Regulation of c-fos gene transcription and myeloid cell differentiation by acute myeloid leukemia 1 and acute myeloid leukemia-MTG8, a chimeric leukemogenic derivative of acute myeloid leukemia 1

Stable depletion of RUNX1-ETO in Kasumi-1 cells induces expression and enhanced proteolytic activity of Cathepsin G and Neutrophil Elastase

The oncogenic fusion protein RUNX1-ETO is a product of the t(8;21) translocation and consists of the hematopoietic transcriptional master regulator RUNX1 and the repressor ETO. RUNX1-ETO is found in 10–15% of acute myeloid leukemia and interferes with the expression of genes that are essential for myeloid differentiation. The neutrophil serine protease Cathepsin G is one of the genes suppressed by RUNX1-ETO, but little is known about its impact on the regulation of other lysosomal proteases. By lentiviral transduction of the t(8;21) positive cell line Kasumi-1 with an RUNX1-ETO specific shRNA, we analyzed long-term effects of stable RUNX1-ETO silencing on cellular phenotypes and target gene expression. Stable anti RUNX1-ETO RNAi reduces both proliferation and apoptosis in Kasumi-1 cells. In addition, long-term knockdown of RUNX1-ETO leads to an upregulation of proteolytic activity in Kasumi-1 cells, which may be released in vitro upon cell lysis leading to massive degradation of cellular proteins. We therefore propose that protein expression data of RUNX1-ETO-silenced Kasumi-1 cells must be analyzed with caution, as cell lysis conditions can heavily influence the results of studies on protein expression. Next, a mass spectrometry-based approach was used to identify protease cleavage patterns in RUNX1-ETO-depleted Kasumi-1 cells and Neutrophil Elastase has been identified as a RUNX1-ETO candidate target. Finally, proteolytic activity of Neutrophil Elastase and Cathepsin G was functionally confirmed by si/shRNA-mediated knockdown in Kasumi-1 cells.

Heparanase is required for activation and function of macrophages

The emerging role of heparanase in tumor initiation, growth, metastasis, and chemoresistance is well recognized and is encouraging the development of heparanase inhibitors as anticancer drugs. Unlike the function of heparanase in cancer cells, very little attention has been given to heparanase contributed by cells composing the tumor microenvironment. Here we used a genetic approach and examined the behavior and function of macrophages isolated from wild-type (WT) and heparanase-knockout (Hpa-KO) mice. Hpa-KO macrophages express lower levels of cytokines (e.g., TNFα, IL1-β) and exhibit lower motility and phagocytic capacities. Intriguingly, inoculation of control monocytes together with Lewis lung carcinoma (LLC) cells into Hpa-KO mice resulted in nearly complete inhibition of tumor growth. In striking contrast, inoculating LLC cells together with monocytes isolated from Hpa-KO mice did not affect tumor growth, indicating that heparanase is critically required for activation and function of macrophages. Mechanistically, we describe a linear cascade by which heparanase activates Erk, p38, and JNK signaling in macrophages, leading to increased c-Fos levels and induction of cytokine expression in a manner that apparently does not require heparanase enzymatic activity. These results identify heparanase as a key mediator of macrophage activation and function in tumorigenesis and cross-talk with the tumor microenvironment.

Proteomics of AML1/ETO Target Proteins: AML1–ETO Targets a C/EBP–NM23 Pathway

Introduction

The rational design of targeted therapies for acute myeloid leukemia (AML) requires the discovery of novel protein pathways in the systems biology of a specific AML subtype. We have shown that in the AML subtype with translocation t(8;21), the leukemic fusion protein AML1–ETO inhibits the function of transcription factors PU.1 and C/EBPα via direct protein–protein interaction. In addition, recently using proteomics, we have also shown that the AML subtypes differ in their proteome, interactome, and post-translational modifications.

Methods

We, therefore, hypothesized that the systematic identification of target proteins of AML1–ETO on a global proteome-wide level will lead to novel insights into the systems biology of t(8;21) AML on a post-genomic functional level. Thus, 6 h after inducible expression of AML1–ETO, protein expression changes were identified by two-dimensional gel electrophoresis and subsequent mass spectrometry analysis.

Results

Twenty-eight target proteins of AML1–ETO including prohibitin, NM23, HSP27, and Annexin1 were identified by MALDI-TOF mass spectrometry. AML1–ETO upregulated the differentiation inhibitory factor NM23 protein expression after 6 h, and the NM23 mRNA expression was also elevated in t(8;21) AML patient samples in comparison with normal bone marrow. AML1–ETO inhibited the ability of C/EBP transcription factors to downregulate the NM23 promoter. These data suggest a model in which AML1–ETO inhibits the C/EBP-induced downregulation of the NM23 promoter and thereby increases the protein level of differentiation inhibitory factor NM23.

