IgG-mediated down-regulation of IgE bound to mast cells: a potential novel mechanism of allergen-specific desensitization.

BACKGROUND: Allergen-specific IgGs are known to inhibit IgE-mediated mast cell degranulation by two mechanisms, allergen-neutralization and engagement of the inhibitory FcγRIIB recruiting the phosphatase SHIP-1. Here we unravel an additional mechanism of IgG-mediated mast cell desensitization in mice: down-regulation of allergen-specific IgE. METHODS: Mast cells were loaded in vitro and in vivo with monoclonal IgE antibodies specific for Fel d1 and exposed to immune complexes consisting of Fel d1-specific IgG antibodies recognizing different epitopes. Down regulation of IgE was followed by flow cytometry. RESULTS: Mast cells loaded with 2 different IgE antibodies efficiently internalized the IgE antibodies if exposed to recombinant Feld d1. In contrast, no down-regulation occurred if mast cells were loaded with IgE antibodies exhibiting a single specificity before stimulation with recombinant Fel d1 [corrected]. Interestingly, however, IgEs of a single specificity were rapidly down-regulated in vitro and in vivo in the presence of Fel d1-specific monoclonal IgGs recognizing another epitope on Fel d1. Despite FceRI-internalization, little calcium flux or mast cell degranulation occurred. FcγRIIB played a dual role in the process since it enhanced IgE internalization and prevented cellular activation as documented by the inhibited calcium flux and mast cell degranulation. Similar observations were made in the presence of low concentrations of IgEs recognizing several epitopes on Fel d1. CONCLUSION: We demonstrate here that Fel d1-specific IgG antibodies interact with FcγRIIB which (i) promotes IgE internalization; and (ii) inhibits mast cell activation. These results broaden our understanding of allergen-specific desensitization and may provide a mechanism for long-term desensitization of mast cells by selective removal of long-lived IgE antibodies on mast cells.

Development of zinc finger domains for recognition of the 5'-ANN-3' family of DNA sequences and their use in the construction of artificial transcription factors.

In previous studies we have developed Cys(2)-His(2) zinc finger domains that specifically recognized each of the 16 5'-GNN-3' DNA target sequences and could be used to assemble six-finger proteins that bind 18-base pair DNA sequences (Beerli, R. R., Dreier, B., and Barbas, C. F., III (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 1495--1500). Such proteins provide the basis for the construction of artificial transcription factors to study gene/function relationships in the post-genomic era. Central to the universal application of this approach is the development of zinc finger domains that specifically recognize each of the 64 possible DNA triplets. Here we describe the construction of a novel phage display library that enables the selection of zinc finger domains recognizing the 5'-ANN-3' family of DNA sequences. Library selections provided domains that in most cases showed binding specificity for the 3-base pair target site that they were selected to bind. These zinc finger domains were used to construct 6-finger proteins that specifically bound their 18-base pair target site with affinities in the pm to low nm range. When fused to regulatory domains, these proteins containing various numbers of 5'-ANN-3' domains were capable of specific transcriptional regulation of a reporter gene and the endogenous human ERBB-2 and ERBB-3 genes. These results suggest that modular DNA recognition by zinc finger domains is not limited to the 5'-GNN-3' family of DNA sequences and can be extended to the 5'-ANN-3' family. The domains characterized in this work provide for the rapid construction of artificial transcription factors, thereby greatly increasing the number of sequences and genes that can be targeted by DNA-binding proteins built from pre-defined zinc finger domains.

Chemically regulated zinc finger transcription factors.

Ligand-dependent transcriptional regulators were generated by fusion of designed Cys(2)-His(2) zinc finger proteins and steroid hormone receptor ligand binding domains. To produce novel DNA binding domains, three-finger proteins binding specific 9-base pair sequences were constructed from modular building blocks. Fusion of these zinc finger proteins to a transcriptional activation domain and to modified ligand binding domains derived from either the estrogen or progesterone receptors yielded potent ligand-dependent transcriptional regulators. Together with optimized minimal promoters, these regulators provide 4-hydroxytamoxifen- or RU486-inducible expression systems with induction ratios of up to 3 orders of magnitude. These inducible expression systems are functionally independent, and each can be selectively switched on within the same cell. The potential use of zinc finger-steroid receptor fusion proteins for the regulation of natural promoters was also explored. A gene-specific six-finger protein binding an 18-base pair target sequence was converted into a ligand-dependent regulator by fusion with either two estrogen receptor ligand binding domains or one ecdysone receptor and one retinoid X receptor ligand binding domain. These single-chain receptor proteins undergo an intramolecular rearrangement, rather than intermolecular dimerization and are functional as monomers. Thus, the ability to engineer DNA binding specificities of zinc finger proteins enables the construction of ligand-dependent transcriptional regulators with potential for the regulation of virtually any desired artificial or natural promoter. It is anticipated that the novel chemically regulated gene switches described herein will find many applications in applied and basic research, where the specific modulation of gene expression can be exploited.

