Biomedical subjects

A Ullrich

Publications and source records attributed to A Ullrich.

At least 199 records · Page 11Linked to original sources

PubMed1993

SH2 domains exhibit high-affinity binding to tyrosine-phosphorylated peptides yet also exhibit rapid dissociation and exchange.

src homology 2 (SH2) domains of intracellular signaling molecules such as phospholipase C-gamma and phosphatidylinositol 3'-kinase-associated protein p85 represent recognition motifs for specific phosphotyrosine-containing regions on activated growth factor receptors. The binding of SH2 domains to activated growth factor receptors controls the interaction with signaling molecules and the regulation of their activities. In this report, we describe the kinetic parameters and binding affinities of SH2 domains of p85 toward short phosphotyrosine-containing peptides with the amino acid sequence motif YMXM, derived from a major insulin receptor substrate, IRS-1, by using real time biospecific interaction analysis (BIAcore). Associations were specific and of very high affinity, with dissociation constants of 0.3 to 3 nM, between phosphopeptides and the two separate SH2 domains contained within p85. Nonphosphorylated peptides showed no measurable binding, and the interactions were specific for the primary sequence very close to the phosphotyrosine residue. Moreover, the interactions between phosphopeptides and SH2 domains of other signaling molecules were of much lower affinity. Interestingly, the binding of the SH2 domains to the tyrosine-phosphorylated peptides was of high affinity as a result of a very high on rate, of 3 x 10(7) to 40 x 10(7)/M/s; at the same time, the rate of dissociation, of 0.11 to 0.19/s, was rapid, allowing for rapid exchange of associating proteins with the tyrosine phosphorylation sites.

Amino Acid Sequence↗

PubMed1993

Characterization of hemopoietic cell populations from human cord blood expressing c-kit.

Human cord blood or bone marrow cells expressing the CD34 surface antigen include a population of pluripotent progenitors. We identified and isolated a subpopulation of cells coexpressing CD34 and c-kit, a transmembrane receptor with tyrosine kinase activity. Novel monoclonal antibodies (16A6, 14A3, 3D6) directed against the extracellular domain of c-kit were used for immunofluorescence labeling and sorting of low-density mononuclear cells (MNCs) from umbilical cord blood and bone marrow. The frequency of c-kit-labeled MNCs from cord blood (mean 5.0% +/- 2.1%, n = 16) was similar to that from adult bone marrow (mean 3.7% +/- 1.3%, n = 4). On average, 1.4% of CD34-positive cells were recorded in cord blood and 2.1% in bone marrow MNCs. Roughly 60% of CD34-positive cells coexpressed c-kit. The ability of CD34+/c-kit+ cells to form multilineage colonies (CFU-GEMM) was assayed after sorting with an antibody that did not show any significant effect on c-kit ligand (RL) or granulocyte/macrophage colony-stimulating factor (GM-CSF)-induced colony formation. For CD34+/c-kit+ cells, we found a 20- to 50-fold enrichment as against total MNCs, and a 2-fold enrichment if compared with the CD34+/c-kit-population. To study expression of c-kit in lymphocytic precursors, monoclonal anti-CD7 or anti-CD10 antibodies were used simultaneously. In contrast to CD34-expressing cells, however, no consistent double-labeled subpopulation of lymphocytic cells was detected. Furthermore, coexpression of CD38 (73% +/- 14%, n = 4) or CD33 (29% +/- 12%, n = 5) on a majority of c-kit-positive cells showed their lineage commitment to erythropoiesis and granulocytopoiesis.

ADP-ribosyl Cyclase↗

PubMed1992

The carboxy terminal 110 amino acid portion of the insulin receptor is important for insulin signalling to pyruvate dehydrogenase.

Insulin-like growth factor 1 (IGF-1) had no detectable effect on pyruvate dehydrogenase (PDH) in NIH 3T3 cells that stably overexpressed normal human IGF-1 receptors. Insulin stimulated PDH activity 3-4--fold in cells that overexpressed normal human insulin receptors, but not in cells expressing TMI receptors, composed of the ligand binding domain of the insulin receptor coupled to the transmembrane and intracellular components of the IGF-1 receptor, or CEX receptors, in which the carboxy terminal 110 amino acid portion of the insulin receptor was exchanged for the corresponding portion of the IGF-1 receptor. In contrast, insulin stimulated glucose uptake in the control cell line and in each of the chimeric receptor-expressing lines with similar dose-response characteristics. These findings suggest the carboxy terminal portion of the IR may play a role in mediating the stimulation of PDH activity.

