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Biomedical subjects

K Calame

Publications and source records attributed to K Calame.

At least 19 recordsLinked to original sources
PubMed2000

Commitment of B lymphocytes to a plasma cell fate is associated with Blimp-1 expression in vivo.

B lymphocyte-induced maturation protein-1 (Blimp-1) is a transcriptional repressor that is sufficient to trigger terminal differentiation in the B cell lymphoma BCL-1. In this study, we have determined the expression pattern of Blimp-1 in vivo in primary and secondary lymphoid organs of humans and immunized mice. Blimp-1 is expressed in plasma cells derived from either a T-independent or T-dependent response in plasma cells that have undergone isotype switching and those resulting from secondary immunization. Blimp-1 is also present in long-lived plasma cells residing in the bone marrow. However, Blimp-1 was not detected in memory B cells. This expression pattern provides further evidence of a critical role for Blimp-1 in plasma cell development, supporting earlier studies in cultured lines. Significantly, Blimp-1 was also found in a fraction (4-15%) of germinal center B cells in murine spleen and human tonsils. Blimp-1 expression in the germinal center is associated with an interesting subset of cells with a phenotype intermediate between germinal center B cells and plasma cells. In the mouse, Blimp-1(+) germinal center B cells peak at day 12 postimmunization and disappear soon thereafter. They are not apoptotic, some are proliferating, they express germinal center markers peanut agglutinin or CD10 but not Bcl-6, and most express CD138 (syndecan-1), IRF4, and cytoplasmic Ig. Together, these data support a model in which B cell fate decisions occur within the germinal center and Blimp-1 expression is critical for commitment to a plasma cell, rather than a memory cell, fate.

Animals↗
PubMed2000

v-Abl utilizes multiple mechanisms to drive G1/S progression in fibroblasts.

Transformation of 3T3 fibroblasts by the v-Abl tyrosine kinase replaces mitogenic and adhesion signals normally required for cell cycle progression. A 3T3 cell line conditionally transformed with v-Abl has been used to study v-Abl's effects on cell cycle in the context of either serum depletion or absence of adhesion signals. We show that E2F-dependent mRNAs, encoding proteins required for cell cycle progression, are induced by v-Abl. In addition, we identify two previously unknown targets of v-Abl signaling: (1) cyclin D1 and D2 mRNAs are induced upon v-Abl activation; and (2) the CDK inhibitor p27 is decreased upon v-Abl activation.

3T3 Cells↗
PubMed2000

BLIMP-1: trigger for differentiation of myeloid lineage.

B lymphocyte-induced maturation protein-1 (BLIMP-1 or PRDI-BF1) is induced when bone marrow-derived progenitors differentiate in response to macrophage-colony stimulating factor (M-CSF) and is present in peripheral blood monocytes and granulocytes. BLIMP-1 is also induced during differentiation of U937 and HL-60 cells into macrophages or granulocytes. Induction of BLIMP-1 mRNA during macrophage differentiation of U937 and HL-60 shows a biphasic pattern. Overexpression of BLIMP-1 is sufficient to initiate macrophage differentiation of U937 cells whereas blocking endogenous BLIMP-1 inhibits differentiation. One target of BLIMP-1-dependent transcriptional repression in U937 cells is c-myc, providing an explanation for cessation of cell division. Thus BLIMP-1 is a key regulator of terminal differentiation in two separate hematopoietic lineages: myeloid cells and B lymphocytes.

Cell Differentiation↗
PubMed2000

BLIMP-I mediates extinction of major histocompatibility class II transactivator expression in plasma cells.

