Daily regeneration of rat adrenocortical cells: circadian and zonal variations in cytogenesis

Enucleation-induced rat adrenal gland regeneration: expression profile of selected genes involved in control of adrenocortical cell proliferation

Enucleation-induced adrenal regeneration is a highly controlled process; however, only some elements involved in this process have been recognized. Therefore, we performed studies on regenerating rat adrenals. Microarray RNA analysis and QPCR revealed that enucleation resulted in a rapid elevation of expression of genes involved in response to wounding, defense response, and in immunological processes. Factors encoded by these genes obscure possible priming effects of various cytokines on initiation of regeneration. In regenerating adrenals we identified over 100 up- or downregulated genes involved in adrenocortical cell proliferation. The changes were most significant at days 2-3 after enucleation and their number decreased during regeneration. For example, expression analysis revealed a notable upregulation of the growth arrest gene, Gadd45, only 24 hours after surgery while expression of cyclin B1 and Cdk1 genes was notably elevated between days 1-8 of regeneration. These changes were accompanied by changes in expression levels of numerous growth factors and immediate-early transcription factors genes. Despite notable differences in mechanisms of adrenal and liver regeneration, in regenerating adrenals we identified genes, the expression of which is well recognized in regenerating liver. Thus, it seems legitimate to suggest that, in the rat, the general model of liver and adrenal regeneration demonstrate some degree of similarity.

Functional zonation of the rat adrenal cortex: the development and maintenance

The adrenal cortex of mammals consists of three concentric zones, i.e., the zona glomerulosa (zG), the zona fasciculata (zF), and the zona reticularis (zR), which secrete mineralocorticoids, glucocorticoids, and adrenal androgens, respectively. In 1994, we identified immunohistochemically a new zone between zG and zF of the rat adrenal gland. The zone appeared to be devoid of any significant endocrine functions specific to adrenocortical zones, therefore, we designated the zone as "undifferentiated cell zone (zU)". Further, BrdU (5-bromo-2'-deoxyuridine)-incorporating cells (cells in S-phase) were concentrated at the outer region and the inner region of zU, and these cells proliferated and migrated bidirectionally: toward zG centrifugally and toward zF centripetally. We proposed that cells in and around zU are stem/progenitor cells of the rat adrenal cortex, maintaining functional zonation of the adrenal cortex. The view is consistent with observations reported recently that Sonic hedgehog (Shh), an important factor in embryonic development and adult stem cell maintenance, exists in zU of the rat adrenal gland and the Shh-containing cells seem to migrate bidirectionally.

Interpreting Stress Responses during Routine Toxicity Studies

Stress often occurs during toxicity studies. The perception of sensory stimuli as stressful primarily results in catecholamine release and activation of the hypothalamic–pituitary–adrenal (HPA) axis to increase serum glucocorticoid concentrations. Downstream effects of these neuroendocrine signals may include decreased total body weights or body weight gain; food consumption and activity; altered organ weights (e.g., thymus, spleen, adrenal); lymphocyte depletion in thymus and spleen; altered circulating leukocyte counts (e.g., increased neutrophils with decreased lymphocytes and eosinophils); and altered reproductive functions. Typically, only some of these findings occur in a given study. Stress responses should be interpreted as secondary (indirect) rather than primary (direct) test article–related findings. Determining whether effects are the result of stress requires a weight-of-evidence approach. The evaluation and interpretation of routinely collected data (standard in-life, clinical pathology, and anatomic pathology endpoints) are appropriate and generally sufficient to assess whether or not changes are secondary to stress. The impact of possible stress-induced effects on data interpretation can partially be mitigated by toxicity study designs that use appropriate control groups (e.g., cohorts treated with vehicle and subjected to the same procedures as those dosed with test article), housing that minimizes isolation and offers environmental enrichment, and experimental procedures that minimize stress and sampling and analytical bias.

This article is a comprehensive overview of the biological aspects of the stress response, beginning with a Summary (Section 1) and an Introduction (Section 2) that describes the historical and conventional methods used to characterize acute and chronic stress responses. These sections are followed by reviews of the primary systems and parameters that regulate and/or are influenced by stress, with an emphasis on parameters evaluated in toxicity studies: In-life Procedures (Section 3), Nervous System (Section 4), Endocrine System (Section 5), Reproductive System (Section 6), Clinical Pathology (Section 7), and Immune System (Section 8). The paper concludes (Section 9) with a brief discussion on Minimizing Stress-Related Effects (9.1.), and a final section explaining why Parameters routinely measured are appropriate for assessing the role of stress in toxicology studies (9.2.).

