Anterior Pituitary Cell Line AtT-20 Research Articles

Differential regulation of the cloned kappa and mu opioid receptors

To directly compare the regulation of the cloned κ and μ opioid receptor, we expressed them in the same cells, the mouse anterior pituitary cell line AtT-20. The coupling of an endogenous somatostatin receptor to adenylyl cyclase and an inward rectifier K + current has been well characterized in these cells, enabling us to do parallel studies comparing the regulation of both the κ and the μ receptor to this somatostatin receptor. We show that the κ receptor readily uncoupled from the K + current and from adenylyl cyclase after a 1 h pretreatment with agonist, as indicated by the loss in the ability of the agonist to induce a functional response. The desensitization of the κ receptor was homologous, as the ability of somatostatin to mediate inhibition of adenylyl cyclase or potentiation of the K + current was not altered by κ receptor desensitization. The μ receptor uncoupled from the K + current but not adenylyl cyclase after a 1 h pretreatment with agonist. Somatostatin was no longer able to potentiate the K + current after μ receptor desensitization, thus this desensitization was heterologous. Interestingly, pretreatment with a somatostatin agonist caused uncoupling of the μ receptor but not the κ receptor from the K + current. These results show that in the same cell line, after a 1 h pretreatment with agonist, the κ receptor displays homologous regulation, whereas the μ receptor undergoes only a heterologous form of desensitization. μ Receptor desensitization may lead to the alterations of diverse downstream events, whereas κ receptor regulation apparently occurs at the level of the receptor itself. Broad alterations of non-opioid systems by the μ receptor could be relevant to the addictive properties of μ agonists. Comparison of κ and μ receptor regulation may help define the properties of the μ receptor which are important in the development of addiction, tolerance, and withdrawal to opioid drugs. These are the first studies to directly compare the coupling of the κ and μ receptors to two different effectors in the same mammalian expression system.

Secretion in AtT-20 cells stably transfected with soluble synaptotagmins.

Synaptotagmin (p65) is an integral membrane secretory vesicle-specific protein with two cytoplasmic repeats homologous to the C2 regulatory domain of protein kinase C. Synaptotagmin has been implicated in the regulation of neurotransmitter release from nerve growth factor-differentiated PC12 cells and from synapses in Drosophila, Caenorhabditis elegans, and squid. To address the function of synaptotagmin in endocrine cells, fragments of rat synaptotagmin I were stably expressed in the mouse anterior pituitary cell line AtT-20. The logic of these experiments is that the fragments may interfere with the endogenous synaptotagmin machinery, thus producing a dominant-negative phenotype. Transfected cells expressed the expected fragments that were comprised of either the first C2 repeat, the second C2 repeat, or the entire cytoplasmic domain. The fragments were localized to both soluble and membrane-associated cellular fractions, despite the absence of the transmembrane domain. The second C2 repeat was shown to coimmunoprecipitate with endogenous synaptotagmin, suggesting that protein-protein interactions are mediating the membrane association of the fragments. These fragments had no effect on the targeting of regulated secretory vesicles or on regulated secretion as assayed by the release of ACTH and [3H]choline. Constitutive secretion assayed by the release of glycosaminoglycan side chains was also unaffected, as was the endocytic pathway monitored by the uptake and clearance of transferrin. These data suggest either the existence of a redundant pathway in secretion or that regulated membrane traffic in endocrine cells does not require synaptotagmin.

Phorbolester stimulates the activity of human corticotropin-releasing hormone gene promoter via 3',5'-cyclic adenosine monophosphate response element in transiently transfected chicken macrophages

We previously characterized the transcriptional activity of the human CRH (hCRH) gene promoter in the mouse anterior pituitary cell line AtT20. Here, we show that phorbolester stimulated through the cAMP response element (CRE), located between -227 and -220 relative to the putative cap site, about 2.6-fold the expression of a chimeric hCRH-chloramphenicol-acetyl-transferase gene which was transiently transfected into chicken macrophages (HD11 cell line). The induction was dependent on the cell types, and was not observed when AtT20 were used as target cells. Elevated mouse c-fos protein expressed from a cotransfecting plasmid pRSVfos or human c-jun protein expressed from RSVjun led to 2.1-fold and 3.1-fold stimulation of the activity of the hCRH promoter. Tetradecanoyl-phorbol-13-acetate stimulated the c-fos and c-jun transactivation by a factor of 10 and 2.6, indicating a modification of c-fos and c-jun is required for the transactivation induced by protein kinase C activation. Mutation within the cAMP response element abolished this transcriptional activation. This result suggests an involvement of the protein kinase C pathway in the regulation of the CRH gene expression.

