Cholera-toxin and corticotropin modulation of inositol phosphate accumulation induced by vasopressin and angiotensin II in rat glomerulosa cells

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

Potentiation of [Ca2+]i response to angiotensin III by cAMP in cortical thick ascending limb

BACKGROUND

In the rat cortical thick ascending limb (CTAL), intracellular Ca2+ ([Ca2+]i) responses to angiotensin II (Ang II) and angiotensin III (Ang III) were mediated by the Ang II subtype 1A receptor (AT1A-R), whereas the arginine vasopressin (AVP)-dependent cAMP accumulation involved the vasopressin receptor type 2 (V2-R). This work was performed in CTAL to investigate the crosstalk between these two receptors by studying their transduction pathways.

METHODS

The cAMP-dependent pathway was activated by 10 minutes of prestimulation with either forskolin, CTP-cAMP or AVP, and Ang II/Ang III-induced [Ca2+]i responses were assessed.

RESULTS

Pretreatment with 5 micromol/L forskolin significantly enhanced the [Ca2+]i response induced by 10-7 mol/L either Ang II or Ang III. Analysis of dose-response curves to Ang III in forskolin-treated CTAL demonstrated that the maximal [Ca2+]i response was significantly increased without altering the EC50. In Ca2+-free medium, the forskolin-induced potentiation of the [Ca2+]i response to Ang III was weaker but always present, suggesting that this effect was not only due to intracellular Ca2+ release but also to extracellular Ca2+ influx. Furthermore, the fact that the forskolin-induced potentiation of the [Ca2+]i response to Ang III was blocked by 10 micromol/L H-89, a specific protein kinase A (PKA) inhibitor, indicated that this effect occurred via activation of PKA. Finally, the potentiation of the [Ca2+]i response to Ang III also was observed following pretreatment with 100 micromol/L CTP-cAMP or 10-7 mol/L AVP.

CONCLUSIONS

In CTAL, there is a positive crosstalk between the adenylyl cyclase and phosphoinositide pathways mediated by V2- and AT1A-R, respectively, through activation of PKA.

Forskolin inhibits 5-hydroxytryptamine-induced phosphoinositide hydrolysis and Ca+2 Mobilisation in canine cultured aorta smooth muscle cells

The effect of forskolin on 5-hydroxytryptamine (5-HT)-induced inositol phosphate (IP) and Ca2+ mobilisation was investigated in canine cultured aorta smooth muscle cells (ASMCs). Pretreatment of ASMCs with forskolin attenuated 5-HT-induced IP accumulation and Ca2+ mobilisation in a time- and concentration-dependent manner. The half-maximal effects (pEC50) of forskolin to attenuate IP and Ca2+ responses to 5-HT occurred at concentrations of 6.28 and 6.64, respectively. Pretreatment of ASMCs with cholera toxin caused a similar inhibition on 5-HT-induced responses. Even after treatment with forskolin for 24 h, the 5-HT-induced responses were still inhibited. The inhibitory effect of forskolin resulted from both a depression of the maximal response and a shift to the right of the concentration-effect curves of 5-HT in these responses. The water-soluble forskolin analogue L-858051 [7-deacetyl-7beta-(gamma-N-methylpiperazino)-butyryl forskolin] significantly inhibited the 5-HT-stimulated IP accumulation. In contrast, the addition of 1,9-dideoxy forskolin, an inactive forskolin analogue, had little effect on IP response. Moreover, SQ-22536 [9-(tetrahydro-2-furanyl)-9-H-purin-6-amine], an inhibitor of adenylate cyclase, and both H-89 [N-(2-aminoethyl)-5-iosquinolinesulphonamide] and HA-1004 [N-(2-guanidinoethyl)-5-iosquinolinesulphonamide], inhibitors of cAMP-dependent protein kinase (PKA), attenuated the ability of forskolin to inhibit the 5-HT-stimulated accumulation of IP in ASMCs. These results indicate that activation of cAMP/PKA might inhibit the 5-HT-stimulated IP accumulation and consequently reduce Ca2+ mobilisation, or inhibit both responses independently.

Angiotensin II increases macrophage-mediated modification of low density lipoprotein via a lipoxygenase-dependent pathway

The molecular and cellular mechanisms by which hypertension enhances atherosclerosis are poorly understood. Angiotensin II (Ang II) has been implicated in the regulation of cellular lipoxygenases (LO), which are thought to play a role in atherogenesis by inducing oxidative modification of low density lipoprotein (LDL). We sought to test the hypothesis that Ang II would stimulate murine macrophage LO activity (which has both 12- and 15-LO activity). Competitive binding studies revealed the presence of Ang II AT1 receptors on mouse peritoneal macrophages (MPM) and J-774 cells, but not on the RAW cell line. Valsartan, a specific AT1 receptor antagonist inhibited Ang II binding, whereas PD 123319, an AT2 receptor antagonist did not. Incubation of MPM or J-774 cells with Ang II (10 pM to 1 microM) for 24 h led to a 2.5-3.5-fold increase in LO activity, measured as generated 13-HODE or 12(S)-HETE. This stimulation was inhibited by valsartan, but not by PD 123319. In contrast, Ang II did not stimulate LO activity in RAW macrophages. Semiquantitative reverse transcriptase-polymerase chain reaction showed a 2-3-fold increase in LO mRNA in MPM, but not in RAW cells after treatment with Ang II. Ang II also induced an increase in 12-LO protein. In addition, pretreatment of J-774 cells with Ang II increased in a dose-dependent manner the ability of the cells to modify LDL, resulting in greater chemotactic activity for monocytes, typical of minimally modified LDL. This stimulation was inhibited by AT1 receptor blockade. In summary, these data suggest that Ang II increases macrophage LO activity via AT1 receptor-mediated mechanisms and this further increases the ability of the cells to generate minimally oxidized LDL. These studies provide a link between hypertension and the associated increased atherosclerosis observed in hypertensive patients.

Inhibition of the T-type Ca2+ current by the dopamine D1 receptor in rat adrenal glomerulosa cells: requirement of the combined action of the G betagamma protein subunit and cyclic adenosine 3',5'-monophosphate

Modulation of ionic Ca2+ currents by dopamine (DA) could play a pivotal role in the control of steroid secretion by the rat adrenal glomerulosa cells. In the present study, we report that DA decreases the T-type Ca2+ current amplitude in these cells. The use of pharmacological agonists and antagonists reveals that this effect is mediated by activation of the D1-like receptors. Modulation by cAMP is complex inasmuch as preincubation of the cells with 8-Br-cAMP or the specific adenylyl cyclase inhibitor, 2',3'-dideoxyadenosine, have no effect per se, but prevent the DA-induced inhibition. The inhibitory effect of DA was abolished by addition of GDPbetaS to the pipette medium but not by pertussis toxin. If a cell is dialyzed with medium containing G alpha(s)-GDP, the inhibitory effect is reduced and cannot be recovered by the addition of GTPgammaS, indicating that the alpha(s) is not involved, but rather the betagamma-subunit. Indeed, DA-induced inhibition was mimicked by G betagamma in the pipette and 8-Br-cAMP in the bath. Similarly, G betagamma release from the activation of the AT1 receptor of angiotensin II did affect the current amplitude only in the presence of 8-Br-cAMP in the bath. The mitogen-activated protein kinase cascade, which can be activated by receptors coupled to Gs, was not involved as shown by the lack of activation of p42mapk by DA and the absence of effect of the mitogen-activated protein kinase inhibitor, PD 098059, on the DA-induced inhibition. Because the binding of G betagamma-subunits to various effectors involves the motif QXXER, we therefore tested the effect of the QEHA peptide on the inhibition of the T-type Ca2+ current induced by DA. The peptide, added to the medium pipette (200 microM), abolished the effect of DA. We conclude that the presence of the G betagamma and an increase in cAMP concentration are both required to inhibit the T-type Ca2+ current in rat adrenal glomerulosa cells.