Positive and negative regulation of endogenous genes by designed transcription factors.

Gene regulation by imposed localization was studied by using designed zinc finger proteins that bind 18-bp DNA sequences in the 5' untranslated regions of the protooncogenes erbB-2 and erbB-3. Transcription factors were generated by fusion of the DNA-binding proteins to repression or activation domains. When introduced into cells these transcription factors acted as dominant repressors or activators of, respectively, endogenous erbB-2 or erbB-3 gene expression. Significantly, imposed regulation of the two genes was highly specific, despite the fact that the transcription factor binding sites targeted in erbB-2 and erbB-3 share 15 of 18 nucleotides. Regulation of erbB-2 gene expression was observed in cells derived from several species that conserve the DNA target sequence. Repression of erbB-2 in SKBR3 breast cancer cells inhibited cell-cycle progression by inducing a G(1) accumulation, suggesting the potential of designed transcription factors for cancer gene therapy. These results demonstrate the willful up- and down-regulation of endogenous genes, and provide an additional means to alter biological systems.

Toward controlling gene expression at will: selection and design of zinc finger domains recognizing each of the 5'-GNN-3' DNA target sequences.

We have taken a comprehensive approach to the generation of novel DNA binding zinc finger domains of defined specificity. Herein we describe the generation and characterization of a family of zinc finger domains developed for the recognition of each of the 16 possible 3-bp DNA binding sites having the sequence 5'-GNN-3'. Phage display libraries of zinc finger proteins were created and selected under conditions that favor enrichment of sequence-specific proteins. Zinc finger domains recognizing a number of sequences required refinement by site-directed mutagenesis that was guided by both phage selection data and structural information. In many cases, residues not expected to make base-specific contacts had effects on specificity. A number of these domains demonstrate exquisite specificity and discriminate between sequences that differ by a single base with >100-fold loss in affinity. We conclude that the three helical positions -1, 3, and 6 of a zinc finger domain are insufficient to allow for the fine specificity of the DNA binding domain to be predicted. These domains are functionally modular and may be recombined with one another to create polydactyl proteins capable of binding 18-bp sequences with subnanomolar affinity. The family of zinc finger domains described here is sufficient for the construction of 17 million novel proteins that bind the 5'-(GNN)6-3' family of DNA sequences. These materials and methods should allow for the rapid construction of novel gene switches and provide the basis for a universal system for gene control.

Toward controlling gene expression at will: specific regulation of the erbB-2/HER-2 promoter by using polydactyl zinc finger proteins constructed from modular building blocks.

To create a universal system for the control of gene expression, we have studied methods for the construction of novel polydactyl zinc finger proteins that recognize extended DNA sequences. Elsewhere we have described the generation of zinc finger domains recognizing sequences of the 5'-GNN-3' subset of a 64-member zinc finger alphabet. Here we report on the use of these domains as modular building blocks for the construction of polydactyl proteins specifically recognizing 9- or 18-bp sequences. A rapid PCR assembly method was developed that, together with this predefined set of zinc finger domains, provides ready access to 17 million novel proteins that bind the 5'-(GNN)6-3' family of 18-bp DNA sites. To examine the efficacy of this strategy in gene control, the human erbB-2 gene was chosen as a model. A polydactyl protein specifically recognizing an 18-bp sequence in the 5'-untranslated region of this gene was converted into a transcriptional repressor by fusion with Kr uppel-associated box (KRAB), ERD, or SID repressor domains. Transcriptional activators were generated by fusion with the herpes simplex VP16 activation domain or with a tetrameric repeat of VP16's minimal activation domain, termed VP64. We demonstrate that both gene repression and activation can be achieved by targeting designed proteins to a single site within the transcribed region of a gene. We anticipate that gene-specific transcriptional regulators of the type described here will find diverse applications in gene therapy, functional genomics, and the generation of transgenic organisms.

