HIF1alpha induces the recruitment of bone marrow-derived vascular modulatory cells to regulate tumor angiogenesis and invasion

Development of hypoxic regions is an indicator of poor prognosis in many tumors. Here, we demonstrate that HIF1alpha, the direct effector of hypoxia, partly through increases in SDF1alpha, induces recruitment of bone marrow-derived CD45+ myeloid cells containing Tie2+, VEGFR1+, CD11b+, and F4/80+ subpopulations, as well as endothelial and pericyte progenitor cells to promote neovascularization in glioblastoma. MMP-9 activity of bone marrow-derived CD45+ cells is essential and sufficient to initiate angiogenesis by increasing VEGF bioavailability. In the absence of HIF1alpha, SDF1alpha levels decrease, and fewer BM-derived cells are recruited to the tumors, decreasing MMP-9 and mobilization of VEGF. VEGF also directly regulates tumor cell invasiveness. When VEGF activity is impaired, tumor cells invade deep into the brain in the perivascular compartment.

AZD2171: a highly potent, orally bioavailable, vascular endothelial growth factor receptor-2 tyrosine kinase inhibitor for the treatment of cancer

Inhibition of vascular endothelial growth factor-A (VEGF) signaling is a promising therapeutic approach that aims to stabilize the progression of solid malignancies by abrogating tumor-induced angiogenesis. This may be accomplished by inhibiting the kinase activity of VEGF receptor-2 (KDR), which has a key role in mediating VEGF-induced responses. The novel indole-ether quinazoline AZD2171 is a highly potent (IC50 < 1 nmol/L) ATP-competitive inhibitor of recombinant KDR tyrosine kinase in vitro. Concordant with this activity, in human umbilical vein endothelial cells, AZD2171 inhibited VEGF-stimulated proliferation and KDR phosphorylation with IC50 values of 0.4 and 0.5 nmol/L, respectively. In a fibroblast/endothelial cell coculture model of vessel sprouting, AZD2171 also reduced vessel area, length, and branching at subnanomolar concentrations. Once-daily oral administration of AZD2171 ablated experimental (VEGF-induced) angiogenesis in vivo and inhibited endochondral ossification in bone or corpora luteal development in ovary; physiologic processes that are highly dependent upon neovascularization. The growth of established human tumor xenografts (colon, lung, prostate, breast, and ovary) in athymic mice was inhibited dose-dependently by AZD2171, with chronic administration of 1.5 mg per kg per day producing statistically significant inhibition in all models. A histologic analysis of Calu-6 lung tumors treated with AZD2171 revealed a reduction in microvessel density within 52 hours that became progressively greater with the duration of treatment. These changes are indicative of vascular regression within tumors. Collectively, the data obtained with AZD2171 are consistent with potent inhibition of VEGF signaling, angiogenesis, neovascular survival, and tumor growth. AZD2171 is being developed clinically as a once-daily oral therapy for the treatment of cancer.

Dicer dependent microRNAs regulate gene expression and functions in human endothelial cells

Dicer is a key enzyme involved in the maturation of microRNAS (miRNAs). miRNAs have been shown to be regulators of gene expression participating in the control of a wide range of physiological pathways. To assess the role of Dicer and consequently the importance of miRNAs in the biology and functions of human endothelial cells (EC) during angiogenesis, we globally reduced miRNAs in ECs by specific silencing Dicer using siRNA and examined the effects on EC phenotypes in vitro. The knockdown of Dicer in ECs altered the expression (mRNA and/or protein) of several key regulators of endothelial biology and angiogenesis, such as TEK/Tie-2, KDR/VEGFR2, Tie-1, endothelial nitric oxide synthase and IL-8. Although, Dicer knockdown increased activation of the endothelial nitric oxide synthase pathway it reduced proliferation and cord formation of EC in vitro. The miRNA expression profile of EC revealed 25 highly expressed miRNAs in human EC and using miRNA mimicry, miR-222/221 regulates endothelial nitric oxide synthase protein levels after Dicer silencing. Collectively, these results indicate that maintenance and regulation of endogenous miRNA levels via Dicer mediated processing is critical for EC gene expression and functions in vitro.

