Membrane-bound carboxypeptidase E facilitates the entry of eosinophil cationic protein into neuroendocrine cells

Dissecting carboxypeptidase E: properties, functions and pathophysiological roles in disease

Since discovery in 1982, carboxypeptidase E (CPE) has been shown to be involved in the biosynthesis of a wide range of neuropeptides and peptide hormones in endocrine tissues, and in the nervous system. This protein is produced from pro-CPE and exists in soluble and membrane forms. Membrane CPE mediates the targeting of prohormones to the regulated secretory pathway, while soluble CPE acts as an exopeptidase and cleaves C-terminal basic residues from peptide intermediates to generate bioactive peptides. CPE also participates in protein internalization, vesicle transport and regulation of signaling pathways. Therefore, in two types of CPE mutant mice, Cpe^fat/Cpe^fat and Cpe knockout, loss of normal CPE leads to a lot of disorders, including diabetes, hyperproinsulinemia, low bone mineral density and deficits in learning and memory. In addition, the potential roles of CPE and ΔN-CPE, an N-terminal truncated form, in tumorigenesis and diagnosis were also addressed. Herein, we focus on dissecting the pathophysiological roles of CPE in the endocrine and nervous systems, and related diseases.

Molecular imaging of heparan sulfate expression with radiolabeled recombinant eosinophil cationic protein predicts allergic lung inflammation in a mouse model for asthma

Heparan sulfate proteoglycans (HSPGs) are glycoproteins consisting of a core protein to which linear heparan sulfate (HS) side chains are covalently attached. These HS side chains mediate a variety of biologic functions involved in inflammation. Radionuclide imaging of HS side chains in tissues with inflammation may be used for the stratification of patients who would most likely benefit from HSPG-targeting therapy. The goal of this study was to evaluate the feasibility of in vivo radionuclide imaging of HS side chain expression in a mouse model of asthma using the recombinant eosinophil cationic protein (rECP).

METHODS

rECP was radioiodinated with (125)I or (123)I using the Chloramine-T method. The 50% inhibitory concentration value for (125)I-labeled rECP was determined in a competitive cell-binding assay using Beas-2B cells. The binding of radiolabeled rECP to HS side chains was evaluated both in vitro and in vivo. The biodistribution of radiolabeled rECP was assessed in asthma mice or in control mice using SPECT imaging, ex vivo biodistribution measurements, and microautoradiography.

RESULTS

The 50% inhibitory concentration value for (125)I-rECP was 7.4 ± 0.1 nM. The loss of HS side chains substantially inhibited the cellular and tissue uptake of (125)I- or (123)I-rECP, indicating that HS side chains of HSPGs are required for (125)I- or (123)I-eosinophil cationic protein binding and uptake both in vitro and in vivo. SPECT imaging demonstrated an appreciably higher accumulation of radioactivity in the lungs of asthma mice than in those of control mice. Ex vivo biodistribution studies also confirmed that there was at least a 4-fold increase in the lung-to-muscle ratio of asthma mice, compared with control mice. The accumulation of radiolabeled rECP was linearly correlated with leukocyte infiltration.

CONCLUSION

This study illustrates the feasibility of using radiolabeled rECP for the visualization of HS side chains of HSPGs and the evaluation of allergic lung inflammation in living subjects. Our data indicate that radiolabeled rECP is a novel imaging agent for HS side chains of HSPGs in predicting allergic lung inflammation in living mice.

New roles of carboxypeptidase E in endocrine and neural function and cancer

Carboxypeptidase E (CPE) or carboxypeptidase H was first discovered in 1982 as an enkephalin-convertase that cleaved a C-terminal basic residue from enkephalin precursors to generate enkephalin. Since then, CPE has been shown to be a multifunctional protein that subserves many essential nonenzymatic roles in the endocrine and nervous systems. Here, we review the phylogeny, structure, and function of CPE in hormone and neuropeptide sorting and vesicle transport for secretion, alternative splicing of the CPE transcript, and single nucleotide polymorphisms in humans. With this and the analysis of mutant and knockout mice, the data collectively support important roles for CPE in the modulation of metabolic and glucose homeostasis, bone remodeling, obesity, fertility, neuroprotection, stress, sexual behavior, mood and emotional responses, learning, and memory. Recently, a splice variant form of CPE has been found to be an inducer of tumor growth and metastasis and a prognostic biomarker for metastasis in endocrine and nonendocrine tumors.

