Activation of the Erythropoietin Receptor Is Not Required for Internalization of Bound Erythropoietin

Hypoxia-induced signaling in the cardiovascular system: pathogenesis and therapeutic targets

Hypoxia, characterized by reduced oxygen concentration, is a significant stressor that affects the survival of aerobic species and plays a prominent role in cardiovascular diseases. From the research history and milestone events related to hypoxia in cardiovascular development and diseases, The "hypoxia-inducible factors (HIFs) switch" can be observed from both temporal and spatial perspectives, encompassing the occurrence and progression of hypoxia (gradual decline in oxygen concentration), the acute and chronic manifestations of hypoxia, and the geographical characteristics of hypoxia (natural selection at high altitudes). Furthermore, hypoxia signaling pathways are associated with natural rhythms, such as diurnal and hibernation processes. In addition to innate factors and natural selection, it has been found that epigenetics, as a postnatal factor, profoundly influences the hypoxic response and progression within the cardiovascular system. Within this intricate process, interactions between different tissues and organs within the cardiovascular system and other systems in the context of hypoxia signaling pathways have been established. Thus, it is the time to summarize and to construct a multi-level regulatory framework of hypoxia signaling and mechanisms in cardiovascular diseases for developing more therapeutic targets and make reasonable advancements in clinical research, including FDA-approved drugs and ongoing clinical trials, to guide future clinical practice in the field of hypoxia signaling in cardiovascular diseases.

Role of hormones in bone remodeling in the craniofacial complex: A review

Background

Diseases such as periodontitis and osteoporosis are expected to rise tremendously by 2050. Bone formation and remodeling are complex processes that are disturbed in a variety of diseases influenced by various hormones.

Objective

This study aimed to review and present the roles of various hormones that regulate bone remodeling of the craniofacial complex.

Methods

A literature search was conducted on PubMed and Google Scholar for studies related to hormones and jawbone. Search strategies included the combinations ("name of hormone" + "dental term") of the following terms: "hormones", "oxytocin", "estrogen", "adiponectin", "parathyroid hormone", "testosterone", "insulin", "angiotensin", "cortisol", and "erythropoietin", combined with a dental term "jaw bone", "alveolar bone", "dental implant", "jaw + bone regeneration, healing or repair", "dentistry", "periodontitis", "dry socket", "osteoporosis" or "alveolitis". The papers were screened according to the inclusion criteria from January 1, 2000 to March 31, 2021 in English. Publications included reviews, book chapters, and original research papers; in vitro studies, in vivo animal, or human studies, including clinical studies, and meta-analyses.

Results

Bone formation and remodeling is a complex continuous process involving many hormones. Bone volume reduction following tooth extractions and bone diseases, such as periodontitis and osteoporosis, cause serious problems and require a great understanding of the process.

Conclusion

Hormones are with us all the time, shape our development and regulate homeostasis. Newly discovered effects of hormones influencing bone healing open the possibilities of using hormones as therapeutics to combat bone-related diseases.

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 Induces an Epithelial to Mesenchymal Transition-Like Process in Mammary Epithelial Cells MCF10A

Anemia is associated with chemotherapy treatment in cancer patients. Erythropoietin (EPO) has been used to treat anemia of cancer patients, because it stimulates erythropoiesis. However, treatment of breast cancer patients with EPO has been associated with poor prognosis and decrease of survival. Epithelial to mesenchymal transition (EMT) is a process by which epithelial cells are transdifferentiated to a mesenchymal state. It has been implicated in tumor progression, because epithelial cells acquire the capacity to execute the multiple steps of invasion/metastasis process. However, the role of EPO on EMT process in human mammary epithelial cells has not been studied. In the present study, we demonstrate that EPO promotes a decrease of E-cadherin expression, an increase of N-cadherin, vimentin, and Snail2 expression, activation of FAK and Src kinases and an increase of MMP-2 and MMP-9 secretions. Moreover, EPO induces an increase of NFκB DNA binding activity, an increase of binding of p50 and p65 NFκB subunits to Snail1 promoter, migration, and invasion in mammary non-tumorigenic epithelial cells MCF10A. In summary, these findings demonstrate, for the first time, that EPO induces an EMT-like process in mammary non-tumorigenic epithelial cells. J. Cell. Biochem. 118: 2983-2992, 2017. © 2017 Wiley Periodicals, Inc.

