Alteration in Gs-mediated signal transduction in S49 lymphoma cells treated with inhibitors of microtubules

Cardiovascular protection associated with cilostazol, colchicine and target of rapamycin inhibitors

ABSTRACT

An alteration in extracellular matrix production by vascular smooth muscle cells is a crucial event in the pathogenesis of vascular diseases such as aging-related, atherosclerosis and allograft vasculopathy. The human target of rapamycin (TOR) is involved in the synthesis of extracellular matrix by vascular smooth muscle cells. TOR inhibitors reduce arterial stiffness, blood pressure, and left ventricle hypertrophy and decrease cardiovascular risk in kidney graft recipients and patients with coronary artery disease and heart allograft vasculopathy. Other drugs that modulate extracellular matrix production such as cilostazol and colchicine have also demonstrated a beneficial cardiovascular effect. Clinical studies have consistently shown that cilostazol confers cardiovascular protection in peripheral vascular disease, coronary artery disease, and cerebrovascular disease. In patients with type 2 diabetes, cilostazol prevents the progression of subclinical coronary atherosclerosis. Colchicine reduces arterial stiffness in patients with Familial Mediterranean Fever and patients with coronary artery disease. Pathophysiological mechanisms underlying the cardioprotective effect of these drugs may be related to interactions between the cytoskeleton, TOR signaling and cyclic AMP synthesis that remain to be fully elucidated. Adult vascular smooth muscle cells exhibit a contractile phenotype and produce little extracellular matrix. Conditions that upregulate extracellular matrix synthesis induce a phenotypic switch toward a synthetic phenotype. TOR inhibition with rapamycin reduces extracellular matrix production by promoting the change to the contractile phenotype. Cilostazol increases the cytosolic level of cyclic AMP, which in turn leads to a reduction in extracellular matrix synthesis. Colchicine is a microtubule-destabilizing agent that may enhance the synthesis of cyclic AMP.

Colchicine use is associated with decreased prevalence of myocardial infarction in patients with gout

OBJECTIVE

The ability of antiinflammatory strategies to alter cardiovascular risk has not been rigorously examined. Colchicine is an antiinflammatory agent that affects macrophages, neutrophils, and endothelial cells, all of which are implicated in the pathogenesis of cardiovascular disease. We examined whether colchicine use was associated with a reduced risk of myocardial infarction (MI) in patients with gout.

METHODS

We conducted a retrospective, cross-sectional study of all patients with an International Classification of Diseases, 9th ed, code for gout in the electronic medical record (EMR) of the New York Harbor Healthcare System Veterans Affairs network and ≥ 1 hospital visit between August 2007 and August 2008. Hospital pharmacy data were used to identify patients who had filled at least 1 colchicine prescription versus those who had not. Demographics and CV comorbidities were collected by EMR review. The primary outcome was diagnosis of MI. Secondary outcomes included all-cause mortality and C-reactive protein (CRP) level.

RESULTS

In total, 1288 gout patients were identified. Colchicine (n = 576) and no colchicine (n = 712) groups had similar baseline demographics and serum urate levels. Prevalence of MI was 1.2% in the colchicine versus 2.6% in the no-colchicine group (p = 0.03). Colchicine users also had fewer deaths and lower CRP levels, although these did not achieve statistical significance. Colchicine effects persisted when allopurinol users were excluded from the analysis.

CONCLUSION

In this hypothesis-generating study, gout patients who took colchicine had a significantly lower prevalence of MI and exhibited trends toward reduced all-cause mortality and lower CRP level versus those who did not take colchicine.

Microtubules modulate melatonin receptors involved in phase-shifting circadian activity rhythms: in vitro and in vivo evidence

MT1 melatonin receptors expressed in Chinese hamster ovary (CHO) cells remain sensitive to a melatonin re-challenge even following chronic melatonin exposure when microtubules are depolymerized in the cell, an exposure that normally results in MT1 receptor desensitization. We extended our findings to MT2 melatonin receptors using both in vitro and in vivo approaches. Using CHO cells expressing human MT2 melatonin receptors, microtubule depolymerization prevents the loss in the number of high potency states of the receptor when compared to melatonin-treated cells. In addition, microtubule depolymerization increases melatonin-induced PKC activity but not PI hydrolysis via Gi proteins similar to that shown for MT1Rs. Furthermore, microtubule depolymerization in MT2-CHO cells enhances the exchange of GTP on Gi-proteins using a photoaffinity analog of GTP. To test whether microtubules are capable of modulating melatonin-induced phase-shifts, microtubules are depolymerized specifically within the suprachiasmatic nucleus of the hypothalamus (SCN) of the Long Evans rat and the efficacy of melatonin to phase shift their circadian activity rhythms was assessed and compared to animals with intact SCN microtubules. We find that microtubule depolymerization in the SCN using either Colcemid or nocodazole enhances the efficacy of 10 pm melatonin to phase-shift the activity rhythms of the Long Evans rat. No enhancement occurs in the presence of beta-lumicolchicine, the inactive analog of Colcemid. Taken together, these data suggest that microtubule dynamics can modulate melatonin-induced phase shifts of circadian activity rhythms which may explain, in part, why circadian disturbances occur in individuals afflicted with diseases associated with microtubule disturbances.

