A 3'-flanking NF-kappaB site mediates developmental silencing of the human zeta-globin gene

The role of globins in cardiovascular physiology

Globin proteins exist in every cell type of the vasculature, from erythrocytes to endothelial cells, vascular smooth muscle cells, and peripheral nerve cells. Many globin subtypes are also expressed in muscle tissues (including cardiac and skeletal muscle), in other organ-specific cell types, and in cells of the central nervous system. The ability of each of these globins to interact with molecular oxygen (O₂) and nitric oxide (NO) is preserved across these contexts. Endothelial α-globin is an example of extra-erythrocytic globin expression. Other globins, including myoglobin, cytoglobin, and neuroglobin are observed in other vascular tissues. Myoglobin is observed primarily in skeletal muscle and smooth muscle cells surrounding the aorta or other large arteries. Cytoglobin is found in vascular smooth muscle but can also be expressed in non-vascular cell types, especially in oxidative stress conditions after ischemic insult. Neuroglobin was first observed in neuronal cells, and its expression appears to be restricted mainly to the central and peripheral nervous systems. Brain and central nervous system neurons expressing neuroglobin are positioned close to many arteries within the brain parenchyma and can control smooth muscle contraction and, thus, tissue perfusion and vascular reactivity. Overall, reactions between NO and globin heme-iron contribute to vascular homeostasis by regulating vasodilatory NO signals and scaveging reactive species in cells of the mammalian vascular system. Here, we discuss how globin proteins affect vascular physiology with a focus on NO biology, and offer perspectives for future study of these functions.

The NF-κB p65/miR-23a-27a-24 cluster is a target for leukemia treatment

p65 is a transcription factor that is involved in many physiological and pathologic processes. Here we report that p65 strongly binds to the miR-23a-27a-24 cluster promoter to up-regulate its expression. As bone marrow-derived cells differentiate into red blood cells in vitro, p65/miR-23a-27a-24 cluster expression increases sharply and then declines before the appearance of red blood cells, suggesting that this cluster is negatively related to erythroid terminal differentiation. Bioinformatic and molecular biology experiments confirmed that the miR-23a-27a-24 cluster inhibited the expression of the erythroid proteome and contributed to erythroleukemia progression. In addition, high level of the p65/miR-23a-27a-24 cluster was found in APL and AML cell lines and in nucleated peripheral blood cells from leukemia patients. Furthermore, anti-leukemia drugs significantly inhibited the expression of the p65/miR-23a-27a-24 cluster in leukemia cells. Administration of the p65 inhibitor parthenolide significantly improved hematology and myelogram indices while prolonging the life span of erythroleukemia mice. Meanwhile, stable overexpression of these three miRNAs in mouse erythroleukemia cells enhanced cell malignancy. Our findings thus connect a novel regulation pathway of the p65/miR-23a-27a-24 cluster with the erythroid proteome and provide an applicable approach for treating leukemia.

Hydroxyurea-inducible SAR1 gene acts through the Giα/JNK/Jun pathway to regulate γ-globin expression

Hydroxyurea (HU) is effectively used in the management of β-hemoglobinopathies by augmenting the production of fetal hemoglobin (HbF). However, the molecular mechanisms underlying HU-mediated HbF regulation remain unclear. We previously reported that overexpression of the HU-induced SAR1 gene closely mimics the known effects of HU on K562 and CD34(+) cells, including γ-globin induction and cell-cycle regulation. Here, we show that HU stimulated nuclear factor-κB interaction with its cognate-binding site on the SAR1 promoter to regulate transcriptional expression of SAR1 in K562 and CD34(+) cells. Silencing SAR1 expression not only significantly lowered both basal and HU-elicited HbF production in K562 and CD34(+) cells, but also significantly reduced HU-mediated S-phase cell-cycle arrest and apoptosis in K562 cells. Inhibition of c-Jun N-terminal kinase (JNK)/Jun phosphorylation and silencing of Giα expression in SAR1-transfected K562 and CD34(+) cells reduced both γ-globin expression and HbF level, indicating that activation of Giα/JNK/Jun proteins is required for SAR1-mediated HbF induction. Furthermore, reciprocal coimmunoprecipitation assays revealed an association between forcibly expressed SAR1 and Giα2 or Giα3 proteins in both K562 and nonerythroid cells. These results indicate that HU induces SAR1, which in turn activates γ-globin expression, predominantly through the Giα/JNK/Jun pathway. Our findings identify SAR1 as an alternative therapeutic target for β-globin disorders.

