Myc stimulates B lymphocyte differentiation and amplifies calcium signaling

The activities of MYC, MNT and the MAX-interactome in lymphocyte proliferation and oncogenesis

The MYC family of proteins plays essential roles in embryonic development and in oncogenesis. Efforts over the past 30 years to define the transcriptional activities of MYC and how MYC functions to promote proliferation have produced evolving models of MYC function. One picture that has emerged of MYC and its partner protein MAX is of a transcription factor complex with a seemingly unique ability to stimulate the transcription of genes that are epigenetically poised for transcription and to amplify the transcription of actively transcribed genes. During lymphocyte activation, MYC is upregulated and stimulates a pro-proliferative program in part through the upregulation of a wide variety of metabolic effector genes that facilitate cell growth and cell cycle progression. MYC upregulation simultaneously sensitizes cells to apoptosis and activated lymphocytes and lymphoma cells have pro-survival attributes that allow MYC-driven proliferation to prevail. For example, the MAX-interacting protein MNT is upregulated in activated lymphocytes and was found to protect lymphocytes from MYC-dependent apoptosis. Here we review the activities of MYC, MNT and other MAX interacting proteins in the setting of T and B cell activation and oncogenesis. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Overexpression of AIOLOS inhibits cell proliferation and suppresses apoptosis in Nalm-6 cells

The AIOLOS gene is important in the control of mature B-lymphocyte differentiation and proliferation. Previous research has shown that deregulated AIOLOS expression is associated with adult B-cell acute lymphoblastic leukemia (ALL) and chronic lymphocytic leukemia in human patients. However, the function of AIOLOS in childhood B-cell precursor (BCP)-ALL is not fully understood. In the present study, Nalm-6 cells were divided into three groups: the untransfected control (UT), the lentiviral vector control (Lenti-Mock) and the AIOLOS-overexpressing (Lenti-AIOLOS) group. Lenti-AIOLOS Nalm-6 cells were constructed by lentiviral transduction, followed by cell proliferation assay, cell-cycle analysis and apoptosis assay, to evaluate the effects of AIOLOS on proliferation, cell cycle distribution and apoptosis of Nalm-6 cells in vitro. Moreover, the expression levels of genes associated with apoptosis and the cell cycle, as well as the transcription factors IKZF1 and NF-κB, were investigated by quantitative reverse transcription-polymerase chain reaction and western blot analysis. The results showed that the proliferation of Nalm-6 cells in the Lenti-AIOLOS group was reduced by 16% on day 8 compared with cells in the UT group (P>0.05). The reduction peaked at 29% on day 10 (P<0.05). The percentage of Nalm-6 cells in the G0/G1 phase increased from 70.4 (UT) to 84.1% (Lenti-AIOLOS) (P<0.01), and the S-phase cells decreased from 20.3 (UT) to 11.7% (Lenti-AIOLOS) (P<0.01). Total apoptotic cells significantly decreased in AIOLOS-transfected Nalm-6 cells (10.75%) compared with those in the Lenti-Mock (17.00%) or UT group (19.05%) (P<0.01). In particular, the difference between the groups in the percentage of late apoptotic cells was significant (2.85 vs. 7.95%; P<0.01). In addition, overexpression of AIOLOS resulted in upregulation of BCL-2 and downregulation of CCND3, BAX, IKZF1 and NF-κB. No changes were detected on C-MYC and P27. Our findings indicate that lentivirus-mediated overexpression of AIOLOS in Nalm-6 cells could inhibit cell proliferation, suppress cell apoptosis and arrest the cell cycle at the G0/G1 phase in vitro.

Control of vertebrate development by MYC

The study of MYC has led to pivotal discoveries in cancer biology, induced pluripotency, and transcriptional regulation. In this review, continuing advances in our understanding of the function of MYC as a transcription factor and how its transcriptional activity controls normal vertebrate development and contributes to developmental disorders is discussed.

Proliferation rate of somatic cells affects reprogramming efficiency

The discovery of induced pluripotent stem (iPS) cells provides not only new approaches for cell replacement therapy, but also new ways for drug screening. However, the undefined mechanism and relatively low efficiency of reprogramming have limited the application of iPS cells. In an attempt to further optimize the reprogramming condition, we unexpectedly observed that removing c-Myc from the Oct-4, Sox-2, Klf-4, and c-Myc (OSKM) combination greatly enhanced the generation of iPS cells. The iPS cells generated without c-Myc attained salient pluripotent characteristics and were capable of producing full-term mice through tetraploid complementation. We observed that forced expression of c-Myc induced the expression of many genes involved in cell cycle control and a hyperproliferation state of the mouse embryonic fibroblasts during the early stage of reprogramming. This enhanced proliferation of mouse embryonic fibroblasts correlated negatively to the overall reprogramming efficiency. By applying small molecule inhibitors of cell proliferation at the early stage of reprogramming, we were able to improve the efficiency of iPS cell generation mediated by OSKM. Our data demonstrated that the proliferation rate of the somatic cell plays critical roles in reprogramming. Slowing down the proliferation of the original cells might be beneficial to the induction of iPS cells.

