Autologous extracellular cytochrome c is an endogenous ligand for leucine-rich alpha2-glycoprotein and beta-type phospholipase A2 inhibitor

Crystal structure of LRG1 and the functional significance of LRG1 glycan for LPHN2 activation

The serum glycoprotein leucine-rich ɑ-2-glycoprotein 1 (LRG1), primarily produced by hepatocytes and neutrophils, is a multifunctional protein that modulates various signaling cascades, mainly TGFβ signaling. Serum LRG1 and neutrophil-derived LRG1 have different molecular weights due to differences in glycosylation, but the impact of the differential glycan composition in LRG1 on its cellular function is largely unknown. We previously reported that LRG1 can promote both angiogenic and neurotrophic processes under hyperglycemic conditions by interacting with LPHN2. Here, we determined the crystal structure of LRG1, identifying the horseshoe-like solenoid structure of LRG1 and its four N-glycosylation sites. In addition, our biochemical and cell-biological analyses found that the deglycosylation of LRG1, particularly the removal of glycans on N325, is critical for the high-affinity binding of LRG1 to LPHN2 and thus promotes LRG1/LPHN2-mediated angiogenic and neurotrophic processes in mouse tissue explants, even under normal glucose conditions. Moreover, the intracavernous administration of deglycosylated LRG1 in a diabetic mouse model ameliorated vascular and neurological abnormalities and restored erectile function. Collectively, these data indicate a novel role of LRG1 glycans as molecular switches that can tune the range of LRG1's cellular functions, particularly the LRG1/LPHN2 signaling axis.

Paradoxical Roles of Leucine-Rich α2-Glycoprotein-1 in Cell Death and Survival Modulated by Transforming Growth Factor-Beta 1 and Cytochrome c

Leucine-rich α₂-glycoprotein-1 (LRG1) has been shown to impact both apoptosis and cell survival, pleiotropic effects similar to one of its known ligands, transforming growth factor-beta 1 (TGF-β1). Recent studies have given insight into the TGF-β1 signaling pathways involved in LRG1-mediated death versus survival signaling, i.e., canonical or non-canonical. Interaction of LRG1 with another ligand, extracellular cytochrome c (Cyt c), promotes cell survival, at least for lymphocytes. LRG1 has been shown to bind Cyt c with high affinity, higher than it binds TGF-β1, making it sensitive to small changes in the level of extracellular Cyt c within a microenvironment that may arise from cell death. Evidence is presented here that LRG1 can bind TGF-β1 and Cyt c simultaneously, raising the possibility that the ternary complex may present a signaling module with the net effect of signaling, cell death versus survival, determined by the relative extent to which the LRG1 binding sites are occupied by these two ligands. A possible role for LRG1 should be considered in studies where extracellular effects of TGF-β1 and Cyt c have been observed in media supplemented with LRG1-containing serum.

Intracellular leucine-rich alpha-2-glycoprotein-1 competes with Apaf-1 for binding cytochrome c in protecting MCF-7 breast cancer cells from apoptosis

Leucine-rich alpha-2-glycoprotein-1 (LRG1) has been shown to compete with apoptosis activating factor-1 (Apaf-1) for binding cytochrome c (Cyt c) and could play a role in inhibition of apoptosis. Employing MCF-7 breast cancer cells, we report that intracellular LRG1 does protect against apoptosis. Thus, cells transfected with the lrg1 gene and expressing higher levels of LRG1 were more resistant to hydrogen peroxide-induced apoptosis than parental cells, while cells in which LRG mRNA was knocked down by short hairpin (sh) RNA-induced degradation were more sensitive. The amount of Cyt c co-immunoprecipitated with Apaf-1 from the cytosol of apoptotic cells was inversely related to the level of LRG1 expression. In lrg1-transfected cells partially-glycosylated LRG1 was found in the cytosol and there was an increase in cytosolic Cyt c in live lrg1-transfected cells relative to parental cells. However, apoptosis was not spontaneously induced because Cyt c was bound to LRG1 and not to Apaf-1. Cyt c was the only detectable protein co-immunoprecipitated with LRG1. Following hydrogen peroxide treatment degradation of LRG1 allowed for induction of apoptosis. We propose that intracellular LRG1 raises the threshold of cytoplasmic Cyt c required to induce apoptosis and, thus, prevents onset of the intrinsic pathway in cells where Cyt c release from mitochondria does not result from committed apoptotic signaling. This mechanism of survival afforded by LRG1 is likely to be distinct from its extracellular survival function that has been reported by several research groups.

