Adrenomedullin Stimulates Cell Proliferation and DNA Synthesis in Rat Adrenal Zona Glomerulosa (ZG) Via Activation of Tk-Mapk Signalling Pathways
Aim: To investigate the in vivo effect of adrenomedullin (ADM) on the proliferative activity of the rat adrenal cortex, using an intact left gland in situ perfusion model. Adrenal glands were exposed to ADM and other chemicals, added to the perfusion medium, for 180 min. Results: ADM concentration-dependently increased the mitotic index and [3H]thymidine incorporation into DNA in the ZG (maximally effective [ADM] being 10-8 M), but not in inner adrenocortical layers. Both the calcitonin gene-related peptide (CGRP) type 1receptor antagonist CGRP(8-37) and the ADM C terminal fragment ADM(22-52) equipotently counteracted the effect of 10-8 M ADM. The adenylate cyclase (AC) inhibitor SQ-22536 (10-4 M), and the protein kinase (PK)A inhibitor H-89 (10-5 M) did not affect ADM-elicited increase in DNA synthesis, although counteracting the ZG proliferogenic action of 10-9 M ACTH. The phospholipase (PL) C inhibitor U-73122 (10-5 M), and the PKC inhibitor calphostin-C (10-5 M), which significantly inhibited the ZG proliferogenic effect of 10-9 M angiotensin-II (Ang II) were also uneffective on ADM response. Both ADM and Ang II (10-7M) activated MAPK kinase (by 30 and 70%) and the phosphorylation of ERK-1 and ERK-2 kinase (by 30% and by 40%), respectively. Activation of ERK-1 and ERK-2 occurred via a MEK kinase-dependent mechanism, since it was abolished by the MEK inhibitor PD-98059. No such effects of ADM were seen in cells from inner adrenocortical layers. The growth-promoting effect of ADM was unaffected by the PLA2 inhibitor AACOCF3 (10-5 M), the COX inhibitor indomethacin (10-5 M) and the mixed COX/lipoxygenase (LOX) inhibitor phenidone (10-5 M). In contrast, it was abolished by either the tyrosine kinase (TK) inhibitor tyrphostin-23 (10-5 M) or by PD-98059 (104 M). Conclusions: 1) ADM exerts a mitogenic effect in the rat ZG, through ADM(22-52)-sensitive CGRP1 receptors; 2) this effect involves activation of the TKMAPK cascade, but not of the AC/PKA-, PLC/PKC-, and COX- or LOX-dependent signaling pathways.