Activin receptor-like kinase (ALK)1 is an antagonistic mediator of lateral TGFbeta/ALK5 signaling

Transforming growth factor-beta (TGFbeta) regulates the activation state of the endothelium via two opposing type I receptor/Smad pathways. Activin receptor-like kinase-1 (ALK1) induces Smad1/5 phosphorylation, leading to an increase in endothelial cell proliferation and migration, while ALK5 promotes Smad2/3 activation and inhibits both processes. Here, we report that ALK5 is important for TGFbeta/ALK1 signaling; endothelial cells lacking ALK5 are deficient in TGFbeta/ALK1-induced responses. More specifically, we show that ALK5 mediates a TGFbeta-dependent recruitment of ALK1 into a TGFbeta receptor complex and that the ALK5 kinase activity is required for optimal ALK1 activation. TGFbeta type II receptor is also required for ALK1 activation by TGFbeta. Interestingly, ALK1 not only induces a biological response opposite to that of ALK5 but also directly antagonizes ALK5/Smad signaling.

Angiogenesis selectively requires the p110alpha isoform of PI3K to control endothelial cell migration

Phosphoinositide 3-kinases (PI3Ks) signal downstream of multiple cell-surface receptor types. Class IA PI3K isoforms couple to tyrosine kinases and consist of a p110 catalytic subunit (p110alpha, p110beta or p110delta), constitutively bound to one of five distinct p85 regulatory subunits. PI3Ks have been implicated in angiogenesis, but little is known about potential selectivity among the PI3K isoforms and their mechanism of action in endothelial cells during angiogenesis in vivo. Here we show that only p110alpha activity is essential for vascular development. Ubiquitous or endothelial cell-specific inactivation of p110alpha led to embryonic lethality at mid-gestation because of severe defects in angiogenic sprouting and vascular remodelling. p110alpha exerts this critical endothelial cell-autonomous function by regulating endothelial cell migration through the small GTPase RhoA. p110alpha activity is particularly high in endothelial cells and preferentially induced by tyrosine kinase ligands (such as vascular endothelial growth factor (VEGF)-A). In contrast, p110beta in endothelial cells signals downstream of G-protein-coupled receptor (GPCR) ligands such as SDF-1alpha, whereas p110delta is expressed at low level and contributes only minimally to PI3K activity in endothelial cells. These results provide the first in vivo evidence for p110-isoform selectivity in endothelial PI3K signalling during angiogenesis.

Direct Evidence of Endothelial Oxidative Stress With Aging in Humans

Aging is associated with impaired vascular endothelial function, as indicated in part by reduced endothelium-dependent dilation (EDD). Decreased EDD with aging is thought to be related to vascular endothelial cell oxidative stress, but direct evidence is lacking. We studied 95 healthy men: 51 young (23+/-1 years) and 44 older (63+/-1 years). EDD (brachial artery flow-mediated dilation) was approximately 50% lower in older versus young men (3.9+/-0.3% versus 7.6+/-0.3%, P<0.01; n=42 older/n=51 young). Abundance of nitrotyrosine (quantitative immunofluorescence), an oxidatively modified amino acid and marker of oxidative stress, was higher in endothelial cells (ECs) obtained from the brachial artery (1.25+/-0.12 versus 0.61+/-0.11 nitrotyrosine intensity/human umbilical vein EC [HUVEC] intensity, P=0.01; n=11 older/n=11 young) and antecubital veins (0.55+/-0.04 versus 0.34+/-0.03, P<0.05; n=19 older/n=17 young) of older men. Flow-mediated dilation was inversely related to arterial EC nitrotyrosine expression (r=-0.62, P=0.01; n=22). In venous samples, EC expression of the oxidant enzyme NAD(P)H oxidase-p47(phox) was higher in older men (0.71+/-0.05 versus 0.57+/-0.05 NAD[P]H oxidase-p47(phox) intensity/HUVEC intensity, P<0.05; n=19 older/n=18 young), whereas xanthine oxidase and the antioxidant enzymes cytosolic and mitochondrial superoxide dismutase and catalase were not different between groups. Nuclear factor-kappaB p65, a component of the redox-sensitive nuclear transcription factor nuclear factor-kappaB, was elevated in both arterial (0.73+/-0.07 versus 0.53+/-0.05 NF-kappaB p65 intensity/HUVEC intensity, P<0.05; n=9 older/n=12 young) and venous (0.65+/-0.07 versus 0.34+/-0.05, P<0.01; n=13 older/n=15 young) EC samples of older men and correlated with nitrotyrosine expression (r=0.51, P<0.05 n=16). These results provide direct support for the hypothesis that endothelial oxidative stress develops with aging in healthy men and is related to reductions in EDD. Increased expression of NAD(P)H oxidase and nuclear factor-kappaB may contribute to endothelial oxidative stress with aging in humans.

