Prostaglandin E1-mediated, cyclic AMP-independent, down-regulation of Gs alpha in neuroblastoma x glioma hybrid cells

Up-regulation of μ-, δ- and κ-opioid receptors in concanavalin A-stimulated rat spleen lymphocytes

Regulation of μ-, δ- and κ-opioid receptor protein level in spleen lymphocytes when stimulated by mitogen is not known. To answer the question whether these cells do express opioid receptor (OR) proteins, primary, fresh rat spleen lymphocytes were prepared and stimulated for 48 h with mitogenic dose of Con A. The unstimulated lymphocytes did not express μ- and δ-OR proteins in detectable amounts, however, stimulation with Con A resulted in appearance of clearly detectable immunoblot signals of both μ-OR and δ-OR. κ-OR were detected already in primary cells and increased 2.4-fold in Con A-stimulated cells. These results were supported by data obtained by flow cytometry analysis indicating a dramatic increase in number of μ-, δ- and κ-OR expressing cells after mitogen stimulation. The newly synthesized μ-, δ- and κ-OR in Con A-stimulated spleen lymphocytes were present in the cells interior and not functionally mature, at least in terms of their ability to enhance activity of trimeric G proteins determined by three different protocols of agonist-stimulated, high-affinity [³⁵S]GTPγS binding assay. The up-regulation of μ-, δ- and κ-OR was associated with specific decrease of their cognate trimeric G proteins, G_i1α/G_i2α; the other Gα and Gβ subunits were unchanged. The level of β-arrestin-1/2 was also decreased in Con A-stimulated splenocytes. We conclude that up-regulation of OR expression level in spleen lymphocytes by Con A proceeds in conjunction with down-regulation of their intracellular signaling partners, G_i1α/G_i2α proteins and β-arrestin-1/2. These regulatory proteins are expressed in high amounts already in unstimulated cells and decreased by mitogen stimulation.

Long-term agonist stimulation of IP prostanoid receptor depletes the cognate G(s)alpha protein in membrane domains but does not change the receptor level

Iloprost (IP) stimulation (1 microM, 2 h) of Flag-epitope-tagged human IP prostanoid receptor (FhIPR) expressed in HEK293 cells resulted in specific decrease of endogenous G(s)alpha protein in detergent-insensitive, caveolin-enriched, membrane domains (DIMs). Receptor protein FhIPR, caveolin, G(i)alpha and GPI-linked, domain markers CD55 and CD59 were unchanged. The same result was obtained in HEK293 cells expressing FhIPR-G(s)alpha fusion protein. The endogenous G(s)alpha decreased, but the level of Flag-hIPR-G(s)alpha protein did not change. The specific depletion of domain-bound pool of G(s)alpha as consequence of iloprost stimulation was also demonstrated in membrane domains prepared according to alkaline treatment plus sonication protocol (detergent-free procedure of Song et al.). Our data further indicated that in control, quiescent cells only a very small amount of IP prostanoid receptor was present in DIMs together with large amount of its cognate G(s)alpha protein. Expressed in quantitative terms, DIMs contained 30-40% of the total cellular amount of G proteins whereas the content of IP prostanoid receptors was 1-3%. The dominant portion (>95%) of FhIPR as well as FhIPR-G(s)alpha was localised in high-density area of the gradient containing detergent-solubilised proteins. FhIPR and FhIPR-G(s)alpha distribution was similar to that of transmembrane plasma membrane (PM) markers (CD147, MHCI, CD29, Tapa1, the alpha subunit of Na,K-ATPase, transmembrane form of CD58 and CD44). All these proteins are known to be fully solubilised by detergent and thus unable to float in density gradient. Our data indicate that (i) long-term agonist stimulation of IP prostanoid receptor is associated with preferential decrease of its cognate G protein G(s)alpha from membrane domains; receptor level is unchanged. (ii) Very small fraction (1-3%) of total cellular amount of receptors is recovered in DIMs together with roughly 40% of G proteins. These data suggest a "supra-stoichiometric" arrangement of G proteins and corresponding receptors in DIMs.

Biochemical changes contributing to functional quiescence in lacrimal gland acinar cells after chronic ex vivo exposure to a muscarinic agonist

Profound secretory dysfunction can be associated with relatively modest lymphocytic infiltration of the lacrimal and salivary glands of Sjögren's syndrome (SjS) patients. SjS patients' sera contain autoantibodies to M3 muscarinic acetylcholine receptors (MAChR) that have variously been reported to have agonistic and antagonistic effects. We sought to identify consequences of chronic agonist stimulation by maintaining acinar cells from rabbit lacrimal glands for 20 h in the presence or absence of 10 microM carbachol (CCh). Exposure to CCh diminished the cells' ability to elevate cytosolic Ca2+ and secrete beta-hexosaminidase in response to acute stimulation with 100 microM CCh, but it enhanced their secretory responses to phenylephrine and ionomycin. Secretory vesicles appeared normal by electron microscopy, but confocal fluorescence microscopy revealed depletion of the secretory vesicle membrane marker, rab3D, and decreased ability to recruit secretory transport vesicles in response to acute 100 microM CCh. Additionally, the apical cortical actin cytoskeleton was disrupted and diminished compared to the basal-lateral cortical network. Subcellular fractionation analyses revealed that total membrane phase protein content was increased. The contents of beta-hexosaminidase and MAChR relative to total protein were not significantly altered, and MAChR abundance in the plasma membrane fraction was increased as the result of redistribution from endomembrane pools. However, relative cellular contents of the heterotrimeric guanosine triphosphate (GTP)-binding proteins, Gq and G11, were decreased. Additional biochemical changes included decreased contents of 47 kDa Gs and Gi3, protein kinase Calpha and rab3D and polymeric immunoglobulin (Ig) receptors; internalization of Na,K-ATPase from the plasma membranes to endomembrane compartments and decreased content of beta-hexosaminidase in the lysosomes. The observations demonstrate that chronic exposure to a MAChR agonist induces refractoriness to optimal stimulation, without causing receptor downregulation, by downregulating postreceptor-signalling mediators and effectors. The cells' secretory mechanisms for IgA and electrolytes also appear to be impaired, as does their ability to properly sort proteins to the lysosomes.

Vascular beta-adrenergic receptor adenylyl cyclase system in maturation and aging

