Storage and secretion of beta-endorphin and related peptides by mouse pituitary tumor cells: regulation by glucocorticoids

Guanabenz ameliorates disease in vanishing white matter mice in contrast to sephin1

OBJECTIVE

Vanishing white matter (VWM) is a leukodystrophy, characterized by stress-sensitive neurological deterioration and premature death. It is currently without curative treatment. It is caused by bi-allelic pathogenic variants in the genes encoding eukaryotic initiation factor 2B (eIF2B). eIF2B is essential for the regulation of the integrated stress response (ISR), a physiological response to cellular stress. Preclinical studies on VWM mouse models revealed that deregulated ISR is key in the pathophysiology of VWM and an effective treatment target. Guanabenz, an α2-adrenergic agonist, attenuates the ISR and has beneficial effects on VWM neuropathology. The current study aimed at elucidating guanabenz's disease-modifying potential and mechanism of action in VWM mice. Sephin1, an ISR-modulating guanabenz analog without α2-adrenergic agonistic properties, was included to separate effects on the ISR from α2-adrenergic effects.

METHODS

Wild-type and VWM mice were subjected to placebo, guanabenz or sephin1 treatments. Effects on clinical signs, neuropathology, and ISR deregulation were determined. Guanabenz's and sephin1's ISR-modifying effects were tested in cultured cells that expressed or lacked the α2-adrenergic receptor.

RESULTS

Guanabenz improved clinical signs, neuropathological hallmarks, and ISR regulation in VWM mice, but sephin1 did not. Guanabenz's effects on the ISR in VWM mice were not replicated in cell cultures and the contribution of α2-adrenergic effects on the deregulated ISR could therefore not be assessed.

INTERPRETATION

Guanabenz proved itself as a viable treatment option for VWM. The exact mechanism through which guanabenz exerts its ameliorating impact on VWM requires further studies. Sephin1 is not simply a guanabenz replacement without α2-adrenergic effects.

60 YEARS OF POMC: Biosynthesis, trafficking, and secretion of pro-opiomelanocortin-derived peptides

Pro-opiomelanocortin (POMC) is a prohormone that encodes multiple smaller peptide hormones within its structure. These peptide hormones can be generated by cleavage of POMC at basic residue cleavage sites by prohormone-converting enzymes in the regulated secretory pathway (RSP) of POMC-synthesizing endocrine cells and neurons. The peptides are stored inside the cells in dense-core secretory granules until released in a stimulus-dependent manner. The complexity of the regulation of the biosynthesis, trafficking, and secretion of POMC and its peptides reflects an impressive level of control over many factors involved in the ultimate role of POMC-expressing cells, that is, to produce a range of different biologically active peptide hormones ready for action when signaled by the body. From the discovery of POMC as the precursor to adrenocorticotropic hormone (ACTH) and β-lipotropin in the late 1970s to our current knowledge, the understanding of POMC physiology remains a monumental body of work that has provided insight into many aspects of molecular endocrinology. In this article, we describe the intracellular trafficking of POMC in endocrine cells, its sorting into dense-core secretory granules and transport of these granules to the RSP. Additionally, we review the enzymes involved in the maturation of POMC to its various peptides and the mechanisms involved in the differential processing of POMC in different cell types. Finally, we highlight studies pertaining to the regulation of ACTH secretion in the anterior and intermediate pituitary and POMC neurons of the hypothalamus.

Reciprocal regulation of calcium dependent and calcium independent cyclic AMP hydrolysis by protein phosphorylation

The hydrolysis of cyclic nucleotide second messengers takes place through multiple cyclic nucleotide phosphodiesterases (PDEs). The significance of this diversification is not fully understood. Here we report the differential regulation of low K(m) Ca2+-activated (PDE1C) and Ca2+-independent, rolipram-sensitive (PDE4) PDEs by protein phosphorylation in the neuroendocrine cell line AtT20. Incubation of cells with 8-(4-chlorophenylthio)-cyclic AMP (CPT-cAMP) enhanced PDE4 and reduced PDE1C activity. These effects were blocked by H89 indicating mediation by cAMP-dependent protein kinase (PKA), furthermore in broken cell preparations PKA produced the same reciprocal changes of PDE activities. Calyculin A, an inhibitor of protein phosphatases 1 and 2 A, stimulated PDE4 and enhanced the inhibitory effect of CPT-cAMP on PDE1C. The reduction of PDE1C activity was characterized by a marked attenuation of the activation by Ca2+/calmodulin. Stimulation of PDE4 activity by CPT-cAMP or calyculin A was attributable to PDE4D3 and these effects could also be reproduced in human embryonic kidney cells expressing epitope-tagged PDE4D3. Together, these data show reciprocal regulation of PDE1C and PDE4D by PKA, which represents a novel scheme for plasticity in intracellular signalling.

A direct inhibitory action of prostaglandins upon ACTH secretion at the late stages of the 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 prostaglandins upon the late stages of the adrenocorticotrophin (ACTH) secretory pathway. 2. Calcium (1 nM - 100 microM), guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S) (1 - 100 microM) and mastoparan (1 and 10 microM) all stimulated ACTH secretion from permeabilized AtT-20 cells in a concentration-dependent manner. GTP-gamma-S and mastoparan stimulated ACTH secretion from permeabilized cells in the absence of calcium. Co-incubation with prostaglandins E(1) and E(2) (PGE(1), PGE(2)) (10 microM) but not prostaglandin F(2 alpha) (PGF(2 alpha)) (10 microM) significantly inhibited calcium-, GTP-gamma-S and mastoparan-evoked secretion by 30 - 50%. 3. The effects of PGE(1) and PGE(2) upon GTP-gamma-S (100 microM)-, calcium (10 microM)- and mastoparan (10 microM)-evoked secretion were concentration-dependent. PGE(1) significantly inhibited GTP-gamma-S- and calcium-evoked secretion at concentrations of PGE(1) above 1 microM but mastoparan-evoked secretion only at the highest concentration of PGE(1) investigated (10 microM). PGE(2) was much more potent than PGE(1) and significantly inhibited GTP-gamma-S- and calcium-evoked secretion at 10 nM and above and mastoparan-evoked secretion above 1 microM. 4. The inhibitory effects of PGE(1) and PGE(2) upon calcium-, GTP-gamma-S- and mastoparan-stimulated ACTH secretion from permeabilized cells were pertussis toxin (PTX) sensitive. 5. In intact cells PGE(1), PGE(2) and PGF(2 alpha) (1 nM - 10 microM) acting singly had little or no effect upon ACTH secretion. However, only PGE(2) (1 nM - 10 microM) significantly inhibited corticotrophin-releasing factor-41 (CRF-41) (100 nM)-evoked secretion in a concentration dependent manner. 6. The present study finds that prostaglandins of the E series exert an inhibitory action, via a pertussis toxin-sensitive GTP-binding (G)-protein, in the late stages of the ACTH secretory pathway distal to the G-exocytosis (Ge)/calcium point of control.