Identification, description and structural analysis of beta phospholipase A2 inhibitors (sbβPLIs) from Latin American pit vipers indicate a binding site region for basic snake venom phospholipases A2

Several snake species possess, in their circulating blood, endogenous PLA₂ inhibitors (sbPLIs) with the primary function of natural protection against toxic enzymes from homologous and heterologous venoms. Among the three structural classes of sbPLIs – named α, β, and γ − the β class (sbβPLIs) is the least known with only four identified sequences, so far. The last class of inhibitors encompass molecules with leucine rich repeats (LRRs) motifs containing repeating amino acid segments. In the present study, we identified and characterized putative sbβPLIs from the liver and venom glands of six Latin American pit vipers belonging to Bothrops and Crotalus genera. The inhibitor from Crotalus durissus terrificus snakes (CdtsbβPLI) was chosen as a reference for the construction of the first in silico structural model for this class of inhibitors, using molecular modeling and molecular dynamics simulations. Detailed analyses of the electrostatic surface of the CdtsbβPLI model and protein-protein docking with crotoxin B from homologous venoms predict the interacting surface between these proteins.

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Transcripts of phospholipases A₂ inhibitors from the β-class (sbβPLIs) were identified in Latin American pit vipers.

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Structural features of sbβPLIs were compared and discussed, including their characteristic leucine-rich repeats (LRRs).

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One sbβPLI (CdtsbβPLI) was chosen for the in silico construction of the first structural model of a sbβPLI.

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A possible mechanism of interaction between sbβPLIs and basic snake venom PLA₂s was suggested.

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Docking predictions between CdtsbβPLI and crotoxin B highlighted the amino acids residues at the interaction surfaces.

Leucine-rich alpha-2-glycoprotein-1 is up-regulated in colorectal cancer and is a tumor promoter

Background

Leucine-rich α-2-glycoprotein-1 (LRG1) is differentially expressed in many kinds of diseases including cancer, however, it has not been thoroughly studied yet.

Purpose

The objective of this study was to detect the expression and potential mechanism of LRG1 in colorectal cancer (CRC). In our study, we examined LRG1 levels in CRC tissue and plasma with quantitative real-time polymerase chain reaction and enzyme-linked immunosorbent assay, respectively. The effect of LRG1 on cancer cells was detected with transwell and MTT assays.

Results

The average plasma LRG1 level in CRC was significantly higher than in polyp group (P=0.002) and healthy controls (P<0.001). Second, plasma LRG1 was positively associated with CA19-9 (r=0.133, P=0.039) and neutrophil ratio (r=0.403, P<0.001). Third, plasma LRG1 of stage IV patients was dramatically different from that of stage I, stage II or stage III patients (P<0.001). LRG1 mRNA expression levels were about 2-fold higher in CRCs compared to normal tissues (P<0.001). And levels of plasma LRG1 were found to be a risk factor in CRC in univariate survival analysis of colorectal prognosis (P=0.013, hazard ratio [HR]=1.803, 95% CI: 1.521-2.137), and multivariate analysis showed that LRG1 was an independent risk factor (P<0.001, HR=1.492, 95% CI: 1.223-1.820). The patients with higher plasma LRG1 value presented with poorer outcome (P=0.013). Functional experiments showed that LRG1 could promote the invasion and growth ability of cells. LRG1 was increased in plasma and tissue compared with that of controls and LRG1 may predict prognosis of CRC patients and LRG1 maybe a tumor promoter.

Conclusion

LRG1 is increased in CRC patients and might serve as a tumor promoter.

