Expression of human 21-hydroxylase (P450c21) in bacterial and mammalian cells: a system to characterize normal and mutant enzymes

Cytochrome P450c21 (steroid 21-hydroxylase) is a key enzyme in the synthesis of cortisol, whose deficiency is the cause of a common genetic disease, congenital adrenal hyperplasia. We have expressed P450c21 (steroid 21-hydroxylase) in E. coli and mammalian cells. In E. coli, P450c21 cDNA was cloned into a T7 expression vector to produce a large amount of P450c21 fusion protein, which enabled antiserum production. In mammalian cells, a plasmid containing full-length P450c21 cDNA (phc21) was constructed and transfected into COS-1 cells to produce active P450c21, which was detected by immunoblotting and 21-hydroxylase activity assay. This system was used to assay mutations involved in the disease. Ile172 of phc21 corresponding to the site of mutation in some cases of the disease was mutagenized to become Asn, Leu, His, or Gln. Mutant as well as normal P450c21 was produced when their cDNAs were transfected into COS-1 cells. The mutant proteins, however, had greatly reduced 21-hydroxylase activities. Therefore, missense mutation at Ile172 resulted in inactivation of the enzyme, but not in repression of enzyme synthesis. The Leu for Ile substitution at amino acid 172 did not result in partial restoration of enzymatic activity, indicating that hydrophobicity at this residue may not play a role in its function.

Structure, sequence, chromosomal location, and evolution of the human ferredoxin gene family

Ferredoxin is an iron-sulfur protein that serves as an electron transport intermediate for mitochondrial cytochromes P450 involved in steroid, vitamin D, and bile acid metabolism. We cloned and characterized the human ferredoxin gene family, which includes two expressed genes and two pseudogenes. Sequence analysis of this gene family revealed that it encodes only one protein product. The expressed genes were assigned to chromosome 11 and pseudogenes to chromosomes 20 and 21 by identifying single-copy probes from each gene segment and hybridizing them to DNA from rodent-human hybrid cells. The pseudogenes lacked introns and contained numerous mutations, including insertion, deletion, and substitution which rendered them inactive. They were 96% and 85% homologous to the expressed gene, yet they were only 78% homologous with each other. The intronless nature, higher diversity among themselves, and distinct chromosomal location of the pseudogenes suggests that they arose by independent, retroposon-mediated events.

Analysis of anabolic steroids using GC/MS with selected ion monitoring

This study describes the use of gas chromatography/mass spectrometry with selected ion monitoring to screen 18 anabolic steroids banned by the International Olympic Committee. These anabolic steroids are analyzed in two fractions depending on their excretion pattern: nonconjugated (free) or conjugated fraction. The wet procedure of extracting steroids from urine consists of an initial isolation of lipophilic compounds on a column packed with Amberlite XAD-2 resin, followed by enzymatic hydrolysis with beta-glucuronidase from Escherichia coli. After extraction, the hydrolyzed steroids are derivatized to the corresponding trimethylsilyl ethers. The derivatized steroids are analyzed by gas chromatography/mass spectrometry with selected ion monitoring of their characteristic ions. It takes 12 and 26 min to run GC/MS and edit the raw data for nonconjugated and conjugated fractions respectively.

A missense mutation at Ile172----Asn or Arg356----Trp causes steroid 21-hydroxylase deficiency