LL37 and hBD-3 elevate the β-1,3-exoglucanase activity of Candida albicans Xog1p, resulting in reduced fungal adhesion to plastic

The opportunistic fungus Candida albicans causes oral thrush and vaginal candidiasis, as well as candidaemia in immunocompromised patients including those undergoing cancer chemotherapy, organ transplant and those with AIDS. We previously found that the AMPs (antimicrobial peptides) LL37 and hBD-3 (human β-defensin-3) inhibited C. albicans viability and its adhesion to plastic. For the present study, the mechanism by which LL37 and hBD-3 reduced C. albicans adhesion was investigated. After AMP treatment, C. albicans adhesion to plastic was reduced by up to ~60% and was dose-dependent. Our previous study indicated that LL37 might interact with the cell-wall β-1,3-exoglucanase Xog1p, which is involved in cell-wall β-glucan metabolism, and consequently the binding of LL37 or hBD-3 to Xog1p might cause the decrease in adhesion. For the present study, Xog1p(41-438)-6H, an N-terminally truncated, active, recombinant construct of Xog1p and Xog1p fragments were produced and used in pull-down assays and ELISA in vitro, which demonstrated that all constructs interacted with both AMPs. Enzymatic analyses showed that LL37 and hBD-3 enhanced the β-1,3-exoglucanase activity of Xog1p(41-438)-6H approximately 2-fold. Therefore elevated Xog1p activity might compromise cell-wall integrity and decrease C. albicans adhesion. To test this hypothesis, C. albicans was treated with 1.3 μM Xog1p(41-438)-6H and C. albicans adhesion to plastic decreased 47.7%. Taken together, the evidence suggests that Xog1p is one of the LL37/hBD-3 targets, and elevated β-1,3-exoglucanase activity reduces C. albicans adhesion to plastic.

Leukocyte-cancer cell fusion: initiator of the warburg effect in malignancy?

The causes of metastasis remain unknown, however it has been proposed for nearly a century that metastatic cells are generated by fusion of tumor cells with tumor-associated leukocytes such as macrophages. Indeed, regardless of cell or tissue origin, when cancer cells in the original in situ tumor transform to malignant, invasive cells, they generally become aneuploid and begin to express molecules and traits characteristic of activated macrophages. This includes two key features of malignancy: chemotactic motility and the use of aerobic glycolysis as a metabolic energy source (the Warburg effect). Here we review evidence that these phenomena can be well-explained by macrophage-cancer cell fusion, as evidenced by studies of experimental macrophage-melanoma hybrids generated in vitro and spontaneous host-tumor hybrids in animals and more recently humans. A key finding to emerge is that experimental and spontaneous cancer cell hybrids alike displayed a high degree of constitutive autophagy, a macrophage trait that is expressed under hypoxia and nutrient deprivation as part of the Warburg effect. Subsequent surveys of 21 different human cancers from nearly 2,000 cases recently revealed that the vast majority (~85%) exhibited autophagy and that this was associated with tumor proliferation and metastasis. While much work needs to be done, we posit that these findings with human cancers could be a reflection of widespread leukocyte-cancer cell fusion as an initiator of metastasis. Such fusions would generate hybrids that express the macrophage capabilities for motility and survival under adverse conditions of hypoxia and nutrient deprivation, while at the same time maintaining the deregulated mitotic cycle of the cancer cell fusion partner.

