Expression of multiple genes regulating cell cycle and apoptosis in differentiating hematopoietic cells is dependent on iron

OBJECTIVE

Iron plays critical roles in many biological processes including hematopoietic cell growth and differentiation. Iron is essential for the differentiation of HL-60 promonocytes. HL-60 cells stimulated with phorbol myristate acetate (PMA) undergo G1/S phase cell-cycle arrest and differentiate to monocyte/macrophages. With iron deprivation, PMA-induced HL-60 cells bypass differentiation and undergo apoptosis. To investigate the molecular basis underlying this observation, we used commercially available gene microarrays to evaluate expression of multiple genes involved in the regulation of cell cycling and apoptosis.

METHODS

We treated HL-60 cells with PMA +/- desferrioxamine (DF), a potent iron chelator, to produce iron deprivation. Cells were cultured for 48 hours, and cDNA was prepared and radiolabeled with alpha-(32)P dCTP, then hybridized to gene arrays containing specific cDNA fragments.

RESULTS

Expression of 11 of 43 genes was inhibited greater than 50% by iron deprivation. These genes were Rb; p21 (WAF1/CIP1); bad; cdk2; cyclins A, D3, E1; c-myc; egr-1; iNOS; and FasL. For each gene the microarray results were confirmed by RT-PCR and/or Northern or Western blotting. Nuclear transcription assays indicated that the role of iron in Rb expression was to support gene transcription. Addition of ferrioxamine (iron saturated DF) instead of DF to PMA-induced cells did not affect gene expression, indicating that diminished expression was due to iron deprivation, not nonspecific toxicity.

CONCLUSION

Iron supports expression of multiple cell cycle-regulatory and apoptosis-related genes during HL-60 cell differentiation, and, in this way, is involved in regulation of a critical cell decision point-the decision to pursue a differentiation-related or apoptotic pathway.

Expression of different NF-kappaB pathway genes in dendritic cells (DCs) or macrophages assessed by gene expression profiling

NF-kappaB/Rel transcription factors have been implicated in the differentiation of monocytes to either dendritic cells (DCs) or macrophages, as well as in the maturation of DCs from antigen-processing to antigen-presenting cells. Recent studies of the expression pattern of Rel proteins and their inhibitors (IkappaBs) suggest that their regulation during this differentiation process is transcriptional. To investigate differential gene expression between macrophages and DCs, we used commercially available gene microarrays (GEArray KIT), which included four of the NF-kappaB/Rel family genes (p50/p105, p52/p100, RelB, and c-rel) and 32 additional genes either in the NF-kappaB signal transduction pathway or under transcriptional control of NF-kappaB/Rel factors. To generate macrophages and DCs, human adherent peripheral blood monocytes were cultured with M-CSF or GM-CSF + IL-4 respectively for up to 8 days. DCs (and in some experiments, macrophages) were treated with lipopolysaccharide (LPS) for the last 48 h of culture to induce maturation. Cells were harvested after 7 days, cDNA was prepared and radiolabeled with alpha-(32)P-dCTP, then hybridized to gene arrays containing specific gene probes. beta-actin and GAPDH or PUC18 oligonucleotides served as positive or negative controls, respectively. The expression of all four NF-kappaB/Rel family genes examined was significantly upregulated in maturing DCs compared to macrophages. The strongest difference was observed for c-rel. RT-PCR determinations of c-rel, RelB, and p105 mRNAs confirmed these observations. Among the 32 NF-kappaB/Rel pathway genes, 14 were upregulated in mature DCs compared to macrophages. These genes were IkappaBalpha, IKK-beta, NIK, ICAM-1, P-selectin, E-selectin, TNF-alpha, TNFR2, TNFAIP3, IL-1alpha, IL-1R1, IL-1R2, IRAK, and TANK. By contrast, only mcp-1 (monocyte chemotactic protein 1) was upregulated in macrophages compared to DCs. NF-kappaB pathway genes upregulated in DCs compared to macrophages were constitutively expressed in monocytes then selectively downregulated during macrophage but not DC differentiation. LPS did not induce expression of most of these genes in macrophages but LPS did induce upregulation of IL-8 in mature macrophages. We conclude that NF-kappaB/Rel family genes, especially c-rel, are selectively expressed during differentiation of monocytes towards DCs. Moreover, this differential expression is associated both with activation of different NF-kappaB signal transduction pathways in DCs and macrophages and with expression of a unique subset of genes in DCs that are transcriptionally targeted by NF-kappaB/Rel factors. The results illustrate the ability of the NF-kappaB pathway to respond to differentiation stimuli by activating in a cell-specific manner unique signalling pathways and subsets of NF-kappaB target genes.

