Fine tuning of adrenocortical functions by locally produced growth factors.

Adult adrenal cortex is a differentiated endocrine tissue that undergoes permanent regeneration. Proliferation of the progenitor cells is restricted to the outer quarter of the cortex (Zajicek et al. 1986, Mc Nicol & Duffy 1987). After this initial proliferative step, adrenocytes are displaced centripetally, until they reach the junction to the medulla (the whole process takes about 100 days in the rat) (Zajicek et al. 1986). Half of the cells die on the way and the remainder are eliminated by apoptosis in the reticularis zone (Wyllie et al. 1973). During this displacement, adrenocytes acquire three distinct functional and morphological phenotypes characterized by different patterns of steroidogenic enzyme gene expression. Younger cells differentiate into glomerulosa cells, acquiring the capacity to synthesize aldosterone. Further along the way, adrenocytes become fasciculata cells, expressing P450c17 (steroid 17á-hydroxylase) and synthesizing cortisol and/or corticosterone. In the innermost zone, the oldest adrenocytes acquire the reticularis phenotype, characterized in primates by the ability to synthesize adrenal androgens (dehydroepiandrosterone). Subcapsular remnants obtained after adrenal enucleation are able to regenerate differentiated cortical tissue with functional fasciculata and reticularis zones, even when grafted in ectopic locations (Skelton 1959, Belloni et al. 1990). The subcapsular glomerulosa zone thus probably contains what may be considered the adrenocyte stem cells. Two fairly recent reports suggest, however, that this may not be the only location of stem cells. Mitani et al. (1994) identified a layer of cells between the zona glomerulosa and the zona fasciculata that do not express aldosterone synthase and 11â-hydroxylase and proposed that it represents the layer of adrenocyte stem cells. These cells were not isolated and therefore their potential for conversion into differentiated adrenocytes with a glomerulosa or fasciculata phenotype has not been proven. More recently, Thomas et al. (1997) succeeded in reconstituting a fully differentiated and vascularized adrenocortical organoid by transplanting clones of cells derived from a single bovine fasciculata cell, in the renal capsule of scid mice. Some 25% of the clones transplanted were able to successfully form cortisol-secreting tissue, suggesting that some but not all of the fasciculata cells that possess the capacity to form a cell clone in vitro can be considered stem cells. Several experimental approaches over the last few decades have clearly established that the pituitary hormone adrenocorticotrophin (ACTH) is the primary regulator of both fetal adrenal development and adult adrenal cortex homoeostasis. Hypophysectomy of adult animals results in a dramatic regression of the adrenocortical tissue caused by a massive deletion of the fasciculata and reticularis zones through apoptosis while the glomerulosa zone remains intact and functional (Nussdorfer & Mazzochi 1972, Carsia et al. 1996). More delicate removal of the sole corticotrope and melanotrope cell populations from the pituitary gland, induced by ganciclovir treatment of transgenic mice expressing the herpes simplex virus thymidine kinase transgene under the control of the pro-opiomelanocortin promoter, results in a similar regression of the adrenal cortex tissue (Allen et al. 1995). Besides its trophic action, ACTH is also a major regulator of adrenal cortex function, i.e. steroidogenesis. It stimulates the synthesis of cortisol by two distinct mechanisms. ACTH binding to its adenylate cyclase-coupled receptor MC2-R acutely increases steroid biosynthesis by stimulating cholesterol esterase, thereby releasing free cholesterol from stored cholesterol ester pools and making substrate available to the steroidogenic enzymes in the cell (Stocco & Clark 1996). ACTH is also able to chronically increase steroidogenesis by stimulating the transcription of several key cAMP-responsive genes that encode steroidogenic enzymes (Miller 1988, Simpson & Waterman 1988) and of the mitochondrial protein StAR (Steroidogenic acute regulatory protein), an essential component of the intramitochondrial cholesterol-transfer machinery (Clark et al. 1995). In vitro studies of the proliferative action of ACTH using adrenocortical cell subpopulations from the glomerulosa or the fasciculata zones have generated controversial results. Some authors report a decrease in cell number and 7