A mutated retinoic acid receptor-alpha exhibiting dominant-negative activity alters the lineage development of a multipotent hematopoietic cell line

Effects of RARα ligand binding domain mutations on breast fibroepithelial tumor function and signaling

Point mutations in the ligand binding domain of retinoic acid receptor alpha (RARα) are linked to breast fibroepithelial tumor development, but their role in solid tumorigenesis is unclear. In this study, we assessed the functional effects of known RARα mutations on retinoic acid signaling using biochemical and cellular assays. All tested mutants exhibited reduced transcriptional activity compared to wild-type RARα and showed a dominant negative effect, a feature associated with developmental defects and tumor formation. X-ray crystallography revealed that the mutants maintained structural integrity, with altered co-activator recruitment explaining the loss of transcriptional function. Transcriptomics and cell growth assays demonstrated that mutant RARα proteins conferred resistance to ligand-induced growth inhibition in phyllodes tumor cells. Although the mutations impair RARα's response to retinoic acid, some mutants could be partially reactivated with synthetic agonists. These findings provide insights into how RARα mutations may contribute to tumorigenesis.

Retinoic acid receptor activity is required for the maintenance of type 1 innate lymphoid cells

Group 1 innate lymphoid cells (G1-ILCs) are innate immune effectors critical for the response to intracellular pathogens and tumors. G1-ILCs comprise circulating natural killer (NK) cells and tissue-resident type 1 ILCs (ILC1s). ILC1s mainly reside in barrier tissues and provide the initial sources of interferon-γ (IFN-γ) to prime the protecting responses against infections, which are followed by the response of recruited NK cells. Despite such distribution differences, whether local environmental factors influence the behavior of NK cells and ILC1s is unclear. Here, we show that the signaling of retinoic acid (RA), active metabolites of vitamin A, is essential for the maintenance of ILC1s in the periphery. Mice expressing RARα403, a truncated form of retinoic acid receptor α (RARα) that exerts dominant negative activity, in a lymphoid cell- or G1-ILC-specific manner showed remarkable reductions of peripheral ILC1s while NK cells were unaffected. Lymphoid cell-specific inhibition of RAR activity resulted in the reduction of PD-1+ ILC progenitors (ILCPs), but not of common lymphoid progenitors (CLPs), suggesting the impaired commitment and differentiation of ILC1s. Transcriptome analysis revealed that RARα403-expressing ILC1s exhibited impaired proliferative states and declined expression of effector molecules. Thus, our findings demonstrate that cell-intrinsic RA signaling is required for the homeostasis and the functionality of ILC1s, which may present RA as critical environmental cue targeting local type 1 immunity against infection and cancer.

Cutting Edge: The Expression of Transcription Inhibitor GFI1 Is Induced by Retinoic Acid to Rein in Th9 Polarization

IL-9, produced mainly by specialized T cells, mast cells, and group 2 innate lymphoid cells, regulates immune responses, including anti-helminth and allergic responses. Polarization of naive CD4 T cells into IL-9-producing T cells (Th9s) is induced by IL-4 and TGF-β1 or IL-1β. In this article, we report that the transcription factor growth factor-independent 1 transcriptional repressor (GFI1) plays a negative role in mouse Th9 polarization. Moreover, the expression of GFI1 is controlled by liganded RARα, allowing GFI1 to mediate the negative effect of retinoic acid on IL-9 expression. The Gfi1 gene has multiple RARα binding sites in the promoter region for recruiting nuclear coactivator steroid receptor coactivator-3 and p300 for histone epigenetic modifications in a retinoic acid-dependent manner. Retinoic acid-induced GFI1 binds the Il9 gene and suppresses its expression. Thus, GFI1 is a novel negative regulator of Il9 gene expression. The negative GFI1 pathway for IL-9 regulation provides a potential control point for Th9 activity.

T Cell-Intrinsic Vitamin A Metabolism and Its Signaling Are Targets for Memory T Cell-Based Cancer Immunotherapy

Memory T cells play an essential role in infectious and tumor immunity. Vitamin A metabolites such as retinoic acid are immune modulators, but the role of vitamin A metabolism in memory T-cell differentiation is unclear. In this study, we identified retinol dehydrogenase 10 (Rdh10), which metabolizes vitamin A to retinal (RAL), as a key molecule for regulating T cell differentiation. T cell-specific Rdh10 deficiency enhanced memory T-cell formation through blocking RAL production in infection model. Epigenetic profiling revealed that retinoic acid receptor (RAR) signaling activated by vitamin A metabolites induced comprehensive epigenetic repression of memory T cell-associated genes, including TCF7, thereby promoting effector T-cell differentiation. Importantly, memory T cells generated by Rdh deficiency and blocking RAR signaling elicited potent anti-tumor responses in adoptive T-cell transfer setting. Thus, T cell differentiation is regulated by vitamin A metabolism and its signaling, which should be novel targets for memory T cell-based cancer immunotherapy.

An immune-shift induced by lycopene; from an eosinophil-dominant type towards an eosinophil/neutrophil-co-dominant type of airway inflammation

Lycopene as the main carotenoid from tomatoes is known to have beneficial effects on various inflammatory diseases. In mice, lycopene ameliorates asthma symptoms and in human asthmatic patients serum lycopene levels are reduced. To further investigate the immunomodulatory effect of lycopene, first, we used a ragweed pollen extract (RWE)-induced asthma model in mice. In a second approach, we established a RWE-induced asthma model in gerbils, because of a more human-like carotenoid absorption in these animals. In RWE-sensitized/RWE-challenged gerbils (C+) following a basal diet, mainly the number of eosinophils in the broncho-alveolar lavage (BAL) significantly increased, comparable to RWE-sensitized/PBS-challenged gerbils (C-). In RWE-sensitized/PBS-challenged gerbils with lycopene-supplementation (L-), an elevated number of mainly neutrophils, in addition to eosinophils, was detected compared to C-, whereas in RWE-sensitized/RWE-challenged animals with lycopene-supplementation (L+), mainly increased neutrophil numbers in BAL were detected compared to C+. Furthermore, using LC-MS, we determined an array of eicosanoids/docosanoids in the lungs and observed that 5-, 8-lipoxygenase (LOX) and cyclooxygenase (COX) pathways were significantly increased after intranasal RWE-challenge in sensitized mice and just by tendency in gerbils. In PBS- and RWE-challenged animals, lycopene-supplementation significantly raised COX-pathway metabolites. In conclusion, we found that lycopene-supplementation resulted in an increased inflammatory influx of neutrophils in combination with increased COX-pathways metabolites. This pro-inflammatory, pro-neutrophil activity induced by lycopene might be an important shift from allergic asthma towards an inflammatory symptomatic asthma type, though with the potential for resolution.

Epithelium intrinsic vitamin A signaling co-ordinates pathogen clearance in the gut via IL-18

Intestinal epithelial cells (IECs) are at the forefront of host-pathogen interactions, coordinating a cascade of immune responses to protect against pathogens. Here we show that IEC-intrinsic vitamin A signaling restricts pathogen invasion early in the infection and subsequently activates immune cells to promote pathogen clearance. Mice blocked for retinoic acid receptor (RAR) signaling selectively in IECs (stop^ΔIEC) showed higher Salmonella burden in colonic tissues early in the infection that associated with higher luminal and systemic loads of the pathogen at later stages. Higher pathogen burden in stop^ΔIEC mice correlated with attenuated mucosal interferon gamma (IFNγ) production by underlying immune cells. We found that, at homeostasis, the intestinal epithelium of stop^ΔIEC mice produced significantly lower amounts of interleukin 18 (IL-18), a potent inducer of IFNγ. Regulation of IL-18 by vitamin A was also observed in a dietary model of vitamin A supplementation. IL-18 reconstitution in stop^ΔIEC mice restored resistance to Salmonella by promoting epithelial cell shedding to eliminate infected cells and limit pathogen invasion early in infection. Further, IL-18 augmented IFNγ production by underlying immune cells to restrict pathogen burden and systemic spread. Our work uncovers a critical role for vitamin A in coordinating a biphasic immune response to Salmonella infection by regulating IL-18 production by IECs.

Epithelial cells line the intestinal lumen, forming a barrier between the body and dietary and microbial contents in the lumen. Apart from absorbing nutrients from diet, these epithelial cells help mediate a stable, symbiotic relationship between commensal bacteria and the immune cells. During infection, they help co-ordinate the immune response to counter the infection. How dietary micronutrients, such as vitamin A, inform epithelial cell function during infection is poorly understood. Using a model where epithelial cells in the gut cannot respond to vitamin A signals, we find that epithelial vitamin A signaling promotes resistance to Salmonella infection. We show that, vitamin A increases the production of a key cytokine, interleukin 18, by epithelial cells. IL-18 promotes shedding of infected epithelial cells to reduce the pathogen invasion while also inducing the production of interferon gamma by immune cells to mediate pathogen clearance. Thus, epithelial cells dynamically respond to dietary vitamin A to regulate interleukin 18 production and potentiate resistance to infection.