The objective of this study was to determine how maturation and aging affects beta (beta)-adrenergic receptor (AR) control of arterial vasorelaxation. Left ventricular (LV) hemodynamics and arterial vasorelaxation in thoracic artery segments were studied in Brown Norway, Fisher 344 cross rats at 6 weeks, 6 months, and 23 months of age. We defined changes in maturation as occurring between 6 weeks and 6 months of age and changes in aging as occurring between 6 months and 23 months of age. With maturation, isoproterenol resulted in a downward shift in heart rate and an upward shift in both LV dP/dt and peripheral vascular resistance responses. Similar changes were noted with aging except for the downward shift in LV dP/dt isoproterenol response. There was a dose-dependent increase in arterial vasorelaxation in response to isoproterenol in all age groups, but the 6-week-old animals had a 5-fold (P<0.01) increase in vasorelaxation compared to other age groups. The isoproterenol-induced arterial vasorelaxation response was not altered by removal of the endothelium. The vasodilatory responses to nitroglycerin, acetylcholine, and adenosine were diminished (P<0.05) with aging. The vasorelaxation responses to forskolin and IBMX were unchanged with maturation and diminished with aging. Incubation of arterial rings in cholera toxin resulted in a reduction in relaxation only in arteries from 6-week-old rats. Maturation resulted in no change in beta -AR density [20.2+/-0.7 v. 18.5+/-0.5 fmol/mg protein, P=n.s., 6 weeks (n=2, 18 aortas were combined v 6-month-old rats)]. With maturation, there was no change in G alpha(i)level. However, beta ARK1 levels were increased (55. 4+/-2.1 v. 40.8+/-0.4, arbitrary densitometry units) and G alpha(s)levels were decreased (29.5+/-0.8 v. 49.9+/-1.9, arbitrary densitometry units). Aging resulted in no change in beta -AR density (15.3+/-1.7 v. 18.5+/-0.5 fmol/mg membrane protein), but decreases in basal, isoproterenol-, naF-, and forskolin-stimulated AC activities. Compared to 6 week data, 23-month-old rats exhibited no change in either G alpha(i)or beta ARK1, however, G alpha(s) was decreased. In summary, beta -AR-stimulated arterial vasorelaxation is depressed during maturation and aging. Since there is no change in beta -AR density but a decrease in G alpha(s)and in basal/stimulated AC activities, the defect in beta -AR signaling during maturation and aging is probably a post receptor defect, i.e. possibly in the receptor-G protein coupling.

Albuterol-induced downregulation of Gsalpha accounts for pulmonary beta(2)-adrenoceptor desensitization in vivo

The aim of the present study was to develop a chronic in vivo model of pulmonary beta(2)-adrenoceptor desensitization and to elucidate the nature and molecular basis of this state. Subcutaneous infusion of rats with albuterol for 7 days compromised the ability of albuterol, given acutely, to protect against acetylcholine-induced bronchoconstriction. The bronchoprotective effect of prostaglandin E(2), but not forskolin, was also impaired, indicating that the desensitization was heterologous and that the primary defect in signaling was upstream of adenylyl cyclase. beta(2)-Adrenoceptor density was reduced in lung membranes harvested from albuterol-treated animals, and this was associated with impaired albuterol-induced cyclic adenosine monophosphate (cAMP) accumulation and activation of cAMP-dependent protein kinase ex vivo. Gsalpha expression was reduced in the lung and tracheae of albuterol-treated rats, and cholera toxin-induced cAMP accumulation was blunted. Chronic treatment of rats with albuterol also increased cAMP phosphodiesterase activity and G protein-coupled receptor kinase-2, but the extent to which these events contributed to beta(2)-adrenoceptor desensitization was unclear given that forskolin was active in both groups of animals and that desensitization was heterologous. Collectively, these results indicate that albuterol effects heterologous desensitization of pulmonary Gs-coupled receptors in this model, with downregulation of Gsalpha representing a primary molecular etiology.

Neuronal signaling systems and ethanol dependence

In recent years there have been remarkable developments toward the understanding of the molecular and/or cellular changes in the neuronal second-messenger pathways during ethanol dependence. In general, it is believed that the cyclic adenosine 3',5'-monophosphate (cAMP) and the phosphoinositide (PI) signal-transduction pathways may be the intracellular targets that mediate the action of ethanol and ultimately contribute to the molecular events involved in the development of ethanol tolerance and dependence. Several laboratories have demonstrated that acute ethanol exposure increases, whereas protracted ethanol exposure decreases, agonist-stimulated adenylate cyclase activity in a variety of cell systems, including the rodent brain. Recent studies indicate that various postreceptor events of the cAMP signal transduction cascade (i.e., Gs protein, protein kinase A [PKA], and cAMP-responsive element binding protein [CREB]) in the rodent brain are also modulated by chronic ethanol exposure. The PI signal-transduction cascade represents another important second-messenger system that is modulated by both acute and chronic ethanol exposure in a variety of cell systems. It has been shown that protracted ethanol exposure significantly decreases phospholipase C (PLC) activity in the cerebral cortex of mice and rats. The decreased PLC activity during chronic ethanol exposure may be caused by a decrease in the protein levels of the PLC-beta 1 isozyme but not of PLC-delta 1 or PLC-gamma 1 isozymes in the rat cerebral cortex. Protein kinase C (PKC), which is a key step in the PI-signaling cascade, has been shown to be altered in a variety of cell systems by acute or chronic ethanol exposure. It appears from the literature that PKC plays an important role in the modulation of the function of various neurotransmitter receptors (e.g., gamma-aminobutyrate type A [GABAA], N-methyl-D-aspartate [NMDA], serotonin2A [5-HT2A], and 5-HT2C, and muscarinic [m1] receptors) resulting from ethanol exposure. The findings described in this review article indicate that neuronal-signaling proteins represent a molecular locus for the action of ethanol and are possibly involved in the neuro-adaptational mechanisms to protracted ethanol exposure. These findings support the notion that alterations in the cAMP and the PI-signaling cascades during chronic ethanol exposure could be the critical molecular events associated with the development of ethanol dependence.

Selective formation of Gsalpha-MHC I complexes after desensitization of human platelets with iloprost

Prolonged treatment of human platelets with the adenylate cyclase-stimulating prostacyclin analog iloprost leads to reduction in cAMP formation. Previous studies have demonstrated that this may be ascribed to modification of both receptor and Gsalpha function rather than of the catalytic component of adenylate cyclase [Mollner, S., Deppisch, H. & Pfeuffer, T. (1992) Eur. J. Biochem. 210, 539-544]. Iloprost-induced desensitization was accompanied by the formation of a Gsalpha-containing 90-kDa product in membranes treated with the bifunctional cross-linker 1,6-bismaleimidohexane. The cAMP-inducing prostanoid PGD2, which does not promote desensitization, did not cause formation of the 90-kDa species either. The long-term effect of the common G-protein activator [AlF4]- on human platelet adenylate cyclase was shown in many respects to be comparable with that of iloprost. However, [AlF4]- treatment also failed to induce the 90-kDa species, showing that different mechanisms of desensitization were operating. Treatment of the cross-linked 90-kDa complex with PNGase F demonstrated the glycoprotein nature of the Gsalpha-associated component. The 90-kDa cross-linked product was purified by consecutive immunoaffinity chromatography and preparative PAGE to apparent homogeneity. Analysis of the purified protein by MS suggested that, besides Gsalpha, the heavy chain of MHC I (HLA-A2) was part of the complex. This was confirmed by coprecipitation of Gsalpha by the monoclonal anti-(MHC I) antibody W6/32.