Vascular endothelium as a contributor of plasma sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P), an abundant lipid mediator in plasma, regulates vascular and immune cells by activating S1P receptors. In this report, we investigated the mechanisms by which high plasma S1P levels are maintained in mice. We found that plasma S1P turns over rapidly with a half-life of approximately 15 minutes, suggesting the existence of a high-capacity biosynthetic source(s). Transplantation of bone marrow from wild-type to Sphk1(-/-)Sphk2(+/-) mice restored plasma S1P levels, suggesting that hematopoietic cells are capable of secreting S1P into plasma. However, plasma S1P levels were not appreciably altered in mice that were thrombocytopenic, anemic, or leukopenic. Surprisingly, reconstitution of Sphk1(-/-)Sphk2(+/-) bone marrow cells into wild-type hosts failed to reduce plasma S1P, suggesting the existence of an additional, nonhematopoietic source for plasma S1P. Adenoviral expression of Sphk1 in the liver of Sphk1(-/-) mice restored plasma S1P levels. In vitro, vascular endothelial cells, but not hepatocytes, secreted S1P in a constitutive manner. Interestingly, laminar shear stress downregulated the expression of S1P lyase (Sgpl) and S1P phosphatase-1 (Sgpp1) while concomitantly stimulating S1P release from endothelial cells in vitro. Modulation of expression of endothelial S1P lyase with small interfering RNA and adenoviral expression altered S1P secretion, suggesting an important role played by this enzyme. These data suggest that the vascular endothelium, in addition to the hematopoietic system, is a major contributor of plasma S1P.

Regulation of ROS signal transduction by NADPH oxidase 4 localization

Reactive oxygen species (ROS) function as intracellular signaling molecules in a diverse range of biological processes. However, it is unclear how freely diffusible ROS dictate specific cellular responses. In this study, we demonstrate that nicotinamide adenine dinucleotide phosphate reduced oxidase 4 (Nox4), a major Nox isoform expressed in nonphagocytic cells, including vascular endothelium, is localized to the endoplasmic reticulum (ER). ER localization of Nox4 is critical for the regulation of protein tyrosine phosphatase (PTP) 1B, also an ER resident, through redox-mediated signaling. Nox4-mediated oxidation and inactivation of PTP1B in the ER serves as a regulatory switch for epidermal growth factor (EGF) receptor trafficking and specifically acts to terminate EGF signaling. Consistent with this notion, PTP1B oxidation could also be modulated by ER targeting of antioxidant enzymes but not their untargeted counterparts. These data indicate that the specificity of intracellular ROS-mediated signal transduction may be modulated by the localization of Nox isoforms within specific subcellular compartments.

Vascular protection: superoxide dismutase isoforms in the vessel wall

Blood vessels express 3 isoforms of superoxide dismutase (SOD): cytosolic or copper-zinc SOD (CuZn-SOD), manganese SOD (Mn-SOD) localized in mitochondria, and an extracellular form of CuZn-SOD (EC-SOD). Because there are no selective pharmacological inhibitors of individual SOD isoforms, the functional importance of the different SODs has been difficult to define. Recent molecular approaches, primarily the use of genetically-altered mice and viral-mediated gene transfer, have allowed investigators to begin to define the role of specific SOD isoforms in vascular biology. This review will focus mainly on the role of individual SODs in relation to endothelium under normal conditions and in disease states. This area is important because reactive oxygen species and superoxide anion are thought to play major roles in changes in vascular structure and function in pathophysiology.

