Isolation of a cDNA for adrenodoxin reductase (ferredoxin-NADP+ reductase). Implications for mitochondrial cytochrome P-450 systems

FDXR regulates TP73 tumor suppressor via IRP2 to modulate aging and tumor suppression

Ferredoxin reductase (FDXR) is a mitochondrial flavoprotein that initiates electron transport from NADPH to several cytochromes P450 via two electron carriers, ferredoxin 1 (FDX1) and FDX2. FDXR is the sole ferredoxin reductase in humans and plays a critical role in steroidogenesis and biosynthesis of heme and iron-sulfur clusters. However, much less is known about the role of FDXR in cancer. Here, we show that FDXR plays a role in tumorigenesis by modulating expression of the tumor suppressor p73. By using genetically modified mouse models, we recently showed that mice deficient in either Fdxr or Trp73 had a shorter lifespan and were prone to spontaneous tumors as compared with wild-type (WT) mice. Interestingly, compound Trp73 ^+/- ;Fdxr ^+/- mice lived longer and developed fewer tumors when compared with Fdxr ^+/- or Trp73 ^+/- mice. Moreover, we found that cellular senescence was increased in Trp73 ^+/- and Fdxr ^+/- mouse embryonic fibroblasts (MEFs), which was further increased in Trp73 ^+/- ;Fdxr ^+/- MEFs, as compared with that in WT MEFs. As FDXR is regulated by p73, we examined whether there was a feedback regulation between p73 and FDXR. Indeed, we found that Trp73 expression was decreased by loss of Fdxr in MEFs and that FDXR is required for p73 expression in multiple human cancer cell lines independent of p53. Mechanistically, we found that loss of FDXR, via FDX2, increased expression of iron-binding protein 2 (IRP2), which subsequently repressed TP73 mRNA stability. We also showed that TP73 transcript contained an iron response element in its 3'UTR, which was required for IRP2 to destabilize TP73 mRNA. Together, these data reveal a novel regulation of p73 by FDXR via IRP2 and that the FDXR-p73 axis plays a critical role in aging and tumor suppression. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Mechanisms of Mitochondrial Redox Signaling in Psychosocial Stress-Responsive Systems: New Insights into an Old Story

SIGNIFICANCE

Psychosocial stress is associated with alterations in serum glucocorticoids and cytokines, such as interleukin-6 (IL-6) and IL-1β, which functionally interact. However, the molecular mechanisms and physiological relationship between the two systems within the context of stress exposure are not well characterized. Recent Advances: Extracellular IL-6, which stimulates the release of cortisol from the zona fasciculata of the adrenal cortex, mediates its intracellular effects by tyrosine phosphorylation of the transcription factor signal transducer and activator of transcription 3 (STAT3). Mitochondrial electron transfer reactions are involved in both STAT3-driven ATP production in oxidative respiration and adrenocortical steroid biosynthesis.

CRITICAL ISSUES

The role of STAT3 in oxidative respiration and steroidogenesis suggests that it integrates both nuclear and mitochondrial actions, thereby preserving main steps of glucocorticoid biosynthesis in the adrenal gland under psychosocial stress. This review discusses the notion that these two pathways are together simultaneously involved in protection against chronic stressors.

FUTURE DIRECTIONS

Linking the function of cytokines and main components of the hypothalamic-pituitary-adrenal (HPA) axis to molecular mechanisms of mitochondrial redox signaling will be essential for a better understanding of the relevant stress-responsive systems engaged in stress vulnerability. Antioxid. Redox Signal. 28, 760-772.

