PINK1 and BECN1 relocalize at mitochondria-associated membranes during mitophagy and promote ER-mitochondria tethering and autophagosome formation

Mitophagy is a highly specialized process to remove dysfunctional or superfluous mitochondria through the macroautophagy/autophagy pathway, aimed at protecting cells from the damage of disordered mitochondrial metabolism and apoptosis induction. PINK1, a neuroprotective protein mutated in autosomal recessive Parkinson disease, has been implicated in the activation of mitophagy by selectively accumulating on depolarized mitochondria, and promoting PARK2/Parkin translocation to them. While these steps have been characterized in depth, less is known about the process and site of autophagosome formation upon mitophagic stimuli. A previous study reported that, in starvation-induced autophagy, the proautophagic protein BECN1/Beclin1 (which we previously showed to interact with PINK1) relocalizes at specific regions of contact between the endoplasmic reticulum (ER) and mitochondria called mitochondria-associated membranes (MAM), from which the autophagosome originates. Here we show that, following mitophagic stimuli, autophagosomes also form at MAM; moreover, endogenous PINK1 and BECN1 were both found to relocalize at MAM, where they promoted the enhancement of ER-mitochondria contact sites and the formation of omegasomes, that represent autophagosome precursors. PARK2 was also enhanced at MAM following mitophagy induction. However, PINK1 silencing impaired BECN1 enrichment at MAM independently of PARK2, suggesting a novel role for PINK1 in regulating mitophagy. MAM have been recently implicated in many key cellular events. In this light, the observed prevalent localization of PINK1 at MAM may well explain other neuroprotective activities of this protein, such as modulation of mitochondrial calcium levels, mitochondrial dynamics, and apoptosis.

Candidate genes for Parkinson disease: Lessons from pathogenesis

Parkinson disease (PD) is a multifactorial neurodegenerative disease characterized by the progressive loss of specific neuronal populations and accumulation of Lewy bodies in the brain, leading to motor and non-motor symptoms. In a small subset of patients, PD is dominantly or recessively inherited, while a number of susceptibility genetic loci have been identified through genome wide association studies. The discovery of genes mutated in PD and functional studies on their protein products have provided new insights into the molecular events leading to neurodegeneration, suggesting that few interconnected molecular pathways may be deranged in all forms of PD, triggering neuronal loss. Here, we summarize the most relevant findings implicating the main PD-related proteins in biological processes such as mitochondrial dysfunction, misfolded protein damage, alteration of cellular clearance systems, abnormal calcium handling and altered inflammatory response, which represent key targets for neuroprotection.

PINK1 protects against cell death induced by mitochondrial depolarization, by phosphorylating Bcl-xL and impairing its pro-apoptotic cleavage

Mutations in the PINK1 gene are a frequent cause of autosomal recessive Parkinson's disease (PD). PINK1 encodes a mitochondrial kinase with neuroprotective activity, implicated in maintaining mitochondrial homeostasis and function. In concurrence with Parkin, PINK1 regulates mitochondrial trafficking and degradation of damaged mitochondria through mitophagy. Moreover, PINK1 can activate autophagy by interacting with the pro-autophagic protein Beclin-1. Here, we report that, upon mitochondrial depolarization, PINK1 interacts with and phosphorylates Bcl-xL, an anti-apoptotic protein also known to inhibit autophagy through its binding to Beclin-1. PINK1-Bcl-xL interaction does not interfere either with Beclin-1 release from Bcl-xL or the mitophagy pathway; rather it protects against cell death by hindering the pro-apoptotic cleavage of Bcl-xL. Our data provide a functional link between PINK1, Bcl-xL and apoptosis, suggesting a novel mechanism through which PINK1 regulates cell survival. This pathway could be relevant for the pathogenesis of PD as well as other diseases including cancer.