A paxillin tyrosine phosphorylation switch regulates the assembly and form of cell-matrix adhesions

Diverse cellular processes are carried out by distinct integrin-mediated adhesions. Cell spreading and migration are driven by focal complexes; robust adhesion to the extracellular matrix by focal adhesions; and matrix remodeling by fibrillar adhesions. The mechanism(s) regulating the spatio-temporal distribution and dynamics of the three types of adhesion are unknown. Here, we combine live-cell imaging, labeling with phosphospecific-antibodies and overexpression of a novel tyrosine phosphomimetic mutant of paxillin, to demonstrate that the modulation of tyrosine phosphorylation of paxillin regulates both the assembly and turnover of adhesion sites. Moreover, phosphorylated paxillin enhanced lamellipodial protrusions, whereas non-phosphorylated paxillin was essential for fibrillar adhesion formation and for fibronectin fibrillogenesis. We further show that focal adhesion kinase preferentially interacted with the tyrosine phosphomimetic paxillin and its recruitment is implicated in high turnover of focal complexes and translocation of focal adhesions. We created a mathematical model that recapitulates the salient features of the measured dynamics, and conclude that tyrosine phosphorylation of the adaptor protein paxillin functions as a major switch, regulating the adhesive phenotype of cells.

Matrix metalloproteinase-9 is required for tumor vasculogenesis but not for angiogenesis: role of bone marrow-derived myelomonocytic cells

Tumor vasculature is derived from sprouting of local vessels (angiogenesis) and bone marrow (BM)-derived circulating cells (vasculogenesis). By using a model system of transplanting tumors into an irradiated normal tissue to prevent angiogenesis, we found that tumors were unable to grow in matrix metalloproteinase-9 (MMP-9) knockout mice, but tumor growth could be restored by transplantation of wild-type BM. Endothelial progenitor cells did not contribute significantly to this process. Rather, CD11b-positive myelomonocytic cells from the transplanted BM were responsible for tumor growth and the development of immature blood vessels in MMP-9 knockout mice receiving wild-type BM. Our results suggest that MMP-9 could be an important target for adjunct therapy to enhance the response of tumors to radiotherapy.

Endothelial nitric oxide synthase uncoupling impairs endothelial progenitor cell mobilization and function in diabetes

Uncoupling of the endothelial nitric oxide synthase (eNOS) resulting in superoxide anion (O(2)(-)) formation instead of nitric oxide (NO) causes diabetic endothelial dysfunction. eNOS regulates mobilization and function of endothelial progenitor cells (EPCs), key regulators of vascular repair. We postulate a role of eNOS uncoupling for reduced number and function of EPC in diabetes. EPC levels in diabetic patients were significantly reduced compared with those of control subjects. EPCs from diabetic patients produced excessive O(2)(-) and showed impaired migratory capacity compared with nondiabetic control subjects. NOS inhibition with N(G)-nitro-l-arginine attenuated O(2)(-) production and normalized functional capacity of EPCs from diabetic patients. Glucose-mediated EPC dysfunction was protein kinase C dependent, associated with reduced intracellular BH(4) (tetrahydrobiopterin) concentrations, and reversible after exogenous BH(4) treatment. Activation of NADPH oxidases played an additional but minor role in glucose-mediated EPC dysfunction. In rats with streptozotocin-induced diabetes, circulating EPCs were reduced to 39 +/- 5% of controls and associated with uncoupled eNOS in bone marrow. Our results identify uncoupling of eNOS in diabetic bone marrow, glucose-treated EPCs, and EPCs from diabetic patients resulting in eNOS-mediated O(2)(-) production. Subsequent reduction of EPC levels and impairment of EPC function likely contributes to the pathogenesis of vascular disease in diabetes.

Hydrogen peroxide is the major oxidant product of xanthine oxidase

Xanthine oxidase (XO) is a critical source of reactive oxygen species (ROS) in inflammatory disease. Focus, however, has centered almost exclusively on XO-derived superoxide (O(2)(*-)), whereas direct H(2)O(2) production from XO has been less well investigated. Therefore, we examined the relative quantities of O(2)(*-) and H(2)O(2) produced by XO under a range (1-21%) of O(2) tensions. At O(2) concentrations between 10 and 21%, H(2)O(2) accounted for approximately 75% of ROS production. As O(2) concentrations were lowered, there was a concentration-dependent increase in H(2)O(2) formation, accounting for 90% of ROS production at 1% O(2). Alterations in pH between 5.5 and 7.4 did not affect the relative proportions of H(2)O(2) and O(2)(*-) formation. Immobilization of XO, by binding to heparin-Sepharose, further enhanced relative H(2)O(2) production by approximately 30%, under both normoxic and hypoxic conditions. Furthermore, XO bound to glycosaminoglycans on the apical surface of bovine aortic endothelial cells demonstrated a similar ROS production profile. These data establish H(2)O(2) as the dominant (70-95%) reactive product produced by XO under clinically relevant conditions and emphasize the importance of H(2)O(2) as a critical factor when examining the contributory roles of XO-catalyzed ROS in inflammatory processes as well as cellular signaling.