Conservation of the Enzyme-Coenzyme Interfaces in FAD and NADP Binding Adrenodoxin Reductase-A Ubiquitous Enzyme

FAD and NAD(P) together represent an ideal pair for coupled redox reactions in their capacity to accept two electrons and their redox potentials. Enzymes that bind both NAD(P) and FAD represent large superfamilies that fulfill essential roles in numerous metabolic pathways. Adrenodoxin reductase (AdxR) shares Rossmann fold features with some of these superfamilies but remains in a group of its own in the absence of sequence homology. This article documents the phylogenetic distribution of AdxR by examining whole genome databases for Metazoa, Plantae, Fungi, and Protista, and determines the conserved structural features of AdxR. Scanning these databases showed that most organisms have a single gene coding for an AdxR ortholog. The sequence identity between AdxR orthologs is correlated with the phylogenetic distance among metazoan species. The NADP binding site of all AdxR orthologs showed a modified Rossmann fold motif with a GxGxxA consensus instead of the classical GxGxxG at the edge of the first βα-fold. To examine the hypothesis that enzyme-coenzyme interfaces represent the conserved regions of AdxR, the residues interfacing FAD and NADP were identified and compared with multiple-sequence alignment results. Most conserved residues were indeed found at sites that surround the interfacing residues between the enzyme and the two coenzymes. In contrast to protein-protein interaction hot-spots that may appear in isolated patches, in AdxR the conserved regions show strict preservation of the overall structure. This structure maintains the precise positioning of the two coenzymes for optimal electron transfer between NADP and FAD without electron leakage to other acceptors.

Leukemia inhibitory factor stimulates steroidogenesis of rat immature Leydig cells via increasing the expression of steroidogenic acute regulatory protein

Leukemia inhibitory factor (LIF) has many physiological roles. However, its effects on Leydig cell development are still unclear. Rat immature and adult Leydig cells were cultured with different concentrations of LIF alone or in combination with luteinizing hormone (LH) for 24 h. LIF (1 and 10 ng/ml) significantly increased androgen production in immature Leydig cells, but had no effects on testosterone production in adult Leydig cells. Further studies revealed that LIF dose-dependently increased Star and Hsd17b3 expression levels in immature Leydig cells. Gene microarray revealed that the upregulation of anti-oxidative genes and Star might contribute to LIF-induced androgen production. In conclusion, LIF has stimulatory effects on androgen production in rat immature Leydig cells.

Effects of Etomidate on the Steroidogenesis of Rat Immature Leydig Cells

BACKGROUND

Etomidate is a rapid hypnotic intravenous anesthetic agent. The major side effect of etomidate is the reduced plasma concentration of corticosteroids, leading to the abnormal reaction of adrenals. Cortisol and testosterone biosynthesis has similar biosynthetic pathway, and shares several common steroidogenic enzymes, such as P450 side chain cleavage enzyme (CYP11A1) and 3β-hydroxysteroid dehydrogenase 1 (HSD3B1). The effect of etomidate on Leydig cell steroidogenesis during the cell maturation process is not well established.

METHODOLOGY

Immature Leydig cells isolated from 35 day-old rats were cultured with 30 μM etomidate for 3 hours in combination with LH, 8Br-cAMP, 25R-OH-cholesterol, pregnenolone, progesterone, androstenedione, testosterone and dihydrotestosterone, respectively. The concentrations of 5α-androstanediol and testosterone in the media were measured by radioimmunoassay. Leydig cells were cultured with various concentrations of etomidate (0.3-30 μM) for 3 hours, and total RNAs were extracted. Q-PCR was used to measure the mRNA levels of following genes: Lhcgr, Scarb1, Star, Cyp11a1, Hsd3b1, Cyp17a1, Hsd17b3, Srd5a1, and Akr1c14. The testis mitochondria and microsomes from 35-day-old rat testes were prepared and used to detect the direct action of etomidate on CYP11A1 and HSD3B1 activity.

RESULTS AND CONCLUSIONS

In intact Leydig cells, 30 μM etomidate significantly inhibited androgen synthesis. Further studies showed that etomidate also inhibited the LH- stimulated androgen production. On purified testicular mitochondria and ER fractions, etomidate competitively inhibited both CYP11A1 and HSD3B1 activities, with the half maximal inhibitory concentration (IC50) values of 12.62 and 2.75 μM, respectively. In addition, etomidate inhibited steroidogenesis-related gene expression. At about 0.3 μM, etomidate significantly inhibited the expression of Akr1C14. At the higher concentration (30 μM), it also reduced the expression levels of Cyp11a1, Hsd17b3 and Srd5a1. In conclusion, etomidate directly inhibits the activities of CYP11A1 and HSD3B1, and the expression levels of Cyp11a1 and Hsd17b3, leading to the lower production of androgen by Leydig cells.

