Thromboxane A(2) receptor mediated activation of the mitogen activated protein kinase cascades in human uterine smooth muscle cells

Thromboxane A receptor-mediated cell proliferation, survival and gene expression in oligodendrocytes

Thromboxane A(2) receptors (TP) were previously localized to discrete regions in the rat brain on myelinated fiber tracts and oligodendrocytes (OLGs). The present studies extended these findings and investigated the effects of TP signaling on cell proliferation, survival, and gene expression in OLG progenitor cells (OPCs) and OLGs. It was found that the TP agonist, U46619 stimulated the proliferation of OPCs and promoted the survival of mature OLGs. Examination of the early gene expression events involved in OPC proliferation, revealed that c-fos expression was substantially increased by U46619 stimulation. Treatment of OPCs or OLGs with U46619 caused activation of the mitogen-activated protein kinases (MAPK) ERK 1/2. In OPCs this activation was blocked by inhibition of src. However, in OLGs this phosphorylation was not only blocked by inhibition of src but also by inhibition of protein kinase C (PKC). Furthermore, U46619 was found to increase CREB phosphorylation in both OPCs and OLGs. Similar to ERK 1/2 activation, there was a divergence in the mechanism of the TP-mediated CREB response for each cell type. Specifically, U46619 activation was attenuated by src and protein kinase A (PKA) inhibition in OPCs, whereas in OLGs this effect was blocked by inhibition of src, PKA as well as by inhibition of PKC. Collectively, these results provide the first demonstration that TP-activated nuclear signaling events are involved in the proliferation of OPCs, the survival of mature OLGs, and the stimulation of gene expression.

Thromboxane Prostanoid Receptor Signals Through GiProtein to Rapidly Activate Extracellular Signal–Regulated Kinase in Human Airways

We showed previously that activation of the thromboxane prostanoid (TP) receptor causes human airway smooth muscle (HASM) cells to proliferate, suggesting a role in airway remodeling. This study aimed at determining the molecular mechanisms underlying this mitogenic action. We found that the MEK inhibitor PD98059 significantly affected agonist-induced DNA synthesis of HASM cells, which suggests that extracellular signal-regulated kinases (ERK) are involved. ERK activation by the agonist U46619 was rapid, sensitive to pertussis toxin, and significantly abrogated by the tyrosine kinase inhibitors genistein and PP1. Stimulation of the TP receptor was also found to translocate phosphorylated ERK into the nucleus. TP receptor was found to activate Ras, as demonstrated by inhibition of ERK activation and DNA synthesis by Clostridium sordellii lethal toxin, and by the ability of U46619 to increase RasGTP. Finally, [(3)H]thymidine incorporation and ERK phosphorylation were also affected by prior treatment with protein kinase C inhibitor GF109203X, although to different extents. In conclusion, in HASM cells TP receptor, predominantly coupled to G(i/o) proteins, activates the Ras/ERK pathway to induce mitogenesis, probably with the involvement of nonreceptor tyrosine kinases and protein kinase C.

Les isoprostanes : un rôle physiopathologique potentiel en pathologie vasculaire

PURPOSE

Isoprostanes are arachidonic acid metabolites produced by a free-radical-dependent mechanism.

CURRENT KNOWLEDGE AND KEY POINTS

The 15-series F2-isoprostanes and some 15-series E2-isoprostanes mediate vasoconstriction in different vascular beds and species. Furthermore, 15-F2t-IsoP induces smooth muscle cells mitogenesis and monocyte adhesion to endothelial cells. In clinical studies, 15-F2t-IsoP levels are increased in vascular disorders involving atherosclerosis, ischemia-reperfusion and inflammation.

FUTURE PROSPECTS AND PROJECTS

Whether the effects observed in vitro are observed consistently in vivo at physiological concentrations and whether these effects contribute to pathological states in vivo is still debated.

Isoprostanes: an emerging role in vascular physiology and disease?

