Construction and function of fusion enzymes of the human cytochrome P450scc system

Light-driven progesterone production by InP-(M. neoaurum) biohybrid system

Progesterone is one of the classical hormone drugs used in medicine for maintaining pregnancy. However, its manufacturing process, coupled with organic reagents and poisonous catalysts, causes irreversible environmental pollution. Recent advances in synthetic biology have demonstrated that the microbial biosynthesis of natural products, especially difficult-to-synthesize compounds, from building blocks is a promising strategy. Herein, overcoming the heterologous cytochrome P450 enzyme interdependency in Mycolicibacterium neoaurum successfully constructed the CYP11A1 running module to realize metabolic conversion from waste phytosterols to progesterone. Subsequently, progesterone yield was improved through strategies involving electron transfer and NADPH regeneration. Mutant CYP11A1 (mCYP11A1) and adrenodoxin reductase (ADR) were connected by a flexible linker (L) to form the chimera mCYP11A1-L-ADR to enhance electron transfer. The chimera mCYP11A1-L-ADR, adrenodoxin (ADX), and ADR-related homolog ARH1 were expressed in M. neoaurum, showed positive activity and produced 45 mg/L progesterone. This electron transfer strategy increased progesterone production by 3.95-fold compared with M. neoaurum expressing mCYP11A1, ADR, and ADX. Significantly, a novel inorganic-biological hybrid system was assembled by combining engineered M. neoaurum and InP nanoparticles to regenerate NADPH, which was increased 84-fold from the initial progesterone titer to 235 ± 50 mg/L. In summary, this work highlights the green and sustainable potential of obtaining synthetic progesterone from sterols in M. neoaurum.

Design of Artificial Enzymes Bearing Several Active Centers: New Trends, Opportunities and Problems

Harnessing enzymes which possess several catalytic activities is a topic where intense research has been carried out, mainly coupled with the development of cascade reactions. This review tries to cover the different possibilities to reach this goal: enzymes with promiscuous activities, fusion enzymes, enzymes + metal catalysts (including metal nanoparticles or site-directed attached organometallic catalyst), enzymes bearing non-canonical amino acids + metal catalysts, design of enzymes bearing a second biological but artificial active center (plurizymes) by coupling enzyme modelling and directed mutagenesis and plurizymes that have been site directed modified in both or in just one active center with an irreversible inhibitor attached to an organometallic catalyst. Some examples of cascade reactions catalyzed by the enzymes bearing several catalytic activities are also described. Finally, some foreseen problems of the use of these multi-activity enzymes are described (mainly related to the balance of the catalytic activities, necessary in many instances, or the different operational stabilities of the different catalytic activities). The design of new multi-activity enzymes (e.g., plurizymes or modified plurizymes) seems to be a topic with unarguable interest, as this may link biological and non-biological activities to establish new combo-catalysis routes.

TSPO: an emerging role in appetite for a therapeutically promising biomarker

There is accumulating evidence that an obesogenic Western diet causes neuroinflammatory damage to the brain, which then promotes further appetitive behaviour. Neuroinflammation has been extensively studied by analysing the translocator protein of 18 kDa (TSPO), a protein that is upregulated in the inflamed brain following a damaging stimulus. As a result, there is a rich supply of TSPO-specific agonists, antagonists and positron emission tomography ligands. One TSPO ligand, etifoxine, is also currently used clinically for the treatment of anxiety with a minimal side-effect profile. Despite the neuroinflammatory pathogenesis of diet-induced obesity, and the translational potential of targeting TSPO, there is sparse literature characterizing the effect of TSPO on appetite. Therefore, in this review, the influence of TSPO on appetite is discussed. Three putative mechanisms for TSPO's appetite-modulatory effect are then characterized: the TSPO–allopregnanolone–GABA_AR signalling axis, glucosensing in tanycytes and association with the synaptic protein RIM-BP1. We highlight that, in addition to its plethora of functions, TSPO is a regulator of appetite. This review ultimately suggests that the appetite-modulating function of TSPO should be further explored due to its potential therapeutic promise.

Recombinant Yarrowia lipolytica strains for the heterologous expression of multi-component enzyme systems: Expression of mammalian steroidogenic proteins

New Yarrowia lipolytica strains for the co-expression of steroidogenic mammalian proteins were obtained in this study. For this purpose, a two-step approach for constructing recombinant strains that permits the simple introduction of several expression cassettes encoding heterologous proteins into the yeast genome was successfully applied. This study tested two series of integrative multi-copy expression vectors containing cDNAs for the mature forms of P450scc system components (cytochrome P450scc (CYP11A1), adrenodoxin reductase, adrenodoxin, or fused adrenodoxin-P450scc) or for P45017α (CYP17A1) under the control of the isocitrate lyase promoter pICL1, which were constructed using the basic plasmids p64PT or p67PT (rDNA or the long terminal repeat (LTR) zeta of Ylt1 as integration targeting sequences and ura3d4 as a multi-copy selection marker). This study demonstrated the integration of up to three expression vectors containing different heterologous cDNA via their simultaneous transformation into haploid recipient strains. Additionally, further combinations of the different expression cassettes in one strain were obtained by subsequent diploidisation using selected haploid multi-copy transformants. Thus, recombinant strains containing three to five different expression cassettes were obtained, as demonstrated by Southern blotting. Expression of P450scc system proteins was identified by western blotting. The presented method for recombinant strain construction is a useful tool for the heterologous expression of multi-component enzyme systems in Y. lipolytica.

Clinical and molecular characterization of thirty Chinese patients with congenital lipoid adrenal hyperplasia

Congenital lipoid adrenal hyperplasia (LCAH), as the most severe form of congenital adrenal hyperplasia (CAH), is caused by mutations in the steroidogenic acute regulatory protein (STAR). Affected patients were typically characterized by adrenal insufficiency in the first year of life and present with female external genitalia regardless of karyotype. Non-classic LCAH patients usually present from 2 to 4 years old with glucocorticoid deficiency and mild mineralocorticoid deficiency, even develop naturally masculinized external genitalia at birth when they have 46,XY karyotype. We described thirty patients from unrelated Chinese families, including three non-classic LCAH ones. Four novel mutations were reported, including c.556A > G, c.179-15G > T, c.695delG and c.306 + 3_c.306 + 6delAAGT. The c.772C > T is the most common STAR mutation in Chinese population, suggesting a possibility of founder effect. Enzymatic activity assay combined with clinical characteristics showed a good genotype-phenotype correlation in this study. Residual STAR activity more than 20 % may be correlated with non-classic LCAH phenotype. We support the perspective that onset age may be affected by multiple factors and masculinization should be the main weighting factor for diagnosis of non-classic LCAH. Compared with 46,XX LCAH patients, less 46,XY ones were found in our report. A less comprehensive inspection and an easy diagnosis due to classical phenotype both would reduce the possibility of 46,XY LCAH patients to be referred to specialists or geneticists.

Advanced Understanding of the Electron Transfer Pathway of Cytochrome P450s

Cytochrome P450s are heme-thiolate enzymes that participate in carbon source assimilation, natural compound biosynthesis and xenobiotic metabolism in all kingdoms of life. P450s can catalyze various reactions by using a wide range of organic compounds, thus exhibiting great potential in biotechnological applications. The catalytic reactions of P450s are driven by electron equivalents that are sourced from pyridine nucleotides and delivered by cognate or matching redox partners (RPs). The electron transfer (ET) route from RPs to P450s involves one or more redox center-containing domains. As the rate of ET is one of the main determinants of P450 efficacy, an in-depth understanding of the P450 ET pathway should increase our knowledge of these important enzymes and benefit their further applications. Here, the various P450 RP systems along with current understanding of their ET routes will be reviewed. Notably, state-of-the-art structural studies of the two main types of self-sufficient P450 will also be summarized.

