Inositol 1,4,5-trisphosphate receptor subtypes in adrenal glomerulosa cells

Nitric oxide modulates NMDA receptor through a negative feedback mechanism and regulates the dynamical behavior of neuronal postsynaptic components

Nitric oxide (NO) is known to be an important regulator of neurological processes in the central nervous system which acts directly on the presynaptic neuron and enhances the release of neurotransmitters like glutamate into the synaptic cleft. Calcium influx activates a cascade of biochemical reactions to influence the production of nitric oxide in the postsynaptic neuron. This has been modeled in the present work as a system of ordinary differential equations, to explore the dynamics of the interacting components and predict the dynamical behavior of the postsynaptic neuron. It has been hypothesized that nitric oxide modulates the NMDA receptor via a feedback mechanism and regulates the dynamic behavior of postsynaptic components. Results obtained by numerical analyses indicate that the biochemical system is stimulus-dependent and shows oscillations of calcium and other components within a limited range of concentration. Some of the parameters such as stimulus strength, extracellular calcium concentration, and rate of nitric oxide feedback are crucial for the dynamics of the components in the postsynaptic neuron.

The role of lipid second messengers in aldosterone synthesis and secretion

Second messengers are small rapidly diffusing molecules or ions that relay signals between receptors and effector proteins to produce a physiological effect. Lipid messengers constitute one of the four major classes of second messengers. The hydrolysis of two main classes of lipids, glycerophospholipids and sphingolipids, generate parallel profiles of lipid second messengers: phosphatidic acid (PA), diacylglycerol (DAG), and lysophosphatidic acid versus ceramide, ceramide-1-phosphate, sphingosine, and sphingosine-1-phosphate, respectively. In this review, we examine the mechanisms by which these lipid second messengers modulate aldosterone production at multiple levels. Aldosterone is a mineralocorticoid hormone responsible for maintaining fluid volume, electrolyte balance, and blood pressure homeostasis. Primary aldosteronism is a frequent endocrine cause of secondary hypertension. A thorough understanding of the signaling events regulating aldosterone biosynthesis may lead to the identification of novel therapeutic targets. The cumulative evidence in this literature emphasizes the critical roles of PA, DAG, and sphingolipid metabolites in aldosterone synthesis and secretion. However, it also highlights the gaps in our knowledge, such as the preference for phospholipase D-generated PA or DAG, as well as the need for further investigation to elucidate the precise mechanisms by which these lipid second messengers regulate optimal aldosterone production.

Mitochondrial cAMP and Ca2+ metabolism in adrenocortical cells

The biological effects of physiological stimuli of adrenocortical glomerulosa cells are predominantly mediated by the Ca²⁺ and the cAMP signal transduction pathways. The complex interplay between these signalling systems fine-tunes aldosterone secretion. In addition to the well-known cytosolic interactions, a novel intramitochondrial Ca²⁺-cAMP interplay has been recently recognised. The cytosolic Ca²⁺ signal is rapidly transferred into the mitochondrial matrix where it activates Ca²⁺-sensitive dehydrogenases, thus enhancing the formation of NADPH, a cofactor of steroid synthesis. Quite a few cell types, including H295R adrenocortical cells, express the soluble adenylyl cyclase within the mitochondria and the elevation of mitochondrial [Ca²⁺] activates the enzyme, thus resulting in the Ca²⁺-dependent formation of cAMP within the mitochondrial matrix. On the other hand, mitochondrial cAMP (mt-cAMP) potentiates the transfer of cytosolic Ca²⁺ into the mitochondrial matrix. This cAMP-mediated positive feedback control of mitochondrial Ca²⁺ uptake may facilitate the rapid hormonal response to emergency situations since knockdown of soluble adenylyl cyclase attenuates aldosterone production whereas overexpression of the enzyme facilitates steroidogenesis in vitro. Moreover, the mitochondrial Ca²⁺-mt-cAMP-Ca²⁺ uptake feedback loop is not a unique feature of adrenocortical cells; a similar signalling system has been described in HeLa cells as well.

Signaling Interactions in the Adrenal Cortex

The major physiological stimuli of aldosterone secretion are angiotensin II (AII) and extracellular K(+), whereas cortisol production is primarily regulated by corticotropin (ACTH) in fasciculata cells. AII triggers Ca(2+) release from internal stores that is followed by store-operated and voltage-dependent Ca(2+) entry, whereas K(+)-evoked depolarization activates voltage-dependent Ca(2+) channels. ACTH acts primarily through the formation of cAMP and subsequent protein phosphorylation by protein kinase A. Both Ca(2+) and cAMP facilitate the transfer of cholesterol to mitochondrial inner membrane. The cytosolic Ca(2+) signal is transferred into the mitochondrial matrix and enhances pyridine nucleotide reduction. Increased formation of NADH results in increased ATP production, whereas that of NADPH supports steroid production. In reality, the control of adrenocortical function is a lot more sophisticated with second messengers crosstalking and mutually modifying each other's pathways. Cytosolic Ca(2+) and cGMP are both capable of modifying cAMP metabolism, while cAMP may enhance Ca(2+) release and voltage-activated Ca(2+) channel activity. Besides, mitochondrial Ca(2+) signal brings about cAMP formation within the organelle and this further enhances aldosterone production. Maintained aldosterone and cortisol secretion are optimized by the concurrent actions of Ca(2+) and cAMP, as exemplified by the apparent synergism of Ca(2+) influx (inducing cAMP formation) and Ca(2+) release during response to AII. Thus, cross-actions of parallel signal transducing pathways are not mere intracellular curiosities but rather substantial phenomena, which fine-tune the biological response. Our review focuses on these functionally relevant interactions between the Ca(2+) and the cyclic nucleotide signal transducing pathways hitherto described 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.