MYC on the path to cancer

The MYC oncogene contributes to the genesis of many human cancers. Recent insights into its expression and function have led to therapeutic opportunities. MYC's activation by bromodomain proteins could be inhibited by drug-like molecules, resulting in tumor inhibition in vivo. Tumor growth can also be curbed by pharmacologically uncoupling bioenergetic pathways involving glucose or glutamine metabolism from Myc-induced cellular biomass accumulation. Other approaches to halt Myc on the path to cancer involve targeting Myc-Max dimerization or Myc-induced microRNA expression. Here the richness of our understanding of MYC is reviewed, highlighting new biological insights and opportunities for cancer therapies.

Disruption of Fnip1 reveals a metabolic checkpoint controlling B lymphocyte development

The coordination of nutrient and energy availability with cell growth and division is essential for proper immune cell development and function. By using a chemical mutagenesis strategy in mice, we identified a pedigree that has a complete block in B cell development at the pre-B cell stage resulting from a deletion in the Fnip1 gene. Enforced expression of an immunoglobulin transgene failed to rescue B cell development. Whereas essential pre-B cell signaling molecules were activated normally in Fnip1-null pre-B cells, the metabolic regulators AMPK and mTOR were dysregulated, resulting in excessive cell growth and enhanced sensitivity to apoptosis in response to metabolic stress (pre-B cell receptor crosslinking, oncogene activation). These results indicate that Folliculin-interacting protein 1 (Fnip1) is vital for B cell development and metabolic homeostasis and reveal a metabolic checkpoint that may ensure that pre-B cells have sufficient metabolic capacity to support division, while limiting lymphomagenesis caused by deregulated growth.

The many roles of IL-7 in B cell development; mediator of survival, proliferation and differentiation

Interleukin-7 (IL-7) plays several important roles during B cell development including aiding in; the specification and commitment of cells to the B lineage, the proliferation and survival of B cell progenitors; and maturation during the pro-B to pre-B cell transition. Regulation and modulation of IL-7 receptor (IL-7R) signaling is critical during B lymphopoiesis, because excessive or deficient IL-7R signaling leads to abnormal or inhibited B cell development. IL-7 works together with E2A, EBF, Pax-5 and other transcription factors to regulate B cell commitment, while also functions to regulate Ig rearrangement by modulating FoxO protein activation and Rag enhancer activity. Suppressor of cytokine signaling (SOCS) proteins are inhibitors of cytokine activation and, in B cells, function to fine tune IL-7R signaling; ensuring that appropriate IL-7 signals are transmitted to allow for efficient B cell commitment and development.

CXCL13 activation of c-Myc induces RANK ligand expression in stromal/preosteoblast cells in the oral squamous cell carcinoma tumor-bone microenvironment

CXC chemokine ligand-13 (CXCL13) has been implicated in oral squamous cell carcinoma (OSCC) tumor progression and osteolysis. The tumor necrosis factor family member RANKL (receptor activator of NF-κB ligand), a critical bone resorbing osteoclastogenic factor, has an important role in cancer invasion of bone/osteolysis. Here, we show high-level expression of CXCL13 in primary human OSCC tumor specimens; however, human bone marrow-derived stromal (SAKA-T) and murine preosteoblast (MC3T3-E1) cells produce at very low level. Recombinant CXCL13 (0-15 ng/ml) dose dependently induced CXCR5 expression in SAKA-T and MC3T3-E1 cells. Conditioned media obtained from OSCC cell lines increased the RANKL expression and an antibody against the CXCL13 specific receptor, CXCR5 markedly decreased RANKL expression in these cells. Furthermore, CXCL13 increased hRANKL-Luc promoter activity. Superarray screening identified c-Myc and NFATc3 transcription factors upregulated in CXCL13-stimulated SAKA-T cells. Immunohistochemical analysis of OSCC tumors that developed in athymic mice demonstrated RANKL and NFATc3 expression in tumor and osteoblast cells, however, showed p-c-Myc expression specific to osteoblastic cells at the tumor-bone interface. We further identified NFATc3 expression, but not c-Myc activation in primary human OSCC tumor specimens compared with adjacent normal tissue. Also, CXCL13 significantly increased p-ERK1/2 in SAKA-T and MC3T3-E1 cells. siRNA suppression of c-Myc expression markedly decreased CXCL13-induced RANKL and NFATc3 expression in preosteoblast cells. Chromatin-immuno precipitation assay confirmed p-c-Myc binding to the hRANKL promoter region. In summary, c-Myc activation through CXCL13-CXCR5 signaling axis stimulates RANKL expression in stromal/preosteoblast cells. Thus, our results implicate CXCL13 as a potential therapeutic target to prevent OSCC invasion of bone/osteolysis.