Class II transactivator (CIITA), a coactivator required for class II major histocompatibility complex (MHC) transcription, is expressed in B cells but extinguished in plasma cells. This report identifies B lymphocyte-induced maturation protein I (BLIMP-I), a transcriptional repressor that is capable of triggering plasma cell differentiation, as a developmentally regulated repressor of CIITA transcription. BLIMP-I represses the B cell-specific promoter of the human gene that encodes CIITA (MHC2TA) in a binding site-dependent manner. Decreased CIITA correlates with increased BLIMP-I during plasma cell differentiation in cultured cells. Ectopic expression of BLIMP-I represses endogenous mRNA for CIITA and the CIITA targets, class II MHC, invariant chain and H2-DM (the murine equivalent of HLA-DM) in primary splenic B cells as well as 18-81 pre-B cells. Thus, the BLIMP-I program of B cell differentiation includes loss of antigen presentation via extinction of CIITA expression.

Animals↗
PubMed2000

p53 deficiency increases transformation by v-Abl and rescues the ability of a C-terminally truncated v-Abl mutant to induce pre-B lymphoma in vivo.

Abelson murine leukemia virus (A-MuLV) is an acute transforming retrovirus that preferentially transforms early B-lineage cells both in vivo and in vitro. Its transforming protein, v-Abl, is a tyrosine kinase related to v-Src but containing an extended C-terminal domain. Many mutations affecting the C-terminal portion of the molecule block the pre-B-transforming activity of v-Abl without affecting the fibroblast-transforming ability. In this study we have determined the abilities of both wild-type and C-terminally truncated (p90) forms of v-Abl to transform cells from p53(-/-) mice. Lack of p53 increases the susceptibility of bone marrow cells to transformation by v-Abl by a factor of more than 7 but does not alter v-Abl's preference for B220(+) IgM(-) pre-B cells. p53-deficient mice have earlier tumor onset, more rapid tumor progression, and decreased survival time following A-MuLV infection, but all of the tumors are pre-B lymphomas. Thus, p53-dependent pathways inhibit v-Abl transformation but play no role in conferring preferential transformation of pre-B cells. Surprisingly, the C-terminally truncated form of v-Abl (p90) transforms pre-B cells very efficiently in mice lacking p53, thus demonstrating that the C terminus of v-Abl does not determine preB tropism but is necessary to overcome p53-dependent inhibition of transformation.

Animals↗
PubMed2000

Repression of c-myc is necessary but not sufficient for terminal differentiation of B lymphocytes in vitro.

The importance of c-myc as a target of the Blimp-1 repressor has been studied in BCL-1 cells, in which Blimp-1 is sufficient to trigger terminal B-cell differentiation. Our data show that Blimp-1-dependent repression of c-myc is required for BCL-1 differentiation, since constitutive expression of c-Myc blocked differentiation. Furthermore, ectopic expression of cyclin E mimicked the effects of c-Myc on both proliferation and differentiation, indicating that the ability of c-Myc to drive proliferation is responsible for blocking BCL-1 differentiation. However, inhibition of c-Myc by a dominant negative form was not sufficient to drive BCL-1 differentiation. Thus, during Blimp-1-dependent plasma cell differentiation, repression of c-myc is necessary but not sufficient, demonstrating the existence of additional Blimp-1 target genes.

B-Lymphocytes↗
PubMed2000

Transcriptional repression by blimp-1 (PRDI-BF1) involves recruitment of histone deacetylase.

B-lymphocyte-induced maturation protein (Blimp-1) is a transcriptional repressor that is considered to be a master regulator of terminal B-cell development because it is sufficient to trigger differentiation in the BCL(1)-cell model. Transcription of the c-myc gene is repressed by Blimp-1 during B-cell differentiation. In this study, we have explored the mechanism by which Blimp-1 represses transcription by using Gal4-fusion protein assays and assays in which Blimp-1 represses the natural c-myc promoter. The results show that Blimp-1 represses the c-myc promoter by an active mechanism that is independent of the adjacently bound activator YY1. Blimp-1 contains two regions that independently associate with histone deacetylase (HDAC) and endogenous Blimp-1 in nuclear extracts binds in vitro to the c-myc Blimp-1 site in a complex containing HDAC. The functional importance of recruiting HDAC for Blimp-1-dependent repression of c-myc transcription is supported by two experiments. First, the HDAC inhibitor tricostatin A inhibits Blimp-1-dependent repression in cotransfection assays. Second, a chromatin immunoprecipitation assay shows that expression of Blimp-1 causes deacetylation of histone H3 associated with the c-myc promoter, and this deacetylation depends on the Blimp-1 binding site in the c-myc promoter.

