Enzyme-activated inhibitors of steroidal hydroxylases

Cytochrome P450 monooxygenases (CYP450) of the steroid biosynthetic pathways are highly substrate specific in comparison to the variable specificities of hepatic CYP450 enzymes. Both groups of enzymes catalyze the reductive cleavage of molecular oxygen with transfer of oxygen to the substrate to form hydroxylated derivatives. Those steroids formed in endocrine tissues represent highly specific endocrine/autocrine hormones with enhanced biological potency, while hepatic hydroxylation of steroids reduces their endocrine bioactivities and enhances urinary elimination. Changes of the hormonal milieu of endocrine and peripheral tissues are associated with the development of hyperplastic and/or malignant conditions. Hormone deprivation induces regression of endocrine dependent growth via apoptosis and may also alter growth of hormone insensitive cells by the induction of negative growth factors. Biosynthetic CYP450 enzymes of those steroids that mediate specific disease processes are potential therapeutic targets for selective intervention. This objective can be accomplished by the design of specific pseudo-substrate analogs that will be activated during enzyme-directed catalysis to produce a reactive functional group in the enzyme's active site that will either tightly or irreversibly bind and inactivate the host enzyme. The CYP450 enzymes that hydroxylate the C19 carbon of androgens (aromatase) and the C18 carbon of corticosterone (aldosterone synthase) were selected as target enzymes because they are terminal enzymes of biosynthetic pathways which hydroxylate specific angular methyl groups. Hypersecretion of their respective hormonal products, estrogens and aldosterone, are associated with specific disease conditions. Substrate analogs containing ethynyl, vinyl, or nitrile groups attached to the C19 or C18 methyl groups were enzyme-activated inhibitors. The ethynyl analogs, 19-acetylenic androstenedione (Plomestane) and 18-acetylenic deoxycorticosterone, had nanomolar inhibitory constants (Ki values) and were irreversible inactivators of their target enzymes in animal models.

Aldosterone biosynthesis and action in vascular cells

In view of the hypothetical possibility that the vascular renin-angiotensin system (RAS) might include aldosterone biosynthesis and action in the vasculature, we have undertaken a study to identify aldosterone released into the perfusion circuit from the rat mesenteric artery, and to investigate the effects of an angiotensin converting enzyme inhibition (ACEI) on aldosterone production from the vasculature. After 30 min equilibration, 240 mL of perfusate was collected and subjected to reverse-phase HPLC and subsequent mass spectrometry. Mass spectra corresponding to authentic corticosterone and aldosterone were obtained from the samples of mesenteric artery perfusate. Production of aldosterone in the mesenteric artery was not changed by adrenalectomy, although it was reduced in the arterial perfusate from rats pretreated with ACEI. By RT-PCR the expression of CYP 11B2 and mineralocorticoid receptor genes were demonstrated in both vascular endothelial and smooth muscle cells. These studies constitute indirect evidence supporting our hypothesis that locally produced aldosterone in the vascular tissue acts on vascular tone and remodeling via a paracrine or autocrine manner.

Inborn errors of aldosterone biosynthesis in humans

Corticosterone methyl oxidase (CMO) type I and type II deficiencies are inborn errors at the penultimate and ultimate steps in the biosynthesis of aldosterone in humans. Recently, steroid 18-hydroxylase (P450C18), or aldosterone synthase (P450aldo), was shown to be a multifunctional enzyme catalyzing these two steps of aldosterone biosynthesis, i.e., the conversion of corticosterone to 18-hydroxycorticosterone and the subsequent conversion of 18-hydroxycorticosterone to aldosterone. This observation suggests that CMO I and CMO II deficiencies are derived from two different mutations in the P450C18 gene (CYP11B2). To elucidate whether or not this is the case, we performed molecular genetic studies on CYP11B2 of both types of patients. Nucleotide sequence analysis has indicated that the gene of CMO I deficient patients is completely inactivated by a frameshift to form a stop codon due to a 5-bp nucleotide deletion in exon 1. Sequence analysis of CYP11B2 of CMO II deficient patients has revealed two point mutations, CGG-->TGG (Arg181-->Trp) in exon 3 and GTG-->GCG (Val386-->Ala) in exon 7. CYP11B1, the gene for steroid 11 beta-hydroxylase (P45011 beta) which was previously postulated to be the target for CMO II deficiency, is not impaired in these two types of patients. Expression studies using the corresponding mutant cDNAs have shown that CMO I deficient patients are null mutants with a complete lack of P450C18 whereas CMO II deficient patients are leaky mutants with an altered P450C18 activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldosterone: the minority hormone of the adrenal cortex

