Molecular cloning of complementary DNA for the alpha subunit of the G protein that stimulates adenylate cyclase

The Molecular Pharmacology of G Protein Signaling Then and Now: A Tribute to Alfred G. Gilman

The recent, unfortunate death of Alfred G. ("Al") Gilman, M.D., Ph.D., represents a sad signpost for an era spanning over 40 years in molecular pharmacology. Gilman's discoveries, influence, and persona were dominant forces in research and training in pharmacology. Here, we review the progression of ideas and knowledge that spawned early work by Gilman and collaborators (among them, one of the authors) and later efforts (including those of the other author) that have recently yielded a comprehensive and precise structural understanding of fundamental topics in pharmacology: the binding of ligands to G protein-coupled receptors (GPCRs) and the interaction of GPCRs with heterotrimeric G proteins and effector molecules. Those data provide new and important insights into the molecular basis that underlies affinity and efficacy, two of the most important features of drug action, which represent the latest chapter in the saga that Al Gilman's work helped launch.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

RGS3 controls T lymphocyte migration in a model of Th2-mediated airway inflammation

T cell migration toward sites of antigen exposure is mediated by G protein signaling and is a key function in the development of immune responses. Regulators of G protein signaling (RGS) proteins modulate G protein signaling; however, their role in the regulation of adaptive immune responses has not been thoroughly explored. Herein we demonstrated abundant expression of the Gi/Gq-specific RGS3 in activated T cells, and that diminished RGS3 expression in a T cell thymoma increased cytokine-induced migration. To examine the role of endogenous RGS3 in vivo, mice deficient in the RGS domain (RGS3(ΔRGS)) were generated and tested in an experimental model of asthma. Compared with littermate controls, the inflammation in the RGS3(ΔRGS) mice was characterized by increased T cell numbers and the striking development of perivascular lymphoid structures. Surprisingly, while innate inflammatory cells were also increased in the lungs of RGS3(ΔRGS) mice, eosinophil numbers and Th2 cytokine production were equivalent to control mice. In contrast, T cell numbers in the draining lymph nodes (dLN) were reduced in the RGS3(ΔRGS), demonstrating a redistribution of T cells from the dLN to the lungs via increased RGS3(ΔRGS) T cell migration. Together these novel findings show a nonredundant role for endogenous RGS3 in controlling T cell migration in vitro and in an in vivo model of inflammation.

Increased expression of adenylyl cyclase 3 in pancreatic islets and central nervous system of diabetic Goto-Kakizaki rats: a possible regulatory role in glucose homeostasis

Adenylyl cyclase 3 (AC3) is expressed in pancreatic islets of the Goto-Kakizaki (GK) rat, a spontaneous animal model of type 2 diabetes (T2D), and also exerts genetic effects on the regulation of body weight in man. In addition to pancreatic islets, the central nervous system (CNS) plays an important role in the pathogenesis of T2D and obesity by regulating feeding behavior, body weight and glucose metabolism. In the present study, we have investigated AC3 expression in pancreatic islets, striatum and hypothalamus of GK rats to evaluate its role in the regulation of glucose homeostasis. GK and Wistar rats at the age of 2.5 mo were used. A group of GK rats were implanted with sustained insulin release chips for 15 d. Plasma glucose and serum insulin levels were measured. AC3 gene expression levels in pancreatic islets, striatum and hypothalamus were determined by using real-time RT-PCR. Results indicated that plasma glucose levels in Wistar rats were found to be similar to insulin-treated GK rats, and significantly lower compared with non-treated GK rats. AC3 expression levels in pancreatic islets, striatum and hypothalamus of GK rats were higher compared with Wistar rats, while the levels were intermediate in insulin-treated GK rats. The AC3 expression display patterns between pancreatic islets and striatum-hypothalamus were similar. The present study thus provides the first evidence that AC3 is overexpressed in the regions of striatum and hypothalamus of brain, and similarly in pancreatic islets of GK rats suggesting that AC3 plays a role in regulation of glucose homeostasis via CNS and insulin secretion.

Silver spoons and other personal reflections

The intent is to tell a story-hopefully one that is at various times serious, light-hearted, or provocative-that describes my life in biomedical science, especially focusing on the 50 years from 1961 (as a college senior) to the present.

Non-canonical functions of RGS proteins

Regulators of G protein signalling (RGS) proteins are united into a family by the presence of the RGS domain which serves as a GTPase-activating protein (GAP) for various Galpha subunits of heterotrimeric G proteins. Through this mechanism, RGS proteins regulate signalling of numerous G protein-coupled receptors. In addition to the RGS domains, RGS proteins contain diverse regions of various lengths that regulate intracellular localization, GAP activity or receptor selectivity of RGS proteins, often through interaction with other partners. However, it is becoming increasingly appreciated that through these non-RGS regions, RGS proteins can serve non-canonical functions distinct from inactivation of Galpha subunits. This review summarizes the data implicating RGS proteins in the (i) regulation of G protein signalling by non-canonical mechanisms, (ii) regulation of non-G protein signalling, (iii) signal transduction from receptors not coupled to G proteins, (iv) activation of mitogen-activated protein kinases, and (v) non-canonical functions in the nucleus.

