Nitric oxide synthesis is involved in arterial haptoglobin expression after sustained flow changes

Haptoglobin: From hemoglobin scavenging to human health

Haptoglobin (Hp) belongs to the family of acute-phase plasma proteins and represents the most important plasma detoxifier of hemoglobin (Hb). The basic Hp molecule is a tetrameric protein built by two α/β dimers. Each Hp α/β dimer is encoded by a single gene and is synthesized as a single polypeptide. Following post-translational protease-dependent cleavage of the Hp polypeptide, the α and β chains are linked by disulfide bridge(s) to generate the mature Hp protein. As human Hp gene is characterized by two common Hp1 and Hp2 alleles, three major genotypes can result (i.e., Hp1-1, Hp2-1, and Hp2-2). Hp regulates Hb clearance from circulation by the macrophage-specific receptor CD163, thus preventing Hb-mediated severe consequences for health. Indeed, the antioxidant and Hb binding properties of Hp as well as its ability to stimulate cells of the monocyte/macrophage lineage and to modulate the helper T-cell type 1 and type 2 balance significantly associate with a variety of pathogenic disorders (e.g., infectious diseases, diabetes, cardiovascular diseases, and cancer). Alternative functions of the variants Hp1 and Hp2 have been reported, particularly in the susceptibility and protection against infectious (e.g., pulmonary tuberculosis, HIV, and malaria) and non-infectious (e.g., diabetes, cardiovascular diseases and obesity) diseases. Both high and low levels of Hp are indicative of clinical conditions: Hp plasma levels increase during infections, inflammation, and various malignant diseases, and decrease during malnutrition, hemolysis, hepatic disease, allergic reactions, and seizure disorders. Of note, the Hp:Hb complexes display heme-based reactivity; in fact, they bind several ferrous and ferric ligands, including O₂, CO, and NO, and display (pseudo-)enzymatic properties (e.g., NO and peroxynitrite detoxification). Here, genetic, biochemical, biomedical, and biotechnological aspects of Hp are reviewed.

Involvement of placental growth factor upregulated via TGF-β1-ALK1-Smad1/5 signaling in prohaptoglobin-induced angiogenesis

A potential role of haptoglobin in arterial restructuring has been suggested, and our previous study demonstrated that prohaptoglobin, the precursor of haptoglobin, stimulates endothelial angiogenesis. However, the mechanisms underlying the angiogenic effects of prohaptoglobin are still unclear. Here, we investigated angiogenic signaling induced by prohaptoglobin using human umbilical vein endothelial cells. Prohaptoglobin upregulated the expression of placental growth factor (PlGF), vascular endothelial growth factor (VEGF)-A, and VEGF receptor 1 and 2, and also induced cell migration and tube network formation. PlGF knockdown attenuated these angiogenic effects of prohaptoglobin. Furthermore, a transcription factor profiling assay indicated that Smad is involved in PlGF expression in response to prohaptoglobin. Transforming growth factor-β1 (TGF-β1) expression and Smad1/5 phosphorylation were also induced by prohaptoglobin treatment. Blockade of TGF-β1 signaling using the TGF-β receptor kinase inhibitor LY2109761 or Smad1/5 siRNA reduced the prohaptoglobin-induced PlGF expression and in vitro tube formation. Knockdown of the TGF-β receptor ALK1, but not ALK5, with a specific siRNA blocked the Smad1/5 phosphorylation and PlGF expression induced by prohaptoglobin. These findings suggest that the angiogenic effects of prohaptoglobin are dependent on PlGF and mediated via a TGF-β1-ALK1-Smad1/5–PlGF/VEGFR1–VEGF-A/VEGFR2 signaling pathway.

Proteomic and bioinformatic discovery of biomarkers for diabetic nephropathy

Diabetes is associated with numerous metabolic and vascular risk factors that contribute to a high rate of micro-vascular and macro-vascular disorders leading to mortality and morbidity from diabetic complications. In this case, the major cause of death in overall diabetic patients results from diabetic nephropathy (DN) or renal failure. The risk factors and mechanisms that correspond to the development of DN are not fully understood and so far, no specific and sufficient diagnostic biomarkers are currently available other than micro- or macroalbuminuria. Therefore, this review describes current and novel protein biomarkers in the context of DN as well as probable proteins biomarkers associated with pathological processes for the early stage of DN via proteomics data together with bioinformatics. In addition, the mechanisms involved in early development of diabetic vascular disorders and complications resulting from glucose induced oxidative stress will also be explored.