Effect of forskolin on bradykinin-induced calcium mobilization in cultured canine tracheal smooth muscle cells

The effects of increases in intracellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) on bradykinin (BK)-induced generation of inositol phosphates (IPs) and Ca2+ mobilization were investigated in canine cultured tracheal smooth muscle cells (TSMCs). Pretreatment of TSMCs with either forskolin or dibutyryl cyclic AMP attenuated BK-stimulated responses. The inhibitory effects of these agents produced both a depression of the maximal response and a shift to the right of the concentration-response curves of BK. The water-soluble forskolin analogue L-858051, 7-deacetyl-7 beta-(r-N-methylpiperazino)-butyryl forskolin, significantly attenuated BK-stimulated IPs accumulation, while 1,9-dideoxy forskolin, an inactive forskolin, had little effect on IPs response. Moreover, SQ-22536, 9-(tetrahydro-2-furanyl)-9-H-purin-6-amine, an inhibitor of adenylate cyclase, and both H-89, N-(2-aminoethyl)-5-isoquinolinesulfonamide, and HA-1004, N-(2-guanidinoethyl)-5-isoquinolinesulfonamide, inhibitors of cyclic AMP-dependent protein kinase (PKA), reversed the ability of forskolin to attenuate BK-stimulated IPs accumulation. The KD and Bmax, values of the BK receptor for [3H]BK binding were not significantly changed by forskolin treatment for 30 min and 4 h. The AlF4(-)-induced IPs accumulation was attenuated by forskolin, indicating that G protein(s) are directly activated by AlF4- and uncoupled to phospholipase C by forskolin treatment. These results suggest that activation of cyclic AMP/PKA might inhibit the BK-stimulated PI breakdown and consequently reduce the [Ca2+]i increases or inhibit independently both responses, which is distal to the BK receptor in canine cultured TSMCs.

Mutation of Asn111 in the third transmembrane domain of the AT1A angiotensin II receptor induces its constitutive activation

A preliminary model of the rat AT1A angiotensin II (AII) receptor (Joseph, M. P., Maigret, B., Bonnafous J.-C., Marie, J., and Scheraga, H. A. (1995) J. Protein Chem. 14, 381-398) has predicted an interaction between Asn111 located in transmembrane domain (TM) III and Tyr292 (TM VII) in the nonactivated receptor; a disruption of this interaction upon AII activation would allow Tyr292 to interact with the conserved Asp74 (TM II). The previous verification that Tyr292 is essential for receptor coupling to phospholipase C (Marie, J., Maigret, B., Joseph, M. P., Larguier, R., Nouet, S., Lombard, C., and Bonnafous, J.-C. (1994) J. Biol. Chem. 269, 20815-20818) prompted us to check the possible alterations in receptor properties upon Asn111 --> Ala mutation. The mutated receptor (N111A) displayed: (i) strong constitutive activity, with amplification of the maximal phospholipase C response to AII; (ii) agonist behavior of the AT2-specific ligand CGP 42112A, [Sar1, Ile8]AII, and [Sar1,Ala8]AII, antagonists of the wild-type receptor; (iii) inverse agonism behavior of the non-peptide ligands DuP 753, LF 7-0156, and LF 8-0129. The results are discussed in the light of the allosteric ternary complex models and other described examples of constitutive activation of G protein-coupled receptors.

Angiotensin II receptors in the human adrenal gland

In the present study, we have characterized distribution and pharmacological properties of angiotensin II (Ang II) receptors in human adrenals frozen immediately after removal. Autoradiographic studies indicate that Ang II receptors are present throughout the gland. Co-incubations with DUP 753, a specific antagonist of the AT1 receptor, and with PD 123319, a specific antagonist of the AT2 receptor, reveal that Ang II receptors are mainly of type 2. The AT1 receptors are detected after 16 weeks of gestation at the periphery of the gland. Competition studies and Scatchard analysis reveal a homogenous population of high affinity AT2 binding sites (Kd = 0.68 +/- 0.1 nM). Binding capacities decrease from 1080 +/- 304 fmol/mg protein at 14 weeks to 275 +/- 55 fmol/mg protein at 21 weeks. These results differ from those obtained in adult glands where autoradiographic studies reveal that the AT1 receptors are found mainly in the zona glomerulosa and AT2 receptors mainly in the medulla. These data suggest that the AT2 receptors could be involved in the morphological or functional differentiation of the human fetal adrenal gland.

Angiotensin II induction of neurite outgrowth by AT2 receptors in NG108-15 cells. Effect counteracted by the AT1 receptors

In the present study, 3-day treatment of nondifferentiated NG108-15 cells with 100 nM angiotensin II (Ang II) induces morphological differentiation of neuronal cells characterized by the outgrowth of neurites. These morphological changes are correlated with an increase in the level of polymerized tubulin and in the level of the microtubule-associated protein, MAP2c. Mediation by the AT2 receptor may be inferred since: (a) these cells contain only AT2 receptors; (b) the effects are mimicked by CGP 42112 (an AT2 receptor agonist); (c) they are not suppressed by the addition of DUP 753 (an AT1 receptor antagonist); and (d) are abolished by co-incubation with PD 123319 (an AT2 receptor antagonist). Application of Ang II in dibutyryl cAMP-differentiated cells (which contain both types of receptors) induces neurite retraction, an effect mediated by the AT1 receptor. These results indicate that the AT2 receptor of Ang II induces neuronal differentiation, which is initiated through an increase in the levels of MAP2c associated with tubulin. Moreover, our results demonstrate that the AT1 receptor inhibit the process of differentiation induced by dibutyryl cAMP, whereas the AT2 receptors potentiate this effect, illustrating negative cross-talk interaction between the two types of Ang II receptors.

Regulation of 5-hydroxytryptamine-induced calcium mobilization by cAMP-elevating agents in cultured canine tracheal smooth muscle cells

The effects of increases in cellular adenosine 3'5'-cyclic monophosphate (cAMP) on 5-hydroxytryptamine-(5-HT-) induced generation of inositol phosphates (IPs) and increases in intracellular Ca2+ ([Ca2+]i) were investigated using canine cultured tracheal smooth muscle cells (TSMCs). Cholera toxin and forskolin induced concentration- and time-dependent cAMP formation with half-maximal effects (-logEC50) produced at concentrations of 7.0 +/- 0.5 and 4.9 +/- 0.4 respectively. Pretreatment of TSMCs with either forskolin or dibutyryl cAMP inhibited 5-HT-stimulated responses. Even after treatment for 24h, these agents still inhibited the 5-HT-induced Ca2+ mobilization. The inhibitory effects of these agents produced both depression of the maximal response and a shift to the right of the concentration response curves of 5-HT. The water-soluble forskolin analogue L-858051 [7-deacetyl-7beta-(gamma-N-methylpiperazino)-butyryl forskolin] significantly inhibited the 5-HT-stimulated accumulation of IPs. In contrast, the addition of 1,9-dideoxy forskolin, an inactive forskolin analogue, had little effect on this response. Moreover, SQ-22536 [9-(tetrahydro-2-furanyl)-9-H-purin-6-amine], an inhibitor of adenylate cyclase, and both H-89 [N-(2-aminoethyl)-5-isoquinolinesulphonamide] and HA-1004[N-(2-guanidinoethyl)-5-isoquinolinesulphonamide], inhibitors of cAMP-dependent protein kinase (PKA), attenuated the ability of forskolin to inhibit the 5-HT-stimulated accumulation of IPs. These results suggest that activation of cAMP/PKA was involved in these inhibitory effects of forskolin. The AlF4--induced accumulation of IPs was inhibited by forskolin, suggesting that G protein(s) are directly activated by AlF4-- and uncoupled from phospholipase C by forskolin treatment. These results suggest that activation of cAMP/PKA might inhibit the 5-HT-stimulated phosphoinositide breakdown and consequently reduce the [Ca2+]i increase or inhibit both responses independently.

