A Novel BTB/POZ Transcriptional Repressor Protein Interacts With the Fanconi Anemia Group C Protein and PLZF

ZBTB Transcription Factors: Key Regulators of the Development, Differentiation and Effector Function of T Cells

The development and differentiation of T cells represents a long and highly coordinated, yet flexible at some points, pathway, along which the sequential and dynamic expressions of different transcriptional factors play prominent roles at multiple steps. The large ZBTB family comprises a diverse group of transcriptional factors, and many of them have emerged as critical factors that regulate the lineage commitment, differentiation and effector function of hematopoietic-derived cells as well as a variety of other developmental events. Within the T-cell lineage, several ZBTB proteins, including ZBTB1, ZBTB17, ZBTB7B (THPOK) and BCL6 (ZBTB27), mainly regulate the development and/or differentiation of conventional CD4/CD8 αβ⁺ T cells, whereas ZBTB16 (PLZF) is essential for the development and function of innate-like unconventional γδ⁺ T & invariant NKT cells. Given the critical role of T cells in host defenses against infections/tumors and in the pathogenesis of many inflammatory disorders, we herein summarize the roles of fourteen ZBTB family members in the development, differentiation and effector function of both conventional and unconventional T cells as well as the underlying molecular mechanisms.

Chase M, Johannsen E, Kieff E, Cahir-McFarland E. RNAs induced by Epstein-Barr virus nuclear antigen 2 in lymphoblastoid cell lines

Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA2) association with RBP-Jkappa is essential for regulation of virus and cell gene transcription and B lymphocyte transformation into infinitely proliferating lymphoblastoid cells (LCLs). To identify EBNA2-regulated cell genes in LCLs, an EBV recombinant that expresses EBNA2 with its C terminus fused in frame to a 4-hydroxytamoxifen (4HT)-dependent mutant estrogen receptor (E2HTF) was used to transform primary B lymphocytes to LCLs. In the presence of 4HT, E2HTF expression level and effects on the LMP1 promoter in transfected BJAB lymphoblasts were similar to EBNA2. In 4HT-supplemented medium, E2HTF EBV recombinant infected LCLs were also similar to EBNA2 LCLs in outgrowth but required higher serum and a restricted range of cell concentrations for consistent growth. In medium without 4HT, E2HTF localized to the cytoplasm, c-myc levels substantially decreased within 6 h, cells stopped growing, and levels of other EBNAs and LMP1 remained stable for 24 h. Over this 24-h period, 30 cell RNAs decreased 2-fold, and 51 other RNAs decreased 1.5-fold. These RNAs encode proteins important in cell adhesion or signaling, transcription, RNA processing, cell-cycle regulation, and survival. Real-time RT-PCR confirmed EBNA2-dependent expression of eight RNAs.

The transcription factor Pokemon: a new key player in cancer pathogenesis

Learning how critical cell regulatory pathways are controlled may lead to new opportunities for cancer treatment. We recently identified the transcription factor Pokemon as a central regulator of the important tumor suppressor ARF. Pokemon is overexpressed in multiple human cancers and cells lacking Pokemon are refractory to oncogenic transformation. These findings suggest that Pokemon may offer an effective new target for cancer therapeutics.