A subcellular analysis of genetic modulation of PINK1 on mitochondrial alterations, autophagy and cell death

Mutations in the PTEN-induced putative kinase1 (PINK1) represent the second most frequent cause of autosomal recessive Parkinson's disease. The PINK1 protein mainly localizes to mitochondria and interacts with a variety of proteins, including the pro-autophagy protein beclin1 and the ubiquitin-ligase parkin. Upon stress conditions, PINK1 is known to recruit parkin at the surface of dysfunctional mitochondria and to activate the mitophagy cascade. Aim of this study was to use a simple and highly reproducible catecholamine cell model and transmission electron microscopy to characterize whether PINK1 could affect mitochondrial homeostasis, the recruitment of specific proteins at mitochondria, mitophagy and apoptosis. Samples were analyzed both in baseline conditions and following treatment with methamphetamine (METH), a neurotoxic compound which strongly activates autophagy and produces mitochondrial damage. Our data provide robust sub-cellular evidence that the modulation of PINK1 levels dramatically affects the morphology and number of mitochondria and the amount of cell death. In particular, especially upon METH exposure, PINK1 is able to increase the total number of mitochondria, concurrently recruit beclin1, parkin and ubiquitin and enhance the clearance of damaged mitochondria. In the absence of functional PINK1 and upon autophagy stress, we observe a failure of the autophagy system at large, with marked accumulation of dysfunctional mitochondria and dramatic increase of apoptotic cell death. These findings highlight the strong neuroprotective role of PINK1 as a key protein in the surveillance and regulation of mitochondrial homeostasis.

A variant in the carboxyl-terminus of connexin 40 alters GAP junctions and increases risk for tetralogy of Fallot

GJA5 gene (MIM no. 121013), localized at 1q21.1, encodes for the cardiac gap junction protein connexin 40. In humans, copy number variants of chromosome 1q21.1 have been associated with variable phenotypes comprising congenital heart disease (CHD), including isolated TOF. In mice, the deletion of Gja5 can cause a variety of complex CHDs, in particular of the cardiac outflow tract, corresponding to TOF in many cases. In the present study, we screened for mutations in the GJA5 gene 178 unrelated probands with isolated TOF. A heterozygous nucleotide change (c.793C>T) in exon 2 of the gene leading to the p.Pro265Ser variant at the carboxyl-terminus of the protein was found in two unrelated sporadic patients, one with classic anatomy and one with pulmonary atresia. This GJA5 missense substitution was not observed in 1568 ethnically-matched control chromosomes. Immunofluorescent staining and confocal microscopy revealed that cells expressing the mutant protein form sparse or no visible gap-junction plaques in the region of cell-cell contact. Moreover, analysis of the transfer of the gap junction permanent tracer lucifer yellow showed that cells expressing the mutant protein have a reduced rate of dye transfer compared with wild-type cells. Finally, use of a zebrafish model revealed that microinjection of the GJA5-p.Pro265Ser mutant disrupts overall morphology of the heart tube in the 37% (22/60) of embryos, compared with the 6% (4/66) of the GJA5 wild-type-injected embryos. These findings implicate GJA5 gene as a novel susceptibility gene for TOF.

The Parkinson-associated protein PINK1 interacts with Beclin1 and promotes autophagy

Mutations in the PINK1 gene cause autosomal recessive Parkinson's disease. The PINK1 gene encodes a protein kinase that is mitochondrially cleaved to generate two mature isoforms. In addition to its protective role against mitochondrial dysfunction and apoptosis, PINK1 is also known to regulate mitochondrial dynamics acting upstream of the PD-related protein Parkin. Recent data showed that mitochondrial Parkin promotes the autophagic degradation of dysfunctional mitochondria, and that stable PINK1 silencing may have an indirect role in mitophagy activation. Here we report a new interaction between PINK1 and Beclin1, a key pro-autophagic protein already implicated in the pathogenesis of Alzheimer's and Huntington's diseases. Both PINK1 N- and C-terminal are required for the interaction, suggesting that full-length PINK1, and not its cleaved isoforms, interacts with Beclin1. We also demonstrate that PINK1 significantly enhances basal and starvation-induced autophagy, which is reduced by knocking down Beclin1 expression or by inhibiting the Beclin1 partner Vps34. A mutant, PINK1(W437X), interaction of which with Beclin1 is largely impaired, lacks the ability to enhance autophagy, whereas this is not observed for PINK1(G309D), a mutant with defective kinase activity but unaltered ability to bind Beclin1. These findings identify a new function of PINK1 and further strengthen the link between autophagy and proteins implicated in the neurodegenerative process.

