Phosphorylation of phospholipase C-coupled receptors

Use of CRISPR/Cas9-edited HEK293 cells reveals that both conventional and novel protein kinase C isozymes are involved in mGlu5a receptor internalization

The internalization of G protein-coupled receptors (GPCRs) can be regulated by protein kinase C (PKC). However, most tools available to study the contribution of PKC isozymes have considerable limitations, including a lack of selectivity. In this study, we generated and characterized human embryonic kidney 293A (HEK293A) cell lines devoid of conventional or novel PKC isozymes (ΔcPKC and ΔnPKC) and employ these to investigate the contribution of PKC isozymes in the internalization of the metabotropic glutamate receptor 5 (mGlu₅). Direct activation of PKC and mutation of rat mGlu_5a Ser⁹⁰¹, a PKC-dependent phosphorylation site in the receptor C-tail, both showed that PKC isozymes facilitate approximately 40% of the receptor internalization. Nonetheless, we determined that mGlu_5a internalization was not altered upon the loss of cPKCs or nPKCs. This indicates that isozymes from both classes are involved, compensate for the absence of the other class, and thus fulfill dispensable functions. Additionally, using the Gαq/11 inhibitor YM-254890, GPCR kinase 2 and 3 (GRK2 and GRK3) knock-out cells and a receptor containing a mutated putative adaptor protein complex 2 (AP-2) interaction motif, we demonstrate that internalization of rat mGlu_5a is mediated by Gαq/11 proteins (77% of the response), GRK2 (27%) and AP-2 (29%), but not GRK3. Our PKC knock-out cell lines expand the repertoire of knock-out HEK293A cell lines available to research GPCR pharmacology. Moreover, since pharmacological tools to study PKC isozymes generally lack specificity and/or potency, we present the PKC knock-out cell lines as more specific research tools to investigate PKC-mediated aspects of cell biology.

Physiological application of nanoparticles in calcium-related proteins and channels

Nanoparticles (NPs) have been studied as therapeutic drug-delivery agents for promising clinical trial outcomes. Nanomaterial-based drugs can transfer conventional drugs to target lesions, such as tumors, with increasing efficiency by enhancing drug-cell interaction or drug absorption. Although they are favorable as efficient drug transfer systems, NPs also exhibit cytotoxicity that affects nonpathological regions. Here, we review the basic information behind NP-induced Ca²⁺ signaling and its participation in channel physiology and pathology. NPs are observed to demonstrate inhibitory or active effects on Ca²⁺ signaling. Thus, understanding Ca²⁺ signaling by NPs as a key mechanism in signal transduction will progress the application of nano-drugs in various diseases without deleterious effect.

Estradiol and angiotensin II crosstalk in hydromineral balance: Role of the ERK1/2 and JNK signaling pathways

The angiotensin II (ANGII) receptor AT1 plays an important role in the control of hydromineral balance, mediating the dipsogenic and natriorexigenic effects and neuroendocrine responses of ANGII. While estradiol (E2) is known to modulate several actions of ANGII in the brain, the molecular and cellular mechanisms of the interaction between E2 and ANGII and its physiological role in the control of body fluids remain unclear. We investigated the influence of E2 (40 μg/kg) pretreatment and extracellular-signal-regulated kinase (ERK1/2) and c-Jun N-terminal kinase (JNK) cell signaling on the dipsogenic and natriorexigenic effects, as well as the neuroendocrine responses to angiotensinergic central stimulation in ovariectomized rats (OVX). We showed that the inhibitory effect of E2 on ANGII-induced water and sodium intake requires the ERK1/2 and JNK signaling pathways. On the other hand, E2 pretreatment prevents the ANGII-induced phosphorylation of ERK and JNK in the lamina terminalis. E2 therapy decreased oxytocin (OT) and vasopressin (AVP) secretion and decreased ERK1/2 phosphorylation in the supraoptic and paraventricular nuclei (SON and PVN, respectively). We found that the AVP secretion induced by ANGII required ERK1/2 signaling, but OT secretion did not involve ERK1/2 signaling. Taken together, these results demonstrate that E2 modulates ANGII-induced water and sodium intake and AVP secretion by affecting the ERK1/2 and JNK pathways in the lamina terminalis and ERK1/2 signaling in the hypothalamic nuclei (PVN and SON) in OVX rats.

Transmembrane signaling in Saccharomyces cerevisiae as a model for signaling in metazoans: state of the art after 25 years

In the very first article that appeared in Cellular Signalling, published in its inaugural issue in October 1989, we reviewed signal transduction pathways in Saccharomyces cerevisiae. Although this yeast was already a powerful model organism for the study of cellular processes, it was not yet a valuable instrument for the investigation of signaling cascades. In 1989, therefore, we discussed only two pathways, the Ras/cAMP and the mating (Fus3) signaling cascades. The pivotal findings concerning those pathways undoubtedly contributed to the realization that yeast is a relevant model for understanding signal transduction in higher eukaryotes. Consequently, the last 25 years have witnessed the discovery of many signal transduction pathways in S. cerevisiae, including the high osmotic glycerol (Hog1), Stl2/Mpk1 and Smk1 mitogen-activated protein (MAP) kinase pathways, the TOR, AMPK/Snf1, SPS, PLC1 and Pkr/Gcn2 cascades, and systems that sense and respond to various types of stress. For many cascades, orthologous pathways were identified in mammals following their discovery in yeast. Here we review advances in the understanding of signaling in S. cerevisiae over the last 25 years. When all pathways are analyzed together, some prominent themes emerge. First, wiring of signaling cascades may not be identical in all S. cerevisiae strains, but is probably specific to each genetic background. This situation complicates attempts to decipher and generalize these webs of reactions. Secondly, the Ras/cAMP and the TOR cascades are pivotal pathways that affect all processes of the life of the yeast cell, whereas the yeast MAP kinase pathways are not essential. Yeast cells deficient in all MAP kinases proliferate normally. Another theme is the existence of central molecular hubs, either as single proteins (e.g., Msn2/4, Flo11) or as multisubunit complexes (e.g., TORC1/2), which are controlled by numerous pathways and in turn determine the fate of the cell. It is also apparent that lipid signaling is less developed in yeast than in higher eukaryotes. Finally, feedback regulatory mechanisms seem to be at least as important and powerful as the pathways themselves. In the final chapter of this essay we dare to imagine the essence of our next review on signaling in yeast, to be published on the 50th anniversary of Cellular Signalling in 2039.

Gonadotrophin-releasing hormone (GnRH) and GnRH agonists: mechanisms of action

The hypothalamic decapeptide gonadotrophin-releasing hormone (GnRH) binds to specific receptors on pituitary gonadotrophs. These receptors belong to the family of G protein-coupled receptors. Their activation leads to phosphoinositide breakdown with generation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and diacylglycerol. These second messengers initiate Ca2+ release from intracellular stores and activation of protein kinase C, both of which are important for gonadotrophin secretion and synthesis. Prolonged activation of GnRH receptors by GnRH leads to desensitization and consequently to suppressed gonadotrophin secretion. This is the primary mechanism of action of agonistic GnRH analogues. By contrast, GnRH antagonists compete with GnRH for receptors on gonadotroph cell membranes, inhibit GnRH-induced signal transduction and consequently gonadotrophin secretion. These compounds are free of agonistic actions, which might be beneficial in certain clinical applications.