Inhibition of the interactions between eosinophil cationic protein and airway epithelial cells by traditional Chinese herbs

Background

The eosinophil cationic protein (ECP) is cytotoxic to bacteria, viruses, parasites and mammalian cells. Cells are damaged via processes of pore formation, permeability alteration and membrane leaking. Some clinical studies indicate that ECP gathers in the bronchial tract of asthma sufferers, damages bronchial and airway epithelial cells, and leads to in breathing tract inflammation; therefore, prevention of the cytotoxicity caused by ECP may serve as an approach to treat airway inflammation. To achieve the purpose, reduction of the ECP-cell interactions is rational. In this work, the Chinese herbal combinative network was generated to predict and identify the functional herbs from the pools of prescriptions. It was useful to select the node herbs and to demonstrate the relative binding ability between ECP and Beas-2B cells with or withour herb treatments.

Results

Eighty three Chinese herbs and prescriptions were tested and five effective herbs and six prescription candidates were selected. On the basis of effective single-herbal drugs and prescriptions, a combinative network was generated. We found that a single herb, Gan-cao, served as a node connecting five prescriptions. In addition, Sheng-di-huang, Dang-guei and Mu-tong also appeared in five, four and three kinds of prescriptions, respectively. The extracts of these three herbs indeed effectively inhibited the interactions between ECP and Beas-2B cells. According to the Chinese herbal combinative network, eight of the effective herbal extracts showed inhibitory effects for ECP internalizing into Beas-2B cells. The major components of Gang-cao and Sheng-di-huang, glycyrrhizic acid and verbascose, respectively, reduced the binding affinity between ECP and cells effectively.

Conclusions

Since these Chinese herbs reduced the binding affinity between ECP and cells and inhibited subsequent ECP entrance into cells, they were potential for mitigating the airway inflammation symptoms. Additionally, we mentioned a new concept to study the Chinese herbs using combinative network in the field of systems biology. The functional single herbs could be identified from the set of prescriptions.

Contactin-associated protein (Caspr) 2 interacts with carboxypeptidase E in the CNS

To identify proteins interacting with the intracellular domain of the neural cell adhesion molecule contactin-associated protein 2 (Caspr2), yeast two-hybrid screening was performed. We identified carboxypeptidase E (CPE) as a Caspr2-interacting candidate protein. Glutathione S-transferase pull-down and immunoprecipitation analyses indicated that Caspr2 was associated with CPE in vitro and in vivo. Both Caspr2 and CPE were expressed predominantly in the CNS. Immunohistochemical analyses revealed that both Caspr2- and CPE-like immunoreactivities were found to co-localize in the apical dendrites and cell bodies of rat cortical neurons. In subcellular localization analysis, Caspr2- and CPE-like immunoreactivities were co-migrated in the fractions of Golgi/ER. Additionally, in COS-7 cells co-transfected with CPE and Caspr2 cDNAs, Caspr2- and CPE-immunoreactivities were co-localized in both Golgi and membrane, whereas it was only observed in Golgi of either COS-7 cell transfected with CPE or Caspr2 cDNA alone. It is known that the membrane-bound form of CPE functions as a sorting receptor of prohormones in the trans-Golgi network. Taken together, our data suggest that CPE may be a key molecule to regulate Caspr2 trafficking to the cell membrane.

How peptide hormone vesicles are transported to the secretion site for exocytosis

Post-Golgi transport of peptide hormone-containing vesicles from the site of genesis at the trans-Golgi network to the release site at the plasma membrane is essential for activity-dependent hormone secretion to mediate various endocrinological functions. It is known that these vesicles are transported on microtubules to the proximity of the release site, and they are then loaded onto an actin/myosin system for distal transport through the actin cortex to just below the plasma membrane. The vesicles are then tethered to the plasma membrane, and a subpopulation of them are docked and primed to become the readily releasable pool. Cytoplasmic tails of vesicular transmembrane proteins, as well as many cytosolic proteins including adaptor proteins, motor proteins, and guanosine triphosphatases, are involved in vesicle budding, the anchoring of the vesicles, and the facilitation of movement along the transport systems. In addition, a set of cytosolic proteins is also necessary for tethering/docking of the vesicles to the plasma membrane. Many of these proteins have been identified from different types of (neuro)endocrine cells. Here, we summarize the proteins known to be involved in the mechanisms of sorting various cargo proteins into regulated secretory pathway hormone-containing vesicles, movement of these vesicles along microtubules and actin filaments, and their eventual tethering/docking to the plasma membrane for hormone secretion.

