Complete structure of the human gene for the vitamin D 1alpha-hydroxylase, P450c1alpha

The rate-limiting, hormonally regulated step in the biosynthesis of the biologically active form of vitamin D, 1,25(OH)2D, is its 1alpha-hydroxylation in the kidney by the mitochondrial P450 enzyme, P450c1alpha. We have recently cloned the human P450c1alpha cDNA and shown that this enzyme is the factor disrupted in vitamin D-dependent rickets, type 1 (VDDR-1). To facilitate the analysis of further patients with VDDR-1 and to permit studies of the regulation of the gene for P450c1alpha, we have used PCR-based tactics to clone the gene. Southern blotting studies indicate that there is only one copy of this gene in the human genome. The complete sequence of all exons and introns show that the gene consists of 9 exons spanning only 5 kb; the entire protein-coding region can be PCR-amplified as a single 4-kb fragment. The transcriptional start site, located by primer extension and S1 nuclease protection, lies 62-bp upstream from the ATG transitional start codon. Analysis of rodent/human somatic cell hybrid DNAs show that this gene lies on chromosome 12. Although the gene is substantially smaller than the human genes for other mitochondrial enzymes, its intron/exon organization is very similar, especially to that of P450scc. This indicates that although the mitochondrial P450 enzymes retain only 30%-40% amino acid sequence identity, they all belong to a single evolutionary lineage.

Cloning of human 25-hydroxyvitamin D-1 alpha-hydroxylase and mutations causing vitamin D-dependent rickets type 1

The secosteroid hormone, 1,25-dihydroxyvitamin D [1,25(OH)2D], plays a crucial role in normal bone growth, calcium metabolism, and tissue differentiation. The key step in the biosynthesis of 1,25(OH)2D is its 1 alpha-hydroxylation from 25-hydroxyvitamin D (25-OHD) in the kidney. Because its expression in the kidney is very low, we cloned and sequenced cDNA for 25-OHD-1 alpha-hydroxylase (P450c1 alpha) from human keratinocytes, in which 1 alpha-hydroxylase activity and mRNA expression can be induced to be much greater. P450c1 alpha mRNA was expressed at much lower levels in human kidney, brain, and testis. Mammalian cells transfected with the cloned P450c1 alpha cDNA exhibit robust 1 alpha-hydroxylase activity. The identity of the 1,25(OH)2D3 product synthesized in transfected cells was confirmed by HPLC and gas chromatography-mass spectrometry. The gene encoding P450c1 alpha was localized to chromosome 12, where the 1 alpha-hydroxylase deficiency syndrome, vitamin D-dependent rickets type 1 (VDDR-1), has been localized. Primary cultures of human adult and neonatal keratinocytes exhibit abundant 1 alpha-hydroxylase activity, whereas those from a patient with VDDR-1 lacked detectable activity. Keratinocyte P450c1 alpha cDNA from the patient with VDDR-1 contained deletion/frameshift mutations either at codon 211 or at codon 231, indicating that the patient was a compound heterozygote for two null mutations. These findings establish the molecular genetic basis of VDDR-1, establish a novel means for its study in keratinocytes, and provide the sequence of the key enzyme in the biological activation of vitamin D.

Long-term outcome in children and adolescents after transsphenoidal surgery for Cushing's disease