Agonist-mediated downregulation of G alpha i via the alpha 2-adrenergic receptor is targeted by receptor-Gi interaction and is independent of receptor signaling and regulation

One mechanism of long-term agonist-promoted desensitization of alpha2AR function is downregulation of the cellular levels of the alpha subunit of the inhibitory G protein, Gi. In transfected CHO cells expressing the human alpha2AAR, a 40.1 +/- 3.3% downregulation of Galphai2 protein occurred after 24 h of exposure of the cells to epinephrine, which was not accompanied by a decrease in Galphai2 mRNA. The essential step that targets Gi for degradation by agonist occupancy of the receptor was explored using mutated alpha2AAR lacking specific structural or functional elements. These consisted of 5HT1A receptor and beta2AR sequences substituted at residues 113-149 of the second intracellular loop and 218-235 and 355-371 of the N- and C-terminal regions of the third intracellular loop (altered Gi and Gs coupling), deletion of Ser296-299 (absent GRK phosphorylation), and substitution of Cys442 (absent palmitoylation and receptor downregulation). Of these mutants, only those with diminished Gi coupling displayed a loss of agonist-promoted Gi downregulation, thus excluding Gs coupling and receptor downregulation, palmitoylation, and phosphorylation as necessary events. Furthermore, coupling-impaired receptors consisting of mutations in the second or third loops ablated Gi downregulation, suggesting that a discreet structural motif of the receptor is unlikely to represent a key element in the process. While pertussis toxin ablated Gi downregulation, blocking downstream intracellular consequences of alpha2AAR activation or mimicking these pathways by heterologous means failed to implicate cAMP/adenylyl cyclase, phospholipase C, phospholipase D, or MAP kinase pathways in alpha2AAR-mediated Gi downregulation. Taken together, agonist-promoted Gi downregulation requires physical alpha2AAR-Gi interaction which targets Gi for degradation in a manner that is independent of alpha2AAR trafficking, regulation, or second messengers.

Agonist-induced internalization of the G protein G11alpha and thyrotropin-releasing hormone receptors proceed on different time scales

Using a combination of confocal immunofluorescence microscopy and subcellular fractionation, we demonstrate for the first time active internalization, trafficking, and down-regulation of a G protein alpha subunit subsequent to agonist occupation of a receptor. This proceeds on a much slower time scale than internalization of the corresponding receptor. In intact E2M11 HEK293 cells that express high levels of murine G11alpha and the rat thyrotropin-releasing hormone (TRH) receptor, the immunofluorescence signal of G11alpha was restricted almost exclusively to the plasma membrane. Exposure to TRH (10 microM) resulted first in partial relocation of G11alpha to discrete, segregated patches within the plasma membrane (10-60 min). Further exposure to TRH caused internalization of G11alpha to discrete, punctate, intracellular bodies (2-4 h) and subsequently to a virtually complete loss of G11alpha from plasma membranes and the cells (8-16 h). Short-term treatment with TRH followed by wash-out of the ligand allowed G11alpha immunofluorescence to be restored to the plasma membrane within 12 h. In subcellular membrane fractions, G11alpha was centered on plasma membranes, and this was not altered by up to 1-2 h of incubation with TRH. Further exposure to TRH (2-4 h) resulted in transfer of a significant portion of G11alpha to light-vesicular and cytosol fractions. At longer time intervals (4-16 h), an overall decrease in G11alpha content was observed.

Selective activation of a chimeric Gi1/Gs G protein alpha subunit by the human IP prostanoid receptor: analysis using agonist stimulation of high affinity GTPase activity and [35S]guanosine-5'-O-(3-thio)triphosphate binding

A FLAG-tagged form of the human IP prostanoid receptor was expressed stably in HEK 293 cells. This bound [3H]iloprost with high affinity and stimulated cAMP production when exposed to agonist. Iloprost produced weak stimulation of GTPase activity and [35S]guanosine-5'-O-(3-thio)triphosphate binding in membranes of these cells. Pretreatment of cells with pertussis toxin did not modify iloprost-mediated stimulation, but this was blocked by cholera toxin. The effects of iloprost were not increased by coexpression of either Gsalpha or Gi1alpha. In contrast, coexpression of a chimeric G protein alpha subunit in which the carboxyl-terminal six amino acids of Gi1alpha were altered to those of Gsalpha resulted in robust stimulation by iloprost. Because the chimeric G protein alpha subunit (Gi1/Gs6alpha) is not a substrate for either pertussis or cholera toxin, pretreatment of cells coexpressing the IP prostanoid receptor and Gi1/Gs6alpha with a mixture of these toxins resulted in resolution of the signal derived from activation of the chimeric G protein. Agonist-stimulated [35S]guanosine-5'-O-(3-thio)triphosphate binding and GTPase activity assays are the most commonly used strategies to examine interactions between G protein-coupled receptors and G proteins. These usually are not appropriate for receptors such as the IP prostanoid receptor that interact with G proteins with low rates of guanine nucleotide exchange and hydrolysis. Chimeric G proteins such as Gi1/Gs6alpha that allow appropriate receptor contacts to be converted to the higher nucleotide turnover rates typical of the Gi family G proteins can overcome this and offer a novel means to examine agonist function at such receptors.

Cholera toxin effects on body temperature changes induced by morphine

The present study evaluates the influence of cholera toxin and its B-subunit on thermic responses to morphine in the rats. The holotoxin (1 microg/rat) and the B-subunit (5 microg) were administered ICV and three days later rats were challenged ICV with morphine and tested for changes of body temperature. Cholera toxin, but not its B-subunit, modified the time course of the hyperthermic response induced by a low dose of morphine (2.5 microg), converted the hypothermia due to a higher dose of morphine (18 microg) to a consistent hyperthermia and only partially reduced the greater hypothermia induced by 36 microg of morphine. Cholera toxin-induced modifications of thermic responses to morphine were paralleled with a decreased Gs(alpha) immunoreactivity and a reduced ability for the toxin to catalyse the "in vitro" ADP-ribosylation of Gs(alpha) in hypothalamic membranes. In contrast, at the same time when morphine-induced effects on body temperature were assessed, no changes in pertussis toxin-mediated ADP-ribosylation of Gi(alpha)/Go(alpha), or basal adenylate cyclase activity, or binding of mu-opioid receptor selective ligand [3H]-DAMGO were observed in hypothalamic areas from rats treated with cholera toxin. These findings suggest that adaptative events secondary to prolonged activation of Gs(alpha) play a role in the modifications of thermic responses to morphine induced by CTX.

Nitric oxide donor SIN-1 mediated down-regulation of the G-protein alpha-subunit in C6 glioma cells

In C6 glioma cells, the nitric oxide (NO) donor 3-morpholinosynonimine hydrochloride (SIN-1) (0.5 mM) produced a significant decrease in the stimulatory G-protein alpha subunit (G alpha(s)) levels. Northern hydridization did not detect any differences in G alpha(s) mRNA levels after SIN-1 treatment. Furthermore SIN-1 increased endogenous and cholera toxin-catalyzed ADP-ribosylation of G alpha(s). 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) (0.5mM), a NO scavenger, had no effect on endogenous or cholera toxin-catalyzed ADP-ribosylation of G alpha(s), but reversed the increase in endogenous and cholera toxin-catalyzed ADP-ribosylation of G alpha(s) induced by SIN-1. These results suggest that increasing ADP-ribosylation may be involved in SIN-1 mediated G alpha(s) down-regulation.

Overexpression of G(s)alpha in NG108-15, neuroblastomaXglioma cells: effects on receptor regulation of the stimulatory adenylyl cyclase cascade

NeuroblastomaXglioma hybrid, NG108-15 cells were stably transfected with an epitope tagged variant of G(s)alpha (HA-G(s)alpha). The introduced HA-G(s)alpha was able to interact with the IP prostanoid receptor and was able to stimulate adenylyl cyclase activity as measured by an enhanced capacity of membrane extracts to reconstitute NaF-dependent adenylyl cyclase activity to membranes of S49 lymphoma cyc- cells. Despite this, neither the maximal stimulation nor the potency of agonist ligands at the IP prostanoid, A2 adenosine or secretin receptors was altered substantially compared to the parental cells although the basal adenylyl cyclase activity was increased. These data indicate that cellular levels of G(s)alpha do not limit signal transduction capacity in NG108-15 cells, whereas enhanced expression of adenylyl cyclase allows greater maximal cAMP generation following receptor activation (MacEwan, D.J., Kim, G.D. and Milligan, G. (1996) Biochem. J. 318, 1033-1039).