Characterization of Two Novel Variants of the Steroidogenic Acute Regulatory Protein Identified in a Girl with Classic Lipoid Congenital Adrenal Hyperplasia

Congenital adrenal hyperplasia (CAH) consists of several autosomal recessive disorders that inhibit steroid biosynthesis. We describe a case report diagnosed with adrenal insufficiency due to low adrenal steroids and adrenocorticotropic hormone excess due to lack of cortisol negative feedback signaling to the pituary gland. Genetic work up revealed two missense variants, p.Thr204Arg and p.Leu260Arg in the STAR gene, inherited by both parents (non-consanguineous). The StAR protein supports CYP11A1 enzyme to cleave the side chain of cholesterol and synthesize pregnenolone which is metabolized to all steroid hormones. We used bioinformatics to predict the impact of the variants on StAR activity and then we performed functional tests to characterize the two novel variants. In a cell system we tested the ability of variants to support cholesterol conversion to pregnenolone and measured their mRNA and protein expression. For both variants, we observed loss of StAR function, reduced protein expression and categorized them as pathogenic variants according to guidelines of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology. These results fit the phenotype of the girl during diagnosis. This study characterizes two novel variants and expands the list of missense variants that cause CAH.

The Adrenal Lipid Droplet is a New Site for Steroid Hormone Metabolism

Steroid hormones play essential roles for living organisms. It has been long and well established that the endoplasmic reticulum (ER) and mitochondria are essential sites for steroid hormone biosynthesis because several steroidogenic enzymes are located in these organelles. The adrenal gland lipid droplet (LD) proteomes from human, macaque monkey, and rodent are analyzed, revealing that steroidogenic enzymes are also present in abundance on LDs. The enzymes found include 3β-hydroxysteroid dehydrogenase (HSD3B) and estradiol 17β-dehydrogenase 11 (HSD17B11). Analyses by Western blot and subcellular localization consistently demonstrate that HSD3B2 is localized on LDs. Furthermore, in vitro experiments confirm that the isolated LDs from HeLa cell stably expressing HSD3B2 or from rat adrenal glands have the capacity to convert pregnenolone to progesterone. Collectively, these data suggest that LDs may be important sites of steroid hormone metabolism. These findings may bring novel insights into the biosynthesis and metabolism of steroid hormones and the development of treatments for adrenal disorders.

Pubertal and Adult Testicular Functions in Nonclassic Lipoid Congenital Adrenal Hyperplasia: A Case Series and Review

Lipoid congenital adrenal hyperplasia (LCAH) is caused by mutations in STAR and characterized by a defect in steroidogenesis and lipid droplet accumulation in steroidogenic cells. Patients with 46,XY and classic LCAH will typically present with female-type external genitalia. However, those with nonclassic LCAH will have masculinized external genitalia. The rarity of the nonclassic form has precluded the clarification of the long-term outcomes of testicular function in nonclassic LCAH. We report the cases of three adult males with nonclassic LCAH in whom primary adrenal insufficiency had been diagnosed at 5 days, 4 years, and 5 years of age. All exhibited complete male external genitalia and had completed pubertal development without androgen replacement. The endocrinological data showed preserved gonadal function in patients 1 and 2 and hypergonadotropic hypogonadism in patient 3. Semen analyses showed normozoospermia in patient 1 and mild oligozoospermia in patient 2. Electron microscopic analysis of a testicular biopsy specimen from patient 2 at 13 years of age revealed prominent lipid accumulation in the cytosol of Leydig cells. Patients 1 and 2 shared the same compound heterozygous mutations in STAR (p.Glu258* and p.Arg272Cys). Patient 3 possessed a heterozygous dominant-negative mutation in STAR (p.Gly22_Leu59del). A functional assay of a variant STAR-Arg272Cys determined the residual activity as 35% of the wild-type STAR. The results from the present case series and a review of four previously reported adult cases indicate that testosterone synthesis can be preserved in most males with nonclassic LCAH to complete pubertal development and induce germ cell maturation despite lipid accumulation in the Leydig cells.

Characterization of the CYP11A1 Nonsynonymous Variant p.E314K in Children Presenting With Adrenal Insufficiency

CONTEXT

Cholesterol side-chain cleavage enzyme (P450scc), encoded by CYP11A1, catalyzes the first step of steroidogenesis. Complete P450scc deficiency leads to primary adrenal insufficiency (PAI) and 46,XY disordered sexual development. Partial impairment can cause variable adrenal and gonadal dysfunction.

OBJECTIVE

Our aim was to evaluate the effects of the CYP11A1 variant p.E314K, identified in patients with PAI, specifically on P450scc enzyme stability and function.

PATIENTS AND METHODS

We studied four boys from two unrelated families presenting with PAI during childhood (3.6 to 9 years old). All patients were compound heterozygous for c.940G>A (p.E314K), a CYP11A1 nonsynonymous variant likely to be pathogenic by some but not all in silico prediction models, and c.835delA (p.I79Yfs*10), a known pathogenic variant. HEK293T cells were transfected with wild type (WT) and p.E314K mutant vectors, and a cycloheximide chase assay was performed to analyze protein stability. Pregnenolone production was assayed from cells expressing WT and p.E314K-F2 fusion proteins.

RESULTS

Two boys experienced spontaneous puberty but then developed evidence of primary gonadal failure at 14 and 18 years old. Two boys had testicular adrenal rest tumor (TART), detected by ultrasound at ages 8.6 and 16 years. Compared with WT, mutant protein synthesis was reduced (P = 0.0006) with increased protein turnover, and mutant P450scc half-life was decreased by ~50%. p.E314K mutant P450scc retained 60% of WT enzymatic activity (P = 0.007).

CONCLUSIONS

The CYP11A1 p.E314K variant impairs P450scc stability and is a possible cause of PAI in childhood. Pathogenic CYP11A1 variants potentially affect both adrenal and gonadal function, and male patients may develop TART.

Predicted Benign and Synonymous Variants in CYP11A1 Cause Primary Adrenal Insufficiency Through Missplicing

Primary adrenal insufficiency (PAI) is a potentially life-threatening condition that can present with nonspecific features and can be difficult to diagnose. We undertook next generation sequencing in a cohort of children and young adults with PAI of unknown etiology from around the world and identified a heterozygous missense variant (rs6161, c.940G>A, p.Glu314Lys) in CYP11A1 in 19 individuals from 13 different families (allele frequency within undiagnosed PAI in our cohort, 0.102 vs 0.0026 in the Genome Aggregation Database; P < 0.0001). Seventeen individuals harbored a second heterozygous rare disruptive variant in CYP11A1 and two had very rare synonymous changes in trans (c.990G>A, Thr330 = ; c.1173C>T, Ser391 =). Although p.Glu314Lys is predicted to be benign and showed no loss-of-function in an Escherichia coli assay system, in silico and in vitro studies revealed that the rs6161/c.940G>A variant, plus the c.990G>A and c.1173C>T changes, affected splicing and that p.Glu314Lys produces a nonfunctional protein in mammalian cells. Taken together, these findings show that compound heterozygosity involving a relatively common and predicted "benign" variant in CYP11A1 is a major contributor to PAI of unknown etiology, especially in European populations. These observations have implications for personalized management and demonstrate how variants that might be overlooked in standard analyses can be pathogenic when combined with other very rare disruptive changes.

Measurement of Mitochondrial Cholesterol Import Using a Mitochondria-Targeted CYP11A1 Fusion Construct

All animal membranes require cholesterol as an essential regulator of biophysical properties and function, but the levels of cholesterol vary widely among different subcellular compartments. Mitochondria, and in particular the inner mitochondrial membrane, have the lowest levels of cholesterol in the cell. Nevertheless, mitochondria need cholesterol for membrane maintenance and biogenesis, as well as oxysterol, steroid, and hepatic bile acid production. Alterations in mitochondrial cholesterol have been associated with a range of pathological conditions, including cancer, hepatosteatosis, cardiac ischemia, Alzheimer's, and Niemann-Pick Type C Disease. The mechanisms of mitochondrial cholesterol import are not fully elucidated yet, and may vary in different cell types and environmental conditions. Measuring cholesterol trafficking to the mitochondrial membranes is technically challenging because of its low abundance; for example, traditional pulse-chase experiments with isotope-labeled cholesterol are not feasible. Here, we describe improvements to a method first developed by the Miller group at the University of California to measure cholesterol trafficking to the inner mitochondrial membrane (IMM) through the conversion of cholesterol to pregnenolone. This method uses a mitochondria-targeted, ectopically expressed fusion construct of CYP11A1, ferredoxin reductase and ferredoxin. Pregnenolone is formed exclusively from cholesterol at the IMM, and can be analyzed with high sensitivity and specificity through ELISA or radioimmunoassay of the medium/buffer to reflect mitochondrial cholesterol import. This assay can be used to investigate the effects of genetic or pharmacological interventions on mitochondrial cholesterol import in cultured cells or isolated mitochondria.