Congenital adrenal hyperplasia (CAH) is a common recessive genetic disease caused mainly by steroid 21-hydroxylase (P450c21) deficiency. Many forms of CAH exist resulting from various mutations of the CYP21B gene. We sequenced CYP21B cDNA from a normal person and its genes from a patient with simple virilizing CAH. When comparing several CYP21B sequences, we found it was polymorphic. In the patient, a single base substitution replaced Ile172 (ATC) with Asn (AAC) in one allele while Arg356 (CGG) was converted to Trp (TGG) in the other. A normal P450c21 cDNA clone was transfected into COS-1 cells to produce 21-hydroxylase activity toward its substrates, progesterone and 17-hydroxyprogesterone. Mutants corresponding to Asn172 or Trp356 mutation were constructed by site-directed mutagenesis of the normal c21 cDNA clone. They failed to produce active enzyme toward either substrate upon transfection into COS-1 cells, demonstrating that these mutations caused CAH. Aligning sequences with other P450s, Ile172 could be located in the membrane anchoring domain and Arg356 in the substrate-binding site of P450c21. Both mutations are present in the CYP21A1P pseudogene, suggesting that they may be transferred from CYP21A1P by gene conversion events.

The 5'-region of the P450XIA1 (P450scc) gene contains a basal promoter and an adrenal-specific activating domain

The first step to the synthesis of all steroids is catalyzed by P450scc. We constructed nine deletion mutants of the 5'-region of the P450scc gene and connected them to a CAT reporter gene to assay transcriptional activity of the P450scc promoter. A short 145 bp fragment stimulated transcription by two fold. This DNA was active in all cells tested irrespective of their tissue origin and steroidogenic activity. DNA at -145/-573 of the upstream region did not increase transcription any further. DNA including 2500 bp of the upstream region stimulated transcription by 10 fold only in adrenal Y-1 cells. Hence in the -145 region contains a low level P450scc promoter and the 2500 bp DNA possesses an adrenal specific enhancing element.

Analysis of the human adrenodoxin promoter: evidence for its activity

Adrenodoxin is an iron-sulfur protein serving as an electron transfer intermediate in the mitochondrial cytochrome P450 system. To study its transcriptional regulation we construct a human adrenodoxin genomic clone which includes 333 bp of DNA upstream of the mRNA start site. This DNA contains a TATA box and two GC boxes. When we place it in front of the CAT reporter gene and transfect it into the recipient cell lines, it directs transcription of the CAT gene. This DNA in reverse orientation does not show any transcriptional activity. This promoter element functions in three mammalian cell lines: JEG-3, COS-1, and Y-1.

Cloning and structure of the human adrenodoxin gene

Adrenodoxin is an iron-sulfur protein that serves as an electron transport intermediate for all mitochondrial forms of cytochrome P450. To facilitate studying the regulation of adrenodoxin, we have cloned and determined the structure of the human adrenodoxin gene. It spans more than 20 kb, containing four exons and three introns. The first exon encodes the 60-amino-acid signal peptide, directing transport of the protein into the inner mitochondrial matrix. The mature peptide of 124 amino acids is encoded by the other three exons. The third exon encodes the portion of the protein containing the iron-sulfur center and a domain which binds other components of the electron transport chain. The transcriptional start sites were determined by primer extension and S1 nuclease mapping. The 5'-flanking region of this gene contains canonical promoters including a TATA box at nucleotide position -30 and two GC boxes at nucleotide positions -60 and -100. The sequence at nucleotides -234 to -252 is also highly homologous to the glucocorticoid-responsive element and the estrogen-responsive element.

Assignment of the functional gene for human adrenodoxin to chromosome 11q13----qter and of adrenodoxin pseudogenes to chromosome 20cen----q13.1

Adrenodoxin is a small iron/sulfur protein serving as an electron-transport intermediate for all mitochondrial forms of cytochrome P450. Southern blots of normal genomic DNA cleaved with six restriction endonucleases probed with full-length human adrenodoxin cDNA revealed complex patterns indicating the presence of multiple adrenodoxin genes. Southern blots of DNA from a panel of mouse/human somatic cell hybrids identified cross-hybridizing adrenodoxin DNA in two loci, chromosome 11q13----qter and chromosome 20cen----q13.1. Examination of adrenodoxin clones from a genomic DNA library in phage lambda revealed some clones bearing gene fragments interrupted by introns and other clones bearing processed pseudogenes. By probing the mouse/human hybrids with unique intronic DNA and by correlating restriction maps of the phage clones with that of uncloned genomic DNA, we show that the authentic transcribed adrenodoxin gene lies on chromosome 11, while pseudogenes lie on chromosome 20.

