Abstract
The complement system represents an effective arsenal of innate immunity as well as an interface between innate and adaptive immunity. Activation of the complement system culminates with the assembly of the C5b-9 terminal complement complex on cell membranes, inducing target cell lysis. Translation of this sequence of events into a malignant setting has traditionally afforded C5b-9 a strict antitumoral role, in synergy with antibody-dependent tumor cytolysis. However, in recent decades, a plethora of evidence has revised this view, highlighting the tumor-promoting properties of C5b-9. Sublytic C5b-9 induces cell cycle progression by activating signal transduction pathways (e.g., Gi protein/ phosphatidylinositol 3-kinase (PI3K)/Akt kinase and Ras/Raf1/ERK1) and modulating the activation of cancer-related transcription factors, while shielding malignant cells from apoptosis. C5b-9 also induces Response Gene to Complement (RGC)-32, a gene that contributes to cell cycle regulation by activating the Akt and CDC2 kinases. RGC-32 is expressed by tumor cells and plays a dual role in cancer, functioning as either a tumor promoter by endorsing malignancy initiation, progression, invasion, metastasis, and angiogenesis, or as a tumor suppressor. In this review, we present recent data describing the versatile, multifaceted roles of C5b-9 and its effector, RGC-32, in cancer.
Introduction
Carcinogenesis in human somatic cells involves a series of genetic and epigenetic alterations that culminate in the generation of a malignant tissue fully prepared to elude most anticancer defense strategies. To date, there are seven key alterations in cancerous cells: self-sufficiency in growth signals; insensitivity to growth suppressors; evasion of apoptosis, enabling replicative immortality; sustained angiogenesis; tissue invasion (metastasis) () and the presence of cancer-related inflammation (CRI) (). Essential orchestrators of CRI are the tumor-associated inflammatory cells (macrophages, fibroblasts, T cells and myeloid-derived suppressor cells) and their secreted chemokines and cytokines, along with complement activation within the tumor microenvironment (, ).
The complement system represents an effective arsenal of innate immunity as well as an interface between innate and adaptive immunity. An ancestral instrument in fighting invasive pathogens and efficient clearance of debris, the complement system can be activated by the classical, alternative, or lectin pathway, all of which unite at the level of C3 activation. All three pathways lead to the membrane attack complex formation and to cell lysis. Activation of the terminal complement proteins C5 to C9 generates membrane-inserted complexes C5b-7, C5b-8, and finally C5b-9, the so-called membrane attack complex (MAC) (, ).
Evidence supporting complement activation, in association with C5b-9 deposits during the antitumoral response exists for a variety of human malignancies (). Niculescu et al. provided the first immunohistochemical support for the presence of C5b-9 deposits (along with IgG, C3 and C4 deposits) in a human cancerous tissue, namely breast carcinoma (). Thereafter, numerous studies demonstrated C5b-9 deposition indicating complement activation in human thyroid (), ovarian (), endometrial (), gastric (–), liver (), colon, renal, and lung carcinomas (), as well as in human osteosarcoma (), medulloblastoma (), glioma (, ), and gastrointestinal stromal tumor () tissues. High levels of soluble C5b-9 were also detected in the ascitic fluid of ovarian cancer (). Elevated circulating C9 protein levels have been reported in the serum or plasma of colon () and gastric () adenocarcinoma, oral squamous cell carcinoma () and squamous cell lung cancer () patients.
C5b-9 has been shown to possess antitumoral properties, acting in synergy with monoclonal antibody (mAb)-based immunotherapies, many of which use complement activation and C5b-9 as an effector to kill tumor cells (, ). In this context, the mAb triggers C5b-9 assembly on cells leading to tumor destruction. Nevertheless, in recent decades, a plethora of evidence has brought about a conceptual switch in this paradigm () and exposed the tumor-promoter properties of C5b-9.
Here, we summarize the available data concerning the complex and versatile role of C5b-9, and that of its pivotal effector RGC-32, in cancer.
Effects of Lytic C5b-9 on Tumor Cells
Successful achievement of cell lysis during complement-dependent cytotoxicity (CDC) requires the formation of multiple C5b-9 complexes on the cell surface (). Once malignant Ehrlich ascites cells already bearing C5b-8 complexes are exposed to C9, a rapid and extensive ATP depletion, coupled with leakage of the adenine nucleotides ATP, ADP, and AMP, precedes cell death. Other prelytic events include the loss of mitochondrial membrane potential with consequent defective ATP synthesis and a vigorous Ca2+ influx, which initiate necrotic cell death ().
