Both proteasomes and lysosomes degrade the activated erythropoietin receptor

Growth hormone is involved in GATA1 gene expression via STAT5B in human erythroleukemia and monocytic cell lines

GATAs are a family of transcription factors consisting of six members. Particularly, GATA1 and GATA2 have been reported to promote the development of erythrocytes, megakaryocytes, eosinophils, and mast cells. However, little information is available on the extracellular ligands that promote GATA1 expression. We evaluated whether growth hormone (GH) is an extracellular stimulator that participates in the signal transduction of GATAs, focusing on GATA1 expression in hematopoietic cell lineages. We used a reporter assay, RT-PCR, real-time quantitative PCR, and western blotting to evaluate GH-induced expression of GATA1 and GATA2 in the human erythroleukemic cell line K562 and the non-erythroid cell line U937. GATA1 expression in these hematopoietic cell lines increased at the transcriptional and protein levels in the presence of GH, and was inhibited by a STAT5 specific inhibitor. Cells transfected with activated STAT5B showed increased expression of GATA1. We identified functional STAT5B consensus sequences as binding site-158 bp from the transcription starting site in the GATA1 promoter region. These results suggest that GH directly induces GATA1 expression via GHR/JAK/STAT5 and is related to hematopoietic cell proliferation.

Canonical and Non-Canonical Functions of Erythropoietin and Its Receptor in Mature Nucleated Erythrocytes of Western Clawed Frog, Xenopus tropicalis

Enucleated erythrocytes are characteristic of adult mammals. In contrast, fish, amphibians, reptiles, birds, and fetal mammals possess nucleated erythrocytes in their circulation. Erythroid maturation is regulated by erythropoietin (EPO) and its receptor (EPOR), which are conserved among vertebrates. In mammals, EPOR on the erythroid progenitor membrane disappears after terminal differentiation. However, in western clawed frog, Xenopus tropicalis, mature erythrocytes maintain EPOR expression, suggesting that they have non-canonical functions of the EPO-EPOR axis rather than proliferation and differentiation. In this study, we investigated the non-canonical functions of EPOR in Xenopus mature erythrocytes. EPO stimulation of peripheral erythrocytes did not induce proliferation but induced phosphorylation of intracellular proteins, including signal transducer and activator of transcription 5 (STAT5). RNA-Seq analysis of EPO-stimulated peripheral erythrocytes identified 45 differentially expressed genes (DEGs), including cytokine inducible SH2 containing protein gene (cish) and suppressor of cytokine signaling 3 gene (socs3), negative regulators of the EPOR-Janus kinase (JAK)-STAT pathway. These phosphorylation studies and pathway analysis demonstrated the activation of the JAK-STAT pathway through EPO-EPOR signaling in erythrocytes. Through comparison with EPO-responsive genes in mouse erythroid progenitors obtained from a public database, we identified 31 novel EPO-responsive genes indicating non-canonical functions. Among these, we focused on ornithine decarboxylase 1 gene (odc1), which is the rate-limiting enzyme in polyamine synthesis and affects hematopoietic progenitor differentiation and the endothelial cell response to hypoxic stress. An EPO-supplemented culture of erythrocytes showed increased odc1 expression followed by a decrease in polyamine-rich erythrocytes, suggesting EPO-responsive polyamine excretion. These findings will advance our knowledge of the unknown regulatory systems under the EPO-EPOR axis and functional differences between vertebrates' nucleated and enucleated erythrocytes.

Luspatercept stimulates erythropoiesis, increases iron utilization, and redistributes body iron in transfusion-dependent thalassemia

Luspatercept, a ligand-trapping fusion protein, binds select TGF-β superfamily ligands implicated in thalassemic erythropoiesis, promoting late-stage erythroid maturation. Luspatercept reduced transfusion burden in the BELIEVE trial (NCT02604433) of 336 adults with transfusion-dependent thalassemia (TDT). Analysis of biomarkers in BELIEVE offers novel physiological and clinical insights into benefits offered by luspatercept. Transfusion iron loading rates decreased 20% by 1.4 g (~7 blood units; median iron loading rate difference: -0.05 ± 0.07 mg Fe/kg/day, p< .0001) and serum ferritin (s-ferritin) decreased 19.2% by 269.3 ± 963.7 μg/L (p < .0001), indicating reduced macrophage iron. However, liver iron content (LIC) did not decrease but showed statistically nonsignificant increases from 5.3 to 6.7 mg/g dw. Erythropoietin, growth differentiation factor 15, soluble transferrin receptor 1 (sTfR1), and reticulocytes rose by 93%, 59%, 66%, and 112%, respectively; accordingly, erythroferrone increased by 51% and hepcidin decreased by 53% (all p < .0001). Decreased transfusion with luspatercept in patients with TDT was associated with increased erythropoietic markers and decreasing hepcidin. Furthermore, s-ferritin reduction associated with increased erythroid iron incorporation (marked by sTfR1) allowed increased erythrocyte marrow output, consequently reducing transfusion needs and enhancing rerouting of hemolysis (heme) iron and non-transferrin-bound iron to the liver. LIC increased in patients with intact spleens, consistent with iron redistribution given the hepcidin reduction. Thus, erythropoietic and hepcidin changes with luspatercept in TDT lower transfusion dependency and may redistribute iron from macrophages to hepatocytes, necessitating the use of concomitant chelator cover for effective iron management.

Local YB-1, Epo, and EpoR concentrations in fractured bones: results from a porcine model of multiple trauma

Little is known about the impact of multiple trauma (MT)-related systemic hypoxia on osseous protein concentration of the hypoxia transcriptome. To shed light on this issue, we investigated erythropoietin (Epo), erythropoietin receptor (EpoR), and Y-box binding protein 1 (YB-1) concentrations in the fracture zone in a porcine MT + traumatic hemorrhage (TH) model. Sixteen male domestic pigs were randomized into two groups: an MT + TH group and a sham group. A tibia fracture, lung contusion, and TH were induced in the MT + TH group. The total observation period was 72 h. YB-1 concentrations in bone marrow (BM) were significantly lower in the fracture zone of the MT + TH animals than in the sham animals. Significant downregulation of BM-localized EpoR concentration in both unfractured and fractured bones was observed in the MT + TH animals relative to the sham animals. In BM, Epo concentrations were higher in the fracture zone of the MT + TH animals compared with that in the sham animals. Significantly higher Epo concentrations were detected in the BM of fractured bone compared to that in cortical bone. Our results provide the first evidence that MT + TH alters hypoxia-related protein concentrations. The impacts of both the fracture and concomitant injuries on protein concentrations need to be studied in more detail to shed light on the hypoxia transcriptome in fractured and healthy bones after MT + TH.

