Characterisation of the rat and mouse homologues of gC1qBP, a 33 kDa glycoprotein that binds to the globular 'heads' of C1q

The C1q and gC1qR axis as a novel checkpoint inhibitor in cancer

Understanding at the molecular level of the cell biology of tumors has led to significant treatment advances in the past. Despite such advances however, development of therapy resistance and tumor recurrence are still unresolved major challenges. This therefore underscores the need to identify novel tumor targets and develop corresponding therapies to supplement existing biologic and cytotoxic approaches so that a deeper and more sustained treatment responses could be achieved. The complement system is emerging as a potential novel target for cancer therapy. Data accumulated to date show that complement proteins, and in particular C1q and its receptors cC1qR/CR and gC1qR/p33/HABP1, are overexpressed in most cancer cells and together are involved not only in shaping the inflammatory tumor microenvironment, but also in the regulation of angiogenesis, metastasis, and cell proliferation. In addition to the soluble form of C1q that is found in plasma, the C1q molecule is also found anchored on the cell membrane of monocytes, macrophages, dendritic cells, and cancer cells, via a 22aa long leader peptide found only in the A-chain. This orientation leaves its 6 globular heads exposed outwardly and thus available for high affinity binding to a wide range of molecular ligands that enhance tumor cell survival, migration, and proliferation. Similarly, the gC1qR molecule is not only overexpressed in most cancer types but is also released into the microenvironment where it has been shown to be associated with cancer cell proliferation and metastasis by activation of the complement and kinin systems. Co-culture of either T cells or cancer cells with purified C1q or anti-gC1qR has been shown to induce an anti-proliferative response. It is therefore postulated that in the tumor microenvironment, the interaction between C1q expressing cancer cells and gC1qR bearing cytotoxic T cells results in T cell suppression in a manner akin to the PD-L1 and PD-1 interaction.

Role of gC1qR as a modulator of endothelial cell permeability and contributor to post-stroke inflammation and edema formation

Ischemic stroke is a leading cause of death and disability worldwide. A serious risk of acute ischemic stroke (AIS) arises after the stroke event, due to inflammation and edema formation. Inflammation and edema in the brain are mediated by bradykinin, the formation of which is dependent upon a multi-ligand receptor protein called gC1qR. There are currently no preventive treatments for the secondary damage of AIS produced by inflammation and edema. This review aims to summarize recent research regarding the role of gC1qR in bradykinin formation, its role in inflammation and edema following ischemic injury, and potential therapeutic approaches to preventing post-stroke inflammation and edema formation.

gC1qR: A New Target for Cancer Immunotherapy

Although breakthroughs in cancer treatment have been achieved, immunotherapy yields only modest benefits in most patients. There is still a gap in clarifying the immune evasiveness and immune-resistance mechanisms. Identifying other candidate targets for cancer immunotherapy is therefore a clear unmet clinical need. The complement system, a pillar of innate immunity, has recently entered the limelight due to its immunoregulatory functions in the tumor microenvironment (TME). In particular, gC1qR, a receptor for globular heads of C1q, serves as a promising new target and has attracted more attention. gC1qR, also named P32/C1qBP/HABP1, is a multifunctional protein that is overexpressed in various cancers and holds prognostic value. It regulates the tumorigenic, progression and metastatic properties of tumor cells through several downstream signaling pathways, including the Wnt/β-catenin, PKC-NF-κB and Akt/PKB pathways. A few preclinical experiments conducted through gC1qR interventions, such as monoclonal antibody, chimeric antigen receptor T-cell (CAR-T) therapy, and tumor vaccination, have shown encouraging results in anticancer activity. The efficacy may rely on the regulatory role on the TME, induction of tumor cells apoptosis and antiangiogenic activity. Nevertheless, the current understanding of the relationship between cancer immunotherapy and gC1qR remains elusive and often contradictory, posing both opportunities and challenges for therapeutic translation in the clinic. In this review, we focus on the current understanding of gC1qR function in cancer immunology and highlight the vital roles in regulating the TME. We also examines the rationale behind targeting gC1qR and discusses the potential for translating into clinical practice.