Structural determinants of human ζ-globin mRNA stability

Background

The normal accumulation of adult α and β globins in definitive erythrocytes is critically dependent upon processes that ensure that the cognate mRNAs are maintained at high levels in transcriptionally silent, but translationally active progenitor cells. The impact of these post-transcriptional regulatory events on the expression of embryonic ζ globin is not known, as its encoding mRNA is not normally transcribed during adult erythropoiesis. Recently, though, ζ globin has been recognized as a potential therapeutic for α thalassemia and sickle-cell disease, raising practical questions about constitutive post-transcriptional processes that may enhance, or possibly prohibit, the expression of exogenous or derepresssed endogenous ζ-globin genes in definitive erythroid progenitors.

Methods

The present study assesses mRNA half-life in intact cells using a pulse-chase approach; identifies cis-acting determinants of ζ-globin mRNA stability using a saturation mutagenesis strategy; establishes critical 3′UTR secondary structures using an in vitro enzymatic mapping method; and identifies trans-acting effector factors using an affinity chromatographical procedure.

Results

We specify a tetranucleotide 3′UTR motif that is required for the high-level accumulation of ζ-globin transcripts in cultured cells, and formally demonstrate that it prolongs their cytoplasmic half-lives. Surprisingly, the ζ-globin mRNA stability motif does not function autonomously, predicting an activity that is subject to structural constraints that we subsequently specify. Additional studies demonstrate that the ζ-globin mRNA stability motif is targeted by AUF1, a ubiquitous RNA-binding protein that enhances the half-life of adult β-globin mRNA, suggesting commonalities in post-transcriptional processes that regulate globin transcripts at all stages of mammalian development.

Conclusions

These data demonstrate a mechanism for ζ-globin mRNA stability that exists in definitive erythropoiesis and is available for therapeutic manipulation in α thalassemia and sickle-cell disease.

NF-kappaB regulation of YY1 inhibits skeletal myogenesis through transcriptional silencing of myofibrillar genes

NF-kappaB signaling is implicated as an important regulator of skeletal muscle homeostasis, but the mechanisms by which this transcription factor contributes to muscle maturation and turnover remain unclear. To gain insight into these mechanisms, gene expression profiling was examined in C2C12 myoblasts devoid of NF-kappaB activity. Interestingly, even in proliferating myoblasts, the absence of NF-kappaB caused the pronounced induction of several myofibrillar genes, suggesting that NF-kappaB functions as a negative regulator of late-stage muscle differentiation. Although several myofibrillar promoters contain predicted NF-kappaB binding sites, functional analysis using the troponin-I2 gene as a model revealed that NF-kappaB-mediated repression does not occur through direct DNA binding. In the search for an indirect mediator, the transcriptional repressor YinYang1 (YY1) was identified. While inducers of NF-kappaB stimulated YY1 expression in multiple cell types, genetic ablation of the RelA/p65 subunit of NF-kappaB in both cultured cells and adult skeletal muscle correlated with reduced YY1 transcripts and protein. NF-kappaB regulation of YY1 occurred at the transcriptional level, mediated by direct binding of the p50/p65 heterodimer complex to the YY1 promoter. Furthermore, YY1 was found associated with multiple myofibrillar promoters in C2C12 myoblasts containing NF-kappaB activity. Based on these results, we propose that NF-kappaB regulation of YY1 and transcriptional silencing of myofibrillar genes represent a new mechanism by which NF-kappaB functions in myoblasts to modulate skeletal muscle differentiation.

