Rapid turnover of GATA-2 via ubiquitin-proteasome protein degradation pathway

Role of the pioneer transcription factor GATA2 in health and disease

The transcription factor GATA2 is involved in human diseases ranging from hematopoietic disorders, to cancer, to infectious diseases. GATA2 is one of six GATA-family transcription factors that act as pioneering transcription factors which facilitate the opening of heterochromatin and the subsequent binding of other transcription factors to induce gene expression from previously inaccessible regions of the genome. Although GATA2 is essential for hematopoiesis and lymphangiogenesis, it is also expressed in other tissues such as the lung, prostate gland, gastrointestinal tract, central nervous system, placenta, fetal liver, and fetal heart. Gene or transcriptional abnormalities of GATA2 causes or predisposes patients to several diseases including the hematological cancers acute myeloid leukemia and acute lymphoblastic leukemia, the primary immunodeficiency MonoMAC syndrome, and to cancers of the lung, prostate, uterus, kidney, breast, gastric tract, and ovaries. Recent data has also linked GATA2 expression and mutations to responses to infectious diseases including SARS-CoV-2 and Pneumocystis carinii pneumonia, and to inflammatory disorders such as atherosclerosis. In this article we review the role of GATA2 in the etiology and progression of these various diseases.

A COP1-GATA2 axis suppresses AR signaling and prostate cancer

Androgen receptor (AR) signaling is crucial for driving prostate cancer (PCa), the most diagnosed and the second leading cause of death in male patients with cancer in the United States. Androgen deprivation therapy is initially effective in most instances of AR-positive advanced or metastatic PCa. However, patients inevitably develop lethal castration-resistant PCa (CRPC), which is also resistant to the next-generation AR signaling inhibitors. Most CRPCs maintain AR expression, and blocking AR signaling remains a main therapeutic approach. GATA2 is a pioneer transcription factor emerging as a key therapeutic target for PCa because it promotes AR expression and activation. While directly inhibiting GATA2 transcriptional activity remains challenging, enhancing GATA2 degradation is a plausible therapeutic strategy. How GATA2 protein stability is regulated in PCa remains unknown. Here, we show that constitutive photomorphogenesis protein 1 (COP1), an E3 ubiquitin ligase, drives GATA2 ubiquitination at K419/K424 for degradation. GATA2 lacks a conserved [D/E](x)xxVP[D/E] degron but uses alternate BR1/BR2 motifs to bind COP1. By promoting GATA2 degradation, COP1 inhibits AR expression and activation and represses PCa cell and xenograft growth and castration resistance. Accordingly, GATA2 overexpression or COP1 mutations that disrupt COP1-GATA2 binding block COP1 tumor-suppressing activities. We conclude that GATA2 is a major COP1 substrate in PCa and that COP1 promotion of GATA2 degradation is a direct mechanism for regulating AR expression and activation, PCa growth, and castration resistance.

The Il6 -39 kb enhancer containing clustered GATA2- and PU.1-binding sites is essential for Il6 expression in murine mast cells

Mast cells serve as a first-line defense of innate immunity. Interleukin-6 (IL-6) induced by bacterial lipopolysaccharide (LPS) in mast cells plays a crucial role in antibacterial protection. The zinc finger transcription factor GATA2 cooperatively functions with the ETS family transcription factor PU.1 in multiple mast cell activities. However, the regulatory landscape directed by GATA2 and PU.1 under inflammation remains elusive. We herein showed that a large proportion of GATA2-binding peaks were closely located with PU.1-binding peaks in distal cis-regulatory regions of inflammatory cytokine genes in mast cells. Notably, GATA2 and PU.1 played crucial roles in promoting LPS-mediated inflammatory cytokine production. Genetic ablation of GATA2-PU.1-clustered binding sites at the Il6 -39 kb region revealed its central role in LPS-induced Il6 expression in mast cells. We demonstrate a novel collaborative activity of GATA2 and PU.1 in cytokine induction upon inflammatory stimuli via the GATA2-PU.1 overlapping sites in the distal cis-regulatory regions.

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GATA2- and PU.1-binding peaks are closely located in distal enhancers of cytokine genes

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GATA2 and PU.1 play crucial roles in promoting LPS-mediated cytokine induction

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The Il6 -39 kb enhancer containing GATA2 and PU.1 motifs are crucial for Il6 induction

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GATA2 inhibitor exerts anti-inflammatory effects via reducing cytokine induction

Proteasome-Mediated Regulation of GATA2 Expression and Androgen Receptor Transcription in Benign Prostate Epithelial Cells

GATA2 has been shown to be an important transcription factor together with androgen receptor (AR) in prostate cancer cells. Less is known about GATA2 in benign prostate epithelial cells. We have investigated if GATA2 exogenous expression in prostate epithelial basal-like cells could induce AR transcription or luminal differentiation. Prostate epithelial basal-like (transit amplifying) cells were transduced with lentiviral vector expressing GATA2. Luminal differentiation markers were assessed by RT-qPCR, Western blot and global gene expression microarrays. We utilized our previously established AR and androgen-dependent fluorescence reporter assay to investigate AR activity at the single-cell level. Exogenous GATA2 protein was rapidly and proteasome-dependently degraded. GATA2 protein expression was rescued by the proteasome inhibitor MG132 and partly by mutating the target site of the E3 ligase FBXW7. Moreover, MG132-mediated proteasome inhibition induced AR mRNA and additional luminal marker gene transcription in the prostate transit amplifying cells. Different types of intrinsic mechanisms restricted GATA2 expression in the transit amplifying cells. The appearance of AR mRNA and additional luminal marker gene expression changes following proteasome inhibition suggests control of essential cofactor(s) of AR mRNA expression and luminal differentiation at this proteolytic level.

Impairment of the Hypothalamus-Pituitary-Thyroid Axis Caused by Naturally Occurring GATA2 Mutations In Vitro

The transcription factor GATA2 regulates gene expression in several cells and tissues, including hematopoietic tissues and the central nervous system. Recent studies revealed that loss-of-function mutations in GATA2 are associated with hematological disorders. Our earlier in vitro studies showed that GATA2 plays an essential role in the hypothalamus–pituitary–thyroid axis (HPT axis) by regulating the genes encoding prepro-thyrotropin-releasing hormone (preproTRH) and thyroid-stimulating hormone β (TSHβ). However, the effect of GATA2 mutants on the transcriptional activity of their promoters remains unelucidated. In this study, we created five human GATA2 mutations (R308P, T354M, R396Q, R398W, and S447R) that were reported to be associated with hematological disorders and analyzed their functional properties, including transactivation potential and DNA-binding capacity toward the preproTRH and the TSHβ promoters. Three mutations (T354M, R396Q, and R398W) within the C-terminal zinc-finger domain reduced the basal GATA2 transcriptional activity on both the preproTRH and the TSHβ promoters with a significant loss of DNA binding affinity. Interestingly, only the R398W mutation reduced the GATA2 protein expression. Subsequent analysis demonstrated that the R398W mutation possibly facilitated the GATA2 degradation process. R308P and S447R mutants exhibited decreased transcriptional activity under protein kinase C compared to the wild-type protein. In conclusion, we demonstrated that naturally occurring GATA2 mutations impair the HPT axis through differential functional mechanisms in vitro.

