Signal characteristics of G protein-transactivated EGF receptor

A signaling mechanism from G alpha q-protein-coupled metabotropic glutamate receptors to gene expression: role of the c-Jun N-terminal kinase pathway

Galphaq-protein-coupled group I metabotropic glutamate receptors (mGluRs) are densely expressed in brain neurons and are actively involved in various cellular activities. In this study, we investigated the role of group I mGluRs in regulating the c-Jun N-terminal kinase (JNK)/stress-activated protein kinase in cultured neurons. We found that selective activation of mGluR5 induced a rapid and transient phosphorylation of JNK. In a series of studies to determine the mechanisms, we found that the conventional mGluR5-associated signaling pathways (inositol-1,4,5-triphosphate-mediated Ca2+ release and activation of protein kinase C) were not involved in the mGluR5 regulation. Instead, ligand stimulation of mGluR5 caused a dynamic transactivation of the epidermal growth factor (EGF) receptor, which in turn triggered a downstream signaling pathway to upregulate JNK phosphorylation. Furthermore, the mGluR5-dependent JNK activation specifically activated c-Jun, but not activating transcription factor-2 or JunD, and increased activator protein-1 (AP-1)-mediated endogenous transcriptional activity. Together, we identified a novel mGluR5-to-nucleus communication through the EGF/JNK pathway, which functions to regulate AP-1-mediated transcription.

Direct involvement of G protein alpha(q/11) subunit in regulation of muscarinic receptor-mediated sAPPalpha release

The G(q/11) protein-coupled receptors, such as muscarinic (M1 & M3) receptors, have been shown to regulate the release of a soluble amyloid precursor protein (sAPPalpha) produced from alpha-secretase processing. However, there is no direct evidence for the precise characteristics of G proteins, and the signaling mechanism for the regulation of G(q/11) protein-coupled receptor-mediated sAPPalpha release is not clearly understood. This study examined whether the muscarinic receptor-mediated release of sAPPalpha is directly regulated by Galpha(q/11) proteins. The HEK293 cells were transiently cotransfected with muscarinic M3 receptors and a dominant-negative minigene construct of the G protein alpha subunit. The sAPPalpha release in the media was measured using an antibody specific for sAPP. The sAPPalpha release enhancement induced by muscarinic receptor stimulation was decreased by a G(q/11) minigene construct, whereas it was not blocked by a control minigene construct (the Galpha carboxy peptide in random order, Galpha(q)R) or Galpha(i) constructs. This indicated a direct role of the Galpha(q/11) protein in the regulation of muscarinic M3 receptor-mediated sAPPalpha release. We also investigated whether the transactivation of the epidermal growth factor receptor (EGFR) by a muscarinic agonist could regulate the sAPPalpha release in SH-SY5Y cells. Pretreatment of a specific EGFR kinase inhibitor, tyrophostin AG1478 (250 nM), blocked the EGF-stimulated sAPPalpha release, but did not block the oxoM-stimulated sAPPalpha release. This demonstrated that the transactivation of the EGFR by muscarinic receptor activation was not involved in the muscarinic receptor-mediated sAPPalpha release.

G-protein-coupled receptor agonists activate endogenous phospholipase Cepsilon and phospholipase Cbeta3 in a temporally distinct manner

Phospholipase Cepsilon (PLCepsilon) is one of the newest members of the phosphatidylinositol-specific phospholipase C (PLC) family. Previous studies have suggested that G-protein-coupled receptors (GPCRs) stimulate phosphoinositide (PI) hydrolysis by activating PLCbeta isoforms through G(q) family G proteins and Gbetagamma subunits. Using RNA interference to knock down PLC isoforms, we demonstrate that the GPCR agonists endothelin (ET-1), lysophosphatidic acid (LPA), and thrombin, acting through endogenous receptors, couple to both endogenous PLCepsilon and the PLCbeta isoform, PLCbeta3, in Rat-1 fibroblasts. Examination of the temporal activation of these PLC isoforms, however, reveals agonist- and isoform-specific profiles. PLCbeta3 is activated acutely within the first minute of ET-1, LPA, or thrombin stimulation but does not contribute to sustained PI hydrolysis induced by LPA or thrombin and accounts for only part of ET-1 sustained stimulation. PLCepsilon, on the other hand, predominantly accounts for sustained PI hydrolysis. Consistent with this observation, reconstitution of PLCepsilon in knockdown cells dose-dependently increases sustained, but not acute, agonist-stimulated PI hydrolysis. Furthermore, combined knockdown of both PLCepsilon and PLCbeta3 additively inhibits PI hydrolysis, suggesting independent regulation of each isoform. Importantly, ubiquitination of inositol 1,4,5-trisphosphate receptors correlates with sustained, but not acute, activation of PLCepsilon or PLCbeta3. In conclusion, GPCR agonists ET-1, LPA, and thrombin activate endogenous PLCepsilon and PLCbeta3 in Rat-1 fibroblasts. Activation of these PLC isoforms displays agonist-specific temporal profiles; however, PLCbeta3 is predominantly involved in acute and PLCepsilon in sustained PI hydrolysis.

Molecular determinants of P2Y2 nucleotide receptor function: implications for proliferative and inflammatory pathways in astrocytes

In the mammalian nervous system, P2 nucleotide receptors mediate neurotransmission, release of proinflammatory cytokines, and reactive astrogliosis. Extracellular nucleotides activate multiple P2 receptors in neurons and glial cells, including G protein-coupled P2Y receptors and P2X receptors, which are ligand-gated ion channels. In glial cells, the P2Y2 receptor subtype, distinguished by its ability to be equipotently activated by ATP and UTP, is coupled to pro-inflammatory signaling pathways. In situ hybridization studies with rodent brain slices indicate that P2Y2 receptors are expressed primarily in the hippocampus and cerebellum. Astrocytes express several P2 receptor subtypes, including P2Y2 receptors whose activation stimulates cell proliferation and migration. P2Y2 receptors, via an RGD (Arg-Gly-Asp) motif in their first extracellular loop, bind to alphavbeta3/beta5 integrins, whereupon P2Y2 receptor activation stimulates integrin signaling pathways that regulate cytoskeletal reorganization and cell motility. The C-terminus of the P2Y2 receptor contains two Src-homology-3 (SH3)-binding domains that upon receptor activation, promote association with Src and transactivation of growth factor receptors. Together, our results indicate that P2Y2 receptors complex with both integrins and growth factor receptors to activate multiple signaling pathways. Thus, P2Y2 receptors present novel targets to control reactive astrogliosis in neurodegenerative diseases.

Endothelial protein C receptor-dependent inhibition of migration of human lymphocytes by protein C involves epidermal growth factor receptor

The protein C pathway is an important regulator of the blood coagulation system. Protein C may also play a role in inflammatory and immunomodulatory processes. Whether protein C or activated protein C affects lymphocyte migration and possible mechanisms involved was tested. Lymphocyte migration was studied by micropore filter assays. Lymphocytes that were pretreated with protein C (Ceprotin) or activated protein C (Xigris) significantly reduced their migration toward IL-8, RANTES, MCP-1, and substance P, but not toward sphingosine-1-phosphate. The inhibitory effects of protein C or activated protein C were reversed by Abs against endothelial protein C receptor and epidermal growth factor receptor. Evidence for the synthesis of endothelial protein C receptor by lymphocytes is shown by demonstration of receptor mRNA expression and detection of endothelial protein C receptor immunoreactivity on the cells' surface. Data suggest that an endothelial protein C receptor is expressed by lymphocytes whose activation with protein C or activated protein C arrests directed migration. Exposure of lymphocytes to protein C or activated protein C stimulates phosphorylation of Tyr845 of epidermal growth factor receptor, which may be relevant for cytoprotective effects of the protein C pathway.

Induction of interleukin-8 secretion and activation of ERK1/2, p38 MAPK signaling pathways by thrombin in dermal fibroblasts

It was reported that thrombin could induce IL-8 secretion from human dermal fibroblasts (HDFs) through activation of proteinase activated receptor (PAR)-1. However, little is known of intracellular signaling pathways involved in the event. In the present study, expression of PARs in primarily cultured HDFs was determined by flow cytometry analysis and reverse transcription polymerase chain reaction (RT-PCR), levels of IL-8 were determined by using ELISA and signaling pathways were examined by using Western blot. It was found that HDFs express PAR-1 and PAR-3, and thrombin induces approximately 7.4-fold increase in IL-8 secretion from HDFs. Hirudin and a PAR-1 blocking antibody completely abolish the action of thrombin. It was also found that PD98059, a mitogen-activated protein kinase (MAPK) pathway inhibitor and U0126, an inhibitor of extracellular signal-regulated kinase (ERK) blocks thrombin-induced phosphorylation of ERK1/2 and IL-8 secretion, indicating the involvement of MAPK/ERK signaling pathway in thrombin-induced IL-8 secretion. p38 MAPK pathway appears also being involved as SB203580, a selective inhibitor of p38 MAPK inhibit phosphorylation of p38 MAPK and thrombin-induced IL-8 secretion. Furthermore, Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) pathway, but not phosphotidylinositol 3-kinase (PI3K)/Akt signaling pathway may also be activated by thrombin. In conclusion, thrombin potently induce IL-8 release via PAR-1 from HDFs. Thrombin elicited IL-8 release is predominantly conducted through MAPK/ERK and p38 MAPK signaling pathways. Discovery of the signaling pathways of thrombin in HDFs may help to understand the role of thrombin in inflammation and tissue remodeling.

