Overexpression of serotonin4 receptors in cisapride-responsive adrenocorticotropin-independent bilateral macronodular adrenal hyperplasia causing Cushing's syndrome.

The serotonin4 (5-HT4) receptor agonists cisapride and/or metoclopramide have been shown to stimulate cortisol secretion in some patients with ACTH-independent bilateral macronodular adrenal hyperplasias (AIMAH) causing Cushing's syndrome. In the present study, we have investigated quantitatively and qualitatively the expression of the 5-HT4 receptor in both normal adrenal cortex and tissues removed from six patients (P1-P6) with cisapride-responsive AIMAH and Cushing's syndrome. Real-time quantitative PCR assay revealed that the 5-HT4 receptor was overexpressed in four of the six hyperplasias studied when compared with normal adrenal cortex. In these tissues, 5-HT4 receptor mRNA expression was 3 to 16 times higher than in normal glands, likely explaining the abnormal in vivo cortisol response to cisapride. Characterization of 5-HT4 receptor splice variants by RT-PCR in both hyperplastic and normal adrenals showed that the variants present in the two hyperplasias that did not overexpress the 5-HT4 receptor, i.e. P2 and P5, could also be detected in the normal adrenal tissue. In addition, sequencing of the full-length cDNAs encoding 5-HT4 receptors in hyperplasias P2 and P5 did not reveal any mutation. Taken together, our results show an overexpression of the 5-HT4 receptor in cisapride-responsive AIMAH. However, in two cases, the level of expression of the receptor in the hyperplastic adrenal cortex was similar to that of normal adrenal gland. The enhanced sensitivity of these two tissues to 5-HT4 receptor agonists was not due to ectopic expression of 5-HT4 receptor isoforms or to the occurrence of somatic gain-of-function mutation of the receptor.