Understanding the ligand-receptor-G protein ternary complex for GPCR drug discovery

Understanding the ternary complex between G protein-coupled receptors (GPCRs), cognate G proteins, and their ligands is an important landmark for drug discovery. Yet, little is known about the specific interactions between GPCRs and G proteins. For a better perspective on the ternary complex dynamics, we adapted a beta(2)-adrenergic receptor(beta(2)AR)-tetGs(alpha) reconstitution system and found evidence that for efficient coupling of the beta(2)AR to Gs does not require specific interactions between the betagamma-subunits and the beta(2)AR. Our results demonstrate that specific interactions between betagamma and the beta(2)AR are not required for G protein activation but likely serve to anchor Gs(alpha) to the plasma membrane. Our results also suggests that the advantages of analysis of G protein activation by using beta(2)AR receptor-tetGs(alpha) system in vitro at the close proximity of the receptor may constitute a simple screening system that avoids false positives and potentially adapted to screen drugs for other GPCRs.

Pseudohypoparathyroidism type I and Albright's hereditary osteodystrophy with a proximal 15q chromosomal deletion in mother and daughter

A 33-year-old woman and her 71-year-old mother were both found to have pseudohypoparathyroidism type I with Albright's hereditary osteodystrophy associated with a cytogenetic deletion of the proximal part of one chromosome 15, resembling that found in Prader-Willi syndrome. As there are overlapping clinical features between these two syndromes a causal relationship cannot be excluded. However, molecular analyses with 10 probes from this region did not detect any uniparental disomy or deletion, features frequently found in Prader-Willi syndrome.

G-Proteins and GPCRs: From the Beginning

From the point of view of a participant observer, I tell the discovery stories of trimeric G-proteins and GPCRs, beginning in the 1970s. As in most such stories, formidable obstacles, confusion, and mistakes make eventual triumphs even more exciting. Because these pivotally important signaling molecules were discovered before the recombinant DNA revolution, today's well-trained molecular biologist may find it amazing that we learned anything at all.

Signaling through Gs alpha is required for the growth and function of neuromuscular synapses in Drosophila

Although synapses are assembled in a highly regulated fashion, synapses once formed are not static structures but continue to expand and retract throughout the life of an organism. One second messenger that has been demonstrated to play a critical role in synaptic growth and function is cAMP. Here, we have tested the idea that signaling through the heterotrimeric G protein, Gs, plays a coincident role with increases in intracellular Ca(+2) in the regulation of adenylyl cyclases (ACs) during synaptic growth and in the function of synapses. In larvae containing a hypomorphic mutation in the dgs gene encoding the Drosophila Gs alpha protein, there is a significant decrease in the number of synaptic boutons and extent of synaptic arborization, as well as defects in the facilitation of synaptic transmission. Microscopic analysis confirmed that Gs alpha is localized at synapses both pre- and postsynaptically. Restricted expression of wild-type Gs alpha either pre- or postsynaptically rescued the mutational defects in bouton formation and defects in the facilitation of synaptic transmission, indicating that pathways activated by Gs alpha are likely to be involved in the reciprocal interactions between pre- and postsynaptic cells required for the development of mature synapses. In addition, this Gs alpha mutation interacted with fasII, dnc, and hyperexcitability mutants in a manner that revealed a coincident role for Gs alpha in the regulation of cAMP and FASII levels required during growth of these synapses. Our results demonstrate that Gs alpha-dependent signaling plays a role in the dynamic cellular reorganization that underlies synaptic growth.

Regulation of RGS proteins by chronic morphine in rat locus coeruleus

The present study explored a possible role for RGS (regulators of G protein signalling) proteins in the long term actions of morphine in the locus coeruleus (LC), a brainstem region implicated in opiate physical dependence and withdrawal. Morphine influences LC neurons through activation of micro -opioid receptors, which, being Gi/o-linked, would be expected to be modulated by RGS proteins. We focused on several RGS subtypes that are known to be expressed in this brain region. Levels of mRNAs encoding RGS2, -3, -4, -5, -7, -8 and -11 are unchanged following chronic morphine, but RGS2 and -4 mRNA levels are increased 2-3-fold 6 h following precipitation of opiate withdrawal. The increases in RGS2 and -4 mRNA peak after 6 h of withdrawal and return to control levels by 24 h. Immunoblot analysis of RGS4 revealed a striking divergence between mRNA and protein responses in LC: protein levels are elevated twofold following chronic morphine and decrease to control values by 6 h of withdrawal. In contrast, levels of RGS7 and -11 proteins, the only other subtypes for which antibodies are available, were not altered by these treatments. Intracellular application of wild-type RGS4, but not a GTPase accelerating-deficient mutant of RGS4, into LC neurons diminished electrophysiological responses to morphine. The observed subtype- and time-specific regulation of RGS4 protein and mRNA, and the diminished morphine-induced currents in the presence of elevated RGS4 protein levels, indicate that morphine induction of RGS4 could contribute to aspects of opiate tolerance and dependence displayed by LC neurons.

Genetic analysis of the Drosophila Gs(alpha) gene

One of the best understood signal transduction pathways activated by receptors containing seven transmembrane domains involves activation of heterotrimeric G-protein complexes containing Gs(alpha), the subsequent stimulation of adenylyl cyclase, production of cAMP, activation of protein kinase A (PKA), and the phosphorylation of substrates that control a wide variety of cellular responses. Here, we report the identification of "loss-of-function" mutations in the Drosophila Gs(alpha) gene (dgs). Seven mutants have been identified that are either complemented by transgenes representing the wild-type dgs gene or contain nucleotide sequence changes resulting in the production of altered Gs(alpha) protein. Examination of mutant alleles representing loss-of-Gs(alpha) function indicates that the phenotypes generated do not mimic those created by mutational elimination of PKA. These results are consistent with the conclusion reached in previous studies that activation of PKA, at least in these developmental contexts, does not depend on receptor-mediated increases in intracellular cAMP, in contrast to the predictions of models developed primarily on the basis of studies in cultured cells.

