Erythroid K562 Cells Research Articles

Release of iron from ferritin requires lysosomal activity

How ferritin-Fe becomes available for cell functions is unknown. Our previous studies with rat hepatoma cells indicated ferritin had to be degraded to release its Fe. In these studies, we investigated whether this occurs in other cell types and whether lysosomes are required. Release of ferritin-Fe was induced with desferoxamine (DFO) in (59)Fe-preloaded hepatoma, Caco2, and erythroid K562 cells and measured by rocket immunoelectrophoresis and autoradiography. The half-lives for ferritin-(59)Fe and protein were parallel (23, 16, and 11 h for the hepatic, Caco2, and K562 cells, respectively). Co-treatment with 180 microM Fe, leupeptin, chymostatin, or chloroquine markedly decreased rates of ferritin-Fe release and ferritin degradation. Lactacystin had no effect except for a small one in erythroid cells. Fractionation of hepatoma cell lysates on iodixanol gradients showed rapid depletion of cytosolic ferritin by DFO treatment but no accumulation in lysosomes. We conclude that regardless of cell type, release of Fe from ferritin occurs mainly through lysosomal proteolysis.

Peptidomics: A modern approach to biodiversity of peptides

Abstract Murine myelomonocytes WEHI-3 and human erythroid leukemia K562 cells were shown to release peptides into the surrounding medium. Four N-terminal sequences of the peptides derived from unknown protein precursors were identified in the conditioned medium of WEHI-3 culture. Twelve N-terminal sequences of the peptides released by K562 cells were established. K562 cells were shown to release long hemoglobin α- and ε-chains fragments, as well as peptides derived from carbonic anhydrase XI, melanophilin, cadherin EGF LAG G-type receptor 2, aldolase A, melastatin 1, peptide homologous to ryanodine receptor fragment, and a few peptides derived from the proteins with unknown function. Comparative analysis of the peptides released by the intact erythroid leukemia K562 cells and the same cells after differentiation (induced by guanosine monophosphate, GMP, and accompanied by start of hemoglobin biosynthesis), has demonstrated that the level of six components, including all established hemoglobin fragments, carbonic anhydrase fragment, and the fragment of the unknown protein significantly increased in the course of differentiation. The spectrum of the peptides released by K562 cells strongly differed from that of the mature erythrocytes: (1) in contrast to the mature erythrocytes, K562 cells release long α-globin fragments and the fragment of ε-globin; (2) erythrocytes almost exclusively release the products of hemoglobin proteolysis; in the case of K562 cells the fragments of enzymes, receptors, and other functional proteins were found. So, the concentration and the content of the individual peptides released by the cells depend on the differentiation state. The comparative analysis of the dynamics of peptide secretion by human erythrocytes performed in the presence and the absence of the 3 % glucose has shown that the rate of the peptide release strongly depends on cell metabolism. In summary, peptidomic studies of cell cultures provide valuable information on the mechanisms of peptide pool formation in tissues and the whole organism.

Sumoylation of p45/NF-E2: Nuclear Positioning and Transcriptional Activation of the Mammalian β-Like Globin Gene Locus

NF-E2 is a transcription activator for the regulation of a number of erythroid- and megakaryocytic lineage-specific genes. Here we present evidence that the large subunit of mammalian NF-E2, p45, is sumoylated in vivo in human erythroid K562 cells and in mouse fetal liver. By in vitro sumoylation reaction and DNA transfection experiments, we show that the sumoylation occurs at lysine 368 (K368) of human p45/NF-E2. Furthermore, p45 sumoylation enhances the transactivation capability of NF-E2, and this is accompanied by an increase of the NF-E2 DNA binding affinity. More interestingly, we have found that in K562 cells, the beta-globin gene loci in the euchromatin regions are predominantly colocalized with the nuclear bodies promyelocytic leukemia protein (PML) oncogenic domains that are enriched with the PML, SUMO-1, RNA polymerase II, and sumoylatable p45/NF-E2. Chromatin immunoprecipitation assays further showed that the intact sumoylation site of p45/NF-E2 is required for its binding to the DNase I-hypersensitive sites of the beta-globin locus control region. Finally, we demonstrated by stable transfection assay that only the wild-type p45, but not its mutant form p45 (K368R), could efficiently rescue beta-globin gene expression in the p45-null, erythroid cell line CB3. These data together point to a model of mammalian beta-like globin gene activation by sumoylated p45/NF-E2 in erythroid cells.