PINK1: one protein, multiple neuroprotective functions

Mutations in the PINK1 gene are the second most frequent cause of autosomal recessive parkinsonism after Parkin, and can represent a risk factor towards sporadic Parkinson’s disease. The PINK1 protein product has been implicated in several functions, mostly aimed at protecting neuronal cells against different types of stress. Growing evidence indicates that PINK1 interplays with other proteins mutated in familial forms of Parkinson’s disease, such as Parkin, DJ-1 and α-synuclein. These interactions contribute to the definition of a complex scenario in which the mainteinance of mitochondrial homeostasis and the clearance of misfolded proteins and damaged organelles represent key neuroprotective processes. In this review, we summarize the current knowledge on PINK1 characteristics and functions, and analyze the network of functional relationships that link PINK1 to other Parkinson’s disease-related proteins.

Late onset sporadic Parkinson's disease caused by PINK1 mutations: clinical and functional study

Homozygous or compound heterozygous mutations in the PINK1 gene represent the second most frequent cause of autosomal recessive parkinsonism after Parkin. The phenotype differs from idiopathic Parkinson's disease for earlier onset, slower disease progression, and better response to therapy. Indeed, the rare patients with onset above 50 years are usually relatives of early-onset probands. Here, we report the first occurrence of compound heterozygous PINK1 mutations in a sporadic patient with a phenotype indistinguishable from idiopathic Parkinson's disease (PD), with onset in the late seventh decade, rapid progression and good response to levodopa that waned with time. Both mutations (p.A244G and p.V317I) were found to abolish the protective effect of wild-type PINK1 against staurosporine-induced apoptosis. These findings further expand the clinical spectrum of PINK1-related parkinsonism to include late onset, typical PD, and underline the existing difficulties in discriminating between mendelian parkinsonism and idiopathic PD.

Heterochromatic gene repression of the retinoic acid pathway in acute myeloid leukemia

Alteration of lineage-specific transcriptional programs for hematopoiesis causes differentiation block and promotes leukemia development. Here, we show that AML1/ETO, the most common translocation fusion product in acute myeloid leukemia (AML), counteracts the activity of retinoic acid (RA), a transcriptional regulator of myelopoiesis. AML1/ETO participates in a protein complex with the RA receptor alpha (RARalpha) at RA regulatory regions on RARbeta2, which is a key RA target gene mediating RA activity/resistance in cells. At these sites, AML1/ETO recruits histone deacetylase, DNA methyltransferase, and DNA-methyl-CpG binding activities that promote a repressed chromatin conformation. The link among AML1/ETO, heterochromatic RARbeta2 repression, RA resistance, and myeloid differentiation block is indicated by the ability of either siRNA-AML1/ETO or the DNA methylation inhibitor 5-azacytidine to revert these epigenetic alterations and to restore RA differentiation response in AML1/ETO blasts. Finally, RARbeta2 is commonly silenced by hypermethylation in primary AML blasts but not in normal hematopoietic precursors, thus suggesting a role for the epigenetic repression of the RA signaling pathway in myeloid leukemogenesis.

A minicircuitry comprised of microRNA-223 and transcription factors NFI-A and C/EBPalpha regulates human granulopoiesis

MicroRNAs play important roles in cell differentiation by acting as translational inhibitors of specific target genes. Here we show that human granulocytic differentiation is controlled by a regulatory circuitry involving miR-223 and two transcriptional factors, NFI-A and C/EBPalpha. The two factors compete for binding to the miR-223 promoter: NFI-A maintains miR-223 at low levels, whereas its replacement by C/EBPalpha, following retinoic acid (RA)-induced differentiation, upregulates miR-223 expression. The competition by C/EBPalpha and the granulocytic differentiation are favored by a negative-feedback loop in which miR-223 represses NFI-A translation. In line with this, both RNAi against NFI-A and ectopic expression of miR-223 in acute promyelocytic leukemia (APL) cells enhance differentiation, whereas miR-223 knockdown inhibits the differentiation response to RA. Altogether, our data indicate that miR-223 plays a crucial role during granulopoiesis and point to the NFI-A repression as an important molecular pathway mediating gene reprogramming in this cell lineage.

