Signal transduction through the EGF receptor transfected in IL-3-dependent hematopoietic cells

Proliferation and Differentiation of Murine Myeloid Precursor 32D/G-CSF-R Cells

Understanding of the hematopoietic stem and progenitor cell biology has important implications for regenerative medicine and the treatment of hematological pathologies. Despite the most relevant data that can be acquired using in vivo models or primary cultures, the low abundance of hematopoietic stem and progenitor cells considerably restricts the pool of suitable techniques for their investigation. Therefore, the use of cell lines allows sufficient production of biological material for the performance of screenings or assays that require large cell numbers. Here we present a detailed description, readout, and interpretation of proliferation and differentiation assays which are used for the investigation of processes involved in myelopoiesis and neutrophilic differentiation. These experiments employ the 32D/G-CSF-R cytokine dependent murine myeloid cell line, which possesses the ability to proliferate in the presence of IL-3 and differentiate in G-CSF. We provide optimized protocols for handling 32D/G-CSF-R cells and discuss major pitfalls and drawbacks that might compromise the described assays and expected results. Additionally, this article contains protocols for lentiviral and retroviral production, titration, and transduction of 32D/G-CSF-R cells. We demonstrate that genetic manipulation of these cells can be employed to successfully perform functional and molecular studies, which can complement results obtained with primary hematopoietic stem and progenitor cells or in vivo models.

EGF receptor activates MET through MAPK to enhance non-small cell lung carcinoma invasion and brain metastasis

MET amplification as a mechanism of acquired resistance to EGF receptor (EGFR)-targeted therapies in non-small cell lung carcinoma (NSCLC) led to investigation of novel combinations of EGFR and MET kinase inhibitors. However, promiscuous interactions between MET and ERBB family members have made it difficult to evaluate the effects of MET on EGFR signaling, both independent of drug treatment and in the context of drug resistance. We addressed this issue by establishing a 32D model cell system wherein ERBBs or MET are expressed alone and in combination. Using this model, we determined that EGFR signaling is sufficient to induce MET phosphorylation, although MET activation is enhanced by coexpression of ERBB3. EGFR-MET cross-talk was not direct, but occurred by a combined regulation of MET levels and intermediary signaling through mitogen-activated protein kinases (MAPK). In NSCLCs harboring either wild-type or mutant EGFR, inhibiting EGFR or MAPK reduced MET activation and protein levels. Furthermore, MET signaling promoted EGFR-driven migration and invasion. Finally, EGFR-MET signaling was enhanced in a highly metastatic EGFR-mutant cell subpopulation, compared with the indolent parental line, and MET attenuation decreased the incidence of brain metastasis. Overall, our results establish that EGFR-MET signaling is critical for aggressive behavior of NSCLCs and rationalize its continued investigation as a therapeutic target for tumors harboring both wild-type and mutant EGFR at early stages of progression.

Insulin receptor substrate 1 expression enhances the sensitivity of 32D cells to chemotherapy-induced cell death

The adapters IRS1 and IRS2 link growth factor receptors to downstream signaling pathways that regulate proliferation and survival. Both suppress factor-withdrawal-induced apoptosis and have been implicated in cancer progression. However, recent studies suggest IRS1 and IRS2 mediate differential functions in cancer pathogenesis. IRS1 promoted breast cancer proliferation, while IRS2 promoted metastasis. The role of IRS1 and IRS2 in controlling cell responses to chemotherapy is unknown. To determine the role of IRS1 and IRS2 in the sensitivity of cells to chemotherapy, we treated 32D cells lacking or expressing IRS proteins with various concentrations of chemotherapeutic agents. We found that expression of IRS1, in contrast to IRS2, enhanced the sensitivity of 32D cells to chemotherapy-induced apoptosis. When IRS2 was expressed with IRS1, the cells no longer showed enhanced sensitivity. Expression of IRS1 did not alter the expression of pro- and anti-apoptotic proteins; however, 32D-IRS1 cells expressed higher levels of Annexin A2. In 32D-IRS1 cells, IRS1 and Annexin A2 were both located in cytoplasmic and membrane fractions. We also found that IRS1 coprecipitated with Annexin A2, while IRS2 did not. Decreasing Annexin A2 levels reduced 32D-IRS1 cell sensitivity to chemotherapy. These results suggest IRS1 enhances sensitivity to chemotherapy in part through Annexin A2.

Integrin signal masks growth-promotion activity of HB-EGF in monolayer cell cultures

The extracellular environment and tissue architecture contribute to proper cell function and growth control. Cells growing in monolayers on standard polystyrene tissue culture plates differ in their shape, growth rate and response to external stimuli, compared with cells growing in vivo. Here, we showed that the EGFR (epidermal growth factor receptor) ligand heparin-binding EGF-like growth factor (HB-EGF) strongly stimulated cell growth in nude mice, but not in cells cultured in vitro. We explored the effects of HB-EGF on cell growth under various cell culture conditions and found that growth promotion by HB-EGF was needed in three-dimensional (3D) or two-dimensional (2D) culture systems in which cell-matrix adhesion was reduced. Under such conditions, cell growth was extremely suppressed in the absence of HB-EGF, but markedly potentiated in the presence of HB-EGF. When the integrin signal was reduced using antibodies or knockout of either integrin β1 or focal adhesion kinase (FAK), cells showed HB-EGF-dependent growth. We also showed that EGF, transforming growth factor-α (TGFα) or ligands of other receptor tyrosine kinases (RTKs) stimulated cell growth in 3D culture, but not in tissue culture plates. These results indicate that the integrin signal was sufficient to support cell growth in 2D tissue culture plates without addition of the growth factor, whereas stimulation by growth factors was clearly demonstrated in culture systems in which integrin signals were attenuated.

A cell-based high-throughput screen for epidermal growth factor receptor pathway inhibitors

Epidermal growth factor receptor (EGFR) is a valid drug target for development of target-based therapeutics against non-small-cell lung cancer. In this study, we established a high-throughput cell-based assay to screen for compounds that may inhibit EGFR activation and/or EGFR-mediated downstream signaling pathway. This drug screening platform is based on the characterization of an EGFR-transfected 32D cell line (32D-EGFR). The expression of EGFR in 32D cells allowed cell proliferation in the presence of either epidermal growth factor (EGF) or interleukin 3 (IL-3) and provided a system for both screening and counterscreening of EGFR pathway-inhibitory compounds. After the completion of primary and secondary screenings in which 32D-EGFR cells were grown under the stimulation of either EGF or IL-3, 9 of 20,000 compounds were found to selectively inhibit the EGF-dependent proliferation, but not the IL-3-dependent proliferation, of 32D-EGFR cells. Subsequent analysis showed that 3 compounds of the 9 initial hits directly inhibited the kinase activity of recombinant EGFR in vitro and the phosphorylation of EGFR in H1299 cells transfected with EGFR. Thus, this 32D-EGFR assay system provides a promising approach for identifying novel EGFR and EGFR signaling pathway inhibitors with potential antitumor activity.

Epidermal growth factor receptor plays a significant role in hepatocyte growth factor mediated biological responses in mammary epithelial cells

Breast cancers often have deregulated hepatocyte growth factor (HGF) and c-Met signaling that results in increased tumor growth and invasion. Elucidating the mechanism responsible for HGF/c-Met action in breast cancer progression has been difficult as c-Met communicates with a number of secondary receptors that can lead to various pathological outcomes. Understanding how these secondary receptors facilitate HGF/c-Met cellular responses will aid in the development of better therapeutic treatment options for breast cancer patients with elevated HGF signaling. In the present study it was shown that the epidermal growth factor receptor (EGFR) plays a significant role in HGF/c-Met mediated biological activities indicative of advanced tumor pathology, including enhanced proliferation and invasion. The clinically relevant EGFR inhibitor gefitinib was used to determine the role of EGFR in HGF-induced proliferation and motility in several mammary carcinoma cells including PyVmT, MDA-MB-231 and 4T1. Our analyses indicated that EGFR inhibition significantly blocked HGF activation of c-Met and EGFR and that inhibition of these pathways mitigated HGF induced proliferation and motility. The data indicate that this inhibition was not through a direct effect of gefitinib on c-Met, but that EGFR is necessary for c-Met activation in the assays performed. These results provide a novel mechanism of action for EGFR as a mediator of HGF signaling thereby linking EGFR to the oncogenic potential of c-Met in mammary carcinomas cells.

