Aminopeptidase a is a constituent of activated pericytes in angiogenesis

New Insights in ATP Synthesis as Therapeutic Target in Cancer and Angiogenic Ocular Diseases

Lactate and ATP formation by aerobic glycolysis, the Warburg effect, is considered a hallmark of cancer. During angiogenesis in non-cancerous tissue, proliferating stalk endothelial cells (ECs) also produce lactate and ATP by aerobic glycolysis. In fact, all proliferating cells, both non-cancer and cancer cells, need lactate for the biosynthesis of building blocks for cell growth and tissue expansion. Moreover, both non-proliferating cancer stem cells in tumors and leader tip ECs during angiogenesis rely on glycolysis for pyruvate production, which is used for ATP synthesis in mitochondria through oxidative phosphorylation (OXPHOS). Therefore, aerobic glycolysis is not a specific hallmark of cancer but rather a hallmark of proliferating cells and limits its utility in cancer therapy. However, local treatment of angiogenic eye conditions with inhibitors of glycolysis may be a safe therapeutic option that warrants experimental investigation. Most types of cells in the eye such as photoreceptors and pericytes use OXPHOS for ATP production, whereas proliferating angiogenic stalk ECs rely on glycolysis for lactate and ATP production. (J Histochem Cytochem XX.XXX-XXX, XXXX).

Comparison of the Behavior of Perivascular Cells (Pericytes and CD34+ Stromal Cell/Telocytes) in Sprouting and Intussusceptive Angiogenesis

Perivascular cells in the pericytic microvasculature, pericytes and CD34+ stromal cells/telocytes (CD34+SCs/TCs), have an important role in angiogenesis. We compare the behavior of these cells depending on whether the growth of endothelial cells (ECs) from the pre-existing microvasculature is toward the interstitium with vascular bud and neovessel formation (sprouting angiogenesis) or toward the vascular lumen with intravascular pillar development and vessel division (intussusceptive angiogenesis). Detachment from the vascular wall, mobilization, proliferation, recruitment, and differentiation of pericytes and CD34+SCs/TCs, as well as associated changes in vessel permeability and functionality, and modifications of the extracellular matrix are more intense, longer lasting over time, and with a greater energy cost in sprouting angiogenesis than in intussusceptive angiogenesis, in which some of the aforementioned events do not occur or are compensated for by others (e.g., sparse EC and pericyte proliferation by cell elongation and thinning). The governing mechanisms involve cell-cell contacts (e.g., peg-and-socket junctions between pericytes and ECs), multiple autocrine and paracrine signaling molecules and pathways (e.g., vascular endothelial growth factor, platelet-derived growth factor, angiopoietins, transforming growth factor B, ephrins, semaphorins, and metalloproteinases), and other factors (e.g., hypoxia, vascular patency, and blood flow). Pericytes participate in vessel development, stabilization, maturation and regression in sprouting angiogenesis, and in interstitial tissue structure formation of the pillar core in intussusceptive angiogenesis. In sprouting angiogenesis, proliferating perivascular CD34+SCs/TCs are an important source of stromal cells during repair through granulation tissue formation and of cancer-associated fibroblasts (CAFs) in tumors. Conversely, CD34+SCs/TCs have less participation as precursor cells in intussusceptive angiogenesis. The dysfunction of these mechanisms is involved in several diseases, including neoplasms, with therapeutic implications.

Novel Treatments and Technologies Applied to the Cure of Neuroblastoma

Neuroblastoma (NB) is the most common extracranial solid tumour in childhood, accounting for approximately 15% of all cancer-related deaths in the paediatric population1. It is characterised by heterogeneous clinical behaviour in neonates and often adverse outcomes in toddlers. The overall survival of children with high-risk disease is around 40-50% despite the aggressive treatment protocols consisting of intensive chemotherapy, surgery, radiation therapy and hematopoietic stem cell transplantation2,3. There is an ongoing research effort to increase NB's cellular and molecular biology knowledge to translate essential findings into novel treatment strategies. This review aims to address new therapeutic modalities emerging from preclinical studies offering a unique translational opportunity for NB treatment.

Systemic cell therapy for muscular dystrophies : The ultimate transplantable muscle progenitor cell and current challenges for clinical efficacy

The intrinsic regenerative capacity of skeletal muscle makes it an excellent target for cell therapy. However, the potential of muscle tissue to renew is typically exhausted and insufficient in muscular dystrophies (MDs), a large group of heterogeneous genetic disorders showing progressive loss of skeletal muscle fibers. Cell therapy for MDs has to rely on suppletion with donor cells with high myogenic regenerative capacity. Here, we provide an overview on stem cell lineages employed for strategies in MDs, with a focus on adult stem cells and progenitor cells resident in skeletal muscle. In the early days, the potential of myoblasts and satellite cells was explored, but after disappointing clinical results the field moved to other muscle progenitor cells, each with its own advantages and disadvantages. Most recently, mesoangioblasts and pericytes have been pursued for muscle cell therapy, leading to a handful of preclinical studies and a clinical trial. The current status of (pre)clinical work for the most common forms of MD illustrates the existing challenges and bottlenecks. Besides the intrinsic properties of transplantable cells, we discuss issues relating to cell expansion and cell viability after transplantation, optimal dosage, and route and timing of administration. Since MDs are genetic conditions, autologous cell therapy and gene therapy will need to go hand-in-hand, bringing in additional complications. Finally, we discuss determinants for optimization of future clinical trials for muscle cell therapy. Joined research efforts bring hope that effective therapies for MDs are on the horizon to fulfil the unmet clinical need in patients.

Single-cell transcriptomics of murine mural cells reveals cellular heterogeneity

Mural cells (MCs) wrap around the endothelium, and participate in the development and homeostasis of vasculature. MCs have been reported as heterogeneous population morphologically and functionally. However, the transcriptional heterogeneity of MCs was rarely studied. In this study, we illustrated the transcriptional heterogeneity of MCs with different perspectives by using publicly available single-cell dataset GSE109774. Specifically, MCs are transcriptionally different from other cell types, and ligand-receptor interactions of different cells with MCs vary. Re-clustering of MCs identified five distinct subclusters. The heterogeneity of MCs in tissues was reflected by MC coverage, various distribution of MC subclusters, and ligand-receptor interactions of MCs and parenchymal cells. The transcriptomic diversity of MCs revealed in this article will help facilitate further research into MCs.

Surface markers: An identity card of endothelial cells

All endothelial cells have the common characteristic that they line the vessels of the blood circulatory system. However, endothelial cells display a large degree of heterogeneity in the function of their location in the vascular tree. In this article, we have summarized the expression patterns of a number of well-accepted endothelial surface markers present in normal microvascular endothelial cells, arterial and venous endothelial cells, lymphatic endothelial cells, tumor endothelial cells, and endothelial precursor cells.

Overcoming Biological Barriers in Neuroblastoma Therapy: The Vascular Targeting Approach with Liposomal Drug Nanocarriers

Neuroblastoma is a rare pediatric cancer characterized by a wide clinical behavior and adverse outcome despite aggressive therapies. New approaches based on targeted drug delivery may improve efficacy and decrease toxicity of cancer therapy. Furthermore, nanotechnology offers additional potential developments for cancer imaging, diagnosis, and treatment. Following these lines, in the past years, innovative therapies based on the use of liposomes loaded with anticancer agents and functionalized with peptides capable of recognizing neuroblastoma cells and/or tumor-associated endothelial cells have been developed. Studies performed in experimental orthotopic models of human neuroblastoma have shown that targeted nanocarriers can be exploited for not only decreasing the systemic toxicity of the encapsulated anticancer drugs, but also increasing their tumor homing properties, enhancing tumor vascular permeability and perfusion (and, consequently, drug penetration), inducing tumor apoptosis, inhibiting angiogenesis, and reducing tumor glucose consumption. Furthermore, peptide-tagged liposomal formulations are proved to be more efficacious in inhibiting tumor growth and metastatic spreading of neuroblastoma than nontargeted liposomes. These findings, herein reviewed, pave the way for the design of novel targeted liposomal nanocarriers useful for multitargeting treatment of neuroblastoma.

