An epidermal growth factor receptor/Gab1 signaling pathway is required for activation of phosphoinositide 3-kinase by lysophosphatidic acid

The Role of GAB1 in Cancer

GRB2-associated binder 1 (GAB1) is the inaugural member of the GAB/DOS family of pleckstrin homology (PH) domain-containing proteins. Upon receiving various stimuli, GAB1 transitions from the cytoplasm to the membrane where it is phosphorylated by a range of kinases. This event recruits SH2 domain-containing proteins like SHP2, PI3K's p85 subunit, CRK, and others, thereby activating distinct signaling pathways, including MAPK, PI3K/AKT, and JNK. GAB1-deficient embryos succumb in utero, presenting with developmental abnormalities in the heart, placenta, liver, skin, limb, and diaphragm myocytes. Oncogenic mutations have been identified in the context of cancer. GAB1 expression levels are disrupted in various tumors, and elevated levels in patients often portend a worse prognosis in multiple cancer types. This review focuses on GAB1's influence on cellular transformation particularly in proliferation, evasion of apoptosis, metastasis, and angiogenesis-each of these processes being a cancer hallmark. GAB1 also modulates the resistance/sensitivity to antitumor therapies, making it a promising target for future anticancer strategies.

Macropinocytosis: mechanisms and regulation

Macropinocytosis is defined as an actin-dependent but coat- and dynamin-independent endocytic uptake process, which generates large intracellular vesicles (macropinosomes) containing a non-selective sampling of extracellular fluid. Macropinocytosis provides an important mechanism of immune surveillance by dendritic cells and macrophages, but also serves as an essential nutrient uptake pathway for unicellular organisms and tumor cells. This review examines the cell biological mechanisms that drive macropinocytosis, as well as the complex signaling pathways - GTPases, lipid and protein kinases and phosphatases, and actin regulatory proteins - that regulate macropinosome formation, internalization, and disposition.

Identification of a cross-talk between EGFR and Wnt/beta-catenin signaling pathways in HepG2 liver cancer cells

EGFRis a transmembrane receptor tyrosine kinase involved in regulating cell proliferation, differentiation and survival. EGFR is actively pursued as a therapeutic target because its aberrant expression or activity has been reported in several cancers. Several studies have reported the nuclear localization of the EGFR in various cell types, however, its exact nuclear functions are not clear yet. In this study, we have generated GFP fusion constructs of EGFR and its mutants to analyze their subcellular localizationin normal and cancer cells and impact of its sub-cellular location on its various activities using immunoblotting, confocal microscopy, reporter assays, loss-of-function EGFR mutants, and EGFR specific small molecule inhibitors. We show that EGFR is involved in modulating TCF dependent β-catenin transcriptional activity in HepG2 cells in a similar fashion as IGF1R tyrosine kinase. Moreover, we show that cytoplasmic and nuclear functions are two independent activities of EGFR.

Human Cytomegalovirus miR-US5-2 Downregulation of GAB1 Regulates Cellular Proliferation and UL138 Expression through Modulation of Epidermal Growth Factor Receptor Signaling Pathways

Human cytomegalovirus (HCMV) causes significant disease in immunocompromised individuals, including transplant patients. HCMV establishes latency in hematopoietic stem cells in the bone marrow. The mechanisms governing latency and reactivation of viral replication are complex and not fully understood. HCMV-encoded miRNAs are small regulatory RNAs that reduce protein expression. In this study, we found that the HCMV miRNA miR-US5-2 targets the epidermal growth factor receptor (EGFR) adaptor protein GAB1 which directly affects downstream cellular signaling pathways activated by EGF. Consequently, miR-US5-2 blocks the EGF-mediated proliferation of human fibroblasts. Early growth response gene 1 (EGR1) is a transcription factor activated by EGFR signaling that regulates expression of HCMV UL138. We show that miR-US5-2 regulates UL138 expression through GAB1-mediated downregulation of the signaling pathways that lead to EGR1 expression. These data suggest that miR-US5-2, through downregulation of GAB1, could play a critical role during reactivation from latency by reducing proliferation and UL138 expression.

Regulation of epidermal growth factor (EGF) receptor (EGFR) signaling is critical for the replication of human cytomegalovirus (HCMV) as well as latency and reactivation in CD34⁺ hematopoietic progenitor cells. HCMV microRNAs (miRNAs) provide a means to modulate the signaling activated by EGF through targeting components of the EGFR signaling pathways. Here, we demonstrate that HCMV miR-US5-2 directly downregulates the critical EGFR adaptor protein GAB1 that mediates activation and sustained signaling through the phosphatidylinositol 3-kinase (PI3K) and MEK/extracellular signal-regulated kinase (ERK) pathways and cellular proliferation in response to EGF. Expression of HCMV UL138 is regulated by the transcription factor early growth response gene 1 (EGR1) downstream of EGFR-induced MEK/ERK signaling. We show that by targeting GAB1 and attenuating MEK/ERK signaling, miR-US5-2 indirectly regulates EGR1 and UL138 expression, which implicates the miRNA in critical regulation of HCMV latency.

IMPORTANCE Human cytomegalovirus (HCMV) causes significant disease in immunocompromised individuals, including transplant patients. HCMV establishes latency in hematopoietic stem cells in the bone marrow. The mechanisms governing latency and reactivation of viral replication are complex and not fully understood. HCMV-encoded miRNAs are small regulatory RNAs that reduce protein expression. In this study, we found that the HCMV miRNA miR-US5-2 targets the epidermal growth factor receptor (EGFR) adaptor protein GAB1 which directly affects downstream cellular signaling pathways activated by EGF. Consequently, miR-US5-2 blocks the EGF-mediated proliferation of human fibroblasts. Early growth response gene 1 (EGR1) is a transcription factor activated by EGFR signaling that regulates expression of HCMV UL138. We show that miR-US5-2 regulates UL138 expression through GAB1-mediated downregulation of the signaling pathways that lead to EGR1 expression. These data suggest that miR-US5-2, through downregulation of GAB1, could play a critical role during reactivation from latency by reducing proliferation and UL138 expression.

CRF1 Receptor Signaling via the ERK1/2-MAP and Akt Kinase Cascades: Roles of Src, EGF Receptor, and PI3-Kinase Mechanisms

In the present study, we determined the cellular regulators of ERK1/2 and Akt signaling pathways in response to human CRF₁ receptor (CRF₁R) activation in transfected COS-7 cells. We found that Pertussis Toxin (PTX) treatment or sequestering Gβγ reduced CRF₁R-mediated activation of ERK1/2, suggesting the involvement of a G_i-linked cascade. Neither G_s/PKA nor G_q/PKC were associated with ERK1/2 activation. Besides, CRF induced EGF receptor (EGFR) phosphorylation at Tyr¹⁰⁶⁸, and selective inhibition of EGFR kinase activity by AG1478 strongly inhibited the CRF₁R-mediated phosphorylation of ERK1/2, indicating the participation of EGFR transactivation. Furthermore, CRF-induced ERK1/2 phosphorylation was not altered by pretreatment with batimastat, GM6001, or an HB-EGF antibody indicating that metalloproteinase processing of HB-EGF ligands is not required for the CRF-mediated EGFR transactivation. We also observed that CRF induced Src and PYK2 phosphorylation in a Gβγ-dependent manner. Additionally, using the specific Src kinase inhibitor PP2 and the dominant-negative-SrcYF-KM, it was revealed that CRF-stimulated ERK1/2 phosphorylation depends on Src activation. PP2 also blocked the effect of CRF on Src and EGFR (Tyr⁸⁴⁵) phosphorylation, further demonstrating the centrality of Src. We identified the formation of a protein complex consisting of CRF₁R, Src, and EGFR facilitates EGFR transactivation and CRF₁R-mediated signaling. CRF stimulated Akt phosphorylation, which was dependent on G_i/βγ subunits, and Src activation, however, was only slightly dependent on EGFR transactivation. Moreover, PI3K inhibitors were able to inhibit not only the CRF-induced phosphorylation of Akt, as expected, but also ERK1/2 activation by CRF suggesting a PI3K dependency in the CRF₁R ERK signaling. Finally, CRF-stimulated ERK1/2 activation was similar in the wild-type CRF₁R and the phosphorylation-deficient CRF₁R-Δ386 mutant, which has impaired agonist-dependent β-arrestin-2 recruitment; however, this situation may have resulted from the low β-arrestin expression in the COS-7 cells. When β-arrestin-2 was overexpressed in COS-7 cells, CRF-stimulated ERK1/2 phosphorylation was markedly upregulated. These findings indicate that on the base of a constitutive CRF₁R/EGFR interaction, the G_i/βγ subunits upstream activation of Src, PYK2, PI3K, and transactivation of the EGFR are required for CRF₁R signaling via the ERK1/2-MAP kinase pathway. In contrast, Akt activation via CRF₁R is mediated by the Src/PI3K pathway with little contribution of EGFR transactivation.

