Intracellular expression of single chain antibodies reverts ErbB-2 transformation

Applying Antibodies Inside Cells: Principles and Recent Advances in Neurobiology, Virology and Oncology

To interfere with cell function, many scientists rely on methods that target DNA or RNA due to the ease with which they can be applied. Proteins are usually the final executors of function but are targeted only indirectly by these methods. Recent advances in targeted degradation of proteins based on proteolysis-targeting chimaeras (PROTACs), ubiquibodies, deGradFP (degrade Green Fluorescent Protein) and other approaches have demonstrated the potential of interfering directly at the protein level for research and therapy. Proteins can be targeted directly and very specifically by antibodies, but using antibodies inside cells has so far been considered to be challenging. However, it is possible to deliver antibodies or other proteins into the cytosol using standard laboratory equipment. Physical methods such as electroporation have been demonstrated to be efficient and validated thoroughly over time. The expression of intracellular antibodies (intrabodies) inside cells is another way to interfere with intracellular targets at the protein level. Methodological strategies to target the inside of cells with antibodies, including delivered antibodies and expressed antibodies, as well as applications in the research areas of neurobiology, viral infections and oncology, are reviewed here. Antibodies have already been used to interfere with a wide range of intracellular targets. Disease-related targets included proteins associated with neurodegenerative diseases such as Parkinson's disease (α-synuclein), Alzheimer's disease (amyloid-β) or Huntington's disease (mutant huntingtin [mHtt]). The applications of intrabodies in the context of viral infections include targeting proteins associated with HIV (e.g. HIV1-TAT, Rev, Vif, gp41, gp120, gp160) and different oncoviruses such as human papillomavirus (HPV), hepatitis B virus (HBV), hepatitis C virus (HCV) and Epstein-Barr virus, and they have been used to interfere with various targets related to different processes in cancer, including oncogenic pathways, proliferation, cell cycle, apoptosis, metastasis, angiogenesis or neo-antigens (e.g. p53, human epidermal growth factor receptor-2 [HER2], signal transducer and activator of transcription 3 [STAT3], RAS-related RHO-GTPase B (RHOB), cortactin, vascular endothelial growth factor receptor 2 [VEGFR2], Ras, Bcr-Abl). Interfering at the protein level allows questions to be addressed that may remain unanswered using alternative methods. This review addresses why direct targeting of proteins allows unique insights, what is currently feasible in vitro, and how this relates to potential therapeutic applications.

Molecular Oncogenesis of Prostate Adenocarcinoma: Role of the Human Epidermal Growth Factor Receptor 2 (HER-2/neu)

The potential mechanisms involving the genesis and growth of androgen-independent prostate cancer include super-expression of the androgen receptor (AR), in an attempt to compensate for the low androgenic plasma levels and mutations of this specific receptor, which could determine resistance to anti-androgenic therapy. However, most advanced prostate tumors have no mutations or amplifications of the AR, suggesting a potential role of non-androgenic growth factors, including epidermal growth factor (EGF), transforming growth factor α, insulin-like growth factor (IGF-1) and fibroblast growth factor. More specifically, these factors, and their receptors like EGFR (HER-1) and HER-2/neu, through paracrine and autocrine mechanisms, may contribute to the proliferation and growth of prostate cancer. Free full text available at www.tumorionline.it

Role of 3'-5'-cyclic adenosine monophosphate on the epidermal growth factor dependent survival in mammary epithelial cells

Epidermal growth factor (EGF) has been suggested to play a key role in the maintenance of epithelial cell survival during lactation. Previously, we demonstrated that EGF dependent activation of PI3K pathway prevents apoptosis in confluent murine HC11 cells cultured under low nutrient conditions. The EGF protective effect is associated with increased levels of the antiapoptotic protein Bcl-XL. Here, we identify the EGF-dependent mechanism involved in cell survival that converges in the regulation of bcl-X expression by activated CREB. EGF induces Bcl-XL expression through activation of a unique bcl-X promoter, the P1; being not only the PI3K/AKT signaling pathway but also the increase in cAMP levels and the concomitant PKA/CREB activation necessary for both bcl-XL upregulation and apoptosis avoidance. Results presented in this work suggest the existence of a novel connection between the EGF receptor and the adenylate cyclase that would have an impact in preventing apoptosis under low nutrient conditions.

Specific in vivo knockdown of protein function by intrabodies

Intracellular antibodies (intrabodies) are recombinant antibody fragments that bind to target proteins expressed inside of the same living cell producing the antibodies. The molecules are commonly used to study the function of the target proteins (i.e., their antigens). The intrabody technology is an attractive alternative to the generation of gene-targeted knockout animals, and complements knockdown techniques such as RNAi, miRNA and small molecule inhibitors, by-passing various limitations and disadvantages of these methods. The advantages of intrabodies include very high specificity for the target, the possibility to knock down several protein isoforms by one intrabody and targeting of specific splice variants or even post-translational modifications. Different types of intrabodies must be designed to target proteins at different locations, typically either in the cytoplasm, in the nucleus or in the endoplasmic reticulum (ER). Most straightforward is the use of intrabodies retained in the ER (ER intrabodies) to knock down the function of proteins passing the ER, which disturbs the function of members of the membrane or plasma proteomes. More effort is needed to functionally knock down cytoplasmic or nuclear proteins because in this case antibodies need to provide an inhibitory effect and must be able to fold in the reducing milieu of the cytoplasm. In this review, we present a broad overview of intrabody technology, as well as applications both of ER and cytoplasmic intrabodies, which have yielded valuable insights in the biology of many targets relevant for drug development, including α-synuclein, TAU, BCR-ABL, ErbB-2, EGFR, HIV gp120, CCR5, IL-2, IL-6, β-amyloid protein and p75NTR. Strategies for the generation of intrabodies and various designs of their applications are also reviewed.

ErbB-2 signaling plays a critical role in regulating androgen-sensitive and castration-resistant androgen receptor-positive prostate cancer cells

While androgen deprivation therapy (ADT) reduces tumor burden, autocrine growth factor loops such as human epidermal growth factor receptor 2 (HER2/ErbB-2/neu) have been proposed to contribute to prostate cancer (PCa) survival and relapse. However, the role of ErbB-2 in regulating androgen-sensitive (AS) and castration-resistant (CR) cell proliferation remains unclear. Here, we determined the role of ErbB-2 in PCa progression and survival under steroid-reduced conditions using two independent PCa cell progression models. In AR-positive androgen-independent (AI) PCa cells that exhibit the CR phenotype, ErbB-2 was constitutively activated, compared to corresponding AS PCa cells. In AS LNCaP C-33 cells, androgen-induced ErbB-2 activation through ERK1/2 mediates PCa cell proliferation. Further, the ErbB-2-specific but not EGFR-specific inhibitor suppresses basal and androgen-stimulated cell proliferation and also blocks ERK1/2 activation. ErbB-2 ectopic expression and cPAcP siRNA transfection of LNCaP C-33 cells each increases ErbB-2 tyrosine phosphorylation, correlating with increased AI PSA secretion and cell proliferation. Conversely, trapping ErbB-2 by transfected endoplasmic reticulum-targeting ScFv5R expression vector abolished DHT-induced LNCaP C-33 cell growth. Moreover, inhibition of ErbB-2 but not EGFR in AI LNCaP C-81 and MDA PCa2b-AI PCa cells significantly abolished AI cell growth. In contrast to androgens via ErbB-2/ERK1/2 signaling in AS PCa cells, the inhibition of ErbB-2 abrogated AI cell proliferation by inhibiting the cell survival protein Akt in those AI cells. These results suggest that ErbB-2 is a prominent player in mediating the ligand-dependent and -independent activation of AR in AS and AI/CR PCa cells respectively for PCa progression and survival.

