Oxidized adrenodoxin acts as a competitive inhibitor of cytochrome P450scc in mitochondria from the human placenta

1,25-Dihydroxyvitamin D3 and 20-Hydroxyvitamin D3 Upregulate LAIR-1 and Attenuate Collagen Induced Arthritis

Vitamin D plays a crucial role in regulation of the immune response. However, treatment of autoimmune diseases with 1,25-dihydroxyvitamin D3 [1,25(OH)₂D3] doses sufficient to be effective is prohibitive due to its calcemic and toxic effects. We use the collagen-induced arthritis (CIA) model to analyze the efficacy of the noncalcemic analog of vitamin D, 20S-hydroxyvitamin D3 [20S(OH)D3], as well as 1,25(OH)₂D3, to attenuate arthritis and explore a potential mechanism of action. Mice fed a diet deficient in vitamin D developed a more severe arthritis characterized by enhanced secretion of T cell inflammatory cytokines, compared to mice fed a normal diet. The T cell inflammatory cytokines were effectively suppressed, however, by culture of the cells with 20S(OH)D3. Interestingly, one of the consequences of culture with 1,25(OH)₂D3 or 20S(OH)D3, was upregulation of the natural inhibitory receptor leukocyte associated immunoglobulin-like receptor-1 (LAIR-1 or CD305). Polyclonal antibodies which activate LAIR-1 were also capable of attenuating arthritis. Moreover, oral therapy with active forms of vitamin D suppressed arthritis in LAIR-1 sufficient DR1 mice, but were ineffective in LAIR-1^−/− deficient mice. Taken together, these data show that the effect of vitamin D on inflammation is at least, in part, mediated by LAIR-1 and that non-calcemic 20S(OH)D3 may be a promising therapeutic agent for the treatment of autoimmune diseases such as Rheumatoid Arthritis.

20S-Hydroxyvitamin D3, a Secosteroid Produced in Humans, Is Anti-Inflammatory and Inhibits Murine Autoimmune Arthritis

The ability to use large doses of vitamin D3 (D3) to chronically treat autoimmune diseases such as rheumatoid arthritis (RA) is prohibitive due to its calcemic effect which can damage vital organs. Cytochrome P450scc (CYP11A1) is able to convert D3 into the noncalcemic analog 20S-hydroxyvitamin D3 [20S(OH)D3]. We demonstrate that 20S(OH)D3 markedly suppresses clinical signs of arthritis and joint damage in a mouse model of RA. Furthermore, treatment with 20S(OH)D3 reduces lymphocyte subsets such as CD4+ T cells and CD19+ B cells leading to a significant reduction in inflammatory cytokines. The ratio of T reg cells (CD4+CD25+Foxp3+ T cells) to CD3+CD4+ T cells is increased while there is a decrease in critical complement-fixing anti-CII antibodies. Since pro-inflammatory cytokines and antibodies against type II collagen ordinarily lead to destruction of cartilage and bone, their decline explains why arthritis is attenuated by 20(OH) D3. These results provide a basis for further consideration of 20S(OH)D3 as a potential treatment for RA and other autoimmune disorders.

Gonadal transcriptome analysis of wild contaminated female European eels during artificial gonad maturation

Since the early 1980s, the population of European eels (Anguilla anguilla) has dramatically declined. Nowadays, the European eel is listed on the red list of threatened species (IUCN Red List) and is considered as critically endangered of extinction. Pollution is one of the putative causes for the collapse of this species. Among their possible effects, contaminants gradually accumulated in eels during their somatic growth phase (yellow eel stage) would be remobilized during their reproductive migration leading to potential toxic events in gonads. The aim of this study was to investigate the effects of organic and inorganic contaminants on the gonad development of wild female silver eels. Female silver eels from two sites with differing contamination levels were artificially matured. Transcriptomic analyses by means of a 1000 candidate gene cDNA microarray were performed on gonads after 11weeks of maturation to get insight into the mechanisms of toxicity of contaminants. The transcription levels of several genes, that were associated to the gonadosomatic index (GSI), were involved in mitotic cell division but also in gametogenesis. Genes associated to contaminants were mainly involved in the mechanisms of protection against oxidative stress, in DNA repair, in the purinergic signaling pathway and in steroidogenesis, suggesting an impairment of gonad development in eels from the polluted site. This was in agreement with the fact that eels from the reference site showed a higher gonad growth in comparison to contaminated fish.