Taxol normalizes the impaired agonist-induced beta2-adrenoceptor internalization in splenocytes from GRK2+/- mice

G protein-coupled receptor kinase 2 (GRK2) is involved in the agonist-induced desensitization of beta2-adrenoceptors. In addition, GRK2 is capable of binding and phosphorylating tubulin. Interestingly, microtubule dynamics profoundly affect agonist-induced internalization of beta2-adrenoceptors. Here, we analyzed agonist-induced beta2-adrenoceptor internalization and signaling in splenocytes from GRK2+/- mice that have a approximately 50% lower level of GRK2 protein compared to wild type (WT) mice. In addition, we investigated the role of microtubule stability in these processes. Splenocytes from GRK2+/- mice express approximately 50% less beta2-adrenoceptors on the cell surface and show impaired agonist-induced beta2-adrenoceptor internalization. Disruption of microtubules using colchicine reduces agonist-induced beta2-adrenoceptor internalization in cells from WT, but not in cells from GRK2+/- mice. Importantly, increasing tubulin stability by taxol almost completely restores the defective agonist-induced beta2-adrenoceptor internalization in cells from GRK2+/- animals, without affecting WT cells. Despite lower surface receptor numbers, cells of GRK2+/- mice show normal beta2-adrenoceptor agonist-induced cAMP responses. Although interfering with microtubule stability has major effects on agonist-induced receptor internalization in GRK2+/- cells, microtubule dynamics do not influence cAMP responses. Our data suggest that cells with low GRK2 adapt to the lower GRK2 level by decreasing the number of beta2-adrenoceptors on the cell surface. In addition, the cellular GRK2 level determines the extent of agonist-induced beta2-adrenoceptor internalization via a mechanism involving microtubule stability. Importantly, however, normalization of agonist-induced receptor internalization by taxol is not sufficient to alter receptor signaling.

Modulation of melatonin receptors and G-protein function by microtubules

Chronic melatonin exposure produces microtubule rearrangements in Chinese hamster ovary (CHO) cells expressing the human MT1 melatonin receptor while at the same time desensitizing MT1 receptors. Because microtubule rearrangements parallel MT1 receptor desensitization, we tested whether microtubules modulate receptor responsiveness. We determined whether depolymerization of microtubules by Colcemid, which prevents melatonin-induced outgrowths in MT1-expressing CHO cells, also prevents MT1 receptor desensitization by affecting G(alpha)-GTP exchange on G-proteins. In this study, we found that depolymerization of microtubules in MT1 receptor expressing cells, prevented melatonin-induced receptor desensitization reflected by an increase in the number of high potency sites when compared with melatonin-treated cells. Further examination of the mechanism(s) underlying this desensitization suggested that these effects occurred at the level of G-proteins. Depolymerization of microtubules during melatonin-induced desensitization, attenuated forskolin-induced cAMP accumulation, the opposite of which usually occurs following melatonin exposure alone. Concomitant to this attenuation in the forskolin response was a reduction in the amount of G(i alpha) protein coupled to MT1 receptors and an increase in [32P] azidoanilido GTP incorporation into G(i) proteins. These data are consistent with the findings that microtubule depolymerization did not affect MT1/G(q) coupling nor did it affect melatonin-induced phosphoinositide hydrolysis following melatonin exposure. However, interestingly, microtubule depolymerization enhanced melatonin-induced protein kinase C activation that was blocked in the presence of pertussis toxin. These data demonstrate that microtubule dynamics can modulate melatonin receptor function through their actions on G(i) proteins and impact on downstream signaling cascades.

Microtubules and actin microfilaments regulate lipid raft/caveolae localization of adenylyl cyclase signaling components

Microtubules and actin filaments regulate plasma membrane topography, but their role in compartmentation of caveolae-resident signaling components, in particular G protein-coupled receptors (GPCR) and their stimulation of cAMP production, has not been defined. We hypothesized that the microtubular and actin cytoskeletons influence the expression and function of lipid rafts/caveolae, thereby regulating the distribution of GPCR signaling components that promote cAMP formation. Depolymerization of microtubules with colchicine (Colch) or actin microfilaments with cytochalasin D (CD) dramatically reduced the amount of caveolin-3 in buoyant (sucrose density) fractions of adult rat cardiac myocytes. Colch or CD treatment led to the exclusion of caveolin-1, caveolin-2, beta1-adrenergic receptors (beta1-AR), beta2-AR, Galpha(s), and adenylyl cyclase (AC)5/6 from buoyant fractions, decreasing AC5/6 and tyrosine-phosphorylated caveolin-1 in caveolin-1 immunoprecipitates but in parallel increased isoproterenol (beta-AR agonist)-stimulated cAMP production. Incubation with Colch decreased co-localization (by immunofluorescence microscopy) of caveolin-3 and alpha-tubulin; both Colch and CD decreased co-localization of caveolin-3 and filamin (an F-actin cross-linking protein), decreased phosphorylation of caveolin-1, Src, and p38 MAPK, and reduced the number of caveolae/mum of sarcolemma (determined by electron microscopy). Treatment of S49 T-lymphoma cells (which possess lipid rafts but lack caveolae) with CD or Colch redistributed a lipid raft marker (linker for activation of T cells (LAT)) and Galpha(s) from lipid raft domains. We conclude that microtubules and actin filaments restrict cAMP formation by regulating the localization and interaction of GPCR-G(s)-AC in lipid rafts/caveolae.

Caveolin regulates microtubule polymerization in the vascular smooth muscle cells

Microtubule and caveolin have common properties in intracellular trafficking and the regulation of cellular growth. Overexpression of caveolin in vascular smooth muscle cells increased the polymer form of microtubule without changing in the total amount of tubulin, and downregulation of caveolin decreased the polymer form of microtubule. Fractionation of cellular proteins followed by immunodetection as well as immunostaining of caveolin and microtubule revealed that caveolin and a portion of microtubule were co-localized in caveolar fractions. A caveolin scaffolding domain peptide, which mimics caveolin function, did not alter the polymerization of microtubule in vitro, but dramatically inhibited the depolymerization of microtubule induced by stathmin, a microtubule destabilizing protein, which was also found in caveolar fractions. Accordingly, it is most likely that caveolin increased the polymer form of microtubule through the inhibition of a microtubule destabilizer, stathmin, suggesting a novel role of caveolin in regulating cellular network and trafficking.

