Prostaglandin E1 high affinity binding sites of rat thymocytes. Specificity and blockade by non-steroidal antiinflammatory drugs and localization in a plasma membrane-enriched fraction

Characterization of the signal transduction of prostaglandin E receptor EP1 subtype in cDNA-transfected Chinese hamster ovary cells

We examined the signal transduction of mouse prostaglandin E receptor EP1 subtype using Chinese hamster ovary cells stably expressing the cloned EP1. Sulprostone, an EP1 agonist, induced a rapid increase in intracellular Ca2+ concentration in the EP1-expressing cells. Most of the increase was abolished by removal of extracellular Ca2+, and was insensitive to U-73122, a phospholipase C inhibitor. Sulprostone stimulated phosphatidylinositol hydrolysis, but this stimulation was abolished by removal of extracellular Ca2+, indicating that EP1-stimulated phosphatidylinositol hydrolysis is the result of extracellular Ca2+ influx. Thus, the signal transduction of EP1 is extracellular Ca2+ entry through a pathway independent of phospholipase C activation. We further examined the regulation of the signal transduction of EP1 having potential phosphorylation sites for either protein kinase C or protein kinase A. Short-term exposure of the cells to 12-O-tetradecanoylphorbol 13-acetate (TPA) completely suppressed the sulprostone-induced increase in intracellular Ca2+ concentration, while forskolin or dibutyryl cAMP did not affect it, suggesting that protein kinase C but not protein kinase A is involved in the regulation of the EP1 signal transduction. Furthermore, long-term exposure to TPA decreased PGE2 protein kinase A is involved in the regulation of the EP1 signal transduction. Furthermore, long-term exposure to TPA decreased PGE2 binding activity of EP1 due to the reduction of the EP1 mRNA level. Protein kinase C induces short- and long-term desensitization of EP1.

Expression of prostaglandin G/H synthase (cyclooxygenase) during murine fetal thymic development

Fetal thymic lobes in organ culture have been shown to have the capacity to metabolize [14C]arachidonic acid (AA) to prostaglandins (PGs), including 6-ketoPGF1 alpha, PGF2 alpha, PGE2, and PGA2. Inhibition of AA metabolism results in inhibition of growth and Thy 1 expression during thymic organ culture. We report herein that freshly-isolated fetal thymic lobes also have the capacity to metabolize [14C]AA to PGs and HETEs at Days 14 and 16 of prenatal murine development. RNA encoding phospholipase A2, which liberates arachidonic acid from membrane phospholipids, and cyclooxygenase (prostaglandin G/H synthase), the first enzyme involved in the conversion of AA to PGs, are expressed during thymic development. We have localized the cyclooxygenase protein to stromal cells in the fetal and adult thymus. Exogenous AA or an analogue of PGI2 (iloprost) stimulated growth of fetal thymocytes in organ culture. These findings, together with our studies of the morphology of thymic lobes cultured with inhibitors of arachidonate metabolism, support the hypothesis that PGs are required for thymocyte proliferation during thymic development.

Effect of apoprotein cross-linking on the metabolism of human HDL3 in rat

Apo E-free human high-density lipoprotein (HDL3) was labeled with 125I in apoprotein and with 3H in cholesteryl linoleyl ether (a non-hydrolyzable analogue of cholesteryl ester). The labeled HDL3 was modified by cross-linking of apoproteins with dimethylsuberimidate (DMS) to inhibit binding to HDL specific receptors. The control and the DMS HDL3 were characterized with respect to their rate of clearance from rat blood, in vivo binding to major rat organs and in vitro binding to purified rat liver plasma membranes. Both 125I and 3H labels from control HDL3 were cleared from rat blood monoexponentially, but 3H at a faster rate than 125I (3H t1/2 = 3.0-4.1 h; 125I t1/2 = 7.0-7.7 h). This difference is consistent with reports of the nonendocytotic selective uptake of HDL-associated cholesteryl ester. DMS modification did not affect the rate of 3H clearance whereas it increased the rate of 125I clearance (HDL3 t1/2 = 7.7 h; DMS HDL3 t1/2 = 4.1 h). Both in vivo binding to rat organs and in vitro binding to rat liver membranes confirmed that DMS modification inhibited the specific binding of HDL, but also suggested that the modification produced saturable binding of HDL to a separate class of sites. Thus, the present data do not rule out the involvement of direct HDL-cell interaction in the selective uptake of HDL cholesteryl ester. However, results suggest that the binding of HDL to its specific cell surface sites is not necessary for this uptake.

