Dual role of calcium in steroidogenesis in the isolated adrenal cell of rat

Multilimbed membrane guanylate cyclase signaling system, evolutionary ladder

One monumental discovery in the field of cell biology is the establishment of the membrane guanylate cyclase signal transduction system. Decoding its fundamental, molecular, biochemical, and genetic features revolutionized the processes of developing therapies for diseases of endocrinology, cardio-vasculature, and sensory neurons; lastly, it has started to leave its imprints with the atmospheric carbon dioxide. The membrane guanylate cyclase does so via its multi-limbed structure. The inter-netted limbs throughout the central, sympathetic, and parasympathetic systems perform these functions. They generate their common second messenger, cyclic GMP to affect the physiology. This review describes an historical account of their sequential evolutionary development, their structural components and their mechanisms of interaction. The foundational principles were laid down by the discovery of its first limb, the ACTH modulated signaling pathway (the companion monograph). It challenged two general existing dogmas at the time. First, there was the question of the existence of a membrane guanylate cyclase independent from a soluble form that was heme-regulated. Second, the sole known cyclic AMP three-component-transduction system was modulated by GTP-binding proteins, so there was the question of whether a one-component transduction system could exclusively modulate cyclic GMP in response to the polypeptide hormone, ACTH. The present review moves past the first question and narrates the evolution and complexity of the cyclic GMP signaling pathway. Besides ACTH, there are at least five additional limbs. Each embodies a unique modular design to perform a specific physiological function; exemplified by ATP binding and phosphorylation, Ca²⁺-sensor proteins that either increase or decrease cyclic GMP synthesis, co-expression of antithetical Ca²⁺ sensors, GCAP1 and S100B, and modulation by atmospheric carbon dioxide and temperature. The complexity provided by these various manners of operation enables membrane guanylate cyclase to conduct diverse functions, exemplified by the control over cardiovasculature, sensory neurons and, endocrine systems.

Integrative Signaling Networks of Membrane Guanylate Cyclases: Biochemistry and Physiology

This monograph presents a historical perspective of cornerstone developments on the biochemistry and physiology of mammalian membrane guanylate cyclases (MGCs), highlighting contributions made by the authors and their collaborators. Upon resolution of early contentious studies, cyclic GMP emerged alongside cyclic AMP, as an important intracellular second messenger for hormonal signaling. However, the two signaling pathways differ in significant ways. In the cyclic AMP pathway, hormone binding to a G protein coupled receptor leads to stimulation or inhibition of an adenylate cyclase, whereas the cyclic GMP pathway dispenses with intermediaries; hormone binds to an MGC to affect its activity. Although the cyclic GMP pathway is direct, it is by no means simple. The modular design of the molecule incorporates regulation by ATP binding and phosphorylation. MGCs can form complexes with Ca²⁺-sensing subunits that either increase or decrease cyclic GMP synthesis, depending on subunit identity. In some systems, co-expression of two Ca²⁺ sensors, GCAP1 and S100B with ROS-GC1 confers bimodal signaling marked by increases in cyclic GMP synthesis when intracellular Ca²⁺ concentration rises or falls. Some MGCs monitor or are modulated by carbon dioxide via its conversion to bicarbonate. One MGC even functions as a thermosensor as well as a chemosensor; activity reaches a maximum with a mild drop in temperature. The complexity afforded by these multiple limbs of operation enables MGC networks to perform transductions traditionally reserved for G protein coupled receptors and Transient Receptor Potential (TRP) ion channels and to serve a diverse array of functions, including control over cardiac vasculature, smooth muscle relaxation, blood pressure regulation, cellular growth, sensory transductions, neural plasticity and memory.

