Activation of erythropoietin receptor in the absence of hormone by a peptide that binds to a domain different from the hormone binding site

Ameliorative effects of Portulaca oleracea L. (purslane) and its active constituents on nervous system disorders: A review

Nowadays, the global interest in the use of herbal medicines and their main components in developing novel effective medications with fewer adverse effects is rising. Precise medicinal plants have potential advantageous applications for several neurodegenerative disorders. Portulaca oleracea L. (purslane) belongs to the Portulacaceae Juss family. In folk medicine, it has been used as a febrifuge, antiseptic, vermifuge, and in treating arthritis, burns, cough, headache, intestine, stomach, liver disorders, as well as shortness of breath. Pharmacological investigations have also disclosed its antioxidant, anti-inflammatory, anti-apoptotic, immunomodulatory, antidepressant, anxiolytic, and neuroprotective properties. The current work prepared an updated and broad literature review on purslane highlighting its therapeutic effects on some nervous system disorders. It has been shown that P. oleracea and its active constituents have considerable neuroregenerative, neuroprotective, and antinociceptive properties. Accordingly, our team classified and discussed the outcomes of some nervous system disorders comprising Alzheimer's disease, Parkinson's disease, depression, epilepsy, anxiety, psychosis, drug dependence, hypoxia, and pain; although, additional preclinical and clinical assessments are necessary to reinforce the beneficial effects of purslane on nervous system disorders.

Discovery and Characterization of Nonpeptidyl Agonists of the Tissue-Protective Erythropoietin Receptor

Erythropoietin (EPO) and its receptor are expressed in a wide variety of tissues, including the central nervous system. Local expression of both EPO and its receptor is upregulated upon injury or stress and plays a role in tissue homeostasis and cytoprotection. High-dose systemic administration or local injection of recombinant human EPO has demonstrated encouraging results in several models of tissue protection and organ injury, while poor tissue availability of the protein limits its efficacy. Here, we describe the discovery and characterization of the nonpeptidyl compound STS-E412 (2-[2-(4-chlorophenoxy)ethoxy]-5,7-dimethyl-[1,2,4]triazolo[1,5-a]pyrimidine), which selectively activates the tissue-protective EPO receptor, comprising an EPO receptor subunit (EPOR) and the common β-chain (CD131). STS-E412 triggered EPO receptor phosphorylation in human neuronal cells. STS-E412 also increased phosphorylation of EPOR, CD131, and the EPO-associated signaling molecules JAK2 and AKT in HEK293 transfectants expressing EPOR and CD131. At low nanomolar concentrations, STS-E412 provided EPO-like cytoprotective effects in primary neuronal cells and renal proximal tubular epithelial cells. The receptor selectivity of STS-E412 was confirmed by a lack of phosphorylation of the EPOR/EPOR homodimer, lack of activity in off-target selectivity screening, and lack of functional effects in erythroleukemia cell line TF-1 and CD34(+) progenitor cells. Permeability through artificial membranes and Caco-2 cell monolayers in vitro and penetrance across the blood-brain barrier in vivo suggest potential for central nervous system availability of the compound. To our knowledge, STS-E412 is the first nonpeptidyl, selective activator of the tissue-protective EPOR/CD131 receptor. Further evaluation of the potential of STS-E412 in central nervous system diseases and organ protection is warranted.

Interferon gamma peptidomimetic targeted to hepatic stellate cells ameliorates acute and chronic liver fibrosis in vivo

Hepatic stellate cells play a crucial role in the pathogenesis of hepatic fibrosis. Thus, pharmacological inhibition of pro-fibrotic activities of these cells might lead to an effective therapy for this disease. Among the potent anti-fibrotics, interferon gamma (IFNγ), a proinflammatory cytokine, is highly efficacious but it failed in clinical trials due to the poor efficacy and multiple adverse effects attributed to the ubiquitous IFNγ receptor (IFNγR) expression. To resolve these drawbacks, we chemically synthesized a chimeric molecule containing (a) IFNγ signaling peptide (IFNγ peptidomimetic, mimγ) that retains the agonistic activities of IFNγ but lacks an extracellular receptor recognition sequence for IFNγR; coupled via heterobifunctional PEG linker to (b) bicyclic platelet derived growth factor beta receptor (PDGFβR)-binding peptide (BiPPB) to induce internalization into the stellate cells that express PDGFβR. The synthesized targeted IFNγ peptidomimetic (mimγ-BiPPB) was extensively investigated for its anti-fibrotic and adverse effects in acute and chronic CCl4-induced liver fibrosis models in mice. Treatment with mimγ-BiPPB, after the onset of disease, markedly inhibited both early and established hepatic fibrosis as reflected by a reduced intrahepatic α-SMA, desmin and collagen-I mRNA expression and protein levels. While untargeted mimγ and BiPPB had no effect, and native IFNγ only induced a moderate reduction. Additionally, no off-target effects, e.g. systemic inflammation, were found with mimγ-BiPPB, which were substantially observed in mice treated with native IFNγ. The present study highlights the beneficial effects of a novel BiPPB mediated cell-specific targeting of IFNγ peptidomimetic to the disease-inducing cells and therefore represents a highly potential therapeutic approach to treat fibrotic diseases.

