Biosynthesis, intracellular processing and secretion of haptoglobin in cultured rat hepatocytes

Proteome profiling of low grade serous ovarian cancer

Background

Serous carcinoma, the subtype of ovarian cancer has the highest occurrence and mortality in women. Proteomic profiling using mass spectrometry (MS) has been used to detect biomarkers in tissue s obtained from patients with ovarian cancer.

Thus, this study aimed at analyzing the interactome (protein-protein interaction (PPI)) and (MS) data to inspect PPI networks in patients with Low grade serous ovarian cancer.

Methods

For proteome profiling in Low grade serous ovarian cancer, 2DE and mass spectrometry were used. Differentially expressed proteins which had been determined in Low grade serous ovarian cancer and experimental group separately were integrated with PPI data to construct the (QQPPI) networks.

Results

Six Hub-bottlenecks proteins with significant centrality values, based on centrality parameters of the network (Degree and between), were found including Transgelin (TAGLN), Keratin (KRT14), Single peptide match to actin, cytoplasmic 1(ACTB), apolipoprotein A-I (APOA1), Peroxiredoxin-2 (PRDX2), and Haptoglobin (HP).

Discussion

This study showed these six proteins were introduced as hub-bottleneck protein. It can be concluded that regulation of gene expression can have a critical role in the pathology of Low-grade serous ovarian cancer.

Neurotoxicity of microglial cathepsin D revealed by secretome analysis

Microglia-driven inflammatory responses have both neuroprotective and neurotoxic effects in the CNS. The excessive and chronic activation of microglia, however, may shift the balance towards neurotoxic effects. In this regard, proteins secreted from activated microglia likely play a key role in the neurotoxic effects. To characterize secreted proteins of activated microglia, conditioned media obtained from BV-2 mouse microglia cells were analyzed by two-dimensional gel electrophoresis or liquid chromatography coupled with tandem mass spectrometry. Among many proteins identified in the secretome of activated microglia, an aspartic endoprotease cathepsin D has been found to mediate microglial neurotoxicity based on the following results: (i) the expression of cathepsin D protein was markedly increased in lipopolysaccharide/interferon-γ-stimulated microglia compared with resting microglia as determined by western blot analysis of conditioned media; (ii) knockdown of cathepsin D expression in microglia using short hairpin RNA diminished the neurotoxicity in the coculture of microglia and neuroblastoma cells and (iii) recombinant procathepsin D protein exerted cytotoxic effects toward cultured neurons. In conclusion, cathepsin D appears to play a central role in the microglial neurotoxicity, and could be a potential biomarker or drug target for the diagnosis and treatment of neurodegenerative diseases that are associated with excessive microglial activation and subsequent neurotoxic inflammation.

Identification of human zonulin, a physiological modulator of tight junctions, as prehaptoglobin-2

Increased intestinal permeability (IP) has emerged recently as a common underlying mechanism in the pathogenesis of allergic, inflammatory, and autoimmune diseases. The characterization of zonulin, the only physiological mediator known to regulate IP reversibly, has remained elusive. Through proteomic analysis of human sera, we have now identified human zonulin as the precursor for haptoglobin-2 (pre-HP2). Although mature HP is known to scavenge free hemoglobin (Hb) to inhibit its oxidative activity, no function has ever been ascribed to its uncleaved precursor form. We found that the single-chain zonulin contains an EGF-like motif that leads to transactivation of EGF receptor (EGFR) via proteinase-activated receptor 2 (PAR(2)) activation. Activation of these 2 receptors was coupled to increased IP. The siRNA-induced silencing of PAR(2) or the use of PAR(2)(-/-) mice prevented loss of barrier integrity. Proteolytic cleavage of zonulin into its alpha(2)- and beta-subunits neutralized its ability to both activate EGFR and increase IP. Quantitative gene expression revealed that zonulin is overexpressed in the intestinal mucosa of subjects with celiac disease. To our knowledge, this is the initial example of a molecule that exerts a biological activity in its precursor form that is distinct from the function of its mature form. Our results therefore characterize zonulin as a previously undescribed ligand that engages a key signalosome involved in the pathogenesis of human immune-mediated diseases that can be targeted for therapeutic interventions.

