Adrenomedullin and its receptors are expressed in the zona glomerulosa of human adrenal gland: evidence that ADM enhances proliferation and decreases apoptosis in cultured ZG cells

Adrenomedullin (ADM) is a hypotensive peptide, that derives from the proteolytic cleavage of pro(p)ADM and acts through two subtypes of receptors, called L1-receptor (L1-R) and calcitonin receptor-like receptor (CRLR). CRLR may function as a calcitonin gene-related peptide or a selective ADM receptor depending on the expression of the subtype 1 or the subtypes 2 and 3 of a family of proteins, named receptor-activity-modifying protein (RAMPs). Reverse transcription (RT)-polymerase chain reaction (PCR) consistently allowed the detection of pADM mRNA in zona glomerulosa (ZG), but not zona fasciculata-reticularis (ZF/R) cells of eight human adrenal cortexes. Despite the rather high level of pADM mRNA, immunocytochemistry and radioimmunoassay showed that the expression of ADM as protein in the ZG was weak. However, ZG cells expressed peptidyl-glycine α-amidating monooxigenase, the enzyme converting immature ADM to the mature peptide, thereby suggesting their potential ability to produce active ADM. RT-PCR demonstrated the presence in ZG, but not ZF/R cells of the specific mRNAs of L1-R, CRLR and RAMPs1-3. Semiquantitative PCR showed that L1-R expression was higher than that of CRLR, while the level of expression of the three RAMPs was nearly the same. ADM (10-8 M) inhibited both angiotensin-II (10-9 M)- and K+ (10-2 M)-stimulated aldosterone secretion from cultured ZG cells, without affecting basal production. ADM (10-8 M) also increased proliferative activity and lowered apoptotic deletion of cultured cells. All the effect of ADM were reversed by the ADM-receptor antagonist ADM22-52. In conclusion our study provides evidence that i) human ZG cells express ADM and ADM receptors of both L1 and CRLR/PAMP2,3 subtypes; and ii) through the activation of these receptors, ADM exerts an aldosterone antisecretagogue action and a growth promoting effect on cultured ZG cells, the latter an effect which includes both stimulation of proliferation and inhibition of apoptosis. Taken together, these findings make it likely that endogenous ADM system plays a potentially important role in the paracrine/ autocrine regulation of human adrenal cortex.