Effect of cAMP elevating agents on carbachol-induced phosphoinositide hydrolysis and calcium mobilization in cultured canine tracheal smooth muscle cells

The effects of increases in intracellular adenosine 3',5'-cyclic monophosphate (cAMP) on carbachol-induced generation of inositol phosphates (IPs) and increases in intracellular Ca2+ ([Ca2+]i) were investigated in canine cultured tracheal smooth muscle cells (TSMCs). The cAMP elevating agents, cholera toxin (CTX) and forskolin, induced concentration- and time-dependent cAMP formation with half-maximal effects (-logEC50) at concentrations of 7.6 +/- 1.3 g/ml and 4.8 +/- 0.9 M, respectively. Forskolin caused a concentration-dependent inhibition of carbachol-induced increase in [Ca2+]i with half-maximal inhibition (-logEC50) at 5.2 +/- 0.7 M. Pretreatment of TSMCs with either CTX (10 micrograms/ml, 4 h), forskolin (10-100 microM, 30 min), or dibutyryl cAMP (1 mM, 30 min) inhibited carbachol-stimulated Ca2+ mobilization and IPs accumulation. The inhibitory effects of these agents produced both depression of the maximal response and a shift to the right of the concentration-response curve of carbachol without changing the EC50 values. After treatment with forskolin for 24 h, carbachol-induced IPs accumulation and Ca2+ mobilization were close to those of control group. SQ-22536 [9-(tetrahydro-2-furanyl)-9H-purin-6-amine, 10 microM], an inhibitor of adenylate cyclase, and HA-1004 [N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride, 50 microM], an inhibitor of cAMP-dependent protein kinase (PKA), attenuated the ability of forskolin to inhibit carbachol-induced IPs accumulation. Moreover, the inactive analogue of forskolin, 1,9-dideoxy forskolin, did not inhibit these responses evoked by carbachol, suggesting that activation of cAMP/PKA was involved in these inhibitory effects of forskolin. The KD and Bmax values of the muscarinic receptor (mAChR) for [3H]-N-methyl scopolamine binding were not significantly changed by forskolin treatment for 30 min and 24 h, suggesting that the inhibitory effect of forskolin is distal to the mAChR. The locus of this inhibition was further investigated by examining the effect of forskolin treatment on AIF4(-)-stimulated IPs accumulation in canine TSMCs. The AIF4(-)-induced response was inhibited by forskolin, supporting the notion that G protein(s) are directly activated by AIF4- and uncoupled to phospholipase C by forskolin treatment. We conclude that cAMP elevating agents inhibit carbachol-stimulated generation of IPs and Ca2+ mobilization in canine cultured TSMCs. Since generation of IPs and increases in [Ca2+]i are very early events in the activation of mAChRs, attenuation of these events by cAMP elevating agents might well contribute to the inhibitory effect of cAMP on tracheal smooth muscle formation.

Dual effects of fluoroaluminate on activation of calcium influx and inhibition of agonist-induced calcium mobilization in rat glomerulosa cells

Results presented in this study demonstrate that, in rat glomerulosa cells, fluoroaluminate (AlF4-) alone stimulates both cAMP accumulation (maximal stimulation 10-fold, ED50, 24 mM) and total inositol phosphate accumulation (maximal stimulation 12-fold, ED50 14 mM). Despite a transient accumulation of Ins(1,4,5)P3 after AlF4- stimulation, no rapid and transient intracellular calcium mobilization was observed. In contrast to angiotensin II (Ang II) or vasopressin (AVP), AlF4- induces only a slow and sustained increase in intracellular Ca2+. We demonstrate that this increase results from a Ca2+ influx mediated by cAMP-protein kinase A (PKA) pathway since preincubation with H-89, a potent PKA inhibitor, inhibits this influx. Moreover, a short preincubation (15 min at 37 degrees C) of cells with AlF4- or ACTH prevents the initial release of Ca2+ from intracellular stores induced by Ang II, but does not affect the amount of InsPs accumulated under Ang II stimulation. This rapid inhibition of Ang II action is mediated by ACTH- or AlF4(-)-stimulated cAMP production since pretreatment with H-89 leads to a complete reversal. cAMP most likely acts at the level of Ins(1,4,5)P3 receptors since an increase in intracellular cAMP blunts the calcium response induced by addition of exogenous Ins(1,4,5)P3 to permeabilized cells. These results point out that, in rat glomerulosa cells, activation of the cAMP pathway can induce a rapid desensitization of the phospholipase C pathway by acting downstream of inositol phosphate accumulation.

Angiotensin II receptor subtypes and contractile responses in portal vein smooth muscle

The selective biphenylimidazole and tetrahydroimidazopyridine antagonists exemplified by losartan (DuP 753) and PD 123319 have been shown to bind selectively to angiotensin AT1 and AT2 receptor subtypes, respectively. To characterize which subtypes of angiotensin II receptors are expressed in mammalian portal vein smooth muscle, we performed, using both membrane and strip preparations, [3H]angiotensin II binding experiments and then contraction experiments to investigate the functional relevance of these binding sites. Specific binding of [3H]angiotensin II was of high affinity, saturable and reversible. Specific binding of [3H]angiotensin II was completely displaced by angiotensin II and the peptide antagonist [Sar1,Ile8]angiotensin II. The inhibition of [3H]angiotensin II binding by losartan (2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)biphe nyl-4-yl)- methyl]imidazole, potassium salt) and DuP 532 (2-n-propyl-4-pentafluoroethyl-1-[(2'-(1H-tetrazol-5-yl)biph enyl-4-yl)- methyl]imidazole-5-carboxylic acid) was biphasic and LIGAND curve-fitting analysis revealed two populations of specific binding sites. One subpopulation represented 75% of the total binding and showed high affinity for angiotensin II, losartan and DuP 532, but low affinity for the peptide angiotensin AT2 receptor antagonist CGP 42112A (N-alpha-nicotinoyl-Tyr-Lys-[N-alpha-CBZ-Arg]-His-Pro-Ile-OH) and thus appeared identical to the cloned angiotensin AT1 receptor subtype. The remaining 25% of the sites showed nearly 1000-fold lower affinity for losartan, 6500-fold lower affinity for DuP 532 and high affinity for PD 123319 (S-1-[[4-(dimethylamino)-3-methylphenyl]methyl]-5-diphenylacetyl- 4,5,6,7-tetrahydro-1H-imidazo-[4,5-c] pyridine-6-carboxylic acid, difluoroacetate monohydrate) and CGP 42112A, with values of Ki in the same range (nM) as those found for losartan and DuP 532 at angiotensin AT1 binding sites. These sites appear to be angiotensin AT2 receptors. Only the angiotensin AT1 receptor subtype interacted with G-proteins, as indicated by the 80% inhibition of [3H]angiotensin II binding in the presence of guanosine 5'-O-(3-thiophosphate) or fluoroaluminates. Although the angiotensin II-induced contraction was completely inhibited by losartan with a pA2 value of 8.8, PD 123319 reduced the angiotensin II-induced contraction by 20-25%, indicating that both angiotensin AT1 and AT2 receptor subtypes are functional in portal vein smooth muscle.

