Control of steroid synthesis in adrenal fasciculata cells

Impact of a novel homozygous mutation in nicotinamide nucleotide transhydrogenase on mitochondrial DNA integrity in a case of familial glucocorticoid deficiency

BACKGROUND

Familial Glucocorticoid Deficiency (FGD) is a rare autosomal recessive disorder that is characterized by isolated glucocorticoid deficiency. Recently, mutations in the gene encoding for the mitochondrial nicotinamide nucleotide transhydrogenase (NNT) have been identified as a causative gene for FGD; however, no NNT activities have been reported in FGD patients carrying NNT mutations.

METHODS

Clinical, biochemical and molecular analyses of lymphocytes from FDG homozygous and heterozygous carriers for the F215S NNT mutation.

RESULTS

In this study, we described an FGD-affected Japanese patient carrying a novel NNT homozygous mutation (c.644T>C; F215S) with a significant loss-of-function (NNT activity = 31% of healthy controls) in peripheral blood cells' mitochondria. The NNT activities of the parents, heterozygous for the mutation, were 61% of controls.

CONCLUSIONS

Our results indicated that (i) mitochondrial biogenesis (citrate synthase activity) and/or mtDNA replication (mtDNA copy number) were affected at ≤60% NNT activity because these parameters were affected in individuals carrying either one or both mutated alleles; and (ii) other outcomes (mtDNA deletions, protein tyrosine nitration, OXPHOS capacity) were affected at ≤30% NNT activity as also observed in murine cerebellar mitochondria from C57BL/6J (NNT^-/-) vs. C57BL/6JN (NNT^+/+) substrains.

GENERAL SIGNIFICANCE

By studying a family affected with a novel point mutation in the NNT gene, a gene-dose response was found for various mitochondrial outcomes providing for novel insights into the role of NNT in the maintenance of mtDNA integrity beyond that described for preventing oxidative stress.

Inhibitory effect of protopanaxatriol ginseng metabolite M4 on the production of corticosteroids in ACTH-stimulated bovine adrenal fasciculata cells

AIMS

We investigated the pharmacological effects of saponins isolated from ginseng root and their metabolites, which occur by hydrolysis of the sugar moieties connecting the aglycone of saponins in the digestive tract, on the production of corticosteroids in bovine adrenal fasciculata cells in vitro.

MAIN METHODS

The levels of corticosteroids produced from adrenal corticotropic hormone (ACTH)-stimulated bovine adrenal fasciculata cells were determined under the presence or absence of ginseng saponins (ginsenosides) and their metabolites using fluorometry, gas-chromatography-mass spectrometry, and sweeping-micellar electrokinetic capillary chromatography.

KEY FINDINGS

An end metabolite of the protopanaxatriol saponins in ginseng, 20(s)-protopanaxatriol (M4), strongly reduced ACTH-stimulated cortisol production. M4 significantly inhibited the production of cortisol induced by different stimuli, alamethicin, dibutyryl cyclic AMP, forskolin, and 22(R)-hydroxycholesterol, a membrane-permeable cholesterol. However, it did not affect the production of cortisol by either pregnenolone, a precursor of cortisol synthesis, or cyclic AMP. Furthermore, M4 significantly inhibited the production of pregnenolone, progesterone, deoxycorticosterone, cortisol, and corticosterone in a dose-dependent manner.

SIGNIFICANCE

Results strongly suggest that protopanaxatriol saponins in ginseng are prodrugs metabolized in the digestive tract so that the end metabolite, M4, produces inhibitory activity of corticosteroid production in the adrenal fasciculata cells in vivo. The results also suggest that M4 inhibits the conversion from cholesterol to pregnenolone because the production of pregnenolone was reduced.

Influence of 5-fluorouracil on ferredoxin reductase mRNA splice variants in colorectal carcinomas

5-Fluorouracil (5-FU) is a frequently used antitumor drug. Recently, it has been shown that mRNA and protein levels of the ferredoxin reductase gene (gene, FDXR; protein, FR) increase drastically after 5-FU treatment in various cell lines including colorectal cancer. The induction is mediated by p53 and enhanced reactive oxygen species (ROS)-associated apoptosis. Thus, knowledge about FDXR expression in human tissue and expression of the known splice variants is critical for understanding this finding. A sensitive and specific reverse transcriptase polymerase chain reaction (RT-PCR) assay for quantification of FDXR mRNA levels including the splice variants, a biological active variant (-18 bp) and an inactive variant (+18 bp), was developed and used to measure mRNAs after 5-FU chemotherapy in colorectal tissues of 40 cancer patients prior to and after treatment with 5-FU for 14 days. Before treatment, the great majority of normal tissues expressed the splice variants in a 100:1 ratio in favor of the -18-variant similar to what has been reported for other tissues. In tumors, the mRNA levels of total FDXR and splice variants were approximately 2-fold higher compared to the normal tissue. After 5-FU treatment, levels of the +18-variant increased 17-fold in tumors and 31-fold in normal tissues, clearly shifting the ratio towards the +18-form. 5-FU-mediated -18-variant induction (>1) in normal (12/17) and tumor tissues (12/16) was apparently associated with response, while a balanced ratio (0.1-2) was associated with 5-FU resistance (n=5) based on the histological evaluation of the tissues.

