ATP-dependent proteolysis of hemoglobin alpha chains in beta-thalassemic hemolysates is ubiquitin-dependent

Long-term efficacy and safety of mitapivat in non-transfusion-dependent α- or β-thalassaemia: An open-label phase 2 study

Non-transfusion-dependent thalassaemia (NTDT) can result in serious complications and comorbidities that can impact patients' quality of life. Mitapivat, a first-in-class, oral, small-molecule allosteric activator of red blood cell pyruvate kinase, is under investigation in adults with thalassaemia. Through its mechanism of action, mitapivat increases adenosine triphosphate, leading to improvements in red blood cell health, ineffective erythropoiesis and haemolysis. An open-label, multicentre, phase 2 study (NCT03692052) is evaluating mitapivat 100 mg twice daily in adults with NTDT. We previously reported a statistically significant haemoglobin response (a ≥1.0 g/dL increase in haemoglobin concentration from baseline at ≥1 assessments between Weeks 4 and 12 [inclusive]) during the 24-week core period. Here, we report efficacy and safety results up to Week 156 and to data cut-off date respectively. Of 20 patients enrolled, 17 continued in the extension period. Median change from baseline in haemoglobin concentration at Week 156 was 1.2 g/dL. Patients receiving mitapivat demonstrated sustained improvements in haemoglobin concentrations and markers of erythropoietic activity, haemolysis and iron homeostasis. Five patients (29%) had a grade ≥3 treatment-emergent adverse event; none were considered treatment related. Treatment with mitapivat was well tolerated, with a safety profile consistent with previous studies of mitapivat in pyruvate kinase deficiency.

Beyond ATP: Metabolite Networks as Regulators of Physiological and Pathological Erythroid Differentiation

Hematopoietic stem cells (HSCs) possess the capacity for self-renewal and the sustained production of all mature blood cell lineages. It has been well established that a metabolic rewiring controls the switch of HSCs from a self-renewal state to a more differentiated state, but it is only recently that we have appreciated the importance of metabolic pathways in regulating the commitment of progenitors to distinct hematopoietic lineages. In the context of erythroid differentiation, an extensive network of metabolites, including amino acids, sugars, nucleotides, fatty acids, vitamins, and iron, is required for red blood cell (RBC) maturation. In this review, we highlight the multifaceted roles via which metabolites regulate physiological erythropoiesis as well as the effects of metabolic perturbations on erythroid lineage commitment and differentiation. Of note, the erythroid differentiation process is associated with an exceptional breadth of solute carrier (SLC) metabolite transporter upregulation. Finally, we discuss how recent research, revealing the critical impact of metabolic reprogramming in diseases of disordered and ineffective erythropoiesis, has created opportunities for the development of novel metabolic-centered therapeutic strategies.

Quality control of protein complex assembly by the ubiquitin-proteasome system

The majority of human proteins operate as multimeric complexes with defined compositions and distinct architectures. How the assembly of these complexes is surveyed and how defective complexes are recognized is just beginning to emerge. In eukaryotes, over 600 E3 ubiquitin ligases form part of the ubiquitin-proteasome system (UPS) which detects structural characteristics in its target proteins and selectively induces their degradation. The UPS has recently been shown to oversee key quality control steps during the assembly of protein complexes. We review recent findings on how E3 ubiquitin ligases regulate protein complex assembly and highlight unanswered questions relating to their mechanism of action.

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.

Oxidative stress in β-thalassaemia and sickle cell disease

Sickle cell disease and β-thalassaemia are inherited haemoglobinopathies resulting in structural and quantitative changes in the β-globin chain. These changes lead to instability of the generated haemoglobin or to globin chain imbalance, which in turn impact the oxidative environment both intracellularly and extracellularly. The ensuing oxidative stress and the inability of the body to adequately overcome it are, to a large extent, responsible for the pathophysiology of these diseases. This article provides an overview of the main players and control mechanisms involved in the establishment of oxidative stress in these haemoglobinopathies.

α-Globin as a molecular target in the treatment of β-thalassemia

The thalassemias, together with sickle cell anemia and its variants, are the world's most common form of inherited anemia, and in economically undeveloped countries, they still account for tens of thousands of premature deaths every year. In developed countries, treatment of thalassemia is also still far from ideal, requiring lifelong transfusion or allogeneic bone marrow transplantation. Clinical and molecular genetic studies over the course of the last 50 years have demonstrated how coinheritance of modifier genes, which alter the balance of α-like and β-like globin gene expression, may transform severe, transfusion-dependent thalassemia into relatively mild forms of anemia. Most attention has been paid to pathways that increase γ-globin expression, and hence the production of fetal hemoglobin. Here we review the evidence that reduction of α-globin expression may provide an equally plausible approach to ameliorating clinically severe forms of β-thalassemia, and in particular, the very common subgroup of patients with hemoglobin E β-thalassemia that makes up approximately half of all patients born each year with severe β-thalassemia.