Over-expression of NCS-1 in AtT-20 cells affects ACTH secretion and storage

The effect of over-expressing neuronal calcium sensor 1 (NCS-1) upon stimulated adrenocorticotrophin (ACTH) secretion was studied in AtT-20 cells. Stably-transfected AtT-20 cell lines over-expressing NCS-1 were obtained and compared to wild type AtT-20 cells. Corticotrophin releasing factor (CRF-41)-stimulated ACTH secretion from NCS-1 over-expressing cells was significantly reduced from that obtained in wild type AtT-20 cells. The effects of other stimulants of ACTH secretion from wild type AtT-20 cells were not attenuated in NCS-1 over-expressing cells. Calcium, guanosine 5'-O-(3'-thiotriphosphate) (GTP-gamma-S) and mastoparan stimulated ACTH secretion from permeabilised wild type AtT-20 and NCS-1 over-expressing AtT-20 cells with significantly greater ACTH secretion obtained in NCS-1 over-expressing cells. This study shows that in intact cells over-expression of NCS-1 reduces exocytotic ACTH release, while in permeabilised cells increases ACTH release. NCS-1 has multiple cellular targets and that directly and indirectly via these targets acts to increase the releasable ACTH pool while inhibiting CRF-41 stimulus-secretion coupling.

Association of B lymphocyte antigen receptor polypeptides with multiple chaperone proteins

The B cell antigen receptor (BCR) is comprised of four different polypeptides, immunoglobulin (Ig) heavy chain, Ig light chain, and the two signaling subunits of this receptor, Ig-alpha and Ig-beta. These four chains must assemble correctly in the endoplasmic reticulum (ER) before the BCR can be transported to the cell surface. The roles of the different chaperone proteins in mediating the assembly of mIg with the Ig-alpha/beta are not fully understood. To gain insights into the roles of chaperone proteins in BCR assembly, we have generated transfected non-lymphoid cell lines that express various intermediate assembled forms of the BCR and used them to examine the interactions of chaperone proteins with subunits of the BCR. We examined the interactions of BiP (GRP78), GRP94 and calnexin with the mu heavy chain, lambda light chain, Ig-alpha and Ig-beta. We report for the first time that Ig-alpha associates with GRP94 and that this interaction increases dramatically when other BCR chains are co-expressed. In contrast, the mu heavy chain interacts strongly with BiP (GRP78) when expressed by itself but this interaction is reduced when the lambda light chain is expressed, with the resulting mu(lambda) complexes interacting with GRP94 and calnexin. Thus, our data are consistent with the idea that there is an ordered association of BCR components with different protein chaperones during BCR assembly.

Effects of phospholipase A(2) activating peptides upon GTP-binding protein-evoked adrenocorticotrophin secretion

A GTP-binding protein (G-protein), termed G-exocytosis (Ge), mediates the effects of calcium ions in the late stages of the adrenocorticotrophin (ACTH) secretory pathway. An activator of Ge, mastoparan, also stimulates phospholipase A(2) and so a comparison of other phospholipase A(2)-activating peptides, melittin and phospholipase A(2)-activating peptide was made with mastoparan to assess whether phospholipase A(2)activation was an important component of Ge-evoked secretion. All three peptides stimulated ACTH secretion in the effective absence of calcium ions from permeabilised cells, actions potentiated by a phospholipase A(2)inhibitor. Ca(2+)-evoked secretion from permeabilised cells was similarly potentiated by a phospholipase A(2) inhibitor. Furthermore, arachidonic acid inhibited Ca(2+)- and Ge-evoked ACTH secretion, an action blocked by the cyclo-oxygenase inhibitor ibuprofen. This study suggests that the products of phospholipase A(2)-generated arachidonic metabolism may exert an inhibitory action on the late post-Ca(2+) stages of the ACTH secretory pathway and that prostaglandins may be the active agents in this capacity.

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

Heterotrimeric GTP-binding (G) proteins, termed Ge, have a role in the late stages of the adrenocorticotrophin (ACTH) secretory pathway in the mouse AtT-20/D16-16 anterior pituitary tumour cell line. The wortmannin sensitivity of Ge-controlled mechanisms in AtT-20 cells was investigated to provide information on the possible mechanisms linking Ge with secretion. Permeabilised cells exposed to calcium ions (10(-9) to 10(-3) M), guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) (10(-8) to 10(-4) M) and mastoparan (10(-8) to 10(-5) M) demonstrated a significant and concentration-dependent stimulation of ACTH secretion from non-stimulated levels for all three agents. Coincubation with wortmannin (10(-5) M) significantly inhibited both calcium-independent and -stimulated secretion. The effect of wortmannin was concentration-dependent being maximal at 10(-6) M. The study shows that wortmannin inhibits both calcium-independent and -stimulated secretion from permeabilised AtT-20 cells indicating a role for phosphatidylinositol-3 kinase in determining the size of the readily releasable pool of ACTH and/or in mediating calcium/Ge-evoked secretion from this pool.