A brief history of adrenal research: steroidogenesis - the soul of the adrenal

The adrenal is a small gland that escaped anatomic notice until the 16th century, and whose essential role in physiology was not established until the mid 19th century. Early studies were confounded by failure to distinguish the effects of the cortex from those of the medulla, but advances in steroid chemistry permitted the isolation, characterization and synthesis of many steroids by the mid 20th century. Knowledge of steroid structures, radiolabeled steroid conversions, and the identification of accumulated urinary steroids in diseases of steroidogenesis permitted a generally correct description of the steroidogenic pathways, but one confounded by the failure to distinguish species-specific differences. The advent of cloning technologies and molecular genetics rapidly corrected and clarified the understanding of steroidogenic processes. Our laboratory in San Francisco was one of several contributing to this effort, focusing on human steroidogenic enzymes, the genetic disorders in their biosynthesis and the transcriptional and post-translational mechanisms regulating enzyme activity.

Crystal structure analysis of Bacillus subtilis ferredoxin-NADP(+) oxidoreductase and the structural basis for its substrate selectivity

Bacillus subtilis yumC encodes a novel type of ferredoxin-NADP+ oxidoreductase (FNR) with a primary sequence and oligomeric conformation distinct from those of previously known FNRs. In this study, the crystal structure of B. subtilis FNR (BsFNR) complexed with NADP+ has been determined. BsFNR features two distinct binding domains for FAD and NADPH in accordance with its structural similarity to Escherichia coli NADPH-thioredoxin reductase (TdR) and TdR-like protein from Thermus thermophilus HB8 (PDB code: 2ZBW). The deduced mode of NADP+ binding to the BsFNR molecule is nonproductive in that the nicotinamide and isoalloxazine rings are over 15 Å apart. A unique C-terminal extension, not found in E. coli TdR but in TdR-like protein from T. thermophilus HB8, covers the re-face of the isoalloxazine moiety of FAD. In particular, Tyr50 in the FAD-binding region and His324 in the C-terminal extension stack on the si- and re-faces of the isoalloxazine ring of FAD, respectively. Aromatic residues corresponding to Tyr50 and His324 are also found in the plastid-type FNR superfamily of enzymes, and the residue corresponding to His324 has been reported to be responsible for nucleotide specificity. In contrast to the plastid-type FNRs, replacement of His324 with Phe or Ser had little effect on the specificity or reactivity of BsFNR with NAD(P)H, whereas replacement of Arg190, which interacts with the 2'-phosphate of NADP+, drastically decreased its affinity toward NADPH. This implies that BsFNR adopts the same nucleotide binding mode as the TdR enzyme family and that aromatic residue on the re-face of FAD is hardly relevant to the nucleotide selectivity.

Studies of interaction of homo-dimeric ferredoxin-NAD(P)+ oxidoreductases of Bacillus subtilis and Rhodopseudomonas palustris, that are closely related to thioredoxin reductases in amino acid sequence, with ferredoxins and pyridine nucleotide coenzymes

Ferredoxin-NADP(+) oxidoreductases (FNRs) of Bacillus subtilis (YumC) and Rhodopseudomonas palustris CGA009 (RPA3954) belong to a novel homo-dimeric type of FNR with high amino acid sequence homology to NADPH-thioredoxin reductases. These FNRs were purified from expression constructs in Escherichia coli cells, and their steady-state reactions with [2Fe-2S] type ferredoxins (Fds) from spinach and R. palustris, [4Fe-4S] type Fd from B. subtilis, NAD(P)(+)/NAD(P)H and ferricyanide were studied. From the K(m) and k(cat) values for the diaphorase activity with ferricyanide, it is demonstrated that both FNRs are far more specific for NADPH than for NADH. The UV-visible spectral changes induced by NADP(+) and B. subtilis Fd indicated that both FNRs form a ternary complex with NADP(+) and Fd, and that each of the two ligands decreases the affinities of the others. The steady-state kinetics of NADPH-cytochrome c reduction activity of YumC is consistent with formation of a ternary complex of NADPH and Fd during catalysis. These results indicate that despite their low sequence homology to other FNRs, these enzymes possess high FNR activity but with measurable differences in affinity for different types of Fds as compared to other more conventional FNRs.