The role of Src in solid and hematologic malignancies

c-Src was the first protooncogene described and was among the first molecules in which tyrosine kinase activity was documented. c-Src has been defined as a common modular structure that participates in much of the crosstalk between the cytoplasmic protein tyrosine kinases and tyrosine kinase receptors. Understanding the structure and function of this important class of protein kinases and elucidating the molecular signaling events mediated by c-Src are important not only for identifying the critical pathways but also for designing new strategies to block or inhibit the action of these kinases. Despite the large amount of information available on c-Src, its precise functions in cancer remain to be elucidated. Recently, there has been renewed interest in c-Src as a molecular target for cancer therapy, and multiple c-Src inhibitors are entering clinical trials. In this review, the authors describe the function and expression of c-Src in human malignancies and the novel c-Src inhibitors and their potential applications for cancer treatment.

Transactivation of epidermal growth factor receptor is involved in leptin-induced activation of janus-activated kinase 2 and extracellular signal-regulated kinase 1/2 in human gastric cancer cells

Leptin is known to act as a growth factor through the Janus-activated kinase (JAK)/signal transducer and activator of transcription signaling pathway as well as the mitogen-activated protein kinase pathway. In this study, we showed a novel signal transduction pathway using two human gastric cancer cell lines, MKN28 and MKN74. Both gastric cancer cells expressed leptin and its receptors (Ob-R) at the protein level. We found that leptin, even at as low as 0.1 ng/mL, induced significant tyrosine phosphorylation of epidermal growth factor receptor (EGFR). Time-course experiments revealed that phosphorylation was maximal after 5 minutes of stimulation and declined thereafter. We also revealed that tyrosine phosphorylation of EGFR induced by leptin was significantly attenuated by two inhibitors, an EGFR tyrosine kinase inhibitor, AG1478, and a broad-spectrum matrix metalloproteinase inhibitor, GM6001. This indicates that the pathway of EGFR transactivation induced by leptin is dependent on proteolytically released EGFR ligands. Leptin induced JAK2 activation and extracellular signal-regulated kinase (ERK) 1/2 activation in these gastric cancer cells, both of which occurred after the peak of EGFR transactivation. Pretreatment of gastric cancer cells with AG1478 significantly reduced the degree of phosphorylation of both JAK2 and ERK1/2. These findings indicate the involvement of EGFR transactivation in the activation of JAK2 and ERK1/2. Our results reveal that EGFR transactivation is involved in the leptin signaling pathway in gastric cancer cells, which extends the physiologic action of leptin beyond its central effects in the hypothalamus to regulate body weight.

Muscarinic M2 receptors mediate transactivation of EGF receptor through Fyn kinase and without matrix metalloproteases

Transactivation of epidermal growth factor receptor (EGFR) by G protein-coupled receptors (GPCRs) has been attributed to the activation of matrix metalloproteases (MMPs) and the release of EGF family ligands such as HB-EGF. This mode of transactivation leads to signalling downstream of EGFR which is indistinguishable from that induced by the ligand. Here we provide evidence that in the COS-7 cell model EGFR transactivation via the muscarinic M2 receptor (M2R) is independent of MMPs and results in an incomplete EGFR signalling including ERK and Akt but not PLCgamma1. Using dominant-negative mutants of c-Src and Fyn and Src-deficient SYF cells as well as by co-immunoprecipitation studies, we can demonstrate that the M2R-mediated transactivation of EGFR specifically involves Fyn but not c-Src or Yes. This specific role of Fyn can be verified in SH-SY5Y human neuroblastoma cells with endogenously expressed M2 receptors.

GPCR-induced migration of breast carcinoma cells depends on both EGFR signal transactivation and EGFR-independent pathways

The epidermal growth factor receptor (EGFR) plays a key role in the regulation of important cellular processes under normal and pathophysiological conditions such as cancer. In human mammary carcinomas the EGFR is involved in regulating cell growth, survival, migration and metastasis and its activation correlates with the lack of response in hormone therapy. Here, we demonstrate in oestrogen receptor-positive and -negative human breast cancer cells and primary mammary epithelial cells a cross-communication between G protein-coupled receptors (GPCRs) and the EGFR. We present evidence that specific inhibition of ADAM15 or TACE blocks GPCR-induced and proHB-EGF-mediated EGFR tyrosine phosphorylation, downstream mitogenic signalling and cell migration. Notably, activation of the PI3K downstream mediator PKB/Akt by GPCR ligands involves the activity of sphingosine kinase (SPHK) and is independent of EGFR signal transactivation. We conclude that GPCR-induced chemotaxis of breast cancer cells is mediated by EGFR-dependent and -independent signalling pathways, with both parallel pathways having to act in concert to achieve a complete migratory response.

Role of p38 mitogen-activated kinase and c-Jun terminal kinase in migration response to lysophosphatidic acid and sphingosine-1-phosphate in glioma cells

A potential role for 1-oleoyl-sn-glycero-3-phosphate or lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) in the regulation of malignant diseases has been widely considered. In this study, we found that in transformed astroglial cells, the expression profile of lysophospholipid receptor mRNA and the action modes of LPA and S1P on cell motility were changed: there was a change in the acquisition of the ability of LPA to stimulate cell migration and a change in the migratory response to S1P from stimulation through S1P(1) to inhibition through S1P(2). LPA-induced cell migration was almost completely inhibited by either pertussis toxin, LPA(1) receptor antagonists including Ki16425 (3-(4-[4-([1-(2-chlorophenyl)ethoxy]carbonyl amino)-3-methyl-5-isoxazolyl] benzylsulfonyl)propanoic acid) or an inhibitor of phosphatidylinositol 3-kinase (PI3K) wortmannin. The LPA-induced action was also suppressed, although incompletely, by several specific inhibitors for intracellular signaling pathways including Rac1, Cdc42, p38 mitogen-activated protein kinase (p38MAPK) and c-Jun terminal kinase (JNK), but not extracellular signal-regulated kinase. Nearly complete inhibition of migration response to LPA, however, required simultaneous inhibition of both the p38MAPK and JNK pathways. Inhibition of Rac1 suppressed JNK but not p38MAPK, while the activity of p38MAPK was abolished by a dominant-negative form of Cdc42. These findings suggest that, in glioma cells, the PI3K/Cdc42/p38MAPK and PI3K/Rac1/JNK pathways are equally important for LPA(1) receptor-mediated migration.

Lysophosphatidic acid-induced effects in human colon carcinoma DLD1 cells are partially dependent on transactivation of epidermal growth factor receptor

BACKGROUND

Lysophosphatidic acid (LPA) is a lipid mediator of diverse effects on various cells. LPA is well known to induce phosphorylation of the epidermal growth factor receptor (EGFR), which is termed transactivation, in some cell types. In this study, we investigated the contribution of EGFR transactivation in LPA-induced responses in colon cancer DLD1 cells.

MATERIALS AND METHODS

Immunoprecipitation was performed to investigate whether LPA induced EGFR phosphorylation. Then, we investigated LPA-induced migration and IL-8 secretion in DLD1 cells. Migration was measured in a modified Boyden chamber and IL-8 secretion was measured by ELISA. In these experiments we used an EGFR inhibitor, AG1478 or matrix metalloproteinase (MMP) inhibitor, GM6001.

RESULTS

Immunoprecipitation analysis revealed that LPA induced a significant level of tyrosine phosphorylation of EGFR in DLD1 cells. The LPA-induced phosphorylation of EGFR was almost completely abrogated by either AG1478 or GM6001. LPA induced significant migration and IL-8 secretion in DLD1, both of which were significantly inhibited by AG1478 or GM6001. However, the inhibitory effects were only partial (migration; 29% +/- 2%, 32 +/- 13% inhibition, IL-8 secretion; 33% +/- 1%, 26% +/- 5% inhibition, respectively).

CONCLUSION

These results clearly indicate that LPA acts upstream of EGFR and compensates the EGF signal and antagonism of the EGF signal cannot completely block tumor progression in colon cancer cells. Blockade of the LPA signal may have clinical significance in the treatment of colon cancer.

