The mechanism by which cortisol is produced in adrenal Cushing's syndrome, when ACTH is suppressed, was previously unknown and was referred to as being "autonomous." More recently, several investigators have shown that some cortisol and other steroid-producing adrenal tumors or hyperplasias are under the control of ectopic (or aberrant, illicit, inappropriate) membrane hormone receptors. These include ectopic receptors for gastric inhibitory polypeptide (GIP), beta-adrenergic agonists, or LH/hCG; a similar outcome can result from altered activity of eutopic receptors, such as those for vasopressin (V1-AVPR), serotonin (5-HT4), or possibly leptin. The presence of aberrant receptors places adrenal cells under stimulation by a trophic factor not negatively regulated by glucocorticoids, leading to increased steroidogenesis and possibly to the proliferative phenotype. The molecular mechanisms responsible for the abnormal expression and function of membrane hormone receptors are still largely unknown. Identification of the presence of these illicit receptors can eventually lead to new pharmacological therapies as alternatives to adrenalectomy, now demonstrated by the long-term control of ectopic P-AR- and LH/hCGR-dependent Cushing's syndrome by propanolol and leuprolide acetate. Further studies will potentially identify a larger diversity of hormone receptors capable of coupling to G proteins, adenylyl cyclase, and steroidogenesis in functional adrenal tumors and probably in other endocrine and nonendocrine tumors.

Abnormal responsiveness of adrenocortical cells to gastric inhibitory polypeptide (GIP) in food-dependent Cushing's syndrome suggested that adrenal expression of ectopic, overexpressed, or mutated GIP receptor (GIPR) underlies this syndrome. The expression of GIPR was studied by RT-PCR in human adrenal tissues from two patients with GIP-dependent Cushing's syndrome (adenoma, bilateral hyperplasia), five fetal or adult controls, one patient with Cushing's disease, and four patients with non-food-dependent cortisol-secreting adenomas or bilateral hyperplasias and compared to that in normal pancreas. Hybridization of the RT-PCR-amplified ribonucleic acids with the human GIPR complementary DNA showed an overexpression of GIPR in the adrenals of the two GIP-dependent Cushing's syndrome patients compared to that in normal adrenal tissues (2-3 orders of magnitude) or pancreas (10-fold); no signal could be seen in adrenal adenomas or macronodular hyperplasia from cases of non-food-dependent Cushing's syndrome. No mutation of the GIPR was identified by sequencing the full-length receptor in GIP-dependent adrenal tissue. New alternative spliced isoforms of the GIPR were found, but are identical in GIP-dependent and normal adrenal tissues. Incubation of adrenal cells with GIP stimulates cortisol secretion in GIP-dependent, but not in normal fetal, adult, or non-food-dependent Cushing's syndrome, adrenals. We conclude that the GIPR overexpression and its coupling to steroidogenesis underlie GIP-dependent Cushing's syndrome.

Gastric inhibitory polypeptide (GIP)-dependent Cushing's syndrome has been reported to occur either in unilateral adrenal adenoma or in bilateral macronodular adrenal hyperplasia. A 33-yr-old woman with Cushing's syndrome was found to have two 2.5- to 3-cm nodules in the right adrenal on computed tomography scan; the left adrenal appeared normal except for the presence of a small 0.8 x 0.6-cm nodule. Uptake of iodocholesterol was limited to the right adrenal. Plasma morning cortisol was 279 nmol/L fasting and 991 nmol/L postprandially, and ACTH remained suppressed. Plasma cortisol increased after oral glucose (202%) or a lipid-rich meal (183%), but not after a protein-rich meal (95%) or iv glucose (93%); the response to oral glucose was blunted by pretreatment with 100 microg octreotide, sc. Plasma cortisol and GIP levels were positively correlated (r = 0.95; P = 0.0001); cortisol was stimulated by the administration of human GIP iv (225%), but not by GLP-1, insulin, TRH, GnRH, glucagon, arginine vasopressin, upright posture, or cisapride orally. A right adrenalectomy was performed; GIP receptor messenger ribonucleic acid was overexpressed in both adrenal nodules and in the adjacent cortex. Histopathology revealed diffuse macronodular adrenal hyperplasia without internodular atrophy. Three months after surgery, fasting plasma ACTH and cortisol were suppressed, but cortisol increased 3.6-fold after oral glucose, whereas ACTH remained suppressed; this was inhibited by octreotide pretreatment, suggesting that cortisol secretion by the left adrenal is also GIP dependent. We conclude that GIP-dependent nodular hyperplasia can progress in an asynchronous manner and that GIPR overexpression is an early event in this syndrome.

. Angiotensin II (AII) actions are mediated by two distinct types of receptors: AT1, which includes two subtypes, AT1A and AT1B, and AT2. AII produces vasoconstriction on the vascular wall acting directly on smooth muscle cells via AT1 receptors. AII receptors have recently been demonstrated on endothelial cells. But the pharmacological characteristics of these receptors and the intracellular signal pathways coupled to them remain unclear. . The aim of this work was to characterize the AII receptor subtypes in rat aortic endothelial cells (RAEC) in primary culture and to evaluate the signal pathways coupled to these receptors by measuring the activation of phospholipase C (PLC) and phospholipase A2 (PLA2). . Labelled AII bound to RAEC in a specific, saturable manner. Scatchard analysis showed a Kd of 1.87±0.49 nM and a Bmax of 50.2±10.9 × 103 sites per cell. AII was displaced by the AT1‐specific antagonist, DuP753 with a Ki of 17.37 ± 1.49 nM, but not by the AT2 receptor analogues CGP42771B or PD123177. These data were confirmed by the finding of AT1 mRNA in endothelial cells. Analysis of RNA expression by RT‐PCR showed the presence of both subtypes, AT1A and AT1B, in endothelial cells, whereas smooth muscle cells express only AT1A. . The activation of PLC and PLA2 in response to AII was evaluated by measuring inositol phosphate production and arachidonic acid release, respectively. Both were enhanced by AII in a dose‐dependent manner, and inhibited by DuP753, but not by PD123177. . We conclude that AT1 receptors are expressed by endothelial cells in primary culture and that phospholipase C and phospholipase A2 are activated via this receptor.