In comparison with the glucocorticoids cortisol and corticosterone, the mineralocorticoid aldosterone is a minority hormone of the mammalian adrenal cortex, and its proper function is dependent upon protective physiological mechanisms. These include a particular site of aldosterone synthesis in zona glomerulosa cells as well as a complex multifactorial control system, which adapts aldosterone production to acute and chronic changes in body sodium and potassium contents, irrespective of pituitary ACTH secretion. In the course of the last few years, an important element of these mechanisms has been identified in the form of the enzyme involved in the final steps of aldosterone biosynthesis. In species such as the human, rat, and mouse, the conversion of deoxycorticosterone to aldosterone is catalyzed by an isozyme (CYP11B2) of cytochrome P450(11 beta) (CYB11B1). The gene encoding this enzyme is expressed only in the zona glomerulosa. Its transcription is enhanced by sodium deficiency and potassium intake, but is suppressed by long-term administration of high doses of ACTH. In contrast, the gene encoding CYP11B1, i.e., the major (non-aldosterone-producing) type of the enzyme, is expressed mainly in the zona fasciculata, and its expression depends on physiological concentrations of ACTH. In other animal species (cattle, pig), the major forms of cytochrome P450(11) beta have an inherent aldosterone-synthesizing activity, which is, however, selectively suppressed in mitochondria of zona fasciculata cells by yet unknown mechanisms.

Dissecting hypertension: the role of the 'new genetics'

The tools of molecular genetics have recently been applied to hypertension, a common multifactorial disorder, with some success. Glucocorticoid-suppressible hyperaldosteronism, an inherited form of human hypertension due to the dominant inheritance of a chimaeric steroid 11 beta-hydroxylase/aldosterone synthase gene, has given an insight into the possible genetic factors involved in essential hypertension. Study of the aldosterone synthase and steroid 11 beta-hydroxylase genes has shown the presence of polymorphisms in both of these genes in human subjects; further studies may demonstrate genetic mutations with pathophysiological effects in patients with essential hypertension.

Inhibition of adrenal cytochromes P450 by 1-aminobenzotriazole in vitro. Selectivity for xenobiotic metabolism

Studies were done to determine the effects of a P450 suicide inhibitor, 1-aminobenzotriazole (ABT), on adrenal steroid and xenobiotic metabolism. Incubation of guinea pig adrenal microsomes with ABT plus an NADPH-generating system caused a time-dependent decline in total P450 concentrations. The maximal decrease in P450 levels was approximately 35% and was accompanied by an equimolar decrease in heme content. Western blot analyses indicated that ABT had no effect on P450 apoprotein levels. Benzphetamine (BZ) N-demethylase and benzo[a]pyrene (BP) hydroxylase activities were inhibited almost completely by microsomal incubations with ABT. In contrast, neither steroid 17 alpha-hydroxylase nor 21-hydroxylase activity was affected by ABT. The steroid-induced type I spectral change in adrenal microsomes also was not affected by ABT, whereas that induced by BZ was eliminated. Similar studies with adrenal mitochondria indicated that ABT had no effect on mitochondrial P450 concentrations or on mitochondrial steroid metabolism. The results demonstrate that the in vitro actions of ABT on adrenal cytochromes P450 are highly selective for those isozymes that catalyze xenobiotic metabolism. Therefore, ABT should serve as a useful probe for further characterization of adrenal xenobiotic-metabolizing P450 isozymes.

Enhanced adrenal renin and aldosterone biosynthesis during sodium restriction in TGR (mREN2)27

