Subsets of epidermal growth factor receptors during activation and endocytosis

Predictive data-driven modeling of C-terminal tyrosine function in the EGFR signaling network

The epidermal growth factor receptor (EGFR) has been studied extensively because of its critical role in cellular signaling and association with disease. Previous models have elucidated interactions between EGFR and downstream adaptor proteins or showed phenotypes affected by EGFR. However, the link between specific EGFR phosphorylation sites and phenotypic outcomes is still poorly understood. Here, we employed a suite of isogenic cell lines expressing site-specific mutations at each of the EGFR C-terminal phosphorylation sites to interrogate their role in the signaling network and cell biological response to stimulation. Our results demonstrate the resilience of the EGFR network, which was largely similar even in the context of multiple Y-to-F mutations in the EGFR C-terminal tail, while also revealing nodes in the network that have not previously been linked to EGFR signaling. Our data-driven model highlights the signaling network nodes associated with distinct EGF-driven cell responses, including migration, proliferation, and receptor trafficking. Application of this same approach to less-studied RTKs should provide a plethora of novel associations that should lead to an improved understanding of these signaling networks.

Epidermal growth factor receptor and integrins meet redox signaling through P66shc and Rac1

This review examines the concerted role of Epidermal Growth Factor Receptor (EGFR) and integrins in regulating Reactive oxygen species (ROS) production through different signaling pathways. ROS as such are not always deleterious to the cells but they also act as signaling molecules, that regulates numerous indespensible physiological fuctions of life. Many adaptor proteins, particularly Shc and Grb2, are involved in mediating the downstream signaling pathways stimulated by EGFR and integrins. Integrin-induced activation of EGFR and subsequent tyrosine phosphorylation of a class of acceptor sites on EGFR leads to alignment and tyrosine phosphorylation of Shc, PLCγ, the p85 subunit of PI-3 K, and Cbl, followed by activation of the downstream targets Erk and Akt/PKB. Functional interactions between these receptors result in the activation of Rac1 via these adaptor proteins, thereby leading to Reactive Oxygen Species. Both GF and integrin activation can produce oxidants independently, however synergistically there is increased ROS generation, suggesting a mutual cooperation between integrins and GFRs for redox signalling. The ROS produced further promotes feed-forward stimulation of redox signaling events such as MAPK activation and gene expression. This relationship has not been reviewed previously. The literature presented here can have multiple implications, ranging from looking at synergistic effects of integrin and EGFR mediated signaling mechanisms of different proteins to possible therapeutic interventions operated by these two receptors. Furthermore, such mutual redox regulation of crosstalk between EGFR and integrins not only add to the established models of pathological oxidative stress, but also can impart new avenues and opportunities for targeted antioxidant based therapeutics.

Phosphorylation of REPS1 at Ser709 by RSK attenuates the recycling of transferrin receptor

RalBP1 associated EPS domain containing 1 (REPS1) is conserved from Drosophila to humans and implicated in the endocytic system. However, an exact role of REPS1 remains largely unknown. Here, we demonstrated that mitogen activated protein kinase kinase (MEK)-p90 ribosomal S6 Kinase (RSK) signaling pathway directly phosphorylated REPS1 at Ser709 upon stimulation by epidermal growth factor (EGF) and amino acid. While REPS2 is known to be involved in the endocytosis of EGF receptor (EGFR), REPS1 knockout (KO) cells did not show any defect in the endocytosis of EGFR. However, in the REPS1 KO cells and the KO cells reconstituted with a non-phosphorylatable REPS1 (REPS1 S709A), the recycling of transferrin receptor (TfR) was attenuated compared to the cells reconstituted with wild type REPS1. Collectively, we suggested that the phosphorylation of REPS1 at S709 by RSK may have a role of the trafficking of TfR.

Emerging Kinase Therapeutic Targets in Pancreatic Ductal Adenocarcinoma and Pancreatic Cancer Desmoplasia

Kinase drug discovery represents an active area of therapeutic research, with previous pharmaceutical success improving patient outcomes across a wide variety of human diseases. In pancreatic ductal adenocarcinoma (PDAC), innovative pharmaceutical strategies such as kinase targeting have been unable to appreciably increase patient survival. This may be due, in part, to unchecked desmoplastic reactions to pancreatic tumors. Desmoplastic stroma enhances tumor development and progression while simultaneously restricting drug delivery to the tumor cells it protects. Emerging evidence indicates that many of the pathologic fibrotic processes directly or indirectly supporting desmoplasia may be driven by targetable protein tyrosine kinases such as Fyn-related kinase (FRK); B lymphoid kinase (BLK); hemopoietic cell kinase (HCK); ABL proto-oncogene 2 kinase (ABL2); discoidin domain receptor 1 kinase (DDR1); Lck/Yes-related novel kinase (LYN); ephrin receptor A8 kinase (EPHA8); FYN proto-oncogene kinase (FYN); lymphocyte cell-specific kinase (LCK); tec protein kinase (TEC). Herein, we review literature related to these kinases and posit signaling networks, mechanisms, and biochemical relationships by which this group may contribute to PDAC tumor growth and desmoplasia.

Impact of USP8 Gene Mutations on Protein Deregulation in Cushing Disease

CONTEXT

Cushing disease (CD) is a rare disorder with severe sequels and incompletely understood pathogenesis. The underlying corticotroph adenomas harbor frequently somatic mutations in the ubiquitin-specific peptidase 8 (USP8) gene. These mutations render USP8 hyperactive and prevent client proteins from degradation.

OBJECTIVE

To investigate the impact of USP8 mutations on proteins deregulated in CD.

DESIGN

One hundred eight pituitary adenomas (75 corticotroph [58 USP8 wild type (WT) and 17 USP8 mutated], 14 somatotroph, and 19 nonfunctioning) were investigated by immunohistochemistry. All evaluated proteins [USP8, arginine vasopressin receptor 1b and 2, corticotropin-releasing hormone receptor, cAMP response element-binding protein (CREB), p27/kip1, cyclin E, heat shock protein 90 (HSP90), orphan nuclear receptor 4, epidermal growth factor receptor, histone deacetylase 2, glucocorticoid receptor, cyclin-dependent kinase 5 and Abelson murine leukemia viral oncogene homolog 1 enzyme substrate 1] were known to be deregulated in CD. Furthermore, AtT20 cells were transfected with USP8 to investigate the expression of possible downstream proteins by immunoblot.

RESULTS

Whereas most of the investigated proteins were not differentially expressed, the cell-cycle inhibitor p27 was significantly reduced in USP8 mutated corticotroph adenoma (H-score 2.0 ± 1.0 vs 1.1 ± 1.1 in WT adenomas; P = 0.004). In contrast, the chaperone HSP90 was expressed higher (0.5 ± 0.4 vs 0.2 ± 0.4; P = 0.29), and the phosphorylation of the transcription factor CREB was increased in USP8 mutated adenomas (1.30.5 ± 0.40.9 vs 0.70.5 ± 0.40.7; P = 0.014). Accordingly, AtT20 cells transfected with the USP8 P720R mutant had higher phosphorylated CREB (pCREB) levels than WT transfected cells (1.3 ± 0.14 vs 1 ± 0.23; P = 0.13).

CONCLUSIONS

We could demonstrate that USP8 mutations are associated with deregulation of p27/kip1, HSP90, and pCREB. These findings suggest that these proteins are direct or indirect clients of USP8 and could therefore be potential targets for therapeutic approaches in patients with CD.

Signaling pathways involved in human sperm hyperactivated motility stimulated by Zn2

To fertilize the egg, sperm cells must reside in the female reproductive tract for several hours during which they undergo chemical and motility changes collectively called capacitation. During capacitation, the sperm develop a unique type of motility known as hyperactivated motility (HAM). The semen contains Zn²⁺ in millimolar concentrations, whereas in the female reproductive tract the concentration is around 1 µM. In this study, we characterize the role of Zn ²⁺ in human sperm capacitation focusing on its effect on HAM. Western blot analysis revealed the presence of G protein-coupled receptor 39 (GPR39) type Zn-receptor localized mainly in the sperm tail. Zn ²⁺ at micromolar concentration stimulates HAM, which is mediated by a cascade involving GPR39-AC-cAMP-PKA-Src-EGFR and phospholipase C. Both the transmembrane adenylyl cyclase (AC) and the soluble-AC are involved in the stimulation of HAM by Zn ²⁺ . The development of HAM is precisely regulated by cyclic adenosine monophosphate, in which relatively low concentration (5-10 µM) stimulated HAM, whereas at 30 µM no stimulation occurred. A similar response was seen when different concentrations of Zn ²⁺ were added to the cells; low Zn ²⁺ stimulated HAM, whereas at relatively high Zn ²⁺ , no effect was seen. We further demonstrate that the Ca ²⁺ -channel CatSper involved in Zn ²⁺ -stimulated HAM. These data support a role for extracellular Zn ²⁺ acting via GPR39 to regulate signaling pathways in sperm capacitation, leading to HAM induction.