A bispecific transmembrane antibody simultaneously targeting intra- and extracellular epitopes of the epidermal growth factor receptor inhibits receptor activation and tumor cell growth

Epidermal growth factor receptor (EGFR) plays an important role in essential cellular processes such as proliferation, survival and migration. Aberrant activation of EGFR is frequently found in human cancers of various origins and has been implicated in cancer pathogenesis. The therapeutic antibody cetuximab (Erbitux) inhibits tumor growth by binding to the extracellular domain of EGFR, thereby preventing ligand binding and receptor activation. This activity is shared by the single chain antibody fragment scFv(225) that contains the same antigen binding domain. The unrelated EGFR-specific antibody fragment scFv(30) binds to the intracellular domain of the receptor and retains antigen binding upon expression as an intrabody in the reducing environment of the cytosol. Here, we used scFv(225) and scFv(30) domains to generate a novel type of bispecific transmembrane antibody termed 225.TM.30, that simultaneously targets intra- and extracellular EGFR epitopes. Bispecific 225.TM.30 and related membrane-anchored monospecific 225.TM and TM.30 proteins carrying extracellular scFv(225) or intracellular scFv(30) antibody fragments linked to a transmembrane domain were expressed in EGFR-overexpressing tumor cells using a doxycycline-inducible retroviral system. Induced expression of 225.TM.30 and 225.TM, but not TM.30 reduced EGFR surface levels and ligand-induced EGFR activation, while all three molecules markedly inhibited tumor cell growth. Co-localization of 225.TM with EGFR was predominantly found on the cell surface, while interaction with 225.TM.30 and TM.30 proteins resulted in the redistribution of EGFR to perinuclear compartments. Our data demonstrate functionality of this novel type of membrane-anchored intrabodies in tumor cells and suggest distinct modes of action of mono- and bispecific variants.

Selection of antibodies that regulate phenotype from intracellular combinatorial antibody libraries

A method is presented that uses combinatorial antibody libraries to endow cells with new binding energy landscapes for the purpose of regulating their phenotypes. Antibodies that are expressed in cells infected with a lentiviral combinatorial antibody library are selected directly for function rather than only for binding. The potential diversity space can be very large because more than one lentivirus can infect a single cell. Thus, the initial combinatorial diversity of ~1.0 × 10(11) members generated by the random association of antibody heavy and light chains is greatly increased by the reassortment of the antibody Fv domains themselves inside cells. The power of the system is illustrated by its ability to select unusual antibodies. Here, the selected antibodies are potent erythropoietin agonists whose ontogeny depends on recombination at the protein level of pairs of antibodies expressed in the same cell to generate heterodimeric bispecific antibodies. The obligate synergy between the different binding specificities of the antibody's monomeric subunits appears to replicate the asymmetric binding mechanism of authentic erythropoietin.

Beyond natural antibodies: the power of in vitro display technologies

In vitro display technologies, best exemplified by phage and yeast display, were first described for the selection of antibodies some 20 years ago. Since then, many antibodies have been selected and improved upon using these methods. Although it is not widely recognized, many of the antibodies derived using in vitro display methods have properties that would be extremely difficult, if not impossible, to obtain by immunizing animals. The first antibodies derived using in vitro display methods are now in the clinic, with many more waiting in the wings. Unlike immunization, in vitro display permits the use of defined selection conditions and provides immediate availability of the sequence encoding the antibody. The amenability of in vitro display to high-throughput applications broadens the prospects for their wider use in basic and applied research.

Polyomavirus middle T-antigen is a transmembrane protein that binds signaling proteins in discrete subcellular membrane sites

Murine polyomavirus middle T-antigen (MT) induces tumors by mimicking an activated growth factor receptor. An essential component of this action is a 22-amino-acid hydrophobic region close to the C terminus which locates MT to cell membranes. Here, we demonstrate that this sequence is a transmembrane domain (TMD) by showing that a hemagglutinin (HA) tag added to the MT C terminus is exposed on the outside of the cells, with the N terminus inside. To determine whether this MT TMD is inserted into the endoplasmic reticulum (ER) membrane, we added the ER retention signal KDEL to the MT C terminus (MTKDEL). This mutant protein locates only in the ER, demonstrating that MT does insert into membranes solely at this location. In addition, this ER-located MT failed to transform. Examination of the binding proteins associated with the MTKDEL protein demonstrated that it associates with PP2A and c-Src but fails to interact with ShcA, phosphatidylinositol 3-kinase (PI3K), and phospholipase C-γ1 (PLC-γ1), despite being tyrosine phosphorylated. Additional mutant and antibody studies show that MT binding to PP2A is probably required for MT to efficiently exit the ER and migrate to the plasma membrane though the TMD also plays a role in this relocation. Overall, these data, together with previous publications, illustrate that MT associates with signaling proteins at different sites in its maturation pathway. MT binds to PP2A in the cytoplasm, to c-Src at the endoplasmic reticulum, and to ShcA, PI3K, and PLC-γ1 at subsequent locations en route to the plasma membrane.

Targeting antibodies to the cytoplasm

A growing number of research consortia are now focused on generating antibodies and recombinant antibody fragments that target the human proteome. A particularly valuable application for these binding molecules would be their use inside a living cell, e.g., for imaging or functional intervention. Animal-derived antibodies must be brought into the cell through the membrane, whereas the availability of the antibody genes from phage display systems allows intracellular expression. Here, the various technologies to target intracellular proteins with antibodies are reviewed.

Intracellular activation of interferon regulatory factor-1 by nanobodies to the multifunctional (Mf1) domain

IRF-1 is a tumor suppressor protein that activates gene expression from a range of promoters in response to stimuli spanning viral infection to DNA damage. Studies on the post-translational regulation of IRF-1 have been hampered by a lack of suitable biochemical tools capable of targeting the endogenous protein. In this study, phage display technology was used to develop a monoclonal nanobody targeting the C-terminal Mf1 domain (residues 301-325) of IRF-1. Intracellular expression of the nanobody demonstrated that the transcriptional activity of IRF-1 is constrained by the Mf1 domain as nanobody binding gave an increase in expression from IRF-1-responsive promoters of up to 8-fold. Furthermore, Mf1-directed nanobodies have revealed an unexpected function for this domain in limiting the rate at which the IRF-1 protein is degraded. Thus, the increase in IRF-1 transcriptional activity observed on nanobody binding is accompanied by a significant reduction in the half-life of the protein. In support of the data obtained using nanobodies, a single point mutation (P325A) involving the C-terminal residue of IRF-1 has been identified, which results in greater transcriptional activity and a significant increase in the rate of degradation. The results presented here support a role for the Mf1 domain in limiting both IRF-1-dependent transcription and the rate of IRF-1 turnover. In addition, the data highlight a route for activation of downstream genes in the IRF-1 tumor suppressor pathway using biologics.