Diabetic macular oedema: under-represented in the genetic analysis of diabetic retinopathy

Diabetic retinopathy, a complication of both type 1 and type 2 diabetes, is a complex disease and is one of the leading causes of blindness in adults worldwide. It can be divided into distinct subclasses, one of which is diabetic macular oedema. Diabetic macular oedema can occur at any time in diabetic retinopathy and is the most common cause of vision loss in patients with type 2 diabetes. The purpose of this review is to summarize the large number of genetic association studies that have been performed in cohorts of patients with type 2 diabetes and published in English-language journals up to February 2017. Many of these studies have produced positive associations with gene polymorphisms and diabetic retinopathy. However, this review highlights that within this large body of work, studies specifically addressing a genetic association with diabetic macular oedema, although present, are vastly under-represented. We also highlight that many of the studies have small patient numbers and that meta-analyses often inappropriately combine patient data sets. We conclude that there will continue to be conflicting results and no meaningful findings will be achieved if the historical approach of combining all diabetic retinopathy disease states within patient cohorts continues in future studies. This review also identifies several genes that would be interesting to analyse in large, well-defined cohorts of patients with diabetic macular oedema in future candidate gene association studies.

Establishment of an antibody specific for cancer-associated haptoglobin: a possible implication of clinical investigation

We previously found that the serum level of fucosylated haptoglobin (Fuc-Hpt) was significantly increased in pancreatic cancer patients. To delineate the mechanism underlying this increase and develop a simple detection method, we set out to generate a monoclonal antibody (mAb) specific for Fuc-Hpt. After multiple screenings by enzyme-linked immunosorbent assay (ELISA), a 10-7G mAb was identified as being highly specific for Fuc-Hpt generated in a cell line as well as for Hpt derived from a pancreatic cancer patient. As a result from affinity chromatography with 10-7G mAb, followed by lectin blot and mass spectrometry analyses, it was found that 10-7G mAb predominantly recognized both Fuc-Hpt and prohaptoglobin (proHpt), which was also fucosylated. In immunohistochemical analyses, hepatocytes surrounding metastasized cancer cells were stained by the 10-7G mAb, but neither the original cancer cells themselves nor normal hepatocytes exhibited positive staining, suggesting that metastasized cancer cells promote Fuc-Hpt production in adjacent hepatocytes. Serum level of Fuc-Hpt determined with newly developed ELISA system using the 10-7G mAb, was increased in patients of pancreatic and colorectal cancer. Interestingly, dramatic increases in Fuc-Hpt levels were observed at the stage IV of colorectal cancer. These results indicate that the 10-7G mAb developed is a promising antibody which recognizes Fuc-Hpt and could be a useful diagnostic tool for detecting liver metastasis of cancer.

Dysfunctional HDL in diabetes mellitus and its role in the pathogenesis of cardiovascular disease

Coronary artery disease, the leading cause of death in the developed and developing countries, is prevalent in diabetes mellitus with 68% cardiovascular disease (CVD)-related mortality. Epidemiological studies suggested inverse correlation between HDL and CVD occurrence. Therefore, low HDL concentration observed in diabetic patients compared to non-diabetic individuals was thought to be one of the primary causes of increased risks of CVD. Efforts to raise HDL level via CETP inhibitors, Torcetrapib and Dalcetrapib, turned out to be disappointing in outcome studies despite substantial increases in HDL-C, suggesting that factors beyond HDL concentration may be responsible for the increased risks of CVD. Therefore, recent studies have focused more on HDL function than on HDL levels. The metabolic environment in diabetes mellitus condition such as hyperglycemia-induced advanced glycation end products, oxidative stress, and inflammation promote HDL dysfunction leading to greater risks of CVD. This review discusses dysfunctional HDL as one of the mechanisms of increased CVD risks in diabetes mellitus through adversely affecting components that support HDL function in cholesterol efflux and LDL oxidation. The dampening of reverse cholesterol transport, a key process that removes cholesterol from lipid-laden macrophages in the arterial wall, leads to increased risks of CVD in diabetic patients. Therapeutic approaches to keep diabetes under control may benefit patients from developing CVD.

Role of Haptoglobin in Health and Disease: A Focus on Diabetes

In Brief Prospective identification of individuals with diabetes who are at greatest risk for developing complications would have considerable public health importance by allowing appropriate resources to be focused on those who would benefit most from aggressive intervention. Haptoglobin (Hp) is an acute-phase protein that is crucial for the elimination of free hemoglobin and the neutralization of oxidative damage. In the past two decades, associations have been made between polymorphisms in Hp and complications arising from diabetes. Individuals with polymorphism in Hp have been shown to have significantly higher risk of developing cardiovascular disease. This review summarizes the current literature on the role of Hp in health and disease, with a focus on diabetes.