Mutational activation of ErbB2 reveals a new protein kinase autoinhibition mechanism

Autoinhibition plays a key role in the control of protein kinase activity. ErbB2 is a unique receptor-tyrosine kinase that does not bind ligand but possesses an extracellular domain poised to engage other ErbBs. Little is known about the molecular mechanism for ErbB2 catalytic regulation. Here we show that ErbB2 kinase is strongly autoinhibited, and a loop connecting the alphaC helix and beta4 sheet within the kinase domain plays a major role in the control of kinase activity. Mutations of two Gly residues at positions 776 and 778 in this loop dramatically increase ErbB2 catalytic activity. Kinetic analysis demonstrates that mutational activation is due to approximately 10- and approximately 7-fold increases in ATP binding affinity and turnover number, respectively. Expression of the activated ErbB2 mutants in cells resulted in elevated ligand-independent ErbB2 autophosphorylation, ErbB3 phosphorylation, and stimulation of mitogen-activated protein kinase. Molecular modeling suggests that the ErbB2 kinase domain is stabilized in an inactive state via a hydrophobic interaction between the alphaC-beta4 and activation loops. Importantly, many ErbB2 human cancer mutations have been identified in the alphaC-beta4 loop, including the activating G776S mutation studied here. Our findings reveal a new kinase regulatory mechanism in which the alphaC-beta4 loop functions as an intramolecular switch that controls ErbB2 activity and suggests that loss of alphaC-beta4 loop-mediated autoinhibition is involved in oncogenic activation of ErbB2.

Intra- and extracellular signaling by endothelial neuregulin-1

Suppression of tumor growth by inhibition of ErbB receptor signaling is well documented. However, relatively little is known about the ErbB signaling system in the regulation of angiogenesis, a process necessary for tumor growth. We have previously shown that heparin-binding EGF-like growth factor (HB-EGF) is expressed by vascular endothelial cells (EC) and promotes endothelial recruitment of vascular smooth muscle cells (SMC). To assess whether other members of the EGF-family regulate angiogenesis, the expression of 10 EGF-like growth factors in primary ECs and SMCs was analyzed. In addition to HB-EGF, neuregulin-1 (NRG-1) was expressed in ECs in vitro and in vivo. Endothelial NRG-1 was constitutively processed to soluble extracellular and intracellular signaling fragments, and its expression was induced by hypoxia. NRG-1 was angiogenic in vivo in mouse corneal pocket and chicken chorioallantoic membrane (CAM) assays. However, consistent with the lack of NRG-1 receptors in several primary EC lines, NRG-1 did not directly stimulate cellular responses in cultured ECs. In contrast, NRG-1 promoted EC responses in vitro and angiogenesis in CAM in vivo by mechanisms dependent on VEGF-A and VEGFR-2. These results indicate that NRG-1 is expressed by ECs and regulates angiogenesis by mechanisms involving paracrine up-regulation of VEGF-A.

The epidermal growth factor receptor gene family as a target for therapeutic intervention in numerous cancers: what's genetics got to do with it?

Over the past 30 years, a relatively simple growth factor and its cognate receptor have provided seminal insights into the understanding of the genetic basis of cancer, as well as growth factor signalling. The epidermal growth factor (EGF), its cognate receptor (EGFR) and related family members have been shown to be important in normal, as well as the malignant growth of many cell types including: glioblastomata, astrocytomas, medulloblastomata, non-small cell lung carcinoma (NSCLC) and breast cancer. This review summarises the history of the EGFR gene and the v-ErbB oncogene, as well as diverse approaches developed to inhibit EGFR activity. The two most advanced therapies use either small-molecule cell membrane permeable kinase inhibitors or antibodies which prevent receptor activation. Recent clinical trials indicate that certain NSCLC patients have mutations in the EGFR gene which makes them more responsive to kinase inhibitors. These mutations appear to enhance the ability of the ligand to activate EGFR activity and also prolong the binding of the EGFR inhibitor to the kinase domain. Evidence to date suggests that these EGFR mutations in NSCLC occur more frequently in Japan than in the western hemisphere. Although these mutations are correlated with enhanced efficacy to the inhibitors in NSCLC, they can not explain or predict the sensitivity of many other cancer patients to the beneficial effects of the EGFR kinase inhibitors or antibody mediated therapy. As with as other small-molecule kinase inhibitors and susceptible diseases (e.g., imatinib and chronic myeloid leukaemia), resistance to EGFR inhibitors has been reported recently, documenting the requirement for development of multi-pronged therapeutic approaches. EGFR kinase inhibitors are also being evaluated as adjuvants in hormonal therapy of breast cancer - especially those which overexpress EGFR. Genetically engineered antibodies specific for the EGFR family member ErbB2 have been developed which show efficacy in the treatment of primary, and prevent the relapse of, breast cancer. Clearly, the EGF/EGFR signalling cascade has, and continues to play, an important role in the development of novel anticancer targeted therapies.

Epidermal growth factor receptor activation in glioblastoma through novel missense mutations in the extracellular domain

BACKGROUND

Protein tyrosine kinases are important regulators of cellular homeostasis with tightly controlled catalytic activity. Mutations in kinase-encoding genes can relieve the autoinhibitory constraints on kinase activity, can promote malignant transformation, and appear to be a major determinant of response to kinase inhibitor therapy. Missense mutations in the EGFR kinase domain, for example, have recently been identified in patients who showed clinical responses to EGFR kinase inhibitor therapy.

METHODS AND FINDINGS

Encouraged by the promising clinical activity of epidermal growth factor receptor (EGFR) kinase inhibitors in treating glioblastoma in humans, we have sequenced the complete EGFR coding sequence in glioma tumor samples and cell lines. We identified novel missense mutations in the extracellular domain of EGFR in 13.6% (18/132) of glioblastomas and 12.5% (1/8) of glioblastoma cell lines. These EGFR mutations were associated with increased EGFR gene dosage and conferred anchorage-independent growth and tumorigenicity to NIH-3T3 cells. Cells transformed by expression of these EGFR mutants were sensitive to small-molecule EGFR kinase inhibitors.

CONCLUSIONS

Our results suggest extracellular missense mutations as a novel mechanism for oncogenic EGFR activation and may help identify patients who can benefit from EGFR kinase inhibitors for treatment of glioblastoma.

Epidermal Growth Factor Receptor (EGFR) Antibody Down-regulates Mutant Receptors and Inhibits Tumors Expressing EGFR Mutations

Activating mutations in the kinase domain of the EGF receptor have been reported in non-small cell lung cancer. The majority of tumors expressing these mutants are sensitive to ATP mimetics that inhibit the EGFR tyrosine kinase. The effect of antibodies that bind to the ectodomain of the receptor is less clear. We report herein the effects and mechanisms of action of the antibody cetuximab in lung cancer cells that naturally express receptor mutations and in ErbB-null 32D hematopoietic cells transfected with mutant EGFR. Treatment with cetuximab down-regulated EGFR levels and inhibited cell growth both in vitro and in vivo. This was associated with inhibition of ligand-independent EGFR signaling. These effects were seen in 32D cells arguing the growth inhibitory action was not because of the blockade of autocrine ligand action. Both antibody-induced EGFR down-regulation and inhibition of growth required receptor dimerization as monovalent Fab fragments only eliminated receptor levels or reduced cell proliferation in the presence of antihuman IgG. Finally, cetuximab inhibited growth of H1975 lung cancer cells and xenografts, which expressed L858R/T790M EGFR and were resistant to EGFR tyrosine kinase inhibitors. These data suggest that cetuximab is an effective therapy against mutant EGFR-expressing cancer cells and thus can be considered in combination with other anti-EGFR molecules.

ErbB2 receptor tyrosine kinase signaling mediates early demyelination induced by leprosy bacilli

Demyelination is a common pathologic feature in many neurodegenerative diseases including infection with leprosy-causing Mycobacterium leprae. Because of the long incubation time and highly complex disease pathogenesis, the management of nerve damage in leprosy, as in other demyelinating diseases, is extremely difficult. Therefore, an important challenge in therapeutic interventions is to identify the molecular events that occur in the early phase before the progression of the disease. Here we provide evidence that M. leprae-induced demyelination is a result of direct bacterial ligation to and activation of ErbB2 receptor tyrosine kinase (RTK) signaling without ErbB2-ErbB3 heterodimerization, a previously unknown mechanism that bypasses the neuregulin-ErbB3-mediated ErbB2 phosphorylation. MEK-dependent Erk1 and Erk2 (hereafter referred to as Erk1/2) signaling is identified as a downstream target of M. leprae-induced ErbB2 activation that mediates demyelination. Herceptin (trastuzumab), a therapeutic humanized ErbB2-specific antibody, inhibits M. leprae binding to and activation of ErbB2 and Erk1/2 in human primary Schwann cells, and the blockade of ErbB2 activity by the small molecule dual ErbB1-ErbB2 kinase inhibitor PKI-166 (ref. 11) effectively abrogates M. leprae-induced myelin damage in in vitro and in vivo models. These results may have implications for the design of ErbB2 RTK-based therapies for both leprosy nerve damage and other demyelinating neurodegenerative diseases.