Mitochondrial proteases act on STARD3 to activate progesterone synthesis in human syncytiotrophoblast

BACKGROUND

STARD1 transports cholesterol into mitochondria of acutely regulated steroidogenic tissue. It has been suggested that STARD3 transports cholesterol in the human placenta, which does not express STARD1. STARD1 is proteolytically activated into a 30-kDa protein. However, the role of proteases in STARD3 modification in the human placenta has not been studied.

METHODS

Progesterone determination and Western blot using anti-STARD3 antibodies showed that mitochondrial proteases cleave STARD3 into a 28-kDa fragment that stimulates progesterone synthesis in isolated syncytiotrophoblast mitochondria. Protease inhibitors decrease STARD3 transformation and steroidogenesis.

RESULTS

STARD3 remained tightly bound to isolated syncytiotrophoblast mitochondria. Simultaneous to the increase in progesterone synthesis, STARD3 was proteolytically processed into four proteins, of which a 28-kDa protein was the most abundant. This protein stimulated mitochondrial progesterone production similarly to truncated-STARD3. Maximum levels of protease activity were observed at pH7.5 and were sensitive to 1,10-phenanthroline, which inhibited steroidogenesis and STARD3 proteolytic cleavage. Addition of 22(R)-hydroxycholesterol increased progesterone synthesis, even in the presence of 1,10-phenanthroline, suggesting that proteolytic products might be involved in mitochondrial cholesterol transport.

CONCLUSION

Metalloproteases from human placental mitochondria are involved in steroidogenesis through the proteolytic activation of STARD3. 1,10-Phenanthroline inhibits STARD3 proteolytic cleavage. The 28-kDa protein and the amino terminal truncated-STARD3 stimulate steroidogenesis in a comparable rate, suggesting that both proteins share similar properties, probably the START domain that is involved in cholesterol binding.

GENERAL SIGNIFICANCE

Mitochondrial proteases are involved in syncytiotrophoblast-cell steroidogenesis regulation. Understanding STARD3 activation and its role in progesterone synthesis is crucial to getting insight into its action mechanism in healthy and diseased syncytiotrophoblast cells.

Adrenodoxin: the archetype of vertebrate-type [2Fe-2S] cluster ferredoxins

Adrenodoxin is probably the best characterized member of the vertebrate-type [2Fe-2S]-cluster ferredoxins. It has been in the spotlight of scientific interest for many years due to its essential role in mammalian steroid hormone biosynthesis, where it acts as electron mediator between the NADPH-dependent adrenodoxin reductase and several mitochondrial cytochromes P450. In this review we will focus on the present knowledge about protein-protein recognition in the mitochondrial cytochrome P450 system and the modulation of the electron transfer between Adx and its redox partners, AdR and CYP(s). We also intend to point out the potential biotechnological applications of Adx as a versatile electron donor to different cytochromes P450, both in vitro and in vivo. Finally we will address the comparison between the mammalian cytochrome P450-associated adrenodoxin and ferredoxins involved in iron-sulfur-cluster biosynthesis. Despite their different functions, these proteins display an amazing similarity regarding their primary sequence, tertiary structure and biophysical features.

Electron paramagnetic resonance and Mössbauer spectroscopy of intact mitochondria from respiring Saccharomyces cerevisiae