Regulation of Hoxb2 by APL-associated PLZF protein

The PLZF gene is translocated in a subset of all-trans-retinoic acid resistant acute promyelocytic leukaemia (APL) cases, encodes a DNA binding transcription factor and is expressed highly in haematopoietic progenitor cells as well-developing central nervous system (CNS). The spatially restricted and temporally dynamic pattern of PLZF expression in the developing CNS suggested that it might play a role in the circuitry regulating hindbrain segmentation. We have now identified a PLZF binding site (PLZF-RE) in an enhancer region of Hoxb2 that itself is required for directing high-level expression in rhombomers 3 and 5 of the developing hindbrain. The wild-type r3/r5 enhancer linked to a heterologous promoter was responsive to regulation by PLZF, and this activity was lost in variants containing a mutated PLZF-RE. Compared with the wild-type protein, the binding of the APL-associated reciprocal RARalpha-PLZF fusion to PLZF-RE was much stronger, suggesting that the N-terminal PLZF sequences missing from the fusion may play a role in the regulation of DNA binding. Consistent with this, the N-terminal POZ domain was required for cooperative binding of PLZF to a multimerized PLZF-RE. In the context of the r3/r5 enhancer, the PLZF-RE cooperated for PLZF binding with an additional A/T-rich motif positioned downstream of the PLZF-RE. This A/T motif was previously shown to be essential for the regulation of Hoxb2 expression in r3 and r5 in cooperation with another Krüppel-like zinc finger protein Krox 20. The presence of both the PLZF-RE and the A/T-rich motif was required for a maximal effect of PLZF on a heterologous promoter and was essential in vivo to direct the expression of a lacZ reporter in the chick neural tube. Hence, both PLZF and Krox20 cooperate with a common A/T motif in mediating in vivo activity of the Hoxb2 enhancer. Our findings indicate that Hoxb2 is a direct target for regulation by PLZF in the developing CNS and suggest that deregulation of Hox gene expression may contribute to APL pathogenesis.

Translocations of the RARalpha gene in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) has been recognized as a distinct clinical entity for over 40 years. Although relatively rare among hematopoietic malignancies (approximately 10% of AML cases), this disease has attracted a particularly good share of attention by becoming the first human cancer in which all-trans-retinoic acid (ATRA), a physiologically active derivative of vitamin A, was able to induce complete remission (CR). ATRA induced remission is not associated with rapid cell death, as in the case of conventional chemotherapy, but with a restoration of the 'normal' granulocytic differentiation pathway. With this remarkable medical success story APL has overnight become a paradigm for the differentiation therapy of cancer. A few years later, excitement with APL was further enhanced by the discovery that a cytogenetic marker for this disease, the t(15:17) reciprocal chromosomal translocation, involves a fusion between the retinoic acid receptor alpha (RARalpha) gene and a previously unknown locus named promyelocytic leukemia (PML). Consequence of this gene rearrangement is expression of the PML-RARalpha chimeric oncoprotein, which is responsible for the cellular transformation as well as ATRA response that is observed in APL. Since this initial discovery, a number of different translocation partner genes of RARalpha have been reported in rarer cases of APL, strongly suggesting that disruption of RARalpha underlies its pathogenesis. This article reviews various rearrangements of the RARalpha gene that have so far been described in literature, functions of the proteins encoded by the different RARalpha partner loci, and implications that these may have for the molecular pathogenesis of APL.

Growth-suppressive effects of BPOZ and EGR2, two genes involved in the PTEN signaling pathway

Defects in PTEN, a tumor suppressor, have been found in cancers arising in a variety of human tissues. To elucidate the tumor-suppressive function of this gene, we have been analysing expression profiles of cancer cells after introduction of exogenous PTEN. Those experiments identified 99 candidate genes that were transcriptionally transactivated. Among them, we report here the further analyses of eight genes, EGR2/Krox-20, BPOZ, APS, HCLS1/HS1, DUSP1/MKP1, NDRG1/Drg1/RTP, NFIL3/E4BP4, and a novel gene (PINK1, PTEN-induced putative kinase). Expression of six of them (PINK1, EGR2, HCLS1, DUSP1, BPOZ, and NFIL3) was decreased in ovarian tumors compared with corresponding normal tissues. Colony-formation assays using plasmid clones designed to express each gene indicated that EGR2 and BPOZ were able to suppress growth of cancer cells significantly; in particular, cancer-cell lines stably expressing BPOZ grew more slowly than control cells containing mock vector. Flow cytometry suggested that over-expression of BPOZ inhibited progression of the cell cycle at the G(1)/S transition. Anti-sense oligonucleotides for BPOZ or EGR2 effectively inhibited their expression, and cell growth was accelerated. Therefore both genes appear to be novel candidates as mediators of the PTEN growth-suppressive signaling pathway.