Mouse Erythropoietin Receptor Research Articles

Single methyl groups can act as toggle switches to specify transmembrane Protein-protein interactions.

Transmembrane domains (TMDs) engage in protein-protein interactions that regulate many cellular processes, but the rules governing the specificity of these interactions are poorly understood. To discover these principles, we analyzed 26-residue model transmembrane proteins consisting exclusively of leucine and isoleucine (called LIL traptamers) that specifically activate the erythropoietin receptor (EPOR) in mouse cells to confer growth factor independence. We discovered that the placement of a single side chain methyl group at specific positions in a traptamer determined whether it associated productively with the TMD of the human EPOR, the mouse EPOR, or both receptors. Association of the traptamers with the EPOR induced EPOR oligomerization in an orientation that stimulated receptor activity. These results highlight the high intrinsic specificity of TMD interactions, demonstrate that a single methyl group can dictate specificity, and define the minimal chemical difference that can modulate the specificity of TMD interactions and the activity of transmembrane proteins.

Solution structure of the transmembrane domain of the mouse erythropoietin receptor in detergent micelles.

Erythropoiesis is regulated by the erythropoietin receptor (EpoR) binding to its ligand. The transmembrane domain (TMD) and the juxtamembrane (JM) regions of the EpoR are important for signal transduction across the cell membrane. We report a solution NMR study of the mouse erythropoietin receptor (mEpoR) comprising the TMD and the JM regions reconstituted in dodecylphosphocholine (DPC) micelles. The TMD and the C-terminal JM region of the mEpoR are mainly α-helical, adopting a similar structure to those of the human EpoR. Residues from S216 to T219 in mEpoR form a short helix. Relaxation study demonstrates that the TMD of the mEpoR is rigid whilst the N-terminal region preceding the TMD is flexible. Fluorescence spectroscopy and sequence analysis indicate that the C-terminal JM region is exposed to the solvent. Helix wheel result shows that there is hydrophilic patch in the TMD of the mEpoR formed by residues S231, S238 and T242, and these residues might be important for the receptor dimerization.

Abstract 908: Development of a cell proliferation assay to be used as a read-out system for determining the in vivo potency of bevacizumab in neutralizing the biological activity of VEGF in cancer patients

Abstract INTRODUCTION: Blocking of vascular endothelial growth factor (VEGF) by the monoclonal antibody bevacizumab or by VEGF-Trap is a validated clinical approach to block tumor angiogenesis. For these anti-angiogenic therapeutics there is still a major lack of knowledge on biological determinants that are predictive for clinical benefit in cancer patients. We reasoned that a test that would assess the inhibition of the biological activity of VEGF by bevacizumab in the patients’ plasma or serum during therapy would serve as a biomarker for response. We tested such an approach by measuring the inhibition of VEGF-dependent cell proliferation by patients’ serum using the Ba/F3-R2 cell line, which is a murine pre-B lymphocyte cell line stably transfected with a fusion protein, consisting of the extracellular domain of hVEGFR2 and the transmembrane and cytoplasmic domain of the mouse erythropoietin receptor. When grown in medium devoid of mIL-3, this cell line is solely dependent on VEGF for proliferation and survival. METHODS: Ba/F3-R2 cells (licensed by the Ludwig Institute for Cancer Research, New York, NY) were seeded at a density of 200.000 cells/mL and cultured for 72 hours in the presence of either A)medium containing hVEGF pre-incubated for 1 hour with various concentrations of bevacizumab (Avastin, Roche /Genentech) or B) naïve serum or serum ( 5 or 10%) collected after one cycle of bevacizumab from 7 patients with metastatic colorectal cancer that was pre-incubated with VEGF. Serum VEGF concentrations of these patients were previously determined by ELISA Cell proliferation was subsequently quantified with the cell proliferation agent WST-1 (Roche). RESULTS: The optimal VEGF concentration to study bevacizumab-mediated modulation of VEGF dose-dependent proliferation of Ba/F3-R2 cells was 1.25 ng/ml. At this concentration, Ba/F3-R2 cell proliferation was inhibited for 50-80% using a clinically effective bevacizumab concentration of 10 μg/mL. Co-incubation of VEGF with naïve serum samples did not have any inhibitory effect on Ba/F3-R2 cell proliferation, while 30-90% inhibition of Ba/F3-R2 cell proliferation was observed when VEGF was co-incubated with serum of 7 patients who had received one cycle of bevacizumab treatment. Furthermore, a significant negative correlation (R2 = 0.85, p = 0.0030) was found between free -VEGF concentrations in serum after one cycle of treatment with bevacizumab and proliferation inhibition of Ba/F3-R2 cells. CONCLUSIONS: We have developed an assay that can be used to reproducibly determine the efficacy of bevacizumab in neutralizing the biological activity of VEGF both in vitro and in patient-derived serum samples before and during treatment with bevacizumab. We propose that this assay could serve as a potential biomarker to predict response to bevacizumab and possibly other agents that target VEGF. Citation Format: Madelon Q. Wentink, Henk J. Broxterman, Tanja D. de Gruijl, Roberto Pili, Hans J. van der Vliet, Arjan W. Griffioen, Henk M.W. Verheul. Development of a cell proliferation assay to be used as a read-out system for determining the in vivo potency of bevacizumab in neutralizing the biological activity of VEGF in cancer patients. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 908. doi:10.1158/1538-7445.AM2014-908

