Epidermal growth factor receptor and cancer: control of oncogenic signalling by endocytosis

Cell-surface metalloprotease ADAM12 is internalized by a clathrin- and Grb2-dependent mechanism

ADAM12 (A Disintegrin And Metalloprotease 12), a member of the ADAMs family of transmembrane proteins, is involved in ectodomain shedding, cell-adhesion and signaling, with important implications in cancer. Therefore, mechanisms that regulate the levels and activity of ADAM12 at the cell-surface are possibly crucial in these contexts. We here investigated internalization and subsequent recycling or degradation of ADAM12 as a potentially important regulatory mechanism. Our results show that ADAM12 is constitutively internalized primarily via the clathrin-dependent pathway and is subsequently detected in both early and recycling endosomes. The protease activity of ADAM12 does not influence this internalization mechanism. Analysis of essential elements for internalization established that proline-rich regions in the cytoplasmic domain of ADAM12, previously shown to interact with Src-homology 3 domains, were necessary for proper internalization. These sites in the ADAM12 cytoplasmic domain interacted with the adaptor protein growth factor receptor-bound protein 2 (Grb2) and knockdown of Grb2 markedly reduced ADAM12 internalization. These studies establish that internalization is indeed a mechanism that regulates ADAM cell surface levels and show that ADAM12 internalization involves the clathrin-dependent pathway and Grb2.

Dynamin II function is required for EGF-mediated Stat3 activation but not Erk1/2 phosphorylation

Signalling from receptor tyrosine kinases is elicited by ligand binding which initiates the activation of many downstream signalling cascades. Endocytosis has been widely accepted as one mechanism in which cells inactivate signalling by internalising and subsequently degrading activated receptors. However, it is now evident that endocytosis of signalling receptors is important in initiation and sustaining downstream signalling. We and others have previously shown that epidermal growth factor receptor (EGFR) overexpression and activation of signal transducer and activator of transcription 3 (Stat3) are associated with tumourigenicity. Here, we examine the role of endocytosis in EGFR signal attenuation and differential signalling. Inhibition of dynamin II (Dyn II), a GTPase required for endocytosis, with a small molecular weight inhibitor, led to reduced EGF-mediated Stat3 phosphorylation and transcriptional activity in the A431 and HN5 human tumour cell lines. However, Dyn II inhibition had minimal effect on EGF-mediated EGFR and Erk1/2 phosphorylation, which is often regarded responsible for the tumourigenic function of the EGFR. Interestingly, this effect on Stat3 activation was not due to reduced EGFR/Stat3 association. Likewise, cells transfected with Dyn II siRNA or stably transfected with Dyn II shRNA had reduced EGF-mediated phospho-Stat3 levels but similar EGF-mediated phospho-EGFR and phospho-Erk1/2 levels compared with controls. Dyn II siRNA also reduced Stat3 transcriptional reporter activity and inhibits Stat3 accumulating into the nucleus. Taken together, our data suggest that the activation status of Stat3 and Erk1/2 and the sustainability of these signals are potentially due to the spatial and temporal control of the EGFR within the cell. This notion may have implications on therapeutic targeting and efficacy when using inhibitors to proteins either regulating endocytosis and/or signalling.

Targeting Stat3 and Smad7 to restore TGF-β cytostatic regulation of tumor cells in vitro and in vivo