Molecular signature of retinoic acid treatment in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia characterized by a block of differentiation at the promyelocytic stage. APL patients respond to pharmacological concentrations of all-trans retinoic acid (RA) and disease remission correlates with terminal differentiation of leukemic blasts. The PML/RAR oncogenic transcription factor is responsible for both the pathogenesis of APL and for its sensitivity to RA. In order to identify physiological targets of RA therapy, we analysed gene expression profiles of RA-treated APL blasts and found 1056 common target genes. Comparing these results to those obtained in RA-treated U937 cell lines revealed that transcriptional response to RA is largely dependent on the expression of PML/RAR. Several genes involved in the control of differentiation and stem cell renewal are early targets of RA regulation, and may be important effectors of RA response. Modulation of chromatin modifying genes was also observed, suggesting that specific structural changes in local chromatin domains may be required to promote RA-mediated differentiation. Computational analysis of upstream genomic regions in RA target genes revealed nonrandom distribution of transcription factor binding sites, indicating that specific transcriptional regulatory complexes may be involved in determining RA response.

Histone Deacetylase Inhibitor Valproic Acid Enhances the Cytokine-Induced Expansion of Human Hematopoietic Stem Cells

Ex vivo amplification of human hematopoietic stem cells (HSC) without loss of their self-renewing potential represents an important target for transplantation, gene and cellular therapies. Valproic acid is a safe and widely used neurologic agent that acts as a potent inhibitor of histone deacetylase activities. Here, we show that valproic acid addition to liquid cultures of human CD34+ cells isolated from cord blood, mobilized peripheral blood, and bone marrow strongly enhances the ex vivo expansion potential of different cytokine cocktails as shown by morphologic, cytochemical, immunophenotypical, clonogenic, and gene expression analyses. Notably, valproic acid highly preserves the CD34 positivity after 1 week (range, 40-89%) or 3 weeks (range, 21-52%) amplification cultures with two (Flt3L + thrombopoietin) or four cytokines (Flt3L + thrombopoietin + stem cell factor + interleukin 3). Moreover, valproic acid treatment increases histone H4 acetylation levels at specific regulatory sites on HOXB4, a transcription factor gene with a key role in the regulation of HSC self-renewal and AC133, a recognized marker gene for stem cell populations. Overall, our results relate the changes induced by valproic acid on chromatin accessibility with the enhancement of the cytokine effect on the maintenance and expansion of a primitive hematopoietic stem cell population. These findings underscore the potentiality of novel epigenetic approaches to modify HSC fate in vitro.

Targeting fusion protein/corepressor contact restores differentiation response in leukemia cells

The AML1/ETO and PML/RARalpha leukemia fusion proteins induce acute myeloid leukemia by acting as transcriptional repressors. They interact with corepressors, such as N-CoR and SMRT, that recruit a multiprotein complex containing histone deacetylases on crucial myeloid differentiation genes. This leads to gene repression contributing to generate a differentiation block. We expressed in leukemia cells containing PML/RARalpha and AML1/ETO N-CoR protein fragments derived from fusion protein/corepressor interaction surfaces. This blocks N-CoR/SMRT binding by these fusion proteins, and disrupts the repressor protein complex. In consequence, the expression of genes repressed by these fusion proteins increases and differentiation response to vitamin D3 and retinoic acid is restored in previously resistant cells. The alteration of PML/RARalpha-N-CoR/SMRT connections triggers proteasomal degradation of the fusion protein. The N-CoR fragments are biologically effective also when directly transduced by virtue of a protein transduction domain. Our data indicate that fusion protein activity is permanently required to maintain the leukemia phenotype and show the route to developing a novel therapeutic approach for leukemia, based on its molecular pathogenesis.