MAPK4 promotes prostate cancer by concerted activation of androgen receptor and AKT

Prostate cancer (PCa) is the second leading cause of cancer death in American men. Androgen receptor (AR) signaling is essential for PCa cell growth/survival and remains a key therapeutic target for lethal castration-resistant PCa (CRPC). GATA2 is a pioneer transcription factor crucial for inducing AR expression/activation. We recently reported that MAPK4, an atypical MAPK, promotes tumor progression via noncanonical activation of AKT. Here, we demonstrated that MAPK4 activated AR by enhancing GATA2 transcriptional expression and stabilizing GATA2 protein through repression of GATA2 ubiquitination/degradation. MAPK4 expression correlated with AR activation in human CRPC. Concerted activation of both GATA2/AR and AKT by MAPK4 promoted PCa cell proliferation, anchorage-independent growth, xenograft growth, and castration resistance. Conversely, knockdown of MAPK4 decreased activation of both AR and AKT and inhibited PCa cell and xenograft growth, including castration-resistant growth. Both GATA2/AR and AKT activation were necessary for MAPK4 tumor-promoting activity. Interestingly, combined overexpression of GATA2 plus a constitutively activated AKT was sufficient to drive PCa growth and castration resistance, shedding light on an alternative, MAPK4-independent tumor-promoting pathway in human PCa. We concluded that MAPK4 promotes PCa growth and castration resistance by cooperating parallel pathways of activating GATA2/AR and AKT and that MAPK4 is a novel therapeutic target in PCa, especially CRPC.

Inherited GATA2 Deficiency Is Dominant by Haploinsufficiency and Displays Incomplete Clinical Penetrance

PURPOSE

Germline heterozygous mutations of GATA2 underlie a variety of hematological and clinical phenotypes. The genetic, immunological, and clinical features of GATA2-deficient patients with mycobacterial diseases in the familial context remain largely unknown.

METHODS

We enrolled 15 GATA2 index cases referred for mycobacterial disease. We describe their genetic and clinical features including their relatives.

RESULTS

We identified 12 heterozygous GATA2 mutations, two of which had not been reported. Eight of these mutations were loss-of-function, and four were hypomorphic. None was dominant-negative in vitro, and the GATA2 locus was found to be subject to purifying selection, strongly suggesting a mechanism of haploinsufficiency. Three relatives of index cases had mycobacterial disease and were also heterozygous, resulting in 18 patients in total. Mycobacterial infection was the first clinical manifestation in 11 patients, at a mean age of 22.5 years (range: 12 to 42 years). Most patients also suffered from other infections, monocytopenia, or myelodysplasia. Strikingly, the clinical penetrance was incomplete (32.9% by age 40 years), as 16 heterozygous relatives aged between 6 and 78 years, including 4 older than 60 years, were completely asymptomatic.

CONCLUSION

Clinical penetrance for mycobacterial disease was found to be similar to other GATA2 deficiency-related manifestations. These observations suggest that other mechanisms contribute to the phenotypic expression of GATA2 deficiency. A diagnosis of autosomal dominant GATA2 deficiency should be considered in patients with mycobacterial infections and/or other GATA2 deficiency-related phenotypes at any age in life. Moreover, all direct relatives should be genotyped at the GATA2 locus.

Arsenite suppresses the transcriptional activity of EVI1 through the binding to CCHC-type Zn finger domain

Transcription factor EVI1 is essential for normal hematopoiesis in embryos but is aberrantly elevated in bone marrow cells of myelodysplastic syndrome (MDS) patients. EVI1 and its downstream GATA-2 appear to be a possible therapeutic target of MDS. Here we found that treatment of EVI1-expressing K562 cells with arsenite (As(III)) reduced the mRNA and protein levels of EVI1 and GATA-2. A gel shift assay using the nuclear extract of K562 cells showed that As(III) suppressed the DNA-binding activity of EVI1. The DNA-binding activity of the recombinant EVI1 protein was also suppressed by As(III) but was recovered by excess amounts of dithiothreitol, suggesting the involvement of cysteine residues of EVI1. Since the 7th Zn finger domain of EVI1, having a motif of CCHC, is known to be involved in DNA-binding, the synthetic peptide of 7th Zn finger domain was reacted with As(III) and subjected to MALDI-TOF-MS analysis. The results showed that As(III) binds to this peptide via three cysteine residues. As(III)-induced reduction of the DNA-binding activity of the recombinant EVI1 was abolished by the mutations of each of three cysteine residues to alanine in the 7th Zn finger domain. These results demonstrate that As(III) causes the down-regulation of EVI1 and GATA-2 by inhibiting the transcriptional activity of EVI1 through the binding to the cysteine residues of CCHC-type Zn finger domain.

Absolute Quantification of Transcription Factors Reveals Principles of Gene Regulation in Erythropoiesis

Dynamic cellular processes such as differentiation are driven by changes in the abundances of transcription factors (TFs). However, despite years of studies, our knowledge about the protein copy number of TFs in the nucleus is limited. Here, by determining the absolute abundances of 103 TFs and co-factors during the course of human erythropoiesis, we provide a dynamic and quantitative scale for TFs in the nucleus. Furthermore, we establish the first gene regulatory network of cell fate commitment that integrates temporal protein stoichiometry data with mRNA measurements. The model revealed quantitative imbalances in TFs' cross-antagonistic relationships that underlie lineage determination. Finally, we made the surprising discovery that, in the nucleus, co-repressors are dramatically more abundant than co-activators at the protein level, but not at the RNA level, with profound implications for understanding transcriptional regulation. These analyses provide a unique quantitative framework to understand transcriptional regulation of cell differentiation in a dynamic context.

Liganded T3 receptor β2 inhibits the positive feedback autoregulation of the gene for GATA2, a transcription factor critical for thyrotropin production

The serum concentration of thyrotropin (thyroid stimulating hormone, TSH) is drastically reduced by small increase in the levels of thyroid hormones (T3 and its prohormone, T4); however, the mechanism underlying this relationship is unknown. TSH consists of the chorionic gonadotropin α (CGA) and the β chain (TSHβ). The expression of both peptides is induced by the transcription factor GATA2, a determinant of the thyrotroph and gonadotroph differentiation in the pituitary. We previously reported that the liganded T3 receptor (TR) inhibits transactivation activity of GATA2 via a tethering mechanism and proposed that this mechanism, but not binding of TR with a negative T3-responsive element, is the basis for the T3-dependent inhibition of the TSHβ and CGA genes. Multiple GATA-responsive elements (GATA-REs) also exist within the GATA2 gene itself and mediate the positive feedback autoregulation of this gene. To elucidate the effect of T3 on this non-linear regulation, we fused the GATA-REs at -3.9 kb or +9.5 kb of the GATA2 gene with the chloramphenicol acetyltransferase reporter gene harbored in its 1S-promoter. These constructs were co-transfected with the expression plasmids for GATA2 and the pituitary specific TR, TRβ2, into kidney-derived CV1 cells. We found that liganded TRβ2 represses the GATA2-induced transactivation of these reporter genes. Multi-dimensional input function theory revealed that liganded TRβ2 functions as a classical transcriptional repressor. Then, we investigated the effect of T3 on the endogenous expression of GATA2 protein and mRNA in the gonadotroph-derived LβT2 cells. In this cell line, T3 reduced GATA2 protein independently of the ubiquitin proteasome system. GATA2 mRNA was drastically suppressed by T3, the concentration of which corresponds to moderate hypothyroidism and euthyroidism. These results suggest that liganded TRβ2 inhibits the positive feedback autoregulation of the GATA2 gene; moreover this mechanism plays an important role in the potent reduction of TSH production by T3.

