THYROTROPIN-RELEASING-HORMONE GENE-EXPRESSION IN THE ANTERIOR-PITUITARY - IV - EVIDENCE FOR PARACRINE AND AUTOCRINE REGULATION

Disulfiram (Dis), an inhibitor of peptidyl-glycine alpha-amidating mon ooxygenase, the enzyme responsible for the production of alpha-amidate d peptides from their immediate, glycine-extended precursors was used to investigate the paracrine effects of TRH on anterior pituitary (AP) hormone secretion. It reduces the production of TRH without directly affecting the classical pituitary hormones, none of which is amidated. Dis (8 mu M) decreased the accumulation of TRH accompanied by an equi molar increase in TRH-Gly levels, indicating that pro-TRH biosynthesis persisted. TRH and TSH release into the medium was significantly lowe red, whereas other pituitary hormones were unaffected. In contrast, de xamethasone (10 nM), which up-regulates TRH gene expression in this sy stem, increased TRH (+89.5%) and TSH (+61.3%) secretion. The combinati on of dexamethasone and Dis further diminished the release of TRH (-73 %) and TSH (-40.3%) observed with Dis alone, indicating that TRH synth esized within the AP regulates TSH secretion. Dis significantly elevat ed prepro-TRH (25-50) and pro-TRH messenger RNA levels, suggesting tha t reduced TRH formation leads to increased pro-TRH biosynthesis and th at TRH regulates its own secretion. Thus, TRH synthesized by cultured AP cells not only stimulates TSH release through a paracrine effect, b ut has a negative feedback on its own biosynthesis by an autocrine mec hanism.