Role of Rac1 in regulation of NOX5-S function in Barrett's esophageal adenocarcinoma cells

We have shown that a novel NADPH oxidase isoform, NOX5-S, is the major isoform of NADPH oxidases in an esophageal adenocarcinoma (EA) cell line, FLO, and is overexpressed in Barrett's mucosa with high-grade dysplasia. NOX5-S is responsible for acid-induced reactive oxygen species production. In this study, we found that mRNA levels of NOX5-S were significantly higher in FLO EA cells than in the normal human esophageal squamous cell line HET-1A or in a Barrett cell line, BAR-T. The mRNA levels of NOX5-S were also significantly increased in EA tissues. The data suggest that NOX5-S may be important in the development of EA. Mechanisms of functional regulation of NOX5-S are not fully understood. We show that small G protein Rac1 was present in HET-1A cells, BAR-T cells, and EA cell lines FLO and OE33. Rac1 protein levels were significantly higher in FLO and OE33 cells than in HET-1A or BAR-T cells. Knockdown of Rac1 with Rac1 small interfering RNA significantly decreased acid-induced increase in H(2)O(2) production in FLO EA cells. Overexpression of constitutively active Rac1 significantly increased H(2)O(2) production, an increase that was blocked by knockdown of NOX5-S. By immunofluorescence staining and immunoprecipitation, we found that NOX5-S was present in the cytosol of FLO EA cells and colocalized with Rac1 and SERCA1/2 Ca(2+)-ATPase which is located in the endoplasmic reticulum membrane. We conclude that Rac1 may be important in activation of NOX5-S in FLO EA cells.

Bile acid reflux contributes to development of esophageal adenocarcinoma via activation of phosphatidylinositol-specific phospholipase Cgamma2 and NADPH oxidase NOX5-S

Gastroesophageal reflux disease complicated by Barrett's esophagus (BE) is a major risk factor for esophageal adenocarcinoma (EA). However, the mechanisms of the progression from BE to EA are not fully understood. Besides acid reflux, bile acid reflux may also play an important role in the progression from BE to EA. In this study, we examined the role of phosphatidylinositol-specific phospholipase C (PI-PLC) and a novel NADPH oxidase NOX5-S in bile acid-induced increase in cell proliferation. We found that taurodeoxycholic acid (TDCA) significantly increased NOX5-S expression, hydrogen peroxide (H(2)O(2)) production, and cell proliferation in EA cells. The TDCA-induced increase in cell proliferation was significantly reduced by U73122, an inhibitor of PI-PLC. PI-PLCbeta1, PI-PLCbeta3, PI-PLCbeta4, PI-PLCgamma1, and PI-PLCgamma2, but not PI-PLCbeta2 and PI-PLCdelta1, were detectable in FLO cells by Western blot analysis. Knockdown of PI-PLCgamma2 or extracellular signal-regulated kinase (ERK) 2 mitogen-activated protein (MAP) kinase with small interfering RNAs (siRNA) significantly decreased TDCA-induced NOX5-S expression, H(2)O(2) production, and cell proliferation. In contrast, knockdown of PI-PLCbeta1, PI-PLCbeta3, PI-PLCbeta4, PI-PLCgamma1, or ERK1 MAP kinase had no significant effect. TDCA significantly increased ERK2 phosphorylation, an increase that was reduced by U73122 or PI-PLCgamma2 siRNA. We conclude that TDCA-induced increase in NOX5-S expression and cell proliferation may depend on sequential activation of PI-PLCgamma2 and ERK2 MAP kinase in EA cells. It is possible that bile acid reflux present in patients with BE may increase reactive oxygen species production and cell proliferation via activation of PI-PLCgamma2, ERK2 MAP kinase, and NADPH oxidase NOX5-S, thereby contributing to the development of EA.

Significance of autoimmunity to non-desmoglein targets in pemphigus

The pathogenesis of pemphigus vulgaris (PV) is a highly controversial, "hot" topic that has received considerable enrichment in recent years by both clinical and basic researchers. On the one hand, the classical view of desmogleins (Dsg) as main targets of this autoimmune disease is supported by the characterization of pathogenic anti-Dsg3 antibodies from both patients and animal models. On the other hand, fundamental doubt has been raised towards this monopathogenic view by several independent factors: (1) pemphigus lesions can be induced in Dsg3-knockout (KO) mice; (2) pemphigus sera contain multiple autoantibodies against different adhesion molecules and also cholinergic receptors; (3) experimental inhibition of PV IgG induced acantholysis can be obtained by interference with different signaling cascades regulating both calcium homeostasis and apoptosis; and (4) cholinergic agonists exhibit anti-acantholytic activity both in vitro and in vivo. The field is open for controlled clinical trials and further basic research to unfold the true story of the pemphigus enigma and provide the basis for a better treatment of pemphigus patients.

Arrestin 3 mediates endocytosis of CCR7 following ligation of CCL19 but not CCL21

Internalization of ligand bound G protein-coupled receptors, an important cellular function that mediates receptor desensitization, takes place via distinct pathways, which are often unique for each receptor. The C-C chemokine receptor (CCR7) G protein-coupled receptor is expressed on naive T cells, dendritic cells, and NK cells and has two endogenous ligands, CCL19 and CCL21. Following binding of CCL21, 21 +/- 4% of CCR7 is internalized in the HuT 78 human T cell lymphoma line, while 76 +/- 8% of CCR7 is internalized upon binding to CCL19. To determine whether arrestins mediated differential internalization of CCR7/CCL19 vs CCR7/CCL21, we used small interfering RNA (siRNA) to knock down expression of arrestin 2 or arrestin 3 in HuT 78 cells. Independent of arrestin 2 or arrestin 3 expression, CCR7/CCL21 internalized. In contrast, following depletion of arrestin 3, CCR7/CCL19 failed to internalize. To examine the consequence of complete loss of both arrestin 2 and arrestin 3 on CCL19/CCR7 internalization, we examined CCR7 internalization in arrestin 2(-/-)/arrestin 3(-/-) murine embryonic fibroblasts. Only reconstitution with arrestin 3-GFP but not arrestin 2-GFP rescued internalization of CCR7/CCL19. Loss of arrestin 2 or arrestin 3 blocked migration to CCL19 but had no effect on migration to CCL21. Using immunofluorescence microscopy, we found that arrestins do not cluster at the membrane with CCR7 following ligand binding but cap with CCR7 during receptor internalization. These are the first studies that define a role for arrestin 3 in the internalization of a chemokine receptor following binding of one but not both endogenous ligands.

Gonadotropin-releasing hormone inhibits pituitary adenylyl cyclase-activating polypeptide coupling to 3',5'-cyclic adenosine-5'-monophosphate pathway in LbetaT2 gonadotrope cells through novel protein kinase C isoforms and phosphorylation of pituitary adenylyl cyclase-activating polypeptide type I receptor

Gonadotrope cells are primarily regulated by GnRH but are also targets of the pituitary adenylyl cyclase-activating polypeptide (PACAP). Although it has been reported that reciprocal interactions between both neuropeptides contribute to regulation of gonadotrope function, the underlying mechanisms remain poorly understood. In this study, we reevaluated PACAP coupling to the cAMP pathway in LbetaT2 gonadotrope cells and analyzed GnRH effect on PACAP signaling. We established that PACAP38 markedly increases intracellular cAMP levels (EC50 of 4.7 +/- 1.3 nm) through the PACAP type 1 receptor (PAC1-R), as evidenced by pharmacological and RT-PCR studies. Interestingly, although GnRH couples to cAMP pathway in LbetaT2 cells, the effects of both neuropeptides were not synergistic. Instead, the GnRH agonist (GnRHa) triptorelin rapidly and strongly inhibited (70% inhibition as early as 5 min) PACAP38-induced cAMP production. Inhibition was calcium independent, mimicked by the phorbol ester phorbol 12-myristate 13-acetate, and blocked by the protein kinase C (PKC) inhibitor bisindoylmaleimide, indicating that GnRHa inhibitory action relies on PKC. Selective down-regulation of both conventional and novel PKC prevented a GnRHa effect, whereas pharmacological inhibition of conventional PKC only was ineffective, strongly suggesting the involvement of novel PKC isoforms. GnRHa did not inhibit forskolin- or cholera toxin-stimulated cAMP accumulation, suggesting that PAC1-R is the predominant target of GnRH. Accordingly, we demonstrated for the first time that GnRH increases PAC1-R phosphorylation through PKC, providing a potential molecular mechanism which may account for GnRH inhibitory effect.