GPIHBP1 is responsible for the entry of lipoprotein lipase into capillaries

The lipolytic processing of triglyceride-rich lipoproteins by lipoprotein lipase (LPL) is the central event in plasma lipid metabolism, providing lipids for storage in adipose tissue and fuel for vital organs such as the heart. LPL is synthesized and secreted by myocytes and adipocytes, but then finds its way into the lumen of capillaries, where it hydrolyzes lipoprotein triglycerides. The mechanism by which LPL reaches the lumen of capillaries has remained an unresolved problem of plasma lipid metabolism. Here, we show that GPIHBP1 is responsible for the transport of LPL into capillaries. In Gpihbp1-deficient mice, LPL is mislocalized to the interstitial spaces surrounding myocytes and adipocytes. Also, we show that GPIHBP1 is located at the basolateral surface of capillary endothelial cells and actively transports LPL across endothelial cells. Our experiments define the function of GPIHBP1 in triglyceride metabolism and provide a mechanism for the transport of LPL into capillaries.

TRPV4 channels mediate cyclic strain-induced endothelial cell reorientation through integrin-to-integrin signaling

Cyclic mechanical strain produced by pulsatile blood flow regulates the orientation of endothelial cells lining blood vessels and influences critical processes such as angiogenesis. Mechanical stimulation of stretch-activated calcium channels is known to mediate this reorientation response; however, the molecular basis remains unknown. Here, we show that cyclically stretching capillary endothelial cells adherent to flexible extracellular matrix substrates activates mechanosensitive TRPV4 (transient receptor potential vanilloid 4) ion channels that, in turn, stimulate phosphatidylinositol 3-kinase-dependent activation and binding of additional beta1 integrin receptors, which promotes cytoskeletal remodeling and cell reorientation. Inhibition of integrin activation using blocking antibodies and knock down of TRPV4 channels using specific small interfering RNA suppress strain-induced capillary cell reorientation. Thus, mechanical forces that physically deform extracellular matrix may guide capillary cell reorientation through a strain-dependent "integrin-to-integrin" signaling mechanism mediated by force-induced activation of mechanically gated TRPV4 ion channels on the cell surface.

AMPKalpha2 deletion causes aberrant expression and activation of NAD(P)H oxidase and consequent endothelial dysfunction in vivo: role of 26S proteasomes

RATIONALE

AMP-activated protein kinase (AMPK) is an energy sensor and ubiquitously expressed in vascular cells. Recent studies suggest that AMPK activation improves endothelial function by counteracting oxidative stress in endothelial cells. How AMPK suppresses oxidative stress remains to be established.

OBJECTIVE

The aim of this study is to examine the effects of AMPK in regulating NAD(P)H oxidase, oxidative stress, and endothelial function.

METHODS AND RESULTS

The markers of oxidative stress, NAD(P)H oxidase subunit expression (gp91(phox), p47(phox), p67(phox), NOX1 to -4), NAD(P)H oxidase-mediated superoxide production, 26S proteasome activity, IkappaBalpha degradation, and nuclear translocation of nuclear factor (NF)-kappaB (p50 and p65) were examined in cultured human umbilical vein endothelial cells and mouse aortas isolated from AMPKalpha2 deficient mice. Compared to the wild type, acetylcholine-induced endothelium-dependent relaxation was significantly impaired in parallel with increased production of oxidants in AMPKalpha2(-/-) mice. Further, pretreatment of aorta with either superoxide dismutase (SOD) or tempol or apocynin significantly improved acetylcholine-induced endothelium-dependent relaxation in AMPKalpha2(-/-) mice. Analysis of aortic endothelial cells from AMPKalpha2(-/-) mice and human umbilical vein endothelial cells expressing dominant negative AMPK or AMPKalpha2-specific siRNA revealed that loss of AMPK activity increased NAD(P)H oxidase subunit expression (gp91(phox), p47(phox), p67(phox), NOX1 and -4), NAD(P)H oxidase-mediated superoxide production, 26S proteasome activity, IkappaBalpha degradation, and nuclear translocation of NF-kappaB (p50 and p65), whereas AMPK activation by AICAR or overexpression of constitutively active AMPK had the opposite effect. Consistently, we found that genetic deletion of AMPKalpha2 in low-density lipoprotein receptor knockout (LDLr(-/-)) strain markedly increased 26S proteasome activity, IkappaB degradation, NF-kappaB transactivation, NAD(P)H oxidase subunit overexpression, oxidative stress, and endothelial dysfunction, all of which were largely suppressed by chronic administration of MG132, a potent cell permeable proteasome inhibitor.