Cushing's disease refers specifically to an ACTH-producing pituitary adenoma that stimulates excess cortisol production. Transsphenoidal surgery is the treatment of choice in children and adolescents, but disparate cure rates have been reported, ranging from 50-98%. The discrepancies in cure rate are due primarily to the technical success of the surgery and the length and method of follow-up. We studied 42 consecutive children and adolescents (age, < or = 18 yr) who underwent transsphenoidal exploration for the primary treatment of Cushing's disease at University of California-San Francisco from 1974-1993. Only 7 patients had persistent disease, defined as evidence of Cushing's disease within 6 months of surgery, yielding an initial remission rate of 83%. We comprehensively evaluated 26 of the 35 patients who experienced an initial remission, including testing of the ACTH-adrenocortical axis. The mean duration of follow-up is 7.2 yr (range, 1.5-13.6 yr). Seven experienced a relapse of Cushing's disease, yielding a net remission rate of 73%. Relapses occurred an average of 4.2 yr postoperatively (range, 0.75-6.2 yr). Five patients experienced relapse within 5 yr of surgery, whereas 2 relapsed more than 5 yr postoperatively. Repeat transsphenoidal surgery was performed in 8 patients with persistent or recurrent disease, and 6 of these remain in remission. Low serum or urinary cortisol measurements within the first post-operative week predicted remission of Cushing's disease, but were not necessarily predictive of long-term cure. Hypercortisolism had significant effects on bone metabolism, as reflected by both diminished bone density in the majority of patients examined and decreased growth rate. Both parameters improved after surgical care, although they did not fully normalize. We conclude that transsphenoidal surgery is a safe and effective treatment for pediatric Cushing's disease, but long-term surveillance is necessary to detect possible recurrences.

Evaluating family practice residencies: a new method for qualitative assessment

BACKGROUND AND OBJECTIVES

This study reports on a novel qualitative method for evaluating family practice training programs. Previous evaluation techniques have generally been quantitative in nature and have limited their scope to a few isolated elements of residency education.

METHODS

A guest faculty, working in conjunction with local faculty, conducted a site analysis of an East Coast and a West Coast family practice residency. Multiple qualitative techniques were used, including participant observation, focus groups, long interviews, and analysis of key texts. Program strengths and weaknesses were analyzed, and a discrepancy model was used to compare program goals and ideals to the actual training realities. The analysis used a process of immersion/crystallization, and triangulation of the multiple data sources was achieved through repeated comparisons.

RESULTS

This report focuses on the process of the evaluations, rather than on their content. In general, the sites have achieved most of their objectives, but notable limitations are present at both programs. This is particularly apparent in terms of multiple demands on faculty, the lack of a shared vision, and program isolation.

CONCLUSIONS

Significant lessons were learned from these initial assessments, which can be used to further refine the method. Comprehensive qualitative reviews may provide unexpected insights and identify program limitations and strengths.

The genetic and functional basis of isolated 17,20-lyase deficiency

Human male sexual differentiation requires production of fetal testicular testosterone, whose biosynthesis requires steroid 17,20-lyase activity. Patients with putative isolated 17,20-lyase deficiency have been reported. The existence of true isolated 17,20-lyase deficiency, however, has been questioned because 17 alpha-hydroxylase and 17,20-lyase activities are catalyzed by a single enzyme, microsomal cytochrome P450c17, and because the index case of apparent isolated 17,20-lyase deficiency had combined deficiencies of both activities. We studied two patients with clinical and hormonal findings suggestive of isolated 17,20-lyase deficiency. We found two patients homozygous for substitution mutations in CYP17, the gene encoding P450c17. When expressed in COS-1 cells, the mutants retained 17 alpha-hydroxylase activity but had minimal 17,20-lyase activity. Substrate competition experiments suggested that the mutations did not alter the enzyme's substrate-binding capacity, but co-transfection of cells with P450 oxidoreductase, the electron donor used by P450c17, indicated that the mutants had a diminished ability to interact with redox partners. Computer-graphic modelling of P450c17 suggests that both mutations lie in or near the redox-partner binding site, on the opposite side of the haem from the substrate-binding pocket. These mutations alter electrostatic charge distribution in the redox-partner binding site, so that electron transfer for the 17,20-lyase reaction is selectively lost or diverted to uncoupling reactions. These are the first proven cases of isolated 17,20-lyase deficiency, and they demonstrate a novel mechanism for loss of enzymatic activity.

