Solubilization of prostacyclin membrane receptors from human platelets

A Review of Prostanoid Receptors: Expression, Characterization, Regulation, and Mechanism of Action

Prostaglandin signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain to cell survival. Disruption of normal prostanoid signaling is implicated in numerous disease states. Prostaglandin signaling is facilitated by G-protein-coupled, prostanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of prostanoid receptors with particular emphasis on human isoforms.

Endogenous inhibitory mechanisms and the regulation of platelet function

The response of platelets to changes in the immediate environment is always a balance between activatory and inhibitory signals, the cumulative effect of which is either activation or quiescence. This is true of platelets in free flowing blood and of their regulation of haemostasis and thrombosis. In this review, we consider the endogenous inhibitory mechanisms that combine to regulate platelet activation. These include those derived from the endothelium (nitric oxide, prostacyclin, CD39), inhibitory receptors on the surface of platelets (platelet endothelial cell adhesion molecule-1, carcinoembryonic antigen cell adhesion molecule 1, G6b-B - including evidence for the role of Ig-ITIM superfamily members in the negative regulation of ITAM-associated GPVI platelet-collagen interactions and GPCR-mediated signalling and in positive regulation of "outside-in" integrin α(IIb)β(3)-mediated signalling), intracellular inhibitory receptors (retinoic X receptor, glucocorticoid receptor, peroxisome proliferator-activated receptors, liver X receptor), and emerging inhibitory pathways (canonical Wnt signalling, Semaphorin 3A, endothelial cell specific adhesion molecule, and junctional adhesion molecule-A).

Amino acid residues conferring ligand binding properties of prostaglandin I and prostaglandin D receptors. Identification by site-directed mutagenesis

Using chimeras of the mouse prostaglandin (PG) I receptor (mIP) and the mouse PGD receptor (mDP), we previously revealed that the cyclopentane ring recognition by these receptors is specified by a region from the first to third transmembrane domain of each receptor; recognition by this region of mIP is broad, accommodating the D, E, and I types of cyclopentane rings, whereas that of mDP binds the D type of PGs alone (Kobayashi, T., Kiriyama, M., Hirata, T., Hirata, M., Ushikubi, F., and Narumiya, S. (1997) J. Biol. Chem. 272, 15154-15160). In the present study, we performed a more detailed chimera analysis, and narrowed the domain for the ring recognition to a region from the first transmembrane domain to the first extracellular loop. One chimera with the replacement of the second transmembrane domain and the first extracellular loop of mDP with that of mIP bound only iloprost. The amino acid substitutions in this chimera suggest that Ser(50) in the first transmembrane domain of mIP confers the broad ligand recognition of mIP and that Lys(75) and Leu(83) in the second transmembrane domain of mDP confer the high affinity to PGD(2) and the strict specificity of ligand binding of mDP, respectively.

Expression of prostacyclin receptors in luteinizing hormone-releasing hormone immortalized neurons: role in the control of hormone secretion

PGs of the E series are involved in the control of LHRH secretion. The present experiments were conducted to clarify whether PGI2 (prostacyclin) might be also involved in such a control, using multiple methodological approaches on immortalized LHRH-secreting neurons. A RT-PCR procedure to detect mouse PGI2 receptor (IP) messenger RNA was first applied, and the results obtained showed the presence of a specific transcript in two cell lines of immortalized LHRH neurons (GT1-1 and GN11 cell lines). Receptor binding assays on membrane preparations from GT1-1 cells showed the presence of a single specific and saturable class of binding sites (Kd = 4.6 nM; 10,000 sites/cell) for [3H]iloprost, a stable analog of PGI2. Competition experiments showed that the binding sites labeled by [3H]iloprost possess the pharmacological characteristics of IP receptors. In functional studies, PGI2 and its analogs, iloprost and cicaprost, were able to stimulate LHRH release from the GT1-1 cells with elevated potencies (EC50 = 0.6-4.3 nM); PGE1 was only slightly less active (EC50 = 28.5 nM), whereas PGE2, considered the major PG involved in LHRH secretion, was poorly effective (EC50 = 921 nM). The relative potencies (EC50) of these compounds in stimulating the intracellular accumulation of cAMP were in line with their LHRH-releasing activities. In conclusion, these results indicate that immortalized LHRH-secreting neurons express IP receptors through which PGI2 may exert relevant effects on LHRH release.

