Gq and G11 are concurrently activated by bombesin and vasopressin in Swiss 3T3 cells

How Gastrin-Releasing Peptide Opens the Spinal Gate for Itch

Spinal transmission of pruritoceptive (itch) signals requires transneuronal signaling by gastrin-releasing peptide (GRP) produced by a subpopulation of dorsal horn excitatory interneurons. These neurons also express the glutamatergic marker vGluT2, raising the question of why glutamate alone is insufficient for spinal itch relay. Using optogenetics together with slice electrophysiology and mouse behavior, we demonstrate that baseline synaptic coupling between GRP and GRP receptor (GRPR) neurons is too weak for suprathreshold excitation. Only when we mimicked the endogenous firing of GRP neurons and stimulated them repetitively to fire bursts of action potentials did GRPR neurons depolarize progressively and become excitable by GRP neurons. GRPR but not glutamate receptor antagonism prevented this action. Provoking itch-like behavior by optogenetic activation of spinal GRP neurons required similar stimulation paradigms. These results establish a spinal gating mechanism for itch that requires sustained repetitive activity of presynaptic GRP neurons and postsynaptic GRP signaling to drive GRPR neuron output.

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Spinal itch relay requires effective communication from GRP to GRP receptor neurons

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Single action potentials in GRP neurons fail to release sufficient GRP

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Only burst firing releases enough GRP to prime GRP receptor neurons for activation

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GRP acts as a volume transmitter probably explaining why itch is hard to localize

Loss of epithelial Gq and G11 signaling inhibits TGFβ production but promotes IL-33-mediated macrophage polarization and emphysema

Heterotrimeric guanine nucleotide-binding protein (G protein) signaling links hundreds of G protein-coupled receptors with four G protein signaling pathways. Two of these, one mediated by G_q and G₁₁ (G_q/11) and the other by G₁₂ and G₁₃ (G_12/13), are implicated in the force-dependent activation of transforming growth factor-β (TGFβ) in lung epithelial cells. Reduced TGFβ activation in alveolar cells leads to emphysema, whereas enhanced TGFβ activation promotes acute lung injury and idiopathic pulmonary fibrosis. Therefore, precise control of alveolar TGFβ activation is essential for alveolar homeostasis. We investigated the involvement of the G_q/11 and G_12/13 pathways in epithelial cells in generating active TGFβ and regulating alveolar inflammation. Mice deficient in both Gα_q and Gα₁₁ developed inflammation that was primarily caused by alternatively activated (M2-polarized) macrophages, enhanced matrix metalloproteinase 12 (MMP12) production, and age-related alveolar airspace enlargement consistent with emphysema. Mice with impaired G_q/11 signaling had reduced stretch-mediated generation of TGFβ by epithelial cells and enhanced macrophage MMP12 synthesis but were protected from the effects of ventilator-induced lung injury. Furthermore, synthesis of the cytokine interleukin-33 (IL-33) was increased in these alveolar epithelial cells, resulting in the M2-type polarization of alveolar macrophages independently of the effect on TGFβ. Our results suggest that alveolar G_q/11 signaling maintains alveolar homeostasis and likely independently increases TGFβ activation in response to the mechanical stress of the epithelium and decreases epithelial IL-33 synthesis. Together, these findings suggest that disruption of G_q/11 signaling promotes inflammatory emphysema but protects against mechanically induced lung injury.

Gq rather than G11 preferentially mediates nociceptor sensitization

Background

The G_q/11-protein signaling mechanism is essential throughout the nervous system, but little is known about the contribution of the individual G-protein GPCR signaling branches towards nociceptor activation and their specific role on nociceptor sensitization. We aimed to unravel the contribution of the G_q/11-signaling pathway towards nociceptor activation via a variety of classical inflammatory mediators signalling via different G-protein GPCRs and investigated the specific contribution of the individual G_q and G₁₁ G-Proteins in nociceptors.

Findings

Using different transgenic mouse lines, lacking Gα_q, Gα₁₁ or both α-subunit of the G-proteins in primary nociceptive neurons, we analyzed the mechanical- and heat-sensitivity upon application of different GPCR-agonists that are known to play an important role under inflammatory conditions (e.g. ATP, Glutamate, Serotonin etc.). We found that the G_q/11-GPCR signaling branch constitutes a primary role in the manifestation of mechanical allodynia and a minor role in the development of thermal hyperalgesia. Moreover, with respect to the mediators used here, the G_q-protein is the principle G-protein among the G_q/11-protein family in nociceptive neurons leading to nociceptor sensitization.

Conclusions

Our results demonstrate that the G_q/11 signaling branch plays a primary role in nociceptor sensitization upon stimulation with classical GPCR ligands, contributing primarily towards the development of mechanically allodynia. Moreover, the deletion of the individual G-proteins led to the finding that the G_q-protein dominates the signalling machinery of the G_q/11 family of G-proteins in nociceptive neurons.

Neurobehavioral phenotyping of G(αq) knockout mice reveals impairments in motor functions and spatial working memory without changes in anxiety or behavioral despair

Many neurotransmitters, hormones, and sensory stimuli elicit their cellular responses through the targeted activation of receptors coupled to the G_αq family of heterotrimeric G proteins. Nevertheless, we still understand little about the consequences of loss of this signaling activity on brain function. We therefore examined the effects of genetic inactivation of Gnaq, the gene that encode for G_αq, on responsiveness in a battery of behavioral tests in order to assess the contribution of G_αq signaling capacity in the brain circuits mediating expression of affective behaviors (anxiety and behavioral despair), spatial working memory, and locomotor output (coordination, strength, spontaneous activity, and drug-induced responses). First, we replicated and extended findings showing clear motor deficits in G_αq knockout mice as assessed on an accelerating rotarod and the inverted screen test. We then assessed the contribution of the basal ganglia motor loops to these impairments, using open field testing and analysis of drug-induced locomotor responses to the psychostimulant cocaine, the benzazepine D₁ receptor agonists SKF83822 and SKF83959, and the NMDA receptor antagonist MK-801. We observed significant increases in drug-induced locomotor activity in G_αq knockout mice from the dopaminergic agonists but not MK-801, indicating that basal ganglia locomotor circuitry is largely intact in the absence of G_αq. Additionally, we observed normal phenotypes in both the elevated zero maze and the forced swim test indicating that anxiety and depression-related circuitry appears to be largely intact after loss of Gnaq expression. Lastly, use of the Y-maze revealed spatial memory deficits in G_αq knockout mice, indicating that receptors signaling through G_αq are necessary in these circuits for proficiency in this task.

SNPs in genes encoding G proteins in pharmacogenetics

Heterotrimeric guanine-binding proteins (G proteins) transmit signals from the cell surface to intracellular signal cascades and are involved in various physiological and pathophysiological processes. Polymorphisms in the genes GNB3 (encoding the Gβ3 subunit), GNAS (encoding the Gαs subunit) and GNAQ (encoding the Gαq subunit) have been the primary focus of investigation. Polymorphisms in these genes could be associated with different complex phenotypes underlining that alterations in G-protein signaling can cause multiple disorders. G proteins present a point of convergence or ‘bottleneck’ between various receptors and effectors, thus making them a sensible tool for pharmacogenetic studies. The pharmacogenetic studies performed to date mostly demonstrate an association between G-protein polymorphisms and response to therapy or occurrence of adverse drug effects. Therefore, polymorphisms in genes encoding G-protein subunits may help to individualize drug treatment in various diseases with regard to both efficacy and safety.

Conditional mutagenesis of G-protein coupled receptors and G-proteins

The G-protein-coupled receptor signaling system, consisting of a huge variety of receptors as well as of many G-proteins and effectors, operates in every cell and is involved in many physiological and pathological processes. The versatility of this system and the involvement of specific components makes G-protein-coupled receptors and their signaling pathways ideal targets for pharmacological interventions. Classical mouse knockout models have often provided important preliminary insights into the biological roles of individual receptors and signaling pathways and they are routinely used in the process of target validation. The recent development of efficient conditional mutagenesis techniques now allows a much more detailed analysis of G-protein-mediated signaling transduction processes. This review summarizes some of the areas in which progress has recently been made by applying conditional mutagenesis of genes coding for G-proteins and G-protein-coupled receptors.