Mitochondria from respiring cells were isolated under anaerobic conditions. Microscopic images were largely devoid of contaminants, and samples consumed O(2) in an NADH-dependent manner. Protein and metal concentrations of packed mitochondria were determined, as was the percentage of external void volume. Samples were similarly packed into electron paramagnetic resonance tubes, either in the as-isolated state or after exposure to various reagents. Analyses revealed two signals originating from species that could be removed by chelation, including rhombic Fe(3+) (g = 4.3) and aqueous Mn(2+) ions (g = 2.00 with Mn-based hyperfine). Three S = 5/2 signals from Fe(3+) hemes were observed, probably arising from cytochrome c peroxidase and the a(3):Cu(b) site of cytochrome c oxidase. Three Fe/S-based signals were observed, with averaged g values of 1.94, 1.90 and 2.01. These probably arise, respectively, from the [Fe(2)S(2)](+) cluster of succinate dehydrogenase, the [Fe(2)S(2)](+) cluster of the Rieske protein of cytochrome bc (1), and the [Fe(3)S(4)](+) cluster of aconitase, homoaconitase or succinate dehydrogenase. Also observed was a low-intensity isotropic g = 2.00 signal arising from organic-based radicals, and a broad signal with g (ave) = 2.02. Mössbauer spectra of intact mitochondria were dominated by signals from Fe(4)S(4) clusters (60-85% of Fe). The major feature in as-isolated samples, and in samples treated with ethylenebis(oxyethylenenitrilo)tetraacetic acid, dithionite or O(2), was a quadrupole doublet with DeltaE (Q) = 1.15 mm/s and delta = 0.45 mm/s, assigned to [Fe(4)S(4)](2+) clusters. Substantial high-spin non-heme Fe(2+) (up to 20%) and Fe(3+) (up to 15%) species were observed. The distribution of Fe was qualitatively similar to that suggested by the mitochondrial proteome.

Interaction between mitochondrial CYP27B1 and adrenodoxin: role of arginine 458 of mouse CYP27B1

A molecular modeling study of CYP27B1 suggests that Arg458 of mouse CYP27B1 is involved in interaction with adrenodoxin (ADX). Thus, we generated CYP27B1 mutants R458K and R458Q and revealed their enzymatic properties. Substrate-induced difference spectra and K(m) values for 1alpha-hydroxylation of 25(OH)D3 indicate that the replacement of Arg458 with Lys or Gln does not affect substrate binding. However, these mutants showed remarkable decreases of both kcat values and the ratio of product formation to NADPH oxidation (coupling efficiency). A high K(m) value of R458Q for ADX concentration and a decrease of rate constant of the first electron transfer seem reasonable considering that the conversion from Arg to noncharged Gln abolishes salt-bridge formation with the acidic residue of ADX. On the other hand, R458K showed atypical kinetics for ADX concentration with Hill's constant of 2.0 and high catalytic activity at high ADX concentration by increase of coupling efficiency. These results suggest that conformational change of R458K by binding the two ADX molecules is essential for 1alpha-hydroxylation of 25(OH)D3. On the other hand, binding one ADX molecule is sufficient for the conformational change of the wild-type CYP27B1, judging from its Michaelis-Menten-type kinetics for ADX concentration with high coupling efficiency. These results suggest that ADX functions as an effector for the oxygen transfer reaction in addition to being an electron donor for CYP27B1.

Light-induced reduction of bovine adrenodoxin via the covalently bound ruthenium(II) bipyridyl complex: intramolecular electron transfer and crystal structure

Bovine adrenodoxin (Adx) plays an important role in the electron-transfer process in the mitochondrial steroid hydroxylase system of the bovine adrenal cortex. Using electron paramagnetic resonance (EPR) spectroscopy, we showed that photoreduction of the [2Fe-2S] cluster of Adx via (4'-methyl-2,2'-bipyridine)bis(2,2'-bipyridine)ruthenium(II) [Ru(bpy)2(mbpy)] covalently attached to the protein surface can be used as a new approach to probe the "shuttle" hypothesis for the electron transfer by Adx. The 1.5 A resolution crystal structure of a 1:1 Ru(bpy)2(mbpy)-Adx(1-108) complex reveals the site of modification, Cys95, and allows to predict the possible intramolecular electron-transfer pathways within the complex. Photoreduction of uncoupled Adx, mutant Adx(1-108), and Ru(bpy)2(mbpy)-Adx(1-108) using safranin T as the mediating electron donor suggests that two electrons are transferred from the dye to Adx. The intramolecular photoreduction rate constant for the ruthenated Adx has been determined and is discussed according to the predicted pathways.

