Lycopene Triggers Nrf2-AMPK Cross Talk to Alleviate Atrazine-Induced Nephrotoxicity in Mice

Atrazine (ATR), an environmental persistent and bioaccumulative herbicide, has been associated with environmental nephrosis. Lycopene (LYC) exhibits important properties of nephroprotection, but there are limited data on the specific underlying mechanism. The primary objective of this study was to explore the therapeutic effect of LYC on ATR-induced nephrotoxicity in mice. The mice were divided randomly into 6 groups and treated as follows: control group (C), 5 mg/kg LYC group (L), 50 mg/kg ATR group (A1), 200 mg/kg ATR group (A2), 50 mg/kg ATR plus 5 mg/kg LYC group (A1+L), and 200 mg/kg ATR plus 5 mg/kg LYC group (A2+L) by oral gavage administration for 21 days. We found that pretreatment with LYC significantly suppressed the ATR-induced renal tubular epithelial cell swelling. Furthermore, LYC mitigated ATR-induced dysregulation of oxidative stress markers by reducing MDA, H₂O₂ levels, and increasing SOD, GPx, CAT concentration, and Nrf2 activation. Moreover, LYC activated the autophagic flux by a detectable change in autophagy-related genes (Beclin-1 and ATGs) and proteins (p62/SQSTM) and by the formation of autophagic vacuole (AV) and LC3 aggregation, in parallel with AMPK activation (pAMPK/AMPK). Herein, ATR-up-regulated nuclear factor erythroid 2-related factor 2 (Nrf2) expression and Nrf2-regulated redox genes, including quinoneoxidoreductase-1 (NQO1) and heme oxidase-1 (HO1), whereas LYC down-regulated those of the above genes. In addition, LYC suppressed ATR-induced activation of autophagy (increased LC3II/LC3I, ATGs, Beclin1, and p62, in parallel with increased AMPK activation). Collectively, our findings identified a cross talk between AMPK-activated autophagy and the Nrf2 signaling pathway in LYC-mediated nephroprotection against ATR-induced toxicity in mice kidney.

Slit-Dependent Endocytic Trafficking of the Robo Receptor Is Required for Son of Sevenless Recruitment and Midline Axon Repulsion

Understanding how axon guidance receptors are activated by their extracellular ligands to regulate growth cone motility is critical to learning how proper wiring is established during development. Roundabout (Robo) is one such guidance receptor that mediates repulsion from its ligand Slit in both invertebrates and vertebrates. Here we show that endocytic trafficking of the Robo receptor in response to Slit-binding is necessary for its repulsive signaling output. Dose-dependent genetic interactions and in vitro Robo activation assays support a role for Clathrin-dependent endocytosis, and entry into both the early and late endosomes as positive regulators of Slit-Robo signaling. We identify two conserved motifs in Robo's cytoplasmic domain that are required for its Clathrin-dependent endocytosis and activation in vitro; gain of function and genetic rescue experiments provide strong evidence that these trafficking events are required for Robo repulsive guidance activity in vivo. Our data support a model in which Robo's ligand-dependent internalization from the cell surface to the late endosome is essential for receptor activation and proper repulsive guidance at the midline by allowing recruitment of the downstream effector Son of Sevenless in a spatially constrained endocytic trafficking compartment.

The exquisite regulation of PLD2 by a wealth of interacting proteins: S6K, Grb2, Sos, WASp and Rac2 (and a surprise discovery: PLD2 is a GEF)

Phospholipase D (PLD) catalyzes the conversion of the membrane phospholipid phosphatidylcholine to choline and phosphatidic acid (PA). PLD's mission in the cell is two-fold: phospholipid turnover with maintenance of the structural integrity of cellular/intracellular membranes and cell signaling through PA and its metabolites. Precisely, through its product of the reaction, PA, PLD has been implicated in a variety of physiological cellular functions, such as intracellular protein trafficking, cytoskeletal dynamics, chemotaxis of leukocytes and cell proliferation. The catalytic (HKD) and regulatory (PH and PX) domains were studied in detail in the PLD1 isoform, but PLD2 was traditionally studied in lesser detail and much less was known about its regulation. Our laboratory has been focusing on the study of PLD2 regulation in mammalian cells. Over the past few years, we have reported, in regards to the catalytic action of PLD, that PA is a chemoattractant agent that binds to and signals inside the cell through the ribosomal S6 kinases (S6K). Regarding the regulatory domains of PLD2, we have reported the discovery of the PLD2 interaction with Grb2 via Y169 in the PX domain, and further association to Sos, which results in an increase of de novo DNA synthesis and an interaction (also with Grb2) via the adjacent residue Y179, leading to the regulation of cell ruffling, chemotaxis and phagocytosis of leukocytes. We also present the complex regulation by tyrosine phosphorylation by epidermal growth factor receptor (EGF-R), Janus Kinase 3 (JAK3) and Src and the role of phosphatases. Recently, there is evidence supporting a new level of regulation of PLD2 at the PH domain, by the discovery of CRIB domains and a Rac2-PLD2 interaction that leads to a dual (positive and negative) effect on its enzymatic activity. Lastly, we review the surprising finding of PLD2 acting as a GEF. A phospholipase such as PLD that exists already in the cell membrane that acts directly on Rac allows a quick response of the cell without intermediary signaling molecules. This provides only the latest level of PLD2 regulation in a field that promises newer and exciting advances in the next few years.

