人白血病细胞系P2X7受体的表达及其介导的细胞内钙浓度变化

采用半定量RT-PCR和流式细胞术,在基因和蛋白水平研究了白血病细胞系U937、HL60和Ramos细胞P2X7受体的表达。荧光染料Fura-2／AM负载后,用荧光分光光度计测定P2X7受体激动剂三磷酸腺苷(adenosine 5′-triphosphate,ATP)和苯甲酰苯甲酸ATP(2′,3′-O-(4-benzoyl)benzoyl-ATP,BzATP)刺激前后细胞内钙离子浓度的变化,以确认其功能。结果表明：U937和HL60细胞系表达P2X7受体的mRNA和蛋白,Ramos不表达;在激动剂的刺激下,可引发U937和HL60细胞胞内钙浓度的显著升高,但对Ramos没有作用。当去除胞外钙离子时,ATP和BzATP刺激均不能引起U937和HL60细胞胞内钙离子浓度的升高。提示U937和HL60细胞表达P2X7受体的基因和功能蛋白,Ramos细胞则不表达该受体。     

Inhibitory effects of U73122 and U73343 on Ca2+ influx and pore formation induced by the activation of P2X7 nucleotide receptors in mouse microglial cell line

P2X7 receptors are ATP-gated ion channels and play important roles in microglial functions in the brain. Activation of P2X7 receptors by ATP or its agonist BzATP induces Ca2+ influx from extracellular space, followed by the formation of non-selective membrane pores that is permeable to larger molecules, such as fluorescent dye. To determine whether phospholipase C (PLC) is involved in the activation of P2X7 receptors in microglial cells, U73122, a specific inhibitor of PLC, and its inactive analogue U73343 were examined on ATP and BzATP-induced channel and pore formation in an immortalized C57BL/6 mouse microglial cell line (MG6-1). ATP induced both a transient and a sustained increase in the intracellular Ca2+ concentration ([Ca2+]i) in MG6-1 cells, whereas BzATP evoked only a sustained increase. U73122, but not U73343, inhibited the transient [Ca2+]i increase involving Ca2+ release from intracellular stores through PLC activation. In contrast, both U73122 and U73343 inhibited the sustained [Ca2+]i increase either prior or after the activation of P2X7 receptor channels by ATP and BzATP. In addition, these U-compounds inhibited the influx of ethidium bromide induced by ATP and BzATP, suggesting possible PLC-independent blockage of the process of P2X7-associated channel and pore formations by U-compounds in C57BL/6 mouse microglial cells.     

Ca2+ store depletion and endoplasmic reticulum stress are involved in P2X7 receptor-mediated neurotoxicity in differentiated NG108-15 cells

P2X7 receptor (P2X7R) activation by extracellular ATP triggers influx of Na(+) and Ca(2+), cytosolic Ca(2+) overload and consequently cytotoxicity. Whether disturbances in endoplasmic reticulum (ER) Ca(2+) homeostasis and ER stress are involved in P2X7R-mediated cell death is unknown. In this study, a P2X7R agonist (BzATP) was used to activate P2X7R in differentiated NG108-15 neuronal cells. In a concentration-dependent manner, application of BzATP (10-100 μM) immediately raised cytosolic Ca(2+) concentration ([Ca(2+)]i) and caused cell death after a 24-h incubation. P2X7R activation for 2 h did not cause cell death but resulted in a sustained reduction in ER Ca2+ pool size, as evidenced by a diminished cyclopiazonic acid-induced Ca(2+) discharge (fura 2 assay) and a lower fluorescent signal in cells loaded with Mag-fura 2 (ER-specific Ca(2+)-fluorescent dye). Furthermore, P2X7R activation (2 h) led to the appearance of markers of ER stress [phosphorylated α subunit of eukaryotic initiation factor 2 (p-eIF2α) and C/EBP homologous protein (CHOP)] and apoptosis (cleaved caspase 3). Xestospongin C (XeC), an antagonist of inositol-1,4,5-trisphosphate (IP3) receptor (IP3R), strongly inhibited BzATP-triggered [Ca(2+)]i elevation, suggesting that the latter involved Ca(2+) release via IP3R. XeC pretreatment not only attenuated the reduction in Ca(2+) pool size in BzATP-treated cells, but also rescued cell death and prevented BzATP-induced appearance of ER stress and apoptotic markers. These novel observations suggest that P2X7R activation caused not only Ca(2+) overload, but also Ca(2+) release via IP3R, sustained Ca(2+) store depletion, ER stress and eventually apoptotic cell death.     

