Origins of zonation: the adrenocortical model of tissue development and differentiation

Steroid biomarkers in human adrenal disease

Adrenal steroidogenesis is a robust process, involving a series of enzymatic reactions that facilitate conversion of cholesterol into biologically active steroid hormones under the stimulation of angiotensin II, adrenocorticotropic hormone and other regulators. The biosynthesis of mineralocorticoids, glucocorticoids, and adrenal-derived androgens occur in separate adrenocortical zones as a result of the segregated expression of steroidogenic enzymes and cofactors. This mini review provides the principles of adrenal steroidogenesis, including the classic and under-appreciated 11-oxygenated androgen pathways. Several adrenal diseases result from dysregulated adrenal steroid synthesis. Herein, we review growing evidence that adrenal diseases exhibit characteristic modifications from normal adrenal steroid pathways that provide opportunities for the discovery of biomarker steroids that would improve diagnosis and monitoring of adrenal disorders.

Functional Zonation of the Adult Mammalian Adrenal Cortex

The standard model of adrenocortical zonation holds that the three main zones, glomerulosa, fasciculata, and reticularis each have a distinct function, producing mineralocorticoids (in fact just aldosterone), glucocorticoids, and androgens respectively. Moreover, each zone has its specific mechanism of regulation, though ACTH has actions throughout. Finally, the cells of the cortex originate from a stem cell population in the outer cortex or capsule, and migrate centripetally, changing their phenotype as they progress through the zones. Recent progress in understanding the development of the gland and the distribution of steroidogenic enzymes, trophic hormone receptors, and other factors suggests that this model needs refinement. Firstly, proliferation can take place throughout the gland, and although the stem cells are certainly located in the periphery, zonal replenishment can take place within zones. Perhaps more importantly, neither the distribution of enzymes nor receptors suggest that the individual zones are necessarily autonomous in their production of steroid. This is particularly true of the glomerulosa, which does not seem to have the full suite of enzymes required for aldosterone biosynthesis. Nor, in the rat anyway, does it express MC2R to account for the response of aldosterone to ACTH. It is known that in development, recruitment of stem cells is stimulated by signals from within the glomerulosa. Furthermore, throughout the cortex local regulatory factors, including cytokines, catecholamines and the tissue renin-angiotensin system, modify and refine the effects of the systemic trophic factors. In these and other ways it more and more appears that the functions of the gland should be viewed as an integrated whole, greater than the sum of its component parts.

Bone Morphogenetic Protein-4 (BMP4): A Paracrine Regulator of Human Adrenal C19 Steroid Synthesis

Bone morphogenetic proteins (BMPs) comprise one of the largest subgroups in the TGF-β ligand superfamily. We have identified a functional BMP system equipped with the ligand (BMP4), receptors (BMP type II receptor, BMP type IA receptor, also called ALK3) and the signaling proteins, namely the mothers against decapentaplegic homologs 1, 4, and 5 in the human adrenal gland and the human adrenocortical cell line H295R. Microarray, quantitative RT-PCR, and immunohistochemistry confirmed that BMP4 expression was highest in the adrenal zona glomerulosa followed by the zona fasciculata and zona reticularis. Treatment of H295R cells with BMP4 caused phosphorylation of the mothers against decapentaplegic and a profound decrease in synthesis of the C19 steroids dehydroepiandrosterone (DHEA), DHEA sulfate, and androstenedione. Administration of BMP4 to cultures of H295R cells also caused a profound decrease in the mRNA and protein levels of 17α-hydroxylase/17,20-lyase (CYP17A1 and P450c17, respectively) but no significant effect on the mRNA levels of cholesterol side-chain cleavage cytochrome P450 (CYP11A1) or type 2 3β-hydroxysteroid dehydrogenase (HSD3B2). Furthermore, Noggin (a BMP inhibitor) was able to reverse the negative effects of BMP4 with respect to both CYP17A1 transcription and DHEA secretion in the H295R cell line. Collectively the present data suggest that BMP4 is an autocrine/paracrine negative regulator of C19 steroid synthesis in the human adrenal and works by suppressing P450c17.

DACH1, a zona glomerulosa selective gene in the human adrenal, activates transforming growth factor-β signaling and suppresses aldosterone secretion