Abelson tyrosine kinase and Calmodulin interact synergistically to transduce midline guidance cues in the Drosophila embryonic CNS

Calmodulin and Abelson tyrosine kinase are key signaling molecules transducing guidance cues at the Drosophila embryonic midline. A reduction in the signaling strength of either pathway alone induces ectopic midline crossing errors in a few segments. When Calmodulin and Abelson signaling levels are simultaneously reduced, the frequency of ectopic crossovers is synergistically enhanced as all segments exhibit crossing errors. But as the level of signaling is further reduced, commissures begin to fuse and large gaps form in the longitudinal connectives. Quantitative analysis suggests that the level of Abelson activity is particularly important. Like Calmodulin, Abelson interacts with son-of-sevenless to increase ectopic crossovers suggesting all three contribute to midline repulsive signaling. Axons cross the midline in almost every segment if Frazzled is co-overexpressed with the Calmodulin inhibitor, but the crossovers induced by the Calmodulin inhibitor itself do not require endogenous Frazzled. Thus, Calmodulin and Abelson tyrosine kinase are key signaling molecules working synergistically to transduce both midline attractive and repulsive cues. While they may function downstream of specific receptors, the emergence of commissural and longitudinal connective defects point to a novel convergence of Calmodulin and Abelson signaling during the regulation of actin and myosin dynamics underlying a guidance decision.

TGF-beta activates Erk MAP kinase signalling through direct phosphorylation of ShcA

Erk1/Erk2 MAP kinases are key regulators of cell behaviour and their activation is generally associated with tyrosine kinase signalling. However, TGF-beta stimulation also activates Erk MAP kinases through an undefined mechanism, albeit to a much lower level than receptor tyrosine kinase stimulation. We report that upon TGF-beta stimulation, the activated TGF-beta type I receptor (TbetaRI) recruits and directly phosphorylates ShcA proteins on tyrosine and serine. This dual phosphorylation results from an intrinsic TbetaRI tyrosine kinase activity that complements its well-defined serine-threonine kinase function. TGF-beta-induced ShcA phosphorylation induces ShcA association with Grb2 and Sos, thereby initiating the well-characterised pathway linking receptor tyrosine kinases with Erk MAP kinases. We also found that TbetaRI is tyrosine phosphorylated in response to TGF-beta. Thus, TbetaRI, like the TGF-beta type II receptor, is a dual-specificity kinase. Recruitment of tyrosine kinase signalling pathways may account for aspects of TGF-beta biology that are independent of Smad signalling.

Prolidase-independent mechanism of camptothecin-induced inhibition of collagen biosynthesis in cultured human skin fibroblasts

The present study was undertaken to evaluate the mechanism of campthotecin (CPT)-induced deregulation of collagen metabolism in cultured human skin fibroblast. It has been found that CPT strongly induced inhibition of collagen biosynthesis. The mechanism of this phenomenon was found to be independent of prolidase activity, an enzyme that plays an important role in enhancement of collagen biosynthesis at post-translational level. In fact, the enzyme activity was found to be stimulated by CPT. Increase in the enzyme activity was accompanied by increase in the expression of beta(1) integrin receptor and some beta(1) integrin-dependent signalling proteins, Sos, MAPK (ERK(1), ERK(2)) and transcription factor NF-kappaB. Since activation of beta(1) integrin induces NF-kappaB that inhibits collagen gene transcription, therefore the mechanism of CPT-dependent inhibition of collagen biosynthesis may be related to beta(1) integrin-dependent stimulation of NF-kappaB. Supporting evidence comes from experiments showing that specific MEK/ERK inhibitor (UO126) inhibited CPT-induced up-regulation of prolidase activity while it had no effect on CPT-induced inhibition of collagen biosynthesis and activation of NF-kappaB. The data suggest that CPT induces inhibition of collagen biosynthesis in cultured human skin fibroblasts by stimulation of NF-kappaB signalling.

Son of sevenless directly links the Robo receptor to rac activation to control axon repulsion at the midline

Son of sevenless (Sos) is a dual specificity guanine nucleotide exchange factor (GEF) that regulates both Ras and Rho family GTPases and thus is uniquely poised to integrate signals that affect both gene expression and cytoskeletal reorganization. Here, using genetics, biochemistry, and cell biology, we demonstrate that Sos is recruited to the plasma membrane, where it forms a ternary complex with the Roundabout receptor and the SH3-SH2 adaptor protein Dreadlocks (Dock) to regulate Rac-dependent cytoskeletal rearrangement in response to the Slit ligand. Intriguingly, the Ras and Rac-GEF activities of Sos can be uncoupled during Robo-mediated axon repulsion; Sos axon guidance function depends on its Rac-GEF activity, but not its Ras-GEF activity. These results provide in vivo evidence that the Ras and RhoGEF domains of Sos are separable signaling modules and support a model in which Robo recruits Sos to the membrane via Dock to activate Rac during midline repulsion.

