Assignment of C1QBP encoding the C1q globular domain binding protein (gC1q-R) to human chromosome 17 band p13.3 by in situ hybridization

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.

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.

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.

cC1qR/CR and gC1qR/p33: observations in cancer

The survival and growth of a primary tumor depends, by and large, on three major events: immune evasion, angiogenesis and metastasis. Tumor cells are "modified self", and as such express a plethora of modified surface antigens capable of inducing antibody production. Anti-tumor cell antibodies should, in theory, activate complement resulting in cell destruction. But this is not the case. Akin to many pathogenic microorganisms whose survival depends on evading the immune system, cancer cells have also evolved diverse mechanisms to prevent host mediated cell destruction by either retaining critical regulatory molecules or by hijacking host proteins to ensure their survival. Although immune evasion, angiogenesis and metastasis are complex biological processes involving a myriad of tumor associated proteins, enzymes, and cytokines, C1qRs can, nonetheless play an important role in all or part of these processes. Although both cC1qR/CR and gC1qR are expressed by all somatic cells, with the exception of red blood cells, both are highly upregulated on almost all types of tumors. It is not surprising therefore that blockade of C1qR on tumor cells inhibits their proliferation suggesting the significance of C1qRs in tumor growth and progression. Interestingly, the two C1q receptors: cC1qR/CR and gC1qR play a differential role in carcinogenesis. While gC1qR promotes tumor cell survival by enhancing angiogenesis and metastasis and also by contributing to the hypercoagulable and prothrombotic microenvironment, cC1qR/CR expression represents a pro-phagocytic "eat-me" signal through which cC1qR/CR expressing tumor cells are tagged for destruction by macrophages. The data accumulated to date therefore identify gC1qR and cC1qR/CR as potential targets for the design of either protein-based, antibody-based or chemical based therapeutic intervention that could be used to enhance conventional anti-cancer therapy. The inhibition of tumor cell proliferation by monoclonal antibody recognizing the C1q site on gC1qR, as well as the identification of agents such as anthracyclin that enhance cC1qR/CR expression on tumor cells, are indeed steps in the right direction.

Bivariate Linkage between Acylation-Stimulating Protein and BMI and High-Density Lipoproteins

OBJECTIVE

Given the importance of visceral adiposity in the metabolic syndrome, whether levels of adipokines have shared genetic effects (pleiotropy) with aspects of the metabolic syndrome should be addressed. Acylation-stimulating protein (ASP), an adipose-derived protein, influences lipid metabolism, obesity, and glucose use. Therefore, our objective was to examine the genetic regulation of ASP and associated pleiotropic effects.

RESEARCH METHODS AND PROCEDURES

We assayed serum ASP levels in 435 Mexican Americans participating in the San Antonio Family Heart Study and performed univariate and bivariate variance components analysis.

RESULTS

Additive genetic heritability of ASP was 26% (p = 0.0004). Bivariate genetic analysis detected significant genetic correlations between ASP and several lipid measures but not between ASP and adiposity or diabetes measures. We detected two potential quantitative trait loci influencing ASP levels. The strongest signal was on chromosome 17 near marker D17S1303 [log of the odds ratio (LOD) = 2.7]. The signal on chromosome 15 reached its peak near marker D15S641 (LOD = 2.1). Both signals localize in regions reported to harbor quantitative trait loci influencing obesity and lipid phenotypes in this population. Bivariate linkage analysis yielded LODs of 4.7 for ASP and BMI on chromosome 17 and 3.2 for ASP and high-density lipoprotein2a on chromosome 15.

DISCUSSION

Given these findings, there seems to be a significant genetic contribution to variation in circulating levels of ASP and an interesting pattern of genetic correlation (i.e., pleiotropy) with other risk factors associated with the metabolic syndrome.

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.

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.

The splicing factor-associated protein, p32, regulates RNA splicing by inhibiting ASF/SF2 RNA binding and phosphorylation

The cellular protein p32 was isolated originally as a protein tightly associated with the essential splicing factor ASF/SF2 during its purification from HeLa cells. ASF/SF2 is a member of the SR family of splicing factors, which stimulate constitutive splicing and regulate alternative RNA splicing in a positive or negative fashion, depending on where on the pre-mRNA they bind. Here we present evidence that p32 interacts with ASF/SF2 and SRp30c, another member of the SR protein family. We further show that p32 inhibits ASF/SF2 function as both a splicing enhancer and splicing repressor protein by preventing stable ASF/SF2 interaction with RNA, but p32 does not block SRp30c function. ASF/SF2 is highly phosphorylated in vivo, a modification required for stable RNA binding and protein-protein interaction during spliceosome formation, and this phosphorylation, either through HeLa nuclear extracts or through specific SR protein kinases, is inhibited by p32. Our results suggest that p32 functions as an ASF/SF2 inhibitory factor, regulating ASF/SF2 RNA binding and phosphorylation. These findings place p32 into a new group of proteins that control RNA splicing by sequestering an essential RNA splicing factor into an inhibitory complex.

Structure and function of gC1q-R: a multiligand binding cellular protein

gC1q-R is a 33 kDa, single chain, highly acidic protein, which was first isolated from membrane preparation of Raji cells and now appears to be ubiquitously distributed. Although, gC1q-R was originally identified as a protein which binds to the globular "heads" of C1q, recent evidence suggests that the molecule is in fact a multiligand binding, multifunctional protein with affinity for diverse ligands which at best are functionally related. These molecules include: thrombin, vitronectin, and high molecular weight kininogen. The gC1q-R molecule, which is identical to the transcription factors SF2 and the Tat-associated protein, or TAP, is the product of a single gene localized on chromosome 17p13.3 in human, and chromosome 11 in mouse, and is encoded by an approximately 1.5-1.6 kb mRNA. The full length cDNA encodes a primary translation protein of 282 residues and the 'mature' or membrane form of the protein isolated from Raji cells corresponds to residues 74-282 and is presumed to be generated by a site-specific cleavage and removal of the highly basic, 73-residues long, N-terminal segment during post-translational processing. The translated amino acid sequence does not predict for the presence of a conventional sequence motif compatible with a transmembrane segment and does not have a consensus site for a GPI anchor. However, there is strong evidence which indicates that gC1q-R is expressed both inside the cell and on the membrane. First, certain mAbs raised against gC1q-R react moderately with intact Raji cells in suspension and this binding increases when the cells are first bound to poly-L-lysine coated surfaces and then fixed with glutaraldehyde. Second, surface labeling of cells using the membrane impermeable sulfo-NHS-LC-biotin shows that gC1q-R on the surface incorporates biotin whereas intracellular gC1q-R does not. In addition, the membrane expression of gC1q-R can be upregulated with inflammatory cytokines such as INF-gamma, TNF-alpha, or LPS. These results suggest, that gC1q-R, is localized both as an intracellular and as a cell surface protein and may have important biological functions in both compartments of the cell.

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.