Among the isoprostanes, the 15-series F2-isoprostanes and 15-E2t-IsoP mediate vasoconstriction in different vascular beds and species. In addition, 15-F2t-IsoP induces smooth muscle cells mitogenesis and monocyte adhesion to endothelial cells. In clinical studies, 15-F2t-IsoP levels are increased in some vascular disorders involving atherosclerosis, ischemia-reperfusion and inflammation. Whether the same effects observed in vitro are observed consistently in vivo at physiological concentrations and whether these effects contribute to pathological states in vivo is still debated.

The α, but Not the β, Isoform of the Human Thromboxane A2 Receptor Is a Target for Nitric Oxide-mediated Desensitization

In humans, thromboxane A2 signals through two thromboxane A2 receptor (TP) isoforms termed TP alpha and TP beta. Signaling by TP alpha, but not TP beta, is subject to prostacyclin-induced desensitization mediated by direct protein kinase (PK) A phosphorylation where Ser329 represents the phosphotarget (Walsh, M. T., Foley, J. F., and Kinsella, B. T. (2000) J. Biol. Chem. 275, 20412-20423). In the current study, the effect of the vasodilator nitric oxide (NO) on intracellular signaling by the TP isoforms was investigated. The NO donor 3-morpholinosydnonimine, HCl (SIN-1) and 8-bromo-guanosine 3',5'-cyclic monophosphate (8-Br-cGMP) functionally desensitized U46619-mediated calcium mobilization and inositol 1,4,5-trisphosphate generation by TP alpha whereas signaling by TP beta was unaffected by either agent. NO-mediated desensitization of TP alpha signaling occurred through a PKG-dependent, PKA- and PKC-independent mechanism. TP alpha, but not TP beta, was efficiently phosphorylated by PKG in vitro and underwent NO/PKG-mediated phosphorylation in whole cells. Deletion/site-directed mutagenesis and metabolic labeling studies identified Ser331 as the target residue of NO-induced PKG phosphorylation of TP alpha. Although TP alpha S331A was insensitive to NO/PKG-desensitization, similar to wild type TP alpha its signaling was fully desensitized by the prostacyclin receptor agonist cicaprost occurring through a PKA-dependent mechanism. Conversely, signaling by TP alpha S329A was insensitive to cicaprost stimulation whereas it was fully desensitized by NO/PKG signaling. In conclusion, TP alpha undergoes both NO- and prostacyclin-mediated desensitization that occur through entirely independent mechanisms involving direct PKG phosphorylation of Ser331, in response to NO, and PKA phosphorylation of Ser329, in response to prostacyclin, within the unique carboxyl-terminal tail domain of TP alpha. On the other hand, signaling by TP beta is unaffected by either NO or prostacyclin.

Involvement of Rho-kinase in contraction of guinea-pig aorta induced by prostanoid EP3 receptor agonists

1. The mechanism of contraction of guinea-pig isolated aorta induced by the prostanoid EP(3) receptor agonist sulprostone (0.1-300 nM) has been investigated. In 60% of the experiments, the sulprostone log concentration-response curve (maximum=15-40% of 100 nM U-46619 response; low-responders) was unaffected by the removal of extracellular Ca(2+), blockade of L-type Ca(2+) channels with nifedipine and depletion of internal Ca(2+) stores. In the remaining preparations (35-65% of 100 nM U-46619 response; high-responders), contractions to higher sulprostone concentrations showed a nifedipine-sensitive component, which was enhanced by charybdotoxin. 2. In Ca(2+)-free Krebs solution, established contractions to 300 nM sulprostone were abolished by the Rho-kinase inhibitors H-1152, Y-27632 and HA-1077 (IC(50) values=190, 770 and 2030 nM). The PKA/Rho-kinase inhibitor H-89 (10 nM-10 micro M) caused enhancement progressing to inhibition. The selective PKC inhibitor Ro 32-0432 (3 micro M) had no effect, while staurosporine, recently shown to be a potent Rho-kinase inhibitor, abolished sulprostone responses (IC(50) approximately 47 nM), but its action was slow. The MAP kinase inhibitors SB 202190, SB 203580 and PD 80958 produced little inhibition. 3. In normal Krebs solution, H-1152 and Y-27632 abolished established contractions to 300 nM sulprostone and 1 micro M phenylephrine, and partially inhibited 10 micro M phenylephrine and 50 mM K(+) responses. 4. The results are discussed in relation to the reported potencies of the protein kinase inhibitors in enzyme assays. Activation of the Rho-kinase pathway appears to be a primary mechanism of contraction induced by EP(3) receptor agonists in guinea-pig aorta.

