G protein-coupled receptor-induced sensitization of phospholipase C stimulation by receptor tyrosine kinases

Varying Stimulation Parameters to Improve Cortical Plasticity Generated by VNS-tone Pairing

Pairing vagus nerve stimulation (VNS) with movements or sounds can direct robust plasticity in motor or auditory cortex, respectively. The degree of map plasticity is influenced by the intensity and pulse width of VNS, number of VNS-event pairings, and the interval between each pairing. It is likely that these parameters interact, influencing optimal implementation of VNS pairing protocols. We varied VNS intensity, number of stimulations, and inter-stimulation interval (ISI) to test for interactions among these parameters. Rats were implanted with a vagus nerve stimulating cuff and randomly assigned to one of three treatment groups to receive 20 days of VNS paired with a 9-kHz tone: (1) Fast VNS: 50 daily pairings of 400-µA VNS with a 30-s ISI; (2) Dispersed VNS: 50 daily pairings of 400-µA VNS with a 180-s ISI; and (3) Standard VNS: 300 daily pairings of 800-µA VNS with a 30-s ISI. Following 20 days of VNS-tone pairing, multi-unit recordings were conducted in primary auditory cortex (A1) and receptive field properties were analyzed. Increasing ISI (Dispersed VNS) did not lead to an enhancement of cortical plasticity. Reducing the current intensity and number of stimulations (Fast VNS) resulted in robust cortical plasticity, using 6 times fewer VNS pairings than the Standard protocol. These findings reveal an interaction between current intensity, stimulation number, and ISI and identify a novel VNS paradigm that is substantially more efficient than the previous standard paradigm.

Autoantibodies to muscarinic acetylcholine receptors found in patients with primary biliary cirrhosis

Background

Autoantibodies to the human muscarinic acetylcholine receptor of the M3 type (hmAchR M3) have been suggested to play an etiopathogenic role in Sjögren's syndrome. Primary biliary cirrhosis (PBC) often is associated with this syndrome. Therefore, we studied the co-presence of hmAchR M3 autoantibodies in patients with PBC.

Methods

Frequency of hmAchR M3 autoantibodies was assessed by Western blotting analysis as well as by an ELISA using a 25-mer peptide of the 2^nd extracellular loop of hmAchR M3. Co-localization of hmAchR M3/PBC-specific autoantibodies was studied by confocal laser scanning microscopy. Finally, sera from patients with PBC as well as from healthy controls were tested.

Results

Western blotting analysis as well as results from ELISA testing revealed a significantly enhanced IgG reactivity in PBC patients in contrast to healthy controls. Co-localization of autoantibodies with the hmAchR M3 receptor-specific autoantibodies was observed in 10 out of 12 PBC-patients but none of the 5 healthy controls. Antibodies of the IgM type were not found to be affected.

Conclusions

For the first time, our data demonstrate the presence of autoantibodies to the hmAchR M3 in PBC patients. These findings might contribute to the understanding of the pathogenesis of this disease. Further studies have to focus on the functionality of hmAchR M3 autoantibodies in PBC patients.

Crosstalk between VEGFR2 and muscarinic receptors regulates the mTOR pathway in serum starved SK-N-SH human neuroblastoma cells

Muscarinic acetylcholine receptors (mAchRs) are guanosine nucleotide-binding protein (G protein) coupled receptors that crosstalk with receptor tyrosine kinases (RTKs) to signal mitogenic pathways. In particular, mAchRs are known to couple with RTKs for several growth factors to activate the mammalian target of rapamycin (mTOR)/Akt pathway, a regulator of protein synthesis. The RTK for the vascular endothelial growth factor (VEGF), VEGFR2, can signal protein synthesis but whether it cooperates with mAchRs to mediate mTOR activation has not been demonstrated. Using serum starved SK-N-SH neuroblastoma cells, we show that the muscarinic receptor agonists carbachol and pilocarpine enhance the activation of the mTOR substrate p70 S6 Kinase (S6K) and its target ribosomal protein S6 (S6) in a VEGFR2 dependent manner. Treatments with carbachol increased VEGFR2 phosphorylation, suggesting that mAchRs stimulate VEGFR2 transactivation to enhance mTOR signaling. Inhibitor studies revealed that phosphatidylinositol 3 kinase resides upstream from S6K, S6 and Akt phosphorylation while protein kinase C (PKC) functions in an opposing fashion by positively regulating S6K and S6 phosphorylation and suppressing Akt activation. Treatments with the phosphatase inhibitors sodium orthovanadate and okadaic acid increase S6, Akt and to a lesser extent S6K phosphorylation, indicating that tyrosine and serine/threonine dephosphorylation also regulates their activity. However, okadaic acid elicited a far greater increase in phosphorylation, implicating phosphatase 2A as a critical determinant of their function. Finally, pilocarpine but not carbachol induced a time and dose dependent cell death that was associated with caspase activation and oxidative stress but independent of S6K and S6 activation through VEGFR2. Accordingly, our findings suggest that mAchRs crosstalk with VEGFR2 to enhance mTOR activity but signal divergent effects on survival through alternate mechanisms.

