Function of RARalpha during the maturation of neutrophils

Use of tamibarotene, a potent and selective RARα agonist, in combination with azacitidine in patients with relapsed and refractory AML with RARA gene overexpression

Tamibarotene-based therapy is a novel targeted approach for the treatment of relapsed/refractory (R/R) acute myeloid leukemia (AML) with retinoic acid receptor alpha (RARA) gene overexpression. Approximately, 50% of higher-risk myelodysplastic syndrome (MDS) patients and approximately 30% of AML patients are positive for RARA overexpression using a blood-based biomarker test that measures RARA expression in peripheral blasts. A phase 2 study investigating the activity of tamibarotene in patients with RARA overexpression was conducted in patients with AML and MDS (NCT02807558). In 28 patients with R/R AML and RARA overexpression treated with tamibarotene in combination with azacitidine, the median overall survival was 5.9 months. In 21 response-evaluable patients, the complete remission/complete remission with incomplete hematologic recovery (CR/CRi) rate was 19%, and median time to initial CR/CRi was 1.2 months. The favorable safety profile and preliminary clinical activity support the development of combination therapies with tamibarotene in myeloid malignancies with RARA overexpression.

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

Retinoic acid receptor regulation of decision-making for cell differentiation

All-trans retinoic acid (ATRA) activation of retinoic acid receptors (RARs) is crucial to an organism’s proper development as established by findings for mouse foetuses from dams fed a vitamin A-deficient diet. ATRA influences decision-making by embryonic stem (ES) cells for differentiation including lineage fate. From studies of knockout mice, RARα and RARγ regulate haematopoiesis whereby active RARα modulates the frequency of decision-making for myeloid differentiation, but is not essential for myelopoiesis, and active RARγ supports stem cell self-renewal and maintenance. From studies of zebrafish embryo development, active RARγ plays a negative role in stem cell decision-making for differentiation whereby, in the absence of exogenous ATRA, selective agonism of RARγ disrupted stem cell decision-making for differentiation patterning for development. From transactivation studies, 0.24 nM ATRA transactivated RARγ and 19.3 nM (80-fold more) was needed to transactivate RARα. Therefore, the dose of ATRA that cells are exposed to in vivo, from gradients created by cells that synthesize and metabolize, is important to RARγ versus RARα and RARγ activation and balancing of the involvements in modulating stem cell maintenance versus decision-making for differentiation. RARγ activation favours stemness whereas concomitant or temporal activation of RARγ and RARα favours differentiation. Crosstalk with signalling events that are provoked by membrane receptors is also important.

Targeting the Retinoic Acid Pathway to Eradicate Cancer Stem Cells

All-trans retinoic acid is a morphogen during embryogenesis and a teratogen. Cancer is an error of development, and the retinoic acid receptors (RAR) for all-trans retinoic acid play a role in cancer. Expression of the cytosolic aldehyde dehydrogenases, which mediate the last step to the synthesis of all-trans retinoic acid, is deregulated in various human cancers. Inhibiting these enzymes using a variety of agents reduced the proliferation of lung cancer cells, reduced the proliferation and induced apoptosis of ovarian, prostate, squamous, and uterine cancer cells, and sensitised breast, colorectal and ovarian cancer cells to chemotherapeutic agents. RARγ is an oncogene within some cases of AML, cholangiocarcinoma, colorectal cancer, clear cell renal cell carcinoma, hepatocellular carcinoma, pancreatic ductal adenocarcinoma, prostate cancer, and ovarian cancer. Pan-RAR and RARγ antagonist inhibition of the action of RARγ led to necroptosis of human prostate and pediatric brain tumour cancer stem cells. Treatment of hepatocellular carcinoma cells with the flavenoid acacetin, which interferes with the action of RARγ, decreased cell growth and induced apoptosis. Targeting the retinoic acid pathway is promising regarding the development of new drugs to eradicate cancer stem cells.

Lessons to cancer from studies of leukemia and hematopoiesis

The starting point to describing the origin and nature of any cancer must be knowledge about how the normal counterpart tissue develops. New principles to the nature of hematopoietic stem cells have arisen in recent years. In particular, hematopoietic stem cells can “choose” a cell lineage directly from a spectrum of the end-cell options, and are, therefore, a heterogeneous population of lineage affiliated/biased cells. These cells remain versatile because the developmental trajectories of hematopoietic stem and progenitor cells are broad. From studies of human acute myeloid leukemia, leukemia is also a hierarchy of maturing or partially maturing cells that are sustained by leukemia stem cells at the apex. This cellular hierarchy model has been extended to a wide variety of human solid tumors, by the identification of cancer stem cells, and is termed the cancer stem cell model. At least, two genomic insults are needed for cancer, as seen from studies of human childhood acute lymphoblastic leukemia. There are signature mutations for some leukemia’s and some relate to a transcription factor that guides the cell lineage of developing hematopoietic stem/progenitor cells. Similarly, some oncogenes restrict the fate of leukemia stem cells and their offspring to a single maturation pathway. In this case, a loss of intrinsic stem cell versatility seems to be a property of leukemia stem cells. To provide more effective cures for leukemia, there is the need to find ways to eliminate leukemia stem cells.

Investigating the genetic architecture of disease resilience in pigs by genome-wide association studies of complete blood count traits collected from a natural disease challenge model

Background

Genetic improvement for disease resilience is anticipated to be a practical method to improve efficiency and profitability of the pig industry, as resilient pigs maintain a relatively undepressed level of performance in the face of infection. However, multiple biological functions are known to be involved in disease resilience and this complexity means that the genetic architecture of disease resilience remains largely unknown. Here, we conducted genome-wide association studies (GWAS) of 465,910 autosomal SNPs for complete blood count (CBC) traits that are important in an animal’s disease response. The aim was to identify the genetic control of disease resilience.

Results

Univariate and multivariate single-step GWAS were performed on 15 CBC traits measured from the blood samples of 2743 crossbred (Landrace × Yorkshire) barrows drawn at 2-weeks before, and at 2 and 6-weeks after exposure to a polymicrobial infectious challenge. Overall, at a genome-wise false discovery rate of 0.05, five genomic regions located on Sus scrofa chromosome (SSC) 2, SSC4, SSC9, SSC10, and SSC12, were significantly associated with white blood cell traits in response to the polymicrobial challenge, and nine genomic regions on multiple chromosomes (SSC1, SSC4, SSC5, SSC6, SSC8, SSC9, SSC11, SSC12, SSC17) were significantly associated with red blood cell and platelet traits collected before and after exposure to the challenge. By functional enrichment analyses using Ingenuity Pathway Analysis (IPA) and literature review of previous CBC studies, candidate genes located nearby significant single-nucleotide polymorphisms were found to be involved in immune response, hematopoiesis, red blood cell morphology, and platelet aggregation.

