The angiogenic regulator CD13/APN is a transcriptional target of Ras signaling pathways in endothelial morphogenesis

The epigenetic reprogramming of poorly aggressive melanoma cells by a metastatic microenvironment

A dynamic, complex relationship exists between tumor cells and their microenvironment, which plays a pivotal role in cancer progression, yet remains poorly understood. Particularly perplexing is the finding that aggressive melanoma cells express genes associated with multiple cellular phenotypes, in addition to their ability to form vasculogenic-like networks in three-dimensional matrix - called vasculogenic mimicry, which is illustrative of tumor cells plasticity. This study addressed the unique epigenetic effect of the microenvironment of aggressive melanoma cells on the behavior of poorly aggressive melanoma cells exposed to it. The data show significant changes in the global gene expression of the cells exposed to 3-D matrices preconditioned by aggressive melanoma cells, including the acquisition of a vasculogenic cell phenotype, upregulation of ECM remodeling genes, and increased invasive ability - indicative of an epigenetic, microenvironment-induced reprogramming of poorly aggressive melanoma cells. However, this epigenetic effect was completely abrogated when a highly cross-linked collagen matrix was used, which could not be remodeled by the aggressive melanoma cells. These findings offer an unique perspective of the inductive properties associated with an aggressive melanoma microenvironment that might provide new insights into the epigenetic regulation of tumor cell plasticity and differentiation, as well as mechanisms that could be targeted for novel therapeutic strategies.

CD13/Aminopeptidase N overexpression by basic fibroblast growth factor mediates enhanced invasiveness of 1F6 human melanoma cells

CD13/Aminopeptidase N (CD13) is known to play an important role in tumour cell invasion. We examined whether basic fibroblast growth factor (bFGF) is involved in the regulation of CD13 expression in human melanoma cells. 1F6 human melanoma cells were stably transfected with constructs encoding either the 18 kDa (18kD) or all (ALL) bFGF isoform proteins. We observed highly increased CD13 mRNA and protein expression in the 1F6 clones regardless of the overexpression of either the 18kD or all isoform proteins. Neutral aminopeptidase activity was increased five-fold and could be inhibited by bestatin and the CD13-neutralising antibody WM15. The enhanced invasion through Matrigel, but not migration in a wound assay, was efficiently abrogated by both bestatin and WM15. Upregulation of CD13 expression was the result of increased epithelial and myeloid promoter activity up to 4.5-fold in 1F6-18kD and 1F6-ALL clones. Interestingly, in a panel of human melanoma cell lines, a significant correlation (r²=0.883, P<0.05) between bFGF and CD13 mRNA and protein expression was detected. High bFGF and CD13 expression were clearly related with an aggressive phenotype. Taken together, our data indicate that high bFGF expression upregulates CD13 expression in human melanoma cells by activating both the myeloid and the epithelial CD13 promoter. In addition, we show that high bFGF and CD13 expression results in enhanced invasive capacity and metastatic behaviour of human melanoma cells.

Prostate-specific membrane antigen regulates angiogenesis by modulating integrin signal transduction

The transmembrane peptidase prostate-specific membrane antigen (PSMA) is universally upregulated in the vasculature of solid tumors, but its functional role in tumor angiogenesis has not been investigated. Here we show that angiogenesis is severely impaired in PSMA-null animals and that this angiogenic defect occurs at the level of endothelial cell invasion through the extracellular matrix barrier. Because proteolytic degradation of the extracellular matrix is a critical component of endothelial invasion in angiogenesis, it is logical to assume that PSMA participates in matrix degradation. However, we demonstrate a novel and more complex role for PSMA in angiogenesis, where it is a principal component of a regulatory loop that is tightly modulating laminin-specific integrin signaling and GTPase-dependent, p21-activated kinase 1 (PAK-1) activity. We show that PSMA inhibition, knockdown, or deficiency decreases endothelial cell invasion in vitro via integrin and PAK, thus abrogating angiogenesis. Interestingly, the neutralization of beta(1) or the inactivation of PAK increases PSMA activity, suggesting that they negatively regulate PSMA. This negative regulation is mediated by the cytoskeleton as the disruption of interactions between the PSMA cytoplasmic tail and the anchor protein filamin A decreases PSMA activity, integrin function, and PAK activation. Finally, the inhibition of PAK activation enhances the PSMA/filamin A interaction and, thus, boosts PSMA activity. These data imply that PSMA participates in an autoregulatory loop, wherein active PSMA facilitates integrin signaling and PAK activation, leading to both productive invasion and downregulation of integrin beta(1) signaling via reduced PSMA activity. Therefore, we have identified a novel role for PSMA as a true molecular interface, integrating both extracellular and intracellular signals during angiogenesis.

