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The family of fibroblast growth factor receptors (FGFRs) is encoded by four distinct genes. FGFR1 and FGFR4 are both expressed during myogenesis, but whereas the function of FGFR1 in myoblast proliferation has been documented, the role of FGFR4 remains unknown. Here we report on a new splice form of FGFR4 cloned from primary cultures of mouse satellite cells. This form, named FGFR4(−16), lacks the entire exon 16, resulting in a deletion within the FGFR kinase domain. Expression of FGFR4(−16) coincided with that of wildtype FGFR4 in all FGFR4-expressing tissues examined. Moreover, expression of both FGFR4 forms correlated with the onset of myogenic differentiation, as determined in mouse C2C12 cells and in the inducible myogenic system of 10T½-MyoD-ER cell line. Both endogenous and overexpressed forms of FGFR4 exhibited N-glycosylation. In contrast to FGFR1, induced homodimerization of FGFR4 proteins did not result in receptor tyrosine phosphorylation. Surprisingly, coexpression of FGFR4 forms and a chimeric FGFR1 protein resulted in FGFR4 tyrosine phosphorylation, raising the possibility that FGFR4 phosphorylation might be enabled by a heterologous tyrosine kinase activity. Collectively, the present study reveals novel characteristics of mouse FGFR4 gene products and delineates their expression pattern during myogenesis. Our findings suggest that FGFR4 functions in a distinctly different manner than the prototype FGFR during myogenic differentiation.

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3606 Poster Board III-542 The Mpl proto-oncogene plays a basic role in megakaryocytic development and platelet production as well as performs substantial function in hematopoietic stem cell (HSC) homeostasis and self-renewal. Several, mutations in Mpl have been shown to be associated with the myeloproliferative disorders. In an attempt to identify cellular factors that could cooperate with Mpl signaling leading to increases in cell survival and proliferation, we performed retroviral insertional mutagenesis using MSCV based vector coding for a drug dependent, dimerizable, fusion protein which contains the cytoplasmic domain of Mpl. Such vector construct can interrupt gene structure through insertion, and has an intact long terminal repeats which can activate adjacent genes. We used that vector construct to transduce the human leukemia cell line K562. After transduction, the endogenous transforming BCR-ABL kinase was blocked using Imatinib, and cells dependent on Mpl signaling were selected by the addition of dimerizing drug, AP20187. In the absence of Mpl signaling the cells underwent erythroid differentiation and died. Cells that acquired proliferative advantage and were dependent on Mpl function were expected to point to mutations synergistic with Mpl signaling. Cloning of retroviral integration sites (RIS) from the selected populations would allow to identify overrepresented clones. Indeed, overrepresented RIS clones were found in several independent transductions. Such overrepresented clones were not observed in an independent experiment where RIS were cloned from cells transduced with the same vector and sorted but not selected on Imatinib and AP20187. RIS from the overrepresented clones pointed to candidate Mpl cooperating genes. AGTRAP a member of renin-angiotensin system (RAS) was identified among the candidate genes. Multiple lines of evidence implicate RAS in hematopoietic cell proliferation and differentiation. RAS components, including renin, angiotensin-converting enzyme(ACE), angiotensin II, angiotensin type 1 and type 2 receptors (AT1 andAT2), and tetrapeptide N-Acetyl-Ser-Asp-Lys-Pro (AcSDKP an inhibitory hematopoietic peptide) have been isolated in the bone marrow as well as in circulating hematopoietic cells. Angiotensin II was found to interact with the AT1 receptor to stimulate erythroid differentiation and increase hematopoietic progenitor cell proliferation. Moreover, stimulation of AT1/AT2 receptors was found to exert an stimulatory/inhibitory action on JAK-STAT pathway, which is directly linked to physiologic functions of erythropoietin, thrombopoietin and other cytokines. Finally, ACE was implicated in enhancing the recruitment of primitive stem cells into the S-phase by degrading AcSDKP. We have tested the AGTRAP function in hematopoietic cells. For that purpose we overexpressed AGTRAP protein in the K562 cells and in K562 cells driven by Mpl signaling. AGTRAP overexpression stimulated the growth of plain K562 cells decreasing the cells division time from 16 to 12 hours. Importantly, it synergized with Mpl signaling to promote the proliferation rate of Mpl dependent K562 cells (expressing the dimerizable construct of Mpl activated with AP20187 while the transforming Bcr-Abl protein was inactivated with Imatinib) decreasing the cells division time from 13 to 9.6 hours. The survival of Mpl dependent K562 cells over-expressing AGTRAP was still dependent on Mpl signaling and on Jak2 function as tested by withdrawal of AG20187 and by treatment with Jak2 inhibitor AG490. Overexpression of AGTRAP protein over-sensitized the UT7/TPO cells to treatment with recombinant human megakaryocyte growth and development factor (rhMGDF), increasing growth rate of the cells at low rhMGDF concentrations. The cells proliferation rate was increased by 22.5% and 39.3% at 1/100 and 1/1000 of optimal concentrations of rhMGDF respectively. There was no growth advantage from AGTRAP expression at optimal concentration of rhMGDF, however in the presence of AGTRAP the UT7/TPO cells were able to proliferate in the absence of rhMGDF altogether. Surprisingly, the overexpression of AGTRAP negatively regulated the growth of HEL cells, known to carry a homozygous transforming mutation of Jak2V617F. In conclusion our data implicate the RAS member AGTRAP in hematopoietic cell proliferation and survival and point to its synergistic effect with the proliferation promoting function of Mpl. Disclosures: No relevant conflicts of interest to declare.

