Phosphorylation of the complement component, C9, by an ecto-protein kinase of human leukemic cells

Casein Kinases 2-dependent phosphorylation of the placental ligand VAR2CSA regulates Plasmodium falciparum-infected erythrocytes cytoadhesion

Placental malaria is characterized by the massive accumulation and sequestration of infected erythrocytes in the placental intervillous blood spaces, causing severe birth outcomes. The variant surface antigen VAR2CSA is associated with Plasmodium falciparum sequestration in the placenta via its capacity to adhere to chondroitin sulfate A. We have previously shown that the extracellular region of VAR2CSA is phosphorylated on several residues and that the phosphorylation enhances the adhesive properties of CSA-binding infected erythrocytes. Here, we aimed to identify the kinases mediating this phosphorylation. We report that Human and Plasmodium falciparum Casein Kinase 2α are involved in the phosphorylation of the extracellular region of VAR2CSA. We notably show that both CK2α can phosphorylate the extracellular region of recombinant and immunoprecipitated VAR2CSA. Mass spectrometry analysis of recombinant VAR2CSA phosphorylated by recombinant Human and P. falciparum CK2α combined with site-directed mutagenesis led to the identification of residue S1068 in VAR2CSA, which is phosphorylated by both enzymes and is associated with CSA binding. Furthermore, using CRISPR/Cas9 we generated a parasite line in which phosphoresidue S1068 was changed to alanine. This mutation strongly impairs infected erythrocytes adhesion by abolishing VAR2CSA translocation to the surface of infected erythrocytes. We also report that two specific CK2 inhibitors reduce infected erythrocytes adhesion to CSA and decrease the phosphorylation of the recombinant extracellular region of VAR2CSA using either infected erythrocytes lysates as a source of kinases or recombinant Human and P. falciparum casein kinase 2. Taken together, these results undoubtedly demonstrate that host and P. falciparum CK2α phosphorylate the extracellular region of VAR2CSA and that this post-translational modification is important for VAR2CSA trafficking and for infected erythrocytes adhesion to CSA.

Purinergic Signaling in Glioma Progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X₇, is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

Complement C5b-9 and Cancer: Mechanisms of Cell Damage, Cancer Counteractions, and Approaches for Intervention

The interactions of cancer cells with components of the complement system are highly complex, leading to an outcome that is either favorable or detrimental to cancer cells. Currently, we perceive only the "tip of the iceberg" of these interactions. In this review, we focus on the complement terminal C5b-9 complex, known also as the complement membrane attack complex (MAC) and discuss the complexity of its interaction with cancer cells, starting with a discussion of its proposed mode of action in mediating cell death, and continuing with a portrayal of the strategies of evasion exhibited by cancer cells, and closing with a proposal of treatment approaches targeted at evasion strategies. Upon intense complement activation and membrane insertion of sufficient C5b-9 complexes, the afflicted cells undergo regulated necrotic cell death with characteristic damage to intracellular organelles, including mitochondria, and perforation of the plasma membrane. Several pro-lytic factors have been proposed, including elevated intracellular calcium ion concentrations and activated JNK, Bid, RIPK1, RIPK3, and MLKL; however, further research is required to fully characterize the effective cell death signals activated by the C5b-9 complexes. Cancer cells over-express a multitude of protective measures which either block complement activation, thus reducing the number of membrane-inserted C5b-9 complexes, or facilitate the elimination of C5b-9 from the cell surface. Concomitantly, cancer cells activate several protective pathways that counteract the death signals. Blockage of complement activation is mediated by the complement membrane regulatory proteins CD46, CD55, and CD59 and by soluble complement regulators, by proteases that cleave complement proteins and by protein kinases, like CK2, which phosphorylate complement proteins. C5b-9 elimination and inhibition of cell death signals are mediated by caveolin and dynamin, by Hsp70 and Hsp90, by the mitochondrial stress protein mortalin, and by the protein kinases PKC and ERK. It is conceivable that various cancers and cancers at different stages of development will utilize distinct patterns of these and other MAC resistance strategies. In order to enhance the impact of antibody-based therapy on cancer, novel precise reagents that block the most effective protective strategies will have to be designed and applied as adjuvants to the therapeutic antibodies.