Conclusions

This study helps to improve our understanding of the genetic basis of CBC traits collected before and after exposure to a polymicrobial infectious challenge and provides a step forward to improve disease resilience.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07835-4.

Regulation of murine B lymphopoiesis by stromal cells

B lymphocytes are crucial for the body's humoral immune response, secreting antibodies generated against foreign antigens to fight infection. Adult murine B lymphopoiesis is initiated in the bone marrow and additional maturation occurs in the spleen. In both these organs, B lymphopoiesis involves interactions with numerous different non-hematopoietic cells, also known as stromal or microenvironment cells, which provide migratory, maturation, and survival signals. A variety of conditional knockout and transgenic mouse models have been used to identify the roles of distinct microenvironment cell types in the regulation of B lymphopoiesis. These studies have revealed that mesenchymal lineage cells and endothelial cells comprise the non-hematopoietic microenvironment cell types that support B lymphopoiesis in the bone marrow. In the spleen, various types of stromal cells and endothelial cells contribute to B lymphocyte maturation. More recently, comprehensive single cell RNA-seq studies have also been used to identify clusters of stromal cell types in the bone marrow and spleen, which will aid in further identifying key regulators of B lymphopoiesis. Here, we review the different types of microenvironment cells and key extrinsic regulators that are known to be involved in the regulation of murine B lymphopoiesis in the bone marrow and spleen.

Endogenous and combination retinoids are active in myelomonocytic leukemias

Retinoid therapy transformed response and survival outcomes in acute promyelocytic leukemia (APL) but has demonstrated only modest activity in non-APL forms of acute myeloid leukemia (AML). The presence of natural retinoids in vivo could influence the efficacy of pharmacologic agonists and antagonists. We found that natural RXRA ligands, but not RARA ligands, were present in murine MLL-AF9-derived myelomonocytic leukemias in vivo and that the concurrent presence of receptors and ligands acted as tumor suppressors. Pharmacologic retinoid responses could be optimized by concurrent targeting of RXR ligands (e.g., bexarotene) and RARA ligands (e.g., all-trans retinoic acid), which induced either leukemic maturation or apoptosis depending on cell culture conditions. Co-repressor release from the RARA:RXRA heterodimer occurred with RARA activation, but not RXRA activation, providing an explanation for the combination synergy. Combination synergy could be replicated in additional, but not all, AML cell lines and primary samples, and was associated with improved survival in vivo, although tolerability of bexarotene administration in mice remained an issue. These data provide insight into the basal presence of natural retinoids in leukemias in vivo and a potential strategy for clinical retinoid combination regimens in leukemias beyond APL.

Retinoic Acid Receptors in Acute Myeloid Leukemia Therapy

Retinoic acid (RA) signaling pathways regulate fundamental biological processes, such as cell proliferation, development, differentiation, and apoptosis. Retinoid receptors (RARs and RXRs) are ligand-dependent transcription factors. All-trans retinoic acid (ATRA) is the principal endogenous ligand for the retinoic acid receptor alpha (RARA) and is produced by the enzymatic oxidation of dietary vitamin A, whose deficiency is associated with several pathological conditions. Differentiation therapy using ATRA revolutionized the outcome of acute promyelocytic leukemia (APL), although attempts to replicate these results in other cancer types have been met with more modest results. A better knowledge of RA signaling in different leukemia contexts is required to improve initial designs. Here, we will review the RA signaling pathway in normal and malignant hematopoiesis, and will discuss the advantages and the limitations related to retinoid therapy in acute myeloid leukemia.

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.

Mesenchymal lineage cells and their importance in B lymphocyte niches

Early B lymphopoiesis occurs in the bone marrow and is reliant on interactions with numerous cell types in the bone marrow microenvironment, particularly those of the mesenchymal lineage. Each cellular niche that supports the distinct stages of B lymphopoiesis is unique. Different cell types and signaling molecules are important for the progressive stages of B lymphocyte differentiation. Cells expressing CXCL12 and IL-7 have long been recognized as having essential roles in facilitating progression through stages of B lymphopoiesis. Recently, a number of other factors that extrinsically mediate B lymphopoiesis (positively or negatively) have been identified. In addition, the use of transgenic mouse models to delete specific genes in mesenchymal lineage cells has further contributed to our understanding of how B lymphopoiesis is regulated in the bone marrow. This review will cover the current understanding of B lymphocyte niches in the bone marrow and key extrinsic molecules and signaling pathways involved in these niches, with a focus on how mesenchymal lineage cells regulate B lymphopoiesis.

Retinoic Acid Receptor γ Activity in Mesenchymal Stem Cells Regulates Endochondral Bone, Angiogenesis, and B Lymphopoiesis

Retinoic acid receptor (RAR) signaling regulates bone structure and hematopoiesis through intrinsic and extrinsic mechanisms. This study aimed to establish how early in the osteoblast lineage loss of RARγ (Rarg) disrupts the bone marrow microenvironment. Bone structure was analyzed by micro-computed tomography (μCT) in Rarg-/- mice and mice with Rarg conditional deletion in Osterix-Cre-targeted osteoblast progenitors or Prrx1-Cre-targeted mesenchymal stem cells. Rarg-/- tibias exhibited less trabecular and cortical bone and impaired longitudinal and radial growth. The trabecular bone and longitudinal, but not radial, growth defects were recapitulated in Prrx1:RargΔ/Δ mice but not Osx1:RargΔ/Δ mice. Although both male and female Prrx1:RargΔ/Δ mice had low trabecular bone mass, males exhibited increased numbers of trabecular osteoclasts and Prrx1:RargΔ/Δ females had impaired mineral deposition. Both male and female Prrx1:RargΔ/Δ growth plates were narrower than controls and their epiphyses contained hypertrophic chondrocyte islands. Flow cytometry revealed that male Prrx1:RargΔ/Δ bone marrow exhibited elevated pro-B and pre-B lymphocyte numbers, accompanied by increased Cxcl12 expression in bone marrow cells. Prrx1:RargΔ/Δ bone marrow also had elevated megakaryocyte-derived Vegfa expression accompanied by smaller sinusoidal vessels. Thus, RARγ expression by Prrx1-Cre-targeted cells directly regulates endochondral bone formation and indirectly regulates tibial vascularization. Furthermore, RARγ expression by Prrx1-Cre-targeted cells extrinsically regulates osteoclastogenesis and B lymphopoiesis in male mice. © 2018 American Society for Bone and Mineral Research.