Inhibitors of histone deacetylases induce tumor-selective apoptosis through activation of the death receptor pathway

Histone deacetylases (HDACs) regulate transcription and specific cellular functions, such as tumor suppression by p53, and are frequently altered in cancer. Inhibitors of HDACs (HDACIs) possess antitumor activity and are well tolerated, supporting the idea that their use might develop as a specific strategy for cancer treatment. The molecular basis for their selective antitumor activity is, however, unknown. We investigated the effects of HDACIs on leukemias expressing the PML-RAR or AML1-ETO oncoproteins, known to initiate leukemogenesis through deregulation of HDACs. Here we report that: (i) HDACIs induce apoptosis of leukemic blasts, although oncogene expression is not sufficient to confer HDACI sensitivity to normal cells; (ii) apoptosis is p53 independent and depends, both in vitro and in vivo, upon activation of the death receptor pathway (TRAIL and Fas signaling pathways); (iii) TRAIL, DR5, FasL and Fas are upregulated by HDACIs in the leukemic cells, but not in normal hematopoietic progenitors. These results show that sensitivity to HDACIs in leukemias is a property of the fully transformed phenotype and depends on activation of a specific death pathway.

Acute myeloid leukemia fusion proteins deregulate genes involved in stem cell maintenance and DNA repair

Acute myelogenous leukemias (AMLs) are genetically heterogeneous and characterized by chromosomal rearrangements that produce fusion proteins with aberrant transcriptional regulatory activities. Expression of AML fusion proteins in transgenic mice increases the risk of myeloid leukemias, suggesting that they induce a preleukemic state. The underlying molecular and biological mechanisms are, however, unknown. To address this issue, we performed a systematic analysis of fusion protein transcriptional targets. We expressed AML1/ETO, PML/RAR, and PLZF/RAR in U937 hemopoietic precursor cells and measured global gene expression using oligonucleotide chips. We identified 1,555 genes regulated concordantly by at least two fusion proteins that were further validated in patient samples and finally classified according to available functional information. Strikingly, we found that AML fusion proteins induce genes involved in the maintenance of the stem cell phenotype and repress DNA repair genes, mainly of the base excision repair pathway. Functional studies confirmed that ectopic expression of fusion proteins constitutively activates pathways leading to increased stem cell renewal (e.g., the Jagged1/Notch pathway) and provokes accumulation of DNA damage. We propose that expansion of the stem cell compartment and induction of a mutator phenotype are relevant features underlying the leukemic potential of AML-associated fusion proteins.

Common themes in the pathogenesis of acute myeloid leukemia

The pathogenesis of acute myeloid leukemia is associated with the appearance of oncogenic fusion proteins generated as a consequence of specific chromosome translocations. Of the two components of each fusion protein, one is generally a transcription factor, whereas the other partner is more variable in function, but often involved in the control of cell survival and apoptosis. As a consequence, AML-associated fusion proteins function as aberrant transcriptional regulators that interfere with the process of myeloid differentiation, determine a stage-specific arrest of maturation and enhance cell survival in a cell-type specific manner. The abnormal regulation of transcriptional networks occurs through common mechanisms that include recruitment of aberrant co-repressor complexes, alterations in chromatin remodeling, and disruption of specific subnuclear compartments. The identification and analysis of common and specific target genes regulated by AML fusion proteins will be of fundamental importance for the full understanding of acute myeloid leukemogenesis and for the implementation of disease-specific drug design.

Oligomerization of ETO is obligatory for corepressor interaction

Nearly 40% of cases of acute myelogenous leukemia (AML) of the M2 subtype are due to a chromosomal translocation that combines a sequence-specific DNA binding protein, AML1, with a potent transcriptional repressor, ETO. ETO interacts with nuclear receptor corepressors SMRT and N-CoR, which recruit histone deacetylase to the AML1-ETO oncoprotein. SMRT-N-CoR interaction requires each of two zinc fingers contained in C-terminal Nervy homology region 4 (NHR4) of ETO. However, here we show that polypeptides containing NHR4 are insufficient for interaction with SMRT. NHR2 is also required for SMRT interaction and repression by ETO, as well as for inhibition of hematopoietic differentiation by AML1-ETO. NHR2 mediates oligomerization of ETO as well as AML1-ETO. Fusion of NHR4 polypeptide to a heterologous dimerization domain allows strong interaction with SMRT in vitro. These data support a model in which NHR2 and NHR4 have complementary functions in repression by ETO. NHR2 functions as an oligomerization domain bringing together NHR4 polypeptides that together form the surface required for high-affinity interaction with corepressors. As nuclear receptors also interact with corepressors as dimers, oligomerization may be a common mechanism regulating corepressor interactions.