Ecto-protein kinase CK2, the neglected form of CK2

Ecto-protein kinases, including protein kinase CK2 (former name, casein kinase 2), have been the focus of research for more than 30 years. At the beginning of the ecto-kinase research their identification was performed with substrates and inhibitors whose specificity under the current knowledge was rather limited. Since all currently known ecto-kinases, including ecto-CK2, have intracellular counterparts, one has to exclude that an ecto-localization originates from intracellular counterparts after cell damage. Protein kinase CK2 is involved in cellular key processes such as cell cycle progression, inhibition of apoptosis, DNA damage repair, differentiation and many other processes. CK2 is composed of two catalytic CK2α or CK2α' subunits and two non-catalytic CK2β subunits. Progress in the ecto-kinase and in particular ecto-CK2 studies was made with the use of transfected tagged CK2 subunits, which allowed to follow their individual transport and localization on the cell surface after transfection. Furthermore, immunofluorescence studies with antibodies against CK2 subunits as well as affinity chromatography with a binding partner of CK2 subunits have improved ecto-kinase research. The use of new and more specific inhibitors as well as of substrates, which do not cross the plasma membrane, have further improved the specificity for ecto-CK2. From the various substrates of ecto-CK2, it can be concluded that ecto-CK2 plays a role in Alzheimer disease, cell adhesion, platelet aggregation, immune response and cellular signalling. New tools and techniques, to study ecto-CK2 activity, are required to identify new substrates and thereby new functional implications for ecto-CK2.

Proteoform Profile Mapping of the Human Serum Complement Component C9 Revealing Unexpected New Features of N-, O-, and C-Glycosylation

The human complement C9 protein (∼65 kDa) is a member of the complement pathway. It plays an essential role in the membrane attack complex (MAC), which forms a lethal pore on the cellular surface of pathogenic bacteria. Here, we charted in detail the structural microheterogeneity of C9 purified from human blood serum, using an integrative workflow combining high-resolution native mass spectrometry and (glyco)peptide-centric proteomics. The proteoform profile of C9 was acquired by high-resolution native mass spectrometry, which revealed the co-occurrence of ∼50 distinct mass spectrometry (MS) signals. Subsequent peptide-centric analysis, through proteolytic digestion of C9 and liquid chromatography (LC)-tandem mass spectrometry (MS/MS) measurements of the resulting peptide mixtures, provided site-specific quantitative profiles of three different types of C9 glycosylation and validation of the native MS data. Our study provides a detailed specification, validation, and quantification of 15 co-occurring C9 proteoforms and the first direct experimental evidence of O-linked glycans in the N-terminal region. Additionally, next to the two known glycosylation sites, a third novel, albeit low abundant, N-glycosylation site on C9 is identified, which surprisingly does not possess the canonical N-glycosylation sequence N-X-S/T. Our data also reveal a binding of up to two Ca²⁺ ions to C9. Mapping all detected and validated sites of modifications on a structural model of C9, as present in the MAC, hints at their putative roles in pore formation or receptor interactions. The applied methods herein represent a powerful tool for the unbiased in-depth analysis of plasma proteins and may advance biomarker discovery, as aberrant glycosylation profiles may be indicative of the pathophysiological state of the patients.

Ecto-protein kinases and phosphatases: an emerging field for translational medicine

Progress in translational research has led to effective new treatments of a large number of diseases. Despite this progress, diseases including cancer and cardiovascular disorders still are at the top in death statistics and disorders such as osteoporosis and osteoarthritis represent an increasing disease burden in the aging population. Novel strategies in research are needed more than ever to overcome such diseases. The growing field of extracellular protein phosphorylation provides excellent opportunities to make major discoveries of disease mechanisms that can lead to novel therapies. Reversible phosphorylation/dephosphorylation of sites in the extracellular domains of matrix, cell-surface and trans-membrane proteins is emerging as a critical regulatory mechanism in health and disease. Moreover, a new concept is emerging from studies of extracellular protein phosphorylation: in cells where ATP is stored within secretory vesicles and released by exocytosis upon cell-stimulation, phosphorylation of extracellular proteins can operate as a messenger operating uniquely in signaling pathways responsible for long-term cellular adaptation. Here, we highlight new concepts that arise from this research, and discuss translation of the findings into clinical applications such as development of diagnostic disease markers and next-generation drugs.