Autocrine, paracrine and juxtacrine signaling by EGFR ligands

Receptor and cytoplasmic protein tyrosine kinases play prominent roles in the control of a range of cellular processes during embryonic development and in the regulation of many metabolic and physiological processes in a variety of tissues and organs. The epidermal growth factor receptor (EGFR) is a well-known and versatile signal transducer that has been highly conserved during evolution. It functions in a wide range of cellular processes, including cell fate determination, proliferation, cell migration and apoptosis. The number of ligands that can activate the EGF receptor has increased during evolution. These ligands are synthesized as membrane-anchored precursor forms that are later shed by metalloproteinase-dependent cleavage to generate soluble ligands. In certain circumstances the membrane anchored isoforms as well as soluble growth factors may also act as biologically active ligands; therefore depending on the circumstances these ligands may induce juxtacrine, autocrine, paracrine and/or endocrine signaling. In this review, we discuss the different ways that EGFR ligands can activate the receptor and the possible biological implications.

cAMP-response element-binding protein mediates prostaglandin F2alpha-induced hypertrophy of vascular smooth muscle cells

Prostaglandin F(2alpha) (PGF(2alpha)) is a vasoactive factor that causes constriction and hypertrophy of vascular smooth muscle cells (VSMCs). However, the mechanism of PGF(2alpha)-induced hypertrophy is largely unknown. Cyclic AMP-response element (CRE)-binding protein (CREB), the best characterized stimulus-induced transcription factor, activates transcription of target genes with CRE and promotes cell growth. We examined the role of CREB in PGF(2alpha)-induced hypertrophy of VSMCs. PGF(2alpha) induced phosphorylation of CREB at serine 133, which is a critical marker of activation, after 5-10min of stimulation in a dose-dependent manner. Pharmacological inhibition of extracellular signal-regulated protein kinase and p38 mitogen-activated protein kinase (p38-MAPK) suppressed PGF(2alpha)-induced CREB phosphorylation. Inhibition of epidermal growth factor receptor (EGFR) and mitogen- and stress-activated protein kinase-1 also suppressed PGF(2alpha)-induced CREB phosphorylation. Overexpression of dominant-negative form of CREB (AdCREB M1), of which serine 133 was replaced with alanine, inhibited PGF(2alpha)-induced c-fos mRNA expression as well as hypertrophy of VSMCs [hypertrophy index (microg/10(4)cell); control 8.13, PGF(2alpha) 9.85, AdCREB M1 7.91, and AdCREB M1+PGF(2alpha) 8.43]. These results suggest that PGF(2alpha) activated CRE-dependent gene transcription through EGFR transactivation, and the CREB pathway plays a critical role in PGF(2alpha)-induced hypertrophy of VSMCs.

Composition and function of g protein-coupled receptor signalsomes controlling mitogen-activated protein kinase activity

Seven membrane-spanning G protein-coupled receptors (GPCRs) function as ligand-activated guanine nucleotide exchange factors for heterotrimeric guanine nucleotide-binding (G) proteins that relay extracellular stimuli by activating intracellular effector enzymes or ion channels. Recent work, however, has shown that GPCRs also participate in numerous other protein-protein interactions that generate intracellular signals in conjunction with, or even independent of, G-protein activation. Nowhere has the importance of protein complex assembly in GPCR signaling been demonstrated more clearly than in the control of the spatial and temporal activity of the extracellular signal-regulated kinase (ERK1/2) mitogen-activated protein (MAP) kinase cascade. ERK1/2 activation by GPCRs often involves cross talk with classical receptor tyrosine kinases or focal adhesion complexes, which scaffold the assembly of a Ras activation complex. Even more surprising is the phenomenon of G protein-independent signaling using beta-arrestins, proteins originally characterized for their role in homologous GPCR desensitization, as scaffolds for the assembly of a multiprotein signalsome directly upon the GPCR. Although both forms of signaling lead to MAP kinase activation, the pathways appear to be functionally, as well as mechanistically, distinct. Transactivated receptor tyrosine kinases mediate rapid and transient MAP kinase activation that favors nuclear translocation of the kinases and transcriptional activation. In contrast, beta-arrestin-dependent signaling produces a slower and more sustained increase in MAP kinase activity that is often restricted to the cytosol. Together, these highly organized signaling complexes dictate the location, duration, and ultimate function of GPCR-stimulated MAP kinase activity.

Thrombin induces DNA synthesis and phosphoinositide hydrolysis in airway smooth muscle by activation of distinct receptors

Chronic airway inflammation induces numerous structural changes of the airways involving hypertrophy and hyperplasia of airway smooth muscle (ASM). Thrombin has been identified in the bronchoalveolar lavage fluid of asthmatic subjects and displays potent bronchoconstrictor and mitogenic activity towards ASM. This study has addressed which proteinase-activated receptors (PARs) and signalling pathways are involved in mediating distinct effects of thrombin. Using cultured bovine tracheal smooth muscle (BTSM) cells as a model system, thrombin stimulated a marked increase in [3H]inositol phosphate ([3H]InsPs) accumulation, which was fully mimicked by a selective PAR1 activating peptide. In contrast, PAR1, PAR2, PAR3 and PAR4 activating peptides were unable to replicate the ability of thrombin to stimulate DNA synthesis as assessed by [3H]thymidine incorporation. Further investigation demonstrated that the mitogenic effect of thrombin did not involve stimulation of PDGF secretion but did involve activation of PDGF or EGF receptors and a G(i/o)-dependent activation of phosphoinositide 3-kinase. Thrombin, but not the PAR1, PAR2, PAR3 or PAR4 activating peptides was able to stimulate PtdIns(3,4,5)P3 mass accumulation. PAR3 antisense oligonucleotides substantially inhibit thrombin-stimulated [3H]thymidine incorporation and PtdIns(3,4,5)P3 generation but had no effect on thrombin-induced phosphoinositide hydrolysis. These data indicate that while PI hydrolysis and Ca2+ mobilisation induced by thrombin operates via PAR1-dependent activation of phospholipase C, phosphoinositide 3-kinase activation and DNA synthesis occurs via a distinct proteinase-activated receptor pathway, possibly involving PAR3.

The tobacco-specific carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone stimulates proliferation of immortalized human pancreatic duct epithelia through beta-adrenergic transactivation of EGF receptors

PURPOSE

Pancreatic ductal adenocarcinoma is an aggressive smoking-associated human cancer in both men and women. The nicotine-derived 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is thought to contribute to the development of these neoplasms in smokers through genotoxic effects. However, NNK has been recently identified as an agonist for both beta(1)- and beta(2)-adrenergic receptors. Binding of NNK to these receptors stimulates proliferation of pulmonary and pancreatic adenocarcinomas cells in vitro and in hamster models. The goal of this study was to elucidate the NNK effects on the signal transduction pathways downstream of both beta(1)- and beta(2)-adrenergic receptors in immortalized human pancreatic HPDE6-c7 cells.

METHODS

The HPDE6-c7 cells are developed from normal pancreatic duct epithelial cells which are the putative cells of origin of pancreatic ductal adenocarcinoma. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazoliumbromide (MTT) cell proliferation assays, Western blot and cyclic AMP assays were employed to demonstrate the effects of NNK and other beta(1)- and beta(2)-adrenergic agonists and antagonist treatments on these cells.

RESULTS

MTT cell proliferation assays demonstrated that NNK and the classic beta-adrenergic agonist, isoproterenol, increased cell proliferation in HPDE6-c7 cells. Western blot and cyclic AMP assays demonstrated that NNK treatments also resulted in: (1) transactivation of the epidermal growth factor receptor, EGFR, (2) an increase in intracellular cyclic AMP accumulation, and (3) phosphorylation of mitogen-activated protein kinase, Erk1/2. The proliferative response to NNK and isoproterenol were inhibited by the use of beta-blockers (propranolol), and the inhibitors of adenylyl cyclase (SQ 22536), EGFR-specific tyrosine kinase (AG 1478) and Erk (PD 98059).

CONCLUSION

These findings suggest that the NNK -mediated beta-adrenergic receptor transactivation of the EGFR and phosphorylation of Erk1/2 in immortalized human pancreatic duct epithelial cells as a novel mechanism might contribute to the development of tobacco-associated pancreatic carcinogenesis.

Lysophosphatidic acid transactivates both c-Met and epidermal growth factor receptor, and induces cyclooxygenase-2 expression in human colon cancer LoVo cells

AIM: To examine whether lysophosphatidic acid (LPA) induces phosphorylation of c-Met and epidermal growth factor receptor (EGFR), both of which have been proposed as prognostic markers of colorectal cancer, and whether LPA induces cyclooxygenase-2 (COX-2) expression in human colon cancer cells.

METHODS: Using a human colon cancer cell line, LoVo cells, we performed immunoprecipitation analysis, followed by Western blot analysis. We also examined whether LPA induced COX-2 expression, by Western blot analysis.

RESULTS: Immunoprecipitation analysis revealed that 10 µmol/L LPA induced tyrosine phosphorylation of c-Met and EGFR in LoVo cells within a few minutes. We found that c-Met tyrosine phosphorylation induced by LPA was not attenuated by pertussis toxin or a matrix metalloproteinase inhibitor, in marked contrast to the results for EGFR. In addition, 0.2-40 µmol/L LPA induced COX-2 expression in a dose-dependent manner.

CONCLUSION: Our results suggest that LPA acts upstream of various receptor tyrosine kinases (RTKs) and COX-2, and thus may act as a potent stimulator of colorectal cancer.