Signaling pathways involved in human sperm hyperactivated motility stimulated by Zn2

To fertilize the egg, sperm cells must reside in the female reproductive tract for several hours during which they undergo chemical and motility changes collectively called capacitation. During capacitation, the sperm develop a unique type of motility known as hyperactivated motility (HAM). The semen contains Zn²⁺ in millimolar concentrations, whereas in the female reproductive tract, the concentration is around 1 µM. In this study, we characterize the role of Zn²⁺ in human sperm capacitation focusing on its effect on HAM. Western blot analysis revealed the presence of GPR39-type Zn-receptor localized mainly in the sperm tail. Zn²⁺ at micromolar concentration stimulates HAM, which is mediated by a cascade involving GPR39-adenylyl cyclase (AC)-cyclic AMP (cAMP)-protein kinase A-tyrosine kinase Src (Src)-epidermal growth factor receptor and phospholipase C. Both the transmembrane AC and the soluble-AC are involved in the stimulation of HAM by Zn²⁺ . The development of HAM is precisely regulated by cAMP, in which relatively low concentration (5-10 µM) stimulated HAM, whereas at 30 µM no stimulation occurred. A similar response was seen when different concentrations of Zn²⁺ were added to the cells; low Zn²⁺ stimulated HAM, whereas at relatively high Zn²⁺ , no effect was seen. We further demonstrate that the Ca²⁺ -channel CatSper involved in Zn²⁺ -stimulated HAM. These data support a role for extracellular Zn²⁺ acting via GPR39 to regulate signaling pathways in sperm capacitation, leading to HAM induction.

Identification of novel pathways linking epithelial-to-mesenchymal transition with resistance to HER2-targeted therapy

Resistance to human epidermal growth factor receptor 2 (HER2)-targeted therapies in the treatment of HER2-positive breast cancer is a major clinical problem. To identify pathways linked to resistance, we generated HER2-positive breast cancer cell lines which are resistant to either lapatinib or AZD8931, two pan-HER family kinase inhibitors. Resistance was HER2 independent and was associated with epithelial-to-mesenchymal transition (EMT), resulting in increased proliferation and migration of the resistant cells. Using a global proteomics approach, we identified a novel set of EMT-associated proteins linked to HER2-independent resistance. We demonstrate that a subset of these EMT-associated genes is predictive of prognosis within the ERBB2 subtype of human breast cancers. Furthermore, targeting the EMT-associated kinases Src and Axl potently inhibited proliferation of the resistant cells, and inhibitors to these kinases may provide additional options for the treatment of HER2-independent resistance in tumors.

Increased EGFR Phosphorylation Correlates with Higher Programmed Death Ligand-1 Expression: Analysis of TKI-Resistant Lung Cancer Cell Lines

Despite the recent development of immunotherapies that target programmed death-1 (PD-1) or programmed death ligand-1 (PD-L1) in non-small cell lung cancer (NSCLC) treatment, these therapies are less effective in NSCLC patients with epidermal growth factor receptor (EGFR) mutations. However, the molecular mechanisms underlying this lower efficacy of immunotherapies in EGFR mutant lung cancers are still unclear. In this study, we analyzed PD-L1 protein expression in lung cancer cell lines with EGFR mutations prior to and after acquisition of resistance to EGFR tyrosine kinase inhibitors (TKIs). We found that parental lung cancer cell lines harboring EGFR mutations showed negative (PC9 and H3255 cells) and positive (HCC827 cells) staining for PD-L1 by immunohistochemistry. Comparing PD-L1 expression between EGFR-TKI resistant cell lines and their parental cells, we found that increased phosphorylation of EGFR was related to increased expression of PD-L1. Increased phosphorylation of EGFR was accompanied by the T790M secondary mutation. Acquired resistance cells with MET amplification or EGFR loss both showed decreased phosphorylation of EGFR and decreased PD-L1 expression. Our results indicate that lung cancer cell lines with EGFR mutations (parental cells) do not harbor high PD-L1 protein expression. In addition, EGFR phosphorylation affects PD-L1 expression after acquisition of resistance to EGFR-TKIs.

RhoC mediates epidermal growth factor-stimulated migration and invasion in head and neck squamous cell carcinoma

Epidermal growth factor receptor (EGFR) is overexpressed in head and neck squamous cell carcinoma (HNSCC) where it has been shown to promote tumor cell invasion upon phosphorylation. One mechanism by which EGFR promotes tumor progression is by activating signal cascades that lead to loss of E-cadherin, a transmembrane glycoprotein of the cell-cell adherence junctions; however mediators of these signaling cascades are not fully understood. One such mediator, RhoC, is activated upon a number of external stimuli, such as epidermal growth factor (EGF), but its role as a mediator of EGF-stimulated migration and invasion has not been elucidated in HNSCC. In the present study, we investigate the role of RhoC as a mediator of EGF-stimulated migration and invasion in HNSCC. We show that upon EGF stimulation, EGFR and RhoC were strongly activated in HNSCC. This resulted in activation of the phosphatidylinositol 3-Kinase Akt pathway (PI3K-Akt), phosphorylation of GSK-3β at the Ser⁹ residue, and subsequent down regulation of E-cadherin cell surface expression resulting in increased tumor cell invasion. Knockdown of RhoC restored E-cadherin expression and inhibited EGF-stimulated migration and invasion. This is the first report in HNSCC demonstrating the role RhoC plays in mediating EGF-stimulated migration and invasion by down-regulating the PI3K-Akt pathway and E-cadherin expression. RhoC may serve as a treatment target for HNSCC.

Microglia Activate Migration of Glioma Cells through a Pyk2 Intracellular Pathway

Glioblastoma is one of the most aggressive and fatal brain cancers due to the highly invasive nature of glioma cells. Microglia infiltrate most glioma tumors and, therefore, make up an important component of the glioma microenvironment. In the tumor environment, microglia release factors that lead to the degradation of the extracellular matrix and stimulate signaling pathways to promote glioma cell invasion. In the present study, we demonstrated that microglia can promote glioma migration through a mechanism independent of extracellular matrix degradation. Using western blot analysis, we found upregulation of proline rich tyrosine kinase 2 (Pyk2) protein phosphorylated at Tyr579/580 in glioma cells treated with microglia conditioned medium. This upregulation occurred in rodent C6 and GL261 as well as in human glioma cell lines with varying levels of invasiveness (U-87MG, A172, and HS683). siRNA knock-down of Pyk2 protein and pharmacological blockade by the Pyk2/focal-adhesion kinase (FAK) inhibitor PF-562,271 reversed the stimulatory effect of microglia on glioma migration in all cell lines. A lower concentration of PF-562,271 that selectively inhibits FAK, but not Pyk2, did not have any effect on glioma cell migration. Moreover, with the use of the CD11b-HSVTK microglia ablation mouse model we demonstrated that elimination of microglia in the implanted tumors (GL261 glioma cells were used for brain implantation) by the local in-tumor administration of Ganciclovir, significantly reduced the phosphorylation of Pyk2 at Tyr579/580 in implanted tumor cells. Taken together, these data indicate that microglial cells activate glioma cell migration/dispersal through the pro-migratory Pyk2 signaling pathway in glioma cells.

Inhibitory mechanism of Korean Red Ginseng on GM-CSF expression in UVB-irradiated keratinocytes

Background

UV-irradiated keratinocytes secrete various proinflammatory cytokines. UV-induced skin damage is mediated by growth factors and proinflammatory cytokines such as granulocyte macrophage colony stimulating factor (GM-CSF). In a previous study, we found that the saponin of Korean Red Ginseng (SKRG) decreased the expression of GM-CSF in UVB-irradiated SP-1 keratinocytes. In this study, we attempted to find the inhibitory mechanism of SKRG on UVB-induced GM-CSF expression in SP-1 keratinocytes.

Methods

We investigated the inhibitory mechanism of SKRG and ginsenosides from Panax ginseng on UVB-induced GM-CSF expression in SP-1 keratinocytes.

Results

Treatment with SKRG decreased the expression of GM-CSF mRNA and protein induced by irradiation of UVB in SP-1 keratinocytes. The phosphorylation of ERK was induced by UVB at 10 min, and decreased with SKRG treatment in SP-1 keratinocytes. In addition, treatment with SKRG inhibited the UVB-induced phosphorylation of epidermal growth factor receptor (EGFR), which is known to be an upstream signal of ERK. From these results, we found that the inhibition of GM-CSF expression by SKRG was derived from the decreased phosphorylation of EGFR. To identify the specific compound composing SKRG, we tested fifteen kinds of ginsenosides. Among these compounds, ginsenoside-Rh3 decreased the expression of GM-CSF protein and mRNA in SP-1 keratinocytes.

