The epidermal growth factor receptor is coupled to a pertussis toxin-sensitive guanine nucleotide regulatory protein in rat hepatocytes

Growth factor-dependent phosphorylation of Gαi shapes canonical signaling by G protein-coupled receptors

A long-standing question in the field of signal transduction is how distinct signaling pathways interact with each other to control cell behavior. Growth factor receptors and G protein-coupled receptors (GPCRs) are the two major signaling hubs in eukaryotes. Given that the mechanisms by which they signal independently have been extensively characterized, we investigated how they may cross-talk with each other. Using linear ion trap mass spectrometry and cell-based biophysical, biochemical, and phenotypic assays, we found at least three distinct ways in which epidermal growth factor affected canonical G protein signaling by the Gi-coupled GPCR CXCR4 through the phosphorylation of Gαi. Phosphomimicking mutations in two residues in the αE helix of Gαi (tyrosine-154/tyrosine-155) suppressed agonist-induced Gαi activation while promoting constitutive Gβγ signaling. Phosphomimicking mutations in the P loop (serine-44, serine-47, and threonine-48) suppressed Gi activation entirely, thus completely segregating growth factor and GPCR pathways. As expected, most of the phosphorylation events appeared to affect intrinsic properties of Gαi proteins, including conformational stability, nucleotide binding, and the ability to associate with and to release Gβγ. However, one phosphomimicking mutation, targeting the carboxyl-terminal residue tyrosine-320, promoted mislocalization of Gαi from the plasma membrane, a previously uncharacterized mechanism of suppressing GPCR signaling through G protein subcellular compartmentalization. Together, these findings elucidate not only how growth factor and chemokine signals cross-talk through the phosphorylation-dependent modulation of Gαi but also how such cross-talk may generate signal diversity.

Heterotrimeric G protein signaling without GPCRs: The Gα-binding-and-activating (GBA) motif

Heterotrimeric G proteins (Gαβγ) are molecular switches that relay signals from 7-transmembrane receptors located at the cell surface to the cytoplasm. The function of these receptors is so intimately linked to heterotrimeric G proteins that they are named G protein-coupled receptors (GPCRs), showcasing the interdependent nature of this archetypical receptor-transducer axis of transmembrane signaling in eukaryotes. It is generally assumed that activation of heterotrimeric G protein signaling occurs exclusively by the action of GPCRs, but this idea has been challenged by the discovery of alternative mechanisms by which G proteins can propagate signals in the cell. This review will focus on a general principle of G protein signaling that operates without the direct involvement of GPCRs. The mechanism of G protein signaling reviewed here is mediated by a class of G protein regulators defined by containing an evolutionarily conserved sequence named the Gα-binding-and-activating (GBA) motif. Using the best characterized proteins with a GBA motif as examples, Gα-interacting vesicle-associated protein (GIV)/Girdin and dishevelled-associating protein with a high frequency of leucine residues (DAPLE), this review will cover (i) the mechanisms by which extracellular cues not relayed by GPCRs promote the coupling of GBA motif-containing regulators with G proteins, (ii) the structural and molecular basis for how GBA motifs interact with Gα subunits to facilitate signaling, (iii) the relevance of this mechanism in different cellular and pathological processes, including cancer and birth defects, and (iv) strategies to manipulate GBA-G protein coupling for experimental therapeutics purposes, including the development of rationally engineered proteins and chemical probes.

Receptor tyrosine kinases activate heterotrimeric G proteins via phosphorylation within the interdomain cleft of Gαi

The molecular mechanisms by which receptor tyrosine kinases (RTKs) and heterotrimeric G proteins, two major signaling hubs in eukaryotes, independently relay signals across the plasma membrane have been extensively characterized. How these hubs cross-talk has been a long-standing question, but answers remain elusive. Using linear ion-trap mass spectrometry in combination with biochemical, cellular, and computational approaches, we unravel a mechanism of activation of heterotrimeric G proteins by RTKs and chart the key steps that mediate such activation. Upon growth factor stimulation, the guanine-nucleotide exchange modulator dissociates Gαi•βγ trimers, scaffolds monomeric Gαi with RTKs, and facilitates the phosphorylation on two tyrosines located within the interdomain cleft of Gαi. Phosphorylation triggers the activation of Gαi and inhibits second messengers (cAMP). Tumor-associated mutants reveal how constitutive activation of this pathway impacts cell's decision to "go" vs. "grow." These insights define a tyrosine-based G protein signaling paradigm and reveal its importance in eukaryotes.

Membrane Recruitment of the Non-receptor Protein GIV/Girdin (Gα-interacting, Vesicle-associated Protein/Girdin) Is Sufficient for Activating Heterotrimeric G Protein Signaling

GIV (aka Girdin) is a guanine nucleotide exchange factor that activates heterotrimeric G protein signaling downstream of RTKs and integrins, thereby serving as a platform for signaling cascade cross-talk. GIV is recruited to the cytoplasmic tail of receptors upon stimulation, but the mechanism of activation of its G protein regulatory function is not well understood. Here we used assays in humanized yeast models and G protein activity biosensors in mammalian cells to investigate the role of GIV subcellular compartmentalization in regulating its ability to promote G protein signaling. We found that in unstimulated cells GIV does not co-fractionate with its substrate G protein Gα_i3 on cell membranes and that constitutive membrane anchoring of GIV in yeast cells or rapid membrane translocation in mammalian cells via chemically induced dimerization leads to robust G protein activation. We show that membrane recruitment of the GIV "Gα binding and activating" motif alone is sufficient for G protein activation and that it does not require phosphomodification. Furthermore, we engineered a synthetic protein to show that recruitment of the GIV "Gα binding and activating" motif to membranes via association with active RTKs, instead of via chemically induced dimerization, is also sufficient for G protein activation. These results reveal that recruitment of GIV to membranes in close proximity to its substrate G protein is a major mechanism responsible for the activation of its G protein regulatory function.

A novel noncanonical signaling pathway for the μ-opioid receptor

The µ-opioid receptor (OPRM1) signals as a classic G protein-coupled receptor by activating heterotrimeric Gi/Go proteins resulting in adenylyl cyclase (AC) inhibition. Such AC inhibition is desensitized after prolonged agonist treatment. However, after receptor desensitization, the intracellular cAMP level remains regulated by OPRM1, as demonstrated by the intracellular cAMP level increase or AC superactivation upon removal of an agonist or addition of an antagonist. We now demonstrate that such intracellular cAMP regulation is mediated by a novel noncanonical signaling pathway resulting from OPRM1 being converted to a receptor tyrosine kinase (RTK)-like entity. This noncanonical OPRM1 signaling is initiated by the receptor recruiting and activating Src kinase within the receptor complex, leading to phosphorylation of the OPRM1 Tyr(336) residue. Phospho-Tyr(336) serves as the docking site for growth factor receptor-bound protein/son of sevenless, leading to the recruitment and activation of the Ras/Raf-1 and subsequent phosphorylation and activation of AC5/6 by Raf-1. Such sequence of events was established by the absence of Ras/Raf1 recruitment and activation by the OPRM1-Y336F mutant, by the presence of Src kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) or the absence of Src activity, by the presence of specific Raf-1 inhibitor GW5074 (5-iodo-3-[(3,5-dibromo-4-hydroxyphenyl) methylene]-2-indolinone) or the absence of Raf-1, or by the dominant negative RasN17 mutant. Src together with Ras activates Raf1 which was established by the inability of the Raf1-Tyr(340/341) mutant to activate AC. Hence, the phosphorylation of OPRM1 at Tyr(336) by Src serves as the trigger for the conversion of a classic Gi/Go-coupled receptor into an RTK-like entity, resulting in a noncanonical pathway even after the original Gi/Go signals are blunted.

