Members of the nuclear factor kappa B family transactivate the murine c-myb gene

MYB: A Key Transcription Factor in the Hematopoietic System Subject to Many Levels of Control

MYB is a master regulator and pioneer factor highly expressed in hematopoietic progenitor cells (HPCs) where it contributes to the reprogramming processes operating during hematopoietic development. MYB plays a complex role being involved in several lineages of the hematopoietic system. At the molecular level, the MYB gene is subject to intricate regulation at many levels through several enhancer and promoter elements, through transcriptional elongation control, as well as post-transcriptional regulation. The protein is modulated by post-translational modifications (PTMs) such as SUMOylation restricting the expression of its downstream targets. Together with a range of interaction partners, cooperating transcription factors (TFs) and epigenetic regulators, MYB orchestrates a fine-tuned symphony of genes expressed during various stages of haematopoiesis. At the same time, the complex MYB system is vulnerable, being a target for unbalanced control and cancer development.

FOXO1 Confers Maintenance of the Dark Zone Proliferation and Survival Program and Can Be Pharmacologically Targeted in Burkitt Lymphoma

The FOXO1 transcription factor plays a central role in the proliferation and survival of B cells at several stages of differentiation. B cell malignancies, with exception of classical Hodgkin lymphoma, maintain expression of FOXO1 at levels characteristic for their non-malignant counterparts. Extensive expression profiling had revealed that Burkitt lymphoma (BL) show many characteristics of the dark zone (DZ) germinal center (GC) B cell program. Here we show that FOXO1 knockdown inhibits proliferation of human BL cell lines. The anti-proliferative effect of the FOXO1 knockdown is associated with the repression of the DZ B cell program including expression of MYB, CCND3, RAG2, BACH2, and CXCR4. In addition, the induction of signaling pathways of the light zone (LZ) program like NF-κB and PI3K-AKT was observed. Using a rescue experiment we identified downregulation of the proto-oncogene MYB as a critical factor contributing to the antiproliferative effect of FOXO1 knockdown. In an attempt to estimate the feasibility of pharmacological FOXO1 repression, we found that the small molecular weight FOXO1 inhibitor AS1842856 induces cell death and growth arrest in BL cell lines at low concentrations. Interestingly, we found that overactivation of FOXO1 also induces growth inhibition in BL cell lines, indicating the importance of a tight regulation of FOXO1 activity in BL.

Leprosy drug clofazimine activates peroxisome proliferator-activated receptor-γ and synergizes with imatinib to inhibit chronic myeloid leukemia cells

Leukemia stem cells contribute to drug-resistance and relapse in chronic myeloid leukemia (CML) and BCR-ABL1 inhibitor monotherapy fails to eliminate these cells, thereby necessitating alternate therapeutic strategies for patients CML. The peroxisome proliferator-activated receptor-γ (PPARγ) agonist pioglitazone downregulates signal transducer and activator of transcription 5 (STAT5) and in combination with imatinib induces complete molecular response in imatinib-refractory patients by eroding leukemia stem cells. Thiazolidinediones such as pioglitazone are, however, associated with severe side effects. To identify alternate therapeutic strategies for CML we screened Food and Drug Administration-approved drugs in K562 cells and identified the leprosy drug clofazimine as an inhibitor of viability of these cells. Here we show that clofazimine induced apoptosis of blood mononuclear cells derived from patients with CML, with a particularly robust effect in imatinib-resistant cells. Clofazimine also induced apoptosis of CD34⁺38^- progenitors and quiescent CD34⁺ cells from CML patients but not of hematopoietic progenitor cells from healthy donors. Mechanistic evaluation revealed that clofazimine, via physical interaction with PPARγ, induced nuclear factor kB-p65 proteasomal degradation, which led to sequential myeloblastoma oncoprotein and peroxiredoxin 1 downregulation and concomitant induction of reactive oxygen species-mediated apoptosis. Clofazimine also suppressed STAT5 expression and consequently downregulated stem cell maintenance factors hypoxia-inducible factor-1α and -2α and Cbp/P300 interacting transactivator with Glu/Asp-rich carboxy-terminal domain 2 (CITED2). Combining imatinib with clofazimine caused a far superior synergy than that with pioglitazone, with clofazimine reducing the half maximal inhibitory concentration (IC₅₀) of imatinib by >4 logs and remarkably eroding quiescent CD34⁺ cells. In a K562 xenograft study clofazimine and imatinib co-treatment showed more robust efficacy than the individual treatments. We propose clinical evaluation of clofazimine in imatinib-refractory CML.

The NF-κB Activating Pathways in Multiple Myeloma

Multiple myeloma(MM), an incurable plasma cell cancer, represents the second most prevalent hematological malignancy. Deregulated activity of the nuclear factor kappaB (NF-κB) family of transcription factors has been implicated in the pathogenesis of multiple myeloma. Tumor microenvironment-derived cytokines and cancer-associated genetic mutations signal through the canonical as well as the non-canonical arms to activate the NF-κB system in myeloma cells. In fact, frequent engagement of both the NF-κB pathways constitutes a distinguishing characteristic of myeloma. In turn, NF-κB signaling promotes proliferation, survival and drug-resistance of myeloma cells. In this review article, we catalog NF-κB activating genetic mutations and microenvironmental cues associated with multiple myeloma. We then describe how the individual canonical and non-canonical pathways transduce signals and contribute towards NF-κB -driven gene-expressions in healthy and malignant cells. Furthermore, we discuss signaling crosstalk between concomitantly triggered NF-κB pathways, and its plausible implication for anomalous NF-κB activation and NF-κB driven pro-survival gene-expressions in multiple myeloma. Finally, we propose that mechanistic understanding of NF-κB deregulations may provide for improved therapeutic and prognostic tools in multiple myeloma.

Advances in the pathophysiology and treatment of relapsed/refractory Hodgkin's lymphoma with an emphasis on targeted therapies and transplantation strategies

Hodgkin’s lymphoma (HL) is highly curable with first-line therapy. However, a minority of patients present with refractory disease or experience relapse after completion of frontline treatment. These patients are treated with salvage chemotherapy followed by autologous stem cell transplantation (ASCT), which remains the standard of care with curative potential for refractory or relapsed HL. Nevertheless, a significant percentage of such patients will progress after ASCT, and allogeneic hematopoietic stem cell transplantation remains the only curative approach in that setting. Recent advances in the pathophysiology of refractory or relapsed HL have provided the rationale for the development of novel targeted therapies with potent anti-HL activity and favorable toxicity profile, in contrast to cytotoxic chemotherapy. Brentuximab vedotin and programmed cell death-1-based immunotherapy have proven efficacy in the management of refractory or relapsed HL, whereas several other agents have shown promise in early clinical trials. Several of these agents are being incorporated with transplantation strategies in order to improve the outcomes of refractory or relapsed HL. In this review we summarize the current knowledge regarding the mechanisms responsible for the development of refractory/relapsed HL and the outcomes with current treatment strategies, with an emphasis on targeted therapies and hematopoietic stem cell transplantation.