Galpha(olf) is necessary for coupling D1 and A2a receptors to adenylyl cyclase in the striatum

In the brain, dopamine and adenosine stimulate cyclic AMP (cAMP) production through D1 and A2a receptors, respectively. Using mutant mice deficient in the olfactory isoform of the stimulatory GTP-binding protein alpha subunit, Galpha(olf), we demonstrate here the obligatory role of this protein in the adenylyl cyclase responses to dopamine and adenosine in the caudate putamen. Responses to dopamine were also dramatically decreased in the nucleus accumbens but remained unaffected in the prefrontal cortex. Moreover, in the caudate putamen of mice heterozygous for the mutation, the amounts of Galpha(olf) were half of the normal levels, and the efficacy of dopamine- and CGS 21680 A(2) agonist-stimulated cAMP production was decreased. Together, these results identify Galpha(olf) as a critical parameter in the responses to dopamine and adenosine in the basal ganglia.

Deficiency of the alpha-subunit of the stimulatory G protein and severe extraskeletal ossification

Progressive osseous heteroplasia (POH) is a rare disorder characterized by dermal ossification beginning in infancy followed by increasing and extensive bone formation in deep muscle and fascia. We describe two unrelated girls with typical clinical, radiographic, and histological features of POH who also have findings of another uncommon heritable disorder, Albright hereditary osteodystrophy (AHO). One patient has mild brachydactyly but no endocrinopathy, whereas the other manifests brachydactyly, obesity, and target tissue resistance to thyrotropin and parathyroid hormone (PTH). Levels of the alpha-subunit of the G protein (Gsalpha) were reduced in erythrocyte membranes from both girls and a nonsense mutation (Q12X) in exon 1 of the GNAS1 gene was identified in genomic DNA from the mildly affected patient. Features of POH and AHO in two individuals suggest that these conditions share a similar molecular basis and pathogenesis and that isolated severe extraskeletal ossification may be another manifestation of Gsalpha deficiency.

Chimeric melatonin mt1 and melatonin-related receptors. Identification of domains and residues participating in ligand binding and receptor activation of the melatonin mt1 receptor

Melatonin receptors bind and become activated by melatonin. The melatonin-related receptor, despite sharing considerable amino acid sequence identity with melatonin receptors, does not bind melatonin and is currently an orphan G protein-coupled receptor. To investigate the structure and function of both receptors, we engineered a series of 14 chimeric receptor constructs, allowing us to determine the relative contribution of each transmembrane domain to ligand binding and receptor function. Results identified that when sequences encoding transmembrane domains 1, 2, 3, 5, or 7 of the melatonin mt(1) receptor were replaced by the corresponding domains of the melatonin-related receptor, the resultant chimeric receptors all displayed specific 2-[(125)I]iodomelatonin binding. Replacement of sequences incorporating transmembrane domains 4 or 6, however, resulted in chimeric receptors that displayed no detectable 2-[(125)I]iodomelatonin binding. The subsequent testing of a "reverse" chimeric receptor in which sequences encoding transmembrane domains 4 and 6 of the melatonin-related receptor were replaced by the corresponding melatonin mt(1) receptor sequences identified specific 2-[(125)I]iodomelatonin binding and melatonin-mediated modulation of cyclic AMP levels. To further investigate these findings, site-directed mutagenesis was performed on residues within transmembrane domain 6 of the melatonin mt(1) receptor. This identified Gly(258) (Gly(6.55)) as a critical residue required for high affinity ligand binding and receptor function.

Alteration of spinal protein kinase C expression and kinetics in morphine, but not clonidine, tolerance

Antinociceptive synergism between spinally administered morphine and clonidine decreases to an additive interaction in morphine- and clonidine-tolerant mice. Spinally administered protein kinase C (PKC) inhibitors also decrease the synergism to addition. To determine whether chronic morphine or clonidine treatment alters spinal PKC activity, the present studies measured PKC activity and expression of PKC isoform proteins in spinal cord cytosol and membrane fractions. Mice were treated for 4 days with either placebo pellets, morphine pellets, s.c. saline, or s.c. clonidine. Morphine pellet-implanted mice were tolerant to morphine-induced tail flick antinociception, but not cross-tolerant to clonidine. Clonidine-pretreated mice were tolerant to clonidine, but not cross-tolerant to morphine. Induction of morphine tolerance produced a 2-fold lower Km value for PKC (8.24 +/- 1.67 microM in placebo pellet vs 4.43 +/- 1.24 microM in morphine pellet) in cytosol, but not membrane fractions from spinal cord. Vmax values were not different. No difference in Km or Vmax values was found between proteins from saline- and clonidine-pretreated animals. Immunoreactive cPKCalpha, betaI, and gamma isoforms decreased 14, 26, and 17%, respectively, in cytosol from morphine-tolerant animals. No difference in PKC isoforms was found in the membranes or in fractions from clonidine-tolerant mice. Morphine tolerance, but not clonidine tolerance, enhanced PKC activity while decreasing protein expression.