Tenascin-X deficiency is associated with Ehlers-Danlos syndrome

The tenascins are a family of large extracellular matrix proteins with at least three members: tenascin-X (TNX), tenascin-C (TNC, or cytotactin) and tenascin-R (TN-R, or restrictin). Although the tenascins have been implicated in a number of important cellular processes, no function has been clearly established for any tenascin. We describe a new contiguous-gene syndrome, involving the CYP21B and TNX genes, that results in 21-hydroxylase deficiency and a connective-tissue disorder consisting of skin and joint hyperextensibility, vascular fragility and poor wound healing. The connective tissue findings are typical of the Ehlers-Danlos syndrome (EDS). The abundant expression of TNX in connective tissues is consistent with a role in EDS, and our patient's skin fibroblasts do not synthesize TNX protein in vitro or in vivo. His paternal allele carries a novel deletion arising from recombination between TNX and its partial duplicate gene, XA, which precludes TNX synthesis. Absence of TNX mRNA and protein in the proband, mapping of the TNX gene and HLA typing of this family suggest recessive inheritance of TNX deficiency and connective-tissue disease. Although the precise role of TNX in the pathogenesis of EDS is uncertain, this patient's findings suggest a unique and essential role for TNX in connective-tissue structure and function.

Two proximal activating protein-1-binding sites are sufficient to stimulate transcription of the ovine follicle-stimulating hormone-beta gene

FSH is an important regulator of mammalian gametogenesis and the female reproductive cycle. Although little is known about the transcriptional regulation of the beta-subunit (the rate-limiting subunit of FSH synthesis), sequence analysis of the ovine FSHbeta promoter has revealed a number of potential activating protein-1 (AP-1; Jun/Fos)-binding sites. To determine whether the gene encoding the beta-subunit of ovine FSH (oFSHbeta) is responsive to AP-1 transcriptional complexes, chimeric constructs containing deleted portions of the oFSHbeta promoter fused to a luciferase reporter were transiently transfected along with c-Jun and c-Fos expression constructs into JAR cells. Analysis of these deletion constructs revealed that the proximal promoter of oFSHbeta is highly stimulated by c-Jun and c-Fos proteins (typically 20-fold with a reporter construct containing oFSHbeta sequences from -215 to +759 bp). This stimulation was lost when a similar construct containing sequences from -84 to +759 bp was tested. Transcriptional start site analysis using reverse transcription-PCR verified that the transcriptional initiation of the -215-bp deletion construct, with or without cotransfected c-Jun/c-Fos, was the same as that observed in vivo. Computer analysis of oFSHbeta sequences from -215 to +1 bp identified four putative AP-1-like elements, located at -155, -120, -83, and -10 bp. Gel retardation experiments using oligonucleotides corresponding to the four putative AP-1-like sites revealed that only -120 and -83 sites in oFSHbeta bound AP-1 proteins in vitro. Site-directed mutagenesis of the -120 and -83 sites showed that each element was required for stimulation by c-Jun and c-Fos proteins as well as 12-O-tetradecanoyl phorbol-13-acetate in transient transfection assays. Finally, immunocytochemical dual labeling was used to show that more than 75% of all FSHbeta-containing cells in ovine pituitary sections from cycling ewes contained nuclear c-Jun, JunB, JunD, and Fos proteins. These data, taken together, show that oFSHbeta transcription can be stimulated by c-Jun and c-Fos proteins via two functionally linked AP-1-like sites in the oFSHbeta proximal promoter and that these sites are likely to be important regulators of FSH production in vivo.

Spontaneous feminization in a 46,XX female patient with congenital lipoid adrenal hyperplasia due to a homozygous frameshift mutation in the steroidogenic acute regulatory protein

The most severe form of congenital adrenal hyperplasia (CAH) is lipoid CAH. It was once thought that this disease was due to mutations in the cholesterol side-chain cleavage enzyme system, thus eliminating the ability to convert cholesterol to pregnenolone, causing a complete absence of steroid hormone production. We recently showed that lipoid CAH is due to mutations in the steroidogenic acute regulatory (StAR) protein, thus preventing acutely stimulated adrenal and gonadal responses to tropic stimulation. However, this lesion may permit low levels of StAR-independent steroidogenesis to persist until the accumulation of intracellular lipid deposits destroys steroidogenic capacity. This model would predict that the steroidogenic cells of the ovaries of affected 46,XX females should remain undamaged until puberty, at which time low levels of StAR-independent estrogen biosynthesis should be detectable. We describe a 15.5-yr-old 46,XX female with a classic history of lipoid CAH who underwent spontaneous feminization and cyclical vaginal bleeding beginning at age 13. Genetic analysis of the patient and her parents showed that she was homozygous for the novel StAR frameshift mutation 261delT. This is the first adolescent female with lipoid CAH who has undergone spontaneous feminization and who has been analyzed genetically. Finding an inactive StAR gene in this patient confirms our two-hit model of the pathogenesis of lipoid CAH, in which loss of StAR activity initially preserves StAR-independent steroidogenesis, which is lost only after cells undergo chronic tropic stimulation and subsequent damage from accumulation of cholesterol esters.

