Pathology and gene mutations of aldosterone-producing lesions

The human adrenal cortex secretes aldosterone and cortisol as major corticosteroids. For their production, CYP11B2 and CYP11B1 catalyze the last steps in the syntheses of aldosterone and cortisol, respectively. In our previous study, CYP11B2 was the first successfully purified from rat adrenals and human clinical samples and then was proved to be aldosterone synthase. We demonstrated the immunohistochemistry for CYP11B2 of both rats and humans and applied it clinically to visualize the functional histology of aldosterone-producing adenoma (APA) causing primary aldosteronism (PA). We discovered aldosterone-producing cell clusters (APCCs) and possible APCC-to-APA transitional lesions (pAATLs) and further visualized aldosterone-producing lesions for rare forms of PA including familial hyperaldosteronism type 3 and novel non-familial juvenile PA. Here we review the history of our research on aldosterone-producing lesions.

Immunohistochemistry of aldosterone synthase leads the way to the pathogenesis of primary aldosteronism

Our group previously purified human and rat aldosterone synthase (CYP11B2 and Cyp11b2, respectively) from their adrenals and verified that it is distinct from steroid 11β-hydroxylase (CYP11B1 or Cyp11b1), the cortisol- or corticosterone-synthesizing enzyme. We now describe their distributions immunohistochemically with specific antibodies. In rats, there is layered functional zonation with the Cyp11b2-positive zona glomerulosa (ZG), Cyp11b1-positive zona fasciculata (ZF), and Cyp11b2/Cyp11b1-negative undifferentiated zone between the ZG and ZF. In human infants and children (<12 years old), the functional zonation is similar to that in rats. In adults, the adrenal cortex remodels and subcapsular aldosterone-producing cell clusters (APCCs) replace the continuous ZG layer. We recently reported possible APCC-to-APA transitional lesions (pAATLs) in 2 cases of unilateral multiple adrenocortical micro-nodules. In this review, we present 4 additional cases of primary aldosteronism, from which the extracted adrenals contain pAATLs, with results of next generation sequencing for these lesions. Immunohistochemistry for CYP11B2 and CYP11B1 has become an important tool for the diagnosis of and research on adrenocortical pathological conditions and suggests that APCCs may be the origin of aldosterone-producing adenoma.

Adrenal nodularity and somatic mutations in primary aldosteronism: one node is the culprit?

CONTEXT

Somatic mutations in genes that influence cell entry of calcium have been identified in aldosterone-producing adenomas (APAs) of adrenal cortex in primary aldosteronism (PA). Many adrenal glands removed for suspicion of APA do not contain a single adenoma but nodular hyperplasia.

OBJECTIVE

The objective of the study was to assess multinodularity and phenotypic and genotypic characteristics of adrenals removed because of the suspicion of APAs.

DESIGN AND METHODS

We assessed the adrenals of 53 PA patients for histopathological characteristics and immunohistochemistry for aldosterone (P450C18) and cortisol (P450C11) synthesis and for KCNJ5, ATP1A1, ATP2B3, and CACNA1D mutations in microdissected nodi.

RESULTS

Glands contained a solitary adenoma in 43% and nodular hyperplasia in 53% of cases. Most adrenal glands contained only one nodule positive for P450C18 expression, with all other nodules negative. KCNJ5 mutations were present in 22 of 53 adrenals (13 adenoma and nine multinodular adrenals). An ATP1A1 and a CACNA1D mutation were found in one multinodular gland each and an ATP2B3 mutation in five APA-containing glands. Mutations were always located in the P450C18-positive nodule. In one gland two nodules containing two different KCNJ5 mutations were present. Zona fasciculata-like cells were more typical for KCNJ5 mutation-containing nodules and zona glomerulosa-like cells for the other three genes.

CONCLUSIONS

Somatic mutations in KCNJ5, ATP1A1, or CACNA1D genes are not limited to APAs but are also found in the more frequent multinodular adrenals. In multinodular glands, only one nodule harbors a mutation. This suggests that the occurrence of a mutation and nodule formation are independent processes. The implications for clinical management remain to be determined.

Different effects of progesterone and estradiol on chimeric and wild type aldosterone synthase in vitro

BACKGROUND

Familial hyperaldosteronism type I (FH-I) is caused by the unequal recombination between the 11beta-hydroxylase (CYP11B1) and aldosterone synthase (CYP11B2) genes, resulting in the generation of a CYP11B1/B2 chimeric gene and abnormal adrenal aldosterone production. Affected patients usually show severe hypertension and an elevated frequency of stroke at a young age. Aldosterone levels rise during pregnancy, yet in pregnant women with FH-1, their hypertensive condition either remains unchanged or may even improve. The purpose of this study was to investigate in vitro whether female sex steroids modulate the activity of chimeric (ASCE) or wild type (ASWT) aldosterone synthase enzymes.

METHODS

We designed an in vitro assay using HEK-293 cell line transiently transfected with vectors containing the full ASCE or ASWT cDNAs. Progesterone or estradiol effects on AS enzyme activities were evaluated in transfected cells incubated with deoxycorticosterone (DOC) alone or DOC plus increasing doses of these steroids.

RESULTS

In our in vitro model, both enzymes showed similar apparent kinetic parameters (Km = 1.191 microM and Vmax = 27.08 microM/24 h for ASCE and Km = 1.163 microM and Vmax = 36.98 microM/24 h for ASWT; p = ns, Mann-Whitney test). Progesterone inhibited aldosterone production by ASCE- and ASWT-transfected cells, while estradiol demonstrated no effect. Progesterone acted as a competitive inhibitor for both enzymes. Molecular modelling studies and binding affinity estimations indicate that progesterone might bind to the substrate site in both ASCE and ASWT, supporting the idea that this steroid could regulate these enzymatic activities and contribute to the decay of aldosterone synthase activity in chimeric gene-positive patients.

CONCLUSIONS

Our results show an inhibitory action of progesterone in the aldosterone synthesis by chimeric or wild type aldosterone synthase enzymes. This is a novel regulatory mechanism of progesterone action, which could be involved in protecting pregnant women with FH-1 against hypertension. In vitro, both enzymes showed comparable kinetic parameters, but ASWT was more strongly inhibited than ASCE. This study implicates a new role for progesterone in the regulation of aldosterone levels that could contribute, along with other factors, to the maintenance of an adequate aldosterone-progesterone balance in pregnancy.

Selenium compound protects corneal epithelium against oxidative stress

The ocular surface is strongly affected by oxidative stress, and anti-oxidative systems are maintained in corneal epithelial cells and tear fluid. Dry eye is recognized as an oxidative stress-induced disease. Selenium compound eye drops are expected to be a candidate for the treatment of dry eye. We estimated the efficacy of several selenium compounds in the treatment of dry eye using a dry eye rat model. All of the studied selenium compounds were uptaken into corneal epithelial cells in vitro. However, when the selenium compounds were administered as eye drops in the dry eye rat model, most of the selenium compounds did not show effectiveness except for Se-lactoferrin. Se-lactoferrin is a lactoferrin that we prepared that binds selenium instead of iron. Se-lactoferrin eye drops suppressed the up-regulated expression of heme oxygenase-1, cyclooxygenase-2, matrix metallopeptidase-9, and interleukin-6 and also suppressed 8-OHdG production in the cornea induced by surgical removal of the lacrimal glands. Compared with Se-lactoferrin, apolactoferrin eye drops weakly improved dry eye in high dose. The effect of Se-lactoferrin eye drops on dry eye is possibly due to the effect of selenium and also the effect of apolactoferrin. Se-lactoferrin is a candidate for the treatment of dry eye via regulation of oxidative stress in the corneal epithelium.