Immunohistochemical expression of aminopeptidase N (CD13) in human lung squamous cell carcinomas, with special reference to Bestatin adjuvant therapy

Bestatin, a specific inhibitor of aminopeptidase N (CD13), has been reported to prolong survival time in patients with completely resected stage I lung squamous cell carcinoma. Considering the antitumor mechanism of Bestatin, it is interesting to know whether CD13 is expressed in human lung squamous cell carcinoma. The immunohistochemical expression of CD13 was examined in human lung carcinoma and the question of whether CD13 was immunohistochemically expressed in the interstitial tissue was investigated, mainly in the fibroblasts and blood vessels, surrounding the tumor nests of various kinds of non-small cell lung cancers, especially of squamous cell carcinomas. In Japanese squamous cell carcinoma of the lung, 38 (61.3%) out of 62 cancers were positively stained in the same manner on immunohistochemistry for CD13. The area of interstitial tissue positively stained for CD13 varied depending on the case. To confirm the cell nature of the interstitial tissue with CD13 positivity, double immunohistochemistry using CD34 and alpha-smooth muscle actin was performed. Double immunohistochemistry showed that the majority of CD13-positive cells were slender fibroblastic cells around the blood vessels and some endothelial cells.

Incadronate inhibits aminopeptidase N expression in prostatic PC-3 cells

Bone metastasis is an important cause of morbidity in advanced prostate cancer. Bisphosphonates are widely used for the treatment and prevention of osteoporosis, but recently have been observed to be effective in controlling prostate cancer metastasis. Since aminopeptidase N (AP-N) is known to be involved in the metastasis of prostate cancer, we investigated the effect of bisphosphonate on AP-N expression. Incadronate induced inhibition of AP-N mRNA and protein expression in PC-3 cells. The inhibitory effect of AP-N mRNA expression was also observed in the cells treated with pravastatin and other nitrogen-containing bisphosphonates, which inhibit the key enzyme in the isoprenoid biosynthesis pathway. The decrease of AP-N mRNA expression induced by incadronate was inhibited by co-incubation with geranylgeranyl diphosphate (GGPP). Moreover, GGTI-286 treatment also resulted in reduced AP-N mRNA expression. The translocation of small G protein Rap1 from the cytosol to the membrane was inhibited by incadronate and pravastatin, respectively. These above results indicate that the decrease in AP-N expression elicited by bisphosphonate is related to the inhibition of the mevalonate pathway.

Neurofibroma-associated growth factors activate a distinct signaling network to alter the function of neurofibromin-deficient endothelial cells

Genetic inactivation of tumor suppressor genes initiates human cancers. However, interaction of accessory cells with the tumor-initiating cell within the microenvironment is often required for tumor progression. This paradigm is relevant to understanding neurofibroma development in neurofibromatosis type I patients. Somatic inactivation of the Nf1 tumor suppressor gene, which encodes neurofibromin, is necessary but not sufficient to initiate neurofibroma development. In contrast, neurofibromas occur with high penetrance in mice in which Nf1 is ablated in Schwann cells in the context of a heterozygous mutant (Nf1+/-) microenvironment. Neurofibromas are highly vascularized, and recent studies suggest that Nf1+/- mice have increased angiogenesis in vivo. However, the function of neurofibromin in human endothelial cells (ECs) and the biochemical mechanism by which neurofibromin regulates neoangiogenesis are not known. Utilizing Nf1+/- mice, primary human ECs and endothelial progenitor cells harvested from NF1 patients, we identified a discrete Ras effector pathway, which alters the proliferation and migration of neurofibromin-deficient ECs in response to neurofibroma-derived growth factors both in vitro and in vivo. Thus, these studies identify a unique biochemical pathway in Nf1+/- ECs as a potential therapeutic target in the neurofibroma microenvironment.

