Domain-specific interactions between the p185(neu) and epidermal growth factor receptor kinases determine differential signaling outcomes

Focal adhesion kinase-related proline-rich tyrosine kinase 2 and focal adhesion kinase are co-overexpressed in early-stage and invasive ErbB-2-positive breast cancer and cooperate for breast cancer cell tumorigenesis and invasiveness

Early cancer cell migration and invasion of neighboring tissues are mediated by multiple events, including activation of focal adhesion signaling. Key regulators include the focal adhesion kinase (FAK) and FAK-related proline-rich tyrosine kinase 2 (Pyk2), whose distinct functions in cancer progression remain unclear. Here, we compared Pyk2 and FAK expression in breast cancer and their effects on ErbB-2-induced tumorigenesis and the potential therapeutic utility of targeting Pyk2 compared with FAK in preclinical models of breast cancer. Pyk2 is overexpressed in tissues from early and advanced breast cancers and overexpressed with both FAK and epidermal growth factor receptor-2 (ErbB-2) in a subset of breast cancer cases. Down-regulation of Pyk2 in ErbB-2-positive, FAK-proficient, and FAK-deficient cells reduced cell proliferation, which correlated with reduced mitogen-activated protein kinase (MAPK) activity. In contrast, Pyk2 silencing had little impact on cell migration and invasion. In vivo, Pyk2 down-regulation reduced primary tumor growth induced by a metastatic variant of ErbB-2-positive MDA 231 breast cancer cells but had little effect on lung metastases in contrast to FAK down-regulation. Dual reduction of Pyk2 and FAK expression resulted in strong inhibition of both primary tumor growth and lung metastases. Together, these data support the cooperative function of Pyk2 and FAK in breast cancer progression and suggest that dual inhibition of FAK and Pyk2 is an efficient therapeutic approach for targeting invasive breast cancer.

Predicting the efficacy of trastuzumab-based therapy in breast cancer: current standards and future strategies

Breast cancer is the most common female malignancy in many industrialized countries. Approximately one fourth of all women diagnosed with early breast cancer present with tumors that are characterized by erbB2 amplification. While the associated Her-2/neu receptor overexpression results in a high risk of relapse and poor prognosis, these tumors also represent a target for a selective monoclonal antibody therapy with trastuzumab (Herceptin). The combination of trastuzumab with chemotherapy has led to a considerable reduction of recurrences and to a significant reduction in breast cancer mortality both in the adjuvant and metastatic setting. Unfortunately, despite Her-2/neu overexpression, not all patients equally benefit from trastuzumab treatment, and almost all women with metastatic breast cancer eventually progress during antibody therapy. Moreover, trastuzumab is burdened with cardiotoxicity, thus increasing the risk of symptomatic congestive heart failure. In addition, the marginal costs for a 1 year therapy of trastuzumab-based therapy, which is currently considered to be the most effective treatment regimen in the adjuvant setting, may amount for up to US$ 40.000. Testing for erbB2 oncogene amplification by fluorescence in situ hybridization (FISH) and chromogenic in situ hybridization (CISH), respectively, and staining for Her-2/neu receptor overexpression by immunohistochemistry (IHC) represent the current standard for determining patient eligibility for trastuzumab-based therapy. However, while the negative predictive value of these assays for predicting the absence of benefit from trastuzumab-based therapy is sufficiently high, their positive predictive value remains insufficient, i.e. only a proportion of patients selected by these tests substantially benefit from trastuzumab-containing regimen. Accordingly, over the last years a number of biomarkers have been evaluated in their potential to predict response to trastuzumab-based therapies. These include markers auf activation of Her-2/neu (e.g., tyrosine phosphorylated Her-2/neu in tissue and cleaved Her-2/neu extracellular domain in serum) and its dimerization partners (e.g., EGFR), respectively, but also components of Her-2/neu-induced downstream signaling pathways that are crucial for the growth inhibitory effects of trastuzumab (e.g., PTEN and PI3K). Other parameters, such as topoisomerase-II alpha and c-myc co-amplifications, have also been identified as potentially useful predictors of response to trastuzumab-based chemotherapy regimen. While the benefit of these predictive biomarkers in the metastatic setting is currently explored, their usefulness in the adjuvant setting is still largely unknown. It is, however, undisputable that, within the group of Her-2/neu overexpressing tumors, further response predictors are needed in order to minimize trastuzumab-associated side effects, and to reduce the considerable societal costs that are associated with trastuzumab-based treatment regimen.

