ErbB3 is involved in activation of phosphatidylinositol 3-kinase by epidermal growth factor

RNA interference against hepatic epidermal growth factor receptor has suppressive effects on liver regeneration in rats

Liver regeneration after a two-thirds partial hepatectomy (PHx) is a complex process requiring interaction and cooperation of many growth factors and cytokines and cross talk between multiple pathways. Along with hepatocyte growth factor and its receptor MET (HGF-MET), the epidermal growth factor receptor (EGFR) signaling pathway is activated within 60 minutes after PHx. To investigate the role of EGFR in liver regeneration, we used two EGFR-specific short hairpin silencing RNAs to inhibit EGFR expression in regenerating normal rat liver. Suppression of EGFR mRNA and protein was evident in treated rats. There was also a demonstrable decrease but not complete elimination of bromo-deoxyuridine incorporation and mitoses at 24 hours after PHx. In addition, we observed up-regulation of MET and Src as well as activation of the ErbB-3-ErbB-2-PI3K-Akt pathway and down-regulation of STAT 3, cyclin D1, cyclin E1, p21, and C/EBP beta. The decrease in the ratio of C/EBP alpha to C/EBP beta known to occur after PHx was offset in shEGFR-treated rats. Despite suppression of hepatocyte proliferation lasting into day 3 after PHx, liver weight restoration occurred. Interestingly, hepatocytes in shEGFR-treated rats were considerably larger when compared with ScrRNA-treated controls. The data indicate that although the MET and EGFR pathways are similar, the contributions made by MET and EGFR are unique and are not compensated by each other or other cytokines.

Phosphatidylinositol 3-kinase (PI3K) pathway activation in bladder cancer

The phosphatidylinositol 3-kinase (PI3K) pathway is a critical signal transduction pathway that regulates multiple cellular functions. Aberrant activation of this pathway has been identified in a wide range of cancers. Several pathway components including AKT, PI3K and mTOR represent potential therapeutic targets and many small molecule inhibitors are in development or early clinical trials. The complex regulation of the pathway, together with the multiple mechanisms by which it can be activated, make this a highly challenging pathway to target. For successful inhibition, detailed molecular information on individual tumours will be required and it is already clear that different tumour types show distinct combinations of alterations. Recent results have identified alterations in pathway components PIK3CA, PTEN, AKT1 and TSC1 in bladder cancer, some of which are significantly related to tumour phenotype and clinical behaviour. Co-existence of alterations to several PI3K pathway genes in some bladder tumours indicates that these proteins may have functions that are not related solely to the known canonical pathway.

An activated ErbB3/NRG1 autocrine loop supports in vivo proliferation in ovarian cancer cells

Ovarian cancer is a leading cause of death from gynecologic malignancies. Treatment for advanced-stage disease remains limited and, to date, targeted therapies have been incompletely explored. By systematically suppressing each human tyrosine kinase in ovarian cancer cell lines by RNAi, we found that an autocrine signal-transducing loop involving NRG1 and activated ErbB3 operates in a subset of primary ovarian cancers and ovarian cancer cell lines. Perturbation of this circuit with ErbB3-directed RNAi decreased cell growth in three-dimensional culture and resulted in decreased disease progression and prolonged survival in a xenograft mouse model of ovarian cancer. Furthermore, a monoclonal ErbB3-directed antibody (MM-121) also significantly inhibited tumor growth in vivo. These findings identify ErbB3 as a potential therapeutic target in ovarian cancer.

An ErbB3 antibody, MM-121, is active in cancers with ligand-dependent activation

ErbB3 is a critical activator of phosphoinositide 3-kinase (PI3K) signaling in epidermal growth factor receptor (EGFR; ErbB1), ErbB2 [human epidermal growth factor receptor 2 (HER2)], and [hepatocyte growth factor receptor (MET)] addicted cancers, and reactivation of ErbB3 is a prominent method for cancers to become resistant to ErbB inhibitors. In this study, we evaluated the in vivo efficacy of a therapeutic anti-ErbB3 antibody, MM-121. We found that MM-121 effectively blocked ligand-dependent activation of ErbB3 induced by either EGFR, HER2, or MET. Assessment of several cancer cell lines revealed that MM-121 reduced basal ErbB3 phosphorylation most effectively in cancers possessing ligand-dependent activation of ErbB3. In those cancers, MM-121 treatment led to decreased ErbB3 phosphorylation and, in some instances, decreased ErbB3 expression. The efficacy of single-agent MM-121 was also examined in xenograft models. A machine learning algorithm found that MM-121 was most effective against xenografts with evidence of ligand-dependent activation of ErbB3. We subsequently investigated whether MM-121 treatment could abrogate resistance to anti-EGFR therapies by preventing reactivation of ErbB3. We observed that an EGFR mutant lung cancer cell line (HCC827), made resistant to gefitinib by exogenous heregulin, was resensitized by MM-121. In addition, we found that a de novo lung cancer mouse model induced by EGFR T790M-L858R rapidly became resistant to cetuximab. Resistance was associated with an increase in heregulin expression and ErbB3 activation. However, concomitant cetuximab treatment with MM-121 blocked reactivation of ErbB3 and resulted in a sustained and durable response. Thus, these results suggest that targeting ErbB3 with MM-121 can be an effective therapeutic strategy for cancers with ligand-dependent activation of ErbB3.

HER3 comes of age: new insights into its functions and role in signaling, tumor biology, and cancer therapy

The human epidermal growth family (HER) of tyrosine kinase receptors underlies the pathogenesis of many types of human cancer. The oncogenic functions of three of the HER proteins can be unleashed through amplification, overexpression, or mutational activation. This has formed the basis for the development of clinically active targeted therapies. However, the third member HER3 is catalytically inactive, not found to be mutated or amplified in cancers, and its role and functions have remained shrouded in mystery. Recent evidence derived primarily from experimental models now seems to implicate HER3 in the pathogenesis of several types of cancer. Furthermore, the failure to recognize the central role of HER3 seems to underlie resistance to epidermal growth factor receptor (EGFR)- or HER2-targeted therapies in some cancers. Structural and biochemical studies have now greatly enhanced our understanding of signaling in the HER family and revealed the previously unrecognized activating functions embodied in the catalytically impaired kinase domain of HER3. This renewed interest and mechanistic basis has fueled the development of new classes of HER3-targeting agents for cancer therapy. However, identifying HER3-dependent tumors presents a formidable challenge and the success of HER3-targeting approaches depends entirely on the development and power of predictive tools.

HOXB9, a gene overexpressed in breast cancer, promotes tumorigenicity and lung metastasis

The mechanisms underlying tumoral secretion of signaling molecules into the microenvironment, which modulates tumor cell fate, angiogenesis, invasion, and metastasis, are not well understood. Aberrant expression of transcription factors, which has been implicated in the tumorigenesis of several types of cancers, may provide a mechanism that induces the expression of growth and angiogenic factors in tumors, leading to their local increase in the tumor microenvironment, favoring tumor progression. In this report, we demonstrate that the transcription factor HOXB9 is overexpressed in breast carcinoma, where elevated expression correlates with high tumor grade. HOXB9 induces the expression of several angiogenic factors (VEGF, bFGF, IL-8, and ANGPTL-2), as well as ErbB (amphiregulin, epiregulin, and neuregulins) and TGF-ss, which activate their respective pathways, leading to increased cell motility and acquisition of mesenchymal phenotypes. In vivo, HOXB9 promotes the formation of large, well-vascularized tumors that metastasize to the lung. Thus, deregulated expression of HOXB9 contributes to breast cancer progression and lung metastasis by inducing several growth factors that alter tumor-specific cell fates and the tumor stromal microenvironment.

Structural analysis of the catalytically inactive kinase domain of the human EGF receptor 3

The kinase domain of human epidermal growth factor receptor (HER) 3/ErbB3, a member of the EGF receptor (EGFR) family, lacks several residues that are critical for catalysis. Because catalytic activity in EGFR family members is switched on by an allosteric interaction between kinase domains in an asymmetric kinase domain dimer, HER3 might be specialized to serve as an activator of other EGFR family members. We have determined the crystal structure of the HER3 kinase domain and show that it appears to be locked into an inactive conformation that resembles that of EGFR and HER4. Although the crystal structure shows that the HER3 kinase domain binds ATP, we confirm that it is catalytically inactive but can serve as an activator of the EGFR kinase domain. The HER3 kinase domain forms a dimer in the crystal, mediated by hydrophobic contacts between the N-terminal lobes of the kinase domains. This N-lobe dimer closely resembles a dimer formed by inactive HER4 kinase domains in crystal structures determined previously, and molecular dynamics simulations suggest that the HER3 and HER4 N-lobe dimers are stable. The kinase domains of HER3 and HER4 form similar chains in their respective crystal lattices, in which N-lobe dimers are linked together by reciprocal exchange of C-terminal tails. The conservation of this tiling pattern in HER3 and HER4, which is the closest evolutionary homolog of HER3, might represent a general mechanism by which this branch of the HER receptors restricts ligand-independent formation of active heterodimers with other members of the EGFR family.