An approach to p32/gC1qR/HABP1: a multifunctional protein with an essential role in cancer

P32/gC1qR/HABP1 is a doughnut-shaped acidic protein, highly conserved in eukaryote evolution and ubiquitous in the organism. Although its canonical subcellular localization is the mitochondria, p32 can also be found in the cytosol, nucleus, cytoplasmic membrane, and it can be secreted. Therefore, it is considered a multicompartmental protein. P32 can interact with many physiologically divergent ligands in each subcellular location and modulate their functions. The main ligands are C1q, hyaluronic acid, calreticulin, CD44, integrins, PKC, splicing factor ASF/SF2, and several microbial proteins. Among the functions in which p32 participates are mitochondrial metabolism and dynamics, apoptosis, splicing, immune response, inflammation, and modulates several cell signaling pathways. Notably, p32 is overexpressed in a significant number of epithelial tumors, where its expression level negatively correlates with patient survival. Several studies of gain and/or loss of function in cancer cells have demonstrated that p32 is a promoter of malignant hallmarks such as proliferation, cell survival, chemoresistance, angiogenesis, immunoregulation, migration, invasion, and metastasis. All of this strongly suggests that p32 is a potential diagnostic molecule and therapeutic target in cancer. Indeed, preclinical advances have been made in developing therapeutic strategies using p32 as a target. They include tumor homing peptides, monoclonal antibodies, an intracellular inhibitor, a p32 peptide vaccine, and p32 CAR T cells. These advances are promising and will allow soon to include p32 as part of targeted cancer therapies.

Complement C1q Binding Protein (C1QBP): Physiological Functions, Mutation-Associated Mitochondrial Cardiomyopathy and Current Disease Models

Complement C1q binding protein (C1QBP, p32) is primarily localized in mitochondrial matrix and associated with mitochondrial oxidative phosphorylative function. C1QBP deficiency presents as a mitochondrial disorder involving multiple organ systems. Recently, disease associated C1QBP mutations have been identified in patients with a combined oxidative phosphorylation deficiency taking an autosomal recessive inherited pattern. The clinical spectrum ranges from intrauterine growth restriction to childhood (cardio) myopathy and late-onset progressive external ophthalmoplegia. This review summarizes the physiological functions of C1QBP, its mutation-associated mitochondrial cardiomyopathy shown in the reported available patients and current experimental disease platforms modeling these conditions.

Overexpression of Multifunctional Protein p32 Promotes a Malignant Phenotype in Colorectal Cancer Cells

p32 is a multifunctional and multicompartmental protein that has been found upregulated in numerous adenocarcinomas, including colorectal malignancy. High levels of p32 expression have been correlated with poor prognosis in colorectal cancer. However, the functions performed by p32 in colorectal cancer have not been characterized. Here we show that p32 is overexpressed in colorectal cancer cell lines compared to non-malignant colon cells. Colon cancer cells also display higher nuclear levels of p32 than nuclear levels found in non-malignant cells. Moreover, we demonstrate that p32 regulates the expression levels of genes tightly related to malignant phenotypes such as HAS-2 and PDCD4. Remarkably, we demonstrate that knockdown of p32 negatively affects Akt/mTOR signaling activation, inhibits the migration ability of colon malignant cells, and sensitizes them to cell death induced by oxidative stress and chemotherapeutic agents, but not to cell death induced by nutritional stress. In addition, knockdown of p32 significantly decreased clonogenic capacity and in vivo tumorigenesis in a xenograft mice model. Altogether, our results demonstrate that p32 is an important promoter of malignant phenotype in colorectal cancer cells, suggesting that it could be used as a therapeutic target in colorectal cancer treatment.

Anti gC1qR/p32/HABP1 Antibody Therapy Decreases Tumor Growth in an Orthotopic Murine Xenotransplant Model of Triple Negative Breast Cancer