The opposing transcriptional functions of Sin3a and c-Myc are required to maintain tissue homeostasis

How the proto-oncogene c-Myc balances the processes of stem cell self-renewal, proliferation and differentiation in adult tissues is largely unknown. We explored c-Myc’s transcriptional roles at the Epidermal Differentiation Complex (EDC) a locus essential for skin maturation. Binding of c-Myc can simultaneously recruit (Klf4, Ovo1-1) and displace (C/EBPα, Mxi1 and Sin3A) specific sets of differentiation-specific transcriptional regulators to EDC genes. We found that Sin3A causes de-acetylation of c-Myc protein to directly repress c-Myc activity. In the absence of Sin3A, genomic recruitment of c-Myc to the EDC is enhanced, and re-activation of c-Myc-target genes drives aberrant epidermal proliferation and differentiation. Simultaneous deletion of c-Myc and Sin3A reverts the skin phenotype to normal. Our results identify how the balance of two transcriptional key regulators can maintain tissue homeostasis via a negative feedback loop.

Myc roles in hematopoiesis and leukemia

Hematopoiesis is a process capable of generating millions of cells every second, as distributed in many cell types. The process is regulated by a number of transcription factors that regulate the differentiation along the distinct lineages and dictate the genetic program that defines each mature phenotype. Myc was first discovered as the oncogene of avian leukemogenic retroviruses; it was later found translocated in human lymphoma. From then on, evidence accumulated showing that c-Myc is one of the transcription factors playing a major role in hematopoiesis. The study of genetically modified mice with overexpression or deletion of Myc has shown that c-Myc is required for the correct balance between self-renewal and differentiation of hematopoietic stem cells (HSCs). Enforced Myc expression in mice leads to reduced HSC pools owing to loss of self-renewal activity at the expense of increased proliferation of progenitor cells and differentiation. c-Myc deficiency consistently results in the accumulation of HSCs. Other models with conditional Myc deletion have demonstrated that different lineages of hematopoietic cells differ in their requirement for c-Myc to regulate their proliferation and differentiation. When transgenic mice overexpress c-Myc or N-Myc in mature cells from the lymphoid or myeloid lineages, the result is lymphoma or leukemia. In agreement, enforced expression of c-Myc blocks the differentiation in several leukemia-derived cell lines capable of differentiating in culture. Not surprising, MYC deregulation is recurrently found in many types of human lymphoma and leukemia. Whereas MYC is deregulated by translocation in Burkitt lymphoma and, less frequently, other types of lymphoma, MYC is frequently overexpressed in acute lymphoblastic and myeloid leukemia, through mechanisms unrelated to chromosomal translocation, and is often associated with disease progression.

Inhibition of the plasma membrane Ca2+ pump by CD44 receptor activation of tyrosine kinases increases the action potential afterhyperpolarization in sensory neurons

The cytoplasmic Ca(2+) clearance rate affects neuronal excitability, plasticity, and synaptic transmission. Here, we examined the modulation of the plasma membrane Ca(2+) ATPase (PMCA) by tyrosine kinases. In rat sensory neurons grown in culture, the PMCA was under tonic inhibition by a member of the Src family of tyrosine kinases (SFKs). Ca(2+) clearance accelerated in the presence of selective tyrosine kinase inhibitors. Tonic inhibition of the PMCA was attenuated in cells expressing a dominant-negative construct or shRNA directed to message for the SFKs Lck or Fyn, but not Src. SFKs did not appear to phosphorylate the PMCA directly but instead activated focal adhesion kinase (FAK). Expression of constitutively active FAK enhanced and dominant-negative or shRNA knockdown of FAK attenuated tonic inhibition. Antisense knockdown of PMCA isoform 4 removed tonic inhibition of Ca(2+) clearance, indicating that FAK acts on PMCA4. The hyaluronan receptor CD44 activates SFK-FAK signaling cascades and is expressed in sensory neurons. Treating neurons with a CD44-blocking antibody or short hyaluronan oligosaccharides, which are produced during injury and displace macromolecular hyaluronan from CD44, attenuated tonic PMCA inhibition. Ca(2+)-activated K(+) channels mediate a slow afterhyperpolarization in sensory neurons that was inhibited by tyrosine kinase inhibitors and enhanced by knockdown of PMCA4. Thus, we describe a novel kinase cascade in sensory neurons that enables the extracellular matrix to alter Ca(2+) signals by modulating PMCA-mediated Ca(2+) clearance. This signaling pathway may influence the excitability of sensory neurons following injury.