Site-directed mutations (Asp405Ile and Glu124Ile) in cytochrome P450scc: effect on adrenodoxin binding

Cytochrome P450scc, mitochondrial adrenodoxin (Adx), and adrenodoxin reductase (AdR) are an essential components in a steroid hydroxylation system. In particular, mytochondrial cytochrome P450scc enzyme catalyses the first step in steroid hormones biosynthesis, represented by the conversion of cholesterol to pregnenolone. In order to study the effect of single mutations on the Adx binding a model of bovine cytochrome P450scc, previously optimized by molecular modeling, was utilized. It was hypothesized by molecular docking that two residues (Asp405 and Glu124) are involved in Adx binding. By site-directed mutagenesis, two mutants of cytochrome P450scc (Asp405Ile and Glu124Ile) expressed in Escherichia coli, were realized by replacing with isoleucines. The site-directed mutations effect on Adx binding was evaluated by differential spectral titration. The apparent dissociation constant values for Asp405Ile and Glu124Ile cytochrome P450scc show that the mutated residues seem to be at the interaction domain with Adx or at least close to it, as predicted by molecular modeling study. Finally, the engineered enzymes were characterized by biochemical and biophysical techniques such as circular dichroism (CD), UV/Vis spectroscopy, and electrochemical analysis.

Molten globule structure and steroidogenic activity of N-218 MLN64 in human placental mitochondria

Progesterone synthesis by the human placenta requires the conversion of mitochondrial cholesterol to pregnenolone by cytochrome P450scc. Most steroidogenic tissues use the steroidogenic acute regulatory protein (StAR) to deliver cholesterol to the inner mitochondrial membrane where P450scc is located, but StAR is not expressed in the human placenta. However, the human placenta does express MLN64, which has a C-terminal domain homologous to StAR that can also transport cholesterol. We investigated the ability of bacterially expressed N-218 MLN64 and N-62 StAR to transport cholesterol between artificial membranes and to its inner membrane site of use in placental mitochondria. Urea denaturation experiments show that N-218 MLN64 undergoes a pH-dependent and denaturant-dependent structural transition to a molten globule state, as reported previously for N-62 StAR. N-218 MLN64 stimulated cholesterol transfer between artificial phospholipid vesicles with an initial rate of 6.5 mol/min.mol N-218 MLN64. Both N-218 MLN64 and N-62 StAR stimulated cholesterol transfer to the inner mitochondrial membrane, as evidenced by a 6-fold stimulation of pregnenolone synthesis with saturating transporter. This stimulation was seen only after the endogenous cholesterol in the steroidogenic pool of the isolated mitochondria was first depleted. No stimulation was observed by N-218 MLN64 or N-62 StAR when 20alpha-hydroxycholesterol was added as substrate for P450scc, confirming that these proteins stimulate P450scc activity by enhancing cholesterol transport. MLN64 levels in placental JEG-3 cells were unresponsive to stimulation by 8-bromo-cAMP over 24 h. These data show that human N-218 MLN64 and N-62 StAR have similar biophysical and functional properties and are able to stimulate steroidogenesis in a human placental system, which normally lacks StAR. The results reveal that with saturating MLN64, steroidogenesis by placental mitochondria proceeds at near-maximal rate.

Electrochemical behaviour of human adrenodoxin on a pyrolytic graphite electrode

Adrenodoxin (Adx) functions as a redox protein in the delivery of electrons to all mitochondrial cytochromes P450. In order to further characterize the human form of this protein, direct electrochemistry of human adrenodoxin (Hadx) has been observed for the first time on a pyrolytic graphite electrode (PGE) modified with poly-L-lysine. A single well-defined redox wave was observed with a midpoint potential of -448+/-3 mV vs. Ag/AgCl (sat. KCl) at scan rates of 10 mV/s and over the pH range 4.0-8.0. At slow scan rates, the reduction process was close to being electrochemically reversible whereas, at faster scan rates, only quasi-reversibility was observed. A correlation was observed between the peak separation (DeltaE) for the cyclic voltammograms and pH over a wide range of scan rates. The variation of DeltaE with pH was at a minimum (optimum reversibility) at pH 7.0 for all scan rates tested. This correlation may suggest that the direct electrochemistry method could possibly provide a means for determining protein or enzyme activity. The electron transfer rate constant, k(s), was determined to be 0.28 s(-1) at pH 7.0 and a small pH dependence was observed. The results obtained in this study demonstrate the facile nature of direct electron transfer for human adrenodoxin, and provide an estimate of the midpoint reduction potential at a pyrolytic graphite electrode via electrostatic immobilisation.