Diamond Blackfan Anemia: An RPS19 Transcript Variant Specifically Interacts with Nuclear Proteins.

Diamond Blackfan Anemia (DBA) is a rare congenic anemia characterized by reduced erythrocyte precursors. Mutations in the ribosomal protein S19 (RPS19) gene are found in about 20% of patients, although it is unclear how this relates to the pathophysiology of the disease. Two transcripts have been reported for human RPS19 in the NCBI database (BC000023 and BC018616), which only differ in the length of the 5′UTR. Both transcripts predict translation to the same protein product and we therefore speculate that the extended 5′UTR may be of importance. While the short (569bp) transcript is well studied, the long (873bp) transcript has not been investigated. We determined the size and tissue distribution as a basic characterization of the long RPS19 transcript. Furthermore, we examined proteins interacting with the different RPS19 transcript variants. Two methods were used determine the size of long RPS19 transcript. Ribonuclease protection assays and 5′-RACE indicate a transcript length consistent with the reported sequence with a 5′UTR of 372bp. Interestingly the 5′UTR predicted in the long transcript contains sequence for a previously reported promoter, suggesting a second novel promoter. Tissue distribution was determined using reverse transcriptase PCR on isolated RNA from a panel of tissues including heart, bone marrow, fetal liver, liver, brain, fetal brain, muscle, kidney, uterus, adrenal gland, lungs, placenta, prostate testes, ovary, thymus, colon, lymphocytes and from K562 and HeLa cell lines. Analysis indicated a ubiquitously expression of the long RPS19 transcript. In order to investigate interactions between the RPS19 transcripts and regulatory proteins, a UV cross-linking assay was performed using in vitro transcribed RNA representing both short and long transcripts. A protein of 55 kDa from nuclear fractions prepared from the erythroid cell lines K562, UT-7 and Cos-1 cells was found to bind to the long RPS19 transcript but not to the short RPS19 transcript. Our results provide evidence for an additional long, RPS19 transcript containing an extended 5′UTR that is ubiquitously transcribed and indicates the existence of a second promoter for RPS19. We further show that a nuclear protein, present in two different erythroid cell lines, interacts with the long RPS19 transcript indicating a function for the extended 5′UTR. This data suggests additional regulatory pathways involving RPS19 mRNA which may be of relevance to the understanding of the molecular mechanisms behind DBA.

Knockdown of ABCme Impairs Heme Biosynthesis as Revealed by Integrating of RNAi and the LiveCell® Array.