Chronic antidepressant treatment prevents accumulation of gsalpha in cholesterol-rich, cytoskeletal-associated, plasma membrane domains (lipid rafts)

Previous studies demonstrated that Gsalpha migrates from a Triton X-100 (TTX-100) insoluble membrane domain to a TTX-100 soluble membrane domain in response to chronic treatment with the antidepressants desipramine and fluoxetine. Antidepressant treatment also causes a Gsalpha redistribution in cells as seen by confocal microscopy. The current studies have focused on examining the possibility that the association between Gsalpha and the plasma membrane and/or cytoskeleton is altered in response to antidepressant treatment, and that this is relevant to both Gsalpha redistribution and the increased coupling between Gsalpha and adenylyl cyclase seen after chronic antidepressant treatment. Chronic treatment of C6 cells with two fuctionally and structurally distinct antidepressants, desipramine and fluoxetine, decreased the Gsalpha content of TTX-100 insoluble membrane domains by as much as 60%, while the inactive fluoxetine analog LY368514 had no effect. Disruption of these membrane domains with the cholesterol chelator methyl-beta-cyclodextrin altered the localization of many proteins involved in the cAMP signaling cascade, but only Gsalpha localization was altered by antidepressant treatment. In addition, microtubule disruption with colchicine elicited the movement of Gsalpha out of detergent-resistant membrane domains in a manner identical to that seen with antidepressant treatment. The data presented here further substantiate the role of Gsalpha as a major player in antidepressant-induced modification of neuronal signaling and also raise the possibility that an interaction between Gsalpha and the cytoskeleton is involved in this process.

Knock-down of RGS4 and beta tubulin in CHO cells expressing the human MT1 melatonin receptor prevents melatonin-induced receptor desensitization

Previously, it has been shown that chronic melatonin exposure in MT1-CHO cells results in receptor desensitization while at the same time producing drastic morphological changes. The addition of a depolymerizing agent during the melatonin pretreatment period prevents MT1 receptor desensitization and the changes in cellular morphology. The lack of morphological change in the presence of a depolymerizing agent is easily explained by the inability of the microtubules to polymerize, however, the prevention of receptor desensitization is a little more complex and may involve G-protein activation. The goal of this study was to determine whether melatonin-induced MT1 receptor desensitization is regulated by proteins known to regulate G-protein activation states, beta-tubulin and RGS4,using anti sense knockdown approaches. The expression of RGS4 mRNA in CHO cells was confirmed using RT PCR and successful knockdown of each was confirmed by western blot analysis or quantitative PCR. Pretreatment of MT1-CHO cells, transfected with the nonsense probes and exposed to melatonin, resulted in a desensitization of the receptor, an increase in forskolin-induced cAMP accumulation, an increase in 2-[125I]-iodomelatonin binding and no change in the affinity of melatonin for the MT1 receptor. However, knockdown of either beta-tubulin or RGS4 in MT1-CHO cells followed by pretreatment with melatonin attenuated the desensitization of melatonin receptors, decreased total 2-[125I]-iodomelatonin binding, and did not affect neither the forskolin response nor the affinity of melatonin for the MT1 receptor. Perhaps RGS4 and beta-tubulin modulate Galpha-GDP and Galpha-GTP states thus modulating MT1 melatonin receptor function.

The role of microtubules in guard cell function

Guard cells are able to sense a multitude of environmental signals and appropriately adjust the stomatal pore to regulate gas exchange in and out of the leaf. The role of the microtubule cytoskeleton during these stomatal movements has been debated. To help resolve this debate, in vivo stomatal aperture assays with different microtubule inhibitors were performed. We observed that guard cells expressing the microtubule-binding green fluorescent fusion protein (green fluorescent protein::microtubule binding domain) fail to open for all major environmental triggers of stomatal opening. Furthermore, guard cells treated with the anti-microtubule drugs, propyzamide, oryzalin, and trifluralin also failed to open under the same environmental conditions. The inhibitory conditions caused by green fluorescent protein::microtubule binding domain and these anti-microtubule drugs could be reversed using the proton pump activator, fusicoccin. Therefore, we conclude that microtubules are involved in an upstream event prior to the ionic fluxes leading to stomatal opening. In a mechanistic manner, evidence is presented to implicate a microtubule-associated protein in this putative microtubule-based signal transduction event.

Insulin increased cAMP phosphodiesterase activity antagonizing metabolic actions of glucagon in rat hepatocytes cultured with herbimycin A

The baseline activity of cyclic nucleotide phosphodiesterase 4 was markedly lowered by primary culture of rat hepatocytes with herbimycin A for 4 h [Eur. J. Biochem. 260 (1999) 398-408.]. We now report that insulin added to this preparation of hepatocytes, which had been completely freed of herbimycin, increased the thus lowered phosphodiesterase activity, consequently antagonizing glucagon-induced production of cAMP and activation of glycogen phosphorylase. The insulin receptor beta-subunits and alpha-tubulin were tyrosine-phosphorylated upon the addition of insulin. The phosphorylation of alpha-tubulin afforded conditions unfavorable for microtubule assembly that is responsible for phosphodiesterase inhibition. These effects of insulin observed in herbimycin-pretreated hepatocytes were not inhibited by wortmannin that actually abolished insulin-induced activation of phosphatidylinositol 3-kinase (PtdIns 3-kinase) under the same conditions. The physiological significance of the insulin action not mediated by PtdIns 3-kinase in herbimycin-pretreated hepatocytes is discussed.