HER/ErbB receptor interactions and signaling patterns in human mammary epithelial cells

BACKGROUND

Knowledge about signaling pathways is typically compiled based on data gathered using different cell lines. This approach implicitly assumes that the cell line dependence is not important. However, different cell lines do not always respond to a particular stimulus in the same way, and lack of coherent data collected from closely related cellular systems can be detrimental to the efforts to understand the regulation of biological processes. To address this issue, we created a clone library of human mammary epithelial (HME) cells that expresses different levels of HER2 and HER3 receptors in combination with endogenous EGFR/HER1. Using our clone library, we have quantified the receptor activation patterns and systematically tested the validity of the existing hypotheses about the interaction patterns between HER1-3 receptors.

RESULTS

Our study identified HER2 as the dominant dimerization partner for both EGFR and HER3. Contrary to earlier suggestions, we find that lateral interactions with HER2 do not lead to strong transactivation between EGFR and HER3, i.e., EGFR activation and HER3 activation are only weakly linked in HME cells. We also find that observed weak transactivation is uni-directional where stimulation of EGFR leads to HER3 activation whereas HER3 stimulation does not activate the EGFR. Repeating our experiments at lower cell confluency established that cell confluency is not a major factor in the observed interaction patterns. We have also quantified the dependence of the kinetics of Erk and Akt activation on different HER receptors. We found that HER3 signaling makes the strongest contribution to Akt activation and that, stimulation of either EGFR or HER3 leads to significant Erk activation.

CONCLUSION

Our study shows that clone cell libraries can be a powerful resource in systems biology research by making it possible to differentiate between various hypotheses in a consistent cellular background. Using our constructed clone library we profiled the cell signaling patterns to establish the role of HER2 in the crosstalk between EGFR and HER3 receptors in HME cells. Our results for HME cells show that the weak linkage between EGFR and HER3 pathways can lead to distinct downstream cellular signaling patterns in response to the ligands of these two receptors.

Tumor-specific apoptosis caused by deletion of the ERBB3 pseudo-kinase in mouse intestinal epithelium

Pharmacologic blockade of EGFR or the closely related receptor ERBB2 has modest efficacy against colorectal cancers in the clinic. Although the upregulation of ERBB3, a pseudo-kinase member of the EGFR/ERBB family, is known to contribute to EGFR inhibitor resistance in other cancers, its functions in normal and malignant intestinal epithelium have not been defined. We have shown here that the intestinal epithelium of mice with intestine-specific genetic ablation of Erbb3 exhibits no cytological abnormalities but does exhibit loss of expression of ERBB4 and sensitivity to intestinal damage. By contrast, intestine-specific Erbb3 ablation resulted in almost complete absence of intestinal tumors in the ApcMin mouse model of colon cancer. Unlike nontransformed epithelium lacking ERBB3, intestinal tumors lacking ERBB3 had reduced PI3K/AKT signaling, which led to attenuation of tumorigenesis via a tumor-specific increase in caspase-3-mediated apoptosis. Consistent with the mouse data, which suggest that ERBB3-ERBB4 heterodimers contribute to colon cancer survival, experimentally induced loss of ERBB3 in a KRAS mutant human colon cancer cell line was associated with loss of ERBB4 expression, and siRNA knockdown of either ERBB3 or ERBB4 resulted in elevated levels of apoptosis. These results indicate that the ERBB3 pseudo-kinase has essential roles in supporting intestinal tumorigenesis and suggest that ERBB3 may be a promising target for the treatment of colorectal cancers.

Expression of a phosphorylated p130(Cas) substrate domain attenuates the phosphatidylinositol 3-kinase/Akt survival pathway in tamoxifen resistant breast cancer cells

Elevated expression of p130(Cas)/BCAR1 (breast cancer anti estrogen resistance 1) in human breast tumors is a marker of poor prognosis and poor overall survival. Specifically, p130(Cas) signaling has been associated with antiestrogen resistance, for which the mechanism is currently unknown. TAM-R cells, which were established by long-term exposure of estrogen (E(2))-dependent MCF-7 cells to tamoxifen, displayed elevated levels of total and activated p130(Cas). Here we have investigated the effects of p130(Cas) inhibition on growth factor signaling in tamoxifen resistance. To inhibit p130(Cas), a phosphorylated substrate domain of p130(Cas), that acts as a dominant-negative (DN) p130(Cas) molecule by blocking signal transduction downstream of the p130(Cas) substrate domain, as well as knockdown by siRNA was employed. Interference with p130(Cas) signaling/expression induced morphological changes, which were consistent with a more epithelial-like phenotype. The phenotypic reversion was accompanied by reduced migration, attenuation of the ERK and phosphatidylinositol 3-kinase/Akt pathways, and induction of apoptosis. Apoptosis was accompanied by downregulation of the expression of the anti-apoptotic protein Bcl-2. Importantly, these changes re-sensitized TAM-R cells to tamoxifen treatment by inducing cell death. Therefore, our findings suggest that targeting the product of the BCAR1 gene by a peptide which mimics the phosphorylated substrate domain may provide a new molecular avenue for treatment of antiestrogen resistant breast cancers.

Novel anticancer targets: revisiting ERBB2 and discovering ERBB3

Aberrant receptor expression or functioning of the epidermal growth factor receptor (Erbb) family plays a crucial part in the development and evolution of cancer. Inhibiting the signalling activity of individual receptors in this family has advanced the treatment of a range of human cancers. In this Review we re-evaluate the role of two important family members, ERBB2 (also known as HER2) and ERBB3 (also known as HER3), and explore the mechanisms of action and preclinical and clinical data for new therapies that target signalling through these pivotal receptors. These new therapies include tyrosine kinase inhibitors, antibody-chemotherapy conjugates, heat-shock protein inhibitors and antibodies that interfere with the formation of ERBB2-ERBB3 dimers.

PTEN deficiency in a luminal ErbB-2 mouse model results in dramatic acceleration of mammary tumorigenesis and metastasis

Overexpression and/or amplification of the ErbB-2 oncogene as well as inactivation of the PTEN tumor suppressor are two important genetic events in human breast carcinogenesis. To address the biological impact of conditional inactivation of PTEN on ErbB-2-induced mammary tumorigenesis, we generated a novel transgenic mouse model that utilizes the murine mammary tumor virus (MMTV) promoter to directly couple expression of activated ErbB-2 and Cre recombinase to the same mammary epithelial cell (MMTV-NIC). Disruption of PTEN in the mammary epithelium of the MMTV-NIC model system dramatically accelerated the formation of multifocal and highly metastatic mammary tumors, which exhibited homogenous pathology. PTEN-deficient/NIC-induced tumorigenesis was associated with an increase in angiogenesis. Moreover, inactivation of PTEN in the MMTV-NIC mouse model resulted in hyperactivation of the phosphatidylinositol 3'-kinase/Akt signaling pathway. However, like the parental strain, tumors obtained from PTEN-deficient/NIC mice displayed histopathological and molecular features of the luminal subtype of primary human breast cancer. Taken together, our findings provide important implications in understanding the molecular determinants of mammary tumorigenesis driven by PTEN deficiency and ErbB-2 activation and could provide a valuable tool for testing the efficacy of therapeutic strategies that target these critical signaling pathways.

Vaccinia-induced epidermal growth factor receptor-MEK signalling and the anti-apoptotic protein F1L synergize to suppress cell death during infection

F1L is a functional Bcl-2 homologue that inhibits apoptosis at the mitochondria during vaccinia infection. However, the extent and timing of cell death during ΔF1L virus infection suggest that additional viral effectors cooperate with F1L to limit apoptosis. Here we report that vaccinia growth factor (VGF), a secreted virulence factor, promotes cell survival independently of its role in virus multiplication. Analysis of single and double knockout viruses reveals that VGF acts synergistically with F1L to protect against cell death during infection. Cell survival in the absence of F1L is dependent on VGF activation of the epidermal growth factor receptor. Furthermore, signalling through MEK kinases is necessary and sufficient for VGF-dependent survival. We conclude that VGF stimulates an epidermal growth factor receptor-MEK-dependent pro-survival pathway that synergizes with F1L to counteract an infection-induced apoptotic pathway that predominantly involves the BH3-only protein Bad.

System theoretical investigation of human epidermal growth factor receptor-mediated signalling

The partitioning of biological networks into coupled-functional modules is being increasingly applied for developing predictive models of biological systems. This approach has the advantage that predicting a system-level response does not require a mechanistic description of the internal dynamics of each module. Identification of the input-output characteristics of the network modules and the connectivity between the modules provide the necessary quantitative representation of system dynamics. However, the determination of the input-output relationships of the modules is not trivial; it requires the controlled perturbation of module inputs and systematic analysis of experimental data. In this report, the authors apply a system theoretical analysis approach to derive the time-dependent input-output relationships of the functional module for the human epidermal growth factor receptor (HER) mediated Erk and Akt signalling pathways. Using a library of cell lines expressing endogenous levels of epidermal growth factor receptor (EGFR) and varying levels of HER2, the authors show that a transfer function-based representation can be successfully applied to quantitatively characterise information transfer in this system.