Aberrant Splicing Is the Pathogenicity Mechanism of the p.Glu314Lys Variant in CYP11A1 Gene

Context: The cholesterol side chain cleavage enzyme (CYP11A1) catalyzes the conversion of cholesterol to pregnenolone, the first rate-limiting step of steroidogenesis. CYP11A1 mutations are associated with primary adrenal insufficiency (PAI) as well as disorders of sex development (DSD) in 46,XY patients. Objective: To define the pathogenicity mechanism for the p.Glu314Lys variant, previously reported, and found in four additional patients with CYP11A1 deficiency. Subjects and Methods: DNA of four patients presenting with delayed PAI and/or 46,XY DSD were studied by Sanger or Massively Parallel sequencing. Three CYP11A1 mutations were characterized in vitro and in silico, and one by mRNA analysis on testicular tissue. Results: All patients were compound heterozygous for the previously described p.Glu314Lys variant. In silico studies predicted this mutation as benign with no effect on splicing but mRNA analysis found that it led to incomplete exon 5 skipping. This mechanism was confirmed by minigene experiment. The protein carrying this mutation without exon skipping should conserve almost normal activity, according to in vitro studies. Two other mutations found in trans, the p.Arg120Gln and p.Arg465Trp, had similar activity compared to negative control, consistent with the in silico studies. Conclusions: We provide biological proof that the p. Glu314Lys variant is pathogenic due to its impact on splicing and seems responsible for the moderate phenotype of the four patients reported herein. The present study highlights the importance of considering the potential effect of a missense variant on splicing when it is not predicted to be disease causing.

Disorders in the initial steps of steroid hormone synthesis

Steroidogenesis begins with cellular internalization of low-density lipoprotein particles and subsequent intracellular processing of cholesterol. Disorders in these steps include Adrenoleukodystrophy, Wolman Disease and its milder variant Cholesterol Ester Storage Disease, and Niemann-Pick Type C Disease, all of which may present with adrenal insufficiency. The means by which cholesterol is directed to steroidogenic mitochondria remains incompletely understood. Once cholesterol reaches the outer mitochondrial membrane, its delivery to the inner mitochondrial membrane is regulated by the steroidogenic acute regulatory protein (StAR). Severe StAR mutations cause classic congenital lipoid adrenal hyperplasia, characterized by lipid accumulation in the adrenal, adrenal insufficiency, and disordered sexual development in 46,XY individuals. The lipoid CAH phenotype, including spontaneous puberty in 46,XX females, is explained by a two-hit model. StAR mutations that retain partial function cause a milder, non-classic disease characterized by glucocorticoid deficiency, with lesser disorders of mineralocorticoid and sex steroid synthesis. Once inside the mitochondria, cholesterol is converted to pregnenolone by the cholesterol side-chain cleavage enzyme, P450scc, encoded by the CYP11A1 gene. Rare patients with mutations of P450scc are clinically and hormonally indistinguishable from those with lipoid CAH, and may also present as milder non-classic disease. Patients with P450scc defects do not have the massive adrenal hyperplasia that characterizes lipoid CAH, but adrenal imaging may occasionally fail to distinguish these, necessitating DNA sequencing.

Thirty-Eight-Year Follow-Up of Two Sibling Lipoid Congenital Adrenal Hyperplasia Patients Due to Homozygous Steroidogenic Acute Regulatory (STARD1) Protein Mutation. Molecular Structure and Modeling of the STARD1 L275P Mutation

Objective: Review the impact of StAR (STARD1) mutations on steroidogenesis and fertility in LCAH patients. Examine the endocrine mechanisms underlying the pathology of the disorder and the appropriate therapy for promoting fertility and pregnancies.

Design: Published data in the literature and a detailed 38-year follow-up of two sibling LCAH patients. Molecular structure and modeling of the STARD1 L275P mutation.

Setting: University hospital.

Patients: Patient A (46,XY female phenotype) and patient B (46,XX female) with LCAH bearing the L275P mutation in STARD1.

Interventions: Since early-age diagnosis, both patients underwent corticoid replacement therapy. Patient A received estrogen therapy at pubertal age. Clomiphene therapy was given to Patient B to induce ovulation. Pregnancies were protected with progesterone administration.

Main Outcome Measures: Clinical and molecular assessment of adrenal and gonadal functions.

Results: Both patients have classic manifestations of corticosteroid deficiency observed in LCAH. Time of onset and severity were different. Patient A developed into a female phenotype due to early and severe damage of Leydig cells. Patient B started a progressive pubertal development, menarche and regular non-ovulatory cycle. She was able to have successful pregnancies.

Conclusions: Understanding the molecular structure and function of STARD1 in all steroidogenic tissues is the key for comprehending the heterogeneous clinical manifestations of LCAH, and the development of an appropriate strategy for the induction of ovulation and protecting pregnancies in this disease.

Steroids and genes related to steroid biosynthesis in the female giant freshwater prawn, Macrobrachium rosenbergii

The giant freshwater prawn, Macrobrachium rosenbergii, is important to many Asian countries due to its high economic value as an aquaculture product. With demand increasing, there is requirement for a better understanding of the biosynthetic components that regulate its growth and reproduction, including steroids, in order to help increase production. Vertebrate-type steroids and their receptors were identified in crustaceans and implicated in reproduction. In this study, we presented the sex steroids estradiol and progesterone by LC-MS/MS in female M. rosenbergii, and reveal steroidogenic-related genes by in silico analysis of de novo assembled transcriptomes. Comparative analysis with other species was performed to confirm their putative role, as well as tissue-specific and quantitative gene expression. We reveal 29 transcripts that encode for steroidogenic-related proteins, including steroidogenic enzymes, a nuclear steroid hormone receptors, and a steroidogenic factor. Moreover, we identified for the first time the presence of steroidogenic factor 1, StAR-related lipid transfer protein, estradiol receptor- and progesterone-like protein in M. rosenbergii. Those targeted for gene expression analysis (3 beta-hydroxysteroid dehydrogenase, 17 beta-hydroxysteroid dehydrogenase, estrogen sulfotransferase and progesterone receptor-like) showed widespread expression within many tissues, and at relatively high levels in the central nervous system (CNS) during ovarian maturation. In summary, we provide further evidence for the existence of steroidogenic pathways in crustaceans, which may be useful for advancing prawn aquaculture.

An Outer Mitochondrial Translocase, Tom22, Is Crucial for Inner Mitochondrial Steroidogenic Regulation in Adrenal and Gonadal Tissues

After cholesterol is transported into the mitochondria of steroidogenic tissues, the first steroid, pregnenolone, is synthesized in adrenal and gonadal tissues to initiate steroid synthesis by catalyzing the conversion of pregnenolone to progesterone, which is mediated by the inner mitochondrial enzyme 3β-hydroxysteroid dehydrogenase 2 (3βHSD2). We report that the mitochondrial translocase Tom22 is essential for metabolic conversion, as its knockdown by small interfering RNA (siRNA) completely ablated progesterone conversion in both steroidogenic mouse Leydig MA-10 and human adrenal NCI cells. Tom22 forms a 500-kDa complex with mitochondrial proteins associated with 3βHSD2. Although the absence of Tom22 did not inhibit mitochondrial import of cytochrome P450scc (cytochrome P450 side chain cleavage enzyme) and aldosterone synthase, it did inhibit 3βHSD2 expression. Electron microscopy showed that Tom22 is localized at the outer mitochondrial membrane (OMM), while 3βHSD2 is localized at the inner mitochondrial space (IMS), where it interacts through a specific region with Tom22 with its C-terminal amino acids and a small amino acid segment of Tom22 exposed to the IMS. Therefore, Tom22 is a critical regulator of steroidogenesis, and thus, it is essential for mammalian survival.

