Erythropoietin gene expression in human, monkey and murine brain

Tissue-specific regulation of erythropoietin production in the murine kidney, brain, and uterus

Erythropoietin (Epo) produced by the kidney regulates erythropoiesis. Recent evidence suggests that Epo in the cerebrum prevents neuron death and Epo in the uterus induces estrogen (E(2))-dependent uterine angiogenesis. To elucidate how Epo expression is regulated in these tissues, ovariectomized mice were given E(2) and/or exposed to hypoxia, and the temporal patterns of Epo mRNA levels were examined. Epo mRNA levels in the kidney and cerebrum were elevated markedly within 4 h after exposure to hypoxia. Although the elevated level of Epo mRNA in the kidney decreased markedly within 8 h despite continuous hypoxia, the high level in the cerebrum was sustained for > or = 24 h, indicating that downregulation operates in the kidney but not in the brain. E(2) transiently induced Epo mRNA in the uterus but not in the kidney and cerebrum. Interestingly, the uterine Epo mRNA was hypoxia inducible only in the presence of E(2). Thus Epo expression appears to be regulated in a tissue-specific manner, endorsing the tissue-specific functions of Epo.

Retinoic acid stimulates erythropoietin gene transcription in embryonal carcinoma cells through the direct repeat of a steroid/thyroid hormone receptor response element half-site in the hypoxia-response enhancer

We have previously reported that expression of the erythropoietin (Epo) gene in mouse embryonal cells was not induced by hypoxia, although hypoxia induced other hypoxia-inducible genes. This study identifies retinoic acid (RA) as an inducer for Epo production in the embryonal carcinoma cell lines P19 and F9. RA induced Epo production through the transcriptional activation of the Epo gene in an oxygen-independent manner. With the use of reporter assays in P19 cells, it is shown that a direct repeat of the nuclear hormone receptor-binding motif separated by a 2-bp spacer (DR-2) in the hypoxia-response enhancer was responsible for the transcriptional activation by RA. Electrophoretic mobility shift assays show that nuclear extracts from P19 cells contained RA receptor complexes that bound to DR-2. In human hepatoma Hep3B cells, an orphan receptor, hepatocyte nuclear factor-4, strongly augmented hypoxic induction of the Epo gene in cooperation with hypoxia-inducible factor-1 (HIF-1) by binding to DR-2, whereas in P19 cells, the interaction of RA receptors with DR-2 was sufficient for RA-induced transcriptional activation of the Epo gene without the requirement of the HIF-1 site. These results suggest that DR-2 regulates expression of the Epo gene by acting as the binding site for different transcription factors in different types of cells.

Retinoic acid stimulates erythropoietin gene transcription in embryonal carcinoma cells through the direct repeat of a steroid/thyroid hormone receptor response element half-site in the hypoxia-response enhancer

We have previously reported that expression of the erythropoietin (Epo) gene in mouse embryonal cells was not induced by hypoxia, although hypoxia induced other hypoxia-inducible genes. This study identifies retinoic acid (RA) as an inducer for Epo production in the embryonal carcinoma cell lines P19 and F9. RA induced Epo production through the transcriptional activation of the Epo gene in an oxygen-independent manner. With the use of reporter assays in P19 cells, it is shown that a direct repeat of the nuclear hormone receptor-binding motif separated by a 2-bp spacer (DR-2) in the hypoxia-response enhancer was responsible for the transcriptional activation by RA. Electrophoretic mobility shift assays show that nuclear extracts from P19 cells contained RA receptor complexes that bound to DR-2. In human hepatoma Hep3B cells, an orphan receptor, hepatocyte nuclear factor-4, strongly augmented hypoxic induction of the Epo gene in cooperation with hypoxia-inducible factor-1 (HIF-1) by binding to DR-2, whereas in P19 cells, the interaction of RA receptors with DR-2 was sufficient for RA-induced transcriptional activation of the Epo gene without the requirement of the HIF-1 site. These results suggest that DR-2 regulates expression of the Epo gene by acting as the binding site for different transcription factors in different types of cells.

Nitric oxide prevents cardiovascular disease and determines survival in polyglobulic mice overexpressing erythropoietin

Nitric oxide (NO) induces vasodilatatory, antiaggregatory, and antiproliferative effects in vitro. To delineate potential beneficial effects of NO in preventing vascular disease in vivo, we generated transgenic mice overexpressing human erythropoietin. These animals induce polyglobulia known to be associated with a high incidence of vascular disease. Despite hematocrit levels of 80%, adult transgenic mice did not develop hypertension or thromboembolism. Endothelial NO synthase levels, NO-mediated endothelium-dependent relaxation and circulating and vascular tissue NO levels were markedly increased. Administration of the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) led to vasoconstriction of peripheral resistance vessels, hypertension, and death of transgenic mice, whereas wild-type siblings developed hypertension but did not show increased mortality. L-NAME-treated polyglobulic mice revealed acute left ventricular dilatation and vascular engorgement associated with pulmonary congestion and hemorrhage. In conclusion, we here unequivocally demonstrate that endothelial NO maintains normotension, prevents cardiovascular dysfunction, and critically determines survival in vivo under conditions of increased hematocrit.