The role of complement in tumor growth

Complement is a central part of the immune system that has developed as a first defense against non-self cells. Neoplastic transformation is accompanied by an increased capacity of the malignant cells to activate complement. In fact, clinical data demonstrate complement activation in cancer patients. On the basis of the use of protective mechanisms by malignant cells, complement activation has traditionally been considered part of the body's immunosurveillance against cancer. Inhibitory mechanisms of complement activation allow cancer cells to escape from complement-mediated elimination and hamper the clinical efficacy of monoclonal antibody-based cancer immunotherapies. To overcome this limitation, many strategies have been developed with the goal of improving complement-mediated effector mechanisms. However, significant work in recent years has identified new and surprising roles for complement activation within the tumor microenvironment. Recent reports suggest that complement elements can promote tumor growth in the context of chronic inflammation. This chapter reviews the data describing the role of complement activation in cancer immunity, which offers insights that may aid the development of more effective therapeutic approaches to control cancer.

Identification of interconnected markers for T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is a complex disease, resulting from proliferation of differentially arrested immature T cells. The molecular mechanisms and the genes involved in the generation of T-ALL remain largely undefined. In this study, we propose a set of genes to differentiate individuals with T-ALL from the nonleukemia/healthy ones and genes that are not differential themselves but interconnected with highly differentially expressed ones. We provide new suggestions for pathways involved in the cause of T-ALL and show that network-based classification techniques produce fewer genes with more meaningful and successful results than expression-based approaches. We have identified 19 significant subnetworks, containing 102 genes. The classification/prediction accuracies of subnetworks are considerably high, as high as 98%. Subnetworks contain 6 nondifferentially expressed genes, which could potentially participate in pathogenesis of T-ALL. Although these genes are not differential, they may serve as biomarkers if their loss/gain of function contributes to generation of T-ALL via SNPs. We conclude that transcription factors, zinc-ion-binding proteins, and tyrosine kinases are the important protein families to trigger T-ALL. These potential disease-causing genes in our subnetworks may serve as biomarkers, alternative to the traditional ones used for the diagnosis of T-ALL, and help understand the pathogenesis of the disease.

Purinergic signaling in glioma progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X(7), is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

Extracellular phosphorylation and phosphorylated proteins: not just curiosities but physiologically important

Mining of the literature and high-throughput mass spectrometry data from both healthy and diseased tissues and from body fluids reveals evidence that various extracellular proteins can exist in phosphorylated states. Extracellular kinases and phosphatases (ectokinases and ectophosphatases) are active in extracellular spaces during times of sufficiently high concentrations of adenosine triphosphate. There is evidence for a role of extracellular phosphorylation in various physiological functions, including blood coagulation, immune cell activation, and the formation of neuronal networks. Ectokinase activity is increased in some diseases, including cancer, Alzheimer's disease, and some microbial infections. We summarize the literature supporting the physiological and pathological roles of extracellularly localized protein kinases, protein phosphatases, and phosphorylated proteins and provide an analysis of the available mass spectrometry data to annotate potential extracellular phosphorylated proteins.

Increase of Fas-induced apoptosis by inhibition of extracellular phosphorylation of Fas receptor in Jurkat cell line

Apoptosis signalling through the Fas pathway requires several steps of aggregation of the Fas receptor in the membrane, including aggregation that may occur in the absence of Fas ligand. Association of Fas domains is determinant to signal transmission following Fas ligand binding to a specific domain. The domains involved in Fas aggregation are located in its extracellular region and contain three potential protein kinase C-binding motifs. We therefore studied the possibility that phosphorylation of the extracellular region of Fas might be implicated in the regulation of Fas-mediated apoptosis. Inhibition experiments of extracellular phosphorylation were performed in human Jurkat T leukemia cells with K252b, an impermeant protein-kinase inhibitor. Extracellular phosphorylation of Fas receptor was related to ecto-kinase, as assessed by the [gamma-(32)P] ATP labelling of Fas-116 kDa aggregates, suppressed by K252b inhibitor which significantly increased the sensitivity to Fas-mediated apoptosis. Ecto-PKC involvement was demonstrated by bisindolylmaleimide VIII, a selective inhibitor of protein kinase C which significantly increased both Fas aggregation in the membrane and Fas-mediated apoptosis and by the addition of the PKC pseudo-substrate 19-36 which inhibited the phosphorylation of 116 kDa Fas aggregates. These data support a role for Fas phosphorylation in the decreased sensitivity to apoptosis in the Jurkat T leukemia cell line.

