Translocation of alpha subunits of stimulatory guanine nucleotide-binding proteins through stimulation of the prostacyclin receptor in mouse mastocytoma cells

Serotonergic pre-treatments block in vitro serotonergic phase shifts of the mouse suprachiasmatic nucleus circadian clock

The suprachiasmatic nucleus (SCN) contains a circadian clock that maintains its time-generating and phase-modulating capacities in vitro. Previous studies report clear differences in the ability of serotonergic stimuli to phase-shift the SCN clock when applied directly to the SCN either in vivo or in vitro: while mice and rat circadian clocks are readily phase-shifted by serotonin (5-HT) or 5-HT agonists applied in vitro, hamster and mice circadian clocks respond inconsistently to 5-HT agonists injected directly into the SCN in vivo. Here we have investigated one possible explanation for these differences: that the SCN isolated in vitro experiences reduced endogenous 5-HT signaling, which increases clock sensitivity to subsequent 5-HT stimulation. For these experiments we treated mouse SCN brain slices with low concentrations of compounds that increase serotonin signaling: 5-HT, a 5-HT agonist (8-OH-DPAT), the 5-HT precursor, l-tryptophan, or the 5-HT re-uptake inhibitor, fluoxetine. Pretreatment with each of these substances completely blocked subsequent phase-shifts induced by mid-subjective day treatment with either 5-HT or 8-OH-DPAT, while they did not block phase-shifts induced by the adenylate cyclase activator, forskolin. Time-course data on l-tryptophan-induced inhibition are consistent with this treatment inducing receptor internalization, while timing of the recovery from inhibition is consistent with receptor reinsertion. Together these data support the hypothesis that SCN clock sensitivity to serotonergic phase modulation is affected by the amount of prior serotonin signaling present in the SCN, and that this signaling alters the density of surface 5-HT receptors on SCN clock neurons.

Activation-induced subcellular redistribution of G alpha(s) is dependent upon its unique N-terminus

The heterotrimeric G protein subunit, alpha(s), can move reversibly from plasma membranes to cytoplasm in response to activation by GPCRs or activating mutations. We examined the importance of the unique N-terminus of alpha(s) in this translocation in cultured cells. alpha(s) contains a single site for palmitoylation in its N-terminus, and this was replaced by different plasma membrane targeting motifs. These N-terminal alpha(s) mutants were targeted properly to plasma membranes, capable of coupling activated GPCRs to effectors, and able to constitutively stimulate cAMP production when they also contained an activating mutation. However, when activated by a constitutively activating mutation or by agonist-activated beta-AR, these N-terminal alpha(s) mutants failed, for the most part, to undergo redistribution from plasma membranes to cytoplasm, as assayed by immunofluorescence microscopy, or from a particulate to soluble fraction, as assayed by subcellular fractionation. These results highlight the importance of the extreme N-terminus of alpha(s) and its single site of palmitoylation for facilitating activation-induced translocation and provide insight into the mechanism of this G protein trafficking event.

Membrane-Bound and cytosolic forms of heterotrimeric G proteins in young and adult rat myocardium: influence of neonatal hypo- and hyperthyroidism

Membrane and cytosolic fractions prepared from ventricular myocardium of young (21-day-old) hypo- or hyperthyroid rats and adult (84-day-old) previously hypo- or hyperthyroid rats were analyzed by immunoblotting with specific anti-G-protein antibodies for the relative content of Gs alpha, Gi alpha/Go alpha, Gq alpha/G11 alpha, and G beta. All tested G protein subunits were present not only in myocardial membranes but were at least partially distributed in the cytosol, except for Go alpha2, and G11 alpha. Cytosolic forms of the individual G proteins represented about 5-60% of total cellular amounts of these proteins. The long (Gs alpha-L) isoform of Gs alpha prevailed over the short (Gs alpha-S) isoform in both crude myocardial membranes and cytosol. The Gs alpha-L/Gs alpha-S ratio in membranes as well as in cytosol increased during maturation due to a substantial increase in Gs alpha-L. Interestingly, whereas the amount of membrane-bound Gi alpha/Go alpha and Gq alpha/G11 alpha proteins tend to lower during postnatal development, cytosolic forms of these G proteins mostly rise. Neonatal hypothyroidism reduced the amount of myocardial Gs alpha and increased that of Gi alpha/Go alpha proteins. By contrast, neonatal hyperthyroidism increased expression of Gs alpha and decreased that of Gi alpha and G11 alpha in young myocardium. Changes in G protein content induced by neonatal hypo- and hyperthyroidism in young rat myocardium were restored in adulthood. Alterations in the membrane-cytosol balance of G protein subunits associated with maturation or induced by altered thyroid status indicate physiological importance of cytosolic forms of these proteins in the rat myocardium.

