Porcine gonadal and placental isozymes of aromatase cytochrome P450: sub-cellular distribution and support by NADPH-cytochrome P450 reductase

19-hydroxy Steroids in the Aromatase Reaction: Review on Expression and Potential Functions

Scientific evidence related to the aromatase reaction in various biological processes spanning from mid-1960 to today is abundant; however, as our analytical sensitivity increases, a new look at the old chemical reaction is necessary. Here, we review an irreversible aromatase reaction from the substrate androstenedione. It proceeds in 3 consecutive steps. In the first 2 steps, 19-hydroxy steroids are produced. In the third step, estrone is produced. They can dissociate from the enzyme complex and either accumulate in tissues or enter the blood.

In this review, we want to highlight the potential importance of these 19-hydroxy steroids in various physiological and pathological conditions. We focus primarily on 19-hydroxy steroids, and in particular on the 19-hydroxyandrostenedione produced by the incomplete aromatase reaction. Using a PubMed database and the search term “aromatase reaction,” 19-hydroxylation of androgens and steroid measurements, we detail the chemistry of the aromatase reaction and list previous and current methods used to measure 19-hydroxy steroids.

We present evidence of the existence of 19-hydroxy steroids in brain tissue, ovaries, testes, adrenal glands, prostate cancer, as well as during pregnancy and parturition and in Cushing’s disease. Based on the available literature, a potential involvement of 19-hydroxy steroids in the brain differentiation process, sperm motility, ovarian function, and hypertension is suggested and warrants future research.

We hope that with the advancement of highly specific and sensitive analytical methods, future research into 19-hydroxy steroids will be encouraged, as much remains to be learned and discovered.

Sequence comparisons of cytochrome P450 aromatases from Australian animals predict differences in enzymatic activity and/or efficiency†

Aromatase (P450arom, CYP19A1) is the terminal enzyme in the synthesis of the steroid hormone family of estrogens. Not surprisingly, this enzyme has structural similarities between the limited number of species studied thus far. This study examined the structure of aromatases from four diverse Australian species including a marsupial (tammar wallaby; Macropus eugenii), monotreme (platypus; Ornithorhynchus anatinus), ratite (emu; Dromaius novaehollandiae) and lizard (bearded dragon; Pogona vitticeps). We successfully built homology models for each species, using the only crystallographically determined structure available, human aromatase. The amino acid sequences showed high amino acid sequence identity to the human aromatase: wallaby 81%, platypus 73%, emu 75% and bearded dragon at 74%. The overall structure was highly conserved among the five species, although there were non-secondary structures (loops and bends) that were variable and flexible that may result in some differences in catalytic activity. At the N-terminal regions, there were deletions and variations that suggest that functional distinctions may be found. We found that the active sites of all these proteins were identical, except for a slight variation in the emu. The electrostatic potential across the surfaces of these aromatases highlighted likely variations to the protein-protein interactions of these enzymes with both redox partner cytochrome P450 reductase and possibly homodimerization in the case of the platypus, which has been postulated for the human aromatase enzyme. Given the high natural selection pressures on reproductive strategies, the relatively high degree of conservation of aromatase sequence and structure across species suggests that there is biochemically very little scope for changes to have evolved without the loss of enzyme activity.

Steroidogenic enzyme activities in the pre- and post-parturient equine placenta

Steroidogenic enzymes in placentas shape steroid hormone profiles in the maternal circulation of each mammalian species. These include 3β-hydroxysteroid dehydrogenase/Δ5-4 isomerase (3βHSD) and 17α-hydroxylase/17,20-lyase cytochrome P450 (P450c17) crucial for progesterone and androgen synthesis, respectively, as well as aromatase cytochrome P450 (P450arom) that converts Δ4-androgens to estrogens. 5α-reductase is another important enzyme in equine placentas because 5α-dihydroprogesterone (DHP) sustains pregnancy in the absence of progesterone in the second half of equine pregnancy. DHP and its metabolites decline dramatically days before foaling, but few studies have investigated placental enzyme activity before or at parturition in mares. Thus, key enzyme activities and transcript abundance were investigated in equine placentas at 300 days of gestation (GD300) and post-partum (term). Equine testis was used as a positive control for P450c17 activity. Substrates were incubated with microsomal preparations, together with enzyme inhibitors, and products were measured by liquid chromatography tandem mass spectrometry or radiometric methods (aromatase). Equine placenta expressed high levels of 3βHSD, 5α-reductase and aromatase, and minimal P450c17 activity at GD300 compared with testis (600-fold higher). At foaling, 3βHSD and aromatase activities and transcript abundance were unchanged but 5α-reductase (and P450c17) was no longer detectable (P < 0.05) and transcript was decreased. Trilostane inhibited 3βHSD significantly more in testis than placenta, suggesting possible existence of different 3βHSD isoforms. Equine placentas have significant capacity for steroid metabolism by 5α-reductase, 3βHSD and aromatase but little for androgen synthesis lacking P450c17. Declining pre-partum 5α-reduced pregnane concentrations coincide with selective loss of placental 5α-reductase activity and expression at parturition in horses.