Epidermal-growth-factor receptor and metalloproteinases mediate thromboxane A2-dependent extracellular-signal-regulated kinase activation

The signalling pathways that link G-protein-coupled receptors to mitogen-activated protein kinases involve receptor and non-receptor tyrosine kinases and protein kinase C (PKC). We explored the pathways that are implicated in the thromboxane (TX) A(2)-dependent activation of extracellular-signal-regulated protein kinase (ERK) and the role of the two TX receptor (TP) isoforms, TP alpha and TP beta. ERK activation by IBOP, a TX analogue, was dependent on epidermal-growth-factor receptor (EGFR) in TP alpha- or TP beta-transfected cells and in human aortic smooth muscle cells (hASMCs), since AG1478, a selective inhibitor of tyrosine phosphorylation of the EGFR, strongly blocked ERK and EGFR phosphorylation. In addition, EGFR transactivation leading to ERK activation involved matrix metalloproteinases (MMPs), since BB2516, an inhibitor of MMP, decreased ERK and EGFR phosphorylation in TP alpha- or TP beta-transfected cells. Moreover, we showed that both isoforms activate ERK phosphorylation in an Src-kinase-dependent manner, whereas PKC was mainly implicated in ERK activation and EGFR phosphorylation by TP beta. In hASMCs, we showed that ERK activation depended on both pertussis-sensitive and -insensitive G alpha-proteins. We demonstrated further that EGFRs, PKC, Src kinase and MMPs are involved in ERK activation by TX. The results of the present study highlight a role for MMPs and PKC in EGFR transactivation triggered by the TPs and demonstrate this mechanism for the first time in primary cells, i.e. hASMCs.

Enhancement of alpha2-adrenoceptor-mediated vasoconstriction by the thromboxane-mimetic U46619 in the porcine isolated ear artery: role of the ERK-MAP kinase signal transduction cascade

1. Alpha(2)-adrenoceptor-mediated contractions in porcine blood vessels can be enhanced in the presence of the thromboxane-mimetic U46619, and forskolin. The aim of this study was to determine the role of U46619 in the enhanced contractions, and to determine whether signalling through the ERK-MAP kinase pathway is involved. 2. Responses to the alpha(2)-adrenoceptor agonist UK14304 (1 micro M) were increased from 22+/-3% of the response to 60 mM KCl to 68+/-12% (n=8, mean+/-s.e.m.) in the presence of a low concentration of U46619 (< 20% of the 60 mM KCl response). 3. Both the direct and the U46619-enhanced UK14304 responses were inhibited by 50 microM PD98059, an inhibitor of the ERK-MAP kinase pathway. UK14304-induced contractions were associated with an increase in ERK2 phosphorylation, indicating an increased activity. In the presence of U46619, there was an enhanced phosphorylation of ERK2. U46619 on its own had no effect on ERK phosphorylation. 4. Both the direct and enhanced UK14304 contractions were inhibited in the absence of extracellular calcium. These conditions also prevented the increase in ERK2 phosphorylation. This indicates a role for calcium influx in the enhanced contractions. 5. In conclusion, this study demonstrates that precontraction with the thromboxane-mimetic U46619 enhances alpha(2)-adrenoceptor-mediated vasoconstriction through the enhancement of the ERK-MAP kinase pathway, and influx of extracellular calcium.

