Reversibility of differentiation and proliferative capacity in avian myelomonocytic cells transformed by tsE26 leukemia virus

The novel tool of cell reprogramming for applications in molecular medicine

Recent discoveries in the field of stem cell biology have enabled scientists to “reprogram” cells from one type to another. For example, it is now possible to place adult skin or blood cells in a dish and convert them into neurons, liver, or heart cells. It is also possible to literally “rejuvenate” adult cells by reprogramming them into embryonic-like stem cells, which in turn can be differentiated into every tissue and cell type of the human body. Our ability to reprogram cell types has four main implications for medicine: (1) scientists can now take skin or blood cells from patients and convert them to other cells to study disease processes. This disease modeling approach has the advantage over animal models because it is directly based on human patient cells. (2) Reprogramming could also be used as a “clinical trial in a dish” to evaluate the general efficacy and safety of newly developed drugs on human patient cells before they would be tested in animal models or people. (3) In addition, many drugs have deleterious side effects like heart arrhythmias in only a small and unpredictable subpopulation of patients. Reprogramming could facilitate precision medicine by testing the safety of already approved drugs first on reprogrammed patient cells in a personalized manner prior to administration. For example, drugs known to sometimes cause arrhythmias could be first tested on reprogrammed heart cells from individual patients. (4) Finally, reprogramming allows the generation of new tissues that could be grafted therapeutically to regenerate lost or damaged cells.

Situational awareness: regulation of the myb transcription factor in differentiation, the cell cycle and oncogenesis

This review summarizes the mechanisms that control the activity of the c-Myb transcription factor in normal cells and tumors, and discusses how c-Myb plays a role in the regulation of the cell cycle. Oncogenic versions of c-Myb contribute to the development of leukemias and solid tumors such as adenoid cystic carcinoma, breast cancer and colon cancer. The activity and specificity of the c-Myb protein seems to be controlled through changes in protein-protein interactions, so understanding how it is regulated could lead to the development of novel therapeutic strategies.

Impaired adult myeloid progenitor CMP and GMP cell function in conditional c-myb-knockout mice

The differentiation of myeloid progenitors to mature, terminally differentiated cells is a highly regulated process. Here, we showed that conditional disruption of the c-myb proto-oncogene in adult mice resulted in dramatic reductions in CMP, GMP and MEP myeloid progenitors, leading to a reduction of neutrophils, basophils, monocytes and platelets in peripheral blood. In addition, c-myb plays a critical role at multiple stages of myeloid development, from multipotent CMP and bipotent GMP to unipotent CFU-G and CFU-M progenitor cells. c-myb controls the differentiation of these cells and is required for the proper commitment, maturation and normal differentiation of CMPs and GMPs. Specifically, c-myb regulates the precise commitment to the megakaryocytic and granulo-monocytic pathways and governs the granulocytic-monocytic lineage choice. c-myb is also required for the commitment along the granulocytic pathway for early myeloid progenitor cells and for the maturation of committed precursor cells along this pathway. On the other hand, disruption of the c-myb gene favors the commitment to the monocytic lineage, although monocytic development was abnormal with cells appearing more mature with atypical CD41 surface markers. These results demonstrate that c-myb plays a pivotal role in the regulation of multiple stages in adult myelogenesis.

Historical origins of transdifferentiation and reprogramming

Transcription factor-induced reprogramming of specialized cells into other cell types and to pluripotency has revolutionized our thinking about cell plasticity, differentiation, and stem cells. The recent advances in this area were enabled by the confluence of a number of experimental breakthroughs that took place over the past 60 years. In this article, I give a historical and personal perspective of the events that set the stage for our current understanding of cellular reprogramming.

The promoter regions of the Myb-regulated Adora2B and Mcm4 genes co-localize with origins of DNA replication

Background

The retroviral oncogene v-myb encodes a transcription factor (v-Myb) which is responsible for the transformation of myelomonocytic cells by avian myeloblastosis virus (AMV). v-Myb is thought to exert its biological effects by deregulating the expression of specific target genes. We have recently demonstrated that the chicken Gas41 gene, whose promoter co-localizes with an origin of DNA replication, is a bona fide Myb target gene. Because of this finding we have asked whether other Myb-regulated genes are also associated with DNA replication origins.

Results

We show that the promoter region of the chicken adenosine receptor 2B gene (Adora2B), a known Myb-target gene, acts as a DNA replication origin. Furthermore, we have examined known replication origins for the presence of Myb binding sites. We found that the intergenic region between the genes for the minichromosome maintenance 4 protein (Mcm4) and the catalytic subunit of DNA-dependent protein kinase (Prkdc), whose human counterpart has been identified as a replication origin, contains a number of Myb binding sites. Our data show that this region also acts as an origin of replication in chicken cells. Interestingly, we found that the chicken Mcm4 gene is also Myb-regulated.

Conclusion

Our work identifies the chicken Mcm4 gene as a novel Myb target gene and presents evidence for the co-localization of two novel origins of DNA replication with Myb-regulated genes. Our work raises the possibility that a fraction of Myb target gene promoters is associated with DNA replication origins.

SUMO modification regulates MafB-driven macrophage differentiation by enabling Myb-dependent transcriptional repression

During the execution of differentiation programs, lineage-specific transcription factors are in competition with antagonistic factors that drive progenitor proliferation. Thus, the myeloid transcription factor MafB promotes macrophage differentiation of myeloid progenitors, but a constitutively active Myb transcription factor (v-Myb) can maintain proliferation and block differentiation. Little is known, however, about the regulatory mechanisms that control such competing activities. Here we report that the small ubiquitin-like protein SUMO-1 can modify MafB in vitro and in vivo on lysines 32 and 297. The absence of MafB SUMO modification increased MafB-driven transactivation and macrophage differentiation potential but inhibited cell cycle progression and myeloid progenitor growth. Furthermore, we observed that direct repression of MafB transactivation by v-Myb was strictly dependent on MafB SUMO modification. Consequently, a SUMOylation-deficient MafB K32R K297R (K32,297R) mutant could specify macrophage fate even after activation of inducible Myb alleles and resist their differentiation-inhibiting activity. Our findings suggest that SUMO modification of MafB affects the balance between myeloid progenitor expansion and terminal macrophage differentiation by controlling MafB transactivation capacity and susceptibility to Myb repression. SUMO modification of lineage-specific transcription factors may thus modulate transcription factor antagonism to control tissue homeostasis in the hematopoietic system.

