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

Paracrinal regulation of neutrophil functions by coronaviral infection in iPSC-derived alveolar type II epithelial cells

Coronaviruses (CoVs) are enveloped single-stranded RNA viruses that predominantly attack the human respiratory system. In recent decades, several deadly human CoVs, including SARS-CoV, SARS-CoV-2, and MERS-CoV, have brought great impact on public health and economics. However, their high infectivity and the demand for high biosafety level facilities restrict the pathogenesis research of CoV infection. Exacerbated inflammatory cell infiltration is associated with poor prognosis in CoV-associated diseases. In this study, we used human CoV 229E (HCoV-229E), a CoV associated with relatively fewer biohazards, to investigate the pathogenesis of CoV infection and the regulation of neutrophil functions by CoV-infected lung cells. Induced pluripotent stem cell (iPSC)-derived alveolar epithelial type II cells (iAECIIs) exhibiting specific biomarkers and phenotypes were employed as an experimental model for CoV infection. After infection, the detection of dsRNA, S, and N proteins validated the infection of iAECIIs with HCoV-229E. The culture medium conditioned by the infected iAECIIs promoted the migration of neutrophils as well as their adhesion to the infected iAECIIs. Cytokine array revealed the elevated secretion of cytokines associated with chemotaxis and adhesion into the conditioned media from the infected iAECIIs. The importance of IL-8 secretion and ICAM-1 expression for neutrophil migration and adhesion, respectively, was demonstrated by using neutralizing antibodies. Moreover, next-generation sequencing analysis of the transcriptome revealed the upregulation of genes associated with cytokine signaling. To summarize, we established an in vitro model of CoV infection that can be applied for the study of the immune system perturbations during severe coronaviral disease.

Emerging role of interactions between tumor angiogenesis and cancer stem cells

Tumor angiogenesis and cancer stem cells (CSCs) are two major hallmarks of solid tumors. They have long received attention for their critical roles in tumor progression, metastasis and recurrence. Meanwhile, plenty of evidence indicates the close association between CSCs and tumor vasculature. CSCs are proven to promote tumor angiogenesis, and the highly vascularized tumor microenvironment further maintains CSCs growth in return, thereby forming a hard-breaking vicious circle to promote tumor development. Hence, though monotherapy targeting tumor vasculature or CSCs has been extensively studied over the past decades, the poor prognosis has been limiting the clinical application. This review summarizes the crosstalk between tumor vasculature and CSCs with emphasis on small-molecule compounds and the associated biological signaling pathways. We also highlight the importance of linking tumor vessels to CSCs to disrupt the CSCs-angiogenesis vicious circle. More precise treatment regimens targeting tumor vasculature and CSCs are expected to benefit future tumor treatment development.

Prognostic Value of Preoperative Serum Leucine Aminopeptidases in Hepatocellular Carcinoma Patients Who Underwent Liver Transplantation

Background

Leucine aminopeptidases (LAPs) have been reported to be involved in tumor cell proliferation, invasion and angiogenesis. However, the relationship between serum leucine aminopeptidases and prognosis of hepatocellular carcinoma (HCC) patients who underwent liver transplantation (LT) was not yet reported. We aimed to evaluate the prognostic value of preoperative serum leucine aminopeptidases in these patients.

Methods

Clinical data of 106 HCC patients who underwent LT were retrospectively analyzed. The sex ratio, age, HBV infection, Child-Pugh stage, preoperative tumor therapy, AFP, the largest tumor size, tumor number, Edmondson grading, macro- and micro-vascular invasion of patients with different serum LAP level and compositions of patients who met the criteria of Milan, UCSF or Hangzhou were compared using the chi-square test. The Kaplan-Meier method was performed in survival analysis and the log rank test was used in survival comparison.

Results

Serum LAPs were correlated with alpha-fetoprotein (AFP), the largest tumor size, tumor number and macro-vascular invasion. Patients with serum LAPs > 87 U/L showed significantly poorer disease-free survival (DFS) and overall survival (OS) than those with serum LAPs ≤ 87 U/L. Univariate analysis indicated that serum LAPs, AFP, the largest tumor size, tumor number, and macro- and micro-vascular invasion were all associated with DFS and OS. Multivariate analysis showed that serum LAPs, macro-vascular invasion and the largest tumor size were independently correlated with DFS and OS. Serum LAPs could also distinguish prognosis between patients with different status of AFP, the largest tumor size, tumor number, and macro- and micro-vascular invasion, as well as patients within and beyond selection criteria, such as Milan, University of California, San Francisco and Hangzhou criteria.

Conclusion

Elevated preoperative serum LAPs were associated with advanced tumor stage and aggressive biological behavior, and thus a poor outcome, which could be a prognostic marker for HCC patients who underwent LT.

CD13/Aminopeptidase N Is a Potential Therapeutic Target for Inflammatory Disorders

CD13/aminopeptidase N is a widely expressed ectoenzyme with multiple functions. As an enzyme, CD13 regulates activities of numerous cytokines by cleaving their N-terminals and is involved in Ag processing by trimming the peptides bound to MHC class II. Independent of its enzymatic activity, cell membrane CD13 functions by cross-linking-induced signal transduction, regulation of receptor recycling, enhancement of FcγR-mediated phagocytosis, and acting as a receptor for cytokines. Moreover, soluble CD13 has multiple proinflammatory roles mediated by binding to G-protein-coupled receptors. CD13 not only modulates development and activities of immune-related cells, but also regulates functions of inflammatory mediators. Therefore, CD13 is important in the pathogenesis of various inflammatory disorders. Inhibitors of CD13 have shown impressive anti-inflammatory effects, but none of them has yet been used for clinical therapy of human inflammatory diseases. We reevaluate CD13's regulatory role in inflammation and suggest that CD13 could be a potential therapeutic target for inflammatory disorders.

Profiling the Surfaceome Identifies Therapeutic Targets for Cells with Hyperactive mTORC1 Signaling

Aberrantly high mTORC1 signaling is a known driver of many cancers and human disorders, yet pharmacological inhibition of mTORC1 rarely confers durable clinical responses. To explore alternative therapeutic strategies, herein we conducted a proteomics survey to identify cell surface proteins upregulated by mTORC1. A comparison of the surfaceome from Tsc1-/-versus Tsc1+/+ mouse embryonic fibroblasts revealed 59 proteins predicted to be significantly overexpressed in Tsc1-/- cells. Further validation of the data in multiple mouse and human cell lines showed that mTORC1 signaling most dramatically induced the expression of the proteases neprilysin (NEP/CD10) and aminopeptidase N (APN/CD13). Functional studies showed that constitutive mTORC1 signaling sensitized cells to genetic ablation of NEP and APN, as well as the biochemical inhibition of APN. In summary, these data show that mTORC1 signaling plays a significant role in the constitution of the surfaceome, which in turn may present novel therapeutic strategies.