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.

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.

An oxidized metabolite of linoleic acid increases intracellular calcium in rat adrenal glomerulosa cells

EKODE, an epoxy-keto derivative of linoleic acid, was previously shown to stimulate aldosterone secretion in rat adrenal glomerulosa cells. In the present study, we investigated the effect of exogenous EKODE on cytosolic [Ca(2+)] increase and aimed to elucidate the mechanism involved in this process. Through the use of the fluorescent Ca(2+)-sensitive dye Fluo-4, EKODE was shown to rapidly increase intracellular [Ca(2+)] ([Ca(2+)](i)) along a bell-shaped dose-response relationship with a maximum peak at 5 microM. Experiments performed in the presence or absence of Ca(2+) revealed that this increase in [Ca(2+)](i) originated exclusively from intracellular pools. EKODE-induced [Ca(2+)](i) increase was blunted by prior application of angiotensin II, Xestospongin C, and cyclopiazonic acid, indicating that inositol trisphosphate (InsP(3))-sensitive Ca(2+) stores can be mobilized by EKODE despite the absence of InsP(3) production. Accordingly, EKODE response was not sensitive to the phospholipase C inhibitor U-73122. EKODE mobilized a Ca(2+) store included in the thapsigargin (TG)-sensitive stores, although the interaction between EKODE and TG appears complex, since EKODE added at the plateau response of TG induced a rapid drop in [Ca(2+)](i). 9-oxo-octadecadienoic acid, another oxidized derivative of linoleic acid, also increases [Ca(2+)](i), with a dose-response curve similar to EKODE. However, arachidonic and linoleic acids at 10 microM failed to increase [Ca(2+)](i) but did reduce the amplitude of the response to EKODE. It is concluded that EKODE mobilizes Ca(2+) from an InsP(3)-sensitive store and that this [Ca(2+)](i) increase is responsible for aldosterone secretion by glomerulosa cells. Similar bell-shaped dose-response curves for aldosterone and [Ca(2+)](i) increases reinforce this hypothesis.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Isoform diversity of the inositol trisphosphate receptor in cell types of mouse origin

Previous reports suggested the expression of four or five different Ins(1,4,5)P3 receptor [Ins(1,4,5)P3R] isoforms in mouse cells [Ross, Danoff, Schell, Snyder and Ullrich (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 4265-4269; De Smedt, Missiaen, Parys, Bootman, Mertens, Van Den Bosch and Casteels (1994) J. Biol. Chem. 269, 21691-21698]. To explore this diversity further, we have isolated and sequenced partial clones of two Ins(1,4,5)P3R mRNAs from the mouse embryonic C3H10T1/2 cell line. These clones showed between 94.2 and 94.9% sequence identity with the corresponding rat Ins(1,4,5)P3R-II and Ins(1,4,5)P3R-III isoforms. Based on these newly obtained sequences we have determined the relative expression of the different Ins(1,4,5)P3R mRNAs in cultured cells and in animal tissues of mouse origin by a ratio reverse transcriptase polymerase chain reaction (RT-PCR). Ins(1,4,5)P3R-I was very prominent in brain and cerebellum and Ins(1,4,5)P3R-II in epithelia such as kidney as well as in both cardiac and skeletal muscle. Ins(1,4,5)P3R-III was highly expressed in all cultured cell types and in tissues with high cell turnover, e.g. testis. The prominent expression of Ins(1,4,5)P3R-I and Ins(1,4,5)P3R-III in A7r5 and C3H10T1/2 cells respectively was confirmed by immunoblot analysis and was compatible with a lower threshold for Ins(1,4,5)P3-induced Ca2+ release in the former cell type. Screening of a large number of mouse cell lines and tissues revealed the presence of Ins(1,4,5)P3R-I as well as of the Ins(1,4,5)P3R-II and Ins(1,4,5)P3R-III isoforms which were identified in the present study, but in contrast with previous reports there was no evidence for more isoform diversity.