Conclusions

Proteomic pathway discovery can identify novel functional pathways in AML, such as the AML1–ETO–C/EBP–NM23 pathway, as the main step towards a systems biology and therapy of AML.

Leukemogenic AML1-ETO fusion protein increases carcinogen-DNA adduct formation with upregulated expression of cytochrome P450-1A1 gene

OBJECTIVE

AML1-ETO fusion protein is a product of chromosome translocation t(8;21) frequently occurred in acute myeloid leukemia (AML), but its sole expression appears to fail to cause overt leukemia in vivo. In this study, we investigated whether AML1-ETO expression impinged on action of chemical carcinogens-DNA adduct formation.

MATERIALS AND METHODS

AML1-ETO fusion protein was conditionally induced in engineered U937-A/E 9/14/18 cells. The formation of polycyclic aromatic hydrocarbon (PAH)-DNA adducts and the expression of PAH-metabolizing enzymes cytochrome P450 (CYP) 1A1 and arylhydrocarbon receptor (AhR) were detected by Western blot and/or quantitative RT-PCR. Luciferase reporter system was used to detect the regulation of AML1-ETO on CYP1A1 transcription.

RESULTS

Our results showed that AML1-ETO induction significantly increased the formation of carcinogen benzopyrene-DNA adducts in leukemic cells. In line with the effect, we also found that AML1-ETO induction upregulated CYP1A1 expression, which was dependent on AML1-binding motif in the promotor of CYP1A1 gene. Additionally, AML1-ETO protein also increased AhR expression, a ligand-activated transcription factor that mediates PAHs-induced CYP1A1 gene expression.

CONCLUSION

These data, combined with its inhibitory effect on DNA repair as reported previously, propose that the presence of AML1-ETO increases the susceptibility of cells to chemical carcinogens, which favors the development of additional genetic alterations.

Williams-Beuren syndrome critical region-5/non-T-cell activation linker: a novel target gene of AML1/ETO

The chromosomal translocation t(8;21) fuses the AML1 (RUNX1) gene on chromosome 21 and the ETO gene on chromosome 8 in human acute myeloid leukemias (AMLs), resulting in expression of the chimeric transcription factor AML1/ETO. AML1/ETO-mediated dysregulation of target genes critical for hematopoietic differentiation and proliferation is thought to contribute to the leukemic phenotype. Several mechanisms, including recruitment of histone deacetylases (HDACs) to AML1 target genes, may be responsible for altered gene expression. We used an ecdysone-inducible expression system in the human monoblastic U-937 cell line to isolate genes that were differentially expressed upon induction of AML1/ETO expression. By representational difference analysis (cDNA-RDA), we identified 26 genes whose expression levels were significantly modulated following AML1/ETO induction for 48 h. None of these genes has previously been described as a target of AML1, ETO or AML1/ETO. One gene downregulated by AML1/ETO in vitro, Williams Beuren syndrome critical region 5 (WBSCR5), was expressed in primary t(8;21)-negative AML blasts but not in primary t(8;21)-positive AML blasts, strongly implying a role of this gene in the phenotype of t(8;21)-positive AML. Four upregulated and four downregulated genes were further studied with all-trans-retinoic acid (ATRA), an inducer of differentiation of U-937 cells, and Trichostatin A (TSA), an HDAC inhibitor. Three out of eight genes including WBSCR5 were regulated during ATRA-induced monocytic differentiation of U-937 cells, however, none of them antagonistically, upon both ATRA treatment and AML1/ETO induction. AML1/ETO-associated dysregulation of gene expression was not mediated by a TSA-sensitive mechanism. The identified genes provide a useful model to study the mechanism by which the AML1/ETO fusion protein exerts its function in transcriptional dysregulation in AML. The possible role of WBSCR5 in normal and malignant hematopoiesis warrants further study.

