Ubiquitin-mediated processes in erythroid cell maturation

Extracellular ubiquitin promotes hepatoma metastasis by mediating M2 macrophage polarization via the activation of the CXCR4/ERK signaling pathway

Background

Stored red blood cell (RBC) transfusion has been shown to enhance the risk of cancer recurrence. However, the underlying mechanism remains unknown. At our lab, we have demonstrated that the extracellular ubiquitin (eUb) released by aged RBCs could promote tumor metastasis in a melanoma mouse model. This study aimed to confirm the pro-tumor effect of eUb on hepatocellular carcinoma (HCC) and explore the related immunoregulatory mechanisms.

Methods

Forty HCC tissue specimens and the corresponding adjacent nontumor and normal liver tissues were collected. Two human hepatoma cell lines (MHCC-97H and HepG2.2.15), one murine hepatoma cell line (Hepa1-6), and one human monocyte cell line (THP-1) were adopted in this study. The coculture of hepatoma cells with macrophages was initiated with Transwell inserts. Cell migration in vitro was detected by Transwell and wound-healing assays, while in vivo tumor metastasis was measured by luciferase assay and H&E staining. Macrophage polarization was measured by flow cytometry, immunofluorescence, ELISA, qPCR, and Western blot. Protein expression was detected by Western blot, and immunoprecipitation was used to confirm the interaction between Ub and CXCR4 (CXC chemokine receptor type 4).

Results

Ub and CXCR4 were significantly upregulated in HCC tissues, and a positive correlation existed between them. In vitro, the migration of hepatoma cells was not affected by eUb directly, but their metastatic abilities were enhanced after coculture with the macrophages pretreated with eUb. Meanwhile, eUb promoted hepatoma cell metastasis in the lung in vivo and increased the ratio of M2 macrophages in the lung tissues and peripheral blood of tumor-bearing mice. Furthermore, the eUb-induced M2 macrophage polarization was related to the activation of the CXCR4/ERK (extracellular regulated protein kinase) signaling pathway.

Conclusions

Extracellular ubiquitin promoted hepatoma metastasis through M2 macrophage polarization via the activation of the CXCR4/ERK signaling pathway, indicating that a personalized transfusion strategy is needed for the treatment of HCC patients. Neutralizing Ub in stored RBC units could lessen the detrimental clinical outcomes induced by the transfusion of stored RBCs.

Extracellular Ubiquitin is the Causal Link between Stored Blood Transfusion Therapy and Tumor Progression in a Melanoma Mouse Model

Background: The transfusion of blood that has been stored for some time was found to be associated with transfusion-related immune modulation (TRIM) responses in cancer patients, which could result in poor clinical outcomes, such as tumor recurrence, metastasis and reduced survival rate. Given the prior observation of the positive correlation between ubiquitin content in whole blood and storage duration by the investigators of the present study, it was hypothesized that this could be the causal link behind the association between the transfusion of stored blood and poor cancer prognosis.

Methods: In the present study, a melanoma mouse model was used to study the potential clinical impact of ubiquitin present in stored blood on cancer prognosis through a variety of cell biology methods, such as flow cytometry and immunohistochemistry.

Results: Both extracellular ubiquitin and the infusion of stored mice blood that comprised of ubiquitin reduced the apoptotic rate of melanoma cells, promoted lung tumor metastasis and tumor progression, and reduced the long-term survival rate of melanoma mice. In addition, the upregulation of tumor markers and tumorigenic TH2 cytokine generation, as well as reduced immune cell numbers, were observed in the presence of ubiquitin.

Conclusions: The present findings provide novel insights into the role of ubiquitin in immune regulation in a melanoma mouse model, and suggest ubiquitin as the causal link between allogeneic blood transfusion therapy and poor cancer prognosis.