Differential effects of inhibin on gonadotropin stores and gonadotropin-releasing hormone binding to pituitary cells from cycling female rats

Numerous studies of rat pituitaries have reported that inhibin suppresses the synthesis and release of FSH and decreases the release of LH. The latter effect seems to be related to the down-regulation of receptors for GnRH. The studies reported here identified cellular changes behind the inhibitory effects of inhibin on gonadotropes to learn whether its effects are mediated by changes in subtypes of gonadotropes. Cell populations from diestrous day 2 and proestrous (morning) rats were collected, dispersed to single cell populations, and plated in medium containing either recombinant 32-kDa inhibin or porcine follicular fluid for 24 h. GnRH binding was detected by exposing the cells to a biotinylated analog (Bio-GnRH) for 10 min before fixation, followed by avidin-peroxidase labeling protocols to detect the biotin on the analog. In parallel fields, the cells were further identified by immunolabeling for LH or FSH beta-subunits or for GH with a different colored reaction product. The most striking changes were seen in cells from proestrous rats. Inhibin reduced the percentages of Bio-GnRH target cells in the population by 60% and the area and density of Bio-GnRH label on the remaining cells. Inhibin reduced the percentages of FSH cells by 30% and caused nearly a 60% reduction in the binding of Bio-GnRH by this cell type (from 83% of FSH cells to 32% of FSH cells). Inhibin also reduced the area of FSH cells and the density of FSH stores. Inhibin's effects on LH cells were limited to a reduction in the area of the cells and the density of LH stores, but not the number of LH cells. In addition, it reduced the percentages of LH cells with Bio-GnRH receptors from 84% to 40%. When cells with GH were analyzed, inhibin had no effect on their percentages, areas, or GH stores. In populations from proestrous rats, inhibin reduced the percentages of GH cells with Bio-GnRH binding from 38% to 21%. These data suggest that inhibin's target cell is the abundant multihormonal gonadotrope that contains LH, FSH, and GH and predominates during proestrus. Inhibin's effects are most severe on FSH cells, which suggests that it may either selectively affect FSH synthesis and stores in bihormonal gonadotropes and/or affect monohormonal FSH cells. Thus, mechanisms behind its inhibitory effects include 1) a reduction in the percentage of Bio-GnRH target cells, 2) a reduction in the area of Bio-GnRH-binding sites on individual cells, and 3) a reduction in the stores of FSH and the percentages of FSH cells. These last effects are consistent with known reductions in FSH synthesis. The effects of inhibin on LH secretion may be secondary to the effects on Bio-GnRH receptors in bihormonal gonadotropes.

Pathophysiology, genetics, and treatment of hyperandrogenism

Presenting symptoms of hirsutism and virilism often signal a disorder of androgen biosynthesis, especially one of the forms of adrenal hyperplasia. The genetics and physiology of the various disorders are reviewed, emphasizing those that results in increased adrenal androgen production. All of these disorders can be diagnosed genetically, permitting family counseling, and all can be treated successfully with appropriate hormonal replacement therapy. Premature adrenarche is not caused by an enzymatic disorder; its origins remain obscure but may be an early harbinger of the polycystic ovary syndrome.