Human adrenodoxin: cloning of three cDNAs and cycloheximide enhancement in JEG-3 cells

Adrenodoxin is an iron-sulfur protein serving as an electron transport intermediate for two mitochondrial steroidogenic cytochromes P450. We have cloned and sequenced three human adrenal adrenodoxin cDNAs. The longest 5'-untranslated region was 131 bases long, and the coding sequences, identical in all three clones, predict a preprotein of 180 amino acids. The 3'-untranslated regions were 235, 596, and 776 bases long due to the presence of alternate polyadenylation sites. RNA transfer blots showed multiple size species of adrenodoxin mRNA consistent with finding multiple polyadenylation sites. Similar sized cross-hybridizing RNA species are found abundantly in the adrenal and testis and to a lesser degree in RNA from human fetal brain, spleen, placenta, kidney, liver, and intestine, as well as in cultured fibroblasts, suggesting the same or a very similar iron-sulfur protein is found in mitochondria of nonsteroidogenic tissues. JEG-3 cells, a transformed progesterone-producing line of trophoblastic origin, accumulate mRNAs for cytochrome P450scc (the mitochondrial cholesterol side-chain cleavage enzyme), adrenodoxin, and the fos oncogene when stimulated with 8-bromo-cyclic AMP. Addition of actinomycin D to such cultures blocked cAMP-induced accumulation of mRNAs for cytochrome P450scc and adrenodoxin. Addition of cycloheximide or puromycin to such cultures substantially reduced basal levels and markedly attenuated the cAMP-induced accumulation of cytochrome P450scc mRNA, but augmented the accumulation of adrenodoxin and fos mRNAs in additive and multiplicative fashions, respectively. These data indicate that the cAMP-induced synthesis of the steroidogenic machinery is not wholly dependent on cycloheximide-sensitive protein mediators.

P450XXI (steroid 21-hydroxylase) gene deletions are not found in family studies of congenital adrenal hyperplasia

Congenital adrenal hyperplasia (CAH) is a common genetic disorder due to defective 21-hydroxylation of steroid hormones. The human P450XXIA2 gene encodes cytochrome P450c21 [steroid 21-monooxygenase (steroid 21-hydroxylase), EC 1.14.99.10], which mediates 21-hydroxylation. The P450XXIA2 gene may be distinguished from the duplicated P450XXIA1 pseudogene by cleavage with the restriction endonuclease Taq I, with the XXIA2 gene characterized by a 3.7-kilobase (kb) fragment and the XXIA1 pseudogene characterized by a 3.2-kb fragment. Restriction endonuclease mapping by several laboratories has suggested that deletion of the P450XXIA2 gene occurs in about 25% of patients with CAH, as their genomic DNA lacks detectable 3.7-kb Taq I fragments. We have cloned human P450c21 cDNA and used it to study genomic DNA prepared from 51 persons in 10 families, each of which includes 2 or more persons with CAH. After Taq I digestion, apparent deletions are seen in 7 of the 20 alleles of the probands; using EcoRI, apparent deletions are seen in 9 of the 20 alleles. However, the apparently deleted alleles seen with Taq I do not coincide with those seen with EcoRI. Furthermore, studies with Bgl II, EcoRI, Kpn I, and Xba I yield normal patterns with at least two enzymes in all cases. Since all probands yielded normal patterns with at least two of the five enzymes used, we conclude that the P450XXIA2 gene "deletions" widely reported in CAH patients probably represent gene conversions, unequal crossovers, or polymorphisms rather than simple gene deletions.