Retinoic Acid, Leaky Gut, and Autoimmune Diseases

A leaky gut has been observed in a number of autoimmune diseases including type 1 diabetes, multiple sclerosis, inflammatory bowel disease, and systemic lupus erythematosus. Previous studies from our laboratory have shown that lupus mice also bear a leaky gut and that the intestinal barrier function can be enhanced by gut colonization of probiotics such as Lactobacillus spp. Retinoic acid (RA) can increase the relative abundance of Lactobacillus spp. in the gut. Interestingly, RA has also been shown to strengthen the barrier function of epithelial cells in vitro and in the absence of probiotic bacteria. These reports bring up an interesting question of whether RA exerts protective effects on the intestinal barrier directly or through regulating the microbiota colonization. In this review, we will discuss the roles of RA in immunomodulation, recent literature on the involvement of a leaky gut in different autoimmune diseases, and how RA shapes the outcomes of these diseases.

Regulation of Expression of CEBP Genes by Variably Expressed Vitamin D Receptor and Retinoic Acid Receptor α in Human Acute Myeloid Leukemia Cell Lines

All-trans-retinoic acid (ATRA) and 1α,25-dihydroxyvitamin D (1,25D) are potent inducers of differentiation of myeloid leukemia cells. During myeloid differentiation specific transcription factors are expressed at crucial developmental stages. However, precise mechanism controlling the diversification of myeloid progenitors is largely unknown, CCAAT/enhancer-binding protein (C/EBP) transcription factors have been characterized as key regulators of the development and function of the myeloid system. Past data point at functional redundancy among C/EBP family members during myeloid differentiation. In this study, we show that in acute myeloid leukemia (AML) cells, high expression of vitamin D receptor gene (VDR) is needed for strong and sustained upregulation of CEBPB gene, while the moderate expression of VDR is sufficient for upregulation of CEBPD in response to 1,25D. The high expression level of the gene encoding for retinoic acid receptor α (RARA) allows for high and sustained expression of CEBPB, which becomes decreased along with a decrease of RARA expression. Expression of CEBPB induced by ATRA is accompanied by upregulated expression of CEBPE with similar kinetics. Our results suggest that CEBPB is the major VDR and RARA-responsive gene among the CEBP family, necessary for expression of genes connected with myeloid functions.

Role of Vitamin A/Retinoic Acid in Regulation of Embryonic and Adult Hematopoiesis

Vitamin A is an essential micronutrient throughout life. Its physiologically active metabolite retinoic acid (RA), acting through nuclear retinoic acid receptors (RARs), is a potent regulator of patterning during embryonic development, as well as being necessary for adult tissue homeostasis. Vitamin A deficiency during pregnancy increases risk of maternal night blindness and anemia and may be a cause of congenital malformations. Childhood Vitamin A deficiency can cause xerophthalmia, lower resistance to infection and increased risk of mortality. RA signaling appears to be essential for expression of genes involved in developmental hematopoiesis, regulating the endothelial/blood cells balance in the yolk sac, promoting the hemogenic program in the aorta-gonad-mesonephros area and stimulating eryrthropoiesis in fetal liver by activating the expression of erythropoietin. In adults, RA signaling regulates differentiation of granulocytes and enhances erythropoiesis. Vitamin A may facilitate iron absorption and metabolism to prevent anemia and plays a key role in mucosal immune responses, modulating the function of regulatory T cells. Furthermore, defective RA/RARα signaling is involved in the pathogenesis of acute promyelocytic leukemia due to a failure in differentiation of promyelocytes. This review focuses on the different roles played by vitamin A/RA signaling in physiological and pathological mouse hematopoiesis duddurring both, embryonic and adult life, and the consequences of vitamin A deficiency for the blood system.

Retinoic acid induces hypersegmentation and enhances cytotoxicity of neutrophils against cancer cells

Hypersegmentation of nuclei is considered a distinct characteristic of the antitumoral phenotype of neutrophils. Retinoic acid, a metabolite of retinol, reorganizes and induces segmentation of the nucleus during the differentiation of neutrophils. However, the role of retinoic acid in the phenotype polarization of neutrophils has not been fully established. Here, we investigated the effect of retinoic acid on phenotype polarization of neutrophils. Retinoic acid-induced the hypersegmentation of human neutrophils via retinoic acid receptors and mTOR pathways. Retinoic acid-induced hypersegmented neutrophils enhanced neutrophil extracellular traps (NETs) formation in response to phorbol-12-myristate 13-acetate (PMA) and fMLP (N-Formylmethionine-leucyl-phenylalanine) stimulation, and increased cytotoxicity against various tumor cells. Moreover, retinoic acid treatment attenuated tumor growth in a murine model of tumor. Taken together, these results suggests that retinoic acid induces the phenotype polarization of neutrophils to exert antitumor effects.

Vaccine-induced modulation of gene expression in turbot peritoneal cells. A microarray approach

We used a microarray approach to examine changes in gene expression in turbot peritoneal cells after injection of the fish with vaccines containing the ciliate parasite Philasterides dicentrarchi as antigen and one of the following adjuvants: chitosan-PVMMA microspheres, Freund́s complete adjuvant, aluminium hydroxide gel or Matrix-Q (Isconova, Sweden). We identified 374 genes that were differentially expressed in all groups of fish. Forty-two genes related to tight junctions and focal adhesions and/or actin cytoskeleton were differentially expressed in free peritoneal cells. The profound changes in gene expression related to cell adherence and cytoskeleton may be associated with cell migration and also with the formation of cell-vaccine masses and their attachment to the peritoneal wall. Thirty-five genes related to apoptosis were differentially expressed. Although most of the proteins coded by these genes have a proapoptotic effect, others are antiapoptotic, indicating that both types of signals occur in peritoneal leukocytes of vaccinated fish. Interestingly, many of the genes related to lymphocytes and lymphocyte activity were downregulated in the groups injected with vaccine. We also observed decreased expression of genes related to antigen presentation, suggesting that macrophages (which were abundant in the peritoneal cavity after vaccination) did not express these during the early inflammatory response in the peritoneal cavity. Finally, several genes that participate in the inflammatory response were differentially expressed, and most participated in resolution of inflammation, indicating that an M2 macrophage response is generated in the peritoneal cavity of fish one day post vaccination.