The morphologic and biochemical changes induced by lytic MAC attack do share some features with those seen in apoptosis (nucleolar changes), although most features correspond more closely to necrotic changes (loss of volume control and defective mitochondria) (, ). The main biochemical changes include Bid cleavage, caspase activation, and activation of extracellular DNases (–). The impact of lytic C5b-9 on the malignant signaling pathways is multifaceted, since CDC has been documented to use several necrotic cell death pathways involving the receptor-interacting protein kinase 1 (RIPK1), receptor-interacting protein kinase 3 (RIPK3) and mixed-lineage kinase domain-like protein (MLKL) (), in concert with the effectors JNK and Bid. The RIPK1/RIPK3/MLKL pathway closely resembles TNF-alpha-induced necroptosis ().
Tumor cells have developed complement resistance through C5b-9 removal (), expression of membrane complement-regulatory proteins (mCRPs) and other cell surface-protective molecules, and secretion of soluble complement inhibitors (, ). The ability of a cell to survive an initial complement-mediated membrane attack affords its resistance against future attacks ().
The use of mAb-based immunotherapies that stimulate the destruction of tumor cells by CDC has received a lot of interest (). The quest for optimal efficacy in CDC has incited many research teams. For instance, Diebolder et al. have shown that IgG hexamerization after antigen binding leads to more effective complement activation and fixation, and thus a more potent CDC (). Narrow C5b-8 pores formed without C9 are sufficient for CDC due to efficient antibody-mediated hexamer formation (). By neutralizing mCRP expression on leukemia cells, Mamidi et al. were the first to achieve both enhanced CDC and improved complement-dependent cellular cytotoxicity by monocyte-derived macrophages and macrophages induced by two anti-CD20 antibodies (rituximab and ofatumumab) and one anti-CD52 antibody (alemtuzumab) (). Of late, the miR-200b, miR-200c, and miR-217 microRNAs have been recognized as potential regulators of mortalin as well as CD46 and CD55 expression in leukemia/ lymphoma and have been observed to coordinate the quantity of C5b-9 deposited on target cells ().
Sublytic C5b-9 Induces Tumor Cell Proliferation and Transcriptional Activation in Malignant Cells
Sublytic levels of C5b-9 assembly in the membrane of malignant cells generate several different biological responses: activation of signal transduction pathways, proliferation, and modulation of apoptosis (, ) (Figure 1).
Figure 1
One of the first investigations of C5b-9 looked at the generation of signal messengers in Ehrlich carcinoma cells by the sublytic terminal complement complexes (TCC) C5b-9, C5b-8, and C5b-7 and identified the signal messengers involved in eliminating TCC from the cell surface (
One of the pioneer studies on sublytic C5b-9-induced tumor cell proliferation through mitotic signaling (
Among the signal transduction networks regulating cancer progression that have been found to function downstream of sublytic C5b-9 are p38/MAPK/JNK1 and JAK1/STAT3 (
Of note, activation of activating protein 1 (AP-1) transcription factor has also been documented following C5b-9 treatment of lymphoblastoid B-cell lines (
In smooth muscle cells (SMC), sublytic assault by MAC stimulates release of insulin-like growth factor-1 (
Non-lethal C5b-9 activates cell cycle by directly influencing major cell cycle regulators: in aortic SMC, sublytic C5b-9 increases the activity of the cyclin-dependent kinases CDK4 and CDK2, whereas in endothelial cells it increases the levels of cell division cycle protein 2 (CDC2), cyclin D1, and proliferating cell nuclear antigen (PCNA) (
Sublytic C5b-9 Protects Tumor Cells From Apoptosis
Sublytic complement-induced protection against TNF-α-mediated apoptosis accompanies the induction of the anti-apoptotic proteins Bcl-2 and Bcl-xL, along with suppressing the TNF-α-induced decrease in the amount of Bcl-2 and Bcl-xL (
It should be noted, however, that data also exist in support of the ability of sublytic C5b-9 to activate various molecules that potentially contribute to programed cellular death. In lung epithelial cells, MAC insertion has been observed to induce Ca2+ influx, leading to mitochondrial overload and loss of mitochondrial transmembrane potential. These changes prompt NLPR3 inflammasome activation, as well as IL-1β production, cytoplasmatic cytochrome c release and caspase activation (
C5b-9 and Angiogenesis
Although initiated by cellular destruction and hypoxia, the propagation of the vascular network in a malignant environment is sustained by upregulation of pro-angiogenic factors (e.g. vascular endothelial growth factor [VEGF], TGF-α, TGF-β, TNF-α, EGF, fibroblast growth factor [FGF]) and downregulation of negative angiogenic regulators (IL-10, IL-12, angiopoietin-2, angiotensin) (72).