Platelet prostanoid receptors

Prostaglandins (PGs) and thromboxanes are important modulators of platelet activation, and there is strong evidence to support the existence of distinct thromboxane, prostacyclin, PGD2 and PGE2 receptors on the platelet plasma membrane. In this review, each of these platelet prostanoid receptors is discussed in detail, with respect to their receptor pharmacology, molecular biology and signal transduction, and as to any therapeutic implications of the development of specific agonists and/or antagonists. In addition, it considers the possibility that there are separate vascular receptors for 8-epi PGF2 alpha, which are not present on the platelet.

Identification of domains conferring ligand binding specificity to the prostanoid receptor. Studies on chimeric prostacyclin/prostaglandin D receptors

To identify domains conferring ligand binding specificity to prostanoid receptors, we constructed a series of chimeric receptors by successively replacing the regions from the carboxyl-terminal tail of mouse prostacyclin (prostaglandin I (PGI)) receptor (mIP) with the corresponding regions of the mouse PGD receptor (mDP). The mIP receptor expressed in COS 7 cells bound [3H]iloprost, a PGI2 analog, and [3H]PGE1 with Kd values of 13 and 27 nM, respectively. This receptor did not bind [3H]PGD2, [3H]PGE2, and [3H]PGF2alpha. The mDP receptor bound only [3H]PGD2 with a Kd value of 43 nM. The chimeric IPN-VII/DPC receptor with replacement of the carboxyl tail of the mIP receptor with that of the mDP receptor showed 12-16-fold higher affinities for [3H]iloprost and [3H]PGE1 than the mIP receptor. The region extending from the sixth transmembrane domain to the carboxyl terminus of the mIP receptor was next replaced with the corresponding region of the mDP receptor. This chimeric IPN-V/DPVI-C receptor acquired the ability to bind [3H]PGD2 and [3H]PGE2 without decreasing the affinities of the mIP receptor to [3H]iloprost and [3H]PGE1. These binding characteristics did not change when the fourth and fifth transmembrane domains of the mIP receptor were further replaced with the corresponding regions of the mDP receptor. However, when the first extracellular to second intracellular loop of the mIP receptor containing the third transmembrane domain was further replaced with those of the mDP receptor, the affinities for [3H]PGE1, [3H]PGE2, and [3H]iloprost were markedly decreased, whereas that for [3H]PGD2 was increased by about 2-fold. [3H]PGF2alpha showed no affinity for the mIP, mDP, and all the chimeric receptors. These results suggest that the sixth to seventh transmembrane domain of the mIP receptor confers the specificity of this receptor to bind selectively to PGE1 and not to PGE2 and that the third transmembrane domain of the mDP receptor confers the selective binding of PGD2 to this receptor.

Synergistic interaction of adenylate cyclase activators and nitric oxide donor SIN-1 on platelet cyclic AMP

The molecular mechanism of the synergistic platelet inhibition by activators of adenylate cyclase and guanylate cyclase in human platelets was investigated. The adenylate cyclase activators iloprost and prostaglandin E1 and the guanylate cyclase activator 3-morpholino-syndnonimine (SIN-1) dose-dependently inhibited thrombin-induced aggregation of washed human platelets. Furthermore, SIN-1 at a concentration inhibiting platelet aggregation by only 10% shifted the IC50 values of iloprost and prostaglandin E1 by one order of magnitude to the left, indicating a synergistic action of adenylate cyclase and guanylate cyclase activators. Iloprost and prostaglandin E1 dose-dependently elevated platelet cAMP without a significant influence on cGMP. In contrast, the platelet cGMP level was dose-dependently elevated by SIN-1. In addiiton, SIN-1 markedly increased cAMP level induced by low concentrations of adenylate cyclase activators (0.1-0.3 nM iloprost or 10-150 nM prostaglandin E1). In contrast, the rise in cAMP induced by higher adenylate cyclase activator concentrations (3 nM iloprost or 30 microM prostaglandin E1) was significantly reduced in the presence of SIN-1. The same biphasic mode of action of SIN-1 was observed with forskolin, an adenylate cyclase stimulator acting receptor independently, indicating a prostacyclin-receptor independent mechanism. The cAMP elevating effect of SIN-1 in the presence of low prostanoid concentrations was completely abolished by piroximone, a selective inhibitor of phosphodiesterase type III. Therefore, the inhibition of phosphodiesterase III by cGMP seems to be the mechanism for the elevation of cAMP levels by SIN-1 in the presence of low concentration of adenylate cyclase activators in human platelets.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning of human prostacyclin receptor cDNA and its gene expression in the cardiovascular system