A role for focal adhesion kinase signaling in tumor necrosis factor-alpha-dependent matrix metalloproteinase-9 production in a cholangiocarcinoma cell line, CCKS1

We have previously reported that tumor necrosis factor-alpha (TNF-alpha) stimulation of CCKS1, a cell line established from cholangiocarcinoma with i.p. dissemination, dramatically increased matrix metalloproteinase-9 (MMP-9) production and tumor invasion. We investigated the role of focal adhesion kinase (FAK) in TNF-alpha-dependent production of MMP-9 in CCKS1 and FAK-null mouse fibroblast cells. TNF-alpha stimulation of CCKS1 or wild-type fibroblasts substantially activated FAK phosphorylation and increased MMP-9 production. In contrast, FAK-null fibroblasts could not respond well to TNF-alpha stimulation. Conditional expression of wild-type FAK in FAK-null cells restored the TNF-alpha-dependent production of MMP-9. TNF-alpha treatment activated the kinase activity of FAK and its phosphorylation especially at Y397 and Y925. Phosphorylated FAK accumulated at focal adhesions and formed a complex with growth factor receptor binding protein 2 and SOS. In contrast, Y397F FAK and Y925F FAK, whose Y397 and Y925 were replaced with phenylalanine, respectively, as well as KD FAK, whose kinase was inactivated, could not restore the MMP-9 production. In addition, small interfering RNA against FAK drastically suppressed the TNF-alpha-dependent production of MMP-9 and inhibited the TNF-alpha-dependent invasion of CCKS1. Taken together, our results suggest the pivotal role of FAK in TNF-alpha-dependent production of MMP-9 and subsequent activation of tumor invasion.

The elucidation of novel SH2 binding sites on PLD2

Our laboratory has recently reported that the enzyme phospholipase D2 (PLD2) exists as a ternary complex with PTP1b and the growth factor receptor bound protein 2 (Grb2). Here, we establish the mechanistic underpinnings of the PLD2/Grb2 association. We have identified residues Y(169) and Y(179) in the PLD2 protein as being essential for the Grb2 interaction. We present evidence indicating that Y(169) and Y(179) are located within two consensus sites in PLD2 that mediate an SH2 interaction with Grb2. This was demonstrated with an SH2-deficient GSTGrb2 R86K mutant that failed to pull-down PLD2 in vitro. In order to elucidate the functions of the two neighboring tyrosines, we created a new class of deletion and point mutants in PLD2. Phenylalanine replacement of Y(169) (PLD2 Y169F) or Y(179) (PLD2 Y179F) reduced Grb2 binding while simultaneous mutation completely abolished it. The role of the two binding sites on PLD2 was found to be functionally nonequivalent: Y(169) serves to modulate the activity of the enzyme, whereas Y(179) regulates total tyrosine phosphorylation of the protein. Interestingly, binding of Grb2 to PLD2 occurs irrespectively of lipase activity, since Grb2 binds to catalytically inactive PLD2 mutants. Finally, PLD2 residues Y(169) and Y(179) are necessary for the recruitment of Sos, but only overexpression of the PLD2 Y179F mutant resulted in increased Ras activity, p44/42(Erk) phosphorylation and enhanced DNA synthesis. Since Y(169) remains able to modulate enzyme activity and is capable of binding to Grb2 in the PLD2 Y179F mutant, we propose that Y(169) is kept under negative regulation by Y(179). When this is released, Y(169) mediates cellular proliferation through the Ras/MAPK pathway.

M-CSF stimulated differentiation requires persistent MEK activity and MAPK phosphorylation independent of Grb2-Sos association and phosphatidylinositol 3-kinase activity

Macrophage colony-stimulating factor (M-CSF) is a physiological regulator of monocyte-macrophage lineage. Ectopic expression of the M-CSF receptor (M-CSFR, or Fms) in murine myeloid cell line FDC-P1 (FD/Fms cells) results in M-CSF-dependent macrophage differentiation. Previously, we observed that M-CSF induces two temporally distinct phases of mitogen-activated protein kinase (MAPK) phosphorylation. Here we show that levels of phosphorylated MAPK kinase MEK1 follow the same kinetics as MAPK phosphorylation, characterized by an early and transient phase (the first 30 min of M-CSF stimulation) and a late and persistent phase from 4 h of stimulation. The MEK inhibitor U0126 strongly inhibited both phases of MAPK phosphorylation as well as FD/Fms cell differentiation, indicating that MAPK may relay M-CSF differentiation signaling downstream of M-CSFR. Treatment of FD/Fms cells with U0126 during the first hour of M-CSF stimulation reversibly blocked the early phase of MAPK phosphorylation but did not affect differentiation. In contrast, U0126 still inhibited FD/Fms cell differentiation when its addition was delayed by 24 h. This demonstrated that late and persistent MEK activity is specifically required for macrophage differentiation to occur. Furthermore, disrupting Grb2-Sos complexes with a specific blocking peptide did not prevent FD/Fms cells differentiation in response to M-CSF, nor did it abolish MAPK phosphorylation. The role of phosphatidylinositol 3-kinase (PI 3-kinase), another potential regulator of the MAPK pathway, was examined using the specific inhibitor LY294002. This compound could not impede FD/Fms cell commitment to macrophage differentiation and did not significantly affect MAPK phosphorylation in response to M-CSF. Therefore, M-CSF differentiation signaling in myeloid progenitor cells is mediated through persistent MEK activity but it is not strictly dependent upon Grb2-Sos interaction or PI 3-kinase activity.

