Vacuolar-type H+-ATPases at the plasma membrane regulate pH and cell migration in microvascular endothelial cells

Microvascular endothelial cells involved in angiogenesis are exposed to an acidic environment that is not conducive for growth and survival. These cells must exhibit a dynamic intracellular (cytosolic) pH (pHcyt) regulatory mechanism to cope with acidosis, in addition to the ubiquitous Na+/H+exchanger and HCO3−-based H+-transporting systems. We hypothesize that the presence of plasmalemmal vacuolar-type proton ATPases (pmV-ATPases) allows microvascular endothelial cells to better cope with this acidic environment and that pmV-ATPases are required for cell migration. This study indicates that microvascular endothelial cells, which are more migratory than macrovascular endothelial cells, express pmV-ATPases. Spectral imaging microscopy indicates a more alkaline pHcytat the leading than at the lagging edge of microvascular endothelial cells. Treatment of microvascular endothelial cells with V-ATPase inhibitors decreases the proton fluxes via pmV-ATPases and cell migration. These data suggest that pmV-ATPases are essential for pHcytregulation and cell migration in microvascular endothelial cells.

The metabolic basis of arginine nutrition and pharmacotherapy

As an essential precursor for the synthesis of proteins and other molecules with enormous biological importance (including nitric oxide, urea, ornithine, proline, polyamines, glutamate, creatine, agmatine, and dimethylarginines), arginine displays remarkable metabolic and regulatory versatility. Evidence available to date provides a sound reason to classify arginine as an essential amino acid for young mammals (including parenterally fed human infants) and as a conditionally essential amino acid for adults under such conditions as trauma, burn injury, massive small-bowel resection, and renal failure. Arginine administration reverses endothelial dysfunction, enhances wound healing, prevents the early stages of tumorigenesis, and improves cardiovascular, reproductive, pulmonary, renal, digestive, and immune functions. Arginine or its effective precursor citrulline may hold great promise as a nutritional or pharmacotherapeutic treatment for a wide array of human diseases.

Tetrahydrobiopterin levels regulate endothelial cell proliferation

Vascular abnormalities, including altered angiogenesis, are major factors contributing to the morbidity and mortality of diabetes. We hypothesized that impaired angiogenesis in diabetes results from decreased tetrahydrobiopterin (BH4)-dependent synthesis of nitric oxide (NO) by endothelial cells (EC). To test this hypothesis, we utilized EC from spontaneously diabetic BB (BBd) and nondiabetes-prone BB (BBn) rats to investigate the link between BH4 and EC proliferation. There were significant decreases in the proliferation rate and expression of proliferating cell nuclear antigen in BBd versus BBn EC, with no evidence of apoptosis in either group. Sepiapterin (a precursor of BH4 via the salvage pathway) increased BH4 synthesis and enhanced proliferation of BBd EC. The stimulating effect of sepiapterin on EC proliferation was attenuated by NG-monomethyl-L-arginine, a NO synthase inhibitor. Reducing BH4 concentrations in BBn EC caused a decrease in proliferation, which was attenuated by a long-acting NO donor. Our results suggest that BH4 levels regulate proliferation of normal EC and that a BH4 deficiency impairs NO-dependent proliferation of BBd EC.

Mechanisms of coronary angiogenesis in response to stretch: role of VEGF and TGF-beta

To test the hypotheses that cyclic stretch of 1) cardiac myocytes produces factors that trigger angiogenic events in coronary microvascular endothelial cells (CMEC) and 2) CMEC enhances the expression of growth factors, cardiac myocytes and CMEC were subjected to cyclic stretch in a Flexercell Strain Unit. Vascular endothelial growth factor (VEGF) but not basic fibroblast growth factor mRNA and protein levels increased approximately twofold in myocytes after 1 h of stretch. CMEC DNA synthesis increased approximately twofold when conditioned medium from stretched myocytes or VEGF protein was added, and addition of VEGF neutralizing antibody blocked the increase. CMEC migration and tube formation increased with the addition of conditioned media but were markedly attenuated by VEGF neutralizing antibody. Myocyte transforming growth factor-beta [corrected] (TGF-beta) increased 2.5-fold after 1 h of stretch, and the addition of TGF-beta neutralizing antibodies inhibited the stretch-induced upregulation of VEGF. Stretch of CMEC increased VEGF mRNA in these cells (determined by Northern blot and RT-PCR) and increased the levels of VEGF protein (determined by ELISA analysis) in the conditioned media. Therefore, cyclic stretch of cardiac myocytes and CMEC appears to be an important primary stimulus for coronary angiogenesis through both paracrine and autocrine VEGF pathways. These data indicate that 1) CMEC DNA synthesis, migration, and tube formation are increased in response to VEGF secreted from stretched cardiac myocytes; 2) VEGF in CMEC subjected to stretch is upregulated and secreted; and 3) TGF-beta signaling may regulate VEGF expression in cardiac myocytes.