Adrenocortical zonation in humans under normal and pathological conditions

CONTEXT

Aldosterone synthase (CYP11B2) and steroid 11 beta-hydroxylase (CYP11B1) catalyze the terminal steps for aldosterone and cortisol syntheses, respectively, thereby determining the functional differentiation of human adrenocortical cells. Little is known, however, about how the cells expressing the enzymes are actually distributed in the adrenals under normal and pathological conditions.

OBJECTIVE

The objective of the study was to determine the localization of CYP11B2 and -B1 in human adrenal specimens by using developed antibodies capable of distinguishing the two enzymes from each other.

RESULTS

Under normal conditions, CYP11B2 was sporadically detected in the zona glomerulosa, whereas CYP11B1 was entirely detected in the zonae fasciculata-reticularis. Adrenocortical cells lacking both enzymes were observed in the outer cortical regions. In addition to conventional zonation, we found a variegated zonation consisting of a subcapsular cell cluster expressing CYP11B2, which we termed aldosterone-producing cell cluster, and a CYP11B1-expressing area. Aldosterone-producing adenomas differed in cell populations expressing CYP11B2 from one another, whereas CYP11B1-expressing and double-negative cells were also intermingled. Adenomas from patients with Cushing's syndrome expressed CYP11B1 entirely but not CYP11B2, resulting in atrophic nontumor glands. The nontumor portions of both types of adenomas bore frequently one or more aldosterone-producing cell clusters, which sustained CYP11B2 expression markedly under the conditions of the suppressed renin-angiotensin system.

CONCLUSION

Immunohistochemistry of the human normal adrenal cortex for CYP11B2 and CYP11B1 revealed a variegated zonation with cell clusters constitutively expressing CYP11B2. This technique may provide a pathological confirmatory diagnosis of adrenocortical adenomas.

Superoxide dismutase analog (Tempol: 4-hydroxy-2, 2, 6, 6-tetramethylpiperidine 1-oxyl) treatment restores erectile function in diabetes-induced impotence

We hypothesized that administration of the superoxide dismutase (SOD) mimetic Tempol (4-hydroxy-2, 2, 6, 6-tetramethylpiperidine 1-oxyl) may reverse diabetes induced ED(erectile dysfunction). To test this hypothesis, ROS related genes (SOD1, SOD2, GPx1, CAT, NOS2, NOS3), erectile functional studies, and immunohistochemical analysis were performed in diabetic rats treated with or without Tempol. Thirty Sprague-Dawley (3–4 months old) rats were divided into 3 groups (n=10 each), 20 with diabetes (diabetic control and Tempol treatment) and 10 healthy controls. Twelve weeks after induction of diabetes by streptozotocin and Tempol treatment, all groups underwent in vivo cavernous nerve stimulation. Rat crura were harvested and expression of antioxidative defense enzymes examined by semi-quantitative RT-PCR. To confirm the RT-PCR results, we performed immunohistochemistry (IHC) for catalase (CAT) and iNOS (NOS2). Nitration of tyrosine groups in proteins was also examined by IHC. Mean intracavernous pressure in the diabetic group was significantly lower than in healthy controls (p<0.001) and was reversed by Tempol treatment (p<0.0108). NOS2 protein expression was significantly increased in diabetic animals compared to healthy controls and Tempol restored NOS2 protein level. Nitrotyrosine was also higher in diabetic animals and though Tempol treatment decreased its formation, it remained higher than that found in healthy controls. This study suggests that Tempol treatment increased erectile function through modulating oxidative stress related genes in diabetic rats. This is the first report about the relationship between diabetes induced erectile dysfunction and oxidative stress, and anti-oxidative therapy using the superoxide dismutase mimetic, Tempol to restore erectile function.

Cytochrome P-450(17alpha) in beta-cells of rat pancreas and its local steroidogenesis

We have found cytochrome P-450(17alpha) in the islets of Langerhans of rat pancreas. Its existence coincided with that of insulin and demarcated those of glucagon and somatostatin, demonstrating the localization in beta-cells. The enzyme has not only 17alpha-hydroxylase activity but also lyase one, which is a prerequisite for androgen biosynthesis. The pancreatic microsomes converted progesterone mainly to androstenedione with a minor production of 17alpha-hydroxyprogesterone. Due to a low activity of the built-in lyase, cytochrome P-450(17alpha) requires a sufficient electron-transfer from P-450 reductase or presence of an activator to promote the C-C bond cleavage. In beta-cells, P-450 reductase was abundant and could efficiently transfer electrons to P-450(17alpha). Actually, inhibition with anti-P-450 reductase or limitation of NADPH preferentially reduced the lyase activity. Androstenedione was accumulated when its further metabolism was suppressed. We also found localization of cytochrome P-450scc and 3beta-hydroxysteroid dehydrogenase in beta-cells. These results indicate that the immediate substrate for androgen formation, progesterone, is intracellularly produced and is converted mainly to androstenedione with support by an efficient electron supply from P-450 reductase. The product was supposed to be further metabolized to the reduced derivatives such as testosterone, 5alpha-androstanedione, and dihydrotestosterone, which would act as local steroids in the islets of Langerhans.

Melanocortin 2 receptor is required for adrenal gland development, steroidogenesis, and neonatal gluconeogenesis

ACTH (i.e., corticotropin) is the principal regulator of the hypothalamus-pituitary-adrenal axis and stimulates steroidogenesis in the adrenal gland via the specific cell-surface melanocortin 2 receptor (MC2R). Here, we generated mice with an inactivation mutation of the MC2R gene to elucidate the roles of MC2R in adrenal development, steroidogenesis, and carbohydrate metabolism. These mice, the last of the knockout (KO) mice to be generated for melanocortin family receptors, provide the opportunity to compare the phenotype of proopiomelanocortin KO mice with that of MC1R-MC5R KO mice. We found that the MC2R KO mutation led to neonatal lethality in three-quarters of the mice, possibly as a result of hypoglycemia. Those surviving to adulthood exhibited macroscopically detectable adrenal glands with markedly atrophied zona fasciculata, whereas the zona glomerulosa and the medulla remained fairly intact. Mutations of MC2R have been reported to be responsible for 25% of familial glucocorticoid deficiency (FGD) cases. Adult MC2R KO mice resembled FGD patients in several aspects, such as undetectable levels of corticosterone despite high levels of ACTH, unresponsiveness to ACTH, and hypoglycemia after prolonged (36 h) fasting. However, MC2R KO mice differ from patients with MC2R-null mutations in several aspects, such as low aldosterone levels and unaltered body length. These results indicate that MC2R is required for postnatal adrenal development and adrenal steroidogenesis and that MC2R KO mice provide a useful animal model by which to study FGD.