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.

Self-organization of polarized cell signaling via autocrine circuits: computational model analysis

Recent studies have suggested that autocrine signaling through epidermal growth factor receptor (EGFR) might be involved in generating or maintaining an intrinsic polarity in tissue cells, possibly via spatial localization of EGFR-mediated signaling. The difficulty of experimental investigation of autocrine signaling makes especially valuable an application of computational modeling for critical hypotheses about the dynamic operation of the underlying signaling circuits, both intracellular and extracellular. Toward this end, we develop and analyze here a spatially distributed dynamic computational model of autocrine EGFR signaling. Under certain conditions, the model spontaneously evolves into a state wherein sustained signaling is spatially localized on smaller than cell dimension, conferring a polarity to the otherwise nonpolar model cell. Conditions of a sufficiently large rate of autocrine EGFR ligand release and of a sufficiently small exogenous ligand concentration are qualitatively consistent with experimental observations of EGFR-mediated migration. Thus, computational analysis supports the concept that autocrine EGFR signaling circuits could play a role in helping generate and/or maintain an intrinsic cell spatial polarity, possibly related to migration as well as tissue organization. We additionally offer particular suggestions for critical nodes in the EGFR signaling circuits governing this self-organization capability.

Adaptive hypersensitivity to estrogen: mechanism for sequential responses to hormonal therapy in breast cancer

Clinical observations demonstrate that women with breast cancer often respond to subsequent endocrine manipulation after resistance to initial hormonal therapy develops. As a mechanistic explanation for these findings, we hypothesized that human breast tumors can adapt in response to the pressure exerted by endocrine therapy with development of hypersensitivity to estradiol. To understand the signaling pathways responsible, we examined estrogenic stimulation of cell proliferation in a model system and provided in vitro and in vivo evidence that long-term deprivation of estradiol (LTED) causes adaptive hypersensitivity. Even though the estrogen receptor alpha (ERalpha) is markedly up-regulated in LTED cells, the enhanced responses to estradiol do not appear to involve mechanisms acting at the level of transcription of estrogen-regulated genes. We found that ERalpha co-opts a classical growth factor pathway and induces rapid nongenomic effects that are enhanced in LTED cells. Estradiol binds to cell membrane-associated ERs, physically associates with the adaptor protein Shc, and induces its phosphorylation. In turn, Shc binds Grb2 and Sos, which result in the rapid activation of mitogen-activated protein kinase. These nongenomic effects of estradiol produced biological effects, as evidenced by Elk-1 activation and by morphological changes in cell membranes. The mechanistic pathways involved in adaptive hypersensitivity suggest that inhibitors of the mitogen-activated protein kinase and phosphatidylinositol-3-OH kinase pathways might prevent the development of adaptive hypersensitivity and allow more prolonged efficacy of endocrine therapies.

Dopamine activates ERKs in alveolar epithelial cells via Ras-PKC-dependent and Grb2/Sos-independent mechanisms

Recently it has been described that dopamine (DA), via dopaminergic type 2 receptors (D(2)R), activates the mitogen-activated protein kinase extracellular signal-regulated kinase (MAPK/ERK) proteins in alveolar epithelial cells (AEC), which results in the upregulation of Na(+)-K(+)-ATPase. In the present report, we used AEC to investigate the signaling pathway that links DA with ERK activation. Incubation of AEC with DA resulted in rapid and transient stimulation of ERK activity, which was mediated by Ras proteins and the serine/threonine kinase Raf-1. Pretreatment of AEC with Src homology 3 binding peptide, which blocks the interaction between Grb2 and Sos, did not prevent DA activation of ERK. Diacylglycerol (DAG)-dependent protein kinase C (PKC) isoenzymes, involved in the DA-mediated activation of ERK proteins as pretreatment with either bisindolylmaleimide or Ro-31-8220, prevented the phosphorylation of Elk-1, and quinpirole, a D(2)R activator, stimulates the translocation of PKCepsilon. Together, the data suggest that DA activated MAPK/ERK via Ras, Raf-1 kinase, and DAG-dependent PKC isoenzymes, but, importantly and contrary to the classical model, this pathway did not involve the Grb2-Sos complex formation.

