Density separation of rat adrenocortical cells: morphology, steroidogenesis, and P-450scc expression in primary culture

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.

Isolation of human adrenocortical aldosterone-producing cells by a novel immunomagnetic beads method

We detected intense CD56 immunostaining in the zona glomerulosa (ZG) and medulla of the normal human adrenal gland and therefore identified CD56, the neural cell adhesion molecule, as a membrane antigen specific for the ZG, aldosterone-producing adenoma (APA), and chromaffin cells. The APA and pheochromocytoma cells, which are histogenetically derived from the ZG and medulla, respectively, also showed intense CD56 immunostaining. Based on these findings we developed a strategy for isolating cells from the ZG and APA using CD56 immunobinding to magnetic beads. Morphology, gene expression studies, and aldosterone measurement confirmed that CD56 positive (+) cells were ZG and APA cells. Analysis of CD56+ cells under light and phase contrast microscopy evidenced that these cells formed clumps, as the ZG cells usually do; with electron microscopy they showed multiple features typical of a steroidogenic phenotype. Expression levels of the CD56 and the aldosterone synthase (CYP11B2) gene were markedly higher in CD56+ cells than CD56- cells (+1600 and +2100% increase, respectively). Moreover, aldosterone secretion was higher (+1380%) from CD56+ cells than from CD56- cells. Hence, this novel methodology allows isolation of a pure population of ZG and APA cells exhibiting multiple characteristics of the aldosterone-producing cells.

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.

Rat glomerulosa cells in primary culture and E. coli lipopolysaccharide action

During endotoxic shock there is a dysfunction of the adrenal gland; both corticosterone and aldosterone secretion are altered. The aim of the present study is to use glomerulosa cells in primary culture as a target of lipopolysaccharide (LPS) action. Glomerulosa cells cultured in basal conditions are able to proliferate; bFGF and ACTH have antagonic effects, bFGF increases proliferation whereas ACTH is antimitogenic. LPS has a biphasic effect, in the short term it is antimitogenic and in the long term increases the proliferation rate. LPS inhibits ACTH-induced corticosterone secretion in a dose-dependent manner in glomerulosa cells in culture similar to that in fasciculata cells, but it does not exert an important direct effect on aldosterone secretion. These results show that LPS exerts different effects in ACTH and ANG II signal transduction pathways and in the two enzymes which catalyze the late step in the steroidogenesis, 11beta-hydroxylase and aldosterone synthase, which could be in agreement with the existence of both enzymes, regulated independently, in rat zona glomerulosa cells.

Differentiated corticosteroid production and regeneration after selective transplantation of cultured and noncultured adrenocortical cells in the adrenalectomized rat

Syngeneic transplantation of adrenocytes was investigated in Lewis rats in regard to differentiated hormone secretion and cortex regeneration after bilateral adrenalectomy as an alternative to steroid substitution.

METHODS

Purified cell suspensions of glomerulosa (density 1.061 +/- 0.001 g/ml) and fasciculata (density 1.034 +/- 0.003 g/ml) cells were obtained by density gradient separation and were transplanted under the kidney capsule either immediately or after a 29-day culture period. Animals were killed after transplantation of cultured glomerulosa (CG-Tx) or cultured fasciculata cells (CF-Tx), noncultured glomerulosa cells (G-Tx) or non-cultured fasciculata cells (F-Tx), or both cell types (GF-Tx) for morphological studies after 30, 120, and 360 days. Plasma samples were drawn for measurement of corticosterone and aldosterone as well as 24 hr-urine for sodium and potassium levels at day 3, 30, 120, and 360 after transplantation.

RESULTS

In primary culture fasciculata cell number remained stationary although glomerulosa cell number increased to almost 10-fold. Vital cortex cells were demonstrated in each explanted graft by histochemistry but only group G-Tx, CG-Tx, and GF-Tx (purified cell suspensions of zona glomerulosa and fasciculata) showed neocortex-like structures. We found plasma (urine) corticosterone to decrease from preoperatively 256-304 ng/ml (226-239 ng/day) in untreated animals to levels about half as high 3 days after transplantation, increasing to normal values in all study groups 30 days after treatment (data given as range). Plasma aldosterone concentrations, 150-180 pg/ml in untreated rats, decreased to nondetectable levels for 1 week after bilateral adrenalectomy. At day 30 group GF-Tx, G-Tx, and CG-Tx showed comparable aldosterone plasma concentrations (104-122 pg/ml); however, levels in F-Tx and CF-Tx were 19-49 pg/ml, and did not increase significantly within the observation period.