Erythropoietin promotes bone formation through EphrinB2/EphB4 signaling

Recent studies have demonstrated that erythropoietin (EPO) has extensive nonhematopoietic biological functions. However, little is known about how EPO regulates bone formation, although several studies suggested that EPO can affect bone homeostasis. In this study, we investigated the effects of EPO on the communication between osteoclasts and osteoblasts through the ephrinB2/EphB4 signaling pathway. We found that EPO slightly promotes osteoblastic differentiation with the increased expression of EphB4 in ST2 cells. However, EPO increased the expression of Nfatc1 and ephrinB2 but decreased the expression of Mmp9 in RAW264.7 cells, resulting in an increase of ephrinB2-expressing osteoclasts and a decrease in resorption activity. The stimulation of ephrinB2/EphB4 signaling via ephrinB2-Fc significantly promoted EPO-mediated osteoblastic differentiation in ST2 cells. EphB4 knockdown through EphB4 shRNA inhibited EPO-mediated osteoblastic phenotypes. Furthermore, in vivo assays clearly demonstrated that EPO efficiently induces new bone formation in the alveolar bone regeneration model. Taken together, these results suggest that ephrinB2/EphB4 signaling may play an important role in EPO-mediated bone formation.

Advances in understanding the pathogenesis of primary familial and congenital polycythaemia

Primary familial and congenital polycythemia (PFCP) is an autosomal-dominant proliferative disorder characterized by erythrocytosis and hypersensitivity of erythroid progenitors to erythropoietin (Epo). Several lines of evidence suggest a causal role of truncated erythropoietin receptor (EpoR) in this disease. In this review, we discuss PFCP in the context of erythrocytosis and EpoR signalling. We focus on recent studies describing mechanisms underlying Epo-dependent EpoR down-regulation. One mechanism depends on internalization mediated through the p85 regulatory subunit of the Phosphoinositide 3-Kinase, and the other utilizes ubiquitin-based proteasomal degradation. Truncated PFCP EpoRs are not properly down-regulated upon stimulation, underscoring the importance of these mechanisms in the pathogenesis of PFCP.

Ligand-induced EpoR internalization is mediated by JAK2 and p85 and is impaired by mutations responsible for primary familial and congenital polycythemia

Epo-induced endocytosis of EpoR plays important roles in the down-regulation of EpoR signaling and is the primary means that regulates circulating Epo concentrations. Here we show that cell-surface EpoR is internalized via clathrin-mediated endocytosis. Both JAK2 kinase activity and EpoR cytoplasmic tyrosines are important for ligand-dependent EpoR internalization. Phosphorylated Y429, Y431, and Y479 in the EpoR cytoplasmic domain bind p85 subunit of PI3 kinase on Epo stimulation and individually are sufficient to mediate Epo-dependent EpoR internalization. Knockdown of p85alpha and p85beta or expression of their dominant-negative forms, but not inhibition of PI3 kinase activity, dramatically impaired EpoR internalization, indicating that p85alpha and p85beta may recruit proteins in the endocytic machinery on Epo stimulation. Furthermore, mutated EpoRs from primary familial and congenital polycythemia (PFCP) patients lacking the 3 important tyrosines do not bind p85 or internalize on stimulation. Addition of residues encompassing Y429 and Y431 to these truncated receptors restored p85beta binding and Epo sensitivity. Our results identify a novel PI3 kinase activity-independent function of p85 in EpoR internalization and support a model that defects of internalization in truncated EpoRs from PFCP patients contribute to Epo hypersensitivity and prolonged signaling.

A transplanted NPVY sequence in the cytosolic domain of the erythropoietin receptor enhances maturation

Activation of the EPO-R [EPO (erythropoietin) receptor] by its ligand EPO promotes erythropoiesis. Low cell surface EPO-R levels are traditionally attributed to inefficient folding mediated by the receptor extracellular domain. In the present study, we addressed the role of the EPO-R intracellular domain in exit from the ER (endoplasmic reticulum) and surface expression. A fusion protein between the thermo-reversible folding mutant of VSVG (vesicular-stomatitis-virus glycoprotein) (VSVGtsO45) and the EPO-R cytosolic domain [VSVG-WT (wild-type)] displayed delayed intracellular trafficking as compared with the parental VSVGtsO45, suggesting that the EPO-R cytosolic domain can hamper ER exit. Although NPXY-based motifs were originally associated with clathrin binding and endocytosis, they may also function in other contexts of the secretory pathway. A fusion protein between VSVGtsO45 and the cytosolic portion of EPO-R containing an NPVY insert (VSVG-NPVY) displayed enhanced glycan maturation and surface expression as compared with VSVG-WT. Notably, the NPVY insert also conferred improved maturation and augmented cell surface EPO-R. Our findings highlight three major concepts: (i) the EPO-R cytosolic domain is involved in ER exit of the receptor. (ii) Sequence motifs that participate in endocytosis can also modulate transport along the secretory pathway. (iii) VSVG-fusion proteins may be employed to screen for intracellular sequences that regulate transport.