Single chain precursor prohaptoglobin promotes angiogenesis by upregulating expression of vascular endothelial growth factor (VEGF) and VEGF receptor2

Prohaptoglobin (proHp) is processed into mature haptoglobin via site-specific cleavage. Although haptoglobin has been well studied, the functions of proHp remain unclear. We investigated the angiogenic action of proHp in endothelial cells, demonstrating that proHp upregulated vascular endothelial growth factor (VEGF) and VEGF receptor 2 (VEGFR2) expression and endothelial sprouting and branching. ProHp-induced sprouting was attenuated by a VEGFR2 inhibitor. Moreover, proHp was detected in sera of cancer patients by immunoprecipitation and Western blot. These findings indicate that proHp promotes angiogenesis via VEGF/VEGFR2 signalling, and serum proHp level may be a useful biomarker for diseases associated with angiogenesis.

Proteomic identification of fucosylated haptoglobin alpha isoforms in ascitic fluids and its localization in ovarian carcinoma tissues from Mexican patients

Background

Ovarian cancer is the most lethal gynecologic disease due to delayed diagnosis, and ascites production is a characteristic of patients in advanced stages. The aim of this study was to perform the proteomic analysis of ascitic fluids of Mexican patients with ovarian carcinoma, in order to detect proteins with a differential expression pattern in the continuing search to identify biomarkers for this disease.

Methods

Samples were collected from 50 patients from the Instituto Nacional de Cancerología of México under informed consent and with approval of the bioethics and scientific committees. After elimination of abundant proteins (Albumin/IgGs) samples were processed for 2D electrophoresis and further protein identification by Mass Spectrometry (MALDI-TOF). Molecules of interest were followed by western blot and lectin binding assays, and their tissue location by histo-immunofluorescence and confocal analysis.

Results and discussion

An area with a differential expression pattern among samples was located in the 2D gels. Identified proteins were 6 alpha 1 isoforms and 1 alpha 2 isoform of Haptoglobin, and 2 isoforms of Transthyretin. While Transthyretin isoforms were constitutively expressed in all samples, clear differences in the expression pattern of Haptoglobin alpha isoforms were found. Moreover, increased levels of fucosylation of Haptoglobin alpha isoforms analyzed in 40 samples by Aleuria aurantia lectin binding by 1D overlay assay showed a positive correlation with advanced stages of the disease. Tissue detection of Haptoglobin and its fucosylated form, by histo-immunofluorescence in biopsies of ovarian cancer, also showed a correlation with ovarian cancer progression. Moreover, results show that fucosylated Haptoglobin is produced by tumor cells.

Conclusions

Increased numbers of highly fucosylated Haptoglobin alpha isoforms in ascitic fluids and the presence of fucosylated Haptoglobin in tumor tissues of ovarian cancer Mexican patients associated with advanced stages of the disease, reinforce the potential of fucosylated Haptoglobin alpha isoforms to be characterized as biomarkers for disease progression.

Haptoglobin polymorphism in individuals with type 2 diabetic microangiopathy

BACKGROUND

Haptoglobin is an acute phase protein with antioxidant and immunomodulatory properties. Gene polymorphism may be a risk factor for diabetic vascular disease in Iranian population.

AIMS

The study investigates the existence or not of an association between haptoglobin genotypes and prevalence of diabetic microangiopathy in individuals with type 2 diabetic microangiopathy.

MATERIALS AND METHODS

We included 206 type 2 diabetic patients (<5 years duration) categorized into two groups according to the presence or absence of diabetic microvascular complications. The cases of interest were diabetic neuropathy, retinopathy, and nephropathy identified during clinical and or laboratory examination. In addition, 114 age- and sex-matched individuals were selected to serve as a control group. Haptoglobin genotyping was done using an amplification gel electrophoresis.

RESULTS

The frequency of haptoglobin phenotype 2-1 in diabetic patients was 70/206 (33.9%) as compared with 54/114 (47.3%) in nondiabetics (P = 0.01). However, the frequency of haptoglobin phenotype 2-2 was greater in diabetics (126/114, 61.1%) than in those without diabetes (56/114, 49.1%; P = 0.02). Patients with diabetic microangiopathy; however, did not differ significantly between haptoglobin phenotype groups.

