Cell-death-inducing monoclonal antibodies raised against DT40 tumor cells: identification of chicken transferrin receptor as a novel cell-death receptor

Antibodies Targeting the Transferrin Receptor 1 (TfR1) as Direct Anti-cancer Agents

The transferrin receptor 1 (TfR1), also known as cluster of differentiation 71 (CD71), is a type II transmembrane glycoprotein that binds transferrin (Tf) and performs a critical role in cellular iron uptake through the interaction with iron-bound Tf. Iron is required for multiple cellular processes and is essential for DNA synthesis and, thus, cellular proliferation. Due to its central role in cancer cell pathology, malignant cells often overexpress TfR1 and this increased expression can be associated with poor prognosis in different types of cancer. The elevated levels of TfR1 expression on malignant cells, together with its extracellular accessibility, ability to internalize, and central role in cancer cell pathology make this receptor an attractive target for antibody-mediated therapy. The TfR1 can be targeted by antibodies for cancer therapy in two distinct ways: (1) indirectly through the use of antibodies conjugated to anti-cancer agents that are internalized by receptor-mediated endocytosis or (2) directly through the use of antibodies that disrupt the function of the receptor and/or induce Fc effector functions, such as antibody-dependent cell-mediated cytotoxicity (ADCC), antibody-dependent cell-mediated phagocytosis (ADCP), or complement-dependent cytotoxicity (CDC). Although TfR1 has been used extensively as a target for antibody-mediated cancer therapy over the years, interest continues to increase for both targeting the receptor for delivery purposes and for its use as direct anti-cancer agents. This review focuses on the developments in the use of antibodies targeting TfR1 as direct anti-tumor agents.

[Analysis of Target Molecules towards Anti-cancer Therapeutic Antibodies]

Target molecules of existing anti-cancer therapeutic monoclonal antibodies (mAbs) are divided into 1) receptor-type tyrosine kinases, such as human epidermal growth factor receptor (HER) family, 2) differentiation antigens, such as CD20 (Rituxan target), 3) angiogenesis-related molecules, and 4) immune checkpoint molecules (PD-1, etc.). We have recently reported a novel therapy targeting lymphangiogenesis, but not angiogenesis, using an anti-LYVE-1 (lymphatic vessel endothelial hyaluronan receptor 1) mAb. At present, many transporters are not considered to be target molecules for the cancer therapy; however, our study strongly suggested that the inhibition of cancer metabolism by mAbs against amino acid transporters will play a significant role in future cancer therapies. Most anti-cancer therapeutic mAbs bind cell-surface molecules on viable cancer cells: therefore, it is necessary to produce mAbs recognizing epitopes on the extracellular domains of native and non-denatured proteins. We concluded that viable cancer cells or cells transfected with cDNA encoding target proteins are suitable immunogens for the production of anti-cancer therapeutic mAbs. We introduce our efforts to develop seeds for therapeutic mAbs using whole cancer cells and transfectants as the immunogen. As many target candidates in the future are multi-pass membrane proteins, such as 12-pass amino acid transporter proteins belonging to the solute carrier (SLC) family, and their possible immunogenic extracellular regions are small, the production of specific mAbs is highly difficult. In this review, we summarize the successful preparation and characterization of mAbs recognizing the extracellular domain of oncoproteins, including transporters.

The Megalobrama amblycephala transferrin and transferrin receptor genes: molecular cloning, characterization and expression during early development and after Aeromonas hydrophila infection

Both transferrin (TF) and transferrin receptor (TFR) play vital roles in iron homeostasis, which is essential for cellular growth and survival. Besides, TF and TFR are also reported to be involved in immune response against bacterial infection. In the present study, we cloned and characterized the Tf and TfR genes in Megalobrama amblycephala. The M. amblycephala Tf gene contained 17 exons and 16 introns, encoding 651 amino acids, while the M. amblycephala TfR gene contained 18 exons and 17 introns, encoding 768 amino acids. In healthy fish, Tf mRNA was most abundant in the liver, and TfR was highly expressed in the blood and brain. During early development, the expression of Tf increased from 12 hpf (hour post fertilization) to 26 hpf, followed by a diminution at 32 hpf, then increased significantly to the peak level at 2 dph (day post hatching). The expression pattern of TfR was similar to that of Tf, fluctuating from 0 hpf to 32 hpf and dramatically increasing to the peak at 2 dph. Additionally, both Tf and TfR genes responded to Aeromonas hydrophila infection, by increasing their expression in the liver, spleen and kidney at both mRNA and protein levels, indicating that they were involved in M. amblycephala immune response. Immunohistochemical analysis and Prussian blue staining verified the internalization of TF-receptor system with bound-iron in the liver of M. amblycephala.