Stromal cell-derived factor-1alpha (SDF-1alpha/CXCL12) stimulates ovarian cancer cell growth through the EGF receptor transactivation

Ovarian cancer (OC) is the leading cause of death in gynecologic diseases in which there is evidence for a complex chemokine network. Chemokines are a family of proteins that play an important role in tumor progression influencing cell proliferation, angiogenic/angiostatic processes, cell migration and metastasis, and, finally, regulating the immune cells recruitment into the tumor mass. We previously demonstrated that astrocytes and glioblastoma cells express both the chemokine receptor CXCR4 and its ligand stromal cell-derived factor-1 (SDF-1), and that SDF-1alpha treatment induced cell proliferation, supporting the hypothesis that chemokines may play an important role in tumor cells' growth in vitro. In the present study, we report that CXCR4 and SDF-1 are expressed in OC cell lines. We demonstrate that SDF-1alpha induces a dose-dependent proliferation in OC cells, by the specific interaction with CXCR4 and a biphasic activation of ERK1/2 and Akt kinases. Our results further indicate that CXCR4 activation induces EGF receptor (EGFR) phosphorylation that in turn was linked to the downstream intracellular kinases activation, ERK1/2 and Akt. In addition, we provide evidence for cytoplasmic tyrosine kinase (c-Src) involvement in the SDF-1/CXCR4-EGFR transactivation. These results suggest a possible important "cross-talk" between SDF-1/CXCR4 and EGFR intracellular pathways that may link signals of cell proliferation in ovarian cancer.

β-Arrestin 2 Expression Determines the Transcriptional Response to Lysophosphatidic Acid Stimulation in Murine Embryo Fibroblasts

G protein-coupled receptors often employ novel signaling mechanisms, such as transactivation of epidermal growth factor (EGF) receptors or G protein-independent signals transmitted by beta-arrestins, to control the activity of extracellular signal-regulated kinases 1 and 2 (ERK1/2). In this study we investigated the role of beta-arrestins in lysophosphatidic acid (LPA) receptor-stimulated ERK1/2 activation using fibroblast lines derived from wild type, beta-arrestin 1, beta-arrestin 2, and beta-arrestin 1/2 knock-out mice. LPA stimulation produced robust ERK1/2 phosphorylation in all four backgrounds. In cells lacking beta-arrestin 2, >80% of LPA-stimulated ERK1/2 phosphorylation was mediated by transactivated EGF receptors. In contrast, ERK1/2 activation in cells expressing beta-arrestin 2 was predominantly EGF receptor-independent. Introducing FLAG epitope-tagged beta-arrestin 2 into the beta-arrestin 1/2 null background restored EGF receptor-independent ERK1/2 activation, indicating that beta-arrestin 2 expression confers ERK1/2 activation via a distinct mechanism. To determine the contributions of beta-arrestin 2, transactivated EGF receptors, and ERK1/2 to LPA-stimulated transcriptional responses, we employed gene expression arrays containing cDNA markers for G protein-coupled receptor-mediated signaling. In the beta-arrestin 1/2 null background, 1 h of exposure to LPA significantly increased transcription of seven marker genes. Six of these responses were EGF receptor-dependent, and two required ERK1/2 activation. In beta-arrestin 2 expressing cells, three of the seven LPA-stimulated transcriptional responses observed in the beta-arrestin 1/2 null background were lost. The four residual responses were independent of EGF receptor transactivation, but all were ERK1/2-dependent. These data indicate that beta-arrestin 2 functions both to attenuate EGF receptor transactivation-dependent signaling and to promote a distinct subset of ERK1/2-mediated responses to LPA receptor activation.

The epidermal growth factor receptors and their family of ligands: their putative role in atherogenesis

The epidermal growth factor receptor is a member of type-I growth factor receptor family with tyrosine kinase activity that is activated following the binding of multiple cognate ligands. Several members of the EGF family of ligands are expressed by cells involved in atherogenesis. EGF receptor mediated processes have been well characterised within epithelial, smooth muscle and tumour cell lines in vitro, and the EGF receptor has been identified immunocytochemically on intimal smooth muscle cells within atherosclerotic plaques. There is also limited evidence for the expression of the EGF receptor family on leukocytes, although their function has yet to be clarified. In this review, we will discuss the biological functions of this receptor and its ligands and their potential to modulate the function of cells involved in the atherosclerotic process.

Leumorphin has an anti-apoptotic effect by activating epidermal growth factor receptor kinase in rat pheochromocytoma PC12 cells

Endogenous opioid peptides, found in the central and peripheral nervous systems, perform neuromodulatory roles, and display a wide range of functional and pharmacological properties in vitro and in vivo. In this study, we investigated the effects of prodynorphin gene products on intracellular signaling events and cell survival in rat pheochromocytoma PC12 cells. Leumorphin, but not other prodynorphin gene products including dynorphin A, beta-neoendorphin and rimorphin (dynorphin B), increased cell viability in PC12 cells. The cytoprotective effect of leumorphin was dependent on the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways, but was insensitive to both naloxone, a general antagonist of the opioid receptor, and nor-binaltorphimine, a specific antagonist of the kappa opioid receptor. Moreover, a competition-binding assay clearly revealed that leumorphin had another binding site(s) in addition to that for the kappa opioid receptor. Interestingly, leumorphin induced activation of the epidermal growth factor receptor via a Src-dependent mechanism, which was proved to be responsible for the increased survival response. Flow cytometric and microscopic analysis showed that leumorphin rescued cells from serum deprivation-induced apoptosis. Collectively, we suggest that leumorphin prevents apoptosis via epidermal growth factor receptor-mediated activation of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways, which occur independent of the kappa opioid receptor.

Prostaglandin E2 Activates Mitogen-Activated Protein Kinase/Erk Pathway Signaling and Cell Proliferation in Non–Small Cell Lung Cancer Cells in an Epidermal Growth Factor Receptor–Independent Manner

Cyclooxygenase 2 (COX-2) overexpression is found in a wide variety of human cancers and is linked to all stages of tumorigenesis. Elevated tumor COX-2 expression is associated with increased angiogenesis, tumor invasion, suppression of host immunity and promotes tumor cell resistance to apoptosis. Previous reports have linked the COX-2 product prostaglandin E2 (PGE2) to the abnormal activation of the mitogen-activated protein kinase/Erk kinase pathway. Here we show that PGE2 is able to rapidly stimulate Erk phosphorylation in a subset of non-small cell lung cancer (NSCLC) cell lines. This effect is not evident in bronchial epithelial cells. In contrast to previous reports in colon cancer, we found that Erk activation as well as cellular proliferation induced by PGE2 was not inhibited by pretreatment of the cells with epidermal growth factor receptor (EGFR) inhibitors. Activation of the Erk pathway by PGE2 was also resistant to src kinase inhibitors but sensitive to the protein kinase C inhibition. PGE2 effects are mediated through four G protein-coupled receptors. Selective inhibition of EP receptors revealed the possible involvement of Ca2+-dependent signaling in PGE2-mediated activation of Erk. Our data indicate the presence of an EGFR-independent activation of the mitogen-activated protein kinase/Erk pathway by PGE2 in NSCLC cells. These findings provide evidence for the possible link between tumor COX-2 overexpression and elevated Erk-mediated cancer cell proliferation and migration. Importantly, these findings suggest that COX-2 overexpression may contribute to EGFR inhibitor resistance in NSCLC.

Bradykinin-induced p42/p44 MAPK phosphorylation and cell proliferation via Src, EGF receptors, and PI3-K/Akt in vascular smooth muscle cells

In our previous study, bradykinin (BK) exerts its mitogenic effect through Ras/Raf/MEK/MAPK pathway in vascular smooth muscle cells (VSMCs). In addition to this pathway, the non-receptor tyrosine kinases (Src), EGF receptor (EGFR), and phosphatidylinositol 3-kinase (PI3-K) have been implicated in linking a variety of G-protein coupled receptors to MAPK cascades. Here, we investigated whether these different mechanisms participating in BK-induced activation of p42/p44 MAPK and cell proliferation in VSMCs. We initially observed that BK- and EGF-dependent activation of Src, EGFR, Akt, and p42/p44 MAPK and [3H]thymidine incorporation were mediated by Src and EGFR, because the Src inhibitor PP1 and EGFR kinase inhibitor AG1478 abrogated BK- and EGF-dependent effects. Inhibition of PI3-K by LY294002 attenuated BK-induced Akt and p42/p44 MAPK phosphorylation and [3H]thymidine incorporation, but had no effect on EGFR phosphorylation, suggesting that EGFR may be an upstream component of PI3-K/Akt and MAPK in these responses. This hypothesis was supported by the tranfection with dominant negative plasmids of p85 and Akt which significantly attenuated BK-induced Akt and p42/p44 MAPK phosphorylation. Pretreatment with U0126 (a MEK1/2 inhibitor) attenuated the p42/p44 MAPK phosphorylation and [3H]thymidine incorporation stimulated by BK, but had no effect on Akt activation. Moreover, BK-induced transactivation of EGFR and cell proliferation was blocked by matrix metalloproteinase inhibitor GM6001. These results suggest that, in VSMCs, the mechanism of BK-stimulated activation of p42/p44 MAPK and cell proliferation was mediated, at least in part, through activation of Src family kinases, EGFR transactivation, and PI3-K/Akt.