3T3 Cells↗

PubMed1992

Expression of c-kit receptor in normal and transformed human nonlymphoid tissues.

The protooncogene c-kit encodes a tyrosine kinase with a molecular weight of 145,000, highly related to the platelet derived growth factor/colony stimulating factor receptors. Mutations of the murine gene result in impairment of hematopoiesis, gametogenesis, and of the melanocyte cell lineage. In order to elucidate c-kit functions in development and oncogenesis we have analyzed immunohistochemically its expression in human normal and transformed nonlymphoid tissues. The receptor has been detected in spermatogonia, melanocytes, and unexpectedly, in astrocytes, renal tubules, parotid cells, thyrocytes, and breast epithelium. While the gene product is expressed in seminoma, lung tumors, and melanoma of low invasiveness, no detectable levels have been detected in thyroid and breast carcinomas, astrocytomas, and invasive melanomas. In breast tumors these findings were confirmed by paired, Northern blot analysis of RNA preparations from normal and transformed tissue. The present results demonstrate that the c-kit receptor plays a role in the development of a larger spectrum of cell lineages. Furthermore, on the basis of the transformation associated changes, we speculate that, while in some cell types, c-kit expression positively regulates mitogenesis and is selected for in neoplastic transformation, in other tissues the c-kit pathway is involved in morphogenesis and differentiation and is, therefore, negatively selected in the course of tumor progression.

Adult↗

PubMed1992

Breast cancer is associated with loss of the c-kit oncogene product.

The proto-oncogene c-kit encodes a tyrosine kinase receptor related to the PDGF/CSF-1 receptors. Mutations of this gene result in impairment of hematopoiesis, melanogenesis and gametogenesis. Using monoclonal antibodies to the c-kit gene product, we have analyzed its expression in normal and transformed human tissues. Unexpectedly, the receptor was found to be expressed in normal mammary epithelium. While in benign breast lesions, the c-kit gene product was detected at variable levels in 82% of the instances, in primary tumors, no product could be identified in 87% of the cases. This phenotype is maintained in metastatic foci. These findings were confirmed by paired Northern blot analysis of RNA preparations from normal and tumor tissues. These results demonstrate that the c-kit receptor may also be involved in the growth control of mammary epithelium and that this function may be impaired following malignant transformation and de-differentiation.

Blotting, Northern↗

PubMed1992

High-efficiency expression/cloning of epidermal growth factor-receptor-binding proteins with Src homology 2 domains.

Src homology 2 domains bind to tyrosine-phosphorylated growth factor receptors and are found in proteins that serve as substrates for tyrosine kinases, such as phospholipase C-gamma 1 and ras GTPase-activating protein. We have previously described the cloning of phosphatidylinositol 3'-kinase-associated p85 from expression libraries with the tyrosine-phosphorylated epidermal growth factor receptor as a probe. We have now modified this technique by using T7 polymerase-based expression libraries, which significantly improves sensitivity of the method. In one screening of such a library, we identified five different murine Src homology 2 domain-containing proteins, which we call GRBs (growth factor receptor-bound proteins). Two of these proteins represented the tyrosine kinase fyn and the mouse homologue of phospholipase C-gamma 1, whereas two genes encoded proteins similar to v-crk and NCK. We also isolated the gene for GRB-7, which encodes a protein of 535 amino acids. In addition to a Src homology 2 domain, GRB-7 also has a region of similarity to the noncatalytic domain of ras GTPase-activating protein and is highly expressed in liver and kidney. Use of this expression/cloning system should increase our ability to identify downstream modulators of growth factor action.

Amino Acid Sequence↗

PubMed1992

Progression of human cutaneous melanoma is associated with loss of expression of c-kit proto-oncogene receptor.

Mutations at the white spotting (w) locus in mice have deleterious effects on germ cells, melanocytes and hematopoietic stem cells. The w locus encodes the c-kit tyrosine-kinase receptor whose ligand is the product of the SI locus. Using monoclonal antibodies (MAb(s)) to the extracellular domain, we have evaluated the expression of c-kit in normal and transformed melanocytes. This cell lineage synthesizes a receptor with a mw of 145 kDa. The gene product is expressed in epidermal melanocytes and in a fraction of nevocytic and blue nevi. In primary melanomas, loss of the receptor is observed in more invasive lesions. Only 30% of the metastatic lesions express detectable levels of the receptor. These findings demonstrate that the c-kit product is down-regulated in melanocytes following malignant transformation. The functional relevance of this modulation remains to be evaluated.

