, a key step in tube maturation and stabilization.

Mutations in 2 distinct genetic pathways result in cerebral cavernous malformations in mice

Cerebral cavernous malformations (CCMs) are a common type of vascular malformation in the brain that are a major cause of hemorrhagic stroke. This condition has been independently linked to 3 separate genes: Krev1 interaction trapped (KRIT1), Cerebral cavernous malformation 2 (CCM2), and Programmed cell death 10 (PDCD10). Despite the commonality in disease pathology caused by mutations in these 3 genes, we found that the loss of Pdcd10 results in significantly different developmental, cell biological, and signaling phenotypes from those seen in the absence of Ccm2 and Krit1. PDCD10 bound to germinal center kinase III (GCKIII) family members, a subset of serine-threonine kinases, and facilitated lumen formation by endothelial cells both in vivo and in vitro. These findings suggest that CCM may be a common tissue manifestation of distinct mechanistic pathways. Nevertheless, loss of heterozygosity (LOH) for either Pdcd10 or Ccm2 resulted in CCMs in mice. The murine phenotype induced by loss of either protein reproduced all of the key clinical features observed in human patients with CCM, as determined by direct comparison with genotype-specific human surgical specimens. These results suggest that CCM may be more effectively treated by directing therapies based on the underlying genetic mutation rather than treating the condition as a single clinical entity.

Blood vessel tubulogenesis requires Rasip1 regulation of GTPase signaling

Cardiovascular function depends on patent blood vessel formation by endothelial cells (ECs). However, the mechanisms underlying vascular "tubulogenesis" are only beginning to be unraveled. We show that endothelial tubulogenesis requires the Ras interacting protein 1, Rasip1, and its binding partner, the RhoGAP Arhgap29. Mice lacking Rasip1 fail to form patent lumens in all blood vessels, including the early endocardial tube. Rasipl null angioblasts fail to properly localize the polarity determinant Par3 and display defective cell polarity, resulting in mislocalized junctional complexes and loss of adhesion to extracellular matrix (ECM). Similarly, depletion of either Rasip1 or Arhgap29 in cultured ECs blocks in vitro lumen formation, fundamentally alters the cytoskeleton, and reduces integrin-dependent adhesion to ECM. These defects result from increased RhoA/ROCK/myosin II activity and blockade of Cdc42 and Rac1 signaling. This study identifies Rasip1 as a unique, endothelial-specific regulator of Rho GTPase signaling, which is essential for blood vessel morphogenesis.

Molecular basis for endothelial lumen formation and tubulogenesis during vasculogenesis and angiogenic sprouting

Many studies reveal a fundamental role for extracellular matrix-mediated signaling through integrins and Rho GTPases as well as matrix metalloproteinases (MMPs) in the molecular control of vascular tube morphogenesis in three-dimensional (3D) tissue environments. Recent work has defined an endothelial cell (EC) lumen signaling complex of proteins that controls these vascular morphogenic events. These findings reveal a signaling interdependence between Cdc42 and MT1-MMP to control the 3D matrix-specific process of EC tubulogenesis. The EC tube formation process results in the creation of a network of proteolytically generated vascular guidance tunnels in 3D matrices that are utilized to remodel EC-lined tubes through EC motility and could facilitate processes such as flow-induced remodeling and arteriovenous EC sorting and differentiation. Within vascular guidance tunnels, key dynamic interactions occur between ECs and pericytes to affect vessel remodeling, diameter, and vascular basement membrane matrix assembly, a fundamental process necessary for endothelial tube maturation and stabilization. Thus, the EC lumen and tube formation mechanism coordinates the concomitant establishment of a network of vascular tubes within tunnel spaces to allow for flow responsiveness, EC-mural cell interactions, and vascular extracellular matrix assembly to control the development of the functional microcirculation.