Pericellular proteases in angiogenesis and vasculogenesis

Pericellular proteases play an important role in angiogenesis and vasculogenesis. They comprise (membrane-type) matrix metalloproteinases [(MT-)MMPs], serine proteases, cysteine cathepsins, and membrane-bound aminopeptidases. Specific inhibitors regulate them. Major roles in initiating angiogenesis have been attributed to MT1-matrix metalloproteinase (MMP), MMP-2, and MMP-9. Whereas MT-MMPs are membrane-bound by nature, MMP-2 and MMP-9 can localize to the membrane by binding to alphavbeta3-integrin and CD44, respectively. Proteases switch on neovascularization by activation, liberation, and modification of angiogenic growth factors and degradation of the endothelial and interstitial matrix. They also modify the properties of angiogenic growth factors and cytokines. Neovascularization requires cell migration, which depends on the assembly of protease-protein complexes at the migrating cell front. MT1-MMP and urokinase (u-PA) form multiprotein complexes in the lamellipodia and focal adhesions of migrating cells, facilitating proteolysis and sufficient support for endothelial cell migration and survival. Excessive proteolysis causes loss of endothelial cell-matrix interaction and impairs angiogenesis. MMP-9 and cathepsin L stimulate the recruitment and action of blood- or bone-marrow-derived accessory cells that enhance angiogenesis. Proteases also generate fragments of extracellular matrix and hemostasis factors that have anti-angiogenic properties. Understanding the complexity of protease activities in angiogenesis contributes to recognizing new targets for stimulation or inhibition of neovascularization in disease.

Aminopeptidase N (APN/CD13) is selectively expressed in vascular endothelial cells and plays multiple roles in angiogenesis

Proteolytic enzyme-mediated degradation of the extracellular matrix (ECM) is crucial for the formation of both tumor metastasis and angiogenesis. Recently, several reports have suggested that aminopeptidases are involved in this process, but precisely how is largely unknown. We found here that aminopeptidase N (APN/CD13) was selectively expressed in vascular endothelial cells including human umbilical vein endothelial cells (HUVEC) and human aortic endothelial cells (HAEC), and was not detectable in a majority of normal cells and tumor cell lines we examined. RNA interference (RNAi) of APN resulted in the inhibition of capillary tube formation of HUVEC on Matrigel. APN siRNA suppressed the migration of HUVEC through a fibronectin-coated Transwell membrane, and reduced the cellular adhesion to Matrigel and various adhesion molecules including type IV collagen, type I collagen and fibronectin. These findings suggest that APN is a multifunctional protein with important roles in vascular endothelial morphogenesis during angiogenesis.

Triton-polyacrylamide gel electrophoresis and leucine aminopeptidase activity staining detect Triton-slowed bands including high-molecular-mass aminopeptidase N (CD13) isoform in cholestatic patient sera

BACKGROUND

Western blotting of aminopeptidase N (APN) detects a high-molecular-mass isoform (260 kDa) [M. Kawai, Y. Otake, Y. Hara High-molecular-mass isoform of aminopeptidase N/CD13 in serum from cholestatic patients. Clin Chim Acta 330 (2003) 141-149] in cholestatic patient serum but is time-consuming.

METHODS

Human sera were electrophoresed on polyacrylamide gel containing Triton-X100 (Triton-PAGE) and stained with leucine-B-naphthylamide (LAP-staining). The stained bands were eluted from the gel, treated with N- and O-glycosidase if necessary, and analyzed by Western blotting [M. Kawai, Y. Otake, Y. Hara High-molecular-mass isoform of aminopeptidase N/CD13 in serum from cholestatic patients. Clin Chim Acta 330 (2003) 141-149].

