Activity of the Nurr1 carboxyl-terminal domain depends on cell type and integrity of the activation function 2

Dimer-specific potentiation of NGFI-B (Nur77) transcriptional activity by the protein kinase A pathway and AF-1-dependent coactivator recruitment

The NGFI-B (Nur77) subfamily of orphan nuclear receptors (NRs), which also includes Nurr1 and NOR1, bind the NurRE regulatory element as either homo- or heterodimers formed between subfamily members. These NRs mediate the activation of pituitary proopiomelanocortin (POMC) gene transcription by the hypothalamic hormone corticotropin-releasing hormone (CRH), an important link between neuronal and endocrine components of the hypothalamo-pituitary-adrenal axis. CRH effects on POMC transcription do not require de novo protein synthesis. We now show that CRH signals activate Nur factors through the cyclic AMP/protein kinase A (PKA) pathway. CRH and PKA rapidly increase nuclear DNA binding activity of NGFI-B dimers but not monomers. Accordingly, CRH- or PKA-activated Nur factors enhance dimer (but not monomer) target response elements. We also show that p160/SRC coactivators are recruited to Nur dimers (but not to monomers) and that coactivator recruitment to the NurRE is enhanced in response to CRH. Moreover, PKA- and coactivator-induced potentiation of NGFI-B activity are primarily exerted through the N-terminal AF-1 domain of NGFI-B. The TIF2 (SRC-2) glutamine-rich domain is required for this activity. Taken together, these results indicate that Nur factors behave as endpoint effectors of the PKA signaling pathway acting through dimers and AF-1-dependent recruitment of coactivators.

Hormone-dependent repression of the E2F-1 gene by thyroid hormone receptors

Thyroid hormone induces differentiation of many different tissues in mammals, birds, and amphibians. The different tissues all differentiate from proliferating precursor cells, and the normal cell cycle is suspended while cells undergo differentiation. We have investigated how thyroid hormone affects the expression of the E2F-1 protein, a key transcription factor that controls G1- to S-phase transition. We show that during thyroid hormone-induced differentiation of embryonic carcinoma cells and of oligodendrocyte precursor cells, the levels of E2F-1 mRNA and E2F-1 protein decrease. This is caused by the thyroid hormone receptor (TR) regulating the transcription of the E2F-1 gene. The TR binds directly to a negative thyroid hormone response element, called the Z-element, in the E2F-1 promoter. When bound, the TR activates transcription in the absence of ligand but represses transcription in the presence of ligand. In addition, liganded TR represses transcription of the S-phase-specific DNA polymerase alpha, thymidine kinase, and dihydropholate reductase genes. These results suggest that thyroid hormone-induced withdrawal from the cell cycle takes place through the repression of S-phase genes. We suggest that this is an initial and crucial step in thyroid hormone-induced differentiation of precursor cells.

The constitutive and inducible expression of Nurr1, a key regulator of dopaminergic neuronal differentiation, in human neural and non-neural cell lines

