Baboon cytochrome P450 17alpha-hydroxylase/17,20-lyase (CYP17).

Human cytochrome P450 17alpha-hydroxylase (CYP17) catalyses not only the 17alpha-hydroxlation of pregnenolone and progesterone and the C17,20-side chain cleavage (lyase) of 17alpha-hydroxypregnenolone, necessary for the biosynthesis of C21-glucocorticoids and C19-androgens, but also catalyses the 16alpha-hydroxylation of progesterone. In efforts to understand the complex enzymology of CYP17, structure/function relationships have been reported previously after expressing recombinant DNAs, encoding CYP17 from various species, in nonsteroidogenic mammalian or yeast cells. A major difference between species resides in the lyase activity towards the hydroxylated intermediates and in the fact that the secretion of C19-steroids take place, in some species, principally in the gonads. Because human and higher primate adrenals secrete steroids, CYP17 has been characterized in the Cape baboon, a species more closely related to humans, in an effort to gain a further understanding of the reactions catalysed by CYP17. Baboon and human CYP17 cDNA share 96% homology. Baboon CYP17 has apparent Km and V values for pregnenolone and progesterone of 0.9 micro m and 0.4 nmol.h-1.mg protein-1 and 6.5 micro m and 3.9 nmol.h-1.mg protein-1, respectively. Baboon CYP17 had a significantly higher activity for progesterone hydroxylation relative to pregnenolone. No 16alpha-hydroxylase and no lyase activity for 17alpha-hydroxyprogesterone. Sequence analyses showed that there are 28 different amino acid residues between human and baboon CYP17, primarily in helices F and G and the F-G loop.     

Novel mutation of the CYP17 gene in two unrelated patients with combined 17alpha-hydroxylase/17,20-lyase deficiency: demonstration of absent enzyme activity by expressing the mutant CYP17 gene and by three-dimensional modeling

The CYP17 gene, located on chromosome 10q24-q25, encodes the cytochrome P450c17 enzyme. Mutations of this gene cause the 17alpha-hydroxylase/17,20-lyase deficiency, which is a rare, autosomal recessive form of congenital adrenal hyperplasia. Approximately 50 different mutations of the CYP17 gene have been described, of which some mutations have been identified in certain ethnic groups. In this study, we present the clinical history, hormonal findings and mutational analysis of two patients from unrelated families, who were evaluated for hypertension, hypokalemia and sexual infantilism. In the first patient, who was a 37-year-old female, additional studies showed a large myelolipoma in the left adrenal gland, and a smaller tumor in the right adrenal gland. In the second patient, who was a 31-year-old phenotypic female, clinical work-up revealed a 46,XY kariotype, absence of ovaries and presence of testes located in the inner opening of both inguinal canals. Analysis of the CYP17 gene by polymerase chain reaction amplification and direct sequencing demonstrated a novel homozygous mutation of codon 440 from CGC (Arg) to TGC (Cys) in both patients. The effect of this novel mutation on 17alpha-hydroxylase/17,20-lyase activity was assessed by in vitro studies on the mutant and wild-type P450c17 generated by site-directed mutagenesis and transfected in nonsteroidogenic COS-1 cells. These studies showed that the mutant P450c17 protein was produced in transfected COS-1 cells, but it had negligible 17alpha-hydroxylase and 17,20-lyase activities. In addition, three-dimensional computerized modeling of the heme-binding site of the P450c17 enzyme indicated that replacement of Arg by Cys at amino acid position 440 predicts a loss of the catalytic activity of the enzyme, as the mutant enzyme containing Cys440 fails to form a hydrogen bond with the propionate group of heme, which renders the mutant enzyme unable to stabilize the proper position of heme. Based on these findings we conclude that expressing the CYP17 gene with functional analysis, combined with three-dimensional computerized modeling of the heme-binding site of the protein provide feasible tools for molecular characterizing of functional consequences of the novel CYP17 mutation on enzyme function.     