ErbB-1 and ErbB-2 acquire distinct signaling properties dependent upon their dimerization partner.

The different epidermal growth factor (EGF)-related peptides elicit a diverse array of biological responses as the result of their ability to activate distinct subsets of ErbB receptor dimers, leading to the recruitment of different intracellular signaling networks. To specifically examine dimerization-dependent modulation of receptor signaling, we constructed NIH 3T3 cell lines expressing ErbB-1 and ErbB-2 singly and in pairwise combinations with each other ErbB family member. This model system allowed the comparison of EGF-activated ErbB-1 with ErbB-1 activated by Neu differentiation factor (NDF)-induced heterodimerization with ErbB-4. In both cases, ErbB-1 coupled to the adaptor protein Shc, but only when activated by EGF was it able to interact with Grb2. Compared to the rapid internalization of EGF-activated ErbB-1, NDF-activated ErbB-1 showed delayed internalization characteristics. Furthermore, the p85 subunit of phosphatidylinositol kinase (PI3-K) associated with EGF-activated ErbB-1 in a biphasic manner, whereas association with ErbB-1 transactivated by ErbB-4 was monophasic. The signaling properties of ErbB-2 following heterodimerization with the other ErbB receptors or homodimerization induced by point mutation or monoclonal antibody treatment were also analyzed. ErbB-2 binding to peptides containing the Src homology 2 domain of Grb2 or p85 and the phosphotyrosine binding domain of Shc varied according to the mode of receptor activation. Finally, tryptic phosphopeptide mapping of both ErbB-1 and ErbB-2 revealed that receptor phosphorylation is dependent on the dimerization partner. Differential receptor phosphorylation may, therefore, be the basis for the differences in the signaling properties observed.

Neu differentiation factor induces ErbB2 down-regulation and apoptosis of ErbB2-overexpressing breast tumor cells.

Neu differentiation factor (NDF), a member of the epidermal growth factor (EGF)-related peptide family, activates ErbB2 via heterodimerization with the NDF receptors ErbB3 and ErbB4. In a similar fashion, EGF receptor (EGFR) agonists induce heterodimers of EGFR and ErbB2. In this paper, we show that the ErbB2-overexpressing breast tumor cells SKBR3, AU565, and MDA-MB453 are growth inhibited by NDF. Cells with elevated levels of ErbB2 but little or no NDF receptors (SKOV3 and MDA-MB361) or cells with low levels of ErbB2 (T47D and MCF7) are not growth inhibited. None of the EGFR agonists tested (EGF, beta-cellulin, or heparin-binding EGF) inhibited growth of ErbB2-overexpressing cells. These results suggest that formation of an ErbB2/NDF receptor heterodimer, but not of an ErbB2/EGFR heterodimer, promotes growth inhibition. In addition, NDF caused a down-regulation of ErbB2 but not of ErbB3. The mechanism underlying the inhibitory effect was examined further in SKBR3 cells, which are 95% growth inhibited by NDF. A G2-M arrest is seen 24 h after NDF treatment, and increased apoptosis is detectable from day 2 onward. The results demonstrate for the first time that NDF induces apoptosis of tumor cells overexpressing ErbB2.

ErbB-2, the preferred heterodimerization partner of all ErbB receptors, is a mediator of lateral signaling.

We have analyzed ErbB receptor interplay induced by the epidermal growth factor (EGF)-related peptides in cell lines naturally expressing the four ErbB receptors. Down-regulation of cell surface ErbB-1 or ErbB-2 by intracellular expression of specific antibodies has allowed us to delineate the role of these receptors during signaling elicited by: EGF and heparin binding EGF (HB-EGF), ligands of ErbB-1; betacellulin (BTC), a ligand of ErbB-1 and ErbB-4; and neu differentiation factor (NDF), a ligand of ErbB-3 and ErbB-4. Ligand-induced ErbB receptor heterodimerization follows a strict hierarchy and ErbB-2 is the preferred heterodimerization partner of all ErbB proteins. NDF-activated ErbB-3 or ErbB-4 heterodimerize with ErbB-1 only when no ErbB-2 is available. If all ErbB receptors are present, NDF receptors preferentially dimerize with ErbB-2. Furthermore, EGF- and BTC-induced activation of ErbB-3 is impaired in the absence of ErbB-2, suggesting that ErbB-2 has a role in the lateral transmission of signals between other ErbB receptors. Finally, ErbB-1 activated by all EGF-related peptides (EGF, HB-EGF, BTC and NDF) couples to SHC, whereas only ErbB-1 activated by its own ligands associates with and phosphorylates Cbl. These results provide the first biochemical evidence that a given ErbB receptor has distinct signaling properties depending on its dimerization.