Presence of glutamine:fructose-6-phosphate amidotransferase for glucosamine-6-phosphate synthesis in endothelial cells: effects of hyperglycaemia and glutamine

AIMS/HYPOTHESIS

Recent studies show that glucosamine infusion impairs endothelium-dependent blood flow in normoglycaemic rats. The pathophysiological relevance of this finding, however, depends on whether de novo glucosamine synthesis occurs in endothelial cells. The aim of this study was to test the hypothesis of whether glutamine:fructose-6-phosphate amidotransferase (the first and key regulatory enzyme in hexosamine synthesis) is present for endothelial glucosamine synthesis.

METHODS

Bovine venular, bovine aortic, human microvascular, human umbilical vein, and rat coronary microvascular endothelial cells were used to measure glutamine:fructose-6-phosphate amidotransferase activity. To determine glucosamine-6-phosphate synthesis in intact cells, they were incubated for 1 h in Krebs bicarbonate buffer containing 5, 15 or 30 mmol/l [U-14C]glucose and 0.5, 2 or 4 mmol/l glutamine. The [14C]Glucosamine-6-phosphate and its end products ([14C]UDP-N-acetylglucosamine and [14C]UDP-Nacetylgalactosamine) were separated by HPLC.

RESULTS

There were high glutamine:fructose-6-phosphate amidotransferase activities in all endothelial cells studied. Exposure of cells to 15 to 30 mmol/l glucose or 2 to 4 mmol/l glutamine increased enzyme activity. Glucosamine-6-phosphate, UDP-N-acetylglucosamine and UDP-N-acetylgalactosamine syntheses increased with increasing extracellular concentrations of glucose from 5 to 30 mmol/l or of glutamine from 0.5 to 4 mmol/l.

CONCLUSION/INTERPRETATION

Our results show the presence of glutamine:fructose-6-phosphate amidotransferase for de novo glucosamine synthesis in endothelial cells and the modulation of this pathway by hyperglycaemia and glutamine. As glucosamine inhibits endothelial nitric oxide synthesis, these findings could have important implications for impaired endothelium-dependent relaxation and vascular dysfunction in diabetes mellitus.

Glutamine metabolism to glucosamine is necessary for glutamine inhibition of endothelial nitric oxide synthesis

L-Glutamine is a physiological inhibitor of endothelial NO synthesis. The present study was conducted to test the hypothesis that metabolism of glutamine to glucosamine is necessary for glutamine inhibition of endothelial NO generation. Bovine venular endothelial cells were cultured for 24 h in the presence of 0, 0.1, 0.5 or 2 mM D-glucosamine, or of 0.2 or 2 mM L-glutamine with or without 20 microM 6-diazo-5-oxo-L-norleucine (DON) or with 100 microM azaserine. Both DON and azaserine are inhibitors of L-glutamine:D-fructose-6-phosphate transaminase (isomerizing) (EC 2.6.1.16), the first and rate controlling enzyme in glucosamine synthesis. Glucosamine at 0.1, 0.5 and 2 mM decreased NO production by 34, 45 and 56% respectively compared with controls where glucosamine was lacking. DON (20 microM) and azaserine (100 microM) blocked glucosamine synthesis and prevented the inhibition of NO generation by glutamine. Neither glutamine nor glucosamine had an effect on NO synthase (NOS) activity, arginine transport or cellular tetrahydrobiopterin and Ca(2+) levels. However, both glutamine and glucosamine inhibited pentose cycle activity and decreased cellular NADPH concentrations; these effects of glutamine were abolished by DON or azaserine. Restoration of cellular NADPH levels by the addition of 1 mM citrate also prevented the inhibiting effect of glutamine or glucosamine on NO synthesis. A further increase in cellular NADPH levels by the addition of 5 mM citrate resulted in greater production of NO. Collectively, our results demonstrate that the metabolism of glutamine to glucosamine is necessary for the inhibition of endothelial NO generation by glutamine. Glucosamine reduces the cellular availability of NADPH (an essential cofactor for NOS) by inhibiting pentose cycle activity, and this may be a metabolic basis for the inhibition of endothelial NO synthesis by glucosamine.

Regulatory role of arginase I and II in nitric oxide, polyamine, and proline syntheses in endothelial cells

Endothelial cells (EC) metabolize L-arginine mainly by arginase, which exists as two distinct isoforms, arginase I and II. To understand the roles of arginase isoforms in EC arginine metabolism, bovine coronary venular EC were stably transfected with the Escherichia coli lacZ gene (lacZ-EC, control), rat arginase I cDNA (AI-EC), or mouse arginase II cDNA (AII-EC). Western blots and enzymatic assays confirmed high-level expression of arginase I in the cytosol of AI-EC and of arginase II in mitochondria of AII-EC. For determining arginine catabolism, EC were cultured for 24 h in DMEM containing 0.4 mM L-arginine plus [1-(14)C]arginine. Urea formation, which accounted for nearly all arginine consumption by these cells, was enhanced by 616 and 157% in AI-EC and AII-EC, respectively, compared with lacZ-EC. Arginine uptake was 31-33% greater in AI-EC and AII-EC than in lacZ-EC. Intracellular arginine content was 25 and 11% lower in AI-EC and AII-EC, respectively, compared with lacZ-EC. Basal nitric oxide (NO) production was reduced by 60% in AI-EC and by 47% in AII-EC. Glutamate and proline production from arginine increased by 164 and 928% in AI-EC and by 79 and 295% in AII-EC, respectively, compared with lacZ-EC. Intracellular content of putrescine and spermidine was increased by 275 and 53% in AI-EC and by 158 and 43% in AII-EC, respectively, compared with lacZ-EC. Our results indicate that arginase expression can modulate NO synthesis in bovine venular EC and that basal levels of arginase I and II are limiting for endothelial syntheses of polyamines, proline, and glutamate and may have important implications for wound healing, angiogenesis, and cardiovascular function.