Classic and Variants APLs, as Viewed from a Therapy Response

Most acute promyelocytic leukemia (APL) are caused by PML-RARA, a translocation-driven fusion oncoprotein discovered three decades ago. Over the years, several other types of rare X-RARA fusions have been described, while recently, oncogenic fusion proteins involving other retinoic acid receptors (RARB or RARG) have been associated to very rare cases of acute promyelocytic leukemia. PML-RARA driven pathogenesis and the molecular basis for therapy response have been the focus of many studies, which have now converged into an integrated physio-pathological model. The latter is well supported by clinical and molecular studies on patients, making APL one of the rare hematological disorder cured by targeted therapies. Here we review recent data on APL-like diseases not driven by the PML-RARA fusion and discuss these in view of current understanding of "classic" APL pathogenesis and therapy response.

Strategies to generate functionally normal neutrophils to reduce infection and infection-related mortality in cancer chemotherapy

Neutrophils form an essential part of innate immunity against infection. Cancer chemotherapy-induced neutropenia (CCIN) is a condition in which the number of neutrophils in a patient's bloodstream is decreased, leading to increased susceptibility to infection. Granulocyte colony-stimulating factor (GCSF) has been the only approved treatment for CCIN over two decades. To date, CCIN-related infection and mortality remain a significant concern, as neutrophils generated in response to administered GCSF are functionally immature and cannot effectively fight infection. This review summarizes the molecular regulatory mechanisms of neutrophil granulocytic differentiation and innate immunity development, dissects the biology of GCSF in myeloid expansion, highlights the shortcomings of GCSF in CCIN treatment, updates the recent advance of a selective retinoid agonist that promotes neutrophil granulocytic differentiation, and evaluates the benefits of developing GCSF biosimilars to increase access to GCSF biologics versus seeking a new mode to fundamentally advance GCSF therapy for treatment of CCIN.

Retinoic Acid Regulates Endothelial β-catenin Expression and Pericyte Numbers in the Developing Brain Vasculature

The acquisition of brain vascular properties, like tight junctions and pericytes, to form the blood-brain barrier (BBB) is crucial for a properly functioning central nervous system (CNS). Endothelial WNT signaling is a known driver of brain vascular development and BBB properties, however, it is unclear how endothelial WNT signaling is regulated. We recently showed that mouse embryos with disruptions in endothelial retinoic acid (RA) signaling have ectopic WNT signaling in the brain vasculature. Using immunohistochemistical analysis, we show that increased vascular WNT signaling in RA mutants (Pdgfbi^cre; dnRAR403-flox and Rdh10 mutants) is associated with elevated expression of the WNT transcriptional effector, β-catenin, in the brain endothelium. In vitro immunocytochemistry and proximity ligation studies in brain endothelial cells reveal that RA, through its receptor RARα, regulates β-catenin expression in brain endothelial cells via transcriptional suppression and phosphorylation events that targets β-catenin for proteasomal degradation, the latter dependent on PKCα. We find that one function of RA in regulating vascular WNT signaling is to modulate the pericyte numbers in the developing brain vasculature. RA-mediated regulation of vascular WNT signaling could be needed to prevent over-recruitment of pericytes that might impair endothelial-pericyte interactions crucial for vascular stability.

Diverse Functions of Retinoic Acid in Brain Vascular Development

As neural structures grow in size and increase metabolic demand, the CNS vasculature undergoes extensive growth, remodeling, and maturation. Signals from neural tissue act on endothelial cells to stimulate blood vessel ingression, vessel patterning, and acquisition of mature brain vascular traits, most notably the blood-brain barrier. Using mouse genetic and in vitro approaches, we identified retinoic acid (RA) as an important regulator of brain vascular development via non-cell-autonomous and cell-autonomous regulation of endothelial WNT signaling. Our analysis of globally RA-deficient embryos (Rdh10 mutants) points to an important, non-cell-autonomous function for RA in the development of the vasculature in the neocortex. We demonstrate that Rdh10 mutants have severe defects in cerebrovascular development and that this phenotype correlates with near absence of endothelial WNT signaling, specifically in the cerebrovasculature, and substantially elevated expression of WNT inhibitors in the neocortex. We show that RA can suppress the expression of WNT inhibitors in neocortical progenitors. Analysis of vasculature in non-neocortical brain regions suggested that RA may have a separate, cell-autonomous function in brain endothelial cells to inhibit WNT signaling. Using both gain and loss of RA signaling approaches, we show that RA signaling in brain endothelial cells can inhibit WNT-β-catenin transcriptional activity and that this is required to moderate the expression of WNT target Sox17. From this, a model emerges in which RA acts upstream of the WNT pathway via non-cell-autonomous and cell-autonomous mechanisms to ensure the formation of an adequate and stable brain vascular plexus.

SIGNIFICANCE STATEMENT

Work presented here provides novel insight into important yet little understood aspects of brain vascular development, implicating for the first time a factor upstream of endothelial WNT signaling. We show that RA is permissive for cerebrovascular growth via suppression of WNT inhibitor expression in the neocortex. RA also functions cell-autonomously in brain endothelial cells to modulate WNT signaling and its downstream target, Sox17. The significance of this is although endothelial WNT signaling is required for neurovascular development, too much endothelial WNT signaling, as well as overexpression of its target Sox17, are detrimental. Therefore, RA may act as a "brake" on endothelial WNT signaling and Sox17 to ensure normal brain vascular development.

Potential role for all-trans retinoic acid in nonpromyelocytic acute myeloid leukemia

All-trans retinoic acid (ATRA) has been very successful in the subtype of acute myelogenous leukemia known as acute promyelocytic leukemia due to targeted reactivation of retinoic acid signaling. There has been great interest in applying this form of differentiation therapy to other cancers, and numerous clinical trials have been initiated. However, ATRA as monotherapy has thus far shown little benefit in nonacute promyelocytic leukemia acute myelogenous leukemia. Here, we review the literature on the use of ATRA in combination with chemotherapy, epigenetic modifying agents and targeted therapy, highlighting specific patient populations where the addition of ATRA to existing therapies may provide benefit. Furthermore, we discuss the impact of recent whole genome sequencing efforts in leading the design of rational combinatorial approaches.

AML1/ETO accelerates cell migration and impairs cell-to-cell adhesion and homing of hematopoietic stem/progenitor cells

The AML1/ETO fusion protein found in acute myeloid leukemias functions as a transcriptional regulator by recruiting co-repressor complexes to its DNA binding site. In order to extend the understanding of its role in preleukemia, we expressed AML1/ETO in a murine immortalized pluripotent hematopoietic stem/progenitor cell line, EML C1, and found that genes involved in functions such as cell-to-cell adhesion and cell motility were among the most significantly regulated as determined by RNA sequencing. In functional assays, AML1/ETO-expressing cells showed a decrease in adhesion to stromal cells, an increase of cell migration rate in vitro, and displayed an impairment in homing and engraftment in vivo upon transplantation into recipient mice. Our results suggest that AML1/ETO expression determines a more mobile and less adherent phenotype in preleukemic cells, therefore altering the interaction with the hematopoietic niche, potentially leading to the migration across the bone marrow barrier and to disease progression.

Cerebrovascular defects in Foxc1 mutants correlate with aberrant WNT and VEGF-A pathways downstream of retinoic acid from the meninges

Growth and maturation of the cerebrovasculature is a vital event in neocortical development however mechanisms that control cerebrovascular development remain poorly understood. Mutations in or deletions that include the FOXC1 gene are associated with congenital cerebrovascular anomalies and increased stroke risk in patients. Foxc1 mutant mice display severe cerebrovascular hemorrhage at late gestational ages. While these data demonstrate Foxc1 is required for cerebrovascular development, its broad expression in the brain vasculature combined with Foxc1 mutant's complex developmental defects have made it difficult to pinpoint its function(s). Using global and conditional Foxc1 mutants, we find 1) significant cerebrovascular growth defects precede cerebral hemorrhage and 2) expression of Foxc1 in neural crest-derived meninges and brain pericytes, though not endothelial cells, is required for normal cerebrovascular development. We provide evidence that reduced levels of meninges-derived retinoic acid (RA), caused by defects in meninges formation in Foxc1 mutants, is a major contributing factor to the cerebrovascular growth defects in Foxc1 mutants. We provide data that suggests that meninges-derived RA ensures adequate growth of the neocortical vasculature via regulating expression of WNT pathway proteins and neural progenitor derived-VEGF-A. Our findings offer the first evidence for a role of the meninges in brain vascular development and provide new insight into potential causes of cerebrovascular defects in patients with FOXC1 mutations.