Laminin acts via beta 1 integrin signalling to alter cholinergic regulation of L-type Ca(2+) current in cat atrial myocytes

A perforated patch recording method was used to determine how plating cells on laminin (20 microg ml(-1); >2 h) alters cholinergic regulation of L-type Ca(2+) current (I(Ca,L)) in atrial myocytes. Acetylcholine (ACh; 1 microm)-induced inhibition of basal I(Ca,L) was not different between cells on glass and laminin. However, stimulation of I(Ca,L) elicited by ACh withdrawal was significantly smaller in cells on laminin (10 +/- 2 %) than on glass (48 +/- 5 %) (P < 0.001). Stimulation of I(Ca,L) induced by either spermine-NO (200 microm), milrinone (10 microm), IBMX (100 microm) or forskolin (1 microm) was significantly smaller in cells plated on laminin than on glass. However, stimulation of I(Ca,L) by 100 microm 8-CPT-cAMP or intracellular dialysis with 50 microM cAMP was not different between cells plated on laminin or glass. Basal, forskolin- and IBMX-stimulated cAMP content was significantly smaller in cells plated on laminin than on glass. Stimulation of I(Ca,L) by ACh withdrawal was significantly smaller in cells plated on an alpha beta 1-integrin antibody (10 +/- 4 %) than on glass (3 +/- 6 %; P < 0.001). In cells on laminin, prior exposure to 100 microg ml-1 YIGSR, a laminin receptor-binding peptide, restored ACh-induced stimulation of I(Ca,L) (58 +/- 14 %)laminin alone (7 +/- 2 %; P < 0. 05). Addition of 20 microm cytochalasin D or 1 microM latrunculin A, agents that prevent actin polymerization, to cells on laminin restored ACh-induced stimulation of I(Ca,L). We conclude that laminin binding to beta 1 integrins acts in association with the actin-based cytoskeleton to attenuate adenylate cyclase activity. As a result, laminin inhibits NO-mediated stimulation of I(Ca,L) elicited by ACh withdrawal. Laminin-integrin signalling may be relevant to changes in autonomic regulation that occur during cardiac development and/or disease.

Microtubule cytoskeleton involvement in muscarinic suppression of voltage-gated calcium channel current in guinea-pig ileal smooth muscle

1. Effects of agents, which affect microtubule polymerization-depolymerization cycle, on Ba2+ current (IBa) flowing through voltage-gated Ca2+ channels and carbachol (CCh)-induced sustained suppression of IBa were examined in whole-cell voltage-clamped smooth muscle cells of guinea-pig ileum. 2. offchicine (100 microM) and vinblastine (100 microM), microtubule depolymerizers, increased the ampitude of IBa. Lumicolchicine (100 microM), an inactive analogue of colchicine, had no effect on IBa. 3. Taxol (1 - 100 microM), a microtubule polymerizer, decreased IBa in a concentration-dependent manner and accelerated the rate of inactivation of IBa. Baccatin III (100 microM), an inactive analogue of taxol, had no effect on IBa. 4. Colchicine (100 microM) and vinblastine (100 microM), but not lumicolchicine (100 microM), decreased or abolished the sustained component of CCh (10 microM)-induced IBa suppression. 5. Pretreatment with taxol (10 - 100 microM) resulted in a concentration-dependent decrease in IBa and the action of CCh on IBa. The inhibitory effects of taxol and CCh on IBa were not additive. 6. Colchicine (100 microM) or taxol (100 microM) had no effect on voltage-gated K+ channel current or CCh-induced non-selective cationic channel current. 7. These results suggest that polymerization of microtubules leads to suppression of Ca2+ channel activity, and that muscarinic sustained suppression of Ca2+ channel current is mediated by a signal transduction element which involves microtubule cytoskeleton.

ANG II-induced Ca(2+) increase in smooth muscle cells from SHR is regulated by actin and microtubule networks

We hypothesized that the cytoskeletal network in vascular smooth muscle cells (VSMC) is critical to the signaling pathways from angiotensin (ANG) II-receptor subtype 1 (AT(1)) activation to intracellular Ca(2+) (Ca(2+)(i)) release from internal stores and Ca(2+) influx. This was tested in spontaneously hypertensive rats (SHR), in which differences were reported in cultured aortic VSMC Ca(2+)(i) regulation and G protein function compared with those in normotensive Wistar-Kyoto (WKY) rats. In cultured aortic VSMC, disorganization of actin filaments with cytochalasin D (2 micromol/l) decreased the ANG II-induced Ca(2+)(i) release from internal stores and the ANG II-induced Ca(2+) influx in SHR in a reversible fashion, whereas it was without effect in WKY rats. On the other hand, blocking the dynamic state of the microtubule network significantly reduced ANG II-induced Ca(2+)(i) release from internal stores but was without effect on Ca(2+) influx in either SHR or WKY rats. This study demonstrates for the first time that, in the SHR, actin filaments play a major role in linking AT(1)-receptor activation to both Ca(2+)(i) release mechanisms and capacitative Ca(2+) influx. Furthermore, a functionally intact microtubule system is a necessary prerequisite for ANG II-induced Ca(2+)(i) release in both strains.

Decreases in cAMP phosphodiesterase activity in hepatocytes cultured with herbimycin A due to cellular microtubule polymerization related to inhibition of tyrosine phosphorylation of alpha-tubulin