Estrogens in Male Physiology

Estrogens have historically been associated with female reproduction, but work over the last two decades established that estrogens and their main nuclear receptors (ESR1 and ESR2) and G protein-coupled estrogen receptor (GPER) also regulate male reproductive and nonreproductive organs. 17β-Estradiol (E2) is measureable in blood of men and males of other species, but in rete testis fluids, E2 reaches concentrations normally found only in females and in some species nanomolar concentrations of estrone sulfate are found in semen. Aromatase, which converts androgens to estrogens, is expressed in Leydig cells, seminiferous epithelium, and other male organs. Early studies showed E2 binding in numerous male tissues, and ESR1 and ESR2 each show unique distributions and actions in males. Exogenous estrogen treatment produced male reproductive pathologies in laboratory animals and men, especially during development, and studies with transgenic mice with compromised estrogen signaling demonstrated an E2 role in normal male physiology. Efferent ductules and epididymal functions are dependent on estrogen signaling through ESR1, whose loss impaired ion transport and water reabsorption, resulting in abnormal sperm. Loss of ESR1 or aromatase also produces effects on nonreproductive targets such as brain, adipose, skeletal muscle, bone, cardiovascular, and immune tissues. Expression of GPER is extensive in male tracts, suggesting a possible role for E2 signaling through this receptor in male reproduction. Recent evidence also indicates that membrane ESR1 has critical roles in male reproduction. Thus estrogens are important physiological regulators in males, and future studies may reveal additional roles for estrogen signaling in various target tissues.

Evolutionary comparisons predict that dimerization of human cytochrome P450 aromatase increases its enzymatic activity and efficiency

Estrogen is an essential vertebrate hormone synthesized from androgens involving multiple hydroxylations, catalyzed by cytochrome P450 aromatase (P450arom or CYP19) enzymes. Despite their importance, very few comparative studies have been conducted on vertebrate and/or mammalian P450arom enzymes, either structurally or functionally. Here we directly compared the human (h-) and porcine gonadal (p(g)-) P450arom, as p(g)-P450arom has very low catalytic efficiency, with a ten-fold higher affinity (Km) for a substrate (androstenedione) and ten-fold reduction in turnover (Vmax). We recombinantly expressed these proteins and compared their interactions on a membrane using a quartz crystal microbalance (QCM) and also with the electron donor protein cytochrome P450 oxidoreductase (CPR). Changes in frequency and dissipation in the QCM supported the h-P450arom forming a homodimer that agreed with the FRET data, but not p(g)-P450arom. Analysis of the X-ray crystal structure of the h-P450arom suggested a likely site of homo-dimerization and found that certain key interacting residues were not conserved in pg-P450arom. Molecular dynamics simulations provide support for the importance of these residues in homo-dimerization. Here we propose that the lower affinity and higher activity with reduced release of intermediate metabolites by the h-P450arom is as a consequence of its ability to form homodimers. The functional implications of dimerization provide an important mechanistic step in the requirement for efficient aromatization.

Pathways and genes involved in steroid hormone metabolism in male pigs: a review and update

This paper reviews state-of-the-art knowledge on steroid biosynthesis pathways in the pig and provides an updated characterization of the porcine genes involved in these pathways with particular focus on androgens, estrogens, and 16-androstenes. At least 21 different enzymes appear to be involved in these pathways in porcine tissues together with at least five cofactors. Until now, data on several porcine genes were scarce or confusing. We characterized the complete genomic and transcript sequences of the single porcine CYP11B gene. We analyzed the porcine AKR1 gene cluster and identified four AKR1C, one AKR1C like genes and one AKR1E2 gene. We provide evidence that porcine AKR1C genes are not orthologous to human AKR1C. A new nomenclature is thus needed for this gene family in the pig. Thirty-two genes are now described: transcript (30+2 characterized in this study) and genomic (complete: 18+1 and partial: 12+1) sequences are identified. However, despite increasing knowledge on steroid metabolism in the pig, there is still no explanation of why porcine testes can produce androstenone and epiandrosterone, but not dihydrotestosterone (DHT), which is also a reduced steroid.

Sequence-function correlation of aromatase and its interaction with reductase

Aromatase is an enzyme required for the conversion of androgens to estrogens. Estrogens are female sex hormones involved in the development and growth of breast tumors. It has been of significant interest to investigate the structure-function relationship of aromatase since its inhibitors have shown great promise in fighting breast cancer. Aromatase belongs to the cytochrome P450 family, and forms an electron-transfer complex with its partner, NADPH-cytochrome P450 reductase (CPR), during the aromatization reaction. Aromatase is found to be widely expressed in vertebrates with unique substrates androstenedione and testosterone, but with various catalytic capacities reflecting species differences in K(m), Vmax, etc. This report will summarize current progress in sequence-function correlation analysis of the aromatase protein family and molecular characterization of the interaction between aromatase and CPR. These studies may lead to a novel field for the development of new inhibitors which interfere with the interaction between aromatase and CPR in order to inhibit the aromatization reaction.