CONCLUSIONS

We conclude that AMPKalpha2 functions as a physiological suppressor of NAD(P)H oxidase and ROS production in endothelial cells. In this way, AMPK maintains the nonatherogenic and noninflammatory phenotype of endothelial cells.

Suppressed NFAT-dependent VEGFR1 expression and constitutive VEGFR2 signaling in infantile hemangioma

Infantile hemangiomas are localized and rapidly growing regions of disorganized angiogenesis. We show that expression of vascular endothelial growth factor receptor-1 (VEGFR1) in hemangioma endothelial cells (hemECs) and hemangioma tissue is markedly reduced compared to controls. Low VEGFR1 expression in hemECs results in VEGF-dependent activation of VEGFR2 and downstream signaling pathways. In hemECs, transcription of the gene encoding VEGFR1 (FLT1) is dependent on nuclear factor of activated T cells (NFAT). Low VEGFR1 expression in hemECs is caused by reduced activity of a pathway involving beta1 integrin, the integrin-like receptor tumor endothelial marker-8 (TEM8), VEGFR2 and NFAT. In a subset of individuals with hemangioma, we found missense mutations in the genes encoding VEGFR2 (KDR) and TEM8 (ANTXR1). These mutations result in increased interactions among VEGFR2, TEM8 and beta1 integrin proteins and in inhibition of integrin activity. Normalization of the constitutive VEGFR2 signaling in hemECs with soluble VEGFR1 or antibodies that neutralize VEGF or stimulate beta1 integrin suggests that local administration of these or similar agents may be effective in hemangioma treatment.

ICAM-1-Mediated, Src- and Pyk2-Dependent Vascular Endothelial Cadherin Tyrosine Phosphorylation Is Required for Leukocyte Transendothelial Migration

Leukocyte transendothelial migration (TEM) has been modeled as a multistep process beginning with rolling adhesion, followed by firm adhesion, and ending with either transcellular or paracellular passage of the leukocyte across the endothelial monolayer. In the case of paracellular TEM, endothelial cell (EC) junctions are transiently disassembled to allow passage of leukocytes. Numerous lines of evidence demonstrate that tyrosine phosphorylation of adherens junction proteins, such as vascular endothelial cadherin (VE-cadherin) and beta-catenin, correlates with the disassembly of junctions. However, the role of tyrosine phosphorylation in the regulation of junctions during leukocyte TEM is not completely understood. Using human leukocytes and EC, we show that ICAM-1 engagement leads to activation of two tyrosine kinases, Src and Pyk2. Using phospho-specific Abs, we show that engagement of ICAM-1 induces phosphorylation of VE-cadherin on tyrosines 658 and 731, which correspond to the p120-catenin and beta-catenin binding sites, respectively. These phosphorylation events require the activity of both Src and Pyk2. We find that inhibition of endothelial Src with PP2 or SU6656 blocks neutrophil transmigration (71.1 +/- 3.8% and 48.6 +/- 3.8% reduction, respectively), whereas inhibition of endothelial Pyk2 also results in decreased neutrophil transmigration (25.5 +/- 6.0% reduction). Moreover, overexpression of the nonphosphorylatable Y658F or Y731F mutants of VE-cadherin impairs transmigration of neutrophils compared with overexpression of wild-type VE-cadherin (32.7 +/- 7.1% and 38.8 +/- 6.5% reduction, respectively). Our results demonstrate that engagement of ICAM-1 by leukocytes results in tyrosine phosphorylation of VE-cadherin, which is required for efficient neutrophil TEM.