Haemopoietic stem cells

Considerable effort has been made in recent years in understanding the mechanisms that govern stem cell generation, proliferation, self-renewal, commitment and lately plasticity. In the development of the haemopoietic system during embryonic and fetal life the notion of different pools of stem cells arising from the endothelium is gaining consensus. Gene expression profiling of populations of stem cells is bringing to light categories of genes important for self-renewal or commitment. Besides the role of transcription factors in lineage decision, the role of soluble factors and transmembrane proteins, very active at the time of embryo development, are taking central stage in the maintenance and in vitro expansion of haemopoietic stem cells (HSCs). The hierarchical model of haemopoietic development is being questioned with reports of lineage switching and plasticity of haemopoietic stem cells to non-haemopoietic cells. Yet the understanding of the overall process is still very fragmented and hypothetical. This is mainly due to the absence of appropriate markers to enable selection of homogeneous stem cell populations and the need to rely on retrospective functional assays, able only to determine the overall behaviour of a population of cells. This review is intended to be an overview of the haemopoietic system and a critical re-visitation of issues such as plasticity and self-renewal important for therapeutic applications of haemopoietic stem cells.

Trichostatin A suppresses transformation by the v-myb oncogene in BM2 cells

BM2 cells are chicken monoblasts transformed by the v-myb oncogene of avian myeloblastosis virus. The constitutively high v-myb expression interferes with the terminal differentiation of BM2 cells, but these cells can be induced to differentiate into macrophage-like cells by phorbol esters. Histone acetylation plays an important role in regulation of transcription and is particularly relevant to the regulation and pathology of hematopoiesis. In the present study, we examined the contribution of elevated histone acetylation to the differentiation of BM2 cells. Inhibition of the activity of endogenous histone deacetylases by trichostatin A (TSA) resulted in histone hyperacetylation causing cell cycle arrest and differentiation of BM2 cells into macrophage polykaryons. TSA did not affect the level of v-Myb protein in BM2 cells, but it downregulated its transcription activation capability. This suggests that chromatin remodeling can be significantly engaged in regulation of proliferation and differentiation of leukemic cells.

The homeodomain protein PRH influences the differentiation of haematopoietic cells

Haematopoiesis involves the differentiation of a self-renewing stem cell into all of the lineages found in circulating blood. Myb-Ets transformed chicken blastoderm cells (MEPs) have many of the characteristics of multipotent haematopoietic cells and represent a useful model system for the study of haematopoiesis. The proline-rich homeodomain protein (PRH) has previously been shown to be expressed in the haematopoietic compartment. In this report we show that PRH mRNA and protein levels are down regulated as MEPs differentiate along the myelomonocytic and erythrocytic lineages. In contrast, PRH mRNA and protein levels remain high as MEPs differentiate toward the thrombocytic lineage. Over-expression of full length PRH in MEPs inhibits their transformation and/or proliferation. However, the over-expression of N-terminally truncated PRH proteins results in normally proliferating cells that are predominantly differentiated along the myelomonocytic and eosinophilic lineages. These results suggest that PRH plays a role in the proliferation and differentiation of haematopoietic cells.

Histone deacetylase inhibitors: inducers of differentiation or apoptosis of transformed cells

Histone deacetylase (HDAC) inhibitors have been shown to be potent inducers of growth arrest, differentiation, and/or apoptotic cell death of transformed cells in vitro and in vivo. One class of HDAC inhibitors, hydroxamic acid-based hybrid polar compounds (HPCs), induce differentiation at micromolar or lower concentrations. Studies (x-ray crystallographic) showed that the catalytic site of HDAC has a tubular structure with a zinc atom at its base and that these HDAC inhibitors, such as suberoylanilide hydroxamic acid and trichostatin A, fit into this structure with the hydroxamic moiety of the inhibitor binding to the zinc. HDAC inhibitors cause acetylated histones to accumulate in both tumor and normal tissues, and this accumulation can be used as a marker of the biologic activity of the HDAC inhibitors. Hydroxamic acid-based HPCs act selectively to inhibit tumor cell growth at levels that have little or no toxicity for normal cells. These compounds also act selectively on gene expression, altering the expression of only about 2% of the genes expressed in cultured tumor cells. In general, chromatin fractions enriched in actively transcribed genes are also enriched in highly acetylated core histones, whereas silent genes are associated with nucleosomes with a low level of acetylation. However, HDACs can also acetylate proteins other than histones in nucleosomes. The role that these other targets play in the induction of cell growth arrest, differentiation, and/or apoptotic cell death has not been determined. Our working hypothesis is that inhibition of HDAC activity leads to the modulation of expression of a specific set of genes that, in turn, result in growth arrest, differentiation, and/or apoptotic cell death. The hydroxamic acid-based HPCs are potentially effective agents for cancer therapy and, possibly, cancer chemoprevention.

MafB is an inducer of monocytic differentiation

The bZip transcription factor MafB is expressed specifically in the myeloid lineage of the hematopoietic system and is up-regulated successively during myeloid differentiation from multipotent progenitors to macrophages. Here we report that this induction reflects an essential role of MafB in early myeloid and monocytic differentiation. We observed that the expression of MafB in transformed chicken hematopoietic precursors dramatically increases the proportion of myeloid colony formation at the expense of multipotent progenitor-type colonies. In addition, the overexpression of MafB in transformed myeloblasts stimulates the rapid formation of macrophages, as judged by morphology, surface marker expression and functional criteria. MafB-induced macrophages exhibit typical levels of phagocytic activity and nitric oxide release after activation by lipopolysaccharide. By contrast, overexpression of the myeloid transcription factor PU.1 in these cells does not induce macrophage differentiation. Furthermore, a dominant-negative allele of MafB inhibits both myeloid colony formation and the differentiation of myeloblasts into macrophages. Taken together, our results indicate that MafB induction is a specific and essential determinant of the monocytic program in hematopoietic cells.

Myb is required for self-renewal in a model system of early hematopoiesis

In hematopoiesis, self-renewal, proliferation, differentiation and apoptosis represent opposing decisions made by stem cells and progenitor cells, which when dysregulated can result in leukaemia. Here, we have investigated the function of Myb proteins in regulating these key cellular decisions, using the cell line FDCP-mix A4 as a model of early hematopoiesis. High concentrations of IL-3 in these cells favour self-renewal over differentiation and apoptosis. However when endogenous Myb activity was inhibited with an inducible dominant interfering protein, self-renewal was replaced by apoptosis and differentiation. Differentiation was to granulocytes and monocyte/macrophages and was closely associated with a G1-S phase block in the cell cycle. As for normal hematopoiesis, cytokine-induced terminal differentiation of FDCP-mix cells is associated with concomitant proliferation prior to its completion. However, when Myb activity was inhibited during this process, proliferation and survival were both reduced, resulting in a much lower yield of mature cells. These results indicate multiple cellular roles of Myb proteins during normal hematopoiesis.