Steroidogenic acute regulatory protein (StAR), a novel mitochondrial cholesterol transporter

Cholesterol is a vital component of cellular membranes, and is the substrate for biosynthesis of steroids, oxysterols and bile acids. The mechanisms directing the intracellular trafficking of this nearly insoluble molecule have received increased attention through the discovery of the steroidogenic acute regulatory protein (StAR) and similar proteins containing StAR-related lipid transfer (START) domains. StAR can transfer cholesterol between synthetic liposomes in vitro, an activity which appears to correspond to the trans-cytoplasmic transport of cholesterol to mitochondria. However, trans-cytoplasmic cholesterol transport in vivo appears to involve the recently-described protein StarD4, which is expressed in most cells. Steroidogenic cells must also move large amounts of cholesterol from the outer mitochondrial membrane to the first steroidogenic enzyme, which lies on the matrix side of the inner membrane; this action requires StAR. Congenital lipoid adrenal hyperplasia, a rare and severe disorder of human steroidogenesis, results from mutations in StAR, providing a StAR knockout of nature that has provided key insights into its activity. Cell biology experiments show that StAR moves large amounts of cholesterol from the outer to inner mitochondrial membrane, but acts exclusively on the outer membrane. Biophysical data show that only the carboxyl-terminal alpha-helix of StAR interacts with the outer membrane. Spectroscopic data and molecular dynamics simulations show that StAR's interactions with protonated phospholipid head groups on the outer mitochondrial membrane induce a conformational change (molten globule transition) needed for StAR's activity. StAR appears to act in concert with the peripheral benzodiazepine receptor, but the precise itinerary of a cholesterol molecule entering the mitochondrion remains unclear.

StAR Search—What We Know about How the Steroidogenic Acute Regulatory Protein Mediates Mitochondrial Cholesterol Import

Cholesterol is the starting point for biosynthesis of steroids, oxysterols and bile acids, and is also an essential component of cellular membranes. The mechanisms directing the intracellular trafficking of this insoluble molecule have received attention through the discovery of the steroidogenic acute regulatory protein (StAR) and related proteins containing StAR-related lipid transfer domains. Much of our understanding of the physiology of StAR derives from studies of congenital lipoid adrenal hyperplasia, which is caused by StAR mutations. Multiple lines of evidence show that StAR moves cholesterol from the outer to inner mitochondrial membrane, but acts exclusively on the outer membrane. The precise mechanism by which StAR's action on the outer mitochondrial membrane stimulates the flow of cholesterol to the inner membrane remains unclear. When StAR interacts with protonated phospholipid head groups on the outer mitochondrial membrane, it undergoes a conformational change (molten globule transition) that opens and closes StAR's cholesterol-binding pocket; this conformational change is required for cholesterol binding, which is required for StAR activity. The action of StAR probably requires interaction with the peripheral benzodiazepine receptor.

Mechanism of StAR's regulation of mitochondrial cholesterol import

The steroidogenic acute regulatory protein (StAR) mediates the acute steroidogenic response by moving cholesterol from the outer to inner mitochondrial membrane, but the mechanism of StAR's action has remained mysterious. We showed that StAR acts on the outer membrane, requires cholesterol binding, and requires the structural change previously described as a pH-dependent molten globule. The current model is that StAR's interaction with protonated phospholipid head groups on the outer mitochondrial membrane induces a molten globule transition needed for StAR to take up cholesterol. Recent data suggest a functional interaction between StAR and the peripheral benzodiazepine receptor (PBR). Whereas many models have suggested that StAR delivers cholesterol to PBR, we suggest that StAR removes cholesterol from the cholesterol-binding domain of PBR and delivers it to the inner mitochondrial membrane.

Pituitary development and physiology

The functions of the pituitary hormones have been relatively well studied; however, understanding the regulation of their synthesis and release have been an ongoing subject of intense research. This review provides an overview of the pituitary cell types and their hormone products. Current understanding of the expression and regulation of the pituitary hormone genes, control of the synthesis and release of the corresponding hormones, and developmental changes are reviewed. This review concludes with a discussion of several of these genes and the genetic disorders with which they are associated.

Lipoproteins regulate expression of the steroidogenic acute regulatory protein (StAR) in mouse adrenocortical cells

The steroidogenic acute regulatory protein (StAR) is required for the movement of cholesterol from the outer to the inner mitochondrial membrane, the site of cholesterol side chain cleavage. Here we describe a novel form of regulation of StAR gene expression in steroidogenic cells. Treatment of Y-1 BS1 adrenocortical cells with either low density lipoprotein (LDL) or high density lipoprotein (HDL) increases expression of endogenous StAR mRNA and protein in a dose-dependent manner. Induction of StAR mRNA by lipoprotein requires basal cAMP-dependent protein kinase, since the inhibitor, R(p)-8-Br-cAMP, inhibited induction of StAR protein by LDL. Likewise, basal StAR expression or LDL induction of StAR protein was not detectable in Y-1 kin-8 cells which are deficient in cAMP-dependent protein kinase. Aminoglutethimide and ketoconazole were used to determine if side chain cleavage of lipoprotein-derived cholesterol is required for induction of StAR mRNA. Treatment with either drug alone induced StAR mRNA expression 1.5-3-fold, while induction of StAR in cells treated with either drug plus LDL, was equal to, or greater than, induction seen with either agent alone, suggesting that lipoprotein does not regulate StAR via generation of an oxysterol intermediate. Both LDL and HDL increased expression of a mouse -966 StAR promoter-reporter construct 1.5-2.5-fold, indicating that regulation occurs at the level of transcription. In contrast, neither lipoprotein was able to induce transcription from a -966 StAR promoter in which the steroidogenic factor-1 site at -135 was abolished, indicating that regulation of StAR transcription by lipoproteins requires steroidogenic factor-1. The regulation of StAR gene expression by lipoproteins may represent a positive feedback circuit which links cholesterol availability with steroidogenic output.