Resveratrol inhibits angiotensin II- and epidermal growth factor-mediated Akt activation: role of Gab1 and Shp2

trans-Resveratrol (RV), a polyphenolic stilbene derivative found in grape skin and other food products, has been proposed to exert beneficial effects in cardiovascular disease. Our group has shown previously that RV inhibits angiotensin II (Ang II)-induced Akt activation and, consequently, vascular smooth muscle cell (VSMC) hypertrophy. In this work, to identify the molecular target of RV, we investigated the impact of RV on early signaling cascades in rat aortic VSMCs triggered by Ang II and epidermal growth factor (EGF). We show that RV does not influence Ang II-mediated transactivation of EGF-receptor but potently inhibits EGF-induced phosphorylation of Akt kinase, suggesting that RV acts downstream of EGF-receptor transactivation in VSMCs. Recent evidence indicates that the adapter molecule Gab1, together with the protein tyrosine phosphatase Shp2, is critically involved in regulating the strength and duration of phosphatidylinositol-3-kinase (PI3K) and Akt activation upon EGF stimulation in fibroblasts. Our results show that stimulation of VSMCs with EGF as well as Ang II leads to a rapid tyrosine phosphorylation of Gab1 and its association with the p85 subunit of PI3K. RV attenuates these processes. Experiments performed in Shp2-deficient fibroblasts revealed that RV does not inhibit EGF-stimulated Akt activation in these cells, suggesting that Shp2 is necessary for the inhibitory effect of RV on the PI3K/Akt pathway. Furthermore, RV treatment activates Shp2. We therefore propose that RV blocks Akt activation in Ang II- and EGF-stimulated VSMCs by activating Shp2, thus preventing interaction between Gab1 and PI3K that is necessary for further signal transduction.

Structure-function relationships among human cathelicidin peptides: dissociation of antimicrobial properties from host immunostimulatory activities

Cathelicidins and other antimicrobial peptides are deployed at epithelial surfaces to defend against infection. These molecules have broad-spectrum killing activity against microbes and can have effects on specific mammalian cell types, potentially stimulating additional immune defense through direct chemotactic activity or induction of cytokine release. In humans, the cathelicidin hCAP18/LL-37 is processed to LL-37 in neutrophils, but on skin it can be further proteolytically processed to shorter forms. The influence of these cathelicidin peptides on keratinocyte function is not known. In the current study, DNA microarray analysis and confirmatory protein analysis showed that LL-37 affects the expression of several chemokines and cytokines by keratinocytes. Analysis of a synthetic peptide library derived from LL-37 showed that antimicrobial activity against bacterial, fungal, and viral skin pathogens resides within specific domains of the parent peptide, but antimicrobial activity does not directly correlate with the ability to stimulate IL-8 production in keratinocytes. IL-8 release was induced by d- and l-amino acid forms of cathelicidin and correlated with membrane permeability, suggesting that highly structure-specific binding to a cell surface receptor is not likely. However, this effect was inhibited by either pertussis toxin or AG1478, an epidermal growth factor receptor tyrosine kinase inhibitor, suggesting that cathelicidin may indirectly stimulate multiple signaling pathways associated with cell surface receptors. Taken together, these observations suggest that proteolytic processing may alter the balance between cathelicidin antimicrobial and host immunostimulatory functions.

{beta}-Arrestin is crucial for ubiquitination and down-regulation of the insulin-like growth factor-1 receptor by acting as adaptor for the MDM2 E3 ligase

The insulin-like growth factor-1 receptor (IGF-1R) plays important roles in physiological growth and aging as well as promoting several crucial functions in cancer cells. However, the molecular mechanisms involved in expression and down-regulation of IGF-1R are still poorly understood. Here we provide evidence that beta-arrestin, otherwise known to be involved in the regulation of G protein-coupled receptors, serves as an adaptor to bring the oncoprotein E3 ubiquitin ligase MDM2 to the IGF-1R. In this way, beta-arrestin acts as a crucial component in the ubiquitination and down-regulation of the receptor. Both MDM2 and beta-arrestin co-immunoprecipitated with the IGF-1R. The beta-arrestin isoform 1 appeared to be more strongly associated with the receptor than isoform 2, and in a molecular context it was 4-fold more efficient in inducing polyubiquitination of IGF-1R, a reaction that required the presence of beta-arrestin and MDM2. Ligand stimulation accelerated IGF-1R ubiquitination. In mouse P6 cells (overexpressing human IGF-1R) absence of beta-arrestin 1, but not of beta-arrestin 2, blocked ubiquitination of IGF-1R. Conversely, in the two studied human melanoma cell lines both beta-arrestin isoforms seemed to be involved in IGF-1R ubiquitination. However, because depletion of beta-arrestin 1 almost completely eliminated degradation, and IGF-1 induced down-regulation of the receptor in these cells, whereas beta-arrestin 2 only had a partial effect, beta-arrestin 1 seems to the more important isoform in affecting the expression of IGF-1R. To our knowledge this is the first study demonstrating a defined molecular role of beta-arrestin with direct relevance to cell growth and cancer.

Cyclooxygenase-2 functions as a downstream mediator of lysophosphatidic acid to promote aggressive behavior in ovarian carcinoma cells

Elevated levels of the bioactive lipid lysophosphatidic acid (LPA) are detectable in the majority of patients with both early- and late-stage ovarian cancer, suggesting that LPA promotes early events in ovarian carcinoma dissemination. LPA contributes to the development, progression, and metastasis of ovarian cancer in part by inducing the expression of genes that contribute to proliferation, survival, or invasion, including cyclooxgenase-2 (COX-2) and matrix metalloproteinase-2 (MMP-2). We have previously shown that LPA promotes proMMP-2 activation and MMP-2-dependent migration and invasion in ovarian cancer cells. The purpose of the current study was to determine whether the effect of LPA on acquisition of the metastatic phenotype in ovarian cancer cells is mediated via a COX-2-dependent mechanism. Immunohistochemical analysis of 173 ovarian tumors showed strong COX-2 immunoreactivity in 63% of tumor specimens, including 50% of borderline tumors. LPA increased COX-2 protein expression in a time- and concentration-dependent manner in two of three immortalized borderline ovarian epithelial cells as well as in four of six ovarian cancer cell lines. This was accomplished by both activation of the Edg/LPA receptor and LPA-mediated transactivation of the epidermal growth factor receptor, which increased COX-2 expression via the Ras/mitogen-activated protein kinase pathway. COX-2 also played a role in LPA-induced invasion and migration, as treatment with the COX-2 specific inhibitor NS-398 reduced LPA-induced proMMP-2 protein expression and activation and blocked MMP-dependent motility and invasive activity. These data show that COX-2 functions as a downstream mediator of LPA to potentiate aggressive cellular behavior.

Lysophospholipids transactivate HER2/neu (erbB-2) in human gastric cancer cells

The ligand-less receptor HER2/neu (erbB-2) has been proposed as a prognostic marker of gastric cancer that correlates with poor clinical outcome, indicating that HER2 signals play an important role in gastric cancer progression. This study demonstrated that two major natural lysophospholipids, lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), induce rapid and transient phosphorylation of HER2 in two human gastric cancer cell lines, MKN28 and MKN74 cells. We also revealed that tyrosine phosphorylation of HER2 induced by both lysophospholipids was significantly attenuated by two inhibitors, an epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor, AG1478, and a broad-spectrum matrix metalloproteinase inhibitor, GM6001. This suggests that the pathway of HER2 transactivation induced by these lysophospholipids is dependent on the proteolytically released EGFR ligands. Our results indicate that LPA and S1P act upstream of HER2 in gastric cancer cells, and thus may act as potent stimulators of gastric cancer.

Clostridium difficile toxin B activates the EGF receptor and the ERK/MAP kinase pathway in human colonocytes

BACKGROUND & AIMS

Clostridium difficile toxin B (TxB) mediates acute inflammatory diarrhea characterized by neutrophil infiltration and intestinal mucosal injury. In a xenograft animal model, TxB was shown to induce interleukin (IL)-8 gene expression in human colonic epithelium. However, the precise mechanisms of this TxB response are unknown. The aim of this study was to investigate the TxB-mediated proinflammatory pathway in colonocytes.

METHODS

The effect of TxB on epidermal growth factor receptor (EGFR), extracellular signal-regulated kinase (ERK) 1/2 signaling pathway and IL-8 gene expression was assessed in nontransformed human colonic epithelial NCM460 cells. TxB regulation of EGFR-ERK1/2 signaling pathways was determined using immunoblot analysis, confocal microscopy, and enzyme-linked immunosorbent assay, whereas IL-8 gene expression was measured by luciferase promoter assay.