The NG2 Proteoglycan in Pericyte Biology

Studies of pericytes have been retarded by the lack of appropriate markers for identification of these perivascular mural cells. Use of antibodies against the NG2 proteoglycan as a pericyte marker has greatly facilitated recent studies of pericytes, emphasizing the intimate spatial relationship between pericytes and endothelial cells, allowing more accurate quantification of pericyte/endothelial cell ratios in different vascular beds, and revealing the participation of pericytes throughout all stages of blood vessel formation. The functional importance of NG2 in pericyte biology has been established via NG2 knockdown (in vitro) and knockout (in vivo) strategies that reveal significant deficits in blood vessel formation when NG2 is absent from pericytes. NG2 influences pericyte proliferation and motility by acting as an auxiliary receptor that enhances signaling through integrins and receptor tyrosine kinase growth factor receptors. By acting in a trans orientation, NG2 also activates integrin signaling in closely apposed endothelial cells, leading to enhanced maturation and formation of endothelial cell junctions. NG2 null mice exhibit reduced growth of both mammary and brain tumors that can be traced to deficits in tumor vascularization. Use of Cre-Lox technology to produce pericyte-specific NG2 null mice has revealed specific deficits in tumor vessels that include decreased pericyte ensheathment of endothelial cells, diminished assembly of the vascular basement membrane, reduced vessel patency, and increased vessel leakiness. Interestingly, myeloid-specific NG2 null mice exhibit even larger deficits in tumor vascularization, leading to correspondingly slower tumor growth. Myeloid-specific NG2 null mice are deficient in their ability to recruit macrophages to tumors and other sites of inflammation. This absence of macrophages deprives pericytes of a signal that is crucial for their ability to interact with endothelial cells. The interplay between pericytes, endothelial cells, and macrophages promises to be an extremely fertile area of future study.

Pericyte-targeting drug delivery and tissue engineering

Pericytes are contractile mural cells that wrap around the endothelial cells of capillaries and venules. Depending on the triggers by cellular signals, pericytes have specific functionality in tumor microenvironments, properties of potent stem cells, and plasticity in cellular pathology. These features of pericytes can be activated for the promotion or reduction of angiogenesis. Frontier studies have exploited pericyte-targeting drug delivery, using pericyte-specific peptides, small molecules, and DNA in tumor therapy. Moreover, the communication between pericytes and endothelial cells has been applied to the induction of vessel neoformation in tissue engineering. Pericytes may prove to be a novel target for tumor therapy and tissue engineering. The present paper specifically reviews pericyte-specific drug delivery and tissue engineering, allowing insight into the emerging research targeting pericytes.

Molecular diagnosis and molecular profiling to detect treatment-resistant ovarian cancer

INTRODUCTION

Epithelial ovarian cancer remains the gynecologic tumor with the highest rate of recurrence after initial optimal cytoreductive surgery followed by adjuvant chemotherapy. Unfortunately, with the development of recurrent ovarian cancer often comes the discovery of chemo-resistant disease. The absence of improvement in long term survival, notwithstanding the use of newer agents as is seen in other cancers, emphasizes the need for improved understanding of the processes that lead to chemo-resistant disease.

AREAS COVERED

This review will cover the following topics: 1. Molecular and cellular mechanisms in platinum and paclitaxel resistance 2. Other molecular mediators of chemo-resistance 3. Expression of stem cell markers in ovarian cancer and relationship to chemo-resistance 4. MicroRNA and long non-coding RNA expression in chemo-resistant ovarian cancer 5. Determination of chromosomal aberrations as markers of chemo-resistance 6. Molecular profiling in chemo-resistant disease. A standard MEDLINE search was performed using the key words; ovarian cancer, chemo-resistant disease, molecular profiling, cancer stem cells and chemotherapy. Expert Commentary: Over the next few years the challenge remains to precisely determine the mechanisms responsible for the onset and maintenance of chemo-resistance and to effectively target these mechanisms.

Cancer targeted therapeutics: From molecules to drug delivery vehicles

The pitfall of all chemotherapeutics lies in drug resistance and the severe side effects experienced by patients. One way to reduce the off-target effects of chemotherapy on healthy tissues is to alter the biodistribution of drug. This can be achieved in two ways: Passive targeting utilizes shape, size, and surface chemistry to increase particle circulation and tumor accumulation. Active targeting employs either chemical moieties (e.g. peptides, sugars, aptamers, antibodies) to selectively bind to cell membranes or responsive elements (e.g. ultrasound, magnetism, light) to deliver its cargo within a local region. This article will focus on the systemic administration of anti-cancer agents and their ability to home to tumors and, if relevant, distant metastatic sites.

Anti-angiogenesis in neuroblastoma

The nature of the angiogenic balance in neuroblastoma is complex, and a spectrum of angiogenesis stimulators and inhibitors have been detected in neuroblastoma tumours. The complex relationships between angiogenic cascade and anti-angiogenic agents in the tumour vascular phase have indicated that anti-angiogenesis can be considered as a strategy for the adjuvant therapy of neuroblastoma. The major goal is to establish if inhibition of angiogenesis is a realistic therapeutic strategy for inhibiting tumour cell dissemination and the formation of metastasis in neuroblastoma.

Multifunctional nanoparticles: cost versus benefit of adding targeting and imaging capabilities

Nanoparticle-based drug delivery systems have been developed to improve the efficacy and reduce the systemic toxicity of a wide range of drugs. Although clinically approved nanoparticles have consistently shown value in reducing drug toxicity, their use has not always translated into improved clinical outcomes. This has led to the development of "multifunctional" nanoparticles, where additional capabilities like targeting and image contrast enhancement are added to the nanoparticles. However, additional functionality means additional synthetic steps and costs, more convoluted behavior and effects in vivo, and also greater regulatory hurdles. The trade-off between additional functionality and complexity is the subject of ongoing debate and the focus of this Review.

Tumor endothelial markers as a target in cancer

INTRODUCTION

Several anti-angiogenic agents have been developed and some of them have been clinically applied in the tumor therapy. Anti-angiogenic therapy faces some hurdles: inherent or acquired resistance, increased invasiveness, and lack of biomarkers. Characterization of tumor endothelial markers may help to target endothelium and to identify potential predictive factors of response to anti-angiogenic therapies. Numerous surrogates, angiogenic and endothelium markers have emerged from recent pre-clinical studies, including physiological and soluble molecules in plasma and from platelets, circulating cells, tumor tissue factors and imaging markers. However, no wholly validated biomarkers currently exist to predict the success or the failure of the anti-angiogenic therapy of cancer. Therefore, the research of suitable and validate biomarkers is currently ongoing.

AREAS COVERED

This review provides an overview of the status of our knowledge concerning tumor endothelial markers, therapeutics targeting, possible resistance mechanisms and predictive value of these biomarkers and discuss future strategies to use and identify them in the anti-angiogenic therapy.

