From the
Renal Unit, ¶AIDS Research Center and Cancer Center, and ||Department of Pathology, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129
Received for publication, February 27, 2003 , and in revised form, April 15, 2003.
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ABSTRACT |
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INTRODUCTION |
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Cytochrome P450 4F3 (CYP4F3) functions to control inflammation by inactivating leukotriene B4 (LTB4) with high specificity (1113), but the mechanisms that regulate CYP4F3 gene expression are not known. LTB4 is a potent chemoattractant of myeloid cells (1417) and has been implicated in the pathogenesis of multiple inflammatory diseases (1821). LTB4 is synthesized from arachidonic acid by the sequential action of 5-lipoxygenase and leukotriene A4 hydrolase, and the chemoattractant activity of LTB4 is mediated by a high affinity G protein-coupled receptor designated BLT1 (22). The major pathway for the catabolism and inactivation of LTB4 in human neutrophils involves microsomal -hydroxylation by CYP4F3 (11). This reaction generates 20-OH LTB4, which can further inhibit LTB4 activities by down-regulating BLT1 (23). The ability of LTB4 to amplify an inflammatory response is, therefore, counterbalanced by the expression of CYP4F3.
Recently we cloned the CYP4F3 gene and demonstrated that alternative splicing generates isoforms that differ in functional properties and tissue distribution (24, 25). Selection of exon 4 generates the neutrophil isoform (CYP4F3A), which has a low Km for LTB4 of <1 µM. Selection of exon 3 instead of exon 4 generates an alternative isoform (CYP4F3B), which has a 44-fold lower efficiency of inactivating LTB4. CYP4F3B is expressed in liver and has a preference for arachidonic acid as a substrate, which it converts to 20-HETE (25). This has significance because 20-HETE is a potent bioactive mediator in certain tissues; it activates protein kinase C and has roles in regulating cell proliferation, vascular tone, and natriuresis (26). These opposing capacities of the CYP4F3 isoforms to generate an active mediator (20-HETE) or inactivate one (LTB4) allow for versatility of function but demand strict controls. CYP4F3 transcription and alternative splicing must be regulated to ensure that the appropriate isoform is generated in the correct setting.
To understand the regulation of CYP4F3 expression, we examined its distribution in maturing populations of human bone marrow cells and identified the splicing pathways. CYP4F3A is expressed in myeloid cells; its expression is coordinate with known myeloid differentiation markers such as CD11b and also increases concomitantly with myeloperoxidase during development. In contrast, CYP4F3B is expressed in lymphocytes, and expression is restricted to a small population (10%) of CD3+ T cells. We determined that alternative promoters regulate lineage-specific expression of CYP4F3 isoforms in myeloid cells, lymphoid cells, and liver. Surprisingly, activity of the myeloid-specific promoter could not be accounted for by known myeloid transcription factors including PU.1 and MZF-1, but it could be activated by ZEB-2 and CtBP. The results suggest new roles for these proteins in myeloid transcription. ZEBs and CtBP are regulated by activated Smads and NAD+, respectively, and therefore have the capacity to integrate various environmental signals during development or inflammation.
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EXPERIMENTAL PROCEDURES |
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Flow CytometryTotal bone marrow mononuclear cells and peripheral blood granulocytes and lymphocytes were stained with allophycocyanin, percyphycoerythrin, or fluoroisothiocyanate-conjugated control IgGs or monoclonal antibodies directed to CD34, CD38, CD33, CD13, CD16, CD15, CD11b, CD14, CD8, and CD3 (BD Biosciences) to define hematopoietic cell subsets in a double or tri-color labeling. Surface-labeled samples were then fixed in 0.1% formaldehyde and permeabilized with 0.1% Triton X-100 to perform intracytoplasmic staining with anti-CYP4F3 antibody. Production, purification, and characterization of polyclonal rabbit antibodies against the C-terminal domain (amino acids 410520) of CYP4F3, which recognize both isoforms (CYP4F3A and CYP4F3B), has been described previously (24). Intracellular staining was performed on unlabeled or surface-labeled cells after fixation and permeabilization. Samples were incubated with 5 µg/ml anti-CYP4F3 polyclonal antibody or 3 µg/ml control rabbit IgG for 30 min at room temperature and were then incubated with 1 µg/ml goat anti-rabbit phycoerythrin-conjugated or fluoroisothiocyanate-conjugated polyclonal antibodies (Sigma). Tri- or four-color-labeled samples were then analyzed by flow cytometry using the FACScalibur instrument and the Cellquest program (BD Biosciences). The data were plotted as single-parameter histograms or bivariate dot plots with logarithmic amplification.
