11 beta-hydroxylation of 18-hydroxy-11-deoxycorticosterone by rat adrenal tissue: zone specificity and effect of sodium and potassium restriction

Hypothesis: aldosterone is synthesized by an alternative pathway during severe sodium depletion. 'A new wine in an old bottle'

1. The last three steps of aldosterone biosynthesis, 11 beta-hydroxylation, 18-hydroxylation and 18-oxidation, have been demonstrated to be catalysed by one enzyme, which is the cytochrome P450(11 beta) (CYP11B) in cow, pig, sheep and bullfrog or cytochrome P450aldo (CYP11B2) in rat, human, mouse and hamster. 2. The related enzyme P450(11 beta) (CYP11B1) from rat, human, mouse and hamster adrenals displays 11 beta-hydroxylation and 18-hydroxylation activities, but not 18-oxidation activity in vitro. No such enzyme has been reported in the cow, pig or sheep to date. 3. Data showing the dissociation of aldosterone secretion from plasma angiotensin II (AngII) levels indicate the presence of other factor(s) that regulate aldosterone biosynthesis in response to changes in body sodium status. Thus, we propose the existence of a 'sodium status factor' that regulates aldosterone biosynthesis in addition to AngII, K+, adrenocorticotropic hormone and atrial natriuretic peptide. 4. We propose that during severe sodium deficiency there is a switch in the aldosterone pathway to a pathway using 18-hydroxy-deoxycorticosterone (18-OH-DOC) rather than corticosterone as an intermediate. This switch may be mediated via the putative 'sodium status factor'. 5. Two models of the hypothesis will be discussed in this paper: (i) a 'one-enzyme' model; and (ii) a 'two-enzyme' model. 6. The one-enzyme model proposes that P450aldo (P450(11 beta) as in the case of the cow, sheep and pig) changes its enzymatic activity during severe sodium deficiency (i.e. switching to the alternative aldosterone biosynthesis pathway). 7. The two-enzyme model proposes that, under normal circumstances, P450aldo synthesizes aldosterone from deoxycorticosterone, while during severe sodium deficiency the P450(11 beta) provides the substrate (i.e. 18-OH-DOC) for the P450aldo.

Functional and expression analysis of ovine steroid 11 beta-hydroxylase (cytochrome P450 11 beta)

In this study, the ovine steroid 11 beta-hydroxylase (P450(11 beta) or CYP11B) cDNA previously reported by us (1) was transfected into COS-7 cells. Using 3H-11-deoxycorticosterone (3H-DOC) as the substrate, and paper partition chromatography for separation of steroid products, the expressed enzyme was shown to catalyse the conversion of DOC to corticosterone (B), 18-hydroxy-11-deoxycorticosterone (18-OH-DOC), 18-hydroxy-corticosterone (18-OH-B), and aldosterone (ALDO). These results suggest that the expressed ovine cDNA exhibited 11 beta-hydroxylase, 18-hydroxylase and aldosterone synthesis activities. The enzymatic activity of the enzyme was further analysed by adding unlabelled steroids to compete with 3H-DOC. The conversion of 3H-DOC to 3H-ALDO was inhibited by the addition of excess DOC, B and 18-OH-DOC, indicating that all these steroids were potential substrates of the enzyme. The results also demonstrated that 18-hydroxylation could occur before 11 beta-hydroxylation with this enzyme. However, the addition of excess cold 18-OH-B had no significant effect on the level of 3H-ALDO that was synthesised. This result could imply that 18-OH-B is not an intermediate involved in the conversion of DOC to aldosterone, or, more likely, the enzyme substrate site is not accessible readily. Our results also indicated that DOC was preferred to 18-OH-DOC as a substrate for the enzyme. We have demonstrated by hybridisation histochemistry using specific oligonucleotide probes that the corresponding P450(11 beta) RNA transcript was present in all zones in the sheep adrenal cortex. In summary, we have shown that the enzyme encoded by the predominant P450(11 beta) cDNA isolated from the sheep adrenocortical cDNA library has all the enzymatic activities to biosynthesise ALDO from DOC. The corresponding transcript of this ovine P450(11 beta) cDNA was located throughout the adrenal cortex and thus the inability of the zonae fasciculata-reticularis to secrete ALDO remains to be understood.