Cyclic stretch activates ERK1/2 via G proteins and EGFR in alveolar epithelial cells

Mechanical stimuli are transduced into intracellular signals in lung alveolar epithelial cells (AEC). We studied whether mitogen-activated protein kinase (MAPK) pathways are activated during cyclic stretch of AEC. Cyclic stretch induced a rapid (within 5 min) increase in extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation in AEC. The inhibition of Na(+), L-type Ca(2+) and stretch-activated ion channels with amiloride, nifedipine, and gadolinium did not prevent the stretch-induced ERK1/2 activation. The inhibition of Grb2-SOS interaction with an SH3 binding sequence peptide, Ras with a farnesyl transferase inhibitor, and Raf-1 with forskolin did not affect the stretch-induced ERK1/2 phosphorylation. Moreover, cyclic stretch did not increase Ras activity, suggesting that stretch-induced ERK1/2 activation is independent of the classical receptor tyrosine kinase-MAPK pathway. Pertussis toxin and two specific epidermal growth factor receptor (EGFR) inhibitors (AG-1478 and PD-153035) prevented the stretch-induced ERK1/2 activation. Accordingly, in primary AEC, cyclic stretch activates ERK1/2 via G proteins and EGFR, in Na(+) and Ca(2+) influxes and Grb2-SOS-, Ras-, and Raf-1-independent pathways.

Molecular events in transmembrane signaling via E-selectin. SHP2 association, adaptor protein complex formation and ERK1/2 activation

E-selectin is a cytokine-inducible adhesion molecule that is expressed by activated endothelial cells at sites of inflammation. In addition to supporting rolling and stable arrest of leukocytes, there is increasing evidence that E-selectin functions in transmembrane signaling into endothelial cells during these adhesive interactions. We have previously shown that adhesion of HL-60 cells (which express ligands for E-selectin), or antibody-mediated cross-linking of E-selectin, results in formation of a Ras/Raf-1/phospho-MEK macrocomplex, extracellular signal-regulated protein kinase (ERK1/2) activation, and c-fos up-regulation. All of these downstream signaling events appear to require an intact cytoplasmic domain of E-selectin. Here we demonstrate that tyrosine 603 in the cytoplasmic domain of E-selectin is required for the E-selectin-dependent ERK1/2 activation. Tyrosine 603 plays an important role in mediating the association of E-selectin with SHP2, and the catalytic domain of SHP2 is, in turn, critical for E-selectin-dependent ERK1/2 activation. An adapter protein complex consisting of Shc.Grb2.Sos bridges between SHP2 and the Ras.Raf.phospho-MEK macrocomplex. These molecular events thus outline a mechanism by which cross-linking of E-selectin by engagement of ligands on adherent leukocytes can initiate a multifunctional signaling pathway in the activated endothelial cell at sites of inflammation.

Basis for signaling specificity difference between Sos and Ras-GRF guanine nucleotide exchange factors

Sos and Ras-GRF are two families of guanine nucleotide exchange factors that activate Ras proteins in cells. Sos proteins are ubiquitously expressed and are activated in response to cell-surface tyrosine kinase stimulation. In contrast, Ras-GRF proteins are expressed primarily in central nervous system neurons and are activated by calcium/calmodulin binding and by phosphorylation. Although both Sos1 and Ras-GRF1 activate the Ras proteins Ha-Ras, N-Ras, and Ki-Ras, only Ras-GRF1 also activates the functionally distinct R-Ras GTPase. In this study, we determined which amino acid sequences in these exchange factors and their target GTPases are responsible for this signaling specificity difference. Analysis of chimeras and individual amino acid exchanges between Sos1 and Ras-GRF1 revealed that the critical amino acids reside within an 11-amino acid segment of their catalytic domains between the second and third structurally conserved regions (amino acids (aa) 828-838 in Sos1 and 1057-1067 in Ras-GRF1) of Ras guanine nucleotide exchange factors. In Sos1, this segment is in helix B, which is known to interact with the switch 2 region of Ha-Ras. Interestingly, a similar analysis of Ha-Ras and R-Ras chimeras did not identify the switch 2 region of Ha-Ras as encoding specificity. Instead, we found a more distal protein segment, helix 3 (aa 91-103 in Ha-Ras and 117-129 in R-Ras), which interacts instead primarily with helix K (aa 1002-1016) of Sos1. These findings suggest that specificity derives from the fact that R-Ras-specific amino acids in the region analogous to Ha-Ras helix 3 prevent a functional interaction with Sos1 indirectly, possibly by preventing an appropriate association of its switch 2 region with helix B of Sos1. Although previous studies have shown that helix B of Sos1 and helix 3 of Ha-Ras are involved in promoting nucleotide exchange on Ras proteins, this study highlights the importance of these regions in establishing signaling specificity.

