Activation of the vitamin D receptor transcription factor stimulates the growth of definitive erythroid progenitors

Erythropoiesis is the process by which red blood cell (RBC) progenitors are produced and undergo terminal differentiation to erythrocytes. During ontogeny, the earliest erythroid progenitors emerge transiently in the yolk sac and differentiate into “primitive” erythroid cells (EryP).^1,2  Two sequential waves of definitive erythroid (EryD) progenitors have been identified: the first wave is transient and yolk sac-derived,³  and the second is stable and generated from hematopoietic stem cells in the fetal liver (FL).^4,5  Within 2 to 3 days after the onset of FL erythropoiesis, EryD progenitors far outnumber EryP in the circulation.⁶  Toward the end of gestation, hematopoietic stem cells from the FL migrate to the bone marrow (BM), which becomes and remains the primary site of hematopoiesis, including the production and differentiation of erythroid progenitors, throughout postnatal life (for recent reviews, see Dzierzak and Philipsen⁷  and Barminko et al⁸ ).

Definitive erythropoiesis produces more than 10¹¹ RBCs per day from erythroid progenitors.⁹  Two distinct EryD progenitors have been defined functionally, using colony assays: the earlier burst-forming unit–erythroid (BFU-E) progenitor undergoes a limited number of self-renewal divisions before giving rise to more mature colony-forming unit–erythroid (CFU-E) progenitors (reviewed in Dzierzak and Philipsen⁷  and Hattangadi et al¹⁰ ). CFU-E maturation is accompanied by 5 to 6 cell divisions that result in the sequential formation of 5 morphologically distinct populations of erythroblasts.¹¹  With each successive division, the cells accumulate hemoglobin and decrease in size; within their nuclei, loss of nucleoli and chromosomal condensation are observed.^2,7,10  Erythroblasts eventually enucleate to form reticulocytes that undergo additional steps in maturation, including clearance of mitochondria and other organelles.^12,13  Flow cytometric strategies have been developed to phenotypically identify these distinct stages of erythroid maturation in the mouse.^11,14-16 

The basal rate of erythropoiesis is determined by the growth and survival of CFU-E progenitors, which are regulated largely by erythropoietin (EPO) and its receptor.^10,17,18  Under conditions of stress such as anemia, acute blood loss, or hypoxia, RBC production must increase above the basal rate, with increased self-renewal of BFU-E in response to EPO and a number of other hormones and cytokines (reviewed in Paulson et al¹⁹  and Socolovsky²⁰ ). The pathways that regulate the growth of early erythroid progenitors remain poorly understood.

In a computational search for transcription factor genes that are differentially regulated in the primitive (embryonic)¹  and definitive (fetal, adult)^21,22  erythroid lineages, we observed that the vitamin D receptor (Vdr) nuclear transcription factor gene is expressed at the fetal and adult, but not the embryonic, stage of erythroid ontogeny. Vdr is related to the glucocorticoid, estrogen, androgen, and other steroid hormone receptors (reviewed in Levin²³ ). Binding of its hormone ligand, vitamin D3, results in formation of heterodimers of Vdr and retinoic acid X receptor (RXR) that translocate into the nucleus.²⁴  Heterodimerization permits high-affinity binding to vitamin D response elements within Vdr target genes.²⁴  The Vdr signaling pathway has been best studied in bone, the immune system, and skin (reviewed in Bouillon et al²⁵ ). Studies on the response to vitamin D signaling in erythropoiesis have employed mostly erythroleukemia cell lines (eg, Moore et al,²⁶  Waki et al,²⁷  and Alon et al²⁸ ) and have reached conflicting conclusions. In one study, calcitriol was reported to inhibit erythroid differentiation in human K562 and mouse erythroleukemia cells, but no effect on proliferation was detected.²⁶  In a second study of human cord blood and TF1 cells, proliferation was enhanced by treatment with calcitriol, but erythroid differentiation was not evaluated.²⁸  In yet a third study, both the proliferation and differentiation of mouse erythroleukemia cells were inhibited by calcitriol.²⁷  Improvement in the anemia associated with chronic renal failure has been reported and may involve an increase in the numbers of BFU-Es.²⁹  The role of this pathway in erythropoiesis remains poorly understood.