Emission of membrane vesicles: roles in complement resistance, immunity and cancer

Complement-mediated cell death is caused by C5b-9, the membrane attack complex (MAC) composed of the five complement proteins C5b, C6, C7, C8, and C9. Assembly of the C5b-9 complex initiates oligomerization of C9 and production of a transmembrane protein channel that inflicts damage to target cells. For protection, cells eliminate the MAC from their surface either by ectocytosis (direct emission of membrane vesicles) or by endocytosis (internalization). The process of ectosome release is rapid and involves cytosolic Ca(2+) and activation of protein kinases, such as protein kinase C (PKC) and extracellular signal-regulated protein kinase (ERK). Recently, the involvement of mortalin (also known as GRP75 and mitochondrial hsp70) in MAC elimination has been suggested. Extracellular application of antibodies directed to mortalin increases cell sensitivity to MAC-mediated lysis. Release of membrane vesicles is ubiquitous and enhanced in apoptotic or tumor cells and upon cell activation. Composition of the ectosomes (also often referred to as microparticles) membrane proteins and lipids appears to be different from those of the original plasma membrane, indicating involvement of a selective sorting process during ectosome formation. Exosomes (unlike ectosomes) are membrane vesicles generated by endocytosis, endosome sorting into perinuclear multivesicular bodies (MVB) and exocytosis of MVBs. Exosomes appear to be different in size and composition from ectosomes. Exosome-associated MAC has also been described. Although research on ectosomes and exosomes is still limited, physiological roles in coagulation, vascular functions, angiogenesis, wound healing and development have been attributed to these shed membrane vesicles. On the other hand, there are indications that elevated levels of ectosomes and exosomes may predispose to morbidity. Membrane vesicles released by cells exposed to complement MAC may play roles in health and disease beyond protection from cell death.

The shape of things to come: an emerging role for protein kinase CK2 in the regulation of cell morphology and the cytoskeleton

Protein kinase CK2 is a highly conserved, pleiotropic, protein serine/threonine kinase that is essential for life in eukaryotes. CK2 has been implicated in diverse cellular processes such as cell cycle regulation, circadian rhythms, apoptosis, transformation and tumorigenesis. In addition, there is increasing evidence that CK2 is involved in the maintenance of cell morphology and cell polarity, and in the regulation of the actin and tubulin cytoskeletons. Accordingly, this review will highlight published evidence in experimental models ranging from yeast to mammals documenting the emerging roles of protein kinase CK2 in the regulation of cell polarity, cell morphology and the cytoskeleton.

Extracellular phosphorylation of C9 by protein kinase CK2 regulates complement-mediated lysis

Ecto-protein kinases (ecto-PK) are expressed on many cell types, both normal and malignant, yet their functions are largely unknown. An ecto-PK capable of phosphorylating the C9 component of the complement system is described. This C9 ecto-PK could be inhibited by TBB, Emodin and DRB, selective inhibitors of protein kinase CK2. Treatment of Raji human B lymphoma cells with these CK2 inhibitors augmented cell killing by Rituximab (anti-CD20 antibodies) and human complement. Analysis of C5b-7-bearing Raji cells showed that extracellular inhibition of the ecto-CK2 enhanced cell lysis by C8 and C9. Blocking of the membrane complement regulator CD59 with monoclonal antibodies further enhanced the effect of the CK2 inhibitors on Raji cell death by complement. C9 ecto-CK2 activity was increased on cancer cells relative to normal fibroblasts and blood cells. Therefore, ecto-CK2 appears to be an additional factor protecting cells from complement-mediated lysis, probably by phosphorylation/inhibition of complement C9.

Coordinate expression of GPEET procyclin and its membrane-associated kinase in Trypanosoma brucei procyclic forms

GPEET procyclin is a major glycosylphosphatidylinositol-anchored protein of procyclic (insect stage) trypanosomes in culture and is heavily phosphorylated in the GPEET pentapeptide repeat. The phosphorylation reaction is a late event and occurs during maturation and transport of GPEET or on the parasite surface by an ecto-protein kinase. Initial biochemical characterization of the GPEET kinase activity now shows that it depends on bivalent cations for maximal activity, is stimulated by sulfhydryl group reagents, and is specific for ATP as phosphoryl donor. No kinase activity is detected in bloodstream form trypanosomes in culture, whereas strong phosphorylation is observed in early procyclic forms. In addition, the GPEET kinase activity is absent from procyclic trypanosomes that have repressed GPEET synthesis but can be induced in these same stocks by conditions, which also induce GPEET expression. However, the presence of an active kinase does not depend on the presence of (functional) GPEET because it can be detected in parasites expressing a non-phosphorylatable GPEET mutant protein and in procyclin null mutant trypanosomes. Interestingly, the presence of the glycosylphosphatidylinositol lipid moiety seems necessary for GPEET to become phosphorylated. Together, the results demonstrate that GPEET and its kinase are expressed during the same life cycle stages and that factors that induce the expression of GPEET in vitro also induce the expression of the GPEET kinase.

