Differential expression of the oncoproteins c-myc and c-myb in human lymphoproliferative disorders

MYB regulates the SUMO protease SENP1 and its novel interaction partner UXT, modulating MYB target genes and the SUMO landscape

SUMOylation is a post-translational modification frequently found on nuclear proteins, including transcription factors (TFs) and coactivators. By controlling the activity of several TFs, SUMOylation may have far-reaching effects. MYB is an example of a developmental TF subjected to SUMO-mediated regulation, through both SUMO conjugation and SUMO binding. How SUMO affects MYB target genes is unknown. Here, we explored the global effect of reduced SUMOylation of MYB on its downstream gene programs. RNA-Seq in K562 cells after MYB knockdown and rescue with mutants having an altered SUMO status revealed a number of differentially regulated genes and distinct gene ontology term enrichments. Clearly, the SUMO status of MYB both quantitatively and qualitatively affects its regulome. The transcriptome data further revealed that MYB upregulates the SUMO protease SENP1, a key enzyme that removes SUMO conjugation from SUMOylated proteins. Given this role of SENP1 in the MYB regulome, we expanded the analysis, mapped interaction partners of SENP1, and identified UXT as a novel player affecting the SUMO system by acting as a repressor of SENP1. MYB inhibits the expression of UXT suggesting that MYB is able not only to control a specific gene program directly but also indirectly by affecting the SUMO landscape through SENP1 and UXT. These findings suggest an autoactivation loop whereby MYB, through enhancing SENP1 and reducing UXT, is itself being activated by a reduced level of repressive SUMOylation. We propose that overexpressed MYB, seen in multiple cancers, may drive this autoactivation loop and contribute to oncogenic activation of MYB.

Establishment of a zebrafish hematological disease model induced by 1,4-benzoquinone

Benzene exposure is associated with various hematological disorders, in particular leukemia. The reactive metabolite of benzene, 1,4-benzoquinone (BQ), generated in bone marrow, is suggested to be a key molecule in mediating benzene-induced hematotoxicity and carcinogenicity. However, its pathogenic role remains largely unknown due to a lack of suitable vertebrate whole-organism models. Here, we present an in vivo study to reveal the effect of BQ exposure on hematotoxicity in zebrafish. From embryonic stages to adulthood, BQ exposure suppressed erythroid and lymphoid hematopoiesis but led to abnormal accumulation of myeloid cells and precursors, which resembles benzene-induced cytopenia and myeloid dysplasia in humans. This myeloid expansion is caused by granulocyte, but not macrophage, lineage, emphasizing the significant role of lineage specificity in BQ-mediated hematopoietic toxicity. Analysis of the c-myb (also known as myb)-deficient mutant cmyb^hkz3 revealed that BQ induced neutrophilia in a c-myb-dependent manner, demonstrating that c-myb is a key intrinsic mediator of BQ hematotoxicity. Our study reveals that BQ causes lineage-specific hematotoxicity in zebrafish from embryonic stages to adulthood. Since c-myb is indispensable for BQ to induce neutrophilia, c-myb could serve as a potential drug target for reversing BQ hematotoxicity.

Summary: Acute exposure to 1,4-benzoquinone leads to lineage-specific hematotoxicity in zebrafish from embryonic stages to adulthood, resembling benzene-induced cytopenia and myeloid dysplasia in humans.

MYB - A regulatory factor in hematopoiesis

MYB is a transcription factor which was identified in birds as a viral oncogene (v-MYB). Its cellular counterpart was subsequently isolated as c-MYB which has three functional domains - DNA binding domain, transactivation domain and negative regulatory domain. c-MYB is essential for survival, and deletion of both alleles of the gene results in embryonic death. It is highly expressed in hematopoietic cells, thymus and neural tissue, and required for T and B lymphocyte development and erythroid maturation. Additionally, aberrant MYB expression has been found in numerous solid cancer cells and human leukemia. Recent studies have also implicated c-MYB in the regulation of expression of fetal hemoglobin which is highly beneficial to the β-hemoglobinopathies (beta thalassemia and sickle cell disease). These findings suggest that MYB could be a potential therapeutic target in leukemia, and possibly also a target for therapeutic increase of fetal hemoglobin in the β-hemoglobinopathies.

c-myb hyperactivity leads to myeloid and lymphoid malignancies in zebrafish

The c-MYB transcription factor is a key regulator of hematopoietic cell proliferation and differentiation, and dysregulation of c-MYB activity often associates with various hematological disorders. Yet, its pathogenic role remains largely unknown due to lack of suitable animal models. Here, we report a detail characterization of a c-myb-gfp transgenic zebrafish harboring c-Myb hyperactivity (named c-myb^hyper). This line exhibits abnormal granulocyte expansion that resembles human myelodysplastic syndrome (MDS) from embryonic stage to adulthood. Strikingly, a small portion of c-myb^hyper adult fish develops acute myeloid leukemia-like or acute lymphoid leukemia-like disorders with age. The myeloid and lymphoid malignancies in c-myb^hyper adult fish are likely caused by the hyperactivity of c-myb, resulting in the dysregulation of a number of cell-cycle-related genes and hyperproliferation of hematopoietic precursor cells. Finally, treatment with c-myb target drug flavopiridol can relieve the MDS-like symptoms in both c-myb^hyper embryos and adult fish. Our study establishes a zebrafish model for studying the cellular and molecular mechanisms underlying c-Myb-associated leukemogenesis as well as for anti-leukemic drug screening.