Enrichment and characterization of specific [3H]PGE2 binding sites in the porcine gastric mucosa

[3H]Prostaglandin E2 (PGE2) binding sites were 10-fold enriched from a porcine fundic mucosal homogenate by differential centrifugation and subsequent discontinuous sucrose gradient separation. PGE2 bound with an activation energy of 66 kJ/mol to a single class of sites with an affinity of 1.5 +/- 0.4 nM and a capacity of 274 +/- 76 fmol/mg protein. There was no indication for any cooperativity between the binding sites. Kinetic analysis revealed a kon of 3 x 10(5) M x s-1 at 30 degrees C and pH 5.5. Dissociation was biphasic with an initial rapid (koff = 10(-3) s-1) and a subsequent slower phase (koff = 4 X 10(-5) s-1), presumably reflecting the existence of two interchangeable forms of [3H]PGE2 binding sites. The rank order of affinity for other prostanoids (PGE2 greater than PGE1 greater than PGA2 greater than iloprost (PGI2 derivative) greater than PGF2 alpha greater than PGB2 greater than PGD2) is discussed against the background of the recently postulated E-type receptors in the stomach.

Presence of glucocorticoid receptors in cultured thymic epithelial cells

We investigated the presence of glucocorticoid receptors (GC) in human thymic epithelial cells grown in primary cultures and in a pure epithelial rat cell line. These GR levels were compared to those determined concomitantly in fresh human thymocytes. The average number of sites were 54,457/cell for males (n = 8) and 58,224/cell for females (n = 8) with mean Kd values of 1.5 and 1.7 X 10(-8) M, respectively, in cultured human epithelial cells. These results are comparable to those obtained for rat thymic epithelial cells. Competition experiments showed that the relative affinities of the steroids tested were in decreasing order: dexamethasone greater than progesterone greater than testosterone and estradiol. This observation is compatible with binding to physiological GR. Moreover, the mean GR value appeared to be approximately 10 times higher for human thymic epithelial cells than for thymocytes. Thus, human epithelial cells as well as thymocytes should be considered as a specific target for glucocorticoid hormones.

Antigen-nonspecific macrophage factors modulating the antibody response in vitro

Antibody response to an antigen involves the co-operation between three types of cells: macrophages, T cells and B cells. The cognate interactions between these cells play a fundamental role in the expression of a specific antibody response, but the last is modulated by antigen-nonspecific soluble factors produced either by macrophages or by T cells. Macrophages elaborate a spectrum of molecules modulating the function of lymphoid cells; among them are IL1 and prostaglandins of the E series, which are respectively enhancer and inhibitor of the antibody response in vitro. These molecules alter T cell and B cell activities through different mechanisms involving activation or inhibition of IL2 production, or alteration of cells surface antigens. However, the cellular events following the fixation of soluble factor on its receptors are not known.

Receptors of prostaglandin E1 in the plasma membrane of normal liver. Effect of membrane fluidity on PGE1 binding

Prostaglandin E1 binding to isolated liver plasma membrane as a function of PGE1 concentration showed saturability of the binding sites at PGE1 concentration of 2.5 X 10(-4) M. Scatchard analysis revealed heterogeneous population of binding sites with a binding capacity of 470 and 990 nmol/mg protein for the higher and lower affinity binding sites respectively. PGE1 binding to liver plasma membrane was progressively and significantly decreased (P less than 0.01) as the incubation temperature was reduced to 22 degrees and 4 degrees C. Procaine at 1 X 10(-3) M concentration showed a significant decrease (P less than 0.01) in the binding capacity of the liver plasma membrane. Colchicine plus cytochalasin-B inhibited PGE1 binding significantly (P less than 0.01) but their inhibition is not equivalent to that of procaine.

