Defective stem cell factor expression in c-myb null fetal liver stroma

Transcriptional regulatory program controlled by MYB in T-cell acute lymphoblastic leukemia

The transcription factor MYB is frequently upregulated in T-cell acute lymphoblastic leukemia (T-ALL), a hematological malignancy originating from T-cell precursors. Here, we demonstrate that MYB plays a crucial role by regulating genes essential for T-ALL pathogenesis. Integrative analysis reveals a long MYB isoform, ENST00000367814.8, which is dominantly expressed and confers a proliferative advantage in T-ALL cells. Rapid depletion of MYB via dTAG-mediated protein degradation affects a large number of genes, which can be classified into early response or late response genes based on their kinetics. Early response genes include many genes involved in hematopoiesis, such as TAL1, RUNX1, GATA3, IKZF2, and CXCR4. Their expression can be recovered at later time-points, suggesting the presence of a negative feedback loop mechanism. In contrast, late response genes, which are continuously downregulated after MYB depletion, includes many genes involved in cell proliferation as well as TAL1 targets, thereby affecting the cellular phenotype.

Erythropoiesis in the mammalian embryo

Red blood cells (RBCs) comprise a critical component of the cardiovascular network, which constitutes the first functional organ system of the developing mammalian embryo. Examination of circulating blood cells in mammalian embryos revealed two distinct types of erythroid cells: large, nucleated "primitive" erythroblasts followed by smaller, enucleated "definitive" erythrocytes. This review describes the current understanding of primitive and definitive erythropoiesis gleaned from studies of mouse and human embryos and induced pluripotent stem cells (iPSCs). Primitive erythropoiesis in the mouse embryo comprises a transient wave of committed primitive erythroid progenitors (primitive erythroid colony-forming cells, EryP-CFC) in the early yolk sac that generates a robust cohort of precursors that mature in the bloodstream and enucleate. In contrast, definitive erythropoiesis has two distinct developmental origins. The first comprises a transient wave of definitive erythroid progenitors (burst-forming units erythroid, BFU-E) that emerge in the yolk sac and seed the fetal liver where they terminally mature to provide the first definitive RBCs. The second comprises hematopoietic stem cell (HSC)-derived BFU-E that terminally mature at sites colonized by HSCs particularly the fetal liver and subsequently the bone marrow. Primitive and definitive erythropoiesis are derived from endothelial identity precursors with distinct developmental origins. Although they share prototypical transcriptional regulation, primitive and definitive erythropoiesis are also characterized by distinct lineage-specific factors. The exquisitely timed, sequential production of primitive and definitive erythroid cells is necessary for the survival and growth of the mammalian embryo.

C-myb Plays an Essential Role in the Protective Function of IGF-1 on Cytotoxicity Induced by Aβ25-35 via the PI3K/Akt Pathway

There have been numerous reports about neurodegenerative diseases, including Alzheimer's disease. Nevertheless, the molecules responsible for the neurodegeneration in Alzheimer's disease are basically unknown. Recent findings indicate that the cellular myeloblastosis (c-myb) regulates neural progenitor cell proliferation. In the current study, the function of insulin-like growth factor-1 (IGF-1) against cell toxicity in SH-SY5Y cells induced by β-amyloid 25-35 (Aβ_25-35) and its molecular mechanism were investigated. It was found that p25 protein production was raised by Aβ_25-35 (25 μM), similar to the increased expression of μ-calpain. The results also showed that Aβ_25-35 reduced c-myb, elevated tau hyper-phosphorylation, and induced death of SH-SY5Y cells. Loss of cell viability and apoptosis of SH-SY5Y cells induced by Aβ_25-35 were attenuated by IGF-1 pretreatment in a dose-dependent manner. In addition, IGF-1 blocked μ-calpain expression, which was induced by Aβ_25-35 and reduced p25 formation and tau hyper-phosphorylation. Moreover, the expression of c-myb in SH-SY5Y cells was increased by combining IGF-1 with Aβ_25-35 or IGF-1 alone. The neuroprotective function of IGF-1 was attenuated in the SH-SY5Y cells, which were transfected with a c-myb small interfering RNA. Furthermore, LY294002, a specific PI3K inhibitor, reduced c-myb expression and abolished IGF-1's protective function in SH-SY5Y cell apoptosis induced by Aβ_25-35. The facts above indicate that c-myb has a role in IGF-1-mediated protection from Aβ_25-35-induced cytotoxicity via the PI3K/Akt pathway.

