ACTH is the major regulator of the body's adaptive response to stress and the physiological stimulus for
glucocorticoid secretion. In addition to the known negative feedback regulation of
ACTH by
glucocorticoids, a hypothalamic
corticotropin release-inhibiting factor (CRIF) that inhibits
ACTH synthesis and secretion has been postulated, but not identified. We previously reported that transfection of
prepro-TRH complementary DNA into the mouse anterior
pituitary tumor cell line AtT-20 results in inhibition of basal and
corticotropin-releasing hormone (CRH)-stimulated
ACTH synthesis and secretion, suggesting that one or more of the cryptic
peptides encoded within the
prepro-TRH precursor has CRIF activity. To narrow the choice of
peptides responsible for CRIF activity, we first deleted specific sequences within the
prepro-TRH complementary DNA and transfected these constructs into AtT-20 cells. Deletion of sequences encoding
amino acids 119-229 resulted in the loss of CRIF activity. Of the
peptides encoded within this region, prepro-TRH-(178-199), a 22-amino
acid peptide, inhibited basal and CRH-stimulated
ACTH synthesis and secretion in cultured primary anterior pituitary cells. As this
peptide is processed from
prepro-TRH in vivo, is found in the external zone of the median eminence, and is secreted from hypothalamic slices in vitro, prepro-TRH-(178-199) fulfills the criteria for a physiological CRIF. The significance of TRH and CRIF sharing a common precursor opens new areas of research in the integrated regulation of pituitary-adrenal and pituitary-thyroid functions.