RESULTS

Triton-PAGE and LAP-staining clearly detected fast bands in all the sera examined. Almost parallel with leucine aminopeptidase activity, slow bands were strongly stained in all 11 cholestatic patients but clearly stained in 3 out of 14 patients with hepatobiliary diseases other than cholestasis. PAGE with various concentrations of Triton showed that Triton slows down slow bands but not fast bands. Western blotting showed that Triton-PAGE-slow bands of cholestasis contained 140 and 260-kDa APN and that fast bands were slightly smaller than monomer-size slow bands after glycosidase treatment.

CONCLUSIONS

Less time-consuming than Western blotting, Triton-PAGE and LAP-staining detect novel APN bands slowed by Triton and partly composed of the high-molecular-mass isoform in cholestasis. The slow bands seem to be homodimers of APN with transmembrane anchors. The polypeptide of the fast band seems to be processed differently from that of the slow band.

Multiple signaling pathways are involved in endothelin-1-induced brain endothelial cell migration

We have observed that the vasoactive peptide endothelin-1 is a potent inducer of migration of primary human brain-derived microvascular endothelial cells. By blocking signal transduction pathways with specific inhibitors, and using dominant negative mutant infections, we have demonstrated that multiple pathways are involved in endothelin-1-induced migration. Absolutely required for migration are protein tyrosine kinase Src, Ras, protein kinase C (PKC), phosphatidylinositol 3-kinase, ERK, and JNK; partial requirements were exhibited by cAMP-activated protein kinase and p38 kinase. Partial elucidation of the signal transduction sequences showed that the MAPKs ERK, JNK, and p38 are positioned downstream of both PKC and cAMP-activated protein kinase in the signal transduction scheme. The results show that human brain endothelial cell migration has distinct characteristics, different from cells derived from other vascular beds, or from other species, often used as model systems. Furthermore, the results indicate that endothelin-1, secreted by many tumors, is an important contributor to tumor-produced proangiogenic microenvironment. This growth factor has been associated with increased microvessel density in tumors and is responsible for endothelial cell proliferation, migration, invasion, and tubule formation. Because many signal transduction pathways investigated in this study are potential or current targets for anti-angiogenesis therapy, these results are of critical importance for designing physiological antiangiogenic protocols.

CD13 in cell adhesion: aminopeptidase N (CD13) mediates homotypic aggregation of monocytic cells

Homotypic aggregation (HA) of cells plays key roles in physiological and pathological processes, such as embryogenesis, immune responses, angiogenesis, tumor cell invasion, and metastasis. Aminopeptidase N (CD13) has been implicated in most of these phenomena, although its participation has been attributed to its enzymatic activity, while its role as an adhesion molecule has been almost unexplored. Here, we show that certain anti‐CD13 monoclonal antibodies induce HA of monocytic U‐937 cells, independently of their effect on enzymatic activity. The phenomenon is related to binding to a specific site on the CD13 molecule and is independent of integrins. It is abrogated by low temperature, by the glycolysis inhibitor 2‐deoxyglucose, and by inhibitors of tyrosine and mitogen‐activated protein kinases. The inhibitor of microtubule polymerization colchicine has a synergistic effect on CD13‐mediated aggregation, suggesting an inhibitory role of microtubules in this process. Finally, during HA, CD13 actively redistributes to the zones of cell‐cell contact, as determined by live cell imaging studies, demonstrating a direct role of CD13 in the adhesion phenomenon. Together, these data show for the first time the participation of CD13 in monocytic cell adhesion.

Aspirin inhibits thrombin action on endothelial cells via up-regulation of aminopeptidase N/CD13 expression

OBJECTIVE

We hypothesized that aspirin may exhibit its anti-atherosclerotic effects via mechanisms other than cyclooxygenase inhibition in platelets.