Here, we report that activation of Vdr by calcitriol, the most biologically active metabolite of vitamin D3,³⁰  stimulates the growth of mouse FL and BM erythroid progenitors, resulting in a large increase in the numbers of mature red blood cells. Vdr is expressed in EryD progenitors and is downregulated during erythroid maturation. The increase in proliferation of progenitors results, at least in part, from maintenance of erythroid progenitor potential and from delayed maturation. The early progenitor CD71^lo/neg, but not the late progenitor-containing CD71^hi population of Lin^neg cKit⁺ cells, is responsive to calcitriol, independent of its calcemic effects. Vdr and the glucocorticoid receptor can cooperate to regulate the proliferation of early progenitors. Lentiviral shRNA-mediated knockdown of Vdr abrogates the stimulation of early progenitor growth by calcitriol. These findings demonstrate that Vdr has an intrinsic function in early erythroid progenitors and may allow the identification of novel targets for therapeutic intervention. Activation of the Vdr pathway offers a new approach for the expansion of EryD progenitors ex vivo.

Detailed experimental procedures are described in the supplemental Materials.

FL (embryonic day 12.5 [E12.5] unless otherwise indicated) or BM (6-8 weeks old) cells were isolated from CD-1 mice (Charles River Laboratories) as described.^31-33  Lineage depletion of single-cell suspensions was performed using a Lineage Cell Depletion Kit (Miltenyi Biotec Inc.) according to the manufacturer’s instructions. The Mount Sinai School of Medicine Institutional Animal Care and Use Committee approved this study.

To expand erythroid progenitors, cells were cultured (37°C, 5% CO₂) in serum-free progenitor medium (PM): StemSpan SFEM (Stem Cell Technologies) supplemented with human recombinant EPO (0.5 units/mL; Amgen), mouse stem cell factor (100 ng/mL; Thermo Fisher Scientific), mouse insulin-like growth factor (40 ng/mL; Thermo Fisher Scientific), lipid concentrate (1×; Thermo Fisher Scientific), penicillin/streptomycin (1%; Pen/Strep; Thermo Fisher Scientific), and dexamethasone (10⁻⁶ M; Sigma; D2915).

Erythroid maturation was initiated by culturing cells in suspension (2.5 × 10⁵/mL, 37°C, 5% CO₂) in maturation medium (MM): IMDM (Corning) supplemented with recombinant human EPO (2 U/mL), mouse stem cell factor (100 ng/mL), knockout serum replacement (10%; Thermo Fisher Scientific), fetal bovine plasma-derived serum (5%; Animal Technologies), protein-free hybridoma medium II (10%; Thermo Fisher Scientific), glutamine (1×; Thermo Fisher Scientific), and penicillin/streptomycin (1%).³⁴  Cell density was measured daily. At cell concentrations 4.0 × 10⁶/mL or higher, the cultures were split into 1.0 × 10⁶ cells/mL. Culture medium was supplemented with calcitriol (100 nM, or as indicated; Sigma), calcipotriol (100 nM or as indicated; Cayman Chemical), or dexamethasone (100 nM).

To examine Vdr expression during ontogeny and at different stages of erythroid maturation, RNA was isolated from whole or lineage-depleted (Lin^neg) BM or FL, maturing Ter119⁺ FL erythroblasts, and progenitor stage (E8.5) yolk sac EryP^1,32,35  and analyzed using quantitative reverse transcription polymerase chain reaction. Vdr mRNA was detected in both whole and Lin^neg BM and FL, but not in maturing Ter119⁺ FL erythroblasts or in EryP (Figure 1A). In our earlier microarray analysis of developing primitive erythroid cells, Vdr was also not expressed in EryP progenitors (E7.5-8.5) or erythroblasts (E9.5-12.5).¹ 

Expression of Vdr was higher in the progenitor-enriched Lin^neg population than in unfractionated tissue from BM or FL (Figure 1A). To determine whether activation of Vdr signaling influences progenitor potential, Lin^neg E12.5 FL cells were plated in clonogenic assays in the presence or absence of calcitriol and scored for the formation of BFU-E and CFU-E colonies. The numbers of BFU-E were increased by nearly 4-fold when cells were cultured with calcitriol (Figure 1B), suggesting that calcitriol enhances the growth and/or survival of progenitors. Comparable increases in BFU-E numbers were observed for Lin^neg cells isolated from E13.5 or E14.5 FL (Figure 1B). Not only the numbers of BFU-E, but also of CFU-E, were increased (by ∼40%; Figure 1 C). The response to Vdr signaling was not limited to fetal erythropoiesis, as BFU-E and CFU-E colony numbers from calcitriol-treated Lin^neg BM cells were also increased (Figure 1D). We have not detected differences between male and female BM cells in these assays (Figure 1B,D). The response to calcitriol stimulation was dose dependent, as shown for CFU-E from BM (supplemental Figure 1C-D).