Association with HSP90 Inhibits Cbl-Mediated Down-regulation of Mutant Epidermal Growth Factor Receptors

Activating mutations in the epidermal growth factor receptor (EGFR), localized in the activation loop within the kinase domain, have been discovered in non-small cell lung cancers (NSCLC). Most of these mutants are exquisitely sensitive to EGFR tyrosine kinase inhibitors, suggesting that they generate receptor dependence in the cancers that express them. 32D cells stably expressing EGFR-L861Q and EGFR-L858R but not wild-type EGFR exhibited ligand-independent receptor phosphorylation and viability. Ligand-induced receptor down-regulation (LIRD) was impaired in mutant-expressing cells. The EGFR mutants were constitutively associated with the E3 ubiquitin ligase Cbl but did not associate with the adaptor protein CIN85 on the addition of ligand. Inhibition of HSP90 activity with geldanamycin restored Cbl function as indicated by receptor ubiquitination and LIRD. These results suggest that EGFR mutants form defective endocytic complexes. In addition, HSP90 plays a role in maintaining the functional conformation of EGFR mutants and protecting activated receptors from LIRD.

HER2 kinase domain mutation results in constitutive phosphorylation and activation of HER2 and EGFR and resistance to EGFR tyrosine kinase inhibitors

HER2/Neu gene mutations have been identified in lung cancer. Expression of a HER2 mutant containing a G776(YVMA) insertion in exon 20 was more potent than wild-type HER2 in associating with and activating signal transducers, phosphorylating EGFR, and inducing survival, invasiveness, and tumorigenicity. HER2(YVMA) transphosphorylated kinase-dead EGFR(K721R) and EGFR(WT) in the presence of EGFR tyrosine kinase inhibitors (TKIs). Knockdown of mutant HER2 in H1781 lung cancer cells increased apoptosis and restored sensitivity to EGFR TKIs. The HER2 inhibitors lapatinib, trastuzumab, and CI-1033 inhibited growth of H1781 cells and cells expressing exogenous HER2(YVMA). These data suggest that (1) HER2(YVMA) activates cellular substrates more potently than HER2(WT); and (2) cancer cells expressing this mutation remain sensitive to HER2-targeted therapies but insensitive to EGFR TKIs.

Distinctive activation patterns in constitutively active and gefitinib-sensitive EGFR mutants

Mutations in the kinase domain of epidermal growth factor receptor (EGFR) are associated with clinical responsiveness to gefitinib in patients with non-small-cell lung cancers (NSCLC). Recently, we have identified many novel EGFR mutations in NSCLC tissues. In this study, we found that gefitinib could suppress the tyrosine phosphorylation of most EGFR mutants better than the wild-type receptor. However, gefitinib had quite variable growth-suppressive effects on different EGFR mutant-expressing cells. All tested EGFR mutants have high basal phosphorylation at multiple tyrosine residues. Upon EGF stimulation, the mutated EGFRs did not have apparently stronger phosphorylation at tyrosines 845, 992, 1,068, and 1,173 than the wild-type receptor. However, stronger phosphorylation at tyrosine 1,045 was observed in the S768I, L861Q, E709G, and G719S mutants. The E746-A750 deletion mutant was less responsive to EGF than the wild-type and other mutant receptors. The S768I, L861Q, E709G, and G719S mutants were refractory to EGF-induced ubiquitination and had more sustained tyrosine phosphorylation. E709G and G719S also lacked EGF-induced receptor downregulation. Our results indicate that, in addition to sensitivity to gefitinib, EGFR mutations also caused various changes in EGFR's regulatory mechanisms, which may contribute to the constitutive activation of EGFR mutants and oncogenesis in NSCLC.

Proteolytic cleavage and phosphorylation of a tumor-associated ErbB4 isoform promote ligand-independent survival and cancer cell growth

The ErbB1 and ErbB2 receptors are oncogenes with therapeutic significance in human cancer, whereas the transforming potential of the related ErbB4 receptor has remained controversial. Here, we have addressed whether four alternatively spliced ErbB4 isoforms differ in regulating cellular responses relevant for tumor growth. We show that the two tumor necrosis factor-alpha converting enzyme (TACE)-cleavable ErbB4 isoforms (the juxtamembrane [JM]-a isoforms) were overexpressed in a subset of primary human breast cancers together with TACE. The overexpression of the JM-a cytoplasmic (CYT)-2 ErbB4 isoform promoted ErbB4 phosphorylation, survival of interleukin-3-dependent cells, and proliferation of breast cancer cells even in the absence of ligand stimulation, whereas activation of the other three ErbB4 isoforms required ligand stimulation. Ligand-independent cellular responses to ErbB4 JM-a CYT-2 overexpression were regulated by both tyrosine kinase activity and a two-step proteolytic generation of an intracellular receptor fragment involving first a TACE-like proteinase, followed by gamma-secretase activity. These data suggest a novel transforming mechanism for the ErbB4 receptor in human breast cancer that is 1) specific for a single receptor isoform and 2) depends on proteinase cleavage and kinase activity but not ligand activation of the receptor.

Specific pharmacological dimerization of KDR in lentivirally transduced human hematopoietic cells activates antiapoptotic and proliferative mechanisms

Selective and regulatable expansion of transduced cells could augment gene therapy for many disorders. The activation of modified growth factor receptors via synthetic chemical inducers of dimerization allows for the coordinated growth of transduced cells. This system can also provide information on specific receptor-mediated signaling without interference from other family members. Although several receptor subunits have been investigated in this context, little is known about the precise molecular events associated with dimerizer-initiated signaling. We have constructed and expressed an AP20187-regulated KDR chimeric receptor in human TF1 cells and analyzed activation of this gene switch using functional, biochemical, and microarray analyses. When deprived of natural ligands, GM-CSF, interleukin-3, or erythropoietin, AP20187 prevented apoptosis of transduced TF1 cells, induced dose-dependent proliferation, and supported long-term growth. In addition, AP20187 stimulation activated the signaling molecules associated with mitogen-activated protein kinase and phosphatidyl-inositol 3-kinase/Akt pathways. Microarray analysis determined that a number of transcripts involved in a variety of cellular processes were differentially expressed. Notably, mRNAs affiliated with heat stress, including Hsp70 and Hsp105, were up-regulated. Functional assays showed that Hsp70 and Hsp105 protected transduced TF1 cells from apoptosis and premature senescence, in part through regulation of Akt. These observations delineate specific roles for kinase insert domain-containing receptor, or KDR, signaling and suggest strategies to endow genetically modified cells with a survival advantage enabling the generation of adequate cell numbers for therapeutic outcomes.

Preparation and characterization of Alexa Fluor 594-labeled epidermal growth factor for fluorescence resonance energy transfer studies: application to the epidermal growth factor receptor

We have prepared and characterized a new fluorescent derivative of murine epidermal growth factor (EGF), Alexa Fluor 594-labeled EGF (A-EGF), for fluorescence studies of EGF-EGF receptor interactions. We describe the synthesis of this derivative and its physical and biological characterization. The significant overlap between the excitation and the emission spectra of A-EGF makes this probe well suited to fluorescence resonance energy homo-transfer. Using time-resolved fluorescence to examine the oligomeric state of the EGF receptor, we have observed resonance energy homo-transfer of A-EGF bound to EGF receptors in cells, but not of A-EGF bound to EGF receptors in membrane vesicles. Our results, interpreted in the context of recent crystallographic studies of the ligand-binding domains of EGF receptors, suggest that observed fluorescence resonance energy transfer does not result from transfer within receptor dimers, but rather results from transfer within higher-order oligomers. Furthermore, our results support a structural model for oligomerization of EGF receptors in which dimers are positioned head-to-head with respect to the ligand-binding site, consistent with the head-to-head interactions observed between adjacent receptor dimers by X-ray crystallography.

Angiopoietin-regulated recruitment of vascular smooth muscle cells by endothelial-derived heparin binding EGF-like growth factor

Recruitment of vascular smooth muscle cells (SMC) by endothelial cells (EC) is essential for angiogenesis. Endothelial-derived heparin binding EGF-like growth factor (HB-EGF) was shown to mediate this process by signaling via ErbB1 and ErbB2 receptors in SMCs. 1) Analysis of ErbB-ligands demonstrated that primary ECs expressed only HB-EGF and neuregulin-1. 2) Primary SMCs expressed ErbB1 and ErbB2, but not ErbB3 or ErbB4. 3) Consistent with their known receptor specificities, recombinant HB-EGF, but not neuregulin-1, stimulated tyrosine phosphorylation of ErbB1 and ErbB2 and migration in SMCs. 4) Neutralization of HB-EGF or inhibition of ErbB1 or ErbB2 blocked 70-90% of the potential of ECs to stimulate SMC migration. Moreover, 5) angiopoietin-1, an EC effector with a role in recruitment of SMC-like cells to vascular structures in vivo, enhanced EC-stimulated SMC migration by a mechanism involving up-regulation of endothelial HB-EGF. Finally, 6) immunohistochemical analysis of developing human tissues demonstrated that HB-EGF was expressed in vivo in ECs associated with SMCs or pericytes but not in ECs of the hyaloid vessels not associated with SMCs. These results suggest an important role for HB-EGF and ErbB receptors in the recruitment of SMCs by ECs and elaborate on the mechanism by which angiopoietins exert their vascular effects.