CONCLUSIONS

Haptoglobin phenotype 2-2 is considered to be a major susceptibility gene for type 2 diabetic patients. Moreover, haptoglobin phenotype 2-1may be good prognostic factors for the development of diabetes mellitus.

Identification of a specific haptoglobin C-terminal fragment in arthritic synovial fluid and its effect on interleukin-6 expression

Haptoglobin (Hp), a major acute-phase plasma protein, has been found in arthritic synovial fluid (SF). However, the function and structural modifications of Hp in arthritic SF are unknown. To investigate in vivo generation of modified Hp associated with inflammatory disease, we examined a new Hp isoform in SF from patients with rheumatoid arthritis (RA). Specific Hp fragments of 28 000 and 15 000 molecular weight were identified in SF of patients with RA, and the two polypeptides were presumed to be fragments of the Hp β-chain (43 000 MW) produced by cleavage with plasmin. The 15 000 MW fragment, which is a C-terminal region of Hp, was observed at higher frequency and levels in RA than in osteoarthritis. Plasmin activity was also higher in SF of RA patients. A recombinant 15 000 MW Hp fragment up-regulated interlukin-6 expression in monocytic cells. These findings indicate that the C-terminal Hp fragment is generated by plasmin in local inflammatory environments and acts as an inflammatory mediator. They further suggest that a specific Hp fragment might be applied as a novel biomarker for the diagnosis and prognosis of inflammatory diseases such as RA.

Haptoglobin expression in endometrioid adenocarcinoma of the uterus

OBJECTIVE

Elevated serum haptoglobin (Hp) concentrations have been reported in patients with malignant diseases. We have shown that Hp is produced by and localizes only in the stroma and not the epithelium of endometriotic lesions, which share many characteristics of carcinoma. Furthermore, Hp mRNA and protein are found exclusively in the stroma of eutopic endometrium from women with endometriosis and not those without endometriosis. We hypothesized that characteristic patterns of Hp gene expression and protein localization in endometrioid adenocarcinoma of the uterus may provide insight into the clinical utility of Hp as a tumor marker or alternative therapeutic approach.

METHODS

Biopsies of endometrioid adenocarcinoma tumors of the uterus and their adjacent nonaffected endometrium were collected. Normal endometrium was collected from healthy women. Haptoglobin messenger RNA (mRNA) levels were quantified by quantitative polymerase chain reaction (Q-PCR). Haptoglobin protein cell-specific localization was identified by immunohistochemistry.

RESULTS

Haptoglobin mRNA levels were significantly greater (P < .005) in endometrioid adenocarcinoma and adjacent nonaffected endometrial tissues than normal endometrium. No correlation was found between Hp levels and cancer stage (P = .673) or grade (P = .739). Haptoglobin protein localized in both stromal and glandular epithelial cells of endometrioid adenocarcinoma and their adjacent nonaffected tissue but not in control endometrium.

CONCLUSIONS

Our results have identified, for the first time, unique patterns of Hp mRNA expression and protein localization in the stromal and glandular epithelial cells of endometrioid adenocarcinoma of the uterus. We propose that this unique pattern of endometrioid adenocarcinoma Hp expression may be developed as a novel diagnostic marker. Modulation of Hp, with its immunomodulatory and angiogenic properties, may generate novel methods of prevention or treatment for endometrial cancer.

Enhancement of angiogenic and vasculogenic potential of endothelial progenitor cells by haptoglobin

Endothelial progenitor cells (EPCs) were transfected with the haptoglobin (Hp) gene to investigate the effect of Hp on cell function. Hp potentiated the gene expression of various pro-angiogenic factors in the EPCs. The Hp-modified EPCs also increased in vitro tube formation on Matrigel compared with control cells. In hindlimb ischaemia models, Hp-EPCs showed a greater ability for improving blood perfusion and recovery from ischaemic injury. These results indicate that Hp improves EPC function in neovasculogenesis, which suggests that ex vivo modification of EPCs with the Hp gene can be applied to the treatment of vascular damage.