Cytochrome P450c17 (steroid 17 alpha-hydroxylase/17,20 lyase): cloning of human adrenal and testis cDNAs indicates the same gene is expressed in both tissues

P450c17 is the single enzyme mediating both 17 alpha-hydroxylase (steroid 17 alpha-monooxygenase, EC 1.14.99.9) and 17,20 lyase activities in the synthesis of steroid hormones. It has been suggested that different P450c17 isozymes mediate these activities in the adrenal gland and testis. We sequenced 423 of the 509 amino acids (83%) of the porcine adrenal enzyme; based on this partial sequence, a 128-fold degenerate 17-mer was synthesized and used to screen a porcine adrenal cDNA library. This yielded a 380-base cloned cDNA, which in turn was used to isolate several human adrenal cDNAs. The longest of these, lambda hac17-2, is 1754 base pairs long and includes the full-length coding region, the complete 3'-untranslated region, and 41 bases of the 5'-untranslated region. This cDNA encodes a protein of 508 amino acids having a predicted molecular weight of 57,379.82. High-stringency screening of a human testicular cDNA library yielded a partial clone containing 1303 identical bases. RNA gel blots and nuclease S1-protection experiments confirm that the adrenal and testicular P450c17 mRNAs are indistinguishable. These data indicate that the testis possesses a P450c17 identical to that in the adrenal. The human amino acid sequence is 66.7% homologous to the corresponding regions of the porcine sequence, and the human cDNA and amino acid sequences are 80.1 and 70.3% homologous, respectively, to bovine adrenal P450c17 cDNA. Both comparisons indicate that a central region comprising amino acid residues 160-268 is hypervariable among these species of P450c17. Comparison of the amino acid sequence of P450c17 with two other human steroidogenic cytochromes P450 show much greater homology with P450c21 (28.9%), another microsomal enzyme, than with P450scc (12.3%), a mitochondrial enzyme.

Human cholesterol side-chain cleavage enzyme, P450scc: cDNA cloning, assignment of the gene to chromosome 15, and expression in the placenta

Conversion of cholesterol to pregnenolone is mediated by P450scc [cholesterol, reduced-adrenal-ferrodoxin: oxygen oxidoreductase (side-chain-cleaving), EC 1.14.15.67]. RNA from several human adrenal samples was translated in vitro and immunoprecipitated with anti-bovine P450scc, indicating that P450scc mRNA represents about 0.5% of human adrenal mRNA in normal, hypertrophied, and malignant adrenals. A 1626-base-pair human adrenal P450scc cDNA was cloned in bacteriophage lambda gt10. Primer extension data indicated P450scc mRNA is about 1850 bases long and that all adrenal P450scc mRNA has the same 5' end. A full-length clone containing 1821 bases was obtained from a human testis cDNA library to yield the complete sequence. The encoded human preP450scc contains 521 amino acids with a molecular weight of 60189.65. The testis and adrenal sequences were identical; the human cDNA and amino acid sequences are 82% and 72% homologous, respectively, with the bovine sequences. P450scc cDNA was used to probe DNA from a panel of mouse-human somatic cell hybrids, showing that the single human P450scc gene lies on chromosome 15. The human P450scc gene is expressed in the placenta in early and midgestation; primary cultures of placental tissue indicate P450scc mRNA accumulates in response to cyclic AMP.

Assignment of the gene for adrenal P450c17 (steroid 17 alpha-hydroxylase/17,20 lyase) to human chromosome 10

P450c17 is the single enzyme mediating both 17 alpha-hydroxylase and 17,20 lyase activities. We identified several human P450c17 cDNA clones in a human adrenal cDNA library we constructed in lambda gt10. A short clone containing the 3'-terminal 650 bases of the full-length sequence was used to examine Southern blots of DNA from normal persons and from a panel of mouse/human somatic cell hybrid lines. The pattern of hybridization of this cDNA to normal human DNA cut with 8 restriction endonucleases suggests the human genome has two (or more) P450c17 genes. The pattern of hybridization to the somatic cell hybrid cell lines, each containing a limited, known number of human chromosomes, indicates the human adrenal P450c17 gene lies on chromosome 10. The chromosomal locations of the other P450c17 genes could not be determined.