Haeme-regulated degradation of delta-aminolevulinate synthase 1 in rat liver mitochondria

Protein turnover, which occurs at various rates, is critical for the homeostasis of cellular protein levels. However, the proteolysis systems that determine the turnover rate of mitochondrial proteins are largely unknown. Delta-aminolevulinic acid synthase (ALAS) 1, a rate-limiting enzyme in the haeme biosynthesis, is one of the mitochondrial proteins that have a very short lifetime. In this study, to reveal the regulatory mechanisms for ALAS1 degradation, we examined the turnover rates of ALAS1 in rat liver under several conditions. In primary rat hepatocytes, the degradation of ALAS1 was stimulated by haeme, and suppressed by inhibition of haeme biosynthesis. Furthermore, the haeme-stimulated degradation of ALAS1 was observed in the isolated mitochondria. These results suggested that, in mitochondria, there exists an ALAS1 degradation system that is regulated by cellular haeme level and plays a crucial role in the regulation of haeme biosynthesis.

Spatial orientation of mitochondrial processing peptidase and a preprotein revealed by fluorescence resonance energy transfer

Mitochondrial processing peptidase (MPP), which is composed of heterodimeric alpha-MPP and beta-MPP subunits. It specifically recognizes mitochondrial preproteins and removes their basic N-terminal signal prepeptides. In order to elucidate the spatial orientation of the preproteins toward MPP, which has been missed by crystal structures of a yeast MPP including a synthetic prepeptide in its acidic proteolytic chamber, we analysed the fluorescence resonance energy transfer (FRET) between EGFP fused to a yeast aconitase presequence (preEGFP) and regiospecific 7-dietylamino-3-(4'-maleimidyl phenyl)-4-methyl coumarin (CPM)-labelled yeast MPPs. FRET efficiencies of 65 and 55% were observed between the EGFP chromophore and CPM-Ser(84) and -Lys(156) of beta-MPP, respectively, leading to calculated distances between the molecules of 48 and 50 A, respectively. Considering the FRET results and the structural validity based on the crystal structure of the MPP-presequence complex, a plausible model of preEGFP associated with MPP was constructed in silico. The modelled structure indicated that amino acid residues on the C-terminal side of the cleavage site in the preprotein were orientated tail out from the large cavity of MPP and interacted with the glycine-rich loop of alpha-MPP. Thus, MPP orientates preproteins at the specific cleft between the catalytic domain and the flexible glycine-rich loop which seems to pinch the extended polypeptide.

A protein from a parasitic microorganism, Rickettsia prowazekii, can cleave the signal sequences of proteins targeting mitochondria

The obligate intracellular parasitic bacteria rickettsiae are more closely related to mitochondria than any other microbes investigated to date. A rickettsial putative peptidase (RPP) was found to resemble the alpha and beta subunits of mitochondrial processing peptidase (MPP), which cleaves the transport signal sequences of mitochondrial preproteins. RPP showed completely conserved zinc-binding and catalytic residues compared with beta-MPP but barely contained any of the glycine-rich loop region characteristic of alpha-MPP. When the biochemical activity of RPP purified from a recombinant source was analyzed, RPP specifically hydrolyzed basic peptides and presequence peptides with frequent cleavage at their MPP-processing sites. Moreover, RPP appeared to activate yeast beta-MPP so that it processed preproteins with shorter presequences. Thus, RPP behaves as a bifunctional protein that could act as a basic peptide peptidase and a somewhat regulatory protein for other protein activities in rickettsiae. These are the first biological and enzymological studies to report that a protein from a parasitic microorganism can cleave the signal sequences of proteins targeted to mitochondria.

Suppression of fetal testicular cytochrome P450 17 by maternal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin: a mechanism involving an initial effect on gonadotropin synthesis in the pituitary

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the fetal expression of testicular cytochrome P450 17 (CYP17), one of the enzymes necessary for sex steroid synthesis, was studied in Wistar rats. Fetal testicular CYP17 exhibited reduced mRNA and protein levels following exposure of the dams at gestational day 15 to 1 microg/kg TCDD. In support of this, CYP17 activity catalyzed by fetal testis homogenate was also reduced by maternal exposure to TCDD. The reduction in CYP17 expression seemed to be specific for fetal stages, because 7 day-old pups born from TCDD-treated dams did not exhibit any reduction in CYP17. In sharp contrast to the in vivo observations, TCDD failed to reduce CYP17 expression in cultured fetal testis, although CYP17 could be induced by activating cAMP-dependent signaling. To assess the role of pituitary luteinizing hormone (LH) on TCDD-induced reduction in fetal testicular CYP17, a further investigation was performed to examine whether the direct injection of LH into fetuses restores the altered CYP17 expression. The results showed that in utero injection of equine chorionic gonadotropin, an LH-mimicking hormone, completely abolishes the TCDD-produced reduction in fetal CYP17. However, neither the alpha- nor beta-subunits of LH in cultured fetal pituitary was reduced by TCDD. These results suggest that 1) maternal exposure to TCDD impairs the expression of testicular CYP17 in a fetal stage-specific manner; 2) this effect is due, at least partially, to a TCDD-produced reduction in circulating LH; and 3) TCDD exerts such an effect by affecting the upstream mechanism regulating the pituitary synthesis of LH.

Ascorbate stimulates monooxygenase-dependent steroidogenesis in adrenal zona glomerulosa

It is well known that ascorbic acid (Asc) is highly concentrated in the adrenal gland, but its function in the gland is not thoroughly elucidated. We therefore examined the possibility that Asc participates in steroidogenic monooxygenase systems of the adrenal cortex with the aid of the regenerating system including outer mitochondrial membrane cytochrome b (OMb). When Asc availability was limited in rat mutants unable to synthesize Asc, the increase in plasma aldosterone concentration under Na-deficiency was suppressed without effect on plasma corticosterone concentration. Aldosterone formation in the isolated mitochondrial fraction of the zona glomerulosa (zG) of the adrenal cortex was stimulated by the addition of Asc and NADH, while corticosterone formation was not. Consistently zG showed a high level of Asc regeneration activity and was rich in OMb among adrenocortical zones. Taken together, the enhanced aldosterone formation that is catalyzed by one of the steroidogenic monooxygenases, P450aldo, may be supported by Asc with its regenerating system.

Recognition and processing of a nuclear-encoded polyprotein precursor by mitochondrial processing peptidase

The nuclear-encoded protein RPS14 (ribosomal protein S14) of rice mitochondria is synthesized in the cytosol as a polyprotein consisting of a large N-terminal domain comprising preSDHB (succinate dehydrogenase B precursor) and the C-terminal RPS14. After the preSDHB-RPS14 polyprotein is transported into the mitochondrial matrix, the protein is processed into three peptides: the N-terminal prepeptide, the SDHB domain and the C-terminal mature RPS14. Here we report that the general MPP (mitochondrial processing peptidase) plays an essential role in processing of the polyprotein. Purified yeast MPP cleaved both the N-terminal presequence and the connector region between SDHB and RPS14. Moreover, the connector region was processed more rapidly than the presequence. When the site of cleavage between SDHB and RPS14 was determined, it was located in an MPP processing motif that has also been shown to be present in the N-terminal presequence. Mutational analyses around the cleavage site in the connector region suggested that MPP interacts with multiple sites in the region, possibly in a similar manner to the interaction with the N-terminal presequence. In addition, MPP preferentially recognized the unfolded structure of preSDHB-RPS14. In mitochondria, MPP may recognize the stretched polyprotein during passage of the precursor through the translocational apparatus in the inner membrane, and cleave the connecting region between the SDHB and RPS14 domains even before processing of the presequence.