A G protein is involved in the angiotensin AT2 receptor inhibition of the T-type calcium current in non-differentiated NG108-15 cells

In non-differentiated NG108-15 cells, both angiotensin II (Ang II) (100 nM) and CGP 42112 (100 nM) decreased the T-type calcium current amplitude by 24 +/- 2% and 21 +/- 3%, respectively. cGMP is not a mediator of the Ang II effect, since loading of cells with 50 microM cGMP did not prevent the inhibitory effects of Ang II. The effects of Ang II involves a non-identified GTPase activity since incubation with GDP beta S (3 mM) completely reversed the inhibitory effect of Ang II while GTP gamma S mimicked its effect. However, Ang II binding was not affected by GTP gamma S, and the effect of Ang II was not modified in pertussis toxin-treated cells. The inhibitory effect of Ang II on the T-type Ca2+ current involves a phosphotyrosine phosphatase activity since sodium orthovanadate prevented the effects of Ang II, although microcystin-LR, a selective Ser/Thr phosphatase 1 and 2A inhibitor, did not modify the effect of Ang II. These results provide the first evidence of a modulation of membrane conductance by Ang II through the AT2 receptor and demonstrate the involvement of a phosphotyrosine phosphatase and a G protein in the AT2 transduction mechanism in NG108-15 cells. Moreover, our data suggest that phosphotyrosine phosphatase activation is proximal to receptor occupation, since sodium orthovanadate inhibits both GTPase activity and T-type current blockage induced by Ang II or CGP 42112, while GTP gamma S inhibition of the T-type calcium current is not impaired.

Endogenous expression of histamine H1 receptors functionally coupled to phosphoinositide hydrolysis in C6 glioma cells: regulation by cyclic AMP

1. The effects of histamine receptor agonists and antagonists on phospholipid hydrolysis in rat-derived C6 glioma cells have been investigated. 2. Histamine H1 receptor-stimulation caused a concentration-dependent increase in the accumulation of total [3H]-inositol phosphates in cells prelabelled with [3H]-myo-inositol. The rank order of agonist potencies was histamine (EC50 = 24 microM) > N alpha-methylhistamine (EC50 = 31 microM) > 2-thiazolylethylamine (EC50 = 91 microM). 3. The response to 0.1 mM histamine was antagonized in a concentration-dependent manner by the H1-antagonists, mepyramine (apparent Kd = 1 nM) and (+)-chlorpheniramine (apparent Kd = 4 nM). In addition, (-)-chlorpheniramine was more than two orders of magnitude less potent than its (+)-stereoisomer. 4. Elevation of intracellular cyclic AMP accumulation with forskolin (10 microM, EC50 = 0.3 microM), isoprenaline (1 microM, EC50 = 4 nM) or rolipram (0.5 mM), significantly reduced the histamine-mediated (0.1 mM) inositol phosphate response by 37%, 43% and 26% respectively. In contrast, 1,9-dideoxyforskolin did not increase cyclic AMP accumulation and had no effect on the phosphoinositide response to histamine. 5. These data indicate the presence of functionally coupled, endogenous histamine H1 receptors in C6 glioma cells. Furthermore, the results also indicate that H1 receptor-mediated phospholipid hydrolysis is inhibited by the elevation of cyclic AMP levels in these cells.

Elevated intracellular cyclic AMP exerts different modulatory effects on cytosolic free Ca2+ oscillations induced by ADP and ATP in single rat hepatocytes

Single aequorin-injected hepatocytes respond to agonists acting via the phosphoinositide signalling pathway by the generation of oscillations in cytosolic free Ca2+ concentration ([Ca2+]free). The duration of [Ca2+]free transients is characteristic of the stimulating agonist. We have previously reported that ADP and ATP, which are believed to act through a single P(2y)-purinoceptor species, induce very different oscillatory [Ca2+]free responses in the majority of hepatocytes. We have interpreted these data as evidence for two separate Ca(2+)-mobilizing purinoceptors for these nucleotides. We show here that the elevation of intracellular cyclic AMP concentration, by the co-application of either dibutyryl cyclic AMP or 7 beta-desacetyl-7 beta-[gamma-(N-methylpiperazino)butyryl]- forskolin (L858051), exerts different modulatory effects on [Ca2+]free oscillations induced by ADP and ATP in single rat hepatocytes. Elevated intracellular cyclic AMP levels enhance the frequency and peak [Ca2+]free of transients induced by ADP. In contrast, the elevation of intracellular cyclic AMP levels in hepatocytes producing [Ca2+]free oscillations in response to ATP stimulates either an increase in the duration of transients or a sustained rise in [Ca2+]free. The data illustrate a further difference between the oscillatory [Ca2+]free responses of hepatocytes to ADP and ATP, thus further arguing against ADP and ATP acting via a single purinoceptor species.

Cyclic AMP selectively enhances bradykinin receptor synthesis and expression in cultured arterial smooth muscle. Inhibition of angiotensin II and vasopressin response

Bradykinin receptors on vascular smooth muscle may play an important role in regulating the endogenous effects of the vascular kallikrein-kinin system. The present study examined the effect of cyclic nucleotides on bradykinin-stimulated responses in cultured arterial smooth muscle cells. Short term stimulation (1 min) with cyclic AMP produced a variable inhibition of bradykinin-stimulated calcium mobilization which was lost in later passaged cells. However, long-term stimulation (24 h) produced a consistent increase in bradykinin-stimulated calcium mobilization in both early and late passaged cells. Further analysis demonstrated that chronic exposure to cAMP produced a twofold increase in both the number of cell surface bradykinin receptors and in bradykinin-stimulated phosphoinositide hydrolysis. The increase in bradykinin receptors was time dependent (> 7 h) and blocked by protein synthesis inhibitors, suggesting that cAMP enhanced the synthesis of new bradykinin receptors. The increase in bradykinin receptor binding and calcium mobilization was also stimulated by cholera toxin, forskolin, and isobutylmethylxanthine, but not isoproterenol or prostaglandin E2. Of considerable interest, prolonged exposure to cAMP inhibited both angiotensin II and arginine vasopressin-stimulated phosphoinositide hydrolysis and intracellular calcium mobilization. In summary, prolonged treatment with cAMP selectively stimulates the synthesis and expression of bradykinin receptors on arterial smooth muscle while decreasing the responsiveness to vasoconstrictor agonists such as angiotensin II and vasopressin.

Modulatory effect of cyclic AMP on calcium fluxes in FRTL-5 cells

The aim of the present study was to investigate the effect of cAMP on calcium fluxes in Fura 2 loaded thyroid FRTL-5 cells. Preincubating the cells with the phosphodiesterase inhibitor Ro-201724 decreased the ATP-stimulated entry of calcium, while having no effect on the release of sequestered calcium. Pretreatment with forskolin decreased both the release of sequestered calcium and the entry of calcium in response to ATP. We then incubated the cells with phenylisopropyl adenosine (PIA), a P2i-receptor agonist earlier shown to decrease cAMP in FRTL-5 cells. Although we did not observe a decrease in cellular cAMP after PIA, the ATP-evoked calcium response was enhanced. Forskolin decreased calcium entry induced by thapsigargin, a Ca(2+)-ATPase inhibitor, but forskolin had no effect on the thapsigargin-evoked release of sequestered calcium. Addition of calcium to cells stimulated with ATP in a calcium-free buffered resulted in a rapid influx of calcium. This response in [Ca2+]i was decreased in cells pretreated with forskolin. In cells stimulated with thapsigargin, the increase in [Ca2+]i after addition of calcium was inhibited in part by forskolin and enhanced by PIA. The results suggest that cAMP may regulate calcium fluxes in FRTL-5 cells. Furthermore, PIA increased agonist-induced calcium entry through a presently unknown mechanism.