Antibodies, Monoclonal↗

PubMed1992

Epidermal growth factor reduces HER-2 protein level in human ovarian carcinoma cells.

Over-expression of the proto-oncogene HER-2 (c-erbB-2/neu) in ovarian, endometrial, and mammary carcinoma is an indicator of poor prognosis. Interactions between the epidermal growth factor (EGF) receptor and the HER-2 protein have been described. The aim of this study was to elucidate the effects of EGF on HER-2 expression. In the human ovarian carcinoma cell lines HTB-77, OVCAR-3, 2780, SKOV-6, SKOV-8 and 2774, and the human mammary tumor cell line SKBR-3, total cellular p185HER-2 was determined by an ELISA, whereas the surface p185HER-2 was measured with a living-cell RIA. Stimulation of these cell lines with either EGF (0.1-30 nM) or TGF-alpha (0.1-30 nM) led to a significant reduction in p185HER-2 expression. The effect was more pronounced in cells with normal HER-2 expression. A reduction of mRNA levels for p185HER-2 by EGF was observed in OVCAR-3 cells but not in the over-expressing lines HTB-77 and SKBR-3. Interestingly, the EGF-induced effect was not always associated with growth stimulation and was not correlated with the number of EGF binding sites detected by a radioligand assay. Our data indicate that EGF treatment results in a down-regulation of p185HER-2.

Base Sequence↗

PubMed1992

The SH2 and SH3 domain-containing protein GRB2 links receptor tyrosine kinases to ras signaling.

A cDNA clone encoding a novel, widely expressed protein (called growth factor receptor-bound protein 2 or GRB2) containing one src homology 2 (SH2) domain and two SH3 domains was isolated. Immunoblotting experiments indicate that GRB2 associates with tyrosine-phosphorylated epidermal growth factor receptors (EGFRs) and platelet-derived growth factor receptors (PDGFRs) via its SH2 domain. Interestingly, GRB2 exhibits striking structural and functional homology to the C. elegans protein sem-5. It has been shown that sem-5 and two other genes called let-23 (EGFR like) and let-60 (ras like) lie along the same signal transduction pathway controlling C. elegans vulval induction. To examine whether GRB2 is also a component of ras signaling in mammalian cells, microinjection studies were performed. While injection of GRB2 or H-ras proteins alone into quiescent rat fibroblasts did not have mitogenic effect, microinjection of GRB2 together with H-ras protein stimulated DNA synthesis. These results suggest that GRB2/sem-5 plays a crucial role in a highly conserved mechanism for growth factor control of ras signaling.

Adaptor Proteins, Signal Transducing↗

PubMed1992

Differential effects of W mutations on p145c-kit tyrosine kinase activity and substrate interaction.

The c-kit gene, mapped to the dominant white spotting (W) locus of the mouse (Chabot, B., Stephenson, D. A., Chapman, V. M., Besmer, P., and Bernstein, A. (1988) Nature 335, 88-89; Geissler, E. N., Ryan, M. A., and Housman, D. E. (1988) Cell 55, 185-192), encodes a receptor tyrosine kinase, p145c-kit. Germline mutations at the W locus lead to loss of function alterations in p145c-kit, and result in mice with developmental defects of varying severity in the melanocytic, hematopoietic stem cell, and primordial germ cell lineages. To investigate in more detail the effect of W mutations on p145c-kit signaling, three mutations, W42, Wv, and W41, that confer severe, intermediate, and mild phenotypic characteristics, respectively, were introduced into the human p145c-kit tyrosine kinase domain. These mutations attenuated the intrinsic tyrosine kinase activity of the receptor to different degrees. In addition, they had differential effects on the interaction of the p145c-kit substrates, phospholipase C gamma, GTPase-activating protein, and the receptor-binding subunit of phosphatidylinositol 3'-kinase, p85. Notably, the Wv mutation, while retaining significant receptor tyrosine kinase activity, was unable to bind phospholipase C gamma and GTPase-activating protein, but could still associate with p85. These results suggest that the location of W mutations may be an important determinant of the specificity of substrate association and phosphorylation, and may explain, at least in part, the cell type-specific defects associated with certain W alleles.