Stimulating effect of erythropoietin on the release of dopamine and acetylcholine from the rat brain slice

We investigated the effects of erythropoietin (EPO) in rat hippocampal and striatal slices, where EPO receptors have been known to exist. EPO stimulated dopamine release from rat striatal slices. Acetylcholine (ACh) release from rat hippocampal slices was not affected by EPO, but high K(+)-induced ACh release was considerably enhanced by EPO. Nitric oxide (NO) production from the hippocampus and the striatum was not affected by EPO. NO-synthase activity was not changed by EPO in the hippocampus or the striatum. These results suggest that EPO stimulates dopamine- and acetylcholine-release without affecting NO production.

Neuroprotection and Angiogenesis: Dual Role of Erythropoietin in Brain Ischemia

Erythropoietin, originally defined as an erythroid growth factor, is upregulated in the brain under conditions of hypoxia. So far, two functions have been identified for this locally produced cytokine: a direct protective effect on neuronal cells during cerebral ischemia and an indirect protection of brain tissue that could be provided by promoting brain vessel growth.

Production and processing of erythropoietin receptor transcripts in brain

The expression of erythropoietin receptor (EpoR) in brain and neuronal cells, and hypoxia-responsive production of erythropoietin (Epo) in the brain suggests that the function of Epo as a survival or viability factor may extend beyond hematopoietic tissue and erythroid progenitor cells. Epo, produced by astrocytes and neurons, can be induced by hypoxia by severalfold, and in animal models Epo administration is neuroprotective to ischemic challenge. We characterized the human EpoR transcript in brain and neuronal cells to determine its contribution in regulating the Epo response in brain. Screening of a human brain cDNA library and quantitative analysis of EpoR transcripts indicate that the EpoR gene locus is transcriptionally active in brain. In addition to the proximal promoter that is active in hematopoietic cells, a significant proportion of transcripts originates far upstream from the EpoR coding region. Unlike erythroid cells with efficient splicing of EpoR transcripts to its mature form, brain EpoR transcripts are inefficiently or alternately processed with a bias towards the 3' coding region. In human EpoR transgenic mice, anemic stress induces expression of the transgene and endogenous EpoR gene in hematopoietic tissue and brain. In culture of neuronal cells, hypoxia induces EpoR expression and increases sensitivity to Epo. Induction of EpoR expression appears to be a consequence of increased transcription from the upstream region and proximal promoter, and a shift towards increased processing efficiency. These data suggest that in contrast to erythropoiesis where erythroid progenitor cells express high levels of EpoR and are directly responsive to Epo stimulation, the neuroprotective effect of Epo and its receptor may require two molecular events: the induction of Epo production by hypoxia and an increase in EpoR expression in neuronal cells resulting in increased sensitivity to Epo.

Erythropoietin crosses the blood-brain barrier to protect against experimental brain injury

Erythropoietin (EPO), recognized for its central role in erythropoiesis, also mediates neuroprotection when the recombinant form (r-Hu-EPO) is directly injected into ischemic rodent brain. We observed abundant expression of the EPO receptor at brain capillaries, which could provide a route for circulating EPO to enter the brain. In confirmation of this hypothesis, systemic administration of r-Hu-EPO before or up to 6 h after focal brain ischemia reduced injury by approximately 50-75%. R-Hu-EPO also ameliorates the extent of concussive brain injury, the immune damage in experimental autoimmune encephalomyelitis, and the toxicity of kainate. Given r-Hu-EPO's excellent safety profile, clinical trials evaluating systemically administered r-Hu-EPO as a general neuroprotective treatment are warranted.

Nonerythropoietic roles of erythropoietin in the fetus and neonate

Epo was once regarded as a cytokine with only hematopoietic effects. It is now clear that the distributions of Epo and Epo-R are more widespread in the developing human. Epo-R is widely distributed during early fetal development, leading to speculation that Epo acts in concert with other growth factors to optimize growth and development. Areas in which Epo has important recognized effects are on endothelial cells, and in the developing heart, gastrointestinal tract, and brain. It may also be important in the regulation of vascular growth during the menstrual cycle, and in the stimulation of testosterone production in men. Epo and Epo-R are prominent in the brain during fetal development, leading to speculation that they play an important role in neurodevelopment. There are also promising data regarding rEpo as a possible neuroprotective agent in such conditions as hypoxia, because it decreases programmed cell death induced during such adverse conditions. It is unlikely, however, that rEpo crosses the blood-brain barrier in normal premature infants, and it is not clear whether the CNS effects of rEpo, should it cross the blood-brain barrier, are harmful or beneficial in the setting of a developing brain.

Erythropoietin protects cultured cortical neurons, but not astroglia, from hypoxia and AMPA toxicity

In addition to its better-known hemopoietic action, erythropoietin (Epo) has neurotrophic properties and neuroprotective effects in some models of hypoxic-ischemic injury. To define further the cellular mechanisms underlying neuroprotection by Epo, we studied the effects of Epo on hypoxia with glucose deprivation in cultured rat cortical neurons and astroglia and on exposure to excitotoxins in cultured rat cortical neurons. Epo (30 pM) reduced neuronal, but not astroglial, cell death from hypoxia with glucose deprivation, and also attenuated the neurotoxic effect of (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), but not other excitotoxins. Epo appears to protect against cerebral ischemia through a direct effect on neurons that may be mediated in part by AMPA receptors.

