Agonistic antibodies stimulate the kinase encoded by the neu protooncogene in living cells but the oncogenic mutant is constitutively active

Enhancing the Anti-tumor Potency of a Novel Siglec-15 Antibody by Engineering its Fc-mediated Effector Functions

Siglec-15, an inhibitory immune checkpoint, is an emerging target in cancer immunotherapy. Blocking the function of Siglec-15 is an excellent strategy for cancer treatment and antibody blockade has been used to target Siglec-15. However, whether Fc-mediated effector functions contribute to the therapeutic effect of antibodies remains unclear. Herein, we generated a monoclonal antibody, 1-15D1, which had a high binding affinity with Siglec-15 and strongly activated T-cell immune response in vitro. Subsequently, the Fc-mediated effector functions of 1-15D1 were explored in a Siglec-15 humanized mouse model, and further improvement in antitumor efficacy was observed in the mouse IgG2a isotype group. Thus, we demonstrate that the antitumor effects of 1-15D1 were mediated via multiple factors. In addition to the T-cell immune response, 2 novel mechanisms were explored, including the internalization of the cell surface Siglec-15 and Fc-mediated effector functions. In conclusion, our studies not only provide a potential agent for the improvement of cancer immunotherapy but also suggest that a specific role of Fc-mediated immune regulation may improve the therapeutic potency of Siglec-15 monoclonal antibody.

A Cell-Based Internalization and Degradation Assay with an Activatable Fluorescence-Quencher Probe as a Tool for Functional Antibody Screening

For the development of therapeutically potent anti-cancer antibody drugs, it is often important to identify antibodies that internalize into cells efficiently, rather than just binding to antigens on the cell surface. Such antibodies can mediate receptor endocytosis, resulting in receptor downregulation on the cell surface and potentially inhibiting receptor function and tumor growth. Also, efficient antibody internalization is a prerequisite for the delivery of cytotoxic drugs into target cells and is critical for the development of antibody–drug conjugates. Here we describe a novel activatable fluorescence–quencher pair to quantify the extent of antibody internalization and degradation in the target cells. In this assay, candidate antibodies were labeled with a fluorescent dye and a quencher. Fluorescence is inhibited outside and on the surface of cells, but activated upon endocytosis and degradation of the antibody. This assay enabled the development of a process for rapid characterization of candidate antibodies potentially in a high-throughput format. By employing an activatable secondary antibody, primary antibodies in purified form or in culture supernatants can be screened for internalization and degradation. Because purification of candidate antibodies is not required, this method represents a direct functional screen to identify antibodies that internalize efficiently early in the discovery process.

Monoclonal Antibodies against the MET/HGF Receptor and Its Ligand: Multitask Tools with Applications from Basic Research to Therapy

Monoclonal antibodies can be seen as valuable tools for many aspects of basic as well as applied sciences. In the case of MET/HGFR, they allowed the identification of truncated isoforms of the receptor, as well as the dissection of different epitopes, establishing structure-function relationships. Antibodies directed against MET extracellular domain were found to be full or partial receptor agonists or antagonists. The agonists can mimic the effects of the different isoforms of the natural ligand, but with the advantage of being more stable than the latter. Thus, some agonist antibodies promote all the biological responses triggered by MET activation, including motility, proliferation, morphogenesis, and protection from apoptosis, while others can induce only a migratory response. On the other hand, antagonists can inhibit MET-driven biological functions either by competing with the ligand or by removing the receptor from the cell surface. Since MET/HGFR is often over-expressed and/or aberrantly activated in tumors, monoclonal antibodies can be used as probes for MET detection or as "bullets" to target MET-expressing tumor cells, thus pointing to their use in diagnosis and therapy.

Herceptin, a recombinant humanized anti-ERBB2 monoclonal antibody, induces cardiomyocyte death

P53 protein levels are elevated by trastuzumab and the biologically similar rat ERBB2/HER2/NEU antibody; and that this coincides with enhanced apoptosis, increased cleaved caspase-3 levels and diminished cardiac function. We also demonstrate that MDM2 may be a regulatory target of anti-ERBB2 thereby implicating the MDM2/p53 axis as a potential molecular component for the undesirable cardiac outcomes noted with trastuzumab. Finally, we show that these MDM2/p53-mediated events are independent of both the ERK1/2 and Akt systems. In conclusion, our findings suggest that the adverse cardiac events observed with trastuzumab may stem from its negative regulation of MDM2 events which impairs p53 degradation resultantly promoting apoptosis leading to cardiac dysfunction. These observations may have important therapeutic implications since they suggest that anticancer agents that inhibit MDM2 and its downstream actions may curb tumor progression at the expense of increasing cardiac stress.

Agonist monoclonal antibodies against HGF receptor protect cardiac muscle cells from apoptosis

Hepatocyte growth factor (HGF), a pleiotropic cytokine with mitogenic, motogenic, morphogenic, and antiapoptotic effects in various cell types, is a cardioprotective growth factor that can counteract the loss of cardiomyocytes usually observed in cardiac diseases. HGF is a quite unstable molecule in its biologically active heterodimeric form. Since all HGF-induced biological responses are mediated by its high-affinity tyrosine kinase receptor (Met/HGF-R) encoded by the Met gene, we asked whether a monoclonal antibody (MAb) that displays receptor full agonist activity could protect cardiac muscle cell lines from hydrogen peroxide-induced apoptosis. We report that the MAb efficiently inhibited hydrogen peroxide-induced cell shrinkage, DNA fragmentation, annexin V positivity, mitochondrial translocation of bax, and caspase activation. The MAb was thus able to counteract apoptosis evaluated by both morphological and biochemical criteria. The agonist activity of the MAb was mediated by Met/HGF-R, since a Met/HGF-R-specific short hairpin RNA (shRNA) inhibited both activation of transduction pathways and motility triggered by MAb DO-24. The protective antiapoptotic effect of MAb DO-24 was dependent on activation of the ras-MAPK Erk1/2 and phosphatidylinositol 3-kinase (PI3-kinase)-Akt transduction pathways, since it was abrogated by treatments with their specific pharmacological inhibitors, PD-98059 and wortmannin. Moreover, the MAb induced a motogenic, but not mitogenic, response in these cells, mimicking in all aspects the natural ligand HGF but displaying a significant higher stability than HGF in culture. This MAb may thus be a valuable substitute for HGF, being more easily available in a biologically active, highly stable, and purified form.

Persistent elimination of ErbB-2/HER2-overexpressing tumors using combinations of monoclonal antibodies: relevance of receptor endocytosis

Monoclonal antibodies (mAbs) to ErbB-2/HER2 or to its sibling, the epidermal growth factor receptor (EGFR), prolong survival of cancer patients, especially when combined with cytotoxic therapies. However, low effectiveness of therapeutic mAbs and the evolution of patient resistance call for improvements. Here we test in animals pairs of anti-ErbB-2 mAbs and report that pairs comprising an antibody reactive with the dimerization site of ErbB-2 and an antibody recognizing another distinct epitope better inhibit ErbB-2-overexpressing tumors than other pairs or the respective individual mAbs. Because the superiority of antibody combinations extends to tumor cell cultures, we assume that nonimmunological mechanisms contribute to mAb synergy. One potential mechanism, namely the ability of mAb combinations to instigate ErbB-2 endocytosis, is demonstrated. Translation of these lessons to clinical applications may enhance patient response and delay acquisition of resistance.