A new molecular role for iron in regulation of cell cycling and differentiation of HL-60 human leukemia cells: iron is required for transcription of p21(WAF1/CIP1) in cells induced by phorbol myristate acetate

To investigate the role of iron in hematopoiesis, we studied effects of iron deprivation on PMA-induced monocyte/macrophage differentiation in HL-60 cells. Iron deprivation induced by desferrioxamine (DF) blocked PMA-induced differentiation and induced S-phase arrest and apoptosis in up to 60% of cells. Apoptosis was not related to a decrease of bcl-2 or to c-myc overexpression. In the presence of DF, PMA-induced upregulation of the cyclin dependent kinase inhibitor (CDKI), p21(WAF1/CIP1), was blocked and its expression could be restored in the presence of DF by supplementation with ferric citrate. Furthermore, ferrioxamine (iron saturated DF) did not block induction of p21(WAF1/CIP1) indicating that the changes were not due to a nonspecific toxic effect of DF. Similarly, hydroxyurea, an inhibitor of ribonucleotide reductase, did not block p21 expression. p21(WAF1/CIP1) antisense oligonucleotides caused cell cycle alterations similar to DF and p21 overexpression overcame effects of iron deprivation on both cell cycling and differentiation. Therefore, p21 is a key target for the effects of iron deprivation on HL-60 cell cycling and differentiation. Nuclear run-on transcription assays and p21 mRNA half-life studies indicated that iron was required to support transcriptional activation of p21(WAF1/CIP1) after a PMA stimulus. By contrast, iron deprivation did not inhibit expression of a second CDKI, p27(KIP1). These data demonstrate a new role for iron during monocyte/macrophage differentiation. A key role of iron is to allow induction of p21(WAF1/CIP1) in response to a differentiation stimulus subsequently blocking cells at the G(1)/S cell cycle interface and preventing premature apoptosis. This effect of iron is independent of its requirement in supporting the activity of the enzyme, ribonucleotide reductase. Because of the central role of p21(WAF1/CIP1) as regulator of the G(1)/S cell cycle checkpoint this requirement for iron to support p21 expression represents an important mechanism by which iron may modulate hematopoietic cell growth and differentiation. Published 2001 Wiley-Liss, Inc.

Analysis of DNA binding proteins associated with hemin-induced transcriptional inhibition. The hemin response element binding protein is a heterogeneous complex that includes the Ku protein

Hemin inhibits transcription of the tartrate resistant acid phosphatase (TRAP) gene. Using deletion mutagenesis of the mouse TRAP 5'-flanking region, we previously identified a 27-bp DNA segment containing a central GAGGC tandem repeat sequence (the hemin response element [HRE]), which bound nuclear proteins (hemin response element binding proteins [HREBPs]) from hemin-treated cells and appeared to be responsible for mediating transcriptional inhibition in response to hemin. We now have used affinity binding to HRE-derivatized beads to identify four HREBP components with apparent molecular masses of 133-, 90-, 80-, and 37-kD, respectively. The 80- and 90-kD components correspond to the p70 and p80/86 subunits of Ku antigen (KuAg) as documented by partial amino acid microsequencing of tryptic digests and immunologic reactivity. Based on reactivity of the HREBP gel shift band with antibodies to the redox factor protein (ref1) in shift Western experiments, it is shown that the 37-kD component represents ref1. The 133-kD component appeared to be a unique protein. KuAg participation in HREBP complexes was specific as it was present in HREBPs bound to HRE microcircles. Results of depletion/reconstitution experiments suggested that KuAg does not bind alone or directly to HRE DNA, but does so only in conjunction with the 133- and/or 37-kD proteins. We conclude that HREBP is a heterogeneous complex composed of KuAg, ref1, and a unique 133-kD protein. We speculate that the role of heme may be to promote interactions among these components, thereby facilitating HRE binding and downregulation of hemin responsive genes.