The oviduct produces erythropoietin in an estrogen- and oxygen-dependent manner

Previously, we showed that erythropoietin (Epo) is produced in the mouse uterus, where Epo is indispensable for estrogen (E(2))-dependent angiogenesis. Expression of uterine Epo mRNA is stimulated by E(2) and hypoxia. The hypoxic induction requires the presence of E(2). In the present study, we examined other female reproductive organs in the mouse with respect to Epo mRNA expression and its stimuli (E(2) and hypoxia)-induced changes. Although Epo mRNA expression was seen in the ovary and oviduct, the E(2)-induced stimulation of Epo mRNA was found only in the oviduct. The E(2)-induced stimulation in the oviduct was transient and rapidly downregulated. Epo mRNA expression in the oviduct was hypoxia inducible, in both the presence and the absence of E(2). E(2)-dependent production of Epo and its mRNA expression were also found by use of cultured oviducts. The E(2) action is probably mediated through the E(2) receptor, and de novo protein synthesis is not required for E(2) induction of Epo mRNA. In the oviduct, the ampulla and isthmus regions produce Epo.

In vivo evidence that erythropoietin has a neuroprotective effect during subarachnoid hemorrhage

To ascertain in vivo whether recombinant human erythropoietin has a neuroprotective effect on the cortex during subarachnoid hemorrhage, 56 rabbits were divided into the following groups: Group 1 control sham operated plus placebo (n=14; saline solution - NaCl 0.9%); Group 2 control sham operated plus recombinant human erythropoietin (n=14); Group 3 subarachnoid hemorrhage plus placebo (n=14); Group 4 subarachnoid hemorrhage plus recombinant human erythropoietin (n=14; intraperitoneal administration of recombinant human erythropoietin immediately after inducing subarachnoid hemorrhage). In none of the Groups 1 and 2 animals was subarachnoid hemorrhage induced. In Group 3 rabbits, an increase in locomotor activity (open field apparatus) was observed 24, 48 and 72 h after surgery, and the mortality rate was 42.9% within 72 h after surgery, and, no increase in locomotor activity was observed in Group 4 rabbits, which survived for at least 72 h. Our findings suggest that recombinant human erythropoietin may be of benefit in the treatment of subarachnoid hemorrhage.

Erythropoietin in the cerebrospinal fluid of neonates who sustained CNS injury

We previously reported that erythropoietin (Epo) is present in human cerebrospinal fluid (CSF). It is not known whether CSF Epo concentrations change under conditions of CNS injury or, if so, whether this change reflects loss of blood-brain barrier integrity or increased CNS Epo synthesis. We hypothesized that CSF Epo increases in conditions of neural injury including hypoxia, meningitis, and intraventricular hemorrhage (IVH) and that CSF Epo concentrations are independent of plasma Epo concentrations. To test these hypotheses, Epo concentrations were measured in 122 paired CSF and blood samples obtained from neonates and children categorized as follows: 16, asphyxia; 31, meningitis; 11, IVH; 41, controls. Twelve infants treated with recombinant Epo (rEpo) and 11 additional samples from children with miscellaneous neurologic problems were also evaluated. CSF and plasma Epo concentrations were significantly higher in asphyxiated infants than in controls (225.0+/-155.0 versus 4.5+/-0.5 mU/mL; mean +/- SEM, p < 0.05, respectively, in CSF; 1806.7+/-1254 versus 5.2+/-0.5, p < 0.05 in plasma). Neonates with IVH had higher CSF Epo concentrations than controls (p < 0.01) but did not have higher plasma Epo concentrations than controls. Patients with meningitis did not have elevated CSF or plasma Epo concentrations. There was no correlation between CSF and plasma Epo concentrations in infants treated with rEpo. We conclude that Epo is selectively increased in the CSF by hypoxia, less so by IVH, and not at all by meningitis. rEpo treatment does not elevate CSF Epo. These findings suggest that rEpo does not cross the blood-brain barrier and that hypoxia induces increased CNS synthesis of Epo.

A potential role for erythropoietin in focal permanent cerebral ischemia in mice

The present study describes, for the first time, a temporal and spatial cellular expression of erythropoietin (Epo) and Epo receptor (Epo-R) with the evolution of a cerebral infarct after focal permanent ischemia in mice. In addition to a basal expression of Epo in neurons and astrocytes, a postischemic Epo expression has been localized specifically to endothelial cells (1 day), microglia/macrophage-like cells (3 days), and reactive astrocytes (7 days after occlusion). Under these conditions, the Epo-R expression always precedes that of Epo for each cell type. These results support the hypothesis that there is a continuous formation of Epo, with its corresponding receptor, during the active evolution of a focal cerebral infarct and that the Epo/Epo-R system might be implicated in the processes of neuroprotection and restructuring (such as angiogenesis and gliosis) after ischemia. To support this hypothesis, a significant reduction in infarct volume (47%; P < 0.0002) was found in mice treated with recombinant Epo 24 hours before induction of cerebral ischemia. Based on the above, we propose that the Epo/Epo-R system is an endogenous mechanism that protects the brain against damages consequent to a reduction in blood flow, a mechanism that can be amplified by the intracerebroventricular application of exogenous recombinant Epo.

