Expression and purification of recombinant vascular endothelial-statin

We have recently discovered VE-statin (egfl7), a novel protein specifically expressed by endothelial cells during mouse development and in the adult. VE-statin is expected to play an essential role during the formation of new blood vessels. Here, we have expressed the coding sequence for VE-statin in bacteria and designed a protocol for the production and purification of large amounts of the protein. The protocol is based on an affinity purification step followed by a strong cation-exchange chromatography and a refolding procedure adapted to epidermal growth factor domains and also to the high tendency of the protein to precipitate in concentrated solutions. The purified protein is pure as assessed by SDS-PAGE, immunoblot and mass spectrometry, and is biologically active as a repressor of human aortic smooth muscle cell migration induced by platelet-derived growth factor-BB.     

Comparative endocrinology-paracrinology-autocrinology of human adult large vessel endothelial and smooth muscle cells

Summary Endothelial and smooth muscle cells were isolated from human adult large blood vessels to compare their proliferative response to hormones and growth factors. Neural extracts and the medium from differentiated hepatoma cells were used as concentrated sources of required hormones and growth factors that supported both cell types. Active hormones and growth factors were identified from the neural extracts and hepatoma medium by substitution or direct isolation and biochemical characterization. Epidermal growth factor, lipoproteins, and heparin-binding growth factors elicited growth-stimulatory effects on both endothelial and smooth muscle cells. Both types of human vascular cells displayed 7600 to 8600 specific heparin-binding growth factor receptors per cell with a similar apparent dissociation constant (Kd) of 200 to 250 pM. Heparin modified the response of both endothelial and smooth muscle cells to heparin-binding growth factors dependent on the type of heparin-binding growth factor and amount of heparinlike material present. In addition, heparin exerted a growth factor-independent inhibition of smooth muscle cell proliferation. Platelet-derived growth factor, insulinlike growth factors, and glucocorticoid specifically supported proliferation of smooth muscle cells with no apparent effect on endothelial cell proliferation. Growth-factorlike proteinase inhibitors had an impact specifically on endothelial cell proliferation. Transforming growth factor beta was a specific inhibitor of endothelial cells, but had a positive effect on smooth muscle cell proliferation. The results provide a framework for differential control of the two vascular cell types at normal and atherosclerotic blood vessel sites by the balance among positive and negative effectors of endocrine, paracrine and autocrine origin. This research was supported by NIH grants CA37589, HL33847, and AM35310 from the National Institutes of Health, Bethesda, MD; grant 1718 from the Council for Tobacco Research; and a grant from RJR/Nabisco, Inc.     

VE-statin/egfl7 regulates vascular elastogenesis by interacting with lysyl oxidases

We previously characterized VE-statin/egfl7, a protein that is exclusively secreted by endothelial cells and modulates smooth muscle cell migration. Here, we show that VE-statin/egfl7 is the first known natural negative regulator of vascular elastogenesis. Transgenic mice, expressing VE-statin/egfl7 under the control of keratin-14 promoter, showed an accumulation of VE-statin/egfl7 in arterial walls where its presence correlated with an impaired organization of elastic fibres. In vitro, fibroblasts cultured in the presence of VE-statin/egfl7 were unable to deposit elastic fibres due to a deficient conversion of soluble tropoelastin into insoluble mature elastin. VE-statin/egfl7 interacts with the catalytic domain of lysyl oxidase (LOX) enzymes and, in endothelial cells, endogenous VE-statin/egfl7 colocalizes with LoxL2 and inhibits elastic fibre deposition. In contrast, mature elastic fibres are abundantly deposited by endothelial cells that are prevented from producing endogenous VE-statin/egfl7. We propose a model where VE-statin/egfl7 produced by endothelial cells binds to the catalytic domains of enzymes of the LOX family in the vascular wall, thereby preventing the crosslink of tropoelastin molecules into mature elastin polymers and regulating vascular elastogenesis.     

