Programmable messengers: a new theory of hormone action

Organ-specific and light-induced expression of plant genes

Light plays a pivotal role in the development of plants. The photoregulation of plant genes involves recognition of light quality and quantity by phytochrome and other light receptors. Two gene families, rbcS and Cab, which code for abundant proteins active in photosynthesis, the small subunit of ribulose bisphosphate carboxylase and the chlorophyll a/b binding protein, show a 20-to 50-fold increase in transcript abundance in the light. Analyses in calli and transgenic plants of deletions of the rbcS gene and of chimeric constructions has allowed localization of two regions involved in light-induced transcription. One element is confined to a 33-base pair region surrounding the TATA box. In addition, an enhancer-like element contained within a 240-base pair fragment can confer phytochrome-induced transcription and organ specificity on nonregulated promoters.

Caveolin-1 and lipid microdomains regulate Gs trafficking and attenuate Gs/adenylyl cyclase signaling

Lipid rafts and caveolae are specialized membrane microdomains implicated in regulating G protein-coupled receptor signaling cascades. Previous studies have suggested that rafts/caveolae may regulate beta-adrenergic receptor/Galpha(s) signaling, but underlying molecular mechanisms are largely undefined. Using a simplified model system in C6 glioma cells, this study disrupts rafts/caveolae using both pharmacological and genetic approaches to test whether caveolin-1 and lipid microdomains regulate G(s) trafficking and signaling. Lipid rafts/caveolae were disrupted in C6 cells by either short-term cholesterol chelation using methyl-beta-cyclodextrin or by stable knockdown of caveolin-1 and -2 by RNA interference. In imaging studies examining Galpha(s)-GFP during signaling, stimulation with the betaAR agonist isoproterenol resulted in internalization of Galpha(s)-GFP; however, this trafficking was blocked by methyl-beta-cyclodextrin or by caveolin knockdown. Caveolin knockdown significantly decreased Galpha(s) localization in detergent insoluble lipid raft/caveolae membrane fractions, suggesting that caveolin localizes a portion of Galpha(s) to these membrane microdomains. Methyl-beta-cyclodextrin or caveolin knockdown significantly increased isoproterenol or thyrotropin-stimulated cAMP accumulation. Furthermore, forskolin- and aluminum tetrafluoride-stimulated adenylyl cyclase activity was significantly increased by caveolin knockdown in cells or in brain membranes obtained from caveolin-1 knockout mice, indicating that caveolin attenuates signaling at the level of Galpha(s)/adenylyl cyclase and distal to GPCRs. Taken together, these results demonstrate that caveolin-1 and lipid microdomains exert a major effect on Galpha(s) trafficking and signaling. It is suggested that lipid rafts/caveolae are sites that remove Galpha(s) from membrane signaling cascades and caveolins might dampen globally Galpha(s)/adenylyl cyclase/cAMP signaling.

Inhibition of proteasome activity blocks the ability of TNF alpha to down-regulate G(i) proteins and stimulate lipolysis

Prolonged treatment of rat adipocytes with TNF alpha increases lipolysis through a mechanism mediated, in part, by down-regulation of inhibitory G proteins (G(i)). Separately, down-regulation of G(i) by prolonged treatment with an A(1)-adenosine receptor agonist, N(6)-phenylisopropyl adenosine (PIA) increases lipolysis. To investigate the role of proteolysis in TNF alpha and PIA-mediated G(i) down-regulation and stimulation of lipolysis, we used the protease inhibitors lactacystin (proteasome inhibitor) and calpeptin (calpain inhibitor). Rat adipocytes were preincubated for 1 h with lactacystin (10 microM) or calpeptin (50 microM), before 30-h treatment with either TNF alpha (50 ng/ml) or PIA (300 nM). We then measured lipolysis (glycerol release), abundance of alpha-subunits of G(i)1 and G(i)2 in plasma membranes (Western blotting) and protease activities (in specific fluorogenic assays). TNF alpha and PIA stimulated lipolysis approximately 2-fold and caused G(i) down-regulation. Although neither lactacystin nor calpeptin affected basal lipolysis, lactacystin completely inhibited both TNF alpha and PIA-stimulated lipolysis (the 50% inhibitory concentration was approximately 2 microM), whereas calpeptin had no effect. Similarly, lactacystin but not calpeptin blocked both PIA and TNF alpha-induced G(i) down-regulation. These findings provide further evidence that the chronic lipolytic effect of TNF alpha and PIA is secondary to G(i) down-regulation and suggest that the mechanism involves proteolytic degradation mediated through the proteasome pathway.

Agonist-induced translocation of Gq/11alpha immunoreactivity directly from plasma membrane in MDCK cells

Both Gsalpha and Gqalpha are palmitoylated and both can move from a crude membrane fraction to a soluble fraction in response to stimulation with agonists. This response may be mediated through depalmitoylation. Previous studies have not demonstrated that endogenous guanine nucleotide-binding regulatory protein (G protein) alpha-subunits are released directly from the plasma membrane. We have examined the effect of agonist stimulation on the location of Gq/11alpha immunoreactivity in Madin-Darby canine kidney (MDCK) cells. Bradykinin (BK; 0.1 microM) caused Gq/11alpha, but not Gialpha, to rapidly translocate from purified plasma membranes to the supernatant. AlF and GTP also caused translocation of Gq/11alpha immunoreactivity from purified plasma membranes. BK caused translocation of Gq/11alpha immunoreactivity in intact cells from the basal and lateral plasma membranes to an intracellular compartment as assessed by confocal microscopy. Thus Gq/11alpha is released directly from the plasma membrane to an intracellular location in response to activation by an agonist and direct activation of G proteins. G protein translocation may be a mechanism for desensitization or for signaling specificity.

