Adrenomedullin gene expression and its different regulation in human adrenocortical and medullary tumors

Adrenomedullin (ADM) is a polypeptide originally discovered in a human pheochromocytoma and is also present in normal adrenal medulla. It has been proposed that ADM could be involved in the regulation of adrenal steroidogenesis via paracrine mechanisms. Our aim was to find out if ADM gene is expressed in adrenocortical tumors and how ADM gene expression is regulated in adrenal cells. ADM mRNA was detectable by Northern blotting in most normal and hyperplastic adrenals, adenomas and carcinomas. The average concentration of ADM mRNA in the hormonally active adrenocortical adenomas was about 80% and 7% of that in normal adrenal glands and separated adrenal medulla respectively. In adrenocortical carcinomas, the ADM mRNA concentration was very variable, but on average it was about six times greater than that in normal adrenal glands. In pheochromocytomas, ADM mRNA expression was about ten times greater than that in normal adrenals and three times greater than in separated adrenal medulla. In primary cultures of normal adrenal cells, a protein kinase C inhibitor, staurosporine, reduced ADM mRNA accumulation in a dose- and time-dependent fashion (P < 0.01), whereas it simultaneously increased the expression of human cholesterol side-chain cleavage enzyme (P450 scc) gene (a key gene in steroidogenesis). In cultured Cushing's adenoma cells, adrenocorticotropin, dibutyryl cAMP ((Bu)2cAMP) and staurosporine inhibited the accumulation of ADM mRNA by 40, 50 and 70% respectively (P < 0.05), whereas the protein kinase C activator, 12-O-tetradecanoyl phorbol 13-acetate (TPA), increased it by 50% (P < 0.05). In primary cultures of pheochromocytoma cells, treatment with (Bu)2cAMP for 1 and 3 days increased ADM mRNA accumulation two- to threefold (P < 0.05). Our results show that ADM mRNA is present not only in adrenal medulla and pheochromocytomas, but also in adrenocortical neoplasms. Both protein kinase A- and C-dependent mechanisms regulate ADM mRNA expression in adrenocortical and pheochromocytoma cells supporting the suggested role for ADM as an autocrine or paracrine (or both) regulator of adrenal function.