The G protein-gated atrial K+ channel is stimulated by three distinct Gi alpha-subunits

Inhibition of overexpression of Giα proteins and nitroxidative stress contribute to sodium nitroprusside-induced attenuation of high blood pressure in SHR

We earlier showed that vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) exhibit enhanced expression of Giα proteins which was attributed to the decreased levels of nitric oxide (NO), because elevation of the intracellular levels of NO by NO donors; sodium nitroprusside (SNP) and S-Nitroso-N-acetyl-DL-penicillamine (SNAP), attenuated the enhanced expression of Giα proteins. Since the enhanced expression of Giα proteins is implicated in the pathogenesis of hypertension, the present study was undertaken to investigate if treatment of SHR with SNP could also attenuate the development of high blood pressure (BP) and explore the underlying molecular mechanisms. Intraperitoneal injection of SNP at a concentration of 0.5 mg/kg body weight twice a week for 2 weeks into SHR attenuated the high blood pressure by about 80 mmHg without affecting the BP in WKY rats. SNP treatment also attenuated the enhanced levels of superoxide anion (O₂^- ), hydrogen peroxide (H₂ O₂ ), peroxynitrite (ONOO^- ), and NADPH oxidase activity in VSMC from SHR to control levels. In addition, the overexpression of different subunits of NADPH oxidase; Nox-1, Nox-2, Nox-4, P^22phox , and P^47phox , and Giα proteins in VSMC from SHR were also attenuated by SNP treatment. On the other hand, SNP treatment augmented the decreased levels of intracellular NO, eNOS, and cGMP in VSMC from SHR. These results suggest that SNP treatment attenuates the development of high BP in SHR through the elevation of intracellular levels of cGMP and inhibition of the enhanced levels of Giα proteins and nitroxidative stress.

Enhanced expression of Giα proteins contributes to the hyperproliferation of vascular smooth muscle cells from spontaneously hypertensive rats via MAP kinase- and PI3 kinase-independent pathways

Vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) exhibit hyperproliferation, enhanced MAP kinase (MAPK) activity, and overexpression of Giα proteins. This study was undertaken to examine whether the overexpression of Giα proteins contributes to the hyperproliferation of VSMC of SHR through MAPK signaling. The hyperproliferation of VSMC from SHR in the absence and presence of angiotensin II was restored towards those in Wistar-Kyoto (WKY) rats levels by pertussis toxin (PT) treatment. In addition, siRNA knockdown of Giα proteins also resulted in the attenuation of hyperproliferation towards control levels. The overexpression of Giα proteins was also inhibited by MAPK and PI3 kinase (PI3K) inhibitors. In addition, the hyperproliferation and enhanced phosphorylation of ERK1/2 and Akt in VSMC from SHR were attenuated towards WKY levels by the inhibitors of MAPK, PI3K, c-Src, and antioxidants, whereas PT was unable to attenuate the enhanced phosphorylation of ERK1/2 and Akt. Furthermore, 8Br-cAMP and forskolin also attenuated the hyperproliferation of VSMC from SHR. These results suggest that the hyperproliferation of VSMC from SHR may be attributed to the enhanced expression of Giα proteins and increased activation of MAPK and PI3 kinase. However, Giα-mediated hyperproliferation may not be mediated through MAPK- and PI3 kinase-dependent pathways and may involve decreased levels of intracellular cAMP.

Quantitative analysis of mammalian GIRK2 channel regulation by G proteins, the signaling lipid PIP2 and Na+ in a reconstituted system

GIRK channels control spike frequency in atrial pacemaker cells and inhibitory potentials in neurons. By directly responding to G proteins, PIP₂ and Na⁺, GIRK is under the control of multiple signaling pathways. In this study, the mammalian GIRK2 channel has been purified and reconstituted in planar lipid membranes and effects of Gα, Gβγ, PIP₂ and Na⁺ analyzed. Gβγ and PIP₂ must be present simultaneously to activate GIRK2. Na⁺ is not essential but modulates the effect of Gβγ and PIP₂ over physiological concentrations. Gα_i1(GTPγS) has no effect, whereas Gα_i1(GDP) closes the channel through removal of Gβγ. In the presence of Gβγ, GIRK2 opens as a function of PIP₂ mole fraction with Hill coefficient 2.5 and an affinity that poises GIRK2 to respond to natural variations of PIP₂ concentration. The dual requirement for Gβγ and PIP₂ can help to explain why GIRK2 is activated by G_i/o, but not G_q coupled GPCRs.

DOI:http://dx.doi.org/10.7554/eLife.03671.001

Though every cell in the body is surrounded by a membrane, there are a number of ways that molecules can pass through this membrane to either enter or leave the cell. Proteins from the GIRK family form channels in the membranes of mammalian cells, and when open these channels allow potassium ions to flow through the membrane to control the membrane's voltage.

GIRK channels are found in the heart and in the central nervous system, and can be activated in a variety of ways. Sodium ions and molecules called ‘signaling lipids’ can regulate the activation of GIRK channels. These channels can also be caused to open by G proteins: proteins that are found inside cells and that help to transmit signals from the outside of a cell to the inside. Three G proteins—called Gα, Gβ, and Gγ—work together in a complex that functions a bit like a switch. When switched on, the Gα subunit is separated from the other two subunits (called Gβγ); and both parts can then activate different signaling pathways inside the cell.

The Gβγ subunits and a signaling lipid have been known to regulate the opening of GIRK channels for a number of years, but these events have only been studied in the context of living cells. The specific role of each molecule, and whether the Gα subunit can also regulate the GIRK channels, remains unknown. Now Wang et al. have produced one type of mouse GIRK channel, called GIRK2, in yeast cells, purified this protein, and added it into an artificial membrane. This ‘reconstituted system’ allowed the regulation of a GIRK channel to be investigated under more controlled conditions than in previous experiments.

Wang et al. found that the Gβγ subunits and the signaling lipid both need to be present to activate the GIRK2 channel. Sodium ions were not essential, but promoted further opening when Gβγ and the signaling lipid were already present. When locked in its ‘on’ state, the Gα subunit had no effect on GIRK2, but adding Gα locked in the ‘off’ state closed these channels by removing the Gβγ proteins.

The findings of Wang et al. suggest that it should be possible to use a similar reconstituted system to investigate what allows different G proteins to activate specific signaling pathways.

DOI:http://dx.doi.org/10.7554/eLife.03671.002

cAMP attenuates the enhanced expression of Gi proteins and hyperproliferation of vascular smooth muscle cells from SHR: role of ROS and ROS-mediated signaling

We previously showed that angiotensin II (ANG II)-induced overexpression of inhibitory G proteins (Gi) was attenuated by dibutyryl-cAMP (db-cAMP) in A10 vascular smooth muscle cells (VSMC). Since enhanced levels of endogenous ANG II contributed to the overexpression of Gi protein and hyperproliferation of VSMC from spontaneously hypertensive rats (SHR), the present study was therefore undertaken to examine if cAMP could also attenuate the overexpression of Gi proteins and hyperproliferation of VSMC from SHR and to explore the underlying molecular mechanisms responsible for this response. The enhanced expression of Giα proteins in VSMC from SHR and Nω-nitro-L-arginine methyl ester hypertensive rats was decreased by db-cAMP. In addition, enhanced inhibition of adenylyl cyclase by inhibitory hormones and forskolin-stimulated adenylyl cyclase activity by low concentration of GTPγS in VSMC from SHR was also restored to Wistar-Kyoto (WKY) levels by db-cAMP. Furthermore, db-cAMP also attenuated the hyperproliferation and the increased production of superoxide anion, NAD(P)H oxidase activity, overexpression of Nox1/Nox2/Nox4 and p47phox proteins, increased phosphorylation of PDGF-receptor (R), EGF-R, c-Src, and ERK1/2 to control levels. In addition, the protein kinase A (PKA) inhibitor reversed the effects of db-cAMP on the expression of Nox4 and Giα proteins and hyperproliferation of VSMC from SHR to WKY levels, while stimulation of the exchange protein directly activated by cAMP did not have any effect on these parameters. These results suggest that cAMP via PKA pathway attenuates the overexpression of Gi proteins and hyperproliferation of VSMC from SHR through the inhibition of ROS and ROS-mediated transactivation of EGF-R/PDGF-R and MAPK signaling pathways.