Cytomegalovirus pp71 protein is expressed in human glioblastoma and promotes pro-angiogenic signaling by activation of stem cell factor

Glioblastoma multiforme (GBM) is a highly malignant primary central nervous system neoplasm characterized by tumor cell invasion, robust angiogenesis, and a mean survival of 15 months. Human cytomegalovirus (HCMV) infection is present in >90% of GBMs, although the role the virus plays in GBM pathogenesis is unclear. We report here that HCMV pp71, a viral protein previously shown to promote cell cycle progression, is present in a majority of human GBMs and is preferentially expressed in the CD133+, cancer stem-like cell population. Overexpression of pp71 in adult neural precursor cells resulted in potent induction of stem cell factor (SCF), an important pro-angiogenic factor in GBM. Using double immunofluorescence, we demonstrate in situ co-localization of pp71 and SCF in clinical GBM specimens. pp71 overexpression in both normal and transformed glial cells increased SCF secretion and this effect was specific, since siRNA mediated knockdown of pp71 or treatment with the antiviral drug cidofovir resulted in decreased expression and secretion of SCF by HCMV-infected cells. pp71- induced upregulation of SCF resulted in downstream activation of its putative endothelial cell receptor, c-kit, and angiogenesis as measured by increased capillary tube formation in vitro. We demonstrate that pp71 induces a pro-inflammatory response via activation of NFΚB signaling which drives SCF expression. Furthermore, we show that pp71 levels and NFKB activation are selectively augmented in the mesenchymal subtype of human GBMs, characterized by worst patient outcome, suggesting that HCMV pp71-induced paracrine signaling may contribute to the aggressive phenotype of this human malignancy.

The MYB oncogene can suppress apoptosis in acute myeloid leukemia cells by transcriptional repression of DRAK2 expression

RNA interference-mediated suppression of MYB expression promoted apoptosis in the AML cell line U937, without affecting expression of the anti-apoptotic MYB target BCL2. This was accompanied by up-regulation of the pro-apoptotic gene DRAK2 and stimulation of caspase-9 activity. Moreover, RNA interference-mediated suppression of DRAK2 in U937 cells alleviated apoptosis induced by MYB down-regulation. Finally ChIP assays showed that in U937 cells MYB binds to a conserved element upstream of the DRAK2 transcription start site. Together, these findings identify a novel mechanism by which MYB suppresses apoptosis in an AML model cell line.

Colorectal cancer mouse models: integrating inflammation and the stroma

Sequences of molecular events that initiate and advance the progression of human colorectal cancer (CRC) are becoming clearer. Accepting that these events, once they are in place, accumulate over time, rapid disease progression might be expected. Yet CRC usually develops slowly over decades. Emerging insights suggest that the tumor cell microenvironment encompassing fibroblasts and endothelial and immune cells dictate when, whether, and how malignancies progress. Signaling pathways that affect the microenvironment and the inflammatory response seem to play a central role in CRC. Indeed, some of these pathways directly regulate the stem/progenitor cell niche at the base of the crypt; it now appears that the survival and growth of neoplastic cells often relies upon their subverted engagement of these pathways. Spurned on by the use of gene manipulation technologies in the mouse, dissecting and recapitulating these complex molecular interactions between the tumor and its microenvironment in the gastrointestinal (GI) tract is a reality. In parallel, our ability to isolate and grow GI stem cells in vitro enables us, for the first time, to complement reductionist in vitro findings with complex in vivo observations. Surprisingly, data suggest that the large number of signaling pathways underpinning the reciprocal interaction between the neoplastic epithelium and its microenvironment converge on a small number of common transcription factors. Here, we review the separate and interactive roles of NFκB, Stat3, and Myb, transcription factors commonly overexpressed or excessively activated in CRC. They confer molecular links between inflammation, stroma, the stem cell niche, and neoplastic cell growth.