In vivo therapeutic synergism of anti-epidermal growth factor receptor and anti-HER2 monoclonal antibodies against pancreatic carcinomas

PURPOSE

Pancreatic carcinoma is highly resistant to therapy. Epidermal growth factor receptor (EGFR) and HER2 have been reported to be both dysregulated in this cancer. To evaluate the in vivo effect of binding both EGFR and HER2 with two therapeutic humanized monoclonal antibodies (mAb), we treated human pancreatic carcinoma xenografts, expressing high EGFR and low HER2 levels.

EXPERIMENTAL DESIGN

Nude mice, bearing xenografts of BxPC-3 or MiaPaCa-2 human pancreatic carcinoma cell lines, were injected twice weekly for 4 weeks with different doses of anti-EGFR (matuzumab) and anti-HER2 (trastuzumab) mAbs either alone or in combination. The effect of the two mAbs, on HER receptor phosphorylation, was also studied in vitro by Western blot analysis.

RESULTS

The combined mAb treatment significantly inhibited tumor progression of the BxPC-3 xenografts compared with single mAb injection (P = 0.006) or no treatment (P = 0.0004) and specifically induced some complete remissions. The two mAbs had more antitumor effect than 4-fold greater doses of each mAb. The significant synergistic effect of the two mAbs was confirmed on the MiaPaCa-2 xenograft and on another type of carcinoma, SK-OV-3 ovarian carcinoma xenografts. In vitro, the cooperative effect of the two mAbs was associated with a decrease in EGFR and HER2 receptor phosphorylation.

CONCLUSIONS

Anti-HER2 mAb has a synergistic therapeutic effect when combined with an anti-EGFR mAb on pancreatic carcinomas with low HER2 expression. These observations may open the way to the use of these two mAbs in a large panel of carcinomas expressing different levels of the two HER receptors.

Compensatory ErbB3/c-Src signaling enhances carcinoma cell survival to ionizing radiation

EGFR and ErbB2 are two members of the ErbB family of receptor Tyr Kinases identified as therapeutic targets for treating carcinomas. Breast carcinoma cells express different complements and variable proportions of ErbB receptor Tyr kinases, which activate unique and redundant signaling cascades that are essential for cell survival. Previously it was shown that a COOH-terminal truncation mutant of the EGFR (EGFR-CD533) blocks EGFR dependent signals and radiosensitizes breast carcinoma cells. In this study the effects of EGFR-CD533 and an analogous truncation mutant of ErbB2 (ErbB2-CD572) on ErbB receptor family dimerization and signaling are further investigated. Using adenoviral vectors in breast carcinoma cell lines with variable ErbB expression profiles, we demonstrate different effects for each deletion mutant. EGFR-CD533 blocks ligand stimulation of EGFR, ErbB2, and ErbB4, but is associated with a compensatory Tyr kinase activity resulting in phosphorylation of ErbB3. In contrast, ErbB2-CD572 produces a weaker, non-specific pattern of ErbB receptor family inhibition, based upon the ErbB expression pattern of the cell type. Investigation of the compensatory Tyr kinase activity associated with EGFR-CD533 expression identified an ErbB3/c-Src signaling pathway that regulates expression of anti-apoptotic Bcl family proteins. This signaling is active in the T47D cell line, which inherently over-express ErbB3, absent in MDA-MB231 cells, which have low ErbB3 expression levels, and is restored in a MDA-MB231 cell line engineered to over-express ErbB3. Furthermore we demonstrate that ErbB3/c-Src signaling is radio-protective, and that its elimination through pharmacologic inhibition of c-Src enhances radiation-induced apoptosis. In summary, these studies identify a novel ErbB3/c-Src survival signal and point to ErbB3 expression levels as an important variable in therapeutic targeting of ErbB receptors in breast carcinoma cells.

Her-2/neu and EGFR tyrosine kinase activation predict the efficacy of trastuzumab-based therapy in patients with metastatic breast cancer

