The retinoic acid receptor is a type II nuclear receptor involved with transcriptional regulation. In a murine corticotrope tumor cell line, retinoic acid has been shown to reduce ACTH secretion and pro-opiomelanocortin (POMC) synthesis. Further, there is evidence that, with prolonged treatment, retinoic acid induces increased caspase-3 activity and cell death in ACTH-secreting cells. It has also demonstrated inhibition of corticosterone production and cell proliferation in adrenal cortex cells [69]. Retinoic acid had demonstrated reduced ACTH secretion and prevented tumor growth in mice with implanted with tumoral corticotropes [70]. In a 6-month study in dogs with spontaneous corticotrope tumors, retinoic acid reduced cortisol excess and improved associated symptoms [71].