Prostaglandin-concentration-dependent desensitization of adenylate cyclase in human erythroleukaemia (HEL) cells is abolished by pertussis toxin and enhanced by induction by dimethyl sulphoxide

Role of G proteins and ERK activation in hemin-induced erythroid differentiation of K562 cells

Heterotrimeric G proteins which couple extracellular signals to intracellular effectors play a central role in cell growth and differentiation. The pluripotent erythroleukemic cell line K562 that acquires the capability to synthesize hemoglobin in response to a variety of agents can be used as a model system for erythroid differentiation. Using Western blot analysis and RT-PCR, we studied alterations in G protein expression accompanying hemin-induced differentiation of K562 cells. We demonstrated the presence of G(alpha s), G(alpha i2) and G(alpha q) and the absence of G(alpha i1), G(alpha o) and G(alpha 16) in K562 cells. We observed the short form of G(alpha s) to be expressed predominantly in these cells. Treatment of K562 cells with hemin resulted in an increase in the levels of G(alpha s) and G(alpha q). On the other hand, the level of G(alpha i2) was found to increase on the third day after induction with hemin, followed by a decrease to levels lower of those of uninduced cells. The mitogen-activated protein kinase ERK1/2 pathway is crucial in the control of cell proliferation and differentiation. Both Gi- and Gq-coupled receptors stimulate MAPK activation. We therefore examined the phosphorylation of ERK1/2 during hemin-induced differentiation of K562 cells. Using anti-ERK1/2 antibodies, we observed that ERK2 was primarily phosphorylated in K562 cells. ERK2 phosphorylation increased gradually until 48 h and returned to basal values by 96 h following hemin treatment. Our results suggest that changes in G protein expression and ERK2 activity are involved in hemin-induced differentiation of K562 cells.

Concomitant regulation of Ca2+ mobilization and G13 expression in human erythroleukemia cells

In human erythroleukemia (HEL) cells, stimulation of alpha2-adrenoceptors by adrenaline or neuropeptide Y Y1 receptors by neuropeptide Y, concomitantly inhibit cAMP accumulation and stimulate mobilization of Ca2+ from intracellular stores via pertussis toxin-sensitive G-proteins. Treatment of HEL cells in chemically-defined, serum-free medium with 1.25% dimethylsulfoxide (DMSO) for 4 days, increased alpha2-adrenoceptor number by 120%, while the neuropeptide Y receptor number was not significantly changed. In DMSO-treated HEL cells, Ca2+ elevations by adrenaline or neuropeptide Y were significantly reduced by 28% and 57%, respectively, while basal Ca2+ and elevations by thrombin or thapsigargin were not significantly altered. Adrenaline and neuropeptide Y-induced inhibition of forskolin-stimulated cAMP accumulation was not significantly altered upon DMSO treatment. While immunodetectable alpha-subunits of Gi2 were not significantly changed by DMSO treatment, those of Gi3 were reduced by 27%. Inactivation of pertussis toxin substrates by pertussis toxin treatment and inhibition of adrenaline or neuropeptide Y stimulated Ca2+ elevations were linearly correlated. These data are compatible with the idea that, in HEL cells, alpha2-adrenoceptors and neuropeptide Y receptors couple to inhibition of adenylyl cyclase via Gi2 while they couple to Ca2+ elevations via Gi3.

Co-expression of prostaglandin receptors with opposite effects: a model for homeostatic control of autocrine and paracrine signaling

Prostaglandins are ubiquitous autocrine mediators that exert their effects through a number of G protein-coupled receptors. Many organs and tissues express many of the prostaglandin receptors, and prostaglandins have diverse effects on individual organs and tissues. In some cases, several prostaglandin receptors are expressed on a single cell type. Co-expressed prostaglandin receptors frequently appear to have opposite actions, suggesting homeostatic control of prostaglandin effects. Co-expression of opposing receptors provides a molecular mechanism for weak or partial agonism and explains the action of a drug as a mixed agonist/antagonist. The physiological relevance of co-expressed opposing receptors for a single agonist perhaps can be explained in terms of the difference between endocrine and autocrine mediators. Endocrine hormones are generally produced by cells distant from their site of action so that they are diluted to an elevated but stable concentration by the time they reach their target cells. In contrast, autocoids are produced by the same cell type on which they act and may reach transiently high levels at their sites of action. The presence of a second type of receptor that negates the action of the first receptor would tend to buffer cellular responses to transient extremes of agonist concentration. The slow onset of inhibition would also allow for time-dependent buffering, providing additional control over autocoid release and effect. The mechanism is relevant to other autocrine and paracrine mediators including neurotransmitters, which reach transiently high concentrations in the synaptic cleft.