gC1qR is highly expressed in breast cancer and plays a role in cancer cell proliferation. This study explored therapy with gC1qR monoclonal antibody 60.11, directed against the C1q binding domain of gC1qR, in a murine orthotopic xenotransplant model of triple negative breast cancer. MDA231 breast cancer cells were injected into the mammary fat pad of athymic nu/nu female mice. Mice were segregated into three groups (n = 5, each) and treated with the vehicle (group 1) or gC1qR antibody 60.11 (100 mg/kg) twice weekly, starting at day 3 post-implantation (group 2) or when the tumor volume reached 100 mm³ (group 3). At study termination (d = 35), the average tumor volume in the control group measured 895 ± 143 mm³, compared to 401 ± 48 mm³ and 701 ± 100 mm³ in groups 2 and 3, respectively (p < 0.05). Immunohistochemical staining of excised tumors revealed increased apoptosis (caspase 3 and TUNEL staining) in 60.11-treated mice compared to controls, and decreased angiogenesis (CD31 staining). Slightly decreased white blood cell counts were noted in 60.11-treated mice. Otherwise, no overt toxicities were observed. These data are the first to demonstrate an in vivo anti-tumor effect of 60.11 therapy in a mouse model of triple negative breast cancer.

The C1q Receptors: Focus on gC1qR/p33 (C1qBP, p32, HABP-1)1

In the past several years, a number of C1q binding surface proteins or receptors have been described. This is not of course surprising considering the complexity of the C1q molecule and its ability to bind to a wide range of cellular and plasma proteins via both its collagen-like [cC1q] region and its heterotrimeric globular heads [gC1q] each of which in turn is capable of binding a specific ligand. However, while each of these "receptor" molecules undoubtedly plays a specific function within its restricted microenvironment, and therefore merits full attention, this review nonetheless, will singularly focus on the structure and function of gC1qR-a multi-functional and multi-compartmental protein, which plays an important role in inflammation, infection, and cancer. Although first identified as a receptor for C1q, gC1qR has been shown to bind to a plethora of proteins found in plasma, on the cell surface and on pathogenic microorganisms. The plasma proteins that bind to gC1qR are mostly blood coagulation proteins and include high molecular weight kininogen [HK], Factor XII [Hageman factor], fibrinogen, thrombin [FII], and multimeric vitronectin. This suggests that gC1qR can play an important role in modulating not only of fibrin formation, particularly at local sites of immune injury and/or inflammation, but by activating the kinin/kallikrein system, it is also able to generate, bradykinin, a powerful vasoactive peptide that is largely responsible for the swelling seen in angioedema. Another important function of gC1qR is in cancer, where it has been shown to play a role in tumor cell survival, growth and metastatic invasion by interacting with critical molecules in the tumor cell microenvironment including those of the complement system and kinin system. Finally, by virtue of its ability to interact with a growing list of pathogen-associated molecules, including bacterial and viral ligands, gC1qR is becoming recognized as an important pathogen recognition receptor [PRR]. Given the numerous roles it plays in a growing list of disease settings, gC1qR has now become a potential target for the development of monoclonal antibody-based and/or small molecule-based therapies.

Soluble gC1qR in Blood and Body Fluids: Examination in a Pancreatic Cancer Patient Cohort

BACKGROUND

gC1qR is a multifunctional cellular protein that has been linked to inflammation and cancer. gC1qR is highly upregulated in adenocarcinomas as compared to normal tissue counterparts, and soluble gC1qR (sgC1qR) has been detected in vitro in the pericellular milieu of proliferating malignant cells.

AIM

The present study explored the tissue expression of gC1qR in pancreatic cancer by immunohistochemistry, and the presence of sgC1qR in vivo, by examining blood and malignant effusions from patients with metastatic pancreatic adenocarcinoma.

METHODS

Tissue expression of gC1qR by pancreatic adenocarcinoma was visualized by immunohistochemistry. SgC1qR was quantified in serum from healthy volunteers (n=20) and pancreatic cancer patients (n=34), as well as in malignant pleural (n=23) and peritoneal effusions (n=27), using a newly developed, sensitive immunocapture sandwich ELISA.

RESULTS

Overexpression of gC1qR was confirmed in pancreatic adenocarcinoma compared to nonmalignant pancreatic tissue. Moreover, increased serum levels of sgC1qR (0.29 ± 0.22 ng/ml) were noted in patients with metastatic pancreatic cancer compared to healthy controls (0.15 ± 0.10 ng/ml) (mean ± S.D.) (p=0.035). In 11 of 16 patients for whom sequential samples were available, serum sgC1qR levels rose with disease progression, and paralleled changes in tumor biomarkers, CEA and CA19.9. In addition to blood, sgC1qR was detected in malignant pleural (0.55 ± 0.47 ng/ml) and peritoneal effusions (0.57 ± 0.38 ng/ml).

