Defining the role of Ikaros as a tumor suppressor and repressor of Notch target gene expressionPublic Deposited
Inactivation of tumor suppressors genes, which encode regulatory proteins critical for maintaining normal cellular function, is a common occurrence in cancer. Ikaros is a hematopoietic-specific zinc finger protein that functions as a differentiation regulator and has properties of a tumor suppressor. Ikaros functions to regulate gene expression as a component of chromatin remodeling complexes, which serve to maintain and alter the accessibility of DNA, thereby facilitating or preventing gene expression. Ikaros null mice develop T cell leukemia with 100% entrance and deregulation of Ikaros expression has been observed in some forms of human leukemia, including acute lymphoblastic leukemia (ALL) and chronic myelocytic leukemia (CML). Using an ex vivo retroviral transduction system to restore Ikaros activity to an Ikaros null leukemia T cell line, JE131, we show that expression of Ikaros causes growth arrest at the G0/G1 phase of the cell cycle, an increase in expression of p27Kip1 and induction of T cell differentiation markers, such as CD4 and CD8. Restoration of Ikaros activity also results in a global increase in histone acetylation on histone H3. Our work has also focused on defining the interaction between Ikaros and Notch, which is also commonly deregulated in human leukemia, using the JE131 model system. We show that cleaved Notch1, intracellular notch (ICN), is expressed aberrantly in the JE131 Ikaros null T cell line. However, addition of g-secretase inhibitors, which prevents cleavage of the Notch1 receptor to generate ICN, fails to potently inhibit growth. This demonstrates that loss of Ikaros alone, not expression of a Notch oncogene, is required for the aberrant growth observed in Ikaros null cells. We have identified a Notch1 target gene, Hairy and enhancer of split homolog-1 (Hes1) as a potential target of Ikaros' repressive activity. It is expressed at high levels in JE131 cells, but is rapidly downregulated upon restoration of Ikaros expression. Using chromatin immunoprecipitation, Ikaros is shown binding directly to the Hes1 promoter. Our work suggests that Ikaros functions as a tumor suppressor in part through regulation of Notch target gene expression.