A Review of Romiplostim Mechanism of Action and Clinical Applicability

Thrombocytopenia results from a variety of conditions, including radiation, chemotherapy, autoimmune disease, bone marrow disorders, pathologic conditions associated with surgical procedures, hematopoietic stem cell transplant (HSCT), and hematologic disorders associated with severe aplastic anemia. Immune thrombocytopenia (ITP) is caused by immune reactions that accelerate destruction and reduce production of platelets. Thrombopoietin (TPO) is a critical component of platelet production pathways, and TPO receptor agonists (TPO-RAs) are important for the management of ITP by increasing platelet production and reducing the need for other treatments. Romiplostim is a TPO-RA approved for use in patients with ITP in the United States, European Union, Australia, and several countries in Africa and Asia, as well as for use in patients with refractory aplastic anemia in Japan and Korea. Romiplostim binds to and activates the TPO receptor on megakaryocyte precursors, thus promoting cell proliferation and viability, resulting in increased platelet production. Through this mechanism, romiplostim reduces the need for other treatments and decreases bleeding events in patients with thrombocytopenia. In addition to its efficacy in ITP, studies have shown that romiplostim is effective in improving platelet counts in various settings, thereby highlighting the versatility of romiplostim. The efficacy of romiplostim in such disorders is currently under investigation. Here, we review the structure, mechanism, pharmacokinetics, and pharmacodynamics of romiplostim. We also summarize the clinical evidence supporting its use in ITP and other disorders that involve thrombocytopenia, including chemotherapy-induced thrombocytopenia, aplastic anemia, acute radiation syndrome, perisurgical thrombocytopenia, post-HSCT thrombocytopenia, and liver disease.

Thrombopoietin receptor agonists: ten years later

The two thrombopoietin receptor agonists (TPO-RA), eltrombopag and romiplostim, were licensed in the US for treatment of immune thrombocytopenia (ITP) in 2008 and, since then, their use has progressively increased around the world; they are currently used in more than 100 countries. The six largest randomized controlled trials conducted in ITP have used one of these two agents. All studies have demonstrated a platelet response rate between 50-90%, depending on the criteria used, with good safety and tolerability. TPO-RA were shown to be effective in reducing bleeding and the need for concomitant or rescue medication. Many other investigations of their mechanism of effect, prospective and retrospective trials, and studies focusing on toxicity have been performed widening our knowledge of these two agents. Initial concerns on issues such as myelofibrosis have not been confirmed. Only a small number of patients develop moderate-severe reticulin fibrosis and/or collagen fibrosis; however, these are usually reversed after discontinuation of TPO-RA. Studies indicate, however, that TPO-RA may increase the risk of venous thromboembolism. Both TPO-RA are currently approved in patients with chronic ITP aged >1-year who are refractory to at least one other treatment. Eltrombopag has acquired two additional indications: severe aplastic anemia refractory to first-line treatment and hepatitis C patients undergoing treatment with interferon-ribavirin. Despite these wide-ranging studies, important questions still need to be answered. This summary review on TPO-RA will summarize what is known regarding efficacy in ITP, evaluate safety concerns in more depth, and focus on the questions that remain.

Computational Design of Antiangiogenic Peptibody by Fusing Human IgG1 Fc Fragment and HRH Peptide: Structural Modeling, Energetic Analysis, and Dynamics Simulation of Its Binding Potency to VEGF Receptor

Peptibodies represent a new class of biological therapeutics with combination of peptide activity and antibody-like properties. Previously, we discovered a novel peptide HRH that exhibited a dose-dependent angiogenesis-suppressing effect by targeting vascular endothelial growth factor receptors (VEGFRs). Here, we computationally designed an antiangiogenic peptibody, termed as PbHRH, by fusing the HRH peptide to human IgG1 Fc fragment using the first approved peptibody drug Romiplostim as template. The biologically active peptide of Romiplostim is similar with HRH peptide; both of them have close sequence lengths and can fold into a α-helical conformation in free state. Molecular dynamics simulations revealed that the HRH functional domain is highly flexible, which is functionally independent of Fc fragment in the designed PbHRH peptibody. Subsequently, the intermolecular interactions between VEGFR-1 domain 2 (D2) and PbHRH were predicted, clustered and refined into three representatives. Conformational analysis and energetic evaluation unraveled that the PbHRH can adopt multiple binding modes to block the native VEGF-A binding site of VEGFR-1 D2 with its HRH functional domain, although the binding effectiveness of HRH segments in peptibody context seems to be moderately decreased relative to that of free HRH peptide. Overall, it is suggested that integrating HRH peptide into PbHRH peptibody does not promote the direct intermolecular interaction between VEGFR-1 D2 and HRH. Instead, the peptibody may indirectly help to improve the pharmacokinetic profile and bioavailability of HRH.

An insight into the thermodynamic characteristics of human thrombopoietin complexation with TN1 antibody

Human thrombopoietin (hTPO) primarily stimulates megakaryocytopoiesis and platelet production and is neutralized by the mouse TN1 antibody. The thermodynamic characteristics of TN1 antibody-hTPO complexation were analyzed by isothermal titration calorimetry (ITC) using an antigen-binding fragment (Fab) derived from the TN1 antibody (TN1-Fab). To clarify the mechanism by which hTPO is recognized by TN1-Fab the conformation of free TN1-Fab was determined to a resolution of 2.0 Å using X-ray crystallography and compared with the hTPO-bound form of TN1-Fab determined by a previous study. This structural comparison revealed that the conformation of TN1-Fab does not substantially change after hTPO binding and a set of 15 water molecules is released from the antigen-binding site (paratope) of TN1-Fab upon hTPO complexation. Interestingly, the heat capacity change (ΔCp) measured by ITC (-1.52 ± 0.05 kJ mol(-1)  K(-1) ) differed significantly from calculations based upon the X-ray structure data of the hTPO-bound and unbound forms of TN1-Fab (-1.02 ∼ 0.25 kJ mol(-1)  K(-1) ) suggesting that hTPO undergoes an induced-fit conformational change combined with significant desolvation upon TN1-Fab binding. The results shed light on the structural biology associated with neutralizing antibody recognition.

Identification of the residues in the extracellular domain of thrombopoietin receptor involved in the binding of thrombopoietin and a nuclear distribution protein (human NUDC)

Thrombopoietin (TPO) and its receptor (Mpl) have long been associated with megakaryocyte proliferation, differentiation, and platelet formation. However, studies have also shown that the extracellular domain of Mpl (Mpl-EC) interacts with human (h) NUDC, a protein previously characterized as a human homolog of a fungal nuclear migration protein. This study was undertaken to further delineate the putative binding domain on the Mpl receptor. Using the yeast two-hybrid system assay and co-immunoprecipitation, we identified that within the Mpl-EC domain 1 (Mpl-EC-D1), amino acids 102-251 were strongly involved in ligand binding. We subsequently expressed five subdomains within this region with T7 phage display. Enzyme-linked immunosorbent binding assays identified a short stretch of peptide located between residues 206 and 251 as the minimum binding domain for both TPO and hNUDC. A series of sequential Ala replacement mutations in the region were subsequently used to identify the specific residues most involved in ligand binding. Our results point to two hydrophobic residues, Leu(228) and Leu(230), as having substantial effects on hNUDC binding. For TPO binding, mutations in residues Asp(235) and Leu(239) had the largest effect on binding efficacy. In addition, deletion of the conservative motif WGSWS reduced binding capacity for hNUDC but not for TPO. These separate binding sites on the Mpl receptor for TPO and hNUDC raise interesting implications for the cytokine-receptor interactions.