CONCLUSION

This study provides the first evidence for the presence of sgC1qR in vivo. The ability to detect sgC1qR in blood and body fluids will enable further studies to elucidate its pathophysiology in malignancy.

gC1qR expression in normal and pathologic human tissues: differential expression in tissues of epithelial and mesenchymal origin

The gC1qR (i.e., gC1q receptor, gC1q binding protein, p32, p33) is a multifunctional cellular protein that interacts with components of the complement, kinin, and coagulation cascades and select microbial pathogens. Enhanced gC1qR expression has been reported in adenocarcinomas arising in a variety of organs. The present study compared gC1qR expression in normal, inflammatory, dysplastic, and malignant tissue of epithelial and mesenchymal origin. gC1qR expression was visualized in tissue sections by immunohistochemistry using the 60.11 monoclonal antibody (i.e., IgG(1) mouse monoclonal antibody directed against gC1qR) and the UltraVision LP Detection System. Sections were counterstained with hematoxylin and examined by light microscopy. Strongest gC1qR expression was noted in epithelial tumors of breast, prostate, liver, lung, and colon, as well as in squamous and basal cell carcinoma of the skin. However, increased gC1qR staining was appreciated also in inflammatory and proliferative lesions of the same cell types, as well as in normal continuously dividing cells. In contrast, tumors of mesenchymal origin generally stained weakly, with the exception of osteoblasts, which stained in both benign and malignant tissues. The data suggest that increased gC1qR expression may be a marker of benign and pathologic cell proliferation, particularly in cells of epithelial origin, with potential diagnostic and therapeutic applications.

A proteomic approach to identification of plutonium-binding proteins in mammalian cells

Plutonium can enter the body through different routes and remains there for decades; however its specific biochemical interactions are poorly defined. We, for the first time, have studied plutonium-binding proteins using a metalloproteomic approach with rat PC12 cells. A combination of immobilized metal ion chromatography, 2D gel electrophoresis, and mass spectrometry was employed to analyze potential plutonium-binding proteins. Our results show that several proteins from PC12 cells show affinity towards Pu(4+)-NTA (plutonium bound to nitrilotriacetic acid). Proteins from seven different spots in the 2D gel were identified. In contrast to the previously known plutonium-binding proteins transferrin and ferritin, which bind ferric ions, most identified proteins in our experiment are known to bind calcium, magnesium, or divalent transition metal ions. The identified plutonium interacting proteins also have functional roles in downregulation of apoptosis and other pro-proliferative processes. MetaCore™ analysis based on this group of proteins produced a pathway with a statistically significant association with development of neoplastic diseases.

The multicompartmental p32/gClqR as a new target for antibody-based tumor targeting strategies

Tumor-associated cell surface antigens and tumor-associated vascular markers have been used as a target for cancer intervention strategies. However, both types of targets have limitations due to accessibility, low and/or heterogeneous expression, and presence of tumor-associated serum antigen. It has been previously reported that a mitochondrial/cell surface protein, p32/gC1qR, is the receptor for a tumor-homing peptide, LyP-1, which specifically recognizes an epitope in tumor cells, tumor lymphatics, and tumor-associated macrophages/myeloid cells. Using antibody phage technology, we have generated an anti-p32 human monoclonal antibody (2.15). The 2.15 antibody, expressed in single-chain fragment variable and in trimerbody format, was then characterized in vivo using mice grafted subcutaneously with MDA-MB-231 human breast cancers cells, revealing a highly selective tumor uptake. The intratumoral distribution of the antibody was consistent with the expression pattern of p32 in the surface of some clusters of cells. These results demonstrate the potential of p32 for antibody-based tumor targeting strategies and the utility of the 2.15 antibody as targeting moiety for the selective delivery of imaging and therapeutic agents to tumors.

