Signal transduction mechanisms involved in carbachol-induced aldosterone secretion from bovine adrenal glomerulosa cells

M3-subtype muscarinic receptor activation stimulates intracellular calcium oscillations and aldosterone production in human adrenocortical HAC15 cells

A previous body of work in bovine and rodent models shows that cholinergic agonists modulate the secretion of steroid hormones from the adrenal cortex. In this study we used live-cell Ca²⁺ imaging to investigate cholinergic activity in the HAC15 human adrenocortical carcinoma cell line. The cholinergic agonists carbachol and acetylcholine triggered heterogeneous Ca²⁺ oscillations that were strongly inhibited by antagonists with high affinity for the M₃ muscarinic receptor subtype, while preferential block of M₁ or M₂ receptors was less effective. Acute exposure to carbachol and acetylcholine modestly elevated aldosterone secretion in HAC15 cells, and this effect was also diminished by M₃ inhibition. HAC15 cells expressed relatively high levels of mRNA for M₃ and M₂ receptors, while M₁ and M₅ mRNA were much lower. In conclusion, our data extend previous findings in non-human systems to implicate the M₃ receptor as the dominant muscarinic receptor in the human adrenal cortex.

Role of phospholipases in adrenal steroidogenesis

Phospholipases are lipid-metabolizing enzymes that hydrolyze phospholipids. In some cases, their activity results in remodeling of lipids and/or allows the synthesis of other lipids. In other cases, however, and of interest to the topic of adrenal steroidogenesis, phospholipases produce second messengers that modify the function of a cell. In this review, the enzymatic reactions, products, and effectors of three phospholipases, phospholipase C, phospholipase D, and phospholipase A2, are discussed. Although much data have been obtained concerning the role of phospholipases C and D in regulating adrenal steroid hormone production, there are still many gaps in our knowledge. Furthermore, little is known about the involvement of phospholipase A2, perhaps, in part, because this enzyme comprises a large family of related enzymes that are differentially regulated and with different functions. This review presents the evidence supporting the role of each of these phospholipases in steroidogenesis in the adrenal cortex.

Regulation of aldosterone synthesis and secretion

Aldosterone is a steroid hormone synthesized in and secreted from the outer layer of the adrenal cortex, the zona glomerulosa. Aldosterone is responsible for regulating sodium homeostasis, thereby helping to control blood volume and blood pressure. Insufficient aldosterone secretion can lead to hypotension and circulatory shock, particularly in infancy. On the other hand, excessive aldosterone levels, or those too high for sodium status, can cause hypertension and exacerbate the effects of high blood pressure on multiple organs, contributing to renal disease, stroke, visual loss, and congestive heart failure. Aldosterone is also thought to directly induce end-organ damage, including in the kidneys and heart. Because of the significance of aldosterone to the physiology and pathophysiology of the cardiovascular system, it is important to understand the regulation of its biosynthesis and secretion from the adrenal cortex. Herein, the mechanisms regulating aldosterone production in zona glomerulosa cells are discussed, with a particular emphasis on signaling pathways involved in the secretory response to the main controllers of aldosterone production, the renin-angiotensin II system, serum potassium levels and adrenocorticotrophic hormone. The signaling pathways involved include phospholipase C-mediated phosphoinositide hydrolysis, inositol 1,4,5-trisphosphate, cytosolic calcium levels, calcium influx pathways, calcium/calmodulin-dependent protein kinases, diacylglycerol, protein kinases C and D, 12-hydroxyeicostetraenoic acid, phospholipase D, mitogen-activated protein kinase pathways, tyrosine kinases, adenylate cyclase, and cAMP-dependent protein kinase. A complete understanding of the signaling events regulating aldosterone biosynthesis may allow the identification of novel targets for therapeutic interventions in hypertension, primary aldosteronism, congestive heart failure, renal disease, and other cardiovascular disorders.

Acute and chronic regulation of aldosterone production

Aldosterone is the major mineralocorticoid synthesized by the adrenal and plays an important role in the regulation of systemic blood pressure through the absorption of sodium and water. Aldosterone production is regulated tightly by selective expression of aldosterone synthase (CYP11B2) in the adrenal outermost zone, the zona glomerulosa. Angiotensin II (Ang II), potassium (K(+)) and adrenocorticotropin (ACTH) are the main physiological agonists which regulate aldosterone secretion. Aldosterone production is regulated within minutes of stimulation (acutely) through increased expression and phosphorylation of the steroidogenic acute regulatory (StAR) protein and over hours to days (chronically) by increased expression of the enzymes involved in the synthesis of aldosterone, particularly CYP11B2. Imbalance in any of these processes may lead to several disorders of aldosterone excess. In this review we attempt to summarize the key molecular events involved in the acute and chronic phases of aldosterone secretion.