Transforming Growth Factor-β (TGF-β) and Epidermal Growth Factor (EGF) signaling pathways are both independently implicated as key regulators in tumor formation and progression. Here, we report that the tumor-associated overexpression of epidermal growth factor receptor (EGFR) desensitizes TGF-β signaling and its cytostatic regulation through specific and persistent Stat3 activation and Smad7 induction in vivo. In human tumor cell lines, reduction of TGF-β-mediated Smad2 phosphorylation, nuclear translocation and Smad3 target gene activation were observed when EGFR was overexpressed, but not in cells that expressed EGFR at normal levels. We identified Stat3, which is activated specifically and persistently by overexpressed EGFR, as a key signaling molecule responsible for the reduced TGF-β sensitivity. Stable knockdown of Stat3 using small hairpin RNA(shRNA) in Head and Neck (HN5) and Epidermoid (A431) tumor cell lines resulted in reduced growth compared with control shRNA-transfected cells when grown as subcutaneous tumor xenografts. Furthermore, xenografts with Stat3 knockdown displayed increased Smad3 transcriptional activity, increased Smad2 phosphorylation and decreased Smad7 expression compared with control xenografts in vivo. Consistently, Smad7 mRNA and protein expression was also significantly reduced when EGFR activity was blocked by a specific tyrosine kinase inhibitor, AG1478, or in Stat3 knockdown tumors. Similarly, Smad7 knockdown also resulted in enhanced Smad3 transcriptional activity in vivo. Importantly, there was no uptake of subcutaneous HN5 xenografts with Smad7 knockdown. Taken together, we demonstrate here that targeting Stat3 or Smad7 for knockdown results in resensitization of TGF-β's cytostatic regulation in vivo. Overall, these results establish EGFR/Stat3/Smad7/TGF-β signaling axis driving tumor growth, which can be targeted therapeutically.

A Miniaturized Ligand Binding Assay for EGFR

In order to study receptor abundance and its function in solutions or in homogenates from clinical specimen, methods such as sandwich or radioimmunoassays are most commonly employed. For the determination of epidermal growth factor receptor (EGFR), we describe the development of a miniaturized bead-based ligand binding assay using its ligand EGF as immobilized capture reagent. This assay was used to analyze lysates from cell lines, and the ligand-bound EGFR was detected using an EGFR-specific antibody combined with a fluorescence-based reporter system. In a proof-of concept study with lysates from breast biopsies, the assay allowed to classify breast cancer samples in accordance to clinically the relevant EGFR cut-off level. The study suggests that such a ligand binding receptor assay could become an integral part of protein profiling procedures to provide additional information about receptor functionality in addition to its abundance.

Thyroid-specific knockout of the tumor suppressor mitogen-inducible gene 6 activates epidermal growth factor receptor signaling pathways and suppresses nuclear factor-κB activity

BACKGROUND

Mitogen-inducible gene 6 (Mig-6) is a putative tumor suppressor gene and prognostic biomarker in papillary thyroid cancer. We hypothesized that Mig-6 knockout would activate pro-oncogenic signaling in mouse thyrocytes.

METHODS

We performed a thyroid-specific knockout using the Cre/loxP recombinase system.

RESULTS

Four knockout and 4 control mouse thyroids were harvested at 2 months of age. Immunoblotting confirmed Mig-6 ablation in knockout mice thyrocytes. Epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase (ERK) phosphorylation levels were increased in Mig-6 knockout compared to wild-type mice. Total EGFR levels were similar in knockout and wild-type mice. However, EGFR was absent in the caveolae-containing membrane fraction of knockout mice, indicating that Mig-6 depletion is associated with a change in the membrane distribution of EGFR. Although p65 localized to the nucleus in wild-type mice, it was distributed in both cytoplasm and nucleus in knockouts, suggesting that Mig-6 loss decreases p65 activity.

CONCLUSION

Our results confirm the feasibility of targeted, thyroid-specific gene knockout as a strategy for studying the relevance of specific genes in thyroid oncogenesis. We suggest that the loss of Mig-6 alters the membrane distribution of EGFR, which may limit receptor degradation and activate this oncogenic signaling pathway.