Essential roles of Gab1 tyrosine phosphorylation in growth factor-mediated signaling and angiogenesis

Growth factors and their downstream receptor tyrosine kinases (RTKs) mediate a number of biological processes controlling cell function. Adaptor (docking) proteins, which consist exclusively of domains and motifs that mediate molecular interactions, link receptor activation to downstream effectors. Recent studies have revealed that Grb2-associated-binders (Gab) family members (including Gab1, Gab2, and Gab3), when phosphorylated on tyrosine residues, provide binding sites for multiple effector proteins, such as Src homology-2 (SH2)-containing protein tyrosine phosphatase 2 (SHP2) and phosphatidylinositol 3-kinase (PI3K) regulatory subunit p85, thereby playing important roles in transducing RTKs-mediated signals into pathways with diversified biological functions. Here, we provide an up-to-date overview on the domain structure and biological functions of Gab1, the most intensively studied Gab family protein, in growth factor signaling and biological functions, with a special focus on angiogenesis.

Protein tyrosine phosphatase-1B regulates the tyrosine phosphorylation of the adapter Grb2-associated binder 1 (Gab1) in the retina

BACKGROUND

Gab1 (Grb2-associated binder 1) is a key coordinator that belongs to the insulin receptor substrate-1 like family of adaptor molecules and is tyrosine phosphorylated in response to various growth factors, cytokines, and numerous other molecules. Tyrosine phosphorylated Gab1 is able to recruit a number of signaling effectors including PI3K, SHP2 and PLC-γ. In this study, we characterized the localization and regulation of tyrosine phosphorylation of Gab1 in the retina.

RESULTS

Our immuno localization studies suggest that Gab1 is expressed in rod photoreceptor inner segments. We found that hydrogen peroxide activates the tyrosine phosphorylation of Gab1 ex vivo and hydrogen peroxide has been shown to inhibit the protein tyrosine phosphatase PTP1B activity. We found a stable association between the D181A substrate trap mutant of PTP1B and Gab1. Our studies suggest that PTP1B interacts with Gab1 through Tyrosine 83 and this residue may be the major PTP1B target residue on Gab1. We also found that Gab1 undergoes a light-dependent tyrosine phosphorylation and PTP1B regulates the phosphorylation state of Gab1. Consistent with these observations, we found an enhanced Gab1 tyrosine phosphorylation in PTP1B deficient mice and also in retinas treated ex vivo with a PTP1B specific allosteric inhibitor.

CONCLUSIONS

Our laboratory has previously reported that retinas deficient of PTP1B are resistant to light damage compared to wild type mice. Since Gab1 is negatively regulated by PTP1B, a part of the retinal neuroprotective effect we have observed previously in PTP1B deficient mice could be contributed by Gab1 as well. In summary, our data suggest that PTP1B regulates the phosphorylation state of retinal Gab1 in vivo.

Key role of PI3Kγ in monocyte chemotactic protein-1-mediated amplification of PDGF-induced aortic smooth muscle cell migration

BACKGROUND AND PURPOSE

Vascular smooth muscle cell (SMC) migration within the arterial wall is a crucial event in atherogenesis and restenosis. Monocyte chemotactic protein-1/CC-chemokine receptor 2 (MCP-1/CCR2) signalling is involved in SMC migration processes but the molecular mechanisms have not been well characterized. We investigated the role of PI3Kγ in SMC migration induced by MCP-1.

EXPERIMENTAL APPROACHES

A pharmacological PI3Kγ inhibitor, adenovirus encoding inactive forms of PI3Kγ and genetic deletion of PI3Kγ were used to investigate PI3Kγ functions in the MCP-1 and platelet-derived growth factor (PDGF) signalling pathway and migration process in primary aortic SMC.

KEY RESULTS

The γ isoform of PI3K was shown to be the major signalling molecule mediating PKB phosphorylation in MCP-1-stimulated SMC. Using a PI3Kγ inhibitor and an adenovirus encoding a dominant negative form of PI3Kγ, we demonstrated that PI3Kγ is essential for SMC migration triggered by MCP-1. PDGF receptor stimulation induced MCP-1 mRNA and protein accumulation in SMCs. Blockade of the MCP-1/CCR2 pathway or pharmacological inhibition of PI3Kγ reduced PDGF-stimulated aortic SMC migration by 50%. Thus PDGF promotes an autocrine loop involving MCP-1/CCR2 signalling which is required for PDGF-mediated SMC migration. Furthermore, SMCs isolated from PI3Kγ-deficient mice (PI3Kγ(-/-)), or mice expressing an inactive PI3Kγ (PI3Kγ(KD/KD)), migrated less than control cells in response to MCP-1 and PDGF.

CONCLUSIONS AND IMPLICATIONS

PI3Kγ is essential for MCP-1-stimulated aortic SMC migration and amplifies cell migration induced by PDGF by an autocrine/paracrine loop involving MCP-1 secretion and CCR2 activation. PI3Kγ is a promising target for the treatment of aortic fibroproliferative pathologies.

A role for sphingomyelin-rich lipid domains in the accumulation of phosphatidylinositol-4,5-bisphosphate to the cleavage furrow during cytokinesis

Cytokinesis is a crucial step in the creation of two daughter cells by the formation and ingression of the cleavage furrow. Here, we show that sphingomyelin (SM), one of the major sphingolipids in mammalian cells, is required for the localization of phosphatidylinositol-4,5-bisphosphate (PIP(2)) to the cleavage furrow during cytokinesis. Real-time observation with a labeled SM-specific protein, lysenin, revealed that SM is concentrated in the outer leaflet of the furrow at the time of cytokinesis. Superresolution fluorescence microscopy analysis indicates a transbilayer colocalization between the SM-rich domains in the outer leaflet and PIP(2)-rich domains in the inner leaflet of the plasma membrane. The depletion of SM disperses PIP(2) and inhibits the recruitment of the small GTPase RhoA to the cleavage furrow, leading to abnormal cytokinesis. These results suggest that the formation of SM-rich domains is required for the accumulation of PIP(2) to the cleavage furrow, which is a prerequisite for the proper translocation of RhoA and the progression of cytokinesis.

α1-Adrenoceptor-mediated contraction of rat aorta is partly mediated via transactivation of the epidermal growth factor receptor

BACKGROUND AND PURPOSE

High level of plasma catecholamines is a risk factor for vascular diseases such as hypertension and atherosclerosis. Catecholamines induce hypertrophy of vascular smooth muscle through α(1) -adrenoceptors, which in cell culture involves the transactivation of epidermal growth factor receptor (EGFR). We hypothesized that EGFR transactivation was also involved in contractions of rat aorta mediated by α(1) -adrenoceptors.

EXPERIMENTAL APPROACH

Thoracic aorta was isolated from 12-14 week old male Wistar rats. In vitro aortic contractile responses to cumulative doses of phenylephrine were characterized in the absence and presence of the EGFR kinase inhibitors, AG1478 and DAPH, in intact and endothelium-denuded rings. Involvement of signal transduction pathways was investigated by using heparin and inhibitors of Src, matrix metalloproteinase (MMP), extracellular signal-regulated kinase (ERK)1/2 and phosphatidyl inositol 3-kinase (PI3K). Phosphorylation of EGFR and ERK1/2 was measured after short-term phenylephrine or EGF stimulation in aorta segments in the presence of AG1478 and the PI3K inhibitor, wortmannin.

KEY RESULTS

AG1478 and DAPH concentration dependently attenuated phenylephrine-induced contractile responses in intact or endothelium-denuded aortic rings. Inhibition of PI3K (wortmannin and LY294002) but not heparin or inhibitors of Src or MMP, prevented the effect of AG1478 on the responses to phenylephrine. Phenylephrine induced phosphorylation of EGFR, which was partially blocked by AG1478. Phenylephrine also increased phosphorylation of ERK1/2, time-dependently and was blocked by AG1478 and wortmannin.

CONCLUSIONS AND IMPLICATIONS

Contractions of rat thoracic aorta mediated by α(1) -adrenoceptors involved transactivation of EGFR, mediated via a PI3K and ERK1/2 dependent pathway.