Red blood cell proteomics reveal remnant protein biosynthesis and folding pathways in PIEZO1-related hereditary xerocytosis

Hereditary xerocytosis is a dominant red cell membrane disorder characterized by an increased leak of potassium from the inside to outside the red blood cell membrane, associated with loss of water leading to red cell dehydration and chronic hemolysis. 90% of cases are related to heterozygous gain of function mutations in PIEZO1, encoding a mechanotransductor that translates a mechanical stimulus into a biological signaling. Data are still required to understand better PIEZO1-HX pathophysiology. Recent studies identified proteomics as an accurate and high-input tool to study erythroid progenitors and circulating red cell physiology. Here, we isolated red blood cells from 5 controls and 5 HX patients carrying an identified and pathogenic PIEZO1 mutation and performed a comparative deep proteomic analysis. A total of 603 proteins were identified among which 56 were differentially expressed (40 over expressed and 16 under expressed) between controls and HX with a homogenous expression profile within each group. We observed relevant modifications in the protein expression profile related to PIEZO1 mutations, identifying two main "knots". The first contained both proteins of the chaperonin containing TCP1 complex involved in the assembly of unfolded proteins, and proteins involved in translation. The second contained proteins involved in ubiquitination. Deregulation of proteins involved in protein biosynthesis was also observed in in vitro-produced reticulocytes after Yoda1 exposure. Thus, our work identifies significant changes in the protein content of PIEZO1-HX erythrocytes, revealing a "PIEZO1 signature" and identifying potentially targetable pathways in this disease characterized by a heterogeneous clinical expression and contra-indication of splenectomy.

Protection of insect neurons by erythropoietin/CRLF3-mediated regulation of pro-apoptotic acetylcholinesterase

Cytokine receptor-like factor 3 (CRLF3) is a conserved but largely uncharacterized orphan cytokine receptor of eumetazoan animals. CRLF3-mediated neuroprotection in insects can be stimulated with human erythropoietin. To identify mechanisms of CRLF3-mediated neuroprotection we studied the expression and proapoptotic function of acetylcholinesterase in insect neurons. We exposed primary brain neurons from Tribolium castaneum to apoptogenic stimuli and dsRNA to interfere with acetylcholinesterase gene expression and compared survival and acetylcholinesterase expression in the presence or absence of the CRLF3 ligand erythropoietin. Hypoxia increased apoptotic cell death and expression of both acetylcholinesterase-coding genes ace-1 and ace-2. Both ace genes give rise to single transcripts in normal and apoptogenic conditions. Pharmacological inhibition of acetylcholinesterases and RNAi-mediated knockdown of either ace-1 or ace-2 expression prevented hypoxia-induced apoptosis. Activation of CRLF3 with protective concentrations of erythropoietin prevented the increased expression of acetylcholinesterase with larger impact on ace-1 than on ace-2. In contrast, high concentrations of erythropoietin that cause neuronal death induced ace-1 expression and hence promoted apoptosis. Our study confirms the general proapoptotic function of AChE, assigns a role of both ace-1 and ace-2 in the regulation of apoptotic death and identifies the erythropoietin/CRLF3-mediated prevention of enhanced acetylcholinesterase expression under apoptogenic conditions as neuroprotective mechanism.

A Full Target-Mediated Drug Disposition (TMDD) Model to Explain the Changes in Recombinant Human Erythropoietin (rhEpo) Pharmacokinetics in Patients with Different Bone Marrow Integrity Following Hematopoietic Transplantation

Our aim was to build a mechanistic full target-mediated drug disposition (TMDD) model for rhEpo to better understand rhEpo disposition, Epo receptor (EpoR) synthesis, and degradation in hematopoietic transplant patients with four distinct bone marrow conditions. All PK data were analyzed simultaneously using the nonlinear mixed effect modeling approach with NONMEM. The final model was a two-compartmental full TMDD model, which adequately characterizes rhEpo PK in patients and provides insight into the dynamics of free EpoR, rhEpo-EpoR, and total EpoR. The model predicted association rate constant (kon), dissociation rate constant (koff), and internalization rate constant (kint) were 0.0276 pM-1h-1, 0.647 h-1, and 0.255h-1, respectively, which were supported by experimental data. Also, the EpoR degradation rate constant (kdeg) was estimated to be 0.461 h-1. EpoR production rate was estimated to be 37.5 pM/h for adults at pre-ablation baseline and 5.91 pM/h, and 4.19 pM/h in the early post-transplant post-engraftment, and late post-transplant full engraftment. Our model provides extensive information on the dynamics of free EpoR, total EpoR and rhEpo-EpoR, and proven to be more robust and can provide more physiologically relevant binding parameters than previous models.

Enhancing Erythropoiesis by a Phytoestrogen Diarylheptanoid from Curcuma comosa

Erythropoietin (Epo) is widely used for the treatment of anemia; however, non-hematopoietic effects and cancer risk limit its clinical applications. Therefore, alternative molecules to improve erythropoiesis in anemia patients are urgently needed. Here, we investigated the potential effects of a phytoestrogen diarylheptanoid (3R)-1,7-diphenyl-(4E,6E)-4,6-heptadien-3-ol, (ASPP 049) isolated from Curcuma comosa on promoting erythropoiesis. Treatment with C. comosa extract improved anemia symptoms demonstrated by increasing red blood cell numbers, hematocrit, and hemoglobin content in anemic mice. In addition, ASPP 049, the major compound isolated from C. comosa, enhanced the suboptimal Epo dosages to improve erythroid cell differentiation from hematopoietic stem cells, which was inhibited by the estrogen receptor (ER) antagonist, ICI 182,780. Moreover, the ASPP 049-activated Epo-Epo receptor (EpoR) complex subsequently induced phosphorylation of EpoR-mediated erythropoiesis pathways: STAT5, MAPK/ERK, and PI3K/AKT in Epo-sensitive UT-7 cells. Taken together, these results suggest that C. comosa extract and ASPP 049 increased erythropoiesis through ER- and EpoR-mediated signaling cascades. Our findings provide insight into the specific interaction between a phytoestrogen diarylheptanoid and Epo-EpoR in a hematopoietic system for the potential treatment of anemia.

Second-generation non-hematopoietic erythropoietin-derived peptide for neuroprotection

Erythropoietin (EPO) is a well-known erythropoietic cytokine having a tissue-protective effect in various tissues against hypoxic stress, including the brain. Thus, its recombinants may function as neuroprotective compounds. However, despite considerable neuroprotective effects, the EPO-based therapeutic approach has side effects, including hyper-erythropoietic and tumorigenic effects. Therefore, some modified forms and derivatives of EPO have been proposed to minimize the side effects. In this study, we generated divergently modified new peptide analogs derived from helix C of EPO, with several amino acid replacements that interact with erythropoietin receptors (EPORs). This modification resulted in unique binding potency to EPOR. Unlike recombinant EPO, among the peptides, ML1-h3 exhibited a potent neuroprotective effect against oxidative stress without additional induction of cell-proliferation, owing to a differential activating mode of EPOR signaling. Furthermore, it inhibited neuronal death and brain injury under hypoxic stress in vitro and in an in vivo ischemic brain injury model. Therefore, the divergent modification of EPO-derivatives for affinity to EPOR could provide a basis for a more advanced and optimal neuroprotective strategy.