Activation of phosphatidylinositol 3-kinase/Akt signaling by EGF downregulates membranous E-cadherin and β-catenin and enhances invasion in nasopharyngeal carcinoma cells

Dysregulation of E-cadherin and β-catenin function in cell-cell adhesion is common in nasopharyngeal carcinoma (NPC) and correlates with metastatic disease. In this study, we examined the role of EGF-activated phosphatidylinositol 3-kinase (PI3K)-Akt signaling in E-cadherin and β-catenin regulation. We found that reduced membranous E-cadherin and β-catenin expression was positively correlated with Akt phosphorylation in NPC tissues. EGF treatment disrupted cell-cell adhesion and resulted in mesenchymal morphological features in NPC cell lines (TW01, TW04, and TW06). Western blot analysis showed that the E-cadherin protein level was partially reduced in TW04 cells only and the β-catenin levels were not considerably affected upon EGF treatment. In contrast, quantitative real-time RT-PCR showed that the E-cadherin, but not β-catenin, mRNA levels were markedly reduced by EGF in all cell lines. Immunofluorescent staining revealed that E-cadherin and β-catenin appeared to be markedly reduced on the cell surface and more localized in the cytoplasm. Inhibition of PI3K by LY294002 did not abolish the EGF-induced downregulation of E-cadherin protein or mRNA in TW04 cells but moderately increased the β-catenin protein level in TW01 cells and mRNA level in TW06 cells. However, LY294002 substantially restored or increased cell surface E-cadherin and β-catenin in all EGF-treated cell lines, in concordance with the inhibition of cell morphological changes. Moreover, LY294002 significantly blocked EGF-driven cell invasion, correlating with the elevation of membranous E-cadherin and β-catenin levels. In conclusion, EGF-induced epithelial-to-mesenchymal transition may not be only dependent on downregulation of E-cadherin protein/mRNA but also on mislocalization of E-cadherin and β-catenin. The mechanisms involved may be related, at least in part, to the PI3K-Akt pathway.

Primary and secondary therapeutic strategies for EGF receptor pathway inhibition in non-small-cell lung cancer

As diagnostic and therapeutic options increase, strategies for the treatment of non-small-cell lung cancer (NSCLC) are becoming more tailored for specific patient subpopulations and individual patients. The introduction of therapy targeted against the EGF receptor (EGFR) pathway has provided new treatment options for select patients with NSCLC. However, more than half of unselected NSCLC patients will fail to achieve disease stabilization on an EGFR tyrosine kinase inhibitor (TKI), and secondary resistance is observed in virtually all patients who initially respond or achieve disease stabilization. Efforts are underway to identify clinical and molecular predictors in patients who may benefit from treatment with EGFR TKIs. Recent strategies for targeting the EGFR pathway include combining EGFR TKIs with newer agents and developing second-generation irreversible EGFR TKIs, which may be used in patients who have failed treatment with first-generation EGFR TKIs.

Polymorphism of the epidermal growth factor receptor extracellular ligand binding domain: the dimer interface depends on domain stabilization

Epidermal growth factor receptors (EGFRs) and their cytoplasmic tyrosine kinases play important roles in cell proliferation and signaling. The EGFR extracellular domain (sEGFR) forms a dimer upon the binding of ligands, such as epidermal growth factor (EGF) and transforming growth factor α (TGFα). In this study, multiple molecular dynamics (MD) simulations of the 2:2 EGF·sEGFR3-512 dimer and the 2:2 TGFα·sEGFR3-512 dimer were performed in solvent and crystal environments. The simulations of systems comprising up to half a million atoms reveal part of the structural dynamics of which sEGFR dimers are capable. The solvent simulations consistently exhibited a prominent conformational relaxation from the initial crystal structures on the nanosecond time scale, leading to symmetry breaking and more extensive contacts between the two sEGFR monomers. In the crystal control simulation, this symmetry breaking and compaction was largely suppressed by crystal packing contacts. The simulations also provided evidence that the disordered domain IV of sEGFR may act as a stabilizing spacer in the dimer. Thus, the simulations suggest that the sEGFR dimer can take diverse configurations in solvent environments. These biologically relevant conformations of the EGFR signal transduction network can be controlled by contacts among the structural domains of sEGFR and its ligands.