EXPERT OPINION

Anti-angiogenesis is a milestone to improve the treatment of several types of cancer and predictive biomarkers for a response to anti-endothelium therapy are one of the most important challenges for anti-angiogenesis research.

Microvascular remodeling and wound healing: a role for pericytes

Physiologic wound healing is highly dependent on the coordinated functions of vascular and non-vascular cells. Resolution of tissue injury involves coagulation, inflammation, formation of granulation tissue, remodeling and scarring. Angiogenesis, the growth of microvessels the size of capillaries, is crucial for these processes, delivering blood-borne cells, nutrients and oxygen to actively remodeling areas. Central to angiogenic induction and regulation is microvascular remodeling, which is dependent upon capillary endothelial cell and pericyte interactions. Despite our growing knowledge of pericyte-endothelial cell crosstalk, it is unclear how the interplay among pericytes, inflammatory cells, glia and connective tissue elements shape microvascular injury response. Here, we consider the relationships that pericytes form with the cellular effectors of healing in normal and diabetic environments, including repair following injury and vascular complications of diabetes, such as diabetic macular edema and proliferative diabetic retinopathy. In addition, pericytes and stem cells possessing "pericyte-like" characteristics are gaining considerable attention in experimental and clinical efforts aimed at promoting healing or eradicating ocular vascular proliferative disorders. As the origin, identification and characterization of microvascular pericyte progenitor populations remains somewhat ambiguous, the molecular markers, structural and functional characteristics of pericytes will be briefly reviewed.

Matricellular proteins: a sticky affair with cancers

The multistep process of metastasis is a major hallmark of cancer progression involving the cointeraction and coevolution of the tumor and its microenvironment. In the tumor microenvironment, tumor cells and the surrounding stromal cells aberrantly secrete matricellular proteins, which are a family of nonstructural proteins in the extracellular matrix (ECM) that exert regulatory roles via a variety of molecular mechanisms. Matricellular proteins provide signals that support tumorigenic activities characteristic of the metastastic cascade such as epithelial-to-mesenchymal (EMT) transition, angiogenesis, tumor cell motility, proliferation, invasion, evasion from immune surveillance, and survival of anoikis. Herein, we review the current understanding of the following matricellular proteins and highlight their pivotal and multifacted roles in metastatic progression: angiopoietin-like protein 4 (ANGPTL4), CCN family members cysteine-rich angiogenic inducer 61 (Cyr61/CCN1) and CCN6, osteopontin (OPN), secreted protein acidic and rich in cysteine (SPARC), tenascin C (TNC), and thrombospondin-1 and -2 (TSP1, TSP2). Insights into the signaling mechanisms resulting from the interaction of these matricellular proteins and their respective molecular partner(s), as well as their subsequent contribution to tumor metastasis, are discussed. In addition, emerging evidences of their promising potential as therapeutic options and/or targets in the treatment of cancer are also highlighted.

Axitinib modulates hypoxia-induced blood-retina barrier permeability and expression of growth factors

This study investigates the effects of the multikinase inhibitor axitinib on the expression of vascular endothelial growth factor (VEGF) receptors 1/2 (VEGFR-1/2) and platelet-derived growth factor (PDGF) receptor beta (PDGFR-β), hypoxia-induced increased tissue permeability, occludin, zonula occludens protein 1 (ZO-1), VEGF-A, and PDGF expression of human retinal pigment epithelial (RPE) cells and human umbilical vein endothelial cells (HUVECs). Primary human RPE cells and HUVECs were exposed to hypoxia and axitinib. Viability of cells, tissue permeability, and expression of occludin, ZO-1, VEGF, PDGF, VEGFR-1/2 and PDGFR-β, and their mRNAs, were investigated by reverse transcription-polymerase chain reaction, enzyme-linked immunosorbent assay, western blotting, and immunohistochemistry. Treatment with axitinib reduced expression of VEGFR-1/2 and PDGFR-β. Hypoxia decreased cell viability, occludin, and ZO-1 expression and increased tissue permeability, expression, and secretion of VEGF and PDGF. Axitinib significantly reduced hypoxia-induced effects on HUVEC and RPE cells. Our in vitro results suggest that axitinib may have promising properties as a potential treatment for diabetic macular edema.

X-ROS signaling: rapid mechano-chemo transduction in heart

We report that in heart cells, physiologic stretch rapidly activates reduced-form nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) to produce reactive oxygen species (ROS) in a process dependent on microtubules (X-ROS signaling). ROS production occurs in the sarcolemmal and t-tubule membranes where NOX2 is located and sensitizes nearby ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR). This triggers a burst of Ca(2+) sparks, the elementary Ca(2+) release events in heart. Although this stretch-dependent "tuning" of RyRs increases Ca(2+) signaling sensitivity in healthy cardiomyocytes, in disease it enables Ca(2+) sparks to trigger arrhythmogenic Ca(2+) waves. In the mouse model of Duchenne muscular dystrophy, hyperactive X-ROS signaling contributes to cardiomyopathy through aberrant Ca(2+) release from the SR. X-ROS signaling thus provides a mechanistic explanation for the mechanotransduction of Ca(2+) release in the heart and offers fresh therapeutic possibilities.

Brain pericytes: emerging concepts and functional roles in brain homeostasis

Brain pericytes are an important constituent of neurovascular unit. They encircle endothelial cells and contribute to the maturation and stabilization of the capillaries in the brain. Recent studies have revealed that brain pericytes play pivotal roles in a variety of brain functions, such as regulation of capillary flow, angiogenesis, blood brain barrier, immune responses, and hemostasis. In addition, brain pericytes are pluripotent and can differentiate into different lineages similar to mesenchymal stem cells. The brain pericytes are revisited as a key player to maintain brain function and repair brain damage.

To exploit the tumor microenvironment: Passive and active tumor targeting of nanocarriers for anti-cancer drug delivery

Because of the particular characteristics of the tumor microenvironment and tumor angiogenesis, it is possible to design drug delivery systems that specifically target anti-cancer drugs to tumors. Most of the conventional chemotherapeutic agents have poor pharmacokinetics profiles and are distributed non-specifically in the body leading to systemic toxicity associated with serious side effects. Therefore, the development of drug delivery systems able to target the tumor site is becoming a real challenge that is currently addressed. Nanomedicine can reach tumor passively through the leaky vasculature surrounding the tumors by the Enhanced Permeability and Retention effect whereas ligands grafted at the surface of nanocarriers allow active targeting by binding to the receptors overexpressed by cancer cells or angiogenic endothelial cells. This review is divided into two parts: the first one describes the tumor microenvironment and the second one focuses on the exploitation and the understanding of these characteristics to design new drug delivery systems targeting the tumor. Delivery of conventional chemotherapeutic anti-cancer drugs is mainly discussed.

Pericyte deficiencies lead to aberrant tumor vascularizaton in the brain of the NG2 null mouse

Tightly regulated crosstalk between endothelial cells and pericytes is required for formation and maintenance of functional blood vessels. When the NG2 proteoglycan is absent from pericyte surfaces, vascularization of syngeneic tumors growing in the C57Bl/6 mouse brain is aberrant in several respects, resulting in retardation of tumor progression. In the NG2 null mouse brain, pericyte investment of the tumor vascular endothelium is reduced, causing deficiencies in both pericyte and endothelial cell maturation, as well as reduced basal lamina assembly. While part of this deficit may be due to the previously-identified role of NG2 in beta1 integrin-dependent periyte/endothelial cell crosstalk, the ablation of NG2 also appears responsible for loss of collagen VI anchorage, in turn leading to reduced collagen IV deposition. Poor functionality of tumor vessels in NG2 null brain is reflected by reduced vessel patency and increased vessel leakiness, resulting in large increases in tumor hypoxia. These findings demonstrate the importance of NG2-dependent pericyte/endothelial cell interaction in the development and maturation of tumor blood vessels, identifying NG2 as a potential target for anti-angiogenic cancer therapy.