Inhibition of tartrate-resistant acid phosphatase gene expression by hemin and protoporphyrin IX. Identification of a hemin-responsive inhibitor of transcription

Tartrate-resistant acid phosphatase (TRAP) is an iron-containing protein encoded by the same gene that codes for uteroferrin, a placental iron transport protein. In human peripheral mononuclear cells, TRAP expression is inhibited by both hemin (ferric protoporphyrin IX) and protoporphyrin IX. Nuclear run-on assays confirmed that this inhibition occurs at the level of gene transcription. Previous studies with mTRAP deletion mutants showed that the hemin effect was dependent on repressor activity in the mTRAP 5'-flanking region at -1846 bp to -1240 bp relative to ATG (Reddy et al, J Bone Mineral Res 10:601, 1995). We now report that gel shift assays showed a DNA binding protein in nuclear extracts of hemin-treated cells termed hemin response element binding protein (HREBP). Additional studies have localized the HREBP binding region in the mTRAP 5'-flanking DNA to a 27-bp sequence at -1815 to -1789 bp relative to ATG. A tandem repeat sequence, GAGGC;GAGGC, contained within this DNA segment, was shown to be involved in binding of HREBP. Highly homologous sequences are present in the 5'-flanking region of the hTRAP gene. Binding of HREBP to the mTRAP DNA sequence was inhibited by anti-HAP1 antibodies, indicating homology between the hemin-responsive factor and the yeast heme-dependent transcription factor, HAP1. A 607-bp segment of the mTRAP 5'-flanking region containing the candidate hemin response element and surrounding sequences conferred hemin regulation on the viral SV40 promoter. Southwestern blotting experiments probing nuclear extracts of hemin-treated U937 cells with the 27-bp binding sequence showed two protein bands at 37 and 133 kD representing candidate HREBPs. A GENINFO search showed several other mammalian genes with tandem GAGGC motifs in noncoding regions, providing the possibility that additional genes may also be regulated by hemin at the level of transcription. These studies provide the first description of a novel iron/hemin-responsive transcriptional regulatory mechanism in mammalian cells.

Characterization of the mouse tartrate-resistant acid phosphatase (TRAP) gene promoter

Tartrate-resistant acid phosphatase (TRAP) is an iron-binding protein that is highly expressed in osteoclasts. To characterize the regulation of TRAP gene expression, progressive 5' and 3' deletions of a 1.8 kb fragment containing the 5'-flanking sequence were fused to a luciferase reporter gene. Two nonoverlapping regions of this 1.8 kb fragment had promoter activity. The upstream promoter (P1) was located within the region from -881 bp to -463 bp relative to the ATG, while the downstream promoter (P2) was located between -363 bp to -1 bp in a region we have previously shown to be an intron in transcripts originating from the upstream promoter. A putative repressor region for the P2 promoter at -1846 bp to -1240 bp and a putative enhancer region at -962 bp to -881 bp relative to the ATG were identified. PCR analysis of promoter-specific transcription of the TRAP gene in various murine tissues showed that both promoters were active in several tissues. Transferrin-bound iron increased P1 promoter activity 2.5-fold and hemin decreased P1 promoter activity, but neither had any effect on P2 activity. These data show that the transcriptional regulation of the TRAP gene is complex and that iron may play a key role in TRAP gene regulation.