Antiapoptotic activity of Stat5 required during terminal stages of myeloid differentiation

Stat5 is activated by multiple receptors of hematopoietic cytokines. To study its role during hematopoiesis, we have generated primary chicken myeloblasts expressing different dominant-negative (dn) alleles of Stat5. This caused a striking inability to generate mature cells, due to massive apoptosis during differentiation. Bcl-2 was able to rescue differentiating cells expressing dnStat5 from apoptosis, suggesting that during cytokine-dependent differentiation the main function of the protein is to ensure cell survival. Our findings with dnStat5-expressing chicken myeloblasts were confirmed with primary hematopoietic cells from Stat5a/Stat5b-deficient mice. Bone marrow cells from these animals displayed a strong increase in apoptotic cell death during GM-CSF-dependent functional maturation in vitro. The antiapoptotic protein Bcl-x was induced by GM-CSF and IL-3 in a Stat5-dependent fashion. Ectopic expression of Bcl-x rescued Stat5-deficient bone marrow cells from apoptosis, indicating that Stat5 promotes the survival of myeloid progenitor cells through its ability to induce transcription of the bcl-x gene. Finally, the recruitment of myeloid cells to inflammatory sites was found strongly impeded in Stat5-deficient mice. Taken together, our findings suggest that Stat5 may promote cytokine-dependent survival and proliferation of differentiating myeloid progenitor cells in stress or pathological situations, such as inflammation.

v-Myb can transform and regulate the differentiation of melanocyte precursors

The activity of the c-Myb transcription factor is essential for the development of definitive multi- and uni-lineage progenitors of the haemopoietic system. Reflecting this requirement, c-Myb has been oncogenically activated by transduction in the E26 avian retrovirus which elicits an acute leukaemia by transforming haemopoietic progenitors. Here, we report the novel finding that Myb in cooperation with EGF receptor signalling can be used to generate clonally expanded populations of transformed cells which have the phenotype of melanocyte precursors. Through the use of a conditional temperature sensitive mutant of Myb, we show that in the transformed cells Myb regulates commitment to melanocyte differentiation and possibly proliferation. These results add to our understanding of the roles of c-Myb beyond the haemopoietic system and to our knowledge and means of investigating the importance of transcription factors in the melanocyte lineage.

Myb binding proteins: regulators and cohorts in transformation

The c-Myb and v-Myb proteins are transcription factors that regulate cell proliferation and differentiation. Both Myb proteins have been shown to interact with a number of cellular proteins, some of which are transcription factors that cooperate to activate specific promoters, while others regulate the transcriptional activity of Myb in specific contexts. By comparing and analysing the types of proteins that bind Myb, and the conserved domains of Myb that interact with other proteins, conclusions can be drawn regarding the role of specific partner proteins in the regulation of gene expression, cell proliferation and disease.

A novel way to induce erythroid progenitor self renewal: cooperation of c-Kit with the erythropoietin receptor

Red blood cells are of vital importance for oxygen transport in vertebrates. Thus, their formation during development and homeostasis requires tight control of both progenitor proliferation and terminal red cell differentiation. Self renewal (i.e. long-term proliferation without differentiation) of committed erythroid progenitors has recently been shown to contribute to this regulation. Avian erythroid progenitors expressing the EGF receptor/c-ErbB (SCF/TGFalpha progenitors) can be induced to long-term proliferation by the c-ErbB ligand transforming growth factor alpha and the steroids estradiol and dexamethasone. These progenitors have not yet been described in mammals and their factor requirements are untypical for adult erythroid progenitors. Here we describe a second, distinct type of erythroid progenitor (EpoR progenitors) which can be established from freshly isolated bone marrow and is induced to self renew by ligands relevant for erythropoiesis, i.e. erythropoietin, stem cell factor, the ligand for c-Kit and the glucocorticoid receptor ligand dexamethasone. Limiting dilution cloning indicates that these EpoR progenitors are derived from normal BFU-E/CFU-E. For a detailed study, mEpoR progenitors were generated by retroviral expression of the murine Epo receptor in bone marrow erythroblasts. These progenitors carry out the normal erythroid differentiation program in recombinant differentiation factors only. We show that mEpoR progenitors are more mature than SCF/TGFalpha progenitors and also do no longer respond to transforming growth factor alpha and estradiol. In contrast they are now highly sensitive to low levels of thyroid hormone, facilitating their terminal maturation into erythrocytes.

Overexpression of an alternatively spliced form of c-Myb results in increases in transactivation and transforms avian myelomonoblasts

An alternatively spliced form of c-myb exists that encodes an additional 120 amino acids in chicken and 121 amino acids in human and mouse. These amino acids are encoded by an additional exon, termed exon 9A. This exon is not present in v-myb, and proteins containing these amino acids have never been tested for oncogenic transformation. A series of myb constructs was therefore created in order to compare the functions of Myb proteins on the basis of their inclusion or exclusion of the amino acids encoded by exon 9A (E9A). We found that the presence of E9A resulted in a robust increase in transactivation for full-length c-Myb (CCC), as well as the singly truncated derivatives dCC and CCd, while doubly truncated Myb proteins v-Myb (dVd) and dCd did not exhibit any differences in transactivation. The increase in transactivation requires the Myb DNA-binding domain. When the leukemic transformation by the Myb proteins was tested, it was found that cells transformed by dVd resembled monoblasts, while cells transformed by CCC and its derivatives, dCd, dCC, and CCd, resembled myelomonoblasts. Despite differences in the morphology of the hematopoietic cells, the cell surface phenotypes and cell cycle profiles of transformed cells did not change for each pair of Myb proteins in the presence or absence of E9A. Thus, there was no direct correlation between the level of transcriptional activation and the strength of leukemic transformation.