The thyrotropin-releasing hormone-receptor complex and G11alpha are both internalised into clathrin-coated vesicles

It has been proposed that, after agonist binding, the thyrotropin-releasing hormone receptor (TRHR) becomes internalised associated with Gq, as part of a TRH-TRHR-Gq ternary complex [13]. We tested this hypothesis directly by examining the intracellular distribution of the TRHR and Gq/11 after agonist binding. The localisation of the TRH-TRHR complex and Gq/11alpha was studied by the biochemical isolation of clathrin-coated vesicles (CCVs). The internalised TRH-TRHR complex was localised in CCVs. The CCVs, which had internalised [3H]MeTRH, contained 4-fold higher levels of radiolabelled ligand than did CCVs from cells incubated with [3H]MeTRH at 4 degrees C. Like the receptor-ligand (RL) complex, G11alpha also translocated to these endocytic vesicles. For example, CCVs from cells with internalised TRH-TRHR complexes contained G11alpha, whereas CCVs from cells without internalised RL complexes lacked G11alpha. We conclude that, after agonist-induced TRHR-G11alpha coupling, both the TRH-TRHR complex and G11alpha are internalised in CCVs.

Order of application determines the interaction between phorbol esters and GTP-gamma-S in dorsal raphe neurons: evidence that the effect of 5-HT is modified upstream of the G protein Ca channel interaction

Phorbol esters activating protein kinase C (PKC) partially uncouple the inhibitory effect of serotonin (5-HT) from serotonergic neuron Ca2+ current. Presently the site of action of PKC is not known and may be the receptor, G protein, or ion channel. We recorded Ca2+ current from acutely isolated neurons with the use of the patch-clamp technique to study the site of action of PKC. Activation of the G protein with internal guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) occluded the response to 5-HT, but unexpectedly this effect was not reversed by the addition of the phorbol ester phorbol 12-myristate 13-acetate (PMA) despite the voltage-dependent reversal of the effect of GTP-gamma-S by long depolarizing steps to +80 mV. PMA was, however, able to partially reverse 5-HT-induced inhibition of Ca2+ current. The rate of reinhibition of the Ca2+ current (related to the concentration of activated G proteins) by GTP-gamma-S after the addition of PMA at -50 mV was identical to the rate when only GTP-gamma-S was present. By contrast, when cells were exposed first to PMA, and then GTP-gamma-S was perfused into the cell, GTP-gamma-S lost about half of its ability to activate the G protein. The rate of reinhibition of the Ca2+ current by internal GTP-gamma-S was also reduced in cells pretreated with PMA. The original result in which PMA did not reverse the action of GTP-gamma-S suggested that the channel was not the functional site of action of PMA, nor was the site on the G protein that binds to the channel, but it did not rule out the receptor. When the receptor was bypassed, after prior PKC activation, it was found that direct activation of the G protein by a nonhydrolyzable analogue of GTP was reduced; taken as a whole, this indicates that in dorsal raphe, and perhaps other neurons, the site of the critical phosphorylation may be on the G protein and possibly at the GTP binding site.

Reversible palmitoylation of signaling proteins

Palmitoylation is unique among lipid modifications of proteins in that it is reversible and regulable. Recent advances in the study of palmitoylation include the following: the correlation of this modification with the localization of a signaling protein to specific membrane subdomains; the demonstration of a specific protein-protein interaction that is promoted by palmitoylation; and the identification, characterization, and purification of enzymes catalyzing this modification.

Mechanisms of desensitization of vasodilatation induced by platelet-activating factor in hypertensive rats

We found that vasodilator effects of platelet-activating factor (PAF) on the mesenteric arterial bed of the rat were significantly attenuated in spontaneously hypertensive rats (SHR) and renal hypertensive rats (RHR). Perfusion of the mesentery with acetylcholine and PAF caused endothelium-dependent vasodilatation accompanied by an increase in cyclic GMP levels in the mesentery from normotensive Wistar Kyoto rats (WKY). Acetylcholine caused a significant increase in cyclic GMP levels in the effluent in both SHR and RHR, whereas PAF could not increase cyclic GMP levels in SHR and slightly increased cyclic GMP in RHR. Incubating the mesentery with PAF markedly inhibited the vasodilatation induced by PAF, but not acetylcholine or sodium nitroprusside. The cyclic GMP accumulation in the effluent was impaired in the mesenteric arterial bed pretreated with PAF and in that obtained from rats given islet-activating protein (IAP). The PAF-induced vasodilatation was completely reversed by the PAF receptor antagonist, CV-6209 (2-[N-acetyl-N-(2-methyl-3-octadecylcarbamoyl-oxypropoxycarbony l) aminomethyl]-1-ethylpyridinium chloride). These results suggest that (1) attenuated vasodilator effects of PAF and decreased cyclic GMP levels in the mesentery from SHR and RHR are due to desensitization but not to impairment of the endothelium; (2) GTP-binding protein, which is IAP-sensitive, may be involved in PAF-induced vasodilatation and cyclic GMP accumulation; (3) desensitization of the mesentery to PAF in SHR and RHR may be due to PAF receptor and GTP-binding protein uncoupling.