The increase in cellular cAMP concentration during 10-min incubation of rat hepatocytes with glucagon or forskolin was enhanced markedly when the hepatocytes had been cultured for several hours with herbimycin A. This effect of herbimycin was accompanied by inhibition of tyrosine-phosphorylation of cellular proteins including alpha-tubulin, antagonized by coaddition of Na3VO4 plus H2O2, which also antagonized the herbimycin-induced tyrosine phosphorylation, and overcome by the addition to the 10-min incubation medium of a certain inhibitor of cAMP phosphodiesterase (PDE), which caused a huge accumulation of cAMP. The effective PDE inhibitors were 4-[3-(cyclopentyloxy)-4-methoxyphenyl]-2-pyrrolidinone (rolipram) and 4-(3-butyloxy-4-methoxyphenyl)-2-imidazolidinone (Ro-20-1724, a PDE4 inhibitor), in addition to 3-isobutyl-1-methylxanthine (a nonselective inhibitor). Rapid breakdown of the once-accumulated cAMP in cultured hepatocytes during the subsequent incubation without PDE inhibitors was progressively prevented when the concentration of herbimycin was increased from 0.3 to 10 microM during prior culture. This effect of herbimycin to inhibit PDE activity in intact cells was abolished by coaddition of a microtubule-disrupting agent, either colchicine or vinblastine, into the culture, but remained unchanged if the vinblastine-containing medium was further supplemented with taxol, a microtubule-stabilizing agent, which by itself mimicked the effect of herbimycin. None of these agents, which thus affected PDE activity in intact cells, inhibited the PDE activity assayable in the cell lysates. The taxol-like and vinblastine-suppressible action of herbimycin to stimulate microtubular assembly was antagonized by Na3VO4/H2O2, as confirmed by confocal microscopic images of the cells stained with fluorescein-bound anti-(alpha-tubulin). Thus, 4-h culture of hepatocytes with herbimycin inhibits phosphorylation of the C-terminal tyrosine residue of alpha-tubulin, thereby stimulating formation of a microtubular network which is responsible for the inhibition of PDE4 in the intact cells by an unknown mechanism.

Microtubules and signal transduction

Although molecular components of signal transduction pathways are rapidly being identified, how elements of these pathways are positioned spatially and how signals traverse the intracellular environment from the cell surface to the nucleus or to other cytoplasmic targets are not well understood. The discovery of signaling molecules that interact with microtubules (MTs), as well as the multiple effects on signaling pathways of drugs that destabilize or hyperstabilize MTs, indicate that MTs are likely to be critical to the spatial organization of signal transduction. MTs themselves are also affected by signaling pathways and this may contribute to the transmission of signals to downstream targets.

Microtubule assembly is regulated by externally applied strain in cultured smooth muscle cells

Mechanical forces clearly regulate the development and phenotype of a variety of tissues and cultured cells. However, it is not clear how mechanical information is transduced intracellularly to alter cellular function. Thermodynamic modeling predicts that mechanical forces influence microtubule assembly, and hence suggest microtubules as one potential cytoskeletal target for mechanical signals. In this study, the assembly of microtubules was analyzed in rat aortic smooth muscle cells cultured on silicon rubber substrates exposed to step increases in applied strain. Cytoskeletal and total cellular protein fractions were extracted from the cells following application of the external strain, and tubulin levels were quantified biochemically via a competitive ELISA and western blotting using bovine brain tubulin as a standard. In the first set of experiments, smooth muscle cells were subjected to a step-increase in strain and the distribution of tubulin between monomeric, polymeric, and total cellular pools was followed with time. Microtubule mass increased rapidly following application of the strain, with a statistically significant increase (P<0.05) in microtubule mass from 373+/-32 pg/cell (t=0) to 514+/-30 pg/cell (t=15 minutes). In parallel, the amount of soluble tubulin decreased approximately fivefold. The microtubule mass decreased after 1 hour to a value of 437+/-24 pg/cell. In the second set of experiments, smooth muscle cells were subjected to increasing doses of externally applied strain using a custom-built strain device. Monomeric, polymeric, and total tubulin fractions were extracted after 15 minutes of applied strain and quantified as for the earlier experiments. Microtubule mass increased with increasing strain while total cellular tubulin levels remained essentially constant at all strain levels. These findings are consistent with a thermodynamic model which predicts that microtubule assembly is promoted as a cell is stretched and compressional loads on the microtubules are presumably relieved. Furthermore, these data suggest microtubules are a potential target for translating changes in externally applied mechanical stimuli to alterations in cellular phenotype.

Neurotensin enhances agonist-induced cAMP accumulation in PC3 cells via Ca2+ -dependent adenylyl cyclase(s)

A human prostate cancer cell line (PC3) with abundant neurotensin (NT) receptors was used to demonstrate that NT potentiated 3',5'-cyclic adenosine monophate (cAMP) accumulation in response to a variety of stimuli, including both direct forskolin (F) and indirect (prostaglandin, (PGE2), isoproterenol (ISO) and cholera toxin (CTx)) activators of adenylyl cyclase. Several mechanisms were investigated and our results indicated an effect on the rate of cAMP formation and not on degradation or extrusion. For each stimulus, NT enhanced efficacy without altering EC50. The effect of NT did not involve stimulatory G-protein (Gs)-activation or interference with a tonic inhibitory G-protein (Gi)-mediated inhibition. A similar response was obtained when NT was added with the stimulus or given as a two minute pulse which was removed prior to addition of stimulus. The potentiating activity disappeared with a t1,2 of approximately 15 min. NT transiently elevated cellular [Ca2+]i and its effects on cAMP could be mimicked by [Ca2+]i-elevating agents (uridine triphosphate (UTP), thapsigargin and ionomycin). Buffering cellular [Ca2+]i with 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM) inhibited cAMP responses to ISO and F in presence and absence of NT. These data support the idea that NT potentiated cAMP formation in response to a variety of stimuli by facilitating the activation of Ca2+ -dependent adenylyl cyclases.

Cytoskeletal regulation of the signal transduction of prostaglandin EP4 receptor

Prostaglandin (PG) EP4 receptor is coupled to Gs, stimulating adenylate cyclase. We tested whether cytoskeleton modulates the signal transduction of the EP4 receptor. A microtubule depolymerizing agent, colcemid, enhanced the PGE2-induced cAMP formation in the cloned EP4 receptor-expressing Chinese hamster ovary cells, but enhanced neither NaF plus AlCl3 nor forskolin-induced cAMP formation. Other microtubule depolymerizing agents, including colchicine, also induced the enhancement. These effects stemmed from the action of the agents on microtubules, because beta-lumicolchicine, an inactive isomer of colchicine, had no effect. In contrast, the microfilament depolymerizing agents did not affect the PGE2-induced cAMP formation but potentiated the enhancing effect of colcemid. This enhancement by colcemid was not due to the suppression of the desensitization of the EP4 receptor. The enhancing effect of colcemid was also observed in another Gs-coupled PGE receptor subtype, EP2 receptor. These results demonstrate that the state of microtubule assembly modulates the signal transduction of the EP4 receptor in concert with microfilament.