MYB elongation is regulated by the nucleic acid binding of NFκB p50 to the intronic stem-loop region

MYB transcriptional elongation is regulated by an attenuator sequence within intron 1 that has been proposed to encode a RNA stem loop (SLR) followed by a polyU tract. We report that NFκBp50 can bind the SLR polyU RNA and promote MYB transcriptional elongation together with NFκBp65. We identified a conserved lysine-rich motif within the Rel homology domain (RHD) of NFκBp50, mutation of which abrogated the interaction of NFκBp50 with the SLR polyU and impaired NFκBp50 mediated MYB elongation. We observed that the TAR RNA-binding region of Tat is homologous to the NFκBp50 RHD lysine-rich motif, a finding consistent with HIV Tat acting as an effector of MYB transcriptional elongation in an SLR dependent manner. Furthermore, we identify the DNA binding activity of NFκBp50 as a key component required for the SLR polyU mediated regulation of MYB. Collectively these results suggest that the MYB SLR polyU provides a platform for proteins to regulate MYB and reveals novel nucleic acid binding properties of NFκBp50 required for MYB regulation.

Piperlongumine inhibits the proliferation and survival of B-cell acute lymphoblastic leukemia cell lines irrespective of glucocorticoid resistance

Piperlongumine (PL), a pepper plant alkaloid from Piper longum, has anti-inflammatory and anti-cancer properties. PL selectively kills both solid and hematologic cancer cells, but not normal counterparts. Here we evaluated the effect of PL on the proliferation and survival of B-cell acute lymphoblastic leukemia (B-ALL), including glucocorticoid (GC)-resistant B-ALL. Regardless of GC-resistance, PL inhibited the proliferation of all B-ALL cell lines, but not normal B cells, in a dose- and time-dependent manner and induced apoptosis via elevation of ROS. Interestingly, PL did not sensitize most of B-ALL cell lines to dexamethasone (DEX). Only UoC-B1 exhibited a weak synergistic effect between PL and DEX. All B-ALL cell lines tested exhibited constitutive activation of multiple transcription factors (TFs), including AP-1, MYC, NF-κB, SP1, STAT1, STAT3, STAT6 and YY1. Treatment of the B-ALL cells with PL significantly downregulated these TFs and modulated their target genes. While activation of AURKB, BIRC5, E2F1, and MYB mRNA levels were significantly downregulated by PL, but SOX4 and XBP levels were increased by PL. Intriguingly, PL also increased the expression of p21 in B-ALL cells through a p53-independent mechanism. Given that these TFs and their target genes play critical roles in a variety of hematological malignancies, our findings provide a strong preclinical rationale for considering PL as a new therapeutic agent for the treatment of B-cell malignancies, including B-ALL and GC-resistant B-ALL.

NF-κB factors control the induction of NFATc1 in B lymphocytes

In peripheral lymphocytes, the transcription factors (TFs) NF-κB, NFAT, and AP-1 are the prime targets of signals that emerge from immune receptors. Upon activation, these TFs induce gene networks that orchestrate the growth, expansion, and effector function of peripheral lymphocytes. NFAT and NF-κB factors share several properties, such as a similar mode of induction and architecture in their DNA-binding domain, and there is a subgroup of κB-like DNA promoter motifs that are bound by both types of TFs. However, unlike NFAT and AP-1 factors that interact and collaborate in binding to DNA, NFAT, and NF-κB seem neither to interact nor to collaborate. We show here that NF-κB1/p50 and c-Rel, the most prominent NF-κB proteins in BCR-induced splenic B cells, control the induction of NFATc1/αA, a prominent short NFATc1 isoform. In part, this is mediated through two composite κB/NFAT-binding sites in the inducible Nfatc1 P1 promoter that directs the induction of NFATc1/αA by BCR signals. In concert with coreceptor signals that induce NF-κB factors, BCR signaling induces a persistent generation of NFATc1/αA. These data suggest a tight connection between NFATc1 and NF-κB induction in B lymphocytes contributing to the effector function of peripheral B cells.

Selective activity of the histone deacetylase inhibitor AR-42 against leukemia stem cells: a novel potential strategy in acute myelogenous leukemia

Most patients with acute myelogenous leukemia (AML) relapse and die of their disease. Increasing evidence indicates that AML relapse is driven by the inability to eradicate leukemia stem cells (LSC). Thus, it is imperative to identify novel therapies that can ablate LSCs. Using an in silico gene expression-based screen for compounds evoking transcriptional effects similar to the previously described anti-LSC agent parthenolide, we identified AR-42 (OSU-HDAC42), a novel histone deacetylase inhibitor that is structurally similar to phenylbutyrate, but with improved activity at submicromolar concentrations. Here, we report that AR-42 induces NF-κB inhibition, disrupts the ability of Hsp90 to stabilize its oncogenic clients, and causes potent and specific cell death of LSCs but not normal hematopoietic stem and progenitor cells. Unlike parthenolide, the caspasedependent apoptosis caused by AR-42 occurs without activation of Nrf-2-driven cytoprotective pathways. As AR-42 is already being tested in early clinical trials, we expect that our results can be extended to the clinic.