Protein kinase C in erythroid and megakaryocytic differentiation: possible role in lineage determination

Erythroid differentiation of normal human hematopoietic progenitor cells was drastically inhibited by phorbol ester, 12-myristate 13-acetate (PMA), an agent known to activate the class of serine-threonine kinases, protein kinase C (PKC). This inhibition was accompanied by augmented megakaryocytic differentiation as demonstrated by expression of megakaryocyte-specific mRNAs and proteins. These effects of PMA were reversed by two specific antagonists of PKC. Analysis of single colonies transferred from cultures not containing PMA to PMA-containing cultures indicated that, in this system, PMA exerts megakaryocytic differentiating activity directly on cells which may have already initiated a progression toward the erythroid pathway of differentiation. These results suggest that modulation of PKC activity plays a role in erythroid and megakaryocytic differentiation, and may constitute an important selective signal between these pathways during normal blood cell development.

Transcriptional regulation during myelopoiesis

The coordinated production of all blood cells from a common stem cell is a highly regulated process involving successive stages of commitment and differentiation. From analyses of mice deficient in transcription factor genes and from the characterizations of chromosome breakpoints in human leukemias, it has become evident that transcription factors are important regulators of hematopoiesis. During myelopoiesis, which includes the development of granulocytic and monocytic lineages, transcription factors from several families are active, including AML1/CBF beta, C/EBP, Ets, c-Myb, HOX, and MZF-1. Few of these factors are expressed exclusively in myeloid cells; instead it appears that they cooperatively regulate transcription of myeloid-specific genes. Here we discuss recent advances in transcriptional regulation during myelopoiesis.

Cells transformed by a v-Myb-estrogen receptor fusion differentiate into multinucleated giant cells

In order to make conditional alleles of the v-myb oncogene, we constructed and tested avian retroviruses which produce a number of different fusion proteins between v-Myb and the human estrogen receptor (ER). We found that the portion of the ER used in making these fusions profoundly affected their transcriptional activation. However, all the fusions tested were only weakly transforming in embryonic yolk sac assays and there was no direct correlation between the level of transcriptional activation and strength of oncogenic transformation. Nevertheless, transformation by a v-Myb-ER fusion was estrogen dependent, and upon withdrawal of the hormone, monocytic-lineage cells differentiated into multinucleated giant cells. Surprisingly, the withdrawal of estrogen caused a dramatic increase in the stability of the fusion protein, although it remained unable to promote cell growth or block differentiation.

v-Myb of E26 leukemia virus up-regulates bcl-2 and suppresses apoptosis in myeloid cells

Many oncogenes have been shown to be deregulated transcription factors, yet direct target genes mediating cell transformation remain elusive. Here we describe such a target for v-Myb by exploiting a temperature-sensitive mutant of the E26 avian leukemia virus encoding Myb-Ets. Myeloblasts transformed by the mutant differentiate into macrophages or die by apoptosis when shifted to the nonpermissive temperature as a result of inactivation of v-Myb. During this process mRNA of the antiapoptotic oncoprotein Bcl-2 is down-regulated with kinetics similar to those of Mim-1, a differentiation-related protein whose expression is directly regulated by Myb. Forced expression of bcl-2 rescues the cells from apoptosis, without preventing either their withdrawal from the cell cycle or their differentiation. v-Myb appears to act directly on the bcl-2 gene, because a bcl-2 promoter-driven reporter is activated by Myb-Ets and v-Myb-VP16 and requires intact Myb binding sites within the promoter. Surprisingly, inactivation of v-Myb in multipotent progenitors transformed by E26 virus does not induce apoptosis, indicating that bcl-2 regulation by the oncoprotein is required for the transformation of some cell types but not others.

A role for STAT family transcription factors in myeloid differentiation

STAT family transcription factors regulate gene expression in response to a wide variety of cytokines. A transcription factor designated differentiation-induced factor (DIF), activated by treatment of myeloid cells with the differentiating agents interferon-gamma (IFN-gamma), granulocyte-macrophage colony-stimulating factor (GM-CSF), colony-stimulating factor-1 (CSF-1) or during phorbol ester-induced differentiation, was characterized as a 112kDa protein related to, but not identical with known isoforms of STAT 5. Taken together with previously published results, our data suggest an important function for members of the STAT 5 subfamily in regulating gene expression during the process of myeloid differentiation.

Functional analysis of the c-myb proto-oncogene

Targeted mutagenesis studies were initiated to determine the normal biological function of the c-myb proto-oncogene. While heterozygous mice are phenotypically indistinguishable from their wild-type littermates, homozygous mutant fetuses die at approximately 15.5 days of gestation apparently due to anemia, which results from an inability to switch from embryonic yolk sac to fetal liver erythropoiesis. Studies are currently being done to determine the extent of hematopoietic abnormalities in the homozygous mutant fetuses. In vitro assays for hematopoietic colony-forming cells have been used to determine the frequency of both erythroid and myeloid progenitors in the fetal livers of wild-type, heterozygous, and homozygous mutant c-myb fetuses. The reduced number of erythroid progenitors was not unexpected considering the mutant fetus's pale color and reduced hematocrit. The dramatically reduced number of colonies derived from myeloid progenitors in the mutant fetuses in comparison to the number detected in phenotypically normal littermates suggests that expression of the c-myb proto-oncogene is critical for the proliferation and/or differentiation of early hematopoietic progenitors and possibly hematopoietic stem cells. Other possible explanations would include a hematopoietic progenitor migration problem from the yolk sac to the fetal liver or a defect in the microenvironment of the liver. Whether the lymphoid lineage is also adversely affected by the lack of c-myb expression remains to be determined. RT-PCR and Northern blot analyses were used in an attempt to identify downstream genes which may be directly or indirectly regulated by the Myb gene product. While the levels of expression of several genes involved in erythropoiesis (GATA-1, NF-E2, SCL, and EpoR) were reduced in the livers of homozygous mutant fetuses in comparison to phenotypically normal littermates and one gene, Kit ligand (KL), was expressed at higher levels in the mutant livers, these results must be viewed with caution. The livers of the mutant fetuses have been shown to be hypocellular in comparison to those of phenotypically normal littermates (35). It is possible that the Myb gene product is directly or indirectly modulating the expression of these genes. Conversely, the alteration in expression may be due to the reduced number or absence of specific hematopoietic lineages in the livers of the mutant fetuses. Differential display has also been used to identify putative novel genes that are involved in hematopoiesis. Preliminary studies suggest that this may be a powerful methodology to compare the expression pattern of genes in the fetal liver of wild-type, heterozygous, and homozygous mutant littermates at 14.5 days of gestation. To date nearly 60% of the partial cDNAs subcloned analyzed have been shown to be differentially expressed. More importantly, 75% of the differentially expressed cDNAs that have been sequenced appear to encode novel genes. Whether any of these novel genes are involved in the c-myb transcriptional cascade remains to be determined. Overall, analysis of the c-myb mutant fetuses have provided valuable insight into the biological function of this interesting proto-oncogene. The continued analysis of this resource will undoubtedly provide additional information concerning the role of the c-myb gene in hematopoiesis.