Heterotrimeric GTP-binding proteins in the lacrimal acinar cell endomembrane system

Secretagogues accelerate traffic in the lysosomal and basal-lateral pathways, as well as in the regulated apical secretory pathway, of lacrimal acinar cells. It has been proposed that alterations of protein segregation in compartments where these traffic pathways intersect may influence autoimmune responses. Heterotrimeric GTP-binding proteins couple secretagogue receptor ligand binding to activation of intracellular signaling cascades, but they are also suggested to participate in endomembrane traffic phenomena. Distributions of G(o), G(i3), G(q), G(11), and two G(s)isoforms were mapped in reconstituted lacrimal acini by confocal immunofluorescence microscopy and in lysates of the reconstituted acini by analytical subcellular fractionation. All G proteins examined were detected at low levels in isolated compartments (blm(i,j)) believed to represent the basal-lateral plasma membrane. G(i3), G(11), and the G(s)isoforms were concentrated in a series of isolated compartments believed to be related to domains of a basal-lateral endosome with sorting and recycling functions (ble-s/r(i,j,k)), a distinct endosomal compartment with basal-lateral membrane-like composition (e-blml), and domains of the trans-Golgi network believed to be involved in traffic to and from the basal-lateral membrane (tgn-blmr). G(o)and G(q)were concentrated in compartments believed to represent a mixture of immature and mature secretory vesicle membranes (isvm and svm) and domains of the trans-Golgi network compartment believed to mediate traffic to secretory vesicles (tgn-svr) and to pre-lysosomes (tgn-lr). Confocal fluorescence microscopy confirmed the presence of both basal-lateral membrane and intracellular pools of the G proteins. Stimulation with 10 microM carbachol for 20min caused a component of the G(o)to redistribute away from the isvm+svm; components of the G(i3), G(q), and G(s)to redistribute away from the tgn-svr+tgn-lr; and a component of the G(i3)to redistribute away from the ble-blml+tgn-blmr. Thus, these proteins may participate in endomembrane traffic steps activated by cholinergic stimulation in addition to playing their classical roles in plasma membrane signal transduction.

Maturation of rat brain is accompanied by differential expression of the long and short splice variants of G(s)alpha protein: identification of cytosolic forms of G(s)alpha

Distribution of the alpha subunit of the stimulatory G protein (G(s)alpha) was analyzed in membrane and cytosolic (supernatant 200 000 g) fractions from rat cortex, thalamus and hippocampus during the course of post-natal development. In parallel, changes in beta-adrenoceptor density and adenylyl cyclase activity were determined. Long (G(s)alphaL) and short (G(s)alphaS) variants of G(s)alpha were assessed by immunoblotting using specific polyclonal antisera reacting with both G(s)alpha isoforms. Post-natal development was associated with an increase in the total amount of brain G(s)alpha. G(s)alphaL was the dominant isoform of G(s)alpha in the membrane fractions of all studied brain regions and its amount increased markedly between post-natal day (PD) 1 and 90. The level of membrane-bound G(s)alphaS also elevated during post-natal development, but more pronounced changes were found in cytosolic G(s)alphaS. Although only a small amount of G(s)alphaS (much smaller than G(s)alphaL) was detected among soluble proteins shortly after birth, G(s)alphaS prevailed over G(s)alphaL at PD90. The G(s)alphaL/G(s)alphaS ratio decreased, respectively, from 3.2 to 1.2 and from 5.0 to 1.5 in the membrane fractions of cortex and hippocampus, but remained almost constant in thalamus between PD1 and 90. More dramatic changes were found in the cytosolic fractions of all studied brain regions: the G(s)alphaL/G(s)alphaS ratio decreased sharply in cortex (from 14.1 to 0.9), hippocampus (from 3.7 to 0.8), and also in thalamus (from 9.5 to 0.5). These results demonstrate that the membrane-cytosol balance of G(s)alpha proteins alters dramatically during the course of brain development. Both G(s)alphaL and G(s)alphaS were expressed in a region- and age-specific manner, which suggests different roles in the maturation of the brain tissue. A cyc(-) reconstitutive assay of cytosolic G(s)alpha indicated that only approximately 20% of this protein was functional, compared with membrane-bound G(s)alpha, and its ability to reconstitute adenylyl cyclase activity increased during the course of maturation. The number of beta-adrenoceptors increased sharply during early post-natal development but only slightly in adulthood, and both GTP- and isoproterenol-stimulated adenylate cyclase activity reached peak values around PD12.