Smooth muscle progenitor cells from peripheral blood promote the neovascularization of endothelial colony-forming cells

Proangiogenic cell therapy using autologous progenitors is a promising strategy for treating ischemic disease. Considering that neovascularization is a harmonized cellular process that involves both endothelial cells and vascular smooth muscle cells, peripheral blood-originating endothelial colony-forming cells (ECFCs) and smooth muscle progenitor cells (SMPCs), which are similar to mature endothelial cells and vascular smooth muscle cells, could be attractive cellular candidates to achieve therapeutic neovascularization. We successfully induced populations of two different vascular progenitor cells (ECFCs and SMPCs) from adult peripheral blood. Both progenitor cell types expressed endothelial-specific or smooth muscle-specific genes and markers, respectively. In a protein array focused on angiogenic cytokines, SMPCs demonstrated significantly higher expression of bFGF, EGF, TIMP2, ENA78, and TIMP1 compared to ECFCs. Conditioned medium from SMPCs and co-culture with SMPCs revealed that SMPCs promoted cell proliferation, migration, and the in vitro angiogenesis of ECFCs. Finally, co-transplantation of ECFCs and SMPCs induced robust in vivo neovascularization, as well as improved blood perfusion and tissue repair, in a mouse ischemic hindlimb model. Taken together, we have provided the first evidence of a cell therapy strategy for therapeutic neovascularization using two different types of autologous progenitors (ECFCs and SMPCs) derived from adult peripheral blood.     

Apolipoprotein E binding to low density lipoprotein receptor-related protein-1 inhibits cell migration via activation of cAMP-dependent protein kinase A

Smooth muscle cell migration and proliferation contribute to neointimal hyperplasia and vascular stenosis after endothelial denudation. Previous studies revealed that apolipoprotein E (apoE) is an effective inhibitor of platelet-derived growth factor-directed smooth muscle cell migration and proliferation and that the anti-migratory function is mediated via apoE binding to low density lipoprotein receptor-related protein-1 (LRP-1). This study was undertaken to identify the intracellular pathway by which apoE binding to LRP-1 results in inhibition of smooth muscle cell migration. The results showed that apoE increased intracellular cAMP levels 3-fold after 5 min, and the increase was sustained for more than 1 h. As a consequence, apoE also increased protein kinase A (PKA) activity in smooth muscle cells. Importantly, suppression of PKA activity with a cell-permeable peptide inhibitor of PKA abolished the inhibitory effect of apoE on smooth muscle cell migration. These results indicated that apoE inhibition of smooth muscle cell migration is mediated via the activation of cAMP-dependent PKA. Additional experiments revealed that apoE also inhibited fibroblasts migration toward platelet-derived growth factor by a similar mechanism of cAMP-dependent PKA activation. It is noteworthy that apoE failed to increase cAMP levels or inhibit migration of LRP-1-negative mouse embryonic fibroblasts and LRP-1-deficient smooth muscle cells. Taken together, these findings established the mechanism by which apoE inhibits cell migration, i.e. via cAMP-dependent protein kinase A activation as a consequence of its binding to LRP-1.     

Density-dependent endothelial cell production of an inhibitor of smooth muscle cell growth