The aim of the study was to investigate the relationships between tissue renin and the steroid production in the adrenal cortex during dietary sodium restriction in the transgenic rat (TGR) (mREN2)27. Thus the effects of a 1-wk low-sodium intake (0.04% NaCl) were studied in 5-wk-old male TGR (n = 33, systolic blood pressure = 151 +/- 3 mmHg) and in 24 age- and sex-matched outbred normotensive Sprague-Dawley (SD) rats. Measurements of plasma and tissue hormones were obtained at 0, 4, and 7 days of a low-sodium diet. Sodium restriction caused sustained increases of adrenal renin activity (from 28.5 +/- 3.5 to 87.5 +/- 4.5 ng.mg protein-1.h-1 on day 7) and of adrenal renin mRNA (+63 +/- 13 and +43 +/- 7% on days 4 and 7, respectively), whereas plasma renin activity (from 3.3 +/- 0.3 to 4.4 +/- 0.6 ng.ml-1.h-1) and renal renin activity (from 0.85 +/- 0.25 to 0.7 +/- 0.4 microgram.mg protein-1.h-1) did not change. The stimulation of the adrenal renin-angiotensin system was associated with a large increase of the aldosterone synthase cytochrome P-450 mRNA (+165 +/- 35 and +184 +/- 44%, on days 4 and 7) and of plasma aldosterone levels (from 125 +/- 32 to 338 +/- 59 pg/ml, P < 0.01). In SD rats, in spite of a more consistent increase in renal and circulating renin, mineralocorticoid production did not increase significantly. These results demonstrate that the exaggerated biosynthesis of aldosterone in TGR during sodium restriction is associated with an activation of renin in the adrenal cortex but not in the kidney.

Vascular aldosterone. Biosynthesis and a link to angiotensin II-induced hypertrophy of vascular smooth muscle cells

Mineralocorticoids have been suggested to act on blood vessels, leading to increased vasoreactivity and peripheral resistance. However, the site of their production has so far been believed to be only the adrenal cortex. Here, we show direct evidence that vascular cells per se are aldosteronogenic, possessing their own system that responds to the steroid. Using polymerase chain reaction after reverse transcription, the CYP11B2 mRNA encoding the key enzyme for the biosynthesis of aldosterone was detected in both endothelial cells and smooth muscle cells cultivated from human pulmonary artery. The aldosterone receptor (type 1 mineralocorticoid receptor) gene was also found to be expressed in smooth muscle cells and, to a lesser extent, in endothelial cells. CYP11B2 gene expression in smooth muscle cells was stimulated by angiotensin II, the effector peptide of the renin-angiotensin system. Furthermore, the angiotensin II-induced increase in [3H]leucine incorporation in smooth muscle cells was significantly enhanced by aldosterone but inhibited by ZK 91587, a type 1 mineralocorticoid receptor antagonist. This may indicate that vascular aldosterone participates in the angiotensin II-induced hypertrophy of vascular smooth muscle cells. The present study therefore provides the starting point for a novel understanding of the molecular basis of vascular remodeling and hypertension.

Disorders of steroid 11 beta-hydroxylase isozymes

The most active corticosteroids are 11 beta-hydroxylated. Humans have two isozymes with 11 beta-hydroxylase activity that are respectively required for cortisol and aldosterone synthesis. CYP11B1 (11 beta-hydroxylase) is expressed at high levels and is regulated by ACTH, whereas CYP11B2 (aldosterone synthase) is normally expressed at low levels and is regulated by angiotensin II. In addition to 11 beta-hydroxylase activity, the latter enzyme has 18-hydroxylase and 18-oxidase activities and thus can synthesize aldosterone from deoxycorticosterone. Insights into the normal functioning of these enzymes are gained from studies of disorders involving them. Mutations in the CYP11B1 gene cause steroid 11 beta-hydroxylase deficiency, a form of congenital adrenal hyperplasia characterized by signs of androgen excess and by hypertension. Mutations in CYP11B2 result in aldosterone synthase (corticosterone methyloxidase) deficiency, an isolated defect in aldosterone biosynthesis that can cause hyponatremia, hyperkalemia, and hypovolemic shock in infancy and failure to thrive in childhood. These are both recessive disorders. Unequal crossing over between the CYP11B genes can generate a duplicated chimeric gene with the transcriptional regulatory region of CYP11B1 but sufficient coding sequences from CYP11B2 so that the encoded enzyme has aldosterone synthase (i.e. 18-oxidase) activity. This results in aldosterone biosynthesis being regulated by ACTH, a condition termed glucocorticoid-suppressible hyperaldosteronism. This form of genetic hypertension is inherited in an autosomal dominant manner.