Adrenal Mitochondria and Steroidogenesis: From Individual Proteins to Functional Protein Assemblies

The adrenal cortex is critical for physiological function as the central site of glucocorticoid and mineralocorticoid synthesis. It possesses a great degree of specialized compartmentalization at multiple hierarchical levels, ranging from the tissue down to the molecular levels. In this paper, we discuss this functionalization, beginning with the tissue zonation of the adrenal cortex and how this impacts steroidogenic output. We then discuss the cellular biology of steroidogenesis, placing special emphasis on the mitochondria. Mitochondria are classically known as the "powerhouses of the cell" for their central role in respiratory adenosine triphosphate synthesis, and attention is given to mitochondrial electron transport, in both the context of mitochondrial respiration and mitochondrial steroid metabolism. Building on work demonstrating functional assembly of large protein complexes in respiration, we further review research demonstrating a role for multimeric protein complexes in mitochondrial cholesterol transport, steroidogenesis, and mitochondria-endoplasmic reticulum contact. We aim to highlight with this review the shift in steroidogenic cell biology from a focus on the actions of individual proteins in isolation to the actions of protein assemblies working together to execute cellular functions.

Design and improvement of artificial redox modules by molecular fusion of flavodoxin and flavodoxin reductase from Escherichia coli

A variety of fusion proteins between the versatile redox partners flavodoxin (FldA) and flavodoxin reductase (Fpr) from Escherichia coli was constructed with the aim to improve the electron transfer properties. The order in which FldA and Fpr were fused and the linker region between them was varied in a systematic manner. A simple molecular tool, designated "DuaLinX", was developed that facilitated the parallel introduction of flexible glycine-rich and rigid proline-rich linkers between the fusion partners in a single cloning event. The fusion constructs were tested for their ability to transfer electrons to cytochrome c and cytochrome P450 109B1 from Bacillus subtilis. With CYP109B1, the performance of the constructs showed, independent of the domain order, a strong dependency on linker length, whereas with cytochrome c this phenomenon was less pronounced. Constructs carrying linkers of ≥15 residues effectively supported the CYP109B1-catalysed hydroxylation of myristic acid. Constructs carrying proline-rich linkers generally outperformed their glycine-rich counterparts. The best construct, FldA-Fpr carrying linker ([E/L]PPPP)4, supported CYP109B1 activity equally well as equivalent amounts of the non-fused redox partners, while cytochrome c reductase activity was ~2.7-fold improved. Thus, to functionally connect redox partners, rigid proline-rich linkers may be attractive alternatives to the commonly used flexible glycine-rich linkers.

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

Mitochondria-associated endoplasmic reticulum membrane (MAM) regulates steroidogenic activity via steroidogenic acute regulatory protein (StAR)-voltage-dependent anion channel 2 (VDAC2) interaction

Steroid hormones are essential for carbohydrate metabolism, stress management, and reproduction and are synthesized from cholesterol in mitochondria of adrenal glands and gonads/ovaries. In acute stress or hormonal stimulation, steroidogenic acute regulatory protein (StAR) transports substrate cholesterol into the mitochondria for steroidogenesis by an unknown mechanism. Here, we report for the first time that StAR interacts with voltage-dependent anion channel 2 (VDAC2) at the mitochondria-associated endoplasmic reticulum membrane (MAM) prior to its translocation to the mitochondrial matrix. In the MAM, StAR interacts with mitochondrial proteins Tom22 and VDAC2. However, Tom22 knockdown by siRNA had no effect on pregnenolone synthesis. In the absence of VDAC2, StAR was expressed but not processed into the mitochondria as a mature 30-kDa protein. VDAC2 interacted with StAR via its C-terminal 20 amino acids and N-terminal amino acids 221-229, regulating the mitochondrial processing of StAR into the mature protein. In the absence of VDAC2, StAR could not enter the mitochondria or interact with MAM-associated proteins, and therefore steroidogenesis was inhibited. Furthermore, the N terminus was not essential for StAR activity, and the N-terminal deletion mutant continued to interact with VDAC2. The endoplasmic reticulum-targeting prolactin signal sequence did not affect StAR association with the MAM and thus its mitochondrial targeting. Therefore, VDAC2 controls StAR processing and activity, and MAM is thus a central location for initiating mitochondrial steroidogenesis.

STARD6 is expressed in steroidogenic cells of the ovary and can enhance de novo steroidogenesis

STARD6 is a member of the StAR-related lipid transfer (START) domain family of proteins whose function thus far remains obscure. While it recently was shown to facilitate steroidogenesis in a cell-free setting, it has not been localized to steroidogenic cells of normal reproductive tissues. In a recent microarray study, we detected STARD6 mRNA in cultured porcine ovarian granulosa cells which are steroidogenic. In the present study, we examined regulation of STARD6 mRNA in porcine granulosa cultures, and found that it was not regulated by cyclic AMP, but it was reduced by combined knockdown of the transcription factors GATA4 and GATA6. We detected both STARD6 mRNA and protein in fresh granulosa cells and whole antral follicles and different stage corpora lutea of pig. The highest levels were discovered in the mid-luteal phase corpus luteum. Immunolocalization within ovarian tissues indicated robust STARD6 immunoreactivity in steroidogenic cells of the corpus luteum. Relatively lesser amounts of STARD6 signal were found in granulosa cells, theca cells, and oocytes. To test the ability of STARD6 to facilitate de novo steroidogenesis, non-steroidogenic COS-1 cells were co-transfected with components of the P450 cholesterol side-chain cleavage system, enabling them to make pregnenolone, and STARD6. STARD6 increased pregnenolone production by two- to three-fold over the empty vector control. In summary, STARD6 is found in the pig ovary, exhibits the strongest expression in highly steroidogenic luteal cells, and significantly enhances pregnenolone production in transfected COS cells independent of cyclic AMP treatment. Collectively, these findings indicate that STARD6 may contribute to steroidogenesis in ovarian cells, but also suggests other cellular functions that require cholesterol trafficking.

Steroid hormone synthesis in mitochondria

Mitochondria are essential sites for steroid hormone biosynthesis. Mitochondria in the steroidogenic cells of the adrenal, gonad, placenta and brain contain the cholesterol side-chain cleavage enzyme, P450scc, and its two electron-transfer partners, ferredoxin reductase and ferredoxin. This enzyme system converts cholesterol to pregnenolone and determines net steroidogenic capacity, so that it serves as the chronic regulator of steroidogenesis. Several other steroidogenic enzymes, including 3β-hydroxysteroid dehydrogenase, 11β-hydroxylase and aldosterone synthase also reside in mitochondria. Similarly, the mitochondria of renal tubular cells contain two key enzymes participating in the activation and degradation of vitamin D. The access of cholesterol to the mitochondria is regulated by the steroidogenic acute regulatory protein, StAR, serving as the acute regulator of steroidogenesis. StAR action requires a complex multi-component molecular machine on the outer mitochondrial membrane (OMM). Components of this machine include the 18 kDa translocator protein (TSPO), the voltage-dependent anion chanel (VDAC-1), TSPO-associated protein 7 (PAP7, ACBD3), and protein kinase A regulatory subunit 1α (PKAR1A). The precise fashion in which these proteins interact and move cholesterol from the OMM to P450scc, and the means by which cholesterol is loaded into the OMM, remain unclear. Human deficiency diseases have been described for StAR and for all the mitochondrial steroidogenic enzymes, but not for the electron transfer proteins or for the components of the cholesterol import machine.

Varied clinical presentations of seven patients with mutations in CYP11A1 encoding the cholesterol side-chain cleavage enzyme, P450scc

CONTEXT

The cholesterol side-chain cleavage enzyme P450scc, encoded by CYP11A1, converts cholesterol to pregnenolone to initiate steroidogenesis. P450scc deficiency can disrupt adrenal and gonadal steroidogenesis, resembling congenital lipoid adrenal hyperplasia clinically and hormonally; only 12 such patients have been reported previously.

OBJECTIVE

We sought to expand clinical and genetic experience with P450scc deficiency.