Conclusion

Taken together, we found that treatment with SKRG decreased the phosphorylation of EGFR and ERK in UVB-irradiated SP-1 keratinocytes and subsequently inhibited the expression of GM-CSF. Furthermore, we identified ginsenoside-Rh3 as the active saponin in Korean Red Ginseng.

EGF receptor-targeting peptide conjugate incorporating a near-IR fluorescent dye and a novel 1,4,7-triazacyclononane-based (64)Cu(II) chelator assembled via click chemistry

A new Boc-protected 1,4,7-triazacyclononane (TACN)-based pro-chelator compound featuring a "clickable" azidomethylpyridine pendant has been developed as a building block for the construction of multimodal imaging agents. Conjugation to a model alkyne (propargyl alcohol), followed by deprotection, generates a pentadentate ligand, as confirmed by X-ray crystallographic analysis of the corresponding distorted square-pyramidal Cu(II) complex. The ligand exhibits rapid (64)Cu(II)-binding kinetics (>95% radiochemical yield in <5 min) and a high resistance to demetalation. It may thus prove suitable for use in (64)Cu(II)-based in vivo positron emission tomography (PET). The new chelating building block has been applied to the construction of a bimodal (PET/fluorescence) peptide-based imaging probe targeting the epidermal growth factor (EGF) receptor, which is highly overexpressed on the surface of several types of cancer cells. The probe consists of a hexapeptide sequence, Leu-Ala-Arg-Leu-Leu-Thr (designated "D4"), followed by a Cys-β-Ala-β-Ala spacer, then a β-homopropargylglycine residue with the TACN-based chelator "clicked" to its side chain. A sulfonated near-infrared (NIR) fluorescent cyanine dye (sulfo-Cy5) was introduced at the N-terminus to study the EGF receptor-binding ability of the probe by laser-fluorescence spectroscopy. Binding was also confirmed by coimmunoprecipitation methods, and an apparent dissociation constant (Kd) of ca. 10 nM was determined from radioactivity-based measurements of probe binding to two EGF receptor-expressing cell lines (FaDu and A431). The probe is shown to be a biased or partial allosteric agonist of the EGF receptor, inducing phosphorylation of Thr669 and Tyr992, but not the Tyr845, Tyr998, Tyr1045, Tyr1068, or Tyr1148 residues of the receptor, in the absence of the orthosteric EGF ligand. Additionally, the probe was found to suppress the EGF-stimulated autophosphorylation of these latter residues, indicating that it is also a noncompetitive antagonist.

Sustained activation of protein kinase C induces delayed phosphorylation and regulates the fate of epidermal growth factor receptor

It is well established that acute activation of members of the protein kinase C (PKC) family induced by activation of cellular receptors can transduce extracellular stimuli to intracellular signaling. However, the functions of sustained activation of PKC are not well studied. We have previously shown that sustained activation of classical PKC isoforms over 15-60 min induced the formation of the pericentrion, a subset of recycling endosomes that are sequestered perinuclearly in a PKC- and phospholipase D (PLD)-dependent manner. In this study, we investigated the role of this process in the phosphorylation of EGFR on threonine 654 (Thr-654) and in the regulation of intracellular trafficking and fate of epidermal growth factor receptor (EGFR). Sustained stimulation of the angiotensin II receptor induced translocation of the EGFR to the pericentrion, which in turn prevents full access of EGF to the EGFR. These effects required PKC and PLD activities, and direct stimulation of PKC with phorbol esters was sufficient to reproduce these effects. Furthermore, activation of PKC induced delayed phosphorylation of EGFR on Thr-654 that coincided with the formation of the pericentrion and which was dependent on PLD and endocytosis of EGFR. Thus, Thr-654 phosphorylation required the formation of the pericentrion. On the other hand, using a T654A mutant of EGFR, we find that the phosphorylation on Thr-654 was not required for translocation of EGFR to the pericentrion but was required for protection of EGFR from degradation in response to EGF. Taken together, these results demonstrate a novel role for the pericentrion in the regulation of EGFR phosphorylation, which in turn is important for the fates of EGFR.

The Rak/Frk tyrosine kinase associates with and internalizes the epidermal growth factor receptor

Src is the founding member of a diverse family of intracellular tyrosine kinases, and Src has a key role in promoting cancer growth, in part, through its association with receptor tyrosine kinases. However, some Src-related proteins have widely divergent physiological roles, and these proteins include the Rak/Frk tyrosine kinase (Frk stands for Fyn-related kinase), which inhibits cancer cell growth and suppresses tumorigenesis. Rak/Frk phosphorylates and stabilizes the Pten tumor suppressor, protecting it from degradation, and Rak/Frk associates with the retinoblastoma (Rb) tumor suppressor. However, the role of Rak/Frk in receptor-mediated signaling is largely unknown. Here, we demonstrate that Rak/Frk associates with epidermal growth factor receptor (EGFR), increasing in activity and EGFR binding after EGF stimulation, when it decreases the pool of EGFR present at the plasma membrane. EGFR-Rak binding is direct, requires the SH2 and SH3 domains of Rak/Frk for efficient complex formation and is not dependent on the Grb2 adaptor protein. EGFR mutations are associated with increased EGFR activity and tumorigenicity, and we found that Rak/Frk associates preferentially with an EGFR exon 19 mutant, EGFRΔ747-749/A750P, compared with wild-type EGFR. Furthermore, Rak/Frk inhibited mutant EGFR phosphorylation at an activating site and dramatically decreased the levels of EGFRΔ747-749/A750P from the plasma membrane. Taken together, the results suggest that Rak/Frk inhibits EGFR signaling in cancer cells and has elevated activity against EGFR exon 19 mutants.

GnRH-(1-5) transactivates EGFR in Ishikawa human endometrial cells via an orphan G protein-coupled receptor

The decapeptide GnRH is known for its central role in the regulation of the hypothalamo-pituitary-gonadal axis. In addition, it is also known to have local effects within peripheral tissues. The zinc metalloendopeptidase, EC 3.4.24.15 (EP24.15), can cleave GnRH at the Tyr(5)-Gly(6) bond to form the pentapeptide, GnRH-(1-5). The central and peripheral effect of GnRH-(1-5) is different from its parent peptide, GnRH. In the current study, we examined the effect of GnRH-(1-5) on epidermal growth factor receptor (EGFR) phosphorylation and cellular migration. Using the Ishikawa cell line as a model of endometrial cancer, we demonstrate that GnRH-(1-5) stimulates epidermal growth factor release, increases the phosphorylation of EGFR (P < .05) at three tyrosine sites (992, 1045, 1068), and promotes cellular migration. In addition, we also demonstrate that these actions of GnRH-(1-5) are mediated by the orphan G protein-coupled receptor 101 (GPR101). Down-regulation of GPR101 expression blocked the GnRH-(1-5)-mediated release of epidermal growth factor and the subsequent phosphorylation of EGFR and cellular migration. These results suggest that GPR101 is a critical requirement for GnRH-(1-5) transactivation of EGFR in Ishikawa cells.

Central role of the EGF receptor in neurometabolic aging

A strong connection between neuronal and metabolic health has been revealed in recent years. It appears that both normal and pathophysiological aging, as well as neurodegenerative disorders, are all profoundly influenced by this "neurometabolic" interface, that is, communication between the brain and metabolic organs. An important aspect of this "neurometabolic" axis that needs to be investigated involves an elucidation of molecular factors that knit these two functional signaling domains, neuronal and metabolic, together. This paper attempts to identify and discuss a potential keystone signaling factor in this "neurometabolic" axis, that is, the epidermal growth factor receptor (EGFR). The EGFR has been previously demonstrated to act as a signaling nexus for many ligand signaling modalities and cellular stressors, for example, radiation and oxidative radicals, linked to aging and degeneration. The EGFR is expressed in a wide variety of cells/tissues that pertain to the coordinated regulation of neurometabolic activity. EGFR signaling has been highlighted directly or indirectly in a spectrum of neurometabolic conditions, for example, metabolic syndrome, diabetes, Alzheimer's disease, cancer, and cardiorespiratory function. Understanding the positioning of the EGFR within the neurometabolic domain will enhance our appreciation of the ability of this receptor system to underpin highly complex physiological paradigms such as aging and neurodegeneration.