Inhibition of Porcine Aminopeptidase M (pAMP) by the Pentapeptide Microginins

Aminopeptidase M (AMP) inhibition is of interest for several diseases, such as highly vascularized cancer types. AMP can be inhibited by linear pentapeptides isolated from Microcystis aeruginosa LTPNA08 (MG7XX). Porcine AMP inhibition—a model for human AMP—activity was spectrophotometrically measured by the formation of p-nitroanilide from L-leucine-p-nitroanilide substrate by AMP. AMP inhibition by MG770 exhibited comparable inhibition levels to amastatin (IC₅₀ values: 1.20 ± 0.1 μM and 0.98 ± 0.1 μM, respectively), while MG756 was slightly less potent (with IC₅₀ values of 3.26 ± 0.5 μM). Molecular modelling suggests a potential binding mode, based on the interaction with the Zn²⁺ cofactor, where MG770′s extra methyl group contributes to the disturbance of the Zn²⁺ cofactor complex and highlights the importance of hydrophobicity for the site.

Mesenchymal Stem Cells in the Adult Human Liver: Hype or Hope?

Chronic liver diseases constitute a significant economic, social, and biomedical burden. Among commonly adopted approaches, only organ transplantation can radically help patients with end-stage liver pathologies. Cell therapy with hepatocytes as a treatment for chronic liver disease has demonstrated promising results. However, quality human hepatocytes are in short supply. Stem/progenitor cells capable of differentiating into functionally active hepatocytes provide an attractive alternative approach to cell therapy for liver diseases, as well as to liver-tissue engineering, drug screening, and basic research. The application of methods generally used to isolate mesenchymal stem cells (MSCs) and maintain them in culture to human liver tissue provides cells, designated here as liver MSCs. They have much in common with MSCs from other tissues, but differ in two aspects-expression of a range of hepatocyte-specific genes and, possibly, inherent commitment to hepatogenic differentiation. The aim of this review is to analyze data regarding liver MSCs, probably another type of liver stem/progenitor cells different from hepatic stellate cells or so-called hepatic progenitor cells. The review presents an analysis of the phenotypic characteristics of liver MSCs, their differentiation and therapeutic potential, methods for isolating these cells from human liver, and discusses issues of their origin and heterogeneity. Human liver MSCs are a fascinating object of fundamental research with a potential for important practical applications.

CD13 tethers the IQGAP1-ARF6-EFA6 complex to the plasma membrane to promote ARF6 activation, β1 integrin recycling, and cell migration

Cell attachment to the extracellular matrix (ECM) requires a balance between integrin internalization and recycling to the surface that is mediated by numerous proteins, emphasizing the complexity of these processes. Upon ligand binding in various cells, the β1 integrin is internalized, traffics to early endosomes, and is returned to the plasma membrane through recycling endosomes. This trafficking process depends on the cyclical activation and inactivation of small guanosine triphosphatases (GTPases) by their specific guanine exchange factors (GEFs) and their GTPase-activating proteins (GAPs). In this study, we found that the cell surface antigen CD13, a multifunctional transmembrane molecule that regulates cell-cell adhesion and receptor-mediated endocytosis, also promoted cell migration and colocalized with β1 integrin at sites of cell adhesion and at the leading edge. A lack of CD13 resulted in aberrant trafficking of internalized β1 integrin to late endosomes and its ultimate degradation. Our data indicate that CD13 promoted ARF6 GTPase activity by positioning the ARF6-GEF EFA6 at the cell membrane. In migrating cells, a complex containing phosphorylated CD13, IQGAP1, GTP-bound (active) ARF6, and EFA6 at the leading edge promoted the ARF6 GTPase cycling and cell migration. Together, our findings uncover a role for CD13 in the fundamental cellular processes of receptor recycling, regulation of small GTPase activities, cell-ECM interactions, and cell migration.

Preclinical activity of melflufen (J1) in ovarian cancer

Ovarian cancer carries a significant mortality. Since symptoms tend to be minimal, the disease is often diagnosed when peritoneal metastases are already present. The standard of care in advanced ovarian cancer consists of platinum-based chemotherapy combined with cytoreductive surgery. Unfortunately, even after optimal cytoreduction and adjuvant chemotherapy, most patients with stage III disease will develop a recurrence. Intraperitoneal administration of chemotherapy is an alternative treatment for patients with localized disease. The pharmacological and physiochemical properties of melflufen, a peptidase potentiated alkylator, raised the hypothesis that this drug could be useful in ovarian cancer and particularily against peritoneal carcinomatosis. In this study the preclinical effects of melflufen were investigated in different ovarian cancer models. Melflufen was active against ovarian cancer cell lines, primary cultures of patient-derived ovarian cancer cells, and inhibited the growth of subcutaneous A2780 ovarian cancer xenografts alone and when combined with gemcitabine or liposomal doxorubicin when administered intravenously. In addition, an intra- and subperitoneal xenograft model showed activity of intraperitoneal administered melflufen for peritoneal carcinomatosis, with minimal side effects and modest systemic exposure. In conclusion, results from this study support further investigations of melflufen for the treatment of peritoneal carcinomatosis from ovarian cancer, both for intravenous and intraperitoneal administration.

Prostate-specific membrane antigen (PSMA)-mediated laminin proteolysis generates a pro-angiogenic peptide

Prostate-specific membrane antigen (PSMA) is a membrane-bound glutamate carboxypeptidase expressed in a number of tissues. PSMA participates in various biological functions depending on the substrate available in the particular tissue; in the brain, PSMA cleaves the abundant neuropeptide N-acetyl-aspartyl-glutamate to regulate release of key neurotransmitters, while intestinal PSMA cleaves polyglutamated peptides to supply dietary folate. PSMA expression is also progressively upregulated in prostate cancer where it correlates with tumor progression as well as in tumor vasculature, where it regulates angiogenesis. The previous research determined that PSMA cleavage of small peptides generated via matrix metalloprotease-mediated proteolysis of the extracellular matrix protein laminin potently activated endothelial cells, integrin signaling and angiogenesis, although the specific peptide substrates were not identified. Herein, using enzymatic analyses and LC/MS, we unequivocally demonstrate that several laminin-derived peptides containing carboxy-terminal glutamate moieties (LQE, IEE, LNE) are bona fide substrates for PSMA. Subsequently, the peptide products were tested for their effects on angiogenesis in various models. We report that LQ, the dipeptide product of PSMA cleavage of LQE, efficiently activates endothelial cells in vitro and enhances angiogenesis in vivo. Importantly, LQE is not cleaved by an inactive PSMA enzyme containing an active site mutation (E424S). Endothelial cell activation by LQ was dependent on integrin beta-1-induced activation of focal adhesion kinase. These results characterize a novel PSMA substrate, provide a functional rationale for the upregulation of PSMA in cancer cells and tumor vasculature and suggest that inhibition of PSMA could lead to the development of new angiogenic therapies.