Thrombopoietin and its splicing variants: structure and functions in thrombopoiesis and beyond

Since its cloning in 1994, several studies have reported that thrombopoietin (THPO) presents several alternative splicing products that differ from the full-length protein in its 5' UTR, N- or C-terminal regions. Most of these splice variants are evolutionarily conserved and have been detected in different tissues as well as in cell lines. Although the possible functions of the THPO isoforms are still elusive, different clues link them to the peculiar mechanism that regulates THPO production. Moreover, novel fields to explore possible roles of the THPO variants are opened by observations that this hormone can influence the formation of hematopoietic progenitors and its expression occurs in some tumors as well as in tissues not directly related to the thrombopoiesis. In this review, we summarize the structure and functions of THPO through the published evidence on its splicing isoforms and discuss about their involvement with physiopathologic phenomena.

A novel thrombopoietin–stem-cell factor fusion protein possesses enhanced potential in stimulating megakaryocyte proliferation and differentiation

TPO (thrombopoietin) and SCF (stem-cell factor) are functionally related cytokines with overlapping but distinct haematopoietic effects. In the present study, a novel TPO-SCF fusion protein that combined the complementary biological effects of TPO and SCF into a single molecule was expressed in, and purified from, Sf9 [Spodoptera frugiperda (fall armyworm)] insect cells. The specific activity of rhTPO (recombinant human TPO)-SCF in megakaryoblastic Mo7e cell proliferation assays was 2.90+/-0.35 x 10(7) units/micromol, approx. 1.7 times as high as that of rhTPO. The specific activity of rhTPO-SCF in TF-1 cells proliferation assays was 7.10+/-0.95 x 10(6) units/micromol, approx. 1.2 times as high as that of rhSCF (recombinant human SCF). In a megakaryocyte-colony-forming assay using human peripheral-blood CD34(+) cells, the SCF moiety of rhTPO-SCF worked in a synergistic way to augment the colony number and exhibited a higher potential to stimulate megakaryocyte colony growth. According to the results of EMSA (electrophoretic mobility-shift assay) and semi-quantitative RT (reverse transcriptase)-PCR, the synergistic effects of the SCF moiety were also reflected in increased STAT5 (signal transducer and activator of transcription 5) DNA binding and enhanced up-regulation of p21 expression in Mo7e cells treated by rhTPO-SCF, suggesting that rhTPO-SCF could be more potent in promoting megakaryocyte proliferation and differentiation.

New thrombopoietic growth factors

Although development of first-generation thrombopoietic growth factors (recombinant human thrombopoietin [TPO] and pegylated recombinant human megakaryocyte growth and development factor [PEG-rHuMGDF]) was stopped due to development of antibodies to PEG-rHuMGDF, nonimmunogenic second-generation thrombopoietic growth factors with unique pharmacologic properties have been developed. TPO peptide mimetics contain TPO receptor-activating peptides inserted into complementarity-determining regions of Fab (Fab 59), attached to the IgG Fc region (AMG 531), or pegylated (Peg-TPOmp). Orally available, TPO nonpeptide mimetics (eltrombopag, AKR-501) bind and activate the TPO receptor by a mechanism different from TPO and may have an additive effect to TPO. TPO agonist antibodies are monoclonal antibodies activating the TPO receptor but modified in size [TPO minibodies; ie, VB22B sc(Fv)(2)] or immunoglobuln type (domain subclass-converted TPO agonist antibodies; ie, MA01G4G344). All second-generation thrombopoietic growth factors stimulate growth of TPO-dependent cell lines via JAK2/STAT signaling pathways and increase platelet counts in animals. When tested in healthy humans, TPO peptide and nonpeptide mimetics produced a dose-dependent rise in platelet count. AMG 531 and eltrombopag markedly increase platelet counts in patients with immune thrombocytopenic purpura, without significant adverse effects. One or more second-generation thrombopoietic growth factors should soon be clinically available for treating thrombocytopenic disorders.

NMR Structural Studies of Interactions of a Small, Nonpeptidyl Tpo Mimic with the Thrombopoietin Receptor Extracellular Juxtamembrane and Transmembrane Domains

Thrombopoietin (Tpo) is a glycoprotein growth factor that supports hematopoietic stem cell survival and expansion and is the principal regulator of megakaryocyte growth and differentiation. Several small, nonpeptidyl molecules have been identified as selective human Tpo receptor (hTpoR) agonists. To understand how the small molecule Tpo mimic SB394725 interacts and activates hTpoR, we performed receptor domain swap and mutagenesis studies. The results suggest that SB394725 interacts specifically with the extracellular juxtamembrane region (JMR) and the transmembrane (TM) domain of hTpoR. Solution and solid-state NMR structural studies using a peptide containing the JMR-TM sequences showed that this region of hTpoR, unexpectedly, consists of two alpha-helices separated by a few nonhelical residues. SB394725 interacts specifically with His-499 in the TM domain and a few distinct residues in the JMR-TM region and affects several specific C-terminal TM domain residues. The unique structural information provided by these studies both sheds light on the distinctive mechanism of action of SB394725 and provides valuable insight into the mechanism of ligand-induced cytokine receptor activation.

Xanthocillins as thrombopoietin mimetic small molecules

Four xanthocillins (1-4), including a new compound 4, were isolated from cultured marine fungus Basipetospora sp. as thrombopoietin (TPO) mimics. Compounds 1-4 promoted the proliferation of a TPO-sensitive human leukemia cell line, UT-7/TPO, and UT-7/EPO-mpl, genetically engineered to express c-Mpl, a receptor for TPO in dose-dependent manners. However, the proliferation of UT-7/EPO, a parental cell line of UT-7/EPO-mpl that was devoid of TPO receptor, was not affected by them. Thrombopoietic action of compound 1 was nearly as potent as that of TPO, inducing cell proliferation at a concentration ranging from 1 to 100nM. Compound 1 also induced the phosphorylation of several proteins, including Janus kinase 2 (Jak2), signal transducers, and activators of transcription-3 (STAT3) and STAT5 in the UT-7/EPO-mpl cell line, but not in the UT-7/EPO cell line. These data indicated that xanthocillins are putative agonists for c-Mpl, as their cellular actions were analogous to those of TPO.