MT1-MMP- and Cdc42-dependent signaling co-regulate cell invasion and tunnel formation in 3D collagen matrices

Complex signaling events control tumor invasion in three-dimensional (3D) extracellular matrices. Recent evidence suggests that cells utilize both matrix metalloproteinase (MMP)-dependent and MMP-independent means to traverse 3D matrices. Herein, we demonstrate that lysophosphatidic-acid-induced HT1080 cell invasion requires membrane-type-1 (MT1)-MMP-mediated collagenolysis to generate matrix conduits the width of a cellular nucleus. We define these spaces as single-cell invasion tunnels (SCITs). Once established, cells can migrate within SCITs in an MMP-independent manner. Endothelial cells, smooth muscle cells and fibroblasts also generate SCITs during invasive events, suggesting that SCIT formation represents a fundamental mechanism of cellular motility within 3D matrices. Coordinated cellular signaling events are required during SCIT formation. MT1-MMP, Cdc42 and its associated downstream effectors such as MRCK (myotonic dystrophy kinase-related Cdc42-binding kinase) and Pak4 (p21 protein-activated kinase 4), protein kinase Calpha and the Rho-associated coiled-coil-containing protein kinases (ROCK-1 and ROCK-2) coordinate signaling necessary for SCIT formation. Finally, we show that MT1-MMP and Cdc42 are fundamental components of a co-associated invasion-signaling complex that controls directed single-cell invasion of 3D collagen matrices.

Formation of endothelial lumens requires a coordinated PKCepsilon-, Src-, Pak- and Raf-kinase-dependent signaling cascade downstream of Cdc42 activation

In this study, we present data showing that Cdc42-dependent lumen formation by endothelial cells (ECs) in three-dimensional (3D) collagen matrices involves coordinated signaling by PKCepsilon in conjunction with the Src-family kinases (SFKs) Src and Yes. Activated SFKs interact with Cdc42 in multiprotein signaling complexes that require PKCepsilon during this process. Src and Yes are differentially expressed during EC lumen formation and siRNA suppression of either kinase, but not Fyn or Lyn, results in significant inhibition of EC lumen formation. Concurrent with Cdc42 activation, PKCepsilon- and SFK-dependent signaling converge to activate p21-activated kinase (Pak)2 and Pak4 in steps that are also required for EC lumen formation. Pak2 and Pak4 further activate two Raf kinases, B-Raf and C-Raf, leading to ERK1 and ERK2 (ERK1/2) activation, which all seem to be necessary for EC lumen formation. This work reveals a multicomponent kinase signaling pathway downstream of integrin-matrix interactions and Cdc42 activation involving PKCepsilon, Src, Yes, Pak2, Pak4, B-Raf, C-Raf and ERK1/2 to control EC lumen formation in 3D collagen matrices.

Determinants for association and guide RNA-directed endonuclease cleavage by purified RNA editing complexes from Trypanosoma brucei

U-insertion/deletion RNA editing in the single mitochondrion of kinetoplastids, an ancient lineage of eukaryotes, is a unique mRNA maturation process needed for translation. Multisubunit editing complexes recognize many pre-edited mRNA sites and modify them via cycles of three catalytic steps: guide RNA (gRNA)-directed cleavage, insertion or deletion of uridylates at the 3'-terminus of the upstream cleaved piece, and ligation of the two mRNA pieces. While catalytic and many structural protein subunits of these complexes have been identified, the mechanisms and basic determinants of substrate recognition are still poorly understood. This study defined relatively simple single- and double-stranded determinants for association and gRNA-directed cleavage. To this end, we used an electrophoretic mobility shift assay to directly score the association of purified editing complexes with RNA ligands, in parallel with UV photocrosslinking and functional studies. The cleaved strand required a minimal 5' overhang of 12 nt and an approximately 15-bp duplex with gRNA to direct the cleavage site. A second protruding element in either the cleaved or the guide strand was required unless longer duplexes were used. Importantly, the single-stranded RNA requirement for association can be upstream or downstream of the duplex, and the binding and cleavage activities of purified editing complexes could be uncoupled. The current observations together with our previous reports in the context of purified native editing complexes show that the determinants for association, cleavage and full-round editing gradually increase in complexity as these stages progress. The native complexes in these studies contained most, if not all, known core subunits in addition to components of the MRP complex. Finally, we found that the endonuclease KREN1 in purified complexes photocrosslinks with a targeted editing site. A model is proposed whereby one or more RNase III-type endonucleases mediate the initial binding and scrutiny of potential ligands and subsequent catalytic selectivity triggers either insertion or deletion editing enzymes.

