Atrial natriuretic peptide effects in AtT-20 pituitary tumour cells

Homologous and heterologous desensitization of guanylyl cyclase-B signaling in GH3 somatolactotropes

The guanylyl cyclases, GC-A and GC-B, are selective receptors for atrial and C-type natriuretic peptides (ANP and CNP, respectively). In the anterior pituitary, CNP and GC-B are major regulators of cGMP production in gonadotropes and yet mouse models of disrupted CNP and GC-B indicate a potential role in growth hormone secretion. In the current study, we investigate the molecular and pharmacological properties of the CNP/GC-B system in somatotrope lineage cells. Primary rat pituitary and GH3 somatolactotropes expressed functional GC-A and GC-B receptors that had similar EC50 properties in terms of cGMP production. Interestingly, GC-B signaling underwent rapid homologous desensitization in a protein phosphatase 2A (PP2A)-dependent manner. Chronic exposure to either CNP or ANP caused a significant down-regulation of both GC-A- and GC-B-dependent cGMP accumulation in a ligand-specific manner. However, this down-regulation was not accompanied by alterations in the sub-cellular localization of these receptors. Heterologous desensitization of GC-B signaling occurred in GH3 cells following exposure to either sphingosine-1-phosphate or thyrotrophin-releasing hormone (TRH). This heterologous desensitization was protein kinase C (PKC)-dependent, as pre-treatment with GF109203X prevented the effect of TRH on CNP/GC-B signaling. Collectively, these data indicate common and distinct properties of particulate guanylyl cyclase receptors in somatotropes and reveal that independent mechanisms of homologous and heterologous desensitization occur involving either PP2A or PKC. Guanylyl cyclase receptors thus represent potential novel therapeutic targets for treating growth-hormone-associated disorders.

Calcium homeostasis in a clonal pituitary cell line of mouse corticotropes

Calcium homeostasis was studied following a depolarization-induced transient increase in [Ca2+]i in single cells of the clonal pituitary cell line of corticotropes, AtT-20 cells. The KCl-induced increase in [Ca2+]i was blocked in (i) extracellular calcium-deficient solutions, (ii) external cobalt (2.0 mM), (iii) cadmium (200 µM), and (iv) nifedipine (2.0 µM). The mean increase in [Ca2+]i in single cells in the presence of an uncoupler of mitochondrial function [carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 1 µM] was 54 ± 13 nM (n = 9). The increase in [Ca2+]i produced by FCCP was greater either during or following a KCl-induced [Ca2+]i load. However, FCCP did not significantly alter the clearance of calcium during a KCl-induced rise in [Ca2+]i. Fifty percent of the cells responded to caffeine (10 mM) with an increase in [Ca2+]i (191 ± 24 nM; n = 21) above resting levels; this effect was blocked by ryanodine (10 µM). Thapsigargin (2 µM) and 2,5 di(-t-butyl)-1,4 hydroquinone (BuBHQ, 10 µM) produced increases in [Ca2+]i (47 ± 11 nM, n = 6 and 22 ± 4 nM, n = 8, respectively) that increased cell excitability. These results support a role for mitochondria and sarco-endoplasmic reticulum calcium stores in cytosolic [Ca2+]i regulation; however, none of these organelles are primarily responsible for the return of [Ca2+]i to resting levels following this KCl-induced [Ca2+]i load.Key words: calcium homeostasis, intracellular calcium stores, anterior pituitary cells, mitochondria.

Activation of guanylate cyclase by natriuretic peptides in mouse pituitary AtT20 cells is influenced by phosphorylation of ANP

The discovery of free and membrane-bound ectokinases raises the question whether phosphorylation is another mechanism to modulate the action of distinct neuropeptides. Atrial-natriuretic-peptide (ANP) which is widespread found in the central nervous system (CNS) and involved in the modulation of stress reactions and emotional states like anxiety contains a recognition-motif for cAMP-dependent protein kinase A. We investigated the effect of phosphorylation of ANP and C-type natriuretic peptide (CNP), a related peptide without phosphorylation site, on their ability to activate their receptors in mouse pituitary AtT20 cells by measuring the formation of cyclic guanosinmonophosphate (cGMP). Phosphorylation with protein kinase A inactivated ANP. Coincubation experiments adding adenosintriphosphate (ATP), ATP-analogues or inhibitors of protein kinases to the medium pointed to the presence of an intrinsic protein kinase A like ectokinase-activity on AtT20 cells. The activity of CNP was unaffected in these experiments. Phosphorylation by ectokinases may be a physiological mechanism to regulate the biological activity of ANP in different tissues, such as pituitary and CNS.

