Treatment of primary cultures of calf adrenal glomerulosa cells with adrenocorticotropin (ACTH) and phorbol esters: a comparative study of the effects on aldosterone production and ACTH signaling system

60 YEARS OF POMC: Adrenal and extra-adrenal functions of ACTH

The pituitary adrenocorticotropic hormone (ACTH) plays a pivotal role in homeostasis and stress response and is thus the major component of the hypothalamo-pituitary-adrenal axis. After a brief summary of ACTH production from proopiomelanocortin (POMC) and on ACTH receptor properties, the first part of the review covers the role of ACTH in steroidogenesis and steroid secretion. We highlight the mechanisms explaining the differential acute vs chronic effects of ACTH on aldosterone and glucocorticoid secretion. The second part summarizes the effects of ACTH on adrenal growth, addressing its role as either a mitogenic or a differentiating factor. We then review the mechanisms involved in steroid secretion, from the classical Cyclic adenosine monophosphate second messenger system to various signaling cascades. We also consider how the interaction between the extracellular matrix and the cytoskeleton may trigger activation of signaling platforms potentially stimulating or repressing the steroidogenic potency of ACTH. Finally, we consider the extra-adrenal actions of ACTH, in particular its role in differentiation in a variety of cell types, in addition to its known lipolytic effects on adipocytes. In each section, we endeavor to correlate basic mechanisms of ACTH function with the pathological consequences of ACTH signaling deficiency and of overproduction of ACTH.

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

Mechanism of action of ACTH: beyond cAMP

ACTH is the major regulator of adrenal cortex function, having acute and chronic effects on steroid synthesis and secretion. The precise molecular mechanisms by which ACTH stimulates steroid synthesis and secretion, as well as cell hypertrophy, survival, and migration are still poorly understood. Several studies have shown that ACTH action is mediated not only by cyclic adenosine monophosphate (cAMP), but also by calcium (Ca(2+)), both interacting closely through positive feedback loops to enhance steroid secretion. However, in spite of the evidence that ACTH could stimulate other signaling pathways, such as inositol phosphates and diacylglycerol or mitogenic-activated protein kinase pathway (MAPK), none is as potent as cAMP. Recent data indicate that duration and potency of the cAMP production could be modulated by several isoforms of adenylyl cyclases and phosphodiesterases. In addition, calcium is probably not a first second messenger per se; rather, there are several arguments indicating that its increase occurs following cAMP production. Finally, in addition to steroid secretion, ACTH, through cAMP, is a survival factor, protecting cells against apoptosis. All of the effects of ACTH are dependent on cytoskeleton integrity. In summary, after 30 years of intensive research in this field, cAMP remains the first obligatory second messenger of ACTH action. However, recent work emphasizes that cell environment (matrix and cytoskeleton) probably interacts with cAMP to coordinate functions other than steroid secretion.

Effects of staurosporine on ACTH-mediated stimulation of aldosterone production

Incubation of rat adrenal glomerulosa cells with low concentrations (up to 50 nM) of the protein kinase (PKC) inhibitor staurosporine (ST) inhibited aldosterone (ALDO) and cyclic AMP (cAMP) production stimulated by adrenocorticotropic hormone (ACTH) and cholera toxin. Only higher concentrations (1.6 microM) of staurosporine inhibited dibutyryl-cAMP- and forskolin-induced stimulation of aldosterone production. cAMP levels were increased only with low concentrations of the PKC inhibitor. This latter increase was avoided by treatment with a maximal concentration of isobutylmethylxanthine (MIX). Our results suggest that: (1) second messengers other than cAMP are involved in ACTH action; (2) staurosporine inhibits different kinases involved in ACTH action in a dose-dependent manner; (3) the protein kinase inhibited by high concentrations of staurosporine appears to be the cAMP-dependent kinase, PKA; and (4) the protein kinase inhibited by low concentrations of staurosporine remains to be identified. This latter species is suggested as being involved in mediating ACTH-induced activation of Gs.