Peripartal rumen-protected methionine supplementation to higher energy diets elicits positive effects on blood neutrophil gene networks, performance and liver lipid content in dairy cows

BACKGROUND

Main objectives were to determine to what extent Smartamine M (SM) supplementation to a prepartal higher-energy diet could alter neutrophil (PMN) and liver tissue immunometabolic biomarkers, and whether those responses were comparable to those in cows fed a prepartal lower-energy diet (CON).

RESULTS

Twenty-eight multiparous Holstein cows were fed CON (NEL = 1.24 Mcal/kg DM) during d -50 to d -22 relative to calving. From d -21 to calving, cows were randomly assigned to a higher-energy diet (OVE, n = 9; NEL = 1.54 Mcal/kg DM), OVE plus SM (OVE + SM, n = 10; SM = 0.07 % of DM) or remained on CON (n = 9). All cows received the same basal lactation diet (NEL = 1.75 Mcal/kg DM). Supplementation of SM (OVE + SM) continued until 30 d postpartum. Liver biopsies were harvested at d -10, 7, and 21 relative to parturition. Blood PMN isolated at -10, 3, and 21 d relative to calving was used to evaluate gene expression. As expected, OVE increased liver lipid content postpartum; however, cows fed OVE + SM or CON had lower concentrations than OVE. Compared with OVE, cows in CON and OVE + SM had greater DMI postpartum and milk production. Furthermore, cows fed OVE + SM had the greatest milk protein and fat percentage and lowest milk SCC despite having intermediate PMN phagocytic capacity. Adaptations in PMN gene expression in OVE + SM cows associated with the lower SCC were gradual increases from -10 to 21 d in genes that facilitate migration into inflammatory sites (SELL, ITGAM), enzymes essential for reducing reactive oxygen metabolites (SOD1, SOD2), and a transcription factor(s) required for controlling PMN development (RXRA). The greater expression of TLR4 on d 3, key for activation of innate immunity due to inflammation, in OVE compared with CON cows suggests a more pronounced inflammatory state. Feeding OVE + SM dampened the upregulation of TLR4, despite the fact that these cows had similar expression of the pro-inflammatory genes NFKB1 and TNF as OVE. Cows in CON had lower overall expression of these inflammation-related genes and GSR, which generates reduced glutathione, an important cellular antioxidant.

CONCLUSIONS

Although CON cows appeared to have a less stressful transition into lactation, SM supplementation was effective in alleviating negative effects of energy-overfeeding. As such, SM was beneficial in terms of production and appeared to boost the response of PMN in a way that improved overall cow health.

Pathways of retinoid synthesis in mouse macrophages and bone marrow cells

In vivo pathways of natural retinoid metabolism and elimination have not been well characterized in primary myeloid cells, even though retinoids and retinoid receptors have been strongly implicated in regulating myeloid maturation. With the use of a upstream activation sequence-GFP reporter transgene and retrovirally expressed Gal4-retinoic acid receptor α in primary mouse bone marrow cells, we identified 2 distinct enzymatic pathways used by mouse myeloid cells ex vivo to synthesize retinoic acid receptor α ligands from free vitamin A metabolites (retinyl acetate, retinol, and retinal). Bulk Kit(+) bone marrow progenitor cells use diethylaminobenzaldehyde-sensitive enzymes, whereas bone marrow-derived macrophages use diethylaminobenzaldehyde-insensitive enzymes to synthesize natural retinoic acid receptor α-activating retinoids (all-trans retinoic acid). Bone marrow-derived macrophages do not express the diethylaminobenzaldehyde-sensitive enzymes Aldh1a1, Aldh1a2, or Aldh1a3 but instead, express Aldh3b1, which we found is capable of diethylaminobenzaldehyde-insensitive synthesis of all trans-retinoic acid. However, under steady-state and stimulated conditions in vivo, diverse bone marrow cells and peritoneal macrophages showed no evidence of intracellular retinoic acid receptor α-activating retinoids, despite expression of these enzymes and a vitamin A-sufficient diet, suggesting that the enzymatic conversion of retinal is not the rate-limiting step in the synthesis of intracellular retinoic acid receptor α-activating retinoids in myeloid bone marrow cells and that retinoic acid receptor α remains in an unliganded configuration during adult hematopoiesis.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

Retiferols - synthesis and biological activity of a conceptually novel class of vitamin D analogs

INTRODUCTION

The hypothesis that retiferols are a novel class of vitamin D analogs with therapeutic potential has been recently proved. The CD-ring of vitamin D, originated from a steroid precursor, is not necessary for biological activity. The retiferol, disubstituted at C-13, was bound to the ligand-binding domain (LBD) of vitamin D receptor (VDR) just like the vitamin D hormone [1,25-(OH)2D3]. This finding opens the way for retiferols as a novel class of vitamin D therapeutics.

AREAS COVERED

This review presents the concept of retiferols and their structure evolution. Medicinal chemistry and therapeutic perspective of retiferols are reviewed showing how these vitamin D analogs became a source of potential therapeutics.

EXPERT OPINION

Docking experiments and molecular modeling have shown that positioning of vitamin D analog at the LBD of VDR is not disturbed by deletion of a large portion of the vitamin D, exactly as hypothesized. Twenty years of structural modifications have shown that removal of the CD-ring fragment and regioselective methylation results in an almost complete loss of the undesired calcemic activity of retiferol while gaining the agonistic activity comparable to that of 1,25-(OH)2D3.

RARα2 expression confers myeloma stem cell features

We previously demonstrated that RARα2 expression is increased in CD138 selected plasma cells of relapsed multiple myelomas (MMs), and increased expression was linked to poor prognosis in newly diagnosed MM patients. In the present study, we demonstrate that increased RARα2 confers myeloma stem cell features. Higher expression of RARα2 was identified in the multiple myeloma stem cell (MMSC) fraction. Overexpression of RARα2 in bulk MM cell lines resulted in: 1) increased drug resistance; 2) increased clonogenic potential; 3) activation of both Wnt and Hedgehog (Hh) pathways; 4) increased side population and aldehyde dehydrogenase levels; and 5) increased expression of embryonic stem cell genes. The opposite effects were seen with RARα2 knockdown. We demonstrate that RARα2 induces drug resistance by activating the drug efflux pump gene ABCC3 and anti-apoptotic Bcl-2 family members. Inhibition of Wnt signaling or ABCC3 function could overcome drug resistance in RARα2 overexpressing MM cells. We also showed that in the 5TGM1 mouse model, targeting of the Wnt and Hh pathways using CAY10404, cyclopamine, or itraconazole significantly reduced the myeloma tumor burden and increased survival. Targeting RARα2 or its downstream signaling pathways provides a potential strategy to eliminate MMSC.