Shifts in the affinity distribution of one class of seven-transmembrane receptors by activation of a separate class of seven-transmembrane receptors

We have demonstrated previously that activation of thrombin receptors causes increased Galpha(q) coupling to thromboxane A(2) receptors and increased thromboxane A(2) receptor ligand affinity. These results led to the hypothesis that thrombin receptor activation stimulates Galpha(q) redistribution to thromboxane A(2) receptors, thereby shifting them to a higher affinity state. The present study investigated three questions regarding this inter-receptor signaling phenomenon: (i) does activation of thrombin receptors cause a redistribution of thromboxane A(2) receptor subpopulations; (ii) does inter-receptor signaling require that participating receptors couple to the same family of G-protein alpha-subunits; and (iii) does inter-receptor signaling occur in cell types other than platelets? It was found that thrombin receptor activation caused a shift in the thromboxane A(2) receptor binding data from a one-site model to a two-site model (K(i) = 0.5 microM vs K(i) = 10 nM and 1.1 microM for the antagonist 4-[2-[[(4-chlorophenyl)sulfonyl]amino]ethyl]benzeneacetic acid (BM13. 505) and K(i) = 2.5 microM vs K(i) = 29.5 nM and 2.6 microM for the agonist 9,11-dideoxy-9alpha,11alpha-methanoepoxy prostaglandin F(2alpha) (U46619). It also was found that activation of prostaglandin D(2) receptors also caused a shift of prostacyclin receptor binding data from a one-site model (IC(50) = 10.1 nM) to a two-site model (IC(50) = 3.3 and 12.5 nM). The physiological manifestation of this inter-receptor signaling between prostacyclin and prostaglandin D(2) receptors was a synergistic inhibition of human platelet aggregation. Finally, the present results established that activation of endothelial cell thrombin receptors shifts thromboxane A(2) receptor affinity from K(i) = 0.8 microM (control) to K(i) = 0.2 microM (thrombin receptor-activating peptide), indicating that cells other than platelets have the capability to signal between seven-transmembrane receptors.

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.

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

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

Is signal transduction modulated by an interaction between heterotrimeric G-proteins and tubulin?

Although it is generally accepted that tubulin plays an important role in G-protein-mediated signal transduction in a variety of systems, the mechanism of this phenomenon is not completely understood. G-protein-tubulin interaction at the cell membrane and the cytosol, and the influence of such an interaction on cellular signaling are discussed in this review article. Because the diameter of a microtubule is 25 nm and the plasma membrane is 9-11 nm thick, it is not possible for membrane-associated tubulin to assemble into a complete microtubule in the membrane environment. However, tubulin heterodimers may be able to function in the membrane environment as individual heterodimers or as polymers arranged into short protofilaments. At the cell membrane, membrane-associated tubulin may influence hormone-receptor interaction, receptor-G-protein coupling, and G-protein-effector coupling. Structural proteins, such as tubulin, can participate in cellular signaling by communicating through physical forces. By virtue of its interaction with the submembranous network of cytoskeletal proteins, tubulin, when perturbed in one locus, can transmit large changes in conformations to other points. Thus, GTP binding to membrane-associated tubulin might lead to a conformational change in either receptors or G proteins. This may, in turn, influence the binding of an agonist to its receptor. On the other hand, in the cell cytosol, subsequent to agonist-induced translocation of G-proteins from the membrane compartment to the cytosol, G-proteins may affect microtubule formation. In GH3 and AtT-20 cells (stably expressing TRH receptor), transiently transfected with Gq alpha cDNA, soluble tubulin levels decreased in Gq alpha-transfected GH3 and AtT-20 cells, by 33% and 52%, respectively. These results suggest that G-proteins may have a direct effect on the microtubule function in vivo. Because tubulin and G-protein families are ubiquitous and highly conserved, an interaction between these two protein families may occur in vivo, and this, in turn, can have an impact on signal transduction. However, the physiological significance of this interaction remains to be demonstrated.

