Mad1 is a transcriptional repressor of Bcl-6

MXD1 regulates the imatinib resistance of chronic myeloid leukemia cells by repressing BCR-ABL1 expression

Tyrosine kinase inhibitors have achieved unprecedented efficacy in the treatment of chronic myeloid leukemia (CML); however, imatinib resistance has emerged as a major problem in the clinic. Because the overexpression of BCR-ABL1 critically contributes to CML pathogenesis and drug resistance, targeting the regulation of BCR-ABL1 gene expression may be an alternative therapeutic strategy. In this study, we found that the transcriptional repressor MXD1 showed low expression in CML patients and was negatively correlated with BCR-ABL1. Overexpression of MXD1 markedly inhibited the proliferation of K562 cells and sensitized the imatinib-resistant K562/G01 cell line to imatinib, with decreased BCR-ABL1 mRNA and protein expression. Further investigation using reporter gene analysis showed that MXD1 significantly inhibited the transcriptional activity of the BCR-ABL1 gene promoter. Taken together, these data show that MXD1 functions as a negative regulator of BCR-ABL1 expression and subsequently inhibits proliferation and sensitizes CML cells to imatinib treatment.

Differential IL-2 expression defines developmental fates of follicular versus nonfollicular helper T cells

In response to infection, naïve CD4⁺ T cells differentiate into two subpopulations: T follicular helper (T_FH) cells, which support B cell antibody production, and non-T_FH cells, which enhance innate immune cell functions. Interleukin-2 (IL-2), the major cytokine produced by naïve T cells, plays an important role in the developmental divergence of these populations. However, the relationship between IL-2 production and fate determination remains unclear. Using reporter mice, we found that differential production of IL-2 by naïve CD4⁺ T cells defined precursors fated for different immune functions. IL-2 producers, which were fated to become T_FH cells, delivered IL-2 to nonproducers destined to become non-T_FH cells. Because IL-2 production was limited to cells receiving the strongest T cell receptor (TCR) signals, a direct link between TCR-signal strength, IL-2 production, and T cell fate determination has been established.

miR-155 targets histone deacetylase 4 (HDAC4) and impairs transcriptional activity of B-cell lymphoma 6 (BCL6) in the Eμ-miR-155 transgenic mouse model

Multiple studies have established that microRNAs (miRNAs) are involved in the initiation and progression of cancer. Notably, miR-155 is one of the most overexpressed miRNAs in several solid and hematological malignancies. Ectopic miR-155 expression in mice B cells (Eμ-miR-155 transgenic mice) has been shown to induce pre-B-cell proliferation followed by high-grade lymphoma/leukemia. Loss of miR-155 in mice resulted in impaired immunity due to defective T-cell-mediated immune response. Here we provide a mechanistic insight into miR-155-induced leukemogenesis in the Eμ-miR-155 mouse model through genome-wide transcriptome analysis of naïve B cells and target studies. We found that a key transcriptional repressor and proto-oncogene, Bcl6 is significantly down-regulated in Eμ-miR-155 mice. The reduction of Bcl6 subsequently leads to de-repression of some of the known Bcl6 targets like inhibitor of differentiation (Id2), interleukin-6 (IL6), cMyc, Cyclin D1, and Mip1α/ccl3, all of which promote cell survival and proliferation. We show that Bcl6 is indirectly regulated by miR-155 through Mxd1/Mad1 up-regulation. Interestingly, we found that miR-155 directly targets HDAC4, a corepressor partner of BCL6. Furthermore, ectopic expression of HDAC4 in human-activated B-cell-type diffuse large B-cell lymphoma (DLBCL) cells results in reduced miR-155-induced proliferation, clonogenic potential, and increased apoptosis. Meta-analysis of the diffuse large B-cell lymphoma patient microarray data showed that miR-155 expression is inversely correlated with Bcl6 and Hdac4. Hence this study provides a better understanding of how miR-155 causes disruption of the BCL6 transcriptional machinery that leads to up-regulation of the survival and proliferation genes in miR-155-induced leukemias.