Endothelial Grb2-associated binder 1 is crucial for postnatal angiogenesis

OBJECTIVE

Grb2-associated binder 1 (Gab1), a scaffolding adaptor protein, plays an important role in transmitting key signals that control cell growth, differentiation, and function from multiple tyrosine kinase receptors. The study was designed to investigate the role of endothelial Gab1 in angiogenesis and its underlying molecular mechanisms.

METHODS AND RESULTS

Using Cre-Lox recombination technology, we generated endothelial-specific Gab1 knockout (Gab1-ecKO) mice. Gab1-ecKO mice are viable and showed no obvious developmental defects in the vascular system. To analyze the role of Gab1 in postnatal angiogenesis, we used hindlimb ischemia and Matrigel plug models. We found that loss of endothelial Gab1 in mice dramatically impaired postnatal angiogenesis. Gab1-ecKO mice had impaired ischemia-initiated blood flow recovery, exhibited reduced angiogenesis, and were associated with marked limb necrosis. We further observed significant endothelial cell (EC) death in the ischemic hindlimb of Gab1-ecKO mice. Matrigel plug assay showed that hepatocyte growth factor (HGF)-mediated angiogenesis was inhibited in Gab1-ecKO mice. In vitro studies showed that Gab1 was required for HGF-induced EC migration, tube formation, and microvessel sprouting. Mechanistically, HGF stimulated Gab1 tyrosine phosphorylation in ECs, leading to activation of extracellular regulated MAP kinase 1/2 and Akt, which are angiogenic and survival signaling.

CONCLUSIONS

Gab1 is essential for postnatal angiogenesis through mediating angiogenic and survival signaling.

Connective tissue growth factor induction by lysophosphatidic acid requires transactivation of transforming growth factor type β receptors and the JNK pathway

Transforming growth factor β (TGF-β) is a very strong pro-fibrotic factor which mediates its action, at least in part, through the expression of connective tissue growth factor (CTGF/CCN2). Along with these cytokines, the involvement of phospholipids in wound healing and the development of fibrosis has been revealed. Among them, lysophosphatidic acid (LPA) is a novel, potent regulator of wound healing and fibrosis that has diverse effects on many types of cells. We decided to evaluate the effect of LPA together with TGF-β on CTGF expression. We found that myoblasts treated with LPA and TGF-β1 produced an additive effect on CTGF expression. In the absence of TGF-β, the induction of CTGF expression by LPA was abolished by a dominant negative form of the TGF-β receptor type II (TGF-βRII) and by the use of SB 431542, a specific inhibitor of the serine/threonine kinase activity of TGF-βRI, suggesting that CTGF induction is dependent on LPA and requires active TGF-βRs. Moreover, we show that LPA requires Smad-2/3 proteins for the induction of CTGF expression, but not their phosphorylation or their nuclear translocation. The requirement of TGF-βRI for LPA mediated-effects is differential, since treatment of myoblasts with LPA in the presence of SB 431542 abolished the induction of stress fibers but not the induction of proliferation. Finally, we demonstrated that CTGF induction in response to LPA requires the activation of JNK, but not ERK, signaling pathways. The JNK requirement is independent of TGF-βRI-mediated activity. These novel results for the mechanism of action of LPA and TGF-β are important for understanding the role of pro-fibrotic growth factors and phospholipids involved in wound healing and related diseases.

Resveratrol regulates the PTEN/AKT pathway through androgen receptor-dependent and -independent mechanisms in prostate cancer cell lines

The tumor suppressor gene PTEN (phosphatase and tensin homolog deleted on chromosome 10) and the androgen receptor (AR) play important roles in tumor development and progression in prostate carcinogenesis. Among many functions, PTEN negatively regulates the cytoplasmic phosphatidylinositol-3-kinase/AKT anti-apoptotic pathway; and nuclear PTEN affects the cell cycle by also negatively regulating the MAPK pathway via cyclin D. Decreased PTEN expression is correlated with prostate cancer progression. Over-expression of AR and upregulation of AR transcriptional activity are often observed in the later stages of prostate cancer. Recent studies indicate that PTEN regulates AR activity and stability. However, the mechanism of how AR regulates PTEN has never been studied. Furthermore, resveratrol, a phytoalexin enriched in red grapes, strawberries and peanuts, has been shown to inhibit AR transcriptional activity in prostate cancer cells. In this study, we use prostate cancer cell lines to test the hypothesis that resveratrol inhibits cellular proliferation in both AR-dependent and -independent mechanisms. We show that resveratrol inhibits AR transcriptional activity in both androgen-dependent and -independent prostate cancer cells. Additionally, resveratrol stimulates PTEN expression through AR inhibition. In contrast, resveratrol directly binds epidermal growth factor receptor (EGFR) rapidly inhibiting EGFR phosphorylation, resulting in decreased AKT phosphorylation, in an AR-independent manner. Thus, resveratrol may act as potential adjunctive treatment for late-stage hormone refractory prostate cancer. More importantly, for the first time, our study demonstrates the mechanism by which AR regulates PTEN expression at the transcription level, indicating the direct link between a nuclear receptor and the PI3K/AKT pathway.

Adenosine A(1) receptor-mediated transactivation of the EGF receptor produces a neuroprotective effect on cortical neurons in vitro

AIM

To understand the mechanism of the transactivation of the epidermal growth factor receptor (EGFR) mediated by the adenosine A(1) receptor (A(1)R).

METHODS

Primary cultured rat cortical neurons subjected to oxygen-glucose deprivation (OGD) and HEK293/A(1)R cells were treated with the A(1)R-specific agonist N(6)-cyclopentyladenosine (CPA). Phospho-EGFR, Akt, and ERK1/2 were observed by Western blot. An interaction between EGFR and A(1)R was detected using immunoprecipitation and immunocytochemistry.

RESULTS

The A(1)R agonist CPA causes protein kinase B (Akt) activation and protects primary cortical neurons from oxygen-glucose deprivation (OGD) insult. A(1)R and EGFR co-localize in the membranes of neurons and form an immunocomplex. A(1)R stimulation induces significant EGFR phosphorylation via a PI3K and Src kinase signaling pathway; this stimulation provides a neuroprotective effect in cortical neurons. CPA leads to sustained phosphorylation of extracellularly regulated kinases 1 and 2 (ERK1/2) in cortical neurons, but only to transient phosphorylation in HEK 293/A(1)R cells. The response to the A(1)R agonist is mediated primarily through EGFR transactivation that is dependent on pertussis toxin (PTX)-sensitive G(i) protein and metalloproteases in HEK 293/A(1)R.

CONCLUSION

A(1)R-mediated EGFR transactivation confers a neuroprotective effect in primary cortical neurons. PI3 kinase and Src kinase play pivotal roles in this response.Acta Pharmacologica Sinica (2009) 30: 889-898; doi: 10.1038/aps.2009.80.

Lysophosphatidic acid signaling in airway epithelium: role in airway inflammation and remodeling

Lysophosphatidic acid (LPA), a potent bioactive phospholipid, induces diverse cellular responses, including cell proliferation, migration, and cytokine release. LPA can be generated intracellularly and extracellularly through multiple synthetic pathways by action of various enzymes, such as phospholipase A(1/2) (PLA(1/2)), phospholipase D (PLD), acylglycerol kinase (AGK), and lysophospholipase D (lysoPLD). Metabolism of LPA is regulated by a family of lipid phosphate phosphatases (LPPs). Significant amounts of LPA have been detected in various biological fluids, including serum, saliva, and bronchoalveolar lavage fluid (BALF). The most significant effects of LPA appear to be through activation of the G-protein-coupled receptors (GPCRs), termed LPA(1-6). LPA regulates gene expression through activation of several transcriptional factors, such as nuclear factor-kappaB (NF-kappaB), AP-1, and C/EBPbeta. In addition to GPCRs, cross-talk between LPA receptors and receptor tyrosine kinases (RTKs) partly regulates LPA-induced intracellular signaling and cellular responses. Airway epithelial cells participate in innate immunity through the release of cytokines, chemokines, lipid mediators, other inflammatory mediators and an increase in barrier function in response to a variety of inhaled stimuli. Expression of LPA receptors has been demonstrated in airway epithelial cells. This review summarizes our recent observations of the role of LPA/LPA-Rs in regulation of airway epithelium, especially in relation to the secretion of pro- and anti-inflammatory mediators and regulation of airway barrier function.