Conditional c-myb knockout in adult hematopoietic stem cells leads to loss of self-renewal due to impaired proliferation and accelerated differentiation

Hematopoietic stem cells (HSCs) have a unique capacity to undergo self-renewal and multi-lineage differentiation to provide a lifetime supply of mature blood cells. By using conditional knockout technology, we disrupted the c-myb proto-oncogene specifically in adult bone marrow (BM) to demonstrate that this transcription factor is a regulator of self-renewal and multi-lineage differentiation of adult HSCs. Targeted disruption of the c-myb gene resulted in a critical depletion of the HSC pool. In addition, BM hematopoiesis in adult mice was impaired, resulting in profound reductions of various hematopoietic lineages including neutrophilic, monocytic, B lymphoid, erythroid, and, unexpectedly, megakaryocytic cells. Serial BM transplantation into lethally irradiated recipient mice indicated an essential role for c-myb in the self-renewal process. Furthermore, in vitro functional assays demonstrated that deletion of the c-myb gene leads to a slightly reduced proliferative capacity and an aberrant and accelerated differentiation of HSCs. In addition to long-term HSCs, functional studies also show that c-myb plays a critical role in short-term HSCs and multi-potential progenitors. Collectively, our data indicate a critical role for c-myb in adult BM hematopoiesis and in self-renewal and multi-lineage differentiation of adult HSCs.

MYB function in normal and cancer cells

The transcription factor MYB has a key role as a regulator of stem and progenitor cells in the bone marrow, colonic crypts and a neurogenic region of the adult brain. It is in these compartments that a deficit in MYB activity leads to severe or lethal phenotypes. As was predicted from its leukaemogenicity in several animal species, MYB has now been identified as an oncogene that is involved in some human leukaemias. Moreover, recent evidence has strengthened the case that MYB is activated in colon and breast cancer: a block to MYB expression is overcome by mutation of the regulatory machinery in the former disease and by oestrogen receptor-alpha (ERalpha) in the latter.

c-Myb is required for neural progenitor cell proliferation and maintenance of the neural stem cell niche in adult brain

Ongoing production of neurons in adult brain is restricted to specialized neurogenic niches. Deregulated expression of genes controlling homeostasis of neural progenitor cell division and/or their microenvironment underpins a spectrum of brain pathologies. Using conditional gene deletion, we show that the proto-oncogene c-myb regulates neural progenitor cell proliferation and maintains ependymal cell integrity in mice. These two cellular compartments constitute the neurogenic niche in the adult brain. Brains devoid of c-Myb showed enlarged ventricular spaces, ependymal cell abnormalities, and reduced neurogenesis. Neural progenitor cells lacking c-Myb showed a reduced intrinsic proliferative capacity and reduction of Sox-2 and Pax-6 expression. These data point to an important role for c-Myb in the neurogenic niche of the adult brain.

The Cxcl12, periostin, and Ccl9 genes are direct targets for early B-cell factor in OP-9 stroma cells

The development of blood cells from hematopoietic stem cells in the bone marrow is dependent on communication with bone marrow stroma cells, making these cells central for the appropriate regulation of hematopoiesis. To identify transcription factors that may play a role in gene regulation in stroma cells, we performed comparative gene expression analysis of fibroblastic NIH3T3 cells, unable to support hematopoiesis in vitro, and OP-9 stroma cells, highly efficient in this regard. These experiments revealed that transcription factors of the early B cell factor (EBF) family were highly expressed in OP-9 cells as compared with the NIH3T3 cells. To identify potential targets genes for EBF proteins in stroma cells, we overexpressed EBF in fibroblasts and analyzed the pattern of induced genes by microarray analysis. This revealed that EBF was able to up-regulate expression of among others the Cxcl12, Ccl9, and Periostin genes. The identification of relevant promoters revealed that they all contained functional EBF binding sites able to interact with EBF in OP-9 cells. Furthermore, ectopic expression of a dominant negative EBF protein or antisense EBF-1 RNA in OP-9 stroma cells resulted in reduced expression of these target genes. These data suggest that EBF proteins might have dual roles in hematopoiesis acting both as intrinsic regulators of B-lymphopoiesis and as regulators of genes in bone marrow stroma cells.