Mutations at the domain interface of GSalpha impair receptor-mediated activation by altering receptor and guanine nucleotide binding

G protein alpha subunits consist of two domains, a GTPase domain and a helical domain. Receptors activate G proteins by catalyzing replacement of GDP, which is buried between these two domains, with GTP. Substitution of the homologous alphai2 residues for four alphas residues in switch III, a region that changes conformation upon GTP binding, or of one nearby helical domain residue decreases the ability of alphas to be activated by the beta-adrenergic receptor and by aluminum fluoride. Both sets of mutations increase the affinity of alphas for the beta-adrenergic receptor, based on an increased amount of high affinity binding of the beta-adrenergic agonist, isoproterenol. The mutations also decrease the rate of receptor-mediated activation and disrupt the ability of the beta-adrenergic receptor to increase the apparent affinity of alphas for the GTP analog, guanosine 5'-O-(3-thiotriphosphate). Simultaneous replacement of the helical domain residue and one of the four switch III residues with the homologous alphai2 residues restores normal receptor-mediated activation, suggesting that the defects caused by mutations at the domain interface are due to altered interdomain interactions. These results suggest that interactions between residues across the domain interface are involved in two key steps of receptor-mediated activation, promotion of GTP binding and subsequent receptor-G protein dissociation.

Phospholipase C-delta1 and oxytocin receptor signalling: evidence of its role as an effector

Although the oxytocin receptor modulates intracellular Ca2+ ion levels in myometrium, the identities of signal molecules have not been clearly clarified. Our previous studies on oxytocin receptor signalling demonstrated that 80 kDa Ghalpha is a signal mediator [Baek, Kwon, Lee, Kim, Muralidhar and Im (1996) Biochem. J. 315, 739-744]. To elucidate the effector in the oxytocin receptor signalling pathway, we evaluated the oxytocin-mediated activation of phospholipase C (PLC) by using solubilized membranes from human myometrium and a three-component preparation containing the oxytocin receptor-Ghalpha-PLC-delta1 complex. PLC-delta1 activity in the three-component preparation, as well as PLC activity in solubilized membranes, was increased by oxytocin in the presence of Ca2+ and activated Ghalpha (GTP-bound Ghalpha). Furthermore the stimulated PLC-delta1 activity resulting from activation of Ghalpha via the oxytocin receptor was significantly attenuated by the selective oxytocin antagonist desGly-NH2d(CH2)5[Tyr(Me)2,Thr4]ornithine vasotocin or GDP. Consistent with these observations, co-immunoprecipitation and co-immunoadsorption of PLC-delta1 in the three-component preparation by anti-Gh7alpha antibody resulted in the PLC-delta1 being tightly coupled to activated Ghalpha on stimulation of the oxytocin receptor. These results indicate that PLC-delta1 is the effector for Ghalpha-mediated oxytocin receptor signalling.

Reproductive dysfunction in women with Albright's hereditary osteodystrophy

Most individuals with Albright's hereditary osteodystrophy (AHO) have deficient expression or function of G(s alpha), the alpha subunit of the guanine nucleotide binding protein that stimulates adenylyl cyclase, and are resistant to parathyroid hormone (PTH) and other hormones that act via stimulation of adenylyl cyclase. To determine the incidence and etiology of ovarian dysfunction in women with AHO, we examined the reproductive history and hypothalamic-pituitary-ovarian axis in 17 affected women aged 17-43 yr. All patients had typical PTH resistance and an approximately 50% reduction in erythrocyte G(s alpha) activity, (0.43 +/- 0.03 vs. 0.92 +/- 0.08 for normal control subjects, P < 0.001). Fourteen of the 17 patients (76%) were oligomenorrheic or amenorrheic, more than half had delayed or incomplete sexual development, and only two had a history of earlier pregnancy. Most women were mildly hypoestrogenic, with normal to slightly elevated serum gonadotropin levels. Computer analysis of 24 hour LH measurement showed that the frequency of LH peaks/24 h in AHO women varied widely, but as a group they were not statistically different from a group of normal women studied in the early follicular phase. Administration of 100 microg synthetic GnRH produced normal FSH and LH responses. We conclude that reproductive dysfunction is common in women with AHO and probably represents partial resistance to gonadotropins.

The stability of the agonist beta2-adrenergic receptor-Gs complex: evidence for agonist-specific states

A restricted version of the ternary complex model for receptor-G protein complex formation has recently been proposed. Known as the two-state model, this model proposes that in the context of agonist and G protein interactions, only two thermodynamic states exist for the receptor: active (R*) and inactive (R). One form of this model suggests that only the R* state of the receptor is capable of interacting with and subsequently activating G proteins. We directly tested the kinetic aspects of a strict two-state receptor model in a cell line containing the native beta2-adrenergic receptor that is capable of inducing Gs expression. We examined adenylyl cyclase activity in the presence of limiting GTP levels and concluded that there exists a different rate of heterotrimer dissociation (i.e., HR*G yields HR* + G*) for different beta2-agonists. This finding is inconsistent with a strict two-state model in which R* is a characteristic of the receptor that is independent of the identity of the agonist. It implies that agonist activation of adenylyl cyclase is more complicated than a simple two-state model.