Heterotrimeric G protein signaling outside the realm of seven transmembrane domain receptors

Heterotrimeric G proteins, consisting of the guanine nucleotide-binding Galpha subunits with GTPase activity and the closely associated Gbeta and Ggamma subunits, are important signaling components for receptors with seven transmembrane domains (7TMRs). These receptors, also termed G protein-coupled receptors (GPCRs), act as guanine nucleotide exchange factors upon agonist stimulation. There is now accumulating evidence for noncanonical functions of heterotrimeric G proteins independent of 7TMR coupling. Galpha proteins belonging to all 4 subfamilies, including G(s), G(i), G(q), and G(12) are found to play important roles in receptor tyrosine kinase signaling, regulation of oxidant production, development, and cell migration, through physical and functional interaction with proteins other than 7TMRs. Association of Galpha with non-7TMR proteins also facilitates presentation of these G proteins to specific cellular microdomains. This Minireview aims to summarize our current understanding of the noncanonical roles of Galpha proteins in cell signaling and to discuss unresolved issues including regulation of Galpha activation by proteins other than the 7TMRs.

Galpha(i1) and Galpha(i3) are required for epidermal growth factor-mediated activation of the Akt-mTORC1 pathway

The precise mechanism whereby epidermal growth factor (EGF) activates the serine-threonine kinase Akt and the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) remains elusive. Here, we report that the alpha subunits of the heterotrimeric guanine nucleotide-binding proteins (G proteins) Galpha(i1) and Galpha(i3) are critical for this activation process. Both Galpha(i1) and Galpha(i3) formed complexes with growth factor receptor binding 2 (Grb2)-associated binding protein 1 (Gab1) and the EGF receptor (EGFR) and were required for the phosphorylation of Gab1 and its subsequent interaction with the p85 subunit of phosphatidylinositol 3-kinase in response to EGF. Loss of Galpha(i1) and Galpha(i3) severely impaired the activation of Akt and of p70 S6 kinase and 4E-BP1, downstream targets of mTORC1, in response to EGF, heparin-binding EGF-like growth factor, and transforming growth factor alpha, but not insulin, insulin-like growth factor, or platelet-derived growth factor. In addition, ablation of Galpha(i1) and Galpha(i3) largely inhibited EGF-induced cell growth, migration, and survival and the accumulation of cyclin D1. Overall, this study suggests that Galpha(i1) and Galpha(i3) lie downstream of EGFR, but upstream of Gab1-mediated activation of Akt and mTORC1, thus revealing a role for Galpha(i) proteins in mediating EGFR signaling.

Heterotrimeric G proteins and the single-transmembrane domain IGF-II/M6P receptor: functional interaction and relevance to cell signaling

The G protein-coupled receptor (GPCR) family represents the largest and most versatile group of cell surface receptors. Classical GPCR signaling constitutes ligand binding to a seven-transmembrane domain receptor, receptor interaction with a heterotrimeric G protein, and the subsequent activation or inhibition of downstream intracellular effectors to mediate a cellular response. However, recent reports on direct, receptor-independent G protein activation, G protein-independent signaling by GPCRs, and signaling of nonheptahelical receptors via trimeric G proteins have highlighted the intrinsic complexities of G protein signaling mechanisms. The insulin-like growth factor-II/mannose-6 phosphate (IGF-II/M6P) receptor is a single-transmembrane glycoprotein whose principal function is the intracellular transport of lysosomal enzymes. In addition, the receptor also mediates some biological effects in response to IGF-II binding in both neuronal and nonneuronal systems. Multidisciplinary efforts to elucidate the intracellular signaling pathways that underlie these effects have generated data to suggest that the IGF-II/M6P receptor might mediate transmembrane signaling via a G protein-coupled mechanism. The purpose of this review is to outline the characteristics of traditional and nontraditional GPCRs, to relate the IGF-II/M6P receptor's structure with its role in G protein-coupled signaling and to summarize evidence gathered over the years regarding the putative signaling of the IGF-II/M6P receptor mediated by a G protein.

Tuberin: a stimulus-regulated tumor suppressor protein controlled by a diverse array of receptor tyrosine kinases and G protein-coupled receptors

Tuberin, a tumor suppressor protein, is involved in various cellular functions including survival, proliferation, and growth. It has emerged as an important effector regulated by receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs). Regulation of tuberin by RTKs and GPCRs is highly complex and dependent on the type of receptors and their associated signaling molecules. Apart from Akt, the first kinase recognized to phosphorylate and inactivate tuberin upon growth factor stimulation, an increasing number of kinases upstream of tuberin have been identified. Furthermore, recruitment of different scaffolding adaptor components to the activated receptors appears to play an important role in the regulation of tuberin activity. More recently, the differential regulation of tuberin by various G protein family members have also been intensively studied, it appears that G proteins can both facilitate (e.g., G(i/o)) as well as inhibit (e.g., G(q)) tuberin phosphorylation. In the present review, we attempt to summarize our emerging understandings of the roles of RTKs, GPCRs, and their cross-talk on the regulation of tuberin.

Single Transmembrane Spanning Heterotrimeric G Protein-Coupled Receptors and Their Signaling Cascades

Heptahelical of serpentine receptors such as the adrenergic receptors are well known to mediate their actions via heterotrimeric GTP-binding proteins. Likewise, receptors that traverse the cell membrane once have been shown to mediate their biological actions by activating several different mechanisms including stimulation of their intrinsic tyrosine kinase activities or the kinase activities of other proteins. Some of these single transmembrane receptors have an intrinsic guanylyl cyclase activity and can stimulate the cyclic GMP second messenger system; however, over the last few years, several studies have shown the involvement of heterotrimeric GTP-binding proteins in mediating signals that eventually culminate in the biological actions of single transmembrane spanning receptors and proteins. These receptors include the receptor tyrosine kinases that mediate the actions of growth factors such as epidermal growth factor, insulin, insulin-like growth factor as well as receptors for atrial natiuretic hormone or the zona pellucida protein (ZP3) and integrins. In this review, the significance of the coupling of the single transmembrane spanning receptors to G proteins has been highlighted by providing several examples of the concept that signaling via these receptors may involve the activation of multiple signaling cascades.