PATIENTS AND METHODS

We sequenced candidate genes in 7 children with adrenal insufficiency who lacked disordered sexual development. P450scc missense mutations were recreated in the F2 vector, which expresses the fusion protein P450scc-Ferredoxin Reductase-Ferredoxin. COS-1 cells were transfected, production of pregnenolone was assayed, and apparent kinetic parameters were calculated. Previously described P450scc mutants were assayed in parallel.

RESULTS

Four of five Bedouin children in one kindred were compound heterozygotes for mutations c.694C>T (Arg232Stop) and c.644T>C (Phe215Ser). Single-nucleotide polymorphism analysis confirmed segregation of these mutations. The fifth kindred member and another Bedouin patient presented in infancy and were homozygous for Arg232Stop. A patient from Fiji presenting in infancy was homozygous for c.358T>C (Arg120Stop). All mutations are novel. As assayed in the F2 fusion protein, P450scc Phe215Ser retained 2.5% of wild-type activity; previously described mutants Leu141Trp and Ala269Val had 2.6% and 12% of wild-type activity, respectively, and Val415Glu and c.835delA lacked detectable activity.

CONCLUSIONS

Although P450scc is required to produce placental progesterone required to maintain pregnancy, severe mutations in P450scc are compatible with term gestation; milder P450scc mutations may present later without disordered sexual development. Enlarged adrenals usually distinguish steroidogenic acute regulatory protein deficiency from P450scc deficiency, but only DNA sequencing is definitive.

Role of mitochondria in steroidogenesis

Adrenal gonadal, placental and brain mitochondria contain several steroidogenic enzymes, notably the cholesterol side chain cleavage enzyme, P450scc, which is the enzymatic rate-limiting step in steroidogenesis which determines cellular steroidogenic capacity. Even before this step, the access of cholesterol to this enzyme system is both rate-limiting and the site of acute regulation via the steroidogenic acute regulatory protein (StAR) which interacts with a complex multi-component 'transduceosome' on the outer mitochondrial membrane (OMM). The components of the transduceosome include the 18 kDa translocator protein (TSPO), the voltage-dependent anion channel (VDAC-1), TSPO-associated protein 7 (PAP7, ACBD3 for acyl-CoA-binding-domain 3), and protein kinase A regulatory subunit 1α (PKAR1A). The precise fashion in which these proteins interact and move cholesterol from the OMM to P450scc, and the means by which cholesterol is loaded into the OMM, remain unclear. Human deficiency diseases have been described for StAR and for P450scc. Mitochondria also contain several 'downstream' steroidogenic enzymes.

Niemann-Pick Type C2 protein contributes to the transport of endosomal cholesterol to mitochondria without interacting with NPC1

Mitochondrial cholesterol is maintained within a narrow range to regulate steroid and oxysterol synthesis and to ensure mitochondrial function. Mitochondria acquire cholesterol through several pathways from different cellular pools. Here we have characterized mitochondrial import of endosomal cholesterol using Chinese hamster ovary cells expressing a CYP11A1 fusion protein that converts cholesterol to pregnenolone at the mitochondrial inner membrane. RNA interference-mediated depletion of the voltage-dependent anion channel 1 in the mitochondrial outer membrane or of Niemann-Pick Type C2 (NPC2) in the endosome lumen decreased arrival of cholesterol at the mitochondrial inner membrane. Expression of NPC2 mutants unable to transfer cholesterol to NPC1 still restored mitochondrial cholesterol import in NPC2-depleted cells. Transport assays in semi-permeabilized cells showed nonvesicular cholesterol trafficking directly from endosomes to mitochondria that did not require cytosolic transport proteins but that was reduced in the absence of NPC2. Our findings indicate that NPC2 delivers cholesterol to the perimeter membrane of late endosomes, where it becomes available for transport to mitochondria without requiring NPC1.

Resveratrol stimulates cortisol biosynthesis by activating SIRT-dependent deacetylation of P450scc

In the human adrenal cortex, cortisol is synthesized from cholesterol by members of the cytochrome P450 superfamily and hydroxysteroid dehydrogenases. Both the first and last steps of cortisol biosynthesis occur in mitochondria. Based on our previous findings that activation of ACTH signaling changes the ratio of nicotinamide adenine dinucleotide (NAD) phosphate to reduced NAD phosphate in adrenocortical cells, we hypothesized that pyridine nucleotide metabolism may regulate the activity of the mitochondrial NAD(+)-dependent sirtuin (SIRT) deacetylases. We show that resveratrol increases the protein expression and half-life of P450 side chain cleavage enzyme (P450scc). The effects of resveratrol on P450scc protein levels and acetylation status are dependent on SIRT3 and SIRT5 expression. Stable overexpression of SIRT3 abrogates the cellular content of acetylated P450scc, concomitant with an increase in P450scc protein expression and cortisol secretion. Mutation of K148 and K149 to alanine stabilizes the expression of P450scc and results in a 1.5-fold increase in pregnenolone biosynthesis. Finally, resveratrol also increases the protein expression of P450 11β, another mitochondrial enzyme required for cortisol biosynthesis. Collectively, this study identifies a role for NAD(+)-dependent SIRT deacetylase activity in regulating the expression of mitochondrial steroidogenic P450.

Alteration in accumulated aldosterone synthesis as a result of N-terminal cleavage of aldosterone synthase

Aldosterone synthase (AS) regulates blood volume by synthesizing the mineralocorticoid aldosterone. Overproduction of aldosterone in the adrenal gland can lead to hypertension, a major cause of heart disease and stroke. Aldosterone production depends upon stimulation of AS expression by the renin-angiotensin system, which takes 12 h to reach full effect, and then 24 h to subside. However, this promoter-dependent regulation of aldosterone production fails to explain phenomena such as rapid-onset hypertension that occurs quickly and then subsides. Here, we investigate the fate of AS after expression and how these events relate to aldosterone production. Using isolated mitochondria from steroidogenic cells and cell-free synthesized AS, we first showed that the precursor form of AS translocated into the matrix of the mitochondria, where it underwent cleavage by mitochondrial processing peptidase to a mature form approximately 54 kDa in size. Mature AS seemed to translocate across the inner mitochondrial membrane a second time to finally reside in the intermembrane space. Unprocessed N-terminal AS has 2-fold more activity than physiological levels. These results show how the subcellular mechanisms of AS localization relate to production of aldosterone and reveal a rapid, promoter-independent regulation of aldosterone production.

High allele frequency of the p.Q258X mutation and identification of a novel mis-splicing mutation in the STAR gene in Korean patients with congenital lipoid adrenal hyperplasia

OBJECTIVE

Steroidogenic acute regulatory (STAR) protein plays a crucial role in steroidogenesis, and mutations in the STAR gene cause congenital lipoid adrenal hyperplasia (CLAH). This study investigated the STAR mutation spectrum and functionally analyzed a novel STAR mutation in Korean patients with CLAH.

METHODS

Mutation analysis of STAR was carried out in 25 unrelated Korean CLAH patients. A region of STAR comprising exons 4-7 was cloned from human genomic DNA into an expression vector, followed by site-directed mutagenesis and transient expression in COS7 cells. The splicing pattern was analyzed by in vitro transcription, and each transcript was functionally characterized by measuring pregnenolone production in COS7 cells cotransfected with the cholesterol side chain cleavage system.

RESULTS

Mutation p.Q258X was identified in 46 of 50 alleles (92%); mutation c.653C>T was detected in two alleles (4%); and mutations p.R182H and c.745-6_810del were found in one allele (2%). Reverse transcriptase-PCR products amplified from a patient heterozygous for compound c.653C>T and c.745-6_810del mutation revealed multiple alternatively spliced mRNAs. In vitro expression analysis of a minigene consisting of exons 4-7 containing the c.653C>T yielded two transcripts in which exon 6 or exons 5 and 6 were skipped. The encoded proteins exhibited defective pregnenolone-producing ability. The c.745-6_810del mutation led to full and partial intron retention.

CONCLUSIONS

p.Q258X is the most common STAR mutation in Korea. A previously reported c.653C>T variant was found to cause aberrant splicing at the mRNA level, resulting in perturbation of STAR function. The c.745-6_810del mutation also resulted in aberrant splicing.