Effects of erythropoietin on neuronal activity

Recently, erythropoietin (EPO) receptors and synthesis of EPO have been identified in the brain. To clarify the effects of EPO on neuronal cells, we investigated the effects of EPO on Ca2+ uptake, intracellular Ca2+ concentration, membrane potential, cell survival, release and biosynthesis of dopamine, and nitric oxide (NO) production in differentiated PC12 cells, which possess EPO receptors. EPO (10(-12)-10(-10) M) increased 45Ca2+ uptake and intracellular Ca2+ concentration in PC12 cells in a dose-related manner; these increases were inhibited by nicardipine (1 microM) or anti-EPO antibody (1:100 dilution). EPO induced membrane depolarization in PC12 cells. After a 5-day culture without serum and nerve growth factor (NGF), viable cell number decreased to 50% of that of the control cells cultured with serum and NGF. EPO (10(-13)-10(-10) M) increased the number of viable cells cultured without serum and NGF; this increase was blunted by nicardipine or anti-EPO antibody. Incubation with EPO (10(-13)-10(-10) M) stimulated mitogen-activated protein kinase activity in PC12 cells. EPO (10(-13)-10(-10) M) increased dopamine release from PC12 cells and tyrosine hydroxylase activity; these increases were sensitive to nicardipine or anti-EPO antibody. Following a 4-h incubation with EPO (10(-14)-10(-10) M), NO production was increased, which was blunted by nicardipine and anti-EPO antibody. In contrast, maximal NO synthase activity was not changed by EPO. These results suggest that EPO stimulates neuronal function and viability via activation of Ca2+ channels.

Erythropoietin modulates intracellular calcium in a human neuroblastoma cell line

1. Recent investigations have shown that the glycoprotein erythropoietin (Epo) and its specific receptor (EpoR) are present in the mammalian brain including human, monkey and mouse. These findings suggest a local action of Epo in the nervous system. The aim of this study was to elucidate a possible functional interaction of Epo with neuronal cells. 2. To examine the influence of externally applied Epo on Ca2+ homeostasis the human neuroblastoma cell line SK-N-MC was chosen as a suitable in vitro model for undifferentiated neuronal cells. 3. Expression of the EpoR in SK-N-MC cells was detected by reverse transcription-PCR, Western blot and immunofluorescence analysis. 4. Patch-clamp studies of SK-N-MC cells confirmed the expression of T-type Ca2+ channels, whose peak macroscopic current was increased by the addition of recombinant human Epo (rhEpo) to the bathing medium. 5. Confocal laser scanning microscopy analysis of SK-N-MC cells confirmed a transient increase in intracellular free [Ca2+] in response to externally applied rhEpo. 6. The transient response to Epo was dependent on external Ca2+ and remained even after depletion of internal Ca2+ stores by caffeine or thapsigargin. However, after depletion the response to Epo was absent when cells were superfused with the T-type Ca2+ channel blocker flunarizine. 7. This study demonstrates that Epo can interact with neuronal cells by affecting Ca2+ homeostasis through an increase in Ca2+ influx via plasma membrane T-type voltage-dependent Ca2+ channels.

Oxygen tension modulates beta-globin switching in embryoid bodies

Little is known about the factors influencing the hemoglobin switch in vertebrates during development. Inasmuch as the mammalian conceptus is exposed to changing oxygen tensions in utero, we examined the effect of different oxygen concentrations on beta-globin switching. We used an in vitro model of mouse embryogenesis based on the differentiation of blastocyst-derived embryonic stem cells to embryoid bodies (EBs). Cultivation of EBs at increasing oxygen concentrations (starting at 1% O2) did not influence the temporal expression pattern of embryonic (betaH1) globin compared to the normoxic controls (20% O2). In contrast, when compared to normoxically grown EBs, expression of fetal/adult (betamaj) globin in EBs cultured at varying oxygen concentrations was delayed by about 2 days and persisted throughout differentiation. Quantitation of hemoglobin in EBs using a 2,7-diaminofluorene-based colorimetric assay revealed the appearence of hemoglobin in two waves, an early and a late one. This observation was verified by spectrophotometric analysis of hemoglobin within single EBs. These two waves might reflect the switch of erythropoiesis from yolk sac to fetal liver. Reduced oxygenation is known to activate the hypoxia-inducible factor-1 (HIF-1), which in turn specifically induces expression of a variety of genes among them erythropoietin (EPO). Although EBs increased EPO expression upon hypoxic exposure, the altered beta-globin appearance was not related to EPO levels as determined in EBs overexpressing EPO. Since mRNA from both mouse HIF-1alpha isoforms was detected in all EBs tested at different differentiation stages, we propose that HIF-1 modulates beta-globin expression during development.

Systemic hypoxia changes the organ-specific distribution of vascular endothelial growth factor and its receptors

Vascular endothelial growth factor (VEGF) plays a key role in physiological blood vessel formation and pathological angiogenesis such as tumor growth and ischemic diseases. Hypoxia is a potent inducer of VEGF in vitro. Here we demonstrate that VEGF is induced in vivo by exposing mice to systemic hypoxia. VEGF induction was highest in brain, but also occurred in kidney, testis, lung, heart, and liver. In situ hybridization analysis revealed that a distinct subset of cells within a given organ, such as glial cells and neurons in brain, tubular cells in kidney, and Sertoli cells in testis, responded to the hypoxic stimulus with an increase in VEGF expression. Surprisingly, however, other cells at sites of constitutive VEGF expression in normal adult tissues, such as epithelial cells in the choroid plexus and kidney glomeruli, decreased VEGF expression in response to the hypoxic stimulus. Furthermore, in addition to VEGF itself, expression of VEGF receptor-1 (VEGFR-1), but not VEGFR-2, was induced by hypoxia in endothelial cells of lung, heart, brain, kidney, and liver. VEGF itself was never found to be up-regulated in endothelial cells under hypoxic conditions, consistent with its paracrine action during normoxia. Our results show that the response to hypoxia in vivo is differentially regulated at the level of specific cell types or layers in certain organs. In these tissues, up- or down-regulation of VEGF and VEGFR-1 during hypoxia may influence their oxygenation after angiogenesis or modulate vascular permeability.