Heme synthesis begins and ends in the mitochondrion, whilst intermediate steps are performed in the cytosol. Consequently, heme precursors must be transported across the mitochondrial membranes, however, the transporters of these intermediates have not been identified. Conditions such as sideroblastic anemia, porphyria and MDS are all characterized by abnormal heme synthesis and accumulation of heme intermediates and iron. Thus, impaired function of mitochondrial transporters is thought to play a key role in these conditions.We have previously reported that over-expression of murine ABCme increases hemoglobin production; however the identity of the heme intermediate transported by ABCme is as yet unclear (Shirihai et al. 2000).We have used lentivirus delivered shRNA, harboring a GFP marker, to knockdown ABCme mRNA in human K562 erythroid cells. Interpreting RNAi experiments in heterogeneous cell populations, such as differentiating cells, can be problematic. Specific challenges which must be addressed are: 1) transgene and RNAi expression is hard to achieve in erythroid cells; 2) cell to cell expression efficiency is variable; 3) differentiation is a stochastic process; 4) erythroid cells are non-adherent rendering imaging of living cells difficult. Taken together, these factors lead to pronounced cellular heterogeneity, which may mask biological effects. Furthermore, the correlation of RNAi expression and heme synthesis requires the use of benzidine staining, which results in protein oxidation and elimination of all fluorescent signals. To address these challenges we developed a new assay that takes advantage of a novel array which corrals living cells into individual wells. The LiveCell® Array is a densely packed array of micron-sized wells, each accommodating an individual cell, embedded within a microscope slide. Sequential measurements of multiple cellular activities can be taken and the images analyzed using Metamorph image analysis software.Using this system we identified a population of cells which expressed high levels of RNAi by screening for GFP. The stage of differentiation of each cell was determined using antibodies against extra-cellular makers (i.e. Glycophorin A) conjugated to fluorescent moieties thus identifying maturing subpopulations of cells. To measure heme content, benzidine was subsequently applied and light absorbance was quantized at the resolution of the individual cell. Heme content was then cross correlated with the differentiation and expression fluorescent markers. By integrating RNAi with the LiveCell® Array we have found that knockdown of ABCme results in reduced heme production in differentiating erythroid cells. Further analysis of heme intermediates using this model is being pursued in order to identify the substrate of ABCme.

The Long Terminal Repeat (LTR) of ERV-9 Human Endogenous Retrovirus Binds to NF-Y in the Assembly of an Active LTR Enhancer Complex NF-Y/MZF1/GATA-2

The solitary ERV-9 long terminal repeat (LTR) located upstream of the HS5 site in the human beta-globin locus control region exhibits prominent enhancer activity in embryonic and erythroid cells. The LTR enhancer contains 14 tandemly repeated subunits with recurrent CCAAT, GTGGGGA, and GATA motifs. Here we showed that in erythroid K562 cells these DNA motifs bound the following three transcription factors: ubiquitous NF-Y and hematopoietic MZF1 and GATA-2. These factors and their target DNA motifs exhibited a hierarchy of DNA/protein and protein/protein binding affinities: NF-Y/CCAAT > NF-Y/GATA-2 > NF-Y/MZF1 > MZF1/GTGGGGA; GATA-2/GATA. Through protein/protein interactions, NF-Y bound at the CCAAT motif recruited MZF1 and GATA-2, but not Sp1 and GATA-1, and stabilized their binding to the neighboring GTGGGGA and GATA sites to assemble a novel LTR enhancer complex, NF-Y/MZF1/GATA-2. In the LTR-HS5-epsilonp-GFP plasmid integrated into K562 cells, mutation of the CCAAT motif in the LTR enhancer to abolish NF-Y binding inactivated the enhancer, closed down the chromatin structure of the epsilon-globin promoter, and silenced transcription of the green fluorescent protein gene. The results indicated that NF-Y bound at the CCAAT motifs assembled a robust LTR enhancer complex, which could act over the intervening DNA to remodel the chromatin structure and to stimulate the transcription of the downstream gene locus.

Protein Kinase A-dependent Phosphorylation of Lutheran/Basal Cell Adhesion Molecule Glycoprotein Regulates Cell Adhesion to Laminin α5

Lutheran (Lu) blood group and basal cell adhesion molecule (B-CAM) antigens reside on two glycoprotein (gp) isoforms Lu and Lu(v13) that belong to the Ig superfamily and differ only by the size of their cytoplasmic tail. Lu/B-CAM gps have been recognized as laminin alpha5 receptors on red blood cells and epithelial cells in multiple tissues. It has been shown that sickle red cells exhibit enhanced adhesion to laminin alpha5 when intracellular cAMP is up-regulated by physiological stimuli such as epinephrine and that this signaling pathway is protein kinase A- and Lu/B-CAM-dependent. In this study, we analyzed the relationship between the phosphorylation status of Lu/B-CAM gps and their adhesion function to laminin alpha5. We showed that Lu isoform was phosphorylated in sickle red cells as well as in erythroleukemic K562 and epithelial Madin-Darby canine kidney cells and that this phosphorylation is enhanced by different stimuli of the PKA pathway. Lu gp is phosphorylated by glycogen synthase kinase 3 beta, casein kinase II, and PKA at serines 596, 598, and 621, respectively. Alanine substitutions of serines 596 and 598 abolished phosphorylation by glycogen synthase kinase 3 beta and casein kinase II, respectively, but had no effect on adhesion of K562 cells to laminin under flow conditions. Conversely, mutation of serine 621 prevented phosphorylation by PKA and dramatically reduced cell adhesion. Furthermore, stimulation of K562 cells by epinephrine increased Lu gp phosphorylation by PKA and enhanced adhesion to laminin. It is postulated that modulation of the phosphorylation state of Lu gp might be a critical factor for the sickle red cells adhesiveness to laminin alpha5 in sickle cell disease.