Early steps in steroidogenesis: intracellular cholesterol trafficking

Steroid hormones are made from cholesterol, primarily derived from lipoproteins that enter cells via receptor-mediated endocytosis. In endo-lysosomes, cholesterol is released from cholesterol esters by lysosomal acid lipase (LAL; disordered in Wolman disease) and exported via Niemann-Pick type C (NPC) proteins (disordered in NPC disease). These diseases are characterized by accumulated cholesterol and cholesterol esters in most cell types. Mechanisms for trans-cytoplasmic cholesterol transport, membrane insertion, and retrieval from membranes are less clear. Cholesterol esters and "free" cholesterol are enzymatically interconverted in lipid droplets. Cholesterol transport to the cholesterol-poor outer mitochondrial membrane (OMM) appears to involve cholesterol transport proteins. Cytochrome P450scc (CYP11A1) then initiates steroidogenesis by converting cholesterol to pregnenolone on the inner mitochondrial membrane (IMM). Acute steroidogenic responses are regulated by cholesterol delivery from OMM to IMM, triggered by the steroidogenic acute regulatory protein (StAR). Chronic steroidogenic capacity is determined by CYP11A1 gene transcription. StAR mutations cause congenital lipoid adrenal hyperplasia, with absent steroidogenesis, potentially lethal salt loss, and 46,XY sex reversal. StAR mutations initially destroy most, but not all steroidogenesis; low levels of StAR-independent steroidogenesis are lost later due to cellular damage, explaining the clinical findings. Rare P450scc mutations cause a similar syndrome. This review addresses these early steps in steroid biosynthesis.

Inner mitochondrial translocase Tim50 interacts with 3β-hydroxysteroid dehydrogenase type 2 to regulate adrenal and gonadal steroidogenesis

In the adrenals, testes, and ovaries, 3β-hydroxysteroid dehydrogenase type 2 (3βHSD2) catalyzes the conversion of pregnenolone to progesterone and dehydroepiandrostenedione to androstenedione. Alterations in this pathway can have deleterious effects, including sexual development impairment, spontaneous abortion, and breast cancer. 3βHSD2, synthesized in the cytosol, is imported into the inner mitochondrial membrane (IMM) by translocases. Steroidogenesis requires that 3βHSD2 acts as both a dehydrogenase and isomerase. To achieve this dual functionality, 3βHSD2 must undergo a conformational change; however, what triggers that change remains unknown. We propose that 3βHSD2 associates with IMM or outer mitochondrial membrane translocases facing the intermembrane space (IMS) and that this interaction promotes the conformational change needed for full activity. Fractionation assays demonstrate that 3βHSD2 associated with the IMM but did not integrate into the membrane. Through mass spectrometry and Western blotting of mitochondrial complexes and density gradient ultracentrifugation, we show that that 3βHSD2 formed a transient association with the translocases Tim50 and Tom22 and with Tim23. This association occurred primarily through the interaction of Tim50 with the N terminus of 3βHSD2 and contributed to enzymatic activity. Tim50 knockdown inhibited catalysis of dehydroepiandrostenedione to androstenedione and pregnenolone to progesterone. Although Tim50 knockdown decreased 3βHSD2 expression, restoration of expression via proteasome and protease inhibition did not rescue activity. In addition, protein fingerprinting and CD spectroscopy reveal the flexibility of 3βHSD2, a necessary characteristic for forming multiple associations. In summary, Tim50 regulates 3βHSD2 expression and activity, representing a new role for translocases in steroidogenesis.

Characterization of novel StAR (steroidogenic acute regulatory protein) mutations causing non-classic lipoid adrenal hyperplasia

Context

Steroidogenic acute regulatory protein (StAR) is crucial for transport of cholesterol to mitochondria where biosynthesis of steroids is initiated. Loss of StAR function causes lipoid congenital adrenal hyperplasia (LCAH).

Objective

StAR gene mutations causing partial loss of function manifest atypical and may be mistaken as familial glucocorticoid deficiency. Only a few mutations have been reported.

Design

To report clinical, biochemical, genetic, protein structure and functional data on two novel StAR mutations, and to compare them with published literature.

Setting

Collaboration between the University Children's Hospital Bern, Switzerland, and the CIBERER, Hospital Vall d'Hebron, Autonomous University, Barcelona, Spain.

Patients

Two subjects of a non-consanguineous Caucasian family were studied. The 46,XX phenotypic normal female was diagnosed with adrenal insufficiency at the age of 10 months, had normal pubertal development and still has no signs of hypergonodatropic hypogonadism at 32 years of age. Her 46,XY brother was born with normal male external genitalia and was diagnosed with adrenal insufficiency at 14 months. Puberty was normal and no signs of hypergonadotropic hypogonadism are present at 29 years of age.

Results

StAR gene analysis revealed two novel compound heterozygote mutations T44HfsX3 and G221S. T44HfsX3 is a loss-of-function StAR mutation. G221S retains partial activity (∼30%) and is therefore responsible for a milder, non-classic phenotype. G221S is located in the cholesterol binding pocket and seems to alter binding/release of cholesterol.

Conclusions

StAR mutations located in the cholesterol binding pocket (V187M, R188C, R192C, G221D/S) seem to cause non-classic lipoid CAH. Accuracy of genotype-phenotype prediction by in vitro testing may vary with the assays employed.

Partial defect in the cholesterol side-chain cleavage enzyme P450scc (CYP11A1) resembling nonclassic congenital lipoid adrenal hyperplasia

CONTEXT

The cholesterol side-chain cleavage enzyme (P450scc), encoded by the CYP11A1 gene, converts cholesterol to pregnenolone to initiate steroidogenesis. Genetic defects in P450scc cause a rare autosomal recessive disorder that is clinically indistinguishable from congenital lipoid adrenal hyperplasia (lipoid CAH). Nonclassic lipoid CAH is a recently recognized disorder caused by mutations in the steroidogenic acute regulatory protein (StAR) that retain partial function.

OBJECTIVE

We describe two siblings with hormonal findings suggesting nonclassic lipoid CAH, who had a P450scc mutation that retains partial function.

PATIENTS AND METHODS

A 46,XY male presented with underdeveloped genitalia and partial adrenal insufficiency; his 46,XX sister presented with adrenal insufficiency. Hormonal studies suggested nonclassic lipoid CAH. Sequencing of the StAR gene was normal, but compound heterozygous mutations were found in the CYP11A1 gene. Mutations were recreated in the F2 plasmid expressing a fusion protein of the cholesterol side-chain cleavage system. P450scc activity was measured as Vmax/Km for pregnenolone production in transfected COS-1 cells.

RESULTS

The patients were compound heterozygous for the previously described frameshift mutation 835delA and the novel missense mutation A269V. When expressed in the P450scc moiety of F2, the A269V mutant retained 11% activity of the wild-type F2 protein.

CONCLUSIONS

There is a broad clinical spectrum of P450scc deficiency. Partial loss-of-function CYP11A1 mutation can present with a hormonal phenotype indistinguishable from nonclassic lipoid CAH.

The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders

Steroidogenesis entails processes by which cholesterol is converted to biologically active steroid hormones. Whereas most endocrine texts discuss adrenal, ovarian, testicular, placental, and other steroidogenic processes in a gland-specific fashion, steroidogenesis is better understood as a single process that is repeated in each gland with cell-type-specific variations on a single theme. Thus, understanding steroidogenesis is rooted in an understanding of the biochemistry of the various steroidogenic enzymes and cofactors and the genes that encode them. The first and rate-limiting step in steroidogenesis is the conversion of cholesterol to pregnenolone by a single enzyme, P450scc (CYP11A1), but this enzymatically complex step is subject to multiple regulatory mechanisms, yielding finely tuned quantitative regulation. Qualitative regulation determining the type of steroid to be produced is mediated by many enzymes and cofactors. Steroidogenic enzymes fall into two groups: cytochrome P450 enzymes and hydroxysteroid dehydrogenases. A cytochrome P450 may be either type 1 (in mitochondria) or type 2 (in endoplasmic reticulum), and a hydroxysteroid dehydrogenase may belong to either the aldo-keto reductase or short-chain dehydrogenase/reductase families. The activities of these enzymes are modulated by posttranslational modifications and by cofactors, especially electron-donating redox partners. The elucidation of the precise roles of these various enzymes and cofactors has been greatly facilitated by identifying the genetic bases of rare disorders of steroidogenesis. Some enzymes not principally involved in steroidogenesis may also catalyze extraglandular steroidogenesis, modulating the phenotype expected to result from some mutations. Understanding steroidogenesis is of fundamental importance to understanding disorders of sexual differentiation, reproduction, fertility, hypertension, obesity, and physiological homeostasis.