Erythropoietin mRNA Expression in Human Fetal and Neonatal Tissue

Based on animal experiments, a switch of the erythropoietin (EPO) production site from the liver in the fetus to the kidneys in the adult has been postulated. To study the switch in humans, we have quantitated EPO mRNA expression in liver, kidney, spleen, and bone marrow of human fetuses and neonates by means of a competitive polymerase chain reaction (PCR). Tissue samples from 66 routine postmortem examinations were obtained. EPO mRNA was expressed in 97% of the tissue specimen derived from the liver (n = 66) and in 93% of those from the kidneys (17 weeks of gestation until 18 months after birth; n = 59). For the first time the EPO gene was found expressed in vivo in human spleen (96% of 64 samples) and in fetal and neonatal bone marrow (81% of 21 samples). EPO mRNA expression in the kidneys increased significantly beyond 30 weeks of gestation (P < .05). Although there was a slight decrease in EPO mRNA content per g liver tissue towards birth, the liver accounted for about 80% of the total body EPO mRNA. The contribution of the spleen and bone marrow were minor compared with liver and kidneys. Our results indicate that in humans the liver is the primary site of EPO gene expression not only in fetal, but also in neonatal life. A significant increase of renal EPO mRNA expression after 30 weeks of gestation might indicate the beginning switch.

© 1998 by The American Society of Hematology.

Coexpression of Erythropoietin and Vascular Endothelial Growth Factor in Nervous System Tumors Associated With von Hippel-Lindau Tumor Suppressor Gene Loss of Function

Hemangioblastomas are highly vascular tumors of the central nervous system that overexpress the hypoxia-inducible gene, vascular endothelial growth factor (VEGF), as a consequence of mutational inactivation of the von Hippel-Lindau tumor suppressor gene (VHL). Previous reports showed that hemangioblastomas can also express erythropoietin (Epo), which is also hypoxia-inducible. However, Epo expression in hemangioblastomas was observed only in individual cases, and the analyses were mainly based on indirect determination of erythropoiesis-stimulating activity. Therefore, we analyzed a series of 11 hemangioblastomas for Epo, VEGF, and VHL expression by Northern blot analysis and compared the results with normal brain and glioblastomas. Surprisingly, we observed Epo mRNA expression in all hemangioblastoma specimens analyzed, but in none of four glioblastomas. In contrast, VEGF mRNA was expressed in all hemangioblastomas and all glioblastomas. In situ hybridization revealed neoplastic stromal cells as Epo- and VEGF-producing cells in hemangioblastomas. These results suggest that in the nonhypoxic microenvironment of hemangioblastoma, Epo, similar to VEGF, might be negatively regulated by the VHL gene product.

© 1998 by The American Society of Hematology.

Erythropoietin mRNA Expression in Human Fetal and Neonatal Tissue

Based on animal experiments, a switch of the erythropoietin (EPO) production site from the liver in the fetus to the kidneys in the adult has been postulated. To study the switch in humans, we have quantitated EPO mRNA expression in liver, kidney, spleen, and bone marrow of human fetuses and neonates by means of a competitive polymerase chain reaction (PCR). Tissue samples from 66 routine postmortem examinations were obtained. EPO mRNA was expressed in 97% of the tissue specimen derived from the liver (n = 66) and in 93% of those from the kidneys (17 weeks of gestation until 18 months after birth; n = 59). For the first time the EPO gene was found expressed in vivo in human spleen (96% of 64 samples) and in fetal and neonatal bone marrow (81% of 21 samples). EPO mRNA expression in the kidneys increased significantly beyond 30 weeks of gestation (P &lt; .05). Although there was a slight decrease in EPO mRNA content per g liver tissue towards birth, the liver accounted for about 80% of the total body EPO mRNA. The contribution of the spleen and bone marrow were minor compared with liver and kidneys. Our results indicate that in humans the liver is the primary site of EPO gene expression not only in fetal, but also in neonatal life. A significant increase of renal EPO mRNA expression after 30 weeks of gestation might indicate the beginning switch.

© 1998 by The American Society of Hematology.

Coexpression of Erythropoietin and Vascular Endothelial Growth Factor in Nervous System Tumors Associated With von Hippel-Lindau Tumor Suppressor Gene Loss of Function

Hemangioblastomas are highly vascular tumors of the central nervous system that overexpress the hypoxia-inducible gene, vascular endothelial growth factor (VEGF), as a consequence of mutational inactivation of the von Hippel-Lindau tumor suppressor gene (VHL). Previous reports showed that hemangioblastomas can also express erythropoietin (Epo), which is also hypoxia-inducible. However, Epo expression in hemangioblastomas was observed only in individual cases, and the analyses were mainly based on indirect determination of erythropoiesis-stimulating activity. Therefore, we analyzed a series of 11 hemangioblastomas for Epo, VEGF, and VHL expression by Northern blot analysis and compared the results with normal brain and glioblastomas. Surprisingly, we observed Epo mRNA expression in all hemangioblastoma specimens analyzed, but in none of four glioblastomas. In contrast, VEGF mRNA was expressed in all hemangioblastomas and all glioblastomas. In situ hybridization revealed neoplastic stromal cells as Epo- and VEGF-producing cells in hemangioblastomas. These results suggest that in the nonhypoxic microenvironment of hemangioblastoma, Epo, similar to VEGF, might be negatively regulated by the VHL gene product.