Membrane guanylate cyclase is a beautiful signal transduction machine: overview

This article is a sequel to the four earlier comprehensive reviews which covered the field of membrane guanylate cyclase from its origin to the year 2002 (Sharma in Mol Cell Biochem 230:3-30, 2002) and then to the year 2004 (Duda et al. in Peptides 26:969-984, 2005); and of the Ca(2+)-modulated membrane guanylate cyclase to the year 1997 (Pugh et al. in Biosci Rep 17:429-473, 1997) and then to 2004 (Sharma et al. in Curr Top Biochem Res 6:111-144, 2004). This article contains three parts. The first part is "Historical"; it is brief, general, and freely borrowed from the earlier reviews, covering the field from its origin to the year 2004 (Sharma in Mol Cell Biochem, 230:3-30, 2002; Duda et al. in Peptides 26:969-984, 2005). The second part focuses on the "Ca(2+)-modulated ROS-GC membrane guanylate cyclase subfamily". It is divided into two sections. Section "Historical" and covers the area from its inception to the year 2004. It is also freely borrowed from an earlier review (Sharma et al. in Curr Top Biochem Res 6:111-144, 2004). Section "Ca(2+)-modulated ROS-GC membrane guanylate cyclase subfamily" covers the area from the year 2004 to May 2009. The objective is to focus on the chronological development, recognize major contributions of the original investigators, correct misplaced facts, and project on the future trend of the field of mammalian membrane guanylate cyclase. The third portion covers the present status and concludes with future directions in the field.

Human chorionic gonadotropin (hCG) increases cytosolic free calcium in adult rat Leydig cells

Regulation of steroidogenesis by luteinizing hormone is mediated by cAMP and calcium. We have investigated changes in cytosolic free calcium ([Ca2+]i) in Leydig cells by using Fura-2 as the fluorescent calcium indicator. Purified Leydig cells were plated on polylysine coated glass coverslips, cultured for 24 h in DMEM/F12 and loaded with Fura-2 at 37 degrees C. [Ca2+]i measurements were made fluorometrically by placing coverslips into 3 ml cuvettes with PBS+calcium. Addition of hCG increased [Ca2+]i gradually after a lag of about 2-3 min and plateaued by 5-6 min. The plateau level was sustained for at least 15 min. Absence of external Ca2+ in the medium or presence of diltiazem or nicardipine or cobalt chloride abolished the rise. Addition of BAY K 8644 or KCl caused a small but significant increase of [Ca2+]i. 8-Br-cAMP, forskolin or cholera toxin produced a gradual rise in [Ca2+]i that plateaued after 5-6 min similar to that observed with hCG. The action of hCG was inhibited by protein kinase A inhibitor (20-residues peptide) but not by protein kinase C inhibitor (staurosporine). We conclude that binding of hCG to its receptors would transmit the signal through G proteins to adenylyl cyclase to increase cAMP which would increase Ca2+ influx into cytosol across plasma membrane Ca2+ channels. Therefore, it appears that the primary action of hCG is to increase cytosolic cAMP which would then regulate [Ca2+]i as well as steroidogenesis.

The actions of calcitonin on the TM3 Leydig cell line and on rat Leydig cell-enriched cultures

Studies demonstrating calcitonin receptors on Leydig cells have suggested that these cells may be one of the many sites affected by this peptide. To investigate this possibility, the effect of synthetic salmon calcitonin on the TM3 Leydig cell line (derived from immature mouse Leydig cells) and on primary Leydig cell-enriched preparations was examined. Synthetic salmon calcitonin stimulated the conversion of [3H]adenine to [3H]cyclic AMP in TM3 cells. In addition, the hormone stimulated the basal secretion of testosterone in both TM3 cell- and Leydig cell-enriched cultures and potentiated the action of hCG on Leydig cell-enriched cultures. Synthetic salmon calcitonin also increased the concentration of androgen and estrogen receptors in cultured TM3 Leydig cells by 2- and 4-fold, respectively, when added to the culture medium (1 micrograms/ml). The fact that 8-bromo-cyclic AMP decreased both androgen and estrogen receptor concentrations suggested that the effect of calcitonin on sex steroid receptors is not mediated by its effect on cyclic AMP in these cells. The possibility that the action of calcitonin on steroid receptors might be mediated by another messenger such as calcium (Ca2+) was therefore considered. Progressively lowering the concentration of Ca2+ in the culture medium of the cells from 1.5 mM to less than 0.01 mM decreased the concentration of both androgen and estrogen receptors. Returning the Ca2+ concentration to normal levels (1.5 mM) restored steroid receptor levels. Receptor levels were also decreased when the extracellular Ca2+ concentration was lowered to 0.5 mM, and treatment with the Ca2+ ionophore, A23187 (1 microM), restored receptor levels to normal. The calcium channel blocker, verapamil, decreased the androgen receptor concentration but unexpectedly increased the concentration of estrogen receptors. It was concluded that calcitonin stimulates cAMP formation and testosterone secretion, and increases the concentration of sex steroid receptors. These observations provide evidence that the previously demonstrated calcitonin receptors on Leydig cells may be coupled to several biologic responses in this cell type.

