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

Abstract

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 ,B-adrenergic receptor antagonist to AtT-20 membranes nor did it decrease basal cyclic AMP formation even in the presence ofexcess phosphodiesterase inhibitor, indicating that the reduction of cyclic AMP levels by SRIF did not involve either an interference with (-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.The AtT-20 mouse pituitary cell line has been extensively used to investigate the regulation ofsynthesis, storage, and secretion of the corticotropin (ACTH)/P-endorphin family of peptides (1-8). Recent studies in our laboratory have demonstrated the responsiveness ofthis clonal cell line to synthetic corticotropinreleasing factor (CRF) (9), to f32-adrenergic agonists, and to vasoactive-intestinal peptide (VIP) (unpublished data). All three substances increase cyclic AMP formation in the tumor cells and the combination of any two of the agents on cyclic AMP synthesis is additive (unpublished MN). Goat anti-rabbit immunoglobulin was from Cappel Laboratories (Cochranville, PA). All other chemicals were of reagent grade.Cell Culture Methods. AtT-20/D16-16 cells were grown in DME medium containing 10% fetal calf serum at 37°C in a humidified atmosphere of 10% C02/90% air as described (9).Incubation Procedure. Cells were plated in 35-mm diameter culture dishes (Costar) at an initial density of 1.5-2 x 105 cells per well and were used 5-7 days after subculturing (60-80% confluency). Before use, cells were equilibrated for 60 min at 37°C in 1 ml of DME medium/25 mM Hepes, pH 7.4, containing 2% fetal calf serum and bacitracin at 3 ,ug/ml. For ACTH secretion studies, the medium was aspirated and replaced with 1 ml of the same medium containing added test agent(s), and the cells were incubated for an additional 60 min. An aliquot of the medium was removed and centrifuged to remove detached cells and debris. The supernatant was frozen at -20°C until used for ACTH measurement. For cyclic AMP studies, cells were preincubated for 30 min with 0.5 mM IBMX in fresh medium. This medium then was aspirated and r...

Selective suppression of insulin-induced proliferation of cultured human hepatoma cells by somatostatin.

“…Recently, it was estimated that the halflife of infused SRIF in the plasma of normal human subjects is only -2 min (7). However, SRIF has also been identified in pancreas (18) Since both insulin and SRIF act through their own specific receptors on the cell surface (23), the selective inhibition ofinsulin but not T3-induced cell proliferation by SRIF may result from the interference of the membrane-bound signal transduction systems ofhormones (24). It has been reported that SRIF inhibits multi receptor-mediated adenylate cyclase activity through the inhibitory guanine nucleotide binding protein (25).…”

Somatostatin system: molecular mechanisms regulating anterior pituitary hormones

“…SRIF treatment resulted in Ca 2C -dependent redistribution of cytoplasmic microfilaments, without affecting intracellular somatotroph GH content (Shimada et al 1990), in addition to reduced association of exocytosis-associated RAB3B and SNARE proteins (Matsuno et al 2003 Adenylate cyclase/cAMP/PKA signalling SRIF inhibits pituitary adenylate cyclase/cAMP/PKA signaling, thereby inhibiting pituitary hormone synthesis and cell growth. SRIF inhibits cAMP production and ACTH secretion induced by CRH, forskolin, isoproterenol, vasoactive intestinal polypeptide (VIP), and cholera toxin in AtT-20 cells (Heisler et al 1982). Similarly, SRIF inhibits cAMP and GH production induced by GHRH stimulation in primary pituitary cells (Bilezikjian & Vale 1983).…”

Lithium induces corticotropin secretion and desensitization in cultured anterior pituitary cells.

“…ACTH secretion by these cells is responsive to a number of stimulatory and inhibitory neurotransmitters, hormones, and toxins (27). These include corticotropin-releasing factor (CRF) (28), isoproterenol (29), potassium (30), somatostatin (30,31), dexamethasone (28,29,32), forskolin (31,33), and phorbol esters (32,34). The effects of several of these agents appear to be mediated by changes in cellular cyclic AMP (27,33,35), but certain effects (of potassium, for example) are not well accounted for by changes in cyclic AMP and seem to involve calcium (35).…”