Calcium stimulates steroidogenesis in permeabilized bovine adrenal cortical cells

Histidine trinucleotide binding protein 2: from basic science to clinical implications

Histidine triad nucleotide-binding protein 2 (HINT2) is a dimeric protein that belongs to the histidine triad protein superfamily, predominantly expressed in the liver, pancreas, and adrenal gland, and localised to the mitochondrion. HINT2 binds nucleotides and catalyses the hydrolysis of nucleotidyl substrates. Moreover, HINT2 has been identified as a key regulator of multiple biological processes, including mitochondria-dependent apoptosis, mitochondrial protein acetylation, and steroidogenesis. Genetic manipulation has provided new insights into the physiological roles of HINT2 in several processes, such as inhibition of cancer progression, regulation of hepatic lipid metabolism, and protective effects on the cardiovascular system. The current review outlines the background and functions of HINT2. In addition, it summarises research progress on the correlation between HINT2 and human malignancies, hepatic metabolic diseases, and cardiovascular diseases, with an attempt to provide new research directions emerging in this field and to unveil the therapeutic value of HINT2 as a target in the combat of human diseases.

The ER-mitochondria couple: In life and death from steroidogenesis to tumorigenesis

Steroidogenesis is a multistep process where interorganelle communications between the endoplasmic reticulum and mitochondria are critical. These intimate interactions physically occur through the Mitochondria-Associated ER membranes called MAMs. MAMs play important roles in mitochondrial morphology and in many cellular functions ranging from lipid metabolism, to calcium signaling and apoptosis together with a critical effect on steroidogenesis. Moreover, our recent characterization of new MAM resident proteins in adrenocortical cells extends the function of MAM in the mechanism of resistance of cancer cells to apoptotic stimuli and offers new perspectives in targeted therapeutic approaches for adrenocortical tumorigenesis.

Of yeast, mice and men: MAMs come in two flavors

The past decade has seen dramatic progress in our understanding of membrane contact sites (MCS). Important examples of these are endoplasmic reticulum (ER)-mitochondria contact sites. ER-mitochondria contacts have originally been discovered in mammalian tissue, where they have been designated as mitochondria-associated membranes (MAMs). It is also in this model system, where the first critical MAM proteins have been identified, including MAM tethering regulators such as phospho-furin acidic cluster sorting protein 2 (PACS-2) and mitofusin-2. However, the past decade has seen the discovery of the MAM also in the powerful yeast model system Saccharomyces cerevisiae. This has led to the discovery of novel MAM tethers such as the yeast ER-mitochondria encounter structure (ERMES), absent in the mammalian system, but whose regulators Gem1 and Lam6 are conserved. While MAMs, sometimes referred to as mitochondria-ER contacts (MERCs), regulate lipid metabolism, Ca²⁺ signaling, bioenergetics, inflammation, autophagy and apoptosis, not all of these functions exist in both systems or operate differently. This biological difference has led to puzzling discrepancies on findings obtained in yeast or mammalian cells at the moment. Our review aims to shed some light onto mechanistic differences between yeast and mammalian MAM and their underlying causes.

Reviewers: This article was reviewed by Paola Pizzo (nominated by Luca Pellegrini), Maya Schuldiner and György Szabadkai (nominated by Luca Pellegrini).

T-Type Calcium Channel: A Privileged Gate for Calcium Entry and Control of Adrenal Steroidogenesis

Intracellular calcium plays a crucial role in modulating a variety of functions such as muscle contraction, hormone secretion, gene expression, or cell growth. Calcium signaling has been however shown to be more complex than initially thought. Indeed, it is confined within cell microdomains, and different calcium channels are associated with different functions, as shown by various channelopathies. Sporadic mutations on voltage-operated L-type calcium channels in adrenal glomerulosa cells have been shown recently to be the second most prevalent genetic abnormalities present in human aldosterone-producing adenoma. The observed modification of the threshold of activation of the mutated channels not only provides an explanation for this gain of function but also reminds us on the importance of maintaining adequate electrophysiological characteristics to make channels able to exert specific cellular functions. Indeed, the contribution to steroid production of the various calcium channels expressed in adrenocortical cells is not equal, and the reason has been investigated for a long time. Given the very negative resting potential of these cells, and the small membrane depolarization induced by their physiological agonists, low threshold T-type calcium channels are particularly well suited for responding under these conditions and conveying calcium into the cell, at the right place for controlling steroidogenesis. In contrast, high threshold L-type channels are normally activated by much stronger cell depolarizations. The fact that dihydropyridine calcium antagonists, specific for L-type channels, are poorly efficient for reducing aldosterone secretion either in vivo or in vitro, strongly supports the view that these two types of channels differently affect steroid biosynthesis. Whether a similar analysis is transposable to fasciculata cells and cortisol secretion is one of the questions addressed in the present review. No similar mutations on L-type or T-type channels have been described yet to affect cortisol secretion or to be linked to the development of Cushing syndrome, but several evidences suggest that the function of T channels is also crucial in fasciculata cells. Putative molecular mechanisms and cellular structural organization making T channels a privileged entry for the "steroidogenic calcium" are also discussed.

Mitochondria-associated endoplasmic reticulum membrane (MAM) regulates steroidogenic activity via steroidogenic acute regulatory protein (StAR)-voltage-dependent anion channel 2 (VDAC2) interaction

Steroid hormones are essential for carbohydrate metabolism, stress management, and reproduction and are synthesized from cholesterol in mitochondria of adrenal glands and gonads/ovaries. In acute stress or hormonal stimulation, steroidogenic acute regulatory protein (StAR) transports substrate cholesterol into the mitochondria for steroidogenesis by an unknown mechanism. Here, we report for the first time that StAR interacts with voltage-dependent anion channel 2 (VDAC2) at the mitochondria-associated endoplasmic reticulum membrane (MAM) prior to its translocation to the mitochondrial matrix. In the MAM, StAR interacts with mitochondrial proteins Tom22 and VDAC2. However, Tom22 knockdown by siRNA had no effect on pregnenolone synthesis. In the absence of VDAC2, StAR was expressed but not processed into the mitochondria as a mature 30-kDa protein. VDAC2 interacted with StAR via its C-terminal 20 amino acids and N-terminal amino acids 221-229, regulating the mitochondrial processing of StAR into the mature protein. In the absence of VDAC2, StAR could not enter the mitochondria or interact with MAM-associated proteins, and therefore steroidogenesis was inhibited. Furthermore, the N terminus was not essential for StAR activity, and the N-terminal deletion mutant continued to interact with VDAC2. The endoplasmic reticulum-targeting prolactin signal sequence did not affect StAR association with the MAM and thus its mitochondrial targeting. Therefore, VDAC2 controls StAR processing and activity, and MAM is thus a central location for initiating mitochondrial steroidogenesis.

Neurosteroids, stress and depression: potential therapeutic opportunities

Neurosteroids are potent and effective neuromodulators that are synthesized from cholesterol in the brain. These agents and their synthetic derivatives influence the function of multiple signaling pathways including receptors for γ-aminobutyric acid (GABA) and glutamate, the major inhibitory and excitatory neurotransmitters in the central nervous system (CNS). Increasing evidence indicates that dysregulation of neurosteroid production plays a role in the pathophysiology of stress and stress-related psychiatric disorders, including mood and anxiety disorders. In this paper, we review the mechanisms of neurosteroid action in brain with an emphasis on those neurosteroids that potently modulate the function of GABA(A) receptors. We then discuss evidence indicating a role for GABA and neurosteroids in stress and depression, and focus on potential strategies that can be used to manipulate CNS neurosteroid synthesis and function for therapeutic purposes.