Role of epidermal growth factor receptor transactivation in endothelin-1-induced enhanced expression of Gi protein and proliferation in A10 vascular smooth muscle cells

We have recently shown that vasoactive peptides such as angiotensin II (Ang II) and endothelin-1 (ET-1) increase the expression of Gi proteins and the proliferation of A10 vascular smooth muscle cells (VSMC) through mitogen-activated protein (MAP) kinase – phosphoinositide (PI) 3-kinase pathways. This study was intended to examine the implication of epidermal growth factor receptor (EGFR) activation in ET-1-induced enhanced expression of Gi proteins and proliferation of A10 VSMC, and to further investigate the underlying mechanisms responsible for these increases. Cell proliferation was determined by [3H]thymidine incorporation and the expression of Gi proteins; extracellular signal-regulated kinases 1 and 2 (ERK1/2) and EGFR phosphorylation was determined by Western blotting. Treatment of A10 VSMC with ET-1 enhanced the expression of Gi proteins, which was attenuated by BQ123 and BQ788, antagonists of ETA and ETB receptor respectively. In addition, ET-1 enhanced the phosphorylation of EGFR in A10 VSMC, which was restored to the control levels by EGFR inhibitor and ETA and ETB receptor antagonists. Furthermore, ET-1 also augmented the proliferation and ERK1/2 phosphorylation of A10 VSMC, which were restored to the control levels by inhibition of EGFR. These data suggest that ET-1 transactivates EGFR, which, through MAP kinase signaling, may contribute to the enhanced expression of Gi proteins and thus increased proliferation of A10 VSMC.

Implication of multiple signaling pathways in the regulation of angiotensin II induced enhanced expression of Giα proteins in vascular smooth muscle cells

We have previously shown that A10 vascular smooth muscle cells (VSMC) exposed to angiotensin II (Ang II) exhibited overexpression of Giα proteins. In the present study, we examined the involvement of different signaling pathways in regulating Ang II induced enhanced expression of Giα proteins in VSMC by using pharmacological inhibitors. Ang II induced increased expression of Giα proteins in A10 VSMC was markedly attenuated by actinomycin D, losartan (an AT(1) receptor antagonist), dibutyryl cAMP, phospholipase C (PLC) inhibitor U73122, protein kinase C (PKC) inhibitors staurosporine and GP109203X, but not by PD123319 (an AT(2) receptor antagonist). In addition, BAPTA-AM and TMB-8 (chelators of intracellular Ca(2+)); and nifedipine (a blocker of L-type Ca(2+) channels) significantly inhibited Ang II induced enhanced expression of Giα proteins. On the other hand, extracellular Ca(2+) chelation using EGTA did not affect the Ang II evoked enhanced levels of Giα proteins. Furthermore, pretreatment of A10 VSMC with calmidazolium (an inhibitor of calmodulin), or KN93 (an inhibitor of CaM kinase), or genistein (an inhibitor of protein tyrosine kinase, PTK), also attenuated the increased levels of Giα proteins induced by Ang II. These results suggest that Ang II induced enhanced expression of Giα proteins may be regulated by different signaling pathways through AT(1) receptors in A10 VSMC.

Regulators of G-protein signaling and their Gα substrates: promises and challenges in their use as drug discovery targets

Because G-protein coupled receptors (GPCRs) continue to represent excellent targets for the discovery and development of small-molecule therapeutics, it is posited that additional protein components of the signal transduction pathways emanating from activated GPCRs themselves are attractive as drug discovery targets. This review considers the drug discovery potential of two such components: members of the "regulators of G-protein signaling" (RGS protein) superfamily, as well as their substrates, the heterotrimeric G-protein α subunits. Highlighted are recent advances, stemming from mouse knockout studies and the use of "RGS-insensitivity" and fast-hydrolysis mutations to Gα, in our understanding of how RGS proteins selectively act in (patho)physiologic conditions controlled by GPCR signaling and how they act on the nucleotide cycling of heterotrimeric G-proteins in shaping the kinetics and sensitivity of GPCR signaling. Progress is documented regarding recent activities along the path to devising screening assays and chemical probes for the RGS protein target, not only in pursuits of inhibitors of RGS domain-mediated acceleration of Gα GTP hydrolysis but also to embrace the potential of finding allosteric activators of this RGS protein action. The review concludes in considering the Gα subunit itself as a drug target, as brought to focus by recent reports of activating mutations to GNAQ and GNA11 in ocular (uveal) melanoma. We consider the likelihood of several strategies for antagonizing the function of these oncogene alleles and their gene products, including the use of RGS proteins with Gα(q) selectivity.

Transactivation of epidermal growth factor receptor by enhanced levels of endogenous angiotensin II contributes to the overexpression of Giα proteins in vascular smooth muscle cells from SHR

We earlier showed that the increased expression of Gi proteins exhibited by vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) was attributed to the enhanced levels of endogenous endothelin. Since the levels of angiotensin II (Ang II) are also enhanced in VSMC from SHR, the present study was undertaken to examine the role of enhanced levels of endogenous Ang II in the overexpression of Giα proteins in VSMC from SHR and to further explore the underlying mechanisms responsible for this increase. The enhanced expression of Giα-2 and Giα-3 proteins in VSMC from SHR compared to WKY was attenuated by the captopril, losartan and AG1478, inhibitors of angiotensin converting enzyme, AT(1) receptor and epidermal growth factor receptor (EGFR) respectively as well as by the siRNAs of AT1, cSrc and EGFR. The enhanced inhibition of forskolin-stimulated adenylyl cyclase activity by low concentrations of GTPγS (receptor-independent functions) and of inhibitory responses of hormones on adenylyl cyclase activity (receptor-dependent functions) in VSMC from SHR was also attenuated by losartan. Furthermore, the enhanced phosphorylation of EGFR in VSMC from SHR was also restored to control levels by captopril, losartan, PP2, a c-Src inhibitor and N-acetyl-L-cysteine (NAC), superoxide anion (O(2)(-)) scavenger, whereas enhanced ERK1/2 phosphorylation was attenuated by captopril and losartan. Furthermore, NAC also restored the enhanced phosphorylation of c-Src in SHR to control levels. These results suggest that the enhanced levels of endogenous Ang II in VSMC from SHR, transactivate EGFR, which through MAP kinase signaling, enhance the expression of Giα proteins and associated adenylyl cyclase signaling.

Enhanced levels of endogenous endothelin-1 contribute to the over expression of Giα protein in vascular smooth muscle cells from SHR: Role of growth factor receptor activation

We earlier showed that vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) exhibit increased expression of Gi proteins. Since the levels of endothelin-1 (ET-1) are enhanced in VSMC from SHR, we undertook the present study to examine the implication of endogenous ET-1 and the underlying mechanisms in the enhanced expression of Giα proteins in VSMC from SHR. The enhanced expression of Giα-2 and Giα-3 proteins in VSMC from SHR was inhibited by ET(A) and ET(B) receptor antagonists, BQ123 and BQ788 respectively. In addition, these antagonists also attenuated the enhanced inhibition of forskolin-stimulated adenylyl cyclase activity by low concentrations of GTPγS and by inhibitory hormones in VSMC from SHR compared to WKY. Furthermore, AG1295, AG1024 and PP2, inhibitors of platelet derived growth factor receptor (PDGFR), insulin-like growth factor 1 receptor (IGF-1R) and c-Src respectively, inhibited the enhanced expression of Giα protein and the enhanced phosphorylation of PDGFR and IGF-1R in VSMC from SHR to WKY levels. In addition, NAD(P)H oxidase inhibitor DPI and N-acetylcysteine (NAC), a scavenger of superoxide anion (O₂⁻) also inhibited the enhanced phosphorylation of PDGFR and IGF-1R and c-Src in VSMC from SHR to control levels. Furthermore, the augmented phosphorylation of ERK1/2 in VSMC from SHR was attenuated by BQ123 and BQ788, growth factor receptors inhibitors and PP2. These results suggest that the enhanced levels of endogenous ET-1 in VSMC from SHR increase oxidative stress, which through c-Src-mediated activation of growth factor receptors and associated MAP kinase signaling, contribute to the enhanced expression of Giα proteins.