Regulation of the cardiac L-type calcium channel in L6 cells by arginine-vasopressin

L-type Ca2+ channel activity was measured in L6 cells as nifedipine-sensitive barium (Ba2+; 5 mM) influx in a depolarizing salt solution containing 140 mM KCl. Addition of AVP (arginine-vasopressin) during Ba2+ uptake reduced the rate of Ba2+ influx by 60-100%; this was followed by a gradual restoration of the initial rate of Ba2+ uptake. Blockade of PKC (protein kinase C) by pretreatment with 10 muM bisindolylmaleimide did not affect the initial inhibition of Ba2+ influx, but completely abolished the recovery phase. The effect of AVP was half-maximal at 10 nM AVP and was blocked by the V1a receptor antagonist d-(CH2)(5)-Tyr(Me)-AVP. Activation of G(alphas) by isoprenaline or cholera toxin antagonized the actions of AVP on Ba2+ uptake. This protection persisted in the presence of the PKA (protein kinase A) inhibitor KT5720, and was not mimicked by agents that increase cAMP. Inhibition of Ba2+ influx was also elicited by ATP and ET (endothelin 1) with an order of effectiveness ET<ATP<AVP. Each of these agents has been reported to act through G(q)-coupled receptors. We conclude that activation of G(q)-coupled receptors produces a rapid inhibition of the cardiac L-type Ca2+ channel, which is subsequently overcome by activation of PKC.

Levels of G-protein alpha q/11 subunits and of phospholipase C-beta(1-4), -gamma, and -delta1 isoforms in postmortem human brain caudate and cortical membranes: potential functional implications

The levels of expression of G-protein alpha(q/11) (Galpha(q/11)) subunits and PLC-beta(1-4), -gamma, and -delta(1) isoforms were quantified by Western blot analysis in order to establish their contribution to the patterns of PLC functioning reported here. Quantitative measurements of the levels of Galpha(q/11) subunits in each region were obtained by comparison with known amounts of Escherichia coli expressed recombinant Galpha(q) subunits. Quantitative analysis indicated that Galpha(q/11) subunits are abundant polypeptides in human brain, with values ranging from about 1200 ng/mg in cerebral cortex to close to 900 ng/mg of membrane protein in caudate. In cerebral cortical membranes, the PLC-beta(1) isoform was more abundant than in caudate membranes. The highest levels of PLC-beta(2) expression were detected in caudate membranes. PLC-beta(3) was little expressed, and there were no significant differences in the relative values between both brain regions. Finally, the levels of the PLC-beta(4) isoform were significantly lower in caudate than in cortical membranes. It is concluded that although most of these data represent relative, not absolute, measures of protein levels within these regions, they contribute nonetheless to the significant differences observed in signaling capacities through the PLC system in both human brain regions.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

Distinct roles of Galpha(q) and Galpha11 for Purkinje cell signaling and motor behavior

G-protein-coupled metabotropic glutamate group I receptors (mGluR1s) mediate synaptic transmission and plasticity in Purkinje cells and, therefore, critically determine cerebellar motor control and learning. Purkinje cells express two members of the G-protein G(q) family, namely G(q) and G11. Although in vitro coexpression of mGluR1 with either Galpha11 or Galpha(q) produces equally well functioning signaling cascades, Galpha(q)- and Galpha11-deficient mice exhibit distinct alterations in motor coordination. By using whole-cell recordings and Ca2+ imaging in Purkinje cells, we show that Galpha(q) is required for mGluR-dependent synaptic transmission and for long-term depression (LTD). Galpha11 has no detectable contribution for synaptic transmission but also contributes to LTD. Quantitative single-cell RT-PCR analyses in Purkinje cells demonstrate a more than 10-fold stronger expression of Galpha(q) versus Galpha11. Our findings suggest an expression level-dependent action of Galpha(q) and Galpha11 for Purkinje cell signaling and assign specific roles of these two G(q) isoforms for motor coordination.

Insulin reduces the requirement for EGFR transactivation in bombesin-induced DNA synthesis

The binding of bombesin to its cognate G-protein coupled receptor stimulates quiescent Swiss 3T3 cells to re-initiate DNA synthesis and cell division. Addition of a non-mitogenic concentration of insulin dramatically potentiates bombesin-induced cell proliferation. We examined whether bombesin-induced EGFR transactivation mediates synergistic cell proliferation induced by bombesin and insulin. Treatment with selective EGFR tyrosine kinase inhibitors blocked EGFR transactivation, DNA synthesis, the transition of cells from quiescence into the cell cycle, and the expression of cyclins D1 and E induced by bombesin alone. In contrast, the inhibitors prevented cell cycle progression to a much lesser degree in cells stimulated with the combination of bombesin and insulin. Our results indicate that EGFR transactivation does not mediate synergistic cell proliferation induced by bombesin and insulin, and imply that insulin compensates for the requirement for EGFR transactivation in bombesin-induced DNA synthesis.

Galardin (GM 6001), a broad-spectrum matrix metalloproteinase inhibitor, blocks bombesin- and LPA-induced EGF receptor transactivation and DNA synthesis in rat-1 cells

Matrix metalloproteinases (MMPs) have been implicated in the transactivation of the epidermal growth factor receptor (EGFR) induced by G-protein coupled receptor (GPCR) agonists. Although EGFR phosphorylation and downstream signaling have been shown to be dependent on MMP activity in many systems, a role for MMPs in GPCR-induced DNA synthesis has not been studied in any detail. In this study we utilized the broad-spectrum matrix metalloproteinase inhibitor, galardin (Ilomastat, GM 6001), to study the mechanism of bombesin- or LPA-induced EGFR transactivation and the role of MMPs in early and late response mitogenic signaling in Rat-1 cells stably transfected with the bombesin/GRP receptor (BoR-15 cells). Addition of galardin to cells stimulated with bombesin or LPA specifically inhibited total EGFR phosphorylation, as well as site-specific phosphorylation of tyrosine 845, a putative Src phosphorylation site, and tyrosine 1068, a typical autophosphorylation site. Galardin treatment also inhibited extracellular signal-regulated kinase (ERK) activation induced by bombesin or LPA, but not by EGF. In addition, galardin inhibited bombesin- or LPA-induced DNA synthesis in a dose dependent manner, when stimulated by increasing concentrations of bombesin, and when added after bombesin stimulation. Furthermore, addition of galardin post-bombesin stimulation indicated that by 3 h sufficient accumulation of EGFR ligands had occurred to continue to induce transactivation despite an inhibition of MMP activity. Taken together, our results suggest that MMPs act as early as 5 min, and up to around 3 h, to mediate GPCR-induced EGFR transactivation, ERK activation, and stimulation of DNA synthesis.

G-proteins as transducers in transmembrane signalling

The G-protein-mediated signalling system has evolved as one of the most widely used transmembrane signalling mechanisms in mammalian organisms. All mammalian cells express G-protein-coupled receptors as well as several types of heterotrimeric G-proteins and effectors. G-protein-mediated signalling is involved in many physiological and pathological processes. This review summarizes some general aspects of G-protein-mediated signalling and focusses on recent data especially from studies in mutant mice which have elucidated some of the cellular and biological functions of heterotrimeric G-prtoteins.

Pasteurella multocida toxin facilitates inositol phosphate formation by bombesin through tyrosine phosphorylation of G alpha q

The intracellularly acting Pasteurella multocida toxin (PMT) is a potent mitogen that stimulates Gq-dependent formation of inositol trisphosphate. We show that PMT, a nontoxic mutant of PMT (PMTC1165S), and bombesin each stimulate time-dependent phosphorylation of G alpha q at tyrosine 349. Although PMT and PMTC1165S each cause phosphorylation of G alpha q, only the wild-type toxin activates Gq. Pretreatment of cells with wild-type or mutant PMT potentiated the formation of inositol phosphates stimulated by bombesin equally. These data show that PMT potentiates bombesin receptor signaling through tyrosine phosphorylation of Gq and distinguishes between the two proposed models of Gq activation, showing that tyrosine phosphorylation is not linked to receptor uncoupling.

