The interferon consensus sequence binding protein is a member of the interferon regulatory factor family of transcription factors (referred to as Icsbp or Irf8). The first described functions for Icsbp involved regulation of phagocyte and B-cell effector genes, including genes encoding components of the phagocyte NADPH-oxidase, Toll-like receptors and interleukin receptors. However, subsequent studies in murine models and human disease indicated that Icsbp also functions as a myeloid leukemia suppressor. For example, decreased Icsbp expression is found in chronic myeloid leukemia (CML) in association with uncontrolled disease, drug resistance and progression to blast crisis (BC). Decreased Icsbp expression is also found in the bone marrow of subjects with some subtypes of acute myeloid leukemia (AML). Consistent with this clinical correlative data, IRF8−/− mice exhibit a myeloproliferative neoplasm that is similar to CML and progresses to BC over time. However, the mechanism for decreased Icsbp expression in leukemia is not known, although preliminary studies indicate that DNA-methylation of the IRF8 locus is not altered. Therefore, in these studies, we investigate the effects of Bcr-abl on IRF8 transcription. This is clinically relevant, because previous studies in our laboratory identified a set of Icsbp-target-genes that contribute to the pathogenesis of CML. We find that Bcr-abl decreases expression of Icsbp mRNA and protein in a kinase dependent manner. Since it is unlikely that Bcr-abl directly binds to the promoter to regulate gene transcription, we hypothesized that Bcr-abl regulates IRF8 through an intermediary transcription factor. In this study, we determine that Stat5 negatively regulates IRF8 transcription through a proximal promoter cis-element. We also find that Stat5 repression activity is necessary for Bcr-abl dependent regulation of IRF8. Bcr-abl is known to phosphorylate and activate Stat5 in CML. In our studies, we find that Stat5 protein (but not mRNA) is also increased in Bcr-abl+ cells. Stat5 is a known substrate for calpain; a serine protease. We previously demonstrated that Icsbp regulates calpain protease activity through repression of the gene encoding Gas2; an endogenous calpain inhibitor. Consistent with this, our current studies demonstrate that Stat5 protein stability is increased in Bcr-abl+ cells in an Icsbp/Gas2/calpain-dependent manner. These results identify novel mechanisms by which Bcr-abl-kinase activity controls a positive feedback loop that leads to decreased Icsbp expression and stabilization of Stat5 protein. These studies suggest that targeting Gas2/calpain might be a novel therapeutic approach to CML.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.