Identification of disease-associated proteins by proteomic approach in ankylosing spondylitis

Ankylosing spondylitis (AS) is a chronic systemic inflammatory disorder of the axial skeleton and shows significant inherited susceptibility. Auto-immune responses have been traditionally considered as a putative pathogenetic event in AS. However, no consistent self-antigen has been identified to responsible for the disorders in AS to this day. In this study, serum protein profiles of AS patients and healthy controls from a large Chinese AS family were investigated by two dimensional electrophoresis analysis. A group of four highly expressed protein spots was observed in all AS patients' profiles and subsequently identified as isoforms of haptoglobin precursor (pre-Hp) by ESI-Q-TOF MS/MS. Increased expression of haptoglobin precursor were also observed in sera of sporadic AS patients. Moreover, bioinformatics analysis revealed epitopes derived from haptoglobin precursor with high affinity binding to HLA-B( *)2705, a primary subtype associated with AS. These results indicate that pre-Hp may be involved in the pathogenesis of AS.

Prohaptoglobin is proteolytically cleaved in the endoplasmic reticulum by the complement C1r-like protein

Many secretory proteins are synthesized as proforms that become biologically active through a proteolytic cleavage in the trans-Golgi complex or at a later stage in the secretory pathway. Haptoglobin (Hp) is unusual in that it is cleaved in the endoplasmic reticulum before it enters the Golgi. Here, we present evidence that the recently discovered complement C1r-like protein (C1r-LP) mediates this cleavage. C1r-LP has not previously been shown to possess proteolytic activity, despite its homology to trypsin-like Ser proteinases. We demonstrate that coexpression of the proform of Hp (proHp) and C1r-LP in COS-1 cells effected cleavage of proHp in the endoplasmic reticulum. This cleavage depended on proteolytic activity of C1r-LP because mutation of the putative active-site Ser residue abolished the reaction. Furthermore, incubation of affinity-purified C1r-LP and proHp led to the cleavage of the latter protein. ProHp appeared to be cleaved at the expected site because substitution of Gly for Arg-161 blocked the reaction. C1r-LP showed specificity for proHp, in that it did not cleave the proform of complement C1s, a protein similar to Hp particularly around the cleavage site. C1r-LP accounts for at least part of the endogenous proHp-cleavage activity because suppression of the C1r-LP expression by RNA interference reduced the cleavage of proHp by up to 45% in the cells of a human hepatoma cell line (HepG2).

Proteomic-based identification of haptoglobin-1 precursor as a novel circulating biomarker of ovarian cancer

Screening for specific biomarkers of early-stage detection of ovarian cancer is a major health priority due to the asymptomatic nature and poor survival characteristic of the disease. We utilised two-dimensional gel electrophoresis (2DE) to identify differentially expressed proteins in the serum of ovarian cancer patients that may be useful as biomarkers of this disease. In this study, 38 ovarian cancer patients at different pathological grades (grade 1 (n=6), grade 2 (n=8) and grade 3 (n=24)) were compared to a control group of eight healthy women. Serum samples were treated with a mixture of Affigel-Blue and protein A (5 : 1) for 1 h to remove high abundance protein (e.g. immunoglobulin and albumin) and were displayed using 11 cm, pH 4–7 isoelectric focusing strips for the first dimension and 10% acrylamide gel electrophoresis for the second dimension. Protein spots were visualised by SYPRO-Ruby staining, imaged by FX-imager and compared and analysed by PDQuest software. A total of 24 serum proteins were differentially expressed in grade 1 (P<0.05), 31 in grade 2 (P<0.05) and 25 in grade 3 (P<0.05) ovarian cancer patients. Six of the protein spots that were significantly upregulated in all groups of ovarian cancer patients were identified by nano-electrospray quadrupole quadrupole time-of-flight mass spectrometry (n-ESIQ(q)TOFMS) and matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI-TOFMS) as isoforms of haptoglobin-1 precursor (HAP1), a liver glycoprotein present in human serum. Further identification of the spots at different pathological grades was confirmed by Western blotting using monoclonal antibody against a haptoglobin epitope contained within HAP1. Immunohistochemical localisation of HAP1-like activity was present in malignant ovarian epithelium and stroma but strong immunostaining was present in blood vessels, areas with myxomatous stroma and vascular spaces. No tissue localisation of HAP1-like immunoreactivity was observed in normal ovarian surface epithelium. These data highlight the need to assess circulating concentration of HAP1 in the serum of ovarian cancer patients and evaluate its potential as a biomarker in the early diagnosis of ovarian cancer.