Phospholipase D2 mediates acute aldosterone secretion in response to angiotensin II in adrenal glomerulosa cells

In primary bovine adrenal glomerulosa cells, the signaling enzyme phospholipase D (PLD) is suggested to mediate priming, the enhancement of aldosterone secretion after pretreatment with and removal of angiotensin II (AngII), via the formation of persistently elevated diacylglycerol (DAG). To further explore PLD's role in priming, glomerulosa cells were pretreated with an exogenous bacterial PLD. Using this approach, phosphatidic acid (PA) is generated on the outer, rather than the inner, leaflet of the plasma membrane. Although PA is not readily internalized, the PA is nonetheless rapidly hydrolyzed by cell-surface PA phosphatases to DAG, which efficiently flips to the inner leaflet and accesses the cell interior. Pretreatment with bacterial PLD resulted in priming upon subsequent AngII exposure, supporting a role of DAG in this process, because the increase in DAG persisted after exogenous PLD removal. To determine the PLD isoform mediating aldosterone secretion, and presumably priming, primary glomerulosa cells were infected with adenoviruses expressing GFP, PLD1, PLD2, or lipase-inactive mutants. Overexpressed PLD2 increased aldosterone secretion by approximately 3-fold over the GFP-infected control under basal conditions, with a significant enhancement to about 16-fold over the basal value upon AngII stimulation. PLD activity was also increased basally and upon stimulation with AngII. In contrast, PLD1 overexpression had little effect on aldosterone secretion, despite the fact that PLD activity was enhanced. In both cases, the lipase-inactive PLD mutants showed essentially no effect on PLD activity or aldosterone secretion. Our results suggest that PLD2 is the isoform that mediates aldosterone secretion and likely priming.

The role of calcium influx pathways in phospholipase D activation in bovine adrenal glomerulosa cells

The steroid hormone aldosterone maintains sodium homeostasis and is therefore important in the control of blood volume and pressure. Angiotensin II (AngII) and elevated extracellular potassium concentrations ([K(+)](e)), the prime physiologic regulators of aldosterone secretion from adrenal glomerulosa cells, activate phospholipase D (PLD) in these cells. The role of Ca(2+) in the activation by these agents is unknown, although nitrendipine, a voltage-dependent Ca(2+) channel antagonist, does not inhibit AngII-elicited PLD activation, despite the fact that this compound blocked elevated [K(+)](e)-stimulated PLD activity. PLD activation triggered by AngII was also unaffected by the T-type calcium channel inhibitor nickel. Nevertheless, Ca(2+) influx was required for AngII-induced PLD activation in both primary cultures of bovine adrenal glomerulosa cells and a glomerulosa cell model, the NCI H295R adrenocortical carcinoma cell line. The involvement of store-operated Ca(2+) (SOC) influx and Ca(2+) release-activated Ca(2+) (CRAC) influx pathways in PLD activation was investigated using thapsigargin, an endoplasmic reticulum Ca(2+) pump inhibitor that empties the store to induce SOC influx, and the SOC inhibitor YM-58483 (BTP2), as well as a CRAC inhibitor, tyrphostin A9. In bovine glomerulosa cells, tyrphostin A9 inhibited AngII-induced PLD activation without affecting elevated [K(+)](e)-stimulated enzyme activity. On the other hand, differences were observed between the bovine adrenal glomerulosa and H295R cells in the involvement of Ca(2+) influx pathways in PLD activation, with the involvement of the SOC pathway suggested in the H295R cells. In summary, our results indicate that Ca(2+) entry only through certain Ca(2+) influx pathways is linked to PLD activation.

Parathyroid hormone effects on signaling pathways in endothelial cells vary with peptide concentration

We have previously reported that parathyroid hormone (PTH) has specific effects on a human umbilical vein endothelial cell line. Further studies were performed to characterize the signaling cascades initiated by PTH. We report that PTH induced the appearance of voltage sensitive calcium channels. Furthermore, PTH increased ceramide but not diacylglycerol content. Since elevations in [Ca(2+)](i) and phospholipid turnover are signals for the activation of protein kinase C (PKC), the cells were screened for PKC isoforms. PTH induced a redistribution of the PKCepsilon to the particulate fractions of cell homogenates. In summary, PTH induced PKC translocation through a calcium-phospholipid pathway in an endothelial cell line.