Phorbolester stimulates the activity of human corticotropin-releasing hormone gene promoter via 3',5'-cyclic adenosine monophosphate response element in transiently transfected chicken macrophages.

We previously characterized the transcriptional activity of the human CRH (hCRH) gene promoter in the mouse anterior pituitary cell line AtT20. Here, we show that phorbolester stimulated through the cAMP response element (CRE), located between -227 and -220 relative to the putative cap site, about 2.6-fold the expression of a chimeric hCRH-chloramphenicol-acetyl-transferase gene which was transiently transfected into chicken macrophages (HD11 cell line). The induction was dependent on the cell types, and was not observed when AtT20 were used as target cells. Elevated mouse c-fos protein expressed from a cotransfecting plasmid pRSVfos or human c-jun protein expressed from RSVjun led to 2.1-fold and 3.1-fold stimulation of the activity of the hCRH promoter. Tetradecanoyl-phorbol-13-acetate stimulated the c-fos and c-jun transactivation by a factor of 10 and 2.6, indicating a modification of c-fos and c-jun is required for the transactivation induced by protein kinase C activation. Mutation within the cAMP response element abolished this transcriptional activation. This result suggests an involvement of the protein kinase C pathway in the regulation of the CRH gene expression.

Immunomodulatory ionophore copolymers, T150R1 and T130R2, induce corticotropin from anterior pituitary cells.

T150R1 is a synthetic copolymer with Na+ ionophore activity. We demonstrated previously that T150R1, when injected into mice, produces rapid thymic involution with depletion of cortical thymocytes. Elevated serum ACTH and corticosterone levels, as well as abrogation of the effects of T150R1 on the thymus by adrenalectomy and hypophysectomy, suggested a pituitary-mediated mechanism. In this work, we investigated the ability of T150R1, and of the related ionophore copolymer T130R2, to stimulate ACTH in vitro from the mouse anterior pituitary cell line AtT-20. Copolymer-induced ACTH release was dose-, time-, and temperature-dependent. Hormone induction peaked at 30 degrees C for T150R1 and 37 degrees C for T130R2. The temperature dependence of ACTH release paralleled that of ionophore activity measured in red blood cells, providing evidence that the ability to induce ACTH is related to the ionophore property of the copolymers. Peak ionophore activity and hormonal release occurred at the temperatures when the copolymers form partially soluble complexes which interact optimally with cell membranes. Cotreatment with exogenous phospholipase C inhibited the effects of T150R1, which suggests that the enzyme either blocks the insertion of T150R1 into the cell membrane or that the phospholipase C-induced increase in intracellular calcium inhibits the ionophore activity of T150R1. These data support an ionophore mechanism for copolymer-induced ACTH release in which changes in the physicochemical structure of the copolymers may affect their interaction with cell membranes. The data also suggest that direct stimulation of pituitary ACTH accounts for at least some of the in vivo immunomodulatory effects of T150R1.

Characterization of a neuronal interleukin-1 receptor and the corresponding mRNA in the mouse anterior pituitary cell line AtT-20.

Interleukin-1 (IL-1) mediates numerous responses on the mouse anterior pituitary cell line AtT-20. We have studied the ligand binding properties of the IL-1 receptors (IL-1R) of the AtT-20 cells and found that they possess 1.5 x 10(3) receptors/cell with a Kd of 0.15 nM for [125I]IL-1 alpha. Using oligonucleotide primers, which define a 236 bp region of the cDNA coding for the cytosolic part of the mouse T-cell IL-1R, and cDNA prepared from AtT-20 cells, we obtained by polymerase chain reaction (PCR) a DNA fragment which was shown to possess an identical sequence to that of the corresponding region of the mouse T-cell IL-1R. Thus the AtT-20 pituitary cell line IL-1 receptor appears to be similar to those found on mouse T-cells and fibroblasts.