Desensitization of beta-adrenergic receptors in adipocytes causes increased insulin sensitivity of glucose transport

To determine the effect of desensitization of adipocyte beta-adrenergic receptors on insulin sensitivity, rats were continuously infused with isoproterenol (50 or 100 micrograms.kg-1.h-1) for 3 days by osmotic minipumps. Epididymal adipocytes were isolated. The cells from treated animals were desensitized to isoproterenol, as determined by response of lipolysis (glycerol release). Binding of [125I]iodocyanopindolol was decreased by approximately 80% in adipocyte plasma membranes isolated from treated rats, indicating that beta-adrenergic receptors were downregulated. Cellular concentrations of Gn alpha and Gi alpha were not altered. Insulin sensitivity was determined by measuring the effect of insulin on glucose transport (2-deoxy-[3H]glucose uptake). Cells from the isoproterenol-infused rats were markedly more sensitive to insulin than those from control rats. This was evidenced by an approximately 50% increase in maximal glucose transport rate in cells from the high-dose isoproterenol-treated rats and by an approximately 40% decrease in the half-maximal effective concentration of insulin in both groups. 125I-labeled insulin binding to adipocytes was not altered by the isoproterenol infusions, indicating that desensitization of beta-adrenergic receptors results in tighter coupling between insulin receptors and stimulation of glucose transport.

Regulation of cellular Gs alpha levels and basal adenylyl cyclase activity by expression of the beta 2-adrenoceptor in neuroblastoma cell lines

Mouse neuroblastoma x rat glioma hybrid NG 108-15 and mouse neuroblastoma x embryonic hamster brain NCB20 cells were transfected with a construct containing a human beta 2 adrenoceptor cDNA under the control of the beta actin promoter. Clones were selected on the basis of resistance to geneticin sulphate and those expressing a range of levels of the receptor expanded for further study. Membranes from a clone of NG108-15 cells expressing high levels of the receptor (beta N22) but not one expressing only low levels of the receptor (beta N17) exhibited a markedly elevated adenylyl cyclase activity when measured in the presence of Mg2+ compared to wild type cells. This was not due to elevated levels of the adenylyl cyclase catalytic moiety however as there was no difference in these membranes when the adenylyl cyclase activity was measured in the presence of Mn2+. The elevated basal activity was partially reversed by addition of the beta-adrenoceptor antagonist propranolol. Agonist activation of beta N22 but not beta N17 cells led to a large selective down-regulation of cellular Gs alpha levels which was independent of the generation of cyclic AMP. Isoprenaline stimulation of adenylyl cyclase activity and of the specific high affinity binding of [3H] forskolin was achieved with substantially greater potency (some 30 fold) in beta N22 cell membranes than in beta N17. By contrast agonist activation of the endogenously expressed IP prostanoid receptor caused stimulation of adenylyl cyclase and stimulation of high affinity [3H] forskolin binding which was equipotent in each of beta N22, beta N17 and wild type NG108-15 cells. Agonist activation of the IP prostanoid receptor caused an equivalent degree of Gs alpha down-regulation in each cell type. Expression of an epitope tagged variant of Gs alpha in NG108-15 cells resulted in prostanoid agonist-induced down-regulation of this polypeptide in a manner indistinguishable from that of wild type Gs alpha. Isolation of clones of NCB20 cells expressing high levels of the beta 2 adrenoceptor also resulted in a specific agonist-induced down-regulation of Gs alpha.

Degradation of G11 alpha/Gq alpha is accelerated by agonist occupancy of alpha 1A/D, alpha 1B, and alpha 1C adrenergic receptors

Cells of clones of rat 1 fibroblasts transfected to express the molecularly defined alpha 1A/D, alpha 1B, or alpha 1C adrenoreceptors and prelabeled with myo-[3H]inositol were each shown to generate high levels of inositol phosphates when exposed to the alpha 1 adrenoreceptor agonist phenylephrine. Maintained exposure of each of these cells to phenylephrine resulted in a large down-regulation of the receptors and also a marked down-regulation of cellular levels of both of the phosphoinositidase C-linked G-proteins Gq alpha and G11 alpha. To examine the mechanism of phenylephrine-induced down-regulation of Gq alpha and G11 alpha, pulse-chase 35S-amino acid labeling experiments were performed with each of the alpha 1A/D, alpha 1B, and alpha 1C adrenoreceptor-expressing cell lines. The rate of degradation of G11 alpha/Gq alpha, which was adequately modeled by a monoexponential with half-life between 33 and 40 h in each of the cell lines in the absence of agonist, was accelerated substantially (some 4-fold) in the presence of phenylephrine. By contrast, the rate of degradation of the G-protein Gi2 alpha, which would not be anticipated to be activated by members of the alpha 1 adrenoreceptor family, was unaltered by the presence of phenylephrine. Levels of mRNA encoding Gq alpha and G11 alpha were not substantially altered by exposure of the cells to phenylephrine in any of the cell lines studied.

Differential interaction with and regulation of multiple G-proteins by the rat A3 adenosine receptor

Interaction of the rat A3 adenosine receptor (A3AR) with G-proteins has been assessed using a stably transfected Chinese hamster ovary cell system. The non-selective AR agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased the labeling of a 41-kDa membrane protein by 4-azidoanilido-[alpha-32P]guanosine 5'-triphosphate (AA-[32P]GTP), a photolabile GTP analogue. Subsequent immunoprecipitation of Gi alpha-subunits indicated that NECA stimulated incorporation of label into both Gi alpha-2 and Gi alpha-3. Additional experiments revealed an A3AR stimulation of label into Gq and/or G11 alpha-subunits, albeit to a lesser degree than that elicited by endogenous P2U purinergic receptors. No interaction with Gs could be detected. Sustained cellular exposure to NECA induced A3AR desensitization and specific down-regulation of Gi alpha-3 and G-protein beta-subunits without changing levels of Gi alpha-2, Gs alpha, or Gq+11 alpha-subunits. Therefore the A3AR can interact with Gi alpha-2, Gi alpha-3, and, to some extent, Gq-like proteins, but sustained agonist exposure down-regulates only one of the G-proteins with which it interacts. This is the first description of the differing specificities of A3AR/G-protein coupling versus down-regulation in situ and provides a potential mechanism by which the A3AR could elicit the heterologous desensitization of signaling events mediated by Gi3.