Re-Engineering Erythropoietin as an IgG Fusion Protein That Penetrates the Blood−Brain Barrier in the Mouse

Erythropoietin (EPO) is a potent neuroprotective agent that could be developed for the treatment of multiple brain disorders. However, EPO does not cross the blood-brain barrier (BBB). A brain-penetrating form of EPO, specific for the mouse, was engineered by fusion of the 166 amino acid EPO to the carboxyl terminus of the heavy chain of a chimeric monoclonal antibody (mAb) against the mouse transferrin receptor (TfR), and this new fusion protein is designated cTfRMAb-EPO. The fusion protein was expressed in stably transfected Chinese hamster ovary cells and purified by protein G chromatography. The fusion protein was homogeneous on SDS-PAGE and Western blotting, and bound the mouse EPO receptor (EPOR) with high affinity, ED50 = 0.33 ± 0.04 nM. The cTfRMAb-EPO fusion protein was radiolabeled by tritiation and injected intravenously (iv) into adult mice for measurements of the plasma pharmacokinetics and brain uptake. The (3)H-fusion protein was rapidly cleared from the blood with a clearance rate of 5.9 ± 0.3 mL/min/kg, which is 14-fold faster than the clearance of EPO in the mouse. The cTfRMAb-EPO fusion protein penetrated the BBB in vivo, as shown by the capillary depletion method. The brain uptake of the cTfRMAb-EPO fusion protein was 2.0 ± 0.1% injected dose/g of brain following iv administration. The high level of brain uptake of the fusion protein enables pharmacologic increases in exogenous EPO in the mouse brain following the systemic injection of the cTfRMAb-EPO fusion protein. In conclusion, EPO has been re-engineered as an IgG fusion protein that binds dual receptors: the mouse TfR, to enable penetration of the BBB, and the mouse EPOR, to produce neuroprotection in brain behind the intact BBB.

Erythropoietin receptor signaling regulates both erythropoiesis and megakaryopoiesis in vivo

Transgenic expression of a gain-of-function truncated mouse erythropoietin receptor gene ( EpoR) leads to expansion of the HSC pool in response to human erythropoietin (Epo). We have re-examined this observation using a knock-in mouse model, wherein the mouse EpoR gene was replaced in its proper genetic locus by a single copy of either a wild-type human or a polycythemia-inducing truncated human EPOR gene. Bone marrow cells obtained from knock-in mice were transplanted together with competitor bone marrow cells in a model that allows tracking of erythroid, platelet, and leukocyte contributions by each genotype. Secondary transplants were also performed. Stem/progenitor cells were identified phenotypically and isolated for colony-forming assays to evaluate cytokine responsiveness by cells with the wild-type human or truncated human EPOR gene. Augmented Epo signaling increased erythroid repopulation post-transplant as expected, but had no effect on short-term or long-term leukocyte repopulation. However, the wild-type human EPOR knock-in mouse showed decreases in both erythroid and platelet repopulation compared to marrow cells from the mutant human EPOR knock-in mouse or normal B6 animals. These results provide evidence supporting a role for Epo signaling in megakaryopoiesis in vivo and suggest a role for Epo signaling early in hematopoietic development.