Regulation of mRNAs encoding the steroidogenic acute regulatory protein and cholesterol side-chain cleavage enzyme in the elasmobranch interrenal gland

The rate-limiting and regulated step in steroidogenesis, the conversion of cholesterol to pregnenolone, is facilitated by the steroidogenic acute regulatory protein (StAR) and cytochrome P450 cholesterol side-chain cleavage (P450scc). We have isolated cDNAs encoding StAR and P450scc from the Atlantic stingray, Dasyatis sabina, and characterized the steroidogenic activity of the encoded proteins using a heterologous expression system. Green monkey kidney (COS-1) cells cotransfected with D. sabina StAR and human P450scc/adrenodoxin reductase/adrenodoxin fusion (F2) constructs produced significantly more pregnenolone than cells transfected with the F2 construct alone. COS-1 cells transfected with a modified F2 construct (F2DS) in which human P450scc is replaced by D. sabina P450scc had higher rates than cells transfected with D. sabina P450scc alone. In other vertebrates, the stress peptide adrenocorticotropic hormone (ACTH) elicits its effects on corticosteroidogenesis in part through regulation of StAR and P450scc mRNAs. In vitro incubation of D. sabina interrenal tissue with porcine ACTH significantly increased intracellular cAMP and corticosteroid production. As demonstrated by quantitative PCR, ACTH also induced significant increases in mRNA abundance of both StAR and P450scc. Our results suggest that, as in higher vertebrates, chronic ACTH-induced glucocorticoid synthesis in elasmobranchs is mediated by regulation of primary steroidogenic mRNAs. This study is the first to demonstrate steroidogenic activity of an elasmobranch P450scc protein and express a composite elasmobranch steroidogenic pathway in a heterologous cell line. Also, the regulation of StAR and P450scc mRNAs has not previously been demonstrated in elasmobranch fishes.

Clinical, genetic, and functional characterization of four patients carrying partial loss-of-function mutations in the steroidogenic acute regulatory protein (StAR)

CONTEXT

Nonclassic congenital lipoid adrenal hyperplasia (lipoid CAH) is a recently recognized disorder caused by mutations in the steroidogenic acute regulatory protein (StAR) that retain partial function. Affected individuals can present with a phenotype of late onset adrenal insufficiency with only mild or minimally disordered sexual development.

OBJECTIVES

The aim was to delineate the clinical spectrum of StAR mutations and correlate phenotype with StAR activity.

PATIENTS

Four patients had nonclassic/atypical lipoid CAH. Adrenal insufficiency was manifested at birth in two patients and at 11 months and 4 yr in the other two. Three were 46,XY with underdeveloped genitalia.

METHODS

The StAR gene was sequenced, mutations were recreated in expression vectors, and StAR activity was measured as pregnenolone production in COS-1 cells cotransfected with the cholesterol side-chain cleavage system. StAR mutants were expressed as N-62 StAR in bacteria, and purified proteins were tested for activity with isolated steroidogenic mitochondria and for cholesterol-binding capacity.

RESULTS

DNA sequencing identified mutations on all alleles. Missense mutations were R188C, G221D, L260P, and F267S; we also tested R192C described by others. The respective activities of R188C, R192C, G221D, L260P, and F267S were 8.0, 39.4, 2.4, 3.1, and 6.1% of wild-type in transfected cells, and 12.8, 54.8, 6.3, 1.8, and 9.5% with isolated mitochondria. Cholesterol binding capacities of R188C, R192C, G221D, L260P, and F267S were 6.7, 55.3, 10.2, 4.6, and 20.9%. These data are correlated to the three-dimensional structure of StAR.

CONCLUSIONS

There is a broad clinical spectrum of StAR mutations; StAR activities in vitro correlate well with clinical phenotypes.

Steroidogenic acute regulatory protein has a more open conformation than the independently folded smaller subdomains

The acute steroidogenic response, which produces steroids in response to stress, requires the steroidogenic acute regulatory protein (StAR). StAR, a mitochondrial matrix protein, acts on the outer mitochondrial membrane (OMM) to facilitate the movement of cholesterol from the outer to inner mitochondrial membrane via an unknown mechanism. The N-terminal sequence was reported to be nonessential for activity. We show that alteration of the StAR amino-terminal sequence does not change the thermodynamic stability of StAR but offers protection from proteolytic degradation. A longer association between StAR and the OMM strengthens the interaction with cholesterol. Far-UV CD spectra showed that the smaller fragments of StAR domains were less alpha-helical compared to N-62 StAR but were structured as determined by limited proteolysis followed by mass spectrometry. The START domain consisting of amino acids 63-193 also exhibited protease protection for amino acids 84-193. The Stern-Volmer quenching constant (K(SV)) of the N-62 StAR protein is 12.1 x 10(5) M(-1), with all other START fragments having significantly smaller K(SV) values ranging from 6 to 10 x 10(5) M(-1), showing that N-62 StAR has a more open conformation. Only N-62 StAR protein is stabilized with cholesterol having an increased DeltaH value of -5.6 +/- 0.3 kcal/mol at 37 degrees C. These findings demonstrate a mechanism in which StAR is stabilized at the OMM by cholesterol to initiate its massive import into mitochondria.

Structural diversity of cytochrome P450 enzyme system

Cytochrome P450 enzyme system consists of P450 and its NAD(P)H-linked reductase or reducing system, and catalyses monooxygenation reactions. The most prevalent type in eukaryotic organisms is 'microsomes type', which consists of membrane-bound P450 and NADPH-P450 reductase. The second type is 'mitochondria type', in which P450 is bound to the inner membrane while the reducing system consisting of an NADPH-linked flavoprotein and a ferredoxin-type iron-sulphur protein is soluble in the matrix space. The third type is 'bacteria type', in which both P450 and the reducing system are soluble in the cytoplasm. In addition to these three types, several forms of P450-reductase fusion proteins have been found in prokaryotic organisms. On the other hand, some P450s catalyse the re-arrangement of the oxygen atoms in the substrate molecules that does not require the supply of reducing equivalents for the reaction. A peculiar P450, P450nor, receives electrons directly from NADH for the reduction of nitric oxide.

MLN64 mediates egress of cholesterol from endosomes to mitochondria in the absence of functional Niemann-Pick Type C1 protein

Niemann-Pick Type C (NPC) disease is a fatal, neurodegenerative disorder, caused in most cases by mutations in the late endosomal protein NPC1. A hallmark of NPC disease is endosomal cholesterol accumulation and an impaired cholesterol homeostatic response, which might affect cholesterol transport to mitochondria and, thus, mitochondrial and cellular function. This study aimed to characterize mitochondrial cholesterol homeostasis in NPC disease. Using wild-type and NPC1-deficient Chinese hamster ovary cells, stably transfected with a CYP11A1 complex to assess mitochondrial cholesterol import by pregnenolone production, we show that cholesterol transport to the mitochondrial inner membrane is not affected by loss of NPC1. However, mitochondrial cholesterol content was higher in NPC1-deficient than in wild-type cells. Cholesterol transport to the mitochondrial inner membrane increased markedly upon exposure of cholesterol-deprived cells to lipoproteins, indicating transport of endosomal cholesterol to mitochondria. Reduction of endosomal metastatic lymph node protein 64 (MLN64) by RNA interference decreased cholesterol transport to the mitochondrial inner membrane and reduced mitochondrial cholesterol levels in NPC1-deficient cells, suggesting that MLN64 transported cholesterol to mitochondria even in the absence of NPC1. In summary, this study describes a transport pathway for endosomal cholesterol to mitochondria that requires MLN64, but not NPC1, and that may be responsible for increased mitochondrial cholesterol in NPC disease.