Caveolin-1 alters Ca(2+) signal duration through specific interaction with the G alpha q family of G proteins

Caveolae are membrane domains having caveolin-1 (Cav1) as their main structural component. Here, we determined whether Cav1 affects Ca(2+) signaling through the Galpha(q)-phospholipase-Cbeta (PLCbeta) pathway using Fischer rat thyroid cells that lack Cav1 (FRTcav(-)) and a sister line that forms caveolae-like domains due to stable transfection with Cav1 (FRTcav(+)). In the resting state, we found that eCFP-Gbetagamma and Galpha(q)-eYFP are similarly associated in both cell lines by Forster resonance energy transfer (FRET). Upon stimulation, the amount of FRET between Galpha(q)-eYFP and eCFP-Gbetagamma remains high in FRTcav(-) cells, but decreases almost completely in FRTcav(+) cells, suggesting that Cav1 is increasing the separation between Galpha(q)-Gbetagamma subunits. In FRTcav(-) cells overexpressing PLCbeta, a rapid recovery of Ca(2+) is observed after stimulation. However, FRTcav(+) cells show a sustained level of elevated Ca(2+). FRET and colocalization show specific interactions between Galpha(q) and Cav1 that increase upon stimulation. Fluorescence correlation spectroscopy studies show that the mobility of Galpha(q)-eGFP is unaffected by activation in either cell type. The mobility of eGFP-Gbetagamma remains slow in FRTcav(-) cells but increases in FRTcav(+) cells. Together, our data suggest that, upon stimulation, Galpha(q)(GTP) switches from having strong interactions with Gbetagamma to Cav1, thereby releasing Gbetagamma. This prolongs the recombination time for the heterotrimer, thus causing a sustained Ca(2+) signal.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.

Actions of gonadotropin-releasing hormone analogues in pituitary gonadotrophs and their modulation by ovarian steroids

Recently, GnRH antagonists (GnRHant) like cetrorelix and ganirelix have been introduced in protocols of controlled ovarian hyperstimulation for assisted reproductive techniques to prevent premature luteinizing hormone (LH) surges. Here we tested, whether the actions of cetrorelix and the GnRH agonist (GnRHag) triptorelin in gonadotrophs are dependent on the steroid milieu. Furthermore, we characterized the actions of cetrorelix and triptorelin on LH secretion and the total LH pool. Female rat pituitary cells were treated either with 0.1 nM triptorelin for 1, 2, 4 and 6 days or for 1, 3, 5 and 6 h or with 1, 10 or 100 nM cetrorelix for 1, 2, 3 and 5 h or for 10 min. Cells were stimulated for 3h with different concentrations of GnRH (10 pM-1 microM). For analysis of the total LH pool, which is composed of stored and released LH, cells were lysed with 0.1% Triton X-100 at -80 degrees C overnight. To test, whether the steroid milieu affects the actions of cetrorelix and triptorelin, cells were incubated for 52 h with 1 nM estradiol (E) alone or with combinations of 100 nM progesterone (P) for 4 or 52 h, respectively. Cells were then treated with 0.1 nM triptorelin for 9 h or 1 nM cetrorelix for 3 h and stimulated for 3 h with different concentrations of GnRH (10 pM-1 microM). The suppressive effect of triptorelin on LH secretion was fully accomplished after 3 h of treatment, for cetrorelix only 10 min were sufficient. The concentration of cetrorelix must be at least equimolar to GnRH to block LH secretion. Cetrorelix shifted the EC50s of the GnRH dose-response curve to the right. Triptorelin suppressed total LH significantly (from 137 to 36 ng/ml) after 1 h in a time-dependent manner. In contrast, only high concentrations of cetrorelix increased total LH. In steroid treated cells the suppressive effects of triptorelin were more distinct. One nanomolar cetrorelix suppressed GnRH-stimulated LH secretion of cells not treated with steroids from 10.1 to 3.5 ng/ml. In cells, additionally treated with estradiol alone or estradiol and short-term progesterone, LH levels were higher (from 3.5 to 5.4 or 4.5 ng/ml, respectively). In cells co-treated with estradiol and progesterone for 52 h LH secretion was only suppressed from 10.1 to 9.5 ng/ml. Steroid treatments diminished the suppressive effect of cetrorelix on LH secretion. In conclusion, the depletion of the total LH pool contributes to the desensitizing effect of triptorelin. The actions of cetrorelix and triptorelin are dependent on the steroid milieu.

Once and future signaling: G protein-coupled receptor kinase control of neuronal sensitivity

G protein-coupled receptors (GPCRs) are the most numerous class of cell surface receptor, and substances acting through GPCRs mediate many critical signaling events and physiological processes. GPCR sensitivity and signaling is dynamic, responding rapidly to adjust to changes in the ambient level of stimulation of target cells. One important mediator of such receptor sensitivity is the family of GPCR kinases (GRKs). Like heterotrimeric G proteins, GRKs recognize agonist-bound, activated receptors, and this recognition promotes catalytic activation of GRKs, resulting in the preferential phosphorylation of activated receptors. GRK-phosphorylated receptors are then targeted by arrestin proteins, which bind to phosphorylated receptors. Arrestin-bound receptors are uncoupled from heterotrimeric G proteins, resulting in decreased sensitivity to further receptor stimulation (desensitization). Arrestin-bound receptors are also accelerated into internalization pathways and linked to distinct arrestin-mediated signaling pathways. GRKs thus serve as gatekeepers for receptors, terminating some signaling pathways and initiating others. One major outstanding question concerning GRKs understanding the mechanisms by which any particular receptor subtype (of the 800 or so in the body) is regulated by a specific GRK(s), and the consequences of this specificity. An understanding of this regulatory specificity could allow targeting of GRK function to ameliorate diseases involving GPCR dysregulation.

Evidence for LHRH-receptor expression in human airway epithelial (Calu-3) cells and its role in the transport of an LHRH agonist

PURPOSE

To determine whether LHRH-receptor is expressed in Calu-3, a human bronchial epithelial cell line, and to further determine whether this receptor plays a role in the transport of deslorelin, an LHRH agonist.

METHODS

Using cultured monolayers of Calu-3 grown at air-interface, the presence and localization of LHRH-receptors in Calu-3 cells was determined using immunochemical methods. To determine the mechanisms of deslorelin transport, the directionality [apical-basolateral (A-B) and basolateral-apical (B-A)] of deslorelin transport across Calu-3 monolayers and the effects of temperature (37 degrees C and 4 degrees C) and an energy depletor (2,4-dinitrophenol) were investigated. To determine the role of LHRH-receptor in deslorelin transport across Calu-3 monolayers, the influence of an LHRH-receptor antisense oligonucleotide on the LHRH-receptor expression and deslorelin transport was studied. Also, the effect of a competing LHRH agonist, buserelin, on deslorelin transport was determined.

