Pertussis toxin blocks somatostatin's inhibition of stimulated cyclic AMP accumulation in anterior pituitary tumor cells

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.

Non-genomic mechanisms of glucocorticoid inhibition of adrenocorticotropin secretion: possible involvement of GTP-binding protein

We investigated non-genomic mechanisms of glucocorticoid negative feedback regulation on pituitary corticotroph cells using the AtT20 mouse corticotroph tumor cell line. A synthetic glucocorticoid dexamethasone (100 nM) potently suppressed forskolin-induced cAMP generation, adrenocorticotropin (ACTH) secretion, and proopiomelanocortin gene expression. When de novo gene expression was inhibited by actinomycin D (1 microM), dexamethasone still suppressed cAMP efflux and ACTH release, although less potently. Interestingly, under the same conditions, pretreatment of the cells with pertussis toxin (50 ng/ml) completely abolished the suppressive effect of dexamethasone on both parameters. These results suggest that non-genomic and genomic mechanisms are involved in the glucocorticoid negative regulation of ACTH expression, and a pertussis toxin-sensitive GTP-binding protein might, at least partly, participate in the non-genomic effect.

Somatostatin activates particulate guanylate cyclase in cultured rat mesangial cells

Although the ability of somatostatin (ST) to relax cultured rat mesangial cells has recently been described, the intimate cellular mechanisms responsible for this effect have not been adequately clarified. The present experiments were designed to test the hypothesis that cyclic GMP (cGMP) could be involved in the genesis of this relaxation. ST increased cGMP synthesis by cultured rat mesangial cells, in basal conditions and in the presence of isobutylmethylxanthine or zaprinast. This effect was dose-dependent, with a threshold value of about 1 nM and a maximal response at ST concentrations between 0.1 and 1 microM. This increased cGMP synthesis was dependent on the stimulation by ST of a particulate guanylate cyclase, as the synthesis of cGMP by a particulate membrane fraction obtained from the cells increased in the presence of ST. When the cGMP-specific phosphodiesterase of mesangial cells was blocked with zaprinast, the ST-dependent relaxation, assessed both by morphological and biochemical criteria, significantly increased with respect to the experiments performed without zaprinast. These results support a role for cGMP in the ST-dependent relaxation of cultured rat mesangial cells. The increased cGMP synthesis appears to be the consequence of the activation of some form of particulate guanylate cyclase.

Somatostatin modulation of adenosine receptor coupled G-protein subunits in the caudate nucleus of the rat

5'-(N-ethylcarboxamido)-adenosine (NECA) and N-[(R)-(phenylisopropyl)]-adenosine (PIA) were incubated in an adenylate cyclase assay of a particulate fraction of caudate-putamen tissue of the rat in order to examine the effect of somatostatin on adenosine receptors coupled adenylate cyclase subunits in vitro. Somatostatin was able to inhibit the enhancement of cyclic AMP formation induced by NECA in the presence of the hydrolysable guanine nucleotide guanosine-triphosphate. The adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine as well as the somatostatin receptor antagonist cyclo (7-aminoheptanoyl-Phe-D-Trp-Lys-O-benzyl-Thr) did not influence somatostatin induced inhibition of NECA-activated adenylate cyclase. Somatostatin did not modulate the effect mediated by the A-1 adenosine receptor agonist PIA. Both pertussis toxin and cholera toxin activated striatal adenylate cyclase acting on the guanine nucleotide regulatory subunit of the enzyme. The stimulation induced by pertussis toxin was antagonized by somatostatin, while in presence of cholera toxin somatostatin enhanced cyclic AMP formation. These results suggest that somatostatin acts through a stimulatory as well as an inhibitory guanine nucleotide regulatory protein subtype to affect probably postsynaptic A-2 adenosine receptor coupled adenylate cyclase activity.