Erythroid Cell-Specific α-Globin Gene Regulation by the CP2 Transcription Factor Family

We previously demonstrated that ubiquitously expressed CP2c exerts potent erythroid-specific transactivation of alpha-globin through an unknown mechanism. This mechanism is reported here to involve specific CP2 splice variants and protein inhibitor of activated STAT1 (PIAS1). We identify a novel murine splice isoform of CP2, CP2b, which is identical to CP2a except that it has an additional 36 amino acids encoded by an extra exon. CP2b has an erythroid cell-specific transcriptional activation domain, which requires the extra exon and can form heteromeric complexes with other CP2 isoforms, but lacks the DNA binding activity found in CP2a and CP2c. Transcriptional activation of alpha-globin occurred following dimerization between CP2b and CP2c in erythroid K562 and MEL cells, but this dimerization did not activate the alpha-globin promoter in nonerythroid 293T cells, indicating that an additional erythroid factor is missing in 293T cells. PIAS1 was confirmed as a CP2 binding protein by the yeast two-hybrid screen, and expression of CP2b, CP2c, and PIAS1 in 293T cell induced alpha-globin promoter activation. These results show that ubiquitously expressed CP2b exerts potent erythroid cell-specific alpha-globin gene expression by complexing with CP2c and PIAS1.

The HS2 Enhancer of the β-globin Locus Control Region Initiates Synthesis of Non-coding, Polyadenylated RNAs Independent of a cis-linked Globin Promoter

The HS2 enhancer in the beta-globin locus control region (LCR) regulates transcription of the globin genes 10-50 kb away. Earlier studies show that a transcription mechanism initiated by the HS2 enhancer through the intervening DNA in the direction of the cis-linked promoter and gene mediates long-range enhancer function. Here, we further analyzed the enhancer-initiated RNAs and their mode of transcription from the HS2 enhancer in the endogenous genome of erythroid K562 cells, in plasmids integrated into K562 cells and in purified DNA used as template in in vitro transcription reactions. We found that the HS2 enhancer was able to initiate transcription autonomously in the absence of a cis-linked globin promoter. The enhancer-initiated, intergenic RNAs were different from the mRNA synthesized at the promoter in several aspects. The enhancer RNAs were synthesized not from a defined site but from multiple sites both within and as far as 1 kb downstream of the enhancer. The enhancer RNAs did not appear to contain a normal cap structure at the 5' ends. They were polyadenylated at multiple sites within 3 kb downstream of their initiation sites and were therefore shorter than 3 kb in lengths. The enhancer RNAs remained in discrete spots within the nucleus and were not processed into mRNA or translated into proteins. These particular features of enhancer-initiated transcription indicate that the transcriptional complex assembled by the enhancer was different from the basal transcription complex assembled at the promoter. The results suggest that in synthesizing non-coding, intergenic RNAs, the enhancer-assembled transcription complex could track through the intervening DNA to reach the basal promoter complex and activate efficient mRNA synthesis from the promoter.