Characterization of neutrophils and macrophages from ex vivo-cultured murine bone marrow for morphologic maturation and functional responses by imaging flow cytometry

Neutrophils and macrophages differentiate from common myeloid progenitors in the bone marrow, where they undergo nuclear morphologic changes during maturation. During this process, both cell types acquire critical innate immune functions that include phagocytosis of pathogens, and for neutrophils the release of nuclear material called nuclear extracellular traps (NETs). Primary cells used to study these functions are typically purified from mature mouse tissues, but bone marrow-derived ex vivo cultures provide more abundant numbers of progenitors and functionally mature cells. Routine analyses of these cells use conventional microscopy and flow cytometry, which present limitations; microscopy is laborious and subjective, whereas flow cytometry lacks spatial resolution. Here we describe methods to generate enriched populations of neutrophils or macrophages from cryopreserved mouse bone marrow cultured ex vivo, and to use imaging flow cytometry that combines the resolution of microscopy with flow cytometry to analyze cells for morphologic features, phagocytosis, and NETosis.

Signalling Through Retinoic Acid Receptors is Required for Reprogramming of Both Mouse Embryonic Fibroblast Cells and Epiblast Stem Cells to Induced Pluripotent Stem Cells

We previously demonstrated that coexpressing retinoic acid (RA) receptor gamma and liver receptor homolog-1 (LRH1 or NR5A2) with OCT4, MYC, KLF4, and SOX2 (4F) rapidly reprograms mouse embryonic fibroblast cells (MEFs) into induced pluripotent stem cells (iPSCs). Here, we further explore the role of RA in reprogramming and report that the six factors (6F) efficiently and directly reprogram MEFs into integration-free iPSCs in defined medium (N2B27) in the absence of feeder cells. Through genetic and chemical approaches, we find that RA signalling is essential, in a highly dose-sensitive manner, for MEF reprogramming. The removal of exogenous RA from N2B27, the inhibition of endogenous RA synthesis or the expression of a dominant-negative form of RARA severely impedes reprogramming. By contrast, supplementing N2B27 with various retinoids substantially boosts reprogramming. In addition, when coexpressed with LRH1, RA receptors (RARs) can promote reprogramming in the absence of both exogenous and endogenously synthesized RA. Remarkably, the reprogramming of epiblast stem cells into embryonic stem cell-like cells also requires low levels of RA, which can modulate Wnt signalling through physical interactions of RARs with β-catenin. These results highlight the important functions of RA signalling in reprogramming somatic cells and primed stem cells to naïve pluripotency. Stem Cells 2015;33:1390-1404.

An ATRActive future for differentiation therapy in AML

The success of all-trans retinoic acid (ATRA) therapy in acute promeylocytic leukemia (APL) has spawned numerous attempts to translate the paradigm of differentiation therapy to non-APL acute myelocytic leukemia (AML). However, the results of clinical trials have been overall disappointing. In this review we discuss the mechanism of retinoic acid signaling and the results of major clinical trials that have attempted to incorporate ATRA into AML regimens. We discuss recent evidence that indicate that the retinoic acid signaling pathway may be dysfunctional in AML. Preliminary studies suggest that targeting the pathways that modify retinoic acid receptor activity may reactivate the dormant retinoic acid-signaling pathway. Such strategies may revive the ability of ATRA to induce myeloid differentiation and apoptosis in non-APL AML.

Would eating carrots protect your liver? A new role involving NKT cells for retinoic acid in hepatitis

Retinoic acid (RA), which is the biologically active form of vitamin A, acts through the nuclear hormone receptor RAR (RA receptor) to induce either gene activation or repression. RA production and its effects have been linked to macrophages, dendritic cells, T and B cells, and iNKT cells in the immune system and play pro- as well as anti-inflammatory roles depending on the cell type and the immune context. In this issue of the European Journal of Immunology, Lee et al. [Eur. J. Immunol. 2012. 42: 1685-1694] show that RA ameliorates Con A-induced murine hepatitis by selectively downmodulating IFN-γ and IL-4 production in disease-causing NKT cells in the liver. Remarkably, this effect is restricted to this liver disease model and does not apply to αGalCer-induced murine liver injury, which is driven by other cytokines. The study identifies retinoid signaling as an important endogenous mechanism controlling immune reactions and also as a potential pharmacological target for treatment of hepatic liver injury. Furthermore, the study by Lee et al. provides additional support for the concept of metabolic regulation of immune function.

Nuclear hormone receptors enable macrophages and dendritic cells to sense their lipid environment and shape their immune response

A key issue in the immune system is to generate specific cell types, often with opposing activities. The mechanisms of differentiation and subtype specification of immune cells such as macrophages and dendritic cells are critical to understand the regulatory principles and logic of the immune system. In addition to cytokines and pathogens, it is increasingly appreciated that lipid signaling also has a key role in differentiation and subtype specification. In this review we explore how intracellular lipid signaling via a set of transcription factors regulates cellular differentiation, subtype specification, and immune as well as metabolic homeostasis. We introduce macrophages and dendritic cells and then we focus on a group of transcription factors, nuclear receptors, which regulate gene expression upon receiving lipid signals. The receptors we cover are the ones with a recognized physiological function in these cell types and ones which heterodimerize with the retinoid X receptor. These are as follows: the receptor for a metabolite of vitamin A, retinoic acid: retinoic acid receptor (RAR), the vitamin D receptor (VDR), the fatty acid receptor: peroxisome proliferator-activated receptor γ (PPARγ), the oxysterol receptor liver X receptor (LXR), and their obligate heterodimeric partner, the retinoid X receptor (RXR). We discuss how they can get activated and how ligand is generated and eliminated in these cell types. We also explore how activation of a particular target gene contributes to biological functions and how the regulation of individual target genes adds up to the coordination of gene networks. It appears that RXR heterodimeric nuclear receptors provide these cells with a coordinated and interrelated network of transcriptional regulators for interpreting the lipid milieu and the metabolic changes to bring about gene expression changes leading to subtype and functional specification. We also show that these networks are implicated in various immune diseases and are amenable to therapeutic exploitation.

Rapid and efficient reprogramming of somatic cells to induced pluripotent stem cells by retinoic acid receptor gamma and liver receptor homolog 1

Somatic cells can be reprogrammed to induced pluripotent stem cells (iPSCs) by expressing four transcription factors: Oct4, Sox2, Klf4, and c-Myc. Here we report that enhancing RA signaling by expressing RA receptors (RARs) or by RA agonists profoundly promoted reprogramming, but inhibiting it using a RAR-α dominant-negative form completely blocked it. Coexpressing Rarg (RAR-γ) and Lrh-1 (liver receptor homologue 1; Nr5a2) with the four factors greatly accelerated reprogramming so that reprogramming of mouse embryonic fibroblast cells to ground-state iPSCs requires only 4 d induction of these six factors. The six-factor combination readily reprogrammed primary human neonatal and adult fibroblast cells to exogenous factor-independent iPSCs, which resembled ground-state mouse ES cells in growth properties, gene expression, and signaling dependency. Our findings demonstrate that signaling through RARs has critical roles in molecular reprogramming and that the synergistic interaction between Rarg and Lrh1 directs reprogramming toward ground-state pluripotency. The human iPSCs described here should facilitate functional analysis of the human genome.

Inhibition of aldehyde dehydrogenase expands hematopoietic stem cells with radioprotective capacity

Hematopoietic stem cells (HSCs) are enriched for aldehyde dehydrogenase (ALDH) activity and ALDH is a selectable marker for human HSCs. However, the function of ALDH in HSC biology is not well understood. We sought to determine the function of ALDH in regulating HSC fate. Pharmacologic inhibition of ALDH with diethylaminobenzaldehyde (DEAB) impeded the differentiation of murine CD34(-)c-kit(+)Sca-1(+)lineage(-) (34(-)KSL) HSCs in culture and facilitated a ninefold expansion of cells capable of radioprotecting lethally irradiated mice compared to input 34(-)KSL cells. Treatment of bone marrow (BM) 34(-)KSL cells with DEAB caused a fourfold increase in 4-week competitive repopulating units, verifying the amplification of short-term HSCs (ST-HSCs) in response to ALDH inhibition. Targeted siRNA of ALDH1a1 in BM HSCs caused a comparable expansion of radioprotective progenitor cells in culture compared to DEAB treatment, confirming that ALDH1a1 was the target of DEAB inhibition. The addition of all trans retinoic acid blocked DEAB-mediated expansion of ST-HSCs in culture, suggesting that ALDH1a1 regulates HSC differentiation via augmentation of retinoid signaling. Pharmacologic inhibition of ALDH has therapeutic potential as a means to amplify ST-HSCs for transplantation purposes.