Membrane sialidase NEU3 is highly expressed in human melanoma cells promoting cell growth with minimal changes in the composition of gangliosides

NEU3 is a membrane sialidase specific for gangliosides. Its increased expression and implication in some cancers have been reported. Here, we analyzed NEU3 expression in malignant melanoma cell lines and its roles in the cancer phenotypes. Quantitative RT-PCR revealed that high levels of the NEU3 gene were expressed at almost equivalent levels with those in colon cancers. To examine the effects of overexpression of NEU3, NEU3 cDNA-transfectant cells were established using a melanoma cell line SK-MEL-28 and its mutant N1 lacking GD3. SK-MEL-28 sublines overexpressing both the NEU3 gene and NEU3 enzyme activity showed no changes in both cell growth and ganglioside expression, while N1 cells showed a mild increase in cell proliferation with increased phosphorylation of the EGF receptor and neo-synthesis of Gb3 after NEU3 transfection. In contrast, NEU3 silencing resulted in a definite reduction in cell growth in a melanoma line MeWo, while ganglioside patterns underwent minimal changes. Phosphorylation levels of ERK1/2 with serum stimulation decreased in the NEU3-silenced cells. All these results suggest that NEU3 is highly expressed to enhance malignant phenotypes including apoptosis inhibition in malignant melanomas.

EGF promotes the shedding of soluble E-cadherin in an ADAM10-dependent manner in prostate epithelial cells

During the progression of prostate cancer, the epithelial adhesion molecule E-cadherin is cleaved from the cell surface by ADAM15 proteolytic processing, generating an extracellular 80kDa fragment referred to as soluble E-cadherin (sE-cad). Contrary to observations in cancer, the generation of sE-cad appears to correlate with ADAM10 activity in benign prostatic epithelium. The ADAM10-specific inhibitor INCB8765 and the ADAM10 prodomain inhibit the generation of sE-cad, as well as downstream signaling and cell proliferation. Addition of EGF or amphiregulin (AREG) to these untransformed cell lines increases the amount of sE-cad shed into the conditioned media, as well as sE-cad bound to EGFR. EGF-associated shedding appears to be mediated by ADAM10 as shRNA knockdown of ADAM10 results in reduced shedding of sE-cad. To examine the physiologic role of sE-cad on benign prostatic epithelium, we treated BPH-1 and large T immortalized prostate epithelial cells (PrEC) with an sE-cad chimera comprised of the human Fc domain of IgG(1), fused to the extracellular domains of E-cadherin (Fc-Ecad). The treatment of untransformed prostate epithelial cells with Fc-Ecad resulted in phosphorylation of EGFR and downstream signaling through ERK and increased cell proliferation. Pre-treating BPH-1 and PrEC cells with cetuximab, a therapeutic monoclonal antibody against EGFR, decreased the ability of Fc-Ecad to induce EGFR phosphorylation, downstream signaling, and proliferation. These data suggest that ADAM10-generated sE-cad may have a role in EGFR signaling independent of traditional EGFR ligands.

Translationally controlled tumor protein induces human breast epithelial cell transformation through the activation of Src

Translationally controlled tumor protein (TCTP) is implicated in cell growth and malignant transformation. TCTP has been found to interact directly with the third cytoplasmic domain of the α subunit of Na,K-ATPase, but whether this interaction has a role in tumorigenesis is unclear. In this study, we examined TCTP-induced tumor progression signaling networks in human breast epithelial cells, using adenoviral infection. We found that TCTP (a) induces Src release from Na,K-ATPase α subunit and Src activation; (b) phosphorylates tyrosine residues 845, 992, 1086, 1148 and 1173 on anti-epidermal growth factor receptor (EGFR); (c) activates PI3K (phosphatidylinositol 3-kinase )-AKT, Ras-Raf-MEK-ERK1/2, Rac-PAK1/2, MKK3/6-p38 and phospholipase C (PLC)-γ pathways; (d) enhances NADPH oxidase-dependent reactive oxygen species (ROS) generation; (e) stimulates cytoskeletal remodeling and cell motility and (f) upregulates matrix metalloproteinase (MMP) 3 and 13. These findings suggest that TCTP induces tumorigenesis through distinct multicellular signaling pathways involving Src-dependent EGFR transactivation, ROS generation and MMP expression.

Galectin-3 regulates MUC1 and EGFR cellular distribution and EGFR downstream pathways in pancreatic cancer cells

MUC1 is a transmembrane glycoprotein which is typically expressed at the apical membrane of normal epithelial cells. In cancer cells, the over-expression of MUC1 and its aberrant localization around the cell membrane and in the cytoplasm favours its interaction with different protein partners such as epidermal growth factor receptor (EGFR) and can promote tumour proliferation through the activation of oncogenic signalling pathways. Our aims were to study the mechanisms inducing MUC1 cytoplasmic localization in pancreatic cancer cells, and to decipher their impact on EGFR cellular localization and activation. Our results showed that galectin-3, an endogenous lectin, is co-expressed with MUC1 in human pancreatic ductal adenocarcinoma, and that it favours the endocytosis of MUC1 and EGFR. Depletion of galectin-3 by RNA interference increased the interaction between MUC1 and EGFR, EGFR and ERK-1,2 phosphorylation, and translocation of EGFR to the nucleus. On the contrary, silencing of galectin-3 led to a decrease of cyclin-D1 levels and of cell proliferation. The galectin-3-dependent regulation of MUC1/EGFR functions may represent an interesting mechanism modulating the EGFR-stimulated cell growth of pancreatic cancer cells.

Targeting erbB receptors

Our work is concerned with the origins and therapy of human cancers. Members of the epidermal growth factor receptor (EGFR) family of tyrosine kinases, also known as erbB or HER receptors, are over expressed and/or activated in many types of human tumors and represent important therapeutic targets in cancer therapy. Studies from our laboratory identified targeted therapy as a way to treat cancer. Rational therapeutics targeting and disabling erbB receptors have been developed to reverse the malignant properties of tumors. Reversal of the malignant phenotype, best seen with disabling the HER2 receptors using monoclonal antibodies is a distinct process from that seen with blocking of ligand binding to cognate receptors as has been done for EGFr receptors. Here we review the mechanisms of action deduced from a number of approaches developed in our laboratory and elsewhere, including monoclonal antibodies, peptide mimetics, recombinant proteins and small molecules. The biochemical and biological principles which have been uncovered during these studies of disabling HER2 homomeric or HER2-EGFr heteromeric receptors will help the development of novel and more efficient therapeutics targeting erbB family receptors.

Phosphoproteomics-based modeling defines the regulatory mechanism underlying aberrant EGFR signaling

Background

Mutation of the epidermal growth factor receptor (EGFR) results in a discordant cell signaling, leading to the development of various diseases. However, the mechanism underlying the alteration of downstream signaling due to such mutation has not yet been completely understood at the system level. Here, we report a phosphoproteomics-based methodology for characterizing the regulatory mechanism underlying aberrant EGFR signaling using computational network modeling.

Methodology/Principal Findings

Our phosphoproteomic analysis of the mutation at tyrosine 992 (Y992), one of the multifunctional docking sites of EGFR, revealed network-wide effects of the mutation on EGF signaling in a time-resolved manner. Computational modeling based on the temporal activation profiles enabled us to not only rediscover already-known protein interactions with Y992 and internalization property of mutated EGFR but also further gain model-driven insights into the effect of cellular content and the regulation of EGFR degradation. Our kinetic model also suggested critical reactions facilitating the reconstruction of the diverse effects of the mutation on phosphoproteome dynamics.

Conclusions/Significance

Our integrative approach provided a mechanistic description of the disorders of mutated EGFR signaling networks, which could facilitate the development of a systematic strategy toward controlling disease-related cell signaling.

HER1 signaling mediates extravillous trophoblast differentiation in humans

This study examines the role of HER1 signaling in the differentiation of proliferative extravillous trophoblast (EVT) into invasive EVT. Using the JAR choriocarcinoma cell line and placental villous explants as experimental models and immunohistochemical assessment of protein markers of EVT differentiation (downregulation of HER1 and Cx40 and upregulation of HER2 and alpha1 integrin), we show that the ability of decidual conditioned medium (DCM) to induce HER1/2 switching was abrogated in the presence of the HER1 antagonist, AG1478. Similarly, epidermal growth factor (EGF) treatment resulted in the downregulation of HER1 and an upregulation of HER2 expression, whereas co-incubation of EGF with AG1478 inhibited this response. However, EGF did not downregulate Cx40 or induce migration of EVT. In contrast, heparin-binding epidermal-like growth factor (HBEGF) stimulated dose-dependent JAR cell migration, which was inhibited by both AG1478 and AG825 (HER2 antagonist). Western blot analysis of HER1 activation demonstrated that HBEGF-mediated phosphorylation of the HER1 Tyr992 and Tyr1068 sites, while EGF activated the Tyr1045 site. Moreover, HBEGF induced a stronger and more sustained activation of both the mitogen-activated protein kinase and phosphoinositol 3 kinase (PIK3) signaling pathways. Migration assays using a panel of signaling pathway inhibitors demonstrated that the HBEGF-mediated migration was dependent on the PIK3 pathway. These results demonstrate that HBEGF-mediated HER1 signaling through PIK3 is an important component of EVT invasion.