UBE2O remodels the proteome during terminal erythroid differentiation

During terminal differentiation, the global protein complement is remodeled, as epitomized by erythrocytes, whose cytosol is ~98% globin. The erythroid proteome undergoes a rapid transition at the reticulocyte stage; however, the mechanisms driving programmed elimination of preexisting cytosolic proteins are unclear. We found that a mutation in the murine Ube2o gene, which encodes a ubiquitin-conjugating enzyme induced during erythropoiesis, results in anemia. Proteomic analysis suggested that UBE2O is a broad-spectrum ubiquitinating enzyme that remodels the erythroid proteome. In particular, ribosome elimination, a hallmark of reticulocyte differentiation, was defective in Ube2o-/- mutants. UBE2O recognized ribosomal proteins and other substrates directly, targeting them to proteasomes for degradation. Thus, in reticulocytes, the induction of ubiquitinating factors may drive the transition from a complex to a simple proteome.

Extracellular ubiquitin: immune modulator and endogenous opponent of damage-associated molecular pattern molecules

Ubiquitin is a post-translational protein modifier and plays essential roles in all aspects of biology. Although the discovery of ubiquitin introduced this highly conserved protein as a molecule with extracellular actions, the identification of ubiquitin as the ATP-dependent proteolysis factor 1 has focused subsequent research on its important intracellular functions. Little attention has since been paid to its role outside of the cell. During recent years, multiple observations suggest that extracellular ubiquitin can modulate immune responses and that exogenous ubiquitin has therapeutic potential to attenuate exuberant inflammation and organ injury. These observations have not been integrated into a comprehensive assessment of its possible role as an endogenous immune modulator. This review recapitulates the current knowledge about extracellular ubiquitin and discusses an emerging facet of its role in biology during infectious and noninfectious inflammation. The synopsis of these data along with the recent identification of ubiquitin as a CXCR4 agonist suggest that extracellular ubiquitin may have pleiotropic roles in the immune system and functions as an endogenous opponent of DAMPs. Functions of extracellular ubiquitin could constitute an evolutionary conserved control mechanism aimed to balance the immune response and prevent exuberant inflammation. Further characterization of its mechanism of action and cellular signaling pathways is expected to provide novel insights into the regulation of the innate immune response and opportunities for therapeutic interventions.

CXC chemokine receptor 4 is a cell surface receptor for extracellular ubiquitin

Ubiquitin is one of the most highly conserved proteins in eukaryotes and plays major biological roles as a post-translational protein modifier. Ubiquitin is also a natural constituent of plasma, and several lines of evidence suggest that extracellular ubiquitin is an immune modulator with anti-inflammatory properties. In addition, ubiquitin treatment has been shown to limit inflammation and reduce organ injury in various disease models and species in vivo. However, its mechanism of action is unknown. Here we show that extracellular ubiquitin is a natural CXC chemokine receptor 4 (CXCR4 and CD184) agonist. Extracellular ubiquitin promotes intracellular Ca(2+) flux and reduces cAMP levels through a G protein-coupled receptor that signals via a Galpha(i/o) protein in THP1 cells. Toll-like receptor 4 stimulation reduces ubiquitin-binding sites, which enabled identification of four Galpha(i/o) PCRs as ubiquitin receptor candidates. Overexpression of candidate genes in HEK293 cells, gene silencing in THP1 cells, competition binding, and signaling studies with the CXCR4 agonist stromal cell-derived factor-1alpha (chemokine (CXC motif) ligand 12) and inhibitor AMD3100 identify CXCR4 as a functional ubiquitin receptor. Our finding uncovers a fundamentally new aspect of the role of ubiquitin in biology, has implications for the understanding of CXCR4-mediated events, and is expected to facilitate development of new therapeutic avenues for a variety of diseases.

