Stimulation of the adenylate cyclase activity of rabbit bone marrow immature erythroblasts by erythropoietin and haemin

Role of cyclic nucleotides in fetal hemoglobin induction in cultured CD34+ cells

OBJECTIVE

In vivo, several drugs have been shown to increase fetal hemoglobin (HbF), including 5-azacytidine (AZA), sodium butyrate (SB), and hydroxyurea (HU). Studies in K562 cells suggest that cyclic guanosine monophosphate (cGMP) is required for HbF induction; however, the role of cyclic nucleotides in HbF induction in primary erythroid cultures has not been established.

METHODS

CD34-selected peripheral blood monocytes cultured in a semi-solid serum-free system that mimics in vivo F-cell production are utilized to explore the role of cyclic adenosine monophosphate (cAMP) and cGMP in HbF induction in response to HU, AZA, and SB.

RESULTS

In serum-free CD34 cultures, HU, SB, and AZA all markedly stimulate FNRBC production up to 30-fold, associated with induction of gamma-globin mRNA and total HbF protein. Guanylate cyclase inhibition results in only minimal blunting of HbF induction by each agent. In contrast, adenylate cyclase inhibition markedly reduces HU, SB, and AZA-mediated FNRBC induction and gamma-globin mRNA induction. The adenylate cyclase activator forskolin modestly induces FNRBC production and augments the action of standard induction agents. HU, AZA, and SB, however, fail to significantly stimulate adenylate cyclase themselves.

CONCLUSIONS

In human CD34(+) cultures, cAMP production is required for full induction of HbF by HU, SB, and AZA, while perturbation of cGMP production has only minimal effects. These findings are in marked contrast to data in K562 cells where cGMP production is critical for HbF induction while cAMP stimulation blunts HbF response, and suggest that these agents may share a common induction pathway.

cAMP/PKA-mediated regulation of erythropoiesis

The role of cyclic AMP (cAMP) as second messenger in erythropoiesis has been suggested in the early 1980s. However, careful analysis showed that cAMP is not generated in direct response to the main erythropoiesis-controlling cytokines such as erythropoietin (Epo). As a result, cAMP disappeared from the central stage in research of erythropoiesis. Instead, other signal transduction pathways, including the Ras/extracellular regulated kinase (ERK)-pathway, the phosphatidylinositol 3-kinase (P13K) and the signal transducer and activator of transcription (STAT5)-pathways, have been found and explored. In concert, these signaling pathways control the transcriptional machinery of erythroid cells. Although cAMP is not directly generated in response to Epo stimulation, it has recently been demonstrated that increased cAMP-levels and in particular the cAMP-dependent protein kinase A (PKA) can modulate erythroid signal transduction pathways. In some cases, like the ERK-signaling pathway, PKA affects signal transduction by regulating the balance between specific phosphatases and kinases. In other cases, such as the STAT5 pathway, PKA enhances Epo signaling by inducing recruitment of additional co-regulators of transcription. In addition to STAT5, PKA also activates other transcription factors that are required for erythroid gene expression. This review discusses the impact of cAMP/PKA on Epo-mediated signaling pathways and summarizes the role of cAMP in malignant erythropoiesis.

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.

Modulation of Calcium Channels in Human Erythroblasts by Erythropoietin

Erythropoietin (Epo) induces a dose-dependent increase in intracellular free Ca2+ ([Ca2+]i ) in human erythroblasts, which is dependent on extracellular Ca2+ and blocked by high doses of nifedipine or Ni2+. In addition, pretreatment of human erythroblasts with mouse antihuman erythropoietin receptor antibody but not mouse immunopure IgG blocked the Epo-induced [Ca2+]i increase, indicating the specificity of the Ca2+ response to Epo stimulation. In this study, the erythropoietin-regulated calcium channel was identified by single channel recordings. Use of conventional whole cell patch-clamp failed to detect Epo-induced whole cell Ca2+ current. To minimize washout of cytosolic constituents, we next used nystatin perforated patch, but did not find any Epo-induced whole cell Ca2+ current. Using Ba2+ (30 mmol/L) as charge carrier in cell-attached patches, we detected single channels with unitary conductance of 3.2 pS, reversal potential of +72 mV, and whose unitary current (at +10 mV) increased monotonically with increasing Ba2+ concentrations. Channel open probability did not appreciably change over the voltage range (−50 to +30 mV) tested. Epo (2 U/mL) increased both mean open time (from 4.27 ± 0.75 to 11.15 ± 1.80 ms) and open probability (from 0.26 ± 0.06 to 2.56 ± 0.59%) of this Ba2+-permeable channel. Our data strongly support the conclusion that the Epo-induced [Ca2+]i increase in human erythroblasts is mediated via Ca2+ entry through a voltage-independent Ca2+ channel.