Leucine Rich α-2 Glycoprotein: A Novel Neutrophil Granule Protein and Modulator of Myelopoiesis

Leucine-rich α2 glycoprotein (LRG1), a serum protein produced by hepatocytes, has been implicated in angiogenesis and tumor promotion. Our laboratory previously reported the expression of LRG1 in murine myeloid cell lines undergoing neutrophilic granulocyte differentiation. However, the presence of LRG1 in primary human neutrophils and a role for LRG1 in regulation of hematopoiesis have not been previously described. Here we show that LRG1 is packaged into the granule compartment of human neutrophils and secreted upon neutrophil activation to modulate the microenvironment. Using immunofluorescence microscopy and direct biochemical measurements, we demonstrate that LRG1 is present in the peroxidase-negative granules of human neutrophils. Exocytosis assays indicate that LRG1 is differentially glycosylated in neutrophils, and co-released with the secondary granule protein lactoferrin. Like LRG1 purified from human serum, LRG1 secreted from activated neutrophils also binds cytochrome c. We also show that LRG1 antagonizes the inhibitory effects of TGFβ1 on colony growth of human CD34⁺ cells and myeloid progenitors. Collectively, these data invoke an additional role for neutrophils in innate immunity that has not previously been reported, and suggest a novel mechanism whereby neutrophils may modulate the microenvironment via extracellular release of LRG1.

Isolation and biochemical characterization of a gamma-type phospholipase A2 inhibitor from Macropisthodon rudis snake serum

A novel phospholipaseA₂ (PLA₂) inhibitory protein (PLI) was purified from the serum of Macropisthodon rudis, a non-venomous snake mainly found in southern China. The molecular mass of the purified PLI was 160 kDa as determined by Superdex 200HR; however, the PLI protein had only one subunit of 25.4 kDa as determined by 12% SDS-PAGE, indicating an oligomeric protein. PLI cDNA obtained by PCR from the liver of Macropisthodon rudis, revealed 549 bps coding for a mature protein of 183 amino acid residues. Based on an amino acid sequence alignment with venomous and non-venomous snakes, this inhibitor was determined to be in the γ type family of PLI. In vitro experiments showed that PLIγ inhibited enzymatic, inflammatory, and antibacterial activities of snake venom PLA₂ isolated from Agkistrodon acutus.

[Relationships between venomous function and innate immune function]

Venomous function is investigated in relation to innate immune function in two cases selected from scorpion venom and serpent venom. In the first case, structural analysis of scorpion toxins and defensins reveals a close interrelation between both functions (toxic and innate immune system function). In the second case, structural and functional studies of natural inhibitors of toxic snake venom phospholipases A2 reveal homology with components of the innate immune system, leading to a similar conclusion. Although there is a clear functional distinction between neurotoxins, which act by targeting membrane ion channels, and the circulating defensins which protect the organism from pathogens, the scorpion short toxins and defensins share a common protein folding scaffold with a conserved cysteine-stabilized alpha-beta motif of three disulfide bridges linking a short alpha helix and an antiparallel beta sheet. Genomic analysis suggests that these proteins share a common ancestor (long venom toxins were separated from an early gene family which gave rise to separate short toxin and defensin families). Furthermore, a scorpion toxin has been experimentally synthetized from an insect defensin, and an antibacterial scorpion peptide, androctonin (whose structure is similar to that of a cone snail venom toxin), was shown to have a similar high affinity for the postsynaptic acetylcholine receptor of Torpedo sp. Natural inhibitors of phospholipase A2 found in the blood of snakes are associated with the resistance of venomous snakes to their own highly neurotoxic venom proteins. Three classes of phospholipases A2 inhibitors (PLI-α, PLI-β, PLI-γ) have been identified. These inhibitors display diverse structural motifs related to innate immune proteins including carbohydrate recognition domains (CRD), leucine rich repeat domains (found in Toll-like receptors) and three finger domains, which clearly differentiate them from components of the adaptive immune system. Thus, in structure, function and phylogeny, venomous function in both vertebrates and invertebrates are clearly interrelated with innate immune function.

Clinicopathological Significance of Leucine-Rich α2-Glycoprotein-1 in Sera of Patients With Pancreatic Cancer

OBJECTIVES

Leucine-rich α2-glycoprotein-1 (LRG-1) is an inflammatory protein. Serum LRG-1 levels can reportedly be used as a cancer biomarker for several types of carcinoma. In the present study, we investigated the clinical usefulness of serum LRG-1 levels as a biomarker of pancreatic cancer.

METHODS

A total of 124 patients with pancreatic cancer, 35 patients with chronic pancreatitis (CP), and 144 healthy volunteers were enrolled in the study. Serum LRG-1 levels were assayed by enzyme-linked immunosorbent assay. Immunohistochemistry was used to examine LRG-1 expression in pancreatic cancer tissues.