A bi-directional carboxypeptidase E-driven transport mechanism controls BDNF vesicle homeostasis in hippocampal neurons

Anterograde transport of brain-derived neurotrophic factor (BDNF) vesicles from the soma to neurite terminals is necessary for activity-dependent secretion of BDNF to mediate synaptic plasticity, memory and learning, and retrograde BDNF transport back to the soma for recycling. In our study, overexpression of the cytoplasmic tail of the carboxypeptidase E (CPE) found in BDNF vesicles significantly reduced localization of BDNF in neurites of hippocampal neurons. Live-cell imaging showed that the velocity and distance of movement of fluorescent protein-tagged CPE- or BDNF-containing vesicles were reduced in both directions. In pulldown assays, the CPE tail interacted with dynactin along with kinesin-2 and kinesin-3, and cytoplasmic dynein. Competition assays using a CPE tail peptide verified specific interaction between the CPE tail and dynactin. Thus, the CPE cytoplasmic tail binds dynactin that recruits kinesins or dynein for driving bi-directional transport of BDNF vesicle to maintain vesicle homeostasis and secretion in hippocampal neurons.

Carboxypeptidase E cytoplasmic tail-driven vesicle transport is key for activity-dependent secretion of peptide hormones

Vesicular transport of peptide hormones from the cell body to the plasma membrane for activity-dependent secretion is important for endocrine function, but how it is achieved is unclear. Here we uncover a mechanism in which the cytoplasmic tail of transmembrane carboxypeptidase E (CPE) found in proopiomelanocotin (POMC)/ACTH vesicles interacts with microtubule-based motors to control transport of these vesicles to the release site in pituitary cells. Overexpression of the CPE tail in live cells significantly reduced the velocity and distance of POMC/ACTH- and CPE-containing vesicle movement into the cell processes. Biochemical studies showed that the CPE tail interacted with dynactin, which, in turn, recruited microtubule plus-end motors kinesin 2 and kinesin 3. Overexpression of the CPE tail inhibited the stimulated secretion of ACTH from AtT20 cells. Thus, the CPE cytoplasmic tail interaction with dynactin-kinesin 2/kinesin 3 plays an important role in the transport of POMC vesicles for activity-dependent secretion.

A heparan sulfate-facilitated and raft-dependent macropinocytosis of eosinophil cationic protein

Eosinophil cationic protein (ECP), a human RNAseA superfamily member, highly implicated in asthma pathology, is toxic to bronchial epithelial cells following its endocytosis. The mechanism by which ECP is internalized into cells is poorly understood. In this study, we show that cell surface-bound heparan sulfate proteoglycans serve as the major receptor for ECP internalization. Removal of cell surface heparan sulfate by heparinases or reducing glycan sulfation by chlorate markedly decreased ECP binding to human bronchial epithelial Beas-2B cells. In addition, ECP uptake and associated cytotoxicity were reduced in glycosaminoglycan-defective cells compared with their wild-type counterparts. Furthermore, pharmacological treatment combined with siRNA knockdown identified a clathrin- and caveolin-independent endocytic pathway as the major route for ECP internalization. This pathway is regulated by Rac1 and ADP-ribosylating factor 6 GTPases. It requires cholesterol, actin cytoskeleton rearrangement and phosphatidylinositol-3-kinase activities, and is compatible with the characteristics of raft-dependent macropinocytosis. Thus, our results define the early events of ECP internalization and may have implications for novel therapeutic design for ECP-associated diseases.

