Ca2+ signalling in K562 human erythroleukaemia cells: effect of dimethyl sulphoxide and role of G-proteins in thrombin- and thromboxane A2-activated pathways

TRPM7-mediated spontaneous Ca2+ entry regulates the proliferation and differentiation of human leukemia cell line K562

Continuous Ca2+ influx is essential to maintain intracellular Ca2+ homeostasis and its dysregulation leads to a variety of cellular dysfunctions. In this study, we explored the functional roles of spontaneous Ca2+ influx for the proliferation and differentiation of a human erythromyeloid leukemia cell line K562. mRNA/protein expressions were assessed by the real-time RT-PCR, western blotting, and immunocytochemical staining. Intracellular Ca2+ concentration ([Ca2+ ]i ) and ionic currents were measured by fluorescent imaging and patch clamping techniques, respectively. Cell counting/viability and colorimetric assays were applied to assess proliferation rate and hemoglobin synthesis, respectively. Elimination of extracellular Ca2+ decreased basal [Ca2+ ]i in proliferating K562 cells. Cation channel blockers such as SK&F96365, 2-APB, Gd3+ , and FTY720 dose dependently decreased basal [Ca2+ ]i . A spontaneously active inward current (Ispont ) contributive to basal [Ca2+ ]i was identified by the nystatin-perforated whole-cell recording. Ispont permeated Ca2+ comparably to Na+ , and was greatly eliminated by siRNA targeting TRPM7, a melastatin member of the transient receptor potential (TRP) superfamily. Consistent with these findings, TRPM7 immune reactivity was detected by western blotting, and immunofluorescence representing TRPM7 was found localized to the K562 cell membrane. Strikingly, all these procedures, that is, Ca2+ removal, TRPM7 blockers and siRNA-mediated TRPM7 knockdown significantly retarded the growth and suppressed hemin-induced γ-globin and hemoglobin syntheses in K562 cells, respectively, both of which appeared associated with the inhibition of ERK activation. These results collectively suggest that spontaneous Ca2+ influx through constitutively active TRPM7 channels may critically regulate both proliferative and erythroid differentiation potentials of K562 cells.

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.

Neuromedin U: a Myb-regulated autocrine growth factor for human myeloid leukemias

The c-myb proto-oncogene has been implicated in leukemogenesis, but possible mechanisms remain ill defined. To gain further insight to this process, we used transcript profiling in K562 cells expressing a dominant-negative Myb (MERT) protein. A total of 105 potential Myb gene targets were identified. Neuromedin U (NmU), a peptide affecting calcium transport, underwent the greatest expression change ( approximately 5-fold decrease). To verify a linkage between c-myb and NmU, their mRNA levels were quantitated using real-time polymerase chain reaction in primary acute myeloid leukemia (AML) and acute lymphoid leukemia (ALL), as well as normal hematopoietic cells. We found that c-myb was elevated in AML and ALL samples, but NmU expression was increased only in AML cells. Significantly, only AML cells expressed the cognate receptor of NmU, NMU1R, suggesting the presence of a novel autocrine loop. We examined this possibility in detail. Exogenous NmU "rescued" growth suppression in K562-MERT cells and stimulated the growth of primary AML cells. Short interfering RNA "knockdown" of NmU in K562 cells arrested cell growth. Exposing Indo-1-labeled K562 cells to NmU induced an intracellular Ca(++) flux consistent with engagement of the NMU1R. Combined, these results suggest that NmU expression is related to Myb and that the NmU/NMU1R axis constitutes a previously unknown growth-promoting autocrine loop in myeloid leukemia cells.