The contribution of gC1qR/p33 in infection and inflammation

Human gC1qR/p33 is a multi-compartmental and multi-functional cellular protein expressed on a wide range of tissues and cell types including lymphocytes, endothelial cells, dendritic cells, and platelets. Although originally isolated as a receptor for C1q by virtue of its affinity (K(d)=15-50 nM), and specificity for the globular heads of this molecule, a large body of evidence has now been accumulated which shows that in addition to C1q, gC1qR can serve as a receptor for diverse proinflammatory ligands including proteins of the plasma kinin-forming system, most notably high molecular weight kininogen (HK; K(d)=9 nM). In addition, gC1qR has been reported to recognize and bind a number of functional antigens of viral and bacterial origin. It is its ability to interact with microbial antigens and its potential to serve as a cellular protein for bacterial attachment and/or entry that has been the focus of our laboratory in the past few years. On the surface of activated platelets, gC1qR has been shown to serve as a binding site for Staphylococcus aureus and this binding is mediated by protein A. Since the binding of S. aureus to platelets is postulated to play a major role in the pathogenesis of endocarditis, gC1qR may provide a suitable surface for the initial adhesion of the bacterium. Recent data also demonstrate that the exosporium of Bacillus cereus, a member of a genus of aerobic, Gram-positive, spore-forming rod-like bacilli, which includes the deadly Bacillus anthracis, contains a binding site for gC1qR. Therefore, by virtue of its ability to recognize plasma proteins such as C1q and HK, as well as bacterial and viral antigens, cell-surface gC1qR not only is able to generate proinflammatory byproducts from the complement and kinin/kallikrein systems, but also can be an efficient vehicle and platform for a plethora of pathogenic microorganisms.

Direct binding of hepatitis C virus core to gC1qR on CD4+ and CD8+ T cells leads to impaired activation of Lck and Akt

Complement plays a pivotal role in the regulation of innate and adaptive immunity. It has been shown that the binding of C1q, a natural ligand of gC1qR, on T cells inhibits their proliferation. Here, we demonstrate that direct binding of the hepatitis C virus (HCV) core to gC1qR on T cells leads to impaired Lck/Akt activation and T-cell function. The HCV core associates with the surface of T cells specifically via gC1qR, as this binding is inhibited by the addition of either anti-gC1qR antibody or soluble gC1qR. The binding affinity constant of core protein for gC1qR, as determined by BIAcore analysis, is 3.8 x 10(-7) M. The specificity of the HCV core-gC1qR interaction is confirmed by reduced core binding on Molt-4 T cells treated with gC1qR-silencing small interfering RNA and enhanced core binding on GPC-16 guinea pig cells transfected with human gC1qR. Interestingly, gC1qR is expressed at higher levels on CD8(+) than on CD4(+) T cells, resulting in more severe core-induced suppression of the CD8(+)-T-cell population. Importantly, T-cell receptor-mediated activation of the Src kinases Lck and ZAP-70 but not Fyn and the phosphorylation of Akt are impaired by the HCV core, suggesting that it inhibits the very early events of T-cell activation.

The impact of alpha-lipoic acid, coenzyme Q10 and caloric restriction on life span and gene expression patterns in mice

We evaluated the efficacy of three dietary interventions started at middle age (14 months) to retard the aging process in mice. These were supplemental alpha-lipoic acid (LA) or coenzyme Q(10) (CQ) and caloric restriction (CR, a positive control). LA and CQ had no impact on longevity or tumor patterns compared with control mice fed the same number of calories, whereas CR increased maximum life span by 13% (p <.0001) and reduced tumor incidence. To evaluate these interventions at the molecular level, we used microarrays to monitor the expression of 9977 genes in hearts from young (5 months) and old (30 months) mice. LA, CQ, and CR inhibited age-related alterations in the expression of genes involved in the extracellular matrix, cellular structure, and protein turnover. However, unlike CR, LA and CQ did not prevent age-related transcriptional alterations associated with energy metabolism. LA supplementation lowered the expression of genes encoding major histocompatibility complex components and of genes involved in protein turnover and folding. CQ increased expression of genes involved in oxidative phosphorylation and reduced expression of genes involved in the complement pathway and several aspects of protein function. Our observations suggest that supplementation with LA or CQ results in transcriptional alterations consistent with a state of reduced oxidative stress in the heart, but that these dietary interventions are not as effective as CR in inhibiting the aging process in the heart.