Ligand- and kinase activity-independent cell survival mediated by the epidermal growth factor receptor expressed in 32D cells

To investigate the intrinsic activities of the epidermal growth factor receptor and the role of its kinase domain in these functions within a cellular environment lacking endogenous ErbB protein expression, wild-type EGF receptor (WT-EGFR) and two kinase-impaired mutants, D813A and K721R, were expressed in 32D murine hematopoietic cells, a line which is normally dependent on interleukin 3 (IL3) for growth and survival. Addition of EGF in the absence of IL3 stimulates receptor autophosphorylation and, in the presence of serum, mitosis in cells expressing WT-EGFR, but not in cells expressing D813A or K721R. Unexpectedly, cells expressing WT-EGFR or K721R exhibited IL3-independent survival in the presence of fetal bovine serum; parental 32D cells and cells expressing D813A did not survive, apparently undergoing apoptosis in the absence of IL3, whether or not serum was present. Addition of EGF did not prevent the apoptosis of WT-EGFR or K721R cells in serum-free medium. Activation of Akt was not necessary to mediate the prosurvival activity of EGF receptor expression. These results suggest that the EGF receptor can mediate the prevention of apoptosis independently of both receptor-ligand binding and receptor kinase activity, and this activity is disrupted by the D813A mutation.

Mitogenic synergy through multilevel convergence of hepatocyte growth factor and interleukin-4 signaling pathways

Hepatocyte growth factor (HGF) regulates various physiological and developmental processes in concert with other growth factors, cytokines and hormones. We examined interactions between cell signaling events elicited by HGF and the cytokine interleukin (IL)-4, in the IL-3-dependent murine myeloid cell line 32D transfected with the human HGF receptor, c-Met. HGF was a potent mitogen in these cells, and prevented apoptosis in response to IL-3 withdrawal. IL-4 showed modest anti-apoptotic activity, but no significant mitogenic activity. IL-4 synergistically enhanced HGF-stimulated DNA synthesis, whereas only additive prevention of apoptosis was observed. IL-4 did not enhance HGF-dependent tyrosine phosphorylation of c-Met or Shc. In contrast, HGF-stimulated activation of MAP kinases was enhanced by IL-4, suggesting that the IL-4 and HGF signaling pathways converge upstream of these events. Although phosphatidylinositol 3-kinase (PI3K) inhibitors diminished HGF-induced mitogenesis, anti-apoptosis, and MAP kinase activation, IL-4 enhanced HGF signaling persisted even in the presence of these inhibitors. IL-4 enhancement of HGF signaling was partially blocked in 32D/c-Met cells treated with inhibitors of MEK1 or c-Src kinases, completely blocked by expression of a catalytically inactive mutant of Janus kinase 3 (Jak3), and increased in 32D/c-Met cells overexpressing STAT6. Our results suggest that the IL-4 and HGF pathways converge at multiple levels, and that IL-4-dependent Jak3 and STAT6 activities modulate signaling events independent of PI3K to enhance HGF-dependent mitogenesis in myeloid cells, and possibly other common cellular targets.

A stopped-flow fluorescence anisotropy method for measuring hormone binding and dissociation kinetics with cell-surface receptors in living cells

We have developed a system for extending stopped-flow analysis to the kinetics of ligand capture and release by cell surface receptors in living cells. While most mammalian cell lines cannot survive the shear forces associated with turbulent, stopped-flow mixing, we determined that 32D cells, murine hematopoietic precursor cells, can survive rapid mixing, even at the high flow rates necessary to achieve dwell times as short as 10 msec. In addition, 32D cells do not express any member of the ErbB family of receptors, providing a null background for studying this receptor family. We have established a series of stable, monoclonal 32D-derived cell lines that express the epidermal growth factor (EGF) receptor, ErbB2, or a combination of both at different ratios. Using these cell lines and a homogeneous fluorescent derivative of H22Y-mEGF modified with fluorescein at the amino terminus (F-EGF), we have measured association and dissociation of F-EGF with its receptor. Association was measured by following the time-dependent changes in fluorescence anisotropy after rapidly mixing cells at various cell densities with F-EGF at 1-15nM. Dissociation was measured both by chase experiments in which unlabeled EGF was mixed with cells pre-equilibrated with F-EGF or by dilution of cells equilibrated with F-EGF. Comparison of these dissociation experiments demonstrated that little or no ligand-induced dissociation occurs in the chase dissociation experiments. For each cell line, data from a series of association experiments and dilution dissociation experiments were subjected to global analysis using a two independent receptor-class model. Our analysis is consistent with the presence of two distinct receptor populations, even in cells bearing only the EGF receptor. Increasing the relative expression of ErbB2 leads to an increase in the fraction of high affinity class receptors observed, without altering the total number of EGF binding sites.

Dissociation of heparan sulfate and receptor binding domains of hepatocyte growth factor reveals that heparan sulfate-c-met interaction facilitates signaling

Hepatocyte growth factor (HGF) is a secreted, heparan sulfate (HS) glycosaminoglycan-binding protein that stimulates mitogenesis, motogenesis, and morphogenesis in a wide array of cellular targets, including hepatocytes and other epithelial cells, melanocytes, endothelial cells, and hematopoietic cells. NK1 is an alternative HGF isoform that consists of the N-terminal (N) and first kringle (K1) domains of full-length HGF and stimulates all major HGF biological activities. Within NK1, the N domain retains the HS binding properties of full-length HGF and mediates HS-stimulated ligand oligomerization but lacks significant mitogenic or motogenic activity. In contrast, K1 does not bind HS, but it stimulates receptor and mitogen-activated protein kinase activation, mitogenesis, and motogenesis, demonstrating that structurally distinct and dissociable domains of HGF are the primary mediators of HS binding and receptor activation. Despite the absence of HS-K1 binding, K1 mitogenic activity in HS-negative cells is strictly dependent on added soluble heparin, whereas K1-stimulated motility is not. We also found that, like the receptors for fibroblast growth factors, the HGF receptor c-Met binds tightly to HS. These data suggest that HS can facilitate HGF signaling through interaction with c-Met that is independent of HGF-HS interaction and that the recruitment of specific intracellular effectors that mediate distinct HGF responses such as mitogenesis and motility is regulated by HS-c-Met interaction at the cell surface.

Identification of anaplastic lymphoma kinase as a receptor for the growth factor pleiotrophin

Pleiotrophin (PTN) is a secreted growth factor that induces neurite outgrowth and is mitogenic for fibroblasts, epithelial, and endothelial cells. During tumor growth PTN can serve as an angiogenic factor and drive tumor invasion and metastasis. To identify a receptor for PTN, we panned a phage display human cDNA library against immobilized PTN protein as a bait. From this we isolated a phage insert that was homologous to an amino acid sequence stretch in the extracellular domain (ECD) of the orphan receptor tyrosine kinase anaplastic lymphoma kinase (ALK). In parallel with PTN, ALK is highly expressed during perinatal development of the nervous system and down-modulated in the adult. Here we show in cell-free assays as well as in radioligand receptor binding studies in intact cells that PTN binds to the ALK ECD with an apparent Kd of 32 +/- 9 pm. This receptor binding is inhibited by an excess of PTN, by the ALK ECD, and by anti-PTN and anti-ECD antibodies. PTN added to ALK-expressing cells induces phosphorylation of both ALK and of the downstream effector molecules IRS-1, Shc, phospholipase C-gamma, and phosphatidylinositol 3-kinase. Furthermore, the growth stimulatory effect of PTN on different cell lines in culture coincides with the endogenous expression of ALK mRNA, and the effect of PTN is enhanced by ALK overexpression. From this we conclude that ALK is a receptor that transduces PTN-mediated signals and propose that the PTN-ALK axis can play a significant role during development and during disease processes.