Common somatic mutations in CACNAID and ATP1A1 may define a subgroup of smaller, zona glomerulosa (ZG)-like aldosterone-producing adenomas. We have therefore sought signature ZG genes, which may provide insight into the frequency and pathogenesis of ZG-like aldosterone-producing adenomas. Twenty-one pairs of zona fasciculata and ZG and 14 paired aldosterone-producing adenomas from 14 patients with Conn's syndrome and 7 patients with pheochromocytoma were assayed by the Affymetrix Human Genome U133 Plus 2.0 Array. Validation by quantitative real-time polymerase chain reaction was performed on genes >10-fold upregulated in ZG (compared with zona fasciculata) and >10-fold upregulated in aldosterone-producing adenomas (compared with ZG). DACH1, a gene associated with tumor progression, was further analyzed. The role of DACH1 on steroidogenesis, transforming growth factor-β, and Wnt signaling activity was assessed in the human adrenocortical cell line, H295R. Immunohistochemistry confirmed selective expression of DACH1 in human ZG. Silencing of DACH1 in H295R cells increased CYP11B2 mRNA levels and aldosterone production, whereas overexpression of DACH1 decreased aldosterone production. Overexpression of DACH1 in H295R cells activated the transforming growth factor-β and canonical Wnt signaling pathways but inhibited the noncanonical Wnt signaling pathway. Stimulation of primary human adrenal cells with angiotensin II decreased DACH1 mRNA expression. Interestingly, there was little overlap between our top ZG genes and those in rodent ZG. In conclusion, (1) the transcriptome profile of human ZG differs from rodent ZG, (2) DACH1 inhibits aldosterone secretion in human adrenals, and (3) transforming growth factor-β signaling pathway is activated in DACH1 overexpressed cells and may mediate inhibition of aldosterone secretion in human adrenals.

Transcriptome profiling reveals differentially expressed transcripts between the human adrenal zona fasciculata and zona reticularis

CONTEXT

The human adrenal zona fasciculata (ZF) and zona reticularis (ZR) are responsible for the production of cortisol and 19-carbon steroids (often called adrenal androgens), respectively. However, the gene profiles and exact molecular mechanisms leading to the functional phenotype of the ZF and ZR are still not clearly defined. In the present study, we identified the transcripts that are differentially expressed in the ZF and ZR.

OBJECTIVE

The objective of the study was to compare the transcriptome profiles of ZF and ZR.

DESIGN AND METHODS

ZF and ZR were microdissected from 10 human adrenals. Total RNA was extracted from 10 ZF/ZR pairs and hybridized to Illumina microarray chips. The 10 most differentially expressed transcripts were studied with quantitative RT-PCR (qPCR). Immunohistochemistry was also performed on four zone-specific genes.

RESULTS

Microarray results demonstrated that only 347 transcripts of the 47 231 were significantly different by 2-fold or greater in the ZF and ZR. ZF had 195 transcripts with 2-fold or greater increase compared with its paired ZR, whereas ZR was found to have 152 transcripts with 2-fold or greater higher expression than in ZF. Microarray and qPCR analysis of transcripts encoding steroidogenic enzymes (n = 10) demonstrated that only 3β-hydroxysteroid dehydrogenase, steroid sulfotransferase, type 5 17β-hydroxysteroid dehydrogenase, and cytochrome b5 were significantly different. Immunohistochemistry and qPCR studies confirmed that the ZF had an increased expression of lymphoid enhancer-binding factor 1 and nephroblastoma overexpressed, whereas ZR showed an increased expression of solute carrier family 27 (fatty acid transporter) (SLC27A2), member 2 and TSPAN12 (tetraspanin 12) CONCLUSION: Microarray revealed several novel candidate genes for elucidating the molecular mechanisms governing the ZF and ZR, thereby increasing our understanding of the functional zonation of these two adrenocortical zones.

Lessons from the gene expression pattern of the rat zona glomerulosa

We recently identified hundreds of transcripts with differential expression in rat zona glomerulosa (zG) and zona fasciculata. Although the genes up-regulated in the zG may be playing important roles in aldosterone production, the relationship between most of these genes and aldosterone production has not been uncovered. Because aldosterone, in the presence of a high sodium diet, is now considered a significant cardiovascular risk factor, in this review we performed gene ontology and pathway analyses on the same microarray data to better define the genes that may influence zG function. Overall, we identified a number of genes that may be involved in aldosterone production through transforming growth factor β (TGF-β), WNT, calcium, potassium, and ACTH signaling pathways. The list of genes we present in the current report may become an important tool for researchers working on primary aldosteronism and aldosterone-related cardiovascular diseases.

Transcriptome analysis reveals differentially expressed transcripts in rat adrenal zona glomerulosa and zona fasciculata

In mammals, aldosterone is produced in the zona glomerulosa (zG), the outermost layer of the adrenal cortex, whereas glucocorticoids are produced in adjacent zona fasciculata (zF). However, the cellular mechanisms controlling the zonal development and the differential hormone production (i.e. functional zonation) are poorly understood. To explore the mechanisms, we defined zone-specific transcripts in this study. Eleven-week-old male rats were used and adrenal tissues were collected from zG and zF using laser-capture microdissection. RNA was isolated, biotin labeled, amplified, and hybridized to Illumina microarray chips. The microarray data were compared by fold change calculations. In zG, 235 transcripts showed more than a 2-fold up-regulation compared to zF with statistical significance. Similarly, 231 transcripts showed up-regulation in zF. The microarray findings were validated using quantitative RT-PCR and immunohistochemical staining on selected transcripts, including Cyp11b2 (zG/zF: 214.2x), Rgs4 (68.4x), Smoc2 (49.3x), and Mia1 (43.1x) in zG as well as Ddah1 (zF/zG 16.2x), Cidea (15.5x), Frzb (9.5x), and Hsd11b2 (8.3x) in zF. The lists of transcripts obtained in the current study will be an invaluable tool for the elucidation of cellular mechanisms leading to zG and zF functional zonation.