Molecular basis of apparent isolated 17,20-lyase deficiency: compound heterozygous mutations in the C-terminal region (Arg(496)----Cys, Gln(461)----Stop) actually cause combined 17 alpha-hydroxylase/17,20-lyase deficiency.

The molecular defect in a reported case of isolated 17,20-lyase deficiency in a 46XY individual has been elucidated. The patient was found to be a compound heterozygote, carrying two different mutant alleles in the CYP17 gene. One allele contains a point mutation of arginine (CGC) to cysteine (TGC) at amino acid 496 in exon 8. The second allele contains a stop codon (TAG) in place of glutamine (CAG) at position 461 in exon 8 which is located 19 amino acids to the carboxy-terminal side of the P-450(17) alpha heme binding cysteine. COS-1 cells transfected with cDNAs containing one or the other of these mutations showed dramatically reduced 17 alpha-hydroxylase and 17,20-lyase activities relative to cells transfected with the wild type P-450(17) alpha cDNA. While the in vitro data in COS 1 cells can explain the patient's physical phenotype, with female external genitalia, it was somewhat discordant with the clinical expression of isolated 17,20-lyase deficiency with relative preservation of 17 alpha-hydroxylase activity in vivo. In addition to the expression studies of these two examples of mutants in the C-terminal region of cytochrome P-450(17) alpha, a third mutant cDNA construct containing a 4-base duplication at codon 480 previously found in patients with combined 17 alpha-hydroxylase/17,20-lyase deficiency was also expressed in COS-1 cells. This expressed protein was completely inactive with respect to both activities, supporting the biochemical findings in serum and in vitro biochemical data obtained using a testis from the patient. The results from these patients clearly indicate the importance of the C-terminal region of human P-450(17) alpha in its enzymatic activities.     

Deletion within the CYP17 gene together with insertion of foreign DNA is the cause of combined complete 17 alpha-hydroxylase/17,20-lyase deficiency in an Italian patient.

The molecular basis of 17 alpha-hydroxylase/17,20-lyase deficiency syndrome in a 14-yr-old 46,XY Italian patient was investigated by amplification, subcloning, and sequencing of specific exonic sequences from genomic DNA samples. A homozygous mutation, consisting of a 518-basepair (bp) deletion combined with a 469-bp insertion, was identified in the CYP17 gene of the patient. The deletion spans much of exon II, the whole intron 2, and a portion of exon III. A part (156 bp) of the inserted sequence shows 95.5% identity to the nuclear antigen-binding site on Marek disease virus DNA and sequences found in rearranged mitochondrial DNA of rat hepatoma cells. A similar degree of sequence identity (99%) was also found between the above sequences and part of the lac operon of E. coli. The inserted sequence is lacking the BamHI site in intron 2 of CYP17 and contains an in-frame stop codon (TAA). Thus, the mutated gene encodes a truncated nonfunctional steroid hydroxylase, giving rise to symptoms associated with complete combined 17 alpha-hydroxylase/17,20-lyase deficiency. The family history revealed that the patient is the child of a consanguineous marriage and has two genotypically and phenotypically female sisters also suffering from symptoms of the disease. Investigation of genomic DNA from these sisters revealed that in each case both CYP17 alleles contained the same mutation. On the other hand, the parents were found to be heterozygous for this mutation. The insertion could not be found in DNA from normal individuals or in the CYP17 gene of other Italian patients with the 17 alpha-hydroxylase deficiency syndrome.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulation of 17 alpha-hydroxylase/C17,20-lyase activity in porcine theca cells