Role of calcium ion in acth-induced steroidogenesis in humans

In order to examine the role of calcium ion in ACTH-induced steroidogenesis in humans, we carried out infusion of a pharmacological dose of ACTH (4.2 micrograms/kg) and a physiological dose of ACTH (0.0084 microgram/kg) for 120 min, and infusion of dibutyryl cyclic AMP (DBcAMP) [0.33 mg/kg/min] for 20 min, in 22 normal subjects with or without verapamil treatment (360 mg/day, orally) for 5 days. The subjects were pretreated with 1.0 mg of dexamethasone and 5.0 mg of enalapril daily for 2 days before each infusion test to inhibit endogenous ACTH and angiotensin II. Following infusion of 0.0084 microgram/kg of ACTH, plasma levels of corticosterone (P less than 0.02) and cortisol (P less than 0.01) were significantly increased; with chronic verapamil treatment plasma levels of corticosterone (P less than 0.05) and cortisol (P less than 0.02) were significantly lower than those without verapamil. On the other hand, 4.2 micrograms/kg of ACTH for 120 min significantly increased the plasma levels of several steroid hormones, although there were no differences between the infusion with and without verapamil. Infusion of DBcAMP for 20 min significantly increased plasma levels of corticosterone (P less than 0.02) and cortisol (P less than 0.01), but verapamil did not affect the steroidogenic response to the DBcAMP infusion. The present results suggest that steroidogenesis induced by a physiological dose of ACTH differs from that after a pharmacological dose of ACTH or DBcAMP, and is mediated mainly by calcium ion as an intracellular messenger in man.

Role of calcium ion in hormone-stimulated lipolysis

Using the flask-incubated fat cell system, the effects of Ca2+ removal from the incubation medium on the lipolytic system were studied. The removal of Ca2+ resulted in a total abolition of the lipolytic response and the increased cyclic AMP accumulation produced by ACTH. The lipolytic response to isoproterenol and forskolin were reduced approximately 40% by Ca2+ removal, but cyclic AMP accumulation was not altered in the presence of either of these agents using a Ca2+-free medium. The lipolytic response to the dibutyryl analog of cyclic AMP was also reduced by omission of Ca2+ from the incubation medium. It is concluded the Ca2+ is required for the interaction of ACTH with its receptor and the resultant activation of adenylate cyclase. Ca2+ also is required at some step in the lipolytic process distal to cyclic AMP.

The stimulation by calcium and its inhibition by ADP of cholesterol side-chain cleavage activity in adrenal mitochondria

Cholesterol side-chain cleavage activity (cholesterol SCC) in a mitochondrial preparation is increased by calcium in a sigmoidal manner. A 5-10-fold increase is obtained and an effect may be seen at 20 microM CaCl2. ADP inhibits the stimulation by calcium with a shift of the sigmoid curve to the right and 10 microM ADP results in a 4-fold increase in the amount of CaCl2 required to obtain one-half the maximal stimulation value. The inhibition is specific for ADP and inhibition by ATP is due to the formation of ADP. The characteristics of the calcium-ADP modulation are such that a suitable ADP-inhibited level of cholesterol sidechain cleavage activity will be stimulated by a given increment of calcium to a greater extent than in the absence of the added ADP. Steroid 11 beta-hydroxylation is also stimulated in a sigmoidal manner by calcium and this stimulation is inhibited by ADP. The 11 beta-hydroxylation, however, is less sensitive to calcium and ADP so that changes in cholesterol SCC are obtained at concentration of calcium and ADP where minimal effects on 11 beta-hydroxylation are found. Calmodulin-like activity is present in the mitochondrial preparation. No evidence, however, for a role for calmodulin in the calcium-ADP effects could be obtained, but the possibility of its involvement cannot be excluded. The calcium-ADP modulations are of a magnitude and take place at sufficiently low concentrations to suggest a physiological role in the regulation of mitochondrial steroidogenesis.