Flow cytometric analysis of STAT5 phosphorylation and CD95 expression in CD4+ T lymphocytes treated with recombinant human erythropoietin

Erythropoietin receptor (EPO-R) appears on the cell surface in the early stages of erythropoiesis. It also has been found on human T and B lymphocytes and monocytes suggesting that EPO could directly influence these cells. Moreover, earlier reports have shown that treatment with recombinant human (rh) EPO in chronic renal failure (CRF) patients improves interleukin-2 production and restores CD4+ T lymphocyte functions. We decided to investigate possibility of direct action of rhEPO on these cells in vitro by phosphorylated signal transducer and activator of transcription 5 (pSTAT5) detection and changes in CD95 antigen expression observation. Flow cytometry was used for detection of pSTAT5 and CD95 expression in CD4+ T lymphocytes treated with rhEPO. Our results show that presence of rhEPO in cell culture of lymphocytes stimulated with anti-CD3 antibody increases percentage of CD4+ T lymphocytes expressing pSTAT5. Stimulating effect of rhEPO was dose dependent. RhEPO presence also increases CD95 expression on these cells but still activated T lymphocytes are resistant to CD95-mediated apoptosis. These observations show that EPO is able to directly influence CD4+ T lymphocytes' signaling pathways.

A diversity of antibody epitopes can induce signaling through the erythropoietin receptor

Stimulation of red cell production through agonism of the erythropoietin receptor (EpoR) has historically been accomplished through administration of erythropoietin (EPO), the native ligand. The short half-life of EPO has led to the development of a variety of other agonists, including antibodies. It is of considerable interest to understand how these agents might activate the EpoR and whether or not it is important to bind in a manner similar to the native ligand. The binding epitopes of a panel of eight agonistic, single-chain antibody (scFv-Fc) constructs were determined through scanning alanine mutagenesis as well as more limited arginine mutagenesis of the receptor. It was found that while some of these constructs bound to receptor epitopes shared by the ligand, others bound in completely unique ways. The use of a panel of agonists and scanning mutagenesis can define the critical binding regions for signaling; in the case of the EpoR, these regions were remarkably broad.

Erythropoietin does not attenuate cytokine production and inflammation in microglia--implications for the neuroprotective effect of erythropoietin in neurological diseases

Erythropoietin is a hematopoietic cytokine which is also produced in the brain under hypoxia. Since this pathology is associated with glial cell activation and release of cytotoxic molecules, we investigated the expression of EPO receptors (EPO-R) and effects of erythropoietin on microglial cell functions in vitro using RT-PCR, Western immunoblotting, nitric oxide measurement, tumor necrosis factor-alpha-(TNF-alpha)-ELISA and gel shift assay analyses. Furthermore, we examined if erythropoietin could modulate proliferation of microglia. As shown by reverse transcription-polymerase chain reaction and immunocytochemistry, rat microglial cells and the murine microglia cell line BV-2 express the EPO-R. However, EPO showed no effect on the release of the proinflammatory mediators' nitric oxide and TNF-alpha. Moreover, EPO was not able to reduce the LPS (lipopolysaccharide) stimulated translocation of the proinflammatory transcription factor NF-kappaB into the nucleus of murine microglia, but induced (3)H-thymidine incorporation into DNA of microglial cells. These results show that microglia are target cells for erythropoietin which possesses mitogenic, but not anti-inflammatory effects on microglia. Therefore, the well-documented neuroprotective effects of erythropoietin could not be ascribed to an anti-inflammatory effect on microglia.