Embryonic data and ultrastructural analyses suggest that the primitive endothelium signals undifferentiated mesenchymal cells to migrate to the forming blood vessel and subsequently regulates mural cell growth and behavior. Upon maturation of the blood vessel, chemotactic and mitogenic signals are apparently diminished and differentiated smooth muscle cells normally remain quiescent. This homeostasis is seemingly upset in conditions which lead to pathologies characterized by smooth muscle cell hyperplasia such as atherosclerosis. By culturing endothelial cells at different densities, we attempted to re-create the various stages of vascular development. Whereas media conditioned by sparse endothelial cells stimulate smooth muscle cells, media conditioned by dense endothelial cell cultures are inhibitory. Culture of sparse smooth muscle cells in media conditioned for 3 days by postconfluent endothelial cell cultures leads to dose-dependent and reversible smooth muscle cell inhibition. Furthermore, in the presence of the endothelial cell-derived inhibitor, smooth muscle cells are rendered refractory to mitogens such as fibroblast growth factor and platelet-derived growth factor. The inhibitory activity is not attributable to the well-characterized inhibitors of smooth muscle cell growth, transforming growth factor type-β, prostaglandin I₂, or heparan sulfate proteoglycan. Partial characterization of the inhibitory conditioned media suggests that the active molecule is smaller than 1,000 da, and stable to boiling as well as proteinase K and heparinase digestion. These findings support the concept that there is intercellular communication between endothelial cells and smooth muscle cells and provide evidence for a novel endothelial cell-derived smooth muscle cell growth inhibitor.     

Hepatoma-derived growth factor is a pulmonary endothelial cell-expressed angiogenic factor

Hepatoma-derived growth factor (HDGF) was previously identified as a developmentally regulated cardiovascular and renal gene that is mitogenic for vascular smooth muscle and aortic endothelial cells. As reciprocal interactions of smooth muscle and endothelial cells are necessary for vascular formation, we examined whether HDGF plays a role in angiogenesis. According to immunohistochemistry, HDGF was highly expressed in endothelial cells of nonmuscularized, forming blood vessels of the fetal lung. HDGF was also expressed in endothelial cells of small (20 microm) mature arteries and veins. By Western immunoblotting, HDGF was highly expressed by human pulmonary microvascular endothelial cells in vitro. Adenoviral overexpression of HDGF was mitogenic for human pulmonary microvascular endothelial cells in serum-free medium, stimulating a 1.75-fold increase in bromodeoxyuridine (BrdU) uptake and a twofold increase in cell migration. With the chick chorioallantoic membrane (CAM), a biologic assay for angiogenesis, exogenous recombinant HDGF significantly stimulated blood vessel formation and a dose-dependent reorganization of cells within the CAM into a more compact, linear alignment reminiscent of tube formation. According to double immunostaining for endothelial cells with a transforming growth factor-betaII receptor antibody and BrdU as a marker of cell proliferation, exogenous HDGF selectively stimulated endothelial cell BrdU uptake. HDGF also activated specific ERK1/2 signaling and did not overlap with VEGF SAPK/JNK, Akt-mediated pathways. We conclude that HDGF is a highly expressed vascular endothelial cell protein in vivo and is a potent endothelial mitogen and regulator of endothelial cell migration by mechanisms distinct from VEGF.     

Role of lipoproteins in growth of human adult arterial endothelial and smooth muscle cells in low lipoprotein-deficient serum

Recently improved culture conditions for human adult arterial endothelial and smooth muscle cells from a wide variety of donors have been used to study the effects of lipoproteins on proliferation of both cell types in low serum culture medium. Optimal growth of endothelial and smooth muscle cells in an optimal nutrient medium (MCDB 107) containing epidermal growth factor, a partially purified fraction from bovine brain, and 1% (v/v) lipoprotein-deficient serum was dependent on either high- or low-density lipoprotein. High- and low-density lipoprotein stimulated cell growth by three- and five-fold, respectively, over a 6-day period. Optimal stimulation of both endothelial and smooth muscle cell growth occurred between 20 and 60 micrograms/ml of high- and low-density lipoproteins, respectively. No correlation between the activation of 3-hydroxyl-3-methylglutaryl coenzyme. A reductase activity and lipoprotein-stimulated cell proliferation was observed. Lipid-free total apolipoproteins or apolipoprotein C peptides from high-density lipoprotein were partially effective and together with oleic acid effectively replaced native high-density lipoprotein for the support of endothelial cell growth. In contrast, apolipoproteins or apolipoprotein C peptides from high-density lipoprotein alone or with oleic acid had no effect on smooth muscle cell proliferation. The results suggest a functional role of high- and low-density lipoproteins and apolipoproteins in the proliferation of human adult endothelial and smooth muscle cells.     