Expression, purification, and characterization of a novel recombinant fusion protein, rhTPO/SCF, in Escherichia coli

Thrombopoietin (TPO) is the principal regulatory cytokine of megakaryopoiesis and thrombopoiesis and promotes all aspects of megakaryocyte development. Stem cell factor (SCF) is mainly a pleiotropic cytokine acting on hematopoiesis by promoting the survival and proliferation of hematopoietic stem cells and has a potent synergistic effect on megakaryopoiesis in the presence of TPO. Here, we report the construction, expression, and purification of a novel recombinant human thrombopoietin/stem cell factor (rhTPO/SCF) fusion protein, which consists of a truncated human thrombopoietin (1-157 a.a.) plus a truncated human stem cell factor (1-145 a.a.), linked by a peptide (GGGGSPGGSGGGGSGG). The TPO/SCF gene was cloned into the Escherichia coli expression vector pET28a and expressed in BL21(DE3) strain. The rhTPO/SCF constituted up to 6% of the total bacterial protein. Co-expression with E. coli chaperones, Trigger Factor (TF) and GroES/GroEL, and lowering cultivation temperature cooperatively improved the solubility of expressed rhTPO/SCF, resulting in about fourfold increase in the yield soluble rhTPO/SCF. The rhTPO/SCF was purified to homogeneity using anion exchange followed by metal affinity chromatography. Western blot analysis confirmed the identity of the purified protein. rhTPO/SCF stimulated a dose-dependent cell proliferation in both TF1 and Mo7e cell lines.

Determination of Thrombopoietin-Derived Peptides Recognized by Both Cellular and Humoral Immunities in Healthy Donors and Patients with Thrombocytopenia

Thrombopoietin (TPO) is a cytokine that promotes megakaryocytopoiesis and thrombopoiesis and is considered a drug suitable for patients with thrombocytopenia. However, unexpected severe thrombocytopenia has developed in some healthy individuals participating in phase I clinical trials with a pegylated recombinant human megakaryocyte growth factor (PEG-rHuMGDF) that contained the first 163 amino acids of endogenous TPO, which resulted in hampering the further development of clinical trials. Autoimmune responses to PEG-rHuMGDF, which cross-reacted with endogenous TPO, were suggested to be involved in this rare but severe adverse event, although the immunogenic epitopes have not yet been determined. To better understand the molecular basis of such autoimmune reactions, we investigated the reactivity of 18 TPO-derived peptides with HLA-A2-binding motifs to plasma and T cells, both from patients with thrombocytopenia (n=24) and from healthy donors (HDs) (n=24). Four peptides, including those possessing amino acids in receptor-binding sites, were preferentially reactive to plasma from at least 20% of the patients, whereas one peptide at position 101-109 was equally reactive to those of the patients and the HDs. Each of the five peptides had the ability to induce peptide-specific cytotoxic T lymphocytes (CTLs) in both groups, albeit with less frequency among the patients. More important, each of these five peptides had the ability to induce HLA-A2-restricted and peptide-specific CTL activity reactive to cells that produce TPO. These results may provide new insights to gain a better understanding of autoimmune reactions to TPO.

Monitoring and quantification of inclusion body formation in Escherichia coli by multi-parameter flow cytometry

Multi-parameter flow cytometry was used to monitor the formation of promegapoietin (PMP) inclusion bodies during a high cell density Escherichia coli fed-batch fermentation process. Inclusion bodies were labelled with a primary antibody and then with a secondary fluorescent antibody. Using this method it was possible to detect PMP inclusion body formation with a high specificity and it was possible to monitor the increased accumulation of the protein with process time (6-48 mg PMP/g CDW) whilst highlighting population heterogeneity.

Thrombopoietin complements G(i)- but not G(q)-dependent pathways for integrin {alpha}(IIb){beta}(3) activation and platelet aggregation

Binding of thrombopoietin (TPO) to the cMpl receptor on human platelets potentiates aggregation induced by a number of agonists, including ADP. In this work, we found that TPO was able to restore ADP-induced platelet aggregation upon blockade of the G(q)-coupled P2Y1 purinergic receptor but not upon inhibition of the G(i)-coupled P2Y12 receptor. Moreover, TPO triggered platelet aggregation upon co-stimulation of G(z) by epinephrine but not upon co-stimulation of G(q) by the thromboxane analogue U46619. Platelet aggregation induced by TPO and G(i) stimulation was biphasic, and cyclooxygenase inhibitors prevented the second but not the first phase. In contrast to ADP, TPO was unable to induce integrin alpha(IIb)beta(3) activation, as evaluated by binding of both fibrinogen and PAC-1 monoclonal antibody. However, ADP-induced activation of integrin alpha(IIb)beta(3) was blocked by antagonists of the G(q)-coupled P2Y1 receptor but was completely restored by the simultaneous co-stimulation of cMpl receptor by TPO. Inside-out activation of integrin alpha(IIb)beta(3) induced by TPO and G(i) stimulation occurred independently of thromboxane A(2) production and was not mediated by protein kinase C, MAP kinases, or Rho-dependent kinase. Importantly, TPO and G(i) activation of integrin alpha(IIb)beta(3) was suppressed by wortmannin and Ly294002, suggesting a critical regulation by phosphatidylinositol 3-kinase. We found that TPO did not activate phospholipase C in human platelets and was unable to restore ADP-induced phospholipase C activation upon blockade of the G(q)-coupled P2Y1 receptor. TPO induced a rapid and sustained activation of the small GTPase Rap1B through a pathway dependent on phosphatidylinositol 3-kinase. In ADP-stimulated platelets, Rap1B activation was reduced, although not abolished, upon blockade of the P2Y1 receptor. However, accumulation of GTP-bound Rap1B in platelets activated by co-stimulation of cMpl and P2Y12 receptor was identical to that induced by the simultaneous ligation of P2Y1 and P2Y12 receptor by ADP. These results indicate that TPO can integrate G(i), but not G(q), stimulation and can efficiently support integrin alpha(IIb)beta(3) activation platelet aggregation by an alternative signaling pathway independent of phospholipase C but involving the phosphatidylinositol 3-kinase and the small GTPase Rap1B.

Expression of the soluble extracellular domain of human thrombopoietin receptor using a maltose-binding protein-affinity fusion system

The thrombopoietin (TPO) receptor (Mpl) belongs to the family of ligand-dependent cytokine receptors and plays a functional role in regulating platelet production. The signaling capacity largely depends on the binding of TPO to the extracellular domains of the TPO receptor (Mpl-EC). Because the expression level of Mpl in human tissue is very low, studies on the functional and spatial characteristics of its ligand-binding sites have been limited. In the present study, we report the expression and purification of Mpl-EC as a fusion with the maltose-binding protein (MBP), designated MBP-Mpl-EC. MBP-Mpl-EC was expressed in the cytoplasm of Escherichia coli as a soluble fusion protein. Specific binding of TPO to purified MBP-Mpl-EC was demonstrated by a dot-blot assay and surface plasmon resonance. We conclude that bacterial expression of MBP-Mpl-EC yields large amounts of protein with correct folding and that it can be used for further structure and function analyses.

