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Overexpression of proto-oncogene c-jun and constitutive activation of the Jun N-terminal kinase (JNK) signaling pathway have been implicated in the leukemic transformation process. However, c-jun expression and the role of the JNK signaling pathway have not been investigated in primary acute myeloid leukemia (AML) cells with frequently observed balanced rearrangements such as t(8;21). In the present study, we report elevated c-jun mRNA expression in AML patient bone marrow cells with t(8;21), t(15;17) or inv(16), and a high correlation in mRNA expression levels of AML1-ETO and c-jun within t(8;21)-positive AML patient cells. In myeloid U937 cells, c-jun mRNA and protein expression increase upon inducible expression of AML1-ETO. AML1-ETO transactivates the human c-jun promoter through the proximal activator protein (AP-1) site by activating the JNK pathway. Overexpression of JNK-inhibitor JIP-1 and chemical JNK inhibitors reduce the transactivation capacity of AML1-ETO on the c-jun promoter and the proapoptotic function of AML1-ETO in U937 cells. An autocrine mechanism involving granulocyte-colony stimulating factor (G-CSF) and G-CSF receptor (G-CSF-R) might participate in AML1-ETO mediated JNK-signaling, because AML1-ETO induces G-CSF and G-CSF-R expression, and G-CSF-R-neutralizing antibodies reduce AML1-ETO-induced JNK phosphorylation. These data suggest a model in which AML1-ETO induces proto-oncogene c-jun expression via the proximal AP-1 site of the c-jun promoter in a JNK-dependent manner.

The transcription factor PU.1 plays a pivotal role in normal myeloid differentiation. PU.1(-/-) mice exhibit a complete block in myeloid differentiation. Heterozygous PU.1 mutations were reported in some patients with acute myeloid leukemia (AML), but not in AML with translocation t(8;21), which gives rise to the fusion gene AML1-ETO. Here we report a negative functional impact of AML1-ETO on the transcriptional activity of PU.1. AML1-ETO physically binds to PU.1 in t(8;21)(+) Kasumi-1 cells. AML1-ETO binds to the beta(3)beta(4) region in the DNA-binding domain of PU.1 and displaces the coactivator c-Jun from PU.1, thus down-regulating the transcriptional activity of PU.1. This physical interaction of AML1-ETO and PU.1 did not abolish the DNA-binding capacity of PU.1. AML1-ETO down-regulates the transactivation capacity of PU.1 in myeloid U937 cells, and the expression levels of PU.1 target genes in AML French-American-British (FAB) subtype M2 patients with t(8;21) were lower than in patients without t(8;21). Conditional expression of AML1-ETO causes proliferation in mouse bone marrow cells and inhibits antiproliferative function of PU.1. Overexpression of PU.1, however, differentiates AML1-ETO-expressing Kasumi-1 cells to the monocytic lineage. Thus, the function of PU.1 is down-regulated by AML1-ETO in t(8;21) myeloid leukemia, whereas overexpression of PU.1 restores normal differentiation.

Recent results indicate that interactions of transcription factors with other nuclear proteins play an important role in stem cell development, lineage commitment and differentiation in the haematopoietic system, and the pathogenesis of myeloid leukaemias. High-throughput proteomics by mass spectrometric analysis of gel-separated proteins can identify multi-protein complexes and changes in the expression of multiple proteins simultaneously. This review describes an application of proteomic methods (2D gel electrophoresis (GE) and mass spectrometry (MS)), which can be used to identify regulated protein targets of transcription factors important in myeloid differentiation and leukaemia. This global high-throughput functional proteomics approach could lead to new insights into the network of protein-protein interactions and target proteins involved in myeloid stem cell development and leukaemia as well as provide new targets for rational pathogenesis-based therapies of leukaemia and cancer.

The article discusses the application of the micro-RNA molecule miR-22 in Hematopoiesis. It discusses its application with regard to it being an oncogenic tumor suppressor or a tumor-suppressive oncogene. Topics discussed include research studies with regard to the matter, discoveries of micro-RNAs and approval of drugs based on micro-RNA.

CCAAT/Enhancer Binding Protein Alpha (C/EBPα) is a key transcription factor involved in the adipocyte differentiation. Here for the first time we demonstrate that E6AP, an E3 ubiquitin ligase inhibits adipocyte differentiation in 3T3-L1 cells as revealed by reduced lipid staining with oil red. Knock down of E6AP in mouse 3T3L1 preadipocytes is sufficient to convert them to adipocytes independent of external hormonal induction. C/EBPα protein level is drastically increased in E6AP deficient 3T3L1 preadipocytes while inverse is observed when wild type E6AP is over expressed. We show that transient transfection of wild type E6AP downregulates C/EBPα protein expression in a dose dependent manner while catalytically inactive E6AP-C843A rather stabilizes it. In addition, wild type E6AP inhibits expression of proadipogenic genes while E6AP-C843A enhances them. More importantly, overexpression of E6AP-C843A in mesenchymal progenitor cells promotes accumulation of lipid droplets while there is drastically reduced lipid droplet formation when E6AP is over expressed. Taken together, our finding suggests that E6AP may negatively control adipogenesis by inhibiting C/EBPα expression by targeting it to ubiquitin-proteasome pathway for degradation. [ABSTRACT FROM AUTHOR]