Adrenocorticotropin activates glycerol-3-phosphate acyltransferase in bovine adrenal glomerulosa cells

1. The mechanism whereby ACTH activates the synthesis of diacylglycerol (DAG) in isolated adrenal glomerulosa cells was investigated. 2. ACTH activates glycerol-3-phosphate acyltransferase (G3PAT) in intact and cell-free preparations of adrenal glomerulosa cells. Whereas activation of G3PAT by ACTH was observed in homogenates and membrane fractions, no activation was observed by angiotensin II (AII) at the same concentration. 3. ACTH effects were mimicked by nonspecific phospholipase C (PLC). 4. Our preliminary results suggest that ACTH activation of G3PAT may account for ACTH-induced increases in DAG via de novo synthesis of phosphatidic acid (PA).

Role of specific isoforms of protein kinase C in angiotensin II and lipoxygenase action in rat adrenal glomerulosa cells

Evidence indicates that the lipoxygenase (LO) pathway of arachidonic acid is a key mediator of angiotensin II (AII)-induced aldosterone synthesis in adrenal glomerulosa cells. Although protein kinase C (PKC) may play a role in AII action, the precise PKC isoforms involved and whether LO products can activate PKC is not clear. We therefore evaluated the effect of AII and LO products such as 12- and 15-hydroxyeicosatetraenoic acids (HETEs) on PKC activation in isolated rat adrenal glomerulosa cells. PKC activity was measured by the phosphorylation of a PKC specific peptide while the PKC isoforms were identified by Western immunoblotting using antibodies that recognize the alpha, beta, gamma or epsilon isoforms of PKC. Treatment of the cells for 15 min with AII (10[-8]M) or the LO products 12- or 15-HETE caused a marked increase in PKC activity in membrane fractions with reciprocal decreases in the cytosolic PKC activity. Rat glomerulosa cells expressed only the alpha, and epsilon isoforms of PKC. AII increased membrane bound levels of both PKC-alpha and -epsilon (1.9- and 1.5-fold, respectively), whereas the LO products predominantly activated PKC-epsilon. Reciprocal decreases in immunoreactive cytosolic PKC levels were seen. AII-induced aldosterone synthesis was blocked by H-7 and retinal as well as by a PKC-specific pseudosubstrate inhibitor, PKC(19-36). These results suggest that AII and LO pathway-induced actions in the adrenal glomerulosa may be mediated by specific PKC isoforms.

Substance P and adrenocorticotropic hormone do not affect T-lymphocyte adhesion to vascular endothelium or surface expression of adhesion receptors

Substance P (SP) and adrenocorticotropic hormone (ACTH) are peptides that have been shown to have both neurological and immunological effects. Because of the demonstrated effects upon immune function, we examined the effects of these peptides on T-lymphocyte adhesion to vascular endothelium and surface adhesion receptor expression. Neither the adhesion assays nor the expression assays showed any statistically significant effect of SP (10 microM) or ACTH (1 microM) for any incubation period used. We conclude that, while SP and ACTH have a variety of immunomodulatory effects, direct modulation of T-lymphocyte adhesion to vascular endothelium is probably not one of them.

A model for studying regulation of aldosterone secretion: freshly isolated cells or cultured cells?

Practically all studies relating to zona glomerulosa function have been performed either with freshly isolated cells or with cells used after 2 or 3 days in culture. This study compares the step-by-step response (binding, second messenger production and aldosterone response) of isolated glomerulosa cells vs cells maintained in primary culture to the main stimuli of aldosterone secretion. One day in culture induces a decrease of 77 and 65% in the basal level of corticosterone and aldosterone secretions, compared to that observed in freshly isolated cells. In these conditions, the cells become more sensitive to most of their stimuli, but not all: e.g. important differences are noted in the dose-response of aldosterone secretion to adrenocorticotropin (ACTH), which is often shifted to a lower concentration sensitivity in cultured cells. For example, 0.1 nM ACTH stimulates steroid secretion by three-fold in isolated cells while 1 pM ACTH already induces a 25 and nine-fold increase, respectively, in corticosterone and aldosterone output in cultured cells. Moreover, some stimuli such as isoproterenol do not have any effect in isolated cells but do stimulate steroid secretion in cultured cells. In contrast, other stimuli, such as serotonin or DA (via DA2 receptors) act preferentially in freshly isolated cells. The main observation derived from this study is that glomerulosa cells, under appropriate conditions, are able to respond to their main secretagogues even after 4 days in culture. At this time, glomerulosa cells maintain their ultrastructural characteristics and functional properties and, aside from a few exceptions, demonstrate higher sensitivity to their known stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of ET-1 potentiation of angiotensin II stimulation of aldosterone production