Adaptive evolution of mammalian aromatases: lessons from Suiformes

Estrogen synthesis evolved in chordates to control reproduction. The terminal enzyme in the cascade directly responsible for estrogen synthesis is aromatase cytochrome P450 (P450arom) encoded by the CYP19 gene. Mammals typically have a single CYP19 gene but pigs, peccaries and other Suiformes have two or more resulting from duplication in a common ancestor. Duplication of CYP genes in the steroid synthetic cascade has occurred for only one other enzyme, also terminal, 11beta-hydroxylase P450 (P450c11). P450arom and P450c11 share common substrates and even physiological functions as possible remnants from a common P450 progenitor, perhaps an ancestral P450arom, which is supported by phylogenetic analysis. Conserved tissue-specific expression patterns of P450arom paralogs in placenta and gonads of pigs and peccaries suggest how functional adaptation may have proceeded divergently and influenced adopted reproductive strategies including ovulation rate and litter size. Data suggest that the porcine placental paralog evolved catalytically to protect female conceptuses from testosterone produced by male siblings; the gonadal paralog to synthesize a novel, nonaromatizable testosterone metabolite (1OH-testosterone) that may increase ovulation rate. This would represent a coevolution facilitating litter bearing as pigs diverged from peccaries. Evidence of convergence between the peccary CYP19 genes and lower tissue expression may therefore represent initiation of loss of the functional paralogs. Studies on the Suiforme aromatases provide insights into the evolution of the steroidogenic cascade and metabolic pathways in general, how it translates into physiological adaptations (altered reproductive strategies for instance), and how duplicated genes become stabilized or disappear from genomes.

Porcine hypothalamic aromatase cytochrome P450: isoform characterization, sex-dependent activity, regional expression, and regulation by enzyme inhibition in neonatal boars

Domestic pigs have three CYP19 genes encoding functional paralogues of the enzyme aromatase cytochrome P450 (P450arom) that are expressed in the gonads, placenta, and preimplantation blastocyst. All catalyze estrogen synthesis, but the gonadal-type enzyme is unique in also synthesizing a nonaromatizable biopotent testosterone metabolite, 1OH-testosterone (1OH-T). P450arom is expressed in the vertebrate brain, is higher in males than females, but has not been investigated in pigs, to our knowledge. Therefore, these studies defined which of the porcine CYP19 genes was expressed, and at what level, in adult male and female hypothalamus. Regional expression was examined in mature boars, and regulation of P450arom expression in neonatal boars was investigated by inhibition of P450arom with letrozole, which is known to reprogram testicular expression. Pig hypothalami expressed the gonadal form of P450arom (redesignated the "gonadal/hypothalamic" porcine CYP19 gene and paralogue) based on functional analysis confirmed by cloning and sequencing transcripts. Hypothalamic tissue synthesized 1OH-T and was sensitive to the selective P450arom inhibitor etomidate. Levels were 4-fold higher in male than female hypothalami, with expression in the medial preoptic area and lateral borders of the ventromedial hypothalamus of boars. In vivo, letrozole-treated neonates had increased aromatase activity in hypothalami but decreased activity in testes. Therefore, although the same CYP19 gene is expressed in both tissues, expression is regulated differently in the hypothalamus than testis. These investigations, the first such studies in pig brain to our knowledge, demonstrate unusual aspects of P450arom expression and regulation in the hypothalamus, offering promise of gaining better insight into roles of P450arom in reproductive function.

Molecular characterization of aromatase

Aromatase is an estrogen synthetase. Estrogens are female sex hormones involved in the development and growth of breast tumors. Elucidation of the structure-function relationship of aromatase has been of significant interest since its inhibitors have shown great promise in fighting breast cancer. Aromatase belongs to the cytochrome P450 family and forms an electron-transfer complex with its partner, NADPH-cytochrome P450 reductase. Because of the membrane-bound character and heme-binding instability, no crystal structure of aromatase has been reported so far. Much remains to be investigated, including the 3-dimensional structure of aromatase, interaction between aromatase and reductase, catalytic mechanism of estrogen synthesis by aromatase, and the binding mechanism of aromatase inhibitors. This review presents current knowledge about structural and functional characteristics of aromatase to address unsolved mysteries about this enzyme.

