Downregulation Of PU Research Articles

Molecular Analysis of the Interaction between the Hematopoietic Master Transcription Factors GATA-1 and PU.1

GATA-1 and PU.1 are transcription factors that control erythroid and myeloid development, respectively. The two proteins have been shown to function in an antagonistic fashion, with GATA-1 repressing PU.1 activity during erythropoiesis and PU.1 repressing GATA-1 function during myelopoiesis. It has also become clear that this functional antagonism involves direct interactions between the two proteins. However, the molecular basis for these interactions is not known, and a number of inconsistencies exist in the literature. We have used a range of biophysical methods to define the molecular details of the GATA-1-PU.1 interaction. A combination of NMR titration data and extensive mutagenesis revealed that the PU.1-Ets domain and the GATA-1 C-terminal zinc finger (CF) form a low affinity interaction in which specific regions of each protein are implicated. Surprisingly, the interaction cannot be disrupted by single alanine substitution mutations, suggesting that binding is distributed over an extended interface. The C-terminal basic tail region of CF appears to be sufficient to mediate an interaction with PU.1-Ets, and neither acetylation nor phosphorylation of a peptide corresponding to this region disrupts binding, indicating that the interaction is not dominated by electrostatic interactions. The CF basic tail shares significant sequence homology with the PU.1 interacting motif from c-Jun, suggesting that GATA-1 and c-Jun might compete to bind PU.1. Taken together, our data provide a molecular perspective on the GATA-1-PU.1 interaction, resolving several issues in the existing data and providing insight into the mechanisms through which these two proteins combine to regulate blood development.

Thalidomide derivative CC-4047 inhibits osteoclast formation by down-regulation of PU.1

AbstractCC-4047, an immunomodulatory analog of thalidomide, inhibits multiple myeloma with unknown effects on the human osteoclast lineage. Early osteoclast progenitors are of hematopoietic origin and differentiate into mature bone resorbing multinucleated osteoclasts. We investigated the effects of CC-4047 and thalidomide on human osteoclastogenesis, using in vitro receptor activator of NFκ-B ligand/macrophage colony-stimulating factor–stimulated bone marrow cell cultures. Treating bone marrow cultures with CC-4047 for 3 weeks decreased osteoclast formation accompanied by complete inhibition of bone resorption. The inhibitory effect was similar when cultures were treated for 3 weeks or for only the first week (90% inhibition), indicating that CC-4047 inhibits early stages of osteoclast formation. Inhibition of osteoclastogenesis by CC-4047 was mediated by a shift of lineage commitment to granulocyte colony-forming units at the expense of granulocyte-macrophage colony-forming units. Further studies revealed that this shift in lineage commitment was mediated through down-regulation of PU.1. Treatment with thalidomide resulted in significantly less potent inhibition of osteoclast formation and bone resorption. These results provide evidence that CC-4047 blocks osteoclast differentiation during early phases of osteoclastogenesis. Therefore, CC-4047 might be a valuable drug for targeting both tumors and osteoclastic activity in patients with multiple myeloma and other diseases associated with osteolytic lesions.

Thalidomide Derivative CC-4047 Inhibits Osteoclast Formation by down Regulation of PU.1.

CC-4047 (Actimid) is an immunomodulatory analog of thalidomide that has stronger anti-myeloma and anti-angiogenic activity than thalidomide, but its effects on human osteoclast lineage are unknown. Early osteoclast progenitors are of hematopoietic origin and progressively differentiate into mature bone resorbing multinucleated osteoclasts. We investigated the effects of CC-4047 and thalidomide on human osteoclastogenesis, using in vitro receptor activator of NFκ-B ligand/M-CSF stimulated culture system of bone marrow cells. Three weeks of treatment of primary bone marrow cultures with 100 μM CC-4047 decreased osteoclast formation accompanied by complete inhibition of bone resorption. Interestingly, osteoclast formation was also inhibited when cultures were treated with CC-4047 only for the first week (90% inhibition). In contrast, inhibitory effect was greatly diminished when the drug was given for only the last week (25% inhibition), indicating that inhibition of osteoclast formation is an early event. The inhibitory effect of CC-4047 on osteoclastogenesis was not induced by cell death, but by a shift of lineage commitment to granulocyte-CFU at the expense of GM-CFU that are osteoclast progenitors. Further studies revealed that this shift is mediated through down regulation of the transcription factor PU.1, which is critical for early osteoclast formation. In contrast to CC-4047, thalidomide was a significantly less potent inhibitor of osteoclast formation and bone resorption. These results provide the first evidence that CC-4047 blocks osteoclast differentiation at the early phase of osteoclastogenesis. Therefore, CC-4047 might be a valuable drug targeting both the tumor and osteoclastic activity in patients with multiple myeloma and potentially other diseases associated with the development of osteolytic lesions.