EP1- and FP-mediated cross-desensitization of the alpha (alpha) and beta (beta) isoforms of the human thromboxane A2 receptor

1. Heterologous desensitization or intermolecular cross-talk plays a critical role in regulating intracellular signalling by diverse members of the G-protein-coupled receptor superfamily. We have previously established that the alpha and beta isoforms of the human thromboxane A(2) receptor (TP) undergo differential desensitization of signalling in response to 17 phenyl trinor prostaglandin (PG)E(2), an agonist of the EP(1) subtype of the PGE(2) receptor (EP) family. 2. Herein, we investigated the molecular basis of TPalpha and TPbeta desensitization in human embryonic kidney (HEK) 293 cells and in renal mesangial cells in response to 17 phenyl trinor PGE(2) and in response to the PGF(2alpha) receptor (FP) agonist PGF(2alpha), and sought to identify the target site(s) of those desensitizations. 3. Our results demonstrated that TPalpha and TPbeta receptors are subject to desensitization in response to both EP(1) and FP receptor activation and that these effects are mediated by direct protein kinase (PK)C phosphorylation of the individual TP isoforms within their unique carboxyl-terminal (C)-tail domains. 4. Moreover, deletion/site-directed mutagenesis and metabolic labelling studies identified Thr(337), within TPalpha, and Thr(399), within TPbeta, as the specific target residues for PKC phosphorylation and EP(1)- and FP-mediated desensitization of TPalpha and TPbeta signalling, respectively. 5. Hence, in conclusion, while the TPalpha and TPbeta diverge within their C-tail domains, they have evolved to share a similar mechanism of PKC-induced phosphorylation and desensitization in response to EP(1) and FP receptor activation, though it occurs at sites unique to the individual TP isoforms.

Structure and regulation of the mDot1 gene, a mouse histone H3 methyltransferase

The nucleotide sequence data reported have been deposited in the DDBJ, EMBL, GenBank(R) and GSDB Nucleotide Sequence Databases under accession numbers AY196089, AY196090, AY376663, AY377920 and AY376664. Recently, a new class of histone methyltransferases that plays an indirect role in chromatin silencing by targeting a conserved lysine residue in the nucleosome core was described, namely the Dot1 (disruptor of telomeric silencing) family [Feng, Wang, Ng, Erdjument-Bromage, Tempst, Struhl and Zhang (2002) Curr. Biol. 12, 1052-1058; van Leeuwen, Gafken and Gottschling (2002) Cell (Cambridge, Mass.) 109, 745-756; Ng, Feng, Wang, Erdjument-Bromage, Tempst, Zhang and Struhl (2002) Genes Dev. 16, 1518-1527]. In the present study, we report the isolation, genomic organization and in vivo expression of a mouse Dot1 homologue (mDot1). Expressed sequence tag analysis identified five mDot1 mRNAs (mDot1a-mDot1e) derived from alternative splicing. mDot1a and mDot1b encode 1540 and 1114 amino acids respectively, whereas mDot1c-mDot1e are incomplete at the 5'-end. mDot1a is closest to its human counterpart (hDot1L), sharing 84% amino acid identity. mDot1b is truncated at its N- and C-termini and contains an internal deletion. The five mDot1 isoforms are encoded by 28 exons on chromosome 10qC1, with exons 24 and 28 further divided into two and four sections respectively. Alternative splicing occurs in exons 3, 4, 12, 24, 27 and 28. Northern-blot analysis with probes corresponding to the methyltransferase domain or the mDot1a-coding region detected 7.6 and 9.5 kb transcripts in multiple tissues, but only the 7.6 kb transcript was evident in mIMCD3-collecting duct cells. Transfection of mDot1a-EGFP constructs (where EGFP stands for enhanced green fluorescent protein) into human embryonic kidney (HEK)-293T or mIMCD3 cells increased the methylation of H3-K79 but not H3-K4, -K9 or -K36. Furthermore, DMSO induced mDot1 gene expression and methylation specifically at H3-K79 in mIMCD3 cells in a time- and dose-dependent manner. Collectively, these results add new members to the Dot1 family and show that mDot1 is involved in a DMSO-mediated signal-transduction pathway in collecting duct cells.