PPARα Agonist-Induced Rodent Tumors: Modes of Action and Human Relevance

Widely varied chemicals--including certain herbicides, plasticizers, drugs, and natural products--induce peroxisome proliferation in rodent liver and other tissues. This phenomenon is characterized by increases in the volume density and fatty acid oxidation of these organelles, which contain hydrogen peroxide and fatty acid oxidation systems important in lipid metabolism. Research showing that some peroxisome proliferating chemicals are nongenotoxic animal carcinogens stimulated interest in developing mode of action (MOA) information to understand and explain the human relevance of animal tumors associated with these chemicals. Studies have demonstrated that a nuclear hormone receptor implicated in energy homeostasis, designated peroxisome proliferator-activated receptor alpha (PPARalpha), is an obligatory factor in peroxisome proliferation in rodent hepatocytes. This report provides an in-depth analysis of the state of the science on several topics critical to evaluating the relationship between the MOA for PPARalpha agonists and the human relevance of related animal tumors. Topics include a review of existing tumor bioassay data, data from animal and human sources relating to the MOA for PPARalpha agonists in several different tissues, and case studies on the potential human relevance of the animal MOA data. The summary of existing bioassay data discloses substantial species differences in response to peroxisome proliferators in vivo, with rodents more responsive than primates. Among the rat and mouse strains tested, both males and females develop tumors in response to exposure to a wide range of chemicals including DEHP and other phthalates, chlorinated paraffins, chlorinated solvents such as trichloroethylene and perchloroethylene, and certain pesticides and hypolipidemic pharmaceuticals. MOA data from three different rodent tissues--rat and mouse liver, rat pancreas, and rat testis--lead to several different postulated MOAs, some beginning with PPARalpha activation as a causal first step. For example, studies in rodent liver identified seven "key events," including three "causal events"--activation of PPARalpha, perturbation of cell proliferation and apoptosis, and selective clonal expansion--and a series of associative events involving peroxisome proliferation, hepatocyte oxidative stress, and Kupffer-cell-mediated events. Similar in-depth analysis for rat Leydig-cell tumors (LCTs) posits one MOA that begins with PPARalpha activation in the liver, but two possible pathways, one secondary to liver induction and the other direct inhibition of testicular testosterone biosynthesis. For this tumor, both proposed pathways involve changes in the metabolism and quantity of related hormones and hormone precursors. Key events in the postulated MOA for the third tumor type, pancreatic acinar-cell tumors (PACTs) in rats, also begin with PPARalpha activation in the liver, followed by changes in bile synthesis and composition. Using the new human relevance framework (HRF) (see companion article), case studies involving PPARalpha-related tumors in each of these three tissues produced a range of outcomes, depending partly on the quality and quantity of MOA data available from laboratory animals and related information from human data sources.

Increased blood pressure in mice lacking cytochrome P450 2J5

The cytochrome P450 (CYP) enzymes participate in a wide range of biochemical functions, including metabolism of arachidonic acid and steroid hormones. Mouse CYP2J5 is abundant in the kidney where its products, the cis-epoxyeicosatrienoic acids (EETs), modulate sodium transport and vascular tone. To define the physiological role of CYP2J5 in the kidney, knockout mice were generated using a conventional gene targeting approach. Cyp2j5 (-/-) mice develop normally and exhibit no overt renal pathology. While renal EET biosynthesis was apparently unaffected by the absence of CYP2J5, deficiency of this CYP in female mice was associated with increased blood pressure, enhanced proximal tubular transport rates, and exaggerated afferent arteriolar responses to angiotensin II and endothelin I. Interestingly, plasma 17beta-estradiol levels were reduced in female Cyp2j5 (-/-) mice and estrogen replacement restored blood pressure and vascular responsiveness to normal levels. There was no evidence of enhanced estrogen metabolism, or altered expression or activities of steroidogenic enzymes in female Cyp2j5 (-/-) mice, but their plasma levels of luteinizing hormone and follicle stimulating hormone were inappropriately low. Together, our findings illustrate a sex-specific role for CYP2J5 in regulation of blood pressure, proximal tubular transport, and afferent arteriolar responsiveness via an estrogen-dependent mechanism.

Engineering and assaying of cytochrome P450 biocatalysts

Cytochrome P450s constitute a highly fascinating superfamily of enzymes which catalyze a broad range of reactions. They are essential for drug metabolism and promise industrial applications in biotechnology and biosensing. The constant search for cytochrome P450 enzymes with enhanced catalytic performances has generated a large body of research. This review will concentrate on two key aspects related to the identification and improvement of cytochrome P450 biocatalysts, namely the engineering and assaying of these enzymes. To this end, recent advances in cytochrome P450 development are reported and commonly used screening methods are surveyed.

Placental Expression and Molecular Characterization of Aromatase Cytochrome P450 in the Spotted Hyena (Crocuta crocuta)