Angiotensin II priming of aldosterone secretion with agents that enhance Ca(2+) influx

We have previously shown that angiotensin II (AngII) is able to prime, or sensitize, the secretory response of cultured bovine adrenal glomerulosa cells to the Ca(2+) channel agonist, BAY K8644. We examined the ability of AngII to prime glomerulosa cells to an elevated extracellular K(+) level, a physiological agonist that also triggers Ca(2+) influx. K(+) (9 mM) elicited enhanced secretion in AngII-primed cells compared to those with no prior exposure to the hormone, suggesting that AngII can sensitize glomerulosa cells to respond to increases in extracellular K(+). The potential involvement of protein kinase C (PKC) in priming was investigated by determining whether enhanced Ca(2+) influx could maintain the AngII-induced phosphorylation of the endogenous PKC substrate, myristoylated, alanine-rich C kinase substrate (MARCKS). Incubation with the AngII antagonist, saralasin, for 30 min following an AngII exposure reduced the AngII-induced increase in MARCKS phosphorylation. 100 nM BAY K8644 together with saralasin was unable to maintain AngII-stimulated MARCKS phosphorylation. On the other hand, phosphorylation of the steroidogenic acute regulatory (StAR) protein was sustained with saralasin exposure, both in the presence and absence of BAY K8644. This observation suggests that persistent StAR phosphorylation/activation may play a role in priming.

Sustained phospholipase D activation in response to angiotensin II but not carbachol in bovine adrenal glomerulosa cells

We have demonstrated previously that in bovine adrenal glomerulosa cells, phospholipase D (PLD) activity can indirectly result in the generation of sn-1,2-diacylglycerol (DAG) through its production of phosphatidic acid (PA) and the subsequent action of PA phosphohydrolase. Furthermore, the PLD-generated DAG can trigger aldosterone secretion. Therefore, we characterized PLD activation by two agonists, angiotensin II (Ang II) and carbachol, to determine if the activity of the enzyme might underlie sustained aldosterone secretion. We determined that Ang II-induced PLD activation occurred via the angiotensin-1 receptor (AT1), and that a specific AT1 antagonist, losartan, inhibited this activation, whereas the same concentration of the AT2-specific antagonist, PD 123319, had no effect. Ang II activated PLD with a dose dependence similar to that observed for aldosterone secretion, with slight increases in activity induced by 0.1 nM Ang II and maximal activation at 10 nM. We also found that Ang II induced a sustained activation of PLD, but that the effect of carbachol, a stable analogue of acetylcholine, was transient; PLD activity increased within 5 min of exposure to carbachol but then ceased by 15 min. Higher carbachol concentrations were also unable to sustain PLD activation. These results suggest that the Ang II-elicited activation of PLD is associated with a sustained increase in aldosterone secretion from glomerulosa cells and further provide the first evidence, to our knowledge, of differences in the kinetics of PLD activation in response to two physiologically relevant agonists. Finally, we speculate that this disparity correlates with different functional responses induced by the two agents.

Novel effect of insulin: insulin-stimulated Na+ transport is mediated by hydrolysis of phosphoinositides

Previous studies showed that insulin stimulation of electrogenic Na+ transport in renal epithelial cells is mediated by a calcium-dependent signal transduction mechanism. The present study was performed to determine whether the insulin-induced increase in intracellular Ca2+ (Cai2+) was mediated by hydrolysis of phosphatidylinositol and release of inositol trisphosphate. Experiments were conducted with cultured A6 cells, derived from Xenopus Laevis, grown on permeable supports. Addition of insulin resulted in 2 to 3 fold increases in inositol trisphosphate and a 50% increase in 1,2 diacylglycerol within 10s, which corresponded to the time-course, previously reported, of insulin stimulated increases in Na+ transport and Cai2+. Further studies showed that aldosterone, previously shown to stimulate an increase in 1,4,5-inositol trisphosphate at onset of the rise in Na+ transport, also increased DAG levels during the initial phase of stimulation of Na+ transport. These studies provide the first evidence that a biological response induced by insulin is mediated by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) which results in two products, inositol trisphosphate which causes the release of Ca2+ from intracellular stores and 1,2 diacylglycerol. In addition this study provides further support for the proposal that a common signal transduction mechanism mediates electrogenic Na+ transport by multiple agonists.