RESULTS

TxB activates EGFR and ERK1/2 phosphorylation with subsequent release of IL-8 from human colonocytes. Pretreatment with either the EGFR tyrosine kinase inhibitor, AG1478, or an EGFR-neutralizing antibody blocked both TxB-induced EGFR and ERK activation. By using neutralizing antibodies against known ligands of EGFR, we found that the activation of EGFR and ERK1/2 phosphorylation was mediated by transforming growth factor-alpha (TGF-alpha). Inhibition of matrix metalloproteinase (MMP) decreased TGF-alpha secretion and TxB-induced EGFR and ERK activation. Inhibition of MMP, EGFR, and ERK activation significantly decreased TxB-induced IL-8 expression.

CONCLUSIONS

TxB signals acute proinflammatory responses in colonocytes by transactivation of the EGFR and activation of the ERK/MAP kinase pathway.

Thromboxane Prostanoid Receptor Signals Through GiProtein to Rapidly Activate Extracellular Signal–Regulated Kinase in Human Airways

We showed previously that activation of the thromboxane prostanoid (TP) receptor causes human airway smooth muscle (HASM) cells to proliferate, suggesting a role in airway remodeling. This study aimed at determining the molecular mechanisms underlying this mitogenic action. We found that the MEK inhibitor PD98059 significantly affected agonist-induced DNA synthesis of HASM cells, which suggests that extracellular signal-regulated kinases (ERK) are involved. ERK activation by the agonist U46619 was rapid, sensitive to pertussis toxin, and significantly abrogated by the tyrosine kinase inhibitors genistein and PP1. Stimulation of the TP receptor was also found to translocate phosphorylated ERK into the nucleus. TP receptor was found to activate Ras, as demonstrated by inhibition of ERK activation and DNA synthesis by Clostridium sordellii lethal toxin, and by the ability of U46619 to increase RasGTP. Finally, [(3)H]thymidine incorporation and ERK phosphorylation were also affected by prior treatment with protein kinase C inhibitor GF109203X, although to different extents. In conclusion, in HASM cells TP receptor, predominantly coupled to G(i/o) proteins, activates the Ras/ERK pathway to induce mitogenesis, probably with the involvement of nonreceptor tyrosine kinases and protein kinase C.

Phosphotyrosine Signaling Networks in Epidermal Growth Factor Receptor Overexpressing Squamous Carcinoma Cells

Overexpression and enhanced activation of the epidermal growth factor (EGF) receptor are frequent events in human cancers that correlate with poor prognosis. Anti-phosphotyrosine and anti-EGFr affinity chromatography, isotope-coded muLC-MS/MS, and immunoblot methods were combined to describe and measure signaling networks associated with EGF receptor activation and pharmacological inhibition. The squamous carcinoma cell line HN5, which overexpresses EGF receptor and displays sustained receptor kinase activation, was used as a model system, where pharmacological inhibition of EGF receptor kinase by erlotinib markedly reduced auto and substrate phosphorylation, Src family phosphorylation at EGFR Y845, while increasing total EGF receptor protein. Diverse sets of known and poorly described functional protein classes were unequivocally identified by affinity selection, comprising either proteins tyrosine phosphorylated or complexed therewith, predominantly through EGF receptor and Src family kinases, principally 1) immediate EGF receptor signaling complexes (18%); 2) complexes involved in adhesion and cell-cell contacts (34%); and 3) receptor internalization and degradation signals. Novel and known phosphorylation sites could be located despite the complexity of the peptide mixtures. In addition to interactions with multiple signaling adaptors Grb2, SHC, SCK, and NSP2, EGF receptors in HN5 cells were shown to form direct or indirect physical interactions with additional kinases including ACK1, focal adhesion kinase (FAK), Pyk2, Yes, EphA2, and EphB4. Pharmacological inhibition of EGF receptor kinase activity by erlotinib resulted in reduced phosphorylation of downstream signaling, for example through Cbl/Cbl-B, phospholipase Cgamma (PLCgamma), Erk1/2, PI-3 kinase, and STAT3/5. Focal adhesion proteins, FAK, Pyk2, paxillin, ARF/GIT1, and plakophillin were down-regulated by transient EGF stimulation suggesting a complex balance between growth factor induced kinase and phosphatase activities in the control of cell adhesion complexes. The functional interactions between IGF-1 receptor, lysophosphatidic acid (LPA) signaling, and EGF receptor were observed, both direct and/or indirectly on phospho-Akt, phospho-Erk1/2, and phospho-ribosomal S6.

EGF receptor activity is essential for adhesion-induced stress fiber formation and cofilin phosphorylation

Integrin-mediated cell adhesion induces activation of the EGF receptor tyrosine kinase independently of the soluble growth factor ligand. EGFR activation is instrumental for subsequent activation of additional signaling pathways in adherent cells, including the Ras-MAP kinase pathway and the phosphatidylinositol 3-kinase/Akt pathway. We demonstrate here that integrin-dependent EGFR activation is also essential for adhesion-induced formation of actin stress fibers, focal adhesion localization and tyrosine phosphorylation of the adapter protein paxillin, as well as transcriptional activation of the serum response factor. All these events are known to be mediated by the small GTPase RhoA. EGFR activity was not found to regulate the activity status of RhoA, however. Instead, we found that EGFR activity is required for integrin-induced phosphorylation of cofilin. Cofilin is an actin-binding protein, which, when unphosphorylated, stimulates depolymerization and severing of actin filaments. Thus, in the absence of the kinase activity of the EGFR, cofilin remains dephosphorylated and depolymerizes actin filaments, rendering cells unable to respond to RhoA signaling. These studies demonstrate adhesion-dependent regulation of cofilin phosphorylation, and identify a novel role for EGFR in integrin signaling.

Phorbol 12-myristate 13-acetate induces epidermal growth factor receptor transactivation via protein kinase Cdelta/c-Src pathways in glioblastoma cells

Both the epidermal growth factor receptor (EGFR) and protein kinase C (PKC) play important roles in glioblastoma invasive growth; however, the interaction between the EGFR and PKC is not well characterized in glioblastomas. Treatment with EGF stimulated global phosphorylation of the EGFR at Tyr(845), Tyr(992), Tyr(1068), and Tyr(1045) in glioblastoma cell lines (U-1242 MG and U-87 MG). Interestingly, phorbol 12-myristate 13-acetate (PMA) stimulated phosphorylation of the EGFR only at Tyr(1068) in the two glioblastoma cell lines. Phosphorylation of the EGFR at Tyr(1068) was not detected in normal human astrocytes treated with the phorbol ester. PMA-induced phosphorylation of the EGFR at Tyr(1068) was blocked by bisindolylmaleimide (BIM), a PKC inhibitor, and rottlerin, a PKCdelta-specific inhibitor. In contrast, Go 6976, an inhibitor of classical PKC isozymes, had no effect on PMA-induced EGFR phosphorylation. Furthermore, gene silencing with PKCdelta small interfering RNA (siRNA), siRNA against c-Src, and mutant c-Src(S12C/S48A) and treatment with a c-Src inhibitor (4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d]pyrimidine) abrogated PMA-induced EGFR phosphorylation at Tyr(1068). PMA induced serine/threonine phosphorylation of Src, which was blocked by both BIM and rottlerin. Inhibition of the EGFR with AG 1478 did not significantly alter PMA-induced EGFR Tyr(1068) phosphorylation, but completely blocked EGF-induced phosphorylation of the EGFR. The effects of PMA on MAPK phosphorylation and glioblastoma cell proliferation were reduced by BIM, rottlerin, the MEK inhibitor U0126, and PKCdelta and c-Src siRNAs. Taken together, our data demonstrate that PMA transactivates the EGFR and increases cell proliferation by activating the PKCdelta/c-Src pathway in glioblastomas.

Complexity in the vascular permeability factor/vascular endothelial growth factor (VPF/VEGF)-receptors signaling

The adult vasculature results from a network of vessels that is originally derived in the embryo by vasculogenesis, a process whereby vessels are formed de novo from endothelial cell (EC) precursors, known as angioblasts. During vasculogenesis, angioblasts proliferate and come together to form an initial network of vessels, also known as the primary capillary plexus. Sprouting and branching of new vessels from the preexisting vessels in the process of angiogenesis remodel the capillary plexus. Normal angiogenesis, a well-balanced process, is important in the embryo to promote primary vascular tree as well as an adequate vasculature from developing organs. On the other hand, pathological angiogenesis which frequently occurs in tumors, rheumatoid arthritis, diabetic retinopathy and other circumstances can induce their own blood supply from the preexisting vasculature in a route that is close to normal angiogenesis. Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) is perhaps the most important of pro-angiogenic cytokine because of its ability to regulate most of the steps in the angiogenic cascade. The main goal of this review article is to discuss the complex nature of the mode of action of VPF/VEGF on vascular endothelium. To this end, we conclude that more research needs to be done for completely understanding the VPF/VEGF biology with relation to angiogenesis.