Histone deacetylase-targeted treatment restores retinoic acid signaling and differentiation in acute myeloid leukemia

Histone deacetylase (HDAC)-dependent transcriptional repression of the retinoic acid (RA)-signaling pathway underlies the differentiation block of acute promyelocytic leukemia. RA treatment relieves transcriptional repression and triggers differentiation of acute promyelocytic leukemia blasts, leading to disease remission. We report that transcriptional repression of RA signaling is a common mechanism in acute myeloid leukemias (AMLs). HDAC inhibitors restored RA-dependent transcriptional activation and triggered terminal differentiation of primary blasts from 23 AML patients. Accordingly, we show that AML1/ETO, the commonest AML-associated fusion protein, is an HDAC-dependent repressor of RA signaling. These findings relate alteration of the RA pathway to myeloid leukemogenesis and underscore the potential of transcriptional/differentiation therapy in AML.

PML/RAR alpha fusion protein expression in normal human hematopoietic progenitors dictates myeloid commitment and the promyelocytic phenotype

The role of fusion proteins in acute myeloid leukemia (AML) is well recognized, but the leukemic target cell and the cellular mechanisms generating the AML phenotype are essentially unknown. To address this issue, an in vitro model to study the biologic activity of leukemogenic proteins was established. Highly purified human hematopoietic progenitor cells/stem cells (HPC/HSC) in bulk cells or single cells are transduced with retroviral vectors carrying cDNA of the fusion protein and the green fluorescent protein (GFP), purified to homogeneity and induced into multilineage or unilineage differentiation by specific hematopoietic growth factor (HGF) combinations. Expression of PML/RAR alpha fusion protein in human HPC/HSC dictates the acute promyelocytic leukemia (APL) phenotype, largely through these previously unreported effects: rapid induction of HPC/HSC differentiation to the promyelocytic stage, followed by maturation arrest, which is abolished by retinoic acid; reprogramming of HPC commitment to preferential granulopoietic differentiation, irrespective of the HGF stimulus (transduction of single sibling HPC formally demonstrated this effect); HPC protection from apoptosis induced by HGF deprivation. A PML/RAR alpha mutated in the co-repressor N-CoR/histone deacetylase binding region lost these biologic effects, showing that PML/RAR alpha alters the early hematopoietic program through N-CoR-dependent target gene repression mechanisms. These observations identify the cellular mechanism underlying development of the APL phenotype, showing that the fusion protein directly dictates the specific lineage and differentiation stage of leukemic cells. (Blood. 2000;96:1531-1537)

Oligomerization of RAR and AML1 transcription factors as a novel mechanism of oncogenic activation

RAR and AML1 transcription factors are found in leukemias as fusion proteins with PML and ETO, respectively. Association of PML-RAR and AML1-ETO with the nuclear corepressor (N-CoR)/histone deacetylase (HDAC) complex is required to block hematopoietic differentiation. We show that PML-RAR and AML1-ETO exist in vivo within high molecular weight (HMW) nuclear complexes, reflecting their oligomeric state. Oligomerization requires PML or ETO coiled-coil regions and is responsible for abnormal recruitment of N-CoR, transcriptional repression, and impaired differentiation of primary hematopoietic precursors. Fusion of RAR to a heterologous oligomerization domain recapitulated the properties of PML-RAR, indicating that oligomerization per se is sufficient to achieve transforming potential. These results show that oligomerization of a transcription factor, imposing an altered interaction with transcriptional coregulators, represents a novel mechanism of oncogenic activation.

