Identification of a New Gene Expressed Specifically in Early Mouse Embryos. (mouse embryo/gene expression/e5.5.)

Retinoic acid-stimulated sequential phosphorylation, PML recruitment, and SUMOylation of nuclear receptor TR2 to suppress Oct4 expression

We previously reported an intricate mechanism underlying the homeostasis of Oct4 expression in normally proliferating stem cell culture of P19, mediated by SUMOylation of orphan nuclear receptor TR2. In the present study, we identify a signaling pathway initiated from the nongenomic activity of all-trans retinoic acid (atRA) to stimulate complex formation of extracellular signal-regulated kinase 2 (ERK2) with its upstream kinase, mitogen-activated protein kinase kinase (MEK). The activated ERK2 phosphorylates threonine-210 (Thr-210) of TR2, stimulating its subsequent SUMOylation. Dephosphorylated TR2 recruits coactivator PCAF and functions as an activator for its target gene Oct4. Upon phosphorylation at Thr-210, TR2 increasingly associates with promyelocytic leukemia (PML) nuclear bodies, becomes SUMOylated, and recruits corepressor RIP140 to act as a repressor for its target, Oct4. To normally proliferating P19 stem cell culture, exposure to a physiological concentration of atRA triggers a rapid nongenomic signaling cascade to suppress Oct4 gene and regulate cell proliferation.

SUMOylation of Tr2 orphan receptor involves Pml and fine-tunes Oct4 expression in stem cells

The Tr2 orphan nuclear receptor can be SUMOylated, resulting in the replacement of coregulators recruited to the regulatory region of its endogenous target gene, Oct4. UnSUMOylated Tr2 activates Oct4, enhancing embryonal carcinoma-cell proliferation, and is localized to the promyelocytic leukemia (Pml) nuclear bodies. When its abundance is elevated, Tr2 is SUMOylated at Lys238 and seems to be released from the nuclear bodies to act as a repressor. SUMOylation of Tr2 induces an exchange of its coregulators: corepressor Rip140 replaces coactivator Pcaf, which switches Tr2 from an activator to a repressor. This involves dynamic partitioning of Tr2 into Pml-containing and Pml-free pools. These results support a model where SUMOylation-dependent partitioning and differential coregulator recruitment contribute to the maintenance of a homeostatic supply of activating, as opposed to repressive, Tr2, thus fine-tuning Oct4 expression and regulating stem-cell proliferation.

Ligand-independent orphan receptor TR2 activation by phosphorylation at the DNA-binding domain

In a previous report we demonstrated protein kinase C (PKC)-mediated phosphorylation of the ligand-binding domain (LBD) of orphan nuclear receptor TR2. In this report, we provide the evidence of PKC-mediated phosphorylation of the DNA-binding domain (DBD) of TR2. Two PKC target sites were predicted within the DBD, at Ser-170 and Ser-185, but only Ser-185 was confirmed by MS. Phosphorylation of DBD facilitated DNA binding of the TR2 receptor and its recruiting of coactivator p300/CBP-associated factor (P/CAF). Ser-185 was required for DNA binding, whereas both Ser-170 and Ser-185 were necessary for receptor interaction with P/CAF. The P/CAF-interacting domain of TR2 was located in its DBD. A double mutant (Ser-170 and Ser-185) of TR2 significantly lowered the activation of its target gene RARbeta2. This study provides the first evidence for ligand-independent activation of TR2 orphan receptor through PTM at the DBD, which enhanced its DNA-binding ability and interaction with coactivator P/CAF.

Protein kinase C-mediated phosphorylation of orphan nuclear receptor TR2: Effects on receptor stability and activity

In vivo metabolic labeling showed that orphan nuclear receptor TR2 could be phosphorylated. Systematic studies were conducted using specific kinases/phosphatase inhibitors to determine the enzymes responsible for TR2 phosphorylation and the effects of TR2 phosphorylation on its protein stability and activation of its target gene. The data showed that protein kinase C (PKC)-mediated phosphorylation enhanced the activating ability of TR2 on target gene RARbeta as well as its stability through protection from proteosome-mediated degradation. Several PKC-mediated potential serine/threonine phosphorylation sites on TR2 protein were predicted from the computer analysis using NetPhos software (http://us.expasy.org) and were commensurate by in vitro phosphorylation of purified TR2 protein using PKC enzyme. Two phosphorylation sites at Ser-461 and Ser-568 were identified by LC-ESI-MS/MS. Point mutations at Ser-568 or Ser-461 were prepared and evaluated for their biological activity. Ser-568, but not Ser-461, mutation significantly reduced PKC-mediated TR2 protein stability and its transcriptional activity.