RESULTS

Immunofluorescence studies indicated the predominance of LHRH-receptor in Calu-3 cells at the apical and lateral surfaces. Western blot and RT-PCR studies further confirmed the expression of LHRH-receptor in Calu-3 cells. Deslorelin transport across Calu-3 monolayers was vectorial, with the cumulative A-B transport (1.79 +/- 0.29%) at the end of 240 min being higher than the B-A transport (0.34 +/- 0.11%). Low temperature as well as 2,4-dinitrophenol abolished this directionality. LHRH-receptor antisense oligonucleotide decreased the receptor expression at the mRNA and protein level and reduced the A-B deslorelin transport by 55 +/- 4%, without affecting the B-A transport, suggesting a role for LHRH-receptor in the vectorial transport of deslorelin. In addition, buserelin reduced the A-B deslorelin transport by 56 +/- 5% without affecting the B-A transport.

CONCLUSIONS

Taken together, our results provide evidence that deslorelin is transported across the respiratory epithelium via the LHRH-receptor.

Oleate promotes the proliferation of breast cancer cells via the G protein-coupled receptor GPR40

Evidence from epidemiological studies and animal models suggests a link between high levels of dietary fat intake and risk of breast cancer. In addition, obesity, in which circulating lipids are elevated, is associated with increased risk of various cancers. Relative to this point, we previously showed that oleate stimulates the proliferation of breast cancer cells and that phosphatidylinositol 3-kinase plays a role in this process. Nonetheless, questions remain regarding the precise mechanism(s) by which oleate promotes breast cancer cell growth. Pharmacological inhibitors of the GTP-binding proteins G(i)/G(o), phospholipase C, Src, and mitogenic-extracellular signal-regulated kinase 1/2 (MEK 1/2) decreased oleate-induced [3H]thymidine incorporation in the breast cancer cell line MDA-MB-231. In addition, oleate caused a rapid and transient rise in cytosolic Ca2+ and an increase in protein kinase B phosphorylation. Overexpressing in these cells the G protein-coupled receptor GPR40, a fatty acid receptor, amplified oleate-induced proliferation, whereas silencing the GPR40 gene using RNA interference decreased it. Overexpressing GPR40 in T47D and MCF-7 breast cancer cells that are poorly responsive to oleate allowed a robust proliferative action of oleate. The data indicate that the phospholipase C, MEK 1/2, Src, and phosphatidylinositol 3-kinase/protein kinase B signaling pathways are implicated in the proliferative signal induced by oleate and that these effects are mediated at least in part via the G protein-coupled receptor GPR40. The results suggest that GPR40 is implicated in the control of breast cancer cell growth by fatty acids and that GPR40 may provide a link between fat and cancer.

Regulation of KCNQ2/KCNQ3 current by G protein cycling: the kinetics of receptor-mediated signaling by Gq

Receptor-mediated modulation of KCNQ channels regulates neuronal excitability. This study concerns the kinetics and mechanism of M₁ muscarinic receptor–mediated regulation of the cloned neuronal M channel, KCNQ2/KCNQ3 (Kv7.2/Kv7.3). Receptors, channels, various mutated G-protein subunits, and an optical probe for phosphatidylinositol 4,5-bisphosphate (PIP₂) were coexpressed by transfection in tsA-201 cells, and the cells were studied by whole-cell patch clamp and by confocal microscopy. Constitutively active forms of Gα_q and Gα₁₁, but not Gα₁₃, caused a loss of the plasma membrane PIP₂ and a total tonic inhibition of the KCNQ current. There were no further changes upon addition of the muscarinic agonist oxotremorine-M (oxo-M). Expression of the regulator of G-protein signaling, RGS2, blocked PIP₂ hydrolysis and current suppression by muscarinic stimulation, confirming that the G_q family of G-proteins is necessary. Dialysis with the competitive inhibitor GDPβS (1 mM) lengthened the time constant of inhibition sixfold, decreased the suppression of current, and decreased agonist sensitivity. Removal of intracellular Mg²⁺ slowed both the development and the recovery from muscarinic suppression. When combined with GDPβS, low intracellular Mg²⁺ nearly eliminated muscarinic inhibition. With nonhydrolyzable GTP analogs, current suppression developed spontaneously and muscarinic inhibition was enhanced. Such spontaneous suppression was antagonized by GDPβS or GTP or by expression of RGS2. These observations were successfully described by a kinetic model representing biochemical steps of the signaling cascade using published rate constants where available. The model supports the following sequence of events for this G_q-coupled signaling: A classical G-protein cycle, including competition for nucleotide-free G-protein by all nucleotide forms and an activation step requiring Mg²⁺, followed by G-protein–stimulated phospholipase C and hydrolysis of PIP₂, and finally PIP₂ dissociation from binding sites for inositol lipid on the channels so that KCNQ current was suppressed. Further experiments will be needed to refine some untested assumptions.

The C-terminal tail of the M3-muscarinic receptor possesses anti-apoptotic properties

This study investigates the mechanisms by which the muscarinic receptor gene family can protect against apoptosis. Chinese hamster ovary cells transfected with human muscarinic receptor subtypes underwent apoptotic cell death following treatment with the DNA-damaging agent etoposide. Apoptosis was significantly reduced following muscarinic receptor stimulation of cells that were transfected with receptor subtypes that couple to the Gq/11/phospholipase C pathway, namely M1, M3, and M5. No protection was detected in cells transfected with the Gi-coupled M2 and M4 receptors. Further analysis of the Gq/11-coupled M3 receptor revealed that truncation of the carboxyl-tail (Delta 565-M3 mutant) removed the ability of the receptor to protect against etoposide-induced cell death. This mutation did not affect the ability of the receptor to signal through the phospholipase C pathway. Furthermore, activation of the Delta 565-M3 receptor resulted in robust activation of the extracellular-regulated kinase (ERK) and c-Jun kinase (JNK). The Delta 565-M3 receptor mutant also underwent agonist-driven phosphorylation in a similar manner to the wild-type receptor indicating that the anti-apoptotic effect of the M3 receptor is independent of receptor phosphorylation. Consistent with this was the fact that two M3-muscarinic receptor mutants deficient in agonist-induced receptor phosphorylation were capable of producing a full anti-apoptotic response. We conclude that the anti-apoptotic response of the muscarinic receptor family was confined to the Gq/11-coupled members of this family. The direct involvement of Gq/11/phospholipase C signaling and the ERK-1/2 and JNK pathways together with receptor phosphorylation in the anti-apoptotic response were eliminated. Mutation of a poly-basic region within the short C-terminal tail of the M3-muscarinic receptor inhibited the ability of the receptor to induce an anti-apoptotic response. We conclude that the conserved poly-basic region in the C-terminal tail of the M1, M3, and M5 receptors contributes to the ability of these receptors to mediate protection against apoptotic cell death.

5-HT2A and 5-HT2C receptors and their atypical regulation properties

The 5-HT(2A) and 5-HT(2C) receptors belong to the G-protein-coupled receptor (GPCR) superfamily. GPCRs transduce extracellular signals to the interior of cells through their interaction with G-proteins. The 5-HT(2A) and 5-HT(2C) receptors mediate effects of a large variety of compounds affecting depression, schizophrenia, anxiety, hallucinations, dysthymia, sleep patterns, feeding behaviour and neuro-endocrine functions. Binding of such compounds to either 5-HT(2) receptor subtype induces processes that regulate receptor sensitivity. In contrast to most other receptors, chronic blockade of 5-HT(2A) and 5-HT(2C) receptors leads not to an up- but to a (paradoxical) down-regulation. This review deals with published data involving such non-classical regulation of 5-HT(2A) and 5-HT(2C) receptors obtained from in vivo and in vitro studies. The underlying regulatory processes of the agonist-induced regulation of 5-HT(2A) and 5-HT(2C) receptors, commonly thought to be desensitisation and resensitisation, are discussed. The atypical down-regulation of both 5-HT(2) receptor subtypes by antidepressants, antipsychotics and 5-HT(2) antagonists is reviewed. The possible mechanisms of this paradoxical down-regulation are discussed, and a new hypothesis on possible heterologous regulation of 5-HT(2A) receptors is proposed.