Post-translational control of Myc function during differentiation

Myc proteins are deeply involved in multiple biological processes including cell proliferation, growth, metabolism, apoptosis, differentiation, and tumorigenesis. Paradoxically, Myc proteins have been found to be capable of both inhibiting and facilitating differentiation depending on the biological context. Recently we identified a new mode of Myc regulation in differentiating muscle cells in which c-Myc protein is proteolytically cleaved by calcium-dependent calpains in the cytoplasm. This cleavage serves two purposes. First, it inactivates the transcriptional function of Myc by removing its C-terminus, a region responsible for the interaction of Myc with Max and DNA. Second, it alters cytoskeletal architecture and accelerates muscle differentiation through the activity of the remaining N-terminal cleavage product (termed Myc-nick). Here we discuss the roles and regulation of full-length Myc and Myc-nick in terminal differentiation and propose a model in which calpain-mediated cleavage of Myc operates as a functional switch.

Role of helix-loop-helix proteins during differentiation of erythroid cells

Helix-loop-helix (HLH) proteins play a profound role in the process of development and cellular differentiation. Among the HLH proteins expressed in differentiating erythroid cells are the ubiquitous proteins Myc, USF1, USF2, and TFII-I, as well as the hematopoiesis-specific transcription factor Tal1/SCL. All of these HLH proteins exhibit distinct functions during the differentiation of erythroid cells. For example, Myc stimulates the proliferation of erythroid progenitor cells, while the USF proteins and Tal1 regulate genes that specify the differentiated phenotype. This minireview summarizes the known activities of Myc, USF, TFII-I, and Tal11/SCL and discusses how they may function sequentially, cooperatively, or antagonistically in regulating expression programs during the differentiation of erythroid cells.

Transcriptional mechanisms regulating Ca(2+) homeostasis

Ca(2+) is a dynamic cellular secondary messenger which mediates a vast array of cellular responses. Control over these processes is achieved via an extensive combination of pumps and channels which regulate the concentration of Ca(2+) within not only the cytosol but also all intracellular compartments. Precisely how these pumps and channels are regulated is only partially understood, however, recent investigations have identified members of the Early Growth Response (EGR) family of zinc finger transcription factors as critical players in this process. The roles of several other transcription factors in control of Ca(2+) homeostasis have also been demonstrated, including Wilms Tumor Suppressor 1 (WT1), Nuclear Factor of Activated T cells (NFAT) and c-myc. In this review, we will discuss not only how these transcription factors regulate the expression of the major proteins involved in control of Ca(2+) homeostasis, but also how this transcriptional remodeling of Ca(2+) homeostasis affects Ca(2+) dynamics and cellular responses.

Pre-B-cell leukemias in Btk/Slp65-deficient mice arise independently of ongoing V(D)J recombination activity

The adapter protein Slp65 and Bruton's tyrosine kinase (Btk) are key components of the precursor-B (pre-B) cell receptor (pre-BCR) signaling pathway. Slp65-deficient mice spontaneously develop pre-B-cell leukemia, expressing high levels of the pre-BCR on their cell surface. As leukemic Slp65-deficient pre-B cells express the recombination activating genes (Rag)1 and Rag2, and manifest ongoing immunoglobulin (Ig) light-chain rearrangement, it has been hypothesized that deregulated recombinase activity contributes to malignant transformation. In this report, we investigated whether Rag-induced DNA damage is involved in oncogenic transformation of Slp65-deficient B cells. We employed Btk/Slp65 double-deficient mice carrying an autoreactive 3-83μδ BCR transgene. When developing B cells in their bone marrow express this BCR, the V(D)J recombination machinery will be activated, allowing for secondary Ig light-chain gene rearrangements to occur. This phenomenon, called receptor editing, will rescue autoreactive B cells from apoptosis. We observed that 3-83μδ transgenic Btk/Slp65 double-deficient mice developed B-cell leukemias expressing both the 3-83μδ BCR and the pre-BCR components λ5/VpreB. Importantly, such leukemias were found at similar frequencies in mice concomitantly deficient for Rag1 or the non-homologous end-joining factor DNA-PKcs. We therefore conclude that malignant transformation of Btk/Slp65 double-deficient pre-B cells is independent of deregulated V(D)J recombination activity.

Myc-nick: a cytoplasmic cleavage product of Myc that promotes alpha-tubulin acetylation and cell differentiation

The Myc oncoprotein family comprises transcription factors that control multiple cellular functions and are widely involved in oncogenesis. Here we report the identification of Myc-nick, a cytoplasmic form of Myc generated by calpain-dependent proteolysis at lysine 298 of full-length Myc. Myc-nick retains conserved Myc box regions but lacks nuclear localization signals and the bHLHZ domain essential for heterodimerization with Max and DNA binding. Myc-nick induces alpha-tubulin acetylation and altered cell morphology by recruiting histone acetyltransferase GCN5 to microtubules. During muscle differentiation, while the levels of full-length Myc diminish, Myc-nick and acetylated alpha-tubulin levels are increased. Ectopic expression of Myc-nick accelerates myoblast fusion, triggers the expression of myogenic markers, and permits Myc-deficient fibroblasts to transdifferentiate in response to MyoD. We propose that the cleavage of Myc by calpain abrogates the transcriptional inhibition of differentiation by full-length Myc and generates Myc-nick, a driver of cytoplasmic reorganization and differentiation.