Glucosamine inhibits inducible nitric oxide synthesis

Glucosamine is widely used in Europe for treatment of arthritis in humans. Based on recent findings that excess production of nitric oxide (NO) by inducible NO synthase (iNOS) mediates the pathogenesis of arthritis, we hypothesized that glucosamine may inhibit NO synthesis. To test this hypothesis, we used an in vivo rat model of lipopolysaccharide (LPS)-induced inflammation. Intravenous administration of d-glucosamine (0.5 mmol/kg) 6 h before, at the time of, and 6 h after intraperitoneal LPS injection (1 mg/kg) decreased urinary excretion of nitrate by 31 and 48%, respectively, at days 1 and 2 post LPS administration. When cultured macrophages were treated with LPS (1 microg/ml) to induce iNOS expression, addition of 0.1, 0.5, 1, and 2 mM d-glucosamine decreased NO production by 18, 38, 60, and 89%, respectively. Glucosamine had no effect on cellular arginine, NADPH or tetrahydrobiopterin concentrations, but dose-dependently suppressed iNOS protein expression. Similar decreases in iNOS protein occurred in spleen, lung, and peritoneal macrophages of glucosamine-treated rats. These studies demonstrate that glucosamine is a novel inhibitor of inducible NO synthesis via inhibition of iNOS protein expression, and provide a biochemical basis for the use of glucosamine in treating chronic inflammatory diseases such as arthritis.

Modulation of tumor angiogenesis by stem cell factor

Mast cells accumulate within solid tumors and can release many angiogenic factors, suggesting that they may modulate vascularization of tumors. Stem cell factor (SCF) stimulates mast cell migration, proliferation, and degranulation and therefore may influence mast cell behavior within tumors. We investigated the contribution of SCF to tumor angiogenesis by manipulating its level in mammary tumors. Sense or antisense cDNA fragments of rat SCF were ligated into an episomal expression vector. Ethylnitrosourea-induced rat mammary tumor cell lines were transfected with vector containing either control (no insert, C-P), sense (S-P), or antisense (AS-P) SCF DNA. The functional nature of the transfectants was confirmed by measuring SCF in cell lysates and conditioned media. Immunohistochemical analysis of the tumors induced in Berlin-Druckrey rats by these transfected cells demonstrated that mast cell number and microvascular density were significantly higher in S-P tumors and significantly lower in AS-P tumors, compared with C-P tumors. The expression of von Willebrand factor, an endothelial cell marker, showed a similar pattern. AS-P tumors were significantly smaller than either C-P or S-P tumors. These data suggest that SCF modulates tumor growth and angiogenesis via the involvement of mast cells.

Arginine nutrition and cardiovascular function

L-Arginine (Arg) is the substrate for the synthesis of nitric oxide (NO), the endothelium-derived relaxing factor essential for regulating vascular tone and hemodynamics. NO stimulates angiogenesis, but inhibits endothelin-1 release, leukocyte adhesion, platelet aggregation, superoxide generation, the expression of vascular cell adhesion molecules and monocyte chemotactic peptides, and smooth muscle cell proliferation. Arg exerts its vascular actions also through NO-independent effects, including membrane depolarization, syntheses of creatine, proline and polyamines, secretion of insulin, growth hormone, glucagon and prolactin, plasmin generation and fibrinogenolysis, superoxide scavenging and inhibition of leukocyte adhesion to nonendothelial matrix. Compelling evidence shows that enteral or parenteral administration of Arg reverses endothelial dysfunction associated with major cardiovascular risk factors (hypercholesterolemia, smoking, hypertension, diabetes, obesity/insulin resistance and aging) and ameliorates many common cardiovascular disorders (coronary and peripheral arterial disease, ischemia/reperfusion injury, and heart failure). Dietary Arg supplementation may represent a potentially novel nutritional strategy for preventing and treating cardiovascular disease.

Rapid determination of nitrite by reversed-phase high-performance liquid chromatography with fluorescence detection