Nur-related factor 1 (Nurr1), nerve growth factor-induced gene B (NGFI-B) and neuron-derived orphan receptor-1 (NOR-1) constitute the orphan nuclear receptor subfamily of transcription factors. Previous studies showed that midbrain dopaminergic neuronal precursor cells failed to differentiate in Nurr1-deficient mice. To investigate a role of Nurr1 in human neuronal function, Nurr1 mRNA expression was studied in human neural cell lines by RT-PCR and northern blot analysis. Nurr1, NGFI-B and NOR-1 mRNA were coexpressed in all human neural and nonneural cell lines under the serum-containing culture condition, except for SK-N-SH neuroblastoma, in which Nurr1 mRNA was undetectable. The levels of Nurr1, NGFI-B and NOR-1 mRNA were elevated markedly in NTera2 teratocarcinoma-derived neurons (NTera2-N), a model of differentiated human neurons, following a 1.5 or 3 h-exposure to 1 mM dibutyryl cyclic AMP or 100 nm phorbol 12-myristate 13-acetate. NGFI-B mRNA levels were also elevated in NTera2-N cells by exposure to 100 ng/mL brain-derived neurotrophic factor (BDNF). To identify Nurr1-target genes, the mRNA expression of 27 genes potentially involved in dopaminergic neuronal differentiation and survival, including BDNF, glia-derived neurotrophic factor, their receptors, tyrosine hydroxylase and alpha-synuclein, were studied in HEK293 cells following overexpression of Nurr1. None of these genes examined, however, showed significant changes. These results indicate that Nurr1, NGFI-B and NOR-1 mRNA are expressed constitutively in various human neural and non-neural cell lines under the serum-containing culture condition, and their levels are up-regulated in human neurons by activation of protein kinase A or protein kinase C pathway, although putative coactivators expressed in dopaminergic neuronal precursor cells might be required for efficient transcriptional activation of Nurr1-target genes.

Requirements for heterodimerization between the orphan nuclear receptor Nurr1 and retinoid X receptors

The nuclear receptor nurr1 is a transcription factor involved in the development and maintenance of neurons synthesizing the neurotransmitter dopamine. Although the lack of nurr1 expression has dramatic consequences for these cells either in terms of differentiation or survival, the mechanisms by which nurr1 controls gene transcription still remain unclear. In the intent to understand better the modalities of action of this nuclear receptor, we have undertaken a systematic analysis of the transcriptional effects and DNA binding properties of nurr1 as a monomer or when forming dimers with the different isotypes of the retinoic X receptor (RXR). Here, we show that nurr1 acts as a gene activator independently of RXR and through an AF2-independent mechanism. In addition, heterodimerization with RXR is isotype-specific, involves multiple domains in the C-terminal region of nurr1, and requires RXR binding to DNA. RXR(alpha)-nurr1 and RXRgamma-nurr1 heterodimers bind direct repeat response elements and display no specific requirements with respect to half-site spacing. However, the retinoid responsiveness of DNA-bound heterodimers requires the reiteration of at least three nurr1 binding sites, thereby limiting retinoid-induced nurr1 transcriptional activity to specific direct response elements.

Defining requirements for heterodimerization between the retinoid X receptor and the orphan nuclear receptor Nurr1

Nurr1, an orphan nuclear receptor mainly expressed in the central nervous system, is essential for the development of the midbrain dopaminergic neurons. Nurr1 binds DNA as a monomer and exhibits constitutive transcriptional activity. Nurr1 can also regulate transcription as a heterodimer with the retinoid X receptor (RXR) and activate transcription in response to RXR ligands. However, the specific physiological roles of Nurr1 monomers and RXR-Nurr1 heterodimers remain to be elucidated. The aim of this study was to define structural requirements for RXR-Nurr1 heterodimerization. Several amino acid substitutions were introduced in both Nurr1 and RXR in the I-box, a region previously shown to be important for nuclear receptor dimerization. Single amino acid substitutions introduced in either Nurr1 or RXR abolished heterodimerization. Importantly, heterodimerization-deficient Nurr1 mutants exhibited normal activities as monomers. Thus, by introducing specific amino acid substitutions in Nurr1, monomeric and heterodimeric properties of Nurr1 can be distinguished. Interestingly, substitutions in the RXR I-box differentially affected heterodimerization with Nurr1, retinoic acid receptor, thyroid hormone receptor, and constitutive androstane receptor demonstrating that the dimerization interfaces in these different heterodimers are functionally unique. Furthermore, heterodimerization between RXR and Nurr1 had a profound influence on the constitutive activity of Nurr1, which was diminished as a result of RXR interaction. In conclusion, our data show unique structural and functional properties of RXR-Nurr1 heterodimers and also demonstrate that specific mutations in Nurr1 can abolish heterodimerization without affecting other essential functions.

