Direct interaction of PU.1 with oncogenic transcription factors reduces its serine phosphorylation and promoter binding.

The homeostasis of hematopoiesis in the bone marrow is governed by a small number of key transcription factors, including PU.1, GATA-1 and c/EBPα. PU.1, a member of the E-twenty-six family of transcription factors, is indispensable for normal hematopoiesis. Inactivation of PU.1 induces acute leukemia in mice. Recent data suggest that the leukemia-associated fusion protein pro-myelocytic leukemia/retinoic acid receptor alpha (PML/RARα) inhibits PU.1, but the mechanism mediating this inhibition is unclear. Here, we investigated the mechanisms by which the fusion proteins PML/RARα and pro-myelocytic leukemia zinc finger/RARα (PLZF/RARα) (X-RARα) interfere with the function of PU.1. We found that X-RARα proteins functionally inactivate PU.1 by reducing its promoter-binding capacity, resulting in a reduction in PU.1-dependent transcriptional transactivation. In fact, X-RARα proteins directly interact with PU.1, leading to both the sequestration of PU.1 from its target promoters and a reduction in its serine phosphorylation, which is crucial for its promoter binding and transcriptional activity. We found that the functional inactivation of PU.1 could be overcome by the forced overexpression of PU.1 in PML/RARα- or PLZF/RARα-positive murine hematopoietic progenitor cells; evidently, this overexpression rescued the leukemic differentiation block induced by X-RARα proteins. Our data thus provide strong evidence that X-RARα proteins functionally inhibit PU.1, shedding light on the mechanism by which X-RARα proteins induce leukemogenesis.

The t(6;9) associated DEK/CAN fusion protein targets a population of long-term repopulating hematopoietic stem cells for leukemogenic transformation.

The t(6;9)-positive acute myeloid leukemia (AML) is classified as a separate clinical entity because of its early onset and poor prognosis. The hallmark of t(6;9) AML is the expression of the DEK/CAN fusion protein. The leukemogenic potential of DEK/CAN has been called into question, because it was shown to be unable to block the differentiation of hematopoietic progenitors. We found that DEK/CAN initiated leukemia from a small subpopulation within the hematopoietic stem cell (HSC) population expressing a surface marker pattern of long-term (LT) HSC. The propagation of established DEK/CAN-positive leukemia was not restricted to the LT-HSC population, but occurred even from more mature and heterogeneous cell populations. This finding indicates that in DEK/CAN-induced leukemia, there is a difference between 'leukemia-initiating cells' (L-ICs) and 'leukemia-maintaining cells' (L-MCs). In contrast to the L-IC cells represented by a very rare subpopulation of LT-HSC, the L-MC seem to be represented by a larger and phenotypically heterogeneous cell population.

Acute promyelocytic leukemia: PML/RARalpha and the leukemic stem cell.

Acute promyelocytic leukemia (APL) is distinguished from other acute myeloid leukemias (AMLs) by cytogenetic, clinical, as well as biological characteristics. The hallmark of APL is the t(15;17), which leads to the expression of the PML/RARalpha fusion protein. PML/RARalpha is the central leukemia-inducing lesion in APL and is directly targeted by all trans retinoic acid (t-RA) as well as by arsenic, both compounds able to induce complete remissions. This review focuses on potential stem cell involvement in APL outlining the knowledge about the APL-initiating stem cell and the influence of PML/RARalpha on the biology of the hematopoietic stem cell. Moreover, the importance of the blockage of t-RA signaling by the PML/RARalpha for the pathogenesis of APL is discussed, taking the relevance of the t-RA signaling pathway for the global hematopoiesis into account.

Deregulated expression of prostate apoptosis response gene-4 in less differentiated lymphocytes and inverse expressional patterns of par-4 and bcl-2 in acute lymphocytic leukemia.

INTRODUCTION: Prostate apoptosis response gene-4, known as par-4, is a new proapoptotic factor functionally required but not sufficient for apoptosis. Since there is evidence from prostate cancer cells that par-4 is involved in regulation of bcl-2 we assessed expression of par-4 and bcl-2 in different populations of normal and neoplastic lymphocytes. MATERIALS AND METHODS: Expression of par-4 mRNA and protein in different subpopulations of normal and neoplastic lymphocytes was assessed by reverse transcription polymerase chain reaction and Western blot. RESULTS: Par-4 mRNA was not detectable in lymphocytes of healthy volunteers (n = 10), but was present in the majority of samples of chronic lymphocytic leukemia (n = 30), chronic lymphocytic leukemia/prolymphocytic leukemia (n = 6) and acute lymphocytic leukemia (n = 10). Par-4 protein was expressed unanimously in samples of mononuclear cells from healthy volunteers and patients with CLL, but less frequently in immature lymphocytes, including neoplastic cells of CLL/PLL and ALL. The decreased frequency of par-4 expression in immature subpopulations was confirmed by results on lymphocytic cell lines at various stages of maturation. Comparing the expressional patterns of par-4 and bcl-2 there was an inverse relationship of both proteins in ALL and different lymphocytic cell lines, indicating a functional relationship of par-4 and bcl-2. CONCLUSIONS: This study establishes par-4 as a factor expressed in the majority of normal and neoplastic lymphocytic cells, demonstrating a decreased frequency of protein expression in less differentiated lymphocytes and an inverse expressional pattern of par-4 and bcl-2 in lymphocytic cell lines and ALL.