The role of PTEN signaling perturbations in cancer and in targeted therapy

The PTEN tumor suppressor was discovered by its homozygous deletion and other mutations in cancer. Since then, PTEN has been shown to be a non-redundant, evolutionarily conserved phosphatase whose function affects diverse cellular progresses such as cell cycle progression, cell proliferation, chemotaxis, apoptosis, aging, muscle contractility, DNA damage response, angiogenesis and cell polarity. In accordance with its ability to influence multiple crucial cellular processes, PTEN has a major role in the pathogenesis of numerous diseases such as diabetes, autism and almost every cancer examined. This review will discuss the diverse ways in which PTEN signaling is modified in cancer, and how these changes correlate with and might possibly affect the action of targeted chemotherapy.

PI3-K/Akt-dependent activation of cAMP-response element-binding (CREB) protein in Jurkat T leukemia cells treated with TRAIL

We recently demonstrated the activation of phosphatidylinositol 3-kinase (PI3-K/Akt) survival pathway in Jurkat T leukemia cells known for their sensitivity to the tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)/Apo2L cytotoxic action. The present investigation was done to elucidate the role of cAMP-response element-binding (CREB) protein in this system. Jurkat T cells were treated with 100-1,000 ng/ml TRAIL for time intervals up to 24 h in the presence or absence of selective pharmacologic inhibitors of PI3-K/Akt (LY294002) or p38 MAPK (SB253580) pathways. Upon TRAIL treatment, a dose-dependent increase in the percentage of apoptotic cells as well as in caspase-3 activity was observed. A further enhancement of apoptotic cell death was obtained with the use of CREB1 siRNA technology, as demonstrated by flow cytometry. Western blot analysis showed a high constitutive level of CREB phosphorylation at Ser(133) in Jurkat T cells under normal serum culture conditions. Under low serum culture conditions, an early (within 1 h) and transient increase in CREB phosphorylation was detected in response to both TRAIL doses and reduced upon pre-treatment with LY294002 or SB253580, demonstrating the PI3-K/Akt- and p38 MAPK-dependency of this effect. The parallel analysis in immune fluorescence demonstrated the nuclear translocation of the phosphorylated form upon treatment with 100 ng/ml TRAIL, whereas the immune labeling was mainly detectable in the cytoplasm compartment upon the higher more cytotoxic dose. These results let us hypothesize that CREB activation can be an important player in the complex cross-talk among pro- and anti-apoptotic pathways in this peculiar cell model.

Cell confluence regulates hepatocyte growth factor-stimulated cell morphogenesis in a beta-catenin-dependent manner

Following organ injury, morphogenic epithelial responses can vary depending on local cell density. In the present study, the role of cell confluence in determining the responsiveness of renal epithelial cells to the dedifferentiating morphogenic signals of hepatocyte growth factor (HGF) was examined. Increasing confluence resulted in a greater tendency of cells to organize into epithelial tubes and a significant decrease in migratory responsiveness to HGF. Analysis of downstream signaling revealed that the HGF receptor c-Met was equally activated in confluent and nonconfluent cells following HGF stimulation but that phosphoinositide 3-kinase-dependent activation of Akt and Rac were selectively diminished in confluent cells. In nonconfluent cells treated with HGF, the high level of Akt activation resulted in inhibitory phosphorylation of glycogen synthase kinase 3beta (GSK-3beta) and increased beta-catenin nuclear signaling. In contrast, confluent cells, in which HGF-stimulated Akt activation was diminished, displayed less inhibitory phosphorylation of GSK-3beta and less nuclear signaling by beta-catenin. Overexpression of beta-catenin (SA), which cannot be phosphorylated by GSK-3beta and targeted for ubiquitination, significantly increased migration in fully confluent cells. Thus, cells maintained at high confluence selectively downregulate signaling events such as Rac activation and beta-catenin-dependent transcription that would otherwise promote cell dedifferentiation and migration.

Activation of Gab1 in pancreatic acinar cells: Effects of gastrointestinal growth factors/hormones on stimulation, phosphospecific phosphorylation, translocation and interaction with downstream signaling molecules

The scaffolding/adapter protein, Gab1, is a key signaling molecule for numerous stimuli including growth factors and G protein-coupled-receptors (GPCRs). A number of questions about Gab1 signaling remain and little is known about the ability of gastrointestinal (GI) hormones/neurotransmitters/growth factors to activate Gab1. Therefore, we examined their ability to activate Gab1 and explored the mechanisms involved using rat pancreatic acini. HGF and EGF stimulated total Gab1 tyrosine phosphorylation (TyrP) and TyrP of Gab1 phospho-specific sites (Y307, Y627), but not other pancreatic growth factors, GI GPCRs (CCK, bombesin, carbachol, VIP, secretin), or agents directly activating PKC or increasing Ca2+. HGF-stimulated Y307 Gab1 TyrP differed in kinetics from total and Y627. Neither GF109203X, nor inhibition of Ca2+ increases altered HGF's effect. In unstimulated cells>95% of Gab1 was cytosolic and HGF stimulated a 3-fold increase in membrane Gab1. HGF stimulated equal increases in pY307 and pY627 Gab1 in cytosol/membrane. HGF stimulated Gab1 association with c-Met, Grb2, SHP2, PI3K, Shc, Crk isoforms and CrkL, but not with PLCgamma1. These results demonstrate that only a subset of pancreatic growth factors (HGF/EGF) stimulates Gab1 signaling and no pancreatic hormones/neurotransmitters. Our results with Gab1 activation with different growth factors, the role of PKC, and its interaction with distant signaling molecules suggest the cellular mechanisms of Gab1 signaling show important differences in different cells. These results show that Gab1 activation plays a central role in HGF's ability to stimulate intracellular transduction cascades in pancreatic acinar cells and this action likely plays a key role in HGF's ability to alter pancreatic cell function (i.e., growth/regeneration).

The adaptor protein Gab1 couples the stimulation of vascular endothelial growth factor receptor-2 to the activation of phosphoinositide 3-kinase

Phosphoinositide 3-kinase (PI3K) mediates essential functions of vascular endothelial growth factor (VEGF), including the stimulation of endothelial cell proliferation and migration. Nevertheless, the mechanisms coupling the receptor VEGFR-2 to PI3K remain obscure. We observed that the Grb2-bound adapter Gab1 is tyrosine-phosphorylated and relocated to membrane fractions upon VEGF stimulation of endothelial cells. We could detect the PI3K regulatory subunit p85 in immunoprecipitates of endogenous Gab1, and vice versa, and measure a Gab1-associated lipid kinase activity upon VEGF stimulation. Furthermore, transfection of the Gab1-YF3 mutant lacking all p85-binding sites strongly repressed PI3K activation measured in vitro. Moreover, Gab1-YF3 severely decreased the cellular amount of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) generated in response to VEGF. Furthermore, adenoviral expression of Gab1-YF3 suppressed both Akt phosphorylation and recovery of wounded human umbilical vein endothelial cell monolayers, a VEGF-dependent process involving cell migration and proliferation under PI3K control. Transfection of other Gab1 mutants, lacking Grb2-binding sites or the pleckstrin homology (PH) domain, also prevented Akt activation, further demonstrating Gab1 involvement in PI3K activation. These mutants were also used to show that interactions with both Grb2 and PtdIns(3,4,5)P3 mediate Gab1 recruitment by VEGFR-2. Importantly, Gab1 mobilization was impaired by (i) PI3K inhibitors, (ii) deletion of Gab1 PH domain, (iii) PTEN (phosphatase and tensin homolog deleted on chromosome 10) overexpression to repress PtdIns(3,4,5)P3 production, and (iv) overexpression of a competitor PH domain for PtdIns(3,4,5)P3 binding, which altogether demonstrated that PI3K is also an upstream regulator of Gab1. Gab1 thus appears as a primary actor in coupling VEGFR-2 to PI3K/Akt, recruited through an amplification loop involving PtdIns(3,4,5)P3 and its PH domain.