GATA-1 reprograms avian myelomonocytic cell lines into eosinophils, thromboblasts, and erythroblasts

The transcription factor GATA-1 is expressed in early hematopoietic progenitors and specifically down-regulated in myelomonocytic cells during lineage determination. Our earlier observation that the differentiation of Myb-Ets-transformed chicken hematopoietic progenitors into myeloblasts likewise involves a GATA-1 down-regulation, whereas expression is maintained in erythroid, thrombocytic, and eosinophilic derivatives, prompted us to study the effect of forced GATA-1 expression in Myb-Ets-transformed myeloblasts. We found that the factor rapidly suppresses myelomonocytic markers and induces a reprogramming of myeloblasts into cells resembling either transformed eosinophils or thromboblasts. In addition, we observed a correlation between the level of GATA-1 expression and the phenotype of the cell, intermediate levels of the factor being expressed by eosinophils and high levels by thromboblasts, suggesting a dosage effect of the factor. GATA-1 can also induce the formation of erythroblasts when expressed in a myelomonocytic cell line transformed with a Myb-Ets mutant containing a lesion in Ets. These cells mature into erythrocytes following temperature-inactivation of the Ets protein. Finally, the factor can reprogram a v-Myc-transformed macrophage cell line into myeloblasts, eosinophils, and erythroblasts, showing that the effects of GATA-1 are not limited to Myb-Ets-transformed myeloblasts. Our results suggest that GATA-1 is a lineage-determining transcription factor in transformed hematopoietic cells, which not only activates lineage-specific genetic programs but also suppresses myelomonocytic differentiation. They also point to a high degree of plasticity of transformed hematopoietic cells.

Regulation of the T-cell receptor delta enhancer by functional cooperation between c-Myb and core-binding factors

A T-cell-specific transcriptional enhancer lies within the J delta 3-C delta intron of the human T-cell receptor (TCR) delta gene. The 30-bp minimal enhancer element denoted delta E3 carries a core sequence (TGTGGTTT) that binds a T-cell-specific factor, and that is necessary but not sufficient for transcriptional activation. Here we demonstrate that the transcription factor c-Myb regulates TCR delta enhancer activity through a binding site in delta E3 that is adjacent to the core site. Both v-Myb and c-Myb bind specifically to delta E3. The Myb site is necessary for enhancer activity, because a mutation that eliminates Myb binding abolishes transcriptional activation by the delta E3 element and by the 370-bp TCR delta enhancer. Transfection of cells with a c-Myb expression construct upregulates delta E3 enhancer activity, whereas treatment of cells with an antisense c-myb oligonucleotide inhibits delta E3 enhancer activity. Since intact Myb and core sites are both required for delta E3 function, our data argue that c-Myb and core binding factors must cooperate to mediate transcriptional activation through delta E3. Efficient cooperation depends on the relative positioning of the Myb and core sites, since only one of two overlapping Myb sites within delta E3 is functional and alterations of the distance between this site and the core site disrupt enhancer activity. Cooperative regulation by c-Myb and core-binding factors is likely to play an important role in the control of gene expression during T-cell development.

Oncogenic truncation of the first repeat of c-Myb decreases DNA binding in vitro and in vivo

Oncogenic activation of c-Myb in both avian and murine systems often involves N-terminal truncation. In particular, the first of three DNA-binding repeats in c-Myb has been largely deleted during the genesis of the v-myb oncogenes of avian myeloblastosis virus and E26 avian leukemia virus. This finding suggests that the first DNA-binding repeat may have an important role in cell growth control. We demonstrate that truncation of the first DNA-binding repeat of c-Myb is sufficient for myeloid transformation in culture, but deletion of the N-terminal phosphorylation site and adjacent acidic region is not. Truncation of the first repeat decreases the ability of a Myb-VP16 fusion protein to trans activate the promoter of a Myb-inducible gene (mim-1) involved in differentiation. Moreover, truncation of the first repeat decreases the ability of the Myb protein to bind DNA both in vivo and in vitro. These results suggest that N-terminal mutants of c-Myb may transform by regulating only a subset of those genes normally regulated by c-Myb.

Regulation of the T-cell receptor delta enhancer by functional cooperation between c-Myb and core-binding factors

A T-cell-specific transcriptional enhancer lies within the J delta 3-C delta intron of the human T-cell receptor (TCR) delta gene. The 30-bp minimal enhancer element denoted delta E3 carries a core sequence (TGTGGTTT) that binds a T-cell-specific factor, and that is necessary but not sufficient for transcriptional activation. Here we demonstrate that the transcription factor c-Myb regulates TCR delta enhancer activity through a binding site in delta E3 that is adjacent to the core site. Both v-Myb and c-Myb bind specifically to delta E3. The Myb site is necessary for enhancer activity, because a mutation that eliminates Myb binding abolishes transcriptional activation by the delta E3 element and by the 370-bp TCR delta enhancer. Transfection of cells with a c-Myb expression construct upregulates delta E3 enhancer activity, whereas treatment of cells with an antisense c-myb oligonucleotide inhibits delta E3 enhancer activity. Since intact Myb and core sites are both required for delta E3 function, our data argue that c-Myb and core binding factors must cooperate to mediate transcriptional activation through delta E3. Efficient cooperation depends on the relative positioning of the Myb and core sites, since only one of two overlapping Myb sites within delta E3 is functional and alterations of the distance between this site and the core site disrupt enhancer activity. Cooperative regulation by c-Myb and core-binding factors is likely to play an important role in the control of gene expression during T-cell development.

