Coup d'Etat: an orphan takes control

Abstract

Chicken ovalbumin upstream promoter transcription factors (COUP-TFs) belong to the steroid/thyroid hormone receptor superfamily. Cloning of their cDNAs demonstrated the existence of two distinct but related genes: COUP-TFI (EAR-3, NR2F1) and COUP-TFII (ARP-1, NR2F2). They are referred to as orphan receptors because ligands for COUP-TFs have yet to be identified. Since 1998, extensive studies have demonstrated their physiological importance in cell-fate specification, organogenesis, angiogenesis, and metabolism, as well as a variety of diseases. In this article, we will comprehensively review the biological functions of COUP-TFII and its underlying mechanism in various developmental processes and diseases. In addition, we will briefly summarize some of the current findings of COUP-TFI.

Fig. 1.

Involvement of COUP-TFII in physiological and pathological processes during mouse development. CDH, Congenital diaphragmatic hernia.

Fig. 2.

Model showing arterial, venous, and lymphatic cell fate specification in the developing embryos. In an arterial-fated angioblast, upon VEGF stimulation, the VEGFR-2/Np-1 signaling activates Notch signaling pathway, leading to the induction of arterial marker EphrinB2 and the establishment of arterial phenotype. In contrast, COUP-TFII promotes venous cell fate by suppressing the expression of Np-1 and Notch signaling molecules in a venous-fated angioblast. As development proceeds, a subset of Sox18 and COUP-TFII-expressing venous endothelial cells turn on Prox1 expression, thus inducing the activation of VEGFR-3/Nrp2 signaling. Both COUP-TFII and Prox1 are required for suppressing blood endothelial cell fate and promoting lymphatic endothelial cell fate. [Adapted from a model presented in Kume T: Histol Histopathol 25:637–646, 2010 (105).]

Fig. 3.

The expression domain of COUP-TFII in mouse hearts. A–D, Immunofluorescent staining of E19.5 hearts with antibodies against COUP-TFII, venous marker EphB4, arterial markers Notch1 and Jag1, and smooth muscle cell marker α-smooth muscle actin (α-SMA) showed that COUP-TFII is expressed in the endothelium of EphB4-positive subepicardial vessels (veins) and the smooth muscle cells of Notch1 and Jag1-expressing intramyocardial vessels (arteries). 4',6-diamidino-2-phenylindole is for nuclei staining. E, The visualization of the coronary venous system by using Tie2Cre transgenic mouse line to label COUP-TFII in the endothelium. F, SM22αCre transgenic mouse line was used to label COUP-TFII in the smooth muscle cells. Whole-mount X-gal staining of SM22Cre; COUP-TFII ^F/+ hearts at P0 clearly marks the coronary arterial system. G and H, Histological sections of hearts (from panels E and F), showing that COUP-TFII is expressed in the endothelial cells of the coronary veins and the smooth muscle cells of the coronary arteries. A, Artery; LA, Left atrium; LCA, left cardiac artery; LCV, left cardiac vein; LV, left ventricle; MCV, major caudal cardiac vein; RA, right atrium; RCA, right cardiac artery; RCV, right cardiac vein; RV, right ventricle; V, vein.

Fig. 4.

Coronary venous identity was altered in the absence of COUP-TFII. Transverse sections of control COUP-TFII^F/F and CRE-ER^T2; COUP-TFII^F/F mouse hearts at E18.5 (TAM administrated at E15.5) were analyzed using antibodies against COUP-TFII (A and B), EphB4 (C and D), and Jag1 (E and F). When COUP-TFII is deleted, EphB4 is down-regulated (arrows) and Jag1 is ectopically expressed in the cardiac veins (arrowheads). A, Artery; V, vein.

Fig. 5.

Increased activity of Notch signaling by loss of COUP-TFII. Quantitative RT-PCR analysis of different factors related to Notch signaling in testes from control and COUP-TFII mutant mice. Bar graph shows fold changes of each molecule in COUP-TFII mutants relative to controls (n = 8). The dotted line indicates the expression levels of those genes in control mice. *, P < 0.05; **, P < 0.01.

Fig. 6.

Model depicting COUP-TFII functions in tumor angiogenesis. In endothelial cells, the absence of COUP-TFII results in the elevated expression of VEGFR-1, a decoy receptor that sequesters VEGF ligand. This leads to attenuation of VEGFR-2 signaling and endothelial cell sprouting in the COUP-TFII-deficient mice. Additionally, endothelial cell proliferation is impaired due to decreased expression of E2F1. In pericytes, the absence of COUP-TFII results in reduced Ang-1 that serves as a paracrine signal to activate Tie2 receptors on the endothelial cells. Thus, COUP-TFII regulates angiogenesis through growth signaling, VEGF/VEGFR-2, and Ang-1/Tie2 signaling pathway within tumor microenvironment.