The role of mitochondrial Ca(2+) and NAD(P)H in the control of aldosterone secretion

The mineralocorticoid hormone aldosterone is synthesized in the zona glomerulosa of the adrenal cortex. Glomerulosa cells respond to the physiological stimuli, elevated extracellular [K(+)] and angiotensin II, with an intracellular Ca(2+) signal. Cytosolic Ca(2+) facilitates the transport of the steroid-precursor cholesterol to mitochondria and, after a few hours, it also induces the transcription of aldosterone synthase. Therefore, the cytosolic Ca(2+) signal is regarded as the most important short and long-term mediator of aldosterone secretion. However, cytosolic Ca(2+) is also taken up by mitochondria and, in turn, the mitochondrial Ca(2+) response activates mitochondrial dehydrogenases resulting in stimulation of respiration and increase in reduced pyridine nucleotides. Since both cholesterol side-chain cleavage and all of the hydroxylation steps of steroid synthesis require NADPH as a cofactor, the importance of cytosolic Ca(2+) - mitochondrial Ca(2+) coupling and of appropriate NADPH supply in respect to hormone production can be assumed. However, the importance of the mitochondrial factors has been neglected so far. Here, after summarizing earlier findings we provide new results obtained through modifying mitochondrial Ca(2+) uptake by knocking down p38 MAPK or OPA1 and overexpressing S100G, supporting the notion that mitochondrial Ca(2+) and reduced pyridine nucleotides are facilitating factors for both basal and stimulated steroid production.

Mitochondrial matrix calcium is an activating signal for hormone secretion

Mitochondrial Ca(2+) signals have been proposed to accelerate oxidative metabolism and ATP production to match Ca(2+)-activated energy-consuming processes. Efforts to understand the signaling role of mitochondrial Ca(2+) have been hampered by the inability to manipulate matrix Ca(2+) without directly altering cytosolic Ca(2+). We were able to selectively buffer mitochondrial Ca(2+) rises by targeting the Ca(2+)-binding protein S100G to the matrix. We find that matrix Ca(2+) controls signal-dependent NAD(P)H formation, respiration, and ATP changes in intact cells. Furthermore, we demonstrate that matrix Ca(2+) increases are necessary for the amplification of sustained glucose-dependent insulin secretion in β cells. Through the regulation of NAD(P)H in adrenal glomerulosa cells, matrix Ca(2+) also acts as a positive signal in reductive biosynthesis, which stimulates aldosterone secretion. Our dissection of cytosolic and mitochondrial Ca(2+) signals reveals the physiological importance of matrix Ca(2+) in energy metabolism required for signal-dependent hormone secretion.

Luteinizing hormone (LH) acts through PKA and PKC to modulate T-type calcium currents and intracellular calcium transients in mice Leydig cells

LH increases the intracellular Ca(2+) concentration ([Ca(2+)](i)) in mice Leydig cells, in a process triggered by calcium influx through T-type Ca(2+) channels. Here we show that LH modulates both T-type Ca(2+) currents and [Ca(2+)](i) transients through the effects of PKA and PKC. LH increases the peak calcium current (at -20mV) by 40%. A similar effect is seen with PMA. The effect of LH is completely blocked by the PKA inhibitors H89 and a synthetic inhibitory peptide (IP-20), but only partially by chelerythrine (PKC inhibitor). LH and the blockers induced only minor changes in the voltage dependence of activation, inactivation or deactivation of the currents. Staurosporine (blocker of PKA and PKC) impaired the [Ca(2+)](i) changes induced by LH. A similar effect was seen with H89. Although PMA slowly increased the [Ca(2+)](i) the subsequent addition of LH still triggered the typical transients in [Ca(2+)](i). Chelerythrine also does not avoid the Ca(2+) transients, showing that blockage of PKC is not sufficient to inhibit the LH induced [Ca(2+)](i) rise. In summary, these two kinases are not only directly involved in promoting testosterone synthesis but also act on the overall calcium dynamics in Leydig cells, mostly through the activation of PKA by LH.

A calcium-induced calcium release mechanism supports luteinizing hormone-induced testosterone secretion in mouse Leydig cells

Leydig cells are responsible for the synthesis and secretion of testosterone, processes controlled by luteinizing hormone (LH). Binding of LH to a G protein-coupled receptor in the plasma membrane results in an increase in cAMP and in intracellular Ca2+ concentration ([Ca2+]i). Here we show, using immunofluorescence, that Leydig cells express ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP3Rs). Measurements of intracellular calcium changes using the fluorescent calcium-sensitive dye fluo-3 and confocal microscopy show that both types of receptors are involved in a calcium-induced calcium release (CICR) mechanism, which amplifies the initial Ca2+ influx through plasma membrane T-type calcium channels (CaV3). The RyRs and IP3Rs are functional, as judged from both their activation by caffeine and IP3 and block by ryanodine and 2-aminoethoxydiphenyl borate (2-APB), respectively. RyRs are the principal players involved in the release of Ca2+ from the endoplasmic reticulum, as evidenced by the fact that global Ca2+ changes evoked by LH are readily blocked by 100 μM ryanodine but not by 2-APB or xestospongin C. Finally, steroid production by Leydig cells is inhibited by ryanodine but not by 2-APB. These results not only broaden our understanding of the role played by calcium in Leydig cells but also show, for the first time, that RyRs have an important role in determining testosterone secretion by the testis.

Possible contribution of 2-aminoethoxydiphenyl-borate-sensitive Ca2+ mobilization to adrenocorticotropin-induced glucocorticoid synthesis in rat adrenocortical cells

Cytoplasmic calcium ([Ca(2+)](i)) provided through voltage-dependent Ca(2+) channels (VDCC) plays an important role in adrenocorticotropin (ACTH)-induced steroidogenesis in adrenocortical cells. To identify alternative mechanisms for [Ca(2+)](i) supply, we investigated the 2-aminoethoxydiphenyl borate (2APB)-sensitive pathway as one of the possible signaling pathways involved in [Ca(2+)](i) supply for ACTH-induced steroidogenesis. In monolayers of cultured rat adrenal fasciculate and reticularis cells, ACTH at 10(-11) M stimulated corticosterone synthesis without increasing intracellular cAMP, and corticosterone synthesis was decreased by 10 microM 2APB by 51.8% (6.71 +/- 0.97 vs. 3.23 +/- 0.05 ng/mL/4 hours; p<0.05). Furthermore, 2APB significantly decreased the 10(-11) M ACTH-stimulated [Ca(2+)](i). ACTH increased the intracellular inositol-1,4,5-trisphosphate (IP3) content with a peak at 10(-13) M ACTH, which illustrates the possibility that ACTH activates IP3/diacylglycerol- dependent protein kinase C signal transduction. However, the difference in ACTH concentrations between that responsible for the IP3 increase and steroidogenesis without elevated cAMP, suggest a hypothesis that IP3 is not required for steroidogenesis, but does involve an unknown messenger, which stimulates the release of Ca(2+) from the ER or the subsequent store-operated Ca(2+) entry (SOCE). The pregnenolone concentration in the culture medium was increased by ACTH, which was significantly suppressed by 2APB, showing that the 2APB-sensitive Ca(2+) supply affects cholesterol transport into the mitochondrial membrane via steroidogenic acute regulatory protein. Therefore, the SOCE may contribute to ACTH-induced steroidogenesis in the mitochondrial region. In conclusion, the [Ca(2+)](i) used for steroidogenesis may be derived from a 2APB-sensitive pathway and via VDCCs, particularly at physiological concentrations of ACTH. We suggest that ACTH receptors activate steroidogenesis via inositol triphosphate, or an unknown downstream messenger, which could be inhibited by 2APB.