Expression of Pitx2 in stromal cells is required for normal hematopoiesis

Although the expression of Pitx2, a bicoid family homeodomain transcription factor, is highly regulated during hematopoiesis, its function during this process was not documented; we thus studied hematopoiesis in Pitx2-null mice. We found that Pitx2(-/-) embryos display hypoplastic livers with reduced numbers of hematopoietic cells, but these cells had normal hematopoietic potential, as evidenced by colony-forming assays, immature progenitor cell assays, and long-term repopulation assays. Because the microenvironment is also crucial to the development of normal hematopoiesis, we established Pitx2(-/-) and Pitx2(+/+) stromas from fetal liver and studied their hematopoietic supportive capacity. We showed that the frequency of cobblestone area-forming cells was 4-fold decreased when using Pitx2(-/-) stromal cells compared with Pitx2(+/+) stromal cells, whatever the Pitx2 genotype of hematopoietic cells tested in this assay. This defect was rescued by expression of Pitx2 into Pitx2(-/-) fetal liver stromal cells, demonstrating a major and direct role of Pitx2 in the hematopoietic supportive capacity of fetal liver stroma. Finally, we showed a reduced capacity of MS5 stromal cells expressing Pitx2 RNAi to support human hematopoiesis. Altogether these data showed that Pitx2 has major functions in the hematopoietic supportive capacity of fetal liver and adult bone marrow stromal cells.

c-myb Heterozygous Mice Are Hypersensitive to 5-Fluorouracil and Ionizing Radiation

Hypersensitivity to chemo- and radiotherapy employed during cancer treatment complicates patient management. Identifying mutations in genes that compromise tissue recovery would rationalize treatment and may spare hypersensitive patients undue tissue damage. Genes that govern stem cell homeostasis, survival, and progenitor cell maintenance are of particular interest in this regard. We used wild-type and c-myb knock-out mice as model systems to explore stem and progenitor cell numbers and sensitivity to cytotoxic damage in two radiosensitive tissue compartments, the bone marrow and colon. Because c-myb null mice are not viable, we used c-myb heterozygous mice to test for defects in stem-progenitor cell pool recovery following γ-radiation and 5-fluorouracil treatment, showing that c-myb+/− mice are hypersensitive to both agents. While apoptosis is comparable in mutant and wild-type mice following radiation exposure, the crypt beds of c-myb+/− mice are markedly depleted of proliferating cells. Extrapolating from these data, we speculate that acute responses to cytotoxic damage in some patients may also be attributed to compromised c-myb function.

Targeting c-Myb expression in human disease

c-Myb is a transcription factor employed in the haematopoietic system and gastrointestinal tract to regulate the exquisite balance between cell division, differentiation and survival. In its absence, these tissues either fail to form, or show aberrant biology. Mice lacking a functional c-myb gene die in utero by day 15 of development. When inappropriately expressed, as is common in leukaemia and epithelial cancers of the breast, colon and gastro-oesophagus, c-Myb appears to activate gene targets of key importance to cancer progression and metastasis. These genes include cyclooxygenase-2 (COX-2), Bcl-2, BclX(L) and c-Myc, which influence diverse processes such as angiogenesis, proliferation and apoptosis. The clinical potential for blocking c-Myb expression in malignancies is based upon strong preclinical data and some trial-based evidence. The modest clinical experience to date has been with haematopoietic malignancies, but other disease classes may be amenable to similar interventions. The frontline agents to achieve this are nuclease-resistant oligodeoxynucleotides (ODNs), which are proving to be acceptable therapeutic reagents in terms of tolerable toxicities and delivery. Nevertheless, further effort must be focused on improving their efficacy, eliminating non-specific toxicity and optimising delivery. Optimisation issues aside, it would appear that anti-c-Myb therapies will be used with most success when combined with other agents, some of which will be established cytotoxic and differentiation-inducing drugs. This review will explore the future strategic use of ODNs in vivo, focusing on a wide spectrum of diseases, including several beyond the haematopoietic malignancies, in which c-Myb appears to play a role.