Integrated protein quality-control pathways regulate free α-globin in murine β-thalassemia

Cells remove unstable polypeptides through protein quality-control (PQC) pathways such as ubiquitin-mediated proteolysis and autophagy. In the present study, we investigated how these pathways are used in β-thalassemia, a common hemoglobinopathy in which β-globin gene mutations cause the accumulation and precipitation of cytotoxic α-globin subunits. In β-thalassemic erythrocyte precursors, free α-globin was polyubiquitinated and degraded by the proteasome. These cells exhibited enhanced proteasome activity, and transcriptional profiling revealed coordinated induction of most proteasome subunits that was mediated by the stress-response transcription factor Nrf1. In isolated thalassemic cells, short-term proteasome inhibition blocked the degradation of free α-globin. In contrast, prolonged in vivo treatment of β-thalassemic mice with the proteasome inhibitor bortezomib did not enhance the accumulation of free α-globin. Rather, systemic proteasome inhibition activated compensatory proteotoxic stress-response mechanisms, including autophagy, which cooperated with ubiquitin-mediated proteolysis to degrade free α-globin in erythroid cells. Our findings show that multiple interregulated PQC responses degrade excess α-globin. Therefore, β-thalassemia fits into the broader framework of protein-aggregation disorders that use PQC pathways as cell-protective mechanisms.

Protein quality control during erythropoiesis and hemoglobin synthesis

Erythrocytes must regulate hemoglobin synthesis to limit the toxicities of unstable free globin chain subunits. This regulation is particularly relevant in β-thalassemia, in which β-globin deficiency causes accumulation of free α-globin, which forms intracellular precipitates that destroy erythroid precursors. Experimental evidence accumulated over more than 40 years indicates that erythroid cells can neutralize moderate amounts of free α-globin through generalized protein quality control mechanisms, including molecular chaperones, the ubiquitin-proteasome system, and autophagy. In many ways, β-thalassemia resembles protein aggregation disorders of the nervous system, liver, and other tissues, which occur when levels of unstable proteins overwhelm cellular compensatory mechanisms. Information gained from studies of nonerythroid protein aggregation disorders may be exploited to further understand and perhaps treat β-thalassemia.

Oxidative crosslinking, spectrin and membrane interactions of hemoglobin mixtures in HbEbeta-thalassemia

The authors have studied the interactions of intact hemoglobin mixtures of HbE and HbA, with the major erythroid membrane skeletal protein, spectrin and tailor-made phospholipids membranes containing aminophospholipids to understand the role of spectrin and phospholipids of erythrocytes in the overall pathophysiology of the hemoglobin disorders. Hemoglobin mixtures were isolated and purified from the peripheral blood samples of HbE carriers and different HbEbeta thalassemia patients, taken for diagnosis. Spectrin binding was studied by fluorescence and oxidative crosslinking, by SDS-PAGE. Membrane perturbation experiments were carried out to study the leakage of the self-quenching fluorophore, carboxyfluorescein, entrapped in the phospholipid vesicles. Hemoglobin mixtures with elevated levels of HbE showed stronger interactions with spectrin reflected in the decrease in binding dissociation constant from 17 to 5 muM upon increase in HbE% from about 30 to 90% in the hemolysates. The yield of the spectrin crosslinked complexes of such hemoglobin mixtures also increased with increase in HbE levels. Presence of ATP/Mg and DPG were found to decrease the overall yield of such complexes and the binding affinity of hemoglobins to spectrin. HbE rich hemolysates also induced greater leakage of entrapped carboxyfluorescein (CF) from phospholipid membranes containing aminophospholipids. Results from this study indicate the roles of skeletal proteins and aminophospholipids, particularly under oxidative stress conditions to be important in the premature destruction of erythrocytes in hemoglobin disorders, e.g. HbEbeta-thalassaemia.

Detection of a thalassemic alpha-chain variant (Hemoglobin Groene Hart) by reversed-phase liquid chromatography

BACKGROUND

Hemoglobin (Hb) Groene Hart [alpha119 (H2)Pro-->Ser (alpha1)], also known as Hb Bernalda, is a nondeletional alpha-thalassemic Hb variant that is frequent in southern Italy and North Africa. This variant is not supposed to be produced in the erythrocytes of carriers. The alpha-thalassemic behavior of this variant has been explained as an impaired interaction between the alpha-globin chain and the alpha-Hb-stabilizing protein.

METHODS

To separate globin chains, we developed a modified reversed-phase liquid chromatography (RPLC) procedure that uses acetonitrile-water solvents containing up to 3 mL/L trifluoroacetic acid. After RPLC, we characterized the isolated globin chains by electrospray ionization (ESI) mass spectrometry (MS) and analyzed their tryptic peptides with matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS and nano-LC-ESI-MS/MS.