Ca(2+)-Calmodulin regulation of testicular androgen production in Mozambique tilapia (Oreochromis mossambicus)

The Ca(2+)-Calmodulin (CaM) signaling pathway has previously been shown to be involved in the regulation of teleost fish ovarian steroidogenesis. However, a putative role of CaM in testicular steroidogenesis and potential targets has not been examined. To examine whether basal steroidogenesis is modulated by Ca(2+) and CaM levels in the testis of Mozambique tilapia (Oreochromis mossambicus) we have incubated testicular fragments in vitro under different conditions and analyzed steroid output. Calcium-free medium with or without EGTA did not affect testicular basal 11-ketotestosterone (11-KT) and testosterone (T) secretion. However, addition of 80microM the CaM inhibitor W7 significantly reduced basal 11-KT, T and androstenedione secretion. Interestingly, the decreased androgen production by 80microM of W7 was accompanied by increased 11-desoxicortisol output and by the activation of cortisol synthesis in the testis, the latter undetected in untreated tissues. However, production of 17,20alpha-dihydroxy-4-pregnen-3-one was unaltered by W7. This suggests that C17,20 desmolase, 21-hydroxylase and possibly 11beta-hydroxysteroid dehydrogenase are targets for CaM. In addition, androgen production was also found to be regulated by the level of cAMP since incubations with forskolin (FK) significantly increased 11-KT and T output. A cross-talk between the cAMP and Ca(2+)-CaM signaling pathways was detected since W7 administration also decreased FK stimulated androgen production. Altogether, these data show that both basal and cAMP stimulated androgen levels were modulated by intracellular Ca(2+)-dependent CaM and that possibly Ca(2+)-CaM determines the shift in steroidogenesis from C21 steroids to androgens.

Hint2 is expressed in the mitochondria of H295R cells and is involved in steroidogenesis

Hint2 belongs to the superfamily of histidine triad hydrolase enzymes. Recently, it has been shown to influence the mitochondria-dependent apoptosis occurring in hepatocytes, but its mechanism of action is still obscure. Here, we demonstrate that Hint2 is expressed in the mitochondria of H295R cells and in normal adrenals, and that this protein is involved in steroidogenesis. The presence of Hint2 in H295R cells was revealed by RT-PCR and by immunoblot analysis of subcellular fractions. The protein appeared associated with mitochondrial membranes, probably facing the interior of the organelle. Hint2 overexpression in H295R cells had no effect on pregnenolone secretion elicited by angiotensin II or K+, whereas protein silencing with specific small interfering RNA resulted in a marked reduction of the steroidogenic response. The duration of the mitochondrial calcium signal induced by angiotensin II was also reduced upon Hint2 down-regulation with small interfering RNA, but not affected after its overexpression, suggesting that under basal conditions, Hint2 is optimally expressed, and not rate limiting in steroidogenesis. Moreover, Hint2 also appeared involved in Ca2+-independent pathways leading to steroid formation. Indeed, pregnenolone formation in response to either forskolin or a hydroxyl analog of cholesterol was markedly reduced after Hint2 silencing. Calcium-dependent and calcium-independent actions of Hint2 on steroidogenesis could be related to its ability to maintain a favorable mitochondrial potential. In conclusion, these data suggest that, in H295R cells, Hint2 is required for an optimal steroidogenic response, possibly because of a particular signalling function exerted within the mitochondria and that still remains to determine at the molecular level.

Mitochondrial function in Leydig cell steroidogenesis

The first and rate-limiting step in the biosynthesis of steroid hormones is the transfer of cholesterol into mitochondria, which is facilitated by the steroidogenic acute regulatory (StAR) protein. Recent studies of Leydig cell function have focused on the molecular events controlling steroidogenesis; however, few studies have examined the importance of the mitochondria. The purpose of this investigation was to determine which aspects of mitochondrial function are necessary for Leydig cell steroidogenesis. MA-10 tumor Leydig cells were treated with 8-bromo-cAMP (cAMP) and site-specific mitochondrial disrupters, pro-oxidants, and their effects on progesterone synthesis, StAR expression, mitochondrial membrane potential (delta psi(m)) and ATP synthesis were determined. Dissipating delta psi(m) with CCCP inhibited progesterone synthesis, even in the presence of newly synthesized StAR protein. The electron transport inhibitor antimycin A significantly reduced cellular ATP, inhibited steroidogenesis, and reduced StAR protein expression. The F0/F1 ATPase inhibitor oligomycin reduced cellular ATP and inhibited progesterone synthesis and StAR protein expression, but had no effect on delta psi(m). Disruption of pH with nigericin significantly reduced progesterone production and StAR protein, but had minimal effects on delta psi(m). Sodium arsenite at low concentrations inhibited StAR protein but not mRNA expression and inhibited progesterone without disrupting delta psi(m). The mitochondrial Ca2+ inhibitor Ru360 also inhibited StAR protein expression. These results demonstrate that delta psi(m), ATP synthesis, delta pH and [Ca2+]mt are all required for steroid biosynthesis, and that mitochondria are sensitive to oxidative stress. These results suggest that mitochondria must be energized, polarized, and actively respiring to support Leydig cell steroidogenesis and alterations in the state of mitochondria may be involved in regulating steroid biosynthesis.

Volume-sensitive Cl- current in bovine adrenocortical cells: comparison with the ACTH-induced Cl- current

In a previous study performed on zona fasciculata (ZF) cells isolated from calf adrenal glands, we identified an ACTH-induced Cl- current involved in cell membrane depolarization. In the present work, we describe a volume-sensitive Cl- current and compare it with the ACTH-activated Cl- current. Experiments were performed using the whole-cell patch-clamp recording method, video microscopy and cortisol-secretion measurements. In current-clamp experiments, hypotonic solutions induced a membrane depolarization to -22 mV. This depolarization, correlated with an increase in the membrane conductance, was sensitive to different Cl- channel inhibitors. In voltage-clamp experiments, hypotonic solution induced a membrane current that slowly decayed and reversed at -21 mV. This ionic current displayed no time dependence and showed a slight outward rectification. It was blocked to variable extent by different conventional Cl- channel inhibitors. Under hypotonic conditions, membrane depolarizations were preceded by an increase in cell volume that was not detected under ACTH stimulation. It was concluded that hypotonic solution induced cell swelling, which activated a Cl- current involved in membrane depolarization. Although cell volume change was not observed in the presence of ACTH, biophysical properties and pharmacological profile of the volume-sensitive Cl- current present obvious similarities with the ACTH-activated Cl- current. As compared to ACTH, hypotonic solutions failed to trigger cortisol production that was weakly stimulated in the presence of high-K+ solution. This shows that in ZF cells, membrane depolarization is not a sufficient condition to fully activate secretory activities.

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.

The effect of cytoplasmic Ca2+ signal on the redox state of mitochondrial pyridine nucleotides

As first observed in rat adrenal glomerulosa cells, cytoplasmic Ca(2+) signal, induced by K(+), angiotensin II or vasopressin, evokes an increase in the level of reduced mitochondrial pyridine nucleotides, NADH and NADPH. Prostaglandin F(2)alpha and extracellular ATP exert similar effects in rat ovarian luteal cells. This coupling of cytoplasmic Ca(2+) concentration and mitochondrial metabolism occurs also when the stimuli are applied at physiological concentration and under conditions when no formation of high-Ca(2+) perimitochondrial microdomains may be presumed. We present evidence that low submicromolar Ca(2+) signals in the cytoplasm can increase mitochondrial Ca(2+) concentration and activate mitochondrial dehydrogenation processes. Several observations support the assumption that intramitochondrial Ca(2+) signals play a significant role in the stimulation of steroid hormone production.