Down-stream regions of the POZ-domain influence the interaction of the t(11;17)-associated PLZF/RARalpha fusion protein with the histone-deacetylase recruiting co-repressor complex.

INTRODUCTION: Acute promyelocytic leukemia (APL) patients with t(15;17)(PML/RARalpha positive) achieve remission upon t-RA treatment, whereas patients with t(11;17)(PLZF/RARalpha positive) do not. Both APL translocation products bind to the histone deacetylase (HD)-recruiting nuclear co-repressor complex (HD-NCR) in a ligand-dependent manner through their RARalpha portion. Differently to PML/RARalpha, PLZF/RARalpha also binds the HD-NCR in a ligand-independent manner through the PLZF portion of the fusion protein (PLZF#), which seems to be crucial for the t-RA resistance of t(11;17) APL patients. MATERIALS AND METHODS: The t-RA sensitivity of U937 cells was tested by the nitro-blue tetrazolium reduction (NBT) assay and by analysis of t-RA-induced type II transglutaminase activity. The interaction between HD-NCR and PLZF/RARalpha was investigated by in vitro binding assays. RESULTS: (i) Deletions in PLZF# convert PLZF/RARalpha from a repressor to an activator of t-RA response in U937 cells; (ii) the effect of PLZF/RARalpha on t-RA-signaling is regulated by the POZ-domain and its down-stream regions of PLZF#; (iii) there are additional binding sites for HD-NCR in PLZF# and (iv) PLZF# not only directly binds but also regulates the binding of PLZF/RARalpha to the HD-NCR. CONCLUSIONS: At least two different mechanisms responsible for the aberrant recruitment of HD-NCR complexes by PLZF# are regulating the different t-RA-sensitivity of the PLZF/RARalpha and PML/RARalpha positive APL blasts: one is related to the direct binding of the different members of the HD-NCR complex to PLZF#; the other is an enforcing effect of PLZF# on the affinity of the PLZF/RARalpha fusion protein to the HD-NCR complex.

BCR-ABL mediates arsenic trioxide-induced apoptosis independently of its aberrant kinase activity.

In the prechemotherapy era arsenic derivatives were used for treatment of chronic myelogenous leukemia, a myeloproliferative disorder characterized by the t(9;22) translocation, the Philadelphia chromosome (Ph+). In acute promyelocytic leukemia response to arsenic trioxide (As2O3) has been shown to be genetically determined by the acute promyelocytic leukemia-specific t(15;17) translocation product PML/RARalpha. Hence, we reasoned that As2O3 might have a selective inhibitory effect on proliferation of BCR-ABL-expressing cells. Here, we report that: (a) As2O3 induced apoptosis in Ph+ but not in Ph- lymphoblasts; (b) enforced expression of BCR-ABL in U937 cells dramatically increased the sensitivity to As2O3; (c) the effect of As2O3 was independent of BCR-ABL kinase activity; and (d) As2O3 reduced proliferation of chronic myelogenous leukemia blasts but not of peripheral CD34+ progenitors. In summary, these data establish As2O3 as a tumor cell-specific agent, making its clinical application in Ph+ leukemia feasible.

PIC-1/SUMO-1-modified PML-retinoic acid receptor alpha mediates arsenic trioxide-induced apoptosis in acute promyelocytic leukemia.

Fusion proteins involving the retinoic acid receptor alpha (RARalpha) and PML or PLZF nuclear protein are the genetic markers of acute promyelocytic leukemia (APL). APLs with PML-RARalpha or PLZF-RARalpha fusion protein differ only in their response to retinoic acid (RA) treatment: the t(15;17) (PML-RARalpha-positive) APL blasts are sensitive to RA in vitro, and patients enter disease remission after RA treatment, while those with t(11;17) (PLZF-RARalpha-positive) APLs do not. Recently it has been shown that complete remission can be achieved upon treatment with arsenic trioxide (As2O3) in PML-RARalpha-positive APL, even when the patient has relapsed and the disease is RA resistant. This appears to be due to apoptosis induced by As2O3 in the APL blasts by poorly defined mechanisms. Here we report that (i) As2O3 induces apoptosis only in cells expressing the PML-RARalpha, not the PLZF-RARalpha, fusion protein; (ii) PML-RARalpha is partially modified by covalent linkage with a PIC-1/SUMO-1-like protein prior to As2O3 treatment, whereas PLZF-RARalpha is not; (iii) As2O3 treatment induces a change in the modification pattern of PML-RARalpha toward highly modified forms; (iv) redistribution of PML nuclear bodies (PML-NBs) upon As2O3 treatment is accompanied by recruitment of PIC-1/SUMO-1 into PML-NBs, probably due to hypermodification of both PML and PML-RARalpha; (v) As2O3-induced apoptosis is independent of the DNA binding activity located in the RARalpha portion of the PML-RARalpha fusion protein; and (vi) the apoptotic process is bcl-2 and caspase 3 independent and is blocked only partially by a global caspase inhibitor. Taken together, these data provide novel insights into the mechanisms involved in As2O3-induced apoptosis in APL and predict that treatment of t(11;17) (PLZF-RARalpha-positive) APLs with As2O3 will not be successful.