Galphaq and Galpha11 proteins mediate endothelin-1 signaling in neural crest-derived pharyngeal arch mesenchyme

Endothelin-A (ET(A)) is a G-protein-coupled receptor expressed in the neural crest-derived mesenchyme of the pharyngeal arches during craniofacial development. Targeted deletion of the ET(A) receptor or its ligand endothelin-1 (ET-1) causes cleft palate and hypoplasia of the mandible, otic cup, and tympanic ring. Previously we showed that Galpha(q)/Galpha(11)-null mice die around E11.0, whereas Galpha(q)((-/-))Galpha(11)((+/-)) mice survive to birth with hypomorphic phenotypes similar to, but less severe than, ET(A) or ET-1-null mice. To determine whether ET-1 signaling is transduced by Galpha(q)/Galpha(11) proteins, we examined the expression patterns of several ET-1 dependent and independent transcription factors in Galpha(q)/Galpha(11)-deficient embryos. Expression of genes encoding the ET-1-dependent transcription factors Dlx3, Dlx6, dHAND, and eHAND was specifically downregulated in the pharyngeal arches of Galpha(q)/Galpha(11)-deficient mice. In contrast, pharyngeal arch expression of the homeobox gene Msx1, which is not regulated by ET-1 signaling, was maintained in these embryos. We conclude that the Galpha(q) and Galpha(11) proteins serve as the intracellular mediators of ET-1 signaling in the pharyngeal arch mesenchyme.

Functional specificity of G alpha q and G alpha 11 in the cholinergic and glutamatergic modulation of potassium currents and excitability in hippocampal neurons

In hippocampal and other cortical neurons, action potentials are followed by a slow afterhyperpolarization (sAHP) generated by the activation of small-conductance Ca(2+)-activated K(+) channels and controlling spike frequency adaptation. The corresponding current, the apamin-insensitive sI(AHP), is a well known target of modulation by different neurotransmitters, including acetylcholine (via M(3) receptors) and glutamate (via metabotropic glutamate receptor 5, mGluR(5)), in CA1 pyramidal neurons. The actions of muscarinic and mGluR agonists on sI(AHP) involve the activation of pertussis toxin-insensitive G-proteins. However, the pharmacological tools available so far did not permit the identification of the specific G-protein subtypes transducing the effects of M(3) and mGluR(5) on sI(AHP). In the present study, we used mice deficient in the Galpha(q) and Galpha(11) genes to investigate the specific role of these G-protein alpha subunits in the cholinergic and glutamatergic modulation of sI(AHP) in CA1 neurons. In mice lacking Galpha(q), the effects of muscarinic and glutamatergic agonists on sI(AHP) were nearly abolished, whereas beta-adrenergic agonists acting via Galpha(s) were still fully effective. Modulation of sI(AHP) by any of these agonists was instead unchanged in mice lacking Galpha(11). The additional depolarizing effects of muscarinic and glutamatergic agonists on CA1 neurons were preserved in mice lacking Galpha(q) or Galpha(11). Thus, Galpha(q), but not Galpha(11), mediates specifically the action of cholinergic and glutamatergic agonists on sI(AHP), without affecting the modulation of other currents. These results provide to our knowledge one of the first examples of the functional specificity of Galpha(q) and Galpha(11) in central neurons.

Neuropeptide-induced androgen independence in prostate cancer cells: roles of nonreceptor tyrosine kinases Etk/Bmx, Src, and focal adhesion kinase

The bombesin/gastrin-releasing peptide (GRP) family of neuropeptides has been implicated in various in vitro and in vivo models of human malignancies including prostate cancers. It was previously shown that bombesin and/or neurotensin (NT) acts as a survival and migratory factor(s) for androgen-independent prostate cancers. However, a role in the transition from an androgen-dependent to -refractory state has not been addressed. In this study, we investigate the biological effects and signal pathways of bombesin and NT on LNCaP, a prostate cancer cell line which requires androgen for growth. We show that both neurotrophic factors can induce LNCaP growth in the absence of androgen. Concurrent transactivation of reporter genes driven by the prostate-specific antigen promoter or a promoter carrying an androgen-responsive element (ARE) indicate that growth stimulation is accompanied by androgen receptor (AR) activation. Furthermore, neurotrophic factor-induced gene activation was also present in PC3 cells transfected with the AR but not in the parental line which lacks the AR. Given that bombesin does not directly bind to the AR and is known to engage a G-protein-coupled receptor, we investigated downstream signaling events that could possibly interact with the AR pathway. We found that three nonreceptor tyrosine kinases, focal adhesion kinase (FAK), Src, and Etk/BMX play important parts in this process. Etk/Bmx activation requires FAK and Src and is critical for neurotrophic factor-induced growth, as LNCaP cells transfected with a dominant-negative Etk/BMX fail to respond to bombesin. Etk's activation requires FAK, Src, but not phosphatidylinositol 3-kinase. Likewise, bombesin-induced AR activation is inhibited by the dominant-negative mutant of either Src or FAK. Thus, in addition to defining a new G-protein pathway, this report makes the following points regarding prostate cancer. (i) Neurotrophic factors can activate the AR, thus circumventing the normal growth inhibition caused by androgen ablation. (ii) Tyrosine kinases are involved in neurotrophic factor-mediated AR activation and, as such, may serve as targets of future therapeutics, to be used in conjunction with current antihormone and antineuropeptide therapies.

Reduced phospholipase C-beta activity and isoform expression in the cerebellum of TS65Dn mouse: a model of Down syndrome

Agonist- and guanine-nucleotide-stimulated phospholipase C-beta (PLC) activity was characterized in crude plasma membrane preparations from cerebral cortex, hippocampus and cerebellum of Ts65Dn mice, a model for Down syndrome, and their control littermates. The levels of expression of PLC-beta((1-4)) isoforms and G-protein alpha(q/11) subunits were also quantified by Western blot analysis to establish their contribution to the patterns of PLC functioning. PLC activity regulated by G-proteins and muscarinic and 5-HT(2) receptors presented a regional distribution in both control and Ts65Dn mice. In both groups of mice, the intensity of PLC responses to maximal activation by calcium followed the sequence cerebellum > cortex > hippocampus. Both basal and maximal PLC activities, however, were significantly lower in cerebellar membranes of Ts65Dn than in control mice. This difference was mostly revealed in crude plasma membranes prepared from cerebellum at the level of G-protein-dependent-PLC activity because the concentration-response curve to GTPgammaS showed a reduction of the maximal effect in Ts65Dn mice, with no change in sensitivity (EC(50)). Western blot analysis showed a heterogeneous distribution of PLC-beta((1-4)) isoforms in both groups of mice. The levels of PLC-beta4 isoform, however, were significantly lower in the cerebellum of Ts65Dn than in control mice. We conclude that the cerebellum of Ts65Dn mice has severe deficiencies in PLC activity stimulated by guanine nucleotides, which are specifically related to a lower level of expression of the PLC-beta4 isoform, a fact that may account for the neurological phenotype observed in this murine model of Down syndrome.

Protein kinase D potentiates DNA synthesis and cell proliferation induced by bombesin, vasopressin, or phorbol esters in Swiss 3T3 cells

We examined whether protein kinase D (PKD) overexpression in Swiss 3T3 cells potentiates the proliferative response to either the G protein-coupled receptor agonists bombesin and vasopressin or the biologically active phorbol ester phorbol 12,13-dibutyrate (PDBu). In order to generate Swiss 3T3 cells stably overexpressing PKD, cultures of these cells were infected with retrovirus encoding murine PKD and green fluorescent protein (GFP) expressed as two separate proteins translated from the same mRNA. GFP was used as a marker for selection of PKD-positive cells. PKD overexpressed in Swiss 3T3 cells was dramatically activated by cell treatment with bombesin or PDBu as judged by in vitro kinase autophosphorylation assays and exogenous substrate phosphorylation. Concomitantly, these stimuli induced PKD phosphorylation at Ser(744), Ser(748), and Ser(916). PKD activation and phosphorylation were prevented by exposure of the cells to protein kinase C-specific inhibitors. Addition of bombesin, vasopressin, or PDBu to cultures of Swiss 3T3 cells overexpressing PKD induced a striking increase in DNA synthesis and cell number compared with cultures of Swiss 3T3-GFP cells. In contrast, stimulation of DNA synthesis in response to epidermal growth factor, which acts via protein kinase C/PKD-independent pathways, was not enhanced. Our results demonstrate that overexpression of PKD selectively potentiates mitogenesis induced by bombesin, vasopressin, or PDBu in Swiss 3T3 cells.