Modulation of Calcium Channels in Human Erythroblasts by Erythropoietin

Erythropoietin (Epo) induces a dose-dependent increase in intracellular free Ca2+ ([Ca2+]i ) in human erythroblasts, which is dependent on extracellular Ca2+ and blocked by high doses of nifedipine or Ni2+. In addition, pretreatment of human erythroblasts with mouse antihuman erythropoietin receptor antibody but not mouse immunopure IgG blocked the Epo-induced [Ca2+]i increase, indicating the specificity of the Ca2+ response to Epo stimulation. In this study, the erythropoietin-regulated calcium channel was identified by single channel recordings. Use of conventional whole cell patch-clamp failed to detect Epo-induced whole cell Ca2+ current. To minimize washout of cytosolic constituents, we next used nystatin perforated patch, but did not find any Epo-induced whole cell Ca2+ current. Using Ba2+ (30 mmol/L) as charge carrier in cell-attached patches, we detected single channels with unitary conductance of 3.2 pS, reversal potential of +72 mV, and whose unitary current (at +10 mV) increased monotonically with increasing Ba2+ concentrations. Channel open probability did not appreciably change over the voltage range (−50 to +30 mV) tested. Epo (2 U/mL) increased both mean open time (from 4.27 ± 0.75 to 11.15 ± 1.80 ms) and open probability (from 0.26 ± 0.06 to 2.56 ± 0.59%) of this Ba2+-permeable channel. Our data strongly support the conclusion that the Epo-induced [Ca2+]i increase in human erythroblasts is mediated via Ca2+ entry through a voltage-independent Ca2+ channel.

Mitogenic effect of erythropoietin on neonatal rat cardiomyocytes: signal transduction pathways

The mitogenic effect of recombinant human erythropoietin (rHuEpo) on primary cultures of neonatal rat cardiac myocytes was observed. rHuEpo triggered a dose-dependent increase in myocyte proliferation. The hormone effect over optimally grown control culture 1 day after addition was maximum with 0.5 U/ml and was inhibited with anti-rHuEpo. Inhibitors of enzymatic pathways known to be involved in the cytokines intracellular mechanism such as genistein (tyrosine kinase inhibitor), 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (phospholipase C [PLC] inhibitor), and 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine (protein kinase C [PKC] inhibitor) prevented the mitogenic action of rHuEpo. Also the inhibition of Na(+)-K(+)-ATPase activity by ouabain blunted the stimulatory action of rHuEpo on cell proliferation. The mitogenic action of the hormone was correlated with cardiac membrane paranitrophenylphosphatase (pNPPase) and PKC activity, since concentrations of rHuEpo that stimulate DNA synthesis increased pNPPase and PKC activity. Moreover, the enzymatic inhibition of tyrosine kinase, PLC, and PKC attenuated the stimulatory action of rHuEpo upon cardiac pNPPase activity. In this paper we demonstrate a non-hematopoietic action of rHuEpo showing both mitogenic and enzymatic effect upon primary myocyte cell culture and on pNPPase activity of neonatal rat heart. These effects are related to the capacity of rHuEpo to stimulate Na(+)-K(+)-ATPase activity and appear to be secondary to the activation of tyrosine kinase and PKC, indicating that in the rHuEpo mediated mitogenic action on cardiomyocytes involves the activation of the same enzymatic pathways that have been described by other cytokines in different tissues.

Ion channels in human erythroblasts. Modulation by erythropoietin

To investigate the mechanism of intracellular Ca2+ ([Cai]) increase in human burst-forming unit-erythroid-derived erythroblasts by erythropoietin, we measured [Cai] with digital video imaging, cellular phosphoinositides with high performance liquid chromatography, and plasma membrane potential and currents with whole cell patch clamp. Chelation of extracellular free Ca2+ abolished [Cai] increase induced by erythropoietin. In addition, the levels of inositol-1,4,5-trisphosphate did not increase in erythropoietin-treated erythroblasts. These results indicate that in erythropoietin-stimulated cells, Ca2+ influx rather than intracellular Ca2+ mobilization was responsible for [Cai] rise. Both Ni2+ and moderately high doses of nifedipine blocked [Cai] increase, suggesting involvement of ion channels. Resting membrane potential in human erythroblasts was -10.9 +/- 1.0 mV and was not affected by erythropoietin, suggesting erythropoietin modulated a voltage-independent ion channel permeable to Ca2+. No voltage-dependent ion channel but a Ca(2+)-activated K+ channel was detected in human erythroblasts. The magnitude of erythropoietin-induced [Cai] increase, however, was insufficient to open Ca(2+)-activated K+ channels. Our data suggest erythropoietin modulated a voltage-independent ion channel permeable to Ca2+, resulting in sustained increases in [Cai].