Antioxidant Protective Mechanisms against Reactive Oxygen Species (ROS) Generated by Mitochondrial P450 Systems in Steroidogenic Cells

Mitochondrial P450 type enzymes catalyze central steps in steroid biosynthesis, including cholesterol conversion to pregnenolone, 11beta and 18 hydroxylation in glucocorticoid and mineralocorticoid synthesis, C-27 hydroxylation of bile acids, and 1alpha and 24 hydroxylation of 25-OH-vitamin D. These monooxygenase reactions depend on electron transfer from NADPH via FAD adrenodoxin reductase and 2Fe-2S adrenodoxin. These systems can function as a futile NADPH oxidase, oxidizing NADPH in absence of substrate, and leak electrons via adrenodoxin and P450 to O(2), producing superoxide and other reactive oxygen species (ROS). The degree of uncoupling depends on the P450 and steroid substrate. Studies with purified proteins and overexpression in cultured cells show consistently that adrenodoxin, but not reductase, is responsible for ROS production that can lead to apoptosis. In the ovary and corpus luteum, antioxidant enzyme activities superoxide dismutase, catalase, and glutathione peroxidase parallel steroidogenesis. Antioxidant beta-carotene, alpha-tocopherol, and ascorbate can protect against oxidative damages of P450 systems. In testis Leydig cells, steroidogenesis is associated with aging of the steroidogenic capacity.

Uterine and placental expression of steroidogenic genes during rodent pregnancy

The ontogeny and functional role of steroidogenesis during mammalian gestation is poorly understood. This review provides a summary of our recent findings on the spatio-temporal expression of key steroidogenic genes controlling progesterone synthesis in the uterus during mouse pregnancy. We have shown that onset of cholesterol side chain cleavage cytochrome P450 (P450scc) and a newly identified isoform of murine 3beta-hydroxysteroid dehydrogenase/isomerase type VI (3betaHSD VI) expression occurs upon decidualization of the uterine wall induced by implantation. This unexpected early expression of the enzymes in the maternal decidua is terminated at mid-pregnancy when the steroidogenic ability reappears in the extraembryonic giant cells at the time of placentation. The giant cells express another protein indispensable for steroid hormone synthesis in the adrenal and gonads, Steroidogenic Acute Regulatory (StAR) protein. Unlike the human placenta, the steroidogenic genes are not expressed in the cells of the mature mouse placenta during the second half of gestation. Finally, our studies suggest that transcriptional regulation of P450scc is mediated by a non-SF-1 protein that substitutes SF-1 functions in the extraembryonic cells. Collectively, the results of the present study suggest that, during early phases of pregnancy, local progesterone synthesis in the maternal decidua and the trophoblast layers surrounding the embryonal cavity is important for successful implantation and/or maintenance of pregnancy. We propose that the local production of progesterone acts as an immunosuppressant at the maternofetal interface preventing the rejection of the fetal allograft.

Involvement of the flavin si-face tyrosine on the structure and function of ferredoxin-NADP+ reductases

In ferredoxin-NADP(+) reductase (FNR), FAD is bound outside of an anti-parallel beta-barrel with the isoalloxazine lying in a two-tyrosine pocket. To elucidate the function of the flavin si-face tyrosine (Tyr-89 in pea FNR) on the enzyme structure and catalysis, we performed ab initio molecular orbital calculations and site-directed mutagenesis. Our results indicate that the position of Tyr-89 in pea FNR is mainly governed by the energetic minimum of the pairwise interaction between the phenol ring and the flavin. Moreover, most of FNR-like proteins displayed geometries for the si-face tyrosine phenol and the flavin, which correspond to the more negative free energy theoretical value. FNR mutants were obtained replacing Tyr-89 by Phe, Trp, Ser, or Gly. Structural and functional features of purified FNR mutants indicate that aromaticity on residue 89 is essential for FAD binding and proper folding of the protein. Moreover, hydrogen bonding through the Tyr-89 hydroxyl group may be responsible of the correct positioning of FAD and the substrate NADP(+)