Role of the ubiquitin ligase Fbw7 in cancer progression

Fbw7 is a member of F-box family proteins, which constitute one subunit of Skp1, Cul1, and F-box protein (SCF) ubiquitin ligase complex. SCF(Fbw7) targets a set of well-known oncoproteins, including c-Myc, cyclin E, Notch, c-Jun, and Mcl-1, for ubiquitylation and degradation. Fbw7 provides specificity of the ubiquitylation of these substrate proteins via recognition of a consensus phosphorylated degron. Through regulation of several important proteins, Fbw7 controls diverse cellular processes, including cell-cycle progression, cell proliferation, differentiation, DNA damage response, maintenance of genomic stability, and neural cell stemness. As reduced Fbw7 expression level and loss-of-function mutations are found in a wide range of human cancers, Fbw7 is generally considered as a tumor suppressor. However, the exact mechanisms underlying Fbw7-induced tumor suppression is unclear. This review focuses on regulation network, biological functions, and genetic alteration of Fbw7 in connection with its role in cancer development.

GSK3 regulates the expressions of human and mouse c-Myb via different mechanisms

Background

c-Myb is expressed at high levels in immature progenitors of all the hematopoietic lineages. It is associated with the regulation of proliferation, differentiation and survival of erythroid, myeloid and lymphoid cells, but decreases during the terminal differentiation to mature blood cells. The cellular level of c-Myb is controlled by not only transcriptional regulation but also ubiquitin-dependent proteolysis. We recently reported that mouse c-Myb protein is controlled by ubiquitin-dependent degradation by SCF-Fbw7 E3 ligase via glycogen synthase kinase 3 (GSK3)-mediated phosphorylation of Thr-572 in a Cdc4 phosphodegron (CPD)-dependent manner. However, this critical threonine residue is not conserved in human c-Myb. In this study, we investigated whether GSK3 is involved in the regulatory mechanism for human c-Myb expression.

Results

Human c-Myb was degraded by ubiquitin-dependent degradation via SCF-Fbw7. Human Fbw7 ubiquitylated not only human c-Myb but also mouse c-Myb, whereas mouse Fbw7 ubiquitylated mouse c-Myb but not human c-Myb. Human Fbw7 mutants with mutations of arginine residues important for recognition of the CPD still ubiquitylated human c-Myb. These data strongly suggest that human Fbw7 ubiquitylates human c-Myb in a CPD-independent manner. Mutations of the putative GSK3 phosphorylation sites in human c-Myb did not affect the Fbw7-dependent ubiquitylation of human c-Myb. Neither chemical inhibitors nor a siRNA for GSK3β affected the stability of human c-Myb. However, depletion of GSK3β upregulated the transcription of human c-Myb, resulting in transcriptional suppression of γ-globin, one of the c-Myb target genes.

Conclusions

The present observations suggest that human Fbw7 ubiquitylates human c-Myb in a CPD-independent manner, whereas mouse Fbw7 ubiquitylates human c-Myb in a CPD-dependent manner. Moreover, GSK3 negatively regulates the transcriptional expression of human c-Myb but does not promote Fbw7-dependent degradation of human c-Myb protein. Inactivation of GSK3 as well as mutations of Fbw7 may be causes of the enhanced c-Myb expression observed in leukemia cells. We conclude that expression levels of human and mouse c-Myb are regulated via different mechanisms.

The ubiquitous nature of cancer: the role of the SCF(Fbw7) complex in development and transformation

The ubiquitin-proteasome system (UPS) is a multi-subunit pathway that allows for ubiquitin modification of proteins and leads to either degradation or other non-proteolytic processes such as trafficking or transcriptional activation. Given its role as a regulator of cellular homeostasis it is not surprising that members of the UPS are frequently aberrantly expressed in a number of disease states including cancer. This review will focus on one member of the UPS, the F-box protein, Fbw7 (also known as Sel-10, Ago, hCDC4) and mechanisms by which Fbw7 interacts with its substrates in the context of development and tumorigenesis will be discussed. In addition, antagonists of this pathway as well as current and future therapeutics for the UPS will be examined.