Her-2/neu overexpression in human breast cancer leads to an aggressive biological behavior and poor prognosis. Although the anti-Her-2/neu antibody trastuzumab (Herceptin(R)) has become a valuable therapeutic option for patients with Her-2/neu-overexpressing breast cancer, many patients do not benefit from this therapy. To evaluate the effect of receptor activation on tumor response, we have investigated the phosphorylation status of Her-2/neu and EGFR in 46 Her-2/neu-overexpressing tumor samples from trastuzumab-treated metastatic breast cancer patients by immunohistochemistry. Activated (p)tyr-1248 Her-2/neu was detected in 9 of 46 breast cancers (20%), and activated (p)tyr-845 and (p)tyr-1173 EGFR were both present in 6 tumors (13%) while EGFR was present in 16 cases (35%). ptyr-1248 Her-2/neu showed a trend to correlate with increased response to trastuzumab (p = 0.063), while ptyr-845, ptyr-1173 EGFR and EGFR did not. The presence of ptyr-1248 Her-2/neu and ptyr-845 or ptyr-1173 EGFR, however, was a strong predictor of both response to trastuzumab-based treatment (OR = 8.0, p = 0.021 and OR = 8.0, p = 0.021) and clinical benefit (OR = 5.47, p = 0.041 and OR = 6.22, p = 0.028 multivariate logistic regression analysis). Furthermore, ptyr-845 EGFR and ptyr-1248 Her-2/neu were both independent predictors of progression-free survival (RR = 0.21, p = 0.01 and RR = 0.45, p = 0.026, multivariate analysis). Patients with ptyr-845 EGFR positive tumors also tended toward increased overall survival (RR = 0.17, p = 0.082). Taken together, we have demonstrated that the determination of activated EGFR improves the utility of ptyr-1248 Her-2/neu staining in predicting the clinical outcome of patients undergoing trastuzumab treatment. We hypothesize that the activation state of both Her-2/neu and EGFR are key determinants for trastuzumab efficacy.

Differential localization of ErbB receptor ensembles influences their signaling in hippocampal neurons

Our studies indicate that ErbB complexes participate in both survival and synaptic plasticity signals of hippocampal neurons but in a manner that depends on the subcellular localization of the receptor ensembles. Using dissociated hippocampal cultures, we found that neurons, rather than glial cells, are the primary targets of ErbB receptor ligands such as epidermal growth factor and heregulin. Further investigation demonstrated that ErbB receptors distribute differentially in hippocampal neurons with the epidermal growth factor receptor confined to neural cell bodies and the p185(c-neu) and ErbB4 receptors distributed to both neural soma and neurites. Activation of ErbB receptor and downstream signaling molecules were observed in neurites only after heregulin stimulation. The receptor complex which mediated neurite located signals was the p185(c-neu)/ErbB4 heterodimer. Colocalization of p185(c-neu), but not epidermal growth factor receptor, with postsynaptic density protein 95 suggests that the heregulin signaling contributes to synapse specific activities. However, the epidermal growth factor receptor complex mediates physiological survival signals, as neuronal survival was enhanced by epidermal growth factor, rather than heregulin. Collectively, these studies indicate that different ErbB ensembles localize to different locations on the neuron to mediate distinct signals and functions.

Serum EGFR levels and efficacy of trastuzumab-based therapy in patients with metastatic breast cancer

The antibody trastuzumab inhibits signal transduction in Her-2/neu overexpressing human breast cancer. However, the activation of co-expressed EGFR has also been show to additionally modulate the anti-tumoural effects of this drug. Similar to Her-2/neu, the extra cellular binding region of EGFR is believed to be proteolytically released from the cell surface upon receptor activation and can be detected in patients' serum (sEGFR). Considering the biological significance of an interaction between EGFR and Her-2/neu signalling in other human malignancies, we have investigated if trastuzumab treatment would affect sEGFR in 33 patients with Her-2/neu overexpressing metastatic breast cancer. We detected EGFR expression in 33% of Her-2/neu overexpressing breast tumours. In contrast to serum Her-2/neu (ECD) levels, which were correlated with the degree of Her-2/neu expression (P=0.048, Mann-Whitney test), we did not detect significant differences between sEGFR serum levels in EGFR expressing or non-expressing tumours. Furthermore, sEGFR serum levels were not correlated with clinical parameters such as response or clinical benefit rates, and no association was found between increased sEGFR levels and progression-free survival or overall survival. While we have previously observed a selective and significant decrease of ECD levels in patients who derived a clinical benefit from trastuzumab treatment during the first weeks of treatment, we were unable to find similar alterations in sEGFR concentrations. We therefore conclude that the measurement of systemic sEGFR levels in addition to ECD serum concentrations do not allow the prediction of clinical course of trastuzumab-treated patients more accurately.

ErbB receptors: directing key signaling networks throughout life

The epidermal growth factor (EGF)-related peptides bind the ErbB receptors, inducing the formation of different homo- and heterodimers. Receptor dimerization promotes activation of the intrinsic kinase, leading to phosphorylation of specific tyrosines located in the ErbB's cytoplasmic region. These phosphorylated residues serve as docking sites for a variety of signaling molecules whose recruitment stimulates intracellular signaling cascades, which ultimately control diverse genetic programs. Particular ligand-receptor complexes have essential roles in embryonic development as well as in the adult. Finally, ErbB receptors are being pursued as therapeutic targets because aberrant ErbB activity has been observed in many human cancers. In this review, we discuss these data in more detail, illustrating the importance of tightly regulated ErbB signaling throughout life.

