Phorbol ester-induced expression of the common, low-affinity binding site for primary prostanoids in vascular smooth muscle cells

Channel modulators affect PGE(2) binding to bovine aortic endothelial cells

PGE(2), PGF(2alpha) and the thromboxane agonist U-46619 bind to bovine aortic endothelial cells and compete on the same binding site with similar affinity. In addition, binding remains unaffected by prolonged exposure to the ligand. These characteristics differ significantly from those of any known G-coupled prostaglandin receptor. Binding of PGE(2) to the cells is reduced in the presence of the cyclic nucleotides cGMP and cAMP, and is unaffected by protein kinase inhibitors. Removal of permeable cyclic nucleotides from the cell medium results in a fast and complete restoration of PGE(2) binding to the cells, suggesting that both cyclic nucleotides reduce PGE(2) binding by a reversible interaction with the prostaglandin-binding site, without the involvement of second messenger-activated protein kinases. Our data further show that binding of prostaglandins to bovine aortic endothelial cells is sensitive to heavy metals and to activators and blockers of calcium, ATP-sensitive K(+) and chloride channels. Nickel, a specific cyclic nucleotide-gated (CNG) channel activator, decreases PGE(2) binding and so do the CNG channel activators Rp-8-Br-PET-cGMPS and Sp-8-Br-PET-cGMPS. On the other hand, the calcium channel blockers pimozide, diltiazem as well as LY-83,583, a guanylate cyclase inhibitor, which were reported to block CNG channels, enhance PGE(2) binding. The sensitivity of PGE(2) binding to selective CNG channel modifying agents, as well as the rapid and reversible interaction with cyclic nucleotides, may suggest that the common low-affinity prostanoid-binding site on bovine aortic endothelial cells is associated with a molecular entity, which possess several properties of a CNG channel.

Amino acid determinants in cyclooxygenase-2 oxygenation of the endocannabinoid 2-arachidonylglycerol

The endocannabinoid, 2-arachidonylglycerol (2-AG), is an endogenous ligand for the central (CB1) and peripheral (CB2) cannabinoid receptors and has been shown to be efficiently and selectively oxygenated by cyclooxygenase (COX)-2. We have investigated 2-AG/COX-2 interactions through site-directed mutagenesis. An evaluation of more than 20 site-directed mutants of murine COX-2 has allowed for the development of a model of 2-AG binding within the COX-2 active site. Most strikingly, these studies have identified Arg-513 as a critical determinant in the ability of COX-2 to efficiently generate prostaglandin H(2) glycerol ester, explaining, in part, the observed isoform selectivity for this substrate. Mutational analysis of Leu-531, an amino acid located directly across from Arg-513 in the COX-2 active site, suggests that 2-AG is shifted in the active site away from this hydrophobic residue and toward Arg-513 relative to arachidonic acid. Despite this difference, aspirin-treated COX-2 oxygenates 2-AG to afford 15-hydroxyeicosatetraenoic acid glycerol ester in a reaction analogous to the C-15 oxygenation of arachidonic acid observed with acetylated COX-2. Finally, the differences in substrate binding do not alter the stereospecificity of the cyclooxygenase reaction; 2-AG-derived and arachidonic acid-derived products share identical stereochemistry.

Rational design of antitumor prostaglandins with high biological stability

microolecular design can overcome the metabolic instability of Delta7-PGA1, while maintaining its antitumor potency. Saturation of the C(13)-C(14) double bond enhances the biological stability but decreases the antiproliferative activity. Configurational inversion of the isomerase-sensitive C(12) stereocenter from the natural S to the unnatural R geometry not only enhances biological stability but also significantly suppresses the growth of the tumor cells. The 12R derivatives markedly increase the induction of p21, a Cdk inhibitor, leading to sharp cell cycle arrest at the G1 phase at a dose level so low that at this dose Delta7-PGA1 methyl ester scarcely exerts an effect. These conspicuous biological properties lead to long-term suppression of tumor cell growth. The structure-stability relationship demonstrates that the stability of prostaglandins (PGs) is crucially controlled by the C(12) configuration and is unaffected by the geometry of the hydroxy-bearing C(15). The successful design of antitumor PGs resistant to enzymatic metabolism provides a new strategy applicable to creating a useful PG for cancer chemotherapy.