Possible pathogenetic role of vascular, immunological and genetic factors in certain gastoenterological disorders

A coeliakia és a gyulladásos bélbetegségek (IBD) változatos klinikai megjelenését befolyásoló faktorok jelentős része ismeretlen. A haptoglobin (Hp) molekula három fő fenotípusa eltérő antioxidáns, scavenger és immunmoduláns tulajdonsággal rendelkezik. Nagyszámú coeliakiás és IBD-betegben vizsgáltuk a Hp-polimorfizmus megoszlását és a betegség klinikai megjelenésével való lehetséges kapcsolatát. A Hp-fenotípusokat szérumból nátrium-dodecil-szulfát poliakrilamid gélelektroforézis (SDS-PAGE) és immunblottal határoztuk meg, amely egyértelműen azonosítja a genotípust is. Coeliakiában a kontrollpopulációhoz képest a Hp 2-1 fenotípus előfordulása szignifikánsan gyakoribb volt, míg a Hp 2-2 ritkább. A Hp 2-2 esetén azonban a súlyos, malabszorpcióval jelentkező klinikai betegség kockázata magasabb volt, míg a silent betegségé jóval alacsonyabb. Crohn-betegekben (CD) a Hp 2-1 fenotípus esetén a gyulladásos betegségforma előfordulása szignifikánsan gyakoribb volt a másik két Hp-fenotípushoz viszonyítva, míg a stenotizáló formáé ritkább. Primer szklerotizáló cholangitisben a Hp 1-1 fenotípus nem fordult elő. A Hp szerepét az immunreakciókban betöltött modulálóhatásai és eltérő térszerkezete magyarázhatja. A mikrobiális és autoantigének elleni szerológiai válasz segítséget jelenthet az IBD differenciáldiagnosztikájában, és összefüggést mutat a betegség fenotípusával. Nagyszámú IBD-betegben vizsgáltuk az anti- Saccharomyces cerevisiae (ASCA) és külső membránporin elleni antitestek (anti-Omp) enzimmel kapcsolt immunoszorbens assay (ELISA), illetőleg az atípusos perinukleáris antineutrophil citoplazmatikus antitest (P-ANCA) (indirekt immunfluoreszcencia) előfordulási gyakoriságát és a betegség fenotípusával való lehetséges kapcsolatukat. A szerológiai markerek hasznosnak bizonyultak az IBD differenciáldiagnosztikájában. CD-ben az ASCA- és az anti-Omp-pozitivitás a vékonybél-érintettség és a nem gyulladásos betegségforma szempontjából logisztikus regressziós analízisben is független rizikófaktornak bizonyult, míg a műtéti kockázat tekintetében nem. A nem gyulladásos típusú vékonybél-CD és az antimikrobiális antitest pozitivitásainak száma, illetőleg az antitesttiterek között szintén pozitív korrelációt találtunk (szerológiai-dózis hatás). Továbbá ASCA és az anti-Omp-antitestek jelenléte kapcsolatot mutatott a nucleotid oligomerizációs domén (NOD2/CARD15) genotípussal, és előfordulási gyakoriságuk szignifikánsan nőtt a NOD2/CARD15 mutációk számának növekedésével, ami gén-dózis hatásra utal és alátámasztja a mikrobákkal szembeni megváltozott érzékelés szerepét a CD patogenezisében.

Circulating haptoglobin is an independent prognostic factor in the sera of patients with epithelial ovarian cancer

OBJECTIVE

This study was conducted to evaluate the prognostic significance of haptoglobin levels in the overall survival of patients presenting with various stages of epithelial ovarian cancer.

MATERIALS AND METHODS

We employed an in-house sandwich enzyme-linked immunosorbent assay method to determine the concentrations of preoperative haptoglobin and C-reactive protein (CRP) in sera samples obtained from 66 malignant tumors, 60 benign tumors, and 10 normal healthy women.

RESULTS

Levels of serum haptoglobin significantly correlated with tumor type (P < .001) and International Federation of Gynecology and Obstetrics stage (P < .05). A significant correlation was observed between clinical stage and patient survival (r = 5.99, P = .026). Our data also indicated that elevated serum haptoglobin levels were associated with poor outcome for overall survival using both univariate and multivariate analyses (P = .048 and P = .036 respectively). Using Pearson's correlation, we have noted that serum CRP concentrations significantly correlated with haptoglobin levels (r(2) = 0.22, P < .001). Immunohistochemical findings and Western blot analyses were compatible with sera levels of haptoglobin in which a higher intensity of staining occurred in late-stage epithelial ovarian cancers.

CONCLUSION

This study provides evidence that preoperative serum levels of haptoglobin could serve as an independent prognostic factor in patients presenting with epithelial ovarian cancer.