Intracellular expression of a single-chain antibody directed to the EGFR leads to growth inhibition of tumor cells.

A gene encoding a single-chain antibody (scFv) which specifically binds the epidermal growth factor receptor (EGFR) has been constructed from hybridoma cells producing the R1 monoclonal antibody. The gene, designated scFv-R1R, was introduced into EGFR transformed NIH3T3 cells via retroviral infection. scFv-R1R was directed to the lumen of the endoplasmic reticulum (ER) where it bound the extracellular domain of the receptor inhibiting its appearance on the plasma membrane. In these cells, EGF induced tyrosine phosphorylation of the EGFR and several substrates was greatly reduced. Furthermore, intracellular retention of EGFR caused a partial inhibition in the transformed growth of the cells. Intracellular expression of receptor tyrosine kinase directed scFvs is a novel approach for affecting tumor cell growth. We have recently shown that scFv-5R directed to ErbB2, another member of the ErbB family, blocks the anchorage independent growth of ErbB2 transformed cells. In order to examine the effects of scFv-R1R and scFv-5R on the long-term growth of tumor cells overexpressing either EGFR or ErbB2, retroviruses encoding the two scFvs were used to infect various human tumor cell lines. Intracellular expression of the scFvs resulted in a marked inhibition of stable colony formation in some of the cell lines. In general, inhibition was observed when overexpressed receptor was targeted. However, in some cases expression of both scFvs was incompatible with long term cell growth suggesting that heterodimers of ErbB2 and EGFR are essential for the growth of some human tumor cell lines.

Neu differentiation factor (Heregulin) activates a p53-dependent pathway in cancer cells.

Previously we reported that neu differentiation factor (NDF)/heregulin (HRG) elevates tyrosine phosphorylation of its receptors erbB-3, erbB-4, and erbB-2 (through heterodimer formation). We also showed that both NDF/HRG and antibodies to erbB-2 can arrest growth and induce differentiation in breast cancer cells. In this study, we report on the mechanism of NDF/HRG-induced cellular effects. We show that NDF/HRG and antibodies to erbB-2 receptors up-regulate expression of p53 by stabilizing the protein. This is accompanied by up-regulation of the p53 inducible gene, p21CIP1/WAF1, in a variety of cell lines: MCF7 and their derivatives (MCF7/HER2, MN1 and MCF-7-puro), ZR75T and LnCap cells. The induction of p21 is further enhanced when cells are treated with both NDF/HRG and DNA-damaging chemotherapeutic agents (i.e. doxorubicin). The NDF/HRG mediated induction of p21 is dependent on wildtype p53, as it fails to occur in cells expressing dominant negative p53 (MDD2). Furthermore, p21 induction is capable of inactivating cdk2 complexes as measured by Histone H1 phosphorylation assays. Finally, we show that in primary cultures of breast and other cancers, p21 is significantly induced in response to NDF/HRG treatment. Collectively, these observations suggest that the mechanism of breast cancer cell growth inhibition and differentiation via erbB receptors activation is through a p53-mediated pathway.

Epidermal growth factor-related peptides activate distinct subsets of ErbB receptors and differ in their biological activities.