Beta-adrenergic signalling in neoplastic lung type 2 cells: glucocorticoid-dependent and -independent defects

Tumorigenic mouse lung-derived type 2 cell lines have large reductions in both beta-adrenergic-stimulated cAMP production and ligand binding to beta-adrenergic receptors. These tumorigenic cells are also relatively insensitive to glucocorticoids. Because glucocorticoids regulate both beta-adrenergic receptor expression and receptor coupling to the stimulatory guanine nucleotide binding protein Gs interactions between the glucocorticoid and beta-adrenergic signalling systems were examined. This study demonstrates that beta-adrenergic ligand binding and agonist sensitivity are increased in a tumorigenic cell line stably expressing a normal glucocorticoid receptor transgene. However, although the transfected tumour cells and non-tumorigenic cells have similar amounts and affinities of beta-adrenergic agonist and antagonist binding, similar amounts of Gs subunits and similar forskolin-stimulated adenylyl cyclase activities, the former remain much less isoproterenol responsive. Competition binding studies demonstrate that tumour cell beta-adrenergic receptors have both high- and low-affinity agonist binding but are functionally uncoupled from Gs. This uncoupling may involve an alteration in Gs, as guanine nucleotides exhibit a reduced ability to stimulate adenylyl cyclase. Thus, some aspects of tumorigenic cell dysfunction in beta-adrenergic signalling can be ameliorated by interactions with the glucocorticoid pathway, but additional defects are also involved.

Evidence that phospholipase delta1 is the effector in the Gh (transglutaminase II)-mediated signaling

A new class of GTP-binding protein transglutaminase II (Gh) couples to a 69-kDa phospholipase C (PLC). An 8-amino acid region (Leu665-Lys672) of the alpha-subunit of Gh (Galphah) is involved in interaction and activation of PLC, an observation that has now been used to characterize the 69-kDa PLC further. A 20-amino acid peptide corresponding to Leu654-Leu673 of Galphah was used to prepare an affinity resin. On incubation with a partially purified PLC preparation from rat liver membranes, the affinity resin-bound approximately69- and 85-kDa proteins were recognized by an antibody to the 69-kDa PLC. Both purified 69-kDa PLC and PLC-delta1 bound to the affinity resin; moreover, antibodies to PLC-delta1 recognized the 69-kDa PLC, and antibodies to the 69-kDa PLC recognized PLC-delta1. A synthetic peptide corresponding to Leu661-Lys672 of Galphah inhibited the binding of PLC-delta1 to the affinity resin and also stimulated PLC-delta1. Reconstitution of PLC-delta1 with GTPgammaS (guanosine 5'-3-O-(thio)triphosphate)-activated Gh resulted in activation of PLC-delta1. Antibodies to Galphah also coimmunoprecipitated PLC-delta1 upon activation of Gh. These findings indicate that PLC-delta1 is the effector of Gh-mediated signaling.

Mediation of glucocorticoid receptor function by the activation of latent transforming growth factor beta 1 in MG-63 human osteosarcoma cells

We analyzed glucocorticoid receptor function using ligand binding assays, DNA band-shift analysis and trans-activation of the murine mammary tumor virus-thymidine kinase-chloramphenicol acetyltransferase reporter gene in transiently transfected MG-63 human osteosarcoma cells. Dexamethasone increased the distribution of MG-63 cells in the G1/G0 phase of the cell cycle, thus decreasing the rate of DNA synthesis and cell growth. Its effect on MG-63 cell growth was neutralized by RU486 and anti-transforming growth factor beta 1 (TGF beta 1) antibody. In addition, (i) dexamethasone increased the levels of active TGF beta 1 in MG-63-conditioned media without significantly altering the expression of TGF beta 1 mRNA in MG-63 cells and (ii) TGF beta 1 inhibited proliferation of MG-63 cells. Therefore, we conclude that glucocorticoid receptor function is mediated by the activation of latent-TGF beta 1 in MG-63 osteosarcoma cells.

Mediation of glucocorticoid receptor function by transforming growth factor beta I expression in human PC-3 prostate cancer cells

We investigated the role of glucocorticoids in controlling the proliferation of androgen-independent PC-3 human prostate cancer cells via the action of transforming growth factor beta 1 (TGF beta 1). The presence of glucocorticoid receptor (GR) in PC-3 cells was detected by immunoblotting analysis using a rabbit anti-GR polyclonal antibody against the synthetic human GR peptide (hGR383-393). In PC-3 cells, GR bound radiolabeled dexamethasone with an affinity similar to wild-type GR. In addition, GR-ligand complex bound radiolabeled DNA as detected by DNA band-shift analysis on gel electrophoresis and trans-activated the mouse mammary tumor virus-thymidine kinase-chloramphenicol acetyltransferase chimeric gene in transiently transfected PC-3 cells. Dexamethasone (0.1 up to 100 nM) and TGF beta 1 (0.5 up to 50 ng/ml) inhibited PC-3 cell proliferation. TGF beta 1 and dexamethasone both increased the distribution of PC-3 cells into the G1/G0 phase of the cell cycle. Platelet-derived growth factor (PDGF) stimulated the proliferation of PC-3 cells and overcame dexamethasone's inhibition of PC-3 cell growth. Dexamethasone's inhibition (10(-7) M) of PC-3 cell growth was completely neutralized by RU 486 (10(-6)M) and partly neutralized by anti-TGF beta 1 polyclonal antibody. Furthermore, dexamethasone up modulated the expression of TGF beta 1 mRNA in PC-3 cells. Because dexamethasone's inhibition was neutralized at least in part by an anti-TGF beta 1 polyclonal antibody and dexamethasone up modulated the expression of TGF beta 1 mRNA in PC-3 cells, we conclude that GR function in human PC-3 prostate cancer cells is mediated at least in part by TGF beta 1 expression.