Emerging roles for G protein-gated inwardly rectifying potassium (GIRK) channels in health and disease

G protein-gated inwardly rectifying potassium (GIRK) channels hyperpolarize neurons in response to activation of many different G protein-coupled receptors and thus control the excitability of neurons through GIRK-mediated self-inhibition, slow synaptic potentials and volume transmission. GIRK channel function and trafficking are highly dependent on the channel subunit composition. Pharmacological investigations of GIRK channels and studies in animal models suggest that GIRK activity has an important role in physiological responses, including pain perception and memory modulation. Moreover, abnormal GIRK function has been implicated in altering neuronal excitability and cell death, which may be important in the pathophysiology of diseases such as epilepsy, Down's syndrome, Parkinson's disease and drug addiction. GIRK channels may therefore prove to be a valuable new therapeutic target.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Role of oxidative stress in high glucose-induced decreased expression of Gialpha proteins and adenylyl cyclase signaling in vascular smooth muscle cells

We have recently shown that aorta from streptozotocin (STZ)-induced diabetic rats and A10 vascular smooth muscle cells (VSMCs) exposed to high glucose exhibited decreased levels of inhibitory guanine nucleotide regulatory protein (Gi)alpha proteins. In the present studies, we investigated the implication of oxidative stress in the hyperglycemia/diabetes-induced decreased expression of the Gialpha protein and adenylyl cyclase signaling in VSMCs by using antioxidants. The levels of Gialpha proteins were significantly decreased in A10 VSMCs exposed to high glucose and in aortic VSMCs from STZ-diabetic rats compared with control cells and were restored to control levels by antioxidants. In addition, (111)Mn-tetralis(benzoic acid porphyrin) and uric acid, scavengers of peroxynitrite, and NG-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase but not catalase, also restored the high glucose-induced decreased expression of Gialpha proteins to the control levels in A10 VSMCs. Furthermore, the enhanced production of superoxide anion (O2-) and increased activity of NADPH oxidase in these cells were also restored to control levels by diphenyleneiodonium, an inhibitor of NADPH oxidase. In addition, the diminished inhibition of adenylyl cyclase activity by inhibitory hormones and forskolin-stimulated adenylyl cyclase activity by low concentrations of GTPgammaS as well as the enhanced stimulation of adenylyl cyclase by stimulatory agonists in hyperglycemic cells were restored to control levels by antioxidant treatments. These results suggest that high glucose-induced decreased levels of Gialpha proteins and associated signaling in A10 VSMCs may be attributed to the enhanced oxidative stress due to augmented levels of peroxynitrite and not to H2O2.

Cyclic GMP modulates the expression of Gi protein and adenylyl cyclase signaling in vascular smooth muscle cells

We have recently shown that the nitric oxide (NO) donor, SNAP, decreased the expression of Gialpha proteins and associated functions in vascular smooth muscle cells. Because NO stimulates soluble guanylyl cyclase and increases the levels of guanosine 3\',5\'-cyclic monophosphate (cGMP), the present studies were undertaken to investigate whether cGMP can also modulate the expression of Gi proteins and associated adenylyl cyclase signaling. A10 vascular smooth muscle cells (VSMCs) and primary cultured cells from aorta of Sprague Dawley rats were used for these studies. The cells were treated with 8-bromoguanosine 3\',5\'-cyclic monophosphate (8BrcGMP) for 24 h and the expression of Gialpha proteins was determined by immunobloting techniques. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation for [alpha-32P]ATP. Treatment of cells with 8-BrcGMP (0.5 mM) decreased the expression of Gialpha-2 and Gialpha-3 by about 30-45%, which was restored towards control levels by KT5823, an inhibitor of protein kinase G. On the other hand, the levels of Gsalpha protein were not altered by this treatment. The decreased expression of Gialpha proteins by 8Br-cGMP treatment was reflected in decreased Gi functions. For example, the inhibition of forskolin (FSK)-stimulated adenylyl cyclase activity by low concentrations of GTPgammaS (receptor-independent Gi functions) was significantly decreased by 8Br-cGMP treatment. In addition, exposure of the cells to 8Br-cGMP also resulted in the attenuation of angiotensin (Ang) II- and C-ANP4-23 (a ring-deleted analog of atrial natriuretic peptide [ANP])-mediated inhibition of adenylyl cyclase activity (receptor-dependant functions of Gi). On the other hand, Gsalpha-mediated stimulations of adenylyl cyclase by GTPgammaS, isoproterenol and FSK were significantly augmented in 8Br-cGMP-treated cells. These results indicate that 8Br-cGMP decreased the expression of Gialpha proteins and associated functions in VSMCs. From these studies, it can be suggested that 8Br-cGMP-induced decreased levels of Gi proteins and resultant increased levels of cAMP may be an additional mechanism through which cGMP regulates vascular tone and thereby blood pressure.

Peroxynitrite inhibits the expression of G(i)alpha protein and adenylyl cyclase signaling in vascular smooth muscle cells

We previously showed that S-nitroso-N-acetylpenicillamine, a nitric oxide donor, decreased the levels and functions of G(i)alpha proteins by formation of peroxynitrite (ONOO(-)) in vascular smooth muscle cells (VSMC). The present studies were undertaken to investigate whether ONOO(-) can modulate the expression of G(i)alpha protein and associated adenylyl cyclase signaling in VSMC. Treatment of A-10 and aortic VSMC with ONOO(-) for 24 h decreased the expression of G(i)alpha-2 and G(i)alpha-3, but not G(s)alpha, protein in a concentration-dependent manner; expression was restored toward control levels by (111)Mn-tetralis(benzoic acid porphyrin) and uric acid, but not by 1H[1,2,4]oxadiazole[4,3-a]quinoxaline-1-one (ODQ) and KT-5823. cGMP levels were increased by approximately 50% and 150% by 0.1 and 0.5 mM ONOO(-), respectively, and attenuated toward control levels by ODQ. In addition, 0.5 mM ONOO(-) attenuated the inhibition of adenylyl cyclase by ANG II and C-type atrial natriuretic peptide (C-ANP(4-23)), as well as the inhibition of forskolin-stimulated adenylyl cyclase activity by GTPgammaS, whereas, the G(s)-mediated stimulations were augmented. In addition, 0.5 mM ONOO(-) decreased phosphorylation of ERK1/2 and p38 MAP kinase and enhanced JNK phosphorylation but did not affect AKT1/3 phosphorylation. These results suggest that ONOO(-) decreased the expression of G(i) proteins and associated functions in VSMC through a cGMP-independent mechanism and may involve the MAP kinase signaling pathway.

In vitro effects of 17 beta-oestradiol on the sensitivity of receptors coupled to adenylate cyclase on striatal neurons in primary culture

Pretreatment of intact striatal neurons from the mouse embryo in primary culture with 17 beta-oestradiol (10(-9) M), 24 hours) enhanced the stimulation of adenylate cyclase activity induced by either dopamine (D1 receptors), isoproterenol, serotonin or 2-chloroadenosine (maximal effective concentrations) but suppressed inhibitory responses evoked by agonists of D2-dopaminergic or enkephalin (mu and delta) receptors. Binding studies indicated that some of these effects are (beta 1) or are not (D1 and D2) associated with changes in the number of receptors. Similar effects were partially seen with testosterone but not with 17 alpha-oestradiol, progesterone or dexamethasone and those induced by 17 beta-oestradiol were abolished when cells were exposed to inhibitors of mRNA transcription (alpha-amanitin) or protein synthesis (cycloheximide). Modifications in the properties of Gs or Go,i proteins were postulated because the number of adenylate cyclase catalytic subunits was not affected by 17 beta-oestradiol pretreatment. Results of ADP-ribosylation experiments with cholera toxin or pertussis toxin and of immunoblot experiments with anti-G alpha o and anti-G beta sera led us to suggest that 17 beta-oestradiol induces qualitative modifications in Go,i proteins leading to a stabilization of the associated form of the heterotrimer G alpha o,i beta gamma. In fact, pretreatment with pertussis toxin (which impairs G alpha o,i beta gamma dissociation) mimics the effects of 17 beta-oestradiol on responses of adenylate cyclase to stimulatory and inhibitory agonists.

Expansion of signal transduction by G proteins. The second 15 years or so: from 3 to 16 alpha subunits plus betagamma dimers

The first 15 years, or so, brought the realization that there existed a G protein coupled signal transduction mechanism by which hormone receptors regulate adenylyl cyclases and the light receptor rhodopsin activates visual phosphodiesterase. Three G proteins, Gs, Gi and transducin (T) had been characterized as alphabetagamma heterotrimers, and Gsalpha-GTP and Talpha-GTP had been identified as the sigaling arms of Gs and T. These discoveries were made using classical biochemical approaches, and culminated in the purification of these G proteins. The second 15 years, or so, are the subject of the present review. This time coincided with the advent of powerful recombinant DNA techniques. Combined with the classical approaches, the field expanded the repertoire of G proteins from 3 to 16, discovered the superfamily of seven transmembrane G protein coupled receptors (GPCRs) -- which is not addressed in this article -- and uncovered an amazing repertoire of effector functions regulated not only by alphaGTP complexes but also by betagamma dimers. Emphasis is placed in presenting how the field developed with the hope of conveying why many of the new findings were made.