Signal transduction by the erythropoietin receptor: evidence for the activation of phospholipases A2 and C

Erythropoietin (Ep) is the peptide growth factor whose actions on the erythroid progenitor cell induce terminal differentiation. However, the intracellular signaling system that is activated by Ep is poorly understood. Our previous studies have implicated the lipoxygenase metabolites of arachidonic acid in the actions of Ep. In this study, we report an early (30 s to 5 min) increase in levels of two lipoxygenase metabolites: leukotriene B4 (LTB4; 3- to 5-fold) and 12-hydroxyeicosatetraenoic acid (12-HETE; 2-fold). These responses were blocked by an antibody to Ep, by lipoxygenase inhibitors, or by 1,6-di[O-(carbamoyl)cyclohexanone oxime]hexane (RHC80267), an inhibitor of diacylglycerol (DAG) lipase. RHC 80267 also significantly inhibited Ep-mediated proliferation. Ep induced the release of [3H]arachidonic acid from cellular phospholipids at 5 min and also increased DAG accumulation at 1 min with a maximum increase of 68.2% over control seen at 30 min. No increase in levels of inositol trisphosphate or phosphatidic acid was observed in response to Ep. Taken together, these data suggest that the signal transduction pathway of the Ep receptor includes the activation of phospholipases A2 and C, resulting in the liberation of DAG and arachidonate and the subsequent formation of LTB4 and 12-HETE.

Induction of tyrosine phosphorylation by the erythropoietin receptor correlates with mitogenesis

A role for tyrosine phosphorylation in the signal-transducing mechanisms of several hematopoietic growth factors has been hypothesized. To extend these observations, we have examined the effects of erythropoietin (Epo) on tyrosine phosphorylation in an Epo-responsive cell that was obtained by transfecting the murine erythropoietin receptor (EpoR) into an interleukin-3 (IL-3)-dependent cell line. By two-dimensional analysis of phosphotyrosine-containing proteins isolated with a monoclonal antibody (1G2) against phosphotyrosine, Epo and IL-3 were found to rapidly induce tyrosine phosphorylation of comparable substrates of 92, 70, and 56 kDa. In addition, Epo uniquely induced phosphorylation of a 72-kDa substrate while IL-3 uniquely induced phosphorylation of a 140-kDa substrate. Immunoprecipitation and mixing experiments indicated that the 72-kDa substrate may represent a small fraction of the EpoR. To explore the significance of tyrosine phosphorylation, we generated two mutants of the EpoR that lacked 108 or 146 amino acids at their carboxyl termini. In addition we constructed an internally deleted mutant that lacked 20 amino acids in a region of sequence homology with the IL-2 receptor beta chain. Although all mutants were expressed at comparable levels and had comparable binding affinities for Epo, only the mutant lacking 108 amino acids at the carboxyl terminus retained significant mitogenic activity or the ability to induce tyrosine phosphorylation.

Induction of tyrosine phosphorylation by the erythropoietin receptor correlates with mitogenesis

A role for tyrosine phosphorylation in the signal-transducing mechanisms of several hematopoietic growth factors has been hypothesized. To extend these observations, we have examined the effects of erythropoietin (Epo) on tyrosine phosphorylation in an Epo-responsive cell that was obtained by transfecting the murine erythropoietin receptor (EpoR) into an interleukin-3 (IL-3)-dependent cell line. By two-dimensional analysis of phosphotyrosine-containing proteins isolated with a monoclonal antibody (1G2) against phosphotyrosine, Epo and IL-3 were found to rapidly induce tyrosine phosphorylation of comparable substrates of 92, 70, and 56 kDa. In addition, Epo uniquely induced phosphorylation of a 72-kDa substrate while IL-3 uniquely induced phosphorylation of a 140-kDa substrate. Immunoprecipitation and mixing experiments indicated that the 72-kDa substrate may represent a small fraction of the EpoR. To explore the significance of tyrosine phosphorylation, we generated two mutants of the EpoR that lacked 108 or 146 amino acids at their carboxyl termini. In addition we constructed an internally deleted mutant that lacked 20 amino acids in a region of sequence homology with the IL-2 receptor beta chain. Although all mutants were expressed at comparable levels and had comparable binding affinities for Epo, only the mutant lacking 108 amino acids at the carboxyl terminus retained significant mitogenic activity or the ability to induce tyrosine phosphorylation.