An epigenetic chromatin remodeling role for NFATc1 in transcriptional regulation of growth and survival genes in diffuse large B-cell lymphomas

The nuclear factor of activated T cells (NFAT) family of transcription factors functions as integrators of multiple signaling pathways by binding to chromatin in combination with other transcription factors and coactivators to regulate genes central for cell growth and survival in hematopoietic cells. Recent experimental evidence has implicated the calcineurin/NFAT signaling pathway in the pathogenesis of various malignancies, including diffuse large B-cell lymphoma (DLBCL). However, the molecular mechanism(s) underlying NFATc1 regulation of genes controlling lymphoma cell growth and survival is still unclear. In this study, we demonstrate that the transcription factor NFATc1 regulates gene expression in DLBCL cells through a chromatin remodeling mechanism that involves recruitment of the SWItch/Sucrose NonFermentable chromatin remodeling complex ATPase enzyme SMARCA4 (also known as Brahma-related gene 1) to NFATc1 targeted gene promoters. The NFATc1/Brahma-related gene 1 complex induces promoter DNase I hypersensitive sites and recruits other transcription factors to the active chromatin site to regulate gene transcription. Targeting NFATc1 with specific small hairpin RNA inhibits DNase I hypersensitive site formation and down-regulates target gene expression. Our data support a novel epigenetic control mechanism for the transcriptional regulation of growth and survival genes by NFATc1 in the pathophysiology of DLBCL and suggests that targeting NFATc1 could potentially have therapeutic value.

Ikaros and Aiolos inhibit pre-B-cell proliferation by directly suppressing c-Myc expression

Pre-B-cell expansion is driven by signals from the interleukin-7 receptor and the pre-B-cell receptor and is dependent on cyclin D3 and c-Myc. We have shown previously that interferon regulatory factors 4 and 8 induce the expression of Ikaros and Aiolos to suppress pre-B-cell proliferation. However, the molecular mechanisms through which Ikaros and Aiolos exert their growth inhibitory effect remain to be determined. Here, we provide evidence that Aiolos and Ikaros bind to the c-Myc promoter in vivo and directly suppress c-Myc expression in pre-B cells. We further show that downregulation of c-Myc is critical for the growth-inhibitory effect of Ikaros and Aiolos. Ikaros and Aiolos also induce expression of p27 and downregulate cyclin D3 in pre-B cells, and the growth-inhibitory effect of Ikaros and Aiolos is compromised in the absence of p27. A time course analysis further reveals that downregulation of c-Myc by Ikaros and Aiolos precedes p27 induction and cyclin D3 downregulation. Moreover, downregulation of c-Myc by Ikaros and Aiolos is necessary for the induction of p27 and downregulation of cyclin D3. Collectively, our studies identify a pre-B-cell receptor signaling induced inhibitory network, orchestrated by Ikaros and Aiolos, which functions to terminate pre-B-cell expansion.

MIR-23A microRNA cluster inhibits B-cell development

OBJECTIVE

The transcription factor PU.1 (encoded by Sfpi1) promotes myeloid differentiation, but it is unclear what downstream genes are involved. Micro RNAs (miRNAs) are a class of small RNAs that regulate many cellular pathways, including proliferation, survival, and differentiation. The objective of this study was to identify miRNAs downstream of PU.1 that regulate hematopoietic development.

MATERIALS AND METHODS

miRNAs that change expression in a PU.1-inducible cell line were identified with microarrays. The promoter for an miRNA cluster upregulated by PU.1 induction was analyzed for PU.1 binding by electrophoretic mobility shift and chromatin immunoprecipitation assays. Retroviral transduction of hematopoietic progenitors was performed to evaluate the effect of miRNA expression on hematopoietic development in vitro and in vivo.

RESULTS

We identified an miRNA cluster whose pri-transcript is regulated by PU.1. The pri-miRNA encodes three mature miRNAs: miR-23a, miR-27a, and miR-24-2. Each miRNA is more abundant in myeloid cells compared to lymphoid cells. When hematopoietic progenitors expressing the 23a cluster miRNAs were cultured in B-cell-promoting conditions, we observed a dramatic decrease in B lymphopoiesis and an increase in myelopoiesis compared to control cultures. In vivo, hematopoietic progenitors expressing the miR-23a cluster generate reduced numbers of B cells compared to control cells.

CONCLUSIONS

The miR-23a cluster is a downstream target of PU.1 involved in antagonizing lymphoid cell fate acquisition. Although miRNAs have been identified downstream of PU.1 in mediating development of monocytes and granulocytes, the 23a cluster is the first downstream miRNA target implicated in regulating development of myeloid vs lymphoid cells.