Selection and characterization of antibodies from phage display libraries against internalizing membrane antigens

Macromolecular delivery systems require high target cell specificity and efficient intracellular uptake. Monoclonal antibodies (mAbs) have been shown to successfully meet these needs and should, due to their biological nature and thus minimal toxicity and limited immunogenicity, be optimal delivery vehicles for various macromolecules (e.g., toxins, drugs, oligonucleotides). Such antibodies could be retrieved from phage display libraries by carefully designed selection and screening methods. In this chapter, we provide protocols for the isolation of phage-derived antibodies reactive to cell surface receptors, which upon binding will induce receptor-mediated internalization of the antibody/receptor complexes. In addition, a protocol describing the identification of target antigens by immunoprecipitation (ip) of cell lysates and preparation of gel plugs for subsequent MALDI-TOF analysis is included. Furthermore, we suggest several techniques that could be employed to confirm the specificity as well as the drug delivery potential of isolated clones.

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.

Mimotope vaccines: epitope mimics induce anti-cancer antibodies

Mimotopes are epitope-mimicking structures. When applied for immunizations they induce desired antibody specificities exclusively based on the principle of molecular mimicry. This is important as antibodies directed against tumor-associated antigens may harbor diverse biological effects depending on their epitope specificity. Thus they may inhibit or promote tumor growth. This review gives an update on different vaccination strategies based on the mimotope concept.

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.

Anti-erbB2 treatment induces cardiotoxicity by interfering with cell survival pathways

Introduction

Cardiac dysfunction is among the serious side effects of therapy with recombinant humanized anti-erbB2 monoclonal antibody. The antibody blocks ErbB-2, a receptor tyrosine kinase and co-receptor for other members of the ErbB and epidermal growth factor families, which is over-expressed on the surface of many malignant cells. ErbB-2 and its ligands neuregulin and ErbB-3/ErbB-4 are involved in survival and growth of cardiomyocytes in both postnatal and adult hearts, and therefore the drug may interrupt the correct functioning of the ErbB-2 pathway.

Methods

The effect of the rat-anti-erbB2 monoclonal antibody B-10 was studied in spontaneously beating primary myocyte cultures from rat neonatal hearts. Gene expression was determined by RT-PCR (reverse transcription polymerase chain reaction) and by rat stress-specific microarray analysis, protein levels by Western blot, cell contractility by video motion analysis, calcium transients by the FURA fluorescent method, and apoptosis using the TUNEL (terminal uridine nick-end labelling) assay.

Results

B-10 treatment induces significant changes in expression of 24 out of 207 stress genes analyzed using the microarray technique. Protein levels of ErbB-2, ErbB-3, ErbB-4 and neuregulin decreased after 1 day. However, both transcription and protein levels of ErbB-4 and gp130 increased several fold. Calreticulin and calsequestrin were overexpressed after three days, inducing a decrease in calcium transients, thereby influencing cell contractility. Apoptosis was induced in 20% cells after 24 hours.

Conclusion

Blocking ErbB-2 in cultured rat cardiomyocytes leads to changes that may influence the cell cycle and affects genes involved in heart functions. B-10 inhibits pro-survival pathways and reduces cellular contractility. Thus, it is conceivable that this process may impair the stress response of the heart.

Quantitative transcriptional control of ErbB receptor signaling undergoes graded to biphasic response for cell differentiation

ErbB receptor ligands, epidermal growth factor (EGF) and heregulin (HRG), induce dose-dependent transient and sustained intracellular signaling, proliferation, and differentiation of MCF-7 breast cancer cells, respectively. In an effort to delineate the ligand-specific cell determination mechanism, we investigated time course gene expressions induced by EGF and HRG that induce distinct cellular phenotypes in MCF-7 cells. To analyze independently the effects of ligand dosage and time for gene expression, we developed a statistical method for estimating the two effects. Our results indicated that signal transduction pathways convey quantitative properties of the dose-dependent activation of ErbB receptor to early transcription. The results also implied that moderate changes in the expression levels of a number of genes, not the predominant regulation of a few specific genes, might cooperatively work at the early stage of the transcription for determining cell fate. However, the EGF- and HRG-induced distinct signal durations resulted in the ligand-oriented biphasic induction of proteins after 20 min. The selected gene list and HRG-induced prolonged signaling suggested that transcriptional feedback to the intracellular signaling results in a graded to biphasic response in the cell determination process and that each ErbB receptor is inextricably responsible for the control of amplitude and duration of cellular biochemical reactions.

Neuregulin activates erbB2-dependent src/FAK signaling and cytoskeletal remodeling in isolated adult rat cardiac myocytes

Cardiac myocyte erbB2 expression is required for maintenance of normal cardiac structure and function, though its role in cardiac cellular physiology is incompletely understood. We tested the hypothesis that erbB2 signaling modulates focal adhesion formation via activation of a src/FAK pathway using adult rat ventricular myocytes in primary culture. The erbB ligand neuregulin-1Beta (NRG-1Beta) induced phosphorylation of Src at Y416 and Y215, and FAK at Y861. Using antibody and pharmacological inhibitor strategies, we found that FAK activation was erbB2- and Src-dependent, but independent of PI3-kinase/Akt pathway. Furthermore, NRG-1Beta stimulated the formation of a multiprotein complex between erbB2, FAK, p130(CAS) and paxillin within 30 min, and induced lamellipodia with longitudinal elongation of the myocytes within days. The extension of lamellipodia resulted in restoration of cell-to-cell contact between isolated myocytes, allowing for synchronous beating. These effects of NRG-1Beta were prevented by a src inhibitor as well as an antibody to erbB2. These results suggest the potential role of NRG-1Beta/erbB2/Src/FAK signaling in the maintenance and repair of electrical and mechanical coupling in cardiomyocytes.