Opposite effects of monokines (interleukin-1 and tumor necrosis factor) on proliferation and heparin-binding (fibroblast) growth factor binding to human aortic endothelial and smooth muscle cells

Summary Heparin-binding (fibroblast) growth factors (HBGF) are mitogens for both human aortic endothelial and smooth muscle cells. Under similar conditions, both vascular cells display similar numbers of specific HBGF binding sites with similar apparent affinity for HBGF. The monokines, interleukin-1 and tumor necrosis factor, inhibit endothelial cell growth and stimulate smooth muscle cell growth. The opposite mitogenic effects correlate with reduction and increase in HBGF receptor number displayed by endothelial and smooth muscle cells, respectively. These results suggest that the two monokines may depress endothelial cell regeneration and augment smooth muscle cell hyperplasia by differential modulation of the HBGF receptor in the two vascular cell types. This work was supported by US Public Health Service grants DK35310 and HL33487. H. S. is a visiting scientist from Takeda Chemical Industries, Ltd., Central Research Division, Juso-Honmachi-2, Yodogawa-ku, Osaka 532, Japan.     

Polarized secretion of a platelet-derived growth factor-like chemotactic factor by endothelial cells in vitro

Cultured bovine aortic endothelial cells secrete a potent migration-stimulating factor for vascular smooth muscle cells (SMCs) and adventitial fibroblasts. Vascular pericytes are 20-fold less responsive, and endothelial cells themselves do not respond at all. Checkerboard analysis of SMC migration in a micro-chemotaxis chamber assay shows that the factor is chemotactic. Chemotactic activity for SMCs and adventitial fibroblasts is specifically inhibited by antibodies against platelet-derived growth factor. Endothelial cells cultured on nitrocellulose filters secrete the platelet-derived growth factor-like factor almost exclusively into the basal compartment. We suggest that this factor plays an important role in the recruitment of vascular wall cells during the morphogenesis of blood vessels and pathological conditions, such as atherosclerosis.     

PDGF and microvessel wall remodeling in adult rat lung: imaging PDGF-AA and PDGF-Rα molecules in progenitor smooth muscle cells developing in experimental pulmonary hypertension

Smooth muscle cells are mostly absent from the walls of microvessels in the adult lung but develop in large numbers as part of the pathology of human and experimental pulmonary hypertensions (PHs). We have previously shown, in an in vivo model of experimental PH, that mesenchymal (interstitial) fibroblasts and intermediate cells are the progenitors of these cells. Although smooth muscle cell development is a defining pathophysiological feature of human PH, little is known about the angiogenic signaling molecules responsible. Here, we report data for platelet-derived growth factor AA (PDGF-AA) and PDGF-Rα, two components of an important signaling pathway for fibroblast and myofibroblast proliferation and migration. Using antibodies linked to protein-A gold and high-resolution imaging techniques, we analyzed the expression of these molecules as smooth muscle cells developed from progenitor cell populations and in endothelial cells of the same microvessels. PDGF-AA was highly expressed by each cell type in control lung. As PH developed, the number of antigenic sites for PDGF-AA decreased with time. PDGF-Rα expression levels in the control lung were low, relative to the ligand, and fell in PH. These data show, for the first time, a marked phenotypic shift in expression levels of the PDGF-AA isoform and its receptor tyrosine kinase in the progenitor smooth muscle cells developing in the microvessels of the adult hypertensive lung.These studies were supported by NIH 5RO1HL070866 and 5RO1HL064240.     