Formation of PML/RAR alpha high molecular weight nuclear complexes through the PML coiled-coil region is essential for the PML/RAR alpha-mediated retinoic acid response

Retinoic Acid (RA) treatment induces disease remission of Acute Promyelocytic Leukaemia (APL) patients by triggering terminal differentiation of neoplastic cells. RA-sensitivity in APL is mediated by its oncogenic protein, which results from the recombination of the PML and the RA receptor alpha (RAR alpha) genes (PML/RAR alpha fusion protein). Ectopic expression of PML/RAR alpha into haemopoietic cell lines results in increased response to RA-induced differentiation. By structure-function analysis of PML/RAR alpha-mediated RA-differentiation, we demonstrated that fusion of PML and RAR alpha sequences and integrity of the PML dimerization domain and of the RAR alpha DNA binding region are required for the effect of PML/RAR alpha on RA-differentiation. Indeed, direct fusion of the PML dimerization domain to the N- or C-terminal extremities of RAR alpha retained full biological activity. All the biologically active PML/RAR alpha mutants formed high molecular weight complexes in vivo. Functional analysis of mutations within the PML dimerization domain revealed that the capacity to form PML/RAR alpha homodimers, but not PML/RAR alpha-PML heterodimers, correlated with the RA-response. These results suggest that targeting of RAR alpha sequences by the PML dimerization domain and formation of nuclear PML/RAR alpha homodimeric complexes are crucial for the ability of PML/RAR alpha to mediate RA-response.

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.

Aberrant recruitment of the nuclear receptor corepressor-histone deacetylase complex by the acute myeloid leukemia fusion partner ETO

Nuclear receptor corepressor (CoR)-histone deacetylase (HDAC) complex recruitment is indispensable for the biological activities of the retinoic acid receptor fusion proteins of acute promyelocytic leukemias. We report here that ETO (eight-twenty-one or MTG8), which is fused to the acute myelogenous leukemia 1 (AML1) transcription factor in t(8;21) AML, interacts via its zinc finger region with a conserved domain of the corepressors N-CoR and SMRT and recruits HDAC in vivo. The fusion protein AML1-ETO retains the ability of ETO to form stable complexes with N-CoR/SMRT and HDAC. Deletion of the ETO C terminus abolishes CoR binding and HDAC recruitment and severely impairs the ability of AML1-ETO to inhibit differentiation of hematopoietic precursors. These data indicate that formation of a stable complex with CoR-HDAC is crucial to the activation of the leukemogenic potential of AML1 by ETO and suggest that aberrant recruitment of corepressor complexes is a general mechanism of leukemogenesis.

Caspases mediate retinoic acid-induced degradation of the acute promyelocytic leukemia PML/RARalpha fusion protein

All-trans-retinoic acid (RA) treatment induces morphological remission in acute promyelocytic leukemia (APL) patients carrying the t(15;17) and expressing the PML/RARalpha product by inducing terminal differentiation of the leukemic clone. RA treatment induces downregulation of PML/RARalpha and reorganization of the PML-nuclear bodies. These events have been proposed to be essential for the induction of APL cell differentiation by RA. Here, we show that in the APL-derived NB4 cell line as well as in myeloid precursor U937 cells expressing the PML/RARalpha (U937/PR9) and in blasts from APL patients, the PML/RARalpha fusion protein is cleaved by a caspase 3-like activity induced by RA treatment. In fact, a caspase 3-like activity is detectable in PML/RARalpha expressing cells after RA treatment, and selective caspase inhibitor peptides are able to prevent the RA-induced degradation of the fusion protein in vivo and in vitro. Using recombinant caspases and PML/RARalpha deletion mutants we mapped a caspase 3 cleavage site (Asp 522) within the alpha-helix region of the PML component of the fusion protein. The extent of PML/RARalpha cleavage directly correlates with the ability of RA to restore the normal PML nuclear bodies (NBs) pattern. However, RA-induced differentiation is not prevented by the persistence of the fusion product and occurs in the absence of normally structured PML NBs. These results indicate that PML/RARalpha is directly involved in conferring RA sensitivity of APL cells and that the RA-induced reassembly of PML NBs is the consequence of the disappearance of PML/RARalpha.