RESULTS

RPLC detected an abnormal peak with a retention time substantially greater than that of the wild-type alpha(A)-globin chain. We identified this variant as Hb Groene Hart and found it in the hemolysates of 11 unrelated patients (1 homozygote, 9 heterozygotes, and 1 heterozygote associated with the -alpha(3.7) deletion). These patients possessed abnormal hematologic features suggesting an alpha-thalassemia phenotype. Molecular modeling suggested that the increase in hydrophobicity was due to opening of the GH interhelical segment following replacement of amino acid residue 119 with a nonhelix breaker residue.

CONCLUSIONS

This method allows the detection of Hb variants at low concentrations, and adjusting the composition of the organic solvents enables the method to identify Hb variants with large changes in hydrophobicity.

Spectrin interactions with globin chains in the presence of phosphate metabolites and hydrogen peroxide: implications for thalassaemia

We have shown the differential interactions of the erythroid skeletal protein spectrin with the globin subunits of adult haemoglobin (HbA); these indicate a preference for alpha-globin over that for beta-globin and intact HbA in an adenosine 5'-triphosphate (ATP)-dependent manner. The presence of Mg/ATP led to an appreciable decrease in the binding affinity of the alpha-globin chain to spectrin and the overall yield of globin-spectrin cross-linked complexes formed in the presence of hydrogen peroxide. Similar effects were also seen in the presence of 2-,3-diphosphoglycerate (2,3 DPG), the other important phosphate metabolite of erythrocytes. The binding affinity and yield of cross-linked high molecular weight complexes (HMWCs) formed under oxidative conditions were significantly higher in alpha-globin compared with intact haemoglobin, HbA and the beta-globin chain. The results of this study indicate a possible correlation of the preferential spectrin binding of the alpha-globin chain over that of the beta-globin in the haemoglobin disorder beta-thalassaemia.

Impaired binding of AHSP to α chain variants: Hb Groene Hart illustrates a mechanism leading to unstable hemoglobins with α thalassemic like syndrome

Alpha hemoglobin stabilizing protein (AHSP) is a small protein of 102 residues induced by GATA-1, Oct-1- and EKLF. It is synthesized at a high level in the red blood cell precursors and acts as a chaperone protecting the alpha hemoglobin (alpha-Hb) chains against precipitation. AHSP and alpha-Hb form a heterodimer complex. In the absence of AHSP, alpha-Hb oxidizes and precipitates within the erythrocyte precursors of the bone marrow leading to apoptosis and defective erythropoiesis. In vitro the binding of AHSP to ferrous alpha-Hb accelerates oxidation of the heme iron in alpha-Hb, but the complex is more resistant to protein unfolding. AHSP could act as a modulating factor in beta-thalassemia. Recent studies showed more severe thalassemic syndromes in patients with decreased levels of AHSP and in one patient who carried a structurally abnormal AHSP. Some alpha-Hb variants with structural abnormality located in the contact area between alpha-Hb and AHSP exhibit an instability and a thalassemic like syndrome. We suggest that this could result from a disturbed interaction between alpha-Hb variants and AHSP. To study this interaction, we constructed the pGEX-alpha-AHSP vector that co-expressed human alpha-Hb and AHSP. Using this approach, we investigated the alpha42 (C7), alpha104 (G11) and alpha119 (H2) sites, where variants with some thalassemic features have been described. Results obtained with recombinant Groene Hart alpha-Hb and Diamant alpha-Hb, in which proline 119 is replaced by a serine and a leucine, respectively, showed clearly an impaired interaction with AHSP. In contrast, the alpha mutants at the sites 42 and 104 exhibit a normal interaction with AHSP. The CO rebinding kinetics of the AHSP/alpha-Hb(42mutant) complexes were similar to those previously obtained with the AHSP/alpha-Hb(WT) complex, which shows a modified rate that is intermediate to the classical Hb allosteric states.

ALDH3A1: a corneal crystallin with diverse functions

Aldehyde dehydrogenase 3A1 (ALDH3A1) comprises a surprisingly high proportion (5-50% depending on species) of the water-soluble protein of the mammalian cornea, but is present little if at all in the cornea of other species. Mounting experimental evidence demonstrates that this abundant corneal protein plays an important role in the protection of ocular structures against oxidative damage. Corneal ALDH3A1 appears to protect against UV-induced oxidative stress through a variety of biological functions such as the metabolism of toxic aldehydes produced during the peroxidation of cellular lipids, the generation of the antioxidant NADPH, the direct absorption of UV-light, the scavenging of reactive oxygen species (ROS), and the possession of chaperone-like activity. With analogies to the abundant, multifunctional, and taxon-specific lens crystallins, mammalian ALDH3A1 has been considered a corneal crystallin, suggesting that it may contribute to the optical properties of the cornea as well. Recent studies have also revealed a novel role for ALDH3A1 in the regulation of the cell cycle. ALDH3A1-transfected HCE cells have increased population-doubling time, decreased plating efficiency, and reduced DNA synthesis, most likely due to a profound inhibition of cyclins and cyclin-dependent kinases. We have proposed that the ALDH3A1-induced reduction in cell growth may contribute to protection against oxidative stress by extending time for DNA and cell repair. Taken together, the multiple roles of ALDH3A1 against oxidative stress in addition to its contributions to the optical properties of the cornea are consistent with the idea that this specialized protein performs diverse biological functions as do the lens crystallins.