Intracellular transport of calcium from plasma membrane to mitochondria in adrenal H295R cells: implication for steroidogenesis

Angiotensin II and extracellular potassium stimulate aldosterone production in adrenal glomerulosa cells by mobilizing the calcium messenger system. This response requires calcium influx across the plasma membrane, followed by calcium uptake into the mitochondria. It has been proposed that calcium is transported to the mitochondria via the lumen of the endoplasmic reticulum, acting as a kind of intracellular calcium pipeline. This hypothesis has been tested in the present study by measuring intramitochondrial calcium variations in H295R cells with a new fluorescent calcium probe, ratiometric pericam. Calyculin A, a protein phosphatase inhibitor, induced the formation of a large cortical layer of actin filaments, removing the peripheral endoplasmic reticulum away from the plasma membrane and thereby physically uncoupling the calcium channels from the pipeline. The mitochondrial calcium response to potassium was markedly reduced after calyculin treatment, but that of AngII was unaffected. Under the same conditions, potassium-stimulated pregnenolone and aldosterone production was significantly reduced, whereas the steroidogenic response to AngII remained unchanged. The inhibitory action of calyculin A on the responses to potassium was not mediated by a modification of the calcium channel activity and was not accompanied by a reduction of the cytosolic calcium response. It therefore appears that, in H295R cells, the organization of the actin cytoskeleton at the cell periphery influences the steroidogenic action of potassium, but not the response to angiotensin II. The response to potassium is proposed to be dependent on the endoplasmic reticulum-mediated transfer of calcium entering through plasma membrane calcium channels to the mitochondria.

Reactive oxygen disrupts mitochondria in MA-10 tumor Leydig cells and inhibits steroidogenic acute regulatory (StAR) protein and steroidogenesis

Reactive oxygen species (ROS) are involved in a variety of pathophysiological conditions of the testis, and oxidative stress is known to inhibit ovarian and testicular steroidogenesis. The site of ROS-mediated inhibition of steroidogenesis in the corpus luteum and MA-10 tumor Leydig cells was shown to be the hormone-sensitive mitochondrial cholesterol transfer step. The purpose of this study was to examine the effects of ROS on steroidogenic acute regulatory (StAR) protein in MA-10 cells and determine the extent to which MA-10 cell mitochondria are sensitive to oxidative stress. cAMP-stimulated progesterone production was inhibited in a dose-dependent manner in MA-10 cells exposed to H(2)O(2). StAR protein, but not mRNA levels, was decreased in parallel to changes in progesterone production. Even at the highest concentrations of H(2)O(2) tested, there was no effect on P450 side-chain cleavage enzyme protein levels. Oxidative stress from exposure to exogenous xanthine oxidase and xanthine resulted in the inhibition of both progesterone production and StAR protein expression. The mature 30- and 32-kDa intramitochondrial forms of StAR were decreased relative to the 37-kDa extramitochondrial precursor form of StAR, indicating that the ROS-mediated inhibition of StAR protein was due, in part, to the inhibition of mitochondrial import and processing. Vital staining with the fluorescent dye tetramethylrhodamine ethyl ester was used to visualize changes in the mitochondrial electrochemical gradient-dependent membrane potential (Deltapsim). ROS caused a significant dissipation of Deltapsi(m) and time-dependent loss of tetramethylrhodamine ethyl ester fluorescence. The inhibitory effects of H(2)O(2) were transient. There was no evidence for ROS-induced cell death, and following H(2)O(2) removal in the presence of continuous treatment with 8-bromo-cAMP, StAR protein levels and progesterone production were restored. In addition, there was no loss of cell viability following treatment with H(2)O(2) or xanthine/xanthine oxidase as determined by trypan blue exclusion. H(2)O(2) did not cause a significant decrease in total cellular ATP levels. These data indicate that oxidative stress-mediated perturbation of the mitochondria and dissipation of Deltapsi(m) results in the inhibition of StAR protein expression and its import, processing, and cholesterol transfer activity. These findings confirm earlier studies demonstrating the requirement for maintenance of an intact Deltapsi(m) for StAR protein function in cholesterol transport. The significant reduction in the 32- to 30-kDa mature forms of StAR, cessation of cholesterol transport, and loss of Deltapsi(m) are consistent with mitochondrial perturbation because of oxidative stress. This mechanism likely contributes to a host of pathophysiological events evident in testicular disorders such as infection, reperfusion injury, aging, cryptorchidism, and varicocele.

Repression of DAX-1 and induction of SF-1 expression. Two mechanisms contributing to the activation of aldosterone biosynthesis in adrenal glomerulosa cells

Angiotensin II (Ang II) and adrenocorticotropic hormone stimulate aldosterone biosynthesis in the zona glomerulosa of the adrenal cortex through induction of the expression of the steroidogenic acute regulatory (StAR) protein, which promotes intramitochondrial cholesterol transfer. To understand the mechanism of this induction of the StAR protein, we have examined the effect of Ang II and forskolin, a mimicker of adrenocorticotropic hormone action, on two transcription factors known to modulate StAR gene expression in opposite ways, DAX-1 and SF-1, in bovine adrenal glomerulosa cells in primary culture. Ang II markedly inhibited DAX-1 protein expression in a time- and concentration-dependent manner (to 38.7 +/- 12.9% of controls at 3 nm after 6 h, p < 0.01), an effect that required de novo protein synthesis and ERK2/1 activation. This effect was associated with a concomitant decrease in DAX-1 mRNA and an increase in mitochondrial StAR protein levels. Similarly, forskolin dramatically repressed DAX-1 protein and mRNA expression (to 19.6 +/- 1.8 and 50.3 +/- 4.7% of controls, respectively, p < 0.01). Neither Ang II nor forskolin affected DAX-1 protein and mRNA stability. The aldosterone response to Ang II was markedly reduced (to 59 +/- 4% of controls, p < 0.01) in transiently transfected cells overexpressing DAX-1. Whereas Ang II was without effect on SF-1 expression, forskolin significantly increased SF-1 protein and mRNA levels in a cycloheximide-sensitive manner (to 167.4 +/- 16.6 and 173.1 +/- 25.1% of controls after 6 h, respectively, p < 0.01). These results demonstrate that the balance between repressor and inducer function of DAX-1 and SF-1 are of critical importance in the regulation of adrenal aldosterone biosynthesis.

ACTH-induced Cl(-) current in bovine adrenocortical cells: correlation with cortisol secretion

ACTH has been shown to depolarize bovine adrenal zona fasciculata cells by inhibiting a K(+) current. The effects of this hormone on such cells have been reexamined using perforated and standard patch recording methods. In current clamp experiments, ACTH (10 nM) induced a membrane depolarization to -36 +/- 1 mV (n = 56), which was mimicked by forskolin (10 microM) or by 8-(4-chlorophenylthio)-cAMP (8 mM). ACTH-induced membrane depolarizations were associated in the majority of cells with an increase in membrane conductance. In the other cells, these membrane responses could occur without change or could be correlated with a transient or with a continuous Cs(+)-sensitive decrease in membrane conductance. The depolarizations associated with an increase in membrane conductance were depressed by Cl(-) current inhibitors diphenylamine-2-carboxylic acid (DPC; 1 mM), anthracene-9-carboxylic acid (9-AC; 1 mM), DIDS (400 microM), verapamil (100 microM), and glibenclamide (20 microM). In voltage-clamped Cs(+)-loaded cells, ACTH activated a time-independent current that displayed an outward rectification and reversed at -21.5 mV +/- 2 (n = 6). This current, observed in the presence of internal EGTA (5 mM), was depressed in low Cl(-) external solution and was inhibited by DPC, 9-AC, DIDS, 5-nitro-2-(3-phenylpropylamino)benzoic acid, verapamil, and glibenclamide. ACTH-stimulated cortisol secretion was blocked by Cl(-) channel inhibitors DIDS (400 microM) and DPC (1 mM). The present results reveal that, in addition to inhibiting a K(+) current, ACTH activates in bovine zona fasciculata cells a Ca(2+)-insensitive, cAMP-dependent Cl(-) current. This Cl(-) current is involved in the ACTH-induced membrane depolarization, which seems to be a crucial step in stimulating steroidogenesis.