BACKGROUND

Prostacyclin elicits a potent vasodilation and inhibition of platelet aggregation through binding to its membrane receptor. The impairment of prostacyclin receptor activity is implicated in various human cardiovascular diseases. In the present study, we succeeded in the isolation and characterization of human prostacyclin receptor cDNA and elucidated its gene expression in human tissues.

METHODS AND RESULTS

We isolated a cDNA clone encoding the human prostacyclin receptor from a human lung cDNA library. The isolated cDNA clone encodes a 386-amino acid protein with seven putative transmembrane domains, which belongs to the G protein-coupled receptor superfamily. [3H]iloprost, a prostacyclin receptor agonist, specifically bound to the receptor transiently expressed in COS-7 cells. The binding was inhibited in the rank order of iloprost = cicaprost, another prostacyclin receptor agonist, > prostaglandin E1 (PGE1) >> PGE2, PGF2 alpha, PGD2, STA2. In addition, iloprost dose-dependently stimulated cAMP generation in these COS-7 cells. These results are consistent with the characteristics of the human prostacyclin receptor. Northern blotting analysis on human tissues revealed that prostacyclin receptor mRNA is abundantly expressed in the aorta, lung, atrium, ventricle, and kidney.

CONCLUSIONS

We cloned human prostacyclin receptor cDNA and elucidated its abundant gene expression in the human cardiovascular system. The present study will lead to better understanding of the significance of prostacyclin in humans and further facilitate the clinical application of prostacyclin.

Cloning and expression of a cDNA for the human prostacyclin receptor

A functional cDNA for the human prostacyclin receptor was isolated from a cDNA library of CMK cells, a human megakaryocytic leukaemia cell line. The cDNA encodes a protein consisting of 386 amino acid residues with seven putative transmembrane domains and a deduced molecular weight of 40,956. [3H]Iloprost specifically bound to the membrane of CHO cells stably expressing the cDNA with a Kd of 3.3 nM. This binding was displaced by unlabelled prostanoids in the order of iloprost = cicaprost >> carbacyclin > prostaglandin E1 (PGE1) > STA2. PGE2, PGD2 and PGF 2 alpha did not inhibit it. Iloprost in a concentration-dependent manner increased the cAMP level and generated inositol trisphosphate in these cells, indicating that this human receptor can couple to multiple signal transduction pathways.

Solubilization and characterization of PGE2 receptor in porcine ciliary epithelium

PGE2 binding sites or receptors of porcine ciliary nonpigmented epithelial (NPE) and pigmented epithelial (PE) membranes were solubilized with detergents (CHAPS and Triton X100). From the Scatchard plots of PGE2 binding to CHAPS-solubilized proteins, the Kd and Bmax values were calculated to be 35 nM and 470 fmol/mg protein for NPE protein and 65 nM and 430 fmol/mg protein for PE protein, respectively. On the basis of the Kd and Bmax values, the solubilized receptor proteins correspond to PGE2 binding sites of the membranes which have previously been shown to be coupled to adenylate cyclase inhibition. For both NPE and PE proteins, the order of binding potency was PGE2 > PGF2 alpha > PGD2. By gel filtration chromatography of NPE and PE proteins, the molecular mass of the major PGE2 binding peak was estimated to be about 150 KDa when solubilized in CHAPS and 46 KDa for Triton X100 extracts. The Bmax values of membrane-associated binding proteins were increased by GTP, indicating a close association of the PGE2 binding sites with a GTP-binding protein. However, GTP did not affect the Bmax values of detergent-solubilized receptor proteins.