Targeted expression of a conditional oncogene in hematopoietic cells of transgenic mice

We have produced two lines of transgenic mice in which the expression of temperature-sensitive SV-40 large T antigen is targeted to bone marrow megakaryocytes via the platelet factor 4 (PF4) tissue-specific promoter. The progeny of these transgenic mice were observed for about 3 mo, and no malignancies were detected over this period of time. The offspring of these transgenic mice, 6- to 12-wk of age, served as a source of bone marrow cells, which upon in vitro cultivation at the permissive temperature yielded immortalized cell lines (MegT). At the permissive temperature, MegT cells exhibit the characteristics of early 2N and 4N megakaryocytes which include the presence of specific gene products such as PF4, glycoprotein IIb, acetylcholinesterase, and CD45 as well as the absence of molecular markers of other cell lineages such as the macrophage marker Mac-1, the T helper cell marker CD4, the mast cell marker IgE, the T cell marker CD2 or the erythroid cell marker alpha-globin. The inactivation of the oncogene by a shift of temperature from 34 degrees to 39.5 degrees C produces a reduction in the frequency of the 2N cells, in conjunction with the appearance of 8N and 16N cells, consisting of 27 and 3% of total cells, respectively. Thus, we have generated hematopoietic cell lines that are trapped in the early stages of megakaryocyte commitment, but able to undergo part of the normal program of terminal differentiation.

Oncogenic truncation of the first repeat of c-Myb decreases DNA binding in vitro and in vivo

Oncogenic activation of c-Myb in both avian and murine systems often involves N-terminal truncation. In particular, the first of three DNA-binding repeats in c-Myb has been largely deleted during the genesis of the v-myb oncogenes of avian myeloblastosis virus and E26 avian leukemia virus. This finding suggests that the first DNA-binding repeat may have an important role in cell growth control. We demonstrate that truncation of the first DNA-binding repeat of c-Myb is sufficient for myeloid transformation in culture, but deletion of the N-terminal phosphorylation site and adjacent acidic region is not. Truncation of the first repeat decreases the ability of a Myb-VP16 fusion protein to trans activate the promoter of a Myb-inducible gene (mim-1) involved in differentiation. Moreover, truncation of the first repeat decreases the ability of the Myb protein to bind DNA both in vivo and in vitro. These results suggest that N-terminal mutants of c-Myb may transform by regulating only a subset of those genes normally regulated by c-Myb.

Ectopic expression of genes during chicken limb pattern formation using replication defective retroviral vectors

A gene transfer method to ectopically express genes during chicken limb pattern formation using replication defective retroviral vectors has been established. Spherical non-proliferating (mitomycin C treated) aggregates of clonal retrovirus producing cells were grafted directly into developing chicken wing buds. The cell aggregates had to be placed in direct contact with the highly proliferative cells of the wing bud to promote efficient in vivo infection of embryonic cells by the released retroviral particles. The majority of grafts resulted in widespread expression of a reporter gene (encoding bacterial beta-galactosidase) during limb pattern formation and early limb bud outgrowth without affecting morphogenesis. This method provides a novel approach to study the effects of ectopic gene expression on limb pattern formation. Possible future applications to study other developmental processes are discussed.

v-myb blocks granulocyte colony-stimulating factor-induced myeloid cell differentiation but not proliferation

To understand the effects of v-myb expression on mammalian hematopoietic cell differentiation, we have constructed a retroviral vector which can efficiently express v-myb gene product in mammalian cells. Infection of interleukin-3-dependent murine progenitor cell line 32D Cl3, which undergoes terminal differentiation to mature granulocytes in the presence of granulocyte colony-stimulating factor (GCSF), with this recombinant retrovirus does not abrogate its requirement of interleukin-3 for growth. However, expression of v-myb in these cells blocks their ability to differentiate in response to GCSF. Instead, the v-myb-infected cells proliferate indefinitely in the presence of GCSF. 32D Cl3 cells infected with empty vector carrying only the neomycin resistance gene responded to the addition of GCSF in a manner identical to that of the uninfected cells and underwent terminal differentiation into granulocytes. These results suggest that oncogenic forms of myb gene bring about transformation by blocking the differentiation signal derived by cytokines while promoting the proliferative signal transduction pathways.

Thyroid hormone receptor/c-erbA: control of commitment and differentiation in the neuronal/chromaffin progenitor line PC12

The c-erbA proto-oncogenes encode nuclear receptors for thyroid hormone (T3), a hormone intimately involved in mammalian brain maturation. To study thyroid hormone receptor (TR) action on neuronal cells in vitro, we expressed the chicken c-erbA/TR alpha-1 as well as its oncogenic variant v-erbA in the adrenal medulla progenitor cell line PC12. In the absence of T3, exogenous TR alpha-1 inhibits NGF-induced neuronal differentiation and represses neuron-specific gene expression. In contrast, TR alpha-1 allows normal differentiation and neuronal gene expression to occur in the presence of T3. Finally, TR alpha-1-expressing cells become NGF-responsive for proliferation when T3 is absent, but NGF-dependent for survival in presence of T3. A similar differentiation induction by NGF plus T3 was observed in a central nervous system-derived neuronal cell line (E 18) expressing exogenous TR alpha-1. Together with the finding that TR alpha-1 constitutively blocked dexamethasone-induced differentiation of PC12 cells into the chromaffin pathway, these results suggest that TR alpha-1 plays an important role in regulating commitment and maturation of neuronal progenitors. In contrast, the v-erbA oncogene, a mutated, oncogenic version of TR alpha-1, partially but constitutively inhibited NGF-induced neuronal differentiation of PC12 cells and potentiated dexamethasone-induced chromaffin differentiation, giving rise to an aberrant "interlineage" cell phenotype.

v-myb blocks granulocyte colony-stimulating factor-induced myeloid cell differentiation but not proliferation

To understand the effects of v-myb expression on mammalian hematopoietic cell differentiation, we have constructed a retroviral vector which can efficiently express v-myb gene product in mammalian cells. Infection of interleukin-3-dependent murine progenitor cell line 32D Cl3, which undergoes terminal differentiation to mature granulocytes in the presence of granulocyte colony-stimulating factor (GCSF), with this recombinant retrovirus does not abrogate its requirement of interleukin-3 for growth. However, expression of v-myb in these cells blocks their ability to differentiate in response to GCSF. Instead, the v-myb-infected cells proliferate indefinitely in the presence of GCSF. 32D Cl3 cells infected with empty vector carrying only the neomycin resistance gene responded to the addition of GCSF in a manner identical to that of the uninfected cells and underwent terminal differentiation into granulocytes. These results suggest that oncogenic forms of myb gene bring about transformation by blocking the differentiation signal derived by cytokines while promoting the proliferative signal transduction pathways.