In vitro three dimensional collagen matrix models of endothelial lumen formation during vasculogenesis and angiogenesis

Discovery and comprehension of detailed molecular signaling pathways underlying endothelial vascular morphogenic events including endothelial lumen formation are key steps in understanding their roles during embryonic development, as well as during various disease states. Studies that used in vitro three-dimensional (3D) matrix endothelial cell morphogenic assay models, in conjunction with in vivo studies, have been essential to identifying molecules and explaining their related signaling pathways that regulate endothelial cell morphogenesis. We present methods to study molecular mechanisms controlling EC lumen formation in 3D collagen matrices. In vitro models representing vasculogenesis and angiogenesis, whereby EC lumen formation and tube morphogenesis readily occur, are described. We also detail different methods of gene manipulation in ECs and their application to analyze critical signaling events regulating EC lumen formation.

Preferential interaction of a 25kDa protein with an A6 pre-mRNA substrate for RNA editing in Trypanosoma brucei

Mitochondrial gene expression in kinetoplastids is controlled after transcription, potentially at the levels of RNA maturation, stability and translation. Among these processes, RNA editing by U-insertion/deletion catalysed by multi-subunit editing complexes is best characterised at the molecular level. Nevertheless, mitochondrial RNA metabolism overall remains poorly understood, including the potential regulatory factors that may interact with the relevant catalytic molecular machines and/or RNA substrates. Here we report on a approximately 25kDa polypeptide in mitochondrial extracts that exhibits a preferential "zero-distance" photo-crosslinking interaction with an A6 pre-mRNA model substrate for RNA editing containing a single [(32)P] at the first editing site. The approximately 25kDa polypeptide purified away from editosomes upon ion-exchange chromatography and glycerol gradient sedimentation. Competition assays with homologous and heterologous transcripts suggest that the preferential recognition of the A6 substrate is based on relatively low-specificity RNA-protein contacts. Our mapping and substrate truncation analyses suggest that the crosslinking activity primarily targeted a predicted stem-loop region containing the first editing sites. Consistent with the notion that pre-mRNA folding may be required, pre-annealing with guide RNA abolished crosslinking. Interestingly, this preferential protein interaction with the A6 substrate seemed to require adenosine 5'-triphosphate but not hydrolysis. As in other biological systems, fine regulation in vivo may be brought about by transient networks of relatively low-specificity interactions in which multiple auxiliary factors bind to mRNAs and/or editing complexes in unique higher-order assemblies.

RNA editing complex interactions with a site for full-round U deletion in Trypanosoma brucei

Trypanosome U insertion and U deletion RNA editing of mitochondrial pre-mRNAs is catalyzed by multisubunit editing complexes as directed by partially complementary guide RNAs. The basic enzymatic activities and protein composition of these high-molecular mass complexes have been under intense study, but their specific protein interactions with functional pre-mRNA/gRNA substrates remains unknown. We show that editing complexes purified through extensive ion-exchange chromatography and immunoprecipitation make specific cross-linking interactions with A6 pre-mRNA containing a single 32P and photoreactive 4-thioU at the scissile bond of a functional site for full-round U deletion. At least four direct protein-RNA contacts are detected at this site by cross-linking. All four interactions are stimulated by unpaired residues just 5' of the pre-mRNA/gRNA anchor duplex, but strongly inhibited by pairing of the editing site region. Furthermore, competition analysis with homologous and heterologous transcripts suggests preferential contacts of the editing complex with the mRNA/gRNA duplex substrate. This apparent structural selectivity suggests that the RNA-protein interactions we observe may be involved in recognition of editing sites and/or catalysis in assembled complexes.