Hematopoietic growth factor receptors

The molecular cloning for most of the hematopoietic growth factor receptors has been achieved over the past few years and revealed that they can by assigned to two discrete receptor families, namely the hematopoietic growth factor superfamily (HRS) and the receptor tyrosine kinase family (RTK). The members of the HRS, including granulocyte-macrophage colony-stimulating factor receptor (GM-CSF-R), interleukin 3 receptor (IL-3-R), granulocyte CSF receptor (G-CSF-R) and erythropoietin receptor (Epo-R), share a common binding domain and the absence of a tyrosine kinase domain in their cytoplasmic portion. In some cases (e.g., GM-CSF-R), the high-affinity receptor structure is obtained through the association of the low-affinity binding chain (alpha chain) with an accessory protein (beta chain). It is conceivable that this protein might also represent the common subunit shared by GM-CSF-R and by IL-3-R when they are co-expressed to form the putative GM-CSF-R/IL-3-R complex. Although tyrosine phosphorylation following ligand receptor activation seems to be a common event in the HRS, its role in the signal transduction mechanisms is unknown. Due to the structural analogies among the members of this family any new insight into one particular receptor member, such as its subunit structure and its signal transduction pathways, will be generalizable to the other family members. The subclass III of the RTK family, including the CSF-1-R and c-kit, is characterized by an additional insert into the kinase domain that recognizes and binds protein substrates. Ligand induced activation of the kinase domain and its signaling potential are mediated by receptor oligomerization which stabilizes interactions between adjacent cytoplasmic domains and leads to activation of kinase function by molecular interaction. Interestingly, the receptors included in this subclass are the products of well known cellular proto-oncogenes. A large variety of structural alteration found in receptor-derived oncogene products may lead to constitutive activation of receptor signals that, consequently, result in the subversion of the mechanisms controlling the cell growth.

Transmembrane signaling during erythropoietin- and dimethylsulfoxide-induced erythroid cell differentiation

Erythropoietin is a glycoprotein factor which specifically regulates the proliferation and differentiation of erythroid progenitor cells. We have investigated here the biochemical mechanisms of erythroid differentiation on mouse erythroleukemia SKT6 cells which can be induced to differentiate either with erythropoietin or dimethyl sulfoxide (Me2SO). cAMP-elevating agents, such as forskolin and 3-isobutyl-1-methyl-xanthine, caused spontaneous erythroid differentiation, and these agents showed the stimulatory effects on erythropoietin- or Me2SO-induced differentiation. An adenylate cyclase inhibitor, 2',5'-dideoxyadenosine, blocked erythropoietin-induced differentiation. The intracellular cAMP level was rapidly increased by addition of erythropoietin but not by Me2SO. These observations suggest that erythroid differentiation induced by erythropoietin is mediated, at least in part, through the cAMP-dependent pathway. When the effect of erythropoietin and Me2SO on the intracellular Ca2+ level was examined using fura 2, no acute change was observed. Measurements of the levels of inositol 1,4,5-trisphosphate and diacylglycerol following stimulation with erythropoietin or Me2SO showed that phosphatidylinositol turnover did not change significantly after erythropoietin stimulation but decreased gradually after Me2SO induction. Taken together, these results indicate that a complex signaling network including the cAMP-dependent pathway is involved in the erythroid differentiation process.

Erythropoietin receptor. Subunit structure and activation

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Rapid activation by erythropoietin of protein kinase C in nuclei of erythroid progenitor cells

The glycoprotein hormone erythropoietin (Ep) regulates the proliferation and differentiation of erythroid progenitor cells by a signal transduction system which is not well understood. It has recently been reported that prolactin, a mitogen and trophic hormone for liver, will activate a nuclear protein kinase C in hepatocytes. As similarities exist in the actions of Ep and prolactin in their target cells, we tested the hypothesis that Ep could activate protein kinase C in nuclei isolated from erythroid progenitor cells. In a pure population of such nuclei, Ep induced a rapid, time- and dose-dependent increase in phosphorylation of endogenous nuclear substrate which could be blocked by inhibitors of protein kinase C or by antibody to Ep. Other known activators of protein kinase C were also effective in this system. These findings show that Ep may exert its effects by a novel signalling pathway, the activation of a nuclear protein kinase C.

