Icsbp/Irf8 Cooperates with Tel to Regulate Fas Induced Apoptosis in Myeloid Leukemia.

Abstract 395

The interferon consensus sequence binding protein (Icsbp) is an interferon regulatory transcription factor (also known as Irf8) which functions as an anti-oncogene for chronic myeloid leukemia (CML) and acute myeloid leukemia (AML). In previous studies we used chromatin immunoprecipitation based high through put screening approaches to identify Icsbp target genes which explain these effects. The identified set of target genes was enriched for genes encoding proteins which regulate cytokine induced proliferation, beta catenin activity, and Fas induced apoptosis. In further investigations of the latter category, we found that Icsbp represses transcription of PTPN13; the gene encoding Fas associated phosphatase 1 (Fap1). Fap1 binds directly to the C-terminus of Fas, resulting in Fas-dephosphorylation and antagonizing Fas-induced apoptosis. We found that repression of PTPN13 by Icsbp increased during myelopoiesis, resulting in decreased Fap1 expression and increased sensitivity to Fas-induced apoptosis as differentiation proceeds. In the current studies, we investigated the mechanism for Icsbp induced PTPN13 repression. We identified a multi-protein repression complex which includes Icsbp, the ets protein Tel, and Hdac3 which binds to a PTPN13 cis element in a manner requiring recognition of the binding site by Tel. We found that histone deacetylase activity of Hdac3 was essential for PTPN13 repression by this complex. Tel was first identified due to participation in a leukemia associated chromosomal translocation with the gene encoding the PdgfRβ. This translocation is characteristic of Chronic Myelomonocytic Leukemia (CMMoL) and results in expression of a fusion protein with the protein-protein interaction domain of Tel and the kinase domain of PdgfRβ. Tel/PdgfRβ fusion protein exhibits constitutive tyrosine kinase activity due to autophosphorylation and activation of fusion protein dimers. This results in transduction of proliferative signals in the absence of cytokine. In this work we identify another function for the Tel/PdgfRβ fusion protein which may also contribute to the pathogenesis of leukemia. We find that Tel/PdgfRβ fusion protein interacts with wild type Tel protein in the cell (expressed by the allele not involved in the translocation), consistent with results of previous investigators. More importantly, we find that interaction with Tel/PdgfRβ renders Tel unavailable for binding to the PTPN13 cis element, which prevents assembly of the Tel/Icsbp/Hdac3 repression complex. The net result is increased PTPN13 promoter activity and Fap1 expression in cells expressing Tel/PdgfRβ fusion protein. This increase in Fap1 expression contributes to resistance to Fas-induced apoptosis in these cells. Therefore, our studies identify a previously unrecognized mechanism for apoptosis resistance in CMMoL and a potential target for therapeutic intervention in this myelodysplastic/ myeloproliferative disorder.