The major source of ovarian androgen is the theca cells. Androgens are produced by the conversion of progestins by the 17 alpha-hydroxylase/C17,20 lyase enzymatic system (lyase). The 3 beta-hydroxysteroid dehydrogenase and aromatase enzymes in the theca cells are modulated by gonadotropins as well as by steroids produced locally. Therefore, the combined effects of hCG plus progesterone, estradiol, or dihydrotestosterone (DHT) on microsomal lyase activity in theca cells from large and medium-sized follicles were determined. Theca cells (3 x 10(6) cells/6 ml/well) were cultured in Medium 199 (M199) containing only insulin (10 micrograms/ml) and transferrin (5 micrograms/ml). At 24 h, theca cells were treated with M199, hCG (15 ng/ml), progesterone, estradiol, or DHT (100 ng/ml) or a combination of hCG + one of the three steroids. Media were removed at various times of culture (27-72 h) and levels of androgen determined by RIA. Microsomes were incubated with 1 microCi [3H]progesterone +0.5 mM NADPH and radioactive conversion products were measured after purification by thin layer chromatography. Administration of progesterone, estradiol, or DHT alone had little effect on lyase activity in theca cells from medium-sized follicles whereas the addition of hCG alone significantly increased lyase activity in these cells. However, concomitant addition of any steroid with hCG inhibited the increase in lyase activity after the addition of hCG alone. Theca cells from large porcine follicles had a higher basal level of lyase activity compared to theca cells from the smaller follicles. Lyase activity in theca cells from large follicles was enhanced by progesterone; estradiol was inhibitory. DHT initially stimulated lyase activity in theca cells from large follicles, but was inhibitory later in culture. In contrast to its marked effect on theca cells from medium follicles, hCG had only a small effect on lyase activity in theca cells from large follicles. Thus, thecal lyase activity increased as the follicle matured, providing more androgen substrate for the production of estrogen. Lyase activity in theca cells of medium follicles appears to be regulated predominantly by gonadotropin from the pituitary while intraovarian regulation of lyase activity by steroids may be more important in larger follicles.     

Inhibition of 17alpha-hydroxylase/C17,20-lyase (CYP17) from rat testis by green tea catechins and black tea theaflavins

In the course of screening for 17alpha-hydroxylase/C17,20-lyase inhibitors from food ingredients, the methanol soluble fraction of green tea and black tea, which were expected to be rich in catechin and theaflavin content, showed potent inhibitory activity. (-)-Epigallocathechin gallate and theaflavin 3-O-gallate with a pirogallol moiety significantly inhibited C17,20-lyase activity on IC50 values of 24.5 microM and 11.5 microM respectively. They had potent cytotoxicity against human prostate cancer LNCaP cells (IC50=28.1 microM and 37.4 microM).     

Immunohistochemistry and in situ hybridization of P-45017 alpha (17 alpha-hydroxylase/17,20-lyase).

Cytochrome P-45017 alpha catalyzes both 17 alpha-hydroxylation and 17,20-side-chain cleavage in steroidogenesis and lies at a key branch point in the pathways of steroid hormone biosynthesis. To obtain information on the precise localization of P-45017 alpha in swine testis, ovary, and adrenal, we undertook the simultaneous detection of P-45017 alpha mRNA and protein by combining immunohistochemistry with in situ hybridization. In situ hybridization was performed on 4% paraformaldehyde-fixed, paraffin-embedded sections by employing either a 39-base oligomer or a cDNA insert (1.7 KB) of porcine testis P-45017 alpha as DNA probe. Immunohistochemical study was performed by employing anti-P-45017 alpha. Hybridization signals were obtained in Leydig cells of the testis, theca interna of the ovarian follicle, and zona fasciculata reticularis cells of the adrenal cortex. Oligonucleotide probing yielded lower background signal than the cDNA probe. No specific signals were obtained in seminiferous tubules of the testis, medulla, and zona glomerulosa of the adrenal, and in membrana granulosa and interstitial cells of the ovary. Hybridization signals were obtained in the cells where immunoreactivity of the enzyme was observed by immunohistochemistry, except for some Leydig cells of the testis and theca interna cells of the ovary in which only immunoreactivity but not hybridization signal was observed. The present study provided detailed information about the precise cellular localization of P-45017 alpha expression at both the protein and mRNA levels in swine adrenal glands and gonads. This approach of simultaneous immunohistochemistry and in situ hybridization analysis of steroidogenic enzymes can be applied in the future to tissues exhibiting abnormal steroid metabolism and should contribute to a better understanding of steroidogenesis.     