Interferon-γ suppresses activin A/NF-E2 induction of erythroid gene expression through the NF-κB/c-Jun pathway

Interferon (IFN)-γ is a proinflammatory cytokine that is linked to erythropoiesis inhibition and may contribute to anemia. However, the mechanism of IFN-γ-inhibited erythropoiesis is unknown. Activin A, a member of the transforming growth factor (TGF)-β superfamily, induces the erythropoiesis of hematopoietic progenitor cells. In this study, a luciferase reporter assay showed that IFN-γ suppressed activin A-induced ζ-globin promoter activation in K562 erythroblast cells in a dose-dependent manner. Activin A reversed the suppressive effect of IFN-γ on the luciferase activity of ζ-globin promoter in a dose-dependent manner. IFN-γ also suppressed the activation of activin A-induced α-globin promoter. IFN-γ reduced the mRNA expression of α-globin, ζ-globin, NF-E2p45, and GATA-1 induced by activin A. The results also showed that IFN-γ induced c-Jun expression when NF-κBp65 and c-Jun bound to two AP-1-binding sites on the c-Jun promoter. The luciferase activity of α-globin and ζ-globin promoters were enhanced by wild-type c-Jun and eliminated by dominant-negative (DN) c-Jun. The suppressive effects of IFN-γ on the mRNA expression of α-globin and ζ-globin were absent in cells expressing DN c-Jun. The ability of NF-E2 to enhance activin A-induced ζ-globin promoter activation decreased when c-Jun was present, and IFN-γ treatment further enhanced the decreasing effect of c-Jun. Chromatin immunoprecipitation revealed that NF-E2p45 bound to the upstream regulatory element (HS-40) of the α-globin gene cluster in response to activin A, whereas c-Jun eliminated this binding. These results suggest that IFN-γ modulates NF-κB/c-Jun to antagonize activin A-mediated NF-E2 transcriptional activity on globin gene expression.

Piperlongumine inhibits proliferation and survival of Burkitt lymphoma in vitro

Piperlongumine (PL), a pepper plant alkaloid from Piper longum, kills solid tumor cells in a highly selective, potent fashion. To evaluate whether PL may have similar effects on malignant blood cells, we determined the efficacy with which PL inhibits the B-lymphocyte derived neoplasm, Burkitt lymphoma (BL). Low micromolar concentrations of PL (IC(50) = 2.8 μM × 8.5 μM) curbed growth and survival of two EBV(+) BL cell lines (Daudi, Raji) and two EBV BL cell lines (Ramos, DG-75), but left normal peripheral blood B-lymphocytes unharmed. PL-dependent cytotoxicity was effected in part by reduced NF-κB and MYC activity, with the former being caused by inhibition of IκBα degradation, nuclear translocation of p65, and binding of NF-κB dimers to cognate DNA sequences in gene promoters. In 4 of 4 BL cell lines, the NF-κB/MYC-regulated cellular target genes, E2F1 and MYB, were down regulated, while the stress sensor gene, GADD45B, was up regulated. The EBV-encoded oncogene, LMP-1, was suppressed in Daudi and Raji cells. Considering that NF-κB, MYC and LMP-1 play a crucial role in the biology of many blood cancers including BL, our results provide a strong preclinical rationale for considering PL in new intervention approaches for patients with hematologic malignancies.

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.

MYB is essential for mammary tumorigenesis

MYB oncogene upregulation is associated with estrogen receptor (ER)-positive breast cancer, but disease requirements for MYB function in vivo have not been explored. In this study, we provide evidence of a critical requirement for MYB functions in models of human and murine breast cancer. In human breast cancer, we found that MYB expression was critical for tumor cell growth both in vitro and in vivo in xenograft settings. In transgenic knockout mice, tissue-specific deletion of the murine MYB gene caused a transient defect in mammary gland development that was reflected in delayed ductal branching and defective apical bud formation. In mouse mammary tumor virus (MMTV)-NEU mice where tumors are initiated by activation of HER2, MYB deletion was sufficient to abolish tumor formation. In the more aggressive MMTV-PyMT model system, MYB deletion delayed tumorigenesis significantly. Together, the findings in these transgenic knockout models implied that MYB was critical during an early window in mammary development when it was essential for tumor initiation, even though MYB loss did not exert a lasting impact upon normal mammary function. Two important MYB-target genes that promote cell survival, BCL2 and GRP78/BIP, were each elevated compared with nontransformed mammary epithelial cells, thereby promoting survival as confirmed in colony formation assays in vitro. Taken together, our findings establish a role for MYB at the hub of ER- and HER2-dependent pathways in mammary carcinogenesis.

Regulation and function of NF-kappaB transcription factors in the immune system

The mammalian Rel/NF-kappaB family of transcription factors, including RelA, c-Rel, RelB, NF-kappaB1 (p50 and its precursor p105), and NF-kappaB2 (p52 and its precursor p100), plays a central role in the immune system by regulating several processes ranging from the development and survival of lymphocytes and lymphoid organs to the control of immune responses and malignant transformation. The five members of the NF-kappaB family are normally kept inactive in the cytoplasm by interaction with inhibitors called IkappaBs or the unprocessed forms of NF-kappaB1 and NF-kappaB2. A wide variety of signals emanating from antigen receptors, pattern-recognition receptors, receptors for the members of TNF and IL-1 cytokine families, and others induce differential activation of NF-kappaB heterodimers. Although work over the past two decades has shed significant light on the regulation of NF-kappaB transcription factors and their functions, much progress has been made in the past two years revealing new insights into the regulation and functions of NF-kappaB. This recent progress is covered in this review.

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.

Critical roles for c-Myb in hematopoietic progenitor cells

While it has long been known that the transcription factor c-Myb is an essential regulator of hematopoiesis, its precise molecular targets have remained elusive. Cell line studies suggest that c-Myb promotes proliferation and at the same time inhibits differentiation, however the early lethality of c-Myb deficient embryos precluded analysis of its role in adult hematopoiesis. Here we review insights derived from recently developed mouse models of c-Myb deficiency that are viable as adults. These studies reveal a complex array of functions for c-Myb in multiple hematopoietic cell types that will redefine our understanding of this crucial transcription factor.

STAP-2 negatively regulates both canonical and noncanonical NF-kappaB activation induced by Epstein-Barr virus-derived latent membrane protein 1

The signal-transducing adaptor protein 2 (STAP-2) is a recently identified adaptor protein that contains a pleckstrin homology (PH) and Src homology 2 (SH2)-like domains, as well as a proline-rich domain in its C-terminal region. In previous studies, we demonstrated that STAP-2 binds to MyD88 and IKK-alpha or IKK-beta and modulates NF-kappaB signaling in macrophages. In the present study, we found that ectopic expression of STAP-2 inhibited Epstein-Barr virus (EBV) LMP1-mediated NF-kappaB signaling and interleukin-6 expression. Indeed, STAP-2 associated with LMP1 through its PH and SH2-like domains, and these proteins interacted with each other in EBV-positive human B cells. We found, furthermore, that STAP-2 regulated LMP1-mediated NF-kappaB signaling through direct or indirect interactions with the tumor necrosis factor receptor (TNFR)-associated factor 3 (TRAF3) and TNFR-associated death domain (TRADD) proteins. STAP-2 mRNA was induced by the expression of LMP1 in human B cells. Furthermore, transient expression of STAP-2 in EBV-positive human B cells decreased cell growth. Finally, STAP-2 knockout mouse embryonic fibroblasts showed enhanced LMP1-induced cell growth. These results suggest that STAP-2 acts as an endogenous negative regulator of EBV LMP1-mediated signaling through TRAF3 and TRADD.