Retinoic acid regulates the development of a gut-homing precursor for intestinal dendritic cells

The vitamin A metabolite retinoic acid (RA) regulates intestinal immune responses through immunomodulatory actions on intestinal dendritic cells (DCs) and lymphocytes. Here, we show that RA also controls the generation of gut-tropic migratory DC precursors, referred to as pre-mucosal DCs (pre-μDCs). Pre-μDCs express the gut trafficking receptor α4β7 and home preferentially to the intestines. They develop in the bone marrow (BM), can differentiate into CCR9⁺ plasmacytoid DCs as well as conventional DCs (cDCs), but preferentially give rise to CD103⁺ intestinal cDCs. Generation of pre-μDCs in vivo in the BM or in vitro is regulated by RA and RA receptor α (RARα) signaling. The frequency of pre-μDCs is reduced in vitamin A-deficient animals and in animals treated with RAR inhibitors. The results define a novel vitamin A-dependent, RA-regulated developmental sequence for DCs and identify a targeted precursor for CD103⁺ cDCs in the gut.

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.

The significance of low PU.1 expression in patients with acute promyelocytic leukemia

BACKGROUND

Although the importance of the hematopoietic transcription factor PU.1 in acute myeloid leukemia (AML) has been demonstrated, the expression of PU.1 in acute promyelocytic leukemia (APL) patient samples awaits further investigation. The current study used APL patient samples to assess the expression pattern of PU.1 in the initiation and progression of APL.

FINDINGS

We used real-time RT-PCR to compare PU.1 expression between de novo APL patient samples and normal blood specimens, and the results indicated that PU.1 expression was significantly lower in newly diagnosed APL patient samples as compared to normal hematopoietic cells. Further evidence showed a significant inverse correlation between the expression level of PML-RARα and that of PU.1. In addition, we analyzed the correlation between PML-RARα and PU.1 expression in a large population of AML patients retrieved from the expression profiles. The results showed that PU.1 expression was lower in patients with APL than other AML subtypes and there was also a trend towards increasing PU.1 expression from AML-M0 to AML-M5, with the exception of AML-M3 (APL). These observations suggested that PU.1 expression was reduced by PML-RARα in APL patients. Furthermore, we measured PU.1 expression in APL-initiating cells isolated from de novo APL patients by side population cell analysis and found that suppression of PU.1 expression occurred concurrently with PML-RARα expression, indicating the pivotal role of PU.1 in APL initiation.

CONCLUSION

Our findings provide evidence that low PU.1 expression in APL patients is required for disease initiation and progression.

No evidence for PML-RARa bcr1 fusion gene in colorectal cancer

Colorectal cancer is the third most prevalent cancer and a leading cause of cancer death. Metastatic colorectal cancer patients are treated with anti-EGFR monoclonal antibodies in combination with chemotherapy; however, the efficiency is only 10-20% of such patients. An increasing amount of data has demonstrated that response to anti-EGFR therapies is confined to patients with KRAS and BRAF wild type tumors but still some of these patients are non responders to this treatment. The presence of oncogenic deregulation of different members of EGFR downstream signaling or crosstalk molecules could predict the lack of response in these patients. In this study, 40 wild type KRAS and BRAF colorectal tumors were analyzed to elucidate whether PML-RARa bcr1 fusion gene may play a role in colorectal carcinogenesis. Specifically we want to determine if this fusion could be responsible for the inability to respond to anti-EGFR monoclonal antibodies. Here, for the first time it is reported, that PML-RARa bcr1 fusion is not responsible for colorectal tumor development and also, this translocation is not predicting the lack of efficacy of anti-EGFR therapies in wild type KRAS and BRAF colorectal cancer patients. These results also suggest that PML-RARa is unlikely to be a promising target for adjuvant therapy in colorectal cancer patients.

Cloning and characterization of retinoid X receptor (RXR) isoforms in the rock shell, Thais clavigera

The organotin compounds tributyltin (TBT) and triphenyltin (TPT) belong to a diverse group of widely distributed environmental pollutants that induce imposex in gastropods. These organotins have high affinity for retinoid X receptor (RXR), which is a transcription factor activated by retinoids, such as 9-cis retinoic acid (9cRA), in vertebrates. However, the molecular mechanisms underlying the regulation of RXR by retinoids and organotins have not been clarified in gastropods. We isolated two isoforms of RXR cDNAs, RXR isoform 1 (TcRXR-1) and RXR isoform 2 (TcRXR-2), in the rock shell Thais clavigera. The deduced amino acid sequences of TcRXR-1 and TcRXR-2 are highly homologous with those of other gastropods. These TcRXR isoforms displayed 9cRA-dependent activation of transcription in a reporter gene assay using COS-1 cells. The transcriptional activity of TcRXR-2, the encoded protein of which has five additional amino acids in the T-box of the C domain, was significantly lower than that of TcRXR-1. Decreases of the transcriptional activity by TcRXR-1 were observed when more than equal amount of TcRXR-2 fused expression vector was existed in a co-transfection assay. Immunoblot analysis showed several shifted bands for TcRXR isoforms resulting from phosphorylation. Mutation of potential phosphorylation sites from serine to alanine in the A/B domain of TcRXR-1 showed that, in the S89A/S103A mutant, there was a band shift and significantly higher transcriptional activity than in the controls when stimulated with 9cRA. Our findings could contribute to a better understanding of the role of interactions between RXR and retinoids and organotins, not only in the induction mechanism of imposex in gastropods but also in the endocrinology of mollusks.