The association of metalloendopeptidase EC 3.4.24.15 at the extracellular surface of the AtT-20 cell plasma membrane

Endopeptidase EC 3.4.24.15 (EP24.15) is a soluble, neuropeptide-degrading metalloenzyme, widely expressed in the brain, pituitary and gonads. For the physiological metabolism of neuropeptides, the enzyme should be located extracellularly, either associated with the plasma membrane or in the extracellular milieu. Western immunoblot analyses of crude cytosolic and post-nuclear membrane fractions prepared by differential centrifugation revealed a slightly smaller molecular mass ( approximately 2 kDa) for EP24.15 in the post-nuclear membrane fraction. This smaller EP24.15 species was also present in an enriched fraction of plasma membrane prepared by Percoll gradient centrifugation. To ascertain whether EP24.15 is associated with the extracellular surface of plasma membrane, two sets of experiments were carried out. First, Western immunoblot analysis of AtT-20 cells treated with the membrane-impermeable, thiol-cleavable cross-linker, 3, 3'-dithio-bis(sulpho-succinimidyl-propionate) (DTSSP), indicated an extracellular membrane association. After cross-linking and thiol-reduction, a distinct band corresponding to EP24.15 was significantly diminished under non-reducing conditions. Second, immunocytochemical studies performed at 4 degrees C on non-permeabilized AtT-20 cells (i.e., non-fixed to prevent antibody internalization), indicated that EP24.15 was expressed on the surface of the AtT-20 cells. We furthermore determined that EP24.15 enzymatic activity is present on the extracellular surface of the cell discernable from the secreted enzyme. These results suggest that the EP24.15 is associated with the extracellular surface of the AtT-20 cell plasma membrane and is enzymatically active. Taken together, the results are consistent with a putative role in the degradation of neuropeptides acting at the external cell surface.

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.

Growth-associated protein-43 (GAP-43) facilitates peptide hormone secretion in mouse anterior pituitary AtT-20 cells

The neuronal growth-associated protein (GAP)-43 (neuromodulin, B-50, F1), which is concentrated in the growth cones of elongating axons during neuronal development and in nerve terminals in restricted regions of the adult nervous system, has been implicated in the release of neurotransmitter. To study the role of GAP-43 in evoked secretion, we transfected mouse anterior pituitary AtT-20 cells with the rat GAP-43 cDNA and derived stably transfected cell lines. Depolarization-mediated beta-endorphin secretion was greatly enhanced in the GAP-43-expressing AtT-20 cells without a significant change in Ca2+ influx; in contrast, expression of GAP-43 did not alter corticotropin-releasing factor-evoked hormone secretion. The transfected cells also displayed a flattened morphology and extended processes when plated on laminin-coated substrates. These results suggest that AtT-20 cells are a useful model system for further investigations on the precise biological function(s) of GAP-43.

Proopiomelanocortin, its derived peptides, and the skin

Proopiomelanocortin (POMC) is a protein synthesized predominately in the pituitary gland but also in a variety of other tissues, including the skin. Through enzyme-mediated cleavage that varies among cell types, POMC can give rise to at least eight distinct peptides whose biologic roles are incompletely delineated. Although blood-borne pituitary-derived bioactivity for the skin was first recognized 80 years ago and the responsible neuropeptides isolated 20-40 years ago, our understanding of POMC-derived peptides in skin is still rapidly evolving. In particular, recent work in cultured human and murine skin-derived cells has demonstrated POMC gene expression as well as modulation of POMC and many of its derived peptides in response to physiologic signals including ultraviolet irradiation and cytokines. Immunoreactivity for these peptides has also been detected in normal skin and hair follicles, strongly suggesting cutaneous synthesis in vivo. Candidate autocrine or paracrine functions include enhanced melanogenesis, immunomodulation, and effects on cell cycle regulation and differentiated function in both the epidermis and its appendages. This article reviews recent data concerning POMC gene expression and regulation, protein processing, signal transduction, and biologic functions relevant to cutaneous biology.

Characterization of a corticotropin releasing hormone responsive region in the murine proopiomelanocortin gene

The most potent, physiologic activator of proopiomelanocortin (POMC) gene transcription is corticotropin releasing hormone (CRH) and increased intracellular cAMP is critical for this effect. The 5'-flanking region of the murine POMC gene has several potential binding sites for regulatory proteins. To characterize the region between nucleotides -141 and -106, which includes a TRE-like site and an adjacent AP-2 consensus sequence, and to study its role in signal-transcription coupling, gel mobility shift assays and transient expression of CAT chimeras were performed. In transient transfections of AtT-20 cells with pCATp-141/-106, CRH treatment led to significant increases in CAT expression compared with CRH treatment of cells transfected with the enhancerless vector. However, no response to direct activation of cAMP dependent protein kinase or protein kinase C was detected. Despite the high homology of the sequence -137/-131 to the consensus AP-1 binding site (TRE), the nuclear factor(s) in AtT-20 cells binding to this region appears to be different than authentic AP-1 since neither a competitor oligonucleotide having the authentic TRE sequence nor antibodies against Jun or Fos affected the gel shift pattern of a probe having the -137/-131 sequence. We conclude that the -141 to -106 region of the murine POMC gene contains a functional CRH responsive element and that second messenger systems that transduce the CRH signal to this element do not exert their actions solely through activation of PKA or PKC.

Early glucocorticoid induction of calmodulin and its suppression by corticotropin-releasing factor in pituitary corticotrope tumor (AtT20) cells

Glucocorticoids inhibit stimulus-evoked ACTH secretion by the rapid induction of new protein(s) that suppress intracellular free calcium signals. The present study examined whether the calcium receptor protein, calmodulin, is induced by glucocorticoids in the mouse pituitary corticotrope tumor (AtT20 D16:16) cell line. Treatment of AtT20 D16:16 cells with the synthetic glucocorticoid dexamethasone markedly (up to 10-fold) increased the level of a single (approximately 1.6kb) calmodulin mRNA 90 min after the application of steroid. Puromycin applied 15 min before and during dexamethasone treatment blocked the induction of this mRNA, suggesting that additional glucocorticoid induced transcription factor proteins may be required for enhanced calmodulin gene transcription. A two-fold increase in the intensity of an approximately 18K immunoreactive calmodulin protein band was detected by immunoblotting at 90 min after dexamethasone administration. Corticotropin releasing factor, added for 30 min at the start of steroid treatment, prevented the increase of calmodulin mRNA, as well as the suppression of corticotropin releasing factor-evoked ACTH release caused by dexamethasone. These data suggest that calmodulin may be involved in the early phase of glucocorticoid inhibition of pituitary ACTH release.