Mechanism of and requirement for estrogen-regulated MYB expression in estrogen-receptor-positive breast cancer cells

MYB (the human ortholog of c-myb) is expressed in a high proportion of human breast tumors, and that expression correlates strongly with estrogen receptor (ER) positivity. This may reflect the fact that MYB is a target of estrogen/ER signaling. Because in many cases MYB expression appears to be regulated by transcriptional attenuation or pausing in the first intron, we first investigated whether this mechanism was involved in estrogen/ER modulation of MYB. We found that this was the case and that estrogen acted directly to relieve attenuation due to sequences within the first intron, specifically, a region potentially capable of forming a stem-loop structure in the transcript and an adjacent poly(dT) tract. Secondly, given the involvement of MYB in hematopoietic and colon tumors, we also asked whether MYB was required for the proliferation of breast cancer cells. We found that proliferation of ER(+) but not ER(-) breast cancer cell lines was inhibited when MYB expression was suppressed by using either antisense oligonucleotides or RNA interference. Our results show that MYB is an effector of estrogen/ER signaling and provide demonstration of a functional role of MYB in breast cancer.

Aberrant NF-kappaB signaling in lymphoma: mechanisms, consequences, and therapeutic implications

The transcription factor NF-kappaB is a tightly regulated positive mediator of T- and B-cell development, proliferation, and survival. The controlled activity of NF-kappaB is required for the coordination of physiologic immune responses. However, constitutive NF-kappaB activation can promote continuous lymphocyte proliferation and survival and has recently been recognized as a critical pathogenetic factor in lymphoma. Various molecular events lead to deregulation of NF-kappaB signaling in Hodgkin disease and a variety of T- and B-cell non-Hodgkin lymphomas either up-stream or downstream of the central IkappaB kinase. These alterations are prerequisites for lymphoma cell cycling and blockage of apoptosis. This review provides an overview of the NF-kappaB pathway and discusses the mechanisms of NF-kappaB deregulation in distinct lymphoma entities with defined aberrant pathways: Hodgkin lymphoma (HL), diffuse large B-cell lymphoma (DLBCL), mucosa-associated lymphoid tissue (MALT) lymphoma, primary effusion lymphoma (PEL), and adult T-cell lymphoma/leukemia (ATL). In addition, we summarize recent data that validates the NF-kappaB signaling pathway as an attractive therapeutic target in T- and B-cell malignancies.

CD40 ligand-mediated activation of the de novo RelB NF-kappaB synthesis pathway in transformed B cells promotes rescue from apoptosis

CD40, a tumor necrosis factor receptor family member, is expressed on B lymphocytes. Interaction between CD40 and its ligand (CD40L), expressed on activated T lymphocytes, is critical for B cell survival. Here, we demonstrate that CD40 signals B cell survival in part via transcriptional activation of the RelB NF-kappaB subunit. CD40L treatment of chronic lymphocytic leukemia cells induced levels of relB mRNA. Similarly, CD40L-mediated rescue of WEHI 231 B lymphoma cells from apoptosis induced upon B cell receptor (surface IgM) engagement led to increased relB mRNA levels. Recently, we characterized a new de novo synthesis pathway for the RelB NF-kappaB subunit, induced by the cytomegalovirus IE1 protein, in which binding of p50/p65 NF-kappaB and c-Jun/Fra-2 AP-1 complexes to the relB promoter works in synergy to potently activate transcription (Wang, X., and Sonenshein, G. E. (2005) J. Virol. 79, 95-105). CD40L treatment of WEHI 231 cells caused induction of AP-1 family members Fra-2, c-Jun, JunD, and JunB. Cotransfection of Fra-2 with the Jun AP-1 subunits and p50/c-Rel NF-kappaB led to synergistic activation of the relB promoter. Ectopic expression of relB or RelB knockdown using small interfering RNA demonstrated the important role of this subunit in control of WEHI 231 cell survival and implicated activation of the anti-apoptotic factors Survivin and manganese superoxide dismutase. Thus, CD40 engagement of transformed B cells activates relB gene transcription via a process we have termed the de novo RelB synthesis pathway, which protects these cells from apoptosis.

Rapamycin-induced inhibition of HTLV-I LTR activity is rescued by c-Myb

Background

Rapamycin is an immunosuppressive which represses translation of transcripts harbouring a polypyrimidine motif downstream of the mRNA cap site through the mammalian target of rapamycin complex. It inhibits the abnormal autologous proliferation of T-cell clones containing a transcriptionally active human T-lymphotropic virus, type I (HTLV-I) provirus, generated from infected subjects. We showed previously that this effect is independent of the polypyrimidine motifs in the viral long terminal repeat (LTR) R region suggesting that HTLV-I transcription, and not translation, is being affected. Here we studied whether rapamycin is having an effect on a specific transcription factor pathway. Further, we investigated whether mRNAs encoding transcription factors involved in HTLV-I transcriptional activation, specifically CREB, Ets and c-Myb, are implicated in the rapamycin-sensitivity of the HTLV-I LTR.

Results

An in vitro analysis of the role of SRE- and NF-κB-mediated transcription highlighted the latter as rapamycin sensitive. Over-expression of c-Myb reversed the rapamycin effect.