Multidisciplinary utilization of dimethyl sulfoxide: pharmacological, cellular, and molecular aspects

DMSO is an amphipathic molecule with a highly polar domain and two apolar methyl groups, making it soluble in both aqueous and organic media. It is one of the most common solvents for the in vivo administration of several water-insoluble substances. Despite being frequently used as a solvent in biological studies and as a vehicle for drug therapy, the side-effects of DMSO (undesirable for these purposes) are apparent from its utilization in the laboratory (both in vivo and in vitro) and in clinical settings. DMSO is a hydrogen-bound disrupter, cell-differentiating agent, hydroxyl radical scavenger, intercellular electrical uncoupler, intracellular low-density lipoprotein-derived cholesterol mobilizing agent, cryoprotectant, solubilizing agent used in sample preparation for electron microscopy, antidote to the extravasation of vesicant anticancer agents, and topical analgesic. Additionally, it is used in the treatment of brain edema, amyloidosis, interstitial cystitis, and schizophrenia. Several systemic side-effects from the use of DMSO have been reported, namely nausea, vomiting, diarrhea, hemolysis, rashes, renal failure, hypertension, bradycardia, heart block, pulmonary edema, cardiac arrest, and bronchospasm. Looking at the multitude of effects of DMSO brought to light by these studies, it is easily understood how many researchers working with DMSO (or studying one of its specific effects) might not be fully aware of the experiences of other groups who are working with it but in a different context.

Group I metabotropic glutamate receptor signaling via Galpha q/Galpha 11 secures the induction of long-term potentiation in the hippocampal area CA1

Heterotromeric G-proteins of the Gq family are thought to transduce signals from group I metabotropic glutamate receptors (mGluRs) in central neurons. We investigated roles of this cascade in hippocampal long-term potentiation (LTP) by using null-mutant mice lacking the alpha subunit of Gq (Galphaq) or G11 (Galpha11). We found no obvious abnormalities in the morphology, layer structure, expression of NMDA receptors, and basic parameters of excitatory synaptic transmission in the hippocampus of Galphaq mutant mice. We used theta burst stimulation (TBS) (3-10 burst trains at 5 Hz; each train consisted of five stimuli at 100 Hz) to induce LTP at Schaffer collateral to CA1 pyramidal cell synapses. Conventional TBS with 10 burst trains induced robust LTP in wild-type, Galphaq mutant, and Galpha11 mutant mice. Weak TBS with three burst trains consistently induced LTP in wild-type mice. In contrast, the same weak TBS was insufficient to induce LTP in Galphaq and Galpha11 mutant mice. In wild-type mice, the LTP by weak TBS was abolished by inhibiting group I mGluR or protein kinase C (PKC) but not by blocking muscarinic acetylcholine receptors. Prior activation of group I mGluR by an agonist significantly enhanced the LTP by weak TBS in wild-type mice. However, this priming effect was absent in Galphaq mutant mice. These results indicate that the signaling from group I mGluR to PKC involving Galphaq/Galpha11 does not constitute the main pathway for LTP, but it secures LTP induction by lowering its threshold in the hippocampal area CA1.

Group I metabotropic glutamate receptor signaling via Galpha q/Galpha 11 secures the induction of long-term potentiation in the hippocampal area CA1