Presence of α-smooth muscle actin-positive endothelial cells in the luminal surface of adult aorta

α-Smooth muscle actin-positive endothelial cells have not been found in adult aortic endothelium except valve leaflets. Here, using en face immunostaining method, we identified α-smooth muscle actin-positive endothelial cells in the luminal surface of rat, mouse and human thoracic aortas. These cells express both endothelial markers and definite smooth muscle cell markers and were only occasionally observed in thoracic aorta of wild type mice and rats. Their density did not increase with aging. Given that α-smooth muscle actin-positive endothelial cells express low level of vascular endothelial-cadherin that is important for the maintenance of cell contact, these cells were frequently detected in the thoracic aorta of 5-week-old apolipoprotein-E deficient mice. In 20- to 24-week-old apolipoprotein-E deficient mice, marked accumulation of α-smooth muscle actin-positive endothelial cells was observed especially in the luminal surface of atheromatous plaques. Our findings indicate the existence of α-smooth muscle actin-positive endothelial cells in adult aortic endothelium and the possible association with progression of atherosclerosis.     

Low density lipoprotein receptor-related protein mediates apolipoprotein E inhibition of smooth muscle cell migration.

This research was undertaken to identify the cell surface receptor responsible for mediating apolipoprotein E (apoE) inhibition of platelet-derived growth factor (PDGF)-directed smooth muscle cell migration. Initial studies revealed the expression of the low density lipoprotein receptor (LDLR), the LDL receptor-related protein (LRP), the very low density lipoprotein receptor (VLDL), and apoE receptor-2 in mouse aortic smooth muscle cells. Smooth muscle cells isolated from LDLR-null, VLDL-null, and apoE receptor-2-null mice were responsive to apoE inhibition of PDGF-directed smooth muscle cell migration, suggesting that these receptors were not involved. An antisense RNA expression knockdown strategy, utilizing morpholino antisense RNA against LRP, was used to reduce LRP expression in smooth muscle cells to assess the role of this receptor in apoE inhibition of cell migration. Results showed that apoE was unable to inhibit PDGF-directed migration of LRP-deficient smooth muscle cells. The role of LRP in mediating apoE inhibition of PDGF-directed smooth muscle cell migration was confirmed by experiments showing that antibodies against LRP effectively suppressed apoE inhibition of PDGF-directed smooth muscle cell migration. Taken together, these results document that apoE binding to LRP is required for its inhibition of PDGF-directed smooth muscle cell migration.     

Phosphatidylinositol 3-kinase but not tuberin is required for PDGF-induced cell migration

The loss of function of the tumor suppressor gene TSC2 and its protein product tuberin promotes the development of benign lesions by stimulating cell growth, although the role of tuberin in regulating cell migration and metastasis has not been characterized. In addition, the role of phosphatidylinositol 3-kinase (PI 3-kinase), an important signaling event regulating cell migration, in modulating tuberin-deficient cell motility remains unknown. Using a tuberin-deficient rat smooth muscle cell line, ELT3, we demonstrate that platelet-derived growth factor (PDGF) stimulates cell migration by 3.2-fold, whereas vascular endothelial growth factor (VEGF), transforming growth factor (TGF)-alpha, and basic fibroblast growth factor (bFGF) increase migration by 2.1-, 2.1-, and 2.6-fold, respectively. Basal and PDGF-induced migration in tuberin-deficient ELT3, ELT4, and ERC15 cells was not significantly different from that of tuberin-positive transformed rat kidney epithelial 2, airway smooth muscle, and pulmonary arterial vascular smooth muscle cells. Expression of tuberin in tuberin-deficient ELT3 cells also had little effect on cell migration. In parallel experiments, the role of PI 3-kinase activation in ELT3 cell migration was investigated. LY-294002, a PI 3-kinase inhibitor, decreased PDGF-induced migration in a concentration-dependent manner with an IC(50) of approximately 5 microM. LY-294002 also abrogated ELT3 cell migration stimulated by bFGF and TGF-alpha but not by VEGF and phorbol 12-myristate 13-acetate. Furthermore, transient expression of constitutively active PI 3-kinase (p110*) was sufficient to induce ELT3 cell migration. However, the migration induced by p110* was less than that induced by growth factors, suggesting other signaling pathways are also critically important in modulating growth factor-induced cell migration. These data suggest that PI 3-kinase is required for growth factor-induced cell migration and loss of tuberin appears to have little effect on cell migration.     