Ubiquitin-dependent lysosomal degradation of the HNE-modified proteins in lens epithelial cells

4-hydroxynonenal (HNE), a highly reactive lipid peroxidation product, may adversely modify proteins. Accumulation of HNE-modified proteins may be responsible for pathological lesions associated with oxidative stress. The objective of this work was to determine how HNE-modified proteins are removed from cells. The data showed that alphaB-crystallin modified by HNE was ubiquitinated at a faster rate than that of native alphaB-crystallin in a cell-free system. However, its susceptibility to proteasome-dependent degradation in the cell-free system did not increase. When delivered into cultured lens epithelial cells, HNE-modified alphaB-crystallin was degraded at a faster rate than that of unmodified alphaB-crystallin. Inhibition of the lysosomal activity stabilized HNE-modified alphaB-crystallin, but inhibition of the proteasome activity alone had little effect. To determine if other HNE-modified proteins are also degraded in a ubiquitin-dependent lysosomal pathway, lens epithelial cells were treated with HNE and the removal of HNE-modified proteins in the cells was monitored. The levels of HNE-modified proteins in the cell decreased rapidly upon removal of HNE from the medium. Depletion of ATP or the presence of MG132, a proteasome/lysosome inhibitor, resulted in stabilization of HNE-modified proteins. However, proteasome-specific inhibitors, lactacystin-beta-lactone and epoxomicin, could not stabilize HNE-modified proteins in the cells. In contrast, chloroquine, a lysosome inhibitor, stabilized HNE-modified proteins. The enrichment of HNE-modified proteins in the fraction of ubiquitin conjugates suggests that HNE-modified proteins are preferentially ubiquitinated. Taken together, these findings show that HNE-modified proteins are degraded via a novel ubiquitin and lysosomal-dependent but proteasome-independent pathway.

Expression of alpha-hemoglobin stabilizing protein gene during human erythropoiesis

alpha-Hemoglobin stabilizing protein (AHSP) is an abundant, erythroid-specific protein that forms a stable complex with free alpha-hemoglobin but not with beta-hemoglobin or hemoglobin A. As such, AHSP is required for normal erythropoiesis, probably acting by blocking the deleterious effects of free alpha-hemoglobin precipitation. In order to study the levels of expression of the AHSP gene during the different phases of erythropoiesis, we carried out a two-phase liquid culture of erythroid cells and real-time quantitative polymerase chain reaction. Blood from control volunteers was cultured with erythropoietin to stimulate differentiation. The different stages of erythropoiesis were confirmed by morphologic and flow cytometric analysis. The results showed a progressive increase in AHSP gene expression following the expression of alpha-globin gene, during maturation of the red blood cell precursors, confirming the probable important function of this protein during normal erythropoiesis.

4-Hydroxy-2-nonenal-mediated impairment of intracellular proteolysis during oxidative stress. Identification of proteasomes as target molecules

Oxidative stress is associated with important pathophysiological events in a variety of diseases. It has been postulated that free radicals and lipid peroxidation products generated during the process may be responsible for these effects because of their ability to damage cellular components such as membranes, proteins, and DNA. In the present study, we provide evidence that oxidative stress causes a transient impairment of intracellular proteolysis via covalent binding of 4-hydroxy-2-nonenal (HNE), a major end product of lipid peroxidation, to proteasomes. A single intraperitoneal treatment with the renal carcinogen, ferric nitrilotriacetate, caused oxidative stress, as monitored by accumulation of lipid peroxidation products and 8-hydroxy-2'-deoxyguanosine, in the kidney of mice. In addition, transient accumulation of HNE-modified proteins in the kidney was also found by competitive enzyme-linked immunosorbent assay and immunohistochemical analyses. This and the observation that the HNE-modified proteins were significantly ubiquitinated suggested a crucial role of proteasomes in the metabolism of HNE-modified proteins. In vitro incubation of the kidney homogenates with HNE indeed resulted in a transient accumulation of HNE-modified proteins, whereas the proteasome inhibitor significantly suppressed the time-dependent elimination of HNE-modified proteins. We found that, among three proteolytic activities (trypsin, chymotrypsin, and peptidylglutamyl peptide hydrolase activities) of proteasomes, both trypsin and peptidylglutamyl peptide hydrolase activities in the kidney were transiently diminished in accordance with the accumulation of HNE-modified proteins during oxidative stress. The loss of proteasome activities was partially ascribed to the direct attachment of HNE to the protein, based on the detection of HNE-proteasome conjugates by an immunoprecipitation technique. These results suggest that HNE may contribute to the enhanced accumulation of oxidatively modified proteins via an impairment of ubiquitin/proteasome-dependent intracellular proteolysis.