© 1998 by The American Society of Hematology.

Tissue distribution of erythropoietin and erythropoietin receptor in the developing human fetus

OBJECTIVE

Erythropoietin receptors (Epo-R) have been demonstrated on several nonhematopoietic cell types in animal models and in cell culture. Our objective was to determine the tissue distribution and cellular specificity of erythropoietin (Epo) and its receptor in the developing human fetus.

STUDY DESIGN

The expression of Epo and Epo-R mRNA was ascertained by RT-PCR for organs ranging in maturity from 5 to 24 weeks postconception. The cellular location of protein immunoreactivity was then determined using specific antiEpo and antiEpo-R antibodies. Antibody specificity was established by Western analysis.

RESULTS

mRNA for Epo and Epo-R was found in all organs in the first two trimesters. Immunolocalization of Epo was limited to the liver parenchymal cells, kidney interstitial cells and proximal tubules, neural retina of the eye, and adrenal cortex. As development progressed, immunoreactivity in the kidney became more prominent. In contrast, immunoreactivity for Epo-R was widespread throughout the body, in cell types including endothelial cells, myocardiocytes, macrophages, retinal cells, cells of the adrenal cortex and medulla, as well as in small bowel, spleen, liver, kidney, and lung.

CONCLUSIONS

The distribution of Epo and its receptor is more widespread in the developing human than was initially postulated. Epo-R is expressed on many cell types during early fetal development, leading us to speculate that Epo acts in concert with somatic growth and development factors during this period. Further investigation is required to understand the nonhematopoietic role of Epo during human development.

Estrogen-dependent production of erythropoietin in uterus and its implication in uterine angiogenesis

Although erythropoietin (Epo) has been shown to possess in vitro angiogenic activity, its physiological significance has not been demonstrated. Normally angiogenesis does not occur actively in adults but an exception is the female reproductive organ. In the uterine endometrium, angiogenesis takes place actively for supporting the endometrial growth that occurs during transition from the diestrus to estrous stage. This transition is under control of 17beta-estradiol (E2), an ovarian hormone, and can be mimicked by injection of E2 to ovariectomized (OVX) mouse. Thus, the uterus is a pertinent site to examine the Epo function in angiogenesis. We found that Epo protein and its mRNA were produced in an E2-dependent manner, when the uterus from OVX mouse was cultured in vitro. The de novo protein synthesis was not needed for E2 induction of Epo mRNA. Administration of E2 to OVX mouse induced a rapid and transient increase in Epo mRNA in the uterus. Injection of Epo into the OVX mouse uterine cavity promoted blood vessel formation in the endometrium. Furthermore, injection of the soluble Epo receptor capable of binding with Epo into the uterine cavity of non-OVX mouse in diestrus stage inhibited the endometrial transition to proestrus stage, whereas heat-inactivated soluble Epo receptor allowed the transition to occur. These results, combined with our finding that the endothelial cells in uterine endometrium express Epo receptor, strongly suggest that Epo is an important factor for the E2-dependent cyclical angiogenesis in uterus.

Mouse Hypoxia-Inducible Factor-1α Is Encoded by Two Different mRNA Isoforms: Expression From a Tissue-Specific and a Housekeeping-Type Promoter

Hypoxic induction of erythropoietin (Epo) and other oxygen-dependent genes is mediated by the hypoxia-inducible factor-1 (HIF-1), a heterodimeric transactivator consisting of an α and a β subunit. We previously found that the mouse gene encoding HIF-1α harbors two alternative first exons (I.1 and I.2), giving rise to two different HIF-1α mRNA isoforms. Here, we show by RNase protection analysis that the exon I.1-derived mRNA isoform is differentially expressed in mouse tissues, being highest in kidney, tongue, stomach, and testis, but undetectable in liver, whereas the exon I.2 mRNA isoform is ubiquitously expressed. Sequence and methylation analysis showed that, in contrast to exon I.1, exon I.2 resides within a region showing typical features of a CpG island, known to be associated with the 5′ end of housekeeping genes. We identified a 232-bp minimal exon I.2 promoter that strongly induced reporter gene expression in mouse L929 fibroblasts and Hepa1 hepatoma cells. In contrast to L929 cells, the exon I.1 promoter was inactive in Hepa1 cells and hypoxic exposure (1% O2) markedly reduced exon I.2 promoter activity in Hepa1 cells. Prolonged exposure of mice to hypoxia (7.5% O2 for up to 72 hours) also caused a decrease in liver HIF-1α mRNA, whereas aldolase mRNA levels increased. These findings might be related to the relatively low Epo levels in the adult liver.

