Cyclic AMP-dependent signaling aberrations in macronodular adrenal disease

Abstract

The adrenal glands are a major source of steroid hormone biosynthesis. In normal physiology, the pituitary hormone corticotropin (ACTH) regulates the secretion of glucocorticoids via its G protein-coupled receptor (ACTHR), the product of the MC2R gene. Aldosterone is another major product of the adrenal gland; its regulation is controlled mainly by the renin-angiotensin system, although ACTH plays a role, too, especially under certain pathological conditions. The adrenal gland also secretes lesser amounts of androgens and intermediate metabolites of all these steroids. Unregulated secretion of any of these hormones can be caused by tumors, adrenocortical adenomas or carcinomas, and/or bilateral (or, rarely, unilateral) hyperplasia. Cortisol-producing hyperplasia of the adrenal glands is caused by two distinct syndromes, both of which have been directly or indirectly associated with protein kinase A signaling: (i) primary pigmented nodular adrenocortical disease (PPNAD) (a micronodular form of bilateral adrenal hyperplasia), either isolated (rarely) or in the context of Carney complex, is caused (in most cases) by mutations of the PRKAR1A gene; and (ii) ACTH-independent macronodular adrenal hyperplasia (AIMAH), or massive macronodular adrenal disease (MMAD), has been associated with aberrant (ectopic) expression, and presumably regulation, of various G protein-coupled receptors. AIMAH is a rare, sporadic condition affecting predominantly middle-aged men and women with an almost equal ratio (the latter in contrast to other forms of endogenous Cushing's syndrome). Some familial cases of AIMAH have also been described, and it appears that the pathophysiological phenomena underlying AIMAH may be present in the far more common, sporadic adrenocortical tumors and, perhaps, in the nodular growth detected in the adrenal glands of the elderly in the general population. Thus, the study of ectopic receptor expression and cAMP-dependent PKA activity in AIMAH may have wider implications for adrenal and, indeed, endocrine tumorigenesis.