The effects of elevated cyclic AMP levels on histamine-H1-receptor-stimulated inositol phospholipid hydrolysis and calcium mobilization in the smooth-muscle cell line DDT1MF-2

The effects of raising cyclic AMP levels, by forskolin stimulation, beta-adrenoceptor activation or cyclic AMP phosphodiesterase inhibition, on inositol phospholipid hydrolysis and increases in intracellular free [Ca2+] ([Ca2+]i) elicited by a range of agonists have been investigated in the hamster vas deferens smooth-muscle cell line DDT1MF-2. Isoprenaline (log [EC50 (M)] = -7.7 +/- 0.2), forskolin and the type IV cyclic AMP phosphodiesterase inhibitor rolipram elicited significant increases in the accumulation of cyclic [3H]AMP. Pretreatment with forskolin (10 microM) attenuated histamine (100 microM)- and N6-cyclopentyladenosine (CPA; 300 nM)-induced release of intracellular Ca2+, observed when cells are stimulated in Ca(2+)-free buffer containing 0.1 mM EGTA. Forskolin had no effect on ATP (100 microM)- or bradykinin (1 microM)-stimulated release of intracellular Ca2+. Histamine-induced intracellular Ca2+ release was also inhibited by pretreatment with rolipram (100 microM) or the membrane-permeant cyclic AMP analogue (Sp)-adenosine 3',5'-monophosphothioate (100 microM). Isoprenaline (1 microM) pretreatment (in the presence of 10 microM rolipram, a concentration which on its own did not decrease the histamine response) attenuated histamine-induced intracellular Ca2+ release. Forskolin inhibited histamine (100 microM)- and CPA (100 nM) stimulated accumulation of [3H]-inositol phosphates, but was without effect on ATP or bradykinin responses. Addition of forskolin (in the presence of 100 microM rolipram) after the cells had been stimulated with histamine (in experiments initiated in Ca(2+)-free buffer) inhibited the rise in [Ca2+]i observed when extracellular Ca2+ (2 mM) was re-applied (owing to receptor-mediated Ca2+ influx). Finally, the refilling of intracellular Ca2+ stores (after receptor-mediated Ca2+ influx is blocked by mepyramine) can be demonstrated in the presence of raised cyclic AMP levels.

T-type calcium channels in adrenal glomerulosa cells: GTP-dependent modulation by angiotensin II

With the use of whole-cell and single-channel current recordings, we have examined in more detail the site of action of angiotensin II (AII) on multiple populations of voltage-gated calcium channels in bovine adrenal glomerulosa cells. AII (10 nM) enhances whole-cell T-type calcium channel current and increases the activity of single T-type calcium channels in cell-attached patch recordings. The AII-induced enhancement of whole-cell calcium channel currents is dependent on the presence of internal GTP and can be inhibited by the competitive AII-receptor antagonist saralasin (1 microM). These results show that AII augments the T-type calcium channel current in bovine adrenal glomerulosa cells.

Different modulatory effects of elevated cyclic AMP on cytosolic Ca2+ spikes induced by phenylephrine or vasopressin in single rat hepatocytes

We show here, by aequorin measurements in single isolated rat hepatocytes, that elevation of cyclic AMP, by dibutyryl cyclic AMP, forskolin or glucagon, has different effects on oscillations in cytosolic concentration of free Ca2+ ('free Ca') induced by phenylephrine or vasopressin. Elevated cyclic AMP does not itself induce free Ca oscillations, but enhances both the peak free Ca and the frequency of spikes induced by phenylephrine. In contrast, elevated cyclic AMP has no effect on peak free Ca of vasopressin-induced spikes, but markedly prolongs the falling phase, with the result that spiking frequency (peak to peak) falls, although the period between spikes of resting free Ca is usually decreased. The data provide another example of receptor-specific information being retained in the oscillator mechanism, with implications for models of the hepatocyte calcium oscillator.

Effects of an inhibitor of adenylate cyclase on acrosome reaction induced by protein kinase C activators

To determine whether the adenylate cyclase and the protein kinase C pathways act independently to modulate the human sperm acrosome reaction, we studied the effects of 2'-O-methyladenosine (adenylate cyclase inhibitor) on acrosome reactions induced by protein kinase C activators (phorbol diesters and synthetic diacylglycerols) or an adenylate cyclase stimulator (forskolin:FR). Fifty aliquots of capacitated spermatozoa were divided into 5 groups (A, B, C, D, and E), each containing 10 samples. One control aliquot (CN) and five experimental aliquots (EX1, EX2, EX3, EX4, and EX5) were prepared from each sample. Phorbol 12-myristate, 13-acetate (PMA, 10 mumol/L), 4 beta-phorbol 12,13-didecanoate (PDD, 0.1 mumol/L), 1-oleoyl-2-acetyl-sn-glycerol (OAG, 50 mumol/L), 1,2-dioctanoyl-sn-glycerol (DOG, 50 mumol/L), or FR (10 mumol/L) was added to each of the experimental aliquots in groups A, B, C, D, and E, respectively. Increasing concentrations of 2'-O-methyladenosine were added to aliquots EX2, EX3, EX4, and EX5. After an incubation period of 25 min at 37 degrees C, it was found that the percentage of acrosome-reacted spermatozoa (%ARS) was significantly and dose-dependently decreased by 2'-O-methyladenosine concentrations of 1 mM or more. Within each group, the %ARS was significantly higher in EX1 aliquots than in CN aliquots. The reduction of acrosome reactions induced by protein kinase C activators by the adenylate cyclase inhibitor suggests that the protein kinase C pathway interacts with the adenylate cyclase pathway to modulate the human sperm acrosome reaction.

Cholera toxin but not pertussis toxin inhibits angiotensin II-enhanced contractions in the rat portal vein

Angiotensin II (Ang II)-enhanced phasic contractions in the rat portal vein were concentration dependently inhibited by cholera toxin (0.1-10 micrograms/ml) and dibutyryl cyclic AMP (0.1-1 mM), but not by pertussis toxin (1 micrograms/ml), which suggests that Gi is not involved in the Ang II signal transduction pathway. It also seems likely that the effect of cholera toxin is due to its ability to increase cyclic AMP production through Gs.