Characterisation of the effects of natriuretic peptides upon ACTH secretion from the mouse pituitary

The involvement of natriuretic peptides in the regulation of ACTH secretion in mice hemi-pituitary preparations was investigated. Atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) all inhibited CRF (10(-9) M)-evoked ACTH secretion over a concentration range of 10(-12)-10(-10) M and also stimulated cyclic GMP accumulation over a concentration range of 10 (-8)-10(-5) M. CNP was the most effective both in the inhibition of ACTH secretion and in the stimulation of cyclic GMP accumulation. Coincubation of hemi-pituitaries with 8bromo-cyclic GMP (10(-4) M) completely inhibited CRF (10(-9) M)-evoked ACTH secretion. Northern blot analysis revealed that all three major isoforms of the natriuretic peptide receptors are expressed in the mouse pituitary. These results demonstrate that natriuretic peptides do inhibit CRF-stimulated ACTH secretion from mouse pituitary preparations. A role for cGMP in mediating this effect on hormone secretion is indicated but the discrepancy between the efficacies of natriuretic peptides in inhibiting the secretory response and stimulating cyclic GMP accumulation suggest a more complicated stimulus-secretion coupling pathway is in operation.

Identification of cGMP-dependent protein kinase and its specific substrates in the anterior pituitary

In the anterior pituitary, cGMP is produced in response to a number of stimuli, but intracellular events distal to cGMP production are obscure. Since cGMP-dependent protein kinase (PKG) is a major effector of cGMP actions in other tissues we have determined whether PKG and its specific substrates might be present and responsive to external signals in the ovine anterior pituitary. Photoaffinity labelling with [32P]cGMP revealed a specific 78 kDa protein in ovine anterior pituitary that comigrated with purified bovine lung PKG-I. PKG in protein extracts from anterior pituitary or cultured anterior pituitary cells was enriched by DEAE ion-exchange chromatography and assayed for activity. Both tissue and cultured cells had a relatively high PKG activity by comparison with aortic smooth muscle (known high activity) and brain (known low activity). Subcellular distribution studies showed that in anterior pituitary, aortic and brain, PKG activity was present in both cytosol and triton-extracted membrane fractions, while in platelets the activity was associated with only the membrane fraction. To determine if this PKG might be responsive to extracellular signals an activity ratio assay was used. Incubation of cultured cells with atrial natriuretic peptide (ANP) and sodium nitroprusside, activators of membrane and cytosolic guanylate cyclases respectively, increased the activity of PKG. To determine events distal to PKG activation, a search for potential substrates of PKG was performed. Few substrates were detectable upon addition of purified PKG to tissue lysates due to the high background activity of endogenous protein kinases in the anterior pituitary. However, 19 substrates of PKG were detected in heat-stable and 14 in acid-soluble protein extracts of the anterior pituitary, in which background phosphorylation was almost abolished. After partial purification through Q-Sepharose ion-exchange chromatography some of these proteins were preferentially phosphorylated by addition of PKG-I, while the others were additionally substrates of exogenous cAMP-dependent protein kinase (PKA) or Ca2+ and phospholipid-dependent protein kinase (PKC). A 132-kDa substrate showed an identical phosphopeptide map to a PKG substrate previously described in vascular smooth muscle and platelets. These data demonstrate for the first time the presence of functional PKG activity and multiple PKG substrates in the anterior pituitary where they may play a role in mediating the intracellular actions of cGMP.