Effect of cortisol on cyclic AMP production stimulated by 1-24 ACTH in adrenal cortex membranes

The effect of cortisol on cyclic AMP production by crude bovine adrenal cortex membrane preparations has been investigated. Results demonstrate that in the presence of cortisol, cyclic AMP production in response to 1-24 ACTH was enhanced up to a concentration of about 74 microM cortisol. At higher concentrations the effect was reversed. Cortisol had no effect on cyclic AMP production in the absence of 1-24 ACTH, and cyclic AMP production was completely inhibited when 5 mM EDTA was added to the incubation tubes with the cortisol.

Effects of stimulation on the steroid profile formed by rat adrenal capsule tissue incubated in vitro

A characteristic of the response of non-dispersed rat adrenal capsule tissue (mainly zona glomerulosa) to ACTH stimulation is that corticosterone and aldosterone production is increased whereas 18-hydroxy-deoxycorticosterone (18-OH-DOC) is not. The effects of potential second messengers on the steroid profile were compared with those of ACTH and K+ ions in adrenal capsule incubations. ACTH stimulated corticosterone, 18-hydroxycorticosterone and aldosterone, but not 18-OH-DOC. In contrast, K+ (5.9 mM) stimulated 18-OH-DOC as well as the other capsule products. Compared with controls incubated with EDTA, the addition of Ca2+ and Mg2+ ions invariably stimulated aldosterone and 18-hydroxycorticosterone, while the effects on corticosterone were variable, and 18-OH-DOC production was unaltered. Addition of dibutyryl cyclic AMP (1 mM and 0.3 mM) or cyclic GMP (1mM) stimulated all products. The results show that Ca2+ ions and dibutyryl cAMP may have slightly different effects in that the cyclic nucleotides can stimulate all products whereas Ca2+ and Mg2+ ions preferentially support aldosterone and 18-hydroxycorticosterone. However, none of the potential intracellular stimulants studied fully reproduce the characteristic response of non-dispersed tissue to ACTH in which the secretion of corticosterone and 18-hydroxy-DOC is disassociated.

Regulation of ovarian ornithine decarboxylase. Role of Calcium ions in enzyme induction in isolated swine granulosa cells in vitro

When swine granulosa cells were cultured in chemically defined medium selectively deficient in Ca2+, the dose-dependent stimulation of ornithine decarboxylase (EC 4.1.1.17) activity in response to prostaglandin E2, L-epinephrine or the somatomedin, multiplication-stimulating activity, was attenuated markedly. Putative calcium influx blockers, verapamil and diltiazem, also inhibited hormone-stimulated enzymic activity. Similar inhibitory effects were exerted by divalent (cobalt) or trivalent (lanthanum) cations believed to compete with calcium for extracellular binding sites. The suppressive effects of extracellular calcium deprivation were time-dependent (suggesting gradual depletion of intracellular calcium stores), and could be mimicked by the intracellular antagonist of calcium action, trifluoperazine. The mechanism(s) subserving diminished hormonal induction of enzyme activity could not be accounted for by alterations in cell viability, general protein synthesis, half-life of decay of enzyme activity (measured in the presence of cycloheximide), or apparent Km of ornithine decarboxylase. Ca2+ and/or calcium antagonists did not modify enzyme activity in cell-free preparations. These observations implicate Ca2+ in the hormonal induction of a discrete cytosolic enzyme in isolated intact ovarian cells.