Functional study on vasodilator effects of prostaglandin E2 in the newborn pig cerebral circulation

Cerebral vascular reactivity to prostaglandin E2 was investigated in newborn pigs using closed cranial windows. Exogenous prostaglandin E2 is a dilator of pial arterioles that elevates cyclic AMP in cortical cerebrospinal fluid. Pial arterioles are less sensitive to prostaglandin E2 than to the prostacyclin receptor agonist iloprost, but their maximal responses to the dilator prostanoids are similar. The cerebrovascular effects of prostaglandin E2 and iloprost are not additive. Pretreatment with either iloprost or prostaglandin E2 decreases pial arteriolar responsiveness to iloprost without affecting responses to isoproterenol. The homologous desensitization of pial arterioles suggests that auto- and cross-tachyphylaxis in vascular effects of iloprost and prostaglandin E2 occur at the receptor level. Indomethacin, which selectively inhibits prostacyclin receptor-mediated responses in cerebral vascular smooth muscle, greatly reduces the vascular responses to prostaglandin E2. These results suggest that vasodilator effects of prostaglandin E2 in the newborn cerebral circulation are mediated via prostacyclin receptors coupled to adenylyl cyclase.

Inhibition of ADP-ribosyltransferase increases synthesis of Gs alpha in neuroblastoma x glioma hybrid cells and reverses iloprost-dependent heterologous loss of fluoride-sensitive adenylate cyclase

Exposure of NG108-15 cells to 50 mM nicotinamide [an inhibitor of mono(ADP-ribosyl)transferase] for 18 hr led to an increase in membrane associated Gs alpha measured either as cholera toxin substrate or by immunoblotting with a specific antiserum. Prolonged exposure of NG108-15 cells to iloprost is followed by homologous loss of iloprost sensitivity, and heterologous loss of fluoride-dependent activation of adenylate cyclase. Nicotinamide reversed the loss of fluoride sensitivity, but failed to restore iloprost-dependent activation of adenylate cyclase. These results with nicotinamide in NG108-15 cells contrasted with those from platelets, which also exhibit heterologous desensitization of fluoride sensitivity following prolonged exposure to iloprost. Treatment of platelets with 50 mM nicotinamide for 18 hr led to an increase of 75.0 +/- 19.4% in the amount of membrane associated cholera toxin substrate. However, there was no associated increase in the abundance of Gs alpha as determined by immunoblotting. Furthermore, in platelets there was no restoration by nicotinamide of the iloprost-dependent loss of fluoride-sensitive adenylate cyclase activity. It follows that heterologous desensitization in platelets is accompanied by inactivation of Gs alpha, which is retained within the plasma membrane in its inactive state. The nicotinamide-dependent increase in the abundance of membrane associated cholera toxin substrate and immunoreactive Gs alpha in NG108-15 cells is associated with an increase of 72.0 +/- 20.3% in the levels of mRNA encoding Gs alpha. The capacity of nicotinamide to increase the abundance of membrane associated Gs alpha was reversed when the cells were cultured in the presence of 20 micrograms/mL cycloheximide. These results suggest that the ability of nicotinamide to increase the abundance of Gs alpha in NG108-15 cells is mediated by de novo protein synthesis.

Biochemical approaches to examine the specificity of interactions between receptors and guanine nuclotide binding proteins

It is now appreciated both that G-protein-linked receptors and signal transducing heterotrimeric G-proteins consist of large multi-member superfamilies and that regulation of a signal transduction cascade can be produced by a variety of means following activation of a G-protein by a receptor. To begin to unravel the complexities of this regulation it is clearly important to be able to define the molecular identity of the G-protein or G-proteins activated by a receptor and to assess the quantitative importance of such interactions for the integration of signals produced by a receptor agonist. Substantial progress has been made towards these goals in recent years and the purpose of this short review will be to discuss the use and potential limitations of some of the currently most widely used approaches.

Receptor availability defines the extent of agonist-mediated G-protein down-regulation in neuroblastoma x glioma hybrid cells transfected to express the beta 2-adrenoceptor

Sustained exposure of neuroblastoma x glioma hybrid, NG108-15, cells transfected to express the human beta 2-adrenoceptor (clone beta N22) to isoprenaline or iloprost (an agonist at the endogenously expressed IP prostanoid receptor) resulted in a substantial and selective down-regulation of the alpha subunit of the G-protein Gs. Treatment of these cells with the irreversible beta-adrenoceptor antagonist bromoacetyl alprenolol menthane diminished both the potency and the maximal ability of isoprenaline but not of iloprost to cause Gs alpha down-regulation. These results demonstrate that the extent of agonist-mediated Gs alpha down-regulation is dependent upon the availability of receptor to agonist.

Equivalent regulation of wild type and an epitope-tagged variant of Gs alpha by the IP prostanoid receptor following expression in neuroblastoma x glioma hybrid, NG108-15, cells

NG108-15 cells were transfected to stably express a haemagglutinin epitope-tagged variant of the long isoform of Gs alpha. Clone BST15 expressed this polypeptide at similar levels to the endogenous long isoform of Gs alpha. Treatment of clone BST15 with the IP prostanoid receptor agonist iloprost resulted in down-regulation of both forms of Gs alpha with both dose-effect curves to iloprost and time courses of loss of the two forms of Gs alpha being indistinguishable. These results demonstrate that the IP prostanoid receptor interacts with and regulates the epitope-tagged variant of Gs alpha in an equivalent manner to the unmodified protein and indicates that the epitope-tagged polypeptide can be used to analyse mechanisms of receptor regulation of cellular G-protein levels.

Agonist-induced transfer of the alpha subunits of the guanine-nucleotide-binding regulatory proteins Gq and G11 and of muscarinic m1 acetylcholine receptors from plasma membranes to a light-vesicular membrane fraction

A clone of a Chinese hamster ovary (CHO) cell line expressing high levels of the human muscarinic M1 acetylcholine (Hm1) receptor undergoes a substantial agonist-specific down-regulation of both Hm1 receptors and the alpha subunits of the guanine-nucleotide-binding (G)-proteins Gq and G11 which is accompanied by a desensitization of inositol-phospholipid-specific-phospholipase-C response [Mullaney, I., Dodd, M. W., Buckley, N. J. & Milligan, G. (1993) Biochem. J. 289, 125-131]. To examine early events in this process, the effect of agonist on subcellular distribution of Gq alpha and G11 alpha and of Hm1 receptors was assessed after short-term and long-term treatment with carbachol. Short-term (30 min) incubation with carbachol (1 mM) induced a simultaneous transfer of a proportion of both Gq alpha and G11 alpha and Hm1 receptors from plasma membranes to distinct light vesicular membranes. The total number of receptors and of Gq alpha and G11 alpha in each cell remained unchanged under these conditions. A similar transfer was noted for the G-protein Gs alpha but not for intrinsic plasma membrane markers. The plasma membrane, as well as light vesicular membrane, pool of Gs alpha subunit was unaffected by further sustained incubation with carbachol (16 h), whereas Hm1 receptors and both Gq alpha and G11 alpha proteins were down-regulated to 25% and 40%, respectively, when compared with untreated cells. Such observations support the idea that down-regulation of both the Hm1 receptor and its associated inositol-phospholipid-specific-phospholipase-C-linked G-proteins is produced by two sequential steps. The first is a transfer of signal-transducing polypeptides from the plasma membrane to a non-plasma membrane light vesicle fraction. The second step is represented by an agonist-specific down-regulation pathway. Both the Hm1 receptor and Gq alpha/G11 alpha appear to follow similar sequestration and down-regulation patterns.