Identification of three isoforms of the InsP3 receptor in human myometrial smooth muscle

The mechanisms responsible for the mobilisation of Ca2+ from intracellular stores sensitive to inositol trisphosphate (InsP3) were studied in saponin-permeabilised human myometrial cells in which the sarcoplasmic reticulum was pre-loaded with 45Ca2+. InsP3-induced 45Ca2+ release was measured over the InsP3 concentration range of 100 nM to 100 microM and showed a graded response. InsP3-induced 45Ca2+ release was inhibited by heparin (20-40 microg/ml) but not significantly affected by caffeine. The Ca2+ sensitivity of InsP3-induced Ca2+ release was measured under conditions which were designed to exclude interference with Ca2+ released by the ryanodine receptor/channel complex. The data showed a bell-shaped relationship with the InsP3 receptor (InsP3R) functional at 10 nM, becoming maximally activated at 300 nM but inhibited at 10 microM Ca2+. Messenger RNA encoding for three isoforms of InsP3R, type I, II and type III, was shown to be present. The relative expression levels of these messengers were obtained by ratio-PCR analysis and the levels of expression of the different isoforms were found to differ between individual patients.

The inositol triphosphate receptor family

Specific receptors on intracellular membranes mediate the Ca2+ mobilization induced by the second messenger molecule D-myo-inositol 1,4,5-triphosphate (IP3). Most cell types appear to contain multiple receptor isoforms. The review summarizes recent progress on IP3 receptor biology with a particular emphasis on distinctive structural and regulatory features of the individual isoforms.

Heteroligomers of type-I and type-III inositol trisphosphate receptors in WB rat liver epithelial cells

We have previously shown that a 222-kDa polypeptide co-immunoprecipitates together with the type-I myoinositol 1,4,5-trisphosphate receptor (IP3R) in WB rat liver epithelial cell extracts, when the immunoprecipitation is carried out with a type-I isoform specific antibody (Joseph, S. K. (1994) J. Biol. Chem. 269, 5673-5679). Utilizing isoform-specific antibodies raised to unique sequences within the COOH-terminal region of IP3 receptors, we now report that the co-immunoprecipitating 222-kDa polypeptide is the type-III IP3R isoform and that type-III IP3R antibodies (Abs) can co-immunoprecipitate the type-I IP3R isoform. Co-immunoprecipitation of IP3R isoforms was not due to cross-reactivity of the antibodies for the following reasons: (a) on immunoblots the type-III antibodies did not cross-react with type-I IP3R and vice versa; (b) inclusion of the COOH-terminal type-III peptide had no effect on the ability of type-I IP3R Ab to co-immunoprecipitate the type-III IP3R but blocked the ability of type-III IP3R Ab to coimmunoprecipitate the type-I isoform; and (c) crude hepatocyte lysates contain undetectable amounts of type-III IP3R, and immunoprecipitation with type-III IP3R Ab does not co-immunoprecipitate any other isoforms. However, type-I and type-II IP3R isoforms were co-immunoprecipitated by their respective antibodies in hepatocyte lysates. Sucrose density gradient analysis of WB cell lysates indicated that the co-immunoprecipitating fraction is exclusively located at the density expected for tetrameric receptors, suggesting that co-immunoprecipitation was not a reflection of the nonspecific aggregation of IP3R isoforms. Phosphorylation of either type-I or type-III immunoprecipitates by protein kinase A indicated that only the type-I IP3R could be phosphorylated in vitro. Fractionation of WB cell membranes and immunofluorescence studies showed that the type-I and type-III isoforms have very similar sub-cellular localizations. We conclude that the WB cell contains both type-I and type-III IP3R isoforms and that a proportion of these receptors exist as heterotetramers.

Type I, II, and III inositol 1,4,5-trisphosphate receptors are unequally susceptible to down-regulation and are expressed in markedly different proportions in different cell types

The type I inositol 1,4,5-trisphosphate (InsP3) receptor can be rapidly depleted from cells during stimulation of phosphoinositide hydrolysis because its degradation is accelerated (Wojcikiewicz, R. J. H., Furuichi, T., Nakade, S., Mikoshiba, K., and Nahorski, S. R. (1994) J. Biol. Chem. 269, 7963-7969). The present study examines the regulatory properties of type II and III InsP3 receptors. Initially, the relative abundance of InsP3 receptors was defined in a range of cell types by quantitative immunoblotting. These studies showed that the proportions in which type I, II, and III InsP3 receptors are expressed differs greatly and that some cells (for example, AR4-2J rat pancreatoma cells) express all three receptors. Analysis of the effects of cholecystokinin and bombesin on AR4-2J cells showed that each of the InsP3 receptors could be down-regulated during activation of phosphoinositide hydrolysis, but that depletion of the type II receptor was limited. Such a discrepancy was also seen in rat cerebellar granule cells and was found to result from the type II receptor being relatively resistant to degradation. In conclusion, type I, II, and III receptors can all be down-regulated, but with different characteristics. As the relative abundance of InsP3 receptors is extremely variable, the extent to which activation of the down-regulatory process alters intracellular signaling will vary depending on which InsP3 receptors are expressed.