Regulation of protein kinase C (PKC) expression by iron: effect of different iron compounds on PKC-beta and PKC-alpha gene expression and role of the 5'-flanking region of the PKC-beta gene in the response to ferric transferrin

We have studied effects of ferric transferrin (FeTF), ferric lactoferrin (FeLF), ferric complexes of pyridoxal- or salicylaldehyde-isonicotinoyl hydrazone, (Fe-PIH, Fe-SIH), and ferric ammonium citrate (FAC) on expression of protein kinase C (PKC) mRNA transcripts in a variety of cultured cell lines. FeTF supported an increase of PKC-beta mRNA transcripts in T-lymphoblastoid (CCRF-CEM; Jurkat), B-lymphoblastoid (Daudi; Raji), promyelocyte (HL-60), erythroleukemia (K562), and monocyte (U937) cell lines. By contrast, FeLF, Fe-PIH, and Fe-SIH did not support an increase of PKC-beta mRNA transcripts in any of these cell lines. Furthermore, FAC supported an increase of PKC-beta mRNA transcripts in HL-60, K562, and U937 cells only. Preincubation of cells with desferrioxamine (DF), a cell-permeable iron chelator, abolished the increments of PKC-beta mRNA observed in response to FeTF or FAC. In contrast to results with PKC-beta, neither FeTF nor FAC caused an increase of PKC-alpha transcripts in any cell line. To locate iron-responsive DNA regulatory elements of the PKC-beta gene, we prepared genetic constructs containing various portions of the human PKC-beta 5'-flanking DNA linked to the firefly luciferase gene. Constructs were cotransfected with the neomycin resistance plasmid, Pwl-neo, into HRE H9 cells, and stable transfectants were selected in G418. Treatment with FeTF of transfectants bearing chimeric gene constructs with 2,200 bp of the PKC-beta 5'-flanking region increased luciferase activity and mRNA transcripts 2.5-fold. This increase was blocked by DF. Neither luciferase activity nor mRNA increased with FeTF in stable transfectants bearing constructs with 342 bp or 587 bp of the PKC-beta 5'-flanking region. These data provide direct confirmation that iron is involved in regulation of PKC-beta but not PKC-alpha gene expression in many cell lines. The form in which iron is presented to these cell lines appears to affect its availability for this function, and cells vary in their capabilities to use nontransferrin iron to support PKC-beta gene expression. Finally, transcriptional upregulation of PKC-beta by FeTF is mediated by DNA sequences located between -2200 bp and -587 bp in the 5'-flanking region of the human PKC-beta gene.

Transcriptional regulation of the tartrate-resistant acid phosphatase (TRAP) gene by iron

Tartrate-resistant acid phosphatase (TRAP) was first identified in cells from patients with hairy cell leukaemia. Subsequently, it has been found in other leukaemias, B-lymphoblastoid cell lines, osteoclasts and subsets of normal lymphocytes, macrophages, and granulocytes. Recent data indicate that TRAP and porcine uteroferrin, a placental iron-transport protein, represent a single gene product. However, the intracellular role of TRAP is unknown. We used a full-length human placental TRAP cDNA probe to examine TRAP expression in human peripheral mononuclear cells (PMCs). TRAP mRNA increased 50-75-fold after 24 h in unstimulated PMC cultures. Cell-fractionation experiments indicated that monocytes were the main cell population accounting for increased TRAP mRNA transcripts, and this was confirmed by histochemical staining for TRAP enzyme activity. Because expression of other iron-binding and -transport proteins is controlled by iron availability, we examined the role of iron in regulating TRAP expression. Increase of TRAP mRNA transcripts in PMCs was inhibited by 50 microM desferrioxamine, a potent iron chelator. The 5' flanking region of the TRAP gene was cloned from a mouse genomic library. In preliminary transient transfection experiments, it was determined that the 5'-flanking region of the TRAP gene contained iron-responsive elements. Therefore, a series of stably transfected HRE H9 cell lines was developed bearing genetic constructs containing various segments of the murine TRAP 5' promoter region driving a luciferase reporter gene. Treatment of transfectants with 100 micrograms/ml iron-saturated human transferrin (FeTF) was performed to assess iron responsiveness of the constructs. Constructs containing a full-length TRAP promoter (comprising base pairs -1846 to +2) responded to FeTF with a 4-5-fold increase of luciferase activity whereas constructs containing only base pairs -363 to +2 of the TRAP promoter did not respond. Constructs containing 1240 or 881 bp of the TRAP promoter gave only a 1.5- to 2-fold increase of luciferase activity with FeTF. In all cases, increase of luciferase activity was blocked by desferrioxamine. Cells transfected with another luciferase construct driven by a simian virus 40 promoter did not show any increase of luciferase activity with FeTF. These data indicate that expression of TRAP is regulated by iron and that this regulation is exerted at the level of gene transcription. The transfection experiments also suggest that the region of the TRAP 5'-flanking sequence between base pairs -1846 and -1240 contains an iron regulatory element.

