IRF8 Regulates Acid Ceramidase Expression to Mediate Apoptosis and Suppresses Myelogeneous Leukemia

Abstract

IFN regulatory factor 8 (IRF8) is a key transcription factor for myeloid cell differentiation and its expression is frequently lost in hematopoietic cells of human myeloid leukemia patients. IRF8-deficient mice exhibit uncontrolled clonal expansion of undifferentiated myeloid cells that can progress to a fatal blast crisis, thereby resembling human chronic myelogeneous leukemia (CML). Therefore, IRF8 is a myeloid leukemia suppressor. Whereas the understanding of IRF8 function in CML has recently improved, the molecular mechanisms underlying IRF8 function in CML are still largely unknown. In this study, we identified acid ceramidase (ACDase) as a general transcription target of IRF8. We demonstrated that IRF8 expression is regulated by IRF8 promoter DNA methylation in myeloid leukemia cells. Restoration of IRF8 expression repressed A-CDase expression, resulting in C16 ceramide accumulation and increased sensitivity of CML cells to FasL-induced apoptosis. In myeloid cells derived from IRF8-deficient mice, A-CDase protein level was dramatically increased. Furthermore, we demonstrated that IRF8 directly binds to the A-CDase promoter. At the functional level, inhibition of A-CDase activity, silencing A-CDase expression, or application of exogenous C16 ceramide sensitized CML cells to FasL-induced apoptosis, whereas overexpression of A-CDase decreased CML cells' sensitivity to FasL-induced apoptosis. Consequently, restoration of IRF8 expression suppressed CML development in vivo at least partially through a Fas-dependent mechanism. In summary, our findings determine the mechanism of IRF8 downregulation in CML cells and they determine a primary pathway of resistance to Fasmediated apoptosis and disease progression. Cancer Res; 71(8); 2882-91. Ó2011 AACR.

Regulation of the Interferon regulatory factor-8 (IRF-8) Tumor Suppressor Gene by the Signal Transducer and Activator of Transcription 5 (STAT5) Transcription Factor in Chronic Myeloid Leukemia

“…In murine models, IRF-8 inhibits BCR-ABL ϩ cell proliferation by inducing expression of genes such as Cdkn4b, Blimp-1, and Mets-1 (36,37). IRF-8 also promotes apoptosis by repressing transcription of anti-apoptotic genes such as BCL2 and ASAH1 (21,38). In addition, IRF-8 suppresses WNT/␤-catenin signaling, which has been implicated in the development of CML (39,40).…”

“…An interaction between WT1 and STAT3 has been reported; it is possible that WT1 and STAT5 could similarly act to regulate IRF-8 expression (42). Alternatively, Hu et al (38) have demonstrated that the IRF-8 promoter is methylated in CML cell lines and primary blasts, resulting in decreased expression. Thus, FIGURE 5.…”

IRF8 acts in lineage-committed rather than oligopotent progenitors to control neutrophil vs monocyte production

“…10,27 This mechanism is defective in CML leukemic cells that exhibit reduced IRF8 expression, rendering them resistant to apoptosis. 10 Our observation that IRF8-deficient mice accumulate CD11b In conclusion, our ability to separate oligopotent GMPs from their lineage-committed progeny has revealed that IRF8 regulates neutrophil and monocyte production independently in lineage-committed progenitors (GPs and MPs), rather than acting in oligopotent GMPs to instruct monocytic fate. This progenitor isolation approach will also ) were isolated from bone marrow and their morphology was assessed using May-Grünwald Giemsa staining.…”

“…[1][2][3][4][5][6][7][8][9] In humans, IRF8 mutations are associated with immunodeficiency, and decreased IRF8 expression has been observed in both acute myelogenous leukemia (AML) and CML patients. [10][11][12] Previous studies have shown that IRF8 promotes monocyte and DC production but limits neutrophil production. 1,4,[6][7][8][13][14][15] Thus, it is widely thought to act in oligopotent progenitors (common myeloid progenitors [CMPs], and/or granulocyte-monocyte progenitors [GMPs]) to instruct the adoption of monocyte/DC fate and suppress granulocyte fate.…”

Regulation of myelopoiesis by the transcription factor IRF8

“…It was reported that suppressing IRF8 expression by BCR-ABL involves STAT5 and Irf8 promoter DNA methylation in primary CML cells and in BCR-ABL-positive cell lines [75,76]. On the other hand, in another study that used BCR-ABL-transduced mouse bone marrow progenitor cells, inhibitors of either STAT5 or DNA methylation failed to rescue Irf8 expression [74].…”