Epidermal growth factor (EGF) receptor kinase-independent signaling by EGF

The ErbB family of receptors, which includes the epidermal growth factor receptor (EGFR), ErbB2, ErbB3, and ErbB4, mediate signaling by EGF-like polypeptides. To better understand the role of the EGFR tyrosine kinase, we analyzed signaling by a kinase-inactive EGFR (K721M) in ErbB-devoid 32D cells. K721M alone exhibited no detectable signaling capacity, whereas coexpression of K721M with ErbB2, but not ErbB3 or ErbB4, resulted in EGF-dependent mitogen-activated protein kinase (MAPK) activation. The kinase activity, but not tyrosine phosphorylation, of ErbB2 was required for EGF-induced MAPK activation. The presence of tyrosine phosphorylation sites in K721M was not a requisite for signaling, indicating that transphosphorylation of K721M by ErbB2 was not an essential mechanism of receptor activation. Conversely, the mutated kinase domain of K721M (residues 648-973) and tyrosine phosphorylation of at least one of the receptors were necessary. EGF was found to activate the pro-survival protein kinase Akt in stable cell lines expressing K721M and ErbB2 but, unlike cells expressing wild-type EGFR, was incapable of activating signal transducers and activators of transcription (STAT) or driving cell proliferation. These results demonstrate that EGFR-ErbB2 oligomers are potent activators of MAPK and Akt, and this signaling does not require EGFR kinase activity.

Activity of the farnesyl protein transferase inhibitor SCH66336 against BCR/ABL-induced murine leukemia and primary cells from patients with chronic myeloid leukemia

BCR/ABL, the oncoprotein responsible for chronic myeloid leukemia (CML), transforms hematopoietic cells through both Ras-dependent and -independent mechanisms. Farnesyl protein transferase inhibitors (FTIs) were designed to block mutant Ras signaling, but they also inhibit the growth of transformed cells with wild-type Ras, implying that other farnesylated targets contribute to FTI action. In the current study, the clinical candidate FTI SCH66336 was characterized for its ability to inhibit BCR/ABL transformation. When tested against BCR/ABL-BaF3 cells, a murine cell line that is leukemogenic in mice, SCH66336 potently inhibited soft agar colony formation, slowed proliferation, and sensitized cells to apoptotic stimuli. Quantification of activated guanosine triphosphate (GTP)-bound Ras protein and electrophoretic mobility shift assays for AP-1 DNA binding showed that Ras effector pathways are inhibited by SCH66336. However, SCH66336 was more inhibitory than dominant-negative Ras in assays of soft agar colony formation and cell proliferation, suggesting activity against targets other than Ras. Cell cycle analysis of BCR/ABL-BaF3 cells treated with SCH66336 revealed G2/M blockade, consistent with recent reports that centromeric proteins that regulate the G2/M checkpoint are critical farnesylated targets of FTI action. Mice injected intravenously with BCR/ABL-BaF3 cells developed acute leukemia and died within 4 weeks with massive splenomegaly, elevated white blood cell counts, and anemia. In contrast, nearly all mice treated with SCH66336 survived and have remained disease-free for more than a year. Furthermore, SCH66336 selectively inhibited the hematopoietic colony formation of primary human CML cells. As an oral, nontoxic compound with a mechanism of action distinct from that of ABL tyrosine kinase inhibition, FTI SCH66336 shows promise for the treatment of BCR/ABL-induced leukemia.

UVB-induced epidermal growth factor receptor phosphorylation is critical for downstream signaling and keratinocyte survival

We have recently shown that UVB radiation activates epidermal growth factor receptor (EGFR)/extracellular regulated kinase 1 and 2 (ERK1/2) and p38 signaling pathways in keratinocytes. However, the functional relevance of these processes for downstream signaling and cell survival remains to be determined. The specific EGFR inhibitor PD153035 markedly decreased UVB-induced phosphorylation of EGFR, ERK1/2 and shc, whereas p38 activation was unaffected. PD153035 pretreatment followed by UVB reduced clonogenic potential and enhanced peroxide production, apoptosis and cell death. Our data suggest that ligand-independent phosphorylation of EGFR and likely dependent downstream signaling pathways regulate cellular defense mechanisms important for cell survival following oxidative stress.

ERbB-2 expression is rate-limiting for epidermal growth factor-mediated stimulation of ovarian cancer cell proliferation

Over-expression of the ErbB-2 proto-oncogene frequently coincides with an aggressive clinical course of certain human adenocarcinomas. The ErbB-2 receptor is a member of the ErbB family of growth factor receptors, and within this complex signaling network, ErbB-2-containing heterodimers are preferentially formed. To assess whether ErbB-2 is a critical component in epidermal growth factor (EGF)-mediated stimulation of tumor cell proliferation, we used as a model SK-OV-3 ovarian cancer cells, which over-express EGF receptor (EGFR) and ErbB-2 receptors. In these cells, we reduced ErbB-2 mRNA and protein expression by transfection with ErbB-2-targeted hammerhead ribozymes and generated cell lines expressing different levels of ErbB-2. In SK-OV-3 cells, ErbB-2 expression conferred a growth advantage and soft agar experiments revealed that ErbB-2 was rate-limiting for anchorage-independent growth. The induction of colony formation by EGF was completely abrogated in ErbB-2-depleted cells, despite unchanged expression levels and tyrosine phosphorylation of the EGFR. The duration of EGF-mediated c-Fos mRNA up-regulation was decreased in parallel with loss of ErbB-2 expression. Furthermore, the rate of spontaneous apoptosis was increased in ErbB-2-depleted cells. Our results demonstrate that in human ovarian cancer cells the EGFR-ErbB-2 heterodimer, and not the EGFR homodimer, can be rate-limiting for EGF-mediated proliferation, thus suggesting that the oncogenic activity of ErbB-2 in human tumors is due in part to its ability to increase the growth response to stroma-derived EGF-like growth factors.

Multiple growth factor induction of a murine early response gene that complements a lethal defect in yeast ribosome biogenesis

Identification of the transcriptionally activated targets of receptor tyrosine kinases is critical to understanding biologic programs directing both normal and neoplastic growth. To elucidate these molecular processes, we identified genes induced by a potent mesenchymal mitogen, platelet-derived growth factor (PDGF). Using differential display reverse transcription-polymerase chain reaction technology, we isolated a novel growth factor-induced cDNA, San5. San5 transcript induction occurred within 60 min in NIH 3T3 fibroblasts and proceeded in the presence of cycloheximide. Maximal induction of the San5 transcript occurred between 8 and 16 h, concurrent with passage of fibroblasts through G(1). San5 message was potently induced by PDGF AA and BB and acidic and basic fibroblast growth factors, all strong activators of fibroblast proliferation, but not by epidermal growth factor and interleukin-4. In a murine hematopoietic progenitor cell line, San5 transcript induction strictly correlated with [(3)H]thymidine uptake. Isolation and sequencing of the murine San5 cDNA revealed amino acid sequence homology to yeast Nop5p, a nucleolar protein required for pre-rRNA processing and ribosome assembly. Strikingly, SAN5 was able to rescue a nop5 null mutant, implicating SAN5 in the process of ribosome biogenesis. Consistent with this result, SAN5 was localized to the nucleolus in both yeast and mouse. Thus, San5 may provide a link between growth factor receptor activation and the cellular translational machinery.

Expression of hematopoietic growth factor receptors on early hematopoietic precursors: detection and regulation

Since the original isolation of colony-stimulating factors from human serum, conditioned medium of murine or human cell lines, or freshly isolated human mononuclear cells, a revolutionary explosion of ideas has occurred in our understanding of molecular controls of the hematopoietic stem cell self-renewal and differentiation. With the availability of techniques of molecular cloning in the early 1 980s, the first hematopoietically activated cytokines led to molecular clones expressed in bacteria, yeast, or mammalian cellular systems. There then followed a development of techniques leading to the molecular cloning and expression of many hematopoietic growth factors and their receptors, as well as the primary, secondary, and tertiary molecules in signal transduction into activation of specific genes for differentiation or self-renewal. The clinical use of these factors in the diagnosis, treatment, and incorporation into new cell therapies for a variety of diseases is a subject of current interest.