RESULTS

Serum LRG-1 levels were significantly increased in patients with pancreatic cancer compared with CP patients and healthy volunteers. The LRG-1 levels increased with progressive clinical stages of pancreatic cancer. Receiver operator curve analysis showed that a combination of carbohydrate antigen 19-9 and LRG-1 resulted in a higher area under the curve for the diagnosis of pancreatic cancer. Positive staining was observed in all cases of pancreatic cancer, but positive signal was scarcely detected in tissues from CP patients or normal surrounding tissue.

CONCLUSIONS

These results suggest that serum LRG-1 is a promising biomarker for pancreatic cancer.

Brain localization of leucine-rich α2-glycoprotein and its role

OBJECTIVES

We have previously reported that the level of leucine-rich alpha-2-glycoprotein (LRG) expression is specifically increased in cerebrospinal fluid (CSF) of idiopathic normal pressure hydrocephalus (INPH). The objective of this study is to examine the localization of LRG - the cerebral areas where it is expressed.

METHOD

The histological sections of autopsied brain specimens from ten subjects, five adult cases (mean age 43.6 years; range 34-50 years) and five senile cases (mean age 76.0 years; range 67-88 years) were prepared, multistained with antibodies against human LRG, glial fibrillary acidic protein (GFAP), CD31, and aquaporin-4 (AQP4), and reviewed for the expression sites of LRG.

RESULTS

Immunostains of GFAP and LRG were compared in standard brain specimens from elderly patients. The results indicated that LRG is distributed throughout the entire brain, with especially high expression in the deep cerebral cortex. In addition, the cells that express LRG showed similar morphology to astrocytes. Double staining of CD31 and LRG revealed a significant expression of LRG in the pericapillary regions. The expression was observed in resident astrocytes, as well as in the capillary vessel to which astrocytic processes grow and adhere. When age-related comparisons were made between senile and adult specimens, LRG expression increased with age.

CONCLUSION

LRG expression in resident astrocytes increased with age.

Biomarker discovery in serum from patients with carotid atherosclerosis

BACKGROUND

Blood-based biomarkers of atherosclerosis have been used to identify patients at high risk for developing stroke. We hypothesized that patients with carotid artery disease would have a distinctive proteomic signature in blood as compared to a healthy control population without carotid artery disease. In order to discover protein biomarkers associated with increased atherosclerotic risk, we used two different strategies to identify biomarkers from patients with clinically defined atherosclerosis who were undergoing endarterectomy for atherosclerotic carotid artery disease. These patients were compared with healthy matched controls.

METHODS

Serum was obtained from patients undergoing endarterectomy (EA; n = 38) and compared to a group of age-matched healthy controls (n = 40). Serum was fractionated using anion exchange chromatography and three different surface-enhanced laser desorption/ionization (SELDI) chip surfaces and then evaluated with mass spectrometry (MS) and two-dimensional difference gel electrophoresis (2D-DIGE).

RESULTS

A random forest (RF) analysis of the SELDI-MS protein peak data distinguished these two groups with 69.2% sensitivity and 73.2% specificity. Four unique SELDI peaks (4.2, 4.4, 16.7 and 28 kDa, all p< 0.01) showed the greatest influence in the RF model. The EA patients with a history of prior clinical atherosclerotic plaque rupture manifested as either stroke or transient ischemic attack (symptomatic; n = 16) were compared to patients with carotid atherosclerosis but no clinical evidence of plaque rupture (asymptomatic; n = 22). Analysis of the SELDI spectra did not separate these two patient subgroups. A subgroup analysis using 2D-DIGE images obtained from albumin-depleted serum comparing symptomatic (n = 10) to asymptomatic EA patients (n = 10) found 4 proteins that were differentially expressed (p < 0.01) in the symptomatic patients. These proteins were identified as α(1)-antitrypsin, haptoglobin and vitamin D binding protein that were downregulated and α(2)-glycoprotein precursor that was upregulated in the symptomatic EA group.

CONCLUSIONS

SELDI-MS data analysis of fractionated serum suggests that a distinct protein signature exists in patients with carotid atherosclerosis compared to age-matched healthy controls. Identification of 4 proteins in a subset of patients with symptomatic and asymptomatic carotid atherosclerosis suggests that these and other protein biomarkers may assist in identifying high-risk patients with carotid atherosclerosis.