Characterization of erythropoietin receptor on erythropoietin-unresponsive mouse erythroleukemia cells

A membrane receptor for erythropoietin was identified in various erythropoietin-unresponsive mouse erythroleukemia cells. Scatchard analyses of the binding of human 125I-labeled erythropoietin to T3C1-2-0, K-1, GM86 and 707 cells showed the presence of a single class of binding sites with apparent Kd values of 0.27-0.78 nM, which are slightly higher than those of erythropoietin-responsive cells. The number of binding sites varied from 110 to 930 per cell. Crosslinking of 125I-erythropoietin to its binding sites with disuccinimidyl suberate revealed the existence of a single binding protein with molecular mass of 63 kDa. No binding sites with higher molecular mass, as observed in erythropoietin-responsive cells, were detected, nor was any specific binding observed to the non-erythroid hematopoietic cell or to the human erythroleukemia cells examined.

Independent activation of adenylate cyclase by erythropoietin and isoprenaline

The possibility that catecholamines modulate the erythropoietin-induced increase in production of cyclic AMP was investigated by examining the effect of erythropoietin and/or L-isoprenaline on the activity of the plasma membrane adenylate cyclase of anaemic rabbit bone marrow erythroblasts. Membranes isolated from cells cultured in the presence of both hormones exhibited both the transient stimulation of basal activity characteristic of erythropoietin action and the loss of the in vitro response to L-isoprenaline, concomitant with the loss of beta-adrenergic receptors, characteristic of L-isoprenaline stimulation. The presence of erythropoietin during cell culture with L-isoprenaline had no effect on the desensitization or number of beta-adrenergic receptors. The stimulation of adenylate cyclase by erythropoietin was observed also in the presence of the beta-antagonist propranolol, when both were added either to whole cells or to isolated membranes. We conclude that these two hormones activate adenylate cyclase independently of each other, via different receptors, with little evidence of cross-modulation.

Fructose 2,6-bisphosphate and glucose 1,6-bisphosphate in rabbit erythroid cells during differentiation

Fructose 2,6-bisphosphate concentration and 6-phosphofructo-2-kinase activity markedly decrease during differentiation of rabbit erythroid cells, being higher in erythroblasts (654 +/- 97 pmol/10(9) cells; 238 +/- 81 U mu/10(9) cells) than in reticulocytes (40 +/- 15 pmol/10(9) cells; 11 +/- 3 U mu/10(9) cells) and much higher than in mature erythrocytes (10 +/- 0.8 pmol/10(9) cells; 2 +/- 1 U mu/10(9) cells). The enzymatic activities involved in glucose 1,6-bisphosphate metabolism also decrease, but the levels of aldohexose 1,6-bisphosphates remain essentially constant during differentiation of erythroid cells.

The role of cAMP and calcium in the stimulation of proliferation of immature erythroblasts by erythropoietin

The hypothesis that cAMP or calcium are the second messengers of erythropoietin (Epo) was tested on fractionated, Epo-responsive immature erythroblasts from anemic rabbit bone marrow by examining whether the proliferative effects of the hormone could be mimicked by agents that increase the intracellular concentration of cAMP or Ca2+. None of the compounds tested (including 10(-6)-10(-4) M db-cAMP, forskolin, isoprenaline or 10(-7)-10(-6) M of the calcium ionophore A23187) alone or in combination could either initiate or potentiate the mitogenic action of the hormone. Furthermore, addition of 0.2 U/ml erythropoietin produced no permanent or transient increase in the uptake of 45Ca2+ by erythroblasts at 37 degrees C. However, cells cultured with imidazole or cordycepin (which reduce the level of intracellular cAMP), or with the calcium chelator EGTA, or the drugs verapamil or TMB-8 (which interfere with the utilization of extracellular or intracellular calcium) showed a decreased stimulation of DNA synthesis by Epo. Finally, the tumour promoter phorbol ester TPA could partially mimic the action of Epo when added to cultures containing more immature progenitor cells. We conclude then that an artificial increase in the cytoplasmic concentration of either cAMP or Ca2+ is not sufficient to elicit the proliferation of Epo-responsive cells.

Murine erythroleukaemia cells (Friend cells) possess high-affinity binding sites for erythropoietin

Murine erythroleukaemia cells represent erythroid precursors blocked near the CFU-E or proerythroblast stage. In contrast to their non-leukaemic equivalents, neither their proliferation nor their differentiation seems to be affected by erythropoietin. However, we show in this paper that both uncommitted and committed, benzidine-positive, cells bind iodinated erythropoietin. The binding is of high affinity (Kd = 490 +/- 160 pM) and reversible with a half-life of the complex of 77 +/- 19 min. The number of binding sites is low (300-600 per cell). In contrast the haematopoietic non-erythroid cell lines HL 60 and L 1210 and the myeloid-erythroid human cell line K 562 do not exhibit specific binding. If these binding sites represent true hormone receptors, their presence on a permanent cell line should facilitate erythropoietin receptor purification.