EGF receptor function is required in late G(1) for cell cycle progression induced by bombesin and bradykinin

We examined the role of epidermal growth factor (EGF) receptor (EGFR) tyrosine kinase activation in G protein-coupled receptor (GPCR) agonist-induced mitogenesis in Swiss 3T3 and Rat-1 cells. Addition of EGFR tyrosine kinase inhibitors (e.g., tyrphostin AG-1478) abrogated bombesin-induced extracellular signal-regulated kinase (ERK) activation in Rat-1 cells but not in Swiss 3T3 cells, indicating the importance of cell context in determining the role of EGFR in ERK activation. In striking contrast, treatment with tyrphostin AG-1478 markedly (~70%) inhibited DNA synthesis induced by bombesin in both Swiss 3T3 and Rat-1 cells. Similar inhibition of bombesin-induced DNA synthesis in Swiss 3T3 cells was obtained using four structurally different inhibitors of EGFR tyrosine kinase. Furthermore, kinetic analysis indicates that EGFR function is necessary for bombesin-induced mitogenesis in mid-late G(1) in both Swiss 3T3 and Rat-1 cells. Our results indicate that EGFR kinase activity is necessary in mid-late G(1) for promoting the accumulation of cyclins D1 and E and implicate EGFR function in the coupling of GPCR signaling to the activation of the cell cycle.

Bombesin and substance P analogues differentially regulate G-protein coupling to the bombesin receptor. Direct evidence for biased agonism

Substance P analogues including [d-Arg1,d-Phe5,d-Trp7,9,Leu11]substance P (SpD) act as "broad spectrum neuropeptide antagonists" and are potential anticancer agents that inhibit the growth of small cell lung cancer cells in vitro and in vivo. However, their mechanism of action is controversial and not fully understood. Although these compounds block bombesin-induced mitogenesis and signal transduction, they also have agonist activity. The mechanism underlying this agonist activity was examined. SpD binds to the ligand-binding site of the bombesin/gastrin-releasing peptide receptor and blocks the bombesin-stimulated increase in [Ca2+]i within the same concentration range that causes sustained activation of c-Jun N-terminal kinase and extracellular signal-regulated protein kinase (ERK). The activation of c-Jun N-terminal kinase by SpD and bombesin is blocked by dominant negative inhibition of G(alpha12). The ERK activation by SpD is pertussis toxin-sensitive in contrast to ERK activation by bombesin, which is pertussis toxin-insensitive but dependent on epidermal growth factor receptor phosphorylation. SpD does not simply act as a partial agonist but differentially modulates the activation of the G-proteins G(alpha12), G(i), and G(q) compared with bombesin. This unique ability allows the bombesin receptor to couple to G(i) and at the same time block receptor activation of G(q). Our results provide direct evidence that SpD is acting as a "biased agonist" and that this has physiological relevance in small cell lung cancer cells. This validation of the concept of biased agonism has important implications in the development of novel pharmacological agents to dissect receptor-mediated signal transduction and of highly selective drugs to treat human disease.

Neurotensin excitation of serotonergic neurons in the rat nucleus raphe magnus: ionic and molecular mechanisms

To understand the cellular and molecular mechanisms by which neurotensin (NT) induces an analgesic effect in the nucleus raphe magnus (NRM), whole-cell patch-clamp recordings were performed to investigate the electrophysiological effects of NT on acutely dissociated NRM neurons. Two subtypes of neurons, primary serotonergic and secondary non-serotonergic cells, were identified from acutely isolated NRM neurons. During current-clamp recordings, NT depolarized NRM serotonergic neurons and evoked action potentials. Voltage-clamp recordings showed that NT excited serotonergic neurons by enhancing a voltage-insensitive and non-selective cationic conductance. Both SR48692, a selective antagonist of subtype 1 neurotensin receptor (NTR-1), and SR 142948A, a non-selective antagonist of NTR-1 and subtype 2 neurotensin receptor (NTR-2), failed to prevent neurotensin from exciting NRM serotonergic neurons. NT-evoked cationic current was inhibited by the intracellular administration of GDP-beta-S. NT failed to induce cationic currents after dialyzing serotonergic neurons with the anti-G(alphaq/11) antibody. Cellular Ca(2+) imaging study using fura-2 showed that NT induced the calcium release from the intracellular store. NT-evoked current was blocked after the internal perfusion of heparin, an IP(3) receptor antagonist, or BAPTA, a fast Ca(2+) chelator. It is concluded that neurotensin enhancement of the cationic conductance of NRM serotonergic neurons is mediated by a novel subtype of neurotensin receptors. The coupling mechanism via G(alphaq/11) proteins is likely to involve the generation of IP(3), and subsequent IP(3)-evoked Ca(2+) release from intracellular stores results in activating the non-selective cationic conductance.

Fuel and hormone regulation of phospholipase C beta 1 and delta 1 overexpressed in RINm5F pancreatic beta cells

The mechanism by which glucose and other fuels stimulate phosphoinositide-specific phospholipase C (PLC) in pancreatic islet beta cells is not known. Previous studies have suggested that glucose may couple to PLC beta 1 and PLC delta 1. To determine directly if fuels activate these PLC isozymes, clones stably overexpressing PLC beta 1 or PLC delta 1 were generated in the fuel-sensitive beta cell line RINm5F, and secretagogue regulation of these PLC isoforms was determined. Overexpression of PLC beta 1 or PLC delta 1 significantly increased PLC activity in isolated cell fractions, consistent with overexpression of active PLC isoforms in these clones. In paired experiments, stimulation of inositol phosphate (IP) accumulation by the fuel glyceraldehyde was enhanced in clones overexpressing PLC beta 1, in parallel with the G-protein alpha subunit activator, AlF(4)(-), suggesting a coupling between glyceraldehyde and this PLC isoform. In contrast, overexpression of PLC delta 1 had no effect on glyceraldehyde- or AlF(4)(-)-stimulated IP accumulation. Similarly, IP accumulation stimulated by ionomycin was enhanced in PLC beta 1, but not PLC delta 1 clones, indicating that increases in intracellular free calcium [Ca(2+)](i) can regulate PLC beta 1 but not PLC delta 1 overexpressed in this cell line. Interestingly, [Arg(8)] vasopressin-stimulated, but not carbachol-stimulated, IP accumulation was significantly increased in clones overexpressing either PLC beta 1 or PLC delta 1. These studies illustrate unique pathways coupling diverse secretagogues to specific PLC isoforms in islet beta cells, and demonstrate that glyceraldehyde can activate PLC beta 1 but not PLC delta 1; whereas, vasopressin, but not carbachol, can stimulate either isoform.

Neurotensin excites periaqueductal gray neurons projecting to the rostral ventromedial medulla

Microinjection of neurotensin into the midbrain periaqueductal gray (PAG) produces a potent and naloxone-insensitive analgesic effect. To test the hypothesis that neurotensin induces the analgesic effect by activating the PAG-rostral ventromedial medulla (RVM) descending antinociceptive pathway, PAG neurons that project to RVM (PAG-RVM) were identified by microinjecting DiI(C18), a retrograde tracing dye, into the rat RVM. Subsequently, fluorescently labeled PAG-RVM projection neurons were acutely dissociated and selected for whole cell patch-clamp recordings. During current-clamp recordings, neurotensin depolarized retrogradely labeled PAG-RVM neurons and evoked action potentials. Voltage-clamp recordings indicated that neurotensin excited PAG-RVM neurons by opening the voltage-insensitive and nonselective cation channels. Both SR 48692, a selective NTR-1 antagonist, and SR 142948A, a nonselective antagonist of NTR-1 and NTR-2, failed to prevent neurotensin from exciting PAG-RVM neurons. Neurotensin failed to evoke cationic currents after internally perfusing PAG-RVM projection neurons with GDP-beta-S or anti-G(alpha q/11) antiserum. Cellular Ca(2+) fluorescence measurement using fura-2 indicated that neurotensin rapidly induced Ca(2+) release from intracellular stores of PAG-RVM neurons. Neurotensin-evoked cationic currents were blocked by heparin, an IP(3) receptor antagonist, and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a fast chelator of Ca(2+). These results suggest that by activating a novel subtype of neurotensin receptors, neurotensin depolarizes and excites PAG-RVM projection neurons through enhancing Ca(2+)-dependent nonselective cationic conductance. The coupling mechanism via G(alpha q/11) proteins is likely to involve the production of IP(3), and subsequent IP(3)-evoked Ca(2+) release leads to the opening of nonselective cation channels.