A genetically encoded fluorescent sensor of ERK activity

The activity of the ERK has complex spatial and temporal dynamics that are important for the specificity of downstream effects. However, current biochemical techniques do not allow for the measurement of ERK signaling with fine spatiotemporal resolution. We developed a genetically encoded, FRET-based sensor of ERK activity (the extracellular signal-regulated kinase activity reporter, EKAR), optimized for signal-to-noise ratio and fluorescence lifetime imaging. EKAR selectively and reversibly reported ERK activation in HEK293 cells after epidermal growth factor stimulation. EKAR signals were correlated with ERK phosphorylation, required ERK activity, and did not report the activities of JNK or p38. EKAR reported ERK activation in the dendrites and nucleus of hippocampal pyramidal neurons in brain slices after theta-burst stimuli or trains of back-propagating action potentials. EKAR therefore permits the measurement of spatiotemporal ERK signaling dynamics in living cells, including in neuronal compartments in intact tissues.

Oxidative stress activates protein tyrosine kinase and matrix metalloproteinases leading to blood-brain barrier dysfunction

The blood-brain barrier (BBB) formed by brain microvascular endothelial cells (BMVEC) regulates the passage of molecules and leukocytes in and out of the brain. Oxidative stress is a major underlying cause of neurodegenerative and neuroinflammatory disorders and BBB injury associated with them. Using human BMVEC grown on porous membranes covered with basement membrane (BM) matrix (BBB models), we demonstrated that reactive oxygen species (ROS) augmented permeability and monocyte migration across BBB. ROS activated matrix metalloproteinases (MMP-1, -2, and -9) and decreased tissue inhibitors of MMPs (TIMP-1 and -2) in a protein tyrosine kinase (PTK)-dependent manner. Increase in MMPs and PTK activities paralleled degradation of BM protein and enhanced tyrosine phosphorylation of tight junction (TJ) protein. These effects and enhanced permeability/monocyte migration were prevented by inhibitors of MMPs, PTKs, or antioxidant suggesting that oxidative stress caused BBB injury via degradation of BM protein by activated MMPs and by PTK-mediated TJ protein phosphorylation. These findings point to new therapeutic interventions ameliorating BBB dysfunction in neurological disorders such as stroke or neuroinflammation.

Activation of TRPV1 by dietary capsaicin improves endothelium-dependent vasorelaxation and prevents hypertension

Some plant-based diets lower the cardiometabolic risks and prevalence of hypertension. New evidence implies a role for the transient receptor potential vanilloid 1 (TRPV1) cation channel in the pathogenesis of cardiometabolic diseases. Little is known about impact of chronic TRPV1 activation on the regulation of vascular function and blood pressure. Here we report that chronic TRPV1 activation by dietary capsaicin increases the phosphorylation of protein kinase A (PKA) and eNOS and thus production of nitric oxide (NO) in endothelial cells, which is calcium dependent. TRPV1 activation by capsaicin enhances endothelium-dependent relaxation in wild-type mice, an effect absent in TRPV1-deficient mice. Long-term stimulation of TRPV1 can activate PKA, which contributes to increased eNOS phosphorylation, improves vasorelaxation, and lowers blood pressure in genetically hypertensive rats. We conclude that TRPV1 activation by dietary capsaicin improves endothelial function. TRPV1-mediated increase in NO production may represent a promising target for therapeutic intervention of hypertension.

Biochemistry and pathophysiology of intravascular and intracellular lipolysis

All organisms use fatty acids (FAs) for energy substrates and as precursors for membrane and signaling lipids. The most efficient way to transport and store FAs is in the form of triglycerides (TGs); however, TGs are not capable of traversing biological membranes and therefore need to be cleaved by TG hydrolases ("lipases") before moving in or out of cells. This biochemical process is generally called "lipolysis." Intravascular lipolysis degrades lipoprotein-associated TGs to FAs for their subsequent uptake by parenchymal cells, whereas intracellular lipolysis generates FAs and glycerol for their release (in the case of white adipose tissue) or use by cells (in the case of other tissues). Although the importance of lipolysis has been recognized for decades, many of the key proteins involved in lipolysis have been uncovered only recently. Important new developments include the discovery of glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), the molecule that moves lipoprotein lipase from the interstitial spaces to the capillary lumen, and the discovery of adipose triglyceride lipase (ATGL) and comparative gene identification-58 (CGI-58) as crucial molecules in the hydrolysis of TGs within cells. This review summarizes current views of lipolysis and highlights the relevance of this process to human disease.