Inhibition of phosphatidylcholine-specific phospholipase C downregulates HER2 overexpression on plasma membrane of breast cancer cells

Introduction

Overexpression on plasma membrane of human epidermal growth factor receptor 2 (HER2) is reported in 25% to 30% of breast cancers. Heterodimer formation with cognate members of the epidermal growth factor receptor (EGFR) family, such as HER3 and EGFR, activates abnormal cell-signalling cascades responsible for tumorigenesis and further transcriptional HER2 gene upregulation. Targeting the molecular mechanisms controlling HER2 overexpression and recycling may effectively deactivate this feedback-amplification loop. We recently showed that inactivation of phosphatidylcholine-specific phospholipase C (PC-PLC) may exert a pivotal role in selectively modulating the expression on the membrane of specific receptors or proteins relevant to cell function. In the present study, we investigated the capability of PC-PLC inhibition to target the molecular mechanisms controlling HER2 overexpression on the membrane of breast cancer cells by altering the rates of its endocytosis and lysosomal degradation.

Methods

Localization on the membrane and interaction of PC-PLC with HER2, EGFR, and HER3 were investigated on HER2-overexpressing and HER2-low breast cancer cell lines, by using confocal laser scanning microscopy, flow cytometry, cell-surface biotinylation, isolation of lipid rafts, and immunoprecipitation experiments. The effects of the PC-PLC inhibitor tricyclodecan-9-yl-potassium xanthate (D609) on HER2 expression on the membrane and on the levels of overall HER2, HER2-HER3, and HER2-EGFR contents were monitored in the HER2-overexpressing SKBr3 cells, after either transient or continuous receptor engagement with anti-HER2 monoclonal antibodies, including trastuzumab. Changes of HER2 expression and cell proliferation were examined in SKBr3, BT-474, and MDA-MB-453 cells continuously exposed to D609 alone or combined with trastuzumab.

Results

PC-PLC selectively accumulates on the plasma membrane of HER2-overexpressing cells, where it colocalizes and associates with HER2 in raft domains. PC-PLC inhibition resulted in enhanced HER2 internalization and lysosomal degradation, inducing downmodulation of HER2 expression on the membrane. Moreover, PC-PLC inhibition resulted in strong retardation of HER2 reexpression on the membrane and a decrease in the overall cellular contents of HER2, HER2-HER3, and HER2-EGFR heterodimers. The PC-PLC inhibitor also induced antiproliferative effects, especially in trastuzumab-resistant cells.

Conclusions

The results pointed to PC-PLC inhibition as a potential means to counteract the tumorigenic effects of HER2 amplification and complement the effectiveness of current HER2-targeting therapies.

The Ste20-like kinase SLK is required for ErbB2-driven breast cancer cell motility

The Ste20-like kinase, SLK, is involved in the control of cell motility through its effects on actin reorganization and focal adhesion turnover. Here we investigated the role of SLK in chemotaxis downstream of the tyrosine kinase receptor, HER2/ErbB2/Neu, which is frequently overexpressed in human breast cancers. Our results show that SLK is required for the efficient cell migration of human and mouse mammary epithelial cell lines in the presence of the Neu activator, heregulin, as a chemoattractant. SLK activity is stimulated by heregulin treatment or by overexpression of activated Neu. Phosphorylation of tyrosine 1201 or tyrosines 1226/7 on Neu is a key event for SLK activation and cell migration, and cancer cell invasion mediated by these tyrosines is inhibited by kinase-inactive SLK. Signaling pathway inhibitors show that Neu-mediated SLK activation is dependent on MEK, PI3K, PLCgamma and Shc signaling. Furthermore, heregulin-stimulated SLK activity requires signals from the focal adhesion proteins, FAK and src. Finally, phospho-FAK analysis shows that SLK is required for Neu-dependent focal adhesion turnover. Together, these studies define an interaction between Neu and SLK signaling in the regulation of cancer cell motility.

Intracellular expression of a single-chain antibody directed against type IV collagenase inhibits the growth of lung cancer xenografts in nude mice

It was documented that type IV collagenase with two subtypes of 72 ku/MMP-2 and 92 ku/MMP-9 plays an important role in tumor invasion and metastasis. The endoplasmic reticulum (ER)-retained, single chain Fv antibody fragment (scFv) was used to inhibit the function of type IV collagenase. For expression in mammalian cells, the assembled scFv M97 gene with ER retention signal encoding 6 additional amino acids (SEKDEL) was reamplified by PCR. The amplified fragments were cloned into the pcDNA3.1 vector. The resulting plasmid was sequenced and then introduced into PG cells, a highly metastatic human lung cancer cell line, by lipofectAMINE method. The result of intrabody gene therapy showed that type IV collegenase expression was down regulated significantly as measured by ELISA. The biological behavior of PG cell, such as the ability of in vitro invasion through Matrigel, colony formation on soft agar, was also inhibited by scFv M97 transfection. Animal experiments in a xenograft model of human lung cancer showed that scFv M97 transfection significantly prolonged the survival time of nude mice. The results indicate that intracellular antibody technology represents a novel and efficient way to abrogate selectively the activity of type IV collagenase.

Intracellular antibodies (intrabodies) and their therapeutic potential

Combining exquisite specificity and high antigen-binding affinity, intrabodies have been used as a biotechnological tool to interrupt, modulate, or define the functions of a wide range of target antigens at the posttranslational level. An intrabody is an antibody that has been designed to be expressed intracellularly and can be directed to a specific target antigen present in various subcellular locations including the cytosol, nucleus, endoplasmic reticulum (ER), mitochondria, peroxisomes, plasma membrane and trans-Golgi network (TGN) through in frame fusion with intracellular trafficking/localization peptide sequences. Although intrabodies can be expressed in different forms, the most commonly used format is a singlechain antibody (scFv Ab) created by joining the antigen-binding variable domains of heavy and light chain with an interchain linker (ICL), most often the 15 amino acid linker (GGGGS)(3) between the variable heavy (VH) and variable light (VL) chains. Intrabodies have been used in research of cancer, HIV, autoimmune disease, neurodegenerative disease, and transplantation. Clinical application of intrabodies has mainly been hindered by the availability of robust gene delivery system(s) including target cell directed gene delivery. This review will discuss several methods of intrabody selection, different strategies of cellular targeting, and recent successful examples of intrabody applications. Taking advantage of the high specificity and affinity of an antibody for its antigen, and of the virtually unlimited diversity of antigen-binding variable domains available for molecular targeting, intrabody techniques are emerging as promising tools to generate phenotypic knockouts, to manipulate biological processes, and to obtain a more thorough understanding of functional genomics.

Targeting the function of the HER2 oncogene in human cancer therapeutics

The year 2007 marks exactly two decades since human epidermal growth factor receptor-2 (HER2) was functionally implicated in the pathogenesis of human breast cancer (Slamon et al., 1987). This finding established the HER2 oncogene hypothesis for the development of some human cancers. An abundance of experimental evidence compiled over the past two decades now solidly supports the HER2 oncogene hypothesis. A direct consequence of this hypothesis was the promise that inhibitors of oncogenic HER2 would be highly effective treatments for HER2-driven cancers. This treatment hypothesis has led to the development and widespread use of anti-HER2 antibodies (trastuzumab) in clinical management resulting in significantly improved clinical antitumor efficacies that have transformed the clinical practice of oncology. In the shadows of this irrefutable clinical success, scientific studies have not yet been able to mechanistically validate that trastuzumab inhibits oncogenic HER2 function and it remains possible that the current clinical advances are a consequence of the oncogene hypothesis, but not a translation of it. These looming scientific uncertainties suggest that the full promise of the treatment hypothesis may not yet have been realized. The coming decade will see a second generation of HER2-targeting agents brought into clinical testing and a renewed attempt to treat HER2-driven cancers through the inactivation of HER2. Here, I review the development of treatments that target HER2 in the context of the HER2 oncogene hypothesis, and where we stand with regards to the clinical translation of the HER2 oncogene hypothesis.

