Structure of genes encoding steroidogenic enzymes

Inhibition of Cholesterol Biosynthesis Reduces γ-Secretase Activity and Amyloid-β Generation

Amyloid-β (Aβ) is one of major molecules contributing to the pathogenesis of Alzheimer's disease (AD). Aβ is derived from amyloid-β protein precursor (AβPP) through sequential cleavages by β- and γ-secretases. Regulation of these components is thought to be an important factor in Aβ generation during the pathogenesis of AD. AβPP, β-secretase, and γ-secretase reside in lipid rafts, where cholesterol regulates the integrity and flexibility of membrane proteins and Aβ is generated. However, the relationship between cholesterol and Aβ generation is controversial. In this study, we aimed to elucidate the direct effects of cholesterol depletion on AβPP processing using AY9944, which blocks the last step of cholesterol biosynthesis and thus minimizes the unknown side effects of upstream inhibitors, such as HMG-CoA reductase inhibitors. Treatment with AY9944 decreased γ-secretase activity and Aβ generation. These results suggested that changes in membrane composition by lowering cholesterol with AY9944 affected γ-secretase activity and Aβ generation, which is associated with AD pathogenesis.

Restart of steroidogenesis in dogs during recrudescence of testicular function following downregulation with a GnRH-agonist implant

To date, no details are available concerning the restart of steroidogenesis following the downregulation of testicular endocrine and germinative function by gonadotrophin-releasing hormone (GnRH)-agonist implants. This restart was assessed by determining the expression of steroidogenic acute regulatory (StAR) protein, cytochrome P450 side-chain cleavage enzyme (P450scc) and cytochrome P450 17α-hydroxylase,17,20-lyase (P450c17). The re-establishment of steroidogenesis was initiated by the removal of the GnRH-agonist implant (18.5 mg azagly nafarelin, Gonazon) at 5 months after treatment. Testes were removed at 3-week intervals (weeks 0-24) and four groups were formed according to the stage of spermatogenesis as revealed by the most developed germ cells observed (developmental group [DG] spermatocytes to DG elongated spermatids). Five dogs served as untreated controls. Positive immunostaining for StAR, P450scc and P450c17 was restricted to Leydig cells. Western blot indicated the specifity of the respective antibodies with hints of a expression of canine-specific P450scc and P450c17 proteins. A significant effect of group was observed for a percentage of the immunopositive area (PIA) as an indicator of active Leydig cells for StAR (P<0.05), P450scc (P<0.001) and P450c17 (P<0.001), with PIA being lowest for the DG spermatocytes. With regard to the strength of the immunopositive signal, a significant effect of group was found for P450scc (P<0.01) and P450c17 (P<0.05), with the lowest intensity being observed in DG spermatocytes. At the mRNA level, the upregulation from DG spermatocytes to DG round spermatids was clearly evident but was only significant for P450scc (P<0.05). Thus, downregulation affects the whole cascade of steroidogenesis, whereas withdrawal of inhibition results in a rapid restart, in part indicating a rebound phenomenon.

Role of cholesterol in APP metabolism and its significance in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is a complex multifactorial neurodegenerative disorder believed to be initiated by accumulation of amyloid β (Aβ)-related peptides derived from proteolytic processing of amyloid precursor protein (APP). Research over the past two decades provided a mechanistic link between cholesterol and AD pathogenesis. Genetic polymorphisms in genes regulating the pivotal points in cholesterol metabolism have been suggested to enhance the risk of developing AD. Altered neuronal membrane cholesterol level and/or subcellular distribution have been implicated in aberrant formation, aggregation, toxicity, and degradation of Aβ-related peptides. However, the results are somewhat contradictory and we still do not have a complete understanding on how cholesterol can influence AD pathogenesis. In this review, we summarize our current understanding on the role of cholesterol in regulating the production/function of Aβ-related peptides and also examine the therapeutic potential of regulating cholesterol homeostasis in the treatment of AD pathology.

Down regulation of gene related sex hormone synthesis pathway in mouse testes by miroestrol and deoxymiroestrol

Miroestrol and deoxymiroestrol are phytoestrogens isolated from tuberous root of Pueraria candollei var. mirifica. Modulatory effects of miroestrol and deoxymiroestrol on enzymes involved in sex-hormone synthesis pathway in male C57BL/6 mice were investigated using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). Miroestrol and deoxymiroestrol suppressed the expressions of 3β-HSD, 17β-HSD1, and CYP17 while CYP19 mRNA expression was slightly decreased. In addition, the expression of 17β-HSD2 was induced in correlation with those did by estradiol. These observations supported that miroestrol and deoxymiroestrol could exhibit the same effect as estradiol regarding regulation of testicular gene related sex hormone synthesis pathway.