Liver cancer stem cell markers: Progression and therapeutic implications

Cancer stem cells (CSCs) are a small subpopulation in cancer, have been proposed to be cancer-initiating cells, and have been shown to be responsible for chemotherapy resistance and cancer recurrence. The identification of CSC subpopulations inside a tumor presents a new understanding of cancer development because it implies that tumors can only be eradicated by targeting CSCs. Although advances in liver cancer detection and treatment have increased the possibility of curing the disease at early stages, unfortunately, most patients will relapse and succumb to their disease. Strategies aimed at efficiently targeting liver CSCs are becoming important for monitoring the progress of liver cancer therapy and for evaluating new therapeutic approaches. Herein, we provide a critical discussion of biological markers described in the literature regarding liver cancer stem cells and the potential of these markers to serve as therapeutic targets.

First-in-human, phase I/IIa clinical study of the peptidase potentiated alkylator melflufen administered every three weeks to patients with advanced solid tumor malignancies

PURPOSE

Melflufen (melphalan flufenamide, previously designated J1) is an optimized and targeted derivative of melphalan, hydrolyzed by aminopeptidases overexpressed in tumor cells resulting in selective release and trapping of melphalan, and enhanced activity in preclinical models.

METHODS

This was a prospective, single-armed, open-label, first-in-human, dose-finding phase I/IIa study in 45 adult patients with advanced and progressive solid tumors without standard treatment options. Most common tumor types were ovarian carcinoma (n = 20) and non-small-cell lung cancer (NSCLC, n = 11).

RESULTS

In the dose-escalating phase I part of the study, seven patients were treated with increasing fixed doses of melflufen (25-130 mg) Q3W. In the subsequent phase IIa part, 38 patients received in total 115 cycles of therapy at doses of 30-75 mg. No dose-limiting toxicities (DLTs) were observed at 25 and 50 mg; at higher doses DLTs were reversible neutropenias and thrombocytopenias, particularly evident in heavily pretreated patients, and the recommended phase II dose (RPTD) was set to 50 mg. Response Evaluation Criteria In Solid Tumors (RECIST) evaluation after 3 cycles of therapy (27 patients) showed partial response in one (ovarian cancer), and stable disease in 18 patients. One NSCLC patient received nine cycles of melflufen and progressed after 7 months of therapy.

CONCLUSIONS

In conclusion, melflufen can safely be given to cancer patients, and the toxicity profile was as expected for alkylating agents; RPTD is 50 mg Q3W. Reversible and manageable bone marrow suppression was identified as a DLT. Clinical activity is suggested in ovarian cancer, but modest activity in treatment of refractory NSCLC.

CD13 regulates anchorage and differentiation of the skeletal muscle satellite stem cell population in ischemic injury

CD13 is a multifunctional cell surface molecule that regulates inflammatory and angiogenic mechanisms in vitro, but its contribution to these processes in vivo or potential roles in stem cell biology remains unexplored. We investigated the impact of loss of CD13 on a model of ischemic skeletal muscle injury that involves angiogenesis, inflammation, and stem cell mobilization. Consistent with its role as an inflammatory adhesion molecule, lack of CD13 altered myeloid trafficking in the injured muscle, resulting in cytokine profiles skewed toward a prohealing environment. Despite this healing-favorable context, CD13(KO) animals showed significantly impaired limb perfusion with increased necrosis, fibrosis, and lipid accumulation. Capillary density was correspondingly decreased, implicating CD13 in skeletal muscle angiogenesis. The number of CD45-/Sca1-/α7-integrin+/β1-integrin+ satellite cells was markedly diminished in injured CD13(KO) muscles and adhesion of isolated CD13(KO) satellite cells was impaired while their differentiation was accelerated. Bone marrow transplantation studies showed contributions from both host and donor cells to wound healing. Importantly, CD13 was coexpressed with Pax7 on isolated muscle-resident satellite cells. Finally, phosphorylated-focal adhesion kinase and ERK levels were reduced in injured CD13(KO) muscles, consistent with CD13 regulating satellite cell adhesion, potentially contributing to the maintenance and renewal of the satellite stem cell pool and facilitating skeletal muscle regeneration.

Evaluation of (99m)Tc-probestin SPECT as a novel technique for noninvasive imaging of kidney aminopeptidase N expression

Aminopeptidase N (APN; CD13; EC 3.4.11.2) is a zinc-dependent membrane-bound exopeptidase that catalyzes the removal of N-terminal amino acids from peptides. APN is known to be highly expressed on renal cortical proximal tubules. APN expression levels are markedly decreased under the influence of nephrotoxins and in the tumor regions of renal cancers. Thus, molecular imaging of kidney APN expression could provide pathophysiological information about kidneys noninvasively. Probestin is a potent APN inhibitor and binds to APN. Abdominal SPECT imaging was conducted at 1 h postinjection of (99m)Tc-probestin in a group of 12 UPII-SV40T transgenic and wild-type mice. UPII-SV40T mice spontaneously develop urothelial carcinoma in situ and invasive transitional cell carcinoma (TCC) that invade kidneys. Histopathology and immunohistochemistry analysis were used to confirm the presence of tumor and to evaluate APN expression in kidney. Radioactivity in normal tissue regions of renal cortex was clearly visible in SPECT images, whereas tumor regions of renal cortex displayed significantly lower or no radioactivity uptake. Histopathological analysis of kidney sections showed normal morphology for both renal pelvic and cortical regions in wild-type mice and abnormal morphology in some transgenic mice. Proliferating cell nuclear antigen staining confirmed the presence of tumor in those abnormal regions. Immunohistochemical analysis of kidney sections using anti-CD13 antibody showed significantly lower APN expression in tumor regions compared to normal regions. Results obtained in this study demonstrate the potential use of (99m)Tc-probestin SPECT as a novel technique for noninvasive imaging of kidney APN expression.