A truncated isoform of c-Mpl with an essential C-terminal peptide targets the full-length receptor for degradation

Thrombopoietin and its cognate receptor c-Mpl are the primary regulators of megakaryopoiesis and platelet production. They also play an important role in the maintenance of hematopoietic stem cells. Here, we have analyzed the function of a truncated Mpl receptor isoform (Mpl-tr), which results from alternative splicing. The mpl-tr variant is the only alternate mpl isoform conserved between mouse and humans, suggesting a relevant function in regulating Mpl signaling. Despite the presence of a signal peptide and the lack of a transmembrane domain, Mpl-tr is retained intracellularly. Our results provide evidence that Mpl-tr exerts a dominant-negative effect on thrombopoietin-dependent cell proliferation and survival. We demonstrate that this inhibitory effect is due to down-regulation of the full-length Mpl protein. The C terminus of Mpl-tr, consisting of 30 amino acids of unique sequence, is essential for the suppression of thrombopoietin-dependent proliferation and Mpl protein down-regulation. Cathepsin inhibitor-1 (CATI-1), an inhibitor of cathepsin-like cysteine proteases, counteracts the effect of Mpl-tr on Mpl protein expression, suggesting that Mpl-tr targets Mpl for lysosomal degradation. Together, these data suggest a new paradigm for the regulation of cytokine receptor expression and function through a proteolytic process directed by a truncated isoform of the same receptor.

Structure of the receptor-binding domain of human thrombopoietin determined by complexation with a neutralizing antibody fragment

The cytokine thrombopoietin (TPO), the ligand for the hematopoietic receptor c-Mpl, acts as a primary regulator of megakaryocytopoiesis and platelet production. We have determined the crystal structure of the receptor-binding domain of human TPO (hTPO(163)) to a 2.5-A resolution by complexation with a neutralizing Fab fragment. The backbone structure of hTPO(163) has an antiparallel four-helix bundle fold. The neutralizing Fab mainly recognizes the C-D crossover loop containing the species invariant residue Q111. Titration calorimetric experiments show that hTPO(163) interacts with soluble c-Mpl containing the extracellular cytokine receptor homology domains with 1:2 stoichiometry with the binding constants of 3.3 x 10(9) M(-1) and 1.1 x 10(6) M(-1). The presence of the neutralizing Fab did not inhibit binding of hTPO(163) to soluble c-Mpl fragments, but the lower-affinity binding disappeared. Together with prior genetic data, these define the structure-function relationships in TPO and the activation scheme of c-Mpl.

Sugar coating extends half-lives and improves effectiveness of cytokine hormones

Recombinant human erythropoietin (rhEPO) is an effective and widely used therapeutic agent that is produced by bioengineering. Modification of the rhEPO protein by glycoengineering increased its already abundant N-glycosylation, which enhances its erythropoietic activity in vivo by decreasing its metabolic clearance. Elliott et al. recently reported increased in vivo activities of thrombopoietin (Mpl ligand) and leptin following carbohydrate addition to both, which suggests that such glycoengineering could be applied to a variety of hormones, cytokines and growth factors.

Thrombopoietin: a tool for understanding thrombopoiesis

Although first proposed to be the primary regulator of platelet production 45 years ago, the gene for thrombopoietin was cloned only within the last decade. Since then, our understanding of megakaryocyte and platelet production has increased substantially, and it is now appreciated that in addition to its critical role in regulating thrombopoiesis, the hormone affects multiple aspects of hematopoiesis, including playing a non-redundant role in stem cell survival, self-renewal and expansion. In addition to this greater physiological understanding of thrombopoietin biology, the molecular mechanisms by which the hormone affects cell survival and proliferation are coming under increased scrutiny. At least four signaling pathways have been identified that play important and non-overlapping roles in stem cell and megakaryocyte growth and development, potentially providing new strategies to therapeutically intervene in hematopoiesis. This review will focus on our current understanding of these processes.

The glycan domain of thrombopoietin (TPO) acts in trans to enhance secretion of the hormone and other cytokines

Thrombopoietin (TPO), the primary regulator of platelet production, is composed of an amino-terminal 152 amino acids, sufficient for activity, and a carboxyl-terminal region rich in carbohydrates (183 residues) that enhances secretion of the molecule. Full-length TPO is secreted at levels 10-20-fold greater than truncated TPO. By introducing into mammalian cells a novel cDNA encoding the TPO secretory leader linked to its carboxyl-terminal domain (TPO glycan domain (TGD)), we tested whether TGD could function in trans to enhance secretion of TPO. The artificial TGD was secreted, inactive in proliferation assays, and did not inhibit TPO activity. However, when co-transfected with a cDNA encoding truncated TPO, TGD enhanced secretion 4-fold, measured by specific bioassay and immunoassay. TGD also enhanced secretion of granulocyte monocyte colony-stimulating factor and stem cell factor but did not affect the production of erythropoietin, interleukin-3, growth hormone, or of full-length TPO. To localize TGD function, we added an endoplasmic reticulum (ER) retention signal to TGD and, separately, deleted the secretory leader. Deletion of the secretory leader attenuated the secretory function of TGD, whereas addition of the ER retention signal did not alter its function. To investigate the physiologic role of TGD in folding and proteasomal protection, we tested full-length and truncated TPO in assays of protein refolding, and we examined protein stability in the presence of proteasome inhibitors. We found that truncated TGD re-folds readily and that proteasome-mediated degradation contributes to the poor secretion of truncated TPO. We conclude that TGD enhances secretion of TPO and can additionally function as an inter-molecular chaperone, in part because of its ability to prevent degradation of the hormone. The cellular location of TGD action is likely to be within the ER or earlier in the secretory pathway.

Identification of a pharmacophore for thrombopoietic activity of small, non-peptidyl molecules. 1. Discovery and optimization of salicylaldehyde thiosemicarbazone thrombopoietin mimics

High-throughput screening has resulted in the discovery of thiosemicarbazone thrombopoietin mimics. A shared pharmacophore hypothesis between this series and a previously identified class, the pyrazol-4-ylidenehydrazines, led to the rapid optimization of both potency and efficacy of the thiosemicarbazones. The application of high-throughput chemistry and purification techniques allowed for the rapid elucidation of structure-activity relationships.

Identification of a pharmacophore for thrombopoietic activity of small, non-peptidyl molecules. 2. Rational design of naphtho[1,2-d]imidazole thrombopoietin mimics

The invention of a new class of naphtho[1,2-d]imidazole thrombopoietin mimics based on a pharmacophore hypothesis for small-molecule thrombopoietic agonists is discussed. Parallel array synthesis and purification techniques allowed for the rapid exploration of structure-activity relationships within this class and for the improvement in TPO mimetic potencies and efficacies.

Mono-N-terminal poly(ethylene glycol)-protein conjugates

A site-directed method of joining proteins to poly(ethylene glycol) is presented which allows for the preparation of essentially homogeneous PEG-protein derivatives with a single PEG chain conjugated to the amine terminus of the protein. This selectivity is achieved by conducting the reductive alkylation of proteins with PEG-aldehydes at lower pH. Working examples demonstrating the application of this method to improve the delivery characteristics and therapeutic value of several proteins are provided.

