Continuous treatment of bestatin induces anti-angiogenic property in endothelial cells

Aminopeptidase N/CD13 Crosslinking Promotes the Activation and Membrane Expression of Integrin CD11b/CD18

The β2 integrin CD11b/CD18, also known as complement receptor 3 (CR3), and the moonlighting protein aminopeptidase N (CD13), are two myeloid immune receptors with overlapping activities: adhesion, migration, phagocytosis of opsonized particles, and respiratory burst induction. Given their common functions, shared physical location, and the fact that some receptors can activate a selection of integrins, we hypothesized that CD13 could induce CR3 activation through an inside-out signaling mechanism and possibly have an influence on its membrane expression. We revealed that crosslinking CD13 on the surface of human macrophages not only activates CR3 but also influences its membrane expression. Both phenomena are affected by inhibitors of Src, PLCγ, Syk, and actin polymerization. Additionally, after only 10 min at 37 °C, cells with crosslinked CD13 start secreting pro-inflammatory cytokines like interferons type 1 and 2, IL-12p70, and IL-17a. We integrated our data with a bioinformatic analysis to confirm the connection between these receptors and to suggest the signaling cascade linking them. Our findings expand the list of features of CD13 by adding the activation of a different receptor via inside-out signaling. This opens the possibility of studying the joint contribution of CD13 and CR3 in contexts where either receptor has a recognized role, such as the progression of some leukemias.

Development of Hydroxamate Derivatives Containing a Pyrazoline Moiety as APN Inhibitors to Overcome Angiogenesis

Aminopeptidase N (APN) was closely associated with cancer invasion, metastasis, and angiogenesis. Therefore, APN inhibitors have attracted more and more attention of scientists as antitumor agents. In the current study, we designed, synthesized, and evaluated one new series of pyrazoline-based hydroxamate derivatives as APN inhibitors. Moreover, the structure-activity relationships of those were discussed in detail. 2,6-Dichloro substituted compound 14o with R₁ = CH₃, showed the best capacity for inhibiting APN with an IC₅₀ value of 0.0062 ± 0.0004 μM, which was three orders of magnitude better than that of the positive control bestatin. Compound 14o possessed both potent anti-proliferative activities against tumor cells and potent anti-angiogenic activity. At the same concentration of 50 μM, compound 14o exhibited much better capacity for inhibiting the micro-vessel growth relative to bestatin in the rat thoracic aorta ring model. Additionally, the putative interactions of 14o with the active site of APN are also discussed. The hydroxamate moiety chelated the zinc ion and formed four hydrogen bonds with His²⁹⁷, Glu²⁹⁸ and His³⁰¹. Meanwhile, the terminal phenyl group and another phenyl group of 14o interacted with S₂' and S₁ pockets via hydrophobic effects, respectively.

Leucine ureido derivatives as aminopeptidase N inhibitors using click chemistry. Part II

Aminopeptidase N (APN) has been proved to be deeply associated with cancer angiogenesis, metastasis and invasion. Therefore, APN gains increasing attention as a promising anti-tumor target. In the current study, we report the design, synthesis, biological evaluation and structure-activity relationship of one new series of leucine ureido derivatives containing the 1,2,3-triazole moiety. Among them, compound 31f was identified as the best APN inhibitor with IC₅₀ value being two orders of magnitude lower than that of the positive control bestatin. Compound 31f possessed selective cytotoxicity to several tumor cell lines over the normal cell line human umbilical vein endothelial cells (HUVECs). Notably, when combined with 5-fluorouracil (5-Fu), 31f exhibited synergistic anti-proliferation effect against several tumor cell lines. At the same concentration, 31f exhibited much better anti-angiogenesis activities than bestatin in the HUVECs capillary tube formation assay and the rat thoracic aorta rings test. In the in vitro anti-invasion assay, 31f also exhibited superior potency over bestatin. Moreover, considerable in vivo antitumor potencies of 31f alone or in combination with 5-Fu were observed without significant toxic signs in a mouse heptoma H22 tumor transplant model.