Redistribution of G(q/11)alpha in the pituitary gonadotrope in response to a gonadotropin-releasing hormone agonist

In the present study, we took advantage of high-resolution multilaser confocal microscopy to examine the distribution of the alpha-subunit of the guanyl nucleotide binding protein subfamily G(q/11) (G(q/11)alpha). Dispersed cultures of pituitary cells were prepared from female weanling rats, fixed, permeabilized, and then stained with monoclonal antiserum (mouse) to the gonadotrope-specific form of secretogranin (SIIp), which was then tagged with Texas Red. Accordingly, the subpopulation of gonadotropes (approximately 15% of total cells) could be identified against a background of other pituitary cell types. G(q/11)alpha was localized with antiserum made in rabbit, then tagged with fluorescein. Hoechst 33258 nuclear stain was also used in some experiments for topological reference. The data indicate localization of the G(q/11)alpha in a cellular region near the plasma membrane and external to the border of the layer occupied by secretory granules. In the absence of activation, there were an average of six clusters of G(q/11)alpha in a section 1 microm thick and through the center of the cell. This corresponds to an average of 60 clusters per cell, assuming a mean gonadotrope diameter of 10 microm. Following continuous treatment with 0.1 microg/ml Buserelin, a metabolically stable GnRH agonist, the average number of clusters increased to 200/cell after 40 min and remained approximately constant for 120 min. This increase was blocked by the protein synthesis inhibitor, cycloheximide. In response to Buserelin, there was an additional increase in the number of clusters inside the cell in the area occupied by the secretory granules and in the perinuclear area. Prolonged (24 h) treatment with Buserelin, sufficient to provoke the onset of desensitization, did not significantly change total numbers of G(q/11)alpha clusters, although more were located in the peripheral compartment, an increase that occurred at the expense of the cytoplasmic compartment. Redistribution of the G(q/11)alpha family may be functionally significant, because this moiety may be rate limiting at the site of regulation of signal transduction.

G protein beta1gamma2 subunits promote microtubule assembly

alpha and betagamma subunits of G proteins are thought to transduce signals from cell surface receptors to intracellular effector molecules. Galpha and Gbetagamma have also been implicated in cell growth and differentiation, perhaps due to their association with cytoskeletal components. In this report Gbetagamma is shown to modulate the cytoskeleton by regulation of microtubule assembly. Specificity among betagamma species exists, as beta1gamma2 stimulates microtubule assembly, and beta1gamma1 is without any effect. Furthermore, a mutant beta1gamma2, beta1gamma2(C68S), which does not undergo prenylation and subsequent carboxyl-terminal processing on the gamma subunit, does not stimulate the formation of microtubules. beta immunoreactivity was detected exclusively in the microtubule fraction after assembly in the presence of beta1gamma2, suggesting a preferential association with microtubules rather than soluble tubulin. Crude microtubule fractions from ovine brain contain Gbetagamma, and electron microscopy reveals a specific association with microtubules. The decoration of microtubules by Gbetagamma appears to be strikingly similar to the periodic pattern observed for microtubule-associated proteins, suggesting a similar site of activation of microtubule assembly by both agents. It is suggested that reformation of the cytoskeleton represents an additional cellular process mediated by Gbetagamma.

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).

G-protein function is reduced in hypertension

A functional impairment in vasodilator tone may be important in the pathogenesis and/or maintenance of elevated peripheral vascular resistance in hypertension. Previous studies of hypertensive subjects have demonstrated impaired beta-adrenergic-mediated vasodilation paralleling a reduction in lymphocyte beta-adrenergic-stimulated adenylyl cyclase activity. We have suggested that this impairment is related to a defect in G-protein function. To determine whether this defect alters the coupling between the G-protein complex and adenylyl cyclase, we performed [3H]forskolin binding studies in lymphocytes from hypertensive subjects, older normotensive subjects, and younger normotensive control subjects. Maximal specific [3H]forskolin binding was used as an index of adenylyl cyclase binding sites. Gpp(NH)p-, NaF/AlCl3-, and isoproterenol-stimulated binding were used as indices of G-protein/adenylyl cyclase coupling. In the absence of other stimulators, maximal [3H]forskolin binding was not significantly different among groups. However, both Gpp(NH)p- and isoproterenol-stimulated [3H]forskolin binding were significantly decreased in lymphocytes from hypertensive subjects. Overall, Gpp(NH)p- and isoproterenol-stimulated [3H]forskolin binding were significantly inversely correlated with blood pressure. No differences in NaF/AlCl3-stimulated [3H]forskolin binding were detected between groups. These studies indicate that G-protein/adenylyl cyclase coupling is impaired in lymphocytes from younger hypertensive subjects and may contribute to the blood pressure-related defect in beta-adrenoceptor-stimulated adenylyl cyclase activity.