Rho GTPase-dependent transformation by G protein-coupled receptors

G protein coupled receptors (GPCRs) constitute the largest family of cell surface receptors, with more than 1000 members, and are responsible for converting a diverse array of extracellular stimuli into intracellular signaling events. Most members of the family have defined roles in intermediary metabolism and generally perform these functions in well-differentiated cells. However, there is an increasing awareness that some GPCRs can also regulate proliferative signaling pathways and that chronic stimulation or mutational activation of receptors can lead to oncogenic transformation. Activating mutations in GPCRs are associated with several types of human tumors and some receptors exhibit potent oncogenic activity due to agonist overexpression. Additionally, expression screening analyses for novel oncogenes identified GPCRs whose expression causes the oncogenic transformation of NIH3T3 mouse fibroblasts. These include Mas, G2A, and the PAR-1 thrombin receptor. In this review we summarize the signaling and transforming properties of these GPCR oncoproteins. What has emerged from these studies is the delineation of a GTPase cascade where transforming GPCRs cause aberrant growth regulation via activation of Rho family small GTPases.

Pleiotropic effects of Pasteurella multocida toxin are mediated by Gq-dependent and -independent mechanisms. involvement of Gq but not G11

Pasteurella multocida toxin (PMT) is a highly potent mitogen for a variety of cell types. PMT has been shown to induce various cellular signaling processes, and it has been suggested to function through the heterotrimeric G-proteins G(q)/G(11). To analyze the role of G(q)/G(11) in the action of PMT, we have studied the effect of the toxin in Galpha(q)/Galpha(11) double-deficient fibroblasts as well as in fibroblasts lacking only Galpha(q) or Galpha(11). Interestingly, formation of inositol phosphates in response to PMT was exclusively dependent on Galpha(q) but not on the closely related Galpha(11). Although Galpha(q)/Galpha(11) double-deficient and Galpha(q)-deficient cells did not respond with any production of inositol phosphates to PMT, PMT was still able to induce various other cellular effects in these cells, including the activation of Rho, the Rho-dependent formation of actin stress fibers and focal adhesions, as well as the stimulation of c-Jun N-terminal kinase and extracellular signal-regulated kinase. These data show that PMT leads to a variety of cellular effects that are mediated only in part by the heterotrimeric G-protein G(q).

Cell cycle-dependent coupling of the vasopressin V1a receptor to different G proteins

Arginine vasopressin (AVP) regulates biological processes by binding to G protein-coupled receptors. In Swiss 3T3 fibroblasts, expressing the V(1a) subtype of vasopressin receptors, AVP mobilizes calcium from intracellular stores. In proliferating cells, the AVP-induced increase in intracellular calcium concentration ([Ca(2+)](i)) was mediated by G proteins of the G(q) family, which are insensitive to pertussis toxin (PTX) pretreatment of the cells. In quiescent cells, the AVP-induced increase in [Ca(2+)](i) was partially PTX-sensitive, suggesting an involvement of G(i) proteins. We confirmed this by photoaffinity labeling of G proteins in Swiss 3T3 cell membranes activated by AVP. In Swiss 3T3 cells arrested in the G(0)/G(1) phase of the cell cycle, the AVP-induced increase in [Ca(2+)](i) was also partially PTX-sensitive but was PTX-insensitive in cells arrested in other phases of the cell cycles. The blocking effect of PTX pretreatment in G(0)/G(1) cells was mimicked by microinjection of antisense oligonucleotides suppressing the expression of the Galpha(i3) subunits. These results were confirmed by microinjection of antibodies directed against the C terminus of G protein alpha-subunits. The data presented indicate that in Swiss 3T3 fibroblasts synchronized in the G(0)/G(1) phase of the cell cycle the V(1a) receptor couples to G(q/11) and G(i3) to activate the phospholipase C-beta, leading to release of intracellular calcium.

[D-Arg(1),D-Trp(5,7,9),Leu(11)]Substance P inhibits bombesin-induced mitogenic signal transduction mediated by both G(q) and G(12) in Swiss 3T3cells

Substance P (SP) analogues including [d-Arg(1),d-Trp(5,7,9), Leu(11)]SP are broad spectrum neuropeptide antagonists and potential anticancer agents, but their mechanism of action is not fully understood. Here, we examined the mechanism of action of [d-Arg(1), d-Trp(5,7,9),Leu(11)]SP as an inhibitor of G protein-coupled receptor (GPCR)-mediated signal transduction and cellular DNA synthesis in Swiss 3T3 cells. Addition of [d-Arg(1),d-Trp(5,7,9), Leu(11)]SP, at 10 micrometer, caused a striking rightward shift in the dose-response curves of DNA synthesis induced by bombesin, bradykinin, or vasopressin and markedly inhibited the activation of p42(mapk) (ERK-2) and p44(mapk) (ERK-1) induced by these GPCR agonists. In addition, this SP analogue also prevented the protein kinase C-dependent activation of protein kinase D induced by these agonists. [d-Arg(1),d-Trp(5,7,9),Leu(11)]SP, at a concentration (10 micrometer) that inhibited these G(q)-mediated events, also prevented GPCR agonist-induced responses mediated through the G proteins of the G(12) subfamily. These include bombesin-induced assembly of focal adhesions, formation of parallel arrays of actin stress fibers, increase in the tyrosine phosphorylation of focal adhesion kinase (FAK), p130(Cas), and paxillin, and formation of a complex between FAK and Src. We conclude that [d-Arg(1),d-Trp(5,7,9),Leu(11)]SP acts as a mitogenic antagonist of neuropeptide GPCRs blocking signal transduction via both G(q) and G(12).

Bombesin and platelet-derived growth factor induce association of endogenous focal adhesion kinase with Src in intact Swiss 3T3 cells

Stimulation of quiescent Swiss 3T3 cells with bombesin induces a rapid increase in the formation of complexes between focal adhesion kinase (FAK) and Src family members, which can be extracted with a buffer containing Triton, deoxycholate, and SDS but not with a buffer containing Triton alone. An increase in complex formation between FAK and Src in response to bombesin could be detected within 1 min, reached a maximum after 10 min, and declined toward base-line levels after 60 min of bombesin treatment. Bradykinin, endothelin, and lysophosphatidic acid also stimulated FAK-Src complex formation. Bombesin stimulated FAK/Src association through a Ca(2+) and phosphatidylinositol 3'-kinase-independent pathway that requires the integrity of the actin filament network and is partly dependent on functional protein kinase C. Treatment with the selective Src kinase inhibitor PP-2 inhibited both FAK activation and phosphorylation of FAK at Tyr(577) induced by bombesin in intact cells. Platelet-derived growth factor at low concentrations (1-10 ng/ml) also induced FAK-Src complex formation via a pathway that depended on the integrity of the actin cytoskeleton and phosphatidylinositol 3'-kinase. Thus, G protein-coupled receptor agonists and platelet-derived growth factor promote complex formation between endogenous FAK and Src in attached cells through different signal transduction pathways.

Distribution of signaling molecules involved in vasopressin-induced Ca2+ mobilization in rat hepatocyte multiplets

In freshly isolated rat hepatocyte multiplets, Ca2+ signals in response to vasopressin are highly organized. In this study we used specific probes to visualize, by fluorescence and confocal microscopy, the main signaling molecules involved in vasopressin-mediated Ca2+ responses. V1a receptors were detected with a novel fluorescent antagonist, Rhm8-PVA. The Galphaq/Galpha11, PLCbeta3, PIP2, and InsP3 receptors were detected with specific antibodies. V1a vasopressin receptors and PIP2 were associated with the basolateral membrane and were not detected in the bile canalicular domain. Galphaq/Galpha11, PLCbeta3, and InsP3 receptors were associated with the basolateral membrane and also with other intracellular structures. We used double labeling, Western blotting, and drugs (cytochalasin D, colchicine) known to disorganize the cytoskeleton to demonstrate the partial co-localization of Galphaq/Galpha11 with F-actin.