Nano-delivery system targeting to cancer stem cell cluster of differentiation biomarkers

Cancer stem cells (CSCs) are one of the most important origins of cancer progression and metastasis. CSCs have unique self-renewal properties and diverse cell membrane receptors that induced the resistance to the conventional chemotherapeutic agents. Therefore, the therapeutic removal of CSCs could result in the cancer cure with lack of recurrence and metastasis. In this regard, targeting CSCs in accordance to their specific biomarkers is a talented attitude in cancer therapy. Various CSCs surface biomarkers have been described, which some of them exhibited similarities on different cancer cell types, while the others are cancer specific and have just been reported on one or a few types of cancers. In this review, the importance of CSCs in cancer development and therapeutic response has been stated. Different CSCs cluster of differentiation (CD) biomarkers and their specific function and applications in the treatment of cancers have been discussed, Special attention has been made on targeted nano-delivery systems. In this regard, several examples have been illustrated concerning specific natural and artificial ligands against CSCs CD biomarkers that could be decorated on various nanoparticulated drug delivery systems to enhance therapeutic index of chemotherapeutic agents or anticancer gene therapy. The outlook of CSCs biomarkers discovery and therapeutic/diagnostic applications was discussed.

Inhibition of p38 MAPK Phosphorylation Is Critical for Bestatin to Enhance ATRA-Induced Cell Differentiation in Acute Promyelocytic Leukemia NB4 Cells

Bestatin has been known as an immunomodulating agent in anti-leukemia treatment. The mechanism by which Bestatin enhances all-trans retinoic acid (ATRA)-induced cell differentiation of acute promyelocytic leukemia (APL) cells is generally attributed to inhibition of cell surface CD13/aminopeptidase N activity. Bestatin also exerts its biological activities besides its ability to inhibit aminopeptidase N enzymatic activity. This article provides data to support an alternative mechanism regarding an important role of inhibition of p38 mitogen-activated protein kinase (MAPK) signal pathway in Bestatin's anti-leukemia effect. Bestatin enhanced ATRA-induced differentiation and inhibited ATRA-driven phosphorylation of p38 MAPK in ATRA-sensitive APL NB4 cells. In contrast, Bestatin could not reverse the differentiation block in ATRA-resistant APL MR2 cells, in which ATRA was unable to induce phosphorylation of p38 MAPK. Moreover, CD13 ligation with anti-CD13 antibody WM-15 resulted in phosphorylation of p38 MAPK, reduced the inhibition of Bestatin on the phosphorylation of p38 MAPK, and completely abolished the enhancement of Bestatin on ATRA-inducing differentiation in NB4 cells. This study shows that inhibition of p38 MAPK phosphorylation is critical for Bestatin to enhance ATRA-induced cell differentiation in ATRA-sensitive APL NB4 cells. Results suggested that pharmacological inhibition of the p38 MAPK pathway might enhance ATRA-dependent differentiation.

Identification of APN/CD13 as the target antigen of FU3, a human monoclonal antibody that recognizes malignant fibrous histiocytoma

Malignant fibrous histiocytoma (MFH), a high-grade, undifferentiated sarcoma, is highly aggressive, resistant to radiochemotherapy and associated with poor prognosis. There are no specific immunohistochemical markers for its diagnosis. The MFH cell line SFT7913 served as and immunogen for the generation of the FU3 monoclonal antibody in our laboratory. FU3 reacted strongly with MFH cells and with perivascular mesenchymal cells. In this study, we demonstrated that the antigen recognized by FU3 was identical to aminopeptidase N (APN/CD13) using FU3 immunoaffinity chromatography and N-terminal amino acid sequencing. Frequent (80%) and high-grade (>50% of cells) expression of APN/CD13 was observed in MFH, although low-grade expression was seen in some other sarcomas. Moreover, small interfering RNA (siRNA) that specifically targets APN/CD13 significantly suppressed MFH cell invasion in vitro. The newly developed monoclonal antibody FU3 specifically recognizes CD13 on MFH cells. Decreased expression of CD13, mediated by siRNA-mediated knockdown, attenuated the invasive capacity of MFH cells. Thus, results indicate that APN/CD13 could be an important diagnostic biomarker and therapeutic target for MFH.