The small G protein Rac1 activates phospholipase Cdelta1 through phospholipase Cbeta2

Rac1, which is associated with cytoskeletal pathways, can activate phospholipase Cbeta2 (PLCbeta2) to increase intracellular Ca(2+) levels. This increased Ca(2+) can in turn activate the very robust PLCdelta1 to synergize Ca(2+) signals. We have previously found that PLCbeta2 will bind to and inhibit PLCdelta1 in solution by an unknown mechanism and that PLCbeta2.PLCdelta1 complexes can be disrupted by Gbetagamma subunits. However, because the major populations of PLCbeta2 and PLCdelta1 are cytosolic, their regulation by Gbetagamma subunits is not clear. Here, we have found that the pleckstrin homology (PH) domains of PLCbeta2 and PLCbeta3 are the regions that result in PLCdelta1 binding and inhibition. In cells, PLCbeta2.PLCdelta1 form complexes as seen by Förster resonance energy transfer and co-immunoprecipitation, and microinjection of PHbeta2 dissociates the complex. Using PHbeta2 as a tool to assess the contribution of PLCbeta inhibition of PLCdelta1 to Ca(2+) release, we found that, although PHbeta2 only results in a 25% inhibition of PLCdelta1 in solution, in cells the presence of PHbeta2 appears to eliminates Ca(2+) release suggesting a large threshold effect. We found that the small plasma membrane population of PLCbeta2.PLCdelta1 is disrupted by activation of heterotrimeric G proteins, and that the major cytosolic population of the complexes are disrupted by Rac1 activation. Thus, the activity of PLCdelta1 is controlled by the amount of bound PLCbeta2 that changes with displacement of the enzyme by heterotrimeric or small G proteins. Through PLCbeta2, PLCdelta1 activation is linked to surface receptors as well as signals that mediate cytoskeletal pathways.

Activation of epidermal growth factor receptor inhibits KCNQ2/3 current through two distinct pathways: membrane PtdIns(4,5)P2 hydrolysis and channel phosphorylation

KCNQ2/3 currents are the molecular basis of the neuronal M currents that play a critical role in neuron excitability. Many neurotransmitters modulate M/KCNQ currents through their G-protein-coupled receptors. Membrane PtdIns(4,5)P2 hydrolysis and channel phosphorylation are two mechanisms that have been proposed for modulation of KCNQ2/3 currents. In this study, we studied regulation of KCNQ2/3 currents by the epidermal growth factor (EGF) receptor, a member of another family of membrane receptors, receptor tyrosine kinases. We demonstrate here that EGF induces biphasic inhibition of KCNQ2/3 currents in human embryonic kidney 293 cells and in rat superior cervical ganglia neurons, an initial fast inhibition and a later slow inhibition. Additional studies indicate that the early and late inhibitions resulted from PtdIns(4,5)P2 hydrolysis and tyrosine phosphorylation, respectively. We further demonstrate that these two processes are mutually dependent. This study indicates that EGF is a potent modulator of M/KCNQ currents and provides a new dimension to the understanding of the modulation of these channels.

Dynamic phospholipid signaling by G protein-coupled receptors

G protein-coupled receptors (GPCRs) control a variety of fundamental cellular processes by regulating phospholipid signaling pathways. Essential for signaling by a large number of receptors is the hydrolysis of the membrane phosphoinositide PIP(2) by phospholipase C (PLC) into the second messengers IP(3) and DAG. Many receptors also stimulate phospholipase D (PLD), leading to the generation of the versatile lipid, phosphatidic acid. Particular PLC and PLD isoforms take differential positions in receptor signaling and are additionally regulated by small GTPases of the Ras, Rho and ARF families. It is now recognized that the PLC substrate, PIP(2), has signaling capacity by itself and can, by direct interaction, affect the activity and subcellular localization of PLD and several other proteins. As expected, the synthesis of PIP(2) by phosphoinositide 5-kinases is tightly regulated as well. In this review, we present an overview of how these signaling pathways are governed by GPCRs, explain the molecular basis for the spatially and temporally organized, highly dynamic quality of phospholipid signaling, and point to the functional connection of the pathways.