Measurement of nitrite and nitrate, the stable oxidation products of nitric oxide (NO), provides a useful tool to study NO synthesis in vivo and in cell cultures. A simple and rapid fluorometric HPLC method was developed for determination of nitrite through its derivatization with 2,3-diaminonaphthalene (DAN). Nitrite, in standard solution, cell culture medium, or biological samples, readily reacted with DAN under acidic conditions to yield the highly fluorescent 2,3-naphthotriazole (NAT). For analysis of nitrate, it was converted to nitrite by nitrate reductase, followed by the derivatization of nitrite with DAN to form NAT. NAT was separated on a 5-microm reversed-phase C18 column (150X4.6 mm, I.D.) guarded by a 40-microm reversed-phase C18 column (50x4.6 mm, I.D.), and eluted with 15 mM sodium phosphate buffer (pH 7.5) containing 50% methanol (flow-rate, 1.3 ml/min). Fluorescence was monitored with excitation at 375 nm and emission at 415 nm. Mean retention time for NAT was 4.4 min. The fluorescence intensity of NAT was linear with nitrite or nitrate concentrations ranging from 12.5 to 2,000 nM in water, cell culture media, plasma and urine. The detection limit for nitrite and nitrate was 10 pmol/ml. Because NAT is well separated from DAN and other fluorescent components present in biological samples, our HPLC method offers the advantages of high sensitivity and specificity as well as easy automation for quantifying picomole levels of nitrite and nitrate in cell culture medium and biological samples.

A cortisol surge mediates the enhanced expression of pig intestinal pyrroline-5-carboxylate synthase during weaning

Citrulline synthesis from glutamine is enhanced remarkably in enterocytes of weanling pigs, but the molecular mechanism(s) involved are not known. The objective of this study was to determine whether a cortisol surge mediates the enhanced expression of intestinal citrulline-synthetic enzymes during weaning. Jejunal enterocytes were prepared from 29-d-old weanling pigs treated with or without metyrapone (an inhibitor of cortisol synthesis), or from age-matched unweaned pigs. The mRNA levels and activities of phosphate-dependent glutaminase (PDG), pyrroline-5-carboxylate synthase (P5CS), ornithine aminotransferase (OAT), carbamoyl-phosphate synthase I (CPS-I) and ornithine carbamoyltransferase (OCT) were determined. The mRNA levels for PDG, P5CS, OAT and OCT were 139, 157, 102 and 55% higher, respectively, in weanling pigs compared with suckling pigs. The activities of PDG and P5CS were 38 and 692% higher, respectively, in weanling pigs compared with unweaned pigs, but the activities of OAT, CPS-I and OCT did not differ between these two groups of pigs. The effects of metyrapone administration to weanling pigs were as follows: 1) prevention of a cortisol surge, 2) abolition of the increases in both mRNA levels and activity of P5CS, 3) no alteration in the mRNA levels and activities of PDG and CPS-I, 4) increases in the mRNA levels for OAT (216%) and OCT (39%) and in OAT activity (30%), and 5) prevention of the increase in intestinal synthesis of citrulline from glutamine. These results suggest that increased P5CS activity reflects in large part the increased levels of P5CS mRNA and is responsible for the increased synthesis of citrulline from glutamine in enterocytes of weanling pigs; these increases may be mediated by a cortisol surge during weaning that can be blocked by metyrapone administration.

Impaired nitric oxide production in coronary endothelial cells of the spontaneously diabetic BB rat is due to tetrahydrobiopterin deficiency

Endothelial cells (EC) from diabetic BioBreeding (BB) rats have an impaired ability to produce NO. This deficiency is not due to a defect in the constitutive isoform of NO synthase in EC (ecNOS) or alterations in intracellular calcium, calmodulin, NADPH or arginine levels. Instead, ecNOS cannot produce sufficient NO because of a deficiency in tetrahydrobiopterin (BH(4)), a cofactor necessary for enzyme activity. EC from diabetic rats exhibited only 12% of the BH(4) levels found in EC from normal animals or diabetes-prone animals which did not develop disease. As a result, NO synthesis by EC of diabetic rats was only 18% of that for normal animals. Increasing BH(4) levels with sepiapterin increased NO production, suggesting that BH(4) deficiency is a metabolic basis for impaired endothelial NO synthesis in diabetic BB rats. This deficiency is due to decreased activity of GTP-cyclohydrolase I, the first and rate-limiting enzyme in the de novo biosynthesis of BH(4). GTP-cyclohydrolase activity was low because of a decreased expression of the protein in the diabetic cells.

Glutamine metabolism in endothelial cells: ornithine synthesis from glutamine via pyrroline-5-carboxylate synthase

L-Glutamine (the most abundant free amino acid in plasma and the body) is a potent inhibitor of endothelial NO synthesis. However, little is known about glutamine metabolism in endothelial cells (EC). As an initial step toward understanding the role of glutamine in endothelial physiology, the present study was conducted to quantify glutamine catabolism in microvascular, aortic and venous EC. For metabolic studies, EC were incubated for 1 h in Krebs bicarbonate buffer containing 5 mM glucose and 0.5-4 mM L-[U-(14)C]-glutamine. For enzymological studies, cell extracts and mitochondrial fractions were prepared to determine the activities of glutamine-degrading enzymes. Our results reveal extensive hydrolysis of glutamine to glutamate and ammonia in a concentration-dependent manner via phosphate-dependent glutaminase in all EC studied. In addition, both metabolic and enzymological evidence indicate a novel pathway for endothelial synthesis of ornithine from glutamine via pyrroline-5-carboxylate synthase. This new knowledge of glutamine metabolism may pave a new path for understanding the physiological role of glutamine in vascular function.