Enhanced expression of Gialpha protein and adenylyl cyclase signaling in aortas from 1 kidney 1 clip hypertensive rats

We have previously shown the augmented levels of Gialpha-2 and Gialpha-3 proteins (isoforms of inhibitory guanine nucleotide regulatory protein (G-protein)), and not of Gsalpha, in the hearts and aortas of spontaneously and experimentally induced hypertensive rats. The increased expression of Gialpha and blood pressure was restored toward WKY levels by captopril treatment, suggesting a role for angiotensin (Ang) II in the enhanced expression of Gialpha protein and blood pressure. This study was undertaken to investigate whether 1 kidney 1 clip (1K-1C) hypertensive rats that exhibit enhanced levels of Ang II also express enhanced levels of Gialpha proteins. Aortas from 1K-1C hypertensive rats were used. The expression of G-proteins was determined at protein levels with immunoblotting techniques, using specific antibodies for different isoforms of G-proteins. The levels of Gialpha-2 and Gialpha-3 proteins were significantly higher in aortas from 1K-1C hypertensive rats than in control rats; Gsalpha levels were unchanged. The inhibitory effect of low concentrations of guanosine 5'-[gamma-thio]triphosphate (GTPgammaS) on forskolin (FSK)-stimulated adenylyl cyclase (AC) activity was significantly enhanced in aortas from 1K-1C hypertensive rats; the inhibitory effect of C-ANP(4-23), which specifically interacts with the atrial natriuretic peptide (ANP)-C receptor, and Ang II on AC was attenuated. GTPgammaS, isoproterenol, glucagon, NaF, and FSK stimulated the AC activity in aortas from control and hypertensive rats to varying degrees; however, the stimulations were significantly lower in hypertensive rats than in control rats. These data suggest that aortas from 1K-1C hypertensive rats exhibit enhanced expression of Gialpha proteins and associated functions.

Nitric oxide modulates Gi-protein expression and adenylyl cyclase signaling in vascular smooth muscle cells

We have previously shown that treatment of rats with the nitric oxide (NO) synthase inhibitor N6-nitro-L-arginine methyl ester for 4 weeks resulted in the augmentation of blood pressure and enhanced levels of Gialpha proteins. The present studies were undertaken to investigate if NO can modulate the expression of Gi proteins and associated adenylyl cyclase signaling. A10 vascular smooth muscle cells (VSMC) and primary cultured cells from aorta of Sprague-Dawley rats were used for these studies. The cells were treated with S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitroprusside (SNP) for 24 h and the expression of Gialpha proteins was determined by immunobloting techniques. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation for [alpha-32P]ATP. Treatment of cells with SNAP (100 microM) or SNP (0.5 mM) decreased the expression of Gialpha-2 and Gialpha-3 by about 25-40% without affecting the levels of Gsalpha proteins. The decreased expression of Gialpha proteins was reflected in decreased Gi functions (receptor-independent and -dependent) as demonstrated by decreased or attenuated forskolin-stimulated adenylyl cyclase activity by GTPgammaS and inhibition of adenylyl cyclase activity by angiotensin II and C-ANP4-23, a ring-deleted analog of atrial natriuretic peptide (ANP) that specifically interacts with natriuretic peptide receptor-C (NPR-C) in SNAP-treated cells. The SNAP-induced decreased expression of Gialpha-2 and Gialpha-3 proteins was not blocked by 1H[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one, an inhibitor of soluble guanylyl cyclase, or KT5823, an inhibitor of protein kinase G, but was restored toward control levels by uric acid, a scavenger of peroxynitrite and Mn(111)tetralis (benzoic acid porphyrin) MnTBAP, a peroxynitrite scavenger and a superoxide dismutase mimetic agent that inhibits the production of peroxynitrite, suggesting that NO-mediated decreased expression of Gialpha protein was cGMP-independent and may be attributed to increased levels of peroxynitrite. In addition, Gsalpha-mediated stimulation of adenylyl cyclase by GTPgammaS, isoproterenol, and forskolin was significantly augmented in SNAP-treated cells. These results indicate that NO decreased the expression of Gialpha protein and associated functions in VSMC by cGMP-independent mechanisms. From these studies, it can be suggested that NO-induced decreased levels of Gi proteins and resultant increased levels of cAMP may be an additional mechanism through which NO regulates blood pressure.

The −318 C>G Single-Nucleotide Polymorphism in GNAI2 Gene Promoter Region Impairs Transcriptional Activity through Specific Binding of Sp1 Transcription Factor and Is Associated with High Blood Pressure in Caucasians from Italy

Inhibiting Galpha subunit 2 protein, which is encoded by the GNAI2 gene, is suggested to be pathogenic for essential hypertension and/or insulin resistance. The aim of this study was to determine whether GNAI2 variations modulate the risk for these abnormalities. Seven single-nucleotide polymorphisms (SNP) at the GNAI2 locus were identified. Because of either low allelic frequency or unlikely biologic relevance (i.e., synonymous or intronic), six SNP were not studied further. The -318C>G SNP (allelic frequency 6%) in the promoter region was studied for association with adiposity, systolic BP (SBP) and diastolic BP, fasting insulin and glucose, and lipids levels in 655 nondiabetic Caucasians from Italy. As compared with individuals who carry the C/C genotype, G carriers (i.e., individuals who carry either the G/G or the C/G genotype) had higher SBP (117.8 +/- 16 versus 113.6 +/- 12.6 mmHg; P = 0.010) and were at increased risk for hypertension (odds ratio 2.2; 95% confidence interval 1.1 to 4.5). Compared with the C, the G allele had 2.5-fold reduced transcriptional activity in transfected HEK293 cells. As predicted by the TRANSFAC database, competition with YY1 or Sp1 transcription factors specifically reduced the binding of HeLa cell nuclear proteins to -318C or -318G allele, respectively, as indicated by shifted electrophoretic mobility. A "supershift" of the nuclear proteins/-318G allele complex was observed after anti-Sp1 was added but not anti-YY1 antibody. The GNAI2 -318 C>G SNP impairs transcriptional activity through specific binding of Sp1 and is associated with high SBP in Caucasians from Italy.

Oxidative stress contributes to the enhanced expression of Gialpha proteins and adenylyl cyclase signaling in vascular smooth muscle cells from spontaneously hypertensive rats

OBJECTIVE

We have previously shown an enhanced expression of Gialpha proteins in spontaneously hypertensive rats (SHR) that precedes the development of hypertension. Since oxidative stress has been shown to be increased in SHR, the present studies were undertaken to examine the role of oxidative stress in enhanced expression of Gialpha proteins in SHR.

METHODS

Aortic vascular smooth muscle cells (VSMC) from 12-week-old SHR and Wistar-Kyoto (WKY) rats were used for the present studies. The levels of inhibitory guanine nucleotide regulatory proteins (Gialpha-2 and Gialpha-3) and stimulatory proteins (Gsalpha) were determined by western blotting techniques. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation from [alpha-32P]ATP.

RESULTS

VSMC from SHR exhibited enhanced expression of Gialpha-2 and Gialpha-3 proteins as compared with age-matched WKY rats; however, the levels of Gsalpha proteins were not different between the two groups. The levels of superoxide anion (O2-) were also increased in SHR as compared with WKY rats, and losartan, an AT1 receptor antagonist, restored the enhanced levels to control WKY rat levels. Treatment of VSMC with antioxidants such as N-acetyl-L-cysteine (NAC) or diphenyleneiodonium (DPI) for 24 h decreased the enhanced expression of Gialpha-2 and Gialpha-3 proteins in a concentration-dependent manner in VSMC from SHR. In addition, the inhibition of forskolin-stimulated enzyme activity by low concentrations of GTPgammaS (receptor-independent Gi functions) and C-ANP4-23-mediated inhibition of adenylyl cyclase (receptor-dependent Gi functions) that were significantly enhanced in SHR were restored to WKY rat levels by NAC and DPI treatments. Similarly, diminished stimulation of adenylyl cyclase by GTPgammaS, isoproterenol and sodium fluoride in SHR was also restored towards control WKY rat levels by NAC and DPI treatments. Furthermore, PD98059, a selective inhibitor of mitogen-activated protein kinase, was able to restore the enhanced expression of Gialpha proteins in VSMC from SHR towards WKY rat levels. In addition, the enhanced activity of extracellular signal-regulated kinase 1/2 in SHR as compared with WKY rats, as demonstrated by enhanced phosphorylation of extracellular signal-regulated kinase 1/2, was also restored to WKY rat levels by NAC or DPI.

CONCLUSIONS

These results suggest that enhanced levels of Gialpha proteins and associated functions in SHR may be attributed to the enhanced oxidative stress present in SHR, which exerts its effects through the mitogen-activated protein kinase signaling pathway.

Losartan-induced attenuation of blood pressure in L-NAME hypertensive rats is associated with reversal of the enhanced expression of Gi alpha proteins

OBJECTIVE

We have previously reported that hearts from N-[omega]-nitro-L-arginine methyl ester (L-NAME)-induced hypertensive rats exhibited an enhanced expression of Gi proteins. Since, losartan, an AT1 receptor antagonist, has been shown to attenuate the L-NAME-induced increase in blood pressure, we undertook the present studies to evaluate whether losartan-induced decreased blood pressure in this model of hypertension is associated with attenuation of enhanced expression of Gi proteins and adenylyl cyclase signalling.