Activation of the G(i) heterotrimeric G protein by ANCA IgG F(ab')2 fragments is necessary but not sufficient to stimulate the recruitment of those downstream mediators used by intact ANCA IgG

Anti-neutrophil cytoplasm autoantibodies (ANCA) are implicated in the pathogenesis of systemic vasculitis. Intact ANCA IgG activate superoxide generation in cytokine-primed neutrophils after binding their antigens and co-engaging Fcgamma receptors (FcgammaR). The contribution of antigen binding via ANCA F(ab')(2) fragments to signaling has been unclear. This study shows that both ANCA IgG and F(ab')(2) fragments of ANCA IgG induce significant GTPase activity, which could be blocked with pertussis toxin and anti-G(i) protein antibodies. Pertussis toxin inhibited ANCA IgG-induced superoxide generation but was without effect on superoxide production after conventional FcgammaR ligation. ANCA F(ab')(2) fragments did not induce superoxide generation. ANCA IgG activated PI 3-kinase-generating PIP(3), activated protein kinase B (PKB), and p21(ras); activation of each mediator was inhibited with pertussis toxin, but PI3K and PKB were not activated by ANCA IgG F(ab')(2) fragments. Intact ANCA IgG induced tyrosine phosphorylation, whereas F(ab')(2) fragments did not, and ANCA IgG-mediated superoxide generation was inhibited with genistein. Both genistein and pertussis toxin together completely abrogated the ANCA-induced oxidative burst. Genistein also inhibited ANCA IgG-induced PIP(3) generation and p21(ras) activation. These data implicate a novel ANCA IgG stimulated signaling pathway that involves both F(ab')(2)-mediated antigen binding and Fc-mediated FcgammaR ligation in cooperative interactions between G(i) proteins and tyrosine kinases that facilitates activation of downstream mediators.

The enzymatic activity of phosphoglucose isomerase is not required for its cytokine function

PGI is a housekeeping gene encoding phosphoglucose isomerase (PGI) a glycolytic enzyme that also functions as a cytokine (autocrine motility factor (AMF)/neuroleukin/maturation factor) upon secretion from the cell and binding to its 78 kDa seven-transmembrane domain receptor (gp78/AMF-R). PGI contains a CXXC motif, characteristic of redox proteins and possibly evolutionarily related to the CC and CXC motif of the chemokine gene family. Using site-directed mutagenesis, single- and double-deletion (CXC, CC) mutants were created by deleting amino acids 331 and 332 of human PGI, respectively. The mutant proteins lost their enzymatic activity; however, neither of the deletions augmented the proteins' binding affinity to the receptor and all maintained cytokine function. The results demonstrate that the enzymatic activity of PGI is not essential for either receptor binding or cytokine function of human PGI.

Specific involvement of G(alphai2) with epidermal growth factor receptor signaling in rat hepatocytes, and the inhibitory effect of chronic ethanol

We have previously shown that chronic alcohol consumption inhibits liver regeneration by impairing epidermal growth factor receptor (EGFR)-operated phospholipase C-(gamma1) (PLC-(gamma1)) activation and the resultant rise in intracellular [Ca(2+)](i). In hepatocytes, activation of PLC-(gamma1) by EGFR requires involvement of a pertussis toxin-sensitive inhibitory guanine nucleotide-binding regulatory protein (G(alphai)) as an intermediate. In the present study, we first identified the G(alphai) protein isoform associated with the activated EGFR, and then examined whether the toxic effect of alcohol on EGFR signaling and liver cell proliferation was exerted on this association. In cultured hepatocytes from control rats, EGF rapidly induced association between EGFR and G(alphai2) but not other G(alphai) isoforms. In hepatocytes from rats fed alcohol for 16 weeks, EGF failed to stimulate this association of G(alphai2) with the EGFR. The impairment of EGFR-G(alphai2) complex formation caused by alcohol was associated with a decreased level of G(alphai2) in the plasma membrane fraction (approximately 50% control). Pertussis toxin, an inhibitor of G(alphai) function, produced an analogous disruption of the association between G(alphai2) and the EGFR, as well as inhibiting EGF-induced DNA synthesis. It is concluded that, in hepatocytes, G(alphai2) is specific among G(alphai) isoforms in coupling activation of the EGFR to other signaling pathways that control cell proliferation. Impaired coupling of G(alphai2) of EGFR could contribute to the mechanism by which chronic alcohol exposure inhibits liver regeneration.

Cell surface beta-1,4-galactosyltransferase-I activates G protein-dependent exocytotic signaling

ZP3 is a protein in the mammalian egg coat (zona pellucida) that binds sperm and stimulates acrosomal exocytosis, enabling sperm to penetrate the zona pellucida. The nature of the ZP3 receptor/s on sperm is a matter of considerable debate, but most evidence suggests that ZP3 binds to beta-1,4-galactosyltransferase-I (GalTase) on the sperm surface. It has been suggested that ZP3 induces the acrosome reaction by crosslinking GalTase, activating a heterotrimeric G protein. In this regard, acrosomal exocytosis is sensitive to pertussis toxin and the GalTase cytoplasmic domain can precipitate G(i) from sperm lysates. Sperm from mice that overexpress GalTase bind more soluble ZP3 and show accelerated G protein activation, whereas sperm from mice with a targeted deletion in GalTase have markedly less ability to bind soluble ZP3, undergo the ZP3-induced acrosome reaction, and penetrate the zona pellucida. We have examined the ability of GalTase to function as a ZP3 receptor and to activate heterotrimeric G proteins using Xenopus laevis oocytes as a heterologous expression system. Oocytes that express GalTase bound ZP3 but did not bind other zona pellucida glycoproteins. After oocyte maturation, ZP3 or GalTase antibodies were able to trigger cortical granule exocytosis and activation of GalTase-expressing eggs. Pertussis toxin inhibited GalTase-induced egg activation. Consistent with G protein activation, both ZP3 and anti-GalTase antibodies increased GTP-gamma[(35)S] binding as well as GTPase activity in membranes from eggs expressing GalTase. Finally, mutagenesis of a putative G protein activation motif within the GalTase cytoplasmic domain eliminated G protein activation in response to ZP3 or anti-GalTase antibodies. These results demonstrate directly that GalTase functions as a ZP3 receptor and following aggregation, is capable of activating pertussis toxin-sensitive G proteins leading to exocytosis.

Coupling of heterotrimeric Gi proteins to the erythropoietin receptor

To identify new proteins involved in erythropoietin (Epo) signal transduction, we purified the entire set of proteins reactive with anti-phosphotyrosine antibodies from Epo-stimulated UT7 cells. Antisera generated against these proteins were used to screen a lambdaEXlox expression library. One of the isolated cDNAs encodes Gbeta2, the beta2 subunit of heterotrimeric GTP-binding proteins. Gbeta and Galpha(i) coprecipitated with the Epo receptor (EpoR) in extracts from human and murine cell lines and from normal human erythroid progenitor cells. In addition, in vitro Gbeta associated with a fusion protein containing the intracellular domain of the EpoR. Using EpoR mutants, we found that the distal part of the EpoR (between amino acids 459-479) was required for Gi binding. Epo activation of these cells induced the release of the Gi protein from the EpoR. Moreover in isolated cell membranes, Epo treatment inhibited ADP-ribosylation of Gi and increased the binding of GTP. Our results show that heterotrimeric Gi proteins associate with the C-terminal end of the EpoR. Receptor activation leads to the activation and dissociation of Gi from the receptor, suggesting a functional role of Gi protein in Epo signal transduction.