Ribosomal synthesis of nonstandard peptidesThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Systems and Chemical Biology, and has undergone the Journal's usual peer review process

It is well known that standard peptides, which comprise proteinogenic amino acids, can act as specific chemical probes to target proteins with high affinity. Despite this fact, a number of peptide drug leads have been abandoned because of their poor cell permeability and protease instability. On the other hand, nonstandard peptides isolated as natural products often exhibit remarkable pharmaco-behavior and stability in vivo. Although it is likely that numerous nonstandard therapeutic peptides capable of recognizing various targets could have been synthesized, enzymes for nonribosomal peptide syntheses are complex; therefore, it is difficult to engineer such modular enzymes to build nonstandard peptide libraries. Here we describe an emerging technology for the synthesis of nonstandard peptides that employs an integrated system of reconstituted cell-free translation and flexizymes. We summarize the historical background of this technology and discuss its current and future applications to the synthesis of nonstandard peptides and drug discovery.

A potent erythropoietin-mimicking human antibody interacts through a novel binding site

Recombinant human erythropoietin (rHu-EPO) is used to treat anemia by activating the erythropoietin receptor (EPOR) in erythroid progenitor cells, leading to proliferation and differentiation into mature red blood cells. To allow less frequent dosing, a hyperglycosylated version of EPO has been developed with a longer half-life. In principle, an agonistic antibody targeting EPOR would offer an even longer half-life, support robust monthly dosing, and, unlike EPO products, reduce the risk of pure red cell aplasia. The efficiency of signaling and corresponding potency of previously reported antibody mimics are generally suboptimal compared with EPO and not suitable for clinical use. Here we describe a potent, fully human, agonistic antibody (ABT007) targeting EPOR that supports potent, more sustained, and less pulsatile elevation of hematocrit in a human EPOR–expressing transgenic mouse model compared with standard doses of rHu-EPO while requiring less frequent dosing. Resolution of the crystal structure of the EPOR extracellular domain (ECD) complexed to the ABT007 Fab fragment, determined at 0.32 nm, identifies a binding site that is consistent with a novel mechanism of receptor activation based on a unique antibody-imposed conformational change. These results demonstrate that a symmetric molecule can serve as a potent activator of the EPOR.

Post-ischemic treatment with erythropoietin or carbamylated erythropoietin reduces infarction and improves neurological outcome in a rat model of focal cerebral ischemia

BACKGROUND AND PURPOSE

Recombinant human erythropoietin (rhEPO; Epoetin-alpha; PROCRITtrade mark) has been shown to exert neuroprotective and restorative effects in a variety of CNS injury models. However, limited information is available regarding the dose levels required for these beneficial effects or the neuronal responses that may underlie them. Here we have investigated the dose-response to rhEPO and compared the effects of rhEPO with those of carbamylated rhEPO (CEPO) in a model of cerebral stroke in rats.

EXPERIMENTAL APPROACH

Rats subjected to embolic middle cerebral artery occlusion (MCAo) were treated with rhEPO or CEPO, starting at 6 h and repeated at 24 and 48 h, after MCAo. Cerebral infarct volumes were assessed at 28 days and neurological impairment at 7, 14, 21 and 28 days, post-MCAo.

KEY RESULTS

rhEPO at dose levels of 500, 1150 or 5000 IU kg(-1) or CEPO at a dose level of 50 microg kg(-1) significantly reduced cortical infarct volume and reduced neurologic impairment. All doses of rhEPO, but not CEPO, produced a transient increase in haematocrit, while rhEPO and CEPO substantially reduced the number of apoptotic cells and activated microglia in the ischemic boundary region.

CONCLUSIONS AND IMPLICATIONS

These data indicate that rhEPO and CEPO have anti-inflammatory and anti-apoptotic effects, even with administration at 6 h following embolic MCAo in rats. Taken together, these actions of rhEPO and CEPO are likely to contribute to their reduction of neurologic impairment following cerebral ischemia.