Endothelin-1 (ET-1) exerts the following two types of aldosterone-stimulating actions on glomerulosa cells: ET-1-mediated direct stimulation of aldosterone secretion (per se effect) and potentiation of the aldosterone secretion to angiotensin II (ANG II; potentiation effect). The role of Ca2+ and protein kinase C (PKC) systems in these two effects was investigated. Incubations of calf cultured adrenal zona glomerulosa cells in low-Ca2+ media or in the presence of the Ca2+ channel antagonist verapamil reduced the aldosterone secretion to ET-1. When cells were preincubated with ET-1 in a low-Ca2+ media or in the presence of the Ca2+ channel antagonist verapamil, washed, and incubated in media with normal Ca2+, ANG II showed potentiation of ANG II-stimulated aldosterone secretion. The PKC inhibitors H-7 and staurosporine did not decrease ET-1-stimulated aldosterone secretion, but they inhibited the potentiation effect of ET-1 on ANG II-mediated aldosterone secretion. Adrenocorticotropic hormone desensitization or prolonged phorbol ester stimulation of PKC resulting in desensitization also resulted in the abolition of the ET-1-mediated ANG II potentiation of aldosterone secretion. The PKC inhibitors did not affect ANG II-stimulated aldosterone secretion. We conclude that ET-1 exerts a direct stimulation of aldosterone secretion through a mechanism dependent on Ca2+ and potentiates ANG II-mediated aldosterone stimulation through a mechanism involving PKC.

Stimulation of aldosterone production by hemin in calf adrenal glomerulosa cell cultures

Aldosterone production from 11-deoxycorticosterone was stimulated by hemin in primary cultures and homogenates of calf adrenal zona glomerulosa, in a time- and dose-dependent fashion. The ferrochelatase inhibitor 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine (DDC) blocked the stimulation of aldosterone mediated by adrenocorticotropin (ACTH). Addition of hemin after treatment with DDC partially restored ACTH action. These results suggest that hemin may play a role in regulation of aldosterone production.

Human P450scc gene transcription is induced by cyclic AMP and repressed by 12-O-tetradecanoylphorbol-13-acetate and A23187 through independent cis elements

Long-term regulation of mammalian steroid hormone synthesis occurs principally by transcriptional regulation of the gene for the rate-limiting cholesterol side-chain cleavage enzyme P450scc. Adrenal steroidogenesis is regulated primarily by two hormones: adrenocorticotropin, which works via cyclic AMP (cAMP) and protein kinase A, and angiotensin II, which works via Ca2+ and protein kinase C. Forskolin and 8-bromo-cAMP stimulated, while prolonged treatment with a phorbol ester (12-O-tetradecanoylphorbol-13-acetate [TPA]) and a calcium ionophore (A23187) additively suppressed accumulation of endogenous P450scc mRNA in transformed murine adrenal Y1 cells. In Y1 cells transfected with 2,327 base pairs of the human P450scc promoter fused to the bacterial gene for chloramphenicol acetyltransferase (CAT), forskolin increased CAT activity 900% while combined TPA plus A23187 reduced CAT activity to 15% of the control level. Forskolin induced the P450scc promoter as rapidly as a promoter containing two cAMP-responsive elements fused to a simian virus 40 promoter, a system known to respond directly to cAMP. Basal expression was increased by sequences between -89 and -152 and was increased further by sequences between -605 and -2327. This upstream region also conferred inducibility by cAMP. TPA plus A23187 transiently increased CAT activity before repressing it, reflecting the complex actions of angiotensin II in vivo. Repression by prolonged treatment with TPA plus A23187 was mediated by multiple elements between -89 and -343. Induction of CAT activity by forskolin was not diminished by treatment with TPA plus A23187, nor were the regions of the promoter responsible for regulation by the two pathways coisolated. Thus, the human gene for P450scc is repressed by TPA plus A23187 by mechanisms and sequences independent of those that mediate induction by cAMP.