Suppression of epidermal growth factor-induced phospholipase C activation associated with actin rearrangement in rat hepatocytes in primary culture

Hepatocytes maintained in primary culture for periods of 1 to 24 hours exhibited a rapid decline in epidermal growth factor (EGF)-induced activation of phospholipase C (PLC), as was evident in a loss of EGF-induced inositol 1,4,5-trisphosphate (IP(3)) formation and mobilization of Ca(2+) from intracellular Ca(2+) stores. The loss of PLC activation was not the result of a decrease in EGF receptor or phospholipase C-gamma1 (PLCgamma1) protein levels, nor the result of a loss of tyrosine phosphorylation of these proteins, but was associated with a decrease in EGF-induced translocation of PLCgamma1 to the Triton-insoluble fraction, presumably reflecting binding to the actin cytoskeleton. Disruption of F-actin by treatment of cultured hepatocytes with cytochalasin D recovered the EGF-induced IP(3) formation and Ca(2+) mobilization to the same level and with the same dose-response relationship as was obtained in freshly isolated cells. Analysis of PLCgamma1 colocalization with F-actin by confocal microscopy showed that PLCgamma1 was mostly distributed diffusely in the cytosol, both in freshly plated cells and in cells in culture for 24 hours, despite marked differences in actin structures. EGF stimulation caused a modest redistribution of PLCgamma1 and a detectable increase in colocalization with cortical actin structures in freshly plated cells or in cytochalasin D-treated cells, but in cells that had been maintained and spread in culture only a limited PLCgamma1 relocation was detected to specific actin-structure associated with lamellipodia and membrane ruffles. We conclude that actin cytoskeletal structures can exert negative control over PLCgamma1 activity in hepatocytes and the interaction of the enzyme with specific actin structures dissociates PLCgamma1 tyrosine phosphorylation from activation of its enzymatic activity.

Association of heterotrimeric G(i) with the insulin-like growth factor-I receptor. Release of G(betagamma) subunits upon receptor activation

The insulin-like growth factor-I receptor (IGF-IR) is a key regulator of cell proliferation and survival. Activation of the IGF-IR induces tyrosine autophosphorylation and the binding of a series of adaptor molecules, thereby leading to the activation of MAPK. It has been demonstrated that pertussis toxin, which inactivates the G(i) class of GTP-binding proteins, inhibits IGF-I-mediated activation of MAPK, and a specific role for G(betagamma) subunits in IGF-I signaling was shown. In the present study, we have investigated the role of heterotrimeric G(i) in IGF-IR signaling in neuronal cells. Pertussis toxin inhibited IGF-I-induced activation of MAPK in rat cerebellar granule neurons and NG-108 neuronal cells. G(alphai) and G(beta) subunits were associated with IGF-IR immunoprecipitates. Similarly, in IGF-IR-null mouse embryo fibroblasts transfected with the human IGF-IR, G(i) was complexed with the IGF-IR. G(alphas) was not associated with the IGF-IR in any cell type. IGF-I induced the release of the G(beta) subunits from the IGF-IR but had no effect on the association of G(alphai). These results demonstrate an association of heterotrimeric G(i) with the IGF-IR and identify a discrete pool of G(betagamma) subunits available for downstream signaling following stimulation with IGF-I.

Epidermal growth factor-induced heterologous desensitization of the luteinizing hormone/choriogonadotropin receptor in a cell-free membrane preparation is associated with the tyrosine phosphorylation of the epidermal growth factor receptor

Epidermal growth factor (EGF) attenuated hCG-stimulated adenylyl cyclase activity in rat luteal and follicular membranes. H7, an equipotent serine/threonine protein kinase inhibitor of cAMP-dependent protein kinases, cGMP-dependent protein kinases, and lipid-dependent protein kinase C, did not effect the ability of EGF to decrease hCG-responsive adenylyl cyclase activity, suggesting that a serine/threonine phosphorylation event catalyzed by these kinases was not critically involved in EGF-induced desensitization. Likewise, pertussis toxin-catalyzed ADP-ribosylation of a 40-kDa luteal membrane protein, which exhibited immunoreactivity with an antibody against Gi alpha, did not hinder the ability of EGF to attenuate hCG-stimulated adenylyl cyclase activity, indicating that Gi did not mediate EGF-induced desensitization. Rather, EGF-induced heterologous desensitization of LH/CG receptor in ovarian membranes was closely associated with the specific and prominent tyrosine phosphorylation of the 170-kDa EGF receptor. Both EGF-stimulated autophosphorylation of EGF receptor and EGF-induced LH/CG receptor desensitization were attenuated by genistein, a tyrosine kinase inhibitor. These results suggest that tyrosine phosphorylation of the 170-kDa EGF receptor is a necessary component of the signaling pathway in EGF-induced heterologous desensitization of the LH/CG receptor.

Activation of phospholipase D signaling pathway by epidermal growth factor in osteoblastic cells

The receptor-mediated activation of phospholipase D (PLD) is a major signaling pathway in several cell systems. This study determined the effects of epidermal growth factor (EGF) on PLD activity in normal rat osteoblastic cells. Primary cultures were obtained from fetal rat calvaria by sequential collagenase digestion and seeded in BGJb media supplemented with 10% fetal calf serum. PLD activity was assayed by the transphosphatidylation reaction in [H3]myristic acid (5 microCi/ml)-labeled cells treated with EGF in the presence of 5% ethanol and measuring the production of phosphatidylethanol (PEtOH). Lipids were extracted and separated by thin-layer chromatography, detected by iodine staining, and the areas of interest were scraped off and transferred to vials for scintillation counting. EGF significantly increased PEtOH production in a dose-dependent manner and at short (10-60 s) and long (up to 30 minutes) incubation periods (p < 0.05). Phosphatidic acid levels were also significantly increased (p < 0.05) compared with unstimulated controls, but the levels were approximately 60% less than those of PEtOH. 4b-phorbol 12-myristate, 13-acetate (PMA) also produced a significant increase in PEtOH levels when compared with unstimulated control cultures, but when PMA was added together with EGF, the production of PEtOH was reduced about 30%. Pretreatment of cells with the protein kinase C (PKC) inhibitor H-7 caused a significant increase in PEtOH levels, compared with cells stimulated with EGF alone. Preincubation of cells with pertussis toxin produced a partial decrease in PEtOH levels. This study demonstrates that EGF activates the PLD signaling cascade in normal rat osteoblastic cells and that the pathway appears to involve, at least in part, a PKC- and Gi protein-dependent mechanism.

MAPkinase: a second site of G-protein regulation of B-cell activation via the antigen receptors

Ligation of the antigen receptors on B cells transduces transmembrane signals leading to the induction of DNA synthesis. We now show that a pertussis toxin-sensitive heterotrimeric G-protein(s) of the Gi class plays a key role in the regulation of surface immunoglobulin (sIg)-mediated DNA synthesis in B cells. This site of G-protein regulation is distinct from that we have previously reported to govern the coupling of the antigen receptors on B cells to the phospholipase C-mediated hydrolysis of phosphatidylinositol-4,5-bisphosphate. We have, moreover, identified a candidate target for this new G-protein regulation by showing that mitogen-activating protein kinase (MAPkinase) activity, which plays a key role in the transduction of sIg-mediated proliferative signals in B cells, is abrogated by pre-exposure to pertussis toxin that covalently modifies and inactivates heterotrimeric G-proteins of the Gi class. Furthermore, our data suggest that this pertussis toxin-sensitive G-protein couples the antigen receptors to MAPkinase activation, at least in part, by regulating sIg-coupling to Lyn, Syk and perhaps Blk and Fyn activity, results consistent with studies in other systems which show that classical G-protein-coupled receptors recruit such protein tyrosine kinases to tranduce MAPkinase activation. Interestingly, however, this G-protein plays no apparent role in the control of up-regulation of major histocompatibility complex class II expression on B cells, suggesting that such G-protein-regulated-tyrosine kinase and MAPkinase activation is not required for the induction of this biological response following antigen receptor ligation.