Arginine nutrition in development, health and disease

As a precursor of nitric oxide, polyamines and other molecules with enormous biologic importance, L-arginine plays versatile key roles in nutrition and metabolism. Arginine is an essential amino acid in the fetus and neonate, and is conditionally an essential nutrient for adults, particularly in certain disease conditions. L-Arginine administration is beneficial in improving reproductive, cardiovascular, pulmonary, renal, gastrointestinal, liver and immune functions, and in facilitating wound healing. The effect of L-arginine in treating many common health problems is unique among amino acids, and offers great promise for improved health and well-being in the future.

Glucocorticoids mediate the enhanced expression of intestinal type II arginase and argininosuccinate lyase in postweaning pigs

Arginine metabolism is enhanced in the small intestine of weanling pigs, but the molecular mechanism(s) involved is not known. The objectives of this study were to determine the following: 1) whether glucocorticoids play a role in induction of intestinal arginine metabolic enzymes during weaning; 2) whether the induction of enzyme activities was due to increases in corresponding mRNA levels; and 3) the identity of the arginase isoform(s) expressed in the small intestine. Jejunum was obtained from 29-d-old weaned pigs that were or were not treated with 17-beta-hydroxy-11beta-(4-dimethylaminophenyl)17alpha-(prop- 1-ynyl)es tra-4,9-dien-3-one (RU486, an antagonist of glucocorticoid receptors), or from age-matched suckling pigs. Activities and mRNA levels for type I and type II arginases, argininosuccinate synthase (ASS) and argininosuccinate lyase (ASL) were determined. Activities of arginase, ASL and ASS increased by 635, 56 and 106%, respectively, in weanling pigs, compared with suckling pigs. RU486 treatment attenuated the increase in arginase activity by 74% and completely prevented the ASL induction in weanling pigs, but had no effect on ASS activity. Pig intestine expresses both type I and type II arginases. On the basis of immunoblot analyses, there was no significant difference in levels of intestinal type I arginase among these three groups of pigs, indicating that changes in arginase activity were due only to type II arginase. The mRNA levels for type II arginase and ASL increased by 135 and 198%, respectively, in weanling pigs compared with suckling pigs, and this induction was completely prevented by RU486. In contrast, ASS mRNA levels did not differ between suckling and weanling pigs. These results suggest that intestinal type II arginase, ASS and ASL are regulated differentially at transcriptional and post-translational levels and that glucocorticoids play a major role in the induction of type II arginase and ASL mRNAs in the small intestine of weanling pigs.

VEGF upregulates ecNOS message, protein, and NO production in human endothelial cells

Vascular endothelial growth factor (VEGF) is an endothelium-specific secreted protein that potently stimulates vasodilation, microvascular hyperpermeability, and angiogenesis. Nitric oxide (NO) is also reported to modulate vascular tone, permeability, and capillary growth. Therefore, we hypothesized that VEGF might regulate endothelial production of NO. The production of nitrogen oxides by human umbilical vein endothelial cells (HUVECs) was measured after 1, 12, 24, and 48 h of incubation with VEGF. VEGF treatment resulted in both an acute (1 h) and chronic (> 24 h) stimulation of NO production. Furthermore, Western and Northern blotting revealed a VEGF-elicited, dose-dependent increase in the cellular content of endothelial cell nitric oxide synthase (ecNOS) message and protein that may account for the chronic upregulation of NO production elicited by VEGF. Finally, endothelial cells pretreated with VEGF for 24 h and subsequently exposed to A-23187 for 1 h produced NO at approximately twice the rate of cells that were not pretreated with VEGF. We conclude that VEGF upregulates ecNOS enzyme and elicits a biphasic stimulation of endothelial NO production.

L-glutamine inhibits nitric oxide synthesis in bovine venular endothelial cells

This study was conducted to test the hypothesis that L-glutamine has differential effects on nitric oxide (NO) synthesis from L-arginine in bovine venular endothelial cells (EC) stimulated by A23187 (a Ca++ ionophore) and receptor-mediated vasodilators (bradykinin and substance P). EC were cultured at 37 degrees C for 24 h in the presence of 0.4 mM L-arginine and 0.0 to 2.0 mM L-glutamine with or without 1 microM A23187, 1 microM bradykinin or 10 microM substance P. The release of nitrite and nitrate by EC was used as an indicator of NO synthesis. A23187, bradykinin or substance P increased NO synthesis from L-arginine by EC in the presence or absence of L-glutamine. The addition of L-glutamine (0.5 and 2 mM) markedly increased intracellular concentrations of L-glutamine, L-glutamate and L-aspartate and decreased NO synthesis by EC in a concentration-dependent manner in the presence or absence of A23187, bradykinin or substance P. L-Glutamine had no effect on L-arginine uptake by EC or on intracellular L-arginine concentration. Neither L-glutamine nor its glutaminase metabolites (ammonia, L-glutamate and L-aspartate) had any effect on endothelial NO synthase activity. Taken together, these results suggest that the inhibition by L-glutamine of NO synthesis from L-arginine is unlikely to result from an effect of L-glutamine on L-arginine transport or NO synthase activity. Although the mechanism involved remains unknown, regulation of the arginine-NO pathway by L-glutamine may have pharmacologic and therapeutic implications in such conditions as inflammation and septic shock by inhibiting NO generation from L-arginine in endothelial cells.