Cyclic nucleotide phosphodiesterase 3B is a downstream target of protein kinase B and may be involved in regulation of effects of protein kinase B on thymidine incorporation in FDCP2 cells

Wild-type (F/B), constitutively active (F/B*), and three kinase-inactive (F/Ba-, F/Bb-, F/Bc-) forms of Akt/protein kinase B (PKB) were permanently overexpressed in FDCP2 cells. In the absence of insulin-like growth factor-1 (IGF-1), activities of PKB, cyclic nucleotide phosphodiesterase 3B (PDE3B), and PDE4 were similar in nontransfected FDCP2 cells, mock-transfected (F/V) cells, and F/B and F/B- cells. In F/V cells, IGF-1 increased PKB, PDE3B, and PDE4 activities approximately 2-fold. In F/B cells, IGF-1, in a wortmannin-sensitive manner, increased PKB activity approximately 10-fold and PDE3B phosphorylation and activity ( approximately 4-fold), but increased PDE4 to the same extent as in F/V cells. In F/B* cells, in the absence of IGF-1, PKB activity was markedly increased ( approximately 10-fold) and PDE3B was phosphorylated and activated (3- to 4-fold); wortmannin inhibited these effects. In F/B* cells, IGF-1 had little further effect on PKB and activation/phosphorylation of PDE3B. In F/B- cells, IGF-1 activated PDE4, not PDE3B, suggesting that kinase-inactive PKB behaved as a dominant negative with respect to PDE3B activation. Thymidine incorporation was greater in F/B* cells than in F/V cells and was inhibited to a greater extent by PDE3 inhibitors than by rolipram, a PDE4 inhibitor. In F/B cells, IGF-1-induced phosphorylation of the apoptotic protein BAD was inhibited by the PDE3 inhibitor cilostamide. Activated PKB phosphorylated and activated rPDE3B in vitro. These results suggest that PDE3B, not PDE4, is a target of PKB and that activated PDE3B may regulate cAMP pools that modulate effects of PKB on thymidine incorporation and BAD phosphorylation in FDCP2 cells.

Epidermal growth factor (EGF) suppresses staurosporine-induced apoptosis by inducing mcl-1 via the mitogen-activated protein kinase pathway

Overexpression of epidermal growth factor receptor (EGFR) and establishment of transforming growth factor alpha (TGF alpha)/EGF autocrine system are frequently detected in tumor cells. In addition to mitogenic ability, we demonstrate in this report that EGF protects a human esophageal carcinoma (CE) cell line, CE81T/VGH, from staurosporine-induced apoptosis. The anti-apoptotic signal of EGF is alleviated by a MEK inhibitor PD98059 or an ERK2 dominant negative mutant but not by a phosphatidylinositol-3'-kinase (PI-3K) inhibitor wortmannin. Furthermore, v-raf blocks apoptosis induced by staurosporine. This evidence implies that the survival signal of EGF is mediated via the Raf-MEK-ERK pathway but not the PI3-K pathway. The survival effect of EGF is coincident with the induction of mcl-1, an antiapoptotic gene in the bcl-2 family. PD98059 also suppresses the induction of Mcl-1 by EGF, implying that EGF may up-regulate Mcl-1 via the MAP kinase pathway. Overexpression of mcl-1 is sufficient to protect against apoptosis, while transfection of a mcl-1 antisense plasmid causes cell death. The expression of mcl-1 antisense plasmid also suppresses the anti-apoptotic effect of EGF. Taken together, these results indicate that EGF may up-regulate Mcl-1 through the MAP kinase pathway to suppress apoptosis.

The tetraspanin CD9 associates with transmembrane TGF-alpha and regulates TGF-alpha-induced EGF receptor activation and cell proliferation

Transforming growth factor-alpha (TGF-alpha) is a member of the EGF growth factor family. Both transmembrane TGF-alpha and the proteolytically released soluble TGF-alpha can bind to the EGF/TGF-alpha tyrosine kinase receptor (EGFR) and activate the EGFR-induced signaling pathways. We now demonstrate that transmembrane TGF-alpha physically interacts with CD9, a protein with four membrane spanning domains that is frequently coexpressed with TGF-alpha in carcinomas. This interaction was mediated through the extracellular domain of transmembrane TGF-alpha. CD9 expression strongly decreased the growth factor- and PMA- induced proteolytic conversions of transmembrane to soluble TGF-alpha and strongly enhanced the TGF- alpha-induced EGFR activation, presumably in conjunction with increased expression of transmembrane TGF-alpha. In juxtacrine assays, the CD9-induced EGFR hyperactivation by transmembrane TGF-alpha resulted in increased proliferation. In contrast, CD9 coexpression with transmembrane TGF-alpha decreased the autocrine growth stimulatory effect of TGF-alpha in epithelial cells. This decrease was associated with increased expression of the cdk inhibitor, p21(CIP1). These data reveal that the association of CD9 with transmembrane TGF-alpha regulates ligand-induced activation of the EGFR, and results in altered cell proliferation.

Effects of luteolin and quercetin, inhibitors of tyrosine kinase, on cell growth and metastasis-associated properties in A431 cells overexpressing epidermal growth factor receptor

1. Flavonoids display a wide range of pharmacological properties including anti-inflammatory. Anti-mutagenic, anti-carcinogenic and anti-cancer effects. Here, we evaluated the effects of eight flavonoids on the tumour cell proliferation, cellular protein phosphorylation, and matrix metalloproteinase (MMPs) secretion. 2. Of the flavonoids examined, luteolin (Lu) and quercetin (Qu) were the two most potent agents, and significantly inhibited A431 cell proliferation with IC50 values of 19 and 21 micronM, respectively. 3. The epidermal growth factor (EGF) (10 nM) promoted growth of A431 cells (+25+/-4.6%) and mediated epidermal growth factor receptor (EGFR) tyrosine kinase activity and autophosphorylation of EGFR were inhibited by Lu and Qu. At concentration of 20 micronM, both Lu and Qu markedly decreased the levels of phosphorylation of A431 cellular proteins, including EGFR. 4. A431 cells treated with Lu or Qu exhibited protuberant cytoplasmic blebs and progressive shrinkage morphology. Lu and Qu also time-dependently induced the appearance of a ladder pattern of DNA fragmentation, and this effect was abolished by EGF treatment. 5. The addition of EGF only marginally diminished the inhibitory effect of luteolin and quercetin on the growth rate of A431 cells, treatment of cellular proteins with EGF and luteolin or quercetin greatly reduced protein phosphorylation, indicating Lu and Qu may act effectively to inhibit a wide range of protein kinases, including EGFR tyrosine kinase. 6. EGF increased the levels of matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9), while Lu and Qu appeared to suppress the secretion of these two MMPs in A431 cells. 7. Examination of the relationship between the chemical structure and inhibitory effects of eight flavonoids reveal that the double bond between C2 and C3 in ring C and the OH groups on C3' and C4' in ring B are critical for the biological activities. 8. This study demonstrates that the inhibitory effects of Lu and Qu, and the stimulatory effects of EGF, on tumour cell proliferation, cellular protein phosphorylation, and MMP secretion may be mediated at least partly through EGFR. This study supports the idea that Lu and Qu may have potential as anti-cancer and anti-metastasis agents.

Contact-dependent growth inhibition and apoptosis of epidermal growth factor (EGF) receptor-expressing cells by the membrane-anchored form of heparin-binding EGF-like growth factor

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) transduces mitogenic signals through the EGF receptor (EGFR). There are two forms of HB-EGF, the membrane-anchored form (pro-HB-EGF) and the soluble form (sHB-EGF). We studied the biological activity of pro-HB-EGF by using a model in which pro-HB-EGF-expressing effector cells was co-cultured with EGFR-expressing target cells. The DER cell, an EGFR-expressing derivative of the interleukin-3-dependent hematopoietic 32D cell line, grows well in the presence of EGF or sHB-EGF without IL-3. When DER cells were co-cultured on a monolayer of Vero-H cells overexpressing pro-HB-EGF, growth inhibition and subsequent apoptosis were induced in the DER cells even in the presence of excess amounts of EGF or sHB-EGF. Such growth inhibition of DER cells was abrogated when specific antagonists for pro-HB-EGF were added in the culture medium or when direct contact of DER cells with Vero-H cells was prevented, indicating that pro-HB-EGF is involved in this inhibitory effect. Pro-HB-EGF-induced apoptosis of DER cells was also observed even in the presence of IL-3. This rules out the possibility of simple competition between soluble EGFR ligands and pro-HB-EGF. Moreover, 32D cells expressing EGFR mutant composed of the extracellular and the transmembrane domain of EGFR and the cytoplasmic domain of erythropoietin receptor did not undergo apoptosis by co-culture with Vero-H cells, indicating that the inhibitory signal induced by pro-HB-EGF-expressing Vero-H cells is mediated to DER cells via EGFR and that the cytoplasmic domain of EGFR is essential for pro-HB-EGF-induced apoptosis. From these results, we concluded that pro-HB-EGF has unique biological activity through cell-cell contact that is distinct from the activity of sHB-EGF.