Small-molecule agonists for the glucagon-like peptide 1 receptor

The peptide hormone glucagon-like peptide (GLP)-1 has important actions resulting in glucose lowering along with weight loss in patients with type 2 diabetes. As a peptide hormone, GLP-1 has to be administered by injection. Only a few small-molecule agonists to peptide hormone receptors have been described and none in the B family of the G protein coupled receptors to which the GLP-1 receptor belongs. We have discovered a series of small molecules known as ago-allosteric modulators selective for the human GLP-1 receptor. These compounds act as both allosteric activators of the receptor and independent agonists. Potency of GLP-1 was not changed by the allosteric agonists, but affinity of GLP-1 for the receptor was increased. The most potent compound identified stimulates glucose-dependent insulin release from normal mouse islets but, importantly, not from GLP-1 receptor knockout mice. Also, the compound stimulates insulin release from perfused rat pancreas in a manner additive with GLP-1 itself. These compounds may lead to the identification or design of orally active GLP-1 agonists.

Multivalency—a way to enhance binding avidities and bioactivity—preliminary applications to EPO

Multivalency has advantages over monovalency for binding interactions and even for activity. In particular, avidity is higher since the off-rate of a multivalent species is much slower than that of a monomer. This is particularly profitable for ligand-binding receptors that require dimerization for activity, such as the receptor of erythropoietin (EPOR). Peptides that mimic the action of erythropoietin (EPO) have been described with no sequence similarity with the human hormone: erythropoietin mimetic peptide (EMP) and EPO receptor peptide (ERP). These two peptides have similar activity but interact through different sites on the EPOR. Here, we describe the construction of several new synthetic homo- and hetero-dimers based on EMP-ERP sequences. To link the monomeric molecules together, several monodisperse polyamide linkers of different lengths were synthesized with dialdehyde functionalities. Chemoselective oxime chemistry was used to obtain homogeneous constructs. Certain chemical incompatibilities were dealt with via a protection approach. The oximes are stable under normal conditions and so lend themselves to biological testing.

Reversal of antigen-dependent signaling by two mutations in antibody/receptor chimera: implication of inverse agonism in cytokine receptor superfamily

Understanding the receptor activation mechanism is essential for the rational design of pharmacologically active ligand molecules. However, the activation mechanism of most cytokine receptors remains still unclear, and while agonism and antagonism have been described for ligand-mimetic peptides, there has been no report of inverse agonism that has been characterized for G protein-coupled receptors (GPCRs). To explore the activation mechanism of cytokine receptors, here we tried to investigate how agonism and antagonism could be altered by randomizing antibody variable region of an antibody/cytokine receptor chimera recognizing hen egg lysozyme (HEL) as an agonist. Based on our previous finding that the co-expression of V(H)-gp130 and V(L)-erythropoietin receptor (EpoR) chimeras transduced strict and efficient HEL-dependent cell growth signal, a V(H)-gp130 library encoding four randomized CDR2 residues was retrovirally infected to IL-3-dependent Ba/F3 cells already transfected with V(L)-EpoR. The selection without IL-3 resulted in a clonal expansion of the transduced cells, and interestingly some of which showed HEL dose-dependent growth suppression. Our results clearly indicate that agonism and antagonism of the antibody/cytokine receptor chimera can be readily switched by a subtle modification of the ligand binding domain as well as that of GPCRs, also implying the existence of inverse agonism in cytokine receptor superfamily.

Drugs for increasing oxygen and their potential use in doping: a review

Blood oxygenation is a fundamental factor in optimising muscular activity. Enhancement of oxygen delivery to tissues is associated with a substantial improvement in athletic performance, particularly in endurance sports. Progress in medical research has led to the identification of new chemicals for the treatment of severe anaemia. Effective and promising molecules have been created and sometimes used for doping purposes. The aim of this review is to present methods, and drugs, known to be (or that might be) used by athletes to increase oxygen transport in an attempt to improve endurance capacity. These methods and drugs include: (i) blood transfusion; (ii) endogenous stimulation of red blood cell production at altitude, or using hypoxic rooms, erythropoietins (EPOs), EPO gene therapy or EPO mimetics; (iii) allosteric effectors of haemoglobin; and (iv) blood substitutes such as modified haemoglobin solutions and perfluorochemicals. Often, new chemicals are used before safety tests have been completed and athletes are taking great health risks. Such new chemicals have also created the need for new instrumental strategies in doping control laboratories, but not all of these chemicals are detectable. Further progress in analytical research is necessary.