Solubilization and purification of the prostaglandin E2 receptor from cardiac sarcolemma

A prostaglandin E2 (PGE2) receptor was solubilized and isolated from cardiac sarcolemma membranes. Its binding characteristics are almost identical to those of the membrane bound receptor. [3H]PGE2 binding to solubilized and membrane bound receptor was sensitive to elevated temperature and no binding was observed in the absence of NaCl. No significant effects of DTT, ATP, Mg2+, Ca2+ or of changes in buffer pH were observed on [3H]PGE2 binding to either solubilized or membrane-bound receptor. Unlabelled PGE1 displaced over 90% of [3H]PGE2 from the CHAPS-solubilized receptor. PGD2, PGI2, PGF2 alpha and 6-keto-PGF1 alpha were not effective in displacing [3H]PGE2 from the receptor. Scatchard analysis of [3H]PGE2 binding to CHAPS-solubilized receptor revealed the presence of two types of PGE2 binding sites with Kd of 0.33 +/- 0.05 nM and 3.00 +/- 0.27 nM and Bmax of 0.5 +/- 0.04 and 2.0 +/- 0.1 pmol/mg of protein. The functional PGE2 receptor was isolated from CHAPS-solubilized SL membrane using two independent methods: first by a WGA-Sepharose chromatography and second by sucrose gradient density centrifugation. Receptor isolated by these two methods bound [3H]PGE2. Unlabelled PGE1 and PGE2 displaced [3H]PGE2 from the purified receptor. Scatchard analysis of [3H]PGE2 binding to purified receptor revealed the presence of the two binding sites as observed for the membrane bound and CHAPS-solubilized receptor. SDS-polyacrylamide gel electrophoresis of the purified receptor fractions revealed the presence of a protein band of M(r) of approx. 100,000. This 100-kDa was photolabelled with [3H]azido-PGE2, a photoactive derivative of PGE2. We propose that this 100-kDa protein is a cardiac PGE2 receptor.

Hypothesis for the receptors of human blood platelet aggregation and its inhibition by structure-activity relationship

We tried to clarify the size and the common charge distribution of the inhibition or stimulation of human platelet aggregation by structure-activity relationship. Numerous inhibiting and stimulating agents were able to enter the receptors. Inhibitory receptor had recess of 14 x 12.5 A in diameter. Stimulatory receptor had recess of 11 x 12 A in diameter. In the recess, there were three charges, two negative and one positive in the inhibitory receptor, and one negative and two positive in the stimulatory receptor, respectively. Charge distributions and conformation of inhibiting or stimulating agents were similar for the inhibitory agents, prostaglandin I2 (PGI2), PGD2, PGE1 adenosine and isoproterenol and conformation of the stimulating agents, thromboxane A2 (TXA2), platelet activating factor (PAF), adenosine diphosphate (ADP) and adrenaline. Each molecule had 3-10 inhibiting and stimulating conformations. The ratio of the number of conformations for inhibition and stimulating of platelet aggregation was highest for PGI2 which showed the strongest inhibitory activity. TXA2 was opposite in both respects.

Prostacyclin analogues inhibit tissue factor expression in the human monocytic cell line THP-1 via a cyclic AMP-dependent mechanism

Increased expression of tissue factor procoagulant by peripheral blood monocytes has been implicated in a number of thrombotic disorders. The present studies were undertaken to determine whether stable analogues of prostacyclin, a potent endothelium-derived platelet inhibitor and vasodilator, could inhibit tissue factor expression by human monocytic cells. Exposure of monocytic tumor THP-1 cells to 100 ng/ml endotoxin, 2 units/ml interleukin-1 beta, or 5 ng/ml tumor necrosis factor-alpha for 4 hours led to increased tissue factor procoagulant activity. Preincubation for 30 minutes with iloprost, ciprostene, and carbacyclin led to a dose-dependent inhibition of tissue factor expression induced by all three challenging agents. Iloprost was the most potent: 50% inhibition occurred at 5 nM, a concentration close to the reported dissociation constant for iloprost binding to the platelet prostacyclin receptor. An orally active analogue, cicaprost, was equally effective against endotoxin-induced tissue factor expression. Carbacyclin and ciprostene were 100 times less potent. Iloprost prevented the endotoxin-induced expression of tissue factor antigen on the surface of THP-1 cells, as determined by flow cytometry. Iloprost (500 pM-50 nM) increased intracellular levels of cyclic AMP. This effect was potentiated by isobutylmethylxanthine, an inhibitor of phosphodiesterase. The inhibitory effects of iloprost on tissue factor expression were also potentiated by isobutylmethylxanthine and mimicked by forskolin and dibutyryl cyclic AMP but not dibutyryl cyclic GMP. These results suggest that prostacyclin may play a role in downregulating tissue factor expression in monocytes, at least in part via elevation of intracellular levels of cyclic AMP.