Terminal megakaryocytic differentiation of TF-1 cells is induced by phorbol esters and thrombopoietin and is blocked by expression of PML/RARalpha fusion protein

We have analyzed the differentiation program of growth factor-dependent TF-1 erythroleukemia cells as well as clones with inducible expression of the APL-specific PML/RARalpha protein. We have shown that TF-1 cells may be induced to megakaryocytic differentiation by phorbol ester (phorbol dibutyrate, PDB) addition, particularly when combined with thrombopoietin (Tpo). RT-PCR studies showed that Tpo induces Tpo receptor (TpoR or c-mpl), whose expression was further potentiated by PDB addition. When the cells are induced with both PDB and Tpo erythropoietin receptor (EpoR) expression was inhibited. In the absence of Zn2+-induced PML/RARalpha expression, PDB and Tpo induced megakaryocytic differentiation of TF-1 MTPR clones as observed in 'wild-type' TF-1 cells. Conversely, when PML/RARalpha expression was induced by Zn2+, PDB and Tpo treatment of these clones caused only a reduced level of megakaryocytic differentiation. These observations indicate that: (1) TF-1 cells as well as other erythroleukemic cells, possess the capacity to differentiate to megakaryocytic cells when grown in the presence of protein kinase (PKC) activators and more efficiently when combined with Tpo; (2) the PML/RARalpha gene has a wide capacity to interfere with the program of hematopoietic differentiation, including megakaryocytic differentiation. Finally, we also observed that PML/RARalpha expression in TF-1 cells induces an up-modulation of interleukin-3 receptor, c-kit and c-mpl, a phenomenon which may offer these cells a growth advantage.

Fusion proteins of the retinoic acid receptor-alpha recruit histone deacetylase in promyelocytic leukaemia

The transforming proteins of acute promyelocytic leukaemias (APL) are fusions of the promyelocytic leukaemia (PML) and the promyelocytic leukaemia zinc-finger (PLZF) proteins with retinoic acid receptor-alpha (RARalpha). These proteins retain the RARalpha DNA- and retinoic acid (RA)-binding domains, and their ability to block haematopoietic differentiation depends on the RARalpha DNA-binding domain. Thus RA-target genes are downstream effectors. However, treatment with RA induces differentiation of leukaemic blast cells and disease remission in PML-RARalpha APLs, whereas PLZF-RARa APLs are resistant to RA. Transcriptional regulation by RARs involves modifications of chromatin by histone deacetylases, which are recruited to RA-target genes by nuclear co-repressors. Here we show that both PML-RARalpha and PLZF-RARalpha fusion proteins recruit the nuclear co-repressor (N-CoR)-histone deacetylase complex through the RARalpha CoR box. PLZF-RARalpha contains a second, RA-resistant binding site in the PLZF amino-terminal region. High doses of RA release histone deacetylase activity from PML-RARalpha, but not from PLZF-RARalpha. Mutation of the N-CoR binding site abolishes the ability of PML-RARalpha to block differentiation, whereas inhibition of histone deacetylase activity switches the transcriptional and biological effects of PLZF-RARalpha from being an inhibitor to an activator of the RA signalling pathway. Therefore, recruitment of histone deacetylase is crucial to the transforming potential of APL fusion proteins, and the different effects of RA on the stability of the PML-RARalpha and PLZF-RARalpha co-repressor complexes determines the differential response of APLs to RA.

Effects on differentiation by the promyelocytic leukemia PML/RARalpha protein depend on the fusion of the PML protein dimerization and RARalpha DNA binding domains

The block of terminal differentiation is a prominent feature of acute promyelocytic leukemia (APL) and its release by retinoic acid correlates with disease remission. Expression of the APL-specific PML/RARalpha fusion protein in hematopoietic precursor cell lines blocks terminal differentiation, suggesting that PML/ RARalpha may have the same activity in APL blasts. We expressed different PML/RARalpha mutants in U937 and TF-1 cells and demonstrated that the integrity of the PML protein dimerization and RARalpha DNA binding domains is crucial for the differentiation block induced by PML/RARalpha, and that these domains exert their functions only within the context of the fusion protein. Analysis of the in vivo dimerization and cell localization properties of the PML/RARalpha mutants revealed that PML/RARalpha--PML and PML/RARalpha--RXR heterodimers are not necessary for PML/RARalpha activity on differentiation. We propose that a crucial mechanism underlying PML/RARalpha oncogenic activity is the deregulation of a transcription factor, RARalpha, through its fusion with the dimerization interface of another nuclear protein, PML.