Conclusion

The sensitivity of HTLV-I transcription to rapamycin may be effected through an NF-κB-pathway associated with the rapamycin-sensitive mTORC1 cellular signalling network.

cAMP differentially regulates gamma-globin gene expression in erythroleukemic cells and primary erythroblasts through c-Myb expression

Our previous studies demonstrated roles of cyclic nucleotides in gamma-globin gene expression. We recently found that, upon activation of the cAMP pathway, expression of the gamma-globin gene is inhibited in K562 cells but induced in adult erythroblasts. Here we show that c-Myb, a proto-oncogene product that plays a role in cell growth and differentiation, is involved in the cAMP-mediated differential regulation of gamma-globin gene expression in K562 cells and primary erythroblasts. Our studies found that c-Myb is expressed at a high level in K562 cells compared to primary erythroblasts, and that c-Myb expression is further increased following the treatment with forskolin, an adenylate cyclase activator. The induction of gamma-globin gene expression was also inhibited in K562 cells by raising the levels of c-Myb expression. Importantly, forskolin-induced erythroid differentiation in K562 cells, as determined by the expression of glycophorins and CD71, suggesting that high-level expression of c-Myb may not be sufficient to inhibit the differentiation of erythroid cells. In contrast, c-Myb was not expressed in adult erythroblasts treated with forskolin and primary erythroblasts may lack the c-Myb-mediated inhibitory mechanism for gamma-globin gene expression. Together, these results show that the cAMP pathway blocks gamma-globin gene expression in K562 cells by increasing c-Myb expression and c-Myb plays a role in defining the mode of response of the gamma-globin gene to fetal hemoglobin inducers in erythroid cells.

Pro-apoptotic role of NF-kappaB: implications for cancer therapy

Nuclear factor-kappaB (NF-kappaB) is generally viewed as anti-apoptotic and oncogenic, leading to a quest for its inhibitors. However, recent evidence suggests that in some situations NF-kappaB may promote apoptosis. Depending on the specific cell type and the stimulus involved, NF-kappaB activation may lead to either anti- or pro-apoptotic response. Both these effects can be mediated by NF-kappaB in a context-dependent manner by selectively regulating its target genes. In this review, we discuss the evidence for NF-kappaB's pro-apoptotic role and explore the possible mechanisms behind it. We emphasize that rather than trying to inhibit NF-kappaB in cancer therapy, agents should be developed to unleash its pro-apoptotic ability.

Growth inhibition and apoptosis in human Philadelphia chromosome-positive lymphoblastic leukemia cell lines by treatment with the dual PPARalpha/gamma ligand TZD18

Treatment of adult Philadelphia chromosome-positive lymphocytic leukemia is rarely successful. We report here the effects of TZD18, a novel dual ligand specific for peroxisome proliferator-activated receptor alpha and gamma (PPARalpha/gamma) on Ph(+) lymphocytic leukemia cell lines BV173, SD1, and SupB-15. Exposure of these cells to TZD18 resulted in growth inhibition in a dose- and time-dependent manner that was associated with G(1) cell cycle arrest. This effect was much stronger than that mediated by the PPARgamma ligand pioglitazone (PGZ), which also belongs to the thiazolidinediones (TZD) class of ligands. However, it may not be mediated through PPARgamma or PPARalpha activation because antagonists of PPARgamma and PPARalpha cannot reverse it. Study of the key regulators of cell cycle progression by Western blot analysis showed that the expression of the cyclin-dependent kinase inhibitor (CDKI) p27(kip1), but not that of p21(cip1), was enhanced, whereas that of c-Myc, cyclin E, cyclin D2, and cyclin-dependent kinases 2 and 4 (CDK-2 and CDK-4) was decreased when these cells were treated with TZD18 (10 or 20 microM). Therefore, the up-regulation of p27(kip1) and the down-regulation of CDK-2 and CDK-4 may, at least in part, account for the G(1) cell cycle arrest. Furthermore, a remarkable induction of apoptosis was observed in the cells treated with this dual ligand. No obvious alteration of bcl-2 protein level occurred, but bax was up-regulated in these TZD18-treated cells. Activation of caspase 8 and caspase 9 by TZD18 was also observed. Importantly, NF-kappaB DNA-binding activity was markedly decreased by the TZD18 treatment. In addition, TZD18 enhanced the growth inhibitory effect of imatinib, a specific tyrosine kinase inhibitor therapeutically used in the treatment of Ph(+) leukemia. Overall, our findings strongly suggest that TZD18 may offer a new therapeutic approach to aid in the treatment of Ph(+) lymphocytic leukemia.

Effects of IKK inhibitor PS1145 on NF-κB function, proliferation, apoptosis and invasion activity in prostate carcinoma cells

A key antiapoptotic transcription factor, nuclear factor kappa-B (NF-kappaB), is known to be critically important for tumor cell growth, angiogenesis and development of metastatic lesions. We and others showed previously that NF-kappaB transcription factor was constitutively activated in androgen-independent prostate carcinoma (PC) cell lines due to the upregulated activity of inhibitor of NF-kappaB kinases (IKK). In this work, using luciferase assay, electrophoretic mobility shift assay and Northern blot analysis of expression of endogenous kappaB-responsive genes, we demonstrate that a novel highly specific small-molecule IKK inhibitor, PS1145, efficiently inhibited both basal and induced NF-kappaB activity in PC cells. We found that PS1145 induced caspase 3/7-dependent apoptosis in PC cells and significantly sensitized PC cells to apoptosis induced by tumor necrosis factor alpha. We also showed that PS1145 inhibited PC cell proliferation. Effects of PS1145 on proliferation and apoptosis correlated with inhibition of interleukin (IL)-6, cyclin D1, D2, inhibitor of apoptosis (IAP)-1 and IAP-2 gene expression and decreased IL-6 protein level. In addition, we found that incubation with PS1145 inhibited the invasion activity of highly invasive PC3-S cells in invasion chamber assay in a dose-dependent manner. Overall, this study provides the framework for development of a novel therapeutic approach targeting NF-kappaB transcription factor to treat advanced PC.

Phosphorylated FADD induces NF-kappaB, perturbs cell cycle, and is associated with poor outcome in lung adenocarcinomas

In an effort to identify a clinical biomarker for lung cancer, we used cDNA microarray and 2D protein analyses to demonstrate that increased Fas-associated death domain (FADD) mRNA and protein were significantly associated with poor survival. Analyses of copy number and sequence of the FADD gene in 24 independent tumors ruled out the existence of an amplified and/or mutated FADD gene in aggressive lung cancers. Immunohistochemistry-based tissue microarray analysis showed that nuclear localization of FADD and elevation of the phosphorylated form of FADD (p-FADD) correlated with poor outcome (P = 0.003). Tumors with increased p-FADD expression showed elevated NF-kappaB (P = 0.004) activation, a frequent molecular alteration associated with tumorigenesis and metastasis in a variety of cancers. To provide a link between p-FADD and NF-kappaB, cell culture studies demonstrated that overexpression of p-FADD leads to an increase in NF-kappaB activity and a decrease in the number of cells in the G2 phase of the cell cycle, compared with cells expressing the nonphosphorylatable form of FADD or the vector control. Furthermore, cDNA microarray analyses of lung tumor samples showed that increased levels of FADD transcripts were significantly correlated with overexpression of cyclins D1 (P < 0.01) and B1 (P < 0.01), genes that are involved in the regulation of cell cycle progression and are inducible by NF-kappaB. These studies demonstrate that induction of NF-kappaB activity and its effects on cell-cycle progression may represent a molecular basis underlying the aggressive tumor behavior associated with elevated p-FADD expression in lung adenocarcinoma.