Effect of acute heat stress on rat adrenal glands: a morphological and stereological study

The morphological and stereological structure of rat adrenal gland was analysed by light microscopy after an acute (60 min) exposure to high ambient temperature (38 degrees C). A significant increase in plasma corticotrophin (ACTH) and serum corticosterone (CORT) concentrations was observed, confirming that acute heat exposure has a strong stressful effect. Under these conditions the adrenal gland mass and volume were decreased, probably as the consequence of adrenal cortex reduction, especially that of the zona fasciculata (ZF). Histological examination revealed that many ZF cells were deprived of lipid droplets. Fibrosis was observed in all parts of the adrenal gland, both cortex and medulla, of heat stressed animals. Mitotic figures were absent in cortical cells after heat exposure, but there were no differences in ZF and zona reticularis (ZR) small blood vessels compared to nonstressed controls.

Angiotensin II early regulated genes in H295R human adrenocortical cells

Evidence for the dysregulation of aldosterone synthesis in cardiovascular pathophysiology has renewed interest in the control of its production. Cellular mechanisms by which angiotensin II (ANG II) stimulates aldosterone synthesis in the adrenal zona glomerulosa are incompletely understood. To elucidate the mechanism of intracellular signaling by ANG II stimulation in the adrenal, we have studied immediate-early regulated genes in human adrenal H295R cells using cDNA microarrays. H295R cells were stimulated with ANG II for 3 h. Gene expression was analyzed by microarray technology and validated by real-time RT-PCR. Eleven genes were found to be upregulated by ANG II. These encode the proteins for ferredoxin, Nor1, Nurr1, c6orf37, CAT-1, A20, MBLL, M-Ras, RhoB, GADD45α, and a novel protein designated FLJ45273 . Maximum expression levels for all genes occurred 3–6 h after ANG II stimulation. This increase was dose dependent and preceded maximal aldosterone production. Other aldosterone secretagogues, K+and endothelin-1 (ET-1), also induced the expression of these genes with variable efficiency depending on the gene and with lower potency than ANG II. ACTH had negligible effect on gene expression except for the CAT-1 and Nurr1 genes. These ANG II-stimulated genes are involved in several cellular functions and are good candidate effectors and regulators of ANG II-mediated effects in adrenal zona glomerulosa.

Glomerulosa function and aldosterone synthesis in the rat

Since its discovery, it has been generally assumed that the primary function of the zona glomerulosa of the adrenal cortex is the secretion of aldosterone. Taking evidence from the rat, and recognising that there is probably considerable species variation, I argue here that the glomerulosa in fact has many functions, including aldosterone synthesis, but is probably only a relatively poor de novo source of steroid. In vitro, the CYP11B2 (aldosterone synthase) of the glomerulosa can and does utilise as substrates products arising from CYP11B1 (11beta-hydroxylase) activity in fasciculata cells. Whether it does in vivo is open to question, but corticosterone and 18-hydroxydeoxycorticosterone are both present in circulating rat plasma at suitable concentrations. Such a mechanism would explain several inconsistencies in the literature, including the anomalous distribution of steroidogenic enzymes in the glomerulosa, the stimulation of CYP11B1 products by aldosterone secretagogues such as potassium ions or angiotensin II, the partial dependence of aldosterone secretion in vivo on an intact pituitary, the sensitivity of aldosterone secretion to tissue disruption in vitro, and the "late pathway" regulation of aldosterone synthesis.

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.

An inverse correlation between expression of a preprocathepsin B-related protein with cysteine-rich sequences and steroid 11beta -hydroxylase in adrenocortical cells

A cDNA encoding a secretory protein hitherto unknown was cloned from mouse adrenocortical cells by subtractive hybridization between the cells without and with expressing steroid 11beta-hydroxylase (Cyp11b-1), a marker for the functional differentiation of cells in the zonae fasciculata reticularis (zFR). The deduced protein consisting of 466 amino acids contained a secretory signal, epidermal growth factor-like repeats, and a proteolytically inactive cathepsin B-related sequence. The amino acid sequence was 89% identical with that of human tubulointerstitial nephritis antigen-related protein. Among the mouse organs examined, adrenal glands prominently expressed its mRNA. The mRNA and its encoded protein were detected in the outer adrenocortical zones that do not express Cyp11b-1, i.e. the zona glomerulosa and the undifferentiated cell zone, while being undetectable in zFR that express Cyp11b-1. The new protein was designated as adrenocortical zonation factor 1 (AZ-1). Clonal lines with different levels of AZ-1 expression were established from Y-1 adrenocortical cells that originally express Cyp11b-1 but little AZ-1. Analyses of the clonal lines revealed that Cyp11b-1 is detected in the clonal lines maintaining little AZ-1 expression and becomes undetectable in those expressing AZ-1. On the other hand, irrespective of the AZ-1 expression, all clones expressed cholesterol side-chain cleavage enzyme, which occurs throughout the cortical zones. These results demonstrated that adrenocortical cells expressing AZ-1 do not express Cyp11b-1, whereas those with little AZ-1 express this zFR marker in vitro and in vivo, implying a putative role of AZ-1 in determining the zonal differentiation of adrenocortical cells.

