ICSBP Activates Transcription of the Gene Encoding Neurofibromin 1 in Differentiating Myeloid Cells.

Previous studies in murine models and human myeloid malignancies indicate that the interferon consensus sequence binding protein (ICSBP) functions as a leukemia tumor suppressor. Previously identified ICSBP target genes include genes encoding gp91phox and p67phox (components of the phagocyte respiratory burst oxidase), the Toll like receptor 4, and other genes involved in mediating the phagocyte inflammatory response. Transcription of these target genes involves interaction of ICSBP with PU.1 and/or interferon regulatory factor 1 (IRF1), and is restricted to mature phagocytic cells. These results predict that ICSBP-deficiency would be associated with terminal differentiation block and immune dysfunction. However, the phenotype of ICSBP −/− mice is dominated by development of a myeloproliferative disorder which evolves to acute myeloid leukemia. Therefore, we used the chromatin immuno-precipitation technique to identify novel ICSBP target genes which might be involved in the ICSBP tumor suppressor effect. We identified the gene encoding neurofibromin 1 (the NF1 gene) as an ICSBP-target-gene. We found that ICSBP activates NF1 transcription during cytokine induced differentiation of myeloid progenitor cells. Because Nf1 protein has ras-GAP activity, increased Nf1-expression down regulates cytokine-induced ras activity, resulting in proliferation arrest during myeloid cell differentiation. In the current studies, we investigate the mechanism by which ICSBP activates NF1-transcription. We find that ICSBP is part of a multi protein complex that binds to and activates a cis element in the NF1 gene. This complex also includes the ets protein PU.1 and interferon regulatory factor 2 (IRF2). We find that tyrosine phosphorylation of ICSBP in response to hematopoietic cytokines is required for assembly of this transcriptional activation complex and therefore NF1-transcription. Identification of NF1 as an ICSBP target gene represents a mechanism for ICSBP-leukemia-suppression. These studies also provide the first indication of the importance of NF1-transcriptional regulation to the control of cytokine-induced proliferation in differentiating myeloid cells.