(1/2293) Leukemia translocation protein PLZF inhibits cell growth and expression of cyclin A.
The PLZF gene was identified by its fusion with the RARalpha locus in a therapy resistant form of acute promyelocytic leukemia (APL) associated with the t(11;17)(q23;q21) translocation. Here we describe PLZF as a negative regulator of cell cycle progression ultimately leading to growth suppression. PLZF can bind and repress the cyclin A2 promoter while expression of cyclin A2 reverts the growth suppressed phenotype of myeloid cells expressing PLZF. In contrast RARalpha-PLZF, a fusion protein generated in t(11;17)(q23;q21)-APL activates cyclin A2 transcription and allows expression of cyclin A in anchorage-deprived NIH3T3 cells. Therefore, cyclin A2 is a candidate target gene for PLZF and inhibition of cyclin A expression may contribute to the growth suppressive properties of PLZF. Deregulation of cyclin A2 by RARalpha-PLZF may represent an oncogenic mechanism of this chimeric protein and contribute to the aggressive clinical phenotype of t(11;17)(q23;q21)-associated APL. (+info)
(2/2293) Three distinct domains in TEL-AML1 are required for transcriptional repression of the IL-3 promoter.
A cytogenetically cryptic (12;21) translocation is the most common molecular abnormality identified in childhood acute lymphoblastic leukemia (ALL), and it generates a chimeric TEL-AML1 protein. Fusion of the Helix-Loop-Helix (HLH) (also called the pointed) domain of TEL to AML1 has been suggested to convert AML1 from a transcriptional activator to a repressor. To define the structural features of this chimeric protein required for repression, we analysed the transcriptional activity of a series of TEL-AML1 mutants on the AML1-responsive interleukin-3 (IL-3) promoter, a potentially relevant gene target. Our results demonstrate that TEL-AML1 represses basal IL-3 promoter activity in lymphoid cells, and deletion mutant analysis identified three distinct domains of TEL-AML1 that are required for repression; the HLH (pointed) motif contained in the TEL portion of TEL-AML1, and both the runt homology domain (Rhd) and the 74 amino acids downstream of the Rhd that are present in the AML1 portion of the fusion protein. Although AML1B (and a shorter AML1 isoform, AML1A) have transcriptional activating activity on the IL-3 promoter, fusion of the AML1 gene to the TEL gene generates a repressor of IL-3 expression. Consistent with this activity, freshly isolated human ALL cells that contain TEL-AML1 do not express IL-3. (+info)
(3/2293) Retinoic acid, but not arsenic trioxide, degrades the PLZF/RARalpha fusion protein, without inducing terminal differentiation or apoptosis, in a RA-therapy resistant t(11;17)(q23;q21) APL patient.
Primary blasts of a t(11;17)(q23;q21) acute promyelocytic leukaemia (APL) patient were analysed with respect to retinoic acid (RA) and arsenic trioxide (As2O3) sensitivity as well as PLZF/RARalpha status. Although RA induced partial monocytic differentiation ex vivo, but not in vivo, As203 failed to induce apoptosis in culture, contrasting with t(15;17) APL and arguing against the clinical use of As203 in t(11;17)(q23;q21) APL. Prior to cell culture, PLZF/RARalpha was found to exactly co-localize with PML onto PML nuclear bodies. However upon cell culture, it quickly shifted towards microspeckles, its localization found in transfection experiments. Arsenic trioxide, known to induce aggregation of PML nuclear bodies, left the microspeckled PLZF/RARalpha localization completely unaffected. RA treatment led to PLZF/RARalpha degradation. However, this complete PLZF/RARalpha degradation was not accompanied by differentiation or apoptosis, which could suggest a contribution of the reciprocal RARalpha/PLZF fusion product in leukaemogenesis or the existence of irreversible changes induced by the chimera. (+info)
(4/2293) Establishment of an inducible expression system of chimeric MLL-LTG9 protein and inhibition of Hox a7, Hox b7 and Hox c9 expression by MLL-LTG9 in 32Dcl3 cells.
The MLL (HRX/ALL-1 gene is frequently disrupted in infantile leukemias and therapy-related leukemias and fused to various translocation partner genes. We previously showed that chimeric MLL proteins localize in the nuclei in a fashion similar to that of MLL protein even if the partner gene encodes a cytoplasmic protein and indicated the importance of the N-terminal portion of MLL common to various MLL translocations. This time we established an inducible expression system for chimeric MLL-LTG9 and truncated N-terminal MLL proteins (MLL-Zf(-)) in 32Dcl3 cells. By utilizing this system, we were able to show inhibition of Hox a7, Hox b7 and Hox c9 genes' expression by induced MLL-LTG9 and MLL-Zf(-). Up-regulation of Hox a7, Hox b7 and Hox c9 was observed when 32Dcl3 cells were cultured with granulocyte colony stimulating factor (G-CSF) in place of interleukin 3 and induction of MLL-LTG9 and MLL-Zf(-) was shown to suppress this upregulation. At the same time, expression of two mammalian Polycomb group genes, M33 and mel-18, which both reportedly affect Hox genes' expression, was not inhibited by MLL-LTG9 and MLL-Zf(-) induction. These results indicate that MLL has an important effect on the expression of at least some Hox genes in hematopoietic cells and suggest that inhibition of the proper expression of Hox genes by chimeric MLL proteins may dysregulate hematopoietic cell differentiation and proliferation, which then can lead to leukemogenesis. (+info)
(5/2293) Use of RhD fusion protein expressed on K562 cell surface in the study of molecular basis for D antigenic epitopes.