Amino Acid Sequence↗

PubMed1992

A panel of monoclonal antibodies for the type I insulin-like growth factor receptor. Epitope mapping, effects on ligand binding, and biological activity.

We obtained 20 mouse monoclonal antibodies specific for human type I insulin-like growth factor (IGF) receptors, using transfected cells expressing high levels of receptors (IGF-1R/3T3 cells) as immunogen. The antibodies immunoprecipitated receptor.125I-IGF-I complexes and biosynthetically labeled receptors from IGF-1R/3T3 cells but did not react with human insulin receptors or rat type I IGF receptors. Several antibodies stimulated DNA synthesis in IGF-1R/3T3 cells, but the maximum stimulation was only 25% of that produced by IGF-I. The antibodies fell into seven groups recognizing distinct epitopes and with different effects on receptor function. All the antibodies reacted with the extracellular portion of the receptor, and epitopes were localized to specific domains by investigating their reaction with a series of chimeric IGF/insulin receptor constructs. Binding of IGF-I was inhibited up to 90% by antibody 24-60 reacting in the region 184-283, and by antibody 24-57 reacting in the region 440-586. IGF-I binding was stimulated up to 2.5-fold by antibodies 4-52 and 16-13 reacting in the region 62-184, and by antibody 26-3 reacting downstream of 283. The latter two groups of antibodies also dramatically stimulated insulin binding to intact IGF-1R/3T3 cells (by up to 50-fold), and potentiated insulin stimulation of DNA synthesis. Scatchard analysis indicated that in the presence of these antibodies, the affinity of the type I IGF receptor for insulin was comparable with that of the insulin receptor. These data indicate that regions both within and outside the cysteine-rich domain of the receptor alpha-subunit are important in determining the affinity and specificity of ligand binding. These antibodies promise to be valuable tools in resolving issues of IGF-I receptor heterogeneity and in studying the structure and function of classical type I receptors and insulin/IGF receptor hybrids.

3T3 Cells↗

PubMed1992

Ligand-induced activation of epidermal growth factor receptor in intact rat mammary adenocarcinoma cells without detectable receptor phosphorylation.

Expression and function of epidermal growth factor receptor (EGFR) was investigated in a metastatic cell clone (MTLn3) derived from the 13762NF rat mammary adenocarcinoma. No receptor phosphorylation could be identified in intact cells or in membrane preparations, while EGF-dependent phosphorylation of substrates occurred in intact cells. Indications for active suppression of receptor phosphorylation came from the fact that EGFRs bound in immunocomplexes or associated with the cytoskeleton of detergent treated cells were able to undergo basal and EGF-induced phosphorylation in vitro. Cross-linking experiments with 125I-EGF, as well as [35S]methionine labeling followed by immunoprecipitation with receptor specific antibodies readily detected in MTLn3 cells the expected 170-kDa EGFR protein. In addition, two proteins with molecular masses of 420-480 and 95 kDa specifically bound 125I-EGF on intact MTLn3 and sparse cultures of A431 cells. Phosphorylation of the 420-480 kDa molecule could be identified in immunocomplexes of EGFRs isolated from MTLn3 and sparse A431 cells, but the 95-kDa receptor molecule was never phosphorylated. While the presence of alternative forms of EGFR in the highly metastatic cell clone MTLn3 was unexpected, our observations of inefficient receptor autophosphorylation are in agreement with other recent reports and suggest that in MTLn3 cells EGFR-mediated signal transduction can be an event independent from receptor autophosphorylation.

Adenocarcinoma↗

PubMed1992

Development of a lethal mast cell disease in mice reconstituted with bone marrow cells expressing the v-erbB oncogene.