Dynamic regulation of GATA2 in fate determination in hematopoiesis: possible approach to hPSC-derived hematopoietic stem/progenitor cells

GATA2, a principal member of the GATA family, plays important roles in the generation and maintenance of hematopoietic stem/progenitor cells. Among the three mRNA transcripts, the distal first exon of GATA2 (IS exon) is specific for hematopoietic and neuronal cells. GATA2 mutants with abnormal expression are often present in acute myeloid leukemia-related familial diseases and myelodysplastic syndrome, indicating the crucial significance of GATA2 in the proper maintenance of blood system functions. This article offers an overview of the regulation dynamics and function of GATA2 in the generation, proliferation, and function of hematopoietic stem cells in both mouse and human models. We acknowledge the current progress in the cell fate determination mechanism by dynamic GATA2 expression. The gene modification approaches for inspecting the role of GATA2 in definitive hematopoiesis demonstrate the potential for acquiring hPSC-derived hematopoietic stem cells via manipulated GATA2 regulation.

Targeting GATA4 for cardiac repair

Various strategies have been applied to replace the loss of cardiomyocytes in order to restore reduced cardiac function and prevent the progression of heart disease. Intensive research efforts in the field of cellular reprogramming and cell transplantation may eventually lead to efficient in vivo applications for the treatment of cardiac injuries, representing a novel treatment strategy for regenerative medicine. Modulation of cardiac transcription factor (TF) networks by chemical entities represents another viable option for therapeutic interventions. Comprehensive screening projects have revealed a number of molecular entities acting on molecular pathways highly critical for cellular lineage commitment and differentiation, including compounds targeting Wnt- and transforming growth factor beta (TGFβ)-signaling. Furthermore, previous studies have demonstrated that GATA4 and NKX2-5 are essential TFs in gene regulation of cardiac development and hypertrophy. For example, both of these TFs are required to fully activate mechanical stretch-responsive genes such as atrial natriuretic peptide and brain natriuretic peptide (BNP). We have previously reported that the compound 3i-1000 efficiently inhibited the synergy of the GATA4-NKX2-5 interaction. Cellular effects of 3i-1000 have been further characterized in a number of confirmatory in vitro bioassays, including rat cardiac myocytes and animal models of ischemic injury and angiotensin II-induced pressure overload, suggesting the potential for small molecule-induced cardioprotection.

Quantitative and qualitative impairments in GATA2 and myeloid neoplasms

GATA2 is a key transcription factor critical for hematopoietic cell development. During the past decade, it became clear that heterozygous germline mutations in the GATA2 gene cause bone marrow failure and primary immunodeficiency syndrome, conditions that lead to a predisposition toward myeloid neoplasms, such as myelodysplastic syndrome, acute myeloid leukemia, and chronic myelomonocytic leukemia. Somatic mutations of the GATA2 gene are also involved in the pathogenesis of myeloid malignancies. Cases with GATA2 gene mutations are divided into two groups, resulting in either a quantitative deficiency or a qualitative defect in the GATA2 protein depending on the mutation position and type. In the former case, GATA2 mRNA expression from the mutant allele is markedly reduced or completely abrogated, and reduced GATA2 protein expression is involved in the pathogenesis. In the latter case, almost equal amounts of structurally abnormal and wildtype GATA2 proteins are predicted to be present and contribute to the pathogenesis. The development of mouse models of these human GATA2-related diseases has been undertaken, which naturally develop myeloid neoplasms.

Transcription factor mutations as a cause of familial myeloid neoplasms

The initiation and evolution of myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) are driven by genomic events that disrupt multiple genes controlling hematopoiesis. Human genetic studies have discovered germline mutations in single genes that instigate familial MDS/AML. The best understood of these genes encode transcription factors, such as GATA-2, RUNX1, ETV6, and C/EBPα, which establish and maintain genetic networks governing the genesis and function of blood stem and progenitor cells. Many questions remain unanswered regarding how genes and circuits within these networks function in physiology and disease and whether network integrity is exquisitely sensitive to or efficiently buffered from perturbations. In familial MDS/AML, mutations change the coding sequence of a gene to generate a mutant protein with altered activity or introduce frameshifts or stop codons or disrupt regulatory elements to alter protein expression. Each mutation has the potential to exert quantitatively and qualitatively distinct influences on networks. Consistent with this mechanistic diversity, disease onset is unpredictable and phenotypic variability can be considerable. Efforts to elucidate mechanisms and forge prognostic and therapeutic strategies must therefore contend with a spectrum of patient-specific leukemogenic scenarios. Here we illustrate mechanistic advances in our understanding of familial MDS/AML syndromes caused by germline mutations of hematopoietic transcription factors.

Domain-specific biological functions of the transcription factor Gata2 on hematopoietic differentiation of mouse embryonic stem cells

The generation of mouse hematopoietic stem cells from hemogenic endothelial cells (HECs) in the aorta/gonad/mesonephros region of developing embryos requires a zinc finger transcription factor Gata2. In the previous study, an enforced expression of Gata2 in vitro promoted the production of HECs from mesodermal cells differentiated from mouse embryonic stem cells (ESCs). Our research group has previously demonstrated that the enforced expression of Gata2 in ESC-derived HECs enhances erythroid and megakaryocyte differentiation and inhibits macrophage differentiation. However, the manner in which the multiple functions of Gata2 are regulated remains unclear. Mouse ESCs differentiate into various types of hematopoietic cells when cocultured with OP9 stromal cells (OP9 system). Using this system and the inducible gene cassette exchange system, which facilitates the establishment of ESCs carrying inducible transgenes under an identical gene expression regulatory unit, the domain-specific functions of Gata2 were systematically dissected in this study. We determined that the N-terminal (amino acid 1-110) region of Gata2 was an erythroid-inducing region, both the middle (amino acid 111-200) and C-terminal (amino acid 413-480) regions were megakaryocyte-inducing regions. Furthermore, the present data strongly suggest that intramolecular antagonistic interactions between each of these regions fine-tune the biological functions of Gata2.

Pseudopodium-enriched atypical kinase 1 mediates angiogenesis by modulating GATA2-dependent VEGFR2 transcription

PEAK1 is a newly described tyrosine kinase and scaffold protein that transmits integrin-mediated extracellular matrix (ECM) signals to facilitate cell movement and growth. While aberrant expression of PEAK1 has been linked to cancer progression, its normal physiological role in vertebrate biology is not known. Here we provide evidence that PEAK1 plays a central role in orchestrating new vessel formation in vertebrates. Deletion of the PEAK1 gene in zebrafish, mice, and human endothelial cells (ECs) induced severe defects in new blood vessel formation due to deficiencies in EC proliferation, survival, and migration. Gene transcriptional and proteomic analyses of PEAK1-deficient ECs revealed a significant loss of vascular endothelial growth factor receptor 2 (VEGFR2) mRNA and protein expression, as well as downstream signaling to its effectors, ERK, Akt, and Src kinase. PEAK1 regulates VEGFR2 expression by binding to and increasing the protein stability of the transcription factor GATA-binding protein 2 (GATA2), which controls VEGFR2 transcription. Importantly, PEAK1-GATA2-dependent VEGFR2 expression is mediated by EC adhesion to the ECM and is required for breast cancer-induced new vessel formation in mice. Also, elevated expression of PEAK1 and VEGFR2 mRNA are highly correlated in many human cancers including breast cancer. Together, our findings reveal a novel PEAK1-GATA2-VEGFR2 signaling axis that integrates cell adhesion and growth factor cues from the extracellular environment necessary for new vessel formation during vertebrate development and cancer.