ErbB-3 expression is associated with E-cadherin and their coexpression restores response to gefitinib in non-small-cell lung cancer (NSCLC)

BACKGROUND

Epidermal growth factor receptor (EGFR) inhibitors are effective in a subset of patients with non-small-cell lung cancer (NSCLC). We previously showed that E-cadherin expression associates with gefitinib activity. Here, we correlated the expressions of ErbB-3 and E-cadherin in NSCLC tumors and cell lines, their effect on response to gefitinib, and induction of both by the histone deacetylase (HDAC) inhibitors vorinostat and SNDX-275.

METHODS

Real-time RT-PCR was carried out on RNA isolated from 91 fresh-frozen NSCLC samples and from 21 NSCLC lines. Protein expression was evaluated with western blot and flow cytometry. Apoptosis was assessed using vibrant apoptosis assay.

RESULTS

Expressions of E-cadherin and ErbB-3 correlated significantly in primary tumors (r = 0.38, P < 0.001) and in cell lines (r = 0.88, P < 0.001). Cotransfection of ErbB-3 and E-cadherin in a gefitinib-resistant cell line showed enhanced apoptotic response to gefitinib. vorinostat and SNDX-275 induced ErbB-3 and E-cadherin in gefitinib-resistant cell lines. When gefitinib-resistant lines were treated with vorinostat and gefitinib, synergistic effects were detected in four of the five lines tested.

CONCLUSION

ErbB-3 and E-cadherin are coexpressed and induced by HDAC inhibitors. For tumors with low ErbB-3 and E-cadherin expressions, the combination of HDAC and EGFR-tyrosine kinase inhibitors increased expression of both genes and produced more than additive apoptotic effect.

HER kinase activation confers resistance to MET tyrosine kinase inhibition in MET oncogene-addicted gastric cancer cells

Tumor cells with genomic amplification of MET display constitutive activation of the MET tyrosine kinase, which renders them highly sensitive to MET inhibition. Several MET inhibitors have recently entered clinical trials; however, as with other molecularly targeted agents, resistance is likely to develop. Therefore, elucidating possible mechanisms of resistance is of clinical interest. We hypothesized that collateral growth factor receptor pathway activation can overcome the effects of MET inhibition in MET-amplified cancer cells by reactivating key survival pathways. Treatment of MET-amplified GTL-16 and MKN-45 gastric cancer cells with the highly selective MET inhibitor PHA-665752 abrogated MEK/mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K)/AKT signaling, resulting in cyclin D1 loss and G(1) arrest. PHA-665752 also inhibited baseline phosphorylation of epidermal growth factor receptor (EGFR) and HER-3, which are transactivated via MET-driven receptor cross-talk in these cells. However, MET-independent HER kinase activation using EGF (which binds to and activates EGFR) or heregulin-beta1 (which binds to and activates HER-3) was able to overcome the growth-inhibitory effects of MET inhibition by restimulating MEK/MAPK and/or PI3K/AKT signaling, suggesting a possible escape mechanism. Importantly, dual inhibition of MET and HER kinase signaling using PHA-665752 in combination with the EGFR inhibitor gefitinib or in combination with inhibitors of MEK and AKT prevented the above rescue effects. Our results illustrate that highly targeted MET tyrosine kinase inhibition leaves MET oncogene-"addicted" cancer cells vulnerable to HER kinase-mediated reactivation of the MEK/MAPK and PI3K/AKT pathways, providing a rationale for combined inhibition of MET and HER kinase signaling in MET-amplified tumors that coexpress EGFR and/or HER-3.

PI3K pathway alterations in cancer: variations on a theme

The high frequency of phosphoinositide 3-kinase (PI3K) pathway alterations in cancer has led to a surge in the development of PI3K inhibitors. Many of these targeted therapies are currently in clinical trials and show great promise for the treatment of PI3K-addicted tumors. These recent developments call for a re-evaluation of the oncogenic mechanisms behind PI3K pathway alterations. This pathway is unique in that every major node is frequently mutated or amplified in a wide variety of solid tumors. Receptor tyrosine kinases upstream of PI3K, the p110 alpha catalytic subunit of PI3K, the downstream kinase, AKT, and the negative regulator, PTEN, are all frequently altered in cancer. In this review, we will examine the oncogenic properties of these genetic alterations to understand whether they are redundant or distinct and propose treatment strategies tailored for these genetic lesions.

The epidermal growth factor receptor family in breast cancer

THE HUMAN EPIDERMAL GROWTH FACTOR RECEPTOR (HER) FAMILY COMPRISES FOUR HOMOLOGOUS MEMBERS: EGFR, HER-2, HER-3, and HER-4. The activation of these receptors triggers a complex series of signal transduction pathways which affect pivotal tumorigenic processes. The deregulation of HER signaling is seen in several human malignancies. HER-2 is now recognized as a key oncogene in breast cancer pathogenesis. Assessment of HER-2 status is of central importance in the prognosis of breast cancer patients. In the light of clinical data suggesting that HER-2 can also be useful as a predictive marker both for trastuzumab and chemotherapy, standardized determination of the HER-2 status in tumors has become more important. Moreover, current data provide evidence for the significance of HER-3 and HER-4 alterations in breast carcinogenesis. Because of the complex interactions among the HER receptors, it is likely that the effect on cell proliferation and tumor growth depends on receptor trans-signaling and thus, the evaluation of the combined expression pattern of all family members is of particular interest. This review presents the current evidence highlighting the role of the family as a whole panel and an update on the role of HER-3 and HER-4 receptors in breast cancer. Moreover, we provide updated data regarding the prognostic value of HER family members giving emphasis to novel methods for the determination of their status, such as real-time polymerase chain reaction. In addition, we review recent therapeutic approaches aimed at targeting the HER family in breast cancer patients.

Modeling oncogene addiction using RNA interference

The clinical efficacy of selective kinase inhibitors suggests that some cancer cells may become dependent on a single oncogene for survival. RNAi has been increasingly used to understand such "oncogene addiction" and validate new therapeutic targets. However, RNAi approaches suffer from significant off-target effects that limit their utility. Here, we combine carefully titrated lentiviral-mediated short hairpin RNA knockdown of the epidermal growth factor receptor (EGFR) with heterologous reconstitution by EGFR mutants to rigorously analyze the structural features and signaling activities that determine addiction to the mutationally activated EGFR in human lung cancer cells. EGFR dependence is differentially rescued by distinct EGFR variants and oncogenic mutants, is critically dependent on its heterodimerization partner ErbB-3, and surprisingly, does not require autophosphorylation sites in the cytoplasmic domain. Quantitative "oncogene rescue" analysis allows mechanistic dissection of oncogene addiction, and, when broadly applied, may provide functional validation for potential therapeutic targets identified through large-scale RNAi screens.

Embryonic requirements for ErbB signaling in neural crest development and adult pigment pattern formation

Vertebrate pigment cells are derived from neural crest cells and are a useful system for studying neural crest-derived traits during post-embryonic development. In zebrafish, neural crest-derived melanophores differentiate during embryogenesis to produce stripes in the early larva. Dramatic changes to the pigment pattern occur subsequently during the larva-to-adult transformation, or metamorphosis. At this time, embryonic melanophores are replaced by newly differentiating metamorphic melanophores that form the adult stripes. Mutants with normal embryonic/early larval pigment patterns but defective adult patterns identify factors required uniquely to establish, maintain or recruit the latent precursors to metamorphic melanophores. We show that one such mutant, picasso, lacks most metamorphic melanophores and results from mutations in the ErbB gene erbb3b, which encodes an EGFR-like receptor tyrosine kinase. To identify critical periods for ErbB activities, we treated fish with pharmacological ErbB inhibitors and also knocked down erbb3b by morpholino injection. These analyses reveal an embryonic critical period for ErbB signaling in promoting later pigment pattern metamorphosis, despite the normal patterning of embryonic/early larval melanophores. We further demonstrate a peak requirement during neural crest migration that correlates with early defects in neural crest pathfinding and peripheral ganglion formation. Finally, we show that erbb3b activities are both autonomous and non-autonomous to the metamorphic melanophore lineage. These data identify a very early, embryonic, requirement for erbb3b in the development of much later metamorphic melanophores, and suggest complex modes by which ErbB signals promote adult pigment pattern development.

The role of ErbB3 and its binding partners in breast cancer progression and resistance to hormone and tyrosine kinase directed therapies

An increasingly important role for the ErbB3 receptor in the genesis and progression of breast cancer is emerging. ErbB3 is frequently overexpressed in breast cancer and coexpression of ErbB2/3 is a poor prognostic indicator. ErbB3 has also been implicated in the development of resistance to antiestrogens such as tamoxifen and ErbB tyrosine kinase inhibitors such as gefitinib. Persistent activation of the AKT pathway has been postulated to contribute to ErbB3-mediated resistance to these therapies. This activation may be due in part to the inappropriate production of the ErbB3 ligand heregulin. ErbB3 binding proteins, which negatively regulate ErbB3 protein levels and the ability of ErbB3 to transmit proliferative signals, also contribute to breast cancer progression and treatment resistance. These proteins include the intracellular RING finger E3 ubiquitin ligase Nrdp1 and the leucine-rich protein LRIG-1 that mediate receptor degradation. Ebp1, another ErbB3 binding protein, suppresses HRG driven breast cancer cell growth and contributes to tamoxifen sensitivity. These studies point to the importance of the evaluation of protein levels and functional activity of ErbB3 and its binding proteins in breast cancer prognosis and prediction of clinical response to treatment.