The role of ErbB2 signaling in the onset of terminal differentiation of oligodendrocytes in vivo

The knock-out analyses of neuregulin and its receptors have indicated that they play essential roles in Schwann cell development. However, the role they play in oligodendrocyte development in vivo has remained unclear, because such knock-out animals die before CNS myelination begins. We examined the role of neuregulin signaling in the CNS by generating transgenic mice that express a dominant-negative mutant of the ErbB2 receptor among oligodendrocytes, using an MBP promoter. The transgenic mice exhibited widespread hypomyelination, resulting from a reduction in oligodendrocyte differentiation. The number of progenitors was conversely increased in the transgenic mice. We report that a reduction in oligodendrocyte differentiation is attributed in part to apoptosis of oligodendrocyte progenitors as they exit the cell cycle. A significant reduction in the number of p27+ oligodendrocyte precursors in the transgenic mice supports this conclusion. Taken together, these data suggest that for oligodendrocyte progenitors, ErbB2 signaling plays a role in governing a properly timed exit from the cell cycle during development into myelinating oligodendrocytes.

ErbB-2 signaling is involved in regulating PSA secretion in androgen-independent human prostate cancer LNCaP C-81 cells

The expression and secretion of prostate-specific antigen (PSA) are regulated by androgens in normal prostate secretory epithelial cells. In prostate cancer patients, the serum PSA level is usually elevated and cancer cells are initially responsive to androgens. However, those cancer cells become androgen-independent after androgen ablation therapy. In hormone-refractory cancer patients, even in an androgen-deprived environment, the circulation level of PSA rebounds and is constitutively elevated through a yet unknown mechanism. Tyrosine phosphorylation of ErbB-2 is involved in regulating the androgen-responsive phenotype of prostate cancer cells, and it is at least partly regulated by the cellular form of prostatic acid phosphatase (PAcP), a prostate-unique protein tyrosine phosphatase. We investigated the ErbB-2 signal pathway in androgen-independent PSA secretion. LNCaP C-81 cells, which are androgen-independent LNCaP cells lacking endogenous PAcP expression with a hypertyrosine phosphorylated ErbB-2, secreted a higher level of PSA in conditioned media than did androgen-sensitive LNCaP C-33 parental cells. A restored expression of cellular PAcP in C-81 cells was concurrent with a decrease in tyrophosphorylation of ErbB-2 and reduction of PSA secretion. Moreover, transient transfection of C-33 cells with the wild-type ErbB-2 or a constitutively active mutant of MEK1 cDNA resulted in an increased level of secreted PSA. The elevation of secreted PSA level by the forced expression of ErbB-2 was inhibited by an MEK inhibitor, PD98059. In C-81 cells, the expression of a dominant negative mutant of ErbB-2 reduced the secreted level of PSA. The inhibition of ErbB-2 or mitogen-activated protein (MAP) kinases by specific inhibitors AG879, AG825, or PD98059 led to a decrease in PSA secretion. Taken together, our data clearly indicate that the ErbB-2 signal pathway via MAP kinases (ERK1/2) is involved in regulating the secretion of PSA by androgen-independent human prostate cancer LNCaP C-81 cells in an androgen-depleted environment.

The crystal structure of a truncated ErbB2 ectodomain reveals an active conformation, poised to interact with other ErbB receptors

ErbB2 does not bind ligand, yet appears to be the major signaling partner for other ErbB receptors by forming heteromeric complexes with ErbB1, ErbB3, or ErbB4. The crystal structure of residues 1-509 of ErbB2 at 2.5 A resolution reveals an activated conformation similar to that of the EGFR when complexed with ligand and very different from that seen in the unactivated forms of ErbB3 or EGFR. The structure explains the inability of ErbB2 to bind known ligands and suggests why ErbB2 fails to form homodimers. Together, the data suggest a model in which ErbB2 is already in the activated conformation and ready to interact with other ligand-activated ErbB receptors.