Protein kinase C-dependent regulation of human erythroleukemia (HEL) cell sphingosine kinase activity

Sphingosine kinase functions in both the catabolism of sphingosine and in signal transduction pathways utilizing sphingosine-1-phosphate. The regulation of sphingosine kinase activity in human erythroleukemia (HEL) cells was investigated by treatment with several bioactive agents. Treatment of HEL cells with phorbol 12-myristate 13-acetate (PMA) caused a time- and concentration-dependent increase in sphingosine kinase activity measured in vitro. Sphingosine kinase activity increased in a phorbol ester- and diacylglycerol-specific manner. Staurosporine and calphostin C, protein kinase C (PKC) inhibitors, blocked the increased in sphingosine kinase activity, suggesting a PKC-dependent regulation. The effects of PMA on sphingosine kinase were dependent on transcription and translation. Purified PKC had no direct effect on sphingosine kinase activity. However, these studies led to the observation that HEL cell sphingosine kinase activity is stimulated in vitro by phosphatidylserine. Interestingly, other inducers of HEL cell differentiation, dimethylsulfoxide and retinoic acid, did not affect sphingosine kinase activity. These results indicate a separate and distinct pathway of PKC-dependent sphingosine kinase activation, and suggest a role for sphingosine kinase in regulation of cell function.

G-protein-coupled receptors in normal human erythroid progenitor cells

Human erythroid progenitor cells were isolated from peripheral blood of healthy donors and amplified in a suspension culture system using recombinant growth factors (stem cell factor, interleukin-3, granulocyte-macrophage colony-stimulating factor and erythropoietin) as well as conditioned medium from a human bone marrow stroma cell line to support cell proliferation. After 6-8 days of culture, the cell population consisted mainly of erythroid colony-forming cells (burst-forming units, BFU-Es and colony-forming units, CFU-Es). In these cells, we studied ligand-induced changes in intracellular Ca2+ concentration ([Ca2+]i) and cAMP formation as the primary effector systems of guanine nucleotide-binding protein (G protein)-coupled receptors. The results confirmed the functional expression of receptors for adenosine (type A2B), prostaglandin E1 and isoprenaline (beta-adrenoceptor), all of which stimulated adenylyl cyclase, as well as for ADP (purinoceptor types P2T and P2U), platelet-activating factor and thrombin all of which caused a transient increase in [Ca2+]i. The efficacy of adenosine and prostaglandin E1 in stimulating cAMP formation was more than 5 times higher than that of isoprenaline, suggesting a low beta-adrenoceptor density. The response to adenosine and isoprenaline decreased by 80 and 55% respectively during maturation into the proerythroblast stage. Similarly, thapsigargin-sensitive intracellular Ca2+ stores and ligand-induced Ca2+ release declined by about 60% during the CFU-E-to-erythroblast transition. The overall functional expression pattern of G protein-coupled receptors differed from that in human erythroleukaemia cell lines or from that in platelets. Primary culture systems for nontransformed cells, such as the one presented here, thus will be indispensable for the study of the functional role of G protein-dependent signalling during haematopoiesis.

Rapid desensitization of adrenaline- and neuropeptide Y-stimulated Ca2+ mobilization in HEL-cells

1. Desensitization of Gs-coupled receptors, the beta 2-adrenoceptor for example, involves rapid and slower components but little is known regarding the existence of rapid desensitization of Gi-coupled receptors and its possible mechanisms. In HEL-cells stimulation of alpha 2A-adrenoceptors by adrenaline or Y1-like neuropeptide Y receptors by neuropeptide Y, transiently mobilizes Ca2+ from intracellular stores via a Gi-protein. We have used this model to study the existence and possible mechanisms of rapid desensitization of a Gi-mediated cellular response. 2. Following stimulation by adrenaline or neuropeptide Y Ca2+ levels returned towards baseline a few minutes after agonist addition and were refractory to a second agonist exposure demonstrating rapid desensitization. Cross-desensitization experiments with neuropeptide Y, adrenaline and moxonidine demonstrated the presence of homologous (both receptors) and heterologous desensitization (neuropeptide Y receptors only), and that the alpha 2A-adrenoceptor desensitization was not specific for phenylethylamine (adrenaline) or imidazoline agonists (moxonidine). 3. The protein kinase C activator, phorbol ester, rapidly desensitized the hormonal Ca2+ responses and inhibitors of protein kinase C enhanced the hormonal responses inconsistently. The tyrosine kinase inhibitor, herbimycin, enhanced Ca2+ mobilization by adrenaline and neuropeptide Y, whereas the protein phosphatase inhibitor, okdadaic acid, did not affect Ca2+ mobilization or its desensitization. 4. In the absence of extracellular Ca2+ the endoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin, reduced hormone-stimulated Ca2+ elevations, demonstrating that mobilization occurs from a thapsigargin-sensitive pool in the endoplasmic reticulum. The inositol phosphate-independent Ca2+release modulator, ryanodine, significantly enhanced adrenaline- and neuropeptide Y-stimulated Ca2+elevations. Blockade of the endoplasmic reticulum Ca2+-ATPase by thapsigargin in the presence of extracellular Ca2+ enhanced hormone-stimulated Ca2+ increases, demonstrating the importance of this enzyme for the termination of the Ca2+ signal.5. It is concluded that adrenaline and neuropeptide Y-stimulated Ca2+ mobilization in HEL-cells occurs from a thapsigargin- and ryanodine-sensitive store in the endoplasmic reticulum and desensitizes rapidly;this appears to involve multiple mechanisms including protein kinases, possibly acting on receptors, and Ca2+ release and sequestration mechanisms.