Targets of the Tal1 transcription factor in erythrocytes: E2 ubiquitin conjugase regulation by Tal1

The Tal1 transcription factor is essential for the development of the hematopoietic system and plays a role during definitive erythropoiesis in the adult. Despite the importance of Tal1 in erythropoiesis, only a small number of erythroid differentiation target genes are known. A chromatin precipitation and cloning approach was established to uncover novel Tal1 target genes in erythropoiesis. The BirA tag/BirA ligase biotinylation system in combination with streptavidin chromatin precipitation (Strep-CP) was used to co-precipitate genomic DNA bound to Tal1. Tal1 was found to bind in the vicinity of 31 genes including the E2-ubiquitin conjugase UBE2H gene. Binding of Tal1 to UBE2H was confirmed by chromatin immunoprecipitation. UBE2H expression is increased during erythroid differentiation of hCD34(+) cells. Tal1 expression activated UBE2H expression, whereas Tal1 knock-down reduced UBE2H expression and ubiquitin transfer activity. This study identifies parts of the ubiquitinylation machinery as a cellular target downstream of the transcription factor Tal1 and provides novel insights into Tal1-regulated erythropoiesis.

Upregulation and maintenance of gap junctional communication in lens cells

The cells of the lens are joined by an extensive network of gap junction intercellular channels consisting of connexins 43, 46, and 50. We have proposed, and experimentally supported, the hypothesis that fibroblast growth factor (FGF) signaling is required for upregulation of gap junction-mediated intercellular coupling (GJIC) at the lens equator. The ability of FGF to increase GJIC in cultured lens cells requires sustained activation of extracellular signal-regulated kinase (ERK). In other cell types, activation of ERK has been shown to block GJIC mediated by connexin43 (Cx43). Why ERK signaling does not block lens cell coupling is not known. Another unresolved issue in lens gap junction regulation is how connexins, synthesized before the loss of biosynthetic organelles in mature lens fiber cells, avoid degradation during formation of the organelle-free zone. We have addressed these questions using serum-free cultures (termed DCDMLs) of primary embryonic chick lens epithelial cells. We show that FGF stimulates ERK in DCDMLs via the canonical Ras/Raf1 pathway, and that the reason that neither basal nor growth factor-stimulated GJIC is blocked by activation of ERK is because it is not mediated by Cx43. In fibroblastic cells, the normally rapid rate of degradation of Cx43 after its transport to the plasma membrane is reduced by treatments that either directly (ALLN; epoxomicin) or indirectly (generation of oxidatively un/mis-folded proteins by arsenic compounds) prevent the ubiquitin/proteasome system (UPS) from acting on its normal substrates. We show here that Cx45.6 and Cx56, the chick orthologs of mammalian Cx50 and Cx46, behave similarly in DCDMLs. When organelles lyse during the maturation of fiber cells, they release into the cytosol a large amount of new proteins that have the potential to saturate the capacity, and/or compromise the function, of the UPS. This would serve to spare gap junctions from degradation during formation of the organelle-free zone, thereby preserving GJIC between mature fiber cells despite the lack of de novo connexin synthesis.

Immunological methods to quantify and characterize proteasome complexes: development and application

The ubiquitin-proteasome pathway plays major roles in all aspects of biology and contributes to various disease processes. Due to the lack of assays that permit proteasome quantification in crude cell extracts, its concentrations in health and disease states as well as the relationship between free 20S core particles (20S) and 26S proteasomes (26S) that consist of 20S singly or doubly capped with 19S regulator complexes (19S) are still largely unknown. Thus, we established a 20S ELISA for the detection of total 20S, and developed a specific 26S ELISA. The latter utilizes the ATP/Mg2+ requirement for 26S stability and shows no cross-reactivity with 20S. Both ELISAs demonstrate intra- and inter-assay variations between 4.9% and 9.4% and recoveries of 105%-109%. Initial application showed that maintenance of the physiological ATP concentration is essential for accurate 26S assessment. Measurements in erythrocyte and peripheral blood mononuclear cell (PBMNC) extracts revealed that the concentrations of 20S were 15-fold and of 26S 130-fold higher in PBMNCs, and suggested that the 26S is the physiological relevant form in PBMNCs (molar ratio 20S/26S 1.1+/-0.4), whereas free 20S is predominant in erythrocytes (molar ratio 20S/26S: 11.5+/-4.0). During storage of packed red blood cell units spontaneous 26S assembly was detectable while specific 26S enzyme activities decreased, indicating that these assays are useful to assess the dynamic interplay between the 20S and 19S. During 26S assay development we further observed that solid phase affinity immobilization (SPAI) of 26S enables quantification of its dissociation into 20S and 19S. Utilizing the SPAI-26S method in combination with the non-hydrolyzable analogue ATP[beta,gamma-NH] and Mg2+ depletion, we provided evidence that ATP binding without hydrolysis via a high affinity binding site (Kd 4-6 microM) as well as ATP binding with hydrolysis via a low affinity binding site that is virtually not saturable under physiological conditions is required to fully stabilize the 26S. Application of these immunological techniques is expected to facilitate proteasome analyses, and may help to better understand its roles in health and disease processes.