Endothelial Gi protein expression is markedly low in human coronary microvessels

Gi protein functionally mediates endothelium-dependent relaxations in large epicardial coronary arteries but not in small coronary arteries, which suggests a different involvement of Gi protein in the endothelium-dependent relaxations between large and small coronary arteries. We previously showed that endothelial Gi protein is present in human epicardial coronary artery. In the present study, we examined the expression of endothelial Gi protein in human coronary microvessels. Immunohistochemical staining with a specific antibody against human Gi protein was performed in intramyocardial coronary microvessels and vasa vasorum from 34 autopsy cases. The immunoreactive levels of the endothelial Gi protein were semiquantitated into four grades (none, 0; slight, +1; moderate, +2; high, +3), and the mean value of the ratings of all endothelial cells was then used as an index of the endothelial Gi protein expression of the vessel. The immunoreactive levels of the endothelial Gi protein were extremely low in intramyocardial coronary microvessels and in vasa vasorum, irrespective of the age of the patients, the presence or absence of coronary risk factors, or the influence of medical treatments. These results may therefore explain in part why endothelium-dependent relaxations in coronary microvessels are not functionally mediated by Gi protein.

Adrenergic desensitization in left ventricle from streptozotocin diabetic swine

Patients with diabetes mellitus that exhibit cardiac pump failure display compromised stroke volume, ejection fraction, and slower rates of rise and fall of left ventricular (LV) dP/dt in the absence of ischemic injury. We hypothesized that diabetic cardiomyopathy may involve decrements in adrenergic sensitivity, with specific molecular alterations in the beta-adrenergic receptor (beta AR)- G protein- adenylyl cyclase (AC) signal transduction system. We assessed the effects of 3 months of streptozotocin-induced diabetes (125 mg/kg i.v.; DIAB, n = 10) on myocardial signal transduction in mini-pigs. DIAB were hyperglycemic compared to controls (CON, n = 10; 20.92 +/- 2.64 v 5.24 +/- 0.35 mM glucose), and had lower fasting insulin levels (6.46 +/- 0.97 v 13.68 +/- 3.91 microU/ml). Transmural LV free wall homogenates from DIAB exhibited similar beta AR density as CON, but decreased cAMP production (pmol cAMP/mg prot.min) using these pharmacological stimulators: 10 microM Isoproterenol plus 100 microM GTP (74 +/- 5 v 97 +/- 11); 100 microM Gpp(NH)p (116 +/- 7 v 161 +/- 17); 10 mM fluoride ion (266 +/- 16 v 324 +/- 25). No differences between DIAB and CON were observed when stimulated by 100 microM forskolin (440 +/- 20 v 429 +/- 33), suggesting no alterations in the catalytic subunit of AC. In DIAB, quantitative immunoblotting indicated slightly depressed levels of Gs (552 +/- 44 v 630 +/- 59 pmol/g ww; NS), but a significant redistribution of alpha s from the sarcolemma to the cytosol (32.7 +/- 0.82% v 25.9 +/- 1.7%). Significantly elevated levels of cardiac Gi were seen in DIAB homogenates compared to CON ventricles (2326 +/- 145 v 1522 +/- 181 pmol/g ww), with no alpha i subunit redistribution. We conclude that despite maintained beta AR density, receptor-dependent and G protein-dependent stimulation of AC is depressed so that streptozotocin-induced diabetic LV is affected by increased cardiac Gi, redistribution of Gs alpha to the cytosol, and an increase in the Gi/Gs ratio. These results help explain depressed catecholamine responsiveness and cardiac performance exhibited by diabetic patients.

Somatostatin induces release of the alpha subunits of pertussis toxin-sensitive G proteins in native membranes and in intact GH4C1 rat pituitary cells

Incubation of GH4C1 rat pituitary cell membranes with the poorly hydrolyzable GTP analogue, GTP gamma S, produces a decrease in the pertussis toxin-catalyzed ADP-ribosylation of 40-kDa protein in the membrane pellet and the release of an alpha-like substrate from the membrane into the supernatant fraction; these effects do not occur with the inactive GDP analogue, GDP beta S. The resolved supernatant fraction from GTP gamma S-stimulated membranes is significantly activated to pertussis toxin-catalyzed [32P]ADP-ribosylation by the addition of purified beta gamma complex. Immunoblot analysis identifies the released pertussis toxin substrate as alpha subunits of Gi2, Gi3, and G(o) in the resolved supernatant. The physiological agonist, somatostatin, also stimulates the release of Gi2 and G(o) alpha subunits but not Gi3 from GH4C1 cell membranes in the presence of a low concentration of GTP gamma S (20 nM). The effects of somatostatin are inhibited by pretreatment of GH4C1 cells with pertussis toxin. Furthermore, the addition of somatostatin to intact GH4C1 cells decreases the level of Gi2 alpha subunits in the crude membrane whereas immunoblot analysis of the 274,000 x g supernatant (cytosolic fraction) clearly shows the presence of Gi2 alpha subunits. These data indicate that pertussis toxin-sensitive G proteins in GH4C1 cells dissociate into alpha subunits and beta gamma complex with the release of the alpha subunits from the membranes upon somatostatin activation.

Involvement of protein kinase C in thrombin-induced translocation of Gi2 alpha from the membrane to the cytosol in mouse mastocytoma P-815 cells

Recently, we reported that in mouse mastocytoma P-815 cells the cytosol contains some factor(s) which promotes the release of GTP-activated Gi2 alpha from the membrane, and that thrombin induces the translocation of Gi2 alpha from the membrane to the cytosol (Takahashi, S., Negishi, M. and Ichikawa, A. (1991) J. Biol. Chem. 266, 5367-5370). Here we investigated the mechanism underlying the thrombin-induced translocation of Gi2 alpha in mastocytoma cells. Thrombin induced a rapid and transient increase in the intracellular Ca2+ concentration ([Ca2+]i) within 1 min, attenuated pertussis toxin-catalyzed ADP-ribosylation of Gi2 in the membrane, and caused the subsequent translocation of Gi2 alpha. Thrombin induced the translocation of protein kinase C from the cytosol to the membrane, and a protein kinase C inhibitor, staurosporine, completely inhibited the thrombin-induced translocation of Gi2 alpha. When cells were treated with thrombin, the ability of the cytosol to release Gi2 alpha from the membrane in the presence of GTP gamma S markedly increased. This stimulatory effect of thrombin on the ability of the cytosol was mimicked by 12-O-tetradecanoylphorbol 13-acetate (TPA), but not by the Ca2+ ionophore, ionomycin. The thrombin- and TPA-induced potentiation of the ability of the cytosol to release Gi2 alpha was completely abolished by staurosporine. Furthermore, phosphorylation of the cytosol by protein kinase C markedly potentiated the ability of the cytosol to release Gi2 alpha. These results together demonstrate that the thrombin-induced translocation of Gi2 alpha is due to enhancement of the ability of the cytosol to release Gi2 alpha via activation of protein kinase C.