Involvement of tyrosine kinase in citrate-stimulated aldosterone production in bovine glomerulosa cells

The present study was designed to assess whether citrate stimulates aldosterone production by isolated bovine adrenal glomerulosa cells in vitro. When the cells were incubated with graded concentrations of citrate up to 4.0 mM, basal aldosterone production was significantly elevated, with a gradual reduction of extracellular ionized calcium concentration. Without citrate, however, adding increasing amounts of calcium chloride to a calcium-free medium did not reproduce the citrate's effect on basal aldosterone production. Genistein, an inhibitor of tyrosine kinases, inhibited the citrate (4 mM)-induced aldosterone production in a dose-dependent manner, with 89.8% of inhibition at a concentration of 10 microM. When the cells were exposed to citrate (4 mM) for 5, 10, and 30 min, tyrosine in Mr 105,000 endogenous protein was dominantly phosphorylated. This study demonstrates for the first time that citrate stimulates aldosterone production in bovine adrenal glomerulosa cells in vitro and also suggests a crucial involvement of protein tyrosine kinase in the steroidogenic action of citrate in the cells.

Calcium ions positively modulate follicle-stimulating hormone- and exogenous cyclic 3',5'-adenosine monophosphate-driven transcription of the P450(scc) gene in porcine granulosa cells

Given the evident modulation of FSH-induced steroidogenesis by Ca2+ in granulosa cells, we here test the hypothesis that Ca2+ controls expression of the enzymatically rate-limiting cytochrome P450(scc) (CYP11A) gene. To test this postulate, we quantitated the ability of Ca2+ to regulate: 1) transcriptional activity of a transiently transfected luciferase reporter gene driven by a 2.32-kb 5'-upstream fragment of the porcine P450(scc) gene promoter region; and 2) accumulation of endogenous P450(scc) transcripts in primary monolayer cultures of porcine granulosa cells. To this end, granulosa cells were stimulated for 4 h with FSH (15 ng/ml, NIDDK-oFSH-20) or 8-Bromo-cAMP (8 Br-cAMP, 1 mM) in serum-free medium containing either 1.8 mM Ca2- or no added Ca2+ with 100 microM EGTA or 100 microM CoCl2. In the presence of extracellular Ca2+, FSH and 8 Br-cAMP stimulated expression of the transfected P450(scc) promoter-reporter fusion construct by 5.6 +/- 1.1 and 3.6 +/- 0.67-fold, respectively over Ca2+-containing unstimulated control (P < or = 0.04, n = 5-6 experiments). The foregoing two agonists augmented 4-h progesterone production by cultured granulosa cells by 1.8 +/- 0.11 and 1.6 +/- 0.16-fold, respectively (P < or = 0.001 for FSH and P < or = 0.01 for 8 Br-cAMP). FSH and 8 Br-cAMP also significantly elevated endogenous P450(scc) transcript levels as measured by homologous solution-hybridization RNase protection assay; i.e. by 3.1 +/- 0.49 and 2.9 +/- 0.45-fold, respectively (P < or = 0.001). In Ca2+-free/EGTA-supplemented medium, basal luciferase reporter-gene activity and endogenous P450(scc) messenger RNA accumulation in granulosa cells declined to 34 +/- 12% and 78 +/- 12%, respectively, of corresponding values in control (unstimulated Ca2+-containing) cultures. Extracellular Ca2+ deprivation inhibited the stimulatory effect of FSH (and 8 Br-cAMP) on P450(scc) promoter-luciferase reporter expression to 58 +/- 30% (and 58 +/- 23%), and restrained endogenous P450(scc) message accumulation to 86 +/- 15% (and 96 +/- 18%) of the value in Ca2+-containing control. Extracellular Ca2+ withdrawal suppressed FSH (and 8 Br-cAMP)-driven progesterone production over 4 h to basal levels but did not alter FSH-stimulated cAMP accumulation by granulosa cells. Ca2+-deprived cells exposed to serum-containing media regained P450(scc) responsiveness to both agonists. Antagonism of cellular uptake of Ca2+ and other divalent cations via administration of cobalt chloride (100 microM) inhibited FSH and 8 Br-cAMP's stimulation of endogenous (but not exogenous promoter-driven) P450(scc) gene expression. In contrast, granulosa-cell concentrations of messenger RNA's encoding sterol-carrier protein-2 (SCP-2) and the low density lipoprotein receptor were not altered by Ca2+ withdrawal. In summary, uptake of extracellular Ca2+ by porcine granulosa cells significantly potentiates transactivation of the endogenously expressed and exogenously transfected P450(scc) gene by FSH and 8 Br-cAMP. The agonistic impact of Ca2+ on P450(scc) promoter activity is requisite downstream of FSH-induced cAMP second-messenger signaling.

Function- and agonist-specific Ca2+signalling: The requirement for and mechanism of spatial and temporal complexity in Ca2+signals

Calcium signals have been implicated in the regulation of many diverse cellular processes. The problem of how information from extracellular signals is delivered with specificity and fidelity using fluctuations in cytosolic Ca2+concentration remains unresolved. The capacity of cells to generate Ca2+signals of sufficient spatial and temporal complexity is the primary constraint on their ability to effectively encode information through Ca2+. Over the past decade, a large body of literature has dealt with some basic features of Ca2+-handling in cells, as well as the multiplicity and functional diversity of intracellular Ca2+stores and extracellular Ca2+influx pathways. In principle, physiologists now have the necessary information to attack the problem of function- and agonist-specificity in Ca2+signal transduction. This review explores the data indicating that Ca2+release from diverse sources, including many types of intracellular stores, generates Ca2+signals with sufficient complexity to regulate the vast number of cellular functions that have been reported as Ca2+-dependent. Some examples where such complexity may relate to neuroendocrine regulation of hormone secretion/synthesis are discussed. We show that the functional and spatial heterogeneity of Ca2+stores generates Ca2+signals with sufficient spatiotemporal complexity to simultaneously control multiple Ca2+-dependent cellular functions in neuroendocrine systems.Key words: signal coding, IP3receptor, ryanodine receptor, endoplasmic reticulum, Golgi, secretory granules, mitochondria, exocytosis.

A role for T-type Ca2+ channels in the synergistic control of aldosterone production by ANG II and K+

Independently, plasma K+ and ANG II stimulate aldosterone secretion from adrenal glomerulosa (AG) cells, but together they synergistically control production. We studied mechanisms to mediate this synergy using bovine AG cells studied under physiological conditions (in 1.25 mM Ca2+ at 37 degrees C). Increasing K+ from 2 to 5 mM caused a potentiation of ANG II-induced aldosterone secretion and a substantial membrane depolarization ( approximately 21 mV). ANG II inhibited a K+-selective conductance in both 2 and 5 mM K+ but caused only a slight depolarization because, under both conditions, membrane potential was close to the reversal potential of the ANG II-induced current. ANG II activated calcium/calmodulin-dependent protein kinase II (CaMKII) equivalently in 2 and 5 mM K+. However, CaMKII activation caused a hyperpolarizing shift in the activation of T-type Ca2+ channels, such that substantially more current was elicited at membrane potentials established by 5 mM K+. We propose that synergy in aldosterone secretion results from K+-induced depolarization and ANG II-induced modulation of T-type channel activation, such that together they promote enhanced steady-state Ca2+ flux.