CD13 promotes mesenchymal stem cell-mediated regeneration of ischemic muscle

Mesenchymal stem cells (MSCs) are multipotent, tissue-resident cells that can facilitate tissue regeneration and thus, show great promise as potential therapeutic agents. Functional MSCs have been isolated and characterized from a wide array of adult tissues and are universally identified by the shared expression of a core panel of MSCs markers. One of these markers is the multifunctional cell surface peptidase CD13 that has been shown to be expressed on human and murine MSCs from many tissues. To investigate whether this universal expression indicates a functional role for CD13 in MSC biology we isolated, expanded and characterized MSCs from bone marrow of wild type (WT) and CD13^KO mice. Characterization of these cells demonstrated that both WT and CD13^KO MSCs expressed the full complement of MSC markers (CD29, CD44, CD49e, CD105, Sca1), showed comparable proliferation rates and were capable of differentiating toward the adipogenic and osteogenic lineages. However, MSCs lacking CD13 were unable to differentiate into vascular cells, consistent with our previous characterization of CD13 as an angiogenic regulator. Compared to WT MSCs, adhesion and migration on various extracellular matrices of CD13^KO MSCs were significantly impaired, which correlated with decreased phospho-FAK levels and cytoskeletal alterations. Crosslinking human MSCs with activating CD13 antibodies increased cell adhesion to endothelial monolayers and induced FAK activation in a time dependent manner. In agreement with these in vitro data, intramuscular injection of CD13^KO MSCs in a model of severe ischemic limb injury resulted in significantly poorer perfusion, decreased ambulation, increased necrosis and impaired vascularization compared to those receiving WT MSCs. This study suggests that CD13 regulates FAK activation to promote MSC adhesion and migration, thus, contributing to MSC-mediated tissue repair. CD13 may present a viable target to enhance the efficacy of mesenchymal stem cell therapies.

An anti-human CD13 monoclonal antibody that suppresses the suppressive function of Treg cells

CD13 (CD13/aminopeptidase N, APN, or CD13/APN) is a widely expressed type II membrane-bound metalloprotease. It is often overexpressed on cancer cells and expressed on CD4(+)CD25(hi) Treg cell subpopulation with higher suppressive ability. It has been determined to be a promising target in cancer diagnosis and therapy. In this study, a functional anti-human CD13 monoclonal antibody, MAb 9E4, was obtained and the specificity of this MAb was verified by flow cytometry. This MAb effectively recognized the CD13 molecule expressed on a series of malignant cell lines. Furthermore, we demonstrated that MAb 9E4 suppresses the suppressive function of Treg cells. This functional anti-human CD13 MAb provides a valuable tool for further study targeting the CD13 positive Treg cells.

The novel alkylating prodrug melflufen (J1) inhibits angiogenesis in vitro and in vivo

Aminopeptidase N (APN) has been reported to have a functional role in tumor angiogenesis and repeatedly reported to be over-expressed in human tumors. The melphalan-derived prodrug melphalan-flufenamide (melflufen, previously designated J1) can be activated by APN. This suggests that this alkylating prodrug may exert anti-angiogenic properties, which will possibly contribute to the anti-tumoral activity in vivo. This work presents a series of experiments designed to investigate this effect of melflufen. In a cytotoxicity assay we show that bovine endothelial cells were more than 200 times more sensitive to melflufen than to melphalan, in HUVEC cells the difference was more than 30-fold and accompanied by aminopetidase-mediated accumulation of intracellular melphalan. In the chicken embryo chorioallantoic membrane (CAM) assay it is indicated that both melflufen and melphalan inhibit vessel ingrowth. Two commercially available assays with human endothelial cells co-cultured with fibroblasts (TCS Cellworks AngioKit, and Essen GFP-AngioKit) also illustrate the superior anti-angiogenic effect of melflufen compared to melphalan. Finally, in a commercially available in vivo assay in mice (Cultrex DIVAA angio-reactor assay) melflufen displayed an anti-angiogenic effect, comparable to bevacizumab. In conclusion, this study demonstrates through all methods used, that melphalan-flufenamide besides being an alkylating agent also reveals anti-angiogenic effects in different preclinical models in vitro and in vivo.

Solid phase synthesis and biological evaluation of probestin as an angiogenesis inhibitor

Probestin is a potent aminopeptidase N (APN) inhibitor originally isolated from the bacterial culture broth. Here, we report probestin synthesis by solid phase peptide synthesis (SPPS) method and evaluated its activity to inhibit angiogenesis using a chicken embryo chorioallantoic membrane (CAM) assay and a CAM tumor xenograft model. Results from these studies demonstrate that probestin inhibits the angiogenic activity and tumor growth.

Design and synthesis of small-molecule fluorescent photoprobes targeted to aminopeptdase N (APN/CD13) for optical imaging of angiogenesis

We report here the synthesis of a nonpeptide, small-molecule fluorescent imaging agent with high affinity to aminopeptidase N (APN/CD13), a key player in a variety of pathophysiological angiogenic processes. On the basis of a recently described lead structure, we synthesized three putative precursor compounds by introducing polyethylene glycol (PEG) spacers comprising amino groups for dye labeling. Different attachment sites resulted in substantial differences in target affinity, cell toxicity, and target imaging performance. In comparison to bestatin, a natural inhibitor of many aminopeptidases, two of our compounds (22, 23) exhibit comparable inhibition potency, while a third (21) does not show any inhibiting effect. Cell binding assays with APN-positive BT-549 and APN-negative BT-20 cells and the final fluorescent probes Cy 5.5-21 and Cy 5.5-23 confirm these findings. The favorable characteristics of Cy 5.5-23 will now be proven in in vivo experiments with murine models of high APN expression and may serve as a tool to better understand APN pathophysiology.

Aminopeptidase N

The third edition of the Handbook of Proteolytic Enzymes aims to be a comprehensive reference work for the enzymes that cleave proteins and peptides, and contains over 850 chapters. Each chapter is organized into sections describing the name and history, activity and specificity, structural chemistry, preparation, biological aspects, and distinguishing features for a specific peptidase. The subject of Chapter 79 is Aminopeptidase N.

Keywords

Actinonin, amastatin, angiogenesis, angiotensin, bestatin, brush border, cancer, CD13, coronavirus, cysteinyl-glycinase, dipeptidyl peptidase IV, enkephalin, glutathione, neprilysin, puromycin, stem cells.