Inhibitory effect of cyclosporin A peptide on rat hepatocytes proliferation induced by mitogens

Treatment of cultured rat hepatocytes with cyclosporin A (0.01-1 microM) for 24, 48, or 72 h in the presence of insulin and epidermal growth factor induced an inhibition on cell proliferation in a time- and concentration-dependent manner, with an IC50 = 0.05 microM CsA corresponding to 48-h treatment. The inhibitory effect of CsA at < or = 0.1 microM doses for 48 h on [3H]thymidine uptake was reversed after withdrawal of the drug and subsequent addition of insulin plus EGF or serum; however, at 1 microM CsA the effect was irreversible and numerous bright small vesicles were observed. The molecular mechanism involved in CsA action in hepatocytes seems to be independent on cAMP and pertussis-toxin sensitive G proteins.

Activation of the calcium-permeable cation channel CD20 by alpha subunits of the Gi protein

When the calcium-permeable cation channel CD20 is expressed in Balb/c 3T3 cells, it is activated by insulin-like growth factor-I (IGF-I) via the IGF-I receptor (Kanzaki, M., Nie, L., Shibata, H., and Kojima, I. (1997) J. Biol. Chem. 272, 4964-4969). The present study was conducted to investigate the role of G proteins in the regulation of the CD20 channel. In the excised patch clamp mode, activation of the CD20 channel by IGF-I required GTP, Mg2+, and ATP in the bath solution, and removal of either GTP or ATP attenuated the activation. Non-hydrolyzable ATP could substitute for ATP, and guanyl-5'-yl thiophosphate blocked the activation of the channel by IGF-I. The CD20 channel was also activated by guanosine 5'-3-O-(thio)triphosphate, and ATP was not required for the activation. Addition of a preparation of Gi/Go holoprotein purified from bovine brain activated the CD20, and the beta-adrenergic receptor kinase peptide did not affect the number of channel openings induced by the G protein. The CD20 channel was stimulated by the GTP-bound form of recombinant Gi2 alpha subunit purified from Sf9 cells. The Gi3 alpha subunit was less effective, and the Gi1 alpha subunit had no effect. Purified recombinant beta1gamma2 subunits did not affect the activity of the channel. Finally, IGF-I-induced activation of CD20 was inhibited by an antibody against Gi2 alpha subunit. These findings indicate that the CD20 channel expressed in Balb/c 3T3 cells is activated by the IGF-I receptor via the alpha subunits of heterotrimeric G proteins.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Tyrosine phosphorylation of GSalpha and inhibition of bradykinin-induced activation of the cyclic AMP pathway in A431 cells by epidermal growth factor receptor

An increasing amount of experimental data suggest that cross-talk exists between pathways involving tyrosine kinases and heterotrimeric G proteins. In a previous study, we demonstrated that bradykinin (BK) increases the intracellular accumulation of cAMP in the human epidermoid carcinoma cell line A431 by stimulating adenylate cyclase activity via a stimulatory G protein (Gsalpha) (Liebmann, C., Graness, A., Ludwig, B., Adomeit, A., Boehmer, A., Boehmer, F.-D., Nürnberg, B., and Wetzker, R. (1996) Biochem. J. 313, 109-118). Here, we present several lines of evidence indicating the ability of epidermal growth factor (EGF) to suppress BK-induced activation of the cAMP pathway in A431 cells via tyrosine phosphorylation of Gsalpha. Gsalpha was specifically immunoprecipitated from A431 cells using the anti-alphas antiserum AS 348. Tyrosine phosphorylation of Gsalpha was detectable in EGF-pretreated cells with monoclonal anti-phosphotyrosine antibodies. Additionally, A431 cells were labeled with [32P]orthophosphate in vivo and treated with EGF, and the resolved immunoprecipitates were subjected to amino acid analysis. The results clearly indicate that EGF induces tyrosine phosphorylation of Gsalpha in A431 cells. Treatment of A431 cells with EGF decreased BK-induced cAMP accumulation in intact cells as well as the stimulation of adenylate cyclase by BK, NaF, and guanyl nucleotides, but not by forskolin. Also, EGF treatment abolished both the BK- and isoprenaline-induced stimulation of guanosine 5'-O-(3-[35S]thiotriphosphate) binding to Gsalpha. In contrast, the BK-evoked, Gq-mediated stimulation of inositol phosphate formation in A431 cells was not affected by EGF pretreatment. Thus, EGF-induced tyrosine phosphorylation of Gsalpha is accompanied by a loss of its susceptibility to G protein-coupled receptors and its ability to stimulate adenylate cyclase via guanyl nucleotide exchange. We propose that Gsalpha may represent a key regulatory protein in the cross-talk between the signal transduction pathways of BK and EGF in A431 cells.

EGF modulation of Na+/H+ antiport in rat hepatocytes: different sensitivity in adult and fetal cells

The modulation by epidermal growth factor (EGF) of the Na+/H+ antiport in fetal and adult rat hepatocytes was studied in nominally HCO3- free solution. EGF (10 nM) activated the antiport in adult rat hepatocytes by 0.22 +/- 0.03 (mean +/- SD;n=10) pH units over basal value, measured with the fluorescent pH-sensitive intracellular probe, 2',7'-bis(carboxyethyl)-5(6)- carboxyfluorescein (BCECF). The effect of EGF was inhibited by amiloride analogue 5-(N-ethyl-N-isopropyl) amiloride (EIPA), by ouabain, inhibitor of the Na+ pump, and by erbstatin analogue, an inhibitor of the tyrosine kinase activity of the EGF receptor. The effect of EGF on Na+/H+ antiport in adult rat hepatocytes appeared to be mediated by both protein kinase C (PKC) and G protein system. No effect of EGF and phorbol 12-myristate 13-acetate, an activator of PKC, on the Na+/H+ antiport was observed in fetal hepatocytes of 20 and 22 days. A different sensitivity of the antiport to high concentrations of amiloride and EIPA suggests that altered amount of the Na+/H+ antiport units or different isoforms could be expressed in fetal compared with adult cells.

Activation of a G protein complex by aggregation of beta-1,4-galactosyltransferase on the surface of sperm

Fertilization is initiated by the species-specific binding of sperm to the extracellular coat of the egg. One sperm receptor for the mouse egg is beta-1,4-galactosyltransferase (GalTase), which binds O-linked oligosaccharides on the egg coat glycoprotein ZP3. ZP3 binding induces acrosomal exocytosis through the activation of a pertussis toxin-sensitive heterotrimeric guanine nucleotide-binding protein (G protein). The cytoplasmic domain of sperm surface GalTase bound to and activated a heterotrimeric G protein complex that contained the Gi alpha subunit. Aggregation of GalTase by multivalent ligands elicited G protein activation. Sperm from transgenic mice that overexpressed GalTase had higher rates of G protein activation than did wild-type sperm, which rendered transgenic sperm hypersensitive to their ZP3 ligand. Thus, the cytoplasmic domain of cell surface GalTase appears to enable it to function as a signal-transducing receptor for extracellular oligosaccharide ligands.