Expression and localization of NOX2 and NOX4 in primary human endothelial cells

Reactive oxygen species (ROS) control the integrity of the vascular endothelium. Our laboratory has recently shown that transduction of human umbilical vein endothelial cells (HUVECs) with an active variant of the small GTPase Rac promotes the production of ROS, ROS-dependent activation of p38 mitogen-activated protein kinase, and loss of vascular/endothelial-cadherin-mediated cell-cell adhesion. Here we show that HUVECs express mRNAs for NOX2 as well as NOX4 mRNA, but not for NOX1 or NOX3. Interestingly, NOX4 was expressed at 100-fold higher levels compared with NOX2. NOX4-green fluorescent protein largely localizes to an intracellular compartment that costained with a marker for the endoplasmic reticulum, and its distribution did not overlap with lysosomes, Weibel-Palade bodies, or mitochondria. The NOX2-regulatory proteins p47(phox) and p67(phox) associated with the actin cytoskeleton and were found in cell protrusions and membrane ruffles, colocalizing with Rac1. This translocation to the cell periphery was promoted by tumor necrosis factor (TNF)-alpha. Finally, scavenging of ROS was found to impair TNF-alpha-induced cytoskeletal rearrangements and the formation of a confluent endothelial monolayer. Together, these data prove the differential mRNA expression of NOX family members in human endothelium and indicate that these NOX proteins and their regulators may be involved in the control of endothelial cell spreading, motility, and cell-cell adhesion.

Chronic treatment with N-acetyl-cystein delays cellular senescence in endothelial cells isolated from a subgroup of atherosclerotic patients

Endothelial senescence may contribute to the pathogenesis of age-related vascular disorders. Furthermore, chronic exposure to risk factors for cardiovascular disease (CVD) accelerates the effects of chronological aging by generating stress-dependent damages, including oxidative stress, therefore promoting stress-induced premature senescence. Our objective was to determine whether a chronic treatment with an antioxidant (N-acetyl-cystein, NAC) could delay senescence of endothelial cells (EC) isolated and cultured from arterial segments of patients with severe coronary artery disease. If EC were considered as one population (n=26), chronic NAC treatment slightly shortened telomere attrition rate associated with senescence but did not significantly delay the onset of endothelial senescence. However, in a subgroup of NAC-treated EC (n=15) cellular senescence was significantly delayed, NAC decreased lipid peroxidation (HNE), activated the catalytic subunit of telomerase (hTERT) and inhibited telomere attrition. In contrast, in another subgroup of EC (n=11) characterized by initial short telomeres, no effect of NAC on HNE and high levels of DNA damages, the antioxidant was not beneficial on senescence, suggesting an irreversible stress-dependent damage. In conclusion, chronic exposure to NAC can delay senescence of diseased EC via hTERT activation and transient telomere stabilization, unless oxidative stress-associated cell damage has become irreversible.

Redox Mechanisms in Blood Vessels

Reactive oxygen species have been implicated in the pathogenesis of virtually every stage of vascular lesion formation, hypertension, and other vascular diseases. We are currently gaining insight into important sources of reactive oxygen species in the vessel wall, including the NADPH oxidases, xanthine oxidase, uncoupled nitric oxide synthase, and mitochondrial sources. Although various reactive oxygen species have pathological roles, some serve as important signaling molecules that modulate vascular tone, growth, and remodeling. In the next several months, a series of articles in Arteriosclerosis, Thrombosis, and Vascular Biology attempt to further elucidate how reactive oxygen species are produced by vascular cells and the roles of these in vascular homeostasis. This series promises to provide a valuable update on a wide variety of issues related to the biochemistry, molecular biology, and physiology of these important and fascinating molecules. Reactive oxygen species have been implicated in the pathogenesis of virtually every stage of vascular lesion formation, hypertension, and other vascular diseases. Upcoming series of articles in Arteriosclerosis, Thrombosis, and Vascular Biology help elucidate how reactive oxygen species are produced by vascular cells and their role in vascular homeostasis.