Sorafenib protects human optic nerve head astrocytes from light-induced overexpression of vascular endothelial growth factor, platelet-derived growth factor, and placenta growth factor

OBJECTIVES

Growth factors, such as vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and placenta growth factor (PlGF) are key players in the development of diabetic retinopathy, age-related macular degeneration, and other retinal neovascular diseases. Glial cells provide a significant source of retinal growth factor production under physiologic and pathologic conditions. Cumulative light exposure has been linked to increased retinal growth factor expression. Previous reports indicate that sorafenib, an oral multikinase inhibitor, might have a beneficial effect on retinal neovascularization. This study was designed to investigate the effects of sorafenib on light-induced overexpression of growth factors in human retinal glial cells.

METHODS

Primary human optic nerve head astrocytes (ONHAs) were exposed to white light and incubated with sorafenib. Viability, expression, and secretion of VEGF-A, PDGF-BB, and PlGF and their mRNA were determined by reverse transcription-polymerase chain reaction, immunohistochemistry, and enzyme-linked immunosorbent assay.

RESULTS

Light exposure decreased cell viability and increased VEGF-A, PDGF-BB, and PlGF expression and secretion. These light-induced effects were significantly reduced when cells were treated with sorafenib at a concentration of 1 microg/ml.

CONCLUSION

Sorafenib significantly reduced light-induced overexpression of VEGF-A, PDGF-BB, and PlGF in primary human ONHAs. Sorafenib has promising properties as a potential supportive treatment for retinal neovascularization.

Differences in tendon graft healing between the intra-articular and extra-articular ends of a bone tunnel

The basic biology of healing between a tendon graft and bone tunnel remains incompletely understood. Distinct variability in the morphological characteristics of the healing tendon-bone attachment site has been reported. We hypothesized that spatial and temporal differences in tendon-to-bone healing exist at different regions of a surgically created bone tunnel. Twenty-four male, Sprague-Dawley rats underwent anterior cruciate ligament (ACL) reconstruction in the left knee using a flexor digitorum longus tendon graft secured using suspensory periosteal fixation. Animals were sacrificed at 4, 7, 11, 14, 21, and 28 days after surgery and prepared for routine histology and immunohistochemical analysis of the healing enthesis at the intra-articular aperture (IAA), mid-tunnel, and extra-articular aperture (EAA). Six animals were used to measure mineral apposition rate (MAR) along the healing bone tunnel by double fluorochrome labeling at 14 and 28 days after surgery. The total area of calcified bone matrix was assessed with von Kossa staining and Goldner-Masson trichrome staining, respectively. The healing tendon-bone interface tissue exhibited a wide chondroid matrix at the IAA, in contrast to a narrow, fibrous matrix at the EAA. There were significantly more osteoclasts at the IAA compared to EAA throughout the study period, except 4 days after surgery (p < 0.05). Collagen continuity between the tendon graft and bone tunnel increased over time, with a more parallel orientation and increased collagen fiber continuity between tendon and bone at the EAA compared to the IAA. MAR was also significantly greater at the EAA at 4 weeks (p < 0.001). Significant differences in healing between the tendon graft and bone exist along the length of bone tunnel secured with suspensory fixation. The etiology of these differences is likely multifactorial in nature, including variable biological and biomechanical environments at different ends of the tunnel. Understanding these differences may ultimately allow surgeons to improve the quality of graft fixation and long-term outcomes after ACL reconstruction.

Does the renin-angiotensin system participate in regulation of human vasculogenesis and angiogenesis?

Several lines of evidence suggest that hypertension and angiogenesis may be related phenomena but a functional link remains elusive. Here, we propose that the renin-angiotensin system (RAS), in addition to its central role in arterial hypertension, also regulates blood vessel formation during normal development and cancer. This mechanistic hypothesis is based on reports of biochemical, genetic, clinical, and epidemiologic data reviewed herein. Species differences between the RAS of rodents and humans likely account for why such a fundamental role in angiogenesis went unrecognized for so long. If proven correct, this hypothesis carries many implications for the medical practices of cardiology, oncology, and neonatology.

Targeting liposomal chemotherapy via both tumor cell-specific and tumor vasculature-specific ligands potentiates therapeutic efficacy

Neuroblastoma, the most common solid tumor of infancy derived from the sympathetic nervous system, continues to present a formidable clinical challenge. Sterically stabilized immunoliposomes (SIL) have been shown to enhance the selective localization of entrapped drugs to solid tumors, with improvements in therapeutic indices. We showed that SIL loaded with doxorubicin (DXR) and targeted to the disialoganglioside receptor GD(2) [aGD(2)-SIL(DXR)] led to a selective inhibition of the metastatic growth of experimental models of human neuroblastoma. By coupling NGR peptides that target the angiogenic endothelial cell marker aminopeptidase N to the surface of DXR-loaded liposomes [NGR-SL(DXR)], we obtained tumor regression, pronounced destruction of the tumor vasculature, and prolonged survival of orthotopic neuroblastoma xenografts. Here, we showed good liposome stability, long circulation times, and enhanced time-dependent tumor accumulation of both the carrier and the drug. Antivascular effects against animal models of lung and ovarian cancer were shown for formulations of NGR-SL(DXR). In the chick embryo chorioallantoic assay, NGR-SL(DXR) substantially reduced the angiogenic potential of various neuroblastoma xenografts, with synergistic inhibition observed for the combination of NGR-SL(DXR) with aGD(2)-SIL(DXR). A significant improvement in antitumor effects was seen in neuroblastoma-bearing animal models when treated with the combined formulations compared with control mice or mice treated with either tumor- or vascular-targeted liposomal formulations, administered separately. The combined treatment resulted in a dramatic inhibition of tumor endothelial cell density. Long-term survivors were obtained only in animals treated with the combined tumor- and vascular-targeted formulations, confirming the pivotal role of combination therapies in treating aggressive metastatic neuroblastoma.

Localization of angiotensin II, the AT1 receptor, angiotensin-converting enzyme, aminopeptidase A, adipocyte-derived leucine aminopeptidase, and vascular endothelial growth factor in the human ovary throughout the menstrual cycle

OBJECTIVE

To assess the expression and cellular distribution of angiotensin II (Ang II), angiotensin type 1 receptor (AT1R), angiotensin-converting enzyme (ACE), aminopeptidase A (APA), adipocyte-derived leucine aminopeptidase (A-LAP), and vascular endothelial growth factor (VEGF) in human ovarian tissue during the menstrual cycle.

DESIGN

Ovarian tissues (n = 52) and corpora lutea (n = 34) were obtained from patients undergoing hysterectomy/oophorectomy, and tissue sections were immunostained for each antigen.

SETTING

University hospital.

PATIENT(S)

Patients undergoing hysterectomy or oophorectomy for benign conditions.

INTERVENTION(S)

Immunostaining of tissue sections using antibodies to each antigen.

MAIN OUTCOME MEASURE(S)

Microscopic evaluation to assess the presence, distribution, and cellular localization.