Neurotensin and EGF induce synergistic stimulation of DNA synthesis by increasing the duration of ERK signaling in ductal pancreatic cancer cells

Neurotensin (NT) and epidermal growth factor (EGF) induced rapid extracellular-regulated protein kinase (ERK) activation through different signaling pathways in the K-Ras mutated human pancreatic carcinoma cell lines PANC-1 and MIA PaCa-2. NT stimulated ERK activation via a protein kinase C (PKC)-dependent (but EGF receptor-independent) pathway in PANC-1 and MIA PaCa-2 cells, whereas EGF promoted ERK activation through a PKC-independent pathway in these cells. Concomitant stimulation of these cells with NT and EGF induced a striking increase in the duration of ERK pathway activation as compared with that obtained in cells treated with each agonist alone. Stimulation with NT + EGF promoted synergistic stimulation of DNA synthesis and anchorage-independent growth. Addition of the MEK inhibitor U0126, either prior to stimulation with NT + EGF or 2 h after stimulation with NT + EGF prevented the synergistic increase in DNA synthesis and suppressed the sustained phase of ERK activation. Furthermore, treatment with the selective PKC inhibitor GF-1 converted the sustained ERK activation in response to NT and EGF into a transient signal and also abrogated the synergistic increase in DNA synthesis. Collectively, our results suggest that the sustained phase of ERK signaling mediates the synergistic effects of NT and EGF on DNA synthesis in pancreatic cancer cells.

GPR4 plays a critical role in endothelial cell function and mediates the effects of sphingosylphosphorylcholine

Angiogenesis is critical for many physiological and pathological processes. We show here that the lipid sphingosylphosphorylcholine (SPC) induces angiogenesis in vivo and GPR4 is required for the biological effects of SPC on endothelial cells (EC). In human umbilical vein EC, down-regulation of GPR4 specifically inhibits SPC-, but not sphingosine-1-phosphate-, or vascular endothelial growth factor (VEGF)-induced tube formation. Re-introduction of GPR4 fully restores the activity of SPC. In microvascular EC, GPR4 plays a pivotal role in cell survival, growth, migration, and tube formation through both SPC-dependent and -independent pathways. The biological effects resulting from SPC/GPR4 interactions involve the activation of both phosphatidylinositol-3 kinase and Akt. Moreover, the effects of SPC on EC require SPC induced trans-phosphorylation and activation of the VEGF receptor 2. These results identify SPC and its receptor, GPR4, as critical regulators of the angiogenic potential of EC.

Transactivation of Trk neurotrophin receptors by G-protein-coupled receptor ligands occurs on intracellular membranes

Neurotrophins, such as NGF and BDNF, activate Trk receptor tyrosine kinases through receptor dimerization at the cell surface followed by autophosphorylation and intracellular signaling. It has been shown that activation of Trk receptor tyrosine kinases can also occur via a G-protein-coupled receptor (GPCR) mechanism, without involvement of neurotrophins. Two GPCR ligands, adenosine and pituitary adenylate cyclase-activating polypeptide (PACAP), can activate Trk receptor activity to increase the survival of neural cells through stimulation of Akt activity. To investigate the mechanism of Trk receptor transactivation, we have examined the localization of Trk receptors in PC12 cells and primary neurons after treatment with adenosine agonists and PACAP. In contrast to neurotrophin treatment, Trk receptors were sensitive to transcriptional and translational inhibitors, and they were found predominantly in intracellular locations particularly associated with Golgi membranes. Biotinylation and immunostaining experiments confirm that most of the transactivated Trk receptors are found in intracellular membranes. These results indicate that there are alternative modes of activating Trk receptor tyrosine kinases in the absence of neurotrophin binding at the cell surface and that receptor signaling may occur and persist inside of neuronal cells.

Redox regulation of protein-tyrosine phosphatases

The protein-tyrosine phosphatases (PTPs) form a large family of signaling proteins with essential functions in embryonic development and adult physiology. The PTPs are characterized by an absolutely conserved catalytic site cysteine with a low pK(a) due to its microenvironment, making it vulnerable to oxidation. PTPs are differentially oxidized and inactivated in vitro and in living cells. Many cellular stimuli induce a shift in the cellular redox state towards oxidation and evidence is accumulating that at least part of the cellular responses to these stimuli are due to specific, transient inactivation of PTPs, indicating that PTPs are important sensors of the cellular redox state.

Oleate promotes the proliferation of breast cancer cells via the G protein-coupled receptor GPR40

Evidence from epidemiological studies and animal models suggests a link between high levels of dietary fat intake and risk of breast cancer. In addition, obesity, in which circulating lipids are elevated, is associated with increased risk of various cancers. Relative to this point, we previously showed that oleate stimulates the proliferation of breast cancer cells and that phosphatidylinositol 3-kinase plays a role in this process. Nonetheless, questions remain regarding the precise mechanism(s) by which oleate promotes breast cancer cell growth. Pharmacological inhibitors of the GTP-binding proteins G(i)/G(o), phospholipase C, Src, and mitogenic-extracellular signal-regulated kinase 1/2 (MEK 1/2) decreased oleate-induced [3H]thymidine incorporation in the breast cancer cell line MDA-MB-231. In addition, oleate caused a rapid and transient rise in cytosolic Ca2+ and an increase in protein kinase B phosphorylation. Overexpressing in these cells the G protein-coupled receptor GPR40, a fatty acid receptor, amplified oleate-induced proliferation, whereas silencing the GPR40 gene using RNA interference decreased it. Overexpressing GPR40 in T47D and MCF-7 breast cancer cells that are poorly responsive to oleate allowed a robust proliferative action of oleate. The data indicate that the phospholipase C, MEK 1/2, Src, and phosphatidylinositol 3-kinase/protein kinase B signaling pathways are implicated in the proliferative signal induced by oleate and that these effects are mediated at least in part via the G protein-coupled receptor GPR40. The results suggest that GPR40 is implicated in the control of breast cancer cell growth by fatty acids and that GPR40 may provide a link between fat and cancer.

EGF receptor transactivation mediates ANG II-stimulated mitogenesis in intestinal epithelial cells through the PI3-kinase/Akt/mTOR/p70S6K1 signaling pathway

The role of epidermal growth factor receptor (EGFR) tyrosine kinase and its downstream targets in the regulation of the transition from the G0/G1 phase into DNA synthesis in response to ANG II has not been previously investigated in intestinal epithelial IEC-18 cells. ANG II induced a rapid and striking EGFR tyrosine phosphorylation, which was prevented by selective inhibitors of EGFR tyrosine kinase activity (e.g., AG-1478) or by broad-spectrum matrix metalloproteinase (MMP) inhibitor GM-6001. Pretreatment of these cells with either AG-1478 or GM-6001 reduced ANG II-stimulated DNA synthesis by approximately 50%. To elucidate the downstream targets of EGFR, we demonstrated that ANG II stimulated phosphorylation of Akt at Ser473, mTOR at Ser2448, p70S6K1 at Thr389, and S6 ribosomal protein at Ser(235/236). Pretreatment with AG-1478 inhibited Akt, p70S6K1, and S6 ribosomal protein phosphorylation. Inhibition of phosphatidylinositol (PI)3-kinase with LY-294002 or mTOR/p70S6K1 with rapamycin reduced [3H]thymidine incorporation by 50%, i.e., to levels comparable to those achieved by addition of either AG-1478 or GM-6001. Utilizing Akt small-interfering RNA targeted to Akt1 and Akt2, Akt protein knockdown dramatically inhibited p70S6K1 and S6 ribosomal protein phosphorylation. In contrast, AG-1478 or Akt gene silencing exerted no detectable inhibitory effect on ANG II-induced extracellular signal-regulated kinase 1/2 phosphorylation in IEC-18 cells. Taken together, our results demonstrate that EGFR transactivation mediates ANG II-stimulated mitogenesis through the PI3-kinase/Akt/mTOR/p70S6K1 signaling pathway in IEC-18 cells.

Sphingosine 1-phosphate transactivates c-Met as well as epidermal growth factor receptor (EGFR) in human gastric cancer cells

Receptor tyrosine kinases (RTKs) are transactivated by the stimulation of G protein-coupled receptors (GPCRs). Sphingosine 1-phosphate (S1P), a ligand of GPCR, is known as a tumor-promoting lipid, but its signaling pathways are not fully understood. We here demonstrated that S1P induces rapid and transient tyrosine phosphorylation of epidermal growth factor receptor (EGFR) and c-Met in gastric cancer cells, both of which have been proposed as prognostic markers of gastric cancers. The pathway of S1P-induced c-Met transactivation is Gi-independent and matrix metalloproteinase-independent, which differs from that of EGFR transactivation. Our results indicate that S1P acts upstream of various RTKs and thus may act as a potent stimulator of gastric cancer.