C/EBPepsilon directs granulocytic-vs-monocytic lineage determination and confers chemotactic function via Hlx

OBJECTIVE

Mutations in the CCAAT enhancer binding protein epsilon (C/EBPepsilon) gene have been identified in the cells of patients with neutrophil specific granule deficiency, a rare congenital disorder marked by recurrent bacterial infections. Their neutrophils, in addition to lacking specific granules required for normal respiratory burst activity, also lack normal phagocytosis and chemotaxis. Although the specific granule deficiency phenotype has been replicated in C/EBPepsilon(-/-) (knockout [KO]) mice, the mechanisms by which C/EBPepsilon mutations act to decrease neutrophil function are not entirely clear.

MATERIALS AND METHODS

In order to determine the role of C/EBPepsilon in neutrophil differentiation and migration, we generated immortalized progenitor cell lines from C/EBPepsilon KO and wild-type mice and performed expression and flow cytometric analysis and functional studies.

RESULTS

Expression of lineage-specific cell surface antigens on our in vitro differentiated cell lines revealed persistent expression of monocytic markers on KO granulocytes. We verified this in primary murine peripheral blood and bone marrow cells. In addition, KO bone marrow had an increase in immature myeloid precursors at the common myeloid progenitor and granulocyte/monocyte progenitor levels, suggesting a critical role for C/EBPepsilon not only in granulocyte maturation beyond the promyelocyte stage, but also in the monocyte/granulocyte lineage decision. We found that restoration of Hlx (H2.0-like homeo box 1) expression, which was decreased in C/EBPepsilon KO cells, rescued chemotaxis, but not the other defects of C/EBPepsilon KO neutrophils.

CONCLUSIONS

We show two new regulatory functions of C/EBPepsilon in myelopoiesis: in the absence of C/EBPepsilon, there is not only incomplete differentiation of granulocytes, but myelopoiesis is disrupted with the appearance of an intermediate cell type with monocyte and granulocyte features, and the neutrophils have abnormal chemotaxis. Restoration of expression of Hlx provides partial recovery of function; it has no effect on neutrophil maturation, but can completely ameliorate the chemotaxis defect in C/EBPepsilon KO cells.

Minireview: Nuclear receptors, hematopoiesis, and stem cells

Nuclear receptors (NRs) regulate a panoply of biological processes, including the function and development of cells within the hematopoietic and immune system, such as erythrocytes, monocytes, and lymphocytes. Significantly less is known regarding the function of NRs in regulating the fate of hematopoietic stem cells (HSCs), the self-renewing, pluripotent cells that give rise to the entirety of the blood and immune systems throughout the lifetime of an individual. Several recent studies suggest, either directly or indirectly, a role for members of the NR family in regulating the differentiation and self-renewal of HSCs, embryonic stem cells, and induced pluripotent stem cells. Herein, we review in detail the function of specific NRs in controlling HSC and other stem cell fate and propose a framework through which these observations can be translated into therapeutic amplification of HSCs for clinical purposes.

Development of macrophages of cyprinid fish

The innate immune responses of early vertebrates, such as bony fishes, play a central role in host defence against infectious diseases and one of the most important effector cells of innate immunity are macrophages. In order for macrophages to be effective in host defence they must be present at all times in the tissues of their host and importantly, the host must be capable of rapidly increasing macrophage numbers during times of need. Hematopoiesis is a process of formation and development of mature blood cells, including macrophages. Hematopoiesis is controlled by soluble factors known as cytokines, that influence changes in transcription factors within the target cells, resulting in cell fate changes and the final development of specific effector cells. The processes involved in macrophage development have been largely derived from mammalian model organisms. However, recent advancements have been made in the understanding of macrophage development in bony fish, a group of organisms that rely heavily on their innate immune defences. Our understanding of the growth factors involved in teleost macrophage development, as well as the receptors and regulatory mechanisms in place to control them has increased substantially. Furthermore, model organisms such as the zebrafish have emerged as important instruments in furthering our understanding of the transcriptional control of cell development in fish as well as in mammals. This review highlights the recent advancements in our understanding of teleost macrophage development. We focused on the growth factors identified to be important in the regulation of macrophage development from a progenitor cell into a functional macrophage and discuss the important transcription factors that have been identified to function in teleost hematopoiesis. We also describe the findings of in vivo studies that have reinforced observations made in vitro and have greatly improved the relevance and importance of using teleost fish as model organisms for studying developmental processes.

Pharmacological manipulation of the RAR/RXR signaling pathway maintains the repopulating capacity of hematopoietic stem cells in culture

The retinoid X receptor (RXR) contributes to the regulation of diverse biological pathways via its role as a heterodimeric partner of several nuclear receptors. However, RXR has no established role in the regulation of hematopoietic stem cell (HSC) fate. In this study, we sought to determine whether direct modulation of RXR signaling could impact human HSC self-renewal or differentiation. Treatment of human CD34(+)CD38(-)lin(-) cells with LG1506, a selective RXR modulator, inhibited the differentiation of HSCs in culture and maintained long-term repopulating HSCs in culture that were otherwise lost in response to cytokine treatment. Further studies revealed that LG1506 had a distinct mechanism of action in that it facilitated the recruitment of corepressors to the retinoic acid receptor (RAR)/RXR complex at target gene promoters, suggesting that this molecule was functioning as an inverse agonist in the context of this heterodimer. Interestingly, using combinatorial peptide phage display, we identified unique surfaces presented on RXR when occupied by LG1506 and demonstrated that other modulators that exhibited these properties functioned similarly at both a mechanistic and biological level. These data indicate that the RAR/RXR heterodimer is a critical regulator of human HSC differentiation, and pharmacological modulation of RXR signaling prevents the loss of human HSCs that otherwise occurs in short-term culture.

Opposing effects of retinoid signaling on astrogliogenesis in embryonic day 13 and 17 cortical progenitor cells

All-trans retinoic acid (RA) is a differentiation factor in many tissues. However, its role in astrogliogenesis has not been extensively studied. Here, we investigated the effect of RA on the regulation of astrogliogenesis at different cortical developmental stages. We prepared rat cortical progenitor cells from embryonic day (E) 13 and E17, which correspond to the beginning of neurogenic and astrogliogenic periods, respectively. Surprisingly, RA promoted astrogliogenesis at E17 but inhibited astrogliogenesis induced by ciliary neurotrophic factor (CNTF) at E13. The inhibitory effect of RA on astrogliogenesis at E13 was not due to premature commitment of progenitors to a neuronal or oligodendroglial lineage. Rather, RA retained more progenitors in a proliferative state. Furthermore, RA inhibition of astrogliogenesis at E13 was independent of STAT3 signaling and required the function of the alpha and beta isoforms of the RA receptors (RAR). Moreover, the differential response of E13 and E17 progenitors to RA was due to differences in the intrinsic properties of these cells that are preserved in vitro. The inhibitory effect of RA on cytokine-induced astrogliogenesis at E13 may contribute to silencing of any potential precocious astrogliogenesis during the neurogenic period.

Mouse neutrophils lacking lamin B-receptor expression exhibit aberrant development and lack critical functional responses

OBJECTIVE

The capacity of neutrophils to eradicate bacterial infections is dependent on normal development and activation of functional responses, which include chemotaxis and generation of oxygen radicals during the respiratory burst. A unique feature of the neutrophil is its highly lobulated nucleus, which is thought to facilitate chemotaxis, but may also play a role in other critical neutrophil functions. Nuclear lobulation is dependent on expression of the inner nuclear envelope protein, the lamin B receptor (LBR), mutations of which cause hypolobulated neutrophil nuclei in human Pelger-Huët anomaly and the "ichthyosis" (ic) phenotype in mice. In this study, we have investigated roles for LBR in mediating neutrophil development and activation of multiple neutrophil functions, including chemotaxis and the respiratory burst.

MATERIALS AND METHODS

A progenitor EML cell line was generated from an ic/ic mouse, and derived cells that lacked LBR expression were induced to mature neutrophils and then examined for abnormal morphology and functional responses.