Expression of human M2 muscarinic receptors in Sf9 cells: characterisation and reconstitution with G-proteins

The gene for the human m2 muscarinic receptor was expressed in Sf9 cells using the baculovirus expression system. As assessed by [3H]NMS binding, Sf9 cells expressed receptor at levels of 3.3 pmoles/mg protein. The receptor was identified on western blots using an anti-muscarinic receptor antibody and was shown to have the pharmacological characteristics of an m2 muscarinic receptor. Membranes from Sf9 cells were examined to identify endogenous G-proteins by immuno-blotting and by ADP-ribosylation, indicating the presence of Gq, and a pertussis-toxin substrate which was not recognised by antibodies raised against the alpha-subunits of Gi1, Gi2, Gi3 or Go. Gsalpha was not detected, neither were there any cholera toxin substrates in Sf9 membranes. Sf9 membranes expressing m2 receptors did not show carbachol-stimulated GTPgammaS binding to endogenous G-proteins; however, when membranes were reconstituted with a mixture of purified Gi and Go, a maximum 8-fold stimulation of GTPgammaS binding was observed in response to carbachol that could be reduced by atropine. These data show that the human muscarinic m2 receptor expressed in Sf9 cells is functional.

Guanine nucleotide binding regulatory proteins: their characteristics and identification

Many biological signals are processed by the binding of chemicals to cell surface receptors. Signals are switched to intracellular language via guanine nucleotide binding regulatory proteins (G-proteins) which are present in all eukaryotic cells. Thus, G-proteins serve as interfaces between receptor-response coupling. Two forms of G-proteins have been reported: conventional G-proteins which are heterotrimeric and consist of alpha, beta, and gamma subunits, and monomeric small molecular weight G-proteins which are generally found as single polypeptides. Recently, high molecular weight G-proteins have also been described. The family of G-proteins contains multiple genes that encode the alpha, beta, or gamma subunits. G-proteins play a pivotal role in excitation-contraction coupling in smooth muscle function and control metabolic and secretory processes. In this review article, we have given a brief overview on the characteristics and methodology for the identification of G-proteins. The heterotrimeric G-proteins are generally identified by Western blotting and ADP-ribosylation with bacterial toxins. The monomeric and high molecular weight G-proteins have been identified by [35S]GTP delta S overlay technique and photoaffinity labeling, respectively. Recently, the use of molecular genetic probes has made it possible to investigate the expression of the message for various G-proteins.

Reduced number and coupling of beta-adrenergic receptors in a modified S49 mouse lymphoma cell line

Long-term culture of S49 wild-type cells in medium containing a high concentration of fetal calf serum leads to a modified (S49m) cell line with a reduced number of beta-adrenergic receptors (R). These S49m cells with a higher rate of proliferation were unable to respond to the beta-adrenergic agonists isoproterenol (ISO) and epinephrine as analysed by measuring adenylate cyclase (ac) activity on purified membranes of these cells. Additionally, no accumulation of cyclic AMP was obtained on S49m intact cells upon stimulation with beta-agonists. Nevertheless, S49m cells were able to respond significantly to the direct activation of the stimulatory guanine nucleotide binding (Gs) protein by aluminium tetrafluoride and sodium fluoride, and to the stimulation of another receptor coupled to the ac system through a Gs protein, by prostaglandin E1 (PGE1). When cloning S49m cells, similar results were obtained upon stimulation with ISO and PGE1 and the cloned cells express the same thy 1.2 and class Id molecules as do S49 cells. The study of S49m cells indicates that they are a beta-adrenergic R-deficient variant distinct from the other variants described for S49 cells.

Alterations in the stimulatory G protein of the rat liver after partial hepatectomy

In adult male rat livers, cAMP generation in response to beta-adrenergic agonists was dramatically stimulated after partial hepatectomy. Quantitation of the alpha subunits of the stimulatory G protein (Gs alpha) using ADP-ribosylation catalyzed by cholera toxin revealed the increment in the amounts of two forms of Gs alpha, Gs alpha-S and Gs alpha-L, during liver regeneration. These increases in the amounts of both Gs alpha proteins were associated with the stimulation in their mRNA levels. In addition, partial hepatectomy gave rise to a shift in the proportion of beta-adrenergic receptor (beta-AR) in the high affinity state produced by beta-AR-Gs complex. The susceptibility of Gs alpha to trypsin was used as a probe for beta-AR-Gs coupling. The GTP-bound forms of both Gs alpha-S and Gs alpha-L were more trypsin-sensitive than their GDP-bound forms. Preincubation of liver plasma membranes prepared from partially hepatectomized rats with the agonist isoproterenol resulted in an enhancement of trypsin-sensitivity of Gs alpha-L, but not Gs alpha-S. This effect was retarded by the addition of the antagonist propranolol. We conclude that the increase in the amount of Gs alpha can be contributed to the rise in beta-response after partial hepatectomy, and suggest that beta-AR is preferentially coupled with Gs alpha-L rather than Gs alpha-S.