Signal transduction by G proteins in cardiac tissues

The role of G proteins in mediating the responses of the heart to circulating catecholamines and to the influences of the autonomic nervous system is of special interest to cardiologists. It is evident that G proteins are essential links in the cascade of biochemical events that ensure when neurotransmitters and hormones interact with receptors on myocardial cells. It is likely [corrected] that dysfunction of G proteins plays a role in cardiovascular pathophysiology. With current methodologies, especially molecular biological and recombinant DNA techniques, and with transgenic animal models that can relate physiological function and specific gene dosage, some cardiovascular diseases may be traced to G protein-related defects.

A substantial proportion of cardiac Gs is not associated with the plasma membrane

The precise interactions between the subunits of Gs (alpha s, beta, gamma) and the plasma membrane remain to be established. If alpha s is associated loosely with the inner membrane, is labile during activation, or is always present to some extent in the cytoplasm, then it should fractionate to the supernatant of a high-speed centrifugation. We identified abundant alpha s (52-66% of total cellular) in the supernatant fraction of right atrial and left ventricular membrane preparations of porcine heart as shown by two distinct measures of alpha s (immunoblotting and ADP ribosylation by cholera toxin). However, functional assays utilizing reconstitution of cardiac alpha s with cyc- S49 membranes revealed that the supernatant fraction contained approximately 16% of total cellular alpha s activity. The alpha s present in the supernatant fraction did not result from contamination by sarcolemmal membrane fragments. We conclude that traditional methods for quantifying alpha s which utilize only detergent extracts from high-speed pellets do not account for a sizable proportion of total cellular alpha s, but that the majority of this population of cardiac alpha s may not be functional, at least with respect to adenylyl cyclase activation.

Evolving concepts of partial agonism

The exact mechanism of receptor activation at the molecular level are still not known, nor do we completely understand the precise factors that distinguish agonist- and partial agonist-induced activation. Nevertheless, recent years have brought forth an explosion of new information regarding beta-adrenergic receptor structure and ligand-induced activation. Partial agonists are likely intermediate in their ability to interact with crucial serine residues (Ser204 and Ser207) on the beta-adrenergic receptor; these interactions allow either incomplete stimulation of the entire receptor population, or full stimulation of only a portion of the entire receptor population. From the work presented by Tota and Schimerlik for the muscarinic cholinergic receptor (another G-protein coupled receptor), it is likely that partial agonists induce or stabilize receptor conformations that have a lower affinity for their G protein compared to receptors stimulated by a full agonist. Molecular cloning of beta-adrenergic receptors and analyses of mutated and chimeric receptors expressed in transfected systems have indicated that domains of the receptor that bind agonists may be different from those with which antagonists interact. Thus, the ability of a partial agonist to interact with these two different domains may be a determinant of efficacy. Agonists alter the sulfhydryl redox status of the beta-adrenergic receptors in the presence of Gs. Disulfide rearrangement has been postulated to provide a structural constraint which biases G-protein-linked receptors in the "ground state" and may be important for stabilizing the active state of the receptor and holding the agonist/receptor/Gs ternary complex in the high-affinity state. Partial agonists induce this state less efficaciously or are less capable of holding the receptor in the active conformation to allow disulfide exchange to take place. The extent of receptor stimulation may dictate which G proteins are activated by a particular receptor, and thus which cellular effectors are stimulated. Alternatively, the level of activation of a receptor may translate into varying states of activation of a particular G protein (stabilized in part by disulfide bonds). Techniques such as fluorescence energy transfer in reconstitution systems or nuclear magnetic resonance spectroscopy should prove useful in distinguishing among these possible mechanisms. Ultimately, as a long-term goal, X-ray crystallography of unoccupied receptors and receptors liganded by partial or full agonists may provide definitive insights. Although definitive answers are not yet possible, the rapid progress in understanding aspects of receptor structure allows a reformulation of ideas regarding the molecular basis of efficacy and partial agonism.(ABSTRACT TRUNCATED AT 400 WORDS)

G protein Gs alpha (GNAS 1), the probable candidate gene for Albright hereditary osteodystrophy, is assigned to human chromosome 20q12-q13.2

Guanine nucleotide-binding proteins, also known as G proteins, mediate intracellular responses to a wide variety of extracellular stimuli. A variety of genes that specify the synthesis of the components of guanine nucleotide proteins have been identified. One of these proteins, termed Gs alpha (GNAS1), is the G protein component of the olfactory signal transduction cascade. Mutations in the GNAS1 gene leading to Gs alpha protein deficiency are known to be associated with pseudohypoparathyroidism Ia (Albright hereditary osteodystrophy) and certain pituitary tumors with acromegaly. Studies on the human--mouse somatic cell hybrids provisionally assigned this gene to chromosome 20. We have now confirmed this localization on chromosome 20 and regionally assigned the GNAS1 gene to 20q12-q13.2 by in situ hybridization.