Differential gene expression in skeletal muscle after induction of heart failure: impact of cytokines on protein phosphatase 2A expression

Different intrinsic alterations of skeletal muscle metabolism and gene expression have been described in chronic heart failure (CHF). As proposed skeletal muscle alterations in CHF may contribute to exercise intolerance and early muscular fatigue. However the exact molecular changes occurring in the skeletal muscle are still unclear. The aim of this study was to characterize the pattern of differential gene expression in an animal model of CHF and to study the regulation of one selected gene. Rats were subjected to LAD ligation or sham operation. mRNA was isolated from musculus quadriceps of both groups and differential gene expression was determined by subtractive hybridization. Quantitative RT-PCR and cell culture experiments were performed to further characterize the changed expression of protein phosphatase 2A (PP2A) in human skeletal muscle biopsies as well as the cytokine dependent regulation of PP2A expression. Out of 800 picked clones differential expression of 24 distinct genes could be identified by sequencing and reverse Northern blotting. PP2A expression demonstrated a significant upregulation in skeletal muscle biopsies from patients with CHF as compared to healthy controls (9.7 +/- 1.9 vs. 4.2 +/- 0.7 arbitrary units; p<0.05). Incubation of rat skeletal muscle myoblasts with a combination of TNF-alpha, IL-1beta, and gamma-IFN caused a 3-fold upregulation of PP2A expression vs. untreated cells. These results suggest that CHF is accompanied by changes in expression of genes involved in energy metabolism, contractility, and apoptosis in the skeletal muscle. The upregulation of PP2A, an important regulator in intracellular signaling and apoptosis, may be due to an increase of inflammatory cytokines.

p32 (gC1qBP) is a general protein kinase C (PKC)-binding protein; interaction and cellular localization of P32-PKC complexes in ray hepatocytes

The aim of this study was to identify cellular proteins that bind protein kinase C (PKC) and may influence its activity and its localization. A 32-kDa PKC-binding protein was purified to homogeneity from the Triton X-100-insoluble fraction obtained from hepatocytes homogenates. The protein was identified by NH(2)-terminal amino acid sequencing as the previously described mature form of p32 (gC1qR). Recombinant p32 was expressed as a glutathione S-transferase fusion protein, affinity-purified, and tested for an in vitro interaction with PKC using an overlay assay approach. All PKC isoforms expressed in rat hepatocytes interacted in vitro with p32, but the binding dependence on PKC activators was different for each one. Whereas PKCdelta only binds to p32 in the presence of PKC activators, PKCzeta and PKCalpha increase their binding when they are in the activated form. Other PKC isoforms such as beta, epsilon, and theta bind equally well to p32 regardless of the presence of PKC activators, and PKCmu binds even better in their absence. It was also found that p32 is not a substrate for any of the PKC isoforms tested, but interestingly, its presence had a stimulatory effect (2-fold for PKCdelta) on PKC activity. We also observed in vivo interaction between PKC and p32 by immunofluorescence and confocal microscopy. A time course of phorbol ester treatment of cultured rat hepatocytes (C9 cells) showed that PKCtheta and p32 are constitutively associated in vivo, whereas PKCdelta activation is required for its association with p32. Our data also showed that phorbol ester treatment induces a transient translocation of p32 from the cytoplasm to the cell nucleus. Together, these findings suggest that p32 may be a regulator of PKC location and function.

C1q and mannose binding lectin engagement of cell surface calreticulin and CD91 initiates macropinocytosis and uptake of apoptotic cells

Removal of apoptotic cells is essential for maintenance of tissue homeostasis, organogenesis, remodeling, development, and maintenance of the immune system, protection against neoplasia, and resolution of inflammation. The mechanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we provide evidence that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the alpha-2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune system. Ingestion of the apoptotic cells through calreticulin/CD91 stimulation is further shown to involve the process of macropinocytosis, implicated as a primitive and relatively nonselective uptake mechanism for C1q- and MBL-enhanced engulfment of whole, intact apoptotic cells, as well as cell debris and foreign organisms to which these molecules may bind.

C1q and mannose binding lectin engagement of cell surface calreticulin and CD91 initiates macropinocytosis and uptake of apoptotic cells

Removal of apoptotic cells is essential for maintenance of tissue homeostasis, organogenesis, remodeling, development, and maintenance of the immune system, protection against neoplasia, and resolution of inflammation. The mechanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we provide evidence that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the alpha-2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune system. Ingestion of the apoptotic cells through calreticulin/CD91 stimulation is further shown to involve the process of macropinocytosis, implicated as a primitive and relatively nonselective uptake mechanism for C1q- and MBL-enhanced engulfment of whole, intact apoptotic cells, as well as cell debris and foreign organisms to which these molecules may bind.