RESULTS

Neutrophils derived from EML-ic/ic cells exhibited nuclear hypolobulation identical to that observed in ichthyosis mice. The ic/ic neutrophils also displayed abnormal chemotaxis, supporting the notion that nuclear segmentation augments neutrophil extravasation. Furthermore, promyelocytic forms of ic/ic cells displayed decreased proliferative responses and produced a deficient respiratory burst upon terminal maturation.

CONCLUSIONS

Our studies of promyelocytes that lack LBR expression have identified roles for LBR in regulating not only the morphologic maturation of the neutrophil nucleus, but also proliferative and functional responses that are critical to innate immunity.

Disruption of retinoic acid receptor alpha reveals the growth promoter face of retinoic acid

BACKGROUND

Retinoic acid (RA), the bioactive derivative of Vitamin A, by epigenetically controlling transcription through the RA-receptors (RARs), exerts a potent antiproliferative effect on human cells. However, a number of studies show that RA can also promote cell survival and growth. In the course of one of our studies we observed that disruption of RA-receptor alpha, RARalpha, abrogates the RA-mediated growth-inhibitory effects and unmasks the growth-promoting face of RA (Ren et al., Mol. Cell. Biol., 2005, 25:10591). The objective of this study was to investigate whether RA can differentially govern cell growth, in the presence and absence of RARalpha, through differential regulation of the "rheostat" comprising ceramide (CER), the sphingolipid with growth-inhibitory activity, and sphingosine-1-phosphate (S1P), the sphingolipid with prosurvival activity.

METHODOLOGY/PRINCIPAL FINDINGS

We found that functional inhibition of endogenous RARalpha in breast cancer cells by using either RARalpha specific antagonists or a dominant negative RARalpha mutant hampers on one hand the RA-induced upregulation of neutral sphingomyelinase (nSMase)-mediated CER synthesis, and on the other hand the RA-induced downregulation of sphingosine kinase 1, SK1, pivotal for S1P synthesis. In association with RA inability to regulate the sphingolipid rheostat, cells not only survive, but also grow more in response to RA both in vitro and in vivo. By combining genetic, pharmacological and biochemical approaches, we mechanistically demonstrated that RA-induced growth is, at least in part, due to non-RAR-mediated activation of the SK1-S1P signaling.

CONCLUSIONS/SIGNIFICANCE

In the presence of functional RARalpha, RA inhibits cell growth by concertedly, and inversely, modulating the CER and S1P synthetic pathways. In the absence of a functional RARalpha, RA-in a non-RAR-mediated fashion-promotes cell growth by activating the prosurvival S1P signaling. These two distinct, yet integrated processes apparently concur to the growth-promoter effects of RA.

Emerging role for microRNAs in acute promyelocytic leukemia

Hematopoiesis is highly controlled by lineage-specific transcription factors that, by interacting with specific DNA sequences, directly activate or repress specific gene expression. These transcription factors have been found mutated or altered by chromosomal translocations associated with leukemias, indicating their role in the pathogenesis of these malignancies. The post-genomic era, however, has shown that transcription factors are not the only key regulators of gene expression. Epigenetic mechanisms such as DNA methylation, posttranslational modifications of histones, remodeling of nucleosomes, and expression of small regulatory RNAs all contribute to the regulation of gene expression and determination of cell and tissue specificity. Deregulation ofthese epigenetic mechanisms cooperates with genetic alterations to the establishment and progression of tumors. MicroRNAs (miRNAs) are negative regulators of the expression of genes involved in development, differentiation, proliferation, and apoptosis. Their expression appears to be tissue-specific and highly regulated according to the cell's developmental lineage and stage. Interestingly, miRNAs expressed in hematopoietic cells have been found mutated or altered by chromosomal translocations associated with leukemias. The expression levels of a specific miR-223 correlate with the differentiation fate of myeloid precursors. The activation of both pathways of transcriptional regulation by the myeloid lineage-specific transcription factor C/EBPalpha (CCAAT/enhancer-binding protein-alpha), and posttranscriptional regulation by miR-223 appears essential for granulocytic differentiation and clinical response of acute promyelocytic leukemia (APL) blasts to all-trans retinoic acid (ATRA). Together, this evidence underlies transcription factors, chromatin remodeling, and miRNAs as ultimate determinants for the correct organization of cell type-specific gene arrays and hematopoietic differentiation, therefore providing new targets for the diagnosis and treatment of leukemias.

A Repressive Epigenetic Domino Effect Confers Susceptibility to Breast Epithelial Cell Transformation: Implications for Predicting Breast Cancer Risk

Retinoic acid (RA) is a master epigenetic regulator that plays a pivotal role in both breast morphogenesis and development. Here, we show for the first time that RA, via the RA receptor alpha (RARalpha), epigenetically regulates in a concerted fashion the transcription of two RA-responsive genes, the RA receptor beta2 (RARbeta2) and the cellular retinol-binding protein 1 (CRBP1). Specifically, an impaired RA signal through RARalpha in human breast epithelial cells triggers a repressive epigenetic domino effect, involving first RARbeta2 and second CRBP1. The phenotype acquired by breast epithelial cells clearly implies that the resistance to RA-mediated growth inhibition precedes the acquisition of morphological epithelial transformation, thus supporting the occurrence of sequential transcriptional silencing of first RARbeta2 and second CRBP1. The identification of this epigenetic network mechanistically linking RARbeta2 and CRBP1 transcription provides the basis for devising more accurate epigenetic tests for the prediction of breast cancer risk.

Dominant negative retinoic acid receptor initiates tumor formation in mice

Background

Retinoic acid suppresses cell growth and promotes cell differentiation, and pharmacological retinoic acid receptor (RAR) activation is anti-tumorigenic. This begs the question of whether chronic physiological RAR activation by endogenous retinoids is likewise anti-tumorigenic.

Results

To address this question, we generated transgenic mice in which expression of a ligand binding defective dominant negative RARα (RARαG303E) was under the control of the mouse mammary tumor virus (MMTV) promoter. The transgene was expressed in the lymphoid compartment and in the mammary epithelium. Observation of aging mice revealed that transgenic mice, unlike their wild type littermates, developed B cell lymphomas at high penetrance, with a median latency of 40 weeks. MMTV-RARαG303E lymphomas were high grade Pax-5+, surface H+L Ig negative, CD69+ and BCL6- and cytologically and phenotypically resembled human adult high grade (Burkitt's or lymphoblastic) lymphomas. We postulated that mammary tumors might arise after a long latency period as seen in other transgenic models of breast cancer. We tested this idea by transplanting transgenic epithelium into the cleared fat pads of wild type hosts, thus bypassing lymphomagenesis. At 17 months post-transplantation, a metastatic mammary adenocarcinoma developed in one of four transplanted glands whereas no tumors developed in sixteen of sixteen endogenous glands with wild type epithelium.

Conclusion

These findings suggest that physiological RAR activity may normally suppress B lymphocyte and mammary epithelial cell growth and that global RAR inactivation is sufficient to initiate a stochastic process of tumor development requiring multiple transforming events. Our work makes available to the research community a new animal resource that should prove useful as an experimental model of aggressive sporadic lymphoma in immunologically uncompromised hosts. We anticipate that it may also prove useful as a model of breast cancer.

Induction of murine leukemia and lymphoma by dominant negative retinoic acid receptor alpha

Acute promyelocytic leukemia (APL) is invariably associated with chromosomal translocation to retinoic acid receptor alpha (RARalpha) locus. In a vast majority of cases, RARalpha translocates to and fuses with the promyelocytic leukemia (PML) gene. It was thought that the fusion protein PML-RARalpha acts as a double dominant negative mutant to inhibit the PML and RARalpha signaling. In an attempt to study the physiological role of retinoic acid in mammary gland development, we created a transgenic model system expressing a dominant negative RARalpha under the regulation of murine mammary tumor viral promoter. We found that the transgene was also targeted to the lymphoid system in addition to mammary gland. Here we showed that dominant negative RARalpha induced acute lymphoblastic leukemia and lymphoma development in the transgenic mice. Retinoic acid blocked tumor development ex vivo through induction of apoptosis. Thus, our results suggested that disruption of RARalpha signaling was the first essential step in the development of APL in vivo.