Chromosome 7 and 19 trisomy in cultured human neural progenitor cells

Background

Stem cell expansion and differentiation is the foundation of emerging cell therapy technologies. The potential applications of human neural progenitor cells (hNPCs) are wide ranging, but a normal cytogenetic profile is important to avoid the risk of tumor formation in clinical trials. FDA approved clinical trials are being planned and conducted for hNPC transplantation into the brain or spinal cord for various neurodegenerative disorders. Although human embryonic stem cells (hESCs) are known to show recurrent chromosomal abnormalities involving 12 and 17, no studies have revealed chromosomal abnormalities in cultured hNPCs. Therefore, we investigated frequently occurring chromosomal abnormalities in 21 independent fetal-derived hNPC lines and the possible mechanisms triggering such aberrations.

Methods and Findings

While most hNPC lines were karyotypically normal, G-band karyotyping and fluorescent in situ hybridization (FISH) analyses revealed the emergence of trisomy 7 (hNPC⁺⁷) and trisomy 19 (hNPC⁺¹⁹), in 24% and 5% of the lines, respectively. Once detected, subsequent passaging revealed emerging dominance of trisomy hNPCs. DNA microarray and immunoblotting analyses demonstrate epidermal growth factor receptor (EGFR) overexpression in hNPC⁺⁷ and hNPC⁺¹⁹ cells. We observed greater levels of telomerase (hTERT), increased proliferation (Ki67), survival (TUNEL), and neurogenesis (β_III-tubulin) in hNPC⁺⁷ and hNPC⁺¹⁹, using respective immunocytochemical markers. However, the trisomy lines underwent replicative senescence after 50–60 population doublings and never showed neoplastic changes. Although hNPC⁺⁷ and hNPC⁺¹⁹ survived better after xenotransplantation into the rat striatum, they did not form malignant tumors. Finally, EGF deprivation triggered a selection of trisomy 7 cells in a diploid hNPC line.

Conclusions

We report that hNPCs are susceptible to accumulation of chromosome 7 and 19 trisomy in long-term cell culture. These results suggest that micro-environmental cues are powerful factors in the selection of specific hNPC aneuploidies, with trisomy of chromosome 7 being the most common. Given that a number of stem cell based clinical trials are being conducted or planned in USA and a recent report in PLoS Medicine showing the dangers of grafting an inordinate number of cells, these data substantiate the need for careful cytogenetic evaluation of hNPCs (fetal or hESC-derived) before their use in clinical or basic science applications.

Multiplexed Cell Signaling Analysis of Metastatic and Nonmetastatic Colorectal Cancer Reveals COX2-EGFR Signaling Activation as a Potential Prognostic Pathway Biomarker

The identification of prognostic determinants of colorectal cancer (CRC), including prediction of occult metastasis, is of urgent consideration, based on the tremendous differences in outcome and survival between patients who present with metastasis or develop metastasis versus those patients with organ-confined or nonrecurrent disease. Currently, a great deal of attention has been focused on using gene expression profiles of tumor specimens as a launch point for prognostic biomarker discovery. In our study, we chose to focus on functional protein-based pathway biomarkers as a new information archive because it is these proteins that form the functional signaling networks that control cell growth, motility, apoptosis, survival, and differentiation. We used reverse-phase protein microarray analysis of laser capture microdissected CRC tumor specimens to profile broad cell signaling pathways from patients who presented with liver metastasis versus patients who remained recurrence free after follow-up. Our results indicate that members of the EGFR and COX2 signaling pathways appear differentially activated in the primary tumors of patients with synchronous metastatic disease. If validated in larger study sets, this pathway defect might be useful as a prognostic clinical tool as well as a guide to potential therapeutic intervention strategies that target occult disease and/or preventative measure.

ERK-dependent threonine phosphorylation of EGF receptor modulates receptor downregulation and signaling

Epidermal growth factor (EGF) signaling is critical in normal and aberrant cellular behavior. Extracellular signal-regulated kinase (ERK) mediates important downstream aspects of EGF signaling. Additionally, EGFR undergoes MEK1-dependent ERK consensus site phosphorylation in response to EGF or cytokines such as growth hormone (GH) and prolactin (PRL). GH- or PRL-induced EGFR phosphorylation alters subsequent EGF-induced EGFR downregulation and signal characteristics in an ERK-dependent fashion. We now use reconstitution to study mutation of the sole EGFR ERK phosphorylation consensus residue, (669)T. CHO-GHR cells, which lack EGFR and express GHR, were stably transfected to express human wild-type or T669A ((669)T changed to alanine) EGFRs at similar abundance. Treatment of cells with GH or EGF caused phosphorylation of WT, but not T669A EGFR, in an ERK activity-dependent fashion that was detected with an antibody that recognizes phosphorylation of ERK consensus sites, indicating that (669)T is required for this phosphorylation. Notably, EGF-induced downregulation of EGFR abundance was much more rapid in cells expressing EGFR T669A vs. WT EGFR. Further, pretreatment with the MEK1/ERK inhibitor PD98059 enhanced EGF-induced EGFR loss in cells expressing WT EGFR, but not EGFR T669A, suggesting that the ERK-dependent effects on EGFR downregulation required phosphorylation of (669)T. In signaling experiments, EGFR T669A displayed enhanced acute (15 min) EGFR tyrosine phosphorylation (reflecting EGFR kinase activity) compared to WT EGFR. Further, acute EGF-induced ubiquitination of WT EGFR was markedly enhanced by PD98059 pretreatment and was increased in EGFR T669A-expressing cells independent of PD98059. These signaling data suggest that ERK-mediated (669)T phosphorylation negatively modulates EGF-induced EGFR kinase activity. We furthered these investigations using a human fibrosarcoma cell line that endogenously expresses EGFR and ErbB-2 and also harbors an activating Ras mutation. In these cells, EGFR was constitutively detected with the ERK consensus site phosphorylation-specific antibody and EGF-induced EGFR downregulation was modest, but was substantially enhanced by pretreatment with MEK1/ERK inhibitor. Collectively, these data indicate that ERK activity, by phosphorylation of a threonine residue in the EGFR juxtamembrane cytoplasmic domain, modulates EGFR trafficking and signaling.

Activation of kinin B receptor triggers differentiation of cultured human keratinocytes

BACKGROUND

Keratinocyte life span is modulated by receptors that control proliferation and differentiation, key processes during cutaneous tissue repair. The kinin B(1) receptor (B(1)R) has been reported in normal and pathological human skin, but so far there is no information about its role in keratinocyte biology.

OBJECTIVES

To determine the consequence of kinin B(1)R stimulation on tyrosine phosphorylation, a key signalling mechanism involved in keratinocyte proliferation and differentiation.

METHODS

Subconfluent primary cultures of human keratinocytes were used to investigate tyrosine phosphorylation, epidermal growth factor receptor (EGFR) transactivation, cell proliferation and keratinocyte differentiation. Cell proliferation was assessed by measuring bromodeoxyuridine incorporation whereas assessment of cell differentiation was based on the expression of filaggrin, cytokeratin 10 (CK10) and involucrin.

RESULTS

The major proteins phosphorylated, after B(1)R stimulation, were of molecular mass 170, 125, 89 and 70 kDa. The 170- and 125-kDa proteins were identified as EGFR and p125(FAK), respectively. Phosphorylation was greatly reduced by GF109203X and by overexposure of keratinocytes to phorbol 12-myristate 13-acetate, indicating the participation of protein kinase C. B(1)R stimulation did not increase [Ca(2+)]i, but triggered EGFR transactivation, an event that involved phosphorylation of Tyr(845), Tyr(992) and Tyr(1068) of EGFR. B(1)R stimulation did not elicit keratinocyte proliferation, but triggered cell differentiation, visualized as an increase of filaggrin, CK10 and involucrin. Blockade of EGFR tyrosine kinase by AG1478, before B(1)R stimulation, produced an additional increase in filaggrin expression.

CONCLUSIONS

The kinin B(1)R may contribute to keratinocyte differentiation and migration by triggering specific tyrosine signalling pathways or by interacting with the ErbB receptor family.