Balance of purines may determine life or death of retinal ganglion cells as A3 adenosine receptors prevent loss following P2X7 receptor stimulation

The purines ATP and adenosine can act as a coordinated team of transmitters. As extracellular adenosine is frequently derived from the enzymatic dephosphorylation of released ATP, the distinct actions of the two purines can be synchronized. In retinal ganglion cells (RGCs), stimulation of the P2X7 receptor for ATP leads to increased intracellular Ca2+ and death. Here we define the contrasting effects of adenosine and identify protective actions mediated by the A3 receptor. Adenosine attenuated the rise in Ca2+ produced by the P2X7 agonist 3'-O-(4-benzoylbenzoyl)ATP (BzATP). Adenosine was also neuroprotective, increasing the survival of ganglion cells exposed to BzATP. The A3 adenosine receptor agonist 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronimide (Cl-IB-MECA) mimicked the inhibition of the Ca2+ rise, whereas the A3 antagonist 3-Ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS-1191) reduced the protective effects of adenosine. Both Cl-IB-MECA and a second A3 receptor agonist IB-MECA reduced the cell loss triggered by BzATP. The actions of BzATP were mimicked by ATPgammaS, but not by ATP. In summary, adenosine can stop the rise in Ca2+ and cell death resulting from stimulation of the P2X7 receptor on RGCs, with the A3 adenosine receptor contributing to this protection. Hydrolysis of ATP into adenosine and perhaps inosine shifts the balance of purinergic action from that of death to the preservation of life.     

Glutamate release and synapsin-I phosphorylation induced by P2X7 receptors activation in cerebellar granule neurons

The present work reports that activation of P2X7 receptor induces synaptic vesicle release in granule neurons and phosphorylation of synapsin-I by calcium-calmodulin-dependent protein kinase II (CaMKII), which in turn modulates secretory event. ATP, in absence of magnesium, induced a concentration-dependent glutamate release with an EC50 value of 1.95 microM. The involvement of P2X7 receptor was suggested when maximal secretory response was significantly reduced by the selective P2X7 antagonist Brilliant Blue G (BBG; 100 nM) and abolished by removing extracellular Ca2+. The involvement of P2X7 receptor on synaptic vesicle release was confirmed by measuring the release of FM 1-43 dye. In this case, pharmacological activation of P2X7 was achieved with the more selective agonist 2'-3'-o-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP; 100 microM) showing a significant FM 1-43 release that was blocked by BBG (100 nM), by Zn2+ ions (100 microM), both P2X7 blockers, but not by suramin (100 microM), antagonist of P2X1, P2X2, P2X3 and P2X5. In addition, BzATP, through P2X7 receptor activation, significantly increased the phosphorylation of synapsin-I, the main presynaptic target of CaMKII. Both effects mediated by BzATP were inhibited by the CaMKII inhibitors KN-62 (10 microM) and KN-93 (10 microM). These results suggest, therefore, that Ca2+ entrance mediated by P2X7 receptor induces glutamate release and in parallel synapsin-I phosphorylation.     

Thrombin chemotactic stimulation of HL-60 cells: studies on thrombin responsiveness as a function of differentiation