METHODS

L-NAME (70 mg/kg body weight) and losartan (10 mg/kg body weight), alone or in combination, were given orally to Sprague-Dawley rats for 4 weeks. The control rats received only plain tap water. The levels of inhibitory guanine nucleotide regulatory proteins (Gi alpha-2 and Gi alpha-3) and stimulatory (Gs alpha) proteins and Gi alpha mRNA in hearts were determined by immunoblotting and Northern blotting, respectively. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation from [32P]ATP.

RESULTS

Systolic blood pressure was enhanced in L-NAME-treated rats compared to control rats (164 +/- 5.2 versus 105 +/- 2 mmHg; n = 30), and was significantly attenuated by losartan treatment (164 +/- 5.2 mmHg versus 120 +/- 2.5 mmHg; n = 30). The expression of Gi alpha-2 and Gi alpha-3 proteins and their mRNA, which was enhanced in L-NAME-treated rats, was reversed by losartan treatment. However, losartan alone did not alter the levels of Gs alpha or Gi alpha proteins. In addition, the stimulatory effects of guanosine 5'-gamma-thiotriphosphate (GTPgammaS), isoproterenol, 5'-N-ethylcarboxamideadenosine (NECA), glucagon, forskolin (FSK) and sodium fluoride (NaF) on adenylyl cyclase, which were diminished in L-NAME-treated rats, were reversed by losartan treatment. Furthermore, the inhibition of forskolin-stimulated enzyme activity by low concentrations of GTPgammaS (receptor-independent Gi functions), which was significantly enhanced in L-NAME-treated rats, was attenuated by losartan treatment. In addition, losartan was able to reverse the attenuated receptor-mediated inhibitions of adenylyl cyclase by oxotremorine and angiotensin II towards control.

CONCLUSIONS

These results suggest the implication of AT1 receptors in enhanced expression of Gi alpha proteins and increased blood pressure in L-NAME-induced hypertension.

Effects of Sodium Selenate Treatment on Altered Responses of Left and Right Atria from Streptozotocin-Induced Diabetic Rats

Experiments were designed to determine whether experimental diabetes alters inotropic and chronotropic effects of adrenergic, adenosinergic, and cholinergic agonists and whether the observed changes are prevented by sodium selenate therapy. Thirty-two rats were divided into four groups of eight subjects each. Diabetes induced by streptozotocin caused significant decreases in isoproterenol-invoked contraction of the left atria with preservation of the right atrial chronotropic responses. The diminished response of the left atrial muscle to isoproterenol did not respond to treatment with sodium selenate. The left atria adenosine-induced direct- and indirect inotropic responses were diminished in the diabetic rats. After treatment with sodium selenate the direct response was completely normalized, but the indirect response was only partially corrected. Adenosine-induced negative chronotropic effects are accompanied by changed responses in diabetic right atria that are corrected after treatment. The carbachol-induced inotropic and chronotropic responses were not altered in tests of the acetylcholine system. We conclude that in diabetic rats, sodium selenate treatment reverses the deficits of adenosine-induced negative inotropic responses of left and right atria, but not those of isoproterenol-induced positive inotropic responses.

Functional M3 muscarinic acetylcholine receptors in mammalian hearts

In contrast to most peripheral tissues where multiple subtypes of muscarinic acetylcholine receptor (mAChR) coexist, with each of them playing its part in the orchestra of parasympathetic innervation, the myocardium has been traditionally considered to possess a single mAChR subtype. Although there is much evidence to support the notion that one receptor subtype (M2) orchestrates myocardial muscarinic transduction, there is emerging evidence that M1 and M3 receptors are also expressed and are of potential physiological, pathophysiological and pharmacological relevance. Clarifying this issue has a profound impact on our thinking about the cholinergic control of the heart function and disease and approaches to new drug development for the treatment of heart disease associated with parasympathetic dysfunction. This review article presents evidence for the presence of the M3 receptor subtype in the heart, and analyzes the controversial data from published pharmacological, functional and molecular studies. The potential roles of the M3 receptors, in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias, are discussed. On the basis of these considerations, we have made some proposals concerning the future of myocardial M3 receptor research.

Redox modulation of Gi protein expression and adenylyl cyclase signaling: role of nitric oxide

Nitric oxide (NO) has been shown to regulate a variety of physiological functions, including vascular tone. The inhibition of NO synthase by N(omega)-nitro-L-arginine methyl ester (L-NAME) has been reported to increase arterial blood pressure. The present studies were undertaken to investigate if the increased blood pressure by L-NAME is associated with enhanced expression of Gi proteins, implicated in the pathogenesis of hypertension. L-NAME was administered orally into Sprague-Dawley rats for a period of 4 weeks. Control rats were given plain tap water only. The systolic blood pressure was enhanced in L-NAME-treated rats as compared with control rats; however, the heart-to-body weight ratio was not different in the two groups. The levels of Gialpha-2 and Gialpha-3 proteins and their mRNA as determined by western and northern blotting, respectively, were significantly augmented in hearts from L-NAME-treated rats, whereas the levels of Gsalpha and Gbeta were unaltered. In addition, the effect of low concentrations of GTPgammaS on forskolin-stimulated adenylyl cyclase activity (receptor-independent functions of Gialpha) was significantly enhanced, whereas the receptor-dependent inhibitions of adenylyl cyclase were completely attenuated in L-NAME-treated rats. Whereas cholera toxin-mediated stimulation of adenylyl cyclase was unaltered in both group of rats, the stimulatory effects of some agonists on adenylyl cyclase activity were diminished in L-NAME-treated rats. These results suggest the implication of NO in the modulation of Gi protein expression and associated adenylyl cyclase signaling.

Cell signal control of the G protein-gated potassium channel and its subcellular localization

G protein-gated inward rectifier K(+) (K(G)) channels are directly activated by the betagamma subunits released from pertussis toxin-sensitive G proteins, and contribute to neurotransmitter-induced deceleration of heart beat, formation of slow inhibitory postsynaptic potentials in neurones and inhibition of hormone release in endocrine cells. The physiological roles of K(G) channels are critically determined by mechanisms which regulate their activity and their subcellular localization. K(G) channels are tetramers of inward rectifier K(+) (Kir) channel subunits, Kir3.x. The combination of Kir3.x subunits in each K(G) channel varies among tissues and cell types. Each subunit of the channel possesses one Gbetagamma binding site. The binding of Gbetagamma increases the number of functional K(G) channels via a mechanism that can be described by the Monod-Wyman-Changeux allosteric model. During voltage pulses K(G) channel current alters time dependently. The K(G) current exhibits inward rectification due to blockade of outward-going current by intracellular Mg(2+) and polyamines. Upon repolarization, this blockade is relieved practically instantaneously and then the current slowly increases further. This slow current alteration is called 'relaxation'. Relaxation is caused by the voltage-dependent behaviour of regulators of G protein signalling (RGS proteins), which accelerate intrinsic GTP hydrolysis mediated by the Galpha subunit. Thus, the relaxation behaviour of K(G) channels reflects the time course with which the G protein cycle is altered by RGS protein activity at each membrane potential. Subcellular localization of K(G) channels is controlled by several distinct mechanisms, some of which have been recently clarified. The neuronal K(G) channel, which contains Kir3.2c, is localized in the postsynaptic density (PSD) of various neurones including dopaminergic neurones in substantia nigra. Its localization at PSD may be controlled by PDZ domain-containing anchoring proteins. The K(G) channel in thyrotrophs is localized exclusively on secretary vesicles, which upon stimulation are rapidly inserted into the plasma membrane and causes hyperpolarization of the cell. This mechanism indicates a novel negative feedback regulation of exocytosis. In conclusion, K(G) channels are under the control of a variety of signalling molecules which regulate channel activity, subcellular localization and thus their physiological roles in myocytes, neurones and endocrine cells.