StAR-A tissue specific acute mediator of steroidogenesis

The rate-limiting and acutely regulated step in steroid hormone biosynthesis is the translocation of cholesterol, the precursor of all steroid hormones, from the mitochondrial outer membrane to the inner membrane, where it is converted to pregnenolone by the cytochrome P450 cholesterol side-chain cleavage enzyme (P450scc). This step has long been known to be dependent upon the de novo synthesis of a labile protein factor, which is required for the intramitochondrial translocation of cholesterol. Recently, the Steroidogenic Acute Regulatory (StAR) protein has been shown to have an indispensable role in acute steroid production and is proposed to be this labile protein factor. Given the fundamental importance of StAR as a key regulator of steroid hormone biosynthesis, the next frontier for researchers is elucidating the molecular mechanisms that control StAR expression and function.

Role of calmodulin-dependent protein kinase II in the acute stimulation of aldosterone production

Acute aldosterone production in adrenocortical cells is highly dependent on calcium (Ca2+) and calmodulin (CaM) activation. To determine the role of calmodulin-dependent protein kinase II (CaM kinase II) in human adrenal aldosterone production, the action of KN93 (a specific CaM kinase II inhibitor) on human adrenocortical H295R cells was examined. The stimulation of aldosterone, production by angiotensin II (Ang II) and potassium (K+) were inhibited by KN93 in a concentration-dependent manner with an IC50 of approximately 0.9 and approximately 0.5 microM, respectively. Aldosterone production was also stimulated by treatment with the calcium channel activator Bay K 8644 (Bay K) (1 microM). This production was inhibited in a concentration-dependent manner by KN93 with an IC50 of between 1 and 3 microM. No inhibition by KN93 (0.3-3 microM) or by the calmodulin inhibitor calmidazolium (0.03-0.3 microM) was observed for 22R-hydroxycholesterol (22R-OHChol) stimulation of aldosterone production. Because 22R-OHChol is a substrate for the cytochrome P450 cholesterol side-chain cleavage enzyme (P450scc) and does not require active transport to the mitochondria, these results indicate that KN93 does not directly inhibit P450scc or later steps leading to aldosterone synthesis. To investigate the site of KN93 action further we examined its effect on agonists induction of steroidogenic acute regulatory (StAR) protein, which was recently shown to regulate the movement of cholesterol from the outer to the inner mitochondrial membranes. Induction of StAR protein in H295R cells by Ang II, or Bay K was not affected by co-treatment with KN93 at concentration which blocked steroidogenesis by 60-80%. These results indicate a direct role of CaM kinase II in Ang II and K+ simulation of aldosterone production and support the hypothesis that CaM kinase II may be involved in the process of cholesterol mobilization to the mitochondria.

Mitochondrial specificity of the early steps in steroidogenesis

Studies in human beings, animals, and cell systems show that the rate-limiting step in steroidogenesis is the conversion of cholesterol to pregnenolone. In the adrenals and gonads, this step is subject to both acute and chronic regulation. Chronic regulation is primarily, but not exclusively at the level of gene transcription, leading to the production of more steroidogenic machinery and thus increasing the cellular capacity for steroidogenesis. Chronic regulation can be inhibited by inhibiting protein synthesis with cycloheximide, but this response varies among various cell types and species. Although the P450scc enzyme system that converts cholesterol to pregnenolone is inherently very slow, the principal site of acute regulation is at the delivery of free cholesterol to mitochondria, rather than at the delivery of reducing equivalents to P450scc. Even when the Vmax of the P450scc system is increased 6-fold by genetic engineering, delivery of cholesterol to the enzyme remains rate-limiting. Targeting of a genetically engineered fusion of the P450scc system to either mitochondria or to the endoplasmic reticulum of non-steroidogenic cells demonstrates that the mitochondrial environment is absolutely required for the conversion of cholesterol to pregnenolone, and that this absolute requirement is not based on either the nature of the available electron donors for P450scc or the availability of substrate. Various factors have been proposed as the essential mediator for the transport of cholesterol into mitochondria to initiate steroidogenesis. A recently identified protein termed Steroidogenic Acute Regulatory protein (StAR) has the necessary properties of enhancing steroidogenesis, rapid cAMP inducibility and rapid cycloheximide sensitivity that characterize the long-sought acute regulator of steroidogenesis. StAR is expressed in steroidogenic tissues exhibiting an acute response but not in steroidogenesis. StAR is expressed in steroidogenic tissues exhibiting an acute response but not in steroidogenic tissues (placenta, brain) that do not exhibit this response. Mutations in StAR are now shown to cause Congenital Lipoid Adrenal Hyperplasia, the last unsolved form of CAH. The actions of StAR can be circumvented by the use of hydroxycholesterols that can freely diffuse into mitochondria, proving that StAR functions as an acute regulator of cholesterol access to mitochondria.