Molecular cloning and characterization of the ovine CYP11B1 promoter

The promoter region of the ovine P45011 beta gene has been cloned and sequenced. This clone contained 2068 bp of the regulatory region and 232 bp of the exon 1. Comparison of this sequence to other P45011 beta promoter sequences revealed six elements that may confer cAMP responsiveness and cell-specific expression to the ovine P45011 beta gene. We obtained consensus sequences for a number of cis-acting elements needed to mediate cAMP responsiveness to the gene. We also report the presence of two new elements, CRE2 and a TRE, located far upstream from the translational site that may participate in the regulation of the ovine P45011 beta gene.

Rapid diagnosis of glucocorticoid suppressible hyperaldosteronism in infants and adolescents

Glucocorticoid suppressible hyperaldosteronism (GSH) is an uncommon form of dominantly inherited hypertension. Presentation with hypertension and complications such as stroke in early life are well recognised. The use of a simple genetic test carried out on blood or placenta facilitates the detection of infants and children with GSH before the development of hypertension, allowing prompt treatment of hypertension if it occurs, and an opportunity to study the effects of growth and environmental influences on the progression of the condition.

A novel cell layer without corticosteroid-synthesizing enzymes in rat adrenal cortex: histochemical detection and possible physiological role

A stratum of cells that did not contain both aldosterone synthase cytochrome P450 (cytochrome P450aldo) and cytochrome P45011 beta was found immunohistochemically between the zona glomerulosa and the zona fasciculata of the rat adrenal cortex. As cytochromes P450aldo and P45011 beta are the enzymes responsible for the biosynthesis of aldosterone and corticosterone, respectively, the cells there are considered to be incapable of synthesizing both aldosterone and corticosterone. Furthermore, the cells are regarded as inert in producing adrenal androgens, because rat adrenal cortex is known to lack steroid 17 alpha-hydroxylase. Thus, the stratum is composed of cells that do not synthesize any of the major corticosteroids in significant quantities. It was 5-10 cells thick under normal feeding conditions, but diminished to 4-5 cells thick when animals were maintained under Na restriction, which is known to stimulate the secretion of angiotensin-II. When the distribution of 5-bromo-2'-deoxyuridine-labeled nuclei in the adrenocortex from BrdU-administered rats was examined, the stained nuclei were concentrated in and around the cell stratum. The pulse-chase experiments showed that the labeled cells migrated out of this layer and into the zonae fasciculata-reticularis. On the basis of these findings, we suggest that the newly discovered cell layer is the progenitor cell zone of the rat adrenal cortex.

The presence of two cytochrome P450 aldosterone synthase mRNAs in the hamster adrenal

We isolated a cDNA from a hamster adrenal cDNA library which was similar in sequence to those of the mouse and rat P450c18 cDNAs. The hamster P450c18 cDNA, however, was shorter than the rat and mouse P450c18 cDNAs at its 5'-end and the peptide leader sequence was absent. From a hamster genomic library we isolated and sequenced the first seven exons and a 5'-flanking region of the first P450c18 gene exon. With this information we were able to generate a P450c18 cDNA containing the peptide leader sequence using the polymerase chain reaction. Northern analyses were performed on adrenals from hamsters maintained on a low sodium diet for 0, 4, 7 and 10 days using a 32P-labeled sequence specific to P450c18; two mRNA bands were found at 2 and 3.4 kb. The intensity of both bands was increased about 3- to 5-fold under sodium restriction compared to controls. A distinct mRNA band of 2.3 kb hybridized with an oligonucleotide specific to P450(11) beta and its intensity did not change following low sodium intake. Immunoblotting analyses were performed using an antibovine adrenal P450(11) beta antibody that does not discriminate between P450(11) beta and P450c18 proteins. Three bands were detected at 52, 48 and 45 kDa in homogenate preparations of entire glands. Furthermore, the 45 kDa protein band was present in homogenates of the zona glomerulosa and absent in homogenates of the zone fasciculata-reticularis. In conclusion, these results show that the hamster adrenals express P450c18 as do mouse, rat and human adrenal glands. Furthermore, two P450c18 mRNAs, which are inducible by a low sodium intake, are present in the hamster adrenal vs one for the rat. The physiological role of these two hamster adrenal mRNA species remains to be elucidated.