Loss of endothelial pertussis toxin-sensitive G protein function in atherosclerotic porcine coronary arteries

Pertussis toxin, an irreversible inhibitor of some G proteins, inhibits endothelium-dependent relaxations to certain agonists in porcine coronary arteries. In the present study, the effects of the toxin were examined on endothelium-dependent and -independent relaxations of hypercholesterolemic and atherosclerotic porcine coronary arteries to assess the functional state of the endothelial pertussis toxin-sensitive G protein. Male Yorkshire pigs were maintained on either a regular diet (control group, n = 7) or a 2% high-cholesterol diet (cholesterol-fed group, n = 7) for 10 weeks. After the initial 2 weeks of maintenance, animals in both groups underwent balloon catheter removal of the endothelium of the left anterior descending or left circumflex coronary arteries. Endothelium-dependent responses were examined in vitro after 10 weeks of maintenance; at this time, a full lining of endothelial cells in both left coronary arteries was confirmed histologically. In arteries with endothelium of the control group (normal responses), pertussis toxin significantly inhibited the endothelium-dependent relaxations to serotonin, UK14304 (a selective alpha 2-adrenergic receptor agonist), and thrombin but not those to ADP, bradykinin, or the calcium ionophore A23187. In previously denuded arteries of the control group (effects of endothelial regeneration alone) or intact arteries of the cholesterol-fed group (effects of hypercholesterolemia alone), the relaxations to serotonin, UK14304, and thrombin were impaired significantly; those relaxations were impaired further in previously denuded arteries of the cholesterol-fed group (effects of atherosclerosis). The inhibitory effects of pertussis toxin were significantly reduced after endothelial regeneration and in hypercholesterolemia and were almost absent in atherosclerosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of guanine-nucleotide-binding regulatory proteins with chemotactic peptide receptors in differentiated human leukemic HL-60 cells

Human leukemic HL-60 cells were differentiated into neutrophil-like cells by treatment with dimethylsulfoxide (Me2SO) or N6,O2'-dibutyryladenosine 3',5'-phosphate (Bt2cAMP), and membrane fractions were prepared from the differentiated cells. Receptors for fMLF (fM,N-formylmethionine) and guanine-nucleotide-binding regulatory proteins (G proteins) serving as the substrate for pertussis toxin (islet-activating protein; IAP) were extracted from cell membranes then reconstituted into phospholipid vesicles. The binding of fMLF to the reconstituted vesicles (or the membranes) was determined with 10 nM [3H] fMLF. In both cases, high-affinity binding to vesicle preparations from the Me2SO- and Bt2cAMP-induced cells was abolished following treatment with IAP, suggesting that fMLF receptors were functionally coupled to IAP-sensitive G proteins in each of the two vesicle types. However, the high-affinity fMLF binding was much higher in vesicle preparations originating from Bt2cAMP-induced cells than in those from Me2SO-induced cells, although the amount of IAP-substrate G protein reconstituted into the each phospholipid vesicles preparation was not significantly different from the other. The G proteins of the two differentiated cells were both identified as inhibitory forms (Gi-2) based on their electrophoretic mobilities and immunoblot analyses. When purified Gi-2 from rat brain was reconstituted into the two IAP-treated vesicles, high-affinity fMLF binding was restored in a similar manner in both. IAP-substrate G proteins partially purified from the two differentiated HL-60 cells were also effective in restoring high-affinity fMLF binding to the IAP-treated vesicles. However, a significant difference was observed that the reconstituted binding was higher with the G-protein-rich fraction from Bt2cAMP-induced cells than with that from Me2SO-induced cells, with each of the two IAP-treated vesicle types. These results suggest that the different high-affinity binding of fMLF observed in the two differentiated HL-60 cells are due to a difference in the property of endogenous G proteins rather than fMLF receptors, though the two G proteins are indistinguishable from each other in terms of the subtype of G protein, Gi-2.

Inhibition of calcium currents in cultured myenteric neurons by neuropeptide Y: evidence for direct receptor/channel coupling

Single channel recordings from rat myenteric plexus neurons demonstrated the presence of two categories of Ca2+ channels. One type of Ca channel had a slope conductance of 27 pS and was sensitive to dihydropyridines while the other channel type had a conductance of 14 pS and was dihydropyridine-insensitive. The 14 pS channel was mostly inactivated at a holding potential of -40 mV, while the 27 pS channel was much more resistant to depolarized holding potentials. A majority of whole-cell current was reprimed by the use of negative holding (-90 mV) potentials, when compared to that obtained at a holding potential of -40 mV. These properties are consistent with N- and L-type Ca channels previously described. In general, the inactivating part of the whole-cell Ca2+ current, selectively reprimed by negative holding potentials, was inhibited by neuropeptide Y (NPY). Depolarization-induced [Ca2+]i transients assessed using fura-2 showed that the inhibitory effects of nitrendipine and NPY were additive. The effects of NPY were abolished by pertussis toxin pretreatment. Single-channel experiments showed that neither the 14 nor the 27 pS Ca channel currents were inhibited by the addition of NPY outside the patch pipette. These results suggest that NPY modulates N-type Ca2+ channels selectively in these neurons and that an easily diffusible second messenger does not appear to participate in receptor/channel coupling.