Proteolytic cleavage of haptoglobin occurs in a subcompartment of the endoplasmic reticulum: evidence from membrane fusion in vitro

The primary translation product of haptoglobin mRNA is a 45-kD polypeptide which is proteolytically cleaved shortly after its synthesis. Previous studies have indicated that the cleavage of this proform of haptoglobin occurs in the ER. In an attempt to characterize the cleaving enzyme, we found that upon incubation of microsomes from rat hepatocytes pulse labeled with [35S]methionine, little cleavage of labeled prohaptoglobin occurred. In contrast, when cells whose cytoplasmic proteins had been released by saponin treatment were incubated, 30-40% of the prohaptoglobin was cleaved. The addition of GTP caused a twofold stimulation, which was abolished by the nonhydrolyzable analog GTP gamma S. With a homogenate of the cells, the addition of GTP resulted in a fourfold stimulation of the degree of cleavage--from 15 to 60%. Differential centrifugation revealed that most of the cleaving activity resided in membranes sedimenting similarly to mitochondria and to a small fraction of the ER. These rapidly sedimenting membranes were therefore prepared from a rat liver homogenate. Upon treatment with high salt, light membranes were released which, when incubated with microsomes of pulse-labeled hepatocytes in the presence of detergent (and in the absence of GTP), induced specific cleavage of prohaptoglobin. The cleaving enzyme had an alkaline pH optimum indicating that it was not of lysosomal origin. These results suggest that cleavage of prohaptoglobin occurs in a subcompartment of the ER. Apparently, the connection between this compartment and the bulk of the ER is broken upon saponin treatment or homogenization but can be reestablished through a process requiring GTP hydrolysis.

Cleavage of the alpha 1-microglobulin-bikunin precursor is localized to the Golgi apparatus of rat liver cells

alpha 1-Microglobulin, a plasma protein with immunoregulatory properties, and bikunin, the light chain of the proteinase inhibitors inter-alpha-inhibitor and pre-alpha-inhibitor, are translated as a precursor protein from the same mRNA. The cosynthesis of alpha 1-microglobulin and bikunin is unique compared to other proproteins such as procomplement components and prohormones, since alpha 1-microglobulin and bikunin have no known functional connection. Different forms of intracellular rat liver alpha 1-microglobulin were isolated and characterized by amino acid sequence analysis, lectin binding and glycosidase treatment. Their subcellular distribution was studied by Nycodenz and sucrose gradient centrifugation, pulse-chase experiments, and electrophoresis with subsequent immunoblotting, using pro-C3 and prohaptoglobin as reference proteins. Two alpha 1-microglobulin-bikunin precursors (40 and 42 kDa), containing one and two N-linked oligosaccharides, respectively, were detected in the endoplasmic reticulum. After transport to the Golgi apparatus, the precursors were cleaved, probably C-terminal to the sequence Arg-Ala-Arg-Arg immediately preceding the bikunin part, yielding free sialylated 28 kDa alpha 1-microglobulin, representing the mature protein. The cleavage was almost complete in phosphatidylinositol 4-kinase-enriched membranes, previously identified as a post-Golgi compartment. A fourth intracellular form of alpha 1-microglobulin, 26 kDa, lacked sialic acid. None of the intracellular forms carried the yellow-brown chromophore associated with alpha 1-microglobulin when purified from serum and urine, suggesting that this chromophore becomes linked to the protein after its secretion from the liver cells.

Comparative effects of diabetes and monensin on the lectin asialoglycoprotein receptor: biosynthesis, structure and function in rats

The hepatic asialoglycoprotein receptor is the first studied mammalian lectin. Modulations in vivo by diabetes and in vitro by the carboxylic ionophore monensin gave rise to similar apparent alterations on its biosynthesis, structure and ligand binding capacity. In normal rats, the receptor (whether purified by ligand or antibody-affinity chromatography) presented a similar pattern in SDS-PAGE analysis, with a major 42-kDa band and two minor ones (49 and 52-54 kDa). In diabetic rats, a new 38-kDa band appeared, but only after antibody-affinity purification. In vitro biosynthesis of the receptor by normal hepatocytes in the presence of 35S-methionine showed that this 38-kDa band was present at the end of a 30-min pulse but decreased during a 180-min chase, in association with an increase in the major 42-kDa band. In diabetic cells, this evolution was retarded. Using a 30-min pulse followed by a 120-min chase in the presence of 100 microM monensin, we showed that this carboxylic ionophore had similar effects on diabetes, leading to a delay in the maturation process of the 42-kDa band and the persistent emergence of the 38-kDa species. Allowing incubation in the presence of 25 or 100 microM monensin, we observed a decrease in the number of ligand binding sites both at the surface (40%) and within the cell (28%). In hepatocytes from diabetic rats, monensin showed no additional effect on the partial diabetes-induced inactivation.