Impaired retinoic acid (RA) signal leads to RARbeta2 epigenetic silencing and RA resistance

Resistance to the growth-inhibitory action of retinoic acid (RA), the bioactive derivative of vitamin A, is common in human tumors. One form of RA resistance has been associated with silencing and hypermethylation of the retinoic acid receptor beta2 gene (RARbeta2), an RA-regulated tumor suppressor gene. The presence of an epigenetically silent RARbeta2 correlates with lack of the RA receptor alpha (RARalpha). Normally, RARalpha regulates RARbeta2 transcription by mediating dynamic changes of RARbeta2 chromatin in the presence and absence of RA. Here we show that interfering with RA signal through RARalpha (which was achieved by use of a dominant-negative RARalpha, by downregulation of RARalpha by RNA interference, and by use of RARalpha antagonists) induces an exacerbation of the repressed chromatin status of RARbeta2 and leads to RARbeta2 transcriptional silencing. Further, we demonstrate that RARbeta2 silencing causes resistance to the growth-inhibitory effect of RA. Apparently, RARbeta2 silencing can also occur in the absence of DNA methylation. Conversely, we demonstrate that restoration of RA signal at a silent RARbeta2 through RARalpha leads to RARbeta2 reactivation. This report provides proof of principle that RARbeta2 silencing and RA resistance are consequent to an impaired integration of RA signal at RARbeta2 chromatin.

Retinoids potentiate peroxisome proliferator-activated receptor gamma action in differentiation, gene expression, and lipid metabolic processes in developing myeloid cells

Nuclear hormone receptors have been shown to be important transcription factors for regulating lipid metabolism in myeloid cells and were also implicated in differentiation processes of the myeloid lineage and macrophages. Peroxisome proliferator-activated receptor gamma (PPARgamma) seems to be a key component of lipid uptake by inducing the scavenger receptor CD36 that mediates oxidized low-density lipoprotein uptake in macrophages. Retinoic acid receptors, on the other hand, were also shown to play important roles in myeloid cell differentiation. In this study, we present evidence for a cross-talk between these two nuclear receptor pathways in myeloid cells. We show that expression level of PPARgamma increases with the degree of monocyte/macrophage commitment during maturation. Activation of PPARgamma leads to the increased expression of maturation markers (e.g., CD14, CD36). It is interesting that retinoid treatment potentiates PPARgamma's ability to induce transcription of its target genes. Retinoid-increased PPARgamma response is sufficient for enhancing lipid uptake. Our data, taken together, indicate that the expression level of PPARgamma increases during monocyte/macrophage development. PPARgamma activity can be enhanced by retinoids at least in part via increasing PPARgamma expression level. These observations can be exploited to enhance therapeutically beneficial PPAR responses in myeloid cells.

Cross talk between retinoic acid signaling and transcription factor GATA-2

All-trans-retinoic acid (RA) stimulates differentiation of normal hematopoietic progenitors and acute myeloid leukemia cells. GATA-2 is a transcription factor expressed in early progenitor cells and implicated in the control of the fate of hematopoietic stem cells and progenitor cells. We have investigated the possibility that the GATA and nuclear hormone receptor pathways are functionally linked through direct protein-protein interaction. Here we demonstrate that in human myeloid KG1 cells, RA receptor alpha (RARalpha), the major RAR expressed in hematopoietic cells, associates with GATA-2. This association is mediated by the zinc fingers of GATA-2 and the DNA-binding domain of RARalpha. As a consequence of this interaction, RARalpha is tethered to the DNA sites that are recognized and bound by GATA-2, and the transcriptional activity of GATA-2 becomes RA responsive. The RA responsiveness of GATA-dependent transcription is eliminated by expression of either a dominant negative form of RARalpha or a GATA-2 mutant that fails to interact with RARalpha. Overexpression of RXRalpha inhibits RARalpha binding to the GATA-2-DNA complex, thus resulting in attenuation of the effects of RARalpha on GATA-2 activity. In addition, inhibition by RA of GATA-2-dependent hematopoietic colony formation in an embryonic stem cell model of hematopoietic differentiation provided biological evidence for functional cross talk between RA and GATA-2-dependent pathways.

Nuclear receptor corepressors

The ability of NR LBDs to transfer repression function to a heterologous DNA binding domain, and the cross-squelching of repression by untethered LBDs, has suggested that repression is mediated by interactions with putative cellular corepressor proteins. The yeast-two hybrid screen for protein interactors has proven to be the key to the isolation and characterization of corepressors. This short review will focus on N-CoR and SMRT.

The molecular pathogenesis of acute promyelocytic leukaemia: implications for the clinical management of the disease

Acute promyelocytic leukaemia (APL) is characterised by chromosomal rearrangements of 17q21, leading to fusion of the gene encoding retinoic acid receptor alpha (RARalpha) to a number of alternative partner genes (X), the most frequent of which are PML (>95%), PLZF (0.8%) and NPM (0.5%). Over the last few years, it has been established that the X-RARalpha fusion proteins play a key role in the pathogenesis of APL through recruitment of co-repressors and the histone deacetylase (HDAC)-complex to repress genes implicated in myeloid differentiation. Paradoxically, the X-RARalpha fusion protein has the potential to mediate myeloid differentiation at pharmacological doses of its ligand (all trans-retinoic acid (ATRA)), which is dependent on the dissociation of the HDAC/co-repressor complex. Arsenic compounds have also been shown to be promising therapeutic agents, leading to differentiation and apoptosis of APL blasts. It is now apparent that the nature of the RARalpha-fusion partner is a critical determinant of response to ATRA and arsenic, underlining the importance of cytogenetic and molecular characterisation of patients with suspected APL to determine the most appropriate treatment approach. Standard protocols involving ATRA combined with anthracycline-based chemotherapy, lead to cure of approximately 70% patients with PML-RARalpha-associated APL. Patients at high risk of relapse can be identified by minimal residual disease monitoring. The challenge for future studies is to improve complete remission rates through reduction of induction deaths, particularly due to haemorrhage, identification of patients at high risk of relapse who would benefit from additional therapy, and identification of a favourable-risk group, for which treatment intensity could be reduced, thereby reducing risks of treatment toxicity and development of secondary leukaemia/myelodysplasia. With the advent of ATRA and arsenic, APL has already provided the first example of successful molecularly targeted therapy; it is hoped that with further understanding of the pathogenesis of the disease, the next decade will yield further improvements in the outlook for these patients.

Genetic selection for modulators of a retinoic-acid-responsive reporter in human cells

We used a genetic screening methodology, a human cell line bearing a retinoic-acid-responsive enhanced GFP reporter, and a flow sorter to recover dominant modulators of reporter expression. Four inducers and three suppressors that were fused to the C terminus of a protein scaffold for stability were isolated and their mechanisms of action studied. Mutagenesis experiments indicated that six of these dominant agents exerted their effects at the protein level. The single cDNA coding fragment that was isolated comprised the central 64-amino-acid section of human cyclophilin B, which contained its peptidyl-prolyl isomerase domain; this cyclophilin fragment repressed expression of the retinoic-acid-responsive reporter. The remaining clones encoded peptides shorter than 30 amino acids unrelated to known gene open reading frames. Genetic epistasis studies between the strongest inducer, R3, and a dominant-negative mutant of RARalpha suggest that the two factors function in the same pathway. Transcript microarray analyses suggest that R3 induced a subset of the retinoid-responsive genes in melanoma cells. Finally, yeast two-hybrid assays and co-immunoprecipitation studies of human cell extracts identified PAT1 as a protein that interacts with R3.

Retinoic acid inhibits in vitro development of mast cells but has no marked effect on mature human skin tryptase- and chymase-positive mast cells

Stem cell factor plays a key role in the development of human mast cells via interaction with Kit receptor. We and other groups have previously shown that a number of cytokines can regulate the stem-cell-factor-dependent development of mast cells in vitro. In this study we investigated the effect of retinoic acid on human mast cells in vitro and in vivo. Retinoids are known to have strong modulatory effects on hematopoietic differentiation. We found that all-trans-retinoic acid, at concentrations as low as 1 nM, inhibits the stem-cell-factor-dependent differentiation of mast cells in vitro. This effect of retinoic acid was found to be on progenitor cells, whereas more mature mast cells were less affected. The use of specific agonists binding either to the RAR or the RXR nuclear receptors indicated involvement of both the RAR/RXR and RXR/RXR pathways in inhibiting mast cell differentiation. In contrast to the effects on mast cell progenitors, retinoic acid had no effect on the number of mature mast cells in skin organ cultures. Furthermore, topical treatment of normal skin with a retinoic-acid-containing cream caused an increase in the number of tryptase-positive mast cells, whereas the numbers of the major cutaneous mast cell type, tryptase- and chymase-positive mast cells, remained unaffected. Our results suggest that retinoic acid suppresses commitment of progenitor cells into the mast cell lineage and/or acts on early mast cell progenitors, whereas mature cutaneous mast cells are less susceptible to retinoic acid.