The orphan nuclear receptor TR2 interacts directly with both class I and class II histone deacetylases

A combination of in vivo and in vitro assays was employed to describe the ligand-independent interaction of the orphan nuclear receptor TR2 and histone deacetylase proteins. The repressive effect of TR2 on transcription of a luciferase reporter driven by a promoter containing a direct repeat-5 (DR5) derived from the human RARbeta gene was suppressed by the addition of the histone deacetylase inhibitor trichostatin A. Immunoprecipitation with FLAG-epitope (MDYKDDDDK)-tagged histone deacetylase proteins was used to demonstrate that TR2 and histone deacetylases 3 or 4 are present in the same immunoprecipitated complex. Deacetylase activity was demonstrated for these coimmunoprecipitates, further confirming the in vivo interaction of TR2 and histone deacetylases. Immunoprecipitation with anti-TR2 antibody was used to demonstrate interaction of TR2 with endogenously expressed histone deacetylases 3 and 4 in COS-1 cells. Dissection of TR2 domains showed that the DNA binding domain of the receptor was responsible for interaction with both histone deacetylases 3 and 4 in glutathione-S-transferase pull-down assays, while the ligand binding domain did not interact. The pull-down data were confirmed with far Western blots that also showed a direct interaction between labeled histone deacetylase proteins and TR2. It is suggested that repression mediated by unliganded TR2 is mediated, in part, by a direct interaction of this receptor with histone deacetylase proteins.

Characterization of the mouse nuclear orphan receptor TR2-11 gene promoter and its potential role in retinoic acid-induced P19 apoptosis

The complete mouse orphan nuclear receptor TR2-11 gene structure and its 5'-untranscribed region were characterized. This gene contains 14 exons, with the first exon encoding only the 5'-untranslated sequence. The regulatory region of this gene was characterized by using reporter assays that define the minimal promoter activity in a sequence 212 nucleotides upstream from the translation initiation site. Furthermore, it was concluded that splicing of intron 1 is required for efficient promoter activity. Reporters driven by this promoter were induced by retinoic acid (RA) in COS-1 cells supplied with exogenous retinoic acid receptor-alpha (RAR(alpha)) and retinoid receptor X-beta (RXR(beta)). Binding of RAR(alpha)/RXR(beta) to the minimal promoter region was demonstrated in gel retardation assays. In P19 cells, both the endogenous TR2-11 gene and the reporters driven by this promoter were induced by RA in a protein synthesis-independent manner, and overexpression of TR2-11 protein resulted in cellular apoptosis in the absence of RA. The regulation of TR2-11 by RA and the implication of TR2 up-regulation in P19 cellular apoptosis are discussed.

Constitutive activation of retinoic acid receptor beta2 promoter by orphan nuclear receptor TR2

The orphan nuclear receptor TR2 functions as a constitutive activator for the endogenous retinoic acid receptor beta2 (RAR(beta2)) gene expression in P19 embryonal carcinoma cells and for reporters driven by the RAR(beta2) promoter in COS-1 cells. The activation of RAR(beta2) by TR2 is mediated by the direct repeat-5 (DR5) element located in the RAR(beta2) promoter. Furthermore, cAMP exerts an enhancing effect on the activation of RAR(beta2) by TR2, which is mediated by the cAMP response element located in the 5'-flanking region of the DR5. The constitutive activation function-1 (AF-1) of TR2 is mapped to amino acid residues 10-30 in its N-terminal A segment. A direct molecular interaction occurs between CREMtau and TR2, detected by co-immunoprecipitation, which is mediated by the N-terminal AB segment of TR2. In gel mobility shift assays, TR2 competes with P19 nuclear factor binding to the RAR(beta2) promoter, and TR2 and CREMtau bind simultaneously to this DNA fragment. The role of TR2 in the early events of RA signaling process is discussed.

Promoter activity of mouse kappa opioid receptor gene in transgenic mouse

The biological activity of mouse kappa opioid receptor (KOR) gene promoter was examined in transgenic mice using a beta-galactosidase (lacZ) reporter strategy for the first time. A lacZ cDNA was inserted at the 5th amino acid in the coding region of a mouse KOR genomic segment containing 3 kb of the 5' regulatory region, to generate a Kor-lacZ fusion gene which was then used to generate transgenic mice. The expression of transgene was demonstrated at the RNA level by reverse transcription-polymerase chain reaction (RT-PCR), and at the protein level by in situ lacZ enzyme assay. From studying three independent transgenic mouse lines that express this transgene, it is concluded that Kor-lacZ expression begins at embryonic day 9.5 (E9.5) and increases in several brain areas and neural tube as embryos develop. At E12.5 and E13.5, Kor-lacZ expression is found primarily in the mantle layer of midbrain, hindbrain and medulla oblongata, cranial ganglion and vagus nerve. At E15.5 and E17.5, the transgene is expressed in eye, ear, neopallial cortex, caudate putamen, lateral ventricle, thalamus, hypothalamus and pons. Therefore, the 3 kb 5' regulatory sequence of the mouse KOR gene is functional in transgenic animals and directs a specific expression pattern recapitulating that of the endogenous KOR gene expression during developmental stages. However, in adult animals, this transgene is only expressed in the brain, indicating that the regulatory information for peripheral expression in the adult is not encoded within this 3 kb upstream sequence.