Angiotensin and calcium signaling in the pituitary and hypothalamus

1) In the rat pituitary, angiotensin type 1B receptors (AT1B) are located in lactotrophs and corticotrophs. 2) Activation of AT1B receptors are coupled to Gq/11 (Guanine protein coupled receptor, or GPCR); they increase phospholipase beta C (PLC) activity resulting in inositol 1,4,5 triphosphate (InsP3) and diacylglycerol (DAG) formation. A biphasic increase in [Ca2+]i triggered by InsP3 and DAG ensues. 3) As many GPCRs, AT1B pituitary receptors rapidly desensitize. 4) This was observed in the generation of InsP3, the mobilization of intracellular Ca(2+), and in prolactin release. Both homologous and heterologous desensitization was evidenced. 5) Desensitization of the angiotensin II type 1 (AT1) receptor in the pituitary shares similarities and differences with endogenously expressed or transfected AT1 receptors in different cell types. 6) In the pituitary hyperplasia generated by chronic estrogen treatment there was desensitization or alteration in angiotensin II (Ang II) evoked intracellular Ca2+ increase, InsP3 generation, and prolactin release. This correlates with a downregulation of AT1 receptors. 7) In particular, in hyperplastic cells Ang II failed to evoke a transient acute peak in [Ca2+]i, which was replaced by a persistent plateau phase of [Ca2+]i increase. 8) Different calcium channels participate in Ang II induced [Ca2+]i increase in control and hyperplastic cells. While spike phase in control cells is dependent on intracellular stores sensitive to thapsigargin, in hyperplastic cells plateau increase is dependent on extracellular calcium influx. 9) Signal transduction of the AT1 pituitary receptor is greatly modified by hyperplasia, and it may be an important mechanism in the control of the hyperplastic process. 10) In the hypothalamus and brain stem there is a predominant expression of AT1A and AT2 mRNA. 11) Ang II acts at specific receptors located on neurons in the hypothalamus and brain stem to elicit alterations in blood pressure, fluid intake, and hormone secretion. 12) Calcium channels play important roles in the Ang II induced behavioral and endocrine responses. 13) Ang II, in physiological concentrations, can activate AT1 receptors to stimulate both Ca2+ release from intracellular stores and Ca2+ influx from the extracellular space to increase [Ca2+]i in polygonal and stellate astroglia of the hypothalamus and brain stem. 14) In primary cell culture of neurons from newborn rat hypothalamus and brain stem, it has also been determined that Ang II elicits an AT1 receptor mediated inhibition of delayed rectifier K(+) current and a stimulation of Ca2+ current. 15) In primary cell cultures derived from the subfornical organ or the organum vasculosum laminae terminalis of newborn rat pups, Ang II produced a pronounced desensitization of the [Ca2+]i response. 16) Hypothalamic and pituitary Ang II systems are involved in different functions, some of which are related. At both levels Ang II signals through [Ca2+]i in a characteristic way.

Regulation of VEGF-induced endothelial cell PAF synthesis: role of p42/44 MAPK, p38 MAPK and PI3K pathways

1. Vascular endothelial growth factor (VEGF) is a potent angiogenic and inflammatory mediator. We have recently shown that this latter effect requires the activation of Flk-1 receptor and subsequent endothelial cell (EC) PAF synthesis. However, the intracellular events that regulate EC PAF synthesis upon Flk-1 stimulation by VEGF remain to be elucidated. 2. Using specific inhibitors and Western blot analysis, we herein report that in bovine aortic endothelial cells (BAEC), VEGF induces the synthesis of PAF through the cascade activation of Flk-1 receptor, phospholipase Cgamma (PLCgamma), protein kinase C (PKC) and p42/44 mitogen-activated protein kinases (MAPK). 3. Moreover, we demonstrate that VEGF-mediated PAF synthesis requires the activation of p38 MAPK, likely by directing the conversion of lyso-PAF to PAF. 4. Interestingly, we observed that VEGF also promoted the activation of the phosphatidyl inositol-3-phosphate kinase (PI3K) pathway, and that its blockade potentiated PAF synthesis following a VEGF treatment. Consequently, it appears that the PI3K pathway acts as a negative regulator of EC PAF synthesis. 5. Taken together, these results allow a better understanding of the intracellular events activated upon EC stimulation by VEGF, and shed a new light on the mechanisms by which VEGF induces PAF synthesis.

Homologous and heterologous uncoupling of muscarinic M(3) and alpha(1B) adrenoceptors to Galpha(q/11) in SH-SY5Y human neuroblastoma cells

1. The present study employed a [(35)S]-GTPgammaS binding protocol in conjunction with immunoprecipitation (IP) of the Galpha subunits to investigate the desensitization of G(q/11)-coupled receptors at the level of the G-protein activation. Membranes from SH-SY5Y cells expressing the recombinant human alpha(1B)-adrenoceptor (alpha(1B)-AR) (and endogenously expressing the M(3) muscarinic acetylcholine receptor (M(3)-AChR)) exhibited G(q/11) activation in a concentration-dependent manner in response to noradrenaline or methacholine. 2. Pre-treatment of intact cells with agonist prior to membrane preparation and use in the [(35)S]-GTPgammaS IP assay demonstrated that both receptors were homologously desensitized by pre-treatment with agonist since the G(q/11) activation in response to a secondary challenge with agonist was markedly reduced. Stimulation of alpha(1B)-AR was effective at heterologously desensitizing the M(3)-AChR. The PKC inhibitor, Ro-31-8220 (10 microM) was ineffective at preventing the agonist-mediated receptor desensitization. 3. [(32)P]P(i)-labelled cells allowed the detection of increases in receptor phosphorylation. Phorbol 12,13 dibutyrate (PDBu) (1 microM) was effective at producing a Ro-31-8220 (10 microM)-sensitive, detectable increase in alpha(1B)-AR but not M(3)-AChR phosphorylation. Noradrenaline (30 microM) stimulated alpha(1B)-AR phosphorylation, which could be partially inhibited by Ro-31-8220 (10 microM). The phosphorylation of M(3)-AChR was increased by methacholine (100 microM) incubation and this effect appeared to be insensitive to Ro-31-8220 (10 microM). 4. These findings demonstrate that [(35)S]-GTPgammaS-Galpha-subunit IP can be used to estimate receptor desensitization as a decline in receptor-G-protein coupling. Both the alpha(1B)-AR and M(3)-AChR undergo rapid homologous desensitization that is associated with an increase in receptor phosphorylation. The heterologous desensitization of M(3)-AChR produced by alpha(1B)-AR stimulation is not associated with a detectable increase in M(3)-AChR phosphorylation, suggesting that receptor phosphorylation is not necessarily a prerequisite for desensitization.

