Thyrotropin-releasing Hormone Stimulation of Adrenocorticotropin Production by Mouse Pituitary Tumor Cells in Culture

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Abstract
ACTH-producing mouse pituitary tumor cells in culture (AtT-20/NYU-1 cells) were found to have binding sites for thyrotropin-releasing hormone (TRH). These putative receptors bound TRH with high affinity; the apparent equilibrium dissociation constant was 3.7 nM. The affinity of the receptors for a series of TRH analogues was similar to those previously reported for TRH-receptor interactions on thyrotropic and mammotropic cells in culture. Like some human pituitary tumors in situ, AtT-20/NYU-1 cells were found to produce the alpha subunit of the glycoprotein hormones (alpha). Alpha accumulation in the medium was constant (3.1 ng/mg cell protein per h) and was not affected by TRH. In contrast, TRH increased the amount of ACTH accumulated in the medium from AtT-20/NYU-1 cells to 190 and 420% of control at 1 and 24 h, respectively. TRH induced a dose-dependent increase in ACTH release during a 30-min incubation; half-maximal stimulation occurred at ∼0.1 nM. TRH had no effect on ACTH release in vitro from anterior pituitary cells derived from normal rats. Because TRH stimulates release of ACTH in some untreated patients with Cushing's disease and Nelson's syndrome as well as pathological states associated with pituitary tumors (but not in normal subjects), AtT-20/NYU-1 cells may serve as an important in vitro model for human pituitary ACTH-secreting adenomas. Moreover, these findings suggest that the primary abnormality in Cushing's disease and Nelson's syndrome, allowing TRH stimulation of ACTH release, may be intrinsic to neoplastic adrenocorticotrophs rather than in neuroregulation of ACTH release.