Retinoid-induced G1 arrest and differentiation activation are associated with a switch to cyclin-dependent kinase-activating kinase hypophosphorylation of retinoic acid receptor alpha

Cell cycle G(1) exit is a critical stage where cells commonly commit to proliferate or to differentiate, but the biochemical events that regulate the proliferation/differentiation (P/D) transition at G(1) exit are presently unclear. We previously showed that MAT1 (ménage à trois 1), an assembly factor and targeting subunit of the cyclin-dependent kinase (CDK)-activating kinase (CAK), modulates CAK activities to regulate G(1) exit. Here we find that the retinoid-induced G(1) arrest and differentiation activation of cultured human leukemic cells are associated with a switch to CAK hypophosphorylation of retinoic acid receptor alpha (RARalpha) from CAK hyperphosphorylation of RARalpha. The switch to CAK hypophosphorylation of RARalpha is accompanied by decreased MAT1 expression and MAT1 fragmentation that occurs in the differentiating cells through the all-trans-retinoic acid (ATRA)-mediated proteasome degradation pathway. Because HL60R cells that harbor a truncated ligand-dependent AF-2 domain of RARalpha do not demonstrate any changes in MAT1 levels or CAK phosphorylation of RARalpha following ATRA stimuli, these biochemical changes appear to be mediated directly through RARalpha. These studies indicate that significant changes in MAT1 levels and CAK activities on RARalpha phosphorylation accompany the ATRA-induced G(1) arrest and differentiation activation, which provide new insights to explore the inversely coordinated P/D transition at G(1) exit.

Retinoids and retinoic acid receptors regulate growth arrest and apoptosis in human mammary epithelial cells and modulate expression of CBP/p300

Retinoids and retinoic acid receptors (RARs) are important mediators of normal epithelial cell homeostasis. To assess the role of retinoids and RARs in regulating growth arrest and apoptosis in benign and malignant mammary epithelial cells, two model systems were developed: 1) RAR function was suppressed in retinoid-sensitive normal human mammary epithelial cells (HMECs) by the dominant-negative retinoic acid receptor, RARalpha403 (DNRAR), and 2) retinoid-resistant MCF-7 breast cancer cells were transduced with a functional RARbeta2. Inhibition of RAR function by the DNRAR in HMECs resulted in retinoid-resistance, increased proliferation, and dysregulated growth when cells were cultured in reconstituted extracellular matrix (rECM). Expression of RARbeta2 in MCF-7 cells resulted in sensitivity to retinoid-induced growth arrest and apoptosis. The CREB-binding protein (CBP) and the homologous protein p300 are tightly regulated, rate-limiting integrators of diverse signaling pathways and are recruited during retinoid-mediated transcriptional activation. The relationship between retinoid receptor expression, growth regulation, and transcriptional regulation of CBP/p300 is poorly understood. Inhibition of RAR function in HMECs by DNRAR suppressed expression of CBP/p300 and expression of RARbeta2 in MCF-7 cells promoted induction of CBP/p300 when cells were treated with 1.0 microM all-trans-retinoic acid (ATRA). These results suggest that ATRA and RARs regulate growth arrest of HMECs and modulate CBP/p300 protein expression. Since CBP and p300 are normally present in limiting amounts, their regulation by ATRA and RARs may be an important element in the control of transcriptional activation of genes regulating growth arrest and apoptosis.

Anticancer effect of retinoic acid via AP-1 activity repression is mediated by retinoic acid receptor alpha and beta in gastric cancer cells

To uncover the mechanisms relating to the anticancer effect of retinoic acids in gastric cancer cells, the mediation of activator protein-1 (AP-1) activity repression by retinoic acid receptors (RARs) was investigated. All-trans retinoic acid (ATRA) inhibited AP-1 activity in BGC-823 cells (RARalpha(+), RARbeta(+)), but not in MKN-45 cells (RARalpha(lo), RARbeta(-)). Transient transfection of RARbeta expression vector into MKN-45 cells significantly resulted in direct repression of AP-1 activity in a receptor concentration-dependent manner, and this could be strengthened by ATRA. Stable transfection of RARbeta into MKN-45 cells directly inhibited cell growth and colony formation, and ATRA also enhanced these effects. Transient transfection of RARalpha into MKN-45 cells however, displayed receptor concentration-dependent AP-1 activity inhibition only in the presence of ATRA. Stable transfection of RARalpha into MKN-45 cells resulted in ATRA-dependent inhibition of cell growth and colony formation. For AP-1 binding activity induced by TPA, the repressive effect of ATRA was only observed in BGC-823 and RARalpha and RARbeta stably transfected MKN-45 cells, but not in intact MKN-45 cells. This indicates the necessity for sufficient cellular RARalpha and/or RARbeta in order for AP-1 activity repression to occur. Deletion of DNA binding domain (DBD) of RARbeta, but not ligand binding domain (LBD), eliminated the anti-AP-1 function of RARbeta. It is therefore concluded that both RARalpha and RARbeta are mediators in the anticancer function of ATRA via AP-1 activity inhibition, and that RARbeta, not RARalpha, can inhibit AP-1 activity to a certain extent directly by itself. Thus DBD, not LBD, is critical for anti-AP-1 activity.

BCR-ABL alters the proliferation and differentiation response of multipotent hematopoietic cells to stem cell factor

Chronic myeloid leukaemia (CML), a hematopoietic stem cell disorder is characterized by the expression of BCR-ABL. To investigate the effects of BCR-ABL on multipotent hematopoietic cells, a temperature sensitive BCR-ABL tyrosine kinase was expressed in the cell line, FDCP-Mix. BCR-ABL mediated an increase in c-kit expression that correlated with an enhanced mitogenic response to SCF. This was not observed in the absence of Bcr-Abl kinase activity or presence of the BCR-ABL inhibitor STI571, which also inhibits c-kit. When cultured in a combination of SCF plus G-CSF the FDCP-Mix cells undergo neutrophilic differentiation over a 7-10 day period. When BCR-ABL was active there was a marked inhibition of cell maturation compared to control cells in which BCR-ABL was either inactive or not present. However, BCR-ABL did not block differentiation as the cells eventually undergo terminal maturation. These data argue that BCR-ABL is directly responsible for the enhanced response to SCF reported in CML progenitor cells. Furthermore, although the primary effect of STI571 is via direct inhibition of BCR-ABL, STI571 additionally reduces the enhanced response to SCF. Thus there are two sites of STI571 action of potential importance in Bcr-Abl expressing cells.

The cytokines IL-3 and GM-CSF regulate the transcriptional activity of retinoic acid receptors in different in vitro models of myeloid differentiation

The disruption of retinoic acid receptor (RAR) activity that characterizes human acute promyelocytic leukemia (APL) is associated with a block to granulocytic differentiation indicating that RARs are critical regulators of normal myeloid differentiation. Nevertheless, how RAR activity might be regulated in the presumably homogenous concentration of retinoids in blood and bone marrow and how these receptors might interact with specific hematopoietic cytokines to regulate normal myeloid differentiation remain unclear. Here, using several cytokine-dependent in vitro models of myeloid development, it was observed that specific hematopoietic cytokines that normally regulate myeloid lineage commitment and differentiation (interleukin-3 and granulocyte-macrophage colony-stimulating factor) trigger the enhancement of both ligand-dependent and ligand-independent transcriptional activity of both endogenous and exogenous (transiently transfected) RARs. This cytokine-mediated enhancement of RAR activity is not associated with any observed changes in expression of the RARs or their respective coactivators/corepressors. These studies define a previously unknown cytokine-RAR interaction during myelopoiesis and suggest that RAR activation might be a critical downstream event following interleukin-3 and granulocyte-macrophage colony-stimulating factor signaling during myeloid differentiation. This observation of ligand-independent activation of RARs that is mediated by certain cytokines represents a new paradigm with respect to how RAR activity might be modulated during hematopoiesis and also suggests a molecular basis for the differential sensitivity of human acute myelogenous leukemia cells to retinoids.

Retinoic acid modulates IL-5 receptor expression and selectively inhibits eosinophil-basophil differentiation of hemopoietic progenitor cells

BACKGROUND

IL-5 plays a central role in eosinophil and basophil differentiation, exerting its effects through the IL-5 receptor (IL-5Ralpha). Currently, little is known concerning regulation of IL-5Ralpha expression in the context of commitment of hemopoietic progenitor cells to the eosinophil and basophil lineages.

OBJECTIVE

Because all-trans retinoic acid (ATRA) is known to modulate some aspects of hemopoietic differentiation, we examined the effects of ATRA on eosinophil-basophil differentiation and IL-5Ralpha expression.

METHODS

Progenitor cells were obtained from bone marrow aspirates and cord blood samples. Enriched populations of CD34(+) cells were isolated by means of positive immunomagnetic selection with MACS beads.