For these studies, we focused on the Lin^neg fraction of FL, which is the major site of definitive erythropoiesis before birth, and in which the majority of hematopoietic progenitors is committed to the erythroid lineage. To dissect the mechanisms by which calcitriol increases progenitor numbers, we next expanded progenitors in liquid medium before plating in methylcellulose. When E12.5 Lin^neg FL cells were cultured for 2 days under progenitor conditions (in PM), cell numbers were increased by ∼3-fold (Figure 2A). Before lineage depletion, the majority of FL cells were Ter119⁺ (∼70%), and a minority were ckit⁺ (∼10%) (data not shown). After lineage depletion, the Ter119⁺ cell population was decreased to ∼28% and the fraction of cKit⁺ cells increased to ∼40%. After 2 days, the frequency of cKit⁺ cells nearly doubled (to ∼77%), with a corresponding decrease in the frequency of Ter119⁺ cells (to ∼12%; Figure 2B). Consistent with the increase in the numbers of cKit⁺ cells, a ∼2-fold increase in Vdr transcription was observed (Figure 2C). To examine the developmental potential of cells cultured in PM, equal numbers of Lin^neg cells were transferred (after 48 hours in PM) to methylcellulose in the presence or absence of calcitriol. Colony numbers were scored after 8 days. Three types of erythroid colonies were identified: BFU-E, CFU-E, and multi-CFU-E (mCFU-E or late BFU-E, cluster of 3-20 CFU-E; supplemental Figure 2A). The distribution of these colony types, but not their total numbers, was altered when Vdr signaling was activated (Figure 2D), with increased numbers of earlier progenitors (BFU-E and mCFU-E) and decreased numbers of the later progenitors (CFU-E). These findings indicate that culture in PM maintains cKit⁺ colony-forming erythroid progenitors that can respond to activation of Vdr by calcitriol.

When progenitors were cultured in suspension under maturation conditions (in MM) and allowed to mature, cell proliferation continued for 6 days, with a ∼60-fold increase in cell number (supplemental Figure 2B), corresponding to ∼5 cell divisions. In the presence of calcitriol, cell numbers increased by 160- to 180-fold (Figure 2E), suggesting that 1 to 2 additional cell divisions took place during the 6-day period. The increase in cell numbers was dose-dependent (supplemental Figure 3). As expected for maturing cells, the numbers of cKit⁺ and Ter119⁺ cells decreased and increased, respectively (supplemental Figure 2C). Cell and nuclear size as well as surface expression of CD44 all decreased as the cells progressed to enucleate (supplemental Figure 2D-G). A maturation delay was observed during the first 4 days of culture in the presence of calcitriol, with lower numbers of Ter119⁺ cells, higher forward scatter (FSC) and CD44 (supplemental Figure 4A-B), and delayed enucleation (Figure 2F; supplemental Figure 4C). By days 6 to 7, staining with Wright-Giemsa and accumulation of hemoglobin protein were similar in treated and untreated cultures (Figure 2G; supplemental Figure 4D).