Gs alpha-dependent and -independent desensitisation of prostanoid IP receptor-activated adenylyl cyclase in NG108-15 cells

NG108-15 mouse neuroblastoma x rat glioma cells were treated with the prostanoid IP receptor agonist iloprost (1 microM) and the time course of changes in the levels of prostanoid IP receptors, adenylyl cyclase activity, and the alpha-subunit of the stimulatory guanine nucleotide binding regulatory protein, Gs, were measured. Incubation of cells with iloprost produced a biphasic time course of desensitisation of prostanoid IP receptor-activated adenylyl cyclase. Parallel analysis of iloprost-induced loss of membrane Gs alpha, NaF-stimulated adenylyl cyclase and [3H]iloprost binding suggested only monophasic curves, with t0.5 values similar to the initial phase of desensitisation of iloprost-stimulated adenylyl cyclase activity. This suggests that the loss of receptor and Gs alpha occur at the same time and account for the initial period of desensitisation due to iloprost pretreatment. Pretreatment of NG108-15 cells with cholera toxin produced a near complete loss of membrane-associated Gs alpha, but the loss of [3H]iloprost binding due to iloprost treatment was not affected by pretreatment with cholera toxin, suggesting that prostanoid IP receptors can be down-regulated in the absence of any coupling to Gs. The second phase of desensitisation of iloprost-stimulated adenylyl cyclase activity, during which there was no further change in NaF-stimulated adenylyl cyclase or in the membrane levels of Gs alpha, was not due to protein kinase A activation, since elevating intracellular cyclic AMP levels with forskolin did not subsequently decrease iloprost-stimulated adenylyl cyclase activity or [3H]iloprost binding. These results demonstrate that iloprost pretreatment of NG108-15 cells induces two distinct phases of desensitisation; an initial desensitisation due to concurrent loss of prostanoid IP receptors and Gs alpha, and then a further desensitisation by an as yet uncharacterized mechanism during which there is no further loss of Gs alpha.

The role of protein kinase A and protein kinase C in prostanoid IP receptor desensitization in NG108-15 cells

Pretreatment of NG108-15 cells for 1 h with 1 microM phorbol 12-myristate,13-acetate produced no significant effect on the subsequent stimulation of adenylate cyclase activity by the IP receptor agonist, iloprost, the adenosine A2 receptor agonist, N-ethylcarboxamidoadenosine (NECA), or sodium fluoride, suggesting that protein kinase C activation does not produce desensitization in this system. Pretreatment of cells with 10 microM iloprost or forskolin for 17 h produced a decrease in the specific binding of [3H]iloprost, consistent with a decrease in IP receptor number. Iloprost pretreatment produced a decrease in responses to iloprost, NECA and sodium fluoride, whereas forskolin pretreatment produced a decrease in subsequent responsiveness to iloprost and NECA, but the response to sodium fluoride remained unaffected. The desensitization produced by forskolin could be completely inhibited by the inhibitor of protein kinase A and protein kinase C, 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7), but H7 had no effect on the desensitization produced by iloprost.

Alterations of adenylyl cyclase-coupled growth hormone-releasing hormone (GHRH) pituitary receptors in different conditions of GHRH deprivation

Previous studies have clearly shown that the progressive decrease of growth hormone (GH) secretion occurring during ageing is coupled with a reduced responsiveness of pituitary GHRH receptors both in terms of GH secretion and activation of the adenylyl cyclase (AC), in the presence of increased basal values of the enzyme. The mechanism(s) subserving the age-associated alterations of GHRH-sensitive AC is likely related to the progressive decrease of hypothalamic GHRH function occurring with ageing. In this context, in old male rats, short-term administration of GHRH decreased the high basal AC activity and enhanced the GHRH-stimulated AC activity. Along this line, we decided to investigate whether experimentally induced abrogation of GHRH function in adult rats would induce the same alterations of AC-coupled GHRH receptors present in aged rats. Passive immunization of male young-adult rats with supra-maximal doses of GHRH antiserum (Ab-GHRH) significantly reduced the AC responsiveness to GHRH, an effect already evident 5 days post-injection and still present at 10 days. At this time interval, the treatment also evoked a significant increase of basal AC levels and of Gs alpha protein in the pituitary and completely blocked the GH-releasing effect of a bolus injection of GHRH. Furthermore, mechanical disruption of brain-pituitary links by complete stereotaxical ablation of the mediobasal hypothalamus induced a significant increase of basal AC levels and Gs alpha protein in the pituitary and a strikingly lower AC responsiveness to GHRH.(ABSTRACT TRUNCATED AT 250 WORDS)

Desensitization of adenylate cyclase responses following exposure to IP prostanoid receptor agonists. Homologous and heterologous desensitization exhibit the same time course

Pretreatment of NG108-15 cells with 0.03-25 microM prostaglandin E1 (PGE1) produced decreases in the maximal stimulation of adenylate cyclase activity produced by iloprost, N-ethylcarboxamidoadenosine and sodium fluoride. The rate of desensitization to all three agents was dependent on the concentration of PGE1 used, but at each concentration of PGE1 the rate of loss of responsiveness to each agent was the same, suggesting that the decreases in responsiveness may be mediated by a single process. Functional desensitization was accompanied by a decrease in the specific binding of [3H]iloprost, consistent with a 75-80% decrease in IP receptor number, with no change in the coupling of the remaining IP receptors to G protein. At each concentration of PGE1 used, the times taken for half maximal decreases in receptor number and functional responsiveness were similar, suggesting that IP receptor down-regulation is a relatively early event in desensitization. IP receptor down-regulation could be inhibited partially by 100 microM chloroquine, suggesting that lysosomal breakdown of receptors may be occurring.

Cyclic AMP regulation of messenger RNA level of the stimulatory GTP-binding protein Gs alpha. Isoproterenol, forskolin and 8-bromoadenosine 3':5'-cyclic monophosphate increase the level of Gs alpha mRNA in cultured astroglial cells

The existence of a cAMP-dependent regulation of the expression of the alpha-subunit of the stimulatory guanine nucleotide-binding protein (Gs) in a well characterized astroglial cells culture was established. The culture of astroglial cells for 3-6 h with isoproterenol (10 microM) or forskolin (10 microM) (a cAMP-inducing agent) increased (200-400%) the response of adenylylcyclase to agents which bypass the receptor; GTP, GTP[S] or forskolin. For prolonged exposure times (15 h or more) to isoproterenol or forskolin, the adenylycyclase activity decreased to the value observed in control cells. The same biphasic response of adenylylcyclase to isoproterenol (10 microM) plus GTP (10 microM) occurred in membrane fractions from cells cultured with forskolin, whereas a diminished response to isoproterenol was observed in isoproterenol-treated cells, indicating that the beta-adrenergic receptor was desensitized. To understand the molecular mechanism of these phenomena, we measured the levels of the alpha subunits of the guanine-nucleotide binding protein (Gs and Gi) by Western-blot analysis. The culture of astroglial cells with isoproterenol or forskolin (3-24 h) resulted in a transient increase of both the Gs alpha and the Gi alpha protein levels, while the level of G beta subunits was unaffected. We also identified Gs alpha protein (about 40% of the total cellular protein) in the supernatant fraction of astroglial cells but its level was not modified by the stimulation of cells by forskolin. The level of Gs alpha mRNA measured by Northern-blot analysis was transiently increased (200%) after stimulation of astroglial cells with isoproterenol or forskolin for an incubation period of 6-9 h, then returned to that of control cells for longer period of time. In addition, the Gs alpha mRNA level was threefold increased when cells were cultured for 2-6 h with 8-bromoadenosine 3':5'-cyclic monophosphate (10 microM), a permeant analogue of cAMP. These results indicate that cAMP induces a time-dependent increase of Gs alpha mRNA. The half-life of Gs alpha protein and Gs alpha mRNA were determined. Pulse-chase studies revealed that the decay of Gs alpha protein was clearly biphasic with an early phase (5-6 h) and a slower second phase (20-25 h) but the treatment of cells with forskolin did not accelerate or slow down the turnover of Gs alpha protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative stoichiometry of the proteins of the stimulatory arm of the adenylyl cyclase cascade in neuroblastoma x glioma hybrid, NG108-15 cells