Specific role for p85/p110beta in GTP-binding-protein-mediated activation of Akt

We prepared CHO (Chinese hamster ovary) cells expressing both IR (insulin receptor) and A1R (A1 adenosine receptor). Treatment of the cells with insulin or PIA [N6-(2-phenylisopropyl)adenosine], a specific A(1)R agonist increased Akt activity in the cells in a PI3K- (phosphoinositide 3-kinase) dependent manner. Transfection of p110beta into the cells augmented the action of PIA with little effect on insulin. Introduction of a pH1 vector producing shRNA (short hairpin RNA) that targets p110beta abolished PIA-induced Akt activation. By contrast, an shRNA probe targeting p110alpha did not impair the effects of PIA. The effect of PIA in p110alpha-deficient cells was attenuated effectively by both Deltap85 and betaARK-CT (beta-adrenergic receptor kinase-C-terminal peptide). A Deltap85-derived protein possessing point mutations in its two SH2 domains did not impair PIA action. These results suggest that tyrosine-phosphorylated proteins and Gbetagamma (betagamma subunits of GTP-binding protein) are necessary for the specific function of p110beta in intact cells. The p110beta-middle (middle part of p110beta) may play an important role in signal reception from GPCRs (GTP-binding-protein-coupled receptor), because transfection of the middle part impaired PIA sensitivity.

BCR-ABL Regulates Phosphatidylinositol 3-Kinase-p110γ Transcription and Activation and Is Required for Proliferation and Drug Resistance

The BCR-ABL oncogene is the hallmark of chronic myeloid leukemia, a clonal hematopoietic stem cell disorder. BCR-ABL displays constitutive tyrosine kinase activity, required for its transformation ability. Although the molecular mechanisms behind this malignancy are not fully understood, a role for phosphatidylinositol (PI) 3-kinase has been repeatedly described. Here we report the specific up-regulation of the class I(B) catalytic subunit of PI 3-kinase (p110gamma) in response to BCR-ABL expression. We demonstrate that this upregulation is due to increased transcription and is dependent on both PI 3-kinase and MEK activity. We performed in vitro kinase activity assays and show that BCR-ABL also leads to increased p110gamma activity and that this activation requires both G protein-coupled receptor and Ras signaling. In addition, by transfection of cells with dominant negative p110gamma, we determined that this specific PI 3-kinase isoform is involved in both proliferation and the apoptosis resistance associated with chronic myeloid leukemia. The data presented here define for the first time the ability of BCR-ABL to alter the expression levels of PI 3-kinase isoforms and also demonstrate a previously unreported link between BCR-ABL and p110gamma.

Dependence of GnRH-induced phosphorylation of CREB and BAD on EGF receptor transactivation in GT1-7 neuronal cells

The hypothalamic neuropeptide, gonadotropin releasing hormone (GnRH), is a primary regulatory factor in the neuroendocrine control of reproduction. The GnRH decapeptide is released in an episodic manner from hypothalamic GnRH neurons, which are known to express GnRH receptors. Here we examined the signaling pathways by which autocrine GnRH stimulation generates cell survival and proliferative signals in hypothalamic GT1-7 cells. Both GnRH and epidermal growth factor (EGF) caused rapid phosphorylation of cyclic AMP response element binding protein (CREB) and BAD. The selective epidermal growth factor receptor (EGF-R) antagonist, AG1478, attenuates the phosphorylation of these proteins by GnRH and EGF. Inhibition of PKC and Src abolished the stimulatory effects of GnRH, but not that of EGF, consistent with a critical role of these signaling molecules upstream of the EGF-R. All of these effects of GnRH were mimicked by phorbol 12 myristate 13-acetate (PMA). Consistent with the prosurvival and mitogenic effects of phosphoinositide 3-kinase/Akt (P13-K/Akt) downstream of the EGF-R, inhibition of P13-K diminished the activation of these proteins following stimulation with GnRH, EGF, and PMA. Overexpression of dominant negative Akt attenuated agonist-induced phosphorylation of BAD, but not that of ERK1/2 and CREB. Moreover, overexpression of wild-type RSK-1 resulted in enhanced basal as well as agonist-induced phosphorylation of CREB and BAD, indicating a critical role of RSK-1 in activating cytosolic as well as nuclear proteins. These data reveal novel signaling mechanisms of GnRH-induced phosphorylation of CREB and BAD in GT1-7 neurons through transactivation of the EGF-R.

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.

Class II phosphoinositide 3-kinase defines a novel signaling pathway in cell migration

The lipid products of phosphoinositide 3-kinase (PI3K) are involved in many cellular responses such as proliferation, migration, and survival. Disregulation of PI3K-activated pathways is implicated in different diseases including cancer and diabetes. Among the three classes of PI3Ks, class I is the best characterized, whereas class II has received increasing attention only recently and the precise role of these isoforms is unclear. Similarly, the role of phosphatidylinositol-3-phosphate (PtdIns-3-P) as an intracellular second messenger is only just beginning to be appreciated. Here, we show that lysophosphatidic acid (LPA) stimulates the production of PtdIns-3-P through activation of a class II PI3K (PI3K-C2β). Both PtdIns-3-P and PI3K-C2β are involved in LPA-mediated cell migration. This study is the first identification of PtdIns-3-P and PI3K-C2β as downstream effectors in LPA signaling and demonstration of an intracellular role for a class II PI3K. Defining this novel PI3K-C2β–PtdIns-3-P signaling pathway may help clarify the process of cell migration and may shed new light on PI3K-mediated intracellular events.

Role of EGF receptor transactivation in phosphoinositide 3-kinase-dependent activation of MAP kinase by GPCRs

Many G protein coupled receptors (GPCRs) cause phosphorylation of MAP kinases through transactivation of the epidermal growth factor receptor (EGF-R), leading to increased cell survival and growth, motility, and migration. Phosphoinositide 3-kinase (PI3K) is one of the important cell survival signaling molecules activated by EGF-R stimulation. However, the extent to which EGF-R transactivation is essential for GPCR agonist-stimulated PI3K activation is not known. Here we examined the mechanism of PI3K activation that elicits GPCR-mediated ERK1/2 activation by pathways dependent and/or independent of EGF-R transactivation in specific cell types. Immortalized hypothalamic neurons (GT1-7 cells) express endogenous gonadotropin-releasing hormone receptors (GnRH-R) and their stimulation causes marked phosphorylation of ERK1/2 and Akt (Ser 473) through transactivation of the EGF-R and recruitment of PI3K. In C9 hepatocytes, agonist activation of AT1 angiotensin II (AT1-R), lysophosphatidic acid (LPA), and EGF receptors caused phosphorylation of Akt through activation of the EGF-R in a PI3K-dependent manner. However, ERK1/2 activation by these agonists in these cells was independent of PI3K activation. In contrast, agonist stimulation of HEK 293 cells stably expressing AT1-R caused ERK1/2 phosphorylation that was independent of EGF-R transactivation but required PI3K activation. LPA signaling in these cells showed partial and complete dependence on EGF-R and PI3K, respectively. These data indicate that GPCR-induced ERK1/2 phosphorylation is dependent or independent of PI3K in specific cell types, and that the involvement of PI3K during ERK1/2 activation is not dependent solely on agonist-induced transactivation of the EGF-R.

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.

PI-3K/Akt and NF-kappaB/IkappaBalpha pathways are activated in Jurkat T cells in response to TRAIL treatment

The aim of this work was to evaluate the involvement of survival pathways in the response of Jurkat T leukaemic cells sensitive to the cytotoxic action of tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)/Apo2L. Jurkat T cells express TRAIL-R2/DR5 and TRAIL-R4/DcR2 receptors and start to die by apoptosis early (3 h) upon TRAIL administration reaching a dose-dependent increase in the percentage of dead cells within 48 h (up to 85-90%). This increase in cell death is accompanied by a dose-dependent significant (P < 0.05) increase in the G0/G1 phase of the cell cycle and reverted by the treatment with a broad inhibitor of caspases, z-VAD-fmk. Co-treatment of the cells with inhibitors of PI-3 kinase (LY294002) and nuclear factor kappa B (NF-kappaB) (SN50) pathways leads to an earlier significantly increased cytotoxicity, respectively in the form of apoptosis and necrosis. Consistently with the data obtained with the pharmacological inhibitors, the activation and nuclear translocation of both PI-3K and NF-kappaB were observed. In summary, our results provide evidence that even in sensitive neoplastic cells TRAIL paradoxically activates pro-survival pathways, which protect against TRAIL-mediated death since their inhibition leads to an earlier and increased cytotoxicity.