Distinct contribution of human cord blood-derived endothelial colony forming cells to liver and gut in a fetal sheep model

Although the vasculogenic potential of circulating and cord blood (CB)-derived endothelial colony-forming cells (ECFC) has been demonstrated in vitro and in vivo, little is known about the inherent biologic ability of these cells to home to different organs and contribute to tissue-specific cell populations. Here we used a fetal sheep model of in utero transplantation to investigate and compare the intrinsic ability of human CB-derived ECFC to migrate to the liver and to the intestine, and to define ECFC's intrinsic ability to integrate and contribute to the cytoarchitecture of these same organs. ECFCs were transplanted by an intraperitoneal or intrahepatic route (IH) into fetal sheep at concentrations ranging from 1.1-2.6 × 10(6) cells/fetus. Recipients were evaluated at 85 days posttransplant for donor (human) cells using flow cytometry and confocal microscopy. We found that, regardless of the route of injection, and despite the IH delivery of ECFC, the overall liver engraftment was low, but a significant percentage of cells were located in the perivascular regions and retained the expression of hallmark endothelial makers. By contrast, ECFC migrated preferentially to the intestinal crypt region and contributed significantly to the myofibroblast population. Furthermore, ECFC expressing CD133 and CD117 lodged in areas where endogenous cells expressed those same phenotypes.

CONCLUSION

ECFC inherently constitute a potential source of cells for the treatment of intestinal diseases, but strategies to increase the numbers of ECFC persisting within the hepatic parenchyma are needed in order to enhance ECFC therapeutic potential for this organ.

The X-ray crystal structure of human aminopeptidase N reveals a novel dimer and the basis for peptide processing

Human aminopeptidase N (hAPN/hCD13) is a dimeric membrane protein and a member of the M1 family of zinc metallopeptidases. Within the rennin-angiotensin system, its enzymatic activity is responsible for processing peptide hormones angiotensin III and IV. In addition, hAPN is also involved in cell adhesion, endocytosis, and signal transduction and it is an important target for cancer therapy. Reported here are the high resolution x-ray crystal structures of the dimeric ectodomain of hAPN and its complexes with angiotensin IV and the peptidomimetic inhibitors, amastatin and bestatin. Each monomer of the dimer is found in what has been termed the closed form in other M1 enzymes and each monomer is characterized by an internal cavity surrounding the catalytic site as well as a unique substrate/inhibitor-dependent loop ordering, which in the case of the bestatin complex suggests a new route to inhibitor design. The hAPN structure provides the first example of a dimeric M1 family member and the observed structural features, in conjunction with a model for the open form, provide novel insights into the mechanism of peptide processing and signal transduction.

Integrin α6(high) cell population functions as an initiator in tumorigenesis and relapse of human liposarcoma

The relapse and resistance to chemo- and radiotherapy are main problems in the treatment of human liposarcoma. It is important to find a functional marker existing in the liposarcoma cells for targeting. In this article, we established a new sub-cell line SW872-S cells with high tumorigenicity from human liposarcoma SW872 cells by repeated inoculation approach. The characteristic of the sub-cell line is linked to the high levels of integrin α6 on the surface. The integrin α6(high) cells show much higher tumor initiation and self-renewal potential in vivo than integrin α6(low) cells do. Targeting integrin α6 with its specific short interfering RNA and antibody significantly inhibits the cell adhesion to laminin and the tumor growth in vitro and in vivo, respectively. Interestingly, integrin α6 marks almost all of the surgical biopsy specimens of patients with liposarcoma relapse. Moreover, integrin α6 is found to coexpress with CD13, which might contribute to the antiapoptosis ability of integrin α6(high) cells. Consistently, integrin α6(high) cells are more sensitive to the CD13 inhibitor bestatin, and 61% of 23 other human tumor cell lines also contain integrin α6(high) CD13(high) subgroup. These results provide evidence, for the first time, to our knowledge, that integrin α6 and CD13 can serve as functional markers of the tumor-initiation subcell population in human liposarcoma as well as other cancers for therapeutic targeting.

Activity screening and structure-activity relationship of the hit compounds targeting APN/CD13

Aminopeptidase N (APN) plays an important role in tumor progression, which participates in the progress such as proliferation, attachment, angiogenesis, and tumor invasion. All of this makes APN as a good chemical therapeutic anti‐tumor target. In this study, a series of chemically synthesized APN inhibitors were tested for the anti‐tumor activities, and three most effective compounds were chosen according to the MTT assay. Then, the enzyme inhibitory, anti‐tumor, specificity, angiogenesis, and invasion were determined to evaluate the activity of these three compounds. All compounds can markedly inhibit the enzyme activity of APN, angiogenesis of endothelial cells, and the invasion of ES‐2 cells. And it had little effect on the viability of K562 which express low level of APN. This data indicated that the tested compounds were APN hit compounds. We also did kinetic assay to determine the inhibition constant and constructed a three‐dimensional quantitative structure–activity relationship model to analyze the structure–activity relationship to direct the further design of novel APN inhibitors as anti‐tumor agents. These data demonstrate that the tested compounds can be developed as novel candidates of anticancer agent.

Enhanced expression of CD13 in vessels of inflammatory and neoplastic tissues

Aminopeptidase-N (CD13) is an important target of tumor vasculature-targeting drugs. The authors investigated its expression by immunohistochemistry with three anti-CD13 monoclonal antibodies (WM15, 3D8, and BF10) in normal and pathological human tissues, including 58 normal, 32 inflammatory, and 149 tumor tissue specimens. The three antibodies stained vessels in most neoplastic tissues, interestingly with different patterns. As a matter of fact, WM15 stained almost all intratumor and peritumor capillaries and only partially large vessels, whereas BF10 and 3D8 reacted with arteries and venules and to a lesser extent with capillaries. These antibodies also stained the stroma in about half of neoplastic tissues. In inflammatory lesions, the three antibodies stained vessels and stroma, whereas in normal tissues, they stained a small percentage of blood vessels. Finally, the three antibodies failed to stain endothelial cells of normal colon, whereas they reacted with activated human umbilical vein endothelial cells and with endothelial cells of colon adenocarcinoma vessels. Overall, WM15 was the most specific antibody for angiogenic tumor vessels, suggesting that it may be a good tool for detecting the CD13 form associated with the tumor vasculature. This finding may be relevant for CD13-mediated vascular targeting therapies.

Aminopeptidase N (CD13) as a target for cancer chemotherapy

The enzyme aminopeptidase N (APN, also known as CD13) is a Zn(2+) dependent membrane-bound ectopeptidase that degrades preferentially proteins and peptides with a N-terminal neutral amino acid. Aminopeptidase N has been associated with the growth of different human cancers and suggested as a suitable target for anti-cancerous therapy. Different approaches have been used to develop new drugs directed to this target, including enzyme inhibitors as well as APN-targeted carrier constructs. This review discusses the prevalence and possible function of APN in malignant diseases, mainly solid tumors, as well as its "drugability" evaluated in preclinical in vivo models, and also provides a brief overview of current clinical trials focused on APN.