Phosphatase and tensin homologue deleted on chromosome 10 deficiency accelerates tumor induction in a mouse model of ErbB-2 mammary tumorigenesis

Loss of the tumor suppressor phosphatase and tensin homologue deleted on chromosome 10 (PTEN) and amplification or elevated expression of ErbB-2 are both involved in human breast cancer. To directly test the importance of these genetic events in mammary tumorigenesis, we have assessed whether mammary-specific disruption of PTEN could cooperate with activation of ErbB-2. Transgenic mice expressing ErbB-2 under the transcriptional control of its endogenous promoter (ErbB-2(KI)) were interbred with mice carrying conditional PTEN alleles and an MMTV/Cre transgene. Loss of one or both PTEN alleles resulted in a dramatic acceleration of mammary tumor onset and an increased occurrence of lung metastases in the ErbB-2(KI) strain. Tumor progression in PTEN-deficient/ErbB-2(KI) strains was associated with elevated ErbB-2 protein levels, which were not due to ErbB-2 amplification or to a dramatic increase in ErbB-2 transcripts. Moreover, the PTEN-deficient/ErbB-2(KI)-derived mouse mammary tumors display striking morphologic heterogeneity in comparison with the homogeneous pathology of the ErbB-2(KI) parental strain. Therefore, inactivation of PTEN would not only have a dramatic effect on ErbB-2-induced mammary tumorigenesis but would also lead to the formation of mammary tumors that, in part, display pathologic and molecular features associated with the basal-like subtype of primary human breast cancer.

The ERBB3 receptor in cancer and cancer gene therapy

ERBB3, a member of the epidermal growth factor receptor (EGFR) family, is unique in that its tyrosine kinase domain is functionally defective. It is activated by neuregulins, by other ERBB and nonERBB receptors as well as by other kinases, and by novel mechanisms. Downstream it interacts prominently with the phosphoinositol 3-kinase/AKT survival/mitogenic pathway, but also with GRB, SHC, SRC, ABL, rasGAP, SYK and the transcription regulator EBP1. There are likely important but poorly understood roles for nuclear localization and for secreted isoforms. Studies of ERBB3 expression in primary cancers and of its mechanistic contributions in cultured cells have implicated it, with varying degrees of certainty, with causation or sustenance of cancers of the breast, ovary, prostate, certain brain cells, retina, melanocytes, colon, pancreas, stomach, oral cavity and lung. Recent results link high ERBB3 activity with escape from therapy targeting other ERBBs in lung and breast cancers. Thus a wide and centrally important role for ERBB3 in cancer is becoming increasingly apparent. Several approaches for targeting ERBB3 in cancers have been tested or proposed. Small inhibitory RNA (siRNA) to ERBB3 or AKT is showing promise as a therapeutic approach to treatment of lung adenocarcinoma.

The HER family and cancer: emerging molecular mechanisms and therapeutic targets

The human epidermal growth factor receptor (HER) family of transmembrane tyrosine kinases regulates diverse cellular functions in response to extracellular ligands. The deregulation of HER signaling through gene amplification or mutation is seen in many human tumors and an abundance of experimental evidence supports the etiological role of these events in cancer pathogenesis. In addition, the fact that they are feasible targets for both antibody and small-molecule therapeutics has made them highly pursued targets for the development of rationally designed anticancer drugs. Several HER-targeting agents have entered clinical practice and this has led to novel discoveries regarding the mechanisms of resistance, which has defined a new generation of challenges for targeted cancer therapies. Here, we review recent advances in our understanding of HER signaling and targeting in cancer.

Acquired resistance to erlotinib in A-431 epidermoid cancer cells requires down-regulation of MMAC1/PTEN and up-regulation of phosphorylated Akt

Erlotinib (Tarceva), an epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor, has clinical activity in advanced lung cancer, but disease that initially responds to erlotinib eventually progresses. The mechanism of this acquired resistance is unclear. We established two erlotinib-resistant pools of A-431 cells, a well-characterized epidermoid cancer cell line that constitutively overexpresses EGFR and is sensitive to erlotinib, by continuous exposure to erlotinib over a 6-month period. The extent of EGFR gene amplification or mutation of the EGFR tyrosine kinase domain was not altered in the resistant cells. Intracellular erlotinib concentrations, determined by liquid chromatography-tandem mass spectrometry, were almost the same in all three cell lines. Immunoprecipitation with EGFR antibody followed by detection with phosphotyrosine antibody revealed that erlotinib effectively reduced EGFR phosphorylation in both parental cells and resistant cells. Erlotinib induced mutated in multiple advanced cancers 1/phosphatase and tensin homologue (MMAC1/PTEN) and suppressed phosphorylated Akt (Ser(473)) but not in the erlotinib-resistant cells. Overexpression of MMAC1/PTEN by transfection with Ad.MMAC1/PTEN or by pharmacologic suppression of Akt activity restored erlotinib sensitivity in both resistant pools. Further, transfection of parental A-431 cells with constitutively active Akt was sufficient to cause resistance to erlotinib. We propose that acquired erlotinib resistance associated with MMAC1/PTEN down-regulation and Akt activation could be overcome by inhibitors of signaling through the phosphatidylinositol 3-kinase pathway.

Enhancement of erbB2 and erbB3 expression during oral oncogenesis in diabetic rats

PURPOSE

The expression of erbB2 and erbB3 receptors was investigated in an experimental model of chemically induced oral carcinogenesis in normal and diabetic (type I) Sprague-Dawley rats.

METHODS

Thirteen diabetic and twelve normal rats developed precancerous and cancerous lesions after 4-nitroquinoline-N-oxide treatment, while six diabetic and six normal animals were used as controls. Sections of biopsies from all animals were classified histologically in the following categories: normal mucosa, hyperplasia, dysplasia, early invasion, well- and moderately-differentiated squamous cell carcinoma. Each section was studied immunohistochemically using monoclonal antibodies against erbB2 and erbB3 proteins and six representative histological regions in each section were analysed.

RESULTS

The erbB2 was expressed at very low levels in normal rats, while in diabetic animals its expression was significantly increased during early invasion (P = 0.04). The erbB3 expression was significantly elevated in well-differentiated carcinoma in normal animals (P = 0.01), while in diabetic animals it was significantly increased during oral mucosal hyperplasia and dysplasia (P = 0.03 and 0.0007, respectively). The comparison of erbB2 expression between diabetic and normal rats revealed significant differences in all stages except for the tumor stage of moderately differentiated carcinoma (P = 0.01, 0.00001, 0.00001, 0.003, and 0.00001). In regard to erbB3 expression, significant differences between diabetic and normal rats existed only in normal, non-cancerous and precancerous stages (P = 0.007, 0.0001, 0.0003).

CONCLUSIONS

It seems that diabetes enhances the expression of both erbB2 and erbB3 in certain stages of oral oncogenesis possibly resulting in promotion of cell proliferation and inhibition of apoptosis.

Targeting HER proteins in cancer therapy and the role of the non-target HER3

Members of the human epidermal growth factor receptor (HER) family have been of considerable interest in the cancer arena due to their potential to induce tumorigenesis when their signalling functions are deregulated. The constitutive activation of these proteins is seen in a number of different common cancer subtypes, and in particular EGFR and HER2 have become highly pursued targets for anti-cancer drug development. Clinical studies in a number of different cancers known to be driven by EGFR or HER2 show mixed results, and further mechanistic understanding of drug sensitivity and resistance is needed to realise the full potential of this treatment modality. Signalling in trans is a key feature of HER family signalling, and the activation of the PI3K/Akt pathway, so critically important in tumorigenesis, is driven predominantly through phosphorylation in trans of the kinase inactive member HER3. An increasing body of evidence shows that HER3 plays a critical role in EGFR- and HER2-driven tumours. In particular, HER3 lies upstream of a critically important tumorigenic signalling pathway with extensive ability for feedback and cross-talk signalling, and targeting approaches that fail to account for this important trans-target of EGFR and HER2 can be undermined by its resiliency and resourcefulness. Since HER3 is kinase inactive, it is not a direct target of kinase inhibitors and not presently an easily drugable target. This review presents the current evidence highlighting the role of HER3 in tumorigenesis and its role in mediating resistance to inhibitors of EGFR and HER2.