An animal model for malignant mastocytosis is described in mice reconstituted with bone marrow cells expressing the v-erbB oncogene. The lethal mast cell disease is characterized by massive infiltration of bone marrow, spleen, and several other visceral organs by connective tissue mast cells, which normally reside in the skin and the peritoneal cavity. As is frequently found in malignant mastocytosis, the v-erbB-induced mast cell disease was accompanied in some primary recipients by an acute myelogenous leukemia (AML) that killed all secondary recipients regardless of whether the AML was already evident in the primary host. The infiltrating mast cells stained strongly positive with berberine sulfate, suggesting that they were terminally differentiated and in vitro they showed only a weak proliferative capacity. The leukemias were clonal but apparently of different origin than the malignant mast cells, implying the transformation of two independent cell populations. Leukemic cells expressed various myeloid-specific markers as well as the B220 antigen, normally associated with the B-cell lineage. However, the Ig heavy chain genes were still in germ line configuration. In culture, these cells proliferated in the absence of exogenous growth factors and had the capacity to differentiate into mature myeloid cells. Preliminary experiments suggest that v-erbB may use parts of a signal transduction pathway normally coupled to the c-kit receptor. The v-erbB-induced malignant mast cell disease should provide a useful animal model for elucidating the cause for malignant mastocytosis in humans and to explore possible therapeutic strategies.

Alpharetrovirus↗

PubMed1992

Distinct alpha-subunit structures of human insulin receptor A and B variants determine differences in tyrosine kinase activities.

Human insulin receptor isoforms (HIR-A and -B) differ in their alpha-subunit structures which result from alternatively spliced precursor mRNAs. This structural difference causes distinct binding affinities for insulin. To determine the impact of the structural difference on receptor signaling, we characterized the tyrosine kinase activity of HIR-A and HIR-B in vitro and determined the insulin stimulated beta-subunit phosphorylation and tyrosine kinase activation in the intact cell. When 32P incorporation in beta-subunits of equal amounts of isolated HIR-A and HIR-B was measured, an increased 32P incorporation in tyrosine residues of the beta-subunit of HIR-B (2.5-fold) compared to that of HIR-A was found after in vitro insulin stimulation. This was paralleled by an increased rate of phosphorylation (2.0-fold) or poly(GluNa,Tyr 4:1). In vitro analysis of Km values for ATP were similar for HIR-A (Km = 14.3 microM +/- 3.8) and HIR-B (Km = 20.2 microM +/- 8.6), whereas the Vmax of HIR-B was significantly increased (HIR-A Vmax = 5.5 mumol/60 min micrograms-1 +/- 1.4, HIR-B Vmax = 42.5 mumol/60 min micrograms-1 +/- 19.2). HPLC analysis of tryptic beta-subunit phosphopeptides revealed identical patterns, suggesting that the difference in kinase activities is not due to an alteration of the phosphorylation-activation cascade within the beta-subunit. However, when cleavage of the alpha-subunit by short-time trypsinization was used to activate the tyrosine kinase, the differences in 32P incorporation between HIR-A and HIR-B were abolished.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Three additional inositol 1,4,5-trisphosphate receptors: molecular cloning and differential localization in brain and peripheral tissues.

Three inositol 1,4,5-trisphosphate receptor (IP3R) cDNAs, designated IP3R-II, -III, and -IV, were cloned from a mouse placenta cDNA library. All three display strong homology in membrane-spanning domains M7 and M8 to the originally cloned cerebellar IP3R-I, with divergences predominantly in cytoplasmic domains. Levels of mRNA for the three additional IP3Rs in general are substantially lower than for IP3R-I, though in the gastrointestinal tract the levels of IP3R-III may be comparable to IP3R-I. Cerebellar Purkinje cells express at least two and possibly three distinct IP3Rs, suggesting heterogeneity of IP3 action within a single cell.

Amino Acid Sequence↗

PubMed1992

Presence of SH2 domains of phospholipase C gamma 1 enhances substrate phosphorylation by increasing the affinity toward the epidermal growth factor receptor.

src homology region 2 and 3 (SH2 and SH3) domains are conserved noncatalytic regions originally described in cytoplasmic tyrosine kinases and subsequently identified in phospholipase C gamma 1 (PLC gamma 1), GTPase-activating protein of ras, and other signaling proteins. Although numerous studies indicate that SH2 domains promote protein-protein interactions by specific binding to tyrosine phosphorylated proteins, the function of SH3 domains is not known. The SH2 domain of PLC gamma 1 binds to certain tyrosine-phosphorylated growth factor receptors, and following phosphorylation on Tyr783 the enzymatic activity of PLC gamma 1 is enhanced, leading to phosphatidylinositol hydrolysis. To determine the functional role of the SH2 domain(s) on substrate phosphorylation in quantitative terms, we have expressed in Escherichia coli PLC gamma 1 constructs encoding the region containing Tyr783 and Tyr771, their two flanking SH2 domains and the SH3 domain, and five different deletion mutants of this region. These six proteins were purified and subjected to quantitative phosphorylation by the epidermal growth factor receptor (EGFR). Analysis of the kinetics of substrate phosphorylation revealed similar Vmax for the phosphorylation of the various mutant proteins. However, the affinity was enhanced for substrates containing SH2 domains: from S0.5 (average apparent Km) of 110 microM to S0.5 of 20 microM with the addition of a single SH2 domain and S0.5 of 3-4 microM for mutants containing two SH2 domains. The presence of the SH3 domain did not influence the apparent Km of substrate phosphorylation. These results demonstrate that the presence of the SH2 domain in PLC gamma 1 lowers the apparent Km (increases the affinity) of substrate phosphorylation by the EGFR, thereby facilitating PLC gamma 1 phosphorylation and activation.