Crystallization of the functional domain of human thrombopoietin using an antigen-binding fragment derived from neutralizing monoclonal antibody

Thrombopoietin (TPO) is a cytokine which primarily stimulates megakaryocytopoiesis and thrombopoiesis. The functional domain of TPO (TPO(163)) consisting of the N-terminal 163 amino acids was prepared and crystallized. Since the crystallization of TPO(163) was unsuccessful using the standard screening methods, a Fab fragment derived from a neutralizing monoclonal antibody was used for crystallization. It was found that the TPO(163)-Fab complex crystallized reproducibly in 0.1 M potassium phosphate buffer pH 6.0 containing 20-25% polyethylene glycol 4000. Thin crystals (0.2 x 0.2 x 0.02 mm) grew in two space groups: P2(1), with unit-cell parameters a = 133.20, b = 46.71, c = 191.47 A, beta = 90.24 degrees, and C2, with unit-cell parameters a = 131.71, b = 46.48, c = 184.63 A, beta = 90.42 degrees. The results of a molecular-replacement analysis indicate that the Fab molecules interact with each other and provide a suitable interface for crystallization.

Inverse relationship of protein concentration and aggregation

PURPOSE

To determine the effect of protein concentration on aggregation induced through quiescent shelf-life incubation or shipping-related agitation.

METHODS

All aggregation was measured by size-exclusion high-performance liquid chromatography. Aggregation was induced by time-dependent incubation under stationary conditions or by agitation caused by shaking, vortexing, or vibration using simulated shipping conditions.

RESULTS

Protein aggregation is commonly a second- or higher-order process that is expected to increase with higher protein concentration. As expected, for three proteins (PEG-GCSF, PEG-MGDF, and OPG-Fc) that were examined, the aggregation increased with higher protein concentration if incubated in a quiescent shelf-life setting. However, aggregation decreased with higher protein concentration if induced by an air/water interface as a result of agitation. This unexpected result may be explained by the rate-limiting effect on aggregation of the air/water interface and the critical nature of the air/ water interface to protein ratio that is greatest with decreased protein concentration. The non-ionic detergent polysorbate 20 enhanced the aggregation observed in the quiescently incubated sample but abrogated the aggregation induced by the air/water interface.

CONCLUSIONS

The effect of protein concentration was opposite for aggregation that resulted from quiescent shelf-life treatment compared to induction by agitation. For motionless shelf-life incubation, increased concentration of protein resulted in more aggregation. However, exposure to agitation resulted in more aggregation with decreased protein concentration. These results highlight an unexpected complexity of protein aggregation reactions.

Thrombopoietin regulates Bcl-xL gene expression through Stat5 and phosphatidylinositol 3-kinase activation pathways

Thrombopoietin (TPO), an essential factor for megakaryopoiesis and thrombopoiesis, works as a survival factor for megakaryocytic lineage cells. However, little is known about the molecular mechanism in detail. We show here that TPO supports the survival of TPO-dependent leukemia cell line UT-7/TPO and normal megakaryocytic progenitors via the induction of Bcl-xL, an anti-apoptotic member of the Bcl-2 family. We further analyzed the signal transduction pathways required for TPO-induced Bcl-xL gene expression. A reporter assay with various lengths of Bcl-x gene promoter revealed that both Stat- and nuclear factor kappa B-binding sites are prerequisites for TPO-induced promoter activity. Consistent with these results, TPO induced the binding of Stat5 and subunits of nuclear factor kappa B, p50, and c-Rel to the Bcl-x gene promoter. AG490, a specific inhibitor for Jak2, and LY294002, a specific inhibitor for phosphatidylinositol (PI) 3-kinase, reduced the protein level of Bcl-xL in UT-7/TPO cells, accompanied by an increase in the ratio of apoptotic cells. Interestingly, LY294002 enhanced the TPO-induced DNA binding activity of Stat5 without affecting the Jak2 activation and tyrosine phosphorylation of Stat5. Concomitantly, confocal microscopy revealed that LY294002 clearly inhibited the nuclear export of Stat5, suggesting that PI 3-kinase regulates the subcellular localization of Stat5. Taken together, our results suggest that both Jak-Stat and PI 3-kinase activation pathways regulate the TPO-induced survival of megakaryocytic cells via Bcl-xL gene expression. In addition, our data suggest possible cross-talk between these two signaling pathways.

Thrombopoietin: a pan-hematopoietic cytokine

The recent discovery of thrombopoietin has enhanced our understanding of both hematopoiesis and platelet production. Thrombopoietin supports hematopoietic stem cell survival and expansion as well as promoting all aspects of megakaryocyte development. The hormone displays many structural similarities to other members of the hematopoietic cytokine family and some notable differences, and regulation of its expression requires both receptor-mediated removal and other mechanisms. Thrombopoietin induces receptor dimerization and tyrosine phosphorylation, and a series of signaling events including activation of JAK/STAT, Shc/Ras/MAPK and PI3K/Akt; these pathways overlap with those induced by other cytokines, but the differences that lead to the unique biological effects of the hormone are gradually being uncovered. Our growing appreciation of how cytokine signaling pathways are translated into megakaryocyte development is discussed.

Mpl ligand increases P2Y1 receptor gene expression in megakaryocytes with no concomitant change in platelet response to ADP

The P2Y(1) receptor is responsible for the initiation of platelet aggregation in response to ADP and plays a key role in thrombosis. Although this receptor is expressed early in the platelet lineage, the regulation of its expression during megakaryocyte differentiation is unknown. In the mouse megakaryocytic cell line Y10/L8057, we detected P2Y(1) mRNA of three sizes (2.5, 4.4, and 7.4 kb). These cells have previously been shown to respond to Mpl ligand, the pivotal regulator of megakaryocytopoiesis, by increasing their expression of differentiation markers. Mpl ligand enhanced levels of P2Y(1) mRNAs in Y10/L8057 cells and this effect was selective: the same cytokine did not increase levels of A2a adenosine receptor mRNA. Although Mpl ligand did not affect the short half-lives of the P2Y(1) mRNAs, it enhanced transcription of the P2Y(1) gene. It also increased cell size and the number of cell surface P2Y(1) receptors, but not P2Y(1) receptor density. Injection of Mpl ligand into mice up-regulated P2Y(1) receptor mRNAs in megakaryocytes, as shown by in situ hybridization. However, platelets isolated from these mice did not exhibit a higher P2Y(1) receptor density or increased reactivity to ADP. This correlates with the finding that Mpl ligand increases GPIIb mRNA in megakaryocytes but not the density of the protein per platelet. Thus, the enhancement of P2Y(1) receptor expression induced by Mpl ligand in megakaryocytes may be an integral feature of their differentiation, whereas clinical use of this compound might not be associated with platelet hyper-reactivity to ADP.

Hydrazinonaphthalene and azonaphthalene thrombopoietin mimics are nonpeptidyl promoters of megakaryocytopoiesis

High-throughput screening for the induction of a luciferase reporter gene in a thrombopoietin (TPO)-responsive cell line resulted in the identification of 4-diazo-3-hydroxy-1-naphthalenesulfonic acids as TPO mimics. Modification of the core structure and adjustment of unwanted functionality resulted in the development of (5-oxo-1,5-dihydropyrazol-4-ylidene)hydrazines which exhibited efficacies equivalent to those of TPO in several cell-based assays designed to measure thrombopoietic activity. Furthermore, these compounds elicited biochemical responses in TPO-receptor-expressing cells similar to those in TPO itself, including kinase activation and protein phosphorylation. Potencies for the best compounds were high for such low molecular weight compounds (MW < 500) with EC(50) values in the region of 1-20 nM.