The oncogene HER2: its signaling and transforming functions and its role in human cancer pathogenesis

The year 2007 marks exactly two decades since Human Epidermal Growth Factor Receptor-2 (HER2) was functionally implicated in the pathogenesis of human breast cancer. This finding established the HER2 oncogene hypothesis for the development of some human cancers. The subsequent two decades have brought about an explosion of information about the biology of HER2 and the HER family. An abundance of experimental evidence now solidly supports the HER2 oncogene hypothesis and etiologically links amplification of the HER2 gene locus with human cancer pathogenesis. The molecular mechanisms underlying HER2 tumorigenesis appear to be complex and a unified mechanistic model of HER2-induced transformation has not emerged. Numerous hypotheses implicating diverse transforming pathways have been proposed and are individually supported by experimental models and HER2 may indeed induce cell transformation through multiple mechanisms. Here I review the evidence supporting the oncogenic function of HER2, the mechanisms that are felt to mediate its oncogenic functions, and the evidence that links the experimental evidence with human cancer pathogenesis.

Escape from HER-family tyrosine kinase inhibitor therapy by the kinase-inactive HER3

Oncogenic tyrosine kinases have proved to be promising targets for the development of highly effective anticancer drugs. However, tyrosine kinase inhibitors (TKIs) against the human epidermal growth factor receptor (HER) family show only limited activity against HER2-driven breast cancers, despite effective inhibition of epidermal growth factor receptor (EGFR) and HER2 in vivo. The reasons for this are unclear. Signalling in trans is a key feature of this multimember family and the critically important phosphatidylinositol-3-OH kinase (PI(3)K)/Akt pathway is driven predominantly through transphosphorylation of the kinase-inactive HER3 (refs 9, 10). Here we show that HER3 and consequently PI(3)K/Akt signalling evade inhibition by current HER-family TKIs in vitro and in tumours in vivo. This is due to a compensatory shift in the HER3 phosphorylation-dephosphorylation equilibrium, driven by increased membrane HER3 expression driving the phosphorylation reaction and by reduced HER3 phosphatase activity impeding the dephosphorylation reaction. These compensatory changes are driven by Akt-mediated negative-feedback signalling. Although HER3 is not a direct target of TKIs, HER3 substrate resistance undermines their efficacy and has thus far gone undetected. The experimental abrogation of HER3 resistance by small interfering RNA knockdown restores potent pro-apoptotic activity to otherwise cytostatic HER TKIs, re-affirming the oncogene-addicted nature of HER2-driven tumours and the therapeutic promise of this oncoprotein target. However, because HER3 signalling is buffered against an incomplete inhibition of HER2 kinase, much more potent TKIs or combination strategies are required to silence oncogenic HER2 signalling effectively. The biologic marker with which to assess the efficacy of HER TKIs should be the transphosphorylation of HER3 rather than autophosphorylation.

Heregulin beta1 drives gefitinib-resistant growth and invasion in tamoxifen-resistant MCF-7 breast cancer cells

INTRODUCTION

Resistance to anti-epidermal growth factor receptor (anti-EGFR) therapies is an emerging clinical problem. The efficacy of anti-EGFR therapies can be influenced by the presence of heregulins (HRGs), which can bind erbB3/4 receptors and can activate alternative signalling pathways. In the present study we have examined whether HRG signalling can circumvent EGFR blockade in an EGFR-positive tamoxifen-resistant MCF-7 (Tam-R) breast cancer cell line.

METHODS

Tam-R cells, incubated with the selective EGFR tyrosine kinase inhibitor gefitinib ('Iressa', ZD1839), were exposed to HRGbeta1 and the effects on erbB receptor dimerization profiles and on activation of associated downstream signalling components were assessed by immunoprecipitation, western blotting and immunocytochemistry. The effects of HRGbeta1 on gefitinib-treated Tam-R cell growth and invasion were also examined, and HRGbeta1 expression levels were assessed in breast cancer tissue by immunohistochemistry to address the potential clinical relevance of such a resistance mechanism.

RESULTS

In Tam-R cells, HRGbeta1 promoted erbB3/erbB2 and erbB3/EGFR heterodimerization, promoted ERK1/2 and AKT pathway activation and increased cell proliferation and invasion. Gefitinib prevented HRGbeta1-driven erbB3/EGFR heterodimerization, ERK1/2 activation and Tam-R cell proliferation, but HRGbeta1-driven erbB3/erbB2 heterodimerization, AKT activation and Tam-R cell invasion were maintained. A combination of gefitinib and the phosphatidylinositol 3-kinase inhibitor LY294002 effectively blocked HRGbeta1-mediated intracellular signalling activity, growth and invasion in Tam-R cells. Similarly, targeting erbB2 with trastuzumab in combination with gefitinib in Tam-R cells reduced HRGbeta1-induced erbB2 and ERK1/2 activity; however, HRGbeta1-driven AKT activity and cell growth were maintained while cell invasion was significantly enhanced with this combination. In clinical tissue all samples demonstrated cytoplasmic tumour epithelial HRGbeta1 protein staining, with expression correlating with EGFR positivity and activation of both AKT and ERK1/2.

CONCLUSION

HRGbeta1 can overcome the inhibitory effects of gefitinib on cell growth and invasion in Tam-R cells through promotion of erbB3/erbB2 heterodimerization and activation of the phosphatidylinositol 3-kinase/AKT signalling pathway. This may have implications for the effectiveness of anti-EGFR therapies in breast cancer as HRGbeta1 is enriched in many EGFR-positive breast tumours.

Inactivation of Akt by the epidermal growth factor receptor inhibitor erlotinib is mediated by HER-3 in pancreatic and colorectal tumor cell lines and contributes to erlotinib sensitivity

Signaling through the receptor for epidermal growth factor receptor (EGFR) is frequently deregulated in solid tumors. Erlotinib (Tarceva, OSI-774, OSI Pharmaceuticals, Inc., Melville, NY) is a low molecular weight, orally bioavailable inhibitor of the EGFR that has been approved for both non-small cell lung cancer and pancreatic cancers. Previous studies have indicated that sensitivity to EGFR antagonists correlated with HER-3 signaling for non-small cell lung cancer. Herein, we have sought to understand the signaling pathways that mediate erlotinib sensitivity for pancreatic and colorectal cancers. In a panel of 12 pancreatic tumor cell lines, we find that EGFR is coexpressed with HER-3 in all cell lines sensitive to erlotinib but not in insensitive cell lines. Erlotinib can block HER-3 phosphorylation in these sensitive cell lines, suggesting that HER-3 is transactivated by EGFR. Knockdown of HER-3 in BxPC3, an erlotinib-sensitive pancreatic tumor cell line, results in inhibition of the phosphorylation for both Akt and S6 and is associated with a decrease in cell proliferation and reduced sensitivity to erlotinib. Therefore, EGFR transactivation of HER-3 mediates Akt signaling and can contribute to erlotinib sensitivity for pancreatic tumors. We extended our analysis to a panel of 13 colorectal tumor cell lines and find that, like pancreatic, HER-3 is coexpressed with EGFR in the most erlotinib-sensitive cell lines but not in erlotinib-insensitive cell lines. These studies suggest that HER-3 could be used as a biomarker to select patients who are most likely to respond to erlotinib therapy.

Baseline gene expression predicts sensitivity to gefitinib in non-small cell lung cancer cell lines

Tyrosine kinase inhibitors (TKI) of the epidermal growth factor receptor (EGFR) produce objective responses in a minority of patients with advanced-stage non-small cell lung cancer (NSCLC), and about half of all treated patients progress within 6 weeks of instituting therapy. Because the target of these agents is known, it should be possible to develop biological predictors of response, but EGFR protein levels have not been proven useful as a predictor of TKI response in patients and the mechanism of primary resistance is unclear. We used microarray gene expression profiling to uncover a pattern of gene expression associated with sensitivity to EGFR-TKIs by comparing NSCLC cell lines that were either highly sensitive or highly resistant to gefitinib. This sensitivity-associated expression profile was used to predict gefitinib sensitivity in a panel of NSCLC cell lines with known gene expression profiles but unknown gefitinib sensitivity. Gefitinib sensitivity was then determined for members of this test panel, and the microarray-based sensitivity prediction was correct in eight of nine NSCLC cell lines. Gene and protein expression differences were confirmed with a combination of quantitative reverse transcription-PCR, flow cytometry, and immunohistochemistry. This gene expression pattern related to gefitinib sensitivity was independent from sensitivity associated with EGFR mutations. Several genes associated with sensitivity encode proteins involved in HER pathway signaling or pathways that interrelate to the HER signaling pathway. Some of these genes could be targets of pharmacologic interventions to overcome primary resistance.

Modeling the effects of HER/ErbB1-3 coexpression on receptor dimerization and biological response

The human epidermal growth factor receptor (HER/ErbB) system comprises the epidermal growth factor receptor (EGFR/HER1) and three other homologs, namely HERs 2-4. This receptor system plays a critical role in cell proliferation and differentiation and receptor overexpression has been associated with poor prognosis in cancers of the epithelium. Here, we examine the effect of coexpressing varying levels of HERs 1-3 on the receptor dimerization patterns using a detailed kinetic model for HER/ErbB dimerization and trafficking. Our results indicate that coexpression of EGFR with HER2 or HER3 biases signaling to the cell surface and retards signal downregulation. In addition, simultaneous coexpression of HERs 1-3 leads to an abundance of HER2-HER3 heterodimers, which are known to be potent inducers of cell growth and transformation. Our new approach to use parameter dependence analysis in experimental design reveals that measurements of HER3 phosphorylation and HER2 internalization ratio may prove to be especially useful for the estimation of critical model parameters. Further, we examine the effect of receptor dimerization patterns on biological response using a simple phenomenological model. Results indicate that coexpression of EGFR with HER2 and HER3 at low to moderate levels may enable cells to match the response of a high HER2 expresser.