RESULT(S)

The luteal tissue is the major site of Ang II, ACE, AT1R, and VEGF, with highest staining intensity found during the midluteal phase and at pregnancy. The AT1R was found in theca cells. The APA was strongly immunolocalized in pericytes. Immunolocalization of AT1R was almost similar to that of VEGF including oocytes in the primordial and intermediate follicles.

CONCLUSION(S)

The expression and distinct pattern of the cellular localization of Ang II and its related proteins in human ovarian tissue during folliculogenesis and in the luteal tissue suggest their roles in the growth and differentiation of theca, granulose, and luteal cells.

Manipulating the angiotensin system--new approaches to the treatment of solid tumours

Angiotensin II (Ang II), a main effector peptide in the renin-angiotensin system (RAS), plays a fundamental role as a vasoconstrictor in controlling cardiovascular function and renal homeostasis. Ang II also acts as a growth promoter or angiogenic factor via type 1 angiotensin II receptors (AT1Rs) in certain tumour cell lines. Recent studies have shown the activation of the local RAS in various tumour tissues, including the abundant generation of Ang II by angiotensin-converting enzyme (ACE) and the upregulation of AT1R expression. Thus, considerable attention has been paid to the role of the RAS in cancer and its blockade as a new approach to the treatment of cancer. There is increasing evidence that the Ang II-AT1R system is involved in tumour growth, angiogenesis and metastasis in experimental models, suggesting the therapeutic potential of an ACE inhibitor and AT1R blocker, both of which have been used as antihypertensive drugs. In addition, specific Ang II-degrading enzymes are expressed in tumours and play a regulatory role in cell proliferation and invasion. This review focuses on the role of the Ang II-AT1R system in solid tumours, particularly in the progression of gynaecological cancer, and presents the clinical potential of manipulating the angiotensin system as a novel and promising strategy for cancer treatment.

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.

Angiogenesis with pericyte abnormalities in a transgenic model of prostate carcinoma

BACKGROUND

Previous studies of the TRansgenic Adenocarcinoma of the Mouse Prostate (TRAMP) model vasculature suggest that, as tumors develop, vessels invade the glandular epithelium. However, changes in the vasculature are difficult to study in conventional thin tissue sections. The authors used a new approach to characterize morphologic and architectural changes of blood vessels and pericytes during tumor development in TRAMP mice.

METHODS

Eighty-micron cryostat sections of normal prostate and three histopathologic stages of TRAMP tumor sections, classified by epithelial cell E-cadherin immunoreactivity, were immunostained with vascular endothelial cell and pericyte receptor antibodies and evaluated by confocal microscopy.

RESULTS

In the normal mouse prostate, capillaries were most abundant in the fibromuscular tunica between the epithelium and smooth muscle of the ductules. In the prostatic intraepithelial neoplasia (PIN) stage, vessels accompanied epithelial cell protrusions into the ductule lumen but remained in the connective tissue at the basal side of the epithelium. Well differentiated tissues had extensive angiogenesis with five times the normal mean vascularity outside ductules. Vessels were of variable diameter, were associated with an increased number of pericytes, and some had endothelial sprouts. Angiogenic blood vessels from poorly differentiated adenocarcinomas were tortuous, variable in caliber, and lacked the normal hierarchy. Pericytes on these vessels had an abnormal phenotype manifested by alpha-smooth muscle actin expression and loose association with endothelial cells. Angiogenesis and loss of vascular hierarchy were also found in human prostate carcinoma.

CONCLUSIONS

Vascular abnormalities, which begin at the PIN stage and intensify in well differentiated and poorly differentiated tumors, may be useful readouts for early detection and treatment assessment in prostate carcinoma.

Contribution of bone marrow-derived pericyte precursor cells to corneal vasculogenesis

PURPOSE

Bone-marrow (BM)-derived hematopoietic precursor cells are thought to participate in the growth of blood vessels during postnatal vasculogenesis. In this investigation, multichannel laser scanning confocal microscopy and quantitative image analysis were used to study the fate of BM-derived hematopoietic precursor cells in corneal neovascularization.

METHODS

A BM-reconstituted mouse model was used in which the BM from enhanced green fluorescent protein (GFP)-positive mice was transplanted into C57BL/6 mice. Basic fibroblast growth factor (bFGF) was used to induce corneal neovascularization in mice. The vasculogenic potential of adult BM-derived cells and their progeny were tested in this in vivo model. Seventy-two histologic sections selected by systematic random sampling from four mice were immunostained and imaged with a confocal microscope and analyzed with image-analysis software.

RESULTS

BM-derived endothelial cells did not contribute to bFGF-induced neovascularization in the cornea. BM-derived periendothelial vascular mural cells (pericytes) were detected at sites of neovascularization, whereas endothelial cells of blood vessels originated from preexisting blood vessels in limbal capillaries. Fifty three percent of all neovascular pericytes originated from BM, and 47% of them originated from preexisting corneoscleral limbus capillaries. Ninety-six percent and 92% of BM-derived pericytes also expressed CD45 and CD11b, respectively, suggesting their hematopoietic origin from the BM.

CONCLUSIONS

Pericytes of new corneal vessels have a dual source: BM and preexisting limbal capillaries. These findings establish BM as a significant effector organ in corneal disorders associated with neovascularization.

Molecular Analysis as a Tool in the Differential Diagnosis of VHL Disease-Related Tumors

Von Hippel-Lindau (VHL) disease is an autosomal dominant tumor syndrome, in which hemangioblastomas (HBs), clear cell renal cell carcinomas (RCCs), and pheochromocytomas are the most frequently encountered tumors. The differential diagnosis of dedifferentiated tumors in general can be difficult, as standard histologic and immunohistochemical investigations do not always allow a definitive diagnosis. We used molecular genetic analysis to resolve the differential diagnosis of sarcomatoid RCC versus pheochromocytoma of a (peri)renal tumor in a VHL patient. Germline mutation analysis identified the C407T mutation, which has been related to a VHL phenotype in which pheochromocytomas are rare. Chromosomal imbalances detected in the tumor by CGH showed a pattern typical for RCCs and not for pheochromocytomas. CGH analysis of the multiple tumors of this VHL patient revealed a comparable karyotype in the metastatic tumors and the (peri)renal tumor. Concordantly, although the germline mutation was detected in all analyzed tumors, LOH 3p was only detected in the (peri)renal mass and most metastases. Overall, based on all genetic data, this tumor corroborated a diagnosis of metastatic sarcomatoid RCC. In line with these observations is the immunopositivity for the RCC-specific RC38 detected in the (peri)renal mass and the metastases that was not detected in pheochromocytomas. The RCC specific marker G250 was uninformative as it stains positive in all types of VHL tumors. This case report illustrates the promising role of genetic analysis in the differential diagnosis of histologically dedifferentiated tumors.

Pathological angiogenesis is reduced by targeting pericytes via the NG2 proteoglycan

The NG2 proteoglycan is expressed by nascent pericytes during the early stages of angiogenesis. To investigate the functional role of NG2 in neovascularization, we have compared pathological retinal and corneal angiogenesis in wild type and NG2 null mice. During ischemic retinal neovascularization, ectopic vessels protruding into the vitreous occur twice as frequently in wild type retinas as in NG2 null retinas. In the NG2 knock-out retina, proliferation of both pericytes and endothelial cells is significantly reduced, and the pericyte:endothelial cell ratio falls to 0.24 from the wild type value of 0.86. Similarly, bFGF-induced angiogenesis is reduced more than four-fold in the NG2 null cornea compared to that seen in the wild type retina. Significantly, NG2 antibody is effective in reducing angiogenesis in the wild type cornea, suggesting that the proteoglycan can be an effective target for anti-angiogenic therapy. These experiments therefore demonstrate both the functional importance of NG2 in pericyte development and the feasibility of using pericytes as anti-angiogenic targets.