Selective eicosanoid formation during HL-60 macrophage differentiation. Regulation of thromboxane synthase

Earlier studies on HL-60 cells induced to differentiate into macrophages by phorbol esters have shown a selective stimulation of thromboxane (Tx) formation from endoperoxide prostaglandin (PG) H2, indicating that Tx synthesis is regulated at the level of Tx synthase (TxS), one of the peripheral enzymes of the PGH-synthase pathway. We now report on the regulation of TxS during HL-60 macrophage differentiation using monoclonal anti-TxS serum and comparing turnover rates of TxS and its biological activity with those of other enzymes of arachidonic acid metabolism. Western-blot analysis, enzyme-linked immunosorbent assay, immunohistochemical staining and [35S]methionine-labeling experiments suggested a phorbol-ester-dependent early induction of synthesis of TxS. [35S]Methionine incorporation into TxS was stimulated within 4 h after initiation of differentiation and was associated with a major rise in the TxS catalytical activity. Pulse-chase experiments showed a half life for the TxS protein of 16.4 h in both control and phorbol-ester-treated cells. The biological half life of TxS was 10.5 h, as determined by PGH2 incorporation into TxB2 after cycloheximide treatment. In contrast, the biological half lives of PGH synthase, prostacyclin synthase and 5-lipoxygenase were significantly shorter and were 3, 2.5 and 2.5 h, respectively. These results reveal that Tx synthesis in macrophages is mediated by at least two distinct mechanisms; a protein-kinase-C-dependent induction of de novo synthesis of TxS and the selective resistance of the enzyme against the activity of protein kinase C.

Oncogene-transformed granulosa cells as a model system for the study of steroidogenic processes

Highly steroidogenic granulosa cell lines were established by transfection of primary granulosa cells from preovulatory follicles with SV40 DNA and Ha-ras oncogene. Progesterone production in these cells was enhanced to levels comparable to normal steroidogenic cells, by prolonged (> 12 h) stimulation with 8-Br-cAMP, forskolin and cholera toxin, which elevate intracellular cAMP. The steroidogenic capacity of individual lines correlated with the expression of the ras oncogene product (p21) and the morphology of the cells. Formation of the steroid hormones was associated with de novo synthesis of the mitochondrial cytochrome P450scc system proteins. Since cholesterol import into mitochondria is essential for steroidogenesis, the expression of the peripheral benzodiazepine receptor (PBR) and the sterol carrier protein 2 was characterized in these cells. The induction of the expression of the genes coding for both proteins appeared to be mediated, at least in part, by cAMP. Stimulation of the PBR by specific agonists enhanced progesterone production in these cells. The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) dramatically suppressed the cAMP-induced steroidogenesis, in spite of enhanced intracellular cAMP levels, suggesting that TPA can modify the effects of cAMP. cAMP stimulation suppressed growth of transformed cells concomitantly with induction of steroidogenesis. The transformed cells lacked receptors for the native stimulants, the gonadotropic hormones. After transfection of the cells with a lutropin (LH) receptor expression plasmid, the LH and hCG response was reconstituted. In these newly established cell lines gonadotropins were able to stimulate the formation of cAMP and progesterone in a dose-dependent manner with an ED₅₀ characteristic of the native receptor. High doses caused desensitization to gonadotropins as observed in normal cells. These newly established oncogene-transformed granulosa cell lines can serve as a useful model to study inducible steroidogenesis and the effect of oncogene expression on this process.

Chicken "erythroid" cells transformed by the Gag-Myb-Ets-encoding E26 leukemia virus are multipotent

The E26 avian leukemia virus encodes a transcriptional activator-type oncoprotein consisting of Gag, Myb, and Ets domains, and transforms early erythroid cells as well as myeloblasts. Surprisingly, we have found that "early erythroid" transformants obtained in culture are multipotent, since they can be induced to differentiate into myeloblasts and eosinophils after superinfection with retroviruses containing kinase-type or ras oncogenes. In addition, TPA is an efficient inducer that generates predominantly eosinophils at low concentrations and myeloblasts at high concentrations. The determination process involves the complete extinction of erythroid/thrombocytic markers and the subsequent activation of myelomonocytic/eosinophilic properties, including the acquisition of specific growth factor requirements. "Erythroleukemic" cells from virus-infected animals were likewise found to be multipotent, making this a unique system to study the genesis of stem cell leukemias and the molecular basis of lineage commitment during hematopoiesis.

Activation of the c-myb locus is insufficient for the rapid induction of disseminated avian B-cell lymphoma

We have previously reported that infection of 9- to 13-day-old chicken embryos with RAV-1 results in rapid development of a novel B-cell lymphoma in which proviral insertion has activated expression of the c-myb gene (E. Pizer and E. H. Humphries, J. Virol. 63:1630-1640, 1989). The biological properties of these B-cell lymphomas are distinct from those associated with the B-cell lymphomas that develop following avian leukosis virus proviral insertion within the c-myc locus. In an extension of this study, more than 200 chickens, infected as 10- to 11-day-old embryos, were examined for development of lymphomas that possess disrupted c-myb loci. Fourteen percent developed disseminated B-cell lymphoma. In the majority of these tumors, the RAV-1 provirus had inserted between the first and second exons that code for p75c-myb. However, insertions between the second and third exons and between the third and fourth exons were also detected. In situ analysis of myb protein expression in tumor tissue revealed morphological features suggesting that the tumor originates in the bursa. Within the bursa, the lymphoma appeared to spread from follicle to follicle without compromising the structural integrity of the organ. Tumor masses in liver demonstrated heterogeneous levels of myb protein suggestive of biologically distinct subpopulations. In contrast to the morbidity data, immunohistological analysis of bursae from 4- to 6-week-old chickens at risk of developing lymphomas bearing altered c-myb loci revealed lesions expressing elevated levels of myb in 16 of 19 birds. The activated myb lymphoma displayed very poor capacity to proliferate outside its original host. Only 1 of 33 in vivo transfers of tumor to recipient hosts established a transplantable tumor. None of the primary tumor tissue nor the transplantable tumor exhibited the capacity for in vitro proliferation. Similar experimental manipulation has yielded in vitro lines established from avian B-cell lymphomas expressing elevated levels of c-myc or v-rel. The dependence on embryonic infection for development of activated-myb lymphoma suggests a requirement for a specific target cell in which c-myb is activated by proviral insertion. It is likely, moreover, that continued tumor development requires elevated expression of myb proteins within a specific cell population in a restricted stage of differentiation.