Differential roles of Akt, Rac, and Ral in R-Ras-mediated cellular transformation, adhesion, and survival

Multiple biological functions have been ascribed to the Ras-related G protein R-Ras. These include the ability to transform NIH 3T3 fibroblasts, the promotion of cell adhesion, and the regulation of apoptotic responses in hematopoietic cells. To investigate the signaling mechanisms responsible for these biological phenotypes, we compared three R-Ras effector loop mutants (S61, G63, and C66) for their relative biological and biochemical properties. While the S61 mutant retained the ability to cause transformation, both the G63 and the C66 mutants were defective in this biological activity. On the other hand, while both the S61 and the C66 mutants failed to promote cell adhesion and survival in 32D cells, the G63 mutant retained the ability to induce these biological activities. Thus, the ability of R-Ras to transform cells could be dissociated from its propensity to promote cell adhesion and survival. Although the transformation-competent S61 mutant bound preferentially to c-Raf, it only weakly stimulated the mitogen-activated protein kinase (MAPK) activity, and a dominant negative mutant of MEK did not significantly perturb R-Ras oncogenicity. Instead, a dominant negative mutant of phosphatidylinositol 3-kinase (PI3-K) drastically inhibited the oncogenic potential of R-Ras. Interestingly, the ability of the G63 mutant to induce cell adhesion and survival was closely associated with the PI3-K-dependent signaling cascades. To further delineate R-Ras downstream signaling events, we observed that while a dominant negative mutant of Akt/protein kinase inhibited the ability of R-Ras to promote cell survival, both dominant negative mutants of Rac and Ral suppressed cell adhesion stimulated by R-Ras. Thus, the biological actions of R-Ras are mediated by multiple effectors, with PI3-K-dependent signaling cascades being critical to its functions.

Differential signaling by alternative HGF isoforms through c-Met: activation of both MAP kinase and PI 3-kinase pathways is insufficient for mitogenesis

HGF/NK2, a naturally occurring truncated HGF isoform, antagonizes the mitogenic and morphogenic activities of full length HGF, but stimulates cell scatter, or the motogenic response to HGF. We studied postreceptor signaling by these HGF isoforms in the human breast epithelial cell line 184B5, and in murine myeloid progenitor 32D cells transfected with c-Met, the human HGF receptor (32D/c-Met). HGF stimulated DNA synthesis in 184B5 and 32D/c-Met cells, while HGF/NK2 was mitogenically inactive, despite the ability of HGF/NK2 to stimulate c-Met autophosphorylation, mitogen-activated protein kinase (MAPK), and phosphatidylinositol 3-kinase (PI3K) in both cell systems. In 184B5 cells, HGF stimulated sustained MAPK activation, while activation by HGF/NK2 declined rapidly. In contrast, both isoforms activated MAPK with rapidly attenuated kinetics in 32D/c-Met cells. In both cell systems the increased motility observed in response to either HGF or HGF/NK2 treatment was more potently blocked by the PI3 kinase inhibitor wortmannin, than by PD98059, an inhibitor of MAPK kinase (MEK1). These data suggest that (1) alternative HGF isoforms signaling through c-Met generate both common and distinct biological responses, (2) the extent and duration of ligand-stimulated c-Met and MAPK activities are dependent on the cellular context and are not predictive of mitogenic signaling, and (3) in at least some HGF target cells, the activation of both MAPK and PI3K signaling pathways is insufficient for mitogenesis elicited through c-Met.

IL-3 and IL-4 Activate Cyclic Nucleotide Phosphodiesterases 3 (PDE3) and 4 (PDE4) by Different Mechanisms in FDCP2 Myeloid Cells

In FDCP2 myeloid cells, IL-4 activated cyclic nucleotide phosphodiesterases PDE3 and PDE4, whereas IL-3, granulocyte-macrophage CSF (GM-CSF), and phorbol ester (PMA) selectively activated PDE4. IL-4 (not IL-3 or GM-CSF) induced tyrosine phosphorylation of insulin-receptor substrate-2 (IRS-2) and its association with phosphatidylinositol 3-kinase (PI3-K). TNF-α, AG-490 (Janus kinase inhibitor), and wortmannin (PI3-K inhibitor) inhibited activation of PDE3 and PDE4 by IL-4. TNF-α also blocked IL-4-induced tyrosine phosphorylation of IRS-2, but not of STAT6. AG-490 and wortmannin, not TNF-α, inhibited activation of PDE4 by IL-3. These results suggested that IL-4-induced activation of PDE3 and PDE4 was downstream of IRS-2/PI3-K, not STAT6, and that inhibition of tyrosine phosphorylation of IRS molecules might be one mechnism whereby TNF-α could selectively regulate activities of cytokines that utilized IRS proteins as signal transducers. RO31-7549 (protein kinase C (PKC) inhibitor) inhibited activation of PDE4 by PMA. IL-4, IL-3, and GM-CSF activated mitogen-activated protein (MAP) kinase and protein kinase B via PI3-K signals; PMA activated only MAP kinase via PKC signals. The MAP kinase kinase (MEK-1) inhibitor PD98059 inhibited IL-4-, IL-3-, and PMA-induced activation of MAP kinase and PDE4, but not IL-4-induced activation of PDE3. In FDCP2 cells transfected with constitutively activated MEK, MAP kinase and PDE4, not PDE3, were activated. Thus, in FDCP2 cells, PDE4 can be activated by overlapping MAP kinase-dependent pathways involving PI3-K (IL-4, IL-3, GM-CSF) or PKC (PMA), but selective activation of PDE3 by IL-4 is MAP kinase independent (but perhaps IRS-2/PI3-K dependent).

A differential requirement for the COOH-terminal region of the epidermal growth factor (EGF) receptor in amphiregulin and EGF mitogenic signaling

The epidermal growth factor receptor (EGFR) mediates the actions of a family of bioactive peptides that include epidermal growth factor (EGF) and amphiregulin (AR). Here we have studied AR and EGF mitogenic signaling in EGFR-devoid NR6 fibroblasts that ectopically express either wild type EGFR (WT) or a truncated EGFR that lacks the three major sites of autophosphorylation (c'1000). COOH-terminal truncation of the EGFR significantly impairs the ability of AR to (i) stimulate DNA synthesis, (ii) elicit Elk-1 transactivation, and (iii) generate sustained enzymatic activation of mitogen-activated protein kinase. EGFR truncation had no significant effect on AR binding to receptor but did result in defective GRB2 adaptor function. In contrast, EGFR truncation did not impair EGF mitogenic signaling, and in c'1000 cells EGF was able to stimulate the association of ErbB2 with GRB2 and SHC. Elk-1 transactivation was monitored when either ErbB2 or a truncated dominant-negative ErbB2 mutant (ErbB2-(1-813)) was overexpressed in cells. Overexpression of full-length ErbB2 resulted in a strong constitutive transactivation of Elk-1 in c'1000 but only slightly stimulated Elk-1 in WT or parental NR6 cells. Conversely, overexpression of ErbB2-(1-813) inhibited EGF-stimulated Elk-1 transactivation in c'1000 but not in WT cells. Thus, the cytoplasmic tail of the EGFR plays a critical role in AR mitogenic signaling but is dispensable for EGF, since EGF-activated truncated EGFRs can signal through ErbB2.

cbl-b inhibits epidermal growth factor receptor signaling

The role of cbl-b in signaling by the epidermal growth factor receptor (EGFR) was studied and compared with c-cbl. We demonstrate in vivo, that cbl-b, like c-cbl, is phosphorylated and recruited to the EGFR upon EGF stimulation and both cbl proteins can bind to the Grb2 adaptor protein. To investigate the functional role of cbl proteins in EGFR signaling, we transfected cbl-b or c-cbl into 32D cells overexpressing the EGFR (32D/EGFR). This cell line is absolutely dependent on exogenous IL-3 or EGF for sustained growth. 32D/EGFR cells overexpressing cbl-b showed markedly inhibited growth in EGF compared to c-cbl transfectants and vector controls. This growth inhibition by cbl-b was the result of a dramatic increase in the number of cells undergoing apoptosis. Consistent with this finding, cbl-b overexpression markedly decreased the amplitude and duration of AKT activation upon EGF stimulation compared to either vector controls or c-cbl overexpressing cells. In addition, the duration of EGF mediated MAP kinase and Jun kinase activation in cells overexpressing cbl-b is shortened. These data demonstrate that cbl-b inhibits EGF-induced cell growth and that cbl-b and c-cbl have distinct roles in EGF mediated signaling.

A metalloprotease-disintegrin, MDC9/meltrin-gamma/ADAM9 and PKCdelta are involved in TPA-induced ectodomain shedding of membrane-anchored heparin-binding EGF-like growth factor

The ectodomains of many proteins located at the cell surface are shed upon cell stimulation. One such protein is the heparin-binding EGF-like growth factor (HB-EGF) that exists in a membrane-anchored form which is converted to a soluble form upon cell stimulation with TPA, an activator of protein kinase C (PKC). We show that PKCdelta binds in vivo and in vitro to the cytoplasmic domain of MDC9/meltrin-gamma/ADAM9, a member of the metalloprotease-disintegrin family. Furthermore, the presence of constitutively active PKCdelta or MDC9 results in the shedding of the ectodomain of proHB-EGF, whereas MDC9 mutants lacking the metalloprotease domain, as well as kinase-negative PKCdelta, suppress the TPA-induced shedding of the ectodomain. These results suggest that MDC9 and PKCdelta are involved in the stimulus-coupled shedding of the proHB-EGF ectodomain.