Inhibition of aldosterone synthesis in rat adrenal cells by nicotine and related constituents of tobacco smoke

Previous work has shown that nicotine and related constituents of tobacco smoke inhibit selected P450 enzymes in the glucocorticoid and sex steroid synthetic pathways. Because aldosterone synthesis is also cytochrome P450 dependent, we hypothesized a similar inhibitory action on aldosterone production. In this study we examined the effects of nicotine, anabasine (a related alkaloid), and cotinine (the major metabolite of nicotine) on in vitro aldosterone synthesis. Freshly isolated rat adrenal cells were assayed for corticosterone and aldosterone production in the basal state and after stimulation with ACTH or angiotensin-II (ANG-II). The addition of nicotine, anabasine, and cotinine in concentrations up to 100 microM did not inhibit stimulated corticosterone production. However, there was a potent dose-dependent inhibitory action of all three tobacco compounds on aldosterone production. The relative inhibitory potency was: cotinine > anabasine > nicotine. When employed at a concentration of 100 microM, the three compounds inhibited ACTH-stimulated aldosterone synthesis by 75%, 44%, and 21%, respectively. ANG-II-stimulated aldosterone synthesis was inhibited by 92%, 78%, and 62%, respectively. The plasma cotinine concentration range attained in tobacco smokers is between 1-10 microM. When tested with [3H]corticosterone and [3H]progesterone as exogenous substrates, 1-10 microM cotinine caused a significant dose-dependent inhibition of ACTH- and ANG-II-stimulated aldosterone synthesis. Cotinine substantially blocked the conversion of corticosterone to 18-hydroxycorticosterone, implicating the 18-hydroxylase or corticosterone 18-methyloxidase-I (CMO-I) step as the major site of inhibition. In summary, our results indicate that tobacco compounds cause direct and specific inhibition of aldosterone synthesis, primarily at the CMO-I step. This enzymatic blockade would be expected to result in activation of the renin-angiotensin system in vivo. We postulate that chronic stimulation of the renin-angiotensin system by this mechanism might contribute to the cardiovascular damage that occurs with long term tobacco use.

[Mental deterioration and hypertension: uncommon manifestation of corticotropin insufficiency and functional carboxymethyl oxidase block]

We report a new case of muscle contractures associated with adrenocortical deficiency. Outstanding features were the diffusion of the contractures, rhabdomyolysis and an encephalopathy which disappeared with hormonal therapy. Endocrinological investigations revealed a functional carboxymethyl oxidase type II defect which could, in part, explain our patient's neuromuscular symptoms.

Immunological analysis of developmental changes in ecdysone 20-mono-oxygenase expression in the cotton leafworm, Spodoptera littoralis

The developmental changes in ecdysone 20-mono-oxygenase during the sixth larval instar of the cotton leafworm, Spodoptera littoralis, were investigated. The specific activity of mitochondrial ecdysone 20-mono-oxygenase in the fat-body exhibited a distinct peak at 72 h, at which time the larvae stop feeding. Immunoblot analyses, using antibodies raised against components of vertebrate mitochondrial steroidogenic enzyme systems [anti-(cytochrome P-450scc), anti-(cytochrome P-450(11) beta), anti-adrenodoxin and anti-(adrenodoxin reductase) antibodies], revealed the presence of specific immunoreactive polypeptides in fat-body mitochondrial extracts. In addition, these antibodies effectively inhibited fat-body mitochondrial ecdysone 20-mono-oxygenase activity. This suggests that the S. littoralis steroid-hydroxylating system(s) may contain polypeptide components analogous to those present in vertebrates. A close correlation between developmental changes in mitochondrial ecdysone 20-mono-oxygenase activity and the abundance of polypeptides (approx. 66 kDa and 50 kDa) recognized by the anti-(cytochrome P-450(11) beta) antibody and a polypeptide (approx. 52 kDa) recognized by the anti-(adrenodoxin reductase) antibody were observed in both fat-body and midgut. These results suggest that developmental changes in the abundance of components of the ecdysone 20-mono-oxygenase system may play an important role in the developmental regulation of the enzyme expression and, hence, of 20-hydroxyecdysone titre.