Gene order in human alpha-globin locus is required for their temporal specific expressions

The human alpha-globin cluster represents a unique model of transcriptional regulation and provides challenges to the current understanding of interactions between distal and proximal regulatory elements. Although the gene proximal regions are believed to possess almost all the necessary elements for temporal and spatial specificity of gene transcription, it is still not clear whether the relative distance of embryonic zeta- and fetal/adult alpha-genes to their distal regulatory element alpha-URE plays any role in transcriptional switching. To investigate the role of gene order in regulating temporal expression, we inverted the entire structure gene region of human alpha-globin locus in a BAC clone bringing alpha-genes closest to alpha-URE and zeta-gene the farthest away. Expression analysis of the reverted locus in transgenic mice showed that alpha-globin genes, now relocated closer to alpha-URE, maintained their expression levels through all developmental stages. However, the zeta-globin gene suffered a total loss at both embryonic and fetal/adult stages. It indicates that proximal location of zeta-globin gene to alpha-URE is necessary for its normal embryonic expression and necessary to prevent embryonic expression of the alpha-globin gene. We proved that, in the human alpha-globin gene cluster, the normal order of structural genes relative to alpha-URE plays a crucial role in the regulation of developmental switching.

Tumor necrosis factor alpha inhibits aclacinomycin A-induced erythroid differentiation of K562 cells via GATA-1

Up-regulation of tumor necrosis factor alpha (TNFalpha) is linked to solid tumors as well as to hematologic disorders including different forms of anemia and multiple myeloma. This cytokine was shown to contribute to inhibition of erythroid maturation mechanisms which are characterized by the expression of specific genes regulated by GATA-1 and NF-E2 transcription factors. Here, we assessed the inhibiting effect of TNFalpha on erythroid differentiation using K562 cells which can be chemically induced to differentiate towards the erythroid pathway by aclacinomycin A, an anthracyclin. Results show that induced hemoglobinization of K562 cells as well as gamma-globin and erythropoietin receptor gene expression are decreased by TNFalpha via the inhibition of GATA-1 at its mRNA and protein expression level. Additionally, both constitutive and induced binding activity of GATA-1 is abolished and induced activation of a GATA-1 driven luciferase reporter construct is inhibited. Altogether, our results provide insight into the molecular mechanisms of inflammation-induced inhibition of erythroid differentiation.

Antisickling effects of an endogenous human α-like globin

Gene replacement or gene reactivation therapies for sickle-cell disease (SCD) typically target the mutant beta(S)-globin subunits of hemoglobin-S (alpha(2)beta(S)(2)) for substitution by nonpathological beta-like globins. Here we show, in vitro and in vivo in a transgenic mouse model of SCD, that the adverse properties of hemoglobin-S can be reversed by exchanging its normal alpha-globin subunits for zeta-globin, an endogenous, developmentally silenced, non-beta-like globin.

Loss of Hmga1 gene function affects embryonic stem cell lympho-hematopoietic differentiation

By interacting with transcription machinery, high-mobility group A 1 (HMGA1) proteins alter the chromatin structure and thereby regulate the transcriptional activity of several genes. To assess their role in development, we studied the in vitro differentiation of embryonic stem (ES) cells that bear one or both disrupted Hmga1 alleles. Here, we report that Hmga1 null ES cells generate fewer T-cell precursors than do wild-type ES cells. Indeed, they preferentially differentiate to B cells, probably consequent to decreased interleukin 2 expression and increased interleukin 6 expression. Moreover, a lack of HMGA1 expression induces changes in hemopoietic differentiation, i.e., a reduced monocyte/macrophage population and an increase in megakaryocyte precursor numbers, erythropoiesis, and globin gene expression. Re-expression of the Hmga1 gene in Hmga1 null ES cells restores the wild-type phenotype. The effect on megakaryocyte/erythrocyte lineages seems, at least in part, mediated by the GATA-1 transcription factor, a key regulator of red blood cell differentiation. In fact, we found that Hmga1-/- ES cells overexpress GATA-1 and that HMGA1 proteins directly control GATA-1 transcription. Taken together, these data indicate that HMGA1 proteins play a prime role in lymphohematopoietic differentiation.