Heterotromeric G-proteins of the Gq family are thought to transduce signals from group I metabotropic glutamate receptors (mGluRs) in central neurons. We investigated roles of this cascade in hippocampal long-term potentiation (LTP) by using null-mutant mice lacking the alpha subunit of Gq (Galphaq) or G11 (Galpha11). We found no obvious abnormalities in the morphology, layer structure, expression of NMDA receptors, and basic parameters of excitatory synaptic transmission in the hippocampus of Galphaq mutant mice. We used theta burst stimulation (TBS) (3-10 burst trains at 5 Hz; each train consisted of five stimuli at 100 Hz) to induce LTP at Schaffer collateral to CA1 pyramidal cell synapses. Conventional TBS with 10 burst trains induced robust LTP in wild-type, Galphaq mutant, and Galpha11 mutant mice. Weak TBS with three burst trains consistently induced LTP in wild-type mice. In contrast, the same weak TBS was insufficient to induce LTP in Galphaq and Galpha11 mutant mice. In wild-type mice, the LTP by weak TBS was abolished by inhibiting group I mGluR or protein kinase C (PKC) but not by blocking muscarinic acetylcholine receptors. Prior activation of group I mGluR by an agonist significantly enhanced the LTP by weak TBS in wild-type mice. However, this priming effect was absent in Galphaq mutant mice. These results indicate that the signaling from group I mGluR to PKC involving Galphaq/Galpha11 does not constitute the main pathway for LTP, but it secures LTP induction by lowering its threshold in the hippocampal area CA1.

Regulation of the human prostanoid TPalpha and TPbeta receptor isoforms mediated through activation of the EP(1) and IP receptors

The intermolecular cross-regulation mediated by the prostanoid IP-receptor (IP)/EP(1) receptor (EP(1)) agonists PGI(2) and 17 phenyl trinor PGE(2) on TP receptor (TP) signalling within platelets was compared to that which occurs to the individual TPalpha and TPbeta receptors over-expressed in human embryonic kidney (HEK) 293 cells. Ligand mediated TP receptor activation was monitored by analysing mobilization of intracellular calcium ([Ca(2+)](i)) following stimulation with the selective thromboxane (TX) A(2) mimetic U46619. Consistent with previous studies, in platelets, PGI(2) acting through endogenous IP receptors completely inhibited U46619-mediated TP receptor signalling in a protein kinase (PK) A-dependent, PKC-independent manner. In HEK 293 cells, PGI(2), acting through endogenous AH6809 sensitive EP(1) rather than IP receptors, and the selective EP(1) receptor agonist 17 phenyl trinor PGE(2) antagonized U46619-mediated signalling by both TPalpha and TPbeta receptors in a PKC-dependent, PKA-independent manner. The maximum response induced by either ligand was significantly (P<0.005) greater for the TPalpha receptor than the TPbeta receptor, pointing to possible physiologic differences between the TP isoforms, although the potency of each ligand was similar for both TP receptors. TP(Delta328), a truncated variant of TP receptor lacking the C-tail sequences unique to TPalpha or TPbeta receptors, was not sensitive to EP(1) receptor-mediated regulation by PGI(2) or 17 phenyl trinor PGE(2) In conclusion, these data confirm that TPalpha and TPbeta receptors are subject to cross regulation by EP(1) receptor signalling in HEK 293 cells mediated by PKC at sites unique to the individual TP receptors and that TPalpha receptor responses are significantly more reduced by EP(1) receptor regulation than those of the TPbeta receptor.

p-Aminobenzoic acid and its metabolite p-acetamidobenzoic acid inhibit agonist-induced aggregation and arachidonic acid-induced [Ca2+]i transients in human platelets

We have previously found that the naturally occurring amine p-aminobenzoic acid (PABA) inhibits the thrombin-induced thromboxane B2 production in human platelets. In this report we show that PABA and its acetylated metabolite p-acetamidobenzoic acid (PACBA) inhibit platelet aggregation induced by agonists such as adenosine diphosphate (ADP) and arachidonic acid (AA). Both substances were equipotent to acetylsalicylic acid regarding inhibition of ADP-induced aggregation and approximately 50% as potent as acetylsalicylic acid regarding arachidonic acid-induced aggregation. Although not significantly inhibiting collagen aggregation, PABA and PACBA reduced the concomitant adenosine triphosphate (ATP) secretion by approximately 30 and 20%, respectively. The antiaggregatory effect does not seem to be mediated through cyclic adenosine monophosphate (cAMP) increase because in our experiments PABA and PACBA did not significantly affect cAMP levels. However, we have found that PABA and PACBA inhibit the intracellular aequorin indicated Ca2+ transient upon arachidonic acid stimulation. Our results describe a hitherto unknown effect of PABA and PACBA on platelet aggregation.