Role of neuregulin-1/ErbB2 signaling in endothelium-cardiomyocyte cross-talk

Neuregulin-1 (NRG-1), a cardioactive growth factor released from endothelial cells, has been shown to be indispensable for the normal function of the adult heart by binding to ErbB4 receptors on cardiomyocytes. In the present study, we have investigated to what extent ErbB2, the favored co-factor of ErbB4 for heterodimerization, participates in the cardiac effects of endothelium-derived NRG-1. In addition, in view of our previously described anti-adrenergic effects of NRG-1, we have studied which neurohormonal stimuli affect endothelial NRG-1 expression and release and how this may fit into a broader frame of cardiovascular physiology. Immunohistochemical staining of rat heart and aorta showed that NRG-1 expression was restricted to the endocardial endothelium and the cardiac microvascular endothelium (CMVE); by contrast, NRG-1 expression was absent in larger coronary arteries and veins and in aortic endothelium. In rat CMVE in culture, NRG-1 mRNA and protein expression was down-regulated by angiotensin II and phenylephrine and up-regulated by endothelin-1 and mechanical strain. CMVE-derived NRG-1 was shown to phosphorylate cardiomyocyte ErbB2, an event prevented by a 24-h preincubation of myocytes with monoclonal ErbB2 antibodies. Pretreating cardiomyocytes with these inhibitory anti-ErbB2 antibodies significantly attenuated CMVE-induced cardiomyocyte hypertrophy and abolished the protective actions of CMVE against cardiomyocyte apoptosis. Accordingly, ErbB2 signaling participated in the paracrine survival and growth controlling effects of NRG-1 on cardiomyocytes in vitro, explaining the cardiotoxicity of ErbB2 antibodies in patients. Cardiac NRG-1 synthesis occurs in endothelial cells adjacent to cardiac myocytes and is sensitive to factors related to the regulation of blood pressure.

EGFR enhances Survivin expression through the phosphoinositide 3 (PI-3) kinase signaling pathway

The ErbB family of receptor tyrosine kinases includes the epidermal growth factor receptor (EGFR), p185/neu/c-erbB2, ErbB3, and ErbB4. Many of these receptors are overexpressed or amplified in various forms of cancers. Previous studies have indicated that activation of erbB molecules contributes to malignant transformation both by promoting cell proliferation through the mitogen-activated protein kinase (MAP kinase) signaling pathway and by preventing apoptosis through the Phosphoinositide 3 kinase (PI-3 kinase) pathway. Disabling erbB receptors converts malignant cells that were resistant to cell death caused by irradiation to cells that are sensitive to apoptosis. Here, we report that an activated form of EGFR can elevate the levels of Survivin, a member of the Inhibitor of Apoptosis Protein (IAP) family implicated in mitotic checkpoint control. Conversely, inactivation of the ErbB receptors reduces the expression levels of Survivin. Furthermore, we found that upregulation of Survivin by EGFR is dependent on the PI-3 kinase pathway but not on the MAP kinase pathway. Indeed, inhibition of PI-3 kinase can diminish Survivin at both the mRNA and the protein levels. Combined with previous findings that Survivin plays a role in control of chromosome segregation and that it is overexpressed in various cancers, our results suggest that EGFR may cause transformation by directly affecting mitosis and increasing chromosome instability.

Neuregulin-induced expression of the acetylcholine receptor requires endocytosis of ErbB receptors

Neuregulin-induced expression of the acetylcholine receptor (AChR) contributes to high concentration of the receptor at the neuromuscular junction (NMJ). Neuregulin-1 activates ErbB tyrosine kinases and subsequently intracellular kinases including Erk that is required for induced AChR expression. Recent studies demonstrate that ligand-induced internalization may regulate signaling of various receptor tyrosine kinases. However, the role of induced ErbB endocytosis in regulating AChR expression was unclear. Here we provide evidence that ErbB tyrosine kinases became rapidly internalized in response to neuregulin. The internalization required the kinase activity of ErbB proteins and involved a clathrin-dependent endocytic pathway. Moreover, neuregulin-induced Erk activation and AChR expression were attenuated when ErbB endocytosis was blocked. These results indicate that ErbB proteins undergo endocytosis in response to neuregulin, and this process is required for neuregulin signaling and induced AChR expression.

Activation and inhibition of anaplastic lymphoma kinase receptor tyrosine kinase by monoclonal antibodies and absence of agonist activity of pleiotrophin

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase that is transiently expressed in specific regions of the central and peripheral nervous systems, suggesting a role in its normal development and function. The nature of the cognate ligands of ALK in vertebrate is still a matter of debate. We produced a panel of monoclonal antibodies (mAbs) directed against the extracellular domain of the human receptor. Two major species of ALK (220 and 140 kDa) were identified in transfected cells, and the use of our mAbs established that the 140-kDa species results from a cleavage of the 220-kDa form. Two mAbs, in the nm range, induced the differentiation of PC12 cells transiently transfected with ALK. In human embryonic kidney 293 cells stably expressing ALK, these two mAbs strongly activated the receptor and subsequently the mitogen-activated protein kinase pathway. We further showed for the first time that activation of ALK also resulted in a specific activation of STAT3. In contrast, other mAbs presented the characteristics of blocking antibodies. Finally, in these cell systems, a mitogenic form of pleiotrophin, a proposed ligand of ALK, failed to activate this receptor. Thus, in the absence of clearly established ligand(s) in vertebrates, the availability of mAbs allowing the activation or the inhibition of the receptor will be essential for a better understanding of the biological roles of ALK.

Integrin alpha5/beta1 expression mediates HER-2 down-regulation in colon cancer cells

HER-2 is constitutively activated and overexpressed in many cancers, and its inhibition in colon cancer cells diminishes tumorigenicity and induces apoptosis. Little is known about the regulation of HER-2 signaling in colon cancer cells. Integrin alpha5/beta1 expression is frequently lost in colorectal cancer cells compared with normal intestinal epithelium, and colon cancer cells lacking integrin alpha5/beta1 expression utilize HER-2 signaling for proliferation and tumorigenicity. Re-expression of integrin alpha5/beta1 in colon cancer cells abrogated their tumorigenicity, but how this occurs is not well known. Stable expression of integrin alpha5/beta1 in colon cancer cells with little or no detectable integrin alpha5/beta1 protein expression resulted in the post-transcriptional down-regulation of HER-2 protein. Integrin alpha5/beta1 was found to interact with HER-2, and the cytoplasmic domain of integrin alpha5/beta1 was sufficient to mediate HER-2 down-regulation. Integrin alpha5/beta1-mediated down-regulation of HER-2 was the result of increased lysosomal targeting. The inhibition of HER-2 signaling represents a potential mechanism by which integrin alpha5/beta1 exerts its tumor suppressor-like activity in colon cancer cells. These results also suggest that a novel function for integrin alpha5/beta1 is the control of HER-2 expression.

ALK receptor tyrosine kinase promotes cell growth and neurite outgrowth

Anaplastic lymphoma kinase (ALK) is a receptor-type protein tyrosine kinase that is expressed preferentially in neurons of the central and peripheral nervous systems at late embryonic stages. To elucidate the role of ALK in neurons, we developed an agonist monoclonal antibody (mAb) against the extracellular domain of ALK. Here we show that mAb16-39 elicits tyrosine phosphorylation of endogenously expressed ALK in human neuroblastoma (SK-N-SH) cells. Stimulation of these cells with mAb16-39 markedly induces the tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1), Shc, and c-Cbl and also their interaction with ALK and activation of ERK1/2. Furthermore, we show that continuous incubation with mAb16-39 induces the cell growth and neurite outgrowth of SK-N-SH cells. These responses are completely blocked by MEK inhibitor PD98059 but not by the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor wortmannin, indicating an essential role of the mitogen-activated protein kinase (MAP kinase) signaling cascade in ALK-mediated growth and differentiation of neurons.