Possible participation of outer mitochondrial membrane cytonchrome B5 in steroidogenesis in zona glomerulosa of rat adrenal cortex

Outer mitochondrial membrane cytochrome b5 (OMb) originally found in rat liver is an isoform of cytochrome b5 (b5) of the endoplasmic reticulum. In contrast to accumulated data on the physiological roles of b5, functions of OMb have not been well characterized except for its involvement in regeneration of ascorbic acid [i.e., in a semidehydroascorbate reductase (SDAR) system]. By using highly specific antibodies against rat OMb, we found immunohistochemically that OMb in the rat adrenal gland was most abundant in the zona glomerulosa (zG) among the three cortical zones, and the expression level was enhanced on angiotensin II-stimulation. SDAR activity was found in zG and inhibited by anti-OMb antibody. Moreover, the increase in plasma aldosterone concentration under Na+ -deficiency was suppressed by limited ascorbic acid (Asc) availability in rat mutants unable to synthesize Asc, while plasma corticosterone concentration was not affected. These data suggest that OMb, present abundantly in zG, participates in aldosterone formation in zG of rat under angiotensin II-stimulation through regeneration of Asc.

Identification of outer mitochondrial membrane cytochrome b5 as a modulator for androgen synthesis in Leydig cells

Outer mitochondrial membrane cytochrome b5 is an isoform of microsomal membrane cytochrome b5. In rat testes the outer mitochondrial membrane cytochrome b5 is present in both mitochondria and microsomes, whereas microsomal membrane cytochrome b5 is undetectable. Outer mitochondrial membrane cytochrome b5 present in the testis was localized in Leydig cells with cytochrome P-45017alpha, which catalyzes androgenesis therein. We therefore analyzed the functions of outer mitochondrial membrane cytochrome b5 in rat testis microsomes by using a proteoliposome system. In a low but physiological concentration of NADPH-cytochrome P-450 reductase and excess amount of progesterone, outer mitochondrial membrane cytochrome b5 stimulated the cytochrome P-45017alpha-catalyzed reactions, 17alpha-hydroxylation and C17-C20 bond cleavage. The effects were different from those by microsomal membrane cytochrome b5 as follows: preferential elevation of the 17alpha-hydroxylase activity by outer mitochondrial membrane cytochrome b5 in an amount-dependent manner versus that of the lyase activity by microsomal membrane cytochrome b5 at the low concentration, and the inhibition of both activities at the high concentration. At a low concentration of progesterone reflecting a physiological cholesterol supply, outer mitochondrial membrane cytochrome b5 elevated primarily the production of 17alpha-hydroxyprogesterone and then facilitated the conversion of the released intermediate to androstenedione. Thus, we demonstrated that outer mitochondrial membrane cytochrome b5 and not microsomal membrane cytochrome b5 functions as an activator for androgenesis in rat Leydig cells.

Recognition of mitochondrial protein precursor lacking arginine at position -2 by mitochondrial processing peptidase: processing of bovine cytochrome P450(SCC) precursor

Mitochondrial processing peptidase (MPP) specifically cleaves off the N-terminal presequence of the mitochondrial protein precursor. Previous studies demonstrated that Arg at position -2 from the cleavage site, which is found among many precursors, plays a critical role in recognition by MPP. We analyzed the structural elements of bovine cytochrome P450 side-chain cleavage enzyme precursor [pre-P450(SCC)], which has Ala at position -2, for recognition by MPP. Replacement of Ala position -2 of pre-P450(SCC) with Arg resulted in an increase in the cleavage rate. Replacement with Gly caused a reduction in the cleavage rate and the appearance of an additional cleavage site downstream of the authentic site. A pre-P450(SCC) mutant with Met at position -2 retained cleavage efficiency equal to that of the wild type. These results indicate that -2 Ala of pre-P450(SCC) is recognized by MPP as a determinant for precise cleavage, and that the amino acid at -2 is required to have a straight methylene chain for interaction with the S(2) site. The preference for distal basic residues, a hydrophobic residue at +1, and hydroxyl residues at +2 and +3, was almost the same as those of the precursors with Arg at -2, indicating that the recognition mechanism of pre-P450(SCC) by MPP is essentially the same as that of the precursors with Arg at position -2.

A proposed common structure of substrates bound to mitochondrial processing peptidase

Mitochondrial processing peptidase (MPP), a metalloendopeptidase consisting of alpha- and beta-subunits, specifically cleaves off the N-terminal presequence of the mitochondrial protein precursor. Structural information of the substrate bound to MPP was obtained using fluorescence resonance energy transfer (FRET) measurement. A series of the peptide substrates, which have distal arginine residues required for effective cleavage at positions -7, -10, -14, and -17 from the cleavage site, were synthesized and covalently labeled with 7-diethyl aminocoumarin-3-carboxylic acid at the N termini and N,N'-dimethyl-N-(iodoacetyl)-N'-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)ethylenediamine (IANBD) at position +4, as fluorescent donor and acceptor, respectively. When the peptides were bound to MPP, substantially the same distances were obtained between the two probes, irrespective of the length of the intervening sequence between the two probes. When 7-diethylamino-3-(4'-maleimidyl phenyl)-4-methyl coumarin was introduced into a single cysteine residue in beta-MPP as a donor and IANBD was coupled either at the N terminus or the +4 position of the peptide substrate as an acceptor, intermolecular FRET measurements also demonstrated that distances of the donor-acceptor pair were essentially the same among the peptides with different lengths of intervening sequences. The results indicate that the N-terminal portion and the portion around the cleavage site of the presequence interact with specific sites in the MPP molecule, irrespective of the length of the intervening sequence between the two portions, suggesting the structure of the intervening sequence is flexible when bound to the MPP.

Glycine-rich region of mitochondrial processing peptidase alpha-subunit is essential for binding and cleavage of the precursor proteins

Mitochondrial processing peptidase, a metalloendopeptidase consisting of alpha- and beta-subunits, specifically recognizes a large variety of mitochondrial precursor proteins and cleaves off amino-terminal extension peptides. The alpha-subunit has a characteristic glycine-rich segment in the middle portion. To elucidate the role of the region in processing functions of the enzyme, deletion or site-directed mutations were introduced, and effects on kinetic parameters and substrate binding of the enzyme were analyzed. Deletion of three residues of the region, Phe(289) to Ala(291), led to a dramatic reduction in processing activity to practically zero. Mutation of Phe(289), Lys(296), and Met(298) to alanine resulted in a decrease in the activity, but these mutations had no apparent effect on interactions between the two subunits, indicating that reduction in processing activity is not due to structural disruption at the interface interacting with the beta-subunit. Although the mutant enzymes, Phe289Ala, Lys296Ala, and Met298Ala, had an approximate 10-fold less affinity for substrate peptides than did that of the wild type, the deletion mutant, delta 289-291, showed an extremely low affinity. Thus, shortening of the glycine-rich stretch led to a dramatic reduction of interaction between the enzyme and substrate peptides and cleavage reaction, whereas mutation of each amino acid in this region seemed to affect primarily the cleavage reaction.

