A trypsin-sensitive protein is required for utilization of exogenous cholesterol for pregnenolone synthesis by placental mitochondria

Impairment of renal steroidogenesis at the onset of diabetes

Accumulating evidence indicates the association between changes in circulating sex steroid hormone levels and the development of diabetic nephropathy. However, the renal synthesis of steroid hormones during diabetes is essentially unknown. Male Wistar rats were injected with streptozotocin (STZ) or vehicle. After one week, no changes in functional or structural parameters related to kidney damage were observed in STZ group; however, a higher renal expression of proinflammatory cytokines and HSP70 was found. Expression of Steroidogenic Acute Regulatory protein (StAR) and P450scc (CYP11A1) was decreased in STZ kidneys. Incubation of isolated mitochondria with 22R-hydroxycholesterol revealed a marked inhibition in CYP11A1 function at the medullary level in STZ group. The inhibition of these first steps of renal steroidogenesis in early STZ-induced diabetes led to a decreased local synthesis of pregnenolone and progesterone. These findings stimulate investigation of the probable role of nephrosteroids in kidney damage associated with diabetes.

Isocitrate dehydrogenase type 2 (IDH2) is part of a multiprotein complex for placental steroidogenesis

BACKGROUND

Human syncytiotrophoblast mitochondria require the activity of the isocitrate dehydrogenase type 2 (IDH2) to obtain reduced coenzymes for progesterone (P4) synthesis. Data from the literature indicate that mitochondrial steroidogenic contact sites transform efficiently cholesterol into P4. In this research, we identified the IDH2 as a member of the steroidogenic contact site and analyzed the steroidogenic role of its activity.

METHOD

Human syncytiotrophoblast mitochondria were isolated by differential centrifugation, and steroidogenic contact sites were obtained by osmotic shock and sucrose gradient ultracentrifugation. In-gel native activity assay, mass spectroscopy, and western blot were used to identify the association of proteins and their activities. P4 was determined by immunofluorescence.

RESULTS

The IDH2 was mainly identified in steroidogenic contact sites, and its activity was associated with a complex of proteins with an apparent molecular mass of ~590 kDa. Mass spectroscopy showed many groups of proteins with several metabolic functions, including steroidogenesis and ATP synthesis. The IDH2 activity was coupled to P4 synthesis since in the presence of Ca²⁺ or Na₂SeO₃, inhibitors of the IDH2, the P4 production decreased.

CONCLUSIONS

The human syncytiotrophoblast mitochondria build contact sites for steroidogenesis. The IDH2, a non-membrane protein, supplies the NADPH required for the synthesis of P4 in a complex (steroidosome) that associate the proteins required to transform efficiently cholesterol into P4, which is necessary in pregnancy to maintain the relationship between mother and fetus.

GENERAL SIGNIFICANCE

The IDH2 is proposed as a check point in the regulation of placental steroidogenesis.

Multiple functions of syncytiotrophoblast mitochondria

The human placenta plays a central role in pregnancy, and the syncytiotrophoblast cells are the main components of the placenta that support the relationship between the mother and fetus, in apart through the production of progesterone. In this review, the metabolic processes performed by syncytiotrophoblast mitochondria associated with placental steroidogenesis are described. The metabolism of cholesterol, specifically how this steroid hormone precursor reaches the mitochondria, and its transformation into progesterone are reviewed. The role of nucleotides in steroidogenesis, as well as the mechanisms associated with signal transduction through protein phosphorylation and dephosphorylation of proteins is discussed. Finally, topics that require further research are identified, including the need for new techniques to study the syncytiotrophoblast in situ using non-invasive methods.

Localization and functional activity of cytochrome P450 side chain cleavage enzyme (CYP11A1) in the adult rat kidney

Cumulative evidence demonstrated effective downstream metabolism of pregnenolone in renal tissue. The aim of this study was to evaluate the expression and functional activity of cytochrome P450 side chain cleavage enzyme (CYP11A1), which converts cholesterol into pregnenolone, in adult rat kidney. Immunohistochemical labeling for CYP11A1 was observed in renal cortex and medulla, on structures identified as distal convoluted tubule and thick ascending limb of Henle's loop, respectively. Immunoblotting analysis corroborated the renal expression of the protein in inner mitochondrial membrane fractions. The incubation of isolated mitochondria with the membrane-permeant cholesterol analogue 22R-hydroxycholesterol resulted in efficient formation of pregnenolone, the immediate precursor for the synthesis of all the steroid hormones. The low progesterone production rate observed in these experiments suggested a poor activity of 3β-hydroxysteroid dehydrogenase enzyme in renal mitochondria. The steroidogenic acute regulatory protein (StAR), involved in the mitochondrial import of cholesterol, was detected in renal tissue at both mRNA and protein level. Immunostaining for StAR showed similar distribution to that observed for CYP11A1. The expression of StAR and CYP11A1 was found to be higher in medulla than in cortex. This enhanced expression of steroidogenesis-related proteins correlated with a greater pregnenolone synthesis rate and higher steroid hormones tissular content measured in medulla. In conclusion, we have established the expression and localization of StAR and CYP11A1 protein, the ability of synthesizing pregnenolone and a region-specific content of sex hormones in the adult rat kidney. These data clearly show that the kidney is a steroid hormones synthesizing organ. It is proposed that the existence in the kidney of complete steroidogenic machinery would respond to a physiological significance.