Adrenocorticotropic hormone and beta-endorphin concentrations in the inferior petrosal sinuses in Cushing's disease and other pituitary diseases

Aim of the present study was the evaluation of ACTH and beta-endorphin-like-immunoreactivity (beta-ELI) in the inferior petrosal sinuses (IPS's) and in the peripheral blood of patients with Cushing's disease (Group 1), with GH- or PRL-secreting adenomas or nontumoral hyperprolactinemia (Group 2). These patients had undergone selective and bilateral simultaneous IPS sampling for diagnostic purposes or for neurosurgical indications. In the patients of Group 1, ACTH and beta-ELI levels were higher in the IPS ipsilateral than in the contralateral to the adenoma and in the periphery (p < 0.001). In the patients of Group 2 ACTH and beta-ELI levels were higher in the IPS's than in the peripheral blood (p < 0.001) and, in the 9 patients with GH- or PRL-secreting adenomas, they were higher in the IPS ipsilateral than in the contralateral to the adenoma and in the periphery (p < 0.05). A significant correlation exists between ACTH and beta-ELI in the periphery (p < 0.01; r = 0.72), in the IPS ipsilateral (p < 0.05; r = 0.54) and contralateral (p < 0.01; r = 0.66) to the adenoma in Group 1, but not in Group 2. In conclusion, higher beta-ELI levels were detected in the IPS's than in the peripheral blood not only in patients with Cushing's disease but also in those with other pituitary diseases not involving ACTH secretion. The absence of correlation between ACTH and beta-ELI in patients not bearing Cushing's disease suggests that in these conditions corticotrophs release ACTH and beta-endorphin in an independent manner.

Atrial natriuretic peptide effects in AtT-20 pituitary tumour cells

Whether atrial natriuretic peptide (ANP)-evoked inhibition of corticotrophin-releasing factor (CRF)-stimulated ACTH secretion was also manifest in ACTH secreting AtT-20 pituitary tumour cells was investigated. ANP stimulated increases in cGMP accumulation at concentrations of the peptide above 10(-8) M which indicates the presence of the ANP receptors on these cells. CRF stimulated a concentration-dependent increase in ACTH secretion from AtT-20 cells which was unaffected by ANP, 8-bromo-cGMP, or sodium nitroprusside (SNP). Calcium stimulated a concentration-dependent increase in ACTH secretion from electrically permeabilised cells which was unaffected by co-incubation with cGMP but potentiated by cAMP. These results reveal the presence of ANP receptors on AtT-20 cells but suggest that an incomplete expression of the stimulus-secretion coupling mechanisms for ANP, at some point after cGMP production, prevents the effects of natriuretic peptides upon ACTH secretion being manifest in these cells.

ACTH secretion from mouse pituitary tumor cells is inhibited by loop diuretic drugs

Stilbene disulfonate and phenylanthranilic acid derivatives block Cl- transport and ACTH secretion from mouse AtT-20 clonal corticotrophs. This study was to determine whether antagonism of Na+/K+/Cl- co-transport by loop diuretics (furosemide or bumetanide) and thereby Cl- entry could block ACTH secretion. Activity of the Na+/K+/Cl- co-transporter (symport) was measured as ouabain-insensitive, loop diuretic-sensitive uptake of 86Rb. Ouabain-insensitive uptake (80% of total 86Rb uptake) was linear over 10 min and was markedly reduced by furosemide. Substitution of Na+ by choline or Cl- by gluconate resulted in an 82 and 55% decrease, respectively, in 86Rb uptake. Furosemide and bumetanide incompletely inhibited 86Rb uptake (maximal inhibition of 60% and 69%, respectively; IC50: 1.6 x 10(-5) and 3 x 10(-6) M, respectively). Forskolin did not affect the activity of the Na+/K+/Cl- co-transporter but both basal and forskolin-stimulated secretion of ACTH were inhibited by furosemide (IC50 of 5 x 10(-4) M; maximal inhibition: 50%). Bumetanide did not affect forskolin-induced cAMP synthesis. Use of cyclamate or gluconate in place of Cl- also resulted in the inhibition of basal and forskolin-stimulated ACTH secretion. The data support the belief that inhibition of Cl- entry into AtT-20 cells by way of an Na+/K+/Cl- co-transporter can result in the inhibition of ACTH secretion but that other anion entry mechanisms may also be coupled to the secretory response in these cells.

Chloride channel blockers inhibit ACTH secretion from mouse pituitary tumor cells

The effect of several chemically related chloride channel blocking drugs was investigated on the adrenocorticotropic hormone (ACTH) secretory process in mouse clonal AtT-20 corticotrophs. When cells were simultaneously exposed to diphenylamine-2-carboxylate (DPC) or related substances (Hoechst compounds 131, 143, and 144) and the adenylate cyclase activator forskolin, ACTH secretion was inhibited by 76-95% [half-maximal inhibitory concentration (IC50) 450, 15, 84, and 32 microM, respectively]. All four compounds also blocked forskolin-stimulated adenosine 3',5'-cyclic monophosphate (cAMP) synthesis in AtT-20 cells by 51-87% (IC50 190, 29, 100, and 130 microM for DPC and compounds 131, 143, and 144, respectively). Pertussis toxin pretreatment of cells caused a partial reversal of DPC-inhibited forskolin-stimulated cAMP formation. The toxin had no effect on inhibition of forskolin-stimulated ACTH secretion by DPC. Secretion of ACTH in response to cAMP-independent stimulants such as the protein kinase C activator 12-O-tetradecanoylphorbol-13-acetate or the calcium channel agonist BAY K 8644 were blocked by compound 131 as was the secretory response to 8-bromoadenosine 3',5'-cyclic monophosphate. These results suggest that phenylanthranilic acids have adenylate cyclase inhibiting action but that the postcyclase activity is more relevant to the ability of these compounds to block ACTH secretion. DPC also blocked 125I efflux (an index of Cl- secretion) from AtT-20 cells. Because an increase in osmotic strength of the culture media reduced forskolin-stimulated ACTH secretion, these data suggest that DPC and related compounds may negatively modulate chloride-dependent osmotically driven ACTH secretion from AtT-20 cells.

Constitutive and basal secretion from the endocrine cell line, AtT-20

A variant of the ACTH-secreting pituitary cell line, AtT-20, has been isolated that does not make ACTH, sulfated proteins characteristic of the regulated secretory pathway, or dense-core secretory granules but retains constitutive secretion. Unlike wild type AtT-20 cells, the variant cannot store or release on stimulation, free glycosaminoglycan (GAG) chains. In addition, the variant cells cannot store trypsinogen or proinsulin, proteins that are targeted to dense core secretory granules in wild type cells. The regulated pathway could not be restored by transfecting with DNA encoding trypsinogen, a soluble regulated secretory protein targeted to secretory granules. A comparison of secretion from variant and wild type cells allows a distinction to be made between constitutive secretion and basal secretion, the spontaneous release of regulated proteins that occurs in the absence of stimulation.