Hematopoietic growth factor mimetics

Hematopoietic growth factors are glycoproteins of 15-70 kDa. Although much clinical success has been obtained using recombinant proteins produced in mammalian cell lines and in microbial fermentation processes, the full-length polypeptides necessarily are expensive to produce, require parenteral administration, and in some cases have provoked detrimental immune responses. With the availability of high throughput biological function and receptor binding assays it has become possible to screen millions, if not billions, of randomly produced organic compounds and relatively short peptides to identify lead compounds for the development of small molecular mimetics of hematopoietic growth factors. Herein the strategies used to screen libraries of small molecules and peptides and the successes in finding mimetics and antagonists for/to erythropoietin, granulocyte colony-stimulating factor, and thrombopoietin are reviewed. Finally, the structural study of mimetic-receptor complexes has provided us with many molecular details of growth factor-induced receptor activation and is likely to yield new insights into the molecular basis of hematopoietic signal transduction.

[Study on interactions between TPO molecules tested with yeast two-hybrid system]

OBJECTIVE

To explore the interactions between TPO molecules and their locations.

METHODS

Full-length TPO, TPO(N) and TPO(C) were inserted respectively into the plasmids of the yeast two-hybrid system. Six different recombinant plasmids were generated. Every 2 recombinant plasmids were transformed into the yeast cell SFY526. The possible interactions between TPOs were then tested by the bioactivity of beta-galactosidase.

RESULTS

There are interactions between full-length TPO themselves TPO(N) themselves and TPO with TPO(N), but not between TPO(C)themselves, TPO with TPO(C) and TPO(N) with TPO(C).

CONCLUSION

Naturally, the molecular interaction domain of TPO may exist in the N terminus but not in the C terminus.

Do the preclinical effects of thrombopoietin correlate with its in vitro properties?

In the short time since its cloning, much has been learned of the in vitro properties of thrombopoietin (TPO). In addition to effects on the differentiation of megakaryocytes, TPO has also been shown to stimulate the proliferation of megakaryocytic progenitor cells, colony-forming units-megakaryocytes (CFU-MK), to act in synergy with interleukin 3 or c-kit ligand and erythropoietin (Epo) to stimulate the development of early and the generation of late erythroid progenitor cells, and to affect the rate of entry into the cell cycle and proliferative capacity of hematopoietic stem cells. An important question posed by these observations, for both TPO and for hematopoietic research in general, is whether the in vitro effects of a cytokine are mirrored by its preclinical and clinical biology. The results of recent studies in mice and nonhuman primates will be presented which have attempted to address this issue. In normal animals, TPO increases the numbers of marrow and spleen CFU-granulocyte/erythroid/macrophage/megakaryocyte, CFU-MK, CFU-GM and BFU-E, but its effects in the peripheral blood are limited to marked increases in the platelet count. The reason for widespread progenitor cell effects, yet stable leukocyte and erythrocyte blood counts, is likely the predominant regulatory effects of G-CSF and Epo; in the absence of elevated levels of the lineage-dominant regulator of each of these cell types, expanded progenitor cell numbers are not translated into increased peripheral blood counts. However, in states of increased blood cell demand such as follows myelosuppressive therapy, elevated levels of Epo and G-CSF allow the effects of TPO on erythropoiesis and myelopoiesis to become manifest. The administration of TPO to myelosuppressed animals is associated with not only greatly expanded hematopoietic progenitor cell recovery, but also improvement in platelet, red cell and leukocyte nadir levels and greatly accelerated recovery of all three cell lineages. These results indicate that the panhematopoietic properties of TPO identified by in vitro culture techniques correlate well with its effects in animals. The results of ongoing clinical trials should soon establish whether these conclusions can be extended to patient care.

Dissection of c-Mpl and thrombopoietin function: studies of knockout mice and receptor signal transduction

The physiological roles and mechanisms of action of thrombopoietin (TPO) and its receptor c-Mpl have been studied through the analysis of mice genetically deficient in these molecules, as well as through the dissection of signaling events utilizing chimeric receptors. The evidence clearly demonstrates that the TPO/c-Mpl system provides dominant control in the regulation of megakaryocytopoiesis. The signaling mechanisms that underlie this process appear to be similar to those noted with other members of the hematopoietic cytokine and cytokine receptor families.

Protein characteristics of thrombopoietin

Thrombopoietin (TPO) was purified from irradiated thrombocytopenic rat plasma. In the process of purification, some biochemical and biological characteristics were investigated. Rat plasma TPO was extremely hydrophobic and exhibited multiple peaks of activity on gel filtration. Both the low and high molecular weight fractions were separately subjected to further purification. Consequently, a rat TPO cDNA was cloned based on the amino acid sequences of purified rat plasma TPO. It revealed that each final purified rat plasma TPO was not a full-length form. In addition, rat hepatocytes and three rat hepatoma cell lines were found to produce rat TPO. Each native TPO derived from cultured cells was also partially purified, and hepatocyte-derived TPOs were shown to be heterogeneous in molecular weight. To study the structure of TPO, various recombinant TPO molecules were generated. Two disulfide bonds (Cys7-Cys151 and Cys29-Cys85) located in the N-terminal domain of TPO have an important effect on its biological activity. The human TPO muteins, sequentially deleted from the C-terminal, were expressed in COS-1 cells. TPO (1-151) was active, but TPO (1-150), which lacks Cys151, did not exhibit TPO activity. These findings indicate that the region essential for TPO activity is the N-terminal domain, which contains two disulfide bonds. Although the role(s) of the C-terminal domain is not clear at present, the potential N-glycosylation in the C-terminal domain is not directly required for exhibiting TPO activity.

Peptide agonists of the thrombopoietin receptor

We have screened a variety of L-amino acid peptide libraries against the extracellular domain of the human thrombopoietin (HuTPO) receptor, c-Mpl. A large number of peptide ligands were recovered and categorized into two families. Peptides from each family compete with the binding of HuTPO and with the binding of peptides from the other familiy. Representative peptides were synthesized and found to activate the full-length HuTPO receptor expressed in Ba/F3 cells to promote proliferation. These peptide families show no apparent homology to the primary sequence of TPO. We have focused our optimization efforts on one of the peptides, a linear 14-mer (IEGPTLRQWLAARA) with an IC50 of 2 nM in a competition binding assay and an EC50 of 400 nM in the proliferation assay. In order to enhance the potency of the compound, we constructed dimeric peptides by linking the carboxy-termini of the 14-mers to a lysine branch. These molecules exhibited slightly higher affinity (0.5 nM) and greatly increased potency (0.1 nM). The EC50 of the dimeric peptide was equivalent to that of the 332 aa form of baculovirus-expressed recombinant HuTPO. As previously shown for the erythropoietin-mimetic peptides, the TPO-mimetic peptides probably activate the TPO receptor by binding and inducing receptor dimerization. This supposition is supported by the observation that covalent dimerization of the peptide enhances its potency by 4,000-fold over that of the monomer. The peptide dimer is also active in stimulating in vitro proliferation of progenitors and maturation of megakaryocytes from human bone marrow, and in promoting an increase in platelet count when administered to normal mice.