AML1-ETO Decreases ETO-2 (MTG16) Interactions with Nuclear Receptor Corepressor, an Effect That Impairs Granulocyte Differentiation

The t(8;21) chromosome abnormality in acute myeloid leukemia targets the AML1 and ETO genes to produce the leukemia fusion protein AML1-ETO. Another member of the ETO family, ETO-2/MTG16, is highly expressed in murine and human hematopoietic cells, bears >75% homology to ETO, and like ETO, contains a conserved MYND domain that interacts with the nuclear receptor corepressor (N-CoR). AML1-ETO prevents granulocyte but not macrophage differentiation of murine 32Dcl3 granulocyte/macrophage progenitors. One possible mechanism is recruitment of N-CoR to aberrantly repress AML1 target genes. We wished to examine another mechanism by which AML1-ETO might impair granulocyte differentiation. We demonstrate that AML1-ETO decreases interactions between ETO-2 and N-CoR. Furthermore, overexpression of ETO-2 relieves AML1-ETO-induced granulocyte differentiation arrest. This suggests that decreased interactions between ETO-2 and N-CoR may contribute to granulocyte differentiation impairment. The MYND domain coimmunoprecipitates with N-CoR and inhibits interactions between ETO-2 and N-CoR, presumably by occupying the ETO-2 binding site on N-CoR. This inhibition of ETO-2 interactions with N-CoR is specific because the MYND domain does not inhibit retinoic acid receptor interactions with N-CoR. To examine the effect of decreasing interactions between ETO-2 and N-CoR in hematopoietic cells, without effects of AML1-ETO such as direct repression of AML1 target genes, the MYND domain was expressed in 32Dcl3 and human CD34+ cells. The MYND domain prevented granulocyte but not macrophage differentiation of both 32Dcl3 and human CD34+ cells, recapitulating this effect of AML1-ETO. In conclusion, decreasing interactions between ETO-2 and N-CoR, an effect of AML1-ETO, inhibits granulocyte differentiation.

The 8;21 translocation in leukemogenesis

A common chromosomal translocation in acute myeloid leukemia (AML) involves the AML1 (acute myeloid leukemia 1, also called RUNX1, core binding factor protein (CBF alpha), and PEBP2 alpha B) gene on chromosome 21 and the ETO (eight-twenty one, also called MTG8) gene on chromosome 8. This translocation generates an AML1-ETO fusion protein. t(8;21) is associated with 12% of de novo AML cases and up to 40% in the AML subtype M2 of the French-American-British classification. Furthermore, it is also reported in a small portion of M0, M1, and M4 AML samples. Despite numerous studies on the function of AML1-ETO, the precise mechanism by which the fusion protein is involved in leukemia development is still not fully understood. In this review, we will discuss structural aspects of the fusion protein and the accumulated knowledge from in vitro analyses on AML1-ETO functions, and outline putative mechanisms of its leukemogenic potential.

AML1 interconnected pathways of leukemogenesis

The AML1 transcription factor, identified by the cloning of the translocation t(8;21) breakpoint, is one of the most frequent targets for chromosomal translocations in leukemia. Furthermore, polysomies and point mutations can also alter AML1 function. AML1, also called CBF alpha 2, PEBP alpha 2 or RUNX1, is thus implicated in a great number of acute leukemias via a variety of pathogenic mechanisms and seems to act either as an oncogene or a tumor suppressor gene. Characterization of AML1 knockout mice has shown that AML1 is necessary for normal development of all hematopoietic lineages and alterations in the overal functional level of AML1 can have a profound effect on hematopoiesis. Numerous studies have shown that AML1 plays a vital role in the regulation of expression of many genes involved in hematopoietic cell development, and the impairment of AML1 function disregulates the pathways leading to cellular proliferation and differentiation. However, heterozygous AML1 mutations alone may not be sufficient for the development of leukemia. A cumulative process of mutagenesis involving additional genetic events in functionally related molecules, may be necessary for the development of leukemia and may determine the leukemic phenotype. We review the known AML1 target genes, AML1 interacting proteins, AML1 gene alterations and their effects on AML1 function, and mutations in AML1-related genes associated with leukemia. We discuss the interconnections between all these genes in cell signaling pathways and their importance for future therapeutic developments.

Common themes in the pathogenesis of acute myeloid leukemia

The pathogenesis of acute myeloid leukemia is associated with the appearance of oncogenic fusion proteins generated as a consequence of specific chromosome translocations. Of the two components of each fusion protein, one is generally a transcription factor, whereas the other partner is more variable in function, but often involved in the control of cell survival and apoptosis. As a consequence, AML-associated fusion proteins function as aberrant transcriptional regulators that interfere with the process of myeloid differentiation, determine a stage-specific arrest of maturation and enhance cell survival in a cell-type specific manner. The abnormal regulation of transcriptional networks occurs through common mechanisms that include recruitment of aberrant co-repressor complexes, alterations in chromatin remodeling, and disruption of specific subnuclear compartments. The identification and analysis of common and specific target genes regulated by AML fusion proteins will be of fundamental importance for the full understanding of acute myeloid leukemogenesis and for the implementation of disease-specific drug design.