Hormonal regulation of P450scc (20,22-desmolase) and P450c17 (17 alpha-hydroxylase/17,20-lyase) in cultured human granulosa cells

Conversion of cholesterol to pregnenolone in man is mediated by a single enzyme, P450scc. To study possible regulation of the single P450scc gene in ovarian steroid synthesis, we incubated human granulosa cells with potential hormonal stimulators, measured P450scc mRNA accumulation by hybridization to 32P-labeled human P450scc cDNA, and compared the results to secretion of progesterone into the culture medium. Primary cultures of human granulosa cells were optimally responsive after 8-14 days of culture. Incubation with hCG (1.0-100 ng/ml), FSH (1.0-50 ng/ml), and (Bu)2cAMP (0.02-2.0 mM) increased P450scc mRNA accumulation and progesterone secretion in dose-dependent fashions. Maximal stimulation increased P450scc mRNA accumulation and progesterone secretion to 490% and 240% of control values, respectively, with hCG, to 166% and 168% with FSH, and to 495% and 380% with (Bu)2cAMP. PRL (to 100 ng/ml), ACTH (10(-6) M), and butyric acid (2 mM) had no significant effect on progesterone secretion or P450scc mRNA accumulation. These data indicate gonadotropin-specific stimulation of cAMP-mediated regulation of P450scc mRNA accumulation in human granulosa cells, presumably mediated by increased P450scc gene transcription. Ovarian estrogen synthesis may require both thecal and granulosa cells, although this two-cell theory of estrogen synthesis is unproven in man. To examine this theory, we probed the same blots used in the experiments described above with 32P-labeled human P450c17 cDNA (P450c17 is the single enzyme mediating both 17 alpha-hydroxylase and 17,20-lyase activities). Only miniscule amounts of P450c17 mRNA were found in the human granulosa cells, and the amounts did not increase in response to any of the above stimuli. These data strongly support the two-cell theory of human ovarian estrogen synthesis.

Structure of a bovine gene for P-450c21 (steroid 21-hydroxylase) defines a novel cytochrome P-450 gene family

P-450c21, a cytochrome P-450 enzyme [steroid 21-monooxygenase (steroid 21-hydroxylase), EC 1.14.99.10], mediates the 21-hydroxylation of glucocorticoid and mineralocorticoid hormones in the adrenal gland. The complete sequence of a bovine P-450c21 gene shows it is 3447 base pairs long and contains 10 exons. The intron/exon organization and encoded amino acid sequence indicate that P-450c21 represents a unique family of genes in the P-450 gene superfamily. Primer extension and S1 nuclease protection experiments identified several cap sites for initiation of transcription; the principal cap site produces mRNA with a 5' untranslated region only 11 bases long. S1 nuclease protection experiments confirm that P-450c21 is actively expressed in the adrenal and the testis, an organ not known to secrete 21-hydroxylated steroids.

Study of cholesterol side-chain cleavage (20,22 desmolase) deficiency causing congenital lipoid adrenal hyperplasia using bovine-sequence P450scc oligodeoxyribonucleotide probes