The BAD protein integrates survival signaling by EGFR/MAPK and PI3K/Akt kinase pathways in PTEN-deficient tumor cells

Tumor cells with mutated PTEN proliferate in an EGFR-independent manner. Induction of PTEN sensitizes cells to EGFR inhibition, and the combination causes synergistic apoptosis. Synergy is due to inhibition of two parallel pathways that phosphorylate the proapoptotic protein BAD at distinct sites. Serine 112 phosphorylation is EGFR/MEK/MAPK dependent, whereas serine 136 phosphorylation is PI3K/Akt dependent. Either phosphorylation is sufficient to sequester BAD to 14-3-3. BAD is released and apoptosis is induced only if both serines are dephosphorylated in response to inhibition of both pathways. Reduction of BAD expression by RNA interference prevents apoptosis in response to pathway inhibition. Thus, BAD integrates the antiapoptotic effects of both pathways. Combined inhibition of EGFR and PI3K signaling may be a useful therapeutic strategy.

Neuregulin-1 enhances survival of human astrocytic glioma cells

Malignant astrocytic gliomas, referred to as astrocytomas, represent the most commonly diagnosed adult primary brain tumor. These tumors are characterized by unrelenting growth that is often resistant to chemotherapy and radiation therapy. Tumor expansion into the healthy surrounding brain tissue produces severe and often fatal consequences. In this study, we examine the potential for the neuregulin-1/erbB receptor signaling cascade to contribute to this process by modulating glioma cell growth. Using antibodies specific for the erbB receptors, we demonstrate the expression patterns for the erbB2, erbB3, and erbB4 receptors in human glioma biopsy samples. We then verify receptor expression in a panel of human glioma cell lines. Next, we investigate the status of the erbB2 and erbB3 receptors in the human glioma cell lines and find that they are constitutively tyrosine-phosphorylated and heterodimerized. Subsequently, we demonstrate that theses same cell lines express membrane bound and released forms of neuregulins, the erbB receptor ligands, suggesting a possible autocrine or paracrine signaling network. Furthermore, we show that exogenous activation of erbB2 and erbB3 receptors in U251 glioma cells by recombinant Nrg-1beta results in enhanced glioma cell growth under conditions of serum-deprivation. This enhancement is due to an increase in cell survival rather than an increase in cell proliferation and is dependent on the activation of erbB2 and phosphatidylinositol-3 kinase (PI3K). Moreover, Nrg-1beta activates an inhibitor of apoptosis, Akt, implying a possible role for this kinase in mediating Nrg-1beta effects in gliomas. This data suggests that glioma cells may use autocrine or paracrine neuregulin-1/erbB receptor signaling to enhance cell survival under conditions where growth would otherwise be limited.

Trans-10,cis-12, not cis-9,trans-11, conjugated linoleic acid decreases ErbB3 expression in HT-29 human colon cancer cells

AIM: To examine whether trans-10,cis-12 CLA (t10c12) or cis-9,trans-11 CLA (c9t11) inhibits heregulin (HRG)-β-stimulated cell growth and HRG-β-ErbB3 signaling in HT-29 cells.

METHODS: We cultured HT-29 cells in the absence or presence of the CLA isomers and/or the ErbB3 ligand HRG-β. MTT assay, [³H]thymidine incorporation, Annexin V staining, RT-PCR, Western blotting, immunoprecipitation, and in vitro kinase assay were performed.

RESULTS: HRG-β increased cell growth, but did not prevent t10c12-induced growth inhibition. T10c12 inhibited DNA synthesis and induced apoptosis of HT-29 cells, whereas c9t11 had no effect. T10c12 decreased the levels of ErbB1, ErbB2, and ErbB3 proteins and transcripts in a dose-dependent manner, whereas c9t11 had no effect. Immunoprecipitation/Western blot studies revealed that t10c12 inhibited HRG-β-stimulated phosphorylation of ErbB3, recruitment of the p85 subunit of phosphoinositide 3-kinase (PI3K) to ErbB3, ErbB3-associated PI3K activities, and phosphorylation of Akt. However, c9t11 had no effect on phospho Akt levels. Neither t10c12 nor c9t11 had any effect on HRG-β-induced phosphorylation of ERK-1/2.

CONCLUSION: These results indicate that the inhibition of HT-29 cell growth by t10c12 may be induced via its modulation of ErbB3 signaling leading to inhibition of Akt activation.

Induction of human NF-IL6beta by epidermal growth factor is mediated through the p38 signaling pathway and cAMP response element-binding protein activation in A431 cells

The CCAAT/enhancer binding protein delta (C/EBPdelta, CRP3, CELF, NF-IL6beta) regulates gene expression and plays functional roles in many tissues, such as in acute phase response to inflammatory stimuli, adipocyte differentiation, and mammary epithelial cell growth control. In this study, we examined the expression of human C/EBPdelta (NF-IL6beta) gene by epidermal growth factor (EGF) stimulation in human epidermoid carcinoma A431 cells. NF-IL6beta was an immediate-early gene activated by the EGF-induced signaling pathways in cells. By using 5'-serial deletion reporter analysis, we showed that the region comprising the -347 to +9 base pairs was required for EGF response of the NF-IL6beta promoter. This region contains putative consensus binding sequences of Sp1 and cAMP response element-binding protein (CREB). The NF-IL6beta promoter activity induced by EGF was abolished by mutating the sequence of cAMP response element or Sp1 sites in the -347/+9 base pairs region. Both in vitro and in vivo DNA binding assay revealed that the CREB binding activity was low in EGF-starved cells, whereas it was induced within 30 min after EGF treatment of A431 cells. However, no change in Sp1 binding activity was found by EGF treatment. Moreover, the phosphatidylinositol 3 (PI3)-kinase inhibitor (wortmannin) and p38(MAPK) inhibitor (SB203580) inhibited the EGF-induced CREB phosphorylation and the expression of NF-IL6beta gene in cells. We also demonstrated that CREB was involved in regulating the NF-IL6beta gene transcriptional activity mediated by p38(MAPK). Our results suggested that PI3-kinase/p38(MAPK)/CREB pathway contributed to the EGF activation of NF-IL6beta gene expression.

Inactivation of ErbB3 by siRNA promotes apoptosis and attenuates growth and invasiveness of human lung adenocarcinoma cell line A549

The ErbB3 receptor and the downstream signaling kinase Akt are implicated in proliferation of lung adenocarcinoma cells. Inhibition by siRNAs to ErbB3 and Akt isoforms 1, 2 and 3 was utilized to investigate the contribution of these molecules to tumor survival, spreading and invasiveness, and the roles of specific Akt isoforms. ErbB3 siRNA stably and dose-dependently suppressed ErbB3 protein for 2 days or more, and reduced cell numbers, by both suppressing cell cycle and causing apoptosis and necrosis. It also inhibited soft agar growth, cell motility and migration, and invasiveness. Akt1, 2 and 3 siRNAs had similar suppressive effects on cell number, apoptosis/necrosis and soft agar growth. However, although Akt1 siRNA had no effect on cell migration or invasion, Akt2 siRNA effectively suppressed both activities, and Akt3 siRNA had moderate effectiveness. In A549 cells, ErbB3 is indicated as having major effects on cell division, survival, motility, migration and invasiveness. All three Akt isoforms are to varying degrees involved in these cell behaviors, with Akt2 especially implicated in migration and invasion. ErbB3 and the Akts are promising targets for therapy, and siRNAs may be useful for this purpose.

ErbB-3 mediates phosphoinositide 3-kinase activity in gefitinib-sensitive non-small cell lung cancer cell lines

Therapies that target the EGF receptor (EGFR), such as gefitinib (IRESSA), are effective in a subset of patients with advanced non-small cell lung cancer (NSCLC). The differences in intracellular signaling networks between gefitinib-sensitive and -resistant NSCLCs remain poorly understood. In this study, we observe that gefitinib reduces phospho-Akt levels only in NSCLC cell lines in which it inhibits growth. To elucidate the mechanism underlying this observation, we compared immunoprecipitates of phosphoinositide 3-kinase (PI3K) between gefitinib-sensitive and -resistant NSCLC cell lines. We observe that PI3K associates with ErbB-3 exclusively in gefitinib-sensitive NSCLC cell lines. Gefitinib dissociates this complex, thereby linking EGFR inhibition to decreased Akt activity. In contrast, gefitinib-resistant cells do not use ErbB-3 to activate the PI3K/Akt pathway. In fact, abundant ErbB-3 expression is detected only in gefitinib-sensitive NSCLC cell lines. Two gefitinib-sensitive NSCLC cell lines with endogenous distinct activating EGFR mutations (L858R and Del747-749), frequently observed in NSCLC patients who respond to gefitinib, also use ErbB-3 to couple to PI3K. Down-regulation of ErbB-3 by means of short hairpin RNA leads to decreased phospho-Akt levels in the gefitinib-sensitive NSCLC cell lines, Calu-3 (WT EGFR) and H3255 (L858R EGFR), but has no effect on Akt activation in the gefitinib-resistant cell lines, A549 and H522. We conclude that ErbB-3 is used to couple EGFR to the PI3K/Akt pathway in gefitinib-sensitive NSCLC cell lines harboring WT and mutant EGFRs.