PIASy antagonizes Ras-driven NSCLC survival by promoting GATA2 SUMOylation

GATA2 regulated transcriptional network has been validated requisite for RAS oncogene-driven non-small cell lung cancer (NSCLC). GATA2 has been reported as a SUMOylated protein. In endothelial cells, its transcriptional activity is attenuated by SUMO-2 conjugation, which is specifically catalyzed by its E3 ligase PIASy. In this study, we found a decreased expression of PIASy in RAS mutant NSCLC cell lines and specimens with RAS mutations. Forced expression of PIASy in NSCLC cells inhibits their viability in vitro, as well as tumorigenesis and growth in vivo. Mechanistically, we demonstrated overexpression of PIASy in A549 cells altered the regulated transcriptional network of GATA2, including proteasome, IL-1-signaling, and Rho-signaling pathways. Forced expression of PIASy resulted in the accumulated SUMOylation of GATA2, attenuating its transcriptional activity in A549 cells. These results collectively suggest that PIASy plays an antagonistic role in RAS-driven NSCLC survival, by enhancing the SUMOylation of GATA2 and inhibiting its transcriptional activity.

In vivo single cell analysis reveals Gata2 dynamics in cells transitioning to hematopoietic fate

Eich et al. reveal the dynamic expression of the Gata2 transcription factor in single aortic cells transitioning to hematopoietic fate by vital imaging of Gata2Venus mouse embryos. Pulsatile expression level changes highlight an unstable genetic state during hematopoietic cell generation.

Cell fate is established through coordinated gene expression programs in individual cells. Regulatory networks that include the Gata2 transcription factor play central roles in hematopoietic fate establishment. Although Gata2 is essential to the embryonic development and function of hematopoietic stem cells that form the adult hierarchy, little is known about the in vivo expression dynamics of Gata2 in single cells. Here, we examine Gata2 expression in single aortic cells as they establish hematopoietic fate in Gata2Venus mouse embryos. Time-lapse imaging reveals rapid pulsatile level changes in Gata2 reporter expression in cells undergoing endothelial-to-hematopoietic transition. Moreover, Gata2 reporter pulsatile expression is dramatically altered in Gata2^+/− aortic cells, which undergo fewer transitions and are reduced in hematopoietic potential. Our novel finding of dynamic pulsatile expression of Gata2 suggests a highly unstable genetic state in single cells concomitant with their transition to hematopoietic fate. This reinforces the notion that threshold levels of Gata2 influence fate establishment and has implications for transcription factor–related hematologic dysfunctions.

The GATA factor revolution in hematology

The discovery of the GATA binding protein (GATA factor) transcription factor family revolutionized hematology. Studies of GATA proteins have yielded vital contributions to our understanding of how hematopoietic stem and progenitor cells develop from precursors, how progenitors generate red blood cells, how hemoglobin synthesis is regulated, and the molecular underpinnings of nonmalignant and malignant hematologic disorders. This thrilling journey began with mechanistic studies on a β-globin enhancer- and promoter-binding factor, GATA-1, the founding member of the GATA family. This work ushered in the cloning of related proteins, GATA-2-6, with distinct and/or overlapping expression patterns. Herein, we discuss how the hematopoietic GATA factors (GATA-1-3) function via a battery of mechanistic permutations, which can be GATA factor subtype, cell type, and locus specific. Understanding this intriguing protein family requires consideration of how the mechanistic permutations are amalgamated into circuits to orchestrate processes of interest to the hematologist and more broadly.

The role of GATA2 in lethal prostate cancer aggressiveness

Advanced prostate cancer is a classic example of the intractability and consequent lethality that characterizes metastatic carcinomas. Novel treatments have improved the survival of men with prostate cancer; however, advanced prostate cancer invariably becomes resistant to these therapies and ultimately progresses to a lethal metastatic stage. Consequently, detailed knowledge of the molecular mechanisms that control prostate cancer cell survival and progression towards this lethal stage of disease will benefit the development of new therapeutics. The transcription factor endothelial transcription factor GATA-2 (GATA2) has been reported to have a key role in driving prostate cancer aggressiveness. In addition to being a pioneer transcription factor that increases androgen receptor (AR) binding and activity, GATA2 regulates a core subset of clinically relevant genes in an AR-independent manner. Functionally, GATA2 overexpression in prostate cancer increases cellular motility and invasiveness, proliferation, tumorigenicity, and resistance to standard therapies. Thus, GATA2 has a multifaceted function in prostate cancer aggressiveness and is a highly attractive target in the development of novel treatments against lethal prostate cancer.

Graded Proteasome Dysfunction in Caenorhabditis elegans Activates an Adaptive Response Involving the Conserved SKN-1 and ELT-2 Transcription Factors and the Autophagy-Lysosome Pathway

The maintenance of cellular proteins in a biologically active and structurally stable state is a vital endeavor involving multiple cellular pathways. One such pathway is the ubiquitin-proteasome system that represents a major route for protein degradation, and reductions in this pathway usually have adverse effects on the health of cells and tissues. Here, we demonstrate that loss-of-function mutants of the Caenorhabditis elegans proteasome subunit, RPN-10, exhibit moderate proteasome dysfunction and unexpectedly develop both increased longevity and enhanced resistance to multiple threats to the proteome, including heat, oxidative stress, and the presence of aggregation prone proteins. The rpn-10 mutant animals survive through the activation of compensatory mechanisms regulated by the conserved SKN-1/Nrf2 and ELT-2/GATA transcription factors that mediate the increased expression of genes encoding proteasome subunits as well as those mediating oxidative- and heat-stress responses. Additionally, we find that the rpn-10 mutant also shows enhanced activity of the autophagy-lysosome pathway as evidenced by increased expression of the multiple autophagy genes including atg-16.2, lgg-1, and bec-1, and also by an increase in GFP::LGG-1 puncta. Consistent with a critical role for this pathway, the enhanced resistance of the rpn-10 mutant to aggregation prone proteins depends on autophagy genes atg-13, atg-16.2, and prmt-1. Furthermore, the rpn-10 mutant is particularly sensitive to the inhibition of lysosome activity via either RNAi or chemical means. We also find that the rpn-10 mutant shows a reduction in the numbers of intestinal lysosomes, and that the elt-2 gene also plays a novel and vital role in controlling the production of functional lysosomes by the intestine. Overall, these experiments suggest that moderate proteasome dysfunction could be leveraged to improve protein homeostasis and organismal health and longevity, and that the rpn-10 mutant provides a unique platform to explore these possibilities.

Essential Role of GATA2 in the Negative Regulation of Type 2 Deiodinase Gene by Liganded Thyroid Hormone Receptor β2 in Thyrotroph

The inhibition of thyrotropin (thyroid stimulating hormone; TSH) by thyroid hormone (T3) and its receptor (TR) is the central mechanism of the hypothalamus-pituitary-thyroid axis. Two transcription factors, GATA2 and Pit-1, determine thyrotroph differentiation and maintain the expression of the β subunit of TSH (TSHβ). We previously reported that T3-dependent repression of the TSHβ gene is mediated by GATA2 but not by the reported negative T3-responsive element (nTRE). In thyrotrophs, T3 also represses mRNA of the type-2 deiodinase (D2) gene, where no nTRE has been identified. Here, the human D2 promoter fused to the CAT or modified Renilla luciferase gene was co-transfected with Pit-1 and/or GATA2 expression plasmids into cell lines including CV1 and thyrotroph-derived TαT1. GATA2 but not Pit-1 activated the D2 promoter. Two GATA responsive elements (GATA-REs) were identified close to cAMP responsive element. The protein kinase A activator, forskolin, synergistically enhanced GATA2-dependent activity. Gel-shift and chromatin immunoprecipitation assays with TαT1 cells indicated that GATA2 binds to these GATA-REs. T3 repressed the GATA2-induced activity of the D2 promoter in the presence of the pituitary-specific TR, TRβ2. The inhibition by T3-bound TRβ2 was dominant over the synergism between GATA2 and forskolin. The D2 promoter is also stimulated by GATA4, the major GATA in cardiomyocytes, and this activity was repressed by T3 in the presence of TRα1. These data indicate that the GATA-induced activity of the D2 promoter is suppressed by T3-bound TRs via a tethering mechanism, as in the case of the TSHβ gene.