Mouse Hypoxia-Inducible Factor-1α Is Encoded by Two Different mRNA Isoforms: Expression From a Tissue-Specific and a Housekeeping-Type Promoter

Hypoxic induction of erythropoietin (Epo) and other oxygen-dependent genes is mediated by the hypoxia-inducible factor-1 (HIF-1), a heterodimeric transactivator consisting of an α and a β subunit. We previously found that the mouse gene encoding HIF-1α harbors two alternative first exons (I.1 and I.2), giving rise to two different HIF-1α mRNA isoforms. Here, we show by RNase protection analysis that the exon I.1-derived mRNA isoform is differentially expressed in mouse tissues, being highest in kidney, tongue, stomach, and testis, but undetectable in liver, whereas the exon I.2 mRNA isoform is ubiquitously expressed. Sequence and methylation analysis showed that, in contrast to exon I.1, exon I.2 resides within a region showing typical features of a CpG island, known to be associated with the 5′ end of housekeeping genes. We identified a 232-bp minimal exon I.2 promoter that strongly induced reporter gene expression in mouse L929 fibroblasts and Hepa1 hepatoma cells. In contrast to L929 cells, the exon I.1 promoter was inactive in Hepa1 cells and hypoxic exposure (1% O2) markedly reduced exon I.2 promoter activity in Hepa1 cells. Prolonged exposure of mice to hypoxia (7.5% O2 for up to 72 hours) also caused a decrease in liver HIF-1α mRNA, whereas aldolase mRNA levels increased. These findings might be related to the relatively low Epo levels in the adult liver.

In vivo evidence that erythropoietin protects neurons from ischemic damage

Erythropoietin (EPO) produced by the kidney and the liver (in fetuses) stimulates erythropoiesis. In the central nervous system, neurons express EPO receptor (EPOR) and astrocytes produce EPO. EPO has been shown to protect primary cultured neurons from N-methyl-D-aspartate (NMDA) receptor-mediated glutamate toxicity. Here we report in vivo evidence that EPO protects neurons against ischemia-induced cell death. Infusion of EPO into the lateral ventricles of gerbils prevented ischemia-induced learning disability and rescued hippocampal CA1 neurons from lethal ischemic damage. The neuroprotective action of exogenous EPO was also confirmed by counting synapses in the hippocampal CA1 region. Infusion of soluble EPOR (an extracellular domain capable of binding with the ligand) into animals given a mild ischemic treatment that did not produce neuronal damage, caused neuronal degeneration and impaired learning ability, whereas infusion of the heat-denatured soluble EPOR was not detrimental, demonstrating that the endogenous brain EPO is crucial for neuronal survival. The presence of EPO in neuron cultures did not repress a NMDA receptor-mediated increase in intracellular Ca2+, but rescued the neurons from NO-induced death. Taken together EPO may exert its neuroprotective effect by reducing the NO-mediated formation of free radicals or antagonizing their toxicity.

Elevated levels of erythropoietin in cerebrospinal fluid of depressed patients

To investigate the role of erythropoietin (EPO) in the central nervous systems, we assayed EPO concentrations in the cerebrospinal fluid (CSF) of patients with depression or old cerebrovascular injuries and controls. Concentrations of EPO in the CSF were significantly higher in 13 patients with depression (3.21+/-0.46 mU/mL) than in 10 patients with old cerebrovascular diseases (1.80+/-0.32 mU/mL, P < 0.01), and in 10 healthy controls (0.98+/-0.26 mU/mL, P < 0.01). Serum EPO concentrations did not differ among these three groups. In the patients with depression, 5 months of treatment with imipramine and/or nortriptyline significantly reduced EPO concentrations in the CSF (1.56+/-0.34 mU/ mL, P < 0.01). Results suggest that the brain of patients with depression may be in an hypoxic state, and that the increased EPO in the CSF may act to limit hypoxia-induced damage to neurons in these patients.

Embryonal Carcinoma P19 Cells Produce Erythropoietin Constitutively But Express Lactate Dehydrogenase in an Oxygen-Dependent Manner

Embryonic stem cells and embryonal carcinoma P19 cells produce erythropoietin (Epo) in an oxygen-independent manner, although lactate dehydrogenase A (LDHA) is hypoxia-inducible. To explore this paradox, we studied the operation of cis-acting sequences from these genes in P19 and Hep3B cells. The Epo gene promoter and 3′ enhancer from P19 cells conveyed hypoxia-inducible responses in Hep3B cells but not in P19 cells. Together with DNA sequencing and the normal transcription start site of P19 Epo gene, this excluded the possibility that the noninducibility of Epo gene in P19 cells was due to mutation in these sequences or unusual initiation of transcription. In contrast, reporter constructs containing LDHA enhancer and promoter were hypoxia inducible in P19 and Hep3B cells, and mutation of a hypoxia- inducible factor 1 (HIF-1) binding site abolished the hypoxic inducibility in both cells, indicating that HIF-1 activation operates normally in P19 cells. Neither forced expression of hepatocyte nuclear factor 4 in P19 cells nor deletion of its binding site from the Epo enhancer was effective in restoring Epo enhancer function. P19 cells may lack an unidentified regulator(s) required for interaction of the Epo enhancer with Epo and LDHA promoters.

Regulated human erythropoietin receptor expression in mouse brain

Erythropoietin (Epo) is known for its role in erythropoiesis and acts by binding to its receptor (EpoR) on the surface of erythroid progenitors. EpoR activity follows the site of hematopoiesis from the embryonic yolk sac to the fetal liver and then the adult spleen and bone marrow. Expression of EpoR has also been observed in selected cells of non-hematopoietic origin, such as the embryonic mouse brain during mid-gestation, at levels comparable to adult bone marrow. EpoR transcripts in brain decrease during development falling by birth to less than 1-3% of the level in hematopoietic tissue. We have now recapitulated this pattern of expression using a human EpoR transgene consisting of an 80-kb human EpoR genomic fragment. The highest level of expression was observed in the embryonic yolk sac and fetal liver, analogous to the endogenous gene, in addition to expression in adult spleen and bone marrow. Although activity of this transgene in brain is initially lower than the endogenous gene, it does exhibit the down-regulation observed for the endogenous gene in adult brain. The expression pattern of hybrid transgenes of an hEpoR promoter fused to beta-galactosidase in 9. 5-day embryos suggested that the hEpoR promoter region between -1778 and -150 bp 5' of the transcription start site is necessary to direct EpoR expression in the neural tube. EpoR expression in the neural tube may be the origin of the EpoR transcripts detected in brain during development. These data demonstrate that both the mouse and human EpoR genes contain regulatory elements to direct significant levels of expression in a developmentally controlled manner in brain and suggest that in addition to its function during erythropoiesis, EpoR may play a role in the development of selected non-hematopoietic tissue.