[3H]iloprost and prostaglandin E2 compete for the same receptor site on cardiac sarcolemmal membranes

We have previously demonstrated that high-affinity PGE receptors are present on purified cardiac sarcolemmal (SL) membrane from bovine heart (Lopaschuk et al. (1989) Circ. Res. 65, 538-545). In this study we determined whether PGI2 receptors are also present on the cardiac SL membrane. Due to the extreme lability of prostacyclin (PGI2) under physiological conditions, the PGI2 analogue, Iloprost was substituted for PGI2. 3H-Iloprost specifically bound to two sites on the SL membrane; one of high affinity (Kd = 0.3 nM, Bmax = 97.0 fmol/mg SL), and one of lower affinity (Kd = 20.6 nM, Bmax = 1589 fmol/mg SL). Competition studies demonstrated that the concentrations of PGE2 and PGE1 necessary to displace 50% of the specific binding of 20 nM [3H]Iloprost on cardiac SL were 15-fold lower than the concentrations of unlabelled Iloprost necessary to displace 50% of binding. In contrast, a 15-fold higher concentration of unlabelled Iloprost was needed to displaced 50% of specific binding of 2 nM [3H]PGE2 compared to the concentrations of PGE1 or PGE2 required to displace 50% of [3H]PGE2 binding. In summary, our results indicate that a prostacyclin receptor is present on the cardiac sarcolemmal membrane, and that PGI2 competes for the same receptor site as PGE2.

Regulation of PGI2 activity by serum proteins: serum albumin but not high density lipoprotein is the PGI2 binding and stabilizing protein in human blood

Although previous studies have shown that serum albumin binds PGI2 and protects it from rapid degradation, it remains debatable whether it is physiologically important due to its low binding affinity for PGI2. We were intrigued by the observations of Yui et al. (J. Clin. Invest. 82 (1988) 803-807) which suggested that apo A-I of the high density lipoprotein (HDL) is the "serum PGI2 stabilizing factor". To clarify this, we carried out experiments to determine the binding kinetics and parameters of HDL and albumin purified from normal pooled human serum. Despite the use of multiple binding assays, we could not detect any binding activity in HDL2, HDL3 or nascent HDL preparations, nor could we demonstrate any PGI2 protecting activity by these molecules. By contrast, purified albumin exhibited essentially identical binding parameters as the native serum from which the albumin was purified. The binding activity of various albumin preparations was not due to the contamination of apo A-I. Computer simulation analysis also failed to provide evidence to support the notion that HDL bound and prolonged PGI2 activity. To determine whether physiological concentrations of albumin influence PGI2 binding to platelet receptors, we measured PGI2 binding to platelet membrane in the absence and presence of albumin. Albumin at 40 mg/ml increased the KD of PGI2 binding to the receptors by 2-3 fold. These findings indicate that albumin plays a major role in protecting PGI2 activity and regulating its availability for platelet PGI2 receptors.

Comparison of vasodilatory prostaglandins with respect to cAMP-mediated phosphorylation of a target substrate in intact human platelets

The recent purification of a vasodilator-stimulated phosphoprotein (VASP) from human platelets and the development of a specific antiserum against VASP made it possible to study the quantitative effects of cAMP-elevating prostaglandins on cAMP-mediated phosphorylation of VASP in intact human platelets. Prostacyclin (PG-I2), prostaglandin-E1 (PG-E1) and the stable prostacyclinanalog Iloprost, all agents used for the treatment of peripheral vascular disease, induced rapid, stoichiometric and reversible phosphorylation of VASP in human platelets mediated by the cAMP-dependent protein kinase. However, there were substantial differences between these three cAMP-elevating prostaglandins with respect to their effects on extent, duration and reversibility of VASP phosphorylation. Maximal VASP phosphorylation was induced both by PG-I2 and Iloprost, but the PG-I2 effect was only of short duration in comparison to that of Iloprost. The extent of PG-E1-induced VASP phosphorylation was less than that observed with PG-I2 and Iloprost. In endothelial cell-platelet coincubations, an endothelial cell-derived, indomethacin-sensitive factor caused a rapid elevation of platelet cAMP level and VASP phosphorylation. These results provided direct evidence that human endothelial cells are capable of producing biologically active quantities of cAMP-elevating prostaglandins sufficient to induce stoichiometric cAMP-mediated protein phosphorylation in human platelets. VASP-phosphorylation induced by PG-I2 and PG-E1 was completely reversible after removal of the prostaglandins whereas this was only partially the case with Iloprost. In addition, evidence is presented that the prostaglandin-regulated adenylate cyclase system but not the cAMP-mediated protein phosphorylation desensitizes in human platelets after prolonged treatment with cAMP-elevating prostaglandins. VASP phosphorylation is proposed as a marker for quantitating aspects of vessel wall-platelet interaction.