Functional assessment of the calcium messenger system in cultured mouse Leydig tumor cells: regulation of human chorionic gonadotropin-induced expression of the steroidogenic acute regulatory protein

The steroidogenic acute regulatory (StAR) protein, a 30-kDa mitochondrial factor, is a key regulator of steroid hormone biosynthesis, facilitating the transfer of cholesterol from the outer to the inner mitochondrial membrane. StAR protein expression is restricted to steroidogenic tissues, and it responds to hormonal stimulation through different second messenger pathways. The present study was designed to explore the mechanisms of extracellular calcium (Ca2+) involved in the hCG-stimulated expression of StAR protein and steroidogenesis in a mouse Leydig tumor cell line (mLTC-1). Extracellular Ca2+ (1.5 mmol/liter) enhanced the hCG (50 microg/liter)-induced increases in StAR messenger RNA (mRNA) and protein levels (1.7 +/- 0.3-fold; 4 h), as monitored by quantitative RT-PCR and immunoblotting. The potentiating effect of Ca2+ on the hCG-stimulated StAR response correlated with the acute progesterone (P) response. In accordance, omission of Ca2+ from the extracellular medium by specific Ca2+ chelators, EDTA or EGTA (4 mmol/liter each), markedly diminished the hCG-stimulated P production. The Ca2+ effect on hCG-induced StAR mRNA expression was dramatically suppressed by 10 micromol/liter verapamil, a Ca2+ channel blocker. The Ca2+-mobilizing agonist, potassium (K+; 4 mmol/liter), greatly increased the hCG responses of StAR expression and P production, which conversely were attenuated by Ca2+ antagonists, further supporting the involvement of intracellular free Ca2+ ([Ca2+]i) in these responses. The interaction of Ca2+ or K+ with hCG accounted for a clear increase in the StAR protein level (1.4-1.8-fold; 4 h) compared with that after hCG stimulation. Inhibition of protein synthesis by cycloheximide (CHX) drastically diminished the hCG-induced StAR protein content, indicating the requirement for on-going protein synthesis for hCG action. The transmembrane uptake of 45Ca2+ was increased by 26% with hCG and was strongly inhibited by verapamil. [Ca2+]i moderately augmented the response to hCG in fura-2/AM-loaded mLTC-1 cells within 30-40 sec, reaching a plateau within 1-3 min. Interestingly, the calcium ionophore (A23187) clearly increased (P < 0.01) StAR mRNA expression, in additive fashion with hCG. Northern hybridization analysis revealed four StAR transcripts at 3.4, 2.7, 1.6, and 1.4 kb, with the 1.6-kb band corresponding to the functional StAR protein; all of them were up-regulated 3- to 5-fold upon hCG stimulation, with a further increase in the presence of Ca2+. The mechanism of the Ca2+ effect on hCG-stimulated StAR expression and P production was evaluated by assessing the involvement of the nuclear orphan receptor, steroidogenic factor 1 (SF-1). Stimulation of hCG significantly elevated (2.1 +/- 0.3-fold) the SF-1 mRNA level, which was further augmented in the presence of Ca2+, whereas EGTA and verapamil completely abolished the increase caused by Ca2+. Cells expressing SF-1 marginally increased StAR expression, but coordinately elevated StAR mRNA levels in response to hCG and hCG plus Ca2+ compared with those in mock-transfected cells. On the other hand, overexpression of the nuclear receptor DAX-1 remarkably diminished (P < 0.0001) the endogenous SF-1 mRNA level as well as hCG-induced StAR mRNA expression. In summary, our results provide evidence that extracellular Ca2+ rapidly increases [Ca2+]i after hCG stimulation, presumably through opening of the transmembrane Ca2+ channel. Neither extracellular Ca2+ nor K+ alone has a noticeable effect on StAR expression and steroidogenesis, whereas they clearly potentiate hCG induction. The Ca2+-mediated increase in hCG involved in StAR expression and P production is well correlated to the levels of SF-1 expression. The stimulatory effect of hCG that rapidly increases [Ca2+]i is responsible at least in part for the regulation of SF-1-mediated StAR expression that consequently regulates steroidogenesis in mouse Leydig tumor cells.

The acute regulation of mineralocorticoid biosynthesis: scenarios for the StAR system

The zona glomerulosa cell of the adrenal cortex produces mineralocorticoids in response to physiological stimuli (angiotensin II and extracellular K(+)) activating the Ca(2+) messenger system. The mechanisms underlying the generation of the Ca(2+) signal have been analyzed extensively and recent developments have contributed to bridging the gap between intracellular signals and activation of the biological function. This article summarizes the current knowledge on the intracellular targets of the Ca(2+) messenger, obtained mainly in bovine glomerulosa cells. Ca(2+) appears to exert a dual effect, both at the intramitochondrial level and at the nuclear level, where it activates steroidogenic acute regulatory protein (StAR) gene transcription.

Regulation of mineralocorticoid biosynthesis by calcium and the StAR protein

In adrenal zona glomerulosa cells, the calcium messenger system is the major signaling mechanism activated by physiological stimulators of aldosterone production. We present here evidence for a dual site of action of the calcium signal: 1) Calcium influx into the mitochondrion is a prerequisite to the activation of steroidogenesis. This calcium entry leads to a rise in mitochondrial calcium concentration and to an increase in intramitochondrial cholesterol transfer and Steroidogenic Acute Regulatory (StAR) protein accumulation in inner mitochondrial membranes. 2) Calcium also exerts a genomic regulatory effect by activating transcription of the StAR gene.

Atrial natriuretic peptide inhibits calcium-induced steroidogenic acute regulatory protein gene transcription in adrenal glomerulosa cells

Atrial natriuretic peptide (ANP) is a potent inhibitor of mineralocorticoid synthesis induced in adrenal glomerulosa cells by physiological agonists activating the calcium messenger system, such as angiotensin II (Ang II) and potassium ion (K+). While the role of calcium in mediating Ang II- and K(+)-induced aldosterone production is clearly established, the mechanisms leading to blockade of this steroidogenic response by ANP remain obscure. We have used bovine adrenal zona glomerulosa cells in primary culture, in which an activation of the calcium messenger system was mimicked by a 2-h exposure to an intracellular high-calcium clamp. The effect of ANP was studied on the following parameters of the steroidogenic pathway: 1) pregnenolone and aldosterone production; 2) changes in cytosolic ([Ca2+]c) and mitochondrial ([Ca2+]m) Ca2+ concentrations, as assessed with targeted recombinant aequorin; 3) cholesterol content in outer mitochondrial membranes (OM), contact sites (CS), and inner membranes (IM); 4) steroidogenic acute regulatory (StAR) protein import into mitochondria by Western blot analysis; 5) StAR protein synthesis, as determined by [35S]methionine incorporation, immunoprecipitation, and SDS-PAGE; 6) StAR mRNA levels by Northern blot analysis with a StAR cDNA; 7) StAR gene transcription by nuclear run-on analysis. While clamping Ca2+ at 950 nM raised pregnenolone output 3.5-fold and aldosterone output 3-fold, ANP prevented these responses with an IC50 of 1 nM and a maximal effect of 90% inhibition at 10 nM. In contrast, ANP did not affect the [Ca2+]c or [Ca2+]m changes occurring under Ca2+ clamp or Ang II stimulation in glomerulosa cells. The accumulation of cholesterol content in CS (139.7 +/- 10.7% of control) observed under high-Ca2+ clamp was prevented by 10 nM ANP (92.4 +/- 4% of control). Similarly, while Ca2+ induced a marked accumulation of StAR protein in mitochondria of glomerulosa cells to 218 +/- 44% (n = 3) of controls, the presence of ANP led to a blockade of StAR protein mitochondrial import (113.3 +/- 15.0%). This effect was due to a complete suppression of the increased [35S]methionine incorporation into StAR protein that occurred under Ca2+ clamp (94.5 +/- 12.8% vs. 167.5 +/- 17.3%, n = 3). Furthermore, while the high-Ca2+ clamp significantly increased StAR mRNA levels to 188.5 +/- 8.4 of controls (n = 4), ANP completely prevented this response. Nuclear run-on analysis showed that increases in intracellular Ca2+ resulted in transcriptional induction of the StAR gene and that ANP inhibited this process. These results demonstrate that Ca2+ exerts a transcriptional control on StAR protein expression and that ANP appears to elicit its inhibitory effect on aldosterone biosynthesis by acting as a negative physiological regulator of StAR gene expression.