Interleukin-1 potentiation of β-endorphin secretion and the dynamics of interleukin-1 internalization in pituitary cells

1. This study demonstrates that human recombinant Interleukin-1 (IL-1) stimulates β-endorphin release and potentiates the secretion of β-endorphin in both a mouse anterior pituitary cell line AtT-20 and rat pituitary cell cultures. 2. In pituitary cell cultures, prolonged treatment with phorbol ester had no effect on IL-1-induced β-endorphin release, but abolished the potentiating effects of IL-1 on vasopressin-induced β-endorphin secretion. 3. The enhancement of CRF-stimulated β-endorphin release by IL-1 was also reduced in normal pituitary cell cultures following depletion of protein kinase C. 4. The late IL-1-induced secretion of β-endorphin does not require the continuous presence of the cytpkine. 5. Incubation of monolayers with 125I-IL-1α (10 −9M) at 8°C and then at 37°C for various times revealed that IL-1α was internalized. There was a progressive increase in the ratio of cytoplasmic to cell-surface-associated 125I-IL-1α. 6. These results indicate that the IL-1-induced β-endorphin release and its potentiation of β-endorphin secretion involves internalization of this cytokine, perhaps via cell surface IL-1 receptors.

Low temperature blocks exit of pro‐opiomelanocortin from the endoplasmic reticulum but not subsequent delivery to the site of prohormone processing

The effect of reduced temperature on the delivery of the prohormone pro-opiomelanocortin (POMC) to the site of prohormone processing was investigated in the mouse anterior pituitary cell line AtT20. At 20 degrees C processing was substantially inhibited and was almost completely arrested at 18 degrees C. Earlier studies with membrane glycoproteins indicated that at these temperatures protein movement was blocked at the level of exit from the Golgi apparatus. In contrast it was found here that the inhibition of processing at reduced temperature was due to the retention of POMC in the endoplasmic reticulum. When POMC was allowed to progress to the Golgi before temperature was reduced, subsequent processing was only slightly retarded by incubation at 18 degrees C. This indicates either that Golgi exit is not inhibited at this temperature, or that the processing apparatus exists in the Golgi. A surprising incidental result was that when held in the endoplasmic reticulum at low temperature POMC is apparently subject to post-translational N-linked glycosylation.

Expression and posttranslational processing of preprodynorphin complementary DNA in the mouse anterior pituitary cell line AtT-20.

A recombinant plasmid containing the rat prodynorphin cDNA was introduced into the mouse anterior pituitary corticotroph cell line AtT-20. These cells normally express and posttranslationally process proopiomelanocortin, but not prodynorphin. Stable transformants were isolated and analyzed for the expression and processing of prodynorphin. The stably transformed AtT-20 cells that expressed a 1.3-kilobase prodynorphin mRNA also expressed prodynorphin protein and processed it to dynorphin peptides. The peptides included leucine-enkephalin, beta-neoendorphin, dynorphin-A8, and dynorphin-B, as identified by gel filtration and reverse phase HPLC followed by RIA using peptide-specific antisera. These results demonstrate that AtT-20 cells efficiently and accurately process prodynorphin at both dibasic sites and monobasic cleavage sites, indicating that the AtT-20 cells contain enzymes capable of cleaving the precursor not only at dibasic residues but also at monobasic residues. The release of prodynorphin-derived peptides paralleled secretion of endogenous proopiomelanocortin-derived peptides when stimulated by CRF, a natural secretagogue for ACTH.

Purification of a putative brain somatostatin receptor.

The brain somatostatin (somatotropin release-inhibiting factor; SRIF) receptor was purified by affinity chromatographic techniques. A protein of 60 kDa could be purified from rat brain. The protein was eluted from a [D-Trp8]SRIF affinity column with either sodium acetate (pH 5.5) or free [D-Trp8]SRIF. The binding of the protein to the affinity column was prevented by free [D-Trp8]SRIF or the stable SRIF analogue SMS 201-996 but not by the inactive somatostatin 28-(1-14). The purified receptor could be covalently labeled by the 125I-labeled SRIF analogue CGP 23996. Excess [D-Trp8]SRIF blocked the binding of 125I-labeled CGP 23996 to the purified receptor, but somatostatin 28-(1-14) did not affect the binding. A 60-kDa protein was also purified from the anterior pituitary cell line AtT-20, which has a high expression of SRIF receptors. In contrast, no 60-kDa protein could be purified from CHO cells, which have no detectable SRIF receptors. These findings present evidence for the purification of the SRIF receptor.

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.