METHODS AND RESULTS

Using enhanced subtraction hybridization analysis, we found in human umbilical vein endothelial cells (HUVECs) that aspirin up-regulates the expression of aminopeptidase N (APN/CD13) mRNA and its surface protein levels in a dose-dependent manner. Enzymatic activity of APN/CD13 on HUVECs was increased approximately 1.5-fold by 1 mmol L(-1) of aspirin, and treatment with bestatin, an inhibitor for APN/CD13 metalloprotease activity, attenuated the enhanced activities of APN/CD13. Since activated thrombin receptor is reported to be inactivated by APN/CD13 in vitro, protective actions of aspirin on HUVECs by thrombin stimulation were examined, resulting in the suppression of endothelin-1 and reactive oxygen species productions in HUVECs. These inhibitory actions of aspirin were partially abrogated by bestatin.

CONCLUSIONS

Aspirin may exert its anti-atherothrombotic effects in part via the inhibition of thrombin action by up-regulating APN/CD13 on endothelial cells.

Addition of an aminopeptidase N-binding sequence to human endostatin improves inhibition of ovarian carcinoma growth

BACKGROUND

Blood vessels in tumors express higher level of aminopeptidase N (APN) compared with normal tissues. It has been reported that peptides that contain asparagine-glycine-arginine (NGR) sequence home to APN in tumor vasculature. Increased expression of APN in tumor vascular endothelium, therefore, offers an opportunity to target NGR peptide-linked therapeutic reagents to tumors.

METHODS

To determine whether an additional NGR sequence could improve endothelial homing and biologic activity, human endostatin was modified genetically to introduce an NGR motif (NGR-endostatin) and was expressed in yeast. In vitro biologic activity of NGR-endostatin was compared with the native protein in endothelial cell proliferation and migration. NGR-modified endostatin was used in tumor localization studies. Finally, the effects of endostatin and NGR-endostatin on tumor growth were determined in two model systems.

RESULTS

Human endostatin has an internal NGR sequence, which is not accessible to bind APN. However, the addition of an NGR-sequence at the amino terminus resulted in strong binding and inhibition of endothelial cell APN. NGR-endostatin showed increased binding to endothelial cells compared with the native protein. Increased binding of endostatin also coincided with improved antiangiogenic properties of endostatin. NGR modification improved tumor localization and, as a consequence, effectively inhibited ovarian carcinoma growth in athymic nude mice.

CONCLUSIONS

These studies demonstrated that human endostatin can be modified genetically to improve its ability to inhibit tumor growth.

Cells of human aminopeptidase N (CD13) transgenic mice are infected by human coronavirus-229E in vitro, but not in vivo

Aminopeptidase N, or CD13, is a receptor for serologically related coronaviruses of humans, pigs, and cats. A mouse line transgenic for the receptor of human coronavirus-229E (HCoV-229E) was created using human APN (hAPN) cDNA driven by a hAPN promoter. hAPN-transgenic mice expressed hAPN mRNA in the kidney, small intestine, liver, and lung. hAPN protein was specifically expressed on epithelial cells of the proximal convoluted renal tubules, bronchi, alveolar sacs, and intestinal villi. The hAPN expression pattern within transgenic mouse tissues matched that of mouse APN and was similar in mice heterozygous or homozygous for the transgene. Primary embryonic cells and bone marrow dendritic cells derived from hAPN-transgenic mice also expressed hAPN protein. Although hAPN-transgenic mice were resistant to HCoV-229E in vivo, primary embryonic cells and bone marrow dendritic cells were infected in vitro. hAPN-transgenic mice are valuable as a source of primary mouse cells expressing hAPN. This hAPN-transgenic line will also be used for crossbreeding experiments with other knockout, immune deficient, or transgenic mice to identify factors, in addition to hAPN, that are required for HCoV-229E infection.