Inhibition of steroidogenesis in rat adrenal cells by 18-ethynyldeoxycorticosterone: evidence for an alternative pathway of aldosterone biosynthesis

The effect of the mechanism-based inhibitor 18-ethynyldeoxycorticosterone (18-E-DOC) on the late steps of the aldosterone biosynthetic pathway was examined in freshly isolated cells of the zona glomerulosa (ZG) and fasciculata (ZF) from rat adrenal glands. ZG synthesis of aldosterone was inhibited by 18-E-DOC in a time- and concentration-dependent manner with a Ki of approximately 0.05 microM. The maximal degree of inhibition of ZG production of aldosterone and 18-hydroxycorticosterone (18-OH-B) was approximately 80%. ZF cells, perhaps surprisingly, were found to secrete 18-OH-B at levels approximately one-third to one-fourth those of ZG cells and the Ki of 18-E-DOC inhibition of 18-OH-B secretion was approximately 10 microM for ZF cells, 200-fold higher than for ZG cells. The inhibitor had no effect on the secretion of corticosterone by either ZG or ZF, and the secretion of 18-hydroxydeoxycorticosterone (18-OH-DOC) by both the ZG and ZF was inhibited only to a minor degree. 18-E-DOC inhibited the biosynthesis of aldosterone by ZG cells incubated with 10 microM added DOC or 18-OH-DOC by approximately 75%, similar to the degree of inhibition of aldosterone biosynthesis from endogenous substrate, whereas ZF biosynthesis of 18-OH-B from either substrate was inhibited by less than 40%. ZF cells do not express aldosterone synthase, the only enzyme known to convert 18-OH-DOC into 18-OH-B. Incubation of MA-10 cells stably transfected with the cDNA of the rat aldosterone synthase with 18-E-DOC resulted in a complete inhibition of the conversion of DOC to aldosterone with a Ki of approximately 0.02 microM. In addition, transfected cells expressing 11beta-hydroxylase convert DOC to 18-OH-B in very small quantities only and cannot convert 18-OH-DOC to 18-OH-B. These data suggest that neither 11beta-hydroxylase nor aldosterone synthase are responsible for the biosynthesis of 18-OH-B by ZF cells from DOC or 18-OH-DOC, that 20% of aldosterone synthesis appears not to be attributable to the actions of aldosterone synthase and that an unknown CYP11B enzyme is also involved in the biosynthesis of 18-OH-B.

Inhibition of aldosterone formation by cortisol in rat adrenal mitochondria

In this work we confirm by a metabolic method the existence of at least two enzymes with 11 beta- and 18-hydroxylase activities in rat adrenal mitochondria. The method was based on the ability of cortisol (F), a foreign alternative substrate, to inhibit competitively metabolite productions from various precursors. F inhibited a) aldosterone (ALDO) production from 11-deoxycorticosterone (DOC) without affecting the yields of corticosterone (B) and 18-hydroxy-11-deoxycorticosterone (18-OHDOC); b) 18-hydroxycorticosterone and aldosterone productions from B (Ki = 2.5 +/- 0.5 microM); and c) ALDO production from 18-OHDOC. These results suggest the existence of two categories of enzymes with both 11 beta- and 18-hydroxylase activities, one comprising those that catalyze the conversions of DOC to B and 18-OHDOC (F-insensitive reactions [FIS]) and the other one comprising the enzymes involved in the conversions of B to 18-OHB and ALDO and that of 18-OHDOC to ALDO (F-sensitive reactions [FS]). The cloned enzymes CYP11B1 and CYP11B2 would pertain respectively to the FIS and FS categories.

Final steps of aldosterone biosynthesis: molecular solution of a physiological problem

The final two steps of aldosterone biosynthesis play a key role in the complex physiological adaptation of aldosterone secretion to changes in sodium and potassium content of the mammalian organism. The nature and identity of the enzyme catalyzing these steps have only recently been established. In the rat as well as in the human adrenal, two types of cytochrome P-450(11 beta) are encoded by two different genes. The major type of the enzyme catalyzes only the conversion of deoxycorticosterone to corticosterone or 18-hydroxy-11-deoxycorticosterone; it is present in all the zones of the adrenal cortex. The second type of the enzyme catalyzes the three steps involved in the conversion of deoxycorticosterone to aldosterone and occurs only in the zona glomerulosa. In rat zona glomerulosa cells, separate control systems independently regulate the expression of the two genes, according to long-term in vivo experiments or to experiments with primary cultures of zona glomerulosa cells. Expression of the non-aldosterone-producing enzyme is induced by ACTH, whereas the expression of the aldosterone-producing enzyme is dependent on the extracellular potassium concentration.