Minimal pre-mRNA substrates with natural and converted sites for full-round U insertion and U deletion RNA editing in trypanosomes

Trypanosome RNA editing by uridylate insertion or deletion cycles is a mitochondrial mRNA maturation process catalyzed by multisubunit complexes. A full-round of editing entails three consecutive steps directed by partially complementary guide RNAs: pre-mRNA cleavage, U addition or removal, and ligation. The structural and functional composition of editing complexes is intensively studied, but their molecular interactions in and around editing sites are not completely understood. In this study, we performed a systematic analysis of distal RNA requirements for full-round insertion and deletion by purified editosomes. We define minimal substrates for efficient editing of A6 and CYb model transcripts, and established a new substrate, RPS12. Important differences were observed in the composition of substrates for insertion and deletion. Furthermore, we also showed for the first time that natural sites can be artificially converted in both directions: from deletion to insertion or from insertion to deletion. Our site conversions enabled a direct comparison of the two editing kinds at common sites during substrate minimization and demonstrate that all basic determinants directing the editosome to carry out full-round insertion or deletion reside within each editing site. Surprisingly, we were able to engineer a deletion site into CYb, which exclusively undergoes insertion in nature.

Expression of mdm-2 protein in neoplastic, preneoplastic, and normal bronchial mucosa specimens: comparative study with p53 expression

Loss of function of the p53 tumor supressor gene is involved in nearly all human cancer. Recently a cellular oncogene product, mdm2, has been shown to bind to p53 and eliminate its ability to function as a transcription factor. mdm2 and p53 immunohistochemical protein expression was studied in tumor tissues, preneoplastic lesions, and normal bronchial mucosa. The specimens were obtained during diagnostic bronchoscopy from 53 patients with lung cancer. In the tumor specimens, p53 nuclear staining was detected in 26 (49%) cases, mdm2 in 11 (20.7%), and simultaneous expression of both proteins in 6 (11.3%) cases. Thirty-five sections with preneoplastic lesions were found in 21 patients. p53 nuclear staining was found in 11 of 35 and mdm2 in 6 of 35 sections. In normal cells, mdm2 positive staining was found in 18 and p53 in 12 specimens. Simultaneous p53 and mdm2 expression was found in 4 specimens. Our results indicate that p53 expression is more frequent than mdm2 expression in lung cancer tissues. Alterations in these proteins are early events and may represent alternative pathways in carcinogenesis.

Immunohistochemical detection of p53 protein in neoplastic, preneoplastic and normal bronchial mucosa specimens obtained during diagnostic bronchoscopy

The expression of p53 protein was evaluated immunochemically in cancer tissue, preneoplastic lesions and normal bronchial mucosa obtained during diagnostic bronchoscopy from 53 patients with lung cancer and 12 patients with benign lung diseases. In lung cancer patients, positive p53 staining was detected in 26/53 (49%) of the tumour specimens. In preneoplastic lesions p53 positive staining was found in 8 of 24 (33.3%) squamous metaplasia, 1 of 4 hyperplasia and 1 of 3 dysplasia lesions. In the same group of patients, 12 cases were found with positive p53 cells in normal bronchial mucosa. In patients with benign diseases, positive p53 staining was found in 1 of 4 cases with squamous metaplasia and in one normal mucosa. Our results provide evidence that somatic genetic alterations may occur in early stages of lung tumorigenesis, raising the possibility that molecular analyses is useful in the early diagnosis of precancerous lesions of the bronchial mucosa, and results indicate that p53 expression can be studied in small tissue specimens obtained during bronchoscopy.