Tyrosine phosphorylation of Grb2 by Bcr/Abl and epidermal growth factor receptor: a novel regulatory mechanism for tyrosine kinase signaling

Growth factor receptor-binding protein-2 (Grb2) plays a key role in signal transduction initiated by Bcr/Abl oncoproteins and growth factors, functioning as an adaptor protein through its Src homology 2 and 3 (SH2 and SH3) domains. We found that Grb2 was tyrosine-phosphorylated in cells expressing BCR/ABL and in A431 cells stimulated with epidermal growth factor (EGF). Phosphorylation of Grb2 by Bcr/Abl or EGF receptor reduced its SH3-dependent binding to Sos in vivo, but not its SH2-dependent binding to Bcr/Abl. Tyr209 within the C-terminal SH3 domain of Grb2 was identified as one of the tyrosine phosphorylation sites, and phosphorylation of Tyr209 abolished the binding of the SH3 domain to a proline-rich Sos peptide in vitro. In vivo expression of a Grb2 mutant where Tyr209 was changed to phenylalanine enhanced BCR/ABL-induced ERK activation and fibroblast transformation, and potentiated and prolonged Grb2-mediated activation of Ras, mitogen-activated protein kinase and c-Jun N-terminal kinase in response to EGF stimulation. These results suggest that tyrosine phosphorylation of Grb2 is a novel mechanism of down-regulation of tyrosine kinase signaling.

The inhibitory role of DOC-2/DAB2 in growth factor receptor-mediated signal cascade. DOC-2/DAB2-mediated inhibition of ERK phosphorylation via binding to Grb2

DOC-2/DAB2 (differentially expressed in ovarian carcinoma-2/disabled 2) appears to be a potential tumor suppressor gene with a growth inhibitory effect on several cancer types. Previously, we have shown that DOC-2/DAB2 suppresses protein kinase C-induced AP-1 activation, which is modulated by serine 24 phosphorylation in the N terminus of DOC-2/DAB2. However, the functional impact of the C terminus of DOC-2/DAB2, containing three proline-rich domains, has not been explored. In this study, we examined this functional role in modulating signaling mediated by peptide growth factor receptor tyrosine kinase, particularly because it involves the interaction with Grb2. Using sequence-specific peptides, we found that the second proline-rich domain of DOC-2/DAB2 is the key binding site to Grb2 in the presence of growth factors. Such elevated binding interrupts the binding between SOS and Grb2, which consequently suppresses downstream ERK phosphorylation. Reduced ERK phosphorylation was restored when the binding between DOC-2/DAB2 and Grb2 was interrupted by a specific peptide or by increasing the expression of Grb2. Furthermore, the C terminus of the DOC-2/DAB2 construct can inhibit the AP-1 activity elicited by growth factors. We conclude that DOC-2/DAB2, a potent negative regulator, can suppress ERK activation by interrupting the binding between Grb2 and SOS that is elicited by peptide growth factors. This study further illustrates that DOC-2/DAB2 has multiple effects on the RAS-mediated signal cascades active in cancer cells.

A critical role for phosphoinositide 3-kinase upstream of Gab1 and SHP2 in the activation of ras and mitogen-activated protein kinases by epidermal growth factor

Although the mechanisms involved in the activation of mitogen-activated protein kinases (MAPK) by receptor tyrosine kinases do not display an obvious role for phosphoinositide 3-kinases (PI3Ks), we have observed in the nontransformed cell line Vero stimulated with epidermal growth factor (EGF) that wortmannin and LY294002 nearly abolished MAPK activation. The effect was observed under strong stimulation and was independent of EGF concentration. In addition, three mutants of class Ia PI3Ks were found to inhibit MAPK activation to an extent similar to their effect on Akt/protein kinase B activation. To determine the importance of PI3K lipid kinase activity in MAPK activation, we have used the phosphatase PTEN and the pleckstrin homology domain of Tec kinase. Overexpression of these proteins, but not control mutants, was found to inhibit MAPK activation, suggesting that the lipid products of class Ia PI3K are necessary for MAPK signaling. We next investigated the location of PI3K in the MAPK cascade. Pharmacological inhibitors and dominant negative forms of PI3K were found to block the activation of Ras induced by EGF. Upstream from Ras, although association of Grb2 with its conventional effectors was independent of PI3K, we have observed that the recruitment of the tyrosine phosphatase SHP2 required PI3K. Because SHP2 was also essential for Ras activation, this suggested the existence of a PI3K/SHP2 pathway leading to the activation of Ras. In addition, we have observed that the docking protein Gab1, which is involved in PI3K activation during EGF stimulation, is also implicated in this pathway downstream of PI3K. Indeed, the association of Gab1 with SHP2 was blocked by PI3K inhibitors, and expression of Gab1 mutant deficient for binding to SHP2 was found to inhibit Ras stimulation without interfering with PI3K activation. These results show that, in addition to Shc and Grb2, a PI3K-dependent pathway involving Gab1 and SHP2 is essential for Ras activation under EGF stimulation.