Metabolic biotinylation of recombinant antibody by biotin ligase retained in the endoplasmic reticulum

Due to its strength and specificity, the interaction between avidin and biotin has been used in a variety of scientific and medical applications ranging from immunohistochemistry to drug targeting. The present study describes two methods for biotinylation of proteins secreted from eukaryotic cells using the Escherichia coli biotin protein ligase. In one system the biotin ligase was co-secreted from the cells along with substrate protein enabling extracellular biotinylation of the tagged protein. In the other system, biotin ligase was engineered to be retained in the endoplasmic reticulum (ER) and metabolically biotinylates the secretory protein as it passes through the ER. An engineered antibody fragment, a diabody with specificity for carcinoembryonic antigen (CEA) was fused to the biotin acceptor domain (123 amino acid) of Propionibacterium shermanii. Coexpression of the fusion protein with ER retained biotin ligase showed higher biotinylation efficiency than biotinylation by co-secreted ligase. Biotinylation of the anti-CEA diabody tagged with a short (15 amino acid, Biotin Avitag) biotin acceptor peptide was also successful. Utilization of ER retained biotin ligase for biotinylation of protein is an attractive alternative for efficiently producing uniformly biotinylated recombinant proteins for a variety of avidin-biotin technologies.

Role of HER2/HER3 co-receptor in breast carcinogenesis

ErbB receptors are essential mediators of cell proliferation and differentiation. Their aberrant activation is associated with the development and severity of many cancers. Homo- and heterodimerization of ErbB receptors result in a wide variety of cellular signal transduction. Dimerization of human epidermal growth-factor receptor (HER)2 and HER3 occurs frequently and is a preferred heterodimer. The HER2/HER3 dimer constitutes a high affinity co-receptor for heregulin, which is capable of potent mitogenic signaling. HER3 is a kinase-defective protein that is phosphorylated by HER2. Tyrosine phosphorylated HER3 is able to directly couple to phosphatidylinositide 3-kinase, a lipid kinase involved in the proliferation, survival, adhesion and motility of tumor cells. The authors' research provides mechanistic evidence that apigenin induces apoptosis by depleting the HER2 protein and, in turn, suppressing the signaling of the HER2/HER3-phosphatidylinositide 3-kinase/Akt pathway. This indicates that inhibition of HER2/HER3 heterodimer function may be an especially effective and unique strategy for blocking the HER2-mediated carcinogenesis of breast cancer cells.

Blocking translocation of cell surface molecules from the ER to the cell surface by intracellular antibodies targeted to the ER

Intracellular antibodies (intrabodies) constitute a potent tool to neutralize the function of target proteins inside specific cell compartments (cytosol, nucleus, mitochondria and ER). The intrabody technology is an attractive alternative to the generation of gene-targeted knockout animals and complements or replaces knockdown techniques such as antisense-RNA, RNAi and RNA aptamers. This article focuses on intrabodies targeted to the ER. Intracellular anti-bodies expressed and retained inside the ER (ER intrabodies) are shown to be highly efficient in blocking the translocation of secreted and cell surface molecules from the ER to the cell surface.The advantage of ER intrabodies over cytoplasmic intrabodies is that they are correctly folded and easier to select. A particular advantage of the intrabody technology over existing ones is the possibility of inhibiting selectively post-translational modifications of proteins.The main applications of ER intrabodies so far have been (i) inactivation of oncogenic receptors and (ii) functional inhibition of virus envelope proteins and virus-receptor molecules on the surface of host cells.In cancer research, the number of in vivo mouse models for evaluation of the therapeutic potential of intrabodies is increasing.In the future, endosomal localized receptors involved in bacterial and viral infections, intracellular oncogenic receptors and enzymes involved in glycosylation of tumour antigens might be new targets for ER intrabodies.

The oncogene HER2: its signaling and transforming functions and its role in human cancer pathogenesis

The year 2007 marks exactly two decades since Human Epidermal Growth Factor Receptor-2 (HER2) was functionally implicated in the pathogenesis of human breast cancer. This finding established the HER2 oncogene hypothesis for the development of some human cancers. The subsequent two decades have brought about an explosion of information about the biology of HER2 and the HER family. An abundance of experimental evidence now solidly supports the HER2 oncogene hypothesis and etiologically links amplification of the HER2 gene locus with human cancer pathogenesis. The molecular mechanisms underlying HER2 tumorigenesis appear to be complex and a unified mechanistic model of HER2-induced transformation has not emerged. Numerous hypotheses implicating diverse transforming pathways have been proposed and are individually supported by experimental models and HER2 may indeed induce cell transformation through multiple mechanisms. Here I review the evidence supporting the oncogenic function of HER2, the mechanisms that are felt to mediate its oncogenic functions, and the evidence that links the experimental evidence with human cancer pathogenesis.

Cancer therapeutic antibodies come of age: targeting minimal residual disease

Ten years after the first clinical application of Rituximab, an anti-CD20 recombinant monoclonal antibody, immunotherapy has become common practice in oncology wards. Thanks to the great diversity of the immune system and the powerful methodology of genetic engineering, the pharmacologic potential of antibody-based therapy is far from exhaustion. The recent application of Trastuzumab, an antibody to a receptor tyrosine kinase, in adjuvant breast cancer therapy marks the beginning of a new phase in cancer treatment. Here we discuss molecular mechanisms of antibody-based therapy, the emerging ability to target minimal disease and the therapeutic potential of combining antibodies with other modalities.

Downregulation of transferrin receptor surface expression by intracellular antibody

To deplete cellular iron uptake, and consequently inhibit the proliferation of tumor cells, we attempt to block surface expression of transferrin receptor (TfR) by intracellular antibody technology. We constructed two expression plasmids (scFv-HAK and scFv-HA) coding for intracellular single-chain antibody against TfR with or without endoplasmic reticulum (ER) retention signal, respectively. Then they were transfected tumor cells MCF-7 by liposome. Applying RT-PCR, Western blotting, immunofluorescence microscopy and immunoelectron microscope experiments, we insure that scFv-HAK intrabody was successfully expressed and retained in ER contrasted to the secreted expression of scFv-HA. Flow cytometric analysis confirmed that the TfR surface expression was markedly decreased approximately 83.4+/-2.5% in scFv-HAK transfected cells, while there was not significantly decrease in scFv-HA transfected cells. Further cell growth and apoptosis characteristics were evaluated by cell cycle analysis, nuclei staining and MTT assay. Results indicated that expression of scFv-HAK can dramatically induce cell cycle G1 phase arrest and apoptosis of tumor cells, and consequently significantly suppress proliferation of tumor cells compared with other control groups. For the first time this study demonstrates the potential usage of anti-TfR scFv-intrabody as a growth inhibitor of TfR overexpressing tumors.