Early contribution of pericytes to angiogenic sprouting and tube formation

Immunostaining with endothelial and pericyte markers was used to evaluate the cellular composition of angiogenic sprouts in several types of tumors and in the developing retina. Confocal microscopy revealed that, in addition to conventional endothelial tubes heavily invested by pericytes, all tissues contained small populations of endothelium-free pericyte tubes in which nerve/glial antigen 2 (NG2) positive, platelet-derived growth factor beta (PDGF beta ) receptor-positive perivascular cells formed the lumen of the microvessel. Perfusion of tumor-bearing mice with FITC-dextran, followed by immunohistochemical staining of tumor vasculature, demonstrated direct apposition of pericytes to FITC-dextran in the lumen, confirming functional connection of the pericyte tube to the circulation. Transplantation of prostate and mammary tumor fragments into NG2-null mice led to the formation of tumor microvasculature that was invariably NG2-negative, demonstrating that pericytes associated with tumor microvessels are derived from the host rather than from the conversion of tumor cells to a pericyte phenotype. The existence of pericyte tubes reflects the early participation of pericytes in the process of angiogenic sprouting. The ability to study these precocious contributions of pericytes to neovascularization depends heavily on the use of NG2 and PDGF beta -receptor as reliable early markers for activated pericytes.

Microarray analysis of in vitro pericyte differentiation reveals an angiogenic program of gene expression

The vasculature consists of endothelial cells (ECs) lined by pericyte/vascular smooth muscle cells (vSMCs). Pericyte/vSMCs provide support to the mature vasculature but are also essential for normal blood vessel development. To determine how pericyte-EC communication influences vascular development, we used the well-established in vitro model of TGFbeta-stimulated differentiation of 10T1/2 cells into pericyte/vSMCs. Microarray analysis was performed to identify genes that were differentially expressed by induced vs. uninduced 10T1/2 cells. We discovered that these cells show an angiogenic program of gene expression, with up-regulation of several genes previously implicated in angiogenesis, including VEGF, IL-6, VEGF-C, HB-EGF, CTGF, tenascin C, integrin alpha5, and Eph receptor A2. Up-regulation of some genes was validated by Western blots and immunocytochemistry. We also examined the functional significance of these gene expression changes. VEGF and IL-6 alone and in combination were important in 10T1/2 cell differentiation. Furthermore, we used a coculture system of 10T1/2 and human umbilical vein ECs (HUVECs), resulting in the formation of cordlike structures by the HUVECs. This cordlike structure formation was disrupted when neutralizing antibodies to VEGF or IL-6 were added to the coculture system. The results of these studies show that factors produced by pericytes may be responsible for recruiting ECs and promoting angiogenesis. Therefore, a further understanding of the genes involved in pericyte differentiation could provide a novel approach for developing anti-angiogenic therapies.

NG2 proteoglycan promotes endothelial cell motility and angiogenesis via engagement of galectin-3 and alpha3beta1 integrin

The NG2 proteoglycan is expressed by microvascular pericytes in newly formed blood vessels. We have used in vitro and in vivo models to investigate the role of NG2 in cross-talk between pericytes and endothelial cells (EC). Binding of soluble NG2 to the EC surface induces cell motility and multicellular network formation in vitro and stimulates corneal angiogenesis in vivo. Biochemical data demonstrate the involvement of both galectin-3 and alpha3beta1 integrin in the EC response to NG2 and show that NG2, galectin-3, and alpha3beta1 form a complex on the cell surface. Transmembrane signaling via alpha3beta1 is responsible for EC motility and morphogenesis in this system. Galectin-3-dependent oligomerization may potentiate NG2-mediated activation of alpha3beta1. In conjunction with recent studies demonstrating the early involvement of pericytes in angiogenesis, these data suggest that pericyte-derived NG2 is an important factor in promoting EC migration and morphogenesis during the early stages of neovascularization.

Regulation of aminopeptidase A expression in cervical carcinoma: role of tumor-stromal interaction and vascular endothelial growth factor

We previously demonstrated that aminopeptidase A (APA), a membrane-bound metallopeptidase degrading bioactive peptides such as angiotensin II (Ang II), is expressed in neoplastic lesions of the uterine cervix, and that its expression is upregulated as the lesion progresses from cervical intraepithelial neoplasms (CIN) toward invasive squamous cell carcinomas (SCC). The present study investigated the regulatory mechanisms involved in APA expression and its potential role in cervical carcinoma. Immunohistochemical staining in high-grade CIN and SCC tissues showed that APA was strongly expressed at the edge of lesions adjacent to cervical stromal cells. Fluorescence-activated cell sorting analysis demonstrated that cell surface APA expression was extremely low in three human SCC cell lines, SiHa, TCS and CaSki, under basal conditions. However, both contact and noncontact cocultures with human cervical fibroblasts resulted in the induction of APA expression in these SCC cells. APA expression was also induced in vivo when TCS cells were subcutaneously inoculated into nude mice. Furthermore, APA expression and enzymatic activity were enhanced by addition of the conditioned medium (CM) from fibroblast culture, but not by heat-treated CM. Among the various cytokines tested, vascular endothelial growth factor (VEGF) significantly increased APA activity, and induction of APA by the fibroblast CM was partly inhibited by anti-VEGF neutralizing antibody. Finally, APA cDNA-transfected APA-overexpressing TCS cells significantly reduced the Ang II-induced cell invasion ability as compared with parental or control vector-transfected TCS cells, although there was no significant difference in cellular proliferation among them. These results suggested the importance of tumor-stromal interaction for the regulation of APA expression in the microenvironment of cervical carcinoma and the potential role for this peptidase in regulating tumor invasion through inactivation of Ang II activity.

Aminopeptidase A is a functional target in angiogenic blood vessels

We show that a membrane-associated protease, aminopeptidase A (APA), is upregulated and enzymatically active in blood vessels of human tumors. To gain mechanistic insight, we evaluated angiogenesis in APA null mice. We found that, although these mice develop normally, they fail to mount the expected angiogenic response to hypoxia or growth factors. We then isolated peptide inhibitors of APA from a peptide library and show that they specifically bind to and inhibit APA, suppress migration and proliferation of endothelial cells, inhibit angiogenesis, and home to tumor blood vessels. Finally, we successfully treated tumor-bearing mice with APA binding peptides or anti-APA blocking monoclonal antibodies. These data show that APA is a regulator of blood vessel formation, and can serve as a functional vascular target.