At birth, the external genitalia of female spotted hyenas (Crocuta crocuta) are the most masculinized of any known mammal, but are still sexually differentiated. Placental aromatase cytochrome P450 (P450arom) is an important route of androgen metabolism protecting human female fetuses from virilization in utero. Therefore, placental P450arom expression was examined in spotted hyenas to determine levels during genital differentiation, and to compare molecular characteristics between the hyena and human placental enzymes. Hyena placental P450arom activity was determined at gestational days (GD) 31, 35, 45, 65 and 95 (term, 110), and the relative sensitivity of hyena and human placental enzyme to inhibition by the specific inhibitor, Letrozole, was also examined. Expression of hyena P450arom in placenta was localized by immuno-histochemistry, and a full-length cDNA was cloned for phylogenetic analysis. Aromatase activity increased from GD31 to a peak at 45 and 65, apparently decreasing later in gestation. This activity was more sensitive to inhibition by Letrozole than was human placental aromatase activity. Expression of P450arom was localized to syncytiotrophoblast and giant cells of mid-gestation placentas. The coding sequence of hyena P450arom was 94% and 86% identical to the canine and human enzymes respectively, as reflected by phylogenetic analyses. These data demonstrate for the first time that hyena placental aromatase activity is comparable to that of human placentas when genital differentiation is in progress. This suggests that even in female spotted hyenas clitoral differentiation is likely protected from virilization by placental androgen metabolism. Decreased placental aromatase activity in late gestation may be equally important in allowing androgen to program behaviors at birth. Although hyena P450arom is closely related to the canine enzyme, both placental anatomy and P450arom expression differ. Other hyaenids and carnivores must be investigated to determine the morphological and functional ancestral state of their placentas, as it relates to evolutionary relationships among species in this important taxonomic group.

Chronic ethanol perturbs testicular folate metabolism and dietary folate deficiency reduces sex hormone levels in the Yucatan micropig

Although alcoholism causes changes in hepatic folate metabolism that are aggravated by folate deficiency, male reproductive effects have never been studied. We evaluated changes in folate metabolism in the male reproductive system following chronic ethanol consumption and folate deficiency. Twenty-four juvenile micropigs received folate-sufficient (FS) or folate-depleted (FD) diets or the same diets containing 40% of energy as ethanol (FSE or FDE) for 14 wk, and the differences between the groups were determined by ANOVA. Chronic ethanol consumption (FSE and FDE compared with FS and FD groups) reduced testis and epididymis weights, testis sperm concentrations, and total sperm counts and circulating FSH levels. Folate deficiency (FD and FDE compared with FS and FSE groups) reduced circulating testosterone, estradiol and LH levels, and also testicular 17,20-lyase and aromatase activities. There was histological evidence of testicular lesions and incomplete progression of spermatogenesis in all treated groups relative to the FS control, with the FDE group being the most affected. Chronic ethanol consumption increased testis folate concentrations and decreased testis methionine synthase activity, whereas folate deficiency reduced total testis folate levels and increased methionine synthase activity. In all pigs combined, testicular methionine synthase activity was negatively associated with circulating estradiol, LH and FSH, and 17,20-lyase activity after controlling for ethanol, folate deficiency, and their interaction. Thus, while chronic ethanol consumption primarily impairs spermatogenesis, folate deficiency reduces sex hormones, and the two treatments have opposite effects on testicular folate metabolism. Furthermore, methionine synthase may influence the hormonal regulation of spermatogenesis.

Characterization of a cetacean aromatase (CYP19) and the phylogeny and functional conservation of vertebrate aromatase

Aromatase (cytochrome P450 19, CYP19, P450arom) is the enzyme responsible for the production of estrogens, hormones critical for development and reproduction. Aromatase was sequenced from a white-sided dolphin (Lagenorhynchus acutus) ovary, transiently transfected into HEK 293 cells, and the expressed protein was characterized for aromatase activity in the presence of androstenedione and testosterone and after exposure to the aromatase inhibitor letrazole. The Kms for androstenedione and testosterone were 63.5 and 75 nM, respectively, values that are very similar to those reported for other mammalian aromatases. A Bayesian phylogenetic analysis of the vertebrate aromatases was performed on the amino acid sequences of aromatases from fish, amphibians, reptiles, birds, and mammals. Based on known species phylogeny, the cetacean aromatase showed an expected grouping with artiodactyls (cow, sheep, and goat). An analysis of functional divergence showed strong conservation of aromatase across the entire protein, which indicates that the observed sequence divergence is functionally neutral.

Cytochrome p450-dependent lipid metabolism in preovulatory follicles

Estrogen biosynthesis and proteolysis are both important processes involved in ovarian follicular development, which may be influenced by cytochrome P450 (CYP)-dependent fatty acid metabolites. However, CYP-dependent lipid metabolism has not been characterized with respect to follicular maturation in vivo. Therefore, follicular fluid was collected in the hours before and after the LH surge in pigs, and concentrations of epoxy, hydroxy, and dihydroxy lipids were measured by liquid chromatography tandem mass spectrometry. Arachidonate oxidation and epoxyeicosatrienoic acid hydrolysis to dihydroxyeicosatrienoic acids (DHETs) were also assessed in thecal and granulosa tissue fractions, and the expression of CYP epoxygenases was evaluated by immunoblots using available antisera. To evaluate soluble epoxide hydrolase (sEH) expression, the porcine sEH was cloned from ovarian tissue, expressed and purified for antibody generation. The follicular fluid oxylipin concentrations ranged from 1-150 nm depending on the compound and estrous stage. The follicular fluid concentrations of CYP-dependent oxylipins increased at estrus, as did sEH expression; however, significant changes in epoxides were not observed, and the 11,12-DHET peak was delayed. The ratio of 14,15-DHET:11,12-DHET across all samples correlated with the log of follicular fluid estradiol concentrations (P < 0.01). Epoxygenase activities were similar in theca and granulosa, varying little with follicular development, whereas the decline of a single CYP2J isoform at ovulation was observed by immunoblots. The sEH activity was higher in granulosa than in theca. Finally, the dynamic changes in follicular CYP-dependent arachidonic acid metabolites and their modulatory function in vascular models suggest roles for these metabolites in follicular maturation, which may include regulation of estradiol biosynthesis and preovulatory remodeling of the follicular wall that should be fully explored in future studies.