Evidence that the ubiquitin proteolytic pathway is involved in the degradation of precipitated globin chains in thalassaemia

Ultrastructural immunocytochemical studies were performed on sections of bone marrow from three patients with beta-thalassaemia major and two patients with haemoglobin H (HbH) disease. Some sections were reacted with either a polyclonal or a monoclonal anti-human-ubiquitin antibody and the reaction visualized using a gold-labelled secondary antibody. The inclusions of precipitated globin chains found within the erythropoietic cells of all five patients reacted much more strongly than the surrounding inclusion-free cytoplasm with both of the anti-ubiquitin antibodies, indicating that the precipitated globin chains were ubiquitinated. A non-specific reaction between the anti-ubiquitin antibodies and the inclusions was excluded by demonstrating that various other antibodies, including a polyclonal anti-human cathepsin D antibody, did not react with the inclusions. The data suggest that the ubiquitin proteolytic pathway is involved in the degradation of precipitated globin chains in alpha- and beta-thalassaemia.

Ubiquitin Aldehyde Increases Adenosine Triphosphate–Dependent Proteolysis of Hemoglobin α-Subunits in β-Thalassemic Hemolysates

Two major causes of the anemia in β-thalassemia are a deficiency in hemoglobin (Hb) β-subunit (and consequently HbA) synthesis and, due to the resulting excess of Hb α-subunits, erythroid cell hemolysis. The hemolytic component might be ameliorated by increasing the intracellular proteolysis of the excess α-subunits. Isolated 3H-labeled α-chains are known to be degraded primarily by the adenosine triphosphate (ATP)- and ubiquitin (Ub)-dependent proteolysis pathway in unfractionated β-thalassemic hemolysates. Our objective was to increase this degradation by targeted intervention. Ub aldehyde (Ubal), a synthetic inhibitor of isopeptidases (proteases that hydrolyze the bond between the Ub polypeptide and its protein adduct), was added to reaction mixtures containing a hemolysate from the blood cells of one of four β-thalassemic donors and 3H-α-chains or 3H-α-globin as a substrate. Optimum enhancement of ATP-dependent degradation occurred at 0.4 to 1.5 μmol/L Ubal and ranged from 29% to 115% for 3H-α-chains and 47% to 96% for 3H-α-globin among the four hemolysates. We suggest that Ubal stimulates 3H-α-subunit proteolysis by inhibition of an isopeptidase(s) that deubiquitinates, or “edits,” Ub-3H-α-subunit conjugates, intermediates in the degradative pathway. In control studies, similarly low Ubal concentrations did not enhance the degradation of 3H-α2β2 (HbA) tetramers or inhibit the activities of methemoglobin reductase and four selected glycolysis pathway enzymes. These and other results may be the basis for a therapeutic approach to β-thalassemia.

Ubiquitin Aldehyde Increases Adenosine Triphosphate–Dependent Proteolysis of Hemoglobin α-Subunits in β-Thalassemic Hemolysates

Two major causes of the anemia in β-thalassemia are a deficiency in hemoglobin (Hb) β-subunit (and consequently HbA) synthesis and, due to the resulting excess of Hb α-subunits, erythroid cell hemolysis. The hemolytic component might be ameliorated by increasing the intracellular proteolysis of the excess α-subunits. Isolated 3H-labeled α-chains are known to be degraded primarily by the adenosine triphosphate (ATP)- and ubiquitin (Ub)-dependent proteolysis pathway in unfractionated β-thalassemic hemolysates. Our objective was to increase this degradation by targeted intervention. Ub aldehyde (Ubal), a synthetic inhibitor of isopeptidases (proteases that hydrolyze the bond between the Ub polypeptide and its protein adduct), was added to reaction mixtures containing a hemolysate from the blood cells of one of four β-thalassemic donors and 3H-α-chains or 3H-α-globin as a substrate. Optimum enhancement of ATP-dependent degradation occurred at 0.4 to 1.5 μmol/L Ubal and ranged from 29% to 115% for 3H-α-chains and 47% to 96% for 3H-α-globin among the four hemolysates. We suggest that Ubal stimulates 3H-α-subunit proteolysis by inhibition of an isopeptidase(s) that deubiquitinates, or “edits,” Ub-3H-α-subunit conjugates, intermediates in the degradative pathway. In control studies, similarly low Ubal concentrations did not enhance the degradation of 3H-α2β2 (HbA) tetramers or inhibit the activities of methemoglobin reductase and four selected glycolysis pathway enzymes. These and other results may be the basis for a therapeutic approach to β-thalassemia.