An extracellular region of the erythropoietin receptor of the subterranean blind mole rat Spalax enhances receptor maturation

Erythropoietic functions of erythropoietin (EPO) are mediated by its receptor (EPO-R), which is present on the cell surface of erythroid progenitors and induced by hypoxia. We focused on EPO-R from Spalax galili (sEPO-R), one of the four Israeli species of the subterranean blind mole rat, Spalax ehrenbergi superspecies, as a special natural animal model of high tolerance to hypoxia. Led by the intriguing observation that most of the mouse EPO-R (mEPO-R) is retained in the endoplasmic reticulum (ER), we hypothesized that sEPO-R is expressed at higher levels on the cell surface, thus maximizing the response to elevated EPO, which has been reported in this species. Indeed, we found increased cell-surface levels of sEPO-R as compared with mEPO-R by using flow cytometry analysis of BOSC cells transiently expressing HA-tagged EPO-Rs (full length or truncated). We then postulated that unique extracellular sEPO-R sequence features contribute to its processing and cell-surface expression. To map these domains of the sEPO-R that augment receptor maturation, we generated EPO-R derivatives in which parts of the extracellular region of mEPO-R were replaced with the corresponding fragments of sEPO-R. We found that an extracellular portion of sEPO-R, harboring the N-glycosylation site, conferred enhanced maturation and increased transport to the cell surface of the respective chimeric receptor. Taken together, we demonstrate higher surface expression of sEPO-R, attributed at least in part to increased ER exit, mediated by an extracellular region of this receptor. We speculate that these sEPO-R sequence features play a role in the adaptation of Spalax to extreme hypoxia.

Increased erythropoietin elimination in fetal sheep following chronic phlebotomy

PURPOSE

To determine by pharmacokinetic (PK) means the role of erythropoietin-receptor (EPO-R) upregulation in fetuses on the elimination of erythropoietin (EPO).

MATERIALS AND METHODS

Six fetal sheep were catheterized at a gestational age of 125-127 days and phlebotomized daily for 6 days. Paired tracer PK studies using recombinant human EPO (rHuEPO) were conducted in the sheep fetuses at baseline and post-phlebotomy, 7 days later. A PK model with Michaelis-Menten elimination was simultaneously fit to the PK data at baseline and post-phlebotomy for each fetus.

RESULTS

Daily phlebotomies reduced the hemoglobin levels from baseline values of 10.8 (5%) (mean (C.V.)) g/dl to a nadir of 4.5 (17%) g/dl post-phlebotomy. The endogenous EPO concentration rapidly increased after the first phlebotomy and remained elevated, although variable, thereafter. The Michaelis-Menten maximal rHuEPO elimination rate parameter, V(max), was significantly greater post-phlebotomy than at baseline (p < 0.05), increasing 1.31 fold. The fetal baseline "linear" clearance at very low concentrations of rHuEPO was determined to be 117 ml/kg/h, similar to that determined in newborn sheep but 2-3 fold higher than that determined in adult sheep.

CONCLUSIONS

The observed increase in V(max) is consistent with an up-regulation of EPO-R due to a positive feedback resulting from the phlebotomy-induced anemia.

Can acidosis and hyperphosphataemia result in increased erythropoietin dosing in haemodialysis patients?

AIM

To evaluate whether factors such as acidosis and hyperphosphataemia that might cause an increased oxygen delivery to tissues could result in increased dosing requirements for intravenous erythropoietin (EPO) administration given to haemodialysis patients.

METHODS

The clinical records of the patients seen at the Hypertension, Nephrology, Dialysis and Transplantation Clinic from December, 2004 through August, 2005 were reviewed to identify patients who had taken intravenous erythropoietin. Two-tailed, Pearson's correlation was performed to determine correlations between any of the parameters. Analysis of variance and stepwise regression for covariance were used to evaluate the relations of demographic and clinical characteristics and laboratory variables. Analysis of covariance and K means cluster analyses were also performed to examine linkage between variables. Kendall's Tau correlation was used for correlations of non-parametric data.