Effects of Corticotrophin Releasing Factor, Muscarine and Somatostatin on Rubidium and Potassium Efflux from Mouse AtT-20 Pituitary Cells

Effects of secretagogues and anti-secretagogues of ACTH secretion on K+ permeability in the clonal pituitary cell line AtT-20 were measured by recording 86Rb or 42K efflux. Efflux was accelerated by the secretagogues K+, corticotrophin, forskolin, isoprenaline, and the Ca-ionophore A23187. Efflux was reduced by the inhibitors somatostatin, muscarine, and oxotremorine, or by removing external Ca. Efflux was also reduced by the K+-channel blocking compound d-tubocurarine but not by tetraethylammonium. Muscarine, oxotremorine, somatostatin, and 0 Ca2+ also reduced intracellular Ca2+ measured by quin-2 fluorescence. It is suggested that most of the resting 86Rb or 42K efflux measured under these conditions occurs via tubocurarine-sensitive Ca2+-dependent K+-channels, and that changes in efflux rate produced by secretagogues or anti-secretagogues are secondary to changes in intracellular Ca2+.

Intracellular calcium and hormone secretion in clonal AtT-20/D16-16 anterior pituitary cells

Intracellular ionized Ca2+ concentration was measured in clonal mouse anterior pituitary tumor cells with the fluorescent Ca2+ indicator Quin-2. In control physiological solution, free cytoplasmic Ca2+ concentration was found to be 139 +/- 11 nM. Replacement of 50 mM NaCl by 50 mM KCl in the extracellular fluid caused a 29 mV depolarization and a 4.2-fold increase in the concentration of free cytoplasmic Ca2+. Under comparable depolarizing conditions, a specific influx of 2.66 nmole of 45Ca2+ per mg protein was detected 1 min after addition of high K+, accompanied by a marked increase in the initial rate of beta-endorphin secretion. In the absence of external Ca2+, depolarization by K+ produced little or no increase in either intracellular free Ca2+ or hormone release. Incubation of AtT-20/D16-16 cells in the secretagogue norepinephrine led to a depolarization accompanied by an increase in spontaneous action potential frequency and a marked elevation in cytosolic Ca2+ concentration. Exposure of cells to somatostatin, an inhibitor of hormone release, led to only transient decreases in burst frequency and no significant reduction in intracellular Ca2+ levels. These results indicate that in addition to intracellular Ca2+, other factors also control secretory activity in AtT-20/D16-16 anterior pituitary cells.

Stimulation by atrial natriuretic factor of cyclic GMP production in cultured anterior and intermediate pituitary tissues: evidence for a major contribution of proliferating nonendocrine cells

Primary cultures of anterior and intermediate pituitary tissues were monitored immunocytochemically for the presence of endocrine and nonendocrine cells and simultaneously tested for their ability to produce cyclic GMP in response to atrial natriuretic factor (ANF). Cells cultured for 3 days and 6 days, in which nonendocrine (vimentin-positive) cells were found to rapidly overgrow the endocrine cells, showed a dramatic elevation in cyclic GMP production stimulated by ANF, with maximum stimulation 300-700% that seen in 1-day cultured cells. Also, ANF-induced accumulation of cyclic GMP in an enriched population of vimentin-positive cells appeared to closely match that triggered in a 3-day culture of anterior pituitary cells, emphasizing the major role played by nonendocrine cells and their ability to synthesize cyclic GMP. In contrast, in the homogeneous population of tumor corticotrophs AtT-20, there was a close relationship between cyclic GMP formation and cell density. It thus appears that contamination of primary cultures of anterior and intermediate pituitary tissues by proliferating nonendocrine cells (mainly fibroblasts), in which ANF-induced accumulation of cyclic GMP may be confused with that of the very secretory cells, leads to overestimation and masking of guanylate cyclase activity of endocrine cells.

Interleukin 1 potentiates the secretion of beta-endorphin induced by secretagogues in a mouse pituitary cell line (AtT-20)

Previous work has shown that corticotropin releasing factor, vasoactive intestinal peptide, phorbol ester, and forskolin cause the secretion of adrenocorticotropic hormone and beta-endorphin from the AtT-20 mouse pituitary cell line. Human recombinant interleukin 1 alpha and 1 beta also stimulated adrenocorticotropic hormone and beta-endorphin secretion from AtT-20 cells in a time- and dose-related manner. The effect appeared only after pretreatment with interleukin 1 (IL-1) for at least 18 hr and was maximum at 24 hr. After pretreatment of the cells over a period of time with IL-1, the secretion induced by corticotropin releasing factor and vasoactive intestinal peptide was increased in more than an additive manner. The enhancement of corticotropin releasing factor-induced beta-endorphin release produced by IL-1 was apparent after 12 hr and reached a maximum at 24 hr. IL-1 did not affect forskolin-induced cAMP generation but enhanced the effect of forskolin on beta-endorphin secretion. This suggests that IL-1 does not induce adenylate cyclase and that forskolin causes the secretion of beta-endorphin by a mechanism independent of cAMP. IL-1 enhanced phorbol ester-induced beta-endorphin secretion. After prolonged treatment with phorbol ester (an activator of protein kinase C), the secretion induced by phorbol ester was abolished as well as the enhancement induced by IL-1. However, prolonged treatment with phorbol ester had no effect on IL-1-induced beta-endorphin secretion. These observations suggest that IL-1 enhances peptide-generated secretion of beta-endorphin by inducing protein kinase C.