Deferoxamine photosensitizes cancer cells in vitro

Effect of the iron chelator deferoxamine (DF) on the production of endogenous porphyrins was studied in adenocarcinoma WiDr cells and erythroid K562 cells in vitro. Porphyrin fluorescence was observed in the cells in vitro incubated with DF. The fluorescence spectra recorded in the cells were similar to that of protoporphyrin IX (PpIX). The amount of PpIX generated by DF was around 5% of the ALA effect. Around 90% of the WiDr cells incubated in vitro with DF (0.5 mM, 24 h) and then exposed to light (400–460 nm, 20 min) were photodynamically inactivated. In conclusion, the present study describes a novel approach of using iron chelating agents without 5-aminolevulinic acid (ALA) to photosensitize cancer cells.

10. The Effect of Globin Locus Control Region (LCR) Elements or the MSCV-LTR on Promoter Trapping by Integrated Lentiviral Vector Genomes

Integrating vectors are desirable for gene therapy of inherited blood disorders to achieve permanent genetic modification and stable expression of transgenes in hematopoietic cells. However, three patients with severe combined immunodeficiency enrolled in a gene therapy trial of a retroviral vector developed leukemia and activation of the LMO2 proto-oncogene by vector integration has been demonstrated in two. Retroviral vectors prefer the transcriptional start sites for integration while lentiviral vector integration sites are distributed throughout the active genes. Thus, the safety profile of integrating vectors is a critical issue. As a strategy to evaluate interaction of regulatory elements in the vectors with genes into which they have integrated, a series of lentiviral promoter trap vectors were designed to compare the effect of regulatory elements within the vector on the potential for promoter trapping. The trapping cassette consists of a splice acceptor followed by the GFP coding sequences and a polyadenylation site. The cassette was inserted immediately upstream of the 3|[prime]| LTR in a reverse orientation in vectors containing either a globin LCR element or the oncoretroviral MSCV-LTR also in a reverse orientation to the lentiviral LTR. As a control, a vector containing the cassette without regulatory elements was also constructed. HeLa or erythroid K562 cells were transduced with these vectors and analyzed for GFP expression by flow cytometry. Transduction frequency was estimated by Southern hybridization and the efficiency of promoter trapping was then calculated as a percentage of GFP expressing cells in transduced cells. Our results showed 5% promoter trapping in HeLa cells transduced with the control vector. This efficiency is similar to that recently reported by others (De Palma et al, Blood 2004 Nov 12). Vectors containing either the globin LCR or the MSCV-LTR gave 15 to 20% trapping efficiency. Similarly, in K562 cells, the percentage of promoter trapping of vectors containing either the globin or MSCV regulatory elements was significantly higher (up to 6-fold) of that of the control vector. To further investigate the effect of the globin LCR or the MSCV-LTR on promoter trapping, we isolated single cell clones from GFP-positive HeLa and K562 cells that had been transduced with a trapping vector. Many clones had lost GFP expression when reanalyzed, but the proportion (43%) in which more than 60% of the cells were positive was higher with vectors containing the globin LCR compared to those containing the MSCV-LTR (27%). In addition, the MFI of globin LCR clones was, on average, slightly higher than that of MSCV-LTR clones (20 and 17, respectively). Eighteen clones having a single integrated proviral genome were identified and 15 integration sites have been mapped to date. Ten integration sites were in genes and 8 of these were in the first intron with the promoter trap in tandem with the natural transcript. Our results to date indicate that the presence and nature of regulatory elements within a lentiviral vector influences the probability of vector genome transcription either by trapping of a natural promoter or by activation of cryptic promoters.