The Human GATA1 Gene Retains a 5' Insulator That Maintains Chromosomal Architecture and GATA1 Expression Levels in Splenic Erythroblasts

GATA1 is a key transcription factor for erythropoiesis. GATA1 gene expression is strictly regulated at the transcriptional level. While the regulatory mechanisms governing mouse Gata1 (mGata1) gene expression have been studied extensively, how expression of the human GATA1 (hGATA1) gene is regulated remains to be elucidated. To address this issue, we generated hGATA1 bacterial artificial chromosome (BAC) transgenic mouse lines harboring a 183-kb hGATA1 locus covering the hGATA1 exons and distal flanking sequences. Transgenic hGATA1 expression coincides with endogenous mGata1 expression and fully rescues hematopoietic deficiency in mGata1 knockdown mice. The transgene exhibited copy number-dependent and integration position-independent expression of hGATA1, indicating the presence of chromatin insulator activity within the transgene. We found a novel insulator element at 29 kb 5' to the hGATA1 gene and refer to this element as the 5' CCCTC-binding factor (CTCF) site. Substitution mutation of the 5' CTCF site in the hGATA1 BAC disrupted the chromatin architecture and led to a reduction of hGATA1 expression in splenic erythroblasts under conditions of stress erythropoiesis. Our results demonstrate that expression of the hGATA1 gene is regulated through the chromatin architecture organized by 5' CTCF site-mediated intrachromosomal interactions in the hGATA1 locus.

Haematopoietic and immune defects associated with GATA2 mutation

Heterozygous familial or sporadic GATA2 mutations cause a multifaceted disorder, encompassing susceptibility to infection, pulmonary dysfunction, autoimmunity, lymphoedema and malignancy. Although often healthy in childhood, carriers of defective GATA2 alleles develop progressive loss of mononuclear cells (dendritic cells, monocytes, B and Natural Killer lymphocytes), elevated FLT3 ligand, and a 90% risk of clinical complications, including progression to myelodysplastic syndrome (MDS) by 60 years of age. Premature death may occur from childhood due to infection, pulmonary dysfunction, solid malignancy and MDS/acute myeloid leukaemia. GATA2 mutations include frameshifts, amino acid substitutions, insertions and deletions scattered throughout the gene but concentrated in the region encoding the two zinc finger domains. Mutations appear to cause haplo-insufficiency, which is known to impair haematopoietic stem cell survival in animal models. Management includes genetic counselling, prevention of infection, cancer surveillance, haematopoietic monitoring and, ultimately, stem cell transplantation upon the development of MDS or another life-threatening complication.

Regulation of GATA-binding protein 2 levels via ubiquitin-dependent degradation by Fbw7: involvement of cyclin B-cyclin-dependent kinase 1-mediated phosphorylation of THR176 in GATA-binding protein 2

A GATA family transcription factor, GATA-binding protein 2 (GATA2), participates in cell growth and differentiation of various cells, such as hematopoietic stem cells. Although its expression level is controlled by transcriptional induction and proteolytic degradation, the responsible E3 ligase has not been identified. Here, we demonstrate that F-box/WD repeat-containing protein 7 (Fbw7/Fbxw7), a component of Skp1, Cullin 1, F-box-containing complex (SCF)-type E3 ligase, is an E3 ligase for GATA2. GATA2 contains a cell division control protein 4 (Cdc4) phosphodegron (CPD), a consensus motif for ubiquitylation by Fbw7, which includes Thr(176). Ectopic expression of Fbw7 destabilized GATA2 and promoted its proteasomal degradation. Substitution of threonine 176 to alanine in GATA2 inhibited binding with Fbw7, and the ubiquitylation and degradation of GATA2 by Fbw7 was suppressed. The CPD kinase, which mediates the phosphorylation of Thr(176), was cyclin B-cyclin-dependent kinase 1 (CDK1). Moreover, depletion of endogenous Fbw7 stabilized endogenous GATA2 in K562 cells. Conditional Fbw7 depletion in mice increased GATA2 levels in hematopoietic stem cells and myeloid progenitors at the early stage. Increased GATA2 levels in Fbw7-conditional knock-out mice were correlated with a decrease in a c-Kit high expressing population of myeloid progenitor cells. Our results suggest that Fbw7 is a bona fide E3 ubiquitin ligase for GATA2 in vivo.

Localization of GATA2 in the nuclear and cytoplasmic regions of ovine conceptuses

GATA transcription factors are emerging as critical regulators in trophoblast development and its gene regulation. The purpose of this study was to examine the expression and cellular localization of GATA2 in ovine conceptuses during the peri-implantation period. In Western blot analyses, GATA2 proteins were found in days 15, 17 and 21 ovine conceptuses (day 0=day of estrus). Using immunohistochemistry and immunofluorescence analyses, we found that GATA2 was localized in days 15, 17 and 21 ovine conceptuses, and more importantly, GATA2 protein was detected in both nuclear and cytoplasmic regions of the trophectoderm. To our knowledge, the present study is the first to demonstrate that GATA2 is localized in two cellular compartments of the trophectoderm in ovine and many other mammalian species, and suggests that the difference in GATA2 location plays a role in the regulation of down-stream genes during the early pregnancy period.

N- and C-terminal transactivation domains of GATA1 protein coordinate hematopoietic program

Transcription factor GATA1 regulates the expression of a cluster of genes important for hematopoietic cell differentiation toward erythroid and megakaryocytic lineages. Three functional domains have been identified in GATA1, a transactivation domain located in the N terminus (N-TAD) and two zinc finger domains located in the middle of the molecule. Although N-TAD is known as a solitary transactivation domain for GATA1, clinical observations in Down syndrome leukemia suggest that there may be additional transactivation domains. In this study, we found in reporter co-transfection assays that transactivation activity of GATA1 was markedly reduced by deletion of the C-terminal 95 amino acids without significant attenuation of the DNA binding activity or self-association potential. We therefore generated transgenic mouse lines that expressed GATA1 lacking the C-terminal region (GATA1-ΔCT). When we crossed these transgenic mouse lines to the Gata1-deficient mouse, we found that the GATA1-ΔCT transgene rescued Gata1-deficient mice from embryonic lethality. The embryos rescued with an almost similar level of GATA1-ΔCT to endogenous GATA1 developed beyond embryonic 13.5 days, showing severe anemia with accumulation of immature erythroid cells, as was the case for the embryos rescued by endogenous levels of GATA1 lacking N-TAD (GATA1-ΔNT). Distinct sets of target genes were affected in the embryos rescued by GATA1-ΔCT and GATA1-ΔNT. We also found attenuated GATA1 function in cell cycle control of immature megakaryocytes in both lines of rescued embryos. These results thus demonstrate that GATA1 has two independent transactivation domains, N-TAD and C-TAD. Both N-TAD and C-TAD retain redundant as well as specific activities for proper hematopoiesis in vivo.