Examination of the effects of increasing Gs protein on beta2-adrenergic receptor, Gs, and adenylyl cyclase interactions

We have examined the effect of increased Gs protein levels on the abilities of three different beta2-agonists to induce GTP shifts and stimulate adenylyl cyclase response in an effort to investigate the kinetic association between the beta2-adrenergic receptor Gs and adenylyl cyclase. Agonist competition binding analysis and adenylyl cyclase concentration-response assays revealed that increases in Gs protein resulted in proportional increases in the areas of the GTP shift and adenylyl cyclase activity. Changes in the magnitude of the GTP shift were evaluated with a novel and straightforward approach for analyzing the GTP shift data that allowed us to determine the proportion of high agonist affinity binding receptor population and the apparent dissociation constant between the agonist bound receptor and Gs, regardless of the Gs protein level or the type of beta2-agonist. Using this method, we concluded that increased Gs results in the accumulation of the receptor population displaying high affinity towards agonist (HRGs) by increasing the number of receptor-Gs complexes (to a receptor:Gs protein ratio of about 0.7 at maximal Gs expression) without affecting the affinity between hormone bound receptor and Gs. Using the Gs protein levels determined with our novel analysis, we ran simulations using the theoretical shuttle model equation that relates the EC50 to available Gs. Fitting the simulations to experimental data required a receptor to catalytic unit ratio of 0.45 and revealed at least two distinct stages for beta2-agonist-stimulated adenylyl cyclase activity, namely, the activation of Gs by the beta2-adrenergic receptor (a step whose rate is dependent on the type of agonist used to stimulate activity), and the activation of adenylyl cyclase by active Gs (a step whose rate is independent of the type of agonist).

Partial constitutive activation of pheromone responses by a palmitoylation-site mutant of a G protein alpha subunit in yeast

G protein alpha subunits are often myristoylated and/or palmitoylated near their amino terminus. The G protein alpha subunit in the yeast Saccharomyces cerevisiae (GPA1 gene product, Gpa1p) is known to be myristoylated, and this modification is essential for G protein activity in vivo. Here we examined whether Gpa1p is palmitoylated and determined the functional consequences of this modification. [3H]-Palmitic acid was incorporated into Gpa1p in cells expressing myc-tagged Gpa1p or Gpa1p-Gst. The label was released upon hydroxylamine treatment. Substitution of the conserved Cys 3 for Ser blocked incorporation of the label (Gpa1pC3S). Palmitoylation was also blocked by a mutation that prevents myristoylation (Gly2Ala), whereas the palmitoylation-site mutation had no effect on myristoylation of Gpa1p. Gpa1pC3S complemented the gpa1 delta mutation in vivo and formed a complex with G beta gamma that was able to undergo nucleotide exchange in vitro. However, basal and pheromone-induced FUSl-lacZ transcription were 2-5-fold higher in the C3S mutant. Pheromone-induced growth arrest was also enhanced by the mutation, but recovery from arrest was not affected. Like wild-type Gpa1p, the C3S mutant was predominantly membrane-associated. Upon Triton X-114 partitioning or high pH treatment, no difference in the membrane-binding properties of the wild-type Gpa1p and the C3S mutant was detected. By sucrose density gradient centrifugation of membranes, however, most of the mutant protein was mislocalized to a non-plasma membrane compartment, whereas G beta gamma localization was unaltered. Taken together, our data suggest that Gpa1p is palmitoylated via a thioester bond at Cys 3 and that palmitoylation plays a role in modulating Gpa1p signaling and membrane localization.

