| Description | The fli-1 gene encodes two isoforms of 51 and 48 kDa, synthesized by alternative translation initiation sites, as mentioned above. Loss of function studies have provided evidence to suggest that both the p51 and p48 isoforms retain the same functional domains and activity (Melet et al., 1996). The functional domains located within the Fli-1 protein include the 5' Ets domain, and a Fli-1-specific region (FLS) referred to as the amino terminal transcriptional activation (ATA) domain, and a 3' Ets and carboxy terminal transcriptional activation (CTA) domain (Figure 2). The 5' Ets domain, sharing 82% sequence identity to Erg and 59-60% to Ets-1 and Ets-2, is located within amino acids 121-196. The FLS, which is absent in the Erg protein, is localized within amino acids 205-292. The 3' Ets domain, sharing 98% homology with Erg, is located within amino acids 277-360 and is responsible for sequence specific DNA-binding activity. The CTA domain, located within amino acids 402-452, is also involved in transcriptional activation and protein-protein interaction. Both the 5' and 3' Ets domain contain sequences of helix 1-loop-helix 2 (H-L-H) secondary structures that are also present in Erg (Rao et al., 1993), while the FLS and CTA domains contain sequences which resemble turn-loop-turn (T-L-T) secondary structures. The structures of the ATA and CTA domains contribute to the transcriptional activity of Fli-1. It has been suggested that the CTA region may serve simultaneously as a transcriptional activator and repressor (Rao et al., 1993). Recently, mice engineered to lack the CTA domain of Fli-1 by homologous recombination were shown to express negligible to low levels of the mutant of Fli-1 mRNA and protein (Spyropoulos et al., 2000). Furthermore, the recombinant Fli-1 protein lacking the CTA domain displayed only 50-60% of the transcriptional activity of wild- type Fli-1, providing further evidence of the CTA domain's involvement in transcriptional activation. The reduced levels of mutant Fli-1 in these mice suggest that the CTA domain also functions to autoregulate Fli-1 expression. NMR spectroscopy analyses have shown that the 3' Ets domain of Fli-1 consists of three alpha-helices and a four stranded beta-sheet that resembles the structures of the class of helix-turn-helix DNA-binding proteins found in the catabolite activator protein of Escherichia coli, as well as those of several eukaryotic DNA binding proteins including H5, HNF-3/forkhead, and the heat shock transcription factor (Liang et al., 1994a; Liang et al., 1994b). A comparison of the Fli-1 3' Ets domain to other structures has suggested that this 3' Ets domain uses a new variation of the winged helix-turn-helix motif for binding to DNA (Liang et al., 1994b).
Fli-1 binds to DNA in a sequence-specific manner, and it has been determined that the optimal DNA binding sequence for Fli-1 is ACCGGAAG/aT/c (Solomon and Kaldis, 1998; Mao et al., 1994; Cui et al., 2009). The bases flanking the core GGA Ets DNA-binding motif synergistically contribute to binding specificity among different Ets transcription factors. Gene promoters containing these Ets sequences have been shown to be transcriptionally regulated by Fli-1, including bcl-2 (Lesault et al., 2002), MDM2 (Truong et al., 2005), gpIX, gpIIb (Bastian et al., 1999), mpl (Deveaux et al., 1996), and recently SHIP-1 (Lakhanpal et al., 2010).
Phosphorylation of both Fli-1 protein isoforms has been predominately detected on serine residues. This phosphorylation is modulated by the concentration of intracellular calcium, similar to Ets-1 and Ets-2, and dephosphorylation is controlled, at least in part, by the phosphatase PP2A (Zhang and Watson, 2005). Post-translational modification by phosphorylation effects Fli-1 DNA binding, protein-protein interaction and transcriptional activation, thereby contributing to the control of gene function (Zhang and Watson, 2005; Asano and Trojanowska, 2009). |
| Expression | Fli-1 is highly expressed in all hematopoietic tissues and endothelial cells, and at a lower level in the lungs, heart and ovaries (Ben-David et al., 1991; Melet et al., 1996; Hewett et al., 2001; Pusztaszeri et al., 2006). The ubiquitous expression of Fli-1 in all endothelial cells (Hewett et al., 2001) suggests a role for Fli-1 in endothelial cell fate and angiogenesis. It has been suggested that Fli-1 is the first dependable nuclear marker of endothelial differentiation (Rossi et al., 2004), is essential for embryonic vascular development (Spyropoulos et al., 2000), and acts as a master regulator establishing the blood and endothelial programmes in the early embryo (Gaikwad et al., 2007; Liu et al., 2008). Moreover immunohistochemical analysis has revealed that Fli-1 expression is a valuable tool in the diagnosis of benign and malignant vascular tumors. |
| Localisation | Similar to other Ets proteins, Fli-1 is a nuclear transcription factor and is generally localized within the nucleus, although Fli-1 protein has also been detected in the cytoplasm of specific cell types (Cui et al., 2009; Pusztaszeri et al., 2006). |
| Function | Fli-1 plays an important role in erythropoiesis. The constitutive activation of fli-1 in erythroblasts leads to a dramatic shift in the Epo/Epo-R signal transduction pathway, blocking erythroid differentiation, activating the Ras pathway, and resulting in massive Epo-independent proliferation of erythroblasts (Tamir et al., 1999; Zochodne et al., 2000). These results suggest that Fli-1 overexpression in erythroblasts alters their responsiveness to Epo and triggers abnormal proliferation by switching the signaling event(s) associated with terminal differentiation to proliferation (Zochodne et al., 2000). The constitutive suppression of Fli-1, mediated through RNA interference or dominant negative protein expression has revealed an essential role for continuous Fli-1 overexpression in the maintenance and survival of the malignant phenotype in both murine and human erythroleukemia (Cui et al., 2009). |
| Homology | Fli-1 is a member of the Ets transcription factor gene family. It is most related to Erg, located on mouse chromosome 16 and human chromosome 21. Similar to Fli-1, Erg is also activated through chromosomal translocation in human cancer. In prostate cancer, TMPRSS2 generates a fusion with ETV1, ETV4 and ETV5, and Erg share 98% homology within the Ets DNA binding domain. The fli-1 gene is conserved in human, mouse, chimpanzee, dog, cow, rat, chicken and zebrafish. |
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| | Figure 2: Fli-1 functional domains. Both human and murine Fli-1 proteins consist of 452 amino acids (aa) which contain the following domains: ATA: amino-terminal transcriptional activation domain, FLS: Fli-1 specific domain, CTA: carboxy-terminal transcriptional activation domain, H-L-H: helix-loop-helix structure, and T-L-T: turn-loop-turn structure. |
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