Regulation of a common, low-affinity binding site for primary prostanoids on bovine aortic endothelial cells

Bovine aortic endothelial cells contain a prostaglandin site which binds with similar low-affinity PGE2, PGF2alpha and the thromboxane agonist U-46619. Treatment of the cells with agents that increase the level of cellular cAMP such as forskolin, a direct activator of adenylate cyclase or IBMX, a phosphodiesterase inhibitor, decreased the binding of PGE2 to the cells. Addition of dibutyryl cAMP to intact cells caused a quick reduction in PGE2 binding with a half time of less than 2 min. The reduction in PGE2 binding was completely reversible after removing the dibutyryl cAMP. The reduction in PGE2 binding after addition of dibutyryl cAMP to the intact cells was also observed after a mechanical disruption of the cells or after permeabilization with digitonin. Incubation of the cells with myristoylated PKI(14-22) amide, a specific protein kinase A inhibitor, resulted in partial suppression of the reduction of PGE2 binding by dibutyryl cAMP. Pretreatment of intact cells for 24 h with 10(-6) M PGE2 or a PKC activator did not reduce the specific binding of [3H]-PGE2. These results suggest that PKA, but not PKC, is involved in a fast reversible regulation of the common prostanoid receptor on bovine endothelial cells.

A common low-affinity binding site for primary prostanoids on bovine aortic endothelial cells

[3H]PGE2 and [3H]PGF2 alpha were shown to bind with similar binding capacity and dissociation constants to bovine aorta endothelial cells. The similarity in the binding parameters suggests that both agonists may bind to the same binding site. Displacement of [3H]PGE2 performed with PGE2, PGF2 alpha or U-46619, a thromboxane agonist, shows that all three prostanoids displaced the bound [3H]PGE2 with comparable potency (IC50 = 10(-7) M). These results indicated that the three different prostanoids, which serve as specific agonists to different prostanoid receptors, also compete for the same binding site in bovine endothelial cells with similar affinity. Comparison of the displacement of [3H]PGE2 or [3H]PGF2 alpha by a number of prostaglandin agonists and antagonists further supports the notion that the natural prostanoids bind with similar affinities to the same binding site. Thus, sulprostone, an EP1/EP3 agonist, displaced bound [3H]PGE2 and [3H]PGF2 alpha with IC50 of about 10(-7) M. On the other hand, thromboxane antagonists (BAY u-3405 and GR-32191B), EP1 specific antagonist (SC-19220) EP1/DP antagonist (AH-6809) and iloprost, a stable prostacyclin agonist, failed to displace bound [3H]PGE2 or [3H]PGF2 alpha at a concentration range of 10(-9)-10(-6) M. Gradual increase of sodium fluoride (NaF), a general activator of G binding proteins, or incubation of permeabilized cells with GTP gamma S resulted in a decrease in [3H]PGE2 binding, suggesting that the binding site represents a low-affinity common prostanoid receptor which, similar to other prostanoid receptors, is probably coupled with G binding proteins.

Okadaic acid induces both augmentation and inhibition of opsonized zymosan-stimulated superoxide production by differentiated HL-60 cells. Possible involvement of dephosphorylation of a cytosolic 21K protein in respiratory burst