NF-kB in development and progression of human cancer

The nuclear factor kB (NF-kB) comprises a family of transcription factors involved in the regulation of a wide variety of biological responses. NF-kB plays a well-known function in the regulation of immune responses and inflammation, but growing evidences support a major role in oncogenesis. NF-kB regulates the expression of genes involved in many processes that play a key role in the development and progression of cancer such as proliferation, migration and apoptosis. Aberrant or constitutive NF-kB activation has been detected in many human malignancies. In recent years, numerous studies have focused on elucidating the functional consequences of NF-kB activation as well as its signaling mechanisms. NF-kB has turned out to be an interesting therapeutic target for treatment of cancer.

Erythroid gene suppression by NF-kappa B

NF-kappa B/Rel transcription factors play essential roles to mediate the immune response and apoptosis, and they have also been implicated in cellular differentiation such as erythropoiesis. To elucidate the possible role(s) of NF-kappa B in erythroid gene regulation and erythropoiesis, we have carried out transient transfection studies of the human embryonic/fetal erythroid cell line K562 and mouse adult erythroid MEL cells. It is shown that tumor necrosis factor-alpha represses the transcription activity directed by either alpha or zeta globin promoter in a dose-dependent manner. Furthermore, different NF-kappa B family members could effectively repress the transfected alpha-like globin promoters in K562 as well as in MEL cells. The involvement of NF-kappa B pathway is supported by the ability of a NF-kappa B-specific, dominant negative mutant to block the tumor necrosis factor-alpha or p65-mediated suppression of the alpha-like globin promoter activities. The suppression appears to be mediated through cis-linked HS-40 enhancer. Finally, stably transfected K562 cells overexpressing p65 contain reduced amounts of the p45/NF-E2 RNA and functional NF-E2 proteins. Our studies have identified a new set of targets of NF-kappa B. We suggest that the relatively high activity of the NF-kappa B pathway in early erythroid progenitors is involved in the suppression of erythroid-specific genes. Later in differentiation, together with other changes, the decline of the amounts of the NF-kappa B family of factors leads to derepression and consequent increase of NF-E2, which in turn would activate a subset of erythroid-specific genes.

NF-kappaB activation in cancer: a challenge for ubiquitination- and proteasome-based therapeutic approach

Nuclear factor-kappa B (NF-kappaB) activation relies primarily on ubiquitin-mediated degradation of its inhibitor IkappaB. NF-kappaB plays an important role in many aspects of tumor development, progression, and therapy. Some types of cancer are characterized by constitutive NF-kappaB activity, whereas in others such activity is induced following chemotherapy. NF-kappaB-harboring tumors are generally resistant to chemotherapy and their eradication requires NF-kappaB inhibition. Here we describe the mechanisms of NF-kappaB activation in normal and tumor cells, review prevalent notions regarding the factor's contribution to tumorigenicity and discuss present and future options for NF-kappaB inhibition in cancer. The ubiquitination-mediated activation of NF-kappaB is intersected by another cancer-associated protein, beta-catenin. We, therefore, compare the related activation pathways and discuss the possibility of differential targeting of the involved ubiquitination machinery.

Identification of a c-myb attenuator-binding factor

Expression of the c-myb proto-oncogene is developmentally regulated at the level of transcription elongation. In pre-B cells, complete c-myb transcripts are produced, whereas transcripts are attenuated near or within a 300-base pair (bp) interval of the first c-myb intron in mature cells. Hypothesizing that transcription attenuation results from a protein complex that physically impedes the progress of RNA polymerase II through the intron, we used electrophoretic mobility shift assays (EMSA) to search for DNA-binding activities that correlated with downregulation of c-myb transcription. We identified a stage-specific DNA binding activity, termed ABF, present in mature B cells but not in pre-B cells. ABF binds to a 15-bp DNA element located within a 300-bp BstEII-XbaI fragment. DMSO-treatment of murine erythroleukemia cells results in rapid downregulation of c-myb transcription and upregulation of ABF DNA binding activity. Thus, ABF binding activity correlates with downregulation of c-myb transcription in two systems. Preliminary biochemical characterization of ABF from mature B cells demonstrates that its primary DNA-binding component is a 64-kDa-protein. We hypothesize that this factor may represent a member of the transcriptional attenuation complex.

c-Myb transcription is activated by protein kinase B (PKB) following interleukin 2 stimulation of Tcells and is required for PKB-mediated protection from apoptosis

During T-cell activation, c-Myb is induced upon interleukin 2 (IL-2) stimulation and is required for correct proliferation of cells. In this paper, we provide evidence that IL-2-mediated induction of the c-myb gene occurs via the phosphoinositide 3-kinase (PI3K) signaling pathway, that protein kinase B (PKB) is the principal transducer of this signal, and that activation of the c-myb promoter can be abolished by deletion of conserved E2F and NF-kappaB binding sites. We show that Myb is required to protect activated peripheral T cells from bcl-2-independent apoptosis and that overexpression of oncogenic v-Myb is antiapoptotic. Overexpression of a Myb dominant-negative transgene abrogates PKB-mediated protection from apoptosis. Taken together, these results suggest that induction of c-myb transcription is an important downstream event for PKB-mediated protection of T cells from programmed cell death.

Human Notch-1 inhibits NF-kappa B activity in the nucleus through a direct interaction involving a novel domain

Notch participates in diverse cell fate decisions throughout embryonic development and postnatal life. Members of the NF-kappaB/Rel family of transcription factors are involved in the regulation of a variety of genes important for immune function. The biological activity of the NF-kappaB transcription factors is controlled by IkappaB proteins. Our previous work demonstrated that an intracellular, constitutively active form of human Notch-1/translocation-associated Notch homologue-1 (Notch(IC)) functions as an IkappaB molecule with specificity for the NF-kappaB p50 subunit and physically interacts with NF-kappaB in T cells. In the current study, we investigated the roles of different domains of Notch(IC) in the regulation of NF-kappaB-directed gene expression and NF-kappaB DNA binding activity. We found that Notch(IC) localizes to the nucleus and that a region in the N-terminal portion of Notch(IC), not the six ankyrin repeats, is responsible for the inhibitory effects of Notch on NF-kappaB-directed gene expression and NF-kappaB DNA binding activity. The N-terminal portion of Notch(IC) inhibited p50 DNA binding and interacted specifically with p50 subunit, not p65 of NF-kappaB. The interaction between Notch and NF-kappaB indicates that in addition to its role in the development of the immune system, Notch-1 may also have critical functions in the immune response, inflammation, viral infection, and apoptosis through control of NF-kappaB-mediated gene expression.