UG4 enhancer-driven GATA-2 and bone morphogenetic protein 4 complementation remedies the CAKUT phenotype in Gata2 hypomorphic mutant mice

During renal development, the proper emergence of the ureteric bud (UB) from the Wolffian duct is essential for formation of the urinary system. Previously, we showed that expression of transcription factor GATA-2 in the urogenital primordium was demarcated anteroposteriorly into two domains that were regulated by separate enhancers. While GATA-2 expression in the caudal urogenital mesenchyme is controlled by the UG4 enhancer, its more-rostral expression is regulated by UG2. We found that anteriorly displaced budding led to obstructed megaureters in Gata2 hypomorphic mutant mice, possibly due to reduced expression of the downstream effector bone morphogenetic protein 4 (BMP4). Here, we report that UG4-driven, but not UG2-driven, GATA-2 expression in the urogenital mesenchyme significantly reverts the uropathy observed in the Gata2 hypomorphic mutant mice. Furthermore, the data show that transgenic rescue by GATA-2 reverses the rostral outgrowth of the UB. We also provide evidence for a GATA-2-BMP4 epistatic relationship by demonstrating that reporter gene expression from a Bmp4 bacterial artificial chromosome (BAC) transgene is altered in Gata2 hypomorphs; furthermore, UG4-directed BMP4 expression in the mutants leads to reduced incidence of megaureters. These results demonstrate that GATA-2 expression in the caudal urogenital mesenchyme as directed by the UG4 enhancer is crucial for proper development of the urinary tract and that its regulation of BMP4 expression is a critical aspect of this function.

Master regulatory GATA transcription factors: mechanistic principles and emerging links to hematologic malignancies

Numerous examples exist of how disrupting the actions of physiological regulators of blood cell development yields hematologic malignancies. The master regulator of hematopoietic stem/progenitor cells GATA-2 was cloned almost 20 years ago, and elegant genetic analyses demonstrated its essential function to promote hematopoiesis. While certain GATA-2 target genes are implicated in leukemogenesis, only recently have definitive insights emerged linking GATA-2 to human hematologic pathophysiologies. These pathophysiologies include myelodysplastic syndrome, acute myeloid leukemia and an immunodeficiency syndrome with complex phenotypes including leukemia. As GATA-2 has a pivotal role in the etiology of human cancer, it is instructive to consider mechanisms underlying normal GATA factor function/regulation and how dissecting such mechanisms may reveal unique opportunities for thwarting GATA-2-dependent processes in a therapeutic context. This article highlights GATA factor mechanistic principles, with a heavy emphasis on GATA-1 and GATA-2 functions in the hematopoietic system, and new links between GATA-2 dysregulation and human pathophysiologies.

GATA-2 mediated regulation of normal hematopoietic stem/progenitor cell function, myelodysplasia and myeloid leukemia

Unremitting blood cell production throughout the lifetime of an organism is reliant on hematopoietic stem cells (HSCs). A rare and relatively quiescent cell type, HSCs are, on entry into cell cycle fated to self-renew, undergo apoptosis or differentiate to progenitors (HPCs) that eventually yield specific classes of blood cells. Disruption of these HSC fate decisions is considered to be fundamental to the development of leukemia. Much effort has therefore been placed on understanding the molecular pathways that regulate HSC fate decisions and how these processes are undermined in leukemia. Transcription factors have emerged as critical regulators in this respect. Here we review the participation of zinc finger transcription factor GATA-2 in regulating normal hematopoietic stem and progenitor cell functionality, myelodysplasia and myeloid leukemia.

Activator protein-2α functions as a master regulator of multiple transcription factors in the mouse liver

AIM

  Activator protein 2α (AP-2α) belongs to the AP-2 family of transcription factors that are involved in the regulation of cell proliferation, differentiation, apoptosis and carcinogenesis and has been suggested to function as a tumor suppressor in many cancers. However, the physiological role of AP-2α in hepatocytes is unknown. The present study is to characterize the expression and function of AP-2α in the liver of conscience mouse.

METHODS

  Exogenous AP-2α was overexpressed in the mouse liver by in vivo gene delivery and changes in transcription factor expression were identified by using protein-DNA arrays and immunoblotting.

RESULTS

  Western blotting and protein/DNA arrays showed that AP-2α is expressed in the nuclei of mouse hepatocytes. Overexpression of AP-2αin vivo significantly suppressed transcription factors AP-1, CREB and c-Myc, and markedly increased CBF, c-Myb, NF-1, Pax-5, RXR, Smad3/4, TR(DR-4), USF-1 and GATA. Among all GATA proteins, only GATA-4 level was dramatically elevated and there was a concomitant loss of phospho-GATA-4. Corresponding changes were detected in upstream kinases Akt, GSK-3β and PKA, which regulates the phosphorylation status and stability of GATA-4 protein.

CONCLUSIONS

  AP-2α is expressed in mouse hepatocytes and it acts as a master regulator of numerous transcription factors in the liver.

Gata2 is required for migration and differentiation of retinorecipient neurons in the superior colliculus

The superior colliculus (SC)/optic tectum of the dorsal mesencephalon plays a major role in responses to visual input, yet regulation of neuronal differentiation within this layered structure is only partially understood. Here, we show that the zinc finger transcription factor Gata2 is required for normal SC development. Starting at embryonic day 15 (E15) (corresponding to the times at which neurons of the outer and intermediate layers of the SC are generated), Gata2 is transiently expressed in the rat embryonic dorsal mesencephalon within a restricted region between proliferating cells of the ventricular zone and the deepest neuronal layers of the developing SC. The Gata2-positive cells are postmitotic and lack markers of differentiated neurons, but express markers for immature neuronal precursors including Ascl1 and Pax3/7. In utero electroporation with Gata2 small hairpin RNAs at E16 into cells along the dorsal mesencephalic ventricle interferes with their normal migration into the SC and maintains them in a state characterized by retention of Pax3 expression and the absence of mature neuronal markers. Collectively, these findings indicate that Gata2 plays a required role in the transition of postmitotic neuronal precursor cells of the retinorecipient layers of the SC into mature neurons and that loss of Gata2 arrests them at an intermediate stage of differentiation.

The role of the GATA2 transcription factor in normal and malignant hematopoiesis

Hematopoiesis involves an elaborate regulatory network of transcription factors that coordinates the expression of multiple downstream genes, and maintains homeostasis within the hematopoietic system through the accurate orchestration of cellular proliferation, differentiation and survival. As a result, defects in the expression levels or the activity of these transcription factors are intimately linked to hematopoietic disorders, including leukemia. The GATA family of nuclear regulatory proteins serves as a prototype for the action of lineage-restricted transcription factors. GATA1 and GATA2 are expressed principally in hematopoietic lineages, and have essential roles in the development of multiple hematopoietic cells, including erythrocytes and megakaryocytes. Moreover, GATA2 is crucial for the proliferation and maintenance of hematopoietic stem cells and multipotential progenitors. In this review, we summarize the current knowledge regarding the biological properties and functions of the GATA2 transcription factor in normal and malignant hematopoiesis.

Regulation of intracellular decorin via proteasome degradation in rat mesangial cells

Decorin (DCN) is a member of small leucine-rich proteoglycan family that neutralizes the bioactivity of transforming growth factor-beta1 (TGF-β1). It has been proven to be a promising anti-fibrotic agent to treat glomerulonephritis. But the underlining mechanism for regulating and degrading intracellular DCN is still not fully understood. In this study, we investigated the roles of ubiquitination in the regulation of cytoplasmic DCN metabolism in rat mesangial cells (MC) by immunoprecipitation and Western blot. The results showed that a proportion of cytoplasmic DCN was ubiquitinated in normal MC and was enhanced in N-glycosylation inhibitor (tunicamycin)-treated MC. After being treated with the proteasome inhibitor MG132, ubiquitinated DCN accumulated and displayed a prolonged half-life, accompanied by decreased TGF-β1 expression and reduced collagen IV mRNA level in MC. This study demonstrated that the stability and function of cytoplasmic DCN can be regulated by ubiquitin-proteasome system (UPS) in MC, which implies that regulating the ubiquitination and degradation of DCN might be a novel approach for modulating MC bioactivity.