Impaired arginine metabolism and NO synthesis in coronary endothelial cells of the spontaneously diabetic BB rat

Arginine metabolism via nitric oxide (NO) synthase and other pathways was studied in coronary endothelial cells (EC) from the spontaneously diabetic BB rat, an animal model of human type I diabetes mellitus (IDDM). EC were prepared from insulin-treated diabetic BB (BBd) and non-diabetes-prone BB (BBn) rats. Basal NO synthesis was studied in EC cultured for 48 h in medium containing 0.4 mM L-arginine. At the end of the culture period, the medium was analyzed for nitrite and nitrate (two major end stable oxidation products of NO), and the cells were used to determine arginine uptake and metabolism and the activities of some arginine-degrading enzymes. For studies of arginine metabolism, cells were incubated at 37 degrees C for 1 h in Krebs-Henseleit bicarbonate buffer (pH 7.4) containing 1 mM L(-)[1-14C]arginine or L(-)[1-14C]ornithine. The rates of production of nitrite plus nitrate by BBd EC were only 15% of those of BBn cells. This impaired NO synthesis in BBd EC was not due to alterations in arginine uptake, NO synthase activity, or intracellular arginine concentrations but might have resulted from a limited intracellular availability of cofactors of NO synthase. In addition to the arginine-NO pathway, arginine was found to be metabolized to urea, ornithine, and, to a much lesser extent, CO2 via arginase and ornithine aminotransferase. The activities of arginase and the formation of ornithine and urea from arginine were decreased by 90% in BBd compared with BBn cells. These results, coupled with the reduced NO synthesis, indicate metabolic defects in arginine metabolism in BBd EC.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased stem cell factor release by hemangioma-derived endothelial cells

BACKGROUND

Capillary hemangiomas, the most common tumors in young children, consist of proliferating capillary vessels and endothelial cells. These tumors also contain large numbers of mast cells, compared with the normal surrounding skin or tissue. We have recently shown that stem cell factor (SCF), the gene product of the murine steel locus, can act as a chemoattractant for mast cells. In this study, we investigated whether SCF might be involved in the recruitment and maintenance of mast cells in hemangiomas.

EXPERIMENTAL DESIGN

Cultured endothelial cells derived from a murine hemangioma were compared with normal vascular endothelial cells for the ability to produce and release SCF, a mitogen for mast cells.

RESULTS

Conditioned medium from hemangioma-derived endothelial cells stimulated the proliferation of cultured mast cells. This proliferative activity was potentiated by interleukin-3. The same conditioned medium was unable to stimulate proliferation of mast cells expressing a defective receptor for SCF. The medium was also unable to stimulate proliferation when it was preincubated with neutralizing antibodies specific for SCF. Immunoprecipitation and Western blot analysis of the conditioned media from hemangioma cells and normal endothelial cells demonstrated the 31,000 molecular weight SCF in hemangioma-conditioned medium only. In addition, proliferative activity for mast cells could not be demonstrated in the conditioned medium of the normal endothelial cells, although Northern blot analysis indicated that both normal and hemangioma-derived endothelial cells express SCF mRNA. Reverse transcriptase-polymerase chain reaction techniques were used to amplify the DNA sequence coding for the proteolytic cleavage site used for release of SCF. Results indicated that both normal and hemangioma-derived endothelial cells express the same transcript for SCF.

CONCLUSIONS

Our data suggest that increased release of SCF is a property of hemangioma-derived endothelial cells that may account for the high numbers of mast cells observed in hemangioma tissue. This increased release of SCF is not due to alternate splicing of SCF transcripts by hemangioma cells.

Insulin stimulates glycolysis and pentose cycle activity in bovine microvascular endothelial cells

Glucose metabolism via the pentose cycle, glycolysis and the Krebs cycle was quantified in bovine microvascular endothelial cells. The major measured end-product of glucose was L-lactate, with relatively small amounts of glucose carbons converted to CO2 and pyruvate. The pentose cycle accounted for less than 4% of the glucose utilized. About 60-70% of the metabolized glucose carbons could not be accounted for by lactate, pyruvate and CO2. Insulin stimulated glycolysis and pentose cycle activity, but had no effect on glucose oxidation via the Krebs cycle. As the pentose cycle is a major source of NADPH which is required for the synthesis of nitric oxide (the endothelium relaxing factor), insulin may play a role in regulating NO generation in endothelial cells by modulating the pentose cycle activity.