RESULTS

There was a significant direct or positive correlation at the 0.01 levels between dry weight, age, intact parathyroid hormone level (PTH), and serum phosphorus and EPO dose. There was an inverse or negative correlation at that level between the serum bicarbonate and urea reduction ratio (URR) with the EPO dose at the same level while there was a weaker correlation but direct correlation between the white blood count (WBC) and EPO dose. There was significant colinearity between serum phosphorus and PTH but serum phosphorus showed a more significant correlation with EPO overall. Stepwise regression analysis for covariance revealed that phosphorus remained significantly correlated with EPO resistance after the removal of the effect of PTH while PTH lost its significance after the effect of phosphorus was removed.

CONCLUSION

Acidosis and hyperphosphataemia are associated with apparent increased erythropoietin dosing requirements. While this study did not evaluate the mechanism of such requirements and indeed many mechanisms might be possible, a rightward shift in the oxygen-haemoglobin dissociation curve resulting in down-regulation of erythropoietin receptors is considered consistent with the data and present knowledge.

TYK2 activity promotes ligand-induced IFNAR1 proteolysis

The type I IFNR (interferon receptor) is a heterodimer composed of two transmembrane chains, IFNAR1 (interferon-alpha receptor 1 subunit) and IFNAR2, which are associated with the tyrosine kinases Tyk2 and Jak1 (Janus kinase 1) respectively. Ligand-induced down-regulation of the type I IFNR is a major mechanism of negative regulation of cellular signalling and involves the internalization and lysosomal degradation of IFNAR1. IFNalpha promotes the phosphorylation of IFNAR1 on Ser535, followed by recruitment of the E3 ubiquitin ligase, beta-TrCP2 (beta-transducin repeats-containing protein 2), ubiquitination of IFNAR1 and proteolysis. The non-catalytic role of Tyk2 in sustaining the steady-state IFNAR1 level at the plasma membrane is well documented; however, little is known about the function of Tyk2 in the steps that precede and succeed serine phosphorylation and ubiquitination of IFNAR1 in response to ligand binding. In the present study, we show that catalytic activation of Tyk2 is not essential for IFNAR1 internalization, but is required for ligand-induced IFNAR1 serine phosphorylation, ubiquitination and efficient lysosomal proteolysis.

Cellular trafficking and degradation of erythropoietin and novel erythropoiesis stimulating protein (NESP)

Erythropoietin (Epo) is essential for the production of mature red blood cells, and recombinant Epo is commonly used to treat anemia, but how Epo is degraded and cleared from the body is not understood. Glycosylation of Epo is required for its in vivo bioactivity, although not for in vitro receptor binding or stimulation of Epo-dependent cell lines; Epo glycosylation actually reduces the affinity of Epo for the Epo receptor (EpoR). Interestingly, a hyperglycosylated analog of Epo, called novel erythropoiesis-stimulating protein (NESP), has a lower affinity than Epo for the EpoR but has greater in vivo activity and a longer serum half-life than Epo. We hypothesize that a major mechanism for degradation of Epo in the body occurs in cells expressing the Epo receptor, through receptor-mediated endocytosis of Epo followed by degradation in lysosomes, and therefore investigated the trafficking and degradation of Epo and NESP by EpoR-expressing cells. We show that Epo and NESP are degraded only by cultured cells that express the EpoR, and their receptor binding, dissociation, and trafficking properties determine their rates of intracellular degradation. Epo binds surface EpoR faster than NESP (k(on) = 5.0 x 10(8) m(-1) min(-1) versus 1.1 x 10(8) m(-1) min(-1)) but dissociates slower (k(off) = 0.029 min(-1) versus 0.042 min(-1)). Surface-bound Epo and NESP are internalized at the same rate (k(in) = 0.06 min(-1)), and after internalization 60% of each ligand is resecreted intact and 40% degraded. Our kinetic model of Epo and NESP receptor binding, intracellular trafficking, and degradation explains why Epo is degraded faster than NESP at the cellular level.