Thrombin, a major procoagulant enzyme and growth factor, is also selectively chemotactic for monocytes and macrophages but not for neutrophils. This effect stands in contrast to other well-known chemotactic agents such as fMet-Leu-Phe, C5a fragments, and LTB4, which stimulate directed cell movement in both cell types, and have important physiological implications. The human leukemic cell line HL-60, which is capable of differentiating either along granulocytic or monocytic lineages, was therefore used to explore the development of this selective monocyte/macrophage chemotactic response to thrombin. Esterolytically inactive DIP-alpha-thrombin, as well as the thrombin-derived chemotactic peptide CB67-129, elicits a dose-dependent chemotactic response in HL-60 cells differentiated to monocytelike cells by treatment with 1,25(OH)2D3 (HL-60/mono), whereas no such response is evident in either undifferentiated HL-60 cells or in cells differentiated into granulocytes by treatment with DMSO (HL-60/gran). Similarly, early events which characterize stimulation of inflammatory cells by chemotactic agents are also evident, but only in monocyte-differentiated cells. In HL-60/mono, thrombin selectively stimulates rapid cytosolic Ca2+ elevation as well as rapid cytoskeletal association of cytosolic actin. Following thrombin stimulation, maximal actin association in these cells occurs within 30 sec (declining to basal levels at the end of 5 min), and maximal Ca2+ elevations are also evident within 15-20 sec, suggesting a temporal relationship between these two events. Thus, the events accompanying stimulation of HL-60/mono by thrombin are characteristic of those seen following stimulation of inflammatory cells by chemotaxins, with a major difference being the selectivity of thrombin as a chemotaxin for cells of macrophage/monocytic lineage. The selective chemotactic responsiveness of HL-60/mono to thrombin appears to relate to the development of specific receptors on these cells as part of monocytic differentiation: HL-60/mono (but HL-60/gran nor undifferentiated HL-60) are capable of significant specific 125-I-labeled alpha-thrombin-binding (ka approximately 20 nM), and possess an estimated 400,000 thrombin-binding sites per cell. Our findings further suggest that the thrombin response of HL-60 and particularly the expression of thrombin receptors on these cells may serve as a useful model system for exploring the biology of monocyte/macrophage differentiation.     

The human SH-SY5Y neuroblastoma cell-line expresses a functional P2X7 purinoceptor that modulates voltage-dependent Ca2+ channel function

Fura-2 imaging of purinergic stimulation of non-differentiated neuronal human SH-SY5Y cells resulted in a rapid elevation in intracellular Ca2+ ([Ca2+]i) that was dependent on extracellular Ca2+. The rank order of agonists (200 micro m) was as follows: 2',3'-O-(4-benzoyl-benzoyl)-ATP (BzATP) > ATP4- > ATP; whereas 2-(methylthio)-ATP, ADP, UTP and alpha,beta-methylene-ATP and beta,gamma-methylene-ATP were ineffective. The response to BzATP was inhibited by pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic-acid (PPADS, 1 micro m), 1-(N,O-bis[5-isoquinolinesulfonyl]-N-methyl-l-tyrosyl)-4-phenylpiperazine (KN-62, 100 nm) and 8-(3-benzamido-4-4-methylbenzamido)-naphthalene-1,3,5-trisulfonic-acid (suramin, 200 micro m). The presence of a P2X7 receptor was confirmed by western blot studies using anti-P2X7. EC50 for BzATP was 212 +/- 6 micro m. BzATP > 30 micro m induced an initial, transient increase in [Ca2+]i before a plateau level was reached. BzATP < 30 micro m only produced a monophasic increase to the plateau level. The transient phase was reduced by the introduction of nimodipine (3 micro m) and to a smaller degree by omega-conotoxin GVIA (1 micro m) despite an almost equal presence of L and N-type Ca2+-channels. In whole-cell voltage-clamp studies at - 90 mV, BzATP (300 micro m) produced a fast activating inward current with a similar pharmacology as observed with Fura-2 imaging. Current clamp studies showed a dose-dependent depolarization to BzATP and ATP4-. BzATP also triggered transmitter release. Thus, the human neuronal SH-SY5Y cell line expresses a functional P2X7 receptor coupled to activation of Ca2+-channels.     

Signal transduction and white cell maturation via extracellular ATP and the P2Y11 receptor

Extracellular ATP promotes a wide range of physiological effects in many tissues. Of particular interest is the effect of ATP on leukaemia-derived HL-60 and NB4 cell lines, which are induced to mature to neutrophil-like cells. The differentiation process appears to be mediated by ATP binding to a cell-surface purinergic P2Y receptor, resulting in the stimulation of adenylyl cyclase, elevation of cAMP levels and activation of protein kinase A. In 1997, a novel ATP-selective P2Y receptor, P2Y11, was cloned and shown to be linked to both cAMP and Ca2+ signalling pathways. The pharmacological profile of ATP analogues used by P2Y11 for cAMP production in transfected cells is reviewed in the present paper and shown to be closely similar to the profiles for cAMP production and differentiation of myeloblastic HL-60 cells and promyelocytic NB4 cells, both of which express P2Y11. Additional data are provided showing that HL-60 mature to neutrophil-like cells in response to extracellular ATP, as measured by upregulation of the N-formyl peptide receptor, N-formyl peptide-mediated actin polymerization and superoxide production. It is proposed that P2Y11 is responsible for the ATP-mediated differentiation of these cells lines and that this receptor may play a role in the maturation of granulocytic progenitors in the bone marrow.     

