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

Regulation of aldosterone and cortisol production by the transcriptional repressor neuron restrictive silencer factor

Aldosterone synthase (CYP11B2) and 11 beta-hydroxylase (CYP11B1) regulate aldosterone and cortisol production, respectively. The expression of these enzymes is promoted by calcium influx through Cav3.2, a T-type calcium channel. Neuron-restrictive silencer factor (NRSF) binds to neuron-restrictive silencer element (NRSE) to suppress the transcription of NRSE-containing genes. We found a NRSE-like sequence in human CYP11B2 and CYP11B1 genes as well as the CACNA1H gene of many mammalian species. The CACNA1H gene encodes the alpha-subunit of Cav3.2. Here we investigated how NRSF/NRSE regulates aldosterone and cortisol synthesis. Inhibition of endogenous NRSF by an adenovirus-expressing dominant-negative NRSF (AD/dnNRSF) increased human CYP11B2 and CYP11B1 mRNA expression, leading to aldosterone and cortisol secretion in human adrenocortical (H295R) cells. In reporter gene experiments, NRSE suppressed luciferase reporters driven by CYP11B2 and CYP11B1 promoters and dnNRSF enhanced them. Moreover, cotransfection of dnNRSF increased luciferase activity of reporter genes after deletion or mutation of NRSE, suggesting that NRSF/NRSE regulates transcription of CYP11B2 and CYP11B1 genes indirectly. AD/dnNRSF augmented mRNA expression of rat CYP11B2 and CYP11B1 genes, neither of which contains a NRSE-like sequence in rat adrenal cells. AD/dnNRSE also significantly increased CACNA1H mRNA in H295R and rat adrenal cells. Efonidipine, a T/L-type calcium channel blocker, significantly suppressed dnNRSF-mediated up-regulation of CYP11B2 and CYP11B1 expression. Moreover, NRSF/NRSE is also involved in angiotensin II- and K(+)-stimulated augmentation of CYP11B2 and CYP11B1 gene transcription. In conclusion, NRSF/NRSE controls aldosterone and cortisol synthesis by regulating CYP11B2 and CYP11B1 gene transcription mainly through NRSF/NRSE-mediated enhancement of the CACNA1H gene.

The site of action of Ca2+ in the activation of steroidogenesis: studies in Ca(2+)-clamped bovine adrenal zona-glomerulosa cells

The Ca(2+)-messenger system plays a crucial role in the regulation of steroid production in adrenal zona-glomerulosa cells, as it is known to mediate the action of both angiotensin II and K+. In the present study we used intact isolated glomerulosa cells in which the cytosolic free Ca2+ concentration ([Ca2+]c) was clamped at various levels with the Ca2+ ionophore ionomycin in order to locate the site(s) of action of Ca2+. By measuring in parallel steroid synthesis and [Ca2+]c, we show that Ca2+ levels (50-860 nM) regulate the production of both pregnenolone (up to 669 +/- 71.1% of the basal production) and aldosterone (up to 301 +/- 42.2%; EC50 = 303 nM). By contrast, Ca2+ did not stimulate the conversion of 11-deoxycorticosterone into aldosterone. Ca2+ modulation did not affect the formation of pregnenolone from freely diffusible analogues of cholesterol, indicating that Ca2+ acts at a step upstream of cholesterol side-chain cleavage. Moreover cycloheximide, an inhibitor of protein translation and of adrenocorticotropin-induced facilitation of intramitochondrial cholesterol transport, the rate-limiting step in steroidogenesis, also blocked Ca(2+)-triggered pregnenolone formation. This is consistent with a model in which Ca2+ promotes cholesterol transfer between mitochondrial membranes. In addition, agents using the cyclic AMP pathway as well as angiotensin II potentiated the steroidogenic response to increases in [Ca2+]c by augmenting both the efficacy and the potency of Ca2+. This effect of angiotensin II did not involve protein kinase C. These results establish a direct link between agonist-induced [Ca2+]c rises and a specific step of the steroidogenic pathway.

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).

Control of bovine placental progesterone synthesis: roles of cholesterol availability and calcium-activated systems

It was previously reported that dispersed bovine placentome secretes progesterone and that the steroidogenic activity of these cells is stimulated by a calcium-mediated, cyclic nucleotide independent mechanism. In the present study, the influence of substrate availability was explored and the roles of calmodulin and protein kinase C in progestin production examined. Incubation of dispersed fetal cotyledon cells with 25-hydroxycholesterol (25-OH-C), a soluble sterol which readily enters cells and is metabolized to steroid hormones, increased progesterone secretion in a dose-dependent manner. The response to 25-OH-C was dependent on the extracellular calcium concentration. Methyl isobutyl xanthine (MIX) alone also increased pregnenolone as well as progesterone secretion, and the combination of 25-OH-C and MIX stimulated progesterone secretion was inhibited by trifluoperazine. The phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), caused no major effects on steroidogenesis but the stimulatory effects of MIX or the ionophore A23187 were enhanced in its presence. These findings suggest that (1) basal progesterone secretion by fetal cotyledon cells is limited by cholesterol availability; (2) MIX increases steroidogenesis in part by increasing the synthesis of pregnenolone, but its actions are expressed independently of cholesterol availability; (3) both calmodulin and protein kinase C may participate in the modulation of bovine placental steroidogenesis.