Interaction between GABAA Receptor β Subunits and the Multifunctional Protein gC1q-R

gamma-Aminobutyric acid type A (GABA(A)) receptors were immunopurified from bovine brain using a monoclonal antibody directed against the alpha1 subunit. Of the several proteins that copurified, a 34-kDa protein was analyzed further. After enrichment and tryptic proteolysis, the resulting fragments were sequenced, and the protein was identified as gC1q-R. Using anti-gC1q-R and anti-GABA(A) receptor antibodies, mutual coimmunoprecipitation could be demonstrated from solubilized rat brain membranes. The stability of this interaction was estimated to be very high. Using the yeast two-hybrid system, various GABA(A) receptor subunit intracellular loop constructs were tested for an interaction with gC1q-R. All beta subunits, but not alpha 1 and gamma 2 subunits, were found to bind to gC1q-R. NH(2)- and COOH-terminally truncated beta 2 subunit loops were used to find the region responsible for the interaction with gC1q-R. A stretch of 15 amino acids containing 7 positively charged residues was identified (amino acids 399--413). This region contains residue Ser-410, which is a protein kinase substrate, and it is known that phosphorylation of this residue leads to an alteration in receptor activity. Localization studies suggested a predominantly intracellular localization. Our observations therefore suggest a tight interaction between gC1q-R and the GABA(A) receptor which might be involved in receptor biosynthesis or modulation of the mature function.

The human gC1qR/p32 gene, C1qBP. Genomic organization and promoter analysis

gC1qR is an ubiquitously expressed cell protein that interacts with the globular heads of C1q (gC1q) and many other ligands. In this study, the 7.8-kilobase pair (kb) human gC1qR/p32 (C1qBP) gene was cloned and found to consist of 6 exons and 5 introns. Analysis of a 1.3-kb DNA fragment at the 5'-flanking region of this gene revealed the presence of multiple TATA, CCAAT, and Sp1 binding sites. Luciferase reporter assays performed in different human cell lines demonstrated that the reporter gene was ubiquitously driven by this 1.3-kb fragment. Subsequent 5' and 3' deletion of this fragment confined promoter elements to within 400 base pairs (bp) upstream of the translational start site. Because the removal of the 8-bp consensus TATATATA at -399 to -406 and CCAAT at -410 to -414 did not significantly affect the transcription efficiency of the promoter, GC-rich sequences between this TATA box and the translation start site may be very important for the promoter activity of the C1qBP gene. One of seven GC-rich sequences in this region binds specifically to PANC-1 nuclear extracts, and the transcription factor Sp1 was shown to bind to this GC-rich sequence by the supershift assay. Primer extension analysis mapped three major transcription start regions. The farthest transcription start site is 49 bp upstream of the ATG translation initiation codon and is in close proximity of the specific SP1 binding site.

Characterization of the murine gene of gC1qBP, a novel cell protein that binds the globular heads of C1q, vitronectin, high molecular weight kininogen and factor XII

gC1qBP is a novel cell protein which was found to interact with the globular heads of C1q, high mol. wt kininogen, factor XII and the heparin-binding, multimeric form of vitronectin. The protein sequence shows no homology to any protein family. This paper describes the genomic organization of mouse gC1qBP and the characterization of its 5' flanking region. The mouse gene consists of six exons separated by five introns, and its total length is approximately 6kb. Exon 1 encodes the putative signal peptide, a long stretch of 70 amino acid residues, and the first four amino acid residues found in the mature gC1qBP. Exons 2-5 encode four very hydrophilic domains, whereas exon 6 encodes a neutral domain. The amino acid sequence responsible for binding to the heparin-binding, multimeric form of vitronectin is located in exon 2. A 1kb DNA fragment upstream of the first initiation codon was sequenced, which contained four potential TATA boxes, seven CAAT boxes, six SP1 sites and various putative transcription factor-binding elements, indicating that the promoter region is in close proximity to the first exon. The mouseC1qbp gene was mapped to chromosome 11, closely linked to D11Mit4 using genomic DNAs from a (C57BL/6J x Mus spretus)F1 x Mus spretus backcross.