Regional differences in the decay kinetics of GABA(A) receptor-mediated miniature IPSCs in the dorsal horn of the rat spinal cord are determined by mitochondrial transport of cholesterol

We examined the possibility of a differential spatial control in the endogenous production of 3alpha5alpha-reduced steroids and its consequences on GABA(A) receptor-mediated miniature IPSCs (mIPSCs) in laminas II and III-IV of the rat spinal cord dorsal horn (DH). Early in postnatal development [younger than postnatal day 8 (P8)], mIPSCs displayed slow decay kinetics in laminas II and III-IV resulting from a continuous local production of 3alpha5alpha-reduced steroids. This was mediated by the tonic activity of the translocator protein of 18 kDa (TSPO), which controls neurosteroid synthesis by regulating the transport of cholesterol across the mitochondrial membrane system. TSPO activity disappeared in laminas III-IV after P8 and was functionally downregulated in lamina II after P15, resulting in a marked reduction of mIPSC duration in these laminas. TSPO-mediated synthesis of 3alpha5alpha-reduced steroids was spatially restricted, because, at P9-P15, when their production was maximal in lamina II, no sign of spillover to laminas III-IV was apparent. Interestingly, after P8, the enzymes necessary for the synthesis of 3alpha5alpha-reduced steroids remained functional in laminas III-IV and could produce such steroids from various precursors or after a single subcutaneous injection of progesterone. Moreover, induction of an acute peripheral inflammation by intraplantar injection of carrageenan, restored a maximal TSPO-mediated neurosteroidogenesis in laminas III-IV. Our results indicate that the decay kinetics of GABA(A) receptor-mediated mIPSCs in the DH of the spinal cord are primarily controlled by 3alpha5alpha-reduced steroids, which can be produced from circulating steroid precursors and/or in a spatially restricted manner by the modulation of the activity of TSPO.

Steroidogenesis in BeWo cells: role of protein kinase A and benzodiazepines

The peripheral benzodiazepine receptor and protein kinase A have been proposed to modulate placental steroidogenesis. Binding of the radioactive benzodiazepine PK 11195 has been observed in membranes isolated from whole human placenta, but the presence of the peripheral benzodiazepine receptors, now called translocator protein, does not seem to be indispensable. We hypothesized that cAMP analogs could induce the translocator protein expression in BeWo cells increasing steroidogenesis in the presence of benzodiazepines. The effect of two benzodiazepines and of 8-Br-cAMP on steroidogenesis in BeWo cells or in isolated human placental mitochondria was studied. Benzodiazepines did not modify progesterone synthesis in either system. Progesterone increased three times in BeWo cells incubated in the presence of 8-Br-cAMP. The translocator protein was not identified by western blot in mitochondria isolated from either the human placenta or BeWo cells but it was present in isolated rat testicular mitochondria. Neither was it observed in isolated mitochondria from BeWo cells incubated with 8-Br-cAMP. An inhibitor of protein kinase A activity, H89, at 25 microM inhibited 90% the steroidogenesis in BeWo cells, even in the presence of 8-Br-cAMP, but protein phosphorylation in mitochondria increased in the presence of H89, suggesting that protein kinase A modulates the phosphorylation cycle of mitochondrial proteins. The results suggest that placental steroidogenesis is regulated via activation of protein kinase A modulated by cAMP.

Contact sites from human placental mitochondria: characterization and role in progesterone synthesis

To understand the functional compartmentalization of human placental mitochondria, we analyzed the composition and steroidogenic activity of contact sites. Several fractions containing contact sites were isolated using osmotic shock treatment and sucrose gradient centrifugation. These fractions contained various proteins and marker enzymes associated with mitochondrial membranes. The fractions containing the cytochrome P450 side chain cleavage system, cholesterol, nicotinamide adenine dinucleotide phosphate-isocitrate dehydrogenase, porin, and adenosine 5(')-triphosphate-diphosphohydrolase activity showed the capacity to synthesize progesterone. Our observations indicate that all necessary elements and enzymes for steroidogenesis are present and functional in placental mitochondrial contact sites. This organization may facilitate the metabolism of cholesterol delivered to the outer mitochondrial membrane into steroid hormones by the inner mitochondrial membrane cholesterol side chain cleavage system.

5'-p-Fluorosulfonylbenzoyl adenosine inhibits progesterone synthesis in human placental mitochondria

The human placental mitochondria have an ATP-diphosphohydrolase (apyrase) activity. In this paper we characterized the effect of 5'-p-fluorosulfonylbenzoyl adenosine (FSBA) on placental apyrase, and its repercussion on progesterone synthesis and oxygen consumption. Apyrase activity was inhibited by FSBA. Nucleosides tri- and diphosphates protected against FSBA inactivation, but divalent cations did not, indicating that FSBA attaches itself to an ATP-binding site of apyrase. In mitochondria, the inactivation of apyrase by FSBA was associated with inhibition of progesterone synthesis. Also, the oxygen consumption induced by ATP but not by ADP, was inhibited, clearly showing that FSBA exclusively inactivated the apyrase in human placental mitochondria. It is concluded that the apyrase activity is closely related to progesterone synthesis, probably associated with the cholesterol transport between mitochondrial membranes.

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.

Calcium modulates the ATP and ADP hydrolysis in human placental mitochondria

This study evaluated the effect of Ca2+ on the extramitochondrial hydrolysis of ATP and ADP by the extramitochondrial ATPase in isolated mitochondria and submitochondrial particles (SMPs) from human term placenta. The effect of different oxidizable substrates on the hydrolysis of ATP and ADP in the presence of sucrose or K+ was evaluated. Ca2+ increased phosphate release from ATP and ADP, but this stimulation showed different behavior depending on the oxidizable substrate present in the incubation media. Ca2+ stimulated the hydrolysis of ATP and ADP in the presence of sucrose. However, Ca2+ did not stimulate the hydrolysis of ADP in the medium containing K+. Ca2+ showed inhibition depending on the respiratory substrate. This study suggests that the energetic state of mitochondria controls the extramitochondrial ATPase activity, which is modulated by Ca2+ and respiratory substrates.

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.