Peptide Aptamers with Binding Specificity for the Intracellular Domain of the ErbB2 Receptor Interfere with AKT Signaling and Sensitize Breast Cancer Cells to Taxol

The ErbB2 receptor tyrosine kinase is overexpressed in approximately 30% of breast tumor cases and its overexpression correlates with an unfavorable prognosis. A major contributor for this course of the disease is the insensitivity of these tumors toward chemotherapy. Monoclonal antibodies, inhibiting the ligand-induced activation of the receptor and tyrosine kinase inhibitors acting on the intrinsic enzymatic activity of the intracellular domain, have been developed as targeted drugs. Both have been shown to be beneficial for breast cancer patients. We targeted a third aspect of receptor function: its association with intracellular signaling components. For this purpose, we selected peptide aptamers, which specifically interact with defined domains of the intracellular part of the receptor. The peptide aptamers were selected from a random peptide library using a yeast two-hybrid system with the intracellular tyrosine kinase domain of ErbB2 as a bait construct. The peptide aptamer AII-7 interacts with high specificity with the ErbB2 receptor in vitro and in vivo. The aptamers colocalized with the intracellular domain of ErbB2 within cells. We investigated the functional consequences of the aptamer interaction with the ErbB2 receptor within tumor cells. The aptamer sequences were either expressed intracellularly or introduced into the cells as recombinant aptamer proteins. The phosphorylation of p42/44 mitogen-activated protein kinase was nearly unaffected and the activation of signal transducers and activators of transcription-3 was only modestly reduced. In contrast, they strongly inhibited the induction of AKT kinase in MCF7 breast cancer cells treated with heregulin, whereas AKT activation downstream of insulin-like growth factor I or epidermal growth factor receptor was not or only slightly affected. High AKT activity is responsible for the enhanced resistance of ErbB2-overexpressing cancer cells toward chemotherapeutic agents. Peptide aptamer interference with AKT activation resulted in the restoration of regular sensitivity of breast cancer cells toward Taxol.

Identification of human alpha-synuclein specific single chain antibodies

Parkinson's disease (PD) is a common neurodegenerative disease of unknown etiology. Evidence suggests a role for protein misfolding in disease pathogenesis. One pathologic feature observed in dopaminergic neurons is the intracytoplasmic eosinophilic inclusions known as Lewy bodies. One component of Lewy bodies, the presynaptic protein, alpha-synuclein forms oligomers and higher order aggregates and is proposed to be involved in dopaminergic neuronal death. In an effort to discriminate between alpha-synuclein conformational forms as well as design potential disruptors of pathogenic misfolding we panned a human phage antibody library for anti-synuclein single chain antibodies (scFvs). We identified six scFvs which recognize different conformers of alpha-synuclein in both an ELISA and Western blot analysis. These scFvs may further our understanding of alpha-synuclein's role in PD.

Controlled activation of ErbB1/ErbB2 heterodimers promote invasion of three-dimensional organized epithelia in an ErbB1-dependent manner: implications for progression of ErbB2-overexpressing tumors

Receptor tyrosine kinases of the ErbB family are implicated in a number of cancers, including that of the breast. ErbB receptors are activated by ligand-induced formation of homodimers and heterodimers. Receptor heterodimerization is thought to play a critical role in breast cancers overexpressing multiple members of the ErbB family. Although coexpression of ErbB receptors is associated with poor patient prognosis, the mechanisms by which receptor heterodimerization regulates tumor progression are not clear, due in part to a lack of methods that allow controlled activation of specific receptor heterodimers in mammary epithelial cells. Here, we report an approach to activate ErbB1-ErbB2 heterodimers in a nontumorigenic breast epithelial cell line, MCF-10A, without interference from endogenous ErbB receptors. Using such a method, we show that whereas both ErbB2 homodimers and ErbB1-ErbB2 heterodimers were equally potent in activating the Ras/mitogen-activated protein kinase pathway, the heterodimers were more potent in activating the phosphoinositide 3'-kinase (PI3K) and phospholipase Cgamma1 pathways than ErbB2 homodimers. We combined the dimerization system with a three-dimensional cell culture approach to show that whereas both ErbB2 homodimers and ErbB1-ErbB2 heterodimers induced disruption of three-dimensional acini-like structures, only heterodimers promoted invasion of cells through extracellular matrix. The ability of heterodimers to induce invasion required the ErbB1 kinase activity and required activation of PI3K, Ras/mitogen-activated protein kinase, and phospholipase Cgamma1 signaling pathways. Thus, we have identified cell invasion as a heterodimer-specific biological outcome and suggest that coexpression of ErbB1 may critically regulate invasive progression of ErbB2-positive breast cancers.

Sequestering ErbB2 in endoplasmic reticulum by its autoinhibitor from translocation to cell surface: An autoinhibition mechanism of ErbB2 expression

ErbB2 is differentially overexpressed in tumor versus host tissues, suggesting that an autoregulation mechanism may modulate the expression of ErbB2 and control cell growth. A truncated ErbB2 extracellular domain, herstatin has been shown to bind to ErbB2 and inhibit the growth of tumor cells expressing ErbB2. In the present study, the interaction of herstatin and ErbB2 in vivo was observed by confocal microscopy. The aggregation of ErbB2 and herstatin was found in endoplasmic reticulum (ER). The decrease of ErbB2 on the cell surface was accompanied with the increased colocalization of ErbB2 and herstatin in the cytoplasm, suggesting that the formation of ErbB2/herstatin complex may prevent transit from ER to cell surface of ErbB2. The formation of ErbB2 and herstatin complex was further confirmed by immunoprecipitation. The results demonstrate that sequestering ErbB2 molecules intracellularly by herstatin may be a possible mechanism of the cell growth inhibition.

In vivo identification of the interaction site of ErbB2 extracellular domain with its autoinhibitor

Direct interference with the transforming potential of ErbB2 has become a subject of great interest. Disruption of critical ErbB2 ectodomain interactions may lead to novel therapeutic approaches for the treatment of various tumors. The ErbB receptor signaling can be inhibited by rationally designed peptide mimetics based on the subdomains of ErbB ectodomain. The mimetics can bind to the ErbB receptor specifically and block inter-receptor interactions, resulting in the growth inhibition of ErbB2-overexpressing cells in vitro. In this study, three-dimensional structure of herstatin, an autoinhibitor of ErbB2 and ErbB2 ectodomain complex was constructed by computer-aided molecular modeling. The binding site on ErbB2 ectodomain for herstatin was determined at S1 domain. The mutants of ErbB2 ectodomain were constructed. The interactions of ErbB2 ectodomain and its mutants with herstatin were analyzed for the first time in living cells that coexpressed herstatin and ErbB2 ectodomain or the mutants. The S1 domain in ErbB2 ectodomain was verified as the interaction site with herstatin by immunoprecipitation, confocal microscopy, and fluorescence resonance energy transfer (FRET). The binding region of herstatin on ErbB2 ectodomain might be a potential target region for the drug design.

Human single-chain antibodies inhibit replication of human immunodeficiency virus type 1 (HIV-1)

The F240 human monoclonal antibody specifically recognizes the disulfide loop-bonded immunodominant epitope of gp41 spanning residues 592-606 and expressed broadly on HIV-1 primary isolates. Despite broad reactivity with native virions and HIV-infected cells, the antibody fails to neutralize infection. However, cytoplasmic expression of single-chain antibody (scFv) directed against gp41 of HIV-1 provides a rationale means to inhibit the maturation of envelope protein. The variable regions of the heavy chain and light chain of human monoclonal antibody were amplified by PCR and linked by a 15 amino acid (GGSGS)3 linker in an orientation of VL-linker-VH and retroviral expression vectors were constructed to simultaneously express F240 scFv and eGFP to facilitate selection of scFv-producing cells. Incorporation of a human immunoglobulin signal sequence directed secretion of the F240 scFv (s-scFv) while an otherwise identical vector lacked this sequence (scFv) resulting in intracellular expression of scFv. Transduced human CD4+ H9 T cells were challenged with HIV. While both secreted and nonsecreted F240 scFv inhibited viral production, secretory F240 scFv was more potent. Thus, this novel approach to direct expression of a nonneutralizing scFv using the Ig signal sequence suggests that targeted therapy using antibodies to conserved, highly expressed epitopes may result in a decrease in viral production due to a reduction of viral assembly and/or transport and expression.