Transcription of the human genes for cytochrome P450scc and P450c17 is regulated differently in human adrenal NCI-H295 cells than in mouse adrenal Y1 cells

Human NCI-H295 cells, which express all of the genes for the steroidogenic enzymes in a hormonally regulated fashion, should be an ideal system in which to study the transcriptional regulation of these genes. Using deletional promoter/reporter constructions for the human P450scc and P450c17 genes, we identified the regions conferring basal and cAMP-induced transcription of these two genes in NCI-H295 human adrenal cells. In the P450scc gene, both basal and cAMP-induced transcriptional activation elements lie within the first 79 bp upstream (-79) from the transcriptional start site. In the P450c17 promoter, both basal and cAMP-responsive elements lie within the first upstream 63 bp, and a second basal element lies between -184 and -206 bp. The locations of these elements are substantially different from the locations of elements that appear to be functionally equivalent when these human gene promoters are transfected into mouse adrenal Y1, mouse testicular MA-10, or human choriocarcinoma JEG-3 cells. These data indicate that the transcriptional regulation of these genes in their native species and cell type differs substantially from their regulation in cells from other species and tissues, and suggests that the results from transfection experiments examining genes for steroidogenic enzymes in heterologous cells may not reflect events in vivo.

The regulation of 17,20 lyase activity

P450c17 is a single microsomal enzyme that catalyzes two distinct steroid biosynthetic activities: 17 alpha-hydroxylase and 17,20 lyase. Human beings have only one gene that encodes only one form of P450c17. Three clinical observations indicated that these were independently regulated activities. First, several cases of isolated 17,20 lyase deficiency were reported, in which 17 alpha-hydroxylase activity was spared. Second, most adrenal steroidogenesis in children stops after 17 alpha-hydroxylation, thus permitting the synthesis of cortisol, whereas most gonadal steroidogenesis proceeds to C19 sex steroids as a result of both activities. Third, the 17,20 lyase activity of the human adrenal is developmentally activated during adrenarche. To catalyze these two activities, P450c17 must receive reducing equivalents from electron donors (redox partners). Previous observations showed that the molar ratio of P450 oxidoreductase to P450c17 was 3-fold higher in the testis than in the adrenal, and that increasing the molar ratio of the redox partner to P450c17 would increase the ratio of 17,20 lyase activity to 17 alpha-hydroxylase. We have recently shown that P450c17 must be phosphorylated on serine and threonine residues by a cAMP-dependent protein kinase to acquire 17,20 lyase activity. We have also recently found two cases of isolated 17,20 lyase deficiency that have mutations of residues in the proposed redox partner binding site. Together, these studies suggest a unified view of the regulation of 17,20 lyase activity. The ratio of 17,20 lyase to 17 alpha-hydroxylase activity of P450c17 is regulated by the availability of reducing equivalents flowing to the enzyme. This can be increased by increasing the molar concentration of electron-donating redox partners, such as P450 oxidoreductase or possibly cytochrome b5, as appears to be the case in the gonads. Alternatively, the affinity of P450c17 for redox partners may be selectively increased by Ser/Thr phosphorylation, or selectively decreased by certain mutations in the redox partner binding site, in either case altering an electrostatic interaction between P450c17 and the redox partner. This model is consistent with all present observations about the biochemistry, genetics, enzymology, and clinical phenomenology of P450c17.

The pathophysiology and genetics of congenital lipoid adrenal hyperplasia

BACKGROUND

Congenital lipoid adrenal hyperplasia results in severe impairment of steroid biosynthesis in the adrenal glands and gonads that is manifested both in utero and postnatally. We recently found mutations in the gene for the steroidogenic acute regulatory protein in four patients with this syndrome, but it was not clear whether all patients have such mutations or why there is substantial clinical variation in these patients.

METHODS

We directly sequenced the gene for steroidogenic acute regulatory protein in 15 patients with congenital lipoid adrenal hyperplasia from 10 countries. Identified mutations were confirmed and recreated in expression vectors, transfected into cultured cells, and assayed for the presence and activity of steroidogenic acute regulatory protein.

RESULTS

Fifteen different mutations in the gene for steroidogenic acute regulatory protein were found in 14 patients; the mutation Gln258Stop was found in 80 percent of affected alleles from Japanese and Korean patients, and the mutation Arg182Leu was found in 78 percent of affected alleles from Palestinian patients. We developed diagnostic tests for these and eight other mutations. Thirteen of the 15 mutations were in exons 5, 6, or 7, and all rendered the steroidogenic acute regulatory protein inactive in functional assays. Some mutants with amino acid replacements were capable of normal mitochondrial processing, indicating that the activity of steroidogenic acute regulatory protein is not associated with its translocation into mitochondria. Steroidogenic cells lacking the protein retained low levels of steroidogenesis. This explains the secretion of some steroid hormones by the ovaries after puberty before affected cells accumulate large amounts of cholesterol esters.