Embryonic cardiomyocyte hypoplasia and craniofacial defects in G alpha q/G alpha 11-mutant mice

Heterotrimeric G proteins of the Gq class have been implicated in signaling pathways regulating cardiac growth under physiological and pathological conditions. Knockout mice carrying inactivating mutations in both of the widely expressed G alpha q class genes, G alpha q and G alpha 11, demonstrate that at least two active alleles of these genes are required for extrauterine life. Mice carrying only one intact allele [G alpha q(-/+);G alpha 11(-/-) or G alpha q(-/-);G alpha 11(-/+)] died shortly after birth. These mutants showed a high incidence of cardiac malformation. In addition, G alpha q(-/-);G alpha 11(-/+) newborns suffered from craniofacial defects. Mice lacking both G alpha q and G alpha 11 [G alpha q(-/-);G alpha 11(-/-)] died at embryonic day 11 due to cardiomyocyte hypoplasia. These data demonstrate overlap in G alpha q and G alpha 11 gene functions and indicate that the Gq class of G proteins plays a crucial role in cardiac growth and development.

Expression of G-protein subtypes in cultured cerebral endothelial cells

This paper describes Western-blotting evidence for the presence of various guanine nucleotide binding proteins, G-proteins in cultured rat cerebral endothelial cells (CECs) and two immortalized cerebral endothelial cell lines, RBE4 and GP8. By using specific antibodies raised against known sequences of appropriate G-protein types that were previously characterized, we demonstrated the presence of Gsalpha, Gi2alpha, Gi3alpha, Gq/11alpha, Goalpha and Gbeta in cell lysates of primary cultures of CECs, and plasma membranes of RBE4 and GP8 cells. The appearance of Goalpha proteins in CECs might be of special importance, since they were not detected in peripheral endothelial cells in previous studies. Isoproterenol and bradykinin displayed significant, dose-dependent stimulation of [35S]GTPgammaS binding above basal values. This assay, reflecting the GDP-GTP exchange reaction on Galpha-subunits by receptor agonists, suggested that there were functional, G-protein coupled beta-adrenergic and bradykinin receptors in these systems. No significant stimulation of [35S]GTP7gammaS binding was noted with serotonin under our experimental conditions. Since stimulation of [35S]GTPgammaS binding by isoproterenol and bradykinin was additive, it was concluded that different Galpha proteins were activated by these two ligands. In analogy to other systems, activation of Gs is most likely by isoproterenol, while Gi and/or Gq/11 proteins might be activated by bradykinin receptors. The possible significance of the receptors and G-proteins detected is being discussed in the functioning of cerebral endothelium, and thus the blood-brain barrier.

Galpha14 and Galphaq mediate the response to trypsin in Xenopus oocytes

Xenopus oocytes respond to trypsin with a characteristic chloride current, virtually indistinguishable from responses mediated by a large number of native and expressed G protein-coupled receptors. We studied the involvement of G proteins of the Galphaq family as possible mediators of this and other G protein-coupled receptor-mediated responses in Xenopus oocytes. We have cloned the third member of the Galphaq family, Xenopus Galpha14, in addition to the previously cloned Xenopus Galphaq and Galpha11 (Shapira, H., Way, J., Lipinsky, D., Oron, Y., and Battey, J. F. (1994) FEBS Lett. 348, 89-92). Amphibian Galpha14 is 354 amino acids long and is 93% identical to its mammalian counterpart. Based on the Galpha14 cDNA sequence, we designed a specific antisense DNA oligonucleotide (antiGalpha14) that, together with antiGalphaq and antiGalpha11, was used in antisense depletion experiments. 24 h after injection into oocytes, either antiGalphaq or antiGalpha14 reduced the response to 1 microg/ml trypsin by 70%, whereas antiGalpha11 had no effect. A mixture of antiGalphaq and antiGalpha14 virtually abolished the response. These data strongly suggest that Galphaq and Galpha14 are the exclusive mediators of the trypsin-evoked response in Xenopus oocytes. Similar experiments with the expressed gastrin-releasing peptide receptor and muscarinic m1 receptor revealed the coupling of Galphaq and Galpha11, but not Galpha14, to these receptors in oocytes. These results confirm the hypothesis that endogenous members of the Galphaq family discriminate among different native receptors in vivo.

Bombesin, vasopressin, lysophosphatidic acid, and sphingosylphosphorylcholine induce focal adhesion kinase activation in intact Swiss 3T3 cells

Treatment of quiescent Swiss 3T3 cells with bombesin rapidly increased focal adhesion kinase (FAK)-associated tyrosine kinase activity in immune complexes. The effect was rapid (maximum at 2.5 min) and dose dependent (half-maximum response at 0.05 nM). Addition of vasopressin, lysophosphatidic acid, and sphingosylphosphorylcholine also elicited a rapid increase in FAK-associated tyrosine kinase activity. Addition of the selective Src inhibitor pyrazolopyrimidine directly to the in vitro kinase assay potently inhibited Src kinase activity induced by bombesin but did not affect the kinase activity of FAK measured by autophosphorylation or by synthetic substrate phosphorylation in paralell assays. In addition, Src activity was not detected in FAK immunoprecipitates using an optimal Src peptide substrate. Thus, agonist-induced tyrosine kinase activity measured in FAK immunoprecipitates is mediated by FAK activation rather than by co-immunoprecipitating Src. Bombesin-induced FAK activation is not dependent either on protein kinase C or Ca2+ mobilization but was completely blocked by treatment with cytochalasin D or by placing the cells in suspension. These findings indicate that FAK activation requires an intact actin cytoskeleton. Our results demonstrate that agonists that act via 7-transmembrane domain receptors stimulate FAK kinase activation.

Mas oncogene signaling and transformation require the small GTP-binding protein Rac

The Mas oncogene encodes a novel G-protein-coupled receptor that was identified originally as a transforming protein when overexpressed in NIH 3T3 cells. The mechanism and signaling pathways that mediate Mas transformation have not been determined. We observed that the foci of transformed NIH 3T3 cells caused by Mas were similar to those caused by activated Rho and Rac proteins. Therefore, we determined if Mas signaling and transformation are mediated through activation of a specific Rho family protein. First, we observed that, like activated Rac1, Mas cooperated with activated Raf and caused synergistic transformation of NIH 3T3 cells. Second, both Mas- and Rac1-transformed NIH 3T3 cells retained actin stress fibers and showed enhanced membrane ruffling. Third, like Rac, Mas induced lamellipodium formation in porcine aortic endothelial cells. Fourth, Mas and Rac1 strongly activated the JNK and p38, but not ERK, mitogen-activated protein kinases. Fifth, Mas and Rac1 stimulated transcription from common DNA promoter elements: NF-kappaB, serum response factor (SRF), Jun/ATF-2, and the cyclin D1 promoter. Finally, Mas transformation and some of Mas signaling (SRF and cyclin D1 but not NF-kappaB activation) were blocked by dominant negative Rac1. Taken together, these observations suggest that Mas transformation is mediated in part by activation of Rac-dependent signaling pathways. Thus, Rho family proteins are common mediators of transformation by a diverse variety of oncogene proteins that include Ras, Dbl family, and G-protein-coupled oncogene proteins.

Impaired motor coordination and persistent multiple climbing fiber innervation of cerebellar Purkinje cells in mice lacking Galphaq

Mice lacking the alpha-subunit of the heterotrimeric guanine nucleotide binding protein Gq (Galphaq) are viable but suffer from ataxia with typical signs of motor discoordination. The anatomy of the cerebellum is not overtly disturbed, and excitatory synaptic transmission from parallel fibers to cerebellar Purkinje cells (PCs) and from climbing fibers (CFs) to PCs is functional. However, about 40% of adult Galphaq mutant PCs remain multiply innervated by CFs because of a defect in regression of supernumerary CFs in the third postnatal week. Evidence is provided suggesting that Galphaq is part of a signaling pathway that is involved in the elimination of multiple CF innervation during this period.

Signal characteristics of G protein-transactivated EGF receptor

The epidermal growth factor receptor (EGFR) tyrosine kinase recently was identified as providing a link to mitogen-activated protein kinase (MAPK) in response to G protein-coupled receptor (GPCR) agonists in Rat-1 fibroblasts. This cross-talk pathway is also established in other cell types such as HaCaT keratinocytes, primary mouse astrocytes and COS-7 cells. Transient expression of either Gq- or Gi-coupled receptors in COS-7 cells allowed GPCR agonist-induced EGFR transactivation, and lysophosphatidic acid (LPA)-generated signals involved the docking protein Gab1. The increase in SHC tyrosine phosphorylation and MAPK stimulation through both Gq- and Gi-coupled receptors was reduced strongly upon selective inhibition of EGFR function. Inhibition of phosphoinositide 3-kinase did not affect GPCR-induced stimulation of EGFR tyrosine phosphorylation, but inhibited MAPK stimulation, upon treatment with both GPCR agonists and low doses of EGF. Furthermore, the Src tyrosine kinase inhibitor PP1 strongly interfered with LPA- and EGF-induced tyrosine phosphorylation and MAPK activation downstream of EGFR. Our results demonstrate an essential role for EGFR function in signaling through both Gq- and Gi-coupled receptors and provide novel insights into signal transmission downstream of EGFR for efficient activation of the Ras/MAPK pathway.