Point Mutation at Single Tyrosine Residue of Novel Oncogene NOK Abrogates Tumorigenesis in Nude Mice

Receptor protein-tyrosine kinases (RPTKs) are tightly regulated during normal cellular processes including cell growth, differentiation, and metabolism. Recently, a RPTK-like molecule named novel oncogene with kinase-domain (NOK) has been cloned and characterized. Overexpression of NOK caused severe cellular transformation as well as tumorigenesis and metastasis in nude mice. In the current study, we generated two tyrosine-->phenylalanine (Y-->F) point mutations (Y327F and Y356F) within the endodomain of NOK that are well conserved in many RPTK subfamilies and are the potential tyrosine phosphorylation sites important for major intracellular signaling. Using BaF3 cells stably expressing the ectodomain of mouse erythropoietin receptor, and the transmembrane and endodomain of NOK (BaF3-E/N), we were able to show that point mutations at either Y327 or Y356 dramatically blocked cellular transformation by NOK as examined by colony formation and cellular DNA synthesis. In addition, tumorigenesis induced by BaF3-E/N was completely abrogated upon the introduction of either single mutation. Importantly, signaling studies revealed that the activation of extracellular signal-regulated kinase was inhibited by Y356F and was significantly reduced by Y327F. Both mutations significantly impaired Akt phosphorylation. Interestingly, both mutations did not affect the kinase activity of NOK. Moreover, apoptotic analysis revealed that both mutations accelerated cell death by activating caspase-3-mediated pathways. Thus, our study shows that these potential tyrosine phosphorylation sites may play critical roles in NOK-mediated tumorigenesis both in vitro and in vivo.

Naturally Occuring N-Terminal Truncated STAT5 Isoforms Are Widely Expressed, Functional, and Have Enhanced Nuclear Localization.

Signal transducer and activator of transcription 5(STAT5) plays major roles in hematopoietic stem cell and progenitor cell development. Our previous studies have shown that mast cells derived from STAT5ab−/− mice in which the first two exons were deleted, had severe defects in interleukin (IL)-3 and stem cell factor (SCF) induced proliferation and survival signaling. However, expression of an endogenous hypomorphic N-terminal truncated form of STAT5 through cryptic start codons found at position 102 and 136 was uncovered, suggesting the possibility of compensation for STAT5-mediated functions. ΔN isoforms of STAT5 were also identified in different wild-type and STAT5ab−/− mouse tissues including significant expression in brain, liver, spleen, lung, kidney, but not in heart. Here we describe that endogenous mast cell STAT5ΔN had surprisingly increased nuclear localization in STAT5ab−/− mast cells even without cytokine stimulation but which increased in response to IL-3 and SCF. In general, the N-terminus is responsible for tetramerization and regulates receptor recognition and phosphatase recruitment. Others have reported that N-terminal truncated mutants of STAT1 and STAT4 failed to migrate into nuclei upon cytokine stimulation. Therefore, the N-terminus of STAT5 functions differently. To further characterize this function of STAT5aΔN136, we co-transfected STAT5aΔN136 with mouse erythropoietin receptor (EPOR) into 293T cells, and subsequently, stimulated the cells with EPO (20 μg/ml). Consistent with the mast cell studies, immunostaining revealed that STAT5aΔN136 had enhanced nuclear retention relative to full-length STAT5a. Both STAT5a and STAT5aΔN136 migrated to the nucleus upon EPO stimulation for 30 minutes. EPO stimulation also showed a similar pattern of phosphorylation, however, STAT5aΔN136 phosphorylation was weaker and more transient, compared to full-length STAT5a. A known constitutively active STAT5aS711F mutant used as a control, showed extremely high and persistent phosphorylation levels. To test for functional activity in mast cells, we used MSCV-based retroviral vectors expressing either STAT5a or STAT5aΔN136 upstream of GFP and transduced STAT5ab−/− mast cells. Initial transduction efficiencies of 20–40% were obtained with all vectors. After 10–25 days of culture, %GFP+ cells rose to 85–100% for both STAT5a and STAT5aΔN136, while the mast cells transduced with an MSCV control vector remained about 20% GFP+. Notably, in mast cells and GP+E86 ecotropic producer cells, overexpession of retroviral full-length STAT5a resulted in detectable STAT5aΔN136 co-expression from the same vector, indicating that effects due to STAT5 expression are potentially modulated by ΔN isoforms generated through alternative internal start codons. Overall, this work indicates that ΔN isoforms of STAT5 are capable of enhanced nuclear localization and can promote growth in STAT5ab−/− mast cells, suggesting that they likely participate in normal signaling functions. Studies to assess transcriptional activity of these isoforms using STAT promoter luciferase constructs is ongoing. A better understanding of the regulation of these natural isoforms of STAT5a and STAT5b and their physiological significance may lead to novel insights into JAK/STAT signaling.

