TR3 Orphan Receptor Is Expressed in Vascular Endothelial Cells and Mediates Cell Cycle Arrest

Abstract

Objective-Endothelial cells play a pivotal role in vascular homeostasis. In this study, we investigated the function of the nerve growth factor-induced protein-B (NGFI-B) subfamily of nuclear receptors comprising the TR3 orphan receptor (TR3), mitogen-induced nuclear orphan receptor (MINOR), and nuclear orphan receptor of T cells (NOT) in endothelial cells. Methods and Results-The mRNA expression of TR3, MINOR, and NOT in atherosclerotic lesions was assessed in human vascular specimens. Each of these factors is expressed in smooth muscle cells, as described before, and in subsets of endothelial cells, implicating that they might affect endothelial cell function. Adenoviral overexpression of TR3 in cultured endothelial cells resulted in decreased [ 3 H]thymidine incorporation, whereas a dominant-negative TR3 variant that inhibits the activity of endogenous TR3-like factors enhanced DNA synthesis. TR3 interfered with progression of the cell cycle by upregulating p27Kip1 and downregulating cyclin A, whereas expression levels of a number of other cell cycle-associated proteins remained unchanged.Conclusions-These findings demonstrate that TR3 is a modulator of endothelial cell proliferation and arrests endothelial cells in the G1 phase of the cell cycle by influencing cell cycle protein levels. We hypothesize involvement of TR3 in the maintenance of integrity of the vascular endothelium. The nerve growth factor-induced protein-B (NGFI-B) subfamily 6 of nuclear receptors (NR4A) belongs to the steroid/thyroid hormone superfamily of transcription factors and comprises TR3 orphan receptor (TR3), mitogen-induced nuclear orphan receptor (MINOR), and nuclear orphan receptor of T cells (NOT). 7 Like other members of the nuclear receptor superfamily, the NGFI-B-like factors contain a central DNA-binding domain, comprising two zinc fingers, that recognizes response elements in the promoters of specific target genes. The amino-terminal domain mediates transactivation, and the carboxy-terminal domain is involved in (hetero)dimerization and ligand binding. At present, the ligands for TR3, MINOR, and NOT are unknown, qualifying these transcription factors as orphan receptors. 8 Several lines of evidence indicate that TR3 and MINOR are involved in T-cell apoptosis. Antisense oligonucleotides directed against TR3 prevent apoptosis in cultured T cells and prostate and lung carcinoma cells. 9 -11 Furthermore, it has been shown that overexpression of TR3 or MINOR in developing T cells of transgenic mice results in massive apoptosis of thymocytes and reduced levels of peripheral T cells, whereas a dominantnegative variant of TR3 inhibits T-cell apoptosis. 12,13 Recently, it has been shown that in response to apoptotic stimuli, TR3 can translocate from the nucleus to the mitochondria to promote cytochrome c release and apoptosis. This proapoptotic effect of TR3 was shown to be independent of its transactivation activity. 14 We have recently shown that TR3 inhibits serumstimulated proliferation of vascular smooth muscle cells (SMCs). Mor...

6-Mercaptopurine, an activator of Nur77, enhances transcriptional activity of HIF-1α resulting in new vessel formation

“…The role of the Nur77 family in the proliferation of endothelial cells is also contradictory. Adenoviral overexpression of Nur77 in cultured endothelial cells resulted in cell-cycle arrest (Arkenbout et al, 2003). In contrast, Zeng et al (2006) showed that TR3/Nur77 is necessary and sufficient for VEGF-A-induced proliferation and survival of cultured HUVECs and for angiogenesis in vivo.…”

The zinc-finger transcription factor, early growth response 3, mediates VEGF-induced angiogenesis

“…Previous studies reported differing effects of manipulation of Nur77 levels in endothelial cells: Arkenbout et al (2003) found that overexpression of Nur77 decreased mitogenic responsiveness, whereas Zeng et al (2006) showed that Nur77 overexpression induced angiogenesis and Nur77 antisense inhibited VEGF-driven angiogenesis. Our results demonstrated that Nur77 siRNA was effective in suppressing VEGF-induced Nur77 mRNA expression and also markedly decreased VEGF-dependent cell proliferation, migration and tubulogenesis, although in contrast to Egr3 knockdown, Nur77 siRNA had little effect on basal or VEGF-dependent cell survival.…”

“…However, studies of Nur77 in endothelial cells have yielded differing results. Adenoviral overexpresion of Nur77 in human umbilical vein endothelial cells (HUVECs) caused cell cycle arrest (Arkenbout et al, 2003), whereas inhibition of Nur77 protein expression suppressed VEGF-induced angiogenesis .…”

The Orphan Nuclear Receptor Nur77 Suppresses Endothelial Cell Activation Through Induction of IκBα Expression

“…In vascular smooth muscle cells, the expression of Nur77 and NOR-1 was significantly induced by atherogenic stimuli, such as platelet-derived growth factor-BB, epidermal growth factor, and ␣-thrombin. 8,9 Overexpression of Nur77 has been shown to inhibit cell proliferation and attenuate vascular injury-induced neointimal formation in vivo. 10,11 NR4A nuclear receptors are also induced in ECs by several stimuli, such as hypoxia, TNF-␣, and vascular endothelial growth factor, and modulate EC growth, survival, and angiogenesis.…”