CONCLUSIONS

The congenital lipoid adrenal hyperplasia phenotype is the result of two separate events, an initial genetic loss of steroidogenesis that is dependent on steroidogenic acute regulatory protein and a subsequent loss of steroidogenesis that is independent of the protein due to cellular damage from accumulated cholesterol esters.

Genetic variation in P450c11AS in Chilean patients with low renin hypertension

Low renin hypertension (LRH), which accounts for 10-20% of patients with idiopathic "essential" hypertension, bears hormonal similarities to mineralocorticoid-induced hypertension, but elevated mineralocorticoid concentrations have not been found. Some patients with LRH have normal, rather than suppressed, plasma aldosterone concentrations, so that the ratio of aldosterone concentration to PRA (Aldo/PRA) is high, suggesting inappropriately increased aldosterone biosynthesis. We characterized the CYP11B2 gene that encodes the aldosterone synthase, P450c11AS, in hypertensive and control populations in a single clinic in Santiago, Chile. We directly sequenced the entire CYP11B2 gene in 12 patients with LRH, 2 high renin hypertensive controls, and 2 normotensive controls. All sequences were identical, except that 8 of 24 LRH alleles encoded arginine rather than lysine at position 173. The Arg173 and Lys173 variants were expressed in transfected MA-10 cells, and their ability to convert deoxycorticosterone to aldosterone was measured; the apparent Michaelis constant (Km) for Lys173 was 2.73 mumol/L; the Km for Arg173 was 2.53 mumol/L. The apparent maximal velocity (Vmax) for Lys173 was 6.5 x 10(-3) micrograms/mL.24 h; the Vmax for Arg173 was 7.8 x 10(-3) micrograms/mL.24 h. The first order rate constant, Vmax/Km was 2.38 for Lys173 and 3.08 for Arg173. As these values were not significantly different, we sought to determine whether Arg173 is a polymorphism linked to LRH. We examined position 173 in 52 unselected patients with idiopathic hypertension and 55 normotensive controls by PCR amplification of CYP11B2 exons 3-5 followed by digestion with Bsu361, which digests the Arg173 sequence, but not the Lys173 sequence. More of the hypertensive alleles (39 of 104, 37.5%) than normotensive alleles (25 of 110, 22.5%) carried Arg173 (chi 2 = 5.57; P < 0.02). Most of the Arg173 alleles (31 of 72, 43.1%) were from hypertensive patients with Aldo/PRA below 30, whereas only 5 of 24 (20.8%) Arg173 alleles were found in patients with Aldo/PRA greater than 30 (chi 2 = 3.79; P = 0.05) Thus, the ARg173 variant of CYP11B2 may be linked to LRH in Chilean patients.

Steroidogenic acute regulatory protein (StAR) retains activity in the absence of its mitochondrial import sequence: implications for the mechanism of StAR action

Steroidogenic acute regulatory protein (StAR) plays a critical role in steroid hormone biosynthesis, presumably by facilitating the delivery of cholesterol to P450scc in the inner mitochondrial membranes. StAR is synthesized as a 37-kDa preprotein that is processed to a 30-kDa mature form by cleavage of an N-terminal mitochondrial import sequence. To identify structural features required for StAR biological activity, we mutated the human StAR cDNA, including the deletion of N- and C-terminal sequences, and examined the ability of the mutants to promote steroidogenesis and enter the mitochondria of transfected COS-1 cells. Deletion of up to 62 residues from the N terminus (N-62) did not significantly affect steroidogenesis-enhancing activity. The N-terminal deletion mutants were associated with mitochondria-enriched fractions, but import and processing were progressively impaired with increasing length of the deletion. Immunogold electron microscopy and in vitro import assays showed that the active N-62 mutant was not imported into the mitochondria. Removal of the 28 C-terminal amino acids (C-28) inactivated StAR. Deletion of the C-terminal 10 amino acids (C-10) reduced steroidogenic activity by 53%, while truncation of the last 4 amino acids had no effect. The C-28 mutant StAR was not efficiently imported into mitochondria or processed, whereas some of the C-10 mutant was processed, indicating that import had occurred. We conclude that in the COS-1 cell system used, StAR does not need to enter into mitochondria to stimulate steroidogenesis and that residues in the C terminus are essential for steroidogenesis-enhancing activity. These findings imply that StAR acts via C-terminal domains on the outside of the mitochondria.

