Analysis of a porcine EP3-receptor: cloning, expression and signal transduction

GR 63799X, an EP3 receptor agonist, induced S phase arrest and 3T6 fibroblast growth inhibition

The importance of arachidonic acid metabolites on the control of cell growth, particularly those derived from cyclooxygenase pathway has long been recognized. Recently, we observed that prostaglandin E(2) (PGE(2)) interaction with EP(1) and EP(4) receptors is involved in serum-induced 3T6 fibroblast growth due to their effect at various levels of the cell cycle machinery. This study shows that prostanoid EP(3) receptor was expressed in 3T6 fibroblast. We studied the role of EP(3) receptor agonist GR 63799X in serum-induced 3T6 cell proliferation. This was concentration-dependent inhibit (IC(50) approximately 10 microM) to a complete inhibition without any cytotoxic or proapoptotic effect. The prostanoid EP(3) receptor agonist treatment decreased the G(0)/G(1) and G(2)/M populations whereas cells were accumulated in S phase. This arrest in S phase was associated with a decrease in cyclin B levels and the enhancement of p21 expression. Our data show that EP(3) agonist decreases cAMP levels in our experimental conditions. Interestingly, the S arrest caused by prostanoid EP(3) receptor agonist seems to be cAMP dependent, at least in part, because forskolin treatment allowed S-arrested cells to progress through cell cycle and consequently growth. Thus, our results suggest that PGE(2) EP(3) receptor interaction may be involved in serum-induced 3T6 fibroblast growth due to their effects on cAMP levels and on cell cycle machinery of the S phase.

Cardiospecific Overexpression of the Prostaglandin EP 3 Receptor Attenuates Ischemia-Induced Myocardial Injury

Background— The generation of prostaglandin E 2 (PGE 2 ) is significantly increased in acute myocardial ischemia and reperfusion. PGE 2 , in addition to other prostaglandins, protects the reperfused ischemic myocardium. It has been hypothesized that this cardioprotection is mediated by E-type prostaglandin receptors of the G i -coupled EP 3 subtype.

Methods and Results— We tested this hypothesis by generating transgenic (tg) mice with cardiospecific overexpression of the EP 3 receptor. According to ligand binding, a 40-fold overexpression of the EP 3 receptor was achieved in membranes prepared from tg hearts compared with wild-type (wt) littermates. In isolated cardiomyocytes from tg mice, the forskolin-induced rise in cAMP was markedly attenuated, indicating coupling of the overexpressed EP 3 receptor to inhibitory G proteins (G i ) with constitutive receptor activity. There was no evidence for EP 3 receptor coupling to G q -mediated protein kinase C signaling. Isolated hearts from tg and wt mice were subjected to 60 minutes of no-flow ischemia and 45 minutes of reperfusion. In tg hearts, ischemic contracture was markedly delayed compared with wt hearts, and the ischemia-induced increase in left ventricular end-diastolic pressure was reduced by 55%. Creatine kinase and lactate dehydrogenase release was significantly decreased by 85% and 73%, respectively, compared with wt hearts.

Conclusions— Constitutive prostaglandin EP 3 receptor signaling exerts a protective effect on cardiomyocytes, which is probably G i mediated and results in a remarkable attenuation of myocardial injury during ischemia and reperfusion. Cardioprotective actions of E-type prostaglandins may be mediated by this receptor subtype.

Activation of IP and EP(3) receptors alters cAMP-dependent cell migration

Migration of vascular smooth cells from the media to the intima essentially contributes to neointima formation after percutaneous transluminal angioplasty and stent implantation. The stable prostacyclin mimetic iloprost has been shown to inhibit neointima formation in experimental restenosis, but it is currently unknown whether this may be caused by an antimigratory effect. Hence, the present study analyses (i) the influence of G(s)-coupled prostacyclin (IP) receptors on cell migration and (ii) verifies whether EP(3) receptors with opposite (i.e., G(i)) coupling may conversely stimulate cell migration. In a modified Boyden chamber model, it was shown that iloprost dose-dependently inhibits the migration of primary human arterial smooth muscle cells, which constitutively express the IP receptor. On the other hand, human arterial smooth muscle cell migration was stimulated by the EP(3) receptor agonist M&B 28.767. To independently study the effects of these receptors, IP or EP(3) receptors were stably overexpressed in chinese hamster ovary cells (CHO-IP and CHO-EP(3)). Chemotaxis of CHO cells transfected with G(s)-coupled IP receptors was concentration-dependently inhibited by iloprost (2-100 nM), while there was no effect of iloprost on mock-transfected CHO. By contrast, CHO-cells that overexpressed EP(3) receptors showed a significant, concentration dependent (1-100 nM) increase of cell migration in presence of the selective EP(3) agonist M&B 28.767. It is concluded that the prostacyclin mimetic iloprost inhibits vascular cell migration, which probably depends on a G(s)-mediated increase of intracellular cAMP. EP(3) receptors conversely stimulate CHO migration.