To understand the details of regulation of guanine-nucleotide-binding-protein-linked transmembrane cellular-signalling cascades, it is important to know the absolute levels of each polypeptide component and the stoichiometry of their interactions. Amounts of the IP prostanoid receptor, the stimulatory G protein of the adenylyl cyclase cascade (Gs alpha) and the functional complex of Gs alpha with adenylyl cyclase, which acts as the cyclic AMP generator, were measured in membranes of neuroblastoma x glioma hybrid, NG108-15, cells. As measured by the specific binding of [3H]prostaglandin E1, the IP prostanoid receptor was present in some 100,000 copies/cell. Gs alpha assessed by quantitative immunoblotting with recombinantly expressed protein, was present in considerably higher levels (1,250,000 copies/cell). However, the maximal formation of a complex of Gs alpha and adenylyl cyclase represented only some 17,500 copies/cell. The previously established 8:1 stoichiometry of concurrent downregulation of Gs alpha and the IP prostanoid receptor in these cells [Adie, E. J., Mullaney, I., McKenzie, F. R. & Milligan, G. (1992) Biochem. J. 285, 529-536] indicates that full-agonist occupation of the receptor should be able to activate some 65% of the expressed Gs. Despite the potential 70-fold excess of Gs alpha over the Gs alpha/adenylyl cyclase complex, IP prostanoid-receptor-agonist-mediated reduction of Gs alpha levels by some 35% resulted in a 25% reduction in the maximal formation of the Gs alpha/adenylyl cyclase complex. Such results demonstrate that adenylyl cyclase is quantitatively the least highly expressed component of this signalling cascade and suggests that much of the cellular Gs alpha may not have access to adenylyl cyclase.

Agonist regulation of cellular G protein levels and distribution: mechanisms and functional implications

Exposure of cells to agonists of receptors linked to G proteins can result in downregulation of cellular levels or redistribution of G proteins from membranes to the cytosol. Agonist-induced reductions in G protein levels have been observed for members of each of the Gs, Gi and Gq families of G proteins, are likely to be dependent upon the level of receptor expression, and are generally restricted to the G protein(s) with which the receptor interacts. The mechanisms responsible, reviewed here by Graeme Milligan, vary with cell type and include both second messenger-dependent and -independent enhanced protein degradation. Agonist-induced reduction in cellular G protein levels can provide one mechanism for the development of sustained heterologous desensitization.

The GTP-binding regulatory proteins, Gs and G(i), are altered in erythrocyte membranes of patients with ischemic heart disease resulting from coronary atherosclerosis

Acute ischemic heart disease is associated with alterations in the cardiac adenylate cyclase system response, although the specificity and mechanism of these events are unknown. We studied the characteristics of inhibitory (G(i)) and stimulatory (Gs) GTP-binding regulatory proteins (G proteins) of adenylate cyclase in erythrocyte membranes of patients (n = 16) with nonacute ischemic heart disease resulting from coronary atherosclerosis. Gs was measured by reconstitution with the resolved catalytic unit of adenylate cyclase and by cholera toxin-catalyzed ADP-ribosylation of a 42-kD protein; G(i) was tested as a 41-kD substrate of pertussis toxin-catalyzed ADP-ribosylation. Gs activity was decreased by 27 +/- 2% in the cholate extract and by 25 +/- 3% in the supernatant of guanosine 5'-(gamma-thio)triphosphate-treated membranes. The amount of cholera toxin substrate was decreased by 33 +/- 3%, and the pertussis toxin substrate was increased by 27 +/- 5% compared with healthy subjects (n = 10). All changes in G-protein characteristics appear to be specific relative to other erythrocyte membrane proteins and hemoglobin. Those patients who have a decreased Gs possess approximately normal Gi, and those with increased G(i) showed no change in Gs. Patients with increased G(i) (normal Gs) exhibited more severe deterioration of their coronary arteries than did patients with decreased Gs (normal G(i)) (P < .05), but these two groups did not differ significantly in serum lipids, hormones, drug therapy, historical data, or baseline assessment (P < 0.05).

Long-term inhibitory effect of cAMP on beta-adrenoceptor acquisition and nonselective attenuation of adenylyl cyclase in hepatocytes

Long-term effects of cAMP on the surface expression of beta-adrenoceptors and adenylyl cyclase activity were investigated in primary cultures of rat hepatocytes. beta-Adrenoceptor density and catecholamine-responsive adenylyl cyclase activity increased during culturing in a biphasic manner, with a plateau of 10-20 h duration occurring approximately 10 h after plating. Treatment of hepatocyte cultures with 8-bromo-cAMP during the plateau period did not affect the density of beta-adrenoceptors. In contrast, addition of 8-bromo-cAMP, 8-chlorophenylthio-cAMP, forskolin or glucagon during a period of active recruitment of surface beta-adrenoceptors resulted in a suppression of the acquisition of beta-adrenoceptors. In both experimental situations there was a partial decrease in hormone-stimulated and basal adenylyl cyclase activity. The results suggest that cAMP exerts at least two types of long-term regulation of adenylyl cyclase in hepatocytes: a suppressive effect on beta-adrenoceptor acquisition, and a partial, nonselective decrease in adenylyl cyclase activity not involving beta-adrenoceptor down-regulation.

An arginine residue is the site of receptor-stimulated, cholera toxin-catalysed ADP-ribosylation of pertussis toxin-sensitive G-proteins

Cholera toxin-catalysed [32P]ADP-ribosylations were performed in the absence of guanine nucleotides on membranes of a clone (1C) of Rat 1 fibroblasts which express high levels of the alpha 2-C10 adrenergic receptor. As well as incorporation of radioactivity into 45,000 and 42,000 M(r) polypeptides which represent forms of Gs alpha, there was weak labelling of a 40,000 M(r) polypeptide(s). Addition of the alpha 2 adrenergic agonist UK14304 to such assays enhanced markedly the incorporation of radioactivity into the 40,000 M(r) polypeptide(s) but did not alter labelling of the forms of Gs. We have previously demonstrated that the 40,000 M(r) polypeptide(s) labelled in such a manner represents a combination of the alpha subunits of Gi2 and Gi3 [Milligan et al. (1991) J. biol. Chem. 266, 6447-6455]. Mercuric acetate treatment of membranes prelabelled with [32P]ADP-ribose by exposure to pertussis toxin and [32P]NAD removed totally the radiolabel from both Gi2 and Gi3. However, cholera toxin-catalysed [32P]ADP-ribosylation of either the alpha subunits of the Gi-subtypes or of forms of Gs was unaffected by such treatment. By contrast, prolonged, but not short-term, exposure to neutral hydroxylamine removed radiolabel incorporated by cholera toxin from the Gi-subtypes and from Gs but did not remove [32P]ADP-ribose incorporated by pertussis toxin from the Gi-subtypes. It is concluded that ADP-ribosylation of pertussis toxin-sensitive G-proteins by cholera toxin, which can be induced by exposure of membranes to agonists for receptors which interact with that G-protein, is at an arginine residue. It is suggested that this residue may be Arg 179 in Gi2 alpha and Arg 178 in Gi3 alpha.