Matching of trastuzumab (Herceptin) epitope mimics onto the surface of Her-2/neu--a new method of epitope definition

As seen with the proto-oncogene Her-2/neu, antibodies targeting different parts of a receptor can have opposing effects. Depending on epitope specificity, in this case, tumor growth can be inhibited--but also enhanced. Therefore, the definition of molecular binding sites is of increasing importance in modern medicine. We here introduce a novel approach for binding site localization, utilizing information obtained by the phage display technique. This is a high throughput screening method for identification of peptide mimics, so called mimotopes, of any binding structure of interest. All target molecules whose structure is available in the RCSB Protein Data Bank can be scanned for mimotope matches on their surface. In this study, we present the matching results of five mimotopes defined for the epitope recognized by trastuzumab (Herceptin), a humanized monoclonal antibody inhibiting tumor growth, on Her-2/neu. The localization thus obtained corresponds to the known trastuzumab epitope. We therefore suggest the algorithm as a novel way of binding site definition, circumventing co-crystallization experiments.

Trastuzumab plus tamoxifen: anti-proliferative and molecular interactions in breast carcinoma

HER2 overexpression has been associated with anti-estrogen resistance in human breast cancer, and it has been suggested that the combined treatment of an anti-HER2 antibody plus tamoxifen has enhanced anti-cancer efficacy in breast cancer. The detailed anti-proliferative interactions between trastuzumab and tamoxifen were analyzed with the isobologram and Chou and Talalay methods, which assess the presence of synergy, addition or antagonism. We used the breast cancer cell lines that are estrogen receptor (ER)-positive and HER2-positive. We also analyzed the molecular changes on the HER2 and (ER) signaling pathways that are induced by trastuzumab plus tamoxifen. In terms of cancer cell proliferation, the simultaneous combination of trastuzumab and tamoxifen on BT-474 cells was more growth inhibitory (44%) than the treatment with trastuzumab (24%) or tamoxifen (31%) alone. Isobologram analysis of simultaneous trastuzumab plus tamoxifen exposure showed, however, that there were antagonistic interactions at an effect level of 30% (IC30). Using Chou and Talalay analysis we also observed antagonistic interactions at lower levels of cell kill, although there were additive effects at highest levels of cell kill. Trastuzumab followed by tamoxifen showed antagonism at all effects levels. Tamoxifen followed by trastuzumab showed antagonism at lower levels of cell kill, and additivity at higher levels of cell kill. Similar interactions were observed using T47D cells. The molecular effects of the combined treatment with trastuzumab plus tamoxifen on the levels of HER2 and ER signaling showed that, with respect to HER2 protein levels, trastuzumab downregulated HER2 by 27%, tamoxifen upregulated HER2 by 40%, and the combination of trastuzumab plus tamoxifen did not induce changes in HER2 respect to control. With respect to HER2 mRNA, trastuzumab upregulated HER2 mRNA to 367%, tamoxifen to 166%, and the combination to 401%. With respect to HER2 phosphorylation, trastuzumab upregulated HER2 phosphorylation to 352%, tamoxifen to 202% and the combination to 633%. Epidermal growth factor receptor levels were not changed by trastuzumab or tamoxifen alone, and were upregulated to 138% by the combination. The protein levels and activity of extracellular recptor kinase were not modified by trastuzumab, tamoxifen or the combination. Finally, estrogen receptor protein and mRNA levels were downregulated to about 50% by trastuzumab, tamoxifen or the combination. Taken together, our results show that in ER-positive breast cancer cells overexpressing HER2, trastuzumab plus tamoxifen have antagonistic interactions when used in combination, and that this antagonism may be related with an increase in HER2 signaling pathways that occurs when tamoxifen is added to trastuzumab.

Bivalent antibody phage display mimics natural immunoglobulin

We report the development of a system for displaying bivalent antibody fragments on M13 bacteriophage in a manner that effectively mimics the binding behavior of natural antibodies. In the "bivalent display" format, two copies of antigen binding sites are displayed on the coat of a single phage particle. Bivalent display was first achieved by the insertion of a dimerization domain, consisting of an IgG1 hinge region and a homodimerizing GCN4 leucine zipper, between a Fab and the C-terminal domain of the M13 gene-3 minor coat protein. In a phagemid-based display system, the resulting "Fab'-zip-phage" particles display bivalent Fabs that resemble natural IgGs. An important functional consequence of bivalent display is an avidity effect, which results in a greatly reduced off-rate for phage bound to immobilized antigen. The avidity effect improved the capture and retention of bivalent Fab'-zip-phage relative to monovalent Fab-phage both with antigen immobilized on plates and with cell surface antigen. To examine the requirements for bivalent display on phage, we systematically trimmed down the dimerization domain and found that a single cysteine was sufficient to confer the same avidity effect conferred by the complete dimerization domain. Bivalent antibody phage display should be useful for many applications. In particular, the technology should aid in the production of antibodies against difficult antigens, and also, in selections that require dimerization for activity.

Development of the Bowman-Birk inhibitor for oral cancer chemoprevention and analysis of Neu immunohistochemical staining intensity with Bowman-Birk inhibitor concentrate treatment

OBJECTIVES/HYPOTHESIS

Cancer chemoprevention is a rapidly evolving approach to reverse or inhibit carcinogenesis, and there is active interest in development of effective chemopreventive agents against head and neck cancers. The retinoids are archetypal chemopreventive agents for oral premalignant lesions. They have significant clinical effect, but widespread use is limited by significant clinical toxicity. The Bowman-Birk Inhibitor is one of several nontoxic compounds exhibiting both potent anticarcinogenic activity and minimal toxicity. The purposes of the study were to summarize the preclinical and clinical development of Bowman-Birk Inhibitor and a Bowman-Birk Inhibitor concentrate against oral premalignant lesions and to evaluate Neu immunohistochemical staining intensity for lesions and simultaneously obtained biopsy specimens of normal-appearing mucosa from the Phase IIa Bowman-Birk Inhibitor concentrate oral leukoplakia chemoprevention trial.

STUDY DESIGN

Part I is a selected literature review. Part II is a retrospective analysis of pathological specimens prospectively obtained from the Phase IIa clinical trial of Bowman-Birk Inhibitor concentrate.

METHODS

Thirty-two sets of biopsy specimens from lesions and uninvolved oral mucosa before and after treatment with Bowman-Birk Inhibitor concentrate in doses ranging from 200 to 1066 chymotrypsin inhibitory units were examined in blinded fashion for Neu immunohistochemical staining intensity using the 3B-5 monoclonal antibody. Staining intensity scores among the lesion and control biopsy specimens before and after Bowman-Birk Inhibitor concentrate treatment were analyzed and compared with previously obtained values for serum Neu, oral mucosal cell Neu, protease activity, and clinical response to treatment.