Identification of Human Aminopeptidase O, a Novel Metalloprotease with Structural Similarity to Aminopeptidase B and Leukotriene A4 Hydrolase

We have cloned and characterized a human brain cDNA encoding a new metalloprotease that has been called aminopeptidase O (AP-O). AP-O exhibits a series of structural features characteristic of aminopeptidases, including a conserved catalytic domain with a zinc-binding site (HEXXHX18E) that allows its classification in the M1 family of metallopeptidases or gluzincins. The structural complexity of AP-O is further increased by the presence of an additional C-terminal domain 170 residues long, which is predicted to have an ARM repeat fold originally identified in the Drosophila segment polarity gene product Armadillo. This ARM repeat domain is also present in aminopeptidase B, aminopeptidase B-like, and leukotriene A4 hydrolase and defines a novel subfamily of aminopeptidases that we have called ARM aminopeptidases. Northern blot analysis revealed that AP-O is mainly expressed in the pancreas, placenta, liver, testis, and heart. Human AP-O was produced in Escherichia coli, and the purified recombinant protein hydrolyzed synthetic substrates used for assaying aminopeptidase activity. This activity was abolished by general inhibitors of metalloproteases and specific inhibitors of aminopeptidases. Recombinant AP-O also cleaved angiotensin III to generate angiotensin IV, a bioactive peptide of the renin-angiotensin pathway with multiple actions on diverse tissues, including brain, testis, and heart. On the basis of these results we suggest that AP-O could play a role in the proteolytic processing of bioactive peptides in those tissues where it is expressed.

Gene expression profiles in rat intestine identify pathways for 1,25-dihydroxyvitamin D(3) stimulated calcium absorption and clarify its immunomodulatory properties

Microarray technology has been used to discover 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) induced gene expression changes in rat small intestine in vivo. Here, we report gene expression changes related to intestinal absorption or transport, the immune system and angiogenesis in response to 1,25-(OH)(2)D(3). Vitamin D deficient rats were intrajugularly given vehicle or vehicle containing 730 ng of 1,25-(OH)(2)D(3)/kg of body weight. Intestinal mRNA was harvested from duodenal mucosa at 15 min, 1, 3, and 6 h post-injection and studied by Affymetrix microarrays. Genes significantly affected by 1,25-(OH)(2)D(3) were confirmed by quantitative RT-PCR with remarkable agreement. The most strongly affected gene in intestine was CYP24 with 97-fold increase at 6 h post-1,25-(OH)(2)D(3) treatment. Intestinal calcium absorption genes: TRPV5, TRPV6, calbindin D(9k), and Ca(2+) dependent ATPase all were up-regulated in response to 1,25-(OH)(2)D(3), supporting the currently accepted mechanism of 1,25-(OH)(2)D(3) induced transcellular calcium transport. However, a 1,25-(OH)(2)D(3) suppression of several intra-/intercellular matrix modeling proteins such as sodium/potassium ATPase, claudin 3, aquaporin 8, cadherin 17, and RhoA suggests a vitamin D regulation of tight junction permeability and paracellular calcium transport. Several other genes related to the immune system and angiogenesis whose expression was changed in response to 1,25-(OH)(2)D(3) provided evidence for an immunomodulatory and anti-angiogenic role of 1,25-(OH)(2)D(3).

Puromycin-insensitive leucyl-specific aminopeptidase (PILSAP) binds and catalyzes PDK1, allowing VEGF-stimulated activation of S6K for endothelial cell proliferation and angiogenesis