CONCLUSIONS

Cells derived from the zona glomerulosa maintain viability, produce both aldosterone and corticosterone, and regenerate a neocortex with cells that histologically resemble both zona glomerulosa and fasciculata cells. They are therefore suitable for adrenocortical transplantation. In contrast, cells derived from the zona fasciculata maintain viability, but do not regenerate zona glomerulosa and do not produce aldosterone. These results suggest that the cell migration model, in which zona glomerulosa cells can acquire the phenotype of zona fasciculata cells as they can migrate centripetally, is more likely the correct explanation of adrenocortical zonation.

Developmental expression and regulation of adrenocortical cytochrome P4501B1 in the rat

A 57-kDa protein whose expression in rat adrenocortical microsomes is increased after weaning has been identified as cytochrome P4501B1 (CYP1B1). Levels of CYP1B1 protein were moderately expressed in late gestation fetuses and on postnatal day 1 (pdl), but were nearly undetectable on pd6 and pd1O. CYP1B1 expression initially increased in the late preweaning period (pd17-19) and again immediately postweaning (pd21-24). The temporal coincidence of CYP1B1 expression and weaning was not due to transition from suckling to solid food, as neonates that were prematurely weaned showed no increase in adrenal CYP1B1 compared with normally weaned littermates. The pattern of CYP1B1 expression paralleled changes in microsomal metabolism of 7,12-dimethylbenz[a]anthracene (DMBA), a marker of CYP1B1 activity. Twice daily injections of ACTH to rat pups (pd3-10) failed to significantly increase the expression of CYP1B1 in pd 10 adrenals, although the injections weakly stimulated steroidogenesis. Adrenocortical cells from pd17 neonates and adult cells, when cultured for 3 days, responded similarly to ACTH induction, although neonates showed more than 4-fold less basal activity. It is concluded that rat adrenal CYP1B1 may be developmentally suppressed, and its expression is independent of diet or the presence of a dam. This suppression is retained in cell culture, but is not due to deficient ACTH signaling. These results may explain the reported resistance of neonatal rat adrenals to the toxic effects of polycyclic aromatic hydrocarbons, which are metabolized by CYP1B1 into mutagenic by-products.

Rapid ras-oncogene-mediated transformation maintains steroidogenic differentiation in adrenocortical parenchymal cells

While its action as a transforming agent is well known, expression of the ras oncogene may also alter tissue-specific differentiation. We have been examining the relationship of transformation and differentiation in steroidogenic cells of the rat. Infection of adrenocortical zona glomerulosa (GLOM) cells with the v-Ki ras containing Kirsten murine sarcoma virus did not induce focus formation. Instead, diffuse cellular multilayers formed from which loosely adherent, refractile cells emerged. After selective passaging these refractile cells, designated KiGLOM, were morphologically transformed, had reduced serum requirements for growth, greatly increased saturation densities, and they rapidly formed tumours in immunosuppressed hosts. In addition, under conditions where normal cells were no longer steroidogenic (ie. after passaging), KiGLOM cells expressed the steroid-specific cholesterol side chain cleavage cytochrome P-450scc and they produced significant, albeit reduced, amounts of corticosterone in comparison with primary GLOM cultures. Additionally, trophic hormone treatment increased steroid production in Ki-GLOM cells and this increase was partially reversed by lovastatin, a pharmacological inhibitor of ras p21 function. Thus, after a morphological selection that removed normal neighbours, v-Ki ras infected cells transformed rapidly while remaining steroidogenic. These results, combined with previous reports of steroidogenic v-Ki ras transformed adrenocortical fibroblasts and ovarian granulosa cells suggest that the ability of the ras oncogene to co-opt signal transduction pathways associated with both growth and differentiation is a common feature of the steroidogenic phenotype.

Influence of cell type on the steroidogenic potential and basal cyclic AMP levels of ras-oncogene-transformed rat cells