Accelerated C5b-9 deposition, accompanied by VEGF, β-FGF, and TGF-β2 release is seen during laser-induced choroidal neovascularization in age-related macular degeneration in CD59-deficient mice (73). Likewise, exposure of retinal pigment epithelium cells to oxidative stress has been found to induce sublytic C5b-9 activation, triggering VEGF secretion via the Src and Ras-Erk pathways (74).
The effects of C5b-9 were later corroborated in cancer cells. In an osteosarcoma epithelial cell line, sublytic C5b-9 activation (via the alternative pathway) instigated production of angiogenic growth factors FGF1 and VEGF-A via the ERK signaling pathway (
RGC-32 and Cancer
The RGC-32 gene was first cloned from rat oligodendrocytes via differential display by Badea and coworkers, in their quest to identify the genes differentially expressed in response to sublytic complement activation (75, 76). RGC-32 fundamentally regulates cellular processes such as the cell cycle, differentiation, wound healing and tumorigenesis (75, 77). It directly binds to cyclin-dependent kinase CDC2 and Akt and stimulates their kinase activity (75, 78).
Various studies have described an aberrant RGC-32 mRNA expression in human cancers: up-regulation in colon (79, 80), ovarian (81, 82), breast (79, 83, 84) and prostate (79) cancers and lymphomas (85, 86) and downregulation in glioblastomas (87), astrocytomas (88), adrenocortical carcinomas (89), and multiple myelomas (90).
We have originally demonstrated a role for RGC-32 deregulation in colon adenocarcinoma, showing that the intensity of RGC-32 immunohistochemical staining corresponded to the increase in the TNM staging of the adenocarcinomas (77). Later, the expression of RGC-32 was shown to be up-regulated in pancreatic cancer tissues and to correlate with TNM stages (91).
Using a gene array and SW480 colon adenocarcinoma cells, we have identified groups of genes that are significantly changed by RGC-32 silencing (77), including genes implicated in chromatin assembly, cell cycle, and RNA processing. We have observed increased lysine acetylation at multiple sites on histones H2B, H3, and H4, and lessened expression of the histone deacetylase SIRT1 upon silencing of RGC-32 expression in SW480 cells (77) (Figure 2). Moreover, an absence of RGC-32 expression induces DNA synthesis and mitosis in colon cancer cells (77). Correspondingly, overexpression of RGC-32 in several cancer cell lines has been shown to delay G2/M cell cycle progression (88).
Figure 2

Molecular mechanisms underlying the role of RGC-32 in oncogenesis. RGC-32 can act both as a tumor suppressor (red inhibitory lines) and a tumor promoter (blue arrows) in a variety of cancers by activating a plethora of molecular pathways. RGC-32 plays an important role in: (a) promoting the TGF-β-induced epithelial-to-mesenchymal transition (EMT), a process in which epithelial cells lose their adhesiveness and gain myofibroblast-like phenotypes, inducing metastasis and cancer progression (80, 91, 92); (b) epigenetic modifications, by inducing histone deacetylases (HDACs), which in turn deacetylate various histone targets such as H2B at lysine 5 (H2BK5), H2BK15, H3K9, and H4K8 and indirectly promote the tri-methylation of H3K27. This in turn may result in transcriptional repression of genes associated with cancer progression (77); (c) cell cycle regulation, in which RGC-32 can promote mitosis by enhancing the activity of kinases crucial for cell cycle progression (93), or induce cell cycle arrest in a p53-dependent manner (88); (d) inhibition of angiogenesis, in which it may behave as a negative feedback regulator of hypoxia-induced signaling pathways (94). The involvement of RGC-32 in these processes might explain its apparent dual role as a tumor suppressor/promoter in the same type of tumor, such as colon cancer.
On the other hand, others have reported that RGC-32 promotes malignant cell proliferation in the colon adenocarcinoma cell line SW480 (80) and in lung adenocarcinoma LTE cells (92). Overexpression of RGC-32 protein in Epstein Barr virus (EBV)-immortalized B cells has been found to disrupt the G2/M checkpoint via CDK1 activation, and RGC-32 has been shown to be indispensable for the growth and survival of lymphoblastoid B cells (86, 93) (Figure 2).