Construction of a P450c27 fusion enzyme: a useful tool for analysis of vitamin D3 25-hydroxylase activity

Liver mitochondrial P450c27, encoded by the CYP27 gene, can catalyse the 25-hydroxylation of vitamin D3 and the 27-hydroxylation of sterols. To facilitate the study of this enzyme in cell culture systems, we engineered a fusion protein consisting of P450c27 coupled to its electron-transport accessory proteins, ferredoxin and ferredoxin reductase, and assessed its enzyme activity by measuring the C-25 and C-27 (side-chain) hydroxylation of 1 alpha-hydroxyvitamin D3 (1 alpha-OH-D3). When incubated with 1 alpha-OH-D3, COS-1 cells transfected with a vector expressing the fusion protein produced 1 alpha,25-(OH)2D2 and 1 alpha,27-(OH)2D3 about four times more efficiently than did cells transfected with three individual components of the fusion. When incubated with the natural substrate, vitamin D3, the efficiency of hydroxylation was lower than that for 1 alpha-OH-D3 but still 1.7-fold higher for the fusion protein than for its individual components. The fusion protein was also able to reproduce qualitatively and quantitatively the activity shown by P450c27 in liver cells in situ. The P450c27-ferredoxin reductase-ferredoxin fusion construct represents a valuable tool for establishing the substrate specificity of this liver cytochrome and for evaluating its potential for activating pro-drug analogues of vitamin D.

Inhibin and estradiol alter gonadotropes differentially in ovine pituitary cultures: changing gonadotrope numbers and calcium responses to gonadotropin-releasing hormone

Both inhibin (IN) and estradiol (E) use separate and distinct mechanisms to decrease the production of FSH 60%-80% while increasing receptors for GnRH (GnRH-Rec) 400%-600% in ovine pituitary cultures. To investigate how these hormones act to create outcomes that appear similar, individual cells in ovine pituitary cultures were analyzed for changes in GnRH-stimulated calcium signaling, GnRH binding, and gonadotropin content under IN or E treatments. Calcium mobilization studies showed that 10 nM GnRH increased intracellular calcium ([Ca++]i) in 9.2% +/- 0.8% of cells in control ovine pituitary cultures. After treatment with 10 nM E for 48 h, there was a small increase in the number of cells responding to GnRH (12.9% +/- 1.4% responded) and a major 5-fold increase in [Ca++]i response to GnRH as compared with untreated cells. By contrast, IN did not alter cellular calcium responses to GnRH but markedly increased the number of cells responding to GnRH (a 3.7-fold increase to 33.8 +/- 2.6%). Cytochemical studies measuring GnRH:GnRH-Rec complexes, FSH, and LH showed that E had no effect on the number of pituitary cells containing FSH, LH, or GnRH-Rec. In contrast, similar studies using IN showed that the number of cells containing GnRH-Rec and LH significantly increased (> 50%), whereas the number of FSH cells decreased (36%), and a lower than normal percentage of the remaining FSH cells carried GnRH-Rec. Thus, the net effect of IN on gonadotropes was to dramatically decrease the ratio of FSH:LH cells that contained GnRH-Rec, from 1:1 (untreated) to 1:8 (IN-treated) and to decrease the percentage of LH/FSH colocalization. In summary, these data indicate that E primarily operates on a fixed, preset number of ovine gonadotropes to inhibit FSH production and increase responsiveness to GnRH. IN, however, seems to change the very nature of ovine gonadotropes by completely turning off FSH synthesis in some cells, lowering GnRH-Rec in other FSH cells, and, finally, inducing LH and GnRH-Rec in newly recruited gonadotropes.