Precise mapping of breakpoints in conserved synteny between human chromosome 1 and pig chromosomes 4, 6 and 9

Previous comparative mapping suggested that at least five pig chromosomes (Sscr4, 6, 9, 10 and 14) share homology with human chromosome 1 (Hsap1). A significant quantitative trait loci (QTL) for fat deposition has been identified on Sscr4 that appears to be near the junction region between Sscr4 and Sscr9 relative to Hsap1. It is of interest to define the boundaries of conserved synteny between pig chromosomes and Hsap1 to use human map information to identify putative comparative positional candidates for this QTL. Eleven genes, including Janus kinase 1 (JAK1), Prostaglandin E receptor3 (PTGER3), urate oxidase (UOX), coagulation factor 3 (F3), vascular cell adhesion molecule 1 (VCAM1), ribosomal protein L5 (RPL5), POU domain, class 2, transcription factor 1 (POU2F1), coagulation factor 5 (F5), Prostaglandin endoperoxide synthase-2 (PTGS2), myosin binding protein H (MYBPH) and Antithrombin III (SERPINC1), were selected to refine the boundaries of the blocks of conserved synteny between Hsap1 and pig chromosomes. Pig sequence tagged sites (STSs) were developed and used to physically map these 11 genes using a somatic cell hybrid panel. Eight loci have been mapped by using fluorescent in situ hybridization (FISH) to improve map resolution. Heterologous FISH was used to refine the location of VCAM1 on human chromosomes. In addition, human yeast artificial chromosomes (YACs) were mapped by heterologous FISH on pig metaphases to refine the boundaries of the regions of homology between Sscr4 and Sscr9 on Hsap1. Results from this study suggest the precise break in conserved synteny on Hsap1 corresponding to the Sscr4/6 and Sscr4/9 transitions are most likely on the Hsap1p22 and Hsap1q24-25 regions, respectively. Further, our data predict that Hsap1q21-24 is a candidate region for the backfat QTL localized to Sscr4.

The cyclooxygenase system participates in functional mdr1b overexpression in primary rat hepatocyte cultures

Overexpression of mdr1-type P-glycoproteins (P-gps) is thought to contribute to primary chemotherapy resistance of untreated hepatocellular carcinoma. However, mechanisms of endogenous multidrug resistance 1 (mdr1) gene activation still remain unclear. Because recent studies have demonstrated overexpression of cyclooxygenase-2 (COX-2) in hepatocytes during early stages of hepatocarcinogenesis, we investigated whether the COX system, which catalyzes the rate-limiting step in prostaglandin synthesis, participates in mdr1 gene regulation. In the present study, primary rat hepatocyte cultures, exhibiting time-dependent mdr1b overexpression, demonstrated basal COX-2 and COX-1 mRNA expression and liberation of prostaglandin E(2) (PGE(2)), indicative of an active COX-dependent arachidonic acid metabolism. PGE(2) accumulation in culture supernatants was further enhanced by arachidonic acid (1mumol/L) and epidermal growth factor (EGF) (16 nmol/L). PGE(2) and prostaglandin F(2alpha) (PGF(2)alpha) (3-6mug/mL), added directly to the culture medium, significantly up-regulated intrinsic mdr1b mRNA overexpression and mdr1-dependent transport activity. Up-regulation was maximal after 3 days of culture. Like prostaglandins, the COX substrate, arachidonic acid, also induced mdr1b gene expression. Apart from this, structurally different COX inhibitors (indomethacin, meloxicam, NS-398) mediated significant inhibition of time-dependent and EGF-induced mdr1b mRNA overexpression, resulting in enhanced intracellular accumulation of the mdr1 substrate, rhodamine 123 (Rho123). Thus, the present data support the conclusion that the release of prostaglandins through activation of the COX system participates in endogenous mdr1b gene regulation. COX-2 inhibition might constitute a new strategy to counteract primary mdr1-dependent chemotherapy resistance.