Gastrin-releasing peptide-induced expression of prostaglandin synthase-2 in Swiss 3T3 cells

Prostaglandins, produced in response to mitogens and cytokines, are potent modulators of gastrointestinal physiology and pathophysiology. We investigated modulation of Prostaglandin synthase 2 (PGS-2) expression by the gastrin-releasing peptide (GRP) receptor in Swiss 3T3 cells. PGS-2 mRNA expression in Swiss 3T3 cells was determined by Northern blot analysis. PGS-2 protein expression in Swiss 3T3 cells was measured by Western blot analysis. GRP caused a transient induction of PGS-2 mRNA in Swiss 3T3 cells that resulted in GRP-dependent expression of PGS-2 protein. Transcriptional activation of PGS-2 by GRP was independent of de novo protein synthesis and was not affected by pertussis toxin. Comparison of signaling pathways used by PMA or EGF to those used by GRP showed that PGS-2 induction by GRP increased under conditions that inhibit PKC activity. Dexamethasone, which blocks PMA and EGF induction of PGS-2, also inhibited GRP-induced accumulation of PGS-2 mRNA. These results show that PGS-2 expression in Swiss 3T3 cells is not only controlled by PKC and receptor tyrosine kinase pathways but also by G-protein coupled receptor signaling pathways.

Stimulatory effect of bombesin on phosphoinositide metabolism in the rat pineal gland

The pineal gland is under complex peptidergic nervous control originating from hypothalamic nuclei. The daily rhythm of bombesin-like peptide in the hypothalamus suggests that this neuropeptide, similarly as other neuropeptides, might be involved in modulation of the physiological activity of the pineal gland. In our experiments we studied the mechanism of signal transduction of bombesin in the isolated pineal glands of rats. The phosphoinositide signalling system was examined by measuring 32P-labelling of phosphatidylinositol (PI), phosphatidylinositol phosphate (PIP) and phosphatidylinositol bisphosphate (PIP2), which reflects phosphoinositide cycle activation. Bombesin induced a significant increase in 32P-labelling of PI, PIP and PIP2. The antagonist of this neuropeptide, (D-Phe12-Leu14)-bombesin, suppressed the increase in 32P-labelling of all phosphoinositides. Bombesin was without effect on cAMP dependent protein phosphorylation. The data indicate that bombesin activates the PI signalling system via specific receptors.

Bombesin, vasopressin, endothelin, bradykinin, and platelet-derived growth factor rapidly activate protein kinase D through a protein kinase C-dependent signal transduction pathway

Protein kinase D (PKD) is a serine/threonine protein kinase that is activated by phorbol esters via protein kinase C in intact cells. To assess the physiological significance of this putative pathway, we examined the regulation of PKD in living cells by mitogenic regulatory peptides and by platelet-derived growth factors (PDGF). Our results demonstrate that bombesin rapidly induces PKD activation in Swiss 3T3 cells, as shown by autophosphorylation and syntide-2 phosphorylation assays. Maximum PKD activation (14-fold above base-line levels) was obtained 90 s after bombesin stimulation. Bombesin also induced PKD activation in Rat-1 cells stably transfected with the bombesin/gastrin releasing peptide (GRP) receptor and in COS-7 cells transiently co-transfected with PKD and bombesin/GRP receptor expression constructs. No inducible kinase activity was demonstrated when COS-7 cells were transfected with a kinase-deficient PKD mutant. Bombesin-mediated PKD activation was prevented by treatment of Swiss 3T3 cells with the protein kinase C inhibitors GF 1092030X and Ro 31-8220. In contrast, these compounds did not inhibit PKD activity when added directly in vitro. Vasopressin, endothelin, and bradykinin also activated PKD in Swiss 3T3 cells through a PKC-dependent pathway. Platelet-derived growth factor-stimulated PKD activation in Swiss 3T3 cells and in porcine aortic endothelial cells stably transfected with PDGF-beta receptors. Treatment with GF 1092030X or Ro 31-8220 inhibited PKD activation induced by PDGF. Thus, our results indicate that PKD is activated by multiple signaling peptides through a protein kinase C-dependent signal transduction pathway in a variety of cell types.

[D-Arg1,D-Trp5,7,9,Leu11]Substance P coordinately and reversibly inhibits bombesin- and vasopressin-induced signal transduction pathways in Swiss 3T3 cells

The novel substance P (SP) analogue, [D-Arg1,D-Trp5,7,9,Leu11]SP like [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP inhibited DNA synthesis induced by bombesin, vasopressin, and bradykinin, but did not interfere with the mitogenic response induced by other growth factors or pharmacological agents in Swiss 3T3 cells. [D-Arg1,D-Trp5, 7,9,Leu11]SP reversibly inhibited bombesin-induced DNA synthesis, causing a 6-fold greater rightward shift in the bombesin dose response than [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP at identical concentrations (10 microM). We found that the new, more potent, SP analogue coordinately and reversibly inhibited bombesin-induced Ca2+ mobilization and protein kinase C (PKC) and mitogen-activated protein (MAP) kinase activation. The dose-response curves for bombesin-induced Ca2+ mobilization and MAP kinase activation were similarly displaced (51- and 40-fold, respectively) by [D-Arg1, D-Trp5,7,9,Leu11]SP. In addition, [D-Arg1,D-Trp5,7,9,Leu11]SP reversibly inhibited bombesin-induced tyrosine phosphorylation of Mr 110,000-130,000 and 70,000-80,000 bands as well as p125 focal adhesion kinase. [D-Arg1,D-Trp5,7,9,Leu11]SP also reversibly and coordinately inhibited vasopressin-induced Ca2+ mobilization, PKC stimulation, MAP kinase activation, tyrosine phosphorylation, and DNA synthesis in Swiss 3T3 cells. Surprisingly, deletion of the terminal Leu of [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP to yield [D-Arg1, D-Phe5,D-Trp7,9]SP1-10 resulted in a selective loss of inhibitory activity of this analogue against bombesin- but not vasopressin-stimulated DNA synthesis, Ca2+ mobilization, and MAP kinase activation. Collectively, these results suggest that SP analogues act at the receptor level to coordinately and reversibly antagonize bombesin- or vasopressin-induced signal transduction in Swiss 3T3 cells.

Specificity of G alpha q and G alpha 11 gene expression in platelets and erythrocytes. Expressions of cellular differentiation and species differences

G alpha q and G alpha 11, members of the Gq family of G-proteins, transduce signals from receptors to the beta isoenzymes of phosphatidyl-inositol-specific phospholipase C (PI-PLC). The receptor specificity of these alpha subunits is unknown. G alpha q and G alpha 11 are ubiquitously expressed in tissues; however, there have been conflicting reports of the presence or absence of G alpha 11 protein in haematopoietic cells. Platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors activate PI-PLC via G alpha q, but the role of G alpha 11 is uncertain. To define their roles in platelet activation we studied G alpha q and G alpha 11 gene expression by immunotransfer blotting and by reverse transcription of mRNA followed by PCR (RT-PCR) and direct sequencing. An antiserum specific for mouse G alpha 11 failed to identify G alpha 11 in dog or human platelets or in dog liver, a tissue known to contain G alpha 11. RT-PCR performed with gene-specific primers demonstrated G alpha q mRNA, but not G alpha 11 mRNA, in normal human and mouse platelets and in thromboxane-sensitive and thromboxane-insensitive dog platelets. Studies of mouse and dog liver and human retina confirmed that the cDNA, primers and probes used could amplify and recognize G alpha 11 in other tissues. However, species-specific oligonucleotide primers and probes were essential to demonstrate G alpha 11, but not G alpha q, mRNA. Compared with mouse cDNA, dog and human G alpha 11 cDNA had twice as many nucleotide substitutions (approx. 12% compared with approx. 6%) as G alpha q, G alpha q mRNA was also found in mature erythrocytes but G alpha 11 mRNA was not identified, whereas both G alpha q and G alpha 11 mRNAs were found in bone marrow stem cells. Therefore G alpha 11 gene expression in haematopoietic cells is linked with cellular differentiation. The lack of G alpha 11 indicates that signal transduction from platelet TXA2/PGH2 receptors to PI-PLC occurs via G alpha q, and that G alpha 11 deficiency is not responsible for defective activation of PI-PLC in thromboxane-insensitive dog platelets. Despite the high degree of similarity that exists between G alpha q and G alpha 11, significantly greater species-specific variation in nucleotide sequence is present in G alpha 11 than in G alpha q. Cellular specificity and species specificity are important characteristics of these Gq family G-proteins.