Cytoskeletal inhibitors impair Ca2+ elevations via neuropeptide Y and other Gi-coupled receptors

Neuropeptide Y, alpha 2-adrenoceptors, thrombin and certain muscarinic acetylcholine receptors can couple to elevations of intracellular free Ca2+ concentrations via Gi-proteins. We have studied the effects of inhibitors of microtubules (colchicine, nocodazole, vinblastine) and microfilaments (cytochalasin B, cytochalasin D) on these effects in human erythroleukemia (HEL) cells. Both types of inhibitors reduced neuropeptide Y-, adrenaline- (via alpha 2A-adrenoceptors) and thrombin-stimulated Ca2+ elevations while the inactive analog beta-lumicolchicine was without inhibitory effects. Similarly, in SK-N-MC cells vinblastine inhibited neuropeptide Y and carbachol (via muscarinic receptors) stimulated Ca2+ elevations. In HEL cells the inhibitory effects of the microfilament inhibitor cytochalasin D and the microtubule inhibitor colchicine were not additive. Colchicine, nocodazole or cytochalasin D did not affect the binding of the agonist neuropeptide Y. On the other hand, neuropeptide Y and thrombin significantly stimulated GTP gamma S binding in the absence but not in the presence of colchicine, vinblastine or cytochalasin B. This was not due to sequestration of G-protein alpha-subunits, since nocodazole did not affect the distribution of immunodetectable Gi alpha 1/2 or Gi alpha 3 between membrane and cytosolic fractions. We conclude that disruption of microfilaments or microtubules impairs Ca2+ elevations by neuropeptide Y and other Gi-coupled receptors by inhibiting receptor/Gi-protein interaction; this does not involve impairment of agonist binding to the receptor or redistribution of Gi-protein alpha-subunits.

Effect of transient overexpression of Gq alpha on soluble and polymerized tubulin pools in GH3 and AtT-20 cells

In order to study Gq-tubulin interaction in the cytosol, GH3 and AtT-20 cells (stably expressing TRH receptor) were transiently transfected with Gq alpha cDNA. Forty-eight hours after transfection, thyrotropin-releasing hormone (TRH)-stimulated prolactin (PRL) secretion by Gq alpha-transfected GH3 cells increased by 90% compared to mock-transfected cells. In addition, using immunocytochemistry it was observed that Gq alpha-specific staining was much more prominent in Gq alpha-transfected GH3 and AtT-20 cells (also transfected with Gq alpha) compared to mock-transfected cells. Thus, transfection resulted in successful overexpression of functional Gq alpha. Forty-eight hours after transfection, cells were processed to obtain soluble and polymerized tubulin fractions. Tubulin levels were determined in these fractions by immunoblotting using polyclonal anti-tubulin antibodies. Compared to mock-transfected cells soluble tubulin levels decreased in Gq alpha-transfected GH3 and AtT-20 cells, by 33 and 52%, respectively. Moreover, compared to mock-transfected cells a 50% reduction in the ratio (an index of the flux between tubulin pools) of soluble and polymerized tubulin levels was observed in Gq alpha-transfected GH3 and AtT-20 cells. To determine whether these effects on tubulin were mediated by Gq directly, we examined the influence of purified Gq on tubulin polymerization. Gq (0.5 microM) inhibited polymerization of crude tubulin (present in GH3 cell cytosol) by 53%. In contrast to its effects on GH3 cell cytosol tubulin, Gq stimulated purified tubulin polymerization by 160%. These results suggest that Gq modulates the polymerization and depolymerization cycles of tubulin and that this modulation is in turn influenced by other unknown cellular components.

Effect of colchicine and taxol on thyrotropin-releasing hormone receptor coupling to G protein in GH(3) cells

The role of membrane-associated tubulin in TRH receptor-G protein coupling was investigated with the use of compounds that influence tubulin function. TRH-stimulated G protein GTPase activity in GH(3) cell membranes was used to determine receptor-G protein coupling. TRH-stimulated GTPase activity was abolished by G(qα) antibody, suggesting that TRH receptor coupling to G(q) results in the activation of G(qα) and the subsequent hydrolysis of GTP. TRH (1 μM) stimulated the enzymatic activity by up to 69 pmol/min/mg protein, and in the presence of 1 μM colchicine the hormone-stimulated activity was only 26 pmol/min/mg protein. Similar inhibition of TRH receptor-G protein coupling was observed with tubulin antibodies and purified tubulin, suggesting that perturbation of membrane-associated tubulin and/or tubulin-G protein interaction by these compounds disrupts receptor-G protein interaction. Next, the events occurring at the initial stages of TRH-mediated signal transduction were correlated to prolactin (PRL) secretion in GH(3) cells. Colchicine (1 μM) and taxol (1 μM) inhibited the basal PRL secretion by 38 and 44%, respectively. In addition, colchicine (1 μM) and taxol (1 μM) significantly inhibited TRH-stimulated PRL secretion. TRH-stimulated PRL secretion in control, colchicine-, and taxol-treated cells was 13.9, 9.1, and 6 ng/mL, respectively. Furthermore, polymerized tubulin levels were decreased by colchicine and increased by taxol. These results suggest that perturbation of the steady state of tubulin-G(q) interaction may disrupt the initial events in TRH-mediated signal transduction.

The stoichiometry of expression of protein components of the stimulatory adenylyl cyclase cascade and the regulation of information transfer

Quantitative analysis of the proteins which compromise the stimulatory arm of the adenylyl cyclase cascade indicate that the adenylyl cyclase catalytic component is usually the least highly expressed. The effects on both potency of agonist ligands and maximal output resulting from targetted alterations in expression levels of each element of this cascade are discussed.

A key role for protein kinase A in homologous desensitization of the beta 2-adrenergic receptor pathway in S49 lymphoma cells