Study of serum haptoglobin and its glycoforms in the diagnosis of hepatocellular carcinoma: a glycoproteomic approach

Increased serum haptoglobin concentration and changes in its glycosylation have been reported in certain cancer types. Information for hepatocellular carcinoma (HCC) has not yet been available. In this study, we aimed to carry out a systematic analysis of serum concentrations of haptoglobin (Hp) and its glycoforms in the patients with HCC and noncancer patients only with chronic liver diseases (CLD) and to examine their clinical values. This study was divided into two major parts, (1) measurement of serum Hp concentration, and investigation of its value in the diagnosis of HCC, and (2) quantitative analysis of Hp glycoforms with alpha-2,6-sialylation and/or alpha-1,6-fucosylation by using lectin affinity purification and 2D gel electrophoresis and investigation of their relationships with tumor stage. The concentrations of serum Hp in HCC patients were significantly higher than those in noncancer patients with CLD. With the use of serum concentrations of Hp and alpha-fetoprotein, a logistic regression (LR) model was developed from the training data set and used to classify the validation cases. At a specificity of 95%, the sensitivity for HCC detection was 79%. Comparing serum concentrations of alpha-2,6-sialylated Hp (S-Hp) and alpha-1,6-fucosylated Hp (F-Hp) between HCC and CLD patients suggests that purification of S-Hp and F-Hp could enrich the glycosylation variants associated with HCC. 2D gel analysis of S-Hp and F-Hp identified a total of 18 glycoforms. A unique pattern of Hp glycoforms comprising both hypersialylated fucosylated and hyposialylated fucosylated species was found in the HCC patients. Serum concentrations of these glycoproteins were significantly higher in the patients with advanced tumors, suggesting their tumor-specific nature. We have shown that serum Hp is a potential biomarker in the diagnosis of HCC. The combined use of Hp and AFP could greatly improve the diagnostic accuracy. A unique pattern of Hp glycoforms with altered sialylation and fucosylation is specific to HCC and associated tumor progression.

In vivo and in vitro studies establishing haptoglobin as a major susceptibility gene for diabetic vascular disease

Hemoglobin (Hb) released during hemolysis is a potent oxidant. Extracorpuscular Hb may enter the vessel wall and mediate low-density lipoprotein oxidation, thereby promoting the development and progression of atherosclerosis. Haptoglobin (Hp) is an antioxidant protein as a result of its ability to bind Hb and block Hb-induced oxidative damage. Hp also facilitates the removal of Hb from the extravascular compartment via the CD163 macrophage scavenger receptor. In man, there are two common alleles for Hp denoted 1 and 2, and correspondingly, three different possible genotypes: Hp1-1, Hp2-1, and Hp2-2. We have recently demonstrated in several longitudinal studies that Hp genotype is an independent risk factor for diabetic vascular complications. Specifically, we have shown that diabetic individuals with Hp2-2 are more likely to develop nephropathy, retinopathy, and cardiovascular disease as compared with those with Hp2-1 or Hp1-1. Mechanistically, we have found significant Hp type differences in the antioxidant and CD163-mediated scavenging and activation functions of the different Hp protein types. Furthermore, we have demonstrated that these functions are modified in the diabetic state. In this review, we focus on the clinical studies associating the Hp polymorphism and diabetic vascular complications, and the molecular basis behind this interaction.

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).

The acute phase protein haptoglobin is locally expressed in arthritic and oncological tissues

Haptoglobin is an acute phase protein known to be highly expressed in the liver. Recently, we showed increased local arterial haptoglobin expression after flow-induced arterial remodelling and found that haptoglobin is involved in cell migration and arterial restructuring probably through accumulation of a temporary gelatin matrix. Since cell migration and matrix turnover are important features in the pathology of arthritis and cancer, we hypothesized that haptoglobin is also locally expressed in arthritic and oncological tissues. In this study, we investigated local haptoglobin expression in arthritic rats (n = 12) using semi-quantitative PCR and Western blotting, and we studied haptoglobin mRNA localization in human kidney tumours (n = 3) using in situ hybridization. The arthritic rats demonstrated an increase of haptoglobin mRNA (2.5-fold, P < 0.001) and protein (2.6-fold, P < 0.001) in the arthritic Achilles tendon. Haptoglobin protein was also increased in the arthritic ankle (2.6-fold, P < 0.001) but not in the non-arthritic knee. In human kidney tumours, tumour and stromal cells produced haptoglobin mRNA. This study shows that the liver protein haptoglobin is, in addition to the artery, also expressed in arthritic and oncological tissues that are recognized for enhanced cell migration and matrix turnover.