Cloning and expression of a prostaglandin E receptor EP3 subtype from human erythroleukaemia cells

Prostaglandins inhibit platelet activation by stimulating intracellular cyclic AMP formation. We have postulated that intracellular cyclic AMP levels in platelets are buffered by a distinct prostaglandin receptor that mediates inhibition of cyclic AMP formation. In order to provide evidence for the model, we have cloned the cDNA coding for a prostaglandin receptor EP3 subtype, which is coupled to inhibition of adenylate cyclase, from the megakaryocytic cell line human erythroleukaemia (HEL) cells. A PCR-generated hybridization probe, produced using primers based on the sequence of the mouse prostaglandin EP3 receptor published by Sugimoto, Namba, Honda, Hayashi, Negishi, Ichikawa and Narumiya [(1992) J. Biol. Chem. 267, 6463-6466], was used to screen a lambda gt11 HEL cell cDNA library. The composite full-length cDNA clone HEP3, generated from the two partial clones pHEP3-7 and pHEP3-5, is 1.6 kb long with an open reading frame coding for 390 amino acids. This clone is 83% identical to the alpha subtype of the mouse EP3 receptor. The full-length construct was transfected into COS-1 cells. The cloned receptor exhibited the properties of a prostaglandin EP3 subtype, inhibiting forskolin-stimulated cyclic AMP formation in response to prostaglandin E2 (PGE2) and binding PGE2 with high specificity and a Kd of 3.2 nM. Radiolabelled PGE2 could be displaced by prostaglandins in the order PGE2 = PGE1 > iloprost = PGD2. Northern blot analysis revealed that the receptor is also present in human kidney.

Prostacyclin-dependent cyclic AMP formation in endothelial cells

The effect of the stable prostacyclin (PGI2) mimetic iloprost on cyclic AMP levels was investigated in cultured porcine aortic endothelial cells. Iloprost (10(-10)-10(-5) mol/l) did not change cyclic AMP levels at passage 1 when endothelial cells were untreated but did so after inhibition of endogenous PGI2 formation by 48 or 72 h treatment with indomethacin or diclofenac (10(-5) mol/l). Iloprost increased cyclic AMP in a concentration-dependent manner and up to 6-fold above control when cells from passage 6 were used. In these cells, basal PGI2 generation was reduced to 20% of that at passage 1. Cyclic AMP stimulation by iloprost (10(-5) mol/l) in passage 6 cells was enhanced, reaching up to 11-fold the control level, when cells were cultured for 48 h in the presence of indomethacin or diclofenac (10(-5) mol/l). Cyclic AMP formation in LLC-PK1 cells, a kidney epithelial cell line without endogenous PGI2 biosynthesis, was markedly (25-fold above basal) stimulated by iloprost and unchanged by pretreatment with indomethacin and diclofenac. The data demonstrate that a continuous basal PGI2 generation occurs in porcine aortic endothelial cells that may be sufficient to completely desensitize PGI2-dependent adenylate cyclase activation, presumably at the receptor or GTP-binding protein level.

Induction of leukotriene C4 synthase activity in differentiating human erythroleukemia cells

Leukotriene (LT)C4 synthase is a membrane-bound, specific glutathione transferase which catalyzes the transformation of LTA4 to LTC4. It was originally shown to be present in rodent mastocytoma and basophilic leukemia cells as well as in macrophages. Recently, expression of human LTC4 synthase was demonstrated in platelets (Söderström, M., et al. (1992) Arch. Biochem. Biophys. 294, 70-74). The present report describes the induction of LTC4 synthase activity during differentiation of human erythroleukemia (HEL) cells by the protein kinase C stimulator 12-O-tetradecanoyl phorbol 13-acetate (TPA), ligands of the steroid-thyroid hormone receptor superfamily: all-trans-retinoic acid (RA) and 1 alpha, 25-dihydroxy-vitamin D3 and in addition dimethylsulfoxide (DMSO). TPA was the most powerful inducer of enzyme activity followed by 1 alpha, 25-dihydroxy-vitamin D3 and DMSO. RA did not induce LTC4 synthase activity.