HS2 Enhancer Function Is Blocked by a Transcriptional Terminator Inserted between the Enhancer and the Promoter

The HS2 enhancer in the beta-globin locus control region regulates transcription of the globin genes 10-50 kb away. How the HS2 enhancer acts over this distance is not clearly understood. Earlier studies show that in erythroid cells the HS2 enhancer initiates synthesis of intergenic RNAs from sites within and downstream of the enhancer, and the enhancer-initiated RNAs are transcribed through the intervening DNA into the cis-linked promoter and gene. To investigate the functional significance of the enhancer-initiated transcription, here we inserted the lac operator sequence in the intervening DNA between the HS2 enhancer and the epsilon-globin promoter in reporter plasmids and integrated the plasmids into erythroid K562 cells expressing the lac repressor protein. We found that the interposed lac operator/repressor complex blocked the elongation of enhancer-initiated transcription through the intervening DNA and drastically reduced HS2 enhancer function as measured by the level of mRNA synthesized from the epsilon-globin promoter. The results indicate that the tracking and transcription mechanism of the HS2 enhancer-assembled transcriptional machinery from the enhancer through the intervening DNA into the cis-linked promoter can mediate enhancer-promoter interaction over a long distance.

Developmental stage differences in chromatin subdomains of the beta-globin locus.

The mammalian beta-globin loci each contain a family of developmentally expressed genes, and a far upstream regulatory element, the locus control region (LCR). In adult murine erythroid cells, the LCR and the transcribed beta-globin genes exist within domains of histone acetylation and RNA polymerase II (pol II) is associated with them. In contrast, the silent embryonic genes lie between these domains within hypoacetylated chromatin, and pol II is not found there. We used chromatin immunoprecipitation and real-time PCR to analyze histone modification and pol II recruitment to the globin locus in human erythroid K562 cells that express the embryonic epsilon-globin gene but not the adult beta-globin gene. H3 and H4 acetylation and H3 K4 methylation were continuous over a 17-kb region including the LCR and the active epsilon-globin gene. The level of modification varied directly with the transcription of the epsilon-globin gene. In contrast, this region in nonerythroid HeLa cells lacked these modifications and displayed instead widespread H3 K9 methylation. pol II was also detected continuously from the LCR to the epsilon-globin gene. These studies reveal several aspects of chromatin structure and pol II distribution that distinguish the globin locus at embryonic and adult stages and suggest that both enhancer looping and tracking mechanisms may contribute to LCR-promoter communication at different developmental stages.

Erythropoietin-induced metallothionein gene expression: role in proliferation of K562 cells.

Recent evidence has demonstrated an appreciable expression of metallothionein (MT) in erythrocytes. However, the induction of the MT protein by hematopoietic growth factors and its subsequent functional significance on clonal expansion or differentiation of erythroid progenitor cells remain elusive. We therefore examined the effects of growth factors erythropoietin (EPO), granulocyte-monocyte colony-stimulating factor (GM-CSF), and interleukin-3 (IL-3) on MT gene expression in erythroid progenitor K562 cell line. EPO, but not IL-3 or GM-CSF, induced a 3-fold increase in MT transcripts in K562 cells. MT induction was associated with EPO-induced cellular proliferation, suggesting a specific role for MT induction by EPO in erythroid progenitor cells. However, EPO- or sodium butyrate-induced differentiation as monitored by hemoglobin formation was inhibited in K562 cells stably transfected with an expression vector containing human MT-IIA gene. This inhibition of differentiation was partially reversed in these cells by an MT antisense phosphorothioate oligonucleotide. Furthermore, the MT-induced inhibition of differentiation was associated with downregulation of EPO receptor transcripts in K562 cells. These data suggest that, among growth factors required for erythropoiesis, EPO is a potent inducer of MT, and that MT may plays a significant role in EPO-induced proliferation, but not in the erythroid-specific differentiation of the progenitor cells.

Dinucleosome DNA of Human K562 Cells: Experimental and Computational Characterizations