In a screen for binding partners of the Epstein-Barr virus transformation-related protein EBNA2, we cloned a novel, evolutionarily conserved protein showing similarity to the Drosophila Parallel Sister Chromatids Protein (PASC). We have named this protein bFriend of EBNA2Q (FOE). Human FOE encodes a protein of 1227 amino acids with a functional bipartite nuclear localization signal, an arginine-rich motif, a putative nuclear export signal as well as with three highly acidic regions and a predicted coiled-coil domain. FOE and EBNA2 coimmunoprecipitate from lymphocyte nuclear extracts. RNA and protein blots show that FOE is expressed in all human tissues. FOE is a nuclear protein with the bulk of the protein associated with the insoluble nuclear fraction biochemically defined as the nuclear matrix. Indirect immunofluorescence and dynamic imaging studies suggest that FOE associates with transcriptionally active nuclear subregions in interphase cells and concentrates at the ends of formed chromosomes during mitosis.

Glycoprotein (GP) IX is a subunit of the von Willebrand receptor, GPIb-V-IX, which mediates adhesion of platelets to the subendothelium of damaged blood vessels. Previous characterization of the GPIX promoter identified a functional Ets site that, when disrupted, reduced promoter activity. However, the Ets protein(s) that regulated GPIX promoter expression was unknown. In this study, transient cotransfection of several GPIX promoter/reporter constructs into 293T kidney fibroblasts with a Fli-1 expression vector shows that the oncogenic protein Fli-1 can transactivate the GPIX promoter when an intact GPIX Ets site is present. In addition, Fli-1 binding of the GPIX Ets site was identified in antibody supershift experiments in nuclear extracts derived from hematopoietic human erythroleukemia cells. Comparative studies showed that Fli-1 was also able to transactivate the GPIb and, to a lesser extent, the GPIIb promoter. Immunoblot analysis identified Fli-1 protein in lysates derived from platelets. In addition, expression of Fli-1 was identified immunohistochemically in megakaryocytes derived from CD34+ cells treated with the megakaryocyte differentiation and proliferation factor, thrombopoietin. These results suggest that Fli-1 is likely to regulate lineage-specific genes during megakaryocytopoiesis.

Glycoprotein (GP) IX is a subunit of the von Willebrand receptor, GPIb-V-IX, which mediates adhesion of platelets to the subendothelium of damaged blood vessels. Previous characterization of the GPIX promoter identified a functional Ets site that, when disrupted, reduced promoter activity. However, the Ets protein(s) that regulated GPIX promoter expression was unknown. In this study, transient cotransfection of several GPIX promoter/reporter constructs into 293T kidney fibroblasts with a Fli-1 expression vector shows that the oncogenic protein Fli-1 can transactivate the GPIX promoter when an intact GPIX Ets site is present. In addition, Fli-1 binding of the GPIX Ets site was identified in antibody supershift experiments in nuclear extracts derived from hematopoietic human erythroleukemia cells. Comparative studies showed that Fli-1 was also able to transactivate the GPIb and, to a lesser extent, the GPIIb promoter. Immunoblot analysis identified Fli-1 protein in lysates derived from platelets. In addition, expression of Fli-1 was identified immunohistochemically in megakaryocytes derived from CD34+ cells treated with the megakaryocyte differentiation and proliferation factor, thrombopoietin. These results suggest that Fli-1 is likely to regulate lineage-specific genes during megakaryocytopoiesis.