In vivo biological effects of various forms of thrombopoietin in a murine model of transient pancytopenia

Thrombopoietin (TPO) is the natural regulator of platelet production in the bone marrow of mammals. This cytokine also seems to play an important role in the development of the erythroid lineage when recovering from anemic conditions. Here we study the effects of various TPO molecules on the recovery of hematopoietic lineages in a mouse model of pancytopenia. Based on previous animal experimentation and clinical experience with other hematopoietic cytokines, we found that daily dosing with TPO augmented the recovery of both the megakaryocyte and erythroid lineages in a mouse model of pancytopenia. However, further experiments showed that no benefit was gained by using more than a single dose of recombinant murine (rm)TPO(335) given 24 h after the initiation of the myelosuppressive treatment. This response to a single dose of rmTPO(335) is dose-dependent. However, the response was attenuated when a truncated, short half-life TPO molecule (rmTPO[153]) was used. Increasing the half-life of the molecule with 10 kDa polyethylene glycol (PEG) does not improve the response. Only when larger PEG molecules (20 kDa or 40 kDa) are linked to the rmTPO(153) is the response to single doses restored to the level of the full-length molecule. These data suggest that, unlike our experience with other cytokines, the commitment of progenitors to a megakaryocytic cell line is accomplished by a single short exposure to TPO.

Biological roles for the second domain of thrombopoietin

Thrombopoietin (TPO) has been isolated from the plasma of animals in several laboratories as several molecular species which vary in molecular weight from 19 kDa to 35 kDa. Upon biochemical analyses, all of these forms appear to represent proteolytic fragments of TPO which share the same amino-terminal sequences and are therefore truncated at their C-terminal ends. Intact recombinant TPO produced in cell culture is a 70 kDa glycoprotein which contains the full polypeptide backbone encoded by the cDNA sequence. A series of deliberate TPO truncations have been constructed by the introduction of stop codons into the cDNA at various positions. The proteins encoded by the truncated cDNAs have been purified to homogeniety for the purpose of detailed biological and biochemical comparisons of such C-terminally truncated TPO proteins with the full-length intact TPO. These comparisons serve to illuminate the possible biological role(s) of the C-terminal domain, which is so far unique in the family of hematopoietic cytokines. The C-terminal domain appears to regulate the specific activity of TPO, to regulate its circulating half-life, and to promote efficient biosynthesis and secretion of the protein.

The role of platelet growth factors in cancer therapy

The use of myeloid growth factors has markedly reduced the complications of chemotherapy-induced neutropenia, however, abrogation of severe thrombocytopenia remains a major clinical problem. Platelet transfusions remain the standard method of preventing or treating thrombocytopenia but are associated with a variety of complications and are a limited resource. A number of cytokines have been clinically investigated for their thrombopoietic activity, the most promising of which is the recently cloned ligand to the hematopoietic growth factor receptor, c-Mpl. The c-Mpl ligand, also referred to as thrombopoietin, megakaryocyte growth and development factor (MGDF) and megapoietin, is a potent lineage-specific agent that promotes growth and maturation of megakaryocytes and their progenitors. It holds promise for clinical use in the treatment of iatrogenic or disease-associated bone marrow failure states and possibly in syndromes of excessive platelet consumption. Early clinical trials assessing the safety and activity of recombinant human MGDF are now underway.

Functional analysis of the C-terminal region of recombinant human thrombopoietin. C-terminal region of thrombopoietin is a "shuttle" peptide to help secretion

Thrombopoietin (TPO) is a cytokine that primarily stimulates megakaryocytopoiesis and thrombopoiesis. TPO has a unique C-terminal tail peptide of about 160 amino acids that consists mostly of hydrophilic residues and contains six N-linked sugar chains. In order to investigate the biological function of the C-terminal domain, two series of mutations were performed. One is systematic truncation from the C terminus. Another is elimination of N-glycosylation sites in the C-terminal domain by Asn to Gln mutations. After the mutant proteins were expressed by mammalian cells, it was found that the elimination of the N-linked sugar sites did not affect the biological activity, whereas truncation of the C-terminal domain resulted in elevation of in vitro activity up to 4-fold. The C-terminal peptide itself was found to inhibit the in vitro activity. Moreover, both the C-terminal truncation and the elimination of the N-glycosylation sites decreased the secretion level progressively down to (1)/(10) that of wild type, and the amount of the mutant left in the cell increased. The N-glycosylation in the C-terminal region was found to be important for secretion of TPO. Among six N-glycosylation sites in the C-terminal region, two locations, Asn-213 and Asn-234, were found to be critical for secretion, and two other locations, Asn-319 and Asn-327, did not affect the secretion.

The glycan domain of thrombopoietin enhances its secretion

Thrombopoietin (TPO) is the major regulator of megakaryocyte development and platelet production. The hormone is structurally characterized by an amino-terminal receptor binding domain (amino acid residues 1-152) predicted to encode a left-handed four-helix bundle structure, and a carboxyl-terminal domain (residues 153-335) that is remarkable for its abundant carbohydrate modification and a lack of homology to other proteins. To investigate the functional role of the carboxyl-terminal glycan domain, we generated truncated forms of murine TPO (TPO1-238, TPO1-174, and TPO1-152) and glycomuteins in which the predicted asparagine (N)-linked sites of glycosylation were sequentially mutated to glutamine (Q), and assayed their secretion and function by comparing them to the native sequence (TPO1-335). Following transient transfection of the corresponding cDNA expression vectors into mammalian cell lines, the secretory efficiencies of the proteins were compared with those of the native hormone. Transfection efficiencies were monitored by cotransfection and reporter gene assay, and TPO secretion was assessed by functional and immunologic assays. We found that full-length TPO was 5-29-fold more efficiently secreted than any of the truncated forms of the hormone in fibroblast and hepatocyte cell lines. Elimination of carboxyl-terminal sites of N-linked glycosylation had a minor impact on secretion of the protein. We conclude that the carboxyl-terminal domain of TPO serves the important role of enhancing secretion of the protein, and in this manner functions as a prosequence.

Pharmacokinetics and hematological effects of the PEGylated thrombopoietin peptide mimetic GW395058 in rats and monkeys after intravenous or subcutaneous administration

GW395058, a potent PEGylated peptide human thrombopoietin receptor (HuTPOr) agonist in vitro, is being evaluated for the treatment of thrombocytopenia. GW395058 shares no sequence homology with TPO. In this report the pharmacokinetics and hematological effects of GW395058 in rats and monkeys are described. Doses eliciting thrombocytosis in rodents (2 or 10 microg/kg s.c.) produced insufficient plasma concentration data for pharmacokinetic parameter estimate calculations. At higher i.v. doses in rats (500, 1,000 or 2,000 microg/kg) serum t1/2 (half-life) values were >20 h, and the area under the concentration time curve increased proportionally with dose. In cynomolgus monkeys GW395058 plasma t1/2 values ranged from 37 to 68 h after s.c. or i.v. dosing, and similar values were observed in rhesus monkeys following s.c. dosing. Rat platelet counts increased following 2 (1.6-fold) or 10 microg/kg (fourfold) s.c. doses. Cynomolgus and rhesus monkey platelet counts did not change significantly at comparable s.c. doses, but did increase slightly (<twofold) in cynomolgus monkeys following a 25 microg/kg s.c. dose and twofold following a 100 microg/kg s.c. dose. Because the plasma t1/2 of GW395058 is long in mouse, rat, dog, and monkey, yet the dose required to double platelet levels in monkeys is 50-fold that required in rats, differences in hematological responses may be due to interspecies differences in the interaction of GW395058 with the TPOr.

