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

Interacting influence of potassium and polychlorinated biphenyl on cortisol and aldosterone biosynthesis

Giving human adrenocortical H295R cells 14 mM KCl for 24 h significantly induced not only aldosterone biosynthesis but also cortisol biosynthesis. Pre-treating the cells with polychlorinated biphenyl 126 (PCB126) further increased potassium-induced aldosterone and cortisol productions in a dose-dependent manner, but all examined concentrations of PCB126 had little effect on the yields of precursor steroids progesterone and 17-OH-progesterone. Subsequent examinations revealed that CYP11B1 and CYP11B2 genes, responsible for the respective final steps of the cortisol and aldosterone biosynthetic pathways, exhibited increased responsiveness to PCB126 under high potassium. While 10(-5) M PCB126 was needed to induce a significant increase in the basal mRNA abundance of either gene, PCB126 could enhance potassium-induced mRNA expression of CYP11B1 at 10(-7) M and CYP11B2 at 10(-9) M. Actually, potassium and PCB126 synergistically upregulated mRNA expression of both genes. Potassium raised the transcriptional rates of CYP11B1 and CYP11B2 probably through a conserved Ad5 cis-element, whereas PCB126 appeared to regulate these two genes at the post-transcriptional level. Positive potassium-PCB126 synergism was also detected in CYP11B2 enzyme activity estimated by aldosterone/progesterone ratio. In contrast, potassium and PCB126 increased CYP11B1 enzyme activity or cortisol/17-OH-progesterone ratio additively. Moreover, potassium improved the time effect of PCB126 on gene expression and enzyme activity of CYP11B2, but not the PCB126 time response of CYP11B1. These data demonstrated that potassium differentially enhanced the potency of PCB126 to induce CYP11B1- and CYP11B2-mediated steroidogenesis.

P450c11B3 mRNA, transcribed from a third P450c11 gene, is expressed in a tissue-specific, developmentally, and hormonally regulated fashion in the rodent adrenal and encodes a protein with both 11-hydroxylase and 18-hydroxylase activities

The rat genome contains four P450c11 genes. One of these (CYP11B1) encodes P450c11 beta, which is the steroid 11 beta-hydroxylase found solely in the adrenal zona fasciculata/reticularis, and is responsible for the conversion of 11-deoxycorticosterone to corticosterone. A second P450c11 gene (CYP11B2) encodes P450c11AS, which is the aldosterone synthase found solely in the adrenal zona glomerulosa. P450c11AS has three activities, 11 beta-hydroxylase, 18-hydroxylase, and 18-oxidase, and is responsible for the conversion of 11-deoxycorticosterone to aldosterone. Recently, two more rat P450c11 genes, P450c11B3 and P450c11B4, were cloned. P450c11B4 appears to be a pseudogene, as two exons are replaced by unrelated DNA. P450c11B3 closely resembles P450c11 beta in mRNA and encoded amino acid sequences, predicting a protein of 498 amino acids. However, the expression of this mRNA and protein have not been demonstrated to date. We now demonstrate that this P450c11B3 mRNA is expressed in the adrenal gland several days after birth and is not expressed during fetal development or in the adult rat adrenal. Like P450c11 beta mRNA, P450c11B3 mRNA is expressed in the zona fasciculata/reticularis and not in the zona glomerulosa. However, the regulation of P450c11B3 mRNA expression is different from that of P450c11 beta mRNA, in that its abundance is decreased by ACTH in a sex-dependent fashion. Transfection of eukaryotic cells with a vector expressing P450c11B3 shows that this form of P450c11 can convert 11-deoxycorticosterone (DOC) to corticosterone and thus has the same enzymatic activity as P450c11 beta. In addition, P450c11B3 can convert DOC to 18-OH DOC and corticosterone to 18-OH corticosterone and thus has 18-hydroxylase activity similar to P450c11AS, but it lacks detectable 18-oxidase activity. Thus, P450c11B3 catalyzes 11 beta- and 18-hydroxylation and thus has a spectrum of activities midway between P450c11 beta and P450c11AS.

Role of adrenal renin in the regulation of adrenal steroidogenesis by corticotropin

The major regulator of mineralocorticoid production in the adrenal is angiotensin II produced by the action of renal renin. The discovery that the rodent adrenal also synthesizes renin and angiotensinogen suggests there is autocrine regulation of mineralocorticoid synthesis. The transgenic rat [TGR(mREN2)27] expresses the Ren-2d gene predominantly in the adrenal. Despite suppressed kidney and plasma renin, these animals develop fulminant hypertension between 5 and 15 weeks of age. Corticosteroid concentrations are significantly elevated during hypertension development. We assessed steroidogenesis in TGR(mREN2)27 rats by analyzing the expression of the mRNAs for three steroidogenic enzymes: P450scc, the rate-limiting step of steroidogenesis; P450c11 beta, which converts deoxycorticosterone to corticosterone in the zona fasciculata/reticularis; and P450c11AS, which converts deoxycorticosterone to aldosterone in the zona glomerulosa. P450c11AS mRNA, but neither P450c11 beta nor P450scc mRNA, was overexpressed in the adrenal gland of TGR(mREN2)27 rats. In situ hybridization with specific probes for P450c11 beta and P450c11AS mRNA localized the former exclusively to the zona fasciculata and the latter to the zona glomerulosa. In TGR(mREN2)27 rats, the size of the adrenal and number of P450c11AS-expressing zona glomerulosa cells were about twice those of a normal Sprague-Dawley rat. Both animals respond to corticotropin similarly; corticotropin had no effect on the expression of P450scc and P45011 beta mRNAs, rendered P450c11AS mRNA undetectable, and simultaneously altered the morphology of the adrenal cortex, resulting in a lack of zona glomerulosa-like cells. Thus, the local renin-angiotensin system has a major effect on the basal expression of P450c11AS mRNA, but little effect on the corticotropin-regulated expression of P450scc, P450c11 beta, and P450c11AS mRNAs.