Epidermal growth factor receptor inhibition in lung cancer: the evolving role of individualized therapy

Non-small cell lung cancer (NSCLC) is the major cause of cancer-related deaths in the USA and worldwide. Most patients present with advanced disease, and treatment options for these patients are generally limited to platinum-based chemotherapy and a few targeted therapies. Targeted agents currently in use for NSCLC inhibit oncogenic receptor tyrosine kinase pathways, such as the epidermal growth factor receptor (EGFR) pathway. While current EGFR-targeted agents, including erlotinib and gefitinib, may result in dramatic responses, they demonstrate efficacy in only a fraction of patients, and resistance to these agents frequently develops. In order to select patients most likely to benefit from blockade of EGFR pathways, investigators have focused on identifying molecular correlates of response to anti-EGFR therapy. New strategies to minimize the risk of resistance to EGFR inhibition have been employed in the development of next-generation EGFR tyrosine kinase inhibitors, such as PF00299804 and BIBW 2992; these include irreversibility of target binding, inhibition of multiple EGFR family receptors, and/or simultaneous inhibition of EGFR and other oncogenic pathways.

KRAS mutations in primary colorectal cancer tumors and related metastases: a potential role in prediction of lung metastasis

Background

KRAS mutations in colorectal cancer primary tumors predict resistance to anti-Epidermal Growth Factor Receptor (EGFR) monoclonal antibody therapy in patients with metastatic colorectal cancer, and thus represent a true indicator of EGFR pathway activation status.

Methodology/Principal Findings

KRAS mutations were retrospectively studied using polymerase chain reactions and subsequent sequencing of codons 12 and 13 (exon 2) in 110 patients with metastatic colorectal tumors. These studies were performed using tissue samples from both the primary tumor and their related metastases (93 liver, 84%; 17 lung, 16%). All patients received adjuvant 5-Fluorouracil-based polychemotherapy after resection of metastases. None received anti-EGFR therapy. Mutations in KRAS were observed in 37 (34%) of primary tumors and in 40 (36%) of related metastases, yielding a 94% level of concordance (kappa index 0.86). Patients with primary tumors possessing KRAS mutations had a shorter disease-free survival period after metastasis resection (12.0 vs 18.0 months; P = 0.035) than those who did not. A higher percentage of KRAS mutations was detected in primary tumors of patiens with lung metastases than in patients with liver metastases (59% vs 32%; p = 0.054). To further evaluate this finding we analyzed 120 additional patients with unresectable metastatic colorectal cancer who previously had their primary tumors evaluated for KRAS mutational status for clinical purposes. Separately, the analysis of these 120 patients showed a tendency towards a higher degree of KRAS mutations in primary tumors of patients with lung metastases, although it did not reach statistical significance. Taken together the group of 230 patients showed that KRAS was mutated significantly more often in the primary tumors of patients with lung metastases (57% vs 35%; P = 0.006).

Conclusions/Significance

Our results suggest a role for KRAS mutations in the propensity of primary colorectal tumors to metastasize to the lung.

Identification of tyrosine-phosphorylated proteins associated with metastasis and functional analysis of FER in human hepatocellular carcinoma cells

BACKGROUND

Aberrant activity of tyrosine-phosphorylated proteins is commonly associated with HCC metastasis. Cell signaling events driven by these proteins are implicated in numerous processes that alter cancer cell behavior. Exploring the activities and signaling pathways of these proteins in HCC metastasis may help in identifying new candidate molecules for HCC-targeted therapy.

METHODS

Hep3B (a nonmetastatic HCC cell line) and MHCC97H (a highly metastatic HCC cell line) were used in this study, and the tyrosine-phosphorylated proteins expressed in these cell lines were profiled by a phosphoproteomics technique based on LC-MS/MS. Protein-protein interaction and functional clustering analyses were performed to determine the activities of the identified proteins and the signaling pathways closely related to HCC metastasis.