RNA Isolation, Reverse Transcription, and Isoform-specific PCR Total RNA was isolated from cells using Tri Reagent (Sigma). Reverse transcription and first-strand cDNA synthesis was performed using the cDNA cycle kit (Invitrogen) with random primers and avian myeloblastosis virus reverse transcriptase. The cDNA was purified by phenol-chloroform extraction and ethanol precipitation. CYP4F3A and CYP4F3B were detected by isoform-specific PCR as described previously (24). Primer pairs specific for ZEB-1 (forward, 5'-CTGAAGAGGACCAGAGGCAG-3', reverse, 5'-CCCAGACTGCGTCACATGTC-3'), ZEB-2 (forward, 5'-CTGAGGAGCTGTCTCGCCTT-3', reverse, 5'-GCTCCTTGGGTTAGCATTTGGTGC-3'), or primers that would bind to CtBP-1 and CtBP-2 (forward, 5'-CCAGGGAGGACCTGGAGAAGTTCA-3'; reverse, 5'-GCCACCTCGCGGATCTGCTC-3') were used in PCR reactions with conditions of 94 °C for 1 min, 62 °C for 1 min, and 72 °C for 1 min for 30 cycles.
5'-Rapid Amplification of cDNA Ends (5'-RACE) AnalysisThe 5'-untranslated region (5'-UTR) sequences of CYP4F3 transcripts in different RNA samples were determined by 5'-RACE (Invitrogen). First-strand cDNA synthesis was performed with primer 1 (5'-CCCCAACCAGCAGCAGGAGCAGCC-3', +79 to +56 relative to adenosine of first methionine). First-round PCR was with the Anchor Primer and primer 2 (5'-ATGCTGCCATTGGCCAAA-3', +48 to +31). Second-round PCR was with the Universal Amplification Primer and primer 3 (5'-GACAGGCTCAGCTGTGGC-3', +20 to +3). The PCR conditions were 94 °C for 1 min, 50 °C for 1 min, and 72 °C for 1 min; 30 cycles were followed by 1 cycle with a 10-min extension time. PCR products were subcloned using a TOPO TA cloning kit (Invitrogen) and sequenced. A minimum of 10 RACE clones was analyzed from each RNA sample.
Construction of Luciferase Reporter Plasmids and Site-directed MutagenesisCYP4F3 promoter-luciferase reporter constructs were made using the pGL3 basic vector (Promega). Defined segments of the 5'-flanking region of the CYP4F3 gene were amplified by PCR. KpnI and NheI sites were added to the 5'-ends of sense and antisense primers, respectively, and PCR products were ligated into the corresponding sites of pGL3 and sequenced. Site-directed mutagenesis was performed using the GeneEditor System (Promega) according to the manufacturer's instructions.
Transient Transfections and Luciferase AssaysHL60 cells were treated with 1.3% Me2SO for 4 days and electroporated with a Gene Pulser (Bio-Rad) at settings of 290 V and 960 microfarads. Each sample (2.3 x 107 cells/ml in complete growth medium plus 1.3% Me2SO, 300 µl/cuvette) was electroporated with 30 µg of pGL3 luciferase reporter plasmid, and 10 µg of -galactosidase expression plasmid (pcDNA3.1/Myc-His-LacZ, Invitrogen) was included as a control for transfection efficiency. HepG2 cells and COS 7 cells were grown to 5060% confluency in 6-well plates and transfected using SuperFect Reagent (Qiagen). HepG2 cells were transfected with 1 µg of luciferase plasmid plus 1 µg of
-galactosidase plasmid/well. COS 7 cells were transfected with a total of 2.5 µg of DNA/well (0.5 µg of luciferase plasmid, 0.5 µg of
-galactosidase, 0.5 µg of CtBP expression vector, and 1 µg of ZEB expression vector); the total DNA added was made up to 2.5 µg with PCDNA3 plasmid (Invitrogen) in control experiments without expression vectors. HepG2 and COS 7 cell lysates were prepared in reporter lysis buffer (Promega) 48 h after transfection. HL60 cell lysates were prepared 1418 h after the addition of DNA. Luciferase activity was measured with a luciferase assay system (Promega) and a luminometer (Monolight 2010, Analytical Luminescence Laboratory) and normalized to the
-galactosidase activity in each sample as determined by a
-galactosidase enzyme assay system (Promega). The results represent the mean of at least three independent experiments. Expression vectors for human ZEB-1 and ZEB-2 (in pCS2 with an N-terminal Myc epitope tag) were generous gifts from Dr. A. Postigo and Dr. D. Dean (5). An expression vector for human CtBP-1 was a generous gift from Dr. R. Goodman (37).