MAD1 and its life as a MYC antagonist: an update

The MYC/MAX/MAD network is of central importance for controlling cell physiology. The network is compiled of transcriptional regulators that form different heterodimers, which can either activate or repress the expression of target genes. Thus these proteins function as a molecular switch to control gene expression. MAD1, a member of this network, acts as a transcriptional repressor. It interacts with MAX to form the OFF position of the switch, antagonizing MYC/MAX complexes that define the ON position. MAD1 regulates cell proliferation and apoptosis through a number of target genes. In addition recent evidence indicates that the expression and activity of MAD1 are regulated at multiple levels. Here the recent developments are summarized, in comparison to MYC, of our understanding how the expression of the MAD1 gene and protein are controlled and what the functional consequences and downstream effectors of MAD1 are, which relay its activity as a transcriptional regulator.

Stochastic modeling of B lymphocyte terminal differentiation and its suppression by dioxin

Background

Upon antigen encounter, naïve B lymphocytes differentiate into antibody-secreting plasma cells. This humoral immune response is suppressed by the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other dioxin-like compounds, which belong to the family of aryl hydrocarbon receptor (AhR) agonists.

Results

To achieve a better understanding of the immunotoxicity of AhR agonists and their associated health risks, we have used computer simulations to study the behavior of the gene regulatory network underlying B cell terminal differentiation. The core of this network consists of two coupled double-negative feedback loops involving transcriptional repressors Bcl-6, Blimp-1, and Pax5. Bifurcation analysis indicates that the feedback network can constitute a bistable system with two mutually exclusive transcriptional profiles corresponding to naïve B cells and plasma cells. Although individual B cells switch to the plasma cell state in an all-or-none fashion when stimulated by the polyclonal activator lipopolysaccharide (LPS), stochastic fluctuations in gene expression make the switching event probabilistic, leading to heterogeneous differentiation response among individual B cells. Moreover, stochastic gene expression renders the dose-response behavior of a population of B cells substantially graded, a result that is consistent with experimental observations. The steepness of the dose response curve for the number of plasma cells formed vs. LPS dose, as evaluated by the apparent Hill coefficient, is found to be inversely correlated to the noise level in Blimp-1 gene expression. Simulations illustrate how, through AhR-mediated repression of the AP-1 protein, TCDD reduces the probability of LPS-stimulated B cell differentiation. Interestingly, stochastic simulations predict that TCDD may destabilize the plasma cell state, possibly leading to a reversal to the B cell phenotype.

Conclusion

Our results suggest that stochasticity in gene expression, which renders a graded response at the cell population level, may have been exploited by the immune system to launch humoral immune response of a magnitude appropriately tuned to the antigen dose. In addition to suppressing the initiation of the humoral immune response, dioxin-like compounds may also disrupt the maintenance of the acquired immunity.

c-Myc overexpression promotes a germinal center-like program in Burkitt's lymphoma

The germinal center (GC) reaction has a pivotal function in human B-cell lymphomagenesis. Genetic aberrations occurring during somatic hypermutation and class switch recombination deregulate key factors controlling B-cell physiology and proliferation. Several human lymphoma entities are characterized by a constitutive GC phenotype and ongoing somatic hypermutation, but the molecular basis for this phenomenon is only partly understood. We have investigated the reasons for a constitutive GC-like program in Burkitt's lymphoma cells. Here, overexpression of c-Myc leads to a centroblast phenotype, promotes high constitutive expression of the key GC factors Bcl-6, E2A and activation-induced cytidine deaminase and contributes to proliferation and somatic hypermutation. Our findings elucidate how the activity of a pivotal transcription factor may freeze B-cell lymphoma cells in a constitutive GC-like state that is even maintained at an extrafollicular location.

The spindle function of CDCA4

In an attempt to discover novel proteins functioning in both interphase nucleus and mitotic spindle as NuMA does, we carried out cDNA library screening with pooled autoimmune antibodies. Among positive clones we found a recently identified transcription regulatory protein (CDCA4) with the distinctive nuclear-mitotic apparatus distribution. CDCA4 localizes at metaphase spindle poles and the midzone in later stages. Additionally, an intensive CDCA4 accumulation parallel to spindle was observed in half of metaphase cells but not in later stages, implying a transient form of CDCA4 binding to midzone from anaphase. Mitotic arrest dissolved CDCA4 from centrosomes but during the spindle recovery, CDCA4 invariably colocalized with the microtubule nucleation foci as a component of microtubule organization center. RNA interference of CDCA4 resulted in significant increase of multinuclei and multipolar spindles, suggesting impaired function in chromosome segregation or cytokinesis. However, the spindle checkpoint and the centrosome cycle appeared not to be affected by such interference. Furthermore, CDCA4 depletion resulted in accelerated cell proliferation, perhaps due to the disruption of CDCA4 nuclear function as a transcription suppressor. Interphase CDCA4 is localized in nucleoli by immunofluorescence, although GFP-CDCA4 expressed in the nucleoplasm. An N-terminal KRKC domain appears to be the nuclear localization signal as identified by sequence alignment and the expression of truncated mutants. Taken together, our results suggested that as a novel nuclearmitotic apparatus protein, CDCA4 is involved in spindle organization from prometaphase. When anaphase begins, CDCA4 may play a different role as a midzone factor involved in chromosome segregation or cytokinesis.