RESULTS

Mean Neu staining score was significantly higher in lesions compared with uninvolved mucosa (P <.001). Pretreatment staining scores for biopsy specimens of lesions and control biopsy specimens of normal-appearing tissues were correlated (Spearman correlation coefficient [r] = 0.375, P =.045), but no correlation between lesion and control biopsy specimen scores was evident after treatment. The change in Neu staining score with Bowman-Birk Inhibitor concentrate treatment in control site biopsy specimens demonstrated an inverse relationship of change in lesion area with Bowman-Birk Inhibitor concentrate treatment (Spearman r = -0.493, P <.007).

CONCLUSION

Bowman-Birk Inhibitor concentrate shows promise to become an effective nontoxic chemopreventive agent based on results of extensive preclinical studies, and Phase I and Phase IIa clinical trials. Bowman-Birk Inhibitor concentrate has dose-related clinical activity against oral leukoplakia and modulates levels of Neu and protease activity. The current investigation identified increased Neu staining intensity in hyperplastic lesions compared with simultaneously obtained biopsy specimens of normal-appearing mucosa both before and after Bowman-Birk Inhibitor concentrate treatment. This finding supports prior observations that increased Neu expression is present in a subset of oral premalignant lesions and head and neck cancers. The trend of increased Neu staining score in control biopsy tissues of subjects exhibiting decreased lesion area following Bowman-Birk Inhibitor concentrate treatment raises questions about the mechanisms of Bowman-Birk Inhibitor concentrate action. One possible explanation is that Bowman-Birk Inhibitor stabilizes the extracellular domain of Neu, thereby preventing receptor truncation and internalization. Further study of modulation of Neu and protease activity by Bowman-Birk Inhibitor concentrate treatment may provide insights into the role of proteases and protease inhibitors in oral premalignant lesions and the mechanisms underlying Bowman-Birk Inhibitor concentrate effects. A Phase IIb randomized, placebo-controlled clinical trial to determine the clinical effectiveness of Bowman-Birk Inhibitor concentrate and further evaluate these candidate biomarkers is under way.

Both the epitope specificity and isotype are important in the antitumor effect of monoclonal antibodies against Her-2/neu antigen

The Her-2/neu oncogene, which encodes a growth factor receptor, was implicated in the malignancy of human adenocarcinomas. Antibodies directed to this molecule have been previously shown to have an antitumor effect in vivo. In an attempt to understand the mechanisms of the antitumor activity, we generated 2 monoclonal antibodies (mAbs), HRO G1 and HRT G1, that recognize different epitopes on Her-2/neu. Both of the mAbs bound HER2/neu on the tumor surface, resulting in phosphorylation of HER2/neu. We also generated IgG2a and IgG2b mAbs from these 2 mAbs, respectively. The results of in vitro studies showed that these anti-Her-2/neu mAbs could not inhibit the growth of the tumor cells that express Her-2/neu molecules by themselves. However, in an antibody-dependent cellular cytotoxicity study using mouse splenocytes as effector cells, HRT mAbs had antitumor activities superior to those of HRO mAbs, indicating that the epitope specificity may also partake in antibody-dependent cellular cytotoxicity with antibody isotype. In a complement-dependent cytotoxicity study, the IgG2a and IgG2b mAbs showed stronger effects than IgG1 isotype mAbs irrespective of the epitope specificities. The results of in vivo studies also showed that HRT mAbs had superior antitumor activity to those of HRO mAbs. The antitumor activity was most prominent in the HRT G2b isotype among HRT mAbs. HRT G1 also showed a moderate antitumor effect, while HRT G2a showed only slight inhibition effect. These data indicate that both the epitope specificity and the differences in Fc region of mAbs could play important roles in the antitumor activities.

Increased fibroblast growth factor-like autoantibodies in serum from a subset of patients with cancer-associated hypercalcemia

Basic fibroblast growth factor (bFGF) is a potent tumor angiogenesis factor which lacks an amino-terminal signal sequence and does not normally circulate in serum from normal subjects. Naturally-occurring autoantibodies which mimicked basic fibroblast growth factor were described in serum from patients with multiple endocrine neoplasia type 1 prolactinoma or sporadic growth-hormone-secreting adenoma associated with increased bFGF. Since bFGF was increased in serum from a variety of cancers, we used endothelial cell proliferation assay(s) to test for bioactivity in the IgG fraction of serum from 56 patients with cancer-associated hypercalcemia, and normal or control subjects. We now report increased IgG-like endothelial cell activity in serum from a hyper prolactinemic subset (4/19 breast cancer; 1/14 renal cancer; 0/23 lung cancer) of cancer-associated hypercalcemic subjects. Highest activity was found in serum from three breast cancer patients who suffered spinal cord compression/metastases. The activity had properties of antiidiotype bFGF antibodies including reaction with anti-human IgG antibodies, and complete neutralization by rabbit antibodies to intact bFGF. The activity in endothelial cells persisted after storage at 0-4 C for 5 yrs; and [prepared by SDS-PAGE and immunoblotting with anti-human IgG] had apparent mol wt corresponding to the heavy chains of IgG. Serum IgG-like activity from 5 of 5 breast cancer patients and 2 of 2 prostate cancer subjects tested [prepared by anti-bFGF antibody, protein-A immunoaffinity, and hydroxyapatite (HA) chromatography] yielded peak HA-adsorbed activity that eluted with 0.4 M sodium phosphate, and was neutralized 70% by antibodies to intact bFGF. Cancer sera mean peak specific activity (12.0 ng-eq bFGF/ug protein) (n = 7) significantly exceeded (P < 0.001) normal sera mean peak specific activity (0.46 ng-eq bFGF/ug protein) (n = 6) in the 0.4 M sodium phosphate eluate fraction from hydroxyapatite columns. These results imply that long-lasting, bioactive FGF-like autoantibodies may arise spontaneously (and contribute to pathophysiology) in subsets of cancer patients with osseous metastases.

Therapeutic efficacy of anti-ErbB2 immunoliposomes targeted by a phage antibody selected for cellular endocytosis

Many targeted cancer therapies require endocytosis of the targeting molecule and delivery of the therapeutic agent to the interior of the tumor cell. To generate single chain Fv (scFv) antibodies capable of triggering receptor-mediated endocytosis, we previously developed a method to directly select phage antibodies for internalization by recovering infectious phage from the cytoplasm of the target cell. Using this methodology, we reported the selection of a panel of scFv that were internalized into breast cancer cells from a nonimmune phage library. For this work, an immunotherapeutic was generated from one of these scFv (F5), which bound to ErbB2 (HER2/neu). The F5 scFv was reengineered with a C-terminal cysteine, expressed at high levels in Escherichia coli, and coupled to sterically stabilized liposomes. F5 anti-ErbB2 immunoliposomes were immunoreactive as determined by surface plasmon resonance (SPR) and were avidly internalized by ErbB2-expressing tumor cell lines in proportion to the levels of ErbB2 expression. F5-scFv targeted liposomes containing doxorubicin had antitumor activity and produced significant reduction in tumor size in xenografted mice compared to nontargeted liposomes containing doxorubicin. This strategy should be applicable to generate immunotherapeutics for other malignancies by selecting phage antibodies for internalization into other tumor types and using the scFv to target liposomes or other nanoparticles.