Proteasomes mediate prolactin-induced receptor down-regulation and fragment generation in breast cancer cells

Prolactin regulates a variety of physiological processes, including mammary gland growth and differentiation, and recent findings support an important role in breast cancer development and progression. However, little is known about the trafficking of its receptor, a member of the cytokine receptor superfamily. In the present study, we examined the effect of ligand on the endogenous "long" isoform of the prolactin receptor in breast cancer cells. We found that prolactin caused rapid and prolonged down-regulation of this receptor. The prolactin-induced increase in degradation was blocked by inhibitors of both proteasomes and lysosomes. However, the ubiquitin-conjugating system was not required for internalization. Prolactin also resulted in the concomitant appearance of a cell-associated prolactin receptor fragment containing the extracellular domain. This latter process required proteasomal, but not metalloprotease, activity, distinguishing it from ectodomain "shedding" of other membrane receptors, which are secreted as binding proteins. The prolactin receptor fragment was labeled by surface biotinylation and independent of protein synthesis. Together, these data indicated that prolactin binding initiates limited proteasomal cleavage of its receptor, generating a cell-associated fragment containing the extracellular domain. Our findings described a new potential mediator of prolactin action and a novel mechanism whereby proteasomes modulate cellular processes.

Both proteasomes and lysosomes degrade the activated erythropoietin receptor

Activation of the erythropoietin receptor (EpoR) after Epo binding is very transient because of the rapid activation of strong down-regulation mechanisms that quickly decrease Epo sensitivity of the cells. Among these down-regulation mechanisms, receptor internalization and degradation are probably the most efficient. Here, we show that the Epo receptor was rapidly ubiquitinated after ligand stimulation and that the C-terminal part of the Epo receptor was degraded by the proteasomes. Both ubiquitination and receptor degradation by the proteasomes occurred at the cell surface and required Janus kinase 2 (Jak2) activation. Moreover, Epo-EpoR complexes were rapidly internalized and targeted to the lysosomes for degradation. Neither Jak2 nor proteasome activities were required for internalization. In contrast, Jak2 activation was necessary for lysosome targeting of the Epo-EpoR complexes. Blocking Jak2 with the tyrphostin AG490 led to some recycling of internalized Epo-Epo receptor complexes to the cell surface. Thus, activated Epo receptors appear to be quickly degraded after ubiquitination by 2 proteolytic systems that proceed successively: the proteasomes remove part of the intracellular domain at the cell surface, and the lysosomes degrade the remaining part of the receptor-hormone complex. The efficiency of these processes probably explains the short duration of intracellular signaling activated by Epo.

Immobilization of erythropoietin to culture erythropoietin-dependent human leukemia cell line

To investigate the effect of immobilized cytokine, erythropoietin (Epo) was immobilized on a culture plate and the Epo-dependent human leukemia cell line UT-7/Epo then was cultured upon the plate. A photo-reactive gelatin was mixed with Epo and the mixture was cast on a plate. The plate was then irradiated with ultraviolet light in the presence or absence of a photo-mask. After washing with water, a micropatterned or unpatterned surface was formed. A leukemia cell line dependent on Epo, UT-7/Epo, was cultured on the sample plate. On the micropatterned surface, apoptosis of cells was induced on the surface without Epo, but was not observed on the Epo-immobilized surface. This result demonstrated that Epo stimulated the cells even after immobilization. Although the activity of immobilized Epo was low, the activity was slightly higher than that achieved by soluble Epo at higher concentration. In addition, the immobilized Epo could be repeatedly used for culture of UT-7/Epo cell. The present study provided a convenient immobilization method and indicated that immobilization of cytokines will be useful for creating an artificial cell culture device.

Receptor activation and 2 distinct COOH-terminal motifs control G-CSF receptor distribution and internalization kinetics

We have studied the intracellular distribution and internalization kinetics of the granulocyte colony-stimulating factor receptor (G-CSF-R) in living cells using fusion constructs of wild-type or mutant G-CSF-R and enhanced green fluorescent protein (EGFP). Under steady-state conditions the G-CSF-R localized predominantly to the Golgi apparatus, late endosomes, and lysosomes, with only low expression on the plasma membrane, resulting from spontaneous internalization. Internalization of the G-CSF-R was significantly accelerated by addition of G-CSF. This ligand-induced switch from slow to rapid internalization required the presence of G-CSF-R residue Trp650, previously shown to be essential for its signaling ability. Both spontaneous and ligand-induced internalization depended on 2 distinct amino acid stretches in the G-CSF-R COOH-terminus: 749-755, containing a dileucine internalization motif, and 756-769. Mutation of Ser749 at position -4 of the dileucine motif to Ala significantly reduced the rate of ligand-induced internalization. In contrast, mutation of Ser749 did not affect spontaneous G-CSF-R internalization, suggesting the involvement of a serine-threonine kinase specifically in ligand-accelerated internalization of the G-CSF-R. COOH-terminal truncation mutants of G-CSF-R, found in severe congenital neutropenia, lack the internalization motifs and were completely defective in both spontaneous and ligand-induced internalization. As a result, these mutants showed constitutively high cell-surface expression.