Downregulation of PU.1 Is Required for Subtype of Multiple Myeloma.

There are several recent reports that certain mutations in critical transcription factors could be associated with leukemogenesis; however, it remains unclear whether such mutations could be linked to the pathogenesis of multiple myeloma (MM). In the present study, we attempted to determine whether mutations in transcription factors could be found in the patients with MM, utilizing MM cell lines and freshly isolated MM cells, purified with CD138 antibody and magnetic beads. When we looked at the expression of Blimp-1, XBP-1, NF-kB (p65 and p50), and PU.1 using semi-quantitative RT-PCR, all except PU.1 were found expressed in all MM cell lines and freshly isolated MM cells examined. All the PCR products were sequenced, but no significant mutations were identified. As examined with RT-PCR, PU.1 was found completely downregulated in 6 of 8 MM cell lines, while only faint to no expression of PU.1 was seen in 12 of 41 freshly isolated MM cells. As PU.1 message was sequenced, no mutations were found in 29 PU.1-positive MM cells or 2 MM cell lines that had relatively high PU.1 expression. Considering that PU.1 is essential for myeloid and lymphoid differentiation and that mice with null mutation in the PU.1 gene have no granulocytes, monocytes/macrophages or B lymphocytes, together with a recent report by others that AML developed in mice with PU.1 downregulation by 20%, we asked whether PU.1 downregulation might be associated with the oncogenesis or maintenance of MM. To examine the effects of PU.1 expression on MM cells, we generated tet-off conditional PU.1 expression model of PU.1-negative U266 MM cell line. Surprisingly the induction of PU.1 expression resulted in cell cycle arrest and apoptosis of those cells. Cell cycle analysis using flow cytometry revealed that PU.1-expressing U266 cells were in the G1 arrest. May-Giemsa staining revealed that such cells were swollen, enlarged, and vacuolous. These changes were completely reverted when the expression of PU.1 was silenced by tetracycline addition. There was no change in expression levels of cell cycle-related genes (p27, cyclin D1, cyclin E) and apoptosis-related genes (Bcl-2, Bcl-XL, and A1), while caspase3 cleavage was observed by 72 hr following PU.1 induction. We are currently examining the mechanism(s) of PU.1 downregulation in MM cells and a downstream cascade of PU.1 expression during cell death.

Dynamic regulation of PU.1 expression in multipotent hematopoietic progenitors

PU.1 is an Ets family transcription factor that is essential for fetal liver hematopoiesis. We have generated a PU.1 gfp reporter strain that allowed us to examine the expression of PU.1 in all hematopoietic cell lineages and their early progenitors. Within the bone marrow progenitor compartment, PU.1 is highly expressed in the hematopoietic stem cell, the common lymphoid progenitor, and a proportion of common myeloid progenitors (CMPs). Based on Flt3 and PU.1 expression, the CMP could be divided into three subpopulations, Flt3+ PU.1hi, Flt3− PU.1hi, and Flt3− PU.1lo CMPs. Colony-forming assays and in vivo lineage reconstitution demonstrated that the Flt3+ PU.1hi and Flt3− PU.1hi CMPs were efficient precursors for granulocyte/macrophage progenitors (GMPs), whereas the Flt3− PU.1lo CMPs were highly enriched for committed megakaryocyte/erythrocyte progenitors (MEPs). CMPs have been shown to rapidly differentiate into GMPs and MEPs in vitro. Interestingly, short-term culture revealed that the Flt3+ PU.1hi and Flt3− PU.1hi CMPs rapidly became CD16/32high (reminiscent of GMPs) in culture, whereas the Flt3− PU.1lo CMPs were the immediate precursors of the MEP. Thus, down-regulation of PU.1 expression in the CMP is the first molecularly identified event associated with the restriction of differentiation to erythroid and megakaryocyte lineages.