Direct evidence for the presence of selective binding sites for (3H) prostaglandin E2 on rat peritoneal macrophages

A method is presented which provides for a simple and rapid determination of PGE2 receptors on viable peritoneal macrophages. Incubation of the harvested cells with (3H)PGE2 revealed specific binding of (3H)PGE2 by use of the Millipore filter assay system. Maximum binding was attained in the presence of 1 mM EDTA. Specific binding was saturable at 65 fmol/mg protein with an equilibrium dissociation constant (Kd) of 3.2 X 10(-8)M. Inhibition of (3H)PGE2 binding with unlabelled prostaglandins revealed a potency series of PGE2 greater than PGE2 greater than PGI2. The PGE2 concentration which displaced 50% of the labelled ligand was 10(-7)M. Comparable kinetic data were obtained for adenylate cyclase stimulation, since the concentration which showed a halfmaximal stimulation of cAMP production was 2 X 10(-7)M of PGE2. Since PGE1 and PGI2 compete with (3H)PGE2 binding in a non-parallel manner compared to PGE2 itself, it is proposed that macrophages possess different types of PG receptors.

Prostaglandin secretion by human thymic epithelium: in vitro effects of steroids

Increasing evidence suggests a role for prostaglandins in the immune response. As steroids have been shown recently to modulate prostaglandin secretion, we have studied the secretion of prostaglandin and the effect of various steroids in culture of human thymus epithelial cells. Using reverse phase high pressure liquid chromatography and radioimmunoassays, we have shown that these cells produce substantial amounts of PGE2 and PGF2 alpha and that this secretion is modulated by steroid hormones. Prostaglandin could represent one of the factors of the thymic environment which respond to steroid hormones.

Down-regulation in vivo of PGE receptors and adenylate cyclase stimulation

Down-regulation in vivo of liver plasma membrane receptors for prostaglandin E (PGE) was investigated in Sprague-Dawley rats using the 16,16-dimethyl analogue of PGE2, This analogue was used for subcutaneous injections because it escapes the rapid pulmonic degradation characteristic of PGE and was recognized well by liver plasma membrane receptors. Following treatment with the analogue, the concentration of PGE receptors was significantly decreased (-37%, P less than 0.001), but the binding affinity was not altered. There was no evidence for carry-through of the analogue into the isolated plasma membrane preparation. It was also demonstrated that GTP decreased the binding affinity between PGE and its receptor. Down-regulation of receptor concentration was associated with a significant decrease (P less than 0.001) in PGE1-stimulated plasma membrane adenylate cyclase activity. These data provide the novel demonstration that rat liver plasma membrane receptor for PGE can be down-regulated in vivo and that this causes a corresponding decrease in PGE-induced plasma membrane adenylate cyclase activity.

Prostaglandin receptor-adenylate cyclase system in plasma membranes of rat liver and ascites hepatomas, and the effect of GTP upon it

1. Adenylate cyclase in plasma membranes from rat liver was stimulated by prostaglandin E1, and to a lesser extent by prostaglandin E2. Prostaglandin F1alpha and A1 did not stimulate the cyclase. The prostaglandin E1-mediated activation was found to require GTP when the substrate ATP concentration was reduced from 3 mM to 0.3 mM in the reaction mixture. Adenylate cyclase of the plasma membranes from rat ascites hepatomas AH-130 and AH-7974 was not stimulated by prostaglandin E1 in the presence or the absence of GTP, although the basal activity of adenylate cyclase as well as its stimulation by GTP alone were similar to normal liver plasma membranes. 2. Liver plasma membranes were found to have two specific binders for [3H] prostaglandin E1 with dissociation constants of 17.6-10(-9) M and 13.6-10(8) M (37 degrees C) and one specific binder for [3H]prostaglandin F2alpha with a dissociation constant of 2.31-10(8) M (37 degrees C). The specific binders for prostaglandin E1 could not be detected in the hepatoma plasma membranes. 3. Binding of [3H] prostaglandin E1 to the liver plasma membranes was exchange by, GTP dGPT, GDP, ATP and GMP-P(N)P, but not by GMP, CGMP, DTTP, UTP or CTP. The increase in the binding of [3H] prostaglandin E1 was found to be due to the increased affinity of the specific binders to prostaglandin F2alpha was not affected by GTP. 4. GTP alone was found to increase V of adenylate cyclase of liver plasma membranes, while GTP plus prostaglandin E1 was found to decrease Km of adenylate cyclase in addition to the increase of V to a further extent.