Electrophoretic Mobility Shift AssaysThe wild type (WT) probe corresponds to a 38-bp region between positions 812 and 775 of the CYP4F3 promoter (5'-CACTCAGTAGGTGCTCTTTAAGAGCAGGTGTCACACAG-3') and includes the putative bipartite ZEB binding site. The mutant probe has each AGGTG sequence converted to CCCGA (5'-CACTCAGTCCCGACTCTTTAAGAGCCCCGATCACACAG-3'). Sense and antisense oligonucleotides were annealed, and double-stranded oligonucleotides were then labeled with [-32P]ATP and T4 polynucleotide kinase (Promega). Nuclear extracts were prepared from COS 7 cells transfected with Myc-tagged ZEB-2 using the nuclear extract kit from ActiveMotif. The DNA binding assay (24-µl final volume) was carried out at 4 °C using a Gelshift Kit from ActiveMotif with buffers A1 and B1. The reaction contained 5 µg of nuclear extract and 100 pg of 32P-labeled double-stranded oligonucleotide (105 cpm). For supershift experiments, a mouse monoclonal antibody to c-Myc (Santa Cruz Biotechnology) was included in the DNA binding assay. A mouse monoclonal antibody to lamin A+B2 (Zymed Laboratories Inc.) was used as a negative control. The reaction mixtures were separated on a 4% polyacrylamide gel prepared in 0.25% TBE buffer, and the gel was then dried and exposed to Kodak X-Omat AR-5 film for 10 h.
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RESULTS |
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We next compared the expression of CYP4F3 in bone marrow to that of known myeloid proteins using a panel of antibodies against surface markers for hematopoietic differentiation and maturation in conjunction with anti-CYP4F3 (Fig. 2). A representative experiment is shown in Fig. 2A. A high percentage of total bone marrow cells are double-positive for CYP4F3 and myeloid markers (for example, 48% CD33+, 37% CD11b+, and 32% CD15+ cells express CYP4F3). Less than 2% of cells are double-positive for CYP4F3 and lymphoid markers such as CD2, CD3, and CD8. Interestingly, this population of lymphocytes expressing CYP4F3 was consistently observed, and it is apparent that some of the cells have a high level of expression. The data presented for total bone marrow cells in Fig. 2A were quantified to determine the expression of each marker protein in the CYP4F3+ population (Fig. 2B). Most CYP4F3 positive cells (>90%) express myeloperoxidase (MPO) and CD33, and a high proportion of CYP4F3 positive cells express other myeloid markers including CD15 (72 ± 2%), CD38 (62% ± 2%), CD11b (54 ± 16%), CD13 (45 ± 12%), CD14 (15 ± 3%), and CD31 (14 ± 2%). In contrast, antigens associated with T and NK lymphocytes (CD2, CD3, CD8, and CD16) or hematopoietic progenitors (CD34) were only expressed in 0.72.5% of the CYP4F3+ cells. An alternative method of evaluating the data is shown in Fig. 2C; cells immunolabeled for specific myeloid and lymphoid surface marker antigens were analyzed to determine the percentage that express CYP4F3. An average of 92% CD15+, 80% CD11b+, 20% CD34+, 15% CD8+, and 10% CD3+ cells were positive for CYP4F3. Overall, the results confirm that CYP4F3 is expressed primarily in myeloid cells but indicate that it is also expressed in a small population of CD34+ progenitors and CD3+ T cells.
To determine whether CYP4F3 is differentially expressed in maturing myeloid cells, we compared CYP4F3 and MPO expression in CD33+ gated populations (Fig. 3). Cells that were negative or low for MPO also had low expression of CYP4F3, and CYP4F3 expression increased in parallel with MPO. This is consistent with the increases in CYP4F3 fluorescence observed as a function of cell granularity (Fig. 1) and confirms that CYP4F3 expression is strongly associated with myeloid differentiation.