Analysis of recognition elements for mitochondrial processing peptidase using artificial amino acids: roles of the intervening portion and proximal arginine

We recently demonstrated, using synthetic peptides modeled on the extension peptide of malate dehydrogenase, that amino acid residues present at the proximal and distal positions relative to the cleavage site are critical determinants for the recognition of substrates by mitochondrial processing peptidase [Niidome et al. (1994) J. Biol. Chem. 269, 24719-24722). While the proximal arginine is unexceptionally located at the -2 position, the position of the distal residue varies among mitochondrial precursor proteins. Between the proximal and distal residues, proline and/or glycine are present in most mitochondrial precursor proteins, and they are considered to play a role in the specific recognition of a substrate by the peptidase. To elucidate the role of the intervening portion, we introduced a non-natural amino acid [2-(2-aminoethoxy)acetic acid] between the distal and proximal residues. We also analyzed the functional elements in the proximal arginine by replacing the residue with various arginine or lysine analogs. The results of kinetic studies indicated that the intervening portion should be flexible for efficient processing, and that the guanidino group of the proximal arginine is recognized by the peptidase through hydrogen and ionic bonds.

Cooperative formation of a substrate binding pocket by alpha- and beta-subunits of mitochondrial processing peptidase

Mitochondrial processing peptidase (MPP) specifically recognizes a large variety of mitochondrial precursor proteins and cleaves off N-terminal extension peptides. The enzyme is a metalloprotease and forms a heterodimer consisting of structurally related alpha- and beta-subunits. To investigate the responsibility of MPP subunits for substrate recognition, we monitored interaction of the fluorescent-labeled peptide substrates with the MPP and its subunits. The specific binding of the peptide to the MPP was confirmed by findings of the direct participation of arginine residues in the binding, which are located at position -2 and the position distal to the cleavage site and are essential for the cleavage reaction. MPP bound the substrate peptides with high affinity only in the dimeric complex, and each subunit monomer had about a 30-fold less affinity than the complex. The individual subunit required arginines at different positions in the peptide for binding, although their affinities were much lower than that of MPP. Fluorescence quenching analysis showed that the peptide bound to MPP was buried in the enzyme. Thus, both subunits of MPP might be required for formation of a substrate binding pocket with multiple subsites lying across them.

Glutamate residues required for substrate binding and cleavage activity in mitochondrial processing peptidase

Mitochondrial processing peptidase, a metalloendopeptidase consisting of alpha- and beta-subunits, specifically recognizes a large variety of mitochondrial precursor proteins and cleaves off N-terminal extension peptides. The enzyme requires the basic amino acid residues in the extension peptides for effective and specific cleavage. To elucidate the mechanism involved in the molecular recognition of substrate by the enzyme, several glutamates around the active site of the rat beta-subunit, which has a putative metal-binding motif, H56XXEH60, were mutated to alanines or aspartates, and effects on kinetic parameters, metal binding, and substrate binding of the enzyme were analyzed. None of mutant proteins analyzed was impaired in dimer formation with the alpha-subunit. Mutation of glutamates at positions 79, 129, and 136, in addition to an active-site glutamate at position 59, resulted in a marked decrease in cleavage efficiency. Together with sequence alignment data, glutamate 136 appears to be involved in metal binding. Glutamate 129 is mostly responsible for the catalysis, as there was a considerable decrease in kcat value by the mutation. Mutation of glutamate 79 led to decrease in kcat value and increase in Km values. Substrate binding experiments using an environmentally sensitive fluorescence probe attached to the peptide showed that the mutation caused a remarkable environmental change at the binding site to the N-terminal region of the substrate peptide and decreased binding of the peptide, thereby suggesting that glutamate 79 participates primarily in substrate binding. Thus, some glutamate residues required for substrate binding and cleavage activity have been identified.

Importance of residues carboxyl terminal relative to the cleavage site in substrates of mitochondrial processing peptidase for their specific recognition and cleavage

We previously identified distal and proximal arginine residues in the N-terminal portion and an aromatic amino acid at position 1 (P1' site3) relative to the cleavage site as important recognition signals in substrates of mitochondrial processing peptidase [Niidome, T., Kitada, S., Shimokata, K., Ogishima, T., and Ito, A. (1994) J. Biol. Chem. 269, 24714-24722; Ogishima, T., Niidome, T., Shimokata, K., Kitada, S., and Ito, A. (1995) ibid. 270, 30322-30326]. To further elucidate the elements required for the specific recognition and cleavage by the enzyme, we synthesized synthetic peptides that possessed only the distal and proximal arginine residues and phenylalanine at the P1' site in a poly alanine sequence, and analyzed the processing reaction toward them. They were not cleaved by the peptidase although they inhibited the peptidase activity. However, when serine was introduced into the C-terminal portions of the sequence, processing was observed. The efficiency of the resultant peptides improved as the number of serine residues was increased. A peptide with serine or histidine at P2' and threonine at P3' was processed most efficiently. These results indicate that the processing reaction catalyzed by the peptidase depends not only on the N-terminal portion but also on the C-terminal portion from the cleavage site in the substrates.

Role of alpha-subunit of mitochondrial processing peptidase in substrate recognition

Mitochondrial processing peptidase is a heterodimer consisting of alpha-mitochondrial processing peptidase (alpha-MPP) and beta-MPP. We investigated the role of alpha-MPP in substrate recognition using a recombinant yeast MPP. Disruption of amino acid residues between 10 and 129 of the alpha-MPP did not essentially impair binding activity with beta-MPP and processing activity, whereas truncation of the C-terminal 41 amino acids led to a significant loss of binding and processing activity. Several acidic amino acids in the region conserved among the enzymes from various species were mutated to asparagine or glutamine, and effects on processing of the precursors were analyzed. Glu353 is required for processing of malate dehydrogenase, aspartate aminotransferase, and adrenodoxin precursors. Glu377 and Asp378 are needed only for the processing of aspartate aminotransferase and adrenodoxin precursors, both of which have a longer extension peptide than the others studied. However, processing of the yeast alpha-MPP precursor, which has a short extension peptide of nine amino acids, was not affected by these mutations. Thus, effects of substitution of acidic amino acids on the processing differed with the precursor protein and depended on length of the extension peptides. alpha-MPP may function as a substrate-recognizing subunit by interacting mainly with basic amino acids at a region distal to the cleavage site in precursors with a longer extension peptide.

Substrate recognition by mitochondrial processing peptidase toward the malate dehydrogenase precursor

Mitochondrial processing peptidase (MPP) cleaves the extension peptides of precursor proteins newly imported into the mitochondria. Using synthetic oligopeptides modeled on the extension peptide of malate dehydrogenase, the critical elements of the substrate for the processing of MPP were determined [Niidome, T., Kitada, S., Shimokata, K., Ogishima, T., and Ito, A. (1994) J. Biol. Chem. 269, 24719-24722; Ogishima, T., Niidome, T., Shimokata, K., Kitada, S., and Ito, A. (1995) ibid. 270, 30322-30326]. In the present study, we constructed mutant precursors and compared the processing reaction with that of the peptide substrates to confirm the validity of use of peptide substrates. In both cases, the arginine residue presents at a proximal (-2) position relative to the processing site proved to be important for the processing. The distal arginine residue at position 7 was replaceable with alanine with no significant loss in cleavage efficiency if the precursor protein contained two consecutive arginine residues at a proximal position, although the arginine residue at a position 7 was indispensable in the model peptide. The proline residue, lying between the distal and proximal arginine residues, which is assumed to break a continuous alpha-helix region in the extension peptide, was needed for the processing. This peptidase has a preference for aromatic amino acids at the P1' site. These results were essentially the same as those obtained with model peptides except for the role of the distal arginine. We also found that amino acids at P2' and P3' sites had some effects on the processing. Thus we concluded that an effective combination of model peptides with precursor proteins is needed for the studies on MPP responsible substrate-recognition mechanisms.