Megakaryocytic programming by a transcriptional regulatory loop: A circle connecting RUNX1, GATA-1, and P-TEFb

Transcription factors originally identified as drivers of erythroid differentiation subsequently became linked to megakaryopoiesis, reflecting the shared parentage of red cells and platelets. The divergent development of megakaryocytic and erythroid progenitors relies on signaling pathways that impose lineage-specific transcriptional programs on non-lineage-restricted protein complexes. One such signaling pathway involves RUNX1, a transcription factor upregulated in megakaryocytes and downregulated in erythroid cells. In this pathway, RUNX1 engages the erythro-megakaryocytic master regulator GATA-1 in a megakaryocytic transcriptional complex whose activity is highly dependent on the P-TEFb kinase complex. The implications of this pathway for normal and pathological megakaryopoiesis are discussed.

GATA factors in human neuroblastoma: distinctive expression patterns in clinical subtypes

Background:

The aim of this study is to elucidate the expression patterns of GATA transcription factors in neuroblastoma and the developing sympathetic nervous system (SNS).

Methods:

GATA-2, -3 and -4 and their cofactor friend-of-GATA (FOG)-2 were investigated in primary neuroblastoma by immunohistochemistry, real-time RT-PCR (n=73) and microarray analysis (n=251). In addition, GATA-2, -3 and FOG-2 expression was determined by northern-blot hybridisation. In the developing murine SNS, Gata-4 and Fog-2 were examined by immunohistochemistry.

Results:

Although Gata-2, -3 and Fog-2 are expressed in the developing nervous system, Gata-4 was not detected. In contrast, protein expression of all factors was observed in human neuroblastoma. Northern-blot hybridisation and real-time RT-PCR suggested specific expression patterns of the four genes in primary neuroblastoma, but did not show unequivocal results. In the large cohort examined by microarrays, a significant association of GATA-2, -3 and FOG-2 expression with low-risk features was observed, whereas GATA-4 mRNA levels correlated with MYCN-amplification.

Conclusion:

The transcription factors GATA-2 and -3, which are essential for normal SNS development, and their cofactor FOG-2 are downregulated in aggressive but not in favourable neuroblastoma. In contrast, upregulation of GATA-4 appears to be a common feature of this malignancy and might contribute to neuroblastoma pathogenesis.

A role for GATA-2 in transition to an aggressive phenotype in prostate cancer through modulation of key androgen-regulated genes

GATA-2, a member of the GATA family of transcription factors, is involved in androgen receptor (AR) signaling, however, little is known regarding its role in prostate cancer. Here, we report that GATA-2 is expressed in a substantial proportion of prostate cancers and that high expression of GATA-2 is associated with biochemical recurrence and distant metastatic progression in a validation set of 203 cancers. In vitro data show that GATA-2 is directly recruited to the promoter region of the AR upon androgen stimulation of LNCaP prostate cancer cells with 5alpha-dihydroxytestosterone (DHT) for 24 h. Ectopic GATA-2 expression causes the induction of AR transcript levels under androgen-depleted conditions (P<0.05). The expression of the AR target gene, AZGP1, is induced upon androgen stimulation and this effect is repressed by GATA-2. In contrast, GATA-2 significantly increases transcript levels of KLK2, which increases further in a time-dependent manner on DHT treatment and in the presence of GATA-2. These results indicate that upregulation of GATA-2 may contribute to the progression to aggressive prostate cancer through modulation of expression of AR and key androgen-regulated genes, one of which, AZGP1, is associated with the progression to metastatic disease.

SCL and associated proteins distinguish active from repressive GATA transcription factor complexes

GATA-1 controls hematopoietic development by activating and repressing gene transcription, yet the in vivo mechanisms that specify these opposite activities are unknown. By examining the composition of GATA-1-associated protein complexes in a conditional erythroid rescue system as well as through the use of tiling arrays we detected the SCL/TAL1, LMO2, Ldb1, E2A complex at all positively acting GATA-1-bound elements examined. Similarly, the SCL complex is present at all activating GATA elements in megakaryocytes and mast cells. In striking contrast, at sites where GATA-1 functions as a repressor, the SCL complex is depleted. A DNA-binding defective form of SCL maintains association with a subset of active GATA elements indicating that GATA-1 is a key determinant for SCL recruitment. Knockdown of LMO2 selectively impairs activation but not repression by GATA-1. ETO-2, an SCL-associated protein with the potential for transcription repression, is also absent from GATA-1-repressed genes but, unlike SCL, fails to accumulate at GATA-1-activated genes. Together, these studies identify the SCL complex as a critical and consistent determinant of positive GATA-1 activity in multiple GATA-1-regulated hematopoietic cell lineages.

Identification of ubiquitinated proteins in Arabidopsis

Ubiquitin (Ub) is a small peptide that is covalently attached to proteins in a posttranslational reaction. Ubiquitination is a precise regulatory system that is present in all eukaryotic organisms and regulates the stability, the activity, the localization and the transport of proteins. Ubiquitination involves different enzymatic activities, in which the E3 ligases catalyze the last step recruiting of the target for labelling with ubiquitin. Genomic analyses have shown that the ubiquitin-proteasome system involves a large number of proteins in plants, as approximately 5% of the total protein belongs to this pathway. In contrast to the high number of E3 ligases of ubiquitin identified, very few proteins regulated by ubiquitination have been described. To solve this, we have undertaken a new proteomic approach aimed to identify proteins modified with ubiquitin. This is based on affinity purification and identification for ubiquitinated proteins using the ubiquitin binding domain (UBA) polypeptide of the P62 protein attached to agarose beads. This P62-agarose matrix is capable of specifically binding ubiquitinated proteins. These bound proteins were digested with trypsin and the peptides separated by HPLC chromatography, spotted directly onto a MALDI target and analyzed by MALDI-TOF/TOF off-line coupled LC/MALDI-MS/MS. A total of 200 putative ubiquitinated proteins were identified. From these we found that several of the putative targets were already described in plants, as well as in other organisms, as ubiquitinated proteins. In addition, we have found that some of these proteins were indeed modified with ubiquitin in vivo. Taken together, we have shown that this approach is useful for identifying ubiquitinated protein in plants.

Protein ubiquitination is modulated by O-GlcNAc glycosylation

During the past two decades, O-GlcNAc modification of cytosolic and nuclear proteins has been intensively studied. Nevertheless, the function of this post-translational modification remains unclear. It has been recently speculated that O-GlcNAc could act as a protective signal against proteasomal degradation, both by modifying target substrates and/or by inhibiting the proteasome itself. In this work, we have investigated the putative relation between O-GlcNAc and the ubiquitin pathway. First, we showed that the level of both modifications increased rapidly after thermal stress but, unlike ubiquitinated proteins, O-GlcNAc-modified proteins failed to be stabilized by inhibiting proteasome function. Increasing O-GlcNAc levels, using glucosamine or PUGNAc, enhanced ubiquitination. Inversely, when O-GlcNAc levels were reduced, using forskolin or glucose deprivation, ubiquitination decreased. Targeted-RNA interference of O-GlcNAc transferase also reduced ubiquitination and moreover halved cell thermotolerance. Finally, we demonstrated that the ubiquitin-activating enzyme E1 was O-GlcNAc modified and that its glycosylation and its interaction with Hsp70 varied according to the conditions of cell culture. Altogether, these results show that O-GlcNAc and ubiquitin are not strictly antagonistic post-translational modifications, but rather that the former might regulate the latter, and also suggest that E1 could be one of the common links between the two pathways.