G protein-coupled receptor kinases 3 and 6 use different pathways to desensitize the endogenous M3 muscarinic acetylcholine receptor in human SH-SY5Y cells

We have investigated the effects of G protein-coupled receptor kinase (GRK) 3 and GRK6 on the phosphorylation and regulation of the M3 muscarinic acetylcholine receptor (mACh) endogenously expressed in SH-SY5Y cells. Overexpression of GRK3 or GRK6 enhanced M3 mACh receptor phosphorylation after high-concentration methacholine (100 microM, 1 min) addition. However, GRK6 was more potent, increasing receptor phosphorylation even after low (3 microM, 1 min) agonist stimulation. Compared with plasmid-transfected control cells expressing equivalent M3 mACh receptor number, GRK3- or GRK6-overexpressing cells exhibited a reduced phospholipase C activity reflected by a lower accumulation of total [3H]inositol phosphates and Ins(1,4,5)P3 mass. In addition, direct stimulation of G protein activation of phospholipase C (by AlF4(-)) was inhibited in GRK3- but not GRK6-overexpressing cells. Guanosine-5'-O-(3-[35S]thio)triphosphate binding and immunoprecipitation of Galpha(q/11) indicated that acute methacholine-stimulated receptor/Galpha(q/11) coupling was unaffected by GRK overexpression. In contrast, agonist pretreatment of cells for 3 min caused M3 mACh receptor uncoupling from Galpha(q/11), which was markedly enhanced by GRK6 overexpression, particularly at lower agonist pretreatment concentrations. However, the increased M3 mACh receptor phosphorylation seen in clones overexpressing GRK3 was not accompanied by increased receptor-Galpha(q/11) uncoupling. Overall, these data suggest that GRK3 and GRK6 use different pathways to desensitize the M3 mACh receptor. GRK6 seems to act as a classical GRK, inducing increased receptor phosphorylation accompanied by an uncoupling of receptor and Galpha(q/11). Conversely, GRK3 may cause desensitization independently of receptor phosphorylation, possibly via Gbetagamma binding and/or direct Galpha(q) binding via its regulator of G protein signaling domain to inhibit phospholipase C activity.

Mechanism of GnRH receptor signaling on gonadotropin release and gene expression in pituitary gonadotrophs

Gonadotropin releasing hormone (GnRH), the first key hormone of reproduction, is synthesized and secreted from the hypothalamus in a pulsatile manner and stimulates pituitary gonadotrophs (5-10% of the pituitary cells) to synthesize and release gonadotropin luteinizing hormone (LH) and follicle stimulating hormone (FSH). Gonadotrophs consist of 60% multihormonal cells (LH+FSH) and 18% LH- and 22% FSH-containing cells. LH and FSH, members of the glycoprotein hormone family, stimulate spermatogenesis, folliculogenesis, and ovulation. Although GnRH plays a pivotal role in gonadotropin synthesis and release, other factors such as gonadal steroids and gonadal peptides exert positive and negative feedback mechanisms, which affect GnRH actions. GnRH actions include activation of phosphoinositide turnover as well as phospholipase D and A2, mobilization and influx of Ca2+, activation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). A complex crosstalk between the above messenger molecules mediates the diverse actions of GnRH. Understanding the signaling mechanisms involved in GnRH actions is the basis for our understanding of basic reproductive functions in general and gonadotropin synthesis and release in particular.

Desensitization of angiotensin II: effect on [Ca2+]i, inositol triphosphate, and prolactin in pituitary cells

Activation of pituitary angiotensin (ANG II) type 1 receptors (AT1) mobilizes intracellular Ca2+, resulting in increased prolactin secretion. We first assessed desensitization of AT1 receptors by testing ANG II-induced intracellular Ca2+ concentration ([Ca2+](i)) response in rat anterior pituitary cells. A period as short as 1 min with 10(-7) M ANG II was effective in producing desensitization (remaining response was 66.8 +/- 2.1% of nondesensitized cells). Desensitization was a concentration-related event (EC(50): 1.1 nM). Although partial recovery was obtained 15 min after removal of ANG II, full response could not be achieved even after 4 h (77.6 +/- 2.4%). Experiments with 5 x 10(-7) M ionomycin indicated that intracellular Ca2+ stores of desensitized cells had already recovered when desensitization was still significant. The thyrotropin-releasing hormone (TRH)-induced intracellular Ca2+ peak was attenuated in the ANG II-pretreated group. ANG II pretreatment also desensitized ANG II- and TRH-induced inositol phosphate generation (72.8 +/- 3.5 and 69.6 +/- 6.1%, respectively, for inositol triphosphate) and prolactin secretion (53.4 +/- 2.3 and 65.1 +/- 7.2%), effects independent of PKC activation. We conclude that, in pituitary cells, inositol triphosphate formation, [Ca2+](i) mobilization, and prolactin release in response to ANG II undergo rapid, long-lasting, homologous and heterologous desensitization.

P2Y2 purinergic and M3 muscarinic acetylcholine receptors activate different phospholipase C-beta isoforms that are uniquely susceptible to protein kinase C-dependent phosphorylation and inactivation

Activation of phospholipase C-beta (PLC-beta) by G protein-coupled receptors typically results in rapid but transient second messenger generation. Although PLC-beta deactivation may contribute to the transient nature of this response, the mechanisms governing PLC-beta deactivation are poorly characterized. We investigated the involvement of protein kinase C (PKC) in the termination of PLC-beta activation induced by endogenous P2Y(2) purinergic receptors and transfected M(3) muscarinic acetylcholine receptors (mAChR) in Chinese hamster ovary cells. Activation of P2Y(2) receptors causes Galpha(q/11) to associate with PLC-beta3, whereas M(3) mAChR activation causes Galpha(q/11) to associate with both PLC-beta1 and PLC-beta3 in these cells. Phosphorylation of PLC-beta3, but not PLC-beta1, is induced by activating either P2Y(2) receptors or M(3) mAChR. We demonstrate that PKC rather than protein kinase A mediates the G protein-coupled receptor-induced phosphorylation of PLC-beta3. The PKC-mediated phosphorylation of PLC-beta3 diminishes the interaction of Galpha(q/11) with PLC-beta3, thereby contributing to the termination PLC-beta3 activity. These findings indicate that the distinct temporal profiles of PLC activation by P2Y(2) receptors and mAChR may arise from the differential activation of PLC-beta1 and PLC-beta3 by the receptors, coupled with a selective PKC-mediated negative feedback mechanism that targets PLC-beta3 but not PLC-beta1.

The expression, regulation and signal transduction pathways of the mammalian gonadotropin-releasing hormone receptor

Normal mammalian sexual maturation and reproductive functions require the integration and precise coordination of hormones at the hypothalamic, pituitary, and gonadal levels. Hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator in this system; after binding to its receptor (GnRHR), it stimulates de novo synthesis and release of gonadotropins in anterior pituitary gonadotropes. Since the isolation of the GnRHR cDNA, the expression of GnRHR mRNA has been detected not only in the pituitary, but also in extrapituitary tissues, including the ovary and placenta. It has been shown that change in GnRHR mRNA is one of the mechanisms for regulating the expression of the GnRHR. To help understand the molecular mechanism(s) involved in transcriptional regulation of the GnRHR gene, the 5' flanking region of the GnRHR gene has recently been isolated. Initial characterization studies have identified several DNA regions in the GnRHR 5' flanking region which are responsible for both basal expression and GnRH-mediated homologous regulation of this gene in pituitary cells. The mammalian GnRHR lacks a C-terminus and possesses a relatively short third intracellular loop; both features are important in desensitization of many others G-protein coupled receptors (GPCRs), Homologous desensitization of GnRHR has been shown to be regulated by various serine-threonine protein kinases including protein kinase A (PKA) and protein kinase C (PKC), as well as by G-protein coupled receptor kinases (GRKs). Furthermore, GnRHR was demonstrated to couple with multiple G proteins (Gq/11, Gs, and Gi), and to activate cascades that involved the PKC, PKA, and mitogen-activator protein kinases. These results suggest the diversity of GnRHR-G protein coupling and signal transduction systems. The identification of second form of GnRH (GnRH-II) in mammals adds to the complexity of the GnRH-GnRHR system. This review summaries our recent progress in understanding the regulation of GnRHR gene expression and the GnRHR signal transduction pathways.Key words: gonadotropin-releasing hormone receptor, transcriptional regulation, desensitization, signal transduction.