Malignant transformation of Slp65-deficient pre-B cells involves disruption of the Arf-Mdm2-p53 tumor suppressor pathway

The adapter protein Slp65 is a key component of the precursor-B (pre-B) cell receptor. Slp65-deficient mice spontaneously develop pre-B cell leukemia, but the mechanism by which Slp65(-/-) pre-B cells become malignant is unknown. Loss of Btk, a Tec-family kinase that cooperates with Slp65 as a tumor suppressor, synergizes with deregulation of the c-Myc oncogene during lymphoma formation. Here, we report that the presence of the immunoglobulin heavy chain transgene V(H)81X prevented tumor development in Btk(-/-)Slp65(-/-) mice. This finding paralleled the reported effect of a human immunoglobulin heavy chain transgene on lymphoma development in Emu-myc mice, expressing transgenic c-Myc. Because activation of c-Myc strongly selects for spontaneous inactivation of the p19(Arf)-Mdm2-p53 tumor suppressor pathway, we investigated whether disruption of this pathway is a common alteration in Slp65(-/-) pre-B cell tumors. We found that combined loss of Slp65 and p53 in mice transformed pre-B cells very efficiently. Aberrations in p19(Arf), Mdm2, or p53 expression were found in all Slp65(-/-) (n = 17) and Btk(-/-)Slp65(-/-) (n = 32) pre-B cell leukemias analyzed. In addition, 9 of 10 p53(-/-)Slp65(-/-) pre-B cell leukemias manifested significant Mdm2 protein expression. These data indicate that malignant transformation of Slp65(-/-) pre-B cells involves disruption of the p19(Arf)-Mdm2-p53 tumor suppressor pathway.

Suppression of B-cell lymphomagenesis by the BH3-only proteins Bmf and Bad

Oncogenic c-Myc is known to balance excessive proliferation by apoptosis that can be triggered by p53-dependent and p53-independent signaling networks. Here, we provide evidence that the BH3-only proapoptotic Bcl-2 family members Bcl-2 modifying factor (Bmf) and Bcl-2 antagonist of cell death (Bad) are potent antagonists of c-Myc-driven B-cell lymphomagenesis. Tumor formation was preceded by the accumulation of preneoplastic pre-B and immature immunoglobulin M-positive (IgM(+)) B cells in hematopoietic organs of Emu-myc/bmf(-/-) mice, whereas Emu-myc/bad(-/-) mice showed an increase of pre-B cells limited to the spleen. Although the loss of Bad had no impact on the tumor immunophenotype, Bmf deficiency favored the development of IgM(+) B cell over pre-B cell tumors. This phenomenon was caused by a strong protection of immature IgM(+) B cells from oncogene-driven apoptosis caused by loss of bmf and c-Myc-induced repression of Bmf expression in premalignant pre-B cells. Steady-state levels of B-cell apoptosis also were reduced in the absence of Bad, in support of its role as a sentinel for trophic factor-deprivation. Loss of Bmf reduced the pressure to inactivate p53, whereas Bad deficiency did not, identifying Bmf as a novel component of the p53-independent tumor suppressor pathway triggered by c-Myc.

Gene regulation and epigenetic remodeling in murine embryonic stem cells by c-Myc

BACKGROUND

The Myc oncoprotein, a transcriptional regulator involved in the etiology of many different tumor types, has been demonstrated to play an important role in the functions of embryonic stem (ES) cells. Nonetheless, it is still unclear as to whether Myc has unique target and functions in ES cells.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate the role of c-Myc in murine ES cells, we mapped its genomic binding sites by chromatin-immunoprecipitation combined with DNA microarrays (ChIP-chip). In addition to previously identified targets we identified genes involved in pluripotency, early development, and chromatin modification/structure that are bound and regulated by c-Myc in murine ES cells. Myc also binds and regulates loci previously identified as Polycomb (PcG) targets, including genes that contain bivalent chromatin domains. To determine whether c-Myc influences the epigenetic state of Myc-bound genes, we assessed the patterns of trimethylation of histone H3-K4 and H3-K27 in mES cells containing normal, increased, and reduced levels of c-Myc. Our analysis reveals widespread and surprisingly diverse changes in repressive and activating histone methylation marks both proximal and distal to Myc binding sites. Furthermore, analysis of bulk chromatin from phenotypically normal c-myc null E7 embryos demonstrates a 70-80% decrease in H3-K4me3, with little change in H3-K27me3, compared to wild-type embryos indicating that Myc is required to maintain normal levels of histone methylation.

CONCLUSIONS/SIGNIFICANCE

We show that Myc induces widespread and diverse changes in histone methylation in ES cells. We postulate that these changes are indirect effects of Myc mediated by its regulation of target genes involved in chromatin remodeling. We further show that a subset of PcG-bound genes with bivalent histone methylation patterns are bound and regulated in response to altered c-Myc levels. Our data indicate that in mES cells c-Myc binds, regulates, and influences the histone modification patterns of genes involved in chromatin remodeling, pluripotency, and differentiation.