Transcriptional regulation of human eosinophil RNases by an evolutionary- conserved sequence motif in primate genome

Background

Human eosinophil-derived neurotoxin (edn) and eosinophil cationic protein (ecp) are members of a subfamily of primate ribonuclease (rnase) genes. Although they are generated by gene duplication event, distinct edn and ecp expression profile in various tissues have been reported.

Results

In this study, we obtained the upstream promoter sequences of several representative primate eosinophil rnases. Bioinformatic analysis revealed the presence of a shared 34-nucleotide (nt) sequence stretch located at -81 to -48 in all edn promoters and macaque ecp promoter. Such a unique sequence motif constituted a region essential for transactivation of human edn in hepatocellular carcinoma cells. Gel electrophoretic mobility shift assay, transient transfection and scanning mutagenesis experiments allowed us to identify binding sites for two transcription factors, Myc-associated zinc finger protein (MAZ) and SV-40 protein-1 (Sp1), within the 34-nt segment. Subsequent in vitro and in vivo binding assays demonstrated a direct molecular interaction between this 34-nt region and MAZ and Sp1. Interestingly, overexpression of MAZ and Sp1 respectively repressed and enhanced edn promoter activity. The regulatory transactivation motif was mapped to the evolutionarily conserved -74/-65 region of the edn promoter, which was guanidine-rich and critical for recognition by both transcription factors.

Conclusion

Our results provide the first direct evidence that MAZ and Sp1 play important roles on the transcriptional activation of the human edn promoter through specific binding to a 34-nt segment present in representative primate eosinophil rnase promoters.

Carboxypeptidase E is required for normal synaptic transmission from photoreceptors to the inner retina

Defects in the gene encoding carboxypeptidase E (CPE) in either mouse or human lead to multiple endocrine disorders, including obesity and diabetes. Recent studies on Cpe-/- mice indicated neurological deficits in these animals. As a model system to study the potential role of CPE in neurophysiology, we carried out electroretinography (ERG) and retinal morphological studies on Cpe-/- and Cpe fat/fat mutant mice. Normal retinal morphology was observed by light microscopy in both Cpe-/- and Cpe(fat/fat) mice. However, with increasing age, abnormal retinal function was revealed by ERG. Both Cpe-/- and Cpe fat/fat animals had progressively reduced ERG response sensitivity, decreased b-wave amplitude and delayed implicit time with age, while maintaining a normal a-wave amplitude. Immunohistochemical staining showed specific localization of CPE in photoreceptor synaptic terminals in wild-type (WT) mice, but in both Cpe-/- and Cpe fat/fat mice, CPE was absent in this layer. Bipolar cell morphology and distribution were normal in these mutant mice. Electron microscopy of retinas from Cpe fat/fat mice revealed significantly reduced spherule size, but normal synaptic ribbons and synaptic vesicle density, implicating a reduction in total number of vesicles per synapse in the photoreceptors of these animals. These results suggest that CPE is required for normal-sized photoreceptor synaptic terminal and normal signal transmission to the inner retina.

Signal peptide of eosinophil cationic protein is toxic to cells lacking signal peptide peptidase

Eosinophil cationic protein (ECP) is a toxin secreted by activated human eosinophils. The properties of mature ECP have been well studied but those of the signal peptide of ECP (ECPsp) are not clear. In this study, several chimeric proteins containing N-terminal fusion of ECPsp were generated, and introduced into Escherichia coli, Pichia pastoris, and human epidermoid carcinoma cell line A431 to study the function of ECPsp. We found that expression of ECPsp chimeric proteins inhibited the growth of E. coli and P. pastoris but not A431 cells. Primary sequence analysis and in vitro transcription/translation of ECPsp have revealed that it is a potential substrate for human signal peptide peptidase (hSPP), an intramembrane protease located in endoplasmic reticulum. In addition, knockdown of the hSPP mRNA expression in ECPsp-eGFP/A431 cells caused the growth inhibitory effect, whereas complementally expression of hSPP in P. pastoris system rescued the cell growth. Taken together, we have demonstrated that ECPsp is a toxic signal peptide, and expression of hSPP protects the cells from growth inhibition.