Vol. 10, No. 3 177 MINIREVIEW The role of the orphan receptor SF-1 in the development and function of the ovary Jaroslaw Mlynarczuk1, Robert Rekawiecki Institute of Animal Reproduction and Food Research of Polish Academy of Science, Olsztyn, Poland Received: 26 October 2009; accepted: 29 October 2010 SUMMARY The development of oocyte and ovulation require a precise synchronization at systemic and local levels. Nuclear receptors are involved in the regulation of these processes. In addition to the well-known nuclear receptors (e.g. receptors for estradiol, progesterone, glucocorticoids), a group of “orphan receptors” are distinguished within a receptor family. The orphan receptors are characterized by a lack of defined physiological ligands. Steroidogenic Factor 1 (SF-1, NR5A1) is a member of the orphan receptor group and is in- volved in the regulation of reproductive processes. The SF-1 structure is similar to that of the steroid receptors but does not have a modulatory domain. The SF-1 as a transcription factor may interact with genes in three main ways: a/ by a mechanism typical for nuclear receptors, encompassing homodimerization of SF-1 units, b/ by a formation heterodimers with other 1Corresponding author: Intitute of Animal Reproduction and Food Research of Polish Academy of Science, Tuwima 10, 10–747 Olsztyn, Poland, [email protected] Copyright © 2010 by the Society for Biology of Reproduction 178 SF-1 in the ovary nuclear receptors, and c/ by action as a monomer. During fetal develop- ment, the SF-1, is responsible for differentiation of the gonads and, during the postnatal period, it is responsible for the increase in the expression of genes involved in steroidogenesis. Knock-out of SF-1 gene leads to a rapid death of newly born mice with symptoms of severe adrenal insuf- ficiency. In humans, SF-1 dysfunction causes an adrenal insufficiency and infertility. Learning of the SF-1 and other orphan receptors’ action mechanisms, will allow the creation of specific drugs, helpful in prevent- ing some diseases of the female reproductive tract. Reproductive Biology 2010 10 3: 177–193. Key words: orphan receptors, SF-1 receptor, reproduction, ovaries. INTRODUCTION The main function of mammalian ovaries is the production of matured female gametes – oocytes. Development and maturation of oocytes is a multistage process which requires the timed action of many regulatory factors at both systemic and local levels. Nuclear receptors (NR), among other receptors, are engaged in the transmission of signals between cells. The NR superfamily of NR is a group of transcription factors which control the gene expression after activation by steroid and thyroid hormones, vitamin D and their deriva- tives, cholesterol and retinoic acid [37]. NR serves as an interface for signals from the whole body to a cell genome [58]. In addition to classical NR [e.g. steroid receptors, thyroid hormone (TR) receptors, vitamin D (VDR) recep- tors], there are numerous orphan receptors [20, 21]. Identification and func- tion of the latter was possible thanks to development of so-called reverse endocrinology [33]. Genes encoding unknown NR or putative response elements were determined during the mapping of the human and animal genomes. There are 48 NR in the human and 49 NR in the mouse genome [2, 41]. A natural ligand for the first discovered orphan receptor – estrogen- related receptor (ERRα; [21]) has not been identified yet. Experimental data indicates that many estrogenic substances (diethylstilbestrol, organochlorine pesticides, phytoestrogens) may be ligands of ERRα [2]. Mlynarczuk & Rekawiecki 179 Nuclear receptors are classified into seven groups (NR0 - NR6) according to their sequence homology and phylogenetic relationships. Steroidogenic factor 1 (SF-1, AD4BP) is one of the NR family and belongs to the NR5 group and NR5A subgroup. Due to this and to the fact that SF-1 was the first receptor discovered in the subgroup, its official name is NR5A1. But its common name (SF-1) is also used in scientific literature. Another member of the NR5 group is liver receptor homologue-1 (LHR-1, NR5A2). The nuclear receptors are also classified according to their physiological ligands and potential function (fig. 1; [2]): 1. “endocrine” receptors: steroid hormone receptors, TR, VDR, retinoic acid receptors (RARs) characterized by high affinity for their ligands (Kd ≥ nM) and high transcriptional activity; 2. “true” orphan receptors: their physiological ligands are unknown, but they may have synthetic ligands. These receptors are often functional inhibitors of transcriptional activity of other NR however, it is unknown whether this inhibition is ligand-dependent or -independent [30]; 3. “adopted” orphan receptors: orphan receptors which were adopted after the discovery of their ligands. Compared to the endocrine receptors, these receptors are characterized by a lower affinity for their ligands and lower transcriptional activity. Within the adopted orphan receptor group, “enigmatic” orphan receptors were further