Transcriptional activation of adrenocortical steroidogenic genes by high potassium or low sodium intake

We have previously shown that the long-term alterations in the intake of sodium and potassium which stimulated aldosterone production in the rat adrenal significantly increased cytochrome P450scc (P450scc) and P45011 beta (P45011 beta) mRNA's and also the mRNA of their electron donor adrenodoxin. In the present study run-on analyses showed an accumulation of nascent RNA in isolated nuclei of zona glomerulosa cells in K(+)-supplemented and Na(+)-depleted rats for P450scc (5- and 6-fold), 3 beta-HSD (3.6- and 2.0-fold) and P45011 beta (6.0- and 6.1-fold), but not for P450c21 (1.4- and 1.1-fold). In contrast, that of adrenodoxin decrease (0.6-fold) in high K+ and remained near control (1.3-fold) in low Na+ intake. Moderate variations in the rate of transcription of P450scc, P450c21, P45011 beta and adrenodoxin genes were observed in the zona fasciculata-reticularis cells of the treated rats. Our results thus demonstrated that positive modulators of aldosterone such as long-term K+ supplementation and Na+ restriction provoked an increase in transcription of the genes encoding key regulatory steroidogenic enzymes of aldosterone biosynthesis in the zona glomerulosa. The rates of transcription of the genes encoding 3 beta-HSD and P450c21, two enzymes catalyzing intermediate steps in the aldosterone pathway, were moderately affected by such treatments. However, according to the known stimulation of adrenodoxin mRNA levels following these treatments, a decreased turnover of the adrenodoxin mRNA rather than initiation of transcription of its gene might be involved in the response to K+ ions, and partially so in the response to Na+ restriction. Finally, the effects of salt-modified intake were mainly restricted to the zona glomerulosa cells, which are solely responsible for aldosterone production.

In vivo regulation of gene expression of enzymes controlling aldosterone synthesis in rat adrenal

We studied the effect of alterations in the intake of sodium and potassium as well as changes in circulating adrenocorticotropin (ACTH) on the expression of the two rate-limiting systems of aldosterone formation in the rat. Low sodium and high potassium intake promoted time-dependent increases in the zona glomerulosa cytochrome P450scc (P450scc) and cytochrome P450c11 (P450c11) protein and mRNA levels, but no changes were found in the zona fasciculata-reticularis. In addition, these responses were associated with markedly elevated transcriptional activities. To further define the contribution of P450c11 and P450c18 (aldosterone synthase) in response to these differing intakes, we evaluated their mRNA levels using gene-specific oligonucleotide probes. P450c18 mRNA was restricted to the zona glomerulosa, whereas P450c11 mRNA was detected in both zona glomerulosa and zona fasciculata-reticularis. Furthermore, only P450c18 mRNA was induced by both low sodium or high potassium intake, as P450c11 mRNA levels remained unchanged. Captopril, an inhibitor of angiotensin-I converting enzyme, abolished the enhancing effects of the low sodium regimen on P450scc and P450c18 mRNA levels. Captopril also suppressed the augmentation of P450c18 mRNA observed with potassium supplementation but had no effect on P450scc mRNA levels. When the hypocholesterolemic drug 4-aminopyrazolopyrimidine (4-APP) was administered to rats for 3 consecurive days, both the level of plasma ACTH and the adrenal content of mRNA encoding P450scc increased 24 h post final injection. The coadministration of dexamethasone with 4-APP prevented these increases. In contrast, the mRNA content of P450c11 remained at control levels. In conclusion, this work demonstrates that variations in the intake of sodium and potassium act on the expression of the CYP11B2 gene, but not on that of the CYP11B1 gene. Moreover angiotensin-II (A-II) is an important factor in this mechanism of action. Both ions also enhance the expression of the CYP11A1 gene. A-II appears to participate in the mechanism of action of the low sodium intake at this level. Another mechanism is postulated for the action of potassium supplementation since captopril did not prevent the increased expression of the CYP11A1 gene. In addition, the fact that 4-APP enhanced the mRNA level of P450scc but not that of P450c, also demonstrates different regulation of the P450s involved at the early and final steps of aldosteroone formation in the rat adrenal zona glomerulosa in vivo.