Role of basic amino acids in the cleavage of synthetic peptide substrates by mitochondrial processing peptidase

Our recent experiments using model peptides of rat malate dehydrogenase (MDH) indicated that a proximal arginine and a distal basic amino acid are important for processing by mitochondrial processing peptidase (MPP). [Niidome, T., Kitada, S., Shimokata, K., Ogishima, T., and Ito, A. (1994) J. Biol. Chem. 269, 24719-24722]. To elucidate if the recognition elements apply to other precursor proteins, we analyzed cleavage of model peptides of human ornithine aminotransferase (OAT). Purified peptidase cleaved peptides that corresponded to N-terminal 1-25 and 3-25 at the correct site (Gly17-Val18) at nearly equal rates. Replacement of Arg16 (-2 position) with lysine or alanine reduced the processing efficiency by 95- and 380-fold, respectively. Either deletion from Met1 to Arg10 or replacement of the basic amino acids between them decreased the processing efficiency considerably. A peptide containing Arg7 in addition to Lys4 and Arg10 was more effective than the control peptide. However, a peptide with one and two consecutive basic amino acids in the distal region had a processing efficiency close to the control peptide. These results indicated that processing of OAT was enhanced by an increase in the number of basic amino acids with a suitable distance between them. In other respects, the processing signal of OAT was essentially the same as that of MDH.

Evidence for 54-kD protein in chicken kidney as a cytochrome P450 with a high molecular activity of 25-hydroxyvitamin D3 1 alpha-hydroxylase

Conversion of 25-hydroxyvitamin D3 (25(OH)D3) to the active vitamin D3, 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) is catalyzed by 25(OH)D3, 1 alpha-hydroxylase(1 alpha-hydroxylase). It has been suggested that this enzyme is cytochrome P450 (P450). We purified 1 alpha-hydroxylase 430-fold from cholate-solubilized kidney mitochondria of vitamin D-deficient chickens by utilizing hydrophobic and ion-exchange column chromatographies. Enzymatic activity was assessed by measuring on HPLC the formation of 1 alpha,25(OH)2D3 from 25(OH)D3 in the assay mixture containing NADPH, adrenodoxin reductase, adrenodoxin as a reducing system. The purified enzyme showed a CO-difference spectrum characteristic of P450. The molecular activity of this preparation was calculated to be 8.7 pmol/min/pmol P450. This value was higher by more than 87-fold than those reported so far. The present preparation was found to contain several proteins on SDS-PAGE. Among them, only the 54-kD protein became undetectable when kidney mitochondria from normal and vitamin D-replete chickens, where 1 alpha-hydroxylase activities were 15 and 0% of that found in vitamin D-deficient chicken, respectively, were used as the starting enzyme sources. Furthermore, the band intensity of the 54-kD protein accounted for the spectrophotometrically determined amount of P450 in the preparation. These results suggest that the 54-kD protein is 1 alpha-hydroxylase.

Analysis of elements in the substrate required for processing by mitochondrial processing peptidase

We have recently demonstrated that synthetic peptides modeled on the extension peptide of malate dehydrogenase can be a good substrate of mitochondrial processing peptidase and that arginine residues present at positions -2 or -3 and distant from the cleavage point were important for recognition by the enzyme (Niidome, T., Kitada, S., Shimokata, K., Ogishima, T., and Ito, A. (1994) J. Biol. Chem. 269, 24719-24722). We further investigated the elements required for substrates of the protease. To analyze the reaction by a more rapid yet quantitative method, we have developed intramolecularly quenched fluorescent substrates. Using the fluorogenic substrates we demonstrated that at least one of the proline and glycine between the distal and proximal arginine residues was also important while other connecting sequences were dispensable. In addition, the protease showed considerable preference for aromatic and, to a lesser extent, hydrophobic amino acids in the P1'-position. These results together with the previous data suggest that the proximal and distal arginine residues, proline and/or glycine between them, and P1' amino acid could be critical determinants for the specific cleavage of the substrates by the protease.

A putative metal-binding site in the beta subunit of rat mitochondrial processing peptidase is essential for its catalytic activity

Mitochondrial processing peptidase (MPP) consists of alpha- and beta-subunits (alpha-MPP and beta-MPP). beta-MPP has a putative metal-binding sequence (HXXEH). To determine whether the sequence of beta-MPP is essential for the enzymatic activity, we individually mutated the histidines and glutamic acid to arginines and glutamine, respectively. The wild-type and mutated beta-MPPs were co-expressed with alpha-MPP in Escherichia coli. All three mutants had completely lost the activity, whereas the lost activity was recovered on the addition of wild-type beta-MPP. The activity of the wild-type enzyme was reduced by the mutant beta-MPPs. We conclude from these observations that the HXXEH region is involved in the formation of the active site and that beta-MPP is the catalytic subunit of MPP.

Localization of P450aldo and P45011 beta in normal and regenerating rat adrenal cortex

A novel layer of cells that do not contain both P450aldo and P45011 beta has been discovered between the zonae glomerulosa and fasciculata of the rat adrenal cortex. Since P450aldo and P45011 beta are the enzymes responsible for the formation of aldosterone and corticosterone, respectively, the cells in that zone are presumably inert in synthesizing both aldosterone and corticosterone, in other words, the layer is composed of cells that have no zone-specific endocrine function as an adrenocortical component. Cytologically, the layer consists of tightly packed cells, which contain a lesser amount of lipid droplet than the cells in the other zones, and appears as a white ring or a white zone in the double immunostaining with anti P450aldo and anti P45011 beta. Upon angiotensin II-stimulation evoked by Na-deficiency, the number of the zona glomerulosa cells expressing P450aldo increases for the initial 2 or 3 days and then the P450aldo-containing zona glomerulosa cells begin to proliferate. Thus angiotensin II serves as a proliferator of the zona glomerulosa cells of the rat adrenal cortex. During the period, the thickness of the white zone decreases for initial 3 days and becomes constant after 5 or 6 days, being about 5% of the total cell number of the adrenal cortex. When localization of replicating cells was examined in the adrenal cortex, they were found to be concentrated in and around the white zone. Then the pulse-chase experiments with BrdU showed that the labeled cells migrated out of the white zone and into the zonae fasciculata and reticularis. The localization of the replicating cells in the regenerating adrenal cortex was also around the region between the zonae glomerulosa and fasciculata. On the basis of these findings, we suggest that the newly discovered cell layer (the white zone) is the stem cell zone of the rat adrenal cortex.