RESULTS

In semisolid methylcellulose cultures of normal human bone marrow, ATRA (10(-6) mol/L) selectively suppressed eosinophil-basophil colony-forming units but had no effect on granulocyte-macrophage colony-forming units. Similarly, ATRA (10(-6) mol/L) inhibited eosinophil-basophil differentiation of cord blood CD34(+) cells in liquid culture, whereas neutrophil differentiation proceeded without impediment. Most importantly, these effects of ATRA (10(-8) to 10(-6) mol/L) on CD34(+) cells were associated with a dose-dependent inhibition of IL-5Ralpha but no change in GM-CSF receptor expression, as detected with flow cytometry.

CONCLUSIONS

These findings indicate that retinoids can differentially regulate expression of IL-5Ralpha, but not GM-CSF receptor, and that these effects have functional consequences in vitro on eosinophil and basophil differentiation.

Multipotent hematopoietic cell lines derived from C/EBPalpha(-/-) knockout mice display granulocyte macrophage-colony-stimulating factor, granulocyte- colony-stimulating factor, and retinoic acid-induced granulocytic differentiation

The transcription factor C/EBPalpha is an important mediator of granulocyte differentiation and regulates the expression of multiple granulocyte-specific genes including the granulocyte-colony-stimulating factor (G-CSF) receptor, neutrophil elastase, and myeloperoxidase. Indeed C/EBPalpha knockout mice display a profound block in granulocyte differentiation. To study this block in granulocytic differentiation in more detail, retroviral vector-mediated transduction of a dominant-negative retinoic acid receptor was used to establish hematopoietic growth factor-dependent, lympho-myeloid progenitor cell lines from the fetal livers of both the C/EBPalpha knockout animals (C/EBPalpha(-/-)) and their heterozygous littermates (C/EBPalpha(+/-)). Surprisingly, the C/EBPalpha(-/-) cell lines displayed significant spontaneous granulocytic differentiation, and this differentiation was markedly enhanced when the cells were stimulated with granulocyte macrophage (GM)-CSF. This GM-CSF-mediated differentiation was associated with the up-regulation of G-CSF receptor mRNA, and the combination of GM-CSF and G-CSF generated more than 95% mature neutrophils in the C/EBPalpha(-/-) cultures. The addition of all-trans retinoic acid also enhanced this granulocytic differentiation of the cultured C/EBPalpha(-/-) cells, indicating that the activated retinoic acid receptors can enhance granulocytic differentiation through a molecular pathway that is independent of C/EBPalpha. These studies clearly indicate that terminal granulocytic differentiation associated with the up-regulation of C/EBPalpha-responsive genes can occur in the absence of C/EBPalpha, and they indicate the existence of multiple independent molecular pathways potentially used by primitive hematopoietic precursors that can lead to the development of mature granulocytes.

Enrichment during transdominant genetic experiments using a flow sorter

BACKGROUND

Flow cytometry, in combination with retroviral expression libraries, is a powerful tool for genetic experimentation in mammalian cells. Expression libraries are transduced into cells engineered with a fluorescent reporter. Sorting for either bright or dim cells allows enrichment for specific inhibitors that alter reporter activity. This strategy has been used to isolate peptides and RNAs that either activate or suppress defined biochemical pathways.

METHODS

Several variables contribute to the enrichment process: (1) the background of the fluorescence bioassay; (2) the mean fluorescence ratio between the induced and noninduced reporter cell populations; (3) the genetic penetrance, or strength, of the inhibitor; and (4) the multiplicity of infection (MOI). An experimental and theoretical analysis, including computer modeling, of these issues in the context of a mammalian cell bioassay was undertaken.

RESULTS

MOI measurements were shown to be problematic. High MOI had little effect on enrichment early in the cycling process but a significant effect at later stages. Penetrance and background were critical throughout the process. Enrichments within about twofold of the theoretical maximum were observed.

CONCLUSIONS

Caution should be exercised in MOI determination because of the danger of significant underestimation. High MOI is potentially advantageous early in the selection process but hinders enrichment in the later rounds. Modeling shows that MOI, assay background and clone penetrance are the principal variables that determine the success of transdominant selections by FACS.

Transcriptional regulation of hemopoiesis

The regulation of blood cell formation, or hemopoiesis, is central to the replenishment of mature effector cells of innate and acquired immune responses. These cells fulfil specific roles in the host defense against invading pathogens, and in the maintenance of homeostasis. The development of hemopoietic cells is under stringent control from extracellular and intracellular stimuli that result in the activation of specific downstream signaling cascades. Ultimately, all signal transduction pathways converge at the level of gene expression where positive and negative modulators of transcription interact to delineate the pattern of gene expression and the overall cellular hemopoietic response. Transcription factors, therefore, represent a nodal point of hemopoietic control through the integration of the various signaling pathways and subsequent modulation of the transcriptional machinery. Transcription factors can act both positively and negatively to regulate the expression of a wide range of hemopoiesis-relevant genes including growth factors and their receptors, other transcription factors, as well as various molecules important for the function of developing cells. The expression of these genes is dependent on the complex interactions between transcription factors, co-regulatory molecules, and specific binding sequences on the DNA. Recent advances in various vertebrate and invertebrate systems emphasize the importance of transcription factors for hemopoiesis control and the evolutionary conservation of several of such mechanisms. In this review we outline some of the key issues frequently identified in studies of the transcriptional regulation of hemopoietic gene expression. In teleosts, we expect that the characterization of several of these transcription factors and their regulatory mechanisms will complement recent advances in a number of fish systems where identification of cytokine and other hemopoiesis-relevant factors are currently under investigation.

Retinoid therapy of childhood cancer

In vitro studies that showed RA could cause growth arrest and differentiation of myelogenous leukemia and neuroblastoma led to clinical trials of retinoids in APL and neuroblastoma that increased survival for both of those diseases. In the case of APL, ATRA has been the drug of choice, and preclinical and clinical data support direct combinations of ATRA with cytotoxic chemotherapy. For neuroblastoma, a phase I study defined a dose of 13-cis-RA, which was tolerable in patients after myeloablative therapy, and a phase III trial that showed postconsolidation therapy with 13-cis-RA improved EFS for patients with high-risk neuroblastoma. Preclinical studies in neuroblastoma indicate that ATRA or 13-cis-RA can antagonize cytotoxic chemotherapy and radiation, so use of 13-cis-RA in neuroblastoma is limited to maintenance after completion of cytotoxic chemotherapy and radiation. A limitation on the antitumor benefit of ATRA in APL is the marked decrease in drug levels that occurs during therapy as a result of induction of drug metabolism, resulting in a shorter drug half-life and decreased plasma levels. Although early studies sought to overcome the pharmacologic limitations of ATRA therapy in APL, the demonstration that ATO is active against APL in RA-refractory patients has led to a focus on studies employing ATO. Use of 13-cis-RA in neuroblastoma has avoided the decreased plasma levels seen with ATRA. It is likely that recurrent disease seen during or after 13-cis-RA therapy in neuroblastoma is due to tumor cell resistance to retinoid-mediated differentiation induction. Studies in neuroblastoma cell lines resistant to 13-cis-RA and ATRA have shown that they can be sensitive, and in some cases collaterally hypersensitive, to the cytotoxic retinoid fenretinide. Fenretinide induces tumor cell cytotoxicity rather than differentiation, acts independently from RA receptors, and in initial phase I trials has been well tolerated. Clinical trials of fenretinide, alone and in combination with ceramide modulators, are in development.

Transcription factors that regulate growth and differentiation of myeloid cells

Recently much progress has been made in our understanding of how myeloid progenitor cells undergo commitment and become mature granulocytes or monocytes/macrophages. Studies of normal and leukemic myeloid cells as well as those of cells derived from mice with targeted disruption showed that a series of transcription factors play a major role in both commitment and maturation of myeloid cells. This is primarily because these transcription factors direct an ordered pattern of gene expression according to a well-defined developmental program. PU.1, an Ets family member, is one of the master transcription factors identified to regulate development of both granulocytes and monocytes/macrophages. Further, C/EBPalpha and C/EBPvarepsilon of the bZip family have important roles in directing granulocytic maturation. A number of additional transcription factors such as AML1, RARalpha, MZF-1, Hox and STAT families of transcription factors, Egr-1 and c-myb etc are shown to play roles in myeloid cell differentiation. Our laboratory has recently obtained evidence that ICSBP, a member of the IRF family, is involved in lineage commitment during myeloid cell differentiation and stimulates maturation of functional macrophages. Future elucidation of pathways and networks through which these transcription factors act in various stages of development would provide a more definitive picture of myeloid cell commitment and maturation.