Inwardly rectifying potassium conductances in AtT-20 clonal pituitary cells

We have detected two inwardly rectifying potassium conductances in AtT-20 clonal corticotrophs, a cell line derived from the mouse pituitary gland. An agonist-independent potassium conductance was activated by voltage steps negative to the reversal potential for potassium (VK) and was completely blocked by 1 mM barium in the bathing solution. The conductance was transient and inactivated completely with a time constant of about 80 ms. Reducing the external sodium concentration from 140 mM to 14 mM attenuated inactivation. In the presence of 100 nM somatostatin an inwardly rectifying conductance, which reversed at potentials close to VK, was also elicited. This conductance exhibited a maximal slope conductance that increased with increasing extracellular potassium. Rectification depends on both voltage and extracellular potassium concentration (Vm-VK). The inward current induced by somatostatin during voltage steps negative to VK was completely blocked by 1 mM extracellular barium, whereas the outward somatostatin-induced current activated at the holding current, which was about 30 mV positive to VK, was unaffected by 1 mM extracellular barium. The muscarinic agonist carbachol (10 microM) also induces an inwardly rectifying conductance of similar magnitude to that induced by somatostatin. Since the agonist-independent potassium current exhibits sodium-dependent inactivation, whereas the hormone-induced current does not inactivate, these currents are probably carried by different populations of potassium channels.

Peptidergic modulation of G-protein coupled cyclic-AMP accumulation in the rat caudate nucleus

Somatostatin, substance P, and vasoactive intestinal polypeptide were incubated in an adenylate cyclase assay with a particulate fraction of caudate-putamen tissue of the rat in order to examine the effect of the neuropeptides on G-protein coupled adenylate cyclase in vitro. Somatostatin induced an enhancement of cyclic AMP formation in presence of guanine nucleotides and cholera toxin but inhibited pertussis toxin and forskolin enzyme stimulation. Pertussis toxin and cholera toxin also depressed forskolin-induced stimulation as described previously. Somatostatin was able to antagonize these inhibitory effects of both toxins. On the contrary, substance P reduced GTP and cholera toxin stimulated striatal adenylate cyclase, without affecting forskolin activation. In our preparation, VIP did not influence basal adenylate cyclase activity or the stimulation by guanine nucleotides, cholera toxin, and pertussis toxin. VIP potently inhibited the enhancement of cyclic AMP formation by forskolin and completely antagonized the inhibitory effect of cholera toxin on forskolin activation. These results suggest that neuromodulatory effects of somatostatin, substance P, and VIP are mediated by the inhibitory as well as stimulatory guanine nucleotide proteins G-i and G-s coupled to an adenylate cyclase system.

Somatostatin acts through G-proteins on dopaminergic adenylate cyclase in the caudate-putamen of the rat

Somatostatin was incubated in an adenylate cyclase assay of a particulate fraction of caudate-putamen tissue of the rat in order to examine the effect of the peptide on D-1 receptor coupled adenylate cyclase in vitro. Somatostatin was able to enhance cyclic AMP formation in the presence of guanylylimidodiphosphate and guanosine-triphosphate. In contrast to this, somatostatin inhibited both dopamine and forskolin-stimulated cyclic AMP accumulation. Pertussis toxin and cholera toxin also depressed forskolin-induced stimulation. Somatostatin was found to antagonize these inhibitory effects of pertussis toxin and cholera toxin. The results suggest that somatostatin acts through a stimulatory as well as an inhibitory guanine nucleotide regulatory protein subtype to affect dopaminergic adenylate cyclase activity.

Iron inhibits D-1 dopamine receptor coupled adenylate cyclase via G-proteins in the caudate nucleus of the rat

The effects of iron ions (Fe(II)sulfate) on basal, forskolin, and dopamine-stimulated activity of adenylate cyclase in membrane preparations from caudate-putamen of the rat have been studied. Iron dose-dependently inhibited both basal and activated adenylate cyclase activity. In contrast to guanylylimidodiphosphate (Gpp(NH)p), guanosine triphosphate (GTP) was found to enhance this inhibitory effect of iron ions. In addition, cholera toxin was able to antagonize the inhibitory effect of iron on forskolin-activated adenylate cyclase. In our preliminary study we suggest an interaction between iron and the guanine nucleotide regulatory subunit. However, further studies are necessary.