ErbB tyrosine kinases and the two neuregulin families constitute a ligand-receptor network

The recently isolated second family of neuregulins, NRG2, shares its primary receptors, ErbB-3 and ErbB-4, and induction of mammary cell differentiation with NRG1 isoforms, suggesting functional redundancy of the two growth factor families. To address this possibility, we analyzed receptor specificity of NRGs by using an engineered cellular system. The activity of isoform-specific but partly overlapping patterns of specificities that collectively activate all eight ligand-stimulatable ErbB dimers was revealed. Specifically, NRG2-alpha [corrected], like NRG1-beta [corrected], emerges as a narrow-specificity ligand, whereas NRG2-beta [corrected] is a pan-ErbB ligand that binds with different affinities to all receptor combinations, including those containing ErbB-1, but excluding homodimers of ErbB-2. The latter protein, however, displayed cooperativity with the direct NRG receptors. Apparently, signaling by all NRGs is funneled through the mitogen-activated protein kinase (MAPK). However, the duration and potency of MAPK activation depend on the identity of the stimulatory ligand-receptor ternary complex. We conclude that the NRG-ErbB network represents a complex and nonredundant machinery developed for fine-tuning of signal transduction.

ErbB2 expression increases the spectrum and potency of ligand-mediated signal transduction through ErbB4

Interleukin 3-dependent murine 32D cells do not detectably express members of the ErbB receptor family and do not proliferate in response to known ligands for these receptors. 32D transfectants were generated expressing human ErbB4 alone (32D.E4) or with ErbB2 (32D.E2/E4). Epidermal growth factor (EGF), neuregulin 1-beta (NRG1-beta), betacellulin (BTC), transforming growth factor-alpha (TGF-alpha), heparin binding-EGF (HB-EGF), and amphiregulin were analyzed for their ability to mediate mitogenesis in these transfectants. 32D.E4 responded mitogenically to NRG1-beta and BTC. Surprisingly, EGF also induced significant DNA synthesis and TGF-alpha was negligibly mitogenic on 32D.E4 cells, whereas HB-EGF and amphiregulin were inactive. Although coexpression of ErbB2 with ErbB4 in 32D.E2/E4 cells did not significantly alter DNA synthesis in response to NRG1-beta or BTC, it greatly enhanced mitogenesis elicited by EGF and TGF-alpha and unmasked the ability of HB-EGF to induce proliferation. EGF-related ligands that exhibited potent mitogenic activity on 32D.E2/E4 cells at low concentrations induced adherence, morphological alterations, and up-regulation of the Mac-1 integrin and FcgammaRII/III at higher concentrations. While 125I-EGF could be specifically crosslinked to both 32D.E4 and 32D.E2/E4 cells, its crosslinking capacity was greatly enhanced in the cotransfected cells. The ability of the various ligands to mediate proliferation and/or adhesion in the two transfectants correlated with their capacity to induce substrate tyrosine phosphorylation and to initiate and sustain activation of mitogen-activated protein kinase. We conclude that the ability of ErbB4 to mediate signal transduction through EGF-like ligands is broader than previously assumed and can be profoundly altered by the concomitant expression of ErbB2.

Epiregulin is a potent pan-ErbB ligand that preferentially activates heterodimeric receptor complexes

The ErbB signaling network consists of four transmembrane receptor tyrosine kinases and more than a dozen ligands sharing an epidermal growth factor (EGF) motif. The multiplicity of ErbB-specific ligands is incompletely understood in terms of signal specificity because all ErbB molecules signal through partially overlapping pathways. Here we addressed the action of epiregulin, a recently isolated ligand of ErbB-1. By employing a set of factor-dependent cell lines engineered to express individual ErbBs or their combinations, we found that epiregulin is the broadest specificity EGF-like ligand so far characterized: not only does it stimulate homodimers of both ErbB-1 and ErbB-4, it also activates all possible heterodimeric ErbB complexes. Consistent with its relaxed selectivity, epiregulin binds the various receptor combinations with an affinity that is approximately 100-fold lower than the affinity of ligands with more stringent selectivity, including EGF. Nevertheless, epiregulin's action upon most receptor combinations transmits a more potent mitogenic signal than does EGF. This remarkable discrepancy between binding affinity and bioactivity is permitted by a mechanism that prevents receptor down-regulation, and results in a weak, but prolonged, state of receptor activation.

Epidermal growth factor and betacellulin mediate signal transduction through co-expressed ErbB2 and ErbB3 receptors

Interleukin-3 (IL-3)-dependent murine 32D cells do not detectably express epidermal growth factor receptors (EGFRs) and do not proliferate in response to EGF, heregulin (HRG) or other known EGF-like ligands. Here, we report that EGF specifically binds to and can be crosslinked to 32D transfectants co-expressing ErbB2 and ErbB3 (32D.E2/E3), but not to transfectants expressing either ErbB2 or ErbB3 individually. [125I]EGF-crosslinked species detected in 32D. E2/E3 cells were displaced by HRG and betacellulin (BTC) but not by other EGF-like ligands that were analyzed. EGF, BTC and HRG also induced receptor tyrosine phosphorylation, activation of downstream signaling molecules and proliferation of 32D.E2/E3 cells. 32D transfectants were also generated which expressed an ErbB3-EGFR chimera alone (32D.E3-E1) or in combination with ErbB2 (32D. E2/E3-E1). While HRG stimulation of 32D.E3-E1 cells resulted in DNA synthesis and receptor phosphorylation, EGF and BTC were inactive. However, EGF and BTC were as effective as HRG in mediating signaling when ErbB2 was co-expressed with the chimera in the 32D.E2/E3-E1 transfectant. These results provide evidence that ErbB2/ErbB3 binding sites for EGF and BTC are formed by a previously undescribed mechanism that requires co-expression of two distinct receptors. Additional data utilizing MDA MB134 human breast carcinoma cells, which naturally express ErbB2 and ErbB3 in the absence of EGFRs, supported the results obtained employing 32D cells and suggest that EGF and BTC may contribute to the progression of carcinomas that co-express ErbB2 and ErbB3.

Possible role of coexpression of CD9 with membrane-anchored heparin-binding EGF-like growth factor and amphiregulin in cultured human keratinocyte growth

CD9 is a protein with 4 transmembrane domains, and functions as a cell surface antigen. We have previously reported that CD9 functions as an up-regulator of membrane-anchored heparin-binding EGF-like growth factor (proHB-EGF) activity, which is a potent mitogen as well as a soluble HB-EGF. Anti-CD9 antibodies can neutralize the juxtacrine activity of proHB-EGF when both CD9 and proHB-EGF are coexpressed. We demonstrated here: (1) the CD9 gene was transcribed and translated in the cultured human keratinocytes; (2) anti-CD9 antibody inhibited the approximately 50% growth of human keratinocytes in culture; (3) CD9 was coprecipitated with proHB-EGF and membrane-anchored amphiregulin (proAR), and (4) the transient coexpression of CD9 with proHB-EGF or proAR in mouse L cells up-regulated their juxtacrine growth factor activities. These results suggest that CD9 would make a heterodimer and/or trimer complex with proHB-EGF and proAR, and might cooperate with proHB-EGF and proAR for human keratinocyte growth in a juxtacrine manner.

Regulation of Bcl-xL expression in human keratinocytes by cell-substratum adhesion and the epidermal growth factor receptor

Cell-substratum adhesion is an essential requirement for survival of human neonatal keratinocytes in vitro. Similarly, activation of the epidermal growth factor receptor (EGF-R) has recently been implicated not only in cell cycle progression but also in survival of normal keratinocytes. The mechanisms by which either cell-substratum adhesion or EGF-R activation protect keratinocytes from programmed cell death are poorly understood. Here we describe that blockade of the EGF-R and inhibition of substratum adhesion share a common downstream event, the down-regulation of the cell death protector Bcl-xL. Expression of Bcl-xL protein was down-regulated during forced suspension culture of keratinocytes, concurrent with large-scale apoptosis. Similarly, EGF-R blockade was accompanied by down-regulation of Bcl-xL steady-state mRNA and protein levels to an extent comparable to that observed in forced suspension culture. However, down-regulation of Bcl-xL expression by EGF-R blockade was not accompanied by apoptosis; in this case, a second signal, generated by passaging, was required to induce rapid and large-scale apoptosis. These findings are consistent with the conclusions that (i) Bcl-xL represents a shared molecular target for signaling through cell-substrate adhesion receptors and the EGF-R, and (ii) reduced levels of Bcl-xL expression through EGF-R blockade lower the tolerance of keratinocytes for cell death signals generated by cellular stress.