In vivo angiogenic phenotype of endothelial cells and pericytes induced by vascular endothelial growth factor-A

VEGF-A is a major angiogenesis and permeability factor. Its cellular effects, which can be used as targets in anti-angiogenesis therapy, have mainly been studied in vitro using endothelial cell cultures. The purpose of the present study was to further characterize these effects in vivo in vascular endothelial cells and pericytes, in an experimental monkey model of VEGF-A-induced iris neovascularization. Two cynomolgus monkeys (Macaca fascicularis) received four injections of 0.5 microg VEGF-A in the vitreous of one eye and PBS in the other eye. After sacrifice at day 9, eyes were enucleated and iris samples were snap-frozen for immunohistochemistry (IHC) and stained with a panel of antibodies recognizing endothelial and pericyte determinants related to angiogenesis and permeability. After VEGF-A treatment, the pre-existing iris vasculature showed increased permeability, hypertrophy, and activation, as demonstrated by increased staining of CD31, PAL-E, tPA, uPA, uPAR, Glut-1, and alphavbeta3 and alphavbeta5 integrins, VEGF receptors VEGFR-1, -2 and -3, and Tie-2 in endothelial cells, and of NG2 proteoglycan, uPA, uPAR, integrins and VEGFR-1 in pericytes. Vascular sprouts at the anterior surface of the iris were positive for the same antigens except for tPA, Glut-1, and Tie-2, which were notably absent. Moreover, in these sprouts VEGFR-2 and VEGFR-3 expression was very high in endothelial cells, whereas many pericytes were present that were positive for PDGFR-beta, VEGFR-1, and NG2 proteoglycan and negative for alpha-SMA. In conclusion, proteins that play a role in angiogenesis are upregulated in both pre-existing and newly formed iris vasculature after treatment with VEGF-A. VEGF-A induces hypertrophy and loss of barrier function in pre-existing vessels, and induces angiogenic sprouting, characterized by marked expression of VEGFR-3 and lack of expression of tPA and Tie-2 in endothelial cells, and lack of alpha-SMA in pericytes. Our in vivo study indicates a role for alpha-SMA-negative pericytes in early stages of angiogenesis. Therefore, our findings shed new light on the temporal and spatial role of several proteins in the angiogenic cascade in vivo.

Regulatory role of membrane-bound peptidases in the progression of gynecologic malignancies

Membrane-bound peptidases play a key role in the control of growth, differentiation, and signal transduction of many cellular systems by degrading bioactive peptides. Thus, abnormal changes in their expression pattern and catalytic function result in altered peptide activation, which contributes to neoplastic transformation or progression. In this review, we describe our recent findings along with work from other groups on the expression and biological functions of membrane-bound peptidases in cancer, focusing on the regulatory roles of three peptidases, aminopeptidase A (APA), neutral endopeptidase (NEP) and placental leucine aminopeptidase (P-LAP), in the progression of gynecologic malignancies. APA, NEP and P-LAP are differentially expressed and localized in various gynecologic malignancies including cervical cancer, endometrial cancer, ovarian cancer and choriocarcinoma in a tumor-type specific pattern. The expression levels are up- or down-regulated depending on histological grade or disease progression. These peptidases play regulatory roles in tumor cell proliferation, invasion or angiogenesis via degradation/inactivation of target peptides such as angiotensin II, endothelin-1 and oxytocin, which act on cancer cells as stimulatory or inhibitory factors. Thus, membrane-bound peptidases may become not only a new diagnostic/prognostic marker, but also a novel molecular target for the treatment of gynecologic malignancies.

Imaging of angiogenesis: from microscope to clinic

Advances in imaging are transforming our understanding of angiogenesis and the evaluation of drugs that stimulate or inhibit angiogenesis in preclinical models and human disease. Vascular imaging makes it possible to quantify the number and spacing of blood vessels, measure blood flow and vascular permeability, and analyze cellular and molecular abnormalities in blood vessel walls. Microscopic methods ranging from fluorescence, confocal and multiphoton microscopy to electron microscopic imaging are particularly useful for elucidating structural and functional abnormalities of angiogenic blood vessels. Magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), ultrasonography and optical imaging provide noninvasive, functionally relevant images of angiogenesis in animals and humans. An ongoing dilemma is, however, that microscopic methods provide their highest resolution on preserved tissue specimens, whereas clinical methods give images of living tissues deep within the body but at much lower resolution and specificity and generally cannot resolve vessels of the microcirculation. Future challenges include developing new imaging methods that can bridge this resolution gap and specifically identify angiogenic vessels. Another goal is to determine which microscopic techniques are the best benchmarks for interpreting clinical images. The importance of angiogenesis in cancer, chronic inflammatory diseases, age-related macular degeneration and reversal of ischemic heart and limb disease provides incentive for meeting these challenges.

Abnormalities in pericytes on blood vessels and endothelial sprouts in tumors

Endothelial cells of tumor vessels have well-documented alterations, but it is less clear whether pericytes on these vessels are abnormal or even absent. Here we report that alpha-smooth muscle actin (alpha-SMA) and desmin-immunoreactive pericytes were present on >97% of blood vessels viewed by confocal microscopy in 100-microm-thick sections of three different spontaneous or implanted tumors in mice. However, the cells had multiple abnormalities. Unlike pericytes on capillaries in normal pancreatic islets, which had desmin but not alpha-SMA immunoreactivity, pericytes on capillary-size vessels in insulinomas in RIP-Tag2 transgenic mice expressed both desmin and alpha-SMA. Furthermore, pericytes in RIP-Tag2 tumors, as well as those in MCa-IV breast carcinomas and Lewis lung carcinomas, had an abnormally loose association with endothelial cells and extended cytoplasmic processes deep into the tumor tissue. alpha-SMA-positive pericytes also covered 73% of endothelial sprouts in RIP-Tag2 tumors and 92% of sprouts in the other tumors. Indeed, pericyte sleeves were significantly longer than the CD31-immunoreactive endothelial cell sprouts themselves in all three types of tumors. All three tumors also contained alpha-SMA-positive myofibroblasts that resembled pericytes but were not associated with blood vessels. We conclude that pericytes are present on most tumor vessels but have multiple abnormalities, including altered expression of marker proteins. In contrast to some previous studies, the almost ubiquitous presence of pericytes on tumor vessels found in the present study may be attributed to our use of both desmin and alpha-SMA as markers and 100-microm-thick tissue sections. The association of pericytes with endothelial sprouts raises the possibility of an involvement in sprout growth or retraction in tumors.

NG2 proteoglycan is expressed exclusively by mural cells during vascular morphogenesis

Immunofluorescence mapping demonstrates that the NG2 proteoglycan is invariably expressed by the mural cell component of mouse neovascular structures. This pattern is independent of the developmental mechanism responsible for formation of the vasculature (vasculogenesis or angiogenesis). Thus, NG2 is expressed in the embryonic heart by cardiomyocytes, in developing macrovasculature by smooth muscle cells, and in nascent microvessels by vascular pericytes. Due to the scarcity of proven markers for developing pericytes, NG2 is especially useful for identification of this cell type. The utility of NG2 as a pericyte marker is illustrated by two observations. First, pericytes are associated with endothelial tubes at an early point in microvessel development. This early interaction between pericytes and endothelial cells has important implications for the role of pericytes in the development and stabilization of microvascular tubes. Second, the pericyte to endothelial cell ratio in developing capillaries varies from tissue to tissue. Because the extent of pericyte investment is likely to affect the physical properties of the vessel in question, it is important to understand the mechanisms that control this process. Additional insight into these and other aspects of vascular morphogenesis should be possible through use of NG2 as a mural cell marker.