Inhibitory effect of the somatostatin analog octreotide on rat pituitary tumor cell (GH3) proliferation in vitro

The effects of somatostatin-14 (SS-14) and the somatostatin-analog octreotide (SMS 201-995, Sandostatin) on proliferation of GH3 pituitary tumor cells were investigated in vitro. SMS 201-995 exerted a significant, but transient, inhibition on GH3 cell growth which reached a maximum at 24 h and was no longer detectable at 48 h. The concentration that evoked the strongest inhibitory effect was 10 nM SMS 201-995, while lower and higher doses resulted in a less pronounced effect. The inhibitory effect SMS 201-995 exerted on cell proliferation was associated with a dose- and time-related reduction in both c-myc and c-fos mRNA levels. SS-14 had no noteworthy influence on either cell proliferation or c-myc and c-fos protooncogene expression. These data demonstrate that SS-analogs transiently inhibit pituitary tumor cell proliferation in vitro.

The effect of vasoactive intestinal polypeptide (VIP) on forskolin stimulated adenylate cyclase in the caudate-putamen of the rat

Vasoactive intestinal polypeptide (VIP) was incubated in an adenylate cyclase assay with a particulate fraction of caudate-putamen (CP) tissue of the rat in order to examine the effect of the peptide on forskolin-activated adenylate cyclase in vitro. Forskolin induced an enhancement of cyclic AMP formation that was mediated by an effect on catalytic subunit and stimulatory guanine nucleotide regulatory protein (Ns). In our preparation, VIP did not influence basal adenylate cyclase activity or the stimulation by dopamine and sodium fluoride but, in the absence of guanylylimidodiphosphate (guanosine 5'-(beta, y-imido)-triphosphate) VIP inhibited the forskolin-stimulation of the enzyme in a noncompetitive manner. Met-encephalin, acting on a D-2 receptor-coupled putative inhibitory guanine nucleotide regulatory protein (Ni), inhibited the adenylate cyclase activity stimulated by forskolin to a slightly greater extent than VIP. When assayed together, these inhibition effects were additive, implying that the peptide receptors are not identical. The Ni-antagonist, MnCl2 completely blocked the inhibition of met-encephalin but had no significant effect on VIP-induced inhibition. In addition, pertussis toxin did not influence the effect of VIP on forskolin-stimulation in contrast to cholera toxin which did antagonize the VIP effect via the stimulatory guanine nucleotide regulatory protein (Ns). Furthermore, specific D-1 and D-2 dopaminergic receptor antagonists alpha(+)-flupentixol and spiperone had no effect on VIP-modulated forskolin-stimulated adenylate cyclase activity. These results suggest that the neuromodulatory effect of VIP is mediated by a Ns distinct from those involved in several adenylate cyclase pools sensitive to stimulation by dopamine and VIP in the rat striatum.

A potassium conductance contributes to the action of somatostatin-14 to suppress ACTH secretion

Somatostatin activates an inwardly rectifying potassium conductance in AtT-20 clonal corticotrophs, a cell line derived from the mouse pituitary gland. The action of somatostatin is blocked by pertussis toxin indicating that a GTP-binding protein couples the somatostatin receptor to the potassium channel. The potassium conductance is depressed by cesium. Cesium also attenuates the suppression of adrenocorticotropin hormone secretion by somatostatin suggesting that the increase in potassium conductance plays a role in this action of somatostatin.