Indirect relationship between vasopressin-induced secretion of ACTH and cyclic nucleotides in cultured anterior pituitary cells

The present study examines whether a correlation exists between cyclic nucleotides and the mechanism of action of arginine vasopressin (AVP) on adrenocorticotropin hormone (ACTH) secretion from pituitary corticotrophs. Incubation of cultured anterior pituitary cells with 3-isobutyl-1-methylxanthine (IBMX) or Rolipram elevated the basal intracellular content of both adenosine 3':5'-cyclic monophosphate (cAMP) and guanosine 3':5'-cyclic monophosphate (cGMP) or cAMP alone, respectively. Both IBMX and Rolipram enhanced the AVP-stimulated secretion of ACTH in cultured anterior pituitary cells, but not in AtT-20 corticotrophs which lack functional AVP receptors. Rolipram was less potent than IBMX in this regard, which suggests a possible involvement of cGMP. In contrast, both drugs showed similar potency to stimulate CRF-induced ACTH secretion. Incubation of pituitary cells with atrial natriuretic factor elevated tissue cGMP levels and increased the ACTH response to AVP. The results of this study show that, although AVP fails to directly affect the levels of cAMP and cGMP in anterior pituitary cells, the stimulatory effect of AVP on ACTH secretion was modulated by the cellular cAMP content.

Evidence that AVP receptors in AtT-20 corticotrophs are not coupled to secretion of POMC-derived peptides

This study reports the presence in AtT-20 corticotrophs of high affinity-low capacity receptors for arginine-vasopressin (AVP), whose binding capacity was considerably enhanced by the divalent metal ion nickel. These binding sites, when analyzed in the presence of nickel, showed high affinity for AVP, vasotocin and oxytocin, but recognized to a lesser extent the V2-agonist 1-deamino-AVP, as well as V1-antagonists. Surprisingly, AVP failed to alter secretion of proopiomelanocortin (POMC)-derived peptides from the cells or corticotropin-releasing factor (CRF)-induced cAMP synthesis, as reported in normal corticotrophs. Exposure of cells to CRF elicited an increase in mRNAPOMC levels, while, in contrast, AVP was without significant effect. It thus appears that in AtT-20 tumor cells, the AVP receptors are not coupled to either the biochemical or biological cellular response.

CRF and cAMP regulation of POMC gene expression in corticotrophic tumor cells

The effects of corticotropin releasing factor (CRF) on pro-opiomelanocortin (POMC) gene expression were investigated in an anterior pituitary corticotrophic tumor cell line, AtT-20/D16-16. The results of mRNA dot blot hybridization assays suggested that CRF, at a concentration of 10(-7) M, positively regulates the expression of the POMC gene in AtT-20 cells in a concentration-dependent fashion. Evaluation of the time course of this effect indicated that CRF had a biphasic mode of action. CRF and alpha-amanitin (inhibitor of RNA polymerase II activity) were also found to affect POMC mRNA levels in a concentration-dependent fashion. Eight-bromo-cyclic adenosine monophosphate (8-Br-cAMP) produced biphasic effects on POMC mRNA levels, supporting evidence of a role for cAMP as a second messenger in the regulation of POMC gene expression. It was also found that alpha-amanitin negatively regulated basal and CRF-stimulated POMC mRNA levels at both the 2 hr and 24 hr time periods, supporting evidence for positive regulation of POMC by CRF at the transcriptional level.

Activation of distinct second messenger systems in anterior pituitary corticotrophic tumor cells alters the phosphorylation states of both shared and distinct cytosolic proteins

The purpose of the present study was to investigate the effects of activation of different second messenger systems on protein phosphorylation in pituitary corticotrophic tumor cells (AtT-20/D16-16). Using two-dimensional gel analysis of cytosolic extracts from AtT-20 cells, several phosphoproteins exhibited alterations in 32P incorporation in response to stimulation of the cells with either forskolin--an activator of adenylate cyclase--or 12-O-tetradecanoyl phorbol-13-acetate (TPA)--a tumor promoting phorbol ester linked to protein kinase C activation. Alterations in phosphorylation levels were seen for phosphoproteins of the following apparent molecular weights and pIs: 87 kDa (pI 4.4-4.6), 67 kDa (pI 4.7-4.9), 43 kDa (pI 4.8-5.0), 39 kDa (pI 4.9-5.1), 33 kDa (pI 4.8-5.0), 19.5 kDa (pI 5.7-5.9), 19 kDa (pI 5.8-6.0), 16 kDa (pI 5.2-5.4) and 14 kDa (pI 5.1-5.3). For individual phosphoproteins, 32P incorporation varied over time and was also modulated by concentrations of Ca2+ and Mg2+ in the incubation medium. Treatment of the cells with forskolin led to statistically significant changes in the phosphorylation states of the 19.5 and 14 kDa proteins. Treatment of the cells with TPA also produced statistically significant changes in the 19.5 and 14 kDa proteins but, in addition, the 87 kDa, the 39 kDa and the 16 kDa phosphoproteins also exhibited significant changes. Alterations in the phosphorylation states of the 19.5 and the 14 kDa proteins were significantly correlated with alterations in beta-endorphin release from the cells. The primary finding of the present study was that activation of distinct second messenger systems can lead to alterations in the phosphorylation states of both shared and distinct phosphoproteins.

Thymosin fraction 5 stimulates secretion of immunoreactive beta-endorphin in mouse corticotropic tumor cells

In addition to reconstituting immune competence, the thymus gland preparation, thymosin fraction 5 (TSN-5), has recently been shown to stimulate secretion of hormones from the hypothalamic-pituitary adrenal axis in vivo and from pituitary corticotropes in vitro. The purpose of the present study was to investigate the effects of TSN-5 on secretion of immunoreactive beta-endorphin (i beta-E) by mouse corticotropic tumor cells. The release of i beta-E by AtT-20 pituitary tumor cells was increased in a dose-dependent manner by concentrations of 30-600 micrograms/ml of TSN-5, whereas concentrations greater than 1,000 micrograms/ml were increasingly less effective in stimulating secretion. TSN-5 (600 micrograms/ml) significantly stimulated i beta-E release within 7 min; maximal secretory responses (up to 275% of control release) occurred by 4 hr. The secretory response of AtT-20 cells to 600 micrograms/ml TSN-5 (37.9 +/- 2.0 vs. 16.1 +/- 1.0 ng i beta-E/ml/4 hr, mean +/- SE) was similar in magnitude to release evoked by 0.1 microM corticotropin-releasing factor (CRF). Combining TSN-5 and CRF treatments increased secretion of i beta-E to nearly 600% of control levels, an effect greater than an additive influence of the two independent treatments. Whereas CRF treatment reduced the levels of i beta-E in AtT-20 cell extracts after 24-hr treatment by 45% (231.8 +/- 24.7 vs. 417.2 +/- 17.8 ng i beta-E/mg protein, CRF vs. vehicle treatments, respectively), TSN-5 did not significantly alter cellular hormone content. Neither TSN-alpha 1 nor TSN-beta 4, two of the component peptides of TSN-5, affected basal or CRF-stimulated release of i beta-E, indicating that an unidentified constituent(s) is corticotropic.(ABSTRACT TRUNCATED AT 250 WORDS)