Identification of a region within the ErbB2/HER2 intracellular domain that is necessary for ligand-independent association

Ligand-independent ErbB2 activation occurs principally by two distinct mechanisms: overexpression and mutation. Overexpression of ErbB2 at the plasma membrane drives receptor self-association in a concentration-dependent manner, which in turn leads to constitutive receptor activation. Subsets of human breast cancers contain a molecular alteration that leads to erbB2 gene amplification and subsequent protein overexpression. Although not recognized to occur in human cancers, mutation can also lead to increased ErbB2 association. A well characterized mutant of the rodent ortholog neu involves substitution of glutamate for valine within the transmembrane domain. In each case, a number of explanations have been proposed to explain the resulting ErbB2 activation. These include stabilization of receptor oligomers, release of negative constraints, and altered receptor conformations. Here we define a short amino acid segment comprising amino acids 966-968 in the intracellular domain that seemingly disrupts receptor-receptor association that is driven either by overexpression or mutation in the transmembrane region. Because of the hydrophobic nature of these amino acids (VVI), we propose that alteration of this segment likely results in a global conformational change in an area that has been proposed previously to be a dimerization motif for ErbB homomeric association.

Herstatin, an autoinhibitor of the human epidermal growth factor receptor 2 tyrosine kinase, modulates epidermal growth factor signaling pathways resulting in growth arrest

Herstatin is an autoinhibitor of the ErbB family consisting of subdomains I and II of the human epidermal growth factor receptor 2 (ErbB-2) extracellular domain and a novel C-terminal domain encoded by an intron. Herstatin binds to human epidermal growth factor receptor 2 and to the epidermal growth factor receptor (EGFR), blocking receptor oligomerization and tyrosine phosphorylation. In this study, we characterized several early steps in EGFR activation and investigated downstream signaling events induced by epidermal growth factor (EGF) and by transforming growth factor alpha (TGF-alpha) in NIH3T3 cell lines expressing EGFR with and without herstatin. Herstatin expression decreased EGF-induced EGFR tyrosine phosphorylation and delayed receptor down-regulation despite receptor occupancy by ligand with normal binding affinity. Akt stimulation by EGF and TGF-alpha, but not by fibroblast growth factor 2, was almost completely blocked in the presence of herstatin. Surprisingly, EGF and TGF-alpha induced full activation of MAPK in duration and intensity and stimulated association of the EGFR with Shc and Grb2. Although MAPK was fully stimulated, herstatin expression prevented TGF-alpha-induced DNA synthesis and EGF-induced proliferation. The herstatin-mediated uncoupling of MAPK from Akt activation was also observed in Chinese hamster ovary cells co-transfected with EGFR and herstatin. These findings show that herstatin expression alters EGF and TGF-alpha signaling profiles, culminating in inhibition of proliferation.

Truncated ErbB2 receptor enhances ErbB1 signaling and induces reversible, ERK-independent loss of epithelial morphology

Shedding of the extracellular domain of the ErbB2 tyrosine kinase receptor and expression of the remaining NH(2)-terminally truncated ErbB2 correlates with lymph node metastases and adverse outcome in human breast cancer. To study the possible signaling from such a truncated receptor, MCF-7 human breast cancer cells expressing NH(2)-terminally truncated ErbB2 (DeltaNErbB2) were compared with cells overexpressing wild-type ErbB2. Expression of DeltaNErbB2 in MCF-7 cells resulted in sustained activation of extracellular signal-regulated kinases (ERK) 1/2, extensive loss of the epithelial morphology, appearance of vesicles and long protrusions as well as pronounced scattering of the cells. Similar alterations were observed upon ErbB2 overexpression but at much lower levels. Employing cell clones with inducible expression of DeltaNErbB2, it was revealed that the morphological changes were fully reversible and depended on continuous expression of DeltaNErbB2 but not on the activation of the ERK1/2 pathway. Interestingly, the expression of DeltaNErbB2 resulted also in the increased expression and phosphorylation of ErbB1 as well as in the prolonged ligand-induced activation of the ErbB1 signaling pathway. In conclusion, constitutive signaling upon expression of the truncated ErbB2 receptor in human breast cancer cells promotes morphological changes indicative of a more motile and aggressive phenotype.

The tyrosine phosphatase SHP-2 is required for mediating phosphatidylinositol 3-kinase/Akt activation by growth factors