Differential effects of ubiquitin aldehyde on ubiquitin and ATP-dependent protein degradation

ATP-dependent proteolysis of 125I-labeled human alpha-globin, bovine alpha-lactalbumin, bovine serum albumin, or chicken lysozyme was assessed in a rabbit reticulocyte extract supplemented with ATP, excess ubiquitin, and variable amounts of ubiquitin aldehyde (Ubal), an inhibitor of many ubiquitin-protein isopeptidases. Low concentrations (0.8 microM) of Ubal increased the ATP-dependent degradation of 125I-alpha-globin by approximately 30% after 2 h at 37 degrees C, had little effect on 125I-lysozyme turnover, and decreased 125I-alpha-lactalbumin or 125I-albumin degradation by approximately 20%. The ATP-dependent degradation of all substrates was inhibited by high concentrations (> 3 microM) of Ubal throughout the incubation (15 min to 2 h); after 2 h, this inhibition ranged from 15% for 125I-alpha-globin to approximately 85% for 125I-alpha-lactalbumin and 125I-albumin. Levels of ubiquitin-125I-protein conjugates were increased significantly with Ubal; with > or = 8.0 microM Ubal, high molecular mass multiubiquitinated conjugates were particularly evident for 125I-alpha-globin and 125I-alpha-lactalbumin. These mixtures also accumulated ubiquitin conjugates with sizes expected for di- through pentaubiquitin oligomers. The results are consistent with the following proposed events: The ATP-dependent degradation of 125I-alpha-lactalbumin or 125I-albumin is probably mediated almost exclusively through polyubiquitinated intermediates. High Ubal concentrations inhibit an isopeptidase(s) which normally disassembles "unanchored" polyubiquitin chains that remain after substrate degradation by the 26S proteasome; these chains accumulate to inhibit further conjugate degradation. Much of the ATP-dependent degradation of 125I-alpha-globin and, to a lesser degree, 125I-lysozyme may occur through alternative structures where ubiquitin monomers or short oligomers are ligated to one or more substrate lysines. For 125I-alpha-globin, even low concentrations of Ubal effectively inhibit deubiquitination of these conjugates to enhance alpha-globin degradation.

Globin chain turnover in reticulocytes from patients with beta (0) -thalassaemia/Hb E disease

beta (0)-Thalassaemia/Hb E disease is the most frequent beta-thalassaemia in Thailand. However, patients have a varying degree of anaemia. The difference in severity is attributed to a differential accumulation of unpaired alpha-globin chains, which is the net result of biosynthesis and catabolism. Turnover of newly synthesized globin chains in reticulocytes from beta(0) -thalassaemia/Hb E patients was determined. Proteolysis was ATP-independent and degraded only 10-15% of the radiolabelled globin during a 4-h incubation period at 37 degrees C and there was no difference in globin turnover in reticulocytes from subjects with mild and severe forms of beta(0)-thalassaemia/Hb E. These results indicate that excess alpha-globin chains do not act as substrates for the reticulocyte proteolytic system and that the difference in severity of anaemia in beta(0)-thalassaemia/Hb E disease cannot be explained by a difference in selective post-translational catabolism.

Degradation of monoubiquitinated alpha-globin by 26S proteasomes

Ubiquitin-125I-alpha-globin conjugate fractions containing either one (Ub1-alpha), or two (Ub2-alpha), or a mixture of three and four (Ub3,4-alpha) molecules of ubiquitin (Ub), covalently linked to one 125I-alpha-globin molecule were isolated after incubation of a proteolysis reaction mixture containing ATP, ubiquitin aldehyde-treated reticulocyte lysate, and human 125I-alpha-globin. Each of the purified conjugate fractions or an identically-purified control sample of unconjugated 125I-alpha-globin was incubated as a substrate in companion proteolysis reaction mixtures containing either purified 26S or 20S rabbit reticulocyte proteasomes. The initial rate of ATP-dependent degradation of the Ub1-alpha conjugate by the 26S proteasomes was approximately 0.44% (1.1 fmol)/min while that of the free 125I-alpha-globin was undetectable. The initial rates of ATP-dependent degradation by the 26S proteasomes of the Ub2-alpha and Ub3,4-alpha conjugates were 2- to-3-fold that of the Ub1-alpha species. Conversely, the degradation of free 125I-alpha-globin and its ubiquitinated conjugates by the 20S proteasomes was not dependent on ATP, nor did it increase with the size of the Ub adduct. Analysis of the products of a reaction mixture with 26S proteasomes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed no conversion of the Ub1-alpha conjugate substrate to higher-molecular-mass conjugates. These results suggest that monobiquitinated alpha-globin can be degraded significantly and specifically by interaction directly with the 26S proteasomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Bovine lens epithelial cells have a ubiquitin-dependent proteolysis system