Arginine residues in the extension peptide are required for cleavage of a precursor by mitochondrial processing peptidase. Demonstration using synthetic peptide as a substrate

Mitochondrial processing peptidase (MPP) specifically recognizes a large variety of mitochondrial precursor proteins and correctly cleaves off the extension peptides. To determine the structure common to all the extension peptides that is required for specific recognition by MPP, we synthesized various oligopeptides of different chain lengths and amino acid sequences, based on the amino acid sequence of the extension peptide of pre-malate dehydrogenase, and determined kinetic parameters of the cleavage reactions. The minimal length of peptides for effective cleavage was 16 amino acid residues consisting of 11 and 5 residues from the cleavage site to the amino- and carboxyl-terminal sides, respectively. Two sets of basic amino acids in the peptide, the distal arginine residue at position -10 and the proximal ones at positions -3 and -2 relative to the cleavage site, were necessary for effective hydrolysis. Of these two, the residue at position -2 was more important for effective cleavage than the one at position -3 and could not be replaced by a lysine residue. The replacement of the distal arginine by lysine had no effect on the cleavage. Our study demonstrates that use of peptides with the proper length is essential for performing kinetic analyses on the cleavage reaction by MPP and that an arginine residue at position -2 to the cleavage site is necessary for the recognition and cleavage of the extension peptide.

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.

Steroid contents and cortical steroidogenic enzymes in non-hyperfunctioning adrenal adenoma

The recent increasing use of ultrasound and computed tomography has revealed numbers of incidentally discovered adrenal tumors. Many studies have focused on their surgical management, but the biological characteristics of these adrenal tumors have remained unclear. Adrenal tumors were resected from 10 patients who underwent gastrectomy or cholecystectomy. No signs or symptoms of adrenal hormone excess or deficiency were evident either before or after the operation. Moreover, after surgery, no major differences in signs and symptoms including blood pressure levels were observed. Before surgery, neurogenic tumors and cysts were excluded by enhanced magnetic resonance imaging. Steroid contents and both the activities and amounts of steroidogenic cytochrome P-450s in the adrenocortical adenomas of these patients were examined. Microscopic examination revealed that the tumors were surrounded by a thin, non-intact capsule; the surrounding cortex was not atrophic and apparently normal; and the cells of both the tumor and adjacent portions were arranged in nests and cords. Measurements of all steroid content (pregnenolone, progesterone, corticosterone, 11-deoxycorticosterone, 18-hydroxydeoxycorticosterone, cortisol, and dehydroepiandrosterone) except aldosterone in 5 resected adrenal tumors were within the normal ranges for the adrenals of 5 patients with renal cell carcinoma. Aldosterone content in tumor portions was significantly lower than in the apparently normal adrenals. Although in both tumor and adjacent portions of another 5 resected adrenal tumors the activities and amounts of cytochrome P-450s (P-450scc, P-450(11 beta), P-450aldo, P-450(17 alpha), and P-450c21) were also within the normal ranges, the activities of P-450scc and P-450(17 alpha) in the tumor portion were greater than those in the adjacent portion.(ABSTRACT TRUNCATED AT 250 WORDS)

Significance of steroidogenic enzymes in the pathogenesis of adrenal tumour

We examined both activities and amounts of steroidogenic cytochrome P-450s at the posttranslational protein level and steroid contents in the adrenocortical adenoma from patients with primary aldosteronism and Cushing's syndrome. Aldosterone synthase cytochrome P-450 (human P-450aldo) was detected in the tumour portion of aldosterone-producing adenoma, but not in the normal control adrenals, at the protein level. Neither the activities nor the amounts of other P-450s in the tumour portion of aldosterone-producing adenoma were significantly different from those in the non-tumour portion in the adenoma and the normal control adrenals. The aldosterone content was significantly elevated, while the androstenedione content was significantly decreased in the tumour portion of the adenoma compared with that in the normal control adrenals. In Cushing's syndrome, both the activities and amounts of P-450(17 alpha) and P-450c21 were significantly elevated in the tumour portion compared with the non-tumour portion of the adenoma and the normal control adrenals, while those of P-450scc and P-450(11 beta) in the tumour portion were not significantly different from the normal control adrenals. The cortisol content was significantly elevated, while the amounts of aldosterone and 18-hydroxydeoxycorticosterone in the tumour portion of the adenoma were significantly decreased compared with those in the normal control adrenals. These results demonstrate that overexpression of P-450aldo in aldosterone-producing adenoma, and those of P-450(17 alpha) and P-450c21 in cortisol-producing adenoma may play some role in the pathogenesis of primary aldosteronism and Cushing's syndrome, respectively.

Molecular cloning of the smaller subunit(P52) of rat liver mitochondrial processing protease

A cDNA encoding the smaller subunit (P52) of mitochondrial processing protease was isolated from a rat liver cDNA library, using cDNA fragment for yeast MAS1 as the probe. The deduced amino acid sequence is highly homologous to those of PEP from Neurospora crassa and MAS1 from Saccharomyces cerevisiae. After in vitro transcription and translation, the precursor peptide was imported into isolated rat liver mitochondria and processed to its mature form.

Zone-specific expression of aldosterone synthase cytochrome P-450 and cytochrome P-45011 beta in rat adrenal cortex: histochemical basis for the functional zonation

Zonal distribution of aldosterone synthase cytochrome P-450 and cytochrome P-45011 beta in rat adrenocortex was investigated immunochemically using specific antibodies to these enzymes. Localization of aldosterone synthase cytochrome P-450 (cytochrome P-450aldo), a recently identified enzyme that converts deoxycorticosterone to aldosterone in rat adrenocortex was strictly confined to two or three outermost cell layers in the zona glomerulosa. In contrast, cytochrome P-45011 beta, which forms corticosterone, but not aldosterone, from deoxycorticosterone, was localized in the zona fasciculata-reticularis and not in the zona glomerulosa. Neither enzyme was detected in the medulla or the capsule. The functional zonation of adrenocortex with respect to aldosterone and corticosterone syntheses is, thus, ascribable to the localization of cytochromes P-450aldo and P-45011 beta in the respective zones. When rats were maintained under Na-depleted conditions for 10 days, the zona glomerulosa cells containing cytochrome P-450aldo proliferated to 10-15 layers, the thickness of which was 5-7-fold that in the nonstimulated rats. Proliferation of the cytochrome P-450aldo-positive cells into the zona fasciculata-reticularis was also observed along with arterial walls. Under these conditions, no significant change in the distribution of cytochrome P-45011 beta was noted. These results indicate that the angiotensin-II stimuli, which had been elicited by the low Na treatment, promoted proliferation of the glomerulosa cells, resulting in increased expression of cytochrome P-450aldo in rat adrenocortex.

Effect of dietary sodium restriction on mRNA for aldosterone synthase cytochrome P-450 in rat adrenals

Changes in the level of mRNA for aldosterone synthase cytochrome P-450 (cytochrome P-450aldo) in rats on dietary sodium restriction were studied by means of Northern and slot blot hybridization using an oligonucleotide probe that allowed differentiation of the message for this enzyme from that for cytochrome P-450(11)beta. These two enzymes have been shown to be highly homologous with each other, exhibiting 88% homology in their nucleotide sequences in the coding region. Upon sodium restriction for 2 weeks, cytochrome P-450aldo mRNA in rat adrenals increased 7-fold, whereas the cytochrome P-450(11) beta mRNA level in the same adrenals did not change significantly. The increase in cytochrome P-450aldo mRNA paralleled that in cytochrome P-450aldo protein, as analyzed by immunoblot technique. These results, together with our previous finding that angiotensin II induced cytochrome P-450aldo in rat adrenocortex [Shibata, H., Ogishima, T., Mitani, F., Suzuki, H., Murakami, M., Saruta, T., & Ishimura, Y. (1991) Endocrinology 128, 2534-2539], suggest that the production of cytochrome P-450aldo is regulated by angiotensin II at the pretranslational level, most likely at the transcriptional level.