Guanine nucleotide sensitivity of [3H]iloprost binding to prostacyclin receptors

A component of the displaceable binding of the stable prostacyclin analogue, [3H]iloprost, to membranes from human platelets and the somatic hybrid cell lines NG108-15 and NCB20, was inhibited by guanine nucleotides. The order of potency of a range of nucleotides for this effect was GTP gamma S greater than GppNHp greater than GTP greater than GDP = GMP; ATP, UTP and CTP were ineffective at concentrations up to 1 mM. In the presence of 100 microM GppNHp, iloprost binding curves were displaced to the right of curves obtained in the absence of guanine nucleotide, and their Hill slopes were greater. This was consistent with a conversion of a minor population of high affinity agonist binding sites to lower affinity sites in the presence of guanine nucleotides. These effects of guanine nucleotides on the binding of the agonist ligand [3H]iloprost were consistent with an interaction with a G protein coupled receptor.

Conformation of receptor-associated PGI2: an investigation by molecular modeling

To elucidate the conformation of receptor-associated prostacyclin (PGI2), we first performed structure-activity correlation analysis of over 200 PGI2 analogues and derived from this analysis several crucial features pertaining to structural requirements for PGI2 activity [Ah-lim Tsai and Kenneth K. Wu, Eicosanoids, 2 (1989) 131-143]. These structural features proved to be useful guidelines for selecting 'model molecules' for further investigations by molecular mechanics. By properly selecting four analogues with either rigid or uniquely oriented alpha-side chain structure for geometric fitting, we succeeded in maximally minimizing the degree of freedom of the carboxylate terminus of PGI2. We were able to define the spatial relationship among the four critical functional groups, i.e., C1-COOH, C6a-O, C11-OH and C15-OH. More information is needed, however, to define the geometry of the omega-side chain, particularly for the moiety beyond C15. Nevertheless, results from structure-activity correlation analysis and molecular modeling provide useful information regarding the conformation of receptor-associated PGI2, which assumes an 'elongated' conformation instead of the traditional 'hairpin' structure.

Autocrine control of adipose cell differentiation by prostacyclin and PGF2 alpha

The mitogenic-adipogenic effect exerted by arachidonic acid, which leads to terminal differentiation of Ob1771 mouse preadipocytes, has been shown to be (i) blocked by cyclooxygenase inhibitors, (ii) mimicked by a stable analogue of prostacyclin (carbaprostacyclin) and (iii) potentiated by PGF2 alpha. Since these prostanoids are known to be synthesized and secreted by preadipocytes, we have proposed that both prostacyclin as the key mediator and PGF2 alpha as a modulator control the expression of terminal events of adipose conversion by means of an autocrine mechanism (Gaillard, D. et al. and Negrel, R. et al. Biochem. J. (1989) 257, 389-397 and 399-405). In order to test this hypothesis, the release of prostacyclin, characterized under the form of its stable degradation product 6-keto-PGF1 alpha, and that of PGF2 alpha have been studied in the culture medium of Ob1771 cells. A striking increase in the release of 6-keto-PGF1 alpha and to a minor degree of PGF2 alpha was observed when cells were exposed to arachidonic acid as shown by using [3H]arachidonic acid prelabelled cells or by radio-immunoassays. Since antagonists of PGF2 alpha and PGI2 receptors were not available, specific antibodies directed against PGF2 alpha and 6 beta-PGI1, another stable analogue of prostacyclin, were added as neutralizing agents in the culture medium. These antibodies were able to counteract the mitogenic-adipogenic effect of arachidonic acid. Prostacyclin and PGF2 alpha thus appear as autocrine mediators in the process of adipose conversion.