Rara haploinsufficiency modestly influences the phenotype of acute promyelocytic leukemia in mice

RARA (retinoic acid receptor alpha) haploinsufficiency is an invariable consequence of t(15;17)(q22;q21) translocations in acute promyelocytic leukemia (APL). Retinoids and RARA activity have been implicated in hematopoietic self-renewal and neutrophil maturation. We and others therefore predicted that RARA haploinsufficiency would contribute to APL pathogenesis. To test this hypothesis, we crossed Rara(+/-) mice with mice expressing PML (promyelocytic leukemia)-RARA from the cathepsin G locus (mCG-PR). We found that Rara haploinsufficiency cooperated with PML-RARA, but only modestly influenced the preleukemic and leukemic phenotype. Bone marrow from mCG-PR(+/-) × Rara(+/-) mice had decreased numbers of mature myeloid cells, increased ex vivo myeloid cell proliferation, and increased competitive advantage after transplantation. Rara haploinsufficiency did not alter mCG-PR-dependent leukemic latency or penetrance, but did influence the distribution of leukemic cells; leukemia in mCG-PR(+/-) × Rara(+/-) mice presented more commonly with low to normal white blood cell counts and with myeloid infiltration of lymph nodes. APL cells from these mice were responsive to all-trans retinoic acid and had virtually no differences in expression profiling compared with tumors arising in mCG-PR(+/-) × Rara(+/+) mice. These data show that Rara haploinsufficiency (like Pml haploinsufficiency and RARA-PML) can cooperate with PML-RARA to influence the pathogenesis of APL in mice, but that PML-RARA is the t(15;17) disease-initiating mutation.

Acute promyelocytic leukaemia: novel insights into the mechanisms of cure

The fusion oncogene, promyelocytic leukaemia (PML)-retinoic acid receptor-α (RARA), initiates acute promyelocytic leukaemia (APL) through both a block to differentiation and increased self-renewal of leukaemic progenitor cells. The current standard of care is retinoic acid (RA) and chemotherapy, but arsenic trioxide also cures many patients with APL, and an RA plus arsenic trioxide combination cures most patients. This Review discusses the recent evidence that reveals surprising new insights into how RA and arsenic trioxide cure this leukaemia, by targeting PML-RARα for degradation. Drug-triggered oncoprotein degradation may be a strategy that is applicable to many cancers.

Low-dose ionizing radiation effects on differentiation of HL-60 cells

The biological effects of low-dose radiation have attracted attention, but data are currently insufficient to fully understand the beneficial role of the phenomenon. In the present study, we have investigated the effects of low doses of gamma-irradiation alone and in combination with all-trans-retinoic acid (RA) on proliferation, apoptosis and differentiation of the human promyelocytic leukemia HL-60 cells. Changes in cell behavior and protein expression were determined with the use of light and fluorescent microscopy, immunocytochemical and Western blot analysis. Low-dose irradiation with 1–100 cGy caused a dose-dependent inhibition of HL-60 cell proliferation, and induced apoptosis and differentiation to granulocytes with an increase in the number of CD15-positive cells. Pre-irradiation with 1–100 cGy for 24 h before treatment with RA promoted apoptosis but did not impair RA-induced differentiation. Both processes were associated with a decrease in the expression of the proliferating cell nuclear antigen (PCNA), BCL-2, c-MYC, and changes in both cytosolic and nuclear levels of protein tyrosine-phosphorylation as well as protein kinase C alpha or beta isoforms. These results demonstrate the beneficial role of low-dose irradiation in modulating leukemia cell proliferation, differentiation and apoptosis.

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.

Transforming growth-interacting factor (TGIF) regulates proliferation and differentiation of human myeloid leukemia cells

Transforming growth-interacting factor (TGIF) is a homeobox transcriptional repressor that has been implicated in holoprosencephaly and various types of cancer. TGIF is expressed in hematopoietic stem cells and modulates TGF-beta and retinoic acid (RA) signaling, both of which play an important role in hematopoiesis. We recently reported that TGIF's levels correlate inversely with survival in patients with acute myelogenous leukemia. Here we present the first direct evidence of a role for TGIF in myelopoiesis. We used short hairpin RNA interference to define the effects of TGIF knockdown on proliferation and differentiation of myeloid leukemia-derived cell lines. Decreased TGIF expression resulted in reduced proliferation and differentiation and lower expression of CEBPbeta, CEBPepsilon, PU.1 and RUNX1, key myeloid transcription factors. Furthermore, TGF-beta signaling was increased and RA signaling was decreased. Further insights into the molecular basis of TGIF's effects were provided by a genome-wide chromatin immunoprecipitation-based elucidation of TGIF target genes. Together, these data suggest that TGIF has an important role myelopoiesis and may regulate the balance between proliferation and differentiation. Reduced TGIF expression could tip the balance toward quiescence thus providing progenitor as well as hematopoietic stem cells protection from anti-cycle agents.

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.

Apoptosis inducing and differentiation enhancement effect of oridonin on the all-trans-retinoic acid-sensitive and -resistant acute promyelocytic leukemia cells

We investigated the effects of oridonin (Ori), a diterpenoid isolated from Rabdosia rubescens, on apoptosis and differentiation of all-trans-retinoic acid (ATRA)-sensitive (NB4) and ATRA-resistant (NB4-R1) cells. The results showed that reactive oxygen species initiates Ori-induced apoptosis. In addition, we found that neither Ori nor ATRA (10 nM) alone induced marked cell differentiation, while co-treatment of these two compounds can induce differentiation of NB4 and NB4-R1 cells which was accompanied by increased RARalpha, C/EBPepsilon or C/EBPbeta. This is the first report to show that RARalpha could be accumulated by Ori which may be useful as a probe to investigate the mechanism of RARalpha catabolism. These results suggest that Ori is a potential candidate for acute promyelocytic leukemia cancer therapy.

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.

Intrinsic Retinoic Acid Receptor α-Cyclin-Dependent Kinase-Activating Kinase Signaling Involves Coordination of the Restricted Proliferation and Granulocytic Differentiation of Human Hematopoietic Stem Cells

Little is known about the mechanisms by which retinoic acid receptor alpha (RAR alpha) mediates the effects of retinoic acid (RA) to coordinate granulocytic proliferation/differentiation (P/D) transition. Cyclin-dependent kinase-activating kinase (CAK) complex, whose activity in phosphorylation of RAR alpha is determined by its targeting subunit ménage à trois 1 (MAT1), regulates G(1) exit, a cell cycle stage when cells commonly commit to proliferation or to differentiation. We previously found that in myeloid leukemia cells, the lack of RA-induced RAR alpha-CAK dissociation and MAT1 degradation suppresses cell differentiation by inhibiting CAK-dependent G(1) exit and sustaining CAK hyperphosphorylation of RAR alpha. This contrasts with our recent findings about the P/D transition in normal primitive hematopoietic cells, where MAT1 degradation proceeds intrinsically together with granulocytic development, in accord with dynamic expression of aldehyde dehydrogenases (ALDHs) 1A1 and 1B1, which catalyze RA synthesis. Blocking ALDH activity inhibits MAT1 degradation and granulocytic differentiation, whereas loss of RAR alpha phosphorylation by CAK induces RA-target gene expression and granulocytic differentiation. These studies suggest that the subversion of RAR alpha-CAK signaling during normal granulopoiesis is crucial to myeloid leukemogenesis and challenges the current paradigm that RA induces cell differentiation solely by transactivating target genes. Disclosure of potential conflicts of interest is found at the end of this article.