We found that okadaic acid (OA), a potent tumor promoter and a phosphatase inhibitor, has a unique opposing effect on opsonized zymosan (Op.-zym.)-elicited O2.- production by differentiated HL-60 cells in a narrow range of concentrations but does not induce any O2.- production by itself. Okadaic acid magnified the O2.- production 2.5-fold at 1.0 microM, while it inhibited it at 2.0 microM or higher concentrations. This effect of OA did not correspond to the changes in the expression of surface receptors (CD11b/CD18, CR3) for Op.-zym., because they were weakly down-regulated by OA at any concentration. Two-dimensional gel electrophoresis revealed that in the absence of OA, Op.-zym. induced rapid dephosphorylation of a cytosolic 21K protein with a very slight increase in phosphorylation of membranous p47phox, which is one of the cytosolic factors required for respiratory burst. In the presence of a stimulatory concentration (1.0 microM) of OA, the Op.-zym.-caused dephosphorylation of the 21K protein was still observed and the phosphorylation of p47phox was enhanced. In the presence of an inhibitory concentration (2.0 or 5.0 microM) of OA, the Op.-zym.-induced dephosphorylation of the 21K protein was strongly inhibited while p47phox was heavily phosphorylated. Acid hydrolysis of the 21K phosphoprotein yielded only phosphoserine as a phosphoamino acid. Furthermore, at least part of the 21K protein seemed to be associated with p67phox and p47phox, because it was co-immunoprecipitated with those cytosolic factors. These results suggest that a cytosolic 21K protein plays an important role in respiratory burst through dephosphorylation by a phosphoserine phosphatase, and that the dephosphorylated 21K protein may work synergistically with the phosphorylated p47phox on the pathway for activation of the respiratory burst oxidase.

FTIR spectral study of intramolecular hydrogen bonding in E-type of 15-keto-prostaglandins in dilute CCl4 solution: structure-activity relationships

FTIR spectra of 15-keto-prostaglandin (PG)E2 (1), 15-keto-PGE1 (2), 13,14-dihydro-15-keto-PGE2 (3), 13,14-dihydro-15-keto-PGE1 (4), and their related compounds were measured in dilute tetrachloromethane solution in order to examine the structure-activity relationships for the 5,6-cis-double bond of the alpha-side chain and the 13,14-trans-double bond and the 15-hydroxyl group of the omega-side chain in PGE2, PGD2, and PGF2 alpha. The spectra were subjected to curve analysis to separate overlapping absorption bands. For compounds 1-4, an intramolecular hydrogen bond involving a 15-membered ring similar to that observed for PGE2, PGD2, and PGF2 alpha was found between the carboxyl and 15-carbonyl groups. The percentages (rho) of the intramolecular hydrogen-bonded molecules with the 15-membered rings in 1-4 and PGE2 were compared with the known binding activities of PGs for various PG receptors, and we found that these activities decrease as the rho values decrease. These results strongly supported our hypothesis that, in PGs, the conformation with the 15-membered ring formed by the intramolecular hydrogen bonds between the carboxyl group of the alpha-side chain and the 15-hydroxyl group of the omega-side chain is a precursory conformation of the active one.

Regulation of insulin-like growth factor I receptors on vascular smooth muscle cells by growth factors and phorbol esters

Insulin-like growth factor I (IGF I) is an important mitogen for vascular smooth muscle cells. To characterize regulation of vascular IGF I receptors, we performed radioligand displacement experiments using rat aortic smooth muscle cells (RASMs). Serum deprivation for 48 hours caused a 40% decrease in IGF I receptor number. Exposure of quiescent RASMs to platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), or angiotensin II (Ang II) caused a 1.5-2.0-fold increase in IGF I receptors per cell. After FGF exposure, there was a marked increase in the mitogenic response to IGF I. IGF I downregulated its receptors in the presence of platelet-poor plasma. Stimulation of protein kinase C (PKC) by exposure of quiescent RASMs to phorbol 12-myristate 13-acetate caused a biphasic response in IGF I binding; there was a 42% decrease in receptor number at 45 minutes and a 238% increase at 24 hours. To determine the role of PKC in growth factor-induced regulation of IGF I receptors, we downregulated PKC by exposing RASMs to phorbol 12,13-dibutyrate (PDBu) for 48 hours. PDGF- and FGF- but not Ang II-mediated upregulation of IGF I receptors was completely inhibited in PDBu-treated cells. Thus, acute PKC activation by phorbol esters inhibits IGF I binding, whereas chronic PKC activation increases IGF I binding. PDGF and FGF but not Ang II regulate vascular IGF I receptors through a PKC-dependent pathway. These data provide new insights into the regulation of vascular smooth muscle cell IGF I receptors in vitro and are of potential importance in characterizing vascular proliferative responses in vivo.