A simple and rapid method for purification of rat haptoglobin for production of antiserum

Plasma from rats with acute inflammatory response was fractionated on Blue Sepharose CL 6B, to separate haptoglobin from albumin and lipoproteins. Affinity chromatography on Blue Sepharose proved to be a convenient method for crude fractionation of plasma. Pure haptoglobin was obtained by the subsequent affinity chromatography on a rabbit-haemoglobin Sepharose column. Minor amounts of rabbit haemoglobin co-eluted from the haemoglobin Sepharose column but did not influence the monospecificity of the antiserum raised in rabbits. By use of the antiserum in single radial immunodiffusion, the concentration of haptoglobin in plasma from normal rats was measured to be 0.5 g/l.

Primary structure of rat liver alkaline phosphatase deduced from its cDNA

Rat liver alkaline phosphatase (ALP) was markedly induced by treatment of rats by bile-duct ligation and colchicine injection. Taking this advantage for enrichment of ALP mRNA, we constructed a lambda gt11 liver cDNA library using polyadenylated RNA prepared from the treated rat liver, and isolated an ALP cDNA clone. The 2165 bp cDNA contained an open reading frame that encodes a 524-amino-acid-residue polypeptide with a predicted molecular mass of 57737 Da. The precursor protein contained a presumed signal peptide of 17 amino acid residues followed by 28 amino acid residues identical with the N-terminal sequence determined from the purified rat liver ALP. It was also confirmed that amino acid sequences of two CNBr-cleavage peptides obtained from liver ALP were contained within the cDNA-encoded protein. Five possible N-linked glycosylation sites were found in the molecule and a highly hydrophobic amino acid sequence at the C-terminus. The deduced polypeptide of rat liver ALP showed 88% homology to that of the human liver-type enzyme in osteosarcoma cells. RNA blot hybridization analysis identified a single species of ALP mRNA with 2.7 kb in both the control and the treated rat livers. An approx. 20-fold increase of the mRNA was detected in the treated liver at 12 h after the onset of stimulation, compared with that in the control liver.

Brefeldin A arrests the intracellular transport of a precursor of complement C3 before its conversion site in rat hepatocytes

The effects of brefeldin A on intracellular transport and posttranslational modification of complement C3 (C3) were studied in primary culture of rat hepatocytes. In the control culture C3 was synthesized as a precursor (pro-C3), which was processed to the mature form with alpha- and beta-subunits before its discharge into the medium. In the presence of brefeldin A the secretion of C3 was strongly blocked, resulting in accumulation of pro-C3. However, after a prolonged interval the mature form of C3 was finally secreted. The results indicate that brefeldin A impedes translocation of pro-C3 to the Golgi complex where pro-C3 is converted to the mature form, but not its proteolytic processing, in contrast to the effects of monensin and weakly basic amines.

Effects of weakly basic amines on proteolytic processing and terminal glycosylation of secretory proteins in cultured rat hepatocytes

We examined the effects of weakly basic amines on the secretion and post-translational modifications of secretory proteins in cultured rat hepatocytes. Weakly basic amines such as methylamine, chloroquine and NH4Cl strongly inhibited not only protein secretion, but also the proteolytic conversion of a proform of complement C3, allowing the precursor to be released into the medium. The amines, however, had no effect on the proteolytic conversion of prohaptoglobin into its subunits. Since available evidence indicates that the conversion of pro-C3 occurs at the Golgi complex while that of prohaptoglobin takes place in the endoplasmic reticulum, it is most likely that the weak bases specifically affect the proteolytic event occurring at the Golgi complex. Electron microscopic observations confirmed that the amines caused morphological changes of the Golgi complex, consisting of dilated cisternae and swollen vacuoles. When the glycosylation of alpha 1-protease inhibitor and haptoglobin was examined, it was found that the amines caused a marked accumulation in the cells of both glycoproteins corresponding to the mature secreted forms. Neuraminidase digestion demonstrated that the glycoproteins accumulating in response to the amines had acquired terminal sialic acid. The results indicate that the amines do not significantly affect terminal glycosylation, in contrast with their definite effect on proteolytic processing, despite the fact that both modifications take place in the Golgi complex.