Human P450scc gene transcription is induced by cyclic AMP and repressed by 12-O-tetradecanoylphorbol-13-acetate and A23187 through independent cis elements

Long-term regulation of mammalian steroid hormone synthesis occurs principally by transcriptional regulation of the gene for the rate-limiting cholesterol side-chain cleavage enzyme P450scc. Adrenal steroidogenesis is regulated primarily by two hormones: adrenocorticotropin, which works via cyclic AMP (cAMP) and protein kinase A, and angiotensin II, which works via Ca2+ and protein kinase C. Forskolin and 8-bromo-cAMP stimulated, while prolonged treatment with a phorbol ester (12-O-tetradecanoylphorbol-13-acetate [TPA]) and a calcium ionophore (A23187) additively suppressed accumulation of endogenous P450scc mRNA in transformed murine adrenal Y1 cells. In Y1 cells transfected with 2,327 base pairs of the human P450scc promoter fused to the bacterial gene for chloramphenicol acetyltransferase (CAT), forskolin increased CAT activity 900% while combined TPA plus A23187 reduced CAT activity to 15% of the control level. Forskolin induced the P450scc promoter as rapidly as a promoter containing two cAMP-responsive elements fused to a simian virus 40 promoter, a system known to respond directly to cAMP. Basal expression was increased by sequences between -89 and -152 and was increased further by sequences between -605 and -2327. This upstream region also conferred inducibility by cAMP. TPA plus A23187 transiently increased CAT activity before repressing it, reflecting the complex actions of angiotensin II in vivo. Repression by prolonged treatment with TPA plus A23187 was mediated by multiple elements between -89 and -343. Induction of CAT activity by forskolin was not diminished by treatment with TPA plus A23187, nor were the regions of the promoter responsible for regulation by the two pathways coisolated. Thus, the human gene for P450scc is repressed by TPA plus A23187 by mechanisms and sequences independent of those that mediate induction by cAMP.

Key role of diacylglycerol-mediated 12-lipoxygenase product formation in angiotensin II-induced aldosterone synthesis

We have shown earlier that the 12-lipoxygenase product of arachidonic acid (AA), 12-hydroxyeicosatetraenoic acid (12-HETE), plays an important role in mediating angiotensin II (AII)-induced aldosterone secretion (J. Clin. Invest. (1987) 80, 1763). In the present study, we have evaluated whether diacylglycerol (DG) is the source of arachidonic acid giving rise to this 12-HETE. Treatment of rat adrenal glomerulosa cells with a DG lipase inhibitor, RHC 80267, which prevents conversion of DG to AA and HETEs, blocked AII-induced aldosterone and 12-HETE formation. In contrast, a DG kinase inhibitor, R59022, which prevents conversion of DG to phosphatidic acid, potentiated AII-induced aldosterone and 12-HETE formation. These two inhibitors block DG metabolism which would be expected to lead to increased DG levels and protein kinase C activity and AII-induced steroidogenesis. However, only R59022 potentiated AII action while RHC 80267 was inhibitory. This suggests that conversion of DG to AA and 12-HETE is important for AII action. Further proof for this was obtained by measuring [3H]AA-labeled DG levels. The combination of the inhibitors significantly potentiated AII-induced DG formation even though this same combination was inhibitory on AII-induced aldosterone and 12-HETE. Thus, the inhibitory effect of RHC 80267 is due to blockade of AA release and not of DG formation. These results suggest that DG plays a dual role in AII action, both as an activator of protein kinase C and as a source of AA for 12-HETE formation.