The undifferentiated cell zone is a stem cell zone in adult rat adrenal cortex

The adrenal cortex of mammals has been known to consist of three morphologically and functionally distinct zones, i.e. the zona glomerulosa (zG), the zona fasciculata (zF) and the zona reticularis (zR), each of which secretes a specific corticosteroid different from those produced by the other two zones. We found previously, however, that an additional zone existed between zG and zF of adult rat adrenal cortex and that the cells in that zone were in a functionally undifferentiated state as an adrenocortical cell [Endocrinology 135, (1994) 431]: they were incapable of synthesizing highly active forms of corticosteroids, such as aldosterone and corticosterone, although they could produce their precursors. Hence, we named the zone as the undifferentiated cell zone (zU) of the adrenal cortex. Here we show that zU and its surroundings, i.e. the innermost portion of zG and the outermost portion of zF are the sites for cell replication in adult rat adrenal cortex and that the cells raised there migrate to other regions. Such cell replications in this region occur regardless of physiological conditions, such as the rise and fall of hormonal stimuli and circadian fluctuation of adrenocortical activities. On the bases of these and other findings previously described, we propose that zU is the stem cell zone of the adult rat adrenal cortex. Our recent success in isolating novel cell lines, which display an undifferentiated phenotype similar to that of zU cells, could facilitate the exploration of molecular mechanisms for the differentiation and development of the adrenocortical cells.

Proliferation of capsular stem cells induced by ACTH in the rat adrenal cortex

Despite great efforts devoted to clarifying the localization of proliferative activity in the adrenal cortex, the agents that stimulate proliferation remain controversial, and the nature of the stem cells from which cortical cells differentiate is incompletely understood. We studied proliferative activity in the rat adrenal cortex using an immunohistochemical method to detect the presence of the Proliferating Cell Nuclear Antigen (PCNA) (an intranuclear enzyme whose synthesis reaches the maximum intensity during the S-phase of the cell cycle). Groups of six rats were subjected to daily intraperitoneal injection of either corticotropin (ACTH1-24--0.2 mg/kg), dexamethasone (Dexa--4 mg/kg) or 0.9% saline for three consecutive days and killed 24 h after the last injection. Adrenal weight was significantly increased by ACTH treatment and reduced by Dexa. Concentrations of endogenous ACTH in plasma were lower in the Dexa group than in controls, and curiously, this was true in the ACTH1-24 treated group as well, probably in consequence of the increased corticosterone levels providing negative feedback at the hypothalamic-pituitary level. Corticosterone levels, as expected, were increased by the ACTH stimulus and reduced by the use of Dexa. Proliferating cell nuclear antigen immunostaining was close to zero in Dexa treated animals and low in controls. In ACTH treated rats, a significantly increased number of cells were positively stained. Positive cells were identified in both in zona glomerulosa (ZG) and zona intermedia (ZI) but many were located in the capsule. Zona fasciculata (ZF) and zona reticularis (ZR) were devoid of staining in all of these cases. We conclude that pharmacological doses of ACTH induce proliferation of capsular fibroblasts. Following descriptions by early 20th century researchers it is possible that these cells may also be stem cells and differentiate into adrenal cortex cells.

Conditionally immortalized adrenocortical cell lines at undifferentiated states exhibit inducible expression of glucocorticoid-synthesizing genes

To facilitate studies on differentiation of adrenocortical cells and regulation of steroidogenic genes, we established cell lines from adrenals of adult transgenic mice harboring a temperature-sensitive large T-antigen gene of simian virus 40. Adrenal glands of the mice exhibited normal cortical zonation including a functionally undifferentiated cell-layer between the aldosterone-synthesizing zona glomerulosa cells and the corticosterone-synthesizing zona fasciculata cells. At a permissive temperature (33 degrees C), established cell lines AcA201, AcE60 and AcA101 expressed steroidogenic genes encoding steroidogenic factor-1, cholesterol side-chain cleavage P450scc, and steroidogenic acute regulatory protein, which are expressed throughout adrenal cortices and gonads. Genes encoding 3 beta-hydroxysteroid dehydrogenase and steroid 21-hydroxylase P450c21, which catalyze the intermediate steps for syntheses of both aldosterone and corticosterone, were inducible in the three cell lines in temperature- and/or dibutyryl cAMP-dependent manners. Notably, these cell lines displayed distinct expression patterns of the steroid 11 beta-hydroxylase P45011 beta gene responsible for the zone-specific synthesis of corticosterone. AcA201 cells expressed the P45011 beta gene at 33 degrees C, showing the property of the zona fasciculata cells, while AcE60 cells expressed it upon a shift to a nonpermissive temperature (39 degrees C). On the other hand, AcA101 expressed the P45011 beta gene at 39 degrees C synergistically with exposure to dibutyryl cAMP. None of these clones express the zona glomerulosa-specific aldosterone synthase P450aldo gene under the conditions we tested. These results show that AcE60 and AcA101 cells display a pattern of the steroidogenic gene expression similar to that of the undifferentiated cell-layer and are capable of differentiating into the zona fasciculata-like cells in vitro.