Complex involvement of pertussis toxin-sensitive G proteins in the regulation of type 1alpha metabotropic glutamate receptor signaling in baby hamster kidney cells

Previously, we demonstrated that the coupling of the metabotropic glutamate receptor mGlu1alpha to phosphoinositide hydrolysis is enhanced by pertussis toxin (PTX) in stably transfected baby hamster kidney cells (BHK). Here, we show that the PTX effect on agonist-stimulated [(3)H]inositol phosphate accumulation can be resolved into two components: an immediate increase in agonist potency, and a more slowly developing increase in the magnitude of the response observed at maximally effective agonist concentrations. Using G(q/11)alpha- and G(i/o)alpha-selective antibodies to immunoprecipitate [(35)S]guanosine-5'-O-(3-thio)triphosphate-bound Galpha proteins, we also show that agonist stimulation of mGlu1alpha in BHK membranes increases specific [(35)S]guanosine-5'-O-(3-thio)triphosphate binding to both G(q/11) and G(i/o) proteins. Preincubation of BHK-mGlu1alpha with L-glutamate (300 microM) results in a progressive loss (60% in 30 min) of L-quisqualate-induced [(3)H]inositol phosphate accumulation (without a change in potency), providing evidence for agonist-induced receptor desensitization. Although such desensitization of mGlu receptor signaling was mimicked by a phorbol ester, agonist-induced phosphorylation of the receptor was not observed and protein kinase C inhibition by Ro 31-8220 did not prevent L-glutamate-mediated desensitization. In contrast, PTX treatment of the cells almost completely prevented L-glutamate-mediated desensitization. Together, these data provide evidence for a multifunctional coupling of mGlu1alpha to different types of G proteins, including PTX-sensitive G(i)-type G proteins. The latter are involved in the negative control of phospholipase C activity while also influencing the rate of desensitization of the mGlu1alpha receptor.

Phosphorylation and regulation of a Gq/11-coupled receptor by casein kinase 1alpha

Agonist-mediated receptor phosphorylation by one or more of the members of the G-protein receptor kinase (GRK) family is an established model for G-protein-coupled receptor (GPCR) phosphorylation resulting in receptor desensitization. Our recent studies have, however, suggested that an alternative route to GPCR phosphorylation may be an operation involving casein kinase 1alpha (CK1alpha). In the current study we investigate the involvement of CK1alpha in the phosphorylation of the human m3-muscarinic receptor in intact cells. We show that expression of a catalytically inactive mutant of CK1alpha, designed to act in a dominant negative manner, inhibits agonist-mediated receptor phosphorylation by approximately 40% in COS-7 and HEK-293 cells. Furthermore, we present evidence that a peptide corresponding to the third intracellular loop of the m3-muscarinic receptor (Ser(345)-Leu(463)) is an inhibitor of CK1alpha due to its ability to both act as a pseudo-substrate for CK1alpha and form a high affinity complex with CK1alpha. Expression of this peptide was able to reduce both basal and agonist-mediated m3-muscarinic receptor phosphorylation in intact cells. These results support the notion that CK1alpha is able to mediate GPCR phosphorylation in an agonist-dependent manner and that this may provide a novel mechanism for GPCR phosphorylation. The functional role of phosphorylation was investigated using a mutant of the m3-muscarinic receptor that showed an approximately 80% reduction in agonist-mediated phosphorylation. Surprisingly, this mutant underwent agonist-mediated desensitization suggesting that, unlike many GPCRs, desensitization of the m3-muscarinic receptor is not mediated by receptor phosphorylation. The inositol (1,4, 5)-trisphosphate response did, however, appear to be dramatically potentiated in the phosphorylation-deficient mutant indicating that phosphorylation may instead control the magnitude of the initial inositol phosphate response.

Selective regulation of Galpha(q/11) by an RGS domain in the G protein-coupled receptor kinase, GRK2

G protein-coupled receptor kinases (GRKs) are well characterized regulators of G protein-coupled receptors, whereas regulators of G protein signaling (RGS) proteins directly control the activity of G protein alpha subunits. Interestingly, a recent report (Siderovski, D. P., Hessel, A., Chung, S., Mak, T. W., and Tyers, M. (1996) Curr. Biol. 6, 211-212) identified a region within the N terminus of GRKs that contained homology to RGS domains. Given that RGS domains demonstrate AlF(4)(-)-dependent binding to G protein alpha subunits, we tested the ability of G proteins from a crude bovine brain extract to bind to GRK affinity columns in the absence or presence of AlF(4)(-). This revealed the specific ability of bovine brain Galpha(q/11) to bind to both GRK2 and GRK3 in an AlF(4)(-)-dependent manner. In contrast, Galpha(s), Galpha(i), and Galpha(12/13) did not bind to GRK2 or GRK3 despite their presence in the extract. Additional studies revealed that bovine brain Galpha(q/11) could also bind to an N-terminal construct of GRK2, while no binding of Galpha(q/11), Galpha(s), Galpha(i), or Galpha(12/13) to comparable constructs of GRK5 or GRK6 was observed. Experiments using purified Galpha(q) revealed significant binding of both Galpha(q) GDP/AlF(4)(-) and Galpha(q)(GTPgammaS), but not Galpha(q)(GDP), to GRK2. Activation-dependent binding was also observed in both COS-1 and HEK293 cells as GRK2 significantly co-immunoprecipitated constitutively active Galpha(q)(R183C) but not wild type Galpha(q). In vitro analysis revealed that GRK2 possesses weak GAP activity toward Galpha(q) that is dependent on the presence of a G protein-coupled receptor. However, GRK2 effectively inhibited Galpha(q)-mediated activation of phospholipase C-beta both in vitro and in cells, possibly through sequestration of activated Galpha(q). These data suggest that a subfamily of the GRKs may be bifunctional regulators of G protein-coupled receptor signaling operating directly on both receptors and G proteins.

Receptor phosphorylation does not mediate cross talk between muscarinic M(3) and bradykinin B(2) receptors

This study examined cross talk between phospholipase C-coupled muscarinic M(3) and bradykinin B(2) receptors coexpressed in Chinese hamster ovary (CHO) cells. Agonists of either receptor enhanced phosphoinositide signaling (which rapidly desensitized) and caused protein kinase C (PKC)-independent, homologous receptor phosphorylation. Muscarinic M(3) but not bradykinin B(2) receptors were also phosphorylated after phorbol ester activation of PKC. Consistent with this, muscarinic M(3) receptors were phosphorylated in a PKC-dependent fashion after bradykinin B(2) receptor activation, but muscarinic M(3) receptor activation did not influence bradykinin B(2) receptor phosphorylation. Despite heterologous phosphorylation of muscarinic M(3) receptors, phosphoinositide and Ca(2+) signaling were unaffected. In contrast, marked heterologous desensitization of bradykinin-mediated responses occurred despite no receptor phosphorylation. This desensitization was associated with a sustained component of muscarinic receptor-mediated signaling, whereas bradykinin's inability to influence muscarinic receptor-mediated responses was associated with rapid and full desensitization of bradykinin responses. Thus the mechanism of functional cross talk most likely involves depletion of a shared signaling component. These data demonstrate that receptor phosphorylation is not a prerequisite for heterologous desensitization and that such desensitization is not obligatory after heterologous receptor phosphorylation.