Regulation of L-arginine synthesis from L-citrulline by L-glutamine in endothelial cells

L-Arginine synthesis from L-citrulline was studied in cultured bovine venular, bovine aortic, human microvascular, and cloned human microvascular endothelial cells (EC). L-Citrulline was actively recycled into L-arginine in all four EC lines, with similar rates between venular and aortic EC. L-Arginine synthesis from L-citrulline was very sensitive to extracellular L-citrulline concentrations in the range normally found in plasma (50-100 microM). L-Glutamine (0.5mM) decreased L-arginine synthesis from L-citrulline, whereas 0.5 mM L-arginine, L-alanine, L-glutamate, or L-lysine had no effect. In contrast to the findings in intact cells, 1 mM L-glutamine had no effect on L-arginine synthesis from L-citrulline in EC lysates. Similarly, L-glutamine (1 mM) had no effect on the conversion of argininosuccinate to arginine in EC lysates. L-Glutamine (0.5 and 1 mM), but not 0.5 mM L-arginine, L-alanine, L-glutamate, or L-lysine, inhibited L-citrulline transport by EC. The inhibition of L-citrulline transport by L-glutamine was dose dependent and competitive in nature. These results suggest that L-glutamine decreased L-arginine synthesis from extracellular L-citrulline by interfering with its transport. Inasmuch as nitric oxide (NO) and L-citrulline are constantly generated from L-arginine, with L-citrulline being actively converted into L-arginine in venular, microvascular, and aortic EC, our data indicate a functioning intracellular arginine-citrulline cycle in these cells. This cycle may function to efficiently scavenge the carbon and alpha-amino group of L-arginine and to maintain a sufficient cellular concentration of L-arginine during prolonged synthesis of NO in EC. (ABSTRACT TRUNCATED AT 250 WORDS)

[The effect of vasoactive factors on the growth of of coronary endothelial cells]

The aim of this study was to evaluate the effect of adenosine and bradykinin on endothelial cell growth at capillary level. DNA synthesis was measured in subconfluent capillary endothelial cells obtained from coronary venules (CVEC) after exposure to adenosine or bradykinin. Our results indicate that adenosine and bradykinin induced a dose-dependent proliferation of CVEC. Maximal effect was observed at the dose 100 microM concentration for adenosine and 0.1 microM concentration for bradykinin.

Mast cells and angiogenesis

Much data exists in the literature to suggest a correlation between mast cell accumulation and angiogenesis. This correlation exists for normal blood vessel growth as well as pathological vessel growth. The recruitment of mast cells to sites of angiogenesis is not completely understood. However, once at the site, mast cell products may act directly on endothelial cells to stimulate their migration and/or proliferation or may act indirectly by degrading connective tissue matrix to provide space for neovascular sprouts to form. Understanding the role of mast cells in angiogenesis may provide avenues for intervening in and manipulating the neovascularization process.

The c-kit receptor ligand functions as a mast cell chemoattractant

Mast cells accumulate at sites of neovascularization, solid tumors, and many immune reactions. Such accumulation requires directed migration of mature mast cells or their precursors. The nature of the chemoattractants that regulate mast cell motility and the identity of the receptors that mediate the chemotactic response are poorly understood. We have tested the ability of stem cell factor (SCF), a mast cell growth factor, to stimulate mast cell migration. Our results show that SCF is a potent mast cell attractant that stimulates directional motility of both mucosal and connective tissue-type mast cells. The activity is potentiated by costimulation with interleukin-3 (IL-3), another mast cell chemoattractant. SCF, a known ligand for the c-kit tyrosine kinase receptor, was unable to stimulate motility in W42 mutant mast cells, which have a defective c-kit tyrosine kinase. However, W42 mast cells were still able to migrate in response to IL-3. These results show that SCF is a chemotactic factor as well as a growth factor and that the c-kit receptor can transduce signals leading to both cell proliferation and increased directional cell motility.

Anoxia-reoxygenation-induced, neutrophil-mediated endothelial cell injury: role of elastase

The aim of this study was to assess the role of neutrophilic elastase in anoxia-reoxygenation-induced, neutrophil-mediated injury to microvascular endothelium. Cultured bovine microvascular endothelial cells were grown to confluence and labeled with 51Cr. The endothelial cells were exposed to a 30-min period of anoxia and subsequently reoxygenated. Endothelial cell injury, quantitated as 51Cr release and cell detachment, was determined 8 h after reoxygenation. Addition of neutrophils upon reoxygenation enhanced the anoxia-reoxygenation-induced increase in 51Cr release and cell detachment. The neutrophil-mediated injury was associated with elastase release from the neutrophils. Four agents were used to inhibit neutrophilic elastase activity: Eglin C, methoxysuccunyl-Ala2-Pro-Val-CH2Cl, L658,758, and a monoclonal antibody against neutrophilic elastase. All elastase inhibitors attenuated the neutrophil-mediated endothelial cell detachment but not 51Cr release. Addition of purified human neutrophilic elastase, at a level that mimicked the release from neutrophils, increased cell detachment in endothelial cells exposed to anoxia-reoxygenation but did not affect 51Cr release. Our results indicate that elastase plays an important role in anoxia-reoxygenation-induced, neutrophil-mediated endothelial cell dysfunction.