The Lin^neg population of FL cells contains cKit⁺ progenitors at different stages of maturation that can be identified prospectively on the basis of their expression of CD71 (Figure 3A); multiple types of hematopoietic progenitors, including early erythroid progenitors, are CD71^lo/neg, whereas late erythroid progenitors are CD71^hi.^14,21,36  To determine which populations of cKit⁺ cells respond to activation by calcitriol, sorted cells were first analyzed for expression of Vdr, which was significantly higher (∼13-fold) in the CD71^lo/neg than in the CD71^hi population (Figure 3B). For these studies, cells were plated at low density (500 cells/mL) to facilitate analysis of single-cell contribution to each colony. The CD71^lo/neg cells produced BFU-E, mCFU-E, and CFU-E, whereas the CD71^hi cells produced mostly CFU-E (Figure 3C). Analysis of colony size (surface area) using ImageJ did not reveal significant differences for the 2 culture conditions (supplemental Figure 5). CD71^lo/neg cells, but not CD71^hi cells, responded to activation of Vdr, with formation of ∼2.5-fold higher numbers of BFU-E (Figure 3C-D). The colonies were strongly pigmented, indicating production of hemoglobin (Figure 3D). When the 2 sorted cell populations were plated directly into MM, treatment with calcitriol again stimulated proliferation of CD71^lo/neg, but not CD71^hi, cells (Figure 3E). Similar to the CD71^hi population, the ckit⁺CD71^med population formed primarily CFU-E and did not increase proliferation in response to calcitriol (supplemental Figure 6B-C).

As observed for unfractionated Lin^neg cells, cKit expression in the CD71^lo/neg population was prolonged, and fewer Ter119⁺ erythroblasts formed in response to calcitriol (supplemental Figure 7A). By day 5, cKit expression was lost and the majority of cells were Ter119⁺ (not shown). However, these Ter119⁺ cells displayed higher FSC and CD44 expression and less enucleation than the untreated cells, suggesting that they were less mature (supplemental Figure 7B-C).

To determine the period over which Vdr signaling can maintain progenitor potential, we cultured cKit⁺CD71^lo/neg and ckit⁺CD71^hi populations in MM in the presence or absence of calcitriol and transferred the cells to methylcellulose cultures after 1, 2, 3, or 4 days (Figure 4A). Both cKit⁺CD71^lo/neg-derived early and late progenitors (Figure 4A) and their surface phenotypes (Figure 4B) were maintained for a longer time and in greater numbers in response to activation of Vdr. CD71^hi progenitor potential was lost after 1 day in the presence or absence of calcitriol (Figure 4A; supplemental Figure 7E).

Calcitriol stimulates Vdr-regulated transcription as well as alterations in Ca²⁺ flux.³⁷  To distinguish between these effects, the CD71^lo/neg population was treated with calcipotriol, a vitamin D3 analog reported to be 100- to 200-fold less potent in its calcemic effects than calcitriol.³⁸  Calcipotriol increased BFU-E numbers by ∼2.5-fold, with a corresponding decrease in the numbers of CFU-E, and increased cell proliferation by ∼2.8-fold (Figure 4C-D), comparable to the effects seen with calcitriol (Figure 3C,E). Therefore, calcium flux is not the primary driver of these effects. As demonstrated by the titration shown in supplemental Figure 7E, the effect of calcipotriol was dose-dependent.

The glucocorticoid receptor (Gr), Similar to Vdr, is a member of the nuclear hormone receptor transcription factor family and has been shown to stimulate the proliferation of cKit⁺CD71^lo/neg cells.²¹  To determine whether the Vdr and Gr signaling pathways can cooperate to modulate erythroid progenitor growth, cKit⁺CD71^lo/neg cells were cultured with or without calcitriol, dexamethasone, or the 2 ligands in combination. After 24 hours, the cells were plated in methylcellulose to evaluate progenitor potential. In the absence of Vdr or Gr ligand, the numbers of BFU-E and mCFU-E decreased as the numbers of CFU-E increased (Figure 4E). Colony numbers were maintained in the presence of either ligand alone (Figure 4E). When the 2 ligands were present together, the numbers of BFU-E and mCFU-E increased (Figure 4E), suggesting that they function cooperatively.

To evaluate the effect of these steroid hormones on proliferation, cKit⁺CD71^lo/neg cells were cultured for up to 9 days in the presence or absence of calcitriol, dexamethasone, or the 2 ligands in combination. Either ligand alone stimulated cell proliferation (calcitriol ∼3-fold; dexamethasone, ∼8-fold; Figure 4F). In combination, calcitriol and dexamethasone increased cell numbers by ∼17-fold (Figure 4F). Although CD71^med cells did not respond to calcitriol alone (supplemental Figure 6B-C), their proliferation was stimulated by dexamethasone and, to an even greater extent, dexamethasone and calcitriol together (Figure 4F). Neither dexamethasone nor calcitriol stimulated proliferation of CD71^hi cells (Figure 4F).