Optical techniques for the molecular imaging of angiogenesis

The process of angiogenesis, an essential hallmark for tumour development as well as for several inflammatory diseases and physiological phenomena, is of growing interest for diagnosis and therapy in oncology. In the context of biochemical characterisation of key molecules involved in angiogenesis, several targets for imaging and therapy could be identified in the last decade. Optical imaging (OI) relies on the visualisation of near infrared (NIR) light, either its absorption and scattering in tissue (non-enhanced OI) or using fluorescent contrast agents. OI offers excellent signal to noise ratios due to virtually absent background fluorescence in the NIR range and is thus a versatile tool to image specific molecular target structures in vivo. This work intends to provide a survey of the different approaches to imaging of angiogenesis using OI methods in preclinical research as well as first clinical trials. Different imaging modalities as well as various optical contrast agents are briefly discussed.

Renin-angiotensin system in human coronavirus pathogenesis

Although initially considered relatively harmless pathogens, human coronaviruses (HCoVs) are nowadays known to be associated with more severe clinical complications. Still, their precise pathogenic potential is largely unknown, particularly regarding the most recently identified species HCoV-NL63 and HCoV-HKU1. HCoVs need host cell proteins to successively establish infections. Proteases of the renin–angiotensin system serve as receptors needed for entry into target cells; this article describes the current knowledge on the involvement of this system in HCoV pathogenesis.

Peptide targeting of platinum anti-cancer drugs

Besides various side effects caused by platinum anticancer drugs, they are not efficiently absorbed by the tumor cells. Two Pt-peptide conjugates; cyclic mPeg-CNGRC-Pt (7) and cyclic mPeg-CNGRC-Pten (8) bearing the Asn-Gly-Arg (NGR) targeting sequence, a malonoyl linker, and low molecular weight miniPEG groups have been synthesized. The platinum ligand was attached to the peptide via the carboxylic end of the malonate group at the end of the peptide. The pegylated peptide is nontoxic and highly soluble in water. Platinum conjugates synthesized using the pegylated peptides are also water-soluble with reduced or eliminated peptide immunogenicity. The choice of carboplatin as our untargeted platinum complex was due to the fact that the malonate linker chelates platinum in a manner similar to that of carboplatin. Cell toxicity assay and competition assay on the PC-3 cells (CD13 positive receptors) revealed selective delivery and destruction of PC-3 cells using targeted Pt-peptide conjugates 7 and 8 significantly more than untargeted carboplatin. Platinum uptake on PC-3 cells was 12-fold more for conjugate 7 and 3-fold more for conjugate 8 compared to that of the untargeted carboplatin, indicating selective activation of the CD13 receptors and delivery of the conjugates to CD13 positive cells. Further analysis on effects of conjugates 7 and 8 on PC-3 cells using caspase-3/7, fluorescence microscopy, and DNA fragmentation confirmed that the cells were dying by apoptosis.

Infarction of tumor vessels by NGR-peptide-directed targeting of tissue factor: experimental results and first-in-man experience

We induced thrombosis of blood vessels in solid tumors in mice by a fusion protein consisting of the extracellular domain of tissue factor (truncated tissue factor, tTF) and the peptide GNGRAHA, targeting aminopeptidase N (CD13) and the integrin alpha(v)beta(3) (CD51/CD61) on tumor vascular endothelium. The designed fusion protein tTF-NGR retained its thrombogenic activity as demonstrated by coagulation assays. In vivo studies in mice bearing established human adenocarcinoma (A549), melanoma (M21), and fibrosarcoma (HT1080) revealed that systemic administration of tTF-NGR induced partial or complete thrombotic occlusion of tumor vessels as shown by histologic analysis. tTF-NGR, but not untargeted tTF, induced significant tumor growth retardation or regression in all 3 types of solid tumors. Thrombosis induction in tumor vessels by tTF-NGR was also shown by contrast enhanced magnetic resonance imaging (MRI). In the human fibrosarcoma xenograft model, MRI revealed a significant reduction of tumor perfusion by administration of tTF-NGR. Clinical first-in-man application of low dosages of this targeted coagulation factor revealed good tolerability and decreased tumor perfusion as measured by MRI. Targeted thrombosis in the tumor vasculature induced by tTF-NGR may be a promising strategy for the treatment of cancer.

Prospective evaluation of aminopeptidase activities in plasma and peripheral organs of streptozotocin-induced diabetic rats

The cleavage of peptides by aminopeptidase enzyme types could be among the mechanisms related to certain disruptions on mediator and modulatory functions in diabetes mellitus. In order to examine this hypothesis, we measured representative aminopeptidase activities in tissues of peripheral organs of control and streptozotocin-diabetic rats. None of the examined aminopeptidase activities differed between diabetics and controls in plasma, ileum, stomach or lung. Soluble and membrane-associated alanyl, and membrane-associated cystyl aminopeptidase activities were higher in the kidney of diabetics. Decreased activity was observed in soluble and membrane-associated aspartyl and soluble dipeptidyl-peptidase IV, while increased activity was observed in soluble alanyl, arginyl, and cystyl aminopeptidases in the pancreas of diabetics. In the jejunum, soluble cystyl aminopeptidase increased in diabetics. Soluble arginyl and type-1-pyroglutamyl aminopeptidase and membrane-associated dipeptidyl-peptidase IV activities increased in the liver of diabetics. Membrane-associated dipeptidyl-peptidase IV and alanyl aminopeptidase activities in the spleen were higher in diabetics than in controls. Membrane-associated alanyl aminopeptidase activity also increased in the heart of diabetics. All these changes in streptozotocin-treated rats were avoided by the administration of insulin. Our comparative analysis of a diverse array of aminopeptidase activities supported the proposal that the regulation of peptide cleavage by these enzyme types is associated with the effects of streptozotocin-diabetes mellitus on peripheral organs.

CD13/APN transcription is regulated by the proto-oncogene c-Maf via an atypical response element

Angiogenic growth factors induce the transcription of the cell surface peptidase CD13/APN in activated endothelial cells of the tumor vasculature. Inhibition of CD13/APN abrogates endothelial invasion and morphogenesis in vitro and tumor growth in vivo suggesting a critical functional role for CD13 in angiogenesis. Experiments to identify the transcription factors responsible for this regulation demonstrated that exogenous expression of the proto-oncogene c-Maf, but not other bZip family members tested, potently activates transcription from a critical regulatory region of the CD13 proximal promoter between -115 and -70 bp which is highly conserved among mammalian species. Using promoter mutation, EMSA and ChIP analyses we established that both endogenous and recombinant c-Maf directly interact with an atypical Maf response element contained within this active promoter region via its basic DNA/leucine zipper domain. However full activity of c-Maf requires the amino-terminal transactivation domain, and site-directed mutation of putative phosphorylation sites within the transactivation domain (serines 15 and 70) shows that these sites behave in a dramatic cell type-specific manner. Therefore, this atypical response element predicts a broader range of c-Maf target genes than previously appreciated and thus impacts its regulation of multiple myeloma as well as endothelial cell function and angiogenesis.