Erythropoietin Signaling Inhibits the Short-Term Marrow Reconstitution.

Murine hematopoietic stem cells (HSCs) transfected with gain-of-function human erythropoietin receptor (EPOR) transgene were reported to have a competitive advantage over wild type mouse HSCs in a bone marrow transplantation (BMT) model (Kirby, Blood ; 95(12):3710, 2000). However, EpoRs may not be normally expressed in early progenitor/stem cells, moreover, whether the Epo/EpoR signaling plays a role on homing and repopulating of hematopoietic progenitor/stem cells is also unknown. Our lab has previously created a mouse model harboring either a knocked-in human wild type (wthEPOR) hypomorphic gene or a mutant human gain-of-function EPOR (mthEPOR) gene into the mouse EPOR locus (Divoky, PNAS ; 98(3):986, 2001). The wthEPOR mice are anemic, while mthEpoR mice are polycythemic. We tested the possible advantage of mthEPOR HSCs in the competitive bone marrow (BM) transplantation assay using C57/Bl6 congenic mice. BM from wthEPOR (CD45.1/45.2 heterozygous or CD45.1) and mthEPOR (CD45.2) mice were co-transplanted (1:1 ratio) into lethally irradiated (137Cs >11Gy split) normal recipients (CD45.1 or CD45.1/45.2). Unexpectedly, in three independent groups the peripheral blood chimerism derived from wthEPOR-bearing cells significantly out-competed the mthEPOR-bearing cells two weeks after BMT (52% vs. 40%, 52% vs. 45% and 50% vs. 42%, n=11, 8 and 6, p<0.01). Furthermore, administration of exogenous Epo (10U/day/mouse) further impaired peripheral blood engraftment (including Ter119+ erythroid lineages) of mthEPOR cells (45% vs. 39%, n=8, p<0.05) though the relative proportion of erythroid cells within the engraftment was increased. Thus, an enhanced Epo/EpoR signaling interferes with the short-term repopulation of hematopoietic progenitors resulting in a decreased engraftment, possibly by interfering with homing. A possible long-term advantage of mthEPOR HSCs is being evaluated in ongoing studies; however, these short term data suggest that Epo administration to BM transplant patients may impede engraftment. [Display omitted]

A Mutant Form of Vascular Endothelial Growth Factor (VEGF) That Lacks VEGF Receptor-2 Activation Retains the Ability to Induce Vascular Permeability

Vascular endothelial growth factor (VEGF) is a major mediator of vasculogenesis and angiogenesis both during development and in pathological conditions. VEGF has a variety of effects on vascular endothelium, including the ability to stimulate endothelial cell mitogenesis, and the potent induction of vascular permeability. These activities are at least in part mediated by binding to two high affinity receptors, VEGFR-1 and VEGFR-2. In this study we have made mutations of mouse VEGF in order to define the regions that are required for VEGFR-2-mediated functions. Development of a bioassay, which responds only to signals generated by cross-linking of VEGFR-2, has allowed evaluation of these mutants for their ability to activate VEGFR-2. One mutant (VEGF0), which had amino acids 83-89 of VEGF substituted with the analogous region of the related placenta growth factor, demonstrated significantly reduced VEGFR-2 binding compared with wild type VEGF, indicating that this region was required for VEGF-VEGFR-2 interaction. Intriguingly, when this mutant was evaluated in a Miles assay for its ability to induce vascular permeability, no difference was found when compared with wild type VEGF. In addition we have shown that the VEGF homology domain of the structurally related growth factor VEGF-D is capable of binding to and activating VEGFR-2 but has no vascular permeability activity, indicating that VEGFR-2 binding does not correlate with permeability activity for all VEGF family members. These data suggest different mechanisms for VEGF-mediated mitogenesis and vascular permeability and raise the possibility of an alternative receptor mediating vascular permeability.