A Novel Role for Gab1 and SHP2 in Epidermal Growth Factor-induced Ras Activation

SHP2 was recently found to down-regulate PI3K activation by dephosphorylating Gab1 but the mechanisms explaining the positive role of the Gab1/SHP2 pathway in EGF-induced Ras activation remain ill defined. Substrate trapping experiments now suggest that SHP2 dephosphorylates other Gab1 phosphotyrosines located within a central region displaying four YXXP motifs. Because these sites are potential docking motifs for Ras-GAP, we tested whether SHP2 dephosphorylates them to facilitate Ras activation. We observed that a Gab1 construct preventing SHP2 recruitment promoted membrane relocation of RasGAP. Moreover, a RasGAP-inactive mutant restored the activation of Ras in cells transfected with SHP2-inactivating Gab1 mutant or in SHP2-deficient fibroblasts, supporting the hypothesis that RasGAP is a downstream target of SHP2. To determine whether Gab1 is a RasGAP-binding partner, a Gab1 mutant deleted of four YXXP motifs was produced. The deletion suppressed RasGAP redistribution and restored the defective Ras activation caused by SHP2-inactivating mutations. Moreover, Gab1 was found to interact with RasGAP SH2 domains, only under conditions where SHP2 is not activated. To identify Ras-GAP-binding sites, Tyr to Phe mutants of Gab1 YXXP motifs were produced. Gab1 constructs mutated on Tyr(317) were severely affected in RasGAP binding and were the most active in compensating for Ras-defective activation and blocking RasGAP redistribution induced by SHP2 inactivation. We have thus localized on Gab1 a Ras-negative regulatory tyrosine phosphorylation site involved in RasGAP binding and showed that an important SHP2 function is to down-regulate its phosphorylation to disengage RasGAP and sustain Ras activation.

Establishment of a human non-small cell lung cancer cell line resistant to gefitinib

The epidermal growth factor receptor (EGFR) tyrosine-kinase inhibitor gefitinib (Iressa, ZD1839) has shown promising activity preclinically and clinically. Because comparative investigations of drug-resistant sublines with their parental cells are useful approaches to identifying the mechanism of gefitinib resistance and select factors that determine sensitivity to gefitinib, we established a human non-small cell lung carcinoma subline (PC-9/ZD) that is resistant to gefitinib. PC-9/ZD cells are approximately 180-fold more resistant to gefitinib than their parental PC-9 cells and PC-9/ZD cells do not exhibit cross-resistance to conventional anticancer agents or other tyrosine kinase inhibitors, except AG-1478, a specific inhibitor of EGFR. PC-9/ZD cells also display significant resistance to gefitinib in a tumor-bearing animal model. To elucidate the mechanism of resistance, we characterized PC-9/ZD cells. The basal level of EGFR in PC-9 and PC-9/ZD cells was comparable. A deletion mutation was identified within the kinase domain of EGFR in both PC-9 and PC-9/ZD, but no difference in the sequence of EGFR cDNA was detected in either cell line. Increased EGFR/HER2 (and EGFR/HER3) heterodimer formations were demonstrated in PC-9/ZD cells by chemical cross-linking and immunoprecipitation analysis in cells unexposed to gefitinib. Exposure to gefitinib increased heterodimer formation in PC-9 cells, but not in PC-9/ZD cells. Gefitinib inhibits EGFR autophosphorylation in a dose-dependent manner in PC-9 cells but not in PC-9/ZD cells. A marked difference in inhibition of site-specific phosphorylation of EGFR was observed at Tyr1068 compared to other tyrosine residues (Tyr845, 992 and 1045). To elucidate the downstream signaling in the PC9/ZD cellular machinery, complex formation between EGFR and its adaptor proteins GRB2, SOS, and Shc was examined. A marked reduction in the GRB2-EGFR complex and absence of SOS-EGFR were observed in PC-9/ZD cells, even though the protein levels of GRB2 and SOS in PC-9 and PC-9/ZD cells were comparable. Expression of phosphorylated AKT was increased in PC-9 cells and inhibited by 0.02 microM gefitinib. But the inhibition was not significant in PC-9/ZD cells. These results suggest that alterations of adaptor-protein-mediated signal transduction from EGFR to AKT is a possible mechanism of the resistance to gefitinib in PC-9/ZD cells. These phenotypes including EGFR-SOS complex and heterodimer formation of HER family members are potential biomarkers for predicting resistance to gefitinib.

Biology of LPA in health and disease

The functions of lysophosphatidic acid (LPA) can be broadly divided into two classes: (1) physiological and (2) pathological roles. The role of LPA in embryonic development can be seen as early as oocyte formation. It continues in postnatal homeostasis, through its ability to impart a level of protection from both stress and local injury, by regulating cellular proliferation, apoptosis, and the reorganization of cytoskeletal fibers. LPA may function as a double-edged sword. While it helps maintain homeostasis against stress and insult, it may also augment the development and spread of pathological processes, including cancers.

Apc deficiency is associated with increased Egfr activity in the intestinal enterocytes and adenomas of C57BL/6J-Min/+ mice

Overexpression of the epidermal growth factor receptor (EGFR) and its increased tyrosine kinase activity are implicated in colorectal cancer (CRC) development and malignant progression. The C57BL/6J-Min/+ (Min/+) mouse is a model for CRC and develops numerous intestinal adenomas. We analyzed the normal mucosa of Min/+ and Apc+/+ (WT) littermate mice together with Apc-null adenomas to gain insight into the roles of Egfr in these intestinal tissues. Protein analyses showed that Egfr activity was highest in the tumors, and also up-regulated in Min/+ relative to WT enterocytes. Expression of ubiquitylated Egfr (Egfr-Ub) was increased in Min/+ enterocytes and tumors. Tumors exhibited increased association of Egfr with clathrin heavy chain (CHC), Gab1, and p85alpha, the regulatory subunit of phosphoinositide 3-kinase (PI3K), and tumors also overexpressed c-Src, PDK1, and Akt. Immunohistochemistry for Akt-p-Ser473 revealed a low level of this active kinase in Min/+ and WT enterocytes and its strong presence in tumors. Prostaglandin E2 (PGE2) is a product of cyclooxygenase-2 (Cox-2) activity that is up-regulated in Min/+ tumors and transactivates Egfr. PGE2 expression was significantly higher in untreated Min/+ tumors and reduced by treatment with the Cox-2 inhibitor, celecoxib. Dietary administration of this NSAID also inhibited Egfr activity in tumors. Increased activation of the EGFR-PI3K-Akt signaling pathway in tumors relative to Apc+/+ and ApcMin/+ enterocytes provides potential opportunities for therapeutic interventions to differentially suppress tumor formation, promotion, progression, and/or recurrence.

Blockade of epidermal growth factor- or heregulin-dependent ErbB2 activation with the anti-ErbB2 monoclonal antibody 2C4 has divergent downstream signaling and growth effects

Due to heterodimerization and a variety of stimulating ligands, the ErbB receptor system is both diverse and flexible, which proves particularly advantageous to the aberrant signaling of cancer cells. However, specific mechanisms of how a particular receptor contributes to generating the flexibility that leads to aberrant growth regulation have not been well described. We compared the utilization of ErbB2 in response to epidermal growth factor (EGF) and heregulin stimulation in colon carcinoma cells. Anti-ErbB2 monoclonal antibody 2C4 blocked heregulin-stimulated phosphorylation of ErbB2 and ErbB3; activation of mitogen-activated protein kinase (MAPK), phosphatidylinositol 3'-kinase (PI3K), and Akt; proliferation; and anchorage-independent growth. 2C4 blocked EGF-mediated phosphorylation of ErbB2 and inhibited PI3K/Akt and anchorage-independent growth but did not affect ErbB1 or MAPK. Immunoprecipitations showed that ErbB3 and Grb2-associated binder (Gab) 1 were phosphorylated and associated with PI3K activity after heregulin treatment and that Gab1 and Gab2, but not ErbB3, were phosphorylated and associated with PI3K activity after EGF treatment. These data show that monoclonal antibody 2C4 inhibited all aspects of heregulin signaling as well as anchorage-independent and monolayer growth. Furthermore, we identify ErbB2 as a critical component of EGF signaling to the Gab1/Gab2-PI3K-Akt pathway and anchorage-independent growth, but EGF stimulation of MAPK and monolayer growth can occur efficiently without the contribution of ErbB2.

Ras activation in response to lysophosphatidic acid requires a permissive input from the epidermal growth factor receptor

The topology of the signalling pathway linking the G-protein-coupled receptor agonist lysophosphatidic acid (LPA) to extracellular-signal-regulated kinase activation remains undeciphered. In the present study, we report that analysis of LPA signals at the level of Ras-GTP formation and Ras nucleotide exchange discriminates true mediatory signals from permissive activities that do not participate in signal relay. Hence, whereas pertussis toxin (PTX) treatment impairs stimulation of nucleotide exchange, epidermal growth factor receptor (EGFR) inhibition does not compromise LPA-induced acceleration of nucleotide exchange, but instead attenuates basal nucleotide turnover on Ras. Our data indicate that LPA activation of Ras proceeds via PTX-sensitive G(i/o)-proteins and requires a permissive input from basal EGFR activity.