Subcellular distribution and characterization of GTP-binding proteins in human neutrophils

The subcellular distribution of GTP binding proteins in human neutrophils and their functional coupling to the N-formylmethionylleucylphenylalanine (FMLP) receptor was characterized to provide insight into mechanisms of cellular activation. Human neutrophils were nitrogen cavitated and fractionated on discontinuous Percoll gradients. Four subcellular fractions were obtained: cytosol, light membranes enriched for plasma membranes, specific granules and azurophilic granules. ADP-ribosylation catalyzed by pertussis toxin (PT) revealed a major substrate of 40 kDa only in plasma membrane and cytosol, and antiserum specific for Gi alpha confirmed the presence of neutrophil Gi alpha in plasma membrane and cytosol and its absence from specific granules. The cytosolic PT substrate was shown to be mostly in monomeric form by molecular sieve chromatography. The rate of the ribosyltransferase reaction was several-fold lower in cytosol compared to plasma membranes, and the extent of ADP-ribosylation was greatly augmented by supplementation with beta gamma subunits in cytosol. ADP-ribosylation catalyzed by cholera toxin (CT) revealed substrates of 52, 43 and 40 kDa in plasma membrane alone. FMLP receptors in plasma membrane were shown to be coupled to the 40 kDa substrate for CT by ligand-modulation of ADP-ribosylation, while FMLP added to specific granules did not induce ribosylation of this substrate even though FMLP receptors were found in high density in this compartment. Both 24 and 26 kDa [32P]GTP binding proteins were found to codistribute with FMLP receptors in specific granules and plasma membranes. Functional evidence for the coupling of GTP binding proteins to the FMLP receptor in specific granules was obtained by modulating [3H]FMLP binding with GTP gamma S, and by accelerating [35S]GTP gamma S binding with FMLP.

The "cellular state": the way to regain specificity and diversity in hormone action

In contrast with the high specificity achieved in the effects of hormones and growth factors by their interaction with a large number of membrane receptors, a loss of information seems to take place due to the small number of second messenger systems. To retain specificity one has to consider the cellular state as defined in a state machine by a pattern of activity. Different molecular mechanisms are considered as possible candidates to establish such states following the initial ligand-receptor interaction and the activation of one or several second messengers.

GTP-binding proteins as possible targets for protein kinase C action

The alpha-subunits of two guanine nucleotide binding proteins Gi and transducin, as well as the beta-subunit of transducin, serve as substrates for phosphorylation by the Ca2+- and phospholipid-dependent protein kinase C (PKC). Phosphorylation of the alpha-subunit of transducin is strictly dependent on its conformation and it is only the inactive form that is subjected to phosphorylation by PKC. This review will focus on the proposition that G proteins may serve as cellular targets for modulatory actions of PKC.

Natural course of the impairment of endothelium-dependent relaxations after balloon endothelium removal in porcine coronary arteries. Possible dysfunction of a pertussis toxin-sensitive G protein

The purposes of the present study were to examine the natural course of the impairment of endothelium-dependent relaxations during a regeneration and tissue repair process after balloon endothelium removal and to elucidate the cellular mechanism(s) underlying it. Twenty-three male Yorkshire pigs underwent balloon endothelium removal along the proximal portion of either the left anterior descending or circumflex coronary artery and were then maintained on a regular chow for 4, 8, 16, or 24 weeks. Endothelium-dependent responses were examined in vitro in rings taken from the control and previously denuded arteries studied in parallel. Morphometric analysis revealed that intimal thickening developed only at the previously denuded area. In the previously denuded arteries with regenerated endothelium, the endothelium-dependent relaxations to UK 14304 (a selective alpha 2-adrenergic agonist), serotonin, and aggregating platelets were impaired 4 weeks after endothelium removal and remained so throughout the study. The endothelium-dependent relaxations to thrombin and adenosine diphosphate became depressed 8 weeks after endothelium removal and those to bradykinin became depressed 16 weeks after endothelium removal, while those to the calcium ionophore A23187 were maintained throughout the study. Endothelium-dependent relaxations to all vasoactive agents were unaltered in the control arteries. In the control arteries, pertussis toxin, an inhibitor of certain G proteins, markedly inhibited the endothelium-dependent relaxations to UK 14304 and serotonin and partially inhibited those to thrombin and aggregating platelets. The responses inhibited by the toxin in control arteries were significantly reduced in the reduced in the previously denuded arteries with regenerated endothelium. The inhibitory effect of pertussis toxin was markedly reduced in those arteries with regenerated endothelium. In quiescent rings, the presence of normal endothelium inhibited the contractions caused by serotonin and aggregating platelets; this endothelium-dependent depression was markedly impaired in the previously denuded arteries throughout the study. Direct relaxation of the coronary smooth muscle to nitric oxide or sodium nitroprusside or direct contraction to KCl or serotonin were comparable between the control and previously denuded arteries. These experiments indicate that endothelium-dependent relaxations progressively worsen after regeneration of the endothelium and that the dysfunction of a pertussis toxin-sensitive G protein partly account for the endothelial dysfunction in the chronic regenerated state.

Membrane-associated binding sites for indoleacetic acid in the rice coleoptile

As described previously, the sensitivity of rice (Oryza sativa L.) coleoptiles to auxin is modulated by oxygen. Under anoxia, coleoptile elongation is insensitive to exogenously applied indole-3-acetic acid (IAA), whereas its sensitivity increases in air in the presence of the exogenous stimulus. Here we report the presence of two independent classes of membrane-bound IAA-binding sites in air-grown coleoptiles. Their binding activity is strictly correlated with the system's sensitivity to IAA. We designate them as site A (high affinity) and site B (low affinity). Site A shows a relatively fast response to anoxia, and is highly specific for auxins. Regulation of site-A binding activity through ATP, whose availability decreases under anoxia, is postulated. A role as auxin carrier is suggested for site B.