Prostanoid receptors: subtypes and signaling

Cyclooxygenases metabolize arachidonate to five primary prostanoids: PGE(2), PGF(2 alpha), PGI(2), TxA(2), and PGD(2). These autacrine lipid mediators interact with specific members of a family of distinct G-protein-coupled prostanoid receptors, designated EP, FP, IP, TP, and DP, respectively. Each of these receptors has been cloned, expressed, and characterized. This family of eight prostanoid receptor complementary DNAs encodes seven transmembrane proteins which are typical of G-protein-coupled receptors and these receptors are distinguished by their ligand-binding profiles and the signal transduction pathways activated on ligand binding. Ligand-binding selectivity of these receptors is determined by both the transmembrane sequences and amino acid residues in the putative extracellular-loop regions. The selectivity of interaction between the receptors and G proteins appears to be mediated at least in part by the C-terminal tail region. Each of the EP(1), EP(3), FP, and TP receptors has alternative splice variants described that alter the coding sequence in the C-terminal intracellular tail region. The C-terminal variants modulate signal transduction, phosphorylation, and desensitization of these receptors, as well as altering agonist-independent constitutive activity.

Reduction of infarct size by selective stimulation of prostaglandin EP(3)receptors in the reperfused ischemic pig heart

We have previously identified prostaglandin EP(3)receptors in left ventricular myocardium. To assess the potential contribution of this receptor subtype to the anti-ischemic properties of E-type prostaglandins (i.e. PGE(1)), two groups of anesthetized open-chest minipigs were subjected to LAD occlusion (1 h) and reperfusion (3 h). In one group, the selective EP(3)receptor agonist M&B 28.767 (2 pmol/kgxmin) was infused into the LAD from 20 min before ischemia until the end of reperfusion. The other group received vehicle. M&B 28.767 did not alter the systemic hemodynamics, but significantly reduced infarct size (tetrazolium staining) and creatine kinase release by 53% and 48%, respectively. Ischemia-induced ventricular arrhythmias were mostly reduced. Further experiments analysed the effects of EP(3)receptor stimulation on normoxic myocardium. PGE(1), an unselective agonist to all EP receptor subtypes, as well as M&B 28.767 (2 pmol/kgxmin of each into the LAD) reduced the action potential duration (epicardial monophasic electrodes) and almost prevented the inotropic response to intravenous isoprenaline. This dual response is consistent with the EP(3)receptor coupling to an inhibitory G protein. This was confirmed in separate experiments with stable Chinese hamster ovary cell transfectants expressing the porcine EP(3)receptor, where M&B 28.767 inhibited the forskolin-induced increase in cAMP in a concentration-dependent manner. It is concluded that the protection of reperfused ischemic myocardium by E-type prostaglandins is mediated by EP(3)receptors, which seems to involve a combined activation of repolarizing membrane currents and an inhibition of deleterious effects caused by ischemia-induced catecholamine release.

Preservation of Gi coupling of a chimeric EP3/I-type prostaglandin (IP) receptor

For the EP3 subtype of prostaglandin E receptors, different C-terminal splice variants are known, which are coupled to distinct heterotrimeric GTP-binding proteins (G-proteins). To test the hypothesis that the C-terminal domain is essential for the G-protein-coupling specificity of the EP3 receptor, we exchanged the carboxyl-terminal tail of a porcine Gi-coupled EP3 receptor isoform for the corresponding C-terminal part of a Gs-coupled prostaglandin receptor. The porcine EP3 receptor was truncated at a lysine (K350) residue at the end of the seventh transmembrane region, representing the splicing site of the different EP3 receptor isoforms. The wild-type C-terminus (37 amino acids) was substituted by the C-terminal tail (89 amino acids) of the human I-type prostaglandin receptor (hIP-R). The G-protein coupling of the resulting chimeric receptor protein was studied in transfected Chinese hamster ovary (CHO) cells. Stimulation of the chimeric receptor protein with the EP3 receptor-specific agonist M&B 28.767 did not increase adenosine 3',5'-cyclic monophosphate (cAMP) formation but did reduce the forskolin-stimulated cAMP formation, indicating Gi coupling. Furthermore, the chimeric receptor did not show constitutive activity as demonstrated for the C-terminally truncated EP3 receptor. Thus, coupling specificity of the EP3 receptor is not exclusively mediated by the carboxyl-terminal tail, and constitutive activity of a C-terminally truncated EP3 receptor can be suppressed by the hIP-R C-terminus.