Voltage dependent calcium and potassium currents in Y-1 adrenocortical cells are unresponsive to ACTH

In this report we use both whole cell and perforated patch clamp recording techniques to characterize calcium and potassium channels in Y-1 adrenocortical cells in order to assess their responsiveness to ACTH. Both transient and long-lasting components of an inward calcium current were identified which were similar to T and L-type Ca2+ currents. With Ba2+ as the charge carrier, the transient current activated at voltages more hyperpolarized than -50 mV with V1/2 for activation at -78.1 mV, and for steady state inactivation at -52.3 mV. The L-type current activated at -20 mV, with a V1/2 for activation at -29.9 mV and steady state inactivation at -44.2 mV. Under perforated patch conditions the response was shifted to more depolarized voltages. Both currents were responsive to agents which usually affect T- or L-type Ca2+ currents. The transient current was completely blocked by 50 microM lanthanum or 200 microM nickel and partially blocked by 300 mM amiloride. Cadmium (100 microM) and nifedipine (300 nM) completely blocked the long-lasting current while omega-conotoxin GVIA (1992 nM) inhibited the current by only 20-25%. The agonist, Bay K 8644 was stimulatory at 50 nM. Both transient and sustained outward potassium currents similar to A-type and delayed rectifier currents, respectively, were present. The transient current demonstrated fast activation at voltages more positive than -10 mV, inactivation with continued depolarization and steady state inactivation at V1/2 = -50 mV. The sustained current activated rapidly and had minimal inactivation with continued depolarization. The transient current was blocked by 5 mM 4AP and the sustained by 25 mM TEA. While Y-1 cells contain both calcium and potassium currents similar to those found in other adrenocortical cells, none of the currents were affected by ACTH or AII, secretagogues which stimulate steroidogenesis. These data, combined with the inability of both Ca2+ and K+ channel blockers to alter ACTH-induced steroidogenesis as reported earlier, suggests that neither calcium nor potassium currents are responsive to ACTH in Y-1 cells.

Submitochondrial distribution of three key steroidogenic proteins (steroidogenic acute regulatory protein and cytochrome p450scc and 3beta-hydroxysteroid dehydrogenase isomerase enzymes) upon stimulation by intracellular calcium in adrenal glomerulosa cells

In adrenal glomerulosa cells, angiotensin II (Ang II) and potassium stimulate aldosterone synthesis through activation of the calcium messenger system. The rate-limiting step in steroidogenesis is the transfer of cholesterol to the inner mitochondrial membrane. This transfer is believed to depend upon the presence of the steroidogenic acute regulatory (StAR) protein. The aim of this study was 1) to examine the effect of changes in cytosolic free calcium concentration and of Ang II on intramitochondrial cholesterol and 2) to study the distribution of StAR protein in submitochondrial fractions during activation by Ca2+ and Ang II. To this end, freshly prepared bovine zona glomerulosa cells were submitted to a high cytosolic Ca2+ clamp (600 nM) or stimulated with Ang II (10 nM) for 2 h. Mitochondria were isolated and subfractionated into outer membranes, inner membranes (IM), and contact sites (CS). Stimulation of intact cells with Ca2+ or Ang II led to a marked, cycloheximide-sensitive increase in cholesterol in CS (to 143 +/- 3. 2 and 151.1 +/- 18.1% of controls, respectively) and in IM (to 119 +/- 5.1 and 124.5 +/- 6.5% of controls, respectively). Western blot analysis revealed a cycloheximide-sensitive increase in StAR protein in mitochondrial extracts of Ca2+-clamped glomerulosa cells (to 159 +/- 23% of controls). In submitochondrial fractions, there was a selective accumulation of StAR protein in IM following stimulation with Ca2+ (228 +/- 50%). Similarly, Ang II increased StAR protein in IM, and this effect was prevented by cycloheximide. In contrast, neither Ca2+ nor Ang II had any effect on the submitochondrial distribution of cytochrome P450scc and 3beta-hydroxysteroid dehydrogenase isomerase. The intramitochondrial presence of the latter enzyme was further confirmed by immunogold staining in rat adrenal fasciculata cells and by immunoblot analysis in MA-10 mouse testicular Leydig cells. These findings demonstrate that under acute stimulation with Ca2+-mobilizing agents, newly synthesized StAR protein accumulates in IM after transiting through CS. Moreover, our results suggest that the import of StAR protein into IM may be associated with cholesterol transfer, thus promoting precursor supply to the two first enzymes of the steroidogenic cascade within the mitochondria and thereby activating mineralocorticoid synthesis.

Sources and sites of action of calcium in the regulation of aldosterone biosynthesis

The role of free calcium as a crucial intracellular messenger in the stimulation of aldosterone biosynthesis by various agonists is well established. Using electropermeabilized or Ca(2+)-clamped adrenal zona glomerulosa (ZG) cells, we have previously shown that Ca2+ entry into the mitochondrial matrix is required for the activation of steroidogenesis. We now describe the use of various strategies to answer the following questions: 1. Which pathway does Ca2+ follow before triggering steroidogenesis? 2. Which step of steroidogenesis is under the control of Ca2+? The first approach combined the patch-clamp method, in the perforated patch configuration, with microfluorimetry of Ca2+; in the second approach, ZG cells were transiently transfected with a chimeric cDNA encoding for the calcium-sensitive photoprotein aequorin linked to a mitochondrial targeting presequence; in a third approach, ZG mitochondria were isolated and fractionated into outer membranes, contact sites and inner membranes and the effect of prior exposure of the ZG cells to a physiologically elevated intracellular calcium concentration or to angiotensin II (Ang II) on cholesterol content was then examined in those three mitochondrial fractions. The results of these combined approaches allow us to propose the following scheme: The source of calcium which is predominantly responsible for mediating the steroidogenic effect of potassium appears to be funneled through the T-type calcium channels to close proximity of the mitochondria. This signal, as well as that triggered by Ang II, appears to be relayed within the mitochondrial matrix. This rise of mitochondrial calcium is associated with a transfer of free cholesterol from the outer to the inner mitochondrial membrane, via the contact sites. Thus the main role of the calcium messenger is to promote intramitochondrial cholesterol transfer and supply to the P450scc enzyme.

Calcium stimulates intramitochondrial cholesterol transfer in bovine adrenal glomerulosa cells

In adrenal glomerulosa cells, angiotensin II (Ang II) stimulates aldosterone synthesis through rises of cytosolic calcium ([Ca2+]c). The rate-limiting step in this process is the transfer of cholesterol to the inner mitochondrial membrane, where it is converted to pregnenolone by the P450 side chain cleavage enzyme. The aim of the present study was to examine the effect of changes in [Ca2+]c and of Ang II on intramitochondrial cholesterol distribution. Freshly prepared bovine zona glomerulosa cells were submitted to a cytosolic Ca2+ clamp (600 nM) or stimulated with Ang II (10 nM). Mitochondria were isolated and subfractionated into outer membranes (OM), inner membranes (IM), and contact sites (CS). Cholesterol content was determined by the cholesterol oxidase assay. Stimulation of intact cells with Ca2+ led to a marked decrease in cholesterol content of OM (to 54 +/- 24% of controls, n = 5) and to a concomitant increase of cholesterol in CS and IM (to 145 +/- 14%, n = 5). When glomerulosa cells were exposed to Ang II, a marked increase of cholesterol in CS occurred (to 172 +/- 16% of controls, n = 5). No significant changes were detected in OM cholesterol, suggesting a stimulation of cholesterol supply to the mitochondria in response to Ang II. Cycloheximide specifically and significantly reduced Ca2+-activated cholesterol transfer to CS and IM. In conclusion, our data indicate that one of the main functions of the Ca2+ messenger is to increase cholesterol supply to the P450 side chain cleavage enzyme by enhancing endogenous intermembrane cholesterol transfer to a mitochondrial site containing the enzymes responsible for the initial steps of the steroidogenic cascade.