Endothelium-dependent inhibition of ergonovine-induced contraction is impaired in porcine coronary arteries with regenerated endothelium

The inhibitory effects of the endothelium against ergonovine-induced contraction were examined in isolated porcine coronary arteries under normal conditions and after endothelial regeneration. Endothelium-dependent responses were examined in vitro in normal Yorkshire pigs (n = 16) and in pigs that had undergone balloon endothelium removal of the left anterior descending coronary artery (LAD) 4 weeks before the study (n = 10). The presence of a complete endothelial lining was confirmed histologically. In rings from normal arteries contracted with prostaglandin F2 alpha in the presence of indomethacin and ketanserin (a 5-HT2-serotonergic blocker), ergonovine caused endothelium-dependent relaxations. They were attenuated by rauwolscine (an alpha 2-adrenergic blocker), inhibited by methiothepin (a combined 5-HT1- and 5-HT2-serotonergic blocker) or by pertussis toxin (an inhibitor of several G proteins) and abolished by oxyhemoglobin (a selective inactivator of endothelium-derived relaxing factor). In quiescent rings from normal arteries, ergonovine caused contractions that were inhibited by the presence of the endothelium; this endothelium-dependent inhibition was abolished by oxyhemoglobin. The direct contractions were not affected by prazosin (an alpha 1-adrenergic blocker), rauwolscine, 6-hydroxydopamine (an agent causing chemical sympathetectomy), or diphenhydramine (an H1-histaminergic blocker) but were inhibited by ketanserin. In rings with regenerated endothelium contracted with prostaglandin F2 alpha, the endothelium-dependent relaxations to ergonovine were reduced significantly and were not inhibited by pertussis toxin. In quiescent rings with regenerated endothelium, the endothelium-dependent inhibition of ergonovine-induced contraction was less.(ABSTRACT TRUNCATED AT 250 WORDS)

S-100a0 protein stimulates the basal (Mg2+-activated) adenylate cyclase activity associated with skeletal muscle membranes

S-100a0 protein, the alpha alpha isoform of the S-100 family, stimulates basal (Mg2+-activated) adenylate cyclase (AC) activity associated with the sarcolemma, longitudinal tubules and terminal cisternae of rat skeletal muscle cells. The stimulatory effect of S-100a0 on AC activity is maximal around 5 microM S-100a0 and half-maximal around 0.2 microM S-100a0. Also, the stimulatory effect is greatest on the AC activity associated with the terminal cisternae than on the other membrane fractions studied. These data are discussed in relation to the subcellular localization of S-100a0 in muscle cells.

G-protein alpha-subunits in cytosolic and membranous fractions of human neutrophils

In plasma membranes of human neutrophils, we identified two major pertussis toxin substrates of 40 kDa Mr with pI values of 5.30 and 5.37. Only the acidic of the two substrates was also present in neutrophil cytosol. Two-dimensional tryptic peptide maps revealed a high degree of homology of cytosolic and particulate substrates. Purified G-protein beta gamma-complex stimulated pertussis toxin-catalyzed [32P]ADP-ribosylation of membranous and cytosolic substrates of neutrophils less than 2-fold and 6-fold, respectively. Hydrodynamic properties of the cytosolic substrate strongly suggested that it exists as a monomer. Purified G-protein beta gamma-complex increased the s20,w value of the cytosolic substrate from 3.3 S to 4.0 S. The GTP analogue, guanosine 5'-O-(3-thiotriphosphate), promoted the release of pertussis toxin substrates from plasma membranes. An antiserum raised against a sequence specific for the Gi2 alpha-subunit reacted with 39-40 kDa proteins in plasma membranes and with an apparently single 40 kDa protein in cytosol. We conclude that neutrophil cytosol contains monomeric Gi2 alpha-subunits which--by interacting with hydrophobic beta gamma-complexes--may reversibly bind to the plasma membrane.

Opioid receptors in magnesium-digitonin-solubilized rat brain membranes are tightly coupled to a pertussis toxin-sensitive guanine nucleotide-binding protein

Opioid receptors solubilized in Mg2+-digitonin (2%, wt/vol) from Mg2+-pretreated rat brain membranes maintain, in addition to high-affinity opioid agonist binding, the modulation by guanine nucleotides. One of the modes of expression of the latter property is an attenuation of agonist binding by guanine nucleotides in the presence of Na+. To investigate the molecular basis of this modulation and to identify the G protein(s) involved, the soluble receptors were [32P]ADP-ribosylated by means of Bordetella pertussis toxin and subjected to molecular size exclusion chromatography. In addition, soluble extracts were chromatographed on lectin and hydrophobic affinity columns. The binding of 35S- and 3H-labelled analogues of GTP was also monitored in the species separated. The oligomeric G protein-coupled opioid receptors and the guanine nucleotide/pertussis toxin-sensitive species showed similar chromatographic properties in all three systems. This indicates that the biochemically functional G protein-opioid receptor complex formed in Mg2+-pretreated membranes in the absence of an agonist is stable in digitonin solution and to chromatographic separation. Further analysis showed that the guanine nucleotide modulation of opioid receptors is via the pertussis toxin substrates with Mr of 41,000 and 39,000, which are identified as Gi and Go alpha subunits, respectively.

The insulin receptor: structure and function

Promising progress in understanding the molecular basis of insulin action has been achieved by demonstrating that the insulin receptor is an insulin-sensitive tyrosine kinase. Here we discuss the structure of this receptor kinase and compare it with receptors for related growth factors. We review the known modes to regulate the receptor kinase activity, either through its autophosphorylation (on tyrosine residues) or through its phosphorylation by other kinases (on serine and threonine residues). We discuss the role of the receptor kinase activity in hormone signal transduction in light of results indicating a reduced kinase activity in insulin-resistant states. Finally, studies to identify natural substrates for the insulin receptor kinase are presented. The possible physiological role of these phosphorylated substrates in mediating insulin action is evaluated.