Cytochrome P450 2A1, 2E1, and 2C9 cDNA-expression by insect cells and partial purification using hydrophobic chromatography

High-level expression of three cloned cytochrome P450 enzymes was accomplished using the baculovirus-insect cell expression system. The amount of enzyme expression was enhanced by cell infections in the presence of medium-supplements containing hemin and by growth in suspension cultures. Human cytochromes P450 2E1 and 2C9 and rat cytochrome P450 2A1 were partially purified from cell extracts using hydrophobic interaction and hydroxyapatite chromatography. The resulting enzymes were of estimated molecular masses similar to those reported previously and analyzed by PAGE. Reconstitution of enzyme activity resulted when the enzymes were incubated together with NADPH-cytochrome P450 reductase, phospholipid, NADPH, and appropriate substrates. The cytochrome P450 activity of the partially purified enzymes was comparable to that of the corresponding enzymes expressed in the vaccinia virus-Hep G2 system. These results provide evidence for a general means of obtaining cytochrome P450 enzymes for mechanistic, immunochemical, and biophysical investigations.

Expression of the steroidogenic acute regulatory (StAR) protein: a novel LH-induced mitochondrial protein required for the acute regulation of steroidogenesis in mouse Leydig tumor cells

The acute response of steroidogenic cells to hormone stimulation is the mobilization of cholesterol from cellular stores and the outer mitochondrial membrane to the inner mitochondrial membrane and the cholesterol side-chain cleavage complex (CSCC) where the first enzymatic reaction occurs. It has been well established that the translocation of cholesterol from the outer to the inner mitochondrial membrane requires de novo protein synthesis and that this process is the rate-limiting, regulated step in steroidogenesis. We have purified a novel mitochondrial protein (named StAR) from the MA-10 mouse Leydig tumor cells which we have previously proposed represents a strong candidate for the newly synthesized regulatory protein in steroidogenesis. The cDNA for StAR was cloned and we have demonstrated that expression of the StAR protein in MA-10 cells in the absence of hormone stimulation results in a 3 fold increase in progesterone production compared to mock transfected cells. These studies indicate a direct indicate a direct relationship between StAR expression and steroidogenesis, therefore, we conclude that StAR is required in the acute regulation of steroidogenesis.

Gonadotropin-releasing hormone agonist as a probe for the pathogenesis and diagnosis of ovarian hyperandrogenism

We have found that women with typical polycystic ovary syndrome have supranormal plasma 17-hydroxyprogesterone responses to a 100-micrograms test dose of the gonadotropin-releasing hormone agonist nafarelin without evidence of hindered estrogen secretion. To understand the basis of this response, we computed the apparent efficiency of the steps in steroid biosynthesis from the pattern of plasma steroids in response to nafarelin. The proximate cause appears to be excessive 17 alpha-hydroxylase activity and high, yet partially down-regulated, 17,20-lyase activity in the delta 4-pathway. These results suggest that this pattern of steroid secretion results from abnormal regulation (dysregulation) of these activities, possibly involving the enzyme cytochrome P450c17. To determine the usefulness of nafarelin testing for the diagnosis of ovarian hyperandrogenism, we then prospectively studied 40 hyperandrogenic women. The plasma 17-PROG response to nafarelin was supranormal in 58% of the women. The responses of 17-PROG to nafarelin and free testosterone to dexamethasone correlated well and were concordant in approximately 85% of cases. Baseline serum luteinizing hormone concentration was elevated in only 48% of cases. To understand ovarian structure-function relationships, we studied another 20 consecutive hyperandrogenic women. Among seven women with polycystic ovaries, five had an elevated LH level, and four of these five (80%) had an elevated 17-PROG response to nafarelin. Conversely, about half of patients with the PCOS-like disorder of ovarian function did not have polycystic ovaries. Ovarian stromal area, but not LH levels, correlated significantly (r = 0.45) with the 17-PROG response to nafarelin. Thus, both stromal hyperplasia and dysregulation of steroidogenesis seem to be manifestations of abnormal intraovarian regulation of cell growth and function. We conclude that a PCOS-like disorder of ovarian function in response to nafarelin testing is found in approximately half of hyperandrogenic women. The pathogenetic implication of our results is that abnormal intraovarian modulation of LH action seems to be a major factor in ovarian hyperandrogenism. The diagnostic implication of our data is that ovarian androgen excess will often be missed by use of common diagnostic criteria for PCOS.