Evidence for heterotrimeric GTP-binding proteins in Toxoplasma gondii

Toxoplasma gondii, an intracellular protozoan parasite, resides within a host-derived vacuole that is rapidly modified by a parasite-secreted membranous tubular network. In this study we investigated the involvement of heterotrimeric G proteins in the secretory pathway of T. gondii. Aluminum fluoride (AlFn), a specific activator of heterotrimeric G proteins, induced secretion from isolated tachyzoites of T. gondii in vitro, as seen by light optics and electron microscopy. In Western blot analyses, antibodies to G protein alpha subunits reacted with 39-42 kDa proteins from T. gondii isolates. Antibodies to G(o) alpha and Gs alpha coupled to the fluorescent probe fluorescein isothiocyanate localized to the paranuclear region of T. gondii. Gi3 alpha immunoprobes were confined to the cytoplasmic matrix of T. gondii and also labeled the parasitophorous vesicle. Fluorescein isothiocyanate-conjugated GA/1, an antipeptide antisera directed toward the GTP binding site common to G protein alpha subunits, was confined to the lateral cytoplasmic domain of the parasites where secretion is most prominent. In time-sequence studies using the GA/1 probe, the immunoreactive material shifted position during invasion of target cell to areas of active secretion.

Light-modulated subcellular localization of the alpha-subunit of GTP-binding protein Gq in crayfish photoreceptors

Gq-type GTP-binding protein (Gq) plays an important role in invertebrate visual phototransduction. The subcellular localization of the alpha subunit of visual Gq in crayfish photoreceptor was investigated immunocytochemically and biochemically to demonstrate the details of the rhodopsin-Gq interaction. The localization of Gq(alpha) changed depending on the light condition. In the dark, Gq(alpha) was localized in the whole rhabdoms as the membrane-bound form. In the light, half of the Gq(alpha) was localized in the cytoplasm as the soluble form. The translocation of Gq(alpha) was reversible. The light-modulated translocation possibly controls the amount of Gq that can be activated by rhodopsin. In vitro hydroxylamine treatment of rhabdomeric membranes suggested that the translocation was regulated by the fatty-acid modification of Gq(alpha).

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

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

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

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

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

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

Two types of prostacyclin receptor coupling to stimulation of adenylate cyclase and phosphatidylinositol hydrolysis in a cultured mast cell line, BNu-2cl3 cells

Prostacyclin (PGI2)-mediated signal transduction was examined in interleukin 3 (IL-3)-dependent BNu-2cl3 mast cells. Iloprost, a stable PGI2 analogue, induced the accumulation of intracellular cAMP and IP3, and an increase in the intracellular Ca2+ concentration. Pretreatment of the cells with a protein kinase C activator, 12-O-tetradecanoyl phorbol 13-acetate, suppressed the iloprost-induced IP3 accumulation and Ca2+ mobilization, but inversely potentiated the cAMP accumulation, suggesting that neither of these signal transduction pathways of iloprosts is the result of a secondary effect of activation of the other. Removal of IL-3 from the culture medium reduced the iloprost-induced IP3 accumulation and Ca2+ mobilization, while it had no effect on the iloprost-induced cAMP accumulation at all. These results taken together suggest that BNu-2cl3 cells express two types of PGI2 receptor; one couples to stimulation of adenylate cyclase, its expression being independent of IL-3, while the other couples to phosphatidylinositol hydrolysis, its expression being dependent on IL-3.

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

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

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.

Modulation of L-type Ca channel activity by P2-purinergic agonist in cardiac cells

The mechanism of enhancement of the L-type Ca current by a P2-purinergic agonist adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S) was studied by recording single channel activity from cell-attached patches on rat isolated ventricular cells using patch pipettes containing 110 mM Ba2+. The application of ATP gamma S to the patch membrane through the pipette solution did not affect single channel activity. The addition of ATP gamma S to the bath containing a depolarizing solution was ineffective due to the voltage dependence of the purinergic stimulation. Bath application of ATP gamma S (100 microM) to control 4-(2-hydroxyethyl)-1-piperazine-ethanesulphonic acid (HEPES) solution increased the amplitude of ensemble average currents both by decreasing the probability of a blank sweep occurring and by increasing the number of openings per non-blank sweep. The single channel conductance (17 pS) was not changed by ATP gamma S. Both activation and inactivation curves were shifted towards hyperpolarized potentials by about 10 mV under P2-purinergic stimulation. Since ATP gamma S increased channel activity when applied via the bath, it must be supposed that a diffusible messenger is involved.