Lens cells must remove obsolete or damaged proteins produced during development, maturation and aging to maintain lens transparency. In reticulocytes removal of abnormal or obsolete proteins is thought to involve a ubiquitin-dependent proteolytic pathway. Two hallmarks of ubiquitin (Ub) dependent proteolysis have previously been demonstrated in lens cell or tissue supernatants: (1) the presence of ubiquitin conjugates, and (2) ATP-dependent proteolysis. Nevertheless, conclusive proof was lacking of a requirement for ubiquitination of substrate proteins for proteolysis. Here we show that in bovine lens epithelial cell (BLEC) supernatant, ATP-dependent proteolysis is also ubiquitin-dependent. Ubiquitin-activating enzyme (E1), the first enzyme in the cascade of ubiquitin ligation, was purified over 3000-fold from a rabbit reticulocyte lysate using Ubiquitin-Sepharose, and showed ATP-PPi exchange activity. Antiserum to E1 was prepared in goats and affinity-purified on Protein G-Sepharose. Western blot analysis revealed that both the goat antiserum and purified antibody (anti-E1(IgG)) recognize specifically E1. Anti-E1(IgG) inhibits 86% of the ATP-dependent degradation of labeled histone H2A in reticulocyte lysate and 75% of the ATP-dependent degradation in BLEC. Upon reconstitution with purified E1, 100% and 80% of proteolysis was restored in reticulocytes and BLEC supernatant, respectively. This confirms that there is a ubiquitin-dependent proteolytic system in lens.

Ubiquitin in health and disease

Studies in recent years have shown that ubiquitin has increasingly important functions in eukaryotic cells; roles which were previously not suspected in healthy and diseased cells. The interplay between molecular pathological and molecular cell biological findings has indicated that ubiquitin may be pivotal in the cell stress response in chronic degenerative and viral diseases. Furthermore, the studies have led to the notion that ubiquitination may not only serve as a signal for nonlysosomal protein degradation but may be a unifying covalent protein modification for the major intracellular protein catabolic systems; these can act to identify proteins for cytosolic proteinases or direct intact and fragmented proteins into the lysosome system for breakdown to amino acids. This unifying role could explain why ubiquitin is restricted to eukaryotic cells, which possess extensive endomembrane systems in addition to a nuclear envelope. Protein ubiquitination is a feature of most filamentous inclusions and certain other intracellular conglomerates that are found in some degenerative and viral diseases. The detection of ubiquitin-protein conjugates is not of great diagnostic importance in these diseases. Protein ubiquitination is not only essential for the normal physiological turnover of proteins but appears to have been adapted as part of an intracellular surveillance system that can be activated by altered, damaged, or foreign proteins and organelles. The purpose of this system is to isolate and eliminate these noxious structures from the cell: as a cytoprotective mechanism this appears to have evolved in the cell akin perhaps to an 'intracellular immune system'. Other heat shock proteins such as hsp 70 may be involved in this process. It is apparent that ubiquitin has a role in embryonic development. Protein ubiquitination is presumably involved in the reorganisation of cytoplasm that accompanies cell differentiation. Ubiquitin is also necessary for the gross intracellular degradative processes which are consequent upon programmed cell death. Cell elimination is of key importance for a number of developmental morphogenetic changes. An understanding of the molecular details of these processes will no doubt provide further insights into the wide ranging roles of ubiquitin in the life process. As it says in the book 'Ubiquitin'; there is no doubt that ubiquitin is a 'lucky' protein. It is lucky in many ways: lucky for scientific progress, lucky for biomedical scientists and lucky for life! If you have not already done so, why don't you get lucky and look for a role for ubiquitin in your experimental system. As Avram Hershko has said "there is plenty to go round"!

Multicatalytic and 26 S ubiquitin/ATP-stimulated proteases in maturing rabbit red blood cells

Rabbit red blood cells of various ages were separated on Percoll gradients and the activities of two large cytosolic proteases were measured. Both the multicatalytic protease (MCP), assayed by hydrolysis of fluorigenic peptides, and the 26 S ubiquitin/ATP-stimulated protease, assayed by degradation of ubiquitin-lysozyme conjugates, declined 3-fold or less during maturation of rabbit reticulocytes to erythrocytes. The ability of MCP to hydrolyze three classes of peptides decreased in parallel indicating that the 20 S protease is not significantly remodeled during red blood cell maturation.

Evidence for ATP and ubiquitin dependent degradation of proteins in cultured bovine lens epithelial cells

Degradation of endogenous lens proteins has been difficult to show under physiological conditions using lens tissue preparations. In contrast, active proteolytic systems in cultured bovine lens epithelial (BLE) cells have been demonstrated previously. BLE cells also contain ubiquitin, a 76 amino-acid polypeptide which is conjugated to proteins in an ATP/Mg(2+)-dependent process prior to their cytosolic proteolysis. In this study, we show that histone H2A, alpha-crystallin, and actin are conjugated to ubiquitin, resulting in higher molecular mass species, which are detected by anti-ubiquitin antibodies. These proteins are also degraded in cell-free assays containing BLE cell supernatants under physiological conditions in an ATP/Mg(2+)-dependent manner. Observation of 125I-labeled proteolytic fragments was made after SDS polyacrylamide gel electrophoresis of the assays. Quantitation of trichloroacetic acid-soluble radiolabeled fragments generated in the presence of ATP/Mg2+ revealed that, with BLE cell supernatant, 25% of the histone H2A was degraded in 3 hr. Proteolysis of alpha-crystallin and actin amounted to 2.3% and 2.9%, respectively. The requirement of ATP/Mg2+ for proteolysis and the observation of ubiquitin conjugation to the same proteins is consistent with the presence of a ubiquitin-dependent proteolytic pathway in BLE cells. Additionally, in this study the BLE cell proteases were even more active on some substrates than the reticulocyte ubiquitin/ATP-dependent proteolytic system.