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Expression of CYP4F3 in Peripheral Blood LeukocytesFlow cytometry was used to compare the expression level of CYP4F3 in different cell types in peripheral blood (Fig. 4). Granulocytes exhibit high fluorescence (Fig. 4A), whereas lymphocytes exhibit low fluorescence similar to negative control staining with IgG (Fig. 4B). When peripheral leukocytes were analyzed for myeloid and lymphoid markers that included CD15, CD14, CD3, and CD8, the results paralleled those in bone marrow in that essentially 100% of CD15+ and CD14+ cells but only 10% of CD3+ and CD8+ cells co-expressed CYP4F3 (not shown). A HL60 promyelocytic leukemia cell line exhibited intermediate fluorescence (Fig. 4C). Analysis of blood from two patients with eosinophilia gave identical results; CYP4F3 was expressed at highest levels in the eosinophils followed by neutrophils and then monocytes (Fig. 4, D and E). Eosinophilia samples were analyzed for CD16, CD45, and CD49D in addition to the markers listed above (not shown).
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Altenative Promoters Regulate Tissue-specific Expression of CYP4F3We have previously shown that functionally distinct splice forms of CYP4F3 exhibit a tissue-specific distribution, with CYP4F3A expressed in neutrophils, and CYP4F3B expressed in liver (24, 25). We suggested that the 5'-UTR of CYP4F3 transcripts is also tissue-specific and might indicate the use of alternative promoters in different cell types (24). To further investigate this possibility, RNA samples from selected cell populations were analyzed by 5'-RACE to determine the sequence of the 5'-UTR and to identify the transcription start sites (Fig. 5A). Similar results were obtained in peripheral blood granulocytes, bone marrow myeloid cells isolated at early (BM-1, CD11b+ CD14) or later (BM-2, CD11b+ CD14+) stages of development, and also HL60 cells; a single transcription initiation site was identified 519 bp upstream of the ATG initiation codon (start site A). The 5'-UTR requires splicing to assume its location upstream of the ATG codon in mature myeloid transcripts, and the only variation observed relates to selection of the 5'-splice donor site. Two alternative splice junctions are used in bone marrow myeloid cells, apparently with similar efficiency, to generate 5'-UTRs of 40 or 34 bp. The 40-bp 5'-UTR was the only form observed in peripheral blood granulocytes. In HepG2 cells, transcription is initiated at a site located 71 bp upstream of the initiation codon (start site B), representative of the liver phenotype (24). The relative positions of transcription start sites A and B are shown schematically in Fig. 5B.
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We investigated whether the use of alternative start sites A and B in myeloid cells and liver was representative of the use of alternative tissue-specific promoters. To test this hypothesis 5'-flanking regions of the CYP4F3 gene were cloned into a luciferase reporter vector and transfected into HL60 cells differentiated with Me2SO or HepG2 cells (Fig. 5C). In HL60 cells, maximum luciferase expression was obtained using a promoter region that extends 350 bp upstream of start site A (construct A6). A different pattern was observed in HepG2 cells; the 350-bp region upstream of start site A has low activity (construct A6), whereas the region between start sites A and B has much higher activity (construct B3). Maximum luciferase expression in HepG2 cells was obtained using a promoter region that extends 630 bp upstream of start site B (construct B6). This construct includes start site A and overlaps the myeloid promoter, but it has low activity in HL60 cells, probably because splicing of exon 1 (the 5'-UTR) is required for luciferase expression in these cells. The splicing process may be impaired due to modification of the 3'-splice junction in promoter-luciferase constructs. Complete deletion of the 3'-splice junction abolishes activity in HL60 cells but has no significant effect in HepG2 cells (compare construct B6 with B4).
Constructs A1-A8 were designed to eliminate a requirement for splicing after transcription initiation from start site A; exon 1of CYP4F3 (the myeloid 5'-UTR) is linked directly to the ATG initiation codon of luciferase. Initially, the two 5'-splice donor sites in exon 1 were excluded (constructs A1-A4). Subsequently, it was determined that the region containing these splice sites (position 500 to 468) was required for full transcriptional activity in HL60 cells (constructs A5-A8). In contrast, the 5'-UTR downstream of start site B is not required for activity in HepG2 cells (compare construct B3 to B5).