SHP-2 is a ubiquitously expressed non-transmembrane tyrosine phosphatase with two SH2 domains. Multiple reverse-genetic studies have indicated that SHP-2 is a required component for organ and animal development. SHP-2 wild-type and homozygous mutant mouse fibroblast cells in which the N-terminal SH2 domain was target-deleted were used to examine the function of SHP-2 in regulating Phosphatidylinositol 3-Kinase (PI3K) activation by growth factors. In addition, SHP-2 and various mutants were introduced into human glioblastoma cells as well as SHP-2(-/-) mouse fibroblasts. We found that EGF stimulation and EGFR oncoprotein (DeltaEGFR) expression independently induced the co-immunoprecipitation of the p85 subunit of PI3K with SHP-2. Targeted deletion of the N-terminal SH2 domain of SHP-2 severely impaired PDGF- and IGF-induced Akt phosphorylation. Ectopic expression of SHP-2 in U87MG gliobastoma cells elevated EGF-induced Akt phosphorylation, and the effect was abolished by mutation of its N-terminal SH2 domain. Likewise, the reconstitution of SHP-2 expression in the SHP-2(-/-) cells enhanced Akt phosphorylation induced by EGF while rescuing that induced by PDGF and IGF. Further lipid kinase activity assays confirmed that SHP-2 modulation of Akt phosphorylation correlated with its regulation of PI3K activation. Based on these results, we conclude that SHP-2 is required for mediating PI3K/Akt activation, and the N-terminal SH2 domain is critically important for a "positive" role of SHP-2 in regulating PI3K pathway activation.

Expression of herstatin, an autoinhibitor of HER-2/neu, inhibits transactivation of HER-3 by HER-2 and blocks EGF activation of the EGF receptor

The four members of the EGF receptor family are capable of homomeric as well as heteromeric interactions. HER-2/neu (erbB-2) dominates as the preferred coreceptor that amplifies mitogenic signaling. An alternative HER-2/neu product, herstatin, consists of a segment of the ectodomain of p185HER-2 and an intron-encoded C-terminus. Recombinant herstatin was found to bind with nM affinity and inhibit p185HER-2. To further examine the impact on receptor activity, herstatin was expressed with various receptor tyrosine kinases. In CHO cells that overexpressed HER-2, herstatin caused a sevenfold inhibition of colony formation that corresponded to a reduction in the tyrosine phosphorylation of p185HER-2. Herstatin also prevented HER-2 mediated transactivation of the kinase impaired HER-3 as reflected in transphosphorylation of HER-3 and heteromers between HER-2 and HER-3. In EGF receptor-overexpressing cells, EGF induction of receptor dimerization and tyrosine phosphorylation were reduced more than 90%, and receptor down-regulation as well as colony formation were also suppressed by coexpression with herstatin. Inhibition was selective for the EGF receptor family since herstatin expression did not reduce tyrosine phosphorylation mediated by the FGF receptor-2 or by insulin-like growth factor -1. Herstatin bound to the EGF receptor as well as to p185HER-2 in pull-down assays suggesting that complex formation may be involved in receptor inhibition. Our findings indicate that herstatin has the capability to negatively regulate combinations of interactions between group I receptor tyrosine kinases that confer synergistic growth signals.

The role of distinct p185neu extracellular subdomains for dimerization with the epidermal growth factor (EGF) receptor and EGF-mediated signaling

The extracellular domain of p185(c-neu) can be viewed as a complex structure of four subdomains, two of which are cysteine-rich subdomains. We have investigated the contribution of these distinct p185(c-neu) extracellular subdomains to p185/epidermal growth factor receptor (EGFR) heteromer formation and EGF-induced heteromeric signaling. Our studies indicate that at least two separate p185 subdomains, a region spanning subdomains I and II and subdomain IV are involved in association of p185 with the EGFR. We also demonstrated that subdomain IV reduced the heteromeric signaling and transforming activities induced by EGF after associating with EGFR. When 126 aa were deleted from subdomain IV, this small subdomain IV-derived fragment could still lead to heterodimers with EGFR and suppress EGF-induced mitogen-activated protein kinase activation and subsequent transformation abilities. These data provide information about trans-inhibitory mechanisms of mutant p185 species and also indicate that both the entire and a part of subdomain IV may represent a therapeutic target for erbB-overexpressing tumors. Finally, these studies define a basic feature of receptor-receptor associations that are determined by cystine-knot containing subdomains.

Biliary cancer growth factor pathways, cyclo-oxygenase-2 and potential therapeutic strategies