Ubiquitin-mediated processes in erythroid cell maturation

Response of the ATP, ubiquitin-dependent system during the enhanced degradation of erythrocyte maturation conforms to the general regulatory features common to several similar but unrelated systems. In erythroid cells enhanced degradation follows three phases: (1) Onset of degradation characterized by an increase in the intracellular concentration of free and conjugated ubiquitin, brought about by reduction in mean cell volume; (2) Active enhanced degradation during cellular remodeling; and (3) Loss of activity as a consequence of spontaneous inactivation of components required for ubiquitin conjugation. The extent of degradative remodeling is probably functionally limited by the loss of these critical ligation enzymes.

Fate of alpha-hemoglobin chains and erythrocyte defects in beta-thalassemia

The fate of alpha-hemoglobin chains and the cause of membrane protein defects in thalassemic erythrocytes have been studied in: (1) human beta-thalassemia syndromes, (2) mouse beta-thalassemia, and (3) normal human erythrocytes loaded with purified alpha-hemoglobin chains. The similarity and differences observed in these three systems underline the importance of insoluble alpha chains and the direct relationship between the amount of these chains and the membrane protein defects. Indeed, in addition to the alpha/non-alpha ratio of globin chain synthesis, the proteolysis and instability of alpha chains are major factors in modulating the cellular defects.

Purification of human erythrocyte proteolytic enzyme responsible for degradation of oxidant-damaged hemoglobin. Evidence for identifying as a member of the multicatalytic proteinase family

Exposure of human red cells to oxidants such as phenylhydrazine, 2,4-dimethylphenylhydrazine and 4-hydrazinobenzoic acid stimulates the proteolysis of hemoglobin as evidenced by the increase in the rate of the free alanine and acid soluble amino groups released. An enzyme responsible for proteolytic degradation of oxidized hemoglobin, was purified from cytosolic fraction of erythrocytes by a DEAE-batch procedure followed by gel-filtration and ion-exchange chromatography. The final enzyme preparation produces a single band in non-denaturing polyacrylamide gel electrophoresis, and eight different bands of 23-32 kDa when subjected to polyacrylamide gel electrophoresis under denaturing conditions. The native enzyme has a molecular mass of about 700 kDa as estimated by gel filtration. The enzyme, unable to hydrolyze native hemoglobin, cleaves phenylhydrazine-treated hemoglobin into small peptides without free amino acid release. In addition, the enzyme shows an endopeptidase activity towards synthetic peptides having a tyrosine or an arginine in the P1 position, whereas it does not hydrolyze shorter peptides and those with a proline in the P1 or P2 position. The proteolytic activity of the enzyme against oxidized hemoglobin is inhibited by chymostatin and p-chloromercuribenzoate, while it is stimulated by N-ethylmaleimide and epoxysuccinylleucylamido-(4-guanidino)butane (E-64). The peptidase activity assayed on succinyl-Leu-Leu-Val-Tyr-MCA is inhibited by chymostatin, hemin, N-ethylmaleimide and p-chloromercuribenzoate. The results obtained show that in human erythrocytes oxidized hemoglobin is cleaved into peptides by a high molecular mass proteinase identified as a member of the multicatalytic proteinase family. It is also suggested that the complete degradation of oxidized hemoglobin to free amino acids requires the involvement of a further proteolytic enzyme(s) which remain(s) to be identified.

Degradation of ornithine decarboxylase in mammalian cells is ATP dependent but ubiquitin independent

Ornithine decarboxylase (ODC), a key enzyme in the biosynthesis of polyamines in mammalian cells is characterized by an extremely short half-life. In the present study, ODC degradation was investigated in 653-1 mouse myeloma cells that overproduce ODC and in ts85 cells that are thermosensitive for conjunction of ubiquitin to target proteins. Addition of 2-deoxyglucose and dinitrophenol (agents that efficiently deplete cellular ATP) to the growth medium of these cells inhibited ODC degradation. In contrast, chloroquine and leupeptin, inhibitors of intralysosomal proteolysis, did not affect ODC degradation. Shifting ts85 cells to 42 degrees C (a non-permissive temperature that inhibited conjugation of ubiquitin to target proteins) did not prevent ODC degradation. The ATP-dependent degradation of ODC in 653-1 cells was inhibited substantially by N alpha-tosyl-L-lysine chloromethane (TosPheMeCl), iodoacetamide and o-phenanthroline. These results suggest that ODC degradation occurs via a non-lysosomal. ATP-requiring and ubiquitin-independent cellular proteolytic mechanism, and that serine proteases and enzymes containing sulphydryl groups and metalloenzyme(s) may be involved in this process.