Calcium ions modulate hormonally stimulated progesterone production in isolated ovarian cells

Swine granulosa-luteal cells incubated in Ca2+-deficient medium (5 muM final Ca2+ concentration) for short time periods produced diminished quantities of progesterone in response to lutropin. Maximally stimulating effects of prostaglandin E2 and L-adrenaline were also impaired significantly. Diminished progesterone production could not be attributed to alterations in protein synthesis or cell viability. Under Ca2+-deprived conditions, the stimulatory actions of cholera toxin, 3-isobutyl-1-methylxanthine and 8-bromo cyclic AMP were also significantly impeded. Administration of a presumptive antagonist of transmembrane Ca2+ influx (verapamil) or of EGTA to chelate extracellular Ca2+, significantly decreased the total cellular content of Ca2+, and antagonized the actions of lutropin. Micromolar concentrations of trifluoperazine mimicked the suppressive effects of Ca2+ deprivation. Conversely, the bivalent-cation ionophore, ionophore A23187, significantly augmented the stimulation of progesterone produced by lutropin. Thus the present observations implicate Ca2+ in the modulation of hormonally stimulated progesterone production in isolated ovarian cells, and suggest that Ca2+ may influence one or more processes distal to, or independent of, cyclic AMP generation. In addition, the susceptibility of progesterone biosynthesis to inhibition by trifluoperazine suggests a possible role for calmodulin in the ovary.

Adrenocorticotrophic hormone and cyclic adenosine monophosphate effect on mouse adrenal cortical cell membrane potential

Adrenocorticotrophic hormone (ACTH) and cyclic adenosine monophosphate (cAMP) both caused a rapid and transient depolarization of the resulting membrane potential of superfused rat adrenal cortical cells. The membrane depolarization to both secretagogues were very similar. The membrane potential changes occurred as early as 0.1 min and were dose dependent in both onset and extent of depolarization.

Steroidogenic action of calcium ions in isolated adrenocortical cells

The corticotropin-induced increase of total intracellular and receptor-bound cyclic AMP in isolated rat adrenocortical cells was strictly dependent on extracellular Ca(2+). A rise in bound cyclic AMP with rising Ca(2+) concentrations was accompanied by a decrease in free cyclic AMP-receptor sites. A Ca(2+)-transport inhibitor abolished the rise in bound cyclic AMP induced by corticotropin. These data suggested that during stimulation by corticotropin some Ca(2+) has to be taken up in order to promote the rise of the relevant cyclic AMP pool. In agreement with this view, adenylate cyclase activity from isolated cells proved also to be dependent on a sub-millimolar Ca(2+) concentration in the presence of corticotropin and GTP. When cells were treated under specific conditions, corticosterone production could be activated by Ca(2+) in the absence of corticotropin (cells primed for Ca(2+)). Ca(2+)-induced steroidogenesis of these cells, in the absence of corticotropin, was also accompanied by an increase in total intracellular and receptor-bound cyclic AMP, as was found previously with corticotropin-induced steroidogenesis in non-primed cells. Calcium ionophores increasing the cell uptake of Ca(2+) were not able, however, to increase the cyclic AMP pools in non-primed cells, unlike corticotropin in nonprimed cells or Ca(2+) in cells primed for Ca(2+). It was concluded that during stimulation by either corticotropin or Ca(2+) a possible cellular uptake of Ca(2+) must be very limited and directed to a specific site which may affect the coupling of the hormone-receptor-adenylate cyclase complex.

Mediatory role of calcium and guanosine 3', 5'-monophosphate in adrenocorticotropin-induced steroidogenesis by adrenal cells

When incubated in a calcium-free medium, isolated rat fasciculata cells showed neither an increase in the concentration of guanocine 3',5'-monophosphate (cyclic GMP) nor an increase in corticosterone production in response to adrenocorticotropic hormone (ACTH). In response to submaximum and maximum steroidogenic concentrations of ACTH, corticosterone formation was directly proportional to increases in calcium concentration ranging from 0 to 2.5 mM. Higher concentration of calcium, however, inhibited maximal ACTH-induced steroidogenesis. In the absence of ACTH, calcium did not stimulate cyclic GMP accumulation and corticosterone formation. ACTH-induced corticosterone synthesis, preceded by an increase in cyclic GMP, was restored when ACTH and calcium were both present in the medium. Cyclic GMP or dibutryl cyclic GMP-induced steroidogenesis was substantially reduced in the absence of calcium, but in contrast to the ACTH effect a significant amount of corticosterone formation occurred without calcium. It is proposed that at the physiological concentrations of the hormone, calcium regulates the transduction of information between hormone receptors and guanylate cyclase.