Cholangiocarcinoma is a hepatic biliary cancer of high morbidity and mortality, whose molecular pathogenesis is unknown. However, there is increasing evidence to suggest that alterations in selected growth factor pathways, including an overexpression of the growth factor receptor tyrosine kinases c-ErbB-2/c-Neu and c-Met, together with possible aberrant autocrine expression of hepatocyte growth factor/scatter factor, the ligand for c-Met, may be playing important roles associated with the development of cholangiocarcinoma in both the human liver and in the furan rat model of cholangiocarcinogenesis. Cyclo-oxygenase-2, whose regulation has been experimentally related to c-ErbB-2/c-Neu as well as to hepatocyte growth factor/scatter factor, and which has been demonstrated to be overexpressed in other cancers of the gastrointestinal tract, has also been observed in preliminary studies to be upregulated in human biliary cancers and in cholangiocarcinoma induced in the furan rat model. Moreover, new data from our laboratory have demonstrated the cyclo-oxygenase-2 inhibitor NS-398 to produce a significant dose-dependent growth inhibition of rat cholangiocarcinoma cells in vitro, as well as to suppress anchorage-independent growth of these cells in soft agar. Based on the data reviewed, we propose that the selective therapeutic targeting of aberrant growth factor receptor tyrosine kinase signaling and of cyclo-oxygenase-2, alone or in combination, has potential to become a useful new approach for the treatment and/or chemoprevention of cholangiocarcinoma. We further propose that the furan rat model may serve as a powerful preclinical model for testing therapeutic and chemopreventative strategies that selectively target c-ErbB-2/c-Neu, cyclo-oxygenase-2, and/or autocrine hepatocyte growth factor/c-Met, aberrantly expressed in cholangiocarcinogenesis.

Inhibition of EGFR-mediated phosphoinositide-3-OH kinase (PI3-K) signaling and glioblastoma phenotype by signal-regulatory proteins (SIRPs)

Several growth factors and cytokines, including EGF, are known to induce tyrosine phosphorylation of Signal Regulatory Proteins (SIRPs). Consistent with the idea that increased phosphorylation activates SIRP function, we overexpressed human SIRPalpha1 in U87MG glioblastoma cells in order to examine how SIRPalpha1 modulates EGFR signaling pathways. Endogenous EGFR proteins are overexpressed in U87MG cells and these cells exhibit survival and motility phenotypes that are influenced by EGFR kinase activity. Overexpression of the SIRPalpha1 cDNA diminished EGF-induced phosphoinositide-3-OH kinase (PI3-K) activation in U87MG cells. Reduced EGF-stimulated activation of PI3-K was mediated by interactions between carboxyl terminus of SIRPalpha1 and the Src homology-2 (SH2)-containing phosphotyrosine phosphatase, SHP2. SIRPalpha1 overexpression also reduced the EGF-induced association between SHP2 and the p85 regulatory subunit of PI3-K. Inhibition of transformation and enhanced apoptosis following gamma-irradiation were observed in SIRPalpha1-overexpressing U87MG cells, and enhanced apoptosis was associated with reduced levels of bcl-xL protein. Furthermore, SIRPalpha1-overexpressing U87MG cells displayed reduced cell migration and cell spreading that was mediated by association between SIRPalpha1 and SHP2. However, SIRPalpha1-overexpressing U87MG clonal derivatives exhibited no differences in cell growth or levels of mitogen-activated protein kinase (MAPK) activation. These data reveal a pathway that negatively regulates EGFR-induced PI3-K activation in glioblastoma cells and involves interactions between SHP2 and tyrosine phosphorylated SIRPalpha1. These results also suggest that negative regulation of PI3-K pathway activation by the SIRP family of transmembrane receptors may diminish EGFR-mediated motility and survival phenotypes that contribute to transformation of glioblastoma cells. Oncogene (2000) 19, 3999 - 4010.

The EGF receptor: a nexus for trafficking and signaling

Ligand binding to the EGF receptor initiates both the activation of mitogenic signal transduction pathways plus trafficking events that relocalize the receptor on the cell surface and within intracellular compartments. The trafficking compartments include caveolae, clathrin-coated pits, and various endosome populations prior to receptor degradation in lysosomes. Evidence is presented that distinct signaling pathways are initiated from these different compartments. These include the Ras/MAP kinase cascade and the PLC-dependent hydrolysis of PI-4,5 P(2). Multiple tyrosine kinase substrates that facilitate EGF receptor trafficking between these various compartments, as well as the participation of phosphoinositides and Ras-like G proteins in the trafficking pathway are also described.