Protein kinase C-mediated phosphorylation of retinal rod outer segment membrane proteins

We have previously reported that the purified GDP-bound alpha-subunit of the GTP-binding protein transducin (TD), present in outer segments of retinal rod cells (ROS), serves as a high affinity substrate (Km = 1 microM) for protein kinase C (PKC) [Zick et al. (1986) Proc. natn. Acad. Sci., U.S.A. 83, 9294-9297]. In the present study we demonstrate that TD-alpha undergoes phosphorylation by PKC when present in its native form in intact ROS membranes. This phosphorylation is inhibited by GTP-gamma-S which activates TD, suggesting that it is only the inactive conformation of TD-alpha that serves as a substrate for PKC. Indeed, both vanadate and AlF4, that confer an active conformation on TD-alpha-GDP, inhibit PKC-mediated phosphorylation of purified TD-alpha-GDP. We demonstrate that the purified beta subunit of TD also serves as an in vitro substrate for PKC. Moreover, following their phosphorylation, both TD-alpha and beta form high affinity complexes with PKC. This is evident from the findings that PKC coprecipitates with both the alpha and beta subunits of TD when the latter are immunoprecipitated by their respective antibodies. PKC phosphorylates additional ROS proteins of 36, 48 and 92 kDa, tentatively identified as rhodopsin, arrestin and the cGMP-phosphodiesterase. Taken together our results strongly suggest that phosphorylation of TD is of physiological relevance and that through phosphorylation of endogenous ROS proteins, PKC could play a key role in regulating phototransduction.

S-100b protein regulates the activity of skeletal muscle adenylate cyclase in vitro

We have investigated the effect of the b isoform of S-100 proteins on adenylate cyclase activity of rat skeletal muscle. S-100b inhibits the adenylate cyclase activity in the presence of Mg2+ (5.0-50 mM), while it activates the same enzyme in the presence of Ca2+ (0.1-1.0 mM) dose-dependently in both cases. S-100b counteracts the stimulatory effect of NaF on adenylate cyclase in the presence of Mg2+ and the inhibitory effect of RMI 12330 A in the presence of Ca2+.

P2-purinoceptor-stimulated phosphoinositide turnover in chick myotubes. Calcium mobilization and the role of guanyl nucleotide-binding proteins

ATP, a trigger of P2-purinoceptor-mediated polyphosphoinositide (PI) turnover in cultured myotubes, increased cytosolic calcium levels in a time- and dose-dependent manner (quin2 fluorescence). The calcium was released from intracellular stores, as acute addition of 5 mM EGTA was without significant effect. Adenosine 5'-(3-thiotriphosphate) and 5'-adenylyl imidodiphosphate also increased intracellular levels of inositol phosphates (InsP) and cytosolic calcium levels. Treatment with cholera or pertussis toxin of myotube cultures did not affect the P2-purinoceptor-mediated InsP increase although PI turnover in permeabilized myotubes was stimulated by guanosine 5'-(3-thiotriphosphate). The results suggest that myotube P2-purinoceptors trigger PI turnover and increase intracellular free calcium levels, via a mechanism insensitive to ADP-ribosylation, by cholera or pertussis toxin of guanyl nucleotide-binding (G) proteins. However, the presence of a phospholipase C-coupled G-protein was otherwise demonstrated.

Photoactivation of intracellular guanosine triphosphate analogues reduces the amplitude and slows the kinetics of voltage-activated calcium channel currents in sensory neurones

The influence of guanine nucleotide analogues on calcium channel currents in cultured rat dorsal root ganglion neurones has been studied using a technique in which the rate of diffusion of the analogues to their site of action is by-passed by photochemical release of the analogues within the neurones. The 1(2-nitrophenyl)ethyl P3-ester derivatives of guanosine 5'-0(3-thio)triphosphate (caged GTP-gamma-S) and 5'-guanylylimidodiphosphate (caged GMP-PNP) were synthesised and found to be completely photolysable by light, yielding free GTP-gamma-S and GMP-PNP. Calcium channel currents were recorded using the whole cell patch technique and either caged GTP-gamma-S or caged GMP-PNP (2 mM) were included in the patch pipette. Stable currents were recorded for 5-10 min, and a single pulse of 300-350 nm irradiation was directed using a liquid light guide onto the recording dish. Calcium channel currents were then recorded every 30-120 s following photochemical release of approximately 20 microM GTP-gamma-S. The peak calcium channel current was reduced by about 70% with a slow time course [t1/2 1.5 +/- 0.2 min (mean +/- SEM); n = 5]. The transient component of the peak current was usually completely abolished, whereas the sustained current measured at the end of the 100 ms depolarising pulse was less affected. Qualitatively similar effects were observed on photolysis of caged GMP-PNP.(ABSTRACT TRUNCATED AT 250 WORDS)

Roles of G protein subunits in transmembrane signalling

A family of proteins called G proteins couples cell surface receptors to a variety of enzymes and ion channels. Since many cells contain several very similar G proteins, an important question is how signals remain specific as they cross the cell membrane.

GTP analogues cause release of the alpha subunit of the GTP binding protein, GO, from the plasma membrane of NG108-15 cells

Incubation of membranes of neuroblastoma x glioma hybrid, NG108-15 cells with GDP beta S followed by immunoblotting of resolved membrane and supernatant fractions with specific anti-peptide antisera showed essentially all of the alpha subunit of Go to be associated with the membrane. Similar experiments with poorly hydrolyzed analogues of GTP caused release of a significant fraction (some 50% within 60 minutes) of Go alpha into the supernatant. This was not mimicked by analogues of ATP. Antisera directed against peptides corresponding to the extreme N and C-termini of GO alpha demonstrated that the released polypeptide was not proteolytically clipped. These experiments show that the alpha subunit of GO need not be invariably bound to the plasma membrane and that guanine nucleotide activation can release the alpha subunit of GO from its site of membrane attachment.