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Short peptides derived from helix C of EPO have a neuroprotective effect.

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Divergent modification of EPO-derived peptides has a differential affinity to EPOR.

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ML1 and its analogs have differential cell protective and proliferative effects.

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ML1-h3 protects neurons by suppressing in vitro oxidative stress.

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ML1-h3 mitigates brain injury in the in vivo mouse ischemic model without hematopoietic effect.

Iron-loaded transferrin potentiates erythropoietin effects on erythroblast proliferation and survival: a novel role through transferrin receptors

Red blood cell production, or erythropoiesis, is a proliferative process that requires tight regulation. Erythropoietin (Epo) is a glycoprotein cytokine that plays a major role in erythropoiesis by triggering erythroid progenitors/precursors of varying sensitivity. The concentration of Epo in bone marrow is hypothesized to be suboptimal, and the survival of erythroid cells has been suggested to depend on Epo sensitivity. However, the key factors that control Epo sensitivity remain unknown. Two types of transferrin receptors (TfRs), TfR1 and TfR2, are known to play a role in iron uptake in erythroid cells. Here, we hypothesized that TfRs may additionally modulate Epo sensitivity during erythropoiesis by modulating Epo receptor (EpoR) signaling. Using an Epo-sensitive UT-7 (UT7/Epo) erythroid cell and human erythroid progenitor cell models, we report that iron-loaded transferrin, that is, holo-transferrin (holo-Tf), synergizes with suboptimal Epo levels to improve erythroid cell survival, proliferation, and differentiation. This is accomplished via the major signaling pathways of erythropoiesis, which include signal transducer and activator of transcription 5 (STAT5), mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK), and phosphoinositide-3-kinase (PI3K)/AKT. Furthermore, we found that this cooperation is improved by, but does not require, the internalization of TfR1. Interestingly, we observed that loss of TfR2 stabilizes EpoR levels and abolishes the beneficial effects of holo-Tf. Overall, these data reveal novel signaling properties of TfRs, which involve the regulation of erythropoiesis through EpoR signaling.

TD-92, a novel erlotinib derivative, depletes tumor-associated macrophages in non-small cell lung cancer via down-regulation of CSF-1R and enhances the anti-tumor effects of anti-PD-1

Recent advances in immune checkpoint inhibition, which augment T-cell immune responses, have highlighted the potential of exploiting one's immune system to combat cancer. However, only a relatively small number of non-small cell lung cancer (NSCLC) patients benefit from immune checkpoint blockade due to the immunosuppressive tumor microenvironment. Therefore, combination immunotherapies are now being developed to achieve maximal therapeutic benefits. In this study, we assessed whether a novel erlotinib derivative, TD-92, which possesses anti-tumor effects across several cancer cell lines, could enhance anti-PD-1 treatment. Our results demonstrated that the combined treatment of anti-PD-1 and TD-92 resulted in a potent anti-tumor response in a Lewis lung carcinoma cancer model, as evidenced by the reduced tumor growth and increased survival. Analysis of immune cell population counts revealed that TD-92 reduced the number of pro-tumorigenic CD11b⁺ F4/80⁺ tumor-associated macrophages, without significantly affecting the total numbers of other major immunocytes. Further experiments showed that TD-92 induced a marked decline in colony stimulating factor 1 receptor (CSF-1R) expression in macrophage cell lines. The results also suggested that c-Cbl-mediated proteasome degradation was involved in TD-92-mediated CSF-1R downregulation. Our data paves the way for the development of additional combination immunotherapies for NSCLC patients.

Role of protein tyrosine phosphatase 1B (PTP1B) in the increased sensitivity of endothelial cells to a promigratory effect of erythropoietin in an inflammatory environment

The proliferation and migration of endothelial cells are vascular events of inflammation, a process which can also potentiate the effects of promigratory factors. With the aim of investigating possible modifications in the activity of erythropoietin (Epo) in an inflammatory environment, we found that Epo at a non-promigratory concentration was capable of stimulating EA.hy926 endothelial cell migration when TNF-α was present. VCAM-1 and ICAM-1 expression, as well as adhesion of monocytic THP-1 cells to endothelial layers were also increased. Structurally modified Epo (carbamylation or N-homocysteinylation) did not exhibit these effects. The sensitizing effect of TNF-α on Epo activity was mediated by the Epo receptor. Inhibition assays targeting the PI3K/mTOR/NF-κB pathway, shared by Epo and TNF-α, show a cross-talk between both cytokines. As observed in assays using antioxidants, cell migration elicited by TNF-α + Epo depended on TNF-α-generated reactive oxygen species (ROS). ROS-mediated inactivation of protein tyrosine phosphatase 1B (PTP1B), involved in Epo signaling termination, could explain the synergistic effect of these cytokines. Our results suggest that ROS generated by inflammation inactivate PTP1B, causing the Epo signal to last longer. This mechanism, along with the cross-talk between both cytokines, could explain the sensitizing action of TNF-α on the migratory effect of Epo.

Management of anemia in patients with cancer: 2019 Italian Association of Medical Oncology (AIOM) guidelines

This article summarizes the latest Italian Medical Oncology Association (AIOM) guidelines on the management of cancer-related and chemotherapy-related anemia with a particular attention to the use of erythropoiesis-stimulating agents and iron supplementation.

The Effect of Size, Maturation, Global Asphyxia, Cerebral Ischemia, and Therapeutic Hypothermia on the Pharmacokinetics of High-Dose Recombinant Erythropoietin in Fetal Sheep

High-dose human recombinant erythropoietin (rEPO) is a promising potential neuroprotective treatment in preterm and full-term neonates with hypoxic-ischemic encephalopathy (HIE). There are limited data on the pharmacokinetics of high-dose rEPO in neonates. We examined the effects of body weight, gestation age, global asphyxia, cerebral ischemia, hypothermia and exogenous rEPO on the pharmacokinetics of high-dose rEPO in fetal sheep. Near-term fetal sheep on gestation day 129 (0.87 gestation) (full term 147 days) received sham-ischemia (n = 5) or cerebral ischemia for 30 min followed by treatment with vehicle (n = 4), rEPO (n = 8) or combined treatment with rEPO and hypothermia (n = 8). Preterm fetal sheep on gestation day 104 (0.7 gestation) received sham-asphyxia (n = 1) or complete umbilical cord occlusion for 25 min followed by i.v. infusion of vehicle (n = 8) or rEPO (n = 27) treatment. rEPO was given as a loading bolus, followed by a prolonged continuous infusion for 66 to 71.5 h in preterm and near-term fetuses. A further group of preterm fetal sheep received repeated bolus injections of rEPO (n = 8). The plasma concentrations of rEPO were best described by a pharmacokinetic model that included first-order and mixed-order elimination with linear maturation of elimination with gestation age. There were no detectable effects of therapeutic hypothermia, cerebral ischemia, global asphyxia or exogenous treatment on rEPO pharmacokinetics. The increase in rEPO elimination with gestation age suggests that to maintain target exposure levels during prolonged treatment, the dose of rEPO may have to be adjusted to match the increase in size and growth. These results are important for designing and understanding future studies of neuroprotection with high-dose rEPO.