Production of thrombopoietin by human carcinomas and its novel isoforms

Thrombocytosis is occasionally seen in patients with carcinomas and has been assumed to be attributable to interleukin-6 or granulocyte-macrophage colony-stimulating factor produced by carcinoma cells. In this study, we clarified whether thrombopoietin (TPO) is involved in carcinoma-associated thrombocytosis. Expression of TPO mRNA was observed in the majority of 27 carcinoma cell lines as determined by reverse transcriptase-polymerase chain reaction (RT-PCR). There were 6 PCR products differing in size; sequence analysis showed the full-length TPO mRNA (TPO-1), 12- and 116-bp deleted variants (TPO-2 and TPO-3, respectively), and 3 novel isoforms (197- and 128-bp deleted forms and a 60-bp insert form of TPO-3; named TPO-4, TPO-5, and TPO-6, respectively). Of 27 lines, 24 expressed TPO-1 mRNA with various other isoforms. Culture supernatants of COS-1 cells transfected with TPO-5 or TPO-6 cDNA did not promote the proliferation of TPO-responsive cells, whereas Western blot analysis on the cell lysates demonstrated TPO-5 but not TPO-6 protein, suggesting poor extracellular secretion (TPO-5) or poor protein synthesis (TPO-6). TPO protein was detected in 10-fold concentrated culture supernatants of cells of these carcinoma lines, with a median concentration of 0.38 fmol/mL as evaluated by enzyme-linked immunosorbent assay. High blood TPO levels were observed with a median value of 3.46 fmol/mL (range, 0.34 to 8.67 fmol/mL) in patients with advanced carcinomas associated with thrombocytosis. These results indicate that thrombocytosis in patients with carcinomas might be caused, at least in part, by TPO produced by carcinoma cells.

Characterization of native human thrombopoietin in the blood of normal individuals and of patients with haematologic disorders

Thrombopoietin (TPO) isolated from thrombocytopenic plasma of various animal species has previously been shown to comprise only truncated forms of the molecule, presumably generated by proteolysis. Native TPO has now been partially purified from normal human plasma by immunoaffinity chromatography and was confirmed to be biologically active. Gel filtration in the presence of SDS revealed that TPO eluted in two peaks: a major peak corresponding to the elution position of fully glycosylated recombinant human TPO (rhTPO) consisting of 332 amino acid residues, and a minor peak corresponding to a smaller molecular size. Immunoblot analysis also revealed that most plasma-derived TPO migrated at the same position as fully glycosylated rhTPO, corresponding to a molecular size of approximately 80 to 100 kDa. Furthermore, the size distribution of circulating TPO in patients with various haematologic disorders did not differ markedly from that of plasma-derived TPO from healthy individuals. These results indicate that the truncation of circulating TPO is not related to disease pathophysiology, and that the predominant form of TPO in blood is a biologically active approximately 80- to 100-kDa species. The size distribution of TPO extracted from normal platelets was similar to that of TPO in plasma; the proportion of truncated TPO was decreased by prior incubation of platelets with hirudin. indicating that the endogenous truncated TPO, at least in platelet extract, was generated by thrombin-mediated cleavage.

Reversibility of heat-induced denaturation of the recombinant human megakaryocyte growth and development factor

PURPOSE

The present study was performed to examine the effect of solution conditions on the reversibility of the thermal denaturation of megakaryocyte growth and development factor (rHuMGDF).

METHODS

Changes in the far UV CD spectra of rHuMGDF with temperature were used to monitor the thermal denaturation of the protein, and the recovery of folded protein following a return to room temperature. The effect of protein concentration, scan rate, and buffer composition on thermal denaturation and on the reversibility were determined. Surface tension measurements were used to determine the effect of this unfolding reaction on the surface adsorption of the protein. Sedimentation velocity was used to assess recovery of native monomer and the size of soluble aggregates. In addition, monomeric protein remaining in solution after incubation at 37 degrees C for 2 weeks in either 10 mM imidazole of 10 mM phosphate was determined.

RESULTS

In phosphate buffer the rHuMGDF irreversibly precipitates upon unfolding under all the conditions examined. In imidazole the unfolding is at least partially reversible, with no visible precipitate seen; the degree of reversibility increased by lowering both protein and salt concentrations, and the amount of time spent at elevated temperature. In order to compare thermal unfolding occuring with different degrees of reversibility, the melting temperature was defined as the temperature at which melting begins. The melting temperature itself is relatively independent of the buffer composition, or experimental conditions. At low protein concentrations the protein stabilizer sucrose had a marginal effect on the thermal transition of rHuMGDF, while at protein concentrations of about 2 mg/ml the inclusion of sucrose increased the apparent melting temperature by about 4 degrees C, to that seen at low protein concentrations, but had little effect on the reversibility of denaturation. Inclusion of 1 or 2 M urea did not affect the reaction. Surface tension measurements of rHuMGDF solutions showed little difference before and after melting, and in the presence or absence of sucrose. When unfolding is irreversible, the MGDF appears to form soluble aggregates of tetramers to 14-mers, while under reversible conditions native monomer is recovered. More monomeric MGDF remained in solution following storage for 2 weeks at 37 degrees C in imidazole than in phosphate, in both the presence and absence of sucrose.

CONCLUSIONS

These results can be explained by assuming that thermal denaturation proceeds as a two-step reaction, the first step being the equilibrium between folded and unfolded states, while the second step is a slow irreversible aggregation. The different buffer systems affect the rate of the aggregation step, but not the intrinsic thermal stability nor the rate of the unfolding step.

PEGylation prevents the N-terminal degradation of megakaryocyte growth and development factor

PURPOSE

Determine the effect of PEGylation on in-vitro degradation for recombinant human Megakaryocyte Growth and Development Factor (rHuMGDF) in the neutral pH range.

METHODS

Degradation products were characterized by cation-exchange HPLC, N-terminal sequencing and mass spectrometry.

RESULTS

The main route of degradation was through non-enzymatic cyclization of the first two amino acids and subsequent cleavage to form a diketopiperazine and des(Ser, Pro)rHuMGDE This reaction was prevented by alkylation of the N-terminus by polyethylene glycol (PEG).

CONCLUSIONS

PEGylation of proteins is commonly performed to achieve increased in-vivo circulation half-lives. For rHuMGDF, an additional advantage of PEGylation was enhanced in-vitro shelf-life stability.