Erythroid gene suppression by NF-kappa B

NF-kappa B/Rel transcription factors play essential roles to mediate the immune response and apoptosis, and they have also been implicated in cellular differentiation such as erythropoiesis. To elucidate the possible role(s) of NF-kappa B in erythroid gene regulation and erythropoiesis, we have carried out transient transfection studies of the human embryonic/fetal erythroid cell line K562 and mouse adult erythroid MEL cells. It is shown that tumor necrosis factor-alpha represses the transcription activity directed by either alpha or zeta globin promoter in a dose-dependent manner. Furthermore, different NF-kappa B family members could effectively repress the transfected alpha-like globin promoters in K562 as well as in MEL cells. The involvement of NF-kappa B pathway is supported by the ability of a NF-kappa B-specific, dominant negative mutant to block the tumor necrosis factor-alpha or p65-mediated suppression of the alpha-like globin promoter activities. The suppression appears to be mediated through cis-linked HS-40 enhancer. Finally, stably transfected K562 cells overexpressing p65 contain reduced amounts of the p45/NF-E2 RNA and functional NF-E2 proteins. Our studies have identified a new set of targets of NF-kappa B. We suggest that the relatively high activity of the NF-kappa B pathway in early erythroid progenitors is involved in the suppression of erythroid-specific genes. Later in differentiation, together with other changes, the decline of the amounts of the NF-kappa B family of factors leads to derepression and consequent increase of NF-E2, which in turn would activate a subset of erythroid-specific genes.

The control of expression of the alpha-globin gene cluster

The alpha-globin gene cluster is located at the very tip of the short arm of chromosome 16. It produces the alpha-like globins, which is combined with the beta-like globins to form hemoglobin, and its mutants cause alpha-thalassemia, which is one of the most common genetic diseases. Its expression shows a tissue and developmental stage specificity that is balanced with that of the beta-globin gene cluster. In this article, we summarize the research on the control of expression of the alpha-globin gene cluster, mainly with respect to the alpha-major regulatory element (alpha-MRE): HS-40, the tissue-specific and developmental control of its expression, and its chromosomal environment. In summary, the alpha-globin gene cluster is expressed in an open chromosomal environment; HS-40, the 5'-flanking sequence, the transcribed region, and the 3'-flanking sequence interact to fully regulate its expression.

Differential regulation of the aldehyde dehydrogenase 1 gene in embryonic chick retina and liver

Aldehyde dehydrogenase (ALDH1) is highly expressed in the dorsal cells of the undifferentiated retina, where it has been proposed to play a role in the formation of a retinoic acid gradient along the ventrodorsal axis. In contrast to the retina, ALDH1 levels increase with differentiation in the liver and remain elevated in the adult tissue. To understand the molecular basis for differential expression of ALDH1 during development, we characterized the ALDH1 transcripts expressed in chick retina and liver. By sequencing, primer extension, and S1 nuclease analysis, we show that retina ALDH1 mRNA has an additional 300 nucleotides of 5'-untranslated sequence resulting from the transcription of two 5' noncoding exons. There is a 24-29-kilobase pair (kb) gap between exons 1 and 2 and a 290-base pair gap between exons 2 and 3. Exon 3, which contains the ALDH1 start codon, represents the first exon of the liver transcript. Using a reporter gene assay, we have identified tissue-specific regulatory elements that govern ALDH1 expression in primary retina and liver cultures. Constructs with >1.6 kb of DNA flanking the 5'-end of exon 1 showed elevated activity in retinal cultures but only basal activity in liver cultures. In contrast, constructs with <1 kb of 5'-flanking DNA were active in both retina and liver cultures. Our results suggest that an important mechanism for the control of ALDH1 transcriptional activity is through the presence of inhibitory elements located 0.7-1.6 kb upstream of the ALDH1 gene. DNase I footprint analysis reveal four sites of protein-DNA interaction within this region, one of which is specific to the liver and corresponds to a NF-kappaB/Rel binding site.