distinguished. Some ligands of the enigmatic orphan receptors have been identified, but the nature of the ligand-dependent activation of these receptors is difficult to associ- ate with any physiological process. SF-1 is an example of an enigmatic orphan receptor with sphingosine as its natural ligand [64, 65]. The main pathway of SF-1 action is typical to that of endocrine receptors such as estradiol (ER) or progesterone (PR) receptors (fig. 3a). Steroid hor- mone receptors and other “endocrine” receptors diffuse through the plasma membrane of their target cells and bind to the specific NR subunits. Then, the ligand-activated subunits dimerize and acquire the transcriptional activity. Nuclear receptors may form homo- (dimerization of the same NR subunits) or heterodimers (dimerization of different NR subunits). After translocation to the nucleus, dimer binds to a specific hormone response element present in the promoter of a target gene and affects the gene’s transcription. 180 SF-1 in the ovary Figure 1. Classification of nuclear receptors. CAR: constitutive androgen receptor [NR1I1]; COUP-TF: chicken ovoalbumin upstream promoter – transcription factor [NR2F]; DAX-1: dosage-sensitive sex reversal, adrenal hypoplasia critical region on chromosome X, gene 1 [NR0B1]; ERR: estrogen related receptor [NR3B]; PNR: photoreceptor cell-specific nuclear receptor [NR2E3]; PPAR: peroxisome proliferator-activated receptor [NR1C], PXR: pregnane X receptor [NR1I2], ROR: RAR-related orphan receptor [NR1F], RXR: retinoid X receptor [NR2B], SF-1: steroidogenic factor-1 [NR5A1]. STRUCTURE OF SF- 1 The existence of SF-1 was predicted in 1984 after the identification of cDNA of bovine 21-hydroxylase and cholesterol side-chain cleavage monooxyge- nase (P450scc; [46, 68]). Two years later the response element for SF-1 was identified in the promoter region of the 21-hydroxylase gene [25, 52]. Next, the protein interacting with the response element for SF-1 was recognized [47, 54], SF-1 cDNA was cloned and the amino acid sequence of SF-1 protein was described [28]. SF-1 is a phylogenetic old structure. As high as 75–85% amino acid se- quence similarity was observed between the SF-1 receptor in different mam- mals and FTZ-F1, a SF-1 homolog discovered in Drosophilla melanogaster [56]. The SF-1 is functionally divided into three domains: DNA binding Mlynarczuk & Rekawiecki 181 Figure 2. Structure of SF-1 and classic nuclear receptor. AF-1/MD: activation func- tion sequence 1/modulatory domain; AF-2: activation function sequence-2; DBD: DNA binding domain, LBD: ligand binding domain; ZF1, ZF2: zinc fingers. domain (DBD, region C), “hinge” region (flexible region, region D) and li- gand binding domain (LBD, region E; fig. 2). In contrast to other NR, SF-1 does not possess A/B region and its modulatory domain (MD) is extremely shortened (fig. 2). Hence, SF-1 does not have the ligand-independent activa- tion function 1 sequence (AF-1). However, the short MD present in SF-1 may be phosphorylated by MAP-kinases [19] and, thus, it may join cofactors (i.e. SOX9 or WT-1) affecting SF-1 transcriptional activity [16, 50]. The DNA binding domain is the most conserved part of SF-1. It con- tains DNA-binding motif composed of two zinc-chelating modules (zinc fingers) that coordinate the interaction between the receptor and hormone response element (HRE). The “hinge” region is partially responsible for homo- or heterodimerization. Miscellaneous cofactors may bind to this region and affect SF-1 transcriptional activity [60]. The ligand binding domain (LBD, domain E) is composed of a ligand binding pocket, di- merization site, activation function 2 sequence (AF-2) and cofactor bind- ing site. The domain mediates dimerization, ligand-induced activation as well as ligand-reversed transcriptional silencing. The enhancement of SF-1 transcriptional activity might be caused by Ptx-1 protein which binds to LBD domain [62]. Because SF-1 does not have a functional 182 SF-1 in the ovary domain A/B and AF-1 sequence (fig. 2), the activation of AF-2 sequence is sufficient for full transcriptional activity of the receptor [55]. In mice, three isoforms of SF-1 (ELP1– ELP3) were found as a result of mRNA SF-1 alternative splicing [51]. The existence of SF-1 isoforms was not confirmed in other mammals. MECHANISM OF SF-1 ACTION More than one pathway may be involved in the regulation of SF-1 tran- scriptional activity. The main pathway starts with the homodimerization of SF-1 units after ligand binding. This is followed by the activation of the transcription of genes containing promoters with the appropriate palindromic response elements [55]. The SF-1 response element is com- posed of two half-palindrome sequences: 5’-AGGTCANNNTGACCT-3’ [5]. Another pathway starts with the heterodimerization of SF-1 with other NR. These heterodimers may be formed by NR characterized by certain transcriptional activity (Retinoid X Receptors-RXR; 9-cis-Retinoid Acid Receptor-RAR) as well as by NR without such activity (DAX-1 of NR0 group; fig.