Two forms of cytochrome P-450(11 beta) in rat zona glomerulosa cells: a short review

Aldosterone, the major mineralocorticoid hormone, is produced exclusively in the zona glomerulosa of the mammalian adrenal cortex. In the rat species, this zonal specificity of aldosterone biosynthesis appears to be due mainly to the existence of a second form of cytochrome P-450(11 beta), which differs from the major form of the enzyme (molecular weight 51,000) by (1) a lower molecular weight (49,000), (2) a broader range of catalytic activities, which include corticosterone methyl oxidation 1 and 2, (3) an exclusive occurrence in the zona glomerulosa, and (4) a crucial dependence on sodium and potassium intake. The 49K form of the enzyme can be induced by potassium ions in vivo (potassium repletion of potassium-deficient rats) or in vitro (primary cell culture). The biosynthesis of this protein is controlled most likely at the level of transcription. According to indirect evidence, ACTH induces only the 51K form of the enzyme in vitro. Prolonged treatment of rats with a high dose of ACTH has a repressive effect on the 49K form of the enzyme.

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.

Adaptation of aldosterone biosynthesis to sodium and potassium intake in the rat

The steroidogenic response of rat adrenal zona glomerulosa to stimulators is variable and depends on the activity of biosynthetic steps involved in the conversion of deoxycorticosterone (DOC) to aldosterone (Aldo). Corticosterone methyl oxidations (CMO) 1 and 2 are stimulated by sodium restriction and suppressed by potassium restriction. These slow alterations are accompanied by the appearance or disappearance of a specific zona glomerulosa mitochondrial protein with a molecular weight of 49,000. Induction of CMO 1 and 2 activities and the appearance of the 49 K protein can also be elicited in vitro by culture of rat zone glomerulosa cells in a medium with a high potassium concentration. The 49 K protein crossreacts with a monoclonal antibody raised against purified bovine adrenal cytochrome P-450(11 beta). The same antibody stains a protein with a molecular weight of 51,000 in rat zona fasciculata mitochondria and in zone glomerulosa mitochondria of rats in which CMO 1 and 2 activities have been suppressed by potassium restriction and sodium loading. The 51 K crossreactive protein was purified to electrophoretic homogeneity by chromatography on octyl-sepharose. In a reconstituted enzyme system, it converted DOC to corticosterone (B) and to 18-hydroxy-11-deoxycorticosterone (18-OH-DOC) but not to 18-hydroxycorticosterone (18-OH-B) or Aldo. A partially purified 49 K protein preparation from zona glomerulosa mitochondria of rats kept on a low-sodium, high-potassium regimen converted DOC to B, 18-OH-DOC, 18-OH-B and Aldo. According to these results, rat adrenal cytochrome P-450(11 beta) exists in two different forms, with both of them capable of hydroxylating DOC in either the 11 beta- of the 18-position, but with only the 49 K form capable of catalyzing CMO 1 and 2. The adaptation of aldosterone biosynthesis to sodium deficiency or potassium intake in rats is due to the appearance of the 49 K form of the enzyme in zona glomerulosa mitochondria.

19-Hydroxylation of 18-hydroxy-11-deoxycorticosterone by adrenal mitochondria prepared from various animal species

We have recently reported that bovine adrenocortical cytochrome P-45011 beta catalyzes 19-hydroxylation of 18-hydroxy-11-deoxycorticosterone (18(OH)DOC) in addition to 11 beta-hydroxylation of the steroid. In this report, we examine the presence of these two activities in 18(OH)DOC and 11 beta- and 18-hydroxylation activities on deoxycorticosterone (DOC) among the adrenal mitochondria prepared from man, ox, pig, rabbit, guinea-pig and rat. The results indicate that these animals could be classified into three groups with respect of these hydroxylation activities. Mitochondria of the first group comprising ox and pig showed rather high 19- and 11 beta-hydroxylation activities on 18(OH)DOC compared to the hydroxylation activities on DOC. Mitochondria prepared from the second group which comprised rabbit, guinea-pig and man showed low 19-hydroxylation activity on 18(OH)DOC, whereas the 11 beta-hydroxylation of 18(OH)DOC well occurred in these species. The last group comprising rat had very low activity both of 11 beta- and 19-hydroxylations when 18(OH)DOC was used as the substrate, whereas both 11 beta- and 18-hydroxylations of DOC were high in rat adrenal mitochondria. No significant difference of these activities could be found between zona glomerulosa cells and zonae fasciculata-reticularis cells of bovine adrenal cortex, and between adrenal mitochondria from spontaneously hypertensive rat and those from WKY normotensive rat.