PML-retinoic acid receptor alpha inhibits PML IV enhancement of PU.1-induced C/EBPepsilon expression in myeloid differentiation

PML and PU.1 play important roles in myeloid differentiation. PML-deficient mice have an impaired capacity for terminal maturation of their myeloid precursor cells. This finding has been explained, at least in part, by the lack of PML action to modulate retinoic acid-differentiating activities. In this study, we found that C/EBPepsilon expression is reduced in PML-deficient mice. We showed that PU.1 directly activates the transcription of the C/EBPepsilon gene that is essential for granulocytic differentiation. The type IV isoform of PML interacted with PU.1, promoted its association with p300, and then enhanced PU.1-induced transcription and granulocytic differentiation. In contrast to PML IV, the leukemia-associated PML-retinoic acid receptor alpha fusion protein dissociated the PU.1/PML IV/p300 complex and inhibited PU.1-induced transcription. These results suggest a novel pathogenic mechanism of the PML-retinoic acid receptor alpha fusion protein in acute promyelocytic leukemia.

RNAi screen identifies UBE2D3 as a mediator of all-trans retinoic acid-induced cell growth arrest in human acute promyelocytic NB4 cells

All-trans retinoic acid (ATRA) has been widely used in differentiation therapy for acute promyelocytic leukemia (APL). ATRA binds to retinoic acid receptor (RAR) and triggers the formation of the transcription coactivator complex, which leads to changes in gene expression, APL cell-cycle arrest and differentiation, and clinical remission. The mechanisms responsible for ATRA's beneficial effects are still ill-defined. Here, we conducted a large-scale, unbiased short hairpin RNA (shRNA) screen aiming to identify mediators of ATRA-induced differentiation and growth arrest of APL cells. Twenty-six proteins were identified. They cover a wide range of cellular functions, including gene expression, intracellular signaling, cell death control, stress responses, and metabolic regulation, indicating the complexity of ATRA-induced cell growth control and differentiation in APL. One of these proteins, the ubiquitin-conjugating enzyme UBE2D3, is up-regulated in ATRA-treated acute promyelocytic NB4 cells. UBE2D3 is physically associated with cyclin D1 and mediates ATRA-induced cyclin D1 degradation. Knocking down UBE2D3 by RNA interference (RNAi) leads to blockage of ATRA-induced cyclin D1 degradation and cell-cycle arrest. Thus, our results highlight the involvement of the ubiquitin-mediated proteolysis pathway in ATRA-induced cell-cycle arrest and provide a novel strategy for modulating ATRA-elicited cellular effects.

Down-regulation of RXRalpha expression is essential for neutrophil development from granulocyte/monocyte progenitors

Neutrophil granulocytes (Gs) represent highly abundant and short-lived leukocytes that are constantly regenerated from a small pool of myeloid committed progenitors. Nuclear receptor (NR) family members are ligand-activated transcription factors that play key roles in cellular proliferation and differentiation processes including myelopoiesis. Retinoid X receptor alpha (RXRalpha) represents the predominant NR types I and II homo- and heterodimerization partner in myeloid cells. Here we show that human myeloid progenitors express RXRalpha protein at sustained high levels during macrophage colony-stimulating factor (M-CSF)-induced monopoiesis. In sharp contrast, RXRalpha is down-regulated during G-CSF-dependent late-stage neutrophil differentiation from myeloid progenitors. Down-regulation of RXRalpha is critically required for neutrophil development since ectopic RXRalpha inhibited granulopoiesis by impairing proliferation and differentiation. Moreover, ectopic RXRalpha was sufficient to redirect G-CSF-dependent granulocyte differentiation to the monocyte lineage and to promote M-CSF-induced monopoiesis. Functional genetic interference with RXRalpha signaling in hematopoietic progenitor/stem cells using a dominant-negative RXRalpha promoted the generation of late-stage granulocytes in human cultures in vitro and in reconstituted mice in vivo. Therefore, our data suggest that RXRalpha down-regulation is a critical requirement for the generation of neutrophil granulocytes.

Downregulation of topoisomerase IIbeta in myeloid leukemia cell lines leads to activation of apoptosis following all-trans retinoic acid-induced differentiation/growth arrest

Among the topoisomerase (topo) II isozymes (alpha and beta), topo IIbeta has been suggested to regulate differentiation. In this study, we examined the role of topo IIbeta in all-trans retinoic acid (ATRA)-induced differentiation of myeloid leukemia cell lines. Inhibition of topo IIbeta activity or downregulation of protein expression enhanced ATRA-induced differentiation/growth arrest and apoptosis. ATRA-induced apoptosis in topo IIbeta-deficient cells involved activation of the caspase cascade and was rescued by ectopic expression of topo IIbeta. Gene expression profiling led to the identification of peroxiredoxin 2 (PRDX2) as a candidate gene that was downregulated in topo IIbeta-deficient cells. Reduced expression of PRDX2 validated at the mRNA and protein level, in topo IIbeta-deficient cells correlated with increased accumulation of reactive oxygen species (ROS) following ATRA-induced differentiation. Overexpression of PRDX2 in topo IIbeta-deficient cells led to reduced accumulation of ROS and partially reversed ATRA-induced apoptosis. These results support a role for topo IIbeta in survival of ATRA-differentiated myeloid leukemia cells. Reduced expression of topo IIbeta induces apoptosis in part by impairing the anti-oxidant capacity of the cell owing to downregulation of PRDX2. Thus, suppression of topo IIbeta and/or PRDX2 levels in myeloid leukemia cells provides a novel approach for improving ATRA-based differentiation therapy.