Protein truncation is required for the activation of the c-myb proto-oncogene

The protein product of the v-myb oncogene of avian myeloblastosis virus, v-Myb, differs from its normal cellular counterpart, c-Myb, by (i) expression under the control of a strong viral long terminal repeat, (ii) truncation of both its amino and carboxyl termini, (iii) replacement of these termini by virally encoded residues, and (iv) substitution of 11 amino acid residues. We had previously shown that neither the virally encoded termini nor the amino acid substitutions are required for transformation by v-Myb. We have now constructed avian retroviruses that express full-length or singly truncated forms of c-Myb and have tested them for the transformation of chicken bone marrow cells. We conclude that truncation of either the amino or carboxyl terminus of c-Myb is sufficient for transformation. In contrast, the overexpression of full-length c-Myb does not result in transformation. We have also shown that the amino acid substitutions of v-Myb by themselves are not sufficient for the activation of c-Myb. Rather, the presence of either the normal amino or carboxyl terminus of c-Myb can suppress transformation when fused to v-Myb. Cells transformed by c-Myb proteins truncated at either their amino or carboxyl terminus appear to be granulated promyelocytes that express the Mim-1 protein. Cells transformed by a doubly truncated c-Myb protein are not granulated but do express the Mim-1 protein, in contrast to monoblasts transformed by v-Myb that neither contain granules nor express Mim-1. These results suggest that various alterations of c-Myb itself may determine the lineage of differentiating hematopoietic cells.

Protein truncation is required for the activation of the c-myb proto-oncogene

The protein product of the v-myb oncogene of avian myeloblastosis virus, v-Myb, differs from its normal cellular counterpart, c-Myb, by (i) expression under the control of a strong viral long terminal repeat, (ii) truncation of both its amino and carboxyl termini, (iii) replacement of these termini by virally encoded residues, and (iv) substitution of 11 amino acid residues. We had previously shown that neither the virally encoded termini nor the amino acid substitutions are required for transformation by v-Myb. We have now constructed avian retroviruses that express full-length or singly truncated forms of c-Myb and have tested them for the transformation of chicken bone marrow cells. We conclude that truncation of either the amino or carboxyl terminus of c-Myb is sufficient for transformation. In contrast, the overexpression of full-length c-Myb does not result in transformation. We have also shown that the amino acid substitutions of v-Myb by themselves are not sufficient for the activation of c-Myb. Rather, the presence of either the normal amino or carboxyl terminus of c-Myb can suppress transformation when fused to v-Myb. Cells transformed by c-Myb proteins truncated at either their amino or carboxyl terminus appear to be granulated promyelocytes that express the Mim-1 protein. Cells transformed by a doubly truncated c-Myb protein are not granulated but do express the Mim-1 protein, in contrast to monoblasts transformed by v-Myb that neither contain granules nor express Mim-1. These results suggest that various alterations of c-Myb itself may determine the lineage of differentiating hematopoietic cells.

A functional c-myb gene is required for normal murine fetal hepatic hematopoiesis

The c-myb proto-oncogene encodes a sequence-specific DNA-binding protein. To better understand its normal biological function, we have altered the c-myb gene by homologous recombination in mouse embryonic stem cells. Resulting homozygous c-myb mutant mice displayed an interesting phenotype. At day 13 of gestation these mice appeared normal, suggesting that c-myb is not essential for early development. By day 15, however, the mutant mice were severely anemic. Analysis indicated that embryonic erythropoiesis, which occurs in the yolk sac, was not impaired by the c-myb alteration. Adult-type erythropoiesis, which first takes place in the fetal liver, was greatly diminished in c-myb mutants, however. Additional hematopoietic lineages were similarly affected. These results are compatible with a role for c-myb in maintaining the proliferative state of hematopoietic progenitor cells.

Mutations in v-myb alter the differentiation of myelomonocytic cells transformed by the oncogene

Chick myelomonocytic cells transformed by the v-myb oncogene-containing viruses E26 and AMV differ in that the former resemble myeloblasts and express the v-myb-regulated granulocyte-specific mim-1 gene, while the latter resemble monoblasts and are mim-1 negative. We constructed a series of AMV-E26 chimeras and localized the critical differences between these viruses to three point mutations within the second repeat of the v-myb DNA binding domain. These three positions are altered in the v-myb protein of AMV relative to the proteins encoded by c-myb or E26 v-myb. Back mutating AMV v-myb at any of these three sites restored the oncogene's ability to activate the mim-1 gene. Surprisingly, two of these changes led to the transformation, in vitro and in vivo, of cells having a promyelocyte-like phenotype. These results indicate that different forms of v-myb impose alternate phenotypes of differentiation on transformed myeloid cells, probably by regulating unique sets of differentiation-specific genes.

Homeobox gene expression plus autocrine growth factor production elicits myeloid leukemia

In the murine myelomonocytic leukemia WEHI-3B, proviral insertions have induced expression of the Hox-2.4 homeobox gene and the gene for the myeloid growth factor interleukin 3 (IL-3). To assess their potential oncogenic role, normal bone marrow cells were infected with retroviruses bearing the genes for IL-3 or IL-3 plus Hox-2.4. Unlike the IL-3 virus, the IL-3/Hox-2.4 virus was highly leukemogenic. Infected cells expressing both genes exhibited retarded differentiation in vitro, generated myelomonocytic cell lines, and provoked a rapid, transplantable myeloid leukemia in vivo. The oncogenic action of Hox-2.4 appears to derive from its ability to impede the IL-3-driven terminal differentiation of myeloid cells. The results suggest that homeobox genes can regulate key differentiation processes such as self-renewal capacity and that their inappropriate expression can be oncogenic.

Transformation by v-myb correlates with trans-activation of gene expression

The v-myb oncogene of avian myeloblastosis virus causes acute myelomonocytic leukemia in chickens and transforms avian myeloid cells in vitro. Its protein product p48v-myb is a nuclear, sequence-specific, DNA-binding protein which activates gene expression in transient DNA transfection studies. To investigate the relationship between transformation and trans-activation by v-myb, we constructed 15 in-frame linker insertion mutants. The 12 mutants which transformed myeloid cells also trans-activated gene expression, whereas the 3 mutants which did not transform also did not trans-activate. This implies that trans-activation is required for transformation by v-myb. One of the transformation-defective mutants localized to the cell nucleus but failed to bind DNA. The other two transformation-defective mutants localized to the cell nucleus and bound DNA but nevertheless failed to trans-activate. These latter mutants define two distinct domains of p48v-myb which control trans-activation by DNA-bound protein, one within the amino-terminal DNA-binding domain itself and one in a carboxyl-terminal domain which is not required for DNA binding.