Recombinant human thrombopoietin: basic biology and evaluation of clinical studies

Thrombocytopenia is a common medical problem for which the main treatment is platelet transfusion. Given the increasing use of platelets and the declining donor population, identification of a safe and effective platelet growth factor could improve the management of thrombocytopenia. Thrombopoietin (TPO), the c-Mpl ligand, is the primary physiologic regulator of megakaryocyte and platelet development. Since the purification of TPO in 1994, 2 recombinant forms of the c-Mpl ligand--recombinant human thrombopoietin (rhTPO) and pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF)--have undergone extensive clinical investigation. Both have been shown to be potent stimulators of megakaryocyte growth and platelet production and are biologically active in reducing the thrombocytopenia of nonmyeloablative chemotherapy. However, neither TPO has demonstrated benefit in stem cell transplantation or leukemia chemotherapy. Other clinical studies have investigated the use of TPO in treating chronic nonchemotherapy-induced thrombocytopenia associated with myelodysplastic syndromes, idiopathic thrombocytopenic purpura, thrombocytopenia due to human immunodeficiency virus, and liver disease. Based solely on animal studies, TPO may be effective in reducing surgical thrombocytopenia and bleeding, ex vivo expansion of pluripotent stem cells, and as a radioprotectant. Ongoing and future studies will help define the clinical role of recombinant TPO and TPO mimetics in the treatment of chemotherapy- and nonchemotherapy-induced thrombocytopenia.

Receptor dimerization in GH and erythropoietin action--it takes two to tango, but how?

The receptors for GH and erythropoietin are members of the cytokine receptor superfamily. They are single membrane-spanning proteins that bind ligand in the extracellular domain and couple to cytosolic JAK tyrosine kinases to initiate signaling. The ligand-engaged GH receptor (GHR) and erythropoietin receptor (EpoR) extracellular domains are believed to exist in a dimerized configuration in which a single ligand molecule engages two receptor extracellular domains. The last several years have witnessed a rapid expansion in our knowledge of the structural and functional details of this dimerization process and have forced a reexamination of how the ligand-containing complexes achieve their conformation. For EpoR, there is good evidence that the unliganded receptor is already a preformed dimer that is activated by a ligand-induced change in the receptor conformation. Owing in some measure to the unavailability of the analogous crystal structure of the unliganded GHR extracellular domain, it is still unknown whether GHR adopts a similar preformed dimer/conformational change in response to GH as is found for EpoR. This review critically examines the state of our knowledge pertaining to GHR and EpoR dimerization, noting differences and similarities between the two.

Structural and functional hot spots in cytokine receptors

The activation of cytokine receptors is a stepwise process that depends on their specific interaction with cognate cytokines, the formation of oligomeric receptor complexes, and the initiation of cytoplasmic phosphorylation events. The recent determination of the structure of extracellular domains of several cytokine receptors allows comparison of their cytokine-binding surfaces. This comparison reveals a common structural framework that supports considerable diversity and adaptability of the binding surfaces that determine both the specificity and the orientation of subunits in the active receptor complex. These regions of the cytokine receptors have been targeted for the development of specific agonists and antagonists. The physical coupling of signaling intermediates to the intracellular domains of their receptors plays a major role in determining biological responses to cytokines. In this review, we focus principally on the receptors for cytokines of the granulocyte-macrophage colony-stimulating factor (GM-CSF) family and, where appropriate, compare them with related cytokine receptors. Several paradigms are beginning to emerge that focus on the ability of the extracellular portion of the cytokine receptor to recognize the appropriate cytokine and on a phosphorylated motif in the intracellular region of the GM-CSF receptor that couples to a specific signaling pathway.

Discovery of cytokine mimics using cell-based systems

The successful cloning and subsequent clinical application of recombinant cytokines and/or growth factors has generated a number of important therapeutics. In contrast to the G-protein-coupled receptors, identification of small-molecule agonists of the cytokine and/or growth factor receptor family has proved difficult. The first small peptides and non-peptidic small-molecule agonists for several receptors have recently been reported. The initial identification and/or crucial characterization of these molecules as true mimics was dependent on the use of cell-based functional assays. This article will review recent cell-based assay technologies that are suitable for HTS and that are being applied to the discovery of novel cytokine and growth factor mimics.

Synthesis and erythropoietin receptor binding affinities of N,N-disubstituted amino acids

N,N-Dicinnamyl, N-benzyl-N-cinnamyl, and N,N-dibenzyl amino acids were prepared and evaluated in an EPO binding assay. Several derivatives of aspartic acid, glutamic acid, and lysine exhibited moderate (10-50 microM) affinity for EBP; 'dimerization' of the most potent analogues by coupling with linear diamines led to EPO competitors having 1-2 microM binding affinities.