I. Introduction II. Nuclear Receptors: General Concepts A. Anatomy of nuclear receptors B. Mechanisms of action III. Orphan Nuclear Receptors A. Definition B. Nomenclature C. Structural and functional diversity IV. Novel Hormone Response Systems: RXR and Its Heterodimeric Partners A. RXR: rexinoids B. PPAR: multiple ligands, multiple functions C. PXR: pregnanes, xenobiotic compounds, and benzoate derivatives D. CAR (constitutive androstane receptor): androstanes and phenobarbital E. LXR: control of cholesterol metabolism by oxysterols F. FXR: bile acids receptor V. Orphans in Search of a Home A. HNF4: diabetes and possible regulation by acyl-coenzyme A (CoA) thioesters B. FTZ-F1: steroidogenesis and sexual development C. Rev-Erb: singular members of the superfamily D. ROR: neuron development and T cell selection E. TR2: the testis receptors F. TLX: forebrain development G. COUP-TF: neurogenesis, angiogenesis, and heart development H. ERR: placenta development and control of lipid metabolism I. NGFI-B: hyp...

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Orphan nuclear receptors: from gene to function.

I. Introduction II. The Initial Identification of SF-1 as a Key Determinant of Steroid Hormone Biosynthesis A. Overview of steroidogenesis B. SF-1 and the regulation of steroidogenesis C. Cloning and structural characterization of SF-1 1. Structural features of SF-1 2. Multiple transcripts are encoded by the gene encoding SF-1 3. The gene encoding SF-1 is evolutionarily conserved in vertebrates and invertebrates III. Characterization of Sites of SF-1 Expression and Identification of Its Target Genes A. Profiles of SF-1 expression 1. Adult steroidogenic tissues 2. Embryonic steroidogenic tissues 3. Other sites of SF-1 expression B. Target genes regulated by SF-1 1. Steroidogenic cells 2. Sertoli cells 3. Gonadotropes 4. VMH IV. The Roles Of SF-1 in Vivo: Targeted Gene Disruption to Create SF-1 Knockout Mice A. General features of the SF-1 knockout mice B. Primary steroidogenic tissues in SF-1 knockout mice C. Pituitary and hypothalamic defects in SF-1 knockout mice V. Perspectives and Future Directions A. ...

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Steroidogenic Factor 1: A Key Determinant of Endocrine Development and Function

The orphan nuclear receptor steroidogenic factor 1 (SF-1, also called Ad4BP and officially designated NR5A1) has emerged as an essential regulator of endocrine development and function. Initially identified as a tissue-specific transcriptional regulator of the cytochrome P450 steroid hydroxylases, SF-1 has considerably broader roles, as evidenced from studies in knockout mice lacking SF-1. The SF-1-knockout mice lacked adrenal glands and gonads and therefore died from adrenal insufficiency within the first week after birth. In addition, SF-1 knockout mice exhibited male-to-female sex reversal of their internal and external genitalia, impaired expression of multiple markers of pituitary gonadotropes, and agenesis of the ventromedial hypothalamic nucleus (VMH). These studies delineated essential roles of SF-I in regulating endocrine differentiation and function at multiple levels, particularly with respect to reproduction. This chapter will review the experiments that established SF-1 as a pivotal, global determinant of endocrine differentiation and function. We next discuss recent insights into the mechanisms controlling the expression and function of SF-1 as well as the current status of research aimed at delineating its roles in specific tissues. Finally, we highlight areas where additional studies are needed to expand our understanding of SF-1 action.

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Steroidogenic factor 1: an essential mediator of endocrine development.

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system.

Since its discovery in the early 1990s, the orphan nuclear receptor SF-1 has been attributed a central role in the development and differentiation of steroidogenic tissues. SF-1 controls the expression of all the steroidogenic enzymes and cholesterol transporters required for steroidogenesis as well as the expression of steroidogenesis-stimulating hormones and their cognate receptors. SF-1 is also an essential regulator of genes involved in the sex determination cascade. The study of SF-1 null mice and of human mutants has been of great value to demonstrate the essential role of this factor in vivo, although the complete adrenal and gonadal agenesis in knock-out animals has impeded studies of its function as a transcriptional regulator. In particular, the role of SF-1 in the hormonal responsiveness of steroidogenic genes promoters is still a subject of debate. This extensive review takes into account recent data obtained from SF-1 haploinsufficient mice, pituitary-specific knock-outs and from transgenic mice experiments carried out with SF-1 target gene promoters. It also summarizes the pros and cons regarding the presumed role of SF-1 in cAMP signalling.

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SF-1 a key player in the development and differentiation of steroidogenic tissues.

Analysis was made on the genomic structure, functions, and expression of the mouse ELP gene, which codes for the embryonal long terminal repeat binding protein. Extensive screening of the cDNA library of embryonal carcinoma cells (EC cells) identified four isoforms of ELP: ELP1 (the original ELP isolate), ELP2, ELP3, and Ad4BP/SF1. Analysis of the genomic sequences revealed that these ELP isoforms were generated by alternative promoter usage and differential splicing. The mRNAs of isoforms initiated at four transcription start sites distributed on three exons. Sequence analysis of the four isoforms identified three polypeptides. The N-terminal portion of ELP1 and ELP2 was longer than ELP3, and Ad4BP/SF1 by 77 aa. The DNA-binding domain and region II were shared by all four isoforms. The C-terminal portion shared by ELP2, ELP3, and Ad4BP/SF1 was 131 aa in length, and that specific to ELP1 was 57 aa in length. The ELP3 and Ad4BP/SF1 isoforms were identical for the coding sequence, but the two differed at the 5' noncoding region. Region II and III domains of nuclear receptors were thought to be involved in ligand-binding and transcriptional activation. ELP1, which lacked region III, functioned as a repressor. The isoforms carrying intact region II and region III functioned as transactivators. Expression of the four isoforms was studied in mouse tissues and in tissue culture cells by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Complex patterns of expression of these isoforms were observed in various tissues. All four ELP isoforms were expressed only in EC cells.

Genomic organization and isoforms of the mouse ELP gene.

The mechanism of repression of transcription by ELP, the embryonal long terminal repeat binding protein, was investigated. ELP represses the Moloney murine leukemia virus long terminal repeat by binding to a site which overlaps with a sequence element for retinoic acid receptor binding. This suggests possible competition of ELP with retinoic acid receptor for the same sequence elements. Oligonucleotides corresponding to ELP and/or retinoic acid receptor binding elements were placed upstream of the SV40 promoter and their effect on gene expression was analyzed by CAT assay. Elements which have affinity to both ELP and retinoic acid receptor were activated by retinoic acid receptor and these activations were repressed by ELP. An ELP binding element without affinity to retinoic acid receptor was insensitive to both activation by retinoic acid receptor and repression by ELP. Furthermore, cellular ELP binding elements and the Moloney leukemia virus long terminal repeat were activated by retinoic acid. These data suggest that one of the mechanism of transcriptional repression by ELP is competition for binding sites with transactivators such as retinoic acid receptors.

Masahiro Okada

Papers

Genomic organization and isoforms of the mouse ELP gene.

Repression of Retinoic Acid-Induced Transactivation by Embryonal LTR Binding Protein.