Fbw7 promotes ubiquitin-dependent degradation of c-Myb: involvement of GSK3-mediated phosphorylation of Thr-572 in mouse c-Myb

Expression of oncoprotein c-Myb oscillates during hematopoiesis and hematological malignancies. Its quantity is not only regulated through transcriptional control but also through the ubiquitin-proteasome pathway, accompanied by phosphorylation, although the mechanisms are poorly understood. In this report, we tried to identify an E3 ubiquitin ligase, which targets c-Myb for ubiquitin-dependent degradation. We found that an F-box protein, Fbw7, interacted with c-Myb, which is mutated in numerous cancers. Fbw7 facilitated ubiquitylation and degradation of c-Myb in intact cells. Moreover, depletion of Fbw7 by RNA interference delayed turnover and increased the abundance of c-Myb in myeloid leukemia cells concomitantly, and suppressed the transcriptional level of gamma-globin, which receives transcriptional repression from c-Myb. In addition, we analysed sites required for both ubiquitylation and degradation of c-Myb. We found that Thr-572 is critical for Fbw7-mediated ubiquitylation in mouse c-Myb using site-directed mutagenesis. Fbw7 recognized the phosphorylation of Thr-572, which was mediated by glycogen synthase kinase 3 (GSK3). In consequence, the c-Myb protein was markedly stabilized by the substitution of Thr-572 to Ala. These observations suggest that SCF(Fbw7) ubiquitin ligase regulates phosphorylation-dependent degradation of c-Myb protein.

The C-MYB locus is involved in chromosomal translocation and genomic duplications in human T-cell acute leukemia (T-ALL), the translocation defining a new T-ALL subtype in very young children

The C-Myb transcription factor is essential for hematopoiesis, including in the T-cell lineage. The C-Myb locus is a common site of retroviral insertional mutagenesis, however no recurrent genomic involvement has been reported in human malignancies. Here, we identified 2 types of genomic alterations involving the C-MYB locus at 6q23 in human T-cell acute leukemia (T-ALL). First, we found a reciprocal translocation, t(6;7)(q23;q34), that juxtaposed the TCRB and C-MYB loci (n = 6 cases). Second, a genome-wide copy-number analysis by array-based comparative genomic hybridization (array-CGH) identified short somatic duplications that include C-MYB (MYB(dup), n = 13 cases of 84 T-ALL, 15%). Expression analysis, including allele-specific approaches, showed stronger C-MYB expression in the MYB-rearranged cases compared with other T-ALLs, and a dramatically skewed C-MYB allele expression in the TCRB-MYB cases, which suggests that a translocation-driven deregulated expression may overcome a cellular attempt to down-regulate C-MYB. Strikingly, profiling of the T-ALLs by clinical, genomic, and large-scale gene expression analyses shows that the TCRB-MYB translocation defines a new T-ALL subtype associated with a very young age for T-cell leukemia (median, 2.2 years) and with a proliferation/mitosis expression signature. By contrast, the MYB(dup) alteration was associated with the previously defined T-ALL subtypes.

Combination of antisense oligonucleotide and low-dose chemotherapy in hematological malignancies

Current conventional chemotherapy for the treatment of hematological malignancies, although quite effective, has associated toxicities to normal tissue and organs, which is still a major dose limiting factor. In addition, high dose chemotherapy followed by autologous stem cell transplantation is limited by tumor cell contamination in the stem cell harvest. The use of conventional chemotherapy alone to purge these tumor cell contaminants is known to damage normal hematopoietic progenitor cells, resulting in delayed engraftment. The combination of antisense oligodeoxynucleotides (ODN) and low doses of chemotherapy offer a potential regiment which may lower the doses of conventional therapeutics required to effectively combat disease, thus lowering cytotoxicity experienced by normal cells. Transient downregulation of genes by ODN treatment, which are involved in the transformation or perpetuation of the cancerous disease state, can remove the growth and survival advantages exploited by tumor cells. Many groups are currently investigating this combination and have produced intriguing results. This review article discusses the current research investigating the combination of antisense ODN therapy with conventional chemotherapy in the treatment of hematological malignancies. Although further improvements in this strategy are required, the results thus far support a future for this strategy in clinical management of hematological malignancy.

Regulation of cell proliferation: late down-regulation of c-myb preceding myelo-monocytic cell differentiation

Expression of the c-myb nuclear oncogene during the cell proliferation and differentiation of HL-60 human promyelocytic leukemia cells was characterized and compared to the expression of c-fos, another nuclear oncogene with transcriptional regulatory activity. During progression through the cell cycle, the amount of c-myb protein increased. The increase was commensurate with total cell size, thus preserving the relative abundance of c-myb protein present at the onset of the cell cycle. In HL-60 cells, the induced metabolic cascade leading to terminal myeloid or monocytic differentiation segregates into two steps occurring over two division cycles. Expression of c-myb did not diverge from the control until late in this metabolic cascade when it declined prior to onset of terminal differentiation. This course of expression was similar for both the retinoic acid induced myeloid or the 1,25-dihydroxy vitamin D2 induced monocytic terminal differentiation of the cells. Bromodeoxyuridine, which induces proliferative arrest but not phenotypic differentiation of these cells, induced the same course of c-myb expression as the inducers of terminal differentiation. The same course of c-myb expression with growth arrest induced by these three different means is consistent with a potential proliferation regulatory role for c-myb in late but not early events leading to terminal differentiation. The dynamics of c-myb expression during this process were qualitatively, but not quantitatively, similar to the course of c-fos expression. Thus, taken with previous results, then amongst the nuclear oncogenes or tumor suppressor genes, c-myc, RB, c-fos, and c-myb, only c-myc and RB expression exhibit early regulation during induced HL-60 cell differentiation.