Inhibition of HER-2/neu kinase impairs androgen receptor recruitment to the androgen responsive enhancer

Advanced prostate cancer invariably recurs despite androgen deprivation therapy. The androgen receptor (AR) likely plays a key role in this progression and in the continued survival and proliferation of prostate cancer cells in the low androgen environment. Cross-talk with growth factor receptors, such as epidermal growth factor receptor (EGFR) family, has been postulated as a potential mechanism to activate AR in recurrent prostate cancer. We have investigated the role of HER-2/neu (ErbB-2) tyrosine kinase in AR function by characterizing the effect of inhibiting endogenous HER-2 activity in LNCaP cells. We used two independent methods, expression of intracellular single-chain antibody against HER-2 and treatment with a novel dual EGFR/HER-2 kinase inhibitor GW572016 (lapatinib). Expression of intracellular HER-2 antibody scFv-5R and treatment with GW572016 inhibited HER-2 signaling. This HER-2 inhibition led to impairment of AR-mediated functions, such as androgen-stimulated growth and the induction of endogenous prostate-specific antigen (PSA) mRNA and protein. Androgen-stimulated recruitment of AR and histone acetylation at the androgen responsive enhancer of the PSA gene, detected by chromatin immunoprecipitation analysis, were impaired by HER-2 inhibition. GW572016 was more potent in its ability to inhibit PSA expression and AR recruitment and histone acetylation than the EGFR-selective kinase inhibitor ZD1839 (gefitinib), consistent with the HER-2 kinase playing the major role in AR regulation. These results show that HER-2 signaling is required for optimal transcriptional activity of AR in prostate cancer cells and suggest that HER-2 inhibition may provide a novel strategy to disrupt AR function in prostate cancer.

Phenotypic knockout of VEGF-R2 and Tie-2 with an intradiabody reduces tumor growth and angiogenesis in vivo

The endothelial cell receptor-tyrosine kinases, VEGF receptor 2 (VEGF-R2) and Tie-2, and their ligands, vascular endothelial growth factor (VEGF) and angiopoietins 1 and 2, respectively, play key roles in tumor angiogenesis. Several studies suggest that the VEGF receptor pathway and the Tie-2 pathway are independent and essential mediators of angiogenesis, leading to the hypothesis that simultaneous interference with both pathways should result in additive effects on tumor growth. In this study, a human melanoma xenograft model (M21) was used to analyze the effects of simultaneous intradiabody depletion of vascular endothelial growth receptor-R2 and Tie-2 on tumor angiogenesis and tumor xenograft growth. The intradiabodies were expressed from recombinant adenovirus delivered through subtumoral injection. Blockade of both VEGF-R2 and Tie-2 pathways simultaneously or the VEGF receptor pathway alone resulted in a significant inhibition of tumor growth and tumor angiogenesis (92.2% and 74.4%, respectively). In addition, immunohistochemical staining of intradiabody-treated tumors demonstrated a decreased number of tumor-associated blood vessels versus control treatment. Previous studies with intrabodies had demonstrated that the Tie-2 receptor pathway was essential for tumor growth. The simultaneous blockade of the VEGF and Tie-2 pathways resulted in effective inhibition of tumor growth and demonstrated the potential of simultaneous targeting of multiple pathways as a therapeutic strategy.

Reorganization of ErbB family and cell survival signaling after Knock-down of ErbB2 in colon cancer cells

The role of the ErbB family in supporting the malignant phenotype was characterized by stable transfection of a single chain antibody (ScFv5R) against ErbB2 containing a KDEL endoplasmic reticulum retention sequence into GEO human colon carcinoma cells. The antibody traps ErbB2 in the endoplasmic reticulum, thereby down-regulating cell surface ErbB2. The transfected cells showed inactivation of ErbB2 tyrosine phosphorylation and reduced heterodimerization of ErbB2 and ErbB3. This resulted in greater sensitivity to apoptosis induced by growth deprivation and delayed tumorigenicity in vivo. Furthermore, decreased heterodimerization of ErbB2 and ErbB3 led to a reorganization in ErbB function in transfected cells as heterodimerization between epidermal growth factor receptor (EGFR) and ErbB3 increased, whereas ErbB3 activation remained almost the same. Importantly, elimination of ErbB2 signaling resulted in an increase in EGFR expression and activation in transfected cells. Increased EGFR activation contributed to the sustained cell survival in transfected cells.

Targeting Tumor Angiogenesis with Adenovirus-Delivered Anti-Tie-2 Intrabody

Inhibition of tumor angiogenesis is a promising approach for cancer therapy. As an endothelial cell–specific receptor kinase expressed almost exclusively on the surface of vascular endothelium, Tie-2 has an important role in tumor angiogenesis. To explore the therapeutic potential of blocking Tie-2 receptor-interaction pathway, an adenoviral vector was used to deliver a recombinant single-chain antibody fragment rabbit intrabody (pAd-2S03) capable of inhibition of both mouse and human Tie-2 surface expression. pAd-2S03 was given to mice with well-established primary tumors, either a human Kaposi's sarcoma (SLK) or a human colon carcinoma (SW1222). The intrabody significantly inhibited growth of both tumors (75% and 63%, respectively) when compared with pAd-GFP control-treated tumors (P < 0.01). Histopathologic analysis of cryosections taken from mice treated with pAd-2S03 revealed a marked decrease in vessel density, which was reduced by >87% in both tumor models when compared with control-treated tumors (P < 0.01). In contrast, human Tie-2-monospecific pAd-1S05 intrabody did not affect the growth of tumors, indicating that the antitumor effect of pAd-2S03 was due to the inhibition of tumor angiogenesis in these murine models. Our results show that the Tie-2 receptor pathway is essential for both SLK sarcoma and SW1222 colon carcinoma xenograft growth. The present study shows the potential utility of antiangiogenic agents that target the endothelium-specific receptor Tie-2 for down-regulation or genetic deletion.

Epigen, the last ligand of ErbB receptors, reveals intricate relationships between affinity and mitogenicity

Four ErbB receptors and multiple growth factors sharing an epidermal growth factor (EGF) motif underlie transmembrane signaling by the ErbB family in development and cancer. Unlike other ErbB proteins, ErbB-2 binds no known EGF-like ligand. To address the existence of a direct ligand for ErbB-2, we applied algorithms based on genomic and cDNA structures to search sequence data bases. These searches reidentified all known EGF-like growth factors including Epigen (EPG), the least characterized ligand, but failed to identify novel factors. The precursor of EPG is a widely expressed transmembrane glycoprotein that undergoes cleavage at two sites to release a soluble EGF-like domain. A recombinant EPG cannot stimulate cells singly expressing ErbB-2, but it acts as a mitogen for cells expressing ErbB-1 and co-expressing ErbB-2 in combination with the other ErbBs. Interestingly, soluble EPG is more mitogenic than EGF, although its binding affinity is 100-fold lower. Our results attribute the anomalous mitogenic power of EPG to evasion of receptor-mediated depletion of ligand molecules, as well as to inefficient receptor ubiquitylation and down-regulation. In conclusion, EPG might represent the last EGF-like growth factor and define a category of low affinity ligands, whose bioactivity differs from the more extensively studied high affinity ligands.