Structural differences in 5'-flanking regions of rat cytochrome P-450aldo and P-450(11) beta genes

Two rat genomic clones, one for cytochrome P-450aldo and the other for P-450(11) beta, were isolated and characterized. The two genes, encoding structurally homologous proteins, were closely similar in their intron-exon organizations. Their 5'-flanking regions, however, contained only a few homologous regions. A putative cyclic AMP responsive element, TGACGTGA, was found in the P-450aldo gene, but this sequence was altered at two positions in the P-450(11) beta gene. S1 nuclease protection assay revealed a single transcription initiation site for the P-450aldo gene, while multiple sites were found for the P-450(11) beta gene. These results suggest that transcriptional regulation of the rat P-450aldo and P-450(11) beta genes is due to differences in the sequences of their 5'-flanking regions.

Aldosterone synthase cytochrome P-450 expressed in the adrenals of patients with primary aldosteronism

A human cytochrome P-450 with aldosterone synthase activity was purified from the mitochondria of an aldosterone-producing adenoma. It was recognized by an anti-bovine cytochrome P-450(11 beta) IgG and by a specific antibody raised against a portion of the CYP11B2 gene product, one of the two putative proteins encoded by human cytochrome P-450(11 beta)-related genes (Mornet, E., Dupont, J., Vitek, A., and White, P. C. (1989) J. Biol. Chem. 264, 20961-20967). A similar and probably the same aldosterone synthase cytochrome P-450 was detected in the adrenal of a patient with idiopathic hyperaldosteronism. These aldosterone synthases were distinguishable from cytochrome P-450(11 beta), the product of another cytochrome P-450(11 beta)-related gene, i.e. CYP11B1, by their catalytic, molecular, and immunological properties and also by their localization. The latter enzyme was unable to produce aldosterone and did not react with the specific antibody against the CYP11B2 gene product. It was present both in tumor and non-tumor portions of the adrenals carrying the adenoma and in normal adrenal cortex. On the other hand, aldosterone synthase cytochrome P-450 localized in the tumor portions of the adrenals or in the adrenal of a patient with idiopathic hyperaldosteronism. Thus aldosterone synthase cytochrome P-450, a distinct species from cytochrome P-450(11 beta), is responsible for the biosynthesis of aldosterone in the human, at least in patients suffering from primary aldosteronism.

Regulation of aldosterone synthase cytochrome P-450 in rat adrenals by angiotensin II and potassium

Changes in the levels of aldosterone synthase cytochrome P-450, a recently identified enzyme in rat adrenals, were studied in response to the renin-angiotensin system and K stimuli. As examined by an immunoblot technique, the zona glomerulosa mitochondria from rats fed on a low Na-normal K diet (8.6 mmol Na+ and 207 mmol K+/kg of diet) or a low Na-high K (0.2 M KCl in drinking water) diet for 4-10 days contained significantly higher amounts of aldosterone synthase cytochrome P-450 than those from rats fed on a normal diet (86 mmol Na+ and 207 mmol K+/kg of diet). Activities of the enzyme were also found to increase by about 10-fold on day 10. In concert with these changes, both plasma renin activity and plasma aldosterone concentration increased, indicating that the renin-angiotensin system was activated in these rats. Feeding with a normal Na-high K diet also induced significantly higher levels of both amount and activity of aldosterone synthase cytochrome P-450 together with an elevated serum K concentration on day 4, though they all decreased to near the control level on the following days. On the other hand, when enalapril malate, an angiotensin I-converting enzyme inhibitor, was administered to the low Na-normal K rats, the increases in the amount and activity of the enzyme as well as in plasma aldosterone concentration were suppressed altogether. However, the enalapril administration to the low Na-high K rats suppressed the increases only partially. These results indicate that the aldosterone synthase cytochrome P-450 is an ultimate target of the regulation of aldosterone biosynthesis by angiotensin II and K.

Molecular cloning of a cDNA encoding aldosterone synthase cytochrome P-450 in rat adrenal cortex

Using an oligonucleotide probe designed on the basis of the N-terminal amino acid sequence of purified rat aldosterone synthase cytochrome P-450 [(1989) J. Biol. Chem. 264, 10935] we have isolated from rat adrenal cDNA library a 2687 base pair cDNA that encodes a protein of 500 amino acid residues. The deduced amino acid sequence contained the regions well conserved among all cytochrome P-450s sequenced to date, and also a portion (residues 25-44) which was identical to the N-terminal peptide sequence of rat aldosterone synthase cytochrome P-450. These results indicate that the cDNA encodes a precursor form of rat aldosterone synthase cytochrome P-450.

Isolation of aldosterone synthase cytochrome P-450 from zona glomerulosa mitochondria of rat adrenal cortex

A cytochrome P-450 capable of producing aldosterone from 11-deoxycorticosterone was purified from the zona glomerulosa of rat adrenal cortex. The enzyme was present in the mitochondria of the zona glomerulosa obtained from sodium-depleted and potassium-repleted rats but scarcely detected in those from untreated rats. It was undetectable in the mitochondria of other zones of the adrenal cortex from both the treated and untreated rats. The cytochrome P-450 was distinguishable from cytochrome P-45011 beta purified from the zonae fasciculata-reticularis mitochondria of the same rats. Molecular weights of the former and the latter cytochromes P-450, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, were 49,500 and 51,500, respectively, and their amino acid sequences up to the 20th residue from the N terminus were different from each other at least in one position. The former catalyzed the multihydroxylation reactions of 11-deoxycorticosterone giving corticosterone, 18-hydroxydeoxycorticosterone, 18-hydroxycorticosterone, and a significant amount of aldosterone as products. On the other hand, the latter catalyzed only 11 beta- and 18-hydroxylation reactions of the same substrate to yield either corticosterone or 18-hydroxydeoxycorticosterone. Thus, at least two forms of cytochrome P-450, which catalyze the 11 beta- and 18-hydroxylations of deoxycorticosterone, exist in rat adrenal cortex, but aldosterone synthesis is catalyzed only by the one present in the zona glomerulosa mitochondria.

Isolation of two distinct cytochromes P-45011 beta with aldosterone synthase activity from bovine adrenocortical mitochondria

Two distinct forms of cytochrome P-45011 beta, with apparent molecular weights of 48,500 (48.5K) and 49,500 (49.5K), have been isolated from bovine adrenocortical mitochondria. Their amino acid sequences up to the 19th position from the N-terminus were only different at the 6th position (Val and Ala for the 48.5K and 49.5K enzymes, respectively). Each sequence was assignable to a distinct cDNA clone for cytochrome P-450(11) beta (Kirita, S., et al. [1988] J. Biochem. 104, 683-686), indicating that the two proteins originate from different genes in bovine adrenocortical cells. Both forms of cytochrome P-450(11) beta were capable of catalyzing aldosterone synthesis as well as the 11 beta- and 18-hydroxylation of 11-deoxycorticosterone. Thus, at least two distinct cytochrome P-450(11) beta species exist in the adrenal cortex and participate in steroidogenesis.