The role of Gab family scaffolding adapter proteins in the signal transduction of cytokine and growth factor receptors

The Grb2-associated binder (Gab) family adapter proteins are scaffolding adapter molecules that display sequence similarity with Drosophila DOS (daughter of sevenless), which is a substrate for the protein tyrosine phosphatase Corkscrew. Gab proteins contain a pleckstrin homology (PH) domain and binding sites for SH2 and SH3 domains. A number of studies in multiple systems have implicated Gab in signaling via many different types of receptors, such as growth factor, cytokine, and antigen receptors, and via oncoproteins. Recent studies of Gab1 and Gab2 knockout mice have clearly indicated an important role for Gabs in vivo. Gab1-deficient mice die as embryos with multiple defects in placental, heart, skin, and muscle development. Gab2-deficient mice are viable, but have a defect in the mast cell lineages and in allergic reactions. Given the apparently central role played by Gab signaling via many receptors, delineating the precise mechanism(s) of Gab-mediated signaling is critical to understanding how cytokines, growth factors, and oncoproteins mediate a variety of biological activities: cell growth, differentiation, survival and malignant transformation.

Norepinephrine activates extracellular-regulated kinase in cortical neurons

BACKGROUND

Previous studies demonstrate that indirect activation of monoamine receptors by antidepressant treatment increases neurotrophic factors that activate the mitogen-activated protein kinase cascade; however, it is also possible that these monoamine receptors influence the mitogen-activated protein kinase pathway independent of neurotrophic factors. The influence of norepinephrine on the phosphorylation of extracellular-regulated protein kinase is characterized.

METHODS

Primary cerebral cortical cultures were prepared from embryonic day 18 rat brains and were subsequently incubated with norepinephrine in the absence or presence of agents acting as noradrenergic receptors or as intracellular signaling proteins. Levels of phosphorylated extracellular-regulated protein kinase were determined by immunoblot.

RESULTS

The results demonstrate that incubation with norepinephrine produces a time- and dose-dependent activation of phosphorylated extracellular-regulated protein kinase and that this increase is dependent on activation of alpha(2)- and beta-adrenergic receptor subtypes. In addition, the results demonstrate that norepinephrine activation of phosphorylated extracellular-regulated protein kinase is dependent on a pertussis toxin-sensitive G protein, a receptor tyrosine kinase, and activation of phosphatidylinositol 3-kinase.

CONCLUSIONS

The findings suggest that activation of the mitogen-activated protein kinase cascade by norepinephrine can occur via a tyrosine kinase-dependent signaling pathway but independent of classical second-messenger or Src-dependent kinases.

Involvement of phospholipase D2 in lysophosphatidate-induced transactivation of platelet-derived growth factor receptor-beta in human bronchial epithelial cells

Lysophosphatidate (LPA) mediates multiple cellular responses via heterotrimeric G protein coupled LPA-1, LPA-2, and LPA-3 receptors. Many G protein-coupled receptors stimulate ERK following tyrosine phosphorylation of growth factor receptors; however, the mechanism(s) of transactivation of receptor tyrosine kinases are not well defined. Here, we provide evidence for the involvement of phospholipase D (PLD) in LPA-mediated transactivation of platelet-derived growth factor receptor-beta (PDGF-R beta). In primary cultures of human bronchial epithelial cells (HBEpCs), LPA stimulated tyrosine phosphorylation of PDGF-R beta and threonine/tyrosine phosphorylation of ERK1/2. The LPA-mediated activation of ERK and tyrosine phosphorylation of PDGF-R beta was attenuated by tyrphostin AG 1296, an inhibitor of PDGF-R kinase, suggesting transactivation of PDGF-R by LPA. Furthermore, LPA-, but not PDGF beta-chain homodimer-induced tyrosine phosphorylation of PDGF-R beta was partially blocked by pertussis toxin, indicating coupling of LPA-R(s) to Gi. Exposure of HBEpCs to LPA activated PLD. Butan-1-ol, which acts as an acceptor of phosphatidate generated by the PLD pathway, blocked LPA-mediated transactivation of PDGF-R beta. This effect was not seen with butan-3-ol, suggesting PLD involvement. The role of PLD1 and PLD2 in the PDGF-R beta transactivation by LPA was investigated by infection of cells with adenoviral constructs of wild type and catalytically inactive mutants of PLD. LPA activated both PLD1 and PLD2 in HBEpCs; however, infection of cells with cDNA for wild type PLD2, but not PLD1, increased the tyrosine phosphorylation of PDGF-R beta in response to LPA. Also, the LPA-mediated tyrosine phosphorylation of PDGF-R beta was attenuated by the catalytically inactive mutant mPLD2-K758R. Infection of HBEpCs with adenoviral constructs of wild type hPLD1, mPLD2, and the inactive mutants of hPLD1 and mPLD2 resulted in association of PLD2 wild type and inactive mutant proteins with the PDGF-R beta compared with PLD1. These results show for the first time that transactivation of PDGF-R beta by LPA in HBEpCs is regulated by PLD2.

The PI 3-kinase-Rac-p38 MAP kinase pathway is involved in the formation of signet-ring cell carcinoma

Signet-ring cell carcinoma is classified in poorly differentiated adenocarcinoma with an aggressive nature and a poor prognosis. We have shown that the activation of PI 3-kinase in highly differentiated adenocarcinomas induces loss of cell-cell contact and formation of vacuoles, giving phenotypes similar to those of signet-ring cell lines. SB203580, a potent p38 MAP kinase inhibitor, blocked this transition, and expression of an active form of MKK6 (MKK6DA), an activator of p38 MAP kinase, gave effects similar to those induced by expression of the active form of PI 3-kinase (BD110), although formation of large vacuoles was not induced. Activation of MKK3, another activator of p38 MAP kinase, was activated in native signet-ring carcinoma cell lines. Anchorage-independent growth of signet-ring cell lines was inhibited by LY294002 or SB203580. These results suggest that p38 MAP kinase is functioning downstream of PI 3-kinase in signaling of the malignant phenotype. Secretion of mucins was enhanced in BD110-expressing cells, but not in MKK6DA-expressing cells, suggesting that secretion of mucins is independent of the MKK6-p38 MAP kinase cascade. Thus, there may be at least two pathways, p38 MAP kinase-dependent and -independent, which are involved in regulation of cell-cell contact and the protein secretion system, respectively.

Macrophage-stimulating protein cooperates with erythropoietin to induce colony formation and MAP kinase activation in primary erythroid progenitor cells

We have shown that Fv2, the Friend virus susceptibility 2 locus, encodes a naturally occurring amino-terminally truncated form of the STK receptor tyrosine kinase (Sf-Stk). Sf-Stk appears to interact with the viral glycoprotein gp55 and drive erythropoietin (Epo)-independent expansion of Friend virus-infected erythroblasts. Presumably, Sf-Stk provides signals that cooperate with EpoR signaling to induce the polyclonal expansion of infected cells. In this report, we show that macrophage-stimulating protein (MSP), the ligand for full-length STK, can also cooperate with Epo to enhance burst-forming units-erythroid (BFU-E) formation. To evaluate the signals induced by MSP/STK in primary erythroid progenitor cells, we adapted a method for the expansion of murine bone marrow mononuclear cells. The expanded progenitor cells express STK and respond to MSP in a colony assay. Furthermore, we demonstrate that low doses of MSP and Epo stimulation of the expanded cells cooperate to induce the phosphorylation of MAP kinase. Using the MEK inhibitor PD98059, we show that the activation of ERK is required for the enhanced BFU-E formation in response to MSP. These findings suggest that MSP has the ability to enhance erythroid colony formation in response to Epo, and that this response is dependent on the ability of MSP to induce the MAP kinase pathway.