Memo mediates ErbB2-driven cell motility

Clinical studies have revealed that cancer patients whose tumours have increased ErbB2 expression tend to have more aggressive, metastatic disease, which is associated with parameters predicting a poor outcome. The molecular basis underlying ErbB2-dependent cell motility and metastases formation, however, still remains poorly understood. In this study, we show that activation of a set of signalling molecules, including MAPK, phosphatidylinositol-3-OH kinase (PI(3)K) and Src, is required for Neu/ErbB2-dependent lamellipodia formation and for motility of breast carcinoma cells. Stimulation of these molecules, however, failed to induce efficient cell migration in the absence of Neu/ErbB2 phosphorylation at Tyr 1201 or Tyr 1227. We describe a novel molecule, Memo (mediator of ErbB2-driven cell motility), that interacts with a phospho-Tyr 1227-containing peptide, most probably through the Shc adaptor protein. After Neu/ErbB2 activation, Memo-defective cells form actin fibres and grow lamellipodia, but fail to extend microtubules towards the cell cortex. Our data suggest that Memo controls cell migration by relaying extracellular chemotactic signals to the microtubule cytoskeleton.

A Multicenter Phase I Gene Therapy Clinical Trial Involving Intraperitoneal Administration of E1A-Lipid Complex in Patients with Recurrent Epithelial Ovarian Cancer Overexpressing HER-2/neuOncogene

PURPOSE

HER-2/neu oncogene is overexpressed in 10-30% of epithelial ovarian cancers and is associated with a poor prognosis. The E1A gene product of adenovirus type 5 down-regulates HER-2/neu and causes tumor regression in animal models. In the current study, we sought to determine the toxicity and biological activity of E1A-lipid complex in ovarian cancer patients.

EXPERIMENTAL DESIGN

A Phase I trial involving intraperitoneal (i.p.) administration of E1A-lipid complex was initiated in ovarian cancer patients to assess biological activity (E1A gene transfer/transcription/translation and HER-2/neu expression) and to determine the maximum tolerated dose. Successive cohorts received E1A-lipid complex at doses of 1.8, 3.6, and 7.2 mg DNA/m(2), given as weekly i.p. infusions for 3 of 4 weeks (each cycle) up to a maximum of six cycles. Peritoneal fluid was sampled at baseline and twice monthly for cellularity, cytology, CA-125, and biological activity

RESULTS

Fifteen patients, with a median age of 57 years (range, 43-81) were recruited. Three (1.8 mg DNA/m(2)), 4 (3.6 mg DNA/m(2)), and 8 patients (7.2 mg DNA/m(2)) received i.p. E1A. A total of 91 infusions (range, 1-18) was administered. Abdominal pain was the dose-limiting toxicity, and the maximum-tolerated dose was 3.6 mg DNA/m(2). E1A gene transfer and expression was observed in all of the patients and at all of the dose levels. HER-2/neu down-regulation could be demonstrated in the tumor cells of 2 patients (18%). There was no correlation between dose and biological activity.

CONCLUSIONS

I.P. EIA-lipid complex gene therapy is feasible and safe. Future studies, either alone or in combination with chemotherapy, particularly in patients with minimal residual disease, should be evaluated.

Enhancement of the p27Kip1-mediated antiproliferative effect of trastuzumab (Herceptin) on HER2-overexpressing tumor cells

The oncogenic activity of the overexpressed HER2 tyrosine kinase receptor requires its localization in the plasma membrane. The antitumor effect of anti-HER2 antibodies (Abs) is mainly dependent on receptor downregulation and comprises p27Kip1-mediated G1 cell cycle arrest. However, one major limitation of anti-HER2 therapy is the reversibility of tumor growth inhibition after discontinuation of treatment caused by the mitogenic signaling associated with cell surface receptor re-expression. We found that the level of p27Kip1 upregulation, inhibition of Cdk2 activity and magnitude of G1 arrest induced by the humanized Ab trastuzumab (Herceptin, HCT) on BT474 and SKBr3 HER2-overexpressing breast cancer cells correlates with the level of cell surface receptor. Thus, continuous exposure of cells to HCT for 72 hr results in downregulation of the cell surface receptor and a concurrent increase in the level of p27Kip1 protein. Discontinuation of Ab exposure after the first 8 hr results in failure to upregulate p27Kip1 and arrest of cell cycle progression. We show that the lysosomotropic amine chloroquine (CQ) augments receptor internalization in HER2-overexpressing cells either pretreated or continuously treated with HCT and leads to an increased and sustained inhibitory effect. The enhanced CQ-dependent loss of functional HER2 from the cell surface resulted in sustained inactivation of the serine/threonine kinase Akt, upregulation of p27Kip1 protein and inhibition of cyclin E/Cdk2 activity. Potentiation of the inhibitory effect of HCT by CQ was directly related to loss of HER2 from the plasma membrane since prevention of Ab-mediated receptor endocytosis by engagement of the receptor with immobilized HCT abrogated the effect of CQ.

Intradiabodies, bispecific, tetravalent antibodies for the simultaneous functional knockout of two cell surface receptors

The specific and high affinity binding properties of intracellular antibodies (intrabodies), combined with their ability to be stably expressed in defined organelles, provides powerful tools with a wide range of applications in the field of functional genomics and gene therapy. Intrabodies have been used to specifically target intracellular proteins, manipulate biological processes, and contribute to the understanding of their functions as well as for the generation of phenotypic knockouts in vivo by surface depletion of extracellular or transmembrane proteins. In order to study the biological consequences of knocking down two receptor-tyrosine kinases, we developed a novel intrabody-based strategy. Here we describe the design, engineering, and characterization of a bispecific, tetravalent endoplasmic reticulum (ER)-targeted intradiabody for simultaneous surface depletion of two endothelial transmembrane receptors, Tie-2 and vascular endothelial growth factor receptor 2 (VEGF-R2). Comparison of the ER-targeted intradiabody with the corresponding conventional ER-targeted single-chain antibody fragment (scFv) intrabodies demonstrated that the intradiabody is significantly more efficient with respect to efficiency and duration of surface depletion of Tie-2 and VEGF-R2. In vitro endothelial cell tube formation assays suggest that the bispecific intradiabody exhibits strong antiangiogenic activity, whereas the effect of the monospecific scFv intrabodies was weaker. These findings suggest that simultaneous interference with the VEGF and the Tie-2 receptor pathways results in at least additive antiangiogenic effects, which may have implications for future drug developments. In conclusion, we have identified a highly effective ER-targeted intrabody format for the simultaneous functional knockout of two cell surface receptors.

The ErbB2/ErbB3 heterodimer functions as an oncogenic unit: ErbB2 requires ErbB3 to drive breast tumor cell proliferation

ErbB2 is a receptor tyrosine kinase whose activity in normal cells depends on dimerization with another ligand-binding ErbB receptor. In contrast, amplification of c-erbB2 in tumors results in dramatic overexpression and constitutive activation of the receptor. Breast cancer cells overexpressing ErbB2 depend on its activity for proliferation, because treatment of these cells with ErbB2-specific antagonistic antibodies or kinase inhibitors blocks tumor cells in the G1 phase of the cell cycle. Intriguingly, loss of ErbB2 signaling is accompanied by a decrease in the phosphotyrosine content of ErbB3. On the basis of these results, it has been proposed that ErbB3 might be a partner for ErbB2 in promoting cellular transformation. To test this hypothesis and directly examine the role of the "kinase dead" ErbB3, we specifically ablated its expression with a designer transcription factor (E3). By infection of ErbB2-overexpressing breast cancer cells with a retrovirus expressing E3, we show that ErbB3 is an essential partner in the transformation process. Loss of functional ErbB2 or ErbB3 has similar effects on cell proliferation and cell cycle regulators. Furthermore, expression of constitutively active protein kinase B rescues the proliferative block induced as a consequence of loss of ErbB2 or ErbB3 signaling. These results demonstrate that ErbB2 overexpression and activity alone are insufficient to promote breast tumor cell division. Furthermore, we identify ErbB3's role, which is to couple active ErbB2 to the phosphatidylinositol 3-kinase/protein kinase B pathway. Thus, the ErbB2/ErbB3 dimer functions as an oncogenic unit to drive breast tumor cell proliferation.