RESULTS

In both cell lines, a total of 247 phosphotyrosine (pTyr) proteins containing 281 pTyr sites were identified without any stimulation. The involvement of almost 30% of these in liver or liver cancer has not been reported previously. Biological process clustering analysis indicated that pTyr proteins involved in cell motility, migration, protein autophosphorylation, cell-cell communication, and antiapoptosis functions were overexpressed during metastasis. Pathway clustering analysis revealed that signaling pathways such as those involved in EGFR signaling, cytokine- and chemokine-mediated signal transduction, and the PI3K and JAK-STAT cascades were significantly activated during HCC metastasis. Moreover, noncanonical regulation of the JNK cascade might also provide new targets for HCC metastasis. After comparing the pTyr proteins that were differentially expressed during HCC cell metastasis, we selected FER, a nonreceptor tyrosine kinase, and validated its role in terms of both expression and function. The data confirmed that FER might play a critical role in the invasion and metastasis of HCC.

CONCLUSION

The identification of pTyr proteins and signaling pathways associated with HCC metastasis could provide useful information for selecting new molecular intervention targets. Moreover, FER might serve as a novel drug target in future HCC therapy.

Desmoid cell motility is induced in vitro by rhEGF

Desmoid tumors are benign but locally invasive myofibroblastic lesions that arise predominantly in the abdominal wall or shoulder and are prone to aggressive local recurrences. A perceived association between desmoid activity and the expression of growth factors during pregnancy or following trauma suggests a cause-and-effect relationship between growth factor stimulation and desmoid invasiveness. We used Boyden Chambers to quantify cell motility in order to determine the effect of growth factor stimulation on desmoid cell migration. Desmoid cell cultures were treated under serum-free conditions with epidermal growth factor (rhEGF) or transforming growth factor alpha (rhTGFalpha). Additional cell cultures were pretreated under serum-free conditions with the EGF receptor (EGFR) inhibitor AG1478, alone or in combination with the TGFbeta1 receptor inhibitor SB431542, and then stimulated with growth factor prior to being assayed for cell motility. The experiments demonstrated a direct dose-dependent relationship between rhEGF stimulation and desmoid motility. In contrast, rhTGFalpha was less effective at inducing cell migration. rhEGF-induced cell migration could be diminished, but not reduced to control levels, by inhibiting EGFR. When EGF and TGFbeta1 receptors were inhibited simultaneously, the level of rhEGF-induced cell migration was reduced significantly beyond the level of cell migration generated by inhibition of EGFR alone.

Epidermal growth factor receptor phosphorylation sites Ser991 and Tyr998 are implicated in the regulation of receptor endocytosis and phosphorylations at Ser1039 and Thr1041

Aberrant expression, activation, and down-regulation of the epidermal growth factor receptor (EGFR) have causal roles in many human cancers, and post-translational modifications including phosphorylation and ubiquitination and protein-protein interactions directly modulate EGFR function. Quantitative mass spectrometric analyses including selected reaction monitoring (also known as multiple reaction monitoring) were applied to the EGFR and associated proteins. In response to epidermal growth factor (EGF) stimulation of cells, phosphorylations at EGFR Ser⁹⁹¹ and Tyr⁹⁹⁸ accumulated more slowly than at receptor sites involved in RAS-ERK signaling. Phosphorylation-deficient mutant receptors S991A and Y998F activated ERK in response to EGF but were impaired for receptor endocytosis. Consistent with these results, the mutant receptors retained a network of interactions with known signaling proteins including EGF-stimulated binding to the adaptor GRB2. Compared with wild type EGFR the Y998F variant had diminished EGF-stimulated interaction with the ubiquitin E3 ligase CBL, and the S991A variant had decreased associated ubiquitin. The endocytosis-defective mutant receptors were found to have elevated phosphorylation at positions Ser¹⁰³⁹ and Thr¹⁰⁴¹. These residues reside in a serine/threonine-rich region of the receptor previously implicated in p38 mitogen-activated protein kinase-dependent stress/cytokine-induced EGFR internalization and recycling (Zwang, Y., and Yarden, Y. (2006) p38 MAP kinase mediates stress-induced internalization of EGFR: implications for cancer chemotherapy. EMBO J. 25, 4195–4206). EGF-induced phosphorylations at Ser¹⁰³⁹ and Thr¹⁰⁴¹ were blocked by treatment of cells with SB-202190, a selective inhibitor of p38. These results suggest that coordinated phosphorylation of EGFR involving sites Tyr⁹⁹⁸, Ser⁹⁹¹, Ser¹⁰³⁹, and Thr¹⁰⁴¹ governs the trafficking of EGF receptors. This reinforces the notion that EGFR function is manifest through spatially and temporally controlled protein-protein interactions and phosphorylations.