Sequence Flexibility in the Polytropic env gp70-Derived Region of the Membrane Glycoprotein (gp55) of Friend Spleen Focus-Forming Virus Affects Its Biological Activity

We previously reported (N. Watanabe, M. Nishi, Y. Ikawa, and H. Amanuma, J. Virol. 65:132-137, 1991) that the mutant Friend spleen focus-forming virus (F-SFFV(MS)), which encodes a mutant gp55 membrane glycoprotein with an ecotropic env gp70 sequence, was nonpathogenic. Here we injected the F-SFFV(MS)-Friend murine leukemia virus (F-MuLV) clone 57 complex into newborn DBA/2 mice. We obtained four groups of pathogenic variant F-SFFV complexes, each showing a different degree of pathogenicity in adult mice and a different gp55 profile. Of these, group 1 variant F-SFFV was particularly interesting, because it was the most frequently obtained and because it produced doublet bands of gp55 (59 and 57 kDa), neither of which reacted with the nonecotropic gp70-specific monoclonal antibody, and because its DNA intermediate did not hybridize with the nonecotropic env-specific probe. Cloning and DNA sequence analysis of the env region of one isolate of the group 1 variant F-SFFV revealed that this virus consisted of two distinct F-SFFV genomes; one (clone 117) differed from the other (clone 118) due to the presence of a 39-bp in-frame deletion. Reconstitution to full-length F-SFFV genomes and a pathogenicity assay showed that each reconstituted F-SFFV was pathogenic, with clone 117 showing a higher degree of pathogenicity than clone 118. Both reconstituted F-SFFVs caused activation of the mouse erythropoietin receptor in the factor-independent cell proliferation assay, although much less efficiently than the wild-type polycythemia-inducing isolate F-SFFVp. Clone 118 produced a gp55 of 59 kDa, while clone 117 produced one of 57 kDa. Clone 118 had a substitution by the F-MuLV clone 57 gp70 sequence, indicating that it was derived from the F-SFFV(MS) env gene by a homologous recombination with the F-MuLV clone 57 env gene. The site of the 39-bp deletion in clone 117 corresponded to the portion of the clone 118 sequence which was unique to the ecotropic env genes. These results indicated the importance for the biological activity of gp55 of the sequences in the gp70 differential region, which are contained in both polytropic and ecotropic env genes.

Secretory production of erythropoietin and the extracellular domain of the erythropoietin receptor by Bacillus brevis: affinity purification and characterization.

Bacillus brevis secretes a large amount of cell wall proteins into the culture medium. For construction of Bacillus brevis expression-secretion vectors of human erythropoietin (EPO) and the extracellular domain of mouse erythropoietin receptor (sEPOR), cDNA for each mature form was inserted into a plasmid containing the promoter region and the signal-peptide encoding region of a cell wall protein. Culture supernatants of transformants were affinity purified using a monoclonal antibody-fixed gel for EPO and and EPO-fixed gel for sEPOR. The affinity purification efficiently removed unwanted proteins, giving samples with sufficiently high purity to analyze amino acid sequences of N-terminal regions and biological activities. Combination of this secretory production and affinity purification may facilitate isolation of a large amount of pure EPO and sEPOR, and is useful for further understanding the molecular mechanism of interaction between EPO and EPOR.

Anti-Erythropoietin Receptor Monoclonal Antibody: Epitope Mapping, Quantification of the Soluble Receptor, and Detection of the Solubilized Transmembrane Receptor and the Receptor-Expressing Cells

A hybridoma cell line producing the monoclonal antibody against erythropoietin receptor (EpoR) was established using the soluble ectodomain of mouse erythropoietin receptor (sEpoR) as an antigen. The monoclonal antibody termed 1G3 bound to the denatured sEpoR. Epitope mapping with peptide library revealed that 1G3 recognized the amino terminal region including the hexapeptide (positions 6 to 11; LeuProAspProLysPhe). The amino acid sequence in this hexapeptide was identical in mice, rats, and humans, and therefore 1G3 bound to EpoR from all of these sources. Using 1G3, we evaluated sEpoR by a sandwich enzyme-linked immunoassay, and EpoR in the solubilized membrane preparation was detected by Western blotting. The cells expressing EpoR were identified with immunochemical staining. We confirmed the presence of EpoR in a neuronal cell line and PC12 cells, and EpoR was expressed in primary cultured hippocampal neurons.