Internalization of the thrombopoietin receptor is regulated by 2 cytoplasmic motifs

Receptor-mediated internalization appears to be the primary mean of regulating the plasma level of thrombopoietin (TPO). However, the processes that regulate Mpl internalization have not previously been described. Using the cytokine-dependent cell line BaF3, we have identified 2 distinct motifs within the cytoplasmic domain of Mpl that underlie ligand-dependent internalization. Removal of the fourth cytoplasmic tyrosine residue by deletion or truncation results in a significant decrease in maximal internalization. The remaining receptor internalization is abrogated by deletion of cytoplasmic residues 54-69, which include the core box2 region (L54L55E56I57L58) and the only dileucine motifs (L54L55 and I57L58) within the cytoplasmic domain of Mpl. Receptor internalization mediated by this latter subdomain does not require Jak2 activation. Furthermore, TPO-stimulated cellular proliferation appears to be directly correlated with receptor internalization, indicating that internalization of the TPO/Mpl complex may be essential for normal signal transduction. Finally, we have demonstrated that upon removal of TPO from the supernatant, Mpl promptly reappears on the cell surface, suggesting that a pool of intracellular Mpl can be rapidly recycled to the cell surface. These data help identify the receptor motifs involved in TPO-induced internalization of Mpl and suggest that Mpl translocation may be necessary for normal cellular proliferation.

Studies with chimeric Mpl/JAK2 receptors indicate that both JAK2 and the membrane-proximal domain of Mpl are required for cellular proliferation

The thrombopoietin (TPO) receptor c-Mpl, like other members of the cytokine receptor superfamily, requires the association and activation of Janus kinases (JAKs) for normal signal transduction. The membrane-proximal portion of the signaling domain, containing conserved box1 and box2 motifs, is sufficient to support the proliferation of cytokine-dependent cell lines and basal megakaryocytopoiesis in vivo. We hypothesized that activation of the JAK2 kinase alone might be sufficient for proliferative signaling. To test this premise, we constructed chimeric receptors in which the extracellular and transmembrane portions of Mpl were fused to the pseudokinase and kinase domains of murine JAK2 kinase. When expressed in the interleukin-3-dependent cell line Ba/F3, the chimeric receptors were appropriately expressed on the cell surface and were able to initiate tyrosine kinase activity upon exposure to TPO. However, chimeric receptors lacking an intact box2 domain of Mpl were unable to support proliferation at any concentration of TPO. Only chimeric receptors containing both JAK2 kinase activity and the box2 region initiated proliferative signaling. Within the box2 motif, we determined that the sequence Glu(56)-Ile(57)-Leu(58) of the Mpl cytoplasmic domain is critical for proliferation of the chimeric receptors. Furthermore, TPO-dependent induction of c-myc transcription is also dependent on this motif. These results indicate that JAK2 activation alone is not sufficient for TPO-induced proliferation and that one or more essential signaling pathways must arise from the cytoplasmic domain of Mpl that includes box2. Although the nature of the signal transduction pathway is not yet known, this second proliferative event is likely to regulate c-myc expression.

Amino acid residues 268-276 of the erythropoietin receptor contain an endocytosis motif and are required for erythropoietin-mediated proliferation