Cloning of ACTH-regulated genes in the adrenal cortex

To search for genes that are induced by ACTH in adrenocortical cells, we screened adrenal cortex cDNA libraries by a differential hybridization method using cDNA probes representing mRNAs from cells with or without ACTH stimulation. Forty clones were identified as ACTH induced (yielding a frequency of about 1/2500 plaques screened), and two clones as ACTH repressed. The cDNAs isolated and sequenced include nuclear genes for microsomal steroidogenic enzymes and novel proteins of yet unidentified functions, and mitochondrial genes encoding subunits of oxidative phosphorylation enzymes. Northern blot analysis of RNA from cells stimulated with ACTH confirmed the induction of these genes by ACTH, yet revealed important differences in the relative responses of the respective mRNAs. The time courses showed the major increase in the initial 6 h; and a decline after 24-36 h. The enhancement of the levels of the mRNAs could be ascribed to transcriptional activation. Since the mitochondrial genome is transcribed as a single polycistronic unit, to account for the > 20-fold differences in the levels of the mitochondrial mRNAs it is necessary to invoke differential stabilities of these mRNAs. The synchronous increase in the expression of both the steroidogenic enzymes and the mitochondrial oxidative phosphorylation system subunits, provides evidence for coregulation of steroidogenic and energy producing capacities of adrenal cells to meet the metabolic needs of steroid hormone production. Suppression of beta-actin gene expression may be related to changes in actin polymerization during ACTH-dependent cytoskeletal reorganization.

cDNA cloning and sequence analysis of the bovine adrenocorticotropic hormone (ACTH) receptor

We isolated five independent cDNAs of nearly 3000 bp for the bovine ACTH receptor by screening adrenal cortex cDNA libraries with a PCR cloned cDNA fragment. The deduced receptor sequence includes 297 residues (M(r) = 33,258) with 81% identity with the human ACTH receptor, and shows seven hydrophobic transmembrane domains. The calculated M(r) of the receptor is smaller than the 40-45 kDa observed in crosslinking studies with labeled ACTH. Since the bovine and human receptors have two glycosylation motifs in the N-terminus, the difference may result from glycosylation of the receptor. Analysis of the sequences of both bovine and human receptors revealed a single protein kinase. A phosphorylation motif located in the third intracellular loop (Ser-209) juxtaposed to a protein kinase C phosphorylation motif (Thr-204). Thus, the involvement of protein kinase A and C pathways in ACTH action may be mediated in part by phosphorylation of the ACTH receptor at these motifs. The 3'-untranslated region of the bovine cDNA is > 2000 bp and includes two inverse repeats giving an extensive and strong secondary structure to the ACTH receptor RNA.