Deletion of a phenylalanine in the N-terminal region of human cytochrome P-450(17 alpha) results in partial combined 17 alpha-hydroxylase/17,20-lyase deficiency

Steroid 17 alpha-hydroxylase and 17,20-lyase activities reside within the same polypeptide chain (cytochrome P-450(17 alpha)), and consequently human 17 alpha-hydroxylase deficiencies are characterized by defects in either or both of these activities. Human mutants having these deficiencies represent an excellent source of material for investigation of P-450(17 alpha) structure-function relationships. The CYP17 gene from an individual having partial combined 17 alpha-hydroxylase/17,20-lyase deficiency has been characterized structurally and the homozygous mutation found to be the deletion of the phenylalanine codon (TTC) at either amino acid position 53 or 54 in exon 1. Reconstruction of this mutation into a human P-450(17 alpha) cDNA followed by expression in COS 1 cells led to production of the same amount of immunodetectable P-450(17 alpha) protein as found with expression of the normal human P-450(17 alpha) cDNA. However, 17 alpha-hydroxylase activity of this mutant protein measured in intact cells was less than 37% of that observed upon expression of the wild-type enzyme, whereas 17,20-lyase activity of the mutant was less than 8% of that observed with the normal enzyme. When estimated in intact cells, the Km for 17 alpha-hydroxylation of progesterone was increased by a factor of 2 in the mutant enzyme, whereas the Vmax was reduced by a factor of 3. In order to estimate the kinetic parameters for the 17,20-lyase reaction, microsomes were isolated from transfected COS 1 cells to enrich for this activity. Surprisingly, the specific activity of the mutant 17 alpha-hydroxylase in microsomes was 3-fold less than that observed in intact cells, indicating that the structure of mutant P-450(17 alpha) was dramatically altered upon disruption of COS 1 cells. Apparently the deletion of a single phenylalanine in the N-terminal region of P-450(17 alpha) alters its folding in such a way that both enzymatic activities are dramatically decreased, leading to the partial combined deficiency observed in this individual.     

Quantitative changes in ovarian 17 alpha-hydroxylase/C17,20-lyase and aromatase activities during the estrous cycle of the hamster

Aromatase activity of the microsomal fraction of ovarian homogenates, measured by a tritium exchange assay using androstenedione (A-dione) as substrate, did not change during the 4-day estrous cycle of the hamster. In contrast, 17 alpha-hydroxylase, measured by a tritium exchange assay with progesterone as substrate, and particularly C17,20-lyase activity, evaluated by acetic acid production from progesterone, drastically decreased on the afternoon of proestrus (Day 4). The latter two activities remained low on Days 1 and 2 but increased dramatically on Day 3 and the morning of Day 4. The serum concentration of A-dione and 17-hydroxyprogesterone reached a peak at 1600 hr of proestrus and then decreased rapidly as the enzyme activities decreased. A-dione levels were undetectable on Day 1 but 17-hydroxyprogesterone levels remained elevated through Day 2. The results are consistent with the view that 17 alpha-hydroxylase and C17,20-lyase are activities of a single cytochrome P-450, as has been shown for testis and adrenal. Increases in hydroxylase and lyase activities occur concomitantly with decreases in the serum concentrations of progesterone, suggesting that the latter steroid may play a role in controlling these enzymes.     

Haploinsufficiency of cytochrome P450 17alpha-hydroxylase/17,20 lyase (CYP17) causes infertility in male mice