Lack of a C-terminal tail in the mammalian gonadotropin-releasing hormone receptor confers resistance to agonist-dependent phosphorylation and rapid desensitization

The mammalian gonadotropin-releasing hormone receptor (GnRH-R) is, at present, the only G-protein-coupled receptor that activates phospholipase C and lacks a C-terminal tail. We have previously demonstrated that this unique structural feature is associated with resistance to rapid desensitization of phosphoinositide signaling in COS-7 and HEK-293 cells (Heding, A., Vrecl, M., Bogerd, J., McGregor, A., Sellar, R., Taylor, P. L., and Eidne, K. A. (1998) J. Biol. Chem. 273, 11472-11477). Using receptors tagged with a nonapeptide of the influenza hemagglutinin protein to enable immunoprecipitation, we now demonstrate that the mammalian GnRH-R is not phosphorylated in an agonist-dependent manner. In contrast, the mammalian thyrotropin-releasing hormone receptor and the African catfish GnRH-R, both of which have a C-terminal tail, are phosphorylated in response to agonist challenge. Furthermore, chimeras of the mammalian GnRH-R with the C-terminal tail of either the mammalian thyrotropin-releasing hormone receptor or the catfish GnRH-R are also phosphorylated in an agonist-dependent manner. Only those receptors having C-terminal tails showed desensitization of phosphoinositide responses within 5-10 min of agonist challenge. We also show that the internalization of all these receptors when expressed transiently in COS-7 cells is similar. This dissociates receptor internalization from rapid desensitization and demonstrates that the lack of a C-terminal tail in the mammalian GnRH-R results in an inability of the receptor to undergo agonist-dependent phosphorylation and that this results directly in a resistance to rapid desensitization.

The tail of the gonadotrophin-releasing hormone receptor: desensitization at, and distal to, G protein-coupled receptors

In recent years a general scheme for the rapid desensitization and cycling of G protein-coupled receptors (GPCRs) has emerged. In this scheme agonist-induced phosphorylation (most often in the receptors' C-terminal tail) causes association with beta-arrestin which not only reduces the efficiency of G-protein activation, but also targets these desensitized receptors for internalization, after which they may be either proteolytically degraded or resensitized and recycled back to the cell surface. Although sustained stimulation of pituitary gonadotrophs with gonadotrophin-releasing hormone (GnRH) is known to cause a pronounced desensitization of GnRH-stimulated gonadotrophin secretion, the discovery that mammalian GnRH receptors do not possess C-terminal tails raised the question of whether receptor desensitization is involved. This review outlines data demonstrating that tail-less mammalian GnRH receptors can be considered as natural desensitization and internalization deficient 'mutants'. This is in stark contrast to non-mammalian GnRH receptors which do possess tails and conform to the general scheme. In the absence of receptor desensitization, post receptor mechanisms take on increasing importance for desensitization of GnRH action via mammalian GnRH receptors. The down regulation of Ins(1,4,5)P3 receptors and consequent desensitization of GnRH effects on cytosolic Ca2+ are discussed as a novel mechanism for such desensitization.

Muscarinic cholinergic receptor signal transduction as a potential target for the developmental neurotoxicity of ethanol

Central nervous system dysfunctions (most notably mental retardation and microcephaly) are among the most significant effects of in utero exposure to ethanol. Ethanol has been shown to cause alterations of both neuronal and glial cells, including cell loss, and changes in their migration and maturation. Here, we propose that one of the potential targets for the developmental neurotoxicity of ethanol may be represented by the signal transduction systems activated by cholinergic muscarinic receptors. Ethanol has been shown to inhibit second messenger systems activated by various G-protein-coupled receptors, including certain subtypes of muscarinic receptors. Although the roles of muscarinic receptors in brain development have not been fully elucidated, two potentially relevant effects have been discovered in the past few years. By activating muscarinic receptors coupled to phospholipid metabolism, acetylcholine can induce proliferation of glial cells, and act as a trophic factor in developing neurons by preventing apoptotic cell death. Ethanol has been shown to inhibit both actions of acetylcholine in vitro. These effects of ethanol may lead to a decreased number of glial cells and to a loss of neurons, which have been observed following in vivo alcohol exposure. In turn, these may be the basis of microencephaly and cognitive disturbances in children diagnosed with Fetal Alcohol Syndrome.

The lack of gonadotrophin-releasing hormone (GnRH) receptor desensitisation in alphaT3-1 cells is not due to GnRH receptor reserve or phosphatidylinositol 4,5-bis-phosphate pool size

The phospholipase C (PLC)-activating gonadotrophin-releasing hormone (GnRH) receptor is thought not to rapidly desensitise in alphaT3-1 cells. This extremely unusual characteristic raises the concern that it might be a feature of the cell type, rather than the receptor per se. Here we have used video imaging to establish whether the effects of endogenous PLC-activating G-protein coupled receptors (GPCRs) on Ca2+ ion concentration [Ca2+]i desensitise in these cells. Oxytocin, endothelin-1, methacholine, and UTP all caused [Ca2+]i increases which underwent rapid homologous desensitisation in that they were transient and responses to repeat stimuli were attenuated whereas subsequent responses to GnRH were not. To test whether receptor reserve obscures functional desensitisation of GnRH receptors, a photoaffinity antagonist (Pant-1), was used to effect a partial and irreversible receptor blockade. UV crosslinking in medium with 1000 nM Pant-1 reduced GnRH receptor number to 20 +/- 5% and reduced maximal buserelin-stimulated [3H]IP(X) accumulation to 57 +/- 5%, demonstrating removal of receptor reserve. In control alphaT3-1 cells the initial rate of GnRH-stimulated [3H]IP(X) accumulation was maintained for at least 5 min and GnRH caused a sustained increase in Ins(1,4,5)P3 mass (confirming the resistance of GnRH receptors to desensitisation) and Pant-1 pre-treatment reduced the magnitude of these responses without altering their temporal profiles. In alphaT3-1 cells stably transfected with recombinant human muscarinic receptors (alphaT3-1/M3), responses to methacholine were characteristic of desensitising GPCRs (transient Ins(1,4,5)P3 and curvilinear [3H]IP(X) responses) and were unaltered by Pant-1. To test the relevance of phospholipid pool size, alphaT3-1/M3 cells were pre-treated with GnRH or methacholine in medium with LiCl (to deplete PtdIns(4,5)P2 pools). These pre-treatments reduced subsequent responses to methacholine and GnRH comparably, indicating access to a shared PtdIns(4,5)P2 pool. Partial depletion of this pool (GnRH pre-treatment in medium with LiCl) reduced the magnitude of the [3H]IP(X) and Ins(1,4,5)P3 responses to methacholine and GnRH, without altering their temporal profiles. Thus the GnRH receptor does not undergo rapid homologous desensitisation in alphaT3-1 cells in spite of the fact that they can desensitise other endogenous (and recombinant) PLC-activating GPCRs, and the lack of desensitisation cannot be attributed to the existence of GnRH receptor reserve or access to an atypically large or rapidly re-cycled PtdIns(4,5)P2 pool. This unique functional characteristic (mammalian GnRH receptors are the only PLC-activating GPCRs known not to rapidly desensitise) almost certainly therefore reflects the atypical structure of these receptors (mammalian GnRH receptors are the only PLC-activating GPCRs known to lack C-terminal tails).

Mechanisms of regulation of neurotensin receptors

Since its discovery in 1973, the neuropeptide neurotensin has been demonstrated to be involved in the control of a broad variety of physiological activities in both the central nervous system and in the periphery. Pharmacological studies have shown that the biological effects elicited by neurotensin result from its specific binding to cell membrane neurotensin receptors that have been characterized in various tissue and in cell preparations. In addition, it is now well documented that most of these responses are subject to rapid desensitization. Such desensitization results in transient responses to sustained peptide applications, or to tachyphylaxis during successive stimulations in the same conditions. More recently, desensitization of neurotensin signalling was investigated at the cellular and molecular levels. In cultured cells, regulation at the second messenger level, receptor internalization, and receptor down-regulation processes have been reported. These are proposed to play a critical role in the control of cell responsiveness to neurotensin. This review aims to compile recent data on the different biochemical processes involved in the regulation of the neurotensin receptor and to discuss the physiological consequences of this regulation in vivo.