Structural analysis of the asparagine-linked oligosaccharides of rat haptoglobin metabolically labeled in a hepatocyte culture system

We analyzed the asparagine-linked oligosaccharide chains of rat haptoglobin which were synthesized and secreted by hepatocytes in primary culture. When the cells were incubated with either [3H]mannose, [3H]galactose, or [3H]fucose, all the radioactive precursors were incorporated into the beta subunit of haptoglobin. [3H]Mannose-labeled haptoglobin was purified from the culture medium by immunoaffinity chromatography, and [3H]oligosaccharides were prepared by strong alkali-borohydride treatment. The oligosaccharides obtained were analyzed by anion-exchange high-performance liquid chromatography, concanavalin-A--Sepharose chromatography and Bio-Gel P-4 chromatography before and after sequential exoglycosidase digestions. The oligosaccharides labeled with [3H]fucose or [3H]galactose were also characterized by the above methods. The results indicate that rat haptoglobin contains two complex-type oligosaccharide chains in each beta subunit; one with a possible structure of ( +/- NeuAc----Gal beta----GlcNAc beta----)3(Man alpha----)2 Man beta----GlcNAc----( +/- Fuc alpha----)GlcNAc and the other with ( +/- NeuAc----Gal beta----GlcNAc beta----Man alpha----)2 Man beta----GlcNAc----( +/- Fuc alpha----)GlcNAc.

Weakly basic amines inhibit the proteolytic conversion of proalbumin to serum albumin in cultured rat hepatocytes

Effects of weak amines on the proteolytic conversion of proalbumin to serum albumin were studied in primary culture of rat hepatocytes. In control culture proalbumin was converted to serum albumin before discharge into the medium. However, in the presence of chloroquine the conversion to serum albumin was inhibited and proalbumin per se was released into medium. A similar inhibition of the processing was also observed in the presence of other amines such as methylamine and NH4Cl. Thus weak amines mimic the carboxylic ionophore monensin with regard to the effect on proalbumin conversion [Oda & Ikehara (1982) Biochem. Biophys. Res. Commun. 105, 766-772]. Since proteolytic conversion of proalbumin is believed to occur at the Golgi complex, these results suggest that weakly basic amines perturb the Golgi complex in addition to lysosomes and endosomes.

Electrophoretic characterization of hepatic alkaline phosphatase released by phosphatidylinositol-specific phospholipase C. A comparison with liver membrane and serum-soluble forms

Alkaline phosphatase was solubilized from plasma membrane of rat liver with butanol-ol, bile acids or sodium deoxycholate, and electrophoretically compared with a soluble form in serum which was derived from the liver. The three enzyme preparations from the plasma membrane migrated at the same position on polyacrylamide-gel electrophoresis in the presence of either Triton X-100 or sodium dodecyl sulphate. The mobility of them, however, was distinctly different from that of the serum-soluble form of the liver-derived alkaline phosphatase. On the other hand, phosphatidylinositol-specific phospholipase C isolated from Bacillus cereus was used to release alkaline phosphatase from plasma membrane. The released alkaline phosphatase was demonstrated to have the same mobility as the serum-soluble form on polyacrylamide-gel electrophoresis in the presence or absence of detergents. The phospholipase C also converted the butan-1-ol-extracted membrane form into the serum-soluble form. The results suggest that release of alkaline phosphatase from the liver into serum is not simply caused by a detergent effect of bile salts, but involves an enzymic hydrolysis of phosphatidylinositol, with which alkaline phosphatase may strongly interact in the membrane.

Biosynthesis and processing of pro-C3, a precursor of the third component of complement in rat hepatocytes: effect of secretion-blocking agents

Biosynthesis and intracellular processing of the third component (C3) of complement were studied in cultured rat hepatocytes. In the control cells, the complement C3 was synthesized as a pro-form, a single polypeptide chain comprising both the alpha- and beta-subunits. Although the cleavage of the pro-form into the subunits was not clearly demonstrable within the cells during pulse-chase periods, all the secreted C3 was the mature processed form. The cells were treated with secretion-blocking agents with different modes of action, colchicine and monensin. Colchicine caused an accumulation of the processed C3 within the cells, whereas monensin blocked the secretion without a significant accumulation of the processed form. The results indicate that the conversion of the C3 pro-form into the subunits takes place in the secretory vesicles just before the secretion.

Characterization of human haptoglobin cDNAs coding for alpha 2FS beta and alpha 1S beta variants

A human liver library, derived from a heterozygous (Hp2-1) donor, has been used to isolate cDNA clones coding for the haptoglobin (Hp) alpha 1S beta and alpha 2FS beta variants. DNA sequencing has shown that the two variants are identical except for the alpha F duplicated segment in Hp alpha 2FS beta. Four nucleotide changes have been found between the phenotypically different F and S regions of the Hp alpha 2 gene, resulting in an Asp,Lys/Asn,Glu substitution.