Myc's other life: stem cells and beyond

Over the last three decades genetic and biochemical studies have revealed the pleiotropic effects of the Myc oncoprotein. While cell line studies have defined the intracellular processes regulated by Myc such as proliferation, differentiation, and metabolic growth, in vivo studies have confirmed these functions, and revealed roles in acquisition and maintenance of stem cell properties. These roles may be partially mediated by Myc's capacity to modify the chromatin landscape on a global scale. Myc also regulates numerous protein-coding transcripts, and many noncoding RNAs (rRNAs, tRNAs, and miRNAs). As Myc activity directly correlates with protein expression, further complexity is provided by post-translational modifications that regulate Myc in normal stem cells or deregulate it in malignant stem cells.

c-Myc-mediated control of cell fate in megakaryocyte-erythrocyte progenitors

It has been found that c-Myc protein plays a critical role in controlling self-renewal versus differentiation in hematopoietic stem cells. We report that c-Myc also controls the fate of megakaryocyte-erythrocyte progenitors through regulating the differentiation of erythroid and megakaryocytic progenitors. In addition to the significant reduction of granulocytes/macrophages and B and T lymphocytes because of the reduction of their corresponding progenitors, we found significantly increased numbers of megakaryocytic progenitors and mature megakaryocytes in bone marrow and spleens of c-Myc-knockout (c-Myc(-/-)) mice. Differentiation of erythrocytes was blocked at the erythroid progenitor stage. This increased megakaryocytopoiesis is a cell-intrinsic defect of c-Myc-mutant hematopoietic stem cells, as shown by transplantation studies. Furthermore, we found that c-Myc is required for polyploidy formation but not for cytoplasmic maturation of megakaryocytes. Megakaryocytes from c-Myc(-/-) mice are significantly smaller in size and lower in ploidy than those of control mice; however, because of the dramatic increase in megakaryocyte number, although fewer platelets are produced by each megakaryocyte, a greater than 3-fold increase in platelet number was consistently observed in c-Myc(-/-) mice. Thus, c-Myc(-/-) mice develop a syndrome of severe thrombocytosis-anemia-leukopenia because of significant increases in megakaryocytopoiesis and concomitant blockage of erythrocyte differentiation and reductions in myelolymphopoiesis.

Reduced c-myc expression levels limit follicular mature B cell cycling in response to TLR signals

The splenic B cell compartment is comprised of two major, functionally distinct, mature B cell subsets, i.e., follicular mature (FM) and marginal zone (MZ) B cells. Whereas MZ B cells exhibit a robust proliferative response following stimulation with the TLR4 ligand LPS, FM B cells display markedly delayed and reduced levels of proliferation to the identical stimulus. The current study was designed to identify a potential mechanism(s) accounting for this differential responsiveness. In contrast to the delay in cell cycle entry, FM and MZ B cells exhibited nearly identical LPS-driven alterations in the expression level of cell surface activation markers. Furthermore, both the NF-kappaB and mTOR signaling cascades were similarly activated by LPS stimulation in FM vs MZ B cells, while inducible activation of ERK and AKT were nearly absent in both subsets. MZ B cells, however, exhibited higher basal levels of phospho-AKT and pS6, consistent with a preactivated status. Importantly, both basal and LPS activation-induced c-myc expression was markedly reduced in FM vs MZ B cells and enforced c-myc expression fully restored the defective proliferative response in FM B cells. These data support a model wherein TLR responses in FM B cells are tightly regulated by limiting c-myc levels, thereby providing an important checkpoint to control nonspecific FM B cell activation in the absence of cognate Ag.

Timing in cellular Ca2+ signaling

Calcium (Ca2+) signals are generated across a broad time range. Kinetic considerations impact how information is processed to encode and decode Ca2+ signals, the choreography of responses that ensure specific and efficient signaling and the overall temporal amplification such that ephemeral Ca2+ signals have lasting physiological value. The reciprocal importance of timing for Ca2+ signaling, and Ca2+ signaling for timing is exemplified by the altered kinetic profiles of Ca2+ signals in certain diseases and the likely role of basal Ca2+ fluctuations in the perception of time itself.

Myc's broad reach

The role of the myc gene family in the biology of normal and cancer cells has been intensively studied since the early 1980s. myc genes, responding to diverse external and internal signals, express transcription factors (c-, N-, and L-Myc) that heterodimerize with Max, bind DNA, and modulate expression of a specific set of target genes. Over the last few years, expression profiling, genomic binding studies, and genetic analyses in mammals and Drosophila have led to an expanded view of Myc function. This review is focused on two major aspects of Myc: the nature of the genes and pathways that are targeted by Myc, and the role of Myc in stem cell and cancer biology.