Genetic diseases of steroid metabolism

All major classes of biologically active steroid hormones (progestins, mineralocorticoids, glucocorticoids, and sex steroids) are synthesized from cholesterol through 11 different bioconversions. With the exception of 5 alpha-reductase, all the enzymes mediating these reactions fall into two classes, cytochromes P450 and short-chain dehydrogenases. Cytochromes P450 are heme-containing membrane-bound proteins with molecular weights of approximately 50,000 that utilize molecular oxygen and electrons from NADPH-dependent accessory proteins to hydroxylate substrates. Short-chain dehydrogenases have molecular weights of 30,000-40,000, have tyrosine and lysine residues at the active site, and remove a hydride from the substrate, transferring the electrons of the hydride to NAD+ or NADP+. In most cases, this reaction is reversible so that the dehydrogenase can also function as a reductase under appropriate conditions. Inherited disorders in enzymes required for steroid biosynthesis have varying effects. Defects that prevent cortisol from being synthesized are referred to collectively as congenital adrenal hyperplasia. Because the enzymes required for cortisol biosynthesis in the adrenal cortex are in many cases required for the synthesis of mineralocorticoids and/or sex steroids, these classes of steroids may also not be synthesized normally. Thus, cholesterol desmolase and 3 beta-hydroxysteroid dehydrogenase deficiencies affect synthesis of all classes of steroids in both the adrenals and gonads. Steroid 21-hydroxylase deficiency, the most common cause (> 90% of cases) of congenital adrenal hyperplasia, can affect both mineralocorticoid and glucocorticoid synthesis, but androgen secretion is usually abnormally high due to shunting of accumulated precursors into this pathway. Excessive secretion of androgens and mineralocorticoids occurs in 11 beta-hydroxylase deficiency (the second most frequent form of congenital adrenal hyperplasia). Mineralocorticoid excess is also seen in 17 alpha-hydroxylase deficiency, but in this disorder sex steroid synthesis is defective. All defects that affect estrogen synthesis (deficiencies of cholesterol desmolase, 3 beta-hydroxysteroid dehydrogenase, 17 alpha-hydroxylase, aromatase, and 17 beta-hydroxysteroid dehydrogenase) are very rare, suggesting that the inability to synthesize placental estrogens may adversely affect fetal survival. A number of enzymes are expressed at sites of steroid action and regulate the amount of active steroid available to steroid receptors. Steroid 5 alpha-reductase converts testosterone to the more active dihydrotestosterone. Deficiency of this activity leads to incomplete development of male genitalia; 17 beta-hydroxysteroid dehydrogenase deficiency has similar phenotypic effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Mitochondrial-genome-encoded RNAs: differential regulation by corticotropin in bovine adrenocortical cells

Differential screening of an adrenal cortex cDNA library for corticotropin (ACTH)-inducible genes led to the isolation of a group of cDNAs representing mitochondrial genes that encode subunits of cytochrome oxidase, ATPase, and NADH dehydrogenase. Northern blot analysis of RNA from cells stimulated by ACTH confirmed the induction of these genes by ACTH yet revealed major differences in the relative responses of the respective mRNAs. The levels of mRNAs for cytochrome oxidase subunit I and ATPase increased 2- to 4-fold and for NADH dehydrogenase subunit 3 increased 20-fold, whereas the levels of the mitochondrial 16S rRNA showed no change within 6 h of ACTH stimulation. These effects of ACTH on mitochondrial mRNA levels probably result from both activation of the H2 transcription unit that encodes mitochondrial mRNAs and alteration of mRNA stability. ACTH also increased the activity of cytochrome oxidase after 12 h of stimulation. Examination of the tissue specificity of expression of five mitochondrial genes showed a wide range of RNA levels among 11 tissues but high correlations between individual RNA levels, consistent with a coordinated expression of the mitochondrial genes, although at different levels in each cell type. Proportionately high levels of mitochondrial mRNAs were found in adrenal cortex, probably reflecting a stimulatory effect of ACTH in vivo. Overall, the results indicate that ACTH enhances the energy-producing capacity of adrenocortical cells.

Steroidogenic enzymes: structure, function, and role in regulation of steroid hormone biosynthesis

In the pathways of steroid hormone biosynthesis there are two major types of enzymes: cytochromes P450 and other steroid oxidoreductases. This review presents an overview of the function and expression of both types of enzymes with emphasis on steroidogenic P450s. The final part of the review on regulation of steroidogenesis includes a description of the normal physiological fluctuations in the steroid output of adrenal cortex and gonads, and provides an analysis of the relative role of enzyme levels in the determination of these fluctuations. The repertoire of enzymes expressed in a steroidogenic cell matches the cell's capacity for the biosynthesis of specific steroids. Thus, steroidogenic capacity is regulated mainly by tissue and cell specific expression of enzymes, and not by selective activation or inhibition of enzymes from a larger repertoire. The quantitative capacity of steroidogenic cells for the biosynthesis of specific steroids is determined by the levels of steroidogenic enzymes. The major physiological variations in enzyme levels, are generally associated with parallel changes in gene expression. The level of expression of each steroidogenic enzyme varies in three characteristics: (a) tissue- and cell-specific expression, determined during tissue and cell differentiation; (b) basal expression, in the absence of trophic hormonal stimulation; and (c) hormonal signal regulated expression. Each of these three types of expression probably represent the functioning of distinct gene regulatory elements. In adult steroidogenic tissues, the levels of most of the cell- and tissue-specific steroidogenic enzymes depend mainly on trophic hormonal stimulation mediated by a complex network of signal transduction systems.