B-cell lymphoma 6 and the molecular pathogenesis of diffuse large B-cell lymphoma

PURPOSE OF REVIEW

The B-cell lymphoma 6 transcriptional repressor is the most commonly involved oncogene in B-cell lymphomas. Sustained expression of B-cell lymphoma 6 causes malignant transformation of germinal center B cells. Understanding the mechanism of action of B-cell lymphoma 6 is crucial for the study of how aberrant transcriptional programming leads to lymphomagenesis and development of targeted antilymphoma therapy.

RECENT FINDINGS

Identification of B-cell lymphoma 6 target genes indicates a critical role for B-cell lymphoma 6 in facilitating a state of physiological genomic instability required for germinal center B cells to undergo affinity maturation, and suggests its contribution to several additional cellular functions. The discovery of several layers of counterregulatory mechanisms reveals how B cells can control and fine-tune the potentially lymphomagenic actions of B-cell lymphoma 6. From the biochemical standpoint, B-cell lymphoma 6 can regulate distinct biological pathways through different cofactors. This observation explains how the biological actions of B-cell lymphoma 6 can be physiologically controlled through separate mechanisms and affords the means for improved therapeutic targeting. The fact that patients with B-cell lymphoma 6-dependent lymphoma can be identified on the basis of gene signatures suggests that therapeutic trials of B-cell lymphoma 6 inhibitors could be personalized to these individuals.

SUMMARY

B-cell lymphoma 6 plays a fundamental role in lymphomagenesis and is an excellent therapeutic target for development of improved antilymphoma therapeutic regimens.

Regulation and functions of Blimp-1 in T and B lymphocytes

B lymphocyte-induced maturation protein-1 (Blimp-1), discovered 16 years ago as a transcriptional repressor of the IFNbeta promoter, plays fundamentally important roles in many cell lineages and in early development. This review focuses on Blimp-1 in lymphocytes. In the B cell lineage, Blimp-1 is required for development of immunoglobulin-secreting cells and for maintenance of long-lived plasma cells (LLPCs). Direct targets of Blimp-1 and the transcriptional cascades Blimp-1 initiates to trigger plasmacytic differentiation are described. Blimp-1 also affects the homeostasis and function of CD4(+), CD8(+), and regulatory CD4(+) T cells, and Blimp-1 levels are highest in antigen-experienced T cells. Blimp-1 attenuates T cell proliferation and survival and modulates differentiation. Roles for Blimp-1 in Th1/Th2 specification, regulatory T cell function, and CD8 differentiation and function are under investigation. Signals that induce Blimp-1 in B cells include Toll-like receptor ligands and cytokines; in T cells, T cell receptors and cytokines induce Blimp-1. In spite of some commonalities, different targets and regulators of Blimp-1 in B and T cells suggest intriguing evolutionary divergence of this regulatory machinery.

Common mechanisms for the regulation of B cell differentiation and transformation by the transcriptional repressor protein BCL-6

The BCL-6 transcriptional repressor protein is a critical regulator of normal B cell differentiation and BCL-6 has recently been shown to act as an oncogene in several mouse model systems. The molecular pathways by which BCL-6 regulates B cell differentiation and also promotes the transformation of primary B cells are undoubtedly related; however, these pathways are poorly understood. The commonly accepted model for BCL-6 function in B cells is that BCL-6 inhibits the terminal differentiation of activated B cells into plasma cells and that deregulation of BCL-6 expression leads to an inhibition of terminal differentiation and continued proliferation. BCL-6 induces a germinal-center phenotype in primary B cells by unknown mechanisms, and can reverse the terminal differentiation of plasma cell tumor lines. BCL-6 can promote the immortalization of primary B cells and can augment telomerase activity. The role of the vast majority of BCL-6 target genes and interacting proteins in normal B cell differentiation and B cell transformation is essentially unresolved and is an important area for future investigation.