The human D antigens, one of the most clinically important blood groups, are presented by RhD protein with a putative 12 transmembrane topology. To understand the molecular basis for the complex antigenic profile of RhD protein, we expressed a series of RhD fusion proteins using different portions of Duffy protein as a tag in erythroleukemic K562 cells. Because the reactivity of monoclonal anti-RhD antibody, LOR15C9, depends mainly on the sequence coded by exon 7 of RhD, we altered DNA sequence corresponding to the amino acid residues 323-331(A) and 350-354(B) in the exon 7. The mutation in region B resulted in a severe reduction in LOR15C9 binding by flow cytometry analysis, suggesting that region B may play an important role in constituting antigen epitopes recognized by LOR15C9. On the other hand, a slight decrease in the antibody binding was observed for the region A mutant, suggesting that the intracellularly located region A may elicit a long distance effect on the formation of exofacial antigen epitopes. In addition, using various monoclonal antibodies against RhD, we compared the antigenic profile of expressed RhD fusion protein with that of endogenous RhD in K562 cells as well as in erythrocytes. (+info)
(6/2293) Constitutive degradation of PML/RARalpha through the proteasome pathway mediates retinoic acid resistance.
PML/RARalpha is the leukemogenetic protein of acute promyelocytic leukemia (APL). Treatment with retinoic acid (RA) induces degradation of PML/RARalpha, differentiation of leukaemic blasts, and disease remission. However, RA resistance arises during RA treatment of APL patients. To investigate the phenomenon of RA resistance in APL, we generated RA-resistant sublines from APL-derived NB4 cells. The NB4.007/6 RA-resistant subline does not express the PML/RARalpha protein, although its mRNA is detectable at levels comparable to those of the parental cell line. In vitro degradation assays showed that the half-life of PML/RARalpha is less than 30 minutes in NB4.007/6 and longer than 3 hours in NB4. Treatment of NB4.007/6 cells with the proteasome inhibitors LLnL and lactacystin partially restored PML/RARalpha protein expression and resulted in a partial release of the RA-resistant phenotype. Similarly, forced expression of PML/RARalpha, but not RARalpha, into the NB4/007.6 cells restored sensitivity to RA treatment to levels comparable to those of the NB4 cells. These results indicate that constitutive degradation of PML/RARalpha protein may lead to RA resistance and that PML/RARalpha expression is crucial to convey RA sensitivity to APL cells. (+info)
(7/2293) Functional domains of the SYT and SYT-SSX synovial sarcoma translocation proteins and co-localization with the SNF protein BRM in the nucleus.
The t(X;18)(p11.2;q11.2) chromosomal translocation commonly found in synovial sarcomas fuses the SYT gene on chromosome 18 to either of two similar genes, SSX1 or SSX2, on the X chromosome. The SYT protein appears to act as a transcriptional co-activator and the SSX proteins as co-repressors. Here we have investigated the functional domains of the proteins. The SYT protein has a novel conserved 54 amino acid domain at the N-terminus of the protein (the SNH domain) which is found in proteins from a wide variety of species, and a C-terminal domain, rich in glutamine, proline, glycine and tyrosine (the QPGY domain), which contains the transcriptional activator sequences. Deletion of the SNH domain results in a more active transcriptional activator, suggesting that this domain acts as an inhibitor of the activation domain. The C-terminal SSX domain present in SYT-SSX translocation protein contributes a transcriptional repressor domain to the protein. Thus, the fusion protein has transcriptional activating and repressing domains. We demonstrate that the human homologue of the SNF2/Brahama protein BRM co-localizes with SYT and SYT-SSX in nuclear speckles, and also interacts with SYT and SYT-SSX proteins in vitro. This interaction may provide an explanation of how the SYT protein activates gene transcription. (+info)
(8/2293) Molecular forceps from combinatorial libraries prevent the farnesylation of Ras by binding to its carboxyl terminus.
INTRODUCTION: Ras is one of the major oncogenes. In order to function properly it has to undergo post-translational processing at its carboxyl terminus. It has been shown that inhibitors of farnesyl transferase, the first enzyme in the processing chain, can suppress the transforming activity of oncogenic Ras. RESULTS: We have identified molecular forceps, branched peptidic molecules, from combinatorial libraries that bind to the carboxyl terminus of Ras and interfere with its farnesylation without inhibiting the farnesyl transferase. The active molecules were selected by a screening against the carboxy-terminal octapeptide of Ras. CONCLUSIONS: The implications of our findings are twofold. First, we demonstrate that it is possible to prevent enzymatic transformations by blocking the enzyme's access to its substrate using a synthetic small molecule to mask the substrate. Second, we show that it is feasible to derive molecules from combinatorial libraries that bind a specific epitope on a protein by selecting these molecules with the isolated peptide epitope. (+info)