Demonstration of an angiotensin II-induced negative feedback effect on aldosterone synthesis in isolated rat adrenal zona glomerulosa cells

Although both angiotensin II (Ang II) and potassium ion (K+) induce marked elevations of cytosolic free calcium concentration, [Ca2+]c, in adrenal zona glomerulosa cells-an effect which is thought to trigger aldosterone synthesis-Ang II is also known to reduce the sustained [Ca2+]c rise induced by K+. We have examined whether this effect of Ang II on the calcium messenger system is reflected at the level of the final biological response, aldosterone synthesis. In superfused isolated rat glomerulosa cells, K+ (8 mM) induced a sustained, 60-fold increase in aldosterone production. In contrast, the maximal response to Ang II (10 nM) amounted to only 10 times the basal production. When added subsequent to K+ stimulation, Ang II provoked an immediate and dramatic drop in aldosterone synthesis, to levels obtained with Ang II alone. Under conditions of maximal K+ stimulation, this effect depended upon Ang II concentration, while the well-known synergistic effect was observed with submaximal concentrations of both agonists. The inhibitory effect of Ang II could be reproduced with dioctanoylglycerol, a selective activator of protein kinase C. By contrast, the aldosterone response to adrenocorticotropic hormone (ACTH) was not affected by Ang II. At submaximal concentrations of ACTH, the steroidogenic effect of Ang II was even additive to that of ACTH. Thus, we have shown that, under conditions of maximal stimulation, Ang II exerts a profound inhibition of steroidogenesis in K(+)-stimulated rat adrenal glomerulosa cells. This counter-regulatory mechanism may ensure adequate levels of aldosterone production in vivo.

Inhibition of low threshold calcium channels by angiotensin II in adrenal glomerulosa cells through activation of protein kinase C

In adrenal glomerulosa cells, low threshold voltage-activated (T-type) calcium channels play a crucial role in coupling physiological variations of extracellular potassium to aldosterone biosynthesis. Angiotensin II markedly reduced the activity of these channels by shifting their activation curve toward positive voltage values. This inhibition of the channels resulted in a marked decrease of the cytosolic free calcium concentration maintained by potassium. This effect was abolished by losartan, a specific antagonist of the angiotensin II AT1 receptor. Hormone action on T-type channels appeared to be mediated by protein kinase C because 1) it was mimicked by phorbol ester and diacylglycerol, and 2) it was significantly reduced by decreasing protein kinase C activity with specific inhibitors such as chelerythrine chloride or a pseudosubstrate of the enzyme, as well as by protein kinase C down-regulation. Similarly, protein kinase C activation reduced the cytosolic calcium response to potassium and the steroidogenic action of this agonist. Low threshold T-type calcium channels therefore appear as potential sites for the modulation of steroidogenesis by protein kinase C in adrenal glomerulosa cells.

Voltage-gated calcium currents have two opposing effects on the secretion of aldosterone

Using Ca2+ channel blockers with different specificities for L- and T-type Ca2+ channels, we have investigated the roles of these two channel types in K(+)-induced aldosterone secretion. In whole cell voltage-clamp experiments, the spider toxin omega-agatoxin-IIIA (omega-Aga-IIIA) completely blocks L-type Ca2+ channels but has no effect on T-type Ca2+ channels. In contrast, Ni2+ and 1,4-dihydropyridines block both L- and T-type Ca2+ channels. Secretion induced by 7 mM extracellular K+ concentration ([K+]o) is unaffected by omega-Aga-IIIA but is strongly inhibited by Ni2+ or the 1,4-dihydropyridine, nitrendipine. This suggests that physiological increases in [K+]o stimulate aldosterone secretion primarily by enhancing Ca2+ entry through T-type Ca2+ channels. Surprisingly, secretion induced by 60 mM [K+]o is enhanced by omega-Aga-IIIA or Ni2+ and is inhibited by the L-type Ca2+ channel activator BAY K 8644. Nitrendipine (1 nM) also stimulates such secretion, although higher concentrations are inhibitory (concentration inhibiting 50% of maximal response approximately 30 nM). If extracellular Ca2+ concentration is reduced from 1.25 to 0.5 mM, secretion induced by 60 mM [K+]o is enhanced, and Ni2+ or low nitrendipine become inhibitory. Together, these results that L-type Ca2+ currents can reduce steroidogenesis and that the role of these currents was previously misconstrued because 1,4-dihydropyridines modify secretion by multiple mechanisms. Thus Ca2+ entry can function as a negative modulator of steroid secretion.

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).

The mitochondrial benzodiazepine receptor: evidence for association with the voltage-dependent anion channel (VDAC)

Specific, high-affinity receptors for numerous drugs have recently been localized to mitochondrial membrane proteins. This review discusses the association of the mitochondrial receptor for benzodiazepines (mBzR) with the voltage-dependent anion channel (VDAC), indicating a possible auxiliary role for VDAC as a putative drug binding protein. The proposed subunit composition of the purified mBzR complex isolated from rat kidney mitochondria includes VDAC, which functions as a recognition site for benzodiazepines (e.g., flunitrazepam), the adenine nucleotide carrier (ADC), and an 18 kDa outer membrane protein identified by covalent labelling with the mBzR antagonists isoquinoline carboxamides (e.g., PK14105).

Effects of ruthenium red on activation of Ca2(+)-dependent cyclic nucleotide phosphodiesterase

Ruthenium red inhibited Ca2(+)-dependent phosphodiesterase (Ca2(+)-PDE) selectively with an IC50 value of 15 microM. Increasing calmodulin concentration in the presence of both 100 microM and 4000 microM Ca2+ completely reversed the inhibition of Ca2(+)-PDE activity by ruthenium red. Ruthenium red-induced inhibition of Ca2(+)-PDE activity was also overcome by increasing the concentration of Ca2+ in the presence of both 200 ng and 2000 ng calmodulin, in sharp contrast to fluphenazine-induced inhibition of Ca2(+)-PDE. These results indicate that ruthenium red has distinct inhibitory mechanism which differs from that of calmodulin antagonists previously reported.

Actin cytoskeleton and calcium-ATPase in the process of abomasal mucus secretion in cattle

The distribution of actin filaments in pyloric gland cells of cattle was studied with respect to their functional significance in the process of exocrine secretion by use of rhodamine-phalloidin labelling and immunogold-electron microscopy based on the biotin-streptavidin bridge technique. Actin concentrates on the filamentous network of the luminal-cell cortex. Membranes of secretory vesicles accumulating in the cell cortex are also labelled for actin. The present results support the concept of a barrier function of cortical microfilaments entrapping vesicles and linking them to the cytoskeleton. In addition, intracellular localization of calcium-ATPase activity was determined. Enzyme activity associated with the microfilamentous cortical matrix is supposed to be of cytoskeletal nature indicating participation of myosin (-like) structures in the dynamic secretion event. Deposition on the interior aspect of secretory vesicle membranes points to an ATPase transporting calcium into these organelles and enabling them to participate via storage of the cation in intracellular calcium homeostasis, thereby influencing the functional architecture of the cortical cytoskeleton.