Identification of a prostacyclin receptor coupled to the adenylate cyclase system via a stimulatory GTP-binding protein in mouse mastocytoma P-815 cells

A stable analogue of prostacyclin, iloprost, specifically bound to 30,000 x g pellet (the membrane fraction) prepared from mouse mastocytoma P-815 cells. The binding was dependent on time, temperature and pH, and absolutely required a divalent cation. The equilibrium dissociation constant and the maximal concentration of the binding site as determined by Scatchard plot analysis were 10.4 nM and 1.12 pmol/mg of protein, respectively. The Hill coefficient was 1.0, indicating a single entity of binding site and no cooperativity. The binding site was highly specific for iloprost among PGs tested (iloprost much greater than PGE1 greater than carbacyclin greater than PGE2). In contrast, the membrane fraction had the binding site specific for PGE2 and PGE1, which was distinct from the prostacyclin receptor. The dissociation of bound [3H]iloprost from the membrane fraction was specifically enhanced by guanine nucleotides. Furthermore, iloprost dose-dependently enhanced the activity of adenylate cyclase in a GTP-dependent manner. These results indicate that a specific prostacyclin receptor is coupled to the adenylate cyclase system via a stimulatory GTP-binding protein in mastocytoma cells.

Asymmetric extraction of membrane lipids by CHAPS

We have characterized and quantitated the lipids which are cosolubilized with serotonin 5-HT1A sites from sheep brain using 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS). Dialysis of the CHAPS extract produced a [3H]8-hydroxy(2-di-n-propylamino)tetralin [( 3H]8-OH-DPAT) binding vesicular preparation of the protein. Quantitative analysis of the lipids present in the CHAPS extract by HPTLC and transmittance-densitometry revealed extraction of phosphatidylethanolamine (PE), phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidyl serine (PS) and phosphatidic acid (PA) in striking preference over cholesterol, galactosylceramides, sulfatides and sphingomyelin. All lipids present in the clear CHAPS-extract were coeluted with the [3H]8-OH-DPAT binding preparation were separated by centrifugation, 95-100% of the [3H]8-OH-DPAT binding protein was retained in the vesicle-containing pellet. The supernatant contained small amounts of cholesterol, PE and PC, but virtually no PS, PI, or PA, whereas the vesicular pellet contained all the lipids mentioned, indicating that PS, PI and PA are more tightly bound to the vesicles than PE, PC and cholesterol. SDS-PAGE analysis of the pellet revealed two major protein bands, at 58 kDa and 33.5 kDa, respectively. Our report outlines a simple and improved densitometric assay used for the first detailed analysis of lipids cosolubilized with an active, membrane protein, and also, a simple assay for CHAPS.

Effects of a stable prostacyclin analogue on platelet activity and on host immunocompetence in mice

The stable prostacyclin (PGI2) analogue, iloprost, is a potent inhibitor of both tumor cell-induced platelet aggregation and of experimental metastasis in mice. To explore possible mechanisms of antimetastatic effect of iloprost, we measured the effect of this drug on both platelet aggregation and immunocompetence in the mouse. Iloprost (4 x 10(-8) M) inhibited platelet aggregation as induced by a mixture of collagen and epinephrine for at least 180 minutes of incubation, and completely reversed platelet aggregation when added during the second wave of aggregation. In addition, aggregation of platelets obtained from iloprost-treated mice (0.2 mg/kg) was completely inhibited for at least 90 minutes of incubation. Moreover, iloprost pretreatment in vivo counteracted tumor cell-induced thrombocytopenia. Thus, mouse platelets were equally sensitive to the inhibitory effect of iloprost on aggregation as platelets of other species including humans. Effects of iloprost on parameters of host immunocompetence that may influence tumor growth and metastasis formation were also evaluated. Iloprost treatment increased significantly macrophage cytostasis to tumor cells, natural killer (NK) lytic activity of spleen cells and T-cell mediated cytotoxicity ex vivo. These results suggested that the antimetastatic effect of iloprost in the mouse may be attributable to multiple mechanisms including inhibition of platelet aggregation and stimulation of certain host immune functions.