Tonic inhibition of adenylate cyclase in cultured hamster Sertoli cells

Sertoli cells cultured from immature hamsters respond to FSH with a dose-related increase in cAMP accumulation. Pertussis toxin acts synergistically with FSH to stimulate cAMP accumulation. This effect of pertussis toxin indicates that Sertoli cell adenylate cyclase is under tonic inhibition due to the activity of the Ni inhibitory transducer. The acetylcholine receptor antagonists atropine or tubocurarine, or the opioid antagonist naltrexone, have no effect on the FSH-induced stimulation of cAMP accumulation, suggesting that neither acetylcholine nor opioids are responsible for the inhibition of Sertoli cell cyclase. While exogenous adenosine is inhibitory, adenosine deaminase augments the ability of FSH to stimulate cAMP accumulation, but not to the level of pertussis toxin. This indicates that the Sertoli cells produce endogenous adenosine that is at least partially responsible for the tonic inhibition of adenylate cyclase. Other possibilities for the tonic inhibition of cyclase include other Sertoli cell products, germ cell products, peritubular cell products or an action of FSH itself through both stimulatory and inhibitory transducers.

Cholinergic inhibition of cAMP accumulation in Sertoli cells cultured from immature hamsters

Acetylcholine inhibits FSH-induced cAMP accumulation in cultured Sertoli cells from immature hamsters. This action of acetylcholine is mimicked by muscarinic cholinergic agonists with a rank order of carbachol greater than acetylcholine greater than arecoline greater than pilocarpine. The carbachol-induced inhibition of stimulated cAMP accumulation is blocked by atropine greater than pirenzepine but not by d-tubocurarine, indicating an apparent muscarinic receptor similar to that found in other peripheral tissues. The fact that pirenzepine is less effective as an inhibitor of the carbachol effect than atropine further defines the muscarinic effect as of the M2 subtype. The ability of carbachol to inhibit FSH-induced cAMP accumulation is blocked by pertussis toxin, which inhibits the action of the Ni inhibitory transducer of adenylate cyclase. These data indicate that cultured Sertoli cells from immature hamsters contain an M2 type muscarinic cholinergic receptor that is negatively coupled to the adenylate cyclase system through the inhibitory Ni transducer.

Thyrotropin effect on the availability of Ni regulatory protein in FRTL-5 rat thyroid cells to ADP-ribosylation by pertussis toxin

Incubation of FRTL-5 rat thyroid cell membranes with [32P]NAD and pertussis toxin results in the specific ADP-ribosylation of a protein of about 40 kDa. This protein has the same molecular mass of the alpha i subunit of the adenylate cyclase regulatory protein Ni and is distinct from proteins ADP-ribosylated by cholera toxin in the same membranes. Prior treatment of FRTL-5 cells with pertussis toxin results in the ADP-ribosylation of Ni, as indicated by the loss of the toxin substrate in the ADP-ribosylation assay performed with membranes prepared from such cells. Preincubation of FRTL-5 cells with thyrotropin causes the same loss; cholera toxin has no such effect. Pertussis toxin, as do thyrotropin and cholera toxin, increases cAMP levels in FRTL-5 cells. Forskolin together with thyrotropin, cholera toxin or pertussis toxin causes a further increase in cAMP levels. Pertussis toxin and thyrotropin are not additive in their ability to increase adenylate cyclase activity, whereas both substances are additive with cholera toxin. A role of Ni in the thyrotropin regulation of the adenylate cyclase activity in thyroid cells is proposed.

Is dopamine-induced inhibition of adenylate cyclase involved in the autoreceptor-mediated negative control of tyrosine hydroxylase in striatal dopaminergic terminals?