PIEZO1 activation delays erythroid differentiation of normal and hereditary xerocytosis-derived human progenitor cells

Hereditary xerocytosis is a dominantly inherited red cell membrane disorder caused in most cases by gain-of-function mutations in PIEZO1, encoding a mechanosensitive ion channel that translates a mechanic stimulus into calcium influx. We found that PIEZO1 was expressed early in erythroid progenitor cells, and investigated whether it could be involved in erythropoiesis, besides having a role in the homeostasis of mature red cell hydration. In UT7 cells, chemical PIEZO1 activation using YODA1 repressed glycophorin A expression by 75%. This effect was PIEZO1-dependent since it was reverted using specific short hairpin-RNA knockdown. The effect of PIEZO1 activation was confirmed in human primary progenitor cells, maintaining cells at an immature stage for longer and modifying the transcriptional balance in favor of genes associated with early erythropoiesis, as shown by a high GATA2/GATA1 ratio and decreased α/β-globin expression. The cell proliferation rate was also reduced, with accumulation of cells in G0/G1 of the cell cycle. The PIEZO1-mediated effect on UT7 cells required calcium-dependent activation of the NFAT and ERK1/2 pathways. In primary erythroid cells, PIEZO1 activation synergized with erythropoietin to activate STAT5 and ERK, indicating that it may modulate signaling pathways downstream of erythropoietin receptor activation. Finally, we studied the in-vitro erythroid differentiation of primary cells obtained from 14 PIEZO1-mutated patients, from 11 families, carrying ten different mutations. We observed a delay in erythroid differentiation in all cases, ranging from mild (n=3) to marked (n=8). Overall, these data demonstrate a role for PIEZO1 during erythropoiesis, since activation of PIEZO1 - both chemically and through activating mutations - delays erythroid maturation, providing new insights into the pathophysiology of hereditary xerocytosis.

The filamin-B–refilin axis – spatiotemporal regulators of the actin-cytoskeleton in development and disease

During development, cycles of spatiotemporal remodeling of higher-order networks of actin filaments contribute to control cell fate specification and differentiation. Programs for controlling these dynamics are hard-wired into actin-regulatory proteins. The filamin family of actin-binding proteins exert crucial mechanotransduction and signaling functions in tissue morphogenesis. Filamin-B (FLNB) is a key player in chondrocyte progenitor differentiation for endochondral ossification. Biallelic loss-of-function mutations or gain-of-function mutations in FLNB cause two groups of skeletal disorders that can be attributed to either the loss of repressive function on TGF-β signaling or a disruption in mechanosensory properties, respectively. In this Review, we highlight a unique family of vertebrate-specific short-lived filamin-binding proteins, the refilins (refilin-A and refilin-B), that modulate filamin-dependent actin crosslinking properties. Refilins are downstream TGF-β effectors in epithelial cells. Double knockout of both refilin-A and refilin-B in mice results in precocious ossification of some axial skeletal elements, leading to malformations that are similar to those seen in FLNB-deficient mice. Based on these findings, we present a model summarizing the role of refilins in regulating the mechanosensory functions of FLNB during skeletal development. We also discuss the possible contribution of refilins to FLNB-related skeletal pathologies that are associated with gain-of-function mutations.

New pathogenic mechanisms induced by germline erythropoietin receptor mutations in primary erythrocytosis

Primary familial and congenital polycythemia is characterized by erythropoietin hypersensitivity of erythroid progenitors due to germline nonsense or frameshift mutations in the erythropoietin receptor gene. All mutations so far described lead to the truncation of the C-terminal receptor sequence that contains negative regulatory domains. Their removal is presented as sufficient to cause the erythropoietin hypersensitivity phenotype. Here we provide evidence for a new mechanism whereby the presence of novel sequences generated by frameshift mutations is required for the phenotype rather than just extensive truncation resulting from nonsense mutations. We show that the erythropoietin hypersensitivity induced by a new erythropoietin receptor mutant, p.Gln434Profs*11, could not be explained by the loss of negative signaling and of the internalization domains, but rather by the appearance of a new C-terminal tail. The latter, by increasing erythropoietin receptor dimerization, stability and cell-surface localization, causes pre-activation of erythropoietin receptor and JAK2, constitutive signaling and hypersensitivity to erythropoietin. Similar results were obtained with another mutant, p.Pro438Metfs*6, which shares the same last five amino acid residues (MDTVP) with erythropoietin receptor p.Gln434Profs*11, confirming the involvement of the new peptide sequence in the erythropoietin hypersensitivity phenotype. These results suggest a new mechanism that might be common to erythropoietin receptor frameshift mutations. In summary, we show that primary familial and congenital polycythemia is more complex than expected since distinct mechanisms are involved in the erythropoietin hypersensitivity phenotype, according to the type of erythropoietin receptor mutation.

Correlated receptor transport processes buffer single-cell heterogeneity

Cells typically vary in their response to extracellular ligands. Receptor transport processes modulate ligand-receptor induced signal transduction and impact the variability in cellular responses. Here, we quantitatively characterized cellular variability in erythropoietin receptor (EpoR) trafficking at the single-cell level based on live-cell imaging and mathematical modeling. Using ensembles of single-cell mathematical models reduced parameter uncertainties and showed that rapid EpoR turnover, transport of internalized EpoR back to the plasma membrane, and degradation of Epo-EpoR complexes were essential for receptor trafficking. EpoR trafficking dynamics in adherent H838 lung cancer cells closely resembled the dynamics previously characterized by mathematical modeling in suspension cells, indicating that dynamic properties of the EpoR system are widely conserved. Receptor transport processes differed by one order of magnitude between individual cells. However, the concentration of activated Epo-EpoR complexes was less variable due to the correlated kinetics of opposing transport processes acting as a buffering system.

Cell surface receptors translate extracellular ligand concentrations to intracellular responses. Receptor transport between the plasma membrane and other cellular compartments regulates the number of accessible receptors at the plasma membrane that determines the strength of downstream pathway activation at a given ligand concentration. In cell populations, pathway activation strength and cellular responses vary between cells. Understanding origins of cell-to-cell variability is highly relevant for cancer research, motivated by the problem of fractional killing by chemotherapies and development of resistance in subpopulations of tumor cells. The erythropoietin receptor (EpoR) is a characteristic example of a receptor system that strongly depends on receptor transport processes. It is involved in several cellular processes, such as differentiation or proliferation, regulates the renewal of erythrocytes, and is expressed in several tumors. To investigate the involvement of receptor transport processes in cell-to-cell variability, we quantitatively characterized trafficking of EpoR in individual cells by combining live-cell imaging with mathematical modeling. Thereby, we found that EpoR dynamics was strongly dependent on rapid receptor transport and turnover. Interestingly, although transport processes largely differed between individual cells, receptor concentrations in cellular compartments were robust to variability in trafficking processes due to the correlated kinetics of opposing transport processes.