Numerous epidermal growth factor (EGF)-related peptide binding members of the ErbB family of receptor tyrosine kinases have been described. While several EGF agonists bind and activate ErbB-1/EGF receptor, neu differentiation factor (NDF) functions as a ligand for ErbB-3 and ErbB-4. However, it is currently unknown which specific subsets of ErbB receptors become activated in response to each of these ligands. The present study addresses this issue using the T47D breast tumor cell line, which expresses moderate levels of all the presently known ErbB receptors. We show that all the EGF agonists, but not NDF, stimulated tyrosine phosphorylation of ErbB-1. In contrast, all the EGF-related factors except amphiregulin were able to induce tyrosine phosphorylation of ErbB-2. The ability to induce tyrosine phosphorylation of ErbB-3 varied dramatically among the different EGF-related peptides. While EGF, transforming growth factor (TGF)-alpha, and amphiregulin only had a moderate effect, NDF dramatically increased the ErbB-3 phosphotyrosine content. Most notably, heparin binding EGF-related growth factor (HB-EGF) and betacellulin (BTC) were more effective than other EGF agonists. Consequently, only NDF, HB-EGF, and BTC significantly stimulated association of phosphatidylinositol kinase activity with ErbB-3. Among the EGF agonists, HB-EGF induced a low level of ErbB-4 tyrosine phosphorylation, while BTC was as efficient as NDF in activating ErbB-4. The BTC activation of ErbB-4 appears to be independent of ErbB-1, as shown by pretreatment of cells with an antibody that inhibits binding of EGF agonists to ErbB-1. As a result of the differential activation of ErbB receptors, most of the EGF-related growth factors had distinguishable biological activities on cultured mammary epithelial cell lines.

Inhibition of signaling from Type 1 receptor tyrosine kinases via intracellular expression of single-chain antibodies.

Members of the Type I/epidermal growth factor receptor (EGFR)-related family of receptor tyrosine kinases have been implicated in the development of human cancer. We have taken a novel approach using the intracellular expression of single chain antibodies (scFv) to specifically inhibit the in vivo action of these receptors. A scFv is a recombinant protein analogous to an Fv domain which is the smallest high affinity binding portion of an antibody. We report here on the expression in mammalian cells of cDNAs encoding scFv-225 and scFv-FRP5 directed against the extracellular domain of, respectively, human EGFR and human ErbB-2. The scFvs were provided with a signal peptide which directs them to the secretory pathway of the cell. scFv-225, which competes with EGF for binding, functions in an autocrine fashion to inhibit EGF-dependent cell growth. scFv-FRP5 was also provided with an endoplasmic reticulum (ER) retention signal and inactivates ErbB-2 in an intracrine fashion, by preventing its appearance on the cell surface.

ErbB-2 is a common auxiliary subunit of NDF and EGF receptors: implications for breast cancer.

Overexpression of the erbB-2 gene contributes to aggressive behavior of various human adenocarcinomas, including breast cancer, through an unknown molecular mechanism. The erbB-2-encoded protein is a member of the ErbB family of growth factor receptors, but no direct ligand of ErbB-2 has been reported. We show that in various cells ErbB-2 can form heterodimers with both EGF receptor (ErbB-1) and NDF receptors (ErbB-3 and ErbB-4), suggesting that it may affect the action of heterologous ligands without the involvement of a direct ErbB-2 ligand. This possibility was addressed in breast cancer cells through either overexpression of ErbB-2 or by blocking its delivery to the cell surface by means of an endoplasmic reticulum-trapped antibody. We report that ErbB-2 overexpression enhanced binding affinities to both EGF and NDF, through deceleration of ligand dissociation rates. Likewise, removal of ErbB-2 from the cell surface almost completely abolished ligand binding by accelerating dissociation of both growth factors. The kinetic effects resulted in enhancement and prolongation of the stimulation of two major cytoplasmic signaling pathways, namely: MAP kinase (ERK) and c-Jun kinase (SAPK), by either ligand. Our results imply that ErbB-2 is a pan-ErbB subunit of the high affinity heterodimeric receptors for NDF and EGF. Therefore, the oncogenic action of ErbB-2 in human cancers may be due to its ability to potentiate in trans growth factor signaling.

Neu differentiation factor activation of ErbB-3 and ErbB-4 is cell specific and displays a differential requirement for ErbB-2.