Nuclear factor kappaB in proliferation, activation, and apoptosis in rat hepatic stellate cells

BACKGROUND/AIMS

Activation of the transcription factor NFkappaB has been demonstrated in activated hepatic stellate cells (HSCs). We investigated the role of NFkappaB in proliferation, in activation, and in TNFalpha-induced apoptosis of HSCs.

METHODS

NFkappaB activation was inhibited using an adenovirus expressing an IkappaB dominant negative protein (Ad5IkappaB) in both quiescent and activated HSCs. Quiescent HSCs were infected with Ad5IkappaB or an adenovirus expressing beta-galactosidase (Ad5LacZ). The cells were cultured for 7 days. HSCs activation was determined by cell morphology, smooth muscle alpha-actin (alpha-sma) expression, and steady-state mRNA levels of alpha1(I) collagen as assessed by Western blot and RNase protection assay, respectively. Proliferation was determined in culture-activated HSCs by 3H-thymidine incorporation and direct cell counting. Apoptosis was analyzed by infecting quiescent or activated HSCs with Ad5IkappaB or Ad5LacZ, and then treating with TNFalpha. Apoptosis was demonstrated by determining cell number, assessing nuclear morphology, TUNEL assay and caspase 3 activity.

RESULTS

After 7 days in culture no differences were noted between the Ad5IkappaB- and the Ad5LacZ-infected cells in the morphology, alpha-sma expression or in alpha1(I) collagen mRNA levels. Ad5IkappaB infection did not modify proliferation in activated HSCs. TNFalpha induced apoptosis only in Ad5IkappaB-infected activated, but not quiescent HSCs. Apoptosis was initially demonstrated 12 h after exposure to TNFalpha. Twenty-four h after the TNFalpha treatment, 60% of the activated HSCs were apoptotic.

CONCLUSION

NFkappaB activity is not required for proliferation or activation of HSCs; however, NFkappaB protects activated HSCs against TNFalpha-induced apoptosis.

Hematotoxicity of the chinese herbal medicine Tripterygium wilfordii hook f in CD34-positive human bone marrow cells

T2, a chloroform/methanol extract of the herb Tripterygium wilfordii Hook f, has been used in China for the treatment of autoimmune and inflammatory diseases for many years. Recent experimental evidence has confirmed that T2 has potent anti-inflammatory and immunosuppressive activity, and a United States Food and Drug Administration-approved clinical trial is currently exploring the efficacy of T2 in the treatment of rheumatoid arthritis. Despite the potential therapeutic benefits of T2, there is ample documentation that T2 is toxic, targeting, among other things, the hematopoietic system, and its use has resulted in cases of leukopenia, thrombocytopenia, and aplastic anemia. This investigation was undertaken to characterize the in vitro effects of T2 on primary human CD34-positive (CD34+) bone marrow cells. Our results demonstrate that T2 has a potent inhibitory effect on the clonogenic response of human bone marrow cells to exogenously added hematopoietic growth factors. The inhibition of colony formation by T2 is not the result of direct cytotoxicity or increased apoptosis and indicates a functional suppression of hematopoiesis. Additional experiments demonstrate that T2 also alters transcriptional regulation in bone marrow cells by inhibiting nuclear factor-kappaB. This transcription factor is found in CD34+ bone marrow cells and has been recently shown to be a requirement for colony formation. These results demonstrate that therapeutic concentrations of T2 exert a significant hematotoxic effect by inhibiting growth factor response in CD34+ bone marrow cells and suggest that inhibition of nuclear factor-kappaB may play a role in the blood dyscrasias encountered with the use of this drug.

Intron 1 rather than 5' flanking sequence mediates cell type-specific expression of c-myb at level of transcription elongation

Previous studies have shown that expression of steady-state c-myb mRNA was regulated primarily by a block in intron 1 during transcription elongation. This study shows that the block site maps approximately 1700 bp from the start of the intron. Studies based on a reporter construct containing c-myb flanking region and intron 1 suggest that the flanking region is not important in the regulation of the cell type-specific expression of c-myb. RNA splicing of intron 1 may enhance the expression in a non-cell type-specific manner. A conserved intron domain comprising the block site is required for defining this site, but this function of the domain is independent of cell type. The cell type-specific regulation of c-myb transcription elongation is mediated by a 5' intron sequence. A mechanism for down regulation of c-myb gene expression by the block to transcription elongation has been proposed.

Transcriptional elongation of c-myb is regulated by NF-kappaB (p50/RelB)

High levels of c-myb expression are necessary for the proliferation of hematopoietic precursor cells whereas down-regulation of c-myb is required for terminal differentiation; this down-regulation occurs through a conditional block to transcriptional elongation in intron I. We previously observed that cAMP analogs prevented the late down-regulation of c-myb during hexamethylene bisacetamide (HMBA)-induced differentiation of murine erythroleukemia (MEL) cells and blocked differentiation; this correlated with the induction of NF-kappaB (p50/RelB) complexes which were shown to bind to NF-kappaB recognition sites flanking the transcriptional pause site of c-myb. We now selected stably-transfected MEL cells which overexpressed p50, RelB or both at levels similar to those induced by cAMP to determine whether these NF-kappaB proteins regulate c-myb expression in intact cells. We demonstrate that transcriptionally active NF-kappaB (p50/RelB) complexes, but not p50 or RelB alone, prevented the early and late down-regulation of c-myb mRNA and increased c-myb transcriptional elongation in HMBA-induced MEL cells. The increase in c-myb expression was sufficient to block erythroid differentiation and allow continuous proliferation of cells in the presence of HMBA. Steady-state c-myb mRNA levels in untreated cells were not affected by overexpression of NF-kappaB, suggesting that p50/RelB specifically modulated the efficiency of transcriptional attenuation during MEL cell differentiation.