Structural basis for multifunctional roles of mammalian aminopeptidase N

Mammalian aminopeptidase N (APN) plays multifunctional roles in many physiological processes, including peptide metabolism, cell motility and adhesion, and coronavirus entry. Here we determined crystal structures of porcine APN at 1.85 Å resolution and its complexes with a peptide substrate and a variety of inhibitors. APN is a cell surface-anchored and seahorse-shaped zinc-aminopeptidase that forms head-to-head dimers. Captured in a catalytically active state, these structures of APN illustrate a detailed catalytic mechanism for its aminopeptidase activity. The active site and peptide-binding channel of APN reside in cavities with wide openings, allowing easy access to peptides. The cavities can potentially open up further to bind the exposed N terminus of proteins. The active site anchors the N-terminal neutral residue of peptides/proteins, and the peptide-binding channel binds the remainder of the peptides/proteins in a sequence-independent fashion. APN also provides an exposed outer surface for coronavirus binding, without its physiological functions being affected. These structural features enable APN to function ubiquitously in peptide metabolism, interact with other proteins to mediate cell motility and adhesion, and serve as a coronavirus receptor. This study elucidates multifunctional roles of APN and can guide therapeutic efforts to treat APN-related diseases.

LYP, a bestatin dimethylaminoethyl ester, inhibited cancer angiogenesis both in vitro and in vivo

Our previous study revealed that LYP, a bestatin dimethylaminoethyl ester, inhibited the growth of human ovarian carcinoma ES-2 xenografts in mice and suppressed aminopeptidase N (APN/CD13) activity more potently than bestatin. In this study, we examined the inhibitory effect of LYP on migration and formation of capillary tube of human umbilical vascular endothelial cells (HUVECs) in vitro and anti-angiogenesis in ES-2 xenografts in mice. LYP did not possess cytotoxicity to HUVEC proliferation according to the MTT assay and trypan blue exclusion assay. However, APN/CD13 activity on cell surface of HUVECs was suppressed in the presence of LYP as measured by quantifying the enzymatic cleavage of the substrate l-leucine-p-nitroanilide. The assays of scratch and transwell chamber showed that LYP significantly inhibited HUVEC migration and invasion through Matrigel coated polycarbonate filters. Capillary tube formation assay revealed that the number of branch points formed by HUVECs on 3-D Matrigel was reduced after incubation with LYP. The anti-angiogenesis of LYP was verified in ES-2 xenografts in mice. The mean vascular density (MVD) and mean vascular luminal diameter (MVLD) were markedly reduced by LYP after two weeks of intravenous injection as evaluated by CD34 immunohistochemical staining. LYP suppression of cancer angiogenesis was greater than that of bestatin. The inhibition of angiogenic molecules may involve in anti-angiogenesis of LYP. The levels of vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF) and transforming growth factor-alpha (TGF-α) were decreased in HUVECs and ES-2 xenografts after treatment with LYP as determined by Western blot analysis. These results indicated that the high efficacy of LYP may partially relate to the inhibition of angiogenesis.

Proprotein convertase PC7 enhances the activation of the EGF receptor pathway through processing of the EGF precursor

Although the processing profile of the membrane-bound epidermal growth factor precursor (pro-EGF) is tissue-specific, it has not been investigated at the cellular level nor have the cognate proteinases been defined. Among the proprotein convertases (PCs), only the membrane-bound PC7, the most ancient and conserved basic amino acid-specific PC family member, induces the processing of pro-EGF into an ∼115-kDa transmembrane form (EGF-115) at an unusual VHPR(290)↓A motif. Because site-directed mutagenesis revealed that Arg(290) is not critical, the generation of EGF-115 by PC7 is likely indirect. This was confirmed by testing a wide range of protease inhibitors, which revealed that the production of EGF-115 is most probably achieved via the activation by PC7 of a latent serine and/or cysteine protease(s). EGF-115 is more abundant at the cell surface than pro-EGF and is associated with a stronger EGF receptor (EGFR) activation, as evidenced by higher levels of phosphorylated ERK1/2. This suggests that the generation of EGF-115 represents a regulatory mechanism of juxtacrine EGFR activation. Thus, PC7 is distinct from the other PCs in its ability to enhance the activation of the cell surface EGFR.

The moonlighting enzyme CD13: old and new functions to target

Aminopeptidase N (CD13) is a widely expressed ectoenzyme with functions that do not always depend on its enzymatic activity: an aspect that has been overlooked. Numerous CD13-targeting tools have been developed in the last few years. Several of them are already undergoing clinical trials, and there are promising reports on the effectiveness of others in animal models of disease. However, their efficacy might be obscured by their effects on unrecognized functions of CD13, resulting in unexpected complications. The purpose of this review is (i) to discuss the various functions ascribed to CD13 and the possible mechanisms behind them and (ii) to consider some of the questions that need to be answered to achieve a better understanding of the biological relevance of these functions, a more precise interpretation of the results obtained after their manipulation and a more rational design of CD13-targeting agents.