Gene array analysis of the effects of chronic adrenocorticotropic hormone in vivo on immature rat adrenal glands

Development of a mature adrenocortical phenotype is a critical event in the transition of mammals from fetal to postnatal life. We previously reported that the functional maturation of the adrenal glands of newborn rats is accelerated by adrenocorticotropic hormone (ACTH). We report here that chronic exposure of neonatal/juvenile rat pups to ACTH in vivo results in significant changes in expression of over 200 genes in the adrenal glands. ACTH significantly upregulated genes associated with cell signaling, gene transcription, cell migration and tissue remodeling. In addition, ACTH significantly downregulated several genes associated with de novo cholesterol biosynthesis and cholesterol trafficking. Finally, ACTH upregulated genes associated with intracellular metabolism and inactivation of glucocorticoids. The results demonstrate that the developmental effects of ACTH alter expression of a broad range of genes involved not solely in steroid synthesis, but in cellular functions related to growth and differentiation of the glands. In addition, the negative effects of ACTH on genes required for cholesterol synthesis and production of active glucocorticoids, suggests a mechanism whereby excessive production of glucocorticoids, which may have deleterious actions on developing structures like the central nervous system, is prevented.

Angiotensin II stimulates protein synthesis and inhibits proliferation in primary cultures of rat adrenal glomerulosa cells

Angiotensin II (Ang II) is one of the most important stimuli of rat adrenal glomerulosa cells. The aim of the present study was to investigate whether Ang II can stimulate cell proliferation and/or hypertrophy and investigate pathways and intracellular targets. A 3-d treatment with Ang II (5-100 nm), through the Ang II type 1 receptor subtype, abolished cell proliferation observed in control cells but increased protein synthesis. Preincubation with PD98059 (a MAPK kinase inhibitor) abolished basal proliferation and had no effect on basal protein synthesis but did reverse the effect of Ang II on protein synthesis. The p38 MAPK inhibitor SB203580 reversed the inhibitory effect on cell proliferation and abolished the increase in protein synthesis, whereas the c-Jun N-terminal kinase inhibitor SP600125 had no effect. Time-course studies revealed that Ang II stimulated phosphorylation of both p42/p44mapk and p38 MAPK but did not activate c-Jun N-terminal kinase. Ang II had no effect on the level of cyclin E expression but increased the expression of the cyclin-dependent kinase, p27Kip1, an effect abolished in cells preincubated with SB203580 and PD98059. In conclusion, in cultured rat glomerulosa cells, a 3-d treatment with Ang II increases protein synthesis, with a concomitant decrease in proliferation. These effects are mediated by both the p42/p44mapk and p38 MAPK pathways, which increase expression of the steroidogenic enzymes, steroidogenic acute regulatory protein and 3beta-hydroxysteroid dehydrogenase and p27Kip1, a protein known to block the cell cycle in G1 phase. Together these results support the key role of Ang II as a stimulus of steroid synthesis rather than a proliferating factor.

Adrenocortical zonation and ACTH

The clear morphological distinction between the cells of the different adrenocortical zones has attracted speculation and experiment to interpret their functions and the ways in which they are regulated. Considerable data have been produced in recent years that has benefited a fuller understanding of the processes of steroidogenesis and of cell proliferation at the molecular level. This now enables the reexamination of earlier concepts. It is evident that there is considerable species variation, and this article, dealing mainly with the rat, reaches conclusions that do not necessarily apply to other mammals. In the rat adrenal, however, the evidence suggests that the greatest differences between the functions of the zones are between the glomerulosa and the fasciculata. Here the sometimes all-or-nothing demarcation in their complement of components associated with steroidogenesis or with cell proliferation suggests a stark division of labor. In this model the fasciculata is the main engine of steroid hormone output and the glomerulosa is the site of cell proliferation, recruitment, and differentiation. Regulating these functions are angiotensin II and other paracrine components that modulate and maintain the glomerulosa, and ACTH, that maintains the fasciculata, and recruits new fasciculata cells by transformation of proliferating glomerulosa cells. Grafted onto this mostly vegetative function of the glomerulosa is CYP11B2, limited to just a fraction of the outer glomerulosa in rats on a normal laboratory diet and generating aldosterone (and 18-hydroxycorticosterone) from precursors whose origin is not, from the evidence summarized here, very clear, but may include the fasciculata, directly or indirectly. The biosynthesis of aldosterone in the rat certainly requires reinterpretation.