The planetary biology of cytochrome P450 aromatases

Background

Joining a model for the molecular evolution of a protein family to the paleontological and geological records (geobiology), and then to the chemical structures of substrates, products, and protein folds, is emerging as a broad strategy for generating hypotheses concerning function in a post-genomic world. This strategy expands systems biology to a planetary context, necessary for a notion of fitness to underlie (as it must) any discussion of function within a biomolecular system.

Results

Here, we report an example of such an expansion, where tools from planetary biology were used to analyze three genes from the pig Sus scrofa that encode cytochrome P450 aromatases–enzymes that convert androgens into estrogens. The evolutionary history of the vertebrate aromatase gene family was reconstructed. Transition redundant exchange silent substitution metrics were used to interpolate dates for the divergence of family members, the paleontological record was consulted to identify changes in physiology that correlated in time with the change in molecular behavior, and new aromatase sequences from peccary were obtained. Metrics that detect changing function in proteins were then applied, including K_A/K_S values and those that exploit structural biology. These identified specific amino acid replacements that were associated with changing substrate and product specificity during the time of presumed adaptive change. The combined analysis suggests that aromatase paralogs arose in pigs as a result of selection for Suoidea with larger litters than their ancestors, and permitted the Suoidea to survive the global climatic trauma that began in the Eocene.

Conclusions

This combination of bioinformatics analysis, molecular evolution, paleontology, cladistics, global climatology, structural biology, and organic chemistry serves as a paradigm in planetary biology. As the geological, paleontological, and genomic records improve, this approach should become widely useful to make systems biology statements about high-level function for biomolecular systems.

Isolation and characterization of goat ovarian aromatase cDNA: assessment of the activity using an intact cell system and placental expression

Goat ovarian follicles produce estrone and estradiol from androgens. The synthesis of C18 estrogens from C19 androgens requires cytochrome P450 aromatase, but little information about this key enzyme is available in the goat. We report here for the first time the cDNA sequence of the goat ovarian aromatase, the activity of the enzyme in a cell system, and its expression in the term goat placenta. A cDNA library from goat ovarian poly(A)+ RNA was constructed. Human aromatase cDNA was selected as probe to screen the library; several clones were isolated, but none was complete. The longest clone was 3.1 kb long, but it lacked the sequence coding for a few amino acids in the NH(2)-terminal. To obtain the missing sequence, we performed reverse amplification of the cDNA end (RACE). Sequence analysis indicated that goat aromatase possessed a very long 3'-untranslated region ( approximately 1790 bp), and a polyadenylation signal (AATAAA) located at position 3320 downstream from the ATG start codon. The coding region of goat cDNA was inserted in an expression vector and transfected into HEK-293 cells that were cultured in presence of [14C]-androstenedione, steroids extracted and further separated by TLC. The transfected cells efficiently transformed [14C]-androstenedione into estrone. This activity was inhibited by 4-hydroxyandrostenedione. We also investigated the presence of mRNA for P450 aromatase in the goat placenta, using reverse transcription-polymerase chain reaction (RT-PCR) and primers derived from the cDNA ovarian sequence and confirmed the expression of the mRNA in term placenta.

Paralogues of porcine aromatase cytochrome P450: a novel hydroxylase activity is associated with the survival of a duplicated gene

The gonadal and placental paralogues of porcine aromatase cytochrome P450 (P450arom) were examined for novel catalytic properties to shed light on the evolutionary survival of duplicated copies of an enzyme critical to reproduction. Recombinant gonadal P450arom catalyzed the formation of a novel metabolite from testosterone, identified by gas chromatography/mass spectrometry and biochemical analyses as 1 beta-hydroxytestosterone (1 beta OH-T), in almost equal proportion to 17beta-estradiol (E(2)). This activity was absent in reactions with the porcine placental paralogue (or other orthologues) of P450arom and was minimal with androstenedione. Incubations with both porcine enzymes and with bovine and human P450arom demonstrated that 1 beta OH-T was not aromatizable, and 1 beta OH-T activated the androgen receptor of prostate cancer cells in vitro. Porcine testicular and follicular granulosa tissues synthesized 1 beta OH-T, which was also detected in testicular venous plasma. These results constitute the first of identification of a novel, perhaps potent, nonaromatizable metabolite of testosterone, whose synthesis (paradoxically) can be definitively ascribed to the activity of the gonadal paralogue of porcine P450arom. It probably represents an evolutionary gain of function associated with fixation and the survival of the genes after CYP19 duplication. Novel activities and adaptive functions may exist among other duplicated vertebrate aromatases.

