Heterotopic expression of the Xl-Fli transcription factor during Xenopus embryogenesis: modification of cell adhesion and engagement in the apoptotic pathway

Continuous Fli-1 expression plays an essential role in the proliferation and survival of F-MuLV-induced erythroleukemia and human erythroleukemia

Erythroleukemia induced by Friend Murine Leukemia Virus (F-MuLV) serves as a powerful tool for the study of multistage carcinogenesis and hematological malignancies in mice. Fli-1, a proto-oncogene and member of the Ets family, is activated through viral integration in F-MuLV-induced erythroleukemia, and is the most critical event in the induction of this disease. Fli-1 aberrant regulation is also observed in human malignancies, including Ewing's sarcoma, which is often linked to expression of the EWS/Fli-1 fusion oncoprotein. Here we examined the effects of Fli-1 inhibition to further elucidate its role in these pathological occurrences. The constitutive suppression of Fli-1, through RNA interference (RNAi), inhibits growth and induces death in F-MuLV-induced erythroleukemia cells. Expression of a dominant negative protein Engrailed (En)/Fli-1 reduces proliferation of EWS/Fli-1-transformed NIH-3T3 cells, and both F-MuLV-induced and human erythroleukemia cells. F-MuLV-induced erythroleukemia cells also display increased apoptosis, associated with reduced expression of bcl-2, a known fli-1 target gene. Introduction of En/Fli-1 into an F-MuLV-infected erythroblastic cell line induces differentiation, as shown by increased alpha-globin expression. These results suggest, for the first time, an essential role for continuous Fli-1 overexpression in the maintenance and survival of the malignant phenotype in murine and human erythroleukemias.

Conserved function of caspase-8 in apoptosis during bony fish evolution

Caspase-8, a member of the caspase family, plays an important role in apoptotic signal transduction in mammals. Here we report the identification and characterization of the caspase-8 (casp8) gene in the zebrafish Danio rerio. The zebrafish casp8 gene has a genomic organization similar to mammalian casp8 genes, consisting of 10 exons. By chromosome mapping, we found that casp8 maps on linkage group 6 (LG6), a zebrafish chromosome segment orthologous to the long arm of human Chr. 2, which carries CASP8. In contrast, the zebrafish casp10-like gene and the cflar gene separately localize on LG9 and LG11, respectively, and these genes form a cluster with CASP8 on the human chromosome. This chromosomal segregation is unique to fish but not other vertebrates. Furthermore, we examined the function of zebrafish Casp8 protein in mammalian cells, and showed that it has pro-apoptotic activity when overexpressed. In addition, this molecule was capable of transmitting apoptotic signals mediated through not only Fas but also the TNF receptor in mouse Casp8-deficient cells. Expression analysis showed that casp8 is maternally expressed, and transcripts continue to be present throughout embryogenesis and into larval stages. These results show that zebrafish casp8 has a structure and function similar to mammalian CASP8 orthologs, and our study suggests that the role of caspase-8 in the apoptotic signal pathway has been conserved over at least 450 million years of vertebrate evolution.

Apoptosis is required during early stages of tail regeneration in Xenopus laevis

The Xenopus tadpole is able to regenerate its tail, including skin, muscle, notochord, spinal cord and neurons and blood vessels. This process requires rapid tissue growth and morphogenesis. Here we show that a focus of apoptotic cells appears in the regeneration bud within 12 h of amputation. Surprisingly, when caspase-3 activity is specifically inhibited, regeneration is abolished. This is true of tails both before and after the refractory period. Programmed cell death is only required during the first 24 h after amputation, as later inhibition has no effect on regeneration. Inhibition of caspase-dependent apoptosis results in a failure to induce proliferation in the growth zone, a mispatterning of axons in the regenerate, and the appearance of ectopic otoliths in the neural tube, in the context of otherwise normal continued development of the larva. Larvae amputated during the refractory stage exhibit a much broader domain of caspase-3-positive cells, suggesting a window for the amount of apoptosis that is compatible with normal regeneration. These data reveal novel roles for apoptosis in development and indicate that a degree of apoptosis is an early and obligate component of normal tail regeneration, suggesting the possibility of the existence of endogenous inhibitory cells that must be destroyed by programmed cell death for regeneration to occur.