Erythropoietin (EPO) promotes viability, proliferation and differentiation of mammalian erythroid progenitor cells via its specific cell surface receptor (EPO-R). We have previously shown that truncated EPO-Rs containing 267 amino acids or less were defective in internalization of (125)I-EPO, whereas internalization via a receptor derivative containing 276 amino acids was unaffected, thus directing focus to the nine amino acid residues FEGLFTTHK at positions 268-276 [Levin, Cohen, Supino, Yoshimura, Watowich, Neumann, FEBS Lett. 427 (1998) 164-170]. Here, a panel of EPO-R mutants was generated to determine the role of these residues in EPO endocytosis, down regulation of cell surface receptors and EPO-mediated signaling. While linking amino acid residues 268-276 to a truncated EPO-R (Delta+9 EPO-R) conferred both ligand uptake and ligand-independent down regulation of the respective receptor from the cell surface, Phe 272 was crucial for EPO endocytosis but not for ligand-independent down regulation. Additional receptor motifs probably play a role in EPO endocytosis and receptor down-regulation, as these processes were not adversely impaired in Delta268-276 EPO-R. A central role of residues 268-276, in particular Phe, was demonstrated by the inability of Delta268-276 and F268,272A EPO-Rs to support EPO-mediated signal transduction.

Oncogene cooperativity in Friend erythroleukemia: erythropoietin receptor activation by the env gene of SFFV leads to transcriptional upregulation of PU.1, independent of SFFV proviral insertion

Cancer is a multi-step, multi-genetic event. Whether oncogenic mutations cooperate with one another to transform cells and how is not well understood. The Friend murine retroviral erythroleukemia model involves mitogenic activation of the erythropoietin receptor (EpoR) by the virus env gene (F-gp55), aberrant over-expression of the transcription factor PU.1, and inactivating mutations in p53. In this report we demonstrate that concurrent expression of F-gp55 and PU.1 in erythroid target cells, in vivo, cooperate to accelerate erythroleukemia induction. Early in the disease, prior to the detection of clonal leukemic cells, activation of the EpoR by F-gp55, but not erythropoietin, resulted in transcriptional upregulation of PU.1 through a trans regulatory mechanism. This could occur in the absence of an integrated provirus within the PU.1 gene locus. The regulation of PU.1 transcription in established erythroleukemia cell lines differed depending upon the level of PU.1 protein present. Our results suggest that the action of F-gp55 contributes to both early and late stages of Friend erythroleukemia and that persistence of F-gp55 expression may be required not only to initiate erythroleukemia but to also maintain erythroleukemia following Friend virus infection.

Growth hormone receptor ubiquitination, endocytosis, and degradation are independent of signal transduction via Janus kinase 2

The ubiquitin-proteasome system is required in growth hormone receptor (GHR) endocytosis. For cytokine receptors, which lack intrinsic tyrosine kinase activity, signal transduction is initiated by the activation of a member of the Janus kinase (JAK) family. Previously, we have shown that GHR and JAK2 tyrosine (de) phosphorylation are regulated via the ubiquitin system. In this study, we examined the role of JAK2-mediated signal transduction in GHR internalization and down-regulation. Mutation of the attachment site for JAK2, box-1, in the GHR cytoplasmic tail resulted in the complete absence of GHR and JAK2 phosphorylation. This modification did not alter the rate and extent of receptor-bound growth hormone internalization as compared with a functional GHR, nor did it change its turnover and transport to the plasma membrane. In addition, the receptor was still normally ubiquitinated and remained dependent on both an intact ubiquitin system and proteasomal action for its internalization. Thus, GHR ubiquitination, endocytosis, and degradation occur independently of GHR signal transduction via JAK2. We conclude that whereas endocytosis and degradation require the ubiquitin system, they are independent of GHR signal transduction.

Coupling of heterotrimeric Gi proteins to the erythropoietin receptor

To identify new proteins involved in erythropoietin (Epo) signal transduction, we purified the entire set of proteins reactive with anti-phosphotyrosine antibodies from Epo-stimulated UT7 cells. Antisera generated against these proteins were used to screen a lambdaEXlox expression library. One of the isolated cDNAs encodes Gbeta2, the beta2 subunit of heterotrimeric GTP-binding proteins. Gbeta and Galpha(i) coprecipitated with the Epo receptor (EpoR) in extracts from human and murine cell lines and from normal human erythroid progenitor cells. In addition, in vitro Gbeta associated with a fusion protein containing the intracellular domain of the EpoR. Using EpoR mutants, we found that the distal part of the EpoR (between amino acids 459-479) was required for Gi binding. Epo activation of these cells induced the release of the Gi protein from the EpoR. Moreover in isolated cell membranes, Epo treatment inhibited ADP-ribosylation of Gi and increased the binding of GTP. Our results show that heterotrimeric Gi proteins associate with the C-terminal end of the EpoR. Receptor activation leads to the activation and dissociation of Gi from the receptor, suggesting a functional role of Gi protein in Epo signal transduction.