Acetylation↗
PubMed1999

Inhibition of v-Abl transformation by p53 and p19ARF.

Tumorigenesis is a multistep process that involves the activation of oncogenes and the inactivation of tumor suppressor genes. The transforming activity of the v-Abl oncogene of Abelson murine leukemia virus (A-MuLV) in immortal cell lines has been well studied, while the effects of v-Abl in primary fibroblasts are less clear. Here we show that v-Abl causes cell cycle arrest in primary mouse embryonic fibroblasts (MEFs) and elevated levels of both p53 and the cyclin-dependent kinase inhibitor p21Cip. p53-/- or p19ARF-/- MEFs were resistant to v-Abl-induced cell cycle arrest. Although wild-type MEFs were resistant to v-Abl transforming activity, p53-/- or p19ARF-/- MEFs were susceptible. The results indicate that loss of p19ARF and p53 function plays an important role during the transformation of primary cells by v-Abl. We suggest that although v-Abl is a potent oncogene, its full potential transforming activity cannot be realized until the ARF-, and p53-dependent growth inhibitory pathway is disabled. We also show that p53 is not the mediator of v-Abl toxicity in immortal fibroblasts and does not determine the susceptibility of immortal fibroblasts to v-Abl transformation.

Abelson murine leukemia virus↗
PubMed1998

Transcription of a minimal promoter from the NF-IL6 gene is regulated by CREB/ATF and SP1 proteins in U937 promonocytic cells.

NF-IL6 is an important transcriptional regulator of genes induced in activated monocytes/macrophages, and NF-IL6 is the only CCAAT/enhancer-binding protein (C/EBP) family member whose steady-state mRNA levels increase upon activation of monocytes (1). We show that increased transcription of the NF-IL6 gene is responsible, at least in part, for induction of NF-IL6 mRNA following activation of U937 promonocytic cells. We have identified a 104-bp minimal promoter region of the NF-IL6 gene that is sufficient for basal and activation-dependent induction of transcription in U937 cells. This region contains binding sites for the cAMP response element-binding protein/activation transcription factor (CREB/ATF) and Sp1 families of transcription factors. Each site is functionally important and contributes independently to transcription of the NF-IL6 gene in U937 cells.

Activating Transcription Factors↗
PubMed1998

Evidence that immunoglobulin VH-DJ recombination does not require germ line transcription of the recombining variable gene segment.

The importance of V(D)J recombination for generating diversity in the immune system is well established, but the mechanisms which regulate V(D)J recombination are still poorly understood. Although transcription of unrearranged (germ line) immunoglobulin and T-cell receptor gene segments often precedes V(D)J recombination and has been implicated in its control, the actual role of germ line transcripts in V(D)J recombination is not known. We used a sensitive reverse transcription-PCR assay to study immunoglobulin VH germ line transcripts in proB lines from RAG-deficient mice. All 10 VH families analyzed were germ line transcribed, and germ line transcription was found in all of the cell lines examined, indicating that active chromatin was present in the VH region. However, not all VH families were germ line transcribed in every cell line, and there was a surprising lack of uniformity in the number and family distribution of germ line VH transcripts in individual lines. When V(D)J recombination was activated by restoration of RAG activity, recombinational activity of endogenous VH genes for which germ line transcription was observed could be compared with those of genes for which it was not observed. This analysis revealed multiple examples of endogenous VH gene segments which were rearranged in cells where their germ line transcription was not detectable prior to RAG expression. Thus, our data provide strong support for the idea that V-(D)J recombination does not require germ line transcription of the recombining variable gene segment.