Erythropoietin enhances Kupffer cell number and activity in the challenged liver

Erythropoietin (EPO) is the main hormone driving mammalian erythropoiesis, with activity mediated via the surface receptor, EPO-R, on erythroid progenitor cells. Recombinant human EPO is currently used clinically for the treatment of anemia in patients with end-stage renal disease, and in certain cancer patients suffering from anemia induced either by the tumor itself or by chemotherapy. EPO-R expression is also detected in non-erythroid cells, including macrophages present in the peritoneum, spleen, and bone marrow (BM). Here we demonstrate that Kupffer cells (KCs) - the liver-resident macrophages - are EPO targets. We show that, in vitro, EPO initiated intracellular signalling and enhanced phagocytosis in a rat KC line (RKC-2) and in sorted KCs. Moreover, continuous EPO administration in mice, resulted in an increased number of KCs, up-regulation of liver EPO-R expression and elevated production of the monocyte chemoattractant CCL2, with corresponding egress of Ly6C^hi monocytes from the BM. In a model of acute acetaminophen-induced liver injury, EPO administration increased the recruitment of Ly6C^hi monocytes and neutrophils to the liver. Taken together, our results reveal a new role for EPO in stimulating KC proliferation and phagocytosis, and in recruiting Ly6C^hi monocytes in response to liver injury.

Direct targets of pSTAT5 signalling in erythropoiesis

Erythropoietin (EPO) acts through the dimeric erythropoietin receptor to stimulate proliferation, survival, differentiation and enucleation of erythroid progenitor cells. We undertook two complimentary approaches to find EPO-dependent pSTAT5 target genes in murine erythroid cells: RNA-seq of newly transcribed (4sU-labelled) RNA, and ChIP-seq for pSTAT5 30 minutes after EPO stimulation. We found 302 pSTAT5-occupied sites: ~15% of these reside in promoters while the rest reside within intronic enhancers or intergenic regions, some >100kb from the nearest TSS. The majority of pSTAT5 peaks contain a central palindromic GAS element, TTCYXRGAA. There was significant enrichment for GATA motifs and CACCC-box motifs within the neighbourhood of pSTAT5-bound peaks, and GATA1 and/or KLF1 co-occupancy at many sites. Using 4sU-RNA-seq we determined the EPO-induced transcriptome and validated differentially expressed genes using dynamic CAGE data and qRT-PCR. We identified known direct pSTAT5 target genes such as Bcl2l1, Pim1 and Cish, and many new targets likely to be involved in driving erythroid cell differentiation including those involved in mRNA splicing (Rbm25), epigenetic regulation (Suv420h2), and EpoR turnover (Clint1/EpsinR). Some of these new EpoR-JAK2-pSTAT5 target genes could be used as biomarkers for monitoring disease activity in polycythaemia vera, and for monitoring responses to JAK inhibitors.

Cytokine Receptor Endocytosis: New Kinase Activity-Dependent and -Independent Roles of PI3K

Type I and II cytokine receptors are cell surface sensors that bind cytokines in the extracellular environment and initiate intracellular signaling to control processes such as hematopoiesis, immune function, and cellular growth and development. One key mechanism that regulates signaling from cytokine receptors is through receptor endocytosis. In this mini-review, we describe recent advances in endocytic regulations of cytokine receptors, focusing on new paradigms by which PI3K controls receptor endocytosis through both kinase activity-dependent and -independent mechanisms. These advances underscore the notion that the p85 regulatory subunit of PI3K has functions beyond regulating PI3K kinase activity, and that PI3K plays both positive and negative roles in receptor signaling. On the one hand, the PI3K/Akt pathway controls various aspects downstream of cytokine receptors. On the other hand, it stimulates receptor endocytosis and downregulation, thus contributing to signaling attenuation.

Lenalidomide Stabilizes the Erythropoietin Receptor by Inhibiting the E3 Ubiquitin Ligase RNF41

In a subset of patients with non-del(5q) myelodysplastic syndrome (MDS), lenalidomide promotes erythroid lineage competence and effective erythropoiesis. To determine the mechanism by which lenalidomide promotes erythropoiesis, we investigated its action on erythropoietin receptor (EpoR) cellular dynamics. Lenalidomide upregulated expression and stability of JAK2-associated EpoR in UT7 erythroid cells and primary CD71+ erythroid progenitors. The effects of lenalidomide on receptor turnover were Type I cytokine receptor specific, as evidenced by coregulation of the IL3-Rα receptor but not c-Kit. To elucidate this mechanism, we investigated the effects of lenalidomide on the E3 ubiquitin ligase RNF41. Lenalidomide promoted EpoR/RNF41 association and inhibited RNF41 auto-ubiquitination, accompanied by a reduction in EpoR ubiquitination. To confirm that RNF41 is the principal target responsible for EpoR stabilization, HEK293T cells were transfected with EpoR and/or RNF41 gene expression vectors. Steady-state EpoR expression was reduced in EpoR/RNF41 cells, whereas EpoR upregulation by lenalidomide was abrogated, indicating that cellular RNF41 is a critical determinant of drug-induced receptor modulation. Notably, shRNA suppression of CRBN gene expression failed to alter EpoR upregulation, indicating that drug-induced receptor modulation is independent of cereblon. Immunohistochemical staining showed that RNF41 expression decreased in primary erythroid cells of lenalidomide-responding patients, suggesting that cellular RNF41 expression merits investigation as a biomarker for lenalidomide response. Our findings indicate that lenalidomide has E3 ubiquitin ligase inhibitory effects that extend to RNF41 and that inhibition of RNF41 auto-ubiquitination promotes membrane accumulation of signaling competent JAK2/EpoR complexes that augment Epo responsiveness. Cancer Res; 76(12); 3531-40. ©2016 AACR.

Design Principles Involving Protein Disorder Facilitate Specific Substrate Selection and Degradation by the Ubiquitin-Proteasome System

The ubiquitin-proteasome system (UPS) regulates diverse cellular pathways by the timely removal (or processing) of proteins. Here we review the role of structural disorder and conformational flexibility in the different aspects of degradation. First, we discuss post-translational modifications within disordered regions that regulate E3 ligase localization, conformation, and enzymatic activity, and also the role of flexible linkers in mediating ubiquitin transfer and reaction processivity. Next we review well studied substrates and discuss that substrate elements (degrons) recognized by E3 ligases are highly disordered: short linear motifs recognized by many E3s constitute an important class of degrons, and these are almost always present in disordered regions. Substrate lysines targeted for ubiquitination are also often located in neighboring regions of the E3 docking motifs and are therefore part of the disordered segment. Finally, biochemical experiments and predictions show that initiation of degradation at the 26S proteasome requires a partially unfolded region to facilitate substrate entry into the proteasomal core.