Mechanisms of resistance to Erbitux (anti-epidermal growth factor receptor) combination therapy in pancreatic adenocarcinoma cells

We previously demonstrated that pancreatic adenocarcinoma BxPC-3 xenografts display resistance to treatment with Erbitux, gemcitabine, and radiation, whereas MIA PaCa-2 xenografts are highly sensitive to the same therapy. Here, we elucidate in vitro mechanisms that may explain the observed differential response of epidermal growth factor receptor (EGFR) expressing pancreatic adenocarcinoma xenografts to Erbitux-based combination therapy in vivo. MIA PaCa-2 and BxPC-3 protein lysates were probed with antibodies to EGFR, ErbB2, ErbB3, and ErbB4. Constitutive ErbB3 activity was visualized by immunoblot analysis using anti-phosphotyrosine antibodies and receptor-specific immunoprecipitates. erbB2 and erbB3 gene expression in both cell lines was quantified with real-time polymerase chain reaction. Erbitux-induced internalization of EGFR was determined by flow cytometry following Erbitux treatment for different incubation times at 0 degrees C and 37 degrees C. MIA PaCa-2 and BxPC-3 protein extracts were also probed with anti-phospho-mitogen-activated protein kinase antibody after stimulation with EGF and in the presence of Erbitux. Although both cell lines expressed EGFR and ErbB2 protein, ErbB3 protein was selectively expressed by BxPC-3 cells, where it also showed evidence of constitutive phosphorylation. There was a 10-fold increase of erbB3 transcript levels in BxPC-3 cells compared with MIA PaCa-2. ErbB4 protein was not detectable in either cell line. Erbitux mediated EGFR internalization in MIA PaCa-2 cells after 2 hours of incubation, whereas it did not promote EGFR internalization in BxPC-3 cells. Likewise, EGF-dependent phosphorylation of MAPK p44/42 was blocked by Erbitux treatment in MIA PaCa-2 but not BxPC-3 cells. Erbitux selectively interfered with EGF-induced MAPK activation in MIA PaCa-2 but not BxPC-3 cells. Persistent MAPK activation and impaired in vitro internalization of EGFR by BxPC-3 pancreatic cancer cells may be due to constitutive ErbB3 signaling, facilitated by heterodimerization with EGFR, which may explain resistance to Erbitux-based combination therapy in vivo.

The docking protein Gab1 is the primary mediator of EGF-stimulated activation of the PI-3K/Akt cell survival pathway

Background

Gab1 is a docking protein that recruits phosphatidylinositol-3 kinase (PI-3 kinase) and other effector proteins in response to the activation of many receptor tyrosine kinases (RTKs). As the autophosphorylation sites on EGF-receptor (EGFR) do not include canonical PI-3 kinase binding sites, it is thought that EGF stimulation of PI-3 kinase and its downstream effector Akt is mediated by an indirect mechanism.

Results

We used fibroblasts isolated from Gab1-/- mouse embryos to explore the mechanism of EGF stimulation of the PI-3 kinase/Akt anti-apoptotic cell signaling pathway. We demonstrate that Gab1 is essential for EGF stimulation of PI-3 kinase and Akt in these cells and that these responses are mediated by complex formation between p85, the regulatory subunit of PI-3 kinase, and three canonical tyrosine phosphorylation sites on Gab1. Furthermore, complex formation between Gab1 and the protein tyrosine phosphatase Shp2 negatively regulates Gab1 mediated PI-3 kinase and Akt activation following EGF-receptor stimulation. We also demonstrate that tyrosine phosphorylation of ErbB3 may lead to recruitment and activation of PI-3 kinase and Akt in Gab1-/- MEFs.

Conclusions

The primary mechanism of EGF-induced stimulation of the PI-3 kinase/Akt anti-apoptotic pathway occurs via the docking protein Gab1. However, in cells expressing ErbB3, EGF and neuroregulin can stimulate PI-3 kinase and Akt activation in a Gab1-dependent or Gab1-independent manner.

New targets for therapy in breast cancer: small molecule tyrosine kinase inhibitors

Over the past several years many advances have been made in our understanding of critical pathways involved in carcinogenesis and tumor growth. These advances have led to the investigation of small molecule inhibitors of the ErbB family of receptor tyrosine kinases across a broad spectrum of malignancies. In this article we summarize the rationale for targeting members of the ErbB family in breast cancer, and review the preclinical and clinical data for the agents that are furthest in development. In addition, we highlight directions for future research, such as exploration of the potential crosstalk between the ErbB and hormone receptor signal transduction pathways, identification of predictive markers for tumor sensitivity, and development of rational combination regimens that include the tyrosine kinase inhibitors.

Targeted expression of the class II phosphoinositide 3-kinase in Drosophila melanogaster reveals lipid kinase-dependent effects on patterning and interactions with receptor signaling pathways

Phosphoinositide 3-kinases (PI3Ks) can be divided into three distinct classes (I, II, and III) on the basis of their domain structures and the lipid signals that they generate. Functions have been assigned to the class I and class III enzymes but have not been established for the class II PI3Ks. We have obtained the first evidence for a biological function for a class II PI3K by expressing this enzyme during Drosophila melanogaster development and by using deficiencies that remove the endogenous gene. Wild-type and catalytically inactive PI3K_68D transgenes have opposite effects on the number of sensory bristles and on wing venation phenotypes induced by modified epidermal growth factor (EGF) receptor signaling. These results indicate that the endogenous PI3K_68D may act antagonistically to the EGF receptor-stimulated Ras-mitogen-activated protein kinase pathway and downstream of, or parallel to, the Notch receptor. A class II polyproline motif in PI3K_68D can bind the Drk adaptor protein in vitro, primarily via the N-terminal SH3 domain of Drk. Drk may thus be important for the localization of PI3K_68D, allowing it to modify signaling pathways downstream of cell surface receptors. The phenotypes obtained are markedly distinct from those generated by expression of the Drosophila class I PI3K, which affects growth but not pattern formation.

Truncated ErbB2 receptor (p95ErbB2) is regulated by heregulin through heterodimer formation with ErbB3 yet remains sensitive to the dual EGFR/ErbB2 kinase inhibitor GW572016

The expression of the NH2 terminally truncated ErbB2 receptor (p95ErbB2) in breast cancer correlates with metastatic disease progression compared with the expression of full-length p185ErbB2. We now show that heregulin (HRG), but not EGF, stimulates p95ErbB2 phosphorylation in BT474 breast cancer cells. Furthermore, phospho-p95ErbB2 forms heterodimers with ErbB3, but not EGFR, while p185ErbB2 heterodimerizes with both EGFR and ErbB3. The predilection of p95ErbB2 to heterodimerize with ErbB3 provides an explanation for its regulation by HRG, an ErbB3 ligand. GW572016, a reversible small molecule inhibitor of EGFR and ErbB2 tyrosine kinases, inhibits baseline p95ErbB2 phosphorylation in BT474 cells and tumor xenografts. Inhibition of p95ErbB2, p185ErbB2, and EGFR phosphorylation by GW572016 resulted in the inhibition of downstream phospho-Erk1/2, phospho-AKT, and cyclin D steady-state protein levels. Increased phosphorylation of p95ErbB2 and AKT in response to HRG was abrogated to varying degrees by GW572016. In contrast, trastuzumab did not inhibit p95ErbB2 phosphorylation or the expression of downstream phospho-Erk1/2, phospho-AKT, or cyclin D. It is tempting to speculate that trastuzumab resistance may be mediated in part by the selection of p95ErbB2-expressing breast cancer cells capable of exerting potent growth and prosurvival signals through p95ErbB2-ErbB3 heterodimers. Thus, p95ErbB2 represents a target for therapeutic intervention, and one that is sensitive to GW572016 therapy.