Effect of ACTH on steroidogenic enzymes in guinea pig fasciculata-glomerulosa cells: changes in activity and mRNA levels

Adrenocorticotropin (ACTH) is known to exert an acute effect on adrenal steroidogenesis as well as long-term effects by regulation of gene expression. In order to further study the long-term action of ACTH, guinea pig fasciculata-glomerulosa (FG) cells in primary culture were treated for up to 72 h with ACTH. The effects of this treatment on steroid secretion, enzyme activity and mRNA levels for steroid enzymes were measured. While the rate of 17-deoxy C-21 steroid secretion decreased over the 72-h period of incubation with ACTH, the 17-hydroxy C-21 steroid secretion rate remained constant for the first 24 h of incubation and declined thereafter; the rate of 4-ene C-19 steroid secretion increased over the 72-h incubation period. ACTH treatment increased 17-hydroxylase and 17,20-lyase activities and the maximal stimulation was reached after 48 h. In contrast, the activity of 21-hydroxylase (P450c21) steadily declined over the 72-h incubation period. ACTH also caused an increase in mRNA levels for P450c21, 17-hydroxylase and 17,20-lyase (P450c17), 3 beta-hydroxysteroid dehydrogenase 4-ene-5-ene-isomerase (3 beta-HSD) and cholesterol side-chain cleavage enzyme (P450scc). The maximal stimulation for the four mRNAs was observed after 18 h of incubation with ACTH, decreasing afterwards except for P450c17 mRNA levels which remained elevated over the 72-h incubation period. Despite the increase in mRNA levels for 3 beta-HSD and P450c21, no increase in their respective enzyme activities was observed and 21-hydroxylase activity even declined over the 72-h incubation period with ACTH, thus suggesting that mechanism(s) other than gene expression alone regulate steroid secretion in FG cells. In conclusion ACTH caused major changes in steroid distribution due to increased 17-hydroxylase and 17,20-lyase activities and decreased 21-hydroxylase activity in FG cells in culture. Moreover, our data revealed major differences in the induction of mRNAs for steroidogenic enzymes and their activities following ACTH treatment.

Succinate stimulation of isocitrate supported deoxycorticosterone metabolism in rat adrenal mitochondria by a synergistic mechanism

The relative effectiveness of succinate and isocitrate in supplying NADPH for cholesterol and deoxycorticosterone (DOC) metabolism by rat adrenal mitochondria has been investigated. As previously seen for cholesterol metabolism, isocitrate supported DOC metabolism at a higher rate than succinate. Maximal rates of DOC metabolism, however, required 10 times more precursor (10mM) than cholesterol metabolism. Addition of DOC to mitochondria inhibited cholesterol metabolism, indicating competition for NADPH between these pathways. Coaddition of these reducing precursors resulted in a substantially greater than additive rate of DOC hydroxylation, but not cholesterol metabolism. The synergistic effect was seen with both 11 beta- and 18 hydroxylation. For both precursors, the synergism was maximal upon addition of only 1 mM of the second precursor. The synergistic effect was far more resistant to added KCN and malonate than succinate supported DOC metabolism, and neither inhibitor affected isocitrate supported DOC metabolism. These results suggest that while cytochromes P450scc and P450(11 beta) use a common supply of NADPH generated by each precursor, there is a pool of NADPH that is only effectively synthesised upon coaddition of precursors and only utilised by cytochrome P450(11 beta). This second NADPH pool may be produced in response to potentiated isocitrate dehydrogenase activity or activation of a different NADPH generating system.

Guanabenz-induced inhibition of aldosterone secretion from isolated rat adrenal glomerulosa cells

The authors examined the effects of the alpha 2-adrenergic agonist guanabenz and other alpha-adrenergic ligands on aldosterone secretion and cyclic nucleotide content in isolated rat adrenal glomerulosa cells. Guanabenz inhibited aldosterone secretion stimulated by potassium, angiotensin II (AII), and adrenocorticotropic hormone (ACTH), exhibiting IC50 values of 35 microM, 43 microM, and 58 microM for stimulation by 10 mM K+, 1 nM AII, and 10 pM ACTH, respectively. Guanabenz did not affect the cGMP content of purified adrenal glomerulosa cells but inhibited ACTH stimulation of cAMP accumulation. Guanabenz inhibition of ACTH-induced cAMP may represent a mechanism for inhibition of aldosterone secretion, however, guanabenz also inhibited aldosterone secretion stimulated by the cAMP analog dibutyryl cAMP. The effect of guanabenz on the early and late pathways of steroidogenesis was tested in the isolated rat glomerulosa cells using 25-OH cholesterol and steroid precursors to aldosterone. Guanabenz inhibited the steroidogenic response to 25-OH cholesterol stimulation of aldosterone secretion but induced a much smaller inhibition of the steroidogenic response to exogenous pregnenolone, progesterone, and 11-deoxycorticosterone. These results suggested that guanabenz inhibited aldosterone secretion primarily through inhibition of the early component of the steroidogenic pathway prior to pregnenolone formation. The effects of guanabenz were not mimicked by other alpha-adrenergic ligands suggesting that these effects of guanabenz were not mediated through activation of alpha-adrenergic receptors.

Heterogeneous pools of cholesterol side-chain cleavage activity in adrenal mitochondria from adrenocorticotropic hormone-treated rats: reconstitution of the isocitrate response with succinate and low concentrations of isocitrate