Copper response to rheumatoid arthritis

Rheumatoid arthritis can be divided into two syndromes, one a potassium deficiency, the other an inappropriate copper requirement seriously affecting the elastin tissues through reduced lysyl oxidase cross linking. The malfunction in copper may arise from the steroids which regulate potassium, which reduces those steroids, and through that, increases the copper response to the needs of the immune system. It is a mechanism which may have evolved to help fight potassium wasting infections.

A potassium-induced mitochondrial protein related to aldosterone biosynthesis

Late steps of aldosterone biosynthesis, i.e., the conversions of corticosterone to 18-hydroxycorticosterone and aldosterone, are catalyzed by a mitochondrial cytochrome P-450. Resumption of potassium intake by potassium-depleted rats resulted within 2 days in a marked stimulation of these conversions, as reflected by increased production of aldosterone and 18-hydroxycorticosterone and decreased outputs of deoxycorticosterone, corticosterone, and 18-hydroxy-11-deoxycorticosterone by incubated capsular portions of the adrenal glands. The stimulation of aldosterone biosynthesis was accompanied by the appearance of a protein with a molecular weight of about 49,000 in the mitochondria of the zona glomerulosa but not of the inner zones of the adrenal cortex. Over 48 h of potassium repletion, the amount of this protein increased in parallel with the activity of the final steps of aldosterone biosynthesis. According to its molecular weight, its zone specificity, and the time course of its appearance, this protein might represent the steroid 18-methyl oxidase (cytochrome P-450CMO for corticosterone methyl oxidase) that catalyzes the conversion of corticosterone to 18-hydroxycorticosterone and aldosterone.

Regulation of 18-hydroxycorticosterone formation in bovine adrenocortical mitochondria

The conversion of deoxycorticosterone to 18-hydroxycorticosterone was examined by using sonicated mitochondrial suspension of bovine adrenocortex. The KM's of deoxycorticosterone for the 11 beta- and 18-hydroxylations were in the same range (1 microM), while the turnover number for the 11 beta-hydroxylation (50 nmol/min/nmol cytochrome P-450) was 6 times as great as that for the 18-hydroxylation (7.3). The KM and turnover number for the 18-hydroxylation of corticosterone were 6 microM and 0.4, respectively. Those for the 11 beta-hydroxylation of 18-hydroxy-11-deoxycorticosterone were 120 microM and 5. When products were analysed in the incubation of deoxycorticosterone with the mitochondrial suspension containing a larger amount of cytochrome P-450, the formation of 18-hydroxycorticosterone was observed in addition to the formation of corticosterone and 18-hydroxy-11-deoxycorticosterone. The kinetic interrelation between the two pathways was further examined together with consideration of the cytochrome P-450-linked hydroxylation system. The result suggests that the pathway via 18-hydroxy-11-deoxycorticosterone substantially participates in the formation of 18-hydroxycorticosterone from deoxycorticosterone. The perturbation of this network by an artificial means, such as the addition of Triton X-100, revealed that the detergent (0.02%) facilitated the production of 18-hydroxycorticosterone from deoxycorticosterone, regardless of its inhibitory effect on the production of corticosterone and 18-hydroxy-11-deoxycorticosterone from deoxycorticosterone. These studies provide an important insight into the regulation mechanism of 18-hydroxycorticosterone formation from the precursors on the mitochondrial level.

Potassium intake and aldosterone biosynthesis: the role of cytochrome P-450

K+ Repletion for 48 h of rats previously kept on a low K+ diet for 2 weeks specifically increased the conversion of corticosterone into aldosterone and 18-hydroxycorticosterone by incubated capsular fractions of rat adrenal tissue. This increase in the activity of the final steps of aldosterone biosynthesis was not accompanied by an increase in capsular adrenal mitochondrial cytochrome P-450 concentration. By contrast, an increased corticosterone-induced absorbance change (BI) was consistently found in capsular adrenal mitochondria upon K+ repletion. In addition, a type I-like absorbance change was induced with 18-hydroxy-11-deoxycorticosterone but not with 18-hydroxycorticosterone. Therefore, K+ repletion of K+ depleted rats specifically increased the binding of corticosterone and possibly 18-hydroxy-11-deoxycorticosterone to the 18-methyl oxidase enzyme complex. Whether this increased binding was due to an increase in enzyme protein concentration or due to a better availability of the substrate to the enzyme, could not be decided from these experiments.