Regulator of G protein signaling Z1 (RGSZ1) interacts with Galpha i subunits and regulates Galpha i-mediated cell signaling

Regulator of G protein signaling (RGS) proteins constitute a family of over 20 proteins that negatively regulate heterotrimeric G protein-coupled receptor signaling pathways by enhancing endogenous GTPase activities of G protein alpha subunits. RGSZ1, one of the RGS proteins specifically localized to the brain, has been cloned previously and described as a selective GTPase accelerating protein for Galpha(z) subunit. Here, we employed several methods to provide new evidence that RGSZ1 interacts not only with Galpha(z,) but also with Galpha(i), as supported by in vitro binding assays and functional studies. Using glutathione S-transferase fusion protein pull-down assays, glutathione S-transferase-RGSZ1 protein was shown to bind (35)S-labeled Galpha(i1) protein in an AlF(4)(-)dependent manner. The interaction between RGSZ1 and Galpha(i) was confirmed further by co-immunoprecipitation studies and yeast two-hybrid experiments using a quantitative luciferase reporter gene. Extending these observations to functional studies, RGSZ1 accelerated endogenous GTPase activity of Galpha(i1) in single-turnover GTPase assays. Human RGSZ1 functionally regulated GPA1 (a yeast Galpha(i)-like protein)-mediated yeast pheromone response when expressed in a SST2 (yeast RGS protein) knockout strain. In PC12 cells, transfected RGSZ1 blocked mitogen-activated protein kinase activity induced by UK14304, an alpha(2)-adrenergic receptor agonist. Furthermore, RGSZ1 attenuated D2 dopamine receptor agonist-induced serum response element reporter gene activity in Chinese hamster ovary cells. In summary, these data suggest that RGSZ1 serves as a GTPase accelerating protein for Galpha(i) and regulates Galpha(i)-mediated signaling, thus expanding the potential role of RGSZ1 in G protein-mediated cellular activities.

Streptozotocin-induced diabetes impairs G-protein linked signal transduction in vascular smooth muscle

The present studies were undertaken to examine if the impaired vascular function observed in diabetes is attributed to the altered levels of G-protein. Diabetes was induced in Sprague Dawley rats by a single intraperitoneal injection of streptozotocin (STZ) (60 mg/kg body wt) and after a period of 5 days, the aorta were used for adenylyl cyclase activity determination and protein quantification. A temporal relationship between the expression of Gialpha proteins and development of diabetes was also examined on day 1, 2, 3, 4 and 5 of injection of STZ. Blood glucose levels were significantly increased from day 1 in STZ-rats as compared to their counterpart control rats and reached to about 20 mM on 3rd day and 30 mM on 5th day. The expression of Gialpha-2 and Gialpha-3 proteins as determined by immunoblotting techniques was decreased by about 70 and 50% respectively in aorta from STZ rats compared to the control rats after 5 days of treatment, whereas 40% decrease in Gialpha-2 and Gialpha-3 was observed after 3rd day of STZ injection. On the other hand, the expression of Gsalpha was unaltered in STZ rats. In addition, the stimulatory effect of cholera toxin (CT) on GTP-mediated stimulation of adenylyl cyclase was not different in STZ as compared to the control group. However, the stimulatory effects of isoproterenol, glucagon, NaF and FSK on adenylyl cyclase activity were significantly enhanced in STZ rats as compared to control rats, whereas basal adenylyl cyclase activity was significantly lower in STZ-rats as compared to control rats. In addition, GTPgammaS inhibited FSK-stimulated adenylyl cyclase activity in concentration-dependent manner (receptor-independent functions of Gialpha) in control rats which was completely attenuated in STZ-rats. In addition, receptor-mediated inhibitions of adenylyl cyclase by angiotensin II, oxotremorine, atrial natriuretic peptide (ANP99-126) and C-ANP4-23 were also attenuated (receptor-dependent functions of Gialpha) in STZ-rats. These results indicate that aorta from diabetic rats exhibit decreased levels of cAMP and decreased expression of Gialpha. The decreased expression of Gialpha may be responsible for the altered responsiveness of adenylyl cyclase to hormonal stimulation and inhibition in STZ-rats. It may thus be suggested that the impaired adenylyl cyclase-Gialpha protein signaling may be one of the possible mechanisms responsible for the impaired vascular functions in diabetes.

Alterations in g-protein-linked signal transduction in vascular smooth muscle in diabetes

The present studies were undertaken to determine the levels of stimulatory and inhibitory guanine nucleotide regulatory proteins (Gs and Gi respectively) and their relationship with adenylyl cyclase activity in aorta from 5-day streptozotocin-induced diabetic (STZ) rats. The levels of Gi alpha-2 as determined by immunoblotting techniques using AS/7 antibody were significantly decreased by about 60% in STZ as compared to control rats, whereas the levels of Gs alpha were not altered. In addition, the stimulatory effect of cholera toxin (CT) on GTP-sensitive adenylyl cyclase was not different in STZ as compared to control rats. On the other hand, the stimulatory effects of GTPgammaS, isoproterenol, glucagon, forskolin (FSK) and sodium fluoride on adenylyl cyclase were enhanced in STZ-rats. Furthermore, GTPgammaS inhibited FSK-stimulated adenylyl cyclase activity in a concentration-dependent manner (receptor independent functions of Gi) in control rats which was almost completely abolished in STZ rats. In addition, receptor-mediated inhibition of adenylyl cyclase by angiotensin II (AII), oxotremorine and atrial natriuretic peptide (ANP) was attenuated in STZ rats. These results suggest that the decreased expression of Gi alpha, but not of Gs alpha, may be responsible for the observed altered responsiveness of adenylyl cyclase to hormonal stimulation and inhibition in STZ-rats. It may thus be suggested that the decreased Gi activity may be one of the possible mechanisms responsible for the impaired vascular functions in diabetes.

Inactivation of enhanced expression of G(i) proteins by pertussis toxin attenuates the development of high blood pressure in spontaneously hypertensive rats

We have previously shown that the enhanced expression of G(i) proteins in spontaneously hypertensive rats (SHR) that precedes the development of high blood pressure may be one of the contributing factors in the pathogenesis of hypertension. In the present study, we demonstrate that the inactivation of G(i) proteins by intraperitoneal injection of pertussis toxin (PT, 1.5 micro g/100 g body wt) into 2-week-old prehypertensive SHR prevented the development of hypertension up to 4 weeks and that, thereafter, it started to increase and reached the same level found in untreated SHR after 6 weeks. A second injection of PT after 4 weeks delayed the increase in blood pressure for another week. The PT-induced decrease in blood pressure in 6-week-old SHR was associated with a decreased level of G(i)alpha-2 and G(i)alpha-3 proteins in the heart, as determined by in vitro ADP ribosylation and immunoblotting. The decreased level of G(i) proteins was reflected in decreased G(i) functions. Furthermore, an augmentation of blood pressure to the same level in PT-treated SHR as found in untreated SHR was associated with enhanced expression and function of G(i). These results indicate that the inactivation of G(i) proteins by PT treatment in prehypertensive SHR attenuates the development of hypertension and suggest that the enhanced levels of G(i) proteins that result in the decreased levels of cAMP and associated impaired cellular functions may be contributing factors in the pathogenesis of hypertension in SHR.

Muscarinic receptors in the mammalian heart

In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.

Cloned delta-opioid receptors in GH(3) cells inhibit spontaneous Ca(2+) oscillations and prolactin release through K(IR) channel activation

Opioid receptors can couple to K(+) and Ca(2+) channels, adenylyl cyclase, and phosphatidyl inositol turnover. Any of these actions may be important in the regulation of neurotransmitter and hormone release from excitable cells. GH(3) cells exhibit spontaneous oscillations of intracellular Ca(2+) concentration ([Ca(2+)](i)) and prolactin release. Activation of cloned delta-opioid receptors stably expressed in GH(3) cells inhibits both spontaneous Ca(2+) signaling and basal prolactin release. The objective of this study was to examine a possible role for K(+) channels in these processes using the patch-clamp technique, fluorescence imaging, and a sensitive ELISA for prolactin. The selective delta receptor agonist [D-Pen(2), D-Pen(2)]enkephalin (DPDPE) inhibited [Ca(2+)](i) oscillations in GH(3) cells expressing both mu and delta receptors (GH(3)MORDOR cells) but had no effect on control GH(3) cells or cells expressing mu receptors alone (GH(3)MOR cells). The inhibition of [Ca(2+)](i) oscillations by DPDPE was unaffected by thapsigargin pretreatment, suggesting that this effect is independent of inositol 1,4,5-triphosphate-sensitive Ca(2+) stores. DPDPE caused a concentration-dependent inhibition of prolactin release from GH(3)MORDOR cells with an IC(50) of 4 nM. DPDPE increased inward K(+) current recorded from GH(3)MORDOR cells but had no significant effect on K(+) currents recorded from control GH(3) cells or GH(3)MOR cells. The mu receptor agonist morphine also had no effect on currents recorded from control cells but activated inward K(+) currents recorded from GH(3)MOR and GH(3)MORDOR cells. Somatostatin activated inward currents recorded from all three cell lines. The DPDPE-sensitive K(+) current was inwardly rectifying and was inhibited by Ba(2+) but not TEA. DPDPE had no effect on delayed rectifier-, Ca(2+)-, and voltage-activated or A-type K(+) currents, recorded from GH(3)MORDOR cells. Ba(2+) attenuated the inhibition of [Ca(2+)](i) and prolactin release by DPDPE, whereas TEA had no effect, consistent with an involvement of K(IR) channels in these actions of the opioid.