This article discusses a study which identified the haematopoietic transcription factors PU.1 and CEBPA as transcriptional regulators of the actin-binding protein CORO1A in patients with acute promyelocytic leukemia (APL) and acute myeloid leukemia (AML). Researchers found that significantly lower CORO1A levels were found in APL and in CEBPA-mutated AML patient samples. They also learned that inhibiting CORO1A markedly impaired neutrophil differentiation of APL cells.

Transcription factor C/EBPa induces normal myeloid differentiation, inactivation of C/EBPa leads to a differentiation block in acute myeloid leukemias (AML), and overexpression of C/EBPa results in AML growth arrest and differentiation. Recent reports suggest that C/EBPa is activated or inactivated via protein-protein interactions. We previously reported that C/EBPa needs to inactivate the proto-oncogene c-Jun via leucine zipper domain interaction in order to induce granulocytic differentiation. We, therefore, hypothesized that c-Jun expression might be elevated in AML and subsequently inactivate C/EBPa. In fact, compared to normal bone marrow mononuclear cells, c-Jun expression is increased in AML patient samples (Affymetrix expression microarray analysis, n=166). c-Jun binds to C/EBPa via the leucine zipper domains and prevents C/EBPa from DNA binding. Inactivation of C/EBPa by c-Jun is necessary for c-Jun to induce proliferation because c-Jun-induced proliferation can be prevented by ectopic overexpression of C/EBPa. The dominant-negative 30-kDa C/EBPa protein, found in AML, fails to downregulate c-Jun mRNA expression in AML patient samples. Thus, our data suggest a model for AML in which c-Jun promotes proliferation and prevents differentiation by inhibiting C/EBPa DNA binding via leucine zipper domain interaction. It might depend on the expression levels of C/EBPa and c-Jun, if inhibition of C/EBPa by c-Jun or if inhibition of c-Jun by C/EBPa is more predominant: proliferation versus differentiation; AML versus normal myeloid development.Oncogene (2003) 22, 4760-4764. doi:10.1038/sj.onc.1206664 [ABSTRACT FROM AUTHOR]

The transcription factor CCAAT/enhancer binding protein α, or C/EBPα, encoded by the CEBPA gene, is crucial for the differentiation of granulocytes. Conditional expression of C/EBPα triggers neutrophilic differentiation, and Cebpa knockout mice exhibit an early block in maturation. Dominant-negative mutations of CEBPA have been found in some patients with acute myeloid leukemia (AML), but not in AML with the t(8;21) translocation which gives rise to the fusion gene RUNX1?CBF2T1 (also known as AML1?ETO) encoding the AML1?ETO fusion protein. RUNX1?CBF2T1 positive-AML blasts had eight-fold lower CEBPA RNA levels and undetectable C/EBPα protein levels compared with other subgroups of AML patients. Conditional expression of RUNX1?CBF2T1 in U937 cells downregulated CEBPA mRNA, protein and DNA binding activity. AML1?ETO appears to suppress C/EBPα expression indirectly by inhibiting positive autoregulation of the CEBPA promoter. Conditional expression of C/EBPα in AML1?ETO-positive Kasumi-1 cells results in neutrophilic differentiation. We suggest that restoring C/EBPα expression will have therapeutic implications in RUNX1?CBF2T1-positive leukemias. [ABSTRACT FROM AUTHOR]

The transcription factor CCAAT enhancer binding protein a (C/EBPα) is a master regulator in granulopoiesis and is frequently disrupted in acute myeloid leukemia (AML). We have previously shown that C/EBPa exerts its effects by regulating microRNAs (miRs) such as miR-223 and miR-34a. Here, we confirm miR-30c as a novel important target of C/EBPα during granulopoiesis. Thus, wild-type C/EBPα-p42 directly upregulates miR-30c expression, whereas C/EBPα-p30, found in AML, does not. miR-30c is downregulated in AML, especially in normal karyotype AML patients with CEBPA mutations. An induced C/EBPα knockout in mice leads to a significant downregulation of miR-30c expression in bone marrow cells. We identified NOTCH1 as a direct target of miR-30c. Finally, a block of miR- 30c prevents C/EBPα-induced downregulation of Notchl protein and leads to a reduced CD11b expression in myeloid differentiation. Our study presents the first evidence that C/EBPα, miR-30c, and Notchl together play a critical role in granulocytic differentiation and AML, and particularly in AML with CEBPA mutations. These data reveal the importance of deregulated miRNA expression in leukemia and may provide novel biomarkers and therapeutic targets in AML. [ABSTRACT FROM AUTHOR]