Structural and functional differences among purified recombinant mammalian aromatases: glycosylation, N-terminal sequence and kinetic analysis of human, bovine and the porcine placental and gonadal isozymes

Recombinant porcine gonadal and placental, and the human and bovine, isozymes of aromatase cytochrome P450 (P450arom) were over-expressed in insect cells, purified and quantified by difference spectroscopy. Human and bovine P450arom exhibited greater apparent molecular size than either porcine isozyme prompting an examination of N-linked glycosylation and amino-terminal peptide sequence. Comparisons of substrate affinities and turnover were also made. In contrast to human and bovine P450arom which are N-linked glycoproteins, neither isozyme of porcine P450arom is glycosylated, explaining in part their lower molecular size. Differences found in N-terminal peptide sequences were unlikely to influence apparent molecular size or enzyme function. Human and bovine P450arom had similar affinities and turnovers for androstenedione (approximately 200 nM, 3/min) and testosterone (approximately 350 nM, 2/min). The porcine isozymes had 10-fold higher affinities but correspondingly lower turnovers, particularly the gonadal P450arom. Overall, the catalytic efficiency (Vmax/Km) was similar for all but porcine gonadal P450arom which was much lower. These data emphasize the structural and functional variability of even the most conserved of proteins among diverse species wherein such differences have previously remained unexpected.

Biochemical assessment of limits to estrogen synthesis in porcine follicles

Limits to estrogen production by early and late preovulatory porcine follicles were assessed by comparing enzymatic capacities for androgen (17,20-lyase) and estrogen (aromatase) synthesis in theca interna and granulosa, support of enzyme activities by the redox partner proteins NADPH-cytochrome P450 oxidoreductase (reductase) and cytochrome b5, and tissue-specific expression and regulation of these proteins. Parameters included follicular fluid (FF) estradiol and progesterone levels, theca and granulosa aromatase and reductase activities, and theca 17,20-lyase activity. Expression of proteins responsible for these activities, aromatase (P450arom) and 17 alpha-hydroxylase/17,20-lyase (P450c17) cytochromes P450, reductase, and for the first time in ovarian tissues cytochrome b5, were examined by Western immunoblot and immunocytochemistry. Theca and granulosa aromatase activities were as much as 100-fold lower than theca 17,20-lyase activity, but aromatase was correlated with only the log of FF estradiol. Granulosa reductase activity was twice that of the theca, and cytochrome b5 expression was clearly identified in both the theca and granulosa layers, as was P450arom, but was not highly correlated with either 17,20-lyase or aromatase activities. Reductase expression did not change with stage of follicular development, but cytochrome b5, P450c17, and P450arom were markedly lower in post-LH tissues. These data indicate that aromatase and not 17,20-lyase must limit porcine follicular estradiol synthesis, but this limitation is not reflected acutely in FF steroid concentrations. Neither reductase nor cytochrome b5 appear to regulate P450 activities, but the expression of cytochrome b5 in granulosa and theca suggests possible alternative roles for this protein in follicular development or function.

Molecular target of endocrine disruption in human luteinizing granulosa cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin: inhibition of estradiol secretion due to decreased 17alpha-hydroxylase/17,20-lyase cytochrome P450 expression

Estradiol (E2) production by human luteinized granulosa cells (hLGC) is inhibited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The molecular target of TCDD toxicity has not been identified. The decrease in E2 is ameliorated by androgen substrate addition and is not associated with changes in aromatase cytochrome P450 (P450arom) activity or protein expression. An antihuman 17alpha-hydroxylase/17,20-lyase cytochrome P450 (P450c17) antisera and a direct radiometric assay of 17,20-lyase activity were used to test the hypothesis that TCDD targets P450c17, thereby decreasing substrate availability for E2 synthesis by hLGC. P450c17 expression and 17,20-lyase activity were detected in hLGC with high levels of E2 secretion. Western immunoblot analysis demonstrated that TCDD treatment of hLGC decreased the expression of P450c17 by as much 50% (P < 0.05). TCDD exposure induced a 65% decrease in 17,20-lyase activity (P < 0.05), but no changes were seen in P450arom or in nicotinamide adenine dinucleotide phosphate (reduced)-cytochrome P450 oxidoreductase (reductase). Furthermore, the decreases in P450c17 and 17,20-lyase were proportional to the inhibition of E2 secretion. We conclude that the molecular target for endocrine disruption of hLGC by TCDD is P450c17, specifically decreasing the supply of androgens for E2 synthesis, and that it does not involve either P450arom or the redox partner protein reductase.