The cKit⁺CD71^lo/neg population of progenitors represents ∼10% of FL Lin^neg cells (Figure 3A). To obtain sufficient numbers of cells for more detailed analyses, we precultured the cells in PM for 2 days in the absence of calcitriol, resulting in a ∼10-fold increase in cell numbers (Figure 5A). The cells remained almost entirely cKit⁺ and Ter119^neg (Figure 5B); the majority of colony-forming cells were erythroid progenitors (Figure 5C). When precultured cells were then plated in colony assays in the presence or absence of calcitriol, the numbers of early progenitors (BFU-E and mCFU-E) increased (57% vs 28%), and numbers of late progenitors (CFU-E) decreased (48% vs 27%; Figure 5C). These results strongly suggest that calcitriol maintains early progenitors at the expense of late progenitors.

When CD71^lo/neg cells were cultured in suspension in MM, calcitriol treatment resulted in increased cell numbers (Figure 5D), cell divisions (dilution of carboxyfluorescein diacetate succinimidyl ester; Figure 5E), and DNA synthesis (incorporation of EdU; Figure 5F). In addition, a transient increase in the numbers of cells in S phase and a decrease in the numbers of cells in G0/G1 were observed (days 2-5; supplemental Figure 8). Consistent with these changes, erythroid maturation was delayed, as evidenced by levels of expression of cKit, Ter119, and CD44; cell size (FSC); and frequency of enucleation (supplemental Figure 9A-C). Activation of Vdr signaling did not appear to influence cell survival, as we did not observe significant changes in expression of annexin V or activated caspase 3/7 (not shown). As expected, calcipotriol and dexamethasone (supplemental Figure 9D-E) each increased the proliferation of CD71^lo/neg cells. Cells precultured in PM before transfer to MM proliferated more when exposed to both calcitriol and dexamethasone (∼7-fold) than when cultured with either ligand alone (∼2.3, calcitriol; ∼3.2, dexamethasone), as observed for cells that were not precultured (Figure 4F).

The Vdr transcription factor regulates gene expression in cells in response to binding of vitamin D3 agonists. Cyp24a1 (1,25-dihydroxyvitamin D3 24-hydroxylase), a well-characterized direct target of Vdr,³⁹  was strongly activated in erythroid progenitors cultured in the presence of calcitriol, but was undetectable in its absence (Figure 6A). In contrast, activation of Vdr by calcitriol blocked the upregulation of erythroid transcription factor genes Gata1, Fog1, and Klf1 (Figure 6B) and the erythroid genes Hbb-a1/2, Hbb-b1, and Alas2 (Figure 6C). To our surprise, the expression of genes associated with erythroid progenitors and known to be downregulated during maturation (Gata2, Hopx, and Gcr)^21,22  was not affected after activation of Vdr. Expression of these genes decreased to nearly undetectable levels within 24 hours, irrespective of the presence of calcitriol (data not shown).

To determine whether the stimulation of erythroid cell proliferation is dependent on Vdr, we used a lentivirus shRNA-mediated knockdown approach. Lin^negcKit⁺CD71^lo/neg cells were plated in PM and transduced with shRNA lentiviruses targeting Vdr (shRNA1 or shRNA2) or with a luciferase shRNA virus and were then allowed to mature by culture in MM, in the presence or absence of calcitriol. Mock transduced and untreated cells served as additional controls. Knockdown of Vdr using shRNA1 or shRNA2 abrogated the calcitriol-mediated increase in cell numbers seen in the controls (Figure 7A). Vdr expression in shRNA1 or shRNA2 lentivirus-transduced cells was reduced to ∼40% of its level in the controls (Figure 7B). Consistent with these findings, Vdr knockdown reversed the delay in maturation in response to calcitriol, as evidenced by control levels of enucleation (Figure 7C) and surface expression of Ter119 (Figure 7D). No differences were observed in cell numbers or in the frequencies of cKit⁺ cells among the various conditions (supplemental Figure 10A-B).