Chondroitin sulfate and cytoplasmic domain-dependent membrane targeting of the NG2 proteoglycan promotes retraction fiber formation and cell polarization

Targeting of the NG2 proteoglycan to cellular retraction fibers was studied by expressing mutant NG2 molecules lacking specific structural elements of the proteoglycan. Both the cytoplasmic domain and the chondroitin sulfate chain of NG2 appear to have roles in sorting NG2 to subcellular microdomains destined to become retraction fibers. Neither of these structural features alone is sufficient to allow optimal targeting of NG2 to retraction fibers, but together they promote efficient localization of the proteoglycan to these sites. This pattern of NG2 sorting seems to be necessary for optimal retraction fiber formation, as cells expressing poorly targeted NG2 mutants are noticeably deficient in their ability to extend retraction fibers. Furthermore, retraction fiber formation correlates strongly with the tendency of cells to assume a polarized morphology with NG2-positive retraction fibers at one pole of the cell and actin-rich lamellipodia at the other. This polarization can be triggered either through engagement of NG2 by the substratum or by exposure to lysophosphatidic acid, a potent activator of the rho GTPase. These results suggest a possible role for NG2 in regulating rho-dependent mechanisms in the trailing processes of motile cells.

Binding of the NG2 proteoglycan to kringle domains modulates the functional properties of angiostatin and plasmin(ogen)

Interactions of the developmentally regulated chondroitin sulfate proteoglycan NG2 with human plasminogen and kringle domain-containing plasminogen fragments have been analyzed by solid-phase immunoassays and by surface plasmon resonance. In immunoassays, the core protein of NG2 binds specifically and saturably to plasminogen, which consists of five kringle domains and a serine protease domain, and to angiostatin, which contains plasminogen kringle domains 1-3. Apparent dissociation constants for these interactions range from 12 to 75 nm. Additional evidence for NG2 interaction with kringle domains comes from its binding to plasminogen kringle domain 4 and to miniplasminogen (kringle domain 5 plus the protease domain) with apparent dissociation constants in the 18-71 nm range. Inhibition of plasminogen and angiostatin binding to NG2 by 6-aminohexanoic acid suggests that lysine binding sites are involved in kringle interaction with NG2. The interaction of NG2 with plasminogen and angiostatin has very interesting functional consequences. 1) Soluble NG2 significantly enhances the activation of plasminogen by urokinase type plasminogen activator. 2) The antagonistic effect of angiostatin on endothelial cell proliferation is inhibited by soluble NG2. Both of these effects of NG2 should make the proteoglycan a positive regulator of the cell migration and proliferation required for angiogenesis.

Expression of aminopeptidase A in human gestational choriocarcinoma cell lines and tissues

Aminopeptidase A (AP-A), a cell-surface metallopeptidase hydrolyzing peptide with N-terminal acidic residues, has been proved to be identical to the B cell differentiation antigen BP-1 and to the kidney differentiation antigen gp160, suggesting recognition of AP-A as a differentiation-related marker on certain normal and transformed cells. AP-A has also been purified from human placenta and been shown to be localized in the trophoblasts. In the present study, we examined the expression and enzymatic activity of AP-A in human gestational choriocarcinoma, a neoplastic transformant from trophoblasts which comprises a heterogenous population of trophoblastic cells in different stages of differentiation. Flow cytometry and immunoblot analysis demonstrated that AP-A was expressed in five choriocarcinoma cell lines which were secreting low or moderate levels of human chorionic gonadotropin (hCG), while two high hCG-secreting cell lines lacked AP-A expression. The AP-A enzymatic activity correlated with cell-surface levels of AP-A and was abrogated by amastatin, an inhibitor of AP-A. Immunohistochemical analysis revealed that AP-A was present in seven of eight choriocarcinoma tissues and was localized on the cell membrane of cytotrophoblastic choriocarcinoma cells, but not on cells with syncytiotrophoblast-like features. These results demonstrate that AP-A is expressed on most choriocarcinomas and its expression is restricted to low hCG-secreting, cytotrophoblastic cells and down-regulated as a function of cell differentiation, suggesting an involvement of AP-A in the differentiation/maturation process of neoplastic trophoblasts.

Vascular endothelial growth factor upregulates the expression of matrix metalloproteinases in vascular smooth muscle cells: role of flt-1

Vascular endothelial growth factor (VEGF) is a critical regulator of angiogenesis that stimulates proliferation, migration, and proteolytic activity of endothelial cells. Although the mitogenic activity of VEGF is endothelial cell specific, recent reports indicate VEGF is able to stimulate chemotaxis and tissue factor production in monocytes. VEGF-stimulated activity in monocytes is mediated by the VEGF receptor flt-1. The purpose of the present study was to investigate the effects of VEGF on another major cell type in the vascular wall, namely, the vascular smooth muscle cell (SMC). Using cultured cells, we showed that VEGF has a minimal mitogenic effect on SMCs, which is in accordance with published data. However, VEGF treatment significantly enhanced production of matrix metalloproteinase (MMP)-1, -3, and -9 by human SMCs. The upregulation of MMP-1 and MMP-9 was pronounced, and the stimulation for MMP-3 was less prominent. Stimulation could be demonstrated at both protein and mRNA levels, as reflected by ELISA, zymography, and Northern blot analysis. To explore the signal transduction pathway for the effect of VEGF on SMCs, we studied the expression of 2 high-affinity VEGF receptors, the kinase insert domain-containing receptor (KDR) and flt-1, in human SMCs. Both reverse transcriptase-polymerase chain reaction and immunoblotting revealed the expression of flt-1. Immunoprecipitation followed by immunoblotting illustrated phosphorylation of the flt-1 receptor after VEGF treatment. Similar methodology failed to detect expression of KDR in human SMCs. These data suggest the role of flt-1 in mediating VEGF-stimulated MMP expression of SMCs. The physiological relevance of MMP upregulation was studied by examining VEGF-stimulated SMC migration through 2 synthetic extracellular matrix barriers, Matrigel and Vitrogen. Our results indicate that VEGF treatment accelerated SMC migration through both barriers, and that this response was blocked by MMP inhibition in Matrigel, which supports a permissive role of MMP in SMC migration. These data are the first to show a direct effect of VEGF on SMCs. SMC-derived MMPs may be an additional source of proteases to digest vascular basement membrane, which is a crucial step in the initial stage of angiogenesis. The MMPs may also contribute to SMC migration in angiogenesis and atherogenesis.

Quantitative analysis of microvascular changes in diffuse astrocytic neoplasms with increasing grade of malignancy

Because glioblastoma multiforme (GBM) frequently shows striking, glomeruloid microvascular proliferation (MVP), this tumor has become a strong candidate for anti-angiogenic therapy. However, the efficacy of anti-angiogenic treatment may rather be determined by the extent of classic angiogenesis with the formation of delicate microvascular sprouts. Therefore, this study differentially quantifies the microvascular changes in supratentorial diffuse astrocytic neoplasms by computerized image analysis of histological sections in which the microvessels were highlighted by a combined anti-collagen IV/MIB-1 staining. Four microvascular parameters (number, area, perimeter, diameter), the cellularity of the glial tissue, and the MIB-1 labeling index were assessed in biopsies of astrocytoma (A, n = 13), anaplastic astrocytoma (AA, n = 14), and GBM (n = 20), and in normal cerebral cortex (n = 7) and white matter (n = 7). In As and AAs, the microvascular parameters were not significantly different from each other, and the microvascular changes were generally limited compared with WM and CX. In contrast, in GBMs the microvascular parameters were highly variable, and their overall mean value was significantly increased compared with As and AAs (ranging from 1.3x for vessel diameter to 3.3x for vessel area). Our study indicates that not only glomeruloid MVP, but also classic angiogenesis, occurs mainly and only locally in GBMs. Thus, this study provides evidence that As and AAs are not good candidates for anti-angiogenic therapy. The efficacy of such therapy for GBMs awaits further evaluation.