A pilot study of the effects of sildenafil on stool characteristics, colon transit, anal sphincter function, and rectal sensation in healthy men

Nitric oxide is an important mediator of gut smooth muscle relaxation and visceral sensation. Sildenafil results in stimulation of the nitric oxide-cyclic GMP pathway. We sought to determine the effects of daily sildenafil administration on colorectal function. Over a 4-week period, sildenafil was administered during weeks 2 and 3. Stool frequency and consistency were assessed daily. Anorectal manometry, rectal sensation, and colon transit testing were performed at the end of weeks 1 and 3. Ten healthy men were studied. No significant differences in segmental or total colon transit time were noted; however, significant changes in stool frequency and trends toward decreased stool consistency were noted during sildenafil use. A trend toward reduced resting anal sphincter pressure was seen after sildenafil. Rectal volumes to first sensation and desire to defecate were significantly increased after sildenafil on test day 2 only. Additionally, volumes to desire to defecate and maximal tolerable volume were significantly increased before sildenafil on test day 2 compared to before sildenafil on test day 1. We conclude that daily administration of sildenafil is well tolerated and results in alterations in colorectal function.

In vitro maturation of nascent reticulocytes to erythrocytes

Most studies of mammalian reticulocyte maturation have used blood reticulocytes.Nascent reticulocytes, as found in bone marrow, have not been available in developmentally synchronized populations. Nascent murine reticulocytes formed in vitro by enucleation of Friend virus–infected erythroblasts were purified and recultured for 110 hours. At 0 hours, all recultured cells were lobulated and contained dense, centralized reticulin. By 110 hours, about 20% to 25% of the cells became biconcave erythrocytes. Most ribosomes and cellular RNAs were degraded within 20 hours, and during that period, heme synthesis declined from a rate equal to that of late erythroblasts to less than 10% of that rate. Many mitochondria appeared normal until they showed outer membrane swelling, degradation, and apparent fusion with intracellular vacuoles at 40 hours of culture. During the period of mitochondrial loss, Bcl-XL, an antiapoptotic protein that accumulates during erythroblast differentiation and maintains mitochondrial membrane integrity, demonstrated progressive decreases and changes consistent with deamidation. Nevertheless, the reticulocytes did not undergo apoptosis, because their apoptotic machinery was degraded. This experimental system that provides a developmentally synchronized population of nascent murine reticulocytes that mature into biconcave erythrocytes in vitro should be useful in further investigations of the cellular events involved in reticulocyte maturation.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Ubiquitin and the enigma of intracellular protein degradation

Contrary to widespread belief, the regulation and mechanism of degradation for the mass of intracellular proteins (i.e. differential, selective protein turnover) in vertebrate tissues is still a major biological enigma. There is no evidence for the conclusion that ubiquitin plays any role in these processes. The primary function of the ubiquitin-dependent protein degradation pathway appears to lie in the removal of abnormal, misfolded, denatured or foreign proteins in some eukaryotic cells. ATP/ubiquitin-dependent proteolysis probably also plays a role in the degradation of some so-called 'short-lived' proteins. Evidence obtained from the covalent modification of such natural substrates as calmodulin, histones (H2A, H2B) and some cell membrane receptors with ubiquitin indicates that the reversible interconversion of proteins with ubiquitin followed by concomitant functional changes may be of prime importance.