Developmentally dynamic histone acetylation pattern of a tissue-specific chromatin domain

We have defined the histone acetylation pattern of the endogenous murine beta-globin domain, which contains the erythroidspecific beta-globin genes. The beta-globin locus control region (LCR) and transcriptionally active promoters were enriched in acetylated histones in fetal liver relative to fetal brain, whereas the inactive promoters were hypoacetylated. In contrast, the LCR and both active and inactive promoters were hyperacetylated in yolk sac. Hypersensitive site two of the LCR was also hyperacetylated in murine embryonic stem cells, whereas beta-globin promoters were hypoacetylated. Thus, the acetylation pattern varied at different developmental stages. Histone deacetylase inhibition selectively increased acetylation at a hypoacetylated promoter in fetal liver, suggesting that active deacetylation contributes to silencing of promoters. We propose that dynamic histone acetylation and deacetylation play an important role in the developmental control of beta-globin gene expression.

Interleukin 1β inhibits interleukin 6–mediated rat γ fibrinogen gene expression

Interleukin (IL)-1β and IL-6 are the 2 major inducers of a group of hepatic genes during acute inflammation; however, each cytokine uses different intracellular signaling molecules. In most instances, the 2 cytokines interact positively to enhance hepatic gene expression, but in one class of acute-phase reactants, which includes fibrinogen, IL-1β exerts a transient inhibitory effect over the IL-6 stimulatory signal. This study explored the effects of IL-1β/nuclear factor κB (NF-κB) and IL-6/signal transducer and activator of transcription 3 (STAT3) combinatory signaling on the transcriptional regulation of the rat γ fibrinogen gene. Northern blot and functional analyses employing luciferase reporter constructs driven by the rat γ fibrinogen promoter demonstrated that IL-1β inhibited the IL-6-mediated transcription of this gene. Exposing primary rat hepatocytes to IL-1β had no effect on IL-6-mediated STAT3 activation; instead, IL-1β-activated NF-κB associated with 2 IL-6 responsive elements (STAT3 binding site) on the rat γ fibrinogen promoter and blocked STAT3 binding to these regions. The competitive binding of NF-κB and STAT3 on the overlapping binding site provides a mechanism for the inhibition by IL-1β of the IL-6-mediated transactivation of rat γ fibrinogen.

Interleukin 1β inhibits interleukin 6–mediated rat γ fibrinogen gene expression

Interleukin (IL)-1β and IL-6 are the 2 major inducers of a group of hepatic genes during acute inflammation; however, each cytokine uses different intracellular signaling molecules. In most instances, the 2 cytokines interact positively to enhance hepatic gene expression, but in one class of acute-phase reactants, which includes fibrinogen, IL-1β exerts a transient inhibitory effect over the IL-6 stimulatory signal. This study explored the effects of IL-1β/nuclear factor κB (NF-κB) and IL-6/signal transducer and activator of transcription 3 (STAT3) combinatory signaling on the transcriptional regulation of the rat γ fibrinogen gene. Northern blot and functional analyses employing luciferase reporter constructs driven by the rat γ fibrinogen promoter demonstrated that IL-1β inhibited the IL-6-mediated transcription of this gene. Exposing primary rat hepatocytes to IL-1β had no effect on IL-6-mediated STAT3 activation; instead, IL-1β-activated NF-κB associated with 2 IL-6 responsive elements (STAT3 binding site) on the rat γ fibrinogen promoter and blocked STAT3 binding to these regions. The competitive binding of NF-κB and STAT3 on the overlapping binding site provides a mechanism for the inhibition by IL-1β of the IL-6-mediated transactivation of rat γ fibrinogen.