The activation function-1 domain of Nur77/NR4A1 mediates trans-activation, cell specificity, and coactivator recruitment

Nur77/NR4A1 is an "orphan member" of the nuclear hormone receptor superfamily. Nur77 and its close relatives Nurr1 and NOR-1 bind as monomers to a consensus binding site, the nerve growth factor induced protein I-B (NGFI-B)-binding response element (NBRE). The Nur77/NURR1/NOR1 nuclear receptors are classified as immediate early response genes which are induced through multiple signal transduction pathways. They have been implicated in cell proliferation, differentiation, and apoptosis. However, the mechanism of coactivation and ligand independent trans-activation remains unclear. Hence we examined the molecular basis of Nur77-mediated cofactor recruitment and activation. We observed that Nur77 trans-activates gene expression in a cell-specific manner, and operates in an activation function-1 (AF-1)-dependent manner. The AB region encodes an uncommonly potent N-terminal AF-1 domain delimited to between amino acids 50 and 160 and is essential for the ligand-independent activation of gene expression. Steroid receptor coactivator-2 (SRC-2) modulates the activity of the N-terminal AF-1 domain. Moreover, SRC-2 dramatically potentiates the retinoid induced RXR-dependent activation of the Nur77 ligand binding domain (LBD). Interestingly, the N-terminal AB region (not the LBD) facilitates coactivator recruitment and directly interacts with SRC, p300, PCAF, and DRIP-205. Consistent with this, homology modeling indicated that the Nur77 LBD coactivator binding cleft was substantially different from that of retinoic acid receptor gamma, a closely related AF-2-dependent receptor. In particular, the hydrophobic cleft characteristic of nuclear receptors was replaced with a much more hydrophilic surface with a distinct topology. This observation accounts for the inability of this nuclear receptor LBD to directly mediate cofactor recruitment. Furthermore, the AF-1 domain physically associates with the Nur77 C-terminal LBD and synergizes with the retinoid X receptor LBD. Thus, the AF-1 domain plays a major role in Nur77-mediated transcriptional activation, cofactor recruitment, and intra- and intermolecular interactions.

Induction of cell cycle arrest and morphological differentiation by Nurr1 and retinoids in dopamine MN9D cells

Dopamine cells are generated in the ventral midbrain during embryonic development. The progressive degeneration of these cells in patients with Parkinson's disease, and the potential therapeutic benefit by transplantation of in vitro generated dopamine cells, has triggered intense interest in understanding the process whereby these cells develop. Nurr1 is an orphan nuclear receptor essential for the development of midbrain dopaminergic neurons. However, the mechanism by which Nurr1 promotes dopamine cell differentiation has remained unknown. In this study we have used a dopamine-synthesizing cell line (MN9D) with immature characteristics to analyze the function of Nurr1 in dopamine cell development. The results demonstrate that Nurr1 can induce cell cycle arrest and a highly differentiated cell morphology in these cells. These two functions were both mediated through a DNA binding-dependent mechanism that did not require Nurr1 interaction with the heterodimerization partner retinoid X receptor. However, retinoids can promote the differentiation of MN9D cells independently of Nurr1. Importantly, the closely related orphan receptors NGFI-B and Nor1 were also able to induce cell cycle arrest and differentiation. Thus, the growth inhibitory activities of the NGFI-B/Nurr1/Nor1 orphan receptors, along with their widespread expression patterns both during development and in the adult, suggest a more general role in control of cell proliferation in the developing embryo and in adult tissues.

The aging process: where are the drug opportunities?

New data support a role for growth hormone secretagogue receptor agonists as rejuvenating agents. Two enzymes critical for the formation of beta-amyloid plaques in Alzheimer's disease have been identified. Estrogen receptor beta continues to emerge as a potential drug target. The orphan nuclear receptor Nurr1 appears to be a target for treatment of Parkinson's disease, and propargylamines are emerging as inhibitors of oxidative damage in neurons.