Pyruvate dehydrogenase complex deficiency caused by ubiquitination and proteasome-mediated degradation of the E1 subunit

Congenital deficiencies of the human pyruvate dehydrogenase (PDH) complex are considered to be due to loss of function mutations in one of the component enzymes. Here we describe a case of PDH deficiency associated with the PDH E1beta subunit (PDHB) gene. The clinical phenotype of the patient was consistent with reported cases of PDH deficiency. Cultured skin fibroblasts demonstrated a 55% reduction in PDH activity and markedly decreased immunoreactivity for PDHB protein, compared with healthy controls. Surprisingly, nucleotide sequence analyses of cDNAs corresponding to the patient PDH E1alpha (PDHA1) and PDHB genes revealed no pathological mutations. Moreover, the relative expression level of PDHB mRNA and the rates of transcription and translation of the PDHB gene were normal. However, PDC activity could be restored in cells from this patient following treatment with MG132, a specific proteasome inhibitor, and normal levels of E1beta could be detected in MG132-treated cells. Similar results were obtained following treatment with Tyr-phostin 23 (Tyr23), a specific inhibitor of epidermal growth factor receptor-protein-tyrosine kinase (EGFR-PTK), which also restored E1beta protein levels to those in cells from healthy subjects or from patients with PDHA1 deficiency. The index patient's cells contained a high basal level of EGFR-PTK activity that correlated with the high level of ubiquitination of cellular proteins, although the total EGFR protein levels were similar to those in cells from Elalpha-deficient subjects and healthy subjects. These data indicate that PDH deficiency in our patient involves a post-translational modification in which EGFR-PTK-mediated tyrosine phosphorylation of the E1beta protein leads to enhanced ubiquitination followed by proteasome-mediated degradation. They also provide a novel mechanism accounting for congenital deficiency of the PDH complex and perhaps other inborn errors of metabolism.

Concise Review: Role and Function of the Ubiquitin-Proteasome System in Mammalian Stem and Progenitor Cells

Highly ordered degradation of cell proteins by the ubiquitin-proteasome system, a sophisticated cellular proteolytic machinery, has been identified as a key regulatory mechanism in many eukaryotic cells. Accumulating evidence reveals that the ubiquitin-proteasome system is involved in the regulation of fundamental processes in mammalian stem and progenitor cells of embryonic, neural, hematopoietic, and mesenchymal origin. Such processes, including development, survival, differentiation, lineage commitment, migration, and homing, are directly controlled by the ubiquitin-proteasome system, either via proteolytic degradation of key regulatory proteins of signaling and gene expression pathways or via nonproteolytic mechanisms involving the proteasome itself or posttranslational modifications of target proteins by ubiquitin or other ubiquitin-like modifiers. Future characterization of the precise roles and functions of the ubiquitin-proteasome system in mammalian stem and early progenitor cells will improve our understanding of stem cell biology and may provide an experimental basis for the development of novel therapeutic strategies in regenerative medicine. Disclosure of potential conflicts of interest is found at the end of this article.

Dynamic regulation of Gata factor levels is more important than their identity

Three Gata transcription factors (Gata1, -2, and -3) are essential for hematopoiesis. These factors are thought to play distinct roles because they do not functionally replace each other. For instance, Gata2 messenger RNA (mRNA) expression is highly elevated in Gata1-null erythroid cells, yet this does not rescue the defect. Here, we test whether Gata2 and -3 transgenes rescue the erythroid defect of Gata1-null mice, if expressed in the appropriate spatiotemporal pattern. Gata1, -2, and -3 transgenes driven by beta-globin regulatory elements, directing expression to late stages of differentiation, fail to rescue erythropoiesis in Gata1-null mutants. In contrast, when controlled by Gata1 regulatory elements, directing expression to the early stages of differentiation, Gata1, -2, and -3 do rescue the Gata1-null phenotype. The dramatic increase of endogenous Gata2 mRNA in Gata1-null progenitors is not reflected in Gata2 protein levels, invoking translational regulation. Our data show that the dynamic spatiotemporal regulation of Gata factor levels is more important than their identity and provide a paradigm for developmental control mechanisms that are hard-wired in cis-regulatory elements.

Cell-cycle-dependent oscillation of GATA2 expression in hematopoietic cells

In vitro manipulation of hematopoietic stem cells (HSCs) is a key issue in both transplantation therapy and regenerative medicine, and thus new methods are required to achieve HSC expansion with self-renewal. GATA2 is a transcription factor controlling pool size of HSCs. Of interest, continuous overexpression of GATA2 does not induce HSC proliferation. In this report, we demonstrate that GATA2 expression, in leukemic and normal hematopoietic cells, oscillates during the cell cycle, such that expression is high in S phase but low in G(1)/S and M phase. GATA2 binding to target Bcl-X gene also oscillates in accordance with GATA2 expression. Using a green fluorescent protein (GFP)-GATA2 fusion protein, we demonstrate cell-cycle-specific activity of proteasome-dependent degradation of GATA2. Immunoprecipitation/immunoblotting analysis demonstrated phosphorylation of GATA2 at cyclin-dependent kinase (Cdk)-consensus motifs, S/T(0)P(+1), and interaction of GATA2 with Cdk2/cyclin A2-, Cdk2/cyclin A2-, and Cdk4/cyclin D1-phosphorylated GATA2 in vitro. Mutants in phosphorylation motifs exhibited altered expression profiles of GFP-GATA2 domain fusion proteins. These results indicate that GATA2 phosphorylation by Cdk/cyclin systems is responsible for the cell-cycle-dependent regulation of GATA2 expression, and suggest the possibility that a cell-cycle-specific "on-off" response of GATA2 expression may control hematopoietic-cell proliferation and survival.

Induction of erythroid-specific genes by overexpression of GATA-2 in K562 cells

GATA transcription factors have been shown to play important roles in hematopoiesis. GATA-2 is expressed in stem and progenitor cells, and has been speculated to control the proliferation and maintain the immaturity of these cells. To examine whether the function of GATA-2 is changeable according to the differentiation stage, we established GATA-2 overexpressing subclones of K562, which is a leukemic cell line committed to the erythroid lineage. Via an increase in the GATA-2 expression level, the expression levels of erythroid-specific genes including alpha-, beta-, and gamma-globin were increased compared to control cells, while the expression level of GATA-1 was unchanged. Expression of the transferrin receptor was also increased in GATA-2 overexpressing K562 cells when examined by flow cytometry. In addition, the heme content of GATA-2 overexpressing K562 cells was more than 2 times higher than control cells. Chromatin immunoprecipitation analysis showed that GATA-2 protein binding to the GATA element in alpha-globin LCR was increased in GATA-2 overexpressing K562 cells. These findings suggest that GATA-2 could induce erythroid-specific genes without competition with GATA-1 when expressed in erythroid-committed cells, and thus further suggest that temporal and spatial regulation may be important for displaying specific functions of GATA-2.

Ubiquitylation in normal and malignant hematopoiesis: novel therapeutic targets

The modification of proteins with ubiquitin is involved in the regulation of various important biological pathways. A crucial step in this process is the modification of specific substrate proteins with ubiquitin by E3 ligases. The ubiquitylation of proteins can result in altered protein function or degradation by the 26S proteasome. Various proteins playing an important role during hematopoiesis are regulated via ubiquitin modification. Recently, alterations in ubiquitylation and proteasomal degradation have been implicated in hematological cancers. Based on these findings, novel therapies that specifically target ubiquitylation or the proteasome are currently being developed. In this review, we will highlight the role of ubiquitylation in normal and malignant hematopoiesis and discuss novel therapeutical approaches that are now tested in various hematological malignancies.