Chimeric tumor necrosis factor receptors with constitutive signaling activity.

Many hormone and cytokine receptors are crosslinked by their specific ligands, and multimerization is an essential step leading to the generation of a signal. In the case of the tumor necrosis factor (TNF) receptors (TNF-Rs), antibody-induced crosslinking is sufficient to trigger a cytolytic effect. However, the quaternary structural requirements for signaling--i.e., the formation of dimers, trimers, or higher-order multimers--have remained obscure. Moreover, it has not been clear whether the 55-kDa or 75-kDa TNF-R is responsible for initiation of cytolysis. We reasoned that an obligate receptor dimer, targeted to the plasma membrane, might continuously signal the presence of TNF despite the actual absence of the ligand. Such a molecule, inserted into an appropriate vector, could be used to project receptor-specific "TNF-like" activity to specific cells and tissues in vivo. Accordingly, we constructed sequences encoding chimeric receptors in which the extracellular domain of the mouse erythropoietin receptor (Epo-R) was fused to the "stem," transmembrane domain, and cytoplasmic domain of the two mouse TNF-Rs. Thus, the Epo-R group was used to drive dimerization of the TNF-R cytoplasmic domain. These chimeric proteins were well expressed in a variety of cell lines and bound erythropoietin at the cell surface. Both the 55-kDa and the 75-kDa Epo/TNF-R chimeras exerted a constitutive cytotoxic effect detected by cotransfection or clonogenic assay. Thus, despite the lack of structural homology between the cytoplasmic domains of the two TNF-Rs, a similar signaling endpoint was observed. Moreover, dimerization (rather than trimerization or higher-order multimerization) was sufficient for elicitation of a biological response.

Structure and Transcription of the Mouse Erythropoietin Receptor Gene

The complete gene encoding the mouse erythropoietin receptor was isolated by using a cDNA probe derived from a mouse erythroleukemia (MEL) cell library. The gene spans approximately 5 kilobases and is present in a single copy per haploid genome. It contains eight exons, and the nucleotide sequence of the coding region from the genomic DNA is identical to the sequence of one of the MEL cDNA clones except for a single amino acid substitution (Leu----Val) at codon 163. There is a cluster of three major transcriptional start sites approximately 150 nucleotides upstream of the initiator ATG codon which is conserved in erythropoietin-dependent and -independent erythroleukemic cells, in MEL cells at different stages of differentiation, and in normal bone marrow cells. The promoter region contains a potential binding site for Sp1, erythroid-specific transcription factor GF-1, and several CACCC boxes, but not typical TATA or CAAT sequences. A fusion gene containing 452 nucleotides of 5' noncoding sequence linked to a promoterless human growth hormone gene directed the transcription of the latter in MEL cells but not in mouse fibroblasts, T cells, B cells, or macrophagelike cells, suggesting that this promoter functions in an erythroid-specific manner.

Structure and transcription of the mouse erythropoietin receptor gene.

The complete gene encoding the mouse erythropoietin receptor was isolated by using a cDNA probe derived from a mouse erythroleukemia (MEL) cell library. The gene spans approximately 5 kilobases and is present in a single copy per haploid genome. It contains eight exons, and the nucleotide sequence of the coding region from the genomic DNA is identical to the sequence of one of the MEL cDNA clones except for a single amino acid substitution (Leu----Val) at codon 163. There is a cluster of three major transcriptional start sites approximately 150 nucleotides upstream of the initiator ATG codon which is conserved in erythropoietin-dependent and -independent erythroleukemic cells, in MEL cells at different stages of differentiation, and in normal bone marrow cells. The promoter region contains a potential binding site for Sp1, erythroid-specific transcription factor GF-1, and several CACCC boxes, but not typical TATA or CAAT sequences. A fusion gene containing 452 nucleotides of 5' noncoding sequence linked to a promoterless human growth hormone gene directed the transcription of the latter in MEL cells but not in mouse fibroblasts, T cells, B cells, or macrophagelike cells, suggesting that this promoter functions in an erythroid-specific manner.