Desensitization of D1 dopamine receptors down-regulates the Gs alpha subunit of G protein in SK-N-MC neuroblastoma cells

The effect of long-term (72 h) treatment with dopamine D1 receptor agonists, SKF 38393 and dopamine on D1 dopamine receptor and G-protein (Gs alpha) was investigated in SK-N-MC neuroblastoma cells. The prolonged treatment of cells with 10 microM SKF 38393 or 10 microM dopamine resulted in a decrease in dopamine D1 receptor by 41 and 81%, respectively, as measured by specific antagonist [3H]SCH 23390 binding. Similarly, the prolonged treatment of SK-N-MC cells with 10 microM SKF 38393 or 10 microM dopamine resulted in a reduction of the level of Gs alpha subunit of G-protein. The results indicate that agonist-induced down-regulation of D1 dopamine receptor may also modulate G-proteins.

Effects of chronic treatment of haloperidol and clozapine on levels of G-protein subunits in rat striatum

Chronic administration of typical neuroleptic drugs, such as haloperidol, causes the supersensitivity of brain dopamine D2 receptor in striatum and limbic regions, while the atypical neuroleptic clozapine does not. In order to understand the mechanism of their action at a molecular level, studies were carried out to assess the effects of chronic treatment of these drugs on the levels of G-proteins in the rat striatum using the Western blot method. Results of the present study demonstrate that the treatment with haloperidol or clozapine, respectively, down-regulate or up-regulate the levels of G proteins. Quantitative immunoblotting, using site-directed specific antisera, demonstrated that chronic treatment with haloperidol down-regulates Gi alpha, Gs alpha, and beta subunits while chronic treatment with clozapine upregulates Gi alpha, Gs alpha, and beta subunits. Neither of these drugs has any effect on the levels of Go alpha.

Plasma-membrane-independent pool of the alpha subunit of the stimulatory guanine-nucleotide-binding regulatory protein in a low-density-membrane fraction of S49 lymphoma cells

We report that compartmentalisation of the stimulatory guanine-nucleotide-binding regulatory protein (Gs) exists in S49 lymphoma cells. In addition to the previously reported cytosolic form of the alpha subunit of Gs (Gs alpha) [Ransnäs, L. A., Svoboda P., Jasper, J. R. & Insel, P. A. (1989) Proc. Natl Acad. Sci. USA 86, 7900-7903], three membrane-bound forms of Gs alpha were identified through rate-zonal centrifugation in sucrose density gradients, Gs alpha-specific anti-peptide serum and an adenylate cyclase complementation assay. The sedimentation profile of the first pool of Gs alpha in the high-density portion of the gradient (1.13-1.16 g/cm3) is identical with that of beta-adrenergic-receptor binding, Na/K-ATPase and adenylate cyclase activity, and may therefore be identified as plasma-membrane fragments. The second pool, which was recovered in the middle portion of the gradient (1.09-1.11 g/cm3), contains a much lower total amount of Gs alpha and correlates with the endoplasmic reticulum (microsomal) enzyme markers, NADPH-cytochrome-c reductase and glucose-6-phosphatase. The identity of the third pool of Gs alpha located at the top of the gradient (1.06-1.08 g/cm3), is unknown. The Golgi apparatus marker, UDPgalactose:N-acetylglucosamine glycosyltransferase, was partially recovered in this area; however, this enzyme was also present in the high-density portion of the gradient. Complete absence of specific adenylate cyclase and Na/K-ATPase activity indicates that this low-density (light) membrane form of Gs alpha is distinct from any plasma-membrane fragments. Furthermore, sedimentation at 100,000 x g proves its particulate (membrane) character. The light membrane form of Gs alpha subunit is functionally active in an adenylate cyclase complementation assay using cyc- membranes devoid of Gs alpha. Overall, our data indicates that a substantial portion of Gs alpha is localized in membrane pools other than plasma membrane.

Cyclic AMP produces desensitization of prostacyclin and adenosine A2 receptors in hybrid cell lines but does not affect Gs function

Prostacyclin and adenosine A2 receptors stimulate adenylate cyclase activity in the related somatic hybrid cell lines NG108-15 and NCB20. The role of cAMP in the desensitization of these receptors has been examined. Pretreatment for 17 h with forskolin or 8-bromo-cAMP had the same effect in both cell lines. There was no change in the response to sodium fluoride or forskolin, suggesting that the function of Gs and adenylate cyclase were unaffected by increased levels of cAMP. Receptor responses were affected however; the maximum response to N-ethylcarboxamidoadenosine (an A2 receptor agonist) was reduced by 30-40%, there was a small but consistent shift to the right of the dose-response curve for iloprost (a stable analogue of prostacyclin) and [3H]iloprost binding studies revealed a loss of prostacyclin receptors. However, the loss of receptor responsiveness was much smaller than that which occurs following pretreatment with prostacyclin or adenosine A2 receptor agonists (Keen et al. (1989) Biochem. Pharmacol. 38, 3827-3833; Kelly et al. (1990) Br. J. Pharmacol. 99, 309-316) suggesting that cAMP may not play a major role in agonist mediated desensitization.

Immunological identification of the alpha subunit of G13, a novel guanine nucleotide binding protein

An antiserum (13CB) was generated against a synthetic peptide, HDNLKQLMLQ, which is predicted to represent the C-terminal decapeptide of the alpha subunit of the novel G-protein, G13. Competitive ELISA indicated that the antiserum reacted with this peptide but that it showed minimal ability to recognize peptides which represent the equivalent regions of the pertussis toxin-insensitive G-proteins, Gq + G11, G12, G15 + G16, GL1 (also called G14) as Gz, and well as other G-proteins. Immunoblots of human platelet membranes with antiserum 13CB identified a single 43-kDa polypeptide, and while this immunoreactivity was abolished by the presence of the cognate peptide it was not modified by the presence of peptides corresponding to the equivalent region of other G-proteins. Immunoreactivity corresponding to G13 alpha was detected in a range of cell types with human platelets having the highest levels of this polypeptide.

Widespread distribution of Gq alpha/G11 alpha detected immunologically by an antipeptide antiserum directed against the predicted C-terminal decapeptide

Antisera were raised to a synthetic peptide which represents the predicted C-terminal decapeptide of the alpha subunit of the G-proteins Gq and G11. Competitive ELISA indicated that antiserum CQ2 displayed strong reactivity against this peptide. Antiserum CQ2 identified an apparently single polypeptide of 42 kDa which was expressed widely. The mobility of this polypeptide in SDS-PAGE was not modified by pretreatment of cells with pertussis toxin, indicating that it was not a substrate for this toxin. Furthermore, the levels and mobility of this polypeptide were unaltered by treatment of cells with cholera toxin, defining that it was not related to Gs alpha.