The mechanisms underlying the development and growth of early erythroid progenitors are poorly understood. Studies in vitro and in vivo have demonstrated that several steroid hormone ligand-activated receptors influence the decision between expansion (glucocorticoid and androgen receptors)^22,40-43  and terminal differentiation (thyroid hormone receptor)^44,45  of erythroid progenitors. VDR is a transcription factor that is activated by binding to its steroid hormone ligand, vitamin D3. The VDR signaling pathway has been largely unexplored in the erythroid lineage, and the targets of signaling by VDR in erythroid progenitors are unknown. Here, we define a function for vitamin D signaling through VDR in EryD (but not EryP) progenitors. Vdr is not expressed in primitive erythroid progenitors (E7.5-8.5) or erythroblasts (E9.5-12.5; Isern et al¹  and this report), and therefore, these cells would not be expected to respond to vitamin D agonists.

Erythroid progenitors are known to respond to a variety of stimuli in vitro and in vivo (reviewed in Hattangadi et al¹⁰  and Paulson et al¹⁹ ). We have found that activation of the murine Vdr pathway by vitamin D3 agonists increases the numbers of BFU-E and CFU-E progenitors from fetal liver and adult bone marrow and delays their maturation. Vdr signaling is active in early erythroid progenitors in FL (E12.5-E14.5) and adult BM, but not in maturing erythroid cells. The expansion of erythroid progenitors results, ultimately, in increased numbers of mature erythroid cells.

We note that definitive erythro-myeloid progenitors are present in the FL at E11.5-12.5; however, their numbers decline thereafter, as hematopoietic stem cell-derived erythroid progenitors expand and differentiate.³  The increase in the numbers of BFU-E colonies in response to calcitriol is essentially the same for Lin^neg cells from E12.5, E13.5, or E14.5 FL. Although some of the responding cells at E12.5 might be erythro-myeloid progenitors, our data clearly indicate that the effect of Vdr activation by calcitriol is not limited to a stage of development when erythro-myeloid progenitors are present. Indeed, erythroid progenitors from adult bone marrow also respond to calcitriol.

The Lin^neg population of hematopoietic cells in FL and BM contains early and late cKit⁺ erythroid progenitors that can be further fractionated on the basis of their expression of CD71.^14,15,21,36  In the discussion that follows, we refer to populations based on their immunophenotype and avoid the terms BFU-E and CFU-E.

The CD71^lo/neg progenitors (here termed early progenitors) are a heterogeneous population, forming nonerythroid colonies, BFU-E, multi-CFU-E, and CFU-E colonies, whereas CD71^hi (late progenitors) form primarily CFU-E colonies (Figure 3C). The primary target of Vdr signaling in fetal liver is the cKit⁺CD71^lo/neg population (early progenitors), but not the cKit⁺CD71^med or cKit⁺CD71^hi populations (late progenitors). In contrast, not only the cKit⁺CD71^lo/neg but also the cKit⁺CD71^med population responds to glucocorticoid signaling (this report and Flygare et al²¹  and Zhang et al²² ). Therefore, the vitamin D pathway functions in a more restricted set of progenitors than does the glucocorticoid pathway. It is important to note that we have evaluated not only changes in immunophenotype and cell numbers in response to calcitriol or dexamethasone but also functional changes in progenitor potential (using colony assays). Although the morphologies of the colonies we scored as CFU-E from any of the 3 CD71 populations were indistinguishable, the differences among these populations in their response to calcitriol vs glucocorticoids strongly suggest there must be functional differences among the target cells.

Treatment of erythroid progenitors from FL with either calcitriol/calcipotriol or dexamethasone increases the numbers of early erythroid progenitors and stimulates cell proliferation during maturation. Therefore, vitamin D agonists may partially substitute for the function of glucocorticoids on early erythroid progenitors. When these steroid hormones are combined, the observed increases are at least additive, and perhaps synergistic. Moreover, although calcitriol alone does not enhance the proliferation of cKit⁺CD71^med cells, its combination with dexamethasone does result in increased cell proliferation, perhaps in part through upregulation of Vdr by Gr.^46,47  The cooperative effects of Vdr and Gr observed in our experiments are reminiscent of the reported synergy between dexamethasone and Ppar-α agonists for CD71^lo/neg progenitors.⁴⁷  We have shown that VDR agonists enhance the proliferation of cKit⁺CD71^lo/neg cells. It is possible that Vdr and Gr have unique functions within certain cell subsets and synergistic functions in other subsets within the cKit⁺CD71^lo/neg population.