Agonist-specific alterations in receptor-phospholipase coupling following inactivation of Gi2alpha gene

Different forms of phospholipase A2, together with pertussis toxin-sensitive G-proteins, [Ca2+]i (intracellular Ca2+ concentration), protein kinase C, calmodulin, protein tyrosine kinases, mitogen-activated protein kinases and Ca2+/calmodulin-dependent protein kinase appear to play a role in agonist-mediated release of arachidonic acid. Here we report that fibroblasts from 14-day-old mouse embryos with inactivated Gi2alpha (alpha-subunit of the heterotrimeric G-protein Gi2) gene display a marked decrease in the ability of lysophosphatidic acid, thrombin and Ca2+ ionophores to release arachidonic acid compared with their normal counterparts. The requirement for Gi2alpha in the release of arachidonic acid following increased [Ca2+]i may be explained by the incomplete translocation of cytosolic phospholipase A2 observed in Gi2alpha-deficient cells. Paradoxically, inactivation of the Gi2alpha gene resulted in up-regulation of bradykinin receptors and their coupling to increased arachidonic acid release, phospholipase C activity and [Ca2+]i. A concomitant increase in basal phospholipase C activity was also observed in the Gi2alpha-deficient cells. These observations establish a pleiotropic and essential role for Gi2alpha in receptor-phospholipase coupling that contrasts with its less obligatory participation in agonist-mediated inhibition of adenylate cyclase.

Specificity of G alpha q and G alpha 11 gene expression in platelets and erythrocytes. Expressions of cellular differentiation and species differences

G alpha q and G alpha 11, members of the Gq family of G-proteins, transduce signals from receptors to the beta isoenzymes of phosphatidyl-inositol-specific phospholipase C (PI-PLC). The receptor specificity of these alpha subunits is unknown. G alpha q and G alpha 11 are ubiquitously expressed in tissues; however, there have been conflicting reports of the presence or absence of G alpha 11 protein in haematopoietic cells. Platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors activate PI-PLC via G alpha q, but the role of G alpha 11 is uncertain. To define their roles in platelet activation we studied G alpha q and G alpha 11 gene expression by immunotransfer blotting and by reverse transcription of mRNA followed by PCR (RT-PCR) and direct sequencing. An antiserum specific for mouse G alpha 11 failed to identify G alpha 11 in dog or human platelets or in dog liver, a tissue known to contain G alpha 11. RT-PCR performed with gene-specific primers demonstrated G alpha q mRNA, but not G alpha 11 mRNA, in normal human and mouse platelets and in thromboxane-sensitive and thromboxane-insensitive dog platelets. Studies of mouse and dog liver and human retina confirmed that the cDNA, primers and probes used could amplify and recognize G alpha 11 in other tissues. However, species-specific oligonucleotide primers and probes were essential to demonstrate G alpha 11, but not G alpha q, mRNA. Compared with mouse cDNA, dog and human G alpha 11 cDNA had twice as many nucleotide substitutions (approx. 12% compared with approx. 6%) as G alpha q, G alpha q mRNA was also found in mature erythrocytes but G alpha 11 mRNA was not identified, whereas both G alpha q and G alpha 11 mRNAs were found in bone marrow stem cells. Therefore G alpha 11 gene expression in haematopoietic cells is linked with cellular differentiation. The lack of G alpha 11 indicates that signal transduction from platelet TXA2/PGH2 receptors to PI-PLC occurs via G alpha q, and that G alpha 11 deficiency is not responsible for defective activation of PI-PLC in thromboxane-insensitive dog platelets. Despite the high degree of similarity that exists between G alpha q and G alpha 11, significantly greater species-specific variation in nucleotide sequence is present in G alpha 11 than in G alpha q. Cellular specificity and species specificity are important characteristics of these Gq family G-proteins.