Heterotrimeric G proteins in heart disease

Guanine nucleotide binding proteins (G proteins) are largely grouped into three classes: heterotrimeric G proteins, ras-like or small molecular weight GTP binding proteins, and others like Gh. In the heart G proteins transduce signals from a variety of membrane receptors to generate diverse effects on contractility, heart rate, and myocyte growth. This central position of G proteins forming a switchboard between extracellular signals and intracellular effectors makes them candidates possibly involved in the pathogenesis of cardiac hypertrophy, heart failure, and arrhythmia. This review focuses primarily on discoveries of heterotrimeric G protein alterations in heart diseases that help us to understand the pathogenesis and pathophysiology. We also discuss the underlying molecular mechanisms of heterotrimeric G protein signalling.Key words: G proteins, signal transduction, adrenergic system, heart failure, hypertrophy.

Immunoprecipitation of high-affinity, guanine nucleotide-sensitive, solubilized mu-opioid receptors from rat brain: coimmunoprecipitation of the G proteins G(alpha o), G(alpha i1), and G(alpha i3)

Antibodies directed against the C-terminal and the N-terminal regions of the mu-opioid receptor were generated to identify the G proteins that coimmunoprecipitate with the mu receptor. Two fusion proteins were constructed: One contained the 50 C-terminal amino acids of the mu receptor, and the other contained 61 amino acids near the N terminus of the receptor. Antisera directed against both fusion proteins were capable of immunoprecipitating approximately 70% of solubilized rat brain mu receptors as determined by [3H][D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin ([3H]DAMGO) saturation binding. The material immunoprecipitated with both of the antisera was recognized as a broad band with a molecular mass between 60 and 75 kDa when screened in a western blot. Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) had an EC50 of 0.4 nM in diminishing [3H]DAMGO binding to the immunoprecipitated pellet. The ratio of G proteins to mu receptors in the immunoprecipitated material was 1:1. When the material immunoprecipitated with affinity-purified antibody was screened for the presence of G protein a subunits, it was determined that G(alpha)o, G(alpha)i1, G(alpha)i3, and to a lesser extent G(alpha)i2, but not G(alpha)s or G(alpha)q11, were coimmunoprecipitated with the mu receptor. Inclusion of GTPgammaS during the immunoprecipitation process abolished the coimmunoprecipitation of G proteins.

M3 muscarinic receptor activation of a delayed rectifier potassium current in canine atrial myocytes

Growing body of evidence indicates that the functional responses of cells to muscarinic acetylcholine receptors (mAChRs) are mediated by multiple receptor subtypes. It is commonly thought that the M2 receptor is the only functional mAChR subtype in the heart and little data regarding the potential roles of other subtypes in cardiac tissues has been reported. In the present study, we provide functional evidence for the presence and physiological function of an M3 receptor in canine atrial myocytes. Using whole-cell patch-clamp techniques, we consistently found that pilocarpine, an mAChR agonist, induced a K+ current similar to but distinct from the classical delayed rectifier K+ current. Same observations were obtained when choline or tetramethylammonium (TMA) was applied to the bath. The currents were abolished by 1 microM atropine. Antagonists selective to M1 (pirenzepine, 100 nM), M2 (methoctramine 100 nM), or M4 (tropicamide 200 nM) receptors failed to alter the currents. Conversely, three different M3-selective inhibitors, p-F-HHSiD (20-200 nM), 4-DAMP methiodide (2-10 nM) and 4-DAMP mustard (4-20 nM), all produced concentration-dependent suppression of the currents. A cDNA fragment representing the M3 receptor was isolated from dog atrial RNA and the mRNA level of this construct was 0.7 +/- 0.1 pg/microg total RNA, as quantified by the competitive RT-PCR methods. Our data strongly suggested that an M3 receptor exists and is coupled to a K+ channel in the heart.

Differential regulation of the cloned kappa and mu opioid receptors

To directly compare the regulation of the cloned kappa and mu opioid receptor, we expressed them in the same cells, the mouse anterior pituitary cell line AtT-20. The coupling of an endogenous somatostatin receptor to adenylyl cyclase and an inward rectifier K+ current has been well characterized in these cells, enabling us to do parallel studies comparing the regulation of both the kappa and the mu receptor to this somatostatin receptor. We show that the kappa receptor readily uncoupled from the K+ current and from adenylyl cyclase after a 1 h pretreatment with agonist, as indicated by the loss in the ability of the agonist to induce a functional response. The desensitization of the kappa receptor was homologous, as the ability of somatostatin to mediate inhibition of adenylyl cyclase or potentiation of the K+ current was not altered by kappa receptor desensitization. The mu receptor uncoupled from the K+ current but not adenylyl cyclase after a 1 h pretreatment with agonist. Somatostatin was no longer able to potentiate the K+ current after mu receptor desensitization, thus this desensitization was heterologous. Interestingly, pretreatment with a somatostatin agonist caused uncoupling of the mu receptor but not the kappa receptor from the K+ current. These results show that in the same cell line, after a 1 h pretreatment with agonist, the kappa receptor displays homologous regulation, whereas the mu receptor undergoes only a heterologous form of desensitization. mu receptor desensitization may lead to the alterations of diverse downstream events, whereas kappa receptor regulation apparently occurs at the level of the receptor itself. Broad alterations of non-opioid systems by the mu receptor could be relevant to the addictive properties of mu agonists. Comparison of kappa and mu receptor regulation may help define the properties of the mu receptor which are important in the development of addiction, tolerance, and withdrawal to opioid drugs. These are the first studies to directly compare the coupling of the kappa and mu receptors to two different effectors in the same mammalian expression system.

Altered expression of inhibitory guanine nucleotide regulatory proteins (Gi-proteins) in experimental hepatocellular carcinoma

Guanine nucleotide regulatory proteins (G-proteins) play an important role in the onset and progression of malignancy. We hypothesized that alterations in inhibitory G-protein (Gi) expression and/or function may contribute to cellular invasion and formation of hepatocellular carcinoma (HCC). H4IIE hepatoma cells were inoculated directly into the liver parenchyma of ACI strain rats, and membranes were prepared from HCC livers and adjacent nonneoplastic livers 12 days following the initial inoculation. Expression of inhibitory Gialpha proteins was determined by Western blot analysis and changes in the functional activity of these proteins confirmed by pertussis toxin catalyzed ADP ribosylation and adenylyl cyclase activity. Inhibitory Gialpha1, Gialpha1/2, and Gialpha3 protein expression was significantly elevated in HCC when compared to adjacent nonneoplastic liver and sham-operated hepatic tissue. Pertussis toxin catalyzed ADP ribosylation of Gialpha substrates was significantly enhanced in HCC concomitant with increased basal and stimulated adenylyl cyclase activity following uncoupling of Gi-proteins with manganese ions. The role of Gi-proteins in cellular proliferation was confirmed using cultured H4IIE cells and normal hepatocytes. In quiescent H4IIE cells, mastoparan (Gialpha activator) increased [3H] thymidine incorporation and cell growth in a dose-dependent manner, whereas both pertussis toxin (a Gi-protein inhibitor) and 8-bromo-cAMP inhibited mitogenesis. In contrast, in isolated cultured hepatocytes, mastoparan inhibited [3H] thymidine incorporation, while pertussis toxin and 8-bromo-cAMP were mitogenic. We conclude that HCC is associated with marked changes in Gialpha-protein expression in vivo and in vitro, direct activation of which leads to increased mitogenesis in H4IIE cells in vitro.

Specific G protein activation and mu-opioid receptor internalization caused by morphine, DAMGO and endomorphin I

Previous studies have shown that the agonist [D-Ala2, N-Me-Phe4, Gly-ol5]enkephalin (DAMGO) but not morphine induces mu-opioid receptor internalization [Arden, J.R., Segredo, V., Wang, Z., Lameh, J., Sadee, W., 1995. J. Neurochem. 65, 1636-1645]. In the present study we investigated the relationship between internalization of the mu-opioid receptor and the specific G proteins activated following treatment with morphine, DAMGO and endomorphin I (Tyr-Pro-Trp-Phe-NH2) (a putative endogenous mu-opioid receptor agonist) in human embryonic kidney (HEK) cells. Endomorphin I and DAMGO, but not morphine, caused mu-opioid receptor internalization. Morphine, DAMGO and endomorphin I each activated Gi1 alpha/Gi2 alpha, Go alpha and Gi3 alpha to a similar extent, but not Gq alpha/G11 alpha or Gs alpha in HEK membranes. Therefore, the three ligands tested differed in their ability to internalize mu-opioid receptors even though they were similar in activating individual G proteins.