Domains for G-protein coupling in angiotensin II receptor type I: studies by site-directed mutagenesis

To delineate domains essential for G-protein coupling in angiotensin II type 1 receptor (AT1), we mutated the receptor cDNA in the putative cytosolic regions and determined consequent changes in the effect of GTP analogs on angiotensin II (Ang II) binding and in inositol trisphosphate production in response to Ang II. Polar residues in targeted areas were replaced by small neutral residues. Mutations in the second cytosolic loop, carboxy terminal region of the third cytosolic loop or deletional mutation in the carboxyl terminal tail simultaneously abolished both the GTP-induced shift to the low affinity form and Ang II-induced stimulation of inositol trisphosphate production. These results suggest that polar residues in the second cytosolic loop, the carboxy terminal region of the third cytosolic loop, and the carboxy terminal cytosolic tail are important for G-protein coupling of AT1 receptor.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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Characterization of biochemical responses of angiotensin II (AT2) binding sites in the rat pheochromocytoma PC12W cells

Rat pheochromocytoma PC12W cell membranes have previously been shown to exclusively contain the AT2 receptor subtype. The present study extended these binding data and explored the functional expression of these binding sites. Our binding competition studies show a potency series of Ang II = Ang III greater than saralasin greater than Ang I = PD123177 much greater than Ang II(1-7) much much greater than losartan. PD123177 (1 microM) completely eliminated [125I]Ang II binding to PC12W cells. Competitive displacement of [125I]Ang II with Ang II shows a dissociation equilibrium constant (Kd) of 1.79 nM and a binding site maximum (Bmax) of 3.97 fmol/mg protein. Investigating several Ang II signal transduction pathways on these cells, we found that Ang II (10(-8) to 10(-6) M) does not affect basal cAMP, cGMP, arachidonic acid release, prostacyclin release, intracellular Ca2+ mobilization or thymidine incorporation in the PC12W cells. Nerve growth factor, cAMP, 5-fluorouridine deoxyriboside modulation of the number of AT2 receptor sites in PC12W cells failed to unmask any Ang II effects on basal cAMP, cGMP and intracellular Ca2+ mobilization. In conclusion, the present study confirms the exclusive presence of AT2 binding sites in the PC12W cells. However, these binding sites are not functionally coupled to common signal transduction pathways.

Mechanisms involved in the interaction of dopamine with angiotensin II on aldosterone secretion in isolated and cultured rat adrenal glomerulosa cells

In a previous study, we have shown that freshly isolated glomerulosa cells possess dopamine (DA) receptors from both DA-1 and DA-2 subclasses, whereas in cultured conditions, cells exhibit dopamine receptors from the DA-1 subclass only. In the present work, we have studied the effect of DA on angiotensin-stimulated glomerulosa cells in these two experimental conditions. Our results demonstrate that in isolated cells, angiotensin II (AT) stimulates inositol phosphate accumulation, calcium influx and steroid secretion. Treatment with pertussis toxin completely blocks AT-stimulated steroid secretion and calcium influx and partially reduces inositol phosphate accumulation. DA alone has no effect on cAMP accumulation. However, in the presence of a specific DA-1 antagonist (SCH 23390), DA reduces intracellular cAMP content. Similarly, DA-like pertussis toxin produces the same inhibitory effects on AT-stimulated cells. The combined influence of DA and pertussis toxin is not additive suggesting that a 'Gi' GTP-binding protein is involved in the DA action. Specific DA antagonists indicate that these inhibitory processes are mediated through the DA-2 receptor subtype. DA may act by decreasing the intracellular calcium concentration since it reduces AT-stimulated Ca2+ influx and that both phospholipase C (PLC) and steroid accumulation are calcium dependent. Yet a direct inhibitory coupling between the DA-2 receptor and PLC may represent a second alternative since DA inhibitory effects are always present when calcium influx is artificially increased or decreased. In cultured cells, we observe an additive effect of DA and AT on aldosterone secretion, which is the result of additive interactions of the second messengers involved, namely cAMP for dopamine and inositol phosphates for angiotensin II. From these studies, we conclude that DA may exert a more versatile effect on aldosterone secretion than previously suspected.

Angiotensin II AT2 receptors do not interact with guanine nucleotide binding proteins

We have studied the effect of GTP gamma S on the affinity and binding kinetics of angiotensin II in plasma membrane particulate prepared from tissues expressing either only AT1 (human renal artery smooth muscle cells), only AT2 (human myometrium and bovine cerebellar cortex) or both angiotensin II receptor subtypes (rat adrenal glomerulosa). We also examined the ability of angiotensin II to stimulate GTP gamma[35S] incorporation in these membrane preparations. In contrast to its effects on angiotensin II binding to the AT1 receptor, GTP gamma S does not affect binding parameters to the AT2 receptor. Moreover, in tissues expressing solely AT2 receptors, angiotensin II was unable to induce GTP gamma[35S] incorporation. These findings indicate that AT2 receptors do not interact with G-proteins and that angiotensin II must therefore mediate some of its effects through G-protein-independent mechanisms.

Cholera toxin-mediated inhibition of signalling in Jurkat cells is followed by, but not due to a loss of T cell receptor complex

Cholera toxin treatment of the human T cell lymphoma Jurkat resulted in inhibition of signalling via the T cell antigen receptor complex (TcR/CD3-complex). Cholera toxin specifically ADP-ribosylated the alpha-subunit of the stimulatory G-protein of the adenylate cyclase (Gs alpha), no other proteins were modified in the intact cells. ADP-ribosylation of Gs alpha and its subsequent activation led to an increase of the cyclic AMP level and in addition, to a drastic reduction of the cell-surface density of the TcR/CD3-complex. Recently, we demonstrated that the effect of cholera toxin at the receptor level is not due to an increased cAMP level (4). As inhibition of signalling is also not cAMP-mediated (8), we examined whether the modulation of the TcR/CD3-complex could be the reason for the interruption of the signalling cascade. Analyzing the time courses of the multiple cholera toxin effects in Jurkat cells at 37 degrees C, the following sequence was found: ADP-ribosylation of Gs alpha--increase of cyclic AMP level--inhibition of signalling via the TcR/CD3-complex--decrease of cell-surface density of the TcR/CD3-complex. Treatment of Jurkat cells at 20 degrees C with cholera toxin resulted in an increase of cyclic AMP and inhibition of signal transduction, while no decrease of TcR/CD3-complex density could be observed. These data imply that receptor loss from the cell-surface is not causative for the inhibition of signalling. More likely, activation of Gs uncouples signal transduction in Jurkat cells via the TcR, which by a so far unknown mechanism is followed by a loss of the receptor from the cell surface.

Cholera toxin impairment of opioid-mediated inhibition of adenylate cyclase in neuroblastoma x glioma hybrid cells is due to a toxin-induced decrease in opioid receptor levels

Cholera toxin treatment (up to 1 microgram/ml, 16 h) of neuroblastoma x glioma hybrid NG108-15 cells produced a decrease of some 35% in both delta opioid receptor-mediated stimulation of high-affinity GTPase activity and inhibition of forskolin-amplified adenylate cyclase. Coincident with these decreases was a down-regulation of some 35% in the delta opioid receptor population. A similar pattern of a decrease in signalling capacity was noted for the alpha 2B-adrenergic receptor in these cells after cholera toxin treatment. Half-maximal effects of cholera toxin on all of the parameters assayed were noted at concentrations between 2 and 5 ng/ml. Neither levels of Gi2, as assessed by immunoblotting with specific antisera, nor the intrinsic activity of the alpha subunit of the guanine-nucleotide-binding protein which acts as the inhibitory G-protein of the adenylate cyclase in these cells, as assessed by guanosine 5'-[beta gamma-imido]triphosphate (Gpp[NH]p)-mediated inhibition of adenylate cyclase, was lowered by cholera toxin treatment. Furthermore, levels of another pertussis toxin-sensitive G-protein (Go) expressed by these cells was also not lowered by cholera toxin treatment. However, as previously noted in other cells [Milligan, Unson & Wakelam (1989) Biochem. J. 262, 643-649], marked down-regulation of the alpha subunit of the stimulatory G-protein (Gs) of the adenylate cyclase cascade was observed in response to cholera toxin treatment. Previous studies [Klee, Milligan, Simonds & Tocque (1985) Mol. Aspects Cell Regul. 4, 117-129] have shown that cholera toxin treatment can result in a decrease in the maximal effectiveness of agonists which function to inhibit adenylate cyclase. These data have been used as evidence to suggest a functional interaction between Gs and 'Gi'. The results provided herein demonstrate that such effects of the toxin can be explained adequately by a decrease in the number of receptors that function to produce inhibition of adenylate cyclase.