Androgen-dependent mammary carcinogenesis in rats transgenic for the Neu proto-oncogene

Transgenic rats were created with overexpression of the Neu proto-oncogene in the mammary gland of both sexes, yet only males developed mammary cancer in an androgen-dependent fashion. Transgenic females only developed mammary cancer if treated with androgens. These tumors were positive for androgen receptor (AR), but negative for estrogen and progesterone receptors. Extensive analysis failed to detect mutations anywhere within the neu transgene from mammary carcinomas. Established mammary carcinomas eventually escaped their dependency on androgens. Transgenic long-term gonadectomized rats did not develop mammary cancer, but Neu overexpression stimulated the growth of their mammary glands. Our results suggest crosstalk between the Neu proto-oncogene and AR signaling pathways in the growth of both the normal and cancerous mammary epithelium.

Identification and characterization of peptides that bind human ErbB-2 selected from a bacteriophage display library

The ErbB-2 receptor, a member of the tyrosine kinase type 1 family of receptors, has been implicated in many human malignancies. The overexpression of ErbB-2 in cancer cells as well as its extracellular accessibility makes it an attractive target for the development of tumor-specific agents. In this study, random peptide bacteriophage display technology was employed to identify peptides that bound the extracellular domain of human ErbB-2. The peptide KCCYSL, most frequently occurring in the affinity-selected phage population, was chemically synthesized and characterized for its binding activities to ErbB-2. The synthetic peptide exhibited high specificity for ErbB-2 and an equilibrium dissociation constant of 30 microM. Peptide binding to ErbB-2 positive human breast and prostate carcinoma cells was visualized in direct cell binding assays. In conclusion, the peptide KCCYSL has the potential to be developed into a cancer imaging or therapeutic agent targeting malignant cells overexpressing the ErbB-2 receptor.

Modulation of anthracycline-induced myofibrillar disarray in rat ventricular myocytes by neuregulin-1beta and anti-erbB2: potential mechanism for trastuzumab-induced cardiotoxicity

BACKGROUND

There is an increased incidence of heart failure in patients treated concurrently with anthracyclines and the chemotherapeutic anti-erbB2 agent trastuzumab (Herceptin). On the basis of our previous studies with recombinant neuregulin-1beta (NRG-1beta), a ligand for the erbB2 receptor tyrosine kinase, we hypothesized that activation of erbB2 by anti-erbB2 versus NRG-1 would cause differential effects on myocyte intracellular signaling as well as anthracycline-induced myofibrillar injury and might potentially account for the clinical toxicity of trastuzumab in the setting of concurrent anthracycline therapy.

METHODS AND RESULTS

We tested this hypothesis using adult rat ventricular myocytes (ARVMs) in culture, assessing myofibrillar structure by immunostaining for myomesin and filamentous actin. Activation of erbB2, extracellular signal-regulated kinase 1/2 (Erk1/2), and Akt was assessed by use of antibodies to phosphorylated activated receptor or kinase detected by immunoblot. ARVMs treated with doxorubicin (0.1 to 0.5 micromol/L) showed a concentration-dependent increase in myofilament disarray. NRG-1beta (10 ng/mL) activated erbB2, Erk1/2, and Akt in ARVMs and significantly reduced anthracycline-induced disarray. In contrast to NRG-1beta, anti-erbB2 (1 microg/mL) caused rapid phosphorylation of erbB2 but not Erk1/2 or Akt, with downregulation of erbB2 by 24 hours. Concomitant treatment of myocytes with anti-erbB2 and doxorubicin caused a significant increase in myofibrillar disarray versus doxorubicin alone.

CONCLUSIONS

NRG-1beta/erbB signaling regulates anthracycline-induced myofilament injury. The increased susceptibility of myofilaments to doxorubicin in the presence of antibody to erbB2 may explain the contractile dysfunction seen in patients receiving concurrent trastuzumab and anthracyclines.

Selection of cell binding and internalizing epidermal growth factor receptor antibodies from a phage display library

The first step in developing a targeted cancer therapeutic is generating a ligand that binds to a receptor which is either tumor specific or sufficiently overexpressed in tumors to provide targeting specificity. For this work, we generated human monoclonal antibodies to the EGF receptor (EGFR), an antigen overexpressed on many solid tumors. Single chain Fv (scFv) antibody fragments were directly selected by panning a phage display library on tumor cells (A431) overexpressing EGFR or Chinese hamster ovary cells (CHO/EGFR cells) transfected with the EGFR gene and recovering endocytosed phage from within the cell. Three unique scFvs were isolated, two from selections on A431 cells and two from selections on CHO/EGFR cells. All three scFv bound native receptor as expressed on a panel of tumor cells and did not bind EGFR negative cells. Phage antibodies and multivalent immunoliposomes constructed from scFv were endocytosed by EGFR expressing cells as shown by confocal microscopy. Native scFv primarily stained the cell surface, with less staining intracellularly. The results demonstrate how phage antibodies binding native cell surface receptors can be directly selected on overexpressing cell lines or transfected cells. Use of a transfected cell line allows selection of antibodies to native receptors without the need for protein expression and purification, significantly speeding the generation of targeting antibodies to genomic sequences. Depending upon the format used, the antibodies can be used to deliver molecules to the cell surface or intracellularly.

c-Cbl is a suppressor of the neu oncogene

A rodent oncogenic mutant of the Neu receptor tyrosine kinase is a useful experimental model because overexpression of the respective receptor, namely HER2/ErbB-2, in human malignancies is associated with relatively aggressive diseases. Here we show that the oncogenic form of Neu is constitutively associated with the product of the c-cbl proto-oncogene and is part of a large complex that includes the phosphoinositide 3-kinase and Shc. Ectopic expression of c-Cbl, a ubiquitin-protein isopeptide ligase specific to activated tyrosine kinases, causes rapid removal of Neu from the cell surface and severely reduces signaling downstream of oncogenic Neu. c-Cbl-induced down-regulation of Neu involves covalent attachment of ubiquitin molecules and requires the carboxyl-terminal domain of Neu. The negative effect of c-Cbl is antagonized by v-Cbl, a virus-encoded oncogenic truncated form of c-Cbl. In an in vivo model, infection of a Neu-transformed neuroblastoma with a c-Cbl-encoding retrovirus caused enhanced down-regulation of Neu and correlated with tumor retardation. Our results implicate c-Cbl in negative regulation of Neu and offer a potential target for treatment of HER2/ErbB-2-positive human malignancies.