The hybrid rat cytochrome P450 containing the first 5 exons of the CYP11B1 and last 4 exons from the CYP11B2 enzyme retains 11 beta-hydroxylase activity, but the alternative hybrid is inactive

Human, mouse and rats have 2 different cytochrome P-450 11 beta-hydroxylases in the adrenal cortex. The classical rat 11 beta-hydroxylase or CYP11B1 enzyme hydroxylates deoxycorticosterone to corticosterone and 18-hydroxydeoxycorticosterone and is located throughout the adrenal. The second aldosterone synthase or CYP11B2 enzyme is located in the zona glomerulosa and converts deoxycorticosterone to corticosterone, 18-hydroxycorticosterone and aldosterone. In rat the coding nucleotide sequence and the deduced amino acid sequences of the CYP11B1 and CYP11B2 genes are homologous by 88% and 83%, respectively. We have constructed two different hybrid cDNAs by exchanging two fragments of the rat CYP11B1 and CYP11B2 at the junction of the 5/6 exon and expressed them in COS 7 cells. The hybrid CYPH11B1 construct containing the first 5 exons of the CYP11B1 when expressed, retains 11 beta-hydroxylase activity, but cannot process corticosterone to 18-hydroxycorticosterone or aldosterone. The hybrid CYPH11B2 construct containing the first 5 exons of the CYP11B2 enzyme when expressed is inactive.

Both low sodium and high potassium intake increase the level of adrenal angiotensin-II receptor type 1, but not that of adrenocorticotropin receptor

Angiotensin-II (AII), a component of the renin-angiotensin system, is the major factor that regulates the formation of aldosterone in the adrenal cortex zona glomerulosa (ZG). The activity of this system is increased by an increase in potassium intake or a decrease in sodium intake. Using immunoblotting analysis, we determined whether these ions affect the expression of type 1 AII receptors (AT1) and compared the results thus obtained with the AT1 receptor mRNA levels. We also studied the interrelation among AII, AT1 receptors, cytochrome P450 aldosterone synthase (P450c18), and plasma aldosterone levels in rats fed a normal diet or a low sodium or high potassium diet with or without captopril, an inhibitor of angiotensin-converting enzyme, for 7 days. The effects of ions on the level of ACTH receptor mRNA were also analyzed. We found that a low sodium intake increased plasma aldosterone levels from 5.5 to 236 ng/dl and led to 2.3- and 3.7-fold increases in the levels of adrenal ZG AT1 receptor protein and AT1 receptor mRNA, whereas a 11.8-fold increase was found in the level of P450c18 mRNA. Captopril almost completely reversed these effects. We have shown that a high potassium intake increased plasma aldosterone levels to 25.9 ng/dl and also led to 1.84- and 1.95-fold increases in the level of ZG AT1 receptor protein and AT1 receptor mRNA, whereas the ZG P450c18 mRNA level was increased 3.5-fold. The plasma aldosterone level of animals fed a high diet of potassium and captopril was still higher than that in control animals at 16.6 ng/dl, and the levels of ZG AT1 receptor and P450c18 mRNAs were only slightly less than those of the high potassium groups, indicating that captopril did not efficiently block aldosterone formation under these conditions. ACTH receptor mRNA levels remain unaffected by either low sodium or high potassium intake. Collectively, these results indicate that the increased aldosterone secretion induced by low sodium or high potassium intake involves concomitant increases in AT1 receptor and P450c18 mRNAs, which are effectively translated into their respective proteins, and that the expression of both proteins is mediated in part by AII.

Separate induction of the two isozymes of cytochrome P-450(11) beta in rat adrenal zona glomerulosa cells

In the rat adrenal cortex, two isozymes of cytochrome P-450(11) beta (CYP11B1 and CYP11B2) have been identified. They are encoded by two different genes with a homology much higher in their coding than in their 5'-flanking regions. CYP11B1 is found in all the zones of the gland and catalyzes a single hydroxylation of deoxycorticosterone (DOC) in the 11 beta- or the 18-position. CYP11B2 is produced exclusively in the zona glomerulosa and catalyzes all three reactions involved in the conversion of DOC to aldosterone. In vivo and in vitro, the expression of the genes encoding CYP11B1 and CYP11B2 is regulated by two separate control systems which appear to operate both independently and interdependently. In vivo zona glomerulosa expression of CYP11B1 was enhanced by ACTH treatment or potassium depletion and was lowered by potassium repletion. CYP11B2 expression disappeared upon potassium depletion or ACTH treatment, but reappeared during potassium repletion. In vitro, only CYP11B1 activity was detectable and responsive to ACTH treatment in zona glomerulosa cells cultured at a potassium concentration of 6.4 mmol/l. Aldosterone biosynthetic activity and mRNA encoding CYP11B2 could be detected only after at least 1 day of exposure to a high extracellular potassium concentration (> or = 12 mmol/l).