Mechanism of corticotropin and cAMP induction of mitochondrial cytochrome P450 system enzymes in adrenal cortex cells

We studied the kinetics of corticotropin (ACTH) induction of mitochondrial cytochromes P450scc and P450c11 and their electron transport proteins, adrenodoxin and adrenodoxin reductase, in bovine adrenal cortex cells in primary culture. The mRNA levels of these enzymes increase and reach a peak within 3-12 h after ACTH addition. The protein levels of adrenodoxin reductase and P450scc show an increase only nearly 24 h after ACTH addition. After ACTH addition, the intracellular level of cAMP reaches maximal levels within 5 min, and then decreases gradually over 60 min. Hence, we examined the effect of a pulse of ACTH or cAMP analogs on enzyme and mRNA levels. Exposure of the cells to ACTH for 1-2 h was sufficient for maximal induction of the enzymes and P450scc mRNA. In contrast, the induction of the enzymes and the mRNA by cAMP analogs or forskolin required the continuous presence of these agents for over 12 h. But, these agents stimulated cortisol secretion to the medium quickly, indicating that they can activate some intracellular processes while not showing any effect on enzyme induction. The absence of any effect of prolonged cAMP pulses on enzyme and mRNA levels weakens the previous hypothesis that cAMP is the sole second messenger for the ACTH induction of steroidogenic enzymes in adrenal cortex cells. The inductive ability of a brief pulse of ACTH indicates that ACTH can rapidly initiate a series of reactions that result in enzyme induction many hours later.

Induction and mitochondrial localization of cytochrome P450scc system enzymes in normal and transformed ovarian granulosa cells

After ovulation of an oocyte, granulosa cells of the ovarian follicle differentiate into luteal cells and become a major factor dedicated to the synthesis of the steroid hormone progesterone. We recently established granulosa cell lines by cotransfection of granulosa cells with SV-40 and Ha-ras oncogene. In these cells progesterone secretion can be induced by cAMP as in normal rat granulosa cells. The induction of progesterone secretion is observed only after approximately 24 h and closely follows the delayed but quantitatively dramatic induction of the mitochondrial cytochrome P450scc which catalyzes the first step in steroid hormone biosynthesis. The mitochondrial P450 system electron transport proteins, adrenodoxin and adrenodoxin reductase, are also induced but adrenodoxin shows a faster induction. Immunofluorescence studies show that the three enzymes are induced in all cells and incorporated into all mitochondria uniformly. Electron microscopic examination using immunogold technique further confirms this and reveals that adrenodoxin is predominantly located on the matrix side of the inner mitochondrial membrane. Thus, adrenodoxin, which is a small highly charged protein, shows a distribution similar to P450scc which is an integral membrane protein. The uniformity of the response of the cells provides further evidence for the homogeneity of the cell line and makes this new granulosa cell line a highly promising system for the study of the molecular mechanisms involved in changes in gene expression during the process of granulosa cell differentiation.

Regulation of tropomyosin expression in the maturing ovary and in primary granulosa cell cultures

