Storage and secretion of β-endorphin and related peptides by mouse pituitary tumor cells: Regulation by glucocorticoids

Abstract

Clonal AtT-20 mouse pituitary tumor cells synthesize and secrete corticotropin, β-lipotropin, and related opioid peptides (endorphins). AtT-20/D16v cells contained approximately 540 pmol/mg protein of β-lipotropin, β-endorphin, γ-endorphin, and α-endorphin in the molar proportions 9, 64, 5, and 22%, respectively. The subcellular distribution of opioid activity and β-endorphin immunoreactivity indicates the localization of endorphins in particulate cytoplasmic organelles. AtT-20/D16v cells and cells of a newly derived subclone secrete β-lipotropin and β-endorphin into culture medium at a basal rate of up to 28 pmol/mg protein/ hr. Treatment of cells with dexamethasone (half-maximally effective concentration 2 n m) reduced basal secretion; part of the inhibitory response occurred rapidly ( t 1 2 approximately 2.4 h) and another part occurred considerably more slowly. The rapid phase of the response was prevented by actinomycin D or cycloheximide. Reduction of intracellular β-endorphin immunoreactivity by dexamethasone followed kinetics resembling the slower phase. Secretion was stimulated 1.8-fold or 7-fold by 1 m m N 6,O 6′ -dibutyryl cyclic AMP or 80 m m KCl, respectively; dexamethasone inhibited secretion stimulated by either compound. It is concluded that (a) AtT-20 cells probably store and secrete β-endorphin-related peptides by mechanisms similar to those of corticotropin-endorphin producing cells of the anterior pituitary gland; and (b) glucocorticoids rapidly inhibit secretion of these peptides by a mechanism that involves increased transcription and translation of protein(s) which inhibit synthesis or increase catabolism of the peptides.