The mechanism of the negative control of tyrosine hydroxylase (TH) activity induced by the stimulation of presynaptic 3,4-dihydroxyphenylethylamine (dopamine, DA) autoreceptors was investigated using rat striatal slices and synaptosomes incubated under control ([ K+] = 4.8 mM) or depolarizing ([ K+] = 60 mM) conditions. The stimulation of DA autoreceptors by 7-hydroxy-2-(di-n-propylamino)tetralin (1 microM 7-OH-DPAT) produced a significant decrease in TH activity extracted from striatal slices maintained under control conditions. This effect was associated with the complete conversion of TH into an enzyme form with a low affinity for its pterin cofactor (Km approximately 0.80 mM). Furthermore, compared to TH extracted from control tissues, that from 7-OH-DPAT-exposed striatal slices was more sensitive to the stimulatory effects of exogenous heparin and cyclic AMP-dependent phosphorylation. Such changes were opposite to those induced by incubating striatal slices with the adenylate cyclase activator forskolin. Indeed, forskolin treatment completely converted TH into an enzyme form with a high affinity for its pterin cofactor (Km approximately 0.16 mM). Such conversion was associated with a shift in the optimal pH for TH activity from 5.8 (control) to 7.2 (forskolin). Under depolarizing conditions, the blockade by (-)-sulpiride of the stimulation of DA autoreceptors by endogenous DA was associated with a marked activation of TH. Modifications of enzymatic characteristics triggered by (-)-sulpiride were then similar to those induced by forskolin treatment. These data suggest that presynaptic DA autoreceptors modulate the activity of TH by controlling the degree of cyclic AMP-dependent phosphorylation of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Pertussis toxin mediates ADP-ribosylation of pituitary membrane proteins

Pertussis toxin catalyzes the ADP-ribosylation of the inhibitory subunit (Ni) of adenylate cyclase. Despite several studies which demonstrate that pertussis toxin influences cyclic AMP accumulation and hormone secretion in normal anterior pituitary cells, the target protein(s) for this toxin in these cells has not been identified. We have examined pertussis toxin mediated ADP-ribosylation in membrane preparations of tumor-derived (235-1, GH4C1, GH3) and normal anterior pituitary cells. Autoradiograms of SDS gels reveal that in the presence of [32P]NAD and pertussis toxin, a 40 kilodalton protein band was labeled in membrane preparations from cells cultured with vehicle. Such labeling was diminished when the cells were exposed to pertussis toxin (35 ng/ml) for 18 hours. Similar results were found in both tumor-derived and normal (monkey and rat) anterior pituitary cells. The pertussis toxin specific band was further resolved into two bands of approximately 39 and 41 kilodaltons. Autoradiograms of two dimensional gels revealed two ADP-ribosylated spots with isoelectric points of 5.7 and 6.2, although the molecular weights appeared identical (approx. 40 kilodaltons). Cholera toxin, which catalyzes the ADP-ribosylation of a 45 kilodalton protein did not prevent labeling of the pertussis toxin-specific band(s) in cells pretreated with cholera toxin. These results suggest that pertussis toxin specifically mediates ADP-ribosylation of the Ni protein in normal anterior pituitary cells.

Bacterial toxins and the role of ADP-ribosylation

Studies on the pathogenesis of "Whooping Cough" and cholera have resulted in the discovery of important pathways in the regulation of cellular metabolism leading to the realization of a complex family of proteins that appear to play central roles in the regulation of hormonal responses and which utilize guanine nucleotides in their mechanism of action. The fact that these bacterial toxins interfere so precisely with the complex regulation of eukaryotic cellular metabolism and the discovery of analogous enzymes within the cytosol of eukaryotic cells suggests that ADP-ribosylation may be an important pathway through which the cell can establish its responsiveness to its environment. Clearly, future work directed towards the role of ADP-ribosylation and towards the mechanisms of the regulation of these endogenous ADP-ribosyltransferases and lyases may provide great insights into the mechanisms of hormone action.

Pertussis toxin potentiates anesthesia-induced renin secretion

The plasma renin concentration was measured after anesthesia in control and in pertussis toxin-treated rats. The anesthetics increased renin secretion and this effect was markedly magnified in rats treated with pertussis toxin. Propranolol partially blocked the increase in plasma renin concentration produced by anesthetics. It is concluded that the adrenergic system is involved in this effect and that pertussis toxin magnifies it by potentiating the beta-adrenergic action.