Animals↗
PubMed1998

Transcriptional regulation during B cell development.

Information is increasingly available concerning the molecular events that occur during primary and antigen-dependent stages of B cell development. In this review the roles of transcription factors and coactivators are discussed with respect to changes in expression patterns of various genes during B cell development. Transcriptional regulation is also discussed in the context of developmentally regulated immunoglobulin gene V(D)J recombination, somatic hypermutation, and isotype switch recombination.

Animals↗
PubMed1997

Repression of c-myc transcription by Blimp-1, an inducer of terminal B cell differentiation.

Transcription of c-myc in plasma cells, which are terminally differentiated B cells, is repressed by plasmacytoma repressor factor. This factor was identified as Blimp-1, known for its ability to induce B cell differentiation. Blimp-1 repressed c-myc promoter activity in a binding site-dependent manner. Treatment of BCL1 lymphoma cells with interleukin-2 (IL-2) plus IL-5 induced Blimp-1 and caused a subsequent decline in c-Myc protein. Ectopic expression of Blimp-1 in Abelson-transformed precursor B cells repressed endogenous c-Myc and caused apoptosis; Blimp-1-induced death was partially overcome by ectopic expression of c-Myc. Thus, repression of c-myc is a component of the Blimp-1 program of terminal B cell differentiation.

Animals↗
PubMed1997

The ZiN/POZ domain of ZF5 is required for both transcriptional activation and repression.

ZF5 is a ubiquitously expressed protein originally identified by its ability to bind and repress the murine c-myc promoter. It contains five C-terminal zinc fingers and a conserved N-terminal ZiN/POZ domain. This motif, found in a growing number of zinc finger proteins, can inhibit DNA binding and mediate dimerization [Bardwell, V.J. and Treisman,R. (1994) Genes Dev., 8,1664-1677]. In the current study, a cyclic amplification and selection of targets (CAST) protocol detected preferred ZF5 binding sites which are highly GC-rich. Binding to these sites by ZF5 depended upon the zinc fingers and was enhanced when the ZiN/POZ domain was removed. Using transient cotransfection assays, ZF5 was shown to activate the HIV-1 LTR and repress the beta-actin promoter. The ZiN/POZ domain was shown to mediate ZF5-dependent transcriptional activation and repression. From these data, we conclude that ZF5 can both activate and repress in the context of different natural promoters and that its ZiN/POZ domain can affect two functions; DNA binding and transcriptional modulation.

3T3 Cells↗
PubMed1997

Induction of c-myc transcription by the v-Abl tyrosine kinase requires Ras, Raf1, and cyclin-dependent kinases.

v-Abl is an oncogenic form of the c-Abl nonreceptor tyrosine kinase. v-Abl induces transcription of c-myc, and c-Myc function is a necessary but not sufficient component of the v-Abl transformation program. Previously we showed that the E2F site in the c-myc promoter is a v-Abl response element and that v-Abl appears to induce c-myc by initiating a phosphorylation cascade that ultimately activates E2F-binding proteins. In this work we have investigated the signaling pathway between the v-Abl tyrosine kinase and activated E2F proteins. We show that the Ras GTPase and Raf1 serine/threonine kinase are required in this pathway. However, in contrast to other aspects of v-Abl signaling, induction of c-myc transcription is independent of the Rac GTPase. Our results also establish a requirement for activated cyclin-dependent kinases (cdks), as v-Abl-dependent induction of c-myc transcription is blocked by cdk inhibitor p21 and induction of c-myc is accompanied by activation of cdk2 and cdk4. Finally, we show that v-Abl-dependent induction of c-myc is accompanied by hyperphosphorylation of pRb, p107, and p130. On the basis of these data, we propose a model for the signaling path from v-Abl to c-myc.

Animals↗