Alpha 2 adrenoceptor-mediated vasoconstriction in porcine palmar lateral vein: role of phosphatidylinositol 3-kinase and EGF receptor transactivation

1 Alpha(2) adrenoceptors cause vasoconstriction in the porcine palmar lateral vein through a mechanism involving the ERK signal transduction cascade, calcium influx, and a Src tyrosine kinase. The aim of the present study was to determine if phosphatidylinositol 3-kinase (PI 3-kinase) and/or epidermal growth factor (EGF) receptor transactivation are also involved. 2 alpha(2) Adrenoceptor-mediated vasoconstriction and ERK2 activation in the porcine palmar lateral vein was inhibited in the presence of either the PI 3-kinase inhibitor LY294002, or the EGF receptor tyrosine kinase inhibitor AG1478 suggesting the involvement of both PI 3-kinase and EGF receptor transactivation. 3 Akt phosphorylation was increased in segments of porcine palmar lateral vein contracted with UK14304 indicating an increase in Akt activation. This is a further indication that PI 3-kinase is involved in alpha(2) adrenoceptor-mediated vasoconstriction. Akt activation was inhibited by the Src tyrosine kinase inhibitor PP2, and removal of extracellular calcium. 4 UK14304 (10 microM) stimulated an increase in intracellular calcium in segments of palmar lateral vein. This was inhibited by removal of extracellular calcium, but not by nifedipine suggesting the rise in calcium is due to influx of calcium through non-L type calcium channels. The increase in calcium was also inhibited by LY294002 indicating that PI 3-kinase is upstream of calcium influx. 5 These data indicate that alpha(2) adrenoceptor-mediated vasoconstriction in the porcine palmar lateral vein is dependent upon stimulation of PI 3-kinase, leading to an influx of calcium. This results in activation of the EGF receptor tyrosine kinase, and finally activation of ERK-MAP kinase.

Characterization of the 5' untranslated region of alpha and beta isoforms of the human thromboxane A2 receptor (TP). Differential promoter utilization by the TP isoforms

In humans, thromboxane (TX) A2 signals through two TXA2 receptor (TP) isoforms, TPalpha and TPbeta, that diverge within their carboxyl terminal cytoplasmic (C) tail regions and arise by differential splicing. The human TP gene contains three exons E1-E3; while E1 exclusively encodes 5' untranslated region (UTR) sequence, E2 and E3 represent the main coding exons. An additional noncoding exon, E1b was identified within intron 1. Additionally, the TP gene contains two promoters P1 and P2 located 5' of E1 and E1b, respectively. Herein, we investigated the molecular basis of the differential expression of the TP isoforms by characterizing the 5' UTR of the TP transcripts. While E1 and E1b were found associated with TP transcript(s), their expression was mutually exclusive. 5' rapid amplification of cDNA ends (5' RACE) established that the major transcription initiation (TI) sites were clustered between -115 and -92 within E1 and at -99 within E1b. While E1 and E1b sequences were identified on TPalpha transcript(s), neither existed on TPbeta transcript(s). More specifically, TPalpha and TPbeta transcripts diverged within E2 and the major TI sites for TPbeta transcripts mapped to -12/-15 therein. Through genetic reporter assays, a previously unrecognized promoter, termed P3, was identified on the TP gene located immediately 5' of -12. The proximity of P3 to the TI site of TPbeta suggests a role for P3 in the control of TPbeta expression and implies that TPalpha and TPbeta, in addition to being products of differential splicing, are under the transcriptional control of distinct promoters.