At the crossroads of steroid hormone biosynthesis: the role, substrate specificity and evolutionary development of CYP17

Cytochrome P450s play critical roles in the metabolism of various bioactive compounds. One of the crucial functions of cytochrome P450s in Chordata is in the biosynthesis of steroid hormones. Steroid 17alpha-hydroxylase/17,20-lyase (CYP17) is localized in endoplasmic reticulum membranes of steroidogenic cells. CYP17 catalyzes the 17alpha-hydroxylation reaction of delta4-C₂₁ steroids (progesterone derivatives) and delta5-C₂₁ steroids (pregnenolone derivatives) as well as the 17,20-lyase reaction producing C₁₉-steroids, a key branch point in steroid hormone biosynthesis. Depending on CYP17 activity, the steroid hormone biosynthesis pathway is directed to either the formation of mineralocorticoids and glucocorticoids or sex hormones. In the present review, the current information on CYP17 is analyzed and discussed.

Congenital adrenal hyperplasia: focus on the molecular basis of 21-hydroxylase deficiency

Congenital adrenal hyperplasia (CAH) is an autosomal recessive disorder caused by defects in one of several steroidogenic enzymes involved in the synthesis of cortisol from cholesterol in the adrenal glands. More than 90% of cases are caused by 21-hydroxylase deficiency, and the severity of the resulting clinical symptoms varies according to the level of 21-hydroxylase activity. 21-Hydroxylase deficiency is usually caused by mutations in theCYP21A2gene, which is located on the RCCX module, a chromosomal region highly prone to genetic recombination events that can result in a wide variety of complex rearrangements, such as gene duplications, gross deletions and gene conversions of variable extensions. Molecular genotyping ofCYP21A2and the RCCX module has proved useful for a more accurate diagnosis of the disease, and prenatal diagnosis. This article summarises the clinical features of 21-hydroxylase deficiency, explains current understanding of the disease at the molecular level, and highlights recent developments, particularly in diagnosis.

Cloning, heterologous expression, and functional characterization of 5-epi-aristolochene-1,3-dihydroxylase from tobacco (Nicotiana tabacum)

Capsidiol is a bicyclic, dihydroxylated sesquiterpene produced by several solanaceous species in response to a variety of environmental stimuli. It is the primary antimicrobial compound produced by Nicotiana tabacum in response to fungal elicitation, and it is formed via the isoprenoid pathway from 5-epi-aristolochene. Much of the biosynthetic pathway for the formation of this compound has been elucidated, except for the enzyme(s) responsible for the conversion of 5-epi-aristolochene to its dihydroxylated form, capsidiol. Biochemical evidence from previous studies with N. tabacum (Whitehead, I. M., Threlfall, D. R., and Ewing, D. F., 1989, Phytochemistry 28, 775-779) and Capsicum annuum Hoshino, T., Yamaura, T., Imaishi, H., Chida, M., Yoshizawa, Y., Higashi, K., Ohkawa, H., Mizutani, J., 1995, Phytochemistry 38, 609-613. suggested that the oxidation of 5-epi-aristolochene to capsidiol was mediated by at least one elicitor-inducible cytochrome P450 hydroxylase. In extending these observations, we developed an in vivo assay for 5-epi-aristolochene hydroxylase activity and used it to demonstrate a dose-dependent inhibition of activity by ancymidol and ketoconazole, two well characterized inhibitors of cytochrome P450 enzymes. Using degenerate oligonucleotide primers designed to the well conserved domains found within most P450 enzymes, including the heme binding domain, cDNA fragments representing four distinct P450 families (CYP71, CYP73, CYP82, and CYP92) were amplified from a cDNA library prepared against mRNA from elicitor-treated cells using PCR. The PCR fragments were subsequently used to isolate full-length cDNAs (CYP71D20 and D21, CYP73A27 and A28, CYP82E1 and CYP92A5), and these in turn were used to demonstrate that the corresponding mRNAs were all induced in elicitor-treated cells, albeit with different induction patterns. Representative, full-length cDNAs for each of the P450s were engineered into a yeast expression system, and the recombinant yeast assessed for functional expression of P450 protein by measuring the CO difference spectra of the yeast microsomes. Only microsomal preparations from yeast expressing the CYP71D20 and CYP92A5 cDNAs exhibited significant CO difference absorbance spectra at 450 nm and were thus tested for their ability to hydroxylate 5-epi-aristolochene and 1-deoxycapsidiol, a putative mono-hydroxylated intermediate in capsidiol biosynthesis. Interestingly, the CYP71D20-encoded enzyme activity was capable of converting both 5-epi-aristolochene and 1-deoxycapsidiol to capsidiol in vitro, consistent with the notion that this P450 enzyme catalyzes both hydroxylations of its hydrocarbon substrate.

The 25(OH)D(3)/1alpha,25(OH)(2)D(3)-24R-hydroxylase: a catabolic or biosynthetic enzyme?