Capsaicin inhibits platelet-activating factor-induced cytosolic Ca2+ rise and superoxide production

Platelet-activating factor (PAF) is an important participant in the inflammatory process. We studied the regulation of PAF activity by capsaicin in human promyelocytic leukemia HL-60 cells. Capsaicin inhibited PAF-induced superoxide production in a concentration-dependent manner. In addition to PAF, the fMLP- and extracellular ATP-induced superoxide productions were inhibited by capsaicin, whereas PMA-induced superoxide production was not affected. In the PAF-stimulated cytosolic Ca2+ increase, capsaicin inhibited in particular the sustained portion of the raised Ca2+ level without attenuation of the peak height. In the absence of extracellular Ca2+, the PAF-induced Ca2+ elevation was not inhibited by capsaicin because capsaicin only inhibited the Ca2+ influx from the extracellular space. In addition, capsaicin did not affect PAF-induced inositol 1,4,5-trisphosphate production, suggesting that phospholipase C activation by PAF is not affected by capsaicin. Store-operated Ca2+ entry (SOCE) induced by thapsigargin was inhibited by capsaicin in a concentration-dependent manner. This capsaicin effect was also observed on thapsigargin-induced Ba2+ and Mn2+ influx. Furthermore, capsaicin's inhibitory effect on the thapsigargin-induced Ca2+ rise overlapped with that of SK&F96365, an inhibitor of SOCE. Both capsaicin and SK&F96365 also inhibited PAF-induced cytosolic superoxide generation in HL-60 cells differentiated by all-trans-retinoic acid. Our data suggest that capsaicin exerts its anti-inflammatory effect by inhibiting SOCE elicited via PLC activation, which occurs upon PAF activation and results in the subsequent superoxide production.     

Multiple signaling pathways of histamine H2 receptors. Identification of an H2 receptor-dependent Ca2+ mobilization pathway in human HL-60 promyelocytic leukemia cells

In order to analyze the complex activities of histamine H2 receptor activation on neutrophils, human HL-60 promyelocytic leukemia cells were differentiated into neutrophils by incubation with dimethyl sufoxide, loaded with the Ca2+-sensitive indicator dyes, indo-1 or fura-2, and the levels of intracellular Ca2+ ([Ca2+]i) measured in a fluorescent-activated cell sorter and fluorimeter, respectively. Histamine increased [Ca2+]i in a dose-dependent manner with a half-maximal concentration (EC50) of approximately 10(-6) to 10(-5) M, which exhibited H2 receptor specificity. Prostaglandin E2 and isoproterenol also induced [Ca2+]i mobilization in HL-60 cells, whereas the cell permeable form of cAMP and forskolin failed to increase [Ca2+]i. Since H2-receptor mediated [Ca2+]i mobilization was not inhibited by reducing the concentration of extracellular Ca2+ nor by the addition of Ca2+ channel antagonists, LaCl3 and nifedipine, [Ca2+]i mobilization is due to the release of Ca2+ from intracellular stores. Furthermore, both 10(-4) M histamine and 10(-6) M fMet-Leu-Phe increased the levels of 1,4,5-inositol trisphosphate. However, histamine-induced mobilization of [Ca2+]i was inhibited by cholera toxin but not by pertussis toxin, whereas the action of fMet-Leu-Phe was inhibited by pertussis toxin but not by cholera toxin. These data suggest that H2 receptors on HL-60 cells are coupled to two different cholera toxin-sensitive G-proteins and activate adenylate cyclase and phospholipase C simultaneously.     