Cloning and characterization of the signal transduction of four splice variants of the human pituitary adenylate cyclase activating polypeptide receptor. Evidence for dual coupling to adenylate cyclase and phospholipase C

Alternative splicing of two exons of the rat pituitary adenylate cyclase activating polypeptide (PACAP) receptor gene generates four major splice variants that are differentially expressed in specific tissues and variably coupled to intracellular second messengers. To evaluate the potential implications of these findings in human physiology, the human PACAP receptor gene was cloned. Alternative splicing about two exons of the gene allowed for four major splice variants that were subsequently identified on cDNA cloning. Each of the four splice variant cDNAs (null, SV-1, SV-2, and SV-3) was stably expressed in NIH/3T3 cells at similar receptor densities. For each splice variant, PACAP (both PACAP-38 and PACAP-27) had similar affinity and potency for stimulating either adenylate cyclase or phospholipase C. However, each receptor splice variant differed in their ligand-stimulated maximal response (efficacy) for total inositol phosphate accumulation with the SV-2 showing the greatest efficacy, followed by the null, SV-1, and SV-3 splice variants. Therefore, unlike the rat, PACAP binds and stimulates signal transduction with nearly equal affinity and potency for each of the receptor splice variants although with varying efficacy for the stimulation of phospholipase C. These results suggest a novel and potentially important mechanism for a single hormone to not only couple to dual signal transduction cascades but also elicit tissue-specific differential activation of phospholipase C in humans.

Bombesin activation of phospholipase C beta 1 in rat acinar pancreatic cells involves the pertussis toxin-sensitive G alpha i3 protein

Bombesin stimulation of inositol 1,4,5-trisphosphate (Ins P3) formation in rat sonicated pancreatic acinar cells was inhibited by an antibody directed against the pertussis toxin (PTX)-sensitive GTP-binding G alpha i3 protein but not by an anti-G alpha q-11 antibody. After solubilization and gel filtration, [125I-Tyr4]bombesin binding sites were recovered in a peak of protein of 67 approximately 90 kDa with a maximal enrichment corresponding to a molecular mass of 83-kDa. Results obtained from the non-hydrolysable GTP analog guanosine-5'-[gamma-thio]triphosphate (GTP gamma S) binding, PTX-stimulated ADP-ribosylation and immunoblotting showed that the 83-kDa fraction contained the G alpha i3 protein but not the G alpha q-11 protein. Furthermore, GTP gamma S increased the bombesin binding dissociation constant (KD) from 0.32 to 0.60 nM, while the anti-G alpha i3 antibody decreased the maximal binding capacity (Bmax) from 50 to 25 fmol/mg protein without affecting the KD. Mixing solubilized bombesin binding sites with a phospholipase C (PLC) preparation from rat pancreas reconstituted a bombesin-stimulated PLC activity which was markedly inhibited by the anti-G alpha i3 antibody but unaffected by the anti-G alpha q-11 antibody. In addition, this stimulation was inhibited by an anti-PLC beta 1 antibody. This result supports the involvement of the PLC beta 1 isoform in bombesin receptor activation.

Bombesin receptors in the brain

We have shown that in the central nervous system BN receptors are closely associated with 5-HT systems. On a subpopulation of dorsal raphe neurons, NMB receptors are able to depolarize cells by reducing gK+. In one of the target regions of the dorsal raphe 5-HT neurons, the SCN, we have also shown that neurons are excited by BN-related peptides. In the SCN, the GRP receptors excite neurons by two different mechanisms: closure of gK+ and opening of an unidentified cation conductance. Expression of human BN receptors from the brain in CHO cells or Xenopus oocytes shows a very similar pharmacological profile to that seen in the rat brain slice preparations. In the CHO cell line, following BN receptor activation, a major second-messenger path involves hydrolysis of PIP2 by phospholipases to yield IP3, which releases Ca2+ from intracellular stores. In the oocyte expression system, a similar second messenger pathway is clearly apparent, and Ca2+-sensitive gCl- represents the last phase in a cascade of events. The final phase of the mechanism of action in the artificial systems does not involve gK+, suggesting a different second messenger cascade to that in neurons. However, the involvement of phospholipases and their phospholipid products have not been excluded in neurons.

G alpha q/11 mediates neurotensin excitation of substantia nigra dopaminergic neurons

Using acutely dissociated substantia nigra pars compacta (SNC) dopaminergic (DA) neurons, our previous studies indicated that neurotensin (NT) excites SNC DA neurons by increasing the cationic conductance and reducing the inwardly rectifying K+ conductance. Further investigation also revealed that pertussis toxin (PTX)- insensitive G-proteins mediate neurotensin modulation of cation and potassium channels. G alpha q and G alpha 11 are widely distributed in various tissues including the brain and likely to mediate PTX-insensitive signal transductions in the nervous system. In this study, two different experiments were conducted to test the hypothesis that G alpha q/11 mediates neurotensin regulation of the cationic and K+ conductances. First, we investigated the expression of G alpha q and G alpha 11 mRNAs in NT-responsive SNC DA neurons by combining whole-cell patch-clamp recordings with single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) assay. After recording NT-evoked membrane currents, the cellular content was harvested from single neurons and used as the template for the subsequent RT-PCR analysis. Both G alpha q and G alpha 11 mRNAs were present in all SNC DA neurons that responded to neurotensin. SNC DA neurons were also internally dialyzed with an antibody directed against the common C-terminus of G alpha q and G alpha 11 during whole-cell recordings. In DA neurons perfused with the anti-G alpha q/11 antiserum, neurotensin failed to evoke inward currents resulting from the opening of cation channels and the closure of inward rectifier K+ channels. It is concluded that NT modulation of cation and inward rectifier K+ channels in SNC DA neurons is transduced by G alpha q and/or G alpha 11.

Ca2+ signalling in K562 human erythroleukaemia cells: effect of dimethyl sulphoxide and role of G-proteins in thrombin- and thromboxane A2-activated pathways

The human leukaemic cell line K562 is a pluripotent stem cell with the potential to mature along a megakaryocytic or erythroid line. In these cells, thrombin and U46619 (9,11-dideoxy-9 alpha, 11 alpha-methanoepoxy prostaglandin F2 alpha), a thromboxane A2 analogue, increased intracellular Ca2+ in a rapid and concentration-dependent manner. The peak transient observed with both thrombin and U46619 was preserved upon stimulation in the absence of extracellular calcium and blunted with phorbol myristate acetate, suggestive of activation of phospholipase C. Short-term treatment with leupeptin abolished the calcium response to thrombin, but did not alter that to U46619. Both pertussis toxin (PT) and DMSO pretreatment inhibited thrombin- but not U46619-stimulated intracellular calcium elevation, indicating that these agonists signal through different G-proteins. Western blot analysis of crude membranes from K562 cells revealed the presence of G12 alpha and G13 alpha; the other known PT-substrates, Gi1 alpha and G0 alpha, were not detected. Consistent with this observation, ADP-ribosylation experiments revealed the presence of two PT substrates which co-migrated with human erythrocyte G12 alpha and G13 alpha. An antibody raised against Gq/11 alpha, a subfamily of G-protein alpha subunits unmodified by PT, specifically recognized 42 kDa protein(s) in K562 cells. PCR amplification of reverse-transcribed K562 RNA followed by DNA sequencing showed that these cells express messages for both Gq alpha and G11 alpha. Treatment of K562 cells with DMSO reduced the levels of thrombin receptor mRNA, without simultaneous changes in the expression of G12 alpha and G13 alpha. We have thus identified Ca(2+)-mobilizing agonists and related G-proteins in K562 cells, together with changes induced by DMSO in this signalling pathway.