Functional roles of angiogenic factors and receptors on non‐endothelial cells in the oropharyngeal cavity of the chukar partridge (Alectoris chukar)

The vascular endothelial growth factor (VEGF) family belong to the platelet‐derived growth factor supergene family and is involved in angiogenesis and mitogenesis. The VEGF–VEGFR system regulates endothelial cell proliferation, migration, vascular permeability, secretion and other non‐endothelial cells functions. To clarify the possible role of endothelial and non‐endothelial cells, VEGF and its receptors, vascular endothelial cell growth inhibitor (VEGI) were immunohistochemically examined in oropharyngeal organs. Ten adult partridges were used in this study and the pharynx and larynx were dissected together with the palate and tongue. VEGI, VEGF and its receptor were highly expressed in luminal epithelial and stromal cells, when compared to glandular epithelial and muscle cells (P < 0.05). Moreover, VEGF, its receptors and VEGI were expressed rather strongly in the endothelial cells of the blood capillaries and in both the endothelial and smooth muscle cells of the large and small blood vessels. In conclusion, VEGF and its receptors (flt1/fms, flk1/KDR and flt4) and VEGI were expressed by various cell groups at varying intensity in the oropharyngeal organs. This demonstrates that they play a critical role in the regulation and maintenance of the functions in cells different from endothelial ones as well as in cell proliferation, differentiation, apoptosis and angiogenesis.     

Adult stem cell homing and differentiation in vitro on composite fibrin matrix

Strategies to generate differentiated cells from haematopoetic progenitor cells will enhance potential use of adult stem cells for therapeutic transplantation or tissue engineering. Transplantation of undifferentiated stem cells into recipient tissue hinges on the hypothesis of a milieu dependent differentiation and it has been suggested that a clot-equivalent scaffold is crucial for these circulating cells to anchor and multiply. Here a natural scaffold, fibrin along with fibronectin, gelatin and growth factors has been used to induce endothelial progenitor cells and smooth muscle progenitor cells to differentiate into endothelial cells and smooth muscle cells, respectively, from peripheral blood mononuclear cells. Characteristics of endothelial cells have been verified by the detection of mRNA for and immunostaining the cells for von Willebrand factor, uptake of acetylated low-density lipoproteins and measurement of released nitric oxide in the culture medium, as nitrite. The specific molecules that characterized smooth muscle cells were alpha smooth muscle actin and calponin, besides deposition of collagen type I and elastin, onto the culture matrix. The adhesive proteins used for the fabrication of endothelial progenitor cells matrix and smooth muscle progenitor cells matrix were the same, but specific differentiation was brought about by modulating the growth factor composition in the matrix and in the culture medium. Both endothelial and smooth muscle cells were consistently developed from 20 ml of human blood.     

New role for tRNA and its fragment purified from human urinary bladder carcinoma conditioned medium: inhibition of endothelial cell growth

The growth of endothelial cells is necessary for angiogenesis, which in turn is required for later steps of tumor progression. In an attempt to purify new modulators of endothelial cell growth from the conditioned medium of human urinary bladder carcinoma cells, we isolated a small and stable oligonucleotide containing 10 to 16 bases. This oligonucleotide inhibited the growth of endothelial cells in vitro and was identified as a fragment of transfer RNA (tRNA). When unfractionated bovine tRNA was added to the cell culture, it specifically inhibited growth of endothelial cells, but not smooth muscle cells, bovine kidney cells, 3T3 fibroblasts, and several cancer cell lines. In contrast, ribosomal RNA, total yeast RNA, and single nucleosides from tRNA hydrolysate had no effect. These results demonstrate a new role for tRNA and its fragment as a selective endothelial cell inhibitor in vitro.