One-thousand-and-one substrates of protein kinase CK2?

CK2 (formerly termed "casein kinase 2") is a ubiquitous, highly pleiotropic and constitutively active Ser/Thr protein kinase whose implication in neoplasia, cell survival, and virus infection is supported by an increasing number of arguments. Here an updated inventory of 307 CK2 protein substrates is presented. More than one-third of these are implicated in gene expression and protein synthesis as being either transcriptional factors (60) or effectors of DNA/RNA structure (50) or translational elements. Also numerous are signaling proteins and proteins of viral origin or essential to virus life cycle. In comparison, only a minority of CK2 targets (a dozen or so) are classical metabolic enzymes. An analysis of 308 sites phosphorylated by CK2 highlights the paramount relevance of negatively charged side chains that are (by far) predominant over any other residues at positions n+3 (the most crucial one), n+1, and n+2. Based on this signature, it is predictable that proteins phosphorylated by CK2 are much more numerous than those identified to date, and it is possible that CK2 alone contributes to the generation of the eukaryotic phosphoproteome more so than any other individual protein kinase. The possibility that CK2 phosphosites play some global role, e.g., by destabilizing alpha helices, counteracting caspase cleavage, and generating adhesive motifs, will be discussed.

Complement resistance of human carcinoma cells depends on membrane regulatory proteins, protein kinases and sialic acid

Nucleated cells employ several strategies to evade killing by homologous complement. We studied complement resistance in the human carcinoma cell lines (CA) T47D (mammary), SKOV3 (ovarian), and PC-3 (prostate) with emphasis on the following mechanisms of defense: 1. Expression and shedding of the membrane complement regulatory proteins (mCRP) CD46, CD55 and CD59; 2. Resistance based on protein phosphorylation; 3. Cell surface expression of sialic acid residues; 4. Desensitization to complement upon exposure to sublytic complement doses. Anti-mCRP antibody blocking experiments demonstrated that CD59 is the main mCRP protecting these CA from complement. Soluble CD59 was also found in supernates of PC-3> SKOV3 > T47D cells. Second, inhibitors of PKC, PKA and MEK sensitized the CA to lysis, thus implicating these protein kinases in CA complement resistance. Third, removal of sialic acid residues with neuraminidase also sensitized CA to lysis. Finally, exposure of CA to sublytic doses of complement conferred on them enhanced resistance to lytic complement doses in a PKC-dependent process. Combined treatment of CA with anti-CD59 antibodies, PD98059 (a MEK inhibitor) and neuraminidase produced a large enhancement in CA sensitivity to complement. Our results show that CD59 and sialic acid residues present on the cell surface, and intracellular processes involving protein phosphorylation act additively to secure CA resistance to complement-mediated lysis. Therefore, the effectiveness of antibody- and complement-based cancer immunotherapy will markedly improve by suppression of the various complement resistance mechanisms.

K562 erythroleukemic cells are equipped with multiple mechanisms of resistance to lysis by complement

Resistance of tumor cells to lysis by complement is generally attributed to several protective mechanisms. The relative impact of these mechanisms in the same tumor cell, however, has not been assessed yet. We have analyzed the interaction of the human erythroleukemia tumor cell line K562 with human complement. K562 cells express the membrane complement regulatory proteins CD59, CD55 and CD46. As shown here for the first time, K562 also spontaneously release the soluble regulators C1 inhibitor, factor H, and soluble CD59. Complement resistance of K562 cells is augmented upon treatment with PMA, TNF or even with sublytic complement. Unlike TNF and sublytic complement, PMA enhanced the expression of membrane-bound CD55 and CD59 and led to increased secretion of soluble CD59. In addition, we show that complement-resistant K562 cells express a membrane-associated proteolytic activity, higher than the complement-sensitive K562/S cells. Treatment of complement-resistant K562 cells with serine protease inhibitors enhance their sensitivity to complement-mediated lysis. Inhibitors of protein kinase C (PKC) also sensitize K562 cells to complement lysis, implicating PKC-mediated signaling in cell resistance to complement. Neutralization of the CD55 and CD59 but not of CD46 regulatory activity with specific antibodies significantly increases complement-mediated K562 cell lysis. Treatment of K562 cells with a mixture of inhibitory reagents results in a significant additive enhancing effect on complement-mediated lysis of K562. In conclusion, K562 cells resist a complement attack by concomitantly using multiple molecular evasion strategies. Future attempts in antibody-based tumor therapy should include strategies to interfere with those resistance mechanisms.

