Regulated endocrine-specific protein-18: a short-lived novel glucocorticoid-regulated endocrine protein

Regulated endocrine-specific protein-18 (RESP18) is an 18-kilodalton endocrine-specific transcript whose expression is regulated by a number of different physiological and pharmacological stimuli in different tissues. RESP18 messenger RNA was identified in all cell types in the anterior pituitary, at levels that varied 2-fold from the lowest (corticotropes and thyrotropes) to the highest (gonadotropes, somatotropes, and mammotropes); the melanotropes of the intermediate pituitary have levels of RESP18 messenger RNA comparable to the highest levels in cells in the anterior pituitary. Mouse RESP18 was cloned and used as the basis for biosynthetic studies on RESP18 in AtT-20 cells, which express RESP18 endogenously; mouse RESP18 was highly homologous to rat RESP18. Pulse-chase biosynthetic labeling studies showed that AtT-20 cells expressed much less RESP18 than the endogenous prohormone, POMC, but that glucocorticoid treatment lowered POMC and raised RESP18 biosynthetic rates so that they were nearly equimolar. Surprisingly, RESP18 was not processed to smaller peptides to any significant extent, nor was RESP18 or any smaller peptide secreted. Newly synthesized RESP18 normally disappeared from AtT-20 cell extracts with a half-life of less than 15 min; the intracellular half-life of RESP18 was increased strikingly after glucocorticoid treatment of the cells. Upon subcellular fractionation, RESP18 was found to be entirely particulate and to cofractionate with markers for the endoplasmic reticulum, rather than with markers for secretory granules, such as POMC and prohormone-processing enzymes. Therefore, RESP18 is a major glucocorticoid-responsive protein in the secretory pathway of corticotropes, but its function may be entirely within the neuroendocrine cell.