Cellular and in vivo activity of JNJ-28871063, a nonquinazoline pan-ErbB kinase inhibitor that crosses the blood-brain barrier and displays efficacy against intracranial tumors

JNJ-28871063 is a potent and highly selective pan-ErbB kinase inhibitor from a novel aminopyrimidine oxime structural class that blocks the proliferation of epidermal growth factor receptor (EGFR; ErbB1)- and ErbB2-overexpressing cells but does not affect the growth of non-ErbB-overexpressing cells. Treatment of human cancer cells with JNJ-28871063 inhibited phosphorylation of functionally important tyrosine residues in both EGFR and ErbB2 and blocked downstream signal transduction pathways responsible for proliferation and survival. A single dose of compound reduced phosphorylation of ErbB2 receptors in tumor-bearing mice, demonstrating target suppression in vivo. Tissue distribution studies show that JNJ-28871063 crosses the blood-brain barrier and penetrates into tumors, where it is able to accumulate to higher levels than those found in the plasma. JNJ-28871063 showed oral antitumor activity in human tumor xenograft models that overexpress EGFR and ErbB2. In an intracranial ErbB2-overexpressing tumor model, JNJ-28871063 extended survival relative to untreated animals. The brain is a primary site of metastasis for EGFR-overexpressing lung cancers and ErbB2-overexpressing breast cancers. Therefore, the ability to penetrate into the brain could be an advantage over existing therapies such as trastuzumab (Herceptin) and cetuximab (Erbitux), which are antibodies and do not cross the blood-brain barrier. These results show that JNJ-28871063 is orally bioavailable, has activity against EGFR and ErbB2-dependent tumor xenografts, and can penetrate into the brain and inhibit ErbB2-overexpressing tumor growth.

Species differences in renal Src activity direct EGF receptor regulation in life or death response to EGF

In rodent models of obstructive nephropathy, exogenous epidermal growth factor (EGF) attenuates tubule cell death in rats and exacerbates cell death in mice. To identify species differences in EGF receptor (EGFR) regulation and signaling, cell lysates were prepared from rat, mouse, and human proximal tubule cells (PTC) and compared by immunoblot analysis for expression and phosphorylation of Src and EGFR. Frozen kidney tissue was also analyzed. Results indicate mouse PTC have constitutive Src- and EGFR-kinase activities not detected in rat or human PTC. Immunoblots of rat, mouse, and human kidney homogenates confirmed this finding in vivo. Src-specific inhibitor PP2 and EGFR kinase inhibitor AG1478 decreased EGF-induced apoptosis in mouse PTC by 74% (P < 0.001) and 70% (P < 0.001), respectively. Expression of a constitutive Src mutant cDNA in rat PTC rendered cells susceptible to EGF-induced death. EGF decreased stretch-induced apoptosis by 66% (P < 0.001) relative to vehicle control in human PTC, similar to rat PTC response. We conclude that elevated Src activity in mouse tubular cells alters downstream EGFR signaling and increases susceptibility to EGF-induced cell death. The unexpected finding that a therapeutic agent (EGF) in rats is detrimental in mice underscores the importance of determining which animal best represents the response of human kidneys to a given agent.

Repulsion of cerebellar granule neurons by chondroitin sulfate proteoglycans is mediated by MAPK pathway

Inhibitory molecules associated with myelin and glial scar formation inhibit axon regeneration after an injury to the central nervous system (CNS). Chondroitin sulfate proteoglycans (CSPGs) that are expressed in the scar contribute to the non-permissive properties of the CNS environment. Here, we employed a spot substrate assay and demonstrated that CSPGs have a repulsive effect on cell bodies as well as neurites of the postnatal cerebellar granule neurons (CGNs) in vitro. Through a brief inhibitor screen, we observed that the effects of CSPGs were abolished in the presence of mitogen-activated protein kinase (MAPK) inhibitor or epidermal growth factor receptor (EGFR) inhibitor. The MAPK pathway was activated in the neurons treated with CSPGs, and this activation was dependent on EGFR. Thus, CSPGs triggered the inhibition of CGNs through the activation of the EGFR-mediated MAPK pathway.

Dual silencing of the EGF and type 1 IGF receptors suggests dominance of IGF signaling in human breast cancer cells

Signaling via the type 1 insulin-like growth factor receptor (IGF1R) confers resistance to EGF receptor (EGFR) inhibitors. It is plausible that reciprocal EGFR compensation could mediate resistance to IGF1R inhibition, prompting us to investigate effects of IGF1R depletion on EGFR signaling in breast cancer cells expressing relatively high (MDA-MB-468) or low (MCF7) EGFR. Transient IGF1R knockdown induced enhanced phosphorylation of the EGFR and its effectors JNK, ERKs and STAT5, but this did not prevent apoptosis induction and inhibition of clonogenic survival following IGF1R knockdown. We used IGF1R shRNA to induce chronic IGF1R depletion, and achieved stable gene silencing in MCF-7 cells; here, EGFR overexpression led to EGFR hyperphosphorylation, again without abrogating survival inhibition after IGF1R knockdown. In both cell lines, dual receptor knockdown prevented EGFR hyperphosphorylation, but induced no greater inhibition of clonogenic survival than IGF1R knockdown alone. These results suggest that the EGFR cannot compensate for IGF1R depletion, and are encouraging for the strategy of IGF1R targeting.

Scalaradial inhibition of epidermal growth factor receptor-mediated Akt phosphorylation is independent of secretory phospholipase A2

The marine natural product 12-epi-scalaradial (SLD) is a specific secretory phospholipase A(2) (sPLA(2)) inhibitor. However, little is known about whether this compound has other pharmacological effects. Here, we revealed a novel effect of SLD on epidermal growth factor receptor (EGFR)-mediated Akt phosphorylation. SLD dose- and time-dependently inhibited epidermal growth factor (EGF)-stimulated Akt phosphorylation, which is required for Akt activation. SLD also blocked the EGF-stimulated membrane translocation of 3-phosphoinositide-dependent protein kinase 1 and inhibited phosphatidylinositol 3-kinase activity. This inhibition is specific for SLD because other phospholipase inhibitors, including sPLA(2) inhibitor thioetheramide-phosphatidylcholine, cytosolic PLA(2) inhibitor arachidonyl trifluoromethyl ketone, cytosolic PLA(2) and Ca(2+)-independent PLA(2) inhibitor methyl arachidonyl fluorophosphonate, phospholipase C inhibitor U73122, and cyclooxygenases inhibitor indomethacin, failed to inhibit EGF-stimulated Akt phosphorylation. Furthermore, arachidonic acid, the main sPLA(2)-catalyzed metabolite, was not able to rescue SLD inhibition of EGF-stimulated Akt phosphorylation. Overexpression of group IIA or group X sPLA(2) did not reverse the inhibitory effect of SLD on Akt phosphorylation, either. Our results demonstrate that SLD inhibits EGFR-mediated Akt phosphorylation, and this novel effect of SLD is independent of sPLA(2).

Fully automated synthesis of (phospho)peptide arrays in microtiter plate wells provides efficient access to protein tyrosine kinase characterization

Background

Synthetic peptides have played a useful role in studies of protein kinase substrates and interaction domains. Synthetic peptide arrays and libraries, in particular, have accelerated the process. Several factors have hindered or limited the applicability of various techniques, such as the need for deconvolution of combinatorial libraries, the inability or impracticality of achieving full automation using two-dimensional or pin solid phases, the lack of convenient interfacing with standard analytical platforms, or the difficulty of compartmentalization of a planar surface when contact between assay components needs to be avoided. This paper describes a process for synthesis of peptides and phosphopeptides on microtiter plate wells that overcomes previous limitations and demonstrates utility in determination of the epitope of an autophosphorylation site phospho-motif antibody and utility in substrate utilization assays of the protein tyrosine kinase, p60^c-src.

Results

The overall reproducibility of phospho-peptide synthesis and multiplexed EGF receptor (EGFR) autophosphorylation site (pY1173) antibody ELISA (9H2) was within 5.5 to 8.0%. Mass spectrometric analyses of the released (phospho)peptides showed homogeneous peaks of the expected molecular weights. An overlapping peptide array of the complete EGFR cytoplasmic sequence revealed a high redundancy of 9H2 reactive sites. The eight reactive phospopeptides were structurally related and interestingly, the most conserved antibody reactive peptide motif coincided with a subset of other known EGFR autophosphorylation and SH2 binding motifs and an EGFR optimal substrate motif. Finally, peptides based on known substrate specificities of c-src and related enzymes were synthesized in microtiter plate array format and were phosphorylated by c-Src with the predicted specificities. The level of phosphorylation was proportional to c-Src concentration with sensitivities below 0.1 Units of enzyme.

Conclusions

The ability of this method to interface with various robotics and instrumentation is highly flexible since the microtiter plate is an industry standard. It is highly scalable by increasing the surface area within the well or the number of wells and does not require specialized robotics. The microtiter plate array system is well suited to the study of protein kinase substrates, antigens, binding molecules, and inhibitors since these all can be quantitatively studied at a single uniform, reproducible interface.