Conversion of cholesterol to pregnenolone is mediated by the cholesterol side-chain cleavage (SCC) enzyme, P450scc. Deficient SCC activity causes congenital lipoid adrenal hyperplasia (also known as 20,22 desmolase deficiency), a potentially lethal defect in the synthesis of all steroid hormones. To probe for possible genetic defects causing this disease we synthesized four oligodeoxyribonucleotides containing 63 to 72 bases corresponding to portions of the bovine complementary DNA (cDNA) sequence for P450scc. The bovine oligonucleotides were labeled and used directly to probe Southern blots of normal human genomic DNA, revealing a pattern indicating there is a single P450scc gene in the human genome. Hybridization to Northern blots of normal human and bovine adrenal messenger RNA indicates that P450scc messenger RNA is about 2.0 kilobases long in both species. Hybridizations of the oligonucleotides to genomic DNA from three unrelated patients with SCC deficiency did not detect a deletion in the human P450scc gene. The bovine sequence oligonucleotides were then used to isolate a human P450scc cDNA clone. The isolated P450scc cDNA fragment contains 818 bases encoding 239 amino acids of the protein, the translation termination signal, and 98 bases of the 3' untranslated region. The sequence of this carboxy-terminal half of the human P450scc protein is 72% homologous with the bovine sequence and contains an additional amino acid not found in bovine P450scc; the human and bovine nucleotide sequences are 81% homologous. Repetition of the genomic DNA blotting studies with the cDNA probe gave the same results obtained with the bovine-sequence oligonucleotide probes, confirming that SCC deficiency is not due to a deletion in the regions of the P450scc hybridizing with the probes. Long, chemically synthesized heterologous sequence oligonucleotides containing unknown numbers of base mismatches with human sequences may thus be used to study human genes so that access to a cDNA is not necessary for such studies.

Cloning and characterization of the bovine gene for steroid 21-hydroxylase (P-450c21)

Steroid 21-hydroxylase activity is required for the synthesis of both cortisol and aldosterone. Using two manually synthesized oligonucleotide probes, we screened a bovine genomic DNA library and identified a phage, lambda E11, carrying the gene for the steroid 21-hydroxylase, P-450c21. The identity of lambda E11 was initially proven by initiating dideoxy sequencing from the two oligonucleotides directly on the full-length, uncleaved phage template. Hybridization of total bovine genomic DNA to lambda E11 restriction fragments indicates the presence of repetitive sequences in or near the P-450c21 gene. Northern blots indicate that the mature mRNA exists in two principal forms of about 2.2 and 2.4 kb in length. Southern blots indicate there are two copies of the gene in the bovine genome. Sequence analysis of a 1141-bp Eco RI fragment of the gene shows three complete exons and a portion of a fourth exon, correctly determining the amino acid sequence of 148 amino acids of this important enzyme. This cloned 1141-bp fragment cross-hybridizes to human genomic DNA, indicating it should be a useful probe for studying the human P-450c21 gene in patients having impaired 21-hydroxylase activity.

Molecular cloning of DNA complementary to bovine adrenal P450scc mRNA

P450scc is the rate-limiting hormonally regulated enzyme that cleaves the cholesterol side chain. Translation of bovine adrenocortical mRNA and immunoprecipitation with rabbit anti-bovine P450scc indicates P450scc mRNA represents 1% of the total. DNA complementary to bovine adrenocortical mRNA was cloned in the PstI site of pBR322 by dC X dG tailing and high-efficiency transformation. A clone containing sequences complementary to P450scc mRNA was identified by hybrid-selected translation only when plasmid DNA was first purified by CsCl gradient centrifugation. As is often the case with hybrid-selected translation, the clone identified contains a small insert.

A clinical trial with cefoperazone in pneumonia and pyelonephritis

15 cases of pneumonia and 15 cases of pyelonephritis were included in a clinical trial of parenteral cefoperazone. The organisms isolated from patients with bacterial pneumonia were: Staphylococcus aureus (6), Klebsiella pneumoniae (5), Diplococcus pneumoniae (3), Pseudomonas species (2), and 6 others. Amongst those with pyelonephritis, Escherichia coli (12), Enterobacter (1), and 3 other pathogens, were isolated. All were sensitive to cefoperazone. The efficacy of cefoperazone in patients with pneumonia was: good in 11 (73%), fair in 3, and poor in 1. In pyelonephritis, 12 (80%) responded well and in the other 3 response was fair. There were no significant side effects or hypersensitivity reactions.