Adenovirus-mediated expression of 5-HT1B receptors in cardiac ventricle myocytes; coupling to inwardly rectifying K+ channels

The 5-HT1B receptor is expressed on nerve terminals where it inhibits neurotransmitter release. When expressed ectopically in fibroblasts, the 5-HT1B receptor inhibits adenylyl cyclase. However, in the central nervous system, the effect of this receptor on neurotransmitter release appears to be cAMP-independent. We therefore investigated alternative effector systems that might be activated by the 5-HT1B receptor. We constructed a recombinant adenovirus that allows expression of high levels of the 5-HT1B receptor in a variety of cells. We chose cardiac ventricle myocytes because they express a muscarinic-gated, inwardly rectifying K+ channel (i[KACh]). In infected ventricle cells, both 5-HT and the muscarinic receptor agonist, carbachol, elicited a similar inwardly rectifying K+ current. The currents elicited by these agonists were pertussis-toxin sensitive and were not additive. These results suggest a common signal transduction pathway for 5-HT1B and muscarinic receptors in ventricle cells.

Insulin resistance in adipocytes after downregulation of Gi subtypes

To determine whether downregulation of Gi proteins is associated with insulin resistance, we incubated isolated adipocytes with N6-(2-phenylisopropyl)adenosine (PIA; an A1-adenosine receptor agonist; 300 nM), prostaglandin E1 (PGE1; 3 microM), or nicotinic acid (1 mM) for 4 days in primary culture. The cells were washed, and the rate of glucose transport (2-deoxy-[3H]glucose uptake) was measured after incubation with various concentrations of insulin for 45 min. Both PIA and PGE1 (which downregulate Gi) decreased the maximal responsiveness of the cells to insulin by approximately 30% and caused a rightward shift in the dose-response curve. By contrast, nicotinic acid (which does not downregulate Gi) did not alter the insulin sensitivity of the cells. Prolonged treatment of adipocytes with either PIA or PGE1 (but not nicotinic acid) rendered the cells completely resistant to the antilipolytic effect of insulin. The ability of insulin to stimulate autophosphorylation of the beta-subunit of the insulin receptor was decreased by approximately 30% in PIA-treated cells, and the dose-response curve was shifted to the right. Similarly, the ability of the receptor to phosphorylate poly(Glu4-Tyr1) was decreased by approximately 35%. This decrease in tyrosine kinase activity of the receptor may account for the decrease in insulin sensitivity of glucose transport but cannot account for the complete loss of antilipolysis. The findings suggest both a direct and indirect involvement of Gi proteins in insulin action.

The heterotrimeric G protein G alpha i2 mediates lysophosphatidic acid-stimulated induction of the c-fos gene in mouse fibroblasts

Lysophosphatidic acid (LPA) utilizes a heterotrimeric guanine nucleotide regulatory (G) protein-coupled receptor to activate the mitogen-activated protein kinase pathway and induce mitogenesis in fibroblasts and other cells. A single cell assay system was used to examine the functional interaction of the LPA receptor with G proteins in intact mouse fibroblasts, by measuring LPA-stimulated induction of the immediate-early gene, c-fos, as read out by a stably expressed fos-lacZ reporter gene. Pretreatment of these cells with pertussis toxin at 100 ng/ml almost completely abolished LPA-stimulated c-fos induction. Western blotting revealed that two pertussis toxin (PTX)-sensitive G proteins, G alpha i2 and G alpha i3, were present in membranes prepared from these cells, and Northern blotting confirmed the absence of message for other PTX-sensitive subunits. Microinjection of an alpha il/alpha i2-specific antibody into living cells decreased LPA-stimulated induction of c-fos by 60%, whereas introduction of antibodies to either alpha i3 or alpha 16, a subtype not present in these cells but used as a control, decreased LPA-stimulated c-fos induction by only 19%. In contrast, the alpha i1/alpha i2-specific antibody had no effect on insulin-induced c-fos expression, which is thought to utilize a G protein-independent mechanism of signaling. In addition, cellular expression of an epitope-tagged PTX-resistant mutant of G alpha i2, but not PTX-resistant G alpha i3, restored LPA-stimulated c-fos induction in cells in which endogenous G protein a subunits were uncoupled from the receptor by pretreatment with PTX. Together, these results provide conclusive in vivo evidence that G alpha i2 is the PTX-sensitive G protein a subunit which mediates LPA-stimulated c-fos induction and perhaps mitogenesis in these cells.

Evidence that a novel somatostatin receptor couples to an inward rectifier potassium current in AtT-20 cells

The recent cloning of five somatostatin receptors has made it possible to begin screening for selective ligands in order to begin characterization of these receptor subtypes expressed endogenously. We have recently reported the characterization of ligands selective for SSTR2 and SSTR5 [Raynor K. et al. (1993) Molec. Pharmac. 43, 838-844; 44, 385-392]. Both of these somatostatin receptor subtypes are endogenously expressed in the mouse pituitary cell line AtT-20 [O'Carroll A.-M. et al. (1992) Molec. Pharmac. 42, 939-946; Patel Y. C. et al. (1994) J. biol. Chem. 269, 1506-1509; Tallent M. et al. (1996) Neuroscience 71, 1073-1081]. Using these selective ligands, as well as other somatostatin analogs, we have characterized the somatostatin receptor which couples to the inward rectifier K+ current in AtT-20 cells. This receptor is sensitive to hexapeptide analogs of somatostatin, but insensitive to octapeptide analogs. This pharmacological profile is distinct from any of the cloned somatostatin receptors and therefore may represent a novel receptor. Somatostatin has been shown to potentiate an inward rectifying K+ channel in many different types of neuronal and non-neuronal cells. The activation of this current is thought to be an important mechanism by which somatostatin inhibits neuronal firing and decreases neurotransmitter and hormone release [Mihara S. et al. (1987) J. Physiol. 390, 335-355]. Therefore, the novel somatostatin receptor coupling to the inward rectifier in AtT-20 cells may be important in somatostatin's role in regulating neurotransmission and hormone release.

Cloning of a G protein-activated inwardly rectifying potassium channel from human cerebellum

Based on sequence homology with the rat atrial G protein-coupled muscarinic potassium channel (GIRK1 or KGA1/KGB1), a human cDNA encoding a G protein-activated inwardly rectifying K+ channel (HGIRK1) was isolated. The cDNA encodes a protein of 501 amino acids and shares 99% identity to rat GIRK1 in its total amino acid sequence. Southern blot analysis of genomic DNA indicates a high degree of conservation among various species. In the human population a useful NlaIII restriction fragment length polymorphism was found in the coding sequence of HGIRK1. Co-expression of HGIRK1 and the 5-HT1A receptor in Xenopus oocytes resulted in opening of the channel upon treatment with serotonin. HGIRK1 currents showed strong inward rectification and could be blocked by extracellular Ba2+. Northern blot analysis shows that HGIRK1 expression in human is most abundant in the brain, while lower levels are round in kidney and heart.

Regulation of a sodium channel-associated G-protein by aldosterone

The action of aldosterone to increase apical membrane permeability in responsive epithelia is thought to be due to activation of sodium channels. This channel is regulated, in part, by G-proteins, but it is not known if this mechanism is regulated by aldosterone. We report that aldosterone stimulates the expression of the 41-kDa alphai3 subunit of the heterotrimeric GTP-binding proteins in A-6 cells. Both mRNA and the total amount of this protein are increased by aldosterone. The G-protein is palmitoylated in response to the steroid, and the newly synthesized subunit is found to co-localize with the sodium channel. Aldosterone stimulation of sodium transport is significantly inhibited by inhibition of palmitoylation. These results suggest that aldosterone regulates sodium channel activity in epithelia through stimulation of the expression and post-translational targeting of a channel regulatory G-protein subunit.

Gi2 alpha protein deficiency: a model of inflammatory bowel disease

Mice deficient for the G protein subunit Gi2 alpha were obtained by gene targeting. They displayed a growth retardation that was apparent at 6 weeks of age. They subsequently developed diffuse colitis with clinical and histopathological features closely resembling those of ulcerative colitis in humans. Seven of 20 Gi2 alpha-deficient mice with colitis also developed adenocarcinomas of the colon. Gi2 alpha-deficient thymocytes displayed two- to fourfold increases in mature CD4+8- and CD4-8+ phenotypes, an approximately threefold increase in high-intensity CD3 staining and enhanced proliferative responses to T-cell receptor stimuli. Stimulation of Gi 2 alpha-deficient peripheral T cells induced a hyperresponsive profile of interleukin-2, tumour necrosis factor, and interferon-gamma production, which may reflect a heightened response of primed cells or a defective negative regulation. We suggest that Gi 2 alpha-deficient mice may represent a useful animal model for dissecting the pathomechanisms of inflammatory bowel disease and also for the development of novel therapeutic strategies.