An in vitro model of ischemia/reperfusion-induced microvascular injury

The major objective of this study was to develop an in vitro model of ischemia/reperfusion (I/R)-induced microvascular injury. Cultured venular endothelial cells were grown to confluency, labeled with 51Cr, and exposed to different durations of anoxia (0.5, 1, 2, 3, and 4 h). 51Cr release and cell detachment (indexes of cell injury) were determined at different times after reoxygenation (1, 2, 4, 6, 8, and 18 h). Because in vivo studies have implicated neutrophils in I/R injury, in some experiments human neutrophils were added to the endothelial cells upon reoxygenation. Periods of anoxia greater than or equal to 2 h resulted in 70-80% 51Cr release and 80-95% cell detachment upon reoxygenation. Under these conditions (near maximal injury), the addition of neutrophils produced negligible effects. Periods of anoxia less than or equal to 1 h resulted in 30-40% 51Cr release and 50-60% cell detachment. Under these conditions (moderate cell injury), addition of neutrophils enhanced endothelial cell injury. Using a 30-min period of anoxia, we also assessed the effects of superoxide dismutase (SOD; 300 U/ml) and allopurinol (20 microM) on anoxia/reoxygenation (A/R)-induced injury in the presence or absence of neutrophils. In the absence of neutrophils, SOD or allopurinol did not protect against A/R-induced injury. However, in the presence of neutrophils, both SOD and allopurinol attenuated the increases in 51Cr release. The results derived using this in vitro model of I/R injury are largely consistent with published in vivo studies. Thus this in vitro model may provide further insights regarding the mechanisms involved in I/R injury.

Mechanisms leading to adenosine-stimulated proliferation of microvascular endothelial cells

This study investigated the mechanisms by which adenosine stimulates proliferation of microvascular endothelial cells. The metabolic byproducts of adenosine, inosine and hypoxanthine were unable to stimulate proliferation. When adenosine uptake was prevented, the stimulation of proliferation was unchanged, suggesting that uptake of adenosine with subsequent incorporation into the nucleotide pool is not the mechanism for increasing proliferation. Treatment of endothelial cells with adenosine analogues, presumably selective for either the A1 or A2 receptor, stimulated proliferation equally. This suggested that adenosine 3', 5'-cyclic monophosphate (cAMP) might not mediate the proliferative response to adenosine. However, radioimmunoassay of cell extracts after treatment with either analogue showed an increase in cAMP. In addition, adenylate cyclase blockade with 2', 5'-dideoxyadenosine prevented the proliferative response brought about by these analogues. These data suggest that the proliferative response to adenosine depends on an increase in cAMP. A 2-h pulse of cholera toxin stimulated endothelial cell proliferation, further supporting a role for cAMP. Pretreatment of endothelial cells with pertussis toxin blocked the stimulation of proliferation, indicating that a Gi or similar G protein is also involved in proliferation. We conclude that the proliferative response to adenosine involves a pertussis toxin-sensitive substrate as well as an increase in cAMP.

Physiology and pathobiology of the microcirculation

The delivery of nutrients to the tissues and the removal of waste products from the tissues is made possible by forcing a stream of blood through an arborizing network of microscopic blood vessels that comprise the microcirculation. The rapidity of the flow stream and, therefore, the rate of nutrient delivery to the tissue, is regulated by the automatic adjustment of the caliber of the precapillary arterioles that serve as the primary loci of vascular resistance. Exchange between the blood stream and the parenchymal cells occurs in capillaries and pericytic venules. Pathologic processes such as inflammation, diabetes, ischemia, and hypertension are characterized by abnormalities in microvascular structure and function.

Adenosine and hypoxia stimulate proliferation and migration of endothelial cells

The proliferation of bovine aortic or coronary venular endothelial cells (EC) in vitro was stimulated by the addition of adenosine (0.5 or 5.0 microM) to the culture medium. Cell counts of adenosine-treated aortic EC were 23-76% and coronary venular EC 19-52% greater than nontreated controls. Because adenosine is known to be released by hypoxic tissues, cell proliferation was quantitated when aortic EC were grown at 2% O2. Cell counts were 41-102% greater under hypoxic conditions than when cells were grown at standard tissue culture conditions (approximately 20% O2). When culture medium conditioned by coronary EC grown at 2% O2 was added to EC growing at standard conditions, cell counts were 24-69% greater than controls with medium conditioned by coronary EC grown at 20% O2. This suggests that hypoxia causes endothelial cells to release a factor(s) into the medium that can stimulate cell proliferation. The addition of the adenosine receptor blocker 8-phenyltheophylline (10(-5) M) prevented the stimulation of proliferation caused by hypoxia-conditioned medium, 2% O2 or 5.0 microM adenosine, suggesting that adenosine mediates its effect via an external membrane receptor. Adenosine also stimulated EC chemotaxis. Taken together, these results suggest that adenosine, released as a result of tissue hypoxia, may act as an angiogenic stimulus for the growth of new blood vessels.

Venular endothelial cells from bovine heart

Coronary venular endothelial cells were isolated by a bead-perfusion technique that allowed the selection of endothelial cells from venules of a specific size. Culture conditions for the microvascular cells were established. Cells grew well in supplemented Dulbecco's modified Eagle's medium. The effect of various substrata on the proliferation of the venular endothelial cells was determined. Matrigel, gelatin, and fibronectin supported high levels of proliferation. Cell shape was correlated with ability of the substratum to support cell proliferation. Cells exhibiting a broad, flattened morphology achieved high levels of proliferation. The formation of vessel meshworks by the coronary venular endothelial cells provides an in vitro model for the study of coronary angiogenesis. Confluent monolayers of these cells can be utilized to examine mechanisms of water and protein transport across coronary venules.