Cholesterol side-chain cleavage in isolated adrenal mitochondria requires unique energy requirements that may determine not only electron transport to P450 but also cholesterol availability. In mitochondria from ACTH-treated rats, two approximately equal pools of reactive cholesterol are indicated by the partial effectiveness of succinate (SU; Type A), and the metabolism of residual cholesterol by 1 mM isocitrate (IC; Type B). Type A metabolism is associated with relatively few initial cholesterol-P450scc complexes and is rapidly and selectively lost when mitochondria are preincubated without an energy source. We now show that cholesterol metabolism supported by IC resolves into equal high and low affinity components (EC50 = 10 and 250 microM) exhibiting, respectively, Type A and Type B characteristics. SU and 50 microM IC, in combination, provided nearly the same activity characteristics as 1 mM IC, including resistance to preincubation and increased turnover of cholesterol-P450scc complexes. Much higher (three to six times) and more sustained pregnenolone formation was seen, with all reductants, following either enhancement of the reactive cholesterol pool or addition of 20-alpha-hydroxycholesterol, indicating that adrenocorticotropic hormone-mitochondria are limited by substrate availability. ATP generation was most effectively supported by SU, and IC was maximally active at 50 microM, emphasizing differences between respiratory and steroidogenic energy requirements. ATP production and the maintenance of uniform suppression after in vivo cycloheximide treatment indicate the integrity of the mitochondrial interaction with all reductants. Inhibitors of SU oxidation (KCN, malonate) strongly inhibited SU-supported cholesterol metabolism but had little effect on SU synergism with IC. Fumarate (but not alpha-ketoglutarate or oxaloacetate) was equally effective as a synergist, but was totally ineffective as a reductant. SU or fumarate, therefore, act by a nonreductive pathway to boost NADPH production from low concentrations of IC. This decrease in apparent Km for IC may be mediated by stimulation of mitochondrial uptake of the reductant through the specific transporters.

Heterogeneous pools of cholesterol side-chain cleavage activity in adrenal mitochondria from ACTH-treated rats: differential responses to different reducing precursors

Side-chain cleavage (SCC) of endogenous cholesterol in adrenal mitochondria isolated from ACTH-treated rats indicates that the size of the reactive cholesterol pool depends on the reducing precursor. At optimal concentrations of reductant, this pool was typically at least 2 times greater for isocitrate than for succinate. Succinate-supported reactions were rapidly completed, were highly sensitive to a 2-min preincubation, and failed to deplete spectrally detected P-450SCC-cholesterol complexes. Cholesterol SCC with 1 mM isocitrate exhibited 2-3 times more fast-phase metabolism, a pronounced slow phase, insensitivity to preincubation, and 60% depletion of spectrally detected cholesterol-P-450SCC complexes. Addition of bovine serum albumin (BSA) and EDTA, either during homogenization or directly to the incubation, prevented preincubation losses in response to succinate and removed most of the difference between succinate and isocitrate activities. This effect of BSA/EDTA was reversed within 5 min by octanoate by a mechanism that was enhanced by Ca2+. These distinct reductant characteristics suggest that only a subpopulation of mitochondria or of pools of activity within individual mitochondria can support cholesterol SCC with succinate while isocitrate is necessary for the remainder. The rapid responses of succinate-supported metabolism to preincubation or to octanoate suggest depletion of a critical factor for cholesterol metabolism. Metabolism of added 20 alpha-hydroxycholesterol or deoxycorticosterone established that NADPH remained fully available after succinate-supported cholesterol metabolism had stopped or after preincubation. Cessation of pregnenolone formation, therefore, results from a failure to supply cholesterol, not inadequate NADPH. The preincubation effect suggests loss of an energy-dependent component that enhances this supply of cholesterol. One possibility tested was that GTP, an activator of intermembrane cholesterol transfer (Xu et al. (1989) J. Biol. Chem. 264, 17674-17680), was being lost. Added GTP slightly activated succinate-supported pregnenolone production but did not prevent preincubation-induced losses. alpha-Ketoglutarate, which can generate matrix GTP, is an effective reductant that, in combination with succinate, prevents preincubation-induced losses.

Effect of lindane on mitochondrial side-chain cleavage of cholesterol in mice

Cholesterol side-chain cleavage of mitochondria, the key rate limiting step in steroid biosynthesis in ovarian tissues, was studied in female Swiss mice fed lindane (gamma-hexachlorocyclohexane) at various doses and over varying periods. The insecticide adversely affected cholesterol side-chain cleavage of the ovary as judged by decreased conversion of this sterol to pregnenolone and subsequently to progesterone in a dose-dependent manner. The formation of pregnenolone was maximally inhibited (75% inhibition) at the highest intake of insecticide with simultaneous inhibition of its conversion to progesterone. At all doses, the rate of inhibition of pregnenolone formation was comparatively higher than its conversion to progesterone. The significant inhibition of cholesterol side-chain cleavage by lindane and resultant decrease in the rate of steroidogenesis in the ovary would account for the observed gonadal hormone deficiency and related reproductive disorders in the lindane-fed mice.

Hormone-sensitive cholesterol ester hydrolase in adrenal tumor cells: activation by corticotropin and tetradecanoyl phorbol acetate

The effects of corticotropin (ACTH) and tetradecanoyl phorbol acetate (TPA) on cholesterol ester hydrolase, intracellular cholesteryl ester concentration and steroid hormone formation were studied in mouse adrenal tumor cells (Y-1) in monolayer culture. Cholesterol ester hydrolase activity increased about 2-fold during 7 min incubation with ACTH, dibutyryl 3',5'-cyclic AMP (dbcAMP) and TPA at maximally effective concentrations; whereas, incubation with phorbol monoacetate had no effect. Long-term exposure to ACTH and dbcAMP markedly lowered intracellular cholesteryl [3H]-oleate concentration and highly increased steroid hormone output, while TPA treatment resulted in lowering cholesteryl [3H]-oleate content without affecting steroid hormone formation. Calcium activated phospholipid-dependent protein kinase C was detected in Y-1 cell cytosol. It is concluded that the mouse adrenal tumor cells in monolayer culture respond to ACTH in a fashion similar to normal adrenocortical cells; whereas, the response to the phorbol ester TPA (possibly mediated through protein kinase C) involves activation of cholesterol ester hydrolase and cholesteryl ester depletion, however, without affecting steroid hormone secretion.