Cytochrome P450 17alpha-hydroxylase/17,20-lyase (CYP17) is critical in determining cortisol and sex steroid biosynthesis. To investigate how CYP17 functions in vivo, we generated mice with a targeted deletion of CYP17. Although in chimeric mice Leydig cell CYP17 mRNA and intratesticular and circulating testosterone levels were dramatically reduced (80%), the remaining testosterone was sufficient to support spermatogenesis as evidenced by the generation of phenotypical black C57BL/6 mice. However, male chimeras consistently failed to generate heterozygous CYP17 mice and after five matings chimeric mice stopped mating indicating a change in sexual behavior. These results suggested that CYP17 deletion caused a primary phenotype (infertility), probably not due to the anticipated androgen imbalance and a secondary phenotype (change in sexual behavior) due to the androgen imbalance. Surprisingly, CYP17 mRNA was found in mature sperm, and serial analysis of gene expression identified CYP17 mRNA in other testicular germ cells. CYP17 mRNA levels were directly related to percent chimerism. Moreover, more than 50% of the sperm from high-percentage chimeric mice were morphologically abnormal, and half of them failed the swim test. Furthermore, 60% of swimming abnormal sperm was devoid of CYP17. These results suggest that CYP17, in addition to its role in steroidogenesis and androgen formation, is present in germ cells where it is essential for sperm function, and deletion of one allele prevents genetic transmission of mutant and wild-type alleles causing infertility followed by change in sexual behavior due to androgen imbalance.     

In vitro and in vivo models for the evaluation of potent inhibitors of male rat 17alpha-hydroxylase/C17,20-lyase

The C(17,20)-lyase is a key enzyme in the biosynthesis of androgens by both the testes and adrenals. A complete inhibition of this enzyme would provide an alternative means of androgen suppression for the treatment of prostatic cancers. In the present study, the inhibitory effects of new non-steroidal compounds were tested in vitro on rat C(17,20)-lyase versus abiraterone, a reference steroidal inhibitor. Their activities were also evaluated in vivo on plasma testosterone (T) and luteinizing hormone (LH) levels and on testes, adrenals, seminal vesicles (SV) and ventral prostate (VP) weights after 3 days of oral treatment to adult male rats (50mg/kg per day p.o.).Inhibition in the nanomolar range was obtained with TX 977, the lead racemate product in this series, and optimization is ongoing based on a slight dissociation observed between its two diastereoisomers, TX 1196-11 (S) and TX 1197-11 (R). These non-steroidal compounds (including YM 55208, a reference competitor) proved to be more active in vivo than abiraterone acetate in this model, but the observed impact on adrenal weight suggests that the specificity of lyase inhibition versus corticosteroid biosynthesis deserves further investigations with this new class of potentially useful agents for the treatment of androgen-dependent prostate cancer.     

Loss of ovarian 17 alpha-hydroxylase/C17,20-lyase activity induced by human chorionic gonadotropin is correlated with in vivo substrate availability

Administration of human chorionic gonadotropin (hCG) to hypophysectomized immature rats caused a rapid reduction in ovarian microsomal 17 alpha-hydroxylase/C17,20-lyase activity (cytochrome P450(17 alpha] with a concomitant large increase in serum progesterone (P4) level. Pretreatment with cycloheximide (Cyclo) or aminoglutethimide (Ag) prevented these effects of hCG, while Actinomycin D (Act-D) or Azastene, an inhibitor of 3-hydroxysteroid dehydrogenase, were ineffective. In ovaries with enzyme activity increased by 48 h exposure to pregnant mare's serum gonadotropin, hCG also caused a large decrease in enzyme activity but only after a lag period of about 2 h: P4 levels were increased simultaneously. Administration of Cyclo. or puromycin (Puro) caused a loss of enzyme activity without changing P4 levels, but both inhibitors prevented some of the loss of activity and rise in P4 induced by hCG. AG and Act D completely inhibited the enzyme reducing action of hCG, as well as the increase in P4 synthesis, in these animals. P4 applied directly onto one ovary of an animal given hCG plus AG reduced enzyme activity by 69%. The results are consistent with the interpretation that increased substrate concentration is one of but not the only important factor in loss of hydroxylase/lyase activity induced by a sudden large increase in luteinizing hormone activity.     