Met5-enkephalin-induced cardioprotection occurs via transactivation of EGFR and activation of PI3K

Our previous studies indicated that opioid-induced cardioprotection occurs via activation of mitochondrial ATP-sensitive K(+) (K(ATP)) channels. However, other elements of the Met(5)-enkephalin (ME) cardioprotection pathway are not fully characterized. In the present study, we investigated the role of tyrosine kinase, MAPK, and phosphatidylinositol 3-kinase (PI3K) signaling in ME-induced protection. Ca(2+)-tolerant, adult rabbit cardiomyocytes were isolated by collagenase digestion and subjected to simulated ischemia for 180 min. ME was administered 15 min before the 180 min of simulated ischemia; blockers were administered 15 min before ME. Cell death was assessed by trypan blue as a function of time. The epidermal growth factor receptor (EGFR) kinase inhibitor AG-1478 (250 nM) blocked ME-induced protection, but the inactive analog AG-9 (100 microM) did not. Treatment with herbimycin (1 microM) completely eliminated ME-induced protection. To verify that ME activates EGFR and to determine the involvement of Src, Western blotting of EGFR was performed after ME administration with and without herbimycin A. ME resulted in herbimycin-sensitive robust phosphorylation of EGFR at Tyr(992) and Tyr(1068). Administration of the selective MAPK inhibitor PD-98059 (10 nM) and the specific MEK1/2 inhibitor U-0126 (10 microM) also inhibited ME-induced cardioprotection. ME-induced ERK1/2 phosphorylation was significantly reduced by PD-98059, the EGFR kinase inhibitor PD-153035 (10 microM), and chelerythrine (2 microM). The PI3K inhibitor LY-294002 (20 microM) abrogated ME-induced protection, and ME-induced Akt phosphorylation at Ser(473) was suppressed by LY-294002, PD-153035, and chelerythrine. We conclude that ME-induced cardioprotection is mediated via Src-dependent EGFR transactivation and activation of the PI3K and MAPK pathways.

EGFR-independent activation of p38 MAPK and EGFR-dependent activation of ERK1/2 are required for ROS-induced renal cell death

2,3,5-Tris-(glutathion-S-yl)hydroquinone (TGHQ), a reactive metabolite of the nephrotoxicant hydroquinone, induces the ROS-dependent activation of MAPKs, followed by histone H3 phosphorylation and oncotic cell death in renal proximal tubule epithelial cells (LLC-PK(1)). Cell death and histone H3 phosphorylation are attenuated by pharmacological inhibition of p38 MAPK or ERK1/2 pathways. Because TGHQ, but not epidermal growth factor (EGF), induces histone H3 phosphorylation and cell death in LLC-PK(1) cells, we hypothesized that there are differences in the mechanisms by which TGHQ and EGF induce activation of the EGF receptor (EGFR). We therefore compared the relative ability of TGHQ, H(2)O(2), and EGF to activate EGFR and MAPKs and found that p38 MAPK activation is EGFR independent, whereas ERK1/2 activation occurs mainly through EGFR activation. TGHQ, H(2)O(2), and EGF induce different EGFR tyrosine phosphorylation profiles that likely influence the subsequent differential kinetics of MAPK activation. We next transfected LLC-PK(1) cells with a dominant negative p38 MAPK-expressing plasmid (pcDNA3-DNp38). TGHQ failed to induce phosphorylation of p38 MAPK and its substrate, MK-2, in pcDNA3-DNp38-transfected cells, indicating loss of function of p38 MAPK. In untransfected, pcDNA3 or pcDNA3-p38 (native)-transfected LLC-PK(1) cells, Hsp27 was intensively phosphorylated after TGHQ treatment, whereas in pcDNA3-DNp38-transfected cells, TGHQ failed to induce Hsp27 phosphorylation. Thus EGFR-independent p38 MAPK and EGFR-dependent ERK1/2 activation by TGHQ lead to the activation of two downstream signaling factors, i.e., histone H3 and Hsp27 phosphorylation, which have in common the potential ability to remodel chromatin.

Molecular mechanism for a role of SHP2 in epidermal growth factor receptor signaling

The Src homology 2-containing phosphotyrosine phosphatase (SHP2) is primarily a positive effector of receptor tyrosine kinase signaling. However, the molecular mechanism by which SHP2 effects its biological function is unknown. In this report, we provide evidence that defines the molecular mechanism and site of action of SHP2 in the epidermal growth factor-induced mitogenic pathway. We demonstrate that SHP2 acts upstream of Ras and functions by increasing the half-life of activated Ras (GTP-Ras) in the cell by interfering with the process of Ras inactivation catalyzed by Ras GTPase-activating protein (RasGAP). It does so by inhibition of tyrosine phosphorylation-dependent translocation of RasGAP to the plasma membrane, to its substrate (GTP-Ras) microdomain. Inhibition is achieved through the dephosphorylation of RasGAP binding sites at the level of the plasma membrane. We have identified Tyr992 of the epidermal growth factor receptor (EGFR) to be one such site, since its mutation to Phe renders the EGFR refractory to the effect of dominant-negative SHP2. To our knowledge, this is the first report to outline the site and molecular mechanism of action of SHP2 in EGFR signaling, which may also serve as a model to describe its role in other receptor tyrosine kinase signaling pathways.

ERBB receptor tyrosine kinases and cellular radiation responses

Ionizing radiation induces in autocrine growth-regulated carcinoma and malignant glioma cells powerful cytoprotective responses that confer relative resistance to consecutive radiation exposures. Understanding the mechanisms of these responses should provide new molecular targets for tumor radiosensitization. ERBB and other receptor Tyr kinases have been identified as immediate early response gene products that are activated by radiation within minutes, as by their physiological growth factor ligands, and induce secondary stimulation of cytoplasmic protein kinase cascades. The simultaneous activation of all receptor Tyr kinases and nonreceptor Tyr kinases leads to complex cytoprotective responses including increased cell proliferation, reduced apoptosis and enhanced DNA repair. Since these responses contribute to cellular radioresistance, ERBB1, the most extensively studied ERBB receptor, is examined as a target for tumor cell radiosensitization. The three methods of ERBB1 inhibition include blockade of growth factor binding by monoclonal antibody against the ligand-binding domain, inhibition of the receptor Tyr kinase-mediating receptor activation, and overexpression of a dominant-negative epidermal growth factor receptor-CD533 that lacks the COOH-terminal 533 amino acids and forms nonfunctional heterodimeric complexes with wild-type receptors. All the three approaches enhance radiation toxicity in vitro and in vivo. The different mechanisms of inhibition have contributed to the understanding of cellular responses to radiation, vary in relative effectiveness and pose different challenges for translation.

Loss of HSulf-1 up-regulates heparin-binding growth factor signaling in cancer

Emerging data suggest that signaling by heparin-binding growth factors is influenced by the sulfation state of N-acetylglucosamine residues of heparan sulfate proteoglycans (HSPGs). Here we report that the recently identified protein HSulf-1, a heparin-degrading endosulfatase, encodes a cell surface-associated enzyme that diminishes sulfation of cell surface HSPGs. The message encoding this enzyme is readily detectable in a variety of normal tissues, including normal ovarian surface epithelial cells, but is undetectable in 5 of 7 ovarian carcinoma cell lines and markedly diminished or undetectable in approximately 75% of ovarian cancers. Similar down-regulation is also observed in breast, pancreatic, renal cells, and hepatocellular carcinoma lines. Re-expression of HSulf-1 in ovarian cancer cell lines resulted in diminished HSPG sulfation, diminished phosphorylation of receptor tyrosine kinases that require sulfated HSPGs as co-receptors for their cognate ligands, and diminished downstream signaling through the extracellular signal-regulated kinase pathway after treatment with fibroblast growth factor-2 or heparin-binding epidermal growth factor. Consistent with these changes, HSulf-1 re-expression resulted in reduced proliferation as well as sensitivity to induction of apoptosis by the broad spectrum kinase inhibitor staurosporine and the chemotherapeutic agent cisplatin. Collectively, these observations provide evidence that HSulf-1 modulates signaling by heparin-binding growth factors, and HSulf-1 down-regulation represents a novel mechanism by which cancer cells can enhance growth factor signaling.