Transcriptional regulation of transferrin receptor expression by cultured lymphoblastoid T cells treated with phorbol diesters

Expression of transferrin receptors (TFR) is required for lymphocyte proliferation. Treatment of lymphoblastic leukemia cell lines with phorbol diester tumor promoters decreases proliferation and induces differentiation. Among changes induced by phorbol diesters is decreased cell surface expression of TFR. To elucidate effects of phorbols on lymphocyte growth and differentiation, we examined TFR expression by measuring 125I-transferrin binding, levels of TFR mRNA by Northern analysis and dot-blot hybridization, and rates of TFR gene transcription by nuclear run-on experiments in CCRF-CEM lymphoblastoid T cells treated with PMA or phorbol dibutyrate. Cell surface expression of TFR was decreased 60 to 85% within 2 min of exposing cells to phorbols and remained decreased for 96 h. Steady state levels of TFR mRNA decreased to less than 30% of control after 48 h. After treating cells with actinomycin D, estimated TFR mRNA t 1/2 was 2.7 h and was unaltered in phorbol-treated cells. Levels of TFR mRNA were not affected by treatment of cells with cycloheximide in either control or phorbol-treated cells. Therefore, post transcriptional mRNA processing by protein factors did not account for decreased TFR mRNA in phorbol-treated cells. Compared to baseline levels, rates of TFR gene transcription in PMA-treated cells increased up to two-fold during the initial 6 h of culture, then decreased over the ensuing 12 h to less than 10% of baseline values. This pattern was not seen in control cultures. Therefore, regulation of TFR gene transcription is a consequence of treating CEM cells with phorbol diesters. Cell surface expression of TFR in phorbol-treated lymphoblastoid T cells may be mediated in part at the level of gene transcription.

Transcriptional regulation of transferrin receptor expression by cultured lymphoblastoid T cells treated with phorbol diesters

Expression of transferrin receptors (TFR) is required for lymphocyte proliferation. Treatment of lymphoblastic leukemia cell lines with phorbol diester tumor promoters decreases proliferation and induces differentiation. Among changes induced by phorbol diesters is decreased cell surface expression of TFR. To elucidate effects of phorbols on lymphocyte growth and differentiation, we examined TFR expression by measuring 125I-transferrin binding, levels of TFR mRNA by Northern analysis and dot-blot hybridization, and rates of TFR gene transcription by nuclear run-on experiments in CCRF-CEM lymphoblastoid T cells treated with PMA or phorbol dibutyrate. Cell surface expression of TFR was decreased 60 to 85% within 2 min of exposing cells to phorbols and remained decreased for 96 h. Steady state levels of TFR mRNA decreased to less than 30% of control after 48 h. After treating cells with actinomycin D, estimated TFR mRNA t 1/2 was 2.7 h and was unaltered in phorbol-treated cells. Levels of TFR mRNA were not affected by treatment of cells with cycloheximide in either control or phorbol-treated cells. Therefore, post transcriptional mRNA processing by protein factors did not account for decreased TFR mRNA in phorbol-treated cells. Compared to baseline levels, rates of TFR gene transcription in PMA-treated cells increased up to two-fold during the initial 6 h of culture, then decreased over the ensuing 12 h to less than 10% of baseline values. This pattern was not seen in control cultures. Therefore, regulation of TFR gene transcription is a consequence of treating CEM cells with phorbol diesters. Cell surface expression of TFR in phorbol-treated lymphoblastoid T cells may be mediated in part at the level of gene transcription.