On the mechanisms of the growth-promoting effect of prostaglandins in hepatocytes: the relationship between stimulation of DNA synthesis and signaling mediated by adenylyl cyclase and phosphoinositide-specific phospholipase C

While many observations indicate that prostaglandins may act as positive regulators of hepatocyte proliferation, the underlying mechanisms are not known. We have examined some of the signal pathways in the growth response induced by prostaglandins in hepatocytes, with particular focus on adenylyl cyclase and phosphoinositide-specific phospholipase C. Adult rat hepatocytes were cultured as primary monolayers in serum-free medium in the presence of EGF and insulin. PGE2 or PGF2 alpha (added 0-3 h after plating) enhanced the incorporation of [3H]-thymidine into DNA (measured at 50 h); at 100 microM the stimulation was about threefold PGI2 and PGD2 also showed significant but smaller stimulatory effects. No significant increase in the level of cyclic AMP (cAMP) was detected in response to any of the prostaglandins. Low concentrations of glucagon (0.1-10 nM), a potent activator of hepatic adenylyl cyclase, or 8-bromo-cAMP (0.1-10 microM) enhanced the DNA synthesis. When 8-bromo-cAMP was used in maximally effective concentrations, no further stimulation was obtained by combining it with glucagon, whereas the effects of PGE2 and 8-bromo-cAMP were completely additive. All the prostaglandins also showed additivity with the effect of glucagon on the DNA synthesis. PGE2, PGF2 alpha, PGI2, and PGD2 increased intracellular inositol-1,4,5-trisphosphate (InsP3), with a relative order of efficacy roughly corresponding to their activity as stimulators of DNA synthesis. Increases in cytosolic free Ca2+, as measured in single cells, were elicited in a majority of the hepatocytes by all these prostaglandins at 1 microM. Supramaximal concentrations of vasopressin, a strong activator of phospholipase C in hepatocytes, acted additively with PGE2 on the DNA synthesis. Pretreatment of the hepatocytes with a concentration of pertussis toxin that prevented the inhibitory effect of PGE2 on glucagon-induced cAMP accumulation did not abolish the ability of PGE2 to stimulate the DNA synthesis. The results do not support a role for adenylyl cyclase activation in the stimulatory effect of prostaglandins on hepatocyte growth. While the data are compatible with an involvement of phosphoinositide-specific phospholipase C in the growth-promoting effect of prostaglandins in cultured rat hepatocytes, they suggest this may not be the sole mechanism.

Pleckstrin inhibits phosphoinositide hydrolysis initiated by G-protein-coupled and growth factor receptors. A role for pleckstrin's PH domains

Pleckstrin is a 40-kDa protein present in platelets and leukocytes that contains two PH domains separated by a 150-residue intervening sequence. Pleckstrin is a major substrate for protein kinase C, but its function is unknown. The present studies examine the effects of pleckstrin on second messenger generation. When expressed in cos-1 or HEK-293 cells, pleckstrin inhibited 1) the G alpha-mediated activation of phospholipase C beta initiated by thrombin, M1-muscarinic acetylcholine, and angiotensin II receptors, 2) the stimulation of phospholipase C beta by constitutively active Gq alpha, 3) the G beta gamma-mediated activation of phospholipase C beta caused by alpha 2A-adrenergic receptors, and 4) the tyrosine phosphorylation-mediated activation of phospholipase C gamma caused by Trk A. However, pleckstrin had no effect on either the stimulation or inhibition of adenylyl cyclase. The inhibition of phosphoinositide hydrolysis caused by pleckstrin was similar in magnitude to that caused by activating protein kinase C with phorbol 12-myristate 13-acetate (PMA). When combined, pleckstrin and PMA had an additive effect, inhibiting phosphoinositide hydrolysis by as much as 90%. Structure-function analysis highlighted the role of pleckstrin's N-terminal PH domain in these events. Although deleting the C-terminal PH domain had no effect, deleting the N-terminal PH domain abolished activity (but not expression) and mutating a highly conserved tryptophan residue within the N-terminal PH domain decreased activity by one-third. Notably, however, a pleckstrin variant in which the N-terminal PH domain was replaced with a second copy of the C-terminal PH domain was nearly as active as native pleckstrin. These results show that: 1) pleckstrin can inhibit pathways leading to both phospholipase C beta- and phospholipase C gamma-mediated phosphoinositide hydrolysis, 2) this inhibition affects activation of phospholipase C beta mediated by either G alpha or G beta gamma, but does not affect the regulation of adenylyl cyclase activity by G alpha or G beta gamma, 3) although pleckstrin is a substrate for protein kinase C, the effects of pleckstrin and PMA are at least partially independent, 4) the inhibition caused by pleckstrin appears to be mediated by the PH domain at the N terminus, rather than the C terminus of the molecule, and 5) location of the two PH domains within the molecule clearly contributes to their individual activity.2+1

Role of heterotrimeric G-proteins in epidermal growth factor signalling

Since in 1986 it was reported that a pertussis toxin-sensitive substrate was involved in the Ca2+ signal induced by epidermal growth factor (EGF) in rat hepatocytes, much evidence accumulated to implicate heterotrimeric G-proteins in EGF action. EGF can also induce a cyclic AMP signal, but while the generation of a Ca2+ signal appears to be quite general in EGF action, the increase in cyclic AMP occurs only in few cell types. In non-transformed cell types these effects appear to involve G-proteins. EGF not only induces cell proliferation but also interacts with hormones in the short-term control of cell function in quiescent cells. Most of the known interactions are on cyclic AMP mediated hormone effects, and in many cases, the interaction between EGF and hormones involves G-proteins. Here we review the evidence accumulated in recent years that implicate G-proteins in EGF action. An understanding of the mechanisms involved may reveal new mechanisms of G-protein regulation and will contribute to our knowledge of EGF function and signal transduction.

Lactogen enhances Nb2 cell GTPase activity after 4 hours incubation

The lactogen receptor has been suggested to associate with one or more G proteins despite the absence of a 7-transmembrane spanning sequence. These studies were designed to determine whether lactogens acutely increase GTP binding to or GTPase activity in Nb2 cell membrane. Incubation of Nb2 cell membrane with either ovine PRL (10 ng/ml) or diluent for 0-1 h resulted in a decrease in total(35)S-GTP binding to both with no difference in GTP binding between PRL- and diluent-treated membranes. There was also no change in(35)S-GTP binding to Nb2 cell membrane incubated with increasing oPRL concentrations (0.001-100 ng/ml) for 60 min. α-(32)P-GTP photoaffinity labelling was used to evaluate changes in GTP binding to specific G proteins. Photoaffinity labelling of α-(32)P-GTP to no G protein was changed after preincubation with oPRL (10 ng/ml) for 0-60 min or with oPRL (0.01-10 ng/ml) for 60 min. Finally, it was determined whether oPRL had any acute effect on GTPase activity, as determined by release of(32)Pi from γ-(32)P-GTP. When Nb2 cell membrane was preincubated for 0-60 min with oPRL (10 ng/ml) or a range of oPRL concentrations (0-10 ng/ml), no change in GTPase activity was observed. However, when Nb2 cells were incubated with lactogen for 0-7 h, GTPase activity in equal quantities of Nb2 cell membrane prepared from those cells increased over time. Increased GTPase activity (64.9-74.4%;P<0.03 compared to 0 h) was observed after 4-7 h incubation with lactogen.In summary, addition of lactogen to Nb2 cell membrane did not acutely increase either GTP binding or GTPase activity. Yet when Nb2 cells were incubated with lactogen for 4 h prior to preparation of membrane, GTPase activity was significantly increased. This evidence, in addition to our previous results showing that 4 h incubation with lactogen increased G protein β subunit concentration and pertussis toxin-stimulated ADP-ribosylation of Gi, support a role for delayed lactogen modulation of one or more G proteins in the Nb2 cell, requiring at least 4 h for maximal effect.