The role of sterol carrier protein 2 in stimulation of steroidogenesis in rat adrenal mitochondria by adrenal cytosol

Cholesterol side-chain cleavage (CSCC) in isolated rat adrenal mitochondria is enhanced by prior corticotropin (ACTH) stimulation in vivo (8-fold). Part of this stimulation is retained in vitro by addition of cytosol from ACTH-stimulated adrenals to mitochondria from unstimulated rats (2.5- to 6-fold). In vivo cycloheximide (CX) treatment fully inhibits the in vivo response and resolves the in vitro cytosolic stimulation into components: (i) ACTH-sensitive, CX-sensitive; (ii) ACTH-sensitive, CX-insensitive; and (iii) ACTH-insensitive, CX-insensitive. These components contribute approximately equally to stimulation by ACTH cytosol. Components (i) and (iii) most probably correspond to previously identified cytosolic constituents steroidogenesis activator peptide and sterol carrier protein 2 (SCP2). SCP2, as assayed by radioimmunoassay or ability to stimulate 7-dehydrocholesterol reductase, was not elevated in adrenal cytosol or other subcellular fractions by ACTH treatment. Complete removal of SCP2 from cytosol by treatment with anti-SCP2 IgG decreased cytosolic stimulatory activity by an increment that was independent of ACTH or CX treatment. Addition of an amount of SCP2, equivalent to that present in cytosol, restored activity to SCP2-depleted cytosol but had no effect alone or when added with intact cytosol, suggesting the presence of a factor in cytosol that potentiates SCP2 action. Pure hepatic SCP2 stimulated CX mitochondrial CSCC 1.5- to 2-fold (EC50 0.7 microM) but was five times less potent than SCP2 in adrenal cytosol. Two pools of reactive cholesterol were distinguished in these preparations characterized, respectively, by succinate-supported activity and by additional isocitrate-supported activity. ACTH cytosol and SCP2 each stimulated cholesterol availability to a fraction of mitochondrial P450scc that was reduced by succinate but failed to stimulate availability to additional P450scc reduced only by isocitrate.

Inhibition of adrenocortical, mitochondrial and microsomal monooxygenases by SU-10'603, a steroid 17 alpha-hydroxylase inhibitor

SU-10'603 is a pyridine derivative that is widely used as a steroid 17 alpha-hydroxylase inhibitor. Studies were done to evaluate its effects in vitro on several other monooxygenases in guinea pig adrenal mitochondrial and microsomal preparations. In adrenal mitochondria, SU-10'603 produced a concentration-dependent inhibition of 11 beta-hydroxylation; 50% inhibition was obtained at a concentration of approximately 0.5 mM. Its potency was similar to that of the 11 beta-hydroxylase inhibitor, metyrapone. SU-10'603 was a more potent inhibitor of cholesterol sidechain cleavage (CSC) than of 11 beta-hydroxylation; a 50% decline in CSC activity was produced by an inhibitor concentration of approximately 0.1 mM. In adrenal microsomal preparations, SU-10'603 had no effect on the rate of 21-hydroxylation of 17 alpha-hydroxyprogesterone. However, SU-10'603 was a potent inhibitor of adrenal microsomal xenobiotic metabolizing monooxygenases (benzo[a]pyrene hydroxylase, benzphetamine demethylase), effecting approximately 50% inhibition of both reactions at a concentration of 0.05 mM. The results indicate that SU-10'603 inhibits several monooxygenases in the guinea pig adrenal cortex and is thus not specific for 17 alpha-hydroxylation.

cDNA sequence of adrenodoxin reductase. Identification of NADP-binding sites in oxidoreductases

Adrenodoxin reductase is an NADP dependent flavoenzyme which functions as the reductase of mitochondrial P 450 systems. We sequenced two adrenodoxin reductase cDNAs isolated from a bovine adrenal cortex cDNA library. The deduced amino acid sequence shows no similarity to the sequence of the microsomal P 450 systems or other known protein sequences. Nonetheless, by sequence analysis and c comparisons with known sequences of dinucleotide-binding folds of two NADP-binding flavoenzymes, two regions of adrenodoxin reductase sequence were identified as the FAD- and NADP-binding sites. These analyses revealed a consensus sequence for the NADP-binding dinucleotide fold (GXGXXAXXXAXXXXXXG, in one-letter amino acid code) that differs from FAD and NAD-binding dinucleotide-fold sequences. In the data base of protein sequences, the NADP-binding-site sequence appears solely in NADP-dependent enzymes, the binding sites of which were not known to date. Thus, this sequence may be used for identification of a certain type of NADP-binding site of enzymes that show no significant sequence similarity.

