Purification of human recombinant GATA-1 from bacteria: implication for protein-protein interaction studies

GATA-1 is a key regulator of terminal erythroid differentiation in mammals and birds. The structural and biochemical studies of human GATA-1 (hGATA-1) are limited by the difficulty of its purification in a sufficient amount. Here we describe the procedure for obtaining pure bacterial recombinant hGATA-1 in an active functional state. We demonstrate that this protein may be successfully used for preparing an affinity column, producing GATA-1-specific rabbit polyclonal antibodies, and studying DNA-protein and protein-protein interactions.

Negative and translation termination-dependent positive control of FLI-1 protein synthesis by conserved overlapping 5' upstream open reading frames in Fli-1 mRNA

The proto-oncogene Fli-1 encodes a transcription factor of the ets family whose overexpression is associated with multiple virally induced leukemias in mouse, inhibits murine and avian erythroid cell differentiation, and induces drastic perturbations of early development in Xenopus. This study demonstrates the surprisingly sophisticated regulation of Fli-1 mRNA translation. We establish that two FLI-1 protein isoforms (of 51 and 48 kDa) detected by Western blotting in vivo are synthesized by alternative translation initiation through the use of two highly conserved in-frame initiation codons, AUG +1 and AUG +100. Furthermore, we show that the synthesis of these two FLI-1 isoforms is regulated by two short overlapping 5' upstream open reading frames (uORF) beginning at two highly conserved upstream initiation codons, AUG -41 and GUG -37, and terminating at two highly conserved stop codons, UGA +35 and UAA +15. The mutational analysis of these two 5' uORF revealed that each of them negatively regulates FLI-1 protein synthesis by precluding cap-dependent scanning to the 48- and 51-kDa AUG codons. Simultaneously, the translation termination of the two 5' uORF appears to enhance 48-kDa protein synthesis, by allowing downstream reinitiation at the 48-kDa AUG codon, and 51-kDa protein synthesis, by allowing scanning ribosomes to pile up and consequently allowing upstream initiation at the 51-kDa AUG codon. To our knowledge, this is the first example of a cellular mRNA displaying overlapping 5' uORF whose translation termination appears to be involved in the positive control of translation initiation at both downstream and upstream initiation codons.

Spi-1/PU.1 is a positive regulator of the Fli-1 gene involved in inhibition of erythroid differentiation in friend erythroleukemic cell lines

Spi-1/PU.1 and Fli-1 are two members of the ETS family of transcription factors whose expression is deregulated by proviral insertion in most erythroleukemic cell lines induced by the spleen focus-forming virus (SFFV) and Friend murine leukemia virus (F-MuLV) components of the Friend viral complex, respectively. In this study, we present evidence that transcription of the Fli-1 gene is positively regulated by Spi-1/PU.1 in SFFV-transformed cell lines: (i) all SFFV-transformed cell lines expressing Spi-1/PU.1 are characterized by a specific pattern of Fli-1 gene transcripts initiated in the -200 region instead of position -400 as reported for F-MuLV-transformed cell lines; (ii) these Fli-1 transcripts initiated in the -200 region are downregulated in parallel with that of Spi-1/PU.1 during hexamethylenebisacetamide (HMBA) induced differentiation; and (iii) Fli-1 transcription is upregulated in SFFV cells lines following stable transfection of a Spi-1/PU.1 expression vector. Furthermore, we found by transient transfection assays that the -270/-41 region of the Fli-1 gene displays promoter activity which is transactivated by Spi-1/PU.1. This promoter is strictly dependent on the integrity of two highly conserved ETS DNA binding sites that bind the Spi-1/PU.1 protein in vitro. Finally, we show that transfection of constitutive or inducible Fli-1 expression vectors in SFFV-transformed cells inhibits their erythroid differentiation induced by HMBA. Overall, these data indicate that Fli-1 is a target gene of the Spi-1/PU.1 transcription factor in SFFV-transformed cell lines. We further suggest that deregulated synthesis of Fli-1 may trigger a common mechanism contributing to erythroleukemia induced by either SFFV or F-MuLV.