Calmodulin and son of sevenless dependent signaling pathways regulate midline crossing of axons in the Drosophila CNS

The establishment of axon trajectories is ultimately determined by the integration of intracellular signaling pathways. Here, a genetic approach in Drosophila has demonstrated that both Calmodulin and Son of sevenless signaling pathways are used to regulate which axons cross the midline. A loss in either signaling pathway leads to abnormal projection of axons across the midline and these increase with roundabout or slit mutations. When both Calmodulin and Son of sevenless are disrupted, the midline crossing of axons mimics that seen in roundabout mutants, although Roundabout remains expressed on crossing axons. Calmodulin and Son of sevenless also regulate axon crossing in a commissureless mutant. These data suggest that Calmodulin and Son of sevenless signaling pathways function to interpret midline repulsive cues which prevent axons crossing the midline.

Systematic analysis of the role of CD19 cytoplasmic tyrosines in enhancement of activation in Daudi human B cells: clustering of phospholipase C and Vav and of Grb2 and Sos with different CD19 tyrosines

CD19 is a coreceptor on B cells that enhances the increase in cytoplasmic calcium and ERK2 activation when coligated with the B cell Ag receptor. Constructs containing point mutations and truncations were expressed in Daudi human B lymphoblastoid cells to systematically determine the requirement for individual CD19 cytoplasmic tyrosines in these responses. Evidence for activity was found for Y330, Y360, and Y421 as well as that previously published for Y391. Precipitates formed with phosphopeptides consisting of CD19 sequences flanking these residues were used to screen for cytoplasmic proteins that mediate signaling. Phosphopeptide Y330 precipitated Grb2 and Sos, whereas phosphopeptides Y391 and Y421 both precipitated Vav and phospholipase C-gamma2. These molecules also were found associated with native CD19. In mapping studies with altered constructs, CD19 Y330 and/or Y360 were necessary for binding Grb2 and Sos. Vav associated with CD19 constitutively in unstimulated cells by a tyrosine-independent mechanism requiring the portion of CD19 encoded by exons 9-12. After B cell Ag receptor stimulation, Vav association was tyrosine-dependent, but binding was influenced by multiple residues. However, when maximally phosphorylated by pervanadate, Y391 and, to a lesser extent, Y421 were sufficient. CD19 Y391 was also both necessary and sufficient for binding phospholipase C-gamma2. Thus, different tyrosines along the CD19 cytoplasmic domain provide scaffolding for the formation of complexes of different signaling molecules.

Ras signaling in the inner medullary cell response to urea and NaCl

The small guanine nucleotide-binding protein Ras, activated by peptide mitogens and other stimuli, regulates downstream signaling events to influence transcription. The role of Ras in solute signaling to gene regulation was investigated in the murine inner medullary collecting duct (mIMCD3) cell line. Urea treatment (100-200 mM), but not sham treatment, increased Ras activation 124% at 2 min; the effect of NaCl did not achieve statistical significance. To determine the contribution of Ras activation to urea-inducible signal transduction, mIMCD3 cells were stably transfected with an expression plasmid encoding a dominant negative-acting N17Ras mutant driven by a dexamethasone-inducible (murine mammary tumor virus) promoter. After 24 h of induction, selected cell lines exhibited sufficient N17Ras overexpression to abolish epidermal growth factor- and hypotonicity-mediated signaling to extracellular signal-regulated kinase (ERK) phosphorylation, as determined by immunoblotting. Conditional N17Ras overexpression inhibited urea- and NaCl-inducible ERK phosphorylation by 40-50%, but only at 15 min, and not 5 min, of treatment. N17Ras induction, however, almost completely inhibited urea-inducible Egr-1 transcription, as quantitated by luciferase reporter gene assay, but failed to influence tonicity-inducible (TonE-mediated) transcription. N17Ras overexpression also blocked urea-inducible expression of the transcription factor Gadd153 but did not influence osmotic or urea-inducible apoptosis. In addition, urea treatment induced recruitment of the Ras activator Sos to the plasma membrane. Taken together, these observations suggest a role for Ras signaling in the IMCD cell response to urea stress.

Stimulation of pancreatic beta-cell proliferation by growth hormone is glucose-dependent: signal transduction via janus kinase 2 (JAK2)/signal transducer and activator of transcription 5 (STAT5) with no crosstalk to insulin receptor substrate-mediated mitogenic signalling