Loading of DNA-binding factors to an erythroid enhancer

The HS-40 enhancer is the major cis-acting regulatory element responsible for the developmental stage- and erythroid lineage-specific expression of the human alpha-like globin genes, the embryonic zeta and the adult alpha2/alpha/1. A model has been proposed in which competitive factor binding at one of the HS-40 motifs, 3'-NA, modulates the capability of HS-40 to activate the embryonic zeta-globin promoter. Furthermore, this modulation was thought to be mediated through configurational changes of the HS-40 enhanceosome during development. In this study, we have further investigated the molecular basis of this model. First, human erythroid K562 cells stably integrated with various HS-40 mutants cis linked to a human alpha-globin promoter-growth hormone hybrid gene were analyzed by genomic footprinting and expression analysis. By the assay, we demonstrate that factors bound at different motifs of HS-40 indeed act in concert to build a fully functional enhanceosome. Thus, modification of factor binding at a single motif could drastically change the configuration and function of the HS-40 enhanceosome. Second, a specific 1-bp, GC-->TA mutation in the 3'-NA motif of HS-40, 3'-NA(II), has been shown previously to cause significant derepression of the embryonic zeta-globin promoter activity in erythroid cells. This derepression was hypothesized to be regulated through competitive binding of different nuclear factors, in particular AP1 and NF-E2, to the 3'-NA motif. By gel mobility shift and transient cotransfection assays, we now show that 3'-NA(II) mutation completely abolishes the binding of small MafK homodimer. Surprisingly, NF-E2 as well as AP1 can still bind to the 3'-NA(II) sequence. The association constants of both NF-E2 and AP1 are similar to their interactions with the wild-type 3'-NA motif. However, the 3'-NA(II) mutation causes an approximately twofold reduction of the binding affinity of NF-E2 factor to the 3'-NA motif. This reduction of affinity could be accounted for by a twofold-higher rate of dissociation of the NF-E2-3'-NA(II) complex. Finally, we show by chromatin immunoprecipitation experiments that only binding of NF-E2, not AP1, could be detected in vivo in K562 cells around the HS-40 region. These data exclude a role for AP1 in the developmental regulation of the human alpha-globin locus via the 3'-NA motif of HS-40 in embryonic/fetal erythroid cells. Furthermore, extrapolation of the in vitro binding studies suggests that factors other than NF-E2, such as the small Maf homodimers, are likely involved in the regulation of the HS-40 function in vivo.

Transcriptional elongation of c-myb is regulated by NF-kappaB (p50/RelB)

High levels of c-myb expression are necessary for the proliferation of hematopoietic precursor cells whereas down-regulation of c-myb is required for terminal differentiation; this down-regulation occurs through a conditional block to transcriptional elongation in intron I. We previously observed that cAMP analogs prevented the late down-regulation of c-myb during hexamethylene bisacetamide (HMBA)-induced differentiation of murine erythroleukemia (MEL) cells and blocked differentiation; this correlated with the induction of NF-kappaB (p50/RelB) complexes which were shown to bind to NF-kappaB recognition sites flanking the transcriptional pause site of c-myb. We now selected stably-transfected MEL cells which overexpressed p50, RelB or both at levels similar to those induced by cAMP to determine whether these NF-kappaB proteins regulate c-myb expression in intact cells. We demonstrate that transcriptionally active NF-kappaB (p50/RelB) complexes, but not p50 or RelB alone, prevented the early and late down-regulation of c-myb mRNA and increased c-myb transcriptional elongation in HMBA-induced MEL cells. The increase in c-myb expression was sufficient to block erythroid differentiation and allow continuous proliferation of cells in the presence of HMBA. Steady-state c-myb mRNA levels in untreated cells were not affected by overexpression of NF-kappaB, suggesting that p50/RelB specifically modulated the efficiency of transcriptional attenuation during MEL cell differentiation.