Reduction in the local expression of complement component 6 (C6) and 7 (C7) mRNAs in oesophageal carcinoma

Differential displays of tumour/normal pair specimens of human oesophagus identified complement component 7 (C7) as being enhanced in normal tissues, but remarkably reduced in carcinoma tissues. In situ hybridisation confirmed the localisation of C7 mRNA in normal oesophageal epithelial cells and its disappearance in tumour cells. When mRNA expressions of other components were examined by means of semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) in 10 tumour/normal pair specimens, significant reductions in C6 and C7 mRNAs were observed, while C3 and C5 mRNAs were enhanced in both normal and tumour tissues. A similar reduction was observed in colon and kidney cancers using the tumour/normal expression array analysis. Gene deletion of C7 was not found in the cell lines by Southern blot analysis. Our findings suggest a possible relationship between oesophageal tumorigenesis and reduced expression of C6 and C7 mRNAs, which is probably caused by a change in gene expression regulation and not by genetic loss of the locus.

Involvement of the ERK mitogen-activated protein kinase in cell resistance to complement-mediated lysis

Sublytic doses of complement desensitize cells and make them resistant to lytic complement doses. This process, named complement-induced protection, requires calcium ion influx, protein kinase C activation and protein synthesis. The involvement of the extracellular signal-regulated kinase, ERK, in cell desensitization by sublytic complement was examined in erythroleukaemia K562 cells and in COS-7 cells. As shown here, ERK is activated in K562 and COS-7 cells within 10 min of sublytic immune attack and then shows a decline and a second peak of activation at 20 min. C7- and C8-deficient human sera have a small effect on ERK activity. However, a significant increase in ERK activation is observed when C7 or C8, respectively, is added back to these sera. Complement-induced ERK activation was blocked in cells treated with GF109203X or Go6976, two selective PKC inhibitors, as well as by treatment with PD098059, an inhibitor of MEK1, the ERK kinase. PD098059 treatment also sensitized K562 cells to complement-mediated lysis and prevented complement-induced protection. COS-7 cells transfected with a dominant-negative MEK plasmid were incapable of undergoing the process of complement-induced protection. In conclusion, cell desensitization by sublytic doses of the complement membrane attack complex involves a signalling cascade that includes PKC-mediated ERK activation.

Complement resistance of tumor cells: basal and induced mechanisms

Clinical and experimental studies have suggested that complement may play a role in tumor cytotoxicity. However, the efficiency of complement-mediated tumor cell lysis is hampered by various protective mechanisms, which may be divided into two categories: basal and induced mechanisms. The basal mechanisms are spontaneously expressed in cells without a need for prior activation, whereas the induced mechanisms develop in cells subjected to stimulation with cytokines, hormones, drugs or with sublytic doses of complement and other pore-formers. Membrane-associated complement regulatory proteins, such as CD55 (DAF, Decay-Accelerating Factor), CD46 (MCP, Membrane Cofactor Protein), CD35 (CR1, Complement Receptor type 1) and CD59, which serve as an important mechanism of self protection and render autologous cells insensitive to the action of complement. appear to be over-expressed on certain tumors. Furthermore, tumor cells secrete several soluble complement inhibitors. Tumor cells may also express proteases that degrade complement proteins, such as C3, or ecto-protein kinases which can phosphorylate complement components, such as C9. Besides this basal resistance, nucleated cells resist, to some extent, complement damage by removing the membrane attack complexes (MAC) from their surface. Several biochemical pathways, including protein phosphorylation, activation of G-proteins and turnover of phosphoinositides have been implicated in resistance to complement. Calcium ion influx and activation of protein kinase C (PKC) and of mitogen-activated protein kinase (MAPK) have also been demonstrated to be associated with the complement-induced enhanced resistance to lysis. The complete elucidation of the molecular mechanisms involved in basal and induced tumor cell resistance will enable the development of strategies for interfering with these evasion mechanisms and the use of the cytotoxic complement system against tumor cells.