Epidermal growth factor receptor abnormalities in the pathogenesis and progression of lung adenocarcinomas

To identify the characteristics and sequence of epidermal growth factor receptor (EGFR) abnormalities relevant to the pathogenesis and progression of lung adenocarcinoma, we performed a precise mapping analysis of EGFR mutation, gene copy number, and total and phosphorylated EGFR protein expression for the same tissue sites. We examined normal bronchial and bronchiolar epithelium (NBE) and tumor tissues obtained from 50 formalin-fixed lung adenocarcinomas, including 24 EGFR-mutant primary tumors with nine corresponding lymph node metastases and 26 wild-type primary tumors. NBE in 12 of 24 (50%) mutant and 3 of 26 (12%) wild-type tumors harbored EGFR mutations; these NBE also showed a lack of EGFR copy number increase and frequent EGFR (69%) and phosphorylated EGFR (33%) overexpression. EGFR mutation and protein overexpression were more frequent in NBE sites within tumors than in NBE sites adjacent to and distant from tumors, suggesting a localized field effect. Sites with high and low EGFR copy numbers were heterogeneously distributed in six of nine primary tumors and in one of eight metastases. EGFR protein overexpression was significantly higher in metastasis sites than in primary tumors. We conclude from our findings that EGFR mutations and protein overexpression are early phenomena in the pathogenesis of lung adenocarcinoma and that EGFR mutation precedes an increase in gene copy number. In EGFR-mutant adenocarcinoma metastases, the higher levels of EGFR overexpression and more homogeneously distributed high gene copy numbers suggest tumor progression. Our findings have important implications for the development of new strategies for targeted chemoprevention and therapy in lung adenocarcinoma using EGFR inhibitors.

Knockdown of GnT-Va expression inhibits ligand-induced downregulation of the epidermal growth factor receptor and intracellular signaling by inhibiting receptor endocytosis

Changes in the expression of N-glycan branching glycosyltransferases can alter cell surface receptor functions, involving their levels of cell surface retention, rates of internalization into the endosomal compartment, and subsequent intracellular signaling. To study in detail the regulation of signaling of the EGF receptor (EGFR) by GlcNAcbeta(1,6)Man branching, we utilized specific siRNA to selectively knockdown GnT-Va expression in the highly invasive human breast carcinoma line MDA-MB231, which resulted in the attenuation of its invasiveness-related phenotypes. Compared to control cells, ligand-induced downregulation of EGFR was significantly inhibited in GnT-Va-suppressed cells. This effect could be reversed by re-expression of GnT-Va, indicating that changes in ligand-induced receptor downregulation were dependent on GnT-Va activity. Knockdown of GnT-Va had no significant effect on c-Cbl mediated receptor ubiquitination and degradation, but did cause the inhibition of receptor internalization, showing that altered signaling and delayed ligand-induced downregulation of EGFR expression resulted from decreased EGFR endocytosis. Similar results were obtained with HT1080 fibrosarcoma cells treated with GnT-Va siRNA. Inhibited receptor internalization caused by the expression of GnT-Va siRNA appeared to be independent of galectin binding since decreased EGFR internalization in the knockdown cells was not affected by the treatment of the cells with lactose, a galectin inhibitor. Our results show that decreased GnT-Va activity due to siRNA expression in human carcinoma cells inhibits ligand-induced EGFR internalization, consequently resulting in delayed downstream signal transduction and inhibition of the EGF-induced, invasiveness-related phenotypes.