The cooperation of RGC-32 with SMAD3, as TGF-β downstream effectors, in the regulation of EMT seen in renal tubular cells (95) indicates a possible involvement of RGC-32 in invasion and metastasis. RGC-32 was shown to influence expression of vimentin, cadherin, and the transcription factors Snail and Slug in pancreatic and colon cancer lines (80, 91) (Figure 2). Also, excessive RGC-32 expression in a colon cancer cell line prompts cytoskeleton reorganization and cell migration (96). Similarly, RGC-32 has been demonstrated to induce EMT and to promote cancer cell migration and invasion in lung adenocarcinoma cells via decreases in the protein level and activity of the matrix metalloproteinases MMP-2 and MMP-9 (92, 97).
Research focusing on the effects of RGC-32 in animal cancer models has yielded contrasting data. Colon cancer tumors lacking RGC-32 that were implanted into nude mice were observed to have a lower growth rate and significantly smaller tumor volumes than did the tumors with intact RGC-32 expression (80). In striking contrast, inoculating RGC-32 into colon cancer tumors placed subcutaneously in mice resulted in a significant tumor growth suppression and decline in angiogenesis (94) (Figure 2).
The thesis of RGC-32 as a functional dyad (tumor suppressor/ promoter) accounts for its contradictory behavior during cancerogenesis: the protein acts in a pleiotropic manner in distinct malignant settings, dependent on the cellular lineage and on the various ligands. For instance, RGC-32 exerts a tumor-suppressive effect in lung adenocarcinomas with wild-type TP53, but a tumor-promoting effect in the tumors carrying TP53 mutations (98). Targeting RGC-32 should be done in conjunction with the role played in specific tumors as well as by using biomarkers that can predict the efficacy of RGC-32 inhibitors in cancer patients. Future studies are needed in order to find effective RGC-32-based drugs.
Conclusions
Considering all the available data, the role of C5b-9 in cancer is indisputably versatile: while it is lethal to tumor cells in a lytic context, when C5b-9 becomes activated to a sublytic level, it instead stimulates tumor growth through several mechanisms. Counteracting these tumor-promoting traits of C5b-9 by therapeutically surmounting CDC resistance in cancer cells and potentiating the antitumoral actions of C5b-9 (and therefore the efficacy of mAb-based immunotherapy) constitutes the next major direction in the field of immuno-oncology.
Statements
Author contributions
HR designed the study. SV, AT, VR, and HR wrote the manuscript. All authors approved the manuscript.
Funding
This work was supported in part by a Veterans Administration Merit Award I01BX001458 (to HR) and by an RO1 NS42011 grant (to HR).
Acknowledgments
We thank Dr. Deborah McClellan for editing this manuscript.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
- AP-1
activator protein 1
- CDC
complement-dependent cytotoxicity
- CDK1
cyclin–dependent kinase 1
- CyB1
cyclin B1
- CyE
cyclin E
- EBV
Epstein-Barr virus
- EGF
epidermal growth factor
- ERK1
extracellular signal-regulated kinase 1
- FGF2
fibroblast growth factor 2
- HIF-1α
hypoxia inducible factor 1 alpha
- IKKα
inhibitor of nuclear factor kappa-B kinase subunit alpha
- JAK1
Janus kinase 1
- MAC
membrane attack complex
- MEK1/2
mitogen activated protein kinase kinase 1
- mTORC2
mammalian target of rapamycin complex 2
- NF-κB
nuclear factor kappa-light-chain-enhancer of activated B cells
- PLK1
polo-like kinase 1
- RGC-32
response gene to complement 32
- RIPK
receptor-interacting protein kinase
- TCC
terminal complement complex
- VEGF
vascular endothelial growth factor
- STAT3
signal transducer and activator of transcription 3.
Abbreviations
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Summary
Keywords
C5b-9, cancer, RGC-32, cell proliferation, apoptosis
Citation
Vlaicu SI, Tatomir A, Rus V and Rus H (2019) Role of C5b-9 and RGC-32 in Cancer. Front. Immunol. 10:1054. doi: 10.3389/fimmu.2019.01054
Received
30 November 2018
Accepted
24 April 2019
Published
09 May 2019
Volume
10 - 2019
Edited by
Trent M. Woodruff, University of Queensland, Australia
Reviewed by
Zvi Fishelson, Tel Aviv University, Israel; Jun Yan, University of Louisville, United States
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Copyright
© 2019 Vlaicu, Tatomir, Rus and Rus.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Horea Rus hrus@umaryland.edu
This article was submitted to Cancer Immunity and Immunotherapy, a section of the journal Frontiers in Immunology
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