RARgamma is critical for maintaining a balance between hematopoietic stem cell self-renewal and differentiation

Hematopoietic stem cells (HSCs) sustain lifelong production of all blood cell types through finely balanced divisions leading to self-renewal and differentiation. Although several genes influencing HSC self-renewal have been identified, to date no gene has been described that, when activated, enhances HSC self-renewal and, when inactivated [corrected] promotes HSC differentiation. We observe that the retinoic acid receptor (RAR)gamma is selectively expressed in primitive hematopoietic precursors and that the bone marrow of RARgamma knockout mice exhibit markedly reduced numbers of HSCs associated with increased numbers of more mature progenitor cells compared with wild-type mice. In contrast, RARalpha is widely expressed in hematopoietic cells, but RARalpha knockout mice do not exhibit any HSC or progenitor abnormalities. Primitive hematopoietic precursors overexpressing RARalpha differentiate predominantly to granulocytes in short-term culture, whereas those overexpressing RARgamma exhibit a much more undifferentiated phenotype. Furthermore, loss of RARgamma abrogated the potentiating effects of all-trans retinoic acid on the maintenance of HSCs in ex vivo culture. Finally, pharmacological activation of RARgamma ex vivo promotes HSC self-renewal, as demonstrated by serial transplant studies. We conclude that the RARs have distinct roles in hematopoiesis and that RARgamma is a critical physiological and pharmacological regulator of the balance between HSC self-renewal and differentiation.

In vivo analysis of the role of aberrant histone deacetylase recruitment and RAR alpha blockade in the pathogenesis of acute promyelocytic leukemia

The promyelocytic leukemia–retinoic acid receptor α (PML-RARα) protein of acute promyelocytic leukemia (APL) is oncogenic in vivo. It has been hypothesized that the ability of PML-RARα to inhibit RARα function through PML-dependent aberrant recruitment of histone deacetylases (HDACs) and chromatin remodeling is the key initiating event for leukemogenesis. To elucidate the role of HDAC in this process, we have generated HDAC1–RARα fusion proteins and tested their activity and oncogenicity in vitro and in vivo in transgenic mice (TM). In parallel, we studied the in vivo leukemogenic potential of dominant negative (DN) and truncated RARα mutants, as well as that of PML-RARα mutants that are insensitive to retinoic acid. Surprisingly, although HDAC1-RARα did act as a bona fide DN RARα mutant in cellular in vitro and in cell culture, this fusion protein, as well as other DN RARα mutants, did not cause a block in myeloid differentiation in vivo in TM and were not leukemogenic. Comparative analysis of these TM and of TM/PML^−/− and p53^−/− compound mutants lends support to a model by which the RARα and PML blockade is necessary, but not sufficient, for leukemogenesis and the PML domain of the fusion protein provides unique functions that are required for leukemia initiation.

Dynamic and reversibility of heterochromatic gene silencing in human disease

In eukaryotic organisms cellular fate and tissue specific gene expression are regulated by the activity of proteins known as transcription factors that by interacting with specific DNA sequences direct the activation or repression of target genes. The post genomic era has shown that transcription factors are not the unique key regulators of gene expression. Epigenetic mechanisms such as DNA methylation, post-translational modifications of histone proteins, remodeling of nucleosomes and expression of small regulatory RNAs also contribute to regulation of gene expression, determination of cell and tissue specificity and assurance of inheritance of gene expression levels. The relevant contribution of epigenetic mechanisms to a proper cellular function is highlighted by the effects of their deregulation that cooperate with genetic alterations to the development of various diseases and to the establishment and progression of tumors.

Once and again: Retinoic acid signaling in the developing and regenerating olfactory pathway

Retinoic acid (RA), a member of the steroid/thyroid superfamily of signaling molecules, is an essential regulator of morphogenesis, differentiation, and regeneration in the mammalian olfactory pathway. RA-mediated teratogenesis dramatically alters olfactory pathway development, presumably by disrupting retinoid-mediated inductive signaling that influences initial olfactory epithelium (OE) and bulb (OB) morphogenesis. Subsequently, RA modulates the genesis, growth, or stability of subsets of OE cells and OB interneurons. RA receptors, cofactors, and synthetic enzymes are expressed in the OE, OB, and anterior subventricular zone (SVZ), the site of neural precursors that generate new OB interneurons throughout adulthood. Their expression apparently accommodates RA signaling in OE cells, OB interneurons, and slowly dividing SVZ neural precursors. Deficiency of vitamin A, the dietary metabolic RA precursor, leads to cytological changes in the OE, as well as olfactory sensory deficits. Vitamin A therapy in animals with olfactory system damage can accelerate functional recovery. RA-related pathology as well as its potential therapeutic activity may reflect endogenous retinoid regulation of neuronal differentiation, stability, or regeneration in the olfactory pathway from embryogenesis through adulthood. These influences may be in register with retinoid effects on immune responses, metabolism, and modulation of food intake.

Sp1 and NF-kappaB transcription factor activity in the regulation of the p21 and FasL promoters during promyelocytic leukemia cell monocytic differentiation and its associated apoptosis

Treatment of human acute promyelocytic leukemia cells with phorbol 12-myristate 13-acetate (PMA) results in growth arrest and differentiation toward monocytes, which subsequently die by apoptosis. However, the relationship between terminal differentiation and apoptosis remains unclear. Here we have studied Sp1 and nuclear factor kappaB (NF-kappaB) transcription factor activity in controlling promoters of cell cycle-regulating (p21/WAF1/CIP1) and cell death (FasL) genes during monocytic differentiation and apoptosis of the human acute promyelocytic leukemia cell lines NB4 and HL-60. Using the electrophoretic mobility shift assay, we observed that PMA treatment of NB4 cells caused an early response in Sp1 binding to the p21 and FasL promoters at 8 h. The firmly adherent cell phenotype, characteristic of differentiated cells, retained Sp1-binding activity to either promoter, but it was often lost completely in detached, apoptotic cells. The association of Sp1 with the p21 promoter during monocytic differentiation correlated with the levels of expressed p21 in the cytoplasmic fraction, as detected by immunoblotting. In HL-60 cells, very weak or no Sp1 binding to either promoter was observed. Low NF-kappaB affinity for its consensus sites and to the FasL promoter was characteristic of apoptotic cells. The results of this study suggest a positive role of Sp1 and NF-kappaB, as regulators of p21 and FasL genes, in leukemic cell survival and monocytic differentiation and a negative role in apoptotic cell death.