Regulation of steroid hydroxylase gene expression is multifactorial in nature

In summary, regulation of steroid hydroxylase gene expression is complex and multifactorial, involving cAMP-dependent and -independent mechanisms required for maintenance of optimal steroidogenic capacity, tissue-specific mechanisms which lead to different steroidogenic pathways in different tissues, and developmental mechanisms which lead to fetal imprinting of steroid hydroxylase expression and which probably overlap with both maintenance and tissue-specific mechanisms. Future studies will involve identification of the trans-acting factors associated with each of these aspects of the multifactorial regulation and characterization of the cis-regulatory elements to which they bind. Such studies will inevitably lead to the identification of genes encoding these trans-acting factors and investigation of their regulation. In this way, it will be possible to work outward from the steroid hydroxylase genes toward the cell surface receptors in order to elucidate the series of events which lead to cAMP-dependent and -independent regulation of steroid hydroxylase gene expression.

Effects of dietary sodium restriction and potassium intake on cholesterol side-chain cleavage cytochrome P-450 and adrenodoxin mRNA levels

We studied the effect of dietary sodium restriction (3 weeks) and high potassium intake (7 days) on transcriptional regulation of cytochrome P-450 cholesterol side chain cleavage (P-450 scc) and adrenodoxin (Adx) in rat adrenal glands. Northern blotting analysis demonstrated that both treatments markedly increased P-450scc and Adx mRNA levels in the zona glomerulosa (Z-G) and the zona fasciculata-reticularis (Z-F-R) compared with controls. The Z-G appears to be more sensitive to variations in electrolytes than does the Z-F-R. The low sodium diet provoked a 2.9-fold increase in P-450scc mRNA level in the Z-G compared to 2.1-fold in the Z-F-R, whereas Adx mRNA levels were enhanced 2.2- and 1.7-fold respectively in these two zones. Restriction of sodium intake provoked significant increases in plasma ACTH, aldosterone and corticosteroids compared with controls. In the Z-G of KCl-loaded rats, we found a 1.6-fold increase in P-450scc and a 2.1-fold increase in Adx mRNA levels, and in the Z-F-R there was a 1.7- and 1.8-fold enhancement. There were no changes in beta-actin mRNA levels upon dietary treatments. These results thus indicate that both sodium depletion and high potassium intake in rats could act at the transcriptional level of P-450scc and Adx, two components of a rate-limiting step in steroidogenesis leading to aldosterone production. In addition, the elevation in plasma ACTH level in response to Na+ restriction suggests a possible involvement of ACTH in the response of the adrenal glands to sodium depletion.

Human adrenodoxin reductase: two mRNAs encoded by a single gene on chromosome 17cen----q25 are expressed in steroidogenic tissues

Adrenodoxin reductase is a mitochondrial flavoprotein that receives electrons from NADPH, thus initiating the electron-transport chain serving mitochondrial cytochromes P450. We have cloned and sequenced two human adrenodoxin reductase cDNAs that differ by the presence of six additional codons in the middle of one clone. The sequence in this region indicates that these six extra codons arise by alternative splicing of the pre-mRNA. Southern blot hybridization patterns of human genomic DNA cut with four restriction enzymes indicate that the human genome has only one gene for adrenodoxin reductase. Analysis of a panel of mouse-human somatic cell hybrids localized this gene to chromosome 17cen----q25. The alternatively spliced mRNA containing the six extra codons represents 10-20% of all adrenodoxin reductase mRNA. The expression of the adrenodoxin reductase gene may be stimulated by pituitary tropic hormones acting through cAMP, but its response is quantitatively much less than the responses of P450scc and adrenodoxin.

Synergistic stimulation of pregnenolone synthesis in rat adrenal mitochondria by n-hexane and cardiolipin

n-Hexane and cardiolipin each stimulate pregnenolone production by isolated rat adrenal mitochondria. Following corticotropin (ACTH) stimulation, mitochondrial cholesterol metabolism exhibits a fast phase lasting 2 min, followed by a 10-fold slower metabolism. ACTH suppression by dexamethazone or cycloheximide (CX) treatment removes this fast phase. n-Hexane, at concentrations approaching 80% of the aqueous solubility limit (approximately 0.08 mM), selectively stimulates the slow phase of metabolism, while cardiolipin (100 microM) stimulates only the fast phase. Other alkanes and ethers are effective. The effect of n-hexane is dependent on mitochondrial integrity, as evidenced by decreased effects in hypoosmotically shocked mitochondria (outer membrane disrupted) and ineffectiveness in sonicated mitochondria (both membranes disrupted). n-Hexane apparently enhances the transfer of outer membrane cholesterol to inner membrane P-450scc. Stimulation by cardiolipin is retained by disrupted mitochondria and may involve enhanced availability of P-450scc to inner membrane cholesterol. When added together, these agents produce more than additive effects on cholesterol metabolism. Preincubation with n-hexane did not increase reactive cholesterol, suggesting that enhanced cholesterol transport occurs only in concert with metabolism of inner membrane cholesterol. Uptake of alkanes into mitochondrial membranes may effect structural changes that facilitate outer to inner membrane cholesterol transfer, but major changes are excluded by the effectiveness of isocitrate as a reductant for P-450scc. In combination, n-hexane and cardiolipin reproduce the effect of the ACTH-sensitive sterol regulatory peptide on mitochondria [R. C. Pedersen and A. C. Brownie (1983) Proc. Natl. Acad. Sci. USA 80, 1882-1886], suggesting that peptide action on adrenal mitochondria may resolve into two analogous components.