The Myeloid Promoter Is Activated by ZEB-2 and CtBPWe initially sought to identify a role for known myeloid transcription factors in the regulation of CYP4F3 gene expression. Potential binding sites were predicted by inspection of the DNA sequence of the myeloid promoter (region 872 to 468) and are summarized in Table I. These include putative sites for the factors MZF-1 (based on the core sequence GGGGA, positions 602, 538, and 515)) and PU.1 (permissible variations of the GAGGAA core sequence, positions 575 and 518). Surprisingly, mutations to these sites did not reduce activity (Table I). We then focused on the region in the 5'-UTR between 500 and 468, which is an active element of the myeloid promoter. This region contains a putative ZEB target site; ZEB-1 (EF1) and ZEB-2 (SIP1) are two-handed zinc finger transcription factors that bind to bipartite CACCT(G) sequences with a variable orientation and spacing of nucleotides (4). Two inverted CACCT sites (AGGTG) with a spacing of 11 bp are located between positions 488 and 468, a location that exactly matches the two splice junctions (Fig. 6A). Furthermore, a similar bipartite motif is observed further upstream between positions 804 and 783, a location that is within a region of the promoter required for activity in HL60 cells (compare constructs A5 and A6 in Fig. 5C).
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The effect of ZEB expression on CYP4F3 promoter activity was measured in COS 7 cells (Fig. 6B). The cells were cotransfected with a ZEB expression vector and construct A6, which contains the myeloid promoter and both bipartite ZEB sites at positions 488 and 804. A 12-fold induction of activity was observed if an expression vector for CtBP was included. No induction of luciferase activity was observed in the absence of CtBP. The response was dependent on the amount of ZEB expression vector used for transfection and decreased in an approximately linear fashion from 1 to 0.25 µg vector. No significant differences were observed in the response to expression vectors for ZEB-1 or ZEB-2. Mutations to the ZEB sites were investigated using constructs A5 and A3, which contain one bipartite site at position 488, or 804, respectively. ZEB and CtBP induce a 6-fold increase in activity of construct A5, and this is abolished by deleting the region 500 to 468 (mut1). ZEB and CtBP induce a 15-fold increase in activity of construct A3, and this can be reduced to 2-fold by a site-specific mutation that converts AGGTG to CCCTG at position 804 (mut2). Construct A3 does not contain any splice junctions, and the effect of mut2 on transcription is, therefore, clearly separable from any effects on splicing. The activity of these mutants in HL60 cells is shown in Table I. Deletion of the ZEB site in exon 1 (500 to 468) results in a 71% decrease in activity of construct A5, and the site-specific mutation at position 804 results in an 88% decrease in activity of construct A3.
The expression of ZEB-1 and ZEB-2 in myeloid cells was analyzed by isoform-specific reverse transcription-PCR (Fig. 7A). Bone marrow CD11b+ cells, peripheral blood granulocytes, and HL60 cells express ZEB-2 but not ZEB-1. HepG2 cells and CD3+ T cells preferentially express ZEB-1, and CtBP was detected in all samples. ZEB-1 and ZEB-2 are highly related and bind to identical DNA sequences, but they may have different transcriptional activity due to differences in their strength of interaction with other proteins (5). A gel retardation assay was then used to demonstrate binding of Myc-tagged ZEB-2 to the CYP4F3 promoter (Fig. 7B). A 32P-labeled oligonucleotide probe corresponding to the region between 812 and 775 of the promoter forms a binding complex with ZEB-2 (Fig. 7B, lane 2, the band indicated by the arrow). Binding is abolished by the addition of a 100-fold molar excess of unlabeled WT oligonucleotide (lane 3) but not by a mutant oligonucleotide which has the AGGTG sequences converted to CCCGA (lane 4). Furthermore, the band disappears after the addition of anti-Myc antibody (lane 5) but not anti-lamin antibody (lane 6). The location of the supershifted band is masked by the presence of a larger complex, but the signal intensity in the region of the larger complex increases after the addition of anti-Myc antibody (lane 5), not anti-lamin antibody (lane 6). Interestingly, the larger complex is disrupted by competition with WT oligonucleotide (compare lanes 2 and 3), but not mutant oligonucleotide (compare lanes 2 and 4), and may be accounted for by an interaction of ZEB-2 with other cellular proteins.
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The distribution of the two isoforms of CYP4F3 containing exon 4 (4F3A) or exon 3 (4F3B) was examined by reverse transcription-PCR using the same RNA samples that were assayed for ZEB-1 and ZEB-2 (Fig. 7A). We previously showed that 4F3A is selectively expressed in peripheral blood granulocytes, whereas 4F3B is selectively expressed in liver (24). All myeloid cell populations analyzed in this study expressed 4F3A and not 4F3B. This includes bone marrow cells isolated at early (BM-1, CD11b+ CD14) or later (BM-2, CD11b+ CD14+) stages of development and HL60 cells. Conversely, the only isoform detected in CD3+ T cells was 4F3B.