Comparison of the hamster and human adrenal P450c17 (17 alpha-hydroxylase/17,20-lyase) using site-directed mutagenesis and molecular modeling

In order to understand the activity specificity of the hamster cytochrome P450 17 alpha-hydroxylase/17,20-lyase (P450c17), we have studied its structure/activity using three hamster P450c17 recombinant mutants (T202N/D240N/D407H). In transiently transfected COS-1 cells, the mutation T202N reduced 17 alpha-hydroxylation of pregnenolone and progesterone to 24 and 44% of wild type (WT), respectively, followed by reduced 17,20-cleavage to 71 and 67%, respectively. On the other hand, the mutation D240N decreased specifically 17,20-lyase activity to 61% of WT when incubated with pregnenolone while the mutation D407H only decreased 17 alpha-hydroxylation to 46% when incubated with progesterone.To comprehend the altered activity profiles of these hamster P450c17 mutants, we have elaborated a 3D model of the hamster P450c17 and compared it to our preceding model of the human P450c17. Analysis of the mutants with this model showed that, without direct contact to the substrates, these mutations transmit structural changes to the active site. By analogy, these results support the concept that any cellular changes modifying the external structure of P450c17, such as phosphorylation, could have influence on its active site and enzymatic activities.     

The C17,20-lyase, 17 alpha-hydroxylase and aromatase activity in rat ovarian granulosa cells stimulated in vivo by gonadotropins

Immature hypophysectomized rats were injected with PMS; some groups received hCG 48h later. The C17,20-lyase activity in the granulosa cells removed from the large preovulatory follicles was estimated by the amount of labelled acetic acid produced from 21 (14C) progesterone or 17-hydroxyprogesterone. 17 alpha-hydroxylase and aromatase activity were measured by the tritium exchange method. Although the granulosa cells contained lyase, it was considerably less than their hydroxylase activity. The remaining tissue, consisting of small follicles and hypertrophied thecal and interstitial tissue, had a great deal more lyase and hydroxylase activity than did the granulosa cells. The results are consistent with the view that granulosa cells can produce estrogen from progesterone and do not require androgen precursors from the theca and/or interstitium.     

4-pregnene-3-one-20 beta-carboxaldehyde: a potent inhibitor of 17 alpha-hydroxylase/C17,20-lyase and of 5 alpha-reductase.

The pregnene derivative, 4-pregnene-3-one-20 beta-carboxaldehyde (22-A) was evaluated as an inhibitor of 17 alpha-hydroxylase/C17,20-lyase in rat testicular microsomes and of 5 alpha-reductase in human prostatic homogenates. The effect of the compound in vivo was studied in adult male rats. The 22-A demonstrated potent and competitive inhibition of 17 alpha-hydroxylase and C17,20-lyase with Ki values 8.48 and 0.41 microM, respectively, significantly below the Km values for these two enzymes (33.75 and 4.55 microM). This compound also showed potent inhibition of 5 alpha-reductase with a Ki value of 15.6 nM (Km for this enzyme is 50 nM). By comparison, ketoconazole, a currently studied 17 alpha-hydroxylase/C17,20-lyase inhibitor for the treatment of prostatic cancer, showed less potent inhibition of 17 alpha-hydroxylase (Ki 39.5 microM) and C17,20-lyase (Ki 3.6 microM) and did not inhibit 5 alpha-reductase. Progesterone which has been reported to inhibit the 17 alpha-hydroxylase/C17,20-lyase, did not significantly reduce the production of testosterone by rat testes in vitro in comparison to controls, while the same concentration of 22-A demonstrated a 42% reduction of testosterone biosynthesis. When the adult male rats were injected s.c. with 22-A at 50 mg/day/kg for a 2 week period, the testosterone concentrations in the rat sera were significantly lower than control values (P less than 0.05), whereas serum corticosterone levels did not change. These results suggest that 22-A is a selective potent inhibitor for 17 alpha-hydroxylase and C17,20-lyase, but is more potent for the C17,20-lyase. The compound also inhibits 5 alpha-reductase, and therefore may reduce biosynthesis of testosterone and dihydrotestosterone effectively. Thus, 22-A may be useful in the treatment of problems associated with the androgen excess and prostatic cancer.