Regulation of extracellular signal-regulated kinase cascades by alpha- and beta-isoforms of the human thromboxane A(2) receptor

Thromboxane A(2) (TXA(2)) stimulates mitogenic growth of vascular smooth muscle. In humans, TXA(2) signals through two TXA(2) receptor (TP) isoforms, termed TPalpha and TPbeta. To investigate the mechanism of TXA(2)-mediated mitogenesis, regulation of extracellular signal-regulated kinase (ERK) signaling was examined in human embryonic kidney 293 cells stably overexpressing the individual TP isoforms. The TXA(2) mimetic 9,11-dideoxy-9alpha,11alpha-methano epoxy prostaglandin F(2alpha) (U46619) elicited concentration- and time-dependent activation of ERK1 and -2 through both TPs with maximal TPalpha- and TPbeta-mediated ERK activation observed after 10 and 5 min, respectively. U46619-mediated ERK activation was inhibited by the TP antagonist [1S-[1alpha,2beta-(5Z)-3beta,4alpha-]]-7-[3-[[2-(phenylamino)carbonyl]hydrazine] methyl]-7-oxabicyclo[-2,2,1-]hept-2yl]-5-heptenoic acid (SQ29,548), and by the mitogen-activated protein kinase kinase inhibitor 2'-amino-3'-methoxyflavone (PD 98059). Although ERK activation through TPalpha was dependent on 2-[1-(dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide (GF 109203X)-sensitive protein kinase (PK) Cs, ERK activation through TPbeta was only partially dependent on PKCs. ERK activation through both TPalpha and TPbeta was dependent on PKA and phosphoinositide 3-kinase (PI3K) class 1(A), but not class 1(B), and was modulated by Harvey-Ras, A-Raf, c-Raf, and Rap1B/B-Raf and also involved transactivation of the epidermal growth factor receptor. Additionally, PKB/Akt was activated through TPalpha and TPbeta in a PI3K-dependent manner. In conclusion, we have defined the key components of TXA(2)-mediated ERK signaling and have established that both TPalpha and TPbeta are involved. TXA(2)-mediated ERK activation through the TPs is a complex event involving PKC-, PKA-, and PI3K-dependent mechanisms in addition to transactivation of the EGF receptor. TPalpha and TPbeta mediate ERK activation through similar mechanisms, although the time frame for maximal ERK activation and PKC dependence differs.

Investigation of a functional requirement for isoprenylation by the human prostacyclin receptor

In the current study, we have established that the human (h) prostacyclin receptor (IP) is isoprenylated in whole cells. Through site directed mutagenesis and generation of the isoprenylation defective hIPSSLC, it was established that while isoprenylation of hIP does not influence ligand binding, it is obligatory for agonist activation of adenylyl cyclase and cAMP generation. Overexpression of GalphaS significantly augmented cAMP generation by the hIP but not by the hIPSSLC. Moreover, GalphaS co-immunoprecipitated with hIP following agonist activation but did not co-immunoprecipitate with hIPSSLC. Whereas hIP mediated concentration-dependent activation of phospholipase C (PLC); the extent of PLC activation by hIPSSLC was impaired compared to hIP. Co-expression of Galphaq significantly augmentated intracellular calcium mobilization by the hIP but not by hIPSSLC. Moreover, whereas Galphaq co-immunoprecipitated with hIP, it failed to co-immunoprecipitate with hIPSSLC. While both the hIP and hIPSSLC underwent agonist-induced internalization, the kinetics and extent of hIPSSLC internalization was impaired compared to hIP. Altering the CAAX motif of the hIP from a farnesyl (-CSLC) to a geranylgeranyl (-CSLL) isoprene acceptor, to generate hIPCSLL, did not affect ligand binding and yielded a receptor that exhibited identical signalling through both Gs- and Gq-coupled effectors to that of hIP. Thus, whereas isoprenylation of hIP does not influence ligand binding, it is functionally imperative in regulating post-receptor events including agonist-activation of adenylyl cyclase, for efficient activation of PLC and for receptor internalization. Though the nature of the isoprenoid attached to hIP does not act as a major determinant, the presence of an isoprenoid group, for example farnesyl or geranylgeranyl, is required for functional receptor-G protein interaction and coupling and for efficient agonist- induced receptor internalization.