Regulation of apoptosis in theXenopus embryo by Bix3

Members of the Bix family of homeobox-containing genes are expressed in the vegetal hemisphere of the Xenopus embryo at the early gastrula stage. Misexpression of at least some of the family members causes activation of mesoderm- and endoderm-specific genes and it is known that some of the proteins, including Bix2 and Bix3, interact with Smad proteins via a motif that is also present in the related protein Mixer. In this paper we study the function of Bix3. Misexpression of Bix3, similar to misexpression of other members of the Bix family, causes the activation of a range of mesendodermal genes, but the spectrum of genes induced by Bix3 differs from that induced by Bix1. More significantly, we find that overexpression of Bix3 also causes apoptosis, as does depletion of Bix3 by use of antisense morpholino oligonucleotides. The ability of Bix3 to causes apoptosis is not associated with its ability to activate transcription and nor with its possession of a Smad interaction motif. Rather, Bix3 lacks a C-terminal motif, which, in Bix1, acts in cis to inhibit apoptosis. Mutation of this sequence in Bix1 causes the protein to acquire apoptosis-inducing activity.

Caspase cleavage of the transcription factor FLI-1 during preB leukemic cell death

Programmed cell death (apoptosis) is a complex phenomenon that is mediated in mammals mainly via the selective cleavage of intracellular proteins by the large family of cysteine aspartate protease caspases. Apoptosis is tightly regulated by the competitive effect of numerous proteins displaying either pro-apoptotic or anti-apoptotic activity. The ETS-family transcription factor FLI-1, frequently associated with malignant transformation, has been shown to display anti-apoptotic activity in several cell types including avian erythroblasts, mouse fibroblasts or lymphoid cells. We show here that apoptosis of murine preB leukemic cells is accompanied with the specific cleavage of FLI-1 by a caspase-like activity. We also demonstrate that the two isoforms of FLI-1 are indeed cleaved at three conserved sites by caspase 3 in vitro. The conservation of these cleavage sites among species suggests that the caspase cleavage of the anti-apoptotic transcription factor FLI-1 may represent a critical step to ensure irreversible cell death.

Cement gland-specific activation of the Xag1 promoter is regulated by co-operation of putative Ets and ATF/CREB transcription factors

The cement gland marks the extreme anterior ectoderm of the Xenopus embryo, and is determined through the overlap of several positional domains. In order to understand how these positional cues activate cement gland differentiation, the promoter of Xag1, a marker of cement gland differentiation, was analyzed. Previous studies have shown that Xag1 expression can be activated by the anterior-specific transcription factor Otx2, but that this activation is indirect. 102 bp of upstream genomic Xag1 sequence restricts reporter gene expression specifically to the cement gland. Within this region, putative binding sites for Ets and ATF/CREB transcription factors are both necessary and sufficient to drive cement gland-specific expression, and cooperate to do so. Furthermore, while the putative ATF/CREB factor is activated by Otx2, a factor acting through the putative Ets-binding site is not. These results suggest that Ets-like and ATF/CREB-like family members play a role in regulating Xag1 expression in the cement gland, through integration of Otx2 dependent and independent pathways.

Ets-1 regulates fli-1 expression in endothelial cells. Identification of ETS binding sites in the fli-1 gene promoter

To understand the role of the Ets-1 transcription factor during angiogenesis, we have overexpressed it in endothelial cells and analyzed the levels of expression of several candidate target genes involved in angiogenesis. The transcripts levels of the ETS transcription factor fli-1 are specifically up-regulated in endothelial cells, which overexpress Ets-1, but not in fibroblasts. Analysis of the promoter of the mouse fli-1 gene reveals that the 1-kb region that comprises the transcription starts and part of exon 1 is responsible for the response of the promoter to Ets-1. The -270/-41 fragment contains two known Spi-1-responding Ets binding sites (EBS), which are also necessary for the activation by Ets-1. In contrast to Spi-1, a third EBS is necessary for the full response of this promoter fragment to Ets-1. The rest of the promoter activity has been located in the -986/-505 region, where three active EBSs have been identified. Furthermore, endogenous Fli-1 was found to be bound to its own gene promoter and to be able to promote the transactivation of its gene. These results suggest that Ets-1 activates an auto-regulatory loop of expression of fli-1 in endothelial cells, a mechanism that could have significant implications for the endothelial cell fate.