Dual mode regulation of migration by lysophosphatidic acid in human gastric cancer cells

Lysophosphatidic acid (LPA), which interacts with at least three G protein-coupled receptors (GPCRs), LPA1/Edg-2, LPA2/Edg-4, and LPA3/Edg-7, is a lipid mediator with diverse effects on various cells. Here, we investigated the expression profiles of LPA receptors and patterns of LPA-induced migration in gastric cancer cells. Northern blot analysis revealed that various gastric cancer cells expressed variable levels of LPA1, LPA2, and LPA3 without a consistent pattern. Using a Boyden chamber assay, LPA markedly increased cell migration of LPA1-expressing cells, the effects of which were almost totally abrogated by Ki16425, an LPA antagonist against LPA1 and LPA3. In contrast, LPA by itself did not significantly induce migration in MKN28 and MKN74 cells, which exclusively expressed LPA2. However, when hepatocyte growth factor (HGF) was placed with LPA in the lower chamber, LPA induced migration of these cells in a dose-dependent manner. Immunoprecipitation analysis revealed that LPA induced transient tyrosine phosphorylation of c-Met in LPA2-expressing cells, which suggests that the transactivation of c-Met by LPA causes a cooperative migratory response with HGF to these cells. Our results indicate that LPA regulates the migration of gastric cancer cells in a receptor-specific manner and suggest that the expression pattern of LPA receptors may affect the metastatic behavior of gastric cancer.

Kinin B2 Receptor-Coupled Signal Transduction in Human Cultured Keratinocytes

Kinins are key pro-inflammatory peptides that exhibit mitogenic effects in tissue-specific cellular systems. Since the life span of the keratinocyte is regulated by receptors that control proliferation and differentiation, and since both processes are affected during wound healing, we have examined the consequence of kinin B2 receptors (B2R) activation in cultured human keratinocytes. Stimulation of keratinocytes by Lys-bradykinin (LBK) induced a rapid and sustained phosphorylation of 42/44 mitogen-activated protein kinase (MAPK) that translocated to the nucleus, and decreased only after 120 min of stimulation. Kinin B1 and B2 receptor (B1R and B2R) antagonists showed that phosphorylation was mainly because of B2R activation. The GF109203X inhibitor almost completely abolished the effect of LBK, suggesting the involvement of protein kinase C in the signal cascade. MAPK phosphorylation was partially dependent on epidermal growth factor receptor transactivation as assessed by the selective inhibitor, AG1478. LBK stimulation did not result in cell proliferation, but produced a rapid c-Fos expression, nuclear translocation of nuclear factor-kappaB, and a moderated (pro)filaggrin synthesis, indicating that it may modulate cell differentiation. Our results support the view that kinins may affect the life span of human keratinocytes and highlight the importance that kinin peptides may have in the pathogenesis and/or progression of skin diseases.

Not so strange bedfellows: G-protein-coupled receptors and Src family kinases

Src family nonreceptor tyrosine kinases are an integral component of the signal transduction apparatus employed by growth factor receptor tyrosine kinases. As such, their role in cellular growth control and malignant transformation has been the subject of intensive investigation. In contrast, classical G-protein-coupled receptor (GPCR) signaling involves activation of second messenger-regulated serine/threonine kinases or ion channels, and is primarily involved in neurotransmission and the short-term regulation of intermediary metabolism. Over the past decade, this strictly dichotomous model of transmembrane signaling has been challenged by the discovery that GPCRs also exert control over cellular growth, proliferation, and differentiation, and do so by stimulating tyrosine phosphorylation cascades. Several mechanisms, from the direct association of Src family kinases with GPCRs or receptor-associated proteins, to the transactivation of receptor tyrosine kinases and focal adhesion complexes by G-protein-mediated signals, permit GPCRs to activate Src family kinases. Conversely, Src activity plays a central role in controlling GPCR trafficking and effects on cell proliferation and cytoskeletal rearrangement. It is now clear that GPCRs and Src family kinases do not belong to separate, exclusive clubs. Rather, these strange bedfellows are intimately involved in multilayered forms of crosstalk that influence a host of cellular processes.

Correlation of Protease-Activated Receptor-1 With Differentiation Markers in Squamous Cell Carcinoma of the Head and Neck and Its Implication in Lymph Node Metastasis

PURPOSE

Protease-activated receptor-1 (PAR-1) is a G-protein-coupled receptor that contributes to multiple signal transduction pathways. Although the functions of PAR-1 in many normal cells, such as platelets and astrocytes, have been well studied, its roles in cancer progression and metastasis have not been fully elucidated, and studies to date appear contradictory.

EXPERIMENTAL DESIGN

To clarify the function of PAR-1 in metastasis of squamous cell carcinoma of the head and neck (SCCHN), we examined PAR-1 expression in clinical specimens by immunohistochemistry and in SCCHN cell lines by immunoblotting. Furthermore, par-1 cDNA-transfected SCCHN cell lines were also used to verify PAR-1-mediated pathway.

RESULTS

The metastatic tumors showed a lower percentage of PAR-1-positive cells (46%) and lower levels of PAR-1 expression (median weight index = 10) than node negative primary tumors (80% and median weight index = 60, respectively). In addition, expression level of PAR-1 positively correlated with levels of keratinocyte differentiation markers keratin-1, -10, and -11. Additional studies using sense and antisense par-1 cDNA-transfected SCCHN cell lines illustrated that the presence of PAR-1 was required for the expression of involucrin, a keratinocyte differentiation marker. PAR-1 expression also contributes to activation of the mitogen-activated protein kinase (MAPK) pathway. Blocking MAPK activation by a mitogen-activated protein/extracellular signal-regulated kinase inhibitor, not by a phosphatidylinositol 3'-kinase inhibitor, reduced level of involucrin, suggesting that regulation of involucrin by PAR-1 is partially through the MAPK signaling pathway.

CONCLUSIONS

Our study suggests that PAR-1 signaling induces differentiation markers in SCCHN cells, and its expression is conversely correlated with cervical lymph node metastasis.

Parathyroid hormone receptor trafficking contributes to the activation of extracellular signal-regulated kinases but is not required for regulation of cAMP signaling

Agonist-mediated activation of the type 1 parathyroid hormone receptor (PTH1R) results in several signaling events and receptor endocytosis. It is well documented that arrestins contribute to desensitization of both G(s)- and G(q)-mediated signaling and mediate PTH1R internalization. However, whether PTH1R trafficking directly contributes to signaling remains unclear. To address this question, we investigated the role of PTH1R trafficking in cAMP signaling and activation of extracellular signal-regulated kinases ERK1/2 in HEK-293 cells. Dominant negative forms of dynamin (K44A-dynamin) and beta-arrestin1 (beta-arrestin1-(319-418)) abrogated PTH1R internalization but had no effect on cAMP signaling; neither acute cAMP production by PTH nor desensitization and resensitization of cAMP signaling were affected. Therefore, PTH1R trafficking is not necessary for regulation of cAMP signaling. PTH-(1-34) induced rapid and robust activation of ERK1/2. A PTHrP-based analog ([p-benzoylphenylalanine1, Ile5,Arg(11,13),Tyr36]PTHrP-(1-36)NH2), which selectively activates the G(s)/cAMP pathway without inducing PTH1R endocytosis, failed to stimulate ERK1/2 activity. Inhibition of PTH1R endocytosis by K44A-dynamin dampened ERK1/2 activation in response to PTH-(1-34) by 69%. Incubation with the epidermal growth factor receptor inhibitor AG1478 reduced ERK1/2 phosphorylation further. In addition, ERK1/2 phosphorylation occurred following internalization of a PTH1R mutant induced by PTH-(7-34) in the absence of G protein signaling. Collectively, these data indicate that PTH1R trafficking and G(q) (but not G(s)) signaling independently contribute to ERK1/2 activation, predominantly via transactivation of the epidermal growth factor receptor.

Src family kinase involvement in rat preglomerular microvascular contractile and [Ca2+]i responses to ANG II

Experiments were performed to investigate the potential role of Src family kinase(s) in the rat afferent arteriolar contractile response to ANG II. The in vitro blood-perfused juxtamedullary nephron technique was employed to monitor afferent arteriolar lumen diameter responses to 1-100 nM ANG II before and during Src family kinase inhibition (10 microM PP2). PP2 did not alter baseline diameter but attenuated ANG II-induced contractile responses by 33 +/- 6%. An inactive analog of PP2 (PP3) had no effect on ANG II-induced afferent arteriolar contraction. The effect of Src kinase inhibition on ANG II-induced intracellular free Ca(2+) concentration ([Ca(2+)](i)) responses was probed in fura 2-loaded preglomerular microvascular smooth muscle cells (PVSMCs) obtained from explants and studied after 3-5 days in culture. In untreated PVSMCs, ANG II evoked peak (Delta = 293 +/- 66 nM) and plateau (Delta = 23 +/- 8 nM) increases in [Ca(2+)](i). In PVSMCs pretreated with PP2, baseline [Ca(2+)](i) was unaltered, but both the peak (Delta = 140 +/- 22 nM) and plateau (Delta = 3 +/- 2 nM) phases of the ANG II response were significantly reduced compared with untreated cells. PP3 did not alter [Ca(2+)](i) responses to ANG II. Immunoprecipitation and Western blot analysis confirmed that 100 nM ANG II increased phosphorylation of c-Src (at Y(416)) in PVSMCs. The phosphorylation response was maximal 1 min after ANG II exposure and was prevented by PP2. We conclude that the preglomerular vasoconstriction evoked by ANG II involves rapid c-Src activation with subsequent effects that contribute to the [Ca(2+)](i) response to the peptide.

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.