Constitutive Expression of PU.1 in Fetal Hematopoietic Progenitors Blocks T Cell Development at the Pro-T Cell Stage

The essential hematopoietic transcription factor PU.1 is expressed in multipotent thymic precursors but downregulated during T lineage commitment. The significance of PU.1 downregulation was tested using retroviral vectors to force hematopoietic precursors to maintain PU.1 expression during differentiation in fetal thymic organ culture. PU.1 reduced thymocyte expansion and blocked development at the pro-T cell stage. PU.1-expressing cells could be rescued by switching to conditions permissive for macrophage development; thus, the inhibition depends on both lineage and developmental stage. An intact DNA binding domain was required for these effects. PU.1 expression can downregulate pre-Tα, Rag-1, and Rag-2 in a dose-dependent manner, and higher PU.1 levels induce Mac-1 and Id-2. Thus, downregulation of PU.1 is specifically required for progression in the T cell lineage.

Type I interferons and IL-12: convergence and cross-regulation among mediators of cellular immunity

Therapeutic use of type I IFN (IFN-α/β) has become common. Many of the diverse diseases targeted are marked by pathogenetic abnormalities in cell-mediated immunity (CMI), these cellular immune responses either causing injury to the host, lacking sufficient vigor for virus or tumor clearance, or both. In general, therapeutic efficacy is limited. It is thus notable that the pleiotropic effects of type I IFN on CMI remain poorly understood. We characterized the effects of type I IFN on the production of IL-12, the central immunoregulatory cytokine of the CD4+ T cell arm of CMI. We show that type I IFN are potent inhibitors of IL-12 production by human monocytes/macrophages. The underlying mechanism involves transcriptional inhibition of the IL-12p40 gene, marked by down-regulation of PU.1 binding activity at the upstream Ets site of the IL-12p40 promoter. Type I IFN have previously been shown to be able to substitute for IL-12 in driving IFN-γ production from T and NK cells. The ability of IFN-α/β to suppress IL-12 production while up-regulating IFN-γ production suggests a possible mechanistic basis for the difficulties of employing these cytokines in diseases involving abnormalities of CMI.

Type I interferons and IL-12: convergence and cross-regulation among mediators of cellular immunity.

Therapeutic use of type I IFN (IFN-alpha/beta) has become common. Many of the diverse diseases targeted are marked by pathogenetic abnormalities in cell-mediated immunity (CMI), these cellular immune responses either causing injury to the host, lacking sufficient vigor for virus or tumor clearance, or both. In general, therapeutic efficacy is limited. It is thus notable that the pleiotropic effects of type I IFN on CMI remain poorly understood. We characterized the effects of type I IFN on the production of IL-12, the central immunoregulatory cytokine of the CD4(+) T cell arm of CMI. We show that type I IFN are potent inhibitors of IL-12 production by human monocytes/macrophages. The underlying mechanism involves transcriptional inhibition of the IL-12p40 gene, marked by down-regulation of PU.1 binding activity at the upstream Ets site of the IL-12p40 promoter. Type I IFN have previously been shown to be able to substitute for IL-12 in driving IFN-gamma production from T and NK cells. The ability of IFN-alpha/beta to suppress IL-12 production while up-regulating IFN-gamma production suggests a possible mechanistic basis for the difficulties of employing these cytokines in diseases involving abnormalities of CMI.

Deregulated expression of the PU.1 transcription factor blocks murine erythroleukemia cell terminal differentiation.

Murine erythroleukemia (MEL) cells are transformed erythroid precursors that are blocked from completing the late stages of erythroid differentiation. A frequent event in the generation of these malignant cells is deregulation of the hematopoietic-specific transcription factor PU.1 (Spi-1) by retroviral insertion of the spleen-focus-forming virus component of Friend virus. During chemically induced reinitiation of MEL cell terminal differentiation, expression of PU.1 is rapidly down-regulated, suggesting that PU.1 might interfere with processes required for terminal differentiation of erythroid precursors. To investigate the role of PU.1 in erythroid differentiation we transfected MEL cells with a PU.1 cDNA controlled by the eucaryotic translation elongation factor EF1 alpha promoter. Deregulated expression of PU.1 blocked chemically induced differentiation and terminal cell division. Deregulated expression of two other protooncogenes, c-myc and c-myb, also has been shown to block MEL differentiation. We present evidence that PU.1 inhibits terminal differentiation at an earlier step than c-Myc and c-Myb. Thus reinitiation of MEL cell terminal differentiation appears to be controlled by an ordered program of turning off several protooncogenes. Down-regulation of PU.1 may be a very early step in this program.