Control of Pim2 kinase stability and expression in transformed human haematopoietic cells

The oncogenic Pim2 kinase is overexpressed in several haematological malignancies, such as multiple myeloma and acute myeloid leukaemia (AML), and constitutes a strong therapeutic target candidate. Like other Pim kinases, Pim2 is constitutively active and is believed to be essentially regulated through its accumulation. We show that in leukaemic cells, the three Pim2 isoforms have dramatically short half-lives although the longer isoform is significantly more stable than the shorter isoforms. All isoforms present a cytoplasmic localization and their degradation was neither modified by broad-spectrum kinase or phosphatase inhibitors such as staurosporine or okadaic acid nor by specific inhibition of several intracellular signalling pathways including Erk, Akt and mTORC1. Pim2 degradation was inhibited by proteasome inhibitors but Pim2 ubiquitination was not detected even by blocking both proteasome activity and protein de-ubiquitinases (DUBs). Moreover, Pyr41, an ubiquitin-activating enzyme (E1) inhibitor, did not stabilize Pim2, strongly suggesting that Pim2 was degraded by the proteasome without ubiquitination. In agreement, we observed that purified 20S proteasome particles could degrade Pim2 molecule in vitro. Pim2 mRNA accumulation in UT7 cells was controlled by erythropoietin (Epo) through STAT5 transcription factors. In contrast, the translation of Pim2 mRNA was not regulated by mTORC1. Overall, our results suggest that Pim2 is only controlled by its mRNA accumulation level. Catalytically active Pim2 accumulated in proteasome inhibitor-treated myeloma cells. We show that Pim2 inhibitors and proteasome inhibitors, such as bortezomib, have additive effects to inhibit the growth of myeloma cells, suggesting that Pim2 could be an interesting target for the treatment of multiple myeloma.

Eliminative signaling by Janus kinases: role in the downregulation of associated receptors

Activation of cytokine receptor-associated Janus kinases (JAKs) mediates most, if not all, of the cellular responses to peptide hormones and cytokines. Consequently, JAKs play a paramount role in homeostasis and immunity. Members of this family of tyrosine kinases control the cytokine/hormone-induced alterations in cell gene expression program. This function is largely mediated through an ability to signal toward activation of the signal transducer and activator of transcription proteins (STAT), as well as toward some other pathways. Importantly, JAKs are also instrumental in tightly controlling the expression of associated cytokine and hormone receptors, and, accordingly, in regulating the cell sensitivity to these cytokines and hormones. This review highlights the enzymatic and non-enzymatic mechanisms of this regulation and discusses the importance of the ambidextrous nature of JAK as a key signaling node that integrates the combining functions of forward signaling and eliminative signaling. Attention to the latter aspect of JAK function may contribute to emancipating our approaches to the pharmacological modulation of JAKs.

Clinical pharmacokinetics and pharmacodynamics of erythropoiesis-stimulating agents

The cloning of the EPO gene in the early 1980s allowed for the development of recombinant erythropoietins and analogues [erythropoiesis-stimulating agents (ESAs)], offering an alternative to transfusion as a method of raising haemoglobin (Hb) levels, which have been used for more than 20 years to treat anaemia in millions of anaemic patients. There are now a number of ESAs available worldwide for the treatment of anaemia, approved for different routes of administration (intravenous and subcutaneous) and dosing intervals (three times weekly, weekly, biweekly and monthly). In this review, we discuss the pharmacokinetic characteristics, including absorption, distribution and elimination processes, across the different ESAs. Incomplete and slow lymphatic absorption, with limited extravascular distribution, and minor contributions of the target-mediated drug disposition to the overall elimination are the common characteristics across the marketed ESA. Additionally, we assess the similarities and differences of ESAs related to pharmacodynamics in the context of the different biomarkers used to monitor the magnitude and duration of the effect, and introduce the concept of the minimum effective concentration of the ESA. The relationship between the minimum effective concentration and the half-life suggests that the time during which drug concentrations are above the minimum effective concentration is the main determinant of ESA efficacy in increasing Hb levels. The tolerance phenomenon and its physiological mechanism and implications for ESA dosing are discussed. Finally, the areas of future clinical pharmacology research are envisioned.

Lyn kinase plays important roles in erythroid expansion, maturation and erythropoietin receptor signalling by regulating inhibitory signalling pathways that control survival

In erythroid cells both positive viability signals and feedback inhibitory signalling require the Src family kinase Lyn, influencing cell survival and their ability to differentiate. This illustrates that Lyn is critical for normal erythropoiesis and erythroid cell development.

Cbl ubiquitination of p85 is essential for Epo-induced EpoR endocytosis

Erythropoietin (Epo) binding to the Epo receptor (EpoR) elicits downstream signaling that is essential for red blood cell production. One important negative regulatory mechanism to terminate Epo signaling is Epo-induced EpoR endocytosis and degradation. Defects in this mechanism play a key role in the overproduction of erythrocytes in primary familial and congenital polycythemia (PFCP). Here we have identified a novel mechanism mediating Epo-dependent EpoR internalization. Epo induces Cbl-dependent ubiquitination of the p85 regulatory subunit of PI3K, which binds to phosphotyrosines on EpoR. Ubiquitination allows p85 to interact with the endocytic protein epsin-1, thereby driving EpoR endocytosis. Knockdown of Cbl, expression of its dominant negative forms, or expression of an epsin-1 mutant devoid of ubiquitin-interacting motifs all compromise Epo-induced EpoR internalization. Mutated EpoRs mimicking those from PFCP patients cannot bind p85, co-localize with epsin-1, or internalize on Epo stimulation and exhibit Epo hypersensitivity. Similarly, knockdown of Cbl also causes Epo hypersensitivity in primary erythroid progenitors. Restoring p85 binding to PFCP receptors rescues Epo-induced epsin-1 co-localization and EpoR internalization and normalizes Epo hypersensitivity. Our results uncover a novel Cbl/p85/epsin-1 pathway in EpoR endocytosis and show that defects in this pathway contribute to excessive Epo signaling and erythroid hyperproliferation in PFCP.

SOCS-1 mediates ubiquitylation and degradation of GM-CSF receptor

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and the related cytokines interleukin (IL)-3 and IL-5 regulate the production and functional activation of hematopoietic cells. GM-CSF acts on monocytes/macrophages and granulocytes, and several chronic inflammatory diseases and a number of haematological malignancies such as Juvenile myelomonocytic leukaemia (JMML) are associated with deregulated GM-CSF receptor (GMR) signaling. The downregulation of GMR downstream signaling is mediated in part by the clearance of activated GMR via the proteasome, which is dependent on the ubiquitylation of βc signaling subunit of GMR via an unknown E3 ubiquitin ligase. Here, we show that suppressor of cytokine signaling 1 (SOCS-1), best known for its ability to promote ubiquitin-mediated degradation of the non-receptor tyrosine kinase Janus kinase 2 (JAK2), also targets GMRβc for ubiquitin-mediated degradation and attenuates GM-CSF-induced downstream signaling.