Interactions between second messengers: cyclic AMP and phospholipase A2- and phospholipase C-metabolites

The article reviews several new findings on the interactions between phospholipase A2- and phospholipase C-derived metabolites and cyclic AMP, in view of the developments recently achieved in studies on intracellular signal transduction. A complex network of multi-directional regulative mechanisms in the airways and inflammatory blood cells is briefly outlined.

Involvement of a pertussis toxin-sensitive G protein in the action of gastrin on gastric parietal cells

The mechanism whereby gastrin triggers phosphoinositide breakdown was investigated in an enriched preparation of isolated rabbit parietal cells (approx. 75%). In a permeabilized preparation of myo-[3H]inositol-labelled cells, GTP[S], a non-hydrolysable GTP analogue, enhanced [3H]inositol trisphosphate ([3H]InsP3 accumulation in a dose-dependent manner; submaximal concentrations of GTP[S] (less than 10 microM), potentiated gastrin-induced [3H]InsP3 release; preincubation for 5 min with GDP[S], a non-hydrolysable GDP analogue, dose-dependently reduced [3H]InsP3 accumulation stimulated by gastrin even in presence of GTP[S]. Exposure of intact parietal cells for 3 h to pertussis toxin (PTx) (200 ng/ml) led to a 15-50% reduction in gastrin-induced [14C]aminopyrine [(14C]AP) uptake (an index of in vitro acid secretion) and [3H]inositol phosphate ([3H]InsP) accumulation. A decrease in the accumulation of the different [3H]inositol phosphate occurred in gastrin-stimulated parietal cells treated with PTx. A rightward shift of gastrin dose-response curves in the presence of PTx was observed for [14C]AP uptake (EC50 values: 0.125 +/- 0.045 nM without PTx and 1.05 +/- 0.63 nM with PTx), for [3H]InsP accumulation (EC50 values: 0.16 +/- 0.08 nM without PTx and 1.56 +/- 0.58 nM with PTx) and [125I]gastrin binding (IC50 values: 0.247 +/- 0.03 nM without PTx and 2.38 +/- 0.56 nM with PTx). In contrast, cholera toxin (CTx) treatment (100 ng/ml) for 3 h was without effect on gastrin-induced [3H]InsP accumulation. CTx induced a pronounced potentiation of gastrin-stimulated [14C]AP uptake; this effect can be mimicked by IBMX (a phosphodiesterase inhibitor) and by forskolin (an activator of adenylyl cyclase). We conclude that: (i) one or more than one G protein appeared to be involved in gastrin receptor coupling to phospholipase C (PL-C); (ii) these G proteins are not substrates for CTx; (iii) one of these appeared to be a PTx-sensitive 'Gi-like' protein which could be involved in hormone-induced acid secretion, (iiii) the potentiating effect of CTx observed on AP uptake stimulated by gastrin suggests the existence of a cooperative effect between cAMP pathway (CTx) and the gastrin-induced phosphoinositide breakdown in acid secretory activity of parietal cells.

Inositol phosphate release and steroidogenesis in rat adrenal glomerulosa cells. Comparison of the effects of endothelin, angiotensin II and vasopressin

Endothelin has steroidogenic activity in adrenal glomerulosa cells, as do two other vasoconstrictor peptides, angiotensin II and vasopressin. The steroidogenic activities of angiotensin II and vasopressin are probably mediated via the phosphatidylinositol-turnover pathway and associated changes in cytosolic Ca2+ concentration. Endothelin caused a steroidogenic response, which was small compared with that to angiotensin II and quantitatively similar to the vasopressin response. Cytosolic free Ca2+ responses were similarly higher to angiotensin II than to either of the other two peptides. However, total inositol phosphate responses to endothelin and angiotensin II were similar when these were measured over 20 min, and were quantitatively greater than the vasopressin response. A detailed study has been made of the phosphatidylinositol-turnover response to endothelin in comparison with responses to angiotensin II and vasopressin. Each of the three peptides produced a rapid and transient rise in Ins(1,4,5)P3 (max. 5-15 s), followed by a slow sustained rise. Ins(1,4,5)P3 was metabolized by both dephosphorylation and phosphorylation pathways, but the relative importance of the two metabolic pathways was different under stimulation by each of the three peptides. These findings show that adrenal glomerulosa cells can distinguish between the stimulation of phosphatidylinositol turnover by three different effectors. These differences in the pathway may be associated with the observed different steroidogenic and Ca2+ responses to the three peptides.

Glutamate-stimulated, guanine nucleotide-mediated phosphoinositide turnover in astrocytes is inhibited by cyclic AMP

The potential for cross-talk between the adenyl cyclase and phosphoinositide (PPI) lipid second messenger system was investigated in astrocytes cultured from neonatal rat brain. Glutamate-stimulated PPI turnover, measured by the formation of total inositol phosphates from myo-[3H]inositol-labeled lipids, was inhibited in a concentration-dependent manner by the elevation of intracellular cyclic AMP levels produced either by stimulation of the isoproterenol receptor linked to adenyl cyclase or by its direct activation by forskolin. N6,2'-O-Dibutyryl cyclic AMP, an analogue that can also activate cyclic AMP-dependent kinase, inhibited glutamate-stimulated PPI turnover in a concentration-dependent manner as well, a result suggesting that cyclic AMP-dependent kinase is involved in mediating the inhibition. Inclusion of an inhibitor of cyclic AMP-dependent kinase, 1-(5-isoquinolinesulfonyl)-2 methylpiperazine dihydrochloride or N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride, blocked the cyclic AMP-mediated inhibition in a concentration-dependent manner, a finding further supporting this hypothesis. The site of inhibition of the phosphoinositol lipid pathway by cyclic AMP was probed using a digitonin-permeabilized cell system. Guanosine 5'-O-(3-thiotriphosphate), a nonhydrolyzable analogue of GTP, stimulated PPI turnover and potentiated glutamate-stimulated PPI turnover, and guanosine 5'-O-(3-thiodiphosphate) inhibited glutamate-stimulated PPI turnover in these cells, results providing evidence that glutamate receptors are coupled to phospholipase C by a guanine nucleotide binding protein in astrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

The G protein coupling T cell antigen receptor/CD3-complex and phospholipase C in the human T cell lymphoma Jurkat is not a target for cholera toxin

Intact Jurkat cells could be stimulated by monoclonal antibodies against the Tcell antigen receptor complex (OKT3 directed against the CD3 complex, BMA031 directed against constant framework epitopes in the alpha/beta heterodimer). The accumulation of inositol phosphates was inhibited by prior incubation of the cells with cholera holotoxin. The inhibitory effect of cholera toxin (CT) was not cAMP mediated because forskolin (a direct activator of adenylate cyclase) did not mimic the inhibitory effect. When measuring phospholipase C (PLC) in a cell-free assay system by using [3H]inositol-labeled membranes, the enzyme could be stimulated by the poorly hydrolyzable GTP analogue guanosine 5'-O-(thiotriphosphate (GTP gamma S). Both anti-receptor antibodies augmented the GTP gamma S stimulatory effect, while the antibodies alone had no stimulatory capacity. In membranes from CT-pretreated cells, whereas the antibodies lost their stimulatory effect on PLC as in untreated cells, whereas the antibodies lost their stimulatory capacity in the presence of GTP gamma S. These data imply that CT exerts its inhibitory effect on signaling by acting at the receptor level while the PLC regulating G protein is not a target for CT-mediated alterations. This assumption is supported by the finding that in intact Jurkat cells CT, which ADP ribosylated only the alpha-subunit of the stimulatory G protein of the adenylate cyclase, led to a loss of the T cell antigen receptor complex from the cell surface as demonstrated by a decrease of receptor density using flow cytometry analysis. Receptor loss could not be achieved by forskolin treatment or incubation of the cells with the binding subunit of the toxin alone.