Transcription in Non-myeloid CellsA cluster of sites essential for transcription in HepG2 cells was identified in a region of the hepatic promoter 4583 bp upstream of start site B (Table I). These include a site for the hepatic factor HNF4 (position 155) and two sites for the HNF3/FREAC family of factors (positions 137 and 123), which are active in a variety of tissues including liver and kidney (27). We previously reported (24) the presence of two TATA-like sequences at positions 549 (upstream of myeloid start site A) and 105 (upstream of hepatic start site B). However, mutations at these sites have low or minimal effect on the activity of promoter constructs (Table I), and the alternative promoters in the CYP4F3 gene may, therefore, belong to a class that does not depend on a TATA box. The hepatic and myeloid promoters overlap between positions 710 and 468, and we identified a single-bp mutation at position 632, which selectively reduces transcription in HepG2 cells. The mutation disrupts a consensus site for AP4 (CAGCTG). The location of transcription factor sites shown to be important for CYP4F3 expression in HL60 and HepG2 cells are shown in Fig. 8A.
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Peripheral blood CD3+ T lymphocytes were isolated to high purity (>99%) by flow cytometry, and 5'-RACE was performed on RNA preparations to determine the transcription start site of CYP4F3 in these cells. The sequences of 10 5'-RACE clones were identical and identify a 19-bp 5'-UTR with the sequence AATCTAAGTGCTTACCAGG. This indicates a novel transcription start site (designated start site C) located 219 bp upstream of the ATG initiation codon, intermediate between start sites A and B (Fig. 8A). It is separated from the myeloid transcription start site A by 300 bp, raising the possibility that distinct promoter elements are recruited to enable expression in lymphoid cells. This was not characterized in detail. Transcription initiation from start site C requires splicing to link the 5'-UTR to the ATG initiation codon and is independent of transcription initiation from start site B.
The splicing pathways of CYP4F3 in myeloid cells, liver, and T cells are shown schematically in Fig. 8B. Selection of exon 4 is the critical event that determines CYP4F3 function in myeloid cells. Recently we cloned the CYP4F3 gene (24) and showed that a unique feature of exon 4 is a 25-bp repeat that duplicates the 5'-splice donor sequence (Fig. 8C). This is shown here for comparison with the myeloid 5'-UTR (exon 1), which contains two alternative 5'-splice junctions. The sequence at the 5'-splice junction of exon 4 is an inverted TACCTG, and this is duplicated to create a sequence resembling a bipartite ZEB binding motif. For example, ZEB site selection experiments demonstrate that nucleotide usage at positions 24 of the CACCT(G) consensus is essentially 100%, but that the C at position 1 can be replaced by T in 14% of cases (28). It is, therefore, possible that similar factors bind to these unique elements in the myeloid 5'-UTR and exon 4, a feature that could enable coupling of myeloid transcription and splicing.
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DISCUSSION |
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The presence of CYP4F3 in eosinophils and its strong association with myeloid differentiation in bone marrow have not previously been shown but are consistent with its functional involvement with LTB4 regulation. LTB4 is a chemoattractant for human eosinophils (17), and eosinophils isolated from patients with hypereosinophilic syndromes express abundant cytosolic phospholipase A2, 5-lipoxygenase, and BLT1 (29). Interestingly, an average of 20% CD34+ progenitor cells in the bone marrow exhibit CYP4F3 reactivity (Fig. 2C), and this may be due to populations of early myeloid-committed cells that have been identified in previous studies by coexpression of CD34 and CD11b (30). An early onset of CYP4F3 expression in bone marrow would be consistent with a role in maintaining low basal levels of LTB4 in addition to regulating LTB4 activity during inflammation. CYP4F3 may protect against an inappropriate amplification of LTB4 production as developing myeloid cells express BLT1 and LTB4 biosynthetic enzymes.