Selection of tumor-specific internalizing human antibodies from phage libraries

Antibody internalization into the cell is required for many targeted therapeutics, such as immunotoxins, immunoliposomes, antibody-drug conjugates and for targeted delivery of genes or viral DNA into cells. To generate directly tumor-specific internalizing antibodies, a non-immune single chain Fv (scFv) phage antibody library was selected on the breast tumor cell line SKBR3. Internalized phage were recovered from within the cell and used for the next round of selection. After three rounds of selection, 40 % of clones analyzed bound SKBR3 and other tumor cells but did not bind normal human cells. Of the internalizing scFv identified, two (F5 and C1) were identified as binding to ErbB2, and one (H7) to the transferrin receptor. Both F5 and H7 scFv were efficiently endocytosed into SKBR3 cells, both as phage antibodies and as native monomeric scFv. Both antibodies were able to induce additional functional effects besides triggering endocytosis: F5 scFv induces downstream signaling through the ErbB2 receptor and H7 prevents transferrin binding to the transferrin receptor and inhibits cell growth. The results demonstrate the feasibility of selecting internalizing receptor-specific antibodies directly from phage libraries by panning on cells. Such antibodies can be used to target a variety of molecules into the cell to achieve a therapeutic effect. Furthermore, in some instances endocytosis serves as a surrogate marker for other therapeutic biologic effects, such as growth inhibition. Thus, a subset of selected antibodies will have a direct therapeutic effect.

Relevance of p185 HER-2/neu oncoprotein quantification in human primary breast carcinoma

The c-erbB-2 proto-oncogene encodes a transmembrane protein tyrosine kinase receptor of 185 kDa (p185) and has been associated with several types of human cancers. In human breast cancer, overexpression of p185 occurs in 15-30% of cases, correlates with poor prognostic factors and characterizes breast cancers with a more aggressive behavior. Overexpression of p185 is usually associated with c-erbB-2 amplification, though it may occur independently and thus define subpopulations of breast cancers which might be of clinical interest. p185 expression is usually detected by immunohistochemistry (IHC) and few studies have been carried out to evaluate the p185 content of breast cancers with an ELISA technique. In this context, we showed, in 106 breast cancer samples, that p185 was expressed at high levels in 13.2%, intermediate levels in 55.7% and negative ones in 31.1% of cases. All p185 positive samples showed a c-erbB-2 oncogene amplification while none of the p185 negative samples and only 4% of p185 imtermediate samples had an amplification of c-erbB-2. p185 expression is significantly correlated with the negativity of estrogen and progestrone receptors, with high levels of cathepsin D and in some conditions with axillary nodal involvement. Thus, using the p185 ELISA assay, the c-erbB-2 status of breast cancers can be defined and moreover a subset can be discriminated which is characterized by intermediate levels of p185 and absence of c-erbB-2 amplification. The quantitative approach towards p185 in breast cancers affords the possibility of identifying more appropriately patients with high or low risk and thus permits adaptation of therapeutic regimens.

Internalizing antibodies and targeted cancer therapy: direct selection from phage display libraries

Antibody internalization is required for the success of many targeted therapeutics, such as immunotoxins, immunoliposomes, antibody-drug conjugates and for the targeted delivery of genes or viral DNA into cells. Recently, it has become possible to directly select antibody fragments from phage display libraries for internalization into mammalian cells. Here we review the therapeutic applications of internalized antibodies and describe how phage display enables the isolation of internalizing antibodies to novel or known targets.

Inhibitory effects of combinations of HER-2/neu antibody and chemotherapeutic agents used for treatment of human breast cancers

Previous studies have demonstrated a synergistic interaction between rhuMAb HER2 and the cytotoxic drug cisplatin in human breast and ovarian cancer cells. To define the nature of the interaction between rhuMAb HER2 and other classes of cytotoxic drugs, we applied multiple drug effect/combination index (CI) isobologram analysis to a variety of chemotherapeutic drug/rhuMAb HER2 combinations in vitro. Synergistic interactions at clinically relevant drug concentrations were observed for rhuMAb HER2 in combination with cisplatin (CI=0.48, P=0.003), thiotepa (CI=0.67, P=0.0008), and etoposide (CI=0.54, P=0.0003). Additive cytotoxic effects were observed with rhuMAb HER2 plus doxorubicin (CI=1.16, P=0.13), paclitaxel (CI=0.91, P=0.21), methotrexate (CI=1.15, P=0.28), and vinblastine (CI=1.09, P=0.26). One drug, 5-fluorouracil, was found to be antagonistic with rhuMAb HER2 in vitro (CI=2.87, P=0.0001). In vivo drug/rhuMAb HER2 studies were conducted with HER-2/neu-transfected, MCF7 human breast cancer xenografts in athymic mice. Combinations of rhuMAb HER2 plus cyclophosphamide, doxorubicin, paclitaxel, methotrexate, etoposide, and vinblastine in vivo resulted in a significant reduction in xenograft volume compared to chemotherapy alone (P<0.05). Xenografts treated with rhuMAb HER2 plus 5-fluorouracil were not significantly different from 5-fluorouracil alone controls consistent with the subadditive effects observed with this combination in vitro. The synergistic interaction of rhuMAb HER2 with alkylating agents, platinum analogs and topoisomerase II inhibitors, as well as the additive interaction with taxanes, anthracyclines and some antimetabolites in HER-2/neu-overexpressing breast cancer cells demonstrates that these are rational combinations to test in human clinical trials.

Toward selection of internalizing antibodies from phage libraries

Antibodies which bind cell surface receptors in a manner whereby they are endocytosed are useful molecules for the delivery of drugs, toxins, or DNA into the cytosol of mammalian cells for therapeutic applications. Traditionally, internalizing antibodies have been identified by screening hybridomas. For this work, we studied a human scFv (C6.5) which binds ErbB2 to determine the feasibility of directly selecting internalizing antibodies from phage libraries and to identify the most efficient display format. Using wild-type C6.5 scFv displayed monovalently on a phagemid, we demonstrate that anti-ErbB2 phage antibodies can undergo receptor-mediated endocytosis. Using affinity mutants and dimeric diabodies of C6.5 displayed as either single copies on a phagemid or multiple copies on phage, we define the role of affinity, valency, and display format on phage endocytosis and identify the factors that lead to the greatest enrichment for internalization. Phage displaying bivalent diabodies or multiple copies of scFv were more efficiently endocytosed than phage displaying monomeric scFv and recovery of infectious phage was increased by preincubation of cells with chloroquine. Measurement of phage recovery from within the cytosol as a function of applied phage titer indicates that it is possible to select for endocytosable antibodies, even at the low concentrations that would exist for a single phage antibody member in a library of 10(9).