Effect of estradiol on estrogen receptor-alpha gene expression and activity can be modulated by the ErbB2/PI 3-K/Akt pathway

Epidermal growth factor (EGF), insulin-like growth factor-I (IGF-I), and heregulin-beta1 (HRG-beta1), can modulate the expression and activity of the estrogen receptor-alpha (ER-alpha) via the phosphatidylinositol 3-kinase (PI 3-K)/Akt pathway in the ER-alpha-positive breast cancer cell line, MCF-7. Estradiol can also rapidly activate PI 3-K/Akt in these cells (nongenomic effect). The recent study examines whether Akt is involved in the ER-alpha regulation by estradiol (genomic effect). Stable transfection of parental MCF-7 cells with a dominant-negative Akt mutant, as well as the PI 3-K inhibitors wortmannin and LY 294,002, blocked the effect of estradiol on ER-alpha expression and activity by 70-80 and 55-63%, respectively. Stable transfection of MCF-7 cells with a constitutively active Akt mimicked the effect of estradiol. The changes in ER-alpha expression and activity were abrogated in response to estradiol by an arginine to cysteine mutation in the pleckstrin homology (PH) domain of Akt (R25C), suggesting the involvement of this amino acid in the interaction between Akt and ER-alpha. Experiments employing selective ErbB inhibitors demonstrate that the effect of estradiol on ER-alpha expression and activity is mediated by ErbB2 and not by EGFR. Moreover, anchorage-dependent and -independent growth assays, cell cycle and membrane ruffling analyses showed that Akt exerts estrogen-like activity on cell growth and membrane ruffling and that a selective ErbB2 inhibitor, but not anti-ErbB2 antibodies directed to the extracellular domain, can block these effects. In the presence of constitutively active Akt, tamoxifen only partially inhibits cell growth. In contrast, in cells stably transfected with either a dominant-negative Akt or with R25C-Akt, as well as in parental cells in the presence of a selective ErbB2 inhibitor, the effect of estradiol on anchorage-dependent and -independent cell growth was inhibited by 50-75 and 100%, respectively. Dominant-negative Akt inhibited membrane ruffling by 54%; however, R25C-Akt did not have any effect, suggesting that kinase activity plays an important role in this process. Scatchard analysis demonstrated a 67% reduction in estrogen-binding capacity in cells transfected with constitutively active Akt. No change in binding affinity of estradiol to the receptor was observed upon transfection with either Akt mutant. Taken together, our results suggest that estradiol treatment results in binding to membrane ER-alpha and interaction with a heterodimer containing ErbB2, leading to tyrosine phosphorylation. This results in the activation of PI 3-K and Akt. Akt, in turn, may interact with nuclear ER-alpha, altering its expression and activity.

Atypical expression of ErbB3 in myeloma cells: cross-talk between ErbB3 and the interferon-alpha signaling complex

We have previously demonstrated that the responsiveness of multiple myeloma (MM) cells to interferon-alpha (IFN-alpha) stimulation is variable, with an atypical growth response displayed by some cells. Here we report the ability of IFN-alpha to induce tyrosine phosphorylation of a 180 kDa band in the KAS-6/1 MM cell line, which is growth responsive to IFN-alpha. Further characterization demonstrated that this band corresponds to ErbB3. To our knowledge, this is the first report of ErbB3 expression in a cell type of the hematopoietic lineage. Although ErbB receptors have been shown to crosscommunicate with various other receptors, our results show for the first time that the IFN-alpha receptor can crosscommunicate with ErbB3. To address the significance of these observations, we transfected ErbB3-negative DP-6 MM cells with ErbB3 and used siRNA to silence ErbB3 in the KAS-6/1 cell line. Although IFN-alpha transactivated ErbB3 in the DP-6 transfectants, it did not confer growth responsiveness to IFN-alpha. Interestingly, silencing ErbB3 expression in the KAS-6/1 cells decreased the overall growth response to IFN-alpha and to interleukin-6. These results suggest that ErbB3 expression alone does not uniquely confer IFN-alpha growth responsiveness, but instead may amplify proliferation rates in MM cells that have acquired atypical expression of this receptor.

Heregulin-beta1 regulates the estrogen receptor-alpha gene expression and activity via the ErbB2/PI 3-K/Akt pathway

This study examines whether the serine/threonine protein kinase, Akt, is involved in the crosstalk between the ErbB2 and estrogen receptor-alpha (ER-alpha) pathways. Treatment of MCF-7 cells with 10(-9) M heregulin-beta1 (HRG-beta1) resulted in a rapid phosphorylation of Akt and a 15-fold increase in Akt activity. Akt phosphorylation was blocked by inhibitors of phosphatidylinositol 3-kinase (PI 3-K), by antiestrogens, the protein tyrosine kinase inhibitor, genistein, and by AG825, a selective ErbB2 inhibitor; but not by AG30, a selective EGFR inhibitor. Akt phosphorylation by HRG-beta1 was abrogated by an arginine to cysteine mutation (R25C) in the pleckstrin homology (PH) domain of Akt, and HRG-beta1 did not induce Akt phosphorylation in the ER-negative variant of MCF-7, MCF-7/ADR. Transient transfection of ER-alpha into these cells restored Akt phosphorylation by HRG-beta1, suggesting the requirement of ER-alpha. HRG-beta1 did not activate Akt in MCF-7 cells stably transfected with an anti-ErbB2-targeted ribozyme, further confirming a role for ErbB2. Stable transfection of the cells with a dominant negative Akt or with the R25C-Akt mutant, as well as PI 3-K inhibitors, blocked the effect of HRG-beta1 on ER-alpha expression and activity and on the growth of MCF-7 cells. Stable transfection of MCF-7 cells with a constitutively active Akt mimicked the effect of HRG-beta1. Experiments employing selective ErbB inhibitors demonstrate that the effect of HRG-beta1 on ER-alpha expression and activity is also mediated by ErbB2 and not by EGFR, demonstrating that ErbB2 is the primary mediator of the effects of HRG-beta1 on ER-alpha regulation. Taken together, our data suggest that HRG-beta1, bound to the ErbB2 ErbB3 heterodimer, in the presence of membrane ER-alpha, interacts with and activates PI 3-K/Akt. Akt leads to nuclear ER-alpha phosphorylation, thereby altering its expression and transcriptional activity.

Activation of ErbB3-PI3-kinase pathway is correlated with malignant phenotypes of adenocarcinomas

Signet-ring cell carcinomas are malignant dedifferentiated carcinomas, which are frequently found in the stomach. We previously demonstrated that a 200 kDa protein is often constitutively phosphorylated on tyrosine and bound to phosphatidylinositol 3-kinase (PI3-kinase) in signet-ring cell carcinoma cells. In this study, we purified the 200 kDa protein from an extract of NUGC-4 cells, a cell line of signet-ring cell carcinoma, and identified it as ErbB3. ErbB3 was found to be phosphorylated selectively in dedifferentiated adenocarcinoma cell lines among various gastric cancer cell lines. Expression of a constitutively active chimeric receptor consisting of ErbB2 and ErbB3 in HCC2998 cells, a highly differentiated adenocarcinoma cell line, revealed that the signaling triggered by phosphorylation of ErbB3 was important for dedifferentiated phenotypes such as loss of cell-cell interaction and high expression of MUC1/DF3 antigen, a marker of the malignant tumors. Taken together, activation of ErbB3 pathway may contribute to the development of dedifferentiated carcinomas.

Role of PI 3-kinase and PIP3 in submandibular gland branching morphogenesis

The mouse submandibular gland (SMG) epithelium undergoes extensive morphogenetic branching during embryonic development as the first step in the establishment of its glandular structure. However, the specific signaling pathways required for SMG branching morphogenesis are not well understood. Using E13 mouse SMG organ cultures, we showed that inhibitors of phosphatidylinositol 3-kinase (PI 3-kinase), wortmannin and LY294002, substantially inhibited branching morphogenesis in SMG. Branching morphogenesis of epithelial rudiments denuded of mesenchyme was inhibited similarly, indicating that PI 3-kinase inhibitors act directly on the epithelium. Immunostaining and Western analysis demonstrated that the p85 isoform of PI 3-kinase is expressed in epithelium at levels higher than in the mesenchyme. A target of PI 3-kinase, Akt/protein kinase B (PKB), showed decreased phosphorylation at Ser(473) by Western analysis in the presence of PI 3-kinase inhibitors. The major lipid product of PI 3-kinase, phosphatidylinositol 3,4,5-trisphosphate (PIP(3)), was added exogenously to SMG via a membrane-transporting carrier in the presence of PI 3-kinase inhibitors and was found to stimulate cleft formation, the first step of branching morphogenesis. Together, these data indicate that PI 3-kinase plays a role in the regulation of epithelial branching morphogenesis in mouse SMG acting through a PIP(3) pathway.

Nrdp1/FLRF is a ubiquitin ligase promoting ubiquitination and degradation of the epidermal growth factor receptor family member, ErbB3

The epidermal growth factor receptor (EGFR/ErbB) family of receptor tyrosine kinases plays fundamental roles in the regulation of cell survival, proliferation, and differentiation. Here, we present evidence that ErbB3 is degraded by proteasomes, and that Nrdp1 (referred to as FLRF in mice) associates with ErbB3 and stimulates its ubiquitination and degradation by proteasomes. Nrdp1 mRNAs are expressed in a variety of human tissues. The N-terminal half of Nrdp1 possesses an atypical RING finger domain, which is required for enhancing ErbB3 degradation. Its C-terminal half by itself associates with ErbB3 and raises ErbB3 levels in cells, probably by acting as a dominant-negative form of Nrdp1. In cell-free systems, Nrdp1 has ubiquitin ligase (E3) activity and ubiquitinates ErbB3, as well as itself, in the presence of the ubiquitin-carrier protein (E2), UbcH5. These data indicate that Nrdp1 is a RING finger-type of ubiquitin ligase, which promotes degradation of ErbB3 by proteasomes and, thus, may be an important factor influencing cell growth.