The kidney is the major source of the circulating dihydroxylated metabolites of vitamin D, 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] and 24R,25-dihydroxyvitamin D(3) [24R,25(OH)(2)D(3)]. The enzymes which catalyze the production of these two dihydroxylated vitamin D metabolites are the 25(OH)D(3)-1alpha-hydroxylase (1alpha-hydroxylase) and -24R-hydroxylase (24R-hydroxylase), respectively. While there is no controversy regarding the fundamental importance of the 1alpha-hydroxylase in the production of the steroid hormone 1alpha,25(OH)(2)D(3), the biologic significance of the 24R-hydroxylase has been the subject of ongoing discussion. Some hold that it is strictly catabolic, leading to side chain oxidation and cleavage of 25-hydroxylated vitamin D sterols, and others hold that it plays a biosynthetic role in the production of 24R,25(OH)(2)D(3) which has biologic activities distinct from those of 1alpha,25(OH)(2)D(3). The 24R-hydroxylase has properties in common with other multicatalytic steroidogenic enzymes: (1) the enzyme carries out multiple oxidative and carbon-carbon bond cleavages; (2) it utilizes two natural substrates; (3) its regulation varies depending on the cell or tissue in which it occurs. The purpose of this paper is to review the current literature relevant to the characteristics of the 24R-hydroxylase and its regulation in the context of other multicatalytic steroid hydroxylases in order to provide a perspective regarding its possible function as both a catabolic and activating enzyme in the vitamin D endocrine system.

Side-chain cleavage of cholesterol sulfate by ovarian mitochondria

Mitochondria isolated from porcine corpora lutea and from the luteinized ovaries of gonadotropin-treated immature rats were found to efficiently cleave the side-chain of cholesterol sulfate to produce 3 beta-hydroxy-5-pregnen-20-one sulfate (pregnenolone sulfate). When mitochondria were preincubated with cholesterol sulfate, the time-course for the side-chain cleavage of cholesterol sulfate was biphasic. With 200 microM cholesterol sulphate, the initial rate of the reaction was the same as that observed for 25-hydroxycholesterol. This rate was not increased when both cholesterol sulfate and 25-hydroxycholesterol were incubated together. The rate of side-chain cleavage by isolated mitochondria supplied with 75 microM cholesterol sulfate as substrate was inhibited by 97% by aminoglutethimide, a specific inhibitor of cytochrome P-450scc. The slow phase of side-chain cleavage of cholesterol sulfate appeared to be limited by the rate of substrate movement to the mitochondrial site of the reaction. Cholesterol sulfate translocation rates were however up to 8 times greater than those observed for cholesterol when equivalent concentrations of the two substrates were added to the mitochondria. We conclude that cholesterol sulfate is a better substrate than cholesterol for side-chain cleavage by isolated mitochondria and that both reactions are catalysed by the same cytochrome P-450scc enzyme.

Placental steroid hormones

The intent of this review was a selective consideration of recent advances in understanding placental steroidogenesis in humans. While we have omitted material, both intentionally and unintentionally, we hope this discourse presents a flavour of the current molecular endocrinology of placental steroidogenesis. In particular, advances in knowledge as it relates to the enzymes involved in progesterone and oestrogen metabolism have been addressed and correlated with placental development. Finally, because relatively less is known about regulation of steroidogenesis in the human placenta beyond the aspects of growth and differentiation, we have discussed regulation in general terms using recent data obtained in animal species.

Metabolism and toxicity of xenobiotics in the adrenal cortex, with particular reference to 7,12-dimethylbenz(a)anthracene

The adrenal cortex contains high amounts of detoxifying enzymes, as well as generators and protectors of reactive oxygen species. The high content of cytochrome P-450 enzymes in the adrenal cortex together with its remarkable tendency to accumulate hydrophobic substances probably contributes to the extraordinary vulnerability of the gland to a number of xenobiotics. The best studied adrenocorticolytic compounds are the potent carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) and its liver metabolite 7-hydroxymethyl-12-methylbenz(a)anthracene (7-OHM-12-MBA). Adrenocorticolysis generated by these agents in vivo as well as in vitro demonstrates high regioselective requirements and is strongly influenced by the presence of ACTH, steroids, cytochrome P-450 inhibitors and antioxidants. Furthermore, 7-OHM-12-MBA has been demonstrated to uniquely generate selective and massive oxidation of mitochondrial glutathione in cultured rat adrenal cells. The DMBA-induced adrenocorticolysis is thoroughly discussed in this review with particular emphasis on the metabolism of DMBA and the influence of various effectors. A working hypothesis involving a possible peroxidative mechanism is also presented.

Regulation of mRNAs for human steroidogenic enzymes

Steroid hormone synthesis is controlled by two classes of mechanisms. "Acute" regulation entails rapid increases or decreases of steroid synthesis and secretion, principally mediated by rapid changes in the activities of the steroidogenic enzymes and by the availability of the substrate, free cholesterol. "Chronic" regulation entails increases or decreases in the amounts of the steroidogenic enzymes as well as their activities. The amounts of the enzymes are regulated principally by the amounts of the specific mRNAs encoding them. These, in turn, are regulated both transcriptionally and post-transcriptionally. The mRNAs are regulated by hormonal induction, by an ontogenic program, and in a tissue-specific fashion.