We recently cloned the CYP4F3 gene and postulated that alternative promoters regulate transcription in liver and neutrophils because distinct 5'-UTRs are generated in each location (24). Many cytochrome P450s including CYP4F3 are expressed in liver, whereas expression in myeloid cells is unusual. The existence of an alternative promoter for myeloid expression is now confirmed using HL60 cells as a model system. Overlapping but distinct promoter regions were identified in HL-60 and HepG2 cells transfected with luciferase reporter constructs containing 5'-flanking regions of the CYP4F3 gene (Fig. 5). The myeloid promoter identified in HL60 cells extends over a relatively short region (350 bp) upstream of transcription start site A (the myeloid start site), requires exon 1 (50 bp immediately downstream of the start site) for full activity, and a TATA box is not essential. Similar features have been observed in promoters for other myeloid-specific genes (31). The presence of alternative promoters represents another unusual feature of the CYP4F3 gene and is comparable with the situation described for CYP19 (aromatase) expression that has been intensively investigated (32). Transcription of CYP19 is regulated by different promoters in different tissues and results in transcripts with different 5'-UTRs, although the protein encoded by these transcripts is always the same.
The myeloid promoter of CYP4F3 includes potential sites for traditional myeloid transcription factors such as MZF-1 and PU.1, but mutations to these sites did not reduce activity in differentiated HL60 cells. An active region of exon 1 contains a bipartite ZEB site consisting of two inverted CACCT sequences separated by 11 bp (Fig. 6A), and deletion of this region reduces transcriptional activity by 71% (Table I). Interestingly, the location of these inverted CACCT sequences corresponds to the location of two 5'-splice donor sites in the pre-mRNA. A similar bipartite ZEB site is located at position 804, 285 bp upstream of the transcription start site, in another active region of the promoter. Mutagenesis of the site at 804 reduces transcriptional activity by 88% (Table I), and this effect is clearly independent of any potential interference with splicing. Two related ZEB genes have been identified, and both are expressed in hematopoietic cells; ZEB-1 (EF1) is present in T cells, and ZEB-2 (SIP1) is present in B cells (5). Previous studies have focused on the role of ZEB genes in lymphocytes and have not specifically investigated myeloid cells. We used isoform-specific PCR to determine that ZEB-2, but not ZEB-1, is expressed in peripheral blood granulocytes and developing myeloid cells in the bone marrow (Fig. 7A). This is the first indication that ZEB-2 has the capacity to act as a myeloid transcription factor.
ZEB proteins have been characterized primarily as transcriptional repressors that bind to the corepressor CtBP (33). However, ZEB activates vitamin D3 receptor (VDR) gene transcription in colon carcinoma cells, and this activity was confirmed in COS 7 cells transfected with a ZEB expression vector and the VDR promoter (10). We utilized this approach to demonstrate activation of the CYP4F3 myeloid promoter by ZEB and observed a 10 15-fold induction of activity with either ZEB-1 or ZEB-2 if a CtBP expression vector was cotransfected (Fig. 6B). This induction was dependent on the amount of ZEB expression vector used for transfection and could be inhibited by mutagenesis or deletion of the ZEB sites. Activation of the VDR promoter did not require CtBP (10), suggesting that ZEB activity is more sensitive to context in the CYP4F3 promoter. Gel retardation assays demonstrate that ZEB-2 forms a binding complex with CYP4F3 promoter DNA, which is dependent on intact CACCT sites (Fig. 7B). The assays were performed with nuclear extracts from COS 7 cells that were transfected with Myc-tagged ZEB-2. Cotransfection of these cells with CtBP was not required to demonstrate ZEB-2 binding to DNA. The function of CtBP, therefore, seems to be unrelated to DNA binding and more likely involves modification of interactions with other proteins in the promoter complex. The capacity of ZEBs to interact with multiple proteins presents opportunities for CYP4F3 gene regulation. ZEB-2 was originally identified as a Smad-interacting protein and designated SIP1 (34). An inverted Smad3 site (GCCCAGAC) is located at position 629 between the two bipartite ZEB sites in the CYP4F3 promoter, and Smads are activated by transforming growth factor-, which is an active cytokine in myeloid development and inflammation (35, 36).
Recently it was proposed that CtBP can function as a redox sensor for transcription by detecting changes in nuclear NAD+/NADH ratios (37). ZEB-CtBP interactions and transcriptional activity could be enhanced with agents such as CoCl2, which perturb cellular oxidation. Other reports agree that CtBP is regulated by NAD but suggest that NAD+ and NADH are equally effective (38). Total NAD+ and NADH levels may vary in response to cellular energy status or DNA damage, and this may lead to changes in CtBP activity. Both mechanisms of CtBP regulation have the potential to be engaged during inflammation. It is well documented that CtBP acts as a corepressor of a number of transcription factors in addition to ZEBs (9). Our results suggest that CtBP acts as a coactivator in specific settings. Interestingly, Drosophila CtBP can act as a transcriptional corepressor or coactivator in a context-dependent manner (39). It is possible that ZEB/CtBP represses transcription from the myeloid promoter of CYP4F3 in certain cell types such as lymphocytes or that it represses transcription of other genes in myeloid cells.