Disparity between expression of transferrin receptor ligand binding and non-ligand binding domains on human lymphocytes

We compared transferrin receptor (TfR) expression on human peripheral blood lymphocytes (PBL) activated by phorbol myristate acetate (PMA) or L-phytohemagglutinin (LPHA) using two techniques: (1) 125I-iron-saturated transferrin (FeTf) binding, (2) reactivity with monoclonal anti-TfR antibodies--OKT9 and B3/25. These monoclonal antibodies do not block FeTf binding, and therefore bind to TfR domains separate from the ligand binding site. Unstimulated PBL bound fewer than 1,000 molecules of 125I-FeTf per cell, and less than 5% of cells expressed TfR antigens detected by OKT9 or B3/25. 125I-FeTf binding and antibody binding increased in parallel on LPHA-activated PBL. After exposure to LPHA for 72 hr, 125I-FeTf binding increased 100-fold to 10(5) molecules per cell and greater than 50% of cells expressed TfR antigens. By contrast, PMA activation of PBL markedly increased binding of OKT9 and B3/25 but not the binding of 125I-FeTf. Cell surface expression of TfR antigens seen by OKT9 and B3/25 did not differ between LPHA- and PMA-activated PBL. However, after 72 hr with PMA, 125I-FeTf binding increased only 6-fold and consistently remained at less than 10(4) molecules per cell. Therefore, PMA induced a disparity between expression of TfR ligand binding domains and immunological domains at the cell surface. Cell proliferation assessed by fluorescent DNA analysis was similar in cultures stimulated by LPHA or PMA. These data indicate that lymphoid cells may possess a mechanism for modulating TfR expression in which down-regulation of FeTf binding occurs without receptor internalization. Alternatively, it is possible that this observation may reflect a membrane perturbation effect of PMA.

Phorbol diesters and transferrin modulate lymphoblastoid cell transferrin receptor expression by two different mechanisms

Expression of transferrin receptors (TfR) by activated lymphocytes is necessary for lymphocyte DNA synthesis and proliferation. Regulation of TfR expression, therefore, is a mechanism by which the lymphocyte's proliferative potential may be directed and controlled. We studied mechanisms by which lymphoblastoid cells modulate TfR expression during treatment with phorbol diesters or iron transferrin (FeTf), agents which cause downregulation of cell surface TfR. Phorbol diester-induced TfR downregulation occurred rapidly, being detectable at 2 min and reaching maximal decreases of 50% by 15 min. It was inhibited by cold but not by agents that destabilize cytoskeletal elements. Furthermore, this downregulation was reversed rapidly by washing or by treatment with the membrane interactive agent, chlorpromazine. In contrast, FeTf-induced TfR downregulation occurred slowly. Decreased expression of TfR was detectable only after 15 min and maximal downregulation was achieved after 60 min. Although FeTf-induced downregulation also was inhibited by cold, it was inhibited in addition by a group of microtubule destabilizing agents (colchicine, vinblastine, podophyllotoxin) or cytochalasin B, a microfilament inhibitor. Furthermore, FeTf-induced downregulation was not reversed readily by washing or by treatment with chlorpromazine. The inactive colchicine analogues, beta- and gamma-lumicolchicine, did not inhibit FeTf-induced TfR downregulation. Similarly, when cells were pretreated with taxol to stabilize microtubules, colchicine no longer inhibited FeTf-induced downregulation. Therefore, FeTf causes TfR downregulation in lymphoblastoid cells by a cytoskeleton-dependent mechanism. Phorbol diesters cause TfR downregulation by a cytoskeleton-independent mechanism. In other experiments, treatment of cells with both a phorbol diester and FeTf, either simultaneously or sequentially, produced additive effects on TfR expression. These data indicate that TfR expression is regulated by two independent mechanisms in lymphoblastoid cells, and they provide the possibility that downregulation of TfR by different mechanisms may result in different effects in these cells.