Alternate promoters and alternate splicing of human tenascin-X, a gene with 5' and 3' ends buried in other genes

Tenascin-X (TN-X) is an extracellular matrix protein encoded by a large gene that overlaps the steroid 21-hydroxylase (P450c21) gene in the HLA locus on chromosome 6p21.3. This may be the most complex locus in the human genome identified to date, containing 13 overlapping transcription units in 160 kb of DNA. Previous studies determined the sequence of 39 TN-X exons, encoding a 12 kb open reading frame, but the promoter(s) of the gene had not been located. We identify the principal TN-X promoter and a previously unknown 5' untranslated exon that lies more than 10 kb upstream from the previously known exons. This promoter, which is substantially different from the promoter for TN-C, initiates transcription in human fetal adrenal and muscle, but expression in human NCI-H295 adrenocortical carcinoma cells is initiated by two other promoters lying further upstream. One of these is the same as the promoter for a recently identified Creb-related protein (Creb-rp), but transcripts initiated form this promoter in human adrenal NCI-H295 tumor cells are spliced differently from Creb-rp, and are largely retained in the nuclei of these cells. By analogy with the other two members of the tenascin family, TN-C and TN-R, it has been predicted that TN-X should undergo alternate splicing in its fibronectin-like domains. RACE cloning and RNase protection experiments reveal no such alternate splicing. The TN-X gene appears to be unique in having both its 5' and 3' ends buried in other genes.

C-type natriuretic peptide-mediated coronary vasodilation: role of the coronary nitric oxide and particulate guanylate cyclase systems

OBJECTIVES

We tested the hypothesis that C-type natriuretic peptide (CNP) mediates coronary vasodilation through activation of cyclic guanosine monophosphate (cGMP) by way of particulate guanylate cyclase.

BACKGROUND

CNP has known peripheral vasodilator properties, and preliminary data have suggested that it can function as a coronary vasodilator.

METHODS

The actions of CNP were studied in instrumented dogs and in organ chamber rings in the presence and absence of a known antagonist to particulate guanylate cyclase, HS-142-1. Additionally, the actions of HS-142-1 were tested on acetylcholine-mediated coronary vasodilation, and immunohistochemical staining was utilized to localize the presence of CNP in the coronary endothelium.

RESULTS

CNP relaxed isolated coronary arteries with (mean +/- SEM 45.9 +/- 7%*) and without (72.0 +/- 7%*) an endothelium (*p < 0.05 for CNP effect alone, p < 0.05 for endothelium vs. no endothelium with CNP). Intracoronary infusions increased coronary blood flow (baseline, 64.6 +/- 5.1 ml/min; CNP-5, 79.9 +/- 6.1*; CNP-20, 103.3 +/- 13.6* [*p < 0.05 vs. baseline value]) and reduced coronary vascular resistance (baseline, 1.6 +/- 0.3 mm Hg/ml per min; CNP-5, 1.4 +/- 0.3*; CNP-20, 1.2 +/- 0.3*). Intracoronary injections increased coronary blood flow (delta baseline coronary flow, 30 +/- 9* ml/min [*p < 0.05]). HS-142-1 significantly attenuated these increases (delta coronary flow, 30 +/- 9* ml/min [CNP] to 14 +/- 6 [CNP + HS-142-1] [p < 0.05 CNP vs. CNP + HS-142-1]) and the relaxation of organ chamber rings (56 +/- 7% [CNP] to 18 +/- 6% [HS-142-1 + CNP]). Finally, CNP was localized to the coronary endothelium and smooth muscle by immunohistochemical staining.

CONCLUSIONS

CNP functions as a coronary vasodilator through activation of cGMP by way of particulate guanylate cyclase. CNP-mediated coronary vasodilation is attenuated by intracoronary HS-142-1. Intracoronary HS-142-1 does not affect acetylcholine-mediated coronary vasodilation. These observations support a role for exogenous CNP as a potent coronary vasodilator.