Cholera toxin modulation of angiotensin II-stimulated inositol phosphate production in cultured vascular smooth muscle cells

Activation of phospholipase C by angiotensin II in vascular smooth muscle has been postulated to be mediated by an unidentified GTP-binding protein (G-protein). Using a permeabilized preparation of myo-[3H]inositol-labelled cultured vascular smooth muscle cells, we examined the ability of a non-hydrolysable analogue of GTP, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), to stimulate inositol phosphate formation. GTP[S] (5 min exposure) stimulated inositol polyphosphate release by up to 3.8-fold in a dose-dependent manner, with an EC50 (concn. producing half-maximal stimulation) of approx. 50 microM. Inositol bisphosphate (IP2) and inositol trisphosphate (IP3) accumulations were also stimulated by NaF (5-20 mM). Furthermore, angiotensin II-induced inositol phosphate formation could be potentiated by a submaximal concentration of GTP[S] (10 microM), and this treatment appeared to interfere with the normal termination mechanism of the initial hormonal signal. The G-protein mediating angiotensin II-stimulated phospholipase C activation was insensitive to pertussis toxin at an exposure time and concentration which were sufficient to completely ADP-ribosylate all available substrate (100 ng/ml, 16 h). In contrast, a similar incubation with cholera toxin markedly inhibited angiotensin II-stimulated IP2 and IP3 release by 67 +/- 6% and 62 +/- 6% respectively. Cholera toxin appeared to inhibit angiotensin II stimulation of phospholipase C by a dual mechanism: it caused a 45% decrease in angiotensin II receptor number, and also inhibited G-protein transduction as assessed by GTP[S]-stimulated IP2 formation. This latter inhibition may be secondary to an increase in cyclic AMP, since it could be simulated by addition of dibutyryl cyclic AMP. Thus angiotensin II-stimulated inositol phosphate formation is cholera-toxin-sensitive, and is mediated by a pertussis-toxin-insensitive G-protein, which may be involved directly in termination of early signal generation.

Muscarinic receptor-stimulated phosphoinositide turnover in human SK-N-SH neuroblastoma cells: differential inhibition by agents that elevate cyclic AMP

The possibility that an increased intracellular concentration of cyclic AMP (cAMP) can regulate the extent of muscarinic receptor-stimulated phosphoinositide (PPI) turnover in the human neuroblastoma cell line SK-N-SH was examined. Addition of either forskolin (or its water-soluble analog, L-85,8051), theophylline, isobutylmethylxanthine, or cholera toxin, agents that interact with either the catalytic unit of adenylate cyclase, cAMP phosphodiesterase, or the guanine nucleotide binding protein linked to adenylate cyclase activation, resulted in a 45-181% increase in cAMP concentration and a 27-70% inhibition of carbachol-stimulated inositol phosphate release. Through the use of digitonin-permeabilized cells, the site of inhibition was localized to a step at, or distal to, the guanine nucleotide binding protein that regulates phospholipase C activity. In contrast, when intact SK-N-SH cells were exposed to prostaglandin E1, the ensuing increases in cAMP were not accompanied by an inhibition of stimulated PPI turnover. These differential effects of increased cAMP concentrations on stimulated PPI turnover may reflect the compartmentation of cAMP within SK-N-SH cells.

Influence of bacterial toxins and forskolin upon vasopressin-induced inositol phosphate accumulation in WRK 1 cells

The accumulation of inositol phosphates in WRK 1 cells, stimulated with a range of vasopressin concentrations, was diminished by prior exposure to cholera toxin or forskolin, whilst that observed in the presence of maximal concentrations of the hormone was enhanced in pertussis-toxin-treated cells. In the presence of [32P]NAD+, both cholera toxin and pertussis toxin provoked the labelling of peptides with approximate Mrs of 45,000 and 41,000 respectively in the membranes of WRK 1 cells. Exposure to cholera toxin or forskolin for 15-18 h enhanced cyclic AMP accumulation in these cells. The concentrations of these agents which provoked half-maximal cyclic AMP accumulation were similar to those required to diminish receptor-mediated inositol phosphate accumulation by 50%. In contrast, half-maximal ADP-ribosylation of the 45,000Mr peptide needed 100-fold greater concentrations of the toxin than were effective in provoking half-maximal inhibition of inositol phosphate accumulation. Cholera toxin or forskolin also reduced the maximal specific binding, to intact WRK 1 cells, of both [3H][Arg8]vasopressin and the V1a antagonist [3H][beta-mercapto-beta,beta-cyclopentamethylenepropionic acid,O-methyl-Tyr2, Arg8]vasopressin. The kinetics for the loss of this binding capacity following cholera-toxin treatment were very similar to those describing the diminution of vasopressin-stimulated inositol phosphate accumulation in the same cells.

Properties of membranous phospholipase C from WRK1 cell: sensitivity to guanylnucleotides and bacterial toxins

As previously described, WRK1 plasma membrane possesses a vasopressin-sensitive phospholipase C [G. Guillon et al., 1986, FEBS Lett. 196, 155-159]. In the present study, we examined the sensitivity of this enzyme to guanylnucleotides. GTP gamma S induces a time- and dose-dependent stimulation of Ins(1,4,5)P3 and Ins(1,4)P2 accumulation. No accumulation of InsP1, Ins(1,3,4)P3 or Ins(1,3,4,5)P4 occurred under similar conditions. Gpp(NH)p produced the same effect but was less potent. GTP and a nonhydrolyzable analogue of ATP, App(NH)p, were without effect. Calcium also stimulated the phospholipase C activity in a time- and dose-dependent manner. In the absence of calcium, the activity of GTP gamma S was considerably reduced. Physiological calcium concentrations (between 10(-8) and 10(-7) M), allowed maximal GTP gamma S stimulation of phospholipase C activity. In this system, the presence of vasopressin alone did not generate inositol phosphate accumulation. However, this hormone: (i) reduced the lag-time observed during GTP gamma S stimulation, (ii) increased the sensitivity of phospholipase C to GTP and to GTP gamma S, and (iii) did not modify the stimulation of phospholipase C induced by maximal doses of GTP gamma S. Unlike sodium fluoride, GTP gamma S elicited an irreversible activation of phospholipase C. Calcium, GTP gamma S and sodium fluoride stimulated the phospholipase C activity via mechanisms sharing a common step, since their maximal effects were not additive. Cholera toxin treatment, known to produce complete ADP-ribosylation of 'alpha s' subunits, partially reduced the basal and the maximal GTP gamma S-mediated stimulation of phospholipase C activity as well as that caused by vasopressin. This inhibition was not mimicked by treatment with either forskolin or pertussis toxin.

G-proteins and second messengers in mitogenesis

Growth factors stimulate mitogenesis via the generation of signal molecules or events. Transduction of these messages into the cell proceeds by the binding of the mitogen to a specific cell surface receptor which then stimulates the effector system (enzyme, ion channel, etc.) via a guanine nucleotide binding regulatory protein (G-protein). The specificity of a particular G-protein is a function of its unique alpha-subunit. This article reviews the effects of different growth factors upon second messenger generation and discusses the involvement of the different G-proteins in these signal transduction pathways.