Mitogenic signal-transduction pathways have not been well defined in pancreatic beta-cells. In the glucose-sensitive rat beta-cell line, INS-1, glucose (6-18 mM) increased INS-1 cell proliferation (>20-fold at 15 mM glucose). Rat growth hormone (rGH) also induced INS-1 cell proliferation, but this was glucose-dependent in the physiologically relevant concentration range (6-18 mM glucose). The combination of rGH (10 nM) and glucose (15 mM) was synergistic, maximally increasing INS-1 cell proliferation by >50-fold. Moreover, glucose-dependent rGH-induced INS-1 cell proliferation was increased further by addition of insulin-like growth factor 1 (IGF-1; 10 nM) to >90-fold at 12 mM glucose. Glucose metabolism and phosphatidylinositol-3'-kinase (PI3'K) activation were necessary for both glucose- and rGH-stimulated INS-1 cell proliferation. Glucose (>3 mM) independently increased tyrosine-phosphorylation-mediated recruitment of growth-factor-bound protein 2 (Grb2)/murine sons of sevenless-1 protein (mSOS) and PI3'K to insulin receptor substrate (IRS)-1 and IRS-2, as well as SH2-containing protein (Shc) association with Grb2/mSOS and downstream activation of mitogen-activated protein kinase and 70 kDa S6 kinase. Glucose-induced IRS- and Shc-mediated signal transduction was enhanced further by the addition of IGF-1, but not rGH. In contrast, rGH was able to activate Janus kinase 2 (JAK2)/signal transducer and activator of transcription 5 (STAT5) signal transduction at glucose concentrations above 3 mM, but neither glucose independently, nor glucose with added IGF-1, were able to activate the JAK2/STAT5 signalling pathway. Thus rGH-mediated proliferation of beta-cells is directly via the JAK2/STAT5 pathway without engaging the Shc or IRS signal-transduction pathways, although activation of PI3'K may play an important permissive role in the glucose-dependent aspect of rGH-induced beta-cell mitogensis. The additive effect of rGH and IGF-1 on glucose-dependent beta-cell proliferation is therefore reflective of rGH and IGF-1 activating distinctly different mitogenic signalling pathways in beta-cells with minimal crosstalk between them.

Role of mitogen-activated protein kinase/extracellular signal-regulated kinase cascade in gonadotropin-releasing hormone-induced growth inhibition of a human ovarian cancer cell line

Although gonadotropin-releasing hormone agonists (GnRHa) have been used in the therapy of the endocrine-dependent cancers, their biological mechanism remained obscure. We have studied the roles of mitogen-activated protein kinase family in the antiproliferative effect of GnRHa on the Caov-3 human ovarian cancer cell line. Reverse transcription-PCR assays confirmed mRNA for GnRH receptor in Caov-3 cells. In the presence of 1 microM GnRHa, the proliferation of cells was significantly reduced to 76% of controls after 24 h, and the effect was sustained up to 4 days. Although GnRHa had no effect on the activation of the Jun N-terminal kinase (JNK), treatment of Caov-3 cells with GnRHa activated extracellular signal-regulated protein kinase (ERK), and its effect was more than that induced by GnRH. Activation of ERK by GnRHa occurred within 5 min, with the maximum occurring at 3 h and sustained until 24 h. GnRHa also activated ERK kinase (mitogen-activated protein/ERK kinase) and resulted in an increase in phosphorylation of son of sevenless (Sos), and Shc. Furthermore, we examined the mechanism by which GnRHa induced ERK activation. Both pertussis toxin (10 ng/ml), which inactivates Gi/Go proteins, and expression of a peptide derived from the carboxyl terminus of the beta-adrenergic receptor kinase I, which specifically blocks signaling mediated by the betagamma subunits of G proteins, blocked the GnRHa-induced ERK activation. Phorbol 12-myristate 13-acetate (PMA) also induced the ERK activity, but pretreatment of the cultured cells with PMA to down-regulate protein kinase C did not abolish the activation of ERK by GnRHa. Elimination of extracellular Ca2+ by EGTA also did not abolish the activation of ERK by GnRHa. To examine the role of ERK cascade in the antiproliferative effect of GnRHa, PD98059, an inhibitor of mitogen-activated protein/ERK kinase, was used. This inhibitor canceled the antiproliferative effect of GnRHa and apparently reversed the GnRH-induced dephosphorylation of the retinoblastoma protein, the hyperphosphorylation of which is a hallmark of G1-S transition in the cell cycle. These results provide evidence that GnRHa stimulation of ERK activity may be mediated by Gbetagamma protein, not by PMA-sensitive protein kinase C nor extracellular Ca2+ in the Caov-3 human ovarian cancer cell line, suggesting that this cascade may play an important role in the antiproliferative effect of GnRHa.

Presence of up-stream and downstream components of a mitogen-activated protein kinase pathway in the PSD of the rat forebrain

We previously reported the presence of Erk2 type mitogen-activated protein kinase (MAPK) and enrichment of its substrates in the post-synaptic density (PSD) fraction, and suggested a role for MAPK in the synaptic transmission and its modulation [Suzuki, T., Okumura-Noji, K., Nishida, E., ERK2-type mitogen-activated protein kinase (MAPK) and its substrates in post-synaptic density fractions from the rat brain, Neurosci. Res., 22 (1995) 277-285.]. In this paper, synaptic localization of the upstream and downstream components of a MAPK cascade was examined. We found that RSK1, Sos1, N-Shc 66 kDa, N-Shc 52 kDa, and Grb2 were present in the PSD fraction, and cPLA(2) was present in the synaptic plasma membrane fraction. RSK2, Sos2, and N-Shc 46 kDa were not present in the PSD fraction. Post-synaptic localization of RSK1 and Sos1 was confirmed by immunohistochemical examination at the electron microscopic level: the two immunoreactivities were localized in the PSDs, both in the spines and dendrites. These results suggest that all the MAPK cascade components examined were associated with PSD or the synaptic plasma membrane, suggesting the role(s) of the MAPK cascade for synaptic transmission and its regulation at post-synaptic sites.