Review
The Ets family contains transcriptional activators and repressors involved in angiogenesis

https://doi.org/10.1016/S1357-2725(01)00025-5Get rights and content

Abstract

The Ets family contains a growing number of transcriptional activators and inhibitors, which activity is regulated by phosphorylation and protein–protein interactions. Among these factors, Ets1, Erg1 and Fli1 are expressed in endothelial cells during angiogenesis in normal and pathological development. The expression of these transcription factors is regulated by angiogenic factors in cultured endothelial cells, as well as by various stresses occurring during angiogenesis. Transfection experiments and transgenic mice analysis revealed that Ets family members are involved in the transcriptional regulation of endothelial specific genes such as those encoding Tie1 and -2, VEGFR1 and -2 and VE-Cadherin. In vitro studies plead for a role of Ets family members in endothelial cell adhesion, spreading and motility. Gene inactivation experiments show that Ets1 is dispensable for embryonic development. The phenotype of knocked-out embryos indicates that Tel is required for maintenance of the developing vascular network in the yolk sac. Altogether, we suggest that Ets family members act both positively and negatively during the different steps of the angiogenic process. The regulation of the initiation of gene transcription arises from the combined activity of different transcriptional regulators. Therefore very few transcription factors are specific for a physiological process, or a given cell type. The transcriptional network that regulates blood vessel formation involves transcription factors which are expressed in a variety of situations. The Lung Kruppel Like Factor (LKLF) which is required for blood vessel stabilisation during murine development is also expressed in the primitive vertebrae and in the lung of the adult (C.T. Kuo, M.L. Veselits, K.P. Barton, M.M. Lu, C. Clendenin, J.M. Leiden, The LKLF transcription factor is required for normal tunica media formation and blood vessel stabilisation during murine embryogenesis, Genes Dev. 11 (22) (1997) 2996–3006). Scl/Tal1 which is essential for angiogenic remodelling of the yolk sac capillary network (J.E. Visvader, Y. Fujiwara, S.H. Orkin, Unsuspected role for the T-cell leukemia protein SCL/tal-1 in vascular development, Genes Dev. 12 (4) (1998) 473–479), is involved in blood cell development and is also expressed in the developing brain. The EPAS transcription factor which was thought to be endothelial cell specific in the mouse embryo (H. Tian, S.L. McKnight, D.W. Russell, Endothelial PAS domain protein 1 (EPAS1), a transcription factor selectively expressed in endothelial cells, Genes Dev. 11 (1) (1997) 72–82) is also expressed in the liver, kidney and cells of the sympathetic nervous system of the chick embryo (J. Favier, H. Kempf, P. Corvol, J.M. Gasc, Cloning and expression pattern of EPAS1 in the chicken embryo. Colocalization with tyrosine hydroxylase, FEBS Lett. 462 (1-2) (1999) 19–24). Ets1, which expression was originally detected in lymphoid cells of adult tissues, has been the first transcription factor to be identified in endothelial cells during angiogenesis in the embryo (B. Vandenbunder, L. Pardanaud, T. Jaffredo, M.A. Mirabel, D. Stehelin, Complementary patterns of expression of c-etsl, c-myb and c-myc in the blood-forming system of the chick embryo, Development 107 (1989) 265–274 [5]) and in tumours (N. Wernert, M.B. Raes, P. Lassalle, M.P. Dehouck, B. Gosselin, B. Vandenbunder, D. Stehelin, The c-ets 1 proto-oncogene is a transcription factor expressed in endothelial cells during tumor vascularisation and other forms of angiogenesis in man, Am. J. Path. 140 (1992) 119–127 [6]). Since then, the Ets family has extended and this review will emphasise the relationships between these factors and angiogenesis.

Section snippets

A short presentation of Ets family members, their domains and their partners

The proto-oncogene ets1 is the cellular progenitor of v-ets, a viral oncogene found in the genome of the E26 acute leukaemia retrovirus [1], [2]. Ets-1 is the founder of a growing family of transcription factors which includes over 50 members characterised by a conserved DNA-binding domain. Some proteins of the family, such as Ets1, Ets2, Erg, Tel and Fli1 harbour another conserved region named the pointed domain, located in their N-terminal part. The origin of the Ets family seems extremely

Expression patterns of Ets family members during normal and pathological development

A first step towards understanding the biological roles of Ets1 has been the identification of cells in which Ets1 might act. A predominant expression of ets1 has been initially detected in the lymphoid organs of neonatal and adult mice, in discrete T and B cell developmental stages, in lymphoid precursors, in immature NK-like cells and myeloid hematopoietic cells [20].

In situ hybridisation analyses carried out during embryonic development revealed a more widespread expression in a variety of

Control of the expression and activity of Ets members

The expression of Ets1 transcripts is associated in vivo with the activation of endothelial cells and the induction of angiogenesis. In vitro, Ets1 is expressed by proliferating [28] and migrating [27] endothelial cells but not after these cells have reached confluence [28]. In addition, Ets1 is hyper-phosphorylated during early mitosis in T-cell lines, suggesting a regulation of its activity during cell proliferation [42]. Expression of Ets1 in endothelial cells is increased in response to

The involvement of Ets1 in the stress response

The GGAA/T core sequence recognised by Ets factors [53] is present in the sequence recognised by heat-shock factors, and similarities have been found between the DNA-binding domains of heat-shock and of Ets factors [54]. This, together with the regulation of Ets factors activity by stress-kinases, suggests that Ets members are involved in the cell response to stress. The vascular endothelium is one of the prime targets for oxidative stress in a variety of inflammatory conditions, possibly

From promoters studies to the identification of target genes

The role of Ets family members during angiogenesis has been partially addressed by the dissection of the cis-acting elements involved in the regulation of endothelial specific genes. An alternative approach has been to over-express or down-regulate these factors in endothelial cells and to analyse variations in the expression levels of endogenous genes. These methods have allowed the identification of several endothelial specific Ets target genes.

What is the role of Ets members in endothelial cells? Insights from in vitro studies

Ets factors are able to transactivate the genes encoding matrix degrading proteases, cell-to-matrix and cell-to-cell adhesion molecules such as integrins, cadherins and intercellular adhesion molecules. Several studies on the effects of various Ets mutants have indicated that Ets factors could be involved in regulating cell adhesion, spreading and migration. Expression of the DNA binding domains of PU1, Ets1 and Ets2 reverts Ras-transformed 3T3 cells, which become larger and flatter with an

In vivo studies and the proof by KO

The search for Ets target genes provided clues for understanding the molecular mechanisms by which these factors might act. In parallel, various strategies have been carried out to understand the role of each Ets family members in the progression of physiological processes.

Conclusion and directions for future research

In vitro bioassays and experimental tumour models indicate that the angiogenic switch is governed by a balance between inhibitors (thrombospondins, angiostatin or endostatin) and activators (FGF-2, VEGF). The description of the expression patterns of Ets family members and functional studies suggest that these transcription factors may control angiogenesis both positively and negatively (Fig. 3).

The Ets family contains both transcriptional activators and inhibitors. Specific probes and

Acknowledgements

Work in the laboratory was funded by the Conseil Régional du Nord Pas de Calais and the European Regional Development Fund, the Association pour la Recherche sur le Cancer, Ligue Nationale contre le Cancer, Fondation pour la Recherche Médicale, Groupement des Entreprises Françaises dans la Lutte contre le Cancer. FS is ‘Chargé de Recherche de l'Institut National de la Santé et de la Recherche Médicale’. EL is supported by a fellowship from the'Ministère de l'Education Nationale, de la Recherche

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