Dinucleosome formation is the first step in the organization of the higher order chromatin structure. With the ultimate aim of elucidating the dinucleosome structure, we constructed a library of human dinucleosome DNA. The library consists of PCR-amplifiable DNA fragments obtained by treatment of nuclei of erythroid K562 cells with micrococcal nuclease followed by extraction of DNA and adaptor ligation to the blunt-ended DNA fragments. The library was then cloned using a plasmid vector and the sequences of the clones were determined. The dominating clones containing the Alu elements were removed. A total of 1002 clones, which comprised a dinucleosome database, contained 84 and 918 clones from the clones before and after removing Alu elements, respectively. Approximately 70% of the clones were between 300 and 400 bp in size and they were distributed to various locations of all chromosomes except the Y chromosome. The clones containing A 2N 8A 2N 8A 2 or T 2N 8T 2N 8T 2 sequences were classified into three types, Type I (N shape), Type II (V shape) and Type III (M shape) according to DNA curvature plots. The locations of experimentally determined curved DNA segments matched well with the calculated ones though the clones of Types I and III showed additional curved DNA segments as revealed by the curvature plots. The distributions of complementary dinucleotides in the nucleosome DNA, at the ends of the dinucleosome DNA clones, allowed us to predict the positions of the nucleosome dyad axis, and estimate the size of the nucleosome core DNA, 125 nt. The distributions of AA and TT dinucleotides, as well as other RR and YY dinucleotides, showed a periodicity with an average period of 10.4 bases, close to the values observed before. Mapping of nucleosome positions in the dinucleosome database based on the observed periodicity revealed that the nucleosomes were separated by a linker of 7.5+∼10× n nt. This indicates that the nucleosome–nucleosome orientations are, typically, halfway between parallel and antiparallel. Also an important finding is that the distributions of AA/TT and other RR/YY dinucleotides, apparently, reflect both DNA curvature and DNA bendability, cooperatively contributing to the nucleosome formation.

The ERV-9 LTR enhancer is not blocked by the HS5 insulator and synthesizes through the HS5 site non-coding, long RNAs that regulate LTR enhancer function.

A solitary long terminal repeat (LTR) of ERV-9 human endogenous retrovirus is located upstream of the HS5 site in the human beta-globin locus control region and possesses unique enhancer activity in erythroid K562 cells. In cells transfected with plasmid LTR-HS5-epsilonp-GFP, the LTR enhancer activates the GFP reporter gene and is not blocked by the interposed HS5 site, which has been reported to have insulator function. The LTR enhancer initiates synthesis of long RNAs from the LTR promoter through the intervening HS5 site into the epsilon-globin promoter and the GFP gene. Synthesis of the sense, long LTR RNAs is correlated with high level synthesis of GFP mRNA from the epsilon-globin promoter. Mutations of the LTR promoter and/or the epsilon-globin promoter show that (i) the LTR enhancer can autonomously initiate synthesis of LTR RNAs independent of the promoters and (ii) the LTR RNAs are not processed into GFP mRNA or translated into GFP. However, reversing the orientation of the LTR in plasmid (LTR)rev-HS5-epsilonp-GFP, thus reversing the direction of synthesis of LTR RNAs in the antisense direction away from the epsilon-globin promoter and GFP gene drastically reduces the level of GFP mRNA and thus LTR enhancer function. The results suggest that the LTR-assembled transcription machinery in synthesizing non-coding, LTR RNAs can reach the downstream epsilon-globin promoter to activate transcription of the GFP gene.

A promoter mutation in the erythroid-specific 5-aminolevulinate synthase (ALAS2) gene causes X-linked sideroblastic anemia

AbstractX-linked sideroblastic anemia (XLSA) is caused by mutations in the erythroid-specific 5-aminolevulinate synthase gene (ALAS2). XLSA was diagnosed in a 32-year-old woman with a mild phenotype and moderately late onset. Pyridoxine therapy had no effect in the proband, but in her affected son engendered a modest increase in hemoglobin concentration and a 4-fold reduction in ferritin iron. Molecular analysis identified a C to G transversion at nucleotide -206 from the transcription start site, as defined by primer extension, in the proximal promoter region of ALAS2. No other mutations were found in the promoter region, the flanking intronic sequences, the exons, or the 3′ genomic region. The same mutation was found in her affected son but not in any other of her unaffected relatives. The mutation resulted in a 94% loss of activity relative to the wild-type sequence for a luciferase reporter construct containing the proximal 293 nucleotides (nt's) of the ALAS2 promoter when transfected into human erythroid K562 cells. Confirming the mutation's deleterious effect, the ALAS2 mRNA level in the proband's erythroid precursors was reduced 87%. The mutation occurred in or near 3 different putative transcription factor binding sites of unknown erythroid importance. The dramatic decreases in reporter activity and mRNA level suggest that the region of the mutation may bind a novel and important erythroid regulatory element.