A point mutation in the murine Hem1 gene reveals an essential role for Hematopoietic protein 1 in lymphopoiesis and innate immunity

Hem1 (Hematopoietic protein 1) is a hematopoietic cell-specific member of the Hem family of cytoplasmic adaptor proteins. Orthologues of Hem1 in Dictyostelium discoideum, Drosophila melanogaster, and Caenorhabditis elegans are essential for cytoskeletal reorganization, embryonic cell migration, and morphogenesis. However, the in vivo functions of mammalian Hem1 are not known. Using a chemical mutagenesis strategy in mice to identify novel genes involved in immune cell functions, we positionally cloned a nonsense mutation in the Hem1 gene. Hem1 deficiency results in defective F-actin polymerization and actin capping in lymphocytes and neutrophils caused by loss of the Rac-controlled actin-regulatory WAVE protein complex. T cell development is disrupted in Hem1-deficient mice at the CD4⁻CD8⁻ (double negative) to CD4⁺CD8⁺ (double positive) cell stages, whereas T cell activation and adhesion are impaired. Hem1-deficient neutrophils fail to migrate in response to chemotactic agents and are deficient in their ability to phagocytose bacteria. Remarkably, some Rac-dependent functions, such as Th1 differentiation and nuclear factor κB (NF-κB)–dependent transcription of proinflammatory cytokines proceed normally in Hem1-deficient mice, whereas the production of Th17 cells are enhanced. These results demonstrate that Hem1 is essential for hematopoietic cell development, function, and homeostasis by controlling a distinct pathway leading to cytoskeletal reorganization, whereas NF-κB–dependent transcription proceeds independently of Hem1 and F-actin polymerization.

Myc inhibits p27-induced erythroid differentiation of leukemia cells by repressing erythroid master genes without reversing p27-mediated cell cycle arrest

Inhibition of differentiation has been proposed as an important mechanism for Myc-induced tumorigenesis, but the mechanisms involved are unclear. We have established a genetically defined differentiation model in human leukemia K562 cells by conditional expression of the cyclin-dependent kinase (Cdk) inhibitor p27 (inducible by Zn(2+)) and Myc (activatable by 4-hydroxy-tamoxifen). Induction of p27 resulted in erythroid differentiation, accompanied by Cdk inhibition and G(1) arrest. Interestingly, activation of Myc inhibited p27-mediated erythroid differentiation without affecting p27-mediated proliferation arrest. Microarray-based gene expression indicated that, in the presence of p27, Myc blocked the upregulation of several erythroid-cell-specific genes, including NFE2, JUNB, and GATA1 (transcription factors with a pivotal role in erythropoiesis). Moreover, Myc also blocked the upregulation of Mad1, a transcriptional antagonist of Myc that is able to induce erythroid differentiation. Cotransfection experiments demonstrated that Myc-mediated inhibition of differentiation is partly dependent on the repression of Mad1 and GATA1. In conclusion, this model demonstrates that Myc-mediated inhibition of differentiation depends on the regulation of a specific gene program, whereas it is independent of p27-mediated cell cycle arrest. Our results support the hypothesis that differentiation inhibition is an important Myc tumorigenic mechanism that is independent of cell proliferation.

Ca2+ signalling checkpoints in cancer: remodelling Ca2+ for cancer cell proliferation and survival

Increases in cytosolic free Ca2+ ([Ca2+]i) represent a ubiquitous signalling mechanism that controls a variety of cellular processes, including proliferation, metabolism and gene transcription, yet under certain conditions increases in intracellular Ca2+ are cytotoxic. Thus, in using Ca2+ as a messenger, cells walk a tightrope in which [Ca2+]i is strictly maintained within defined boundaries. To adhere to these boundaries and to sustain their modified phenotype, many cancer cells remodel the expression or activity of their Ca2+ signalling apparatus. Here, we review the role of Ca2+ in promoting cell proliferation and cell death, how these processes are remodelled in cancer and the opportunities this might provide for therapeutic intervention.

B cell development: important work for ERK

The B cell receptor (BCR) activates the ERK kinases, but what does ERK do? In this issue of Immunity, Yasuda et al. (2008) show that ERK controls a transcription factor network by which the pre-BCR drives early B cell development.

Erk kinases link pre-B cell receptor signaling to transcriptional events required for early B cell expansion

The pre-B cell receptor (pre-BCR) plays a crucial role in the development of immature B cells. Although certain aspects of proximal pre-BCR signaling have been studied, the intermediate signal transducers and the distal transcription modulators are poorly characterized. Here, we demonstrate that deletion of both Erk1 and Erk2 kinases was associated with defective pre-BCR-mediated cell expansion as well as a block in the transition of pro-B to pre-B cells. Phosphorylation of transcription factors Elk1 and CREB was mediated by Erk, and a dominant-negative mutation in the Erk-mediated phosphorylation sites of Elk1 or CREB suppressed pre-BCR-mediated cell expansion as well as expression of genes including Myc, which is involved in the cell-cycle progression. Together, our results identify a crucial role for Erk kinases in regulating B cell development by initiating transcriptional regulatory network and thereby pre-BCR-mediated cell expansion.