Activation of P2X(7) receptors decreases glutamate uptake and glutamine synthetase activity in RBA-2 astrocytes via distinct mechanisms

Glutamate clearance by astrocytes is critical for controlling excitatory neurotransmission and ATP is an important mediator for neuron-astrocyte interaction. However, the effect of ATP on glutamate clearance has never been examined. Here we report that treatment of RBA-2 cells, a type-2-like astrocyte cell line, with ATP and the P2X(7) receptor selective agonist 3'-O-(4-benzoylbenzoyl) adenosine 5'-triphosphate (BzATP) decreased the Na+-dependent [3H]glutamate uptake within minutes. Mechanistic studies revealed that the decreases were augmented by removal of extracellular Mg2+ or Ca2+, and was restored by P2X7 selective antagonist , periodate-oxidized 2',3'-dialdehyde ATP (oATP), indicating that the decreases were mediated through P2X(7) receptors. Furthermore, stimulation of P2X7 receptors for 2 h inhibited both activity and protein expression of glutamine synthetase (GS), and oATP abolished the inhibition. In addition, removal of extracellular Ca(2+) and inhibition of protein kinase C (PKC) restored the ATP-decreased GS expression but failed to restore the P2X(7)-decreased [3H]glutamate uptake. Therefore, P2X7-mediated intracellular signals play a role in the down-regulation of GS activity/expression. Activation of P2X7 receptors stimulated increases in intracellular Na+ concentration ([Na+](i)) suggesting that the P2X(7)-induced increases in [Na+](i) may affect the local Na+ gradient and decrease the Na+-dependent [3H]glutamate uptake. These findings demonstrate that the P2X7-mediated decreases in glutamate uptake and glutamine synthesis were mediated through distinct mechanisms in these cells.     

A correlation between phorbol diester-induced protein phosphorylation and superoxide anion generation in HL-60 cells during granulocytic maturation

As HL-60 cells matured along the granulocytic pathway, phorbol diester-induced superoxide anion production was compared to phorbol diester-induced protein phosphorylation using an in vitro phosphorylation technique. Maturation was induced by 0, 2, 4, or 6 days incubation with dimethyl sulfoxide (Me2SO). In 0 day Me2SO HL-60 cells, phorbol 12-myristate 13-acetate induced phosphorylation of protein pp29 (Mr = 28,600) and to a lesser extent protein pp76 (Mr = 76,300). With increased time of Me2SO incubation, phorbol 12-myristate 13-acetate induced phosphorylation of pp212 (Mr = 211,800), pp134 (Mr = 134,200), and pp76, whereas the phosphorylation of pp29 did not change appreciably. In close agreement with this increase in protein phosphorylation was the observed increase in phorbol diester-induced superoxide anion formation. Morphological characterization of cells during Me2SO-induced differentiation reveals that these increases in phorbol diester responses are probably attributable to the proportional rise in metamyelocytes, band, and segmented neutrophils. A variety of phorbol diesters increased superoxide anion generation in HL-60 cells differentiated into granulocyte-like cells by 6-day incubation with Me2SO. The structure-activity relationship of these phorbol diester derivatives for protein phosphorylation was strongly correlated to their ability to increase superoxide anion generation. Thus, we propose that phorbol diester-induced phosphorylation of pp212, pp134, and pp76, but not pp29 may play a role in mediating the functional response of phorbol diester-induced superoxide anion generation in HL-60 cells differentiated into mature granulocyte-like cells.     

Benzoyl ATP is an antagonist of rat and human P2Y1 receptors and of platelet aggregation

The effects of 2'- and 3'-O-(4-benzoylbenzoyl)-ATP (BzATP) on intracellular Ca2+ mobilization and cyclic AMP accumulation were investigated using rat brain capillary endothelial cells which express an endogenous P2Y1 receptor, human platelets which are known to express a P2Y1 receptor, and Jurkat cells stably transfected with the human P2Y1 receptor. In endothelial cells, BzATP was a competitive inhibitor of 2-methylthio ADP (2-MeSADP) and ADP induced [Ca2+]i responses (Ki = 4.7 microM) and reversed the inhibition by ADP of adenylyl cyclase (Ki = 13 microM). In human platelets, BzATP inhibited ADP-induced aggregation (Ki = 5 microM), mobilization of intracellular Ca2+ stores (Ki = 6.3 microM), and inhibition of adenylyl cyclase. In P2Y1-Jurkat cells, BzATP inhibited ADP and 2-MeSADP-induced [Ca2+]i responses (Ki = 2.5 microM). It was concluded that BzATP is an antagonist of rat and human P2Y1 receptors and of platelet aggregation. In contrast to other P2Y1 receptor antagonists (A2P5P and A3P5P) which inhibit only ADP-induced Ca2+ mobilization, BzATP inhibits both the Ca2+- and the cAMP-dependent intracellular signaling pathways of ADP. These results provide further evidence that P2Y1 receptors contribute to platelet ADP responses.     