Inhibition of phospholipase C-epsilon by Gi-coupled receptors

We recently reported that several Gs-coupled receptors stimulate phospholipase C (PLC)-epsilon via increased formation of cyclic AMP and subsequent activation of the small GTPase Rap2B by the cyclic AMP-activated exchange factor Epac1. Here we show by studies in HEK-293 and N1E-115 neuroblastoma cells that this stimulation induced by Gs-coupled receptors or the direct adenylyl cyclase activator, forskolin, is potently inhibited by Gi-coupled receptors, known to inhibit cyclic AMP formation. PLC inhibition by the overexpressed M2 muscarinic receptor and the endogenously expressed sphingosine-1-phosphate and delta-opioid receptors was fully pertussis toxin-sensitive and accompanied by a reduction in Rap2B activation induced by Gs-coupled receptors. In contrast, Rap2B activation and PLC stimulation induced by membrane-permeable cyclic AMP analogues, including an Epac-specific activator, or PLC stimulation caused by constitutively active Rap2B were not affected by the Gi-coupled receptors. In summary, our data indicate that Gi-coupled receptors can inhibit PLC-epsilon, most likely by suppressing formation of cyclic AMP required for Epac-mediated Rap2B activation.

IRS-1 and Vascular Complications in Diabetes Mellitus

The expected explosive increase in the number of patients with diabetes mellitus will increase the stress on health care. Treatment is focused on preventing vascular complications associated with the disorder. In order to develop better treatment regimens, the field of research has made a great effort in understanding this disorder. This chapter summarizes the current views on the insulin signaling pathway with emphasis on intracellular signaling events associated with insulin resistance, which lead to the prothrombotic condition in the vasculature of patience with diabetes mellitus.

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

PURPOSE

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

EXPERIMENTAL DESIGN

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

RESULTS

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

CONCLUSIONS

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

From inhibition to excitation: Functional effects of interaction between opioid receptors

Opioids have excitatory effects in multiple regions of the nervous system. Excitation by opioids is generally attributed to inhibition of inhibitory pathways (disinhibition). However, recent studies indicate that opioids can directly excite individual cells. These effects may occur when opioid receptors interact with other G protein coupled receptors, when different subtypes of opioid receptors interact, or when opioids transactivate other receptors such as receptor tyrosine kinases. Changes in the relative level of expression of different receptors in an individual cell may therefore determine its functional response to a given ligand. This phenomenon could represent an adaptive mechanism involved in tolerance, dependence and subsequent withdrawal.

Biology of LPA in health and disease

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

Regulation and cellular roles of phosphoinositide 5-kinases

The membrane phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP(2)), plays a critical role in various, apparently very different cellular processes. As precursor for second messengers generated by phospholipase C isoforms and class I phosphoinositide 3-kinases, PIP(2) is indispensable for cellular signaling by membrane receptors. In addition, PIP(2) directly affects the localization and activity of many cellular proteins via specific interaction with unique phosphoinositide-binding domains and thereby regulates actin cytoskeletal dynamics, vesicle trafficking, ion channel activity, gene expression and cell survival. The activity and subcellular localization of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) isoforms, which catalyze the formation of PIP(2), are actively regulated by membrane receptors, by phosphorylation and by small GTPases of the Rho and ARF families. Spatially and temporally organized regulation of PIP(2) synthesis by PIP5K enables dynamic and versatile PIP(2) signaling and represents an important link in the execution of cellular tasks by Rho and ARF GTPases.