Vdr and vitamin D3 have both interdependent and independent functions.^48-50  Our shRNA knockdown studies revealed that Vdr is necessary for the proliferative response to vitamin D3. Unliganded Vdr is apparently not necessary for the growth of erythroid progenitors, as indicated by the absence of an effect of partial (∼60%) Vdr knockdown in the absence of vitamin D3. However, it is possible that complete Vdr knockdown would reveal a requirement for unliganded Vdr. In any case, these findings point to an erythroid cell-autonomous function for Vdr in response to activation by calcitriol.

Calcipotriol, a vitamin D3 analog with far less potent calcemic effects than calcitriol,³⁷  induces the same changes in the growth of early erythroid progenitors as calcitriol. Therefore, these changes are likely a result of Vdr-regulated transcription, and not to alterations in Ca²⁺ flux. Whether these genes are direct or indirect targets of Vdr remains to be determined. Although vitamin D response elements can be identified within or at various distances from genes encoding erythroid regulators such as Gata1 or Klf1 (not shown), it is not yet known which of these elements function in erythroid progenitors.

Both the glucocorticoid receptor (Gr) and Vdr are steroid hormone receptors. The regulation of erythroid progenitors by glucocorticoids such as dexamethasone has been well documented in the literature (eg, see Flygare et al,²¹  Zhang et al,²²  and von Lindern et al⁵¹ ). Although DNA-binding mutant Gr mice are not anemic, an erythroid defect was uncovered in response to hypoxic stress.⁴¹  As observed for Gr, Vdr is not essential for steady-state erythropoiesis.^52-54  Given the similarity in response of erythroid progenitors to glucocorticoids and to vitamin D agonists, and the additive effects of these steroid hormones, it seems likely that Vdr, similar to Gr, plays a role in stress erythropoiesis. It is worth noting that, as stress erythroid progenitors have unique properties, many of the genes involved in the erythroid recovery from stress have little, if any, effect on steady-state erythropoiesis.^55,56 

Although glucocorticoids are well known to stimulate erythropoiesis, their use in the treatment of anemia is associated with severe adverse effects.^57-59  Several studies have reported an association between vitamin D deficiency and anemia (reviewed in Smith and Tangpricha⁶⁰ ) and have established that vitamin D3 improves anemia in chronic renal failure and other types of anemia.^29,60-62  Vitamin D deficiency is also associated with and is a predictor of anemia in end-stage heart failure.⁶³  Although the results from the present study suggest that vitamin D agonists stimulate erythropoiesis by targeting progenitors, expression of genes known to regulate erythroid progenitors (Gata2, Zfp36l2, Bmi1, and Hopx) was not affected by Vdr signaling. Therefore, other genes must be involved in the Vdr signaling pathway in erythroid progenitors.

In conclusion, these findings support a model in which vitamin D signaling through the Vdr transcription factor stimulates the growth of early erythroid progenitors (see cartoon, supplemental Figure 10). A better understanding of how the growth of erythroid progenitors and the switch from self-renewal to differentiation are controlled might facilitate the development of novel erythropoiesis-stimulating agents)^64,65  with fewer adverse effects. Moreover, combined activation of distinct pathways within progenitors might be achieved at lower concentrations of their agonists to achieve maximum therapeutic benefit, and might also be of value for the development of more efficient systems for the generation ex vivo of RBCs (or their progenitors) for transfusion.^66,67 

The authors thank the Mount Sinai Flow Cytometry Shared Resource Facility for assistance with cell sorting. The authors also thank James Bieker, Nithya Gnanapragasam, Diane Krause, and Merav Socolovsky for helpful discussions.

This work was supported by grants from the National Institutes of Health (R01 HL62248 from the National Heart, Lung, and Blood Institute and DK52191 from the National Institute of Diabetes and Digestive and Kidney Diseases) (M.H.B.) and in part by a training grant from the National Institutes of Health, National Heart, Lung, and Blood Institute (T32 HL094283) (J.B.).

Contribution: J.B. and M.H.B. designed the experiments, wrote the manuscript, and prepared the figures; J.B. performed most of the experiments; and B.M.R., A.E., and A.N.L. assisted with some of the experiments.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Margaret H. Baron, Icahn School of Medicine at Mount Sinai, Box 1079, 1468 Madison Ave, Annenberg 24-04E, New York, NY 10029-6574; e-mail: margaret.baron@mssm.edu.