Induction of cyclo-oxygenase synthesis in human promyelocytic leukaemia (HL-60) cells during monocytic or granulocytic differentiation.

Cyclo-oxygenase (COX) production in human promyelocytic leukaemia (HL-60) cells was studied during monocytic differentiation induced by 1 alpha, 25-dihydroxyvitamin D3 (24 nM; 3 days) or phorbol 12-myristate 13-acetate (100 nM; 1 day), or during granulocytic differentiation induced by retinoic acid (1 microns; 4 days). Undifferentiated or differentiated HL-60 cells were labelled with [35S]methionine, and membrane-bound COX was solubilized and quantified by SDS/PAGE. Immunoprecipitated 35S-labelled COX from cells induced to differentiate into monocytic or granulocytic lineage were clearly detected on the autoradiograms as a protein of approx. 70 kDa molecular size, whereas only a very faint COX band was detected in untreated HL-60 cells. During both monocytic and granulocytic differentiation, COX activity (measured by the conversion of exogenous arachidonic acid into prostaglandin E2) was dramatically increased. In addition, thromboxane synthesis was preferentially enhanced during monocytic differentiation. HL-60 cells, induced to differentiate into the monocytic or granulocytic lineage, provide a useful tool to investigate the cellular mechanisms involved in regulation of the synthesis of individual prostanoid-metabolizing enzymes.     

A lineage-specific Ca(2+)-activated K+ conductance in HL-60 cells.

Cells of the human promyelocytic cell line HL-60 can be controllably induced to terminally differentiate into either granulocytes or monocyte/macrophages. HL-60 promyelocytes and terminally differentiated macrophages express a K(+)-selective ion channel which is activated by intracellular free Ca2+ concentrations above 10(-7) M. Because of its voltage independence, this channel can be distinguished from the voltage- and Ca(2+)-activated family of outward-rectifying channels. The channel is selective for K+ against Na+ and is blocked by Ba2+, thus it may be similar to the Ca(2+)-activated K+ channel previously described in human macrophages. In its sensitivity to block by charybdotoxin, this channel also resembles a Ca(2+)-activated K+ channel of lymphocytes, which plays a role in activation-dependent hyperpolarization. In contrast to promyelocytes and macrophages, functional expression of the Ca(2+)-activated K+ channel is suppressed to nearly undetectable levels in granulocytes derived from HL-60 cells by retinoic acid-induced differentiation. These data suggest that signals which produce elevation of intracellular Ca2+ will hyperpolarize promyelocytes and differentiated macrophages by activating this conductance; however, signals which elevate free Ca2+ in granulocytes must act on other effectors, which may produce a different final influence on membrane potential.     

Functional reconstitution of fMet-Leu-Phe receptor in Xenopus laevis oocytes

fMet-Leu-Phe (fMLP) receptors were functionally reconstituted into Xenopus laevis oocytes by microinjection with RNA isolated from promyelocytic leukemia cells (HL-60) differentiated with 750 microM N6, O2-dibutyryl cyclic adenosine 3',5'-monophosphate. fMLP-induced Ca2+ mobilization was monitored by measurement of photon emission elicited by aequorin coinjected with RNA into albino X. laevis oocytes. Maximal expression of the fMLP receptor was achieved 48 h after microinjection of RNA. Dose-response experiments revealed a K0.5 of 9.5 nM fMLP which is in good agreement with the dissociation constants of the fMLP receptor complex in human neutrophils. Furthermore, the fMLP-induced Ca2+ mobilization in Xenopus oocytes was blocked by the fMLP receptor inhibitor t-butoxycarbonyl-Met-Leu-Phe. Size fractionation of the RNA and microinjection of the individual fractions indicated that messenger RNA for the fMLP receptor is between 1.5 and 2.0 kilobases. Reconstitution of the fMLP receptor into Xenopus oocytes can be employed to isolate the cDNA encoding the fMLP receptor as well as to study the regulation of the fMLP receptor in a functional system.