Activators and target genes of Rel/NF-kappaB transcription factors

The vertebrate transcription factor NF-kappaB is induced by over 150 different stimuli. Active NF-kappaB, in turn, participates in the control of transcription of over 150 target genes. Because a large variety of bacteria and viruses activate NF-kappaB and because the transcription factor regulates the expression of inflammatory cytokines, chemokines, immunoreceptors, and cell adhesion molecules, NF-kappaB has often been termed a 'central mediator of the human immune response'. This article contains a complete listing of all NF-kappaB inducers and target genes described to date. The collected data argue that NF-kappaB functions more generally as a central regulator of stress responses. In addition, NF-kappaB activation blocks apoptosis in several cell types. Coupling stress responsiveness and anti-apoptotic pathways through the use of a common transcription factor may result in increased cell survival following stress insults.

Activators and target genes of Rel/NF-kappaB transcription factors

The vertebrate transcription factor NF-kappaB is induced by over 150 different stimuli. Active NF-kappaB, in turn, participates in the control of transcription of over 150 target genes. Because a large variety of bacteria and viruses activate NF-kappaB and because the transcription factor regulates the expression of inflammatory cytokines, chemokines, immunoreceptors, and cell adhesion molecules, NF-kappaB has often been termed a 'central mediator of the human immune response'. This article contains a complete listing of all NF-kappaB inducers and target genes described to date. The collected data argue that NF-kappaB functions more generally as a central regulator of stress responses. In addition, NF-kappaB activation blocks apoptosis in several cell types. Coupling stress responsiveness and anti-apoptotic pathways through the use of a common transcription factor may result in increased cell survival following stress insults.

An Essential Role for NF-κB in Human CD34+ Bone Marrow Cell Survival

The transcription factor, NF-κB, is important for T-cell activation, B-cell maturation, and human immunodeficiency virus transcription and plays a role in alternatively mediating and protecting against apoptosis in a variety of cell types. However, a role for NF-κB in human CD34+ bone marrow cells has not been described. We provide evidence here that virtually all human CD34+ bone marrow cells express NF-κB that can be activated by exposure to phorbol 12-myristate 13-acetate and a variety of cytokines, eg, tumor necrosis factor , interleukin-3, and granulocyte-macrophage colony-stimulating factor. In addition, we demonstrate that NF-κB may be required for human CD34+bone marrow cell clonogenic function and survival. These results offer insight into a new role for NF-κB in maintaining survival and function in hematopoietic stem and progenitor cells and suggest that proposed strategies involving inhibition of NF-κB activation as an adjunct to cancer chemotherapy should be approached with caution.

Limitation of Tat-associated transcriptional processivity in HIV-infected PBMC

The ability of HIV to match levels of viral mRNA to the activation state of the host cell may play a role in its ability to persist as well as to replicate. This linkage depends on the function of the viral transcriptional regulatory protein, Tat, which increases the efficiency of RNA elongation (transcriptional processivity) in response to cellular activation. To quantify levels of Tat function in vivo, a quantitative competitive RT-PCR assay was developed that reflects levels of TAR leader fragments (nonprocessive transcripts) and viral mRNA (processive transcripts), indicating low or high levels of Tat function, respectively. The abundance of these RNA species was measured in peripheral blood mononuclear cells (PBMC) of 22 HIV-1-positive individuals (CD4(+) T cell counts 63-934/mm3) and in established cell line models of HIV constitutive replication (H9IIIB) and reversible latency (U1 and ACH-2). In PBMC, the level of total viral transcripts ranged over four orders of magnitude; however, nonprocessive transcription predominated: 70% of PBMC samples had a ratio of processive to total transcripts of <0.3 and none of the samples had 100% processivity. The cell line studies revealed that, even in activated H9IIIB cells, nonprocessive transcription dominates and that latently infected cells can have different transcriptional responses to activation. This is the first study that enumerates degrees of transcriptional processivity in the circulating mononuclear cell compartment and the results suggest that limitation of Tat function may be a common phenotype throughout the course of the disease.

Identification of the major positive regulators of c-myb expression in hematopoietic cells of different lineages

The c-myb gene is primarily expressed in hematopoietic cells, and it is overexpressed in many leukemias. The regulation of its expression is of critical importance in hematopoietic cells. We identified the major positive regulatory sites in the 5'-flanking sequence of the human c-myb gene, and we found that the positive regulators differed in cells of different lineages. In the Molt-4 T-cell line, two Ets-like binding sites were required for the expression of c-myb. The 5' site played a minor role in the regulation of c-myb expression, and we demonstrated that a protein of 67 kDa bound to this site. Antibodies against Ets proteins showed no cross-reactivity with this protein. We showed that Ets-1 bound to the 3'-regulatory site in the c-myb promoter by electrophoretic mobility shift assay and antibody studies. Both of these Ets-like binding sites were nonfunctional in the DHL-9 B-cell line and the K562 myeloid cell line. We identified a novel transcription factor of 50.5 kDa that was required for expression of c-myb in these cell lines.

Isolation and characterization of a tobacco mosaic virus-inducible myb oncogene homolog from tobacco

Salicylic acid (SA) plays an important role in signaling the activation of plant defense responses against pathogen attack including induction of pathogenesis-related (PR) proteins. To gain further insight into the SA-mediated signal transduction pathway, we have isolated and characterized a tobacco mosaic virus (TMV)-inducible myb oncogene homolog (myb1) from tobacco. The myb1 gene was induced upon TMV infection during both the hypersensitive response and development of systemic acquired resistance in the resistant tobacco cultivar following the rise of endogenous SA, but was not activated in the susceptible cultivar that fails to accumulate SA. The myb1 gene was also induced by incompatible bacterial pathogen Pseudomonas syringae pv. syringae during the hypersensitive response. Exogenous SA treatment rapidly (within 15 min) activated the expression of myb1 in both resistant and susceptible tobacco cultivars with the subsequent induction of PR genes occurring several hours later. Biologically active analogs of SA and 2,6-dichloroisonicotinic acid (a synthetic functional analog of SA), which induce PR genes and enhanced resistance, also activated the myb1 gene. In contrast, biologically inactive analogs were poor inducers of myb1 gene expression. Furthermore, the recombinant Myb1 protein was shown to specifically bind to a Myb-binding consensus sequence found in the promoter of the PR-1a gene. Taken together, these results suggest that the tobacco myb1 gene encodes a signaling component down-stream of SA that may participate in transcriptional activation of PR genes and plant disease resistance.