Role of G-proteins in ciliary process adenylyl cyclase responses of the albino rabbit eye

After intravitreal injections of cholera or pertussis toxin (CTX or PTX, 0.5 -1 microgram/eye) into the albino rabbit eye, the in vitro responses of ciliary process adenylyl cyclase (AC) to isoproterenol, vasoactive intestinal peptide (VIP), and forskolin (FSK) were increased 21-40% for PTX, but for CTX-injected eyes AC responses to fluoroaluminate, VIP and FSK decreased 70-50%. The increased responses after PTX suggests that this toxin blocked an inhibitory Gi control of AC that is present in the control tissue. However, prolonged (> 24 hr) in vivo exposure to CTX appears to down-regulate the AC enzyme. In contrast to the in vivo findings, AC responsiveness was unaffected by PTX pre-treatment of membranes in vitro, while CTX pre-treatment increased basal activity (+600%), and the FSK response (+30%), but decreased responsiveness to fluoroaluminate, VIP and isoproterenol by 88-56%. Treatment of ciliary process membranes with 32P-NAD and CTX or PTX followed by SDS-PAGE autoradiography of labelled proteins gave two bands for the G-protein alpha-subunits of Gs (45, 56 kDa) and one broad band centered at 40 kDa for Gi-type subunits respectively. Western blots using specific antibodies showed the presence of Gi type I or III, but no detectable Gi type II or Go in rabbit ciliary processes. We conclude that the changes in adenylyl cyclase enzyme responses after intraocular CTX or PTX may not correlate with cAMP levels and intraocular pressure effects. However, the in vitro biochemical data on AC responses and on G-proteins provide evidence for dual regulation of ciliary process AC by activating and inhibitory G-proteins.

Changes in type VI adenylyl cyclase isoform expression correlate with a decreased capacity for cAMP generation in the aging ventricle

We investigated the developmental regulation of the beta-adrenergic receptor-Gs-adenylyl cyclase pathway in myocardial membranes from fetal, neonatal, adult, and mature adult rats by measuring the density of the beta-adrenergic receptor and the activities of the stimulatory guanine nucleotide-binding protein Gs and the adenylyl cyclase enzyme. Total beta-adrenergic receptor content (in femtomoles per milligram protein) was greatest in the fetal (124.4 +/- 20.5 fmol/mg) and neonatal (122.3 +/- 16.1 fmol/mg) stages and gradually decreased in the adult (90.9 +/- 8.0 fmol/mg) and mature adult (70.0 +/- 9.6 fmol/mg) stages. An equivalent pattern was seen for adenylyl cyclase activity: the basal activity of the effector enzyme or that measured in the presence of 0.1 mmol/L isoproterenol with 0.1 mmol/L Gpp(NH)p, 10 mmol/L NaF, or 0.05 mmol/L forskolin was greater in the fetus and the neonate than in the adult and the mature adult. These data suggested that decreased stimulation of the catalytic unit by Gs could be the underlying cause of diminished adenylyl cyclase activity with aging. However, quantification of Gs by reconstitution into S49 cyc- membranes (in picomoles cAMP per microgram for 10 minutes) demonstrated no significant decrease during development from fetus (1.55 +/- 0.1 pmol/microgram) to neonate (1.9 +/- 0.5 pmol/microgram) and subsequent aging to adult (2.6 +/- 0.2 pmol/micrograms) and mature adult (1.9 +/- 0.2 pmol/microgram). When Northern blot analysis was used to characterize the relative amounts of mRNA coding for Gs alpha, no significant differences were seen among the developmental stages studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor regulation of G-protein palmitoylation

Many alpha subunits of heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) are palmitoylated. Exposure of cells to the beta-adrenergic agonist isoproterenol increased incorporation of [3H]palmitate specifically into alpha s, the alpha subunit that mediates stimulation of adenylyl cyclase. Pulse-chase experiments suggested that isoproterenol increased turnover of alpha s-bound palmitate. Mutagenesis of Cys-3 in alpha s or alpha o (a homologous alpha subunit) prevented palmitoylation of these proteins. Differing results were obtained when mutations of Cys-3 in alpha s or alpha o were expressed in cells and assayed for their distribution between soluble and membrane fractions. Some alpha subunits, including alpha o, are myristoylated at the amino-terminal glycine residue. Mutation of this glycine prevented both myristoylation and palmitoylation of alpha o, indicating that myristoylation precedes palmitoylation of dually acylated alpha subunits. The amino-terminal sequences and fatty acylation properties of dually acylated alpha subunits are strikingly similar to those of some members of the Src family of protein-tyrosine kinases. The amino-terminal sequence Met-Gly-Cys-Xaa-Xaa-Ser/Cys shared by these proteins may represent a motif for cotranslational and posttranslational processing that includes myristoylation of the glycine residue and reversible palmitoylation of the cysteine residue.

12-O-tetradecanoyl-phorbol-13-acetate (TPA) counteracts the cAMP up-regulation of the expression of the stimulatory guanine nucleotide binding protein (Gs alpha) and Gs alpha messenger RNA in cultured pig thyroid cells

In this study, we examined whether the protein kinase C (PKC) pathway could interfere with the regulation of Gs protein in porcine thyroid cells. The two days culture of cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) (0.1 microM) alone neither affected adenylyl cyclase activity, nor the level of Gs alpha protein in membranes when compared with control cells. The co-addition of TPA with thyrotropin (TSH) (1 mU/ml) or forskolin (fk) (10 microM) in the culture medium, abolished the stimulatory effects of either agonists on the activation of adenylyl cyclase by fk or [AlF4]- and on the increase of Gs alpha protein. By contrast, TPA had effects neither on the Gi-dependent inhibition of adenylyl cyclase nor on Gi alpha proteins levels. The level of Gs alpha mRNA measured by Northern blot analysis was increased (200%) in TSH- or fk-treated cells and this increase was counteracted by TPA. The effects of TPA observed after 6-9 h of contact with cells were mimicked by mezerezin, a non-phorbol protein kinase C activator and blocked by bisindolylmaleimide a specific protein kinase C inhibitor (GF 109203X). These results suggest that the activation of the PKC pathway prevents the cAMP-dependent up-regulation of Gs alpha protein and Gs alpha mRNA expression.