A monoclonal antibody against meningococcus group B polysaccharides distinguishes embryonic from adult N-CAM

The neural cell adhesion molecules (N-CAM) occur chiefly in two molecular forms that are selectively expressed at various stages of development. Highly sialylated forms prevalent in embryonic and neonatal brain are gradually replaced by less sialylated forms as development proceeds. Here we describe a monoclonal antibody raised against the capsular polysaccharides of meningococcus group B (Men B) which specifically distinguishes embryonic N-CAM from adult N-CAM. This antibody recognizes alpha 2-8-linked N-acetylneuraminic acid units (NeuAc alpha 2-8). Immunoblot together with immunoprecipitation experiments with cell lines or tissue extracts showed that N-CAM are the major glycoproteins bearing such polysialosyl units. Moreover we could not detect any sialoglycolipid reactive with this antibody in mouse brain or in the neural cell lines examined. By indirect immunofluorescence staining this anti-Men B antibody decorated cells such as AtT20 (D16/16), which expressed the embryonic forms of N-CAM, but not cells that expressed the adult forms. In primary cultures this antibody allowed us to follow the embryonic-to-adult conversion in individual cells. In addition, the existence of cross-reactive polysialosyl structures on Men B and N-CAM in embryonic brain cells for caution in efforts to develop immunotherapy against neonatal meningitis.

Atrial natriuretic factor does not affect basal, forskolin- and CRF-stimulated adenylate cyclase activity, cAMP formation or ACTH secretion, but does stimulate cGMP synthesis in anterior pituitary

The report that ANF inhibits basal and CRF-stimulated adenylate cyclase activity in anterior pituitary homogenates suggested that the atrial peptide could inhibit ACTH secretion. This possibility was investigated in the ACTH-secreting AtT-20 mouse pituitary tumor cell line as well as homogenates or primary cell cultures from rat anterior hypophysis. ANF (up to 5 X 10(-7) M) was found to be completely ineffective in stimulating basal, CRF- and/or forskolin-stimulated adenylate cyclase activity, cAMP accumulation and ACTH secretion. Similarly, ANF had no effect on spontaneous or GRF-induced GH release from cells in primary culture. ANF receptors, however, are present in AtT-20 cells and anterior pituitary cells as evidenced by the ability of the peptide to stimulate intracellular cGMP accumulation. The data, therefore, suggests that ANF does not have a negative modulatory action on the secretory function of anterior pituitary. The role of cGMP in any other action(s) of ANF remains unknown.

Lithium induces corticotropin secretion and desensitization in cultured anterior pituitary cells

Lithium stimulated corticotropin (ACTH) secretion by mouse pituitary tumor cells (AtT-20/D16-16) and by normal rat anterior pituitary cells in primary culture. Effects were observed at less than 2 mM LiCl. ACTH secretion was comparable in magnitude to that induced by other secretagogues, was calcium dependent, and was inhibited by somatostatin. Lithium also induced changes in [3H]inositide metabolism; these changes accompanied and were correlated with changes in ACTH secretion. The most prominent and reliable effect was to increase [3H]inositol monophosphate. Other secretagogues had no effect on [3H]inositides in the presence or absence of lithium. Pretreatment with lithium for 3 hr desensitized the cells to the effects of subsequent exposure to lithium. The cells were not desensitized to lithium by pretreatment with other secretagogues, nor were they desensitized by lithium to the effects of corticotropin-releasing factor, high potassium, or forskolin. However, pretreatment with lithium did desensitize the cells to stimulation by phorbol esters. The interaction between lithium and phorbol esters suggests the involvement of inositide metabolism and protein kinase C in the regulation of ACTH secretion and possibly of other hormones or neurotransmitters. It also suggests new avenues of research into the basis of lithium's psychopharmacological effects.

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.

12-O-tetradecanoyl-phorbol-13-acetate-induced ACTH secretion in pituitary tumor cells

The ability of the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) to stimulate secretion of immunoreactive ACTH from a clonal strain of mouse pituitary tumor cells (AtT-20/D16-16), was investigated. The secretory response to TPA was concentration-and time-dependent. TPAs action on AtT-20 cells was cyclic AMP independent. TPA, even at maximally effective concentrations, had an additive effect on ACTH secretion when co-applied with other agonists such as corticotropin releasing factor, vasoactive intestinal peptide, or (1)-isoproterenol. Secretion of ACTH in response to TPA was reduced by lowering extracellular calcium concentration or in the presence of the calcium channel blocker, nifedipine. Indomethacin and dexamethasone, both blocked TPA stimulated secretion; arachidonic acid, in high concentrations, had a small stimulatory effect on ACTH release. The data suggest that TPA induced secretion in pituitary tumour cells may result from the previously reported ability of the phorbol ester to stimulate the activity of phospholipase A2, and to the subsequent biosynthesis of prostaglandins.

Isolation and characterization of alpha-endorphin and gamma-endorphin from single human pituitary glands

alpha-Endorphin and gamma-endorphin, two closely related peptides of the pro-opiomelanocortin family with characteristic biological activities, were purified to homogeneity from single human pituitary glands and chemically identified. Isolation of the peptides was based on size fractionation by Sephadex G-75 chromatography followed by two HPLC steps using reverse-phase and paired-ion reverse-phase systems and was monitored by radioimmunoassay. During the isolation procedure alpha- and gamma-endorphin-sized material behaved chromatographically and immunologically indistinguishably from synthetic alpha- and gamma-endorphin. The amino acid composition and NH2-terminus of isolated peptides demonstrated their identity as authentic alpha-endorphin and gamma-endorphin. Acetylated forms were absent. In addition, evidence is provided that large forms with alpha- and gamma-endorphin immunoreactivity detected during gel filtration are human lipotropin-(1-74) and -(1-75), respectively. The data substantiate that alpha-endorphin and gamma-endorphin exist as endogenous peptides in the human pituitary gland.

