Retinoic acid specifically inhibits ACTH production in human neuroendocrine tumors. (a) Human pituitary adenoma cells from eight patients with Cushing disease were treated with 10 nM or 100 nM retinoic acid (RA) for 24 hours, and ACTH was measured by RIA in the supernatants. Values at the top represent the basal ACTH production of each tumor, and the bars indicate percentages of these values after retinoic acid treatment. (b) NCI-N-592 small-cell lung cancer cells were treated with retinoic acid. After 24 hours, ACTH was measured in the supernatants. (c–e) Normal rat pituitary cells were treated for 24 hours with retinoic acid (RA), 5 μM forskolin (Forsk), 10 nM hydrocortisone (HC), 100 nM thyrotropin-releasing hormone (TRH), 1 μM bromocriptine (BrCr), 100 nM growth hormone–releasing hormone (GHRH), or 100 nM octreotide (Oct). Supernatants were collected and ACTH (c), prolactin (d), and growth hormone (GH) (e) were measured by RIA. Averages of four wells per treatment and the corresponding SEs are shown. *P < 0.001 compared with the corresponding basal values, **P < 0.01 compared with the stimulated values, determined by ANOVA in combination with the Scheffé test. b–e are representative of three independent experiments. (f) COUP-TFI expression was detected in normal human pituitary (n = 3) by immunoperoxidase staining (diaminobenzidine brown color). Inset: negative control. (g) Double immunohistochemistry for COUP-TFI (brown color, filled arrows) and ACTH (vector red color, open arrows). (h) COUP-TFI immunohistochemistry in Cushing adenomas was negative (n = 7).