CORTICOTROPIN RELEASE-INHIBITING FACTOR IS PREPROTHYROTROPIN-RELEASING HORMONE-(178-199)

ACTH is the major regulator of the body's adaptive response to stress and the physiological stimulus for glucocorticoid secretion. In additi on to the known negative feedback regulation of ACTH by glucocorticoid s, a hypothalamic corticotropin release-inhibiting factor (CRIF) that inhibits ACTH synthesis and secretion has been postulated, but not ide ntified. We previously reported that transfection of prepro-TRH comple mentary DNA into the mouse anterior pituitary tumor cell line AtT-20 r esults 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 C RIF activity. To narrow the choice of peptides responsible for CRIF ac tivity, we first deleted specific sequences within the prepro-TRH comp lementary DNA and transfected these constructs into AtT-20 cells. Dele tion 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-stimulate d 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 hypo thalamic slices in vitro, prepro-TRH-(178-199) fulfills the criteria f or a physiological CRIF. The significance of TRH and CRIF sharing a co mmon precursor opens new areas of research in the integrated regulatio n of pituitary-adrenal and pituitary-thyroid functions.