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.

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.

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.

Influence of captopril on adrenal cytochrome P-450s and adrenodoxin expression in high potassium or low sodium intake

To investigate the role of the renin-angiotensin system in steroidogenic enzyme expression, the angiotensin-I converting enzyme inhibitor captopril was administered in conjunction with high potassium (K+) or low (Na+) intake to rats for a 7-day period. Northern blot analysis of adrenocortical zona glomerulosa RNA revealed that sodium restriction markedly increased mRNA production of P-450scc (3.1-fold) and P-450(11 beta) (3.4-fold) as well as of the electron donor adrenodoxin (2.0-fold). Captopril combined to the low Na+ diet led to suppression of these effects and, as also seen with captopril alone, further diminished P-450(11 beta) mRNA levels below controls. These responses were accompanied by parallel changes in respective protein levels of the enzymes as indicated by Western blot analyses. Captopril was also shown to inhibit the K(+)-stimulated levels of P-450(11 beta) mRNA (3.3-fold) and protein (1.4-fold) beneath control values (0.6- and 0.8-fold, respectively). On the other hand, increased P-450scc mRNA and protein levels by K+ loading were not affected by captopril treatment. No response was observed in any steroidogenic enzyme expression in zona fasciculata-reticularis following either diet with or without captopril. Thus, the inhibitory effect of captopril on stimulated steroidogenesis seemed to be mediated in part through transcriptional regulation of P450s. In addition, it appeared that P-450(11 beta) expression might be under the control of the renin-angiotensin system in both high K+ and low Na+ diets as opposed to the K+ stimulation of P-450scc where other mechanisms seemed to be involved.

Zone-specific regulation of two messenger RNAs for P450c11 in the adrenals of pregnant and nonpregnant rats

Adrenal mitochondria possess two steroidogenic cytochrome P450s. P450c11 converts deoxycorticosterone to corticosterone and aldosterone, and P450scc converts cholesterol to pregnenolone. These P450s receive electrons from NADPH via adrenodoxin reductase and adrenodoxin. A single bovine P450c11 protein has 11-hydroxylase, 18-hydroxylase, and 18-oxidase activities, but this series of enzymatic steps may be mediated by more than one enzyme in rats. Enzymatic assays of purified rat mitochondrial proteins have suggested that one enzyme found in all zones of the adrenal cortex has both 11- and 18-hydroxylase activities, whereas another enzyme, found exclusively in the zona glomerulosa, catalyzes 18-hydroxylation and 18-oxidation of corticosterone. We studied the number and zonal distribution of P450c11 mRNA species in the rat adrenal and how these mRNAs are regulated in the adrenals of normal and pregnant rats. Rats synthesize two similar, but distinct, P450c11 mRNAs. One, P450c11A, is found in both the zona glomerulosa and fasciculata/reticularis, whereas the second, P450c11B, is found only in the zona glomerulosa. The abundance of neither P450c11A mRNA nor P450c11B mRNA is affected by a high-salt diet. However, when rats receive a low-salt diet, P450c11A mRNA decreases and P450c11B mRNA increases. Dexamethasone decreases the amount of P450c11A mRNA without affecting P450c11B mRNA. The combination of a high-salt diet and dexamethasone decreases the amount of both mRNAs further to almost undetectable amounts. Rats given a low-salt diet and dexamethasone have a dramatic increase in the abundance of P450c11B mRNA. Thus both forms of P450c11 mRNA are regulated independently in the rat adrenal cortex. In situ hybridization studies show that only the P450c11 found in the zona glomerulosa is regulated by salt treatment in vivo, whereas glucocorticoid treatment in vivo regulates P450c11 in all zones. In the adrenals of pregnant rats, P450c11B is regulated in a similar fashion to its regulation in the nonpregnant rat adrenal, despite major differences in sodium retention and intravascular volume in pregnant and nonpregnant rats. In the pregnant rat, a low-salt diet increases the abundance of P450c11B to a greater degree than in the nonpregnant rat. By contrast, dexamethasone does not diminish the abundance of P450c11A mRNA in the pregnant rat but reduces it to an almost undetectable amount in the nonpregnant rat. Thus, the regulation of glucocorticoid and mineralocorticoid production in the pregnant and nonpregnant rat occurs by different mechanisms.