The inhibitory guanine nucleotide-binding regulatory subunit of adenylate cyclase has an adenylate cyclase-independent modulatory effect on ACTH secretion from mouse pituitary tumor cells

Somatostatin inhibits agonist-stimulated cAMP synthesis and ACTH secretion from mouse pituitary tumor cells. It also decreases basal hormone release without affecting cAMP levels and inhibits ACTH secretion in response to agonists whose action is independent of prior cAMP synthesis. These inhibitory effects are attenuated by pertussis toxin, suggesting that the inhibitory guanine nucleotide-binding regulatory subunit of adenylate cyclase modulates effectors, other than adenylate cyclase, during transduction of negative hormonal signals.

Effect of pertussis toxin on the adrenergic regulation of plasma renin activity

Basal plasma renin activity (PRA) was not modified by pertussis toxin administration. On the contrary, the modulation of PRA by adrenergic amines was markedly affected by the toxin. Administration of epinephrine did not modified PRA in the controls but markedly increased it in toxin-treated rats. This effect of epinephrine was reproduced in control rats when yohimbine was given before the catecholamine. Clonidine decreased PRA to a much more significant extent in control rats than in animals treated with the toxin. Isoproterenol stimulated PRA to a greater level in toxin-treated rats. Our data indicates that pertussis toxin blocks the alpha 2-adrenergic modulation of renin release and magnifies the ability of beta adrenergic activation to stimulate PRA.

Somatostatin desensitization in rat anterior pituitary cells

The possibility that desensitization to the inhibitory effects of somatostatin (SS) might develop following chronic exposure to this tetradecapeptide was examined in cultured rat anterior pituitary cells. Pretreatment with 1 microM SS for 48 h caused a shift in the IC50 of SS to inhibit 3-isobutyl-1-methylxanthine (IBMX) or growth hormone-releasing factor (GRF)-induced growth hormone (GH) and TRH + IBMX-induced thyroid-stimulating hormone (TSH) release by more than 2 orders of magnitude. Refractoriness developed after 12 h of exposure to doses of SS of 10 nM or more and became maximal at 48 h. Restoration of SS responsiveness followed a similar time-course upon removal of the peptide. In superfused cells, 10 nM SS lowered GH secretion rates to less than 5 ng/min within 2 h, but GH release began to rise after 16 h despite the continued presence of SS. However, when somatostatin was delivered in pulses, it remained fully effective for more than 36 h. Somatostatin-28 was also capable of inducing refractoriness in cultured pituitary cells. However, cells made refractory to either SS-14 or SS-28 were not made refractory to the same extent to the other form of somatostatin. These results indicate that the pituitary can become desensitized to the inhibitory actions of somatostatin just as it does to the stimulatory actions of the other hypothalamic releasing hormones.

Stress hormones: their interaction and regulation

Stress stimulates several adaptive hormonal responses. Prominent among these responses are the secretion of catecholamines from the adrenal medulla, corticosteroids from the adrenal cortex, and adrenocorticotropin from the anterior pituitary. A number of complex interactions are involved in the regulation of these hormones. Glucocorticoids regulate catecholamine biosynthesis in the adrenal medulla and catecholamines stimulate adrenocorticotropin release from the anterior pituitary. In addition, other hormones, including corticotropin-releasing factor, vasoactive intestinal peptide, and arginine vasopressin stimulate while the corticosteroids and somatostatin inhibit adrenocorticotropin secretion. Together these agents appear to determine the complex physiologic responses to a variety of stressors.

Cellular mechanisms regulating adrenocorticotropin release

Using a tumor cell line of the mouse anterior pituitary (AtT-20/D16-16), the various factors regulating ACTH release and the intracellular mechanisms mediating this response were investigated. CRF, catecholamines and VIP stimulate ACTH release whereas glucocorticoids and SRIF block secretion. Glucocorticoids block both ACTH synthesis and release. SRIF acts through multiple mechanisms to prevent stimulated ACTH release. Cyclic AMP and Ca++ are important second messengers in the receptor mediated release of ACTH but other mediators may also be involved. The interaction of these various CRF-like substances and inhibitors of ACTH release may result in a fine-tune regulation of corticotroph activity Such regulation may be important in the organism response to stress.