Activation of Neu (ErbB-2) mediated by disulfide bond-induced dimerization reveals a receptor tyrosine kinase dimer interface

Receptor dimerization is a crucial intermediate step in activation of signaling by receptor tyrosine kinases (RTKs). However, dimerization of the RTK Neu (also designated ErbB-2, HER-2, and p185(neu)), while necessary, is not sufficient for signaling. Earlier work in our laboratory had shown that introduction of an ectopic cysteine into the Neu juxtamembrane domain induces Neu dimerization but not signaling. Since Neu signaling does require dimerization, we hypothesized that there are additional constraints that govern signaling ability. With the importance of the interreceptor cross-phosphorylation reaction, a likely constraint was the relative geometry of receptors within the dimer. We have tested this possibility by constructing a consecutive series of cysteine substitutions in the Neu juxtamembrane domain in order to force dimerization along a series of interreceptor faces. Within the group that dimerized constitutively, a subset had transforming activity. The substitutions in this subset all mapped to the same face of a predicted alpha helix, the most likely conformation for the intramembrane domain. Furthermore, this face of interaction aligns with the projected Neu* V664E substitution and with a predicted amphipathic interface in the Neu juxtamembrane domain. We propose that these results identify an RTK dimer interface and that dimerization of this RTK induces an extended contact between juxtamembrane and intramembrane alpha helices.

Human tumor growth suppression by apoptosis induced with anti-ErbB-2 chimeric monoclonal antibody

We established an anti-ErbB-2 mouse-human chimeric monoclonal antibody (MoAb), CH401, which was able to kill cancer cells overexpressing the ErbB-2 protein in vitro. The analysis of the killing mechanism indicated that MoAb CH401 might be the first anti-erbB-2 mouse-human chimeric MoAb which can induce the apoptosis of cancer cells, since morphological changes and DNA fragmentation were recognized in MoAb CH401-treated cells. The ErbB-2 receptor appears to have two opposing functions: acting as a receptor both for a growth factor and for an apoptotic factor. Our results indicate that MoAb CH401 treatment may prove to be very useful for cancer therapy.

Agonistic monoclonal antibodies against the Met receptor dissect the biological responses to HGF

Hepatocyte growth factor, also known as scatter factor, is a pleiotropic cytokine, which stimulates cell motility, invasion, proliferation, survival and morphogenesis, and induces the expression of specific genes by activating its receptor tyrosine kinase. In this work we have isolated, characterized and used as agonists two monoclonal antibodies (mAbs) directed against the extracellular domain of HGF receptor to investigate the requirements for receptor activation and for the different biological responses. The two mAbs display similar affinities, react with epitopes different from the hepatocyte growth factor binding site, and behave as either full or partial agonists. The full agonist mAb (DO-24) triggers all the biological effects elicited by hepatocyte growth factor, namely motility, proliferation, cell survival, invasion, tubulogenesis and angiogenesis. The partial agonist mAb (DN-30) induces only motility. Only the full agonist mAb is able to induce and sustain the expression of urokinase-type plasminogen activator receptor for prolonged periods of time, while both mAbs up-regulate the constitutive expression of urokinase-type plasminogen activator. Both mAbs activate receptor phosphorylation, which, being strictly dependent on mAb bivalence, requires receptor dimerization. Since simple receptor dimerization is not sufficient to trigger full biological responses, we propose that the region on the ss chain of the receptor recognized by the full agonist mAb is crucial for optimal receptor activation.

A hierarchical network of interreceptor interactions determines signal transduction by Neu differentiation factor/neuregulin and epidermal growth factor

The ErbB family includes four homologous transmembrane tyrosine kinases. Whereas ErbB-1 binds to the epidermal growth factor (EGF), both ErbB-3 and ErbB-4 bind to the Neu differentiation factors (NDFs, or neuregulins), and ErbB-2, the most oncogenic family member, is an orphan receptor whose function is still unknown. Because previous lines of evidence indicated the existence of interreceptor interactions, we used ectopic expression of individual ErbB proteins and their combinations to analyze the details of receptor cross talks. We show that 8 of 10 possible homo-and heterodimeric complexes of ErbB proteins can be hierarchically induced by ligand binding. Although ErbB-2 binds neither ligand, even in a heterodimeric receptor complex, it is the preferred heterodimer partner of the three other members, and it favors interaction with ErbB-3. Selective receptor overexpression in human tumor cells appears to bias the hierarchical relationships. The ordered network is reflected in receptor transphosphorylation, ErbB-2-mediated enhancement of ligand affinities, and remarkable potentiation of mitogenesis by a coexpressed ErbB-2. The observed superior ability of ErbB-2 to form heterodimers, in conjunction with its uniquely high basal tyrosine kinase activity, may explain why ErbB-2 overexpression is associated with poor prognosis.

Roles for a cytoplasmic tyrosine and tyrosine kinase activity in the interactions of Neu receptors with coated pits

The neu proto-oncogene product, p185neu (HER2, c-ErbB-2), encodes a cell-surface tyrosine kinase receptor with high oncogenic potential, which correlates with increased tyrosine kinase activity and a rapid receptor internalization rate. To investigate the interactions and signal(s) leading to the endocytosis of Neu receptors, we employed lateral mobility and internalization studies. Fluorescence photobleaching recovery measurements revealed that activation of Neu receptors (induced by mutation or by agonistic antibodies) markedly reduced their mobile fractions. To elucidate the signals involved, other mutants, all carrying a constitutively dimerizing oncogenic mutation, were analyzed. A kinase-negative mutant and a mutant lacking all cytoplasmic tyrosine phosphorylation consensus sequences exhibited high mobile fractions, similar to nonactivated Neu. Retention of a single tyrosine autophosphorylation site (Tyr-1253) out of the five known such sites was sufficient to immobilize a large fraction of the receptor. For all mutants, internalization correlated with receptor immobilization and was blocked by treatments that interfere with coated pit structure, indicating that the immobilization is due to interactions with coated pits. This was supported by the coimmunoprecipitation of alpha-adaptin only with the constitutively activated Neu mutants. We conclude that activated Neu receptors become stably associated with coated pits via plasma membrane adaptor complexes (AP-2). Efficient Neu receptor endocytosis requires activation, a functional kinase domain, and at least one tyrosine autophosphorylation site.

Analysis of c-erbB2 protein content of human glioma cells and tumor tissue

This study was designed to determine whether or not overexpression of the c-erbB2 protein plays a role in the etiology of human gliomas. The c-erbB2 gene codes for a 185 kDa cell membrane glycoprotein (gp185c-erbB2), which is similar to the receptor for epidermal growth factor. In initial studies, four human glioma cell lines (A-172, U118MG, U138MG and SW608) were used to develop techniques for detecting and quantifying gp185c-erbB2, using immunofluorescence microscopy, immunoblot analysis and flow cytometry. A-172 cells were found to have the highest content of gp185c-erbB2. More detailed studies utilizing A-172 cells indicated that cellular gp185c-erbB2 content changed little in response to conditions affecting cellular proliferative status, including serum deprivation, growth in low glucose medium and treatment with dimethyl sulfoxide. Ten human glioma specimens were then analyzed for cellular gp185c-erbB2 fluorescence and DNA content, using A-172 cells as a biological standard. Results indicated that gp185c-erbB2 was expressed at levels comparable to that of A-172 cells in many specimens, and at a very high level in one specimen. These data reiterate the problem of the molecular heterogeneity of human gliomas and indicate that gp185c-erbB2 may have a role in at least a subset of malignant glial tumors.