Binding Sites↗

PubMed1992

Tumor-inhibitory monoclonal antibodies to the HER-2/Neu receptor induce differentiation of human breast cancer cells.

The HER-2/neu protooncogene (also called erbB-2) encodes a tyrosine kinase receptor for a polypeptide growth-regulatory molecule. Amplification and overexpression of the gene have been frequently observed in human adenocarcinomas and correlated with poor prognosis. To explore the potential of antibody therapy directed at the HER-2/Neu receptor, we have raised a panel of murine monoclonal antibodies to the human protein, and tested their effect on the tumorigenic growth of HER-2/neu-transfected fibroblasts in athymic mice. We previously reported that the i.p. injected antibodies either inhibited or accelerated the tumorigenic growth of HER-2/neu transfectants in athymic mice. Here we report that these opposing effects were induced also by i.v. injected antibodies, they lasted over 7 weeks, and were probably mediated by distinct epitopes on the receptor molecule. To understand the cellular mechanisms underlying antibody-induced tumor inhibition, we tested the effect of the monoclonal antibodies on various cultured human breast cancer cells. Our analysis revealed that the tumor-inhibitory antibodies specifically induced phenotypic cellular differentiation that included growth arrest at late S or early G2 phase of the cell cycle, markedly altered cytoplasm and nuclear morphology, synthesis and secretion of milk components (casein and lipids), and translocation of the HER-2/Neu protein to cytoplasmic and perinuclear sites. The extent of cellular differentiation by various antibodies could be correlated with their tumor-inhibitory potential, whereas a tumor-stimulatory monoclonal antibody or control immunoglobulin were completely inactive with respect to cellular differentiation. Taken together, our in vivo and in vitro studies correlate the tumor inhibitory potential of monoclonal antibodies to HER-2/Neu with their capacity to induce cellular differentiation in vitro. This observation may hold promise for immunotherapy of cancers that express the HER-2/neu oncogene.

Animals↗

PubMed1992

Heterodimerization of c-erbB2 with different epidermal growth factor receptor mutants elicits stimulatory or inhibitory responses.

Ligand-induced dimerization of growth factor receptors is crucial for stimulation of their intrinsic protein tyrosine kinase activity promoting receptor autophosphorylation by an intermolecular mechanism. Moreover, the suppressive and negative dominant action of defective epidermal growth factor receptor (EGFR) was shown to be caused by formation of inactive heterodimers with normal EGFR leading to diminished biological signaling. In this report we explore the structural requirements and functional significance of heterodimerization between EGFR and HER2. HER2 (also called c-erbB-2 or neu) is a member of the EGFR family whose natural ligand is still unknown. We show that in response to EGF, wild type EGFR and various EGFR mutants were able to undergo heterodimerization with HER2. Addition of EGF to transfected cells co-expressing HER2 with a kinase negative point mutant of EGFR (K721A) stimulated heterodimer formation, tyrosine phosphorylation of K721A and HER2, and tyrosine phosphorylation of one of their known substrates, phospholipase C gamma. However, the binding of EGF to transfected cells co-expressing HER2 together with another EGFR mutant CD533 (a deletion mutant lacking most of the cytoplasmic domain of EGFR) caused heterodimerization and inhibition of tyrosine kinase activity. It appears therefore that EGF-induced heterodimerization of EGFR and HER2 can promote either stimulatory or inhibitory influences on kinase activity. We propose that the nature of receptor interactions on the cell surface can either activate or inhibit the initiation of growth factor-controlled cellular signaling.

3T3 Cells↗