The acute promyelocytic leukaemia specific PML and PLZF proteins localize to adjacent and functionally distinct nuclear bodies.

Acute promyelocytic leukaemia is characterized by translocations that involve the retinoic acid receptor alpha (RAR alpha) locus on chromosome 17 and the PML locus on 15 or the PLZF locus on 11. The resulting abnormal translocation products encode for PML/RAR alpha or PLZF/RAR alpha fusion proteins. There is increasing experimental evidence that the APL-specific fusion proteins have similar biologic activities on differentiation and survival and that both components of the fusion proteins (PML or PLZF and RAR alpha) are indispensable for these biological activities. The physiologic function of PML or PLZF or whether PML and PLZF contribute common structural or functional features to the corresponding fusion proteins is not known. We report here immunofluorescence studies on the cellular localization of PLZF and PLZF/RAR alpha and compare it with the localization of PML and PML/RAR alpha. PLZF localizes to nuclear domains of 0.3-0.5 microns, approximately 14 per cell in the KG1 myeloid cell line. These PLZF-bodies are morphologically similar to the domains reported for PML (PML-NBs). There is tight spatial relationship between about 30% of PLZ-NBs and PML-NBs: they partially overlap. However, PML and PLZF do not form soluble complexes in vivo. PLZF- and PML-NBs are functionally distinct. Adenovirus E4-ORF3 protein expression alters the structure of the PML-NBs and interferon increases the number of PML-NBs and neither has any effect on PLZF NBs. The localization of PLZF/RAR alpha is different to that of PLZF and RAR alpha. The nuclear distribution pattern of PLZF/RAR alpha is one of hundreds of small dots (microspeckles) less than 0.1 micron. Expression of PLZF/RAR alpha did not provoke disruption of the PML-NBs. Co-expression of PML/RAR alpha and PLZF/RAR alpha in U937 cells revealed apparent colocalization. Overall the results suggest that the PML- and PLZF-NBs are distinct functional nuclear domains, but that they may share common regulatory pathways and/or targeting sequences, as revealed by the common localization of their corresponding fusion proteins.

Opposite effects of the acute promyelocytic leukemia PML-retinoic acid receptor alpha (RAR alpha) and PLZF-RAR alpha fusion proteins on retinoic acid signalling.

Fusion proteins involving the retinoic acid receptor alpha (RAR alpha) and the PML or PLZF nuclear protein are the genetic markers of acute promyelocytic leukemias (APLs). APLs with the PML-RAR alpha or the PLZF-RAR alpha fusion protein are phenotypically indistinguishable except that they differ in their sensitivity to retinoic acid (RA)-induced differentiation: PML-RAR alpha blasts are sensitive to RA and patients enter disease remission after RA treatment, while patients with PLZF-RAR alpha do not. We here report that (i) like PML-RAR alpha expression, PLZF-RAR alpha expression blocks terminal differentiation of hematopoietic precursor cell lines (U937 and HL-60) in response to different stimuli (vitamin D3, transforming growth factor beta1, and dimethyl sulfoxide); (ii) PML-RAR alpha, but not PLZF-RAR alpha, increases RA sensitivity of hematopoietic precursor cells and restores RA sensitivity of RA-resistant hematopoietic cells; (iii) PML-RAR alpha and PLZF-RAR alpha have similar RA binding affinities; and (iv) PML-RAR alpha enhances the RA response of RA target genes (those for RAR beta, RAR gamma, and transglutaminase type II [TGase]) in vivo, while PLZF-RAR alpha expression has either no effect (RAR beta) or an inhibitory activity (RAR gamma and type II TGase). These data demonstrate that PML-RAR alpha and PLZF-RAR alpha have similar (inhibitory) effects on RA-independent differentiation and opposite (stimulatory or inhibitory) effects on RA-dependent differentiation and that they behave in vivo as RA-dependent enhancers or inhibitors of RA-responsive genes, respectively. Their different activities on the RA signalling pathway might underlie the different responses of PML-RAR alpha and PLZF-RAR alpha APLs to RA treatment. The PLZF-RAR alpha fusion protein contains an approximately 120-amino-acid N-terminal motif (called the POZ domain), which is also found in a variety of zinc finger proteins and a group of poxvirus proteins and which mediates protein-protein interactions. Deletion of the PLZF POZ domain partially abrogated the inhibitory effect of PLZF-RAR alpha on RA-induced differentiation and on RA-mediated type II TGase up-regulation, suggesting that POZ-mediated protein interactions might be responsible for the inhibitory transcriptional activities of PLZF-RAR alpha.