Synergy of epidermal growth factor and 12(S)-hydroxyeicosatetraenoate on protein kinase C activation in lens epithelial cells

12(S)-hydroxyeicosatetraenoic acid (12(S)HETE) is a bioactive metabolite of arachidonic acid synthesized by 12-lipoxygenase. The 12-lipoxygenase blocker, baicalein, prevents epidermal growth factor (EGF)-induced activation of protein kinase C (PKC) alpha and beta in lens epithelial cells, whereas supplementation with 12(S)HETE reverses this effect, suggesting that EGF and 12(S)HETE may work together to activate PKC. This study investigates the mechanism of PKCbeta activation by EGF and 12(S)HETE. 12(S)HETE alone directed translocation of PKCbeta through the C1 rather than the C2 domain, without activating phosphoinositide 3-kinase (PI3K) or MAPK signaling or increasing intracellular calcium concentration. In the presence of baicalein, EGF triggered an asymmetric phosphorylation of the EGF receptor initiating signaling through PI3K and MAPK, but not PLCgamma. Together, 12(S)HETE and EGF synergistically increased phosphorylation of PKCbeta in the activation loop and C terminus as well as PKCbeta-specific activity. PI3K inhibitors blocked phosphorylation, but MEK1 inhibitors did not. Microvesicles containing phosphatidylinositol 3,4,5-trisphosphate mimicked the action of EGF on PKCbeta activity in the presence of 12(S)HETE. Kinase-inactive PKCbeta mutations in either activation loop or C terminus were effectively translocated by 12(S)HETE, as was PKCbeta in the presence of chelerythrine or Gö-6983. These findings indicate that unphosphorylated PKCbeta is translocated to the membrane by 12(S)HETE and phosphorylated by EGF-dependent PI3K signaling, to generate catalytically competent PKCbeta.

Epidermal growth factor and thrombin induced proliferation of immortalized human keratinocytes is coupled to the synthesis of Egr-1, a zinc finger transcriptional regulator

The epidermal growth factor (EGF) receptor is highly expressed in HaCaT keratinocytes as shown by Western blotting. Stimulation of HaCaT cells with EGF, and also with the serine protease thrombin, induced DNA synthesis, measured by incorporation of 5-bromo-2'-deoxyuridine into the DNA of proliferating cells. Using antibodies directed against the active form of the EGF receptor, we show that in HaCaT cells EGF and thrombin triggered a rapid activation of the EGF receptor, followed by the phosphorylation and activation of the extracellular signal-regulated protein kinase (ERK). Moreover, EGF and thrombin induced a transient synthesis of the zinc finger transcriptional regulator Egr-1. Proliferation, activation of ERK, and biosynthesis of Egr-1 was completely inhibited in EGF or thrombin-treated HaCaT cells by the MAP kinase kinase inhibitor PD98059 and by AG1487, an EGF receptor-specific tyrosine kinase inhibitor. These data indicate that phosphorylation and activation of both the EGF receptor and ERK are essential for mitogenic signaling via EGF and thrombin. The synthesis of Egr-1 in HaCaT cells as a result of EGF or thrombin stimulation suggests that Egr-1 may be an important "late" part of the EGF and thrombin-initiated signaling cascades. We postulate that Egr-1 may function as a "third messenger" in keratinocytes connecting mitogenic stimulation with changes in gene transcription.

Fluorescent histochemical techniques for analysis of intracellular signaling

Intracellular signaling relies on the orchestrated cooperation of signaling proteins and modules, their intracellular localization, and membrane trafficking. Recently, a repertoire of fluorescence-based techniques, which significantly increases our potential for detailed studies of the involved mechanisms, has been introduced. Microscopic techniques with increased resolution have been combined with improved techniques for detection of signaling proteins. Transfections of fluorescently tagged proteins have allowed in vivo microscopy of their trafficking and interactions with other proteins and intracellular structures. We present an overview of general signaling principles and a description of techniques based on fluorescent microscopy suited for studies of signaling mechanisms.

A mutant EGF-receptor defective in ubiquitylation and endocytosis unveils a role for Grb2 in negative signaling

Ligand-induced desensitization of the epidermal growth factor receptor (EGFR) is controlled by c-Cbl, a ubiquitin ligase that binds multiple signaling proteins, including the Grb2 adaptor. Consistent with a negative role for c-Cbl, here we report that defective Tyr1045 of EGFR, an inducible c-Cbl docking site, enhances the mitogenic response to EGF. Signaling potentiation is due to accelerated recycling of the mutant receptor and a concomitant defect in ligand-induced ubiquitylation and endocytosis of EGFR. Kinetic as well as morphological analyses of the internalization-defective mutant receptor imply that c-Cbl-mediated ubiquitylation sorts EGFR to endocytosis and to subsequent degradation in lysosomes. Unexpectedly, however, the mutant receptor displayed significant residual ligand-induced ubiquitylation, especially in the presence of an overexpressed c-Cbl. The underlying mechanism seems to involve recruitment of a Grb2 c-Cbl complex to Grb2-specific docking sites of EGFR, and concurrent acceleration of receptor ubiquitylation and desensitization. Thus, in addition to its well-characterized role in mediating positive signals, Grb2 can terminate signal transduction by accelerating c-Cbl-dependent sorting of active tyrosine kinases to destruction.

Active EGF receptors have limited access to PtdIns(4,5)P2 in endosomes: implications for phospholipase C and PI 3-kinase signaling

Although prolonged cell signaling is attenuated by internalization and downregulation of active receptors, it is now appreciated that many receptors continue to signal in intracellular compartments. Employing enhanced green fluorescent protein fusion probes, we have investigated the hypothesis that multiple signaling pathways are affected by the differential trafficking of membrane substrates such as PtdIns(4,5)P2. A phosphotyrosine-specific probe, but not a PtdIns(4,5)P2-specific probe, colocalized with internalized EGF as well as transferrin in EGF-stimulated living cells expressing autophosphorylation-competent EGF receptors. Neither probe colocalized with transferrin in the absence of EGF, demonstrating that the reduced level of accessible PtdIns(4,5)P2 in endosomes is constitutive. Finally, a PtdIns(3,4,5)P3-specific probe, which monitors phosphorylation of PtdIns(4,5)P2 by phosphoinositide 3-kinases, was recruited to the plasma membrane but not to EGF- or transferrin-containing endosomes in response to EGF stimulation. These results suggest that while many internalized receptors continue to engage intracellular enzymes, the phospholipase C and phosphoinositide 3-kinase signaling pathways are abrogated by the constitutive lack of accessible PtdIns(4,5)P2 in endosomes.

Temperature dependence of the epidermal growth factor receptor signaling network can be accounted for by a kinetic model

Stimulation of isolated hepatocytes with epidermal growth factor (EGF) causes rapid tyrosine phosphorylation of the EGF receptor (EGFR) and adapter/target proteins, which was monitored with 1 and 2 s resolution at 37, 20, and 4 degrees C. The temporal responses detected for multiple signaling proteins involve both transient and sustained phosphorylation patterns, which change dramatically at low temperatures. To account quantitatively for complex responses, we employed a mechanistic kinetic model of the EGFR pathway, formulated in molecular terms as cascades of protein interactions and phosphorylation and dephosphorylation reactions. Assuming differential temperature dependencies for different reaction groups, such as SH2 and PTB domain-mediated interactions, the EGFR kinase, and the phosphatases, good quantitative agreement was obtained between computer-simulated and measured responses. The kinetic model demonstrates that, for each protein-protein interaction, the dissociation rate constant, k(off), strongly decreases at low temperatures, whereas this decline may or may not be accompanied by a large decrease in the k(on) value. Temperature-induced changes in the maximal activities of the reactions catalyzed by the EGFR kinase were moderate, compared to such changes in the V(max) of the phosphatases. However, strong changes in both the V(max) and K(m) for phosphatases resulted in moderate changes in the V(max)/K(m) ratio, comparable to the corresponding changes in EGFR kinase activity, with a single exception for the receptor phosphatase at 4 degrees C. The model suggests a significant decrease in the rates of the EGF receptor dimerization and its dephosphorylation at 4 degrees C, which can be related to the phase transition in the membrane lipids. A combination of high-resolution experimental monitoring and molecular level kinetic modeling made it possible to quantitatively account for the temperature dependence of the integrative signaling responses.

Mitogenesis and endocytosis: What's at the INTERSECTIoN?

Endocytosis is a regulated physiological process by which cell surface proteins are internalized along with extracellular factors such as nutrients, pathogens, peptides, toxins, etc. The process begins with the invagination of small regions of the plasma membrane which ultimately form intracellullar vesicles. These internalized vesicles may shuttle back to the plasma membrane to recycle the membrane components or they may be targeted for degradation. One role for endocytosis is in the attenuation of receptor signaling. For example, desensitization of activated membrane bound receptors such as G-protein coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs) occurs, in part, through endocytosis of the activated receptor. However, accumulating evidence suggests that endocytosis also mediates intracellular signaling. In this review, we discuss the experimental data that implicate endocytosis as a critical component in cellular signal transduction, both in the initiation of a signal as well as in the termination of a signal. Furthermore, we focus our attention on a recently described adaptor protein, intersectin (ITSN), which provides a link to both the endocytic and the mitogenic machinery of a cell. Thus, ITSN functions at a crossroad in the biochemical regulation of cell function.