Ubiquitin in homeostasis, development and disease

Ubiquitin is the most phylogenetically conserved protein known. This 8,500 Da polypeptide can be covalently attached to cellular proteins as a posttranslational modification. In most cases, the addition of multiple ubiquitin adducts to a protein targets it for rapid degradation by a multisubunit protease known as the 26S proteasome. While the ubiquitin/26S proteasome pathway is responsible for the degradation of the bulk of cellular proteins during homeostasis, it may also be responsible for the rapid loss of protein during the programmed death of certain cells, such as skeletal muscle during insect metamorphosis. In addition, alterations in the expression and regulation of ubiquitin may play significant roles in pathological disorders. For example, dramatic increases in ubiquitin and ubiquitin-protein conjugates are observed in a wide variety of neurodegenerative disorders, including Alzheimer's disease. Patients suffering from the autoimmune disease systemic lupus erythematosus generate antibodies reacting with ubiquitin and ubiquitinated histones. At present, it is not known whether these changes in ubiquitin expression and regulation initiate pathological changes in these diseases or if they are altered as a consequence of these disorders.

Dynamics of ubiquitin conjugation during erythroid differentiation in vitro

To gain insight into the role of ubiquitin-mediated proteolysis in erythroid differentiation, levels of ubiquitin conjugating enzymes (E2s) and ubiquitin conjugates were analyzed during in vitro differentiation of murine erythroleukemic (MEL) cells. After 4 days of culture in the presence of the inducer dimethyl sulfoxide, MEL cells expressed high levels of the erythroid-specific proteins, globin, and band 3. During the same interval, cellular contents (mol/cell) of E2-14K, E2-25K, and E2-35K decreased up to approximately 5-fold; as suggested by results obtained with E2-25K, this reflected a lower level of mRNA in differentiating cells. Concentrations of these E2s changed more modestly during in vitro differentiation, since cellular volume also decreased. Comparison of levels of the three E2s in undifferentiated MEL cells and reticulocytes suggests that their concentrations remain fairly constant during in vivo differentiation of proerythroblasts into reticulocytes. Thus, these components of the ubiquitin-mediated proteolytic pathway are likely to function constitutively during this interval. Two-dimensional Western blots showed a broad spectrum of ubiquitin conjugates, including free multiubiquitin chains, in undifferentiated MEL cells. As seen for several E2s, the concentration of ubiquitin conjugates (including free chains) decreased modestly during in vitro differentiation. E2-20K and E2-230K, which are abundant in reticulocytes, were low or absent in undifferentiated and differentiated MEL cells. In erythroid cells these two E2s are reticulocyte-specific; apparently MEL cells do not differentiate far enough to allow induction of their expression.

Developmentally regulated loss of ubiquitin and ubiquitinated proteins during pollen maturation in maize

Eukaryotic cells typically contain 0.2-1.0% of their total protein as the highly conserved protein ubiquitin, which exists both free and covalently attached to cellular proteins. The attachment of ubiquitin to cellular proteins occurs posttranslationally by a three-enzyme pathway and results in a peptide linkage of the C terminus of ubiquitin either to a lysyl epsilon-amino group of a substrate protein or to a lysyl epsilon-amino group of a previously linked ubiquitin molecule. The multiple conjugation of ubiquitin to substrate proteins via ubiquitin-ubiquitin linkages is thought to be necessary, but not sufficient, for recognition and degradation by a ubiquitin-dependent protease. In higher plant cells the steady-state level of ubiquitinated proteins is generally constant and can be readily detected in all somatic tissues. In contrast, we have found that a developmentally regulated loss of free ubiquitin and ubiquitinated proteins occurs during maize (Zea mays L.) pollen maturation. This dramatic loss of ubiquitin correlates temporally with commitment to the gametophytic developmental program. Northern blot analysis indicates that the loss of ubiquitin is not due to low levels of ubiquitin mRNA, suggesting that a posttranscriptional regulatory mechanism is responsible.