Plasticity of peptide biosynthesis in corticotropes: independent regulation of different steps in processing

Compared to corticotropes in the adult rat anterior pituitary, neonatal corticotropes exhibit a significantly lower extent of conversion of precursor molecules into ACTH and a substantially greater extent of ACTH cleavage into smaller product peptides similar in size to alpha-melanotropin and corticotropin-like intermediate lobe peptide (CLIP). In the present study we examined pro-ACTH/endorphin (PAE; also called POMC) processing in corticotropes at different times during postnatal development to determine when these cells cease cleaving ACTH into smaller peptides and when they cease accumulating large amounts of intact precursor in vivo. The pattern of processing observed in the newborn is maintained through the second week after birth. A dramatic decrease in ACTH cleavage occurs between the second and third postnatal weeks. The extent of precursor cleavage increases toward the adult pattern by the fifth postnatal week. Explants of newborn anterior pituitary were previously shown to exhibit increased cleavage of ACTH into ACTH-(1-13)NH2 and CLIP, a process that was suppressed by glucocorticoids. To determine whether corticotropes from older animals maintained this plasticity and what intercellular interactions might be required, dissociated primary cultures were maintained in complete serum-free medium with or without added glucocorticoids. After 7 days in complete serum-free medium, the cellular content of both intact precursor and peptides the size of CLIP was increased compared to the corresponding in vivo pattern for animals from birth through adulthood. Although corticotropes from younger animals exhibited more pronounced changes when placed in culture, even cultures from adult animals exhibited some ACTH cleavage. For corticotrope cultures prepared from animals up to postnatal day 15, chronic treatment with dexamethasone did not suppress ACTH cleavage activity, although glucocorticoids did suppress ACTH cleavage in cultures from animals older than postnatal day 22. Biosynthetic labeling studies with cultures from 4- to 5-week-old rats demonstrated that the powerful suppressive effect of dexamethasone on the cleavage of newly synthesized ACTH-(1-39) was only evident 24 h after addition of the glucocorticoid to the culture medium. In contrast, removal of dexamethasone allowed cleavage of ACTH to commence within a few hours.