Expression and characterization of a humanized cocaine-binding antibody

The murine immunoglobulin G (IgG) cocaine-binding monoclonal antibody (mAb), GNC92H2, is notable for its exquisite specificity for cocaine, as opposed to chemically-related cocaine metabolites, and for its moderately high affinity (K(d) approximately 200 nM) for cocaine. Recently, we described the crystal structure of a mouse/human chimeric Fab construct at 2.3 A resolution. Herein, we report the successful framework humanization of a single-chain Fv (scFv) GNC92H2 construct without loss of affinity for cocaine. In brief, we compared the mAb GNC92H2 sequence to human antibody sequences, and used structure-based design to incorporate mutations (total = 49) that would humanize the framework region without affecting the overall shape of the binding pocket or the key cocaine-contact residues. The codons of the rationally designed sequence were optimized for E. coli expression, and the gene was synthesized by a de novo PCR reaction using 14 overlapping primers. Expression of the scFv construct was significantly improved in E. coli by fusion to thioredoxin. Intriguingly, this construct apparently refolds to form soluble active antibody in the reducing environment of the cytoplasm. Competitive ELISA and equilibrium dialysis demonstrated comparable binding activity between the humanized scFv and the whole IgG. The successful humanization of mAb GNC92H2 should enhance its potential therapeutic value by reducing its overall. immunogenicity.

Reversion of transformed phenotype in ovarian cancer cells by intracellular expression of anti folate receptor antibodies

The alpha-folate receptor (FR) is selectively overexpressed in 90% of nonmucinous ovarian carcinomas, whereas no expression is detectable in normal ovarian surface epithelium (OSE). Indirect evidence suggests that FR expression is associated with tumor progression and affects cell proliferation. To evaluate better the role of FR, we developed an approach based on intracellular expression of single-chain (sc) antibodies (intrabody) to downmodulate membrane expression of FR in ovary cancer cells. IGROV-1 and SKOV3 ovarian carcinoma cell lines were transfected with an anti-FR intrabody. Transfectants and parental cells were tested for FR, integrins and anti-FR intrabody expression by fluorescence-activated cell sorting (FACS), reverse transcription and polymerase chain reaction (RT-PCR) and/or immunoblotting. Cell growth characteristics and adhesion properties were evaluated in liquid, semisolid and organotypic cultures. The anti-FR scFv inhibited FR expression from 60 to 99%. At physiological concentrations of folate, proliferation varied directly as a function of FR expression. FR downmodulation was accompanied by reduced colony-forming ability in soft agar, morphological change of the cells, significant enhanced adhesion to laminin or Matrigel, a two- to three-fold increase in alpha6beta4 integrin expression, and a marked reduction in laminin production. In three-dimensional organotypic cultures, anti-FR intrabody-transfected IGROV1 cells grew as a single-ordered layer, reminiscent of normal OSE growth in vivo. In conclusion, the anti-FR intrabody reverses the transformed phenotype in ovary cancer cells and may provide an efficient means to inhibit selectively the growth of these cells.

Generation and functional characterization of intracellular antibodies interacting with the kinase domain of human EGF receptor

Intracellular expression of single-chain antibodies (scFvs) represents a promising approach for selective interference with cellular proto-oncogenes such as the epidermal growth factor receptor (EGFR). Previously, we have shown that intrabodies targeted to the lumen of the endoplasmic reticulum prevent the transit of EGFR or the related ErbB2 molecule to the cell surface, thereby inactivating their transforming potential. While intramolecular disulfide bridges important for antibody stability are correctly formed during expression in the secretory pathway, scFvs expressed in the reducing environment of the cytosol are often inactive. To overcome this problem and to generate antibody fragments that interact with the intracellular domain of human EGFR in the cytoplasm, here we have chosen a two-step approach combining classical selection of scFvs by phage display with subsequent expression in yeast. After enrichment of EGFR-specific antibody fragments from a combinatorial library by biopanning, a yeast two-hybrid screen was performed using the intracellular domain of EGFR as bait. Screening of 1.5 x 10(5) preselected scFv plasmids under highly stringent conditions yielded 223 colonies that represented at least five independent scFv clones functional in the intracellular milieu of eukaryotic cells. Interaction of selected antibody fragments with the intracellular domain of EGFR was confirmed in GST pull-down and coimmunoprecipitation experiments. Upon cytoplasmic expression in human tumor cells, scFvs colocalized with EGFR at the plasma membrane demonstrating their functionality in vivo.

Expression of scFv antibodies in Xenopus embryos to disrupt protein function: implications for large-scale evaluation of the embryonic proteome

SUMMARY

We evaluated the use of single-chain antibody (scFv) expression as a tool to disrupt the function of specific proteins in embryos of the frog, Xenopus laevis. The expression of scFvs that recognize the bone morphogenetic protein receptor (ALK3) or the fibroblast growth factor receptor1 (FGFR1) as endoplasmic reticulum-anchored proteins caused distinct developmental defects that were virtually indistinguishable from the defects caused by expression of the dominant negative forms of each receptor. These results demonstrate that scFvs from phage-display libraries can be readily fashioned into effective and specific inhibitors of signaling pathways in developing embryos. In addition, as several effective scFvs against a specific target can be isolated rapidly, this approach represents a valuable new tool for large-scale functional analysis of the embryonic proteome.

Yeast Genetic Methods for the Detection of Membrane Protein Interactions

Due to the pivotal role of membrane proteins in many cellular processes, their direct link to human disease and their often extracellular accessibility towards drugs, an understanding of membrane protein function is desirable. However, the hydrophobic nature of membrane proteins often results in insoluble proteins which makes protein isolation difficult and therefore hinders the determination of protein complex composition and protein function. Recently, several yeast genetic techniques have made the characterisation of interactions among membrane proteins more feasible. Techniques such as the guanine-nucleotide binding protein fusion assay, the reverse Ras recruitment system and the split-ubiquitin system have been fruitful in monitoring known protein interactions and uncovering novel interactions. Since many disease states have altered membrane protein function, one can use these systems to recreate interactions involving disease causing membrane proteins. Once established, screens for small molecules, peptides and/or single chain antibodies which disrupt such interactions can provide insight into the biology of the interaction and thus help guide therapeutical research. In this review, we speculate on the feasibility of using inhibitors of protein interactions as drugs and the adaptation of these techniques to select for inhibitors of defined protein interactions.

Targeting ligand-activated ErbB2 signaling inhibits breast and prostate tumor growth

ErbB2 is a ligand-less member of the ErbB receptor family that functions as a coreceptor with EGFR, ErbB3, and ErbB4. Here, we describe an approach to target ErbB2's role as a coreceptor using a monoclonal antibody, 2C4, which sterically hinders ErbB2's recruitment into ErbB ligand complexes. Inhibition of ligand-dependent ErbB2 signaling by 2C4 occurs in both low- and high-ErbB2-expressing systems. Since the ErbB3 receptor contains an inactive tyrosine kinase domain, 2C4 is very effective in blocking heregulin-mediated ErbB3-ErbB2 signaling. We demonstrate that the in vitro and in vivo growth of several breast and prostate tumor models is inhibited by 2C4 treatment.