Sodium restriction increases aldosterone biosynthesis by increasing late pathway, but not early pathway, messenger ribonucleic acid levels and enzyme activity in normotensive rats

To determine whether changes in dietary sodium intake modify the early and/or late pathways of aldosterone biosynthesis, we studied in Sprague-Dawley rats the effect of sodium restriction on early (conversion of cholesterol to pregnenolone) and late (conversion of corticosterone to aldosterone) pathway activity and on the mRNA levels for the enzymes regulating these steps. Sodium restriction increased basal and angiotensin-II-stimulated aldosterone output from isolated zona glomerulosa cells by 5- to 9-fold. This increase in aldosterone output did not appear to be due to changes in the conversion of cholesterol to pregnenolone or in the mRNA levels of the early pathway enzyme, cholesterol side-chain cleavage cytochrome P-450. In contrast, sodium restriction increased the conversion of corticosterone to aldosterone 10-fold and increased by over 10-fold the mRNA levels of the late pathway enzyme aldosterone synthase. Sodium restriction had no effect on zona glomerulosa levels of 11 beta-hydroxylase mRNA. In two other normotensive rats, Dahl salt-resistant and Wistar Kyoto, sodium restriction again specifically increased aldosterone synthase mRNA without altering 11 beta-hydroxylase or cholesterol side-chain cleavage cytochrome P-450 mRNA levels. Thus, it appears that sodium restriction specifically increases late pathway aldosterone synthase mRNA levels, resulting in an increase in enzyme levels, followed by an increase in late pathway activity and an increase in aldosterone output.

Human NCI-H295 adrenocortical carcinoma cells: a model for angiotensin-II-responsive aldosterone secretion

Excessive secretion of aldosterone from the adrenal results in the most common form of endocrine hypertension. An understanding of the regulatory processes involved in aldosterone synthesis and release is needed to define the biomolecular mechanisms controlling excessive production of aldosterone. However, in vitro studies regarding the regulatory mechanisms of human aldosterone production have been limited because of difficulties in obtaining tissue and the subsequent isolation of aldosterone-secreting glomerulosa cells. Herein we describe an adrenocortical carcinoma cell line, NCI-H295, which provides a suitable angiotensin-II (AII)-responsive model system to investigate the acute and chronic regulation of aldosterone synthesis. The cells were characterized with regard to the effects of AII on second messenger systems, aldosterone release, and levels of aldosterone synthase (P450c18) mRNA. In the presence of lithium, AII caused a rapid, but transient, increase in the production of inositol tris- and bisphosphates, whereas a prolonged gradual accumulation of inositol monophosphate occurred. Treatment with AII resulted in a 4.5-fold increase in total inositol phosphates in a concentration-dependent manner and an increase in intracellular cytoplasmic free Ca2+. Significant increases in aldosterone (3.5-fold) were detected within 1 h of AII addition. Aldosterone release occurred in a concentration-and time-dependent manner. The type 1 AII (AT1) receptor was shown to be responsible for activation of phosphoinositidase-C, increased intracellular free Ca2+, and aldosterone production, as determined by use of the AT1 receptor antagonist DuP753. In addition, AII treatment resulted in a time-dependent increase in levels of P450c18 mRNA, as detected by RNAse protection assay. In summary, NCI-H295 cells provide a valuable model system to define mechanisms regulating human aldosterone production.

Angiotensin increases aldosterone synthase mRNA levels in human NCI-H295 cells

Understanding the regulation of aldosterone secretion has been hampered by the lack of a cell culture system that remains chronically responsive to angiotensin stimulation. NCI-H295 cells, cultured from a human adrenocortical tumor, express the three major pathways of adrenal steroidogenesis and produce small amounts of aldosterone during basal culture. We have determined changes in aldosterone production and in aldosterone synthase (AS, P45011B2) mRNA levels in these cells in response to angiotensin II (AII) and forskolin. Culture of NCI-H295 cells with 10(-7) M AII or with 10(-5) M forskolin stimulated aldosterone production and increased AS mRNA levels, though the effect of AII was greater. When cells were cultured with increasing concentrations of AII from 10(-11) through 10(-8) M, a dose-dependent increase in AS mRNA levels paralleled increases in aldosterone production. In view of these findings, these human adrenocortical cells should be useful for exploring mechanisms regulating aldosterone production.