Granulosa cell differentiation in vitro in response to gonadotropins is characterized by major changes in cell shape, cell aggregation, and the organization of microfilaments. These changes are associated with enhanced steroidogenesis in maturing granulosa-lutein cells. Since nonmuscle tropomyosin isoforms were implicated in stabilizing actin filaments, we studied the organization and expression of tropomyosin in differentiating primary cultures of rat granulosa cells and during ovarian folliculogenesis and luteinization. In unstimulated primary granulosa cell cultures tropomyosin was found mainly along stress fibers. In differentiating cells tropomyosin staining was diffuse with sometimes a subcortical organization. The changes in tropomyosin organization were accompanied by a pronounced decrease in the synthesis, translation in vitro, and mRNA levels of all the rat nonmuscle tropomyosin isoforms, with a greater reduction in the higher molecular weight isoforms than in the smaller isoforms. Similar results were obtained whether cells were stimulated to differentiate with gonadotropins, with cAMP, by culturing cells on an extracellular matrix, or by treatment with cytochalasin B. The effect of cytochalasin B was reversible; upon removal of the drug tropomyosin synthesis increased to near control levels, while that of proteins associated with luteinization decreased drastically. RNA isolated from ovaries with follicles at the preantral, preovulatory stage and from corpora lutea contained decreased tropomyosin mRNA levels during ovarian luteinization when the level of RNA for a key steroidogenic enzyme, cytochrome P-450 cholesterol side chain cleavage (P-450 scc), increased. The results suggest a physiological relevance for the low level of tropomyosin expression in the mechanisms which bring about the morphological and biochemical development and maturation of granulosa cells.

cDNA sequence of adrenodoxin reductase. Identification of NADP-binding sites in oxidoreductases

Adrenodoxin reductase is an NADP dependent flavoenzyme which functions as the reductase of mitochondrial P 450 systems. We sequenced two adrenodoxin reductase cDNAs isolated from a bovine adrenal cortex cDNA library. The deduced amino acid sequence shows no similarity to the sequence of the microsomal P 450 systems or other known protein sequences. Nonetheless, by sequence analysis and c comparisons with known sequences of dinucleotide-binding folds of two NADP-binding flavoenzymes, two regions of adrenodoxin reductase sequence were identified as the FAD- and NADP-binding sites. These analyses revealed a consensus sequence for the NADP-binding dinucleotide fold (GXGXXAXXXAXXXXXXG, in one-letter amino acid code) that differs from FAD and NAD-binding dinucleotide-fold sequences. In the data base of protein sequences, the NADP-binding-site sequence appears solely in NADP-dependent enzymes, the binding sites of which were not known to date. Thus, this sequence may be used for identification of a certain type of NADP-binding site of enzymes that show no significant sequence similarity.

Regulation of steroid hydroxylase gene expression is multifactorial in nature

In summary, regulation of steroid hydroxylase gene expression is complex and multifactorial, involving cAMP-dependent and -independent mechanisms required for maintenance of optimal steroidogenic capacity, tissue-specific mechanisms which lead to different steroidogenic pathways in different tissues, and developmental mechanisms which lead to fetal imprinting of steroid hydroxylase expression and which probably overlap with both maintenance and tissue-specific mechanisms. Future studies will involve identification of the trans-acting factors associated with each of these aspects of the multifactorial regulation and characterization of the cis-regulatory elements to which they bind. Such studies will inevitably lead to the identification of genes encoding these trans-acting factors and investigation of their regulation. In this way, it will be possible to work outward from the steroid hydroxylase genes toward the cell surface receptors in order to elucidate the series of events which lead to cAMP-dependent and -independent regulation of steroid hydroxylase gene expression.

Human adrenodoxin reductase: two mRNAs encoded by a single gene on chromosome 17cen----q25 are expressed in steroidogenic tissues

Adrenodoxin reductase is a mitochondrial flavoprotein that receives electrons from NADPH, thus initiating the electron-transport chain serving mitochondrial cytochromes P450. We have cloned and sequenced two human adrenodoxin reductase cDNAs that differ by the presence of six additional codons in the middle of one clone. The sequence in this region indicates that these six extra codons arise by alternative splicing of the pre-mRNA. Southern blot hybridization patterns of human genomic DNA cut with four restriction enzymes indicate that the human genome has only one gene for adrenodoxin reductase. Analysis of a panel of mouse-human somatic cell hybrids localized this gene to chromosome 17cen----q25. The alternatively spliced mRNA containing the six extra codons represents 10-20% of all adrenodoxin reductase mRNA. The expression of the adrenodoxin reductase gene may be stimulated by pituitary tropic hormones acting through cAMP, but its response is quantitatively much less than the responses of P450scc and adrenodoxin.