Activation of a Gi protein in digitonin/cholate-solubilized membrane preparations of mouse sperm by the zona pellucida, an egg-specific extracellular matrix

Mammalian sperm possess guanine nucleotide-binding regulatory proteins (G proteins) that are involved in signal transduction pathways leading to zona pellucida (ZP)-mediated acrosomal exocytosis. We have previously examined ZP-G protein dynamics in mouse sperm homogenates, as well as cell-free membrane preparations, and our data support the existence of ZP receptor-G protein complexes in sperm membranes. However, the composition of this complex has not been identified due to experimental limitations of the membrane preparations. In the present study, a detergent-solubilized preparation from mouse sperm membranes that retained the signaling properties of cell homogenates and cell-free membrane preparations was developed using buffers containing digitonin and cholate. GTP gamma S, a poorly hydrolyzable analogue of GTP, bound to these solubilized preparations in a specific and concentration-dependent fashion that reached saturation at 100 nM. Incubation of this solubilized membrane preparation with heat-solubilized ZP resulted in an increase in specific GTP gamma S binding in a concentration-dependent manner, with a maximal response at 4-6 ZP/microliters. Mastoparan (50 microM) increased GTP gamma S binding to levels similar to that seen with solubilized ZP. Mastoparan plus ZP stimulated GTP gamma S binding to the same extent as mastoparan or ZP alone. Pertussis toxin completely inhibited ZP-stimulated GTP gamma S binding and decreased mastoparan-stimulated GTP gamma S binding by 50-60%. Purified ZP3, the ZP component that possesses quantitatively all of the sperm binding and acrosomal exocytosis-inducing activities of the intact ZP, stimulated GTP gamma S binding to an extent similar to that of solubilized ZP.(ABSTRACT TRUNCATED AT 250 WORDS)

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

Role of GTP-binding proteins in the polyunsaturated fatty acid stimulated proliferation of mouse mammary epithelial cells

Polyunsaturated fatty acids enhance the proliferation of mouse mammary epithelial cells stimulated by epidermal growth factor (EGF) by modulating the post-receptor signaling pathways. The growth stimulatory effect of these fatty acids is completely inhibited by pertussis toxin, whereas the inhibition of EGF and insulin stimulated growth is only partial. The treatment of cell cultures with 12-O-tetradecanoyl-phorbol-13 acetate (TPA) reverses the growth inhibitory effect of pertussis toxin and fully restores the growth as was in the control cultures untreated with the toxin suggesting a role for PKC in this reversal. It appears that the functions of Gi-proteins are required in the mediation of fatty acid effect on growth. The predominant types of Gi alpha in mammary epithelial cells are Gi alpha 1, Gi alpha 2, and Gi alpha 3. Among these, the levels of Gi alpha 1 and 2 appears to be regulated by steroid hormones. Linoleic acid raises the level of GTP-bound Ras in the cells above the levels induced by EGF. Pertussis toxin reduces the level of Ras-GTP and inhibits phosphorylation of MAP kinase by EGF. It has been speculated that Gi-proteins interact with the receptor bound nucleotide exchange factor and the membrane anchored Raf kinase and constitute two sites for pertussis toxin action. The phosphorylation by PKC may uncouple Gi-protein interaction with these effectors and enable the agonist-induced signals to bypass the inhibitory action of PT on growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of tyrosine kinase inhibition on basal and epidermal growth factor-stimulated human Caco-2 enterocyte sheet migration and proliferation

Mucosal healing requires enterocyte migration (restitution) supplemented by proliferation. Proliferation and migration may be studied independently by thymidine uptake and proliferation-blocked cell migration using human Caco-2 enterocyte monolayers in culture. Since epidermal growth factor (EGF) promotes mucosal healing and the EGF receptor is a tyrosine kinase, we hypothesized that tyrosine kinases might therefore modulate enterocyte migration and proliferation. The tyrosine kinase inhibitors genistein and 2,5-dihydroxymethylcinnamate, which block kinase ATP-binding and substrate-binding sites, respectively, were studied alone and with EGF. Proliferation was blocked with mitomycin. Although each inhibitor decreased basal and EGF-stimulated monolayer expansion when cell proliferation occurred, neither genistein nor 2,5-dihydroxymethylcinnamate decreased migration when proliferation was blocked. However, each inhibitor prevented EGF stimulation of proliferation-blocked migration and thymidine uptake. More substantial inhibition of basal proliferation by genistein correlated with increased protein-linked DNA breaks, which may reflect nonspecific inhibition of DNA topoisomerase activity by genistein. The more specific 2,5-dihydroxymethylcinnamate blocked changes in the alpha 2 integrin subunit organization which may modulate EGF-stimulated migration. Antiproliferative effects of tyrosine kinase inhibitors decrease basal monolayer expansion but true basal enterocyte migration appears independent of tyrosine kinase regulation. However, a specific tyrosine kinase-dependent modulation of cell-matrix interaction inhibits EGF-stimulated migration.

EGF modulates phosphoinositide levels in ovarian granulosa cells stimulated by luteinizing hormone

Hamster granulosa cells were exposed to epidermal growth factor (EGF) and luteinizing hormone (LH) to study cross-talk between second messenger pathways involving tyrosine kinase, cAMP, and phosphoinositides. Granulosa cells from ovarian preovulatory follicles of PMSG-primed hamsters were incubated with various additives in serum-free medium. LH, but not EGF, stimulated inositol phosphate (IP) accumulation; however, when combined with LH, EGF inhibited IP accumulation in a manner that was concentration dependent for both LH and EGF. The inhibitory effects of EGF were significantly reduced by the tyrosine kinase inhibitor genistein and by pertussis toxin suggesting a role for tyrosine kinase and an inhibitory G-protein (Gi) in this system. EGF stimulated an increase in cAMP, but it does not appear to modulate LH-stimulated IP levels via cAMP.

Effects of genistein, tyrphostin, and pertussis toxin on EGF-induced mitogenesis in primary culture and clonal osteoblastic cells

Epidermal growth factor (EGF) has been found to stimulate proliferation in a variety of cell types. The EGF receptor is known to have tyrosine kinase activity [1], however, the role of this signal mechanism has not been established in bone cells. The aim of this study was to determine whether tyrosine kinase activity and G inhibitory (Gi) proteins are involved in EGF-stimulated proliferation in the osteoblastic cell line G292 and in primary culture osteoblasts isolated from neonatal rat calvaria. Cell proliferation was measured by 3H-thymidine incorporation using liquid scintillation spectrometry. EGF stimulates a dose-dependent increase in proliferation of G292 and primary culture cells above control. Genistein was able to inhibit the effects of EGF in the G292 cells. In the primary culture cells, genistein with EGF appeared to enhance proliferation compared with EGF alone or genistein alone. Tyrphostin 25, on the other hand, inhibited the EGF response in both of these cell types. Inactivation of Gi proteins with pertussis toxin was able to inhibit EGF-induced mitogenesis in the neonatal rat osteoblasts but did not appear to specifically inhibit this response in the G292 cells. These results suggest that although both of these osteoblastic cell types increase proliferation in response to EGF, their signal pathways are different.