Action potentials and membrane ion channels in clonal anterior pituitary cells

The electrophysiological properties of the mouse anterior pituitary cell line AtT-20/D16-16 were investigated with intracellular and patch-clamp techniques. Clonal AtT-20/D16-16 cells were found to be electrically excitable, with most cells exhibiting spontaneous bursting action potentials. The mean burst rates varied from 1.4 Hz at -55mV to 8.2 Hz at -25mV, showing an approximately linear frequency-current relationship in the low current range. The bursts consisted of one to several fast Na+ spikes superimposed on a slow pacemaker potential, followed by a Ca2+ spike and a Ca2+-sensitive afterhyperpolarization. Removal of either Na+ or Ca2+ from the bathing medium led to cessation of spontaneous activity and the appearance of arrhythmic firing patterns. Single channel recordings revealed the presence of Ca2+-dependent K+ channels with unitary conductances of approximately equal to 130 pS in physiological medium. These channels were activated by both intracellular Ca2+ and membrane depolarization. Addition of norepinephrine (10 microM) led to increases in burst frequency and beta-endorphin secretion mediated by activation of beta-adrenergic receptors. Our results, in conjunction with previous work, suggest that the Ca2+ that enters the cell during the burst may be involved in hormone secretion.

Corticotropin releasing factor stimulation of protein carboxylmethylation in mouse pituitary tumor cells

A putative role for the protein carboxylmethylase (PCM) enzyme has been suggested in exocytotic secretion. The involvement of 3H-methyl incorporation into protein carboxylmethyl esters during corticotropin releasing factor (CRF)-induced ACTH secretion from AtT-20/D16-16 mouse pituitary cells was investigated. Protein carboxylmethylation and ACTH secretion both increased as a function of extracellular CRF concentration, and both processes were temporally parallel up to 60 min incubation. The less potent [Met(O)21]-CRF also stimulated increases in protein carboxylmethylation and ACTH secretion. The free acid analogue of CRF did not alter either process. A combination of the PCM inhibitors, 3-deazaadenosine and L-homocysteine thiolactone, reduced both CRF-stimulated protein carboxylmethylation and ACTH release. Dexamethasone, known to inhibit ACTH secretion and synthesis, inhibited both CRF-stimulated protein carboxylmethylation and ACTH secretion.

Somatostatin inhibits multireceptor stimulation of cyclic AMP formation and corticotropin secretion in mouse pituitary tumor cells

The AtT-20/D16-16 mouse pituitary tumor cell secretes corticotropin (ACTH) in response to corticotropin-releasing factor (CRF), (-)-isoproterenol, and vasoactive intestinal peptide (VIP). These responses are associated with a rapid increase in cyclic AMP formation. Somatostatin (SRIF) markedly decreases the stimulatory effect of CRF, (-)-isoproterenol, and VIP on both cyclic AMP formation and immunoreactive ACTH secretion. Forskolin and cholera toxin, adenylate cyclase activators, also stimulate cyclic AMP formation and ACTH secretion in AtT-20 cells and these responses are all inhibited by SRIF. The ACTH secretory responses to melittin and to the calcium ionophore A23187, neither of which increases cyclic AMP in AtT-20 cells, were not inhibited by SRIF. SRIF did not affect the binding of a tritiated beta-adrenergic receptor antagonist to AtT-20 membranes nor did it decrease basal cyclic AMP formation even in the presence of excess phosphodiesterase inhibitor, indicating that the reduction of cyclic AMP levels by SRIF did not involve either an interference with beta-adrenergic agonist binding to receptors or stimulation of cyclic AMP degradation. These results indicate that the inhibition of CRF-, (-)-isoproterenol-, and VIP-stimulated ACTH secretion by SRIF may be regulated by its inhibitory action on adenylate cyclase.

Correlation between electrical activity and ACTH/beta-endorphin secretion in mouse pituitary tumor cells

The electrical and secretory activities of mouse pituitary tumor cells (AtT-20/D-16v), which contain and release the ACTH/beta-endorphin family of peptides, were studied by means of intracellular recordings and radioimmunoassays. Injection of depolarizing current pulses evoked action potentials in all cells and the majority (82%) displayed spontaneous action potential activity. Action potentials were found to be calcium-dependent. Barium increased membrane resistance, action potential amplitude and duration, and release of ACTH and beta-endorphin immunoactivity. Isoproterenol increased both action potential frequency and hormone secretion. Raising the external calcium concentration increased the frequency and amplitude of the action potentials and stimulated secretion of ACTH and beta-endorphin immunoactivity. Thus, stimulation of secretory activity in AtT-20 cells was closely correlated with increased electrical activity. However, a complete blockade of action potential activity had no effect on basal hormone secretion in these cells. These results suggest that the mechanisms underlying stimulated hormone secretion are different from those responsible for basal secretory activity. It is proposed that the increased influx of calcium due to the increased action potential frequency initiates the stimulated release of hormone from these cells.

Corticotropin-releasing factor stimulates phospholipid methylation and corticotropin secretion in mouse pituitary tumor cells

The 41-residue synthetic ovine corticotropin-releasing factor (CRF; corticoliberin) has been shown to stimulate release of corticotropin (adrenocorticotropic hormone; ACTH) and beta-endorphin from AtT-20/D16-16 mouse pituitary tumor cells. Phospholipid methylation of phosphatidylethanolamine to phosphatidylcholine with S-adenosylmethionine as methyl donor has been suggested as a possible membrane transduction mechanism for some receptor-induced events. CRF increased phospholipid methylation in pituitary tumor cells at concentrations that also stimulated immunoreactive ACTH secretion, and both processes increased linearly and in parallel with time. The methionine sulfoxide derivative of CRF was less potent than CRF was in stimulating both phospholipid methylation and hormone secretion, and the COOH-terminal free acid analogue of CRF had no effect on either process. CRF-induced increases in phospholipid methylation and ACTH secretion were reduced when cells were treated with the phospholipid methyltransferase inhibitors 3-deazaadenosine and L-homocysteine thiolactone. These CRF-stimulated effects were also blocked by the glucocorticoid dexamethasone. It is suggested that phospholipid methylation may be a CRF receptor-mediated event associated with ACTH release in pituitary tumor cells.