Modulation of phosphoinositide 3-kinase activation by cholesterol level suggests a novel positive role for lipid rafts in lysophosphatidic acid signalling

Methyl-beta-cyclodextrin (MbetaCD) was used to explore a role for cholesterol-enriched plasma membrane microdomains in coupling lysophosphatidic acid (LPA) stimulation to phosphoinositide 3-kinase (PI3K) activation. Cholesterol depletion strongly inhibited the production of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate in Vero cells stimulated with LPA. In agreement, the phosphorylation of Akt/protein kinase B, but not of Erk kinases, was suppressed by MbetaCD. MbetaCD did not interfere with the overall phospholipid metabolism, and its effects were reversed in cholesterol add-back experiments. Finally, PI3K was detected in lipid rafts prepared from control but not MbetaCD-treated cells, suggesting that these microdomains contribute to LPA signalling by compartmentalising component(s) of the PI3K pathway.

Protein kinase C-alpha and protein kinase C-epsilon are required for Grb2-associated binder-1 tyrosine phosphorylation in response to platelet-derived growth factor

Grb2-associated binder-1 (Gab1) is an adapter protein related to the insulin receptor substrate family. It is a substrate for the insulin receptor as well as the epidermal growth factor (EGF) receptor and other receptor-tyrosine kinases. To investigate the role of Gab1 in signaling pathways downstream of growth factor receptors, we stimulated rat aortic vascular smooth muscle cells (VSMC) with EGF and platelet-derived growth factor (PDGF). Gab1 was tyrosine-phosphorylated by EGF and PDGF within 1 min. AG1478 (an EGF receptor kinase-specific inhibitor) failed to block PDGF-induced Gab1 tyrosine phosphorylation, suggesting that transactivated EGF receptor is not responsible for this signaling event. Because Gab1 associates with phospholipase Cgamma (PLCgamma), we studied the role of the PLCgamma pathway in Gab1 tyrosine phosphorylation. Gab1 tyrosine phosphorylation by PDGF was impaired in Chinese hamster ovary cells expressing mutant PDGFbeta receptor (Y977F/Y989F: lacking the binding site for PLCgamma). Pretreatment of VSMC with (a specific PLCgamma inhibitor) inhibited Gab1 tyrosine phosphorylation as well, indicating the importance of the PLCgamma pathway. Gab1 was tyrosine-phosphorylated by phorbol ester to the same extent as PDGF stimulation. Studies using antisense protein kinase C (PKC) oligonucleotides and specific inhibitors showed that PKCalpha and PKCepsilon are required for Gab1 tyrosine phosphorylation. Binding of Gab1 to the protein-tyrosine phosphatase SHP2 and phosphatidylinositol 3-kinase was significantly decreased by PLCgamma and/or PKC inhibition, suggesting the importance of the PLCgamma/PKC-dependent Gab1 tyrosine phosphorylation for the interaction with other signaling molecules. Because PDGF-mediated ERK activation is enhanced in Chinese hamster ovary cells that overexpress Gab1, Gab1 serves as an important link between PKC and ERK activation by PDGFbeta receptors in VSMC.

A function for phosphoinositide 3-kinase beta lipid products in coupling beta gamma to Ras activation in response to lysophosphatidic acid

Although Gbetagamma is thought to mediate mitogen-activated protein kinase (MAPK) activation in response to G protein-coupled receptor stimulation, the mechanisms involved in this pathway have not been clearly defined. Phosphoinositide 3-kinase (PI3K) has been proposed as an early intermediate in this process, but its role has remained elusive. We have observed that dominant negative mutants of p110beta, but not of p110gamma, inhibited MAPK stimulation in response to lysophosphatidic acid (LPA). The role of p110beta was located upstream from Ras. To determine which of the lipid or protein kinase activities of p110beta were important for Ras activation, we produced a mutant p110beta lacking the lipid but not the protein kinase activity. This protein displayed a dominant negative activity similar to a kinase-dead mutant, indicating that p110beta lipid kinase activity was essentially involved in Ras activation. In agreement, overexpression of the lipid phosphatase PTEN was found to specifically inhibit Ras stimulation induced by LPA. In addition, we have observed that the PH domain-containing adapter protein Gab1, which is involved in p110beta activation during LPA stimulation, is also implicated in this pathway downstream of p110beta. Indeed, both membrane redistribution and phosphorylation of Gab1 were reduced in the presence of PI3K inhibitors or dominant negative p110beta. Downstream of Gab1, the tyrosine phosphatase SHP2 was found to mediate Ras activation in response to LPA and to be recruited through PI3K and Gab1, because transfection of Gab1 mutant deficient for SHP2 binding inhibited Ras activation without interfering with PI3K activation. We conclude that LPA-induced Ras activation is mediated by a p110beta/Gab1/SHP2 pathway. Moreover, we present data indicating that p110beta is effectively the target of betagamma in this pathway, suggesting that the p110beta/Gab1/SHP2 pathway provides a novel link between betagamma and Ras by integrating two early events of LPA signaling, i.e. Gbetagamma release and tyrosine kinase receptor transactivation.

Critical role of lysophospholipids in the pathophysiology, diagnosis, and management of ovarian cancer

Lysophosphatidic acid (LPA), the simplest of all phospholipids, exhibits pleiomorphic functions in multiple cell lineages. The effects of LPA appear to be mediated by binding of LPA to specific members of the endothelial differentiation gene (Edg) family of G protein-coupled receptors (GPCR). Edg 2, Edg4, and Edg7 are high affinity receptors for LPA, and Edg1 may be a low affinity receptor for LPA. PSP24 has been shown to be responsive to LPA in Xenopus oocytes, however, its role in mammalian cells is unclear. The specific biochemical events initiated by the different Edg receptors, as well as the biological outcomes of activation of the individual receptors, are only beginning to be determined. LPA levels are consistently elevated in the plasma and ascites of ovarian cancer patients, but not in most other epithelial tumors, with the exception of cervix and endometrium, suggesting that LPA may be of particular importance in the pathophysiology of ovarian cancer. In support of this concept, ovarian cancer cells constitutively and inducibly produce high levels of LPA and demonstrate markedly different responses to LPA than normal ovarian surface epithelium. Edg4 and Edg7 levels are consistently increased in malignant ovarian epithelial cells contributing to the aberrant response of ovarian cancer cells to LPA. Edg2 may represent a negative regulatory LPA receptor inducing apoptosis in ovarian cancer cells. Thus, increased levels of LPA, altered receptor expression and altered responses to LPA may contribute to the initiation, progression or outcome of ovarian cancer. Over 40% of known drugs target GPCR, making LPA receptors attractive targets for molecular therapeutics. Indeed, using the structure-function relationship of LPA in model systems, we have identified selective Edg2 anatgonists, as well as Edg4 and Edg7 agonists. These lead compounds are being assessed in preclinical model systems. Understanding the mechanisms regulating LPA production, metabolism and function could lead to improved methods for early detection and to new targets for therapy in ovarian cancer.

Apoptotic effect of sphingosine 1-phosphate and increased sphingosine 1-phosphate hydrolysis on mesangial cells cultured at low cell density

The lipid mediator sphingosine 1-phosphate (S1P) may alter the proliferation of mesangial cells during pathophysiological processes. Here, S1P stimulated proliferation of rat mesangial cells and phosphorylation of MAPKs at subconfluent cell density. Both effects were inhibited by pertussis toxin treatment. Mesangial cells expressed several S1P receptors of the endothelial differentiation gene family: EDG-1, -3, -5, and -8. Conversely, S1P induced apoptosis at low cell density (2 x 10(4) cells/cm(2)), which was demonstrated by flow cytometry and Hoechst staining. Apoptosis was observed also in quiescent or growing cells and was not reverted by lysophosphatidic acid or platelet-derived growth factor. S1P enhanced phosphorylation of SAPKs. Incubation with [(33)P]S1P, [(3)H]S1P, and [(3)H]sphingosine demonstrated increased S1P hydrolysis, resulting in enhanced intracellular sphingosine levels and decreased S1P levels. A rise in total ceramide levels was also observed; however, ceramide did not originate from [(3)H]sphingosine, and S1P-induced apoptosis was not inhibited by fumonisin B, precluding involvement of de novo ceramide synthesis in apoptosis. Therefore, we suggest that sphingosine accumulation and decreased S1P are primarily responsible for S1P-induced apoptosis. In conclusion, incubation of low-density mesangial cells with S1P results in apoptosis, presumably due to increased S1P hydrolysis.

The gift of Gab

Gab proteins, including mammalian Gab1, Gab2, Gab3, Drosophila DOS and Caenorhabditis elegans Soc1, comprise a growing family of scaffolding/docking molecules involved in multiple signaling pathways mediated by receptor tyrosine kinases (RTKs) and non-RTK receptors. This paper reviews the structure/function relationships of Gab proteins and their biological roles during normal growth, differentiation and development programs.