Genetically heterogeneous origins of idiopathic erythrocytosis

The idiopathic erythrocytosis (IE) group of disorders is defined by an absolute increase in red cell mass and hematocrit without elevation of the megakaryocytic or granulocytic lineages. It is associated with a wide range of serum erythropoietin (Epo) levels and broadly falls into groups of raised/inappropriately normal or low/undetectable Epo levels. A spectrum of molecular defects has been described in association with IE, which reflects the heterogeneity of this disorder. To date the most common identified cause of IE has been mutations in the von Hippel Landau (VHL) protein, which results in aberrant oxygen sensing and dysregulated Epo production. Studying the molecular basis of IE will provide insights into the control of Epo synthesis and Epo-induced signaling pathways.

Erythropoiesis stimulating agents: approaches to modulate activity

Recombinant human erythropoietin (rHuEPO), such as the approved agents epoetin alfa and epoetin beta, has been used successfully for over 20 years to treat anemia in millions of patients. However, due to the relatively short half-life of the molecule (approximately 8 hours), frequent dosing may be required to achieve required hemoglobin levels. Therefore, a need was identified in some anemic patient populations for erythropoiesis stimulating agents with longer half-lives that required less frequent dosing. This need led to the development of second generation molecules which are modified versions of rHuEPO with improved pharma-cokinetic and pharmacodynamic properties such as darbepoetin alfa, a hyperglycosylated analog of rHuEPO, and pegzyrepoetin, a pegylated rHuEPO. Third generation molecules, such as peginesatide, which are peptide mimetics that have no sequence homology to rHuEPO have also recently been developed. The various molecular, pharmacokinetic, and pharmacodynamic properties of these and other erythropoiesis stimulating agents will be discussed in this review.

JAK2 mutants (e.g., JAK2V617F) and their importance as drug targets in myeloproliferative neoplasms

The Janus kinase 2 (JAK2) mutant V617F and other JAK mutants are found in patients with myeloproliferative neoplasms and leukemias. Due to their involvement in neoplasia and inflammatory disorders, Janus kinases are promising targets for kinase inhibitor therapy. Several small-molecule compounds are evaluated in clinical trials for myelofibrosis, and ruxolitinib (INCB018424, Jakafi®) was the first Janus kinase inhibitor to receive clinical approval. In this review we provide an overview of JAK2V617F signaling and its inhibition by small-molecule kinase inhibitors. In addition, myeloproliferative neoplasms are discussed regarding the role of JAK2V617F and other mutant proteins of possible relevance. We further give an overview about treatment options with special emphasis on possible combination therapies.

Tyrosine kinase inhibitors induce down-regulation of c-Kit by targeting the ATP pocket

The stem cell factor receptor (SCF) c-Kit plays a pivotal role in regulating cell proliferation and survival in many cell types. In particular, c-Kit is required for early amplification of erythroid progenitors, while it must disappear from cell surface for the cell entering the final steps of maturation in an erythropoietin-dependent manner. We initially observed that imatinib (IM), an inhibitor targeting the tyrosine kinase activity of c-Kit concomitantly down-regulated the expression of c-Kit and accelerated the Epo-driven differentiation of erythroblasts in the absence of SCF. We investigated the mechanism by which IM or related masitinib (MA) induce c-Kit down-regulation in the human UT-7/Epo cell line. We found that the down-regulation of c-Kit in the presence of IM or MA was inhibited by a pre-incubation with methyl-β-cyclodextrin suggesting that c-Kit was internalized in the absence of ligand. By contrast to SCF, the internalization induced by TKI was independent of the E3 ubiquitin ligase c-Cbl. Furthermore, c-Kit was degraded through lysosomal, but not proteasomal pathway. In pulse-chase experiments, IM did not modulate c-Kit synthesis or maturation. Analysis of phosphotyrosine peptides in UT-7/Epo cells treated or not with IM show that IM did not modify overall tyrosine phosphorylation in these cells. Furthermore, we showed that a T670I mutation preventing the full access of IM to the ATP binding pocket, did not allow the internalization process in the presence of IM. Altogether these data show that TKI-induced internalization of c-Kit is linked to a modification of the integrity of ATP binding pocket.

Signaling pathways of cell proliferation are involved in the differential effect of erythropoietin and its carbamylated derivative

It is now recognized that in addition to its activity upon erythroid progenitor cells, erythropoietin (Epo) is capable of stimulating survival of different non-erythroid cells. Since stimulation of erythropoiesis is unwanted for neuroprotection, Epo-like compounds with a more selective action are under investigation. Although the carbamylated derivative of erythropoietin (cEpo) has demonstrated non-hematopoietic tissue protection without erythropoietic effect, little is known about differential mechanisms between Epo and cEpo. Therefore, we investigated signaling pathways which play a key role in Epo-induced proliferation. Here we show that cEpo blocked FOXO3a phosphorylation, allowing expression of downstream target p27(kip1) in UT-7 and TF-1 cells capable of erythroid differentiation. This is consistent with the involvement of cEpo in slowing down G1-to-S-phase progression compared with the effect of Epo upon cell cycle. In contrast, similar antiapoptotic actions of cEpo and Epo were observed in neuronal SH-SY5Y cells. Inhibition and competition assays suggest that Epo may act through both, the homodimeric (EpoR/EpoR) and the heterodimeric (EpoR/βcR) receptors in neuronal SH-SY5Y cells and probably in the TF-1 cell type as well. Results also indicate that cEpo needs both the EpoR and βcR subunits to prevent apoptosis of neuronal cells. Based on evidence suggesting that cell proliferation pathways were involved in the differential effect of Epo and cEpo, we went forward to studying downstream signals. Here we provide the first evidence that unlike Epo, cEpo failed to induce FOXO3a inactivation and subsequent p27(kip1) downregulation, which is clearly shown in the incapacity of cEpo to induce erythroid cell growth.