A small population of CD3+ T cells (10%) expresses CYP4F3 in bone marrow and peripheral blood, and this appeared to contradict the conclusion that CYP4F3 expression is restricted to hematopoietic cells of myeloid lineage by a myeloid-specific promoter. However, 5'-RACE analysis identified a novel transcription start site in these cells located at position 219 (designated start site C). A distinct lymphoid promoter region may, therefore, be involved. We have previously described two isoforms of CYP4F3 that are generated by alternative splicing (25). Selection of exon 4 generates CYP4F3A, which has a low Km for LTB4 of <1 µM. Selection of exon 3 generates CYP4F3B, which has a preference for arachidonic acid as a substrate, which it converts to 20-HETE. Isoform-specific PCR demonstrated that CYP4F3A is the isoform expressed in myeloid cell populations detected by anti-CYP4F3 in flow cytometry experiments (Fig. 7A). In contrast, the isoform detected in CD3+ T cells was CYP4F3B. LTB4 is functionally active on some lymphocyte populations (40, 41), but generation of 20-HETE by CYP4F3B in T cells may be of greater importance than inactivation of LTB4. The ability of 20-HETE to activate MAP kinase pathways and stimulate cell proliferation (42, 43) could have implications with respect to T cell activation or oncogenesis.
Selection of exon 4 is the critical event that determines CYP4F3 function in myeloid cells (25). CYP4F2 is the only other member of the human CYP4F family to contain homologues of both exons 3 and 4 (other characterized members contain a homologue of exon 3 only), but the latter does not appear to be used as a substrate for splicing. This is unexpected because the CYP4F2 and CYP4F3 genes are highly related. A unique feature of CYP4F3 is a 25-bp repeat that duplicates the 5'-splice donor sequence of exon 4 (Fig. 8C), and this is similar to exon 1 (the myeloid 5'-UTR), where two alternative 5'-splice donor sequences correspond with the location of a bipartite ZEB site. The ZEB transcription factors have a number of unusual features; they can act as transcriptional repressors or activators (6, 7, 8, 10); they interact with multiple proteins including CtBP and Smads (33, 34); they bind with two zinc finger regions to two target sites (4); they can act over long distances and may serve as bridging molecules (44). It is interesting to speculate that they might also play a role in splicing. There is growing evidence that transcription and splicing are coupled in many genes (45), and this would have particular relevance to myeloid CYP4F3 expression and its regulation in oxidative stress and inflammation.
Expression of the CYP4F3 gene involves alternative choices at the level of transcription initiation, transcription termination, and splicing. Alternative splicing of CYP4F3 represents the first recorded example of functionally different enzymes being encoded within a single CYP gene (25); substrate specificity is determined by selection of exon 4 (CYP4F3A) or exon 3 (CYP4F3B). 3'-RACE analysis has identified two transcription termination signals that generate transcripts of 5.034 and 2.339 kb (24, 46), although it is not yet clear whether these transcripts are differentially distributed. In this report we show that alternative promoters regulate CYP4F3 expression in myeloid and hepatic cells. A distinct transcription start site in lymphoid cells suggests that there are at least three tissue-specific promoters in the gene. Regulation of CYP4F3 gene expression is clearly complex and must involve coordinated actions at all levels of mRNA processing to generate the correct isoform in specific locations.
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FOOTNOTES |
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To whom correspondence should be addressed: Renal Unit, Massachusetts General Hospital East, 149 The Navy Yard, 13th St., Charlestown, MA 02129. Tel.: 617-724-4336; Fax: 617-726-5669; E-mail: christma{at}helix.mgh.harvard.edu.
1 The abbreviations used are: CtBP, C-terminal binding protein; CYP, cytochrome P450; LTB4, leukotriene B4; HETE, hydroxyeicosatetraenoic acid; UTR, untranslated region; MPO, myeloperoxidase; RACE, rapid amplification of cDNA ends; WT, wild type; HNF, hepatocyte nuclear factor.
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ACKNOWLEDGMENTS |
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REFERENCES |
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