Angiotensin-induced steroidogenesis in rabbit adrenal: effects of pH and calcium

The effect of variations in pH and Ca2+ on angiotensin II (A-II)-induced steroidogenesis was tested on isolated adrenal glomerulosa cell suspensions. The results show that a reduction in pH from 7.4 to 6.5 produces both a shift to the left of the A-II dose-response curve as well as an increase in maximum steroid production. In contrast, removal of Ca2+ from the incubation medium virtually abolished steroidogenesis to A-II (5 X 10(-9)M(, KCl(10mM) and ACTH (250 microU/ml). The Ca2+ antagonist D-600, however, was less effective than simple removal of Ca2+ as 10(-4) M was required to block the steroidogenic response to these same agonists. The results indicate that the response characteristics of this system to A-II resemble most closely those seen with isolated arterial smooth muscle - especially rabbit aortic strips.

Steroidogenesis in isolated adrenal cells: excitation by calcium

Calcium salts were found to replace ACTH in inducing steroidogenesis in isolated adrenocortical cells from rats. This Ca-specific stimulation occurred when the cation was presented to the cells in the presence of phosphate and carbonate as a counter-ions under conditions which favoured the formation of colloidal calcium. Colloid generation and stabilization was facilitated by the use of calcium buffers and gelatin. Stable soluble or sparingly soluble calcium complexes were inactive. The preparation of cells and metastable calcium solutions is described in detail. The Ca trigger was sensitive to Ca deprivation or inhibitors of Ca transport and could be replced by Sr. The relative role of Ca and cyclic AMP as second messengers is discussed.

The effect of calcium ions on testosterone production in Leydig cells from rat testis

Leydig-cell suspensions, prepared from rat testes, were incubated with different amounts of Ca2+ with and without added luteinizing hormone. The basal testosterone production in the absence of luteinizing hormone was unaffected by the Ca2+ concentration in the incubation medium. The luteinizing hormone-stimulated testosterone production, however, was progressively decreased in the absence of Ca2+ to one-third of that with 2.50 mM-Ca2+. This decrease in luteinizing hormone-stimulated testosterone production was independent of the different concentrations of luteinizing hormone (0-10mug/ml) used and could be restored by the addition of Ca2+ to the incubation medium. The restoration of the stimulation was achieved within 30 min after the addition of Ca2+ to the medium. Activation of cyclic AMP-dependent protein kinase by luteinizing hormone was not decreased by omission of Ca2+ from the incubation medium, suggesting that Ca2+ may be involved in steroidogenesis at a stage beyond the luteinizing hormone receptor-adenylate cyclase-protein kinase system.

Steroidogenesis and extracellular cAMP accumulation in adrenal tumor cell cultures

ACTH stimulated steroidogenesis and cAMP (adenosine 3',5'-monophosphate) accumulation in an adrenocortical mouse tumor cell line (clone Y1) with Kd values which differed by more than one order of magnitude (5.2 X 10(-11) M and 7 X 10(-10) M, respectively). All of the cAMP formed in response to added ACTH appeared extracellularly in 5- or 30-min incubations. ACTH, at 5 and 10 muU/ml, stimulated steroidogenesis to 25% and 40% of maximum activity; and increased the extracellular accumulation of cAMP 1.4-fold and 2.3-fold, respectively. The effects of ACTH appeared to be via an action on intracellular ATP, specific for cAMP and dependent on an ACTH-sensitive adenylate cyclase system. These observations indicate that ACTH increases cAMP accumulation in Y1 cells at virtually all steroidogenic concentrations and suggest that cAMP is an essential component of ACTH-stimulated steroidogenesis.

Mode of action of peptide hormones

The mode of action is discussed of the peptide hormones which trigger neosynthesis of a specific product in their target cells without being involved in any release step. Particular attention is paid to the early events elicited by ACTH in isolated adrenocortical cells. It is shown that extracellular calcium ions at physiological concentrations can serve as first messenger activating steroidogenesis if the isolated cells are pretreated in an appropriate ionic environment. Among other factors the extracellular calcium/phosphate ratio seems to be of importance. A model is proposed where calcium serves as direct messenger in the physiological activation by ACTH, cyclic AMP being a subserving factor maintaining full steroidogenesis.