Puromycin-insensitive leucyl-specific aminopeptidase (PILSAP) plays an important role in angiogenesis by regulating the proliferation and migration of endothelial cells (ECs). Here we characterize the mechanism by which PILSAP regulates the vascular endothelial growth factor (VEGF)–stimulated proliferation of ECs. The specific elimination of PILSAP expression or its enzymatic activity inhibited VEGF-stimulated G1/S transition in ECs. This G1 arrest correlated with reduced cyclin dependent kinase 4/6 (CDK4/6) activity and retinoblastoma (Rb) protein phosphorylation. Analyses of signaling molecules upstream of CDK4/6 revealed that S6 kinase (S6K) activation was affected by PILSAP, whereas that of phosphatidylinositol-3 kinase (PI3K), Akt, and extracellular signal-related kinase 1/2 (ERK1/2) was not. We further demonstrated that PILSAP bound phosphatidylinositol-dependent kinase 1 (PDK1) and removed 9 amino acids from its N-terminus, which allowed S6K to associate with PDK1 and PILSAP upon VEGF stimulation. We constructed mutant PILSAP, which lacked the aminopeptidase activity but bound PDK1. Mutant PILSAP abrogated S6K activation upon VEGF stimulation in a dominant-negative manner. An N-terminal truncated form of PDK1 abolished the dominant-negative effect of mutant PILSAP. Finally, the introduction of a mutated PILSAP gene in ECs inhibited angiogenesis and retarded tumor growth in vivo. These results indicate that PILSAP plays a crucial role in the cell cycle progression of ECs and angiogenesis via the binding and modification of PDK1.

Oncogenic Ras and its role in tumor cell invasion and metastasis

The processes by which cancer cells leave the tumor and enter adjacent tissue is known as invasion, whereas metastasis refers to secondary tumor colonization of tissue at a distance from the primary lesion. These two events are the most lethal of cancer phenomena and the signaling mechanisms that govern them are complex. The Ras signaling pathways are well represented in their involvement in tumor initiation, but considerably less is known about their contribution to invasion and metastasis. In this review, we discuss the current evidence for mutant Ras proteins as significant players in these aspects of cancer progression.

Administration of CD34+ cells after stroke enhances neurogenesis via angiogenesis in a mouse model

Thrombo-occlusive cerebrovascular disease resulting in stroke and permanent neuronal loss is an important cause of morbidity and mortality. Because of the unique properties of cerebral vasculature and the limited reparative capability of neuronal tissue, it has been difficult to devise effective neuroprotective therapies in cerebral ischemia. Our results demonstrate that systemic administration of human cord blood-derived CD34(+) cells to immunocompromised mice subjected to stroke 48 hours earlier induces neovascularization in the ischemic zone and provides a favorable environment for neuronal regeneration. Endogenous neurogenesis, suppressed by an antiangiogenic agent, is accelerated as a result of enhanced migration of neuronal progenitor cells to the damaged area, followed by their maturation and functional recovery. Our data suggest an essential role for CD34(+) cells in promoting directly or indirectly an environment conducive to neovascularization of ischemic brain so that neuronal regeneration can proceed.

Role of Aminopeptidase in Angiogenesis

Microvessels are composed of endothelial cells and surrounding pericytes. Angiogenesis, a neo-vessel formation from pre-existing microvessels, is a complex phenomenon, which requires following sequential steps: detachment of pre-existing pericytes for vascular destabilization, extracellular matrix turnover, migration, proliferation, tube formation by endothelial cells (ECs), and reattachment of pericytes for vascular stabilization. Aminopeptidases regulate the N-terminal modification of proteins and peptides for maturation, activation or degradation, and thereby relate to a variety of biological processes. Recently, three aminopeptidases have been reported to be involved in angiogenesis. They include type 2 methionine aminopeptidase, aminopeptidase N, and adipocyte-derived leucine aminopeptidase/puromycin insensitive leucyl-specific aminopeptidase. This review will focus on the possible role of these aminopeptidases in angiogenesis.

Regulation of endothelial cell differentiation and transformation by H-Ras

Angiogenesis is regulated by growth factors which activate tyrosine kinase receptors leading to the activation of a number of intracellular signaling pathways. The specific function of H-Ras during FGF-2 stimulated endothelial cell differentiation, defined as invasive growth and formation of branching networks in fibrin gels, was investigated by using conditionally immortalized endothelial cell lines induced to express H-Ras mutants. Expression of inhibitory N17Ras did not impair differentiation in response to FGF-2 and TNF-alpha. The farnesyltransferase inhibitor FTI-277 inhibited farnesylation of Ras but did not inhibit differentiation of human microvascular endothelial cells or mouse brain endothelial cells. In contrast, activated V12Ras inhibited endothelial cell differentiation and cells displayed a transformed phenotype with an increased rate of proliferation and loss of contact inhibited growth. Furthermore, V12Ras expressing endothelial cells grew as solid tumors when injected subcutaneously into mice. Our data suggest that, in endothelial cells, H-Ras activity is not required for differentiation. However, this activity must be tightly regulated as aberrant activity can disturb the ability of endothelial cells to undergo differentiation.