Effects of mastoparan upon the late stages of the ACTH secretory pathway of AtT-20 cells

1. The mouse AtT-20/D16-16 anterior pituitary tumour cell line was used as a model system for the study of the effects of mastoparan upon the late stages of the adrenocorticotrophin (ACTH) secretory pathway. 2. Mastoparan (10(-8)-10(-5) M), an activator of heterotrimeric guanosine 5'-triphosphate binding proteins (G-proteins), stimulated ACTH secretion from electrically-permeabilized AtT-20 cells in a concentration-dependent manner in the effective absence of calcium ions with a threshold of 10(-6) M. Guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) (10(-8)-10(-4) M) also stimulated ACTH secretion from electrically-permeabilized AtT-20 cells in a concentration-dependent manner in the effective absence of calcium ions with a threshold of 10(-6) M. This GTP-gamma-S-evoked secretion is consistent with previous studies which demonstrated that a G-protein, termed GE, mediates calcium evoked ACTH secretion from AtT-20 cells. GTP-gamma-S-evoked secretion however was not as great as that obtained in response to mastoparan. 3. Both mastoparan (10(-5) M) and GTP-gamma-S (10(-4) M) stimulated ACTH secretion from electrically-permeabilized AtT20 cells in a time-dependent manner. A time of 30 min was adopted as the standard incubation period for the study of both mastoparan and GTP-gamma-S-stimulated ACTH secretion from permeabilized AtT-20 cells. 4. Mastoparan (10(-8)-10(-5) M) stimulated ACTH secretion from permeabilized AtT-20 cells to the same extent in the presence and absence of the protein kinase C (PKC) inhibitor, chelerythrine chloride (10(-5) M). 5. Mastoparan (10-8 10-5 M)-stimulated ACTH secretion from permeabilized AtT-20 cells was significantly reduced in the presence of guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S, 10-4 M).6. The mastoparan analogue, Mas-7 (10-8-10-5 M) stimulated ACTH secretion from permeabilized AtT-20 cells to a greater extent than mastoparan (10-8 10-5 M) however, the mastoparan analogue Mas-17 (10-8- 10-5 M) had no effect upon ACTH secretion from permeabilized AtT-20 cells.7. Mastoparan (10-8-10-5 M) stimulated ACTH secretion from permeabilized AtT-20 cells in the presence and absence of ATP, normally present in the standard permeabilization medium at a concentration of 5 mM. Mastoparan (10-8- 10-5 M)-stimulated ACTH secretion as well as control secretion was reduced when ATP was omitted.8. The results of the present study demonstrate that mastoparan stimulated ACTH secretion from permeabilized AtT-20 cells and displayed characteristics consistent with calcium ion- and GTP-y-gamma-S-stimulated ACTH secretion from permeabilized AtT-20 cells. This suggests that in permeabilized AtT-20 cells, mastoparan directly activates GE and that this G-protein may be a heterotrimeric G-protein. This study also suggests mastoparan may be a useful alternative to GTP-gamma-S as a means of directly activating GE.

Characterization of natriuretic peptide receptor subtypes in the AtT-20 pituitary tumour cell line

Receptors for the natriuretic peptide family have been characterized in the adrenocorticotrophic hormone (ACTH)-secreting AtT-20 pituitary tumour cell line. Northern blot analysis detected mRNA transcripts for the guanylate cyclase-linked GC-B receptor subtype. There was no evidence for the expression of either guanylate cyclase-linked GC-A receptor or atrial natriuretic peptide (ANP)-C (clearance) receptor mRNAs. Cyclic GMP production in AtT-20 cells was stimulated up to 200-fold by C-type natriuretic peptide (CNP), which was 10- and 20 times as effective as equivalent concentrations of brain natriuretic peptide and ANP respectively. Cyclic GMP dose-response curves to CNP failed to show any signs of saturation even at concentrations up to 30 microM, indicating a relatively low affinity of CNP for the GC-B receptor. Although CNP induced large stimulations in cyclic GMP production, specific binding of [125I-Tyr0]CNP could not be demonstrated in AtT-20 cells. The absence of specific binding with this radiolabelled analogue is possibly due to a reduced affinity for the GC-B receptor, as CNP analogues with N-terminal modifications such as [Tyr0]CNP and [127I-Tyr0]CNP exhibited reduced abilities to stimulate cyclic GMP production in these cells. Despite elevating cyclic GMP levels, CNP had no effect on basal or corticotrophin-releasing factor-stimulating ACTH release from the cells. These results show that the guanylate cyclase-coupled GC-B receptor is the only natriuretic peptide receptor subtype expressed in AtT-20 cells. Although CNP can markedly stimulate cyclic GMP production in these cells, there is incomplete expression of the normal natriuretic peptide-induced inhibitory pathway of ACTH secretion at some point distal to the production of cyclic GMP.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.