Developmental regulation of erythropoietin and erythropoiesis

It is well established that erythropoiesis occurs first in the yolk sac, then in the liver, subsequently moving to the bone marrow and, in rodents, the spleen during development. The origin of the erythropoietic precursors and some factors suggested to be important for the changing location of erythropoiesis are discussed in this review. Until recently, the major site of erythropoietin (Epo) production in the fetus was thought to be the liver, but studies have shown now that the Epo gene is expressed strongly in the fetal kidney, even in the temporary mesonephros. The metanephric Epo mRNA is upregulated by anemia, downregulated by glucocorticoids, and contributes substantially to circulating hormone levels in hemorrhaged ovine fetuses. Other sites of Epo and Epo receptor production, likely to have important actions during development, are the placenta and the brain.

Detection of erythropoietin in human liquor: intrinsic erythropoietin production in the brain

Until now, erythropoietin (EPO) was thought to be produced exclusively in fetal liver and adult kidney and to regulate mammalian erythropoiesis. However, we recently showed that steady state levels of EPO mRNA could be induced up to 100-fold in primary mouse astrocytes cultured under hypoxic conditions, and also reported the presence of mRNA for EPO and its receptor in the brain of mouse, monkey and human. In extending these studies on humans we now show that immunoreactive EPO is present in ventricular cerebrospinal fluid (CSF) of 5 patients with traumatic brain injuries: EPO was found in 15 out of 15 CSF samples. There was no correlation between the serum EPO concentration and the concentration in the CSF. However, EPO concentrations in CSF correlated with the degree of blood-brain-barrier dysfunction. This suggests that EPO does not cross the intact blood-brain-barrier, implying that EPO is produced in the brain itself, most probably by astrocytes in an oxygen-dependent manner. In view that neuronal cells carry the EPO receptor, we propose that EPO acts in a paracrine fashion in the central nervous system and might function as a protective factor against hypoxia-induced damage of neurons.

Insulin-like growth factors and insulin stimulate erythropoietin production in primary cultured astrocytes

Erythropoietin (EPO) is established as a major regulator of erythropoiesis. However, we and others have shown that neurons express erythropoietin receptor (EPO-R), that astrocytes produce EPO and that EPO may act as a neurotrophic factor in the CNS. We also found that EPO production is activated by insulin and insulin-like growth factors (IGFs) in astrocytes in a dose-dependent manner and that IGF-I was the most potent activator. The concentrations required for half-maximal activation were 3 nM IGF-I, 10 nM IGF-II and 100 nM insulin. The oxygen concentration regulates EPO production; hypoxia stimulates EPO production in astrocytes. The stimulatory effect of IGFs and insulin on EPO production in astrocytes was not affected by the oxygen concentration of astrocyte culture. Insulin and IGFs did not increase the total protein synthesis of astrocytes but increased EPO mRNA levels, indicating that EPO production is stimulated at the mRNA level. It appeared that the growth factor-induced accumulation of EPO mRNA in astrocytes was caused by activation of the tyrosine kinase-signal transduction pathway, because tyrosine phosphorylation of receptors for IGF-I and insulin was activated when astrocytes were stimulated by these growth factors.

Erythropoietin receptor is expressed in rat hippocampal and cerebral cortical neurons, and erythropoietin prevents in vitro glutamate-induced neuronal death

Recently, erythropoietin has been shown to be produced by astrocytes and its production is hypoxia-inducible. In the present study, we demonstrated, using a reverse transcription-polymerase chain reaction assay and immunostaining of the cells, that the erythropoietin receptor was expressed in cultured hippocampal and cerebral cortical neurons of day 19 rat embryo. Erythropoietin protected the cultured neurons from glutamate neurotoxicity. Neurons cultured for seven to 10 days were exposed to glutamate for 15 min and after culture for a further 24 h in the absence of glutamate the neuron survival was assayed. Significant protection was observed with erythropoietin from 3 pM (c. 100 pg/ml) in a dose-dependent manner. The protection was completely reversed by co-application of a soluble erythropoietin receptor, an extracellular domain capable of binding with erythropoietin. For exhibition of the neuroprotective effect, exposure of neurons to erythropoietin approximately 8 h prior to exposure to glutamate was required. Experiments with the inhibitors indicated that RNA and protein syntheses were necessary for the protection. However, exposure to erythropoietin for a short period (5 min or less) was sufficient to elicit the protective effect. The protective effect of erythropoietin was blocked by the simultaneous addition of EGTA. These findings and the previous finding that erythropoietin induces a rapid and transient increase in intracellular Ca2+ concentration in neuronal cells suggest that erythropoietin plays a neuroprotective role in brain injury caused by hypoxia or ischemia and that erythropoietin-induced Ca2+ influx from outside of the cells is a critical initial event yielding an enhanced resistance of the neurons to glutamate toxicity.