Alternative splicing of the human cyclin D-binding Myb-like protein (hDMP1) yields a truncated protein isoform that alters macrophage differentiation patterns

We have cloned two novel, alternatively spliced messages of human cyclin D-binding Myb-like protein (hDMP1). The known, full-length protein has been named hDMP1alpha and the new isoforms, hDMP1beta and hDMP1gamma. The hDMP1alpha, -beta, and -gamma splice variants have unique expression patterns in normal hematopoietic cells; hDMP1beta mRNA transcripts are strongly expressed in quiescent CD34+ cells and freshly isolated peripheral blood leukocytes, as compared with hDMP1alpha. In contrast, activated T-cells and developing myeloid cells, macrophages, and granulocytes express low levels of hDMP1beta transcripts, and hDMP1gamma is ubiquitously and weakly expressed. Mouse Dmp1 has been shown to activate CD13/aminopeptidase N (APN) and p19ARF gene expression via binding to canonical DNA recognition sites in the respective promoters. Assessment of CD13/APN promoter responsiveness demonstrated that hDMP1alpha but not hDMP1beta and -gamma, is a transcriptional activator. Furthermore, hDMP1beta was found to inhibit the CD13/APN promoter transactivation ability of hDMP1alpha. Stable, ectopic expression of hDMP1beta and, to a lesser extent hDMP1gamma, reduced endogenous cell surface levels of CD13/APN in U937 cells. Moreover, stable, ectopic expression of hDMP1beta altered phorbol 12-myristate 13-acetate-induced terminal differentiation of U937 cells to macrophages and resulted in maintenance of proliferation. These results demonstrate that hDMP1beta antagonizes hDMP1alpha activity and suggest that cellular functions of hDMP1 may be regulated by cellular hDMP1 isoform levels.

CD13/APN transcription is induced by RAS/MAPK-mediated phosphorylation of Ets-2 in activated endothelial cells

CD13/aminopeptidase N (CD13/APN) is a potent regulator of angiogenesis both in vitro and in vivo and transcription of CD13/APN in endothelial cells is induced by angiogenic growth factors via the RAS/MAPK pathway. We have explored the nuclear effectors downstream of this pathway that are responsible for CD13/APN induction. The response to serum/angiogenic growth factors mapped to a 38-bp region of the CD13/APN promoter containing an Ets-core motif that specifically binds a protein complex from nuclear lysates from activated endothelial cells. This motif and the proteins that target it are functionally relevant because mutation of this sequence abrogates CD13/APN transcription. Analysis of endothelial Ets family members showed that Ets-2, and to a lesser extent Ets-1, transactivate CD13/APN promoter activity via the Ets-core motif, whereas Fli, Erg, and NERF are ineffective. We investigated the possibility that the induction of CD13/APN is mediated by phosphorylation of Ets-2 via RAS/MAPK. A phosphorylation-defective Ets-2 mutant, T72A, failed to transactivate CD13/APN, suggesting that Ets-2 phosphorylation is obligatory for CD13/APN induction. To confirm a role for endogenous Ets-2 in CD13/APN expression, we specifically abrogated Ets-2 mRNA and protein by siRNA knockdown that significantly inhibited CD13/APN transcription. Finally, to assess the relevance of Ets-2 in endothelial cell function, we induced endothelial cells containing Ets-2 siRNA oligonucleotides to form capillary networks. Cells containing the Ets-2 inhibitory small interfering RNAs were completely incapable of forming the organized networks characteristic of endothelial morphogenesis. Thus, the phosphorylation of Ets-2 by RAS/MAPK is a prerequisite for CD13/APN endothelial induction and Ets-2 and its targets play essential roles in endothelial cell function.