Important role of Nox4 type NADPH oxidase in angiogenic responses in human microvascular endothelial cells in vitro

OBJECTIVE

Redox signaling mediated by Nox2-containing NADPH oxidase has been implicated in angiogenic responses both in vitro and in vivo. Because Nox4 type NADPH oxidase is also highly expressed in endothelial cells, we studied the role of Nox4 in angiogenic responses in human endothelial cells in culture.

METHODS AND RESULTS

Inhibition of Nox4 expression by small interfering RNA reduced angiogenic responses as assessed by the tube formation and wound healing assays, in both human microvascular and umbilical vein endothelial cells. Overexpression of wild-type Nox4 enhanced, whereas expression of a dominant negative form of Nox4 suppressed the angiogenic responses in endothelial cells. These effects were mimicked by exogenous H2O2 and the antioxidant compound ebselen, respectively. Overexpression of Nox4 enhanced receptor tyrosine kinase phosphorylation and the activation of extracellular signal-regulated kinase (Erk). Inhibition of the Erk pathway reduced the endothelial angiogenic responses. Nox4 expression also promotes proliferation and migration of endothelial cells, and reduced serum deprivation-induced apoptosis.

CONCLUSIONS

Nox4 type NADPH oxidase promotes endothelial angiogenic responses, at least partly, via enhanced activation of receptor tyrosine kinases and the downstream Erk pathway.

Inhibition of APN/CD13 leads to suppressed progressive potential in ovarian carcinoma cells

Background

Aminopeptidase N (APN/CD13), a 150-kDa metalloprotease, is a multifunctional cell surface aminopeptidase with ubiquitous expression. Recent studies have suggested that APN/CD13 plays an important role in tumor progression of several human malignancies. In the current study, we investigated the role of APN/CD13 in ovarian carcinoma (OVCA) progression.

Methods

We first examined the expression of APN/CD13 at the protein level in a variety of OVCA cell lines and tissues. We subsequently investigated whether there was a correlation between APN/CD13 expression and invasive potential of various OVCA cell lines. Moreover, we investigated the function of APN/CD13 in OVCA cells using bestatin, an APN/CD13 inhibitor, or transfection of siRNA for APN/CD13.

Results

We confirmed that APN/CD13 was expressed in OVCA tissues and cell lines to various extents. There was a positive correlation between APN/CD13 expression and migratory potential in various OVCA cell lines with accordingly enhanced secretion of endogenous MMP-2. Subsequently, we found a significant decrease in the proliferative and migratory abilities of OVCA cells after the addition of bestatin or the inhibition of APN/CD13 expression by siRNA. Furthermore, in an animal model, daily intraperitoneal administration of bestatin after inoculation of OVCA cells resulted in a decrease of peritoneal dissemination and in prolonged survival of nude mice.

Conclusion

The current data indicate the possible involvement of APN/CD13 in the development of OVCA, and suggest that clinical use of bestatin may contribute to better prognosis for ovarian carcinoma patients.

Essential role of placental leucine aminopeptidase in gynecologic malignancy

The M1 aminopeptidase family is important for the maturation or degradation of bioactive peptides by hydrolyzing their N-terminal amino acids. Some investigators have studied aminopeptidase in the maintenance of homeostasis including maintenance of normal pregnancy, memory retention, blood pressure regulation and antigen presentation. However, there are a few reports on the relation between the M1 aminopeptidase family and carcinoma. In addition to its capacity to degrade a range of peptides, placental-leucine aminopeptidase (P-LAP) has novel functions that impact on normal cells and neoplastic cells. P-LAP is the focus of this review.

The Reversion-inducing Cysteine-rich Protein with Kazal Motifs (RECK) Interacts with Membrane Type 1 Matrix Metalloproteinase and CD13/Aminopeptidase N and Modulates Their Endocytic Pathways

The reversion-inducing cysteine-rich protein with Kazal motifs (RECK) is anchored to the cell surface via glycosylphosphatidylinositol. This molecule antagonizes the function of membrane type 1 matrix metalloproteinase (MT1-MMP) to promote proMMP-2 maturation. Here, we attempt to clarify the mechanism underlying RECK functions. First, we found that RECK forms a complex with MT1-MMP and inhibits its proteolytic activity. Notably, RECK increases the amount of MT1-MMP that associates with detergent-resistant membranes during sucrose gradient ultracentrifugation. Furthermore, perturbation of membrane cholesterol significantly affected the function of RECK in suppressing MT1-MMP function. These findings indicate that RECK possibly regulates MT1-MMP function by modulating its behavior on the cell surface as well as by enzymatic action; this prompted us to find another molecule whose behavior in detergent-resistant membranes is influenced by RECK. Subsequently, we found that RECK interacts with CD13/aminopeptidase N. Further, we found that RECK inhibits the proteolytic activity of CD13 in a cholesterol perturbation-sensitive manner. Finally, we examined whether RECK influences the behavior of MT1-MMP and CD13 during their internalization from the cell surface. In the absence of RECK, MT1-MMP and CD13 were internalized along with the markers of clathrin- or caveolae-dependent endocytosis. However, interestingly, in the presence of RECK these molecules were internalized preferentially with an endocytic marker that is neither clathrinnor caveolae-dependent, indicating that RECK modulates endocytic pathways of MT1-MMP and CD13. This modulation was correlated with the accelerated internalization and decay of MT1-MMP and CD13. This study unveils the novel function and target molecules of RECK.

Proteolytic processing of CXCL11 by CD13/aminopeptidase N impairs CXCR3 and CXCR7 binding and signaling and reduces lymphocyte and endothelial cell migration

CXCR3 ligands were secreted by tissue fibroblasts and peripheral blood-derived mononuclear leukocytes in response to interferon-gamma (IFN-gamma) and Toll-like receptor (TLR) ligands. Subsequent purification and identification revealed the presence of truncated CXCL11 variants missing up to 6 amino acids. In combination with CD26/dipeptidyl peptidase IV, the metalloprotease aminopeptidase N (APN), identical to the myeloid cell marker CD13, rapidly processed CXCL11, but not CXCL8, to generate truncated CXCL11 forms. Truncated CXCL11 had reduced binding, signaling, and chemotactic properties for lymphocytes and CXCR3- or CXCR7-transfected cells. CD13/APN-truncated CXCL11 failed to induce an intracellular calcium increase but was still able to bind and desensitize CXCR3 for intact CXCL11 signaling. CXCL11 efficiently bound to CXCR7, but CXCL11 was not able to induce calcium signaling or ERK1/2 or Akt phosphorylation through CXCR7. CD26-truncated CXCL11 failed to attract lymphocytes but still inhibited microvascular endothelial cell (HMVEC) migration. However, further processing of CXCL11 by CD13 resulted in significant reduction of inhibition of HMVEC migration. Taken together, during inflammation or cancer, CXCL11 processing by CD13 may lead to a reduced number of tumor-infiltrating lymphocytes and in a more angiogenic environment.