Towards therapeutic antibodies to membrane oncoproteins by a robust strategy using rats immunized with transfectants expressing target molecules fused to green fluorescent protein

Cell-surface molecules containing growth factor receptors, adhesion molecules and transporter proteins are often over-expressed in various cancer cells, and could be regarded as suitable targets for therapeutic monoclonal antibodies (mAb). Anti-cancer therapeutic mAb are claimed to bind these cell-surface molecules on viable cancer cells: therefore, it is necessary to produce mAb recognizing epitopes on the extracellular domains of native but not denatured proteins. We have experienced difficulty in obtaining mAb bound to viable cancer cells using synthetic peptides or recombinant proteins produced in bacteria as immunogens, although these immunogens are relatively easy to prepare. In this context, we have concluded that viable cancer cells or cells transfected with cDNA encoding target proteins are suitable immunogens for the production of anti-cancer therapeutic mAb. Furthermore, we selected rats as the immunized animals, because of their excellent capacity to generate diverse antibodies. Because many target candidates are multi-pass (type IV) membrane proteins, such as 7-pass G protein-coupled receptors and 12-pass transporter proteins belonging to the solute carrier family, and their possible immunogenic extracellular regions are very small, production of specific mAb was extremely difficult. In this review, we summarize the successful preparation and characterization of rat mAb immunized against the extracellular domain of type I, type II and type IV membrane oncoproteins fused to green fluorescent protein as an approach using reverse genetics, and also introduce the discovery of cell-death-inducing antibodies as an approach using forward genetics and a strategy to produce reshaped antibodies using mimotope peptides as the immunogen.

The cell surface protein gene ecm33+ is a target of the two transcription factors Atf1 and Mbx1 and negatively regulates Pmk1 MAPK cell integrity signaling in fission yeast

The highly conserved fission yeast Pmk1 MAPK pathway plays a key role in cell integrity by regulating Atf1, which belongs to the ATF/cAMP-responsive element-binding (CREB) protein family. We identified and characterized ecm33(+), which encodes a glycosyl-phosphatidylinositol (GPI)-anchored cell surface protein as a transcriptional target of Pmk1 and Atf1. We demonstrated that the gene expression of Ecm33 is regulated by two transcription factors Atf1 and a MADS-box-type transcription factor Mbx1. We identified a putative ATF/CREB-binding site and an RLM1-binding site in the ecm33(+) promoter region and monitored the transcriptional activity of Atf1 or Mbx1 in living cells using a destabilized luciferase reporter gene fused to three tandem repeats of the CRE and six tandem repeats of the Rlm1-binding sequence, respectively. These reporter genes reflect the activation of the Pmk1 pathway by various stimuli, thereby enabling the real-time monitoring of the Pmk1 cell integrity pathway. Notably, the Deltaecm33 cells displayed hyperactivation of the Pmk1 signaling together with hypersensitivity to Ca(2+) and an abnormal morphology, which were almost abolished by simultaneous deletion of the components of the Rho2/Pck2/Pmk1 pathway. Our results suggest that Ecm33 is involved in the negative feedback regulation of Pmk1 cell integrity signaling and is linked to cellular Ca(2+) signaling.

SLP-65 phosphorylation dynamics reveals a functional basis for signal integration by receptor-proximal adaptor proteins

Understanding intracellular signal transduction by cell surface receptors requires information about the precise order of relevant modifications on the early transducer elements. Here we introduce the B cell line DT40 and its genetically engineered variants as a model system to determine and functionally characterize post-translational protein modifications in general. This is accomplished by a customized strategy that combines mass spectrometric analyses of protein modifications with subsequent mutational studies. When applied to the B cell receptor (BCR)-proximal effector SLP-65, this approach uncovered a differential and highly dynamic engagement of numerous newly identified phospho-acceptor sites. Some of them serve as kinase substrates in resting cells and undergo rapid dephosphorylation upon BCR ligation. Stimulation-induced phosphorylation of SLP-65 can be early and transient, or early and sustained, or late. Functional elucidation of conspicuous phosphorylation at serine 170 in SLP-65 revealed a BCR-distal checkpoint for some but not all possible B cell responses. Our data show that SLP-65 phosphorylation acts upstream for signal initiation and also downstream during selective processing of the BCR signal. Such a phenomenon defines a receptor-specific signal integrator.

Simultaneous induction of apoptotic, autophagic, and necrosis-like cell death by monoclonal antibodies recognizing chicken transferrin receptor

Programmed cell death (PCD) is categorized as apoptotic, autophagic, or necrosis-like. Although the possibility that plural (two or three) death signals could be induced by a given stimulus has been reported, the precise mechanisms regulating PCD are not well understood. Recently, we have obtained two anti-chicken transferrin receptor (TfR) monoclonal antibodies (mAbs; D18 and D19) inducing a unique cell death. Although the cell death had several features of apoptosis, autophagic and necrosis-like morphological alterations were simultaneously observed in electron microphotographs. In addition to cells with condensed chromatin and an intact plasma membrane (apoptotic cells), cells having many vacuoles in the cytoplasm (autophagic cells), and enlarged cells with ruptured plasma membranes (necrosis-like cells) were observed in DT40 cells treated with the mAbs, however, the latter two types of dead cells were not detected upon treatment with staurosporine, a typical apoptosis inducer. In autophagic cells, numerous membrane-bound vesicles occupying most of the cytoplasmic space, which frequently contained electron-dense materials from cytoplasmic fragments and organelles, were observed. The simultaneous induction of multiple death signals from a stimulus via the TfR is of great interest to those researching cell death. In addition, activation of caspases was observed in DT40 cells treated with D19, however, the cell death was not inhibited with z-VAD-fmk, a pan-caspase inhibitor, suggesting that at least in part, a caspase-independent pathway is involved in the TfR-mediated cell death.