Rap2B-dependent stimulation of phospholipase C-epsilon by epidermal growth factor receptor mediated by c-Src phosphorylation of RasGRP3

Receptor tyrosine kinase regulation of phospholipase C-epsilon (PLC-epsilon), which is under the control of Ras-like and Rho GTPases, was studied with HEK-293 cells endogenously expressing PLC-coupled epidermal growth factor (EGF) receptors. PLC and Ca(2+) signaling by the EGF receptor, which activated both PLC-gamma1 and PLC-epsilon, was specifically suppressed by inactivation of Ras-related GTPases with clostridial toxins and expression of dominant-negative Rap2B. EGF induced rapid and sustained GTP loading of Rap2B, binding of Rap2B to PLC-epsilon, and Rap2B-dependent translocation of PLC-epsilon to the plasma membrane. GTP loading of Rap2B by EGF was inhibited by chelation of intracellular Ca(2+) and expression of lipase-inactive PLC-gamma1 but not of PLC-epsilon. Expression of RasGRP3, a Ca(2+)/diacylglycerol-regulated guanine nucleotide exchange factor for Ras-like GTPases, but not expression of various other exchange factors enhanced GTP loading of Rap2B and PLC/Ca(2+) signaling by the EGF receptor. EGF induced tyrosine phosphorylation of RasGRP3, but not RasGRP1, apparently caused by c-Src; inhibition of c-Src interfered with EGF-induced Rap2B activation and PLC stimulation. Collectively, these data suggest that the EGF receptor triggers activation of Rap2B via PLC-gamma1 activation and tyrosine phosphorylation of RasGRP3 by c-Src, finally resulting in stimulation of PLC-epsilon.

Growth factor pre‐treatment differentially regulates phosphoinositide turnover downstream of lysophospholipid receptor and metabotropic glutamate receptors in cultured rat cerebrocortical astrocytes

Reactive gliosis is an aspect of neural plasticity and growth factor (GF) stimulation of astrocytes in vitro is widely regarded as a model system to study astrocyte plasticity. Astrocytes express receptors for several ligands including lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P), agonists for the G-protein-coupled lysophospholipid receptors (lpRs). Activation of lpRs by LPA or S1P leads to multiple pharmacological effects including the influx of calcium, phosphoinositide (PI) hydrolysis, phosphorylation of extracellular receptor regulated kinase (ERK), release of arachidonic acid, and induces mitogenesis. Treatment of astrocytes in vitro with a growth factor cocktail (containing epidermal growth factor [EGF], basic fibroblast growth factor [bFGF] and insulin) led to a marked attenuation of lpR-induced PI hydrolysis. In contrast, under identical conditions, GF treatment led to marked potentiation of PI hydrolysis downstream of activation of another abundantly expressed G-protein coupled receptor, mGluR5. Quantitative gene expression analysis of GF-treated or control astrocytes by TaqMan RT-PCR indicated that GF treatment did not change gene expression of lpa1 and s1p1, but increased gene expression of s1p5 which is expressed at very low levels in basal conditions. These results suggest that GF differentially affected PLC activation downstream of mGluR5 versus lpR activation and that the changes in mRNA levels of lpRs do not account for marked attenuation of agonist-induced phosphoinositide turnover.

Roles of PLC-γ2 and PKCα in TPA-induced apoptosis of gastric cancer cells

AIM: To investigate the roles of PLCγ2 and PKCα in TPA-induced apoptosis of gastric cancer cells.

METHODS: Human gastric cancer cell line MGC80-3 was used. Protein expression levels of PLCγ2 and PKCα were detected by Western blot. Protein localization of PLCγ2 and PKCα was shown by immunofluoscence analysis under laser-scanning confocal microscope. Apoptotic morphology was observed by DAPI fluorescence staining, and apoptotic index was counted among 1000 cells randomly.

RESULTS: Treatment of gastric cancer cells MGC80-3 with TPA not only up-regulated expression of PLC-γ2 protein, but also induced PLC-γ2 translocation from the cytoplasm to the nucleus. However, this process was not directly associated with apoptosis induction. Further investigation showed that PKCα translocation from the cytoplasm to the nucleus was correlated with initiation of apoptosis. To explore the inevitable linkage between PLC-γ2 and PKCα during apoptosis induction, PLC inhibitor U73122 was used to block PLC-γ2 translocation, in which neither stimulating PKCα translocation nor inducing apoptosis occurred in MGC80-3 cells. However, when U73122-treated cells were exposed to TPA, not only PLC-γ2, but also PKCα was redistributed. On the other hand, when cells were treated with PKC inhibitor alone, PLC-γ2 protein was still located in the cytoplasm. However, redistribution of PLC-γ2 protein occurred in the presence of TPA, no matter whether PKC inhibitor existed or not.