Critical role of the plasma membrane for expression of mammalian mitochondrial side chain cleavage activity in yeast

Engineered yeast cells efficiently convert ergosta-5-eneol to pregnenolone and progesterone provided that endogenous pregnenolone acetylase activity is disrupted and that heterologous sterol delta7-reductase, cytochrome P450 side chain cleavage (CYP11A1) and 3beta hydroxysteroid dehydrogenase/isomerase (3beta-HSD) activities are present. CYP11A1 activity requires the expression of the mammalian NADPH-adrenodoxin reductase (Adrp) and adrenodoxin (Adxp) proteins as electron carriers. Several parameters modulate this artificial metabolic pathway: the effects of steroid products; the availability and delivery of the ergosta-5-eneol substrate to cytochrome P450; electron flux and protein localization. CYP11A1, Adxp and Adrp are usually located in contact with inner mitochondrial membranes and are directed to the outside of the mitochondria by the removal of their respective mitochondrial targeting sequences. CYP11A1 then localizes to the plasma membrane but Adrp and Adxp are detected in the endoplasmic reticulum and cytosol as expected. The electron transfer chain that involves several subcellular compartments may control side chain cleavage activity in yeast. Interestingly, Tgl1p, a potential ester hydrolase, was found to enhance steroid productivity, probably through both the availability and/or the trafficking of the CYP11A1 substrate. Thus, the observation that the highest cellular levels of free ergosta-5-eneol are found in the plasma membrane suggests that the substrate is freely available for pregnenolone synthesis.

Ca(2+) modulates respiratory and steroidogenic activities of human term placental mitochondria

We investigated the effects of calcium on the oxidative metabolism and steroidogenic activity of human term placental mitochondria. Submicromolar Ca(2+) concentrations stimulated state 3 oxygen consumption with 2-oxoglutarate and isocitrate and activated the 2-oxoglutarate and the NAD-isocitrate dehydrogenases by diminishing their Michaelis-Menten constants. Ca(2+) inhibited NADP-isocitrate dehydrogenase (NADP-ICDH) and the synthesis of progesterone. The NADP-ICDH maximal velocity was threefold higher than that of NAD-ICDH and had a threefold lower K(m) for isocitrate than NAD-ICDH. Isocitrate but not malate or 2-oxoglutarate supported progesterone synthesis. Calcium inhibition of progesterone synthesis was observed with isocitrate but not with malate or 2-oxoglutarate. Tight regulation of NADP-isocitrate dehydrogenase by calcium ions suggests that this enzyme plays an important role in placental mitochondrial metabolism.

Placental cytochrome P450scc (CYP11A1): comparison of catalytic properties between conditions of limiting and saturating adrenodoxin reductase

The mitochondrial side-chain cleavage of cholesterol, catalysed by cytochrome P450scc, is rate-limiting in the synthesis of progesterone by the human placenta. Cytochrome P450scc activity is in turn limited by the concentration of adrenodoxin reductase (AR) in placental mitochondria. In order to better understand which components of the cholesterol side-chain cleavage system are important in the regulation of placental progesterone synthesis, we have examined their effects on P450scc activity with both saturating and limiting concentrations of AR. The present study reveals that decreasing the AR concentration causes a decrease in the K(m) of cytochrome P450scc for cholesterol, facilitating saturation of the enzyme with its substrate. Decreasing AR resulted in P450scc activity becoming less sensitive to changes in P450scc concentration. The adrenodoxin (Adx) concentration in mitochondria from term placentae is near-saturating for P450scc and under these conditions, we found that decreasing AR reduces the K(m) of P450scc for adrenodoxin. Increasing either the cholesterol or P450scc concentration increased the amount of AR required for P450scc to work at half its maximum velocity. A relatively small increase in AR can support considerably higher rates of side-chain cleavage activity when there is a coordinate increase in AR and P450scc concentrations. We conclude from this study that cholesterol is near-saturating for cytochrome P450scc activity in placental mitochondria due to the P450scc displaying a low K(m) for cholesterol resulting from the low and rate-limiting concentration of AR present. This study reveals that it is unlikely that cholesterol or adrenodoxin concentrations are important regulators of placental progesterone synthesis but AR or coordinate changes in AR and P450scc concentrations are likely to be important in its regulation.