Transformation with ras oncogenes causes loss, maintenance or modulation of differentiation, depending on the developmental history of the target cells. In the present study, we examined steps in signal transduction that may underlie some of this variation, using steroidogenic cells of adult rats as the model system. Steroidogenesis in normal cells is regulated by cyclic AMP and protein kinase A (the cAMP/PKA pathway). We showed previously that transformation with v-Ki-ras induces constitutive progesterone secretion in ovarian and adrenocortical cells that are normally steroidogenic (ovarian granulosa and adrenal glomerulosa cells) and also in developmentally related cells that are normally nonsteroidogenic (ovarian surface epithelium and adrenal capsular fibroblasts), but not in unrelated nonsteroidogenic cells, such as muscle fascia fibroblasts and peritoneal mesothelium. In the present study, basal cAMP levels in all transformed ovarian and adrenal cell-lines were increased over basal levels in normal cells, and of transformed muscle fascia and mesothelial cell-lines. As in normal cells, transformation-induced steroidogenesis was stimulated by cAMP and was PKA dependent. A comparison of malignancy-related characteristics showed that transformed cells from nonsteroidogenic organs were more tumorigenic in vivo and less sensitive to growth inhibition by cAMP in vitro than transformed ovarian and adrenocortical cells. The results show that the abnormal, constitutive steroidogenesis induced by the viral form of the Kirsten ras oncogene (v-Ki-ras) in certain cell types is associated with tissue-specific increases in basal cAMP levels. Thus, although the ras oncogenes function primarily through other signal transduction pathways, transformation with ras oncogenes alters PKA-mediated signal transduction in a manner that is developmentally determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Mixed parenchymal-stromal populations of rat adrenocortical cells support the proliferation and differentiation of steroidogenic cells

The mechanisms which regulate adrenocortical steroidogenesis in differentiated parenchymal cells have been studied in great detail. However, the stem cells that are responsible for regeneration of the adult cortex have never been identified or isolated, and their characteristics are unknown. We have developed a tissue culture system that supports the simultaneous proliferation and differentiation of steroidogenic adrenocortical cells. Utilizing density gradient separation, a cell population composed of a mixture of stromal, endothelial and parenchymal cells (MIX) was isolated from the adult rat adrenal cortex. In primary culture, MIX populations formed high saturation density multilayers from which rounded cells emerged. These cells proliferated, contained lipid, and expressed the steroidogenic enzymes delta 5,3 beta-hydroxysteroid dehydrogenase and cholesterol side-chain cleavage cytochrome P-450scc. After selective passaging, rounded MIX-derived cells retained their steroidogenic potential, even in the absence of trophic hormone treatment. In contrast, parenchymal cells obtained from the zonae fasciculata (FASC) and glomerulosa (GLOM) respectively, formed homogeneous monolayers in primary culture, gradually de-differentiated, and no longer responded to trophic hormone treatment after passaging. Thus, primary MIX cultures provided a microenvironment that resulted in the production of adrenocortical cells with stem cell-like qualities. These cultures provide for the first time, a system for the identification of specific inducers that are responsible for both adrenocortical cytogenesis and its associated proliferation and steroidogenic differentiation.

Divergent differentiation of rat adrenocortical cells is associated with an interruption of angiotensin II-mediated signal transduction

The zones of the adrenal cortex contain distinct populations of cells which share a common developmental origin and steroidogenic template. In the rat, zona glomerulosa cells respond to angiotensin II (Ang II) with increased steroidogenesis while zona fasciculata/reticularis cells do not. We have examined Ang II-mediated signal transduction in homogeneous cellular sub-populations derived from either the zona glomerulosa (GLOM) or the zona fasciculata (FASC). In both of these sub-populations Ang II treatment significantly increased the levels of 3H-labelled inositol phosphates as well as the total mass of inositol 1,4,5-triphosphate. In contrast, the two cell types exhibited very different Ang II-mediated changes in free intracellular calcium ([Ca2+]i). Ang II (10 nM), induced [Ca2+]i increases of > 50 nM in 90% of individual GLOM cells (53/58), but in only 28% of FASC cells (11/39). These [Ca2+]i responses occurred after a transient Ang II stimulation ( < 1 min), in the presence of verapamil and in the absence of extracellular calcium, indicating an intracellular release. In small groups of 10-30 cells, stimulation with 1, 10 and 100 nM Ang II induced [Ca2+]i increases of 78, 178 and 215 nM respectively in GLOM cultures compared to only 35, 64, and 65 nM in FASC cultures. Thapsigargin treatment, which releases intracellular calcium in an inositol phosphate independent manner, elicited [Ca2+]i increases in both populations. Importantly, a calcium ionophore induced elevation of [Ca2+]i increased steroidogenesis in both cell types. These results suggest that an interruption of the signaling cascade at the level of intracellular calcium release contributes to the lack of a steroidogenic response to Ang II by the FASC cells. Therefore, in the rat adrenal cortex, divergent differentiation of related cell types may involve alterations within signal transduction pathways distal to initial receptor-mediated events (i.e. inositol phosphate production) and proximal to downstream effector events (i.e. steroidogenesis).