Production and characterization of mouse monoclonal antibodies to wild-type and oncogenic FLI-1 proteins

Mouse monoclonal antibodies were raised against the C-terminal domain of human FLI-1, a member of the ETS family of transcription factors which is involved in various murine and human malignancies. This FLI-1 specific domain is included in the fusion product EWS-FLI-1, an oncogenic variant of FLI-1 expressed in Ewing tumors. Antibodies were screened first by enzyme-linked immunosorbent assay onto recombinant FLI-1-coated plates. Positive clones were then tested for their ability to immunoprecipitate over-expressed EWS-FLI-1 protein. Three monoclonal antibodies were selected and further characterized. One of them, termed 7.3 MoAb, was shown to react with FLI-1 and EWS-FLI-1 in immunoblotting, immunoprecipitation, and immunofluorescence experiments. With all three methods, this antibody not only enabled the detection of overexpressed proteins but also more interestingly, that of endogenously expressed proteins. Furthermore, the 7.3 MoAb can specifically inhibit the interaction of FLI-1 with its DNA-binding site as shown by electrophoretic mobility shift assay. The 7.3 MoAb appears to be specific for FLI-1 because it does not react with ERG, the ETS family member most closely related to FLI-1. This antibody should be a useful tool in the diagnostic evaluation of Ewing tumors and should permit biochemical analyses to study the function of the wild-type FLI-1 protein and of the EWS-FLI-1 fusion protein.

Thrombopoietin-induced expression of the glycoprotein IIb gene involves the transcription factor PU.1/Spi-1 in UT7-Mpl cells

Thrombopoietin (TPO) is the major regulator of proliferation and differentiation of megakaryocytes and their progenitors. These actions can be reproduced in the human megakaryoblastic cell line UT7 into which the murine TPO receptor, c-Mpl, was introduced. In these cells, TPO enhanced the expression of the specific megakaryocytic marker integrin glycoprotein (GP) IIb-IIIa while decreasing the expression of erythroid genes (Porteu, F., Rouyez, M. -C., Cocault, L., Benit, L., Charon, M., Picard, F., Gisselbrecht, S. , Souyri, M., and Dusanter-Fourt, I. (1996) Mol. Cell. Biol. 16, 2473-2482). We have now analyzed the effect of TPO on the transcriptional activity of the GPIIb promoter in these cells. Using transient transfection assays of a series of human GPIIb promoter fragments, we delineated a TPO-responsive element within the previously reported enhancer region of the promoter. Although this enhancer included GATA- and Ets-binding sites (EBSs), we found that only EBS -514 was important for TPO response. We identified PU. 1/Spi-1 as the endogenous Ets transcription factor that strongly and preferentially interacted with this enhancer EBS. This factor did not interact with other proximal EBSs in the GPIIb promoter. We next showed that TPO induced a strong and selective increase of PU. 1/Spi-1 expression and DNA binding activity in UT7-Mpl cells. In contrast, TPO did not affect the expression of Ets-1/2 while weakly increasing the levels of Fli-1. Overexpression of PU.1/Spi-1 was further shown to enhance GPIIb promoter activity in the absence and presence of TPO. Overall, our data indicated that, in UT7-Mpl cells, TPO increased the transcriptional activity of a GPIIb gene in part due to an enhanced expression of an unexpected transcription factor, the Ets family PU.1/Spi-1 factor. To our knowledge, this is the first evidence of a role for the PU.1/Spi-1 factor in the regulation of megakaryocytic genes.

Activation of erythroid-specific promoters during anthracycline-induced differentiation of K562 cells

Anthracycline antitumor drugs such as aclacinomycin (ACM) and doxorubicin (DOX) used in subtoxic concentrations induce erythroid differentiation of the erythroleukemic cell line K562. To elucidate the possible role of erythroid genes of the erythropoietin receptor (EpoR) and the transcription factor GATA-1 in this effect, the regulatory regions of the above genes and human epsilon- and gamma-globin and porphobilinogen deaminase (PBGD) genes were fused to the firefly luciferase gene. The resulting reporter constructs were tested in a transfection assay of the erythroleukemic cell line K562 stimulated to differentiate by treatment with the anthracycline drugs ACM and DOX or hemin (HEM). The results showed activation of the tested promoters after cell treatment with ACM, but not with DOX or HEM. In contrast to the mouse EpoR gene promoter, the activity of the human EpoR gene promoter (-659/-60) in the reporter construct was not modified by addition of the first intron sequence. In ACM-treated K562 cells, EpoR gene promoter activity completely correlated with EpoR and GATA-1 mRNA levels and the degree of erythroid maturation. In addition, ACM strongly activated the erythroid gene promoters that contain GATA binding sites. Nevertheless, less activation was also observed for the GATA-1 gene promoter (-312/-31) lacking any known GATA binding sites. Insertion of the GATA-1 gene enhancer with two canonic GATA binding sites, stimulated the ACM activation effect for EpoR and GATA-1 promoter-containing constructs. Mutation of the enhancer GATA binding sites abolished this effect. All the regulatory regions tested (except gamma-globin promoter) were completely inactive in nonerythroid COS7 cells. These data indicate that (1) two structurally different anthracycline analogues, DOX and ACM, differ in their differentiation mechanisms, and (2) ACM switches on the erythroid program of K562 cells, at least in part because of interaction with a factor(s) that recognizes the GATA binding sites in the promoter region of erythroid genes leading to their activation.