CONCLUSION: PLC-γ2 translocation is critical in transmitting TPA signal to its downstream molecule PKCα. As an effector, PKCα directly promotes apoptosis of MGC80-3 cells. Therefore, protein translocation of PLCγ2 and PKCα is critical event in the process of apoptosis induction.

Mechanisms of cross-talk between G-protein-coupled receptors resulting in enhanced release of intracellular Ca2+

Alteration in [Ca(2+)](i) (the intracellular concentration of Ca(2+)) is a key regulator of many cellular processes. To allow precise regulation of [Ca(2+)](i) and a diversity of signalling by this ion, cells possess many mechanisms by which they are able to control [Ca(2+)](i) both globally and at the subcellular level. Among these are many members of the superfamily of GPCRs (G-protein-coupled receptors), which are characterized by the presence of seven transmembrane domains. Typically, those receptors able to activate PLC (phospholipase C) enzymes cause release of Ca(2+) from intracellular stores and influence Ca(2+) entry across the plasma membrane. It has been well documented that Ca(2+) signalling by one type of GPCR can be influenced by stimulation of a different type of GPCR. Indeed, many studies have demonstrated heterologous desensitization between two different PLC-coupled GPCRs. This is not surprising, given our current understanding of negative-feedback regulation and the likely shared components of the signalling pathway. However, there are also many documented examples of interactions between GPCRs, often coupling preferentially to different signalling pathways, which result in a potentiation of Ca(2+) signalling. Such interactions have important implications for both the control of cell function and the interpretation of in vitro cell-based assays. However, there is currently no single mechanism that adequately accounts for all examples of this type of cross-talk. Indeed, many studies either have not addressed this issue or have been unable to determine the mechanism(s) involved. This review seeks to explore a range of possible mechanisms to convey their potential diversity and to provide a basis for further experimental investigation.

Regulation of muscarinic acetylcholine receptor signaling

Multiple mechanisms regulate the signaling of the five members of the family of the guanine nucleotide binding protein (G protein)-coupled muscarinic acetylcholine (ACh) receptors (mAChRs). Following activation by classical or allosteric agonists, mAChRs can be phosphorylated by a variety of receptor kinases and second messenger-regulated kinases. The phosphorylated mAChR subtypes can interact with beta-arrestin and presumably other adaptor proteins as well. As a result, the various mAChR signaling pathways may be differentially altered, leading to short-term or long-term desensitization of a particular signaling pathway, receptor-mediated activation of the mitogen-activated protein kinase pathway downstream of mAChR phosphorylation, as well as long-term potentiation of mAChR-mediated phospholipase C stimulation. Agonist activation of mAChRs may also induce receptor internalization and down-regulation, which proceed in a highly regulated manner, depending on receptor subtype and cell type. In this review, our current understanding of the complex regulatory processes that underlie signaling of mAChR is summarized.

Proximal events in signaling by plasma membrane estrogen receptors

Estradiol (E2) rapidly stimulates signal transduction from plasma membrane estrogen receptors (ER) that are G protein-coupled. This is reported to occur through the transactivation of the epidermal growth factor receptor (EGFR) or insulin-like growth factor-1 receptor, similar to other G protein-coupled receptors. Here, we define the signaling events that result in EGFR and ERK activation. E2-stimulated ERK required ER in breast cancer and endothelial cells and was substantially prevented by expression of a dominant negative EGFR or by tyrphostin AG1478, a specific inhibitor for EGFR tyrosine kinase activity. Transactivation/phosphorylation of EGFR by E2 was dependent on the rapid liberation of heparin-binding EGF (HB-EGF) from cultured MCF-7 cells and was blocked by antibodies to this ligand for EGFR. Expression of dominant negative mini-genes for Galpha(q) and Galpha(i) blocked E2-induced, EGFR-dependent ERK activation, and Gbetagamma also contributed. G protein activation led to activation of matrix metalloproteinases (MMP)-2 and -9. This resulted from Src-induced MMP activation, implicated using PP2 (Src family kinase inhibitor) or the expression of a dominant negative Src protein. Antisense oligonucleotides to MMP-2 and MMP-9 or ICI 182780 (ER antagonist) each prevented E2-induced HB-EGF liberation and ERK activation. E2 also induced AKT up-regulation in MCF-7 cells and p38beta MAP kinase activity in endothelial cells, blocked by an MMP inhibitor, GM6001, and tyrphostin AG1478. Targeting of only the E domain of ERalpha to the plasma membrane resulted in MMP activation and EGFR transactivation. Thus, specific G proteins mediate the ability of E2 to activate MMP-2 and MMP-9 via Src. This leads to HB-EGF transactivation of EGFR and signaling to multiple kinase cascades in several target cells for E2. The E domain is sufficient to enact these events, defining additional details of the important cross-talk between membrane ER and EGFR in breast cancer.