Chronic dynamic exercise improves a functional abnormality of the G stimulatory protein in cardiomyopathic BIO 53.58 Syrian hamsters

BACKGROUND

The effects of chronic exercise training on myocardial contractility and beta-adrenergic signal transduction in hearts with left ventricular dysfunction have not been determined.

METHODS AND RESULTS

Fourteen-week-old cardiomyopathic BIO 53.58 and normal F1B Syrian hamsters underwent 10 weeks of treadmill training and were compared with 24-week-old BIO 53.58 and F1B untrained controls. Left ventricular isovolumic maximum positive dP/dt and peak developed pressure were significantly lower in BIO 53.58 than in F1B controls. Exercise training improved left ventricular contractile indices in BIO 53.58 but not F1B hamsters. The left ventricular beta-adrenergic receptor number (Bmax) was similar in BIO 53.58 and F1B controls. Basal adenylate cyclase activity (ACA) and ACAs stimulated by isoproterenol, 5'-guanylylimidodiphosphate (GppNHp), sodium fluoride, and forskolin were significantly lower in BIO 53.58 than in F1B controls. The functional activity of stimulatory guanine nucleotide-binding protein (Gs), as determined by reconstitution with S49 lymphoma cyc- cell membranes, was significantly lower in BIO 53.58 controls. After 10 weeks of exercise training, Bmax and basal and isoproterenol-stimulated ACAs were unchanged in either BIO 53.58 or F1B hamsters compared with controls. However, in F1B hamsters, training decreased ACAs stimulated by GppNHp, sodium fluoride, and forskolin, with a reduced functional activity of Gs. In contrast, these ACAs increased significantly in association with an enhanced Gs activity in cardiomyopathic BIO 53.58 hamsters after training.

CONCLUSIONS

Chronic exercise training does not change receptor-mediated beta-adrenergic responsiveness in either F1B or BIO 53.58 hamsters. However, exercise training reduces Gs activity in normal F1B hamsters and improves the functional abnormality of Gs in cardiomyopathic BIO 53.58 hamsters. This improvement may potentially contribute to augmented left ventricular contractility in BIO 53.58 after training.

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)

Vaccinia virus systems for expression of G alpha genes in S49 cells

In this chapter, we describe the use of recombinant VV to express G alpha subunits efficiently in S49 cyc- cells, a murine lymphoma cell line deficient in endogenous Gs alpha activity. Additional studies have shown that VV infection does not interfere with G-protein-coupled signal transduction events. In particular, the VV-expressed rat Gs alpha subunit is able to mediate efficient receptor-dependent and receptor-independent activation of adenylyl cyclase in cyc- cells. This system should therefore be useful for rapidly assessing the activity of modified or chimeric Gs alpha subunits.

Separate GTP binding and GTPase activating domains of a G alpha subunit

Most members of the guanosine triphosphatase (GTPase) superfamily hydrolyze guanosine triphosphate (GTP) quite slowly unless stimulated by a GTPase activating protein or GAP. The alpha subunits (G alpha) of the heterotrimeric G proteins hydrolyze GTP much more rapidly and contain an approximately 120-residue insert not found in other GTPases. Interactions between a G alpha insert domain and a G alpha GTP-binding core domain, both expressed as recombinant proteins, show that the insert acts biochemically as a GAP. The results suggest a general mechanism for GAP-dependent hydrolysis of GTP by other GTPases.

Activation of the alpha subunit of Gs in intact cells alters its abundance, rate of degradation, and membrane avidity

Binding of GTP induces alpha subunits of heterotrimeric G proteins to take on an active conformation, capable of regulating effector molecules. We expressed epitope-tagged versions of the alpha subunit (alpha s) of Gs in genetically alpha s-deficient S49 cyc- cells. Addition of a hemagglutinin (HA) epitope did not alter the ability of wild type alpha s to mediate hormonal stimulation of adenylyl cyclase or to attach to cell membranes. The HA epitope did, however, allow a mAb to immunoprecipitate the recombinant protein (HA-alpha s) quantitatively from cell extracts. We activated the epitope-tagged alpha s in intact cells by: (a) exposure of cells to cholera toxin, which activates alpha s by covalent modification; (b) mutational replacement of arginine-201 in HA-alpha s by a cysteine residue, to create HA-alpha s-R201C; like the cholera toxin-catalyzed modification, this mutation activates alpha s by slowing its intrinsic GTPase activity; and (c) treatment of cells with the beta-adrenoceptor agonist, isoproterenol, which promotes binding of GTP to alpha s, thereby activating adenylyl cyclase. Both cholera toxin and the R201C mutation accelerated the rate of degradation of alpha s (0.03 h-1) by three- to fourfold and induced a partial shift of the protein from a membrane bound to a soluble compartment. At steady state, 80% of HA-alpha s- R201C was found in the soluble fraction, as compared to 10% of wild type HA-alpha s. Isoproterenol rapidly (in < 2 min) caused 20% of HA-alpha s to shift from the membrane-bound to the soluble compartment. Cholera toxin induced a 3.5-fold increase in the rate of degradation of a second mutant, HA-alpha s-G226A, but did not cause it to move into the soluble fraction; this observation shows that loss of membrane attachment is not responsible for the accelerated degradation of alpha s in response to activation. Taken together, these findings show that activation of alpha s induces a conformational change that loosens its attachment to membranes and increases its degradation rate.