Regulation of microsomal P450, redox partner proteins, and steroidogenesis in the developing testes of the neonatal pig

Testicular growth and plasma androgen concentrations increase markedly in the first weeks of neonatal life of pigs. The regulation of steroidogenesis through this period was examined by measuring total microsomal cytochromes P450 (P450), 17alpha-hydroxylase/17,20-lyase P450 (P450c17) and aromatase P450 (P450arom) enzyme activities, and the redox partner proteins nicotinamide adenine dinucleotide phosphate, reduced form (NADPH)-cytochrome P450 reductase (reductase) and cytochrome b(5) in testicular microsomes. Testes were collected from 1-24 d of age, and testicular development was suppressed by a GnRH antagonist in some animals from d 1-14. Both 17/20-lyase and aromatase activities increased from d 1-7 but not thereafter, and 17-20-lyase activity was always at least 200-fold higher than aromatase activity. Reductase decreased in wk 1, then increased to d 24. No changes were seen in cytochrome b(5) expression. GnRH antagonist treatment suppressed plasma LH, testosterone and testes growth to d 14. 17,20-Lyase and aromatase activities in testicular microsomes were reduced by 20% and 50%, respectively. Total microsomal P450 concentration was reduced by 50% on d 7, but there was no effect of treatment on reductase or cytochrome b(5) expression. These data support the hypothesis that the rise in neonatal testicular androgen secretion is more likely due to gonadotropin-stimulated gonadal growth, rather than specific P450c17 expression. Neither P450c17 nor P450arom can account for the decline in total microsomal P450. Reductase and cytochrome b(5) expression appears to be constitutive, but reductase levels saturate both P450c17 and P450arom.

Structural determinants of aromatase cytochrome p450 inhibition in substrate recognition site-1

The porcine gonadal form of aromatase cytochrome P450 (P450arom) exhibits higher sensitivity to inhibition by the imidazole, etomidate, than the placental isozyme. The residue(s) responsible for this functional difference was mapped using chimeragenesis and point mutation analysis of the placental isozyme, and the kinetic analysis was conducted on native and mutant enzymes after overexpression in insect cells. The etomidate sensitivity of the placental isozyme was markedly increased by substitution of the predicted substrate recognition site-1 (SRS-1) and essentially reproduced that of the gonadal isozyme by substitution of SRS-1 and the predicted B helix. A single isoleucine (I) to methionine (M) substitution at position 133 of the placental isozyme (I(133)M) was proven to be the critical residue within SRS-1. Residue 133 is located in the B'-C loop and has been shown to be equally important in other steroid-metabolizing P450s. Single point mutations (including residues 110, 114, 120, 128, 137, and combinations thereof among others) and mutation of the entire B and C helixes were without marked effect on etomidate inhibitory sensitivity. The same mutation (I(133)M) introduced into human P450arom also markedly increased etomidate sensitivity. Mutation of Ile(133) to either alanine (I(133)A) or tyrosine (I(133)Y) decreased apparent enzyme activity, but the I(133)A mutant was sensitive to etomidate inhibition, suggesting that it is Ile(133) that decreases etomidate binding rather than Met(133) increasing enzyme sensitivity. Androstenedione turnover and affinity were similar for the I(133)M mutant and the native placental isozyme. These data suggest that Ile(133) is a contact residue in SRS-1 of P450arom, emphasize the functional conservation that exists in SRS-1 of a number of steroid-hydroxylating P450 enzymes, and suggest that substrate and inhibitor binding are dependent on different contact points to varying degrees.

A comparative approach to structure-function studies of mammalian aromatases

To date, structure--function studies of aromatase cytochrome P450 (P450arom) have been advanced by point mutation analyses utilizing almost exclusively the human enzyme, in conjunction with computer-generated models of the three-dimensional form of the enzyme based on prokaryotic cytochromes P450. Recent studies have identified duplicated isozymes of porcine P450arom, the gonadal and placental forms of which appear to differ substantially in substrate utilization and inhibitor sensitivity. We present a comparative approach to define regions of P450arom responsible for specific functional characteristics using complimentary DNAs encoding the porcine isozymes. Constructs encoding the native and chimeric porcine and human P450arom enzymes were transiently expressed and activity was assessed using the tritiated water assay. Sensitivity to inhibition by the imidazole etomidate was investigated, and P450arom expression was assessed by immunoblot analysis. All constructs yielded active P450arom, suggesting that exchanging entire structural elements does not preclude catalytic function. The activity of the gonadal isozyme was shown to be inhibited by etomidate at concentrations 185 and 300-fold lower than those required to induce a similar inhibition of the placental and human enzymes, respectively. In contrast, there was only a two-fold difference in the sensitivity of the gonadal and placental isozymes to inhibition by CGS16949A. Analysis of chimeric constructs indicated that the sensitivity to etomidate was associated with residues in the B, B' and C helices of the gonadal P450arom encompassing only one of six putative substrate recognition sites. Additionally, sensitivity to etomidate was not correlated with enzyme activity among the chimeric enzymes. Therefore, it appears that residues of the porcine gonadal P450arom that are responsible for etomidate binding may be distinct from those involved in substrate recognition and metabolism. These data support the notion that a comparative approach employing the use of chimeric enzymes provides a useful tool in directing point mutational analysis to determine residues important in inhibitor and perhaps substrate recognition of P450 enzymes such as P450arom. These studies are currently in progress.