Complexity of signal transduction mediated by ErbB2: clues to the potential of receptor-targeted cancer therapy

The erbB2 oncogene belongs to the type I trans-membrane tyrosine kinase family of receptors. Its medical importance stems from its widespread over-expression in breast cancer. This review will focus on the signal transduction through this protein, and explains how the overexpression of erbB2 may result in poor prognosis of breast cancer, and finally it will summerize our current understanding about the therapeutic potential of receptor-targeted therapy in breast cancer. ErbB2 does not have any known ligand which is able to bind to it with high affinity. However the kinase activity of erbB2 can be activated without any ligand, if it is overexpressed, and by heteroassociation with other members of the erbB family (erbB1 or epidermal growth factor receptor, erbB3 and erbB4). This interaction substantially increases the efficiency and diversity of signal transduction through these receptor complexes. In addition, erbB2 forms large scale receptor clusters containing hundreds of proteins. These receptor islands may take part in recruiting cytosolic factors which relay the signal towards the nucleus or the cytoplasm. Overexpression of erbB2 was linked to higher transforming activity, increased metastatic potential, angiogenesis and drug resistence of breast tumor in laboratory experiments. As a corollary of these properties, erbB2 amplification is generally thought to be associated with a poor prognosis in breast cancer patients. These early findings lead to the development of antibodies that down-regulate erbB2. Such a therapeutic approach has already been found effective in experimental tumor models and in clinical trials as well. Further understanding of the importance of erbB2 and growth factor receptors in the transformation of normal cells to malignant ones may once give us a chance to cure erbB2 over-expressing breast cancer.

Sustained mitogen-activated protein kinase activation is induced by transforming erbB receptor complexes

We used a genetic approach to characterize features of mitogen-activated protein kinase (MAPK) activation occurring as a consequence of expression of distinct erbB receptor combinations in transformed human cells. Kinase-deficient erbB proteins reduced epidermal growth factor (EGF)-induced tyrosine phosphorylation of endogenous Shc proteins and also reduced immediate and sustained EGF-induced ERK MAPK activities in human glioblastoma cells, although basal ERK MAPK activities were unaffected. Basal and EGF-induced JNK and p38 MAPK kinase activities were equivalent in parental cancer cells and EGFR-inhibited subclones. When ectopically overexpressed in murine fibroblasts and human glioblastoma cells, a constitutively activated human EGF receptor oncoprotein (deltaEGFR) induced EGF-independent elevation of basal ERK MAPK activity. Basal JNK MAPK kinase activity was also specifically induced by deltaEGFR, which correlated with increased phosphorylation of a 54-kDa JNK2 protein observed in deltaEGFR-containing cells. The JNK activities in response to DNA damage were comparably increased in cells containing wildtype EGFR or deltaEGFR. Consistent with the notion that transforming erbB complexes induce sustained and unregulated MAPK activities, coexpression of p185(neu) and EGFR proteins to levels sufficient to transform murine fibroblasts also resulted in prolonged EGF-induced ERK in vitro kinase activation. Transforming erbB complexes, including EGFR homodimers, deltaEGFR homodimers, and p185(neu)/EGFR heterodimers, appear to induce sustained, unattenuated activation of MAPK activities that may contribute to increased transformation and resistance to apoptosis in primary human glioblastoma cells.

Mutagenesis reveals a role for epidermal growth factor receptor extracellular subdomain IV in ligand binding

The extracellular domain of the epidermal growth factor (EGF) receptor (EGFR) comprises four subdomains (I-IV) and mediates binding of several different polypeptide ligands, including EGF, transforming growth factor-alpha, and heparin-binding EGF. Previous studies have predominantly implicated subdomain III in ligand binding. To investigate a possible role for sequences in subdomain IV, we constructed several mutant EGFRs in which clusters of charged or aromatic amino acids were replaced with alanine. Analysis of stably transfected Chinese hamster ovary cells expressing mutant EGFRs confirmed that they were present on the cell surface at levels approaching that of the wild-type receptor. Although tyrosine phosphorylation of most mutants was markedly induced by EGF, a cluster mutation (mt25) containing four alanine substitutions in the span of residues 521-527 failed to respond. EGF-induced tyrosine phosphorylation of an alternative mutant (DeltaEN) with amino acids 518-589 deleted was also greatly diminished. Larger doses of EGF or heparin-binding EGF induced only weak tyrosine phosphorylation of mt25, whereas the response to transforming growth factor-alpha was undetectable. These results suggest that mt25 might be defective with respect to either ligand binding or receptor dimerization. Quantitative analyses showed that binding of (125)I-EGF to mt25 and DeltaEN was reduced to near background levels, whereas binding of EGF to other cluster mutants was reduced 60-70% compared with wild-type levels. Among the mutants, only mt25 and DeltaEN failed to form homodimers or to transphosphorylate HER2/Neu in response to EGF treatment. Collectively, our results are the first to provide direct evidence that discrete subdomain IV residues are required for normal binding of EGF family ligands. Significantly, they were obtained with the full-length receptor in vivo, rather than a soluble truncated receptor, which has been frequently used for structure/function studies of the EGFR extracellular region.