Erythroid Gene Suppression by NF-κB

NF-kappa B/Rel transcription factors play essential roles to mediate the immune response and apoptosis, and they have also been implicated in cellular differentiation such as erythropoiesis. To elucidate the possible role(s) of NF-kappa B in erythroid gene regulation and erythropoiesis, we have carried out transient transfection studies of the human embryonic/fetal erythroid cell line K562 and mouse adult erythroid MEL cells. It is shown that tumor necrosis factor-alpha represses the transcription activity directed by either alpha or zeta globin promoter in a dose-dependent manner. Furthermore, different NF-kappa B family members could effectively repress the transfected alpha-like globin promoters in K562 as well as in MEL cells. The involvement of NF-kappa B pathway is supported by the ability of a NF-kappa B-specific, dominant negative mutant to block the tumor necrosis factor-alpha or p65-mediated suppression of the alpha-like globin promoter activities. The suppression appears to be mediated through cis-linked HS-40 enhancer. Finally, stably transfected K562 cells overexpressing p65 contain reduced amounts of the p45/NF-E2 RNA and functional NF-E2 proteins. Our studies have identified a new set of targets of NF-kappa B. We suggest that the relatively high activity of the NF-kappa B pathway in early erythroid progenitors is involved in the suppression of erythroid-specific genes. Later in differentiation, together with other changes, the decline of the amounts of the NF-kappa B family of factors leads to derepression and consequent increase of NF-E2, which in turn would activate a subset of erythroid-specific genes.

Induction of fetal hemoglobin synthesis by valproate: modulation of MAP kinase pathways.

Valproate has been found to stimulate fetal hemoglobin (HbF) synthesis in patients with sickle cell disease. In accordance with these clinical observations, we found a moderate induction of HbF synthesis in K562 erythroid cells in vitro. Investigation of the role of the mitogen-activated protein kinase (MAPK) pathways by Western blot analysis and use of specific kinase inhibitors suggests that inhibition of ERK pathway and activation of the p38 pathway may contribute to the HbF-inducing activity of valproate.

Cell-surface expression of RhD blood group polypeptide is posttranscriptionally regulated by the RhAG glycoprotein

In most cases, the lack of Rh in Rhnull red cells is associated with RHAG gene mutations. We explored the role of RhAG in the surface expression of Rh. Nonerythroid HEK293 cells, which lack Rh and RhAG, or erythroid K562 cells, which endogenously express RhAG but not Rh, were transfected with RhD and/or RhAG cDNAs using cytomegalovirus (CMV) promoter–based expression vectors. In HEK293 cells, a low but significant expression of RhD was obtained only when RhAG was expressed at a high level. In K562 cells, as expected from the opposite effects of the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) on erythroid and CMV promoters, the levels of endogenous RhAG and recombinant RhD transcripts were substantially decreased and enhanced upon TPA treatment of RhD-transfected cells (K562/RhD), respectively. However, flow cytometry and fluorescence microscopy analysis revealed a decreased cell-surface expression of both RhAG and RhD proteins. Conversely, TPA treatment of RhAG-transfected cells increased both the transcript and surface expression levels of RhAG. When K562/RhD cells were cotransfected by the RhAG cDNA, the TPA-mediated induction of recombinant RhAG and RhD transcription was associated with an increased membrane expression of both RhAG and RhD proteins. These results demonstrate the role of RhAG as a strictly required posttranscriptional factor regulating Rh membrane expression. In addition, because the postulated 2:2 stoichiometry between Rh and RhAG observed in the native red cell membrane could not be obtained in cotransfected K562 cells, our study also suggests that as yet unidentified protein(s) might be involved for optimal membrane expression of Rh.