Cross signaling, cell specificity, and physiology

The literature on intracellular signal transduction presents a confusing picture: every regulatory factor appears to be regulated by all signal transduction cascades and to regulate all cell processes. This contrasts with the known exquisite specificity of action of extracellular signals in different cell types in vivo. The confusion of the in vitro literature is shown to arise from several causes: the inevitable artifacts inherent in reductionism, the arguments used to establish causal effect relationships, the use of less than adequate models (cell lines, transfections, acellular systems, etc.), and the implicit assumption that networks of regulations are universal whereas they are in fact cell and stage specific. Cell specificity results from the existence in any cell type of a unique set of proteins and their isoforms at each level of signal transduction cascades, from the space structure of their components, from their combinatorial logic at each level, from the presence of modulators of signal transduction proteins and of modulators of modulators, from the time structure of extracellular signals and of their transduction, and from quantitative differences of expression of similar sets of factors.

Stimulation of phospholipase C-epsilon by the M3 muscarinic acetylcholine receptor mediated by cyclic AMP and the GTPase Rap2B

Stimulation of phospholipase C (PLC) by G(q)-coupled receptors such as the M(3) muscarinic acetylcholine receptor (mAChR) is caused by direct activation of PLC-beta enzymes by Galpha(q) proteins. We have recently shown that G(s)-coupled receptors can stimulate PLC-epsilon, apparently via formation of cyclic AMP and activation of the Ras-related GTPase Rap2B. Here we report that PLC stimulation by the M(3) mAChR expressed in HEK-293 cells also involves, in part, similar mechanisms. M(3) mAChR-mediated PLC stimulation and [Ca(2+)](i) increase were reduced by 2',5'-dideoxyadenosine (dd-Ado), a direct adenylyl cyclase inhibitor. On the other hand, overexpression of Galpha(s) or Epac1, a cyclic AMP-regulated guanine nucleotide exchange factor for Rap GTPases, enhanced M(3) mAChR-mediated PLC stimulation. Inactivation of Ras-related GTPases with clostridial toxins suppressed the M(3) mAChR responses. The inhibitory toxin effects were mimicked by expression of inactive Rap2B, but not of other inactive GTPases (Rac1, Ras, RalA, Rap1A, and Rap2A). Activation of the M(3) mAChR induced GTP loading of Rap2B, an effect strongly enhanced by overexpression of Galpha(s) and inhibited by dd-Ado. Overexpression of PLC-epsilon and PLC-beta1, but not PLC-gamma1 or PLC-delta1, enhanced M(3) mAChR-mediated PLC stimulation and [Ca(2+)](i) increase. In contrast, expression of a catalytically inactive PLC-epsilon mutant reduced PLC stimulation by the M(3) mAChR and abrogated the potentiating effect of Galpha(s). In conclusion, our findings suggest that PLC stimulation by the M(3) mAChR is a composite action of PLC-beta1 stimulation by Galpha(q) and stimulation of PLC-epsilon apparently mediated by G(s)-dependent cyclic AMP formation and subsequent activation of Rap2B.

Expression profile analysis of colon cancer cells in response to sulindac or aspirin

Nonsteroidal anti-inflammatory drugs (NSAIDs) have a preventive effect against colorectal cancer. Although inhibition of cyclooxygenase-2 plays a crucial role in the suppression of tumors, precise mechanisms of their action remain to be disclosed. To identify genes involved in the growth-suppressive effect of NSAIDs, we utilized cDNA microarray containing 23,040 genes and analyzed time-dependent alteration of gene expression in response to sulindac or aspirin in NSAIDs-sensitive SW480 and SW948 colon-cancer cells as well as in relatively resistant SNU-C4 cells. Consequently we identified 112 genes with commonly altered expression by sulindac and 176 with commonly altered expression by aspirin in the three lines. Addition of sulindac and that of aspirin altered expression levels of 130 and 140 genes, respectively, in SW480 and SW948 cells but not in SNU-C4 cells. These data may lead to a better understanding of growth-suppressive effects on colonic epithelium, and may provide clues for identifying novel therapeutic and/or preventive molecular targets of colon cancer.