We have used a [3H]forskolin binding assay to assess Gs-adenylyl cyclase interactions in intact wild-type (WT) and kin- S49 cells under conditions that desensitize the beta 2-adrenergic receptor (beta 2-AR) system. This assay provides a measurement of G alpha s-adenylyl cyclase interaction that does not rely on the determination of second messenger accumulation or enzyme activity in broken cells. Kin- S49 cells lack protein kinase A (PKA) activity and provide a unique system in which to study the relative importance of this enzyme in beta 2-AR desensitization. Although both WT and kin- S49 cells display similar kinetics of cAMP accumulation and agonist-induced cell-surface beta 2-AR loss, we found that these cell types exhibited very different extents of desensitization of forskolin binding following agonist treatment. Specifically, 10 microM isoproterenol (37 degrees C, 30 min) induced the loss of 70% of [3H]forskolin binding sites in WT cells but only 30% in kin- cells. This loss of sites in WT cells displayed a t1/2 of approximately 7 min, was agonist concentration-dependent (EC50 approximately 60 nM), was not mimicked by 8-Br-cAMP, and could be blocked by the PKA inhibitor, H89. The difference between WT and kin- cells in agonist-induced desensitization of the beta 2-AR pathway was also noted in studies of cAMP accumulation in cells. In addition, preincubation of intact cells with isoproterenol did not inhibit guanine nucleotide-dependent [3H]forskolin binding in permeabilized cells. Overall, data obtained from [3H]forskolin binding assays demonstrate the involvement of PKA in the agonist-dependent uncoupling of beta 2-AR and Gs; thus we conclude that PKA plays an important role in the homologous desensitization of the beta 2-AR-Gs-adenylyl cyclase pathway in intact cells.

Factors determining specificity of signal transduction by G-protein-coupled receptors. Regulation of signal transfer from receptor to G-protein

Among subfamilies of G-protein-coupled receptors, agonists initiate several cell signaling events depending on the receptor subtype (R) and the type of G-protein (G) or effector molecule (E) expressed in a particular cell. Determinants of signaling specificity/efficiency may operate at the R-G interface, where events are influenced by cell architecture or accessory proteins found in the receptor's microenvironment. This issue was addressed by characterizing signal transfer from R to G following stable expression of the alpha 2A/D adrenergic receptor in two different membrane environments (NIH-3T3 fibroblasts and the pheochromocytoma cell line, PC-12). Receptor coupling to endogenous G-proteins in both cell types was eliminated by pertussis toxin pretreatment and R-G signal transfer restored by reconstitution of cell membranes with purified brain G-protein. Thus, the receptor has access to the same population of G-proteins in the two different environments. In this signal restoration assay, agonist-induced activation of G was 3-9-fold greater in PC-12 as compared with NIH-3T3 alpha 2-adrenergic receptor transfectants. The cell-specific differences in signal transfer were observed over a range of receptor densities or G-protein concentration. The augmented signal transfer in PC-12 versus NIH-3T3 transfectants occurred despite a 2-3-fold lower level of receptors existing in the R-G-coupled state (high affinity, guanyl-5'-yl imidodiphosphate-sensitive agonist binding), suggesting the existence of other membrane factors that influence the nucleotide binding behavior of G-protein in the two cell types. Detergent extraction of PC-12 but not NIH-3T3 membranes yielded a heat-sensitive, macromolecular entity that increased 35S-labeled guanosine 5'-O-(thiotriphosphate) binding to brain G-protein in a concentration-dependent manner. These data indicate that the transfer of signal from R to G is regulated by a cell type-specific, membrane-associated protein that enhances the agonist-induced activation of G.

Age-related changes in beta-adrenergic neuroeffector systems in the human heart

BACKGROUND

Aging decreases cardiac beta-adrenergic responsiveness in model systems and in humans in vivo. The purpose of this study was to comprehensively evaluate the age-related changes in the beta-receptor-G protein-adenylyl cyclase complex in nonfailing human hearts.

METHODS AND RESULTS

Twenty-six nonfailing explanted human hearts aged 1 to 71 years were obtained from organ donors and subjected to pharmacological investigation of beta-adrenergic neuroeffector systems. When the population was subdivided into the 13 youngest and 13 oldest subjects, total beta-receptor density assessed by maximum [125I]ICYP binding (beta max) was reduced in older hearts by 37% in left ventricles and 31% in right ventricles (both P < .05), and the downregulation was confined to the beta 1 subtype (r = .78 left ventricle beta 1 density versus donor age). Older donor hearts exhibited a 3- to 4-fold rightward shift of ICYP-isoproterenol (ISO) competition curves and demonstrated 43% fewer receptors in a high-affinity agonist binding state (P < .05). Older hearts exhibited decreased adenylyl cyclase stimulation by ISO, by zinterol (beta 2-agonist), and by the G protein-sensitive probes forskolin, Gpp(NH)p, and NaF. In contrast, there was no change in response to manganese, a specific activator of the adenylyl cyclase catalytic subunit. Toxin-catalyzed ADP ribosylation in membranes prepared from older versus younger hearts revealed a 29% to 30% reduction (P < .05) with cholera toxin (Gs) but no difference with pertussis toxin (Gi). The systolic contractile response of isolated right ventricular trabeculae to ISO was decreased by 46%, with a 10-fold increase in ISO EC50 in older relative to younger donor hearts.

CONCLUSIONS

There is a profound decrease in cardiac beta-adrenergic responsiveness with aging. This occurs by multiple mechanisms including downregulation and decreased agonist binding of beta 1-receptors, uncoupling of beta 2-receptors, and abnormal G protein-mediated signal transduction.

beta-Lumicolchicine interacts with the benzodiazepine binding site to potentiate GABAA receptor-mediated currents

An analogue of colchicine, beta-lumicolchicine, does not bind tubulin or disrupt microtubules. However, this compound is not pharmacologically completely inactive. beta-Lumicolchicine was found to competitively inhibit [3H]flunitrazepam binding and to enhance muscimol-stimulated 36Cl- uptake in mouse cerebral cortical microsacs. It also markedly potentiated GABA responses in Xenopus oocytes expressing human alpha 1 beta 2 gamma 2S, but not alpha 1 beta 2, GABAA receptor subunits; this potentiation was reversed by the benzodiazepine receptor antagonist flumazenil. These results strongly suggest a direct effect of beta-lumicolchicine on the GABAA receptor/chloride channel complex and caution that it possesses pharmacological effects, despite its inability to disrupt microtubules. Furthermore, beta-lumicolchicine is structurally unrelated to benzodiazepines or quinolines and may provide a novel approach to the synthesis of ligands for this receptor.

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

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

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

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