Regulation of Erythropoietin Receptor Activity in Endothelial Cells by Different Erythropoietin (EPO) Derivatives: An in Vitro Study

In endothelial cells, erythropoietin receptors (EPORs) mediate the protective, proliferative and angiogenic effects of EPO and its analogues, which act as EPOR agonists. Because hormonal receptors undergo functional changes upon chronic exposure to agonists and because erythropoiesis-stimulating agents (ESAs) are used for the long-term treatment of anemia, it is critical to determine the mechanism by which EPOR responsiveness is regulated at the vascular level after prolonged exposure to ESAs. Here, we investigated EPOR desensitization/resensitization in human umbilical vein endothelial cells (HUVECs) upon exposure to three ESAs with different pharmacokinetic profiles, epoetin alpha (EPOα), darbepoetin alpha (DarbEPO) and continuous EPOR activator (CERA). These agonists all induced activation of the transcription factor STAT-5, which is a component of the intracellular pathway associated with EPORs. STAT-5 activation occurred with either monophasic or biphasic kinetics for EPOα/DarbEPO and CERA, respectively. ESAs, likely through activation of the STAT-5 pathway, induced endothelial cell proliferation and stimulated angiogenesis in vitro, demonstrating a functional role for epoetins on endothelial cells. All epoetins induced EPOR desensitization with more rapid kinetics for CERA compared to EPOα and DarbEPO. However, the recovery of receptor responsiveness was strictly dependent on the type of epoetin, the agonist concentration and the time of exposure to the agonist. EPOR resensitization occurred with more rapid kinetics after exposure to low epoetin concentrations for a short period of desensitization. When the highest concentration of agonists was tested, the recovery of receptor responsiveness was more rapid with CERA compared to EPOα and was completely absent with DarbEPO. Our results demonstrate that these three ESAs regulate EPOR resensitization by very different mechanisms and that both the type of molecule and the length of EPOR stimulation are factors that are critical for the control of EPOR functioning in endothelial cells. The differences observed in receptor resensitization after stimulation with the structurally different ESAs are most likely due different control mechanisms of receptor turnover at the intracellular level.

The regulated expression, intracellular trafficking, and membrane recycling of the P2Y-like receptor GPR17 in Oli-neu oligodendroglial cells

GPR17 is a G-protein-coupled receptor that is activated by two classes of molecules: uracil-nucleotides and cysteinyl-leukotrienes. GPR17 is required for initiating the differentiation of oligodendrocyte precursors but has to be down-regulated to allow cells to undergo terminal maturation. Although a great deal has been learned about GPR17 expression and signaling, no information is currently available about the trafficking of native receptors after the exposure of differentiating oligodendrocytes to endogenous agonists. Here, we demonstrate that neuron-conditioned medium induces the transcriptionally mediated, time-regulated expression of GPR17 in Oli-neu, an oligodendrocyte precursor cell line, making these cells suitable for studying the endocytic traffic of the native receptor. Agonist-induced internalization, intracellular trafficking, and membrane recycling of GPR17 were analyzed by biochemical and immunofluorescence assays using an ad hoc-developed antibody against the extracellular N-terminal of GPR17. Both UDP-glucose and LTD(4) increased GPR17 internalization, although with different efficiency. At early time points, internalized GPR17 co-localized with transferrin receptor, whereas at later times it partially co-localized with the lysosomal marker Lamp1, suggesting that a portion of GPR17 is targeted to lysosomes upon ligand binding. An analysis of receptor recycling and degradation demonstrated that a significant aliquot of GPR17 is recycled to the cell surface. Furthermore, internalized GPR17 displayed a co-localization with the marker of the "short loop" recycling endosomes, Rab4, while showing very minor co-localization with the "long loop" recycling marker, Rab11. Our results provide the first data on the agonist-induced trafficking of native GPR17 in oligodendroglial cells and may have implications for both physiological and pathological myelination.

Biology and significance of the JAK/STAT signalling pathways

Since its discovery two decades ago, the activation of the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway by numerous cytokines and growth factors has resulted in it becoming one of the most well-studied intracellular signalling networks. The field has progressed from the identification of the individual components to high-resolution crystal structures of both JAK and STAT, and an understanding of the complexities of the molecular activation and deactivation cycle which results in a diverse, yet highly specific and regulated pattern of transcriptional responses. While there is still more to learn, we now appreciate how disruption and deregulation of this pathway can result in clinical disease and look forward to adoption of the next generation of JAK inhibitors in routine clinical treatment.

The nonhematopoietic effects of erythropoietin in skin regeneration and repair: from basic research to clinical use

Erythropoietin (EPO) is the main regulator of red blood cell production but there exists also a variety of nonhematopoietic properties. More recent data show that EPO is also associated with the protection of tissues suffering from ischemia and reperfusion injury as well as with improved regeneration in various organ systems, in particular the skin. This review highlights the mechanisms of EPO in the different stages of wound healing and the reparative processes in the skin emphasizing pathophysiological mechanisms and potential clinical applications. There is clear evidence that EPO effectively influences all wound-healing phases in a dose-dependent manner. This includes inflammation, tissue, and blood vessel formation as well as the remodeling of the wound. The molecular mechanism is predominantly based on an increased expression of the endothelial and inducible nitric oxide (NO) synthase with a consecutive rapid supply of NO as well as an increased content of vascular endothelial growth factor (VEGF) in the wound. The improved understanding of the functions and regulatory mechanisms of EPO in the context of wound-healing problems and ischemia/reperfusion injury, especially during flap surgery, may lead to new considerations of this growth hormone for its regular clinical application in patients.

Thrombopoietin receptor down-modulation by JAK2 V617F: restoration of receptor levels by inhibitors of pathologic JAK2 signaling and of proteasomes

The constitutively active JAK2 V617F mutant is the major determinant of human myeloproliferative neoplasms (MPNs). We show that coexpression of murine JAK2 V617F and the murine thrombopoietin (Tpo) receptor (TpoR, c-MPL) in hematopoietic cell lines or heterozygous knock-in of JAK2 V617F in mice leads to down-modulation of TpoR levels. Enhanced TpoR ubiquitinylation, proteasomal degradation, reduced recycling, and maturation are induced by the constitutive JAK2 V617F activity. These effects can be prevented in cell lines by JAK2 and proteasome inhibitors. Restoration of TpoR levels by inhibitors could be detected in platelets from JAK2 inhibitor-treated myelofibrosis patients that express the JAK2 V617F mutant, and in platelets from JAK2 V617F knock-in mice that were treated in vivo with JAK2 or proteasome inhibitors. In addition, we show that Tpo can induce both proliferative and antiproliferative effects via TpoR at low and high JAK2 activation levels, respectively, or on expression of JAK2 V617F. The antiproliferative signaling and receptor down-modulation by JAK2 V617F were dependent on signaling via TpoR cytosolic tyrosine 626. We propose that selection against TpoR antiproliferative signaling occurs by TpoR down-modulation and that restoration of down-modulated TpoR levels could become a biomarker for the treatment of MPNs.

Receptor dynamics in signaling

Reliable inter- and intracellular communication is central to both the development and the integrity of multicellular organisms. Key mediators of these processes are cell surface receptors that perceive and convert extracellular cues to trigger intracellular signaling networks and ultimately a phenotypic response. Deregulation of signal transduction leads to a variety of diseases, and aberrations in receptor proteins are very common in various cancer types. Therefore, cell surface receptors have been established as major targets in drug discovery. However, in order to efficiently apply therapeutics, it is crucial to gain knowledge about design principles of receptor signaling. In this chapter, we will discuss signal transduction at the receptor level for examples from different receptor classes.