Neu differentiation factor (NDF)-induced signaling involves the activation of members of the ErbB family of receptor tyrosine kinases. Although ectopic expression of recombinant ErbB receptors has yielded valuable insight into their signaling properties, the biological function and in vivo interplay of these receptors are still poorly understood. We addressed this issue by studying NDF signaling in various human cell lines expressing moderate levels of all known ErbB receptors. NDF-induced phosphorylation of ErbB-2 and ErbB-3 was found in the breast epithelial cell line MCF10A, the breast tumor cell lines T47D and MCF7, and the ovarian tumor cell line OVCAR3. Despite similar expression levels, NDF-induced phosphorylation of ErbB-4 was cell specific and only detected in T47D and OVCAR3 cells. Blocking cell surface expression of ErbB-2 by intracellular expression of a single-chain antibody revealed that in these two cell lines, ErbB-2 significantly enhanced phosphorylation of ErbB-4. Efficient NDF-induced phosphorylation of ErbB-3 was strictly ErbB-2 dependent in the breast tumor cell lines T47D and MCF7, while it was largely ErbB-2 independent in MCF10A and OVCAR3 cells. Consequently, NDF-stimulated intracellular signaling and induction of a biological response displayed a cell-specific requirement for ErbB-2. Thus, while ErbB-2 cooperates with NDF receptors in the breast tumor cell lines, ErbB-2 independent mechanisms seem to prevail in other cellular contexts.

Single-chain antibody-mediated intracellular retention of ErbB-2 impairs Neu differentiation factor and epidermal growth factor signaling.

ErbB-2 becomes rapidly phosphorylated and activated following treatment of many cell lines with epidermal growth factor (EGF) or Neu differentiation factor (NDF). However, these factors do not directly bind ErbB-2, and its activation is likely to be mediated via transmodulation by other members of the type I/EGF receptor (EGFR)-related family of receptor tyrosine kinases. The precise role of ErbB-2 in the transduction of the signals elicited by EGF and NDF is unclear. We have used a novel approach to study the role of ErbB-2 in signaling through this family of receptors. An ErbB-2-specific single-chain antibody, designed to prevent transit through the endoplasmic reticulum and cell surface localization of ErbB-2, has been expressed in T47D mammary carcinoma cells, which express all four known members of the EGFR family. We show that cell surface expression of ErbB-2 was selectively suppressed in these cells and that the activation of the mitogen-activated protein kinase pathway and p70/p85S6K, induction of c-fos expression, and stimulation of growth by NDF were dramatically impaired. Activation of mitogen-activated protein kinase and p70/p85S6K and induction of c-fos expression by EGF were also significantly reduced. We conclude that in T47D cells, ErbB-2 is a major NDF signal transducer and a potentiator of the EGF signal. Thus, our observations demonstrate that ErbB-2 plays a central role in the type I/EGFR-related family of receptors and that receptor transmodulation represents a crucial step in growth factor signaling.

Autocrine inhibition of the epidermal growth factor receptor by intracellular expression of a single-chain antibody.

A gene encoding a single-chain antibody which specifically binds the human epidermal growth factor (EGF) receptor has been constructed and expressed intracellularly. The single-chain antibody is derived from monoclonal antibody 225 which competes with EGF for binding to the extracellular domain of the receptor. The single-chain antibody was provided with a signal peptide to direct it to the secretory pathway and was expressed in EGF receptor transformed NIH/3T3 fibroblasts. EGF induced activation of its receptor was reduced in these cells. In addition, EGF-induced anchorage-independent growth of the cells was inhibited. The data suggest that the single-chain antibody functions in an autocrine fashion to inhibit the activity of the EGF receptor.

Intracellular expression of single chain antibodies reverts ErbB-2 transformation.

We report a novel approach for specific in vivo inactivation of the ErbB-2 receptor tyrosine kinase and suppression of ErbB-2-induced transformation. Genes encoding single chain antibodies that specifically bind to the extracellular domain of human ErbB-2 were constructed and expressed intracellularly in NIH/3T3 fibroblasts transformed by activated ErbB-2. The single chain antibodies are derived from monoclonal antibodies FRP5 and FWP51 (Harwerth, I.M., Wels, W., Marte, B. M., and Hynes, N. E. (1992) J. Biol. Chem. 267, 15160-15167) and are composed of heavy and light chain variable domains connected by a flexible peptide linker. The antibodies were provided with: 1) an N-terminal hydrophobic leader sequence to target their synthesis to the lumen of the endoplasmic reticulum, and 2) a C-terminal retention signal to prevent secretion. When expressed in ErbB-2-transformed cells, the single chain antibodies bound to the receptor and prevented its transit through the endoplasmic reticulum. This resulted in the functional inactivation of the receptor and reversion of the transformed phenotype. This is the first demonstration of a targeted and stable inactivation of a cellular oncoprotein via intracellular antibody expression. The use of such a strategy represents a simple and powerful approach to study the in vivo function of receptors and other cellular proteins.