Effects of transforming growth factor beta 1 on the adenylyl cyclase-cAMP pathway in prostate cancer

We reported previously that MATLyLu rat prostate cancer cells engineered to overproduce transforming growth factor beta 1 (TGF beta 1) produce larger, more metastatic tumors in vivo. We recognized that this ability of TGF beta 1 to act as a positive modulator of prostate tumor behavior might be due to effects of TGF beta 1 on the host and/or on the tumor cells. In this study we demonstrated that the cells themselves respond to endogenously produced TGF beta 1, and that the adenylyl cyclase (AC)-cAMP pathway is affected. TGF beta 1-overproducing cells had lower membrane AC activity, lower intracellular cAMP content, and a lower Gs alpha protein level than did control cells. Prostate cancer cells were growth inhibited by 8-bromo-cAMP or forskolin, agents that elevate intracellular cAMP. Thus, TGF beta 1 overproduction affects the phenotype of the tumor cells, deliberate activation of endogenously produced latent TGF beta 1 is not required (indicating that the cells themselves are capable of activating latent TGF beta 1), and TGF beta 1 overproduction lowers the cellular concentration of the growth inhibitor cAMP. Therefore, TGF beta 1 overproduction could affect tumor behavior in vivo in part via a direct effect on the tumor cells.

Neurotrophin induced cAMP and IP3 responses in PC12 cells. Different pathways

NGF and BDNF elevate cAMP and IP3 levels in membranes of PC12 cells within a subsecond time period. The cAMP formation induced by NGF and BDNF pretreatment for 2 s was reduced by GDP-beta-S and PTX, but not the trkNGFR inhibitor K 252a. NGF, but not BDNF, induced IP3 formation. IP3 formation was reduced by K 252a, but not by GDP-beta-S and PTX. Using p75NGFR expressing, but trkNGFR-deficient PCNA cell membranes, NGF and BDNF induced cAMP formation, but not IP3 formation. We suggest that NGF and BDNF induced cAMP formation is mediated via a p75NGFR/G-protein mediated mechanism, and IP3 formation via a K 252a sensitive pathway.

Different signal transduction by epidermal growth factor may be responsible for the difference in modulation of amino acid transport between fetal and adult hepatocytes

[1-14C]-2-aminoisobutyric acid (AIB) uptake and signal transduction pattern after epidermal growth factor (EGF) stimulation were examined in freshly isolated hepatocytes from 20-day-old fetuses and 3-month-old rats. EGF induced a transient increase of AIB transport after 10 min only in adult animals; the observed unresponsiveness of fetal liver is not dependent on a lack of EGF receptors which are present though to a lesser extent on the plasma membrane in this period. As far as the production of the second messengers, inositol trisphosphate (IP3) and calcium, is concerned, substantial differences were found: EGF increased IP3 production in adult hepatocytes, whereas it had no effect in fetal ones. Moreover, the addition of EGF induced a calcium transient in hepatocytes from adult animals, while there was no increase in fetal cells. The lack of EGF effect on amino acid transport in fetal cells could be due to its inability to produce both IP3 and calcium transients, suggesting that this transduction pathway is not activated during fetal life.

Epidermal growth factor interferes with the effect of adrenaline on glucose production and on hepatic lipase secretion in rat hepatocytes

We studied the interaction of epidermal growth factor (EGF) and adrenaline in the control of several metabolic functions in isolated hepatocytes from fed rats. EGF did not modulate glucose release, urea production or hepatic lipase secretion, but interfered with the stimulatory effect of adrenaline on both glucose and urea production and also with the inhibitory effect of this hormone on hepatic lipase secretion. EGF also interfered with the effect of both angiotensin II and vasopressin on glucose release and on hepatic lipase secretion. While the effect of EGF interfering with the action of adrenaline on glucose release was potentiated in the absence of extracellular calcium, the effect on the inhibition of hepatic lipase secretion was abolished. These results suggest that EGF interfered with catecholamine actions in the liver at a site distal from the generation of the calcium signal.

The role of G-proteins versus protein tyrosine kinases in the regulation of lymphocyte activation

The relative roles of G-proteins and protein tyrosine kinases (PTKs) in the regulation of antigen receptor-mediated signalling in B and T cells is controversial. As they, and the biochemical events they control, are potential targets for intervention in various immune dysfunctions, the resolution of the controversy is of great interest. Here, Margaret Harnett and Kevin Rigley provide a timely assessment of current understanding, and propose a model for the interaction of G-proteins and PTKs in antigen receptor-mediated signalling.

Hepatocyte growth factor induces calcium mobilization and inositol phosphate production in rat hepatocytes

The effects of hepatocyte growth factor (HGF) on intracellular Ca2+ mobilization were studied using fura-2-loaded single rat hepatocytes. Hepatocytes microperfused with different amounts of HGF responded with a rapid concentration-dependent rise in the cytosolic free Ca2+ concentration with a maximum increase of 142% at 80 ng/ml of HGF. The lag period of the Ca2+ response was decreased with increasing HGF concentrations, being 64 +/- 12 s, 42 +/- 6 s, and 14 +/- 2 s, respectively, with 8, 20, and 80 ng/ml of HGF. The detailed pattern of Ca2+ transients, however, was variable. Out of 16 cells tested using 20 ng/ml of HGF, 68% showed sustained oscillatory responses, whereas other cells showed a sustained increase in the cytosolic-free Ca2+ upon exposure to HGF, which was dependent on the presence of extracellular Ca2+. HGF also induced Ca2+ entry across the plasma membrane. Mobilization of Ca2+ by HGF was accompanied by a rapid accumulation of inositol 1,4,5-trisphosphate (Ins 1,4,5-P3). The effects of HGF and epidermal growth factor (EGF) were comparable and partly additive for Ins 1,4,5-P3 production and for the sustained phase of Ca2+ mobilization. Preincubation of cells with 10 microM of genistein to inhibit protein tyrosine kinases abolished the HGF-induced Ca2+ response and also inhibited HGF-induced Ins 1,4,5-P3 production in rat liver cells. These data indicate that early events in the signal transduction pathways mediated by HGF and EGF have in common the requirements for tyrosine kinase activity, Ins 1,4,5-P3 production, and Ca2+ mobilization.

Pertussis toxin blocks activin A-induced production of inositol phosphates in rat hepatocytes

The present study was conducted to examine an involvement of G protein in the action of activin A in rat parenchymal liver cells. Activin A induced a dose-dependent increase in inositol phosphates in cells prelabelled with [3H]inositol. The effect of activin A was completely blocked by pretreatment of the cells with pertussis toxin. In contrast, pertussis toxin had little effect on angiotensin II-induced production of inositol phosphates. Both activin A and angiotensin II inhibited glucagon-mediated production of cAMP. Pretreatment of the cells with pertussis toxin blocked the inhibition induced by both activin A and angiotensin II. In permeabilized cells, activin A augmented production of inositol phosphates. Activin-mediated production of inositol trisphosphate was enhanced by GTP-gamma S and was attenuated by GDP-beta S. These results suggest that a pertussis toxin-sensitive G protein(s) may be involved in the action of activin A in hepatocytes.