Hydrophobic core of the steroidogenic acute regulatory protein for cholesterol transport

The steroidogenic acute regulatory protein (StAR), the first family member of START (StAR-related lipid transport) proteins, plays an essential role by facilitating the movement of cholesterol from the outer to inner mitochondrial membrane. Wild-type and mutant StAR binds cholesterol with similar intensity, but only wild-type StAR can transport it to mitochondria. Here, we report that the hydrophobic core is crucial for biological activity of proteins with START domains. Wild-type StAR increased steroidogenic activity by 7-9-fold compared to mutant R182L StAR, but both of them showed similar near-UV CD spectra. The fluorescence maximum of wild-type StAR is red shifted in comparison to mutant StAR under identical urea concentration. TFE increased the alpha-helical contribution of wild-type StAR more than the mutant protein. Acrylamide quenching for the wild-type protein (K(SV) = 12.0 +/- 0.2-11.2 +/- 0.5 M(-1)) exceeded that of the mutant protein (K(SV) = 4 +/- 0.2 M(-1)). Consistent with these findings, the hydrophobic probe ANS bound wild-type StAR (K(app) = 8.1 x 10(5) M(-1)) to a greater degree than mutant StAR (K(app) = 3.75 x 10(5) M(-1)). Partial proteolysis examined by mass spectrometry suggests that only wild-type StAR has a protease-sensitive C-terminus, but not the mutant. Stopped-flow CD revealed that the time of unfolding of mutant StAR was 0.017 s. In contrast, the wild-type StAR protein is unfolded in 16.3 s. In summary, these results demonstrate that wild-type StAR adopts a very flexible form due to the accommodation of more water molecules, while mutant StAR is generated by an alternate folding pathway making it inactive.

Steroidogenic activity of StAR requires contact with mitochondrial VDAC1 and phosphate carrier protein

The steroidogenic acute regulatory protein (StAR) is required for adrenal and gonadal steroidogenesis and for male sexual differentiation. StAR acts on the outer mitochondrial membrane (OMM) to facilitate movement of cholesterol from the OMM to the inner mitochondrial membrane to be converted to pregnenolone, the precursor of all steroid hormones. The mechanisms of the action of StAR remain unclear; the peripheral benzodiazepine receptor, an OMM protein, appears to be involved, but the identity of OMM proteins that interact with StAR remain unknown. Here we demonstrate that phosphorylated StAR interacts with voltage-dependent anion channel 1 (VDAC1) on the OMM, which then facilitates processing of the 37-kDa phospho-StAR to the 32-kDa intermediate. In the absence of VDAC1, phospho-StAR is degraded by cysteine proteases prior to mitochondrial import. Phosphorylation of StAR by protein kinase A requires phosphate carrier protein on the OMM, which appears to interact with StAR before it interacts with VDAC1. VDAC1 and phosphate carrier protein are the first OMM proteins shown to contact StAR.

Severe combined adrenal and gonadal deficiency caused by novel mutations in the cholesterol side chain cleavage enzyme, P450scc

CONTEXT

Mitochondrial cytochrome P450scc converts cholesterol to pregnenolone in all steroidogenic tissues. Although progesterone production from the fetally-derived placenta is necessary to maintain pregnancy to term, four patients with mutations in the gene encoding P450scc (CYP11A1), have been described, one in a 46,XX female and three in underandrogenized 46,XY individuals, all with primary adrenal failure.

OBJECTIVE

Our aim was to determine whether P450scc mutations might be found in other children and to explore genotype/phenotype correlations.

METHODS AND PATIENTS

We performed mutational analysis of CYP11A1 in individuals with 46,XY disorders of sex development and primary adrenal failure, followed by functional studies of P450scc activity and of P450scc RNA splicing.

RESULTS

Among nine 46,XY infants with adrenal failure and disordered sexual differentiation, two infants had compound heterozygous mutations in CYP11A1. One patient harbored the novel P450scc missense mutations L141W and V415E, which retained 38 and 0% activity, respectively. The other carried a CYP11A1 frameshift mutation c835delA (0% activity) and a splice site mutation [IVS3+(2-3)insT] that prevented correct splicing of P450scc mRNA.

CONCLUSIONS

P450scc deficiency is a recently recognized disorder that may be more frequent than originally thought. The phenotypic spectrum ranges from severe loss-of-function mutations associated with prematurity, complete underandrogenization, and severe, early-onset adrenal failure, to partial deficiencies found in children born at term with clitoromegaly and later-onset adrenal failure. In contradistinction to congenital lipoid adrenal hyperplasia caused by steroidogenic acute regulatory protein mutations, adrenal hyperplasia has not been reported in any of the six patients with P450scc deficiency.

Molecular mechanism of reduction in pregnenolone synthesis by cigarette smoke

Steroidogenic acute regulatory protein (StAR) facilitates the movement of cholesterol from the outer to inner mitochondrial membrane for the synthesis of pregnenolone. Here, we investigated the molecular mechanism of the reduction of pregnenolone synthesis by cigarette smoke condensate (CSC). Pre-exposure or post-exposure of cells with CSC led to reduced pregnenolone synthesis, in a fashion similar to its effect on isolated mitochondria. However, there was no difference in the expression of 30 kDa StAR in cells treated with moderately concentrated CSC by either regimen. The active form of 37 kDa StAR is degraded easily suggesting that the continuous presence of CSC reduces StAR expression. Mitochondrial import of (35)S-methionine-labeled StAR followed by extraction of the StAR-mitochondrial complex with 1% digitonin showed similarly sized complexes in the CSC-treated and untreated mitochondria. Further analysis by sucrose density gradient centrifugation showed a specific complex, "complex 2", in the untreated mitochondria but absent in the CSC-treated mitochondria. Mass spectrometric analysis revealed that complex 2 is the outer mitochondrial protein, VDAC1. Knockdown of VDAC1 expression by siRNA followed by co-transfection with StAR resulted in a lack of pregnenolone synthesis and 37 kDa StAR expression with reduced expression of the intermediate, 32 kDa StAR. Taken together, these results suggest that in the absence of VDAC1, active StAR expression is reduced indicating that VDAC1 expression is essential for StAR activity. In the absence of VDAC1-StAR interaction, cholesterol cannot be transported into mitochondria; thus the interaction with VDAC1 is a mandatory step for initiating steroidogenesis.

Steroidogenic acute regulatory protein (StAR), a novel mitochondrial cholesterol transporter

Cholesterol is a vital component of cellular membranes, and is the substrate for biosynthesis of steroids, oxysterols and bile acids. The mechanisms directing the intracellular trafficking of this nearly insoluble molecule have received increased attention through the discovery of the steroidogenic acute regulatory protein (StAR) and similar proteins containing StAR-related lipid transfer (START) domains. StAR can transfer cholesterol between synthetic liposomes in vitro, an activity which appears to correspond to the trans-cytoplasmic transport of cholesterol to mitochondria. However, trans-cytoplasmic cholesterol transport in vivo appears to involve the recently-described protein StarD4, which is expressed in most cells. Steroidogenic cells must also move large amounts of cholesterol from the outer mitochondrial membrane to the first steroidogenic enzyme, which lies on the matrix side of the inner membrane; this action requires StAR. Congenital lipoid adrenal hyperplasia, a rare and severe disorder of human steroidogenesis, results from mutations in StAR, providing a StAR knockout of nature that has provided key insights into its activity. Cell biology experiments show that StAR moves large amounts of cholesterol from the outer to inner mitochondrial membrane, but acts exclusively on the outer membrane. Biophysical data show that only the carboxyl-terminal alpha-helix of StAR interacts with the outer membrane. Spectroscopic data and molecular dynamics simulations show that StAR's interactions with protonated phospholipid head groups on the outer mitochondrial membrane induce a conformational change (molten globule transition) needed for StAR's activity. StAR appears to act in concert with the peripheral benzodiazepine receptor, but the precise itinerary of a cholesterol molecule entering the mitochondrion remains unclear.