CD13/APN regulates endothelial invasion and filopodia formation

CD13/aminopeptidase N is a transmembrane peptidase that is induced in the vasculature of solid tumors and is a potent angiogenic regulator. Here, we demonstrate that CD13 controls endothelial cell invasion in response to the serum peptide bradykinin by facilitating signal transduction at the level of the plasma membrane. Inhibition of CD13 abrogates bradykinin B(2) receptor internalization, leading to the attenuation of downstream events such as bradykinin-induced activation of Cdc42 and filopodia formation, and thus affects endothelial cell motility. Investigation into mechanisms underlying this block led us to focus on B(2)R internalization via membrane-dependent mechanisms. Membrane disruption by depletion of cholesterol or trypsinization halts B(2)R internalization, invasion, and filopodia formation, which can be recovered with addition of cholesterol. However, this functional recovery is severely impaired in the presence of CD13 antagonists, and the distribution of membrane proteins is disordered in treated cells, suggesting a role for CD13 in plasma membrane protein organization. Finally, exogenous expression of wild-type but not mutant CD13 further alters protein distribution, suggesting peptidase activity is required for CD13's regulatory activity. Therefore, CD13 functions as a novel modulator of signal transduction and cell motility via its influence on specific plasma membrane organization, thus regulating angiogenesis.

Impaired angiogenesis in aminopeptidase N-null mice

Aminopeptidase N (APN, CD13; EC 3.4.11.2) is a transmembrane metalloprotease with several functions, depending on the cell type and tissue environment. In tumor vasculature, APN is overexpressed in the endothelium and promotes angiogenesis. However, there have been no reports of in vivo inactivation of the APN gene to validate these findings. Here we evaluated, by targeted disruption of the APN gene, whether APN participates in blood vessel formation and function under normal conditions. Surprisingly, APN-null mice developed with no gross or histological abnormalities. Standard neurological, cardiovascular, metabolic, locomotor, and hematological studies revealed no alterations. Nonetheless, in oxygen-induced retinopathy experiments, APN-deficient mice had a marked and dose-dependent deficiency of the expected retinal neovascularization. Moreover, gelfoams embedded with growth factors failed to induce functional blood vessel formation in APN-null mice. These findings establish that APN-null mice develop normally without physiological alterations and can undergo physiological angiogenesis but show a severely impaired angiogenic response under pathological conditions. Finally, in addition to vascular biology research, APN-null mice may be useful reagents in other medical fields such as malignant, cardiovascular, immunological, or infectious diseases.

Altered levels of acid, basic, and neutral peptidase activity and expression in human clear cell renal cell carcinoma

Peptides play important roles in cell regulation and signaling in many tissues and are regulated by peptidases, most of which are highly expressed in the kidney. Several peptide convertases have a function in different tumor stages, and some have been clearly characterized as diagnostic and prognostic markers for solid tumors, including renal cancer; however, little is known about their in vivo role in kidney tumors. The present study compares the activity of a range of peptidases in human tumor samples and nontumor tissue obtained from clear cell renal cell carcinoma (CCRCC) patients. To cover the complete spectrum and subcellular distribution of peptide-converting activity, acid, neutral, basic, and omega activities were selected. CCRCC displays a selective and restricted pattern of peptidase activities. Puromycin-sensitive aminopeptidase activity in the tumor increases [tumor (t) = 10,775 vs. nontumor (n) = 7,635 units of peptidase (UP)/mg protein; P < 0.05], whereas aminopeptidase N decreases (t = 6,664 vs. n = 33,381 UP/mg protein; P < 0.001). Aminopeptidase B activity of the particulate fraction in tumors decreases (t = 2,399 vs. n = 13,536 UP/mg protein; P < 0.001) compared with nontumor tissues, and aspartyl-aminopeptidase activity decreases significantly in CCRCC (t = 137 vs. n = 223 UP/mg protein; P < 0.05). Soluble and particulate pyroglutamyl peptidase I activities, aminopeptidase A activity, and soluble aminopeptidase B activity do not vary in renal cancer. The relative expression for the aforementioned peptidases, assayed using quantitative RT-PCR, increases in CCRCC for aminopeptidases B (1.5-fold) and A (19-fold), aspartyl-aminopeptidase (3.9-fold), puromycin-sensitive aminopeptidase (2.5-fold), and pyroglutamyl peptidase I (7.6-fold). Only aminopeptidase N expression decreases in tumors (1.3-fold). This peptidase activity profile in the neoplastic kidney suggests a specific role for the studied convertases and the possible involvement of an intracrine renin-angiotensin system in the pathogenesis of CCRCC.

cNGR: A Novel Homing Sequence for CD13/APN Targeted Molecular Imaging of Murine Cardiac Angiogenesis In Vivo

Objective— Previously, the peptide sequence cNGR has been shown to home specifically to CD13/APN (aminopeptidase N) on tumor endothelium. Here, we investigated the feasibility of selective imaging of cardiac angiogenesis using the cNGR-CD13/APN system.

Methods and Results— CD13/APN induction and cNGR homing were studied in the murine myocardial infarction (MI) model. By real-time polymerase chain reaction (PCR) at 7 days after MI, CD13/APN expression was 10- to 20-fold higher in the angiogenic infarct border zone and the MI area than in non-MI areas. In vivo fluorescence microscopy confirmed specific homing of fluorophore-tagged cNGR to the border zone and MI territory at 4 and 7 days after MI with a local advantage of 2.3, but not at 1 or 14 days after MI. Tissue residence half-life was 9.1±0.3 hours, whereas the half-life in plasma was 15.4±3.4 minutes. Pulse chase experiments confirmed reversible binding of cNGR in the infarct area. Fluorescent labeled cNGR conjugates or antibodies were injected in vivo, and their distribution was studied ex vivo by 2-photon laser scanning microscopy (TPLSM). cNGR co-localized exclusively with CD13/APN and the endothelial marker CD31 on vessels.

Conclusions— In cardiac angiogenesis endothelial CD13/APN is upregulated. It can be targeted specifically with cNGR conjugates. In the heart cNGR binds its endothelial target only in angiogenic areas.