Bilirubin binding to normal and modified human erythrocyte membranes: effect of phospholipases, neuraminidase, trypsin and CaCl2

Binding of bilirubin to human erythrocyte membranes was studied after various enzymatic treatments as well as calcium loading. Whereas phospholipase D treatment of erythrocyte membranes resulted in 23% increase in bilirubin binding, phospholipase C-treated membranes showed remarkable enhancement in bilirubin binding. Polar head groups in general and negatively charged phosphate moieties, in particular, of phospholipids of the membrane appear to inhibit a large amount of bilirubin from binding to the membranes. Neuraminidase treatment of the membranes also led to a slight increase in bilirubin binding as compared to untreated membranes. Membrane proteins and carbohydrates seem to play significant regulatory role in bilirubin binding. However, no direct correlation was found between the increase in bilirubin binding and the amount of carbohydrate released upon tryptic digestion of membranes. Increase in bilirubin binding to trypsin-treated membranes can be ascribed to the increase in free bilirubin concentration in the incubation mixture as a result of tryptic hydrolysis of albumin by the membrane-bound tryptic activity. Calcium-loaded erythrocyte membranes also showed remarkable increase in bilirubin binding as a result of negative charge shielding and calcium-induced hydrophobic aggregation. Taken together, these results suggest the inhibitory role of polar head groups of phospholipids (phosphate in particular), carbohydrate and sialic acid in the binding of bilirubin to erythrocyte membranes.

A new phospholipase-C-calcium signalling pathway mediated by cyclic AMP and a Rap GTPase

Stimulation of phosphoinositide-hydrolysing phospholipase C (PLC) generating inositol-1,4,5-trisphosphate is a major calcium signalling pathway used by a wide variety of membrane receptors, activating distinct PLC-beta or PLC-gamma isoforms. Here we report a new PLC and calcium signalling pathway that is triggered by cyclic AMP (cAMP) and mediated by a small GTPase of the Rap family. Activation of the adenylyl cyclase-coupled beta2-adrenoceptor expressed in HEK-293 cells or the endogenous receptor for prostaglandin E1 in N1E-115 neuroblastoma cells induced calcium mobilization and PLC stimulation, seemingly caused by cAMP formation, but was independent of protein kinase A (PKA). We provide evidence that these receptor responses are mediated by a Rap GTPase, specifically Rap2B, activated by a guanine-nucleotide-exchange factor (Epac) regulated by cAMP, and involve the recently identified PLC-epsilon isoform.

Galpha(i2) enhances insulin signaling via suppression of protein-tyrosine phosphatase 1B

Suppression of the expression of the heterotrimeric G-protein Galpha(i2) in vivo has been shown to provoke insulin resistance, whereas enhanced insulin signaling is observed when Galpha(i2) is overexpressed in vivo. The basis for Galpha(i2) regulation of insulin signaling was explored in transgenic mice with targeted expression of the GTPase-deficient, constitutively active Q205L Galpha(i2) in fat and skeletal muscle. Phosphorylation of insulin receptor and IRS-1 in response to insulin challenge in vivo was markedly amplified in fat and skeletal muscle expressing Q205L Galpha(i2). The expression and activity of the protein-tyrosine phosphatase 1B (PTP1B), but not protein-tyrosine phosphatases SHP-1, SHP-2, and LAR, were constitutively decreased in tissues expressing the Q205L Galpha(i2), providing a direct linkage between insulin signaling and Galpha(i2). The loss of PTP1B expression may explain, in part, the loss of PTP1B activity in the iQ205L transgenic mice. Activation of Galpha(i2) in mouse adipocytes with lysophosphatidic acid was shown to decrease PTP1B activity, whereas pertussis toxin inactivates Galpha(i2), blocks lysophosphatidic acid-stimulated inhibition of PTP1B activity, and blocks tonic suppression of PTP1B activity by Galpha(i2). Elevation of intracellular cAMP in fat cells is shown to increase PTP1B activity, whereas either depression of cAMP levels or direct activation of Galpha(i2) suppresses PTP1B. These data provide the first molecular basis for the interplay between Galpha(i2) and insulin signaling, i.e. activation of Galpha(i2) can suppress both the expression and activity of PTP1B in insulin-sensitive tissues.